Chin. J. Chem. Eng., 15(6) 895—898 (2007)
Hydroxylation of Benzene with Hydrogen Peroxide over Highly
Efficient Molybdovanadophosphoric Heteropoly Acid Catalysts*
ZHANG Fumin(张富民), GUO Maiping(郭麦平), GE Hanqing(葛汉青) and WANG Jun(王军)**
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
Abstract Keggin type molybdovanadophosphoric heteropoly acids, H3+nPMo12－nVnO40 (n＝1—3), were prepared
by a novel environmentally benign method, and their catalytic performances were evaluated via hydroxylation of
benzene to phenol with hydrogen peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile.
Various reaction parameters, such as reaction time, reaction temperature, ratio of benzene to hydrogen peroxide,
concentration of aqueous hydrogen peroxide, ratio of glacial acetic acid to acetonitrile in solvent and catalyst concentration, were changed to obtain an optimal reaction conditions. H3+nPMo12－nVnO40 (n＝1—3) are revealed to be
highly efficient catalyst for hydroxylation of benzene. In case of H5PMo10V2O40, a conversion of benzene of 34.5%
with the selectivity of phenol of 100% can be obtained at the optimal reaction conditions.
Keywords hydroxylation, heteropoly acid, phenol, benzene
1
INTRODUCTION
Heteropoly acids (HPAs) are transition metal
oxygen anion clusters which are prepared with a wide
range of molecular weight, composition and architecture[1]. Among them, the Keggin-type HPAs have
been widely studied as catalyst for acid-catalyzed and
oxidative reactions[2—5], because their redox and
acid-base properties can be tailored by choosing various heteroatoms, and/or substituting the addenda atom
with different ions[6]. Particularly, vanadium-substituted
Keggin-type molybdophosphoric heteropoly acids
(H3+nPMo12－nVnO40·xH2O) have attracted much attention as catalyst for a variety of oxidation reactions.
Hydroxylation of benzene is of great interest
since it leads to phenol, an important chemical industrial feedstock. Most of current studies on direct oxidation of benzene to phenol in liquid phase uses hydrogen peroxide or oxygen gas as the oxidant. The
reaction is usually carried out in acetone, methanol or
acetonitrile using the vanadium- or titanium-containing
heterogeneous catalysts such as Ti/MCM-41[7],
TS-1[8], supported VOx[9,10] and so on. Recently,
iron- and chromium-containing phosphotungstate salts
are reported as efficient catalyst for benzene hydroxylation[11,12]. Vanadium substituted phosphotungstate
and phosphomolybdate are also reported as catalyst
for phenol production under various reaction conditions[13—17]. However, the conversions of benzene
reported in the previous literature using HPAs as catalyst are still comparatively low. This has motivated us
to synthesize the molybdovanadophosphoric acids and
to optimize the reaction conditions so as to improve
the yield of phenol.
Very recently, a new environmentally benign
method was reavealed for the synthesis of the
Keggin-structured heteropoly acids H3+nPMo12－nVnO40
(abbreviated as PMoVn, n＝1—3)[18] by the reaction
of an aqueous slurry which contained stoichiometric
amounts of MoO3, V2O5 and H3PO4. This new method
is significantly different from the conventional procedure using H2SO4 acidification combined with ether
extraction[19]. In this work, using the prepared
PMoVn (n＝1—3) by the environmentally benign
method as catalyst for hydroxylation of benzene to
phenol with hydrogen peroxide as oxidant in a mixed
solvent of glacial acetic acid and acetonitrile, the
various reaction conditions are investigated and it is
confirmed that H3+nPMo12－nVnO40 are highly efficient
catalyst for this reaction.
2
EXPERIMENTAL
NaVO3 and H3PMo12O40 (PMo) were purchased
from Sinopharm Chemical Reagent Co. with AR purity. Molybdovanadophosphoric heteropoly acids
PMoVn (n＝1—3) were synthesized according to the
previous report[18]. For H4PMo11VO40·xH2O (PMoV1),
44.47g MoO3 and 2.82g V2O5 were dissolved in
770ml distilled water, respectively, and the two solutions were mixed and heated to 373K under reflux and
stirring, followed by the addition of the 3.56g aqueous
solution of 85% H3PO4. The slurry was further heated
at 373K for 24h so as to obtain a clear solution of
salmon pink color, followed with a vacuum heating at
327K to collect the orange solid. The final solid was
obtained by recrystallization in water with the initial
concentration of 59% by mass for three times at 277K.
