Selective carbonylation of methanol to dimethyl carbonate by gas-liquid-solid-phase boundary electrolysis

Article abstract of DOI:10.1246/cl.2002.448

Selective and efficient electrochemical carbonylation of MeOH to DMC was performed over PdCl₂/VGCF (vapor grown carbon fiber) anode by utilizing the three-phase boundary electrolysis at 1 atm (CO) and 298 K.

Full text of DOI:10.1246/cl.2002.448

448
Chemistry Letters 2002
Selective Carbonylation of Methanol to Dimethyl Carbonate
by Gas–Liquid–Solid-Phase Boundary Electrolysis
Ichiro Yamanaka,ꢀ Akiyasu Funakawa and Kiyoshi Otsuka
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
Ookayama, Meguro-ku, Tokyo 152-8552
(Received December 10, 2001;CL-011243)
Selective and efficient electrochemical carbonylation of MeOH
to DMC was performed over PdCl2/VGCF (vapor grown carbon
fiber) anode by utilizing the three-phase boundary electrolysis at
1 atm (CO) and 298 K.
Dimethyl carbonate (DMC) is one of the important chemicals
in the current chemical industry because DMC is expected to be a
safe carbonylation reagent to substitute for strong toxic phosgene
and dimethyl sulfate for the synthesis of carbamates, isocyanates,
etc. However, the DMC production of more than 20% is
manufactured by the phosgene method in the world. Parts of the
DMC production are manufactured by the EniChem method1 and
the Ube method2 from CO and MeOH. The two industrial processes
have a great advantage of no use of phosgene, compared with the
phosgene method, but have disadvantages such as severe condi-
tions, multi-steps operations, and so on. Therefore, it has been
desired to develop a new reaction system for the one-step synthesis
of DMC from MeOH and CO under mild conditions.
We have reported the electrochemical carbonylation method
for the DMC synthesis over the PdCl2/graphite anodes in the gas
phase at atmospheric pressure and 343 K (eq 1).3 However, the
performance for the DMC formation was not excellent because
partial pressures (concentrations) of MeOH and CO on the anode
were limited at atmospheric conditions.
Figure 1. Concept of the three-phase boundary electrolysis for
the carbonylation of methanol to dimethyl carbonate.
2CH3OH þ CO ! ðCH3OÞ2CO þ 2Hþ þ 2eÀ
ð1Þ
We have concluded that the increase in the both concentrations of
CO and MeOH at the anode is essential for a drastic advance in the
DMC formation.
We propose a new concept and a new cell structure for the
electrochemical carbonylation to enable to increase the both
concentrations of CO and MeOH at the same time. Our idea is
utilization of the three-phase boundary of the gas phase (CO), liquid
phase (MeOH), and solid phase (anode) for the electrochemical
carbonylation, as shown in Fig. 1. Solution of MeOH and
electrolyte fills a space between gas-diffusion anode and cathode
(2 ml). A high partial pressure of CO (101 kPa), and a high
concentration of MeOH (24.8 mol lÀ1) should be supplied to the
active site at the three-phase boundary.
Figure 2 shows the effect of electrolytes dissolved in MeOH on
the carbonylation over PdCl2/VGCF (vapor grown carbon fiber)
anode at a constant applied voltage of 3.5 V and 298 K. The anode
(1.5 cm2) was prepared by the hot-press method from electro-
catalyst powder (1.1 ꢀmol-Pd/g-VGCF) and PTFE powder.
Products, dimethyl carbonate (DMC), dimethyl oxalate (DMO),
dimethoxymethane (DMM), methyl formate (MF), and CO2 were
determined by GC, HPLC and GC-mass techniqes. When acidic
(HClO4) or basic (NaOH) electrolyte was used for the carbonyla-
tion, formation rate of (DMM þ MF) was very high though DMC
was exactly produced. An electrochemical activation of MeOH in
acidic or basic conditions should be too strong for the carbonylation.
Figure 2. Effects of various electrolytes on the electrochemical
carbonylation of methanol over PdCl2/VGCF anode. T ¼ 298 K,
appl.volt. ¼ 3:5 V, anode;PdCl
(1.1 ꢀmol cmÀ2)/VGCF, CO
2
(101 kPa), cathode;Pt/graphite, He (101 kPa). a) 0.9 mol l À1, b)
0.7 mol lÀ1, c) 0.05 mol lÀ1
.
Thus, MeOH easily oxidized to DMM and MF before the addition of
CO in acidic and basic conditions.3 As you can see clearly, neutral
electrolytes, Et4NBr, Et4NClO4, NH4ClO4 and NaClO4, showed
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
