Highly selective synthesis of biphenyl by the Pd(OAc)2/HPA/O2/AcOH catalyst system

Article abstract of DOI:10.1016/S0022-328X(02)01901-0

A new catalytic system, dimerization of benzene with 100% selectivity and 19% yield under the conditions of 130 C, 10 atm and 4 h. PdHPMo₁₂O ₄₀ itself was also found to act as a catalyst to give 95% of selectivity of biphenyl with a lower yield. A plausible mechanism of this system is also discussed.

Full text of DOI:10.1016/S0022-328X(02)01901-0

Journal of Organometallic Chemistry 664 (2002) 59Á65
/
www.elsevier.com/locate/jorganchem
Highly selective synthesis of biphenyl by the
Pd(OAc)2/HPA/O2/AcOH catalyst system
Makoto Okamoto, Midori Watanabe, Teizo Yamaji ꢀ
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581,
Japan
Received 4 July 2002; received in revised form 6 September 2002; accepted 12 September 2002
Abstract
A new catalytic system, Pd(OAc)2/H3PMo12O40/O2/AcOHÁH2O (2:1), has been found to give biphenyl by the oxidative
/
dimerization of benzene with 100% selectivity and 19% yield under the conditions of 130 8C, 10 atm and 4 h. PdHPMo12O40 itself
was also found to act as a catalyst to give 95% of selectivity of biphenyl with a lower yield. A plausible mechanism of this system is
also discussed.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Biphenyl; Oxidative dimerization; Benzene; Pd(OAc)2; H3PMo12O40
1. Introduction
absorbed oxygen at 84 8C [8]. As seen from these data it
seemed to be difficult to increase the selectivity for
biphenyl because of the preferential hydroxylation of
benzene at a high temperature.
We have reported that biphenyl can be synthesized
with a high selectivity (88%) by the Pd-catalyzed
dimerization of benzene in the presence of MoO2(acac)2
as a cocatalyst and O2 in acetic acid [7].
In continuing study, we have recently investigated the
effect of HPAs of Keggin type containing molybdenum
such as H4SiW12ꢁnMon, H15ꢁnPV12ꢁnMonO40, and
H3PW12ꢁnMonO40 as a cocatalyst in the Pd(OAc)2-
catalyzed dimerization of benzene in the presence of O2
and acetic acid (Eq. (1)) and found that HPAs of Keggin
type containing molybdenum act as an excellent coca-
talyst to obtain biphenyl from benzene in the Pd(OAc)2/
HPA/O2/AcOH catalyst system. One of main purposes
of this research is to develop a process to be applied
industrially with no by-products. It is also our target in
the future to recycle the catalyst which has the same
anion ligand in a catalyst with a solvent. Here we report
the details.
Biphenyl is an important industrial material used as a
heat transfer agent, an antimildew agent or a raw
material of synthetic resins [1,2]. It is presently manu-
factured in a vapor phase dehydrogenative dimerization
of benzene [2]. However, a low temperature liquid phase
synthesis of biphenyl from benzene is also interesting
both scientifically and industrially. The Pd-catalyzed
dimerization of benzene in a liquid phase has been
studied extensively [3Á6] However, there are few reports
/
on the selectivity of biphenyl synthesis in the presence of
O2 and acetic acid except that we reported recently [7].
Kozhevnikov and coworkers reported that the selec-
tivity of biphenyl in PdSO4-catalyzed dimerization of
benzene using a Keggin type HPA, H5PMo10V2O40 as a
cocatalyst by oxygen in AcOHÁ
sulfuric acid to the system. It was 73% on the basis of
/
H2O (80:20) with
ꢀ Corresponding author. Tel./fax: ꢂ
E-mail address: yamaji@cstf.kyushu-u.ac.jp (T. Yamaji).
/
81-92-6424374
0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 9 0 1 - 0

