p-Xylene Oxidation to Terephthalic Acid: A Literature Review Oriented

Review pubs.acs.org/CR

p‑Xylene Oxidation to Terephthalic Acid: A Literature Review Oriented toward Process Optimization and Development Rogério A. F. Tomás,† Joaõ C. M. Bordado,‡ and Joaõ F. P. Gomes*,‡,§ †

ARTLANT, Zona Industrial e Logística de Sines, Zona 2, Lote 2E1, Monte Feio, 7520-064 Sines, Portugal Instituto de Biotecnologia e Bioengenharia/Instituto Superior Técnico (IBB), Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal § Instituto Superior de Engenharia de Lisboa (ISEL), Á rea Departamental de Engenharia Química, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal ‡

S Supporting Information *

5.6.1. Acetic Acid Derivatives 5.6.2. Benzaldehyde and Benzoic Acid Derivatives 5.6.3. Terephthalic Acid Derivatives 5.6.4. Phenol Derivatives 5.6.5. Biphenyl and Benzophenone Derivatives 5.6.6. Anthraquinone and Fluorenone Derivatives 5.6.7. Ester Derivatives 5.7. Catalyst Deactivation 5.7.1. Catalyst Regeneration 6. Operational Optimization Studies 7. Recent Research Trends in Terephthalic Acid Synthesis 7.1. Oxidation in Sub- and Supercritical Water 7.2. Alternative Catalyst Systems and Cocatalysts 7.2.1. Zirconium 7.2.2. Carbon Dioxide Use as Co-oxidant 7.2.3. N-Hydroxyimides 7.2.4. Guanidine 7.3. Oxidation in Ionic Liquids 7.4. Heterogeneous Systems 8. Conclusions Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Polyester History 2.1. AMOCO Process 3. Chemistry Behind the Process 3.1. General Aspects of Hydrocarbon Oxidation 3.2. p-Xylene Autoxidation to Terephthalic Acid 3.3. Catalyst Influence in Autoxidation 3.3.1. Cobalt and Manganese Acetates As Oxidation Catalysts 3.3.2. Cobalt/Manganese/Bromide Catalyst Systems 4. Activity 4.1. Temperature and Pressure Effect on Activity 4.2. Water Effect on Activity 4.3. Catalyst Concentration Effect on Activity 4.4. Cobalt/Manganese and Bromine/Metals Ratios Effect on Activity 4.5. Agitation Effect on Activity 4.6. Alkali Metal Effects on Activity 5. Selectivity 5.1. Carbon Oxides 5.1.1. Carbon Oxides: Solvent Burning 5.1.2. Carbon Oxides: Mechanism of Solvent Burning 5.2. Methyl Bromide 5.3. Methane 5.4. Methyl Acetate 5.5. Selectivity Assessment 5.6. Byproduct Chemistry © XXXX American Chemical Society

A B B C C E F F I P P P R R S S T U U

AE AF AG AG AG AH AH AH AI AI AL AL AM AN AO AP AQ AR AS AT AT AT AT AT AT AU AU AU

1. INTRODUCTION Terephthalic acid is an aromatic carboxylic acid core to polyester fibers production. A large majority of the production of terephthalic acid is via aerobic catalytic oxidation of p-xylene with air in acetic acid medium, catalyzed by cobalt, manganese, and bromide compounds, in a process commonly known as the

W Y Y Z AA AC

Received: July 25, 2012

A

dx.doi.org/10.1021/cr300298j | Chem. Rev. XXXX, XXX, XXX−XXX

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AMOCO process. This work aims to be a review of the chemistry fundamentals and kinetics behind the process of catalytic oxidation of p-xylene, providing insight into the most important aspects relevant for industrial terephthalic acid production operation and optimization and also on the most recent research aiming at alternatives to the existing dominant aerobic catalytic oxidation with the cobalt−manganese− bromine catalyst in acetic acid medium. In the first section, a brief evolution of the industrial process of terephthalic acid is given, followed by general aspects of hydrocarbon oxidation, relevant to the chemistry behind the pxylene oxidation process and the difficulties inherent to the oxidation reaction that conditioned evolution of the catalysts used until discovery of the cobalt/manganese/bromine AMOCO catalyst, together with the respective kinetic mechanism. In the second section, factors that have influence the activity of the catalyst are discussed focusing on process parameters that have more impact on industrial process yield and their optimization. In the third section, selectivity aspects are presented with particular emphasis on the byproducts and the direct influence of the catalyst composition and their formation. A brief review of available references on operational optimization studies is made in the fourth section. Finally, in the fifth section, recent research in p-xylene oxidation is reviewed, presenting alternative catalysts, solvents, and promoters and also heterogeneous catalysis studies, which all aim to be alternatives to the presently used dominant technology.

terephthalate, which readily esterifies to dimethyl terephthalate. The two oxidations could be combined so that p-xylene and methyl p-toluate were oxidized in the same vessel and so could be the two esterification reactions. Currently, direct liquidphase catalytic oxidation processes, consisting of homogeneous liquid-phase oxidation of p-xylene in a solvent with air in the presence of a transition-metal catalyst, is the dominant industrial process for terephthalic acid manufacture. There are variations of this process depending on the operating conditions and catalysts used, but from these, the Mid-Century process, also known as the AMOCO process, is the most widespread used technology.3 2.1. AMOCO Process

The drive behind the research effort to direct oxidation has been the relatively high resistance to oxidation of the p-toluic acid which was first formed. This obstacle was overcome by the discovery of bromide-controlled air oxidation in 1955 by the Mid-Century Corp.4−7 and ICI, with the same patent application date. The Mid-Century process was bought and developed by Standard Oil of Indiana, later AMOCO, with some input from ICI. The process uses acetic acid as solvent, oxygen from compressed air as oxidant, a temperature of about 200 °C, and a combination of cobalt, manganese, and bromide ions as catalyst. Various salts of cobalt and manganese can be used, and the bromine source can be hydrobromic acid, sodium bromide, or tetrabromoethane among others. The preferred system is composed of cobalt and manganese acetates and hydrobromic acid as bromine source. If a bromoalkane is used such as tetrabromoethane, the molecule must solvolyze or be oxidized readily to HBr. An ionic source of bromide is necessary to achieve an active catalyst. A feed mixture of pxylene, acetic acid, and catalyst is continuously fed to the oxidation reactor. The feed mixture also contains water, a byproduct of the reaction. The reactor is operated at 175−225 °C and 15−30 bar. Compressed air is fed to the reactor in excess to provide measurable oxygen partial pressure and achieve high p-xylene conversion. The reaction is highly exothermic, releasing 2 × 108 J/kg of reacted p-xylene. Most of the terephthalic acid precipitates, as it is formed, due to its low solubility in acetic acid. This yields a three-phase system: solid terephthalic acid crystals, solvent with some dissolved terephthalic acid, and vapor consisting mostly of nitrogen, acetic acid, water, and smaller amounts of oxygen and carbon oxides. The heat of reaction is removed by solvent evaporation. Over 98% of the p-xylene is reacted, and the yield to terephthalic acid is greater than 95 mol %. Small amounts of p-xylene and acetic acid are lost, owing to complete oxidation to carbon oxides, and impurities such as oxidation intermediates are present in reactor effluent. The excellent yield and low solvent loss in a single reactor pass account for the near universal selection of this technology for new plants. Oxidation of the methyl groups is approximately consecutive, with two main intermediates p-toluic acid and 4-formylbenzoic acid or as it is customarily referred to 4-carboxybenzaldehyde (4-CBA). 4-CBA acid is troublesome, owing to its structural similarity to terephthalic acid. It cocrystallizes with terephthalic acid and becomes trapped and inaccessible for completion of oxidation.

2. POLYESTER HISTORY Terephthalic acid history is closely related to polyester history. Since the pioneering research of Wallace Carothers on condensation of aliphatic dicarboxylic acids with diols to yield polyesters and the first granted patent in 1941 to J. R. Whinfield and J. T. Dickson, PET manufacture acted as a driving force to terephthalic acid industrial process development.1 The resistance of the p-xylene molecule to conversion to terephthalic acid constituted from the beginning, an intrinsic barrier, around which the several technologies developed themselves to bypass that difficulty. One of the earliest commercially viable routes to terephthalic acid was through the explosive hazardous liquid-phase nitric acid p-xylene oxidation using 30−40% diluted nitric acid as oxidant at temperatures between 160 and 200 °C and pressures from 8.5 to 13.5 bar. Terephthalic acid precipitates are separated and purified in subsequent steps but contaminated with colored and color-forming impurities.2 To overcome this a diversion tactic was used consisting of esterification with methanol to dimethyl terephthalate (DMT); successive crystallizations and distillations were required to yield a product with acceptable quality. For the first few years of PET production, the polymer was all made by a DMT ester interchange route.1 The Dynamit− Nobel process quickly replaced the rather unsatisfactory and hazardous nitric oxidation route to DMT. p-Xylene was oxidized with air, in the absence of solvent in the temperature range of 140−180 °C and pressure between 5−8 bar using a cobalt catalyst, to p-toluic acid, which was esterified by methanol to form methyl p-toluate, which was then oxidized by air to monomethyl terephthalate in the same vessel, which in turn was esterified with methanol to make DMT. The success of this process was that even though p-toluic acid is very resistant to oxidation with a cobalt catalyst, its methyl ester is not; it is rapidly converted to monomethyl B


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to acetophenone and acetaldehyde to acetic acid with air in the temperature range of 102−104 °C for periods of 24 h, catalyzed by metals. Catalysts used were acetates of cobalt, nickel, manganese, vanadium, cerium, iron, and chromium. In the catalyzed oxidation of ethylbenzene yields of acetophenone were higher than those Stephens11 obtained for the uncatalyzed oxidation. The basis of catalytic oxidation−reduction action is electron transfer from the reductant to the catalyst, followed by electron transfer from the catalyst to the oxidant, which is faster than direct electron transfer from the reductant to the oxidant. A necessary, but not sufficient, requirement for such catalysis is that the metal must possess two distinct oxidation states, none of them too stable when compared to the other. Oxidation and reduction of saturated molecules frequently results in generation of free radicals. Transition metals, such as Cu, Co, and Mn, have the ability to catalyze these oxidations. Among such reactions are the autoxidations of hydrocarbons and other organic molecules, initially to hydroperoxides, which proceed by free-radical chain mechanisms in which the most important propagation steps are

The slurry is passed from the reactor to one or more surge vessels where the pressure is reduced. Solid terephthalic acid is then recovered by centrifugation or filtration, and the cake is dried and stored prior to purification. This is typically referred to as crude terephthalic acid (CTA) but is more than 99% pure. Crude terephthalic acid is unsuitable as a feedstock for polyester, primarily owing to the 4-CBA impurity concentration, which acts as chain termination agent in PET manufacture, and so goes through a purification process. In the purification process, crude terephthalic acid is dissolved in hot water and 4-CBA is reduced and recovered to p-toluic acid via catalytic hydrogenation over a palladium catalyst at about 250 °C and recycled to improve the process yield. There are also other color-forming impurities and residual amounts of catalyst metals and bromine. The AMOCO purification process removes 4-CBA to less than 25 ppm and also gives a white powder from the light yellow CTA feed, commonly referred to as purified terephthalic acid (PTA). This process allowed development of a route to polyester from PTA by direct oxidation, which has since become more widely used than the process using DMT. Several other processes for manufacturing TA have been patented, and some of them have been used commercially, but these two remain the most important.1,3

R• + O2 → RO2•

(3)

RO2• + RH → ROOH + R•

(4)

overall:

RH + O2 → ROOH

(5)

Metals catalyze the oxidation, generating free radicals in chain decomposition of hydroperoxides according to the following equations. ROOH + Co2 + → RO• + OH− + Co3 +

(6)

3. CHEMISTRY BEHIND THE PROCESS

ROOH + Co3 + → RO2• + H+ + Co2 +

(7)

3.1. General Aspects of Hydrocarbon Oxidation

overall:

Oxidation is a widely used synthetic route to a large range of chemicals. Published reports of oxidation studies of alkyl aromatics with molecular oxygen are from as early as 1912, when Ciamician and Silber8 studied the light effect on oxidation of toluene, o-xylene, m-xylene, p-xylene, and p-cymene. These hydrocarbons were lit by sunlight for a year in contact with oxygen, obtaining the corresponding carboxylic acids as main products. Isophthalic and terephthalic acids were formed besides m- and p-toluic acids from m-xylene and p-xylene, respectively. Stephens (1926)9,10 carried out oxidation of several aromatic hydrocarbons with oxygen at temperatures around 100 °C in the presence of dimmed light. Hydrocarbons such as xylenes and ethylbenzene were allowed to be in contact with oxygen for periods from 24 up to 60 days obtaining for xylenes monoaldehydes as the major oxidation product and only a fraction of the corresponding monocarboxylic acids. Stephens also accounted for a mechanism in which hydrocarbon undergoes stepwise oxidation with aldehydes and ketones as intermediates, excluding alcohols,11and also the inhibiting effect of water in oxidation.11,12 Evidence for formation of hydroperoxides as intermediates was found only in cyclohexene oxidation.13 Hartman and Seibert succeeded in peroxide isolation in tetralin oxidation in 1932.14 Oxidation of hydrocarbon induced by light is not practical for industrial purposes due to the long induction times usually involved. Action of catalysts is therefore necessary. The combined action of molecular oxygen and metal catalysts is more suitable for industrial use, since the oxidation reaction happens much faster. King et al.15 studied the liquid-phase oxidation of ethylbenzene

2ROOH → RO• + RO2• + H 2O

(8)

followed by RO• + RH → ROH + R•

(9)

The chain reaction is continued by the newly formed radical.16 Indeed, there is in the literature accumulated evidence that hydrocarbon oxidation follows a free-radical chain mechanism.17 Autoxidation is any oxidation that occurs in the presence of air or oxygen forming peroxides and hydroperoxides. A generic commonly accepted mechanism for hydrocarbon autoxidation found in literature reviews is as follows18−21 initiation step RH + initiator → R• + H ‐ initiator

(10)

propagation steps R• + O2 → ROO•

(11)

ROO• + RH → ROOH + R•

(12)

termination steps R• + R′• → R−R′

(13)

ROO• + R′OO• → ROOOOR′

(14)

ROO• + R′• → ROOR′

(15)

RH represents a hydrocarbon from which a hydrogen atom can be abstracted, yielding the free radical R•. ROO• and R′OO• are peroxide radicals and ROOH a hydroperoxide. C


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R• and O2 decreases with increasing resonance stabilization energy of the radicals.23 Considering the equilibrium given by eq 16, for free radicals with a higher resonance stabilization than the peroxy radical, the left-hand side of the equation has increased stability compared to the right-hand side. This means that the energy difference between R• and O2 reactants and the peroxide radical product RO2• is small. The energy barrier that the peroxide radical has to overcome to decompose back to R• and O2 is small. In a temperatures range from 300 to 700 K, Keq decreases from the nonresonance-stabilized methyl radical to the resonance-stabilized benzyl radical. Thus, according to the equilibrium expression of eq 17 for Keq, [ROO•] will be smaller than [R•].24 Consequently, hydroperoxide formation rates, as per eq 12 will be low. Uncatalyzed oxidation will not be feasible for practical purposes. In the case of olefins oxidation, considering eq 12, the dissociation energy of the α C−H bond decreases with increasing substitutions at the double bond, i.e., olefins with higher substitution are more reactive

In the initiation step, the initiator attacks the substrate molecule to generate the chain carrier free radical. At lower temperatures, a long induction period precedes chain initiation if the hydrocarbon is very pure or contains any molecules that act as antioxidants that trap or decompose peroxide radicals, inhibiting chain reaction. This is observed, for example, in the oxidation of impure benzaldehyde with oxygen.22 When impure benzaldehyde is allowed to react with oxygen, an induction period is observed before any oxygen absorption occurs. After a single distillation procedure, both the induction period and the maximum rate were reduced considerably. Further distillations eliminate the induction period, and the maximum reaction rate is achieved immediately at the beginning of the reaction. In addition, impure benzaldehyde oxidation shows both an induction period and a maximum rate greater than that observed for the pure substance, which is an indication of the presence of both inhibitors and catalysts, the latter suggested to be metallic ions. In propagation steps, after the highly reactive free radical is formed it reacts with an oxygen molecule to form a peroxy radical (eq 11) which, by abstracting a hydrogen atom from some other molecule, yields a hydroperoxide (eq 12). The hydroperoxide will decompose in a sequence of further oxidations to yield molecules with carbonyl and carboxyl groups. Propagation steps are the product-controlling steps in the autoxidation sequence. It is suggested in the literature19 that radicals of intermediate stability can enter in equilibrium with oxygen R• + O2 ⇄ ROO•

Similarly, α-hydrogen atom reactivity in toluene, ethylbenzene, and cumene toward a peroxy radical is 0.08, 0.5, and 1.0, which is in accordance with the increasing degree of substitution at the α-carbon atom.19 In benzene derivatives, the nature and position of a substituting group in the ring also plays a role in the molecule’s reactivity toward a peroxy radical. A series of changes in reaction conditions will cause changes in the rate of a chemical reaction or in the position of its chemical equilibrium. If the series of changes affects rates or equilibrium of another chemical reaction in the same way minus the effect of a given substrate, there will be a Gibbs free energy relationship, ΔG, between the two sets of effects. This relationship between free energy and equilibrium constants or reaction rates was first established for the ionization reaction of benzoic acid derivatives with meta and para substituents to each other25

(16)

The equilibrium constant Keq is given by Keq =

[ROO•] [R•][O2 ]

(17)

This equilibrium is important whenever radicals react with oxygen in the presence of the parent hydrocarbon to yield the dialkyl peroxide in preference to the hydroperoxide ROO• + R• → ROOR

(18)

The equilibrium between the radical, oxygen, and peroxy radical may give only a low concentration of the peroxy radical in the presence of alkyl radical, favoring formation of the dialkyl peroxide and not the hydroperoxide. Assuming sequential unimolecular kinetics for propagation steps, the propagation rate, vP, is given by vP = kP[ROO•][RH]

−RT ln K + RT ln K 0 = ΔG =

⎞ A ⎛⎜ B1 + B2 ⎟ 2⎝ ⎠ d D

(21)

K is a rate or equilibrium constant for a substituted reactant, K0 is the corresponding quantity for the unsubstituted reactants, ΔG is the Gibbs free energy or its kinetic analogue, d is the distance from the substituent to the reacting group, D is the dielectric constant of the medium, and A, B1, and B2 are constants independent of the temperature and solvent. Equation 21 can be rearranged to eqs 22 and 23

(19)

The terms in right brackets represent the species concentration, and kP is the propagation rate constant. If vP is fast enough, i.e., vP ≫ vT, where vT is the rate of termination, the chain reaction is viable and products drop out of each cycle via the hydroperoxide molecule ROOH. It is easily understood that a critical issue is then how fast that peroxy radical is formed and reacts with other molecules, as per reaction steps given by eqs 11 and 12, to form the hydroperoxide molecule: if the peroxy radical stability is low, [ROO•] will be low and hydroperoxide product formation will be low. The rate of propagation steps controls the outcome of the autoxidation sequence, with radical stability playing a major role. Factors governing radical stabilization are the ability of delocalizing electrons through resonance stabilization and electron availability of the carbon− hydrogen bond being ruptured. The rate of interaction between

log K = log K 0 + σρ

(22)

log k = log k 0 + σρ

(23)

or

Equations 22 and 23 are two simplified forms of what is known by the Hammett equation, where

σ=−

A 2.303R

(24)

and D


Chemical Reviews ρ=

⎞ 1 ⎛⎜ B1 + B2 ⎟ ⎠ d T⎝ D 2

Review

the polar effect is that p-xylene is much more reactive than ptoluic acid toward a peroxy radical.

