(70g) Investigation of the Pyrolytic Decomposition Pathways of Lignin Model Compounds

Lignin is an abundant biopolymer that constitutes ca. 25-30% of dry biomass, that is rich in aromatic oxygen functional groups (phenols, ethers, alcohols, etc) making it a potential renewable energy and chemical resource that would replace or supplement fossil fuel-based resources.  However it is a very complex cross-linked biopolymer material which makes it challenging to deconvolute and control the ability to form desirable products formed from thermochemically decomposition without energy intensive upgrading processes.    Much of the fundamental pyrolysis behavior that occurs from the thermochemical decay pathways involving oxygen substituted molecules is not understood.  To approach this complicated problem we investigate the simplified model compound that represents 46-60% of the major arylglycerol-beta-aryl ether, β-O-4 interunit linkage, phenethyl phenyl ether (PPE; Ph^αCH₂^βCH₂OPh). We then systematically rebuild the structure with its lignin-like functionalities, i.e., alcohols, ethers, etc., and investigate the pyrolysis decomposition and substituent influences on product selectivity and reaction kinetics.  It has been seen that under liquid phase pyrolysis conditions at ca. 345 °C, decomposition occurs through a free radical chain pathway which involves homolysis of the weak C-O bond, followed by competitive hydrogen abstraction at either the α or β carbon which can be used to describe the product selectivity as α/β, and a rearrangement step that follows the β-hydrogen step.   We have recently investigated the pyrolysis of oxygen substituents on the aromatic positions of PPE, and discovered that substitution on the aromatic positions has an effect on the rate and selectivity. We now seek to understand the influence on the thermodynamic reaction rates and selectivities of substitutuents at the aliphatic alpha and beta carbons.