Elsevier

Electrochimica Acta

Volume 56, Issue 7, 28 February 2011, Pages 2926-2933
Electrochimica Acta

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

https://doi.org/10.1016/j.electacta.2010.12.087Get rights and content

Abstract

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/CH2Cl2) 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 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.

Introduction

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 majority 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 manufactured from phosgene and phenol (Eq. (1)) or dimethyl carbonate and phenol (Eq. (2)).COCl2 + 2C6H5OH  (C6H5O)2CO + 2HCl(CH3O)2CO + 2C6H5OH  (C6H5O)2CO + 2CH3OH

Direct synthesis of DPC has been studied for oxidative carbonylation of phenol with O2 using Pd catalysts. The direct synthetic method is attractive from the viewpoint of green and sustainable chemistry because of its reduced energy consumption (CO2 emission) and environmental safeguards [1]. This catalytic carbonylation is the primary alternative to the phosgene process.

Three decades ago, the stoichiometric carbonylation of phenol and CO to DPC using Pd2+ and trialkylamine at room temperature was reported (Scheme 1), wherein a phenoxide anion (PhO) produced 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 catalytic 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 pressures >6 MPa at 100 °C [3], [4], [5], [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. Therefore, suppression of unfavourable effects of accumulated water is very important in the oxidative carbonylation method.

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 electrochemical oxidation of Pd0 to Pd2+ and the oxidation potential is easily controlled. Mediated electrocarbonylation of phenol to DPC would occur at the Pd2+-anode.

The use of the electrochemical potential for the re-oxidation of a Pd0 catalyst has been reported in pioneer studies on carbonylation of alkynes to unsaturated diesters [7], alkenes to esters [8] and aromatic amines to isocyanates [9]. These indirect electrocarbonylations 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], [11], [12] and electrocarbonylation of methanol to dimethyl carbonate 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]. Electrocarbonylation of methanol in the liquid phase was also improved by use of the Pd/C anode coupled with a Br-mediator [16]. Electrocatalysis 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 unselective 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 electrochemical 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.

Section snippets

Stoichiometric carbonylation

Stoichiometric carbonylation of phenol promoted by amine: stoichiometric carbonylation of phenol was confirmed and studied 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 titration, and were controlled below 20 ppm. Stoichiometric reaction proceeded as follows; (i) introduction of CO into the mixture of

Stoichiometric carbonylation of phenol

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 stoichiometric carbonylation of phenol with PdCl2 were studied and applied to the mediated electrocarbonylation (Table 1). No carbonylation 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.

Conclusions

The mediated electrocarbonylation of phenol to DPC with P(CO) = 1 atm at the [PdCl2/AC + VGCF] anode and 25 °C was studied, although the maximum TON and yield were not excellent. The use of a dry solution (<20 ppm H2O) and the addition of either trialkylamine (Et3N) and phenol or alkaline metal phenoxide (PhOX, X; Li, Na and K) were efficient in DPC formation and a lower electrolysis current (1 mA) was favoured. PhOX worked as the promoter and the supporting electrolyte for the mediated

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