(25)

σ is a substituent constant dependent upon the substituent, and ρ is a reaction constant dependent on the reaction medium and temperature. With ρ values greater than 1, the reaction rate is more sensitive to electronic effects than is the ionization of benzoic acids. If 0 < ρ < 1, electron-withdrawing groups still increase the rate constant but to a lesser extent than for benzoic acids. If ρ is negative, electron-donating groups increase the rate. Small ρ values, 0 < ρ < 1, often mean that the mechanism involves intermediates with little charge separation, such as radicals. High ρ values, both positive and negative, are consistent with a mechanism in which charged intermediates are present.26,27 The Hammett equation is also applicable to oxidation of alkylaryl hydrocarbons, where polar effects in freeradical reactions are present.28 Considering the reaction given in eq 12 in the propagation step of the autoxidation, the reactivity of a benzene derivative toward a peroxy radical can be explained taking into account polar effects and the stability of the radical formed. Considering cumene, the rate of reaction of its para-substituted derivatives shows a pronounced polar effect, as can be seen in Figure 1.

Exceptions to these findings exist. When electron-withdrawing substituents increase the rate or when electrondonating substituents decrease the rate, when compared with the nonsubstituted molecule, a higher resonance effect can explain an enhanced net aralkyl radical stabilization. Any reactions in which free radicals are consumed, resulting in molecules that are not free radicals, are the chain termination steps (reactions steps given by eqs 13−15). These will stop the chain and cause the chemistry to stop or be slow. Products formed through termination reactions may not be necessarily the same or similar to hydrocarbon being oxidized or its hydroxyl, carbonyl, or carboxylic derivatives. Alcohols, ethers, or peroxides may be obtained and even hydrocarbons resulting from condensation of several hydrocarbon radicals. For instance, as mentioned above, when the peroxy radicals are resonance stabilized, reaction with the parent hydrocarbon may yield peroxides as per eq 18 as well as dimer through condensation in eq 13. Ketones may also be obtained through interaction between two primary or secondary peroxide radicals via a cyclic transition state.19

The effect of inhibitors (antioxidants) on hydrocarbon oxidation, with influence also in initiation steps as mentioned above, also is a factor in reaction termination. This effect will depend upon whether a steady supply of free radicals (chain initiators) is being generated in sufficient number to overcome the chain-breaking action of the inhibitor. The view that the action of inhibitors is a relative rather than an absolute effect is substantiated by experimental evidence. For example, compounds, such as phenols or alcohols, which also extend the initiation inhibition period may have little effect on a reaction which has reached a steady state.18

Figure 1. ρσ plot for the peroxy radical attack on substituted cumenes. kx and kH are the rates of reaction of the peroxy radicals with a substituted cumene and cumene, respectively. σ constants from ref 29. Adapted from ref 28.

From Figure 1, it can be seen that electron-withdrawing substituents, like nitro or cyano, decrease the rate when compared with nonsubstituted cumene and an electrondonating substituent, like tert-butyl, has the opposite effect on rate. Oxidation is favored by increasing electron density at the reaction site. This has been interpreted in terms of resonance stabilization in the transition state for the reaction between the peroxy radical and the hydrocarbon carbon−hydrogen bond.19

3.2. p-Xylene Autoxidation to Terephthalic Acid

In p-xylene oxidation, disregarding any byproducts formed in secondary reactions, methyl groups are oxidized as per the sequence of eq 2.3 The mechanism of oxidation is also through free radicals as presented in eqs 11−15, and it is in that mechanism where the limitations lie. Considering attack of an initiator on a p-xylene molecule, the p-methyl benzyl radical would be obtained as per eq 10.

Polar effects have their influence in resonance structure I, which is stabilized by electron-donating substituents and destabilized by electron-withdrawing substituents in R′. Resonance structure II explains the effects of the stability of the ROO• radical upon the reactivity of a carbon−hydrogen bond. Therefore, if both polar and radical stability effects are present, the reactivity of a carbon−hydrogen bond toward a peroxy radical can be very high. One practical consequence of

This radical has the ability to become stabilized through resonance. E


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3.3. Catalyst Influence in Autoxidation

In the presence of small amounts of transition metals the mechanism given by eqs 11−15 of oxidation is strongly modified. The metal performs four important functions.16−21 • First, some transition metals participate in the initiation steps, abstracting hydrogen atoms from hydrocarbons, forming free-radical species, and enhancing the rate. • Second, metals react quickly with the primary formed peroxides via the Haber−Weiss cycle.30,31,18,21

However, the product of radical interaction with O2, as per eq 11, the peroxy p-methyl benzyl radical, is not able to achieve such kind of stabilization. The carbon atom to which the oxygen atom is bonded has all four possible valences occupied, and so the free electron in the peroxy oxygen atom is isolated from the ring double-bond-conjugated system, and thus, the radical cannot form any resonance hybrid, achieving a lower stabilization. The equilibrium is therefore more favorable

toward the reactants in eq 11 and the peroxy radical concentration is low, which according to eqs 17 and 19 causes the propagation rate to be low. As mentioned above, in the oxidation sequence in eq 2, an even slower step is oxidation of p-toluic acid (eq 27). Consider the mechanism given by eqs 10−15: in the oxidation of the first methyl group of the pxylene molecule, the p-methyl benzyl radical is resonance stabilized; in the oxidation of the second methyl group, in the p-carboxyl benzyl radical, the COOH group is an electronwithdrawing group, in opposition to the electron-donating methyl group in the first oxidation. This will cause the electrons of the ring to be attracted toward the COOH group, making them less available to stabilize the CH2• group through resonance structures.

The last step of this scheme, given by eq 37, shows reduction of the metal catalyst to its initial oxidation state, showing that decomposition is self-sustained with only small amounts of metal. • Third, metals also react very rapidly and selectively with peracids, formed from benzaldehydes, and also with higher yields.21 • Fourth, transition-metal complexes have the ability to coordinate to radicals, such as the peroxy p-methyl benzyl, trapping them, preventing them from reacting in termination steps that decompose the radical and decrease the rate, and driving them forward to products.16 3.3.1. Cobalt and Manganese Acetates As Oxidation Catalysts. Homogeneous transition-metal-catalyzed reactions are widely used in Industrial Chemistry. Hydrogenation, isomerization, polymerization, carbonylation, epoxydation, and oxidation are some examples of industrial reactions catalyzed by transition metals.32,33 Cobalt and manganese are metals used in some homogeneous-catalyzed processes. Manganese acetylacetonate and manganese acetate as well as cobalt bromide, cobalt oleate, cobalt naphthenoate, and cobalt acetate are some examples of the catalysts used. In oxidation, in particular, manganese and cobalt acetates are very important catalysts.2,34,35 Morimoto and Ogata proposed the following mechanism for substituted toluene oxidation catalyzed by cobalt acetate in acetic acid.36

The carboxyl group makes p-carboxyl benzyl radical stabilization difficult. The methyl group carbon−hydrogen bond of p-toluic acid is then strengthened. This is true also in catalyzed oxidations. Oxidation of a methyl group to a carboxylic group causes the σpara substituent constant to increase from −0.1 to 0.42, making oxidation of the second methyl group 4.9 times slower. Because p-xylene has the double of oxidizable hydrogen atoms of p-toluic acid, p-xylene is 2 × 4.9 = 9.8 times more reactive.21 The deactivating effect of the carboxylic group is bypassed by converting the acid to a methyl ester as is done in the Dynamit−Nobel process. Ester groups are electron donating and do not deactivate the ring as carboxylic groups do. As a consequence, methyl p-toluate oxidizes to monomethyl terephthalate up to terephthalic acid. Autoxidation of p-xylene to terephthalic has thus two great major constrains: greater stability of the p-methyl benzyl radical toward the p-methyl peroxybenzyl radical and p-toluic acid resistance to oxidation due to the instability of the p-carboxyl benzyl radical. These constrains make autoxidation a process too slow for commercial processes. Air oxidation in commercial processes requires the use of catalysts to achieve higher rates and yields. F

PhCH3 + Co3 + → PhCH 2• + Co2 + + H+

(38)

PhCH 2• + O2 → PhCH 2O2•

(39)


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Figure 2. Product concentration variation over reaction time: symbols, observed; line, calculated. Adapted from ref 43.

PhCH 2O2• + Co2 + → PhCHO + Co3 + + OH−

(40)

PhCHO + Co3 + → PhCO• + Co2 + + H+

(41)

•

PhCO3•

(42)

PhCO3• + PhCH3 → PhCO3H + PhCH 2•

(43)

PhCO3•

(44)

PhCO + O2 →

•

+ PhCHO → PhCO + PhCO3H

PhCO3H + Co2 + → PhCO2• + OH− + Co3 +

(45)

PhCO2• + PhCH3 → PhCO2 H + PhCH 2•

(46)

PhCO2• + PhCHO → PhCO• + PhCO2 H

(47)

K′

2Co(III) ⇄ Co(III)2 K″

d[O2 ] [RH][Co3 +]2 ∝k dt [Co2 +]

Co(III)2 + RH ⇄ {RH+AcO− Co(III)} + Co(II)

(50)

{RH+AcO− Co(III)} → R• + Co(III) + AcOH

(51)

where AcO− and AcOH represent the acetate anion and acetic acid, respectively. During hydrocarbon oxidations in the presence of oxygen the ratio of cobaltic ion to total cobalt tends to reach a steady state value close to 50%. When cobaltous ion is added, cobaltic ion is regenerated to reach 50% again. This has been an argument to suggest that a dimer comprising both cobaltic and cobaltous ions may also be an active catalytic species. As mentioned above, cobaltous ion has been found to have an inhibiting effect; thus, the oxidation rate is expected to decrease unless catalyst regeneration occurs. A possible concerted mechanism for consumption and regeneration for Co(III) is given by37

PhCHO, PhCO2H, and PhCO3H represent benzaldehyde, benzoic acid, and perbenzoic acid, respectively, and PhCO•, PhCO2•, and PhCO3• represent their respective radicals. PhCH2• and PhCH2O2• represent the benzyl radical and peroxy benzyl radicals, respectively. Generation of free radicals can occur either by an electron-transfer mechanism in which an electron is transferred from the aromatic hydrocarbon to the metal catalyst with proton loss or hydrogen atom abstraction from peroxy radicals, which is a propagation step. An induction period may be observed or not depending on the amount of cobaltic species (Co3+) present in the catalyst. It is observed that it is suppressed by addition of benzaldehyde, which has the effect of inducing oxidation of part of the catalyst from the cobaltous (Co2+) to the cobaltic state (Co3+).36,37 Evidence of the absence of an induction period for aromatic hydrocarbons can be found in the literature.38−40 The induction period is due to the low concentration of cobaltic ion and can be understood as the time required for the inhibiting cobaltous ion to be oxidized to cobaltic ion until its concentration is high enough to initiate hydrogen abstraction from the hydrocarbon as per eq 38.41 This is confirmed by some experimentally established hydrocarbon oxidation rate laws which are directly proportional to [Co 3+ ] and [RH] and inversely proportional to [Co2+].37−40,42 −

(49)

The same type of concerted mechanism may account for the activity of manganese as cocatalyst for cobalt RH + O2 + Mn(III)− Co(II) → ROO− + Mn(II)− Co(III) + H+

Mn(II)− Co(III) ⇄ Mn(III)− Co(II)

(53) (54)

Ichikawa et al.43 proposed a similar mechanism to the one given by eqs 38−46, excluding the reactions given by eqs 44 and 46, hydrogen abstraction from the aldehyde. The assumed reaction steps are the same as those in eq 2. Their comparison of the obtained results with the calculations made from the assumed kinetic mechanism shows good results, as shown in Figure 2. In terms of catalyst structure, mixtures of several similar multinuclear complexes have been proposed,44 the most common being the proposed catalyst structure of a binuclear μ-dihydroxy complex for both cobaltic and cobaltous acetates, Figure 3.43 Hendriks et al.45 proposed a mechanism where the species abstracting a hydrogen atom from the hydrocarbon molecule

(48)

The rate law, second order dependent on [Co3+],38−40,42 accounts for the postulated pre-equilibrium where a cobaltic dimer, (Co(III)−Co(III)), is formed, with this dimer being the active species for hydrocarbon hydrogen abstraction in the early stage of the reaction G


Chemical Reviews

Review

an aldehyde and from acid-catalyzed reduction with Co2+ ions. • In the Hendriks et al. mechanism Co2+ ion appears to be more active in generating more nonradical products, especially in peracid decomposition, and the Co3+ ion also appears to be more active, generating both nonradicals and radicals from aldehydes. Hendriks et al.45 found that the oxidation rate of toluene by Co(III) in anaerobic and aerobic conditions follows the same rate equations, and the rate constants under different conditions differ slightly. The product distribution, on the other hand, was found to be considerably different: in anaerobic conditions products were mainly benzyl acetates and benzaldehydes derived from the several toluene derivatives; in aerobic conditions products were mainly benzaldehydes and benzoic acid. The finding that hydrocarbon oxidation by benzylperoxy radicals is not a reaction of any significance led to the conclusion that aerobic oxidation is not a radical chain reaction, in opposition to the typical free-radical chain mechanism proposed by Morimoto and Ogata. A similar conclusion was obtained by Heiba et al.,46 who found that the cobaltic-catalyzed tertiary alkyl benzene oxidation rate was slower than that of primary alkyl benzene. This is in opposition with what was mentioned above for the α-hydrogen atom relative reactivity of 0.08, 0.5, and 1.0 for toluene, ethylbenzene, and cumene in free-radical autoxidations.19 The σ plot data from Hendriks et al.45for the relation between the hydrogen abstraction rate from a substituted toluene by Co3+ and the Hammett substituent constants are shown in Figure 4.

Figure 3. Proposed structure for the catalyst in Teijin process. Adapted from ref 43.

does not involve formation of a dimer, but initiation made by Co3+ involves formation of an intermediate radical−cation. k1(CH3CO2 H)

•

Co3 + + PhCH3 XoooooooooooooooY {(PhCH3+ )(CH3CO2−)} k −1(CH3CO2 H)

+ Co2 + + H+

(55)

Co3 + + {(PhCH3+•)(CH3CO2−)} ⇄ Co3 +{(PhCH3+•)(CH3CO2−)}

(56)

Co3 +{(PhCH3+•)(CH3CO2−)} → Co3 + + PhCH 2• + CH3CO2 H

(57)

CH3CO2 H/H 2O

Co3 + + PhCH 2• ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Co2 + + PhCH 2OCOCH3 /PhCH 2OH + H+

(58)

PhCH 2• + O2 → PhCH 2O2•

(59)

Co3 + + PhCH 2O2• → Co2 + + PhCHO + 0.5O2

(60)

Co3 + + PhCHO → Co2 + + PhCO• + H+

(61)

H 2O

Co3 + + PhCO• ⎯⎯⎯→ Co2 + + PhCO2 H + H+

(62)

PhCO• + O2 → PhCO3•

(63)

H+

Co2 + + PhCO3• ⎯→ ⎯ Co3 + + PhCO3H

PhCO3•

(64) •

+ PhCHO → PhCO3H + PhCO 2H

(65)

+

2Co2 + + PhCO3H ⎯⎯⎯→ 2Co3 + + PhCO2 H + H 2O

(66)

Some of the main differences between the two proposed mechanisms are as follows. • The Morimoto and Ogata mechanism explains benzylic radical formation by direct hydrogen abstraction by the cobaltic ion, and in the mechanism proposed by Hendriks et al. a radical−cation is formed. The Morimoto and Ogata mechanism explains aldehyde formation from Co2+ oxidation and peroxide radical reduction, whereas Hendriks et al. explains aldehyde formation through Co3+ reduction and peroxide radical oxidation. • Acids in the Morimoto and Ogata mechanism are formed from hydrogen atom abstraction from a hydrocarbon molecule by the PhCO2• radical, and the Hendriks et al. mechanism explains it by PhCO• oxidation in the presence of water by reduction of a Co3+ ion and hydroperacid decomposition with Co2+ ions. • Peracids in the Morimoto and Ogata mechanism are formed exclusively from PhCO3• hydrogen abstraction from a hydrocarbon molecule, and in the Hendriks et al. mechanism it is from PhCO3• hydrogen abstraction from

Figure 4. ρσ plot for hydrogen abstraction by Co3+ on substituted toluenes. kx1 and kH are the reaction rates for substituted toluene and toluene, respectively. σ+ constants from ref 47 Adapted from ref 45.

The ρ value found for this series plot is −1.97, a large negative ρ. Like in the case of autoxidation of cumene derivatives previously mentioned, a ρ negative value means that electron-donating groups, like the methyl group, increase the rate constant and electron-withdrawing groups, like the carboxy group, decrease the rate constant. However, unlike the case of autoxidation, ρ has a large value, meaning that the in the transition state positively charged intermediates develop, which is inconsistent with a free-radical mechanism26,46,49,52. In addition, from Figure 4 it can be seen that p-toluic acid (pcarboxy toluene) is less reactive toward oxidation than toluene, even in the presence of a catalyst. Kashima and Kamiya40 also observed that the apparent chain length of p-toluic acid oxidation by cobaltic acetate in acetic acid is much shorter compared to that of p-xylene. This is related with the increased oxidation potential arising from the carboxylic group presence, causing electron transfer from the molecule to the catalyst H


Chemical Reviews

Review

Figure 5. Data fitting for product concentration variation over reaction time according to the Hendriks et al. mechanism: symbols, observed; line, calculated. Adapted from ref 45.

as a solvent that no terephthalic acid yield was observed. They also concluded that Co and Mn action in the Co/Mn/Br catalyst was synergistic, other halide salts of Co and Mn gave low terephthalic acid yields, and finally acetic acid was the most stable solvent. The main discovery was the ability of bromine to increase the rate of oxidation.7 Bromine and bromine and cobalt show activity toward hydrocarbon oxidation, but the most successful combination has proved to be combination of bromine, cobalt, and manganese, the core catalyst of the AMOCO MC method. 3.3.2.1. Catalysis by Bromine. Bromine compounds, in particular, hydrogen bromide, are known hydrocarbon oxidants, according to the mechanism54−56

complex to be harder. This feature has been of great importance in the industrial oxidation of p-xylene to terephthalic acid. The resistance of p-toluic acid toward oxidation accounts for some reported need for use of larger amounts of catalyst in industrial processes, compared with what is usually employed in other autoxidations.48,43 Despite the differences in the mechanisms, the oxidation rate in the Hendriks et al. mechanism, Figure 5, follows the same proportionality as the one found in the Morimoto and Ogata mechanism, given by eq 48, and the fit for periodical variation of each compound, like the one obtained by Ichikawa et al.43 also shows good results. Both mechanisms therefore explain the product formation sequence given by eq 2. The catalyst structure, as proposed by Hendriks et al., in acetic medium is a six-ligand mononuclear complex, rather than a dimer structure, based on magnetic susceptibility measurements. 49 Cobaltous acetate can be represented as CoII(OAc−)2(AcOH)4 and cobaltic acetate by CoIII(OAc−)3(AcOH)3. In solution the acetic acid ligands can be progressively exchanged by water, aldehydes, phenol, and carboxylic acids. Octahedral geometry is the usual geometry for d-block six-coordinated metal complexes, such as cobalt complexes, Figure 6.50 Recent research has shown that in acetic acid solution two forms of mononuclear catalytically active cobaltic complexes may coexist in equilibrium. Reduction of cobaltic species is accompanied by oxidation of the solvent and is given by the following mechanism51 Coα 3 + ⇄ Coβ 3 + → Co2 +

HBr + O2 → Br • + HO2•

(68)

RCH3 + Br• → RCH 2• + HBr

(69)

RCH 2• + O2 → RCH 2OO•

(70)

k

RCH 2OO• + HBr → RCH 2OOH + Br •

(71)

The peroxide, on loss of a water molecule, yields an aldehyde (or ketone). Further oxidation yields a carboxylic acid.

