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

Abstract

A comparison is made of CrOx–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 CrOx on ZrO2–SiO2 via impregnation, and (iii) the co-precipitation of CrOx 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 CrOx and ZrO2 ensures high efficiency of the catalysts. At 500 and 550°C, the most efficient catalyst is CrZr/SiO2 synthesized by depositing CrOx 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 CrOx 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.