Structural and electrochemical characteristics of activated carbon

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Structural and electrochemical characteristics of activated carbon derived from lignin-rich residue Joah Han, So-Yeon Jeong, Jae Hoon Lee, Joon Weon Choi, Jae-Won Lee, and Kwang Chul Roh ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05351 • Publication Date (Web): 25 Dec 2018 Downloaded from http://pubs.acs.org on January 4, 2019

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ACS Sustainable Chemistry & Engineering

Structural and electrochemical characteristics of activated carbon derived from lignin-rich residue

Joah Han,‡,† So-Yeon Jeong,‡,§ Jae Hoon Lee,∆ Joon Weon Choi,¶ Jae-Won Lee,*,§ and Kwang Chul Roh*,† †.

Division of Energy & Environmental Materials, Korea Institute of Ceramic Engineering &

Technology, 101, Soho-ro, Jinju-si, Gyeongsangnam-do 52851, Republic of Korea §.

Department of Forest Products and Technology, Chonnam National University, 77,

Yongbong-ro, Gwangju 61186, Republic of Korea ∆.

Department of Forest Sciences, College of Agriculture and Life Science, Seoul National

University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-921, Republic of Korea ¶.

Graduate School of International Agricultural Technology and Institute of Green-Bio

Science and Technology, Seoul National University, Pyeongchang, Gangwon-do 232-916, Republic of Korea

‡Co-first

author: So-yeon Jeong,

*Corresponding

author. Tel: +82-55-792-2625. E-mail: [email protected] (Kwang Chul Roh),

Tel:+82-62-530-2098. E-mail: [email protected] (Jae-Won Lee)

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Abstract Lignin-rich residue was obtained by sequential acid pretreatment (sulfuric, oxalic, and maleic acid; H-ST, H-OT, and H-MT) and enzymatic hydrolysis (EH). Pretreatment using dicarboxylic acid (oxalic and maleic acid) showed a relatively low solid yield (72.55 and 69.27%) compared with sulfuric acid pretreatment (74.83%). In addition, the enzymatic hydrolysis yield of pretreated biomass differed significantly depending on the acid catalyst used. To investigate structural properties of lignin-rich residue, milled wood lignin (MWL) was extracted. H-MT-EH-MWL and H-OT-EH-MWL were found to have higher Mw and polydispersity values than H-ST-EH-MWL, but the syringyl-to-guaiacyl (S/G) ratio of H-STEH-MWL was the highest. The lignin-rich residue was used to prepare activated carbons (ACs) to make commercially viable energy storage materials. These activated carbons showed commercially viable specific surface areas (SSAs) (>2000 m2/g) and high rate capabilities (>90 % at 50 mA/cm2). H-ST-EH-AC had the highest BET SSA value (2182 m2/g), H-MT-EH-AC had a slightly lower value (2156 m2/g), but H-OT-EH-AC had the lowest value (2079 m2/g). The sp2/sp3 ratio of H-ST-EH-AC (3.8) is higher than the others (H-MT-EH-AC: 3.6 and H-OT-EH-AC: 3.1). Based on the lignin-rich residue structure, it is considered the high S-type lignin content of H-ST-EH can be attributed to the graphitic structure in H-ST-EH-AC.

Keywords: Biomass, Pretreatment, Lignin-rich residue, Lignin structure, Activated carbon, Supercapacitors


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Introduction Climate change regulations have been enforced globally to reduce greenhouse gas emissions and promote the use of renewable energy sources over fossil fuels 1. Biomass has attracted considerable attention as a promising renewable energy resource for use in a wide variety of industries including paper, heat, electricity, and transport because of its intrinsic carbonneutrality and recyclability. In addition, it is environmentally-friendly and found in natural abundance. Biomass consists of cellulose, hemicellulose, and lignin, and these components have been used to produce clean energy sources, such as biofuels and high value-added substances. Pretreatment is required in bioethanol production to enhance the enzymatic hydrolysis efficiency during subsequent hydrolysis, and cellulose has, in particular, been used for producing clean energy sources2. Following pretreatment, the biomass then undergoes enzymatic hydrolysis to produce fermentable sugar, and approximately 50 million tons of lignin-rich residues are produced annually for this process 3. Sulfuric acid is generally used as catalyst in pretreatment due to its low cost and high efficiency in hemicellulose hydrolysis; however, it also produces fermentation inhibitors, causes corrosion of the reactor, and makes it difficult to collect the catalyst. To overcome these problems, substitution with a dicarboxylic acid, such as oxalic and maleic acid, has been proposed 4. Lignin has a high carbon content >60 w.% and a particular aromatic structure that enables its use in making high value-added products, such as carbonaceous materials, surfactants, dispersants, dyes, and paints. The basic constituent of lignin is phenylpropane (C6-C3), which forms a complex three-dimensional structure by forming ether or carbon-carbon bonds. Lignin consists of three types of monolignols: p-coumaryl alcohol (H) without a methoxyl group, coniferyl alcohol (G) with one methoxyl group, and sinapyl alcohol (S) with two methoxyl groups 5. The linkage types of lignin are β-O-4 (40–60%), biphenyl (3.5–25%), β-5



(4–10%), and α-O-4 (3–5%) linkages. However, due to its diverse nature and complex structure, approximately 5% of total lignin is used for high value-added production despite a multitude of applications, and the remaining 95% is burned for power generation, as it is highly exothermic or it is discarded 3. Additional processing steps are therefore required to obtain purified lignin from lignin-rich biomass 6. There have been few reports about the production of high value-added products (such as carbonaceous materials) from lignin-rich biomass without using an extraction process or conducting an investigation into the correlation between the lignin structure and the electrochemical characteristics of lignin-derived carbon. Many studies have reported the use of lignin-based carbons in supercapacitors

7, 8.