(67)

3.3.2. Cobalt/Manganese/Bromide Catalyst Systems. The resistance of p-toluic acid toward oxidation was overcome with a successful catalyst breakthrough obtained by Saffer and Barker 4−6 almost simultaneously with McIntyre and O’Neill.52,53 The first attempts to increase rates in aromatic hydrocarbon oxidation were made with manganese bromide playing the role of catalyst in p-cumene oxidation in acetic acid medium. The overall oxidation yield was found to be no better than that obtained using other manganese salts, but practically no intermediates were found. They also tried all the elements in the periodic table and found Co and Mn to give the highest terephthalic acid yields. They then varied the anion of Mn and discovered that bromide increased both the yield and the rate of oxidation. They then switched to p-xylene as the reagent and found that Mn/Br gave a 77% terephthalic acid yield while Mn(II) acetate gave a very low yield. Then they established that Co/Br gave a 79% yield and that if benzene replaced acetic acid

3.3.2.2. Catalysis by Cobalt and Bromine. Ravens57 studied the oxidation of p-toluic acid by cobalt and manganese bromides in acetic acid. They found a rate law given by − I

d[O2 ] ∝ k[Co2 +][NaBr]1/2 [O2 ]1/2 dt

(77)


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Review

catalysis in solvent media by either cobaltous bromide or a mixture of a cobaltous salt and inorganic bromide as catalyst. At the end of the first stage, all ionic bromide disappeared from solution and the bromine is present in the form of pbromomethyl benzoic acid. The second stage is catalyzed by a cobalt salt and p-bromomethyl benzoic acid with no bromide ions in solution. The role of bromine in the catalytic oxidation of hydrocarbons is explained as to be other than that of reaction initiation by hydrogen abstraction as postulated by Ravens, Bawn, and Wright, since formation of hydrobromic acid from bromide salts as per eq 78 proceeds to a very low extent due to the low equilibrium constant. In fact, the long induction periods observed in cobalt-bromide-catalyzed oxidations lead to the hypothesis that its role is to catalyze propagation steps.59−63 Figure 7 shows the oxygen absorption rate for p-xylene oxidation catalyzed by cobalt dibromide, CoBr2, and cobalt decanoate, CoDe2, and sodium bromide.

The reaction rate has a linear dependence on cobalt acetate concentration at constant sodium bromide concentration and a square root dependence on sodium bromide concentration with constant cobalt concentration dependence. The rate dependence on [NaBr]1/2 holds only up to a bromide:cobalt ratio smaller than 2, where an inflection point occurs, and beyond it, large deviations occur due to structural changes in the cobaltous ion complex. The small initial rate in the absence of one or the other ion shows that the combined action of both catalysts has a beneficial effect on the reaction rate. The

Figure 6. Octahedral representations of CoII(OAc−)2(H2O)4 (1) and CoIII(OAc−)3(H2O)3 (2).

dependence of the reaction rate on [Co2+] and [NaBr] found by independent variation of the catalyst component concentrations is further substantiated by the rate dependence on [CoBr2]3/2, which may also be expressed as [Co2+][NaBr]1/2. The dependence of the reaction rate on [NaBr]1/2 and [O2]1/2 indicates that both oxygen and sodium bromide are involved in the initiation step and that chain termination is by a bimolecular radical−radical reaction. The initiation step by reaction between HBr and O2, as per reaction in eq 68, arises from pre-establishment of the equilibrium NaBr + CH3CO2 H ⇄ HBr + CH3CO2 Na

Figure 7. Oxygen absorption curves in the autoxidation of p-xylene in acetic acid catalyzed with 10−2 M CoBr2 and with a mixture of 10−2 M CoDe2 and 2 × 10−2 M NaBr at 80 °C. Adapted from ref 62.

(78)

The proposed mechanism for p-toluic acid oxidation is given by PhCH3 + Br • → PhCH 2• + HBr •

•

Cobalt dibromide shows an initial higher rate, but it decreases after approximately 30 min, showing that catalyst is becoming deactivated. The rate of the mixture containing CoDe2 and NaBr is kept constant over a large period. Higher cobalt dibromide concentrations show pronounced induction periods, which, on addition of sodium bromide, are eliminated. This shows that cobalt dibromide is not the active catalyst, but cobalt acetate bromide, formed by eq 86, is

(79)

PhCH 2 + O2 → PhCH 2OO

(80)

PhCH 2OO• + Co2 + → PhCHO + OH− + Co3 +

(81)

Co3 + + HBr → Co2 + + H+ + Br •

(82)

2PhCH 2OO• → PhCH 2CH 2Ph + 2O2

(83)

PhCH 2OO• + PhCH 2• → PhCH 2OOCH 2Ph

(84)

2PhCH 2• → PhCH 2CH 2Ph

(85)

CoBr2 + NaCH3COO → CoBrCH3COO + NaBr

(86)

In addition, increasing the ratio between sodium bromide and cobalt increases the rate from a ratio of 0.5 to 1.0 from which the rate is kept constant, as Figure 8 shows. At a ratio of 1.0, the concentration of free HBr is very low and most of the bromide is complexed with cobalt. This confirms that the oxidation rate increase is due to catalysis by cobalt acetate bromide and not by HBr.62 The catalytic mechanism in the propagation step is given by62

where Ph stands for

An important aspect of this mechanism is that p-toluic acid does not enter in its propagation, reaction propagation being through the electron-transfer reaction between a cobaltous ion and peroxide radical, as given by eq 81. This absence of p-toluic acid is accounted for in agreement with the rate law equation given by eq 77. The role of cobalt is that of decomposing peroxide radical and regenerating the mechanism initiating species, Br•, through eq 82. Bawn and Wright58 also studied ptoluic acid oxidation by cobalt bromide in acetic acid medium and divided the oxidation in two stages. The first stage involved

RH

ROOH + (Co)2 → radical ⎯→ ⎯ R• •

R + O2 → RO2 •

J

•

(87) (88)

•

RO2 + RH → ROOH + R

(89)

RO2• + Co2 +BrH → products + (Co2 + + Br•)

(90)

(Co2 + + Br•) + RH → (Co2 +BrH) + R•

(91)


Chemical Reviews

Review

hydrocarbon + dioxygen catalyst = metal(s)/bromide

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ oxygenate + water solvent

(94)

More than 251 alkylaromatic compounds have been oxidized using the method described in eq 94.21 The most common example is oxidation of alkylbenzenes to aromatic carboxylic acids

Figure 8. Steady state rate of oxidation of p-xylene in acetic acid as a function of the NaBr/Co molar ratio. Adapted from ref 62.

RO2• + RO2• → products + O2

When bromine and manganese compounds are added to a reaction catalyzed by cobalt compounds, creating a Co/Mn/Br catalysts system, five important changes result, indicating that new catalytic pathways are available.21 • Catalyst becomes 16 times more active. • Formation of CO2 and CO from secondary reactions decreases by a factor of 10, indicating a more selective reaction, accompanied also by a higher activity. • The steady state concentration of cobaltic ion is reduced to only 0.6%. • The maximum temperature of oxidation achievable is higher than the 150 °C usual for cobalt-catalyzed oxidations with comparable yields. • Long induction times are absent, except for highly purified hydrocarbons. Although Co/Mn/Br catalyst type is most common in the AMOCO MC method, there are other catalyst systems with bromide and with one, two, or three metals, Table 1. The most common catalyst is Co/Mn/Br followed by Co/Br and Mn/Br. Cobalt is present in the majority of the catalysts due to its known ability to rapidly decompose hydroperoxides via the Haber−Weiss cycle,30,31 the primary products of the oxidation process. The new available kinetic pathways for a Co/ Mn/Br MC catalyst type in an aromatic hydrocarbon oxidation can be depicted as shown in Figure 10. In simple terms, bromine acts as promoter, initiating hydrogen abstraction from the hydrocarbon and generating bromide. Bromide ion is oxidized by Mn3+, yielding Mn2+. Mn2+ is oxidized back to Mn3+ by Co3+, yielding Co2+, which, by peroxide decomposition, will be oxidized back to Co3+.

(92)

(Co)2 represents a dimer cobalt species as postulated above for cobalt acetate catalysis,38−40,42 and Co2+BrH represents the cobalt acetate bromide, the active species in this catalytic scheme

Replacement of approximately 20% of cobalt by manganese increases by a factor of 5 the rate of oxidation, showing that a strong catalytic synergy exists, due to the ability of manganese to react with peroxy radicals. This synergy exists up to a liming extent, from where manganese deactivation starts to occur, Figure 9. 3.3.2.3. Catalyst Breakthrough: AMOCO MC Method. The corollary of the experiments with inclusion of bromine was the conclusion that the critical combination of components required to achieve high efficiency in the oxidation of p-xylene to terephthalic acid was a source of bromine and a mixture of cobalt and manganese salts.7 This is the basis of the AMOCO Mid-Century (MC) method

Figure 9. Mixing effect of cobalt with manganese in the rate of oxidation of p-xylene in acetic acid. Adapted from ref 62. K


Chemical Reviews

Review

with n = 1 and 2, denoting monomer or dimer, and the square brackets denote the ligands in the inner coordination sphere of the metal, M. Figure 11a and 11b shows suggested structures for the left-hand side structures with n = 1 and 2, respectively.

Table 1. Examples of Catalyst Systems Using Bromine and One, Two, or Three Metalsa one metal

Co/Br

Mn/Br

two metals

V/Br Co/Mn/Br

Cu/Br Co/Ce/Br

Co/Br/ acetate Ce/Br Co/Cu/Br

Co/Mn/Br/ Cl Co/Ca/Br Co/U/Br Co/Mn/Zr/ Br Ni/Mn/Zr/ Br Co/Mn/Fe/ Br

Mn/Si/Br

Mn/Cu/Br

Co/Mn/Br/ acetate Ni/Mn/Br

Mn/Fe/Br

Co/Te/Br

Co/Ni/Br

Co/Mn/Ce/ Br Co/Mn/Hf/ Br Co/Mn/Mo/ Br

Co/Ce/Zn/ Br Co/Mn/Ru/ Br;

Co/Ce/Zr/Br;

three metals

a

Co/Br/Cl

Co/Mn/Cr/Br

Figure 11. Suggested structures for Co/Mn acetate catalyst in anhydrous acetic acid mixtures. M = Co(II), Co(III), Mn(II), Mn(III). Adapted from ref 65.

Adapted from ref 21.

Addition of water to acetic acid results in acetic acid ligands being exchanged with aqua ligands

The nature of the Co/Mn/Br catalyst in the oxidation of pxylene can be very diverse and complex.64 The reaction mixture may contain, in addition to Co(OAc)2·4H2O and Mn(OAc)2·4H2O other mononuclear species like Co(OAc)Br and Co(OAc)3, multinuclear species like Co3(O)(OAc)x and Mn3(O)(OAc)x, and heteromultinuclear complexes like Co2Mn(O)(OAc)x or CoMn2(O)(OAc)x. Mononuclear complexes are the predominant species in the initial stages of oxidation: they show the ability to abstract a hydrogen atom from p-xylene. Some of these species like cobalt acetate bromide, Co(OAc)Br, are very short lived due to their high reactivity toward p-xylene, arising from their very high redox activities. Cluster heteromultinuclear complexes, found in the later stages, probably play an important role in the oxidations of both p-toluic and 4-carboxybenzaldehyde. Cobalt acetate bromide, Co(OAc)Br, which as previously mentioned is described to be a powerful and active species in aromatic hydrocarbon oxidation, is often studied in anhydrous acetic acid.65 In industrial oxidation of p-xylene, the reaction medium contains 5−15% of water to make the solvent less aggressive toward the materials used in the equipment and associated pipework. In anhydrous acetic acid the predominant species can be described through the right-shifted equilibrium

[M(HOAc)m (H 2O)m − 4 (OAc)2 ]n ⇄ {[M(HOAc)m (H 2O)m − 5 (OAc)]n (OAc)}

(97)

with m ranging from 1 to 4 and decreasing with increasing water concentration. Figure 12a and 12b shows the ligand distribution around the metal in acetic acid/water mixtures. At water concentration close to 10%, which is common in industrial processes, the predominant structure has four aqua and two acetate ligands. The proposed structure is shown in Figure 13. Addition of hydrobromic acid in anhydrous acetic acid results in acid−base neutralization between HBr and acetate ions, leading, predominately, to bromine inner-sphere coordination [M(HOAc)4 (OAc)2 ]n + {[M(HOAc)5 (OAc)](OAc)}n + HBr ⇄ [M(HOAc)4 (OAc)(Br)]n + {[M(HOAc)4 (OAc)](Br)}n + HOAc (98)

Most of the bromide is coordinated in anhydrous acetic but rapidly decreases as water concentration increases. If HBr is added with more than 5% water, as is the case for most industrial processes, little bromide is coordinated and the predominant species is the ion-paired bromide salt

+HOAc

[M(HOAc)4 (OAc)2 ]n XoooooooY {[M(HOAc)5 (OAc)](OAc)}n (96)

Figure 10. Catalytic pathway for MC oxidation of p-toluic acid. Adapted from ref 21. L


Chemical Reviews

Review

Figure 12. Distribution of ligands around cobalt(II) (a) and manganese(II) (b) as a function of water concentration in acetic acid. Adapted from ref 65.

RO2 + [Co(II)− Mn(II)] H+

XooY ROOH + [Co(III)− Mn(II)]

(102)

Peracids react preferentially with metal dimers RO2 H + [Co(II)− Co(II)] H+

Figure 13. Suggested structures for Co/Mn acetate catalyst in an acetic acid solution with water concentration of 10%. M = Co(II), Co(III), Mn(II), Mn(III). Adapted from.65.

XooY ROH + [Co(III)− Co(III)]

(103)

RO2 H + [Co(II)− Mn(II)] {[M(HOAc)m (H 2O)m − 5 (OAc)n ]n (OAc)} + HBr

H+

XooY ROH + [Co(III)− Mn(III)]

⇄ {[M(HOAc)m (H 2O)m − 5 (OAc)]n (Br)} + HOAc

(104)

Ligands were not represented for simplicity. 3.3.2.3.1. Kinetic Mechanism of the AMOCO MC Method. In the literature there are several studies describing p-xylene oxidation, but these were performed at conditions different from those adopted in industrial processes or the models were based on empirical data and not a kinetic mechanism.43,45,61,66−68 Recently, Wang et al.69 proposed a stepwise mechanism through intermediates formed as per eq 105 to explain the oxidation of p-xylene catalyzed by cobalt and manganese acetates and hydrobromic acid in acetic acid at conditions close to those used in an industrial process.

(99)

The proposed structure for a Co/Mn/Br catalyst in acetic acid solution in the presence of water is shown in Figure 14.

Figure 14. Suggested structures for Co/Mn/Br catalyst in an acetic acid mixture solution. M = Co(II), Co(III), Mn(II), Mn(III). Adapted from ref 65.

Bromide is hydrogen bonded to one of the coordinated aqua ligands. The homogeneous Co/Mn/Br catalyst is thus not a single catalyst but a mixture in which several catalytically active species can coexist. In this sense a catalyst is any structure made when its metals are oxidized by oxidants like peroxy radicals or peracids. In such a mixture the oxidants may react with either a monomer or a dimer

The combined effect of Co and Mn is to decompose peroxides to yield the oxygenate product. Mechanism initiation is done by hydrogen abstraction from the hydrocarbon by bromine radicals generated through bromide ion oxidation by the metals

H+

RO2• + [Co(II)] XooY ROOH + [Co(III)]

(100)

•

RO2 + [Co(II)− Co(II)] H+

XooY ROOH + [Co(III)− Co(II)]

(101) M

Mn 2 + + Co3 + → Co2 + + Mn 3 +

(106)

Co3 + + Br − → Co2 + + Br •

(107)

Mn 3 + + Br − → Mn 2 + + Br •

(108)


Chemical Reviews

Review

Br• + HOOCPhCH 2O2• → HOOCPhCH 2O2 Br

3.3.2.3.1.1. Oxidation of p-Xylene to p-Tolualdehyde (PX to TALD).

(127)

Br• + CH3PhCH3 → Br − + CH3PhCH 2• + H+

(109)

3.3.2.3.1.4. Oxidation of 4-Carboxybenzaldehyde to Terephthalic Acid (4-CBA to TA).

CH3PhCH 2• + O2 → CH3PhCH 2O2•

(110)

Br• + HOOCPhCHO → Br − + HOOCPhCO• + H+ (128)

CH3PhCH 2O2• + Co2 + + H+ → CH3PhCHO + Co

3+

+ H 2O

•

HOOCPhCO + O2 → (111)

→ CH3PhCHO + Mn

+ H 2O

→ HOOCPhCOOOH + Co3 + (112)

Br• + CH3PhCH 2• → CH3PhCH 2Br

(113)

Br• + CH3PhCH 2O2• → CH3PhCH 2O2 Br

(114)

•

CH3PhCO + O2 → CH3PhCO3•

+ Co

2+

CH3PhCO3• +

→ HOOCPhCOOOH + Mn 3 +

→ 2HOOCPhCOOH Br• + HOOCPhCO• → HOOCPhCOBr •

Br +

CH3PhCO2 + Mn

2+

+ ke.109[CH3PhCOOOH])

d[PT] = r2 − r3 = r2 − ke112[PT][Br •] dt

(118)

+ ke.122[CH3PhCOOOH])

CH3PhCOOOH + CH3PhCHO → 2CH3PhCOOH

Br• + CH3PhCO3• → CH3PhCO3Br

(121)

ri = kici[Br •] =

(138)

∑ fi ci[Br•] = ε

(139)

kici λ1c1 + λ 2c 2 + λ3c3 + λ4c4 + ε

(140)

λi are rate constants from the reaction mechanism, and c1, c2, c3, and c4 are PX, TALD, PT, and 4-CBA concentrations, respectively. Equation 140 shows that the rate follows a direct proportionality to [Br•], a critical factor to initiate the oxidation reaction. Bromine radical concentration depends on intermediate product concentrations. As these decrease, [Br•] increases, increasing the reaction rate. When the intermediate concentration is very low when compared with ε, ∑j4= 1 λjcj ≪ ε, the rate is first order to the intermediate, which is found in several other kinetic mechanisms.37−40,42 The rate law is also independent of oxygen concentration, which in industrial processes is fed to the oxidation reactor in excess to achieve high conversion of the limiting reactant p-xylene and also of the

(123)

HOOCPhCH 2O2• + Co2 + + H+ (124)

HOOCPhCH 2O2• + Mn 2 + + H+ Br• + HOOCPhCH 2• → HOOCPhCH 2Br

(137)

then the general rate law is given by

(122)

→ HOOCPhCHO + Mn 3 + + H 2O

∑ ki[I•] =

j=1

Br• + HOOCPhCH3 → Br − + HOOCPhCH 2• + H+

→ HOOCPhCHO + Co3 + + H 2O

(136)

4

[I•] =

Product p-toluic acid is yielded through reaction between aldehyde and peracid by the Baeyer−Villiger70 oxidation (eq 119). 3.3.2.3.1.3. Oxidation of p-Toluic Acid to 4-Carboxybenzaldehyde (PT to 4-CBA).

