Preparation of high-performance activated carbons from hemicellulose pre-extracted residues of poplar and their application in VOCs removal

He, J., Zhao, Y., Zhou, Y., and Wu, S. (2023). “Preparation of high-performance activated carbons from hemicellulose pre-extracted residues of poplar and their application in VOCs removal,” BioResources 18(2), 2874-2896.

Abstract

Hemicellulose was pre-extracted from poplar by the KOH extraction method. A series of activated carbons with high VOCs adsorption capacity were prepared using hemicellulose pre-extraction residue (HPR) as carbon precursor and KOH as the activator. The results showed that the pre-extraction using organic solvent (benzene-ethanol mixture) had no significant effect on the hemicellulose removal efficiency and the microporous structure of activated carbons. After the selective pre-extraction of 38.2 to 65.7 wt% hemicellulose from poplar, the final yield of activated carbons only decreased by 1.1 to 2.0%, but the pore structure of activated carbons was greatly improved. A total of 40.7 wt% hemicellulose in poplar was removed under 4 wt% alkali concentration and 3 h KOH treatment. The activated carbons prepared from HPR of poplar gave the highest BET surface area (3066 m²·g^-1) and pore volume (1.32 cm³·g^-1). The pore structures of activated carbons can be controlled to some extent by changing the removal degree of hemicellulose. The activated carbon obtained under the optimized conditions showed excellent adsorption capacity for toluene (733 mg·g^-1) and dichloromethane (184 mg·g^-1). The correlation between adsorption properties and pore structure shows that the adsorption capacities of toluene and dichloromethane were closely related to micropores (＜ 2 nm) and ultramicropores (＜ 0.6 nm) of activated carbon, respectively. The pre-extraction of hemicellulose greatly improved the volatile organic compound (VOC) adsorption capacity of activated carbons by increasing the percentage of micropores.

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Preparation of High-performance Activated Carbons from Hemicellulose Pre-extracted Residues of Poplar and their Application in VOCs Removal

Jiakai He,^a Yuanyuan Zhao,^bYun Zhou,^b and Shubin Wu ^a,*

DOI: 10.15376/biores.18.2.2874-2896

Keywords: Poplar; Hemicellulose; Activated carbon; VOCs adsorption

Contact information: a: State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, P.R. China; b: Guangzhou Paper Group Ltd., Guangzhou, 511462, P.R. China; *Corresponding author: shubinwu@scut.edu.cn

INTRODUCTION

Volatile organic compounds (VOCs) refer to organic compounds with a saturated vapor pressure higher than 133.322 Pa at room temperature and boiling points between 50 and 260 ℃, mainly including various alkanes, olefins, halogenated hydrocarbons, and aromatic compounds. The VOCs are toxic and highly irritating. They are also important precursors of photochemical smog and cause great harm to the environment and human health (Enesca and Cazan 2020; David and Niculescu 2021). Adsorption is the most widely used VOC treatment technology at present (Zhang et al. 2017). The key to adsorption technology is an adsorbent with excellent performance. Activated carbon is widely used in the adsorption and separation of VOCs because of its well-developed pore structure, large specific surface area, and strong specific adsorption capacity. At present, a large number of studies on the removal of VOCs by activated carbon have been reported (Gil et al. 2014; Pui et al. 2019; Zhou et al. 2019; Li et al. 2020; Zhang et al. 2020).

