Effects of Lignin Structure on Hydrodeoxygenation Reactivity of Pine

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Effects of Lignin Structure on Hydrodeoxygenation Reactivity of Pinewood Lignin to Valuable Chemicals Hongliang Wang, Haoxi Ben, Hao Ruan, Libing Zhang, Yunqiao Pu, Maoqi Feng, Arthur Jonas Ragauskas, and Bin Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02563 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 5, 2017

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 a a a a b d b,c 15 Hongliang Wang , Haoxi Ben , Hao Ruan , Libing Zhang , Yunqiao Pu , Maoqi Feng , Arthur J. Ragauskas , and 16 17 Bin Yang a* 18 19 20 21 22 23a Department of Biological Systems Engineering, Washington State University, 2710 Crimson Way, Richland, WA 24 25 99354, USA 26 27b 28 BioEnergy Science Center (BESC), Biosciences Division, Oak Ridge National Laboratory, PO Box 2008 MS6341, 29 30Oak Ridge, TN 37831-6341 USA 31 32c 33 Department of Chemical and Biomolecular Engineering, Department of Forestry, Wildlife, and Fisheries, & Center 34 35for Renewable Carbon, University of Tennessee, Knoxville, TN, USA 36 37d Chemistry & Chemical Engineering Division, Southwest Research Institute, 6220 Culebra Rd. San Antonio, TX 38 39 4078238-5166, USA. 41 42 43 44 45 46*Corresponding Author: Email: [email protected]; Phone: (+1) 509-372-7640; Fax: 509-372-7690 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Effects of Lignin Structure on Hydrodeoxygenation Reactivity of Pinewood Lignin to Valuable Chemicals

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1 2 3 4 5 6 7 8 9 Abstract: Hydrodeoxygenation (HDO) of two dilute acid flowthrough pretreated softwood lignin samples, including 10 11 12residual lignin in pretreated solid residues (ReL) and recovered insoluble lignin in pretreated liquid (RISL), with 13 14apparent different physical and chemical structures, were comprehensively studied. A combination of catalysts (HY 15 16 17zeolite and Ru/Al2O3) was employed to investigate the effects of lignin structures, especially condensed structures, on 18 19the HDO upgrading process. Results indicated that the condensed structure and short side chains in lignin hindered its 20 21HDO conversion under different reaction conditions, including catalyst loading and composition, hydrogen pressure, 22 23 24and reaction time. In addition to lignin structure, HY zeolite was found crucial for lignin depolymerization while 25 26Ru/Al2O3 and relatively high hydrogen pressure (4 MPa) were necessary for upgrading unstable oxy-compounds to 27 28 cyclohexanes at high selectivity (>95 wt%). Since lignin structure essentially affects its reactivity during the HDO 29 30 31conversion, the yield and selectivity of HDO products can be predicted by detailed characterization of lignin structure. 32 33The insights gained from this study in the fundamental reaction mechanisms based on the lignin structure will facilitate 34 35 36upgrading of lignin to high-value products for applications in the production of both fuels and chemicals. 37 38 39 40 41 Lignin reactivity; Softwood lignin; Condensed lignin; Hydrodeoxygenation; Biofuel; 42Keywords: 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 Introduction 4 5 6 Lignin has promising potential to be used to produce a wide variety of bulk and fine chemicals, and advanced fuels 7 8 1-3 The current utilization of lignocellulosic biomass undervalues 9 (e.g. jet fuel) that currently rely on fossil resources. 10 11lignin’s potential to address the world’s energy and chemicals demands.4,5 In addition to the conversion of 12 13 14carbohydrates, integrating the production of valuable biofuels and other co-products from lignin feedstock into the 15 16biorefinery’s output provides a promising opportunity to improve the overall operation efficiency, carbon conversion, 17 18economic viability, and sustainability of biomass biorefinery.1 Therefore, various processes, especially the catalytic 19 20 6-7 21hydrodeoxygenation (HDO) processes, have been developed for the efficient utilization of lignin. Increased market 22 23competitiveness will also attract new biobased companies to contribute to the commercialization of lignin biorefinery. 24 25Due to its sub-structures and/or inter-unit linkages diversity, a variety of chemical compounds can be produced from 26 27 28biomass-derived lignin by using thermochemical process. Due to lignin’s inherent aromatics-based structure, the 29 30production of cyclic hydrocarbons with relatively long carbon chain, which can be used as advanced fuels, such as jet 31 32 fuel, by catalytic HDO is one of the most promising strategies in lignin valorization. Full utilization of lignin holds the 33 34 35key to the next stage of renewable energy production. 36 37 Although lignin holds great potentials in modern lignocellulosic biorefineries, there are multiple challenges to be 38 39 40addressed. One of the biggest hindrances for effective lignin utilization is its intrinsic heterogeneity.8,9 Typically, lignin 41 42 43is constructed from three major monomers, including p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, which 44 45evolve into p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units in the lignin framework (Figure S1). Different 46 47sources and processing methods can result in lignins with obvious differences in chemical structures that may affect the 48 49 50conversion of lignin. Softwoods, e.g. pine and spruce, are the dominant lignocellulosic sources in the northern 51 52hemisphere, and have been the subject of great interest as a renewable resource for biofuel production.10 Softwoods 53 54 usually have high lignin content, and the lignin in softwoods possesses much higher G units ratio (nearly 95% of the 55 56 57total units) as compared to the lignins in the hardwoods and grasses. The lack of methoxyl groups at the ortho position 58 59to the phenolic hydroxyl group in G units facilitates the softwood lignin to produce condensed and branched structures 60 ACS Paragon Plus Environment


