An efficient production of benzene from benzoic acid in subcritical water using a copper(i) oxide catalyst

Article abstract of DOI:10.1039/c4gc01875c

A method combining subcritical water technology and a commercially available copper(i) oxide catalyst has been used for the production of benzene from benzoic acid. The benzene yield reached 91 mol% with 100% selectivity at 350 °C and ～25 MPa in 90 min. This is the first time the reaction from benzoic acid to benzene using an environmentally-friendly, efficient and economical method has been realized. The proposed reaction mechanism indicates that Cu₂O was an effective and stable catalyst, and the process was driven by the unique properties of subcritical water: the high ion product, the enhanced solubilisation of products, and the high diffusivity. This journal is

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Benzene yield from benzoic acid increased greatly to 90 mol%
with 100% selectivity in subcritical water catalyzed by Cu2O.

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An Efficient Production of Benzene from Benzoic Acid
in Subcritical Water Using a Copper (I) Oxide Catalyst
Qingxin Zheng, Masato Morimoto*, and Toshimasa Takanohashi*
reaction from benzoic acid to benzene involving an environmental-
friendly, efficient and economical system.
A method combining subcritical water technology and a
commercially available copper (I) oxide catalyst has been
performed for the production of benzene from benzoic acid.
The benzene yield reached 91 mol% with 100% selectivity at
350°C and ~25 MPa in 90 min. This is the first time to realize
the reaction from benzoic acid to benzene environmental-
friendly, efficient and economical. The proposed reaction
mechanism indicates that Cu2O was an effective and stable
catalyst, and the process was driven by the unique properties
of subcritical water: high ion product, the high solubility of
products and high diffusivity.
The current report describes the combination of SbCW technology
with a commercially available catalyst, Cu2O, for the production of
benzene from benzoic acid (dotted arrow in Scheme. 1). Despite its
simplicity, this idea had not been evaluated previously. Drastic
improvements in reaction performance were observed and a reaction
mechanism was proposed herein based on examination of various
influential factors. The development of an environmentally benign,
efficient and economical method is highly desirable.
The experimental procedure is described in the ESI and Table 1
summarizes the conditions and results. The first factor evaluated was
product selectivity. Previous research has shown that benzoic acid
under hydrothermal conditions can result in the production of
benzene, phenol, biphenyl, and benzophenone.10,
In the current
Produce of benzene from benzoic acid is one of the most basic
reactions in organic chemistry.1 In the past more than 100 years,
various methods have been proposed, as shown in Scheme. 1.
Initially, benzoic acid was heated with a large amount of alkali, such
as soda-lime,2 which proved environmentally harmful. A copper-
quinolone system latter was employed and the reaction rate was
enhanced by the addition of an acid.1, 3 Then Maier et al.4 obtained
benzene yields of 96% on an alumina-supported nickel catalyst at
180°C and 95% yield on a silica gel-supported palladium catalyst at
370°C, in the presence of excess hydrogen. Afterwards in a nitrogen
atmosphere, Takemura et al.3 acquired benzene yield of 86.2% on a
moist cerium cation-exchanged hydrogen Y type zeolite (CeHY) at
370°C after 60 min. While some methods above achieved high
conversion rates, their use of alkali, acid, organic solvents, hydrogen
or expensive catalysts are obvious drawbacks with regard to
environment, cost and natural resource availability. Recently,
hydrothermal methods using subcritical water (SbCW) or
supercritical water (SCW) have also been applied for the production
of benzene from benzoic acid. Hydrothermal method is inherently
environmentally benign,5-7 however, the relatively high stability of
benzoic acid in water greatly limits the efficiency of the reaction and
all the past relevant research shows that the yield of benzene
obtained from benzoic acid by hydrothermal method is quite low.8-12
For example, when benzoic acid was heated at 350°C in SbCW for
500 min, the benzene yield was less than 10%.12 Therefore, it is
significant and challenging to explore a novel method to realize the
12
study, processing benzoic acid at 350°C and 24-28 MPa (entry 5)
resulted in benzene as the sole product, based on the chromatogram
of dichloromethane-recovered products shown in Fig. 1 and the GC-
MS spectra of products dissolved in dichloromethane, hexane, and
decane, respectively, shown in Fig. S3. Thus, the method described
herein exhibited 100% selectivity in the conversion of benzoic acid
to benzene and carbon dioxide. Since no undesired products were
obtained, the molar yields of benzene shown in Table 1 were
determined from those of carbon dioxide, which are
stoichiometrically equivalent.
