Effects of Lignin Structure on Hydrodeoxygenation ... - ACS Publications

Jan 5, 2017 - Chemistry & Chemical Engineering Division, Southwest Research Institute, 6220 Culebra Rd., San Antonio, Texas 78238-5166,. United States...
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Effects of Lignin Structure on Hydrodeoxygenation Reactivity of Pine Wood Lignin to Valuable Chemicals Hongliang Wang,† Haoxi Ben,†,⊥ Hao Ruan,† Libing Zhang,† Yunqiao Pu,‡ Maoqi Feng,∥ Arthur J. Ragauskas,‡,§ and Bin Yang*,†

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†

Department of Biological Systems Engineering, Washington State University, 2710 Crimson Way, Richland, Washington 99354, United States ‡ Biosciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS6341, Oak Ridge, Tennessee 37831-6341, United States § Department of Chemical and Biomolecular Engineering, Department of Forestry, Wildlife, and Fisheries, & Center for Renewable Carbon, University of Tennessee, Knoxville, Tennessee 37996, United States ∥ Chemistry & Chemical Engineering Division, Southwest Research Institute, 6220 Culebra Rd., San Antonio, Texas 78238-5166, United States S Supporting Information *

ABSTRACT: Hydrodeoxygenation (HDO) of two dilute acid flowthrough pretreated softwood lignin samples, including residual lignin in pretreated solid residues (ReL) and recovered insoluble lignin in pretreated liquid (RISL), with apparent different physical and chemical structures, was comprehensively studied. A combination of catalysts (HY zeolite and Ru/Al2O3) was employed to investigate the effects of lignin structures, especially condensed structures, on the HDO upgrading process. Results indicated that the condensed structure and short side chains in lignin hindered its HDO conversion under different reaction conditions, including catalyst loading and composition, hydrogen pressure, and reaction time. In addition to lignin structure, HY zeolite was found crucial for lignin depolymerization, while Ru/Al2O3 and relatively high hydrogen pressure (4 MPa) were necessary for upgrading unstable oxy-compounds to cyclohexanes at high selectivity (>95 wt %). Since the lignin structure essentially affects its reactivity during HDO conversion, the yield and selectivity of HDO products can be predicted by detailed characterization of the lignin structure. The insights gained from this study in the fundamental reaction mechanisms based on the lignin structure will facilitate upgrading of lignin to high-value products for applications in the production of both fuels and chemicals. KEYWORDS: Lignin reactivity, Softwood lignin, Condensed lignin, Hydrodeoxygenation, Biofuel

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INTRODUCTION Lignin has promising potential to be used to produce a wide variety of bulk and fine chemicals and advanced fuels (e.g., jet fuel) that currently rely on fossil resources.1−3 The current utilization of lignocellulosic biomass undervalues lignin’s potential to address the world’s energy and chemicals demands.4,5 In addition to the conversion of carbohydrates, integrating the production of valuable biofuels and other coproducts from lignin feedstock into the biorefinery’s output provides a promising opportunity to improve the overall operation efficiency, carbon conversion, economic viability, and sustainability of biomass biorefinery.1 Therefore, various processes, especially the catalytic hydrodeoxygenation (HDO) processes, have been developed for the efficient utilization of lignin.6,7 Increased market competitiveness will also attract new biobased companies to contribute to the commercialization of lignin biorefinery. Due to its substructures and/or interunit linkages diversity, a variety of chemical compounds can be produced from biomass-derived lignin by using thermochemical © 2017 American Chemical Society

process. Due to lignin’s inherent aromatics-based structure, the production of cyclic hydrocarbons with relatively long carbon chains, which can be used as advanced fuels, such as jet fuel, by catalytic HDO is one of the most promising strategies in lignin valorization. Full utilization of lignin holds the key to the next stage of renewable energy production. Although lignin holds great potentials in modern lignocellulosic biorefineries, there are multiple challenges to be addressed. One of the biggest hindrances for effective lignin utilization is its intrinsic heterogeneity.8,9 Typically, lignin is constructed from three major monomers, including p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which evolve into p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units in the lignin framework (Figure S1). Different sources and processing methods can result in lignins with obvious Received: October 24, 2016 Revised: December 28, 2016 Published: January 5, 2017 1824

DOI: 10.1021/acssuschemeng.6b02563 ACS Sustainable Chem. Eng. 2017, 5, 1824−1830

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ACS Sustainable Chemistry & Engineering

