Understanding Low-Pressure Hydropyrolysis of Lignin Using

Research Article pubs.acs.org/journal/ascecg

Understanding Low-Pressure Hydropyrolysis of Lignin Using Deuterated Sodium Formate Wenqi Li,† Shuai Zhou,‡ Yuan Xue,† Young-Jin Lee,§ Ryan Smith,‡ and Xianglan Bai*,† †

Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, United States Bioeconomy Institute, Iowa State University, Ames, Iowa 50011, United States § Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States ‡

ABSTRACT: In the present study, hydropyrolysis of lignin was investigated by copyrolyzing lignin with sodium formate as the hydrogen donor in order to generate reactive hydrogen atoms under atmospheric pressure. It was found that the free hydrogen atoms released from thermal decomposition of sodium formate promotes the production of phenol, guaiacol, syringol, methyl and ethyl phenols, while reducing the yield of the phenolics with vinyl, propenyl and carbonyl groups. Acetic acid was eliminated by reacting with sodium carbonate byproduct. In comparison to pyrolyzing lignin alone, the pyrolysis-oil produced from copyrolysis of lignin and sodium formate contained a higher fraction of phenolic monomers and a lower fraction of phenolic oligomers both before and after storage. Deuterated sodium formate was copyrolyzed with lignin to investigate the mechanism of hydrogen transfer during low-pressure hydropyrolysis of lignin. Deuterium atoms were found in all of the GC/MS detectable phenolic compounds. As the amount of sodium formate increased, the fraction of deuterated molecules in total number of the compound molecules increased, as did the number of deuterium atoms in the individual deuterated molecules regardless if the compound yield increased or decreased. Among the products, phenol, and nonmethoxy methyl, and ethyl phenols were deuterated most significantly. The cleavage of side chain C−C and C−O bonds of lignin polymer by pyrolysis generates radical intermediates. The free hydrogen atoms in the vicinity not only promoted the bond cleavages but also capped the radical intermediates to avoid the coupling reactions among lignin radicals. Secondary reactions of primary pyrolysis products of lignin were also investigated by pyrolyzing phenolic monomers with deuterated sodium formate. It was found that in addition to thermal cracking, the reactive monomers likely polymerize and then further decompose during pyrolysis. The external hydrogen promotes the secondary reactions to produce more stable secondary products. KEYWORDS: Lignin, Hydropyrolysis, Sodium formate, Deuterium, Hydrogen transfer

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tion.14−16 An effective lignin depolymerization would not only provide an alternative source of biofuels and chemicals but also greatly improve the economic prospect of lignin-producing industries.16 Fast pyrolysis of biomass rapidly converts biomass into energy dense liquid, which can potentially be upgraded to hydrocarbon fuels and chemicals. Lignin-derived pyrolysis oil is a complex mixture of phenolic monomers and oligomers. This pyrolysis oil, rich in phenolic oligomers rather than monomers, is difficult to upgrade as it tends to form large amounts of char and coke during thermal volatilization and catalytic upgrading due to its low volatility and reactivity for deoxygenation. The stability of the pyrolysis oil is also poor, attributed to the reactivity of the phenolic compounds containing various functionalities. Condensation and polymerization of the phenolics occur during storage, which increase the molecular weight of the pyrolysis oil, as well as water content. Reactive free radicals generated from lignin pyrolysis, as well as

INTRODUCTION Renewable energy has received increasing attention due to growing concerns over greenhouse gas emissions and national energy security.1−3 Biomass is an important source of renewable energy because it can be a substitute for fossil fuels in the production of liquid transportation fuels and chemicals.4,5 To date, the conversion of carbohydrates to biofuels and chemicals is largely successful and has already reached commercial scale production.6−10 In comparison, lignin is much more difficult to convert to value-added products. Lignin accounts for 10−35% of lignocellulosic biomass and is the second most abundant biopolymer on the earth after cellulose.11 Isolated lignin is also abundantly available in paper, pulping industries and cellulosic biorefineries as a byproduct.12 Lignin is a randomly cross-linked, three-dimensional polymer biosynthesized from three primary precursor monomers: pcoumaryl alcohol, coniferyl alcohol and sinapyl alcohol.13 Because of its chemical structure, lignin can be a promising source of renewable aromatics. However, the majority of lignin is currently burned as low-quality boiler fuel.12 Lignin is difficult to utilize biologically or thermochemically because of its complex chemical structure and recalcitrance for depolymeriza© XXXX American Chemical Society

Received: June 2, 2017 Revised: August 9, 2017 Published: August 15, 2017 A

DOI: 10.1021/acssuschemeng.7b01748 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Scheme diagrams of Py/GC reactor system and Py-oil collection system.

This method was also applied to biomass and cellulose to produce bio-oil with improved qualities.25,26 Although less convenient than using hydrogen gas, calcium salt recovered from pyrolysis char can be regenerated in their approach. Unfortunately, limited discussions were offered in these studies to explain the underlying reaction mechanisms in molecular level. Future research effort in low-pressure hydropyrolysis of biomass or lignin is expected to increase. In the present study, the role of hydrogen during lowpressure hydropyrolysis of lignin was investigated. To produce reactive hydrogen atoms under low-pressure pyrolysis condition, lignin was copyrolyzed with sodium formate. The product composition was analyzed and the pyrolysis oil was collected to investigate its storage stability. Lignin and ligninderived phenolic model compounds were then copyrolyzed with deuterated sodium formate to track hydrogen atoms in the products to determine the reaction mechanisms of lignin hydropyrolysis.

unsaturated C=C and C=O functionalities are responsible for various secondary reactions.17,18 In addition to pyrolysis oil, lignin also produces over 30% of char that has less value than pyrolysis oil. Pyrolyzing lignin with catalysts, additives and/or reactive gases could improve the product distribution and the selectivity of the products in pyrolysis oil. For example, pyrolysis of lignin with zeolite catalysts deoxygenates phenolic compounds to produce aromatic hydrocarbons. Although this approach also helps to stabilize pyrolysis oil, efficiency of carbon conversion is low and severe coking quickly deactivates the catalyst. Thermal and catalytic conversions of lignin often result in very high yields of solid residue, which is one of the most problematic issues in upgrading lignin. The pyrolysis char accounts for about 40% of lignin and together with catalytic coke, as much as 60% of lignin carbon is lost as solid residue.19,20 It was reported that using H2 as carrier gas could reduce char yield during biomass pyrolysis and increase heating value of pyrolysis oil.21 Catalytic hydropyrolysis of lignin also reduced the coke formation and increased the yield of hydrocarbons.22 Catalytic hydropyrolysis, however, is usually conducted at elevated pressures and an undesired saturation of aromatic rings could occur under such conditions.23 Effective low-pressure hydropyrolysis is highly preferred because it can reduce hydrogen consumption while also eliminating the challenges in operating pressurized reactors. Instead of hydrogen gas, hydrogen donor agents can be used during lignin hydropyrolysis to enable low-pressure operation. Mukkamala et al.24 pyrolyzed a lignin pretreated with both calcium hydroxide and formic acid through a two-step liquid impregnation method. They reported an increasing yield of pyrolysis oil and decreasing char yield from pyrolysis of the pretreated lignin in comparison to pyrolyzing untreated lignin. The formate salt formed during the pretreatment process acted as low-pressure hydrogen donor and the derived free hydrogen atoms effectively improved the quality of pyrolysis products.