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1 2 3 by forming 5-5’ bonds during lignin biosynthesis and conversion processes (Figure S1).11 Such process makes 4 5 6 softwood lignin resistant to be converted by thermochemical or biochemical processes. 7 8 In addition to sources, the processing (or pretreatment) methods of lignin, including isolation and purification, will 9 10 11affect the lignin structure as well. Traditional pretreatment methods, including steam, ammonia or carbon dioxide 12 13 14explosion, acid or alkali hydrolysis, and biological hydrolysis, are either inefficient for sugar release or not very 15 16friendly for lignin harnessing. For example, for steam explosion and diluted acid pretreatments, although both could 17 18reach more than 90% sugar recovery, it was proved that series of lignin condensation occurred at the relatively high 19 20 12,13 Thus, it is critical to develop efficient processing methods that can recover 21reaction temperatures required by them. 22 23sugars in high yield and lignin with high purity and minimal condensation at the same time. Our previous results 24 25indicated that near 100% recovery of sugars and lignin with high purity could be simultaneously achieved from poplar 26 27 28wood through very dilute sulfuric acid flowthrough pretreatment with about 0.05 wt% H2SO4 versus 0.7–3.0 wt% 29 30typical for other dilute acid technologies.14,15 Recently, pinewood was pretreated via the same very dilute acid 31 32 flowthrough pretreatment with high carbohydrate recovery, and mainly three types of lignins were obtained, including 33 34 35residue lignin (ReL), separated Recovered Insoluble Lignin (RISL), and Recovered Soluble Lignin (RSL), as shown in 36 37Figure 1. The RSL was dissolved in water and mainly made up of lignin degraded monomers and dimers, which was 38 39 40found to be negligible (small amount). The RISL was recovered from the flowing solvent, and the ReL was remained in 41 42the pretreatment solid residues. ReL and RISL were found with high purity but significant differences in their chemical 43 44structures, especially the aliphatic linkages and side chains. 45 46 47 Moreover, the structural features and reactivity of lignin-derived reactive intermediates were found imperative to 48 49 50rationally select effective hydrogenolysis and HDO catalyst systems for determining the yield and distribution of the 51 52final products.8,16-18 Abu-Omar et al17 and Song et al19 reported effects of substrate on product yield during lignin 53 54 depolymerization in methanol over Ni/C catalyst. Shu and co-authors recently found that lignin structure had great 55 56 20 57effects on lignin conversion through lignin hydrogenolysis over the synergistic catalyst of CrCl3 and Pd/C. Results 58 59reported by Bouxin and co-authors showed that the proportion of β-O-4 linkages of lignin structure was crucial for both 60 ACS Paragon Plus Environment the yield and the nature of the monomeric products from catalytic lignin depolymerization over Pt/alumina.18 Although

