Synthesis of diphenylmethane from formalin and benzene in a biphasic system with 12-tungstophosphoric acid

Article abstract of DOI:10.1039/b104321h

Heteropoly acids such as H₃PW₁₂O₄₀ are exceptionally active catalysts in the synthesis of diphenylmethane from benzene and formalin (aqueous formaldehyde) in a biphasic system and can be recycled simply by drying the aqueous phase at room temperature.

Full text of DOI:10.1039/b104321h

Synthesis of diphenylmethane from formalin and benzene in a
biphasic system with 12-tungstophosphoric acid
Zhaoyin Hou and Toshio Okuhara*
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan.
E-mail: oku@ees.hokudai.ac.jp
Received (in Cambridge, UK) 16th May 2001, Accepted 23rd July 2001
First published as an Advance Article on the web 16th August 2001
Heteropoly acids such as H3PW12O40 are exceptionally
active catalysts in the synthesis of diphenylmethane from
benzene and formalin (aqueous formaldehyde) in a biphasic
system and can be recycled simply by drying the aqueous
phase at room temperature.
chromatograph (Shimadzu, GC-14B) with a 15 m capillary
column of cross-linked 5% PhMe siloxane (HP-5, USA) for the
aromatic compounds and a Porapak P column for methyl
formate and methanol. The TCD-GC with an APS-201 (Flusin
T) column was also used for HCHO, HCOOH, and H2O. All
liquid acids (except for para-toluenesulfonic acid) were
exclusively present in the aqueous phase. When the reaction
solution was cooled to room temperature after the reaction (2 h),
the solution consisted of two phases, a benzene phase (the
upper) and an aqueous phase (the lower), and a phase boundary
was apparent.
Diphenylmethane (PhCH2Ph) is an important intermediate in
spices, pharmaceutics and other fine chemicals1 and its
alkylated compounds can be used as insulators in place of PCB.2
PhCH2Ph has commercially been synthesized by the Friedel–
Crafts reaction with benzyl chloride and benzene, using
aluminium-amalgam, AlCl3, or ZnCl2 as catalyst.3 However,
both the materials and the catalysts are environmentally
unfriendly. Moreover, harmless solid acids like HY zeolite,4 H-
ZSM-5,5 and sulfated ZrO26 were reported to be of insufficient
activity.
As Table 1 shows, the conversions of HCHO reached over 93
mol% with H3PW12O40 and Sc(CF3SO3)3. It should be
emphasized that only H3PW12O40 gave an appreciable amount
of PhCH2Ph (the yield was 35.3% under these conditions),
which accumulated in the benzene phase. In addition, di-
benzylbenzene ((PhCH2)2C6H4), methyl benzyl ether
(PhCH2OMe), and methyl formate (HCOOMe) were detected
in the benzene phase, and formic acid, methanol, and excess
HCHO in the aqueous phase. For the formation of PhCH2Ph, the
Synthesis of PhCH2Ph by condensation of benzene with
formalin [eqn. (1)] is very attractive, since the direct use
2C6H6 + HCHO (in water) ? PhCH2Ph + H2O
(1)
+
of formalin is economical and the only by-product of the
reaction is water. There are several reports describing reactions
between aromatic compounds and paraformaldehyde (water-
free oligomers of HCHO) catalysed by liquid acids,7 solid acids
like HY zeolite,8 and SiO2-composites of polymer resins.9
However, when formalin was used instead of paraformalde-
