Transiting from Adipic Acid to Bioadipic Acid. 1 ... - ACS Publications

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Transiting from Adipic Acid to Bioadipic Acid. 1, Petroleum-Based Processes Jan C. J. Bart and Stefano Cavallaro* Dipartimento di Ingegneria Elettronica, Chimica e Ingegneria Industriale, Università degli Studi di Messina, Viale F. Stagno D’Alcontres, 31-98166 Sant’Agata di Messina, Italy S Supporting Information *

ABSTRACT: Adipic acid, which is a very important commodity chemical, is traditionally manufactured industrially by an aged and unsustainable multistep process that involves homogeneous catalysts, aggressive oxidants (concentrated nitric acid) and the production of large quantities of the greenhouse gas nitrous oxide. Much research and development into alternative and cleaner process routes to adipic acid have been carried out over the past 70 years. Improved reaction schemes have invoked variations in fossil raw materials (from benzene and phenol to cyclohexane, cyclohexanol, cyclohexanone, cyclohexene, butadiene, and adiponitrile, etc.), oxidants (O2, air, H2O2, t-BuOOH, ozone), catalysts (homogeneous, heterogeneous, phase transfer, biomimetic) and reaction conditions (low temperature, atmospheric pressure, and solvent-, metal-, halide-, and corrosion-free). No single proposed alternative pathwaysome of which are purely academicsimultaneously addresses the problems of petrochemical origin, toxic starting materials or reagents, generation of environmentally incompatible byproducts, use of forcing reaction conditions, and cost in an entirely satisfactory manner, despite very intense efforts. Recently, more benign bio-based reaction pathways have been proposed starting from renewables, such as glucose or vegetable oils (which will be discussed in Part 2 of this series).

1. INTRODUCTION Adipic acid, or hexanedioic acid (C6H10O4, having a molecular weight of MW = 146.14), is an important industrial intermediate; it is used as a starting reagent for polyamide-6,6. There are several commercial production route options for adipic acid (AA), all of which are based on petroleum-derived feedstocks (e.g., cyclohexane, benzene, phenol, butadiene). Most recently, phenol has been virtually eliminated as a feedstock, and cyclohexanebased processes now account for ∼93% of the global production capacity. Therefore, adipic acid production requires transformation of an inert hydrocarbon to a dioxygenate. The conventional process is Du Pont’s nitric acid oxidation of a cyclohexanol/cyclohexanone mixture derived from cyclohexane.1,2 Adipic acid accounts for ∼60% of global cyclohexane consumption. The latter product is mainly obtained by catalytic hydrogenation of benzene, a process that is critically dependent on the availability of low-cost hydrogen from steam crackers; nitric acid is derived from ammonia (the Haber−Bosch process). All current AA manufacturing processes suffer from the high costs associated with multistep operations, as well as from significant environmental pollution problems. Adipic acid production is not compatible with today’s environmental constraints and health safety regulations. Many process improvements for the production of adipic acid have been proposed. It is highly desirable to develop more environmentally responsible and economical methods for the manufacture of adipic acid. In practice, this means meeting societal demands for sustainable processes based on renewable resources and reagents, heterogeneous catalysts, high conversion and selectivity, mild reaction conditions, no solvents, low production costs, and many other factors (see Table S1 in the Supporting Information). Yield, selectivity, and space-time yield are the only metrics for © 2014 American Chemical Society

evaluating reaction efficiency. More sustainable processes often have better economics than conventional ones, because fewer byproducts translate to lower costs for feedstock, separation, and disposal. Higher productivity allows a reduction in reactor size. Despite concerted efforts to develop new process routes based on fossil raw materials, no commercially competitive process has yet displaced the aged adipic acid manufacturing process. Adipic acid chemistry and liquid-phase oxidation of cyclohexane have periodically been reviewed,2−13 and biotechnological production has also been reviewed repeatedly.14,15 This paper examines the prospects for renovation of industrial adipic acid manufacture using petrochemical feedstocks.

2. ADIPIC ACID INDUSTRY STATUS Adipic acid is a high-volume bulk petrochemical commodity, as is reflected in its high consumption volume and low selling price (typically € 1650/t, January 2013).16 It is the most important aliphatic dicarboxylic acid in the chemical industry and ranks as one of the top 50 chemicals in the United States. The quality of adipic acid is generally very uniform, since most applications require high purity. High-purity adipic acid used in polyamide6,6 fibers and some resins accounts for ∼75% of the total output, but lower-grade adipic acid is used for polyurethane production, as a reactant to form polyester polyols, plasticizers, lubricant components, and food acidulant (E 355).12,17 Almost 90% of the domestic U.S. adipic acid is used to produce nylon-6,6. Received: Revised: Accepted: Published: 1

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melt spinning. To manufacture adiponitrile from adipic acid via ammoniation and dehydration, no high-purity polymer grade is required. Adipic acid production qualifies as an industrial gas project under the Clean Development Mechanism (CDM, as of 2005). Other such projects concern N2O from nitric acid production and HFC-23 from HCFC-22 production. The CDM does not directly reduce global greenhouse gas (GHG) emissions, but it does help to achieve a given emission reduction target at a lower cost. In 2009, CDM plants represented only 22% of the global adipic acid production.19 Since January 2007, the average cash cost of adipic acid production has ranged between € 650 and € 1500/t AA, while the net profits from the CDM amount to € 1000/t. Since the profits from producing adipic acid are typically significantly lower than the production costs, the profits from CDM greatly exceed those from the core business and determine a large competitive advantage of CDM plants over non-CDM plants. An economically rational producer can be expected to maximize the adipic acid production of a plant with N2O abatement under the CDM. The operating rates of CDM plants (85%) were unaffected by the decrease in market demand that affected non-CDM plants (60%, as of 2007). Thus, by shifting production in non-CDM plants to CDM plants, CDM incentives lead to undesired carbon leakage.19

Therefore, its production trends are closely correlated with nylon consumption trends. Global demand for adipic acid grew continuously from 2001 to 2007 (up to ∼2.6 Mt with a plant utilization rate of 84%), mainly due to rising demand in China, but dropped sharply (−22%) in 2008/2009 (to below 2.0 Mt with a plant utilization rate of 64%), because of the global economic downturn. The current adipic acid market volume is again ∼2.6 Mt/yr, with an average annual demand growth rate forecast of 3%−3.5%.17 The total production capacity of 23 adipic acid plants (2010) worldwide amounts to almost 3 Mt/yr, as follows: United States (USA), 910 kt; People’s Republic of China (PRC), 650 kt; Germany, 470 kt; France, 320 kt; South Korea, 150 kt; Japan, 125 kt; Singapore, 110 kt; Brazil, 95 kt; Italy, 85 kt; Ukraine, 60 kt; and India, 1 kt.19 Therefore, the global capacity distribution is as follows: East Asia, 1035 kt; European Union (EU), 935 kt; USA, 910 kt; rest of the world (ROW), 85% to olone,1 the rate of cleavage of the C−H bond is high, which favors the formation of significant amounts of CHHP, which decomposes to II and III, and generates a small amount of carboxylic acids as byproducts.64 In a steady-state reactor, the hydroperoxide is no longer the sole primary product as II is formed right from the start. It has been shown that up to 30% of II forms directly from I, with the remainder coming via the hydroperoxide.21 The use of a second stage to remove the hydroperoxide by reaction as it is formed offers high selectivities. Methods to convert CHHP to K and A consist in catalytic decomposition186 or hydrogenation over Pd.35 Cyclohexanone-rich olone can be

industrial application. The corresponding diiron analogue catalyzes oxygenation of cyclohexane to olone at 32 °C with very high efficiency (99%) of H2O2 utilization170 (32% for TS-1).82 A ruthenium-substituted polyoxometalate [WZnRu2III(OH)(H2O)(ZnW9O34)2]11− was inactive toward hydroxylation of secondary carbon atoms.171 Also Cu-based systems have been widely investigated; olone yields are very low (95%) and reduce HNO3 consumption (200 °C. Cyclohexyl nitrite is always present, together with III. The reaction proceeds by nitrosation, with the formation of α-nitrosocyclohexanone as a key intermediate. This product can react in many ways and greatly affects the selectivity and HNO3 consumption of the overall process. Acid-catalyzed isomerization, followed by hydrolysis forms an α-diketone. When α-diketone is oxidized in the presence of suitable V5+ concentrations, AA is formed with a selectivity of >92%.201 Here, the formation of adipic acid is accompanied by the reduction of the regenerable oxides NO and NO2, as follows: VO2 + + HNO3 ↔ VO2+ + NO2 + H+

Figure 9. Transformation of adipomononitrolic acid (AMNA) to adipic acid.

(10)

and/or It is possible to favor certain oxidation routes of cyclohexanol by choosing the right reaction conditions (temperature, HNO3

3VO2 + + HNO3 + H 2O ↔ 3VO2+ + NO + 3H+ 14

(11)

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agricultural activities and industrial HNO3 and adipic acid production. Nitrous oxide is a powerful greenhouse gas (GHG) with a global warming potential (GWP) of 296 CO2 equiv over a 100-year time frame and an atmospheric lifetime of 114 years. N2O is also a dominant ozone-depleting substance.207 The more recently developed awareness of the role of N2O as a potent GHG and ozone depletor204,207 has raised concerns about its release. As a consequence, concerted action has been taken to reduce its impact on the off-gases.62,206,208,209 Nitrous oxide in the adipic acid waste stream can be abated by (i) catalytic destruction;210−212 (ii) thermal decomposition; (iii) conversion to recoverable NO (for nitric acid production); or (iv) recoverage as pure gas (for use as oxidant) (see Table S3 in the Supporting Information). N2O can be recovered and its decomposition heat is reusable; however, a huge amount of energy is required for such high-temperature processes. Thermal destruction operates at over 1000 °C but generates relatively large amounts of NO in high-temperature decomposition. In modern plants outfitted with tail-gas abatement technology, N2O emission amounts to only 0.3 kg/t AA (EcoInvent Databank V2.1). Catalytic techniques to decompose or selectively reduce N2O at ∼500 °C can achieve a total emissions reduction of >99%.206,210 Some plants now even achieve 99.999% abatement.19 N2O emissions from adipic acid production have decreased substantially, compared to 1996. Overall N2O abatement is estimated to have improved from ∼32% in 1990 to ∼90% in 2000.213 All plants in Western industrialized countries voluntarily installed N2O abatement technologies already in the 1990s. According to the global warming treaty signed in Kyoto, all the adipic acid manufacturers (with rather uniform production technology) were required to install N2O abatement systems by the end of 2000, adding cost to the existing technology. Asahi Chemical Industry, BASF, Bayer (now Lanxess), Du Pont (now Invista), and Rhodia (Brazil) employ catalytic or thermal processes to destroy N2O. Solutia and Rhodia (France) recycle N2O back into their production processes (phenol and HNO3, respectively). BASF uses waste N2O as a feedstock for industrial processes, turning the waste into a valuable byproduct. In the presence of a few unabated plants of Chinese producers, an average of ∼88% N2O emission reduction is achieved on a global basis. As a result, at the current state of the art, the production of adipic acid is still associated with ∼36 kg N2O/t AA emission. Despite significant emissions abatement and controls, adipic acid manufacture in the United States released an amount of nitrous oxide equivalent to ∼1.9 Tg CO2 equiv in 2009, corresponding to 99.3% abatement efficiency at full plant utilization rates.214 The contribution of U.S. adipic acid plants to the total N2O emission is 0.7−1.5% (2011 data). At present, nitric acid production (at some 600 plants worldwide, with great variety in plant type) is the largest industrial source of N2O emissions. Clearly, N2O emission should best be prevented rather than cured. N2O emissions from China’s adipic acid manufacture are set to triple by 2020 (an increase of 75 kt/yr), unless decomposition units are installed. N2O emissions from adipic acid production have qualified as projects under the Clean Development Mechanism (CDM) since 2005. During the economic downturn in 2008/2009 AA production partially shifted from plants that installed abatement technology in the 1900s to CDM plants. CDM plants have achieved an average abatement level of 99%. With abatement costs, which are quite low (0.1−4 €/t CO2 equiv), CDM creates considerable revenues for plant operators. This has the

