Comparison of glucose conversion to 5-HMF using different modified mordenites in ionic liquid and biphasic media

Abstract

Modified mordenites have been used in the direct conversion of glucose into 5-hydroxymethylfurfural (5-HMF) in two different media: the ionic liquid 1-butyl-3-methylimidazolium bromide ([BMIM]Br) and a biphasic system composed of water–acetone and ethyl acetate. The results revealed that there are crucial differences in the final catalytic performances. Modified mordenite treated with 1 M NH₄Cl only, with a moderate Si/Al ratio of 11.2 and a high content of strong Brønsted acid sites (TPD_NH3 acidity of 1.39 mmol g⁻¹), possessed the highest catalytic activity resulting in 64% 5-HMF yield with 97% glucose conversion in the ionic liquid system. On the other hand, the mordenite treated with 1 M NH₄Cl and 2.4 M NH₄F, with a total acidity of 1.51 mmol g⁻¹, mainly due to Lewis acid sites, gave 50% 5-HMF yield with 98% conversion in the biphasic system. The presence of strong Lewis acid sites has a major effect on the catalytic activity in aqueous–organic biphasic medium, in agreement with earlier works that showed that the reaction proceeds via Lewis acid-catalyzed isomerization followed by Brønsted acid-catalyzed dehydration. In contrast, the quick adsorption and strong interaction of the ionic liquid with the zeolite, which hinders the diffusion of the products, enhances the role of zeolite mesoporosity in the catalytic activity when the reaction is carried out in ionic liquid solution.