The synthesis of PMoV2 and PMoV3 was similar to
the above procedure except that the different ratios of
the reactants were used to match their molecular formulas. The results of inductively coupled plasma (ICP)
analysis, thermal gravity analysis/differential thermal
analysis (TGA/DTA), infrared spectroscopy (IR),
ultra-violet-visible spectroscopy (UV-Vis) and X-ray
diffraction (XRD) indicated that the prepared samples
were Keggin-type HPAs, as shown by previous study[18].
Received 2006-10-26, accepted 2007-10-12.
* Supported by the National Natural Science Foundation of China (Nos.20306011, 20476046) and the Ph.D. Program Foundation for Chinese Universities (No.20040291002).
** To whom correspondence should be addressed. E-mail: [email protected]
Chin. J. Ch. E. (Vol. 15, No. 6)
896
The liquid phase hydroxylation of benzene was
carried out in a 120ml home-made glass reactor fitted
with a water-cooled condenser and a magnetic stirrer.
In a typical procedure, 0.06mmol of catalyst (approximately accounting for 0.42% by mass in the reaction medium) was added to 40mmol of benzene,
which was dissolved in the mixed solvent of 10ml of
glacial acetic acid and 10ml of acetonitrile, then the
mixture was heated to 338K. 120mmol of hydrogen
peroxide (30% aqueous solution) was added into the
above suspension within 0.5h by a syringe pump, followed by further vigorous stirring of 6h. Using the
mixed solvent for this reaction, a homogeneous phase
was observed and the catalyst was well dissolved in
the reaction media due to the polarity of the solvent.
The reaction mixture was sampled periodically, followed by the gas chromatography (GC) analysis with
a flame ionization detector (FID) and a 30m SE-54
capillary column. The area calibration method based
on GC technique was used to calculate the conversion
of benzene and product selectivity.
3
RESULTS AND DISCUSSION
The prepared three samples PMoV1, PMoV2 and
PMoV3 and two controlled samples NaVO3 and
H3PMo12O40 (PMo) were evaluated as catalysts for
hydroxylation of benzene to phenol. The results are
shown in Table 1. Over all samples, phenol was the
only product detected by GC-MS (Thermo Finnigan)
under the employed reaction conditions. It can be seen
from Table 1 that no phenol was observed when PMo
without substituted vanadium was used as catalyst,
and a comparatively low conversion of benzene was
observed over NaVO3. By contrast, when
vanadium-substituted PMoVn were used as catalyst,
much higher conversion of benzene was obtained.
This implies that vanadium in the Keggen structure of
HPAs is essential for catalyzing the hydroxylation
reaction, which is in accordance with the results in
previous reports[15—17]. Furthermore, the conversion of benzene did not increase monotonically with
the content of substituted vanadium atoms in PMoVn
catalysts, i.e., it increased in the following order:
PMoV3 ＜PMoV1 ＜PMoV2. This order is different
from those in previous reports on the reaction carried
out in acetonitrile or acetic acid[16,17]. However, it is
noteworthy that low effective utilization of H2O2 was
Table 1
observed over all catalysts mostly due to the decomposition of H2O2 along with hydroxylation.
Comparing the TONV data with TON in Table 1,
the activity in terms of TONV did not follow the same
order as that of TON. This suggests that the vanadium
atoms in PMoV1 are more catalytically effective than
those in other catalysts. The high catalytic activity of
vanadium species existing in the Keggin unit may
originate from the cooperative action of molybdenum
framework with vanadium centers[15].
Figure 1 shows the behavior of the benzene conversion as a function of reaction time over the PMoV2
catalyst. The conversion of benzene sharply increased
at the initial reaction stage, and afterwards reached the
maximum value of 34.5% at reaction time of 6h. This
maximum conversion is evidently higher than the previous result with 26% conversion using acetic acid as
solvent in 100min[17], which may be attributed to the
promotion effect of the mixed solvent of glacial acetic
acid and acetonitrile. With increasing the reaction time
further, the conversion of benzene remained nearly
constant due to the depletion of H2O2 mainly by the
self-decomposition.
