Accepted Manuscript

Pyrolytic production of phenolic compounds from the lignin residues of bioe-thanol processes

Kyung A Jung, Seung Han Woo, Seong-Rin Lim, Jong Moon Park

PII: S1385-8947(14)01030-4DOI: http://dx.doi.org/10.1016/j.cej.2014.07.126Reference: CEJ 12494

To appear in: Chemical Engineering Journal

Received Date: 1 May 2014Revised Date: 28 July 2014Accepted Date: 30 July 2014

Please cite this article as: K.A. Jung, S.H. Woo, S-R. Lim, J.M. Park, Pyrolytic production of phenolic compoundsfrom the lignin residues of bioethanol processes, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.07.126

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Pyrolytic production of phenolic compounds

from the lignin residues of bioethanol processes

Kyung A Junga, Seung Han Woob, Seong-Rin Limc, and Jong Moon Parka,d*

a School of Environmental Science and Engineering, POSTECH, 77 Cheongam-ro, Nam-gu,

Pohang 790-784, South Korea

b Department of Chemical and Biological Engineering, Hanbat National University, 125

Dongseodaero, Yuseong-gu, Daejeon 305-719, South Korea

c Department of Environmental Engineering, Kangwon National University, 1

Kangwondaehak-gil, Chuncheon 200-701, South Korea

d Department of Chemical Engineering, Division of Advanced Nuclear Engineering,

POSTECH, 77 Cheongam-ro, Nam-gu, Pohang 790-784, South Korea

* To whom all correspondence should be addressed.

(J. M. Park)

Tel: +82 54 279 2275, Fax: +82 54 279 2699, Email: jmpark@postech.ac.kr

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Abstract

Lignin is the major source of polyphenolic compounds available from natural

biomass. Although most commercial lignin products have been supplied from wood biomass,

the residues from bioethanol production processes could be an additional potential source.

About 75% of lignin inherent in corn stover and rice straw was recoverable from the residues

of bioethanol producing processes. Chemical structures and thermolysis features of the lignin

residues from corn stover and rice straw obtained through acid-alkali pretreatments were

characterized. Due to inherent structural differences, the corn stover- and rice straw lignin

were more reactive and had less thermal stability than the wood-based Kraft lignin. The corn

stover lignin showed the lowest maximum degradation temperature with the highest mass

loss rate in the primary pyrolysis reaction and it was mainly pyrolyzed into monomers of

lignin building blocks with a higher phenol content (10%), unlike the other lignin samples (<

6%).

Keywords: Aromatics; Bioethanol residues; Lignin; Phenolics; Pyrolysis;

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1. Introduction

Lignin is the second largest source of organic raw materials. Lignin accounts for

about 10-41% of the lignocellulosic biomass (40-60% and 7-20% for cellulose and

hemicellulose, respectively). It is the only major source of polyphenolic compounds available

from natural biomass feedstocks [1, 2]. Lignin is a complex and amorphous aromatic

compound comprised of variously linked phenylpropane units, such as hydroxyphenyl (H),

guaiacyl (G), and syringyl (S). The compositional variations in these units depend upon the

type of biomass feedstocks (i.e., hardwood, softwood, and herbaceous biomass) [3].

Until now, most commercial lignin has been supplied from papermaking plants (i.e.

sulfite pulping and Kraft pulping processes) as byproducts [4], but the potential amount of

residual lignin produced from the cellulosic ethanol industry has been increasing as much [5].

While the pulping processes utilize wood-based biomass for lignin production, the bioethanol

industry utilizes various types of biomass feedstock such as crop residues, herbaceous

biomass, and municipal wastes with high availability [6]. As types of crop residue, corn

stover and rice straw are abundant worldwide, and they can be converted into fuel ethanol [7]

and are a vital source of lignin production. In 2012, approximately 875 million tons of corn

stover and 1,005 million tons of rice straw were produced [8]. Thus, lignin that is derived

from corn stover and rice straw is expected to produce ca. 200 million tons worldwide [2, 3].

In the bioethanol producing process, only cellulose and/or hemicellulose in the crop

residue is utilized, and lignin remains a residual byproduct. In the bioethanol process, dilute-

acid and subsequent alkali pretreatments are used to recover fermentable carbohydrates and

remove the lignin component from the biomass [9, 10]. These processes are similar to a Kraft

lignin process guaranteeing higher purity of wood lignin components for the production of

lignin-based chemicals [11]. The first dilute acid step removes almost all of the hemicellulose

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and acid-insoluble components from the biomass and then the next alkali step solubilizes the

rest of the lignin fractions, not cellulose.

While cellulose and hemicellulose are meaningful carbon sources in the biochemical

fuel conversion process, most of the discharged lignin fractions, a non-fermentable

compound to microorganisms, has been utilized for low-quality materials such as soil

amendment, dispersing or binding agents, adsorbents, and boiler fuels, in spite of the fact that

higher value opportunities are available for lignin [11-13]. The main high added-value

applications of lignin can be categorized into three groups: power/fuel, macromolecules, and

aromatics [11, 14]. Among those applications, aromatics commodities that are derived from

lignin such as BTX (benzene, toluene, and xylene), phenol, and vanillin, are expected to offer

a significant market potential with an estimated market value of over $ 130 billion; moreover,

pyrolysis technology has drawn attention to opportunities available for converting lignin into

these high value-added products [11, 15, 16].

