Concluding Remarks

Over the past 4 years, my PhD research has been focused on the utilization of lignocellulosic biomass as a renewable carbon source for the production of high-value chemicals through two approaches: (i) organosolv biomass pretreatment at mild conditions and (ii) the catalytic upgrading of stabilized carbohydrates into building-block molecules and secondary chemicals. Critical challenges linked to biomass processing such as product separation, solvent recovery, kinetic and mass-transfer limitations were addressed. This was done by employing liquid-CO₂, protection group chemistry, flow-through reactor setups and heterogeneous catalysts.

Organosolv biomass pretreatment

The aim of this project was to use mild temperatures and low acid content in solvent-aided integrated biomass depolymerization. The utilization of aprotic polar solvents such as γ-valerolactone (GVL), 1,4-dioxane or tetrahydrofuran (THF) allowed us to extract up to 80% lignin with almost full xylan recovery and cellulose retention in the pretreated substrate. When GVL was used, over 99.5% of the solvent was recovered by liquid-CO₂ extraction, which could significantly improve process economy. This approach generated a carbohydrates stream with a concentration of 185 g L^-1 after enzymatic hydrolysis. Moreover, we found that the addition of formaldehyde during biomass pretreatment prevented the formation of inter-unit C-C linkages in lignin during its extraction, increasing yields of lignin monomers by 6-fold based on native lignin. Following this approach, we used a similar reversible stabilization of xylose and glucose by acetal formation with aldehydes, preventing sugar dehydration to furans and their subsequent degradation. This stabilization strategy pushes back the longstanding kinetic limits of polysaccharide depolymerization vs. dehydration and enabled the recovery of biomass-derived carbohydrates at high yields and concentrations.

Stabilized carbohydrates upgrading

Stabilized carbohydrates are versatile molecules produced almost quantitatively from lignocellulosic biomass, that contrary to unmodified carbohydrates, are volatile and soluble in alkanes. The utilization of stabilized carbohydrates as platform molecules presents an opportunity to produce building block molecules through new reaction pathways. For example, when diformylxylose (formaldehyde stabilized xylose) was used as a starting reagent, similar furfural yields to those obtained with xylose via hydrogen transfer were achieved without Lewis acid addition. Surprisingly, no difference in the reaction kinetics was observed in the presence of a Lewis acid, which suggested that diformylxylose proceeded to furfural through a different dehydration mechanism. In light of these results, we proposed a tandem hydrolysis-dehydration reaction mechanism by a new intermediate identified during operando ¹³C NMR studies.

Currently, my research is centered in the upgrading of aldehyde-stabilized carbohydrates into furans and polyols using heterogeneous catalysts. The goal of this approach is to produce targeted molecules through new, potentially simpler reaction routes compared to those currently established. The formation of novel products through these routes is also considered.

In my future research, I would like to explore the production of hydrogen and liquid fuels from aldehyde-stabilized carbohydrates using bifunctional heterogeneous catalysts, particularly zeolites. The projected reaction pathways include aqueous-phase reforming and cascade processes involving hydrogenolysis and aldol-condensation. Given that aldehyde-stabilized carbohydrates exhibit different reactivity compared to unmodified carbohydrates, they present an opportunity to explore novel upgrading routes.

Teaching Interests:

My teaching interest has been molded by chemical reaction engineering and thermodynamics. Based on my research and teaching experience, I would be well suited for courses in material and energy balances, kinetics and reactor design, and thermodynamics. Additionally, I have particular interests in specialized courses in heterogeneous catalysis and micro-reactors. I also visualize the possibility of developing new courses related to biorefineries.

Website/Contact:

• https://people.epfl.ch/cgi-bin/people?id=244963&op=bio&lang=en&cvlang=en
• ydna.questell@epfl.ch