Figure 1 Influence of reaction time on the conversion of
benzene to phenol over the PMoV2 catalyst
(Conditions: 338K, benzene 40mmol, aqueous H2O2 of 30%
120mmol, catalyst concentration 0.42% by mass,
glacial acetic acid 10ml, acetonitrile 10ml)
The influence of reaction temperature on the
conversion of benzene was investigated over PMoV2
by varying the temperature from 308K to 348K, while
other reaction parameters were kept constant, as
shown in Fig.2. The V-substituted HPAs were reported
to decompose at more than 473K[3], thus, the PMoV2
catalyst is able to maintain its Keggin structure during
Catalytic performances of various catalysts in benzene hydroxylation to phenol
Catalyst
Conversion of benzene, %
Selectivity, %
TON①
TONV②
Efficiency of H2O2③, %
PMo
0
—
—
—
0
NaVO3
10.0
100
4
4
3.3
PMoV1
31.2
100
222
222
10.4
PMoV2
34.5
100
240
120
11.5
PMoV3
28.6
100
194
65
9.5
Note: Reaction conditions: 338K, 6h, benzene 40mmol, aqueous H2O2 (30%) 120mmol, glacial acetic acid 10ml, acetonitrile 10ml,
catalyst concentration 0.42% by mass.
① Turnover number calculated as moles of phenol formed on one mole of catalyst within 6h of reaction time.
② Turnover number calculated as moles of phenol formed on one mole of V in catalyst within 6h of reaction time.
③ Efficiency of H2O2＝100×products (mol of phenol)/total mol of added H2O2.
December, 2007
Hydroxylation of Benzene with Hydrogen Peroxide
Figure 2 Influence of reaction temperature on the
conversion of benzene to phenol over the PMoV2 catalyst
(Conditions: 6h, benzene 40mmol, aqueous H2O2 of 30%
120mmol, catalyst concentration 0.42% by mass,
glacial acetic acid 10ml, acetonitrile 10ml)
benzene hydroxylation in this temperature range. The
conversion of benzene over the PMoV2 catalyst increased steadily with increasing reaction temperature,
and reached the plateau value of 34.5% at 338K.
Table 2 shows the conversion of benzene over
PMoV2 at different catalyst concentrations in reaction
medium. It can be seen that the conversion of benzene
sharply increased with the increase of the concentration of PMoV2 up to 0.42%, corresponding to the
maximum conversion of 34.5% (Table 2, Entry 4).
However, further increasing the concentration of
PMoV2 led to the decrease of the conversion of benzene. The same phenomenon was also observed by
Ishida et al.[20] who studied the supported vanadium
catalysts in liquid phase hydroxylation of benzene
897
using in situ generated hydrogen peroxide. They found
that vanadium loadings had an optimum critical value
and interpreted it based on the assumption that the
catalytically active species was monomeric vanadium
species. It was proposed that the high vanadium loading favored the existence of vanadium in the form of
inactive oxo dimer. The results obtained in the present
work are consistent with this viewpoint, i.e., the excess of PMoV2 used in the reaction medium resulted
in the decrease of benzene conversion.
It has been reported that the addition of acidic
component strongly affected the catalytic reactivity of
hydroxylation reaction with H2O2 as oxidant[21].
Therefore, the effect of the amount of glacial acetic
acid on hydroxylation of benzene was examined, as
indicated in Table 3. When the same reaction was carried out in a mixture of 13.3ml of CH3CN and 6.7ml
of glacial acetic acid, or 6.7ml of CH3CN and 13.3ml
of glacial acetic acid, the product of hydroxylation
was improved by 4%—6% than that in pure acetonitrile. However, the use of more glacial acetic acid
could not further increase the conversion of benzene
(Table 3, Entry 5). A mixed solvent of CH3CN and
glacial acetic acid in a volume ratio of 1︰1 (Table 3,
Entry 2) was the appropriate solvent for this reaction.
Table 3 Effect of the mixed solvent of glacial acetic acid
(HOAc) with CH3CN on the hydroxylation of benzene
to
phenol with H2O2 catalyzed by PMoV2①
Entry
HOAc/CH3CN
(by volume)
Conversion of
benzene, %
TON②
1
0
21.6
150
Table 2 Influence of catalyst concentration on the
conversion of benzene over the PMoV2 catalyst①
2
1︰2
27.2
189
3
1︰1
34.5
240
Entry Catalyst concentration, % Conversion of benzene, %
1
0
0
2
0.14
12.4
3
0.28
19.7
4
0.42
34.5
5
0.56
28.7
6
0.70
22.3
7
0.84
21.6
① Reaction conditions: 338K, 6h, benzene 40mmol, aqueous
H2O2 (30%) 120mmol, glacial acetic acid 10ml, acetonitrile
10ml.
