Catalytic fast pyrolysis is a promising way to convert lignin into fine chemicals and fuels, but current approaches lack selectivity and yield unsatisfactory conversion. Understanding the pyrolysis reaction mechanism at the molecular level may help to make this sustainable process more economic. Reactive intermediates are responsible for product branching and hold the key to unveiling these mechanisms, but are notoriously difficult to detect isomer-selectively. Here, we investigate the catalytic pyrolysis of guaiacol, a lignin model compound, using photoelectron photoion coincidence spectroscopy with synchrotron radiation, which allows for isomer-selective detection of reactive intermediates. In combination with ambient pressure pyrolysis, we identify fulvenone as the central reactive intermediate, generated by catalytic demethylation to catechol and subsequent dehydration. The fulvenone ketene is responsible for the phenol formation. This technique may open unique opportunities for isomer-resolved probing in catalysis, and holds the potential for achieving a mechanistic understanding of complex, real-life catalytic processes.

Fast pyrolysis of lignin is one of the most promising methods to convert the complex and irregular structure of lignin into renewable chemicals and fuel. During pyrolysis the complex set of radical reactions, rearrangements, and eliminations is influenced by temperature, pressure, and the lignin origin and structure. This model compound study aims to understand reaction pathways and how primary intermediates lead to the observed product selectivity. The pyrolysis microreactor directly connected to the gas chromatograph with a mass spectrometer (py-GC/MS) detects the final products, while imaging photoelectron photoion coincidence (iPEPICO) with VUV synchrotron radiation shows primary decomposition radicals. The tested model compounds, diphenylether (DPE) and ortho-methoxyphenol (guaiacol), represent a common lignin linkage and the most present subunit in lignin, respectively. Radical fragments, such as the hydroxycyclopentadienyl radical in guaiacol decomposition, are identified by mass-selected threshold photoelectron spectra (ms-TPES) in excellent agreement with the Franck-Condon simulation. While homolysis produces phenoxy-, phenyl-, and hydroxyphenoxy radicals, which are observed in high vacuum, radically initiated reactions are dominant in ambient conditions and produce recombination and rearrangement products, such as 2-hydroxybenzaldehyde in the case of guaiacol. The degree of substitution plays a dominant role in both the stabilization of the intermediate radical and the following degree of recombination. The recombination of phenoxy radicals is enhanced compared to hydroxy-phenoxy radicals.

Lignin pyrolysis is a promising method for the sustainable production of phenolic compounds from biomass. However, detailed knowledge about the radicals involved in this process and their influence on the molecular products is missing. Herein, we report on the pyrolysis of hard- and softwood Klason lignins under inert gas conditions in a temperature range between 350-550 °C. During the pyrolysis process, the formed radicals were detected by in situ high-temperature electron paramagnetic resonance spectroscopy. The overall formation of volatile products during lignin pyrolysis was determined using thermogravimetric analysis. The volatile molecular products were characterized and quantified using GC-MS analysis. Major differences were observed between hardwood and softwood lignins. Hardwood lignins form more radicals and volatile products than softwood lignins at temperatures from 350 to 450 °C. In the late stages of the pyrolysis process at 550 °C radical quenching reactions become dominant in hardwood lignins. We identified the disproportionation of two semiquinone radicals to quinone and hydroquinone species as the most likely quenching reaction. Our results show that both the pyrolysis temperature and the type of lignin source have a major influence on radical formation and the molecular products during the depolymerization of lignin.