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1 2 3 a few attempts have already been made to correlate lignin structure with its catalytic conversion, direct evidence to 4 5 6 clearly connect the lignin structure with its HDO reactivity is still not revealed. In this study, two types of lignins (ReL 7 8 and RISL) produced from the same source (pinewood) with high purity but obviously different structures were used to 9 10 perform HDO reactions over commercial benchmark catalysts (HY zeolite and Ru/Al2O3). Reaction parameters, 11 12 13including the catalyst loadings and compositions, hydrogen pressures, and the reaction times, were investigated to 14 15provide the evidence of the effect of lignin structure on its HDO conversion, and seek a better understanding of the key 16 17 18factors that affect the yields and distributions of lignin HDO products. 19 20 1. Materials and methods 21 22 232.1. Feedstock and flowthrough pretreatment 24 25 26 Ru/Al2O3, acetyl bromide, acetic acid, ethyl acetate, and n-decane were purchased from Fisher Scientific. Zeolite Y 27 28 29was purchased from Zeolyst International. Detailed properties of these catalysts can be found in the supporting 30 31materials. 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 ACS Paragon Plus Environment


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1 2 3 Figure 1 Flowthrough pretreatment of Lodgepole Pine wood. 4 5 6 Lignin was isolated from Lodgepole Pine sawdust by flowthrough pretreatment. Beetle killed Lodgepole Pine wood 7 8 9 was purchased from Forests Corporation. It was cut and sieved to obtain 40-60 mesh particles as substrate. Before 10 11pretreatment, 0.5 g dry Lodgepole Pine wood was loaded into a flowthrough reactor with 20.5 mL working volume. 12 13 14The reactor was connected to a pump, and then was immersed in a fluidized sand bath. Sulfuric acid solution (0.05 wt%) 15 16at room temperature was pumped through the reactor to purge air and then used to pressurize the reactor to 500 psi. The 17 18loaded biomass was completely wetted by this procedure. The reactor was heated to 240 oC in a 4 kW fluidized sand 19 20 o 21bath (model SBL-2D, Omega engineering, Inc., CT). The temperature of the sand bath was set 15 C higher than the 22 23target reaction temperature. The flow rate was set at 25 mL/min. After 8 min pretreatment, the reactor was immediately 24 25cooled by cold water. Solid residues (mainly residue lignin, ReL) remained in the tubular reactor were dried at room 26 27 28temperature. Hydrolysates were collected and precipitated. The resulting precipitates (mainly insoluble lignin, RISL) 29 30were filtrated and dried at room temperature.6 The schematic diagram of the flowthrough pretreatment including the 31 32 resulted products can be found in Figure 1. 33 34 35 362.2. Lignin recovery and analysis 37 38 Lignin was recovered in hydrolysates as soluble and insoluble portions (see Figure 1). The soluble lignin (RSL) in 39 40 41hydrolysates was evaluated by determining ultra-violet spectroscopic absorbance under 320 nm. The insoluble lignin 42 43 (RISL) in hydrolysates and residue lignin (ReL) remained in the solid residues were collected by precipitation, 44 45 46filtration, DI water washing, and drying at room temperature. Lignin contents in untreated and pretreated solids were 47 48determined by following the NREL K-lignin analysis procedure.21 Lignin recovery as ReL or RISL was calculated as 49 50 51follows: 52 53 Lignin recovery (%) =

× 100% (1) 54 55 56 57 2.3. Purity of insoluble lignin (RISL) and residue lignin (ReL) 58 59 60 ACS Paragon Plus Environment

