Organic Acid-Regulated Lewis Acidity for Selective Catalytic Hydroxymethylfurfural Production from Rice Waste: An Experimental–Computational Study

A series of organic acids, including maleic acid (maleicA), malic acid (malicA), succinic acid (SA), and oxalic acid (OA)/oxalate, was evaluated as green additives to promote the selective Al(III)-catalyzed production of hydroxymethylfurfural (HMF) from food waste. The roles of Al(III)–organic acid interactions in altering the catalytic functions were elucidated on the basis of experimental and computational evidence. The catalytic results showed that the Al/OA and Al/oxalate systems gave the slowest glucose conversion among the studied systems. OA/oxalate had such a high affinity for Al(III), that the Lewis acidity of Al(III) (i.e., ability to accept electron pairs) was unfavorably reduced, which was supported by the theoretical calculations of Gibbs free energies considering the Al(III)–OA complexes the most thermodynamically feasible. When rice waste was used as the substrate, which is rich in glucose-based starch, the addition of maleicA to the Al(III) system enhanced the HMF selectivity. The Lewis acidity of Al(III) was plausibly moderated through the appropriate maleicA–Al(III) coordination, which suppressed the loss of sugars and HMF to side reactions (e.g., polymerization), while enabling the desirable HMF formation reactions (i.e., glucan hydrolysis, glucose isomerization, and fructose dehydration) to proceed. The HMF yields of 35.2 and 17.3 mol % were obtained from rice waste in the Al(III) and Al/maleicA systems, respectively. This study provides mechanistic insights into the complementary roles of organic additives in catalytic conversion, highlighting a simple and environmentally benign approach to design high-performance systems for biomass valorization.