Production of Valuable Chemicals and Fuel Molecules from Lignin Via Fast Pyrolysis: Experimental and Theoretical Studies Using Model Compounds

Abstract

Lignin is the second-most abundant compound in lignocellulosic biomass (up to 30% dry weight) and a major by-product of the pulp and paper industries. Even though it is projected as a primary source of renewable phenolic compounds, its complex and highly-condensed structure with phenyl propane monomers, viz., p-hydroxyphenyl, guaiacyl, and syringyl sub-units, makes lignin conversion challenging. The use of lignin as a source of phenolic compounds is also exacerbated by its wide molecular weight distribution and branching. Pyrolysis and catalytic fast pyrolysis have emerged as promising thermochemical conversion technologies to convert lignin into phenols and aromatic hydrocarbons. Pyrolysis of lignin model compounds is valuable to unravel the mechanism of formation of phenols through the cleavage of specific linkages in lignin, and their secondary gas phase decomposition reactions. This chapter focuses on experimental and theoretical studies of free radical and concerted reactions of lignin model compounds for the production of phenolic and other aromatic compounds. The purpose of this chapter is to provide an essence of fast pyrolysis chemistry of lignin and its model compounds, and the associated reaction kinetics. Challenges in obtaining a mechanistic understanding of lignin pyrolysis are highlighted, and the need for a synergistic combination of experimental and computational studies is emphasized.KeywordsLigninFast pyrolysisCatalytic pyrolysisDensity functional theoryKinetic modelPhenolics