Reductive Catalytic Fractionation of Corn Stover Lignin

Abstract

Reductive catalytic fractionation (RCF) has emerged as an effective biomass pretreatment strategy to depolymerize lignin into tractable fragments in high yields. We investigate the RCF of corn stover, a highly abundant herbaceous feedstock, using carbon-supported Ru and Ni catalysts at 200 and 250 °C in methanol and, in the presence or absence of an acid cocatalyst (H 3 PO 4 or an acidified carbon support). Three key performance variables were studied: (1) the effectiveness of lignin extraction as measured by the yield of lignin oil, (2) the yield of monomers in the lignin oil, and (3) the carbohydrate retention in the residual solids after RCF. The monomers included methyl coumarate/ferulate, propyl guaiacol/syringol, and ethyl guaiacol/syringol. The Ru and Ni catalysts performed similarly in terms of product distribution and monomer yields. The monomer yields increased monotonically as a function of time for both temperatures. At 6 h, monomer yields of 27.2 and 28.3% were obtained at 250 and 200 °C, respectively, with Ni/C. The addition of an acid cocatalysts to the Ni/C system increased monomer yields to 32% for acidified carbon and 38% for phosphoric acid at 200 °C. The monomer product distribution was dominated by methyl coumarate regardless of the use of the acid cocatalysts. The use of phosphoric acid at 200 °C or the high temperature condition without acid resulted in complete lignin extraction and partial sugar solubilization (up to 50%) thereby generating lignin oil yields that exceeded the theoretical limit. In contrast, using either Ni/C or Ni on acidified carbon at 200 °C resulted in moderate lignin oil yields of ca. 55%, with sugar retention values >90%. Notably, these sugars were amenable to enzymatic digestion, reaching conversions >90% at 96 h. Characterization studies on the lignin oils using two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance and gel permeation chromatrography revealed that soluble oligomers are formed via solvolysis, followed by further fragmentation on the catalyst surface via hydrogenolysis. Overall, the results show that clear tradeoffs exist between the levels of lignin extraction, monomer yields, and carbohydrate retention in the residual solids for different RCF conditions of corn stover.

The current and emerging sources of technical lignins and their applications

“…The efficiency of reductive catalytic fractionation was demonstrated in a ‘lignin‐first approach‘ with corn stover as the substrate . The milled corn stover was digested with H 2 and Ni or Ru catalyst in MeOH.…”

A Bibliometric Study of Scientific Publications regarding Hemicellulose Valorization during the 2000–2016 Period: Identification of Alternatives and Hot Topics

“…A promising approach that has been identified is the combination of traditional chemical pretreatments and transition-metal catalysis [128]. In these catalytic fractionation processes, lignocellulosic biomass is treated at high temperatures (160-230 • C) in water, classical organosolv solvent (with methanol and ethanol as the most typical examples), or a mixture of both in the presence of a heterogeneous transition metal-based catalyst, such as Ni, Ru, Rh, Pt, Pd, or Cu [129][130][131][132][133]. These processes can separate all the three main lignocellulose components, where cellulose is retained as a solid and is delignified to a high degree, the hemicellulose is partially solubilized, and lignin is selectively converted to a set of monomers and oligomers that together form a separate phase of "lignin oil" that can be easily fractionated by distillation.…”

Bifunctional Molybdenum Polyoxometalates for the Combined Hydrodeoxygenation and Alkylation of Lignin‐Derived Model Phenolics

“…The first step in lignocellulosic biomass processing involves removing lignin by chemical processes. [11][12][13][14][15][16] RCF is typically performed in the presence of protic solvents at temperatures ranging from 453-523K using supportedt ransition metal catalysts (e.g.,r uthenium, palladium, or nickel) and hydrogen gas or other hydrogen transfer agents (e.g.,m ethanol,i sopropyl alcohol, or formic acid) as reductants. Technical lignin is burned because it contains highly recalcitrant CÀC linkagest hat render the material unsuitablef or selectived epolymerization to monomers.…”