HOOCPhCH 2• + O2 → HOOCPhCH 2O2•

(135)

Applying the steady state approximation to all radical and peracids and considering that the total concentration of radicals, [I•], is approximately equal to a constant, ε, i.e.

(119) (120)

(134)

d[4‐CBA] = r3 − r4 = r3 − [4‐CBA](ke.118[Br•] dt

3+

Br• + CH3PhCO• → CH3PhCOBr

(133)

d[TALD] = r1 − r2 = r1 − [TALD](ke.105[Br •] dt

3+

+ H → CH3PhCOOOH + Mn

→ HOOCPhCO3Br

d[PX] = −r1 = ke.99[PX][Br •] dt

(116)

+

HOOCPhCO3•

(132)

The reaction rates for the most important intermediates, PX, TALD, PT, and 4-CBA, are given by

(117) •

(131)

HOOCPhCOOOH + HOOCPhCHO

(115)

+ H → CH3PhCOOOH + Co

(130)

HOOCPhCO3• + Mn 2 + + H+

Electron-transfer mechanism initiation between metal and hydrocarbon producing a radical cation by reactions given in eqs 55−57, more common in the absence of bromine, was not considered. Attack of peroxide radical on hydrocarbon and solvent molecules was neglected. Also, hydroperoxide formation and decomposition reactions by Co2+ and Mn2+ via the Haber−Weiss cycle were not considered. The only termination reactions considered were those between the bromine radical and the free radicals (eqs 113, 114, 120, 121, 126, 127, 133, and 134). 3.3.2.3.1.2. Oxidation of p-Tolualdehyde to p-Toluic Acid (TALD to PT). Br• + CH3PhCHO → Br − + CH3PhCO• + H+

(129)

HOOCPhCO3• + Co2 + + H+

CH3PhCH 2O2• + Mn 2 + + H+ 3+

HOOCPhCO3•

(125) (126) N


Chemical Reviews

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Figure 15. Evolution of reactants concentration as a function of time: (a) batch experiment at 160 °C and 14 atm; (b) semicontinuous experiment at 160 °C and 18 atm. Adapted from ref 69.

intermediates. Figure 15 shows intermediate concentration evolution as a function of time. The results show good agreement between the data from the batch and semicontinuous experiment and the concentration predicted from eq 140. Figure 15a shows the typical behavior of consecutive reactions and that the methyl groups are oxidized consecutively, in agreement with the sequence given by eq 105. The concentration of p-toluic acid tends to build-up in the experiments, showing the typical difficulty in oxidizing the second methyl group of p-xylene, as confirmed by Figure 15b. The above mechanism and rate law proposed by Wang requires regression from experimental data of 9 parameters. The number of these parameters increases as the number of elementary reactions considered in the mechanism increase. This inevitably increases the complexity of the mathematical model as it requires more and more kinetic parameters determined from experimental data fitting regression. Several authors have adopted simplifications.67,68 Sun et al.71 established a radical chain mechanism that assumes that all participating peroxide free radicals have the same reactivity for removing an αhydrogen from any hydrocarbon intermediate substrate shown in eq 105. It is also assumed that all substrates have the same initiation rate constants. This assumption in particular is cumbersome: historically, development of chemical process technology in the process of terephthalic acid from p-xylene oxidation encountered resistance in the oxidation of p-toluic acid and differences in reactivity have been largely accounted for, as easily seen from a σ-constant plot. Finally, in Sun’s mechanism, differences in the termination rate constants are small and, therefore, negligible. Nevertheless, the first-order kinetic model only relies on six rate constants, and the data fit of concentration changes of reactants for different catalyst concentrations shows, like in the case of Wang’s mechanism, good agreement. The ultimate simplification of the kinetics of p-xylene oxidation is to consider a lumped scheme, as per eq 105, with no initiation, propagation, and termination elementary reactions as in a “classical” reaction chain mechanism, considering only the most relevant intermediates in the overall reaction pathway. The kinetics can be explained in terms of classical kinetics of a consecutive reaction involving four steps. Considering the simplified eq 105 scheme, reaction rates are given by

d[PX] = −k1[PX] dt

d[TALD] = k1[PX] − k 2[TALD] dt

(142)

d[PT] = k 2[TALD] − k 3[PT] dt

(143)

d[4‐CBA] = k 3[PT] − k4[4‐CBA] dt

(144)

On integration with the initial conditions, t = 0, [PX] = [PX]0, [TALD] = [TALD]0, [PT] = [PT]0, and [4-CBA] = [4-CBA]0, rate laws are given by [PX] = e−k1t [PX]0

(145)

k1 [TALD] = (e−k1t − e−k 2t ) [PX]0 k 2 − k1

(146)

k1k 2e−k1t k1k 2e−k 2t [PT] = − [PX]0 (k 3 − k1)(k 2 − k1) (k 3 − k 2)(k 2 − k1) +

k1k 2e−k3t (k 3 − k 2)(k 3 − k1)

(147)

k1k 2k 2(e−k1t − e−k4t ) [4‐CBA] = [PX]0 (k 3 − k1)(k 2 − k1)(k4 − k1) −

k1k 2k 2(e−k 2t − e−k4t ) (k 3 − k 2)(k 2 − k1)(k4 − k 2)

+

k1k 2k 2(e−k3t − e−k4t ) (k 3 − k 2)(k 3 − k1)(k4 − k 3)

(148)

[TA] = [PX]0 − [PX] − [TALD] − [PT] − [4‐CBA] (149) 72

Li and Li adopted a lumped scheme for characterization of p-xylene oxidation catalyzed by Co(OAc)2/Mn(OAc)2 in the presence of CoBr2 and MnBr2 at temperatures of 100, 110, and 120 °C and oxygen pressures of 2 and 10 atm. They obtained a good fit of eqs 145−149 to their experimental results, showing that assuming pseudo-first-order reaction rates for each step of p-xylene oxidation is a reasonable simplification of the reaction mechanism. Wang et al.73 adopted the same lumped kinetic scheme for the oxidation catalyzed with Co(OAc)2/Mn(OAc)2 and HBr in acetic acid in the temperature range from 188 to

(141) O


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197 °C and oxygen partial pressures from 12 to 40 KPa. The rate law is given by ri = kj

Cj 4 (∑i = 1 diCi

+ θ )βj

It can be seen that as temperature increases, the conversion rate of 4-carboxybenzaldehyde to terephthalic acid also increases. Figure 17 shows the trend of 4-carboxybenzaldehyde concentration.

(150)

with β and θ being kinetic parameters. The agreement between experimental data and model prediction based on the lumped scheme is satisfactory, being detailed enough to characterize the distribution of the most important species concentration with different reaction conditions. Lumped kinetic schemes show enough detail to give time function concentrations of the main intermediate species; more detailed mechanisms account for other issues of great importance in industrial processes like formation of byproduct and solvent burning into carbon oxides.

4. ACTIVITY In the AMOCO process, oxidation of p-xylene progresses nearly to completion: over 98% of the p-xylene is reacted and the yield to terephthalic acid is greater than 95 mol %. This presents a problem when monitoring reaction activity in a continuous reactor. A major change in activity does not result in an easily measured decrease in the level of reactant, as pxylene is only present in the reactor contents in small amounts as it quickly reacts with oxygen. Monitoring an intermediate concentration can be a useful method to monitor the activity. Among the several intermediates, considering only those given by eq 105, p-toluic acid and 4-carboxybenzaldehyde can both be, in a first approach, chosen to monitor activity. As expected, p-toluic acid is present at the highest concentration among intermediates in the reaction medium due to its known resistance in oxidizing the second methyl group of p-xylene. Because it is very soluble in the acetic acid solvent, very little precipitates with terephthalic acid. In the purification process, it is removed from terephthalic acid, because of its solubility in water. Historically, p-toluic acid has not been the major concern when assessing the AMOCO process terephthalic acid quality. Because 4-carboxybenzaldehyde is less soluble in acetic acid than p-toluic acid and it has a structure similar to terephthalic acid, part of it coprecipitates with terephthalic acid and is present as an impurity in the CTA. Therefore, it presents itself as the preferred intermediate to monitor the activity, even though p-toluic acid can also be used to assess it.

Figure 17. Concentration of 4-carboxybenzaldehyde vs time at different reaction temperatures. Adapted from ref 69.

At all times, the concentration of 4-carboxybenzaldehyde is smaller at greater temperatures. Temperature has the effect of decreasing the amount of 4-carboxybenzaldehyde as it increases. In an industrial reactor, temperature is indirectly controlled by pressure, showing a direct proportionality between both physical properties. 4.2. Water Effect on Activity

The stoichiometric reaction of p-xylene oxidation with oxygen yields two water molecules. Water is produced mainly as a byproduct in the main reaction but also in other chemical reactions such as oxidation to carbon oxides of both p-xylene and acetic acid. Water is separated from acetic acid by means of fractional or azeotropic distillation. In addition, the water present in the reaction medium may also come as part of the feed streams to the reactor, as an impurity or contamination. Concerning the reaction rate, water has a net decreasing effect,21,48,66 as seen from Figure 18b. From Figure 18a, as water concentration increases, relative rate decreases, showing that water concentration has a decreasing effect on activity. The decreasing effect is due to a progressive substitution of acetate ligands coordinated to cobalt,48 which occurs as water in the medium increases,65 as seen from Figure 19a and 19b. However, the effect of water on activity may not be so direct and is, and it has, in fact, a subtle effect, which needs to be balanced and accounted for. Water may have a positive effect on activity, as74 found for peroxide initiation of hydrocarbons like tetralin, cumene, cyclohexene, and methylcyclohexane. The protons of the water molecule interact with hydroperoxide molecules, favoring splitting of the O−O bond, yielding more radicals. This effect is only visible at small quantities of water, water having an inhibiting effect at higher quantities. Partenheimer75 compared the effect of water in the oxidation of 4-chlorotoluene in anhydrous acetic acid and acetic acid containing 5% water. The initial rate of oxidation is higher in anhydrous acetic acid than the rate with 5% water, but at longer reaction times, the effect is the opposite. Water may have an oxidation-promoting effect due mainly to the interaction with catalyst. During oxidation, benzylic bromides are formed, which are known to be catalytically inactive forms of bromine. In fact, in anhydrous acetic acid the initial rates are high, but as benzylic bromides are formed, the catalyst loses bromine, becoming deactivated, which explains the decreasing rates of oxidation at longer reaction times. In

4.1. Temperature and Pressure Effect on Activity

An Arrhenius plot of the rate constant for the oxidation reaction of 4-carboxybenzaldehyde to terephthalic acid is shown in Figure 16.

Figure 16. Temperature dependence of the rate constant for the oxidation reaction of 4-carboxybenzaldehyde to terephthalic acid. Data from ref 69. P


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Figure 18. Decreasing effect of water in p-xylene oxidation catalyzed by cobalt acetate: (a) data from ref 48; (b) reaction temperature 120 °C and air pressure of 10.3 bar. Adapted from ref 21.

Figure 19. Effect of water on (a) Co/Mn/Br-catalyzed oxidation of 4-chlorotoluene and p-toluic acid; (b) molecular oxygen uptake during Co/Mn/ Br-catalyzed oxidation of 4-chlorotoluene. Adapted from ref 75.

acetic acid with 5% water, the initial rates are lower, due to water inhibition, but at higher reaction times, water hydrolyzes the benzylic bromide, releasing it to regenerate the catalyst, increasing the rate.

inhibitor conversion, and less oxidation of Mn(II) to Mn(IV), leading to less MnO2 formation and faster Br− oxidation, are favored at lower water concentrations; hydrolysis of benzylic bromide, less formation of aryl formate and inhibiting phenol, less carboxylic acid metal precipitation, and higher reactivity of peroxy radicals due to acetic acid H bonding are favored at higher water concentrations.75 Wang et al.76 found that water has the effect of decreasing the rates of the oxidation reactions in the first methyl group and increasing the rates of the oxidation reactions of the second methyl group. Overall, there is an optimum water content in the reaction media above which

The balanced effect of water may be summarized as follows: reactivity of peroxy radicals of aldehydes and alcohols, catalyst

Figure 20. Reaction rate constant kj as a function of water content and batch reaction time as a function of initial water. Data from ref 76. Q


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Figure 21. Experimental and simulated 4-carboxybenzaldehyde concentration at different catalyst concentrations at 191 °C and 40 bar oxygen partial pressure: scatter, experimental; line, data fitting. Br/metals ratio = 0.7. Adapted from ref 73.

consistent with the rate law given by eq 48. Figure 21 shows the time evolution of 4-carboxybenzaldehyde concentration in pxylene air oxidation in acetic acid with different concentrations of Co/Mn/Br catalyst. Similar observations resulted for p-xylene, p-tolualdehyde, and p-toluic acid. In all experiments, the total amount of catalyst varied but the Co/Mn/Br ratio was kept constant, keeping the catalyst composition identical. Drawing a vertical line intercepting the 4 curves at a given time, it is easily seen that 4-carboxybenzaldehyde and the other main intermediates concentration decrease as catalyst concentration increases, showing that the activity of the catalyst has increased, Figure 22.

the global reaction rate is decreased, hence activity, and below which the rate is increased. The reason can be explained in terms of changes in the catalyst structure. In anhydrous acetic acid most of the bromide ions are coordinated to the metal, but this rapidly decreases as water increases. When bromide is coordinated directly to the metal, electron transfer from the Br− ligand to the Mn3+ metallic complex center occurs easily. At higher water concentrations, as aqua ligands progressively substitute bromide ligands, the latter are in the outer coordination sphere of the complex, coordinated through hydrogen bonds to aqua ligands (see Figure 14). This increases the resistance of electron transfer from Br− to the metal cation, as per eqs 107 and 108, decreasing the reaction rate. Also, with increasing number of aqua ligands approach of the organic intermediates and transient species from entering them metallic inner coordination sphere of the catalyst is prevented.77 On the other hand, the oxidation potential of metals is also changed. For the redox pair Co3+/Co2+, the reduction potential decreases from 1.9 V in acetic acid to 1.84 V in water, making reduction of the cobaltic ion to cobaltous ion easier, thus enhancing the reaction rate as water increases. For oxidation of p-xylene and p-tolualdehyde, the control steps are Mn3+ reduction by Br− as per eq 108 and hydrogen abstraction by eqs 109 and 115, respectively. Addition of water increases the resistance of electron transfer from bromide ion to the metal cation, slowing down the oxidation rate. However, for oxidation of p-toluic acid and 4carboxybenzaldehyde oxidation, for the abstraction step, the first methyl has been oxidized to the electron-withdrawing carboxy group. The electron-withdrawing effect decreases the reaction rates of the step given by eq 108 and hydrogen abstraction steps given by eqs 122 and 128, respectively. The carboxyl group makes radical stabilization difficult to occur (see eq 32), and the control step of both p-toluic acid and 4carboxybenzaldehyde oxidation processes shifts to generation of Co3+, controlled by the reaction given by eq 109 and propagation steps given by eqs 123−125 and eqs 129−131, respectively. Addition of water decreases the redox potential of Co2+/Co3+ and Mn2+/Mn3+ and makes the oxidation of Co2+ by Mn3+ as per eq 106 and reactions in the propagation steps easier, resulting in oxidation rate enhancement. When water content exceeds the optimal value wH2O, the resistance of electron transfer becomes the control step once again. Overall, the effect of water on rate is a competition between an inhibiting coordination effect and an electrochemical promoting effect.

Figure 22. Intermediate product concentration variation with total catalyst concentration. Catalyst composition kept constant. Adapted from ref 73.

4.4. Cobalt/Manganese and Bromine/Metals Ratios Effect on Activity

The cobalt/manganese ratio has an overall strong effect on the rate of each main step in the oxidation of p-xylene, but the different steps of eq 105 show different sensitivities.78 The two last steps, oxidation of p-toluic acid and 4-carboxybenzaldehyde, show a higher rate of sensitivity toward the [Co]/[Co + Mn] ratio when compared with the rather insensitive two first steps. The rate constant shows an increasing trend with increasing ratio, up to an inflection maximum, from where it starts to decrease, showing that a minor quantity of manganese metal is required. Because the electron-transfer rate between catalyst and electron depends on the catalyst cluster structure, catalysis activation of the cluster is different at different [Co]/[Co + Mn] ratios. The optimum ratio, the highest achieved rate at a given ratio, shows a decreasing trend with increasing temperature, as shown by Figure 23. At lower temperatures, the optimum ratio is nearly close to 1, meaning that cobalt is the dominant metal species is the catalyst, which accounts for its well-known higher catalytic activity when compared with manganese. As temperature rises, its effect on promoting activity compensates for the lower

4.3. Catalyst Concentration Effect on Activity

Activity normally increases with catalyst concentration up to a maximum value and then decreases.21 This observation is R


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is required even at low amounts to catalyze the reaction properly, and its absence has to be compensated with larger amounts of catalyst. Similar trends on both ratios were found by Kamiya and co-workers,62 as shown in Figures 8 and 9. 4.5. Agitation Effect on Activity

Brill48 reported that the oxidation reaction of p-xylene appears to be independent of rapid stirring, although with very slow agitation both the rate and the yields are strongly affected. As one may expect, a regime of poor mixing will lead to a decrease in the contact between the oxygen molecule and the hydrocarbon radicals. 4.6. Alkali Metal Effects on Activity

Figure 23. Temperature dependence trend of the optimum [Co]/[Co + Mn] ratio. Adapted from ref 78.