Low nitric acid consumption has been the subject of several patents.145,202 Sampson (to Du Pont)145 disclosed a two-stage liquid-phase vanadium−copper-catalyzed, 40−55 wt % HNO3 two-stage oxidation of olone at 75−110 °C/115 °C, wherein the oxidant is removed from the reactor before it is converted completely to N2O and/or N2. This appreciably reduces the total nitric acid consumption. Castellan et al.147,148,202 have specified reaction conditions that lead to the lowest possible consumption (0.6−0.65 kg HNO3/kg AA) at maximum selectivity (up to 97.8%), namely oxidation at T = 25−40 °C, followed by oxidation at T > 75 °C in the presence of a vanadium catalyst, without extraction or separation. Even in this case, AA is formed partly via the diketone and partly via the AMNA intermediate. The optimized reaction conditions, which favor the formation of the intermediate product cyclohexanedione (and NOx) rather than AMNA (although not exclusively), lead to a 20% reduction of specific HNO3 consumption, as well as in a further improvement of the selectivity toward adipic acid (>97%). The specific consumption of HNO3 can be reduced by ∼0.2 kg HNO3/kg AA (from 0.88 kg/kg AA to 0.65 kg/kg AA), which means a considerable economic benefit. In industrial practice, the lowest possible HNO3 consumption at maximum adipic acid selectivity is realized under conditions in which adipic acid is formed partly via cyclohexanedione and partly via an adipomononitrolic acid intermediate. Since the reaction is highly exothermal, an adipic acid plant based on HNO3 oxidation of olone must dispose of an efficient heat exchanging system.7 An aduate heat sink is provided by using a high HNO3/KA volume ratio (15:1). Consequently, the nitric acid oxidation of KA oil to adipic acid can be visualized as a large nitric acid cycle. For a large-scale producer, a captive ammonia oxidation plant is advantageous. The process is capable of high yield and product purity. Radici (Italy) and a Japanese manufacturer are believed to use the oxidation of pure cyclohexanol instead of KA oil. 5.5. Nitrous Oxide Destruction Technologies. The nitric acid oxidation of olone is not a process with economy of energy and raw materials. Adipic acid is associated with a high fossil fuel energy demand and high waste gas streams. The cumulative energy demand (CED) of feedstock and process energy inputs is typically 78.9 GJ/t (Germany, 1995).203 The classical process from KA oil generates nitrous and other nitrogen oxides, nonmethane volatile organics, and CO in the waste gas stream. During the oxidation of olone, nitric acid is reduced to a mixture of N2, NO, NO2, and N2O, with nitrous oxide as the main byproduct,204 formally resulting from eq 8 (for cyclohexanol) and eq 12 for cyclohexanone:

An IPCC default emission factor for adipic acid of 300 kg N2O/t (or 1 mol N2O/mol AA) has been given.205 NOx emissions are a major environmental concern. Although NO and NO2 (∼1 mol NOx/mol AA) in the off-gas can almost completely be recycled or recovered as HNO3 (as used by Rhodia/Chalampé), N2O requires a separate downstream treatment.206 Regardless of the procedure, the process remains substoichiometric, with regard to HNO3. Until recently (the 1990s), nitrous oxide was greatly lost and adipic acid production was held accountable for ∼10% of global anthropogenic nitrous oxide emissions.204 Global N2O emissions largely stem from 15

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Figure 10. Route developed by Solutia, Inc. to recycle nitrogen oxides.

50% AA selectivity were obtained.223 Various catalytic systems for the conversion of olone into adipic acid by air oxidation have been attempted, such as Co(III)/Co(II), Mn(III)/Mn(II), V(V), or Cu(II).11 In the presence of acetic acid and water, Cu(NO3)2 gives 95% conversion of ketone with 72% AA yield at 110 °C after 8 h.11 Asahi has reported an adipic acid selectivity of 77% in pure oxygen at complete cyclohexanone conversion when using Mn(OAc)2/Co(OAc)2 as a catalyst and acetic acid as a solvent at 70 °C under 1 atm pure O2.224 Although this reaction can be used as an industrial process for adipic acid production, the presence of corrosive acetic acid complicates the process and negatively affects the product quality. It should also be considered that Asahi is a producer of cyclohexanol (from cyclohexene), rather than cyclohexanone. Using air instead of HNO3 presents several advantages: (i) cost, (ii) no corrosion, (iii) no recoverage of nitrogen oxides, and (iv) no N2O formation. While no NOx is produced, the need for acetic acid as a solvent leads to other severe corrosion problems which should be avoided. Nitric acid oxidation remediates the selectivity and quality issues but introduces other problems (cfr. Section 5.4). Despite advances in air oxidation for the conversion of hydrocarbon feedstocks to bulk chemicals,225 the last step of adipic acid synthesis has never been replaced. The low yields for the all-air oxidation route to adipic acid, along with the higher investment for conversion of KA mixture in the adipic acid step and acetic acid recovery have led to complete displacement of this technology. Not surprisingly, many alternative routes to adipic acid that eliminate the need to use HNO3 as an oxidant have been attempted (cfr. Sections 4 and 12).

unintended consequence of a lack of any economic incentive to invest in new adipic acid technology. Nitrous oxide has been used for LPO of cyclohexene to cyclohexanone (99% conversion),60,62 as well as for single-step oxidative hydroxylation of benzene to phenol.59,62,215−218 An innovative and sustainable process option (Boreskov Institute of Catalysis (BIC)/Solutia, Inc. technology) to adipic acidnot advanced beyond the pilot stageis reuse of N2O from nitric acid oxidation of olone as a primary oxidant for the conversion of benzene to phenol (AlphOx process)59,62 (see Figure 10). By recycling waste N2O produced in the industrial process, one of the more potent waste streams from the traditional adipic acid manufacture is potentially eliminated, thus turning nitric acid oxidation of olone into a greener process. However, all of the waste associated with nitric acid production still applies. Obviously, integration of adipic acid production with a downstream process that uses N2O as a reactant is feasible, provided that the amount of N2O co-produced fits the requirements. This is often a difficult match industrially and is not met by Solutia.

6. TWO-STEP AIR OXIDATION OF CYCLOHEXANE TO ADIPIC ACID Before the advent of nitric acid oxidation of cyclohexanol/ cyclohexanone, direct air oxidation of cyclohexane to adipic acid219 and to olone164 were described by Loder (to Du Pont) in 1940. Subsequently, several processes were described involving multistep air (oxygen) oxidation of cyclohexane to adipic acid via olone. British Patent No. 941,662 (to Scientific Design Co., Inc.)220 and British/U.S. Patents Nos. 956,779/3,234,271 (to Halcon International, Inc.)221,222 disclose LPO of cyclohexane to adipic acid in a two-step air oxidation process under mild conditions, consisting of (i) oxidation to olone (one/ol ≥1) at 8% to 10% conversion per pass at 125−135 °C and 10 atm using Co naphthenate or acetate as catalyst, and recycling of unreacted cyclohexane; and (ii) further oxidation in acetic acid over a metal (M = Co, Mn) salt at 85° to 90 °C at 7 atm yielding 50%−70% adipic acid after 6 h. A Rohm & Haas plant in the United States, based on autoxidation technology, was abandoned, because of poor product quality.7 Air oxidation for the second step has not found favor, because of a loss of selectivity and the proliferation of secondary products.2 All commercial facilities using this route have long been shut down. Another possibility is to subject olone first to dehydrogenation to form cyclohexanone, which is then catalytically oxidized to AA in air. The information on this process is rather limited, cfr. Section 10. It is possible to replace HNO3 with oxygen to perform the oxidation of olone with catalytic amounts of Co(OAc)2 and Mn(OAc)2 at 70−80 °C using acetic acid, but reported adipic acid yields are lower, compared to the conventional process. In this process, 100% conversion of KA oil would be auspicable in order to avoid recycling. Using Keggin polyoxometalates and acetic acid in air oxidation of cyclohexanone, 99% conversion and

7. DIRECT ONE-STEP CONVERSION OF CYCLOHEXANE TO ADIPIC ACID It is obviously highly desirable to simplify adipic acid manufacturing from cyclohexane by one-step oxidation, using either chemical oxidizing agents such as nitric acid,226 mixtures of nitric acid and nitrogen dioxide or tetroxide,196,227 or, more preferably, chemical oxidizing agents such as molecular oxygen.24 Early endeavors to produce adipic acid from petroleum feedstock and saturated hydrocarbons have resulted in low yields ( 130 °C. In the beginning of the 1970s, only a few methods had succeeded in achieving reasonable yields, even though they have not been adopted industrially. Asahi has reported 88% cyclohexane conversion and 73% adipic acid yield (with glutaric acid as the main byproduct) at 90 °C and 30 atm O2 using a Co(OAc)2 catalyst and acetaldehyde promoter in acetic acid solvent.28,32 Adipic acid formation over Co(OAc)2 is drastically increased by (i) a low reaction temperature (80−90 °C); (ii) cobalt(III); and (iii) a high catalyst concentration (102−103 times exceeding that used in conventional autoxidations). The reaction rate expression indicates that the cyclohexane oxidation reaction over Co(OAc)2 is not a classical free-radical oxidation wherein peroxy and alkoxy radicals serve as a hydrogen abstracting substance, but a catalytic reaction involving an electron transfer mechanism with Co(III) and Co(II) ions.28,31 Control of the reaction rate is achieved by varying the oxidant consumption rate.268 Oxidation of cyclohexane by Co(OAc)3 exhibits an activation energy of ∼10 kcal/mol. No reaction occurs in the absence of a solvent (acetic acid).28 U.S. Patent No. 4,263,453 (to Gulf Research & Development) claims that cyclohexane oxidation at 95 °C and 20 atm in acetic acid, using a high concentration of Co(OAc)2 with butanone-2 and water as additives, improves conversion to 94% and adipic 19

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Table 2. Aerobic Oxygenation of Cyclohexane with 1 atm O2 catalyst

conversion (%)

time (h)

total turnover number, TONa

turnover frequency, TOFb(h−1)

reference

α-SiW11{Fe (OH2)}O395− c γ-SiW10{MnIII(OH2)}2O386− γ-SiW10{FeIII(OH2)}2O386−

2.42 6.4 1.1 65 36 45

4.17 96 96 20 9 6

1159 789 135 130 121 90

439 8.2 1.4 6.5 13 15

169 167 168 277 278 279

III

NHPI/Mn(acac)2 Fe(II)-picolinic acid NHPI/Co(acac)2 a

TON = products (mmol)/catalyst (mmol). bTOF = number of moles of substrate converted per mole of catalyst per hour. cMicrowave-assisted.

substituted molecular sieves, metal (alloy) catalysts, etc. Apart from the inherent technical advantage of catalyst heterogeneity, activity, selectivity and stability of such heterogeneous autoxidation catalysts all require specific development. Shape-selective redox molecular sieves such as CoAlPOs are effective selective oxidation catalysts for a variety of hydrocarbons in oxygen atmosphere at moderate temperature. However, the conversion of cyclohexane often yields olone as the main reaction product with only minor adipic acid formation.10 CoAlPO catalysts for cyclohexane oxidation have been evaluated in the absence and presence of acetic acid as a solvent at moderate O2 partial pressure (ca. 10 atm) and rather low temperatures (130 °C).83,177,280−283 In the presence of acetic acid the intermediate cyclohexanol is converted to the more stable cyclohexylacetate.282,284 Hence, such systems suffer from the inherent disadvantages of requiring acetic acid solvent and a separate hydrolysis step. CoAlPO-5 and CoAlPO-11 molecular sieves show higher selectivity to monooxygenates and improved stability over classical homogeneous cobalt naphthenate as catalysts for low-conversion autoxidation of cyclohexane.83 A critical factor for obtaining active and truly heterogeneous autoxidation catalysts is site isolation of cobalt by its framework substitution. The amount of dissolved cobalt is much lower than in homogeneous catalysts. Heterogenization of cobalt catalysis dramatically affects the product distribution. With a homogeneous catalyst, CHHP is only a minor product; with CoAlPO-5, it is a main product. As a result of the immobilization of Co ions at framework positions, CoAlPO catalysts are quite stable. This contrasts with the deactivation of cobalt naphthenate by formation of precipitates. CoAlPO catalysts can be reused without loss of activity. On the other hand, a cobalt-exchanged zeolite (CoNaY), in which the cobalt ions are very mobile, is catalytically inactive and even strongly inhibits reaction under autoxidation conditions. When acetic acid is added as a promoter,281,282,284 CoAlPO acts as an inhibitor rather than as a catalyst. This is explained by acid leaching of cobalt from its isolated sites and reprecipitation at the surface. In this way, the essential advantage of site isolation is lost. The same holds for catalysts with high cobalt content. CoAlPO-36 (pore aperture 0.65 nm × 0.75 nm) is superior to CoAlPO-5, CoAlPO-11, and CoAPO-18 in cyclohexane oxidation in air at moderate temperature and pressure.177 Framework-substituted cobalt is responsible for the catalytic activity; there is a correlation between the degree of activity and the amount of oxidizable cobalt in the framework. The efficiency of air oxidation over CoAlPO-5 depends on the Co(II)/Co(III) ratio.285 CoAlPO-18, with the highest fraction of oxidizable cobalt, does not show any activity, primarily because of its smaller pores (0.38 nm).177 Conversion and selectivity for adipic acid over CoAlPOs are rather low.177,282 Using CoAlPO-36, a conversion of 9.6% and a turnover number (TON; moles of cyclohexane converted per mole of cobalt in the catalyst) of 166 were reported after 16 h in dry air (15 atm) at 130 °C; the

