MgO/NaX zeolite as basic catalyst for oxidative methylation of toluene with methane

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Abstract

Oxidative methylation of toluene with methane was studied over a series of MgO/NaX zeolite catalysts. The effect of MgO on the zeolite was examined by X-ray diffraction (XRD) and temperature-programmed desorption (TPD) of CO2. The results indicated that the conversion, selectivity and yield of C8 hydrocarbons (ethylbenzene and styrene) are significantly improved by impregnation of maximum 13 wt.% MgO into the zeolite. The latter catalyst also displays a good stability. It is found that the amount of active sites but not their strength depends on the content of MgO in the zeolite. The catalysts possess well preserved crystal structure and low amount of MgO crystal phase.

Introduction

Zeolites were involved as catalysts for basic reactions in recent years [1], [2]. Basic zeolites can be prepared by simple ion exchange with suitable cations, as well as by impregnation with suitable oxides [3], [4]. Among the most appropriate substances which create or improve basicity upon introducing into zeolites are alkali and alkali earth metal compounds [5]. The basicity of the so modified zeolites is associated with an enhanced negative charge of the framework oxygens (generally accepted as basic sites in all types of zeolites) under the influence of appropriate non-framework species which come from the corresponding compounds.

Different base-catalyzed reactions were reported to proceed in the presence of alkali and alkali earth modified zeolites [6], [7], [8]. Previously, we have found alkali modified zeolites to show interesting activity, selectivity and yield for the oxidative conversion of methane and the oxidative methylation of toluene with methane [9], [10]. In the present study our interest was focused on magnesium oxide modified zeolites and their characterization by measuring the catalytic behavior in oxidative methylation of toluene with methane, basicity by TPD of CO2 and crystal phase composition by XRD.

Section snippets

Experimental

A NaX zeolite (Si/Al=1.23) obtained from Union Carbide was used as starting material. In order to establish the optimum content of MgO in the zeolite (with the purpose of obtaining catalysts with high basicity) the parent zeolite was impregnated with different amounts of MgO (7, 13 and 19 wt.%). To produce a catalyst loaded with 7 wt.% MgO, 15 g of NaX were stirred with 0.5 M aqueous solution of magnesium nitrate (51.4 ml). Samples containing 13 and 19 wt.% MgO were obtained in the same way using

Catalysis

The effect of temperature on the parameters characterizing the activity of the catalysts (conversion, selectivity and yield of C8 hydrocarbons) is depicted in Fig. 1 for the sample 13MgO/NaX. The activity increases with the temperature and reaches a maximum at the highest temperature examined. The effect is more pronounced in the range 700–750°C than in the range 750–800°C. Because of that, the study of the catalytic activity as a function of the MgO content was carried out at 750°C.

Conclusions

Catalytic samples obtained by impregnation of NaX zeolite with MgO are tested in the reaction of oxidative methylation of toluene with methane. A dependence of the catalytic activity on the temperature and on the MgO content is established. By means of XRD studies it is demonstrated that the catalysts, irrespectively of their MgO content possess well preserved crystal structure and low amount of MgO crystal phase. By TPD of CO2 it is shown that the content of MgO in the samples determines the

Acknowledgements

This research was supported by the National Scientific Fund of the Bulgarian Ministry of Education and Science under Project No. X-817.

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