Non-ideal solutions for biomass upgrading

Rodriguez Quiroz, Natalia
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University of Delaware
The growing demand for fossil fuels and increasing concerns about the environmental impact of their production and utilization have driven research to explore alternative renewable feedstocks and develop green and efficient technologies for their upgrade to fuels and chemicals. Nonedible lignocellulosic biomass is a promising carbon source owing to its abundance and high chemical functionality. Despite extensive investigation in the field of biomass valorization, critical challenges remain. Homogeneous catalytic systems offer solutions to many of these challenges. Nevertheless, the complex behavior that makes them promising also makes them difficult to understand, hindering advancements towards industrially feasible processes. This thesis aims to provide a fundamental understanding of the molecular interactions governing the reactivity of promising non-ideal aqueous (chapters two and three), biphasic (chapter four), and organic (chapter five) systems capable of alleviating bottlenecks in biomass valorization. ☐ First, we investigate homogeneous metal salt solutions as promising media in lignocellulose upgrading. We find that the role of the metal salts changes from dilute to concentrated solutions to molten salt hydrates (MSH). Dilute solutions of transition metal halide salts exhibit catalytic properties governed by metal-water speciation. The speciation is a secondary effect in more concentrated solutions, where the molecular interactions between the solvent, organic substrates, and salts become essential. These complex interactions change the activity coefficient of the species, evidenced by changes in the solubility of the sugars. These effects are further enhanced in MSH, where the substrate-salt interactions dominate and determine the ability to dissolve and hydrolyze cellulose in solution. ☐ We elaborate on the promising capabilities of MSH in chapter three. Specifically, we study the hydrolysis of cellulose in LiBr AMSH, attaining high glucose yields at low acid concentrations, low temperatures, and short times with potentially considerable economic benefits. Enabled by thermodynamic modeling and detailed kinetic studies, we find that the enhanced hydrolysis of cellulose in LiBr AMSH stems from a salt-induced increase in acidity due to preferential solvation of the salt's cations which draws solvation away from the acid catalyst, increasing the activity. Overall, we reveal that the efficacy of MSHs to hydrolyze cellulose is mainly determined by the effect of the salt on the solution's acidity. ☐ In Chapter 4, we couple fast experimental reaction kinetics, spectroscopy, and multiscale modeling to elucidate the enhancements in the rate and selectivity of fructose dehydration in biphasic systems. We show that apparent insoluble solvents used in biphasic systems can reach significant mutual solubility with water at reaction temperatures, enabling the partition of the sugar and catalyst into the extracting phase. In the organic-rich environment, the dehydration of fructose proceeds faster and more selectively than in water due to increased relative abundance of the reactive furanose isomer, enhanced water-catalyst-substrate interactions driven by nanophase separation, and higher product stability stemming from preferential solvation. We demonstrate that these solvent effects impact other critical biphasic reactions in biomass upgrading and provide qualitative principles for solvent selection. ☐ Finally, Chapter 5 exploits non-polar-organic-rich solvent systems formed upon partitioning the aqueous to the organic phase for the thermodynamically unfavorable Brønsted acid-catalyzed glucose dehydration. In saturated MIBK/water systems, we achieve unprecedented yields of HMF from glucose, catalyzed solely by hydrochloric acid. We reveal that in ketone solvents, glucose undergoes dehydration via an acyclic isomerization to fructose initiated by protonation of the ring oxygen enabled by solvent enhanced catalyst-ring oxygen interactions. Ultimately, we expose the pivotal role of solvents in overcoming glucose’s low reactivity and selectivity by guiding selective protonation of the glucose site leading to HMF formation.
5-HMF , Biomass , Catalysis , Fructose dehydration , Homogeneous catalysis , Metal salts