60
M. Okamoto et al. / Journal of Organometallic Chemistry 664 (2002) 59Á
/
65
ð1Þ
2. Results and discussion
2.1. Efficiency of oxidative dimerization of benzene with
various heteropolyacid cocatalysts
Fig. 1. Effect of composition of H3PW12ꢁn Mon O40. Conditions:
volume of autoclave (100 cm3), Pd(OAc)2 (0.13 mmol), HPA (Mo/
Pdꢃ
0.5), AcOH (3.0 g), benzene (38 mmol), O2 (10 kg cmꢁ2),
/
Table 1 shows representative results of various HPA
cocatalysts. The reaction without a HPA as a cocatalyst
gives a mixture of 1, 2, and 3 (entry 1). We have carried
out the reaction using H3PW12ꢁnMonO40 as a HPA.
From this table, the addition of a HPA as a cocatalyst
retarded the formation of phenol dramatically.
H3PMo12O40 exhibited the best result of 94% (entry 8).
H3PW2Mo10O40 was the second best (entry 7). The
substitution of P with Si in H3PMo12O40 did not change
much the selectivity of biphenyl (entry 11). However, the
use of H4SiW6Mo6O40 and H7PV4Mo8O40 exhibited
inferior results (entries 9 and 10) as compared with the
corresponding H3PW12ꢁnMonO40.
To elucidate the effect on the substitution number of
molybdenum atom in H3PW12ꢁnMonO40, the relation-
ship of the yield of the products against the substitution
number of molybdenum atom in H3PW12ꢁnMonO40
was examined (Fig. 1). As shown in Fig. 1, the yield of
biphenyl increased with increasing the substitution
number of molybdenum atom in H3PW12ꢁnMonO40.
The HPA fully substituted by molybdenum atom
(H3PMo12O40) gave the best result. The selectivity of
130 8C, 4 h.
phenols stayed low, almost constant through the in-
crease of molybdenum ratio.
2.2. Effect of substitution of acidic protons of a
heteropolyacid by a Pd2ꢂ or a tetrabutylammonium
cation
In order to compare the results with proton type
HPAs, we investigated the reactions using HPAs sub-
stituted by a Pd2ꢂ or tetramethylammonium cations.
Table 2 summarizes the results of the effect of
replacing the acidic protons in a HPA by a Pd2ꢂ or
tetrabutylammonium (TBA) cations. As can be seen
from the table PdHPMo12O40 gave the same result with
the corresponding H3PMo12O40 (entry 8). The change of
P to Si in PdHPMo12O40 type HPA afforded almost the
same result (entry 5). The decrease of Mo/Pd ratio in the
system to 0.25 increased the selectivity to 95% (entry 4).
Table 2
Effect of HPAs
a
b
Entry HPA
Product (mmol)
1/(1ꢂ2ꢂ3) (%)
Table 1
Effect of HPAs
a
1
2
3
b
Entry HPA
Product (mmol)
1/(1ꢂ2ꢂ3) (%)
1
2
None
PdHPW6Mo6O40
PdH2SiW6Mo6O40
0.47 0.44 0.03 50
0.41 0.10 0.01 79
0.39 0.05 0.01 87
0.86 0.04 0.01 95
0.88 0.06 0.01 93
0.52 0.05 0.01 90
0.67 0.06 0.01 91
0.82 0.03 0.02 94
0.19 0.01 0.00 95
0.40 0.17 0.02 68
1
2
3
3
c
1
2
None
0.47 0.44 0.03 50
0.17 0.00 0.00 100
4
PdH2SiMo12O40
PdH2SiMo12O40
H3PW11Mo1O40
H3PW9Mo3O40
H3PW8Mo4O40
H3PW6Mo6O40
H3PW4Mo8O40
H3PW2Mo10O40
H3PMo12O40
H4SiW6Mo6O40
H7PV4Mo8O40
H4SiMo12O40
5
d
3
0.34 0.06 0.01
0.39 0.04 0.01
0.34 0.03 0.01
0.40 0.03 0.01
0.55 0.03 0.01
0.80 0.03 0.02
0.42 0.11 0.02
0.79 0.29 0.02
0.71 0.04 0.02
83
89
89
91
93
94
76
72
92
6
PdH2SiMo12O40
PdH2SiMo12O40
e
4
7
5
8
PdHPMo12O40
PdHPMo12O40
(TBA)3PW6Mo6O40
f
6
9
10
7
8
Conditions: volume of autoclave (100 cm3), Pd(OAc)2 (0.13
a
9
mmol), HPA (Mo/Pdꢃ0.5), AcOH (3.0 g), benzene (38 mmol), O2
10
11
(10 kg cmꢁ2), 130 8C, 4 h.
Determined by GC.
b
Conditions: volume of autoclave (100 cm3), Pd(OAc)2 (0.13
Mo/Pdꢃ0.25.
Mo/Pdꢃ0.75.
Mo/Pdꢃ1.0.
a
c
d
mmol), HPA (Mo/Pdꢃ0.5), AcOH (3.0 g), benzene (38 mmol), O2
(10 kg cmꢁ2), 130 8C, 4 h.
e

