Chemical transformations of poplar lignin during co-solvent enhanced

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Chemical transformations of poplar lignin during cosolvent enhanced lignocellulosic fractionation process Xianzhi Meng, Aakash Parikh, Nikhil Nagane, Bhogeswararao Seemala, Rajeev Kumar, Charles M. Cai, Yunqiao Pu, Charles E. Wyman, and Arthur Jonas Ragauskas ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01028 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Sustainable Chemistry & Engineering

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Chemical Transformations of Poplar Lignin during Co-Solvent

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Enhanced Lignocellulosic Fractionation Process

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Xianzhi Meng†, Aakash Parikh‡,§, Bhogeswararao Seemala‡,§, Rajeev Kumar‡, Yunqiao Pu⊥, Phillip Christopher§,#, Charles E Wyman‡,§, Charles M. Cai‡,§, Arthur J. Ragauskas*,†,⊥,∥ †

Department of Chemical & Biomolecular Engineering, University of Tennessee Knoxville, Knoxville, TN 37996, USA ‡ Bourns College of Engineering – Center of Environmental and Research Technology (CE-CERT), University of California, Riverside, CA 92507, USA § Department of Chemical and Environmental Engineering, Bourns College of Engineering, University of California, Riverside, CA 92521, USA # Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106, USA ⊥ Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ∥Department of Forestry, Wildlife, and Fisheries; Center for Renewable Carbon, The University of Tennessee Knoxville, Institute of Agriculture, Knoxville, TN 37996, USA

Corresponding Author: * Arthur J. Ragauskas. Fax: 865-974-7076; Tel: 865-974-2042; E-mail: [email protected]

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Converting lignocellulosic biomass to ethanol is significantly hindered by the innate

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recalcitrance of biomass, and it usually requires a pretreatment stage prior to enzymatic

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hydrolysis. A promising novel pretreatment named Co-Solvent Enhanced Lignocellulosic

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Fractionation (CELF) that involves moderate temperature reactions with a dilute acid in

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THF-water mixture was recently developed to overcome biomass recalcitrance. Detailed

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elucidation of physicochemical structures of the fractionated lignin that is precipitated after

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CELF pretreatment, also called CELF lignin, reveal transformations in its molecular weights,

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monolignol composition, and hydroxyl groups content. Isolated CELF lignin revealed dramatic

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reductions in its molecular weight by up to ~90% compared to untreated native lignin.

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Furthermore, CELF lignin’s β-O-4 interunit linkages were extensively cleaved after CELF

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pretreatment as indicated by a semi-quantitative HSQC NMR analysis. This is further evidenced

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by a 31P NMR analysis showing a significant decrease in aliphatic OH groups due to the

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oxidation of lignin side chains, whereas the content of total phenolic OH groups in CELF lignin

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significantly increased due to drastic cleavage of interunit linkages. In conclusion, CELF process

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generated a uniquely transformed pure lignin feedstock of low content aryl ether linkages, low

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molecular weight, and high amount of phenolic hydroxyl groups, suitable for its development

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into fuels, chemicals, and materials.

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KEYWORDS

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Biomass recalcitrance, Co-Solvent Enhanced Lignocellulosic Fractionation, Antioxidant, Lignin

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Valorization 2


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INTRODUCTION Lignocellulosic biomass represents a promising sustainable platform to fossil resources.1 Long

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term driving forces including environment concerns and energy security have promoted

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worldwide biofuel development.2 Nevertheless, there are still significant challenges associated

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with biofuel commercialization. Biomass recalcitrance represents one of the critical challenges

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for biological biorefining of biomass, therefore it is vital to develop cost-effective pretreatment

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techniques to enhance sugar release performance through enzymatic hydrolysis. Over the past

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decade, various pretreatment technologies have been developed to overcome biomass

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recalcitrance and each pretreatment has its own target, but all have been proven to change the

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plant cell wall structure.3, 4 Lignin, a three dimensional cross-linked polyphenolic polymer, has

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been considered as the most recalcitrant component of the three primary biopolymers present in

