Engineering Aspects of Industrial Liquid-Phase Air Oxidation of

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Ind. Eng. Chem. Res. 2000, 39, 3958-3997

REVIEWS Engineering Aspects of Industrial Liquid-Phase Air Oxidation of Hydrocarbons Akkihebbal Krishnamurthy Suresh Department of Chemical Engineering, IIT Bombay, Powai, Mumbai 400 076, India

Man Mohan Sharma† and Tamarapu Sridhar* Department of Chemical Engineering, Monash University, Clayton, Victoria 3168, Australia

Liquid-phase air oxidation of hydrocarbons, notably p-xylene, cumene, ethylbenzene/isobutane, cyclohexane, and n-butane, is of great scientific, technological, and commercial importance. This state-of-the-art paper covers the chemistry and engineering science aspects of these reactions. The role of uncatalyzed reactions and metal ion and mixed metal ion catalysts with bromide activation is discussed. An analysis is presented for the role of mass transfer in influencing the rate of reaction and selectivity for the desired product. Different types of reactors that are used, notably bubble-column reactors and mechanically agitated reactors, are analyzed, and a simple basis is provided for selection of reactors. Some emerging oxidation systems, notably oxidation of cycloalkenes (cyclohexene/cyclooctene/cyclododecene) and oxidation of isobutane under supercritical conditions, are presented. New strategies for conducting air oxidations, such as in biphasic systems (including fluorous biphasic systems), biocatalysis, photocatalysis, etc., are emerging and illustrate the considerable tailoring of the reaction microenvironment that is becoming possible. In some cases, it may be possible to manipulate chemo-, regio-, and enantioselectivity in these reactions. Contents 1. 2. 3. 4.

Introduction Scope and Structure of the Review Historical Development A Survey of Industrial Hydrocarbon Oxidations 4.1. p-Xylene and Other MC Oxidations 4.2. Oxidation of Cyclohexane, Other Saturated Hydrocarbons, and Terpenes 4.3. LPO-Based Routes to Phenol and Benzylic Alcohols 4.4. The Oxirane Process: Oxidation of iso-Butane and Ethylbenzene 4.5. Acetic Acid from Paraffin Oxidation 5. Mechanism of Hydrocarbon Oxidation 5.1. Initiation 5.2. Propagation 5.3. Termination 5.4. Degenerate Chain Branching 5.5. Overall Kinetic Features Based on the Mechanism 5.6. Catalysis of Organic Oxidations 5.7. Co-oxidations 6. Chemistry of Selected Oxidations

3959 3959 3960 3963 3964 3965

3967 3967 3968 3969 3969 3969 3970 3970 3971 3971 3972 3973

6.1. p-Xylene and Other MC Oxidations 6.2. Oxidation of Cyclohexane 6.3. Oxidation of Cumene 6.4. Oxidation of Isobutane 6.5. Oxidation of Cycloalkenes 6.6. Oxidation of Vinyl Cyclohexene and Vinyl Cyclohexane 6.7. Miscellaneous Oxidations 7. Kinetics of Hydrocarbon Oxidation 7.1. Laboratory Reactors 7.2. Kinetic Models from Laboratory Studies 8. Processing Options 8.1. The Case for Liquid-Phase Air Oxidation 8.2. Reactor Configurations and Materials 9. Role of Mass Transfer in Liquid-Phase Oxidations 9.1. Mass-Transfer Rates at Elevated Temperatures and Pressures and under Actual Oxidation Conditions 9.2. Influence of Mass Transfer on Liquid-Phase Oxidations 10. Rate Oscillations and Other Nonlinear Phenomena

10.1021/ie0002733 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/11/2000

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11. Safety Issues in Organic Oxidations 12. New Developments in Organic Oxidations 12.1. Biphasic Mode of Operation 12.2. The Role of Ultrasound in Oxidation Reactions 12.3. Oxidation in Supercritical Media 12.4. Photochemical Activation of Oxidation Reactions 12.5. Enzyme-Catalyzed Reactions 12.6. Use of O2 + H2 and O2 + CO as Oxidants 12.7. Catalyst Developments 12.8. Stereospecificity in Organic Oxidations 13. Conclusions References

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is, therefore, to review these developments and analyze their implications for future industrial practice in this area.

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2. Scope and Structure of the Review

3989 3990 3990 3991 3991 3992 3992 3993

1. Introduction Processes involving the oxidation of hydrocarbons in the liquid phase, using air or oxygen, are of great importance to industrialized economies because of their role in converting petroleum hydrocarbon feedstocks such as alkanes, olefins, and aromatics into industrial organic chemicals important in the polymer and petrochemical industries. Some notable examples are the oxidations of p-xylene to terephthalic acid and dimethyl terephthalate, of cyclohexane to cyclohexyl hydroperoxide and cyclohexanol/cyclohexanone, of cumene to cumene hydroperoxide, and of iso-butane to tert-butyl hydroperoxide and tert-butyl alcohol. Some of the major inefficiencies in the production of such chemicals can often be traced to the operation of the reactor. Oxidative transformations of functional groups are also basic to organic chemistry and are used extensively in the laboratory and industrial synthesis of a variety of fine organic chemicals. It is therefore not surprising that this class of reactions has spawned much study and research. In recent times, increasing environmental concerns have also acted as a significant driving force of research. The mechanisms that operate in organic oxidations and the theoretical underpinnings of such mechanisms have thus been studied in detail, and extensive treatises on the chemistry of oxidation processes are available.1-3 What is perhaps surprising is that, barring a few exceptions,4,5 an exhaustive review of the engineering aspects of such reactions is lacking. Information about these aspects is still largely to be found scattered in various places, amidst discussions of specific processes. The main objective of the present paper is therefore to bring together and discuss, in a unified manner, the literature on hydrocarbon oxidations, with a strong emphasis on the engineering aspects of relevance to industrial practice. The last few decades have seen several advances in this area, much of it only available in patents. A second objective of this paper * Author to whom correspondence should be addressed. Tel: 613 9905 3427. Fax: 613 9905 9649. E-mail: tam.sridhar@ eng.monash.edu.au. † Permanent address: 502, “Saurabh”, Plot 39, Swastik Park, Chembur, Munibai 400 071, India.

The review addresses hydrocarbon “autoxidations”, that is, oxidations in which molecular oxygen is activated via a free-radical chain process. These are also classified as “homolytic” processes,4 insofar as they originate from the homolytic breakage of a C-H bond. Thus, those processes (“heterolytic”) in which oxygen activation occurs by direct reaction with a reducing metal ion (the Wacker process is an example) are largely excluded from the scope of this review. As already stated, the emphasis is on the engineering aspects, and discussion of the mechanistic aspects is kept brief. Although the attempt is to study hydrocarbon oxidations in a unified manner, individual peculiarities are often important to industrial practice, and these are highlighted where relevant. Examples from various hydrocarbon oxidations are liberally used, but the commercially important oxidations form the center of attention. The commonality of mechanisms and industrial importance both make for an emphasis on processes involving the oxidation of secondary and tertiary carbon atoms. Within this perimeter, both catalyzed and uncatalyzed oxidations are discussed. Air has tended to be the preferred source of oxygen in the past for reasons of both safety and economics, and the focus here is naturally on air oxidations. However, some recent developments on the use of enriched air or even pure oxygen are included as they could be of interest to future designs. Even though gas- and liquid-phase homolytic oxidations share common mechanisms, the reaction rate and selectivity are usually better in the case of liquid-phase oxidations. The reasons, at least partly, have to do with the higher density of the liquid phase. Hence, except in the case of lighter hydrocarbons for which very high pressures become necessary to maintain a liquid phase at reaction temperatures, oxidation in the liquid phase turns out to be the natural choice. Isobutane presents an interesting case for which a minor shift in reaction conditions can make the reaction occur under gas or liquid or supercritical conditions. This case is considered in some detail, but for the rest, it is the liquid-phase oxidations that form the focus here. Finally, emphasis has been placed on the developments of the last two decades or so, and references are made to earlier literature wherever relevant. Historically, a fundamental understanding of the nature and mechanisms of oxidation reactions has proceeded in parallel with industrial practice, as has been the case with so many other areas of human endeavor. We begin our discussion of hydrocarbon oxidations, with a brief historical account in section 3. This is followed, in section 4, by a broad survey of the field of industrial oxidations, which provides the backdrop for further discussion. Because oxidation of hydrocarbons ultimately gives carbon dioxide and water, it is clear that it is partial oxidations that are of greatest interest to the industry, and the success of an industrial oxidation process depends on proper control of the reaction to yield the desired intermediates with reasonable selectivities. The complex chemistry of hydrocarbon oxidations leads to a multiplicity of products even at fairly early stages in the conversion. An understanding of the chemistry is,

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therefore, essential if one is to appreciate the motivation behind much of the research and development in this area. Section 5 describes the salient aspects of the mechanisms that operate in hydrocarbon oxidations. Over the years, several catalysts have found application in such reactions for various reasons, and section 5 includes a discussion of the role of particular catalysts in modifying the reaction mechanism. Notwithstanding the unity of the mechanistic principles that underlie hydrocarbon oxidations, it is often the minor differences among them that are of interest from the point of view of industrial exploitation. Thus, whereas one expects from chemistry that the oxidation of an aldehyde (which occurs as an intermediate in oxidations leading to carboxylic acid) should be rapid, unoxidized aldehydic impurities are a major problem in terephthalic acid manufacture, because the product specifications for fiber-grade terephthalic acid are very stringent. Again, in the oxidation of cymene, the small extent of oxidation of the secondary C atom that occurs in parallel with the desired oxidation of the tertiary C atom has important implications for downstream processing. (The oxidation of ethylbenzene provides another similar example.) The section on general mechanisms is, therefore, followed by a section on the chemistry of commercially important oxidations. This section also treats some important emerging oxidation systems such as cycloalkenes and vinylcyclohexane/vinylcyclohexene. Given the complexity of the chemistry involved in hydrocarbon oxidation, and the fact that one often must estimate the kinetics from rate data in heterogeneous (gas-liquid) systems, the planning and execution of laboratory studies to establish true kinetics is usually a demanding task. Furthermore, the engineer must decide the level of detail at which he needs to establish the kinetics for the purpose at hand. Thus, the engineer is often forced to resort to an empirical approach to establish kinetics, with some basis from the known mechanisms. One therefore sees, in the literature, several lumped kinetic models for oxidation systems. These models and the issues involved in planning and executing kinetic investigations in this area are discussed in section 7. In many hydrocarbon oxidations, the desired intermediates have a tendency to undergo further reactions in the oxidizing medium. The limitations imposed by the chemistry on the selectivity to the desired intermediates that is achieved has often meant that conversions are kept low to minimize the formation of unwanted products. The need to achieve better selectivity at reasonable conversions has naturally been a major driving force for research. This, coupled with other issues such as safety and the need for control, presents a number of process options. Some of the issues involved and the processing options that emerge are considered in section 8. The high temperatures at which hydrocarbon oxidations are carried out and the need to maintain a liquid phase under these conditions results in high-pressure operation. Furthermore, the oxygen for the reaction must be supplied by a process of gas-liquid mass transfer. Thus, the oxidation reactors are usually highpressure gas-liquid contactors such as bubble columns and sparged, agitated reactors. Rational design of such equipment requires a knowledge, under conditions of pressure and temperature, of important mass-transferrelated quantities such as mass-tranfer coefficients, gas

holdup, and interfacial area. Available information on these aspects is summarized in section 9. The kinetics of gas-liquid reactions such as hydrocarbon oxidations are, in general, subject to masstranfer limitations. Under appropriate conditions, the rate as well as the selectivity of such reactions can be influenced by mass transfer. Theories of mass transfer with chemical reaction have been fairly well developed, but there are only a few instances of their application to oxidation reactions. These studies are summarized in section 9.2. The complexity of chemical mechanisms in organic oxidations leads to highly nonlinear kinetics, which, in turn, often result in complex dynamic behavior. Many features of such behavior have been documented in experimental studies. The available information on such aspects is summarized in section 10. Safety is clearly a major consideration in hydrocarbonair contact, both in the design of industrial processes and in the planning of experimental research. Apart from hazardous possibilities of gas-phase oxidation such as autoignition, the dangers of ignition from an external source also deserve attention. Additional considerations assume importance in the design and operation of experimental rigs, which are usually designed so as to provide flexibility in operation and choice of operating parameters. The relevant aspects of this issue are elaborated in section 11. The usual motivating factors such as a search for better rates and selectivities, together with the recent emphasis on cleaner and safer processes, has meant that new processes and processing alternatives are always on the horizon. Section 12 discusses some major new developments in the area of organic oxidations, which hold promise for further development. Alternative catalysis and heterogeneous strategies (whether for catalysis or for simultaneous separation of reaction intermediates) are among the important themes. Much of this progress has appeared in the patent literature. Salient conclusions on the state of our understanding of industrial liquid-phase oxidations are summarizcd in section 13. 3. Historical Development History shows that the major developments in hydrocarbon oxidations have most often been motivated by the need for appropriate feedstocks for the evergrowing polymer industry. In the following section, we seek to examine the development of the more practical aspects of technology, on one hand, and the evolution of theoretical insights, on the other, in this field. Although this historical account does not claim to be exhaustive, it does show the complementary manner in which advances along the two fronts have been made, and it serves to highlight areas in which understanding still lags behind practice. From early days, the functionalization of naturally occurring petroleum components through reaction with air was naturally seen as the simplest way to derive useful chemicals. The research of Semenov (and later Bolland and others) in the first half of the twentieth century clarified the concepts of chain reactions and put the theory of free-radical autoxidations on a firm basis.1,3 Industrial practice also developed alongside. Whereas early processes for the oxidative transformation of petroleum feedstocks involved vapor-phase oxidations (ethylene to ethylene oxide, for example), liquid-


phase oxidation processes began from the 1950s. The Wacker process for the conversion of terminal olefins to carbonyl compounds and the Hock process for the production of phenol from cumene (via the hydroperoxide) were commercialized during this period. The cumene-phenol process was developed, by Distillers Co. in the U.K. and Hercules Powder Co. in the U.S., from a reaction discovered by Hock and Lang during the war. (A parallel development by A. Farkas in the Wilmington research labs of Union Oil Co., which went unpublished because of the company’s interest in naphthenic hydroperoxides, is described by Farkas6.) At that time, the free-radical chemistry of such reactions had just been established. Several major contributions to that field, such as the concept of activation of a tertiary H atom by a phenyl ring (which leads to the specificity of oxidation in this process) and an understanding of the role of impurities, came during the process-development effort.7 Several developments of an engineering nature also played a part in the final commercialization, such as the improvement in rate and selectivity through staging of reactors and the imposition of a temperature profile (with the temperature decreasing as the hydroperoxide concentration increases) on the reactor cascade. The cumene-phenol process accounts for most of the installed capacity for phenol in the world today and is expected to maintain its dominant position as long as the coproduct acetone has a market.8 Homogeneous catalysis of liquid-phase oxidations has played a major role in the development of oxidation processes. The catalytic effect of transition metal ions such as cobalt on oxidations had been known for some time (for example by the Du Pont work on the oxidation of toluene9). Whereas catalysis is important to the industrial process, adventitious catalysis, say by the reactor wall material and impurities in the chemicals used, can be a major problem in the understanding and interpretation of reaction kinetics. Twigg7 describes how, in the kinetic studies leading to the process development for phenol, the most elaborate purification procedures for cumene had to be followed in order to avoid adventitious catalysis. The role of wall catalysis had been recognized in the Russian work on cyclohexane oxidation,10 but the need for passivating reactor surfaces in kinetic work seems to have been recognized and procedures developed in the work of Winkler and Hearne11 on isobutane oxidation. Wall catalysis remains a problem with the interpretation of many kinetic studies reported in the literature and is especially a problem in laboratory reactors with large surface-tovolume ratios. Systematic studies of the industrially important homogeneous catalysis of oxidation processes began in the 1950s.2 The discovery of the (appropriately named) “Mid-Century” catalysts and the subsequent development of the Mid-Century (MC) process for the oxidation of p-xylene to terephthalic acid belong to this period.12 The impetus for the discovery of the Mid-Century catalyst was provided by the need for a cheap source of aromatic acids for the industrial production of aromatic polyesters, in particular, poly(ethylene terephthalate) (PET). In searching for an alternative to esterification (Witten process of 1951) for overcoming the inhibiting effect of the first carboxyl group on further oxidation, Landau and co-workers at Scientific Design discovered the promoting effect of the bromide anion13 and immediately obtained qualitative improvements in yield

over the norm using cobalt catalyst alone. Standard Oil of Indiana (later Amoco) purchased the patent rights to the new catalyst and proceeded to develop the oxidation technology. Landau14 has recently given a fascinating account of this development. In the subsequent decades, many aspects of the action of MidCentury catalysts have been clarified, and the principle has been extended to over 200 other aromatic, alkylaromatic, and other systems.12 In particular, improvements in the technology for purification have today put Pure Terephthalic Acid (PTA) in a dominant position in the world market as the preferred raw material for PET fiber.8 The processes for the oxidation of cyclohexane, like those for the oxidation of p-xylene, were driven by developments in the synthetic fiber industry, as raw materials for the manufacture of Nylon-6 and Nylon6,6 (caprolactam, adipic acid, and hexamethylenediamine) are derived from this route. van de Moesdijk15 has given an interesting account of the development of competing processes for caprolactam up to the late 1970s. Caprolactam came into prominence in the late 1930s when I. G. Farben synthesized a spinnable polymer (Nylon-6 or Perlon) from it. Early processes (1940s) for the manufacture of caprolactam were based on the hydrogenation of phenol [Allied Signal (U.S.) and DSM are still users of this route]: phenol f cyclohexanone f cyclohexanone oxime f caprolactam. Caprolactam production expanded in Europe after the war, when the I. G. Farben process was freed from patent restrictions. In view of long-range forecasts for caprolactam, (former) Dutch State Mines (now DSM) started research on the cyclohexane oxidation route in the 1950s even though the company was operating phenol-based plants. By 1959, the company had the designs of a plant ready, and the DSM caprolactam processes have since been licensed widely by Stamicarbon, a subsidiary of DSM. Other companies in Europe and the U.S. (Inventa, BASF, Snia Viscosa, Du Pont, Rhone Poulenc, Nitrogen Works in Poland, etc.) have also been active in this area and have developed their own processes. Even prior to the advent of Perlon in Europe, Carothers at Du Pont had produced the condensation polymer of adipic acid and hexamethylenediamine, which he called “Nylon”. In the late 1930s, the knowhow for Perlon and Nylon were exchanged between Du Pont and I. G. Farben, and industrial development of both were undertaken. Du Pont’s original development of a commercial adipic acid process dates from 1937. The basis for the industrially applied two-step process, with the oxidation of cyclohexane to cyclohexanol and cyclohexanone as the first step, was laid sometime during the war.16 A single-step process from cyclohexane to adipic acid makes obvious economic sense, and several attempts at developing such a process have been made. Asahi17,18 and Gulf research19 seem to have arrived at a feasible process using activated cobalt catalyst in an acetic acid medium, but these processes have not been commercialized. Patent activity has, however, been rife in the area. PPC ventures have developed a commercial route directly from cyclohexane to adipic acid up to pilot scale (Fluor Daniel offers this process). Also, Twentyfirst century research corporation is very active in this area and has filed as many as 17 patents recently (for example, see Vassiliou et al.20 and Dassel et al.21). Today, practically the entire world output of adipic acid is via the liquid-phase oxidation (LPO) of cyclohexane

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by air, followed by the further oxidation of the products using nitric acid. Air oxidation for the second step has not found favor because of loss of selectivity and proliferation of secondary products.16 The oxidation of cyclohexane must be one of the least efficient of all major industrial chemical processes. Methods of improving the selectivity have naturally been the focus of research effort over the decades. A development that led to a technology and several process plants in the U.S., Japan, and Europe in its wake was the application, by Scientific Design, of Bashkirov’s method of using boric acid to convert the alcohol that forms from the hydroperoxide directly to the boric acid ester of the alcohol to prevent the overoxidation of the alcohol.22 Although this process is still in use by Bayer in cyclohexane oxidation, the boric acid variant of the oxidation process seems to have largely given way to other processes in the following decades, mainly because of the costs of handling the borate ester adducts and recycling the boric acid.23 Although the Flixborough disaster of 1974 opened up important questions of safety24 in cyclohexane oxidation processes, the route continues to occupy a dominant position today in the manufacture of the raw materials for nylon (both Nylon-6 and Nylon-6,6),3 and research interest in the process continues unabated (see, for example, Schuchardt et al.25). Recent developments in the selective hydrogenation of benzene to cyclohexene, however, hold the promise of a much more economical route to cyclohexanol and would seem to have the potential of displacing the direct oxidation of cyclohexane. Although this new route to adipic acid has been commercialized by Asahi, the process has not yet been applied outside Asahi. Yet another illustration of the link between polymer feedstocks and organic oxidation is the development of the oxirane process, in the late 1960s, for the oxidation of propylene to propylene oxide. This process, developed by Atlantic Richfield (ARCO), uses an interesting cooxidation principle to oxidize propylene in an indirect manner. Oxygen is incorporated into the propylene molecule by reaction with a hydroperoxide, which itself is produced by a liquid-phase air oxidation step from a conveniently oxidized substance such as isobutane or ethylbenzene. The features of co-oxidation are a direct result of the free-radical chemistry and had been studied in the 1950s.26 In fact, most oxidation systems, except at very low conversions, ought to be treated as cooxidation systems because of the participation of the products. Deliberate co-oxidation routes have also been attempted for the oxidation of p-xylene to terephthalic acid by using butane,19,27 acetaldehyde, and paraldehyde8 as co-oxidizing substances. However, the commercial success of such a process also depends on the market for the coproduct. The oxirane process accounted for more than half of the production of propylene oxide in the U.S. in 1991.8 ARCO is a world leader in this field. The functionalization of alkanes by treatment with oxygen has an obvious attraction for the industrial chemist and has been a recurring theme. The development of paraffin oxidation began around 1930 in Germany.8 A process that yields a small number of products is obviously desirable, but a multiplicity of products is obtained because of the comparable reactivity of all the methylene groups, and the high reactivity of the primary and secondary reaction products. A substantial selectivity to a mixture of secondary alcohols was

obtained by the use of boric acid in Bashkirov’s method. The process was of importance in Japan, the U.S., and the U.S.S.R. until the late 1970s8 and continues to operate in the C.I.S. The desired specificity could, however, be obtained in small molecules such as n- and isobutane. The Celanese LPO process for acetic acid from n-butane is still operative. The interest in isobutane oxidation in the context of the oxirane process has been mentioned earlier. The pioneering work of Winkler and Hearne11 on this reaction led to a number of patents.28,29 The conflicting demands of rate and selectivity was again the major problem, and efforts to solve this saw the application in the 1980s of the emerging area of reaction in supercritical media.30-32 Interest in isobutane oxidation has been rekindled in recent years33 because of the possibility of its development for the production of the fast-growing petrochemical, methyl tert-butyl ether (MTBE), apart from tert-butyl alcohol. The development of industrial processes naturally required that extensive engineering studies be carried out. Although not much of this was published in the open literature, mention must be made of the work of Steeman et al.,34 who reported the first comprehensive set of data on the oxidation of cyclohexane to the scientific world. These data demonstrated conclusively the importance of backmixing in the reactor. Over the years, reactor and process design aspects have received considerable attention.35-37 However, the models used in the early attempts were rather crude and did not pay adequate attention to the influence of mass transfer, and hence, process development usually involved extensive experimentation at different scales. Although an awareness of the issues involved in mass transport with chemical reaction is evident in the early Russian work on organic oxidations (for example, Berezin et al.10), the first attempt to explicitly acknowledge the role of mass transfer in organic oxidations was probably the work of Hobbs et al.,38 in which the possibility of the reaction becoming mass-tranfer-limited and the bulk becoming starved of oxygen was recognized. These authors made an attempt to explain certain experimental observations that could not be explained on the basis of considerations of chemistry alone by invoking the role of mass transfer, albeit in a qualitative manner. The possibility of reaction within the diffusion film was, however, not considered. Meanwhile, the theory of gasliquid reactions had made significant strides, starting with the work of Higbie in 1935, and hydrocarbon oxidations had been recognized as a major class of reactions of industrial significance falling within the scope of the theory.39 In the 1960s, van de Vusse40-42 applied the theory to complex reactions with kinetics of the type encountered in organic oxidations. However, although one can find studies delving into the question of mass-tranfer limitations in organic oxidations (summaries are available in Doraiswamy and Sharma43), it must be said that the subject has received relatively little attention in the literature and has remained largely inconclusive. One reason for this was probably that engineering descriptions of the kinetics (which were invariably of the “lumped” type) did not often account for features such as induction periods and autocatalysis. Another reason was the lack of data on the important mass-tranfer parameters in systems and under conditions of temperature and pressure which were of industrial interest. Attempts to bring hydrocarbon oxidations within the ambit of the theories of mass transfer with