Apart from hydrogen bromide, other sources of active bromine possible in the AMOCO MC include sodium bromide and potassium bromate.21 Use of sodium bromide as a substitute for hydrogen bromide is desirable in order to minimize corrosion of equipment and pipework. When hydrogen bromide is used as the source of active bromine, sodium hydroxide is added to control corrosion in several points of the plant and necessarily ends in the recycled solvent to the reactor feed lines. Line flushing between vessels is also made with sodium hydroxide whenever a line choke occurs. It is therefore important to assess sodium influence in the p-xylene oxidation chemistry. Jhung and co-workers,80−82 studied the effect of alkali metal addition to p-xylene oxidation media by establishing a comparison of the acceleration and deceleration of oxygen uptake relative to a baseline with no alkali metal addition. As Figure 25 shows, deceleration occurs in the early stage of the reaction and acceleration occurs in the later stage and both increase with the alkali metal concentration, in the studied case, potassium. In addition, the deceleration in the beginning of the reaction is of the same magnitude as the acceleration in the later phase of it. Also, Jhung and co-workers found that the effect of potassium addition is higher when the manganese and bromine contents in the catalyst are low. In fact, the Co/Mn/Br catalyst system is activated the most when alkali metal addition equalizes the atomic concentration of bromine, which, according to Jhung, means the existence of an optimum for its concentration. The acceleration and deceleration effect is due to the alkali metal and not to the basicity of the companying anion and explained by the influence in the coordination of bromine to the catalyst metals throughout the Co−Mn−Br oxidation−reduction catalytic cycle shown in Figure 10. The alkali metal influence on the catalytic cycle

manganese catalytic activity, allowing its content in the catalyst to increase. This trend is consistent with the observation made by Cheng and co-workers, that the [Co]/[Co + Mn] ratio effect on the variation is much more remarkable at lower temperatures. Figure 2479 shows a plot of the variation of the

Figure 24. Catalyst concentration as a function of the bromine/metals at constant activity. Adapted from ref 79.

total catalyst concentration as a function of the bromine/metals ratio to keep activity constant, in this case, the same 4carboxybenzaldehyde in product. It can be seen catalyst function decreases with decreasing bromine/metals over a ratio of 0.3; increasing the ratio has little effect on activity. However, below 0.3, the amount of catalyst required to hold the activity constant increases sharply. This observation is consistent with the known Br role in the mechanism: bromine

Figure 25. Difference in oxygen uptake variation with addition of alkali metals throughout the oxidation reaction of p-xylene. Adapted from ref 81. S


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Figure 26. Influence of the alkali metal in the Co−Mn−Br catalytic cycle. Adapted from ref 80.

5. SELECTIVITY The selectivity of a reaction may be defined as the fraction of the starting material that is converted to the desired product. In a broader view, selectivity can be defined between the desired chemistry and the undesired chemistry. Within the scope of pxylene catalytic oxidation in acetic acid media, one can consider not only the main and parallel reactions in which p-xylene is consumed yielding other products than terephthalic acid but also reactions that consume acetic acid. Degradation of p-xylene to carbon oxides and to aromatic byproducts and acetic acid degradation also to carbon oxides, methyl bromide, methane, and methyl acetate are reactions that contribute to an overall economic yield decrease of a the process. Like in activity monitoring, for selectivity evaluation, 4-carboxybenzaldehyde can be used to establish a baseline to assess the competitive chemistry in p-xylene oxidation. In the AMOCO process,

appears to be independent of the functional groups of the molecule being oxidized, which suggest that the interaction is with the catalyst and not with the organic substrate. Figure 26 shows the influence of the alkali metal in the Co−Mn−Br catalytic cycle. In the early stage of the reaction, rate deceleration is caused by interaction of the alkali metal with the bromine ion, binding it and diverting it from the reduction of Mn3+ ion in the catalytic cycle. With the progress of the reaction are formed other substances; catalyst poisoning species that also interact with bromine, slowing the reaction. In a later stage of the reaction, as the alkali metal progressively interacts with bromine and the poisonous species, the interaction between bromine and the poisonous species will decrease and rate will increase again. T


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conversion of p-xylene to terephthalic acid is nearly 100% complete, so 4-carboxybenzaldehyde as a percentage of terephthalic acid product is normalized against the p-xylene feed rate, thus being a good selectivity indication. This allows byproduct formation also to be normalized against the feed rate of p-xylene and be used as a comparative measure against 4carboxybenzaldehyde as a percentage of terephthalic acid product in selectivity assessment. 5.1. Carbon Oxides

Complete oxidation of a hydrocarbon yields water and carbon dioxide. As seen from eq 105, progressive and partial oxidation of a methyl group in organic molecules yields a carboxylic acid group. If further oxidation is occurs then decarboxylation of the molecule occurs, yielding carbon monoxide and carbon dioxide as products, COx. Decarboxylation occurs not only in the aromatic acids, either intermediate or product, but also in the acetic acid solvent. In acidic medium, acid-catalyzed decarboxylation reactions are dependent on factors such as the ionic strength and substituents on the aromatic ring.83 The mechanism is given by

Figure 27. Carbon oxides formation trend with increasing catalyst concentration during p-xylene oxidation catalyzed by a Co/Mn/Br catalyst at 197 °C. Adapted from ref 79.

observation is consistent with a need for a balance between bromine and metals: metals in their trivalent oxidation state react with bromide ions to form bromide atoms, which will abstract hydrogen atoms from methyl groups to yield radicals; however, competitive degradation of carboxylic groups of both product and solvent molecules to carbon oxides is favored by a high concentration of metals, in particular cobalt, which has a well-known ability to decarboxylate acids.49,84 Co3 + + CH3CO2 H ⇄ Co2 + + CH3CO2• + H+

(154)

Co3 + + CH3CO2•

In dilute acid solution, ipso protonation is the ratedetermining step, whereas in highly acidic solutions, the ratedetermining step is aromatic carbocation decarboxylation. Aromatic acid decarboxylation can also occur by a free-radical pathway, catalyzed by metals and radicals, though hydrogen abstraction from the carboxylic group of the molecule.

CH3CO2 H

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Co2 + + CO2 + CH3CO2 CH3 + H+

(155)

Thus, as a result of these two competitive reactions, one can expect an optimum Br/metals ratio to achieve greater catalyst selectivity with respect to carbon oxides formation. 5.1.1. Carbon Oxides: Solvent Burning. Of particular importance in industrial manufacture of terephthalic acid is acetic acid solvent degradation to carbon oxides, as this is one of the greatest parcels of operational costs. Not only does addition of manganese into the catalyst feed promote oxidation of the hydrocarbon due to the synergistic effect with cobalt, as shown in Figure 10, but also it has been found that it decreases the content of carbon oxides in the oxidation off-gas. 14C isotopic experiments performed by Kenigsberg et al.85 lead to the conclusion that acetic acid degradation into carbon oxides, i.e., solvent burning, has a dependence on the manganese to cobalt ratio. In addition, they also found the following. • Carbon dioxides are formed from both the methyl and the carboxylic groups of the acetic acid molecule. • CO2 is formed in quantities higher than CO: from 3 to 26 times more. • Cobalt has a stronger ability to form COx from the carboxylic group. • Manganese has a stronger ability to form COx from the methyl group. • Radicals from the main reaction also decompose acetic acid, yielding methyl radicals and CO, CO2, formic acid, and formaldehyde, Figure 29. There is a minimum, located toward low [Mn]/[Mn + Co] ratios, where COx formation can be optimized: low cobalt content in the catalyst caused carbon oxides formation to increase, but the same will be expected to happen if the cobalt content becomes too high, Figure 30. In Figure 30, the minimum COx formation corresponds to a high cobalt content, with a general trend of COx formation increasing with increasing manganese. However, Cheng78 and

The benzoyloxyl radicals formed rapidly decarboxylate to yield aryl radicals. The aryl radicals can then react with several other molecules to yield other undesired byproducts, decreasing the overall selectivity. Regarding decarbonylation, very little carbon monoxide, generally less than 0.01%, is formed during the reaction of benzaldehydes described, which is indicative that decarbonylation is not important in aromatic aldehyde autoxidations.77 It may then be expected that most of the CO comes from decarbonylation of acetic acid and not the benzaldehydes. As may be expected, the effect of catalyst increase in the oxidation is to increase COx formation, as shown by Figure 27. The effect of catalyst composition in terms of bromine/ metals ratio is shown in Figure 28. The total amount of carbon oxides moles per feed mole of pxylene shows a minimum for a molar bromine/metals ratio close to 0.3. As the ratio falls below the 0.3 minimum production of carbon oxides increases rapidly. Above the minimum formation of COx shows a less steep trend. This U


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Figure 28. Carbon oxides formation trend with increasing bromine/metals molar ratio during p-xylene oxidation catalyzed by a Co/Mn/Br catalyst at 197 °C. Adapted from ref 79.

Figure 29. Carbon oxides degree of decomposition trend dependence with [Mn]/[Mn + Co] ratio for 14CH3COOH and CH314COOH. Adapted from ref 85.

Figure 31. Carbon oxides generation trend dependence with [Co]/ [Co + Mn]. Adapted from ref 78.

Figure 30. Total COx formation trend dependence with [Mn]/[Mn + Co] ratio for 14CH3COOH and CH314COOH. Adapted from ref 85.

Li et al. showed that the formation rate constants of CO2 and CO vary with process conditions that normally also affect pxylene oxidation. CO2 and CO formation rate constants increase with increasing [Co]/[Mn] ratio and decrease with [Br]/[Co] ratio and decreasing temperature. Water concentration has a promoting effect on COx formation rate constants up to 10−15%, from which it begins to have an inhibiting effect. Increasing temperature, air flow rate, and total metals concentration also increases COx formation rates.86 In later

co-workers found the opposite trend at similar catalyst concentration magnitudes and temperature ranges, as seen from Figure 31. The main difference in the catalyst feed was the source of bromine. Kenigsberg used NaBr, whereas Cheng used HBr as the bromine source. Yet, the bromine to metals ratio used by Kenigsberg was almost three times higher, which suggests that bromine is the differentiating factor, most likely due to changes in catalyst structure. V


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Figure 32. Formation rates of (a) CO2 in batch oxidation experiment, (b) CO in batch oxidation experiment, (c) CO2 in semicontinuous oxidation experiment, and (d) CO in semicontinuous oxidation experiment. Acetic acid/p-xylene weight ratios: (□) 20/1, (○) 10/1, (△) 5/1, (▽) 3/1; () model fitting. Adapted from ref 87.

work, Cheng et al. suggested that rates of solvent burning and the main reaction are related and have a competitive relationship between them.87 The observed rates of COx formation during p-xylene oxidation in batch and semicontinuous experiments are shown in Figure 32a and 32b. Data fitting is given by rCO2 =

2

2

rCO =

2

(156)

0.791NyCO Vr{1 − (yO + yCO + yCO )} 2

2

1 rCO 1−η x

(158)

rCO =

η rCO 1−η x

(159)

and η being within the 0.244−0.252 range. 5.1.2. Carbon Oxides: Mechanism of Solvent Burning. Like the main reaction, solvent burning follows a free-radical oxidation mechanism. Throughout the p-xylene oxidation several radicals are formed which can abstract hydrogen atoms from other reactant, intermediate, product, or solvent molecules. Among these radicals, RCO• and RCOO• radicals will undergo decarbonylation and decarboxylation yielding CO and CO2, respectively

0.791NyCO

Vr{1 − (yO + yCO + yCO )}

rCO2 =

(157)

where rCO2 and rCO are the CO2 and CO formation rates, respectively, N is the air flow rate, Vr is the reaction volume, and yO2, yCO2, and yCO are, respectively, the oxygen, carbon dioxide, and carbon oxide molar fractions in the reactor off-gas. The behavior of the batch experimental curves shows a camel back like shape: an initial sharp increase of COx to a first peak, from which it decreases to a minimum; from this minimum the rate increases again to a new peak, from which it drops at the end of the reaction. In the semicontinuous curves the COx generation rate increases sharply at the beginning of the reaction, after which it shows a relatively stable decreasing plateau throughout most of the reaction time. At the end of the reaction the COx formation rate drops sharply when the pxylene feed stops. The fact that CO and CO2 formation rates show the same behavior either in batch or in semicontinuous experiments leads to the conclusion that the two rates are proportional, i.e., rCO2 = ηrCO, with

RCO• + R′H → RCHO + R′•

(160)

RCO• + O2 → RCOOO•

(161)

RCOO• + R′H → RCOOH + R′•

(162)

RCO• → R• + CO

(163)

RCOO• → R• + CO2

(164)

where R represents any hydrocarbon radical and R′H represents any molecule with an abstractable hydrogen atom including acetic acid I• + CH3COOH → HI + CH3COO•

(165)

I• + CH3COOH → HI + •CH 2COOH

(166)

•

CH 2COOH + O2 → CO2 , CO, H 2O

W

(167)


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Figure 33. Comparison between solvent burning and the main reaction on (a) batch experiment and (b) semicontinuous experiment. Adapted from ref 87.

Figure 34. Relation of terephthalic acid selectivity and rC̅ Ox in (a) batch experiment and (b) semicontinuous experiment. Adapted from ref

CH3COO• → CH3• + CO2

I• represents any radical, like the very strong oxidizing radical OH• or PhCH2OO• but also cobalt or manganese in their trivalent oxidation state, originating from the thermal decomposition of the respective acetates, which is found to be the most important.88 Consumption of I in the burning reaction thus shows a competition relationship between the main reaction and the solvent burning reaction. In the latter, the methyl radical is of particular importance as it yields some the most important byproducts in the MC-AMOCO industrial process, such as methyl bromide, methane, and methyl acetate (169)

CH3• + RH → CH4 + R•

(170)

CH3• + CH3COO− → CH3COOCH3

(171)

CH3•

+ O2 → CO2 , CO, H 2O

.

reaction and the main reaction is shown in Figure 33 for both batch and semicontinuous experiments. The results suggest an interstimulative relationship between both reactions. Considering the scheme in eq 105, the rate of CO2 formation shows an accompanying behavior to both the rates of formation of all intermediates, rall, and formation rates of p-toluic acid and terephthalic acid, r2 + r4, suggesting a proportionality relation. While Call increases, the opportunity that the reactant and intermediates are attacked by the active free radicals and metal ions increases too and the opportunity for solvent being attacked decreases reversely. Therefore, the rate of the burning side reaction, rCO2, will decrease faintly with increasing concentrations of reactant and intermediates, Call. For batch and semicontinuous oxidations, the rate of CO2 formation is, according to Cheng, given by

(168)

CH3• + Br• → CH3Br

87

rCO2 =

(172)

α1(r1 + r2) 4

(∑i = 1 Ci + ε)

+ α2(r1 + r2) (173)

with α1 and α2 being empirical model parameters and rCO2 = ηrCO.

Comparison between the CO2 generation rate and rates of intermediate reaction as per eq 105 for the solvent burning X


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Figure 35. Methyl bromide formation trend at a constant 4-carboxybenzaldehyde with increasing bromine/metals molar ratio during p-xylene oxidation catalyzed by a Co/Mn/Br catalyst at 197 °C. Adapted from ref 79.

Figure 36. Comparison of methane formation with CO2 and CO formation with increasing metal concentration at constant air flow. Adapted from ref 84.

decrease, as it formed one undesired product, and of activity decrease. Methyl bromide, a gas in industrial process operating conditions, is partially lost in the off-gas process and treated as an effluent. The higher the formation of methyl bromide, the lower the amount of bromine present in the catalyst, which has the effect of decreasing its activity and selectivity. Figure 35 indicates that there is an optimum Br/metals ratio which yields a minimum of methyl bromide at a constant 4carboxybenzaldehyde in the product. At low Br/metals ratios, the catalyst is not active enough and so a higher catalyst concentration is required to keep 4-carboxybenzaldehyde constant. However, high absolute catalyst concentrations lead to decarboxylation of solvent and other carboxylic acids present in the reactor. This yields more methyl radicals, which will yield methyl bromide. At high Br/metals ratios, the high concentration of bromine favors formation of methyl bromide as it reacts competitively with the methyl radical, diverting it from other reactions. Optimal conditions for low methyl bromide formation and high catalyst selectivity are a balance between other factors, which includes catalyst concentration and composition.

From the relationship between the main reaction and solvent burning, the following COx formation−terephthalic acid selectivity relation arises as depicted in Figure 34. In batch oxidation, the burning side reaction rC̅ Ox decreased with the increase of p-xylene/acetic acid mass ratio but terephthalic acid selectivity increased. In semicontinuous oxidation, the burning side reaction rate rC̅ Ox increased with the increase of p-xylene feed rate but terephthalic acid selectivity remained relatively constant. In semicontinuous oxidation, rC̅ Ox is lower than that in batch oxidation and terephthalic acid selectivity is slightly higher. One conclusion is that the operation mode affects the p-xylene oxidation process, and therefore, in an industrial continuous reactor there is an optimal operation mode in which low burning side reaction and high terephthalic acid selectivity can be achieved.87 5.2. Methyl Bromide

As mentioned above, methyl bromide is formed in side reactions during oxidation of p-xylene. Its formation is the result of a combination of methyl radicals with bromine, as per the reaction in eq 169. Methyl radicals come from degradation of solvent or other aromatic molecules and bromine from catalyst: therefore, methyl radical formation decreases the overall yield and is an example of both catalyst selectivity

5.3. Methane

As one of the byproducts formed from decarboxylation degradation, methane is formed in eq 170. Methane is insoluble Y


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Figure 37. Comparison of methane formation with carbon dioxide formation with increasing air flow/metal ratio. Adapted from ref 84.

in the reaction media and present in the reaction off-gas. As expected, increasing catalyst concentration causes more decarboxylation to occur and more methyl radicals will be formed, Figure 36. These will abstract hydrogen atoms from other molecules yielding methane. Abstraction of hydrogen from methyl groups of p-xylene is favored by the resonance stabilization of the radical, as per eq 30. Less methane, relative to carbon oxide, is seen as the air flow/metals ratio increases, as shown in Figure 37, and the ratio of carbon dioxide/methane increases. As the air flow/metals ratio decreases, the oxidation media becomes increasingly starved of oxygen and methyl radicals tend to preferentially abstract hydrogen atoms from other molecules to form methane as per eq 170 rather than forming carbon oxide as per eq 172. A competitive behavior appears to exist. In addition, one may expect that increasing the degree of agitation within an industrial reactor would also contribute to formation of methane the same way increasing the air flow/metals ratio.

