Pretreatment of beechwood with polyoxometalate-based catalysts towards the production of polyunsaturated fatty acids by Crypthecodinium cohnii

Abstract

In the present work, a novel OxiOrganosolv pretreatment was developed for the fractionation of beechwood. Τhe process employs a mixture of water and common organic solvents, polyoxometalate complexes (POMs) as homogeneous catalysts and O2 that acts as an oxidative agent enabling their regeneration. Due to their strong oxidizing ability and redox reversibility, POMs can achieve efficient biomass fractionation by catalyzing simultaneously hemicellulose hydrolysis and oxidation of lignin ether bonds that leads to its depolymerization, dissolution in the organic solvent and removal from the cellulose-rich pulp. According to the results, both commercially available phosphomolybdic acid (HPMo) and also the in-house produced Fe- and Cu- exchanged salts successfully promoted delignification and hemicellulose removal compared to the OxiOrganosolv pretreatment in the absence of catalyst. Most importantly, catalytic lignin depolymerization was achieved without the use of mineral acids that are typically used in a traditional organosolv. Evaluation of the enzymatic digestibility of the pulps revealed HPMo as the most favorable catalyst for increased saccharification yields and production of sugar-rich hydrolysates that were further used as carbon sources for fermentation and production of ω-3 polyunsaturated fatty acids (ω-3 PUFAs) from the heterotrophic microalgae Crypthecodinium cohnii. Hydrolysates produced from HPMo and Fe-PMo-assisted OxiOrganosolv pretreatment supported cell growth and led to high accumulation of ω-3 PUFAs, while severe inhibition was observed with the hydrolysates from Cu-PMo pretreated biomass. This study demonstrates the possibility of using POMs as catalysts for efficient biomass delignification with the aim to obtain sugar-rich streams that can be utilized for the production of nutraceuticals.