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 °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.