such as those used commercially in the selective oxidation of cyclohexane, many oligomers of the cobalt salt are present. Shen et al.34 have reported LPO of cyclohexane to dibasic acids with an immobilized cobalt catalyst using glacial acetic acid as a solvent at 85−105 °C and 5−20 atm with cyclohexanone as a co-reactant. μ3-Oxo-bridged Co/Mn cluster complexes, CoMn2(O), exhibit high catalytic activity and selectivity for the aerial oxidation, in the homogeneous liquid phase, of I, II, and III to adipic acid by a non-HNO3 route.274 The cluster complexes are superior to the individual Co and Mn acetates. Yields of adipic acid are comparable to those in the current process using HNO3. Selective oxidation of III achieves high conversions (85−99%) and AA selectivity (76−82%) at 90 °C and 37 atm air pressure, and high TOF (up to 7500 h−1). Adipic acid is also the major product of the oxidation of KA oil. However, succinic acid and glutaric acid are the major products in cyclohexane oxidation. For economical reasons, and also to facilitate purification of the products obtained, it is preferred to work with the lowest possible catalyst concentration (but considering the aforementioned restrictions). In this respect, manganese is an interesting catalyst for cyclohexane oxidation processes. Rhodia262 has disclosed direct LPO of I to adipic acid in oxygen or an oxygencontaining gas environment using manganese and chromium acetates dissolved in the reaction medium (acetic acid) at 80−140 °C. Using a soluble Mn/Cr catalyst in an acetic acid solvent results in a 15.3% conversion of I and a molar ratio of adipic acid/total acids of 77.6% after 170 min at 105 °C and an air pressure of 100 atm. Another Rhodia disclosure claims a lipophilic monocarboxylic (C7−C20) oxidation solvent comprising a step of extracting the dicarboxylic acid, which consists of performing liquid-phase extraction of adipic acid using water.265 According to RPC, Inc. (Atlanta, GA)/Fluor Daniel, their optimized one-step air oxidation technology to adipic acid (with glutaric acid and succinic acid as the main byproducts), essentially still based on Co(OAc)2·4H2O and acetic acid,96,261,268,275,276 reduces capital costs by 30% and operating costs by 20%, compared to the classical DuPont route. Although homogeneous cobalt has been found to be an efficient catalyst under moderate reaction conditions, selectivity and yield of adipic acid alone are not sufficient for commercialization. A formidable challenge is the development of new catalysts performing efficiently with 1 atm O2. As shown in Table 2, transition metal-substituted polyoxometalates (POM) display high activity (to KA oil). With microwave assistance, iron polyoxotungstates catalyze oxygenation of cyclohexane to KA oil with 90%−95% selectivity and very high turnover frequencies (>400 h−1) and turnover numbers (>1000). More recently, oxidation of cyclohexane with various oxidants in the liquid phase and heterogeneous or heterogenized catalysts has actively been pursued using transition metal oxides and complexes incorporated in inorganic matrices, such as active carbon, silica, alumina, zeolites, cation-exchanged or framework 20

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system. Encapsulation of complexes, mainly phthalocyanines, in faujasite-type zeolites minimizes leaching of heterogeneous catalysts. The stability of the metal centers toward leaching may also be improved by silylation. Nanostructured amorphous metals/alloys are superior (slurry) catalysts for aerobic oxidation of cyclohexane under mild conditions. With a sonochemically produced nanostructured cobalt catalyst (20 nm particle size) at room temperature and an O2 partial pressure of 40 atm, a selectivity of 80% to II/III (in 5:1 ratio) was realized at cyclohexane conversions up to 40%.180 Isobutyraldehyde was used as a coreductant, and a catalytic amount of acetic acid was also added. However, also, in this case, the rates are too low for commercial exploitation. It is finally noticed that aerobic oxidation of hydrocarbons (to II and III only) has also been performed using Ru, Fe, and Cu catalysts in the presence of acetaldehyde. Oxidation of I (with 4.5% conversion) over the Cu(OH)2/aldehyde system yields 38% II and 58% III after 24 h in 1 atm O2 at 20 °C.172 A combination of copper salt and a crown ether is even more effective: using CuCl2/18-crown-6 gives II/III with high TON (16 000) and reasonably high yields (71%) with a one/ol ratio of 6.1.173 A simple and efficient system for hydrocarbon oxidation, without using crown ethers, consists of copper complexes derived from copper salts and acetonitrile. Cu(OAc) 2/ acetaldehyde in acetonitrile converts I with molecular oxygen (8 atm air) in autoclave at 70 °C to olone (one/ol = 54:41) with TON = 27 000.172 Despite extensive research, the development of one-step technology for air oxidation of cyclohexane to adipic acid under mild conditions has been less than satisfactory. Many proposed catalytic systems only promote formation of monooxygenates from cyclohexane, few yield the diacid with satisfactory yield (Table 3) and none has been commercialized so far as a replacement for the DuPont two-step air/nitric acid process. Failures to achieve the goal are to be attributed to a variety of reasons: low selectivity and per-pass conversion (lower than in the conventional approach), product distribution, co-product formation (glutaric and succinic acids), reaction conditions (T, p), need for solvents, long process times, catalyst stability, leaching, catalyst recycling, cost, etc. In view of the high corrosivity of acetic acid, the high energy demand for solvent recycle and product workup and lower product quality it is not entirely surprising, given that most one-step adipic acid processes have not been implemented at a commercial scale. No single proposed reaction pathway simultaneously addresses the problems of petrochemical origin (cyclohexane), safety, toxic starting materials or reagents, absence of organic solvents, conversion and selectivity, generation of (environmentally incompatible) byproducts, use of forcing reaction conditions, and costs in an industrially satisfactory manner. Most studies on the direct adipic acid synthesis have only been conducted in a batch mode. Once a reaction has taken place according to the direct synthesis, a complex mixture of two liquid phases (a polar and a nonpolar phase) is present at ambient temperature, along with a solid phase mainly consisting of adipic acid. The two phases are to be recycled to the reactor, partially or totally, with or without further treatment. Bonnet et al. (to Rhodia) described a solventless, continuous process for producing adipic acid in one step from cyclohexane (cfr. Section 7.3). 7.1.1. NHD-Based Innovative Aerobic Oxidation Technology. Liquid-phase aerobic oxidation (or autoxidation) is extensively practiced in the chemical industry, although its efficiency is

principle products were II and III and small quantities of adipic acid (12 mol %).177 The more active MnAlPO-36 catalyst achieved 13% conversion and TON of 233 after 24 h at 130 °C and 15 atm air pressure, yielding 15% II, 47% III, and 33% adipic acid.283 The addition of III as a co-reactant increases the rate of cyclohexane oxidation and the selectivity to adipic acid.280 CrAlPO-5 is a stable, recyclable solid catalyst for selective LPO of both I and II to III using O2 or t-BHP as the oxidant.286 The size and shape of the pores of solid molecular sieves must be carefully selected, to ensure that alkane oxidation takes place in a shape-selective manner in the spatially restricted environment of the catalytically active sites. Thomas et al. have used various MeAlPO molecular sieve catalysts for the autoxidation of cyclohexane in O2, without solvents: CoIII(MnIII)AlPO-36,177 FeIII(MnIII)AlPO-5,240 and FeIIIAlPO-31.53,257 Several of such appropriately designed microporous aluminophosphate catalysts offer a means of yielding adipic acid in air oxidation of cyclohexane84 or n-hexane.85 A relationship has been established between a constrained environment and high selectivity for adipic acid formation. FeAlPO-31 (4 at. % Fe; pore diameter = 0.54 nm) gave 65% selectivity to adipic acid from cyclohexane (6.6% conversion) in dry air at 100 °C after 24 h.84,287 In the same conditions, FeAlPO-5 mainly produces II (50.5%). The selectivity patterns of FeAlPO-31 and FeAlPO-5 differ essentially as a consequence of differences in pore diameter. Costantini et al. (to Rhône-Poulenc)288 have disclosed LPO of cyclohexane (or II and III) to adipic acid using molecular oxygen in a solvent (usually acetic acid) in the presence of a heterogeneous catalyst comprising Mn atoms incorporated in a molecular sieve (metallosilicates, AlPO, SAPO, silicalite) at 80−140 °C and 1−200 atm. In recent years, some other redox molecular sieves as catalyst were reported for the air oxidation of I (mainly to olone), including Ce-AlPO-5,178 Co-ZSM-5,179 Cr-MCM-41, and Cr-MCM-48.289 The conversion over CrAlPO-5 is higher than over the mesoporous catalysts. In these catalyst systems, the conversion of I is low (90%. Au-SBA-15 is very efficient in this reaction with a conversion of I of 18% at 150 °C and 10 atm O2 and a total KA oil selectivity of 93% after 8 h.290 Bi-SBA-15 is an excellent and considerably less-expensive catalyst for the solventless selective oxidation with O2 of I (in 16.9% conversion) to olone (in 93% yield).176 The activity of heterogeneous oxidation catalysts such as oxides or metal cations and complexes incorporated in organic matrices, such as SiO2, Al2O3, active carbon, zeolites, or aluminophosphates, is greatly determined by the choice of the solvent, which determines the polarity of the medium. Reaction conditions are generally milder and the metal complexes more stable than under homogeneous conditions. Because of the high polarity of the products and the hydrophilic character of the zeolites, solvents exert a major influence on product distribution; temperature and concentration effects are considerable.93,291 Various microporous metallosilicates containing different transition metals suffer from leaching of the metal during the reaction. Metal aluminophosphates (MeAlPO, Me = Cr, Co, Fe, Cu, V, Mn) are relatively stable under the reaction conditions after repeated use, provided that the overall conversion in the oxidations is kept relatively low (ca. 10%−12%). This level of conversion avoids leaching by the polar reaction products. Large amounts of (di)carboxylic acids cause metal leaching from the catalysts. The activity of these systems is also greatly affected by the choice of a solvent, which determines the polarity of the 21

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Table 3. Representative One-Step Autoxidations of Cyclohexane to Adipic Acida catalyst system Co(OAc)2 Co(OAc)2 Co(OAc)2 Co(OAc)2, Fe(OAc)2 NHPI/Mn(acac)2, Fe(acac)2, RuCl3 FeIII APO-31 FeIII APO-31 FeCl16Pc-NaX FeT(o-Cl) PP Mn(OAc)2, Co(OAc)2/t-BBA

additive

conditions

solvent

cyclohexane conversion (%)

adipic acid selectivity (%)

reference(s)

cyclohexanone 2-butanone/H2O cyclohexanone cyclohexanone

O2 (30 atm), 90 °C O2 (20 atm), 95 °C O2 (20 atm), 100 °C O2 (20 atm), 95 °C O2 (1 atm), 100 °C

acetic acid acetic acid acetic acid acetic acid acetic acid

88 94 85 71−82 90

73 80.6 77 71−77 84

28 269 37 271 259, 277, 292

air, 100 °C APB (water), 110 °C APB (water), 110 °C O2 air (20 atm), 130 °C

no no no no no

n.g. 88.6 73.7 n.g. 10.4c

65 81.2 55.0 21.4b >70.7

287 53, 257 257 293 265,294

cyclohexanone

a

Abbreviations used in this table: acac, acetylacetonate; APB, acetylperoxyborate; NHPI, N-hydroxyphthalimide; Pc, phthalocyanine; t-BBA, tbutylbenzoic acid; T(o-Cl) PP, meso-tetra(o-chlorophenyl)porphyrin. bAdipic acid yield. cPer pass.

not necessarily high.30,63 Homogeneous catalytic oxidations of cyclohexane using Co or Mn salts present several problems (e.g., oxidative attack on the C−H bonds is slow and requires drastic reaction conditions (20−30 atm)). To overcome these limitations, Ishii et al. developed an efficient catalytic system consisting of moderately inexpensive, nontoxic N-hydroxyphthalimide (NHPI) combined with metal mediators.259,279,292 The use of NHPI with atmospheric oxygen under mild conditions is a considerable improvement. By employing NHPI as an in situ N-oxyl phthalimide (PINO) radical producing catalytically active homogeneous species, novel aerobic oxidation of alkanes, which surpasses the conventional autoxidations in conversion and selectivity, has been achieved under mild conditions and in an environmentally more friendly manner. Using NHPI and Co(OAc)2 as catalysts cyclohexane in benzonitrile yields K/A oil (with a K/A ratio of 10:1) with a per-pass conversion of 12% and 93% selectivity after 16 h in O2 at 75 °C.259 Oxidation by the NHPI/Co(acac)2 system in acetonitrile under mild conditions provides an alternative direct route to cyclohexanone.279 NHPI/ Co/MCBA (m-chlorobenzoic acid) can be used for aerobic oxidation of alcohols (83% yield for III).295 Despite early attempts28 and much catalyst research (see Table 3), one-pot air-based oxidation of cyclohexane to adipic acid still presents a considerable challenge. An important application of the NHPI-catalyzed aerobic oxidation of alkanes is the direct conversion of I in acetic acid solution to adipic acid at 80−100 °C and 1 atm using NHPI in the presence of a transition metal, as Mn(acac)2 and/or Co(OAc)2.259,277,292,295 Conversion of 90% and adipic acid yield of 76% (after 6 h) were reached. Surprisingly, cyclohexanol is hardly formed. The one-step radical-chain oxidation of cyclohexane with NHPI (Figure 12) is the first practical environmentally friendly process for the production of adipic acid in which nitric acid is not used as the oxidant. Daicel Chemical Industries, Ltd. (Osaka) has evaluated the process at a pilot-scale level (30 t/yr) for further commercial application,296 but it apparently gone not easy to be realized. The process is very complex. Drawbacks of Ishii’s radical catalyst are high cost and the requirement for two co-catalysts and an acetic acid solvent, making this process less benign. Another disadvantage of the process is the decomposition of NHPI to phthalimide, which cannot easily be recycled to NHPI. N-hydroxy derivatives (NHDs) are key organocatalysts for selective free-radical aerobic oxidation of organic compounds such as cyclohexane.297 The most commonly employed NHD catalysts are 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), NHPI, and N-hydroxysaccharin (NHS). The latter is capable

