High-yield synthesis of HMF from glucose and fructose by selective catalysis with water-tolerant rare earth metal triflates assisted by choline chloride

The conversion of naturally occurring organic substances into value-added platform chemicals by simple, green, and efficient procedures represents one of the most accessible and sought-after routes towards sustainable chemistry. In the present work, we report the remarkable catalytic activity of rare-earth metal triflates in conjunction with choline chloride, a natural, low-cost, and available organic compound to selectively convert glucose and fructose into hydroxymethylfurfural (HMF). The hypothesized mechanism is based on the initial glycosylation of glucose assisted by scandium(III) triflate and choline chloride to produce a glycoside, which can evolve through an intramolecular rearrangement and subsequent dehydration to produce the final product HMF. A comparison with other types of catalysts is carried out with particular focus on the side reactions. The apparatus consists of a closed biphasic system and the excellent capacity of methyl propyl ketone (MPK) to extract HMF in only one cycle is proved. The process was conducted at 150 °C using 1.5 molar equivalents of choline chloride in which glucose was converted into HMF after three hours using the catalyst in 8% molar quantity, while fructose was converted in one hour employing the catalyst in 4% molar quantity. The best performance was obtained by employing scandium (III) triflate as a catalyst with an yield of 94% and 99% of HMF from glucose or fructose, respectively. We assumed a first-order reaction model for both glucose and fructose conversion into HMF. The R-squared values are greater than 0.9, demonstrating that our kinetic model fitted well with the experimental results. In addition, activation energies are 16.9 kJ mol−1 for glucose and 9.31 kJ mol−1 for fructose due to the longer reaction path of glucose. The catalytic system can be recycled up to five times with a HMF yield of over 80% for glucose and over 90% for fructose, maintaining the same selectivity.