Scheme 1. Illustration of Benzene Production from Benzoic Acid by Different
Methods.
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Table 1. Influence of Catalyst Concentration, Benzoic Acid Concentration, Temperature, Pressure and Holding Time on the Formation of Benzene from
Benzoic Acid.
entry
Cu2O/benzoic acid
(mol %)
benzoic acid/water
(wt %)
temperature
(°C)
holding time
(min)
pressure
(MPa)
yield
(mol %)
number of
replicates
1
2
3
4
5
6
7
8
100
50
10
0
50
50
50
50
1.0
1.0
1.0
1.0
5.0
5.0
1.0
1.0
350
350
350
350
350
350
210
210
60
60
60
60
60
60
60
60
--
86.3 ± 0.6
45.8 ± 0.2
3.0 ± 0.1
2.4 ± 0.1
87.4 ± 0.3
48.2 ± 0.2
0.5 ± 0.2
0.2 ± 0.1
2
2
2
2
3
2
2
2
19.4-21.0
--
--
24.4-27.7
18.4-19.7
1.8-2.0
18.0-19.8
Temperature was an important factor in this reaction. When the
reaction was performed at 210°C and ~2 or 19 MPa for 60 min
(entries 7 and 8, respectively), benzene yields were less than 1
mol%. The temperature claimed in the patent of Pfirmann and
Schubert13 (210°C) had no effect even at high pressure. The yield at
350°C, however, was 45.8%. Thus, the reaction described herein
requires high temperatures for optimum efficiency.
The influence of pressure is evident from entries 5 and 6, which
were obtained at 350°C and ~26 and ~19 MPa, respectively.
Benzene yield increased from 48 to 87% with the increased pressure,
indicating that higher pressure was preferable; i.e., a higher density
of water was beneficial to the overall reaction. This result agrees
with previous reports that, when run at 400°C, the decarboxylation
rate is a function of water density.11 The effect of water density is
due to the unique properties of subcritical water, such as the
concentrations of hydronium/hydroxyl ions (H3O+/OH-) and the
solubility of organic compounds.
The ion product of water (Kw) reflects the degree of water
ionization and increases with increasing density in hydrothermal
Figure 1. Chromatogram obtained by GC/FID analysis of the obtained products.
The inset shows an enlarged region of the chromatogram.
conditions. The densities of water at 350°C and 26 and 19 MPa were
*
630 and 594 Kg/m3 with Kw values of 1.8 × 10-12 and 8.1 × 10-13
,
The necessity and stability of the Cu2O catalyst were also
respectively.14 This corresponds to higher levels of H3O+/OH- at
higher pressures. Our results suggest that the decarboxylation of
benzoic acid prefers higher ion concentrations, since higher
pressures yielded greater degrees of conversion (entries 5 and 6).
The solubility of benzene in SbCW may be another important factor
affected by pressure. The water–benzene mixture at 350°C exists as
a single phase at pressures greater than 20 MPa, but separates into
two phases at pressures ≤ 20 MPa.15 Therefore, it is possible that the
higher solubility of benzene under high-pressure conditions (entry 5)
contributed to the observed higher conversion. The high diffusivity
of SbCW may also facilitate a rapid reaction.
Fig. 2 shows the benzene yields as a function of temperature and
reaction time using 50 mol% of Cu2O to benzoic acid and 5 wt% of
benzoic acid to water (see entry 5). The pressure observed during the
experiment increased with increasing reaction time because the
number of product molecules (benzene and carbon dioxide) was
twice that of the reactants (benzoic acid). The benzene yield
increased with increasing reaction time, reaching as much as 91
mol% at 90 min. This yield is considerably higher than any other
attained in previous reports with no use of hydrogen.
examined. Entry
4
shows that without any catalyst the
decarboxylation of benzoic acid at 350°C and 60 min gave only trace
amounts of benzene, with a 2.4 mol% yield. This is in agreement
with the 5 mol% yield obtained in previous research using identical
conditions.12 This result shows that benzoic acid is almost inert when
subjected to direct decarboxylation in SbCW. In contrast, entries 1-4
show that benzene yield increases linearly with the amount of Cu2O
added to the reaction mixture. Under the conditions listed in entry 5,
the benzene yield reached 87.4 mol%, which is much higher than
that obtained using only SbCW or SCW, and even higher than that
reported yields using a CeHY catalyst in a nitrogen atmosphere.3
Thus, Cu2O is an excellent catalyst for the hydrothermal synthesis of
benzene from benzoic acid. Note that a relatively large amount of
catalyst was used because the commercially available Cu2O particles
had a very low specific surface area of 0.7 m2/g, as shown in Fig. S4.
