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Lignin Valorization through Thermochemical Conversion: Comparison of Hardwood, Softwood and Herbaceous Lignin Shuai Zhou, Yuan Xue, Ashokkumar M Sharma, and Xianglan Bai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01488 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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ACS Sustainable Chemistry & Engineering

Lignin Valorization through Thermochemical Conversion: Comparison of Hardwood, Softwood and Herbaceous Lignin Shuai Zhou,† Yuan Xue,‡ Ashokkumar Sharma,† Xianglan Bai*‡ †

‡

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

States *Corresponding author. Postal address: Iowa State University, 2070 Black Engineering Building, Ames, IA 50011; Tel: +1 515 294 6886; Fax: +1 515 294 3261; E-mail address: [email protected] (X. Bai)

ACS Paragon Plus Environment


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Abstract In the present study, milled wood lignin (MWL) and organosolv lignin isolated from red oak (hardwood), loblolly pine (softwood) and corn stover (herbaceous biomass) were characterized by TGA, elemental analyzer, GPC, FTIR, 2D-HSQC NMR, and then pyrolyzed in the absence and presence of a zeolite catalyst. For all three biomass species, organosolv lignins contained fewer volatiles in comparison to the corresponding MWLs. Red oak lignin was affected most by the organosolv process, evident by the greatest decrease in volatile content and increase in carbon content of the organosolv lignin. Compared to the corresponding MWLs, organosolv lignins produced more char and less phenolic oil upon pyrolysis. Organosolv lignins also convert to catalytic coke and light hydrocarbons in higher selectivity in comparison to the MWLs during catalytic pyrolysis. When pyrolyzed, corn stover MWL produced 16.26 % of phenolic monomers, which is a significantly higher yield compared to 8.61 % from red oak MWL and 9.51 % from loblolly pine MWL. During catalytic pyrolysis, corn stover lignins also produced higher yields of aromatic hydrocarbons in comparison to red oak or loblolly pine derived lignins. Overall, corn stover lignin had the highest potential for volatilization since it retains highly branched polymer structure enriched in tricin, ferulate and coumarate groups.

Keywords: Milled wood lignin, organosolv lignin, pyrolysis, catalyst, phenolic monomers, hydrocarbons


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Introduction Lignin is one of three major compositions of lignocellulosic biomass, accounting for up to 35% of biomass. Lignin is also available as a byproduct from cellulosic industries, such as paper, pulping and biorefineries. Lignin has gained great attention in recent years as it became increasingly available from emerging biorefineries as byproducts. Chemically, lignin is a threedimensional, amorphous polymer biosynthesized through the random crosslinking of three phenylpropane units, which are coniferyl alcohol (G unit), sinapyl alcohol (S unit), and pcoumaryl alcohol (H unit).1 Thus, aromatic compounds derived from lignin could be applied as biofuels, chemicals, or other biobased products. However, effective conversion of lignin to value-added products has yet to be estabilished, despite the enormous research efforts. The irregular molecular structure of lignin, its recalcitrance for thermal and chemical deconstruction, and an incomplete understanding about lignin depolymerization cause lignin to be seriously underutilized.2-4 Fast pyrolysis is a promising pathway to convert biomass because of its robustness, low capital cost and process simplicity.5 During pyrolysis, feedstock is rapidly heated to moderate temperatures in the absence of oxygen, and the arising vapors are quenched to maximize pyrolysis oil yield. Upon pyrolysis, lignin decomposes to phenolic compounds which could be further deoxygenated in the presence of catalyst. When lignocellulosic biomass is pyrolyzed, the phenolic compounds (i.e., pyrolytic lignin) are recovered along with carbohydrate-derived sugars, furans and acids as bio-oil.6 Thus, either crude bio-oil or isolated pyrolytic lignin was analyzed to investigate lignin pyrolysis.7-12 More commonly, pyrolysis kinetics and reaction mechanism of lignin were studied by pyrolyzing isolated lignin.13-16 Isolated lignin was also mixed with cellulose and/or hemicellulose to investigate the pyrolysis behavior of plant biomass or the interactions among different compositions of biomass.17-20 Nevertheless, lignin pyrolysis is still difficult to fully understand because lignin structure is highly dependent on both biomass species and isolation methods. In general, hardwood lignin mainly consists of S and G units and softwood lignin mostly consists of G units.21 Herbaceous lignin, on the other hand, includes H, S and G units. These basic phenolic units form a complex polymer network of natural lignin, linked through various C-O-C and C-C bonds, such as β-O-4, α-O-4, 5-5, 4-O-5, β-5, β-1 and ββ.1 While the relative ratios of these linkages vary with biomass species, β-ether (β-O-4) is the



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most abundant linkage among all of them. The physio-chemical treatment during the isolation process can also greatly alter the natural structure of lignin. Not only the covalent ether and ester bonds between lignin and carbohydrates break, but intra-lignin bonds are also cleaved. Lignin fragmentation is usually followed by repolymerization and condensation, and the severity of structural modification depends on the isolation method.22 Thus, some researchers compared the pyrolysis of lignin isolated using the same method from different biomass species or lignin with same biomass origin but different isolation methods. For example, Wang et al.23 pyrolyzed milled wood lignins derived from different types of woody biomass and analyzed the volatiles using TGA-FTIR. Shen et al.24 pyrolyzed Klason lignin produced from maple, rice straw and rice husk at various temperatures ranged from 550 up to 900 °C. Kalogiannis et al.25 compared the catalytic pyrolysis of Kraft lignin derived from different wood sources and analyzed the bio-oil. Pine-wood lignins isolated using four different methods (i.e., alkali, Klason, organosolv and milled wood) were also recently studied by Wang et al.22 and the pyrolysis kinetics of the lignin samples were compared. Although various types of lignin were studied, comprehensive characterization and comparison of hardwood, softwood and herbaceous lignin in terms of their potentials for value-added products under same conversion conditions is rarely found in literature. It is well-known that biomass pyrolysis is affected by various experimental factors, such as heating rate, reaction temperature, the vapor resident time, as well as how the vapor is recovered after pyrolysis.26 In addition to aforementioned limitation, it should also be noted that the variations between natural lignin and isolated lignin in terms of pyrolysis conversion are not well described in literature for the three basic biomass species. Comparing different forms of lignin derived from different biomass species could provide useful information as increased varieties of lignocellulosic biomass become the feedstocks for biorefineries worldwide. Particularly, herbaceous biomass has become the largest renewable source available for the production of biofuels and bioproducts.27 However, herbaceous biomass-derived lignin was studied less frequently compared to wood-derived lignin. In this study, milled wood lignin (MWL) and organosolv lignin were extracted from hardwood (red oak), softwood (loblolly pine) and herbaceous biomass (corn stover) and characterized. The six different types of lignin were then subjected to non-catalytic and catalytic pyrolysis and the products were analyzed. The milled wood process combines mechanical milling and mild


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organic solvent extraction. Although it is impossible to isolate intact natural lignin from the biomass, MWL is the best type of isolated lignin which can represent the chemical structure and reactivity of natural lignin.13,28 The organosolv process combines organic solvents and dilute acids to isolate lignin from the biomass as a soluble product. Organosolv lignin is considered as the most preferred, high-quality lignin because of its high purity and relatively non-hazardous extraction condition.29 For the same reasons, organosolv lignin is increasingly becoming the major form of lignin from biorefineries.

Materials and methods Biomass preparation for lignin extraction Red oak, loblolly pine and corn stover were ball milled for 30 min at 400 rpm using a planetary ball mill (Retsch PM100) to reduce their particle sizes. The ball milled biomass was dispersed into methanol (100 g biomass / 1 L methanol) and agitated for 24 hours. The methanol extraction was repeated using the solid residue recovered after first extraction. The sample was filtered to remove methanol and vacuum oven dried at 40 °C for 24 hours to obtain extractivefree samples. MWL process: The dry, extractive-free biomass sample was milled in a planetary ball mill for 72 hours. The ball milled sample was extracted by a mixture of 1,4-dioxane and water (96:4, v:v) for 24 hours. The extracted mixture was filtered and the recovered solid residue was extracted again using a fresh mixture of 1, 4-dioxane and water for 24 hours. Both extracts were combined and the solvent was removed in a rotary evaporator at 40 °C to obtain a crude milled lignin containing about 10% residual carbohydrates. The crude milled lignin was purified by dissolving in a 100 mL mixture of acetic acid and water (90:10, v:v). Next the dissolved solution was added dropwise into cold water. The precipitated lignin was centrifuged and freeze-dried, and then further dissolved in a 50 mL mixture of 1, 2-dichloreethane and ethanol (2:1, v:v). The solution was centrifuged to remove the solid and the lignin solution was added



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dropwise in a 1L anhydrous ethyl ether. The precipitated lignin was centrifuged and then vacuum oven dried to obtain pure milled lignin. Organosolv lignin process: Fifteen grams of the extractive-free biomass sample was added to a mixture of ethanol and water (125 ml ethanol and 125 ml water), as well as 1.5 g of sulfuric acid. The solution was transferred into a 500 ml Parr reactor and heated up to 180 °C. After the temperature reached the preset temperature, the reactor was cooled down and the solution was filtered through filter paper (Whatman 42). The filtrated liquid was precipitated in 750 ml water, and the solid residue was collected and dried at 50 °C.

Characterization of lignin Thermal gravimetric analysis (TGA) Proximate analysis was performed in a Mettler Toledo TGA/DSC system (TGA/DSC 1 STARe system, Mettler Toledo). Approximately 10 mg of lignin samples were placed in a crucible, heated in a nitrogen environment up to 105 °C at 10 °C/min, and held for 40 min to determine moisture content. Later, the sample continued to be heated up to 900 °C at 10 °C/min and held at the same temperature for 20 min to determine volatile content. Finally, air was introduced to combust the residue to calculate the fixed carbon and ash content. Elemental analysis (CHNS-O) Elemental analysis was conducted in an elemental analyzer (Vario Micro Cube, Elementar, Germany) using approximately 5 mg of the sample each time. Gel permeation chromatography (GPC) analysis The molecular weight distributions of the lignin samples were determined using a gel permeation chromatograph (GPC) (Ultimate 3000, DIONEX HPLC) equipped with a Shodex Refrative Index (RI) and Diode Array Detector (DAD). Two Agilent PLgel 3µm 100A0 300 x 7.5mm (p/n PL1110-6320) columns were connected in series and maintained at 25°C. Tetrahydrofuran with a flow rate of 1 ml/min was used as the eluent. The GPC column was calibrated using polystryrene


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standards with molecular weight range of 162-38,640 g/mol. An ultraviolet wavelength of 254 nm was used. The lignin samples were acetylated prior to the GPC analysis to increase its solubility in tetrahydrofuran. Briefly, 100 mg of lignin were placed in a glass vial and 3 ml of pyridine and 3 ml of acetic anhydride were also added. The vial with the lignin sample and the solution was heated in an oil bath at 80 °C for 3 hours under continuous agitation. After the reaction was completed, the whole solution was added into 500 ml deionized water to precipitate the acetylated lignin. Precipitated lignin was filtered and washed with deionized water three times to remove the unreacted pyridine and acetic anhydride. Fourier transform infrared spectrometry (FTIR) analysis FTIR (NICOLET iS10 SMART iTR, Thermo Scientific) was applied to investigate the structure of the lignin. Each sample was scanned 32 times from 4000 cm-1 to 650 cm-1 with a resolution of 4 cm-1. The FTIR spectrum was auto-baseline corrected and normalized for comparison. Two-dimensional Heteronuclear Single-Quantum Correlation Nuclear Magnetic Resonance (2D HSQC NMR analysis) [1H13C] 2D-NMR heteronuclear single-quantum coherence (HSQC) spectroscopies of lignin were obtained at 25 °C on a Bruker Biospin Advance 800 MHz spectrometer incorporated with a 5 mm cryogenically cooled z-gradient probe using the Bruker pulse sequence ‘hsqcetgpsisp.2’. A sample concentration of 100 mg lignin per 1 mL of solvent mixture was used. The solvent mixture was composed of dimethyl sulfoxide (DMSO)-d6 and pyridine-d5 (v/v: 4/1). The sample was prepared by dissolving 100 mg of isolated lignin in the solvent mixture and then held in the shaker for 20 min to properly dissolve lignin in the solvent mixture. Initial processing of the HSQC data was performed using Bruker’s Topspin 3.2 software, and later the MestReNova (v10.0.2, Mestrelab Research, S.L.) processing software was used to identify different structural compounds in the isolated lignins by comparing the acquired 2D-NMR signals with the available literature.30-33

