Hydroxylation of benzene to phenol under air and carbon monoxide catalyzed by molybdovanadophosphoric acid

Article abstract of DOI:10.1002/anie.200462769

(Chemical Equation Presented) The simpler, the better: Phenol is synthesized by the direct oxidation of benzene under air (15 atm) and CO (10 atm) with molybdovanadophosphoric acid as the catalyst (see scheme). Activated molecular oxygen serves as the oxidant and phenol is produced in 28% yield. The catalyst can be recovered and reused.

Full text of DOI:10.1002/anie.200462769

Communications
Phenol Synthesis
The reaction of 1 under air (15 atm) and CO (5 atm) with
H7PMo8V4O40·nH2O[12] catalyst in acetic acid/water (9:1; 13.5/
1.5 mL) at 908C for 15 h afforded 2 in 27.3% yield (turnover
number (TON) = 21.8), along with a small amount of 1,4-
benzoquinone (3; 3.4% yield), which was a further oxidation
product of 2 (Table 1, run 2). About 70% of the CO was
Hydroxylation of Benzene to Phenol under Air
and Carbon Monoxide Catalyzed by
Molybdovanadophosphoric Acid**
Masayuki Tani, Takao Sakamoto, Shuichi Mita,
Satoshi Sakaguchi, and Yasutaka Ishii*
Table 1: Oxidation of benzene (1) to phenol (2) under air and CO by
H7PMo8V4O40.[a]
Run
Air:CO
[atm]
AcOH/H2O
[mL]
Conv.
[%][b]
Yield [%][c]
TON
[mol/mol]
Phenols are one of the most important classes of raw
materials in the chemical industry, and a variety of com-
pounds are derived from them, such as resins, dyes, and
pharmaceuticals.[1] In 2000, the worldwide production of
phenol was 6.6 megatons. Most of this phenol is produced by
the cumene process reported by Hock and Lang in 1944.[2]
The direct conversion of benzene to phenol by hydroxylation
with O2 is the most desirable method among phenol syntheses.
However, indirect phenol production through the formation
of cumene hydroperoxide and oxidative decomposition of
benzoic acid are currently employed. The direct synthesis of
phenol from benzene and nitrous oxide has been described,[3]
but this method is only cost-effective when nitrous oxide is
available cheaply as a by-product.[4] Therefore, for a long time
much attention has been paid to the direct synthesis of phenol
by the hydroxylation of benzene with O2, which is referred to
as a dream oxidation in the chemical industry. There have
been many attempts at the direct conversion of benzenes to
phenols with H2O2[5] or O2,[6] under the influence of a reducing
agent, such as H2[7] or CO[8] and transition metals, such as Cu,
Pd, or Pt,[9] or heteropolyoxometalates.[10] Recently, Mizu-
kami et al. reported a one-step conversion of benzene to
phenol with O2 and H2 at a palladium membrane.[7]
2
3
1
2
3
4
5
6
7
8
17.5:2.5
15:5
15:10
10:10
20:0
15:5
15:5
15:5
15:5
13.5/1.5
13.5/1.5
13.5/1.5
13.5/1.5
13.5/1.5
18/2
9/1
4.5/0.5
15.0/0
33.0
41.6
47.4
44.5
n.r.
41.4
41.0
44.7
n.r.
20.9
27.3
28.1
25.9
3.2
3.4
4.0
2.8
16.7
21.8
22.4
20.7
26.9
21.0
18.1
3.5
3.7
4.1
21.5
16.8
14.5
9
10[d]
11[e]
15:5
15:5
13.5/1.5
13.5/1.5
35.8
n.r.
23.3
3.6
18.6
[a] 1 (2 mmol) was reacted under air and CO in aqueous acetic acid in
the presence of AcONa (0.1 mmol) with H7PMo8V4O40·nH2O (25 mmol,
1.25 mol% based on H7PMo8V4O40·24H2O) at 908C for 15 h. [b] n.r. =no
reaction. [c] Based on 1 used. [d] Without NaOAc. [e] Air:H2 =15:5
(atm); H2 was used instead of CO.
recovered in this reaction, and the selectivity for 2 based on
CO consumed was approximately 35%. This is the first
successful conversion of 1 into 2 in high yield by molybdo-
vanadophosphoric acid under mild conditions.[13] It was found
that the yield of 2 was influenced by the partial pressures of
air and CO. For instance, the reaction under high air pressure
(17.5 atm) and low CO pressure (2.5 atm) resulted in a
decrease of the conversion of 1 into 2 (20.9%; run 1). Under a
CO pressure of 10 atm and an air pressure of 15 or 10 atm
(runs 3 and 4, respectively), the results were almost the same
as those for run 2. In the absence of CO, no reaction was
induced (run 5). The reaction was considerably influenced by
the amount of solvent mixture and the water content (runs 6–
9). No reaction took place in anhydrous acetic acid (run 9).
