Unlocking the Anion Effect on Steerable Production of 5-Hydroxymethylfurfural

Homogeneous metal salt catalysts play a pivotal role in industrial production of 5-hydroxymethylfurfural (HMF). Herein, we first proposed the anion effect on steerable production of HMF using metal salts with different anions as catalyst in a biphasic system of tetrahydrofuran (THF)/NaCl aqueous solution (NaCl aq). Notably, the anions affected the catalytic activity of the metal salts, leading to an order of magnitude difference in the HMF yields, i.e., AlBr₃ (74.0 mol %)>AlCl₃ (60.8 mol %)>Al₂(SO₄)₃ (35.2 mol %)>Al(NO₃)₃ (14.9 mol %). The anion effect on steerable production of HMF could be attributed to the proximity effect and electron tension. Anions form close-range interactions with glucose molecules by proximity effect to promote electron transfer, facilitating the isomerization of glucose to fructose. Besides, anions induce a reduction of the electron cloud density of glucose carbon atoms, generating electron tension that rapidly transforms glucose from the ground state to the transition state, thereby increasing the HMF yield. Based on the revelation of anions effect and evaluation of techno-economic process, we expect to provides theoretical guidance for high-throughput screening of metal salt catalysts in industrial biorefinery.