1. INTRODUCTION
ε -Caprolactam is the monomer of nylon-6 (polycaprolactam). The huge demand across the industries, such as packaging, electrical and electronics, consumer goods and appliances, and automotive greatly increase the overall ε -caprolactam consumption in the nylon-6 industry.1,2 The estimated market value ofε -caprolactam is projected to reach $18.6 billion by 2019.3 The industrial manufacture ofε -caprolactam involves a multiple-step transformation of benzene to cyclohexanone and/or cyclohexanol, with the latter can be facilely converted to the former by dehydrogenation, and the transformation of cyclohexanone to ε -caprolactam.1 Cyclohexanol is also a key precursor of adipic acid, one of the monomers for nylon-66 (poly(hexamethylene adipamide)).4 Thomas and Raja have skillfully developed a green one-step process to bridge the gap between cyclohexanone and ε -caprolactam by using redox and acid sites co-existing on the metal-doped nanoporous aluminophosphates (AlPOs) in tandem, thus avoiding the use of corrosive oleum and sulfuric acid and the generation of unwanted ammonium sulfate.5
However, the existing industrial processes for the production of cyclohexanone/cyclohexanol is far from safe, efficient, or atom-economic. Currently, there are three industrialized processes for this purpose, namely, (i) cyclohexane oxidation, (ii) cyclohexene hydration, and (iii) phenol hydrogenation (Scheme 1). The first process, though prevailing, is restricted by a low conversion of 4.5% to sustain a high selectivity of 93% (Table S1). In addition, the cyclohexane/air mixture is of high explosion risk, which has resulted in one of the most devastating accidents in the history of petrochemical industry.6 The second process was developed by Asahi Chemical in the late 1980s. Albeit cyclohexene hydration gives high cyclohexanol selectivity of 99.3%, the reaction is of low efficiency due to the poor miscibility of cyclohexene with water (200 ppm at 298 K7 and 500 ppm at 393 K simulated by Aspen Plus). Moreover, the equilibrium conversion of cyclohexene hydration is only about 12.7%.8,9 For the third process, while phenol hydrogenation to cyclohexanol can be highly selective,10 the production of phenol is complicated, which involves the alkylation of benzene with propylene, cumene oxidation to cumene hydroperoxide, and cleavage of cumene hydroperoxide to phenol.11 Furthermore, analogous to cyclohexane oxidation, cumene oxidation must be kept at a low conversion of 25% and is of high explosion risk.
Figure 1 illustrates the overall atom economy and per pass yield of cyclohexanol (including cyclohexanone for the first process) via these three processes starting from benzene based on the industrial and literature data compiled in Table S1. According to Trost, atom economy is defined as how much of the reactants end up in the product.12 Herein, the reactant refers to benzene, and the product refers to cyclohexanol. The overall atom economy is in the order of 83.7% (Process 1) < 95.5% (Process 3) < 99.3% (Process 2). However, because of the bottlenecks either in selectivity (Processes 1 and 3) or in thermodynamics (Process 2), the per pass yield of cyclohexanone/cyclohexanol is low and is in the order of 3.7% (Process 1) < 5.1% (Process 2) < 14.9% (Process 3). In industry, the cyclohexanol yield is improved by massively circulating the unreacted feedstocks, which greatly adds up to the energy demand.
Aside from three industrial processes mentioned above, other processes, including cyclohexene esterification–transesterification and esterification–hydrolysis, have been widely investigated to produce cyclohexanol. However, for the former process, ether formation and olefin formation are common side reactions during transesterification.13–15 Besides, like esterification reactions, transesterification reactions are typical, equilibrium limited reactions.15,16 These drawbacks greatly increase the cost of product separation, thus inhibiting this process from practical application. For the latter process, the hydrolysis step in particular is very complex due to multiple reactions, phase splitting, and mismatch between the reaction conditions and separation conditions. Furthermore, the energy requirement is very high.17,18 Hence, Freund and co-workers concluded that unless a right catalyst can be developed, this process is economically not viable.19
Herein, we report a novel cyclohexene esterification–hydrogenation process for the production of cyclohexanol (Scheme 2). In this process, cyclohexene obtained from the partial hydrogenation of benzene20 is esterified with acetic acid to cyclohexyl acetate. The latter is then hydrogenated to cyclohexanol. We found that the experimentally determined equilibrium conversion for cyclohexene esterification at the stoichiometric ratio is always ≥68% in the temperature range of 333–373 K, which is substantially higher than that of cyclohexene hydration. And, the hydrogenation of ester to alcohol is not thermodynamically limited and is usually of high conversion and selectivity.21 The additional merit is that the hydrogenation of cyclohexyl acetate simultaneously yields ethanol that is widely used as antiseptic, solvent, and fuel or fuel additive. Therefore, the novel cyclohexene esterification–hydrogenation process devoid of the shortcomings of the existing industrial processes is highly promising to produce cyclohexanol in a safe, efficient, and green manner.