Mesoporous Tantalum Phosphates: Preparation,Characterization And Catalytic Performance For Xylose Dehydration

More and more attention has been paid to biomass resource as it is a kind of renewable alternative to fossil fuels.Lignocellulose is one of the primary components of biomass.Xylose,which is the primary product derived from xylan hemicelluloses,is a cheap monosaccharide in the natural world.Furfural is an important raw material in industry,which is generally produced via dehydration of xylose via acid catalysts.So an important field of study deals with efficient solid acid catalysts for dehydration of xylose.Our previous work indicated that hydrated niobium oxide and tantalum oxide are typical water-tolerant heterogeneous catalysts.In this work,dehydration of xylose to produce furfural on tantalum compound have been studied.Attention has been paid to the preparation of mesoporous tantalum phosphates and its impact on texture properties of catalyst and catalytic performance of xylose dehydration reaction.And explanation are given to clarify the reason from the aspect of side reaction mechanism.A series of mesoporous tantalum phosphates TaOPO₄ with different P/Ta molar ratios were prepared and the impact of P/Ta ratio on the texture properties and acidity of catalysts were studied.The results showed that high phosphate content benefits the formation of mesoporous structures.Samples appeared to be disordered materials with P/Ta molar ratio<0.75.Moreover,more phosphate content improves Br?nsted acid sites ratio as the Br?nsted acid site of TaOPO₄ is due to the presence of P-OH.The Br?nsted to Lewis acid site ratio improved from 0.2 for sample with P/Ta=0.41 to 1.1 for sample with P/Ta=0.89.Compared with hydrated tantalum oxide,mesoporous tantalum phosphate appears to be a better solid acid catalyst as it owns larger specific surface area,narrow pore size distribution in mesoporous range and high Br?nsted acid site ratio.It is known that Br?nsted to Lewis acid site ratio of solid acid catalysts is crucial to the xylose dehydration reaction.Mesoporous tantalum phosphates TaOPO₄ with different P/Ta molar ratios and hydrated tantalum oxide were investigated in the xylose dehydration reaction to study the catalytic performance of these tantalum compound.When P/Ta ratio increased from 0.41 to 0.89,the xylose consumption rate decreased,while the selectivity of furfural showed similar volcano-type dependencies on P/Ta ratio,maximum at P/Ta = 0.84?72.1 mol%?.Hydrated tantalum oxide was founded to be poorer than mesoporous tantalum phosphates for the dehydration of xylose.We correlated xylose consumption rate and furfural selectivity of the TaOPO₄ catalysts with the ratio of Br?nsted acidity to Lewis acidity?B/L?.It was clear that the consumption rate of xylose decreased with increasing B/L ratio,while furfural selectivity showed a volcano-type dependence on the B/L ratio.Although the TaOPO₄-0.89 catalyst showed the highest B/L ratio,the furfural selectivity was lower as TaOPO₄-0.89 had lots of micro-pores.The stability of carbocation can explain why Br?nsted and Lewis acid sites are different in terms of furfural selectivity.To confirm the impact of acid type on furfural selectivity,we did the selectively poisoning experiment.By selectively poisoning Br?nsted acid sites,only the ratio of Br?nsted to Lewis acid sites was changed in the reaction system,which makes the conclusion more convincing.