Direct synthesis of diphenyl carbonate by mediated electrocarbonylation of phenol at Pd2+-supported activated carbon anode

Article abstract of DOI:10.1016/j.electacta.2010.12.087

Mediated electrocarbonylation of phenol to diphenyl carbonate (DPC) at a PdCl₂-supported activated carbon anode in 1 atm CO at 298 K was studied. A dry CH₂Cl₂ or CH₃CN solvent and a galvanostatic electrolysis of 1 mA were necessary for formation of DPC, while the addition of a base and a supporting electrolyte was also essential. A combination of triethylamine (Et₃N) and tetrabutylammonium perchlorate (Bu₄NClO₄) was suitable in various combinations. The addition of 2 equiv. of Et₃N to the electrolyte (C₆H₅OH/Bu₄NClO₄/CH ₂Cl₂) at 1-h intervals was more efficient in the formation of DPC than a single initial addition of the same amount of Et₃N. The yield of DPC was 130% based on Pd and its current efficiency (CE) was 42% for 6 h. The CE of the CO₂ formation was only 3%. Sodium phenoxide (PhONa) showed dual functionality as a base and supporting electrolyte. When the mediated electrocarbonylation was conducted in a C₆H ₅OH/PhONa/CH₃CN electrolyte, DPC was produced in 172% yield and 40% CE for 6 h. The CE of the CO₂ formation was 10%. DPC formed continuously after a single initial addition of 4 equiv. of PhONa. Li or K phenoxide also worked as promoters for the mediated electrocarbonylation of phenol to DPC.

Full text of DOI:10.1016/j.electacta.2010.12.087

Electrochimica Acta 56 (2011) 2926–2933
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Electrochimica Acta
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Direct synthesis of diphenyl carbonate by mediated electrocarbonylation of
phenol at Pd2+-supported activated carbon anode
Toru Murayama, Tomohiko Hayashi, Yuji Arai, Ichiro Yamanaka∗
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-S1-16 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Mediated electrocarbonylation of phenol to diphenyl carbonate (DPC) at a PdCl2-supported activated
carbon anode in 1 atm CO at 298 K was studied. A dry CH2Cl2 or CH3CN solvent and a galvanostatic elec-
trolysis of 1 mA were necessary for formation of DPC, while the addition of a base and a supporting
electrolyte was also essential. A combination of triethylamine (Et3N) and tetrabutylammonium perchlo-
rate (Bu4NClO4) was suitable in various combinations. The addition of 2 equiv. of Et3N to the electrolyte
(C6H5OH/Bu4NClO4/CH2Cl2) at 1-h intervals was more efficient in the formation of DPC than a single ini-
tial addition of the same amount of Et3N. The yield of DPC was 130% based on Pd and its current efficiency
(CE) was 42% for 6 h. The CE of the CO2 formation was only 3%. Sodium phenoxide (PhONa) showed dual
functionality as a base and supporting electrolyte. When the mediated electrocarbonylation was con-
ducted in a C6H5OH/PhONa/CH3CN electrolyte, DPC was produced in 172% yield and 40% CE for 6 h. The
CE of the CO2 formation was 10%. DPC formed continuously after a single initial addition of 4 equiv. of
PhONa. Li or K phenoxide also worked as promoters for the mediated electrocarbonylation of phenol to
DPC.
Received 30 November 2010
Received in revised form
25 December 2010
Accepted 28 December 2010
Available online 11 January 2011
Keywords:
Diphenyl carbonate
Alternative phosgene method
Pd electrocatalysis
Carbonylation
Sustainable chemistry
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Three decades ago, the stoichiometric carbonylation of phenol
and CO to DPC using Pd2+ and trialkylamine at room temperature
The demand for polycarbonates is growing worldwide because
of their use as a transparent thermoplastic with the current rate
of the production being three million tonnes per year. A major-
ity of polycarbonates are manufactured by a phosgene process,
i.e., interfacial polycondensation of bisphenol-A and phosgene. An
alternative to this phosgene process is transesterification using
bisphenol-A and diphenyl carbonate (DPC). DPC is a key material
in the phosgene-free process. However, DPC is currently manufac-
tured from phosgene and phenol (Eq. (1)) or dimethyl carbonate
and phenol (Eq. (2)).
was reported (Scheme 1), wherein a phenoxide anion (PhO−) pro-
duced from phenol and trialkylamine promoted nucleophilic attack
on CO [2]. Phenyl salicylate was produced as a byproduct. Here a
key reaction was re-oxidation of Pd0 to Pd2+ with O2 under cat-
alytic conditions. The rate of the oxidation of Pd0 to Pd2+ with
O2 was very slow; therefore, a Cu2+/Cu+ redox couple was used
in the Wacker oxidation at 100 ◦C. In earlier attempts to achieve
catalytic synthesis of DPC, a redox couple of Mn3+/Mn2+ or a twin-
redox couple of benzoquinone/hydroquinone and Co3+/Co2+ was
reported for the re-oxidation of Pd0 to Pd2+ under CO and O2 pres-
sures >6 MPa at 100 ◦C [3–6]. The carbonylation activity of the Pd
catalyst and the yield of DPC were considerably good in previous
studies; however, water accumulation was a serious problem in
the catalytic carbonylation of phenol with O2. A significant amount
of the water formation in the mixture accelerates the hydrolysis of
DPC to phenol and CO2 and the direct oxidation of CO to CO2. There-
fore, suppression of unfavourable effects of accumulated water is
very important in the oxidative carbonylation method.
COCl2 + 2C6H5OH → (C6H5O)2CO + 2HCl
(1)
(2)
(CH3O)2CO + 2C6H5OH ꢀ (C6H5O)2CO + 2CH3OH
Direct synthesis of DPC has been studied for oxidative carbony-
lation of phenol with O2 using Pd catalysts. The direct synthetic
method is attractive from the viewpoint of green and sustain-
able chemistry because of its reduced energy consumption (CO2
emission) and environmental safeguards [1]. This catalytic car-
bonylation is the primary alternative to the phosgene process.
The present study investigated the use of an electrochemical
potential instead of O2 for the re-oxidation of the Pd2+-catalyst in
the DPC synthesis. Water is not formed during the electrochemi-
cal oxidation of Pd0 to Pd2+ and the oxidation potential is easily
controlled. Mediated electrocarbonylation of phenol to DPC would
occur at the Pd2+-anode.
