Enhancing combustibility of coal tar via 2,5-dimethylfuran-mediated ring expansion: Mechanistic insights from diels-alder/friedel-crafts reactions

This study aimed to enhance the combustibility of coal tar (CT) through a 2,5–dimethylfuran (DMF)-mediated ring enlargement modification, elucidate the reaction mechanism, and validate the industrial potential of modified products as high-density fuels. Catalytic reactions between DMF and CT (using AlCl 3 as the catalyst) were conducted. The modified products were characterized for chemical composition, pyrolysis behavior, and optical properties via GC–MS, TG-DTG/DSC, and UV/PL techniques. Fuel performance was evaluated through hydrogenation experiments, while reaction pathways were analyzed using Gaussian-based density functional theory (DFT) calculations to uncover mechanistic insights. Post-modification, the content of tricyclic and higher aromatic hydrocarbons (TAHs) increased significantly (20 %), with a 75 % reduction in weight loss during Stage I pyrolysis and a distinct endothermic trend in Stage III (attributed to C-O bond cleavage). UV/PL analysis revealed a 200 nm redshift in absorption peaks and a 50 nm delay in fluorescence excitation peaks, indicating increased aromatic ring numbers. Hydrogenation improved product density by 10 % (reaching 0.96 g/mL) and calorific value to 42.0 MJ/kg, confirming its superior performance as a high-density fuel. This work proposed, for the first time, a DMF-mediated ring enlargement strategy to directionally modify CT molecular structures, significantly enhancing combustibility and energy density. Experimental and DFT results systematically revealed the synergistic Diels-Alder (DA) and Friedel-Crafts (FC) alkylation mechanisms between DMF and CT, offering novel insights for high-value utilization of coal tar. The technology also held promise for applications in CT separation and pharmaceutical intermediate synthesis, demonstrating critical industrial relevance.