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

ABSTRACT: 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(O ^t Bu)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-.

SUBMITTER: Zhao Y 

PROVIDER: S-EPMC6645419 | biostudies-literature | 2018 Sep

REPOSITORIES: biostudies-literature