Investigation of H-zeolite and metal-impregnated zeolites as transformation catalysts of glucose to hydroxymethylfurfural

Zeolite is an underutilized natural resource in Indonesia. Zeolite is widely used as a heavy metal adsorbent because of its anionic charge. The adsorbed metals in zeolite can be used as additional Lewis acid sites (catalyst) for chemical reactions. The use of zeolite as a catalyst is supported by its unique pore structure that can produce specific reactions. One chemical reaction that requires Lewis acid sites is the transformation of carbohydrate derivative compounds into 5-hydroxymethylfurfural (HMF). HMF is a multifunctional intermediate compound of polymers, fuels, pure chemicals, and derivatives of other organic compounds. HMF can be formed through the isomerization of glucose into fructose by Lewis acid catalysts and the dehydration of fructose into HMF by Brønsted acids. A bifunctional acid catalyst could allow a one-pot reaction. This study has 3 objectives, namely: a) impregnation of Ende-NTT natural zeolite as a fructose and glucose transformation catalyst into HMF, b) increasing the value and function of natural zeolite from Indonesia, and c) create a new utilization pathway for metal adsorbing zeolites. Ende-NTT natural zeolite was chemically activated by using alkaline activation (NaOH). Activated zeolite was converted to Na-zeolite through saturation with 2 M NaOH. Na-zeolite was then converted to H-zeolite through the ion exchange of Na⁺ with NH₄⁺ ions which were continued by the process of removing NH₃ through heating to obtain H-zeolite. The Lewis acid sites of H-zeolite was increased by impregnating Cu and Mn metal salts. Cr metal salt was used as a comparison in this study. The transformation of glucose and fructose into HMF was carried out by the reflux method using DMSO solvent and identified by using high-performance liquid chromatography. Rietveld analysis showed that Ende-NTT natural zeolite was a mixture of mordenite and clinoptilolite types of zeolite with a ratio of 81:19. CuOx-zeolite catalyst had a higher yield than H-zeolite in the transformation of fructose and glucose into HMF with yields at 69% and 10%, respectively.