•

CH 2CO2 H + O2 → CO, CO2 , CH3OH, CH 2O, HCOOH

Both methyl radical and methanol can react with acetic acid to yield methyl acetate CH3• + Mn + 1 + CH3COOH → CH3COOCH3 + Mn + + H+

CH3OH + CH3COOH → CH3COOCH3 + H 2O

+ CH3COOCH3

(174)

Methyl acetate formation in industrial production of terephthalic acid is of great importance as it is formed mainly by acetic acid degradation, causing solvent loss, which is controlled to a certain extent by recovering it in the process and recycling it to the oxidation reactor, where it is reconverted to acetic acid by hydrolysis.66,90,91 Decarboxylation is initiated by peroxide interaction with the carboxylic or methyl groups of acetic acid, followed by degradation of the resulting radicals, yielding carbon monoxide, carbon dioxide, methanol, formaldehyde, and formic acid.85,88,89 Methanol results from reaction between a methyl radical and water. Oxidation of methanol initially yields formaldehyde, which upon further oxidation yields formic acid. Formic acid can esterify with methanol to yield methyl formate.88

(176)

(180)

Methyl acetate formation depends on the process conditions that lead to acetic acid degradation like water concentration, temperature, p-xylene oxidation rate, catalyst concentration, and composition, as shown in Figures 38 and 39. The role of water in decreasing methyl acetate formation is due to ligand substitution in the catalyst, decreasing its activity49,77 by destabilizing the highest oxidation states of the metals responsible for the decarboxylation reaction and also due to its role as reactant in methyl acetate hydrolysis to acetic acid and methanol, the latter undergoing oxidation to carbon oxides.

2CH3COOH + 0.5O2 → CH3COOCH3 + H 2O + CO2

CH3CO2 H + ROO• → ROOH + •CH 2CO2 H

(179)

2Co(CH3COO)3 → 2Co(CH3COO)2 + CO2

Methyl acetate is one of the main byproducts in the terephthalic acid process.66 Its formation can be described as oxidative decarboxylation of acetic acid89

(175)

(178)

When the metal catalyst is in the form of acetates, the metal in its higher oxidation state can oxidize the acetate ligand to carbon dioxide and methyl acetate49

5.4. Methyl Acetate

CH3CO2 H + ROO• → ROOH + CH3• + CO2

(177)

CH3COOCH3 + H 2O → CH3COOH + CH3OH

(181)

Just like the activity behavior of catalyst with water concentration, methyl acetate formation does not always decrease with increasing water. Roffia found the water concentration optimum to be within the range 12−16%. At lower water concentrations, methyl acetate formation is high mainly due to increased catalyst activity, decomposing solvent molecules. The increase of p-xylene rate in an oxidation reactor is proportional to its feed rate. Increasing the amount of pxylene comparatively increases its competition with acetic acid solvent molecules for reacting with catalyst and peroxides, decreasing methyl acetate formation. An optimized p-xylene/ solvent ratio will contribute to less methyl acetate formation. Like in formation of carbon oxides, increasing the total catalyst Z


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Figure 39. Methyl acetate formation trend with varying bromine to metals ratio. Adapted from ref 89.

bromine/metals ratio decreases COx formation, meaning less methyl radicals and hence less methyl acetate. Also, bromide ions, by coordinating to the catalyst metals, replace acetate ions in the electron-transfer process between ligand and metal in its high oxidation state, hindering RCOO• acyloxy radicals formation and promoting formation of bromine radicals, which are promoters of the main reaction synergistic cycle in detriment of the methyl acetate byproduct formation secondary reaction.89 Increasing the relative amount of acetate ion in catalyst composition also has the effect of increasing methyl acetate formation.92 As seen above,65 as the catalyst is more acetate ligand rich, i.e., less coordinated aqua ligands, its activity increases, yielding more carbon oxides and methyl acetate. 5.5. Selectivity Assessment

As mentioned above, for selectivity evaluation during p-xylene oxidation, monitoring 4-carboxybenzaldehyde in terephthalic acid product is a valuable tool. The optimization goal is to have a high yield of terephthalic acid with a low content of 4carboxybenzaldehyde and simultaneously low burning to carbon oxides. Figure 41 shows CO2 formation vs 4carboxybenzaldehyde content in terephthalic acid product at different temperatures. The shape of the curves shows competition behavior between 4-carboxybenzaldehyde content in terephthalic acid product and unwanted degradation of p-xylene and acetic acid to carbon dioxide. It can be seen that for all the curves carbon dioxide formation increases as 4-carboxybenzaldehyde decreases. Driving 4-carboxybenzaldehyde to low levels requires use of a higher catalyst concentration. This causes carbon oxides formation to be higher, as shown in Figures 28 and 36, in particular, if higher cobalt concentrations are used, as shown in Figures 30 and 31. As an alternative, for achieving a lower 4carboxybenzaldehyde concentration, more severe oxidation conditions, i.e., higher temperatures are required. This will cause more carbon dioxide to be formed, as also confirmed when looking at Figure 42, which shows the effect of temperature on 4-carboxybenzaldehyde and carbon oxides at constant catalyst composition. Typically, for a constant level of burn, an optimal catalyst composition to obtain a minimum 4-carboxybenzaldehyde exists.97 The trade-off is to achieve terephthalic acid product with low 4-carboxybenzaldehyde content suitable for PET manufacture and simultaneously reduce solvent burning, which has a detrimental effect on the manufacturing process economical reliability. The cobalt to manganese ratio and the bromine to metals ratio are essential 4-carboxybenzaldehyde

Figure 38. Methyl acetate formation trends with variation of water concentration. (a) Three levels of p-xylene oxidation rate (mol kg−1 h−1); Co2+, 0.022%; 220 °C; stirring speed, 500 rpm. (b) Three levels of Co2+ concentration (% wt); 220 °C; p-xylene oxidation rate, 3.8 stirring speed, 500 rpm. (c) Three levels of temperature; Co2+, 0.022%; p-xylene oxidation rate, 3.8; stirring speed, 500 rpm. Adapted from ref 89.

concentration also leads to increased methyl acetate formation. Association of these two trends is explained easily by the mechanism of methyl acetate formation which requires methyl radicals, as per eq 178, which in turn are formed during solvent decomposition through eq 175. The effect of increasing temperature increases methyl acetate formation, especially at low water concentrations. This is explained by the higher catalyst activity at higher temperatures, leading to more methyl acetate formation. Methyl acetate formation also depends on the catalyst composition. As shown in Figure 39, methyl acetate formation tends to decrease with increasing bromine/metals ratio. As seen previously, to a certain extent decreasing the AA


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Figure 40. Carbon oxides and methyl acetate formation trends with varying catalyst acetate to metals ratio during p-xylene and pseudocumene oxidation. Adapted from ref 92.

Figure 41. Carbon dioxide formation vs 4-carboxybenzaldehyde present in terephthalic acid product at different temperatures. Black curves on the left vertical axis, and gray curves on the right vertical axis. Adapted from refs 93−95.

Figure 42. Effect of temperature on 4-carboxybenzaldehyde and carbon oxides at constant catalyst composition. Scatter: data points at 190, 175, and 170 °C. Solid line: data trend. Adapted from ref 96.

Figure 43. 4-Carboxybenzaldehyde in terephthalic acid product as a function of the Co/(Co + Mn) ratio trends at different relative levels of burning. Adapted from ref 97.

optimization parameters. Figure 43 shows trends of 4carboxybenzaldehyde at different levels of burning with changing metals ratio in the catalyst. The minimum of 4carboxybenzaldehyde occurs at higher cobalt concentrations, showing its higher selectivity when compared with manganese. However, a further increase causes a steep increase in 4carboxybenzaldehyde, which means that the catalyst is losing its selectivity: the Br/Mn/Co synergy cycle (Figure 10) is being broken. In addition, the loss of selectivity is also accompanied

by an increase in carbon oxides formation, as cobalt is attacking the molecules’ carboxylic group, in particular, that of acetic acid. Figure 44 shows a similar behavior when varying the bromine/metals ratio at different levels of burning. Decreasing the bromine/metals ratio causes 4-carboxybenzaldehyde concentration to increase, because the catalyst loses activity, resembling a bromine-free metal catalyst. Increasing the ratio too much causes the catalyst to be poor in metals and not to be able to catalyze peroxide decomposition reactions properly. AB


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increases as Br/(Co + Mn) decreases. This shows that the catalyst selectivity toward product terephthalic acid worsens as the bromine to metals ratio decreases, forming more carbon oxides. The reason behind this is that catalyst activity decreases as bromine content decreases. To keep 2,6-dicarboxyfluorenone concentration constant a catalyst compensation is necessary. In proportion, cobalt content will necessarily increase, making the catalyst more selective to decarboxylation reactions, thus forming more carbon oxides. Figure 49 shows the carbon oxides formation trend when the inverse of 4-carboxybenzaldehyde concentration in product terephthalic acid changes at different sodium concentrations and constant catalyst feed. As seen in Figures 41 and 48, the inverse relationship between 4-carboxybenzaldehyde and carbon oxides is also maintained in the presence of sodium. From Figure 49, despite the scarcity of experimental data, a trend arises: for constant 4carboxybenzaldehyde concentration in product terephthalic acid, as the sodium concentration increases, carbon oxides formation tends also to increase, which means that higher sodium concentrations tend to worsen the selectivity. This selectivity decrease may be related with decreased activity in the presence of alkali metals in the early stage of the reaction as observed by Jhung and co-workers.80−82 Overall, 4-carboxybenzaldehyde, 2,6-dicarboxyfluorenone, and carbon oxides all have an unfavorable effect on terephthalic acid manufacture, either by having a quality specification decrease effect as high 4carboxybenzaldehyde and 2,6-dicarboxyfluorenone do or by accounting for process economic losses as the burning of solvent does. Driving process conditions to less solvent burn leads to undesired byproducts increase and vice versa. In addition, carbon oxides formation is preceded by degradation not only of solvent but also of carboxylic groups of other molecules in the reaction media, which yields several free radicals. This causes not only solvent and terephthalic acid loss, but the combination of these radical also leads to formation of other undesired byproducts, decreasing the overall selectivity and efficiency of the process and product quality.

Figure 44. 4-Carboxybenzaldehyde in terephthalic acid product as a function of the Br/(Co + Mn) ratio trends at different relative levels of burning. Adapted from ref 97.

Other substances formed in the oxidation process, such as 2,6-dicarboxyfluorenone or 2, 6-DCF, which have a detrimental effect on terephthalic acid color and hence in its quality, may also be used for selectivity monitoring.98 Figure 45 shows the 2,6-dicarboxyfluorenone concentration and carbon oxides formation trends with changing catalyst composition. Increasing COx formation with increasing the cobalt to manganese ratio and with the relative increase of the bromine to metals ratio is a consequence of greater catalyst activity, leading to higher decomposition of carboxylic groups of the molecules within the reaction media. Decreasing the 2,6dicarboxyfluorenone concentration requires both a more active and a more concentrated catalyst, with progressively greater cobalt/manganese ratios and bromine contents, either in a smaller range as shown by Figure 45b or in a wider ratios range as shown by Figure 46. Like in the case of 4-carboxybenzaldehyde, 2,6-dicarboxyfluorenone and carbon oxides show an antagonistic competitive behavior. Figure 47 shows that cobalt-rich catalyst compositions, i.e., increasingly higher Co/Mn, lead to progressively less 2,6-dicarboxyfluorenone at the expense of more COx formation. Figure 48 shows the effect of bromine to metals ratio on selectivity at constant 2,6-dicarboxyfluorenone concentration. The data shows that for a given catalyst composition, allowing the Br/(Co + Mn) to vary to maintain a constant 2,6dicarboxyfluorenone concentration, carbon oxides formation

5.6. Byproduct Chemistry

The extent of selectivity of the oxidation of p-xylene to terephthalic acid will determine the amounts of byproducts formed during the reaction. Byproducts formation arises both

Figure 45. (a) 2,6-Dicarboxyfluorenone concentration trend in reaction effluent slurry, and (b) carbon oxides formation trend with simultaneously changing Br/(Co + Mn) and Co/Mn ratios: cobalt held constant; bromine and manganese varying. Adapted from ref 98. AC


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Figure 46. 2,6-Dicarboxyfluorenone concentration trend in reaction effluent slurry with changing Co/Mn and Br/(Co + Mn) ratios over a wide range. Adapted from ref 98.

Figure 47. Carbon oxides vs 2,6-dicarboxyfluorenone concentration trend (black line read on left vertical and low horizontal axis), and 2,6dicarboxyfluorenone and carbon oxides formation trends with changing Co/Mn ratio (dark gray and light gray lines read on right vertical and upper horizontal axis). Adapted from ref 98.

Figure 48. Carbon oxides vs 2,6-dicarboxyfluorenone concentration at different Br/(Co + Mn) ratios. Adapted from ref 98.

from degradation of the molecules and subsequent combinations among them to form several more products.

As seen previously, acetic acid degradation leads to formation of the methyl radical, which then leads to formation of carbon AD


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Figure 49. Carbon oxides formation with changing 4-carboxybenzaldehyde concentration in product terephthalic acid at different sodium concentrations. Dashed gray lines represent trend extrapolations. Adapted from ref 99.

oxidation media and their concentration tends to build up, unless a process purge is made. It is therefore important to know what the main byproducts formed are.102 5.6.1. Acetic Acid Derivatives. Decomposition of acetic acid and acetate ligands by radicals or metals yields several byproducts via the CH3• , •CH2COOH, and CH3COO• radicals, as shown previously by eqs 160−167 and 175−181, Figure 51.

oxides, methane, formic acid, formaldehyde, methanol, methyl bromide, and methyl acetate. Decarboxylation of the carboxylic group of any other molecule, including both terephthalic acid and its preceding intermediates, also leads to formation of methyl radical with subsequent formation of those products and also to formation of other radicals, which upon further recombination can yield more complex molecules. In addition, the impurities present in p-xylene and in the solvent in commercial manufacture of terephthalic acid are also a source of molecules that can undergo several transformations like full or partial oxidations and subsequent decarboxylation to yield other byproducts. For instance, p-xylene may contain impurities such as toluene, o-xylene, m-xylene, pseudocumene, 1,2,3trimethylbenzene, and 1,3,5-trimethylbenzene. The corresponding carboxylic acids resulting from oxidation of these impurities, i.e., benzoic acid, o-phthalic acid, isophthalic acid, trimellitic acid, hemimellitic acid, and trimesic acid, are then found in the oxidation mother liquor.100 Formation of alkyl radicals throughout the oxidation reaction can also cause catalyst decomposition and deactivation. Considering reaction of the methyl radical with bromine to yield methyl bromide as per eq 169 is the simpler form of bromoalkane formation. This last secondary section is of particular importance as it consumes bromine from the catalyst, deactivating it. Some of the several byproduct pathways are shown in Figure 50. Aside from yield decrease, byproducts represent a quality issue for PET manufacture, as many of them decrease polymerization rate and, being monofunctional, act as chain termination agents, causing the polymer to have an undesired molecular weight and improper color and optical properties.101 Because solvent is recycled, they are always present the

Figure 51. Main byproducts formed from decomposition of acetic acid.

Sun et al. attributes formation of the acetic acid hydroperoxide intermediate HOOCH2CO2H as the source of CO, CO2, formaldehyde, and formic acid.103 As mentioned above, acetic acid degradation together with aromatic acid decarboxylation and decarboxylation are the main sources of CO and CO2. Formation followed by coupling of two •CH2COOH radicals is related also to formation of succinic acid102 2•CH 2COOH → HO2 CCH 2CH 2CO2 H

(182)

Succinic acid can be brominated, yielding bromosuccinic acid, which in turn can lose a HBr molecule to yield maleic acid and fumaric acid isomers. When manganese in the trivalent oxidation state attacks the methyl group of acetic acid generating the carboxymethyl radical, glycolic acid can result from reaction with methane in the presence of water. Glycolic acid can then either react with acetic acid with loss of one water molecule to yield acetoxyacetic acid or be oxidized to glyoxylic acid. The latter can yield oxalic acid upon reaction with a water molecule. All these substances upon decarboxylation and decarbonylation will yield, respectively, CO2 and CO in the off-gas.88

Figure 50. Byproduct chemistry pathways in the oxidation of p-xylene in acetic acid by a Co/Mn/Br catalyst. AE


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This electrophilic radical can add to p-xylene and other aromatic molecules

byproduct chemistry as they yield considerably diverse radicals which on combination with themselves, in nonmainstream chemistry free-radical termination reactions, yield progressively more complex products. Formation of benzoic acid derivatives is preceded by decarbonylation and decarboxylation reactions of carbonyl and carboxylic groups, respectively, and formation of a phenyl radical. Phenyl radicals can abstract hydrogen from other hydrocarbon and form benzoic acid, react with bromine from the catalyst to form p-bromo benzoic acid, and be oxidized to p-hydroxy benzoic acid.79,102

Methyl radicals formed from acetic acid decomposition can also react with other aromatic molecules and radicals forming more byproducts via methyl bromide. They can add to an aromatic ring in Friedel−Crafts-type reactions,104,105 promoted by excessive solvent degradation and poor gas−liquid oxygen transfer and catalyzed by metals, such as iron, from corrosion of equipment. An example of this reaction is formation of trimellitic acid

5.6.2. Benzaldehyde and Benzoic Acid Derivatives. Considering p-tolualdehyde and p-toluic acid to be, respectively, benzaldehyde and benzoic acid derivatives and 4carboxybenzaldehye a derivative of either one of them, their reactions, other than those leading to terephthalic acid, also account for formation of byproducts. Figure 52 shows some of the byproducts formed from benzaldehyde reactions. Together with the common free-radical hydrogen abstraction reaction, decarboxylation reactions play a major role in

Figure 53. Benzoic acid derivatives byproducts formed from decarbonylation and decarboxylation reactions. Adapted from ref 102.

Figure 52. Benzaldehyde byproduct chemistry; X = H, CH3. Adapted from ref 77. AF


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catalyst becomes progressively deactivated and the Br/(Co + Mn) ratio decreases, formation of insoluble and product blackening Mn(IV) compounds, like MnO2, also occurs.76,106 5.6.3. Terephthalic Acid Derivatives. Terephthalic acid can also undergo decarboxylation and bromination reactions to form some of the compounds shown in Figure 55. Evidence of addition of a methyl radical to the benzene ring is given when formation of 2-methyl terephthalic acid and trimellitic acid is found in quantities larger than expected if all the pseudocumene found as a p-xylene contaminant would have its methyl groups oxidized partially or totally to carboxylic groups.102 Trimellitic acid can react with manganese and form manganese trimellitate, which is moderately insoluble in acetic acid/water mixtures and contributes to depletion of manganese from the liquid medium

In the decarbonylation process, when a carboxy benzoyl radical is formed it can react with methyl radicals to form pcarboxy acetophenone

p-Xylene oxidation intermediates can undergo bromination catalyzed by equipment corrosion metals

Another possible route for benzyl bromide formation is through direct interaction of the alkyl aromatic molecule with the bromide ion present in the coordination sphere of the catalyst75 5.6.4. Phenol Derivatives. The importance of phenols in p-xylene oxidation is related to the fact that they are known to be effective oxidation inhibitors.107,108 Decarbonylation or decarboxylation followed by oxidation of phenyl radicals represents the intermediate stage in byproduct phenol synthesis. Bromination then yields substances like 2,4dibromophenol and 2,4,6-tribromophenol. 2-Hydroxy-4-methylbenzaldehyde can be either the result of solvolysis of a brominated compound or a product of oxidation of trialkylbenzene formed as a byproduct or present as an impurity in p-xylene, Figure 56.102

Phenol derivatives can react to form brominated compounds, Figure 54.