∗
Corresponding author. Tel.: +81 3 5734 2144; fax: +81 3 5734 2144.
E-mail address: yamanaka@apc.titech.ac.jp (I. Yamanaka).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.12.087

T. Murayama et al. / Electrochimica Acta 56 (2011) 2926–2933
2927
O
O-C-O
+ CO + Pd2+ + 2R3N
+ Pd0 + 2R3NH+
2
OH
O
OH
C-O
+ R3N
+ R3NH+
O–
OH
Refs. [2,3].
Scheme 1.
The use of the electrochemical potential for the re-oxidation of
a Pd0 catalyst has been reported in pioneer studies on carbony-
lation of alkynes to unsaturated diesters [7], alkenes to esters [8]
and aromatic amines to isocyanates [9]. These indirect electrocar-
bonylations were efficiently performed and produced high product
yields.
Our research group has also reported the Wacker oxidation of
ethylene and propylene at a Pd/C anode applying a fuel cell reaction
[10–12] and electrocarbonylation of methanol to dimethyl carbon-
ate in the gas phase at Pd/graphite and Cu/graphite anodes [13,14].
Electrocarbonylation activity of the Pd/C anode was improved by
applying a three-phase boundary of CO (gas), methanol (liquid)
and electrode (solid). A turnover number (TON) increased to 36
in 1 h and a CO selectivity increased to 90% [15]. Electrocarbony-
lation of methanol in the liquid phase was also improved by use
of the Pd/C anode coupled with a Br−-mediator [16]. Electrocatal-
ysis of gold for carbonylation of methanol was achieved in the
liquid phase at 25 ◦C and the selectivity to dimethyl carbonate
and dimethyl oxalate could be controlled by the anode potential
[17,18]. These electrocarbonylation systems have been applied to
carbonylation of phenol. However, carbonylation products such
as DPC and phenyl salicylate have not been detected, whereas
only black-brown unknown products (tar) have been obtained. It
was concluded, therefore, that a stronger oxidation potential was
unsuitable for the electrocarbonylation of phenol because an uns-
elective oxidation of phenol proceeded and formed tar. We have
recently applied mild electrochemical oxidation conditions to the
carbonylation of phenol and have accomplished the first electro-
chemical synthesis of DPC at P(CO) = 1 atm and 25 ◦C [19]. The
details of the electrocarbonylation of phenol to DPC were studied
and the reaction paths for the DPC formation are discussed in this
study.
Fig. 1. Diagram of one-compartment electrolysis cell for DPC synthesis.
(Et2NPh) and N,N,Nꢀ,Nꢀ-tetramethylethane-1,2-diamine (TMEDA)
were used in this study.
Stoichiometric carbonylation of phenol promoted by PhONa:
stoichiometric carbonylation of phenol by PdCl2 using a solution of
sodium phenoxide (PhONa, 4 equiv.), phenol (0.89 mol dm−3) and
CH3CN (0.030 dm−3, dried over MS-3A) was conducted in a similar
manner as that described above.
Stoichiometric carbonylation of phenol promoted by
CH3CO2Na: stoichiometric carbonylation of phenol by PdCl2
using a solution of CH3CO2Na (2 equiv.), C6H5OH (0.89 mol dm−3
)
and CH3CN (0.030 dm−3, dried over MS-3A) was conducted in a
similar manner.
Stoichiometric carbonylation of cresol promoted by PhONa:
stoichiometric carbonylation of cresol by PdCl2 using a solu-
tion of sodium phenoxide (PhONa, 4 equiv.), 4-CH3–C6H4OH
(0.89 mol dm−3) and CH3CN (0.030 dm−3, dried over MS-3A) was
conducted in a similar manner.
All reagents were purchased from Aldrich Co. and Wako Pure
Chemical Co., and required no further purification.
2. Experimental
2.2. Mediated electrocarbonylation
2.1. Stoichiometric carbonylation
2.2.1. Anode preparation
An electrocatalyst of PdCl2/AC was prepared by impregnation
of PdCl2/2HCl aqueous solution on activated carbon (AC, Wako
Pure Chemical Co.), followed by drying in air at 100 ◦C. An anode
was prepared by the hot-press method, using 30 mg of PdCl2/AC
powder (Pd loading: 11 wt% and 30 ␮mol), 125 mg vapour-grown
carbon-fibre (VGCF, Showa Denko Co.) and 40 mg polytetraflu-
oroethylene (PTFE, Daikin Co.) [13–16]. Hereafter, the prepared
anode is denoted as [PdCl2/AC + VGCF]. The geometric area of the
anode was 5 cm2.
Stoichiometric carbonylation of phenol promoted by amine:
stoichiometric carbonylation of phenol was confirmed and stud-
ied using PdCl2 (1 mmol), phenol (30 equiv.), triethylamine (Et3N,
7 equiv.) and CO (1 atm) in CH2Cl2 (0.030 dm−3, dried over MS-
4A) at 25 ◦C. Water content in the dry CH2Cl2 solvent and in
the reaction mixture were monitored using the Karl Fischer titra-
tion, and were controlled below 20 ppm. Stoichiometric reaction
proceeded as follows; (i) introduction of CO into the mixture of
PdCl2, phenol, tetrabutylammonium perchlorate (Bu4NClO4) and
CH2Cl2 for 1 h and (ii) addition of Et3N and initiation of car-
bonylation by stirring with a magnetic spin bar for 1 h at 25 ◦C.