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MATERIALS AND METHODS

Materials. Organosolv lignin derived from corn stover was provided by Archer Daniels Midland (ADM). The lignin is free of carbohydrate residues. Characterization of raw lignin is as follows: C, H, N, S and O (by difference) contents of the lignin were 62.58, 4.70, 1.63, 0.24% and 30.86%; moisture, volatile, fixed carbon and ash contents are 2.15, 60.97, 36.56 and 0.32%; The average molecular weight (Mw) and number-averaged molecular weight (Mn) of lignin are 3198 and 1421 Da. The herbaceous lignin contains ferulate and pcoumarate groups, in addition to hydroxylphenol (H), syringol (S) and guaiacol (G) groups. The two most abundant linkages in the lignin are β−O−4, β−5 linkages. Methanol, acetone and tetrahydrofuran (THF) were purchased from Fisher Scientific Company. Sodium formate, deuterium sodium formate and chemicals used as calibration standards were purchased from Sigma-Aldrich Company. Pyrolysis. Pyrolysis experiments were performed using a Frontier Lab Tandem μ-Reactor (Rx-3050TR) with an Auto-Shot sampler (AS1020E) pyrolysis system. The reactor system has two furnaces arranged in tandem. The temperatures of the furnaces are controlled B


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ACS Sustainable Chemistry & Engineering Table 1. GC/MS Detected Monomers from Pyrolysis of Lignin without and with Sodium Formate Product Yield (wt %) No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Compound assignment Benzene Acetic acid Toluene Phenol 2-Methoxyphenol 2-Methylphenol 4-Methylphenol 2-Methoxy-4-methylphenol 3,5-Dimethylphenol 4-Ethylphenol 2-Methoxyl-4-ethylphenol 4-Vinylphenol 2-Methoxy-4-vinylphenol 2,6-Dimethoxyphenol trans-Isoeugenol 1,2,4-Trimethoxybenzene Vanillin 1,2,3-Trimethoxy-5-methylbenzene 3′,5′-Dimethoxyacetophenone Phenol, 2,6-dimethoxy-4-(2-propenyl)Ethanone, 1-(4-hydroxy-3,5dimethoxyphenyl)-

Control 0.028 1.989 0.063 0.578 0.540 0.053 0.328 0.460 0.043 0.379 0.240 4.255 2.400 0.711 0.133 0.431 0.141 0.152 0.275 0.554 0.159

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.004 0.166 0.001 0.017 0.023 0.004 0.003 0.017 0.003 0.016 0.019 0.107 0.032 0.031 0.006 0.018 0.015 0.007 0.004 0.014 0.007

1:0.5 0.025 0.286 0.063 0.921 0.893 0.099 0.207 0.238 0.057 0.459 0.249 4.159 2.405 1.006 0.101 0.195 0.090 0.156 0.281 0.461 0.094

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

independently from 40 to 900 °C with 1 °C intervals. In the present study, the temperature of the first furnace was kept at 500 °C, and the temperature of the second reactor was set at 300 °C to prevent condensation of pyrolysis products. The sample was pyrolyzed in the first furnace and the pyrolytic vapors passed through an empty second furnace before entering an online Agilent 7890B/5977A GC/MSD system. Helium was used both as the sweep gas in the micropyrolyzer and the carrier gas in the GC/MS. The GC column was an alloy capillary column (Ultra Alloy-1701). The GC oven was heated from 40 to 280 °C at a heating rate of 6 °C/ min and kept at the final temperature for an additional 10 min. The MS signal was used to identify the reaction products. The products were quantified in the FID. The calibration curves of the products were created using five different concentrations of authentic chemical compounds. For each test, 500 ± 10 μg of lignin or phenolic model compounds were pyrolyzed alone or mixed with natural or deuterated sodium formate and then pyrolyzed. The ratio of lignin to sodium formate ranged from 1:0.5 to 1:4. The ratio of phenolic model compound and sodium formate was 1:1. Collection of Pyrolysis Vapor and Storage. The micropyrolyzer was placed on top of a metal frame with a circular hole in the middle of the frame. The GC needle attached to the bottom of the pyrolyzer reactor was inserted into a rubber stopper that capped an end of a U-shaped glass tube filled with glass beads. The glass tube was cooled using liquid nitrogen bath to condense the pyrolysis oil when the samples were pyrolyzed. Noncondensable gases were released from the open end of the glass tube. The schematic diagrams of the pyrolysis reactor and pyrolysis oil collection system are shown in Figure 1. Lignin or the mixtures of lignin and sodium formate were pyrolyzed in the micropyrolyzer by manual injection. To collect enough pyrolysis oil, ten cups of each type of sample were sequentially pyrolyzed. After all of the samples were pyrolyzed, the wall of the glass tube and the beads were rinsed with 2.5 mL of THF or methanol to collect pyrolysis oil. The pyrolysis oil collected in methanol was placed inside a fume hood to evaporate methanol. After methanol was evaporated, the glass vial containing the pyrolysis oil was sealed and kept at room

1:1

0.002 0.087 0.000 0.019 0.023 0.000 0.001 0.007 0.001 0.000 0.004 0.026 0.018 0.033 0.005 0.006 0.003 0.005 0.001 0.001 0.005

0.027 0.293 0.071 0.970 0.949 0.107 0.229 0.239 0.062 0.474 0.261 4.113 2.343 1.099 0.103 0.206 0.091 0.167 0.277 0.443 0.093

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1:2 0.004 0.142 0.005 0.056 0.085 0.009 0.020 0.022 0.003 0.021 0.017 0.090 0.023 0.094 0.008 0.024 0.012 0.008 0.005 0.032 0.010

0.030 0.043 0.071 1.029 1.021 0.122 0.222 0.212 0.071 0.493 0.267 4.013 2.295 1.191 0.092 0.176 0.080 0.178 0.271 0.416 0.072

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1:3 0.002 0.006 0.002 0.003 0.001 0.007 0.011 0.000 0.006 0.015 0.006 0.042 0.035 0.000 0.000 0.005 0.001 0.004 0.008 0.006 0.000

