Chemical and structural changes associated with Cu-catalyzed alkaline-oxidative delignification of hybrid poplar

Abstract

BackgroundAlkaline hydrogen peroxide pretreatment catalyzed by Cu(II) 2,2′-bipyridine complexes has previously been determined to substantially improve the enzymatic hydrolysis of woody plants including hybrid poplar as a consequence of moderate delignification. In the present work, cell wall morphological and lignin structural changes were characterized for this pretreatment approach to gain insights into pretreatment outcomes and, specifically, to identify the extent and nature of lignin modification.ResultsThrough TEM imaging, this catalytic oxidation process was shown to disrupt cell wall layers in hybrid poplar. Cu-containing nanoparticles, primarily in the Cu(I) oxidation state, co-localized with the disrupted regions, providing indirect evidence of catalytic activity whereby soluble Cu(II) complexes are reduced and precipitated during pretreatment. The concentration of alkali-soluble polymeric and oligomeric lignin was substantially higher for the Cu-catalyzed oxidative pretreatment. This alkali-soluble lignin content increased with time during the catalytic oxidation process, although the molecular weight distributions were unaltered. Yields of aromatic monomers (including phenolic acids and aldehydes) were found to be less than 0.2 % (wt/wt) on lignin. Oxidation of the benzylic alcohol in the lignin side-chain was evident in NMR spectra of the solubilized lignin, whereas minimal changes were observed for the pretreatment-insoluble lignin.ConclusionsThese results provide indirect evidence for catalytic activity within the cell wall. The low yields of lignin-derived aromatic monomers, together with the detailed characterization of the pretreatment-soluble and pretreatment-insoluble lignins, indicate that the majority of both lignin pools remained relatively unmodified. As such, the lignins resulting from this process retain features closely resembling native lignins and may, therefore, be amenable to subsequent valorization.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0300-5) contains supplementary material, which is available to authorized users.

Performance of three delignifying pretreatments on hardwoods: hydrolysis yields, comprehensive mass balances, and lignin properties

“…charkoviensis × caudina cv. NE-19, was obtained from 18-year-old trees grown at the University of Wisconsin Arlington Agricultural Experiment Station (Arlington, WI) and harvested in 2011 and is identical to the feedstock used in our prior work [39,41]. Hybrid poplar logs were initially debarked and chipped prior to subsequent milling.…”

“…Building on these previous studies, the goal of the work presented here was to compare the performance of three pretreatments: (1) an acidic solvolysis pretreatment employing THF and water co-solvents (co-solventenhanced lignocellulosic fractionation, CELF) [28][29][30][31][32] (2) a high-solid loading pretreatment with the ionic liquid cholinium lysinate ([Ch][Lys]) that has the potential to be derived from lignocellulosic biomass [33][34][35][36][37], and (3) two-stage Cu-catalyzed alkaline hydrogen peroxide pretreatment (Cu-AHP) utilizing an alkaline pre-extraction followed by an Cu-catalyzed alkaline-oxidative stage [38][39][40][41][42]. The efficacy of these three pretreatments was evaluated on two different hardwood feedstocks, a hybrid poplar and a eucalyptus, and the impact of pretreatment on hydrolysis yields and lignin properties, including the lignin's susceptibility to depolymerization, was assessed.…”

Alternatives for Chemical and Biochemical Lignin Valorization: Hot Topics from a Bibliometric Analysis of the Research Published During the 2000–2016 Period

“…The composition, structure and functional groups of lignin can be determined by spectroscopic techniques. Lignin characterization has been improved by recent developments in Fourier-transform infrared (FT-IR) and 1D and 2D nuclear magnetic resonance (NMR) spectroscopy [172][173][174][175][176]. Among these methods, 2D NMR spectroscopy is the most powerful tool [177].…”

Nucleophilic Thiols Reductively Cleave Ether Linkages in Lignin Model Polymers and Lignin

“…All lignin used in this study, unless otherwise stated, was isolated via the coppercatalyzed alkaline hydrogen peroxide (Cu-AHP) pretreatment of poplar. [23] This lignin is slightly oxidized at the α position. [24] Lignin reaction conditions were optimized, testing various solvents (Figures S9 and S10), reaction times ( Figure S11), thiols ( Figures S15-S17), and work-up procedures ( Figures S12-S14 and Figure S24) to optimize conditions for depolymerization and analysis.…”

“…With the above reaction parameters optimized, Cu-AHP lignin [23] was stirred in the presence of powdered K 2 CO 3 with either neat BME (Figures 3a and S15), neat DTT ( Figure S15), or neat 1,3-propanedithiol ( Figures S23a,c) at 100°C for 24 h. For molecular weight analysis, mixtures were diluted with water to solubilize the base and lignin (see Supporting Information, page S7). Large decreases in molecular weight were observed when Cu-AHP lignin was depolymerized with BME (Figure 3a, c) and DTT (Figures S15, S16) despite these thiols being less Gel permeation chromatograms of A) Cu-AHP lignin before and after treatment with BME, B) Cu-AHP lignin oxidized with Bobbitt's salt before and after treatment with BME.…”