Cr/SiO2 mesoporous catalysts: Effect of hydrothermal treatment and calcination temperature on the structure and catalytic activity in the gas phase dehydration and dehydrogenation of cyclohexanol

Abstract

Cr/SiO2 mesoporous materials (Cr-MCM-41) were obtained by hydrothermal synthesis. The effect of hydrothermal treatment and calcination temperature on the structure and catalytic behavior of synthesized materials were analyzed. The materials were characterized by XRD, N2 adsorption-desorption, TEM-SEM, TG-DTA, FTIR-HATR, DRUV-vis and XPS. The hydrothermal treatment promoted the incorporation of Cr3+ into the silica network, improving their resistance to oxidation. The calcination in air produces the simultaneous presence of Cr3+ and Cr6+ ions, and their relative amounts depend both on the hydrothermal pretreatment and the calcination temperatures. The Cr3+ ions constituted coordinative unsaturated acid centers responsible of the cyclohexanol to cyclohexene dehydration reaction. Moreover, the Cr6+ ions correspond to either isolated monochromate species linked to the silica structure or to polychromate aggregates, which produce crystalline α-Cr2O3 clusters after calcination in air to 450 °C. The monochromate centers were active for the dehydrogenation reaction. So, although dehydration was the main reaction, the dehydrogenation reaction to cyclohexanone also was observed after calcination in air to 450 °C.