Different sulfated catalysts including SO 4 /TiO 2 −SiO 2 , SO 4 /Ti-SBA-15, SO 4 /ZrO 2 , SO 4 /AC, and SO 4 /SiO 2 were tested in fructose dehydration to 5-hydroxymethylfurfural (HMF). Reactions were carried out in dimethyl sulfoxide (DMSO) at 110°C. Characterization results indicated that no sulfur leaching was observed from SO 4 /ZrO 2 , SO 4 /TiO 2 −SiO 2 , and SO 4 /Ti-SBA-15 catalysts in the reaction tests. The SO 4 /TiO 2 −SiO 2 catalyst had a high amount of strong acid sites and the highest amount of Bronsted sites. The highest selectivity to HMF at high conversion, that is, 89% selectivity at 77% fructose conversion was obtained over this catalyst. It preserved its activity after four times reuse.
■ INTRODUCTIONDiminishing fossil fuel reserves and CO 2 emission problems force people to find alternative and sustainable resources to produce valuable chemicals. Production of these chemicals from biomass is an environmentally friendly process compared to a petroleum based process. 5-Hydroxymethylfurfural (HMF) is one of the key intermediates in order to convert the biomass to valuable chemicals such as polymers, biofuels, and bulk chemicals. 1 The most effective way of producing HMF is by carbohydrate conversion, especially dehydration of fructose.Fructose dehydration reaction occurs on the acid sites of the catalyst. Acid concentration, type of acid sites, and acid strength of the catalyst affect the product distribution significantly. 2−4 It is reported that fructose conversion to intermediates takes place on the Lewis sites, whereas Bronsted sites are responsible for the HMF formation from these intermediates. 5 Various acid catalysts including homogeneous (ionic liquids, mineral and organic acids) and heterogeneous types have been investigated. Because of the product contamination and recovery problems, heterogeneous catalysts are generally preferred. Wide ranges of heterogeneous catalysts (e.g., resins, metal sulfates, metal phosphates, heteropolyacids, zeolites, niobic acid based catalysts) have been tested. 3,6−15 However, no satisfactory yields have been achieved yet. Some of the catalysts have low stabilities due to leaching and some of them are not selective and promote side product formation such as formic acid and levulinic acid. Therefore, there are still studies pursued to find a stable, active, selective, and cheap heterogeneous catalyst for fructose dehydration to HMF.Sulfur ions and sulfate groups create Bronsted and strong acid sites when loaded on a support, such as iron oxide, alumina, titania, and zirconia. They were very active in fructose dehydration. 7,17 However, they leached during the reaction in different solvents. Solvent type also affects HMF yield. Different types of solvents from environmentally benign alcohols and water to the high-boiling-point polar aprotic solvents (dimethyl sulfoxide (DMSO) and dimethylamide (DMA)) have been investigated. High yields of HMF have been achieved by using high-boiling-point solvents such as DMSO and DMA. 9 Zirconia is known as ...