0.033 0.000 0.076 1.067 1.028 0.135 0.223 0.213 0.080 0.541 0.270 3.808 2.171 1.233 0.085 0.168 0.058 0.186 0.257 0.402 0.061

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1:4 0.003 0.000 0.004 0.080 0.043 0.013 0.006 0.003 0.007 0.041 0.020 0.302 0.194 0.040 0.011 0.012 0.020 0.009 0.037 0.045 0.015

0.031 0.000 0.077 1.049 1.014 0.143 0.238 0.217 0.088 0.542 0.266 3.844 2.188 1.186 0.086 0.171 0.054 0.181 0.271 0.404 0.063

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.002 0.000 0.003 0.032 0.049 0.006 0.008 0.008 0.005 0.007 0.003 0.142 0.077 0.063 0.003 0.009 0.005 0.005 0.021 0.008 0.008

temperature for 2 weeks. After storage, THF was added to dissolve the pyrolysis oil. Gel Permeation Chromatography of Pyrolysis Oil. The molecular weight distributions of the fresh pyrolysis oil and the stored pyrolysis oil were determined by conducting gel permeation chromatography (GPC) analysis. The GPC test was performed in a Dionex Ultimate 3000 high performance liquid chromatography system, which was equipped with a Shodex refractive index (RI) and a diode array detector (DAD). The GPC column was calibrated using six polystyrene standard samples with the molecular weights ranging from 162 to 10110 Da.

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RESULTS AND DISCUSSION Product Distribution. Table 1 shows the yields of GC/MS detectable compounds produced from pyrolysis of the organosolv lignin without and with sodium formate. Among the compounds, 4-vinylphenol and 2-methoxy-4-vinylphenol were two major products of lignin and their weight based yields were 4.3% and 2.4% by pyrolyzing lignin alone. Other major phenolic compounds include phenol, guaiacol, syringol, alkylated phenols, phenolic aldehyde, phenolic ketone, trimethoxyl benzene etc. Acetic acid and small quantities of benzene and toluene were also produced. When lignin was copyrolyzed with sodium formate, the yields of the phenols containing unsaturated carbon side chains (i.e., 4-vinylphenol, 2-methoxy-4-vinylphenol, trans-isoeugenol, 2,6-dimethoxy-4-(2propenyl)phenol) and carbonyl groups (i.e., 1-(4-hydroxy-3,5dimethoxyphenyl) ethanone, vanillin) decreased. On the other hand, the yields of most methyl and ethyl phenols (i.e., 2methylphenol, 4-ethyphenol, 2-methoxy-4-ethylphenol and 1,2,3-trimethoxy-5-methylbenzene) increased. Copyrolysis of lignin with sodium formate also increased the yields of simple phenols (i.e., phenol, guaiacol and syringol), as well as benzene and toluene. These results suggest that the presence of sodium formate promotes the formation of compounds with saturated functionalities and enhances cracking reactions. The vinylphenols are preferentially produced from the cleavage of β−O−

C


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Table 2. GC/MS Detected Monomers When Lignin Was Copyrolyzed with Sodium Formate or Stoichiometric Amount of Sodium Carbonate Product yield (wt %)

a

No.

Compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Benzene Acetic acid Toluene Phenol 2-Methoxyphenol 2-Methylphenol 4-Methylphenol 2-Methoxy-4-methylphenol 3,5-Dimethylphenol 4-Ethylphenol 4-Ethyl-2-methoxyphenol 4-Vinylphenol 2-Methoxy-4-vinylphenol 2,6-Dimethoxyphenol trans-Isoeugenol 1,2,4-Trimethoxybenzene Vanillin 1,2,3-Trimethoxy-5-methylbenzene 3′,5′-Dimethoxyacetophenone Phenol, 2,6-dimethoxy-4-(2-propenyl)Ethanone, 1-(4-hydroxy-3,5-dimethoxyphenyl)-

Lignin 0.028 1.989 0.063 0.578 0.540 0.053 0.328 0.460 0.043 0.379 0.240 4.255 2.400 0.711 0.133 0.431 0.141 0.152 0.275 0.554 0.159

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.004 0.166 0.001 0.017 0.023 0.004 0.003 0.017 0.003 0.016 0.019 0.107 0.032 0.031 0.006 0.018 0.015 0.007 0.004 0.014 0.007

w/HCOONa (1:2)

w/Na2CO3 (1:1)a

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.030 ± 0.001

0.030 0.043 0.071 1.029 1.021 0.122 0.222 0.212 0.071 0.493 0.267 4.013 2.295 1.191 0.092 0.176 0.080 0.178 0.271 0.416 0.072

0.002 0.006 0.002 0.003 0.001 0.007 0.011 0.000 0.006 0.015 0.006 0.042 0.035 0.000 0.000 0.005 0.001 0.004 0.008 0.006 0.000

0.056 0.629 0.525 0.059 0.251 0.285 0.041 0.420 0.186 3.279 1.819 0.615 0.092 0.234 0.083 0.108 0.205 0.366 0.073

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.006 0.036 0.011 0.008 0.012 0.005 0.002 0.067 0.005 0.109 0.110 0.011 0.001 0.005 0.001 0.001 0.001 0.001 0.000

Stoichiometric mole equivalence ratio.

with hydrogen increased the concentration of acetic acid in biooil. In the present study, the yield of acetic acid decreased, possibly because the acid reacted with sodium salt. Sodium formate decomposes at 330 °C to produce sodium carbonate, carbon monoxide and hydrogen atoms (eq 1). The produced sodium carbonate could then absorb acetic acid before the acid is evaporated.

4 linkages during pyrolysis of lignin. Particularly, pyrolysis of corn stover lignin produces high yields of vinylphenols because the lignin is rich in coumarate and ferulate groups.27 The vinylphenols could be saturated by hydrogen atoms released from thermal decomposition of sodium formate to become ethylphenols. Alternatively, ethylphenols could be produced from β−O−4 bond cleavage as the primary products when free hydrogen atoms are available. Zhang et al.28 modeled pyrolysis of lignin β−O−4 dimers and found that the activation energy required to form ethylphenols from β−O−4 cleavage is much lower than the energy needed for forming vinylphenols (148.4 vs 274 kJ/mol). However, instead of ethylphenols, vinylphenols are the major products of lignin β−O−4 cleavage because forming ethylphenols requires donor hydrogen whereas the amount of free hydrogen atoms generated during lignin pyrolysis is limited. Accordingly, the reaction pathway that forms ethylphenols was favored when free hydrogen atoms were externally provided. In the presence of external hydrogen, the phenolic aldehydes and ketones could be hydrogenated to form phenolic alcohols. However, the corresponding phenolic alcohols were not detected in the GC/MS chromatogram when lignin and sodium formate were copyrolyzed. This suggests that carbonyl groups might have been hydrodeoxygenated to alkyl groups. The yields of simpler phenols, as well as benzene and toluene, increased, suggesting hydrogen promotes cracking reactions. As the amount of sodium formate increased, the increasing or decreasing trend of the products became more noticeable. Copyrolyzing lignin with sodium formate also significantly reduced the yield of acetic acid. Acetic acid in bio-oil has detrimental effects because the acid is mainly responsible for the corrosiveness of bio-oil and also facilitates bio-oil aging.29 As shown in Table 1, acetic acid completely disappeared when the ratio of lignin to sodium formate increased to 1:3. Previously, Zhang et al.30 reported that pyrolysis of biomass

