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We are grateful to M. Newton (Brookhaven National Laboratory),
D. Phillips (Imperial College London), A. Orr-Ewing (Univ. of Bristol),
D. Clary (Univ. of Oxford), and G. Worth (Univ. of Birmingham) for
comments on the manuscript; to I. Rubtsov and R. Schmehl (Tulane
University) for discussions; and to the Engineering and Physical
Sciences Research Council, Univ. of Sheffield, and Science and
Technology Facilities Council for support.
Materials and Methods
Figs. S1 to S8
29 August 2013; accepted 14 November 2014
conversion of cyclohexane to adipic
acid by ozone and UV light
Kuo Chu Hwang* and Arunachalam Sagadevan
Nitric acid oxidation of cyclohexane accounts for ~95% of the worldwide adipic acid
production and is also responsible for ~5 to 8% of the annual worldwide anthropogenic
emission of the ozone-depleting greenhouse gas nitrous oxide (N2O). Here we report a
N2O-free process for adipic acid synthesis. Treatment of neat cyclohexane, cyclohexanol, or
cyclohexanone with ozone at room temperature and 1 atmosphere of pressure affords
adipic acid as a solid precipitate. Addition of acidic water or exposure to ultraviolet (UV)
light irradiation (or a combination of both) dramatically enhances the oxidative conversion
of cyclohexane to adipic acid.
Adipic acid is a precursor for the synthesis of the nylon-6,6 polymer and, as such, is one of the most important industrial chemical intermediates. More than 3.5 million met- ric tons of adipic acid were produced in
2013, reflecting a ~5% growth rate per year over
the past 5 years (1, 2). Nearly 95% of the world-
wide industrial production of adipic acid em-
ployed the nitric acid oxidation method (3). The
first step is air oxidation of cyclohexane under
high temperatures (125° to 165°C) and high pres-
sure (8 to 15 atm) to produce KA oil (i.e., a mix-
ture of cyclohexanone and cyclohexanol) with
4 to 11% conversion and ~85% selectivity (4, 5).
In the second step, nitric acid is applied as an
oxidant: the conversion is ~100%, and the selec-
tivity for adipic acid is 93 to 95% with some other
short-chain acids as side products (see Fig. 1A).
The process requires the nitric acid–to–KA oil
ratio to be maintained at 40:1. Disadvantages
of the current industrial process include low
overall product yield; corrosion of reaction vessels
by nitric acid; emission of the ozone-depleting
greenhouse gas N2O; and high energy consump-
tion. It was estimated that ~0.3 kg of N2O gas is
formed per kilogram of adipic acid produced
(6, 7). After energy-consuming recovery and re-
cycling, the amount of N2O gas released to the
atmosphere still accounts for ~5 to 8% of annual
anthropogenic N2O emission worldwide (3, 7, 8).
Many efforts have been devoted to developing
more efficient and environmentally friendly pro-
cesses for industrial production of adipic acid
that avoid the emission of N2O. In 1998, Sato et al.
reported a process using H2O2 as an oxidant to
convert cyclohexene to adipic acid in the pres-
ence of a Na2WO4 catalyst and the phase-transfer
reagent [CH3(n-C8H17)3N]HSO4. Although the over-
all adipic acid yield (93%) is very high (9), produc-
tion of 1 mol of adipic acid requires consumption
of 4 to 4.4 mol of H2O2. The price of H2O2 is ~55%
of the price of adipic acid. The requirement of 4
to 4.4 mol of H2O2 for production of 1 mol of
adipic acid is economically infeasible (7). In ad-
dition, other negative factors hinder commercial-
ization of this process, including low availability
Department of Chemistry, National Tsing Hua University,
Hsinchu, Taiwan, Republic of China.
*Corresponding author. E-mail: firstname.lastname@example.org
Fig. 1. Comparison of the industrial process and the method presented herein for production of
adipic acid. (A) Industrial nitric acid process. (B) O3-UV method.