Figure 56. Some of the phenol derivatives formed in p-xylene oxidation. Adapted from ref 102.

5.6.5. Biphenyl and Benzophenone Derivatives. Coupling of aryl radicals, a termination reaction, yields biphenyl derivatives. The presence of these compounds in the mother liquor is evidence of the occurrence of decarbonylation and decarboxylation of aromatic acids and aldehydes. Coupling between phenyl and an aroyl radical is also a termination reaction that yields benzophenones. Formation of an aroyl radical depends on the reaction conditions. If conditions are such that the aroyl radical does not have enough oxygen to form a peroxy radical, it will attack a substrate molecule to yield

Figure 54. Products resulting from bromination of phenol byproduct derivative from benzoic acid. Adapted from ref 102.

Bromination reactions not only represent formation of undesired products but also have the consequence of deactivation of the catalyst. As benzylic bromides are formed, byproduct cascades are started due to conversion of a much more Co/Mn/Br active catalyst to a less active and less selective Co/Mn catalyst, Br/(Co + Mn) decreases, which is also responsible for coloration of the product.76 In addition, as

Figure 55. Some of the terephthalic acid derivatives formed in p-xylene oxidation. Adapted from ref 102. AG


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Figure 57. Formation routes of biphenyl and benzophenone derivatives during p-xylene oxidation: X, Y = H, CH3, CHO, COOH, OH; m, n = 0, 1, 2. Adapted from ref 102.

a benzophenone.101 Other route to benzophenones is through alkylation of an aromatic with a benzyl bromide derivative via Friedel−Crafts alkylation. This yields a biphenylmethane, which upon oxidation yields a benzophenone compound, Figure 57.102 5.6.6. Anthraquinone and Fluorenone Derivatives. Cyclization of o-carboxy benzophenones is the most likely path to yield anthraquinone derivatives. Oxidation of dihydroanthracene derivatives is also a plausible route. Note that the dihydroanthracene may be the reduction product of an anthraquinone recycled to the process. Fluorenones, in particular, 2,6-dicarboxyfluorenone, have a detrimental effect on terephthalic acid color.98 Fluorenone can be formed by two distinct paths. One route may start with decarboxylation of an o-carboxy benzophenone, yielding an o-benzoylphenyl radical, which upon electron bonding completes the cyclization.102,109 The other route is via a biphenyl with a methyl group in the ortho position. Oxidation of the methyl group yields a carboxylic or a carbonyl group, which after formation of an aroyl radical is converted to a ketone group via cyclization. This later route has been employed successfully to synthesize 2,6dicarboxyfluorenone via oxidation of 2,4,5-trimethylbiphenyl to 2,4,5-tricarboxybiphenyl, Figure 58.110 5.6.7. Ester Derivatives. p-Toluic acid and terephthalic acid esterification products are also byproducts found in pxylene oxidation mother liquor. The routes to esterification may involve other byproducts as intermediates such as benzoyl peroxides, phenols, and aroyloxy radicals. Esterification of ptoluic acid and terephthalic acid with 4-(hydroxymethyl)benzoic acid, a minor intermediate found in terephthalic acid,111 is also one possible route for ester formation. Benzyl acetate esters formation is also important as the main reaction is carried out in acetic acid media. Examples of esters formed in p-xylene oxidation in acetic acid together with other byproducts of the families mentioned above are given in Table 2.

5.7. Catalyst Deactivation

A catalyst is usually defined as a substance which increases the rate of a reaction without being consumed and with its properties indefinitely unchanged. However, this is not an absolutely true, as a catalyst undergoes chemical and physical degrading modifications which alter its activity and selectivity. This degradation of catalytic properties is often referred to as catalyst deactivation and despite being more common in heterogeneous catalysis also occurs in homogeneous catalysts, mainly due to the high reactivity of the active species in the metal complex.113 With a Co/Mn/Br catalyst, deactivation is due mainly to reactions of bromine with organic substrate molecules and precipitation of insoluble manganese compounds. The bromination reaction as a form of catalyst deactivation is closely related to formation of byproducts. Coupling between a bromine atom and a methyl radical, as per eq 169, yields methyl bromide which being volatile at normal reaction conditions is removed together with the reaction offgas and is taken this way from the reaction media. Formation of benzylic bromides also accounts for catalyst deactivation despite remaining in the reaction media. Benzylic bromides, as a source of bromine to the Co/Mn/Br catalyst, have little or no catalytic activity in the oxidation of alkyl aromatics.75 Addition of a benzylic bromide to the oxidation media comprising a Co/Mn catalyst and an alkyl aromatic has a negligible effect on the rate of oxidation, Figure 59. When bromine is lost from the catalyst one passes from a high active Co/Mn/Br catalyst to a much less active Co/Mn catalyst. Bromine losses, in particular, to methyl bromide, make the Br/(Co + Mn) ratio decrease. This leads, according to Figures 28 and 35, when at the optimum, to carbon oxides and methyl bromide formation increases. Considering the catalytic cycle given in Figure 10, the redox reactions between manganese and bromine cease and the concentration of Mn3+ ion tends to reach a steady state. The AH


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Figure 58. Formation routes of anthraquinone and fluorenone derivatives during p-xylene oxidation: X, Y = H, CH3, CHO, COOH, CH2OH; m, n = 0, 1, 2. Adapted from ref 102.

organic bromide to the ionic bromide form, causing the reaction rate to increase significantly.75

implication is that because water is present in the oxidation media, disproportionation of Mn3+ can occur 2Mn 3 + → Mn 2 + + Mn 4 +

(189)

Water is known to increase the proportion of Mn4+ formation in acetic acid/water solutions, and more Mn4+ is formed as the water concentration increases, Table 3.114 From Mn4+, MnO2 is formed, a product coloring substance and also insoluble, which by removing manganese from the reaction media also contributes to catalyst deactivation. Manganese trimellitate, as seen previously, has the same effect on the catalyst system. Bromine loss to byproducts, together with manganese precipitation of insoluble compounds, results in a cyclic cascade of events that ends ultimately in a low catalyst activity, Figure 60. 5.7.1. Catalyst Regeneration. The quickest way to regenerate a bromine-depleted Co/Mn/Br catalyst is through addition of hydrobromic acid, but other forms are possible. Benzylic bromides can be reconverted, recovering bromine in its ionic form, thus regenerating the catalyst. Addition of sodium chloride leads to displacement of the covalent bonded

In addition, because in the oxidation media water is present, hydrolysis of benzylic bromide can also occur.This reaction is another possible route for formation of benzylic alcohols.

6. OPERATIONAL OPTIMIZATION STUDIES Optimization of an operating plant deals with reducing costs while maintaining product quality. In a commercial terephthalic acid plant to achieve this objective the variables are many and to better manipulate them understanding of the chemistry behind the process is fundamental. The main goal is to minimize 4-carboxybenzaldehyde in the final product, keeping operational costs within controlled economical limits while AI


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Table 2. High Molecular Weight Byproducts Found in p-Xylene Oxidation Solvent101,102,112

AJ


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Table 2. continued

Table 3. Effect of Water in the Calculated Equilibrium Values of Mn4+ from the Mn3+ Disproportionation in Acetic Acid/Water Mixturesa

a

Figure 59. Effect of different bromine sources on the O2 uptake rate during Co/Mn/Br-catalyzed oxidation of pseudocumene. Adapted from ref 75.

water (vol %)

water (M)

Kd (×106)

Mn as Mn(IV) (%)

1 4.7 9.1 16

0.56 2.61 5.06 8.89

7.6 8 30 800

2.7 2.8 5.2 22

Data from ref 114.

concentrations, water concentration in the mother liquor, rate, impurities, etc.

maximizing production rate. In a simplified manner, this is usually a trade-off between product quality, expressed in 4-CBA content, and production cost, expressed in terms of solvent burning, i.e., COx in reactor off-gas, Figure 61. In a more sophisticated model, solvent burning by itself is not the only cost factor but parcels like catalyst concentration, power, utilities, etc., should also be taken into account and 4CBA is dependent on many process variables, like catalyst

4‐CBA = ϕ([Mn], [Co], [Br], vent O2 %, H 2O%, p‐xylene feed rate, total feed rate, residence time, temperature, ...)

(192)

Construction of models based on process variables allows us to predict 4-CBA in the product and establish which process variables have greater influence in its variation. Selection of AK


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7. RECENT RESEARCH TRENDS IN TEREPHTHALIC ACID SYNTHESIS 7.1. Oxidation in Sub- and Supercritical Water

Water is an excellent environmentally friendly solvent. Because it is nontoxic, readily available, inexpensive, and benign to the environment, it has been the subject of much research as solvent media for oxidation of p-xylene.123 More recently, research has turned to sub- and supercritical water as oxidation media.124,125 Sub- and supercritical water (Tc = 647.3 K; Pc = 221.2 bar]126) are interesting reaction media because their properties vary considerably as temperature increases. As the temperatures rises up to the critical temperature, some of the physical properties of water progressively approach those of organic solvents. Figure 62 shows the dielectric constant,

Figure 60. Schematic cyclic cascade of events leading to catalyst deactivation.

Figure 61. Trade-off between product quality and production cost.

these variables allows their proper manipulation within a product quality and cost policy. Han et al.115,116 uses Multivariate Statistical Process Models to build 4-CBA prediction models based on regression of process variable data. Liu et al.117 use a Fuzzy Neural Network with a reduced number of variables, and Zhang et al.118 use a Fuzzy Support Vector Regression Model built from 10 process variables of industrial data to predict 4-CBA concentration. Mu et al.119,120 using a set of 10 process variables developed a genetic algorithm which optimizes operation profit either by manipulating product quality, i.e., 4-CBA and process throughput, or by operating the process variables sequence in the proper operating order. Hong et al.121 present a Fuzzy Adaptive Immune Algorithm. Gujarathi and Babu122 present a multiobjective optimization algorithm based on effect of six decision variables identified, namely, catalyst concentration, vent oxygen content, % water in solvent, feed xylene rate, temperature of reactor, and total feed rate, also to achieve the same effect of allowing the industrial operator to make the desirable improvement in the process design decisions. It is a difficult and time-consuming method to choose an appropriate set of operation conditions for PTA production by empirical means alone. These models, reflecting the same individual trends observed individually from experimental observations, provide a useful way to manipulate one variable individually or several variables in a given sequence, within preset limits, to aim for a desired 4-CBA concentration and economical profit with a comfortable confidence margin to avoid unnecessary economical cost caused by poor trial and error industrial experiments.

Figure 62. Properties of water as a function of temperature at 25 bar: (a) dielectric constant; (b) terephthalic acid solubility; (c) ion product. Adapted from ref 125. AL


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Table 4. Yields of p-Xylene Oxidation to Terephthalic Acid in Sub- and Supercritical Water media

T (°C)

catalyst

max yield (%)

4-CBA yield (%)

oxidant

ref

subcritical supercritical supercritical supercritical subcritical supercritical supercritical supercritical supercritical supercritical supercritical supercritical supercritical

300 380 400 380 300 380 380 380 380 380 380 380 380

MnBr2 MnBr2 MnBr2 MnBr2 MnBr2 HBr CoBr2 MnBr2/HBr MnBr2/benzoic acid CuBr2 CuBr2/ NiBr2 Cu/Co/Br Cu/Co/NH4/Br

49 ± 8 57 ± 15 >90 95 80 19.1 6.3 22.7 35.6 55.6 59 60 70.5

0.03 ± 0.04

H2O2 H2O2 H2O2 H2O2 O2 O2 O2 O2 O2 O2 O2 O2 O2

127 128 129 130,131 132 133 133 133 133 124 124 124 124

∼0 Co(III) > Mn(III) > Co(II) > Mn (II). During the catalytic cycles shown in Figure 10, cobalt and manganese are present mainly in their divalent forms, and hence, the promotion effect of their trivalent forms is small. Addition of Zr(IV) to Co/Mn/Br will be expected to increase the rate by promoting the benzylic peroxide to the benzaldehyde, as zirconium is present as Zr(IV). Zr(IV) will have stronger coordination with the peroxide, weakening the O−O bond, decomposing it to aldehyde and water, and thus increasing the aldehyde/(alcohol + acetate) ratio, Figure 65.137 Zirconium addition to Co/Mn/Br allows a decrease in the cobalt concentration in the catalyst, but it should be always balanced for optimization purposes. It may not be desirable as cobalt provides other selective pathways, such as avoiding AN


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causing an explosion or fire, and a higher concentration of O2 could be employed in the absence of an inert gas diluent such as nitrogen air. The beneficial effect of CO2 as co-oxidant may be explained by three mechanistic interpretations, independently or by their combination.140 • The presence of CO2 may have a suppressive effect in decarboxylation reactions of intermediates and in product, which would improve the overall yield and appear to enhance the oxidation rate. This effect has also been used to explain the inhibiting role of CO2 as an inert gas in acetic acid decomposition.144,145 • Improvements in oxidation rate and product selectivity might arise from increased solubility of substrate and intermediates and, particularly, increased solubility of oxygen. The effect is due to solvent expansion caused by CO2, more notorious as CO2 pressure is increased further,145 even at a temperature of 160 °C, a low temperature when compared to those normally employed in commercial processes. • The increased solubility of oxygen in the presence of CO2 may indeed account for formation of a catalytically active cyclic peroxocarbonate species, CO42−, on the

Figure 65. Suggested mechanism of metal-catalyzed dehydration of benzoyl peroxide. X = CH3 or COOH. Adapted from ref 137.

addition of peroxy and alkoxy radicals through their decomposition, reducing the concentration of hydroxyl radicals from benzyl hydroperoxide thermal decomposition, and decreasing formation of phenyl formate, one of the possible products of the Baeyer−Villiger reaction. Phenyl formate is undesirable because not only it is a decrease of terephthalic acid yield but also with its hydrolysis it yields its phenol antioxidant derivative.107,108 In addition, phenol can be further oxidized, yielding the undesirable, color-forming quinone product. 7.2.2. Carbon Dioxide Use as Co-oxidant. Carbon dioxide is one of the byproducts formed during p-xylene oxidation and is formed from degradation of both products and acetic acid solvent. CO2 has been given a new role as co-oxidant in homogeneous catalytic oxidation of aromatic hydrocarbons.138−143 Figure 66 shows the beneficial effect of CO2 when it is introduced in the gaseous feed stream. The beneficial effect of CO2 as co-oxidant is clear, as an increase in carboxylic acid in the product and increase in oxygen consumption can be observed, showing that the activity and selectivity toward the carboxylic acid is enhanced, even at milder conditions. In addition to Co/Mn/HBr catalyst systems, CO2 also has beneficial effects as co-oxidant in catalyst systems of the type Co/Mn/HBr/M and Co/Mn/HBr/M−M′, where M stands for an alkali metal or alkaline earth metal and M′ stands for a transition metal, including Zr, Hf, and Mo and lanthanides, such as Ce, Pr, and Sm.140 Oxidation of intermediates, such as p-toluic acid, the rate-determining step, shows a similar behavior, which suggests that the presence of CO2 yields a product with less intermediate impurities, which in turn may reduce the demand in the purification step in a commercial AMOCO process. With the use of CO2, a higher concentration of O2 could be used in the reaction without

Figure 67. Postulated formation mechanism and form of the peroxocarbonate species. Oxygen atoms in bold represent those from the original O2 molecule. M = Co or Mn. Adapted from ref 140.

metal center, via a synergistic interaction between the O2 and the CO2 molecules, as shown in Figure 67.

Figure 66. Effect of CO2 in the liquid-phase oxidation with a Co/Mn/Br catalyst of (a) m-xylene and (b) p-xylene in the presence of 147 ppm of potassium. Adapted from ref 140. AO


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turn generate catalytically active Co(III) species, increasing its concentration, thus promoting oxidation of the aromatic hydrocarbon.37 More recently, the drive to remove corrosive bromine from the catalyst and from the process led to research in transition-metal salt catalytic oxidation of aromatic hydrocarbons promoted by N-hydroxyphthalimide (NHPI) and derivatives.146−149 Oxidation of p-xylene yields up to 84% of terephthalic yield with less than 1% 4-carboxybenzaldehyde yield when 20 mol % NHPI is used together with cobalt and manganese acetates in acetic acid at 150 °C and at a pressure of 30 atm for 3 h.147 Figure 69 shows the postulated mechanism catalyzed by NHPI similar to the classic free-radical oxidation mechanism. It involves initial in situ NHPI decomposition by a free-radical chain initiator such as a peroxide compound or a labile metal complex to the phthalimide N-oxyl (PINO) radical. PINO radical abstracts a hydrogen atom from the hydrocarbon, generating an alkyl radical which will react with the oxygen molecule to yield a peroxy radical. The PINO radical is regenerated back to NHPI through hydrocarbon hydrogen abstraction. Peroxide radicals yield alkyl peroxide molecules, which will be decomposed by the transition-metal complex to the oxygenate product. The mechanism above follows a classical free-radical chain reaction, and when fast oxygen consumption occurs, both NHPI and the PINO radical decompose, decreasing the overall oxidation rate of the substrate.148 This means that to keep a steady oxidation rate NHPI needs to be continuously added to the reaction medium. To reduce the amount of NHPI required, N-acetoxyphthalimide (NAPI) can effectively achieve a similar yield on terephthalic acid (80%) with only 5 mol % together

The key role of the peroxocarbonate species for oxidation of alkyl aromatics such as p-xylene is not clear. It may lie in transferring an oxygen atom bonded to the metal in the peroxocarbonate complex to the p-xylene molecule to form an alcohol and then in assisting abstraction of hydrogen to form aldehyde, forming the p-toluic acid intermediate. The same mechanism is repeated in oxidation of the other methyl group of the p-toluic acid molecule to produce terephthalic acid. However, it may indirectly participate in formation of free radicals by acceleration of the catalytic pathway shown in Figure 10, acting in particular in Co2+ oxidation acceleration, inducing an acceleration in the other steps, Figure 68.140,141

Figure 68. Postulated role of the peroxocarbonate species in the conventional MC oxidation catalytic pathway. Adapted from ref 140.

7.2.3. N-Hydroxyimides. Organic promoters have been used in terephthalic acid synthesis and manufacture.48 In the Teijin process, methyl−ethyl ketone is used as a promoter,2,43 with its principal function to generate peroxy radicals, which in

Figure 69. Postulated mechanism for aerobic oxidation of an aromatic hydrocarbon promoted by NHPI in the presence of a transition-metal catalyst complex. X = H, CH3, CO2H; LnCo: cobalt or transition-metal complex with n ligands L. Adapted from ref 149. AP


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with 0.5 mol % of both cobalt and manganese acetates. NAPI can be prepared according to eq 193.