chemical structures, especially the aliphatic linkages and side chains. Moreover, the structural features and reactivity of ligninderived reactive intermediates were found imperative to rationally select effective hydrogenolysis and HDO catalyst systems for determining the yield and distribution of the final products.8,16−18 Abu-Omar et al.17 and Song et al.19 reported effects of substrate on product yield during lignin depolymerization in methanol over Ni/C catalyst. Shu and coauthors recently found that lignin structure had great effects on lignin conversion through lignin hydrogenolysis over the synergistic catalyst of CrCl3 and Pd/C.20 Results reported by Bouxin and coauthors showed that the proportion of β-O-4 linkages of the lignin structure was crucial for both the yield and the nature of the monomeric products from catalytic lignin depolymerization over Pt/alumina.18 Although a few attempts have already been made to correlate lignin structure with its catalytic conversion, direct evidence to clearly connect the lignin structure with its HDO reactivity is still not revealed. In this study, two types of lignins (ReL and RISL) produced from the same source (pine wood) with high purity but obviously different structures were used to perform HDO reactions over commercial benchmark catalysts (HY zeolite and Ru/Al2O3). Reaction parameters, including catalyst loadings and compositions, hydrogen pressures, and reaction times, were investigated to provide evidence of the effect of lignin structure on its HDO conversion and seek a better understanding of the key factors that affect the yields and distributions of lignin HDO products.

differences in chemical structures that may affect the conversion of lignin. Softwoods, e.g., pine and spruce, are the dominant lignocellulosic sources in the Northern Hemisphere and have been the subject of great interest as a renewable resource for biofuel production.10 Softwoods usually have high lignin content, and the lignin in softwoods possesses much higher G units ratios (nearly 95% of the total units) as compared to the lignins in the hardwoods and grasses. The lack of methoxyl groups at the ortho position to the phenolic hydroxyl group in G units facilitates the softwood lignin to produce condensed and branched structures by forming 5−5′ bonds during lignin biosynthesis and conversion processes (Figure S1).11 Such a process makes softwood lignin resistant to convertion by thermochemical or biochemical processes. In addition to sources, the processing (or pretreatment) methods of lignin, including isolation and purification, will affect the lignin structure as well. Traditional pretreatment methods, including steam, ammonia or carbon dioxide explosion, acid or alkali hydrolysis, and biological hydrolysis, are either inefficient for sugar release or not very friendly for lignin harnessing. For example, for steam explosion and diluted acid pretreatments, although both could reach more than 90% sugar recovery, it was proved that a series of lignin condensation occurred at the relatively high reaction temperatures required by them.12,13 Thus, it is critical to develop efficient processing methods that can recover sugars in high yield and lignin with high purity and minimal condensation at the same time. Our previous results indicated that near 100% recovery of sugars and lignin with high purity could be simultaneously achieved from poplar wood through very dilute sulfuric acid flowthrough pretreatment with about 0.05 wt % H2SO4 versus 0.7−3.0 wt % typical for other dilute acid technologies.14,15 Recently, pine wood was pretreated via the same very dilute acid flowthrough pretreatment with high carbohydrate recovery, and mainly, three types of lignins were obtained, including residue lignin (ReL), separated Recovered Insoluble Lignin (RISL), and Recovered Soluble Lignin (RSL), as shown in Figure 1. The RSL was dissolved in water and mainly made up of lignin-degraded monomers and dimers, which was found to be negligible (small amount). The RISL was recovered from the flowing solvent, and the ReL remained in the pretreatment solid residues. ReL and RISL were found with high purity but with significant differences in their

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MATERIALS AND METHODS

Feedstock and Flowthrough Pretreatment. Ru/Al2O3, acetyl bromide, acetic acid, ethyl acetate, and n-decane were purchased from Fisher Scientific. Zeolite Y was purchased from Zeolyst International. Detailed properties of these catalysts can be found in the Supporting Information. Lignin was isolated from Lodgepole Pine sawdust by flowthrough pretreatment. Beetle-killed Lodgepole Pine wood was purchased from Forests Corporation. It was cut and sieved to obtain 40−60 mesh particles as substrate. Before pretreatment, 0.5 g of dry Lodgepole Pine wood was loaded into a flowthrough reactor with 20.5 mL working volume. The reactor was connected to a pump and then was immersed in a fluidized sand bath. Sulfuric acid solution (0.05 wt %) at room temperature was pumped through the reactor to purge air and then used to pressurize the reactor to 500 psi. The loaded biomass was completely wetted by this procedure. The reactor was heated to 240 °C in a 4 kW fluidized sand bath (model SBL-2D, Omega engineering, Inc., CT). The temperature of the sand bath was set 15 °C higher than the target reaction temperature. The flow rate was set at 25 mL/min. After 8 min pretreatment, the reactor was immediately cooled by cold water. Solid residues (mainly residue lignin, ReL) remaining in the tubular reactor were dried at room temperature. Hydrolysates were collected and precipitated. The resulting precipitates (mainly insoluble lignin, RISL) were filtrated and dried at room temperature.6 The schematic diagram of the flowthrough pretreatment including the resulted products can be found in Figure 1. Lignin Recovery and Analysis. Lignin was recovered in hydrolysates as soluble and insoluble portions (Figure 1). The soluble lignin (RSL) in hydrolysates was evaluated by determining ultraviolet spectroscopic absorbance under 320 nm. The insoluble lignin (RISL) in hydrolysates and residue lignin (ReL) remained in the solid residues were collected by precipitation, filtration, DI water washing, and drying at room temperature. Lignin contents in untreated and pretreated solids were determined by following the NREL Klason lignin analysis procedure.21 Lignin recovery as ReL or RISL was calculated as follows:

Figure 1. Flowthrough pretreatment of Lodgepole Pine wood. 1825

DOI: 10.1021/acssuschemeng.6b02563 ACS Sustainable Chem. Eng. 2017, 5, 1824−1830

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ACS Sustainable Chemistry & Engineering Lignin recovery (%) =

Lignin mass weight in ReL or RISL × 100% Lignin mass weight in untreated feedstock

residue lignin). Product yield was calculated by the effective carbon number (ECN) approach.25,27

(1)

Lignin conversion Carbon content in original lignin − carbon content in residue solid = Carbon content in original lignin

Purity of Insoluble Lignin (RISL) and Residue Lignin (ReL). The purity of RISL and ReL was analyzed by determining UV absorbance of solubilized lignin. The recovered RISL and ReL in the solid phase were subjected to acetyl bromide (AcBr) solubilization followed by acetic acid treatment.22 Typically, 1−2 mg of lignin was mixed with 25 wt % AcBr in 2.5 mL acetic acid in a 15 mL glass bottle. The bottle was sealed with a PTFE-coated silicone cap and placed in an oven at 70 + 0.2 °C for 30 min. The bottle was shaken at 10 min intervals to promote dissolution of the lignin. Then, the mixture was cooled and transferred to a 50 mL flask and mixed with 2.5 mL of 2 M sodium hydroxide and 12 mL of acetic acid. Also, 7.5 M hydroxylamine hydrochloride (0.5 mL) was then added to the combined solution. Acetic acid was added to reach 50 mL of total volume. The UV absorption of the resulting solution at 280 nm was measured against a control solution without a lignin sample. The purity of the lignin samples was calculated using the equation of Morrison:22

Yieldx (wt %) =

Mass n _decane 142

×

areax /ECNx area n _decane/10

× MWx

Mass lignin

(4)

25

Total product yield =

∑ Yieldx x=1

Selectivityx =

Yield of product x × 100% Toal product yields

(5) (6)

where x is oxy-compounds, cyclohexanes, or ring-opened alkanes.

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RESULTS AND DISCUSSIONS Lignin Recovery and Purity. On the basis of the calculation listed in eq 1, the recovery percentages of RSL, RISL, and ReL from flowthrough pretreatment of pine wood were 1.2−3.5, 69.2−78.5, and 22.5−27.3 wt %, respectively. The results indicated that RSL was negligible, and most of the lignin in pine wood pretreated was recovered in the form of RISL. In addition, both the improved acetyl bromide procedure and NREL Klason lignin analysis method were applied to analyze the purity of RISL and ReL. The results showed the purities of RISL and ReL were over 90%. Transmission Electron Microscope Imaging of RISL and ReL. The physical structures of the RISL and ReL samples were examined by STEM imaging of dry lignin samples. The most striking feature in the STEM images of RISL is the presence of discrete spherical balls (Figure 2a). These lignin

Lignin = [3.37 × absorbance/sample concentration (g/L) − 1.05] × 100%

(3)

× 100%

(2)

Characterization of ReL and RISL. Scanning transmission electron microscope (STEM) images were measured on a FEI Titan 80-300 scan/transmission electron microscope operating at 300 kV. Prior to the measurements, lignins were ground and loaded on copper grids. The solid-state cross polarization/magic angle spinning (CP/ MAS) 13C NMR analysis of lignin was performed on a Bruker Avance III 400 MHz spectrometer operating at a frequency of 100.59 MHz for 13 C using a Bruker double-resonance 4 mm MAS probe head at ambient temperature. The samples were packed in a 4 mm ZrO rotor fitted with a Kel-F cap and spun at 8000 Hz. CP/MAS 13C data were acquired with a Bruker CP pulse sequence under the following acquisitions: pulse delay 4 s, contact pulse 2000 μs, and 2k to 4k numbers of scans.23 Hydrodeoxygenation (HDO) Conversion of Lignin. HDO catalysis experiments were carried out in a 100 mL Parr reactor. Typically, 100 mg of softwood lignin, 100 mg of Ru/Al2O3, 200 mg of HY zeolite, and 30 mL of water were added into a Parr reactor. H2 was flushed three times to remove air in the reaction system. Then, the reactor was pressurized with H2 (4 MPa at room temperature), heated to 250 °C, and maintained for different reaction times. After the reaction, the vessel was cooled to room temperature by immersing in cold water. Gases in the reactor were exhausted to an inverted container vessel filled with water to determine the volume and collected for GC analysis. The organic products in the liquid phase were extracted by ethyl acetate.24,25 The solid was dried, weighed, and then analyzed by a TOC analyzer. The organic phase was analyzed by GC and GC-MS. Then, n-decane and vanillin were added to the ethyl acetate phase and used as internal standards for deoxygenated and oxycompounds (lignin HDO products that contain oxygen atoms), respectively.24,26 The response factor for each component was calculated using the effective carbon number (ECN) method.25,27 HDO Products Analysis by GC-MS. The organic extracted samples from the reaction solution (1 μL) were injected with 0.6 mL min−1 of He (carrier gas) into a DB-5 (30 m length × 250 μ m I.D. × 0.25 μ m film thickness, J&W Scientific) capillary column fitted in an Agilent Technologies 7890A GC system set in the splitless mode. The GC oven was programmed to 48 °C for 2 min. Then, it was raised at the rate of 10 °C per min until the temperature reached 200 °C and was held at this temperature for 1 min. After that, the temperature was raised at the rate of 5 °C until the temperature finally reached at 300 °C and held at the final temperature for 5 min. Eluting compounds were detected by an MS (Agilent Technologies 5975C) inert XL EI/ CI MSD with a triple axis detector and compared using NIST libraries. A Shimadzu TOC-V analyzer was used to quantify the total organic carbon of the original lignin and residue solids (including char and

