418
Published on the web April 5, 2010
Direct Synthesis of Diphenyl Carbonate by Electrocarbonylation at a Pd2+-supported Anode
Toru Murayama, Yuji Arai, Tomohiko Hayashi, and Ichiro Yamanaka*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1-S1-43 Ookayama, Meguro-ku, Tokyo 152-8552
(Received January 6, 2010; CL-100014; E-mail: yamanaka@apc.titech.ac.jp)
The first electrochemical synthesis of diphenyl carbonate
was accomplished using triethylamine/tetrabutylammonium
perchlorate/phenol/dichloromethane or sodium phenoxide/phe-
nol/acetonitrile electrolyte at a [PdCl2/activated carbon and
vapor grown carbon fiber] anode, 1 atm CO and 25 °C. Sodium
phenoxide functioned as a promoter, triethylamine, and a
supporting electrolyte, tetrabutylammonium perchlorate, for the
carbonylation.
equiv) were used for the carbonylation. DPC yields decreased in
this order 68% (Et3N) > 47 (i-Pr2EtN) > 18 (n-Bu3N) º 0
(Et2HN). Thus, we chose Et3N and n-Bu4NClO4 to conduct the
electrocarbonylation.
A Pd anode (5 cm2) was prepared from PdCl2 (30 ¯mol)
loaded on activated carbon (PdCl2/AC), vapor grown carbon
fiber (VGCF) as an electron conductor, and a PTFE binder by
hot press.7 Various electrolysis conditions for DPC formation
were studied using the [PdCl2/AC + VGCF] anode. We found
that a galvanostatic electrolysis using a conventional one-
compartment cell with a Pt-coil counter electrode and without a
reference electrode was suitable for the formation of DPC. The
electrolyte was 1000 equiv of PhOH, 2 equiv of Et3N, and 0.1 M
n-Bu4NClO4/CH2Cl2 (30 mL) dried over MS-4A. The solutions
were electrolyzed under galvanostatic conditions with a low
current of 1 mA after introduction of 1 atm CO.
The majority of polycarbonates are manufactured by a
phosgene process, interfacial polycondensation of bisphenol-A
and phosgene. An alternative phosgene process is transester-
ification using bisphenol-A and diphenyl carbonate (DPC). DPC
is the key material for the phosgene-free process of polycar-
bonates.1 However, DPC is manufactured from phosgene and
phenol (PhOH) or dimethyl carbonate and PhOH by an ester
exchange reaction.
Figure 1 shows time courses of the formation of DPC under
different procedures. First, stoichiometric carbonylation of
PhOH by PdCl2/AC in the anode was studied under open
circuit conditions. DPC formed smoothly increasing after
30 min; a final yield of 44% was obtained based on Pd2+. CO2
also formed by stoichiometric oxidation among CO, contami-
nated H2O and Pd2+ (34% yield). Second, galvanostatic
electrolysis of 1 mA was applied at the [PdCl2/AC + VGCF]
anode, and the DPC yield slightly increased to 54% after 2 h.
Then another portion of 2 equiv of Et3N was added to the
solutions; the DPC yield jumped to 73%. Therefore, electrolysis
was performed over a long period of time, adding 2 equiv of
Et3N every 60 min. The DPC yield linearly increased with the
reaction time. A current efficiency (CE) of 42% for the DPC
Three decades ago, the stoichiometric carbonylation of
PhOH and CO to DPC using Pd2+ and triethylamine (Et3N) was
reported.2 Here, a key reaction is the reoxidation of Pd0 to Pd2+
with O2 under the catalytic conditions.3 The oxidation rate of Pd0
to Pd2+ with O2 is very slow; therefore, a redox couple of Mn3+
/
Mn2+, or a twin redox couple of benzoquinone/hydroquinone
and Co3+/Co2+, were reported for the reoxidation of Pd0 to Pd2+
under pressures of CO and O2 > 6 MPa at 100 °C.4-6 The cata-
lytic carbonylation is the primary alternative phosgene process of
the DPC production; however, water accumulation is a problem.
The hydrolysis of DPC to PhOH and CO2 and the direct oxi-
dation of CO to CO2 are accelerated by water. We have reported
electrochemical system for carbonylation of methanol,7-9 but it
could not perform carbonylation of PhOH. We investigated
electrosynthesis of DPC by Pd electrocatalyst in this work.
We confirmed stoichiometric carbonylation using PdCl2
(1 mmol), PhOH (30 equiv), Et3N (7 equiv), and CO (1 atm)
in CH2Cl2 (30 mL, dried by MS-4A) at 25 °C. Products in the
solution (DPC, phenyl salicylate, oxidation products of PhOH)
and CO2 in the outlet gas mixture were analyzed using GC and
HPLC. Experimental error was «5% for each product yield.
150
100
50
Significant yields of DPC (31%) and CO2 (25%), based on Pd2+
,
and a trace of phenyl salicylate were obtained. In addition, black
deposits assumed to be Pd0 were observed after the reaction.
First, we studied the effects of the addition of supporting
electrolytes, tetraalkylammonium perchlorate, chloride, or bro-
mide (R4N-ClO4, -Cl, and -Br), to the stoichiometric carbon-
ylation conditions on the DPC yield. DPC yields decreased in
this order: 47% (n-Bu4NClO4) > 39 (n-Hx4NClO4) > 35 (Et4N-
ClO4) > 31 (no addition) º 0 (n-Bu4NBr and n-Bu4NCl).
Second, the effects of the amount of Et3N were studied on the
DPC yield with n-Bu4NClO4. A higher DPC yield of 68% was
obtained with 2 equiv of Et3N. Furthermore, other amines (2
0
0
1
2
3
4
reaction time / h
Figure 1. Time courses of DPC yield based on Pd by
galvanostatic electrolysis (1 mA) of phenol/Et3N/n-Bu4NClO4/
CH2Cl2 at the [PdCl2/AC + VGCF] anode at 25 °C. Electrolyte:
0.12 M phenol, Et3N, 0.1 M n-Bu4NClO4/CH2Cl2 (30 mL), CO
1 atm. +: open circuit, : initial addition of 2 equiv of Et3N,
intermittent addition of 2 equiv of Et3N at every 1 h, : initial
addition of 8 equiv of Et3N.
:
Chem. Lett. 2010, 39, 418-419
© 2010 The Chemical Society of Japan