The reaction without sodium acetate resulted in a slight
decrease in the yield of 2 (run 10). No reaction took place
with a combination of air and H2 in place of CO as the
reducing agent (run 11). It is thought that CO, which can be
easily obtained from abundant cheap carbon resources, such
as asphalt and pitch, is a superior reducing agent to H2 derived
from methane and naphtha,[14] although the present method
brought about the concurrent formation of phenol and a
stoichiometric amount of CO2.
In the course of our study on the coupling of benzene with
acrylates in the presence of O2 by Pd(OAc)2 combined with
molybdovanadophosphates, we found that a small amount of
phenol is formed as a by-product.[11] Thus, our attention has
been focused on the direct conversion of benzene (1) into
phenol (2), and an efficient simple catalytic method for the
synthesis of 2 from 1 using molybdovanadophosphoric acids
as catalysts has been established [Eq. (1)].
[*] M. Tani, T. Sakamoto, S. Mita, S. Sakaguchi, Prof. Y. Ishii
Department of Applied Chemistry
Faculty of Engineering and High Technology Research Center
Kansai University
Figure 1 shows the time-dependence of the hydroxylation
of 1. The production of 2 increased smoothly with time up to
five hours and then continued at a constant level, while, 2 was
slowly converted into 3 at the same time. The fact that 2 was
converted into the overoxidized products including 3 by its
individual exposure to CO and air in the presence of the
catalyst supports this interpretation.
Suita, Osaka 564-8680 (Japan)
Fax: (+81)6-6339-4026
E-mail: ishii@ipcku.kansai-u.ac.jp
[**] This work was partially supported by MEXT.KAKENHI
(No.15750095).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2586
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DOI: 10.1002/anie.200462769
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Angewandte
Chemie
Table 3: Oxidation of benzene (1) to phenol (2) under air and CO by
recovered catalyst.[a]
Run
Recovery
Yield [%][b]
2
3
1
2
3
4
5
1st
2nd
3rd
4th
5th
27.5
27.5
28.5
28.9
28.0
4.3
5.3
2.0
2.5
3.2
[a] Reactions were carried out under the same conditions as those for
run 2 in Table 1. [b] Based on 1 used. The conversion of 1 was 40–42%.
Ph16OH and Ph18OH. This finding shows that a nucleophilic
attack of H218O to form a cationic benzene intermediate
seems to be unlikely as a major route to 2. Second, the
hydroxylation of 1 under CO and 18O2 instead of 16O2 in acetic
acid and H218O was examined [Eq. (2)].
Figure 1. Time-dependence curve for the hydroxylation of 1 under the
same conditions as run 6 in Table 1.
Table 2 shows the results of the hydroxylation of 1 in the
presence of several molybdovanadophosphoric acids. The
reaction was also promoted by H3+nPMo12ꢀnVnO40·nH2O (n =
Table 2: Oxidation of benzene (1) to phenol (2) under air and CO
catalyzed by several molybdovanadophosphoric acids.[a]
Run
Catalyst
Conv.
[%]
Yield [%][b]
2
3
GC–MS measurement of the resulting phenol indicated
that an approximate 8:2 mixture of Ph18OH and Ph16OH is
formed. The preferential formation of Ph18OH indicates that
gas-phase oxygen (18O2) serves as an oxygen source in this
hydroxylation reaction. Also, it is likely that the 16O atom in
the Ph16OH comes from the oxygen in the H7PMo8-
V4O40·nH2O structure. In fact, it was found that
H7PMo8V4O40 was easily reduced with CO under these
conditions. For example, when H7PMo8V4O40·nH2O
(0.5 mmol) in acetic acid was treated with CO (5 atm, ca.
11 mmol) at 908C for 5 h, about 1 mmol of CO2 was formed,
and the color of the solution changed from orange to dark
green. The formation of CO2 and this color change of the
catalyst solution suggest that H7PMo8V4O40 is easily reduced
to H7PMo8V4O40ꢀx, in which the oxygen is partially reduced as
well. On the other hand, no such color change of the catalyst
was observed when H2 instead of CO was used for the
reduction of H7PMo8V4O40. This is in agreement with the fact
that the hydroxylation of 1 to 2 did not take place when H2
was employed in place of CO (Table 1, run 11).