2HCOONa → Na 2CO3 + 2H + CO

(1)

Previously, Zhang et al.31 used a bed filled with calcium carbonate to remove acetic acid from the pyrolysis vapor of biomass. They reported that acetic acid is adsorbed on the surface of calcium carbonate to form acetate salt. They also stated that the acetate salt is thermally unstable, thus further decomposing at a higher temperature to release ketone. As indicated in Equation 1, every 2 mol of sodium formate would produce 1 mol of sodium carbonate during thermal decomposition. To evaluate the possible effect of sodium carbonate derived from sodium formate, lignin was copyrolyzed with the stoichiometric amount of sodium carbonate and the results are compared with that of copyrolysis of lignin and sodium formate in Table 2. The yield of acetic acid decreased from 2.0% to 0.043% by copyrolyzing lignin with sodium formate, whereas no acetic acid was found among the products when lignin was copyrolyzed with the stoichiometric amount of sodium carbonate. This result supports the hypothesis that the elimination of acetic acid is due to the carbonate salt produced from the thermal decomposition of sodium formate. The yield of acetic acid was slightly higher when lignin was copyrolyzed with sodium formate than with the stoichiometric amount of sodium carbonate, possibly because lignin started to produce acetic acid prior to sodium formate decomposing to sodium carbonate. However, the small difference in the yields could also be a derivation of the experimental data. As shown in Table 2, it was also found that sodium carbonate reduces the yields of D


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ACS Sustainable Chemistry & Engineering vinylphenols, propenylphenol, ketones and aldehydes. The yield of 2-methoxyl-4-ethylphenol also decreased, whereas 4ethylphenol yield increased. These changes are attributed to the effect of sodium cation or carbonate anion on lignin pyrolysis. It was previously suggested that the presence of inorganic salts during pyrolysis of biomass promotes dehydration, decarboxylation, demethoxylation and carbonization reactions, and also lowers the temperature for the maximum mass loss rate.32 Alkali metal ions, such as Na and K, usually have a stronger catalytic effect than alkaline earth metal ions. On the other hand, it was found that the yields of phenol, guaiacol, syringol and alkylated phenols were not affected significantly as they were observed with sodium formate. Di Blasi et al.33 also reported that the effect of carbonate and acetate salts are relatively small compared to other anion forms of salts. The different product distributions of lignin between copyrolysis with sodium formate and copyrolysis with sodium formate in Table 2 suggest that hydrogen atoms released during thermal decomposition of sodium formate are responsible for many of the observed changes in the yield and selectivity of the phenolics. Storage Stability of Pyrolysis Oil. The GPC results of raw lignin and lignin-derived pyrolysis oils in Figure 2a show the molecular weight (MW) distributions of fresh pyrolysis oils produced from lignin or the mixture of lignin with different amounts of sodium formate. The major peak appearing at the left side of the chromatogram (MW ∼ 130 Da) corresponds to phenolic monomers whereas the smaller, broader peak appearing at the right side of the chromatogram (MW ∼ 360 Da) corresponds to phenolic oligomers. In comparison to the pyrolysis oil of lignin, the pyrolysis oils produced from copyrolysis of lignin and sodium formate had an increased intensity of the monomer peak and decreased intensity of the oligomer peak. This trend became even stronger as the amount of sodium formate increased. This result indicates that copyrolysis of lignin with sodium formate suppresses the formation of phenolic oligomers and increases the concentrations of phenolic monomers and other low MW products in the pyrolysis oil. The GPC results of the pyrolysis-oils produced from lignin or the mixture of lignin and sodium formate before and after storage are compared in Figure 2b,c, respectively. After the fresh pyrolysis oils are stored at room temperature for 2 weeks, the intensity of the monomer peak decreased whereas the intensity of the oligomer peak increased in the oils. The oligomer peak also tailed toward the higher MW region in the stored pyrolysis oils, which is attributed to the polymerization of the phenolic monomers and low MW oligomers to higher MW oligomers during storage. For the pyrolysis oil of lignin, the oligomer peak becomes the dominant peak after storage. In comparison, the monomer peak remains the major peak in the stored pyrolysis oil derived from the mixture of lignin and sodium formate. The fresh or stored pyrolysis oils produced from the mixture of lignin and sodium formate both have lower average MWs in comparison to the pyrolysis oils of lignin under the same conditions. Vinylphenols and phenolic aldehydes could polymerize during storage with carboxylic acids in pyrolysis oil catalyzing the polymerization.29,34 Copyrolysis of lignin with sodium formate reduced the yields of the reactive phenols and acetic acid, thus suppressing the polymerization. Copyrolysis of Lignin with Deuterated Sodium Formate. Although reducing acetic acid is helpful, providing free hydrogen atoms during lignin pyrolysis is essential in

Figure 2. Molecular weight distributions of pyrolysis oils based on GPC analysis: (a) raw lignin, and fresh pyrolysis oils produced from lignin pyrolysis and copyrolysis of lignin and sodium formate at different ratios; (b) pyrolysis oils produced from lignin before and after storage; (c) pyrolysis oils produced from the mixture of lignin and sodium formate before and after storage.

improving the quality of pyrolysis oil. To investigate the mechanism of external hydrogen transfer during lignin pyrolysis, deuterated sodium formate was copyrolyzed with lignin and deuterium (D) atoms in the molecules of the pyrolysis products were tracked. It is known that the substitution of an atom in a molecule with its isotope could change the rate of a chemical reaction, which is known as the kinetic isotopic effect. The deuterium isotopic effect on lignin pyrolysis was previously described by Watanabe et al.35 Although the authors reported that the deuterated phenolic dimers have lower conversions than the corresponding natural hydrogen phenolic dimers, D substitution did not change the reaction mechanism or produce new products. Thus, using deuterium to investigate the mechanism of hydrogen transfer among the products is feasible as the reaction kinetics is not the E


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Figure 3. Peaks of m/z = 109, 110, 111, 112 and 113 in mass ion spectra of 2-methyl-phenol are caused by deuterated molecules.