Figure 2. STEM images of RISL (a) and ReL (b).

balls cover a wide range of sizes from 50 to 600 nm. Similar lignin spherical structures were found in previous reports.28,29 Compared to RISL, ReL showed the significantly different physical structure in STEM imaging (Figure 2b). No apparent ball structure was found in the STEM imaging of ReL. Unlike RISL, ReL presented an amorphous cotton-shaped microstructure. The differences between physical structures of RISL and ReL are plausibly due to different chemical structures of these two types of lignins. Solid State CP/MAS 13C NMR Analysis of RISL and ReL. The recovered insoluble lignin (RISL) and residual lignin (ReL) samples were analyzed by using solid state CP/MAS 13C NMR spectroscopy (Figure 3). Both RISL and ReL samples 1826

DOI: 10.1021/acssuschemeng.6b02563 ACS Sustainable Chem. Eng. 2017, 5, 1824−1830

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Figure 3. Solid state CP/MAS 13C NMR analysis of recovered insoluble lignin (RISL) and residual lignin (ReL).

showed typical diagnostic softwood lignin peaks in the aromatic regions. However, two notable differences can be observed in the spectra of these two lignin samples. The RISL sample demonstrates much stronger signal intensity of the peaks between ∼60−85 ppm, while ReL shows higher peak intensity in the aliphatic area (∼15−40 ppm). Since almost 100% carbohydrates were removed from the flowthrough reactor, the extra aliphatic C−O peaks in the RISL sample can be assigned to aliphatic C−O bonds in the lignin structure, which can be the ether linkages, including β-O-4 and α-O-4, or the lignin− carbohydrate complex (LCC).30 These ether bonds in the RISL have relatively low bond dissociation energy and high reactivity, thus can be more favorable for the following HDO upgrading. Compared to RSIL lignin, ReL not only has limited content of such ether linkages but also shows a much higher amount of aliphatic C−C bonds. These aliphatic C−C bonds can present the various short side chains in the aromatic structures of lignin, which can be produced by rearrangements of methoxyl groups and ether bonds (Figure S2). The extra C−C bonds, methyl groups, and other short side chains on aromatic structures of ReL may reduce the active sites in lignin structure and thus build barriers to the HDO upgrading process. Molecular weights of RISL and ReL are 1556 and 1205, respectively, indicating that some weak ether bonds in ReL may have already been destroyed so ReL becomes relatively smaller in size but more condensed. On the contrary, depolymerized lignin fractions as RISL can be immediately washed out through the flowthrough process, thus avoid further condensation reactions among the decomposed lignin fragments under high temperature and acidic conditions. Lignin HDO Conversion. Lignin HDO reactions were carried out in a 100 mL Parr reactor. Our previous research indicated that alkali lignin extracted from corn stover was effectively converted into jet fuel range cyclohexanes with the combined catalysts of HY zeolite and Ru/Al2O3.2 The same catalytic system was applied in this study to examine the effects of lignin structure on its HDO conversion. Both RISL and ReL produced from pine wood by using diluted acid flowthrough pretreatment were used as substrates in the HDO conversion. The HDO products were extracted by ethyl acetate and then were quantitatively analyzed. Three major types of products, including oxy-compounds, cyclohexanes, and ring-opening products, were found in the ethyl acetate extracted HDO product mixtures (Figure 4). Results indicated that the

Figure 4. Hydrodeoxygenation of RISL and ReL in aqueous phase. Reaction conditions: lignin 100 mg, solid acid zeolites (HY) 200 mg, Ru/Al2O3 100 mg, water 30 mL, PH2 = 4 MPa, t = 4 h, T = 250 °C.