Thermochemical conversion of lignin Pyrolysis



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Lignin samples were pyrolyzed in a Frontier tandem micropyrolyzer system (Frontier laboratory, Japan) equipped with an Agilent Gas chromatography (GC) that is connected to mass spectrometry (MS), flame ionization detector (FID) and thermal conductivity detector (TCD) (7890B GC, 5977A MSD). Detailed information about the pyrolyzer system can be found elsewhere.34 Briefly, the reactor system consists of two furnaces, temperatures of which can be controlled from room temperature up to 900 °C. Each time, approximately 500 µg of lignin was placed in a sample cup and then dropped into the preheated furnace for pyrolysis at 500 °C. A catalyst bed can be placed inside of the second furnace to upgrade the pyrolysis vapor leaving the first furnace. The final products leaving the second furnace were swept into an online GC by helium gas. The vapor was then separated into three streams, two of which were sent to two ZB-1701 columns (60 m × 250 µm × 0.25 µm) connected to a mass spectrometer (MS, 5975C, Agilent, USA) for identification, and a FID for quantification. The third stream was swept into a porous layer open tubular column (60 m × 0.320 mm) (GS-GasPro, Agilent, USA) to separate non condensable gas (NCG) and quantified with a TCD. The front inlet split ratio was 50:1 with a total flow rate of 156 mL/min. The GC oven temperature was initially kept at 40 °C for 3 min, increased to 280 °C with a heating rate of 6 °C/min, and then held at 280 °C for another 3 min. The standard phenolic compounds and aromatic hydrocarbon compounds were purchased from Sigma Aldrich. The standard gas samples (CO, CO2, CH4, C2H4, C3H6, and C4H8) diluted in helium were purchased from Praxair, USA. The calibration curves of the compounds were created by injecting the standard compounds to the GC-MS/FID/TCD system with five different concentrations.

Char residue remaining in the sample cup was collected and its carbon content was determined using the elemental analyzer described above. Catalytic pyrolysis Catalytic pyrolysis was also conducted in the tandem micropyrolyzer. HZSM-5 catalyst (CBV 2314, Zeolyst international) was calcined at 500 °C for 6 hours prior to use. The detailed operation procedure can be found elsewhere.35 Ten milligrams of the catalyst were loaded in the catalyst bed with helium as the carrier gas. The volatile products and gases were analyzed and quantified by the GC-MS/FID/TCD. The temperature of the pyrolysis reactor was 500 °C and the catalyst bed was kept at 600 °C. The carbon content in the used catalyst was also determined. The carbon yields of non-catalytic or catalytic pyrolysis products were calculated using following equation: Carbon yield of a product C%


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!"#$

= %$ & ' × 100%

(1)

Carbon selectivity of the pyrolysis vapor for the catalytic conversion products (i.e., aromatic hydrocarbons, light hydrocarbons, coke, CO and CO2) were calculated as Product selectivity % !"#$.

= %$ & !//. . 0! × 100%

(2)

Carbon selectivity of individual aromatic hydrocarbons among the total aromatic hydrocarbons was calculated as Product selectivity % 1/"

= %$ & &$ 1/". × 100%

(3)

Carbon selectivity of the individual light hydrocarbon among the total light aromatic hydrocarbons was calculated through: Product selectivity % '1$ 1/"

= %$ & '1$ 1/". × 100%

(4)

Results and Discussions Proximate and ultimate analyses Proximate and ultimate analyses of the six types of lignin are given in Table S1 (see supplemental information). For all biomass species, organosolv lignin had overall lower volatile contents and higher fixed carbon contents than the corresponding MWLs, indicating that the organosolv lignin is more thermally resistant to volatilization, in comparison to natural lignin. Red oak MWL had the greatest volatile content, which is 72.21 %. The content, however, reduced to 56.51 % in the corresponding organosolv lignin, accompanied by an increase in fixed carbon content from 27.22 % to 41.75 %. On the other hand, the volatile content in corn stover MWL was comparable



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with red oak MWL, and it only moderately decreased from 71.05 % to 66.81 % by the organosolv process. Loblolly pine MWL contained the lowest amount of volatiles (~62 %), but the content was only slightly reduced in the corresponding organosolv lignin. Elemental analysis shows that carbon content increased significantly from the range of 58.88 61.80 % in various MWL to 67.38 - 68.66 % in the respective organosolv lignin whereas oxygen content decreased from 33.19 - 36.08 % to 26.06 - 26.87 %. The same trend was reported in previous studies.22 Natural lignin in biomass is expected to have even higher oxygen content and lower carbon content in comparison to the MWL, thus, is volatilized more easily. GPC Analysis Weight and number average molecular weights (23 456 2 ) and polydispersity index (PDI) of the lignin samples are listed in Table S2. The order of 23 of the MWLs was red oak lignin (i.e., 5363 Da) > loblolly pine lignin (i. e., 4659 Da) > corn stover lignin (i. e., 3858 Da). Red oak MWL has the highest 23 , most likely because of the higher molecular sizes of monolignols containing more methoxyl groups. The 23 s of the organosolv lignins were much lower (< 2000 Da) than that of the MWLs in all three biomass species. The PDI measures the uniformity of the molecular sizes and the values of were below 2 for all three organosolv lignins. Overall, the lignin fragmentation during organosolv extraction was most significant with red oak lignin, followed by loblolly pine, and was least significant with corn stover lignin.

FTIR analysis FTIR analysis results of the lignin samples are given in Figure S1 (a)-(c). Different spectra were compared by defining the band at 1500 cm-1 (i.e., aromatic ring skeleton) as the standard.22 In red oak lignin, the band at 3300 cm-1, which reduced in organosolv lignin, corresponds to the phenolic OH and aliphatic OH groups. The band at 1370 cm-1, which represents phenolic OH and aliphatic C-H in CH3, also decreased. Thus, the trend of the two bands suggests that the phenolic OH decreased during the organosolv process. Dissociation of phenolic OH from the benzene ring is unlikely occurred due to its high bond dissociation energy. However, repolymerization could have occurred at PhO because the bond dissociation energy between PhO and H is much lower (85.5 kcal/mol versus 111 kcal/mol).36 The bands representing methyl


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(2800 cm-1) and methoxyl (2856 cm-1) decreased in organosolv lignin due to demethylation and demethoxylation. The energy requirement for demethoxylation is 97.7 kcal/mol whereas it for demethylation is only 52.8 kcal/mol. The bands at 1700 cm-1 and 1650 cm-1 representing the C=O stretch in ketones, aldehydes, and carboxylic acid groups, decreased significantly in the organosolv process. In organosolv lignin, the decreased bands at 1225 and 1274 cm-1 are related to C-C plus C-O plus and C=O stretching. The band at 1120 cm-1 corresponds to the C-H in plane deformation for S units and the band at 1140 cm-1 is for G units. Both bands decreased due to decreased concentrations of S and G units. The bands at 1085 and 1030 cm-1 represent the C-O deformation in alcohols and aliphatic ethers, and the unconjugated C=O stretch. These bonds decreased mainly due to the cleavage of C-O-C ether bonds, which are abundant in lignin structures (i.e., β-O-4, α-O-4 bonds). The intensities of all other bands relative to that of the aromatic ring vibration became lower in the organosolv lignin, which agrees with the findings of previous studies37, i.e., the organosolv lignin has a more aromatically condensed structure. While the similar changes were observed in the FTIR spectrum of loblolly pine lignin and corn stover lignin, corn stover lignin contained relatively higher concentrations of ketones, aldehydes, and carboxylic acid groups compared to red oak or loblolly pine lignin. For corn stover lignin, the sharp band that appears at 836 cm-1 is due to the H units present in corn stover.

2D HSQC NMR The 2D NMR spectrum of the lignin is compared in Figure S2. The structures of the different groups of lignin were previously described by Luterbacher et al.30 The same structures are also illustrated for identification of the contour peaks appearing at the NMR spectrum. All MWLs were enriched by β-aryl ether linkages as they are predominant linkages in natural lignin. Other detected functional groups include methoxyl groups, S and G groups, benzaldehyde end groups and cinnamaldehyde end groups. Ferulate and p-coumarate groups were also observed in the three MWLs. These groups, which are more abundant in herbaceous biomass, act as the bridge between lignin and polysaccharides in plant. The β-β coupling product shown in the spectrum was from sinapyl p-coumarate conjugates during delignification.30 Tricin, which is a newly discovered unit, is the precursor that starts many lignin chains during biosynthesis.30 Though literature suggests that tricin is mostly found in monocots,38 it was detectable in the MWLs



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derived from all three biomass species. The presence of the above groups indicates that original structures of natural lignin are largely preserved in MWLs. Due to the variation of the building blocks, however, the differences in the NMR spectrum were also found in the MWL derived from red oak, loblolly pine and corn stover. The contour-peak intensity of S lignin was weaker in loblolly pine MWL compared to the red oak lignin due to the lack of S units. In addition to the G unit and S unit, H unit was also detected in corn stover lignin. The intensity of the contour peaks of β-aryl ether groups was overall lower in the corn stover MWL compared to red oak and loblolly pine MWL. The β-β coupling products were found most scarcely in red oak lignin and most abundantly in corn stover lignin. Corn stover MWL also contained the highest concentration of tricin groups and is enriched in ferulate and p-coumarate groups. As expected, fewer natural lignin groups were detected in the 2D-NMR spectrum of the organosolv lignins compared to the corresponding MWLs. The decrease in the contour peaks of β-aryl ether linkages was observed with organosolv lignin due to the ether cleavage and associated polymer fragmentation. Previous studies using phenolic model compounds suggest that the hemolytic cleavage of C-O bonds in the aryl-ether produce reactive free radicals as the intermediates.39,40 These free radical species further react to form new phenolic structures that contain an increased number of thermally stable C-C bonds. For example, the reaction of phenoxyl and benzyl radicals forms benzylphenolic structures.39 The β-aryl ether linkages decreased most in red oak organosolv lignin as β-O-4 linkage accounts for 60% of total linkages in natural hardwood lignin.27 The concentrations of G and S groups also decreased in the red oak organosolv lignin, due to the demethylation and/or demethoxylation of lignin. Tricin groups were still detectable but they were detached from the lignin chain. The contour peaks of ferulate and p-coumarate groups were also largely reduced, likely because these groups were broken into shorter side-chain products (for example, vinyl phenols)41 during the extraction process. The peak of the β-β coupling product was no longer detectable with this lignin. Benzaldehyde end group was less affected by the organosolv process. However, the cinnamaldehyde end-groups showed a slight decrease in red oak organosolv lignin. In the loblolly pine organosolv lignin, S and G units also decreased. Tricin groups were no longer detectable and the peaks of the p-coumarate groups nearly disappeared. Ferulate groups, on the other hand, were still abundantly present. The contour peak of the β-β coupling product


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was also detectable. Similar to red oak lignin, cinnamaldehyde end-groups decreased in the organosolv lignin, and the new peak of an unknown group appeared at the near region. For corn stover organosolv lignin, the benzaldehyde end-group was no longer detectable and the peak of the cinnamaldehyde end-group decreased. The contour peak of H groups became stronger whereas the peaks of S and G groups were reduced; suggesting that S and G units partly converted to H units. On the other hand, the peaks of the β-β coupling products and tricin groups were still detectable, and ferulate and p-coumarate groups also remained in high concentrations. Overall, corn stover organosolv lignin has lignin subunits with longer and branched side-chains with oxygen. Red oak organosolv lignin is likely to have the most condensed aromatic structure with shorter side chains, and loblolly pine organosolv lignin has the intermediate structure.

