Structural and Morphological Changes in Kraft Lignin during

Research Article pubs.acs.org/journal/ascecg

Structural and Morphological Changes in Kraft Lignin during Hydrothermal Carbonization Hanne Wikberg,*,† Taina Ohra-aho,† Filoklis Pileidis,‡ and Maria-Magdalena Titirici*,‡,§ †

VTT Technical Research Centre of Finland Ltd, P.O.Box 1000, Espoo, FI-02044 VTT, Finland School of Engineering and Materials Science, and §Materials Research Institute, Queen Mary University of London, Mile End Road, E14NS, London, United Kingdom

‡

ABSTRACT: In this paper we have investigated the changes in kraft lignin structure and morphology upon hydrothermal carbonization (HTC) at 240 °C with and without the presence of carbonization catalysts. The main conclusion of this study is that the structure of kraft lignin was altered only slightly during the hydrothermal carbonization. The main changes in the kraft lignin’s structure were demethylation and dealkylation as well as cleavage of β-O-4 bonds. The final product had a more thermally stable and complex cross-linked structure. More pronounced changes occurred in the material structure and morphology upon the addition of catalysts. The energy density of the resulting materials was not significantly different from the energy density of the original kraft lignin. KEYWORDS: Hydrothermal carbonization, Kraft lignin, Lignin-derived carbonaceous materials

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INTRODUCTION Hydrothermal carbonization (HTC) is a technology developed in the early twentieth century for converting biomass to coal like materials.1 The possibility of using a sustainable, fast, and simple process makes HTC an interesting alternative for biobased carbon materials generation at a larger scale. Several recent review articles on this topic exist.2−10 HTC is used to produce carbonaceous materials having specific structure and properties. In HTC, wet biomass is processed at moderate temperatures (ca. 180−250 °C) under self-generated pressures at approximately 20−30 bar. As a result of numerous chemical reactions including hydrolysis, dehydration, decarboxylation, polymerization, aromatization, and condensation, a carbonaceous solid fraction accompanied by a liquid phase are formed, while the gas phase is insignificant.5,11 In general, the solid carbonaceous material consists of spherical particles of different size having functional groups on the surface.6 These types of carbonaceous materials have been suggested to be used, for example, as adsorbents for water purification, in energy storage, as catalysts for fuel cells, in heterogeneous catalysts, as solid fuels and as soil fertilizers.2,6,8,12 The composition of the liquid phase depends on the raw material and process conditions. In general, in the case of carbohydrate containing precursors, the liquid phase has a high content of furan derivatives (e.g., 5hydroxymethlfurfural, furfural) and organic acids (e.g., levulinic acid, formic acid). It is commonly known in the literature that, under acidic hydrolytic conditions, the hydrothermal process degrades hexose sugars to hydroxymethyl furfural (HMF) while pentose sugars dehydrate into furfural.13−16 With increasing time, the HMF decomposes, on one hand, into levulinic and formic acid © 2015 American Chemical Society

and, on the other hand, into hydrothermal carbon materials. Furfural forms carbonaceous material having higher content of carbon aromatic structures.6 Cellulose may also carbonize without complete hydrolysis to monosaccharides through intramolecular condensation, dehydration and decarboxylation reactions as described by Falco et al.15 Most of the studies on hydrothermal carbonization have been performed using carbohydrates, cellulose or raw biomass. In the case of raw biomass it was observed that lignin has a strong influence on the final composition of the resulting materials as demonstrated by 13C-NMR studies.15 Unlike carbohydrates, lignin is more complex highly heterogeneous polymer composed of hydroxycinnamyl alcohols, including pcoumaryl, coniferyl, and sinapyl alcohol building blocks (Figure 1). Lignin units are interlinked with various ether and carbon− carbon bonds, in which the β-O-4 ether bond is the most common linkage in plant materials.17 The β-O-4 linkage accounts for approximately 48−60% of the total interunit linkages in native type lignin. The reaction mechanisms of lignin during thermal treatments are totally different from those of carbohydrates. Few studies on lignin during hydrothermal carbonization have been published. Dinjus et al. studied the influence of lignin during the hydrothermal carbonization of biomass and compared it with other precursors.18 They discovered that when using biomass with high lignin content, only soluble constituents were converted and the lignin initial structure highly remained. Received: June 26, 2015 Published: September 27, 2015 2737