Coal is one of the main raw materials for the preparation of activated carbon, but the increasingly serious energy shortage and environmental pollution problems are motivating people to give priority to renewable and sustainable resources as alternatives to fossil energy. Plant fiber biomass has become an important raw material for the preparation of activated carbon because of its high carbon content, low price, and its renewable nature. Production of activated carbon from different plant fiber materials was reported in a range of studies, including wood (Yagmur et al. 2013), bamboo (Deng et al. 2015), bark (Sessa et al. 2022), fruit shell (Magoling and Macalalad 2017), bagasse (Guo et al. 2020), straw (Basta et al. 2009), pomelo peel (Liu et al. 2018), etc. Additionally, a lot of research has focused on optimizing the preparation process of activated carbon to improve VOCs adsorption capacity. Yang et al. (2021) studied the effect of the chemical activation process on the adsorption capacity of VOCs by properly adjusting the pre-carbonization method and the injection method of KOH. Zhu et al. (2018) prepared activated carbon from corncob waste residue and studied the effects of carbonization temperature, impregnation ratio, and carbonization time on the structure and chemical properties of activated carbon. Under the optimized conditions, the specific surface area of the carbon material reached 2022 m²·g^-1, and the toluene adsorption capacity reached 415 mg·g^-1(Zhu et al. 2018). In Batur’s study, activated carbon was produced from defatted black cumin biowaste with ZnCl₂ activation through the response surface methodology. When the VOCs inlet concentration was 20 mg·L^-1, the adsorption capacity of activated carbon with the specific surface area of 1213 m²·g^-1 for benzene, toluene, and xylene was 495 mg·g^-1, 580 mg·g^-1, and 674 mg·g^-1, respectively (Batur and Kutluay 2022). Li et al. prepared straw-based activated carbon with a toluene adsorption capacity of 322 mg·g^-1 and an ethyl acetate adsorption capacity of 240 mg·g^-1 by optimizing the process (Li et al. 2021). In the above studies, only some studies emphasize the importance of the chemical composition of raw materials to the final quality and performance of activated carbon (Wang et al. 2018). In addition, some researchers improved the adsorption capacity of specific VOCs by modifying carbonaceous adsorbent. Yang et al. synthesized MgO/C composite adsorption material, in which MgO basic sites were more present in the interior of AC than on the surface, thus strengthening the adsorption performance of the adsorption material for polar VOCs (Yang et al. 2022a).

When plant fiber biomass is used to prepare carbon materials, the yield is low, although it is an excellent carbon precursor (Wang et al. 2022). This phenomenon occurs because cellulose, hemicellulose, and lignin are the main components of plant fiber biomass. The thermal decomposition of the three components forms the carbonaceous structure of carbon materials, but the contributions of the three components to carbon quality and structure are different (Benoît et al. 2008). In the process of biomass pyrolysis at high temperatures, cellulose, hemicellulose, and lignin decompose at different rates in different temperature ranges (Collard and Blin 2014). Because of its amorphous structure and low molecular weight, hemicellulose is mostly converted into carbon-containing volatile components, such as carbon monoxide, carbon dioxide, and methane, during the preparation of carbon materials, resulting in its low contribution to carbon quality (Zhang et al. 2019b). Therefore, the process of pre-extracting hemicellulose from plant fiber raw materials, and then preparing high-performance activated carbon from the remaining fiber residue, theoretically, is conducive to the realization of high-value utilization of hemicellulose, and will not have a significant impact on subsequent carbon production.

At present, there are many methods to separate hemicellulose from biomass, including acid treatment (Yang et al. 2022b), steam explosion (Yan et al. 2021), hydrothermal extraction (Sun et al. 2022), alkali treatment (Xu et al. 2022), etc. Although acid treatment can effectively separate hemicellulose, it will lead to a certain degree of hydrolysis and this is not conducive to the utilization of hemicellulose as macromolecules. The cost of the steam explosion is high and hydrothermal extraction requires high temperature and pressure. The alkali method (KOH, NaOH, etc.) is the most commonly used chemical method of separating hemicellulose, which can be realized under low temperatures and pressure. Inorganic lye extraction is the most economical and effective method to extract hemicellulose from wood fiber (Yuan et al. 2016). Peng et al. (2012) treated pea stems with KOH solutions of different concentrations and separated alkali-soluble hemicellulose into linear and branched hemicellulose with the aid of iodine complex precipitation. García et al. (2022) used cold alkali extraction to selectively separate hemicellulose from elephant grass and retained 93.0% cellulose and 82.1% lignin in raw materials. It is worth noting that alkali liquor has a strong ability to dissolve xylose, so the alkali method is more suitable for extracting xylan from hardwood. In this study, KOH treatment at room temperature was chosen because of its simple operation, mild conditions, and ability to separate a large amount of xylose. The KOH can be also recovered when hemicellulose in an alkali treatment solution is separated. Wood is an excellent precursor for the preparation of carbon materials. Poplar is one kind of poplar fast-growing wood in China, and it is rich in hemicellulose, so it is used as raw material in this study.