Table 3. Deuterium Distribution in Pyrolysis Products and Effect of Lignin and Sodium Formate Ratio Percentage of compound molecule containing given number of D atom (%) Lignin to sodium formate = 1:0.5 Compounds/D atom number Toluene Phenol 2-Methoxyphenol 2-Methylphenol 4-Methylphenol 2-Methoxy-4-methylphenol 4-Ethylphenol 2-Methoxy-4-ethylphenol 4-Vinylphenol 2-Methoxy-4-vinylphenol 2,6-Dimethoxyphenol trans-Isoeugenol 1,2,4-Trimethoxybenzene Vanillin 1,2,3-Trimethoxy-5-methylbenzene 3′,5′-Dimethoxyacetophenone 2,6-Dimethoxy-4-(2-propenyl)phenol 1-(4-Hydroxy-3,5-dimethoxylphenol) ethanone

0 82 74 86 61 65 88 75 86 93 95 95 95 94 98 87 93 96 85

1 15 21 14 22 24 12 20 14 7 5 5 5 6 2 13 7 4 15

2 3 4

3 1

17 8

3

5

focus of this study. Previously, Ben et al.36 also used deuterium gas to investigate catalytic upgrading of lignin pyrolysis oil. The presence of a D atom in a compound molecule can be determined based on the mass-to-charge ratio (m/z) in the mass spectra of the compound. In the mass spectra, if the molecular ion peak of a pyrolysis product appears at m/z = M, the substitution of an H atom in its molecule by a D atom would cause the peak to appear at m/z = M+1 instead. The mass of corresponding peak would further increase by one mass unit each time when an additional H atom is substituted by a D atom. Taking 2-methylphenol as an example, as shown in Figure 3, the molecular ion peak of the compound has m/z 108. When lignin was copyrolyzed with D-sodium formate, m/z peaks also appeared at 109, 110, 111, 112 and 113, respectively. The highest m/z was 113, suggesting that 2-methyphenol molecule could contain up to five D atoms. The percentages of nondeuterated or deuterated molecules among total molecules of 2-methylphenol, as well as the distribution of D atom among deuterated individual molecules, were calculated using the method previously described by Mullen et al.37 In their method, the hydrogen loss during EI-MS measurement appearing at m/z = M−1 is taken into account by using the intensity ratio of M− 1 peak versus M peak in the pyrolysis products of nonisotropic organic compounds. The detailed calculation method is given in the Supporting Information of their publication. In the present study, the accuracy of the method was further improved

Lignin to sodium formate = 1:4 0 57 49 79 17 34 64 53 78 77 78 75 74 74 83 65 76 79 67

1 23 24 19 20 20 30 27 22 20 20 22 22 24 13 30 21 19 30

2 12 13 2 23 18 5 11 3 2 3 4 2 4 5 3 2 2

3 5 10

4 3 3

5

19 13

17 11

3 3

5

2

1

6

1

1

by considering the presence of 13C isotopic atoms in nature. There are 1.1 13C atoms in every 100 12C atoms in nature. Thus, the probability of 13C atoms appearing in a molecule containing n carbon atoms would be n × 1.1%. The presence of a 13C atom in a natural molecule would increase its molecular weight, causing a m/z = M+1 peak. The peak intensity of m/z = M+1 caused by 13C isotope would be n × 1.1% of m/z = M peak intensity. In our modified method, the 13C contribution to the m/z = M+1 peak was taken into account by calculating the peak intensity ratio of m/z = M to m/z = M+1, in addition to the peak intensity ratio of m/z = M-1 to m/z = M. Since 13C isotope atoms are also present in the molecules with deuterated atoms, both of the peak ratios were used to correct the peak intensities of deuterated molecules of the compound. Again, using the mass spectra of 2-methylphenol as an example, m/z = 108 is the main mass peak, m/z = 109 is due to the 13C isotope, and m/z = 107 is due to the hydrogen atom loss. Thus, the peak intensity ratios of m/z = 107 to m/z = 108, and m/z = 108 to m/z = 109 were calculated for a natural 2-methylphenol molecule based on its standard mass ion spectra. The same peak intensity ratios were assumed for m/z = 108 compared to m/z = 109, and m/z = 109 compared to m/z = 110, respectively, in a 2-methylphenol molecule containing one D atom. For a 2-methylphenol molecule containing two D atoms, the peak intensity ratios of m/z = 109 to m/z = 110, and m/z = 110 to m/z = 111 were corrected. It should be noted that the F


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Scheme 1. Hydrogen Transfer in Phenolic Compounds during Copyrolysis of Lignin and Deuterated Sodium Formatea

a

(a) β−O−4 structure; (b) α−O−4 structure; (c) 4−O−5 and 5−5 structure.

deuterated in the varied compounds. Also, the majority of the deuterated molecules contained one D atom. When lignin is pyrolyzed, β−O−4, α−O−4, 4−O−5, β−5, β−1, β−β and 5−5 linkages connecting two aromatic rings are cleaved and as a result, reactive free radicals are abundantly generated.38−42 Some of the radical intermediates could couple with ligninabstracted free hydrogen atoms to form phenolic monomers. Without free hydrogen atoms, the intermediate radicals could react with each other to form large MW oligomers or eventually form solid residue through cross-linking. When deuterated sodium formate was copyrolyzed with lignin, the radical intermediates could interact with free D atoms instead of other phenolic radicals. During the interaction, the radical end of the intermediates was capped by D atoms to produce deuterated phenolic monomers. The major linkages found in lignin structure are illustrated in Scheme 1, which are β−O−4, α−O−4, 4−O−5 and 5−5 linkages.12 The possible bond cleavages and subsequent D atom substitutions are proposed based on the results shown in Table 3. For 2-methoxyl-4-vinylphenol and 4-vinylphenol, about 5% of the total number of the molecules was deuterated and each deuterated molecule contained one D atom. In a classic β−O− 4 linkage of lignin, the oxygen bond in −Cβ (H2)−O− linkage is cleaved first. Next, a hydrogen atom is abstracted from −Cα (H2)− to form a vinyl group, −Cα (H)=Cβ (H2). If the Cα position is branched, the −C− or −O− bonded to the

hydrogen loss in EI-MS could also occur for a D that was replaced during the copyrolysis of lignin and sodium formate, which would then appear at m/z = M-2 instead of M-1. Monotonic isotope patterns, however, suggest such contribution might be minimal. Thus, D-loss was ignored in the present study, assuming hydrogen loss in EI-MS occurs only for nonreplaced original hydrogen. Deuterium distributions in the individual molecules of pyrolysis products are given in Table 3 for copyrolysis of lignin and deuterated sodium formate at two different ratios (1:0.5 and 1:4). Acetic acid was not listed because its molecules did not contain any D atoms. Benzene was also not included, because its m/z peak intensities in this study were too low for accurate peak value calculation, caused by the low product yield. Also, 3,5-methylphenol was not listed because its GC/MS peak was merged into the peak of a nearby appearing compound in the case of copyrolyzing lignin and deuterated sodium formate. For each compound listed in Table 3, not all of the molecules contain D atoms. The fraction of molecules do not containing D atoms are likely produced solely from thermal cracking of lignin without being affected by external hydrogen. The rest of the molecules contain D atoms because free D atoms donated by sodium formate reacted with the precursor of the compound during pyrolysis. When the ratio of lignin and sodium formate was 1:0.5, 2 to 25% of total numbers of the molecules were G