DOI: 10.1021/acssuschemeng.5b00925 ACS Sustainable Chem. Eng. 2015, 3, 2737−2745

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ACS Sustainable Chemistry & Engineering

both strength requirements and cost objectives.21,22 Lignin has also been widely studied as a raw material for engineering plastics by blending it with natural or synthetic polymers or using lignin’s functional groups to form lignin-based polymers, such as polyurethane, phenol formaldehyde, epoxy resins, and polyesters, via polycondensation reactions.23 In this paper we have examined the possibility to produce lignin-derived carbonaceous materials using catalytic hydrothermal carbonization and looked at the structural and morphological changes occurring during the hydrothermal treatment. As a precursor, we have used softwood kraft lignin extracted from an industrial black liquor by acidification with CO2, purified by acidic washing, and dried before the thermal treatments. The purpose was to understand in depth the structural changes occurring in the lignin structure upon hydrothermal treatment in the presence of different carbonization catalysts (H2SO4, Fe 2+) in order to provide a potential way for lignin valorisation into useful carbon materials. To the best of our knowledge this is the first study looking at the HTC of lignin in the presence of various catalysts.

Figure 1. Monomeric lignin units: (a) p-coumaryl alcohol, (b) coniferyl alcohol, and (c) sinapyl alcohol.

Kang et al. performed hydrothermal carbonization of cellulose, kraft lignin, d-xylose, and wood meal and compared the properties of the resulting products.19 It was reported that lignin hydrothermal carbonization products were made of polyaromatic hydrochar having phenolic hydrochar on the surface. They suggested that this polyaromatic hydrochar was formed in solid−solid conversion of the nondissolved lignin and phenolic hydrochar was formed when the dissolved lignin fragments decomposed to phenolics through hydrolysis and further formed phenolic hydrochar through polymerization. Lu and Berge evaluated the carbonization of pure compounds, mixtures of the pure compounds, and complex feedstocks comprised of the pure compounds.20 Low sulfonate alkali lignin was used as the lignin source. Results indicated that feedstock properties influenced carbonization product properties. Results confirmed that yields associated with lignin were statistically different from those obtained when carbonizing other pure feedstock. Larger solid yield associated with the carbonization of lignin was not necessarily an indication of lignin carbonization/conversion. Indeed, lignin provides an attractive alternative as raw material for HTC since it has high carbon content (over 60%) and it is widely available. Large amounts of lignin are extracted by wood pulping annually. Besides the lignin derived from wood pulping processes, increasing amounts of available lignin in the future will be produced from the production of ethanol from biomass. Even though the utilization of lignin has been studied for decades, relatively few promising possibilities for its utilization have been identified so far and still approximately 98% of the available lignin is burnt to produce energy. Potential high-value products from lignin include for example low-cost carbon fibers, engineering plastics and thermoplastic elastomers, polymeric foams, and membranes, and a variety of fuels and chemicals all currently sourced from petroleum.21 The desire to provide low-cost lightweight materials for automobiles and to improve the economics of biorefineries has led to an increased interest in low-cost carbon fiber manufacture from lignin even though so far no demonstration has yet been made of suitable lignins being processed into carbon fiber that satisfy

