Improving HMF yield using an integrated modeling approach
Date
2015
Authors
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Publisher
University of Delaware
Abstract
Higher global standards of living have propelled demand for petroleum-derived transportation fuels and chemicals to new heights. The negative environmental consequences of petroleum extraction and refining have driven research into alternative pathways to produce these critical compounds. Lignocellulosic biomass can be upgraded to many drop-in replacements for petroleum products via the acid-catalyzed dehydration of the carbohydrate fractions of biomass to furans. Furans, particularly 5-hydroxymethylfurfural (HMF) and furfural, can be used as platforms for a wide range of target molecules including alkanes, aromatics, pharmaceuticals, and polymer precursors. One of the main barriers to realizing the potential of the furan platform has been the non-selective production of furans from sugars, particularly HMF-production from hexose sugars. The present thesis takes a model-based approach to understand the conversion of hexose sugars to HMF and leverages this knowledge to propose strategies for improving HMF production. The reaction kinetics of fructose dehydration in aqueous HCl are modeled, leading to a proposed skeleton mechanism for fructose dehydration that is consistent with both experimental trends and first-principles calculations. The skeleton model explains increases in dehydration rate in mixed solvents based on changes in the amount of water in the system; the reaction kinetics of fructose dehydration in water-dimethylsulfoxide are well-described by this model. Higher temperatures and reduced water content correspond to higher HMF yields. The homogeneous reactor model is then integrated with adsorption isotherms, developed in this thesis, to evaluate HMF yield enhancement in a reactive adsorber. The reactive adsorber performs most effectively at low temperatures. Next, the tandem conversion of glucose to HMF in zeolite beta is elucidated in a model that isolates intrinsic heterogeneous kinetics in H-BEA zeolite from adsorption and homogeneous Brønsted acid chemistry. A balanced number of Brønsted and Lewis acid sites maximizes HMF production from glucose. Finally, the tandem conversion of glucose to HMF using homogeneous CrCl 3 and HCl catalysts is examined. It is shown that multiple kinetic regimes exist for tandem Lewis acid isomerization/Brønsted acid dehydration from glucose to HMF using both bifunctional solid materials (H-BEA) and homogeneous dual acid catalysts (CrCl 3 +HCl). Then using CrCl3 catalyst, dehydration is rate-limiting and HMF yields are improved by the addition of a small amount of HCl. As the amount of HCl increases, isomerization becomes rate-limiting. In this isomerization-limited regime, additional HCl decreases CrCl3 activity, thereby reducing HMF yields.