Reductive catalytic fractionation of black locust bark

Abstract

Tree bark is a major waste stream from the wood processing industries, and thus an alluring feedstock for biorefineries. This contribution explores a catalytic biorefining strategy for black locust (Robinia pseudoacacia) bark. In this Reductive Catalytic Fractionation (RCF) strategy, bark is processed in methanol at elevated temperatures (200–250 °C) in the presence of a heterogeneous hydrogenation catalyst (e.g. Ru/C) and pressurized hydrogen. This enables the (partial) extraction and solvolytic depolymerization of lignin, suberin and hemicellulose which are present in bark. The formed lignin intermediates are effectively stabilized via catalytic hydrogenation, thereby avoiding repolymerization reactions and thus enabling the formation of a highly depolymerized lignin fraction. RCF of black locust bark results in a solid phase (mainly cellulose and residual lignin) and a process liquor. The soluble products were further separated via liquid/liquid extraction into an aqueous phase, containing glycerol and carbohydrate derivatives, and an organic phase, containing phenolic mono-, di- and oligomers as well as suberin-derived, long-chain aliphatic monomers (mainly α,ω-dimethyl esters and ω-OH methyl esters, next to alkanols and methyl esters). A comparison with black locust wood (hardwood) was made, highlighting the presence of suberin and the more condensed lignin structure in bark. Furthermore, this contribution examines the effect of various process parameters (e.g. temperature, reaction time, catalyst type and loading) on the extraction and depolymerization of lignin and suberin polymers. Finally, a heptane extraction was executed on the bark oil (i.e. dried organic phase) as a proof-of-concept separation of the suberin-derived aliphatics on one hand and the lignin phenolics on the other.