Influence of the Carbonization Process on Activated Carbon

Research Article pubs.acs.org/journal/ascecg

Influence of the Carbonization Process on Activated Carbon Properties from Lignin and Lignin-Rich Biomasses Catalina Rodríguez Correa,*,† Moritz Stollovsky,† Tobias Hehr,† Yannik Rauscher,† Birgit Rolli,‡ and Andrea Kruse† †

Department of Conversion Technologies and LCA of Renewable Resources, Institute of Agricultural Engineering, University of Hohenheim, Garbenstrasse 9, 70599 Stuttgart, Germany ‡ Institute for Catalysis Research and Technology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *

ABSTRACT: Lignin-rich biomass (beech wood, pine bark, and oak bark) and four lignins were tested as precursors to produce activated carbon (AC) via a two-step chemical activation with KOH. First, the precursors were carbonized via either pyrolysis or hydrothermal carbonization, with the purpose of evaluating the influence of the carbonization process on the AC properties. Pyrolysis chars (pyrochars) were thermally more stable than hydrothermal carbonization chars (hydrochars); thus, more AC was yielded from pyrochars (AC yield calculated from the char amount). The difference between ACs from hydrochars and pyrochars was small regarding the AC yield calculated from the initial amount of biomass or lignin. Additionally, no considerable differences in terms of total surface area and surface chemistry were found between both ACs. To understand this, the mechanism of the activation was explained as a local alkali-catalyzed gasification. In the case of hydrochar, carbonization reactions occurred simultaneously to the gasification because of their lower thermal stability. Thus, the carbon content and yields of hydrochar ACs were similar to pyrochar ACs, but their microporous surface areas were lower, likely due to condensation of volatile matter. KEYWORDS: Lignin, Pyrolysis, Hydrothermal, Carbonization, Activated Carbon

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or wood) lead to chars with higher fixed carbon contents (Cfix; polyaromatic carbon).2 Chars with a high Cfix have a high thermal stability and a relatively low reactivity. These properties allow the char to withstand the aggressive reactions that occur during activation; thus, ACs from lignin and lignin-rich biomasses have larger surface areas and the yields are higher.3−5 The most common way to carbonize biomass is pyrolysis. Lignin pyrolysis is slow and occurs over a wide temperature range.6 The mechanisms that describe the decomposition and char formation involve the release of oxygen and hydrogen as CO, CO2, H2, and H2O, as well as condensable volatiles formed due to free radical reactions initiated after the cleavage of weak inner bonds (usually the C−O bond in the β-O-4 structure).7 This leads to the formation of single- and multi-phenolic compounds that polymerize further and condense.8 Secondary reactions between volatiles contribute to char formation, including the combination of carbon-centered radicals that form C−C bonds.9 The structure of the solid phase starts

INTRODUCTION Lignin, one of the main biomass components, is an abundant heterogeneous polymer composed of phenylpropane units linked together by ether and C−C bonds. Furthermore, it is an important residue from the pulping process in the paper industry. There are several possibilities to use lignin as sustainable feedstock in the energy and chemical sectors due to its availability and chemical structure. It has also shown promising results as a precursor for activated carbon (AC).1 Activated carbon is an interesting material with important adsorptive properties due to its porosity, high surface area, and surface chemistry. It is frequently used as an adsorbent material in aqueous or gas media. It can also be found in energy storage systems (e.g., electrodes or hydrogen and methane storage units). Its characteristics depend strongly on the precursor material and the carbonization and activation processes. It is produced by a two-step process: First, the precursor is carbonized, followed by either a physical (water steam or CO2) or a chemical (acids or bases) activation. During this last step, the surface area and porosity are developed. In contrast to cellulose-rich biomasses (e.g., grass, straw, or grain husks), lignin or lignin-rich biomasses (e.g., coconut shells, fruit stones, © 2017 American Chemical Society

Received: June 12, 2017 Revised: July 18, 2017 Published: July 28, 2017 8222

DOI: 10.1021/acssuschemeng.7b01895 ACS Sustainable Chem. Eng. 2017, 5, 8222−8233

Research Article

ACS Sustainable Chemistry & Engineering

detailed mechanism of path B has been presented by Kang et al.26 by including the polymerization of dissolved phenolics, which agglomerate on the surface of nondissolved lignin particles and of the hydrochar formed already. Forchheim at al.22 pointed out that not all bonds can be easily cleaved by water, as is the case of ether links; therefore, it can be said that the oligomers found in the mixture are not only a consequence of polymerization, but also of the partial cleaving of lignin. Hu et al.27 explained lignin decomposition through ether bond cleavage at temperatures lower than 280 °C, followed by the formation of monomers that, in turn, react with hydrogen from other places to produce monomeric phenolic compounds or new radicals. These lower temperatures are not enough to cleave stronger bonds; therefore, the solid residues consist mainly of unreacted lignin rich in OH−, CH3−, and CH2− groups, as well as carbonyl groups. The HTC chars (hydrochars) have similar properties to brown coal (e.g., H/ C ratio, heating value), but its degree of aromaticity (and thus, thermal stability) is considerably lower compared to pyrochars.9,28,29 It was observed during the experiments conducted for this work that the mixture containing hydrochars rose inside the reactor (like a muffin during baking) during chemical activation with KOH. Contrarily, the volume of mixture with pyrochars remained unchanged. Lignin also swells under pyrolytic conditions at low temperatures,30 which led to the hypothesis that hydrochar swelling during activation is a consequence of the low (or absent) decomposition of lignin during HTC.25 For this reason, the focus of this work was to explore HTC and pyrolysis as a first step (carbonization) during a two-step activation and investigate their influence on the surface areas. The effect on the surface chemistry of ACs was also of special interest, since lignin hydrochars are richer in oxygen and hydrogen than pyrochars.

