How Inter- and Intramolecular Reactions Dominate the Formation of Products in Lignin Pyrolysis.

One of the key challenges in renewable chemical production is the conversion of lignin, especially by fast pyrolysis. The complexity of the lignin pyrolysis process has hindered the elucidation of the mechanism, inhibiting further industrial implementation. By combining pyrolysis of model compounds (4-phenoxyphenol and 2-methoxy-phenoxybenzene) with lignin bond characteristics both under vacuum and under realistic pressure conditions, the roles of inter- and intramolecular reactions were established. On the one hand, the stable 4-O-5 ether bond enables, without breaking, C-C bond formation and even directly forms naphthalene depending on the position and type of the substituent. p-Benzoquinone intermediates, on the other hand, are highly unstable at ambient pressure and directly decompose into coke and carbon monoxide. The system pressure (radical concentration) plays a crucial role in the dominant reaction mechanism by initiating intramolecular reactions, interfering with intramolecular reactions. H-transfer and recombination reactions suppress the decarbonylation of phenoxy radicals, thus yielding a very different product distribution.