Supercapacitors are most suitable for high-

power energy storage systems, including cold-starting assistants, electric vehicles, backup power systems, and regenerative braking systems, due to their desirable properties such as an excellent rate capability, high power density, long cycle life over 105 cycles, and low maintenance cost 9, 10. Qu et al. (2015) and Salinas-Torres et al. (2016) reported using ligninderived porous carbon with hierarchical structures (made from commercial lignin) in a supercapacitor7, 8. Nevertheless, these studies used commercial lignin, and the correlation between the lignin structure and carbon materials has not been reported. The use of ligninrich residues as carbon precursors has positive environment and the economic effect and is thus beneficial. In addition, value-added products with higher char yield than coconut shells can be generated without additional processing, by producing energy storage system from non-purified linin-rich residues discarded from industry11. In this study, lignin-rich residue was obtained after sequential pretreatment and enzyme hydrolysis. To investigate the effect of acid catalysts on the change in lignin structure during pretreatment, milled wood lignin was extracted and its physical and chemical properties were investigated by derivatization followed by reductive cleavage (DFRC) and nuclear magnetic


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resonance (NMR). In addition, activated carbon was prepared from the lignin-rich residue for use in a supercapacitor, and the structural and electrochemical properties of the activated carbon were investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and cyclic voltammetry.

Methodology Pretreatment and enzymatic hydrolysis of biomass Mixed hardwood chips (Poong Lim Ltd., Co.) (Quercus mongolica, Robinia pseudoacacia L. and Castanea crenata) were used as raw materials and milled to a 20–80 mesh size. Pretreatment was conducted at 170 °C for 60 min. To provide the same acidic condition, 100 mM oxalic acid, 250 mM maleic acid, and 70 mM sulfuric acid were used 12. The reactor was loaded with 50 g (dry weight basis) of biomass and acid solution, for a total solid/liquid ratio of 1:4 (w/w).

Following pretreatment, the biomass was separated and washed prior to

enzymatic hydrolysis, where the 2 g of pretreated biomass (dry weight) was mixed with 20 mL of sodium citrate buffer (50 mM, pH 4.8) and enzymes (Cellic® CTec2; 17.5 FPU/biomass (g)) 2. In this study, the raw material is denoted as “H” (mixed hardwood), and the sulfuric, maleic, and oxalic acid pretreated biomass is referred to as “H-ST”, “H-MT”, and “H-OT”, respectively. Furthermore, “H-ST-EH”, “H-MT-EH”, and “H-OT-EH” denote biomass obtained from enzymatic hydrolysis.

Chemical and crystallinity analysis of biomass The chemical compositions of the raw material, pretreated biomass, and lignin-rich residues were analysed using the NREL method (Laboratory Analytical Procedure-Determination of



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Structural Carbohydrates and Lignin in Biomass), and the crystallinity of the biomass was analysed using an X’Pert PRO Multipurpose X-ray Diffractometer (PANalytical, Netherlands) with 2θ ranging from 5 to 50° at 40 kV and 30 mA 13, 14.

Isolation of milled wood lignin (MWL) and its characterization Milled wood lignins (MWLs) obtained from lignin-rich residues flowing enzymatic hydrolysis were isolated to analyze the effect of the different acid catalysts on the structural changes of lignin15. In order to prepare MWL, biomass was extracted with ethanol/benzene solution (2:1, v/v) for 6 h at 80°C15. The extractive free biomass was milled in a ball mill for 30 min at 500 rpm. After this process, milled wood was mixed with dioxane/water (9.5:0.5, v/v) solution for 48 h. The solution was then separated by centrifugation and evaporated. The residue was dissolved with acetic acid/water solution (9:1, v/v). The solution was then precipitated in distilled water and the precipitated residue separated via centrifugation. Subsequently, the precipitated residue was freeze dried and then dissolved with 1,2dichloroethane/ethanol solution (2:1, v/v). Then, insoluble material was removed via centrifugation and the supernatant was mixed with diethyl ether. Finally, the precipitated material was centrifuged and freeze dried for MWL analysis. In this respect, H-ST-EH-MWL, H-MT-EH-MWL, and H-OT-EH-MWL refer to milled wood lignin obtained from lignin-rich residue. An elemental analysis was performed using an US/CHNS-932 instrument (LECO Corp., USA), and the contents of methoxyl and phenolic hydroxyl groups were investigated using the Baker (1996) and Mansson (1983) methods, respectively

16, 17.

