A comparative study of oxidative methylation of toluene with methane to styrene and ethylbenzene over bi-alkali-promoted MgO, CaO, SrO or BaO

Abstract

The selectivity to styrene and ethylbenzene (C 8 selectivity), their yields (C 8 yield), and the stability with time-on-stream in the oxidative methylation of toluene with methane have been compared for superbasic catalysts prepared by promoting MgO, CaO, SrO or BaO with binary alkali metal compounds. The bi-alkali-promoted substrates are more effective than the mono-alkali promoted systems. The most effective catalytic system (5 mol-% Na + + 5 mol-% Cs +)/CaO gave a toluene conversion as high as 45.0 mol-% and a total C 8 selectivity of 60.0 mol-% (styrene/ethylbenzene = 2.2) resulting in a total C 8 yield of 27%, and operated in a stable manner for 60 h. These are the highest values ever reported in the open literature. The bi-alkali-promoted MgO also showed similar performances but somewhat smaller than those of promoted CaO. In contrast, the bi-alkali-promoted SrO and BaO were less effective under the same conditions, both exhibiting much lower performances than the promoted CaO or MgO. In terms of the toluene conversion, the C 8 selectivity and the stability with time-on-stream, the following sequence was observed for the bi-alkali-promoted substrates: CaO > MgO > BaO > SrO. The relatively high performances of the bi-alkali-promoted CaO or MgO compared to the mono-alkali ones are attributed to the synergistic increase in surface basicity (superbasicity) caused by the enrichment of the surface with the bi-alkali mixtures. Unlike SrO and BaO, which form stable carbonates with the CO 2 formed during the reaction which inhibits the surface basicity, the Ca or Mg carbonates decompose to oxides under the reaction conditions, thereby ensuring a higher catalytic performance. It is shown that there exists an optimum substrate, CaO, which upon promotion with bi-alkali becomes the most effective in the oxidative methylation of toluene.