rearranging through cross-linking reactions, and the aromatic structures begin to grow at temperatures higher than 500 °C. The solid product (pyrochar) is rich in polyaromatic carbon rearranged into turbostratic graphene sheets.10,11 By increasing the temperature and/or the reaction time, the degradation of the charcoal continues by releasing CO and H2, which leads to a solid matrix richer in C.8,12 The chemical composition of lignin depends on its parent biomass: Lignins from grasses consist of coumaryl, guaiacyl, and syringyl alcohol units; softwood lignins are mostly conformed by guaiacyl and that from hardwoods is rich in guaiacyl and syringyl units.9,13 Müller-Hagedorn et al.14 investigated hornbeam wood (hardwood), walnut wood (hardwood), and scots pine (softwood) to understand the differences of the pyrolytic decomposition between wood species. They determined that coniferous (softwood) wood decomposed at higher temperatures than deciduous (hardwood) wood due to its lower concentration of β-O-4 bonds (hardwood has about 60% and softwood 46%), which cleave at lower temperatures compared to other major linkages.15−17 Lignin is dissolved during pulping and precipitated afterward. This leads to a structural change, which, in turn, affects the properties of lignin chars. Jiang et al.18 studied different lignins to understand the influence of the separation method and the plant species on the kinetic constants of pyrolysis. For this, the kinetic parameters of alkali, hydrolytic, organosolv, and Klason lignins were estimated by fitting mass loss curves obtained from thermogravimetric analysis using the Kissinger method. The results showed that the activation energy was dependent on the parent biomass, as well as on the separation method, and that the reaction order depended strongly on the separation method. Hydrothermal carbonization (HTC) has gained more attention over the past few decades as it provides the opportunity to carbonize biomasses with high water content at lower temperatures than pyrolysis. Cellulose and hemicellulose are hydrolyzed and consecutively decompose fast and completely during HTC due mainly to water elimination reactions (eq 1). On the other hand, lignin decomposition is unsubstantial.19−21 4(C6H10O5)n → 2(C12H10O5)n + 10H 2O

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EXPERIMENTAL SECTION

Lignins and Biomass. The precursor materials were two kraft lignins: low sulfonate and alkali kraft (Sigma-Aldrich 471003 and 370959, respectively; CAS number: 8068-05-1), organosolv lignin from Platycodon grandiflorum (Chemical Point UG; CAS number: 8068-03-9), and indulin AT, a purified form of kraft lignin from softwood (MeadWestVaco; CAS number: 8068-05-1). The lignin purity was more than 90%, according to the provider. Beech wood (Fagus sylvatica L.) and bark from pine (Pinus nigra L.) and oak (Quercus robur L.) trees were used. Pine and oak were chosen as examples of softwood and hardwood, respectively. The fiber analyses of the biomasses are presented in Table S1 (Supporting Information). The biomass particle size was smaller than 1 cm for pyrolysis and was milled with a Retsch SM 200 cutting mill to a particle size smaller than 0.5 mm for the HTC. The latter is necessary to enable the fast and complete swelling of the dried biomass in water. Only then, can it be considered a hydrothermal conversion, i.e., a conversion in water. Pyrolysis. Pyrolysis experiments were conducted in a lab-scale reactor under constant N2 flow (10 L/min). The untreated samples were placed in porcelain crucibles inside the reactor, which, in turn, was introduced into a muffle furnace. The reactor was heated to 600 °C with a constant heating rate of 10 °C/min. The holding time at the reaction temperature was 2 h. These parameters were chosen to ensure that the vast majority of the organic compounds were carbonized and that the final product is composed mainly of carbon.31 After the reaction time was completed, the reactor was removed from the furnace and the samples (pyrochars) were cooled to room temperature with gaseous nitrogen at a flow rate higher than 20 L/min. Hydrothermal Carbonization (HTC). The HTC was conducted in high-pressure autoclaves made from stainless steel X5CrNiMo1712-2 with a maximum capacity of 250 mL. Temperature and pressure

(1)

The low HTC temperatures lead to a slow lignin decomposition. At temperatures close to 200 °C in water, ether link hydrolysis dominates over free radical reactions, which occur at higher temperatures and are the main mechanism during pyrolysis.20,22 Consequently, only a partial degradation occurs at hydrothermal conditions. Hydrolysis reactions are catalyzed by H+ and OH− ions present under hydrothermal conditions and lead to the production of singlering compounds (e.g., vanillic and syringic acids, vanillin, coniferyl and sinapyl alcohols, and syringaldehyde).20,23 The increase in hydrolysis reactions as a consequence of the changes in water properties at high temperatures is the basis for all chemical interpretations of HTC. Kruse et al.24,25 proposed two different paths for the decomposition of biomass based on the particle structures: Path A corresponds to the hydrolysis of carbohydrates followed by the polymerization of, e.g., hydroxymethylfurfural and the formation of spherical particles; path B consists of solid-to-solid reactions and the formation of char particles with a similar shape to the precursor. Path B is typically followed by hard (lignin-rich) biomasses. A more 8223


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Iodine and methylene blue are substances commonly used to determine AC adsorption capacity and obtain information on pore size distribution. The iodine number was determined following the ASTM standard 4607-94.35 Regarding methylene blue adsorption, equilibrium tests were conducted at room temperature using 100 mL of a 1000 mg/L methylene blue solution mixed with 0.1 g of AC. The mixture was stirred for 48 h to ensure that the equilibrium was reached.36 The final concentrations Ce (mg/L) were measured with a Hach-Lange DR600 spectrophotometer set at a wavelength of 664 nm. The equilibrium concentration qe (mg/g) was calculated according to eq 2, where C0 (mg/L) is the initial concentration of the methylene blue solution, V (l) is the solution volume, and W (g) is the amount of AC employed for the analysis.