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MATERIALS AND METHODS

Materials. The lignin used in the studies was softwood kraft lignin extracted from industrial black liquor by acidification with CO2, purified by acidic washing, and dried before the thermal treatments. The hydrothermal carbonizations were conducted as follows: 70 g of kraft lignin as 10 w/wt % aqueous suspensions was used in each of the three HTC treatments (see Table 1). Reaction mixtures were made in deionized water as such or with 0.15 M aqueous H2SO4 solution or with or 1.75 × 10−2 molar aqueous FeCl2 solution (2 wt % of FeCl2 with respect to lignin’s weight). Ultra-Turrax disperser was applied for 2 min at 6500 rpm to produce a homogeneous particle suspension. The pH of each suspension was measured. Hydrothermal Carbonization. Reactions were carried out in 1 L reactor (Amar Equipment Pvt, Ltd.) equipped with anchor stirrer bar, pressure gauge and transmitter, ceramic heating band, temperature control, water cooling coil and venting valve for gases. Reaction was recorded with Amar Data Acquisition System software at 30 s logging rate for reactor temperature, pressure, and torque. After purging the reactor with N2, the suspension was placed into the reactor and stirring at 70 rpm was started. The temperature of 240 °C was selected based on the pure kraft lignin’s TGA curve (Figure 9) as a temperature where the structure of kraft lignin starts to degrade effectively. Higher temperatures promote the reactions typical to hydrothermal liquefaction (∼270 °C) where the objective is to achieve an optimal level of energy recovery into biocrude, an oil-like product.10 In this work the main interest was to look at the structural changes in the solid hydrochar. The desired reaction temperature 240 °C was reached in 60 min. This temperature was maintained for 22 h and then the reactor was allowed to cool to 130 °C, after which additional water cooling was applied. Reactor was vented from gases at ambient temperature. Carbonaceous solids were collected and separated from process water in suction. Solids were disintegrated for 2 min with G&B food processor in 600 mL of deionized water and collected in suction. Disintegration and washing treatments were repeated. Solids were washed with absolute alcohol (5 × 100 mL) in suction, and finally dried in vacuum oven at 70 °C. Solids were powdered with a mortar and pestle to make them homogeneous before the analyses.

Table 1. Experimental Data of Hydrothermal Carbonization code

T (°C)

t (h)

catalyst

pH

yield (%)

carbon recovery (%)

DOC (mg/L)

pure KL KL-HTC KL-H2SO4-HTC KL-FeCl2-HTC

240 240 240

22 22 22

H2SO4 FeCl2

3.3 0.9 3.1

61 67 61

67 72 67

4200 5800 5000

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ACS Sustainable Chemistry & Engineering Table 2. Elemental Composition (with Ash Correction), Heating Value, and Tg of the Samples code

C (%)

H (%)

O (%)

S (%)

ash (%)

heating value (MJ kg−1)

Tg (°C)

pure KL KL-HTC KL-H2SO4-HTC KL-FeCl2-HTC

65.0 71.2 70.3 70.6

6.0 5.7 5.5 5.6

27.5 22.2 22.9 22.3

1.7 1.0 1.1 0.9

0.6 0.3 0.8 0.2

26.7 29.2 28.6 29.2

152 175 ND 191

surface and make it inert. An appropriate amount of sample (ca. 5−10 mg) was weighed into a weighed pan. The pans were closed by coldpressing. For the final results, the samples were heated stochastically temperature modulated measurement using heating at the rate of 2 °C/min from 25 to 250 °C. The glass transition temperatures were determined from the midpoint. In order to gain further structural insight, all of the samples were subjected to thermogravimetric analysis (TGA) in a TGA Q500 machine (TA Instruments). Approximately 3 g of each carbonaceous powder was used. The temperature was raised from 20 to 1000 °C, under nitrogen at a constant rate of 10 °C/min. The mass losses of the samples were calculated using the following equation:

Yield denotes in this paper to the amount of recovered solid after HTC and carbon recovery denotes to the recovered carbon in hydrochar based on the equations below: % solid yield =

amount of recovered solid after heat treatment (g) initial amount of solid raw material (g) × 100%

