Efficient 5-hydroxymethylfurfural production in ChCl-based deep eutectic solvents using boric acid and metal chlorides

Bio-based 5-hydroxymethylfurfural (5-HMF) production in DESs has garnered significant attention due to its effectiveness and environmental friendliness. In this study, acidic ChCl-based DESs and ChCl-fructose DES were compared in terms of 5-HMF production in the presence of four acids, namely boric acid, oxalic acid, citric acid, and p-toluenesulfonic acid. Simultaneously, two types of ChCl-fructose and ChCl-glucose DESs were explored in combination with boric acid for 5-HMF production respectively. The 5-HMF yield was optimized by varying parameters such as the mass ratio of ChCl to carbohydrate, catalyst usage, temperature, time, and water content. The results indicated that ChCl-fructose DES exhibits better performance in converting fructose to 5-HMF than acidic ChCl-based DES. A 5-HMF yield of 65.2% was obtained in ChCl-fructose DES (6 : 4 w/w) with 20% water and 80 mg boric acid at 120 °C for 120 minutes. Similarly, a 5-HMF yield of 33.7% from glucose was presented in ChCl-glucose DES (6 : 4 w/w) with 20% water, 120 mg boric acid, and 25 mg CrCl₃·6H₂O at 120 °C for 120 minutes. The addition of an appropriate amount of water is beneficial for promoting 5-HMF production in ChCl-fructose and ChCl-glucose DESs. The high viscosities of ChCl-fructose and ChCl-glucose DESs can be attributed to a strong intermolecular force, resulting in a large mass transfer resistance. Overall, this work provides an efficient and inexpensive approach for producing 5-HMF in ChCl-fructose and ChCl-glucose DESs.