were controlled with internal sensors connected to a data logger RSG 30 (Endress+Hauser AG). The autoclaves were filled to 70% of their volume capacity with a biomass and distilled water mixture with a mass ratio of 20:80. The autoclaves were heated to 220 °C in approximately 1 h. After the temperature was reached, the autoclaves were left to react for an additional 5 h. These parameters were chosen to ensure a complete conversion of the carbohydrates into secondary char (hydrochar).32,33 Subsequently, the autoclaves were quenched to room temperature in cold water. The final pressures ranged between 27 and 51 bar (Table S2, Supporting Information). The solid product was filtered and washed using deionized water until the filtrate reached an electrical conductivity lower than 30 μS/cm. The hydrochar was dried at 105 °C for at least 16 h, and the process water was collected for further characterization. Chemical Activation. Chemical activation was conducted by mixing the pyrochars and hydrochars with solid KOH in nickel crucibles at a char-to-KOH mass ratio of 1:4. The crucibles were placed inside a stainless steel reactor, which, in turn, was placed inside a muffle furnace. The furnace was heated to 600 °C at a constant heating rate of 10 °C/min under a constant N2 flow (10 L/min). This reaction temperature was chosen based on the thermal stability of the pyrochar; the pyrochar was produced at 600 °C. Therefore, it is thermally stable until temperatures up to 600 °C, and the changes measured during activation corresponded only to the activation effect.5 After the reaction temperature was reached, the mixture was left to react for 2 h. Subsequently, the reactor was cooled to room temperature under a N2 flow rate higher than 20 L/min. The product was then mixed with aqueous HCl (2 M) and was left to react for 30 min under constant stirring to neutralize the remaining KOH and other basic compounds formed during activation. The ACs were washed with deionized water using a vacuum filtration system until the filtrate had an electrical conductivity lower than 30 μS/cm. The ACs were dried for more than 16 h at 105 °C and stored for further characterization. Characterization. The elemental composition of the pyrochars, hydrochars, and ACs was measured using an Elementar Vario EL cube for CHNS. The oxygen content was determined by difference (O% = 100 − (C% + H% + N% + S%) − Ash). The ash content was determined following the DIN standards 14775, 51719, and 12902 for the biomass, chars, and ACs, respectively. The ash content of the lignins was determined following the same standard as the chars due to their high thermal stability. The thermogravimetric analysis of the feedstock and of the pyrochars and hydrochars was conducted using a Netzsch STA 449 F5. The samples were heated to 800 °C with a constant heating rate of 10 °C/min in an oxygen-free atmosphere provided by creating a vacuum three times prior to the measurement and by measuring under a constant nitrogen flow of 70 mL/min. The HTC process waters were analyzed with gas chromatography to obtain a better understanding of the precursor decomposition. The organic fraction of the process water was extracted using a solution of 700 mg/L pentadecane in ethyl acetate. Pentadecane is the internal standard. The organic fraction was analyzed in an Agilent gas chromatograph 1530A coupled with a flame ion detector. An Rtx 1MS column (Restek GmbH) was used with He as the carrier gas. The equipment was calibrated with a solution mixture that contained approximately 100 mg/L of cyclopentanone, furfural, methyl cyclopentanone, cyclohexanone, cyclohexanol, 5-methyl furfural, phenol, okresol, guaiacol, p-kresol, 4-ethylphenol, 4-methylguaiacol, catechol, 4ethylguaiacol, 4-methylcatechol, syringol, vanillin, 4-ethylcatechol, and pentadecane. The analysis was conducted by heating the sample from 35 to 180 °C with a constant heating rate of 6 °C/min; subsequently, it was heated with a heating rate of 30 °C/min to 280 °C and was left at this temperature for 10 min. The N2 isotherms were measured in a Quantachrome NOVA 1200 at −196 °C for all ACs, which had been previously degassed at 130 °C for at least 10 h. The apparent surface areas (SBET) were calculated using the Brunauer−Emmett−Teller (BET) method within the modified p/p0 range proposed by Rouquerol et al.34 Micropore areas and volumes were obtained from t-plots calculated in the p/p0 range of 0.2−0.5.

qe =

(C0 − Ce) × V W

(2)

Changes in surface chemistry were studied by means of Fourier transform infrared spectroscopy (FTIR) using a Varian 660-IR spectrometer. The samples were mixed with KBr and pressed into thin wafers, which were scanned between 500 and 4000 cm−1. Acid groups on the surface were determined following the standardized Boehm titration procedure proposed by Goertzen et al.37 and Oickle et al.38 For this, 1.5 g of AC were dried at 105 °C for at least 16 h and mixed with 50.00 mL of three different alkaline solutions (NaOH, Na2CO3, NaHCO3) with a concentration of 0.05 M. The mixtures were agitated for 24 h, followed by a filtration step using folded 8-μm filter paper (Whatmann, 2 V). Aliquots (10 mL) from the filtered solution were acidified with 20 mL of 0.05 M HCl for NaOH and NaHCO3 and with 30 mL for Na2CO3. The new solutions were purged with N2 for 2 h to remove any dissolved CO2, followed by a back-titration with 0.05 M NaOH with a continuous N2 purge. The titration process was conducted in a SI Analytics TitroLine 7000. The functional group concentrations were calculated following the equations presented by Goertzen et al.,37 as well as by assuming that NaOH neutralizes all acidic groups (phenols, lactonic, and carboxylic), while Na2CO3 neutralizes lactonic and carboxylic groups and NaHCO3 only neutralizes carboxylic acids.

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RESULTS AND DISCUSSION Carbonization. The carbonization yield and aromaticity (degree of aromatic condensation; in this case, it was assessed in terms of %C) can be considered as indicators of process efficiency and thermal stability, respectively. Hydrochar yields were, in all cases, considerably higher than the pyrochar yields (Figure 1), but the biomasses yielded the lowest char amounts compared to lignins in both cases. Additionally, the biomasses led to chars with lower carbon contents than lignins (Table 1).

