Lignin is one of the main constituents of terrestrial biomass. It consists of aromatic oxygenates that are linked to an amorphous polymer with various chemical bonds. Large quantities of lignin-rich black liquor are produced during pulping of lignocellulosic biomass, but nearly all of it is burned as a low value fuel because few economically viable options for lignin valorization exist. On the other hand, as the largest natural source of aromatic carbons, lignin has a tremendous potential as a resource for renewable chemicals. To reach this potential, cost-effective processes for isolation, depolymerization and upgrading of lignin are required. Many efforts have been undertaken, but the separation of products and catalysts from solvents is usually prohibitively expensive. The conversion of lignin to phenol is particularly attractive because it provides an alternative path to a bulk chemical with a world market of over 10 million tons per year.<br/><br/>This contribution presents a multistep process including depolymerization, deoxygenation and cracking of lignin-derived compounds to funnel the different building blocks into phenol as a common product. Mechanocatalytic depolymerization of lignin offers the opportunity to convert the solid feedstock with solid catalysts in a solvent-free environment. Intimate solid-solid interactions are induced through the mechanical forces in ball mills. This leads to the formation of highly active, metastable catalytic sites and short-lived hot spots that can reach 1100 K for a limited time. Selective hydrodeoxygenation (HDO) of aromatic oxygenates is achieved over reducible ceria-zirconia catalysts with defect sites that can absorb oxygen atoms from organic reactants. While these catalysts show high HDO activity for multifunctional oxygenates, their activity for removing the last phenol group from an aromatic ring is much lower. Lastly, alkyl side chains are removed from phenolics in zeolite-catalyzed cracking reactions. In contrast to the well-established cracking process of hydrocarbons, the presence of phenol groups results in an additional deactivation route due to chemisorbed species that needs to be mitigated.

Multistep process for converting lignin to phenol

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.