Microporous Humins Synthesized in Concentrated Sulfuric Acid Using 5-Hydroxymethyl Furfural

Selective Synthesis of Levulinic Ester from Furfural Catalyzed by Hierarchical Zeolites

Furfural is a platform molecule that can be catalytically converted using a cascade series of reactions into levulinic esters, essential compounds used as fuel additives. Bifunctional catalysts containing Lewis and Brønsted acid sites such as zeolites are commonly used for these conversions. However, microporous zeolites often present diffusional restriction due to the size similarity of furfural and other molecules to the zeolites’ micropores. Thus, incorporating mesopores in these materials through post-synthetic protocols is a promising pathway to circumventing these limitations. This study presents the creation of hierarchical beta and mordenite using Si or Al removal and their employment in the furfural conversion to isopropyl levulinate (PL). Mordenite zeolite did not produce satisfactory mesopores, while the beta was more efficient in generating them by both acid and alkaline treatments. Beta zeolite treated in an alkaline solution presented larger mesopores (14.9 and 34.0 nm), maintaining a total acidity value close to its parent zeolite and a higher Lewis/Brønsted ratio. The combination of these features led to an improved diffusion of bulkier products and the highest furfural conversion (94%) and PL selectivity (90%), suggesting that a post-modification of beta zeolites produced efficient catalysts for upgrading abundantly available furfural.

Selective Adsorption of CO2 on Zeolites NaK-ZK-4 with Si/Al of 1.8-2.8

Zeolites with appropriately narrow pore apertures can kinetically enhance the selective adsorption of CO₂ over N₂. Here, we showed that the exchangeable cations (e.g., Na⁺ or K⁺) on zeolite ZK-4 play an important role in the CO₂ selectivity. Zeolites NaK ZK-4 with Si/Al = 1.8-2.8 had very high CO₂ selectivity when an intermediate number of the exchangeable cations were K⁺ (the rest being Na⁺). Zeolites NaK ZK-4 with Si/Al = 1.8 had high CO₂ uptake capacity and very high CO₂-over-N₂ selectivity (1190). Zeolite NaK ZK-4 with Si/Al = 2.3 and 2.8 also had enhanced CO₂ selectivity with an intermediate number of K⁺ cations. The high CO₂ selectivity was related to the K⁺ cation in the 8-rings of the α-cage, together with Na⁺ cations in the 6-ring, obstructing the diffusion of N₂ throughout the zeolite. The positions of the K⁺ cation in the 8-ring moved slightly (max 0.2 Å) toward the center of the α-cage upon the adsorption of CO₂, as revealed by in situ X-ray diffraction. The CO₂-over-N₂ selectivity was somewhat reduced when the number of K⁺ cations approached 100%. This was possibly due to the shift in the K⁺ cation positions in the 8-ring when the number of Na⁺ was going toward 0%, allowing N₂ diffusion through the 8-ring. According to in situ infrared spectroscopy, the amount of chemisorbed CO₂ was reduced on zeolite ZK-4s with increasing Si/Al ratio. In the context of potential applications, a kinetically enhanced selection of CO₂ could be relevant for applications in carbon capture and bio- and natural gas upgrading.

Humins from Biorefineries as Thermoreactive Macromolecular Systems

Conversion of lignocellulosic biomass often brings about the formation of several side products. Among these, a black and viscous coproduct known as humins is formed on acidic treatment of polysaccharides. To improve the efficiency of this process from an economical and environmental perspective, new solutions for humins valorization are urgently needed. This work focuses on the comprehensive understanding of humins with special emphasis on their structure/properties relationships. Humins were subjected to different thermal treatments and characterized by means of structural, thermoanalytical, and rheological investigations. The structure and composition of humins are very diverse and depend on the thermochemical conditions. On sufficient heating, humins change into a nonreversible and more branched furanic structure with a relatively high glass-transition temperature (T_g >65 °C). Thus, humins can be easily processed for preparing thermoset-like resins.