RESUMO
A series of CrOx-ZrO2-SiO2 (CrZrSi) catalysts was prepared by a "one-pot" template-assisted evaporation-induced self-assembly process. The chromium content varied from 4 to 9 wt.% assuming Cr2O3 stoichiometry. The catalysts were characterized by XRD, SEM-EDX, temperature-programmed reduction (TPR-H2), Raman spectroscopy, and X-ray photoelectron spectroscopy. The catalysts were tested in non-oxidative propane dehydrogenation at 500-600 °C. The evolution of active sites under the reaction conditions was investigated by reductive treatment of the catalysts with H2. The catalyst with the lowest Cr loading initially contained amorphous Cr3+ and dispersed Cr6+ species. The latter reduced under reaction conditions forming Cr3+ oxide species with low activity in propane dehydrogenation. The catalysts with higher Cr loadings initially contained highly dispersed Cr3+ species stable under the reaction conditions and responsible for high catalyst activity. Silica acted both as a textural promoter that increased the specific surface area of the catalysts and as a stabilizer that inhibited crystallization of Cr2O3 and ZrO2 and provided the formation of coordinatively unsaturated Zr4+ centers. The optimal combination of Cr3+ species and coordinatively unsaturated Zr4+ centers was achieved in the catalyst with the highest Cr loading. This catalyst showed the highest efficiency.