Ind. Eng. Chem. Res., Vol. 39, No. 11, 2000 3963 Table 1. Major Chemical Processes Utilizing Hydrocarbon Oxidation1,8,55 product

capacitya 106 tpy

oxidation step

important processes

main application

purified terephthalic acid (PTA)

11.38 (1995)

p-xylene to terephthalic acid

Amoco Mid-Century

PET (fiber, film, resin)

dimethyl terephthalate (DMT) cumyl hydroperoxide (CHPO)

4.06 (1995)

Witten, BASF, DuPont

PET (fiber, film, resin)

6.5 (1998)

p-xylene to p-toluic acid and monomethyl ester of pTA to DMT cumene to cumyl hydroperoxide

Hock (BP, Kellogg, etc.)

phenol

benzoic acid

0.28 (1995)

toluene to benzoic acid

DSM, Dow

phenol and salts, esters of benzoic acid

-caprolactam

3.7 (1995)

cyclohexane to KA

Nylon-6 (Perlon)

adipic acid

2.2 (1993)

propylene oxide

4 (1993)

cyclohexane to cyclohexanone/ cyclohexanol (KA) and KA to adipic acid iso-butane to TBHP

BASF, Bayer, DuPont, DSM, Stamicarbon Scientific Des. BASF, Bayer, Dupont, Stamicarbon Scientific Des. oxirane

acetic acid

6.0b (1994)

ethylbenzene to hydroperoxide butane/naphtha

oxirane Celanese, BP, UCC

remarks applies also to other alkylaromatics such as m-xylene, pseudocumene, 2,6-dimethylnaphthalene two-step process or one-step process coproduct: acetone also applies to cresols, resorcinol/hydroquinone from cymenes, diisopropylbenzenes also employed in the Snia-Viscosa route to -caprolactam and Henkel route to TA

Nylon-6,6

K:A ratio depends on use of boric acid

propylene glycol, polyols styrene

coproduct: tert-butyl alcohol (gasoline additive) phenyl methyl carbinol (dehydrated to styrene) main route is via carbonylation of methanol

vinyl acetate, cellulose acetate, PTA, solvent, etc.

a Capacities shown are worldwide figures and refer to the year indicated in parentheses. b About 9% of the total production capacity shown is by the oxidation route.

chemical reaction therefore met with limited success. In the last two or three decades, several studies, at Monash University,44-50 Twente,51,52 and other places, have attempted to address these gaps, but much remains to be done. The nonlinear kinetics of organic oxidations, especially in combination with mass-transport influences, can, in principle, lead to a rich variety of dynamic behavior with possible consequences for the safe operation of industrial reactors among other things. Once again, while theoretical aspects have received much attention in this area (see, for example, the review by Gray and Scott53), and experimental studies do show evidences of complex behavior, quantitative application of the theories to hydrocarbon oxidations is still to come. The observations of Hobbs et al.35 on “critical conditions” in the oxidation of methyl ethyl ketone suggest hysteresis-type effects. Because such situations can lead to unstable operation in practice, these authors suggested that industrial reactors should be operated “with a mass-transfer-limited zone present”. A growing appreciation for the interaction between physical transport phenomena and chemical kinetics in organic oxidations has led to an examination of the possibility of using oxygen-rich gases for oxidation. Recent developments in air separation technology have also contributed to the interest in this area. The issues of safe design and operation of oxidation reactors, always of paramount importance in this field, are being reexamined in this context. A quantitative assessment of the costs and benefits for such a changeover to oxygen-rich gases does not yet seem possible without actual experimentation for most hydrocarbon oxidations, although some clues are available from the modeling

and simulation carried out on particular oxidations (see, for example, Suresh et al.,50 Cao et al.,54 and Roby and Kingsley55). 4. A Survey of Industrial Hydrocarbon Oxidations A broad survey of the commercially important processes employing liquid-phase hydrocarbon oxidations is presented in Table 1. The production volumes of the chemicals listed is impressive and serves to underline the importance of liquid-phase oxidations in industry. It is interesting to note the different roles of the oxidation step in these processes. In some of the processes, such as the Amoco process for the manufacture of PTA, the oxidation step leads directly to the product of interest. There are others, such as the process for caprolactam, in which oxidation is the step that produces a key intermediate that is then further processed to the product of interest. In the oxirane process, hydrocarbon oxidation provides a convenient carrier of oxygen for the selective oxidation of propylene to propylene oxide. A choice of hydrocarbons is therefore available (several have been suggested; see Weissermel and Arpe8), and the market for the coproduct determines which hydrocarbon is chosen in a given context. Although treatises on hydrocarbon oxidations underline the similarities that exist in the mechanisms that govern various organic oxidations, from an engineering point of view, it is the differences that exist between hydrocarbons that is often of interest; these differences call for innovative technological developments. Furthermore, depending on the place of the desired product in the reaction sequence (whether as a primary, secondary,

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Table 2. Some Important Process Parameters in the Commercially Important Oxidations1,8,55 reaction conditions hydrocarbon liquid

gas

desired product

temp °C

pressure bar

p-xylene

air

PTA

190-205 15-30

p-xylene

air

p-toluic acid pTA DMT

140-170 4-8

monomethyl air ester of pTA p-xylene + air and DMT monomethyl methanol ester of pTA cyclohexane air KA oil

125-165 8-15

cyclohexane

air

cyclohexanol

125-165 8-15

cumene

air

cumyl 130-140 5 hydroperoxide TBHP 120-140 35 EBHP 120-140 35

iso-butane air ethylbenzene air toluene

air

benzoic acid

butane

air

acetic acid

a

140-240 40 100-200 5-20

110-120 2-3 165 9

175 180

54 15-20

medium acetic acid (90-95%)

catalyst MC cata

conversion 95%

selectivity

reactor type

process

>90%

stirred Amoco reactors, process Ti/hastelloy lined Co and Mn Witten salts process pTSA or 85% Witten other acid process p-xylene 90% countercurr. Du Pont and column one-step rxn prod process Co and Mn 5-8% 80-85% stirred salts reactors Co and Mn 12-13% >90% stirred salts with reactors boric acid CHPO 30% 93-95% bubble initiator column initiator 25% 60% oxirane initiator 15-17% 87% bubble cap oxirane trays Co salts 90% Dow Co acetate 90% Snia viscosa caprolactam process MC cata 99% 96% Amoco product mix Co acetate 10-20 Huls acetic acid uncat UCC

MC catalyst consists of Co and Mn acetate, NH4Br, or tetrabromoethane.

or tertiary intermediate), the process needs tend to be very different. In the manufacture of terephthalic acid, the product of interest forms fairly late in the reaction sequence, and, considering its resistance to oxidation, it may in fact be considered as the end product. In a majority of situations however, the desired product is an intermediate susceptible to further oxidation, and selectivity considerations become extremely important. Table 2 provides some relevant details for the oxidations involved in the processes presented in Table 1. Choice of reaction conditions, as well as the conversion to which the reaction must be allowed to proceed, are often determined by the selectivity-coversion relationships, and significant differences between the various cases stand out. 4.1. p-Xylene and Other MC Oxidations In terms of tonnages, the conversion of p-xylene to PTA and dimethyl terephthalate (DMT) ranks as the highest among the oxidation processes. As raw material for conversion to PET fiber, the use of terephthalic acid lagged behind that of DMT in the past. Although direct esterification of terephthalic acid with ethylene glycol is chemically the simplest method for manufacturing PET, the difficulty of manufacturing terephthalic acid on a large scale at a purity sufficient for polycondensation has prevented its widespread use in the past. The presence of small amounts of 4-carboxybenzaldehyde, a partial oxidation product, would interfere with the polycondensation. The problem here is thus the presence of an intermediate, when the final product is desired. However, with improvements in the purification technology by high-pressure hydrogenation, which converts the aldehyde to the easily separable p-toluic acid, the dominance of terephthalic acid has been continuously

increasing. Thus, the share of PTA in the PET raw materials went up from 29% in 1976 to 55% in 1988. In 1995, the worldwide production of PTA was 11.38 million tonnes and that of DMT, 4.06 million tonnes. In 1998, the share of PTA has further increased. A fascinating account of the development of these technologies from early stages has recently been written by Landau.14 In the oxidation of p-xylene, the oxidation proceeds up to the stage of p-toluic acid (oxidation of one methyl group to -COOH), but for reasons that are still a matter of debate,3 the resulting carboxylic group seems to confer resistance to further oxidation. Technologically, two approaches have been found to circumvent the problem. In the older Witten method, the acid group is esterified with methanol, a procedure that allows oxidation to proceed. In practice, the esterification and the second stage of oxidation are carried out simultaneously. A single-step variant of the process is practiced by BASF, Montecatini, and Du Pont, using countercurrent contacting of the p-xylene (and oxidation products) with a stream of air and methanol. Whereas the two-step process gives a higher selectivity with respect to methanol (80% vs 60-70% in the single-step process8), the selectivity with respect to the hydrocarbon is better in the single-step process (Table 2). In either case, DMT is the product, which can be hydrolyzed to terephthalic acid or used directly in the manufacture of PET by transesterification. In the second method, used in the Amoco (MidCentury) process, the bromide in the catalyst promotes the reaction sufficiently to give reasonable rates of oxidation even beyond the p-toluic acid stage. This method uses less catalyst and provides higher rates12 than the first and is being increasingly preferred. The


major disadvantages of the method are the need for special materials (Table 2) and the loss of acetic acid solvent through decarboxylation. The former is engendered by the use of metal/bromide/acid mixtures. The latter problem is present to some extent whenever acetic acid is used as a solvent and is, in fact, less of a problem with the Mid-Century catalyst than with cobalt alone. The principle of the Amoco process has been successfully applied in the case of other aromatics and alkylaromatics. Thus, m-xylene can be oxidized to isophthalic acid, pseudocumene to trimellitic anhydride and trimellitic acid, mesitylene to trimesic acid, and 2,6-dimethylnaphthalene to naphthalene-2,6-dicarboxylic acid. Isophthalic acid finds application in polyesters and copolyesters, and its application in PET resins is rising at an impressive rate. Trimellitic acid is mainly used for the manufacture of esters for plasticizers. Because of work carried out by Du Pont, trimellitic anhydride is emerging as an important component for the manufacture of extremely heat-resistant polyimides.8 Naphthalene-2,6dicarboxylic acid is now emerging as an important raw material in the manufacture of PEN resins as well as co-polyesters with PTA, which have higher temperature ratings and better barrier properties than PET. The MC method lends itself to batch as well as continuous processing. Amoco has manufacturing capacities of 2 × 105 tons/year of isophthalic acid (continuous process) and about 4.7 × 104 tons/year of trimellitic acid (batch process), both based on the MC process with yields of over 90%.12,56 The catalytic principle of the Mid-Century process and the process details have been discussed comprehensively by Partenheimer.12 Co-oxidation of p-xylene with various other substances, such as acetaldehyde and paraldehyde in acetic acid solution (Toray process), has been attempted with good results; the conditions are milder (100-140 °C), bromide corrosion is not a problem, and a selectivity of over 97% has been claimed. However, a use must be found for the coproduct, acetic acid. Other processes for terephthalic acid have been discussed by Weissermel and Arpe.8 One route involves the LPO of toluene and is discussed later. 4.2. Oxidation of Cyclohexane, Other Saturated Hydrocarbons, and Terpenes The oxidation of cyclohexane is notoriously inefficient. It is therefore interesting that this process, which offers selectivities no higher than 80% even at 4% conversion,23 has not only become established in industry, but also stood the test of time. The alternative route to cyclohexanol and cyclohexanone involves the hydrogenation of phenol, a reaction that can be carried out with a selectivity in excess of 97% at 99% conversion. The process economics still favors cyclohexane oxidation. Practically all of the adipic acid16 and about 63% of the caprolactam57 produced in the world in 1990 used cyclohexane oxidation as the first step. This oxidation, as usually practiced, uses cobalt naphthenate or stearate as the catalyst, is carried out only to very small conversions (usually less than 8%), and produces a mixture of cyclohexanol and cyclohexanone. For caprolactam, a ketone-rich mixture is preferred. The ratio of alcohol to ketone in the oxidation mixture depends mainly on the use of catalyst (the alcohol-to-ketone ratio being about 0.5 in the uncatalyzed oxidation and 1.5 in the presence of cobalt stearate4) and can be made to strongly favor the alcohol by the use of boric acid

(alcohol-to-ketone ratio of 9:1). Castellan et al.16 and Weissermel and Arpe57 have recently summarized the information available on the oxidation of cyclohexane and the manufacture of adipic acid and caprolactam. The former of these references also has an extensive listing of patents on the manufacture of adipic acid and summarizes the status of alternative routes for adipic acid. The main problem in the oxidation of cyclohexane to cyclohexanol and cyclohexanone is one of selectivity and arises from the fact that these products are much more reactive with respect to oxidation than cyclohexane itself in the oxidation medium and under the conditions employed. It is interesting to note, in passing, the widely divergent process needs in the cases of p-xylene and cyclohexane. The problem in the former case is the inhibiting effect of the intermediate (p-toluic acid), whereas in the latter, it is the high reactivity of the intermediate. Industrially, one of two approaches is followed in order to contend with the selectivity problem in the case of cyclohexane oxidation. The first approach is to keep the conversion very low in order to keep the concentration of the products low in the oxidation mixture and, hence, to prevent their overoxidation. The conflicting demands of selectivity on one hand and safety and rate on the other are best reconciled by having several stirred reactors in series (reactor selection issues are discussed in section 8.2 and safety in section 11). The disadvantage of the method is the need to recycle large volumes of unconverted cyclohexane. The second approach (Halcon and IFP) allows higher conversions to be used, as in this case, boric acid is used to direct the reaction of the hydroperoxide toward cyclohexanol and protect the alcohol as its boric acid ester to prevent its over-oxidation. Recovery and recycling of the boric acid used is the problem in this case. In the further conversion of the KA (ketone-alcohol) mixture to adipic acid, catalytic air oxidation under milder conditions (acetic acid medium, Cu or Mn acetate as catalyst, 80-85 °C, 7 bar) has received some attention. Scientific Design has developed a process for a twostage autoxidation route to adipic acid. However, the route using oxidation with nitric acid as the second stage is preferred because of better selectivity, although there are the additional problems of corrosion and recovery of the nitrogen oxides produced. (Also, in recent years, formation of N2O, an important greenhouse gas, has become a major concern with this process.) Weissermel and Arpe8 report that the Rohm & Haas plant in the U.S., based on autoxidation technology, was abandoned because of poor product quality. The low conversion levels in the oxidation of cyclohexane with traditional cobalt catalysts have motivated a large literature on alternative catalysts. Two approaches in particular, which have been targeted for industrial development in cyclohexane oxidation, will briefly be discussed here, although neither seems to have yet reached the stage of commercialization. The first of these is to maximize the selectivity to the hydroperoxide in the first oxidation step and achieve a selective conversion of the hydroperoxide in a second stage by employing milder conditions and special catalysts. Developments in the low-temperature, selective decompositions of cyclohexyl hydroperoxide using homogeneous as well as heterogeneous catalysts has led to a rejuvenation of interest in this approach. DSM developed such a process and applies it on a large scale.

3966


Tolman et al.58 describe a family of active and long-lived catalysts for selective conversion of the hydroperoxide. Ciborowski23 argues the case for this process alternative and discusses the possibilities of utilizing the hydroperoxide. According to this author, overall selectivity in the oxidation step to (cyclohexyl hydroperoxide + cyclohexanol + cyclohexanone) can approach 90% under optimum conditions, with the hydroperoxide constituting up to 80% of these products. Obviously, in the oxidation step, a total exclusion of circumstances that could lead to a decomposition of the hydroperoxide is needed. No transition metal catalyst is used, and passivation of the wall, choice of noncatalytic wall material,59 homogeneous oxidation,59-61 and use of a second phase to remove the hydroperoxide by reaction as it is formed62 are some of the methods that have been claimed to offer high selectivities. The interest in homogeneous oxidation, considering the low solubility of oxygen in cyclohexane, indicates a recognition of the possibility that the reaction can move into masstransfer-limited regimes. Recent studies at Monash University on homogeneous oxidation in a variety of reactor types in aluminum and glass-lined reactors have produced encouraging results.59,61,63 In the second approach, which is relevant to adipic acid manufacture, a direct oxidation to adipic acid is attempted. If reasonable selectivity can be achieved, such an approach would obviously have the advantage of simplicity, as an entire process step is eliminated. Asahi17,18 and Gulf64 have made attempts in this direction, and claims of 70-75% selectivity to adipic acid at conversion levels of 50% and above have been made. The oxidation is carried out in acetic acid medium, with a cobalt salt as catalyst. Typical reaction conditions quoted include temperatures of 70-100 °C and reaction times of 2-6 h. According to the Asahi work, conditions that favor the formation of Co3+ in the reaction medium are to be preferred. They use aldehydes and ketones as promoters for this purpose. Gulf patents claim that the presence of water improves the adipic acid yields substantially. However, neither process seems to have been implemented industrially. In recent years, interest in direct routes to adipic acid would seem to have revived, judging from the large amount of patent activity in the area. The work of Twenty-first century research corporation was cited in section 3; also see Park and Goroff65 and Vassiliou et al.66 The role of water and the solvent-to-hydrocarbon ratio would seem to be important from these patents, as also would a careful regulation of temperature versus holdup time. Some patents (Dassel and Vassiliou,67 for example) deal with unconventional contacting methods, such as contacting liquid in atomized form with a stream of the oxidizing gas. While on the whole, it must be admitted that the results on single-step oxidation to adipic acid have been less than satisfactory,16 some of the principles recognized by the earlier work, such as the need to keep the transition metal in the higher oxidation state,17 are of interest for further development (see, for example, Steinmetz et al.68). Also, some interesting chemistry has been brought out as a result of this research. In particular, we mention the work of Iwahama et al.,69 which demonstrates the oxidation of cyclohexane at 100 °C and atmospheric pressure using N-hydroxyphthalimide as a radical catalyst. The reaction is conducted in acetic acid medium with trace amounts of transition metal salts as acetylacetonates. The manganese salts

appear to yield the best selectivity to adipic acid. Cyclohexanone and adipic acid are the main products. Surprisingly, cyclohexanol is not formed, and it is suggested that it is further converted under the reaction conditions. In another development, Kulsreshtha et al.70 have reported selectivities of 77% at 85% cyclohexane conversion using a Co(III) catalyst at 100 °C. The oxidation of cyclododecane is analogous to the oxidation of cyclohexane and, indeed, has been used by some researchers as a model system for studying the kinetics of cyclohexane oxidation.36 At the same temperatures (150-160 °C) as employed for cyclohexane oxidation, cyclododecane can be oxidized in the liquid phase at atmospheric pressure to a cyclododecanol/ cyclododecanone mixture. Boric acid is used in the industrial process,8 and selectivities of about 80% are achieved at 25-30% conversion. Alcohol-to-ketone ratios are similar to those in the analogous cyclohexane oxidation. Processes are operated by Hu¨ls in Germany and Du Pont in the U.S. Further oxidation with nitric acid is used to convert the alcohol/ketone mixture to 1,12-dodecanedioic acid, a product of some importance in polyamides (Nylon-6,12) and polyesters. 1,12-Dodecanedioic acid is the second most important dicarboxylic acid after adipic acid. This dicarboxylic acid has been attracting additional attention in recent years, as the macrocyclic musk, made via esterification with ethylene glycol, is preferred over polycyclic musk for environmental reasons, and in the new generation of polycarbonates. Cyclododecanol and cyclododecanone are also precursors to lauryl lactam (monomer of Nylon-12), again in analogy with cyclohexane oxidation products. A recent claim by Ube industries71 refers to the liquidphase oxidation of cyclododecane in the presence of mono(2-ethylhexyl) phosphate and di(2-ethylhexyl) phosphate at 160 °C, where 73% selectivity to hydroperoxide at 12% conversion has been realized. A novel feature of this claim is the use of phosphate ester, whose actual role is unclear. The Bashkirov principle of using boric acid to produce alcohol esters, and thereby gain alcohol selectivity, was first developed for the oxidation of n-paraffins as one of the routes to linear secondary alcohols in the C10-C20 range. These alcohols have application in detergent manufacture. This process still operates in Japan and the C.I.S.8 Atmospheric operation at 140-190 °C, in the presence of about 0.1% KMnO4 and 4-5% metaboric acid, to conversion levels of 15-25% is used. The borate esters are later hydrolyzed with NaOH. The oxidation yields sec-alcohols almost exclusively, with a statistical distribution of OH groups. Weissermel and Arpe8 discuss some process variants. Also allied to cyclohexane oxidation is the oxidation of p-menthane and cis-pinane, steps of importance in the manufacture of several flavor intermediates apart from hydroperoxides (which are used in free-radical polymerizations as initiators). Thus, pinane, derived from R-pinene (a plant product), is oxidized to a mixture of cis- and trans-pinane hydroperoxides. This mixture is in turn hydrogenated to a mixture of cis- and transpinanols, which, on pyrolysis, yields linalool.3 Apart from being a product of importance in its own right, linalool can also be readily isomerized to nerol and geraniol using an orthovanadate catalyst. Similarly, limonene can be converted, via air oxidation, to carvone and other products using a cobalt catalyst.72 These authors have also studied the oxidation of R- and