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1 2 3 The purity of RISL and ReL was analyzed by determining UV absorbance of solubilized lignin. The recovered 4 5 6 RISL and ReL in the solid phase were subjected to acetyl bromide (AcBr) solubilization followed by acetic acid 7 8 treatment.22 Typically, 1-2 mg lignin was mixed with 25 wt% AcBr in 2.5 mL acetic acid in a 15 mL glass bottle. The 9 10 bottle was sealed with a PTFE-coated silicone cap and placed in an oven at 70+0.2 ºC for 30 min. The bottle was 11 12 13shaken at 10-min intervals to promote dissolution of the lignin. Then, the mixture was cooled and transferred to a 50 14 15mL flask and mixed with 2.5 mL 2 M sodium hydroxide and 12 mL acetic acid. 7.5 M hydroxylamine hydrochloride 16 17 18(0.5 mL) was then added to the combined solution. Acetic acid was added to reach 50 mL of total volume. The UV 19 20absorption of the resulting solution at 280 nm was measured against a control solution without lignin sample. The 21 22purity of the lignin samples was calculated using the equation of Morrison.22 23 24 25 Lignin = [3.37 × absorbance/sample concentration (g/L) - 1.05]×100% (2) 26 27 28 29 302.4. Characterization of ReL and RISL 31 32 33 STEM images were measured on a FEI Titan 80-300 scan/transmission electron microscope operating at 300 kV. 34 35 13 36Prior to the measurements, lignins were ground and loaded on copper grids. The solid-state CP/MAS C NMR analysis 37 38of lignin was performed on a Bruker Avance III 400 MHz spectrometer operating at a frequency of 100.59 MHz for 13C 39 40 using a Bruker double-resonance 4-mm MAS probe head at ambient temperature. The samples were packed in a 4 mm 41 42 13 43ZrO rotor fitted with a Kel-F cap and spun at 8,000 Hz. CP/MAS C data were acquired with a Bruker CP pulse 44 45sequence under the following acquisitions: pulse delay 4 s, contact pulse 2000 µs, and 2k to 4k numbers of scans.23 46 47 482.5. Hydrodeoxygenation (HDO) conversion of lignin 49 50 51 HDO catalysis experiments were carried out in a 100 mL Parr reactor. Typically, softwood lignin 100 mg, 52 53 54Ru/Al2O3 100 mg, HY zeolite 200 mg, and 30 mL water were added into a Parr reactor. H2 was flushed three times to 55 56remove air in the reaction system. Then the reactor was pressurized with H2 (4 MPa at room temperature) and heated to 57 58 o 59250 C and maintained for different reaction time. After the reaction, the vessel was cooled down to room temperature 60

by immersing in cold water. Gases in the reactor were exhausted to an inverted container vessel filled with water to ACS Paragon Plus Environment


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1 2 3 determine the volume, and collected for GC analysis. The organic products in liquid phase were extracted by ethyl 4 5 24,25 The solid was dried and weighted, and then analyzed by TOC analyzer. The organic phase was analyzed by 6 acetate. 7 8 GC and GC-MS. n-decane and vanillin were added to the ethyl acetate phase and used as internal standards for 9 10 deoxygenated and oxy-compounds (lignin HDO products that contain oxygen atoms), respectively.24,26 The response 11 12 25,27 13factor for each component was calculated using the effective carbon number (ECN) method. 14 15 162.6. HDO products analysis by GC-MS 17 18 19 The organic extracted samples from reaction solution (1 µl ) were injected with 0.6 mL min−1 of He (carrier gas) 20 21 into a DB-5 (30 m length × 250 µ m I.D. × 0.25µ m film thickness, J&W Scientific) capillary column fitted in an 22 23 24Agilent Technologies 7890A GC system set in the splitless mode. The GC oven was programmed to 48 °C for 2 min. 25 26Then it was raised at the rate of 10 °C per min until the temperature reached 200 °C, and was held at this temperature 27 28 29for 1 min. After that, the temperature was raised at the rate of 5 °C until the temperature finally reached at 300 °C and 30 31held at the final temperature for 5 min. Eluting compounds were detected by an MS (Agilent Technologies 5975C) 32 33inert XL EI/CI MSD with a triple axis detector, and compared using NIST libraries. Shimadzu TOC-V Analyzer was 34 35 36used to quantify the total organic carbon of the original lignin and residue solids (including char and residue lignin). 37 38Product yield was calculated by the effective carbon number (ECN) approach.25,27 39 40 41 42 43 ! " " # #$# % %#$ # &! ! " " # '(#)*' (%#) 44 Lignin conversion= × 100% (3) 45 ! " " # #$# % %#$ # 46 47 6788 9_;9< 7BC7D/ FGHD 48 × ×JKL ?@A 7BC7 9_;995 wt%), further 38 39demonstrating that the current catalytic system was capable of converting biomass-derived lignin into advanced 40 41 biofuels. 42 43 44 3.5. Possible reaction channels involved in lignin HDO conversion 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Figure 6 Possible reaction channels involved in lignin HDO conversion 60 ACS Paragon Plus Environment