The frequencies of

β-O-4 linkages in MWLs were determined by derivatization followed by use of the reductive cleavage (DFRC) method 18. In addition, the molecular weight distributions of MWLs were determined using gel permeation chromatography (GPC, ViscotekRImax, Viscotek, UK)


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equipped with a PLgel 3 μm MIXED-D column (300 × 7.5 mm, VARIAN), a PLgel 3 μm MIXED-E column (300 × 7.5 mm, VARIAN), a PLgel 5 μm guard column (50 × 7.5 mm, VARIAN), and a UV-Vis detector (VE3210, Viscotek) 19. A quantitative 13C NMR analysis was conducted by dissolving 100 mg of each MWL sample in 0.65 mL dimethyl sulfoxide (DMSO-d6), and NMR spectra were obtained using a ProPulse 500 MHz spectrometer (Agilent, USA) equipped with an OneNMR probe. 13C NMR analysis was performed with a 30° pulse angle at 125.7 MHz, a 2.0 s relaxation delay, a 1.4 s acquisition time, and 30000 scans, and chromium (III) acetylacetonate (20 μL, 0.01 M) was added as a relaxation agent to reduce the relaxation time. For 2D-HSQC NMR analysis, 100 mg of each MWL sample was dissolved in 0.65 mL of dimethyl sulfoxide (DMSO-d6). The collected complex points were 1K for the 1H dimension with d1 (2 s), 32 scans were conducted, and 128 time increments were used in analysis

20.

To conduct pyrolysis-GC/MS analysis, 1 mL of internal standard

(1.3 mg fluoranthene/1 mL methanol) and 1 mg of each MWL sample were introduced in a quartz tube, and the sample was then pyrolyzed with a Pt coil pyroprobe (Pyroprobe 2000, CDS Analytical Inc., Oxford, PA, USA). The pyroprobe temperature was increased at a rate of 10 °C/ms and maintained under an inert atmosphere (>99.9% He) for 20 s, and the released volatile products were analysed with a GC (Agilent Technologies 7890A) equipped with an FID and DB-5 capillary column (60 m × 0.25 mm ID × 0.25 mm film thickness). The split ratio was 1:200 for sample injection, the temperature of the pyrolyzer interface was set to 250°C, and the GC injection and detection temperatures were set to 320°C and 300°C, respectively. The oven temperature was programmed as follows: maintained at 50°C for 1 min, increased to 130°C at a rate of 3°C /min, increased to 180°C at a rate of 1.5°C /min, increased to 280°C at a rate of 6°C /min for 5 min, and then increased to 320°C for 10 min 21.



Preparation of activated carbon Lignin-rich residues obtained from enzymatic hydrolysis were used as precursor. A biomass mount of 10 g was placed into an alumina crucible and carbonized at 800°C at a heating rate of 3°C/min for 1 h under an inert atmosphere with a N2 flow rate of 300 mL/min. Following this procedure, the carbon derived from the lignin-rich residue was mixed with KOH pellets (C/KOH weight ratio of 1:4 (w/w%)), and the mixed powder was placed in a nickel furnace and heated at 900°C for 1 h under an inert atmosphere (N2 gas flow, heating rate: 2°C/min and flow rate: 300 mL/min). The resulting powder was then cooled to an ambient temperature before washing with a 0.1 M HCl solution. The sample was then washed with distilled water until a pH 7. The activated carbon was finally obtained after filtration and drying overnight at 80°C. H-ST-EH-AC, H-MT-EH-AC, and H-OT-EH-AC denote the various activated carbon obtained from the lignin-rich residues.

Physical characterization of activated carbon The surface morphologies of samples were analysed by field-emission scanning electron microscopy (FE-SEM; JEOL, JSM-6700F, Japan) and the structure was confirmed by highresolution transmission electron microscopy (HR-TEM; JEOL, JEM-2000EX, Japan). Thermogravimetric analysis (TGA; Netzsch, Germany) was then used to measure the constant mass loss of samples after carbonization. Pore texture was detected using a gas analyzer (Belsorp-Mini II, BEL, Japan); the Brunauer–Emmett–Teller (BET) method and non-localized density functional theory (NLDFT) were used to calculate the specific surface area and micro/meso pore distributions from N2 adsorption–desorption isotherms; functional groups on surface were investigated via X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe, ULVAC-PHI Inc., Osaka, Japan); and X-ray diffraction (XRD) was also


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conducted at 40 kV and 200 mA with Cu Kα radiation (λ = 0.154 nm) (Rigaku D/Max 2500/PC, Japan).

Electrochemical characterization of activated carbon Rubber-type electrodes were used to conduct an electrochemical analysis of samples. These rubber-type electrodes were fabricated using the activated carbon, polytetrafluoroethylene (PTFE; D60, Daikin Industries, Japan), and Super P (Imerys Graphite & Carbon, Belgium) as the active material, binder, and conductive agent, respectively. The mixed powder contained a weight ratio of active materials: binder: conductive agent of 90:5:5 (w/w/w %). The rubbertype electrode was used as the working electrode (one electrode mass with binder and conductive agent and electrode density as follows: H-ST-EH-AC: 0.0104 g and 0.52 g/cc, HMT-EH-AC: 0.0106 g and 0.51 g/cc, and H-OT-EH-AC: 0.0103 g and 0.52 g/cc, commercial activated carbon (referred to as ‘YP50F’, which is obtained from coconut shell via steam activation, Kuraray Chemical. Co., Japan): 0.0108 g and 0.54 g/cc) and tetraethylammonium tetra-fluoroborate in acetonitrile (1 M TEABF4 in ACN) was used as the electrolyte in CR2032 coin cells. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using a VSP potentiostat (Biologic, France). CV tests were conducted from 0 to 2.7 V at various scan rates (1–100 mV/s) and impedance was measured at 0 V over the frequency range from 1 mHz to 100 kHz with an amplitude of 5 mV. Additionally, galvanostatic charge–discharge (GCD) measurements were conducted using a GCD tester (Hi-EDLC-16CH, Human Instrument Co., Korea), and the rate capability was confirmed at various current densities ranging from 1 to 50 mA/cm2. The gravimetric specific capacitance (Csp), specific energy, and power were respectively calculated via the following equations, C_sp = 2•I∆t/m∆V, E = (C_sp∙〖∆V〗^2)/2, P = E/∆t



