Eco-Friendly Synthesis of 5-Hydroxymethylfurfural and Its Applications as a Starting Material to Synthesize Valuable Heterocyclic Compounds

The development of new catalysts for converting biomass to high-value chemicals has received much attention for both energy and the environment; thus, designing and developing green and efficient catalysts for biomass conversion is pertinent. In this study, we developed a novel homogeneous catalyst in an eco-sustainable reaction medium of ionic liquids (ILs) to convert carbohydrates to 5-hydroxymethylfurfural (HMF), a renewable building block for biodiesel and other valuable compounds, using experimental and computational tools. Particularly, based on 1,4-diazabicyclo[2.2.2]octane, the synthesized IL, which bears Brønsted acid sites, in combination with 1-ethyl-3-methylimidazolium chloride (or [EMIM]Cl) as a binary IL catalyst/solvent was investigated for the conversion of carbohydrates (i.e., fructose, glucose, sucrose, and cellulose) to HMF. The results reveal that the reaction occurs under simple and mild conditions, in which fructose can be converted with a high yield, even at room temperature. Importantly, the product, HMF, can be easily isolated using liquid–liquid extraction that overcomes the HMF separation challenge posed by using fractional distillation. Last but not the least is that the catalyst with structural integrity can be effectively recovered and reused with a slight decrease in catalytic activity. In addition to the investigation of the operation parameters, the detailed reaction mechanism of the conversion of fructose to HMF was revealed by electronic structure and statistical mechanic calculations. Moreover, the as-prepared HMF can also be converted to high-value heterocyclic compounds such as (5-(di(1H-indol-3-yl)methyl)furan-2-yl)methanol, (5-(3-phenyl-1-(piperidin-1-yl)prop-2-yn-1-yl)furan-2-yl)methanol, ethyl 4-(5-(hydroxymethyl)furan-2-yl)-2-methyl-1,4-dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine-3-carboxylate, and 2-amino-4-(5-(hydroxymethyl)furan-2-yl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile using deep eutectic solvents as reaction media. Therefore, with the novel activities and high sustainability, the designed catalytic system has shown great potential for industrial applications of converting carbohydrates to value-added products.