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1 2 3 The HDO conversion of lignin is a complicated process, involving various kinds of reactions and pathways. On the 4 5 18,26 and results obtained from the current study, we proposed some tentative reaction pathways 6 basis of previous reports 7 8 of lignin HDO conversion (Figure 6). The majority of C-O-C bonds existing in lignin have relatively low bond 9 10 dissociation energy than the C-C bonds, thus they can be facilely disrupted either by acid catalyzed hydrolysis or 11 12 13through metal-catalyzed hydrogenolysis. The cleavage of C-O-C bonds can depolymerize lignin into its oligomers, 14 15dimers, and monomers. In most cases, these products are unstable oxy-compounds under the HDO reaction conditions. 16 17 18They can be converted to each other via further depolymerization or rearrangement reactions. Lignin dimers and 19 20monomers can also be converted to other oxy-compounds via reactions, including alkylation, demethylation, and 21 22demethoxylation etc. Under the combination catalysis of HY and Ru/Al O , all the small molecular oxy-compounds 2 3 23 24 25can be converted into more stable cyclohexanes through deoxygenation and hydrogenation reactions. Ring-opening 26 27products (e.g. linear and branched alkanes) can be generated by hydrogenolysis of cyclohexanes. Most importantly, the 28 29 presence of condensed structures and short side chains is a notable disadvantage for lignin HDO conversion since it can 30 31 32prevent lignin from depolymerization and induce the formation of char. In addition to a certain amount of inherent C-C 33 34bond linkages, lignin condensed structures and short side chains can be also formed during both biomass pretreatment 35 36 37process and lignin conversion process. Thus, the minimization of lignin condensation reactions and rearrangement 38 39reactions during its separation and conversion processes is important for the production of advanced biofuels or high 40 41value-added chemicals from the biopolymer. 42 43 44 3. Conclusion 45 46 47 Lignin’s reactivity plays a crucial role in its commercial applications for the production of fuels and chemicals. 48 49 50Results from this study indicated that condensed structures and short side chains in lignin had significant impacts on 51 52yields of valuable precursors for fuels and chemicals. HY zeolite was found vital during the depolymerization of lignin 53 54 55while Ru/Al2O3 and relatively high hydrogen pressure (i.e. 4 MPa or 6 MPa) were necessary for the upgrading of 56 57unstable oxy-compounds to cyclohexanes with high selectivity (>95 wt%), and then further converted to ring-opened 58 59alkanes. Ru/Al O was found to be an efficient catalyst for the upgrading of aromatic oxy-compounds to more stable 2 3 60 ACS Paragon Plus products such as cyclohexanes. Tentative reaction pathways of Environment lignin HDO conversion were proposed. Overall, in order

