Date of Award
8-2015
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical Engineering
Committee Chair/Advisor
David A. Bruce, Committee Chair
Committee Member
Sarah Harcum
Committee Member
Mark Roberts
Committee Member
Mark Thies
Abstract
Lignocellulosic biomass is the most viable choice for producing high-value chemicals, materials, liquid transportation fuels, and energy from a renewable source. Lignin, in particular, is of interest because it serves as the only significant source of renewable aromatic carbon. Commercially, the Kraft pulping process is the predominant method for separating lignin and hemicellulose (collectively called black liquor) from the cellulose fraction of woody biomass. To enable the production of low molecular weight compounds such phenols and aromatic acids, high-purity, low-ash lignin must first be extracted from the black liquor. This separation is achieved using the Sequential Liquid- Lignin Recovery Process, which takes advantage of the reduced solubility of lignin in acidified black liquor. The resulting lignin product is a mixture of polymers of varying molecular weight that contain guaiacyl, syringyl, and p-hydroxyphenyl units connected through a complex network of ether linkages of varying types. To create high-value chemicals and fuels it is necessary to further breakdown the lignin polymer using a series of selective reaction processes. The primary focus of this work is on the development of optimized heterogeneous catalyst for the hydrodepolymerization of lignin.
Mesoporous SBA-15 and Al-SBA-15 were chosen as the catalyst support structures due to the high surface area and large pore diameter, resulting in highly dispersed active sites that are accessible by large molecules such as lignin and lignin model compounds. A bifunctional lignin hydrogenation catalyst was synthesized through the incorporation of aluminum into the SBA-15 framework, which increased the acidity of the catalyst support, and incipient wetness impregnation of palladium, yielding hydrogenation sites on the catalyst surface. A lignin model compound, 4-phenoxyphenol, which is representative of 4-O-5 type ether linkages in lignin, was used to evaluate catalyst performance and identify reaction pathways and kinetics for reactions important to lignin depolymerization. Cobalt iron oxide (CoFe2O4) magnetic nanoparticles were also incorporated into the catalyst support (AlSiMNP) to improve catalyst separation from the product mixture. These catalyst materials showed good catalytic activity toward the hydrogenation of 4-phenoxyphenol, with conversions as high as 53% and 95% for SBA-15 and Al-SBA-15 catalysts, respectively. It was determined that the presence of aluminum within the framework of the catalyst support enhances the hydrogenation and C-O bond cleavage of phenolic aryl ethers for both magnetic and non-magnetic Al-SBA- 15 based catalyst supports when compared to non-aluminum containing SBA-15 supports. Further, analysis of reaction products indicated preferential hydrogenation of the 4-phenoxyphenol benzene ring compared to the phenol ring as well as 4-O bond cleavage over 1’-O of the 4-phenoxyphenol ether linkage. The kinetic analysis of hydrogenation reactions using 3%Pd/Al-SBA-15 and 3%Pd/AlSiMNP catalyst materials showed that 4-phenoxyphenol hydrogenation exhibited a first-order dependence on 4- phenoxyphenol concentration and was mass transfer limited by the rate of bulk hydrogen diffusion to the catalyst surface. The information herein furthers the understanding of lignin conversion into valuable products and identifies the importance of both acid and hydrogenation catalytic sites for the selective conversion of lignin into lower molecular weight organic products.
Recommended Citation
Carter, Bethany, "Synthesis of Renewable Aromatic Chemicals and Fuels from Biomass Derived Lignin Model Compounds" (2015). All Dissertations. 4289.
https://open.clemson.edu/all_dissertations/4289