conversion of RISL was 57.6%, while the conversion of ReL was less than 10%. ReL exhibited resistance to upgrading by the HDO system in this study, which is consistent with the previous prediction of the lignin activity on the basis of lignin structures characterized by NMR analysis. Results suggest that the high ratio of condensed structures and short side chains in ReL can result in barriers to the depolymerization of lignin. Therefore, it is crucial to prevent the formation of such structures in lignin for the subsequent lignin valorization/ upgrading. Most products from HDO of RISL and ReL have cyclohexanes basic structures with a carbon number range from 9 to 18 in the diesel and jet-fuel range. A number of ringopening products (23−25 wt %), including branched and linear alkanes, were also detected by GC-MS. Some of these products, especially branched alkanes, might be derived from ringopening reactions of cyclohexanes through hydrogenolysis.26,31 In addition, the hydrogenation of C16−C18 fatty acids and esters from some lipophilic polymers, such as cutin or suberin moieties, in the pine wood can also generate linear alkanes typically with carbon numbers higher than nonane.32 On the other hand, no obvious pressure changes were noticed before and after HDO reactions, and the collected gas products were analyzed by GC-FID. Results indicated that the yield of gas products from both two types of lignin was negligibly less than 5 wt %. The conversion of lignin was slightly higher than the yield of all detected products, indicating that the undetectable lignin depolymerized oligomers might be present but could not be extracted by ethyl acetate or analyzed by GC-MS. In addition to the lignin structure, the catalyst is another key factor that can affect the HDO reaction of lignin. A class of bifunctional catalysts (acid + metal) has been extensively investigated and applied in lignin HDO conversion. On the basis of recent research, both components in the bifunctional catalyst are indispensable to achieve efficient HDO conversion of lignin.2,24,33−35 However, very limited reports provided information on respective effects of each component on lignin HDO. In order to gain more insights on such effects, lignin HDO reactions under different loading/ratios of HY and Ru/ 1827

DOI: 10.1021/acssuschemeng.6b02563 ACS Sustainable Chem. Eng. 2017, 5, 1824−1830

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ACS Sustainable Chemistry & Engineering Table 1. Hydrodeoxygenation of RISL under Various Conditionsa,b selectivity (%)

a

run

HY (mg)

Ru/Al2O3 (mg)

conversion (wt %)

product yield (wt %)

oxy-compounds

cyclohexanes

ring-opened alkanes

1 2 3 4 5 6 7c 8d

50 100 200 300 200 100 200 200

100 100 100 100 50 200 100 100

31.2 38.4 57.6 59.5 54.3 49.6 51.3 58.9

9.8 13.2 20.1 21.4 19.6 17.3 18.2 22.3

2.2 3.8 4.5 4.7 15.3 1.4 55.6 3.8

80.3 78.6 72.1 70.4 66.3 72.2 29.2 72.6

17.5 17.6 23.4 24.9 18.4 26.4 15.2 23.6

Reaction conditions: RISL 100 mg, water 30 mL, 4 h, 250 °C. bPH2 = 4 MPa, cPH2 = 1 MPa, dPH2 = 6 MPa.

Al2O3 were investigated in this study. RISL was chosen as the reactant since ReL showed lower activity during the HDO conversion. It was found that lignin conversion was significantly dependent on the loading of HY. As listed in Table 1, runs 1−3, with 100 mg loading of Ru/Al2O3, increasing the loading of HY from 50 to 200 mg significantly improved lignin conversion from 31% to 58%, respectively. Even at lower Ru/Al2O3 loading of 50 mg, lignin conversion reached 54.3% with 200 mg of HY high loading (Table 1, run 5). However, with lower loading of 100 mg of HY, high Ru/Al2O3 loading of 200 mg only resulted in a relatively low lignin conversion of 49.6% (Table 1, run 6). Results indicated that the acid component of HY, as compared with Ru/Al 2 O3 , played a more crucial role in lignin depolymerization. In addition, the product distributions were also affected by the catalyst compositions. High loading of Ru/ Al2O3 not only favored the production of deoxygenated products but also benefited the generation of ring-opening products (Table 1, run 6), which may be due to the enhancement of both hydrogenation and hydrogenolysis reactions under high Ru/Al2O3 loading. Moreover, hydrogen pressure was found important to the selectivity of products, although lignin conversion and product yield were not significantly affected. With the same loading of 200 mg of HY and 100 mg of Ru/Al2O3, when the hydrogen pressure was as low as 1 MPa, the dominant type of products was oxy-compounds with a selectivity of 55.6%, while the selectivity of cyclohexanes reached about 72% at a higher hydrogen pressure of 4 MPa. Increasing hydrogen pressure from 1 to 4 MPa only slightly improved the selectivity of ringopened alkanes. Higher hydrogen pressure at 6 MPa led to the similar selectivity of major products as that at 4 MPa. Results suggested that relatively high hydrogen pressure (i.e., 4 or 6 MPa) was necessary to yield deoxygenated products with high selectivity during HDO conversion of lignin. Lignin conversion, total product yield, and selectivity of products during the RISL HDO conversion are shown in Figure 5. It took about 18 min to reach the target reaction temperature of 250 °C, at which point it was set as time zero. Both lignin conversion and total product yield increased with the reaction time. The obvious gap between lignin conversion and total product yield suggested that a considerable amount of oligomer products existed in the reaction solution. Lignin conversion continuously increased with the reaction time until the reaction time was over 4 h. The slowing of the lignin conversion rate over time plausibly resulted from the inherent heterogeneity of lignin. Ether linkages, such as β-O-4 and α-O4, could be easily disrupted at the initial stage of the reaction because of their relatively low bond dissociation energies.16,32 However, condensed structures (e.g., 5−5′ bonds) were found