% carbon recovery % C in solid product × solid product weight as dry solid = × 100% % C in the original lignin × original lignin weight as dry solid

mass loss % = initial amount % − final amount %

Characterization. Elemental analysis for C, H, S, and O were determined using FLASH 2000 series analyzer. The sample is weighed in tin capsules, placed inside the Thermo Scientific MAS 200R autosampler at a preset time, and then dropped into an oxidation/ reduction reactor kept at a temperature of 900−1000 °C. The exact amount of oxygen required for optimum combustion of the sample is delivered into the combustion reactor at a precise time. The reaction of oxygen with the tin capsule at elevated temperature generates an exothermic reaction which raises the temperature to 1800 °C for a few seconds. At this high temperature, both organic and inorganic substances are converted into elemental gases which, after further reduction, are separated in a chromatographic column and finally detected by a highly sensitive thermal conductivity detector (TCD). The determination of oxygen is done via pyrolysis in the same analyzer. Dissolved organic carbon (DOC) content was measured using the standard SFS-EN 1484. Pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) measurements were performed with a filament pulse pyrolyzer (Pyrola2000, PyrolAB Sweden) connected to a GC/MS instrument (Varian 3800 GC/2000 MS, column J&W, DB-1701, 30 m × 0.25 mm, film 1 μm) with ion trap mass spectrometer (EI 70 eV) detection. A 100 μg sample was weighed accurately and pyrolyzed at 580 °C for 2 s. Peak areas of lignin pyrolysis degradation products were calculated and normalized to 100%. Each sample was analyzed twice. More detailed description of the method is found elsewhere.24 1 H and 13C solid state nuclear magnetic resonance spectroscopy (NMR) experiments have been acquired on a Bruker Avance 300 MHz (7 T) spectrometer using 4 mm zirconia rotors spinning at a MAS frequency of νMAS = 14 kHz. 1H and 13C chemical shifts were referenced relative to tetramethylsilane (TMS; δ = 0 ppm). Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Bruker Tensor-27 spectrometer with attenuated total reflectance unit (ATR) (range 4000 to 400 cm−1, 32 scans), geared with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector, and performed for analysis of surface functionalities. Scanning electron microscopy (SEM) was carried out on a FEI Quanta 3D Environmental SEM (ESEM) to observe the surface morphology of all samples. The samples were mounted on aluminum stubs and coated with gold prior to analysis. Particle size distribution of the samples was measured by laser diffraction (Beckman Coulter LS230, CA) equipped with PIDS detector for particles from 40 to 400 nm. Samples were diluted 1:10 and mixed overnight before each measurement. Differential scanning calorimetry (DSC) measurements were carried out with the Mettler Toledo differential scanning calorimeter model DSC2 system STARe SW 13.00, Mettler Toledo GmbH, Switzerland, under flowing nitrogen (flow rate 80 mL min−1) using 40 μL sealed an aluminum crucible which was pretreated at 500 °C to oxidize the

Heating values were measured using the standard SFS-EN 14918 for gross calorific value.

