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

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 CH3SH catalyzed by HZSM-5 zeolite was chosen as model reaction. In the temperature range of 350-550 degrees C, the alkylation of toluene to produce xylenes was the main reaction involved in the process. The reaction between toluene and CH3SH was compared with the well-known reaction between toluene and CH3OH. Significant similarities exist, notably the ability of both CH3SH and CH3OH 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 CH3OH was reached at 350 degrees C, while that of 67.3% for CH3SH was reached at 450 degrees 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 (CH3OH) and 26.4 kcal/mol (CH3SH)). The high performance in alkylation proved by CH3SH was attributed to its lower consumption in the side reactions (i.e., the formation of light hydrocarbons).