Bu4NClO4 was added as a supporting electrolyte to ascertain
its influence on the carbonylation. Other salts, such as Bu4NBr,
Bu4NCl, Hx4NClO4 (Hx: hexyl) and Et4NClO4, as well as other
amines, diethylamine (Et2NH), diisopropylethylamine (i-Pr2NEt),
tributylamine (Bu3N), triphenylamine (Ph3N), diethylphenylamine
2.2.2. Electrolysis procedure
A one-compartment electrolysis glass cell was used for the car-
bonylation of phenol, as shown in Fig. 1. The [PdCl2/AC + VGCF]
anode was placed on the flat bottom of the cell, and a current
collector made of Au wire and a PTFE rod were attached to the
anode by physical pressing. A Pt-coil counter-electrode was also

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T. Murayama et al. / Electrochimica Acta 56 (2011) 2926–2933
Table 1
placed inside the cell. The distance between the anode and the
counter-electrode was 1 cm. Pure CO (1 atm, dried using a cold trap
at −76 ◦C) was introduced into the electrolysis solutions of phenol
(1.0 mol dm−3)/Bu4NClO4 (0.1 mol dm−3)/CH2Cl2 (0.033 dm−3) for
1 h. Et3N was added to the electrolyte and the solution was elec-
trolyzed under galvanostatic conditions at 1 mA for 4 h at 25 ◦C. A
HZ-5000 electrochemical instrument system (Hokuto Denko Co.)
was employed for the galvanostatic electrolysis. A reference elec-
trode was not used for the galvanostatic electrolysis to prevent
contamination by additional water. Other electrolyte solutions of
Stoichiometric carbonylation of phenol by metal salts at 25 ◦C.
Metal salt
Additives
Product yield (%)a
Entry
Base (equiv.), electrolyte
DPCb
PSc
CO2
1
2
3
4
5
6
7
8
9
PdCl2
Et3N (7), –
Et3N (7), –
Et3N (7), –
Et3N(7), –
Et3N (7), Bu4NClO4
Et3N (7), Bu4NCl
Et3N (7), Bu4NBr
Et3N (7), Et4NClO4
Et3N (7), Hx4NClO4
30.5
14.3
0
0
46.6
0
0
35.3
38.6
0.8
16.0
0
0
0.4
0
0
0.5
0.5
24.6
69.4
0
0
24.9
0
0
21.9
21.9
Pd(OAc)2
HAuCl4
CuCl2
PdCl2
PdCl2
PdCl2
phenol (0.89 mol dm−3), PhONa (4 equiv.) and CH3CN (0.033 dm−3
)
PdCl2
PdCl2
were used for carbonylation. Lithium and potassium phenoxides,
PhOLi and PhOK, were also used instead of PhONa.
Metal salt 0.03 mol dm−3, C6H5OH 1 mol dm−3, electrolyte 0.1 mol dm−3, CH2Cl2
0.030 dm−3, CO 1 atm, reaction time 1 h.
a
Product yield based on Pd2+
Dipheny carbonate.
Phenyl salicylate.
.
2.2.3. Alkali metal phenoxide synthesis and handling
b
c
PhOLi and PhOK were synthesized by a neutralization reaction
between phenol and LiOH, and phenol and KOH, respectively. For
PhOLi synthesis, 0.6 mol dm−3 phenol/H2O solution (0.05 dm−3and
2.0 mol dm−3 LiOH/H2O solution (0.015 dm−3) were well mixed.
This mixture was dried under pressure conditions at 25 ◦C A
white solid powder (3.79 g) was obtained, which was identified
as PhOLi·1.9H2O using the elemental analysis and the Karl Fis-
cher titration. In addition, PhOK·1.6H2O (4.39 g) was obtained in
a similar manner.
PhONa·2H2O (Aldrich), PhOLi·1.9H2O (synthesized) and
PhOK·1.6H2O (synthesized) reagents contained significant quanti-
ties of H2O. Therefore, PhONa, PhOLi and PhOK were dissolved in
dry CH3CN and the solutions were dried using MS-3A. The dried
50 mM PhONa, PhOLi and PhOK/CH3CN solutions (<20 ppm H2O)
were used as reagents in carbonylation study.
formed using a Shimadzu 10VP system (UV–vis detector, DOS-3
column (4∅ × 150 mm), H2O/CH3CN solvent).
The outlet gas mixture (CO, CO2 and H2) was analyzed using an
on-line GC (Shimadzu GC-8A, Porapak-Q column (4∅ × 2 m), TCD
detector, He carrier gas) for CO2 and CO, and another GC (Shi-
madzu GC-8A, Activated Carbon column (4∅ × 2 m), TCD detector,
Ar carrier gas) for H2. Experimental error was 5% for each product
yield.
3. Results and discussion
3.1. Stoichiometric carbonylation of phenol
2.2.4. Open-yield
Stoichiometric carbonylation of phenol to DPC with Pd2+
((PhCN)2PdCl2) has previously been reported in an Et3N/CH2Cl2
solvent at room temperature [2]. The conditions for stoichiomet-
ric carbonylation of phenol with PdCl2 were studied and applied
to the mediated electrocarbonylation (Table 1). No carbonyla-
tion products were confirmed in the stoichiometric reaction of
C6H5OH (1.00 mol dm−3)/CH2Cl2 (0.030 dm−3, dried over MS-4A),
PdCl2 (0.0333 mol dm−3) and CO (1.00 atm) at 25 ◦C. The addition
of 7 equiv. of Et3N against Pd2+ resulted in a significant yield of
DPC (30.5%) based on Pd2+ and a trace of phenyl salicylate, as indi-
cated in entry 1. Black deposits of Pd0 were produced after the
reaction. CO2 (24.6% yield) was produced by the oxidation of CO
with H2O and Pd2+. Et3N promoted the DPC formation by creating
a supply of PhO− (Scheme 1). Et3N also enhanced the CO2 forma-
tion due to stabilization of H+ with Et3N to yield Et3NH+ (Eq. (7)).
The absolute amount of CO2 produced (2.46 × 10−4 mol) was larger
than the H2O content, approximately 0.40 × 10−4 mol (20 ppm) in
0.030 dm−3 CH2Cl2. Therefore additional water contamination of
Et3N could have contributed to the CO2 formation.
Stoichiometric reaction between phenol, CO and Pd2+ in the
[PdCl2/AC + VGCF] anode proceeded under open-circuit conditions.
Hereafter, the yields of DPC and CO2 based on Pd2+ (30 ␮mol) are
denoted as DPC open-yield and CO2 open-yield.
Current efficiency: DPC was formed by 2-electron oxidation (Eq.
(3)), with current efficiency (CE) as defined in Eq. (4).
2C6H5OH + CO → (C6H5O)2CO + 2H+ + 2e−
(DPC yield)(2)(96, 485)(100)
(3)
(4)
CE =
%
charge passed
CE for CO2 formation was calculated using Eqs. 5 and 6, similar
to DPC formation.