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1 2 3 to fully unlock the potential of lignin, a sleep giant in modern biorefinery, lignin itself, catalysts, and reaction 4 5 6 conditions should all be taken into account carefully. 7 8 9 10 11 12Acknowledgements 13 14 We are grateful to the Sun Grant-DOT Award # T0013G-A- Task 8, and the Seattle-based Joint Center for 15 16 17Aerospace Technology Innovation for funding this research. We acknowledge the Bioproducts, Sciences and 18 19 20Engineering Laboratory, Department of Biosystems Engineering at Washington State University and The Boeing 21 22Company. Part of this work was conducted at the William R. Wiley Environmental Molecular Sciences Laboratory 23 24 (EMSL), a national scientific user facility located at the Pacific Northwest National Laboratory (PNNL) and sponsored 25 26 27by the Department of Energy’s Office of Biological and Environmental Research (BER). Oak Ridge National 28 29Laboratory (ORNL) is managed by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. 30 31 Department of Energy. We also thank Drs. Langli Luo, Chongmin Wang for their assistance on STEM testing, and Dr. 32 33 34Yuling Qin and Ms. Marie S. Swita for insightful discussions. 35 36 37References 38 39 40(1) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. 41 A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. 42 43 E. Lignin Valorization: Improving Lignin Processing in the Biorefinery. Science 2014, 344, 709-720. 44 45(2) Wang, H.; Ruan, H.; Pei, H.; Wang, H.; Chen, X.; p Tucker, M.; Cort, J. R.; Yang, B. Biomass-derived Lignin to 46 47 Jet Fuel Range Hydrocarbons via Aqueous Phase Hydrodeoxygenation. Green Chem. 2015, 17, 5131-5135. 48 49(3) Laskar, D. D.; Yang, B.; Wang, H.; Lee, J. Pathways for biomass-derived lignin to hydrocarbon fuels. Biofuel. 50 Bioprod. Bior. 2013, 7, 602-626. 51 52(4) Vispute, T. P.; Zhang, H. Y.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable Chemical Commodity Feedstocks 53 54 from Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330, 1222-1227. 55 56(5) Holladay, J.; White, J.; Bozell, J.; Johnson, D. Top Value-Added Chemicals from Biomass - Volume II-Results of 57 Screening for Potential Candidates from Biorefinery Lignin. Richland, WA, 2007. 58 59 (6) Laskar, D. D.; Tucker, M. P.; Chen, X.; Helms, G. L.; Yang, B. Noble-metal catalyzed hydrodeoxygenation of 60

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1 2 3 4 For Table of Contents Use only 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19Title: 20 21 22Effects of Lignin Structure on Hydrodeoxygenation Reactivity of Pinewood Lignin to Valuable Chemicals 23 24Authors: 25 26 Hongliang Wang, Haoxi Ben, Hao Ruan, Libing Zhang, Yunqiao Pu, Maoqi Feng, Arthur J. Ragauskas, and Bin 27 28 29 30 31Synopsis: 32 33 34Condensed structures and short side chains can obviously hinder lignin from hydrodeoxygenation conversion. 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 ACS Paragon Plus Environment

Yang


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Flowthrough pretreatment of Lodgepole Pine wood 98x75mm (220 x 220 DPI)


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TEM images of RISL (a) and ReL (b) 138x72mm (220 x 220 DPI)



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Solid state CP/MAS 13C NMR analysis of Recovered insoluble lignin (RISL) and Residual lignin (ReL) 160x113mm (220 x 220 DPI)


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Hydrodeoxygenation of RISL and ReL in aqueous phase. Reaction conditions: lignin 100 mg, solid acid zeolites (HY) 200 mg, Ru/Al2O3100 mg, Water 30 mL, PH2 =4 MPa, t=4 h, T=250 oC. 143x111mm (220 x 220 DPI)



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Reaction profile of HDO conversion of RISL. Reaction conditions: lignin 100 mg, solid acid zeolites (HY) 200 mg, Ru/Al2O3 100 mg, water 30 mL, PH2=4 MPa, 250 oC. 128x99mm (220 x 220 DPI)


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Possible reaction channels involved in lignin HDO conversion 165x47mm (220 x 220 DPI)



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Condensed structures and short side chains can obviously hinder lignin from hydrodeoxygenation conversion 270x126mm (150 x 150 DPI)


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Effects of Lignin Structure on Hydrodeoxygenation Reactivity of Pine

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