where C_sp represents the gravimetric specific capacitance in F/g; m is the mass of one electrode (g) (the practical electrode mass of H-ST-EH-AC, H-MT-EH-AC, and H-OT-EHAC is 10 mg); I, ∆t, and ∆V represent the discharge current (A), time (s), and potential window (V), respectively; E is specific energy (Wh/kg); and P is specific power (W/kg) (based on a two electrode system).

Results and discussion Effects of dilute acid pretreatment and enzymatic hydrolysis on biomass The chemical compositions and solid yields of the biomass are shown in Table 1. The solid yield was found to differ depending on the acid catalyst used, even though the same pH was used during the pretreatment. The lowest solid yield of 69.27% occurred with the maleic acid pretreatment. The solid yield of pretreated biomass with dicarboxylic acid catalysts (such as oxalic acid and maleic acid) were relatively lower than that pretreated with sulfuric acid. This result can be understood in terms of the different pKa values of each acid catalyst and the selective degradation of hemicellulose by dicarboxylic acids 22. In general, Maleic and oxalic acid have higher pKa values than sulfuric acid. They also provide two pKa values, which may result in more efficient hydrolysis of the biomass over a range of temperatures and pH values23, 24. Following pretreatment, the biomass contained only 1.22-2.55% xylan, which suggests that it was mostly removed during pretreatment (regardless of catalyst type). The glucan and lignin contents of pretreated biomass ranged from 50.69-59.93% and 34.7337.05%, respectively; these values are higher than those of the raw material and are related to the degradation of hemicellulose. The results are consistent with those of other reports 12. The change in crystallinity of the pretreated biomass is shown in Fig. S1 (a): the degree of crystallinity increased to 56.02–58.90% compared with that of the raw material (33.20%), in


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relation to the degradation of hemicellulose and amorphous regions of cellulose during pretreatment. The highest crystallinity value of 58.90% occurred in H-MT, which also shows that it had the lowest solid yield during pretreatment, H-ST had a crystallinity value of 58.43%, but the highest solid yield, and H-OH had a crystallinity value of 56.02%. These results imply that there was greater degradation of the crystalline region in H-OT than in HMT. Oxalic acid is a considerably stronger organic acid than maleic acid (pKa= 1.87 versus 6.07) and has ionization constants of 6.5×10-2 and 6.1×10-5, which correspond to pKa values of 1.25 and 4.14, respectively

22, 23.

Therefore, in addition to the amorphous regions, it is

considered that some of the crystalline regions of cellulose may be degraded during pretreatment which would thus lead to the low crystallinity values in H-OT 25. In general, hemicellulose and lignin contents are important factors that affect the enzymatic hydrolysis yield 26. As the xylan content in the pretreated biomass used in this study was low (1.37-2.55%), lignin was regarded as the main factor that affected the yield of enzymatic hydrolysis. The high lignin content in pretreated biomass hinders enzymatic hydrolysis due to adsorption of the enzyme into lignin 27. Therefore, the lignin content is an important factor to improve the efficiency of enzymatic hydrolysis. The cellulose to glucose conversion rate during enzymatic hydrolysis after 120 h was 67.94% with oxalic acid, 55.80% with maleic acid, and 38.59% with sulfuric acid (Fig. S1 (b)). Although it was expected that the highest enzymatic hydrolysis yield would be found in the sulfuric acid pretreated biomass, due to the low concentration of lignin in the pretreated biomass (Table 1), the highest yield was actually obtained for the oxalic acid pretreated biomass. This result relates to structural changes occurring in the biomass during pretreatment. Lim et al. (2013) reported that such changes are closely related to enzymatic hydrolysis rate

12.

The results therefore imply that the

enzymatic hydrolysis yield could be significantly correlated with the lignin structure and crystallinity rather than only with the lignin content. In this study, the lignin content was



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found to have no direct effect on enzymatic hydrolysis. The chemical compositions of lignin-rich residues obtained from enzymatic hydrolysis are shown in Table 1, where the glucan and lignin contents are seen to range from 21.19-39.24% and 45.59-62.86%, respectively. The lignin-rich residue obtained from H-OT-EH contained high amounts of lignin and low amounts of glucan; however, a high amount of glucan remained in H-ST-EH due to the low enzymatic hydrolysis yield.