CO + H2O → CO2 + 2H+ + 2e−
(5)
(6)
(CO2 yield)(2)(96, 485)(100)
CE =
%
charge passed
DPC and CO2 formation rates correspond to their CEs in galvano-
static electrolysis. In other words, 1 mA electrolysis with 100% CE
corresponds to a maximum formation rate of 18.7 ␮mol h−1 (DPC
or CO2).
CO + H2O + 2Et3N + Pd2+ → CO2 + 2Et3NH+ + Pd0
(7)
Other metal oxidants, such as Pd(OAc)2, HAuCl4 and CuCl2 were
tested for carbonylation of phenol with the addition of Et3N in
entries 2–4, respectively. As mentioned in the introduction, gold
and copper are active for the electrocarbonylation of methanol
[12,15]. DPC and phenyl salicylate were produced using Pd(OAc)2,
whereas Au3+ and Cu2+ were not efficient in the stoichiometric
carbonylation of phenol. The product selectivity using Pd(OAc)2
differed significantly from that using PdCl2. Counter-anion species
may have affected the carbonylation selectivity, but the total yields
of DPC and phenyl salicylate were very similar in entries 1 and 2.
Next, the effects of adding a supporting electrolyte to the
reaction mixture were studied on stoichiometric carbonylation
of phenol in entries 5–9. The yield of DPC increased from 30.5%
2.3. Product analyses
Products in the solution were analyzed using GC and HPLC tech-
niques. DPC and phenyl salicylate were analyzed using a Shimadzu
GC-2010 (ZB-1 capillary column (0.25∅ × 30 m), FID detector, He
carrier gas) and Agilent EZ Chrom and SS420X. The reaction mix-
ture (5.0 × 10−4 dm−3) was introduced using a micro-syringe and
an external standard solution of phenanthrene (100 ␮mol)/CH3CN
(1.0 × 10−4 dm−3) was added. A 0.05 ␮L sample was then injected
to the GC and analyzed at 150 ◦C.
Other carbonylation or oxidation products of phenol could not
be detected using GC and HPLC analysis. HPLC analysis was per-

T. Murayama et al. / Electrochimica Acta 56 (2011) 2926–2933
2929
Table 2
Effect of amines on stoichiometric carbonylation of phenol by PdCl2 at 25 ◦C.
150
Entry
Base (equiv.)
Product yield (%)a
DPCb PSc
46.6
CO2
100
50
0
5
10
11
12
13
14
15
16
17
Et3N (7)
Et3N (2)
Et3N (1)
Et2NH (2)
i-Pr2NEt (2)
Bu3N (2)
Ph3N (2)
Et2NPh (2)
TMEDA (2)
0.4
0.1
0
24.9
11.4
6.1
22.4
4.2
4.6
0
68.0
41.2
0
(
)
0
47.3
18.4
0
0.3
0
0
0
0
0
0.8
1.1
3.1
PdCl2 0.03 mol dm−3, C6H5OH 1 mol dm−3, n-Bu4NClO4 0.1 mol dm−3, various amine,
CH2Cl2 0.030 dm−3, CO 1 atm, reaction time 1 h.
Product yield based on Pd2+
Dipheny carbonate.
Phenyl salicylate.
.
a
0
1
2
3
4
b
c
Reaction time / h
Fig. 2. Time courses of DPC formation by mediated electrocarbonylation (1 mA)
of C6H5OH/Et3N/Bu4NClO4/CH2Cl2 at the [PdCl2/AC + VGCF] anode at 25 ◦C. Anode
(5 cm2): PdCl2 (30 ␮mol)/AC (30 mg) + VGCF (125 mg) + PTFE (40 mg), Electrolysis
solution: 1.0 mol dm−3 C6H5OH, Et3N 0–8 equiv. 0.1 mol dm−3 Bu4NClO4, CH2Cl2
0.033 dm−3, CO 1 atm. +: open circuit, ꢁ: initial addition of Et3N (2 equiv.) at 0 h
and additional Et3N (2 equiv.) added after open circuit condition as indicated (ꢁ),
ꢁ: addition of Et3N (2 equiv.) twice at 0 and 1 h, ᭹: intermittent addition of Et3N (2
equiv.) at 0, 1, 2 and 3 h, ꢂ: initial addition of Et3N (8 equiv.) at 0 h.
to 46.6% with the addition of Bu4NClO4, but the yield of phenyl
salicylate decreased from 0.8% to 0.4%. On the other hand, the
DPC and phenyl salicylate yields were zero following the addi-
tion of Bu4NBr and Bu4NCl, but several unidentified GC peaks were
observed. Other tetra-alkylammonium perchlorates, Hx4NClO4 and
Et4NClO4, were tested in stoichiometric carbonylation solutions in
entries 8 and 9, giving yields of DPC that were larger than those
without electrolyte (entry 1). Therefore, tetra-alkylammonium per-
chlorate is able to enhance the stoichiometric carbonylation. In
particular, Bu4NClO4 could be an efficient electrolyte for the car-
bonylation.
Furthermore, the effects of different quantities of Et3N on DPC
formation were studied by adding Bu4NClO4 in entries 10–12
(Table 2). The addition of 2 equiv. of Et3N against Pd2+ obtained a
higher DPC yield than the addition of any other quantity of Et3N. The
yields of CO2 decreased as the amounts of Et3N added decreased.
As mentioned above, contamination of H2O and stabilization of H+
by amines (Eq. (3)) should enhance the CO2 formation. Further-
more, 2 equiv. of other amines, such as Et2HN, i-Pr2NEt, Bu3N, Ph3N,
Et2NPh and TMEDA were tested for carbonylation in entries 13–17.
i-Pr2NETt and Bu3N promoted the stoichiometric carbonylation of
phenol significantly in contrast to other amines. Trialkylamines
were suitable additives to enhance the carbonylation; in contrast,
phenylamines were not found to be efficient. Therefore, we selected
2 equiv. of Et3N and 0.1 mol dm−3 of Bu4NClO4 as additives for
mediated electrocarbonylation of phenol at the [PdCl2/AC + VGCF]
anode.
