Distribution of open sites in Sn-Beta zeolite

Abstract

A survey of the open site geometries in Sn-Beta has been completed. Comparing the relative energies of 144 distinct open site structures identifies both T9 and T1 sites as the most stable open sites. However, a key feature of these sites is that the Sn-O-Si bridge which is hydrolyzed is opposite the SnOH, rather than adjacent. This results in geometries in which the SiOH in the open site is significantly more acidic than a surface SiOH or a SiOH defect in the zeolite, as found in adsorption calculations of NH3, pyridine, and acetonitrile. Frequency shifts calculated for acetonitrile are consistent with experimental frequency shifts, and the proposed open site geometry suggests a new assignment for a peak observed experimentally by Harris et al. [1] and Otomo et al. [2] The stabilization of the open site silanol by the nearby Sn generates this unusual Brønsted acidity in the Sn-Beta open site, which highlights the need to consider new reaction mechanisms in the Sn-Beta literature.