Pyrolysis, one of promising thermochemical conversion technologies, can play a vital

role in transforming the lignin residues of the bioethanol processes into value-added products

[17]. In the absence of oxygen, pyrolysis reaction breaks down the large lignin compounds

into condensable lower-molecular-weight compounds, including monolignols, monophenols,

and other poly-substituted phenols [2, 16]. However, due to the significant complexity of the

pyrolysis reactions, the various features and the relative distribution of the aromatic

derivatives attained through lignin pyrolysis would be based on several factors, such as the

type of feedstock, the feedstock preparation methods, and the pyrolysis reaction conditions

[18].

Many studies have been reported on the potential utilization and thermal degradation

of lignin residues, but most of those have focused on either wood-based lignin or synthesized

lignin model compounds as pyrolytic substrates [14, 19-21]. The potential obstacles to the

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pyrolytic utilization of non-wood based bioethanol residual lignin have been informed less

often. This study is the first to investigate the development of a high value-added application

for lignin residues generated from bioethanol processes using crop residues to reinforce

merits and value of future biorefinery system. The lignin residues obtained by dilute acid and

alkali pretreatments of corn stove and rice straw were thermally degraded in a fixed-bed

pyrolysis reactor. The chemical compositions of the lignin compounds and the aromatic

products obtained by the pyrolysis reaction were also compared with commercial Kraft lignin

in order to provide a deeper understanding of the differences in the pyrolytic behavior of

lignin starting from different biomass sources and the differences in the pyrolytic pathways of

the aromatic products that are formed.

2. Materials and Methods

2.1. Raw materials and lignin preparation

Four types of lignin were prepared: corn stover lignin and rice straw lignin as

residual products derived from the bioethanol process, Kraft lignin, purchased from Sigma-

Aldrich Corp., and re-precipitated Kraft lignin, called ReLignin, as pure chemicals. The corn

stover lignin and rice straw lignin were obtained by a sequential process of dilute-acid

hydrolysis and alkali pretreatment, which is a typical lignocellulosic biomass pretreatment for

bioethanol productionn. The corn stover and rice straw, collected from farmlands near

Pohang, South Korea, were rinsed with tap water and air-dried in a venting hood.

Pretreatment of the biomass was conducted at 10% of the biomass-to-solution ratio with 3%

(w/w) of the chemical agents (H2SO4 and NaOH for the dilute-acid and alkali pretreatments,

respectively) at 120 °C for 30 min. After the dilute-acid hydrolysis was completed, the

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insoluble sample was water-washed until the supernatant was neutralized, and then the alkali

pretreatment was conducted under the same reaction conditions. At the end of these stages,

the alkali-soluble lignin was precipitated by using H2SO4 solution [22]. The precipitated solid

phase was oven-dried and finely ground for use in the experiments. To observe the impact

that the pretreatment had on the lignin material, ReLignin was also obtained from raw Kraft

lignin through the same pretreatment process.

2.2. Mass change in the lignocellulosic components after chemical treatments

The mass change of the main components of the lignocellulosic biomass after acid-

and alkali pretreatment was estimated by element and by mixed raw biomass. Pure chemicals

of cellulose, hemicellulose (i.e., xylan), and lignin (i.e., Kraft lignin), which were purchased

from Sigma-Aldrich Co., each underwent dilute-acid hydrolysis and alkali pretreatment, as

mentioned above. The insoluble fractions were determined by weighing the hydrolyzate

residues after filtration and oven-drying. Concerning the corn stover and the rice straw, the

insoluble fractions of the acid- and alkali pretreatment were determined by weighing them

and the Klason lignin method [23] was also applied to each insoluble residue in an effort to

estimate the potential amount of lignin recovered in all of the stages.

2.3. Compositional analysis

The lignin content of the raw biomass, the corn stover and the rice straw, was

estimated based on the Klason lignin (acid-insoluble) method [23] because that approach

allowed the amount of the residual biomass lignin to be recovered and determined after the

raw biomass material passed through the dilute-acid pretreatment. The oven-dried lignin

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samples (0.175 g) were hydrolyzed with 72% H2SO4 solution (3 ml) at 30 °C for 1 h. The

hydrolyzate was diluted to 3% of H2SO4 solution with deionized water, and then was carried

out at 121 °C for 1 h. The Klason lignin of the raw biomass was determined by weighing the

hydrolyzate residues after filtration and oven-drying. The contents of cellulose and

hemicellulose of corn stover and rice straw were determined using methods of Hill et al. [24].

For determination of the insoluble holocellulose content, deionized water (160 ml), glacial

acetic acid (0.5 ml), and 15 % Na2ClO2 (10 ml) were added to the ground biomass (1 g) in a

flask. The flask was heated in a water bath at 75 °C for 4 h in total; and three additions of

acetic acid (0.5 ml) and the Na2ClO2 solution (10 ml) were applied at hourly intervals. After

heating, the flask was cooled to below 10 °C in an ice bath. The delignified sample was

filtered through a glass sinter crucible, washed with 95% ethanol (200 ml), cold deionized

water (200 ml), and acetone (200 ml). The crucible was weighed after oven-drying overnight

at 50 °C. For the cellulose content, the delignified sample was mixed with a 10% NaOH/15%

Na2B4O7 solution (10 ml), and then purged with N2 gas. The flask was placed at 20 °C for 2 h;

and then the sample, transferred to the crucible, was washed with the NaOH/ Na2B4O7

solution (50 ml), deionized water (100 ml), and ethanol (100 ml). The crucible was oven-

dried overnight at 105 °C. The insoluble portion stands for the amount of cellulose in the

biomass; and the amount of hemicellulose was obtained by the difference from the

holocellulose and the cellulose. The carbon, hydrogen, nitrogen, and sulfur contents of the

lignin sample were measured, twice, using a Vario MICRO CHNS analyzer (Elementar

Analysensysteme GmbH, Germany). The oxygen content was estimated by calculating the

difference in the amount of the elements measured in advance. The elemental analysis of the

lignin samples and the raw biomass is given in Table 1.