CO and Pd2+ in the anode proceed under open-circuit conditions.
DPC formed smoothly ceasing after 30 min, giving a DPC open-
yield of 44% [19]. CO2 was also formed by oxidation of CO with
H2O-contaminated solvent and Pd2+, giving a CO2 open-yield of
35%. Next, the mediated electrocarbonylation with 1 mA was con-
ducted at the flesh [PdCl2/AC + VGCF] anode. DPC yield increased
slightly after 30 min and was 54% at 120 min. When an additional 2
equiv. of Et3N was added to the solution under open-circuit condi-
tions, the DPC yield jumped to 73% as indicated (ꢁ). Therefore, the
mediated electrocarbonylation was conducted by adding 2 equiv.
of Et3N twice, at 0 and 1 h. The DPC yield increased linearly after
1 h and reached to a plateau after 2.5 h. When 2 equiv. of Et3N was
added in 1-h intervals (0, 1, 2 and 3 h), the DPC yield increased lin-
early after 4 h. The final DPC yield was 130% at 4 h based on Pd2+
,
which corresponds to a turnover number (TON) of 1.30. Phenyl sal-
icylate and other oxidation products of phenol were not detected
by GC and HPLC techniques. When a total of 8 equiv. of Et3N was
added simultaneously in the initial electrolysis solution, the DPC
yield also increased linearly over 4 h but the yield was lower than
that obtained with the 1-h interval addition of Et3N. The interval
addition of Et3N was more efficient in the mediated electrocarbony-
lation than in initial single addition. These experimental results
suggest that Et3N enhances the mediated electrocarbonylation of
phenol to DPC, but converts to an inactive form during electrolysis.
The CE calculated from the slope of Fig. 2 was 42% for the DPC for-
mation by the interval addition of Et3N. CO2 production increased
slightly using a 3% CE. On the other hand, the CE of DPC formed
by the single addition of 8 equiv. Et3N was 26% and that of the
CO2 formation was 29%. These CE data also clearly indicate that the
interval addition of Et3N was more efficient than the single initial
addition for the DPC formation. The open-yields and the mediated
electrocarbonylation activities of DPC and CO2 are summarized in
Table 3.
3.2. Electrochemical re-oxidation of Pd0 under carbonylation
conditions
If the black deposit of Pd were to oxidize with the electro-
chemical potential under stoichiometric carbonylation conditions,
continuous electrocatalytic synthesis of DPC would occur. The
[PdCl2/AC + VGCF] anode (5 cm2) was applied for the carbonylation.
Various electrolytic conditions for the DPC formation were studied
using the [PdCl2/AC + VGCF] anode. Reaction conditions consisting
of galvanostatic electrolysis of solutions (1 mol dm−3 C6H5OH, 2
equiv. of Et3N (60 ␮mol), 0.1 mol dm−3 Bu4NClO4 and 0.030 dm−3
CH2Cl2 dried over MS-4A), a low electrolysis current of 1 mA, and
the use of a conventional one-compartment cell (Fig. 1) were found
to be efficient in the DPC formation [17]. If a wet CH2Cl2 solvent (ca.
60,000 ppm H2O) was used in these carbonylation conditions, DPC
was not produced, with the primary product being CO2.
As described above, Et3N was efficient for the electrosynthe-
sis of DPC. The influence of substitution of the ethyl group of Et3N
by methyl, benzyl and phenyl groups on the mediated carbonyla-
tion was studied. Fig. 3 shows the mediated electrocarbonylation
of phenol with addition of 2 equiv. of various amines, such as
Me2NEt, MeNEt2, Me2NBn or Me2NPh at 1-h interval. The forma-
tion rates of DPC depended on the amines, such that Et3N, MeNEt2,
Fig. 2 shows the time courses of the DPC formation under differ-
ent conditions. First, the DPC open-yield and CO2 open-yield were
confirmed. As mentioned in Section 2, reactions between phenol,

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T. Murayama et al. / Electrochimica Acta 56 (2011) 2926–2933
150
100
50
0
2
3
4
0
1
Reaction time / h
Fig. 3. Effects of amines on time courses of DPC formation by mediated elec-
trocarbonylation (1 mA) of C6H5OH/amine/Bu4NClO4/CH2Cl2 at [PdCl2/AC + VGCF]
anode at 25 ◦C. Anode (5 cm2): PdCl2 (30 ␮mol)/AC (30 mg) + VGCF (125 mg) + PTFE
(40 mg), Electrolysis solution: 1.0 mol dm−3 C6H5OH, intermittent addition of amine
(2 equiv.) at each 1 h. 0.1 mol dm−3 Bu4NClO4, CH2Cl2 0.033 dm−3, CO 1 atm. ᭹: Et3N,
ꢁ: Me2NEt, ꢁ: Me2NBn, ꢂ: MeNEt2, ♦: Me2NPh.
>Me2NBn > Me2NEt ꢂ Me2NPh. This order may indicate that the
pKa values of their ammonium forms affect the formation rates
of DPC. However, their pKa values in dry CH2Cl2 have not been
reported.
As described above, the carbonylation activity was not suf-
ficient, but it proves that the mediated electrocarbonylation of
phenol to DPC can be accomplished under ambient conditions of
P(CO) = 1 atm and 25 ◦C.
Fig. 4. Effects of alkaline phenoxide (PhOX) on yields of DPC (a) and CO2 (b) by
mediated electrocarbonylation of C6H5OH/PhOX/CH3CN at [PdCl2/AC + VGCF] anode
at 25 ◦C. Electrolysis solution: 0.89 mol dm−3 C6H5OH, PhOX 0.12 mmol, CH3CN
0.033 dm−3, CO 1 atm. Using PhONa, ᭹: DPC, ꢃ: CO2; using PhOLi, ꢃ: DPC, ꢁ: CO2;
using PhOK, ꢄ: DPC, ꢂ: CO2.
This indicates that PhONa can be used instead of Et3N. Next, medi-
ated electrocarbonylation using PhONa was conducted at the flesh
[PdCl2/AC + VGCF] anode with 1 mA for 6 h. PhONa functioned as
a supporting electrolyte and DPC was continuously produced. The
final yield was greater than 172% based on Pd2+ (1.72 TON). The
CE for the DPC formation calculated from the initial slope of Fig. 4
was 40% after 1 h (Table 3). On the other hand, the yield of CO2
increased to 50% within 1 h and slowly increased after 1 h. The CO2
production ceased after 4 h and the final yield of CO2 was 60% after
6 h. The time course of the CO2 formation indicated that water con-
taminated from the electrolyte was consumed by oxidizing CO (Eq.
