p-Sulfonic acid calix[4]arene: A highly efficient organocatalyst for dehydration of fructose to 5-hydroxymethylfurfural

Biomass is an attractive renewable source of carbon since it has a huge potential as a substituent of the scarce fossil fuels and has been widely exploited to obtain important chemicals such as 5-hydroxymethylfurfural (HMF). Fructose, one of the main components of biomass, has been widely explored as precursor for obtaining HMF. Several efforts have been expended in the search for efficient and reproducible methodologies for the synthesis of HMF, a chemical platform important in the production of fuels and other value-added chemicals. In this work, the organocatalyst p-sulfonic acid calix[4]arene (CX4SO(3)H) was for the first time employed as a catalyst for fructose dehydration reaction to produce HMF. Various parameters such as temperature, reaction time, catalyst load, fructose concentration and catalytic efficiency of other Bronsted acids were investigated. H-1 NMR spectroscopy was successfully used as qualitative and quantitative method in all steps of optimization. Using only 1 mol% of CX4SO(3)H as the catalyst (DMSO-i-PrOH 20% V/V, 140 degrees C, 50 mg mL(-1) of fructose), 92% yield HMF could be obtained in a relatively short time (45 min). Thus, the present work was able to produce HMF from fructose with high efficiency and metal-free conditions.