Toluene Methylation by Methyl Mercaptan and Methanol over Zeolites - A Comparative Study

Abstract

In this study, we show that the Friedel-Crafts (FC) alkylation of aromatic hydrocarbons by thiols to form C-C bond is feasible. The gas-phase reaction between toluene and CH 3 SH catalyzed by HZSM-5 zeolite was chosen as model reaction. In the temperature range of 350-550 °C, the alkylation of toluene to produce xylenes was the main reaction involved in the process. The reaction between toluene and CH 3 SH was compared with the well-known reaction between toluene and CH 3 OH. Significant similarities exist, notably the ability of both CH 3 SH and CH 3 OH to generate methoxonium species on the zeolite surface and to methylate the aromatic ring in a typical FC process. The maximum alkylation yield of 41% for CH 3 OH was reached at 350 °C, while that of 67.3% for CH 3 SH was reached at 450 °C. This difference in temperature can be correlated with the energy barriers required for the formation of methoxonium species (i.e., 24.6 kcal/mol (CH 3 OH) and 26.4 kcal/mol (CH 3 SH)). The high performance in alkylation proved by CH 3 SH was attributed to its lower consumption in the side reactions (i.e., the formation of light hydrocarbons).