(3)). The average CE of CO2 formation was 10%, calculated from
the yields between 10 min and 6 h, (Table 3). The formation of H2
was confirmed during electrolysis. The formation rate of H2 was
constant for 6 h and its CE was 68%.
3.3. Mediated electrocarbonylation of phenol to DPC using PhONa
According a previous study, the function of Et3N is to supply
phenoxy anions (PhO−), as shown in Scheme 1 [1]. The nucleophilic
phenoxy anion attacks CO on Pd2+, producing DPC. The supply of
PhO− may be essential for mediated electrocarbonylation of phenol
to DPC. Thus, PhONa was used as a source of PhO− and a supporting
electrolyte instead of Et3N and Bu4NClO4, respectively, although,
PhONa did not dissolve in the phenol/CH2Cl2 solution. Next, CH3CN
dried over MS-3A was applied to carbonylation. The water con-
tent was lower than 20 ppm, as determined using the Karl Fischer
titration.
Fig. 4(a) and (b) shows time courses of carbonylation of
phenol and CO2 formation at the [PdCl2/AC + VGCF] anode
in various solutions, respectively. First, the open-yields of
To understand the function of PhONa, i.e., PhO− and Na+, medi-
ated electrocarbonylation of phenol was studied using PhOLi or
PhOK instead of PhONa. The electrolysis solutions were dried over
MS-3A to <20 ppm H2O. Kinetic curves of the DPC formation in the
two cases were very similar to those of PhONa (Fig. 4). The time
courses of the CO2 yield using PhOLi and PhOK differed from those
using PhONa. However, the yield of CO2 decelerated after 3 h, and
on comparison, the reaction gave yields similar to that obtained
DPC and CO2 were confirmed in
a solution of PhONa (4
equiv.)/C6H5OH (0.89 mol dm−3)/CH3CN (0.033 dm−3), CO (1 atm)
and Pd2+ (30 ␮mol) in the anode. CO2 was immediately produced
by addition of PhONa/CH3CN at 0 min. The open-yields of DPC and
CO2 were considerably high, 50% and 23%, respectively, in 10 min.
Table 3
Summarization of open-yields and CEs of DPC and CO2 formation at [PdCl2/AC + VGCF].
Reactivities
2 eq. ×4 Et3Na
8 eq. Et3Nb
DPC
4 eq. PhONac
DPC
4 eq. PhOLic
DPC
4eq. PhOKc
DPC
DPC
CO2
28
CO2
CO2
CO2
CO2
Open-yield/%d
CE/%e
44
42
43
26
40
29
50
40
23
10
57
41
13
10
50
31
11
10
3
a
2 equiv. Et3N × 4 times addition + Bu4NClO4.
Initial addition of 8 equiv. Et3N + Bu4NClO4.
Initial addition of 4 equiv. PhONa, PhOLi or PhOK.
Yield based on Pd supported on AC.
b
c
d
e
CE calculated from the slope by mediated-electrocarbonylation with 1 mA.

T. Murayama et al. / Electrochimica Acta 56 (2011) 2926–2933
2931
80
60
40
20
(b)
0
8
40
30
20
10
6
4
2
0
(a)
0
0
5
10
15
20
PhONa / equiv.
Fig. 6. Effects of concentration of phenol on (a) mediated electrocarbonylation
of phenol (1 mA) and (b) open-yield at [PdCl2/AC + VGCF] anode at 25 ◦C. Elec-
trolyte: C6H5OH/4 equiv. PhONa/CH3CN (0.033 dm−3), CO 1 atm. Anode (5 cm2):
PdCl2 (30 ␮mol)/AC (30 mg) + VGCF (125 mg) + PTFE (40 mg). Stoichiometric yield,
ꢃ: DPC, ꢂ: CO2; electrochemical formation rate, ᭹: DPC, ꢄ: CO2.
Fig. 5. Effects of quantities of PhONa addition on (a) mediated electrocarbonylation
of phenol (1 mA) and (b) open-yield at [PdCl2/AC + VGCF] anode at 25 ◦C. Electrolyte:
0.89 mol dm−3 C6H5OH/PhONa/CH3CN (0.033 dm−3), CO 1 atm. Anode (5 cm2): PdCl2
(30 ␮mol)/AC (30 mg) + VGCF (125 mg) + PTFE (40 mg), Stoichiometric yield, ꢃ: DPC,
ꢂ: CO2; electrochemical formation rate, ᭹: DPC, ꢄ: CO2.
experiments. The formation rate (CE) of CO2 was almost constant
(15 3% CE).
using PhONa. The open-yields and CEs of DPC and CO2 using PhOLi
or PhOK are listed in Table 3. The alkaline cations, Na+, Li+, or K+,
did not have significant influence on the mediated electrocarbony-
lation of phenol to DPC. Although alkaline cations may have affected
the formation of CO2 at an early stage of the reaction but the final
yields of CO2 were very similar.
Fig. 5b shows the open-yields of DPC and CO2 extrapolated from
time courses of the product formation at 0 min. The DPC open-yield
increased with addition of PhONa and showed a maximum of 56%
at 6 equiv. The DPC open-yield decreased slightly with excess addi-
tions of PhONa. In contrast, the CO2 open-yield was almost constant
with the addition of PhONa.
When the galvanostatic current was increased to 3 mA, the DPC
yield increased slightly, 206% based on Pd2+ (2.06 TON), compared
to that at 1 mA [19]. The CEs of the DPC and the CO2 formations
were only 17% and 11%, respectively. A higher electrolytic current
did not favour the DPC formation at the [PdCl2/AC + VGCF] anode;
no other products or reactions were detected that could account
for the current of approximately 72% CE.
