Lignin accounts for 15-40% of the weight of plant biomass and is the second most abundant natural polymer in the world, surpassed only by cellulose. Converting the strong, long chains of molecules (polymers) in lignin into smaller hydrocarbons (monomers). This is a highly desirable reaction as bio-refineries can use the monomers are the starting blocks for other chemicals, or they can be blended with oil-based products to form “drop-in" fuels.
Lignin is a heavily branched 3D network of subunits (methoxylated phenylpropanoid), held together by C-O and C-C bonds in a completely random order. It is relatively easy to break the C-O bonds when separating the monomers in lignin. However, the presence of stable C-C bonds present from the native lignin, or formed during lignin extraction, prevent full decomposition.
The team of scientists have designed a mesoporous multifunctional catalyst Ru/NbOPO4 that achieves, for the first time, cleavage of both C-C and C-O bonds in lignin in a one-pot process, yielding up to 153% of monocyclic C6~C9 hydrocarbons; 1.5 times the yield achieved using the established route.
A representative structure of a lignin fragment showing various linkages and schematic representation of the one-pot separation of lignin into monocyclic aromatic hydrocarbons, comparing previous studies (below the arrow) with this study (above the arrow).
Professor Yanqin Wang, from the East China University of Science and Technology, said: “Our new catalyst, Ru/NbOPO4, integrates multiple active sites on the functional NbOx support to enable the cleavage of both interunit C-O and C-C linkages in lignin, thus maximising the monocyclic hydrocarbon production."
The team used a technique known as Inelastic Neutron Scattering (INS) on the TOSCA beamline at STFC's ISIS Neutron and Muon Source to confirm the reason for the excellent activity of the catalyst. They found that it originates from a combination of strong adsorption and a synergistic effect between the Ru particles, NbOx species, and acid sites on the catalyst surface. These all promote the dissociation of hydrogen, the strong adsorption of substrates and intermediates, and the partial protonation and activation of adsorbed substrates; driving the entire reaction.
“It is very important to understand the underlying catalytic mechanism. Measurements at TOSCA have revealed important insights into the lignin conversion, providing key evidence to the observed reactivity." Explains researcher Dr Sihai Yang, from the School of Chemistry, University of Manchester.
“Zeolite-based catalysts currently dominate the state-of-the-art petroleum refineries for hydrocarbon cracking, and pyrolysis-based bio-refineries producing small molecule feedstock chemicals. The activity of Ru/NbOPO4 is very different to these conventional zeolite-based catalysts and may have great potential for future lignin upgrading."
TOSCA is a neutron spectrometer optimised for broadband vibrational spectroscopy in the infrared region, and Dr Stewart Parker and Dr Svemir Rudic from ISIS are delighted that the members of the catalysis community from across the globe are able to rely on TOSCA's capabilities to shed light on important scientific questions such as lignin monomer production.
The team is now using their results to develop new, more efficient catalysts to improve biomass conversion.
The full publication, "Breaking the limit of
lignin monomer production via cleavage of interunit carbon-carbon linkages" can be viewed online at the Chem website.
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