289274 Adsorption in Zeolites of Components Encountered in Hexose Dehydration
The synthesis of chemical feedstocks from renewable sources, like biomass, has gained a lot of interest during the recent years. A promising reaction in this field is the acid catalyzed dehydration of carbohydrates, such as fructose and glucose, towards 5-hydroxymethylfurfural (HMF). HMF may serve as starting compound for several useful chemicals. Despite the fact that the dehydration of hexoses to HMF has been studied for many years, researchers are still searching for both the most suitable catalytic system and the best reaction medium. Zeolites have been proposed as advantageous heterogeneous catalysts compared to other materials, due to certain characteristics such as the possibility of tuning acidic and basic properties, hydrophilic and hydrophobic character, and adsorption and shape-selectivity. In fact, several interesting results on the dehydration of fructose and glucose using zeolites have been reported [1, 2]. However, HMF can undergo further rehydration to form levulinic acid and formic acid. Other side reactions are the oligomerizations of fructose and/or HMF leading to the so-called humins. In this regard, the study of the adsorption on the catalyst of the reactants, products and byproducts involved in the dehydration of sugars becomes crucial to understand the reactivity of such systems. Furthermore, zeolites without Bronsted acidity can also be considered as adsorbents that can selectively remove HMF from reaction medium before further degradation. Such a reactive adsorption process can be envisioned as an alternative to the bi-phasic reactive extraction configuration suggested in the literature . In the present work, we studied the adsorption of fructose, glucose, mannose, 5-hydroxymethylfurfural, levulinic acid and formic acid in different zeolites as a function of temperature and zeolite SiO2/Al2O3. The highest HMF loadings were observed for zeolite FAU. However, All zeolites types tested adsorbed higher amounts of HMF than any other component and for each zeolite type HMF adsorption increased with the hydrophobicity of the zeolites (higher SiO2/Al2O3 ratios). The enthalpies of adsorption were also estimated from the adsorption isotherms measured at different temperatures. The strongest adsorption was determined for HMF, followed by levulinic acid. Finally, the competition between fructose and HMF for the actives sites on H-BEA-18 is studied by performing the adsorption experiments with binary mixtures. The adsorption of fructose decreases with increasing the amount of HMF, and it becomes negligible at high HMF concentrations. The results will be analyzed taking into account the effect of adsorbate-adsorbent interactions. Simulations using an adsorptive reactor model showed that using an adsorbent improves the HMF yield. The results will be analyzed taking into account the single and multicomponent adsorption isotherms and will be compared to that of a reactor using an organic phase for extraction.
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