CHARACTERIZATION OF HYDROGEN-BONDING IN ZEOLITES BY PROTON SOLID-STATE NMR-SPECTROSCOPY

Variable-temperature magic-angle spinning proton NMR spectroscopy has characterized the structure and dynamics of hydrogen-bonded adsorption complexes between various adsorbates and the Bronsted acid site in a representative zeolite, H-ZSM5. The active site Bronsted proton chemical shift was found to be extremely sensitive to the amount and type of adsorbate introduced. The adsorbates studied were acetylene, ethylene, carbon monoxide, benzene, ethane, and nitrogen, with emphasis on the first four. Slow-exchange spectra obtained at 123 K for loadings of less than 1 equiv showed resolved peaks for free and complexed Bronsted sites. Low-temperature reverse cross-polarization experiments confirm that the adsorbates hydrogen-bond specifically to the Bronsted site and do not interact with the external silanol groups. Fast-exchange-averaged chemical shifts were observed at 298 K. Quantitative treatment of the loading-dependent chemical shifts for acetylene, ethylene, and carbon monoxide yielded equilibrium constants at 298 K of ca. 7, 7, and 0.7 equiv-1, respectively. The differences in the observed change in chemical shift with type of adsorbate are explained in terms of hydrogen-bond shifts and established theories of shielding contributions from neighboring group anisotropy effects. A semiquantitative treatment of induced shifts for acetylene, ethylene, and benzene adsorption yielded a characteristic hydrogen-bond distance ranging from 2.35 to 2.78 angstrom, which is in agreement with gas-phase model compounds and only slightly larger than distances calculated for model clusters using quantum chemical methods. These results suggest that quantitative interpretation of proton chemical shifts for zeolite catalysts provides information on the strength of hydrogen bonds and suggests geometries of the hydrogen-bonded complexes similar to those observed in the gas phase and calculated for model clusters. Furthermore, benzene undergoes rapid deuterium-hydrogen exchange near 298 K, apparently through a hydrogen-bonded intermediate.