2.4. FTIR analysis

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For the Fourier Transformation Infrared (FTIR) spectra, the oven-dried and finely

ground lignin samples were analyzed using a Nicolet 6700 FTIR spectrometer (Thermo

Scientific., USA) equipped with an Attenuated Total Reflectance (ATR) mode with a

diamond crystal in the region of 4000-500 cm-1. Each sample was scanned 32 times and the

resolution was 4 cm-1. Background spectra were collected in air. The spectra of the lignin

samples were analyzed using OMNIC 8.2 software (Thermo Scientific., USA) to determine

the peak position and heights.

2.5. Thermogravimetric analysis

The thermal decomposition characteristics of each lignin sample were analyzed using

a thermogravimetric analyzer (SII Exstar 600, Seiko Instrumentation Inc., Japan). The

analyzer was controlled by a computer and it was used to collect the following thermal

decomposition data: thermogravimetry (TG) and differential thermogravimetry (DTG). As a

carrier gas, nitrogen (99.99%) flowed at a rate of 100 ml min-1 for the pyrolysis experiments.

Approximately 10-15 mg of the prepared lignin sample was pyrolyzed from 30 °C to 900 °C

at a heating rate of 10 °C min-1.

2.6. Fixed-bed pyrolysis and pyrolysis product analysis

The pyrolysis products of the lignin sample were obtained by a fixed bed pyrolysis

system shown in Figure S1 (in the supporting information). The lignin sample was pyrolyzed

from a room temperature to 800 °C at a heating rate of 10 °C min-1. After the reactions, the

pyrolysis products were separated into three phases: char, bio-oil, and non-condensable gas.

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The vapor products that were released during the pyrolysis reactions were divided into

condensable liquid or non-condensable gas after passing through the condenser at -4 °C. The

methanol-soluble fraction of the liquid products generated by the lignin pyrolysis was

analyzed using a gas chromatography-mass spectrometry (GC-MS) instrument (Varian CP-

3800 GC and Varian 320 MS models, Varian Inc., USA). The GC was equipped with a fused

silica capillary column coated with (5% phenyl)-methylpolysiloxane (DB-5ms, 60 m x 0.25

mm x 0.25 µm, Agilent J&W, USA). The GC oven temperatures were programmed as follow:

from 100 °C (hold for 3.5 min) to 160 °C at a heating rate of 15 °C min-1 (hold for 20 min);

from 160 °C to 200 °C at a heating rate of 15 °C min-1 (hold for 15 min); from 200 °C to

280 °C at a heating rate of 5 °C min-1 (hold for 5 min). The carrier gas, helium (99.999%),

flowed at a constant rate of 0.8 ml min-1. The spectra of each of the lignin pyrolysis products

were identified by retention time and NIST mass spectral libraries using AMDIS32 computer

software.

3. Results and Discussion

3.1. Recovery of residual lignin from a bioethanol process

Figure 1 shows a schematic diagram of the process used to recover lignin residues

from the lignocellulosic bioethanol production. As cellulose, hemicellulose, and lignin, the

main components of lignocellulosic biomass, were differently solubilized in acid and alkali

solutions, sequential acid-alkali pretreatment of the bioethanol process was used to separate

the lignin from the biomass feedstocks. To quantify solubilized phases of each main

component under different acid and alkali pretreatment conditions, the pure chemicals of

cellulose, hemicellulose, and lignin were carried out. After the acid pretreatment, the

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hemicellulose was mostly solubilized in acid solution (i.e., only 0.9% of hemicellulose

remained acid-insoluble); however, 63.9% of the lignin and 97.2% of the cellulose remained

acid-insoluble. The loss of the lignin resulted from presence of the acid-soluble lignin [25].

After the individual alkali pretreatment, both hemicellulose and lignin were significantly

solubilized (the insoluble of 1.0% and 4.2%, respectively) and 67.3% of the cellulose was left

alkali-insoluble. Thus, it points out that the prior acid pretreatment of a bioethanol process

could efficiently remove hemicellulose from the raw biomass because hemicellulose was

entirely soluble in both each acid and alkali solution. However, in the next alkali pretreatment,

a certain amount of alkali-soluble cellulose might be included in lignin residues that were

obtained from the chemical pretreatments, through the re-precipitation process with acid

solution.

Figure 2 and Table 2 show the changes of the Klason lignin content in the solid and

liquid phases of the corn stover and the rice straw after a sequential acid-alkali pretreatment.

The raw materials of corn stover and rice straw consist of 31.2% and 40.5 % cellulose, 23.9%

and 32.7% hemicellulose, as well as 20.1% and 25.6% lignin, respectively. After the acid

pretreatment, the relative lignin fractions increased 32.2 wt.% and 34.9 wt.% in the solid

phases of the corn stover and the rice straw, respectively. Thus, 80.6 wt.% and 81.6 wt.% of

the lignin in the corn stover and the rice straw, respectively, remained in the solid phase after

the acid pretreatment. Of the acid-pretreated lignin in the solid phase, about 92-93% was

solubilized in following the alkali solution and then recovered by acid precipitation. The acid-

soluble and alkali-insoluble lignin content in the corn stover and rice straw accounted for c.a.

25 wt.% of inherent lignin in both materials. Consequently, c.a. 14.9 wt.% and 19.5 wt.% of

the raw corn stover and rice straw biomass, respectively, could be retrieved as the lignin

residue resource from bioethanol producing processes.

