Selective conversion of lignin derivatives to drop-in petrochemical intermediates through molybdenum oxide-based catalysts
Date
2024-12
Authors
Kohler, Andrew Joseph
Major Professor
Advisor
Shanks, Brent H.
Vigil, R. Dennis
Tessonnier, Jean-Philippe
Roling, Luke T.
Sadow, Aaron D.
Committee Member
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Abstract
The efficient separation and conversion of lignin to form valuable commodity products remains the primary challenge in economically utilizing low-cost renewable carbon resources. Traditionally, lignin has often been overlooked in biomass processing to maximize sugar extraction, leading to heavy modification of its structure. This modification typically results in recalcitrant C-C bond formation, making the lignin harder to process and limiting its application to a low-grade boiler feed. However, a change in approach to lignin can pay massive dividends since the general propylphenolic structure of lignin building blocks provides it with excellent potential to be converted into BTEX aromatics and C2-C4 alkenes. Importantly, these molecules represent drop-in replacements for petrochemicals, which would allow for the utilization of the existing and efficient infrastructure and markets to provide a boon to the biorenewable chemical industry.
To first enhance lignin viability, it is critical to consider lignin upfront to prevent the formation of intractable products at the backend. Thus, this dissertation starts with a study on “lignin-first” fractionation from the abundant agricultural residue corn stover using alcohol solvents in a flow-through reactor (Chapter 2). Typical solvolytic lignin extraction methods are hindered by the low stability of the resulting lignin oligomers at extraction conditions, thus requiring immediate reductive stabilization with a noble metal catalyst and high-pressure H2 to prevent recondensation and the formation of refractory C-C bonds. However, the need for this costly reductive step can be obfuscated by carefully controlling the solvent space-time in a flow-through reactor. Notably, increased solvent flow rates limit the exposure of the extracted lignin oligomers in the heated zone help to prevent solvent-induced decomposition of β-O-4 linkages and subsequently minimize the formation of reactive intermediates. Critically, the recovery of these “native-like” lignin oligomers allows for subsequent depolymerization through pyrolysis, which is less capital-intensive than reductive processes and promotes the partial decomposition of the propyl side chains of the monolignol building blocks to lower, more valuable ranges. As such, coupling pyrolysis with immediate hydrodeoxygenation (HDO) using bulk MoO3 allows for the conversion of the obtained lignin oligomers to monoaromatic and C2-C4 aliphatic platform chemicals.
However, despite this improved recovery of valuable aromatic moieties, using bulk MoO3 as a deoxygenation catalyst in this process results in a low selectivity of petrochemically relevant C2-C4 alkenes relative to alkanes. To better understand how these undesired products form and better inform mitigation strategies, detailed kinetic studies were performed for the HDO of linear oxygenated model compounds (Chapter 3). Here, it was indicated through co-feed studies that though alkenes can be sequentially hydrogenated on MoO3, this pathway is blocked before complete monomer deoxygenation as alkenes bind more weakly to the common HDO active site. Instead, alkanes form directly from HDO through hydrogenolysis, the rate of which is enhanced over molecules with fewer electron-donating constituents around the initial functional group. Importantly, this relationship is inverse for the overall rate of HDO as higher electron donation from the constituents is proposed to enhance the rate-determining O-H bond formation step.
Since most lignin-derived aliphatic molecules contain low electron-donating C-O functional groups, further enhancement of HDO and suppression of competitive hydrogenolysis must come through rational catalyst design (Chapter 4). Thus, to further understand how the surface properties affect these competing pathways, a library of catalysts across an array of metal oxide supports (Al2O3, Nb2O5, SiO2, TiO2, and ZrO2) were synthesized with varying degrees of oligomerized MoOx structures. The broad scope of this study revealed that the support and structure effects are not isolated, as they both work to change the adsorption strength of the molybdenum active site, leading to a Sabatier volcano relationship between oxygen binding strength and the rate of HDO. Critically, this combinatorial MoOx structure/support effect does not apply to the selectivity between the alkane and alkene products. Instead, hydrogenolysis is catalyzed at the bridging oxygen between molybdenum and the support, as a Sabatier relationship is formed between the adsorptive properties of the support (pH at the point of zero charge) and alkane selectivity.
This dissertation illustrates an integrated approach to lignin conversion to petrochemical intermediates, wherein the careful design of fractionation, depolymerization, and deoxygenation steps can significantly enhance the recovery of valued hydrocarbons and propel biorenewable chemical production forward.
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dissertation