This laboratory experiment introduces students to an important reaction in biomass valorization and allows them to gain a practical understanding of green chemistry. Acid-catalyzed dehydration reactions of fructose to 5-hydroxymethylfurfural, and thus humins, were performed both in aqueous solvent and without, along with two different catalysts (Amberlyst-15 and alumina). Students were able to compare and analyze the effects of these different conditions using Thin-Layer Chromatography, while grasping concepts of catalysis and circular economy. By observing the formation of humins under some of the reactions tested, the students could evidence systems thinking in humins valorization.
The formation of humins is one drawback of biomass valorization (e.g., hemicellulosic fraction) toward platform chemicals and fuels. Furanic compounds including 5hydroxymethylfurfural (HMF) and 5-methoxymethylfurfural (MMF) can be extracted from humins and valorized. Unlike HMF, reactions with MMF have not been widely studied. In this work, MMF was extracted from humins aiming to its hydrogenation in continuous flow. Based on success from literature studies with HMF hydrogenation, 5% Ru/C and 5% Pd/C catalysts were employed, and both have demonstrated excellent conversion of MMF toward hydrogenation products (100%), with an interesting selectivity switch depending on reaction conditions. Ru-based catalysts were more prone to deactivation as compared to Pd catalysts and were extremely stable after a few hours on stream under the investigated reaction conditions. A reaction mechanism was proposed for MMF hydrogenation.
The acidity of Al-SBA-15 materials functionalized by ball milling with several niobium loadings (0. 25-1 wt.%) as well as with several fluorine loadings (by wet impregnation using NH 4 F as a precursor) was characterized and materials investigated in the esterification of valeric acid to alkyl valerates. The parent Al-SBA-15 support as well as the modified materials loaded with Nb and/or F have been catalysts synthesized characterized by X-ray diffraction (XRD), N 2 physisorption measurements, and diffuse reflection infrared spectroscopy (DRIFT) among others. A special interest was paid on the acidity of the materials that was investigated by temperature-programmed desorption of pyridine. Interestingly, the characterization results for the materials containing fluorine showed up an increase in the acidity strength despite of a reduction in the number of acid sites. The catalytic performance of the as-prepared catalysts was investigated in the microwave-assisted esterification reaction of valeric acid to valerate esters. Thus, while the materials modified with niobium exhibited a lower catalytic activity as compared with the catalytic support (Al-SBA-15), the materials loaded with fluorine either onto Al-SBA-15 or on Nb1%/Al-SBA-15 materials presented enhanced conversion values of valeric acid. Therefore, it can be said that the new acid sites with enhanced strength formed by the incorporation of fluorine boost the esterification of valeric acid with alcohols to form the respective valerate ester.
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