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aromatic ring via C−C bond, Cα (H) of lignin polymer is also linked to two other atoms on the side chain, such as −Cβ−Cα (H)−H, −Cβ−Cα (H)−O− or Cα (H)−H2. When Cβ and O in −Cβ−Cα (H)−O− are both cleaved away, two D atoms could be added to Cα (H) to form deuterated methyl group. Along with the possible D substitutions at the phenolic hydroxyl and two ortho positions of the aromatic ring, an individual molecule of nonmethoxyl methylphenol could accept a maximum of five D atoms when the compound is produced. Ethyl group could accept up to three D atoms (i.e., one D atom at the Cα position and two D atoms at the Cβ position if Cβ was originally branched). Together with the phenolic hydroxyl and two ortho positions of the aromatic ring, an individual molecule of ethylphenol could receive a maximum of six D atoms during its formation from lignin. Because of the occupied ortho positions, the maximum possible number of D atoms in a single methoxylated methyl or ethyl phenols is one fewer (or two fewer for dimethoxy methyl or ethyl phenols) than that of the corresponding nonmethoxyl methyl or ethylphenols. For phenol, the maximum possible number of D atoms in its molecule is four because the phenolic hydroxyl, the two ortho positions and the para position on the aromatic ring could accept D atoms. When the ratio of lignin to sodium formate was 1:4, the amount of free D atoms released from sodium formate increased. As a result, not only did the total amount of the available free hydrogen atoms increased, but the fraction of D atoms in the free hydrogen atoms (i.e., the sum of D atoms and lignin self-abstracted H atoms) also increased. Accordingly, more D atoms were incorporated into the individual molecules of the pyrolysis compounds, despite the fact that the yields of some phenolic compounds decreased by increasing amounts of sodium formate. For example, among 2-methoxy-4-vinylphenol molecules, the percentage of deuterated molecules increased from 5% to 22%. The molecules accounting for 20% of the total number of 2-methoxy-4-vinylphenol molecules contained one D atom in each molecule and the molecules with two D atoms were 2%. For the majority of the phenolic products, the deuterated molecules could contain up to two D atoms with the increased sodium formate amount, suggesting that at least two side chain C−C or C−O linkages cleaved followed by free hydrogen capping. The increasing trend of D atom substitution was most significant in methyl, ethyphenols and phenol molecules. The numbers of D atoms in individual molecules of 2-methylphenol and 4-methylphenol were up to five, 4-ethylphenol molecules were up to six and phenol molecules were up to four. As described above, the highest D atom numbers found in the individual molecules match the maximum possible D atom numbers these compounds could accept during pyrolysis. These results indicate that all the possible C−C and C−O bond cleavages described above occurred in the presence of an increased amount of free hydrogen atoms. In some molecules, all of the cleaving ends of the radical intermediates were capped by D atoms, probably because D atoms were locally abundant when the particular molecules formed from lignin. For 2methylphenol, the number of molecules containing one to five D atoms accounted for 20%, 23%, 19%, 17% and 3% of the total molecules, respectively. Only 17% of 2-methylphenol molecules were left nondeuterated. For phenol, the percentage of nondeuterated molecules also decreased from 74% with the lower sodium formate ratio (1:0.5) to 49% with the higher sodium formate ratio (1:4). The percentages of individual

branched side chain of Cα (H) also has to be cleaved in order to form the vinyl group. In this case, the hydrogen abstraction at Cα position is not needed. In either of these cases, both Cβ and Cα positions in lignin do not need a free hydrogen atom in order to form vinyl group. However, if Cβ position in lignin polymer is also branched (i.e., Cβ (H)−C−, or Cβ (H)−O−), Cβ would accept one free hydrogen atom to form vinyl group. In this case, the free D atom has a chance to enter the vinyl group. A D atom could also present in the phenolic hydroxyl of the monomers. For example, when oxygen is directly attached to an aromatic ring, an aromatic oxygen free radical is produced when the side chain linkage of oxygen, such as β−O−4, α−O− 4, or 4−O−5 cleaves. The aromatic oxygen radical is then capped by a free D atom to form deuterated phenolic hydroxyl. D atoms could also enter the aromatic ring of the vinylphenols through capping the aromatic radicals generated from the side chain cleavage at the ortho positions of the aromatic ring. The possible side chain linkages at the ortho position of aromatic ring include methoxyl, β−5, β−1 and 5−5 linkages. Demethoxylation at the ortho position followed by the hydrogen capping of the aromatic ring radicals is highly possible as demethoxylation is promoted during hydropyrolysis.43 Accordingly, a maximum of four D atoms could appear in the nonmethoxyl vinylphenol (and maximum three D atoms for a single methoxyl vinylphenol). However, the chance of having all four D atoms in a vinylphenol molecule is low because the vinyl group could further react before D atoms occupy other positions of the vinyphenol. For example, the vinylgroup could be saturated by free hydrogen atoms before demethoxylation occurring at the aromatic ring. As shown in Table 3, the percentage of deuterated molecules was higher in guaiacol than that in syringol (15% vs 5%). D substitution is unlikely to occur at two ortho positions of syringol because these positions are fully occupied by original methoxy groups in lignin. In comparison, the nonmethoxy ortho position of guaiacol could have accepted a D atom if a side chain previously exists in this position of lignin polymer and it was cleaved by pyrolysis. In guaiacol and syringol, the phenolic hydroxyl and the para position of the aromatic ring both have an opportunity to accept D atoms. Cleavage of side chain C−O or C−H bond at these positions of lignin polymer followed by hydrogen radical capping form the corresponding phenolic monomers. The percentages of deuterated molecules were lower in propenyl phenol, phenolic aldehydes and ketones because fewer bond cleavages occurred when these compounds are directly formed from lignin polymer structure. Among the products, methyl phenols, ethyl phenols and phenol were deuterated in relatively higher percentages. The numbers of D atoms in their individual molecules were also higher. For 2-methylphenol molecules, 21% of them had one D atom in their individual molecules, and 17% of the total molecules had two D atoms in each molecule. For 4ethylphenol molecules, the percentages of the molecules with one or two D atoms were 20% and 5% of total molecule numbers, respectively. Among 4-methylphenol molecules, the numbers of molecules containing one, two and three D atoms accounted for 24%, 8% and 3% of total number of the compound molecules. Phenol also contained up to three D atoms in its individual molecules. These compounds were deuterated more severely and contained higher numbers of D atoms in their individual molecules, because these compounds could accept D atoms in multiple positions of their molecules when lignin decomposes. In addition to being connected to H


