Quantitative multinuclear MAS NMR studies of zeolites

Abstract

1H, 27Al, and 29Si MAS NMR investigations combined with elemental analysis of beta zeolites dealuminated by different methods, were performed. Because in fully protonated samples the number of Brønsted protons equals the number of tetrahedral Al atoms in the framework, the framework Si/Al ratio of the samples was estimated from 1H MAS NMR in combination with chemical analysis. To quantify the true Si/Al ratio of the zeolite framework, an unambiguous assignment of the peaks due to the Brønsted acid sites in the 1H MAS NMR spectra was made. By studying the spectra of H-beta as well as Cs-exchanged samples, the δ H=5.1–5.9 ppm signal could be assigned to interactions of bridging hydroxyl groups with the zeolite framework and the δ H=3.8–4.2 ppm signal to free Brønsted acid sites. The results obtained from the quantification of the 1H MAS NMR spectra were confirmed by those of the 27Al MAS NMR spectra. In addition, the number of defect sites in the framework was determined by comparing the true Si/Al framework ratio with the Si/Al ratio derived from 29Si MAS NMR.