Hydrodeoxygenation of m-cresol over Pd/Al-SBA-15 catalysts: Effect of Al content on the deoxygenation reaction pathways

Abstract

The effect of aluminium content (x = 5, 20 and 40 wt %) in SBA-15 type silica supports used to obtain Pd/xAl-SBA-15 catalysts was evaluated in the HDO reaction of m-cresol at 300 °C, under atmospheric and high (30 bar) pressure. Aluminium incorporation was performed using the two-step pH-adjustment method, affording the deposition of Al phases over silica previously precipitated. Pd was thereafter deposed using incipient wetness impregnation. Physico-chemical properties of the catalysts were investigated by XRD, N2 physisorption, H2 chemisorption, DRIFTS of adsorbed pyridine, STEM-EDX mapping coupled to HAADF imaging. The acidity (Brønsted and Lewis sites) generated by the incorporation of aluminium at low content (i.e. 5 wt %) onto the SBA-15 structure resulted in the improvement of HDO of m-cresol, with an increase in selectivity toward the deoxygenated products, toluene under atmospheric pressure (38 % for the Pd/SBA vs. 77 % for the Pd/Al5-SBA) and methylcyclohexane under high pressure (14 % for the Pd/SBA vs. 30 % for Pd/Al5-SBA). The selectivity into methylcyclohexanone was also affected by the total pressure: 5 % under atmospheric pressure vs 64 % under 30 bar over Pd/Al5-SBA). However, under atmospheric pressure, at higher Al contents, side reactions catalyzed by acid sites were promoted, such as isomerization and disproportionation reactions, which were not observed under high pressure. Besides that, the increase of quantity of Al leaded to the decrease of the surface accessibility and hence drop on the reactions rate over both pressure conditions. This work demonstrated the efficiency of Pd catalyst in HDO reaction which can be obtained by a precise adjustment of the quantity of Al in the SBA catalytic support.