M. Okamoto et al. / Journal of Organometallic Chemistry 664 (2002) 59Á
/
65
61
However, the same replacement of P to Si gave a better
selectivity in PdH2SiW6Mo6O40 (entry 3).
Table 3
Effect of the weight ratio of H2O against AcOH solvent
a
The increase of Mo/Pd ratio to 0.05Á0.1 decreased
/
b
Entry AcOH (g) H2O (g) Product (mmol)
1/(1ꢂ2ꢂ3) (%)
biphenyl productivity and selectivity (entries 6 and 7).
The change of all protons by TBA cations in
H3PW6Mo6O40 also decreased the selectivity (entry 10).
The most striking result is that PdHPMo12O40 itself
acted as a catalyst to give 95% selectivity of biphenyl
with a lower yield of biphenyl (entry 9). The productiv-
ity ratio of entries 9/entry 8 for biphenyl is 0.23. On the
contrary Pd usage ratio of entries 9/entry 8 is 0.1. This
means that the complex of the type of PdHPMo12O40
can produce as much as 2.3-fold of biphenyl in the case
of entry 9. This suggests that when a proton type HPA
cocatalyst is added to the Pd catalyst it may form a kind
1
2
3
1
2
3
4
3
2
1
0
0
1
2
3
0.80 0.03 0.02 94
2.42 0.08 0.07 94
1.40 0.01 0.02 98
0.00 0.00 0.00
Á/
a
Conditions: volume of autoclave (100 ml), Pd(OAc)2 (0.13 mmol),
H3PMo12O40 (5.5 mmol), PhH (38 mmol), AcOH, H2O, O2 (10 atm),
130 8C, 4 h.
we tried to add H2O to the solvent to increase the
solubility of a HPA in the solvent. The results are shown
in Table 3. Best yield of biphenyl was obtained in a
solvent with AcOH/H2O ratio of 2. Entries 2 and 3 also
showed these effects giving higher yields than the case of
no water in the solvent. When H2O only was used as a
solvent, no biphenyl was formed.
of PdÁHPA complex to act as an active catalyst to
/
facilitate the production of biphenyl.
2.3. Effect of composition of H3PW12ꢁnMonO40 against
TONs of Pd
It is interesting to see the effect of composition of
heteropolyacid on the productivity of Pd. Fig. 2 shows
such results. One can see that the TON ratio in the
presence of H3PW12ꢁnMonO40 and in the absence of it
increases with the increase of n from 1 to 4 and stays
constant until 6, then increases again to 12. However,
the ratio does not exceed 1 in the case of these types of
heteropolyacids up to n 10. This means that selectivity
increase depresses the productivity of biphenyl in case of
a HPA of these types indicating the difference of type of
catalytic mode with or without HPA in the reaction.
H3PMo12O40 acted very effectively.
2.5. Effect of the amount of H3PMo12O40 in AcOHÁ
H2O solvent
/
By adding more HPA in a more soluble solvent, the
productivity of biphenyl may increase. We changed the
amount of HPA added to the solvent of AcOHÁH2O
/
(weight ratio 2:1). These results are shown in Table 4.
When 0.06 mmol amount of HPA is added to the system
it gave all biphenyl and no phenols at all (entry 2). It is a
very surprising fact. When the amount was increased up
to 0.28 mmol the selectivity stayed 100% and the yield
also stayed almost constant. However, the yield de-
creased after that. It may be due to the formation of an
undesired active complex. Fig. 3 shows the yield of
biphenyl versus the amount of the HPA. From this
figure one can see that around 0.2 mmol of HPA is
appropriate for this reaction.
2.4. Effect of weight ratio of H2O against AcOH in a
solvent
We found that a complex formation of Pd with a
HPA might accelerate the reaction. By this information
Table 4
a
Effect of the amount of H3PMo12O40 in AcOHÁH2O solvent
/
b
Entry H3PMo12O40 (mmol) Product (mmol)
1/(1ꢂ2ꢂ3) (%)
1
2
3
1
2
3
4
5
0.01
0.06
0.17
0.28
0.56
0.80 0.03 0.02
94
3.13 0.00 0.00 100
3.63 0.01 0.00 100
3.46 0.01 0.00 100
1.78 0.02 0.00
99
Fig. 2. Effect of composition of H3PW12ꢁn Mon O40. Conditions:
volume of autoclave (100 ml), Pd(OAc)2 (0.13 mmol), H3PW12ꢁn Mo-
a
Conditions: volume of autoclave (100 cm3), Pd(OAc)2 (0.13
n O40 (Mo/Pdꢃ
130 8C, 4 h. TON with a HPA/TON without a HPA.
/
0.5), PhH (38 mmol), AcOH (3.0 g), O2 (10 atm),
mmol), H3PMo12O40, PhH (38 mmol), AcOH (2.0 g), H2O (1.0 g),
O2 (10 kg cmꢁ2), 130 8C, 4 h.
a

