Catalytic synthesis of diethyl carbonate by oxidative carbonylation of ethanol over PdCl2/Cu-HMS catalyst

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Abstract

Direct synthesis of diethyl carbonate (DEC) by oxidative carbonylation of ethanol offers a prospective “green chemistry” strategy to eliminate the phosgene used in the traditional preparation processes. PdCl2/Cu-HMS catalyst has been investigated, which demonstrated excellent selectivity to DEC by oxidative carbonylation of ethanol in the gas-phase reaction. The Si/Cu molar ratio of mesoporous Cu-HMS supports showed a remarkable effect on catalytic activities. An optimized Si/Cu molar ratio existed for catalytic performance, which was about 50/1. From XPS, ICP, XRD, nitrogen physisorption and IR characterization and analysis, it could be concluded that copper species incorporated into HMS frame and was highly dispersed in the frame of silica. The catalytic performances of PdCl2/Cu-HMS were related with both Cu content in the Cu-HMS and the order degree of mesoporous structure for Cu-HMS.

Introduction

Diethyl carbonate (DEC) is recognized as an environmentally benign chemical because of its negligible ecotoxicity and low bioaccumulation and persistence. Because of its high oxygen contents (40.6 wt.%), DEC has been proposed as a replacement for tert-butyl ether (MTBE) [1] as an attractive oxygen-containing fuel additive. A further significant advantage of DEC over other fuel oxygenates such as MTBE is that DEC could slowly decompose to CO2 and ethanol, which have no environmental impact when released into the environment [2]. In addition, the gasoline/water distribution coefficients for DEC are more favorable than those for dimethyl carbonate (DMC) and ethanol [3]. Besides these applications as a fuel additive, DEC is also drawing attention as a safe solvent and an additive in lithium cell electrolyte [3], [4], [5].

Several synthetic routes to DEC have been developed so far, e.g., (i) the phosgene–ethanol process, in which COCl2 is reacted with ethanol, obviously involving dangerous phosgene gas [6]; (ii) the oxidative carbonylation of ethanol over a slurry of CuCl [7]; (iii) reaction between ethyl nitrite (C2H5ONO) and carbon monoxide [8]; (iv) the gas-phase oxidative carbonylation of ethanol using a heterogeneous catalyst [9], [10], [11], [12]; (v) transesterification reaction of DMC and ethanol [13], [14]; (vi) activation of carbon dioxide [15]; (vii) reaction of ethanol with urea over organotin catalysts [16]. Among them, oxidative carbonylation of ethanol in the gas-phase has been deemed as one of the most promising routes for DEC synthesis based on the “green chemistry” principles [1], [9]. Also, it is well known that the structures and component of support play an important role in the structures and activities of supported catalysts. Since 1980s, efforts have been made in the development of supported copper-based catalysts for DMC synthesis. All kinds of catalysts were prepared by impregnating the active carbon, oxides (MnO, ZnO, TiO2, SiO2, and Al2O3) or HMS silica in methanol solution of CuCl2 [9], [17]. Previous results have shown that all of the tested catalysts deactivated quickly because of losing the chlorine anion and the remodel of active copper species on the surface. Recently, HY zeolite and mesoporous MCM-41 silica have been reported as the promising supports for the oxidative carbonylation reaction [18], [19], [20], [21], [22], wherein CuY catalyst and CuCl/MCM-41 catalyst were prepared by the high temperature solid state ion-exchange method. However, in the case of synthesis of DEC by oxidative carbonylation, only active carbon was widely used as the support. Hence, efforts have been dedicated to develop more suitable supports than active carbon for more efficient gas-phase oxidative carbonylation of ethanol to DEC. And it turns out to be highly interesting to further explore the other possible applications of the catalytically attractive materials for DEC synthesis. In our previous work, it mainly reported a new type of catalyst and illuminated the catalytic reaction mechanism by the interaction between Pd2+ loaded on the surface and Cu2+ in Cu-HMS frame [23]. This report is the continuation of that work.

This paper mainly made further investigation on the structural properties of Cu-HMS on the DEC synthesis. It was found that a reasonable amount of PdCl2 loadings was a critical point in the DEC preparation. In addition, the effects of Si/Cu molar ratio of PdCl2/Cu-HMS catalysts on the reaction performances for producing DEC were investigated.

Section snippets

Catalyst preparation

The HMS and Cu-HMS were synthesized following the procedures similar to those proposed by Tanev et al. via a neutral templating pathway using dodecylamine (DDA) as a surfactant [24], [25]. In a typical synthesis, 3.85 g of DDA was dissolved in 130 ml of water, 32.5 ml of ethanol was then added to generate a 40:10 H2O/EtOH solution of the surfactant. The surfactant solution was stirred for 15 min. At the same time, 0.35 g of copper chloride was added in the mixture of 20 ml ethanol and 23 ml tetraethyl

Characterization of HMS materials and catalysts

The binary catalyst systems PdCl2-CuCl2-TBAB employing mesoporous HMS silica as a support for the gas-phase oxidative carbonylation of methanol at atmospheric pressure have exhibited excellent catalytic performance in the reaction of dimethyl carbonate (DMC) synthesis [17]. HMS was reported to demonstrate more advantages than other supports, such as larger pores and higher thermal and hydrothermal stability [26]. However, the loss of the chlorine and the remodel of active copper species on the

Conclusion

A novel catalyst, PdCl2/Cu-HMS, for DEC synthesis was investigated in the gas-phase oxidative carbonylation of ethanol in this work which showed excellent catalytic performances. TEM, nitrogen sorption, XPS, ICP and IR analysis suggested that an ordered mesoporous Cu-HMS with large surface area and pore volume was successfully synthesized, and copper was incorporated into the Cu-HMS frame with favorable mesoporous stability. The improvement of the catalytic activities with the optimized Si/Cu

Acknowledgements

The financial supports from the National Natural Science Foundation of China (NSFC) (grant no. 20876112), and the Science Foundation for Youth of Jiangnan University (no. 2009LQN13) are gratefully acknowledged.

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