(330a) Adsorption of the Compounds Encountered in Monosaccharide Dehydration in Zeolites

Biomass carbohydrates are the most abundant renewable resources available and have the potential to replace petroleum as a source of both energy and chemicals, leading to more environmentally friendly processes and reducing the dependence on crude oil. A promising reaction in this field is the acid-catalyzed dehydration of monosaccharides, such as fructose and glucose, towards 5-hydroxymethylfurfural (HMF), an important intermediate for the production of valuable chemicals.¹When zeolites are used as catalysts the preferential adsorption of reactants, products or byproducts could markedly affect the yield to the desired product. In this regard, studying the adsorption of the compounds present in the reaction media in the zeolite catalyst becomes crucial to understand the reactivity of the system and it is necessary to calculate the intrinsic reaction rate constants from kinetic data. Zeolites can also be used as adsorbents for the selective removal of the HMF from the reaction media (either in-situ or after dehydration). In this respect understanding the effect of the framework type, composition (e.g. Si/Al, extraframework cation), and presence of co-solvent (e.g. dimethylsulfoxide (DMSO)) on HMF adsorption are important for the design of an adsorptive separation process.

Adsorption loadings from solution are usually estimated from changes of the solute concentration assuming that the solution volume is not affected by adsorption. This quantity is called excess adsorption loading and sometimes it is quite different from the real loading. We formulated and used a method for the estimation of the real adsorption loading from the experimentally measured excess adsorption that can be applied to calculate the adsorption isotherms both in the case of single-solute and multi-solute mixtures.

The effect of temperature on the adsorption of fructose, glucose, HMF, levulinic and formic acid from aqueous solutions in the protonated form of zeolite beta with SiO₂/Al₂O₃ratio of 18 (H-BEA) will initially be presented. The limiting heat of adsorption at zero coverage of all components was calculated from isotherms measured at 0, 25 and 40 °C. Adsorption experiments from multi-solute solutions were also carried out and the data were found to be in good agreement with predictions made using Ideal Adsorbed Solution Theory (IAST) and the single solute isotherm experimental data.

Adsorption of HMF was studied in more detail. We examined the effect of the framework type (BEA, MFI, FAU), composition (Si/Al) as well as the addition of DMSO as co-solvent on its adsorption uptake. It was found that in each zeolite type the adsorption uptake of HMF increases with the hydrophobicity (higher Si/Al ratio) of the zeolite. Experimentally measured isotherms were in good agreement with those calculated using Grand Canonical Monte Carlo - Expanded Ensemble (GCMC-EE) simulations. It is found that in the case of HMF/water binary mixtures, zeolites with larger pores (i.e. FAU), have higher HMF adsorption capacity but lower selectivity because large pores allow water adsorption due to favorable H₂O-HMF attraction. Therefore, dealuminated zeolite Y is less selective for the separation of HMF from water than silicalite-1 (siliceous MFI type zeolite) that has smaller pores sizes. In addition, experiments and simulations show that in HMF/DMSO/water ternary mixtures, the HMF loading decreases with increasing DMSO concentration. This decrease is due to the high solubility of HMF in DMSO, and to the fact that in the entropic contribution to the free energy of adsorption dominates over the enthalpic contribution. Analysis of our data shows that the ratio of the energetic interaction in the zeolite to the solvation free energy is a key factor in controlling separation from liquid mixtures.

To conclude, our findings could shed light on the acid catalyzed dehydration of monossacharides and the HMF separation, and serve as a hint to the rational design of microporous adsorbents for liquid-phase separations in biomass processing.

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