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plant cell walls for the conversion of biomass to biofuels.5 With its rich polyaromatic structure,

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lignin has significant potentials to serve as a sustainable feedstock for the production of

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renewable chemicals and fuels.6 Thus, many pretreatments have been developed and modified

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with an effort of removing or at least redistributing lignin across the plant cell wall.7

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Organosolv pretreatment is a process that usually involves addition of organic solvents such as

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ethanol, methanol, or acetone as modifiers or co-solvents to the biomass under specific

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conditions of pressure and temperature in the presence of an acid.7 Recently, tetrahydrofuran

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(THF) has been identified as a new multifunctional solvent for enhancing the pretreatment of

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biomass to obtain higher yields of sugars and other fuel precursors as well as improving the

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overall utilization of biomass for conversion to fuels. Traditionally, THF was used to dissolve 3



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acetylated lignin for molecular weight analysis in biomass research.8 Notably, THF can be

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considered as a renewable solvent as it can be manufactured from maleic anhydride,

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1,4-butanediol or furfural that are catalytically produced from C5 sugars.9-11 THF is also

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inherently biodegradable, thus it is not expected to be environmentally persistent.12 Co-Solvent

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Enhanced Lignocellulose Fractionation (CELF) pretreatment technology was developed as a new

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generation pretreatment that involves reacting biomass with dilute acid in mixtures of

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THF-H2O.13-15 At reaction temperatures of 170 oC or higher, CELF pretreatment could produce

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high yields of fuel precursors including furfural (FF), 5-hydroxymethylfurfural (5-HMF), and

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levulinic acid (LA) directly from biomass.13 For example, employing dilute FeCl3 in CELF

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reaction of hardwood chips resulted in co-production yields of >90% FF and >50% 5-HMF

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simultaneously.16 At milder temperatures below 160 oC, CELF can be used to achieve nearly

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complete recovery (>95%) of fermentable C5 and C6 sugars from biomass if the pretreated

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solids were subsequently treated using enzymes at extremely low dosages.14 Combining CELF

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pretreatment with simultaneous saccharification and fermentation (SSF) dramatically improved

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ethanol fermentation yields at high biomass solids loadings.17 In all cases, between 85-95% of

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the acid insoluble “Klason” lignin can be removed from biomass during CELF pretreatment,

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resulting in the precipitation of a very clean lignin product, also called CELF lignin, from the

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liquid phase after recovery of the THF by low-temperature vacuum distillation.13, 14, 16

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With all of the potential applications of lignin as renewable sources for production of fuels,

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materials, and chemicals, understanding its fundamental characteristics is of significant

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importance.6 During dilute acid or aqueous hydrothermal pretreatments, lignin deposits typically 4


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onto cellulose surface as droplets.18-20 On the other hand, it has been reported that lignin adopts

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extended coil configurations during CELF process and is preferentially solvated by THF as

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determined by all-atom molecular-dynamics (MD) simulation.21 A further replica exchange MD

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simulation study by Smith et al. showed that lignin was a coil regardless of being in the miscible

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or immiscible temperature region, suggesting CLEF pretreatment may be performed at lower

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temperatures and pressures.22 It was reported that CELF pretreatment increased S lignin fraction

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and reduced G lignin in solid residue for poplar and corn stover, and in fact, CELF pretreatment

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almost completely removed corn stover G units.23 However, the detailed structural

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characterization of CELF lignin which is the lignin dissolved in THF, accounting for over 80%

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of lignin in raw material, has not been reported prior to this paper to the best of our knowledge.

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In this study, the structure of poplar lignin extracted from CELF pretreatment at different

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conditions was determined following well-established methodologies including GPC and NMR,

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and the results were compared to the native (untreated) lignin to determine the exact effect of

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CELF process on the chemical structure of lignin.