Figure 1. Comparison of the yields between each carbonization process. 8224


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al.25 Compared to the results of Fang et al.,26 the carbon contents of the hydrochars were significantly lower; however, this can be explained by the considerably shorter reaction times used in the present work. Biomass and lignin decomposition during HTC can also be assessed from the distribution of phenolic compounds in the process water (Table S3, Supporting Information). The compound distribution measured for the process waters of lignins were in agreement with the findings of Pecina et al.39 Although the process waters from beech wood and both barks showed some of the components present in the process waters from lignin (e.g., guaiacol and catechol), the concentrations were considerably lower. Syringol, guaiacol, and 4-methylguaiacol were present at higher concentrations in beech wood and oak bark than in pine bark process waters since these are hardwoods.40 By contrast, syringol was absent in pine bark process water since lignins from softwood are composed mostly of guaiacyl-type nuclei. Furfural was only present in beech wood, as this biomass has the highest cellulose and hemicellulose content of the precursors studied. These polymers are composed by hexoses and pentoses that react into 5hydroxymethylfurfural and furfural, respectively, under hydrothermal conditions.19,41 The furfural formation reaches a maximum at 220 °C and after about 1 h, followed by a series of consecutive reactions that led to products such as formic acid and char.42,43 Other products of the furfural degradation are phenolic compounds, which could explain the absence of furfural in the process waters of the barks.44 It can be assumed that alkali kraft and indulin AT lignins were extracted from softwoods due to the high concentration of guaiacol in the process water compared to that of syringol. The ash content of hydrochars resembled that of the precursors or, in the case of lignins, it was slightly lower. The main reason is mineral leaching during HTC, which was evaluated from the anion concentration in the process waters (Table S4, Supporting Information). Potassium, calcium, magnesium, and sodium salts were found in the highest concentrations during HTC. This was expected since these are the most common minerals in biomass and because their solubility in water, especially at hydrothermal conditions, is high. The process waters of lignins, especially low sulfonate lignin, had a high concentration of sodium and sulfur, which are residues from the pulping process. In contrast to HTC, lower yields of the solid product were observed after pyrolysis. Similarly, the degree of carbonization and ash contents were higher because of the substantial oxygen and hydrogen losses (Table 1). The char yields from the lignins were slightly higher than those from the barks despite the relatively high lignin content in bark. Beech wood yielded the lowest char amount, probably because of its low lignin content (fiber analysis is presented in Table S1, Supporting Information). Pine bark yielded more char than oak bark after both carbonization processes. This is mainly due to its higher lignin content and stability; pine bark lignin is composed mainly of guaiacyl monomers and has a more cross-linked structure, which makes the decomposition process more energy demanding.16 Low sulfonate lignin led to the highest char yields, which can be explained by its high ash content. However, concerning carbon content on an ash-free basis, low sulfonate lignin chars were similar to the chars from the other lignins. Thermogravimetric Analysis (TGA). Aromaticity as an indicator of product quality, more specifically of its thermal

Table 1. Elemental Composition on an Ash-Free-Basis (AF) of Precursor, Chars, and ACs (AC)a CAF (%)

HAF (%)

NAF (%)

SAF (%)

ODIF,AF (%)

Ash (%)

Beech wood Untreated Hydrochar Pyrochar Hydrochar-AC Pyrochar-AC

48.9 63.8 90.8 87.0 87.0

6.5 5.3 2.2 0.7 0.5

0.1 0.2 0.5 0.5 0.6

0.1 0.0 0.1 0.1 0.1

44.4 30.7 6.4 11.6 11.8

0.6 0.2 2.3 0.4 0.3

Oak bark Untreated Hydrochar Pyrochar Hydrochar-AC Pyrochar-AC

50.9 62.3 80.7 84.2 83.6

6.0 5.3 1.9 1.0 1.7

1.0 1.0 1.5 1.0 0.8

0.3 0.1 0.4 0.1 0.2

41.8 31.2 15.6 13.6 13.7

8.0 8.2 19.2 1.5 0.8

Pine bark Untreated Hydrochar Pyrochar Hydrochar-AC Pyrochar-AC

56.9 66.4 68.9 86.9 87.8

6.4 5.2 1.9 0.7 0.6

0.3 0.3 0.5 0.7 0.8

0.0 0.0 0.1 0.1 0.1

36.4 28.1 28.6 11.6 10.7

0.7 0.5 1.8 0.6 0.4

Low sulfonate lignin Untreated Hydrochar Pyrochar Hydrochar-AC Pyrochar-AC

62.4 66.8 91.3 85.3 87.6

6.5 6.5 1.7 0.8 0.7

0.1 0.1 0.5 0.9 0.8

5.4 5.7 3.9 0.7 0.1

25.6 20.9 2.5 12.2 10.8

17.0 20.9 24.9 1.1 0.2

Alkali Kraf t lignin Untreated Hydrochar Pyrochar Hydrochar-AC Pyrochar-AC

66.3 69.4 90.9 88.5 86.9

6.0 5.8 1.7 0.5 0.7

0.3 0.6 0.9 0.7 0.8

1.4 1.1 1.2 0.3 0.1

26.0 23.1 5.4 10.0 11.5

3.0 0.9 6.1 1.1 0.5

Indulin AT Untreated Hydrochar Pyrochar Hydrochar-AC Pyrochar-AC

66.1 69.7 91.8 89.6 85.8

6.0 5.9 2.0 0.7 0.7

0.4 0.5 1.0 0.5 0.7

1.4 1.1 1.4 0.3 0.1

26.0 22.8 3.9 8.9 12.7

2.3 0.6 5.6 0.9 0.2

Organosolv Lignin Untreated Hydrochar Pyrochar Hydrochar-AC Pyrochar-AC

63.9 69.3 90.6 87.0 88.6

5.7 5.8 2.2 0.7 0.6

0.8 0.7 1.2 1.3 0.8

0.1 0.1 0.2 0.1 0.1

29.6 24.0 5.8 10.9 9.9

1.1 0.6 3.0 1.3 0.6

a

Oxygen was calculated by difference (DIF).

This is explained by the high cellulose and hemicellulose contents of the biomass. Cellulose and hemicellulose decomposed faster during pyrolysis and reached the maximum decomposition rate at considerably lower temperatures than lignin. The elemental composition of lignin hydrochars resembled closely that of raw lignin, which is an important factor to argue the low decomposition degree of lignin during HTC. This is also in agreement with the findings of Dinjus et 8225


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Figure 2. Mass loss (m; continuous line) and first derivate of the mass loss (−dm/dT; dotted line) of the precursor (black) as well as the chars obtained from hydrothermal carbonization (blue) and pyrolysis (red).

residue in the crucible. The hydrochar was next, followed by the pyrochar, which showed the highest stability. The differential mass loss thermograms (dTG; dashed lines) showed drastic

stability, was evaluated for the precursors and chars by means of TGA (Figure 2). All samples showed the same trend: The precursor decomposed the fastest, thereby, leaving the lowest 8226


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Figure 3. AC yields calculated based on the initial amount of hydrochar and pyrochar (left) and on the initial amount of biomass (right).

