Influence of surface acidity/basicity of selected metal oxide catalysts and reaction atmospheres on the ketonisation of propionic acid to produce 3-pentanone as a liquid biofuel precursor

Defossilisation of the transportation sector can be achieved via the conversion of renewable biomass into drop-in liquid hydrocarbon-rich fuels. Bio-oils from the pyrolysis of lignocellulosic biomass contain significant proportion of carboxylic acids, which can be upgraded to liquid fuel range precursors via C-C coupling e.g., ketonisation. In this present study, ZrO2, SiO2, and SiO2–ZrO2 were synthesised and used for the ketonisation of propionic acid to 3-pentanone, in a stirred 100 mL batch reactor between 300 ᵒC and 400 ᵒC under 10 bar initial pressure of nitrogen or hydrogen. The order of ketonisation activity by the catalysts was: ZrO2 > SiO2–ZrO2 > SiO2 under both nitrogen and hydrogen atmospheres, based on their different surface acidity/basicity properties. Under nitrogen, ZrO2 catalyst showed high activity and selectivity towards 3-pentanone with the highest yield of 70.3 % at 350 ᵒC. Interestingly, the catalyst gave 12.2 % higher yield of 3-pentanone under hydrogen than nitrogen. This indicated positive influence of hydrogen towards the ketonisation reaction, possibly by preventing formation of intermediates and thus enhancing catalyst stability. Preliminary tests involving mixtures of propionic acid, and a bio-oil sample shows that ZrO2 was still selective toward ketonisation in the presence of other classes of compounds in bio-oils.