The catalytic activity of space versus terrestrial synthesized zeolite Beta catalysts in the Meerwein Ponndorf Verley Reactions: Support for PFAL as the Lewis active site for cis-alcohol selectivity

The Lewis activity of the Meerwein-Ponndorf-Verley (MPV) reactions is hypothesized to be due to partial framework aluminum (PFAl) that is either octahedrally or tri-coordinated. Crystals grown in the free-fall environment of low earth orbit (LEO) are more uniform; that is, have fewer lattice “defects” compared to those grown in a gravity field (i.e., on earth). Therefore, crystals grown in orbit should be less catalytically active relative to their earth grown counterparts. The catalytic activity towards the MPV reaction, and the associated IR and XPS spectrum for zeolite Beta that was synthesized on earth (1g) and aboard the International Space Station (10⁻³–10⁻⁵g) were compared in their as-synthesized forms, and after applying heat treatment protocols designed to stress the crystal structure to generate Lewis acid sites (i.e., tri and octahedrally coordinated PFAl). The activity of the MPV reaction and cis-alcohol selectivity over the heat-treated flight samples was observed to be lower than the identically heat-treated terrestrial zeolite Beta samples. Higher MPV activity as well as cis-alcohol selectivity is related to both a higher number of partial framework Al atoms (PFAl), and a constrained pore volume. As PFAl are created by the destruction of the framework upon heat treatment, flight samples were shown to be thermally more stable with fewer lattice defects and less associated stress in zeolite Beta crystals. The changes observed in the IR spectra, as well as the XPS Al Auger and 2p peaks, of the terrestrial samples support this conclusion. Additionally, the flight samples showed higher tr-alcohol selectivity, which implies more pore volume and less channel blockage. This is consistent with the fact that crystals grown in space have less stress, fewer lattice defects, and thus there are fewer channel obstructions.