Facile Decarbonylation Suppresses the <i>Ortho</i> Effect and Ketene Formation in the Catalytic Pyrolysis of Substituted Benzaldehyde Lignin Model Compounds

Ketene intermediates lead to branching and lower phenol selectivities in the catalytic pyrolysis of lignin model compounds, which makes understanding their formation mechanism key to enable targeted process optimization. While gas-phase pyrolysis of methoxy- and hydroxy-substituted benzaldehydes favors fulvenone ketene formation, it is unclear if the same reaction pathways dominate in the presence of Brønsted acid sites. Thus, we tested if HZSM-5 produces fulvenone utilizing <i>operando</i> photoelectron photoion coincidence spectroscopy. Hydroxybenzaldehydes undergo acid-catalyzed decarbonylation, via oxonium mediated hydrogen transfer reactions, to phenol instead of dehydrogenation to fulvenone. The catalytic pyrolysis of anisaldehydes is initiated by demethylation and decarbonylation to yield anisole or hydroxybenzaldehydes and does not produce ketene either. Subsequently, decarbonylation and demethylation, respectively, lead to phenol and methylated derivatives due to abundant surface methyl groups over HZSM-5. Comparative analysis of the catalytic pyrolysis pathways of methoxyphenols and anisaldehydes, reveals that the chemistry of individual functional groups outcompetes the interactions of the vicinal substituents (<i>ortho</i> effect) in anis- and salicylaldehydes, resulting in the suppression of fulvenone ketene. We discuss how the high reactivity of aldehyde functionalities by decarbonylation may be leveraged to increase selectivities to value-added products.