GaCl_x- or GaAlCl_x- grafted Si-MCM-41: Highly active and moisture insensitive/stable catalyst for the acylation and benzylation of benzene, napthalene and substituted benzenes

Abstract

GaCl_x- and GaAlCl_x-grafted Si-MCM-41 catalysts were prepared by reacting anhydrous GaCl₃ and mixed anhydrous GaCl₃ and AlCl₃, respectively, with the terminal hydroxyl groups of Si-MCM-41 (pore size: 2.5 nm) in a dry non-aqueous reaction medium (CCl₄) under reflux. The catalysts showed high activity for both the acylation and benzylation (with benzoyl chloride and benzyl chloride, respectively) of benzene, toluene, p-xylene, cumene, mesitylene, anisole and naphthalene. Both the catalysts are moisture insensitive or less moisture sensitive; the presence of moisture has a beneficial effect in the acylation over both the catalysts. The catalytically active sites of the GaClx-grafted Si-MCM-41 are (-O-)₂GaCl (i.e. partially chlorided Ga anchored to Si through -O- linkage) species and the equivalent sites of the GaAlCl_x-grafted Si-MCM-41 are (-O-)₂Ga(or Al)Cl and (-O-)₃Ga(or Al) species. The high acylation/benzylation activities of both the catalysts even in the presence of moisture are attributed to their redox properties (Ga³⁺ → Ga¹⁺) rather than to their Lewis acid properties. After the grafting/chemical bonding, the grafted gallium chloride catalyst showed much higher acylation activity but lower benzylation activity than the physically deposited (on Si-MCM-41) or unsupported GaCl₃. The GaAlCl_x-grafted Si-MCM-41 showed higher acylation activity than the GaCl_x-grafted Si-MCM-41, due to a synergetic effect of the Ga and Al species. However, the relative benzylation activity of the two catalysts showed strong dependence upon the presence or absence of electron donating group(s) and also upon the electron donating ability of the group(s) attached to the aromatic nucleus.