β-pinenes. The oxidation of limonene in glacial acetic acid containing lithium chloride in the presence of PdCl2-CuCl2 gives trans-carveyl acetate as the major product, as has been reported by Gusevskaya and Gonsalves.73 Although some of these products can also be directly extracted from plant sources, the synthetic routes mentioned above are far more important. These products are also important as intermediates in the manufacture of other terpene products such as citral, ionones, citronellol, citronellal, menthol, vitamins A and E, and carotenoids. 4.3. LPO-Based Routes to Phenol and Benzylic Alcohols Phenol is another major chemical derived from a liquid-phase oxidation route. In the U.S., it ranks 34th in terms of capacity in the overall list of industrial chemicals.74 The direct oxidation of benzene to phenol is highly unselective, as total oxidation of phenol is preferred to partial oxidation of benzene, and hence, only the indirect manufacturing processes have been implemented.8 Of the five routes that have attained commercial significance, three involve a liquid-phase autoxidation. By far the most important process is the Hock process, first commercialized in 1953. The process is based on the oxidation of cumene to cumene hydroperoxide and the subsequent cleavage of the hydroperoxide in acidic media to phenol and acetone. The BP, Hercules, and Kellogg processes use an air/aqueous emulsion at a pH of 8.5-10.5, and other conditions are shown in Table 2. The table reflects the more recent trend to reduce or even eliminate water from the reaction medium. Alternatively, air/cumene contacting can be used at 120 °C (Hercules process). As oxidation catalysts encourage hydroperoxide decomposition (see discussion on cyclohexane oxidation above), none is used, although substances such as Cu, Mn, and Co salts can reduce the induction period at the start.8 The oxidate is worked up to 65-90% before being subjected to acid cleavage. Cumene was originally produced in World War II as an additive for gasoline. Its potential for making phenol, via neat air oxidation to hydroperoxide and subsequent acid-catalyzed cleavage, was recognized in the 1950s and came to fruition thereafter. Today, more than 90% of the phenol produced in the world, at a level of approximately 6.5 million tons per annum, is produced by this route. Several single-stream plants producing phenol at 200 000 tons per annum exist, and these have real large-size oxidizers. The process is, in theory, applicable to other feedstocks such as ethylbenzene (coproduct acetaldehyde) and cyclohexylbenzene (coproduct cyclohexanone). Cumene is, however, expected to remain the dominant feedstock because of the market for acetone. Another potential possibility is to use secbutyl benzene in place of cumene so that the corresponding hydroperoxide will, on cleavage, yield phenol and methyl ethyl ketone. The other LPO-based routes to phenols8 are those using cyclohexane oxidation (Scientific Design) and toluene oxidation [DSM, Kalama USA (now Goodrich)]. In the former case, the oxidation is carried out until the stage of cyclohexanol/cyclohexanone, which is dehydrogenated to phenol at 400 °C. In the latter, toluene is oxidized to benzoic acid, which is oxidatively decarboxylated to phenol. The first step of toluene oxidation in the second process is of some importance in its own

right, because of the various other uses of benzoic acid (as well as its salts), for example, as an intermediate (to caprolactam in the Snia Viscosa process and to terephthalic acid in the Henkel process) and as an additive in the rubber and food industries. A range of processes are available for the oxidation of toluene, among them the Amoco Mid-Century process discussed earlier. Two other processes are shown in Table 2. In a manner analogous to the Hock process, cymene can be oxidized and cleaved to m- and p-cresol (coproduct acetone). These processes have been operated for over 25 years by Sumitomo and Mitsui.8 Again, 2-isopropylnaphthalene has been converted, via the hydroperoxide, to 2-naphthol (a widely used dye intermediate) and acetone with a yield of about 95%.75 The oxidation takes place at 110 °C. Modified Hock processes are also used in the manufacture, from diisopropylbenzenes, of the more important dihydroxybenzenes, hydroquinone, and resorcinol (combined world capacity of about 85 000 tons/year in 1994). Thus, m-diisopropylbenzene has been converted to resorcinol and p-diisopropylbenzene and to hydroquinone in Japan (Mitsui and Sumitomo) and the U.S. (Goodyear). The presence, in the case of the diisopropylbenzenes, of two hydroperoxide groups after the oxidation leads to a greater number of byproducts and lower rates than are attained with cumene hydroperoxide, requiring modifications to the cumene-phenol process.76 4.4. The Oxirane Process: Oxidation of iso-Butane and Ethylbenzene The oxirane processes, or the “indirect oxidation” routes to propylene oxide, which are of interest in the context of liquid-phase autoxidations, accounted for about 48% of the worldwide propylene oxide (PO) capacity in 1991.8 The importance of the oxirane route in PO manufacture has been increasing through the years. Indirect oxidations exploit the ability of hydroperoxides (or peroxy acids) to selectively transfer their peroxidic oxygen to olefins to form epoxides, while themselves being converted to the alcohol (or the acid). The hydroperoxide or the peroxy acid is produced, either in a preliminary step or in situ; the former has established a dominant position in industry. Although a choice of hydroperoxides/peroxy acids is available (see, for example, Weissermel and Arpe8 and Kahlich et al.77), coproduct economics have meant that the oxirane capacity is split almost equally between isobutane- and ethylbenzene-based plants, the former accounting for about 56% of the capacity. The conditions of oxidation and typical conversion-selectivity figures are shown in Table 2. The aim in the oxidation is to maximize the selectivity to the hydroperoxide. Under the conditions, the hydroperoxide and the alcohol are substantially the only products formed, with small amounts of acetone in the case of isobutane and acetophenone in the case of ethylbenzene. For example, in isobutane oxidation, even at 48% conversion, the total selectivity to tert-butyl hydroperoxide (TBHP) and tert-butyl alcohol (TBA) is 96%, 50% being the selectivity to TBHP. The coproduct tert-butyl alcohol in the isobutane-based process, apart from being a gasoline additive, can be dehydrated to isobutene and further converted to methyl tert-butyl ether (MTBE). In the ethylbenzene-based process, the coproduct phenyl methyl carbinol is converted to styrene. A small amount of primary hydroperoxide of ethylbenzene is also formed, which gives phenyl ethyl

3968


alcohol (which has a high value as an aroma chemical). In the context of the recent interest in the oxidation of cyclohexane to cyclohexyl hydroperoxide, the possibility of using cyclohexyl hydroperoxide to convert propylene to PO in a step analogous to the existing oxirane processes has also been commented upon (e.g., Ciborowski23). Although the interest in isobutane oxidation has been in the context of the oxirane process, possibilities for its development for MTBE production are being actively examined. The spread of unleaded fuels and legislation such as the U.S. Clean Air Act have resulted in increasing demands for MTBE (12th in terms of capacity in the list of industrial chemicals in the U.S. in 199674). According to the projections made in 1993, MTBE was expected to continue its growth at about 15% per year.78 If this growth is sustained, the traditional route based on isobutenesin most crackers, the C4 stream containing isobutene is directly converted to MTBE with methanolsmay not be able to meet the demand because of problems of isobutene availability. It will be necessary to resort to other routes based on TBA and isobutane.79,81 Oxidation of isobutane followed by etherification of TBA with methanol seems to hold promise. Although oxidation with hydrogen peroxide is a research focus (see, for example, Grieken et al.79), H2O2 is expensive. Furthermore, a common disadvantage of all processes based on H2O2 is that, with the current technology, safety considerations require H2O2 facilities to be restricted in size.77 Hence, interest in the autoxidation of isobutane has resurfaced33,82 and is likely to continue. Some of the recent literature (patent and published) that focusses on TBA rather than TBHP in the oxidation of isobutane (for example, Shen et al.83 and Fan et al.84) and patents dealing with the selective conversion of TBHP to TBA (for example, Sanderson et al.85,86) are also indicative of this trend. While one considers the potential of developments cited in the previous paragraph, some recent developments should be borne in mind. Most recently, MTBE has come under a cloud, and its usage in motor gasoline is being phased out. (The state of California has banned the addition of MTBE to gasoline.) ETBE (ethy tert-butyl ether) is a possible replacement for MTBE, as there does not seem to be any objection to its use. TBA as an additive also seems to be safe. TAME (tert-amyl methyl ether) is another gasoline additive with properties similar to MTBE. As with isobutene in cracker streams, isopentene, which is also produced in small amounts during steam cracking, is used for TAME production. However, of relevance in the present context is the possibility of development of isopentane autoxidation for TAME, along lines similar to isobutane oxidation discussed above. The first plant for TAME began production in 1987, and since then, other plants have been put into operation. These plants use the acid-catalyzed reaction of isopentene with methanol.8 4.5. Acetic Acid from Paraffin Oxidation Liquid-phase oxidation routes also account for a significant proportion of acetic acid produced in the world, although they have been superseded in recent years by methanol carbonylation. Acetic acid is one of the most important aliphatic intermediates and the first carboxylic acid used by man. It ranked 33rd in the list of U.S. industrial chemicals according to a recent survey

and was one of the few organics to register a doubledigit growth during 1994-1995.74 In 1994, LPO routes with acetaldehyde and butane/naphtha as feedstocks accounted for about 32% of world production, with the acetaldehyde-based process being the more important (23% of world capacity).57 This, therefore, is a case of autoxidation of an oxygen-containing compound, in contrast to most oxidations discussed so far (with the exception of the oxidation of cyclohexanone). The chemistry is, however, similar, with the free-radical mechanism producing peracetic acid as the primary product. Under mild conditions (no catalyst, ethyl acetate medium, -15 to 40 °C, 25-40 bar), peracetic acid can be recovered as the main product in acetaldehyde oxidation, as in the commercial processes operated by UCC in the U.S., Daicel in Japan, and British Celanese in the U.K. for peracetic acid. In the processes for acetic acid however, solutions of Co and Mn acetates at low concentrations (0.5 wt %) are used; as in other processes, they accelerate the reaction by helping in radical generation and facilitating the decomposition of the peracetic acid. The Hoechst process8 uses oxygen; typical reaction conditions are indicated in Table 2. Careful temperature control is needed to limit degradation reactions and obtain the high selectivities shown. The Rhoˆne-Poulenc/Melle Bezons process uses air instead of oxygen, and although selectivities are similar, the greater amount of inert species in the gases requires washing of the gases to remove acetaldehyde and acetic acid. The byproducts are separated by distillation, and an interesting feature is that they serve as entraining agents for water, so that anhydrous acetic acid is directly obtained. Various routes to the manufacture acetic acid from light paraffin feedstocks have been commercialized, and Weissermel and Arpe8 describe them in some detail. Some of these processes illustrate how, whereas selectivity to the product of interest remains a problem when a straight through-process from the alkane to the acid is attempted, byproduct recovery can contribute to the overall success of the process when reasonable quantities can be recovered. The Hu¨ls process (Table 2) is a typical catalyzed LPO that produces, in addition to acetic acid, other byproducts such as acetaldehyde, acetone, MEK, etc. A feature of this process is that, depending on the demand, MEK can be manufactured at up to 17% of the plant capacity, although at the expense of acetic acid. Normally, byproducts are recycled to simplify product workup. The Distillers-BP process operates in Europe, Japan, and the C.I.S. and uses a mixed feedstock of crude oil distillates in the 15-95 °C boiling range (roughly C4-C7). The process uses an uncatalyzed oxidation, at 160-200 °C and 40-50 bar. Byproduct recovery is important to the profitability of the process. We have surveyed some of the more important LPO processes so far. The unifying thread of the free-radical chemistry (modified in some instances by the action of the catalysts used) that runs through all these processes, as well as the individual peculiarities of the specific processes that lead to highly specific technologies, are both evident in this broad overview. The complex chemistry of these oxidations makes them highly versatile and makes process optimizations toward a number of alternatives from a given feedstock become possible.


5. Mechanism of Hydrocarbon Oxidation The chemistry of liquid-phase autoxidation of hydrocarbons, both catalyzed and uncatalyzed, has been the subject of several monographs and reviews (see, for example, Emanuel et al.,1 Sheldon and Kochi,2 Berezin et al.,10 and Partenheimer12,56). Therefore, only the important features and those relevant to the considerations of this paper will be summarized in this section. The generic features exhibited by hydrocarbon oxidations in the liquid phase suggest similarities in the underlying kinetic mechanisms. Thus, the presence of an induction period, autocatalytic behavior, and zeroorder kinetics with respect to oxygen, are all characteristics commonly observed, irrespective of the hydrocarbon. Again, the multiplicity of products that form indicates a complexity of chemistry, which is occasioned by the fact that the primary products of oxidation are themselves amenable to oxidation by mechanisms very similar to those that apply to the parent hydrocarbon. The free-radical chain theory of autoxidation,1,2 as a general framework within which to interpret the kinetics of hydrocarbon oxidations in the liquid phase, is now generally accepted. According to this theory, oxidation is mediated by free-radical intermediates, and is characterized by the elementary processes of initiation, propagation, and termination. Also, because the hydroperoxide molecules, which are the “links” in this chain mechanism, are often unstable with respect to the formation of free radicals, the degenerate chain-branching reaction is usually important. Often, therefore, the branching mechanism soon overtakes the primary initiation mechanisms in the generation of free radicals. The degenerate branching reactions have been shown to be the cause of several features observed in hydrocarbon oxidations. In what follows, we shall briefly consider the elementary processes in the free-radical chain mechanism. 5.1. Initiation The reaction may be initiated either by the hydrocarbon itself, e.g.

RH + O2 f R* + HO*2

(1)

2RH + O2 f 2R* + H2O2

(2)

or by the decomposition of an initiator such as a peroxide, e.g.

I f I* *

(3) *

RH + I f IH + R

(4)

When no initiator is used, either or both reactions 1 and 2 may be important. For example, reaction 1 has been shown to be prominent in cyclohexane, cumene, and o-xylene, and reaction 2 in the case of tetralin. Cyclohexanol shows evidence of both mechanisms.1,87 These reactions are endothermic and quite slow. Therefore, induction periods are often observed in oxidations without added initiators. Indeed, Sheldon and Kochi2 opine that initiation from the hydrocarbon is kinetically and thermodynamically unfavorable and that initiation in the absence of added initiators is due to the decomposition of adventitious peroxidic impurities. Initiation from the hydrocarbon can be facilitated by the participation of metal ions, as described later, and this factor

could be important in laboratory reactors in which considerable surface-to-volume ratios are present, even though no metal ions may be intentionally added. Use of initiators is favored in fundamental studies where it circumvents problems such as poor reproducibility of induction periods in batch reactors. When used, these initiators are usually peroxides or hydroperoxides with reasonable rates of decomposition in the temperature range 50-150 °C. Rate constant information on some of the initiators commonly used in autoxidations is available from Sheldon and Kochi.2 In industrial processes, if initiators are used, they are usually chosen so as not to complicate the chemistry. Thus, TBHP might be used as an initiator in the oxidation of isobutane;11 being itself a product, it would not lead to products that would otherwise be absent. Similarly, acetaldehyde has been used in studies on the oxidation of cyclohexane to adipic acid, as its oxidation product is acetic acid, which is used as a solvent in these attempts. Whether an initiator is used or not, the mechanisms shown above are of importance only in the initial stages of the reaction and are overtaken even at small conversions by the chain-branching reactions to be discussed later. This explains why reproducible kinetic behavior is observed in the post-induction period even when induction periods themselves may be irreproducible (see, for example, Suresh et al.50 and Suresh88). 5.2. Propagation The following two reactions represent the general chain-propagation mechanism:

R* + O2 f RO*2

(5)

RO*2 + RH f ROOH + R*

(6)

At oxygen partial pressures greater than about 100 Torr, reaction 5 (rate constants > 109 mol L-1 s-1, Sheldon and Kochi2) is usually much faster than reaction 6, so that the R* radicals are effectively scavenged, and the overall reaction shows a zero-order behavior in oxygen. Each occurrence of reaction 6 represents the formation of a “link” in the chain reaction, and the chains are said to be long when the number of occurrences of reaction 6 per initiation event is large. A “kinetic chain length”, which represents the average number of links per chain, can be calculated as the ratio of the rate of the propagation reaction to the rate of the initiation reaction (assuming that each initiation event starts one chain). When the chains are long, hydrocarbon consumption occurs essentially by reaction 6. The kinetic chain length can be related to the efficiency of hydroperoxide formation.3 The rate of reaction 6 depends on the nature of the hydrocarbon as well as on the nature of the radical. The peroxy radicals are relatively stable, and abstract preferentially only the most weakly bound hydrogen atom. Thus, the facility of hydroperoxide formation decreases in the order:2 tertiary C > secondary C > primary C. Thus, for example, in the oxidation of cumene, attack always occurs on the tertiary C in the isopropyl group, with negligible attack on the ring or the methyl carbons. On the other hand, the reactivity of the alkylperoxy radical strongly depends on its structure, being influenced by steric as well as polar effects, in general increasing as the electron-withdrawing capacity of the R-substituent increases. For example, acylperoxy radicals are much

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more reactive than alkylperoxy radicals, thus explaining the high rates and long chain lengths observed in the autoxidation of aldehydes. Thus, autoxidation rates (and selectivity, through chain length) depend not only on the nature of the hydrocarbon itself, but also on the structure of the peroxy radical derived from it. Howard89 has some compilations of propagation rate constants at room temperature. At moderate and high conversions, there are several hydrocarbons and several types of peroxy radicals participating in reactions of the same type as 6 above. Similar is the situation with co-oxidations. In such cases, the C-H bond energies of the hydrocarbons and the relative reactivities of the peroxy radicals together determine the relative rates of oxidation of the different compounds in the oxidizing mixture and the selectivities that result. Propagation (and hence the overall reaction) kinetics at very low oxygen partial pressures, when reaction 5 begins to control the overall propagation rate, are expected to become first-order in oxygen. These situations are not as well studied as the zero-order behavior at higher oxygen partial pressures. It is possible that such a situation arises in industrial reactors, which are often operated with extremely low exit oxygen partial pressures in the interest of safety. The free-radical population would be expected to become more complex with several types (alkyl, alkyloxy, alkylperoxy) of free radicals, and this situation is believed to lead to a much greater number of products than at higher oxygen concentrations, thereby lowering selectivities.2 The concentration of these radicals can vary both spatially and temporally, depending on local oxygen concentrations. Some of the reactions of the other radical classes do, however, lead to similar products as would otherwise form (for example,3 ROH can form from the attack of the primary alkoxy radical on RH, while secondary and tertiary alkoxy radicals can give, through β-scission reactions, aldehydes and ketones respectively). Some data available on the oxidation of isobutane31 suggest higher selectivities to TBHP under low oxygen partial pressures. The isobutane data have been discussed recently by Suresh;90 a summary is given in section 6.4. 5.3. Termination Given the relative abundance of the peroxy type of free radicals under normal circumstances, the dominant mode of termination would be by (“Russell mechanism”) reactions of the type2,3

RO*2 + RO*2 f RO4R f O2 + non-radical products (7) It is possible that mutual termination between different types of radicals becomes important under conditions of extremely low oxygen partial pressures, when types of radicals other than peroxy radicals can also accumulate in solution. The tetroxide that forms in the reaction shown above undergoes decomposition in a manner that depends on its structure. Thus, the tetroxides derived from secondary and primary alkylperoxy radicals decompose by disproportionation to the correponding alcohol and carbonyl compound. However, when chains are long, most of the observed concentrations of alcohols and ketones in the reaction mixture derives

from the decomposition of the hydroperoxide. The termination mechanisms associated with t-alkylperoxy radicals lead to dialkyl peroxides.11,91 In general, the rate constant for termination of primary alkylperoxy radicals is higher than those of secondary alkylperoxy radicals, which are themselves much higher than those of tertiary alkylperoxy radicals. Howard89 presents some data on these rate constants. Under low oxygen concentrations, when alkyl and alkoxy radicals can accumulate to some extent, termination mechanisms involving these species would be expected to become important. 5.4. Degenerate Chain Branching As mentioned earlier, except in the very early stages of the reaction, the main source of free radicals in most liquid-phase organic oxidations are the so-called degenerate branching reactions in which the primary products of the chain mechanism participate. These reactions lead to a number of consequences in the conversion range of industrial interest, such as autocatalysis, a progressive decrease in the kinetic chain length with conversion, and a multiplicity of secondary and tertiary products. The decomposition of hydroperoxides, the primary oxidation products, to radical species can itself be a complex process, as both nonchain and chain mechanisms have been implicated. Three mechanisms have been identified as being important87 in the breakdown of hydroperoxides to radical species. These are a unimolecular homolysis with -O-O- bond breakage, viz.,

ROOH f RO* + *OH

(8)

a bimolecular interaction of a hydroperoxide molecule with the original hydrocarbon, viz.,

ROOH + RH f RO* + R* + H2O

(9)

and a bimolecular mechanism involving association between two hydroperoxide molecules, viz.,

2ROOH f RO* + RO*2 + H2O

(10)

