Highly active and low moisture sensitive supported thallium oxide catalysts for Friedal-Crafts type benzylation and acylation reactions: Strong thallium oxide - support interactions

Abstract

Liquid-phase benzylation of benzene by benzyl chloride (at 80° C) over basic TlOx (without support) and supported TlOx prepared using different low and high surface area commonly used catalyst carriers has been investigated. The catalysts have been characterized by their surface area and also by XRD, FTIR, and XPS. Strong TlO_x-support interactions (which are chemical in nature) have been observed for the catalysts prepared using high surface area (HS), supports, such as Si-MCM-41, silica gel (HS), silica-alumina (HS), alumina (HS), and zirconia (HS), which have surface hydroxyl groups. These catalysts are found to be inactive for benzene benzylation or benzoylation reaction. However, the TlOx supported on sintered low surface area (LS) macroporous catalyst carriers [viz. zirconia (LS), silica (LS), and silica-alumina (LS)], which have no surface hydroxyl groups, showed high benzene benzylation activity. TlOx/zirconia (LS) is a highly promising catalyst for both the benzylation and acylation of benzene and other aromatic compounds and it shows high activity, even in the presence of moisture. The activity of this catalyst for the benzylation of benzene and substituted benzenes is in the following order: benzene > toluene > p-xylene > anisole. The induction period for the benzylation and acylation reactions over the catalyst, however, depends strongly on the moisture present in the catalyst and/or in the reaction mixture. The induction period is drastically reduced by the HCl gas pretreatment of the catalyst before the benzylation or acylation reaction. Only the catalyst that contains Tl₂O₃ shows activity for the benzylation and acylation reactions. A redox mechanism for these reactions over the supported TlOx catalysts has been proposed.