4
2︰1
25.4
177
5
∞
24.3
169
Table 4
Reaction conditions: 338K, 6h, benzene 40mmol, aqueous
H2O2 (30%) 120mmol, catalyst concentration 0.42% by mass,
total volume of solvent 20ml.
② Turnover number calculated as moles of phenol formed on
one mole of PMoV2 in catalysts in 6h.
①
The effects of the amount of H2O2 added into reaction medium on the hydroxylation of benzene were
investigated, as shown in Table 4. The gradual decrease of the amount of H2O2 resulted in the gradual
Effect of the molar ratio of H2O2 to benzene on the hydroxylation of benzene to phenol catalyzed by PMoV2①
Entry
H2O2/benzene (molar ratio)
Concentration of H2O2, %
Conversion of benzene, %
1
0.5
30
5.6
2
1.0
30
13.7
3
2.0
30
24.6
4
3.0
30
34.5
5
4.0
30
31.3
6
3.0
20
33.7
7
3.0
10
29.6
① Reaction conditions: 338K, 6h, benzene 40mmol, glacial acetic acid 10ml, acetonitrile 10ml.
② Turnover number calculated as moles of phenol formed on one mole of PMoV2 in catalysts in 6h.
TON②
39
95
171
240
218
234
206
Chin. J. Ch. E. 15(6) 895 (2007)
Chin. J. Ch. E. (Vol. 15, No. 6)
898
decrease of conversion of benzene, indicating that
increasing of the amount of H2O2 seems to be an effective way to increase the yield of phenol. As known,
the stoichiometric ratio of H2O2 to benzene for the
hydroxylation reaction is 1︰1, while the results show
that the H2O2 needed for the favorable phenol yield is
about three times of its stoichiometry. In fact, in the
reaction of benzene hydroxylation, hydrogen peroxide
consumed in its self-decomposition is much more than
that consumed in the hydroxylation reaction[17,21].
However, when too much aqueous H2O2 (molar ratio
of H2O2/benzene＞4) was added at the start of the
reaction, the reaction mixture did not easily become
homogeneous even at 343K, which resulted in the
decrease of conversion of benzene. The catalytic activities as a function of the concentration of aqueous
H2O2 feed from 10% to 30% by mass are also compared in Table 4. As can be seen, the low initial H2O2
concentration led to slightly lower conversion of benzene.
4
CONCLUSIONS
By experimenting over wide ranges of reaction
parameters for the hydroxylation of benzene into
phenol over the H3+nPMo12-nVnO40 (n＝1—3) catalysts, the optimal conditions are determined, namely
the reaction temperature is 338K, reaction time of 6h,
the catalyst concentration at 0.42% (by mass, the molar ratio of H2O2 to benzene being 3︰1 using 30% by
mass) aqueous H2O2 solution as oxidant, and moreover, the mixed solvent of glacial acetic acid and acetonitrile with the volume ratio of 1︰1 is suitable for
this reaction. In this case, H5PMo10V2O40 shows the
very high benzene conversion of 34.5% with 100%
phenol selectivity. This indicates that the molybdovanadophosphoric heteropoly acids prepared herein are
catalytically highly efficient catalysts for the hydroxylation of benzene.
7
8
9
10
11
12
13
14
15
16
17
REFERENCES
1
2
3
4
5
6
Mizuno, N., Misono, M., “Heterogenous catalysis”,
Chem. Rev. Sci. Eng., 98, 199—217(1998).
Keggin, J.F., “Structure of the molecule of
12-phosphotungstic acid”, Nature, 131, 908—909(1933).
Kozhevnikov, I.V., “Heteropoly acids and related-compounds as catalysts for fine chemical synthesis”, Catal. Rev. Sci. Eng., 37(2), 311—352(1995).
Zhang, F.M., Yuan, C.S., Wang, J., Kong, Y., Zhu, H.Y.,
Wang, C.Y., “Synthesis of fructone over dealmuinated
USY supported heteropoly acid and its salt catalysts”, J.
Mol. Catal. A Chem., 247, 130—137(2006).
Shinachi, S., Matsushita, M., Yamaguchi, K., Mizuno, N.,
“Oxidation of adamantane with 1 atm molecular oxygen
by vanadium-substituted polyoxometalates”, J. Catal.,
233, 81—89(2005).