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RESULTS AND DISCUSSION Softwood kraft lignin was hydrothermally carbonized in the presence of different carbonization catalysts (H2SO4, Fe2+). The aim of using sulfuric acid catalyst during the hydrothermal treatment was to induce lignin degradation and generate components that would undergo polymerization to carbonaceous material.25 Metal ions have been shown to catalyze hydrothermal carbonization and hence iron (II) chloride was used as a Fe 2+ source to catalyze the reaction and to induce the formation of carbon nanomaterials.6,26 In this paper we use sample codes “pure KL” referring to original kraft lignin, “KLHTC” referring to hydrothermally carbonized kraft lignin without the catalyst, “KL-H2SO4-HTC” referring to hydrothermally carbonized kraft lignin with H2SO4 as a catalyst and “KL-FeCl2-HTC” referring to hydrothermally carbonized kraft lignin with FeCl2 as a catalyst. The HTC solid yields are presented in Table 1. The variations in the solid yields after HTC normally originate from factors like feedstock solubility to water, reaction mechanisms occurring in the formation of carbonaceous particles as well as component partitioning to the gas and liquid phase.15,20 It needs to be emphasized that solid yield for water-soluble raw materials is 0% in the beginning of the HTC process, while it is 100% for water insoluble raw materials like kraft lignin. For insoluble raw materials it means that the final recovered solid product is composed of both thermally converted material as well as the original raw material which has not gone through thermal conversion. For lignin, the high yields in HTC are associated with the more thermally stable phenolic structure, which is beneficial for carbonaceous product generation as a result of condensation reactions.19 According to Table 1, highest yield was obtained for KL-H2SO4-HTC (67%). Lignin is a thermally stable biopolymer and only minor fraction of lignin can be dissolved in water at the temperature we used.27 However, the use of H2SO4 as a catalyst decomposed lignin structure enabling dissolution of higher amount of lignin in the liquid phase. These dissolved liquid precursors then polymerized to carbonaceous solid by time increasing the yield. The yields were similar (61%) for KL-FeCl2-HTC and KL-HTC in the time frame we used. 2739


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Figure 2. Example of a carbonization diagram described by Ramke et al.29 and Behrendt et al.30

Figure 3. Distribution of aromatic degradation products in the samples.

KL-HTC (4200 mg/L) contributing 8.1%, 6.9%, and 5.8% of the total original carbon, respectively. The elemental composition is presented in Table 2. Hydrothermal carbonization leads to an increase in the carbon content and a simultaneous decrease in hydrogen and oxygen content due to formation of more condensed structure. The pure kraft lignin contained 65% of carbon and the final carbon content was approximately 70% for all the samples, regardless of the use of catalysts. The content of sulfur decreased during the HTC up to 50% due to the release of sulfur containing components as gaseous volatiles and liquid degradation products. One of the main restrictions for the use of lignin

The carbon recovery in hydrochars is consistent with the yields, being highest for KL-H2SO4-HTC (72%) compared with the other samples (67%) (Table 1). Carbon recovery can be considered high compared with many other thermal conversions technologies.28 The DOC was measured to evaluate the amount of the dissolved organic carbon in liquid phase, Table 1. The DOC values measured after 22 h HTC treatments show that the amount of dissolved organic carbon was slightly higher for catalyzed samples KL-H2SO4-HTC (5800 mg/L) and KLFeCl2-HTC (5000 mg/L) compared with uncatalysed sample 2740


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H2SO4 changed the product distribution more than FeCl2, which did not differ much from uncatalysed HTC sample. In thermal treatments the degradation of lignin is highly dependent on the bond energy of the different linkages present in the lignin structure (Figure 4).35 Due to the low thermal

