Comparison of Fast Pyrolysis Behavior of Cornstover Lignins Isolated

Letter pubs.acs.org/journal/ascecg

Comparison of Fast Pyrolysis Behavior of Cornstover Lignins Isolated by Different Methods Jing Zhang,† Kwang Ho Kim,§ Yong S. Choi,† Ali H. Motagamwala,∥,⊥ James A. Dumesic,‡,∥,⊥ Robert C. Brown,§ and Brent H. Shanks*,†,‡ †

Department of Chemical and Biological Engineering, Iowa State University, 617 Bissell Road, Ames, Iowa 50011, United States Center for Biorenewable Chemicals, Iowa State University, 617 Bissell Road, Ames, Iowa 50011, United States § Bioeconomy Institute, Iowa State University, 617 Bissell Road, Ames, Iowa 50011, United States ∥ Department of Chemical and Biological Engineering, University of Wisconsin, 1415 Engineering Drive, Madison, Wisconsin 53706, United States ⊥ Great Lakes Bioenergy Research Center, 1552 University Ave., Madison, Wisconsin 53726, United States ‡

S Supporting Information *

ABSTRACT: The effect of lignin isolation methods on lignin fast pyrolysis was investigated utilizing an online product analysis methodology to give high mass balance closure for the products. Lignin samples with high purity were isolated from cornstover using three methods: Björkman milling, organosolv extraction, and gamma-valerolactone (GVL) extraction. Although milled wood lignin has been extensively studied, characterization and the fast pyrolysis of milled cornstover lignin has not been previously reported. The current study shows that the milled cornstover lignin contains a higher molecular weight and more aryl ether linkages relative to organosolv and GVL extracted cornstover lignin. During fast pyrolysis, considerably more phenolics, especially those with side chain double bonds derived from aryl ether cleavage, were formed from the milled cornstover lignin concomitant with less char formation. KEYWORDS: Fast pyrolysis, Lignin characterization, Lignin isolation, Milled cornstover lignin, Phenolic compound

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INTRODUCTION

Björkman method was broadly applied to isolated lignin from woody biomass, resulting in milled wood lignin that has been widely acknowledged to be the most closely representative of native lignin. Unfortunately, even this method causes some decomposition of the lignin polymer. Therefore, the structure of lignin depends on both the type of biomass and the manner in which it is isolated from lignocellulose. As the second most abundant component of lignocellulosic biomass, lignin has potential for producing fuels and chemicals. One possible deconstruction approach for converting lignin is fast pyrolysis. Several previous studies have examined the pyrolysis of different types of lignin. Wang et al. reported that

Lignin is an amorphous biopolymer consisting of phenylpropane monomer units with hydroxyl and methoxyl groups substituted on the aromatic ring.1,2 Unlike the uniform structure of cellulose and hemicellulose, lignin has a complicated structure since the number of methoxyl groups on the aromatic ring varies from zero to two and can be coupled in an almost random fashion. The three types of phenylpropane monomers, 4-hydroxylphenylpropane (H), guaiacylpropane (G), and syringylpropane (S), correspond to the inclusion of zero, one, or two methoxy groups on the phenylpropane ring, respectively. The content of H, G, and S units in lignin is different among hardwood, softwood, and herbaceous biomass.3,4 Due to an intimate connection to cellulose and hemicellulose, lignin cannot be isolated from lignocellulose without altering its original structure. The © 2017 American Chemical Society

Received: May 3, 2017 Revised: May 26, 2017 Published: May 31, 2017 5657

DOI: 10.1021/acssuschemeng.7b01393 ACS Sustainable Chem. Eng. 2017, 5, 5657−5661

Letter

ACS Sustainable Chemistry & Engineering

described by Luterbacher et al.11 A Mettler-Toledo Analytical (USA) thermogravimetric analyzer was used to determine the initial decomposition temperature, moisture and ash content for the lignin samples. The molecular weight of the lignin samples was determined by gel permeation chromatography (GPC) using a high-performance liquid chromatography system (Ultimate 3000, Dionex, Sunnyvale, USA) with a Shodex refractive index and diode array detector. Details about the TGA and GPC analyses are given in the Supporting Information. Fourier transform infrared (FTIR) spectrophotometry (Bruker Tensor 37, USA) was performed using dry KBr discs containing 1% of lignin powders. Each spectrum was recorded in the range from 4000 to 400 cm−1 with a resolution of 4 cm−1 over 20 scans. Background subtraction was applied to each spectrum. Fast Pyrolysis of Lignin. The fast pyrolysis experiments were performed using a single-shot micropyrolyzer (Model 2020 iS, Frontier Laboratories, Japan). The micropyrolyzer was connected to a gas chromatograph (GC) system (7890A, Agilent Technologies, USA) for product identification and quantification. The GC system was equipped with three detectors including mass spectrometry (MS), flame ionization detector (FID), and thermal conductivity detector (TCD), which were used for product identification, condensable product quantification, and noncondensable product quantification, respectively. Details about the GC ramping program, product identification, product calibration, and char measurement can be found in previous publications.14,15 In the current study, all three lignin samples were pyrolyzed at four different temperatures (350, 425, 500, and 575 °C). Pyrolysis results are presented based on the average of triplicate trials.

