Solvent-Dipole Deshielding of Hydrated Aluminum Lewis Acids for Enhanced 5-Hydroxymethylfurfural Production from Biomass

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Abstract 5-Hydroxymethylfurfural (HMF) is a key biomass-derived platform chemical for producing furan-based fuels, fine chemicals, and polymers. Its production from glucose in homogeneous catalytic systems, however, is often limited by the solvation of Lewis acid catalytic centers, where coordinating solvent molecules and anions can shield the active sites, restrict substrate access, and diminish HMF yield and selectivity. Here, we demonstrate that polar aprotic cosolvents can function as active components of the catalytic microenvironment rather than passive reaction media. In water-rich AlCl3 solutions, dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMA), and N,N-dimethylformamide (DMF) exert dipole–dipole interactions with Al-coordinated water molecules through hydrogen bonding, distorting the [Al(H2O)6]3+ solvation shell and increasing access to the Lewis acidic Al center without directly coordinating to it. Spectroscopic measurements and electronic-structure calculations reveal that weakening of the Al–O(water) interaction increases in the order of DMF < DMA < DMSO. This solvation-shell deshielding decreases the apparent activation energy for glucose-to-fructose isomerization from 96 to 79 kJ·mol–1 and enables an HMF yield of 90% from glucose at 150 °C. Glucose streams derived from sugarcane bagasse, poplar sawdust, and palm trunk sawdust afford HMF yields exceeding 80%, while the integrated sugarcane-bagasse route achieves a 49 mol % HMF yield based on the cellulose originally present in the biomass. These results establish dipolar cosolvent control of hydrated metal centers as a molecular design principle for homogeneous biomass conversion.