In order to conserve finite fossil fuels and reduce green house gas emission, the development of sustainable energy is an inevitable challenge in the 21st century. Among the alternative energy sources, cellulosic biomass (e.g. energy grasses such as miscanthus or agricultural waste such as corn stover, bagasse, cereal straws, and wood chips) presents several unique advantages: (1) it can produce liquid fuels or chemicals that substitute the existing petroleum-derived ones without requiring significant modification of infrastructure, (2) it is renewable and potentially carbon-neutral in overall life cycle, and (3) it is inexpensive, ubiquitous and faces less land use competition with food compared to edible biomass (sugar, starch and oil). Aiming to contribute to the development of biomass conversion technology, my Ph.D. study was conducted at the Energy Biosciences Institute, a multidisciplinary joint initiative for biomass research between UC Berkeley, Lawrence Berkeley National Laboratory, University of Illinois at Urbana-Champaign, and BP. Specifically, my focus has been the downstream conversion of sugar/sugar derivatives into chemicals, as described below.
Chapter 1: Polyethylene terephthalate (PET) is a plastic in large demand worldwide, due to its utility in the manufacturing of a range of products from beverage containers to synthetic fibers. The key monomer of PET, terephthalic acid, is currently produced by oxidizing p-xylene from crude oil. In an effort to reduce our dependence on fossil fuel, we demonstrated a route to synthesize solely biomass-derived p-xylene as a drop-in replacement. Namely, we explored the feasibility of converting 2,5-dimethylfuran (derived from 5-(hydroxymethyl)furfural, a typical by-product of cellulose hydrolysis) and acrolein (produced from glycerol, a side-product of fatty acid methyl ester biodiesel production) into p-xylene. This synthesis consisted of a sequential Diels-Alder reaction, oxidation, dehydration, and decarboxylation. In particular, the pivotal first step, the Diels-Alder reaction to construct 7-oxabicyclo[2,2,1]hept-2-ene core structure, was studied in detail to provide useful kinetic and thermodynamic data. The concept was realized and the bio-derived p-xylene was obtained in 34 % overall yield over four steps.
Chapter 2: The deoxygenation reaction of sugar moieties is essential for the conversion of cellulosic biomass to chemicals and fuels. While numerous reports focus on the pyrolysis, hydrogenolysis and acid-catalyzed dehydration reaction of biomass, one much less developed deoxygenation pathway is the deoxydehydration (DODH) reaction, which removes two adjacent hydroxyl groups from vicinal diols to afford alkenes. We have developed an oxorhenium-catalyzed DODH reaction using a sacrificial alcohol (e.g. 1-butanol, 3-pentanol) as a recyclable and environmentally friendly solvent/reductant, and successfully applied it to sugars, sugar acids and sugar alcohols. When combined with the alcohol reductant, oxorhenium compounds, namely methyltrioxorhenium (MTO) and perrhenic acid (HReO4), showed much higher activity than other precedented DODH systems and enabled the unstable polyol substrates to undergo clean DODH reactions. Linear polyene products and aromatic compounds were obtained with remarkable selectivity. Mechanistic insights were acquired by studying the isolated Re(V) species as well as by examining the unprecedented modes of DODH on 2-ene-1,4-diol and 2,4-diene-1,6-diol moieties.