the milled hardwood lignin produced more methanol than milled softwood lignin because the former contained more methoxy groups.5 Wang et al. also reported that for a hardwood, lauan, the generated pyrolytic lignin had a lower molecular weight and initial thermal decomposition temperature than its milled wood lignin.6 However, both studies were performed based on thermogravimetric analysis (TGA), which could not provide the high heating rates that characterize fast pyrolysis. Importantly, they also did not quantify the products. Using a pyrolyzer with high heating rates, Obst found softwood lignin primarily produced G-type compounds, while hardwood lignin produced both S- and G-type compounds.7 Additionally, Evans et al. reported vinylphenol and coumaryl alcohol were the dominant volatile products from the fast pyrolysis of herbaceous lignin.8 However, these studies primarily focused on qualitative comparisons or, in a few instances, quantitative comparison of just a few products. Recently, Patwardhan et al. quantified a comprehensive product distribution for the fast pyrolysis of organosolv lignin form cornstover, but no other types of lignin were investigated.9 To our knowledge, there has been no systematic comparison of product distributions from the pyrolysis of cornstover lignins obtained by different isolation methods. We hypothesize that different isolation methods will alter the lignin structure to different degrees, thereby impacting the pyrolysis product distribution. Cornstover was chosen as the lignin source since it is the largest quantity of biomass residue in the United States.10 In the current study, we investigated three types of isolated cornstover lignin obtained from Björkman milling, gamma-valerolactone (GVL) solvent extraction, and organosolv processing. The GVL solvent extraction was recently explored as an approach to isolate native-like lignin from cornstover,11 while organosolv processing has been commercially used to produce lignin. The isolated lignins from these three processes are referred to in this work as milled cornstover lignin (MCL), GVL extracted lignin (GVLL), and organosolv lignin (OSL) respectively.

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RESULTS AND DISCUSSION As shown in Table 1, the MCL and OSL samples were highly pure lignin. As indicated by NMR analysis, GVLL was also a Table 1. Purity, Ash Content, Moisture Content, and Average Molecular Weight for Ligninsa

MATERIALS AND METHODS

lignin type

lignin purity (wt %)

ash content (wt %)

moisture (wt %)

Mnb (Da)

Mwc (Da)

MCL OSL GVLL

97.2 96.5 −

∼0 0.1 0.2

0.7 0.6 0.5

2351 864 827

6740 2185 1910

a

Milled cornstover lignin (MCL), GVL extracted lignin (GVLL), and organosolv lignin (OSL). bNumber-average molecular weight. c Weight-average molecular weight.

Lignin Extraction. Organosolv lignin from cornstover was procured from Archer Daniels Midland (ADM) Company. Milled cornstover lignin was prepared according to the Björkman lignin method.12 This extraction was performed by solubilizing 100 g of cornstover powder in 1 L of methanol for 24 h to remove extractives. The extractive-free cornstover was subjected to ball milling for 96 h, followed by two separate extractions using 1 L of dioxane:water (96:4, v:(v) for 24 h. The extracts were vacuum-dried to obtain crude lignin. Further purification was performed by dissolving the crude lignin in 100 mL of 90% acetic acid followed by precipitation in ice water. The precipitated lignin was separated by centrifugation and freeze-drying. The lignin was then dissolved in 50 mL of 1,2-dichloroethane: ethanol (2:1, v:(v) solution followed by centrifugation. The supernatant was added dropwise in 1 L of ethyl ether and centrifuged. The insoluble portion was freeze-dried and underwent ethyl ether washing three times to obtain MCL. GVLL was obtained by treating the cornstover in a batch reactor with a mixture of GVL, water, and sulfuric acid at 120 °C for 30 min. After cooling, the slurry was filtered and washed with hot GVL. Then, DI water was added to the filtrate to precipitate crude lignin, followed by excessive DI water washing to obtain GVLL. The experimental details have been reported elsewhere.11 Characterization of Lignins. The amount of Klason lignin and acid-insoluble lignin were determined using the NREL LAP TP-51042618 protocol.13 In the current study, the purity of MCL and OSL was taken as the sum of Klason lignin and acid soluble lignin. The purity of GVLL was qualitatively determined by NMR analysis as