Table 1. Solid yield during pretreatment and the chemical compositions of the raw material, pretreated biomass, and lignin-rich residue obtained from enzymatic hydrolysis of the pretreated biomass Biomassa

Glucan (%)

Xylan (%)

Lignin (%)

Solid yield (%)

Raw material (H)

42.65 (0.39)b

18.93 (0.43)

26.25 (0.45)

-

H-ST

50.69 (0.43)

2.55 (0.17)

34.73 (0.82)

74.83 (0.48)

H-MT

59.93 (0.48)

1.22 (0.08)

35.37 (0.36)

69.27 (0.87)

H-OT

55.65 (0.97)

1.37 (0.08)

37.05 (0.76)

72.55 (0.23)

H-ST-EH

39.24 (0.69)

NDc

45.59 (0.52)

66.24 (0.22)

H-MT-EH

24.87 (0.77)

NDc

60.19 (0.85)

44.46 (0.61)

21.19 (0.81)

NDc

62.86 (0.97)

46.74 (0.37)

H-OT-EH a H:

mixed hardwood, ST: sulfuric acid pretreated biomass, MT: maleic acid pretreated biomass, OT: oxalic acid

pretreated biomass, EH: enzymatic hydrolysis. b The parentheses contain the standard deviation values, with the analysis repeated in triplicate. c ND indicates not detectable.

Chemical structure properties of milled wood lignin isolated from lignin-rich biomass Milled wood lignins (MWLs) were isolated from the lignin-rich residue after enzymatic hydrolysis and their chemical and physical properties were investigated; the elemental compositions, functional groups, and molecular weights of the MWLs are presented in Table 2. There are no significant differences between the elemental compositions of MWLs using the different acid catalysts: the carbon, hydrogen, and oxygen contents range from 61.562.0%, 5.6-5.9%, and 32.3-32.6%, respectively, lignin generally consists of 64.4% carbon,


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5.9% hydrogen, and 30% oxygen 28. The results are therefore similar to that of original lignin. Although the methoxyl group (OMe) content is 14.6-15.0%, which is slightly decreased compared to poplar milled wood lignin (PMWL; 15.4%), there are no significant differences between the acid catalysts used 20. In addition, the phenolic hydroxyl group (Phe-OH) content is decreased slightly to 4.2-5.0% compared to that of the raw material (5.1%); this result relates to condensation of the degradation products, which indicates that the acid catalysts selectively cleaved ether bonds in the lignin, such as β-O-4 linkages, during pretreatment 29. GPC analysis was conducted to investigate the average molecular weights (Mw and Mn) and polydispersity indexes (PDIs) of the MWLs. Results show degreases of 18.49%, 26.15%, and 16.48% in the Mw of H-OT-EH-MWL, H-MT-EH-MWL, and H-ST-EH-MWL, respectively, compared to PMWL (10,634 Da). These results are related to lignin fragmentation in the biomass from β-O-4 cleavage during pretreatment and imply a close relationship between the solid yield of the biomass and the acid catalyst employed 20. The polydispersity index (PDIs) of H-OT-EH-MWL, H-MT-EH-MWL, and H-ST-EH-MWL, are 2.6, 2.7, and 2.5, respectively, which are lower than that of PMWL (3.40) and indicate difference in the Mw distribution between the acid catalysts employed. Furthermore, the molecular mass distribution in H-ST-EH-MWL is more uniform than that in the other MWLs.

Table 2. Elemental composition, functional group and molecular weight of milled wood lignin obtained from lignin-rich residue Elemental composition (wt%)

Lignin C

H-STEH61.5 MWL H-MTEH62.0 MWL

Functional groups (wt%)

Klason lignin

Mw (Da)

Mn (Da)

Polydispersity index (PDI) (Mw/Mn)

H

N

S

O

OMeb

Phe-OHc

5.9

0.0

0.3

32.6

14.9 (0.2)a

4.2 (0.2)

95.2 (0.5)

6,474

2,567

2.5

5.6

0.0

0.2

32.4

15.0 (0.2)

4.7 (0.5)

96.0 (0.6)

8,170

3,029

2.7



H-OTEHMWL

61.8

5.8

0.0

0.3

14.6 (0.6)

32.3

5.0 (0.4)

96.9 (0.2)

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6,774

2,558

The parentheses contain the standard deviation with the analysis repeated three times. Phenolic hydroxyl group. a

b

2.6

Methoxyl group.

c

The β-O-4 linkages frequencies in MWLs are shown in Table 3. β-O-4 linkages were selectively degraded and acetylated coniferyl alcohol and sinapyl alcohol were produced as C6-C3 type monomers following the reaction30. The total DFRC products amount obtained was 236.9-354.3 μmol/g, which is slightly lower than that of PMWL (976.6 μmol/g) 20. The DFRC product was lowest for H-MT-EH-MWL, which implies that cleavage of β-O-4 linkages in H-MT-EH-MWL occurred easily compared with MWLs using other acid catalysts. The syringyl to guaiacyl ratio (S/G) is an important factor used to understanding the structural properties of lignin

31,

and the bond type and OMe content in MWL can be

identified indirectly by the S/G ratio 32. G-type lignin is more difficult to degrade than S-type lignin; therefore, high S/G values indicate easy delignification, which is closely related to the degree of coupling in lignin 30. The S/G ratio of the MWLs decreased to 0.6-0.8 compared to that of PMWL (1.4), which indicates that the β-O-4 structures in S-type lignin cleaved much more rapidly than those in the G-type lignin during pretreatment 20. The S/G ratio was highest for H-ST-EH-MWL despite the low DFRC product yield, which implies that the sample consisted of a relatively large amount of S-type lignin with β-O-4 structures compared to the other MWLs. These results show that H-ST-EH-MWL was more easily degraded than the other MWLs.

