Isotope labeling-based studies on the pyrolysis of dimeric β-O-4 lignin model compounds phenylethyl phenyl ether and phenylethanol phenyl ether

Abstract

In this study, the pyrolysis of the β-O-4 lignin model compounds phenylethyl phenyl ether (PPE) and phenylethanol phenyl ether (PEPE) in the presence of H₂O, D₂O and H₂¹⁸O at 500°C was investigated. The pyrolysis products were examined by GC-MS to analyze the pyrolysis process and product formation mechanisms. By comparing the pyrolysis products of PPE and PEPE, the chemical structure effect of C_α-OH was found to decrease the yield of styrene, benzaldehyde, and phenylethanol, while increasing the yield of acetophenone, which facilitated the decomposition of C_β-O and led to an increase of the phenol yield. Analysis of the isotope-labeled PPE pyrolysis products revealed that phenol undergoes deuteration to varying degrees, and the deuterated intermediates undergo intramolecular exchanges to produce deuterium-substituted phenols (C₆H₅OD, C₆H₄DOH, C₆H₄DOD), which verified the presence of the reciprocal isomer 2,4-cyclohexadienone. The product acetophenone from the pyrolysis of PEPE was deuterated to varying degrees, indicating multiple pathways of formation, which was accompanied by the formation of phenol, resulting in a high abundance of acetophenone and phenol products. Phenylethanol was found to be an important intermediate in the formation of phenylacetaldehyde. The order of occurrence of the different reaction intermediates of PPE and PEPE was hypothesized and the free radical reaction was found to play a dominant role. This experiment reveals the evolution of the products of PPE and PEPE during pyrolysis.