TGA analysis The mass loss (TGA) profiles of the lignin samples with increasing temperature are given in Figure 1 (a). Mass loss started at lower temperatures in the MWLs compared to the corresponding organosolv lignins. Red oak MWL and corn stover MWL have similar TGA profiles; both decomposed easier in comparison to loblolly pine MWL. The difference in the TGA profiles between the MWL and organosolv lignin was most dramatic with red oak lignin because a higher amount of β-aryl ether linkages was cleaved. In comparison, least change was found with the TGA profiles of two corn stover lignins. Corn stover lignin originally contains fewer amounts of β-ether linkages and other side chains of the lignin subunits were less affected by the organosolv process. Differential thermogravimetric (DTG) curves of different lignins are given in Figure 1 (b)-(d). In Figure 1 (b), two major DTG peaks (for mass loss rate) were observed for red oak MWL: the peak appears at 268 °C followed by a slightly stronger peak at 340 °C (the peak appears at about 100 °C is due to evaporation of residue moisture). The first DTG peak is related to the devolution of small phenolic compounds from β-ether cleavage, and release of H2O, CO and CO2 from the cleavage of lateral chains in the lignin polymer.23 The second DTG peak is related to the release of large MW phenols and gaseous products that mainly consist of CO, CO2 and CH4. The intensities of the two DTG peaks were comparable to each other, because red oak native



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lignin contains a large amount of thermally labile aryl ether bonds. In comparison, there was a single sharp DTG peak appears at 357 °C for the red oak organosolv lignin, due to its more thermally stable structure. Loblolly pine MWL was more difficult to volatilize, as a broad DTG curve with a left shoulder was observed (Figure 1(c)). After it was isolated in the organosolv process, the DTG curve of the lignin became sharp and narrow, and its peak also shifted to 376 °C from 345 °C with the MWL. For corn stover MWL, the DTG curve had three distinct peaks. The first peak appears at 159 °C could be related to decomposition of ferulate and courmaric groups that are abundantly present in corn stover lignin. Decomposition of these groups could start at temperatures well below 200 °C.41 For the organosolv lignin, the intensity of the first peak became slightly lower because some of the hydroxycinnamate groups were degraded during the organosolv process. The second peak was blended into the big blunt DTG curve due to aryl ether cleavage. Also, the maximum mass-loss rate appeared at slightly a higher temperature in stronger intensity compared to it occurred with the MWL.

Py-GC/MS Table 1 summarized the carbon yields of the pyrolysis products of lignin. The quantified pyrolysis products include GC/MS detectable phenolic monomers, acetic acid, toluene, light gases and pyrolysis char. The sum of the quantified products accounts for 60-70 C% of the lignin. The rest products are mainly GC/MS unidentified phenolic oligomers. Thus, the yields of phenolic oligomers were determined by subtracting the total carbon yields of the quantified products from 100%. In the table, the liquid products indicate the sum of all condensable products that contain carbon atoms. The yields of liquid products were 43 ~ 55.02 % for the lignins. The highest yield was observed from corn stover MWL, while lowest yield was from loblolly pine organosolv lignin. The liquid yields were also higher with the MWLs compared to the corresponding organosolv lignins. The yields of GC/MS detectable phenolic monomers ranged from 6.32 % to 16.28 % for different types of lignin. The lowest monomer yield was found with red oak organosolv lignin, which is even lower than the corresponding value of MWL (i.e., 8.61%). S type phenols were the major


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phenolic compounds, followed by G type phenols. S or G phenolic monomers are produced from monomeric units of lignin by breaking α or β-aryl ether linkage. These phenols could have further cracked into H type phenols.12,42 While the yields of most of the compounds were lower than 1%, 1,2,4-trimethoxylbenzene, phenol, 2,6-dimethoxy-4-(2-propenyl)-, 2,6dimethoxylphenol, creosol, 4-vinylphenol were the major phenolic monomers. The yields of phenolic monomers with longer side chains, such as 3,5-dimethoxy-4-hydroxycinnamaldehyde, 3,5-dimethoxy-4-hydroxyphenylacetic acid, were lower with the organosolv lignin compared to that with the MWL. For loblolly pine lignins, the yields of phenolic monomers were also found to be higher with the MWL than that with organosolv lignin. G type phenols were the major phenolic monomers, which is the character of softwood lignin. The phenolic monomers with relatively higher yields were trans-isoeugenol, creosol, 2-methoxylphenol and 2-methoxy-4-vinylphenols. The yields of phenols with shorter side chains, such as phenol, 2-methoxylphenol, creosol, were higher with the organosolv lignin. The total yield of phenolic monomers was 16.26 % for corn stover MWL and 16.28 % for corn stover organosolv lignin, both significantly higher than the yields from red oak lignin or loblolly pine lignin. H type phenols were the major type of phenolic monomers, followed by G and S type phenols. Two most abundant phenolic monomers were 4-vinylphenol and 2-methoxy-4vinylphenol as the added yields were 9.62 % for the MWL, and 9.32 % for the organosolv lignin. The vinylphenols are produced by β-aryl ether cleavage as well as from degradation of hydroxycinnamates. The yields of GC/MS non-detectable phenolic compounds were between 33.92 and 38.33 % for varied types of lignin, much higher than the yields of the quantified phenolic monomers. Because organosolv lignin has a tendency to produce the phenolic compounds with shorter side chains than the corresponding MWL, it can be speculated that average MW of phenolic oil produced from organosolv lignin (as well as other industrial lignin) is lower than what it would be if derived from natural lignin by pyrolyzing biomass. Previously, we analyzed pyrolysis vapors of corn stover organosolv lignin collected in a cold solvent and reported that the phenolic compounds are mostly monomers and dimers.16 We also reported that the average MW of phenolic-oil produced from pyrolysis of the same lignin is lower than 300 Da.43 Nsimba et al.44



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pyrolyzed an organosolv lignin extracted from woody biomass and reported that average 23 values of four fractionated phenolic oils to be between 212 and 430 Da. Ben et al.45 also reported that the average 23 of the pyrolysis-oil produced from Kraft lignin was 364 Da. In comparison, the average 23 s of the pyrolytic lignin derived from pyrolysis of the plant biomass ranged from 649 to 1319 Da, according to the study conducted by Meier’s group.10 Natural lignin tends to produce phenolic compounds with longer or more branched side chains in comparison to modified lignin, resulting in increased overall 23 of phenolic oil. It has also shown that carboxylic acids (acetic acid, formic acid etc.) among pyrolysis products of biomass promote polymerization of phenolic compounds.15,46 Acetic acid is mostly originated from acetyl groups of hemicellulose. Acetic acid yield was lower with the organosolv lignins because acetyl groups decreased during the extraction process. The yield of acetic acid was highest with corn stover MWL because herbaceous biomass lignin is naturally acetylated.47 Light gases produced from the pyrolysis of lignin mostly consist of CO2, CO and CH4. There was no significant difference in the yields of CO with MWLs derived from different biomass species. Red oak lignin produced an overall higher yield of CH4, due to the high content of methoxyl group. The yield of CO2 was higher with corn stover lignin because the lignin contains more C-O functionalities on side chains. For all cases, the yields of CO and CO2 decreased, whereas the yield of CH4 increased in the organosolv lignins compared to the corresponding MWLs. The yields of CO and CO2 decreased, because the organosolv lignin contains fewer amounts of carbonyl and carboxyl groups, as well as ether bondsin comparison to the corresponding MWL, which was confirmed in the FTIR spectrum. The order of pyrolysis char yield from the MWL was loblolly pine > red oak >corn stover. The char yields were lower with MWLs in comparison to the corresponding organosolv lignins. It is speculated that the char yield is even lower with the lignin in biomass. It is reported that the solid/liquid interaction among cellulose and lignin reduces primary char formation and the interaction among the respective vapor products suppresses secondary char formation.18 Hosoya et al.18 suggested that carbohydrate-derivatives act as H donors whereas lignin-derived phenolic compounds serve as H acceptors to inhibit char formation. The char yield increased in organosolv lignin since the carbon condensed structure is more difficult to volatilize. Loblolly pine organosolv lignin resulted in the highest char yield at 500 °C, containing as much as 50.33 %


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of lignin carbon. The methoxyl group substitution on the ortho position of the phenolic ring enhances C-O cleavage and but also repolymerization.39 The methoxyl groups on phenolic rings are known to promote char formation.48

Catalytic pyrolysis The carbon based product distribution of lignin during the catalytic pyrolysis is given in Table 2. Zeolite is the most commonly used catalyst in biomass conversion due to its cracking and deoxygenation ability in the absence of hydrogen and/or high pressure. HZSM-5 is the most effective zeolite catalyst in terms of deoxygenating biomass, contributed by its well-balanced acidity and shape selectivity.49 A series of reactions, such as deoxygenation, cracking, aromatization, oligomerization and isomerization occur in its active sites.50 The yields of aromatic hydrocarbons were 5.42 ~ 8.67 % when the pyrolysis vapors of the lignins were converted in the ex-situ catalyst bed. Overall, corn stover lignins produced highest amounts of aromatic hydrocarbons, while red oak lignins produced least amounts. Both red oak and loblolly pine organosolv lignins produced lower amounts of aromatic hydrocarbon compared to the corresponding MWLs. However, the yields were similar with corn stover MWL and organosolv lignin. Comparing Table 1 and 2, it was found that the higher the yields of the phenolic monomers are, the more aromatic hydrocarbons are produced. The yields of light hydrocarbons were highest with red oak lignins due to the high methane yields. Corn stover lignin produced intermediate yields of light hydrocarbon gases. Among the gaseous products, the yields of ethylene and propylene were particularly high due to the long branched structure of corn stover lignins. Catalytic coke is the result of polymerization and carbonization of the phenolic compounds at catalyst sites. The coke yields accounted from 13.73 to 20.64 %, lowest with red oak MWL and highest with corn stover organosolv lignin. Since pyrolysis char is solely due to thermal decomposition, the carbon selectivity of the pyrolysis vapors for hydrocarbons, carbon oxides and catalytic coke during catalyst conversion was calculated, and the results are compared in Figure 2. Compared to the corresponding MWLs, the pyrolysis vapors of organosolv lignin had much higher selectivity for forming catalytic coke and lower selectivity for producing CO2 and CO. For red oak or loblolly pine lignins, the



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selectivity for aromatic hydrocarbons was lower, whereas the selectivity for light hydrocarbons was higher with the organosolv lignin, in comparison to the corresponding MWLs. However, the trend was not obvious with corn stover lignins. The selectivity of benzene, toluene, xylene, ethyl benzene and polyaromatic hydrocarbons among total aromatic hydrocarbons is also given in Figure 3. Overall, aromatic hydrocarbons derived from red oak lignins had the highest selectivity for single ring aromatics. Although corn stover lignins produced the higher yields of total aromatics, the products had an increased selectivity for polyaromatic hydrocarbons over single ring aromatics. When converted by zeolite catalyst, most of the phenolic compounds are hardly able to enter the zeolite pores due to their large molecular sizes compared to zeolite pores. Thus, the phenols are likely first adsorbed on the zeolite surface and then converted. The existence of a phenolic pool on the catalytic surface was suggested by To et al.51 Without entering the pores, some of the adsorbed phenolic compounds could be directly deoxygenated at the limited numbers of external active sites to form aromatic hydrocarbons. In a recent study, we have also proven that the active sites on the surface of ZSM-5 catalyst play an important role in cracking and deoxygenating lignin-derived phenolic compounds.34 In addition to the surface deoxygenation, the side chain fragments could also enter the zeolite pores and be deoxygenated. Mullen et al.52 suggested that the aliphatic linkers between lignin units are the major source of aromatic hydrocarbons because only these small molecules are able to enter the zeolite pores and join the hydrocarbon pool. Forming aromatic hydrocarbons through this pathway is more pronounced with the MWLderived pyrolysis vapors because the relatively longer carbon chain lengths of the fragments make them easier to aromatize inside the pores. For organosolv lignin, the cracking fragments preferentially become light hydrocarbon gases due to their short carbon chains. The selectivity of individual hydrocarbon gases among total light hydrocarbons is given in Figure 4 for different types of lignin and the results also support the above theory. Compared to MWLs, the selectivity of methane was higher, whereas the selectivity of propylene was lower with organosolv lignins. The selectivity of propylene was highest with corn stover lignin, because the phenols derived from corn stover lignin have longer and branched side chains than that from red oak lignin or loblolly pine lignin. The relatively higher content of the side chains in corn stover lignin likely contributed to the higher yields of hydrocarbons.


