Oxidative Methylation of Toluene with Methane over Superbasic Catalysts: A Selective Route to Styrene and Ethylbenzene through Alternative Feedstocks

Abstract

The oxidative methylation of toluene with methane to ethylbenzene and styrene has been investigated over various monoalkali-metal-compounds-promoted MgO catalysts and superbasic catalysts prepared by promoting MgO with various binary alkali metal compounds. The experiments have been carried out under atmospheric pressure, at 650-850°C, at a CH4 :O2 : C7H8 : He partial pressure ratio of 14.7 : 2.9 : 1 : 11.8 and at a space velocity of 15,000 cm3 g−1 h−1. The performances of the bialkali-metal-compounds-promoted MgO are much higher than any monoalkali-promoted MgO and are found to strongly depend on the nature of the alkali metal compounds forming the pair, the concentration of the alkali metal ions, and the reaction conditions. The most effective catalytic systems are obtained with MgO promoted with the bialkali metal compounds of LiA + NaA, LiA + CsA, KA + CsA or NaA + CsA (A = SO2−4, OH−, Cl−, CH3COO−, CO2−3 or NO−3) containing total alkali loadings of 10 mol% with equal molar amounts of both alkalis. Any bialkali-promoted system containing rubidium was less effective than those systems. The highest C8-selectivity (57 mol%) and C8-yield (24.2%) with a styrene/ethylbenzene molar ratio of 1.6 were obtained over (5 mol% Na+ + 5 mol% Cs+)/MgO (prepared from the sulfate precursors) at 750°C under the aforementioned conditions. The total C8-selectivity and the total C8-yield over the bialkali-promoted systems remained almost unchanged for a period of 60 h. In contrast, the maximum C8-yield over any monoalkali-promoted system never exceeded 11.6% (styrene/ethylbenzene = 1.4) under the same conditions and the stability with time-on-stream was less prominent. The relationship between the catalytic performances and the physicochemical characteristics of the catalysts revealed by XPS and basicity measurements is explored. The relatively high performances of the bialkali-metal-compounds-promoted MgO are likely to be related to the synergistic increase in the surface basicity (superbasicity) caused by the enrichment of the surface layer with the alkali ions (Na+ and Cs+).