449
very good performances for the DMC formation. The DMC
selectivities based on MeOH were reached to 80%. Among the
neutral electrolytes, NaClO4 was most expected one for the DMC
formation because a high formation rate of 41 ꢀmol hÀ1cmÀ2
(TON ¼ 37 hÀ1), a high current efficiency (CE ¼ 60%) and a high
CO-selectivity (CO-sel ¼ 70%). This performance was drastically
improved compared with the result obtained in our previous work
(TON ¼ 1 hÀ1, CE ¼ 11%, and CO-sel ¼ 14%).3 These results
strongly show the advantage of the new electrolysis cell system for
the carbonylation.
Effects of the concentration of NaClO4 (0.05–0.5 mol lÀ1) on
the carbonylation were studied. The formation rates of DMC
increased with increasing in the concentration of NaClO4,
corresponding to the increase in current density. A higher formation
rate of DMC (TON ¼ 100 hÀ1) was obtained at 0.5 mol lÀ1 but the
formation rates of by-products also increased. Thus, a high CE of
59% and a high CO-sel of 66% were obtained at 0.1 mol lÀ1
.
Figure 3 shows effects of PdCl2 loading over VGCF on the
carbonylation at 3.5 V. The current densities were almost constant
without dependence of PdCl2 loading. The formation rate of DMC
increased with increasing in PdCl2 loading. In contrast, the
formation rate of DMO increased with decreasing in the PdCl2
loading. On the other hand, the formation rates of CO2 and the
oxidation product of MeOH (DMM and MF) increased slightly with
increasing in the PdCl2 loading. A very high TON of 192 hÀ1 for the
DMC formation was obtained at the PdCl2 loading of
0.27 ꢀmol cmÀ2 though the formation rate of DMO was fairly
large. The most effective PdCl2 loading is around 2.1 ꢀmol cmÀ2
because the highest formation rate of DMC, the highest CE of 65%
and the highest CO-sel of 83% were obtained. This PdCl2/VGCF
anode could be repeatedly used for the carbonylation for 3 times.
This fact proposed that Pd species should be held on the VGCF
during the carbonylation. However, we have not obtained any
information for formula and state of Pd species, yet.
Figure 4. The carbonylation of methanol over PdCl2/VGCF
anode as functions of anode potentials. T ¼ 298 K, anode;PdCl 2
(0.1 mol lÀ1)/CH3OH.
(2.1 ꢀmol cmÀ2)/VGCF, CO (101 kPa), electrolyte;NaClO
4
was 1.80 V (vs AgjAgCl). The carbonylation of MeOH began from
1.0 V (AgjAgCl). The formation rate of DMC linearly increased
with increasing in anode potentials until 2.0 V, but decelerated
above 2.0 V. In contrast to the DMC formation, the formation rate of
DMO accelerated above 2.0 V. The formation rates of CO2,
DMM þ MF, and O2 also increased above 2.0 V. These observa-
tions recommend the change of the electrocatalysis of PdCl2/VGCF
anode at around 2.0 V. We didn’t obtain detail information on the
reaction mechanism in this system. On the basis of previous work,3;4
Pd2þ/Pd0 redox should catalyze the electrochemical carbonylation
of MeOH (eq 1) and the electrochemical oxidation of MeOH (eq 2-
3).5 However, we did not observe the formation of DMO (eq 4) over
PdCl2/graphite anode in our previous work.3 Equivalent H2
corresponding to the carbonylation (eq 1 and 4) and the oxidation
(eq 2 and 3) was produced at the cathode. We must continuously
study to clarify the reaction mechanism for the formation of DMC
and DMO over the PdCl2/VGCF anode.
3CH3OH ! ðCH3OÞ2CH2 þ H2O þ 2Hþ þ 2eÀ
2CH3OH þ H2O ! HCO2CH3 þ 4Hþ þ 4eÀ
2CH3OH þ 2CO ! ðCO2CH3Þ2 þ 2Hþ þ 2eÀ
ð2Þ
ð3Þ
ð4Þ
As described so far, the three-phase boundary electrolysis was
effective for the carbonylation of MeOH to DMC under mild
conditions.
A good performance of the DMC production
(40 TON hÀ1, 67% current efficiency, 82% CO-selectivity) over
the PdCl2 (2.1 ꢀmol cmÀ2)/VGCF anode was obtained at 1.8 V
(AgjAgCl) by using the neutral electrolyte (NaClO4 0.1 mol lÀ1).
References
Figure 3. Effects of PdCl2 loading on the carbonylation of
methanol over the PdCl2/VGCF anode. T ¼ 298 K, appl.volt. ¼
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3:5 V, anode;CO (101 kPa), electrolyte;NaClO
CH3OH.
(0.1 mol lÀ1)/
4
2
3
Figure 4 shows the effect of anode potentials on the
carbonylation of MeOH over PdCl2/VGCF anode under potentio-
static conditions. The anode potential at the applied voltage of 3.5 V
4
5