Understanding the reaction mechanism in lignin catalytic fast pyrolysis
Heterogeneous catalytic reaction mechanisms are driven by short-lived transient species, which are notoriously difficult to detect and identify. To move beyond "trial-and-error" approaches in our quest to convert cheap and abundant biomass, such as lignin, into high value fine chemicals or fuels, we have to understand the reaction mechanism. Only then can we design novel catalysts that improve selectivity and conversion, i.e. how much we can produce from a given amount of raw material.
In favorable cases, catalysis intermediates are desorbed from the catalyst surface, which opens up the possibility of their identification using sensitive and selective gas-phase techniques. At the VUV beamline, elusive species can be detected isomer-selectively by sampling of a collision free environment after the reactor using a combination of mass spectrometry and photoelectron spectroscopy. Here, we investigated the zeolite catalyzed pyrolysis of guaiacol, a lignin model compound at 400 to 500 °C.
The photoion mass selected threshold photoelectron spectra exhibit a unique fingerprint of each isomer of a certain mass, depending on its electronic and vibrational structure (see magnifying glass). We found that the fulvenone ketene (molecule in the magnifying glass) is the key reactive species responsible for the formation of phenol (precursor for plastics) and benzene (gasoline). Understanding the reaction mechanism of these processes goes beyond the current “cook-and-look” approaches often used in catalysis. By optimizing the conditions and driving the reaction to pass through a single reactive intermediate, we may fine tune the selectivity and improve the conversion significantly.