Okuhara, T., Mizuno, N., Misono, M., “Catalytic chemistry of heteropoly compounds”, Adv. Catal., 41, 113—
December, 2007
18
19
20
21
252(1996).
Zhang, W.Z., Wang, J.L., Tanev, P.T., Pinnavaia, T.J.,
“Catalytic hydroxylation of benzene over transition-metal substituted hexagonal mesoporous silicas”,
Chem. Commun., (8), 979—980(1996).
Bengoa, J.F., Gallegos, N.G., Marchetti, S.G., Alvarez,
A.M., Cagnoli, M.V., Yeramian, A.A., “Influence of TS-1
structural properties and operation conditions on venzene
catalytic oxidation with H2O2”, Micropor. Mesopor. Mater., 24, 163—172(1998).
Chen, Y.W., Lu, Y.H., “Characteristics of V-MCM-41 and
its catalytic properties in oxidation of benzene”, Ind. Eng.
Chem. Res., 38, 1893—1903(1999).
Das, S.K., Kumar, A.J., Nandrajog, S., “Polymer-supported VO2+ schiff-base catalyst for hydroxylation of benzene”, Tetrahedron Lett., 36, 7909—7912(1995).
Kuznetsova, L.I., Detusheva, L.G., Fedotov, M.A., Likholobov, V.A., “Catalytic properties of heteropoly complexes containing Fe(III) ions in benzene oxidation by
hydrogen peroxide”, J. Mol. Catal. A Chem., 111, 81—
90(1996).
Kuznetsova, N.I., Kuznetsova, L.I., Likholobov, V.A.,
“Catalytic properties of Cr-containing heteropolytungstates in H2O2 participated reactions: H2O2 decomposition and oxidation of unsaturated hydrocarbons with
H2O2”, J. Mol. Catal. A Chem., 108, 135—143(1996).
Seo, Y.J., Mukai, Y., Tagawa, T., Goto, S., “Phenol synthesis by liquid-phase oxidation of benzene with molecular oxygen over iron-heteropoly acid”, J. Mol. Catal.
A Chem., 120, 149—154(1997).
Parsoni, L.C., Cruz, A.T., Buffon, R., Schuchardt, U.,
“Complexes of palladium(II) and platinum(II) with the
7−
PW11O39
heteropolyanion as catalytically active species
in benzene oxidation”, J. Mol. Catal. A Chem., 120,
117—123(1997).
Nomiya, K., Yagishita, K., Nemoto, Y., Kamataki, T.A.,
“Functional action of Keggin-type mono-vanadium(V)substituted heteropolymolybdate as a single species on
catalytic hydroxylation of benzene in the presence of hydrogen peroxide”, J. Mol. Catal. A Chem., 126, 43—
53(1997).
Alekar, N.A, Indira, V., Hallingudi, S.B., Srinivas, D.,
Gopinathan, S., Gopinathan, C., “Kinetics and mechanism of selective hydroxylation of benzene catalysed by
vanadium substituted heteropolymolybdates”, J. Mol.
Catal. A Chem., 164, 181—189(2000).
Zhang, J., Tang, Y., Li, G.Y., Hu, C.W., “Room temperature direct oxidation of benzene to phenol using hydrogen peroxide in the presence of vanadium-substituted
heteropolymolybdates”, Appl. Catal. A Gen., 278, 251—
261(2005).
Zhang, F.M., Guo, M.P., Ge, H.Q., Wang, J., “A new
method for synthesis of molybdovanadophosphoric heteropoly acids and their catalytic activities”, Chem. Ind.
Eng. Prog., 25(10), 1171—1174(2006). (in Chinese)
Tsigdinos, G.A., Hallada, C.J., “Molybdovanadophosphoric acids and their salts (I) Investigation of methods
of preparation and characterization”, Inorg. Chem., 7,
437—441(1968).
Ishida, M.A., Masumoto, Y., Hamada, R., Nishiyama, S.,
Tsuruya, S., Masai, M., “Liquid-phase oxygenation of
benzene over supported vanadium catalysts”, J. Chem.
Soc. Perkin Trans., 2(4), 847—854(1999).
Gao,
F.X.,
Hua,
R.M.,
“Highly
efficient
K7NiV13O38-catalyzed hydroxylation of aromatics with
aqueous hydrogen peroxide (30%)”, Appl. Catal. A Gen.,
270, 223—226(2004).