The amount of the insoluble fractions of both the corn stover and the rice straw

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decreased from 36.2% and 41.1%, respectively, in the alkali pretreatment to 23.5% and

24.7%, respectively, in the acid-alkali pretreatment (Figure 2), because the weakened

lignocellulosic structure of both the corn stover and the rice straw, due to acid pretreatment,

might lead the non-lignin components to be much more solubilized under successive alkali

conditions. The stepwise removal or recovery processes of the pretreatment resulted in a

relative increase in the amount of lignin content (c.a. 55.8% and 55.4% for the corn stover

and the rice straw, respectively) in the alkali liquid phase.

3.2. Chemical structures of different lignin components

Figure 3 shows the FTIR spectra of the four lignin samples, the Kraft lignin, the

ReLignin, the corn stover lignin, and the rice straw lignin, in the region of 2000-600 cm-1.

The band assignments and the wavenumbers are listed in Table 3. Typical patterns of lignin

with wavenumbers at 1595, 1456, and 1420 cm-1 were observed in all four of the lignin

samples; on the other hand, differences in the IR intensities of lignin samples were also

observed at several wavenumbers. In the Kraft lignin, the corn stover lignin, and the rice

straw lignin, the key differences were found to be: 1325 cm-1, C-O stretching of syringyl

units; 1263 cm-1, C-O stretching of guaiacyl unit; 1209 cm-1, C-C, C-O, and C=O stretching

of guaiacyl unit; 1157 cm-1, C=O stretching in ester group of lignin; 1116 cm-1, aromatic C-H

deformation of syringyl unit; and 1026 cm-1, C-O stretching of primary alcohol [21, 26-28].

The Kraft lignin had strong intensities at critical peaks of the guaiacyl unit (1263 cm-

1 and 1209 cm-1), whereas the corn stover lignin and the rice straw lignin demonstrated

relatively keen peaks at 1325 cm-1, 1157 cm-1 and 1116 cm-1 for the C-O stretching of the

syringyl unit, and the ester group of the hydroxylphenyl propane (H) units and the aryl C-H

of the syringyl unit, respectively [29]. These results indicate that these bioethanol residual

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lignin resources were composed of significantly more syringyl (dimethoxyphenolic) units

than the Kraft lignin. The syringol/guaiacol (S/G) ratio of the corn stover lignin and the rice

straw lignin were almost two times higher than the S/G ratio of the Kraft lignin. As shown in

Table 4, the corn stover lignin and the rice straw lignin have higher the S/G ratios than the

Kraft lignin and the ReLignin, due to larger number of syringyl units in the corn stover lignin

and the rice straw lignin. This difference in compositional units could affect the thermal

degradation mechanisms and the product distribution of lignin pyrolysis. Syringyl and

guaiacyl units have different delignification rates under alkali processes, and they also have

disparate thermal decomposition reactivity when they are used to determine various aromatic

derivatives of pyrolysis products [20, 26].

3.3. Thermal degradation of lignin components

Thermal degradation of lignin consist of three stages: (I) the loss of moisture bound

by surface tension at low temperature ranges (near 100 °C), (II) primary pyrolysis at middle

temperature ranges (200-400 °C), and (III) secondary reaction at high temperatures over

700 °C. Bio-oil from lignin pyrolysis involving thermally-degraded aromatic compounds is

mostly produced in the second step, and pyrolysis gases (e.g. H2, CH4, and CO) begin to be

released in the third step at higher temperatures. The TG, DTG, and differential scanning

calorimetry (DSC) curves (Figure 4) of the lignin samples, including the Kraft lignin, the

ReLignin, the corn stover lignin, and the rice straw lignin, show that the corn stover lignin

and the rice straw lignin were more easily degraded in the lower thermal degradation

temperature ranges and had less thermal residue than both the Kraft lignin and the ReLignin.

The Kraft lignin was degraded slowly under all the temperatures, ranging from room

temperature to 900 °C, with a very low mass loss rate (max. 1.50 wt.% °C -1 at 365 °C). This

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lignin started to be pyrolyzed at 178 °C, and the primary pyrolysis continued at a wide range

as the temperature increased toward 520 °C. The primary pyrolysis of the corn stover lignin,

the rice straw lignin, and the ReLignin also occurred at a temperature range that was similar

to the temperature range of the Kraft lignin, but they demonstrated greater mass loss rates.

The corn stover lignin had the highest mass loss rate (3.14 wt.% °C -1 at 350 °C) from the

primary pyrolysis. The rice straw lignin and the ReLignin were largely pyrolyzed at 360 °C

with 2.33 wt.% °C -1 and 387 °C with 1.83 wt.% °C -1, respectively. In the primary pyrolysis,

nearly 50% of the organic matter in the corn stover lignin and the rice straw lignin (48 wt.%

and 52 wt.%, respectively) were decomposed to volatiles; however, the Kraft lignin and the

ReLignin were decomposed by only c.a. 42 wt.%. In previous studies, pyrolysis of the other

wood-based milled lignin was also found to have higher mass loss rates at higher

temperatures (380-396 °C) than the corn stover lignin and the rice straw lignin [21], thus

utilizing bioethanol residual crop lignin could reduce the energy input required for the

pyrolysis process to reach the primary decomposition reaction for value-added chemicals

production.