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MATERIALS AND METHODS

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Feedstocks and Chemicals. Poplar (Populus trichocarpa x deltoids) was provided by

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National Renewable Energy Laboratory (NREL, Golden, CO) and size reduced in a laboratory

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mill (Model 4, Arthur H. Thomas Company, Philadelphia, PA) to obtain less than a 1 mm

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particle size. The composition of untreated poplar was measured to be ~45% glucan, ~14% xylan,

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and ~22% Klason lignin using NREL laboratory analytical procedure. Ferric chloride was

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purchased from Sigma Aldrich (St. Louis, MO, US) and reagent grade THF was purchased from 5



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Fisher Scientific (NJ, US). All the chemicals used in this study were used as received without

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any further purification.

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Co-solvent Enhanced Lignocellulosic Fractionation Pretreatment. CELF pretreatments at

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different severities were performed in a 1 L Parr reactor (236HC Series, Parr instruments

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Company, Moline, IL) equipped with 6-blade impellers operating at 200 rpm driven by an

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electric motor. The Parr reactor was heated using a 4 kW fluidized sand batch (Model SBL-2D,

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Techne Princeton, NJ, US), and the temperature was measured by an in-line thermocouple

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(Omega, K-type).13 The co-solvent mixture was prepared by adding THF to water in increasing

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volumes starting from 1:1 (THF 50% v/v) to 7:1 (THF 87.5% v/v) THF-to-water ratios. Biomass

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and acid catalyst (FeCl3 or H2SO4) loadings based on the total mass of the reaction mixture and

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anhydrous mass were 5 wt% and 1 wt%, respectively. After pretreatment, the reactor was

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quenched in a water bath at room temperature. The hydrolysate was then separated from the

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solids using vacuum filtration through a glass fiber filter paper (Fischer Scientific, Pittsburg,

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PA). The CELF pretreatment conditions applied in this work are shown in Table 1.

137 138 139 140 141 142 143 6


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Table 1. List of pretreatment conditions applied in this work for CELF process of poplar. Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Temperature (oC) 160 180 170 170 180 180 180 180 180 180 180 180 180 180 180

Time (min) 15 20 30 60 20 20 20 20 20 20 20 20 20 15 30

Catalyst loading (M) 0.1 H2SO4 0.1 H2SO4 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.1 FeCl3 0.05 FeCl3 0.15 FeCl3 0.1 FeCl3 0.1 FeCl3

THF: water (v:v) 1:1 4:1 3:1 3:1 1:1 2:1 3:1 4:1 5:1 6:1 7:1 4:1 4:1 3:1 3:1

145 146

CELF lignin isolation. CELF lignin was isolated from the liquid product by room

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temperature vacuum distillation according to literature procedures (Scheme 1).16 Once the THF

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was removed from the aqueous phase, the precipitated sticky dark resinous solid lignin was

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rinsed with water and diethyl ether to remove non-lignin soluble impurities. The purified lignin

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was then dried at 40 oC for 15 h and the dry weight was measured by a moisture content

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analyzer. The composition of isolated lignin was determined according to a NREL analytical

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procedure.24 All the CELF lignin samples were almost carbohydrate free, and presented a high

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total lignin content above 90%. The detailed composition analysis of CELF lignin are presented

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in Table S1 in supporting materials.

7



Biomass THF:Water Acid Catalyst

CELF pretreatment

Solid-liquid Separation

Cellulose rich solids

Vacuum distillation

Liquid hydrolysate

155 156 157

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Water

Precipitated sticky dark resinous lignin

Filtration Filtration and diethyl ether wash

Dried at 40 oC for 15 h

Solid lignin

Dry lignin powder

Scheme 1. Schematic diagram of CELF lignin isolation. Cellulolytic Enzyme Lignin Isolation. Cellulolytic enzyme lignin (CEL) was isolated from

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poplar according to a modified method.25 In brief, poplar was Soxhlet-extracted with

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toluene/ethanol for 8 h. The extractives-free poplar was then ball-milled in a porcelain jar with

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ceramic balls via Retsch PM 100 (Newton, PA) at 600 rpm for 2.5 h followed by enzymatic

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hydrolysis in acetate buffer (pH 4.8, 50 °C) using Cellic CTec 2 cellulase and HTec 2

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hemicellulase (1:1, mass ratio), as the enzymes (150 mg protein loading/g biomass) for 48 h. The

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solid residue was isolated by centrifugation and hydrolyzed again with freshly added buffer and

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enzymes at same loadings. To remove any remaining cellulases, the lignin-rich residue was then

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treated with Streptomyces griseus protease (Sigma-Aldrich) at 37 °C overnight followed by a 10

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min deactivation at 100 °C. After filtration, the solid residue was extracted twice with 96% (v/v)

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p-dioxane/water mixture at room temperature for 48 h. The extracts were combined, rotary

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evaporated, and freeze-dried to recover CEL.

