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

A natural zeolite specimen from Pune, India, was studied using 1D ((29)si, (27)AI) and 2D (Al-27) 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 Si-29 MAS NMR spectra at -86 to -89 ppm were assigned to three single Si(3Al) sites (n = 2-4), the largest one at -95.5 ppm was shown to be composed of the two Si environments, Si-1(2Al) and Si-5(2Al). A complex Al-27 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 Al-27 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 T-3 sites was observed upon dealumination. (C) 2015 Elsevier Inc. All rights reserved.