Pyrolysis Mechanism Study of Lignin Model Compounds by Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry

To investigate the lignin pyrolysis mechanism, two α-O-4 and one completely substituted β-O-4 lignin dimeric model compounds were studied using in situ synchrotron vacuum ultraviolet photoionization time-of-flight mass spectrometry (SVUV PIMS) at 350–500 °C. The collision-reduced vacuum condition, in situ characteristic, and “soft” ionization technique of this reactor system allowed for the direct detection of thermolysis radicals and high-boiling-point compounds. For the α-O-4 compound 4-(benzyloxy)phenol (BOP) pyrolysis, benzyl radical, p-semiquinone radical, toluene, and bibenzyl were confirmed, supplying firm evidence for the free radical dominant mechanism. Experiments of the p-methoxy substituent on the hydroxyl position of BOP show that the p-methoxy substituent can lower the Caromatic–Cα and Oether–Caromatic bond dissociation energies. For β-O-4 compound guaiacylglycerol-β-guaiacyl ether (GGGE), it is inferred that the C–O homolysis mechanism was minor and the concerted reaction dominated in the e...