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.