clean aromatic product except for some GVL residues.11 The residual GVL content was quantified by pyrolyzing it at a relatively low temperature. This revealed the GVL residue to be 17.5 wt % of the GVLL sample. Table 1 also shows that the three lignin samples had low ash and moisture content. Lignin purity was also indicated by the relative absence of pyrolysis products of nonlignin origin, as subsequently described. Table 1 shows the MCL sample had a much higher molecular weight than OSL. Similar trends among isolated lignins from woody biomass have been reported.16 These results suggested that hydrolysis occurred to a lesser extent during Björkman milling than with organosolv processing. Table 1 shows that GVLL had a molecular weight more similar to that of OSL than MCL. Since NMR characterization indicated that GVLL had a lignin structure more closely resembling native lignin with little degradation other than limited β-ether cleavage,11 the molecular weight reduction for GVLL was likely due to cleavage of weak aryl ether bonds. The three delignification methods resulted in different degrees of depolymerization, as suggested by GPC analysis, which might be expected to lead to differences in the hydroxyl 5658


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OSL, attributed to the phenolic hydroxyl group.18,19 A small shoulder at the same location was observed for GVLL, as well. However, this peak was not apparent for MCL. This difference might be explained by hydrolytic breakage of the aromatic ether linkages leading to the formation of phenolic hydroxyl groups. As such, fragmentation of aromatic ether linkages, possibly the most sensitive β ether bonds,2 appeared to occur to a larger extent in the production of OSL and GVLL compared to MCL. Similar to woody biomass, the Bjorkman milling of cornstover appeared to preserve the native ether linkages in lignin to a larger extent thereby suppressing the generation of hydroxyl groups relative to the GVL extraction and organosolv process. TGA analysis results, which approximate the heating rate for slow pyrolysis, are shown in Figure S2. The initial decomposition temperatures for MCL and OSL were nearly identical, at around 200 °C, suggesting the difference in molecular weight between MCL and OSL did not affect their thermal stability. The initial decomposition temperature for GVLL was obscured by interference from the residual GVL solvent in the sample. The char yield from TGA analysis was obtained using the sample weight before introducing oxygen into the system (around 880 °C). For GVLL, the char yield was calculated on a GVL solvent-free basis. As shown in Table S1,

group and ether bond functionalities. Figure S1 shows the FTIR spectra of the three lignins with the magnified spectra for specific peaks shown in Figure 1. As most clearly shown in

Figure 1. FT-IR spectra in the 1400−1000 cm−1 range for the lignin samples: milled cornstover lignin (MCL), GVL extracted lignin (GVLL), and organosolv lignin (OSL).

Figure 1, a stronger signal at 1170 cm−1, attributed to aromatic ether linkage,17 was observed for MCL relative to OSL and GVLL. Additionally, one peak at 1365 cm−1 was observed for

Figure 2. Yield comparison for phenolics with side chain double bonds (PWDB), phenolics without side chain double bonds (PNDB), and char among the three samples, milled cornstover lignin (MCL), GVL extracted lignin (GVLL), and organosolv lignin (OSL), at pyrolysis temperatures of (a) 350 °C, (b) 425 °C, (c) 500 °C, and (d) 575 °C. 5659


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the carbon appeared as char. In contrast, no noticeable char formation occurred for PPPD pyrolysis.20 The only difference between HH and PPPD was that the former has one more phenolic hydroxyl group. Moreover, significantly less char was formed from native lignocellulose in which hemicellulose had been extracted compared to a physical mixture of cellulose and OSL with comparable amounts of cellulose and lignin.15 Since both kinds of cellulose−lignin mixtures were essentially free of minerals and pure cellulose generates very little char, the difference in char yields was attributed to differences in the lignin in the two samples. Zhang et al. hypothesized that this difference was due to the higher hydroxyl content of OSL compared to native lignin.15 Therefore, cleavage of aromatic ether linkages during fast pyrolysis formed significant amounts of PWDB, while hydrolytic cleavage of these linkages during the process of lignin isolation introduced hydroxyl groups that facilitated char formation. Of the lignins tested, MCL undergoes the least amount of aromatic ether linkage hydrolytic cleavage during isolation, which allowed such ether linkages to subsequently be available for cleavage during pyrolysis. This effect resulted in a product containing more PWDB and less char than the other extracted lignins.

