Lignin is a major component of biomass. Converting this valuable feedstock into fine chemicals and fuels can help to achieve CO2 neutrality, which was agreed upon in the European Union to be reached until 2050. Currently, the selectivity and the conversion of this process are low and the reaction is difficult to control, which may be owed to the lack of mechanistic understanding. Reaction pathways are governed by highly reactive and elusive intermediates, which evade detection using standard chemical analysis tools, like gas chromatography mass spectrometry (GC/MS). This calls for sensitive, selective and multiplexed detection tools, such as double imaging photoelectron photoion coincidence (i2PEPICO) as developed at the vacuum ultraviolet beamline of the Swiss Light Source. PEPICO allows to record mass and photoelectron spectra, where the ions and electrons from the same ionization event are directly correlated. This permits to measure photoion mass-selected threshold photoelectron spectra, which enable isomer-selective detection of reactive species, such as radicals and carbenes. In this contribution, we discuss how this state-of-the-art detection tools gained new insights into catalytic pyrolysis (CFP) of lignin model compounds, such as guaiacol and benzenediols over zeolites. While resorcinol and hydroquinone possess similar reactivity, catechol sticks out and can uniquely dehydrate to form the reactive fulvenone intermediate. This ketene rapidly hydrogenates to form resonantly stabilized phenoxy radicals, which undergo a branching towards phenol or yield cyclopentadienyl radicals (c-C5H5) upon decabonylation. The latter radicals are hydrogenated to form cyclopentadiene (c-C5H6), which can dimerize in a Diels-Alder reaction to yield dicyclopentadiene. The dimer can decompose in different pathways to yield fulvene, benzene, toluene and xylenes, or dehydrogenate multiple times to form naphthalene. This pathways provide compelling evidence for coke formation mechanisms, which are responsible for the deactivation of the catalyst. In guaiacol the demethylation is the first step in CFP to afford catechol, while the large abundance of methyl radicals on the catalyst surface leads to a rapid methylation of all reaction products. We will compare these reaction mechanisms with uncatalyzed pyrolysis and discuss the new insights gained, which may help to develop predictive models making this process economically more viable.

New mechanistic insights into lignin catalytic pyrolysis by detection of highly reactive and elusive intermediates

ACS Spring 2022

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