Effect of Deposition Sequence on Catalytic Activity of CrOx–ZrO2–SiO2 in Nonoxidative Propane Dehydrogenation.

A comparison is made of CrO_x–ZrO₂–SiO₂ catalysts (9 wt % chromium oxide based on Cr₂O₃); (Cr + Zr)/Si molar ratio of 0.8) synthesized using different orders of introducing components: (i) the simultaneous precipitation of all components, (ii) the deposition of CrO_x on ZrO₂–SiO₂ via impregnation, and (iii) the co-precipitation of CrO_x and ZrO₂ on SiO₂. The SiO₂ precursors are TEOS in methods (i) and (ii), and SiO₂ produced by calcination of rice husk in (iii). The catalysts are tested in the nonoxidative dehydrogenation of propane in a flow system with a fixed catalyst bed at 500–600°С. The co-precipitation of CrO_x and ZrO₂ ensures high efficiency of the catalysts. At 500 and 550°C, the most efficient catalyst is CrZr/SiO₂ synthesized by depositing CrO_x and ZrO₂ on SiO₂; at 600°C, the best on-stream behavior is exhibited by CrZrSi catalyst synthesized via the simultaneous precipitation of all components. SEM/EDX, XRD, H₂-TPR, and Raman spectroscopy are used to show that in the catalysts synthesized via the co-precipitation of CrO_x and ZrO₂, these components (which form active sites) are uniformly distributed, have close contact, and are adequately dispersed, while Cr⁶⁺ is readily reduced to Cr³⁺ by the hydrogen contained in the reaction medium. [ABSTRACT FROM AUTHOR]