hyde, no reaction occurred under the same conditions,7,9 due to
serious inhibition by water. There is no report detailing a
catalyst effective for the reaction of benzene with formalin. In
this communication, we report that H3PW12O40 is unusually
active and recyclable for this reaction in a biphasic system.
Similar biphasic catalytic systems have been reported for
cyclotrimerization of propionaldehyde catalysed by heteropoly
acid10 and for nitration of benzene by lanthanide triflates.11
The reaction was performed at 433 K in a stainless steel
autoclave (ca. 100 cm3, TAIATSU TECHNO, Japan) with 40
cm3 of benzene (450 mmol), 6.72 cm3 of formalin (Wako
Chem. Co., HCHO 90 mmol, H2O 222 mmol, methanol (as
stabilizer) 18.2 mmol), and the catalyst (2.3–16.6 mol% with
respect to HCHO). The vapor phase was analyzed with a TCD
gas chromatograph (Shimadzu, GC-8A) with an active carbon
column. Each liquid phase was analyzed with an FID gas
following steps are considered:8 H2 COH is generated from
HCHO and the proton attacks the benzene ring to form, initially,
benzyl alcohol. Benzyl alcohol can be transformed to the benzyl
cation via protonation and dehydration. PhCH2Ph is produced
by the attack of the benzyl cation on the benzene ring. With
H3PW12O40 as catalyst, two competitive reactions of HCHO
proceeded: (i) attack of the benzyl cation on benzene to form
PhCH2Ph and (ii) dimerization of HCHO to HCOOMe
(Tichenko reaction).
On the other hand, Sc(CF3SO3)3 efficiently catalysed dimer-
ization of HCHO to HCOOMe, but was inactive for the
formation of PhCH2Ph. para-Toluenesulfonic acid (PTS) was
also active for the dimerization of HCHO, but less active than
H3PW12O40 for PhCH2Ph formation. Mineral acids like H2SO4,
H3PO4, and HNO3 showed no activity for the formation of
PhCH2Ph and CF3COOH was also inactive for this reaction. It
was confirmed that HCOOMe did not react with benzene under
these conditions with any of these catalysts.
A series of other heteropoly acids were examined as catalysts
for the synthesis of PhCH2Ph from benzene and formalin (Table
2). All these heteropoly acids showed activities: H4SiW12O40
and H3PMo12O40 were effective, but H6P2W18O62 and H4Si-
Table 1 Conversion and selectivity of diphenylmethane synthesis from benzene and formalin with various liquid acids
Selectivitya (mol%)
Conversiona Yieldb
Catalyst
(mmol) (mol%)
(mol%) CO
HCOOMe MeOH
HCOOH PhCH2Ph (PhCH2)2CH2 PhCH2OMe MBc (%)
H3PW12O40
Sc(CF3SO3)3
PTSd
HNO3
H3PO4
(4.2)
(2.1)
(12.6)
(12.6)
(4.2)
93.2
95.1
82.2
24.4
18.7
33.7
35.3
0.0
4.1
0.0
0.0
0.0
3.3
0.2
4.2
10.0
0.2
0.6
35.1
83.0
71.8
90.0
68.8
92.0
5.4
12.3
7.2
0.0
12.0
4.0
5.3
4.6
6.3
0.0
19.0
4.0
38.7
0.0
5.9
0.0
0.0
0.0
11.4
0.0
1.5
0.0
0.0
0.0
0.8
0.0
3.0
0.0
0.0
0.0
97.0
100.0
79.8
100.0
100.0
101.0
H2SO4
(6.3)
Reaction conditions: benzene 40 cm3 (450 mmol), formalin 6.72 cm3 (HCHO 90 mmol, H2O 222 mmol, methanol 18.2 mmol), 160 °C for 2 h. a On the basis
of HCHO. b PhCH2Ph (on the basis of HCHO) c Mass balance; 100 3 (total amount of products and remaining HCHO)/(the initial amount of HCHO) d para-
Toluenesulfonic acid.
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Chem. Commun., 2001, 1686–1687
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104321h