In this study, poplar wood was used as raw material and KOH was used to pre-extract hemicellulose from poplar wood at room temperature. While achieving high conversion and utilization of hemicellulose, hemicellulose pre-extracted residues (HPR) were used as carbon precursors and KOH was used as an activator to prepare activated carbon with high VOCs adsorption capacity, which was used to remove VOCs. Firstly, the influence of organic solvent extracts from poplar on the pre-extraction of hemicellulose and subsequent preparation of activated carbon was evaluated. Then, the influence of hemicellulose removal degree in poplar on activated carbon yield and pore structure was investigated. The difference in hemicellulose removal degree was realized by changing alkali treatment conditions. Finally, toluene and dichloromethane were selected to represent VOCs with different molecular properties. The adsorption performance of activated carbon was evaluated by the dynamic adsorption method and the relationship between the pore structure of activated carbon and the adsorption capacity of VOCs was constructed.

EXPERIMENTAL

Materials

Poplar chips (hardwood) were obtained from Shandong Jianghe Paper Co., Ltd. (China). Potassium hydroxide (KOH), benzene, anhydrous ethanol, and other reagents were purchased from Guangzhou Chemical Reagent Co., Ltd. (China).

Preparation of Carbon Precursors

Poplar-based materials were prepared as carbon precursors through different pretreatment:

(a) The poplar powder after air drying, crushing, screening, and drying was marked as P and 40 to 60 mesh standard sieve was used for screening.

(b) Poplar powder (P) was extracted with the benzene-ethanol mixture (2:1, v: v) for 6 h, dried at 105 ℃ for 24 h after air drying, and the raw material after extraction was marked as P_E. The process of pre-extraction of organic solvents is described in detail in the Chinese national standard GB/T 35818 (2018).

(c) 20 wt% KOH solution was added to P and P_E at a solid-liquid ratio of 1:10. The solid-liquid ratio refers to the ratio of the mass (g) of poplar powder to the volume (mL) of KOH solution. After the mixture was treated at 30 ℃ for 3 h, the filtrate (hemicellulose solution) and HPR was obtained by solid-liquid separation. The HPR was washed to neutral with water and dried at 105 ℃ for 24 h and marked PK_20-3 and P_EK_20-3.

(d) Amounts of 4, 7, 10, or 20 wt% KOH solution was added to P at a solid-liquid ratio of 1:10. After the mixture was treated at 30 ℃ for 3 h, the filtrate (hemicellulose solution) and HPR was obtained by solid-liquid separation. The HPR was washed to neutral with water and dried at 105 ℃ for 24 h, marked PK_x-y. Here x represents the concentration of KOH solution and y represents the time of alkali treatment.

Composition Analysis of Carbon Precursors

The holocellulose and Klason-lignin contents were determined according to the procedure detailed in the Chinese national standard GB/T 35818 (2018). α-Cellulose was obtained by treating delignified raw materials with 17.5% sodium hydroxide solution (GB/T 744 (2004)). The hemicellulose content was represented by the difference between holocellulose content and α-cellulose content. It should be noted that the hemicellulose content obtained is an approximate value. Residues refer to other components in raw materials except for cellulose, hemicellulose, and lignin, including extracts and ash. The content of the organic solvent extract was determined by extraction with organic solvent (benzene-ethanol mixture), and the ash content was determined by the remaining sample mass after the raw material was burned in a muffle furnace at 575 ℃ (GB/T 35818 (2018)).

Preparation of Activated Carbons

The carbon precursors (on a dry basis) were placed in a porcelain crucible in a tubular furnace, heated to 500 ℃ in argon gas flow at a heating rate of 10 ℃/min, and then carbonized at 500 ℃ for 1 h. The carbonized solids were obtained after cooling to room temperature. Carbonated solids were impregnated with KOH solution using a weight ratio of carbonated solid/KOH of 1/4 (on a dry basis), and then dried for 24 h in an oven at 105 ℃. The mixture of carbonized solids and KOH was put into a porcelain crucible, then heated to 800 ℃ at a heating rate of 10 ℃/min in an argon atmosphere and activated at 800 ℃ for 1 h. The resultant activated carbons were thoroughly washed with 1 M HCl followed by ultrapure water until the pH was neutral. Finally, the activated carbons were dried at 105 ℃ for 24 h and stored in sealed vials. Activated carbons were labeled as (carbon precursor-AC).