The use of Cu2O particles with a higher specific surface area would
decrease the amount of catalyst required.
The catalyst was almost completely recovered after the
experiment (99%). By using ICP, the fraction of Cu2O leached into
water was determined to be as small as 0.08%. Fig. S5 shows SEM
micrographs of Cu2O particles before and after the experiment. It
appears that particle size increased slightly during the reaction,
possibly due to partial dissolution in SbCW. Figs. S6 and S7 show
the XRD diffractograms and XPS spectra of catalyst, respectively,
indicating that there were no changes in the crystal structure or
valence state of both copper and oxygen. These data indicate that the
Cu2O catalyst is stable in SbCW.
Based on past research on copper-quinoline decarboxylation,16
subcritical water technology, and the results of the current study, a
mechanism is proposed for the synthesis of benzene from benzoic
acid with Cu2O in SbCW, shown in Scheme. 2. The mechanism
involves the following steps: (1) benzoic acid is ionized with water
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complete dissolution of benzene in step 6, which enabled removal of
the product from the catalyst surface. This step was accelerated by
the high diffusivity of SbCW, which would also increase the
diffusion rates of steps 2, 4, and 6. Therefore, high-pressure
conditions yielded greater degrees of conversion. Thus, the method
described herein makes use of all of the advantages of SbCW: high
H3O+/OH- concentrations, high diffusivity, and enhanced solubility
of organic compounds.
We present a novel method for the production of benzene
from benzoic acid using a Cu2O catalyst in subcritical water at
~350°C and ~25 MPa. Without alkali, acid, organic solvents,
hydrogen or expensive catalysts, conversion yields reached >90
mol% in 90min with 100% selectivity. Based on the influence
of factors, a mechanism has been proposed to explain the
reaction process, indicating that Cu2O was an effective and
stable catalyst for the reaction, and the process was driven by
the unique properties of SbCW: a high concentration of
hydronium/hydroxyl ions (high ion product), the high solubility
of products and high diffusivity. This is the first time to realize
the reaction from benzoic acid to benzene in a really “green”
method: environmentally friendly, efficient and economical.
This method is applicable for the sustainable production of
benzene and enlightening for the development of hydrothermal
method in the current organic chemistry.
Figure 2. Yields of benzene as a function of temperature.
This work was supported by JST, Strategic International
Collaborative Research Program (SICORP).
Notes and references
Advanced Fuel Group, Energy Technology Research Institute, National
Institute of Advanced Industrial Science and Technology (AIST), 16-1
Onogawa, Tsukuba 305-8569, Japan.
Email: m.morimoto@aist.go.jp (M. Morimoto);
toshi-takanohashi@aist.go.jp (T. Takanohashi).
†
Electronic Supplementary Information (ESI) available: Experimental
section, yields of benzene as a function of temperature, a schematic
diagram of the reaction apparatus, processes to replace the interior air by
nitrogen and collect the gaseous product, GC-MS data, adsorption-
desorption isotherm, SEM micrographs, XRD diffractograms, and XPS
spectra. See DOI: 10.1039/c000000x/
Scheme 2. Proposed Mechanism for the Reaction from Benzoic Acid to Benzene
Catalyzed by Copper (I) Oxide in Subcritical Water.
to C6H5COO-, (2) the C6H5COO- ion is adsorbed onto the surface of
Cu2O, (3) a π- electron of the aromatic ring is withdrawn by the
positive charge on copper, resulting in an electron transfer from the
carboxyl ion to the aromatic ring, (4) a molecule of carbon dioxide is
released, (5) the negative charge on the aromatic ring is protonated
by H3O+ and benzene is formed with a molecule of water, (6)
benzene is removed from the surface of Cu2O by water. These six
steps therefore control the reaction and are subject to the various
influential factors discussed above. The ionic reaction on the surface
of Cu2O with no thermal decomposition resulted in 100% selectivity
for the production of benzene. The surface of Cu2O is required for
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