The effect of grafting zirconia and ceria onto alumina as a support for silicotungstic acid for the catalytic dehydration of glycerol to acrolein
The effect of ceria and zirconia grafting onto alumina (α and θ-δ phases) as supports for silicotungstic acid for the dehydration of glycerol to acrolein was studied. 30 % Silicotungstic acid (STA) supported on 5 % zirconia/δ,θ-alumina was the best catalyst, producing 85 % selectivity to acrolein at 100 % glycerol conversion, and it showed stable activity without using oxygen as a co-feed. The catalyst produced a STA of 90 g(acrolein) kg(cat) -1 h-1, which was greater than the STA simply supported on δ,θ-alumina, which only demonstrated 75 % selectivity towards acrolein. The effect of grafting on the support material was investigated by means of nitrogen adsorption, ammonia temperature-programmed desorption, thermogravimetric analysis, Raman spectroscopy, and powder X-ray diffraction. A pulsed-field gradient (PFG) NMR technique was also used to study diffusion processes associated with the catalysts. Diffusion studies of the grafted catalysts showed that zirconia contributes to the formation of more tortuous pathways within the pore structure, leading to the diminution of acid strength and making the catalyst less susceptible to coke formation. Grafting zirconia onto δ,θ- alumina as a support for silicotungstic acid (STA) improves the catalytic performance for glycerol dehydration to acrolein. PFG-NMR diffusion studies show that catalyst stability is related to pore structure stability, and that grafting leads to decreased acid strength, more tortuous pore structure, and a decrease in coke formation.