62
M. Okamoto et al. / Journal of Organometallic Chemistry 664 (2002) 59Á65
/
system. Table 5 gives the results. Entry 2 supports that
the formation of phenol is explained by a radical
mechanism. Entry 3 also supports the radical process.
The polyol scavenger did not give a clear-cut result
because of its partial decomposition in the reaction
system (entry 4). Other scavengers (entries 5Á7) decom-
/
posed completely at this temperature and inhibited this
reaction. It is under investigation why the reaction was
stopped by the decomposition of these scavengers.
2.7. 31P-NMR spectra of H3PMo12O40, PdHPMo12O40
and the reaction mixture
Fig. 3. Effect of the amount of H3PMo12O40.
a Conditions: volume of
autoclave (100 cm3), Pd(OAc)2 (0.13 mmol), H3PMo12O40, PhH (38
4
shows the 31P-NMR spectra of HPA
mmol), AcOH (2.0 g), H2O (1.0 g), O2 (10 kg cmꢁ2), 130 8C, 4 h.
Determined by GC.
b
Fig.
(H3PMo12O40), PdHPMo12O40 and the reaction mixture
at room temperature in d-water. They show two types of
2.6. Effect of radical scavengers
peaks, one at around ꢁ
/
3.4Á
/
ꢁ3.6 ppm and the other
/
around ꢁ1.31 to ꢁ1.32 ppm in the former two
complexes. The first peak is responsible for PMo12
species and the second is for PMo9 species. The reaction
/
/
In order to investigate the reaction mechanism we did
the experiments using radical scavengers in this reaction
Fig. 4. 31P-NMR spectra of active species in the reaction (ppm from down field from 85% H3PO4).

M. Okamoto et al. / Journal of Organometallic Chemistry 664 (2002) 59Á
/
65
63
Table 5
Effect of radical scavengers
a
a
Conditions: volume of autoclave (100 cm3), Pd(OAc)2 (0.13 mmol), radical scavenger (1.0 mmol), PhH (38 mmol), AcOH (3.0 g), O2 (10 kg
cmꢁ2), 130 8C, 4 h.
Determined by GC.
b
mixture showed three peaks at ꢁ
/
2.75, ꢁ
/
1.31 (main
sidering all these possibilities we estimated the active
species of this reaction are a variety of PMo9O31 ones
forming the equilibrium depending on the reaction
conditions as given in Fig. 5.
peak), and ꢁ0.25 ppm. These three peaks are respon-
/
sible for P2Mo18, PMo9 and PMo11, respectively. The
striking fact is that the peak for PMo12 species is not
found in the reaction mixture showing that PMo12
species does not exist in the reaction system.
2.8. Possible mechanism and catalytic image of an active
catalyst
By the facts obtained above we estimated the active
species of the reaction in the following way [9].
PMo12O340ꢁ species reacts with water to give PMo9O31-
(H2O)3ꢁ species losing 3MoO3. PMo9O31(H2O)3ꢁ un-
dergoes deprotonation or protonation to give a variety
of PMo9O31 species in Fig. 5. This PMo9O31(H2O)3ꢁ
species gives a P2Mo18O66ꢁ2 species at a low temperature
like room temperature. However, this species is not
observed at high temperature like 130 8C. The
PMo12O34ꢁ0 species also forms PMo11O379ꢁ species. Con-
The attainment of high selectivity of biphenyl by a
HPA containing Mo is estimated in the following way as
depicted by Fu and Ono [10]. The OH radical from
HOOH formed [4] reacts with OH radical of Mo-
polyanion of a HPA to form H2O and oxomolybdenum
species (MoÄ
MoÃ
OH and AcOꢁ. This catalytic cycle is completed as
shown in Fig. 6.
/
O) which then reacts with AcOH to give
/
Fig. 5. The variation of a HPA structure.