Figure 12. One-step radical-chain oxidation of cyclohexane to adipic acid with NHPI as catalyst. Reproduced with permission from ref 295. (Copyright 2001, Wiley−VCH GmbH, Weinheim, Germany.)

of activating a source of oxygen and promoting oxidation of an alkane at even milder reaction temperatures (20−100 °C) at ambient pressure than NHPI.298,299 Table S5 in the Supporting Information shows the performance of various NHD catalysts and co-catalysts in the aerobic oxidation of cyclohexane. These processes are usually carried out in polar solvents, because of the low solubility of NHPI in nonpolar solvents. The use of 4-lauryloxycarbonyl-NHPI allows the possibility to perform the reaction solvent free.300 In view of metal toxicity, metal-free C−H activation is the obvious next target. Xu et al. have reported a biomimetic nonmetal system (NHPI/anthraquinone/HY) with low conversion (15%) and selectivity for cyclohexane oxidation to II and III at 100 °C.301 The same research group has also reported a highly efficient and metal-free aerobic oxidation of cyclohexane by an o-phenanthroline mediated NHPI system302 (see Table S5 in the Supporting Information). 7.1.2. Oxidation of Cyclohexane under Supercritical Conditions. In oxidation reactions, CO2 and H2O are the only solvents that can be used without the formation of any solvent byproducts. Supercritical carbon dioxide (sc-CO 2) is an attractive solvent for catalytic oxidations and might be a suitable replacement for organic solvents, since it combines such benefits as nontoxicity, nonflammability, and the elimination of solvent residues and waste. Conducting chemical reactions under supercritical conditions affords opportunities to manipulate the 22

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7.2. Biomimetic Oxidation of Cyclohexane. Enzymes are capable of oxidizing cyclohexane to cyclohexanol at room temperature with high selectivity.309 Cytochrome P-450 catalyzes alkane hydroxylation either by molecular dioxygen (O2) in the presence of the coenzyme NADPH, or by oxygen atom donors such as H2O2, peracids, PhIO, etc. Simulation of the function of cytochrome P-450 with transition-metal catalysts such as ruthenium, iron and copper has resulted in the discovery of biomimetic catalytic reactions for selective and highly efficient oxidations of nonactivated hydrocarbons using peroxides and molecular oxygen/aldehyde under mild conditions.310 The biomimetic oxygenation model (mainly of academic interest) encompasses three elementary factors: a redox center, a oneelectron transfer chain, and multiple binding sites similar to the surrounding protein environment of the enzyme. The ability of cytochrome P-450 to activate oxygen for hydrocarbon oxidation with high product selectivity at high conversion has motivated many studies involving metalloporphyrin catalysts.311−313 The active species in these systems is presumably a highly reactive oxometallo(V)porphyrin. Iron porphyrins are efficient biomimetic catalysts for alkane hydroxylation, using either various oxygen atom donors or O2 itself in the presence of a reducing agent, according to the following monooxygenase reaction:

reaction environment (solvent properties) by simply adjusting the reaction conditions (T, p). Furthermore, homogeneously catalyzed reactions conducted in supercritical fluids (SCFs) exhibit increased selectivities, as well as accelerated mass transfer and reaction rates, as a consequence of their special physical properties, and allow facile separation of reagents, catalysts, and products by selective precipitation. Reaction rate, selectivity, and reaction equilibrium can be tuned by pressure or by a small amount of co-solvent. With a sc-CO2 medium, some of the diffusion-controlled heterogeneous reactions can be carried out homogeneously by dissolving both the reactant and catalyst in a single fluid phase. By conducting the reaction in pure CO2, the solubility of oxygen is excellent (much better than in water), yet the solubility of the oxidation catalyst is relatively poor at moderate pressures (lower than in conventional organic solvents). Moreover, the low dielectric constant of CO2 provides a suboptimal performance of the catalyst. Consequently, it is often being preferred to use a mixture of CO2 and an organic solvent to ensure a polar environment, as well as high oxygen and catalyst solubility. An advantage of the poor solvent power of CO2 is reduced metal leaching from catalysts. There is extensive literature on the use of CO2 as an environmentally benign solvent for oxidation of cyclohexane.303−306 In these studies, knowledge of the phase behavior is essential toward understanding the effects of temperature and pressure. Srinivas et al.304,305 examined the effects of temperature, pressure, and solvent on the cyclohexane oxidation to olone as the main product in an sc-CO2 medium for homogenizing the initial reaction mixture at T > 127 °C in the absence of added catalyst. The observed low conversions were ascribed to the dilution of the reactants by CO2 (as compared to liquid-phase oxidation) but the lack of catalyst could be invoked equally well. Enhanced conversions and rates have been observed with increasing T, p in the critical region. Cyclohexanone formation increases with an increase in pressure. Aerobic oxidation of I to yield II and III in the presence of a soluble Fe-porphyrin catalyst and acetaldehyde has been efficiently enhanced in pressurized CO2.303 The reaction rate reaches a maximum value at the critical pressure. Hou et al.306 have studied the phase behavior and critical parameters for the selective oxidation of I to olone in compressed CO2, using oxygen as an oxidant. This reaction was conducted with/ without co-solvent in both the two-phase and supercritical region, using a MnAlPO-5 molecular sieve catalyst at 125.2 °C. In the two-phase regime, which more easily allows for the use of air as the primary oxide, the reaction rate was higher, because of enhancements in the mass-transfer rate and moderate operating pressure. The use of air in a single-phase mixture would require a higher pressure and needs extra separation steps to prevent nitrogen buildup (and alteration of the phase behavior) in the system. The addition of a small amount (0.2 mol %) of butyric acid co-solvent to sc-CO2 significantly enhances the conversion and also the selectivity changes considerably. The byproducts of the reaction in sc-CO2 with/without co-solvent are much less than in the case of reactions in liquid solvents or without solvents. Oxidation of I to olone in a continuous process in sc-CO2, using supported metal (M = Fe, Co, Mn) oxide catalysts and zeolite-immobilized metal complexes gives substantially lower yields than the conventional batch process.307 Since new largescale processes for adipic acid are likely to require innovations in several areas, not just an alternative solvent use, it is unlikely that simply addition of CO2 to a conventional process could provide substantial green and economic benefits. Industrial CO2-based oxidation processes have not yet been realized.308

RH + O2 + 2e− + 2H+ → ROH + H 2O

(13)

Most efforts to devise biomimetic catalysts have been devoted to homogeneous metalloporphyrin-based catalysts, following up pioneering work by Groves et al.,314 who used Fe(TPP)Cl (TPP = 5,10,15,20-tetraphenylporphyrin)−PhIO (iodosylbenzene) for the oxidation of I to II. Although the product was obtained only in 8% yield, the lack of further oxidation to III is significant. Groves’ catalyst is rapidly oxidatively destroyed under the reaction conditions. Shape selectivity for the hydroxylation of cyclohexane with iodosobenzene as oxidant has been demonstrated by using various sterically hindered manganese and iron porphyrins such as MnTTPPP(OAc) (TTPPP = 5,10,15,20-tetrakis(2′,4′,6′triphenylphenyl)-porphyrinato).315 The more robust Mn(TDCPP)(Cl) (TDCPP = tetra-2,6-dichlorophenylporphyrin)/ imidazole-H2O2 system achieves hydroxylation of cyclohexane with great efficiency (conversion, 54%; ol/one (3:1) yield, 40%) within 2 h at 20 °C without significant degradation of the catalyst system.316 Low-valent ruthenium-catalyzed cytochrome P-450 type oxidation of alkanes with t-BHP or peracids gives the corresponding ketones and alcohols in good yields.255,256 Typically, Ru/C catalyzed oxidation of cyclohexane with CH3CO3H at room temperature gives 67% III, along with a small amount of II (1%) efficiently with 62% cyclohexane conversion. Ru, Fe, and Cu catalysts (even Fe(0)) are also effective for the oxidation of cyclohexane with O2 in the presence of acetaldehyde:310

These aerobic oxidations are rationalized as follows: RCHO + O2 ⎯⎯⎯⎯→ RCO3H

(15)

Mn + RCO3H → Mn + 2O + RCO2 H

(16)

Mn + 2O + RH → Mn + ROH

(17)

M cat

23

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In analogy, also RuII, MnIII, and CoII metallorporphyrins bearing meso-pentafluorophenyl groups are highly efficient catalysts for the aerobic oxidation of cyclohexane with acetaldehyde and give cyclohexanol/one in 62%−75% yield with high TON (220 mol product/mol catalyst).310,317 Although such systems require stoichiometric reducing agents, ultraviolet (UV)-light-assisted metalloporphyrin-catalyzed oxidation of cyclohexane runs without a sacrifical reagent243 (cfr. Section 7.4). There are only few reports using metalloporphyrins for the direct oxidation of cyclohexane to adipic acid.274,318 Ratnasamy et al.318 have disclosed the preparation of adipic acid from I with molecular oxygen in the presence of porphyrin catalysts and acetonitrile as a solvent; low cyclohexane conversions (6%− 16%) and adipic acid yields (2.5%−7%) exclude industrialization of this procedure. In a further advancement, a product yield of 21.4% has been reported for direct oxidation of I to adipic acid catalyzed by meso-tetra (o-chlorophenyl) iron porphyrin, FeT(o-Cl)PP, in the presence of molecular oxygen (25 atm) in a solvent- and additive-free reaction system after 8 h at 140 °C.293 The TON value observed, ∼24 584, is the highest among those reported for direct aerial oxidation of cyclohexane to diacids (compare values of 293, 190, 594, and 789 for a Co/Mn cluster complex, 274 a FeAlPO-5 zeolite, 319 NHPI/Mn(acac) 2 / Co(OAc)2,277 and {γ-SiW10[Mn(OH)2]2O38}6−,167 respectively. Mono-oxygenases are characterized by site isolation of the active center. Anchoring metalloporphyrins onto a rigid support achieves site isolation and provides catalysts, which imitate the high specificity and activity of the cytochrome P-450 enzyme. In cyclohexane oxidation, Fe(III) and Mn(II)(TMP)-Y (TMP = tetramethylporphyrin) form olone by H2O2 at 50 °C, whereas Fe(III)-Y and Cu(II)-exchanged zeolite Y are completely inactive.320 TMP enters the NaY supercages; the most commonly used ligand of model compounds of cytochrome P-450, TPP, is too large to be accommodated in NaY. The activity enhancement by TMP ligation suggests that the active species is an oxygen adduct of the exchanged metal ion, which has been acknowledged as the active center of biomimetic systems of cytochrome P-450. Supported metalloporphyrin catalysts are also more practical for preparative applications.313 Oxidation of cyclohexane to adipic acid with molecular oxygen, peroxides or iodosobenzene over zeolite-encapsulated metal phthalocyanine (Pc) complexes has been reported extensively.78,79,93,257,291,318,321−323 The parallels between such catalysts and biological oxidation systems have been indicated. Substituents of the phtalocyanine nucleus, the nature and mode of addition of the oxidant, and the solvent strongly influence the results. It has been shown that the concept of using a zeolite as a support for partial oxidation catalyst systems is viable, and that it leads to increased yields and selectivities as a result of zeolite internal structure.321 While a low turnover was reported for FePcY/iodosobenzene (low solubility)321 higher turnover values were obtained with the industrially more appropriate oxygen source t-BHP.78,291 However, in the conditions used by Jacobs et al.291 the FePcY/t-BHP system is characterized by very slow oxidation rates (35% conversion of I at 60 °C after 300 h; appreciable AA formation only after 600 h). Site isolation of the active center created by bonding to a support prevents the formation of less reactive dimers (i.e., deactivation). In fact, major drawbacks of homogeneous metal phtalocyanine catalysts in industrial oxidation processes are aggregation in solution (catalyst deactivation) and low oxidative stability, because of high oxidizability of the Pc nucleus. Both disadvantages can be overcome by substitution of the

