Computational Investigations of Furfural Valorization

Sihang Liu

Research output: Book/ReportPh.D. thesis

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Abstract

The global crisis caused by overreliance on fossil fuels has rung the bell for finding alternatives to energy generation and chemical production. In the past decade, we have seen hopes of resolving the challenge in building a sustainable and circular biomass economy. One of the most important sectors of biomass economy is to efficiently upgrade biomass derivatives into value-added fuels and chemicals. Valorizing furfural, a platform chemical mainly derived from non-food agricultural residues, is regarded as a model process to understand biomass valorization reactions. Though great progress has been achieved in furfural valorization via both conventional thermal reactors and novel electrolyzers, there are still issues concerning elusive reaction mechanisms, poor understanding of reaction interfaces, and suboptimal reaction conditions and setups.
The Thesis begins with a study of gas-phase furfural hydrogenation towards furfuryl alcohol at metal-gas interfaces, with the target to find active and nontoxic catalysts to replace Cr-based catalysts in industry. Based on density functional theory (DFT) calculations, we show that rate-limiting steps vary on different metal surfaces, suggesting that we need to lower corresponding barriers to improve activities on different metal catalysts. Furthermore, we construct a microkinetic model to describe the activity of furfuryl alcohol production from furfural and hydrogen gas. The established activity volcano suggests that Cu-rich alloys, e.g., Cu3Ni, present enhanced activities and can become next-generation catalyst candidates for furfural hydrogenation.
To study the solvent effect on furfural hydrogenation, we then transit from the metal-gas interface to the metal-water interface. We investigate the furfural adsorption at various metal-water interfaces using ab initio molecular dynamics (AIMD) simulations. Different from metal-gas interfaces for our first study, aqueous phase conditions pose strong solvation penalties to furfural adsorption, which we attribute to the displacement of water interacting with the metal surface. To reduce the computational cost in simulating interfacial chemistry, we find that the OH binding energy serves as a simple descriptor to correlate with solvation energies. This helps generalize the understandings of adsorption phenomena in ultrahigh vacuum (UHV) conditions towards solvated systems. Based on the identified solvent effect, we unveil the origin of activity in aqueous phase furfural hydrogenation over strong-binding metal surfaces. 
We then move on to explore the electrification of furfural hydrogenation, i.e., furfural electroreduction, using ‘green’ protons and electrons. We first embark on the investigation of furfural reduction over Cu electrodes, the most studied material for this reaction. Combining grand-canonical DFT (GC-DFT) calculations, microkinetic simulations and experiments, we unveil the possible rate-limiting steps to produce furfuryl alcohol and 2-methyl furan, lying beyond the first proton-coupled electron transfer (PCET). A general pH-potential map to boost selectivity towards more valuable 2-methyl furan product is also proposed. Furthermore, we argue that the surface hydrogenation contributes little to the overall activity on Cu terraces even under very acidic conditions.
Inspired by our experimental collaborations, we further our excursion of furfural reduction to well-defined single atom catalysts. We propose using furfural and hydrogen adsorption energies to classify the selectivity for this reaction at mild conditions. Supported molecular single atom catalysts generally bind furfural weakly and selectively produce the dimer product (hydrofuroin). The formation of the dimer is shown to be limited by the first protonation, while the coupling step is facile in solution. Furthermore, the metal center can be modified on the general metal-nitrogencarbon complex to achieve higher selectivity towards the monomer product (furfuryl alcohol). It is predicted that by modulating the metal center and coordination in the single atom motifs, we can further unleash the potential of those ‘lonely’ active sites in furfural reduction and other biomass electrovalorization reactions. 
The theoretical insights obtained in the Thesis establish a better understanding of furfural (electro-)valorization reactions and provide rationale to design more active and selective catalysts with optimized reaction conditions.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages165
Publication statusPublished - 2023

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