Table 3. Quantitative analysis of derivatization followed by reductive cleavage (DFRC) products of milled wood lignin obtained from lignin-rich residue Lignin

Amount (μmol/g sample)


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Total

S/G a

(Acetylated coniferyl

(Acetylated sinapyl

alcohol)

alcohol)

PMWLb

414.7 (25.7)

561.8 (28.3)

976.6 (52.0)

1.4

H-ST-EH-MWL

195.0 (5.2)

159.3 (16.9)

354.3 (11.7)

0.8

H-MT-EH-MWL

162.8 (11.1)

114.1 (14.4)

276.9 (3.2)

0.7

H-OT-EH-MWL

186.6 (8.8)

120.9 (1.1)

307.4 (9.9)

0.6

a

The average molar ratios of sinapyl alcohol/coniferyl alcohol formed by the cleavage of β-O-4 linkages.

b

Poplar milled wood lignin 20.

13C

NMR analysis of the MWLs was performed to investigate lignin structural changes

occurring with respect to the different acid catalysts used during pretreatment (Fig. S2). β-O4, β-5, and β-β linkages of 82.5-88.0 ppm are observed in the MWLs, and the values of HMT-EH-MWL and H-OT-EH-MWL differed significantly compared with H-ST-EH-MWL 20. Structural changes in the lignin occur with an increased biomass solid yield during pretreatment. The phenyl propane units (C6-C3) of lignin include benzene rings and propyl side chains, and the aromatic region range from 155.0 to 102.0 ppm

33,34.

In this study, a

quantitative comparison was made between the integrated MWL values for regions of protonated aromatics (124–102 ppm), condensed aromatics (140–124 ppm), and oxygenated aromatics (155–140 ppm) (Table 4) 20. Results show a high amount of condensed aromatics in H-MT-EH-MWL but a low amount in H-ST-EH-MWL, which reflects differences between the solid yield when using these pretreatments. A high content of condensed lignin in the biomass hinders enzymatic hydrolysis, due to the increased enzyme adsorption onto the condensed lignin 35. The pretreated biomass with oxalic acid contained relatively low amounts of condensed lignin, and these results are consistent with the Mw value of H-OT-EH-MWL. Consequently,



Page 16 of 34

the enzymatic hydrolysis yield was high compared with that of sulfuric acid, even though the biomass contained a high amount of lignin. The oxygenated aromatic (aromatic C-O) content in H-MT-EH-MWL was low. As H- and G-type lignin consists of low C-O bond levels compared with S-type lignin, results show that the S-type lignin was more easily degraded in H-MT-EH-MWL than in other MWLs.

Table 4. Assignment and quantification of the signals from the 13C NMR spectra of milled wood lignin obtained from lignin-rich residue Region δ (ppm)

Assignment

155–140

Area (arbitrary unit) H-ST-EH-MWL

H-MT-EH-MWL

H-OT-EH-MWL

Aromatic C–O

1.95*

1.89

1.91

140–124

Aromatic C–C

1.73

1.82

1.75

124–102

Aromatic C–H

2.37

2.30

2.34

61.3–58

β-O-4

0.37

0.44

0.54

58–54

OCH3

1.65

1.73

1.92

54–53

β–β, β-5

0.10

0.12

0.20

* Values

are based on a C9 lignin unit.

2D-HSQC NMR analysis was conducted to investigate the aromatic region and sub-units within the MWLs, and the results are shown in Fig. 1. The inter-unit linkages of β-aryl-ether (β-O-4, A), resinol (β-β, B), and phenylcoumaran (β-5, C) can be observed by their crosspeaks. β-5 linkages are identified in H-MT-EH-MWL and H-OT-EH-MWL but not observed in H-ST-EH-MWL, and β-β linkages are detected in H-MT-EH-MWL. These results indicate that lignin condensation was more rapidly with maleic and oxalic acid pretreatments than with sulfuric acid. Furthermore, the degree of condensation is high in H-MT-EH-MWL, which is consistent with 13C NMR data. The cross-peaks in the aromatic region clearly show separate signals for S- and G-type units, whereas H-type units are not observed 17. The S-type units show strong signals for C2,6/H2,6 correlation at δC/δH103.9/6.69 (S2,6), and the G-type


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units show strong correlations at δC/δH 110.9/6.99 (G2), 115.4/6.71 (G5), and 119.6/6.77 (G6). In addition, the small amount of oxidized phenolic S’ units (Cα=O) and S” units (CαOOH) showed correlations at δC/δH 106.4/7.24 (S′2,6) and δC/δH 106.4 /7.19 (S”2,6), respectively, in all MWLs.

Fig. 1. 2D-HSQC spectra of H-ST-EH-MWL (a), (b); H-MT-EH-MWL (c), (d); and H-OT-EH-MWL (e), (f) (left: side chain region, right: aromatic region).



