Zirconium Modification Promotes Catalytic Activity of a Single-Site Cobalt Heterogeneous Catalyst for Propane Dehydrogenation

The effect of Zr modification on the catalytic activity of Co/SiO2 was investigated for nonoxidative propane dehydrogenation. Isolated Zr on SiO2 surface sites were prepared by organometallic synthesis using Zr­(OtBu)4 as a precursor. The resulting Zr/SiO2 support was functionalized with Co2+ ions via strong electrostatic adsorption. Spectroscopic (diffuse reflectance infrared Fourier transform spectroscopy, UV–vis, electron paramagnetic resonance) and microscopic characterization (transmission electron microscopy, scanning transition electron microscopy) results are consistent with single-site cobalt that preferentially associates with the mono-dispersed Zr at a variety of loadings and Co/Zr ratios. The oxidation state of Co in the as-prepared Co/SiO2 and Co–Zr/SiO2 was both +2 with tetrahedral and octahedral geometries, respectively. In situ X-ray absorption near edge structure and extended X-ray absorption fine structure results confirmed that the oxidation state of Co remained as +2 under reaction condition for both Co/SiO2 and Co–Zr/SiO2 samples and both catalysts have tetrahedral Co2+ as the active catalyst. Despite similar Co coordination environments, the catalytic activity and selectivity was significantly improved by the Zr modification of the silica support versus Co/SiO2. This was attributed to the change in oxygen donor ability and Co–O bond strength of the SiO–Zr–O sites of Co–Zr/SiO2 compared with the SiO– ligands in Co/SiO2. These results show that tuning of the support SiO2 oxygen donation ability by use of an anchoring site (e.g., SiO–Zr–O–) can be used to alter both rate and selectivity of propane dehydrogenation with single-site heterogeneous catalysts. These results also show some preference for Co2+ active sites to associate with SiO–Zr–O– sites over SiO–