similar char yields were observed for OSL and GVLL, both of which were higher than for MCL. Fast pyrolysis of the three lignin samples was performed using the Frontier micropyrolyzer in the temperature range from 350 to 575 °C. As shown in Tables S3−S6, mass balances higher than 90% were achieved, which made possible quantitative comparison of the product yields for the lignin samples. For GVLL, the residual solvent did not appear to decompose since 17.5 wt % GVL was observed among the products at all pyrolysis temperatures tested. To correct for the presence of this solvent, the pyrolysis product distribution of GVLL was reported on a GVL solvent-free basis. For all the three lignins, the product yields not derived from lignin (grouped in Table S2) summed to less than 1%, which was further evidence of the high purity of the isolated lignins. Figure 2 and Tables S3−S6 compare the yields of char and total phenolic compounds produced from the three lignin samples for each pyrolysis temperature. Yields of phenolics, char, and gas from the different lignins as a function of temperature are plotted in Figure S3, which showed a decrease in char yield and increase in gas yield for all lignins from 350 to 575 °C, while the yield of phenolics was maximized at 500 °C. Significantly higher yields of total phenolics were observed for MCL compared to GVLL and OSL at all pyrolysis temperatures. Concomitantly, a lower char yield was observed for MCL relative to GVLL and OSL, which was the same tend observed from TGA. The relative change in yields of phenolics as a function of temperature were similar for MCL, OSL, and GVLL, with the highest yield of total phenolics achieved at 500 °C for all cases. By dividing the phenolic products into phenolics with side chain double bonds (PWDB) and phenolics without side chain double bonds (PNDB) (grouped in Table S2), the main difference in the total phenolics yield between MCL and OSL (or GVLL) was attributed to the higher yield of PWDB from MCL, as shown in Figure 2. On the basis of the aforementioned lignin characterization, the major difference between MCL and OSL/GVLL was the molecular weights. Ether bonds, possibly the most fragile β-ether bonds, were cleaved to a greater extent during organosolv and GVL extraction compared to Bjö rkman milling. Ether bonds surviving the Björkman milling process would readily cleave during fast pyrolysis. Our previous work with lignin model compounds provided insights into the β-ether bond cleavage during fast pyrolysis.20 Pyrolysis of 2-phenoxyphenylethanol (PPE), the simplest β-O4 compound, primarily formed acetophenone, benzeneacetaldehyde, and phenol, the first two of which are PWDB accounting for 60 wt % of total pyrolysis products.20 Similarly, pyrolysis of 2-phenoxyphenyl-1,3-propanediol (PPPD) primarily formed acetophenone, benzeneacetaldehyde, benzaldehyde, and phenol, among which PWDB accounted for up to 50 wt % of the yield.20 On the basis of the product distribution, it was proposed that pyrolysis involved concerted reactions in which β-O-4 linkages were initially cleaved by a retro-ene reaction through a cyclic transition state followed by keto−enol tautomerization, demethylation, and hydroxyl group migration. In contrast, hydrolytic cleavage of β-O-4 bonds during the lignin isolation process could introduce hydroxyl groups instead of side chain double bonds. These hydroxyl groups decrease the volatility of the pyrolytic products and increase the tendency of intermolecular dehydration thereby facilitating char formation. For example, during pyrolysis of a p-hydroxy-substituted PPPD, 1-(4-hydroxyphenyl)-2-phenoxypropane-1,3-diol (HH), 34% of

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CONCLUSIONS Pyrolysis of cornstover lignins from three extraction methods was performed. For MCL which had the highest molecular weight, the pyrolytic yield for total phenolics was 40% to 47% higher concomitant with less char formation compared to OSL and GVLL. Aromatic ether bond cleavage during fast pyrolysis formed significant amounts of phenolics with side chain double bonds, which primarily accounted for the higher phenolics and lower char yield from MCL. Therefore, if the goal was to maximize carbon recovery from the pyrolytic deconstruction of lignin, the preferred lignin isolation method would be one that preserved aromatic ether bonds such as with MCL.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01393. Experimental details of TGA and GPC analysis, lignin char yield and weight loss curve from TGA analysis, FTIR spectra of lignin samples in the range of 1400−1000 cm−1, grouping for pyrolysis products, and detailed pyrolysis product distribution from lignin at different temperatures. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1 515 294 1895. E-mail: [email protected] (B.H. Shanks). ORCID

Brent H. Shanks: 0000-0002-1805-415X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Patrick Johnston for his assistance in the gel permeation chromatography analysis. 5660


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REFERENCES

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Comparison of Fast Pyrolysis Behavior of Cornstover Lignins Isolated

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