Correlation of 129Xe NMR shielding data with the pore structures of various aluminophosphate molecular sieves

Abstract

129Xe NMR spectra of two different amounts of xenon sealed inside sample tubes containing various microporous aluminophosphate (AlPO-5, -11, -31, and -41) and silicoaluminophosphate (SAPO-5, -11, -31, and -41) molecular sieves are recorded at different temperatures and analysed. For most cases, the 129Xe shielding tensor of adsorbed xenon is asymmetric and distinct spectral differences between circular and elliptical cross-sections, framework dimensions, and xenon loading levels of sieve channels are detected based on the powder pattern lineshape fittings. The roles of Xe–wall and Xe–Xe interactions and different adsorbant diffusion modes on spectral characteristics are discussed. The shielding tensor axes are associated with the sieve channel planes by comparing the data with previously published limiting case models of lineshapes. Shielding tensor span is found to be a promising indicator of sieve channel dimensions. Small differences between AlPO and SAPO molecular sieves belonging to the same framework structure type are also detected. In AlPO-31, fast exchange effects between Xe in the free intercrystallite volume and Xe physisorbed on the external crystallite surfaces are observed and the intercrystallite cavity sizes are estimated.