Theory studies on pyrolysis mechanism of phenethyl phenyl ether

In order to understand the mechanism of lignin pyrolysis, the pyrolysis of phenethyl phenyl ether (PPE) as dimeric lignin model compound was investigated by density functional theory methods at B3LYP/6-31G(d) level. Ten possible pyrolytic pathways were proposed including two concerted reactions and eight free-radical reactions. The standard thermodynamic and kinetic parameters in each reaction pathway were calculated at different temperatures. The calculation results show that all reactions are endothermic. Both thermodynamic and kinetic analyses support reaction pathways 9 and 10 in concerted mechanism as major reaction channels. The activation energies of rate-determining steps in pathways 9 and 10 are 60.1 kcal/mol and 52.8 kcal/mol respectively. We get four prior pathways from all the reaction pathways of PPE pyrolysis in four different bond rupture ways. Among these four reaction pathways, kinetic analysis prior supports pathway 2 whose rate-determining step is step 1 with an activation energy of 60.6 kcal/mol. It secondly supports pathway 4 whose rate-determining step is step 4 with an activation energy of 68.6 kcal/mol. Then it supports pathway 7 whose rate-determining step is step 12 with an activation energy of 93.6 kcal/mol. At last, kinetic analysis supports pathway 6 whose rate-determining step is step 10 with an activation energy of 96.7 kcal/mol. It can be inferred that major products of lignin pyrolysis are styrene, phenol and the compounds that can be formed by combinations of phenoxy, benzyl, phenyl and hydrogen radical.