Acid-base pairs of acidic ionic liquids-functionalized covalent triazine frameworks with controllable morphology for boosting C–O bond cleavage of carbohydrate
The cleavage of C–O bonds in biomass-derived carbohydrates is of great significance for the conversion of biomass to chemicals and fuels. We demonstrated that various F127-directed covalent triazine frameworks functionalized by sulfonic acid-based ionic liquids ([C 3 N][OTf]-FCTF) were synthesized by a two-step synthesis process. The interesting morphology of spherical nanofiber of the [C 3 N][OTf]-FCTF(0.25)-10 catalyst was obtained by carefully adjusting the usage of both F127 and ethanol in the synthetic system. The 5-hydroxymethylfurfural synthesis reaction parameters of were first optimized using the [C 3 N][OTf]-FCTF(0.25)-10 via response surface methodology. The [C 3 N][OTf]-FCTF(0.25)-10 exhibited high catalytic activity, and outperformed other CTF-based catalysts with different morphologies and commercial solid acids in the conversion of carbohydrates (e.g., fructose, inulin, sucrose and honey). This is due to the advantages of the loose spherical nanofiber, the rich N-containing units, the suitable Brønsted acid nature and the adjacent acid-base pairs on the surface of the catalyst. The high Brønsted acidity of [C 3 N][OTf]-FCTF(0.25)-10 played a positive role in the selective synthesis of levulinic acid from carbohydrates. The theoretical calculations demonstrated that discrepancies in electron cloud distribution between constructed acid-base pairs and N-rich groups in the catalyst could facilitate the cleavage of C–O bonds in fructose molecules. The Fukui function combined with experimental results elucidated the reaction pathways. The robust interaction between –[C 3 N][OTf] groups and frameworks ensured the excellent reusability of [C 3 N][OTf]-FCTF catalysts. This study will provide a novel approach for synthesizing CTF-based solid acids with controllable morphology, enabling the high-value utilization of carbohydrates.
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