The critical role of crystal phases of LaPO4 in controlling the acidic sites for the production of 5-hydroxymethylfurfural from glucose

The production of 5-hydroxymethylfurfural (5-HMF) from saccharide conversion has fallen short of industrial expectations due to low reaction efficiency and high costs. Herein, we fabricated a series of LaPO4 catalysts with controlled crystal structures and tuned acidity via a facile hydrothermal method. The hexagonal LaPO4-120 catalyst exhibited a high glucose conversion of 99.9% and a remarkable 5-HMF yield of 90.1% at 150 °C for 60 min. Additionally, the hexagonal LaPO4-120 catalyst showed promising activities in converting various saccharides including fructose, glucose, cellobiose, sucrose, and inulin. A series of characterization studies and DFT calculations revealed that the hexagonal LaPO4-120 catalyst possessed low-coordination La sites and abundant zeolitic water stored in the open and oxygen-lined channels, which provided Lewis acid sites and a dynamically local Brønsted acid environment to respectively facilitate glucose isomerization and the subsequent dehydration. A plausible reaction mechanism involving a synergetic proton transfer via hydrogen bonding is proposed. This study offers insights for the design of low-cost and heterogeneous catalysts with finely tuned Lewis and Brønsted acid sites for 5-HMF production.