Date of Award
Doctor of Philosophy (PhD)
Fabio H. Ribeiro
Fabio H. Ribeiro
Committee Member 1
Committee Member 2
W. Nicholas Delgass
Committee Member 3
Mahdi Abu- Omar
Biomass, as the only renewable source of carbon, presents a sustainable alternative to liquid fossil fuels for the production of renewable transportation fuels and chemicals. The lignin portion of biomass poses a significant challenge in the upgrading and conversion of lignocellulosic biomass to fuels and chemicals, and in many current biorefineries is not utilized. However, the lignin fraction is a valuable feedstock due to its aromatic structure, lower O:C ratio than cellulose and hemicellulose, and high energy content (up to 40%) and weight fraction (average 25 wt%) of biomass. Several processes (including fast-pyrolysis, fast-hydropyrolysis, and liquid phase processing) are able to process all components of lignocellulosic biomass, including lignin which is converted to aromatic oxygenated compounds. Many challenges still remain in the high-yield conversion of these lignin-derived compounds into usable hydrocarbon products.
This dissertation focuses on production of hydrocarbons for use as both fuels and chemicals via high-pressure catalytic hydrodeoxygenation (HDO) of the lignin-derived fraction of biomass. Catalytic HDO is performed as a second-stage process to upgrade the intermediate lignin-derived oxygenates produced in first-stage processes such as: the H2Bioil fast-hydropyrolysis/HDO process, a single-step lignin depolymerization process (which produces lignin-derived methoxypropylphenols and a solid cellulose/ hemicellulose fraction), or any other process capable of generating lignin-derived oxygenates (such as pyrolysis).
In this work, high-pressure HDO reactions of the methoxypropylphenol compound dihydroeugenol (2-methoxy-4-propylphenol) as well as other phenolic lignin-derived compounds were investigated. These studies were used to gain an understanding of how various catalysts performed at removing specific oxygen functional groups present in lignin (such as etheric methoxy and phenol groups), to investigate the HDO reaction pathways of these catalysts, and to apply this understanding to develop improved HDO catalysts.
A bimetallic platinum and molybdenum catalyst supported on multi-walled carbon nanotubes (5%PtMo/MWCNT) was found to be effective for complete oxygen removal from a variety of lignin-derived compounds. Hydrocarbons were obtained in 100% yield from dihydroeugenol, including 98% yield of the hydrocarbon propylcyclohexane. Hydrogen partial pressure was varied from 0 to 2.36 MPa, and the reaction rate was found to be directly proportional to the hydrogen pressure and the ratio of aromatic to saturated hydrocarbons produced was found to be inversely proportional to the hydrogen pressure. Therefore, there is a tradeoff with hydrogen pressure between rate and production of aromatic hydrocarbons, which are more valuable fuel compounds due to higher octane ratings.
Kinetic experiments on dihydroeugenol were conducted to elucidate the reaction pathway. Three main reaction pathways were identified: HDO, hydrogenation of the aromatic ring, and alkylation. The HDO pathway occurred via a series reaction, with methoxy group removal occurring first followed by removal of the phenol oxygen group. The phenol oxygen removal pathway was shown to involve Pt catalyzed hydrogenation of the aromatic ring to form an alcohol, followed by Mo catalyzed dehydration to form the hydrocarbon products. Alkylation from the methoxy group occurred as a minor side reaction.
A series of PtMo catalysts with varying metal ratios were tested to evaluate the balance of the Pt and Mo metals on steps in the oxygen removal pathway. Catalyst characterization techniques such as chemisorption, scanning transmission electron microscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy were employed to characterize the catalyst structure. Catalyst components identified were: Pt particles, bimetallic PtMo particles, Mo carbide, and Mo oxide phases. Trends in the catalyst structure were compared with kinetics to link structure to steps in the reaction pathway. The kinetic behavior and structure of the catalysts in the PtMo series fell into three categories: a Pt dominating category, an intermediate category where the Pt and Mo behavior was balanced, and a Mo dominating category. The PtMo catalysts in the intermediate category had the optimum balance of Pt to Mo to maintain a high rate (from Pt) and oxygen removal (from Mo).
Based on this work, greater than 50% yield of hydrocarbons was obtained from the lignin fraction of biomass (98% yield of propylcyclohexane from methoxypropylphenols and 54% yield of methoxypropylphenols from the single-step depolymerization of lignin). This effective utilization of the lignin fraction combined with the high yield production of hydrocarbons from the lignin-derived methoxypropylphenols opens a new realm of possibilities for biorefinery scale conversion of biomass to fuels and chemicals. Based on these new lignin processing developments, new synergistic biorefinery concepts are presented. Estimated calculations show that the lignin processes presented here, if used for chemical production, could exceed the annual U.S. demand of propylene and benzene with a fraction left over for fuel production.
Yohe, Sara Lynn, "High-Pressure Vapor-Phase Catalytic Hydrodeoxygenation of Lignin-Derived Compounds to Hydrocarbons on Bimetallic Catalysts in the Range of 0.2-2.4 MPa." (2013). Open Access Dissertations. 28.