Propylene Hydrogenation and Propane Dehydrogenation by a Single-Site Zn2+ on Silica Catalyst

This study reports the highly selective (more than 95%) dehydrogenation of propane to propylene as well as the reverse hydrogenation reaction by silica-supported single-site Zn(II) catalyst. The catalyst is thermally stable at dehydrogenation temperature (550 degrees C and above), and catalytic byproducts are small. In situ UV-resonance Raman, XANES, and EXAFS spectra reveal that tetrahedrally coordinated Zn(II) ions are chemisorbed into the strained three-membered siloxane rings on the amorphous silica surface. Under reaction conditions, the Zn(II) ion loses one Zn-O bond, resulting in a coordinatively unsaturated, 3-coordinate active center. The infrared spectrum of adsorbed pyridine indicates that these are Lewis acid sites. Theoretical calculations based on hybrid density functional theory suggest that the catalyst activates H-H and C-H bonds by a nonredox (metal) mechanism consisting of heterolytic cleavage of C-H bonds, in contrast with the homolytic mechanisms such as oxidative addition/reductive elimination pathways. The computed minority catalytic pathway consists of undesired C-C bond cleavage at Zn(II) site, follows a slightly different mechanism, and has a significantly higher activation energy barrier. These mechanisms are consistent with the high olefin selectivity observed for single-site Zn(II) on SiO2.