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Lignin Molecular Weight Distribution Analysis. The molecular weight analysis was

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performed as previously described.26 Oven-dried lignin samples (~5 mg) were acetylated with a

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2.0 mL acetic anhydride/pyridine (1:1, v/v) mixture at room temperature. After 24 h, ethanol was

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added to the reaction and left for 30 min. The solvent was then subjected to rotary evaporation 8


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under reduced pressure. The whole process was repeated twice until all the acetic acid was

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removed from the solution. The derivatized lignin was then dried at 45 °C overnight in a VWR

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1400E vacuum oven and then dissolved in THF (1 mg/mL) overnight prior to the GPC analysis.

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The molecular weight analysis was analyzed by an Agilent GPC SECurity 1200 system equipped

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with four Waters Styragel columns (HR1, HR2, HR4, and HR6) and a UV detector (270 nm).

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Polystyrene narrow standards were used to prepare the calibration curve, and THF was used as

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the mobile phase at a flow rate of 1.0 mL/min. The injection volume was set at 30 µL. The

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number-average and weight-average molecular weights were calculated using a WinGPC Unity

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software. While polystyrenes are widely used as standards to generate the relative calibration

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curve in GPC analysis, high molecular weight (MW) polystyrene standard and high MW lignin

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are not necessary eluted at the same elution volume, and the molecular weight should be taken

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with a measure of caution.

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Lignin NMR Characterization. Two-dimensional 13C-1H HSQC and 31P NMR experiments

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were both acquired with a Bruker Avance 400 MHz spectrometer according to a previously

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published literature.26 A standard Bruker pulse sequence was used under the following

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conditions: 210 ppm spectral width in F1 (13C) dimension with 256 data points and 11 ppm

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spectral width in F2 (1H) dimension with 1024 data points, a 90° pulse, a 1JC-H of 145 Hz, a 1.5 s

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pulse delay, and 32 scans. ~50 mg of dry lignin samples were dissolved in deuterated DMSO for

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HSQC experiments. Quantitative 31P NMR spectra were acquired using an inverse-gated

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decoupling pulse sequence (Waltz-16), 90° pulse, 25 s pulse delay with 128 scans. ~20.0 mg of

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lignin samples were dissolved in a solvent mixture of pyridine and deuterated chloroform 9



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(1.6/1.0, v/v, 0.50 mL). The lignin solution was then further derivatized with 0.075 mL

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2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP). Chromium acetylacetonate and

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endo-N-hydroxy-5-norbornene-2,3-dicarboximide (NHND) were also added to the solution as

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relaxation agent and internal standard, respectively. All the data was processed using the

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TopSpin 2.1 software (Bruker BioSpin).

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Error Analysis. Error bars shown in each figure represent standard error which is the standard

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deviation of the sampling distribution of a statistic, defined as the standard deviation divided by

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the square root of the sample size – three independent assays unless otherwise specified.

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RESULTS AND DISCUSSION

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Molecular Weight Analysis. Lignin molecular weight is an important index to evaluate the

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physical properties of lignin. The weight-average (Mw) and number-average (Mn) molecular

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weight of CELF lignin samples are presented in Figure 1. The Mn and Mw of CEL isolated from

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untreated poplar were 2850 and 11850 g/mol, respectively. As shown in Fig. 1, CELF lignin

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fraction had significantly lower Mw and Mn compared to CEL regardless of pretreatments

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conditions, suggesting dramatic depolymerization of lignin occurred during the CELF