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The selectivity of carbon oxides was higher with MWLs. This is due to the stronger extent of deoxygenation with MWLs by zeolite. The results could also suggest that MWL-derived pyrolysis vapors contain more oxygenated products than that from the corresponding organosolv lignins. Interestingly, it was noted that the selectivity for catalytic coke was much higher with corn stover organosolv lignin compared to corn stover MWL, despite that the selectivity for aromatic or light hydrocarbons were similar. It could be that higher oxygen content in the pyrolysis vapors derived from corn stover MWL helped thermal desorption of the phenols to suppress coke formation. It was also found that the selectivity of polyaromatics was higher with corn stover lignins than that with red oak lignin or loblolly pine lignins. This is probably related to the higher concentration of phenolic OH in the pyrolysis vapors of corn stover lignin as phenolic OH has strong tendency to polymerize. Vinylphenols are also abundant in pyrolysis vapor derived from corn stover lignin and these compounds are prone to repolymerize due to the reactive C=C bonds.

Conclusions MWLs and organosolv lignins isolated from red oak, loblolly pine and corn stover were characterized and further pyrolyzed in the absence and presence of catalyst. For all biomass species, MWLs decomposed at lower temperatures and produced higher amounts of volatiles in comparison to the corresponding organosolv lignins due to the higher oxygen contents. The organosolv process had the greatest influence on red oak lignin by decreasing the volatile content of MWL from 72.21 % to 56.51 % and increasing carbon content from 58.88 % to 68.16 %. The variation was least noticeable with corn stover organosolv lignin because tricin, ferulate and courmate groups remained after the extraction. Upon pyrolysis, organosolv lignins produced less phenolic oil and more char, in comparison to the corresponding MWLs. Corn stover lignins produced significantly higher yields of phenolic monomers (i.e., > 16 %) and lower yields of char in comparison to the wood-derived lignins. During the catalytic upgrading, pyrolysis vapors of organosolv lignins preferentially form coke and light hydrocarbons compared to MWLderived pyrolysis vapors. The yields of aromatic hydrocarbons were highest with corn stover lignins and lowest with red oak lignin. On the other hand, the selectivity of single ring aromatic



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hydrocarbons followed the opposite trend. Overall, corn stover lignins showed great potential for pyrolytic conversion because of its branched polymer structure.

Acknowledgement The work is partially supported by the CenUSA Bioenergy project. The authors also acknowledge Prof. Robert Brown, Dr. Marjorie Rover, Ryan Smith, Patrick Johnston, and Patrick Hall at the Bioeconomy Institute of Iowa State University for useful discussion and technical support.


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Table 1. Carbon based product distribution of lignin pyrolysis at 500°C Feedstock Isolation method Yield (C %) Light Gases CO CO₂ CH₄ Sum

Red oak MWL Organosolv

Liquid products Phenolic monomers Phenol 2-Methoxyphenol m-cresol p-cresol Phenol, 2-methoxy-3-methyl2-Methoxy-5-methylphenol Creosol Phenol, 2,5-dimethyl4-ethylphenol Phenol, 4-ethyl-2-methoxy4-vinylphenol 2-Methoxy-4-vinylphenol Eugenol Phenol, 2-methoxy-4-propylPhenol, 2,6-dimethoxytrans-Isoeugenol 1,2,4-Trimethoxybenzene Vanillin Benzene, 1,2,3-trimethoxy-5-methylPhenol, 2-methoxy-4-propylApocynin 3',5'-Dimethoxyacetophenone Phenol, 2,6-dimethoxy-4-(2-propenyl)Benzaldehyde, 4-hydroxy-3,5-dimethoxy2-Propanone, 1-(4-hydroxy-3-methoxyphenyl)Ethanone, 1-(4-hydroxy-3,5-dimethoxyphenyl)Desaspidinol 3,5-Dimethoxy-4-hydroxyphenylacetic acid Desaspidinol / 2-Pentanone, 1-(2,4,6trihydroxyphenyl) 3,4,5-Trimethoxyphenylacetic acid 3,5-Dimethoxy-4-hydroxycinnamaldehyde Phenol, 4-(3-hydroxy-1-propenyl)-2-methoxyHomovanillic acid 2-Propenal, 3-(4-hydroxy-3-methoxyphenyl)Total monomers

Loblolly pine MWL Organosolv

Corn stover MWL Organosolv

3.75 3.26 2.17 9.19

2.97 2.17 2.57 7.72

3.42 2.83 1.46 7.71

2.91 2.06 1.7 6.67

3.15 4.24 1.16 8.55

2.54 3.68 1.15 7.37

0.06 0.35 0.07 0.08 0.05 0.01 0.84 0.07 0.2 0.52 0.14 0.1 0.63 0.34 1.43 0.21 0.39 0.21 0.65 0.9 0.38 0.07 0.3 0.13

0.04 0.26 0.05 0.06 0.05 0.01 0.81 0.07 0.19 0.27 0.09 0.1 0.46 0.19 1.37 0.1 0.32 0.13 0.39 0.66 0.15 0.1 0.23 0.07

0.26 0.67 0.13 0.36 0.06 0.01 1.3 0.18 0.07 0.45 0.37 0.85 0.26 0.08 0.12 1.52 0.4 0.63 0.24 0.25 0.13 0.35 0.12 0.07 0.07 -

0.3 0.99 0.11 0.28 0.07 0.02 1.42 0.13 0.05 0.58 0.25 0.64 0.17 0.14 0.18 1.13 0.42 0.32 0.17 0.12 0.1 0.32 0.04 0.09 0.04 -

0.41 0.48 0.09 0.48 0.04 0.64 0.08 0.31 0.18 6.82 2.8 0.12 0.03 0.57 0.44 0.61 0.25 0.19 0.12 0.41 0.73 0.18 0.24 0.07 -

0.8 0.66 0.11 0.54 0.04 0.68 0.07 0.72 0.39 5.62 3.7 0.04 0.03 0.65 0.18 0.62 0.1 0.17 0.06 0.28 0.56 0.05 0.12 0.06 -

0.1 0.08 0.3 8.61

0.09 0.03 0.04 6.32

0.15 0.12 0.31 9.51

0.03 0.07 0.12 8.32

16.26

16.28


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Phenolic oligomers* Acetic acid Toluene Sum (Liquid product)

37.79 1.6 48

38.33 0.07 44.72

34.87 0.53 0.03 44.93

34.6 0.06 0.02 43

36.33 2.41 0.02 55.02

33.92 0.19 0.04 50.42

Char

42.81

47.56

47.36

50.33

36.43

42.21

* Calculated by subtracting the quantified product yields from 100%.



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Table 2. Carbon based product distribution of different lignin during ex-situ catalytic pyrolysis by HZSM-5 zeolite (temperatures: pyrolysis reactor: 500°C, catalyst bed: 600°C.) Feedstock Isolation methods

Red oak MWL

Lob. pine

Corn stover

Organosolv

MWL

Organosolv

MWL

Organosolv

Oxygenates CO

7.66

5.5

6.11

5.33

6.58

4.96

CO₂

4.05

2.87

3.67

2.74

5.45

4.62

Methane

3.27

4.17

2.41

2.88

2.11

2.43

Ethylene

4.33

5

4.31

4.51

5.14

4.26

Propylene

2.44

1.99

2.03

1.95

3.27

2.74

Butene

0.15

0.15

0.13

0.13

0.16

0.16

Total

10.2

11.31

8.88

9.47

10.67

9.59

Benzene

1.58

1.24

1.78

1.49

1.84

1.79

Toluene

2.08

1.56

1.99

1.71

1.79

1.51

Xylene

1.24

0.86

0.95

0.8

0.81

0.68

C₉₊

0.66

0.34

0.75

0.56

0.96

1.01

C₁₀₊

2.07

1.25

2.39

2.48

3.25

3.68

Total

7.63

5.25

7.86

7.04

8.65

8.67

Char

42.81

47.56

47.36

50.33

36.43

42.21

Coke

13.73

17.69

14.06

18.87

17.95

20.64

57.19

52.44

52.64

49.67

63.57

57.79

86.08

90.83

87.93

93.78

85.75

90.7

Olefins + CH₄

Aromatics

Carbon balance


Page 27 of 64

100

(a) TGA

90 80 Red oak-MWL Red oak-Organosolv Loblolly pine-MWL Loblolly pine-organosolv Corn stover-MWL Corn stover-Organosolv

Residue (%)

70 60 50 40 30 20 10 0 0

100

200

300 400 500 Temperature (°C)

600

700

800

0.01

(b) DTG: Red oak 0.008

Mass loss rate (mg/s)

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


MWL Organosolv

0.006

0.004

0.002

0 0

100

200

300

400

500

Temperature (°C)


600

700

800


0.008

(c) DTG: Loblolly pine


MWL 0.006

Organosolv

0.004

0.002

0 0

100

200

300

400

500

600

700

800

Temperature (°C)

0.008

(d) DTG: Corn stover MWL


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0.006 Organosolv 0.004

0.002

0 0

100

200

300

400

500

600

700

800

Temperature (°C)

Figure 1. TGA analysis of MWL and organosolv lignin derived from red oak, loblolly pine and corn stover; (a) comparison of TGA profiles of all lignin; DTG curves of MWL and organosolv lignin derived from (b) red oak, (c) loblolly pine and (d) corn stover.


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40

Carbon selectivity (%)

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Coke light hydrocarbons CO2

Aromatic hydrocarbons CO

35 30 25 20 15 10 5 0 MWL

Organosolv

Red oak

MWL

Organosolv

Loblolly pine

MWL

Organosolv

Corn stover

Figure 2. Carbon selectivity of pyrolysis vapors of different types of lignin for coke, hydrocarbons and carbon oxides when the vapors are converted by HZSM-5 zeolite.



45 40


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Page 30 of 64

35

Benzene

Toluene

Xylene

C9 Aromatics

C10+ Aromatics

30 25 20 15 10 5 0 MWL

Organosolv

Red oak

MWL

Organosolv

Loblolly pine

MWL

Organosolv

Corn stover

Figure 3. Carbon selectivity of individual aromatic hydrocarbons among total aromatic hydrocarbons.


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60 Methane

Ethylene

Propylene

Butene

50


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


40 30 20 10 0 MWL

Organosolv

Red oak

MWL

Organosolv

Loblolly pine

MWL

Organosolv

Corn stover

Figure 4. Carbon selectivity of light hydrocarbons among total light hydrocarbons



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For Table of Contents Use Only Lignin Valorization through Thermochemical Conversion: Comparison of Hardwood, Softwood and Herbaceous Lignin Shuai Zhou, Yuan Xue, Ashokkumar Sharma, Xianglan Bai*

Synopsis: Volatilization of milled wood lignins and organosolv lignins of three biomass species were investigated.


