~(17)O Magic Angle Spinning NMR Studies of Bronsted Acid Sites in Zeolites HY and HZSM-5

摘要

High-resolution ~(17)O/~1H double resonance NMR spectra were obtained for two zeolites,one with a low Si/AI ratio (zeolite HY)and one with a high Si/AI ratio (HZSM-5),to investigate their local structure and Bronsted acidity.Two different oxygen signals,corresponding to Bronsted acid sites in supercages and sodalite cages of zeolite HY were readily resolved in the two-dimensional (2-D)~1H-~(17)O heteronuclear correlation (HETCOR)NMR spectra allowing the ~(17)0 isotropic chemical shift (delta_(cs))and quadrupolar coupling parameters (quadrupolar coupling constant,QCC,and asymmetry parameter,n)for the two oxygen atoms to be extracted.Similar experiments for HZSM-5 showed that the sites in this system are associated with a much larger distribution in NMR parameters than found in HY.~(17)O-~1H rotational echo double resonance (REDOR)NMR was applied to probe the O-H distances in zeolites HY and HZSM-5.Weaker ~(17)O-~1H dephasing was observed for zeolite HZSM-5 in comparison to that of HY,consistent with longer O