A pyrolysis-GC/MS analysis was conducted to investigate the structural properties of MWLs, and a list of pyrolysis products is presented in Table S1. A total of 37 different compounds were identified; these are classified here as syringyl type (S), guaiacyl type (G), phydroxyphenyl type (H), and modified lignin type (ML). Some carbohydrate-derived compounds (C), such as acetic acid, levoglucosan, and hexadecanoic acid, were detected in all of the acid catalyst samples, which imply that the MWLs contained carbohydrates. Creosol (G), syringol (S), and 4-methylsyringol (S) were identified as the main pyrolysis products generated from lignin. 4-propylsyringol (S) and dihydrosyringenine (S) were detected only in H-MT-EH-MWL; and vanillin and conifer aldehyde was detected in H-MTEH-MWL and H-OT-EH-MWL. These results demonstrate that the pyrolysis products differed depending on the acid catalyst used and thus reflect differences in the biomass degradation mechanism of the acid catalysts employed.

Electrochemical properties of activated carbon Activated carbon was prepared from a lignin-rich residue. Fig. S3 (a-f) shows high resolution FE-SEM images of the biomass both before and after carbonization and KOH activation in an inert atmosphere. The lignin-rich residues of all catalysts are found to have an aggregated morphology (Fig. S3 (a-c)). The biomass surfaces change to a porous morphology after carbonization and KOH activation, as shown in Fig. S3 (d-f). Pores with diameters of less than 50 nm are evident on the surfaces and the porosities of the lignin-rich residues increase in accordance with the following process. First, inorganic compounds in the lignin-rich residue were partially removed at temperatures below 400 °C during the carbonization process. This effect was confirmed by thermogravimetric analysis in nitrogen at a heating rate of 3 °C/min until 800 °C, and there were mass losses of 51%, 44%, and 45% in H-ST-EH, HMT-EH, and H-OT-EH at temperatures below 400°C and above 350°C. Subsequently,


Page 18 of 34

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metallic K penetrated the biomass surface and generated a number of pores during the KOH activation process, which can be described using the following stoichiometry reaction that is thermodynamically possible at equilibrium conditions below 730 °C 36, 37: 6KOH + C ↔ 2K + 3H2 + 2K2CO3 The reaction between KOH and C begins at approximately 400 °C, which means that the formation of potassium carbonate (K2CO3) begins at approximately that temperature. K2CO3 decomposes into K2O and CO2 from this reaction ‘K2CO3 → K2O + CO2’ and CO2 and K2CO3 generate carbon monoxide (CO) and metallic K above 800°C 38. The metallic K gases participated in developing pores on the sample surfaces from consumption of carbon. Finally, the activated carbons obtained from the lignin-rich residues were found to have bimodal pore textures comprising mostly micropores with a few mesopores that were formed from the decomposition of oxygen functional groups of lignin 39. The micropores and mesopores made maximum and minimum contributions, respectively, to the surface area 7. As seen in Fig. 2 (a-f), a partially graphitic structure is evident in the HR-TEM images of lignin-rich residues and activated carbon (regardless of the lignin content) after carbonization at 800°C and subsequent KOH activation. In previous work, we reported that among biomass components, lignin affects the graphitic structure of lignin-derived activated carbon10. Although all MWLs had similar carbon contents (61.5–62.0 wt.%), they had different cellulose and lignin contents and lignin structures. Thus, we could imply that lignin structure also more significantly affects graphitic structure than lignin contents because the graphitic structure of the H-ST-EH-AC derived from H-ST-EH-MWL, containing high S-type lignin contents, showed the clearest than others in HR-TEM, although H-OT-EH had the highest lignin contents.



Fig. 2. HR-TEM images of H-ST-EH-C (a), H-MT-EH-C (b), and H-OT-EH-C (c) after carbonization at 800°C; H-ST-EH-AC (d), H-MT-EH-AC (e) and H-OT-EH-AC (f) after carbonization at 800°C and activation at 900°C, respectively.

XPS analysis was conducted to investigate the surface components of the activated carbon (Fig. 3 and Table 5). As seen, a carbon content above 94% is found on the surfaces of all activated carbon, with graphitic structures (sp2 C=C) at 284.4 eV, sp3-hybridised carbon (C-C) at 285.0–285.1 eV, phenolic (C-O) at 286.1–286.2 eV, carbonyl (C=O) at 287.4–287.5 eV, carboxyl or ester groups (O-C=O) at 288.3–288.6 eV and shake-up satellite (π-π* transition) at 289.6–289.7 eV 10. On the basis of the sp2/sp3 ratios shown in Table 4, values above 3.0 were found in all activated carbon, which is similar to the value of amorphous carbon, however, the sp2/sp3 ratio of H-ST-EH-AC (3.8) was higher than that of the others (H-MTEH-AC: 3.6 and H-OT-EH-AC: 3.1) 40. In addition, the shake-up satellites (π-π* transition), which are indicative of a conjugated system, were also the highest (at 4.2) in H-ST-EH-AC, which implies that the activated carbon has a high graphitic sp2 bonding character that could be attributed to the high S-type lignin content of H-ST-EH-MWL 41.


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Fig. 3. Deconvolution of XPS C 1s spectra of H-ST-EH-AC (a), H-MT-EH-AC (b), and H-OT-EH-AC (c) with sp3 hybridized carbon (C-C), graphitic structure (sp2 C=C), phenolic (C-O), carbonyl (C=O), carboxyl groups (O-C=O) and Shake-up satellite (π-π* transition).

Table 5. Surface components on activated carbon derived from lignin-rich residue C (%)*

Graphitic

(C-C)

(sp2)

sp3

C–O

Samples

H-ST-EHAC H-MT-EHAC H-OT-EHAC ‫٭‬

B.E.

