Alkaline Oxidative Pretreatment followed by Reductive Lignin Depolymerization

Transcription

1 Alkaline Oxidative Pretreatment followed by Reductive Lignin Depolymerization Eric L. Hegg Michigan State University Great Lakes Bioenergy Research Center (GLBRC) Michigan Forest Bioeconomy Conference February 12-13,

2 Great Lakes Bioenergy Mission & Goals Sustainable production of fuels & chemicals from energy crops grown on marginal lands Develop high yielding and high value dedicated bioenergy crops Improve efficiency of biomass conversion into fuels and products 2

3 Two-Stage Cu-catalyzed Alkaline Hydrogen Peroxide Pretreatment (Cu-AHP) Stage 1 Alkaline Extraction Raw Biomass (30 o C) NM6 Hybrid Poplar Stage 2 1. NaOH 2. Cu(bpy) 3. H 2 O 2 Extracted Biomass (30 o C) Cu-AHP Biomass Biotechnol. Biofuels 2013, 6, e119 Biotechnol. Bioeng. 2013, 110, Biotechnol. Biofuels 2016, 9, e34

4 Confocal Microscopy Reveals Changes in lignin and cellulose Accessibility Shi-You Ding ACS Sustainable Chem. Eng. 2018, 6, 2932

5 Lignin: An Underutilized Product Cu-AHP released 40% or more of the lignin from poplar Recovered Lignin Biotechnol. Biofuels 2016, 9, e34 Cu-AHP lignin stream is largely unmodified

6 Lignin Stream Is Slightly Oxidized Untreated Hybrid Poplar Cu-AHP Solubilized Lignin In collaboration with John Ralph and Ali Azarpira (UW-Madison)

7 Enzymatic β-o-4 Cleavage with Glutathione

8 Organic Thiol β-o-4 Cleavage without Proteins

9 Biological Vs. Chemical Reduction Requires an enzymatic system to cleave Cleaves soluble lignin dimers Requires a reductase system to regenerate glutathione Can cleave dimers and the lignin polymer Unassisted by proteins or metals Diffusible small molecule redox carrier Readily coupled to electrochemistry

10 % Yield Role of Glutathione in Lig Enzymes PDB: 4PUA, 4MZW hrs Phenol Acetophenone 2-Phenoxyacetophenone

11 % Yield % Yield Simple Lignin Models Are Cleaved via a 2-Step Process Complete Cleavage of 2-Step Process 2 nd Step of Cleavage Rxn hrs Phenol Acetophenone 2-Phenoxyacetophenone 2-step cleavage demonstrated Nucleophilic cleavage product and thioether formation Reductive cleavage product and disulfide formation Initial attack by the thiol on the α-carbon is the rate-determining step hrs Acetophenone Thioether

12 Other Requirements for Cleavage Increasing Reactivity Aryl ketone is not needed Aryl ether is needed Alpha OH must be oxidized (same as enzymatic pathway) Thiol cleavage only occurs at keto aryl ethers

13 Reactivity with Ring Substituents Increasing Reactivity Lignin-like functionality on ring decreases reactivity Thiols can demethylate methoxy group under certain conditions. Does this occur in our system?

14 Reactivity with Ring Substituents Increasing Reactivity Lignin-like functionality on ring decreases reactivity Thiols can demethylate methoxy group under certain conditions. Does this occur in our system? GCMS shows no catechol or methylthiol species

15 Molecular Weight (g/mol) Mass Loss (%) Cu-AHP Lignin Is Depolymerized Molecular weight (Mp) decreases by >60% Lignin polymer abundance decreased Mass balance suggests solubilization of cleavage fragments Molecular Weight of Thiol-Treated Lignin 100 % Mass Loss of Thiol-Treated Lignin Lignin ph9 DTT ph9 BME ph9 0 Neat DTT Neat BME Mn Mw Mp

16 % unit in each sample Lignin Analytical Technique Shows High Release of Monomers Lignin BME DTT 1,3-Propanedithiol Thiophenol S/G Ratio of Thiol-Treated Lignin S/G % Monomer Released from Lignin Polymer 25 Lignin 20 BME DTT 15 1,3-propanedithiol 10 thiophenol 5 0 Syringyl avg Guaiacyl avg HydroxyPhenyl avg Little difference in S/G ratio suggests no preference for S/G cleavage Thioacidolysis uses thiols and acid to measure the S and G content Lignin monomers are released during thiol-treatment

17 Conclusion Cu-AHP is a highly effective pretreatment for woody biomass Lignin generated from Cu-AHP process is uniquely susceptible to depolymerization via thiol-mediate cleavage of the β-o-4 bonds Thiols can cleave oxidized aryl ether bonds without the assistance of proteins or metals in a 2-step process Thiols can cleave whole lignin by diffusing through the polymer

18 Acknowledgments David Hodge Ned Jackson Aditya Bhalla Namita Bansal Grace Klinger Funding: DE-FC02-07ER64494 DE-SC

19 Conclusion Cu-AHP is a highly effective pretreatment for woody biomass Lignin generated from Cu-AHP process is uniquely susceptible to depolymerization via thiol-mediate cleavage of the β-o-4 bonds Thiols can cleave oxidized aryl ether bonds without the assistance of proteins or metals in a 2-step process Thiols can cleave whole lignin by diffusing through the polymer

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21 Cysteine-Type Mediator % yield % yield Glutathione is a tripeptide Cysteine performs the S N 2 Reaction Try cysteine type mediator hrs phenol acetophenone 2-phenoxyacetophenone Solubility issue Only dissolves in water hrs phenol acetophenone 2-phenoxyacetophenone

22 % Yield Which Organic Thiols Are Effective? Small, non-bulky thiols Soluble in organic solvent pk a ~ Phenol hrs Acetophenone 2-Phenoxyacetophenone Increasing Reactivity

23 % yield β-site Requirements Increasing Reactivity S N 2 favors 1 > 2 > 3 sites. Thiol cleavage reaction follows S N 2 rules hrs Phenol Methyl hydroxy may aide in cleavage via hydrogen bonding Dimer