Alkenic hydrocarbons show a linear relationship between the rate and the concentration of the hydroperoxide, suggesting the predominance of mechanism 10. The oxidations of cyclohexane and isobutane show a proportional relationship between the rate and total product concentration, which again can perhaps be explained on the basis of such mechanisms. As an example of the importance of the chainbranching process vis-a`-vis initiation as a source of radicals under oxidation conditions, it can be calculated that, for cyclohexane oxidation at 1500 °C, the conversion at which the two mechanisms become competitive is as low as 0.01% even if reaction 8 alone is considered to operate among the branching mechanisms cited. The situation is not much different in general at other temperatures, as the activation energies of the initiation and chain-branching reactions are usually comparable, being in the range 25-35 kcal/mol. The heat effects associated with the branching reactions are usually small and could be of either sign, depending on the bond energies of the participating bonds. The relative importance of the three types of mechanisms described influences the overall kinetic


form. Mechanism 9 has been observed, for example, in the case of tetralin and cumene and can often take precedence over the mechanism 8 because of bond energy considerations.87 Furthermore, the hydrocarbons participating in these reactions at higher conversions could be the reactants as well as the intermediate products formed during the oxidation. The bimolecular mechanism 10 has been observed with, for example, tertbutyl hydroperoxide and several alkenic hydrocarbons and is generally more important in cases where the hydroperoxides occur in reasonable concentrations. The observed linear dependence of the oxidation rate on the product concentrations with species such as cyclohexane49,92 and isobutane90 is perhaps indicative of this mechanism being important in these cases as well. Apart from the mechanisms discussed, there are others that often play a substantial role in radical generation after the induction period. For example, in the case of isopropyl alcohol, H2O2, which is produced during the oxidation, decomposes to free-radical species. Again, the peroxy radical can itself isomerize and decompose into a smaller radical and a molecular product, a mechanism that explains the occurrence of low-molecular-weight compounds even at early stages in some oxidations.87 Such unimolecular decomposition of the hydroperoxide is often catalyzed by metal surfaces, and this explains, in part, the differences in behavior that are often observed between laboratory reactors made from different materials (for example, the different peroxide levels in cyclohexane oxidation in glass and stainless steel reactors; see Berezin et al.10). It is therefore clear that, to obtain scale-independent kinetics, the influence of wall catalysis is to be recognized and appropriate precautions taken. These aspects are further elaborated in section 7. 5.5. Overall Kinetic Features Based on the Mechanism The elementary processes involved in hydrocarbon oxidations have been established from extensive experimentation. It is therefore possible, in principle, to treat oxidation kinetics directly from the mechanisms presented above. For example, Berezin et al.10 present various kinetic models for different combinations of the elementary processes discussed above. However, there are some difficulties with this approach in treating technical kinetics. First, whereas consensus has been reached on the broad features of the free-radical chain mechanism, sharp controversies remain on various points of detail, which are sometimes important from the industrial viewpoint. Second, sometimes, particularly when catalysts are involved, nonradical pathways become important in intermediate product conversions and add to the complexity of the chemistry. Finally, the chemistry gets complicated anyway, even for simple materials, at the conversions of industrial interest, because of the participation of the primary products in the various elementary processes. Thus, tractable models can be formulated with confidence only for the very initial stages of industrial oxidations. At moderate to high oxygen concentrations and when branching is unimportant, it can be shown2,87 that the rate of hydrocarbon consumption is given by

rRH ) kp[RH]

x( ) rin 2kt

(11)

where rin is the rate of initiation and kp and kt are the rate constants for propagation and termination (eq 7), respectively. The kinetic features thus depend on the initiation mechanism. Some of the features anticipated from eq 11 have been experimentally observed. For example, the rate of hydroperoxide formation in the LPO of methylpentanes, with hydroperoxides being used as initiators, was observed to be proportional to the square root of the hydroperoxide concentration by Farkas and co-workers (cited by Farkas6), consistent with the above equation when the initiation is by homolytic decomposition of the hydroperoxide. If initiation occurs according to reaction 1, one would expect the reaction to show a half-order dependence on oxygen and a 3/2-order dependence on the hydrocarbon. In practice however, the order with respect to the hydrocarbon is difficult to discern at the low conversions at which eq 11 applies, and the order with respect to oxygen is more often seen to be zero, showing perhaps the dominance of other initiation mechanisms even at a fairly early stage. Equation 11 does not explicitly account for the autocatalysis and accounts for it only indirectly if the products influence the rate of initiation, as in the work of Farkas mentioned above. Despite the shortcomings and limited scope of equations of this type, they are useful in comparing the susceptibility to oxidation of various substances. Given that the rate of initiation can often be manipulated, say by adding initiators, the ratio kp/(2kt)1/2 is a measure of the intrinsic tendency of the compound to become oxidized and has been called the oxidizability.2,4 It is thus seen that the ease of oxidation of an organic depends not only on the value of kp, but on the value of kt as well. The significant rate of termination of primary and secondary alkylperoxy radicals is the main reason an otherwise reactive hydrocarbon such as toluene has a rather slow rate of autoxidation.2 Oxidizability values can also be used to rationalize, for example, the high reactivity of benzaldehyde, the moderate reactivity of cumene, and the low reactivity of p-xylene89 under comparable conditions. For a given oxidizability, whereas the rate of reaction is seen to vary directly on rin, the chain length varies inversely as its square root. Because short chains are associated with a proliferation of products and hence with low selectivities, this implies that addition of initiators to increase the rate can result in poor selectivities, a feature often observed. At moderate conversions, when hydroperoxide is the major product, a self-sustaining chain reaction becomes possible with ROOH as the radical source. The following equation has been derived2,3 to describe the rate of hydrocarbon conversion under these circumstances:

rRH ) nk2p

[RH]2 2fkt

(12)

where n is the number of radicals produced by decomposition of one molecule of the hydroperoxide and f is the fraction of RH consumed, which disappears through alkylperoxy radical attack. 5.6. Catalysis of Organic Oxidations It has been pointed out that the most important catalysts used in hydrocarbon oxidations involve transition metals. Cobalt has been the most prominent among

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the transition metals used. According to Partenheimer,12 cobalt performs at least three functions in the oxidation medium: (a) It quickly reacts with the primary peroxides via the Haber-Weiss cycle

ROOH + Co(II) f RO* + Co(III) + -OH

(13)

ROOH + Co(III) f RO*2 + Co(II) + H+

(14)

(b) It acts as a radical initiating species when in the higher (+3) oxidation state (i.e., it generates R* radicals from RH), thereby enhancing the rate by participating in the initiation step (see eq 11). (c) It reacts rapidly and selectively with peracids, which are formed in the oxidation of aldehydes, and thus facilitates intermediate product conversion at advanced stages of oxidation. The first two mechanisms can operate in all cobaltcatalyzed reactions. The ease of reactions 13 and 14 depends on the redox potential of the Co(III)/Co(II) couple; the nature of the ligand (acetate or bromide, etc.) and the solvent also have some influence on this. The reason for the effectiveness of cobalt (and manganese) is the fact that the two oxidation states in its case are of comparable stability, so that reactions 13 and 14 can occur concurrently, and a catalytic process results.2 With other metals (such as copper), alternative routes (to 14) for reducing the metal, such as hydrogen transfer by the solvent, sometimes operate to close the catalytic cycle. The possibilities are discussed in Sheldon and Kochi.2 In other words, the presence of cobalt leads to a higher rate of initiation and a higher rate of decomposition of the hydroperoxide, the primary product in any autoxidation. Both of these features are observed in practice; induction periods and hydroperoxide levels are both smaller in cobalt-catalyzed oxidations. The last property seems to play a particularly important role in oxidations in acetic acid medium as, under these conditions, Co(II) can exist as a dimeric species that provides a nonradical, fast, selective route from the peracid to the acid.12 It is clear from the discussion above that the solvent has some role in catalytic oxidations. It has been pointed out by several authors12,17,68 that the ratio of Co(III) to Co(II) (which is influenced by, among other things, the polarity of the solvent) in the oxidation medium is important. Careful studies, quoted by Sheldon and Kochi,2 have shown that the commencement of the autoxidation virtually coincides with the conversion of cobalt to the III species. [During oxidation, the Co(III) level increases to a maximum and then decreases. The decrease is attributed to the reduction to the II species by the aldehydic species generated in the oxidation. The oxidation state of cobalt is readily recognized by the color of the solution.] The role of the solvent in MC oxidations is described in section 6. The role of promoters such as aldehydes, ketones, etc. in reducing induction periods (Castellan et al.16 give a summary of the use of promoters in cyclohexane oxidation to adipic acid) is also explained by their role in promoting the III species. A phenomenon of some importance associated with the cobalt-catalyzed autoxidations is the “catalystinhibitor conversion”.2 In media of low polarity, cobalt complexes are catalysts at low concentrations but inhibitors at high concentrations, and the transition seems to occur abruptly at some concentrations. This

is why solvents such as acetic acid are used when high catalyst loading is needed. Metal catalysts are usually added in the form of hydrocarbon soluble salts such as stearates, naphthenates, etc. Normally, the ligand only has a marginal influence on the course of the autoxidations. When reactions are carried out in acetic acid, the metal acetate is commonly employed. In nonpolar media, the phenomenon of catalyst deactivation is sometimes observed, as the catalyst is extremely sensitive to polar substances formed during the reaction.2 For example, in the oxidation of cyclohexane, an insoluble precipitate of cobalt adipate is sometimes formed, leading to a decrease in rate.2,10 A striking observation is reported by Suresh et al.,93 who observed the autocatalytic reaction to move from the bulk liquid into the film, and then again from the film back into the bulk, as a result possibly of such catalyst deactivation. These problems are much less significant in a polar protic medium, such as acetic acid. Finally, at high catalyst loadings, the mechanism of oxidation is believed to be different, with the autoxidation being suppressed.17,94 It is possible that the direct attack on the substrate by the metal complex, with the regeneration of the catalyst by reaction with oxygen or peroxidic intermediates (or manganese and bromide, see section 6.1), becomes important in this case. Sheldon and Kochi2 discuss the mechanisms by which the direct interaction of strong metal oxidants with organic substrates can lead to the production of radical intermediates. These oxidations therefore show features not commonly expected from the free-radical mechanism. Typical rate expressions for such mechanisms are given by Carra` and Santacesaria4 and Tanaka;17 these rate expressions explicitly involve the concentrations of the catalyst in its different oxidation states. 5.7. Co-oxidations Consideration of co-oxidations, in which a mixture of two or more substrates is autoxidized, is important for several reasons. First, they form the basis for some industrial production processes (for example, p-xylene and paraldehyde or acetaldehyde; see section 4). Secondly, even other oxidation systems, at moderate to high conversions should, strictly speaking, be considered as co-oxidation systems because of the participation of the products. Third, they provide the chemist with a tool for obtaining quantitative information on the relative reactivities of peroxy radicals on different hydrocarbons. Sheldon and Kochi2 cite several instances where dramatic effects have been observed by the addition of small amounts of a second substrate. The drastic reduction in the rate of oxidation of cumene in the presence of readily oxidizable impurities has been well documented,7,26 and explained on the basis of the rates of the cross-propagation (i.e., between one hydrocarbon and the alkylperoxy radical derived from the other hydrocarbon) and the cross-termination (between alkylperoxy radicals derived from different hydrocarbons) reactions. It is also often observed that a large increase in the rate of oxidation of an unreactive component is obtained in the presence of a small amount of a substance easily attacked by alkylperoxy radicals. Indeed, this is one of the reasons for the autocatalysis commonly observed in organic oxidations. When a material with a low oxidizability is co-oxidized with one of high oxidizability, the former may oxidize faster than the latter if kp ratios favor the former. Methods of


estimating such ratios are available, and Hobbs et al.38 have developed procedures based on co-oxidation considerations to predict product distributions in hydrocarbon autoxidations. 6. Chemistry of Selected Oxidations Having given an overview of the general mechanisms that operate in autoxidations in the previous section, in this section, we will summarize the available information on some oxidations of commercial significance as well as some emerging oxidation systems, to show how the general mechanisms provide a backdrop against which the similarities as well as the differences among specific oxidation reactions can be understood. Once again, where extensive treatises are available, the discussion here is kept brief. 6.1. p-Xylene and Other MC Oxidations The particular kinetic features of these processes are essentially due to the catalyst used, and the relevant chemistry has been comprehensively reviewed recently by Partenheimer.12 Acetic acid solvent is essential to the method. Among other things, Co(II) exists as a dimer in acetic acid/water mixtures, which provides for a nonradical, highly selective, and very fast generation of acids from the peracids that form in these oxidations en route to the final products. A typical solvent composition might be 10% water/90% acetic acid. In the oxidation of p-xylene, a ratio of hydrocarbon to solvent of 1:3 is typically used, and the yield of the final product is about 90% (Table 2). The catalyst contains cobalt, manganese, and bromide (for example, cobalt acetate, manganese acetate, and hydrogen bromide) in solution in acetic acid. This combination offers a high activity as compared to other autoxidation catalysts; it also offers a high selectivity over a wide temperature range. These features make the catalyst suitable for a variety of oxidations. An advantage incidentally arising out of the use of acetic acid solvent is that the carboxylic acid end products are usually insoluble in acetic acid at room temperature and precipitate out, thus simplifying the downstream separation of the reaction product. It is instructive to observe the changes when bromine and manganese are added into a co-catalyzed oxidation of a methylaromatic. Observable effects include a drastic increase in the rate and an equally dramatic decrease in the vent carbon monoxide and carbon dioxide (an indication of the higher selectivity). Induction times are generally reduced, and higher temperatures than applicable to co-catalyzed oxidations become available. The concentration of the higher-valency form of cobalt, Co(III), decreases dramatically, indicating the availability of new catalytic pathways that recycle cobalt to the +2 state very efficiently. According to a model described by Partenheimrer,12 the efficacy of the MC catalyst is due to the fact that the catalytic cycles of cobalt, manganese, and bromide become coupled to produce synergistic results. Thus, the Co(III), which is produced by the oxidation of Co(II) by the hydroperoxide, is recycled to the +2 state by redox reaction with Mn, which, in the process, is oxidized from the +2 to the +3 state. The manganese is, in turn, recycled to the lower oxidation state by reaction with bromide, and finally, the bromide is regenerated with the generation of a hydrocarbon radical of the R* type. Bromine atom being a much better H abstractor from the aromatic methyl

group than Co3+, the net result is a much higher activity due to a higher rate of initiation) and selectivity (presumably due to increased chain length). In general, it can be surmised that the reactive agents, alkylperoxy radicals and bromine atoms, selectively abstract the most weakly bound H. Thus, ring carbons are left untouched; on the other hand, any C atom attached directly is susceptible to attack. Thus, benzylic methyl group is attacked preferentially to aliphatic methyl group (e.g., in acetic acid). The tertbutyl group will be oxidized, but at a much lower rate than the benzylic methyl group. The relative rates of oxidation of methylaromatic compounds in MC oxidations (relative to toluene) have been correlated by the Hammett function

log(k/k0) ) σF

(15)

where k and k0 are the rate constants for the disappearances of the hydrocarbon and toluene, respectively, σ is the Hammett constant characteristic of the substituent on the ring, and F, a constant that depends on the conditions of oxidation (such as solvent, catalyst composition, etc.), has a value of -0.95 for the solvent and catalyst typical of industrial applications. The negative sign indicates that the electron-withdrawing substituents are less active. In the industrially important case of polymethylbenzenes, this correlation anticipates that, once the first methyl group is oxidized, the second methyl group, in a para- position with respect to first, will be much less reactive because of the reduction in the ring electron density. Thus, in the oxidation of p-xylene, p-toluic acid is about 10 times less reactive than p-xylene, and high yields of p-toluic acid can be obtained before oxidation of the second methyl group commences. Alternative explanations have also been advanced for the low reactivity of this intermediate, for example, by Hobbs.3 In any case, the effect is seen to be much more pronounced in cobalt-catalyzed systems (as indicated by the higher value of F for these systems). Thus, if one wants to stop the reaction after the oxidation of a single methyl group, one prefers a cobalt-catalyst system, and if one wants the oxidation of all the methyl groups, one prefers an MC system.12 Although several byproducts have been detected in MC oxidations, most form in less than 0.1% yield and include CO2 and methyl acetate intermediates from the oxidation of subsequent CH3 groups after the first (for example, the troublesome 4-carboxy benzaldehyde in the oxidation of p-xylene). Despite the features mentioned above, some combustion of the solvent acetic acid results, and this is one of the areas in which research efforts are active. With naphthalene derivatives, bromination of the ring to give undesired products is a problem. Another problem that is intrinsic to the method is that water (which is a reaction product) deactivates the MC catalyst. Continuous addition of acetic anhydride has been suggested12 as a way to reduce the problem. However, another intrinsic problem, that of precipitation of the catalyst metal by the product carboxylic acid, is less serious if water is allowed to accumulate. This latter problem can also be contained by a decrease in pH and an increase in bromide concentration. Anything that affects the steady-state concentrations of phenolic radicals, of Co(III) and Mn(III), and of the chemical form of the catalyst is a potential deactivating agent. Thus, phenols, sulfides, primary amines, etc. in trace amounts can deactivate

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the catalyst (phenolic radicals are more long-lived than phenyl). Some metals are strong inhibitors. The reagents that are themselves antioxidants must be masked for oxidation to proceed (acetylation of phenols and amines, etc.). The aromatic acids also directly deactivate the catalyst, but precipitation of acids from the acetic acid contains this problem. It is therefore advisable to choose the minimum possible temperature for the oxidation so that the solubility of acid is kept small. The purity of the precipitated acid also depends on the temperature. For example, 96% terephthalic acid precipitates from pure acetic acid at 200 °C, whereas 99.4% precipitates at 100 °C. 6.2. Oxidation of Cyclohexane Because of the selectivity problems in cyclohexane oxidation, the uncatalyzed oxidation to increase the hydroperoxide is a relevant process option, as discussed in section 4. The role of wall catalysis must be duly considered in such process designs. Berezin et al.10 show that the product spectrum in glass reactors is quite different to that in steady-state reactors. This leads to obvious problems in translating the results from laboratory reactors to commercial-size reactors. Use of aluminum for the wetted surfaces in laboratory reactors is an innovation that has been tried recently with good results.59 Oxidation in glass reactors follows the general radical mechanism outlined above rather closely. The characteristic feature of cyclohexane oxidation is that the conversions are kept deliberately low. It is therefore the chemistry under these low-conversion conditions that is of interest. The kinetic chains are usually reasonably long under such conditions. Cyclohexyl hydroperoxide is the sole primary product and its radical decomposition is responsible for most of the degenerate chain branching. At extremely small conversions, the overall rate shows a square-root dependence on the hydroperoxide concentration, as is to be expected from unimolecular branching and quadratic termination. Some striking differences are observed when oxidation is carried out in a steady-state reactor. The hydroperoxide is no longer the sole primary product; cyclohexanol forms from the start. It has been shown10 that up to 30% of the cyclohexanol forms directly from the cyclohexane, the rest coming via the hydroperoxide. Furthermore, ketone and not the hydroperoxide has been implicated as the product mainly responsible for chain branching. In fact, the hydroperoxide levels in oxidation in steel vessels (as in the case of catalyzed oxidation) are much smaller than those in the case of oxidation in glass vessels. The loss of selectivity with conversion in the case of cyclohexane oxidation is because of the high reactivity of the oxidation products. Both cyclohexanol and cyclohexanone are oxidized many times faster than cyclohexane itself. At equal rates of initiation, the rates of oxidation of pure compounds follow the ratio95 cyclohexane, 1, to cyclohexanol, 40, to cyclohexanone, 27. The case of cyclohexanol is somewhat curious, as its addition at the start of the reaction actually retards the oxidation, whereas if it is added during the oxidation, it has an accelerating effect. This result has been rationalized10 as being due to the fact that, whereas cyclohexanol is more reactive than cyclohexane, the radicals that form from it are less reactive than the cyclohexylperoxy radicals. In the medium of oxidizing cyclohexane, cyclohexanol is converted almost quantitatively to cyclo-

hexanone; hence the beneficial effects of boric acid, which traps cyclohexanol, thereby preventing its further conversion and all of the side reactions that thereby result. Cyclohexanone is also a highly reactive compound in the oxidizing medium; the ratio of the rate constants for its formation and consumption has been estimated as 1:30. Although Berezin et al.10 claim from estimated values of activation energies of these reactions that higher temperatures should favor selectivities to the ketone, no clear evidence of this nature is seen in the available data. Water, which forms late in the reaction, seems to have an inhibiting effect on the reaction and can, under certain conditions, cause a second phase to form. When oxidation is carried out in a solvent medium such as acetic acid, decarboxylation of the solvent is a problem as in the case of MC oxidations. There are few reports of MC catalysts having been tried for cyclohexane oxidation, say in efforts to develop a single-step process to adipic acid. 6.3. Oxidation of Cumene The liquid-phase oxidation of cumene is a unique example of “uncatalyzed” reaction, taken to levels of conversion around 30%, that is commercially practiced.

To avoid problems of leakage and the attendant problems of safety, such oxidations are carried out in largesize bubble-column reactors (sparged reactors) that are jacketed, and in the recent past, even provided with coils inside the reactor. The liquid circulation velocities in such bubble columns are remarkably high, and fairly high values of the heat-transfer coefficient are realized even through the jacket, facilitating the heat removal problem. Typically, oxidation is carried out at around 135 °C (see Table 2) and a pressure at the top of the reactor on the order of 3-4 atm; liquid heights are typically 1015 m. The oxidation is carried out in 4-6-stage oxidizers to improve the selectivity. The bubble-column reactors are staged, as the extent of liquid-phase backmixing in bubble columns is very high. An optimized overall level of conversion is about 30%; above this level, the yield decreases because of side reactions associated with decomposition of the hydroperoxide to the carbinol, cleavage to acetophenone and methanol, etc.

At overall levels of conversion approaching 30%, inevitably some oxidation at -CH3 occurs along with


cleavage, and some formic acid is formed. This formic acid catalyses the decomposition of the hydroperoxide to phenol and acetone, and this, in turn, inhibits the oxidation. To obviate this problem, a small amount of water, containing buffered carbonate-bicarbonate, is used so that formic acid is instantaneously neutralized. Originally, the aqueous-phase volume used was high (even 5%), but in subsequent years, because of a better understanding of the process, this volume has been reduced to only 1-2%, and of late even “dry” oxidations have been adopted, i.e., use of water is rendered unnecessary because of better control of operating conditions. Because the level of conversion of cumene is only up to 30%, the remaining cumene, separated after the hydroperoxide is concentrated to about 80% (and of late even up to 90%), must be recycled and constitutes the major part of the feed to the reactor. This cumene, as well as cumene obtained from cleavage of cumylphenol and R-methyl styrene (AMS) dimers, obtained during acid-catalyzed cleavage of the hydroperoxide to phenol, has R-methylstyrene as an impurity, which gives a “bromine number” to cumene and is detrimental to the oxidation. This side-product must be suitably removed, e.g., via clay-catalyzed reaction to convert AMS to higher-boiling alkylated products, which can be easily removed by fractionation. The cumene-based phenol technology has been adopted for making m- and p- cresols and follows similar plant practices. However, in large plants, additional problems arise, as there is some oxidation at the methyl group as well.

The hydroperoxide on the CH3 group, upon cleavage, will give -CH2OH, and this further complicates the separation train. The oxidation of m- and p- diisopropylbenzene, which can be separated by distillation because of a significant difference in boiling points, eventually leads to dihydroperoxides, which, on cleavage, give m-dihydroxybenzene (resorcinol) and p-dihydroxybenzene (hydroquinone). The array of products that can be formed has parallels with that encountered in manufacturing phenol. However, since the switch over to the second isopropyl group starts when the first group is nearly fully converted to the hydroperoxide, the level of carbinols that are formed will be relatively much higher than those obtained in the case of phenol. This will also be the case for acetophenone-type products. The byproducts do have some commercial value. In the recent past, the dehydrated products of dicarbinols giving R-methyl styrenes have gained some importance, as they have

some unique properties, in some respect like divinyl benzenes, and as they can be converted to some value added substances, via reaction with phenol to make a special bisphenol, via reaction with HCN and conversion to the corresponding amines, or even direct reaction with ammonia to give the corresponding amines, as shown below.

