Molecular design and experimental study on the synergistic catalysis of cellulose into 5-hydroxymethylfurfural with Bronsted-Lewis acidic ionic liquids

As a key renewable chemical for plastics and fine chemicals, the preparation of 5-hydroxymethylfurfural (5-HMF) from cellulose is an important research topic. The conversion of cellulose to 5-HMF in high yields is difficult owing to the recalcitrant and heterogeneous nature of cellulose. In this work, six types of Bronsted-Lewis acidic ionic liquids (ILs) were designed, which can catalyse the conversion of cellulose to 5-HMF in different tandem reaction steps. Cellulose disaccharide is used as a model compound to calculate the structure and frontier orbital energy of the catalytic system by density functional theory, and thus predict catalytic activity. Based on the HOMO-LUMO orbital energy, the order of catalytic activity of Bronsted-Lewis acidic ILs is as follows: [Hnmp]Zn2Cl5 > [Hnmp] ZnCl3 > [HSO3-(CH2)(3)-MIM]Zn2Cl5 > [HSO3(CH2)(3-)MIM]Zn2Cl5 > [HSO3-(CH2)(3)-NEt3]Zn2Cl5 > [HSO3-(CH2)(3)-NEt3]ZnCl3. Subsequently, six types of ILs were synthesised and used to catalyse cellulose conversion into 5-HMF. It was found that [Hnmp]Zn2Cl5 exhibited the highest efficiency with a 5-HMF yield of 39.29%, as predicted. The accuracy of the theoretical calculation was verified. This research has the potential to reduce the cost of catalysts by providing an important theoretical and practical screening method for the directed design of synergistic catalysts in tandem reactions.