The time dependence of the aerobic oxidation of p-xylene catalyzed by NHPI or NAPI combined with cobalt acetate is shown in Figure 70. From the figure it can be seen that p-xylene oxidation in the presence of NAPI is slower and also that the yield in terephthalic acid is higher. It has been suggested that the early stage of p-xylene oxidation NHPI is rapidly decomposed to phthalimide and phthalic anhydride. NAPI is considered to be resistant to this rapid decomposition when violent chain reactions take place, since it is gradually hydrolyzed to NHPI by water present in acetic acid as well as the water resulting from oxidation.146,148 7.2.4. Guanidine. Cheng and co-workers150 studied the effect of guanidine as promoter in Co/Mn/Br p-xylene oxidation. The high basicity of guanidine (pKa = 13.6) and the exceptional “Y-aromacity” resonance stabilization of the 6 π electrons of guanidine and of its conjugated acid ion are responsible for its thermodynamic stability, Figure 71.151 Figure 72 shows the effect of guanidine addition to the Co/ Mn/Br catalyst in the rate constants on stepwise oxidation of pxylene to terephthalic acid, as per eq 48, and on formation of carbon dioxides. Concerning rate constants, guanidine appears to have an inhibiting effect on the first two steps of the oxidation and a clear promoting effect on oxidation of the second methyl group, as happens with the influence of water in the catalyst activity, which is an indication of guanidine influence on catalyst structure. In addition, a clear effect in carbon oxides formation decreases is observed, indicating higher catalyst selectivity when compared with the normal Co/Mn/Br catalyst. Being a

Figure 71. Resonance stabilization of (a) guanidine and (b) guanidine ion.

stronger electron donor than water, guanidine can progressively replace aqua and acetic ligands in the catalyst structure and bridge metal ions, as shown in Figure 73. The Y-shaped aromatic structure disperses the π electrons in the transition state of the complex, stabilizing it, and lowering the activation energy of the electronic transfer reaction in the inner coordination, effectively enhancing the rate. This means that addition of guanidine to the catalyst will make structures II and III in Figure 72 more active than structure I. Coordination of guanidine to the inner coordination sphere is limited, and thus, high outer-sphere formation of the ion pair guanidine ion−acetate ion, {Gua+OAc−}, will occur. This ion pair will go through ion exchange reactions with HBr molecules, either free or bonded to the catalyst, yielding the ion pair {Gua+Br−}. This will cause a depletion of Br− ions on the catalyst coordination sphere (see Figure 14), decreasing its catalytic activity, as formation of bromine free radical via the synergy cycle of Figure 10 is lessened. Concerning formation of carbon oxides, the positive effect of guanidine is also due to an increase of catalyst inner-sphere electron-transfer reaction rates. As these electron-transfer rates increase stationary Co(III) concentration decreases. Cobaltic ions are responsible for decarboxylation reaction of acetic acid as per eqs 154 and 155.

Figure 70. Time-dependence concentration of p-xylene, p-toluic acid, and terephthalic acid in the aerobic oxidation of p-xylene catalyzed by (a) NHPI and (b) NAPI combined with cobalt acetate. Adapted from ref 146. AQ


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Figure 72. Effect of guanidine composition on rate constants (black lines) and in COx generation (gray line). Adapted from ref 150.

fluids at room temperature. They have very low or no measurable vapor pressure and thus at ambient temperature are effectively nonvolatile. Because they consist of at least of one anion and one cation, properties such as melting point, viscosity, density, and hydrophobicity can be adjusted to a particular purpose simply by changing the nature of one of the components of the ion pairs. They are immiscible with some alkane solvents and so can be used in two-phase systems. Their composing anions are often weakly coordinating ligands, such as BF4− and PF6−, and, hence, have the potential to be highly polar yet noncoordinating solvents. They can be expected, therefore, to have a strong rate-enhancing effect on reactions involving cationic intermediates. Ionic liquids can dissolve a wide range of organic inorganic and organometallic compounds and also gases such as H2, CO, and O2, which makes them suitable for catalysis of several reactions.152,153 Ionic liquids can be used in catalysis as either catalyst or solvent or a combination of these, as catalyst activator, or as cocatalyst for reaction, as a catalyst ligand source, or as solvent alone for several reactions, including aerobic oxidation reactions, as they are stable to oxidation.154 Saleh155 patented a process to produce aromatic aldehydes from carbonylation of suitable alkyl aromatic compounds with pyridine or imidazole derivative ionic liquids in a pressure range from 4 to 100 bar and temperature range from 10 to 100 °C. The carbonylation reaction of toluene in those experimental conditions can achieve a conversion of 66% with p-tolualdehyde selectivity up to 89.3%. Subsequent oxidation of p-tolualdehyde with air in acetic acid with a Co/ Mn/Br catalyst provides an alternative route from toluene to terephthalic acid. Howarth156 achieved oxidation of several aromatic aldehydes catalyzed by nickel acetylacetonate in [bmim][PF6] at 60 °C and atmospheric pressure. In oxidation of benzaldehyde, p-tolualdehyde, and terephthaldicarboxyaldehyde, yields of 66%, 63%, and 47% of the respective aromatic acids, benzoic acid, p-toluic acid, and terephthalic acid, were achieved. Earle and Katdare157 patented a process in which toluene is oxidized with nitric acid and air with pyridine or imidazole derivatives ([bmim][NO3]) as the ionic liquid from atmospheric pressure up to 100 bar in the ideal temperature range from 100 to 120 °C. Benzoic acid yield was between 70% and 90%. When p-xylene is used as feedstock in [bmim][OMs], terephthalic acid yield is low, 4.8%, with a low selectivity, 11.5%, with the other products formed in the reaction, p-toluic

Figure 73. Suggested structures for the Co/Mn/Br/guanidine catalyst system. M, M′ = Co(II), Co(III), Mn(II), Mn(III). Adapted from ref 150.

In addition, acetic ligand substitution by guanidine also keeps them away from the metal, avoiding their decarboxylation in the inner sphere of the catalyst. In conclusion, an optimized amount of guanidine promoter will increase the reaction rate and decrease solvent burning.150 7.3. Oxidation in Ionic Liquids

Most of chemical reactions are performed in molecular solvents. Recently, ionic liquids have emerged as alternative solvents for several chemical reactions. Ionic liquids are often AR


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attainable yields, and selectivity close to that used in the conventional MC AMOCO process and are bromine-free catalysts.

acid, with 36.4% yield, and with a higher selectivity of 88.5%. The nature of the ionic liquid also has a decisive influence on the outcome of a reaction, as Earle and co-workers158 have shown for oxidation of toluene with nitric acid, Figure 74.

7.4. Heterogeneous Systems

Like with research in homogeneous systems, research on heterogeneous systems is also motivated by efforts to improve yields, achieve higher selectivity, and replace of hazardous chemicals of softening process conditions. In their 1986 paper, Hronec and Hrabě160 review briefly efforts of hydrocarbons liquid-phase oxidations catalyzed by heterogeneous catalysts, including oxidation of p-xylene over CoY and MnY zeolites, but with very low yields and oxidation in only one methyl group. In the same paper, the results are reported for oxidation of p-xylene in water medium catalyzed by metal oxides such as lead, cerium chromium, silver nickel, manganese, and silver oxides. These metal oxides show little or no activity as catalysts for p-xylene oxidation to p-toluic acid or terephthalic acid in the absence of p-toluic acid. Addition of ptoluic acid increases activity, but only the cobalt and manganese oxides show activity to yield terephthalic acid. With the other metal oxides p-xylene conversion ceases at 5−10%. These observations led the authors to the conclusion that the catalytically active species are not heterogeneous metal oxides but the homogeneous toluate salts. The activity of cobalt and manganese oxides is strongly dependent on calcination temperatures and shows both a critical concentration above which terephthalic acid yield is maximum and optimum water content. Only Co−Cr and Mn−Ce bimetallic catalysts show a higher activity than single metal catalyst, which is ratio dependent, as there are both antagonistic and synergistic effects. Chavan and co-workers161 reported high activity and selectivity toward terephthalic acid when p-xylene is oxidized in acetic acid−water liquid medium at normal industrial process temperature and pressures between 14 and 38 barg using as heterogeneous catalyst neat and μ3-oxo-bridged Co/Mn cluster

Figure 74. Influence of the ionic liquid nature on the outcome of the oxidation of toluene with nitric acid. Adapted from ref 158.

Terephthalic acid can be obtained directly from oxidation of p-xylene using NHPI/O2/HNO3 as oxidative system in [bmim][OMs] at 110 °C and 1 bar.159 The maximum p-xylene conversion achieved was 98%, with a 96% terephthalic acid yield and 98% selectivity. The proposed mechanism is shown in Figure 75. NHPI and nitric acid react to yield the PINO radical, water, and NO2 radical. The PINO radical then abstracts a hydrogen atom from the methyl group of p-xylene, initiating the chain reaction. NO2 radical decomposes the peroxide radical, regenerating HNO3 and yielding the oxygenate product. Ionic liquids present themselves as a promising research field for oxidation of alkyl aromatics to produce alternative routes to aromatic carboxylic acids like terephthalic acid, with the important advantage of use of nonvolatile recyclable solvents,

Figure 75. Proposed mechanism for oxidation of p-xylene to terephthalic acid by the NHPI/O2/HNO3 system in [bmim][OMs] at 110 °C and 1 bar. Adapted from ref 159. AS


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Figure 76. Proposed catalytic oxidation of p-toluic acid over CoMn2(O)−Y catalyst. Adapted from ref 161.

constrains that may also constitute a barrier to a viable, fullscale commercial process.

complexes encapsulated in Zeolite−Y. Using such catalyst, with heteronuclear complexes, 100% conversions of p-xylene were achieved and distributions in the product being between 98.9% and 99.4% for terephthalic acid and 0.01% or less for 4carboxybenzaldehyde. These results were, however, obtained at 38 barg, a much higher pressure that the 14−16 barg used in commercial processes. In commercial processes 4-carboxybenzaldehyde is present in 0.1−0.5% in the crude product before the hydrogenation purification process. The reasoning behind these results is in the low redox potentials shown by the cluster complexes, indicating easier electron transfer between Co2+/ Co3+ redox states in the oxidation and reductions steps of the proposed mechanism of catalytic oxidation, as shown in Figure 76. Due to the close dimensions of catalyst cluster and zeolite, leaching is difficult and hence catalyst is stable and can be separated and recycled. Washing the solid mixture with 0.1 N NaOH is required to recover the carboxylic acids in the form of water-soluble salts, and the catalyst is recovered by filtration. No plugging due to build-up in the catalyst is reported. Indeed, heterogeneous catalyst may be deactivated by solids which block the active sites. This was reported by Kim et al. in their studies of p-xylene oxidation using transition metals supported SBA-15,162 by Das and Chakrabarty in the oxidation catalyzed by Co(III)−oxo cubane clusters,163 and by Bastock et al. using Envirocat EPAC as catalyst with a terephthalic acid yield of only 37.5%.164 This effect constitutes a barrier when development of a commercial process is desired. More recently, patents have been filed claiming development of heterogeneous catalysts consisting of cobalt and manganese heteronuclear complexes encapsulated in a zeolite and conceptual design of a process using solvent consisting of a mixture of p-xylene and water.165−170 Oxidation is done in a series of four reactors which operate at progressively increasing temperature from 256 to 300 °C and 50 bar absolute. Conversion of p-xylene in the four reactors is extremely low, 1.5%, but the formed terephthalic acid remains in solution as well as 4-carboxybenzaldehyde. In commercial processes, once formed terephthalic acid as 4-carboxybenzaldehyde both precipitate, making their separation difficult and creating the need for a purification unit. Remaining in solution, 4carboxybenzaldehyde is converted to terephthalic acid; in addition, because both substances are in the liquid phase, no blocking of the catalyst is expected. Process conditions are maintained such that terephthalic acid is precipitated from solution only when it leaves the reaction system, and the solvent is recycled to the reactors, being undersaturated terephthalic acid. The terephthalic acid product may contain less than 0.005 w/w % of 4-carboxybenzaldehyde. The low pxylene conversions and high temperatures and pressures are

8. CONCLUSIONS In this paper, p-xylene oxidation is reviewed with particular emphasis on the AMOCO MC method, giving a historical evolution of the processes of terephthalic acid manufacture from the early days’ difficulties inherent to the p-xylene resistance toward oxidation to the breakthrough of the AMOCO MC cobalt/manganese/bromine catalyst system. The main state of the art detailed kinetics are presented, which are a useful tool for investigation of mechanisms of new catalyst and byproducts, but a lumped kinetic mechanism proves to account effectively for the main intermediates during p-xylene oxidation to terephthalic acid. The actual chemistry proves to be very complex, and the literature gives evidence that changes in catalyst composition cause changes in its structure which account for different behavior in terms of activity and selectivity, which in turn are important when it is desirable to correlate this change with industrial process control variables. Optimization of an industrial process is thus intimately related with optimization of the catalyst. Notwithstanding its high yield and widespread use, the current dominant process of terephthalic acid manufacture using the AMOCO MC Method catalyst still has some handicaps related with the aggressive nature of its components and required investment in specialized equipment. These handicaps are drives to the study of new catalyst, promoters, and solvents, some of which are presented in this paper, and are an object of interest. Further research efforts and process development are necessary to shift from the current mainstream technology to more environmentally friendly and equally effective technologies.

ASSOCIATED CONTENT S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. AT


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Biographies

Technical University of Lisbon. He worked for several years in Portuguese National Laboratories, supporting industry and developing R&D projects, and is now a Habilitated Coordinating Professor with the Chemical Engineering Departmental Area at ISEL, Lisbon Polytechnic, and a researcher of IBB.

ACKNOWLEDGMENTS R.A.F.T. thanks ARTLANT PTA SA for all the training and support given which allowed this paper to be written and also Instituto Piaget−Pólo de Santo André for logistical support during the literature research. REFERENCES (1) McIntyre, J. E. The Historical Development of Polyesters. In Modern Polyesters: Chemistry and Technology of Polyesters and Copolymers; Scheirs, J., Long, T. E., Eds.; John Wiley & Sons, Ltd.: Chichester, 2003. (2) Raghavendrachar, P.; Ramachandran, S. Ind. Eng. Chem. Res. 1992, 31, 453. (3) Sheehan, R. J. Terephthalic Acid, Dimethyl Terephthalate and Isophthalic Acid. Ullmann’s Encyclopedia of Industrial Chemistry; WileyVCH Verlag GmbH & Co.: Weinheim, 2005; Vol. 35, pp 639−651. (4) Saffer, A.; Barker, R. S. U.S. Patent 2833816, May 6, 1958. (5) Saffer, A.; Barker, R. S. GB Patent 807091, January 7, 1959. (6) Saffer, A.; Barker, R. S. U.S. Patent 3089906, May 14, 1963. (7) Landau, R.; Saffer, A. Chem. Eng. Prog. 1968, 64 (10), 20. (8) Ciamician, G.; Silber, P. Ber. Dtsch. Chem. Ges. 1912, 45 (1), 38. (9) Stephens, H. N. J. Am. Chem. Soc. 1926, 48 (7), 1824. (10) Stephens, H. N. J. Am. Chem. Soc. 1926, 48 (11), 2920. (11) Stephens, H. N. J. Am. Chem. Soc. 1928, 50 (9), 2523. (12) Stephens, H. N. J. Am. Chem. Soc. 1928, 50 (1), 186. (13) Stephens, H. N. J. Am. Chem. Soc. 1928, 50 (2), 568. (14) Hartman, M.; Seibert, M. Helv. Chim. Acta 1932, 15 (1), 1390. (15) King, E. P.; Swann, S., Jr.; Keyes, D. B. Ind. Eng. Chem. 1929, 21 (12), 1227. (16) Halpern, J. Discuss. Faraday Soc. 1968, 46, 7. (17) A General Discussion on Oxidation. Trans. Faraday Soc. 1946, 42, 99. (18) Frank, C. E. Chem. Rev. 1950, 46 (1), 155. (19) Russell, G. A., J. Chem. Educ., 1959, 36(3), 111. (20) Mayo, F. R. Acc. Chem. Res. 1968, 1 (7), 193. (21) Partenheimer, W. Catal. Today 1995, 23 (2), 69. (22) Mulcahy, M. F. R.; Watt, I. C. Proc. R. Soc. London, Ser. A 1953, 216, 10. (23) Bolland, J. L. Q. Rev. Chem. Soc. 1949, 3, 1. (24) Benson, S. W. J. Am. Chem. Soc. 1965, 87 (5), 972. (25) Hammett, L. P. J. Am. Chem. Soc. 1937, 59 (1), 96. (26) Lowry, T. H., Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper Collins: New York, 1987. (27) Hammett, L. P. J. Am. Chem. Soc. 1937, 59 (1), 96. (28) Russell, G. A. J. Am. Chem. Soc. 1956, 78 (5), 1047. (29) Jaffé, H. H. Chem. Rev. 1953, 53 (2), 145. (30) Haber, F.; Weiss, J. Die Naturwiss 1932, 20, 948. (31) Haber, F.; Weiss, J. Proc. R. Soc. London, Ser. A 1934, 147, 332. (32) Hagen, J. Industrial Catalysis. A Practical Approach; Wiley-VCH Verlag GmbH & Co.: Weinheim, 2006. (33) van Leeuwen, P. Homogeneous Catalysis. Understanding the Art, 2nd ed.; Kluwer Academic Publishers: Dordrecht, 2004. (34) Donaldson, J. D.; Beyersmann, D. Cobalt and Cobalt Compounds. Ullmann’s Encyclopedia of Industrial Chemistry; WileyVCH Verlag GmbH & Co.: Weinheim, 2005; Vol. 35, pp 639−651. (35) Reidies, A. H. Manganese Compounds. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co.: Weinheim, 2005; Vol. 35, pp 639−651. (36) Morimoto, T.; Ogata, Y. J. Chem. Soc. (B) 1967, 62. (37) Hanotier, J.; Hanotier-Bridoux, M. J. Mol. Catal. 1981, 12, 133. (38) Kamiya, Y.; Kashima, M. J. Catal. 1972, 25 (3), 326.

Rogério A. F. Tomás holds a B.Sc. degree in Chemical Engineering from the Technical University of Lisbon and a M.Sc. degree in Management and Industrial Strategy also from the Technical University of Lisbon. He was responsible for start-up of the new producing PTA plant from Artlant in Sines, Portugal, and is now responsible for design and start-up of a pilot plant in The Netherlands. He is also studying for his Ph.D. degree on optimization of the PTA production process.

João C. M. Bordado holds a B.Sc. degree in Chemical Engineering and a M.Sc. degree in Chemical Engineering and was awarded his Ph.D. degree in Chemical Engineering, all from the Technical University of Lisbon. After several years working in industry on polymer plants and developing R&D projects, he is now Full Professor with the Chemical Engineering Department of IST, Technical University of Lisbon, and a researcher of IBB.

João F. P. Gomes holds a B.Sc. degree in Chemical Engineering and was awarded his Ph.D. degree in Chemical Engineering, both from the AU


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