Chloridion induced acid sites in covalent organic frameworks for 5‑hydroxymethylfurfural synthesis from fructose

The versatile architecture of covalent organic frameworks (COFs) provides a powerful platform for tailoring their functions. Herein, we demonstrate the molecular engineering of 2D ionic COF nanosheets (iCONs) to reach a family of organic polymeric catalysts with tunable acidity. These solid acidic iCONs are synthesized through Schiff base condensation of the ionic monomer triaminoguanidinium chloride and the aromatic aldehydes with different surface groups. Compared with that in the monomer, the Cl^– in iCON matrix tends to be near the framework H atom, generating a new Brønsted acid site with much short Cl^–∼H⁺ distance that resembles HCl. As a result, these iCONs are highly active in the typical acid reactions of aldol condensation and dehydration of fructose into 5-hydroxymethylfurfural (HMF). The shorter Cl^–∼H⁺ distance, the better acid catalytic activity. The catalyst DHPA-TG_Cl reaches a high HMF yield of above 97 % within a short reaction time of 15 min, providing the turnover frequency (TOF) as high as 155.2 h⁻¹. Facile recycling and stable reusability are also observed. The free energy profiles of these iCONs catalyzing fructose conversion to HMF confirm the function of Cl^–∼H⁺ units in lowering the energy barrier of the rate-determining step for the water release in the HMF synthesis.