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Lignin Valorization through Thermochemical Conversion: Comparison of Hardwood, Softwood and Herbaceous Lignin Shuai Zhou,† Yuan Xue,‡ Ashokkumar Sharma,† Xianglan Bai*‡ †

‡

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

States *Corresponding author. Postal address: Iowa State University, 2070 Black Engineering Building, Ames, IA 50011; Tel: +1 515 294 6886; Fax: +1 515 294 3261; E-mail address: [email protected] (X. Bai)



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Abstract In the present study, milled wood lignin (MWL) and organosolv lignin isolated from red oak (hardwood), loblolly pine (softwood) and corn stover (herbaceous biomass) were characterized by TGA, elemental analyzer, GPC, FTIR, 2D-HSQC NMR, and then pyrolyzed in the absence and presence of a zeolite catalyst. For all three biomass species, organosolv lignins contained fewer volatiles in comparison to the corresponding MWLs. Red oak lignin was affected most by the organosolv process, evident by the greatest decrease in volatile content and increase in carbon content of the organosolv lignin. Compared to the corresponding MWLs, organosolv lignins produced more char and less phenolic oil upon pyrolysis. Organosolv lignins also convert to catalytic coke and light hydrocarbons in higher selectivity in comparison to the MWLs during catalytic pyrolysis. When pyrolyzed, corn stover MWL produced 16.26 % of phenolic monomers, which is a significantly higher yield compared to 8.61 % from red oak MWL and 9.51 % from loblolly pine MWL. During catalytic pyrolysis, corn stover lignins also produced higher yields of aromatic hydrocarbons in comparison to red oak or loblolly pine derived lignins. Overall, corn stover lignin had the highest potential for volatilization since it retains highly branched polymer structure enriched in tricin, ferulate and coumarate groups.

Keywords: Milled wood lignin, organosolv lignin, pyrolysis, catalyst, phenolic monomers, hydrocarbons


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Introduction Lignin is one of three major compositions of lignocellulosic biomass, accounting for up to 35% of biomass. Lignin is also available as a byproduct from cellulosic industries, such as paper, pulping and biorefineries. Lignin has gained great attention in recent years as it became increasingly available from emerging biorefineries as byproducts. Chemically, lignin is a threedimensional, amorphous polymer biosynthesized through the random crosslinking of three phenylpropane units, which are coniferyl alcohol (G unit), sinapyl alcohol (S unit), and pcoumaryl alcohol (H unit).1 Thus, aromatic compounds derived from lignin could be applied as biofuels, chemicals, or other biobased products. However, effective conversion of lignin to value-added products has yet to be estabilished, despite the enormous research efforts. The irregular molecular structure of lignin, its recalcitrance for thermal and chemical deconstruction, and an incomplete understanding about lignin depolymerization cause lignin to be seriously underutilized.2-4 Fast pyrolysis is a promising pathway to convert biomass because of its robustness, low capital cost and process simplicity.5 During pyrolysis, feedstock is rapidly heated to moderate temperatures in the absence of oxygen, and the arising vapors are quenched to maximize pyrolysis oil yield. Upon pyrolysis, lignin decomposes to phenolic compounds which could be further deoxygenated in the presence of catalyst. When lignocellulosic biomass is pyrolyzed, the phenolic compounds (i.e., pyrolytic lignin) are recovered along with carbohydrate-derived sugars, furans and acids as bio-oil.6 Thus, either crude bio-oil or isolated pyrolytic lignin was analyzed to investigate lignin pyrolysis.7-12 More commonly, pyrolysis kinetics and reaction mechanism of lignin were studied by pyrolyzing isolated lignin.13-16 Isolated lignin was also mixed with cellulose and/or hemicellulose to investigate the pyrolysis behavior of plant biomass or the interactions among different compositions of biomass.17-20 Nevertheless, lignin pyrolysis is still difficult to fully understand because lignin structure is highly dependent on both biomass species and isolation methods. In general, hardwood lignin mainly consists of S and G units and softwood lignin mostly consists of G units.21 Herbaceous lignin, on the other hand, includes H, S and G units. These basic phenolic units form a complex polymer network of natural lignin, linked through various C-O-C and C-C bonds, such as β-O-4, α-O-4, 5-5, 4-O-5, β-5, β-1 and ββ.1 While the relative ratios of these linkages vary with biomass species, β-ether (β-O-4) is the



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most abundant linkage among all of them. The physio-chemical treatment during the isolation process can also greatly alter the natural structure of lignin. Not only the covalent ether and ester bonds between lignin and carbohydrates break, but intra-lignin bonds are also cleaved. Lignin fragmentation is usually followed by repolymerization and condensation, and the severity of structural modification depends on the isolation method.22 Thus, some researchers compared the pyrolysis of lignin isolated using the same method from different biomass species or lignin with same biomass origin but different isolation methods. For example, Wang et al.23 pyrolyzed milled wood lignins derived from different types of woody biomass and analyzed the volatiles using TGA-FTIR. Shen et al.24 pyrolyzed Klason lignin produced from maple, rice straw and rice husk at various temperatures ranged from 550 up to 900 °C. Kalogiannis et al.25 compared the catalytic pyrolysis of Kraft lignin derived from different wood sources and analyzed the bio-oil. Pine-wood lignins isolated using four different methods (i.e., alkali, Klason, organosolv and milled wood) were also recently studied by Wang et al.22 and the pyrolysis kinetics of the lignin samples were compared. Although various types of lignin were studied, comprehensive characterization and comparison of hardwood, softwood and herbaceous lignin in terms of their potentials for value-added products under same conversion conditions is rarely found in literature. It is well-known that biomass pyrolysis is affected by various experimental factors, such as heating rate, reaction temperature, the vapor resident time, as well as how the vapor is recovered after pyrolysis.26 In addition to aforementioned limitation, it should also be noted that the variations between natural lignin and isolated lignin in terms of pyrolysis conversion are not well described in literature for the three basic biomass species. Comparing different forms of lignin derived from different biomass species could provide useful information as increased varieties of lignocellulosic biomass become the feedstocks for biorefineries worldwide. Particularly, herbaceous biomass has become the largest renewable source available for the production of biofuels and bioproducts.27 However, herbaceous biomass-derived lignin was studied less frequently compared to wood-derived lignin. In this study, milled wood lignin (MWL) and organosolv lignin were extracted from hardwood (red oak), softwood (loblolly pine) and herbaceous biomass (corn stover) and characterized. The six different types of lignin were then subjected to non-catalytic and catalytic pyrolysis and the products were analyzed. The milled wood process combines mechanical milling and mild


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organic solvent extraction. Although it is impossible to isolate intact natural lignin from the biomass, MWL is the best type of isolated lignin which can represent the chemical structure and reactivity of natural lignin.13,28 The organosolv process combines organic solvents and dilute acids to isolate lignin from the biomass as a soluble product. Organosolv lignin is considered as the most preferred, high-quality lignin because of its high purity and relatively non-hazardous extraction condition.29 For the same reasons, organosolv lignin is increasingly becoming the major form of lignin from biorefineries.

Materials and methods Biomass preparation for lignin extraction Red oak, loblolly pine and corn stover were ball milled for 30 min at 400 rpm using a planetary ball mill (Retsch PM100) to reduce their particle sizes. The ball milled biomass was dispersed into methanol (100 g biomass / 1 L methanol) and agitated for 24 hours. The methanol extraction was repeated using the solid residue recovered after first extraction. The sample was filtered to remove methanol and vacuum oven dried at 40 °C for 24 hours to obtain extractivefree samples. MWL process: The dry, extractive-free biomass sample was milled in a planetary ball mill for 72 hours. The ball milled sample was extracted by a mixture of 1,4-dioxane and water (96:4, v:v) for 24 hours. The extracted mixture was filtered and the recovered solid residue was extracted again using a fresh mixture of 1, 4-dioxane and water for 24 hours. Both extracts were combined and the solvent was removed in a rotary evaporator at 40 °C to obtain a crude milled lignin containing about 10% residual carbohydrates. The crude milled lignin was purified by dissolving in a 100 mL mixture of acetic acid and water (90:10, v:v). Next the dissolved solution was added dropwise into cold water. The precipitated lignin was centrifuged and freeze-dried, and then further dissolved in a 50 mL mixture of 1, 2-dichloreethane and ethanol (2:1, v:v). The solution was centrifuged to remove the solid and the lignin solution was added



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dropwise in a 1L anhydrous ethyl ether. The precipitated lignin was centrifuged and then vacuum oven dried to obtain pure milled lignin. Organosolv lignin process: Fifteen grams of the extractive-free biomass sample was added to a mixture of ethanol and water (125 ml ethanol and 125 ml water), as well as 1.5 g of sulfuric acid. The solution was transferred into a 500 ml Parr reactor and heated up to 180 °C. After the temperature reached the preset temperature, the reactor was cooled down and the solution was filtered through filter paper (Whatman 42). The filtrated liquid was precipitated in 750 ml water, and the solid residue was collected and dried at 50 °C.

Characterization of lignin Thermal gravimetric analysis (TGA) Proximate analysis was performed in a Mettler Toledo TGA/DSC system (TGA/DSC 1 STARe system, Mettler Toledo). Approximately 10 mg of lignin samples were placed in a crucible, heated in a nitrogen environment up to 105 °C at 10 °C/min, and held for 40 min to determine moisture content. Later, the sample continued to be heated up to 900 °C at 10 °C/min and held at the same temperature for 20 min to determine volatile content. Finally, air was introduced to combust the residue to calculate the fixed carbon and ash content. Elemental analysis (CHNS-O) Elemental analysis was conducted in an elemental analyzer (Vario Micro Cube, Elementar, Germany) using approximately 5 mg of the sample each time. Gel permeation chromatography (GPC) analysis The molecular weight distributions of the lignin samples were determined using a gel permeation chromatograph (GPC) (Ultimate 3000, DIONEX HPLC) equipped with a Shodex Refrative Index (RI) and Diode Array Detector (DAD). Two Agilent PLgel 3µm 100A0 300 x 7.5mm (p/n PL1110-6320) columns were connected in series and maintained at 25°C. Tetrahydrofuran with a flow rate of 1 ml/min was used as the eluent. The GPC column was calibrated using polystryrene


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standards with molecular weight range of 162-38,640 g/mol. An ultraviolet wavelength of 254 nm was used. The lignin samples were acetylated prior to the GPC analysis to increase its solubility in tetrahydrofuran. Briefly, 100 mg of lignin were placed in a glass vial and 3 ml of pyridine and 3 ml of acetic anhydride were also added. The vial with the lignin sample and the solution was heated in an oil bath at 80 °C for 3 hours under continuous agitation. After the reaction was completed, the whole solution was added into 500 ml deionized water to precipitate the acetylated lignin. Precipitated lignin was filtered and washed with deionized water three times to remove the unreacted pyridine and acetic anhydride. Fourier transform infrared spectrometry (FTIR) analysis FTIR (NICOLET iS10 SMART iTR, Thermo Scientific) was applied to investigate the structure of the lignin. Each sample was scanned 32 times from 4000 cm-1 to 650 cm-1 with a resolution of 4 cm-1. The FTIR spectrum was auto-baseline corrected and normalized for comparison. Two-dimensional Heteronuclear Single-Quantum Correlation Nuclear Magnetic Resonance (2D HSQC NMR analysis) [1H13C] 2D-NMR heteronuclear single-quantum coherence (HSQC) spectroscopies of lignin were obtained at 25 °C on a Bruker Biospin Advance 800 MHz spectrometer incorporated with a 5 mm cryogenically cooled z-gradient probe using the Bruker pulse sequence ‘hsqcetgpsisp.2’. A sample concentration of 100 mg lignin per 1 mL of solvent mixture was used. The solvent mixture was composed of dimethyl sulfoxide (DMSO)-d6 and pyridine-d5 (v/v: 4/1). The sample was prepared by dissolving 100 mg of isolated lignin in the solvent mixture and then held in the shaker for 20 min to properly dissolve lignin in the solvent mixture. Initial processing of the HSQC data was performed using Bruker’s Topspin 3.2 software, and later the MestReNova (v10.0.2, Mestrelab Research, S.L.) processing software was used to identify different structural compounds in the isolated lignins by comparing the acquired 2D-NMR signals with the available literature.30-33