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Figure 4. Pyrolysis-GC/MS chromatogram of 2-methoxy-4-vinylphenol in absence and presence of sodium formate (1:1 ratio). Arrows indicate the increase or decrease of the compound yield by the presence of sodium formate.

methoxyl-4-vinylphenol is chosen because it is one of the two most abundant phenolic monomers produced upon pyrolyzing the lignin. According to previous research, methoxyl and vinyl groups in the compound promote secondary reactions, including side chain cleavage and polymerization.17,44 The vinylphenol was highly reactive during pyrolysis, evident by the presence of a number of phenolic compounds other than the starting compound in the chromatograms. Some secondary reaction products, such as phenol, guaiacol and 2-methoxyl-4methylphenol can be produced from thermal cracking of the starting compound. However, trans-isoeugenol and vanillin among the products cannot be directly produced from 2methoxy-4-vinylphenol by thermal cracking. These compounds must be produced through an alternative pathway, possibly by the polymerization of 2-methoxy-4-vinylphenol and subsequent decomposition of the oligomers. Trans-isomerizations or other reallocation reactions could also have taken place. Among the products were also 2-methylphenol and 2-methoxyl-4-ethylphenol. 2-methylphenol could be formed from demethoxylation of the vinylphenol followed by alkylation. 2-methoxyl-4ethylphenol is likely the result of vinyl group saturation. Copyrolyzing with sodium formate reduced the peak intensity of the unconverted 2-methyoxy-4-vinylphenol in the chromatogram, indicating that sodium formate promoted a secondary reaction of 2-methyoxy-4-vinylphenol. Among the secondary products, the amounts of phenol, guaiacol and 4-ethylphenol increased, suggesting that both the cracking reactions and hydrogenation were enhanced by sodium formate. On the other hand, the amounts of trans-isoeugenol and vanillin decreased by copyrolyzing with sodium formate, probably because less 2-methoxyl-4-vinylphenol polymerized. The increasing or decreasing trends of the secondary products were similar to the trends of the products when the compounds were produced from copyrolysis of lignin and sodium formate (Table 1).

phenol molecules containing one to four D atoms were 24%, 13%, 10% and 3%, respectively. As shown in Table 1, the yields of phenol or 2-methyphenol also increased with the increasing amount of sodium formate. The increased total amount of free hydrogen atoms enhanced the cleavages of C−C and C−O bond on the aromatic side chains and then subsequently capped the intermediates to produce smaller and/or saturated phenolic molecules. Interestingly, the extent of deuteration was much less significant in methoxylated compounds. In 2-methoxy-4methyphenol, the individual molecules contained up to two D atoms although the compound molecule could potentially accept a maximum of four D atoms. Also, 64% of the compound molecules were not deuterated after pyrolysis. For 2-methoxyl-4-ethylphenol, 78% of the molecules were not deuterated and deuterated molecules contained one D atom, despite that the maximum possible number of D atoms in the compound molecule is five. A similar phenomenon was also observed with guaiacol. Among guaiacol molecules, 79% of total molecules were not deuterated whereas 19% contained one D atom and only 2% contained two D atoms in their individual molecules. It is likely that the phenolic compounds were further demethoxylated and then capped by D atoms to form corresponding nonmethoxyl phenols containing higher numbers of D atoms ahead of D substitutions at other positions of the methoxy containing phenols. Pyrolysis of Model Compounds with Deuterated Sodium Formate. In addition to directly participating in decomposition of lignin to cap the intermediate fragments, external hydrogen could also affect secondary reactions of primary products. Thus, lignin-derived phenolic monomers were pyrolyzed with and without sodium formate to investigate secondary reactions of primary products. Figure 4 shows the GC/MS chromatograms obtained when 2-methoxy-4-vinylphenol was pyrolyzed with and without sodium formate. The 2I


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Figure 5. Comparison of phenol pyrolysis in the absence and presence of deuterated sodium formate (1:1 ratio): (a) GC/MS chromatogram; (b) EI mass spectra.

methoxyl group in the phenolic compounds promotes secondary char formation in the vapor. Previously, Bai et al.45 also reported that the pyrolysis vapor of guaiacol contains oligomers. In comparison to the vinyl and methoxy phenols, phenol, cresol and benzene were stable during pyrolysis and the recovered (evaporated) starting compounds also do not contain D atoms. The comparisons of the GC/MS chromatograms and the mass ion spectra of phenol when it is pyrolyzed alone or copyrolyzed with sodium formate are given in Figure 5 as examples.

D atoms were also found among the secondary reaction products of 2-methoxyl-4-vinylphenol when the vinylphenol was copyrolyzed with deuterated sodium formate (the data is not shown here because of the reason mentioned below). The distributions of D atom in the secondary reaction compounds were similar to those which were observed when the same compounds were produced from copyrolysis of lignin and deuterated sodium formate. For example, individual phenol molecules produced from copyrolysis of the vinylphenol and deuterated sodium formate contained up to four D atoms and 2-methylphenol molecules contained a maximum of five D atoms. If 2-methoxy-4-vinylphenol directly converted to phenol by cracking, only two D atoms would appear in the phenol molecule (i.e., one D atom at the para position for capping dealkylated intermediate and another D atom at the ortho position for capping demethoxylated intermediate). Thus, those individual phenol molecules containing more than two D atoms must be produced from alternative pathways, possibly from the cracking of the polymerized 2-methyoxy-4-vinylphenol. Pyrolysis of guaiacol produced phenol, 2-methylphenol, dimethoxyl benzene as well as other compounds. When copyrolyzed with sodium formate, up to four D atoms were found in the molecules of phenol and 2-methylphenol. If guaiacol is demethoxylated and the aromatic radical intermediate is capped, only one D atom would possibly appear in the phenol molecule. The demethoxylated aromatic-radical intermediate could react with a methyl radical produced from demethylation of another guaiacol molecule to form 2methylphenol. In this route, hydrogen capping does not occur. Thus, the presence of multiple numbers of D atoms in the individual molecules of phenol and 2-methyphenol also confirms that the secondary reactions of guaiacol are not only limited to thermal cracking. Hosoya et al.44 reported that