Figure 4. Schematic representation of the activation mechanism based on the mechanisms for alkali-catalyzed carbon gasification (adapted from various works57−59).

changes after the carbonization processes not only in the number of decomposing components (peaks), but also in the decomposition rates. Raw beech wood, oak bark, and pine bark showed the three typical peaks corresponding to the decomposition of hemicellulose, cellulose, and lignin at around 300, 360, and 430 °C, respectively. The dTGs from the barks showed a consistent behavior with the findings of Jakab et al.,16 who determined that softwoods are thermally more stable than hardwoods because of the condensed structures which exist already. Oak bark presented an additional peak at higher temperatures. This is consistent with carbonate decomposition, especially calcium carbonate, which is contained in large concentrations in oak ashes.45 After HTC, the biomass hydrochars displayed a pronounced peak at around 400 °C, which corresponds to the incomplete decomposition of the unreacted lignin and to the decomposition of the char obtained from carbohydrate decomposition and repolymerization reactions.46,47 Two peaks can be observed in the case of the bark hydrochars. This is a clear separation of the unreacted lignin and char decompositions. Additionally, the shoulder corresponding to the hemicellulose decomposition present in the dTGs of the raw biomasses was absent in the dTGs of the hydrochars. The lignins remained nearly unchanged after HTC, which can be concluded from the mass loss and dTG curves of the hydrochars. These curves were almost superimposed on top of the mass loss and dTG curves of the precursor. The residue amount was also almost equivalent in both cases. This is particularly true for the low sulfonate, kraft, and indulin lignins.

Regarding organosolv lignin, the raw sample presented a double peak between 215 and 260 °C that can be explained by polysaccharide impurities after the separation process (xylan, glucan, galactan, arabinan).48 This behavior was also observed by Jiang et al.18 Organosolv, alkali kraft, and indulin hydrochars showed a higher peak (faster decomposition) rate than the parent material, which can be a consequence of the partial decomposition of the complex three-dimensional molecular structure of lignin after HTC. For this reason, hydrochar can be cleaved more easily at higher temperatures since some of the ether bonds were partially hydrolyzed during HTC. On the other hand, the decomposition peak was shifted to higher temperatures, which indicates an increase in thermal stability. This is supported by the slight carbon increase presented in Table 1. Pyrochars obtained from all the samples showed the highest thermal stability, leading to residues between 79% and 92%, as shown in Figure 2. Additionally, no peaks before 600 °C could be observed since chars were produced at this temperature. At higher temperatures, the decomposition of those unreacted components continues. However, oak bark and low sulfonate lignin pyrochars showed a more substantial decomposition at high temperatures due to ash decomposition. In the case of low-sulfonate lignin, it was the decomposition of Na2CO3 formed during pyrolysis.49 This peak is smaller for lowsulfonate hydrochar since Na+ ions dissolved in the process water during HTC. Characterization of Activated Carbons. Biomass decomposition during HTC leads to almost no pore formation, 8227


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Figure 5. Iodine number (left) and methylene blue adsorption (right).

even at high temperatures.27,50,51 Contrary to hydrochars and due to the extensive decomposition of biomass at higher temperatures, pyrochars can present relatively high surface areas; Brown et al.52 produced biochars from pitch pine at 600 °C with a surface area close to 400 m2/g. Similar results were obtained by Lehmann53 and Braida.54 Pyrochar textural properties can improve cation exchange processes in soils53 or remove some contaminants from aqueous systems,55 but the surface areas and microporosities are insufficient for more advanced applications, such as energy or gas storage systems. A chemical activation process with KOH was conducted to increase the surface area and micropore volumes. The AC yield was calculated based on the direct precursor (chars) and initial feedstock (lignins and biomass). It is possible to observe in Figure 3 that, in terms of the direct precursor, pyrochars led to a higher yield than hydrochars. This can be explained by the thermal instability presented by hydrochars at the activation temperature. On the basis of the mass loss derivate presented in Figure 2, hydrochars begin to decompose at temperatures as low as 200 °C up to 800 °C, and the maximum decomposition rate occurs between 350 and 500 °C. This decomposition is directly related to the low cross-linked aromatic structure of hydrochars, especially for those with the highest cellulose content.56 It also involves the formation of volatiles and a further carbonization of the hydrochar and of the unreacted lignin, leading to an increase in the degree of aromatic condensation. This was confirmed by the carbon increase shown in the elemental composition (Table 1). Contrary to this, pyrochars are thermally stable up to 600 °C in the absence of an oxidizing (activating) agent; therefore, if any decomposition occurs, it is probably related to the reorganization of the carbon structure. When KOH is present during the reaction, it acts as an oxidizing agent as well as a catalyzer, therefore, the activation mechanism can be understood as an analogy to an in situ alkali-catalyzed gasification (Figure 4): When KOH interacts with the surface of a carbon particle, an oxygen exchange reaction occurs. The carbon is oxidized to CO and the potassium is reduced to an intermediate state KH. The alkali compound reacts either with another KH or with a CH group to release molecular hydrogen and form metallic potassium. The metallic potassium, being extremely unstable, reacts further with CO2 (another oxygen exchange reaction), forming potassium oxide. Furthermore, the potassium oxide is reduced by carbon into metallic potassium or converted into potassium carbonate. The potassium carbonate also acts as an “alkali donor,” contributing to the development of the surface area.57,58 Moreover, the reaction between potassium and the

oxygen bonded inside the char leads to K2O or KOH, which are the initial species. In other words, no catalysis or forced gasification occurs in the presence of a high oxygen concentration; it occurs only with carbon. Concerning the yield calculated based on the initial amount of feedstock, it can be observed that the carbonization process does not have a major influence on this property since there were no considerable differences between the yield of pyrolyzed or hydrothermally carbonized lignins or wood. A possible explanation is to understand hydrochar carbonization during activation as a process, which is simultaneous to but independent of the gasification. In other words, if KOH were absent during the thermal process, the final product would be similar or equal to the pyrochar. For this reason, AC can be understood as a thermodynamically metastable product (state of low Gibbs free enthalpy), and the path to get to it might have no influence. Only low sulfonate lignin showed significant differences between the AC yields obtained from hydrochar and pyrochar. This is probably a consequence of the high ash content and composition. Both hydrochar and pyrochar from low sulfonate lignin have approximately the same amount of mineral content; however, the ash composition might be different due to the strong leaching of sodium salts during HTC. During activation, KOH reacts not only with carbon, but also with silicates and aluminates to form potassium silica compounds.60 These compounds are removed during the washing step subsequent to the thermal treatment, lowering the ash content.50,61 This could explain the large difference in yields between pyrochar and hydrochar ACs from low sulfonate lignin. Another possible explanation is the formation of sulfides during activation. This was a consequence of the high sulfur content of the low sulfonate lignin hydrochar compared to the pyrochar.60 Lignins led to generally higher AC yields than biomass because of the intrinsic aromatic structure of lignin compared to that of carbohydrates. When biomass or lignins were exposed to high temperatures, the aromatic compounds underwent a cross-linking process that condensed the lignin structure even more. This led to an activation energy increase for gasification reactions.62 This can also be seen within the biomass group: Pine bark was the biomass with the highest lignin content; therefore, it led to the highest AC yield. Oak bark was next, and beech wood came last since it presented the lowest lignin content. Adsorption Capacity and Surface Area. The mass lost during the activation process is reflected in the increase in surface area and porosity volumes. The porosity distribution 8228


