Silica-Alumina-Supported Acidic Molybdenum Catalysts - TPR and XRD Characterization

Abstract

A series of silica-aluminas with controlled composition has been synthesized and used as supports for Mo oxide catalysts. X-ray diffraction (XRD) and temperature-programmed reduction (TPR) were used to study the role of support composition upon the stability of various oxidic species, including products of solid state reaction between Mo oxide and silica-alumina. In general, the reducibility of Mo increases with increasing Mo loading, regardless of support composition. For a fixed Mo loading, however, extent of reduction increases with SiO 2 content and shows a maximum at a composition SiO 2 : Al 2O 3 = 75 : 25 (wt%). Except for silica-rich catalysts, the average oxidation state of Mo does not reach below +4 at the conditions used for TPR (maximum temperature: 550°C). Most of the support compositions yield XRD-amorphous Mo oxide. Only Mo-rich and silica-rich catalysts (MoO 3 ≥ 12 wt%; SiO 2 ≥ 50 wt%) contain crystalline phases of Mo: orthorhombic MoO 3 on 100% SiO 2, and MoO 3 and Al 2(MoO 4) 3 On silica-aluminas.