Distribution of open sites in Sn-Beta zeolite

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.