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Above 500 °C, only graphite was detected. This behavior was studied further by Tromp et al.66 using high-temperature X-ray diffraction at different temperatures, and they detected that the CnK compounds became thermodynamically unstable at temperatures higher than 875 K. Pyrochar is composed of turbostratic graphene layers that can act like graphite. Regarding hydrochar, the degree of aromatic condensation is low, but it increases during activation, which leads to the formation of graphene sheets and, thus, to the formation of lamellar structures. The low microporosity of hydrochar ACs compared to that of pyrochar ACs can be explained due to the thermal decomposition of hydrochars during activation. The volatiles’ release can lead to pore destruction or they can condensate and repolymerize, which can lead to pore clogging. Consequently, the microporous surface area is reduced. Another possible reason was the leaching of potassium during HTC, which depleted the hydrochars of the intrinsic potassium. As a result, the amount of potassium available during pyrochar activation was slightly higher than during hydrochar activation. The intercalation compounds and the gasification reactions involve mainly the alkali metal (potassium) and carbon; therefore, a high carbon content is necessary to achieve high surface areas. This was also shown by Oh et al.4 and by Hu and Srinivasan,3 where the highest surface areas obtained were achieved when the precursor was carbonized prior to activation. The pore size distribution together with the AC adsorption potentials were evaluated with methylene blue adsorption and iodine number (Figure 5). The maximum methylene blue adsorption capacity for all carbon materials was approximately 800 mg/g. Pyrochar ACs generally presented slightly higher capacities than hydrochars. Hydrochar ACs from beech wood, indulin AT, and alkali kraft lignin showed the highest adsorption potential. These three precursors have in common that the ash content was considerably low compared to the others. The high ash content of low sulfonate lignin can account for the low adsorption potential shown by the ACs from this material, regardless of the carbonization process. However, the mineral leaching during the HTC and during activation explained the higher adsorption of methylene blue for the hydrochar AC than for the pyrochar AC. Concerning the iodine number, pyrochar ACs also presented higher results than hydrochar ACs, which is consistent with the large microporosity calculated from the t-plot. Methylene blue is a large molecule compared to iodine. This gives information on the pore size distributions of the ACs. Different molecular areas have been reported in the literature for both molecules. In the case of methylene blue, the molecular areas range between 1.30 and 1.97 nm2,67,68 and after short contact times, it can access pores larger than 1.04−1.33 nm.69 Regarding iodine, its molecular area is 0.40 nm2; therefore, it can access smaller pores than methylene blue.70 On the basis of this information, ACs from hydrochars and pyrochars have similar pore volumes that are penetrable by methylene blue; on the other hand, pyrochar ACs have a larger number of pores that are accessible for iodine. FTIR Spectra and Surface Analysis. Surface chemistry from the precursors, chars, and ACs was studied with FTIR spectroscopy (the complete set of spectra is presented in Figures S2−S8 ,Supporting Information). The spectra of the precursors presented peaks corresponding to the stretching of O−H and C−H groups at around 3400 and 2900 cm−1, respectively, as well as those consistent with aromatic ring vibrations around 1600 and 1500 cm−1. All lignins presented

and surface areas were evaluated by iodine and methylene blue adsorption at room temperature (Figure 5) and by applying the BET and t-plot models to nitrogen isotherms measured at −196 °C (Table 2). Table 2. Total Surface Areas Calculated after the BET Model and Micropore Areas as Percentage of the Total Area BET surface area (m2/g)

Micropore area/BET area (%)

Precursor

HTC

Pyrolysis

HTC

Pyrolysis

Beech wood Oak bark Pine bark Low sulfonate Alkali Kraft Indulin Organosolv Average Std. dev

2327 1957 2517 2122 2548 2183 2435 2298 268

2474 2271 2489 2123 2031 2314 2538 2305 236

75 75 87 75 63 71 81 75 7

93 92 94 87 92 92 91 92 2

The nitrogen isotherms (Figure S1, Supporting Information) from ACs from pyrochars and hydrochars can be classified as type Ia and Ib, respectively.63 This classification corresponds to microporous materials. The difference is that ACs from pyrochars have a very small external surface area (most of the surface area consists of micropores) and narrow micropores, whereas ACs from hydrochars have wider micropores.64 This was corroborated by the results obtained from the BET and tplot models, as well as from the iodine numbers (Figure 5). The average BET surface area of ACs from pyrochars and hydrochars was approximately 2300 m2/g, and it can be seen from Table 2 that there are no significant differences between the textural properties of the ACs obtained from different carbonizates. Biomasses showed slightly higher surface areas than lignins, which can be explained by the presence of cellulose. Rodriguez Correa et al. showed that cellulose and lignin are the biomass components that contribute most to the surface area development, and when mixed, a larger surface area can be obtained than when the components are activated separately.5 Pine bark, being a softwood, is thermally more stable than oak bark.16 This is also reflected in the higher thermal stability of the pine bark pyrochars and hydrochars. It has been mentioned previously that a higher thermal stability means more polyaromatic carbon, which, in turn, means that the alkali-catalyzed gasification will happen more slowly and in a more organized way. Contrary to this, oak bark chars are less stable and release relatively large quantities of volatiles at the activation temperature (Figure 2). This can lead to pore expansion or clogging, which explains the lower surface areas of oak bark ACs compared to those of pine bark ACs. Another possible reason is the higher hemicellulose content of oak bark, which lowers the surface area.5 Concerning microporosity, an average of 92% of the total surface area of pyrochar ACs correspond to micropores, which is considerably higher than that of hydrochar ACs (75%). Micropore formation can be explained by a mechanism that involves the formation of intercalation compounds of metallic potassium with carbon CnK (lamellar structure).59 According to Nixon et al.,65 these compounds are formed with graphite and an excess of potassium. Their study focused on the temperature dependence of the atomic ratio K/C, and the results showed that K/C decreases with increasing temperature until 500 °C. 8229