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pretreatments. CELF lignin samples also showed relatively lower polydispersity index (PDI)

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compared to that of CEL (4.16). This indicated that CELF pretreatment led to relatively narrower

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molecular weight distributions in soluble lignin. This is in agreement with other data reported

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that organosolv lignin mainly contains small molecular weight fractions compared to untreated

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native lignin.27-30 In order to understand the effect of pretreatment conditions (temperature, time,

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THF/water ratio, acid catalyst, etc.) on molecular weights distribution of CELF lignin, certain 10


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samples were further compared with each other. By comparing sample 7 with 14 and 15, it was

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concluded that extending the pretreatment time from 15 to 30 min at 180 oC did not have any

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significant impact on the lignin molecular weight, indicating that both depolymerization and

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condensation reactions were equally favored during this period. However, extending the

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pretreatment time from 30 to 60 min, it was found to have a significant impact on the lignin

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molecular weight for CELF pretreatment performed at 170 oC, as GPC results indicated that 60

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min pretreatment led to a ~27.1% decrease of lignin molecular weight compared to 30 min

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pretreatment (sample 3 vs. 4). This suggested that cleavage reactions were favored over

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repolymerization under this particular condition. Given the fact that CELF lignin from trial 7, 14,

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and 15 had lower molecular weight than CELF lignin 3 and 4, it can be concluded that higher

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pretreatment temperature appeared to have a more pivotal effect on lignin molecular weight than

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longer pretreatment time. Comparison of sample 2 with 8 led to the conclusion that H2SO4

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tended to generate lower molecular weight lignin than FeCl3. Increasing pretreatment

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temperature from 170 oC to 180 oC (sample 3 vs. 15) caused a ~34.8% decrease of lignin

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molecular weight while increasing the FeCl3 catalyst concentration from 0.05 to 0.15% (sample

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8, 12, and 13) had a neglect effect on lignin molecular weight. Regarding the THF/water ratio,

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relatively lower molecular weight lignin was obtained using 2:1, 3:1 and 7:1 THF/water ratio,

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therefore although the impact is significant, no obvious trend can be obtained (sample 5 – 11) for

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the impact of THF: water ratio. In conclusion, all lignin samples underwent significant

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degradation during CELF process regardless of pretreatment conditions; pretreatment

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temperature, acid catalyst, and THF/water ratio had important effect on lignin molecular weight, 11



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pretreatment time had moderate effect depending on the pretreatment temperature applied while

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FeCl3 concentration had a negligible effect on CELF lignin molecular weight distribution. (a). 2500

(b). 3000 Mn

Mw

2000

Molecular Weight (g/mol)

Molecular Weight (g/mol)

Mn

1500 1000 500 0

Mw

2500 2000 1500 1000 500 0

CELF Lignin 3

CELF Lignin 4

CELF CELF CELF Lignin 7 Lignin 14 Lignin 15

(c). 2500 Mw

CELF Lignin 1

2000 1500 1000 500

CELF Lignin 2


(d). 14000 Molecular Weight (g/mol)

Mn Molecular Weight (g/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

12000 10000 8000 6000 4000 2000 0

0 CELF Lignin 5

CELF Lignin 6


Mn Mw Cellulolytic Enzyme Lignin

238 239

Figure 1. Weight average (Mw) and number average (Mn) molecular weights of CELF lignin and

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CEL lignin. (a). Effect of pretreatment time and temperature. (b). Effect of catalyst and its

241

loading. (c). Effect of THF/water ratio. (d). Molecular weight of CEL. Sample information is

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presented in Table 1.

