Isomer-Dependent Selectivities in the Pyrolysis of Anisaldehyde

The thermal decomposition of the three anisaldehyde isomers was studied by photoelectron photoion coincidence spectroscopy. The pyrolysis products were identified isomer-selectively by their photoion mass-selected threshold photoelectron spectrum in comparison to reference spectra. C–O bond fission at the methoxy group generates methyl and aldehyde phenoxy radicals in the initial fragmentation step for all anisaldehyde isomers. For para and meta isomers, these radicals can produce either 1-formyl-2,4-cyclopentadiene-1-yl radical or phenoxy radicals by low-energy removal of CO from the phenoxy radical (Ph–O) or aldehyde (−CHO) group, respectively. The 1-formyl-2,4-cyclopentadiene-1-yl radical channel may then generate cyclopentadienyl radicals by decarbonylation or fulvenone (c-C₅H₄═C═O), a reactive ketene species in lignin valorization, by hydrogen atom loss. In contrast, the phenoxy radicals only generate cyclopentadienyl radicals by decarbonylation. The ortho-anisaldehyde isomer shows high selectivity toward the phenoxy radical channel as a result of sterically enhanced fast H transfer from the aldehyde to phenoxy groups, which significantly lowers the decarbonylation energy barrier from the aldehyde group and fully suppresses the fulvenone channel. This contrasts with pyrolysis reactions of other bifunctionalized benzene compounds, such as benzenediol and salicylaldehyde, in which only the ortho isomer yields fulvenone. In general, cyclopentadienyl formation is unwanted in catalytic processes, because it is a coke precursor, responsible for catalyst deactivation. This shows how detailed, isomer-specific reaction mechanisms provide insights into selectivity control and extending the catalyst lifetime.