The thermal degradation of lignin that occurred in a full temperature range at low

mass loss rates was the result of the inherent aromatic chemical structures of lignin and the

rigidity with various branches and the main chain molecules of lignin [2, 30]. The ether

linkages (e.g. β-O-4, O-CH3) among pure H, G, and S unit compounds, accounting for over

50% of lignin linkages, are known to be cleaved at 400-500 °C [20, 29]. Nevertheless, the

cleavage reactions between the corn stover-/rice straw lignin and the Kraft Lignin/ReLignin

were different. The cleavage reactions in the corn stover lignin and the rice straw lignin

occurred at lower temperature ranges than the Kraft lignin and the ReLignin, but the former

types of lignin also had larger mass loss rates than the later types. This shows that the corn

stover lignin and the rice straw lignin had weaker thermal stability than the Kraft lignin. The

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higher S/G ratios of the corn stover lignin and the rice straw lignin (see Table 4) also caused

reactive thermolysis because of the additional chances from one more –OCH3 group of a

syringyl unit [20]. On the other hand, the ReLignin had a much higher temperature range for

primary pyrolysis than even the Kraft lignin because acid pretreatment might release more

syringyl units from lignin depolymerization and, thus, could induce more condensed phenolic

units with stronger thermal stability [31].

Unlike primary pyrolysis, the secondary reaction of all of the lignin samples, except

the Kraft lignin, occurred at the temperature range from 725 to 850 °C, which is higher than

the temperature range for the Kraft lignin (650-850 °C). At the higher temperature, the

secondary reaction of the lignin evolved gas components (e.g., CO, CH4, and H2) through the

thermal cracking of the tar residues. These gas components were originated from C=O, –

OCH3, aryl C=C, and aryl C-H portions of the lignin unit structure [2]. At the secondary

reaction, the rice straw lignin degraded faster (0.36 wt.% °C -1 at 806 °C) than the other lignin

samples at higher temperatures. The residues of the corn stover lignin and the rice straw

lignin were less solid (34 and 36 wt.%, respectively) than the residues of the Kraft lignin and

the ReLignin (about 45 wt.%), after reaching 900 °C. The DSC curves (Figure 4c) show that

the primary pyrolysis of lignin was slightly exothermic, but the secondary reaction was

endothermic. The exothermic reaction resulted in high residual portions of the lignin samples

after the primary pyrolysis [2]. The differences in thermal degradation temperatures/rates, and

the amount of solid residues left from the pyrolysis of the lignin samples represent the

variations on chemical structure, flexibility of the polymeric chains, degree of crosslinking,

and the amount of impurities in origin raw biomass [26]. In addition, the monomer

composition of the lignin determines the different thermal energies required to break down

the polymer structure [15]. Thus, under the same pyrolysis conditions, the lignin resources

derived from the bioethanol residual corn stover lignin and the rice straw lignin would require

15

a lower temperature and demand a less energy to produce value-added aromatic compounds

than wood-based lignin sources.

3.4. Phenolic aromatic compounds from pyrolysis of different lignin components

Total ion current (TIC) chromatograms, and a list of compounds of lignin pyrolysis

oils achieved and identified by GC/MS and NIST mass spectral libraries are shown in Figure

5 and the supporting information. The predominant compounds with relatively larger peak

areas in each of the pyrolysis oils, for all four of the lignin samples, are demonstrated in Table

5 and Figure 6. Most of the compounds with larger peak areas of all of the lignin pyrolysis

oils seem to have similar chemical formulas and structures which would be derived from the

phenylpropane (a carbon feature of C6C3) building blocks, such as guaiacyl (G), syringyl (S),

and hydroxyphenolic (H) units of lignin [3]; however, the major compounds and the relative

area (%) of each of the lignin samples varied. These differences are due to the different

content ratio of the phenylpropane building blocks, and the various mechanisms of structural

rearrangement and cleavage [20]. In general, woody biomass lignin (i.e., Kraft lignin)

principally consist of G and S type units, whereas non-woody biomass lignin sources (i.e.,

crop lignin and herbaceous biomass lignin), such as corn stover lignin and rice straw lignin,

have more H type units as well as G and S type units at levels that are comparable to the G

and S levels in the woody biomass lignin. These various combinations of G, S, and H type

units in lignin sources would result in noteworthy effects on the aromatic compounds of

pyrolysis products [3, 29, 32]. Furthermore, the pyrolytic cleavage mechanisms of the β-O-4

linkage, accounts for over 50% of all the lignin linkages, would rely on the side-chain

structure [33].

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The Kraft lignin oil primarily contained C7 and C8 aromatic compounds (c.a. 50%)

and, among them, 2-methoxyphenol (guaiacol) and 2-methoxy-4-methylphenol (creosol)

accounted for 25.1% and 14.4%, respectively. The main formation of 2-methoxyphenol and

2-methoxy-4-methylphenol would primarily occur through the O-CH3 bond homolysis of

syringyl- and guaiacyl-type lignin, respectively. Pyrolysis of syringyl and guaiacyl lignin

compounds could be put through a series of reactions, such as the cleavage of building block

linkages, radical-induced rearrangement, demethoxylation, and radical coupling reactions, in

step-wise and heating-up processes [32]. Even though the Kraft lignin sample had a higher

number of G type units (i.e., a lower S/G ratio), the pyrolysis oils of the Kraft lignin

contained fewer derivatives of the guaiacyl units, but a significant number of guaiacyl

monomers (i.e., 2-methoxyphenol). The amount of 2-methoxyphenol that could be produced

from either block linkage cleavages of the guaiacyl units or aromatic demethoxylation of the

syringyl units after the cleavage was greater than the amount of 2-methoxy-4-methylphenol

that could be formed through the coupling of O-CH3 homolysis products, carbon-centered

radicals and methyl radicals [20]. In the fixed-bed pyrolysis system, most volatile aromatic

compounds that was produced at the primary reaction leave the heating area with the N2 gas

flow, thus, under the following temperature-dependent reactions, variations in the succeeding

aromatic products (e.g., catechols, pyrogallols, cresols, xylenols, phenols, and polycyclic

aromatic hydrocarbon [PAHs]).