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HSQC NMR Analysis. In an effort to further investigate changes in the lignin structure

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during CELF pretreatment, HSQC NMR analysis of selected CELF lignin samples was

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performed. The representative HSQC spectra of CEL and CELF lignin (sample 5, 180 oC, 20 min, 12


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0.1 M FeCl3, 1:1 THF/H2O ratio) are shown in Figure 2. The HSQC NMR spectra showed that

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CELF lignin demonstrated very different structural features compared with poplar CEL. The

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aromatic and aliphatic 13C/1H cross-peaks are assigned according to literature (Table S2).25 As

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expected and consistent with literature, poplar CEL is primarily composed of syringyl (S) and

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guaiacyl (G) units along with considerable amounts of p-hydroxyphenyl benzoate (PB) units.31

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In aromatic regions, the cross peaks associated with S and G units were either reduced in

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intensity or significantly shifted to condensed form in CELF lignin. In particular, the peaks

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associated with condensed G2 and S2/6 were observed at δC/δH 112.4/6.72 ppm and 106.5/6.51

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ppm, respectively, which is consistent with literature.32 In aliphatic regions, signals associated

255

with methoxyl group and β-aryl ether (β-O-4) inter-linkages appeared to be the most prominent

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ones in CEL. The presence of phenylcoumaran (β-5) and resinol subunit (β-β) in CEL were also

257

confirmed by its C-H correlations. However, only trace amount of β-β interunit linkages were

258

detected and the remaining peaks for β-O-4 and β-5 were not apparent in CELF lignin as shown

259

in Fig. 2.

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A semi-quantitative analysis using volume integration of contours in HSQC spectra was

261

further performed to calculate the monolignol compositions (such as S/G ratio) and relative

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abundance of lignin interunit linkages (such as β-O-4, β-β, and β-5). For monolignol

263

compositions of S, G, and PB calculation, S2/6, oxidized and condensed S2/6, G2, condensed G2,

264

PB2/6 contours were used with G2 and condensed G2 integrals doubled. The Cα signals were used

265

for contour integration for the calculation of all the interunit linkages. Table 2 presents the

266

relative contents of lignin subunits as well as interunit linkages for poplar CEL and several 13



267

selected CELF lignin samples. The relative content of total S units (including

268

oxidized/condensed units) decreased while the content of total G units (including condensed

269

units) increased in CELF lignin samples causing a decrease of S/G ratio. The change in PB

270

content upon pretreatment was found to depend on the applied CELF pretreatment conditions.

271

Table 2 also confirms the dramatic cleavage of β-O-4 which is probably the most prominent

272

change in lignin structure following CELF.

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The observed scission of β-O-4, β-β, and β-5 interunit linkages are in part agreement with

274

those observed for conventional organosolv lignin using ethanol, methanol or acetone. It was

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reported that organosolv lignin isolated from ethanol pretreated Miscanthus × giganteus (190 oC

276

and 60 min) had a ~55% decrease of β-O-4 compared with the untreated milled wood lignin

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(MWL).30 Similarly, Hallac et al. reported that ethanol organosolv pretreated Buddleja davidii

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(195 oC and 60 min) had a ~57% decrease in the content of β-O-4 compared to MWL.27 The acid

279

catalyzed cleavage of β-O-4 interunit linkages has been recognized as the major mechanism of

280

lignin depolymerization during acidic organosolv pretreatments.33 However, this type of

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depolymerization is usually accompanied with re-polymerization or condensation reactions,

282

especially under acidic conditions.34 Thus, cleavage of lignin interunit linkages is generally

283

incomplete as there are competing solubilization and condensation pathways through the

284

carbocation intermediate. Herein, we reported a near complete removal of the common lignin

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interunit linkages by an acid-catalyzed organosolv pretreatment using THF as the organic solvent

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at relatively mild temperature (180 oC). The reason that THF can preferentially solvate lignin is

287

that the THF/water mixture can form a “theta” solvent system, in which the solvent-lignin and 14


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lignin-lignin interactions are approximately equivalent in strength, leading to the lignin adopting

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Gaussian random-coil conformations.21 Lignin in this type of coil conformation would not

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self-aggregate, therefore it can be more easily broken down into smaller molecular fractions. The

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reduction of β-5 could be attributed to the formation of stilbenes through the loss of γ-methylol

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groups as formaldehyde.35

15



Page 16 of 26

293 294

Figure 2. The representative HSQC NMR spectra of CEL (Top) and CELF lignin sample 5

295

(Bottom) including aromatic (Left) and aliphatic (Right) regions. S: syringyl; S’: oxidized

