Catalytic fast pyrolysis (CFP) is very a promising way to convert lignin, a stable macromolecule abundant in biomass, into fine chemicals and transportation fuels, but current approaches lack selectivity and yield unsatisfactory conversion. We believe that understanding the CFP reaction mechanism at the molecular level will help to make this sustainable process more economic. Highly reactive intermediates are responsible for product branching and are notoriously difficult to detect isomer-selectively using standard chemical analysis tools such as GC/MS or NMR. However they hold the key to unveiling these mechanisms.
We have investigated the CFP of guaiacol, a lignin model compound, using photoelectron photoion coincidence (iPEPICO) spectroscopy with vacuum ultraviolet (VUV) synchrotron radiation, which allows for isomer-selective detection of reactive intermediates. To identify the isomeric contributions we have recorded photoion mass-selected threshold (near-zero kinetic energy) photoelectron spectra (TPES) at the exit of a catalytic reactor. The vibrational fine structure of these TPES is an isomer-specific fingerprint, which can be used to assign the species with the help of calculated Franck–Condon (FC) factors (see figure). 

In combination with ambient pressure CFP, py-iPEPICO identified the fulvenone ketene (c-C₅H₄=C=O) as the central reactive intermediate, generated by catalytic dehydration of catechol (1,2-benzenediol), which is the demethylation product of guaiacol. This fulvenone ketene is responsible for the formation of e.g. phenol, cresols and cyclopentadiene (see following scheme).[1] 
More broadly, we are convinced that py-iPEPICO opens new opportunities for isomer-resolved probing in catalysis, and holds great potential for achieving a mechanistic understanding of complex, real-life catalytic processes beyond CFP.

[1] P. Hemberger, V.B.F. Custodis, A. Bodi, T. Gerber, J. A. van Bokhoven, Nat. Commun. 2017, accepted.