phthalocyanine hydrogen atoms with electron-withdrawing groups (halogen, nitro, cyano). Halo- and nitro-substituted metal (Me = Fe, Co, Cu, Cr, Mn) phthalocyanines in zeolites X and Y were used as catalysts for oxidation of I to adipic acid under ambient conditions, using both molecular oxygen and alkyl peroxides as oxidants.318 Corma et al.322 reported the oxidation of cyclohexane in acetonitrile over H2O2 and t-BHP using Cu2+phthalocyanine and Co2+-perfluorophthalocyanine incorporated in Y-faujasite and mesoporous MCM-41, respectively. Butylhydroperoxide was found as a more convenient oxidizing agent than hydrogen peroxide. Autoxidation of cyclohexane with copper phthalocyanines encapsulated in zeolites (CuCl16PcNaX, Pc = phthalocyanine) at 70 °C using molecular oxygen and t-BHP as the oxidants with various solvents leads to maximum cyclohexane conversion, adipic acid yield, and turnover frequency (TOF) of 12.7%, 41.1%, and 143.2 h−1 (for O2, methanol) and 27.7%, 22.7%, and 312.1 h−1 (for t-BHP, acetic acid) after 8 h.93 Solvents exert a major influence on the product distribution with II, III, and AA as the main products. With acetonitrile as the solvent succinic and glutaric acid are not formed as byproducts. Encapsulated materials show much higher catalytic efficiencies than neat complexes. Cyclohexane has also been converted to II, III, and adipic acid using FeCl16Pc-NaX in oxygen or air with APB or PAA as an oxidant, yielding as much as 53% AA at 73.7% conversion after 16 h at 110 °C.257 Intrazeolite (NaX) hexadecafluorophthalocyanine (F16Pc) complexes of Ru(II) are stable in conditions of cyclohexane oxidation over t-BHP.79 Conversion of cyclohexane with FePcY/ t-BHP in acetone and dichloromethane at room temperature is relatively high (up to 25%) with efficiencies of 10%−40% (depending on operating mode) and high selectivities to III (95%).78 Cyclohexanol is converted up to 70% with 70% efficiency. Activities attained by FePcY are higher than for the homogeneous FePc complexes, which are oxidatively destroyed under the reaction conditions. LPO of cyclohexane with t-BHP over zeolite-encapsulated and activated carbon-supported FePc catalysts is strongly dependent on the polarity of the solid support material.323 Phthalocyanine and porphyrin metal complexes immobilized on a carrier material were also used for the catalytic decomposition of CHHP in the conversion of cyclohexane to olone.189 Gif systems were designed to emulate the nonheme enzymatic oxidations of alkanes. Cyclohexane oxidation using Barton’s GoAggII system and H2O2 as the oxidant in an inert atmosphere gives 91% efficiency (number of moles of oxidized products per mole of peroxide consumed) and a selectivity of 94% for III and 6% for II after 10 h.233 Using Gif chemistry, saturated hydrocarbons were selectively transformed into ketones/alcohols and alkenes by application of dioxygen and t-BHP using soluble Fe(III) and Cu(II) chelates.324 Cyclohexane hThe s been converted to cyclohexane carboxylic acid using Gif chemistry with Fe(CO)5 and H2O2.74 Gif and GoAgg oxidations are not important to industry. 7.3. Solvent-Free Liquid-Phase Oxidation of Cyclohexane. The activity of a cyclohexane/catalyst/oxidant system is greatly affected by the choice of the solvent, which determines the polarity of the system. The absence of reaction solvents is advantageous for clean industrial processing. Several attempts have been made to develop solvent-free routes for cyclohexane oxidation under mild conditions.253 This generally requires substrate-soluble catalysts. Solvent-free systems for functionalization of saturated hydrocarbons, operating with cyclohexane-soluble Fe(III) and Cu(II) 24

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Figure 13. Schematic flowchart for a one-step adipic acid process from cyclohexane using a lipophilic catalyst. Reproduced with permission from ref 294. (Copyright 2006, The Royal Society of Chemistry, London.)

catalysts, or catalysts in suspension using t-BHP and O2 were first investigated by Barton et al.324 and later also by Schuchardt et al.325 The systems based on iron are more selective for oxygenated products. The presence of t-BHP as an initiator is fundamental for the reactivation of the system. In the presence of cyclohexane-soluble iron and copper catalysts, t-BHP/O2 selectively oxidizes this substrate to olone (with cyclohexene for copper catalysts). At 25 atm O2 at 70 °C for 24 h, conversions with iron catalysts are 9% (TON = 166) with an olone selectivity of 80% selectivity in adipic acid), yielding 95 g AA/L (liquid phase)/h, largely outperforms previously reported catalytic systems for this reaction, such as Co/AcOH catalysts in air or molecular oxygen (13−28 g L−1 h−1)31,32,34 (also see Table 3 and TS of ref 294). The catalytically active species of this oxidation reaction has not been identified. The process meets most of the requirements that are described in Table S1 in the Supporting Information. 25

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catalyst MnTTPPP(OAc) (TTPPP = tetrakis(triphenylphenyl) porphyrin) gave only 19% 1-hexanol.315 Yet, controlled partial oxidation of linear alkanes is easier to achieve with “sacrificial” oxidants, such as H2O2, alkyl hydroperoxides or iodosobenzene, than with O2 or air. Modest terminal selectivities were reported for alkane oxidation with H2/O2 mixtures or preformed H2O2 on zeolites modified by redox-active cations such as Fe-ZSM-5,336 whereas contradictory results were found for n-hexane reactions with H2O2 on vanadium silicalite (VS-2).244,337 For the CoAlPO-18 molecular sieve catalyst, a 7.2% conversion of n-hexane has been reported with 53.5% hexanoic acid selectivity and TON = 121.4 (moles of substrate converted per mole of cobalt in the catalyst) at 100 °C and 15 atm of air after 24 h.329 Overall conversions must be kept at a low level to avoid leaching of the transition metals by polar products (carboxylic acids). However, they are comparable to those in industrially significant oxidations of cyclohexane. The reaction rates are low, reflecting diffusional requirements. The overall performance of CoAlPO-18 is superior to that of most other inorganic catalysts for regioselective oxidation of linear alkanes, with the exception of a Pd/Fe bimetallic catalyst exposed to a gas mixture of H2 and O2 to generate H2O2 in situ.338 For alkane oxidation with O2, Herron et al. reported modest terminal selectivities using framework-substituted microporous aluminophosphates.339 Unprecedented terminal selectivities of 65.5% and 61.3%, respectively, were reported for n-hexane to hexanoic acid as the predominant product on MnAlPO-18 and CoAlPO-18 structures with small 8-ring windows (0.38 nm).329 However, these findings of regioselectivity have remained unconfirmed on catalysts with identical composition and structure.340 n-Hexane oxidation turnover rates (per redox-active Mn center) for MnAlPO-5 and MnAlPO-18 are approximately equal, because the small n-hexane reactants diffuse rapidly to contact the active sites of microporous structures. On the other hand, cyclohexane oxidation turnover rates for MnAlPO-5 exceed those of MnAlPO-18, because the small channels in the latter inhibit access of the bigger size reactants to the Mn active centers.340 The rate of n-hexane reactions with O2 is first order in hexylhydroperoxide (HHP) concentration in microporous MnAlPO-5 and MnAlPO-18 catalysts. CoAlPO-18 and MnAlPO-18 are twice as active in n-hexane oxidation as CoAlPO-11, which is effective in the low-conversion autoxidation of cyclohexane.83 Shape selectivity of n-alkane is crucial in the case of MIIIAlPO18 (MIII = Mn, Co, Fe), but not in MIIIAlPO-36 or MIIIAlPO-5. To achieve oxyfunctionalization of both (methyl) ends of n-hexane, a sufficiently large number of framework CoIII ions should ideally be present on the inner walls of CoAlPO-18, spaced at ∼7−8 Å. A stable CoIIIAlPO-18 microporous catalyst with high Co:P ratio (0.10) achieves double terminal oxyfunctionalization, thereby enabling 9.5% per-pass conversion of n-hexane into adipic acid in substantial amounts (33.6 mol % after 24 h) using air (15 atm) as an oxidant at low temperatures (100 °C).85,287,341 There is no leaching of the Co ions. Cyclohexane is too large to gain access to the MeAlPO-18 active sites. However, using acetic acid as a solvent results in considerable oxidation of both cyclohexane and n-hexane, denoting that the solvent favors both homogeneous and heterogeneous catalysis. CoIIIAlPO-34 is another benign single-site heterogeneous oxidation catalyst for the conversion of n-hexane to adipic acid in oxygen or air.257 For aerial oxidation of cyclohexane to adipic acid, FeAlPO-31 is preferred (65% product selectivity at 100 °C after 24 h).84

The organic solvent- and halide-free oxidation of cyclohexanone with 30% H2O2 (cfr. Section 11) is another example of a green method of adipic acid synthesis.330 7.4. Photocatalyzed Liquid-Phase Oxidation of Cyclohexane. Autoxidation (initiation of the reaction chain) can be accomplished by exposure to light. Photocatalysis is a lowtemperature and green alternative for the direct and selective oxidation of alkanes with molecular oxygen. Photochemical activation of hydrocarbons and photocatalytic selective oxidation of cyclohexane in particular have attracted limited attention.331−333 Room-temperature liquid-phase photocatalyzed oxidation of cyclohexane over TiO2 in an aqueous environment with O2 in the absence of H2O2 does not produce any oxygenates. In the presence of H2O2, which provides the reaction medium with a greater concentration of OH• species, II and III are obtained in 30.1% and 44.0% selectivity, respectively, together with some other products and traces of CHHP, with a total conversion of 4.24% after 2 h.331 Ring opening does not occur. Increased reaction times result in an overall increase in conversion but lower selectivities. The photoassisted selective oxidation of cyclohexane (at λ = 350−450 nm) to cyclohexanone catalyzed by FeIII(tdcpp)(OH) (tdcpp = meso-tetrakis(2,6-dichlorophenyl)porphyrin) under mild conditions (22 °C, 1 atm, 200 Torr O2) does not require consumption of a reducing agent in contrast with most iron porphyrin-based systems (cfr. Section 7.2):243

In the photoinitiated LPO of cyclooctane by oxygen or air in the presence and absence of metal complexes and a solvent reaction, mixtures are formed.333 Frei et al.332,334 have reported photocatalyzed oxidation by O2 of cyclohexane in zeolite cages of NaY (free of acid sites) induced by visible light (λ < 520 nm) at room temperature in the absence of a solvent and photosensitizer. Cyclohexanone (plus water) and its precursor, CHHP, are the exclusive products. The unprecedented complete selectivity persisted even at 40% cyclohexane conversion. The highly selective oxidation pathway differs from all other methods of homogeneous or heterogeneous catalytic oxidation of cyclohexane by O2 in which cyclohexanol is a major co-product.335 The result is also outstanding, considering the usually required low conversions in autoxidation radical chain processes. Scaleup is an unresolved challenge. Technologically, such methods are especially difficult, because strong light sources are needed, as well as solid catalysts that are transparent to light. Since the cyclohexane−oxygen system poorly absorbs light, special sensitizing additives would be required. Photocatalysis for direct and selective oxidation of cyclohexane has not yielded adipic acid.

8. DIRECT ONE-STEP CONVERSION OF N-HEXANE TO ADIPIC ACID Linear alkanes such as n-hexane are notoriously difficult to oxyfunctionalize into desirable products such as hexanoic acid or adipic acid. These saturated hydrocarbons even resist attack by strong oxidizing agents such as boiling nitric acid. Selective oxidation of both terminal methyl groups in n-alkaneswith bond strengths ∼13 kJ/mol stronger than secondary C−H bondsis a demanding task. This reaction is catalyzed by enzymes with nonheme iron active centers. However, replication of their ability did not give the expected results. The oxidation of n-hexane by iodosobenzene (PhIO) on the metalloporphyrin 26

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Figure 14. Proposed reaction pathway for the synthesis of adipic acid via the oxidation of cyclohexene with hydrogen peroxide. Reproduced with permission from ref 355. (Copyright 2008, Elsevier B.V., Amsterdam.)

