Abstract
Lignin is a potential renewable source of oxygenated chemicals and liquid fuels, its reaction pathways are often studied using model compounds. Phenethyl phenyl ether (PPE; PhCH2CH2OPh)) is the simplest model for the most common �-O-4 linkage in lignin. Previously, we developed a computational scheme to calculate the �/�-product selectivity in the pyrolysis of PPE by systematically exploiting error cancellation in the computation of relative rate constants. The �/�-selectivity is defined as the selectivity between the competitive hydrogen abstraction reaction paths on the �- and �-carbon of PPE. We use density functional theory and employ transition state theory where we include diagonal anharmonic correction in the vibrational partition functions for low frequency modes for which a semi-classical expression is used. In this work we investigate the effect of oxygen substituents (hydroxy, methoxy) in the para-position on the phenethyl-ring of PPE on the �/�-selectivities. The total �/�-selectivity increases when substituents are introduced and is larger for the methoxy than the hydroxy substituent. The strongest effect of the substituents is observed for the �-pathway of the hydrogen abstraction by the phenoxyl chain carrying radical for which the rate increases. For the �-pathway and the abstraction by the R-benzyl radical (R=OH,OCH3) the rate decreases with the introduction of the substituents. These findings are compared with results from recent experimental studies.