Fig. 6 shows the effect of varying phenol concentrations from
0 to 0.89 mol dm−3 on the mediated electrocarbonylation of phe-
nol to DPC at the [PdCl2/AC + VGCF] anode. In the electrochemical
reaction shown in Fig. 6a, the formation rate (CE) of DPC sharply
increased with the concentration of phenol and reached a constant
formation rate (ca. 40% CE) above 0.05 mol dm−3. Interestingly, DPC
was produced in significant quantities with 11% CE at 0 mol dm−3
phenol. The formation rate of CO2 was very high, i.e., 75% CE at
0 mol dm−3 phenol. The formation rate of CO2 (CE) decreased with
increasing phenol concentration. These facts indicate that the car-
bonylation of phenol to DPC and the oxidation of CO by water to
CO2 were competitive.
3.4. Influence of reaction conditions on DPC formation
The effects of various PhONa concentrations on mediated elec-
trocarbonylation of phenol to DPC at the [PdCl2/AC + VGCF] anode
were studied. The reaction conditions were the same as that used
for Fig. 4, except for the addition of PhONa. All electrolytic solutions
(PhONa; 0–16.7 equiv.) were dried over MS-3A to <20 ppm H2O.
Electrolysis did not proceeded without the addition of PhONa.
Fig. 5a shows the DPC and CO2 formation rates under the
mediated electrocarbonylation with 1 mA. The formation rates
of products corresponded to their CEs because the current was
constant. The maximum formation rate of DPC or CO2 was
18.7 ␮mol h−1 in the mediated electrocarbonylation with 1 mA, as
described in Section 2. Mediated electrocarbonylation could not
proceed in the steady state at 0 mol dm−3 PhONa because PhONa
functions as a promoter and electrolyte. Therefore, data of the for-
mation rates of DPC and CO2 were not at 0 mol dm−3. The formation
rate (CE) of DPC increased with the PhONa addition and showed a
maximum (40% CE) at 4.0 of equiv. PhONa concentrations higher
than 6 equiv. were not efficient for the DPC formation (<20% CE).
Therefore, the addition of 4.0 equiv. of PhONa was selected in later
In Fig. 6b, DPC open-yield at 0 mol dm−3 phenol was zero.
The DPC open-yield increased with the concentration of phe-
nol and reached a constant yield of approximately 50% above
0.20 mol dm−3. The CO2 open-yield was almost constant at 25% for
all concentrations of phenol.
When no phenol was added (0 mol dm−3), it was observed that
the DPC formation with 11% CE and the lack of DPC with 0%
open-yield contradicted each other. To resolve this discrepancy,
the time courses of the products were studied during the elec-
trolysis of the 4 equiv. of PhONa/CH3CN (0.033 dm−3) solutions at
the [PdCl2/AC + VGCF] anode with 1 mA, as shown in Fig. 7. CO2
was produced at 0 min, 20% open-yield of CO2. However, DPC was
not formed at 0 min, demonstrating that the DPC open-yield was
zero using PhONa/CH3CN solution. The major product, CO2, was
produced linearly during electrolysis, whereas DPC was produced
slowly after 2 h. An induction period was observed during for the
DPC formation from PhONa. This unusual result may be due to phe-

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T. Murayama et al. / Electrochimica Acta 56 (2011) 2926–2933
H2
CO
2e–
300
200
2
OH
Pd2+
2RONa
2RONa
2e–
Pt
Pd0
O
2ROH
2Na+
O-C-O
2ROH. 2Na+
Cathode
Anode
100
0
RONa (Proton accepter):
Sodium Phenoxide, Sodium Acetate, Sodium Cresolate.
Scheme 2.
0
1
2
3
4
5
6
Reaction time /h
The value of pKa of phenol (9.95) is larger than that of CH3CO2H
(4.76); therefore, PhONa should not be produced in the mixture
of phenol and CH3CO2Na in entry 20. When a mixture of C6H5OH
and CH3CO2Na was analyzed using a GC technique, CH3CO2H was
not detected. These facts suggest that CH3CO2Na functions as a H+
acceptor and promotes carbonylation, as indicated in Eq. 9.
Fig. 7. Time courses of DPC and CO2 formation by mediated electrocarbonylation
of PhONa/CH3CN at [PdCl2/AC + VGCF] anode at 25 ◦C. Electrolysis current 1 mA,
electrolysis solution: PhONa 0.12 mmol, CH3CN 0.030 dm−3, CO 1 atm. ᭹: DPC, ꢄ:
CO2.
nol formation during the oxidation of CO with H2O (Eq. (8)). The
accumulated phenol formed DPC by the mediated electrocarbony-
lation after 2 h.
2C6H5OH + CO + Pd2+ + 2CH3CO2Na → (C6H5O)2CO + Pd0
+ 2CH3CO2H + 2Na+
If the Eq. (9) is correct, PhONa also functions as the H+ accepter
in the same way as CH3CO2Na (Eq. (10)).
(9)
CO + H2O + 2PhONa → CO2 + 2PhOH + 2Na+ + 2e−
(8)
3.5. Function of PhONa
2C6H5OH + CO + Pd2+ + 2PhONa → (C6H5O)2CO
+ Pd0 + 2PhOH + 2Na+
(10)
As described in Fig. 6, PhONa did not directly produce DPC by
Pd2+ at the anode and mediated electrocarbonylation. These results
indicate that PhOX (X: Na, Li, K) promotes Pd2+-catalysed the car-
bonylation of phenol to DPC. To obtain more detailed information
regarding the function of PhONa, advanced stoichiometric reac-
tions were studied in the dry CH3CN solvent, as shown in entry
18–22 in Table 4. Entry 18 was the result of a standard stoichio-
metric carbonylation of phenol with PhONa and significant yields
of DPC and CO2 were produced. Entry 19 was the result of a stoichio-
metric reaction using PhONa, CO and PdCl2 and a major product was
CO2 and a trace amount of DPC was produced. Entry 20 is a stoichio-
metric carbonylation of phenol with CH3CO2Na. DPC was produced
in 24% yield. CH3CO2Na functioned as the promoter instead of
PhONa during the formation of DPC. Entry 21 was the result of
carbonylation of p-cresol with PhONa. (4-CH3–C6H4O)2CO was pro-
duced in 72% yield by the stoichiometric carbonylation of p-cresol
with PhONa. DPC was not produced in entry 21. Entry 22 was the
result of carbonylation of phenol with sodium p-cresolate. DPC was
produced in 51% yield by the stoichiometric carbonylation of phe-
nol with sodium p-cresolate. (4-CH3–C6H4O)2CO was not produced
in entry 22.
