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/SiO 2 was investigated for nonoxidative propane dehydrogenation. Isolated Zr on SiO 2 surface sites were prepared by organometallic synthesis using Zr(O t Bu) 4 as a precursor. The resulting Zr/SiO 2 support was functionalized with Co 2+ 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/SiO 2 and Co-Zr/SiO 2 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/SiO 2 and Co-Zr/SiO 2 samples and both catalysts have tetrahedral Co 2+ 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/SiO 2 . This was attributed to the change in oxygen donor ability and Co-O bond strength of the ≡SiO-Zr-O sites of Co-Zr/SiO 2 compared with the ≡SiO- ligands in Co/SiO 2 . These results show that tuning of the support SiO 2 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 Co 2+ active sites to associate with ≡SiO-Zr-O - sites over ≡SiO-.