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Table 3. Distribution of Acidic Sites (oxygen-containing groups) on the Surface Measured with the Boehm titration methoda Carbonization

a

Sample

Total acidic groups (μmol/g)

Pyrolysis

Pine bark Low sulfonate Organosolv R1 Organosolv R2

1.324 1.139 1.275 1.228

Phenolic (μmol/g) 78 53 92 98

Lactonic (μmol/g) 717 853 637 614

Carboxylic (μmol/g) 529 233 546 516

Hydrothermal carbonization

Pine bark Oak bark Alkali Kraft

1.207 1.307 1.214

N.A.b N.A.b 11

N.A.b N.A.b 681

N.A.b N.A.b 522

R1 and R2 denote sample replicates. bMissing information could not be provided due to lack of material for completing the measurements.

the oxygen increase in the ACs compared with the pyrochars, which is probably a consequence of surface oxidation during activation (gasification reactions). These peaks were also present in the hydrochar AC spectra. Hydrochar and pyrochar ACs had roughly the same oxygen content; however, it is difficult to quantify and compare functional groups from FTIR spectra due to effects such as band overlapping and baseline drift. Some samples were analyzed by means of Boehm titration to complement the FTIR results and to identify differences between pyrochar and hydrochar ACs (Table 3). The concentration of acidic groups was roughly the same for the ACs analyzed, and it was observed that the carbonization process had no considerable effect on the distribution of the functional groups. The carbons analyzed showed mostly lactonic and carboxylic groups, which have two oxygen atoms, and a low concentration of phenolic groups. The total acidic groups for hydrochar and pyrochar ACs were approximately 1300 μmol/g. Considering that the total oxygen concentration in the ACs was between 8.9−13.7%, which accounts for 5500 to 8550 μmol O/g, it can be said that the concentration of basic functional groups was predominant or equivalent to the concentration of acid groups. This is consistent with the findings of Cardoso et al.78 and Wu et al.79 This is also supported by the findings of Altenor et al.,80 who stated that carbons with a high concentration of acidic groups, especially carboxylic groups, have a higher adsorption capacity for methylene blue due the basic nature of methylene blue. On the other hand, Yang et al.81 observed an increase in the methylene blue adsorption in ACs obtained at higher carbonization temperatures, despite the decrease in the surface functionalities. There was no correlation (r2 = 0.0368) found in this work between acidic groups and methylene blue adsorption (Figure S9, Supporting Information). Boehm titration as a tool for determining the oxygen groups provided valuable information; however, some studies have shown that not all the oxygen can be measured by this method. An example is the presence of groups within narrow micropores or oxygen bonded within graphene rings (nonreactive).82 This explains the difficulty in determining the exact distribution of acidic groups for some of the samples.

peaks corresponding to C−O stretching of guaiacyl (around 1270 and 1213 cm−1), but only organosolv lignin showed the stretch of coumaryl ester groups at 1694 and 1630 cm−1.71,72 This is consistent with its origin (Platycodon grandiflorum, a herbaceous flowering perennial plant), as lignins from herbaceous plants contain this component in a more significant proportion than hardwoods and softwoods.17 The three biomasses presented the band corresponding to oxygenated functional groups characteristic of cellulose (1030−1060 cm−1), as well as a band at around 1500 cm−1, which represents CC vibrations of skeletal aromatic rings.73 The syringyl peak was found in beech wood (1111 and 1331 cm−1) and oak bark (1094 and 1153 cm−1) FTIR spectra but was absent in the pine bark FTIR spectrum. This is consistent with the lignin composition of the different types of wood: Softwoods are composed mainly of guaiacyl and significantly lower amounts of p-hydroxyphenyl and syringyl. Conversely, hardwoods possess mostly syringyl and guaiacyl in similar proportions, and phydroxyphenyl is present in lower quantities.17,74,75 Both carbonization processes led to a decrease in the functional groups, especially of those related to carbohydrates. Hydrochars showed IR spectra similar to those of the precursor; however, the band between 1030 and 1060 cm−1 was less intense. The bands below 850 cm−1 (C−H groups located in the edges of the aromatic planes) became less intense, indicating fewer substituted aromatic rings. The bands corresponding to guaiacyl and syringly were also present in the spectra of the hydrochar. The same was observed for the bands corresponding to aliphatic C−H groups (between 2880 and 3000 cm−1). This again supports the fact that lignin undergoes a minor decomposition during HTC. Contrary to hydrochars, the bands between 2880 and 3000 cm−1 as well as those below 850 cm−1 were undetectable in the FTIR spectra of pyrochars. On the other hand, bands were observed at around 1550, 1650, and 1750 cm−1, which correspond to oxygen-containing groups (ketones, aldehydes, lactones, or carboxyl groups) and O−H groups, either from adsorbed water or from phenolic groups. The loss of intensity of the bands at 850 cm−1 together and the increase in the band intensity at 1600 cm−1 (related to aromatic rings and C  C vibrations) is a strong indication of increased aromaticity.76,77 The FTIR spectra from hydrochar ACs showed a drastic decrement of functional groups compared to hydrochars; however, some peaks were still observable. The most relevant peaks were those related to CC vibrations around 1500 cm−1. This again corroborates the aromaticity increase that hydrochars undergo during activation. In the case of pyrochar ACs, the spectra showed an increase in the intensity of the peaks corresponding to acidic groups between 1550 and 1780 cm−1 (phenolic, lactonic, carboxylic). This is consistent with