According to Eq. (10), the stoichiometric carbonylation of p-
cresol produced (4-CH3–C6H4O)2CO and did not produce DPC in
entry 21. Sodium p-cresolate also functioned as the H+ accepter
and (4-CH3–C6H4O)2CO was not produced in entry 22.
3.6. Reaction scheme
Based on these experimental observations, we propose the fol-
lowing reaction scheme for the mediated electrocarbonylation of
phenol to DPC at the [PdCl2/AC + VGCF] anode (Scheme 2).
First, the Pd2+-catalysed the stoichiometric carbonylation of
phenol proceeds in which PhONa promotes the carbonylation.
PhONa functions as the H+ accepter because of the following
reasons: (i) CH3CO2Na promotes the stoichiometric carbonyla-
tion, (ii) DPC is not directly produced from PhONa and (iii)
(4-CH3–C6H4O)2CO is produced from the solution of cresol and
PhONa. After the promoting this reaction, PhONa is converted to
phenol and Na+ (Eq. (10)).
Next, Pd0 at AC was electrochemically oxidized to regenerate
Pd2+. Then, H2 was produced by phenol reduction at the Pt-wire
cathode and PhONa was possibly reproduced. Therefore, a single
initial addition of PhONa was efficient in the mediated electrocar-
bonylation of phenol to DPC.
Table 4
Effects of additives on stoichiometric carbonylation phenols in CH3CN by PdCl2 at
25 ◦C.
Product yield (%)a
The addition of Et3N is expected to promote the DPC formation
and to form Et3NH+ (Eq. (12)). However, when a mixture of phenol,
Et3N and CH2Cl2 were added, Et3N and phenol were detected, and
the neutralization of phenol and Et3N to (C6H4O−)(Et3NH+) did not
proceed or was very slow under our reaction conditions.
Entry
Reactant
Promoter
DPCb
(4-CH3–C6H4O)2CO
CO2
18
19
20
21
22
Phenol
–
Phenol
p-cresol
Phenol
PhONa
PhONa
CH3CO2Na
PhONa
4-CH3–C6H4ONa
70.1
1
–
–
29.3
87.1
25.6
20.9
47.7
23.9
0
51.3
–
71.5
0
2C6H5OH + CO + Pd2+ + 2Et3N → (C6H5O)2CO + Pd0
+ 2Et3NH+
(12)
PdCl2 0.03 mol dm−3, C6H5OH or 4-CH3–C6H4OH 1 mol dm−3, promoter (2 equiv),
CH3CN 0.030 dm−3, CO 1 atm, reaction time 1 h.
Et3NH+ was not able to regenerate Et3N and Et3NH+PhO− accu-
mulated because of the formation of H2 at the cathode, as indicated
Product yield based on Pd2+
Dipheny carbonate.
.
a
b

T. Murayama et al. / Electrochimica Acta 56 (2011) 2926–2933
2933
in Eq. (13). Therefore, the addition of Et3N at 1-h intervals was
efficient in the mediated electrocarbonylation.
although the maximum TON and yield were not excellent. The use
of a dry solution (<20 ppm H2O) and the addition of either trialky-
lamine (Et3N) and phenol or alkaline metal phenoxide (PhOX, X; Li,
Na and K) were efficient in DPC formation and a lower electroly-
sis current (1 mA) was favoured. PhOX worked as the promoter and
the supporting electrolyte for the mediated electrocarbonylation of
phenol to DPC. We proposed that PhONa and Et3N functioned as the
H+ accepterfrom phenol but not from asimple baseduring carbony-
lation to produce phenoxide. An indirect electroorganic synthetic
mechanism for the DPC formation, consisting of stoichiometric
carbonylation of phenol to DPC by Pd2+ and the electrochemical
oxidation of Pd0 to Pd2+, was proposed. To develop an efficient
alternative to the phosgene method for DPC production, we must
further accelerate the oxidation rate of Pd0 to Pd2+ (electrolysis
current) while simultaneously suppressing the direct oxidation of
phenol and the formation of a by-product (tar) at the anode.
2C6H5OH + 2Et3NH+ + 2e− → H2 + 2Et3NH+C6H5O−
(13)
As summarized in Table 3, the electrolysis systems using Et3N,
PhONa, PhOK and PhOLi showed very similar CEs for the DPC forma-
tion. The CEs of the combination of DPC and CO2 were ca .50%. The
products accounting for 50% CE could not be identified in the liquid
phase using GC or HPLC techniques. We speculated that an oxida-
tive oligomerization of phenol proceeded and oligomer deposited
on the carbon surface. To achieve a more selective and active car-
bonylation of phenol, the electrocatalysis of the Pd anode must
improve to suppress the unselective oxidation of phenol.
The anode potential was not monitored during the electrol-
ysis to suppress water contamination because dry electrolyte
solutions were essential for our system. In a separate experi-
ment, we measured the [PdCl2/AC + VGCF] anode potential using an
Ag/Ag+ (0.01 mol dm−3 Ag(NO3)/0.1 mol dm−3 Bu4NClO4/CH3CN)
reference electrode. The anode potential was +0.5 V (Ag/Ag+) for
1 mA of electrolysis and the applied voltage between the anode
and the cathode was approximately 3 V.
This reaction system has a strong reductive atmosphere of 1 atm
CO and no oxygen. The AC surface had strong adsorption and reduc-
tion abilities owing to the free electrons. The reduction rate of
Pd2+ by carbonylation can be calculated from stoichiometric reac-
tion rate under the open-circuit condition, as shown in Fig. 4. The
reduction of 30 ␮mol Pd2+ was completed in 10 min, which cor-
responded to 10 mA. Thus, the reduction rate was 10 times faster
than the electrochemical oxidation rate of Pd0 (1 mA). Therefore,
Pd0 is more stable than Pd2+ on the AC surface during the mediated
electrocarbonylation at 1 mA.
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The mediated electrocarbonylation of phenol to DPC with
P(CO) = 1 atm at the [PdCl2/AC + VGCF] anode and 25 ◦C was studied,