296

syringyl. 16


Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


297

Table 2. Semi-quantitative analysis of lignin subunits and interunit linkages in poplar CEL and

298

CELF lignin samples. Lignin samples

CEL

CELF 1

CELF 2

CELF 5

CELF 8

Syringyl

60.9

56.8

44.3

54.2

52.4

Guaiacyl

39.1

43.2

55.7

45.8

47.6

p-hydroxybenzoate 14.3

13.6

17.5

17.7

14.0

S/G ratio

1.6

1.3

0.8

1.2

1.1

β-O-4

61.1

9.2

N/A

N/A

N/A

β-β

4.7

1.9

0.04

0.2

0.07

β-5

5.4

1.8

N/A

N/A

N/A

Lignin subunits (%)

Inter-linkages (%)

299 300

Content (%) expressed as a fraction of S+G. N/A: below detection limit. 31

P NMR Analysis. Lignin OH groups are important characteristics associated with lignin’s

301

negative role in enzymatic hydrolysis of lignocellulosics.36 Quantitatively 31P NMR analysis was

302

used to determine the amount of various types of hydroxyl groups in selected CELF lignin

303

samples, and results are compared to that of the CEL (Figure 3). The chemical shifts and

304

integration regions are summarized in Table S3. Aliphatic OH group is the dominant OH type in

305

poplar CEL which accounts for ~87% of the total free OH groups, and CELF pretreatment

306

resulted in a decrease of the content of aliphatic OH group and an increase of the content of total

17



307

phenolic OH group. Among phenolic OH groups, C5 substituted OH was observed as the most

308

prominent OH type in all the CELF lignin samples. The amount of carboxylic acid group in

309

CELF lignin is also higher than that in CEL, which could be due to the hydrolysis of ester bonds

310

or oxidation of aliphatic OH groups during CELF pretreatment.29, 30 The dramatic increase of the

311

content of phenolic OH could be a result of β-O-4 interunit linkages scission as evidenced by the

312

HSQC analysis. The significant decrease of aliphatic hydroxyl groups could be attributed to the

313

loss of γ-methylol group as formaldehyde and OH groups on Cα,β or the whole side chain in

314

general to from stilbene structures ultimately.27 CELF lignin 1 had the highest amount of

315

aliphatic OH (2.57 mmol/mg) and lowest amount of total phenolic OH (3.06 mmol/mg), possibly

316

because of its low pretreatment severity. Increase in catalyst loading and THF/water ratio were

317

found to have a negligible effect on the lignin OH content. As free phenolic groups are essential

318

for antioxidant activity of lignin, CELF lignin might be suitable for application as antioxidants.37 7 CEL 6

CELF 1 CELF 2

5 OH content (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

CELF 5 CELF 8

4

3

2

1

0

319 18


Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


320

Figure 3. Quantification of different hydroxyl group contents (mmol/g) of CEL and CELF lignin

321

samples determined by 31P NMR spectra.

322

Future CELF lignin Valorization Pathways. Lignin is the most abundant renewable

323

aromatic polymer on earth, offering great potential for high value-added applications in

324

polymeric materials and chemical development.38 Organosolv lignin is usually sulfur-free, rich in

325

functionality, and has limited carbohydrate contamination.39 Following the depolymerization of

326

lignin via CELF process to yield CELF lignin, a solvent-based extraction or solute-based

327

precipitation technique can be applied to further extract the low molecular weight components

328

from the high molecular weight components. The low molecular weight components of CELF

329

lignin could be a preferable candidate as a non-fuel product in polymeric materials. On the other

330

hand, a catalytic oxidative or reductive fragmentation could be applied on the cross-linked high

331

molecular weight fraction of CELF lignin to yield viable fuel, chemical, and pharmaceutical

332

precursors. In addition, CELF lignin could also be considered as a favorable lignin for structural

333

carbon fiber production because of its low content of reactive C-O bonds in interunit linkages

334

such as β-O-aryl ether.40

335

CONCLUSIONS

336

Sustainable pretreatment techniques need to be developed to extract lignin with high yield,