The deliberate desire to make the above R-methyl styrenes would benefit from this route in view of mildness of conditions and high yields. Catalytic dehydrogenation of two isopropyl groups is most unlikely to give a high yield of R-methyl styrenes. 6.4. Oxidation of Isobutane The oxidation of isobutane in the liquid phase to tertbutyl hydroperoxide (TBHP) and tert-butyl alcohol (TBA) is widely practiced. Although products are important themselves (e.g., TBHP as an oxidizing agent and a free-radical initiator and TBA as a gasoline additive), a major application of the process in the past has been in the indirect oxidation process for the manufacture of propylene oxide, wherein TBHP is used to epoxidize propylene.8 The recent interest in the possibility of developing isobutane oxidation for the production of MTBE and some related issues have been discussed in section 4. MTBE can be produced conveniently by etherification of TBA with methanol. The

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production of isobutene from isobutane via oxidation to TBHP, decomposition of TBHP to TBA, and final dehydration, may be cheaper than catalytic dehydrogenation. However, if the TBHP is used for epoxidation, then the economics of producing isobutene from TBA improves dramatically.96 The first comprehensive work on isobutane oxidation was that of Winkler and Hearne,11 who carried out the liquid- and vapor-phase oxidation in a semibatch reactor. The authors do not recommend oxidation in the vapor phase because of the formation of unacceptably large quantities of side products. Hence, it has been suggested that a reaction product consisting essentially of TBHP and TBA can be obtained in high yield by reacting isobutane with molecular oxygen in the liquid phase of a vapor-liquid mixture at a temperature in the range of 373-423 K and a pressure of at least 25 atm. The prominent features of hydrocarbon oxidation, such as induction period, autocatalysis, and catalysis by the metal wall of the reactor, are evident in their work. Their results indicate that, with proper precaution to prevent wall catalysis, the total yield of TBHP and TBA is fairly constant between 90 and 96% at temperatures of 398 K and below, over a fairly wide range of conversion. Winkler and Hearne11 used di-tert-butyl peroxide as an initiator in most of their experiments. Under these circumstances, radical initiation in the initial stages is likely to be by the decomposition of the peroxide

ROOR f 2RO*

(16)

RO* + RH f ROH + R*

(17)

with R* taking part in the normal propagation reactions 5 and 6 (see section 5). If TBHP is the desired product, use of the initiator can have the undesirable consequence of reducing its concentration because of the reaction of the RO* radical with the hydroperoxide to produce the alcohol in a new propagation step.

RO* + ROOH f ROH + ROO*

(18)

In any case, the RO* radical may decompose to acetone and a methyl radical, a possibility that accounts for the presence of acetone in the reaction mixture. The methyl radical so produced leads to other products such as methanol, formic acid, CO, and CO2. The levels of most of these are quite small, as indicated by the high selectivity figures quoted above. If cobalt is used to catalyze the reaction, the consequences of catalyzed decomposition of tert-butyl hydroperoxide ensue, which in this case include, in addition to a faster rate of oxidation and more TBA, higher levels of RO* decomposition products. Winkler and Hearne11 proposed that the rate of the oxidation reaction in the liquid phase can be enhanced by carrying out the oxidation above the critical temperature of isobutane (407.9 K). However, it is necessary that the reaction be conducted in the presence of a relatively high boiling solvent. The use of externally supplied reaction solvents, e.g, organic acids, is discouraged by Winkler and Hearne11 because of the increased complexity of the oxidation reaction and subsequent product separation and recovery. The idea of using additives such as 2-propanol or water for improving the yield, which appears in several patents subsequent to the work of Winkler and Hearne,11 does not seem to

have been established in industrial practice, probably for the same reason. The conditions under which the oxidation of isobutane is normally carried out in industry (temperature of 393408 K and pressure of around 35 bar) are quite close to its critical properties (Tc ) 406.9 K and Pc ) 35 bar). In view of recent interest in studying chemical reactions under supercritical conditions, it can be expected that phenomena such as the clustering effect and the special ionization effects of the supercritical fluid would have a strong influence on the reaction mechanism.84,97 Surprisingly, limited literature is available on isobutane oxidation under supercritical conditions. However, the influence of supercritical conditions was explicitly addressed by researchers from the Shell oil company through patents.30,32 They carried out detailed studies on batch and continuous reactors to identify the effect of the important operating variables on productivity and selectivity under supercritical conditions and concluded that operation in the supercritical phase gives TBHP productivities several times higher than those obtained with the conventional liquid-phase process.30 A temperature range of 423-433 K has been claimed to be the best for high productivities. Baumgartner31 carried out a reaction of isobutane and molecular oxygen in a continuous stirred tank reactor (CSTR) with a residence time from 15 to 70 min, and concluded that operation in supercritical phase at less than 0.04 mol % oxygen concentration leads to TBHP selectivity better than that obtained at a high concentration of oxygen (1% mol/mol) in the feed. This is somewhat counterintuitive, as hydrocarbon oxidations are usually zero-order in oxygen, and if oxygen concentrations are too low for the zeroorder behavior to be observed, one only expects poorer selectivity because of the increased heterogeneity of the radical population. However, as has been pointed out, data in the low-oxygen regime is far too minimal in the literature to permit firm conclusions. Building on the work of Baumgartner,31 Foster32 employed a reactor configuration that keeps isobutane concentration high while keeping product and oxygen concentration low, which also results in better productivities. The proposed reaction system envisages a cascade of several CSTRs with the supercritical isobutane moving from one CSTR to the next in series and the oxygen being fed in a crosscurrent manner. Shah et al.98 have examined the kinetics of isobutane in the liquid phase and under supercritical conditions. The influence of supercritical conditions on the rate and selectivity of the reaction was investigated, and this permits a comparison of liquid-phase and supercriticalphase oxidation. The reactions were also studied in a glass-lined continuous reactor using predissolved oxygen. This mode of operation allows kinetic studies without an intervention of gas-liquid mass transfer and, in addition, eliminates the catalytic effect, if any, of the stainless steel walls. The reaction is autocatalytic, and the selectivity toward the hydroperoxide decreases with an increase in overall conversion. Under supercritical conditions, the rates and selectivity were significantly higher than those obtained in liquid-phase oxidation. In both subcritical and supercritical oxidations, temperature has an adverse effect on selectivity toward tert-butyl hydroperoxide. The temperature and wall material exhibit a strong impact on the reaction kinetics. TBHP has a tendency to decompose and react with isobutane to form TBA. The decomposition is


highly influenced by the wall material, and it is at its maximum in the unpassivated stainless steel reactor. Fan et al.84 have studied the oxidation of supercritical isobutane to TBA and have investigated the influence of silica-titania and palladium-carbon as heterogeneous catalysts. The authors compare the performance of the reaction under gas-, liquid-, and supercriticalphase conditions and conclude that the rate as well as selectivity to TBA are superior in the supercritical case. Some dehydration of TBA to isobutene was also observed in their work, which the authors attribute to the action of acid sites on the catalyst surface. Although their data show the superior rate and selectivity of supercritical oxidation over gas-phase oxidation (as is to be expected), comparison with liquid-phase oxidation is less clear, as the latter was conducted at a lower temperature. Furthermore, contrary to the claims of the authors, their results, in fact, show a smooth transition in rate and selectivity as the temperature is increased at a constant (supercritical) pressure to go from liquid to supercritical state. The promotional effect of the heterogeneous catalyst is, however, clearly demonstrated. Suresh,90 in his analysis of isobutane oxidation under supercritical conditions, has concluded that reaction kinetics in the liquid phase can be extrapolated conveniently in order to predict the kinetics under supercritical conditions. An enhancement in the reaction rate is realized under supercritical conditions. However, the modeling work indicates that this increase arises only because of the increase in temperature employed to bring the reaction mixture to the supercritical state. The fact that the rate expression used for predicting the liquid-phase kinetics works reasonably well under supercritical conditions indicates that the “supercritical” state of the substrate, as such, does not induce any remarkable effect on either the rate of the reaction or the selectivity toward the desired products. 6.5. Oxidation of Cycloalkenes The interesting and diverse chemistry exhibited by the oxidation of saturated hydrocarbons is also seen in the oxidation of unsaturated hydrocarbons. Early studies (see Kamneva and Panfilova99) were concerned with elucidating the structure of the hydroperoxide. Criegee et al.100 demonstrated that cyclohexene hydroperoxide has an open structure with the double bond retained. Van Sickle et al.101 discuss some of the early work. Among the cycloalkenes, cyclopentene is the most reactive, and cyclooctene is the least reactive. The authors have emphasized that many olefins react partly or fully through addition of peroxy radicals to double bonds rather than through hydrogen abstraction (or transfer). The products formed from these oxidations depend on whether the hydrogen abstraction or the addition reactions dominate.

RO*2 + R ) RO2H + R*

(19)

RO*2 + R ) RO2R

(20)

Most of the cycloalkenes give the hydroperoxide as the primary product. However, cyclooctene differs from its analogues in that it gives high yields of the epoxide, presumably through the addition of the peroxy radical.

RO*2 + R ) RO* + epoxide

(21)

Furthermore, in the case of cyclooctene, phase separation occurs even at conversions as low as 0.4%. Figure 1 provides a schematic diagram of the various chemical pathways for cycloalkenes. Murphy et al.,102 in their lucid review of allylic oxofunctionalization of cyclic olefins, point out that allylic oxidation (which preserves the unsaturation and results in R,β-unsaturated ketones and alcohols of cyclic olefins) and epoxidation are two competing processes both in vivo and in vitro. Allylic oxidation (involving free radicals) is most likely in the presence of low-oxidationstate transition metal species, and epoxidation is to be expected in the presence of species such as Ru(VIII), Cr(VI), etc., but the two are often competitive processes in practice. The process that dominates depends, among other things, on the nature of the olefin and the relative stability of the allylic radical formed. With the more complex cyclic olefins, competitive isomerization and structural rearrangements often result in poor selectivity to the desired R,β-unsaturated ketone. The authors illustrate these points with the aid of examples taken from the industrially relevant cases of cyclohexene, isophorone (to ketoisophorone, a key intermediate in the synthesis of carotenoids and flavoring substances), and R-pinene (to verbenone, considered a suitable precursor to taxol). Developments in both homogeneous and heterogeneous catalysis are reviewed. Mahajani et al.103 have examined the kinetics of the oxidation of cyclohexene. Cyclohexene is emerging as an important raw material for cyclohexanol, cyclohexanone, cyclohexene epoxide, cyclohexenol, cyclohexenone, 1,2-cyclohexanediol, and cyclohexadiene.104,105 The successful commercialization of the selective hydrogenation of benzene by Asahi chemicals provides a cost-effective route to this raw material. The oxidation of cyclohexene is, in some respects, similar to that of cyclohexane. The various chemical pathways for cyclohexene are shown schematically in Figure 1. The uncatalyzed oxidation of cyclohexene yields large quantities of the hydroperoxide, and its decomposition yields the alcohol and ketone. The uncatalyzed reaction also yields cyclohexene oxide. Moreover, the oxide appears to be a primary product, but the exact mechanism for its formation from the hydrocarbon is not clear. The kinetic model proposed by Suresh et al.49 for cyclohexane oxidation applies equally well for this reaction. The oxidation of cyclohexene can be taken to much higher levels, approaching 20% conversion, than in the case of cyclohexane, for which conversions of 3-5% are needed to ensure high selectivity. The products obtained from cyclohexene allow a wider range of speciality chemicals to be produced. The hydrogenation of benzene is the only viable route to cyclohexene, and selectivity is only on the order of 50%. In the case of cyclododecene, almost quantitative yields can be obtained by the selective hydrogenation of cyclododecatriene, which, in turn, can be conveniently made by the selective trimerization of butadiene. Similarly, the selective hydrogenation of cyclooctadiene yields cyclooctene. Thus, there may be a distinct advantage in obtaining cyclododecanone and other products, similar to those realized in the oxidation of cyclohexene via the oxidation of cyclododecene rather than from cyclododecane. Interest in this area can be discerned from some patents for the oxidation of cyclododecene106,107 using a ruthenium/cerium system as the catalyst. In the first of these,106 a two-phase system

3978


Figure 1. Prominent chemical pathways in cycloalkene oxidations.

is used with ruthenium tetroxide in the organic phase as the oxidizing agent. The tetroxide is regenerated in the second (aqueous) phase, which contains cerium ions in the +4 state. The latter, it is suggested, can be regenerated electrolytically in a separate step. The second patent107 provides a way of combining these steps in order to achieve simultaneous regeneration of the cerium (+4) species. Van Sickle et al.101 have reported preliminary data on the noncatalytic oxidation of cyclododecene, where substantially high yields of epoxide have been indicated. Recent unpublished results from Monash University,108 carried out in a way to obtain data of commercial importance, clearly establish the formation of the epoxide, in addition to cyclododecenone; there are clear

differences in the rate of oxidation of the cis- and transcyclododecene. The epoxidation of alkenes (cyclic as well as noncyclic) using molecular oxygen in a single step has remained a challenge in the field of oxidation chemistry. Indirect oxidations in two steps (that of propylene to propylene oxide is an example; see section 4) are the rule in industry. Recently, Iwahama et al.109 reported a onepot epoxidation of alkenes (such as octene-1-cyclohexene and cis- and trans-2-octene) using molecular oxygen, using hydrocarbons such as ethylbenzene and tetralin as the hydrocarbon source under mild conditions. The autoxidation of the hydrocarbon was assisted by Nhydroxypthalimide (NHPI), and the epoxidation of the alkene with the resulting hydroperoxide was catalyzed


by Mo(CO)6. Although the rates were generally low under the conditions employed (6-12 h at 60-70 °C), the results are interesting in view of the excellent selectivities reported. With cyclohexene and cyclooctene, selectivities of 74% and 83%, at conversion levels of 80% and 89%, respectively, were obtained. The use of NHPI seems to overcome the mismatch between the much higher rate of oxidation of the hydrocarbon to hydroperoxide as compared to the rate of epoxidation in conventional systems. In the case of adamantane, in the presence of carbon monoxide and oxygen, 1-adamantane carboxylic acid was formed. 6.6. Oxidation of Vinyl Cyclohexene and Vinyl Cyclohexane These reactions are potentially attractive both scientifically and commercially, as these versatile olefinic compounds are available in high yield from butadiene.

In the case of vinyl cyclohexene, the oxidation can take place at the olefinic double bond in the ring, as in the case of cyclohexene, or at other allylic positions. There is really scanty information on autoxidation of this important olefinic compound, although some information is available on Wacker-type oxidation, with cupric chloride-PdCl2 catalyst, to a ketonic compound. Biela et al.110 have studied the autoxidation of this olefin as well as vinyl cyclohexane, which can be obtained by selective ring hydrogenation of vinyl cyclohexene. In the case of vinyl cyclohexane, about 40% of the absorbed oxygen goes to the peroxides. In vinyl cyclohexene, the C-H bonds in position 6 are preferentially attacked, although other allylic positions also undergo oxidation to give the products illustrated below.

6.7. Miscellaneous Oxidations Interest in the epoxidation of alkenes has been mentioned earlier (see section 6.5) because of the importance of such a process from the synthetic and industrial points of view. The work of Iwahama et al.109 was discussed earlier and involves some interesting chemistry. These authors have demonstrated the Mocatalyzed epoxidations of a wide variety of alkenes with hydroperoxides generated in situ by the NHPI-catalyzed aerobic oxidation of hydrocarbons. The selectivity to the epoxide depended, among other things, on the hydrocarbon used, ethylbenzene and tetralin giving good results whereas toluene gave poor results with oct-2ene, probably as a result of competing reactions in the case of toluene. Although Mo(CO)6 was employed as the epoxidation catalyst in most of their work, the epoxidation of allylic alcohol, trans-hex-2-en-1-ol, to epoxy alcohol was achieved in high yield when VO(acac)2 was employed instead, even with very small amounts. It is possible that VO(acac)2 not only catalyzes the epoxidation, but also serves to activate NHPI, a function performed by cobalt acetate in the other oxidations. Another recent development of interest in this area is the direct epoxidation of linear aliphatic olefins by molecular oxygen using Schiff base complexes.111 Possibilities of heterogenizing the catalyst by encapsulation in modified zeolite cages have also been demonstrated, and good selectivities in both configurations have been realized at reasonable conversions. Mayo et al.112 have reported some interesting results on the oxidation of R-methylstyrene (AMS) at 110-160 °C. For temperatures up to 100 °C and adequate oxygen, the principal product is the alternating polyperoxide. This peroxide is reasonably stable at the temperature of oxidation but cleaves cleanly to acetophenone and formaldehyde at higher temperatures and reduced pressure. Between 100 and 120 °C and at atmospheric total pressure, the principal products are AMS oxide and acetophenone; it appears that there was mass-tranfer limitation and that the liquid phase was not saturated with oxygen. 7. Kinetics of Hydrocarbon Oxidation

The relative product distribution with respect to the cisand trans- isomers is not obvious (see also section 12.8). In the case of vinylcyclohexane, the following dominant products are obtained in equimolar amounts:

The other products are shown below.

The determination of kinetics of liquid-phase organic oxidations presents a nontrivial problem. The quantitative analysis of the various products from a reactor is, by itself, a significant challenge. In addition, there are other issues to consider. First, as is obvious from the preceding discussion on the chemistry and kinetic mechanism of these reactions, a detailed treatment of kinetics is usually not feasible; fortunately, it is also probably unnecessary for engineering applications. An engineering approach to kinetics, involving lumped models, is thus usually followed. One postulates a series-parallel reaction network that attempts to capture the essential features of the reaction depending on the purpose for which the kinetic model is intended. Second, because the most convenient way for conducting the reaction is through contact of a liquid, hydrocarbon under the appropriate conditions with a gaseous stream

3980


containing oxygen, the possible influence of masstranfer factors on the observed behavior must be considered carefully in interpretations of rate data. Finally, although one would like laboratory reactors and their operation to be flexible in order to explore all the features of the reaction, considerations of safety impose restrictions on reactor design and operation. These issues have resulted in several innovative approaches to the determination and interpretation of technical kinetics in the field of organic oxidations. 7.1. Laboratory Reactors Early studies considered the reaction to be generally slow enough for intrinsic kinetics to control in any laboratory reactor that provides reasonable mass-tranfer rates. Several arguments can be advanced against such an assumption. First, the reactions are known to exhibit autocatalytic features, and the reaction rate increases as the products accumulate. Therefore, the influence of mass transfer can become important during the course of the reaction, even if the reaction is initially slow and kinetically controlled. Second, at the elevated temperatures and pressures necessary for these reactions, the knowledge of mass-tranfer coefficients, even in laboratory reactors, is far from perfect, and extrapolation from data and correlations under ambient conditions is hazardous.48 For many systems, even properties such as solubility and diffusivity are not documented in the open literature and must be estimated for the conditions of reaction. (The available information on these aspects will be reviewed in section 9.) Thus, one must necessarily consider the implications of mass transfer with chemical reaction in designing kinetic experiments and interpreting data from them. The theories of mass transfer with chemical reaction indicate that gas-liquid reactions can occur in one of several “regimes”, depending on the relative rates of mass transfer and chemical reaction. To determine the kinetics with reasonable accuracy, the reaction must take place in the slow-reaction regime with substantial kinetic control (in which case the overall rate is determined by the reaction rate in the bulk liquid) or in the fast-reaction regime (in which the reaction is complete in a very small region close to the interface). Several “model” reactors suitable for kinetic studies on gasliquid reactions have been described in the literature (such as laminar jet, wetted wall column, stirred contactor, etc), in which the mass-tranfer rates can be characterized with a high degree of confidence (see, for example, Doraiswamy and Sharma43). An appropriate choice of the model reactor must be made, depending on the velocity of the reaction being studied, so that the right regime can be engineered and the reaction kinetics determined. The conditions of organic oxidations (elevated temperature and pressure) usually necessitate considerable design modifications if such model contactors are to be used. Most of the kinetic studies described in the literature have therefore been conducted in miniature versions of industrial equipment, such as sparged and mechanically agitated reactors, bubble columns, etc. Because the mass-tranfer parameters needed to properly interpret reaction data from these reactors are usually not known with confidence (especially under the reaction conditions employed), it is hazardous to infer the regime on the basis of conventional tests such as the effect of agitation speed, etc. For example, the effect of agitation speed on interfacial

area is quite different at low and high speeds in pressurized contactors46,113 and not taking cognizance of such differences can result in erroneous conclusions being drawn. It is therefore preferable in such studies to rely on some direct indicator of the reaction regime. In their studies on cyclohexane oxidation, Suresh et al.49 developed a technique for measuring dissolved oxygen levels during reaction. The presence of measurable levels of dissolved gas shows that the reaction is in the slow-reaction regime with some degree of kinetic control. These authors observed that the reaction starts off being kinetically controlled (with the liquid being saturated with oxygen at the prevailing partial pressure) but gradually moves to other regimes because of autocatalysis. This transition could be followed directly with the help of the observed variation in dissolved oxygen levels, and the part of the experiment in which kinetics had a role in determining the observed absorption rates could be identified. Even at the rates of mass transfer that could be achieved in small mechanically agitated contactors with intense agitation, measurable concentrations of dissolved oxygen could be detected only at conversions less than 5-7%. Therefore, it seems incorrect to assume the absence of mass-tranfer limitations in such reactions in general. An alternative strategy for circumventing the problem of mass-tranfer interferences in kinetic measurements is to eliminate the mass-tranfer step altogether by predissolving enough oxygen in the liquid before the start of the reaction and then to conduct the reaction homogeneously. The zero-order dependence of oxidation rates on oxygen is an advantage here, as the oxygen concentration need not be followed as long as it does not become so small during the reaction as to call into question the zero-order assumption. However, if reasonable conversions must be achieved, acievement of the oxygen requirement necessitates high pressures at the dissolution stage. Suresh et al.49 used this principle to study cyclohexane oxidation in small batch reactors (“microautoclaves”). Oxygen was dissolved by equilibration at high pressures at room temperature, and the temperature was rapidly raised by plunging the reactor in a fluidized bed maintained at the desired reaction temperature. Reaction was stopped by plunging the reactor in a cold-water bath, the oxygen conversion was measured by slow depressurization and measurement of the volume of the oxygen released, and the liquid contents of the reactor were analyzed for hydrocarbon conversion and product profile. In such work, because each experiment gives a single conversion point, it is important to reproduce induction periods exactly in a series of experiments by thorough cleaning between runs. Passivation of the walls by procedures similar to those used by Winkler and Hearne11 may also be necessary to prevent wall catalysis. With sufficient care, however, reliable kinetic information can be obtained. Wen et al.63 and Guo61 have used a similar principle to study cyclohexane oxidation in homogeneous continuous-flow reactors. The cyclohexane was saturated with oxygen at room temperature at the required pressure (depending on the conversion desired), and the saturated liquid was pumped through the reactor. At the outlet of the reactor, the pressure was allowed to decrease, and the flow rates of the gas evolved and the liquid were separately measured. Wen et al.63 used a CSTR constructed from a length of tube and stirred by a magnetic ball moved to and fro by the action of an


