IR spectroscopic studies of the acidic properties of the mesoporous molecular sieve MCM-41

Abstract

The concentration, acid strength and thermal stability of the Brønsted- and Lewis-acid sites in the proton form of the mesoporous molecular sieve MCM-41 were monitored by IR spectroscopy as function of the Al content. The results were compared with those for zeolite H⁺-ZSM-5 with the same elemental composition. The spectra of hydroxy groups in purely siliceous [Si]-MCM-41 and H⁺-[Si, Al]-MCM-41 contain a single band at 3740 cm^–1, assigned to the Si—OH groups, and are independent of the Al content. Sorption of ammonia or pyridine generates bands from the ammonium and pyridinium ions. The concentrations of both Brønsted- and Lewis-acid sites in H⁺-[Si, Al]-MCM-41 are lower than the concentration of four-coordinate aluminium. The Brønsted sites in H⁺-MCM-41 are more prone to dehydroxylation than those in zeolite H⁺-ZSM-5, and the effect increases with the Al content. The acid strength of the Brønsted sites in H⁺-MCM-41 is independent of the Al content and much lower than in zeolite H⁺-ZSM-5.