(especially kraft lignin) in certain applications is the amount of impurities it contains. If the impurities can be effectively reduced or totally removed, it can open up new possibilities for kraft lignin’s utilization. The changes in the atomic C, H, and O composition and the product’s quality as a fuel can be studied using a Van Krevelen diagram in Figure 2. Carbonisation of biomass has been described by Ramke et al.29 and Behrendt et al.30 as a process where biomass, such as wood or peat, converts into carbon. The process includes gradual reduction of the O and H content, described by the O/C and H/C ratios of the resulting material. The conversion proceeds gradually from the upper right corner of the diagram, which is a typical biomass region to lignitic brown coal, brown coal, black coal and finally anthracite. The process is general in character, which means that it also describes the progression of the conversion taking place in the hydrothermal carbonization process. The intensity of the carbonization is shown by the length of the vector. Included in the graph is the progress of hydrothermal carbonization of KL-HTC, KL-H2SO4-HTC, and KL-FeCl2-HTC. It can be seen that the composition of the original kraft lignin converts from a lignitic brown coal-type material into a hydrochar whose composition resembles brown coal. Heating values in Table 2 describe the quality of the produced HTC samples as a fuel. Pure kraft lignin has a heating value of 26.7 MJ/kg which is increased approximately 10% after HTC. This increase is due to the higher carbon and lower oxygen content and hence slightly increased energy density of the produced materials. Lower moisture and ash content are advantageous when considering these materials as renewable energy sources but the increase in the heating values is very small denoting that hydrothermal process is not efficient way to produce solid fuel from kraft lignin. For comparison, the heating value for untreated wood is 18 MJ/kg.31 Py-GC/MS is widely used to characterize biopolymer structure, especially lignin. In pyrolysis, lignin is thermally degraded to a mixture of phenols by heating the sample in an inert atmosphere. The formed products retain the structural information on the original polymer.32 It can be seen from PyGC/MS results that the overall changes in phenolic pyrolysis degradation products were rather small between pure KL and the produced carbonaceous materials (Figure 3). The same degradation products were identified, but the distribution of the phenolic degradation products varied. As a result of HTC treatment, side chain structures with three carbon atoms decreased and the structures containing short side chain structures increased at the same time indicating dealkylation reactions.33 An increase of catechol type structures was also detected in HTC samples referring to demethylation. The use of H2SO4 catalyst (KL-H2SO4-HTC) increased the extent of demethylation reactions shown as the highest amount of catechol structures. The catechol content of FeCl2 catalyzed HTC sample (KL-FeCl2-HTC) was rather similar to the uncatalysed HTC sample (KL-HTC). Phenolic pyrolysis degradation products, which has carbonyl or hydroxyl group in side chain (i.e., vanillin, homovanillin, conireryl alcohols, coniferyl aldehyde structures) are typical degradation products formed from the native β-O-4 linkage.24 However, the amount of β-O-4 linkages in kraft lignin is small compared with native lignins.34 As a result of the HTC treatment, structures typical for native type lignin were clearly decreased indicating cleavage of β-O-4 linkage. Addition of

Figure 4. Typical bond energy [J mol−1] in lignin structure according to the work of Faravelli et al.35 Homolysis of the C−O and O−C bonds during heat treatment of lignin according to the work of Braun et al.37

stability, β-O-4 ether bond is cleaved more easily than the condensed ones36 and the other bonds present in lignin (Figure 4). This supports the Py-GC/MS results indicating the cleavage of β-O-4 bond in hydrothermal carbonization of kraft lignin. The O−CH3 bond has rather similar bond energy than β-O-4 ether bond and demethylation was indeed observed. Even though the bond energies of the side chain structures are slightly higher, the side chains with three carbon atoms were also cleaved to a certain degree during HTC according to the Py-GC/MS results. On the whole, it appears that thermal treatment itself caused more changes in the lignin structure than the use of catalysts. The solid state NMR spectra are presented in Figure 5. The main peak areas can be listed as follows: • 0−70 ppm are sp3 carbons • 110−160 ppm are sp2 carbons (110−118 ppm are βcarbons, 125−130 ppm are CC and 140−150 ppm are α-carbons) • 175−220 ppm are oxygen functionalities Overall, the changes between the samples are minor denoting that the structure of kraft lignin is only slightly modified in thermal treatment. For pure lignin (Pure KL), the peak at 57 ppm attributes to methoxyl groups and the peak at 145−148 ppm are due to aromatic carbons bounded to lignin methoxyl groups. As it is shown, there is a slight decrease in the density of the peak at 57 ppm in HTC-treated samples (KL-HTC, KLH2SO4-HTC, KL-FeCl2-HTC) indicating a cleavage of methyl groups as was also detected by Py-GC/MS measurements. At the same time, there is a small change in the aromatic CC region at 125−130 ppm. In the case of catalyzed HTC-samples, the intensity of CC region is higher than for the pure KL and KL-HTC. The high density of CC bonds in collaboration with the complete degradation and decrease of methyl groups (mainly at 145−148 ppm with low density at 57 ppm) indicates 2741


