Depolymerisation-hydrogenation of condensed tannins as a strategy for generating flavan-3-ol monomers

Abstract

Flavan-3-ol monomers are interesting, functionalised phenolic building blocks. Here, a strategy is presented for producing these flavan-3-ol monomers, starting from condensed tannins present in biomass waste streams such as tree bark. Using purified procyanidin condensed tannins as a reference substrate, a one pot depolymerisation-hydrogenation process was developed. Briefly, procyanidin condensed tannins were processed in methanol at elevated temperature (125-175 °C) in the presence of hydrogen gas and a heterogeneous hydrogenation catalyst (Pd/C). The elevated temperature enables a solvolytic depolymerisation of the condensed tannins, forming reactive intermediates which are immediately stabilised through catalysed hydrogenation. The flavan-3-ol monomers catechin and epicatechin are the two main products, with a combined yield of up to 34 wt%. Furthermore, the role of the hydrogenation catalyst is elucidated, and the effect of reaction temperature, hydrogen pressure and catalyst loading is studied. Finally, the successful depolymerisation-hydrogenation of a crude methanolic larch bark extract demonstrated the feasibility of this process and its robustness against impurities.