For the most part, the ReLignin was thermally converted into 2-methoxy-4-

methylphenol (22.3%) and 2-methoxylphenol (19.1%). Despite the fact that the Kraft lignin

and the ReLignin had the same origin, the bioethanol pretreatment process reversed the main

products (i.e., 2-methoxyphenol and creosol) of in the ReLignin pyrolysis in comparison to

those of the Kraft lignin. The S/G ratio of the ReLignin gently increased (Table 4) because

the acid pretreatment might led to degradation of the β-O-4 linkages and released more

17

syringyl units than guaiacyl units. Yasuda et al. [34] reported that the acid treatment induced

a shift toward higher molecular weight distribution of the Kraft lignin because carbon-carbon

linkages formed during the acid treatment were favorably substituted by hydroxyl groups.

Molecular weight distribution of lignin features the reactivity and physicochemical properties

of lignin. The total number of phenolic hydroxyl groups in the lignin complexes would also

be increased by the acid pretreatment [31]. However, autohydrolysis (hot water only

pretreatment) of lignin with increasing temperatures resulted in a decrease in molecular

weight [35]. Thus, the pyrolysis of the ReLignin could result in an easier pathway by which

to cleave the bond linkages; it could also result in increased coupling of the phenoxy radicals

and the methyl radicals to generate 2-methoxy-4-methylphenol. The greater number of

methylation reactions in the pyrolysis of the ReLignin with the acid pretreatment caused the

C8 aromatic compounds to be in the majority. Compared to the Kraft Lignin, increasing

degree of polymerization of the pyrolysis products might result from higher molecular weight

distribution of the ReLignin produced by the acid pretreatment. This result also points out

that pyrolysis of bioethanol residual lignin sources that have undergone acid pretreatments,

including the corn stover lignin and the rice straw lignin, might be preferably converted into

more radical coupling products than natural lignin sources.

On the other hand, the corn stover lignin was mainly pyrolyzed to 2-methoxylphenol

(11.4%), phenol (10.0%), and 2,6-dimethoxyphenol (syringol, 9.9%). Each compound is the

monomeric form of the G, H, and S units, respectively. The corn stover lignin oil had higher

C6 compounds (12.4% of total areas), mostly composed of phenol, than the other lignin

pyrolysis oils (~7.6%). Phenol could be produced by demethylation of the G units after

homolysis and radical-induced rearrangement, as well as the block linkage cleavage of the H

units [20, 29]. However, as mentioned above, the fixed-bed pyrolysis system had an

insufficient chance of carrying out the step-wise reactions and the formation of phenol would

18

mostly result from the linkage cleavage reaction of the considerable amount of H units in the

corn stover lignin source. The lowest maximum degradation temperature of the corn stover

lignin also supports the finding that the cleavage of the ether linkages of the H units was

preferentially carried out in the primary reaction because the bond dissociation energy of the

aromatic O-CH3/OH bond is higher than those of the O-CH3 and OH linkages [29, 36].

Unlike the Kraft lignin, this competitive initial cleavage reaction of the linkages against

aromatic bonds in the primary stage might lead the pyrolysis of the corn stover lignin to form

three monomers (e.g., phenol, 2-methoxylphenol, and 2,6-dimethoxyphenol) of each building

block at similar major levels, instead of generating step-wise derivatives of the G and S units.

The presence of methyl- and ethyl-phenyl compounds in the corn stover lignin could also be

caused by increased chances for the formation and coupling of methyl- radicals and phenyl

radicals derived from a small number of H units and the competitive initial linkage cleavage.

The rice straw lignin was thermally converted to 3,4-dimethoxyphenol (14.0%), 2-

methoxyphenol (11.6%), and 4-ehtyl-2-methoxyphenol (10.0%), and C8 aromatic compounds

were predominant in the rice straw lignin oil. Similar levels of 3,4-dimethoxyphenol and 2-

methoxyphenol mean that the G and S type units present in the rice straw lignin at

comparable levels. Despite the fact that the rice straw lignin has a similar crop biomass to

corn stover, the rice straw lignin oil had a lower proportion of phenol, but more derivatives of

methoxyphenyl compounds since the rice straw lignin possessed fewer H type units than the

corn stover lignin [37]. Unlike the Kraft lignin and the corn stover lignin, the pyrolysis of the

rice straw lignin produced monomers of both the G and S units as well as ethylated products.

Moreover, 4-ehtyl-2-methoxyphenol could be formed through the coupling of the 2-methoxy-

4-methylphenol radicals and the methyl radicals, which were derivatives of O-CH3 homolysis.

According to the origin of lignin, phenolic compounds of lignin pyrolysis changed in

the relative amount, but not in the compound species. Phenolic compounds are the vital

19

volatiles in the primary pyrolysis stage (200-400 °C) of most lignin feedstocks; however, in

the higher temperature stage, phenolic compounds generated in the small amount and release

of CO, CO2, CH4, and methanol increased [38]. Selective production of phenolic compounds

for specific compound species in high purity is a momentous technology to develop a lignin-

based biorefinery platform. Hence, the types of lignin feedstocks and pyrolysis conditions

would be key factors to obtain the required phenolic products in high purity. This study

shows the feasibility of corn stover lignin as the pyrolysis feedstock for phenol-dominant bio-

oil production. These phenolic compounds in lignin pyrolysis oils could be more refined by a

multi-step separation using alkali, acid and organic solvents to remove a water-soluble phase

including polar carbohydrates, low-molecular-weight acids, ketones, and aldehydes [39].