Thermochemical conversion of lignin Pyrolysis



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Lignin samples were pyrolyzed in a Frontier tandem micropyrolyzer system (Frontier laboratory, Japan) equipped with an Agilent Gas chromatography (GC) that is connected to mass spectrometry (MS), flame ionization detector (FID) and thermal conductivity detector (TCD) (7890B GC, 5977A MSD). Detailed information about the pyrolyzer system can be found elsewhere.34 Briefly, the reactor system consists of two furnaces, temperatures of which can be controlled from room temperature up to 900 °C. Each time, approximately 500 µg of lignin was placed in a sample cup and then dropped into the preheated furnace for pyrolysis at 500 °C. A catalyst bed can be placed inside of the second furnace to upgrade the pyrolysis vapor leaving the first furnace. The final products leaving the second furnace were swept into an online GC by helium gas. The vapor was then separated into three streams, two of which were sent to two ZB-1701 columns (60 m × 250 µm × 0.25 µm) connected to a mass spectrometer (MS, 5975C, Agilent, USA) for identification, and a FID for quantification. The third stream was swept into a porous layer open tubular column (60 m × 0.320 mm) (GS-GasPro, Agilent, USA) to separate non condensable gas (NCG) and quantified with a TCD. The front inlet split ratio was 50:1 with a total flow rate of 156 mL/min. The GC oven temperature was initially kept at 40 °C for 3 min, increased to 280 °C with a heating rate of 6 °C/min, and then held at 280 °C for another 3 min. The standard phenolic compounds and aromatic hydrocarbon compounds were purchased from Sigma Aldrich. The standard gas samples (CO, CO2, CH4, C2H4, C3H6, and C4H8) diluted in helium were purchased from Praxair, USA. The calibration curves of the compounds were created by injecting the standard compounds to the GC-MS/FID/TCD system with five different concentrations.

Char residue remaining in the sample cup was collected and its carbon content was determined using the elemental analyzer described above. Catalytic pyrolysis Catalytic pyrolysis was also conducted in the tandem micropyrolyzer. HZSM-5 catalyst (CBV 2314, Zeolyst international) was calcined at 500 °C for 6 hours prior to use. The detailed operation procedure can be found elsewhere.35 Ten milligrams of the catalyst were loaded in the catalyst bed with helium as the carrier gas. The volatile products and gases were analyzed and quantified by the GC-MS/FID/TCD. The temperature of the pyrolysis reactor was 500 °C and the catalyst bed was kept at 600 °C. The carbon content in the used catalyst was also determined. The carbon yields of non-catalytic or catalytic pyrolysis products were calculated using following equation: Carbon yield of a product C%


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!"#$

= %$ & ' × 100%

(1)

Carbon selectivity of the pyrolysis vapor for the catalytic conversion products (i.e., aromatic hydrocarbons, light hydrocarbons, coke, CO and CO2) were calculated as Product selectivity % !"#$.

= %$ & !//. . 0! × 100%

(2)

Carbon selectivity of individual aromatic hydrocarbons among the total aromatic hydrocarbons was calculated as Product selectivity % 1/"

= %$ & &$ 1/". × 100%

(3)

Carbon selectivity of the individual light hydrocarbon among the total light aromatic hydrocarbons was calculated through: Product selectivity % '1$ 1/"

= %$ & '1$ 1/". × 100%

(4)

Results and Discussions Proximate and ultimate analyses Proximate and ultimate analyses of the six types of lignin are given in Table S1 (see supplemental information). For all biomass species, organosolv lignin had overall lower volatile contents and higher fixed carbon contents than the corresponding MWLs, indicating that the organosolv lignin is more thermally resistant to volatilization, in comparison to natural lignin. Red oak MWL had the greatest volatile content, which is 72.21 %. The content, however, reduced to 56.51 % in the corresponding organosolv lignin, accompanied by an increase in fixed carbon content from 27.22 % to 41.75 %. On the other hand, the volatile content in corn stover MWL was comparable



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with red oak MWL, and it only moderately decreased from 71.05 % to 66.81 % by the organosolv process. Loblolly pine MWL contained the lowest amount of volatiles (~62 %), but the content was only slightly reduced in the corresponding organosolv lignin. Elemental analysis shows that carbon content increased significantly from the range of 58.88 61.80 % in various MWL to 67.38 - 68.66 % in the respective organosolv lignin whereas oxygen content decreased from 33.19 - 36.08 % to 26.06 - 26.87 %. The same trend was reported in previous studies.22 Natural lignin in biomass is expected to have even higher oxygen content and lower carbon content in comparison to the MWL, thus, is volatilized more easily. GPC Analysis Weight and number average molecular weights (23 456 2 ) and polydispersity index (PDI) of the lignin samples are listed in Table S2. The order of 23 of the MWLs was red oak lignin (i.e., 5363 Da) > loblolly pine lignin (i. e., 4659 Da) > corn stover lignin (i. e., 3858 Da). Red oak MWL has the highest 23 , most likely because of the higher molecular sizes of monolignols containing more methoxyl groups. The 23 s of the organosolv lignins were much lower (< 2000 Da) than that of the MWLs in all three biomass species. The PDI measures the uniformity of the molecular sizes and the values of were below 2 for all three organosolv lignins. Overall, the lignin fragmentation during organosolv extraction was most significant with red oak lignin, followed by loblolly pine, and was least significant with corn stover lignin.

FTIR analysis FTIR analysis results of the lignin samples are given in Figure S1 (a)-(c). Different spectra were compared by defining the band at 1500 cm-1 (i.e., aromatic ring skeleton) as the standard.22 In red oak lignin, the band at 3300 cm-1, which reduced in organosolv lignin, corresponds to the phenolic OH and aliphatic OH groups. The band at 1370 cm-1, which represents phenolic OH and aliphatic C-H in CH3, also decreased. Thus, the trend of the two bands suggests that the phenolic OH decreased during the organosolv process. Dissociation of phenolic OH from the benzene ring is unlikely occurred due to its high bond dissociation energy. However, repolymerization could have occurred at PhO because the bond dissociation energy between PhO and H is much lower (85.5 kcal/mol versus 111 kcal/mol).36 The bands representing methyl


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(2800 cm-1) and methoxyl (2856 cm-1) decreased in organosolv lignin due to demethylation and demethoxylation. The energy requirement for demethoxylation is 97.7 kcal/mol whereas it for demethylation is only 52.8 kcal/mol. The bands at 1700 cm-1 and 1650 cm-1 representing the C=O stretch in ketones, aldehydes, and carboxylic acid groups, decreased significantly in the organosolv process. In organosolv lignin, the decreased bands at 1225 and 1274 cm-1 are related to C-C plus C-O plus and C=O stretching. The band at 1120 cm-1 corresponds to the C-H in plane deformation for S units and the band at 1140 cm-1 is for G units. Both bands decreased due to decreased concentrations of S and G units. The bands at 1085 and 1030 cm-1 represent the C-O deformation in alcohols and aliphatic ethers, and the unconjugated C=O stretch. These bonds decreased mainly due to the cleavage of C-O-C ether bonds, which are abundant in lignin structures (i.e., β-O-4, α-O-4 bonds). The intensities of all other bands relative to that of the aromatic ring vibration became lower in the organosolv lignin, which agrees with the findings of previous studies37, i.e., the organosolv lignin has a more aromatically condensed structure. While the similar changes were observed in the FTIR spectrum of loblolly pine lignin and corn stover lignin, corn stover lignin contained relatively higher concentrations of ketones, aldehydes, and carboxylic acid groups compared to red oak or loblolly pine lignin. For corn stover lignin, the sharp band that appears at 836 cm-1 is due to the H units present in corn stover.

2D HSQC NMR The 2D NMR spectrum of the lignin is compared in Figure S2. The structures of the different groups of lignin were previously described by Luterbacher et al.30 The same structures are also illustrated for identification of the contour peaks appearing at the NMR spectrum. All MWLs were enriched by β-aryl ether linkages as they are predominant linkages in natural lignin. Other detected functional groups include methoxyl groups, S and G groups, benzaldehyde end groups and cinnamaldehyde end groups. Ferulate and p-coumarate groups were also observed in the three MWLs. These groups, which are more abundant in herbaceous biomass, act as the bridge between lignin and polysaccharides in plant. The β-β coupling product shown in the spectrum was from sinapyl p-coumarate conjugates during delignification.30 Tricin, which is a newly discovered unit, is the precursor that starts many lignin chains during biosynthesis.30 Though literature suggests that tricin is mostly found in monocots,38 it was detectable in the MWLs



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derived from all three biomass species. The presence of the above groups indicates that original structures of natural lignin are largely preserved in MWLs. Due to the variation of the building blocks, however, the differences in the NMR spectrum were also found in the MWL derived from red oak, loblolly pine and corn stover. The contour-peak intensity of S lignin was weaker in loblolly pine MWL compared to the red oak lignin due to the lack of S units. In addition to the G unit and S unit, H unit was also detected in corn stover lignin. The intensity of the contour peaks of β-aryl ether groups was overall lower in the corn stover MWL compared to red oak and loblolly pine MWL. The β-β coupling products were found most scarcely in red oak lignin and most abundantly in corn stover lignin. Corn stover MWL also contained the highest concentration of tricin groups and is enriched in ferulate and p-coumarate groups. As expected, fewer natural lignin groups were detected in the 2D-NMR spectrum of the organosolv lignins compared to the corresponding MWLs. The decrease in the contour peaks of β-aryl ether linkages was observed with organosolv lignin due to the ether cleavage and associated polymer fragmentation. Previous studies using phenolic model compounds suggest that the hemolytic cleavage of C-O bonds in the aryl-ether produce reactive free radicals as the intermediates.39,40 These free radical species further react to form new phenolic structures that contain an increased number of thermally stable C-C bonds. For example, the reaction of phenoxyl and benzyl radicals forms benzylphenolic structures.39 The β-aryl ether linkages decreased most in red oak organosolv lignin as β-O-4 linkage accounts for 60% of total linkages in natural hardwood lignin.27 The concentrations of G and S groups also decreased in the red oak organosolv lignin, due to the demethylation and/or demethoxylation of lignin. Tricin groups were still detectable but they were detached from the lignin chain. The contour peaks of ferulate and p-coumarate groups were also largely reduced, likely because these groups were broken into shorter side-chain products (for example, vinyl phenols)41 during the extraction process. The peak of the β-β coupling product was no longer detectable with this lignin. Benzaldehyde end group was less affected by the organosolv process. However, the cinnamaldehyde end-groups showed a slight decrease in red oak organosolv lignin. In the loblolly pine organosolv lignin, S and G units also decreased. Tricin groups were no longer detectable and the peaks of the p-coumarate groups nearly disappeared. Ferulate groups, on the other hand, were still abundantly present. The contour peak of the β-β coupling product


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was also detectable. Similar to red oak lignin, cinnamaldehyde end-groups decreased in the organosolv lignin, and the new peak of an unknown group appeared at the near region. For corn stover organosolv lignin, the benzaldehyde end-group was no longer detectable and the peak of the cinnamaldehyde end-group decreased. The contour peak of H groups became stronger whereas the peaks of S and G groups were reduced; suggesting that S and G units partly converted to H units. On the other hand, the peaks of the β-β coupling products and tricin groups were still detectable, and ferulate and p-coumarate groups also remained in high concentrations. Overall, corn stover organosolv lignin has lignin subunits with longer and branched side-chains with oxygen. Red oak organosolv lignin is likely to have the most condensed aromatic structure with shorter side chains, and loblolly pine organosolv lignin has the intermediate structure.

