Lignin is a natural amorphous polymer that acts as the essential glue that gives plants their structural integrity. It is a main constituent of lignocellulosic biomass (15-30% by weight, 40% by energy), together with cellulose and hemicelluloses. The conversion of lignocellulosic biomass to biofuels is primarily focused on converting cellulose and hemicellulose in feedstocks into biofuels. These approaches assume that the lignin fraction will be used primarily for heat and power production. This current vision undervalues lignin's potential to address this nation's high quality liquid fuel requirements. Lignin conversion has significant potential as a source for the sustainable production of fuels and bulk chemicals. Despite the potential, the conversion of lignin to biofuels has proven to be challenging.
Professor Bin Yang of Washington State University believes the fundamental hurdles in lignin-to-hydrocarbon catalytic conversion process could be surmounted if creative manipulation towards lignin reactivity, catalyst selectivity and enhanced scale of transformation is employed. Many unknowns remain which lead to Yang's specific objectives for this EAGER proposal:
(1) To improve understanding of lignin chemistry and reactivity (including chemical properties, solubility, molecular weight, and monolignol composition) in the depolymerization and hydrodeoxygenation reactions that lead to hydrocarbons; (2) To establish state-of-the-art NMR techniques (e.g. HR-MAS NMR) to probe structural features, modification and yields of lignin intermediates and products when processed under hydrogen and with supported metal and solid acid catalysts; (3) To understand the reaction chemistry and kinetics of lignin depolymerization and hydrodeoxygenation to hydrocarbons by screening a wide range of catalyst systems leading to high yield and oxygen removal.
As an example of the needs for this information, to date, virtually no published data has been reported on conversion of the lignin fraction left after ethanol production from lignocellulosics. Co-production of ethanol, diesel and/or jet fuel utilizing all major components of biomass, including lignin and carbohydrates, would significantly improve the total carbon use in biomass, and enhance the overall operational efficiency, carbon conversion rate, economic viability, and sustainability of biofuels production.
The educational component of the project is to continue development of outreach programs for university students and the general public, who will become cognizant of energy use and needs issues. The proposed activities will provide the foundation on which an integrated program that combines research, education and outreach focusing on biofuels and chemicals will be developed at Washington State University.
The key challenges to using biomass derived lignin as a biofuel resource include its intrinsic complex cross-linked polymeric aromatic structure, recovery of the lignin in a reactive state, and efficient depolymerization via selective inter-unit C-O-C bond cleavage, and hydrodeoxygenation of the low molecular weight moieties to aromatic and aliphatic hydrocarbons. Our results suggest that recovery of reactive lignin fragments from pretreatment processes provides a promising platform for conversion to both aromatic and cyclic aliphatic hydrocarbons. The reactivity and structural features of technical lignins with cleavable and/or labile 8–O–4' inter-unit linkages could be considered as the key factors for catalyst selection for HDO of technical lignin in presence of hydrogen. Our studies on heterogeneous catalysts for the conversion of oligomeric technical lignins have identified new combinations of catalyst matrixes with multi-catalytic functionalities suitable for efficiently depolymerizing the lignin polymeric framework into monomeric reactive intermediates and subsequently eliminating oxygen via HDO processes. The examined HDO processes demonstrated the cleavage of 8–O–4' inter-unit linkages with both technical lignins and synthetic model compounds (e.g. monomeric lignin, and oligomers) to produce C7 to C18 reactive intermediates. Our results showed ~85% conversion of technical lignin with ~75% product selectivity for toluene under various HDO conditions in the presence of 5% noble-metal (Ru, Rh and Pt)/Al2O3(or C)-Zeolyst (e.g.,NH4+ Z-Y 57277-14-1) catalyst matrix. Additionally, these multi-functional catalytic matrixes showed good selectivity in conversion of lignin to aromatic hydrocarbons under HDO conditions in aqueous media. Overall, this study opens new catalytic approaches for the production of alkylbenzene derivatives from a wide variety of lignin potentially available at large scale from lignocellulosic biorefineries. As the integration of research and education, the entire project involved participation of K-12 school students, undergraduates, graduate students and postdocs, particularly from underrepresented groups.
