Effective Biomass Fractionation through Oxygen-Enhanced Alkaline–Oxidative Pretreatment

Abstract

The high recalcitrance of plant cell walls is an obstacle for effective chemical or biological conversion into renewable chemicals and transportation fuels. Here, we investigated the utilization of both oxygen (O₂) and hydrogen peroxide (H₂O₂) as co-oxidants during alkaline–oxidative pretreatment to improve biomass fractionation and increase enzymatic digestibility. The oxidative pretreatment of hybrid poplar was studied over a variety of conditions. Employing O₂ in addition to H₂O₂ as a co-oxidant during the two-stage alkaline pre-extraction/copper-catalyzed alkaline hydrogen peroxide (Cu-AHP) pretreatment process resulted in a substantial improvement in delignification relative to using H₂O₂ alone during the second-stage Cu-AHP pretreatment, leading to high overall sugar yields even at H₂O₂ loadings as low as 2% (w/w of the original biomass). The presence of H₂O₂, however, was both critical and synergistic. Performing analogous reactions in the absence of H₂O₂ resulted in approximately 25% less delignification and 30% decrease in sugar yields. The lignin isolated from this dual oxidant second stage had high aliphatic hydroxyl group content and reactivity to isocyanate, indicating that it is a promising substrate for the production of polyurethanes. To test the suitability of the isolated lignin as a source of aromatic monomers, the lignin was subjected to a sequential Bobbitt’s salt oxidation followed by a formic acid-catalyzed depolymerization process. Monomer yields of approximately 17% (w/w) were obtained, and the difference in yields was not significant between lignin isolated from our Cu-AHP process with and without O₂ as a co-oxidant. Thus, the addition of O₂ did not lead to significant lignin crosslinking, a result consistent with the two-dimensional heteronuclear single-quantum coherence NMR spectra of the isolated lignin.