View Article Online
Table 2 Conversion and selectivity of diphenylmethane synthesis from benzene and formalin with heteropoly acids
Selectivitya (mol%)
Conversion
Catalyst
(mmol)
(mol%)
Yield (mol%)
HCOOMe
PhCH2Ph
TONb
H3PW12O40
H4SiW12O40
H3PMo12O40
H6P2W18O62
H4SiMo12O40
(4.2)
(4.2)
(4.2)
(2.1)
(4.2)
93.2
92.8
93.5
72.4
92.0
35.3
25.4
23.4
9.2
34.3
43.6
54.8
68.8
77.1
37.9 (79.9)
27.4 (79.7)
25.1 (71.1)
12.7 (76.1)
3.9 (33.9)
7.1
5.1
4.7
2.8
0.7
3.6
Reaction conditions were the same as given in the footnotes of Table 1. a On the basis of HCHO. The figures in parentheses are the selectivities for PhCH2Ph
among the aromatic compounds (PhCH2Ph + (PhCH2)2CH2 + PhCH2OMe). b Turnover number defined as (mol of PhCH2Ph)/(mol of catalyst).
a
Table 3 Recycled H3PW12O40
drying the aqueous solution at room temperature. The recovered
H3PW12O40 was found to be active in the repeated runs,
Total yield of
aromaticsb
(mmol)
Selectivity (mol%)c
although the conversion of HCHO and yield of aromatics
decreased somewhat. As shown in Table 3, the conversion and
the total yield of aromatics reached plateaus after the fourth run.
These changes in the conversion, yield, and selectivity are due
to the remaining carbonaceous species on the heteropoly acid.
Elemental analysis showed that the amount of carbon on the
used H3PW12O40 was 2.1%, which corresponds to 0.4 mole-
cules of PhCH2Ph per heteropoly anion at the fifth run. IR
spectroscopy of H3PW12O40 revealed that the Keggin structure
was completely retained after repeating the reaction five
times.
In conclusion, H3PW12O40 is a highly active and recyclable
catalyst for use in the synthesis of diphenylmethane from
benzene and formalin in a biphasic system. This is a novel
example for direct use of formalin in clean organic synthesis
with solid acid.
Run
Conversion (%)
PhCH2Ph
TONd
1
2
3
4
5
83.5
74.7
72.2
64.1
63.1
31.4
79.1
76.2
73.4
70.6
66.7
4.5
2.9
2.4
1.7
1.6
23.8 (75.6%)
21.1 (68.4%)
17.8 (56.7%)
17.6 (56.1%)
Reaction conditions: benzene 40 cm3 (450 mmol), formalin 6.72 cm3
(HCHO 90 mmol, H2O 222 mmol, methanol 18.2 mmol), H3PW12O40 4.2
mmol, 160 °C, and 30 min. a Between the runs, the aqueous phase of
H3PW12O40 was dried in the ambient atmosphere. The recovered solid
H3PW12O40 was reused without any treatment. b Total yield of PhCH2Ph,
(PhCH2)2CH2 and PhCH2OMe. The figures in parentheses are the
percentage yield relative to that of the first run. c Selectivity for PhCH2Ph
among the aromatic products on the basis of HCHO. d Turnover number
defined as (mol of PhCH2Ph)/(mol of catalyst).
Mo12O40 were less efficient. With H3PW12O40 as catalyst, when
the ratio of HCHO/benzene was reduced from 1+5 to 1+10,
keeping the amount of benzene (450 mmol) the same, the yield
of PhCH2Ph became higher (42%) with 43% selectivity and
97% conversion. However, the amount of PhCH2Ph produced
was still higher at 1+5 ratio.
IR spectroscopy showed that, while the Keggin structure of
SiMo12O4042 decomposed to some extent after the reaction, the
structures of the other polyanions were retained. The pH values
of the aqueous phases of these heteropoly acids (pH = 0.10,
0.58, 0.61, 0.79, and 0.96 for H3PW12O40, H4SiW12O40,
H3PMo12O40, H6P2W18O62, and H4SiMo12O40, respectively)
were close to those of Sc(CF3SO3)3 (pH = 0.90), H3PO4 (pH =
1.25), HNO3 (pH = 0.65), and H2SO4 (pH = 0.19). Thus the
exceptionally high activities of the heteropoly acids are due to
not only the strong acidity of the solution, but also the soft
basicity of the heteropoly anion, which has been proposed by
Izumi et al.12 The cationic intermediates are probably stabilized
by the heteropoly anions.
Notes and references
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10 S. Sato, C. Sakura, H. Furuta, T. Sodesawa and F. Nozaki, J. Chem. Soc.,
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12 Y. Izumi, K. Matsuo and K. Urabe, J. Mol. Catal., 1983, 18, 299.
The results of five successive reactions are summarized in
Table 3, where used H3PW12O40 was recovered simply by
Chem. Commun., 2001, 1686–1687
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