Synthesis of 4,4′-dimethylbiphenyl through shape-selective methylation of 4-methylbiphenyl with methanol I. Comparison of MgO Modified and Hydrothermally Treated HZSM-5 Zeolite Catalysts

MgO modified and hydrothermally treated HZSM-5 catalysts were prepared and characterized with XRD, TPD, and N₂ physical adsorption. Alkylation of 4-methylbiphenyl (4-MBP) with methanol to prepare 4,4′-dimethylbiphenyl (4,4′-DMBP) was carried out over the prepared catalyst samples under fixed-bed down-flow reaction conditions. Both the MgO-modified and hydrothermally treated HZSM-5 catalysts significantly improved the selectivity of 4,4′-DMBP (≤ 80% over Mg-CBV or 70% over HT-500 vs. only 13% over the parent HZSM-5). The hydrothermally treated HZSM-5 catalysts followed by HCl leaching improved the activity stability. After 1000 min reaction, the 4-MBP conversion was ∼ 8%, and the selectivity of 4,4′-DMBP was ∼ 65%. An obvious decrease of the catalyst acidity was observed either after modifying it by MgO or by hydrothermal treatment, however, the surface area of the micropores of the former decreased and that of the mesopores of the latter increased as examined by low temperature nitrogen adsorption. The high selectivity over ZSM-5 modified with MgO largely resulted from the depression of the secondary reactions of 4,4′-DMBP, e.g., isomerization, dealkylation, and alkylation, as demonstrated by the reaction of 4,4′-DMBP with methanol as the reactants over MgO/HZSM-5 and the parent zeolite as the catalyst. Over hydrothermally treated catalysts, the high selectivity was originated from the depression of isomerization and dealkylation of 4,4′-DMBP.