Tuning the metathesis performance of a molybdenum oxide-based catalyst by silica support acidity modulation and high temperature pretreatment

Molybdenum oxide-based catalysts containing 5 wt% of Mo obtained by simple impregnation of a silica mesoporous support were studied in the olefin metathesis reaction at 50 °C. The effect of support modification as well as high temperature pretreatment of the catalysts before metathesis was evaluated. The acid properties of the base mesoporous silica SBA-1 support with a cubic pore structure were tuned by Al-doping post-preparation procedures using ammonium hexafluoro-aluminate to control the Si/Al ratio. The possibility to control the acidity as well as the difference in the structure of molybdenum oxide sites is discussed. Correlation between the acidity of the supports and the catalytic performance was found. Al-Doping improves the specific activity and propylene conversion. However, an increase in acidity has a negative effect on the selectivity to metathesis products. Furthermore, the effect of high temperature pretreatment of the catalysts on their metathesis performance was investigated. Molybdenum oxide based catalysts supported on pure SBA-1 were pretreated in a flow of inert gas, hydrogen, carbon monoxide and hydrocarbons (ethane and propane) at 650 °C. It has been clearly shown that short pretreatment in carbon monoxide and hydrocarbon atmospheres improves significantly the catalytic performance in comparison with hydrogen pretreatment or conventional pretreatment in inert atmospheres. Based on DFT calculations, the H-assisted mechanism for the formation of active Mo alkylidene species from silica–alumina-supported Mo(iv) oxo species with participation of neighboring Brønsted acid sites is proposed.