4. Conclusion

The typical acid-alkali pretreatments of bioethanol processes could be a suitable by-

pathway to recover lignin resources from lignocellulosic biomass materials, such as corn

stover and rice straw. About 74.1% and 76.2% of the inherent lignin in the corn stover and the

rice straw, respectively, was retrieved through a bioethanol producing process (acid-alkali

pretreatment). Compared to non-lignin components, the relative content of the recoverable

lignin from the corn stover and the rice straw also increased from 20-25% in the biomass

material to c.a. 55% in the following alkali solution of the sequential acid-alkali pretreatment.

The corn stover lignin and the rice straw lignin were pyrolyzed into similar phenol

derivatives (e.g., methoxy- and methylmethoxy- phenols), but in various distribution ratios,

due to their different chemical structural characteristics. The corn stover lignin of bioethanol

residual lignin sources, which are significantly different from wood-based lignin, have high

potential to produce value-added aromatics, especially C6 compounds, with less energy

20

demand. The corn stover lignin was composed of more S units as well as a considerable

amount of H units in comparison to the wood-based Kraft lignin and the ReLignin, which

were G unit-predominant. In comparison to the other lignin samples, the structural feature of

the corn stover lignin led to further reactive thermolysis with the lowest maximum thermal

degradation temperature (350 °C) at the highest mass loss rate (3.14 wt.% °C-1), much more

phenol production, and the smallest solid residue (34%) after reaching 900 °C. Despite

having a similar crop biomass to the corn stover lignin, the rice straw lignin has higher

thermal stability, which caused lower mass loss behavior than was found in the corn stover

lignin. The rice straw lignin had a different pyrolytic pathway, favorable for the production of

C8 aromatic derivatives and methylation products, most likely due to the fact that it has fewer

H units than the corn stover lignin. The pyrolytic utilization of crop biomass-based bioethanol

residual lignin potentially could be a sustainable source of phenolic aromatic production

resulting in their ability to be petrochemical-replaceable. Thus the lignin resources should no

longer be thrown out. Besides, further research for selective aromatics production and

mixture purification in lignin pyrolysis processes could improve the efficiency of the entire

biorefinery process.

21

Acknowledgements

This work was supported by the Advanced Biomass R&D Center (ABC) of Korea

Grant funded by the Ministry of Education, Science and Technology (ABC-2013059453), by

BK21+ program through the National Research Foundation of Korea funded by the Ministry

of Education, Science and Technology, and by the project titled 'Technology Development of

Marine Industrial Biomaterials', funded by the Ministry of Oceans and Fisheries, Korea. The

authors thank Dr. Namchul Jung, and Hyo Jin Lee for technical support.

22

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Table 1. Elemental analysis of the raw biomass and lignin samples.

Samples Carbon Hydrogen Nitrogen Sulfur Oxygen* H/C O/C HHV

†

% in mass molar ratio MJ kg-1

Kraft Lignin 59.9 4.5 0.1 7.4 28.1 0.90 0.35 22.37

Kraft ReLignin 64.7 4.8 0.2 2.5 27.9 0.89 0.32 23.79

Corn stover, raw biomass 44.6 5.3 1.4 0.8 47.9 1.42 0.81 14.16

Corn stover Lignin 58.6 5.4 2.1 4.7 29.3 1.10 0.38 22.76

Rice straw, raw biomass 41.6 5.0 0.6 0.7 52.0 1.45 0.94 11.96

Rice straw Lignin 56.5 4.8 1.2 4.4 33.1 1.02 0.44 20.47

* Calculated by the difference in relative contents.

† Estimated using the Dulong’s formula: HHV (MJ kg-1) = 33.8C + 144.3(H - O/8) + 9.42S

26

Table 2. Change of the insoluble fraction, and the Klason lignin in the solid and liquid phases after chemical pretreatments of the bioethanol process.

Biomass Phase Raw biomass (wt.%) 1st: acid pretreatment (wt.%) 2nd: alkali pretreatment (wt.%)

Biomass Lignin Biomass* Lignin1† Lignin2‡ Biomass* Lignin1† Lignin2‡

Corn stover Solid 100 20.1 50.2 16.2 80.6 23.5 1.29 6.42

Liquid¶ 0 0 49.8 3.9 19.4 26.7 14.9 74.1

Rice straw Solid 100 25.6 59.8 20.9 81.6 24.7 1.38 5.39

Liquid¶ 0 0 40.2 4.73 18.4 35.1 19.5 76.2

* Solid and liquid phases based on raw biomass † Lignin based on raw biomass ‡ Lignin based on initial lignin ¶ Calculated by the difference

27

Table 3. FTIR analysis of the lignin samples.*

No. Wavenumbers

(cm-1

) Band assignment

1 1595 C=O stretching conjugated to the aromatic rings

2 1508 Aromatic ring vibrations

3 1456 C-H bending of methyl and methylene groups

4 1420 C-H deformation in lignin

5 1361 Aliphatic C-H stretching in CH3 and O-H in-plane bending

6 1325 C-O stretching of syringyl units

7 1263 C-O stretching of guaiacyl unit

8 1209 C-C, C-O, and C=O stretching of guaiacyl unit

9 1157 C=O stretching in ester group of lignin

10 1116 Aromatic C-H deformation of syringyl units

11 1074 C-O stretch of secondary alcohols and aliphatic ethers

12 1026 C-O stretching of primary alcohol

13 852 C-H bending

14 830-811 C-H bending of syringyl units

* Referred from Wang et al. [21], Xiao et al. [23], Sammons et al. [26], Kline et al. [27]

28

Table 4. The syringyl and guaiacyl ratios of the lignin samples.

