Efficient preparation of 5-hydroxymethylfurfural from cellulose in a biphasic system over hafnyl phosphates

A series of hafnyl phosphates HfO(PO₄)_x (x = 1.0, 1.5 and 2.0) were prepared by coprecipitation of HfCl₄ with KH₂PO₄, which were explored for efficient production of HMF from cellulose in NaCl-H₂O/THF biphasic system. An excellent HMF yield of 69.8% was achieved over HfO(PO₄)_2.0 at 190 °C for 240 min. Levulinic acid was barely produced during the reaction since the strength of HfO(PO₄)_2.0 is not enough to rehydrate HMF, and humins deposition was almost eliminated due to its poorer HMF adsorption (9.3 mg/g). The results showed that the role of the phosphate group in the catalyst could be in deactivation of unselective Lewis acid sites. Furthermore, high yields of HMF from other carbohydrates such as fructose (94.8%), glucose (90.5%), cellobiose (79.3%), sucrose (86.6%), starch (75.3%), inulin (80.4%), as well as wheat straw (18.6%) could be achieved using HfO(PO₄)_2.0 as a catalyst. During catalyst recyclable test, the catalytic performance of HfO(PO₄)_2.0 is well maintained after five catalytic cycles. Characterization analysis of fresh and recycled HfO(PO₄)_2.0 with SEM, XRD, and FT-IR indicated the catalyst has good stability and robustness.