(HPAs) are often used not only for the oxidation of organic substrates but also for many acid-catalyzed reactions, because they possess the dual catalytic functions of oxidizing ability and strong acidity. Direct conversion of cyclohexene to adipic acid has been pursued in particular by Japanese research groups. The use of aqueous H2O2 in the presence of tris(cetylpyridinium) 12tungstophosphatederived from a heteropoly acid combined with N-cetylpyridinium chlorideunder homogeneous conditions (t-BuOH as the solvent) for the production of carboxylic acids from alkenes is of limited practical value, since it requires very long reaction times (24 h at 80 °C) and leads to a moderate yield (70%) of adipic acid with cyclohexene.89 Oxyfunctionalization procedures, which require toxic and carcinogenic chlorinated hydrocarbon solvents, defeat the environmental advantages of H2O2 as the oxidant. Various processes based on a solvent-free system using 30 wt % H2O2 have been reported. Noyori et al.47,351,359 have first described the efficient and clean direct oxidation of cyclohexene to adipic acid in 90.3% yield under green conditions. The reaction takes place in the presence of catalytic amounts of Na2WO4·2H2O, NH2CH2PO3H2, and [(n-C8H17)3NCH3]HSO4 as an acidic phase-transfer catalyst (PTC) at 75−90 °C, requires 4.4 mol H2O2/mol AA, and generates water as the only byproduct. Slight modifications to the procedure have been proposed.360−362 While the direct conversion of cyclohexene to adipic acid by H2O2 is a step in the right (green) direction, industrial application is prohibitive because the cost of the oxidant exceeds the value of the end-product; moreover, also the PTC catalyst, synthesized from (C8H17)3N and toxic dimethyl sulfate, is not easily available and relatively costly. Employing a similar overall approach, Deng et al.347 replaced the harmful and expensive PTC with a much less expensive peroxotungstate-oxalato complex [W(O)(O2)2L2]2− (where L2 is an organic acid with multifunctional groups), prepared in situ, and obtained adipic acid in 96.6% yield at 94 °C in 20 h by H2O2 oxidation of cyclohexene in a water emulsion under strong stirring and without surfactant. The main byproducts were 1,2-cyclohexanediol and glutaric acid. However, this process also requires substitution of some of the hydrogen peroxide with molecular oxygen for cost reasons. Oxidative cleavage of cyclohexene to adipic acid catalyzed by [(n-C8H17)3NCH3]3 {PO4[W(O)(O2)2]4} in 40% aqueous H2O2 occurs at 85 °C in 87% yield and 99% purity under twophase conditions in the absence of organic solvents.352 Although surfactant-type polyoxometalates352,363 increase the yield of adipic acid, they are relatively expensive if used industrially for adipic acid production and generally not environmentally benign. An oxodiperoxotungsten complex with 8-quinolinol as ligand achieves 89.8% yield of adipic acid from cyclohexene at 90 °C in one step by refluxing for 20 h using 30 wt % H2O2 in the absence of organic solvent.349 Ren et al.350 have recently reported glycine phosphotungstate and phosphomolybdate as highly efficient oxidation catalysts of cyclohexene to adipic acid with 30% H2O2 at 90 °C for 12 h (95.1% yield, 99.8% selectivity). Dawson-type

Despite their greater bond strength, oxygen insertion at terminal C−H bonds appears to be favored by steric effects arising from bulky ligands or small channels. Catalytic n-hexane− O2 reactions have been explored using various Mn-exchanged zeolites with channels of varying size and connectivity, namely 8-membered ZSM-58, 10-membered ZSM-5 and ZSM-57, and 12-membered MOR.342 Initial terminal selectivities (within 0.5 h) of air oxidation of n-hexane at 130 °C and 7 atm are as follows: ZSM-58, 8.4%; ZSM-5, 24%; ZSM-57, 14%; MOR, 9.5%; and uncatalyzed, 8.2%. Oxidation rates for hexanols, hexanal/hexanones, and acids (hexanoic acid and smaller acids) are first order to the HHP concentration on all Mn-zeolite catalysts, except Mn-ZSM-58, on which products form exclusively via noncatalytic autoxidation because of the restricted access of both n-hexane and HHP to Mn cations present within small channels (0.36 nm).

9. SELECTIVE DIRECT OXIDATION OF CYCLOHEXENE TO ADIPIC ACID As indicated in Section 4, adipic acid feedstocks are either cyclohexane, phenol, or cyclohexene, where the former has a cost advantage. With the present ready availability of cyclohexene through the selective hydrogenation of benzene (Asahi Chemical Industry Co., Ltd.), this feedstock has been proposed for alternative adipic acid production by less environmentally destructive methods, compared to the classical route involving cyclohexane. Cyclohexene is commonly produced commercially in 60% yield by partial hydrogenation of benzene at 120−180 °C at a pressure of 30−100 atm using an aqueous-phase ruthenium catalyst.343−346 Partial hydrogenation is problematic, because complete hydrogenation is more favorable. In general, it is difficult to accomplish the uncatalyzed oxidative cleavage of olefinic double bonds with usual oxidants such as hydrogen peroxide. When cyclohexene is oxidized by a strong aqueous solution of H2O2 (commonly 30 wt %) in the presence of appropriate homogeneous, 347−350 phasetransfer,351,352 or heterogeneous catalysts353,354 in organic solvent- and halide-free conditions adipic acid is formed in the absence of N2O. A key requirement for this reaction in an aqueous medium is to ensure close contact of the reagents. Onepot transformation of cyclohexene to adipic acid proceeds via a six-step scheme (Figure 14) involving three types of oxidative reactions (epoxidation of a cycloolefin, dehydrogenations of two secondary alcohols, and regioselective Baeyer−Villiger oxidation of an α-hydroxyketone) and two hydrolytic reactions. Many steps in this scheme are facilitated by acidic conditions. Although hydrogen peroxide is an alternative oxidant for adipic acid synthesis, which transforms cyclohexene with excellent selectivity and activity, the practicality of this green approach is hampered by the cost of H2O2 (theoretically 4 mol H2O2/mol AA). Polyoxometalates and polyperoxometalates are of considerable interest as catalysts for a variety of organic oxidations of alkenes and alcohols with the environmentally acceptable H2O2 under moderate reaction conditions.88,356−358 Heteropoly acids 27

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Table 4. Direct Oxidation (30% H2O2) of Cyclohexene to Adipic Acid Product Distribution (%) catalyst

time (h)

conversion (%)

TOFa

diol

adipic acid

othersb

reference

WSBA-15 WSBA-15 TAPO WMCM-41

13 30 72 24

100 100 100 98c

404 354 105 63

59.5 42.8 30.0

30.0 45.9 30.3

10.5 11.3 39.7 72.0

373 354 374

a

TOF = the number of moles of adipic acid per mole of metal per hour. bThe other products include 2-cyclohexene-ol, 2-cyclohexenone, 3-hydroxycyclohexanone, and cyclohexanedione. cOxidation of cyclooctene to epoxide in a H2O2/t-BHP mixture.

system, TAPO-5 reaches an AA yield of 30% at total conversion after 72 h at 80 °C, but the activity diminishes significantly upon reuse.354 Catalyst deactivation often makes recycling in H2O2based processes problematic, because metal leaching occurs.94,246,355,367,375 Although the yield in adipic acid was not as high as in other synthesis methods,84,347,351 details of the mechanistic pathway of cyclohexene oxidation to adipic acid were revealed. It appears that the cis-cyclohexanediol is formed by a free-radical mechanism, but not the trans-diol. The cis-diol undergoes oxidation more readily than the trans-diol analogue.354 Other potentially interesting catalytic systems for preparing adipic acid from cyclohexene have been reported. Perez et al.348 have proposed a recyclable and environmentally friendly process for the oxidation of cyclohexene by H2O2 in microemulsions with easy recovery of pure adipic acid and reuse of the reaction media. Wilhoit (to Du Pont de Nemours & Co.) described a single-step preparation of adipic acid by electrochemical oxidation of cyclohexene (100% conversion, 80% selectivity) over Ce(IV)/ RuO4.376 For LPO of cyclohexene with N2O (cfr. Section 5.5). Haas et al.377 reported the oxidation of cyclohexene in sc-CO2 and Mo(CO)6 catalyst precursor with t-BHP, but with epoxide and diol formation. Production of adipic acid from cyclohexene oxide with the Jones reagent (CrO3/H2SO4) under mild conditions cannot be used industrially in view of the generation of 1.5 t toxic chromiumcontaining waste per tonne of adipic acid.378 The selective direct oxidation of cyclohexene to adipic acid invariably requires expensive oxidants (H2O2, t-BHP) and/or microstructured catalysts, which are either too costly or complicated to apply to industrial processes. Further progress toward industrialization requires catalysts that can activate dioxygen. Using Co(OAc)3 cyclohexene is oxidized in air 80 times more readly than cyclohexane.28 The (nonindustrially applied) synthesis of adipic acid using benign reagents (H2O2), solvents (H2O), and methods (PTC, catalyst recycling) is often used in teaching the concepts of green chemistry.360−362 9.1. Two-Step Processes for Adipic Acid Production from Cyclohexene. Apart from the more desirable one-step hydroperoxidation of cyclohexene into adipic acid,359 which meets the hurdles described in Section 9 and requires 4 mol H2O2/mol AA, thus rendering the process unsustainable, various two-step pathways can be envisaged that involve cyclohexanol, cyclohexanone, or 1,2-cyclohexanediol (CHD) as intermediates (see Figure 15). The high reactivity of the double bond makes cyclohexene very useful as a raw material for the production of cyclohexanol. Hydration of cyclohexene to II with industrially sufficient catalytic activity (high yield) can be carried out using cation (Ti, Zr, Cr, Mo, W, etc.)-exchanged zeolites at ∼120 °C in the presence of an aqueous acid solution.379 Hydration of cycloolefins is an endothermic reaction. Asahi Chemical Industry has developed an industrial process for the hydration of cyclohexene

heteropolyoxometalate (R4N)4[S2W18O62] (M = W, Mo)/ CH3CN/H2O2−H2O systems have also been used under mild conditions (65−75 °C) in a nonchlorinated solvent for homogeneous LPO of cyclohexene to adipic acid in high yield.364 Various heterogeneous catalysts have been studied for the oxidative cleavage of cyclohexene.354,365−367 Adipic acid is produced in high yield by hydroperoxidation on a faujasiteencapsulated manganese diimine catalyst, cis-[Mn(bpy)2]2+-NaX (bpy, bipyridyl), with TON = 760.365 Many papers have reported cyclohexene epoxidation with H2O2 over various microporous and mesoporous catalysts.238,355,368−371 In the presence of TS-1, cyclohexene is oxidized very slowly by H2O2 as the molecule presents steric restrictions to the mediumpored TS-1 framework. On the other hand, larger-pored Ti-Beta is active for the oxidation of such bulky olefins as cyclohexene (forming glycol ethers), as well as for the oxidation of cyclohexane (forming cyclohexanol almost exclusively).241 Also, various mesostructured catalysts such as Ti-MMM-2, Ce-SBA-15, Ti-AlSBA-15, and TAPO-5 have proved to be active with either H2O2354,355 or t-BHP.366,372 Cyclohexene oxidation over a Ti-containing mesoporous mesophase material (Ti-MMM, pore diameter of 3.4−3.8 nm) with aqueous H2O2 in CH3CN yields cyclohexene oxide and the corresponding diol as the main oxidation products.367 Framework Ti-substituted and Tigrafted MCM-41 mesoporous catalysts were used for LPO of cyclohexene with aqueous H2O2 and t-BHP but with lower intrinsic activity and selectivity than TS-1; no adipic acid was formed.370 However, trimethylsilylation of Ti-MCM-41 and Ti-MCM-48 results in a remarkable enhancement of catalytic activity in oxidation of cyclohexene with H2O2 but again without adipic acid formation.371 Lapisardi et al.372 have reported “one-pot” synthesis of adipic acid from cyclohexene under mild conditions (80 °C, 24 h) over bifunctional Ti-AlSBA mesostructured catalysts in acetonitrile using t-BHP; yields of 88% CHD were reached after 24 h, and 80% AA after 48 h. The diol is a long-lived intermediate. Leaching of active titanium species was negligible. However, the consumption of 4 mol t-BHP/mol AA, according to Figure 14, is economically unsustainable. A reusable oxotungsten-containing silica mesoporous catalyst (WSBA-15) has been described for the direct oxidation (30% H2O2) of cyclohexene to adipic acid (45.9% yield after 30 h at 85 °C) under organic solvent-free conditions.373 Table 4 compares the catalytic performance of organic solvent-free oxidation of cyclohexene to adipic acid with other catalysts. The TOF values for WSBA-15 are clearly superior to those reported for TAPO-5 and WMCM-41. The catalytic activity of microporous titanium-substituted aluminophosphates in cyclohexane oxyfunctionalization and in cyclohexene epoxidation by H2O2 using acetone as a solvent decreases in the order TAPO-5 > TAPO-11 > TAPO-36, with TAPO-5 being almost as active as TS-1.238 Used for hydroperoxidation of cyclohexene to adipic acid in an organic solvent-free 28

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Figure 15. Some alternative routes to adipic acid. Reproduced with permission from ref 10. (Copyright 2001, Elsevier B.V., Amsterdam.)

one-pot epoxidation and hydrolysis (dihydroxylation) of cyclohexene with aqueous H2O2.381 9.2. Oxidative Cleavage of Diols. Osmium(VIII) compounds are the most reliable catalysts for dihydroxylation of alkenes with molecular oxygen as an oxidant. In the presence of K2(OsO2(OH)4) in t-BuOH, cyclohexene can yield the diol in 75% selectivity at 90% conversion at 50 °C and 1 atm O2 after 16 h.46 Oxygen consumption shows that both O atoms are used productively for diol formation. Cyclohexene can also selectively be oxidized to 1,2-cyclohexanediol (CHD) with H2O2 or t-BHP. Dihydroxylation of olefins with H2O2 catalyzed by transitionmetal complexes (e.g., polyoxometalates) and solid catalysts (e.g., zeolites) has been reported by many authors with limited diol selectivities (90%) at 12%−13% conversion per pass (cfr. Section 5.2). FeAlPO-5 is an exceptionally

Figure 17. Green methods for adipic acid manufacture. Reproduced with permission from ref 330. (Copyright 2003, The Royal Society of Chemistry, London.)

