Low-Temperature Modification of Mesoporous MCM-41 Material with Sublimated Aluminum Chloride in Vacuum

Abstract

Treatment of siliceous MCM-41 with aluminum chloride was performed in a vacuum at a temperature of 403 K. The obtained AlCl3/MCM-41 material was characterized by multinuclear solid-state NMR spectroscopy and by application of probe molecules. The results of NMR spectroscopy indicate a reaction of aluminum chloride with vicinal silanol groups at the surface of conditioned MCM-41 and the formation of tetrahedrally coordinated aluminum species bound to chlorine and framework oxygen atoms. In the local structures of 3% of these aluminum atoms, strong Brønsted acid sites are formed causing a 1H MAS NMR signal at 6.0 ppm. According to 13C MAS NMR spectroscopy of acetone-2-13C adsorbed on AlCl3/MCM-41 (241 and 245 ppm), these Brønsted acid sites are characterized by an acid strength higher than that of bridging OH groups in H−ZSM-5 (223 ppm).