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due to CC vibration. These changes show that H2SO4 conditions promote faster degradation of lignin compared with FeCl2 conditions or uncatalysed HTC conditions. The morphology of the materials can be investigated with SEM in Figure 7 showing the images of the pure kraft lignin

Figure 5. Solid state NMR spectra of pure kraft lignin and HTC samples.

that the structure of lignin is more degraded under H2SO4- and FeCl2-catalyzed carbonization reactions. The FT-IR spectra are shown in Figure 6. The spectrum of pure lignin (Pure KL) is characterized by an O−H band at Figure 7. SEM images of (a) pure KL, (b) KL-HTC, (c) KL-H2SO4HTC, and (d) KL-FeCl2-HTC.

(pure KL) and the hydrochars (KL-HTC, KL-H2SO4-HTC, KL-FeCl2-HTC). Complex aggregated and agglomerated structures are formed in HTC, whose size are larger than the size of the initial material. This is in agreement with the particle size measurement results in Figure 8. The use of catalysts changed the morphology and the surface characteristics of the samples. The pure KL has a smooth surface and only few pores (Figure 7a) while KL-HTC (Figure 7b) has a smooth surface covered by smaller particles. In the case of KL-FeCl2-HTC (Figure 7d), the surface is characterized by the existence of large pores which are probably formed as a consequence of the evaporation of volatiles during the HTC process. In the case KL-H2SO4-HTC (Figure 7c), more spherical particles and pores exist. This indicates that the use of H2SO4 as a catalyst increased the degradation reactions by breaking down the lignin aromatic structure and causing evaporation of larger amount of volatiles as observed also from the other results. Particle size distributions for the samples are shown in Figure 8. Number-average particle size describes the quality of the sample while volume average particle size describes the quantity of the sample. Mean volume average particle size increased strongly, especially for uncatalysed sample, indicating particle aggregating and agglomeration in HTC which can also be seen from the SEM images (Figure 7). Mean number-average particle size slightly decreased in HTC, except for KL-H2SO4HTC. Overall, this refers that the HTC process produced large amount of submicron sized carbonaceous particles but on the other hand some of the particles formed large aggregates and agglomerates which affects the volume average particles size enormously. Softening region for in situ lignin has determined to be below 200 °C.38 The glass transition temperature, Tg, for dry lignin is often more difficult to detect than for a synthetic polymer, due the complex structure of lignin and sometimes the only

Figure 6. FT-IR spectra of pure kraft lignin and HTC samples.

3450 cm−1, an intense C−H band at 2930 cm−1 and a band at 2850 cm−1, which are typical vibrations of methoxyl groups. The absorbance of carbonyl groups conjugated with aromatic ring is at 1650 cm−1. Two bands attributed to methoxyl groups appear at 1460 and 1425 cm−1. The absorbance of C−H vibration of aromatic rings appears at 1615 and 1515 cm−1.The secondary aliphatic groups appear at 1218 cm−1 and ether band is at 1045 cm−1. Hydrothermally carbonized lignin (KL-HTC) and FeCl2 catalyzed lignin (KL-FeCl2-HTC) show similarities to the pure lignin (Pure KL) but they have less OH groups. Most significant changes are shown on the sample catalyzed using H2SO4 (KL-H2SO4-HTC). The O−H band at 3450 cm−1 and the methoxyl group vibration are missing indicating demethylation as observed from Py-GC/MS and NMR results. There is also a small band around 1700−1720 cm−1, which is probably 2742


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Figure 8. Particle size distributions as differential number and volume.

