Accessibility in Liquid Media: Cyclodehydration of Hexane-2,5-Diol for the Evaluation of Layered Catalysts

In order to establish layered materials as heterogeneous catalysts in the liquid phase, acidic layered materials are evaluated here through the conversion of hexane-2,5-diol used as a model reaction. Such polyoxygenate reactant is representative of some of the biomass components and the interactions of the reagent or furan product with the interlayers surface is a key to the understanding of these lamellar materials features. Layered HNbMoO6 catalyst is particularly active and stable for the studied reaction compared to several zeolites and its high activity is obtained without any pre-activation step prior to reaction. The influence of temperature and water content in the reaction mixture are also studied for this catalyst. The importance of intercalation in the reaction process is specifically evaluated using Raman spectroscopy and powder X-ray diffraction. On this layered catalyst, the reaction proceeds after intercalation of diol within the interlayer space followed by subsequent cyclodehydration giving rise to 2,5-dimethyltetrahydrofuran with high selectivity. Comparison with related layered materials such as HNbWO6 or H2W2O7 highlights a distinct behavior both in reactivity and intercalation and emphasizes the role of the accessibility to the active sites.