Elsevier

Applied Catalysis A: General

Volume 366, Issue 1, 15 September 2009, Pages 93-101
Applied Catalysis A: General

Oxidative carbonylation of methanol to dimethyl carbonate over CuCl/SiO2–TiO2 catalysts prepared by microwave heating: The effect of support composition

https://doi.org/10.1016/j.apcata.2009.06.042Get rights and content

Abstract

Copper(I) chloride catalysts with a loading of 20 wt%, supported on silica–titania mixed oxides with Si/Ti ratios of 1, 5, 10 and 50 were prepared by conventional and microwave heating methods and tested in the oxidative carbonylation of methanol to dimethyl carbonate (DMC). X-ray diffraction (XRD), nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA) were used to examine the bulk and surface properties of the CuCl/SiO2–TiO2 catalysts. Quantum-chemical calculations were performed to explore the interaction of CuCl with the silica–titania support. Microwave heating showed some significant advantages over the conventional heating method, with markedly reduced preparation temperature and time, and provided improved catalytic activity in the oxidative carbonylation of methanol. The catalytic behavior of CuCl/SiO2–TiO2 in the test reaction studied was strongly dependent on the support composition. Incorporation of tetrahedral Ti(IV) species into the silica matrix could enhance the interaction of copper species with the oxide support. The improved catalytic performance of CuCl/SiO2–TiO2 in the DMC synthesis can be understood by the existence of the strong coordination interactions between the Cu+ centers of CuCl and the bridging oxygen atoms at the Si–O–Ti bonds in the silica–titania support.

Graphical abstract

Quantum-chemical calculations on cluster models indicated that the interactions between CuCl molecules and the Si–O–Ti bonds lead to the formation of the relatively strongly bound O-complexes, in which the bridging oxygen atoms at Si–O–Ti bonds serve as electron-donating ligands to Cu+ centers of CuCl.

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Introduction

Dimethyl carbonate (DMC) has been drawing attention from researchers due to its applications in replacing environmentally unfriendly compounds [1]. It is currently produced via the oxidative carbonylation of methanol on a simple CuCl catalyst in a slurry reactor [2]. Since this process suffers from severe equipment corrosion and catalyst deactivation, several efforts have been made to develop new and more efficient catalyst systems. Homogeneous complex catalysts were studied for DMC synthesis using various base additives [3], [4], [5] or ionic liquids [6] as promoters or co-solvents. However, the use of homogeneous catalysts suffers from the problem of catalyst separation. Heterogenized homogenous catalyst systems with excellent catalytic performance are highly desirable due to the attractive advantages over homogeneous counterparts in liquid-phase reactions. Attempts have been made in this direction by immobilizing CuCl or CuCl2 on polymer supports [7], [8] or mesoporous materials [9]; improved catalytic performance was often obtained. Moreover, a chloride-free catalyst prepared in situ from copper(II) acetate and 2,2′-bipyridine was recently reported that could provide catalytic performance comparable to the CuCl-based system in the DMC synthesis [10].

On the other hand, a continuous vapor-phase process for oxidative carbonylation of methanol is considered more desirable because it has been found almost free from corrosion [11]. Cu(II) chloride-based catalysts such as CuCl2, CuCl2–PdCl2 or CuCl2 promoted by acetates or hydroxides suffered from a strong deactivation due to the loss of the chloride involved in the reaction [12], [13], [14], [15]. There have been attempts to improve the catalyst stability such as immobilization of the active species on functionalized mesoporous silica and replacing chloride by a negatively charged zeolite framework. Yuan et al. [16] reported that CuCl2 supported onto aminofunctionalized MCM-41 and MCM-48 was more active and stable in the DMC yield in comparison with those with CuCl2 supported on nonfunctionalized mesoporous silicas. King [17] showed that Cu-exchanged Y zeolite catalysts, prepared by a solid-state ion exchange method (SSIE), displayed good productivity and selectivity for DMC synthesis with a strongly enhanced stability compared with the usual activated charcoal supported chloride-based catalysts. Various Cu-exchanged zeolites obtained by this method, such as Cu-MCM-41 [18], Cu-X [19], Cu-ZSM-5 [20], as well as Cu-Y/β-SiC [21] have been extensively investigated as potential candidates for vapor-phase DMC synthesis. Detailed experimental and theoretical investigations on the mechanisms of DMC synthesis on Cu-Y have been reported by Bell and his co-workers [22], [23]. Richter et al. [24], [25] recently found that incipient wetness impregnation of zeolite Y with copper(II) nitrate solution and inert activation at 650 °C could lead to chloride-free Cu-zeolite catalysts for the oxidative carbonylation of methanol to DMC in the gas phase at both normal and elevated ambient pressure.