Figure 5. 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 °C.

as barriers to lignin conversion, thus slowing the rate of lignin conversion during the later stage of HDO reaction. Moreover, additional condensed structures and short side chains generated from the intramolecular polymerization of lignin can further decrease its HDO reactivity.32,36 As shown in Figure 5, products distribution significantly changed over time. The initial ethyl acetate extracted products mainly contain monooxy-compounds (e.g., guaiacol, cresol, cyclohexanols) and cyclohexanes. As the reaction proceeded, the selectivity of oxy-compounds dramatically decreased, while that of cyclohexanes increased rapidly, indicating that the primary products of oxy-compounds could be converted to cyclohexanes. Compared to oxy-compounds and cyclohexanes, the selectivity of ring-opening products only slightly increased, plausibly due to hydrogenolysis of cyclohexanes products.25 It should be noted that, after 2 h of reaction, the deoxygenated alkanes within the carbon range of C9−C18 were dominant in the extracted products (>95 wt %), further demonstrating that the current catalytic system was capable of converting biomassderived lignin into advanced biofuels. Possible Reaction Channels Involved in Lignin HDO Conversion. The HDO conversion of lignin is a complicated process, involving various kinds of reactions and pathways. On the basis of previous reports18,26 and results obtained from the current study, we proposed some tentative reaction pathways of lignin HDO conversion (Figure 6). The majority of C−O−C bonds existing in lignin have relatively lower bond dissociation 1828

DOI: 10.1021/acssuschemeng.6b02563 ACS Sustainable Chem. Eng. 2017, 5, 1824−1830

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Figure 6. Possible reaction channels involved in lignin HDO conversion.

energy than the C−C bonds; thus, they can be facilely disrupted either by acid-catalyzed hydrolysis or through metalcatalyzed hydrogenolysis. The cleavage of C−O−C bonds can depolymerize lignin into its oligomers, dimers, and monomers. In most cases, these products are unstable oxy-compounds under the HDO reaction conditions. They can be converted to each other via further depolymerization or rearrangement reactions. Lignin dimers and monomers can also be converted to other oxy-compounds via reactions, including alkylation, demethylation, demethoxylation, etc. Under the combination catalysis of HY and Ru/Al2O3, all the small molecular oxycompounds can be converted into more stable cyclohexanes through deoxygenation and hydrogenation reactions. Ringopening products (e.g., linear and branched alkanes) can be generated by hydrogenolysis of cyclohexanes. Most importantly, the presence of condensed structures and short side chains is a notable disadvantage for lignin HDO conversion since it can prevent lignin from depolymerization and induce the formation of char. In addition to a certain amount of inherent C−C bond linkages, lignin condensed structures and short side chains can be also formed during both the biomass pretreatment process and lignin conversion process. Thus, the minimization of lignin condensation reactions and rearrangement reactions during the separation and conversion processes is important for the production of advanced biofuels or high value-added chemicals from the biopolymer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+1) 509-372-7640. Fax: 509-372-7690. ORCID

Arthur J. Ragauskas: 0000-0002-3536-554X Bin Yang: 0000-0003-1686-8800 Present Address ⊥

H. Ben: Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, P.R. China.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Sun Grant-DOT Award No. T0013G-ATask 8 and the Seattle-based Joint Center for Aerospace Technology Innovation for funding this research. We acknowledge the Bioproducts, Sciences and Engineering Laboratory, Department of Biosystems Engineering at Washington State University and The Boeing Company. Part of this work was conducted at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility located at the Pacific Northwest National Laboratory (PNNL) and sponsored by the Department of Energy’s Office of Biological and Environmental Research (BER). Oak Ridge National Laboratory (ORNL) is managed by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. We also thank Dr. Langli Luo and Dr. Chongmin Wang for their assistance on STEM testing and Dr. Yuling Qin and Ms. Marie S. Swita for insightful discussions.

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CONCLUSION Lignin’s reactivity plays a crucial role in its commercial applications for the production of fuels and chemicals. Results from this study indicated that condensed structures and short side chains in lignin had significant impacts on yields of valuable precursors for fuels and chemicals. HY zeolite was found vital during the depolymerization of lignin, while Ru/ Al2O3 and relatively high hydrogen pressure (i.e., 4 or 6 MPa) were necessary for the upgrading of unstable oxy-compounds to cyclohexanes with high selectivity (>95 wt %), and then further converted to ring-opened alkanes. Ru/Al2O3 was found to be an efficient catalyst for the upgrading of aromatic oxycompounds to more stable products such as cyclohexanes. Tentative reaction pathways of lignin HDO conversion were proposed. Overall, in order to fully unlock the potential of lignin, a sleep giant in modern biorefinery, lignin itself, catalysts, and reaction conditions should all be taken into account carefully.

