Rapid Migration of Oxide-Supported Ga(I) Ions, and Consequences for Catalytic Propane Dehydrogenation

Abstract Ga-containing oxides have been widely investigated as catalysts for acceptorless propane dehydrogenation. During catalyst activation by H2 at elevated temperatures, Ga can migrate. Nevertheless, the PDH active sites have not typically been considered mobile under reaction conditions. Evidence for mobility during activation is indirect and consists largely of changes in the hydroxyl population, observed using IR spectroscopy. In this study, Magic-Angle Spinning NMR and IR spectroscopies, STEM-EDS, and conventional kinetic measurements are enhanced by analysis of the in situ XANES. They establish that Ga(I) is the highly mobile species, and that it can interconvert with active sites. Since the XANES signatures differ significantly for Ga(I)/SiO2 vs Ga(I)/ZSM-5, it was possible using linear combination fitting to monitor and quantify the dynamic redistribution of Ga species in physical mixtures of SiO2 and HZSM-5. Under H2, Ga(III) is reduced to Ga(I), which then diffuses rapidly between SiO2 and ZSM-5 particles whose diameters are ca. 200 μm. This migration can occur in both directions, and is significantly faster than Ga(III) reduction. As a result, a mixture of weakly active Ga/SiO2 and HZSM-5 shows a PDH activity approaching that of highly active Ga/ZSM-5. Thermodynamic analysis of the equilibrium populations on both supports suggests an enthalpic preference for Ga(III) and an entropic preference for Ga(I). Ga ions in both oxidation states are enthalpically more stable on HZSM-5, while Ga association with SiO2 is likely entropically favored. These thermodynamic preferences, in combination with long-range Ga(I) mobility, predict and explain the temperature-dependent redistribution of Ga species between oxidation states, across supports, and even onto catalyst bed diluents under reducing conditions. The high mobility of Ga(I) during catalyst activation and under reaction conditions enables Ga to sample a large range of environments, challenging long-standing assumptions about the ability to precisely synthesize single-site Ga catalysts for alkane dehydrogenation.