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

The concentration, acid strength and thermal stability of the Bronsted- 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 Bronsted- and Lewis-acid sites in H+-[Si, Al]-MCM-41 are lower than the concentration of four-coordinate aluminium. The Bronsted 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 Bronsted sites in H+-MCM-41 is independent of the Al content and much lower than in zeolite H+-ZSM-5.