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Information as mentioned in the text. (PDF)

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REFERENCES

(1) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Lignin Valorization: Improving Lignin Processing in the Biorefinery. Science 2014, 344, 709−720. (2) Wang, H.; Ruan, H.; Pei, H.; Wang, H.; Chen, X.; Tucker, M. P.; Cort, J. R.; Yang, B. Biomass-derived Lignin to Jet Fuel Range Hydrocarbons via Aqueous Phase Hydrodeoxygenation. Green Chem. 2015, 17, 5131−5135. (3) Laskar, D. D.; Yang, B.; Wang, H.; Lee, J. Pathways for biomassderived lignin to hydrocarbon fuels. Biofuels, Bioprod. Biorefin. 2013, 7, 602−626. (4) Vispute, T. P.; Zhang, H. Y.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable Chemical Commodity Feedstocks from Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330, 1222−1227.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02563. 1829

DOI: 10.1021/acssuschemeng.6b02563 ACS Sustainable Chem. Eng. 2017, 5, 1824−1830

Research Article

ACS Sustainable Chemistry & Engineering (5) Holladay, J.; White, J.; Bozell, J.; Johnson, D. Top Value-Added Chemicals from Biomass. Volume IIResults of Screening for Potential Candidates from Biorefinery Lignin; PNNL-16983; Pacific Northwest National Laboratory: Richland, WA, 2007. (6) Laskar, D. D.; Tucker, M. P.; Chen, X.; Helms, G. L.; Yang, B. Noble-metal catalyzed hydrodeoxygenation of biomass-derived lignin to aromatic hydrocarbons. Green Chem. 2014, 16, 897−910. (7) Rinaldi, R.; Jastrzebski, R.; Clough, M. T.; Ralph, J.; Kennema, M.; Bruijnincx, P. C.; Weckhuysen, B. M. Paving the way for lignin valorisation: Recent advances in bioengineering, biorefining and catalysis. Angew. Chem., Int. Ed. 2016, 55, 8164−8215. (8) Saito, T.; Perkins, J. H.; Vautard, F.; Meyer, H. M.; Messman, J. M.; Tolnai, B.; Naskar, A. K. Methanol Fractionation of Softwood Kraft Lignin: Impact on the Lignin Properties. ChemSusChem 2014, 7, 221−228. (9) Vishtal, A. G.; Kraslawski, A. Challenges in industrial applications of technical lignins. BioResources 2011, 6, 3547−3568. (10) Galbe, M.; Zacchi, G. A review of the production of ethanol from softwood. Appl. Microbiol. Biotechnol. 2002, 59, 618−628. (11) Zakzeski, J.; Bruijnincx, P. C.; Jongerius, A. L.; Weckhuysen, B. M. The catalytic valorization of lignin for the production of renewable chemicals. Chem. Rev. 2010, 110, 3552−3599. (12) Trajano, H. L.; Engle, N. L.; Foston, M.; Ragauskas, A. J.; Tschaplinski, T. J.; Wyman, C. E. The fate of lignin during hydrothermal pretreatment. Biotechnol. Biofuels 2013, 6, 110. (13) Martin-Sampedro, R.; Capanema, E. A.; Hoeger, I.; Villar, J. C.; Rojas, O. J. Lignin changes after steam explosion and laccase-mediator treatment of eucalyptus wood chips. J. Agric. Food Chem. 2011, 59, 8761−8769. (14) Yan, L.; Zhang, L.; Yang, B. Enhancement of total sugar and lignin yields through dissolution of poplar wood by hot water and dilute acid flowthrough pretreatment. Biotechnol. Biofuels 2014, 7, 76. (15) Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y.; Holtzapple, M.; Ladisch, M. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 2005, 96, 673−686. (16) Stiefel, S.; Marks, C.; Schmidt, T.; Hanisch, S.; Spalding, G.; Wessling, M. Overcoming lignin heterogeneity: reliably characterizing the cleavage of technical lignin. Green Chem. 2016, 18, 531−540. (17) Klein, I.; Saha, B.; Abu-Omar, M. M. Lignin depolymerization over Ni/C catalyst in methanol, a continuation: effect of substrate and catalyst loading. Catal. Sci. Technol. 2015, 5, 3242−3245. (18) Bouxin, F. P.; McVeigh, A.; Tran, F.; Westwood, N. J.; Jarvis, M. C.; Jackson, S. D. Catalytic depolymerisation of isolated lignins to fine chemicals using a Pt/alumina catalyst: part 1impact of the lignin structure. Green Chem. 2015, 17, 1235−1242. (19) Song, Q.; Wang, F.; Cai, J. Y.; Wang, Y. H.; Zhang, J. J.; Yu, W. Q.; Xu, J. Lignin depolymerization (LDP) in alcohol over nickel-based catalysts via a fragmentation-hydrogenolysis process. Energy Environ. Sci. 2013, 6, 994−1007. (20) Shu, R.; Long, J.; Xu, Y.; Ma, L.; Zhang, Q.; Wang, T.; Wang, C.; Yuan, Z.; Wu, Q. Investigation on the structural effect of lignin during the hydrogenolysis process. Bioresour. Technol. 2016, 200, 14− 22. (21) Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D.; Crocker, D. Determination of Structural Carbohydrates and Lignin in Biomass; National Renewable Energy Laboratory: Denver, CO, 2005 (22) Iiyama, K.; Wallis, A. F. A. Determination of lignin in herbaceous plants by an improved acetyl bromide procedure. J. Sci. Food Agric. 1990, 51, 145−161. (23) Maciel, G. E.; O’Donnell, D. J.; Ackerman, J. J. H.; Hawkins, B. H.; Bartuska, V. J. A 13C NMR study of four lignins in the solid and solution states. Makromol. Chem. 1981, 182, 2297−2304. (24) Zhang, W.; Chen, J.; Liu, R.; Wang, S.; Chen, L.; Li, K. Hydrodeoxygenation of Lignin-Derived Phenolic Monomers and Dimers to Alkane Fuels over Bifunctional Zeolite-Supported Metal Catalysts. ACS Sustainable Chem. Eng. 2014, 2, 683−691.