external magnet, whereas Guo61 used a glass-lined plugflow reactor. The usefulness of these reactors is demonstrated by the internal consistency of the data obtained on the oxidation of cyclohexane from several types of reactors, including the mechanically agitated reactor, batch microautoclaves, CSTR, and PFR. Taken together, these studies49,61,63 represent the most comprehensive studies on a single oxidation reaction of commercial importance. Attention was drawn earlier to the presence of an induction period in organic oxidations and some of the consequences that follow from it. Induction periods can be quite long in uncatalyzed oxidations and may not be quite eliminated even in the presence of some reaction products or dissolved catalysts.49,93,114 The length of an induction period can be influenced by the presence of small levels of impurities, wall catalysis (in steady-state reactors), etc. Variability of induction periods is particularly important in laboratory studies, where the small volumes of reactors employed result in an exaggerated effect of such factors. Although the use of glasslined reactors, or passivation techniques to render the wall noncatalytic if steady-state reactors must be used, is recommended for complete reproducibility of batch data, Suresh et al.49 have shown that good reproducibilty of post-induction-period behavior in batch experiments can be achieved with thorough cleaning of the reactor between experiments, even if induction period variability is not totally eliminated. This also means that, in flow reactors, any differences in induction behavior do not influence steady-state behavior. However, in batch microautoclave studies, in which several experiments are necessary to construct a single time course of the reaction, absolute reproducibility of induction periods is a must. Some authors (for example, Winkler and Hearne11 and Morbidelli et al.115) attempt to eliminate induction periods by the use of free-radical initiators, but (depending on the concentrations employed) such initiators could affect more than just the induction behavior, and it is difficult to be certain that the kinetics one obtains is not itself influenced. Thus, initiators could reduce the chain length (see discussion in section 5) to a point where initiation and termination mechanisms influence the overall behavior more than the propagation reactions. Indeed, the data and model of Morbidelli et al.115 for ethylbenzene oxidation show no autocatalysis, perhaps as a consequence of the use of fairly high levels of azobisisobutyronitrile (AIBN) as an initiator. The presence of an induction period can often be used to advantage in the measurement of various physical parameters in gas-liquid contact under the conditions of reaction. The reaction is slow enough to be neglected under these conditions, and one sees essentially a physical absorption behavior. This was first demonstrated by Suresh et al.,49 who determined oxygen solubilities and mass-tranfer coefficients at reaction temperatures and pressures for the oxygen-cyclohexane system by taking advantage of the induction period. More recently, Tekie et al.116 have also used the same method to determine the mass-tranfer characteristics for the oxygen-cyclohexane system with two types of contactors. Aspects of safety deserve the utmost attention in the design and operation of laboratory equipment for hydrocarbon oxidation studies. Apart from the usual precautions to be taken in the design and operation of

pressure vessels (provision of high pressure and temperature alarms, pressure-relief systems etc.), the dangers inherent in hydrocarbon-air contact must be considered in detail. Furthermore, to serve their purpose adequately, experimental reactors must be built so as to be flexible and allow operation over a wide and varied range of conditions. It is to be ensured that safety is not compromised in such designs. Some of the important considerations are summarized in section 11. 7.2. Kinetic Models from Laboratory Studies It is clear from the discussion of the mechanisms that operate in hydrocarbon oxidations that any attempt to adequately reflect all aspects of the mechanism in a kinetic model is bound to fail, not only because of the inherent detail and complexity of these mechanisms, but also because of the fact that not all pieces of the mechanistic puzzle are available yet. Under such circumstances, therefore, one must be satisfied with empirically derived models of technical kinetics that are meant to address specific modeling or simulation needs, with the understanding that the model or the parameter or both may require modification as the needs change. In literature, one sees several such models. In many such cases, an attempt is made to show that the kinetic model is consistent, or at least not inconsistent, with the basic free-radical mechanism. Lumped kinetics, involving a series-parallel network of reactions have been written for a number of oxidations, in which the lumping follows the logic of process objectives. Thus, in cyclohexane oxidation, if the emphasis is on producing a mixture of cyclohexanol and cyclohexanone in the first stage of air oxidation, a series scheme such as A f B f C might be considered (for example, see Spielman117), where A stands for cyclohexane; B, a mixture of alcohol and ketone; and C, the secondary oxidation products such as acids and other undesired compounds. If the ratio of alcohol to ketone is important, the lump B can be further split up and the kinetics elaborated to include the formation of the ketone from the alcohol. If, on the other hand, the process objective is to stop at the hydroperoxide, then the kinetic model would reflect this by introducing the hydroperoxide as the primary product, the alcohol and ketone (either separately or as a lump) as the secondary product, and acids as the tertiary product. Clearly, the experimental plan to establish such kinetics could also be dictated by the modeling approach. The kinetic models of Morbidelli and co-workers54 for the oxidation of p-xylene and the early models for cyclohexane oxidation34,36,117 and isobutane28 are of this type. Although such models usually incorporate explicitly the observed zero-order behavior of the kinetics with respect to oxygen, autocatalysis, another commonly observed characteristic of most hydrocarbon oxidations, is only accounted for indirectly, if at all. For example, whereas the early models for cyclohexane oxidation do not mention autocatalysis at all, the model for isobutane oxidation proposed by Brejc et al.33 consists of a network of reactions whose kinetics are such that autocatalysis is indirectly the result. Botton et al.,92 looking to include explicitly a term in the concentration of reaction products to explain autocatalysis in cyclohexane oxidation, found that an expression that was first-order with respect to (lumped) products was able to correlate the data well. Suresh et al.49 further elaborated these kinetic expressions by invoking the free-radical mech-

3982


anism and obtained the following expressions for the rates of consumption of cyclohexane (c) and oxygen (o) and the rate of formation of the intermediates i (cyclohexyl hydroperoxide + cyclohexanol + cyclohexanone), respectively, in the uncatalyzed oxidation of cyclohexane:

rc ) ro ) ri )

k01k3cpcL (k01 + k02cp + k3cL) k3cpcL(k01 + k02cp) (k01 + k02cp + k3cL) k3cpcL(k01 - k02cI) (k01 + k02cp + k3cL)

(22)

(23)

(24)

Here, cp, cL, and cI are, respectively, the concentrations of (lumped) products, oxygen, and intermediates. Although the rates are expected (from the mechanism) to be first-order in cyclohexane, because of the low conversions to which cyclohexane oxidation is usually run, the concentration of cyclohexane is essentially constant and does not figure in the rate expressions. The equations were tested by fitting them to data obtained from two types of reactors in the kinetic regime and were found to provide a consistent explanation across both reactor types and a range of oxygen and product concentrations. Recently, Suresh90 has shown that similar principles can be used to obtain rate expressions for the oxidation of isobutane in the liquid and supercritical phases. Mahajani et al.103 have also shown the applicability of the cyclohexane oxidation model above for the oxidation of cyclohexene. It thus appears that some generalization of rate forms for hydrocarbon oxidation can be attempted along these lines. Similar equations have been used to describe catalytic oxidation as well, as at small catalyst concentrations, the qualitative features are similar in catalyzed and uncatalyzed oxidations. An interesting observation made by Suresh et al.93 was that the (Co-) catalyzed oxidation retained all of the features of the uncatalyzed oxidation as described by the equations given above, the effect of the catalyst (1.5 ppm Co added as cobalt naphthenate) being the same as that of a 10 °C increase in temperature. However, at late stages of oxidation (perhaps outside the range of industrial interest), the rate was observed to fall in the catalyzed case, perhaps because the catalyst was precipitated out as the adipate. It would be useful to look for similar correlations in other oxidations as well. 8. Processing Options 8.1. The Case for Liquid-Phase Air Oxidation Although this review concerns liquid-phase air oxidations exclusively, in view of the fact that other types of oxidations (e.g., the use of stoichiometric oxidizing agents and oxidations in the vapor phase) also enjoy a significant presence in the chemical industry, it is not out of place to consider briefly the relative merits and demerits of the various types of oxidations. Let us first consider the use of air (or oxygen) as an oxidizing agent as opposed to stoichiometric reagents such as permanganate, dichromate, etc. The main problem with the latter reagents is the large amounts of byproduct salts that must be disposed of.12 Nitric acid

oxidations usually give good yields of aromatic acids,16,17 but except in the case of adipic acid (where nitric acid oxidation of cyclohexanone has been preferred over the air oxidation in industry), the need to dispose of nitrated byproducts and handle nitrous oxide emissions has led to its displacement by autoxidation methods. In most cases other than adipic acid, autoxidation yields (for example, using the Mid-Century processes) are comparable to or exceed those obtained by using stoichiometric oxidizing agents. There are no reports of the MC method having been tried for adipic acid, as noted in section 4. The next thing to consider is the case for liquid-phase oxidation (LPO) as opposed to gas or vapor-phase oxidation (VPO). VPOs are amenable to solid-phase catalysis; the large surface areas that can be packed into a small mass of the catalyst make for good economy. Any diffusion limitations would also be expected to be much less severe in gas-solid systems as compared to liquid-solid systems. In general, under otherwise comparable conditions, VPO is more economical than LPO. Often, however, poor selectivity prevents VPO from being economical. For example, whereas VPO of oxylene to phthalic anhydride is much preferable to LPO (to phthalic acid followed by dehydration), VPO of cumene is invariably accompanied by cracking (to benzene and propylene). LPO gives the hydroperoxide in the latter case. The poor selectivities of VPOs are often due to the fact that the conditions required are more severe than those required for LPOs (except in the case of lighter hydrocarbons such as isobutane). The temperature required for gas-phase oxidation to take place at appreciable rates (300-500 °C) is usually much higher than that for LPO (140-170 °C).10,12 The other factor that makes for significant differences between LPO and VPO is the large difference in density between the liquid and vapor phases. The higher density of the liquid phase usually makes for better productivities in the case of liquid-phase oxidations. Thus, space time yields are often an order of magnitude higher for LPOs.12 The higher temperatures, low concentrations of hydrocarbon normally employed (typically less than 3% in air12), and low rates also mean that gas-phase processes tend to be more energy intensive. As pointed out above, the question of selectivity is an important one in the consideration of LPO and VPO for a given substance. In general, it is uncommon to find comparisons being made for oxidations in gas and liquid phases at comparable temperatures. Mayo et al.112 have reported radical-initiated oxidations of isobutylene in benzene solution at 80 and 147 °C and several atmospheres of total pressure and compared the results with gas-phase oxidations at 147 and 197 °C and 0.1-0.5 atm total pressure. It is interesting to find that the oxidation reaction occurs mostly by the addition mechanism and that all oxidations give mostly acetone, isobutylene oxide, and a high-boiling residue. Similarly, radicalinitiated oxidations of cyclopentene were studied at 100 °C, at concentrations from 9 M in the neat hydrocarbon to 0.025 M in chlorobenzene and from 0.027 M to 0.056 M in the gas phase. Mayo et al.112 found the rates and products of reaction to be similar in the two phases. The main product is cyclopentenyl hydroperoxide, and it causes autocatalysis and gives secondary products. On the other hand, Bulygin et al.118 find that selectivities are usually much better in LPO. Although, in some cases, the observed differences in selectivity can be attributed once again to density differences, there are


others in which differences in chemistry also play a part. Thus, in the case of isobutane, the differences in the product spectra from gas-phase and liquid-phase processes can be narrowed down by diluting the liquid phase with CCl4 in the latter case and increasing the concentration of the reactant in the former.2 However, Berezin et al.10 have shown that the observed differences in the case of oxidation between LPO and VPO cannot be explained from considerations of density alone and that changes in reaction mechanism must be taken into account. The higher temperature leads to more C-C bond scission than in the LPO case, and hence, more smaller molecules are produced; furthermore, products of combustion, namely, CO and CO2, form in higher amounts. Thus, one loses out on selectivity to the desired compounds. This would probably also explain why gas-phase oxidation has a greater chance of success with short-chain hydrocarbons. In particular cases, spectacular selectivity can be obtained in VPO through the use of highly specific solid catalysts; an example is the conversion of butane to maleic anhydride.12 From the point of view of engineering, LPOs are simpler.12 They are usually carried out in CSTRs or bubble columns, which possess the advantage of superior heat-transfer characteristics for the exothermic LPO reactions. Solid catalytic reactors for VPOs tend to be of the fixed-bed tubular type, and multitubular reactors are required in the interest of efficient heat removal. These are expensive to fabricate. Fluidized beds are used in the oxidation of butane (as well as in the ammoxidation of propene to acrylonitrile) and exhibit excellent heat-transfer characteristics. Catalyst deactivation and replacement can lead to additional constraints on the reactor design. Furthermore, control of LPOs is easier with little possibility of runaways. VPOs exhibit a much more complex behavior than LPOs with cool flames, hot flames, etc. On the other hand, on the issue of safe operation, it must be considered that the hydrocarbon inventory in VPO processes is much smaller than that in LPO processes. LPOs are almost invariably homogeneously catalyzed. Although catalyst recovery presents problems and catalyst cost must be be borne in mind (especially where complex mixtures such as the MC catalyst are involved), relative proportions are easily adjusted in the liquidphase process. Catalyst composition can even be varied within limits during the oxidation. Catalyst concentration is also easily varied. In view of the above, it is not surprising that gasphase oxidations have not been studied in the same detail as LPOs. Of late, reactions in supercritical media have been attracting some attention in the context of organic oxidations.90,97,98,119 The case of isobutane in the liquid vs supercritical phase is referred to in sections 6.4 and 7.2. It was shown that the rate of oxidation in the supercritical phase could be predicted from that in the liquid phase on the basis of the temperature effect. Having considered the case for liquid-phase oxidations, we must next examine the choice of oxidizing gas. In the past, air has invariably been chosen for reasons of cost; in some cases, enriched air (up to 28% oxygen120) has been used also. Because most organic oxidations are zero-order in oxygen down to very low oxygen concentrations, if conditions are such that the kinetic regime prevails, rate and selectivity characteristics would not be seriously affected by the choice of gas. However, because oxidations are autocatalytic, it is possible for

one of the mass-tranfer-limited regimes to arise as the reaction proceeds, and indeed, the relevance of masstranfer factors has been demonstrated in several cases (this is discussed further in section 9). If the bulk liquid can become starved of oxygen, one would, in general, expect that higher oxygen partial pressures would be beneficial. There are other factors also to consider in chosing the oxidizing gas. If substantially pure oxygen can be used under conditions such that most of the oxygen is absorbed, then the volume of gases to be treated before venting decreases. Condensation of the hydrocarbon from the reactor exhaust gases is also facilitated if the concentration of noncondensables is reduced. Thus, there would appear to be several advantages in going to enriched air or even pure oxygen. However, the safety issues must be examined very carefully. Praxair Technology Inc.55,120 has patented what it calls the “Liquid Oxidation Reactor” (LOR), which uses pure oxygen. Through careful design of the reactor internals (which include a draft tube to promote liquid circulation) and mixing devices, it is claimed that the oxygen absorption efficiencies are improved to a point where 90-99% of the oxygen entering is absorbed. Several advantages are claimed, such as operation at lower temperatures and pressures, lower solvent combustion rates (such as of acetic acid in p-xylene oxidation), and lower levels of partially oxidized impurities. It is also claimed that the good mixing conditions inside the reactor and high extents of absorption before the gases leave the reactor ensure that safety is not compromised. Furthermore, appropriate modifications in the reactor hydrodynamics120 make it possible that the heat of reaction is removed by evaporation of the hydrocarbon and that heat-transfer surfaces inside the reactor are rendered unnecessary. In MC oxidations, the carboxylic acid product is often insoluble in the reaction medium and therefore precipitates out on cold surfaces, necessitating a frequent cleaning of such surfaces inside the reactor. Elimination of such surfaces therefore has an advantage. 8.2. Reactor Configurations and Materials Most oxidations are performed under pressure, and it would be useful to compare the benefits and shortcomings of sparged reactors versus mechanically agitated contactors. When the demand on the volumetric mass-tranfer coefficient (kLa) is not very high and heat loads are reasonable, such as in the oxidation of cumene, large-size sparged reactors, with diameters up to 4-5 m, are widely used. In view of the high heat-transfer coefficient, even jacket cooling may prove to be adequate. In any case, coils can be inserted in sparged reactors. Here, the liquid phase is essentially backmixed, but the gas phase is largely in plug flow and, should the rate be dependent on the partial pressure of oxygen, it will provide additional advantage, as in mechanically agitated reactors, backmixing in the gas phase is pronounced. Should there be a large loading of solids, e.g., a product, as in the case of oxidation of p-xylene to terephthalic acid (PTA) in acetic acid medium, unusually high gas velocities will be required for uniform suspension of particles, and this, quite apart from excessive power consumption, may even jeopardize the safety as the partial pressure of oxygen (pO2) at the outlet may not be in the desirable range. Thus, mechanically agitated reactors are used in PTA production

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even though problems of leakage through shaft sealing and shaft mechanical soundness are possible problems. Krishna and co-workers121-123 have recently reviewed the strategies for multiphase reactor selection. They recommend a three-level strategy for reactor selection. At the first level, in the case of gas-liquid systems, they suggest a decision on the dispersion mode (gas-in-liquid, liquid-in-gas, or bicontinuous), based on the reaction locale (as determined from Hatta number calculations). Different dispersion modes are associated with different ranges of the parameter aˆ DA/kL, which determines the volume ratio between the film (i.e., the near-interface region) and the bulk. Some additional considerations are warranted in the case of liquid-phase oxidations because of the peculiarity of the kinetics involved. Thus, a low value of the Hatta number (say, as occurs at low conversions in most cases) would suggest a gas-dispersedin-liquid mode of operation, which is, indeed, the type of operation used in industrial practice (for example, in cyclohexane oxidation). However, one must be careful in extending the argument too far. Given that masstranfer coefficients in such equipment do not vary a great deal, the film-to-bulk volume ratio can be decreased by decreasing the gas holdup and the interfacial area. However, such devices would push the reaction toward the diffusional end of the slow-reaction regime, with the result that, although the reaction occurs in the bulk liquid, it does so at negligible oxygen concentrations with the possible consequence that the chemistry shifts to first-order mechanisms with an attendant loss of selectivity. Furthermore, because of the autocatalytic nature of the kinetics, the reaction could, as conversion increases, take place to a significant extent in the film, and determinations of the reaction locale with no accounting for the autocatalytic kinetics could lead to the wrong conclusions. Aspects of mass transfer with chemical reaction in organic oxidations are discussed further in section 9.2. Liquid-phase backmixing is another consideration that merits attention in the context of hydrocarbon oxidation in view of the kinetic mechanisms involved, particularly when the desired product is an intermediate prone to further oxidation. Selectivity considerations would normally dictate a reactor with minimum backmixing, but the autocatalytic kinetics (particularly in uncatalyzed oxidation) would call for a minimum backmixing for the reaction to get started. Considerations of safety would also require good mixing conditions in the reactor. These diverse requirements can be reconciled by having several mixed reactors (or bubble columns) in series, with the hydrocarbon moving from one stage to the next while the gas is fed to all stages in parallel in a crosscurrent manner. The above considerations revolve mostly around the rate of oxidation. Often, a more important issue is the selectivity of oxidation. If the desired products are the intermediates, as in most oxidations, it becomes important to choose reactors with high mass-tranfer rates so that the reaction can be made to occur in the bulk up to reasonable conversions. This is because the selectivity to the intermediates is known to suffer in consecutive reaction schemes in the event of film reaction. These considerations thus help to rationalize the preponderance of bubbling- and agitated-type contactors for industrial oxidations.