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CONCLUSIONS Copyrolysis of lignin and sodium formate was found to increase the yields of simpler phenolic monomers and alkylated phenols while decreasing the yields of vinyl, propenyl phenols as well as phenolic aldehydes and ketones. Copyrolysis with sodium formate also eliminated acetic acid because acetic acid was reacted with sodium carbonate produced from thermal decomposition of sodium formate. The GPC analysis shows that copyrolysis of lignin and sodium formate suppresses the formation of phenolic oligomers and the resulting pyrolysis oil has an overall lower average MW after storage. When lignin and deuterated sodium formate were copyrolyzed, nearly all the pyrolysis products were partly deuterated. For each compound, both the percentage of deuterated molecule numbers in total molecules and the number of D atoms in individual molecules increased as the amount of sodium formate increased, regardless whether the yield of the compound decreased or increased during copyrolysis. The present study shows that during low-pressure lignin hydropyrolysis, hydrogen enhances the cleavages of side chain C−C and C−O bonds to produce an increased amount of reactive radical intermediates. The externally provided free hydrogen atoms are mixed with lignin J


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(11) Li, C.; Zhao, X.; Wang, A.; Huber, G. W.; Zhang, T. Catalytic Transformation of Lignin for the Production of Chemicals and Fuels. Chem. Rev. 2015, 115, 11559. (12) Pandey, M. P.; Kim, C. S. Lignin depolymerization and conversion: a review of thermochemical methods. Chem. Eng. Technol. 2011, 34 (1), 29. (13) Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A. Liquid fuels, hydrogen and chemicals from lignin: a critical review. Renewable Sustainable Energy Rev. 2013, 21, 506. (14) Ruiz-Dueñas, F. J.; Martínez, Á . T. Microbial degradation of lignin: how a bulky recalcitrant polymer is efficiently recycled in nature and how we can take advantage of this. Microb. Biotechnol. 2009, 2 (2), 164. (15) 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.; et al. Lignin valorization: improving lignin processing in the biorefinery. Science 2014, 344 (6185), 1246843. (16) De Wild, P.; Van der Laan, R.; Kloekhorst, A.; Heeres, E. Lignin valorisation for chemicals and (transportation) fuels via (catalytic) pyrolysis and hydrodeoxygenation. Environ. Prog. Sustainable Energy 2009, 28 (3), 461. (17) Hosoya, T.; Kawamoto, H.; Saka, S. Secondary reactions of lignin-derived primary tar components. J. Anal. Appl. Pyrolysis 2008, 83 (1), 78. (18) Kim, K. H.; Bai, X.; Cady, S.; Gable, P.; Brown, R. C. Quantitative Investigation of Free Radicals in Bio-Oil and their Potential Role in Condensed-Phase Polymerization. ChemSusChem 2015, 8 (5), 894. (19) Wang, K.; Kim, K. H.; Brown, R. C. Catalytic pyrolysis of individual components of lignocellulosic biomass. Green Chem. 2014, 16 (2), 727. (20) Zhou, S.; Xue, Y.; Sharma, A.; Bai, X. L. Lignin Valorization through Thermochemical Conversion: Comparison of Hardwood, Softwood and Herbaceous Lignin. ACS Sustainable Chem. Eng. 2016, 4 (12), 6608. (21) Melligan, F.; Hayes, M.; Kwapinski, W.; Leahy, J. Hydropyrolysis of biomass and online catalytic vapor upgrading with NiZSM-5 and Ni-MCM-41. Energy Fuels 2012, 26 (10), 6080. (22) Jan, O.; Marchand, R.; Anjos, L. C.; Seufitelli, G. V.; Nikolla, E.; Resende, F. L. Hydropyrolysis of Lignin Using Pd/HZSM-5. Energy Fuels 2015, 29 (3), 1793. (23) Bu, Q.; Lei, H.; Zacher, A. H.; Wang, L.; Ren, S.; Liang, J.; Wei, Y.; Liu, Y.; Tang, J.; Zhang, Q.; et al. A review of catalytic hydrodeoxygenation of lignin-derived phenols from biomass pyrolysis. Bioresour. Technol. 2012, 124, 470. (24) Mukkamala, S.; Wheeler, M. C.; van Heiningen, A. R.; DeSisto, W. J. Formate-assisted fast pyrolysis of lignin. Energy Fuels 2012, 26 (2), 1380. (25) Case, P. A.; Wheeler, M. C.; DeSisto, W. J. Formate assisted pyrolysis of pine sawdust for in-situ oxygen removal and stabilization of bio-oil. Bioresour. Technol. 2014, 173, 177. (26) Case, P. A.; Truong, C.; Wheeler, M. C.; DeSisto, W. J. Calcium-catalyzed pyrolysis of lignocellulosic biomass components. Bioresour. Technol. 2015, 192, 247. (27) Mansfield, S. D.; Kim, H.; Lu, F.; Ralph, J. Whole plant cell wall characterization using solution-state 2D NMR. Nat. Protoc. 2012, 7 (9), 1579. (28) Zhang, J.-j.; Jiang, X.-y.; Ye, X.-n.; Chen, L.; Lu, Q.; Wang, X.-h.; Dong, C.-q. Pyrolysis mechanism of a β-O-4 type lignin dimer model compound. J. Therm. Anal. Calorim. 2016, 123 (1), 501. (29) Patwardhan, P. R.; Brown, R. C.; Shanks, B. H. Understanding the fast pyrolysis of lignin. ChemSusChem 2011, 4 (11), 1629. (30) Zhang, H.; Xiao, R.; Wang, D.; He, G.; Shao, S.; Zhang, J.; Zhong, Z. Biomass fast pyrolysis in a fluidized bed reactor under N 2, CO 2, CO, CH 4 and H 2 atm. Bioresour. Technol. 2011, 102 (5), 4258. (31) Zhang, J.; Choi, Y. S.; Shanks, B. H. Tailoring the Composition of Bio-oil by Vapor-Phase Removal of Organic Acids. ChemSusChem 2015, 8 (24), 4256.

self-abstracted hydrogen atoms to cap the radical intermediates of lignin, therefore suppressing the coupling reactions among different lignin radical intermediates to avoid forming high MW products. Among the pyrolysis products, phenol and nonmethoxyl methyl, ethylphenols were most severely deuterated because the formation of these compounds from lignin pyrolysis involves multiple C−C and C−O bond cleavages followed by hydrogen capping at the cleaving ends of the intermediates. Pyrolysis of phenolic model compounds with deuterated sodium formate showed that in addition to thermal cracking, the reactive monomers most likely undergo polymerization followed by cracking. The presence of sodium formate promoted the conversion of the reactive monomers to more stable secondary products by participating in both the pathways.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +1 515 294 6886; Fax: +1 515 294 3261; E-mail: [email protected] (X. Bai). ORCID

Young-Jin Lee: 0000-0002-2533-5371 Xianglan Bai: 0000-0001-6849-2687 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge the research funding supported by ExxonMobil Co. The authors thank Dr. Robert Brown, Mr. Patrick Johnston and Dr. Marjorie Rover at Bioeconomy Institute of Iowa State University for technical support.

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Understanding Low-Pressure Hydropyrolysis of Lignin Using

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