TGA analysis The mass loss (TGA) profiles of the lignin samples with increasing temperature are given in Figure 1 (a). Mass loss started at lower temperatures in the MWLs compared to the corresponding organosolv lignins. Red oak MWL and corn stover MWL have similar TGA profiles; both decomposed easier in comparison to loblolly pine MWL. The difference in the TGA profiles between the MWL and organosolv lignin was most dramatic with red oak lignin because a higher amount of β-aryl ether linkages was cleaved. In comparison, least change was found with the TGA profiles of two corn stover lignins. Corn stover lignin originally contains fewer amounts of β-ether linkages and other side chains of the lignin subunits were less affected by the organosolv process. Differential thermogravimetric (DTG) curves of different lignins are given in Figure 1 (b)-(d). In Figure 1 (b), two major DTG peaks (for mass loss rate) were observed for red oak MWL: the peak appears at 268 °C followed by a slightly stronger peak at 340 °C (the peak appears at about 100 °C is due to evaporation of residue moisture). The first DTG peak is related to the devolution of small phenolic compounds from β-ether cleavage, and release of H2O, CO and CO2 from the cleavage of lateral chains in the lignin polymer.23 The second DTG peak is related to the release of large MW phenols and gaseous products that mainly consist of CO, CO2 and CH4. The intensities of the two DTG peaks were comparable to each other, because red oak native



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lignin contains a large amount of thermally labile aryl ether bonds. In comparison, there was a single sharp DTG peak appears at 357 °C for the red oak organosolv lignin, due to its more thermally stable structure. Loblolly pine MWL was more difficult to volatilize, as a broad DTG curve with a left shoulder was observed (Figure 1(c)). After it was isolated in the organosolv process, the DTG curve of the lignin became sharp and narrow, and its peak also shifted to 376 °C from 345 °C with the MWL. For corn stover MWL, the DTG curve had three distinct peaks. The first peak appears at 159 °C could be related to decomposition of ferulate and courmaric groups that are abundantly present in corn stover lignin. Decomposition of these groups could start at temperatures well below 200 °C.41 For the organosolv lignin, the intensity of the first peak became slightly lower because some of the hydroxycinnamate groups were degraded during the organosolv process. The second peak was blended into the big blunt DTG curve due to aryl ether cleavage. Also, the maximum mass-loss rate appeared at slightly a higher temperature in stronger intensity compared to it occurred with the MWL.

Py-GC/MS Table 1 summarized the carbon yields of the pyrolysis products of lignin. The quantified pyrolysis products include GC/MS detectable phenolic monomers, acetic acid, toluene, light gases and pyrolysis char. The sum of the quantified products accounts for 60-70 C% of the lignin. The rest products are mainly GC/MS unidentified phenolic oligomers. Thus, the yields of phenolic oligomers were determined by subtracting the total carbon yields of the quantified products from 100%. In the table, the liquid products indicate the sum of all condensable products that contain carbon atoms. The yields of liquid products were 43 ~ 55.02 % for the lignins. The highest yield was observed from corn stover MWL, while lowest yield was from loblolly pine organosolv lignin. The liquid yields were also higher with the MWLs compared to the corresponding organosolv lignins. The yields of GC/MS detectable phenolic monomers ranged from 6.32 % to 16.28 % for different types of lignin. The lowest monomer yield was found with red oak organosolv lignin, which is even lower than the corresponding value of MWL (i.e., 8.61%). S type phenols were the major


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phenolic compounds, followed by G type phenols. S or G phenolic monomers are produced from monomeric units of lignin by breaking α or β-aryl ether linkage. These phenols could have further cracked into H type phenols.12,42 While the yields of most of the compounds were lower than 1%, 1,2,4-trimethoxylbenzene, phenol, 2,6-dimethoxy-4-(2-propenyl)-, 2,6dimethoxylphenol, creosol, 4-vinylphenol were the major phenolic monomers. The yields of phenolic monomers with longer side chains, such as 3,5-dimethoxy-4-hydroxycinnamaldehyde, 3,5-dimethoxy-4-hydroxyphenylacetic acid, were lower with the organosolv lignin compared to that with the MWL. For loblolly pine lignins, the yields of phenolic monomers were also found to be higher with the MWL than that with organosolv lignin. G type phenols were the major phenolic monomers, which is the character of softwood lignin. The phenolic monomers with relatively higher yields were trans-isoeugenol, creosol, 2-methoxylphenol and 2-methoxy-4-vinylphenols. The yields of phenols with shorter side chains, such as phenol, 2-methoxylphenol, creosol, were higher with the organosolv lignin. The total yield of phenolic monomers was 16.26 % for corn stover MWL and 16.28 % for corn stover organosolv lignin, both significantly higher than the yields from red oak lignin or loblolly pine lignin. H type phenols were the major type of phenolic monomers, followed by G and S type phenols. Two most abundant phenolic monomers were 4-vinylphenol and 2-methoxy-4vinylphenol as the added yields were 9.62 % for the MWL, and 9.32 % for the organosolv lignin. The vinylphenols are produced by β-aryl ether cleavage as well as from degradation of hydroxycinnamates. The yields of GC/MS non-detectable phenolic compounds were between 33.92 and 38.33 % for varied types of lignin, much higher than the yields of the quantified phenolic monomers. Because organosolv lignin has a tendency to produce the phenolic compounds with shorter side chains than the corresponding MWL, it can be speculated that average MW of phenolic oil produced from organosolv lignin (as well as other industrial lignin) is lower than what it would be if derived from natural lignin by pyrolyzing biomass. Previously, we analyzed pyrolysis vapors of corn stover organosolv lignin collected in a cold solvent and reported that the phenolic compounds are mostly monomers and dimers.16 We also reported that the average MW of phenolic-oil produced from pyrolysis of the same lignin is lower than 300 Da.43 Nsimba et al.44



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pyrolyzed an organosolv lignin extracted from woody biomass and reported that average 23 values of four fractionated phenolic oils to be between 212 and 430 Da. Ben et al.45 also reported that the average 23 of the pyrolysis-oil produced from Kraft lignin was 364 Da. In comparison, the average 23 s of the pyrolytic lignin derived from pyrolysis of the plant biomass ranged from 649 to 1319 Da, according to the study conducted by Meier’s group.10 Natural lignin tends to produce phenolic compounds with longer or more branched side chains in comparison to modified lignin, resulting in increased overall 23 of phenolic oil. It has also shown that carboxylic acids (acetic acid, formic acid etc.) among pyrolysis products of biomass promote polymerization of phenolic compounds.15,46 Acetic acid is mostly originated from acetyl groups of hemicellulose. Acetic acid yield was lower with the organosolv lignins because acetyl groups decreased during the extraction process. The yield of acetic acid was highest with corn stover MWL because herbaceous biomass lignin is naturally acetylated.47 Light gases produced from the pyrolysis of lignin mostly consist of CO2, CO and CH4. There was no significant difference in the yields of CO with MWLs derived from different biomass species. Red oak lignin produced an overall higher yield of CH4, due to the high content of methoxyl group. The yield of CO2 was higher with corn stover lignin because the lignin contains more C-O functionalities on side chains. For all cases, the yields of CO and CO2 decreased, whereas the yield of CH4 increased in the organosolv lignins compared to the corresponding MWLs. The yields of CO and CO2 decreased, because the organosolv lignin contains fewer amounts of carbonyl and carboxyl groups, as well as ether bondsin comparison to the corresponding MWL, which was confirmed in the FTIR spectrum. The order of pyrolysis char yield from the MWL was loblolly pine > red oak >corn stover. The char yields were lower with MWLs in comparison to the corresponding organosolv lignins. It is speculated that the char yield is even lower with the lignin in biomass. It is reported that the solid/liquid interaction among cellulose and lignin reduces primary char formation and the interaction among the respective vapor products suppresses secondary char formation.18 Hosoya et al.18 suggested that carbohydrate-derivatives act as H donors whereas lignin-derived phenolic compounds serve as H acceptors to inhibit char formation. The char yield increased in organosolv lignin since the carbon condensed structure is more difficult to volatilize. Loblolly pine organosolv lignin resulted in the highest char yield at 500 °C, containing as much as 50.33 %


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of lignin carbon. The methoxyl group substitution on the ortho position of the phenolic ring enhances C-O cleavage and but also repolymerization.39 The methoxyl groups on phenolic rings are known to promote char formation.48

Catalytic pyrolysis The carbon based product distribution of lignin during the catalytic pyrolysis is given in Table 2. Zeolite is the most commonly used catalyst in biomass conversion due to its cracking and deoxygenation ability in the absence of hydrogen and/or high pressure. HZSM-5 is the most effective zeolite catalyst in terms of deoxygenating biomass, contributed by its well-balanced acidity and shape selectivity.49 A series of reactions, such as deoxygenation, cracking, aromatization, oligomerization and isomerization occur in its active sites.50 The yields of aromatic hydrocarbons were 5.42 ~ 8.67 % when the pyrolysis vapors of the lignins were converted in the ex-situ catalyst bed. Overall, corn stover lignins produced highest amounts of aromatic hydrocarbons, while red oak lignins produced least amounts. Both red oak and loblolly pine organosolv lignins produced lower amounts of aromatic hydrocarbon compared to the corresponding MWLs. However, the yields were similar with corn stover MWL and organosolv lignin. Comparing Table 1 and 2, it was found that the higher the yields of the phenolic monomers are, the more aromatic hydrocarbons are produced. The yields of light hydrocarbons were highest with red oak lignins due to the high methane yields. Corn stover lignin produced intermediate yields of light hydrocarbon gases. Among the gaseous products, the yields of ethylene and propylene were particularly high due to the long branched structure of corn stover lignins. Catalytic coke is the result of polymerization and carbonization of the phenolic compounds at catalyst sites. The coke yields accounted from 13.73 to 20.64 %, lowest with red oak MWL and highest with corn stover organosolv lignin. Since pyrolysis char is solely due to thermal decomposition, the carbon selectivity of the pyrolysis vapors for hydrocarbons, carbon oxides and catalytic coke during catalyst conversion was calculated, and the results are compared in Figure 2. Compared to the corresponding MWLs, the pyrolysis vapors of organosolv lignin had much higher selectivity for forming catalytic coke and lower selectivity for producing CO2 and CO. For red oak or loblolly pine lignins, the



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selectivity for aromatic hydrocarbons was lower, whereas the selectivity for light hydrocarbons was higher with the organosolv lignin, in comparison to the corresponding MWLs. However, the trend was not obvious with corn stover lignins. The selectivity of benzene, toluene, xylene, ethyl benzene and polyaromatic hydrocarbons among total aromatic hydrocarbons is also given in Figure 3. Overall, aromatic hydrocarbons derived from red oak lignins had the highest selectivity for single ring aromatics. Although corn stover lignins produced the higher yields of total aromatics, the products had an increased selectivity for polyaromatic hydrocarbons over single ring aromatics. When converted by zeolite catalyst, most of the phenolic compounds are hardly able to enter the zeolite pores due to their large molecular sizes compared to zeolite pores. Thus, the phenols are likely first adsorbed on the zeolite surface and then converted. The existence of a phenolic pool on the catalytic surface was suggested by To et al.51 Without entering the pores, some of the adsorbed phenolic compounds could be directly deoxygenated at the limited numbers of external active sites to form aromatic hydrocarbons. In a recent study, we have also proven that the active sites on the surface of ZSM-5 catalyst play an important role in cracking and deoxygenating lignin-derived phenolic compounds.34 In addition to the surface deoxygenation, the side chain fragments could also enter the zeolite pores and be deoxygenated. Mullen et al.52 suggested that the aliphatic linkers between lignin units are the major source of aromatic hydrocarbons because only these small molecules are able to enter the zeolite pores and join the hydrocarbon pool. Forming aromatic hydrocarbons through this pathway is more pronounced with the MWLderived pyrolysis vapors because the relatively longer carbon chain lengths of the fragments make them easier to aromatize inside the pores. For organosolv lignin, the cracking fragments preferentially become light hydrocarbon gases due to their short carbon chains. The selectivity of individual hydrocarbon gases among total light hydrocarbons is given in Figure 4 for different types of lignin and the results also support the above theory. Compared to MWLs, the selectivity of methane was higher, whereas the selectivity of propylene was lower with organosolv lignins. The selectivity of propylene was highest with corn stover lignin, because the phenols derived from corn stover lignin have longer and branched side chains than that from red oak lignin or loblolly pine lignin. The relatively higher content of the side chains in corn stover lignin likely contributed to the higher yields of hydrocarbons.


