Date of Award

August 2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Jesse Q. Bond

Keywords

Biomass conversion, Heterogeneous catalysis, Levulinic Acid, Mechanism of selective oxidation, Oxidative C-C scission, Surface chemistry

Subject Categories

Engineering

Abstract

Levulinic acid (LA) is a platform chemical derived from lignocellulosic biomass. Among the various applications LA has in industrial commodities and specialty chemicals, we previously reported a novel pathway that converts LA to maleic anhydride (MA) in high yields through methyl a-carbon oxidative scission over supported vanadium oxide catalysts. However, the high selectivity of methyl scission during LA oxidation appears to be unexpected according to the trends observed during the analogous oxidative scission of methyl ketones (e.g., 2-butanone and 2-pentanone). The impacts of vanadium oxide structures, metal oxide substrates, and methyl ketone molecular structures on both selectivity and reactivity of the oxidative scission were investigated in order to understand the origins of the high MA yield resulting from LA oxidation. Surprisingly, none of the aforementioned significantly increased the selectivity of methyl scission. However, further analysis of the oxidation route from LA to MA identified a new reaction intermediate—protoanemonin—and clarified its significance for the observed high MA yield.

Reactivity data demonstrated that the rate-limiting step for the oxidative scission of LA over supported vanadium oxide is the methyl scission of protoanemonin to MA. A mechanism study of analogous methyl ketone scission was carried out to investigate the fundamentals of LA oxidation. The observed mechanistic insights suggested that the oxidative scission of methyl ketones involves both Eley-Rideal and Mars van Krevelen mechanisms. Accordingly, this joint mechanism is proposed for the first time. The active site for the methyl ketone oxidative scission was identified using pyridine poisoning, NH3 poisoning, and water co-feeding. The results suggest that adsorbed methyl ketones on acidic sites (Lewis and Bronsted) and redox sites (V-O-M) are able to react with gas-phase oxygen, cleaving into two fragments. The alkyl fragment can form ketones or aldehydes with the oxygen of either. In contrast, the carbonyl fragment can form surface acetate with lattice oxygen and desorb as acetic acid only if the lattice oxygen is in the V-O-M bond bridge.

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Open Access

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