Solid-state 29Si and 27Al NMR studies of natural mesolite

Abstract

A natural zeolite specimen from Pune, India, was studied using 1D (29Si, 27Al) and 2D (27Al) multi-quantum (MQ) solid-state NMR. The signals of silicon and alumina spectra were assigned using empirical relationships between the bilateral Si–O–Al angles in a zeolite framework and the corresponding chemical shifts. While three distinct signals of 29Si MAS NMR spectra at −86 to −89ppm were assigned to three single Sin(3Al) sites (n=2–4), the largest one at −95.5ppm was shown to be composed of the two Si environments, Si1(2Al) and Si5(2Al). A complex 27Al single-Bloch decay spectrum of mesolite could not be properly assigned without resorting to a 3MQ MAS NMR experiment. It was shown that three signals arising from 3 different crystallographic positions of the framework aluminum in mesolite could be discerned using the 27Al 3MQ MAS NMR spectra. The quadrupolar parameters of the three Al sites were obtained. Finally, treatment of mesolite with oxalic acid was carried out, leading to a partially dealuminated sample. Small, albeit selective, removal of aluminum predominantly from the T3 sites was observed upon dealumination.