Applied Catalysis A, General
Efficient oxidation of p-xylene to terephthalic acid by using N,N-
dihydroxypyromellitimide in conjunction with Co-benzenetricarboxylate
Luo Xua, Dawei Chena, Haoran Jianga, Xia Yuana,b,
a College of Chemical Engineering, Xiangtan University, Xiangtan, 411105, China
b National & Local United Engineering Research Centre for Chemical Process Simulation and Intensification, Xiangtan, 411105, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The MOF Co-BTC (BTC = benzenetricarboxylate) has been synthesized by a hydrothermal method, and char-
acterized by means of N2 physical adsorption, X-ray diffraction, scanning electron microscope, thermogravi-
metric analysis, and X-ray photoelectron spectroscopy. The material has multiple crevices, as opposed to a pore
structure, and shows high thermal stability, with Co in the divalent state. It has been used in conjunction with
N,N-dihydroxypyromellitimide to catalyze the oxidation of p-xylene to terephthalic acid, the reaction conditions
for which have been investigated and optimized. At 150 °C, with acetonitrile as solvent instead of acetic acid and
in the absence of corrosive bromine, the conversion of p-xylene reached 100 % and the selectivity for ter-
ephthalic acid exceeded 97 %. Under the optimized conditions, Co-BTC exhibits stronger catalytic activity than
cobalt(II) acetate, and maintains excellent stability during the reaction. The reaction mechanism has been de-
duced, and the roles of N,N-dihydroxypyromellitimide and Co-BTC as synergistic catalysts in the reaction have
been clarified.
p-Xylene oxidation
Terephthalic acid
Co-benzenetricarboxylate
NDHPI
Metal–organic framework
1. Introduction
a hot topic of research.
N-Hydroxyphthalimide (NHPI) and its derivatives are effective
In the polyester industry, terephthalic acid (TA) is an important raw
material used primarily in the manufacture of non-toxic polyethylene
terephthalate (PET), which accounts for about 20 % of the polyester
market. PET is mainly used in machinery parts, fiber materials, and
packaging for foods and medicines.
catalysts for the oxidation of organic compounds by molecular oxygen
under mild conditions [6], and are widely used as initiators for the
oxidation of hydrocarbons. Tashiro et al. [7] used NHPI/Co(OAc)2/Mn
(OAc)2 as a catalyst and acetic acid as a solvent to oxidize PX at 100 °C.
The yield of TA reached 82 % after 14 h of reaction; when the tem-
perature was raised to 150 °C, the yield of TA reached 84 % after 3 h.
Koshino et al. found that the PINO formed by the -NO-H cleavage of
NHPI is more likely to capture a proton from the alkyl group on the
alkyl aromatic hydrocarbon in acetonitrile (MeCN) than in acetic acid;
during the oxidation of PX, the effect of N,N-dihydroxypyromellitimide
(NDHPI) as an initiator is superior to that of NHPI because it bears two
Heterogeneous catalysts have also been used to catalyze the oxi-
dation of PX [9]. Ratnasamy et al. loaded cobalt manganese oxide into a
zeolite to catalyze the oxidation of PX at high temperature (473 K) and
an air pressure of 550 psi. The conversion of PX was 100 %, and the
selectivity for TA was over 98 %, but corrosive bromine was still needed
as a co-catalyst to capture hydrogen from the methyl group [10]. Deka
et al. prepared porous CeO2 as a catalyst to catalyze the oxidation of PX
to TA. Using only water as a solvent under extremely mild conditions
(70 °C, 1 bar O2), the conversion of PX reached 100 % and the
Oxidation of p-xylene (PX) is the main route for obtaining TA. This
process is shown in Scheme 1, and is consistent with a mechanism of
hydrocarbon radical oxidation. In general, the difficulty of hydrocarbon
oxidation lies mainly in the initiation step, that is, the formation of
hydrocarbon radicals by the removal of hydrogen from alkyl groups
[1–4]. However, in the oxidation of PX, p-methyl benzyl radical and p-
TA will also be present [5], which are resistant to oxidation. Due to the
difficult oxidation characteristics of PX, current industrial production of
TA mainly relies on the harsh Amoco process, using cobalt(II) acetate,
manganese(II) acetate, and bromine as catalysts, and acetic acid as
solvent, which is conducted at high temperature (about 200 °C) and
high pressure (1.5–3.0 MPa). The selectivity for TA is over 95 %. Under
the high-temperature operating conditions, corrosive bromine acts as a
co-catalyst, resulting in high production and equipment costs. There-
fore, the quest for an efficient and mild catalyzed route for PX oxida-
tion, leading to TA without the need for acid and bromine, has become
⁎ Corresponding author at: College of Chemical Engineering, Xiangtan University, Xiangtan, 411105, Hunan, China.
Received 21 December 2019; Received in revised form 6 April 2020; Accepted 14 April 2020
0926-860X/©2020ElsevierB.V.Allrightsreserved.