Date of Award
Doctor of Philosophy (PhD)
Micro and Nanoscale Systems
Lignin is a high-volume farm waste and environmental hazard of paper and pulp industries. To promote the utilization of its rich aromatic units into important chemicals and fuels, efforts were intensively made to breakdown lignin structure with a variety of depolymerization processes involving heating, solvent, and catalysts or their combination. Among those processes, ethanolysis in supercritical conditions shows promising performance for its high lignin conversion and little char formation. To improve the yield and selectivity of aromatics, particularly phenols, we examined the important roles of acidity and pore structure of different zeolite catalyst play in this process. Zeolites with close micropores and acidity defined by their crystal structures including Beta, Y, and ZSM-5 were first examined. Zeolites with the same microporous structure but different acidic strength caused by various H-type sites were further evaluated. Comparisons were further made between HZSM-5 and HY zeolites with unique mesoporous structures and their counterparts with exclusive micropores. Despite the complexity of lignin depolymerization and its greatly diversified products, strong acidity was found effective to cleave both the C-O-C and C-C linkages on lignin structure to receive more phenols while mild acidity works mainly in ether bond breakdown. When the diffusion issues of gigantic lignin intermediate and monomer products are severe (e.g., in microporous zeolites), overall yield and selectivity of lignin depolymerization products fall and the pore size of catalyst becomes dominant between the two key factors. Like in many petrochemical reactions involving bulky molecules, hierarchical pore structure also is important to promote mass transport and increase the exposure and utilization of acidic site inside zeolite catalysts. At the presence of mesopores in zeolites, their pore configuration is less sensitive when comparing with the acidity to decide the yield and selectivity to phenols of C8-C11. These findings provide important guidelines on the selection and design of zeolites with appropriate acidity and pore structure to facilitate lignin depolymerization or other cracking processes.
The products of lignin depolymerization are a mixture of various organic compounds including alcohols, ester, phenols, and other large hydrocarbons with high oxygen content (up to 40 wt.%), poor thermal stability, and low heating values (16-19 mJ/kg), insuitable to serve as alternative or replacement to fossil fuel. Hydrotreating step, a classic refinery process to remove oxygen and other unwanted elements in oil by adding hydrogen, is often suggested for the upgrading of bio-oil to increase its C/O ratio, improve its energy density, stability, as well as other required fuel properties. We successfully synthesized new mesoporous zeolites, Meso-ZSM-5, via solid-state crystallization of dry aluminosilicate nanogels. Palladium was further loaded on these zeolites to form a bi-functional catalyst (Pd/Meso-ZSM-5). When used in the hydrodeoxygenation of guaiacol, a major lignin depolymerization compound, Pd/Meso ZSM-5 exhibits superior guaiacol conversion and product distribution when compared with those supported on conventional microporous ZSM-5 counterparts. This is attributed to the improved diffusion and accessibility of active sites inside Meso-ZSM-5 with its unique hierarchically porous structure formed through neighbor nanocrystals connecting at edges. Ring saturated hydrocarbons are largely produced at 200 °C when hydrogenation dominates while alkaylated aromatics become major HDO products as deoxygenation becomes favorable at 250 °C. Unlike the disappointing conversion and severe coking issue over many HDO catalysts, this catalyst shows excellent anti-coking performance at various temperature conditions. These encouraging results demonstrated the great potential of Pd/Meso-ZSM-5 catalyst in bio-oil upgrading processes and may ignite the wide use in emerging renewable energy fields as well as many other reactions in traditional fossil fuel industrials.
Baxter, Nathan Cody, "" (2020). Dissertation. 876.