337

quality, and purity for an integrated biorefinery. Poplar was subjected to a Co-Solvent Enhanced

338

Lignocellulosic Fractionation process, and a new type of lignin stream named CELF lignin was

339

recovered and characterized for its physicochemical structural features. Results showed that

340

CELF lignin presented a decreased molecular weight, aryl ether interunit linkages, aliphatic 19



Page 20 of 26

341

hydroxyl content and increased free phenolic and carboxylic acid hydroxyl groups compared

342

with the untreated native lignin. The scission of β-O-4 ether interunit linkages in CELF lignin is

343

the major mechanisms of lignin depolymerization during CELF pretreatment. CELF process

344

provides a potential value-added lignin stream to the biorefinery process, and it also serve as a

345

unique tool to study lignin chemistry in biomass pretreatments.

346

ASSOCIATED CONTENT

347

Supporting Information

348

The Supporting Information is available free of charge on the ACS Publications website at DOI:

349

Table S1. Composition analysis of CELF lignin samples.

350

Table S2. Assignments of lignin 13C-1H correlation signals observed in the HSQC spectra of

351

lignin samples.

352

Table S3. Typical chemical shifts and integration regions for lignin samples in 31P NMR spectra.

353

Table S4. Weight average (Mw) and number average (Mn) molecular weights of CELF lignin

354

and CEL lignin.

355

Table S5. Quantification of different hydroxyl group contents (mmol/g) of CEL and CELF

356

lignin samples determined by 31P NMR spectra

357

AUTHOR INFORMATION

358

Corresponding Author

359

*

360

Authors’ Contributions

Art J. Ragauskas. Fax: 865-974-7076; Tel: 865-974-2042; E-mail: [email protected].

20


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361

The experiments were performed through contributions of all authors. All authors have approved

362

the final version of the manuscript.

363

Notes

364

The authors declare that they have no competing financial interests.

365

ACKNOWLEDGMENTS

366

The authors acknowledge funding support from Bioenergy Technologies Office (BETO) in the

367

Office of Energy Efficiency and Renewable Energy (EERE) under Award No. DE-EE0007006

368

with U.S. Department of Energy (DOE). The publisher, by accepting the article for publication,

369

acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable,

370

worldwide license to reproduce the published form of this manuscript, or allow others to do so,

371

for United States Government purposes. The Department of Energy will provide public access to

372

these results of federally sponsored research in accord with the DOE Public Access Plan

373

(https://www.energy.gov/downloads/doe-public-access-plan). The views and opinions of the

374

authors expressed herein do not necessarily state or reflect those of the United States

375

Government or any agency thereof. Neither the United States Government nor any agency

376

thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any

377

legal liability or responsibility for the accuracy, completeness, or usefulness of any information,

378

apparatus, product, or process disclosed, or represents that its use would not infringe privately

379

owned rights.

380

Abbreviations

381

CEL: Cellulolytic enzyme lignin 21



382

CELF: Co-Solvent Enhanced Lignocellulosic Fractionation (CELF)

383

FF: Furfural

384

G: Guaiacyl

385

GPC: Gel permeation chromatography

386

HMF: 5- hydroxymethylfurfural

387

LA: levulinic acid

388

MD: Molecular-dynamics

389

MWL: Milled wood lignin

390

Mw: Weight-average molecular weight

391

Mn: Number-average molecular weight

392

NMR: Nuclear magnetic resonance

393

NHND: endo-N-hydroxy-5-norbornene-2,3-dicarboximide

394

NREL: National Renewable Energy Laboratory

395

PB: p-hydroxyphenyl benzoate

396

PDI: Polydispersity index

397

S: Syringyl

398

SSF: Simultaneous saccharification and fermentation

399

TMDP: 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane

400

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528 529 530 531

TABLE OF CONTENT

532 533 534 535 536

SYNOPSIS Co-Solvent Enhanced Lignocellulosic Fractionation pretreatment offers a promising approach for lignin isolation with unique characteristics.

26


Chemical transformations of poplar lignin during co-solvent enhanced

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