Cyclohexanol is oxidized much more rapidly than cyclohexane. Yet, no adipic acid is formed from air oxidation of II over Mn and Cu acetate at 90 °C after 6 h.222 While adipic acid is not formed on the oxidation of II alone under conditions wholly favorable for 30

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cyclohexane. RuCl2(PPh3)3-t-BHP and Ru/C-PAA in benzene or EtOAc give the following results (conversion, selectivity to III/II): 41%, 47%/7%, and and 62%, 67%/1%, respectively.255,256 For the oxidation of cyclohexanone, various oxidants have been considered (HNO3, H2O2, and O2/air). Obtaining adipic acid in high yield is not an easy matter. As mentioned elsewhere (previously, in Section 5.4), 60% HNO3 oxidation of cyclohexanone yields 62%−73% adipic acid at 82−90 °C and 1 atm with lower dicarboxylic acids as byproducts; cyclohexanol requires a lower temperature (55−60 °C) for the reaction, with 83% yield.198,199 For results from using air or oxygen as the oxidant for KA oil, cfr. Section 6. For catalytic oxidative cleavage of C−C bonds of ketones, two types of reactions can be considered: heterolytic processes with aqueous H2O2 and homolytic reactions with O2. Cyclohexanone is easily oxidized by H2O2 in an uncatalyzed reaction at ∼100 °C.394,401 Thermally activated Baeyer−Villiger oxidation of III with H2O2 leads to adipic acid formation. Without catalyst, the thermally activated radical reaction converts III to 43%−55% adipic acid at 30%−40% conversion over H2O2 at different feed ratios after 6 h at 90 °C with ε-caprolactone, 6-hydroxyhexanoic acid, glutaric acid, and succinic acid as byproducts.401 Overoxidation of III is rather slow. The relevant reaction rates are modified when TS-1 is used as a catalyst for the oxidation of III with H2O2.304,401 In this case, the high concentration of hydroxy radicals within pores favors the consecutive formation of diacids (75% adipic acid, 25% glutaric and succinic acids).401 Lactone is very efficiently transformed to diacids. The proper choice of solvent is a tool to control the reaction rates to various products. Although the oxidation of cyclohexanone with H2O2 over H2MoO4 and H2WO4 under homogeneous conditions using acid acetic or t-BuOH as a solvent has been reported, the highest yield of adipic acid was only ca. 50% and the selectivity was rather low.402 Usui et al.330 reported a green method for the oxidation of III to adipic acid in 91% yield at 90 °C, using H2WO4 and a 3.3 molar amount of 30% H2O2 in halide- and organic solvent-free conditions (heterogeneous reaction) (see Figure 17). Although the reaction utilizes H2WO4 as the precatalyst, it is readily oxidized with H2O2 to form H2[WO(O2)2(OH)2],403 which is water-soluble. High reactivity in this heterogeneous reaction requires solvent-free conditions. Solvents (i.e., homogeneous conditions) significantly depress the adipic acid yield from cyclohexanone (to 31% for t-BuOH and 52% for dioxane). While no operational problems are foreseen for large-scale operations, the process is not economical. The oxidation of cyclohexanone to adipic acid with oxygen, according to

oxidizing III in the presence of acetic acid, it can readily be converted catalytically to the diacid in 93% yield at 85−90 °C in air under pressure (10 atm) when in admixture with III.220 Cyclohexanol is dehydrogenated to III with less byproduct formation when using a promoted (Cu,Zn)O catalyst than with conventional (Cu,Mg)O.395 CrAlPO-5 is a heterogeneous catalyst for LPO of II using O2 or t-BHP under mild conditions; at 110 °C and an O2 pressure of 5 atm, 30% conversion of II and 97% selectivity of III were observed after 5 h but no adipic acid.286 Chromium-substituted mesoporous materials Cr-MCM-41 and Cr-MCM-48, which have been used for LPO of II at 110 °C and 10 atm O2 in chlorobenzene solvent, yield 97% III after 5 h; leaching of catalytically active chromium was stronger than that for microporous Cr-AlPO-5.289 At relatively low temperatures, metal catalysts induce little or no C−C bond breaking of organic molecules and overoxidation is avoided. Liquid-phase oxidative dehydrogenation of an aqueous solution of II over Pt/C at 150 °C and 10 atm air pressure yields 45% adipic acid, and other diacids.396 The economy of the process is limited by the low solubility of cyclohexanol in water (3.6% at 20 °C). Selective and economic oxidation of cyclohexanol to adipic acid with air as a clean oxidant under mild reaction conditions still remains an elusive target.

11. LIQUID-PHASE OXIDATION OF CYCLOHEXANONE TO ADIPIC ACID Cyclohexanone (III), which is an important intermediate for the production of caprolactam, can be produced catalytically by air oxidation of cyclohexane (recall Sections 5 and 7.1) or by hydrogenation of phenol in the liquid or vapor phase in one or two steps.101,104,105,397 The former technology accounts for 80% of the global capacity of pure cyclohexanone (2.8 Mt/yr). Because of the severity of the reaction conditions in the second step, the two-step method of phenol hydrogenation to cyclohexanone (hydrogenation to II, followed by dehydrogenation to III) has been replaced by a more-economical and lessintensive single-step process.101 Partial hydrogenation of phenol over Pd/Al2O3 catalysts modified with alkali or alkali earth materials is one of the most selective methods for manufacturing cyclohexanone.105 Cyclohexanone can be also prepared by various other methods (recall Figure 2), such as oxygenation of cyclohexene,52,398 or via the diol by dihydroxylation of cyclohexene with molecular oxygen,46 followed by a base-induced elimination.399 LPO of cyclohexene to cyclohexanone in air at 70 °C using a PdBr2/ CuCl2 catalyst in a microemulsion medium yields III in low yields (30% conversion, 30% selectivity).400 Dehydrogenation of II to III is usually carried out with supported copper catalysts.395 Industrially, the hydrogen obtained as a byproduct in the dehydrogenation process is employed in the hydrogenation of benzene to cyclohexane. Adipic acid production does not need pure cyclohexanone as the intermediate, being based industrially on nitric acid oxidation of cyclohexanol/one (olone) mixtures in various ratios (cfr. Section 5.4). However, if desired, cyclohexanone-rich olone (K/A = 10:1) can be obtained from air oxidation of I over NHPI/ Co(OAc)2 catalysts in benzonitrile.259 It is also of interest to notice that the photocatalytic oxidation of I leads to III at high conversion (cfr. Section 7.4). Ruthenium-catalyzed oxidations of cyclohexane with peroxide efficiently give III with small amounts of II.255,256 Oxo-ruthenium species (RuO) generated from low valent ruthenium and peroxides can be used for oxidation of

is only formally a simple reaction. The product selectivity of air oxidation of III is surprisingly complex, even at low temperatures (80 °C). The major primary product is 2-hydroperoxycyclohexanone, whose decomposition, catalyzed by III, leads to other hydroperoxides, ε-caprolactone, adipic and glutaric acids, adipaldehyde acid, cyclohexane-1,2-dione, and δ-valerolactone.33 Catalysts used for the aerobic cleavage of cyclic ketones under mild conditions include V oxo and dioxo salts and complexes,404 Keggin-type polyoxometalates,405 and heterogenized VO catalysts.406 Table 5 shows some typical catalytic systems used for the 31

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Table 5. Selection of Catalysts for Aerial Oxidation of Cyclohexanone to Adipic Acid (AA)

a

catalyst

solvent, T (°C)

substrate/catalyst molar ratio

conversion (%)

AA selectivity

reference

Pt/carbon/monolith Pb2.62Ru1.38O6.5 Mn(OAc)2/Co(OAc)2a Mn(OAc)2 Mn(NO3)2/Co(NO3)2 H7PMo8V4O40 H5PMo10V2O40 Co/Mn cluster

water, 140 °C 1.5 N NaOH, 35 °C acetic acid, 70 °C acetic acid/CF3COOH, 65 °C acetic acid + catalytic HNO3, 40 °C acetic acid/water, 70 °C acetonitrile/methanol, 60 °C acetic acid/water (MEK), 100 °C

830 12 7 215 50 220 20−600 5000

100 100 100 99.8 97.5 99 98 97.6

21 mol % 68.9 mol % 77 mol % 75 mol % 93.4 mol % 51 mol % 55 mol % 86.6 wt %

407 391 224 408 409 223 410 274

Reactant: ol/one.

Figure 18. Diacid product selectivity of O2 oxidation of cyclohexanone catalyzed by Mn(OAc)2/Co(OAc)2. Reproduced with permission from ref 224. (Copyright 2003, The Chemical Society of Japan.)

acetic acid at 65 °C, yielding 80.5% adipic acid (along with 6.5% glutaric acid and 1.1% succinic acid) at total conversion after 4 h.408 In PCT International Publication No. WO 01/87,815 A2 (to Lonza), Fumagalli et al. disclosed a high conversion of III (97.5%) to AA with high selectivity (93.4%) at 40 °C in 1 atm O2, using Mn(NO3)2/Co(NO3)2/HNO3 (in 1:1:3 mol ratio) in acetic acid for 5 h.409 Also, Asahi has repeatedly expressed interest in the development of a prospective industrial route for adipic acid production from III based on O2 oxidation without the generation of N2O.224,264 Using Mn(OAc)2/Co(OAc)2 in air containing 90%) under industrially interesting conditions (T, t). The microemulsion was mainly composed of cyclohexene as the oily phase, hydrogen peroxide as the aqueous phase, and benzalkonium chlorides (with potential toxicity for aquatic organisms) as the surfactant. Sodium tungstate (Na2WO4) acted as the catalyst. System recyclability was demonstrated. Although microemulsions (water, oil, and surfactant) are described as versatile reaction media for many organic reactions, there is still no industrial process based on microemulsions.

12. ENVIRONMENTALLY DESIGNED PROCESSES TO ADIPIC ACID New strategies for conducting air oxidation of organic substrates such as biphasic systems, biocatalysis, and photocatalysis, are

13. ALTERNATIVE PATHWAYS FOR ADIPIC ACID MANUFACTURE None of the main process routes developed for adipic acid in the past (Table 1) are truly sustainable, both from an economic point

reaction is solvent-dependent. Although the HPA-n/AcOHH2O/O2 system leads to adipic acid, glutaric and succinic acid, as well as CO2, also are formed in considerable quantities. Adipic acid is stable to degradation by the HPA-n/O2 system. However, the selectivity to adipic acid is still low, compared with nitric acid oxidation. Polyoxometalates (POMs) are among the very few systems that can catalyze the oxidation of organic substrates with air at a moderate reaction temperature through a redox-type mechanism.11,77,87,392,413,414 POM-catalyzed oxidation of III to adipic acid with air in a water-only solvent proceeds with a redox mechanism; however, when using an acetic acid co-solvent, a radical-chain automechanism prevails, especially for a very low catalyst-to-cyclohexanone ratio.415 The latter is more selective to adipic acid than the redox mechanism. Recent developments for the activation of O2 based on polyoxometalates are very promising, as shown for α-substituted cycloalkanones in methanol or acetic-water mixture in the liquid phase.405 Adipic acid may be obtained with 100% selectivity at 90%−97% conversion, according to Figure 19, which is an example of a catalytic reaction with atom economy and high selectivity.

Figure 19. Conversion of α-substituted cyclohexanones by activation of dioxygen by polyoxymetalates. R = OH, Me, Ph, H, etc.