9. Role of Mass Transfer in Liquid-Phase Oxidations The analysis of gas-liquid oxidation reactors requires separation of mass-transfer and reaction-kinetics effects and is complicated by the need to consider the coupling between mass transfer and chemical reaction. The theory of simultaneous gas-liquid mass transfer and chemical reaction has been the subject of numerous theoretical and experimental investigations (a comprehensive review is available in Doraiswamy and Sharma43). The behavior of such reactors is dominated by the relative rates of mass transfer and chemical reaction. The location of the reaction and the impact of operating variables on the reaction are also dictated by these rates. Thus, a rational design of gas-liquid reactors is dependent upon an understanding of this complex interaction. Although technical kinetics are usually determined under the (temperature and pressure) conditions of reaction, difficulties arise in prediction of mass-tranfer rates. The oxidation reactions discussed in this review take place at high temperatures and elevated pressures. Our knowledge base on mass transfer almost entirely rests upon experiments conducted under ambient conditions. It is not at all clear that extrapolation of such data to high temperatures and pressures is a valid exercise. In this section, we first review the status of data on mass-tranfer-related parameters and then consider theoretical advances in bringing organic oxidations within the ambit of the theories of mass tranfer with chemical reaction. 9.1. Mass-Transfer Rates at Elevated Temperatures and Pressures and under Actual Oxidation Conditions The mass-tranfer rate is determined by the product of the mass-tranfer coefficient (kL) and the interfacial area per unit volume (a). Experimental techniques have been developed to measure both the product and the interfacial areas.43 The measurement of interfacial area uses either a light-transmission technique46,124 or a model chemical reaction.125,126 The latter technique gives an average interfacial area. Some discrepancies between these techniques have been noticed for bubbling reactors.46 In addition, there are only a few model reactions available for use at high temperatures and pressures.51,52 Despite these difficulties, some preliminary assessment of the effect of temperature and pressure on mass-tranfer parameters is now possible. The mass-tranfer coefficient depends on the diffusion coefficient and on a hydrodynamic parameter (variously described by the theories of mass transfer as a film thickness or a surface renewal rate), which, among other things, would be influenced by properties such as surface tension. Most theories of mass transfer predict that

kL ) D m

(25)

where the exponent m varies between 0.5 and 1. Temperature has a significant effect on the diffusion coefficient.45,127 Hence, one anticipates that the masstranfer coefficient will increase with temperature. Suresh et al.48 present a detailed investigation of the effect of temperature on the mass-tranfer coefficient. For a flatinterface reactor, the exponent m is found to be around 0.6, which is not dissimilar from the surface renewal theory prediction of m ) 0.5. For a bubbling reactor, kL


is found to be nearly independent of temperature. This surprising result is ascribed to the overlap of the diffusion boundary layers around bubbles, which could be significant at large dispersed-phase volume fraction and at high diffusivities. The lowering of surface tension at high temperatures and the presence of any adventitious surface-active impurities may also contribute by suppressing surface renewal rates. Other literature data128,130 suggest various values, with Kishinevsky and Serebryansky131 even suggesting zero. This contradictory state of affairs results at least partly from the method used to change diffusion coefficients. The use of different gases does not give adequate variation in D and could lead to erroneous conclusions. On the other hand, if temperature is varied, concurrent changes in physical properties, especially surface tension, also result. The reduction in surface tension leads to a reduction in the surface renewal rates, and hence, the mass-tranfer coefficient decreases. These counteracting effects, along with the differences between bubbling and flat-interface reactors mentioned earlier, are responsible for the contradictory results in the literature. Under the conditions for organic oxidations, the typical Schmidt number varies from 10 to 300. These are extremely low values and are well out of the range of Schmidt numbers found in the reported studies on mass transfer. It is therefore unwise to rely on such data in the analysis of oxidation reactors. The picture is much clearer on the effect of pressure on mass-tranfer coefficients. Because pressure, at least up to 5 MPa, has only a minor effect on physical properties in the liquid state, one would not expect the liquid-phase mass-tranfer coefficients to depend on pressure. The experimental data130,132 are in agreement with this reasoning. However, the early results of Yoshida and Arakawa133 show a small decrease in the mass-tranfer coefficient with increasing pressure. On the other hand, the gas-phase mass-tranfer coefficient decreases with pressure because of the decrease in the gas-phase diffusion coefficient.134 The situation is very similar with interfacial area. There is a paucity of data on pressure and temperature effects. There is general agreement135,136 that bubble formation at a single orifice is affected by pressure. Higher pressures lead to smaller bubble volumes and higher frequency of formation. A recent review by Oyevaar and Westerterp51 discusses the relevant issues; also see Oyevaar and Westerterp.52 The effect of pressure on interfacial area depends on the type of reactor used. For bubble-column reactors, the bubble sizes formed at the distributor have a major influence. Smaller bubble sizes lead to a reduction in rise velocity, and hence, the gas holdup increases. However, this effect is somewhat modified by the coalescence rate, which depends on physical properties and the hydrodynamic conditions. As a result, a group of investigators137,138 claims that pressure has a significant effect on the gas holdup, whereas Deckwer ct al.139 show that pressure has no effect on gas holdup. Oyevaar and Westerterp51 carefully discuss these contradictory results and show that at least some of these differences could result from the sparger design and from the superficial gas velocity employed. Oyevaar et al.140 show large increases in holdup with pressure, especially if the superficial velocity is high. This paper also presents data on interfacial areas in bubble columns. The results indicate that the area increases with pressure, but the

influence of pressure is not as large as on gas holdup. This implies that the average bubble diameter increases with pressure, which is contrary to the single-orifice data cited earlier. The chemical method used to measure interfacial area gives a mass-tranfer-weighted area. This implies that bubbles, which do not contribute to the mass transfer, are not accounted for. Hence, the geometric area could well be quite different from the mass-tranfer effective area. Thus, different measurement techniques could easily lead to different results.46 Pressure effects in bubble columns are also discussed by Letzel et al.,141 Krishna et al.,142 and Wilkinson et al.143,144 In gas-liquid stirred vessels, the impact of hydrodynamics is even greater. The earliest study of pressure effects on interfacial area in stirred contactors is due to Sridhar and Potter.46 Using the light-transmission technique, their paper demonstrates significant pressure effects under conditions relevant to the oxidation of hydrocarbons. A tentative correlation developed by these authors ascribes the effect of pressure to two sources. One is the relative contribution of the energy input because of the gas (kinetic energy), and the other is an empirical factor depending on gas density. Oyevaar et al.,145 Oyevaar and Westerterp,51 and Oyevaar et al.52 used the chemical method to probe pressure effects in stirred contactors at room temperature. The earlier papers found no pressure effects at low pressures up to 2 MPa. However, the more recent study at higher pressures of 8 MPa52 found a significant pressure effect. These papers also identified the gas density and velocity at the orifice as the dominant parameters. However, the specific form suggested by Sridhar and Potter was not found to be supported by these experiments. Pressure effects were found to be uniquely dependent upon the product of the gas density and the gas velocity at the orifice. When this product exceeds 200 kg/m2s, a significant increase in area is observed. Note, however, that in organic oxidations, the large vapor pressures can cause the gas density to be large even at much smaller pressures. Recently, Tekie et al.116 have conducted experimental investigations on mass transfer under conditions of high temperatures and pressures in the cyclohexane oxidation process in agitated reactors. However, their reactors employ gas-inducing impellers or surface aeration. Such reactors are not common in industry, for reasons discussed under section 8. In summary, mass transfer under conditions obtained in oxidation reactors is determined by several factors. The available data, although insufficient for design purposes, nonetheless indicate the need to adequately allow for these effects in data analysis. 9.2. Influence of Mass Transfer on Liquid-Phase Oxidations The need to account for possible mass-tranfer influences in organic oxidations (considering that these reactions are usually carried out by bubbling an oxygencontaining gas through liquid hydrocarbon) has been generally appreciated even in early literature; the early Russian work10 or the work of Hobbs35 are examples. However, a combination of factors, such as the lack of an adequate theoretical analysis and the lack of adequate information on the magnitudes of mass-tranfer parameters under the conditions of oxidation, has forced researchers to correlate observed rate data in terms of “overall” kinetics in many instances. It is therefore not

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surprising that the “reaction rate parameters” in several published accounts of kinetics are functions of such physical variables as the gas flow rate (see, for example, Steeman et al.34). Even in work where the mass-tranfer step is included in the rate model, the importance of the regime, the locale of reaction, and its influence on the selectivity to the intermediates is often not appropriately brought out. This has been because of certain difficulties with treating liquid-phase oxidation reactions within the well-established framework of gasliquid reactions.39,43 There are several factors that the modeling must address, such as the consecutiveparallel nature of the overall reaction network; the zeroorder behavior with respect to the dissolving species (oxygen), which can change to first-order behavior at low concentrations; and the free-radical chemistry and consequent autocatalysis. In their review of the literature on the selectivity of gas-liquid reactions, Darde et al.146 concluded that there were only a few experimental investigations of selectivity, and even these have led only to partial results. The theoretical analyses, on the other hand, these authors found, involve approximations and often do not consider the concentration profile of the dissolved gas. Although early theoretical work on hydrocarbon oxidations tended to classify these reactions as slow and hence occurring in the slowreaction regime, the autocatalytic nature of the reaction makes such a conclusion hazardous when applied over a conversion range. Mann and co-workers147,148 were the first to consider the effect of autocatalysis within the framework of the film and penetration theories and showed that much larger enhancements result through the accumulation of the product in the diffusion film and consequent acceleration of the reaction there. The most systematic effort to bring organic oxidations within the ambit of the theories of mass transfer and chemical reaction is probably to be found in the case of cyclohexane oxidation. The experimental work of Suresh et al.50 on cyclohexane oxidation clearly showed that the reaction is slow enough at the beginning that dissolved oxygen concentrations in the bulk attain saturation, but that it accelerates as products accumulate, as shown by the declining oxygen levels in the liquid. Ultimately, depending on conditions such as the oxygen partial pressure and conversion level, the reaction can get fast enough to take place partly or wholly in the film, producing enhancement of mass-tranfer coefficients. These authors developed a film theoretic approach to the treatment of cyclohexane oxidation. However, the enhancement factors predicted by the theory (using independently determined kinetics and mass-tranfer parameters) were much smaller than those observed experimentally. The authors speculated that the observed discrepancies could, at least in part, be due to interfering concentration fields around bubbles, caused by the high diffusivities and large gas holdup. They therefore developed50,149 a bubble-swarm model that accounts in an approximate manner for these phenomena, which appeared to provide a better explanation. However, Suresh150 has subsequently shown, taking first-order reactions in general, that swarm effects should not be expected to lead to predictions different from the traditional local-rate models (such as film and penetration theories), provided that the physical masstranfer coefficient, on the basis of which enhancements are calculated, is also determined under similar swarm conditions. Because this is the way in which physical

mass-tranfer coefficients were determined in the work of Suresh et al.,48,50 the whole question would still seem to be somewhat open. It is possible that the mass-tranfer parameters (especially interfacial area) could undergo changes during oxidation because of the changing composition of the liquid (especially, production of products which lead to foaming and so on). Suresh88 does report some visual observations that suggest that such may have been the case. In the case of other oxidations, where some effort has been made to apply film and penetration theories to delineate regimes (Doraiswamy and Sharma43 provide a summary), the role of reaction regime in modifying selectivities is usually inadequately appreciated (see Cao et al.,54 for example). Confusion also results from an insufficient knowledge of mass-tranfer parameters under conditions of reaction. In summary, therefore, much remains to be done in this industrially important and intellectually challenging area. One aspect that has hardly received any attention at all is the change in chemistry that one obtains under oxygen-deficient conditions. For example, Partenheimer12 points out that, with easily oxidized feedstocks such as p-methoxytoluene, insufficient oxygen diffusion rates may result in depletion of dissolved oxygen, leading to dimerization of R* radicals. Low yields and dark-colored products are often the result (there might be a clue here to understand the inferior optical properties of the product in batch as compared to continuous operation in the oxidations of p-xylene, o-xylene, and mesitylene). The way commercial reactors are operated (with negligible oxygen concentrations in the reactor off-gases), such conditions could well prevail in a number of cases, at least in a part of the reaction volume. The kind of selectivities for which the kinetics inherently provide, and the ways in which they are modified by mass-tranfer limitations, are questions of some importance that still await answers. 10. Rate Oscillations and Other Nonlinear Phenomena Considering the complexity of the chemistry that governs organic oxidations, rate laws are invariably nonlinear, and one should not be surprised to see some exotic phenomena associated with nonlinear kinetics. Hobbs et al.35 described some hysteresis-type effects in the oxidation of MEK, in which the reaction came to a standstill as the temperature was lowered to a certain value and could not be re-started until the temperature was raised much beyond this value. This behavior is reminiscent of the “ignition-extinction” behavior that has been theoretically predicted and experimentally observed in such nonlinear chemical systems as exothermic CSTRs (see, for example, Froment and Bischoff151), where they are attributed to temperature feedback effects. Hronec and Ilavsky152 reported oscillations in the isothermal catalyzed oxidation of a mixture of p-xylene and p-toluic acid and of n-dodecane in an air sparged reactor. The oscillations were observed in the concentration of oxygen in the exit gases and were reported as being aperiodic. The authors assumed masstranfer limitations to be absent, as high stirring rates were used, and attributed the oscillations to kinetic phenomena. Jensen153 and Roelofs et al.154 describe oscillations in the air oxidation of benzaldehyde in acetic acid medium, catalyzed by cobalt/bromide. The oscillations were observed as coincident variations in the color


of the solution and the oxidation potential. Oscillations in the dissolved oxygen concentration were also reported, in which the oxygen levels went all the way to zero before coming up to about 25% of saturation. In the work of Jensen,153 the air flow rate was found to be an important parameter affecting the oscillations. Suresh et al.49,114 observed oscillations in dissolved oxygen in their studies of the oxidation of cyclohexane, under both catalyzed and uncatalyzed conditions. As in the case of benzaldehyde oxidation, these oscillations also occurred as the dissolved oxygen declined in the batch oxidations. The evidence that a variety of nonlinear phenomena occur in organic oxidation is thus compelling. However, the explanations for these phenomena are still lacking. Although early analyses of such exotic behavior were more or less restricted to the exothermic CSTRs, in later years, isothermal reactors have also been shown to exhibit a rich array of nonlinear behavior, provided that there is an autocatalytic step (with certain kinetic features) in the reaction mechanism (the extensive work of Gray and co-workers in this area is summarized by Gray and Scott53). Again, many bromate-driven oscillators have been reported, the most celebrated being the Belousov-Zhabotinsky reaction.155 Although it is thus possible that the nonlinearity of the kinetics is solely responsible in the case of organic oxidations also, the role of mass-tranfer limitations also must be keenly examined, particularly as many of the reports seem to suggest that these phenomena are coincidental with low oxygen levels in the liquid. Again, possible changes in reaction chemistry at low oxygen levels also could play a part. Although mechanisms implicating the Co2+ f Co3+ transition have been proposed in the benzaldehyde work cited earlier, such mechanisms cannot explain the observed oscillations in uncatalyzed oxidations. Suresh et al.88,114 investigated the effect of mass transfer through simulations of the kinetic model they had developed earlier, which incorporates the zero-tofirst-order transition at low oxygen concentrations. Their simulations did not show any oscillations under the conditions employed, and they concluded, therefore, that the explanation for the oscillations had to be sought in the kinetics. They proposed a mechanism in which, under the low dissolved oxygen concentrations that occur when some product concentration has been accumulated, the active free radicals are inactivated (either through termination or through conversion to less-active free radicals), thereby lowering oxygen consumption rates and allowing oxygen levels to increase. The reaction then accelerates, consuming oxygen, and the whole cycle repeats. To experimentally test such a hypothesis, they conducted an experiment in which, as the concentration of dissolved oxygen neared zero in a normal oxidation, the oxygen supply was periodically switched with nitrogen and back every 5 min. In these runs, they found that, on switching back to oxygen every time, the dissolved oxygen levels rose nearly to saturation again, before immediately decreasing to zero. It thus appears that, although the reactivity of the liquid is high during oxidation because of the products in solution, a short interruption in oxygen supply would somehow “inactivate” the liquid, thus necessitating a short “induction” period before the original reactivity could be restored. Although the possibility of these and similar nonlinear phenomena is of obvious academic interest, implications for industrial operations are no less significant.

For one thing, aspects of reactor control must consider the variety of dynamic behavior that is possible. It would be interesting to see whether the selectivity of the oxidation is any different under these conditions, because of the change in chemistry associated with low oxygen levels in the liquid. Suresh et al.,114 for example, observed the alcohol-to-ketone ratio to be different under conditions of oscillation. More work, both experimental and theoretical, is clearly needed to clarify these aspects. 11. Safety Issues in Organic Oxidations Oxidations being mediated by highly reactive species such as free radicals, safety considerations are of utmost importance. The dangers of explosion are very real in hydrocarbon-air contact. Although this has long been recognized, there sometimes has been a tendency to regard the dangers involved as being somewhat exaggerated (see Berezin et al.,10 for example). The whole subject was brought into sharp focus by the Flixborough explosion of June 1974 involving a cyclohexane oxidation plant. The evidence presented in that case to the court of enquiry24 and the post-enquiry discussion (see, for example, Mecklenburgh156) contains much valuable information to guide future designs. Kletz157 discusses this and other case histories of major industrial disasters in order to glean the lessons learned. Some of the statistics provided (based on an analysis of about 500 incidents) are of relevance in the present context. Of the incidents in storage and blending areas, about 10% were due to the formation of flammable mixtures in the vapor space. After storage vessels, the equipment most often involved was pressure vessels. In 23% of the cases where the cause was ascribed to ignition, the source was unknown; in about one-third of the cases where the source was known, this was autoignition. Hot surfaces, sparks, and static electricity were among the other common causes. Among primary causes, Kletz157 lists the use of the wrong materials of construction as accounting for 7% of the incidents. In the case of most of the hydrocarbons of relevance to this review, the autoignition temperatures are much higher than the operating temperatures. In the case of cyclohexane, for example, the autoignition temperature is 523 K, whereas oxidation temperatures are on the order of 423 K. Hence, one can assume that the mixtures of air and hydrocarbon resulting under normal conditions are incapable of spontaneous combustion, provided heat transfer is good enough to prevent the formation of local hot spots. However, dangers of ignition from an external source remain. Here, the main difficulty is that the flammability limits for mixtures of hydrocarbon in air are hardly ever available under the conditions of temperature and pressure employed in the oxidations. The range of flammable compositions expands with elevation and temperature, and hence, caution is necessary in using data available under ambient conditions. Designs are usually to be based on very conservative estimates. Exhaust oxygen levels below 8% are generally considered safe. When dry air is used as the oxidizing gas, the usual situation is that the gas-vapor mixture passes through the flammable region as it passes through the reactor and the hydrocarbon vaporizes into the gas bubbles. At the exit of the reactor, the gas-vapor mixture is usually above the upper flammability limit. Good mixing conditions should be provided in the reactor so that good heat-transfer conditions prevail, and this is usually not a problem in agitated

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vessels and bubble columns, although every care must be taken to eliminate possibilities of buildup of static electricity. The possibility of liquid entrainment in the leaving gases must also be considered. Because such liquid is likely to contain reactive hydroperoxides, the design of gas lines should ensure that no dead pockets are possible. Downstream of the reactor, the hydrocarbon must be condensed out of the gas and returned to the reactor, and this is another place where the mixture could pass through the flammability range. Although the reactor designs are usually such that the oxygen level by this stage is very small (in steady-state operation), the design of the condenser is important. For one thing, situations could occur under which the exit oxygen levels are higher than those found in steady operation. Furthermore, in the condenser, there is a case of condensation of the hydrocarbon from its mixture with a noncondensable (nitrogen). It is possible for the condensation to become mass-transfer-controlled and a mist to form. One must then provide for efficient demisting because, apart from the need for loss prevention, this mist can prove to be a hazard. Fires have sometimes been observed in the gas lines, especially during start-up. Fluid backing up into the gas lines can cause this.157 Alexander158 cautions that bad dispersion could lead to a buildup of free radicals, resulting in fires well below auto-ignition temperatures. Gas inlet and sparger designs should therefore ensure that stagnant pools of liquid do not collect at any point. The above are general considerations that apply equally well to industrial and laboratory reactor systems. Although safety issues are addressed in the industrial scenario through methodologies such as HAZOP, the necessity for adequately addressing these issues is no less in laboratory reactors. Furthermore, laboratory reactors sometimes have features in the interest of acquiring useful and comprehensive data that call for additional considerations. Thus, whereas the industrial reactor operates in a narrow window, the laboratory reactor, by its very purpose, must be operated under a variety of conditions. Therefore, safety considerations not only are important in the design, but also must be an essential part of experiment planning. In general, it is good practice to keep the volume of the liquid hydrocarbon handled to a minimum in laboratory reactors. The microautoclaves49 or the flow reactors of the type employed in the studies of Wen,59 Wen et al.,63 and Guo61 (see section 7.1) are ideal in this regard, as the reactor holds only about 30-35 cm3 of liquid. Although the liquid holdup in any experiment is just the reactor volume in the case of microautoclaves, even with flow reactors of similar volumes, an experiment for 10-20 residence times can be carried out with fairly small volumes of liquid. There is the further advantage in such equipment that the oxygen supply can be isolated before the actual experiment, once the liquid is saturated with oxygen at the desired pressure. On the other hand, because pure oxygen is used, adequate precautions are to be taken during the saturation step, although this step is carried out in the cold. The presence of moving parts such as mechanical agitators calls for precautions in preventing an accumulation of static electricity or the generation of hot spots. Despite all of the advantages of small liquid holdup, Wen59 and Guo61 housed their equipment in an enclosure that could be flooded with carbon dioxide in case of an emergency.

The design of gas-liquid contactors, on the other hand, calls for a much greater consideration of the safety issues, as the liquid holdup in these equipment is usually much larger. Thus, the semibatch equipment used in the studics of Suresh et al.49 had a holdup of about 20 dm3 of cyclohexane, about half of which would be at reaction temperature and pressure during an experiment. Furthermore, oxygen or an oxygen-containing gas was continuously passed through the liquid. Sridhar159 and Suresh88 have discussed in detail the safety considerations in the design of such equipment. Although the remarks made above in regard to the good heat- and mass-tranfer conditions inside the reactor (where flammable compositions could occur) are even more likely to be valid in laboratory reactors than in industrial reactors, Suresh et al.49 worked with a system that avoids the formation of flammable mixtures in the reactor altogether by using a nitrogen stream that was presaturated with the hydrocarbon upstream of the reactor and mixing the desired (small) percentage of oxygen into this stream at the reactor inlet. This approach has the additional advantage that vaporization inside the reactor can be neglected in interpreting the data using the theories of mass transfer with chemical reaction. However, it also increases the hydrocarbon holdup in the experimental rig. Suresh et al.48 housed all equipment containing hydrocarbon at or near reaction temperature inside a cabinet filled with carbon dioxide as a precaution. Although industrial reactors usually operate with negligible oxygen levels in the gases leaving the reactor, laboratory reactors must often operate with significant outlet oxygen levels so that absorption rates can be determined independently from gas-phase measurements. Consequently, the comments made earlier in connection with the design of condensers to recycle the hydrocarbon are of even greater importance here. A demister should, therefore, be provided downstream of the condenser. In uncatalyzed oxidations, hydroperoxides can form in nonnegligible concentrations in the reactor and can be carried downstream as entrainment. Care must be taken to avoid dead spots and regions where a buildup of such compounds can take place. 12. New Developments in Organic Oxidations The commercial importance of organic oxidations and the intellectual challenges they pose remain a potent driver for new research. In this section, we illustrate some of the new processing options that have been considered recently. 12.1. Biphasic Mode of Operation Several authors have studied oxidation in two-phase (biphasic) systems. When the second phase is water, the catalyst can be drawn to the organic-water interface by a surface-active complexing agent. An example is provided by Chung et al.160 who oxidized tetralin to R-tetralone with a nickel complex of a surface-active ligand in the presence of an emulsifier. High selectivities are reported, and the reaction can be carried out at 60 °C. The products remain in the organic phase, and the fluorous phase containing the catalyst can be recycled. Parenthetically, we note that R-tetralone can be efficiently dehydrogenated to R-naphthol. Similarly, Launay et al.161 used ruthenium trichloride for the oxidation of cycloalkanes with tert-butyl hydroperoxide