Peak height* S unit G unit S/G ratio

Kraft Lignin 0.345 0.853 0.40

Kraft ReLignin 0.346 0.820 0.42

Corn stover Lignin 0.394 0.509 0.77

Rice straw Lignin 0.241 0.333 0.72

* Calculated using normalized FTIR spectra with peak intensities at 1325 and 1263 cm-1 for syringyl and guaiacyl structure units, respectively.

29

Table 5. Top five compounds with the largest peak areas of GC/MS chromatograms for the Kraft lignin (a), the Kraft ReLignin (b), the corn stover lignin (c), and the rice straw lignin (d).

No. Retention

time (min)

Name Formula Area (%)

(a) Kraft Lignin

4 9.20 Phenol, 2-methoxy- C7H8O2 25.1 10 10.88 Creosol C8H10O2 14.4 14 12.61 Phenol, 4-ethyl-2-methoxy- C9H12O2 5.9 9 10.71 Catechol C6H6O2 5.4 60 52.87 Methyl dehydroabietate C21H30O2 5.0 (b) Kraft ReLignin 8 10.8669 Creosol C8H10O2 22.3 3 9.1376 Phenol, 2-methoxy- C7H8O2 19.1 11 12.5719 Phenol, 4-ethyl-2-methoxy- C9H12O2 9.8 7 10.6883 Catechol C6H6O2 7.1 10 11.9519 1,2-Benzenediol, 3-methyl- C7H8O2 3.1 (c) Corn stover Lignin

5 9.09 Phenol, 2-methoxy- C7H8O2 11.4 1 7.39 Phenol C6H6O 10.0 19 14.28 Phenol, 2,6-dimethoxy- C8H10O3 9.9 16 12.50 Phenol, 4-ethyl-2-methoxy- C9H12O2 7.9 7 10.19 Phenol, 4-ethyl- C8H10O 7.1 (d) Rice straw Lignin

13 14.26 Phenol, 3,4-dimethoxy- C8H10O3 14.0 4 9.07 Phenol, 2-methoxy- C7H8O2 11.6 11 12.48 Phenol, 4-ethyl-2-methoxy- C9H12O2 10.0 25 52.89 9-Octadecenamide, (Z)- C18H35NO 8.1 17 32.35 Ethanone,

1-(4-hydroxy-3,5-dimethoxyphenyl)- C10H12O4 7.1

30

List of Figures

Figure 1. Schematic diagram of a lignin recovery process in the bioethanol production of lignocellulosic biomass.

Figure 2. Changes in the Klason lignin content in the solid and liquid phases after chemical pretreatments of bioethanol processes for the corn stover (a) and the rice straw (b). Change of the Klason lignin contents in solid and liquid phases after chemical pretreatments of a bioethanol process. The liquid phase of “After Acid-Alkali pretreatment” excludes the removed fraction after prior acid pretreatment.

Figure 3. FTIR spectra of the lignin samples in the region of 2000-600 cm-1.

Figure 4. The TG (a), DTG (b), and DSC (c) curves of the Kraft lignin, the Kraft ReLignin, the corn stover lignin, and the rice straw lignin. The numbers in brackets refer to the final mass fraction of the lignin samples.

Figure 5. The GC/MS chromatograms of the pyrolysis oils derived from the Kraft lignin (a), the Kraft ReLignin (b), the corn stover lignin (c), and the rice straw lignin (d). The inserted tables show the relative area (%) of the compounds sorted by carbon numbers. The chemical composition lists with the labeling numbers for each of the lignin samples are available as supporting information.

Figure 6. A stacked column chart for the largest peak areas of the GC/MS chromatograms for the Kraft lignin (a), the Kraft ReLignin (b), the corn stover lignin (c), and the rice straw lignin (d).

31

Figure 1. Schematic diagram of a lignin recovery process in the bioethanol production of lignocellulosic biomass.

32

Figure 2. Changes in the Klason lignin content in the solid and liquid phases after chemical pretreatments of bioethanol processes for the corn stover (a) and the rice straw (b). The liquid phase of “After Acid-Alkali Pretreatment” excludes the removed liquid fraction after the prior acid pretreatment.

(a)

(b)

33

Figure 3. FTIR spectra of the lignin samples in the region of 2000-600 cm-1.

34

Figure 4. The TG (a), DTG (b), and DSC (c) curves of the Kraft lignin, the Kraft ReLignin, the corn stover lignin, and the rice straw lignin. The numbers in brackets refer to the final mass fraction of the lignin samples.

(a)

(c)

(b)

35

Figure 5. The GC/MS chromatograms of the pyrolysis oils derived from the Kraft lignin (a), the Kraft ReLignin (b), the corn stover lignin (c), and the rice straw lignin (d). The inserted tables show the relative area (%) of the compounds sorted by carbon numbers. The chemical composition lists with the labeling numbers for each of the lignin samples are available as supporting information.

(a) (b)

(c)

(d)

36

Figure 6. A stacked column chart for the largest peak areas of the GC/MS chromatograms for the Kraft lignin (a), the Kraft ReLignin (b), the corn stover lignin (c), and the rice straw lignin

(d).

(a) (b)

37

(c)

(d)

38

Highlights

- Corn stover lignin and rice straw lignin were obtained from bioethanol producing processes.

- Their chemical structures and thermolysis features were characterized.

- Phenolic aromatic compounds from pyrolysis of the lignin were analyzed.

- These residual lignin could be utilized as a new source of aromatic production.