64
M. Okamoto et al. / Journal of Organometallic Chemistry 664 (2002) 59Á65
/
Kozhernikov and coworkers [8] used a low tempera-
ture like 84 8C. We used a temperature like 130 8C.
The activity difference of a complex at different
temperatures might have caused the difference of the
best cocatalyst. These are now under investigation.
Fig. 6. Scavenging hydroxy species by Mo(OH) species of a HPA.
3. Conclusion
It was found that biphenyl could be synthesized with
100% selectivity by Pd-catalyzed dimerization of ben-
zene in the presence of H3PMo12O40 as a cocatalyst and
O2 in AcOH/H2O solvent at 130 8C.
It was also found that PdHPMo12O40 itself can act as
a catalyst to give 95% selectivity with a lower yield of
biphenyl.
Fig. 7. Possible mechanism.
4. Experimental
By considering the facts given and above discussion
we would like to postulate the following possible
mechanism and catalytic image of our biphenyl synth-
esis reaction in Figs. 7 and 8.
The main point in the possible reaction mechanism is
that Pd(0) is reoxidized by the help of a HPA polyanion
through the formation of the complex shown in Fig. 8
Pd salts of HPA were gifted generously from Teijin
Ltd. All other reagents were obtained commercially and
used as received.
A typical reaction was carried out as follows: in a 100-
cm3 stainless steel autoclave equipped with a magnetic
stirrer bar, a pressure gauge and a thermocouple were
charged C6H6 (38 mmol), AcOH (3 g), 0.13 mmol of
affording MoÃ
tion of Pd2ꢂ from Pd(0) and attains a high selectivity of
biphenyl by the formation of MoÃOH bond in a HPA
/
OÃ/Pd bonding to facilitate the produc-
Pd(AcO)2 and a HPA (Mo/Pdꢃ0.5) and then oxygen
/
was introduced to the pressure of 10 atm after flashing
two times. The mixture was heated at 130 8C with
stirring for 4 h. After the reaction the products were
analyzed by gas chromatography. The analyses were
performed on a Shimazu GC-14B system with a
/
polyanion to react with a OH radical from HOOH
formed. The increase of properly composed Mo atom in
a HPA may increase the chance of an active catalyst to
decompose OH radical to give a high selectivity of
biphenyl.
SUPELCO WAX capillary column of 60 mꢄ
mmꢄ0.5 mm using naphthalene as an internal standard
under the conditions of injection and detector tempera-
/0.32
/
This Mo atom increase also might give more chances
tures (100Á
200 8C, 2 minꢁ1) and He (220 kPa).
/
to form a PdÃ
/
OÃ
/
Mo bonding to facilitate a formation
31P-NMR spectra were recorded on a JEOL ECP-400
(162 MHz) spectrometer using D2O as a solvent at
25 8C; chemical shifts were expressed in parts per
million downfield from 85% H3PO4.
of Pd(II) from Pd(0) by oxygen molecule.
Therefore, the most productivity and the selectivity
100% of biphenyl by Pd(OAc)2 may be attained by
H3PMo12O40 cocatalyst which has properly composed
Mo atoms for this reaction. The oxidation through PdÃ
OÃMo bonding may be stable but not so fast as a direct
oxidation of Pd(0) by molecular oxygen.
/
/
Acknowledgements
This work was supported by Research Institute of
Innovative Technology for the Earth (RITE 12C31).
Heteropolyacids were generously gifted by Nippon
Inorganic Color and Chemical Co., Ltd.
References
[1] I.S. Popushoi, E.D. Kogan, E.P. Styngach, M.S. Yamko, O.M.
Radul, A.I. Reinbold, Izu. Akad. Nauk. Mold. SSR, Ser. Biol.
Khim. Nauk. 1 (1987) 69.
Fig. 8. Possible mechanistic roles of Pd(OAc)2 and HPA in the
oxidative dimerization of benzene to biphenyl.

M. Okamoto et al. / Journal of Organometallic Chemistry 664 (2002) 59Á
/
65
65
[2] A. Standen, Kirk-Othmer Encyclopedia of Chemical Technology,
vol. 7, 2nd ed., Interscience Publishers, New York, 1972, p.
191.
[6] Y. Fujiwara, I. Moritani, K. Ikegami, R. Tanaka, S. Teranishi,
Bull. Chem. Soc. Jpn. 43 (1970) 863.
[7] M. Okamoto, T. Yamaji, Chem. Lett. (2001) 212.
[8] G.U. Mennenga, A.I. Rudenkov, K.I. Matveev, I.V. Kozhevni-
kov, React. Kinet. Catal. Lett. 5 (1976) 401.
[3] H. Itatani, H. Yoshimoto, Chem. Ind. (1971) 674.
[4] R. Van Helden, G. Verberg, Rec. Trav. Chim. Pays-bas 84 (1965)
1263.
[9] The Chemical Society of Japan, Jikken Kagaku Koza, 4th ed.,
No. 17, Maruzen Co. Ltd., Japan, 1991, p. 462.
[10] Z. Fu, Y. Ono, J. Mol. Catal. A: Chem. 118 (1997) 293.
[5] (a) Y. Ichikawa, T. Yamaji, Japanese Patent 10303, 1975;
(b) Y. Ichikawa, T. Yamaji, Chem. Abstr. 83 (1975) 147271m.