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The selectivity of carbon oxides was higher with MWLs. This is due to the stronger extent of deoxygenation with MWLs by zeolite. The results could also suggest that MWL-derived pyrolysis vapors contain more oxygenated products than that from the corresponding organosolv lignins. Interestingly, it was noted that the selectivity for catalytic coke was much higher with corn stover organosolv lignin compared to corn stover MWL, despite that the selectivity for aromatic or light hydrocarbons were similar. It could be that higher oxygen content in the pyrolysis vapors derived from corn stover MWL helped thermal desorption of the phenols to suppress coke formation. It was also found that the selectivity of polyaromatics was higher with corn stover lignins than that with red oak lignin or loblolly pine lignins. This is probably related to the higher concentration of phenolic OH in the pyrolysis vapors of corn stover lignin as phenolic OH has strong tendency to polymerize. Vinylphenols are also abundant in pyrolysis vapor derived from corn stover lignin and these compounds are prone to repolymerize due to the reactive C=C bonds.

Conclusions MWLs and organosolv lignins isolated from red oak, loblolly pine and corn stover were characterized and further pyrolyzed in the absence and presence of catalyst. For all biomass species, MWLs decomposed at lower temperatures and produced higher amounts of volatiles in comparison to the corresponding organosolv lignins due to the higher oxygen contents. The organosolv process had the greatest influence on red oak lignin by decreasing the volatile content of MWL from 72.21 % to 56.51 % and increasing carbon content from 58.88 % to 68.16 %. The variation was least noticeable with corn stover organosolv lignin because tricin, ferulate and courmate groups remained after the extraction. Upon pyrolysis, organosolv lignins produced less phenolic oil and more char, in comparison to the corresponding MWLs. Corn stover lignins produced significantly higher yields of phenolic monomers (i.e., > 16 %) and lower yields of char in comparison to the wood-derived lignins. During the catalytic upgrading, pyrolysis vapors of organosolv lignins preferentially form coke and light hydrocarbons compared to MWLderived pyrolysis vapors. The yields of aromatic hydrocarbons were highest with corn stover lignins and lowest with red oak lignin. On the other hand, the selectivity of single ring aromatic



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hydrocarbons followed the opposite trend. Overall, corn stover lignins showed great potential for pyrolytic conversion because of its branched polymer structure.

Acknowledgement The work is partially supported by the CenUSA Bioenergy project. The authors also acknowledge Prof. Robert Brown, Dr. Marjorie Rover, Ryan Smith, Patrick Johnston, and Patrick Hall at the Bioeconomy Institute of Iowa State University for useful discussion and technical support.


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Page 56 of 64

Table 1. Carbon based product distribution of lignin pyrolysis at 500°C Feedstock Isolation method Yield (C %) Light Gases CO CO₂ CH₄ Sum

Red oak MWL Organosolv

Liquid products Phenolic monomers Phenol 2-Methoxyphenol m-cresol p-cresol Phenol, 2-methoxy-3-methyl2-Methoxy-5-methylphenol Creosol Phenol, 2,5-dimethyl4-ethylphenol Phenol, 4-ethyl-2-methoxy4-vinylphenol 2-Methoxy-4-vinylphenol Eugenol Phenol, 2-methoxy-4-propylPhenol, 2,6-dimethoxytrans-Isoeugenol 1,2,4-Trimethoxybenzene Vanillin Benzene, 1,2,3-trimethoxy-5-methylPhenol, 2-methoxy-4-propylApocynin 3',5'-Dimethoxyacetophenone Phenol, 2,6-dimethoxy-4-(2-propenyl)Benzaldehyde, 4-hydroxy-3,5-dimethoxy2-Propanone, 1-(4-hydroxy-3-methoxyphenyl)Ethanone, 1-(4-hydroxy-3,5-dimethoxyphenyl)Desaspidinol 3,5-Dimethoxy-4-hydroxyphenylacetic acid Desaspidinol / 2-Pentanone, 1-(2,4,6trihydroxyphenyl) 3,4,5-Trimethoxyphenylacetic acid 3,5-Dimethoxy-4-hydroxycinnamaldehyde Phenol, 4-(3-hydroxy-1-propenyl)-2-methoxyHomovanillic acid 2-Propenal, 3-(4-hydroxy-3-methoxyphenyl)Total monomers

Loblolly pine MWL Organosolv

Corn stover MWL Organosolv

3.75 3.26 2.17 9.19

2.97 2.17 2.57 7.72

3.42 2.83 1.46 7.71

2.91 2.06 1.7 6.67

3.15 4.24 1.16 8.55

2.54 3.68 1.15 7.37

0.06 0.35 0.07 0.08 0.05 0.01 0.84 0.07 0.2 0.52 0.14 0.1 0.63 0.34 1.43 0.21 0.39 0.21 0.65 0.9 0.38 0.07 0.3 0.13

0.04 0.26 0.05 0.06 0.05 0.01 0.81 0.07 0.19 0.27 0.09 0.1 0.46 0.19 1.37 0.1 0.32 0.13 0.39 0.66 0.15 0.1 0.23 0.07

0.26 0.67 0.13 0.36 0.06 0.01 1.3 0.18 0.07 0.45 0.37 0.85 0.26 0.08 0.12 1.52 0.4 0.63 0.24 0.25 0.13 0.35 0.12 0.07 0.07 -

0.3 0.99 0.11 0.28 0.07 0.02 1.42 0.13 0.05 0.58 0.25 0.64 0.17 0.14 0.18 1.13 0.42 0.32 0.17 0.12 0.1 0.32 0.04 0.09 0.04 -

0.41 0.48 0.09 0.48 0.04 0.64 0.08 0.31 0.18 6.82 2.8 0.12 0.03 0.57 0.44 0.61 0.25 0.19 0.12 0.41 0.73 0.18 0.24 0.07 -

0.8 0.66 0.11 0.54 0.04 0.68 0.07 0.72 0.39 5.62 3.7 0.04 0.03 0.65 0.18 0.62 0.1 0.17 0.06 0.28 0.56 0.05 0.12 0.06 -

0.1 0.08 0.3 8.61

0.09 0.03 0.04 6.32

0.15 0.12 0.31 9.51

0.03 0.07 0.12 8.32

16.26

16.28


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Phenolic oligomers* Acetic acid Toluene Sum (Liquid product)

37.79 1.6 48

38.33 0.07 44.72

34.87 0.53 0.03 44.93

34.6 0.06 0.02 43

36.33 2.41 0.02 55.02

33.92 0.19 0.04 50.42

Char

42.81

47.56

47.36

50.33

36.43

42.21

* Calculated by subtracting the quantified product yields from 100%.



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Page 58 of 64

Table 2. Carbon based product distribution of different lignin during ex-situ catalytic pyrolysis by HZSM-5 zeolite (temperatures: pyrolysis reactor: 500°C, catalyst bed: 600°C.) Feedstock Isolation methods

Red oak MWL

Lob. pine

Corn stover

Organosolv

MWL

Organosolv

MWL

Organosolv

Oxygenates CO

7.66

5.5

6.11

5.33

6.58

4.96

CO₂

4.05

2.87

3.67

2.74

5.45

4.62

Methane

3.27

4.17

2.41

2.88

2.11

2.43

Ethylene

4.33

5

4.31

4.51

5.14

4.26

Propylene

2.44

1.99

2.03

1.95

3.27

2.74

Butene

0.15

0.15

0.13

0.13

0.16

0.16

Total

10.2

11.31

8.88

9.47

10.67

9.59

Benzene

1.58

1.24

1.78

1.49

1.84

1.79

Toluene

2.08

1.56

1.99

1.71

1.79

1.51

Xylene

1.24

0.86

0.95

0.8

0.81

0.68

C₉₊

0.66

0.34

0.75

0.56

0.96

1.01

C₁₀₊

2.07

1.25

2.39

2.48

3.25

3.68

Total

7.63

5.25

7.86

7.04

8.65

8.67

Char

42.81

47.56

47.36

50.33

36.43

42.21

Coke

13.73

17.69

14.06

18.87

17.95

20.64

57.19

52.44

52.64

49.67

63.57

57.79

86.08

90.83

87.93

93.78

85.75

90.7

Olefins + CH₄

Aromatics

Carbon balance


Page 59 of 64

100

(a) TGA

90 80 Red oak-MWL Red oak-Organosolv Loblolly pine-MWL Loblolly pine-organosolv Corn stover-MWL Corn stover-Organosolv

Residue (%)

70 60 50 40 30 20 10 0 0

100

200

300 400 500 Temperature (°C)

600

700

800

0.01

(b) DTG: Red oak 0.008


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


MWL Organosolv

0.006

0.004

0.002

0 0

100

200

300

400

500

Temperature (°C)


600

700

800


0.008

(c) DTG: Loblolly pine


MWL 0.006

Organosolv

0.004

0.002

0 0

100

200

300

400

500

600

700

800

Temperature (°C)

0.008

(d) DTG: Corn stover MWL


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

Page 60 of 64

0.006 Organosolv 0.004

0.002

0 0

100

200

300

400

500

600

700

800

Temperature (°C)

Figure 1. TGA analysis of MWL and organosolv lignin derived from red oak, loblolly pine and corn stover; (a) comparison of TGA profiles of all lignin; DTG curves of MWL and organosolv lignin derived from (b) red oak, (c) loblolly pine and (d) corn stover.


Page 61 of 64

40


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


Coke light hydrocarbons CO2

Aromatic hydrocarbons CO

35 30 25 20 15 10 5 0 MWL

Organosolv

Red oak

MWL

Organosolv

Loblolly pine

MWL

Organosolv

Corn stover

Figure 2. Carbon selectivity of pyrolysis vapors of different types of lignin for coke, hydrocarbons and carbon oxides when the vapors are converted by HZSM-5 zeolite.



45 40


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Page 62 of 64

35

Benzene

Toluene

Xylene

C9 Aromatics

C10+ Aromatics

30 25 20 15 10 5 0 MWL

Organosolv

Red oak

MWL

Organosolv

Loblolly pine

MWL

Organosolv

Corn stover

Figure 3. Carbon selectivity of individual aromatic hydrocarbons among total aromatic hydrocarbons.


Page 63 of 64

60 Methane

Ethylene

Propylene

Butene

50


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


40 30 20 10 0 MWL

Organosolv

Red oak

MWL

Organosolv

Loblolly pine

MWL

Organosolv

Corn stover

Figure 4. Carbon selectivity of light hydrocarbons among total light hydrocarbons



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

For Table of Contents Use Only Lignin Valorization through Thermochemical Conversion: Comparison of Hardwood, Softwood and Herbaceous Lignin Shuai Zhou, Yuan Xue, Ashokkumar Sharma, Xianglan Bai*

Synopsis: Volatilization of milled wood lignins and organosolv lignins of three biomass species were investigated.

Supporting Information Table S1. Proximate and elemental analyses of corn stover, red oak, and loblolly pine lignin Table S2. GPC analysis of red oak, loblolly pine and corn stover lignin. Figure S1. FTIR spectrum of MWL and organosolv lignin derived from (a) red oak, (b) loblolly pine and (c) corn stover Figure S2. 2D-HSQC-NMR spectrum of different types of lignin in DMSO-d6 : pyridine-d5 (4:1, v:v). (a) Red oak MWL, (b) Red oak organosolv lignin, (c) Loblolly pine MWL, (d) Loblolly pine organosolv lignin, (e) Corn stover MWL, (f) Corn stover organosolv lignin


Page 64 of 64

Comparison of Hardwood, Softwood and ... - ACS Publications

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