(TBHP). Under reaction conditions, colloidal ruthenium is formed and remains at the interface. Several cycloalkanes were studied, and the corresponding ketone was obtained as the predominant product. ten Brink et al.162 have adopted the biphasic system, using a water-soluble palladium complex of bidentate amine ligands such as bathophenanthroline disulfonate for the Wacker-type conversion of terminal olefins (e.g., hexene-1, octene-1, etc.) to the corresponding ketones. The catalysts are stable and can be recycled. This is an improvement over the conventional Wacker process. Ito et al.163 carried out the Wacker-type air oxidation of styrene and its derivatives to the corresponding acetophenone using (en)Pd(NO3). This complex facilitates the transfer of organic material into the aqueous phase in a reverse phasetransfer catalysis. The structure of the cage is reported in this paper. Dobler et al.164 show that osmiumcatalyzed dihydroxylations are facilitated by operating in a two-phase system. The method is an improvement on the well-known Sharpless dihydroxylations. The use of a two-phase system prevents nonselective oxidation of the diols and permits the recycle of the catalyst. Fluorous biphasic systems have also been developed wherein a perfluorinated solvent replaces water.165 An additional advantage of this system is that, at temperatures around 60 °C, the reaction mixture becomes homogeneous, thereby facilitating rapid reaction. These authors report the oxidation of various olefins to the corresponding ketones in the presence of a palladium catalyst with TBHP as an oxidant. On cooling, the reaction mixture again forms two phases: the organic phase contains the products, and the aqueous phase containing the catalyst can be recycled. Pozzi et al.166 studied the oxidation of alkenes and obtained high conversions and selectivity to the epoxide using a cobalt complex of tetraarylporphyrin. The inertness of the fluorocarbon and the high solubility of oxygen in this phase are additional advantages, and as such, we anticipate that this technique will continue to attract attention. Environmental considerations have created a large interest in aqueous biphasic catalysis.167 However, these systems can generate some aqueous solutions as a waste stream. This has motivated the search for the use of a dense-phase CO2-based system. Homogeneous ruthenium complexes with fluorous phosphines are being developed as catalysts. Pesiri168 demonstrates the epoxidation of alkenes in dense-phase CO2 using oxo vanadiumtriisopropoxide. The rates in supercritical medium are three times faster than those in hexane (see section 12.4). Harada et al.169 have shown how, in the Wacker process for the conversion of R-olefins to the corresponding carbonyl compounds, the use of R-cyclodextrin can make a remarkable change. Thus, C8-C10 olefins give the corresponding methyl ketones, but higher olefins (C12-C14) do not react. It is surprising that oct-2-ene gives poor results. The fundamental aspects pertaining to the prediction of rate and selectivity remain to be studied. Microemulsions are thermodynamically stable and, for sparingly soluble hydrocarbons, may well offer some advantages. There is very little information in the literature, but a recent claim from Bayer170,171 reveals a Wacker process in a microemulsion medium with olefins such as cyclopentene, cyclohexene, etc. The use of phase-transfer catalysis in enhancing air

oxidation of hydrocarbons has received attention. Matienko and co-workers172-175 have studied a number of related issues, in connection with the oxidation of ethylbenzene. These authors carried out the liquidphase oxidation of ethyl benzene with Ni(acac)2 as a catalyst. Macrocyclic polyether 18-crown-6 changes the selectivity, and this can also be realized using quaternary salts such as Me4NBr and n-C16H38Me3NBr. There is also a claim176 for the liquid-phase air oxidation of toluene to benzoic acid using cobalt catalyst and a phase-transfer catalyst like [Me(CH2)-9]2N+Me2Br- at 135-160 °C and 12-15 atm pressure. 12.2. The Role of Ultrasound in Oxidation Reactions The role of free radicals and their impact on reaction pathways has been detailed in section 5. Ultrasonication, including through cavitation,177,178 is known to create free radicals. Thus, we expect that the air oxidation of dissolved organics in wastewater would be favorably influenced by ultrasonic treatment during oxidation. There is scant literature on the oxidation of hydrocarbons under conditions of industrial importance. This is particularly relevant as cavitation can be employed in large-scale operations. Sulman179 has cited the effect of ultrasound on the liquid-phase oxidation of n-tetradecane at 385 K in the presence of 0.3% cobalt stearate as a catalyst. Ultrasound appears to increase the production of oxygenated compounds by 15-30%. The maximum acceleration of the oxidation is reached at a frequency of 300 kHz. The oxidation rate under these conditions does not vary with time, whereas it shows a maximum in the absence of ultrasound. The oxidation of alkenes in the presence of Mo(CO)6 to form enols and epoxides is also cited in this work. 12.3. Oxidation in Supercritical Media A thesis from Clemson University has covered oxidation of alkyl aromatics to aldehydes and acids in supercritical (SC) water (Tc ) 647.3 K, Pc ) 217.6 atm) and has found it to be promising (cited by Haas and Kolis180). The MnBr2-catalyzed reaction of p-xylene giving terephthalic acid has been reported. The oxidation of stilbene with O2 and MoO2(acac)2 or VO(acac)2 gave benzaldehyde and benzoic acid (i.e., cleavage occurred). In SC, water only thermally stable olefins undergo this type of reaction; simple nonconjugated olefins such as 1-octene and cyclohexene decompose in the harsh environment of SC water. In contrast to SC water, conditions in SC CO2 (Tc ) 30.4 K, Pc ) 72.8 atm) are milder and benign. The solvent is environmentally friendly. SC CO2 is an attractive solvent for catalytic oxidations because it is noncoordinating and inert toward any further oxidation. Some reports have appeared on oxidations in supercritical media in the literature. Haas and Kolis180 have reported that Mo catalyst can oxidize toluene to benzaldehyde in 25% yield in SC CO2. There is a claim181 that, when propylene is oxidized under supercritical conditions (T > 91.6 °C, P > 46.1 atm) with silver catalyst, propylene oxide (PO) is obtained. At 3.6% conversion, a 36 mol % selectivity to PO was obtained. However, recently, Dow182,183 has claimed that propylene can be converted to PO by oxygen in a cascade of reactors using dichlorobenzene as a reaction medium.

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These reports have significance in the pursuit of a direct oxidation process for PO from propylene. 12.4. Photochemical Activation of Oxidation Reactions The role of photocatalysis in realizing the selectivity for the desired product in the oxidation of hydrocarbons has attracted attention in recent years. The photocatalytic oxidation of hydrocarbons in the aqueous phase in the presence of finely divided TiO2 has been reported by Gonzalez et al.184 Thus, toluene was oxidized to benzyl alcohol, and at 11.6%, conversion about 90% selectivity to benzaldehyde was obtained. Frei et al.185 have given an overview of photocatalyzed oxidation of hydrocarbons in zeolite cages with examples of toluene to benzaldehyde. The liquid-phase oxidation of isobutane was referred to in section 6.4, for which typically, at about 8% conversion, we get 75% selectivity with respect to the hydroperoxide. Blatter et al.186 have done some ingenious experiments in which isobutane and O2 gas were loaded in a single-photon process (monitored in situ by FTIR). It is noteworthy that the selectivity to the hydroperoxide (HPO) was 98% even when more than 50% of the loaded reactants had reacted. This synthesis in zeolite environments offers an opportunity for in situ use of the HPO as an oxidizing agent without isolation and storage. Optically translucent zeolite membranes will have to be used. Sanjuan et al.187 describe a photocatalytic system consisting of an organic dye trapped in Ti-zeolite pores. The dye absorbs energy from light and generates hydroxyl radicals from water. These radicals react with olefins in the presence of molecular oxygen to form allylic hydroperoxides, which, in turn, facilitate the epoxidation of the alkene at the Ti sites. The photosensitizer is protected from attack by hydroxyl radicals because of its location within the pores of the zeolite. Highly dispersed titanium oxide on silica has been reported to catalyze the photooxidation of propene to propylene oxide using molecular oxygen.188 The catalyzed photooxidation of cyclopentene189 gives the following products:

Bloodworth and Eggelte190 have reported photooxidation, with O2, of cyclopentene, cyclohexene, cycloheptene, and cyclooctene in dichloromethane medium containing tetraphenylporphyrin, using a 400-W sodium lamp, to the corresponding hydroperoxides. Liquidphase air oxidation of saturated hydrocarbons such as decane, cyclooctane, Decalin, etc. under photochemical activation has been reported by Nekhaev et al.191 The catalytic asymmetric dihydroxylation of R-methylstyrene (AMS) by air under visible irradiation is a fascinating example of oxidation of a relatively cheap hydrocarbon, AMS, that is obtained as a byproduct of cumene oxidation (see section 6.3). Although Sharpless’ method of asymmetric dihydroxylation is known, the work of Krief and Colaux-Castillo192 yields a new perspective by using air as an oxidant. Under visible irradiation, this reaction can be conducted in the presence of catalytic amounts of Os(IV), phthalazine dihy-

droquinidine chiral ligands. The optically active dihydroxy compound can be selectively hydrogenated to the optically active monohydroxy (primary) species, which, on carbonylation, should give the optically active 2-aryl propanoic acid. Thus, a new route, which has potential to be commercial, to optically active 2-aryl propanoic acid like S-ibuprofen and S-naproxen, may emerge. Li et al.193 have used vesicles to direct the photosensitized oxidation of olefins either toward the singletoxygen-mediated or the superoxide-radical-anion-mediated products by controlling the status and location of the substrate and sensitizer molecules in the reaction media. 12.5. Enzyme-Catalyzed Reactions In living organisms, enzymes such as cytochrome P450 catalyze the oxidation of various organic compounds. Cytochrome P450 functions to activate molecular oxygen by iron porphyrin to generate an oxo iron porphyrin while transferring oxygen atom to the substrate. The simulation of these enzymatic functions with transition metals has received much attention. In such cases, the oxo metal complex is generated without porphyrins. For example, low-valent ruthenium complex can react with a hydroperoxide, and subsequent cleavage yields an oxo ruthenium (IV) species, which can then be used to oxidize hydrocarbons.194 Adam et al.195 have reported biocatalytic asymmetric hydroxylation of hydrocarbons with the microorganism Bacillus megaterium. This strain carried out the hydroxylation chemoselectively and enantioselectively in the benzylic and nonbenzylic positions of a variety of unfunctionalized aryl alkanes (the phenyl ring was unaffected). Salycilate phenobarbitols, which are potent inducers of cytochrome P450 activity, changed the regioselectivity of the microbial CH insertion without an effect on enantioselectivity. Other enzyme catalysts have also been developed. Alkane hydroxylase from Pseudomonas oleovorans has been used to produce optically pure epoxides from terminal alkenes. The same enzyme also assisted in the regiospecific introduction of oxygen into alkanes. Details of these approaches are contained in a report on the 4th Japanese-Swiss Meeting on Bioprocess Development.196 Dicarboxylic acids such as adipic acid, suberic acid, sebacic acid, azelaic acid, dodecanedioic acid, and brassylic acid are commercially important.197 An important commercial route, for some of these dicarboxylic acids, is through the oxidation of cyclic compounds (adipic acid from cyclohexane, dodecanedioic acid from cyclododecane etc.; see section 4). These acids are useful intermediates in the production of polyamides. For example, the reaction of 1,12-dodecanedioic acid with hexamethylene diamine gives Nylon-6,12, and brassylic acid (1,13-tridecanedioic acid) is used to manufacture the corresponding nylon. Brassylic acid and 1,12-dodecanedioic acid are preferred raw materials for making environmentally friendly musks as aroma compounds through esterification with ethylene glycol. These dicarboxylic acids are useful for making polyesters, and lately, the use of 1,12-dodecanedioic acid for making modified polycarbonates has been reported. It is known that brassylic acid is commercially manufactured (in Japan and China) by the aerobic fermentation route.189,197 Similarly, fermentation of n-dodecane can give 1,12dodecanedioic acid. The engineering aspects of such aerobic fermentation reactions require further attention.


12.6. Use of O2 + H2 and O2 + CO as Oxidants Several unusual oxidations have been reported recently. Researchers at Tosoh in Japan198 have demonstrated the oxidation of benzene to phenol using mixtures of oxygen/hydrogen and oxygen/CO. A palladiumruthenium catalyst was used, and the oxidation was carried out in an acetic acid medium. The rates appear to be promising, and the method has the added advantages that no coproduct acetone is formed, unlike in the case of the process based on the oxidation of cumene. Meiers and Holderich199 have used PddPt in TiSilicalite for a “one-pot” conversion of propylene to propylene oxide using H2 and O2. This probably implies in situ production of hydrogen peroxide, which subsequently epoxidizes propylene. 12.7. Catalyst Developments As in the other areas reviewed in this section, one of the prime motivating forces for catalysis research has been environmental concerns: increased efficiencies lead to reduced effluent volumes, heterogenization facilitates catalyst recycle and reuse. Langhendries et al.200 have recently reviewed developments in providing clean catalytic technology for liquid-phase hydrocarbon oxidations. Low-efficiency processes such as cyclohexane oxidation have naturally been at the forefront of research attention. A development of some interest is the demonstration of catalysis by nanostructured amorphous metals such as cobalt and iron and alloys.201 With cyclohexane oxidation, a selectivity of 80% (to cyclohexanol-cyclohexanone) at a conversion as high as 40% was realized with amorphous cobalt. Isobutyraldehyde was used as a co-reductant, and a catalytic amount of acetic acid was also added. A notable feature was the high ratio of alcohol to ketone (as high as 5:1). The reaction was carried out at room temperature with an oxygen partial pressure of 40 atm. The rates are, however, too low under these conditions for commercial exploitation, and additional studies are needed. Although rates were better at 70 °C, selectivities are not reported at the higher temperature. The catalyst was prepared by a sonochemical method. Attempts to mimic enzyme action using iron catalysts, either in zeolite-type cages or in membranes, have yielded some significant improvements in rate and selectivity for cyclohexane oxidation with tert-butyl hydroperoxide. Vanoppen et al.202 have attempted heterogenization of the conventional cobalt catalyst by incorporating it into the framework of aluminophosphate molecular sieves. This has the effect of keeping the cobalt dispersed so that there is no deactivation due to either precipitation or clustering. The authors obtained selectivities to hydroperoxide + alcohol + ketone that are better than those obtained with homogeneous catalysts at similar conversion levels. Other studies in which similar principles have been applied to other metal catalysts with encouraging results are reviewed by Langhendries et al.200 Functionalized heterogeneous catalysts for the oxidation reaction have also been developed. Chisem et al.203 have developed chemically modified mesoporous silica with metal ions immobilized on a hydrophobic chain. These catalysts are shown to facilitate the epoxidation of cyclohexene with excellent selectivity and also to assist in the oxidation of ethylbenzene to acetophenone. Efficient removal of water is necessary to prevent

catalyst deactivation in the case of oxidation of ethylbenzene. Das and Clark204 have reported a complex form of Co(III) immobilized on a chemically modified silica substrate, which has proved to be useful for conversion of ethylbenzene to acetophenone. Raja and Thomas205 replaced a portion of the aluminum ions in molecular sieves with Mn(III) ions and demonstrated regioselectivity in the air oxidation of linear dodecane. The selectivity arises from the pore dimensions that govern the access of the hydrocarbon. The terminal methyl group in dodecane is able to access the active catalyst sites, whereas the carbon atoms along the dodecane backbone are protected when the pore diameter is small enough. Tailored molecular sieves have been put forward as oxidation catalysts. Recently, Dugal et al.206 designed a heterogeneous catalyst for air oxidation of cyclohexane to adipic acid based on FeAlPO-5 and FeAlPO-31. The latter catalyst showed selectivity to adipic acid as high as 65%, whereas the former gave a maximum selectivity of 31%. This difference is apparently due to cyclohexane being much more confined in the case of FeAlPO-31. Thus, the diffusion of cyclohexane and cyclic intermediates is limited, and hence, further oxidation to linear products such as adipic acid is facilitated. In an earlier work from the same school, high selectivity for cyclohexanol from cyclohexane is obtained over an FeIIIAlPO-5 catalyst. It is even more remarkable that Raja et al.207 have designed a molecular sieve catalyst for the extremely difficult air oxidation of n-hexane to adipic acid. The catalyst is based on a CoIII framework-substituted AlPO. The catalyst has also been found to be effective for the epoxidation of alkenes with air. This addresses the problem of selective oxidation of linear alkanes, particularly in the terminal position, to alcohols and carboxylic acids and can, indeed, change the fundamentals of the business of producing primary alcohols and dicarboxylic acids such as adipic acid. However, problems associated with leaching, long-term stability, the structure of the acid site, and selectivity require further investigation. Chromium-, cobalt-, or vanadium-substituted aluminophosphates have been shown to be active (for example, Kraushaar-Czarnetski208). Vanoppen et al.209 used zeolite Y ion exchange with alkali metal ions to study the oxidation of cyclohexane. Soluble heteropolyacids (HPA) and palladium acetate have been used for the direct oxidation of benzene to phenol, but catalyst deactivation through the irreversible reduction of HPA remains a problem. Passoni et al.210 report on attempts to heterogenize these catalysts by encapsulating them in MCM-41 or microporous AlPO4-VPI-5 molecular sieves. The HPA leaches out of the former support, whereas the molecular sieves retain the catalyst. The poor accessibility of the reactants to the catalyst results in low reaction rates. Along similar lines, Khenkin et al.211 report a vanadomolybdophosphate polyoxometalate supported on mesoporous MCM-41 with which they were able to get product selectivities in the oxidation of alkanes and alkenes similar to those obtained via homogeneous oxidation, although the catalytic activity was somewhat reduced. Lempers and Sheldon (1998) question whether some of these catalysts truly function as heterogeneous catalysts. These authors show that small amounts of leached metal could homogeneously catalyze the reaction and suggest ways of testing these catalysts. However, in the work of Khenkin et al.211 cited

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above, no leakage was observed and the solid catalyst could be recovered and reused without loss in activity. A number of claims have been made in which porphyrin complexes are used as catalysts to convert olefins such as propylene, 1 and 2-butene, butylene, cyclopentene, cyclohexene, cyclooctene, etc. Mitsui Toatsu claims conversion of cyclooctene to cyclooctene epoxide, even at 25 °C. A recent patent from China212 refers to the epoxidation of olefins with oxygen in the presence of a single oxygen acceptor H2A, selected from diazo compounds, quinones, and phenazines, and a transition metal selected from Ti, V, Cr, Mo, W, etc. The oxidized H2A is reduced by hydrogen to regenerate the single oxygen acceptor. Another development of significance in the quest for direct air oxidation routes for olefins to epoxides is the use of N-hydroxyphthalimide (NHPI), on which the work of Iwahama and co-workers (section 6.6) has been discussed earlier. Higashijima213 has reported a novel water-soluble oxidation catalyst that is a ruthenium-substituted heteropolyanion, [SiW11O39Ru(III)(H2O)]5-, which is shown to be effective for the oxidation of, for instance, p-xylene at 200 °C in water as a solvent. At 99% conversion, the products consisted of terephthalic acid (58.8%), p-toluic acid (17.6%), p-toluic aldehyde (0.2%), and carbon dioxide (20%). This development is of interest in the context of the current search for environment-friendly processes and is an example of a “green” oxidation catalyst, in a nonhalogen-containing water solvent system. Methods of synthesis of these heteropolyanion catalysts are also simultaneously attracting a great deal of interest. Higashijima213 claims that their method of hydrothermal synthesis is more effective and practical in comparison with the earlier methods. The anion was salted out as the cesium salt from the hydrothermal solution. Zeolites are interesting supports for oxidation with metal complexes, because the entrapped complexes are prevented from dimerizing and degrading. However, the small pore sizes usually make them unavailable for large complexes. Wang et al.111 have used new zeolites in which the cage size had been expanded as supports for encapsulating their Schiff base complexes that show good activity in oxidizing linear aliphatic olefins to their epoxides (see section 6.7) and have obtained good activity and selectivity with many olefins. As the size of the olefin increases, the catalyst activity decreases, possibly as a result of access limitations in the zeolite cage. The authors have investigated the effect of several parameters such as zeolite type, metal ion, and reaction temperature. In particular, the influence of temperature was found to be complex, indicating interactions among several phenomena. A number of modifications have been claimed for conversion of olefins to ketones with Pd-based catalysts. Idemitsu and Kosan have claimed conversions of 1-butene to methyl ethyl ketone in 1,4-dioxane-water in the presence of PdSO4, H6PV3Mo9O40, and Fe2(SO4)3. Woltinger et al.214 have studied cobalt salophen catalyst encapsulated in zeolites, in the presence of the palladium-quinone system for the air oxidation of 1,3dienes. In the case of 1,3-cyclohexadiene, 1,4-diacetoxy2-cyclohexene in high yield was obtained at room temperature. Chauvet et al.215 have shown that catalysts such as (CH3CN)2PdCl(NO2), unlike the common Wacker cata-

lysts based on PdCl2 and CuCl/CuCl2, convert strained norbornenes to the epoxide. This is an important result as it demonstrates that molecular oxygen can be used for epoxidation with low-valent transition metals from the Pt group and in the complete absence of any peroxidic structure element, albeit for restricted olefins. Olefins such as 1-octene, vinylcyclohexene, styrene, etc. are converted to the corresponding ketone, as would be the case with the usual Wacker catalyst. 12.8. Stereospecificity in Organic Oxidations In the case of several hydrocarbons of industrial importance, such as pinane, menthane, cyclooctene, cyclododecene, etc., there are cis and trans configurations. In the case of cyclooctene for instance, the cis isomer can be obtained predominantly, if not exclusively, by manipulating the catalyst and the conditions of the hydrogenation of cyclooctadiene. It is expected that rates of oxidation of cis and trans isomers, under similar conditions, will be different. There is very little information in the literature on this aspect, which is both scientifically interesting and commercially important. For examrple, in section 12.3, reference was made to the selectivity of oxidation of cis- versus transstilbene, where only the cis isomer reacts. Similarly, Eggersdorfer216 shows that the air oxidation of pinane at 95 °C gives 2-pinane hydroperoxide, which is used as a radical initiator for polymerization reactions. In this reaction the cis isomer reacts more rapidly than the trans isomer. Haas and Kolis180 have reported oxidation of cyclohexene, cyclooctene, 1-octene, vinylcyclohexane, cis- and trans-stilbene, etc., in SC CO2 and Mo(CO)6 catalyst precursor with t-BuOOH. The product can be epoxide or alcohol when 70% aqueous t-BuOOH is used. The highest yields and fastest rates of diol and epoxide formation were observed with cis-alkenes, whereas trans-alkenes were considerably less reactive. transStilbene does not undergo the oxidation reaction, probably because of steric effects. Cyclic olefins such as cyclohexene react relatively very rapidly. cis-Cyclooctene was oxidized in 100% yield to its epoxide with no sign of hydrolysis in contrast to, say, the oxidation of cyclohexene where epoxide is converted to diol as well. The work of Iwahama et al.109 on the epoxidation of alkenes with a hydroperoxide generated in situ was discussed earlier (sections 6.5 and 6.7). A notable feature of the results reported is the excellent stereospecificity of the epoxidation. Thus, trans-oct-2-ene was epoxidized almost exclusively to trans-2,3-epoxyoctane with a selectivity of 88% at 78% conversion. A small amount of the diol was formed (about 4%). Similar results were obtained with the cis isomer as the starting material. These results are in contrast to the metal-catalyzed epoxidations of cis-olefins using an aldehyde and oxygen, in which a mixture of cis- and trans-epoxides is usually obtained. 13. Conclusions Liquid-phase air oxidation of important bulk raw materials like p-xylene, cumene, ethylbenzene/isobutane, cyclohexane, n-butane, etc., to industrially important products, can be analyzed on a fairly rational basis, taking into consideration aspects of chemistry, engineering kinetics, type of reactor, etc.


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Received for review February 21, 2000 Revised manuscript received August 4, 2000 Accepted August 5, 2000 IE0002733

Engineering Aspects of Industrial Liquid-Phase Air Oxidation of

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