Figure 9. TGA curves for pure kraft lignin and HTC samples under N2.

possibility is to detect the range of the change in the curve.28 The Tg values are shown in Table 2. For the pure KL, the Tg value of 152 °C was detected which is in accordance with the literature values. The Tg for KL-HTC was 175 °C and for KLFeCl2-HTC 191 °C. In other words HTC treatments increased the Tg value which can be interpreted to be a sign for the cleavage of the plasticizing side chains, as was also observed from Py-GC/MS results. Another explanation for higher Tg is the formation of the functionalities favoring intermolecular hydrogen bonding or covalent cross-linking which are wellknown to increase the glass transition temperature. The use of catalyst further increased the Tg referring to more extensive cleavage of side chains and more cross-linked structure during HTC. Tg for KL-H2SO4-HTC was not detected which denotes

to even more cross-linked structure making the measurement inaccurate or impossible. It can be concluded that as a consequence of thermal treatment a more cross-linked and hence thermally stable structure having less side chains is formed. TGA curves under N2 gas for the samples are presented in Figure 9. The lowest mass loss is achieved for catalyzed HTC samples KL-H 2 SO 4 -HTC (46.9%) and KL-FeCl 2 -HTC (51.7%) while it is slightly higher for uncatalysed HTC sample KL-HTC (53.5%). Highest mass loss is for pure lignin (67.4%). As it is shown, the majority of the weight loss of lignin-derived carbon materials occurs between 300 and 450 °C, and these materials seem stable below 300 °C. For pure lignin a weight loss of 10−15% occurred between 0 and 100 °C due to water 2743



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removal, whereas this was not happening for the HTC treated samples. At the same temperature range, the weight loss for HTC materials is approximately 5% for all samples. At 300− 450 °C, weight loss for HTC materials is 25−30% and is attributed to volatile removal. At higher temperatures between 500 and 1000 °C, mass loss for KL-H2SO4-HTC is 15−17%, whereas in the case of KL-HTC and KL-FeCl2-HTC it is ∼20%. It is hence clear that hydrothermally carbonized lignin has a more thermally stable structure than pure lignin.

REFERENCES

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CONCLUSIONS Kraft lignin was hydrothermally carbonized with and without different catalysts (H2SO4, Fe 2+). Changes in the structure of lignin caused by hydrothermal processing and further use of catalysts were studied. Overall, the structure of kraft lignin was altered only slightly, as reported also earlier. The main changes in the lignin structure caused by hydrothermal conditions was the cleavage of carbon side chain structures as well as demethylation causing increasing amount of catechol structures. Cleavage of β-O-4 linkages was also observed. Thermally more stable, rigid and cross-linked structure was formed having slightly higher energy density. The use of H2SO4 as a catalyst promoted the faster degradation of lignin under HTC conditions resulting in higher solid yields and carbon recoveries, creating at the same time a more stable carbonaceous structure. The use of Fe2+ as a catalyst had only minor effects on the final structure of the hydrochar compared with the uncatalysed hydrochar. Main Observations. • Hydrothermal conditions used in this study cause only minor changes to the structure of kraft lignin. • Side chain structures with three carbon atom decrease while structures with short side chain increase simultaneously and cleavage of β-O-4 linkage occurred according to Py-GC/MS and DSC results. • Increase in catechol type structures and decrease in methyl groups was observed according to Py-GC/MS, NMR, and FT-IR results. • More stable, rigid, and cross-linked carbon structure was produced according to TGA and DSC results. • Slight increase in carbon content and simultaneous decrease in hydrogen, oxygen, and sulfur content took place according to the elemental composition. • H2SO4 promoted the higher degradation of lignin resulting in higher solid yield and carbon recovery while the use of Fe2+ as a catalyst had only minor effects on the final structure of the hydrochar compared with the uncatalysed hydrochar according to Py-GC/MS, NMR, FT-IR, and SEM results.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: hanne.wikberg@vtt.fi (H.W.). *E-mail: [email protected] (M.-M.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the personnel at VTT Technical Research Centre of Finland Ltd for all the help provided with the analytical measurements and especially M.Sc. Heimo Kanerva for performing the HTC experiments. 2744


Research Article

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Structural and Morphological Changes in Kraft Lignin during

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