An investigation of the adsorption of aromatic hydrocarbons in zeolite Na–Y by solid-state NMR spectroscopy

Abstract

The adsorption of toluene inside zeolite Na–Y was investigated by solid-state NMR spectroscopy. The environment of Na + ions at different sites in Na–Y before and after adsorption was characterized by 23Na magic-angle spinning (MAS) NMR. The information on the dynamic behavior of guest molecules inside the supercage of Na–Y was obtained by analyzing wideline 2H NMR spectra. The effect of loading level and temperature on molecular dynamics was also examined. The cation–sorbate interactions were directly probed by 23Na{ 1H} rotational-echo double-resonance (REDOR) experiments at different temperatures. Molecular Monte Carlo simulations were also performed to assist in the interpretation of the NMR data. 23Na MAS and 23Na{ 1H} REDOR results show that each toluene molecule is facially coordinated to a Na + ion at the SII site in the supercage, forming a π-complex. The adsorption also causes the Na + ions initially located at the SI′ site to slightly shift to a new position within the sodalite cage, but has little effect on the Na + at the SI site. The 2H NMR results indicate that the toluene molecules undergo a 2-site flip around the molecular long axis in addition to the methyl group rotation about its C 3 axis. 23Na MAS spectra suggest that the adsorptive behavior of benzene and p-xylene in Na–Y is similar to that of toluene/Na–Y. 23Na{ 1H} REDOR results further indicate that inside the supercage, the degree of molecular motion follows the order of benzene > toluene > p-xylene.