1
2
3
4
5
H3PMo12O40
H4PMo11V1O40
H5PMo10V2O40
H6PMo9 V3O40
VO(acac)2
n.r.
22.8
36.8
33.6
13.6
21.3
22.2
n.r.
1.2
3.0
2.1
[a] Reactions were carried out under the same conditions as those for
run 2 in Table 1. [b] Based on 1 used.
1–3)[12] to give 2 in yields of 13–22%, but H3PMo12O40·nH2O,
which lacked the vanadium atom, did not catalyze the
hydroxylation. These results indicate that the vanadium
species is a key element for the hydroxylation of benzene,
although an active species is not identified because of the
complexity of the molybdovanadophosphoric acids used as
catalysts. Their 31P NMR spectra (see Supporting Informa-
tion) attest to the existence of many molybdovanadophos-
phoric acids that contain varying compositions of P, Mo, and
V. However, no reaction occurred by using [VO(acac)2]
(acac = acetylacetonate) as a catalyst (run 5). This result will
be discussed later.
From a practical synthetic viewpoint, reuse of the
recovered catalyst is very important. Thus, the hydroxylation
was repeated with recovered catalyst under the same
conditions as those for run 2 in Table 1. The yields of 2 and
3 were almost the same throughout five runs, that is, 27–29%
for 2 and 2–5% for 3, in the 40–42% conversion of 1
(Table 3). The 31P NMR spectrum of the recovered catalyst
was only slightly different from that of the fresh one (see
Supporting Information).
It may be assumed that the oxidation proceeds through a
Fenton-type reaction of 1 with H2O2 generated in situ (CO +
O2 + H2O!H2O2 + CO2), since H2O2 is reported to be
formed with high turnover frequency from CO, O2, and
H2O in the presence of a Pd catalyst.[15] Thus, we examined
the hydroxylation of benzene with H2O2 catalyzed by
molybdovanadophosphoric acid. Treatment of 1 with 35%
H2O2 (1 equiv) in the presence of H6PMo9V3O40 in AcOH at
908C for 8 h produced 2 in low yield (6–7%) based on the
amount of 1 used. However, hydroxylation by H2O2 with
[VO(acac)2] in place of the heteropoly acid afforded 2 in 3–
4% yield, whereas the reaction under CO and air was not
catalyzed by [VO(acac)2] at all (see Table 2, run 5). This fact
Two labeling experiments were carried out to obtain
mechanistic information on the hydroxylation. The reaction
using H218O in place of H216O afforded an 8:2 mixture of
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Communications
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addition, the reaction using 16O2 and H218O gave an 8:2
mixture of Ph16OH and Ph18OH as shown above. If H2O2 is
formed in this reaction, H16O18OH would be generated from
16O2 and H218O, and as a consequence an approximate 1:1
mixture of Ph16OH and Ph18OH would be obtained. There-
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In conclusion, we have developed a simple, direct
synthetic method of producing phenol (2) from benzene (1)
with air in the presence of CO by using molybdovanadophos-
phoric acids as catalysts. This method may provide a promis-
ing alternative route to 2 from 1 in place of the cumene
process, which needs three reaction steps.
Experimental Section
Oxidation of benzene: in a typical procedure, the reaction was carried
out in a teflon autoclave. Benzene (2 mmol) was added to a solution
of H7PMo8V4O40·24H2O (26.5 mmol) and AcONa (0.1 mmol) in
AcOH (13.5 mL) and water (1.5 mL). The mixture was stirred at
908C for 15 h under air (15 atm) and CO (5 atm). After the reaction,
GC and GC–MS analyses were performed. The conversions and
yields of products were estimated from the peak areas, based on the
GC internal standard technique.
Recovery of the catalyst: After the reaction, H2O (5 mL) was
added to the mixture, followed by extraction with iPr2O (40 mL). The
aqueous layer was washed with iPr2O (40 mL), and the combined
aqueous extracts were concentrated by evaporation under reduced
pressure. The green solid obtained was dried in vacuo to give the
recovered catalyst. The 31P NMR spectra of the fresh and recovered
catalysts were similar.
Received: December 1, 2004
Published online: March 22, 2005
Keywords: hydrocarbons · hydroxylation · oxidation · oxygen ·
.
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2588
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www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 2586 –2588