Mixed silica–titania materials have been attracting considerable interest due to their potential applications as catalysts or catalyst supports for a wide variety of reactions [26]. Recently, silica–titania mixed oxides were used as the support material for Pt [27], [28], [29], Er [30], Au [31], Ni [32], Fe–Pt [33] and MoO3 [34] catalysts in oxidation, photocatalysis, hydrogenation and transesterification reactions. It was revealed that the presence of tetrahedral Ti(IV) species in the oxide support has great influences on the dispersion of active components and their catalytic performances. In a previous paper we presented a detailed study of the evolution of the bulk and surface structure of mixed silica–titania systems. We showed that the binary oxide materials with well-controlled surface properties could be obtained by varying several crucial preparation parameters in the sol–gel process [35]. In the present work, we have further investigated the use of silica–titania mixed oxides as supports for the active copper species in order to probe into the correlation between the nature of the oxide support and the catalytic performance of the supported copper(I) chloride catalysts.

Microwave heating has attracted much attention in the preparation of catalysts since it causes rapid and even heating, leading to uniform distribution of the active components in the support. Besides, it can alter the activity and selectivity in certain reactions [36], [37], [38], [39], [40], [41], [42]. Copper(I) chloride is known to absorb microwave radiation well [43]. This indicates that preparation of CuCl in a microwave field might lead to fast and efficient heating, as well as different microstructural products. In this case, the microwave heating is considered to have great potential for preparing the supported copper(I) chloride catalysts.

This article summarizes the results of a work carried out to understand the effects that the structure and surface properties of silica–titania mixed oxides-supported copper(I) chloride catalysts have on oxidative carbonylation reactions. The support composition structurally promotes the catalytic functionalities by modifying the interactions between the active copper species and the oxide support. The catalysts were prepared by conventional and microwave heating methods. The effect of support on the performance of promoted catalysts has been evaluated with oxidative carbonylation of methanol in liquid-phase.

Section snippets

Materials

A series of silica–titania mixed oxides with Si/Ti ratios of 1, 5, 10, and 50 were prepared by the following method described in the previous paper [35]. Pure SiO2 and TiO2 were also obtained by the sol–gel route for comparison. Each material was calcined in air at 550 °C (0.5 °C/min heating rate) for 4 h and was denoted as STx, where x is the Si/Ti molar ratio. The as-prepared STx support was mixed with CuCl (weight ratio: 80/20) that was finely ground using a mortar and pestle. Then the mixtures

Microwave heating behavior of materials

The support ST10, CuCl, and a mixture of ST10 and CuCl (20 wt%) were heated in the microwave oven at output power 3.5 kW, and the temperature was measured with irradiation time. Heating curves are plotted in Fig. 2. The support ST10, whose dielectric constant and dielectric loss do not show much dispersion, does not get heated up much when exposed to microwaves. The regions of high slopes in the temperature–time plots, referred to as “thermal runaways” [53], are observed after a long duration

Conclusions

Microwave dielectric heating is found to be an adequate method for the preparation of high dispersed silica–titania mixed oxide-supported CuCl catalysts. The effect of support on oxidative carbonylation performance for CuCl/SiO2–TiO2 catalysts is significantly dependent on the surface properties of the silica–titania support, due to the different interactions with active phases. Results from XPS and TGA showed that the copper species strongly attached to the silica–titania support were formed

Acknowledgments

This work has been supported by grants from the National Natural Science Foundation of China (20606022) and the National Basic Research Program of China (2005CB221204). The authors would like to thank Riguang Zhang for his help with the quantum-chemical calculations.

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