(25) Zhang, J.; Teo, J.; Chen, X.; Asakura, H.; Tanaka, T.; Teramura, K.; Yan, N. A Series of NiM (M = Ru, Rh, and Pd) Bimetallic Catalysts for Effective Lignin Hydrogenolysis in Water. ACS Catal. 2014, 4, 1574−1583. (26) Singh, S. K.; Ekhe, J. D. Towards effective lignin conversion: HZSM-5 catalyzed one-pot solvolytic depolymerization/hydrodeoxygenation of lignin into value added compounds. RSC Adv. 2014, 4, 27971−27978. (27) Scanlon, J. T.; Willis, D. E. Calculation of Flame Ionization Detector Relative Response Factors Using the Effective Carbon Number Concept. J. Chromatogr. Sci. 1985, 23, 333−340. (28) Sannigrahi, P.; Kim, D. H.; Jung, S.; Ragauskas, A. Pseudo-lignin and pretreatment chemistry. Energy Environ. Sci. 2011, 4, 1306−1310. (29) Donohoe, B. S.; Decker, S. R.; Tucker, M. P.; Himmel, M. E.; Vinzant, T. B. Visualizing lignin coalescence and migration through maize cell walls following thermochemical pretreatment. Biotechnol. Bioeng. 2008, 101, 913−925. (30) Constant, S.; Wienk, H. L.; Frissen, A. E.; de Peinder, P.; Boelens, R.; van Es, D. S.; Grisel, R. J.; Weckhuysen, B. M.; Huijgen, W. J.; Gosselink, R. J.; Bruijnincx, P. C. A. New insights into the structure and composition of technical lignins: a comparative characterisation study. Green Chem. 2016, 18, 2651−2665. (31) Ben, H.; Jarvis, M. W.; Nimlos, M. R.; Gjersing, E. L.; Sturgeon, M. R.; Foust, T. D.; Ragauskas, A. J.; Biddy, M. J. Application of a pyroprobe−Deuterium NMR system: deuterium tracing and mechanistic study of upgrading process for lignin model compounds. Energy Fuels 2016, 30, 2968−2974. (32) Joffres, B.; Nguyen, M.; Laurenti, D.; Lorentz, C.; Souchon, V.; Charon, N.; Daudin, A.; Quignard, A.; Geantet, C. Lignin hydroconversion on MoS 2-based supported catalyst: Comprehensive analysis of products and reaction scheme. Appl. Catal., B 2016, 184, 153−162. (33) Wang, X.; Rinaldi, R. Bifunctional Ni catalysts for the one-pot conversion of Organosolv lignin into cycloalkanes. Catal. Today 2016, 269, 48−55. (34) Singh, S. K.; Ekhe, J. D. Cu−Mo doped zeolite ZSM-5 catalyzed conversion of lignin to alkyl phenols with high selectivity. Catal. Sci. Technol. 2015, 5, 2117−2124. (35) Anaya, F.; Zhang, L.; Tan, Q.; Resasco, D. E. Tuning the acid− metal balance in Pd/and Pt/zeolite catalysts for the hydroalkylation of m-cresol. J. Catal. 2015, 328, 173−185. (36) Joffres, B.; Lorentz, C.; Vidalie, M.; Laurenti, D.; Quoineaud, A.A.; Charon, N.; Daudin, A.; Quignard, A.; Geantet, C. Catalytic hydroconversion of a wheat straw soda lignin: characterization of the products and the lignin residue. Appl. Catal., B 2014, 145, 167−176.

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DOI: 10.1021/acssuschemeng.6b02563 ACS Sustainable Chem. Eng. 2017, 5, 1824−1830