C

B.E.

C

(eV)

(%)

(eV)

(%)

4.8

284.4

62.2

285

16.2

95.6

4.4

284.4

63.1

285.1

94.9

5.1

284.4

58.7

285.1

C 1s

O 1s

95.2

C=O

O–C=O

π-π*

sp2/sp3 ratio

B.E.

C

B.E.

C

B.E.

C

B.E.

C

(eV)

(%)

(eV)

(%)

(eV)

(%)

(eV)

(%)

3.8

286.1

7.9

287.4

6.4

288.3

3.1

289.6

4.2

17.3

3.6

286.2

7.9

287.4

5.3

288.4

2.8

289.6

3.6

18.8

3.1

286.2

10.3

287.5

4.5

288.6

4.8

289.7

2.9

C (%) represents the content of functions in the samples, * Binding Energy

As seen in Fig. 4 (a), the (002) plane peak of H-ST-EH-AC is closer to 26° than that of the other activated carbon, which may be attributed to its partially graphitic structure, as evidenced by XPS and HR-TEM data. This result is also supported by first- and second-order Raman spectra (Fig. 4 (b)). Fig. 4 (b) shows the G band (at 1580 cm−1) associated with the zone-centre phonon of E2g symmetry and sp2-hybridized carbon, and the D band (at 1350 cm−1) corresponding to the K-point phonons of A1g symmetry, which indicate 6-fold aromatic rings by disorder, and 2D band (at 2700 cm-1) of the samples 42. The ID/IG ratio calculated from G and D band intensities shows at degree of disorder in crystalline structures; the value



was found to be relatively low in H-ST-EH-AC (1.04) and H-MT-EH-AC (1.05) compared to H-ST-EH-AC (1.07) 43. In addition, Fig. 4(b) presents magnified Raman spectra and values of Raman spectroscopic parameter; the G peaks (first-order spectra), which indicate a twodimensional ordered structure, are red-shifted in H-ST-EH-AC and are smaller than those of the other activated carbon. These changes in E2g symmetry show the development of organized structures

44.

Overall, these results indicate that H-ST-EH-AC possesses a more

partially graphitic structure than the other activated carbons, which can be attributed to the highest S/G ratio and lowest PDI value of lignin in H-ST-EH, as previously mentioned.

Fig. 4. Wide-angle XRD patterns and Raman spectra of H-ST-EH-AC, H-MT-EH-AC and H-OT-EH-AC.

The BET specific surface area (SSA) and pore properties of the activated carbon are shown in Table 6. H-ST-EH-AC has the highest BET SSA value (2182 m2/g), H-MT-EH-AC has a similar value (2156 m2/g), and H-OT-EH-AC has the lowest value (2079 m2/g). The SSA of activated carbon is inversely related to the lignin content; the lignin-rich residue in H-OT-EH had high amount of lignin and thus low SSA (Table 1 and 6). It is unlikely that the aromatic


Page 22 of 34

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structures contributed to the formation of surface pores due to their chemical inertness, whereas it is considered that glucan within the lignin-rich residue contributed to micropore formation during carbonization and activation

45.

In addition, it could be implied that the

uniform small lignin particles of H-ST-EH, as indicated by PDI and S/G ratio analyses (Table 2 and 3), increased the surface porosity and made the structure partially graphitic after carbonization and KOH activation (H-ST-EH-AC).

Table 6. Yield, BET specific surface area (SSA), and pore texture of activated carbon after carbonization and KOH activation Carbonization Sample

Temperature (°C)

Yield (%)

Activation Temperature (°C)

Yield

SBET

Vtotal

(%)

(m /g)

2

(cm /g)

3

H-ST-EH-AC

800

32

900

60

2182

0.93

H-MT-EH-AC

800

37

900

71

2156

0.92

H-OT-EH-AC

800

37

900

73

2079

0.89

Commercial AC

-

1729

0.80

-

Vmicro 3

Vmeso 3

(cm /g)

(cm /g)

0.86

0.07

(92.5%)

(7.5%)

0.85

0.07

(92.4%)

(7.6%)

0.82

0.07

(92.1%)

(7.9%)

0.64

0.16

(80.0%)

(20.0%)

*SBET: BET specific surface area; Vtotal: total pore volume; Vmicro: micropore volume; Vmeso: mesopore volume.

All samples produced carbonization yield above 30% and activation yield above 60 %, as shown in Table 6. Fig. 5 shows the N2 adsorption–desorption isotherms and pore size distribution of activated carbon. N2 adsorption–desorption isotherms of activated carbon are Type I (as designated by IUPAC), which is similar to zeolite and commercial activated carbon. However, these isotherms indicate that the three activated carbons have a microporous structure, while the sorption isotherm of commercial activated carbon is type I with some contribution of type IV, which indicates a micropore with mesopore texture 45.



Page 24 of 34

H-ST-EH-AC and H-MT-EH-AC have larger total pore volumes than in H-OT-EH-AC, with respective values of 0.93, 0.92, and 0.89 cm3/g. Higher porosity provides a larger number of accessible spaces that enable penetration of electrolyte ions on the surface of supercapacitors. NLDFT shows that all the activated carbons are predominantly composed of micropores

Structural and electrochemical characteristics of activated carbon

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