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CONCLUSIONS The most relevant properties to characterize ACs are surface area, pore size distribution, and surface chemistry. These are strongly related to the carbonization process and the precursor. It was shown in the work presented here that lignins from the paper and pulp industry performed similarly to lignin-rich biomasses and could be converted into high-added value microporous ACs. It was established that pyrolysis and HTC 8230


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tigation of acid-catalyzed and free-radical reaction pathways. J. Anal. Appl. Pyrolysis 1995, 33, 1−19. (8) Faravelli, T.; Frassoldati, A.; Migliavacca, G.; Ranzi, E. Detailed kinetic modeling of the thermal degradation of lignins. Biomass Bioenergy 2010, 34 (3), 290−301. (9) Dorrestijn, E.; Laarhoven, L. J. J.; Arends, I. W. C. E.; Mulder, P. The occurrence and reactivity of phenoxyl linkages in lignin and low rank coal. J. Anal. Appl. Pyrolysis 2000, 54 (1−2), 153−192. (10) Kercher, A. K.; Nagle, D. C. Microstructural evolution during charcoal carbonization by X-ray diffraction analysis. Carbon 2003, 41, 15−27. (11) Williams, P. T.; Besler, S. The influence of temperature and heating rate on the slow pyrolysis of biomass. Renewable Energy 1996, 7 (3), 233−250. (12) Brebu, M.; Vasile, C. Thermal degradation of lignin - A review. Cellul. Chem. Technol. 2010, 44 (9), 353−363. (13) Yoshida, T.; Oshima, Y.; Matsumura, Y. Gasification of biomass model compounds and real biomass in supercritical water. Biomass Bioenergy 2004, 26 (1), 71−78. (14) Müller-Hagedorn, M.; Bockhorn, H.; Krebs, L.; Müller, U. A comparative kinetic study on the pyrolysis of three different wood species. J. Anal. Appl. Pyrolysis 2003, 68−69, 231−249. (15) Saiz-Jimenez, C.; De Leeuw, J. W. Lignin pyrolysis products: Their structures and their significance as biomarkers. Org. Geochem. 1986, 10 (4−6), 869−876. (16) Jakab, E.; Faix, O.; Till, F. Thermal decomposition of milled wood lignins studied by thermogravimetry/mass spectrometry. J. Anal. Appl. Pyrolysis 1997, 40−41, 171−186. (17) Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A. Liquid fuels, hydrogen and chemicals from lignin: A critical review. Renewable Sustainable Energy Rev. 2013, 21, 506−523. (18) Jiang, G.; Nowakowski, D. J.; Bridgwater, A. V. A systematic study of the kinetics of lignin pyrolysis. Thermochim. Acta 2010, 498 (1−2), 61−66. (19) Jing, Q.; Lü, X. Kinetics of non-catalyzed decomposition of Dxylose in high temperature liquid water. Chin. J. Chem. Eng. 2007, 15 (5), 666−669. (20) Kruse, A.; Dahmen, N. Water - A magic solvent for biomass conversion. J. Supercrit. Fluids 2015, 96, 36−45. (21) Funke, A.; Ziegler, F. Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering. Biofuels, Bioprod. Biorefin. 2010, 4 (2), 160−177. (22) Forchheim, D.; Hornung, U.; Kruse, A.; Sutter, T. Kinetic modelling of hydrothermal lignin depolymerisation. Waste Biomass Valorization 2014, 5 (6), 985−994. (23) Fang, Z.; Sato, T.; Smith, R. L.; Inomata, H.; Arai, K.; Kozinski, J. A. Reaction chemistry and phase behavior of lignin in hightemperature and supercritical water. Bioresour. Technol. 2008, 99 (9), 3424−3430. (24) Karayıldırım, T.; Sınağ, A.; Kruse, A. Char and coke formation as unwanted side reaction of the hydrothermal biomass gasification. Chem. Eng. Technol. 2008, 31 (11), 1561−1568. (25) Dinjus, E.; Kruse, A.; Tröger, N. Hydrothermal carbonization 1. influence of lignin in lignocelluloses. Chem. Eng. Technol. 2011, 34 (12), 2037−2043. (26) Kang, S.; Li, X.; Fan, J.; Chang, J. Characterization of hydrochars produced by hydrothermal carbonization of lignin, cellulose, d-xylose, and wood meal. Ind. Eng. Chem. Res. 2012, 51 (26), 9023−9031. (27) Hu, J.; Shen, D.; Wu, S.; Zhang, H.; Xiao, R. Effect of temperature on structure evolution in char from hydrothermal degradation of lignin. J. Anal. Appl. Pyrolysis 2014, 106, 118−124. (28) Xiao, L.-P.; Shi, Z.-J.; Xu, F.; Sun, R.-C. Hydrothermal carbonization of lignocellulosic biomass. Bioresour. Technol. 2012, 118, 619−623. (29) Apaydın-Varol, E.; Pütün, A. E. Preparation and characterization of pyrolytic chars from different biomass samples. J. Anal. Appl. Pyrolysis 2012, 98, 29−36.

are suitable carbonization steps to produce a carbon-rich precursor (char) for the activation. It was also determined that ACs from pyrochars have larger microporous areas than hydrochar ACs. The main reason was attributed to the higher amount of undecomposed lignin present in the hydrochar and to the lower amount of carbon available. A mechanism based on an in situ alkali-catalyzed gasification and the formation of lamellar compounds was used to explain the surface area and microporosity development during activation. From the chemical point of view, it was evident that the interaction between the potassium species and carbon was the important step. Therefore, it can be concluded that, independently of the mechanism, the pore formation is intensified and becomes more frequent using a precursor with a higher carbon content. Despite the differences in microporosity, the ACs obtained via pyrolysis and HTC presented similar properties (C, yield, surface areas, and functional groups). On the basis of this, AC can be understood as a thermodynamically metastable state that can be reached through both carbonization processes. This is important not only for understanding material properties and design but also for process development and upscaling.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01895. Biomass composition, HTC final pressures, characterization of HTC process waters, N2 isotherms, FTIR spectra, correlation between methylene blue and acidic groups, and GC-FID spectra. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 (0) 711 459 ́ 24701 (Catalina Rodriguez Correa). ORCID

Catalina Rodríguez Correa: 0000-0001-7750-2250 Notes

The authors declare no competing financial interest.

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Influence of the Carbonization Process on Activated Carbon

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