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advantage of simplicity, and it would make environmental and economic sense; consequently, many attempts at developing such a process have been made. Compared to the conventional two-step technology, a direct one-step oxidation of I with air or oxygen to adipic acid should lower total investment costs by eliminating the olone production step, nitric acid production, handling/recovery/purification/recycle units, and by simplification of the air abatement and wastewater treatment. Direct, onestep adipic acid synthesis in the presence of solvents, catalysts, initiators, or promoters has been given attention for a long time. Daicel Chemical Industries, Ltd. has shown active interest in the industrial application of one-step aerobic oxidation of cyclohexane in acetic acid using Ishii’s NHPI catalyst (cfr. Section 7.1.1). In a further advancement, Rhodia has disclosed a solventless one-step air oxidation of I (cfr. Section 7.3). Despite good adipic acid yields achieved by various proposed direct synthesis processes (cfr. Sections 7 and 7.1), the oxidation of cyclohexane to adipic acid with oxidants other than nitric acid has not yet reached the commercial level. A valid, more sustainable, alternative to the multistep processes currently in use for adipic acid production (cfr. Section 4) has not yet been found. In many cases, a major problem is the high corrosiveness of acetic acid (the solvent used for autoxidation reactions) and the high energy demand for solvent recycle. It is also the case to mention the extremely complex continuous one-step process technology and control. Most studies in direct oxidation have been conducted in batch mode. After an oxidation reaction has taken place according to the direct synthesis, a mixture of two liquid phases (a polar and a nonpolar phase) is present at ambient temperature, along with a solid phase mainly consisting of adipic acid. Cyclohexane is the main contributor to the nonpolar phase and nitric acid to the polar phase. However, in order to improve reaction rate and reactivity of the air oxidation of cyclohexane to adipic acid, a single phase at the operating temperature is to be attained by carefully adjusting the temperature and pressure in the reaction zone, the gaseous oxidant flow rate, and the feed rate of cyclohexane, solvent, catalyst, and water (if being fed), as well as other parameters.276 As a reminder, a considerable amount of water is being produced by the oxidation of cyclohexane in air. Critical aspects for successful oxidation of cyclohexane are maintenance of a single liquid phase during the time the reaction is taking place and avoidance of catalyst precipitation. This imposes strict control of the water level in the liquid reaction mixture during the steadystate oxidation of I to adipic acid. Water is a strong contributor to the formation of two phases from a single phase. The minimum water level at which the catalyst precipitation occurs decreases as the cyclohexane content and/or catalyst content decrease. Complex direct synthesis process technology and control have been developed for cyclohexane oxidation in the presence of Co(OAc)2·4H2O and acetic acid.276 With further reference to Table 6, it cannot reasonably be expected that the process of high photochemical conversion of cyclohexane (40%) in cages of NaY in air at room temperature (to III rather than adipic acid)332,334 will easily be scaled up (cfr. Section 7.4). There are no similar industrial chemical plants. Alkanes are difficult to oxidize, and oxyfunctionalization on both terminal positions is particularly arduous. Although Thomas et al.85 reported the successful synthesis of adipic acid via the oxidation of n-hexane in air at 100 °C and 15 atm with 9.5% conversion-per-pass and 33.6% selectivity (cfr. Section 8), the results could not be confirmed using MnAlPO-18.340

of view and an environmental point of view. Greening of largescale oxidation processes is actively being pursued. The synthesis of adipic acid is a prime example of efforts to replace current industrial processes with more benign manufacturing modes. The most sustainable approach to adipic acid (also the one with the best economics) combines the use of inexpensive raw materials with a single, direct oxidative transformation under mild conditions using air or O2 as the oxidant. There are several promising developments toward reducing the environmental impact of adipic acid manufacture, as amply illustrated by the results of research reported in the preceding sections. These are composed of the following: (i) use of alternative feedstocks (e.g., cyclohexene, n-hexane), including bioplatform chemicals (see ref 38); (ii) activation of hydrocarbons under milder reaction conditions (lower T, p, solvent-free, metal-free, halogenfree, etc.) and activation of terminal C atoms; (iii) use of environmental friendly oxidants; (iv) new types of homogeneous and heterogeneous catalytic systems; and (v) alternative modes of contacting reactants (cfr. Sections 7.1.2 and 12). Although alternative types of oxidants have widely been proposed, economical considerations impose air or O2 for bulk chemicals. Table 6 shows the main alternative synthetic routes to Table 6. Alternative Chemical Pathways to Adipic Acid • • • • • • • • •

direct air oxidation of cyclohexane (Daicel)a,b solventless, continuous one-step air oxidation of cyclohexane (Rhodia)b photocatalyzed oxidation of cyclohexanec,d one-step air oxidation of n-hexaned two-step process from cyclohexene (Asahi)e single-step cyclohexene (hydroper)oxidationd air oxidation of cyclohexanone in aqueous solvent (Asahi)b oxidative scission of 1,2-cyclohexanediol (Enichem)b,d chemo-enzymatic processesd

a

Pilot plant. bNot yet commercial. cTo cyclohexanone (no adipic acid). dResearch phase. eCommercial.

adipic acid. Many new promising processes are in an advanced state of development. Societal demands for catalyst applications are still growing. In the case of the selective oxidation of cyclohexane, the severe requirements cannot easily be satisfied. Various new classes of catalysts have come to the forefront in recent years and have been applied for adipic acid synthesis. Table 3 shows some promising one-step aerobic autoxidations of I to adipic acid using several new catalyst types: microporous aluminum phosphates (MeAlPOs) (cfr. Section 7.1), biomimetic catalysts (cfr. Section 7.2), and NHD-based catalysts (cfr. Section 7.1.1). Transition-metalion-substituted microporous aluminum phosphates such as FeAlPO-31 for oxidation of cyclohexane to adipic acid with oxygen, CoAlPO-18 for oxidation of n-hexane to AA with oxygen, and TAPO-5 for oxidation of cyclohexene to AA with H2O2 are model systems for single-site heterogeneous catalysts (SSHC) and have carefully been designed. Also nanostructured catalysts have shown efficient aerobic oxidation activity toward cyclohexane under mild conditions (cfr. Section 7.1). However, despite these catalyst developments, the adipic acid yield of the DuPont process is still unbeaten. A single-step process for the conversion of cyclohexane to adipic acid with molecular oxygen would obviously have the 34

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Also Mn-exchanged ZSM-58, ZSM-5, ZSM-57, and MOR only showed small selectivity to diacids.342 With regard to other feedstocks, Asahi Chemical Industries Co., Ltd., which developed the process for the selective hydrogenation of benzene to cyclohexene,343 uses the olefin produced (60 kt/yr) for captive transformation first to II by hydration and then to adipic acid by nitric acid oxidation of the alcohol (cfr. Section 9.1). On the other hand, one-pot transformation of cyclohexene to adipic acid with excellent selectivity and activity using H2O2 (Figure 14) is hampered by the prohibitive cost of the oxidant (cfr. Section 9) and requires further research efforts. Industrialization of a one-step cyclohexene to adipic acid process requires catalysts that can activate oxygen. Cyclohexene is not a widely available feedstock. More recently, Asahi has also disclosed a method for producing adipic acid with high selectivity by oxidizing III with molecular oxygen in aqueous solvent using a (Fe,V)-based catalyst,264 (cfr. Section 11). Industrial followup has not been noticed. The economy of the transformation of 1,2-cyclohexanediol to adipic acid by H2O2 is to be ascertained (cfr. Section 9.1). Attempts to solvent-free conversion in air have only achieved low selectivity. Apart from some economically prohibitive routes to adipic acid using H2O2 as the oxidant,351,359 intense R&D on the development of high-performance industrial catalysts has not resulted in major commercial breakthroughs. Not surprisingly, more rewarding results are now expected from biotechnological approaches to adipic acid.38

promoters and initiators, microemulsions, and slurries or supercritical conditions. Many different oxidants have been evaluated. Despite these global efforts by industry and academia, the much auspicated aerial conversion of cyclohexane to adipic acid still meets severe constraints. The adipic acid industry has also taken its environmental responsabilities seriously by actively developing N2O abatement technology. However, the resulting economic distortions created by crediting mechanisms, connected with industrial gas projects implemented under the Clean Development Mechanism (CDM), could now well act as a deterrent to the introduction of new adipic acid manufacturing technologies.

14. DYNAMICS OF THE ADIPIC ACID INDUSTRY The fact that an industrial process developed at the end of 1930s is still being operated today might induce the uninformed public to suggest that the adipic acid industry has been inactive. The data presented here show a different picture altogether. Oxidation of cyclohexane is the least efficient of all major industrial processes. Obviously, such a low-efficiency process has been at the forefront of much research attention. Since the air oxidation of (cyclo)hexane inherently is extremely difficult, adipic acid manufacture has considered a variety of alternative petrochemical feedstocks, such as cyclohexanol, cyclohexanone, cyclohexene, phenol, butadiene, adiponitrile, THF, syngas, etc. In particular, attention has been focused on cyclohexene. At present, attempts to force a breakthrough toward the use of renewable feedstocks (e.g., glucose) are in full swing. The adipic acid industry has responded adequately to changes in economic conditions. The relatively high cost of phenol in the late 1930s has led to the preferred use of cyclohexane as raw material. Despite changes in the availability and price of hydrocarbons and energy cost, relative process positions have changed only slightly since the mid-1970s. The markedly higher price for benzene or cyclohexane, vis-à-vis butadiene, has reduced the use of cyclohexane-based adipic acid as an intermediate for the manufacture of adiponitrile. Increased energy costs have made major impacts on conventional cyclohexane oxidation technology. Unit energy consumption has decreased through energy recovery procedures. Great value has been placed on evolutionary increase in yield. All tools of the trade have skilfully been applied. A great variety of catalysts have been developed (homogeneous, heterogeneous, shape-selective, nanostructured, immobilized, phase transfer, photocatalysts, biocatalysts, etc.). Reaction conditions have been varied widely from vapor phase to liquid phase, multistep to one-step, batch to continuous, with or without use of solvents,

Table 7. Main Features of Petrochemical (Nitric Acid-Based) Adipic Acid Processing

15. CONCLUSIONS AND PROSPECTS Pathways to adipic acid are broadly based on benzene to cyclohexane, cyclohexene and phenol, on toluene to phenol via benzoic acid, and on butadiene. All commercial processes differ only in the way cyclohexanol is obtained from benzene. All adipic acid pathways are multistep, which often share the same steps. The commodity chemical adipic acid is still being manufactured essentially in an inefficient and unsustainable multistep process dating back to World War II, which involves homogeneous catalysts, aggressive oxidants (concentrated nitric acid), and the production of large quantities of the greenhouse gas nitrous oxide. This process urgently needs to be greened. DuPont’s cyclohexane-based technology meets few of the modern sustainability criteria (see Table 7). Existing adipic

a

effecta

feature description

− − + − − − + + + + − − − +

safety (cfr. Section 3) renewable feedstock inexpensive raw materials minimal use of auxiliary (nontoxic) reagents multistep process operational simplicity relatively mild reaction conditions excellent (carbon) efficiency high selectivity easy recovery of pure adipic acid generation of environmentally incompatible byproducts (N2O) high-energy requirements (CED) straightforward catalyst recycling relatively lowest cost

+: positive; −: negative.

acid plants all use the oxidation of cyclohexanol, with or whithout admixture with cyclohexanone, with nitric acid (rather than air or O2). The safety of cyclohexane oxidation requires many precautions. Adipic acid production is characterized by a high rate of carbon usage, namely >95%, based on cyclohexanol, or even >97%, also considering the commercially useful byproduct glutaric acid. This explains why the process is withstanding the test of time, despite other drawbacks. A typical adipic acid plant currently produces very little waste: off-gases are either reused (NO, NO2) or decomposed (N2O) to innocuous molecules, albeit requiring (often subsidized) end-of-pipe solutions. Adipic acid synthesis benefits from (i) activation of molecular oxygen; (ii) activation of C−H bonds by specifically designed catalysts; and (iii) highly selective catalytic oxygen transfer to 35

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relatively inert chemical substances such as (cyclo)hexane. Oxidation of cyclohexane is strongly depends on the nature of the oxidant, catalyst (and catalyst concentration), initiator, solvent, temperature, and more. Although alternative processes for producing adipic acid have repeatedly been proposed by industrial research laboratories (Amoco, Asahi, BASF, Diacel, DuPont, Enichem, Gulf, Halcon, Rhodia, RPC, etc.) and academic institutions, few of these have commercially been implemented (see Tables 1 and 6). For decades, R&D has worked its way through improving feedstock (from fossil-based to renewables), liquid-phase oxidation of hydrocarbons, reaction conditions for operational simplicity (one-step, ambient temperature and pressure, solventless, metal- and chlorine-free), activation (catalysts), oxidants (air, molecular oxygen), conversion and selectivity, and eco-impact (noxious waste abatement). In particular, the search for processes with fewer steps has been fruitless until now. The final industrial target is developing a simple and efficient system that gives high conversion and selectivity for air oxidation of an inexpensive substrate under mild operating conditions. Various obstacles to greener oxidation have gradually been removed: higher conversion (98%) and selectivities (88%), lower T (