Characterization of Activated Carbons

The N₂ adsorption/desorption isotherms of activated carbons were measured at 77 K using an ASAP 2460 analyzer, and the pore structure parameters were calculated according to the N₂ adsorption/desorption isotherms. The samples were previously outgassed at 300 ℃ for 10 h. The specific surface area of activated carbons was calculated by the BET method, the specific surface area and volume of micropores were obtained by the t-plot method, and the pore size distribution (PSD) was obtained by the BJH method. The micromorphology of the samples was analyzed by scanning electron microscope (SEM, FEI Inspect F50, FEI Company, Hillsboro, OR, USA).

Adsorption Test

The adsorption capacity of poplar-based activated carbon for toluene and dichloromethane was evaluated using the experimental device shown in Fig. 1. Before adsorption, the samples were degassed in a vacuum box at 105 ℃ for 12 h to remove the adsorbed moisture and impurities. The air in the test system was discharged with high-purity nitrogen. The VOCs dynamic adsorption experiment conditions were set as follows: the amount of active carbon was 100 mg (± 1%), the active carbon was fixed in a quartz tube with an inner diameter of 6 mm, the adsorption temperature was controlled at 30 ℃ through the glass fiber heating belt, the concentration of toluene and dichloromethane was 500 ppm (± 2%), and the gas flow rate at the inlet side was controlled at 100 mL/min. The outlet concentration of the gas was detected by a gas chromatograph (GC2002, Kechuang, Shanghai, China).

Fig. 1. Schematic of VOCs dynamic adsorption experimental setup

The breakthrough time (t_b) is defined as the time when the VOCs outlet to inlet concentration ratio (C/C₀) is 5%, and the saturation time (t_s) is defined as the time when the VOCs outlet to inlet concentration ratio (C/C₀) is 95%. The adsorption capacity at t_b and t_s is expressed as breakthrough adsorption capacity (Q_b) and saturated adsorption capacity (Q_s). The adsorption capacity (Q) of activated carbon can be calculated according to the following formula,

(1)

where Q is the adsorption capacity (mg·g^-1), V is the VOCs gas flow rate (mL·min^-1), C₀ is the VOCs gas inlet concentration (g·m^-3), t is the adsorption time (min), C_t is the VOCs gas outlet concentration (g·m^-3) at t (min), and m is the sample mass (g).

The Yoon-Nelson (Y-N) model was used for the nonlinear fitting of the adsorption breakthrough curve. The adsorption rate and 50% adsorbate breakthrough time can be obtained through linear regression analysis. The formula of the Y-N model is as follows (Yoon and Nelson 1984),

(2)

where K_YN is the physical diffusion rate of adsorption process (min⁻¹), τ is the penetration time of 50% adsorbate (min), C₀ is the inlet concentration of VOCs (g·m^-3), C_t is the outlet concentration (g·m^-3) of VOCs at t (min), and t is the adsorption time (min).

RESULTS AND DISCUSSION

Effect of Organic Solvent Extraction on Alkali Extraction and Preparation of Activated Carbons

Before using plant fiber for research, many researchers considered using organic solvent for extraction to eliminate the influence of organic solvent extract. Table 1 shows the contents of holocellulose, α-cellulose, hemicellulose, and Klason-lignin of four carbon precursors (P, PK_20-3, P_E, and P_EK_20-3). It should be pointed out here that the ash content of the poplar used in this study was below 1%. Table 1 shows that the content of holocellulose and hemicellulose in P was 74.5% and 34.0%, and the content of Klason-lignin was 21.7%, which is consistent with previous reports (Yagmur et al. 2013). The composition of PE was close to P, indicating that the composition of poplar had not been noticeably changed by organic solvent extraction. This may be because of the low content (less than 1%) of organic solvent extract in the poplar used, and the fact that the extract mainly was present in the parenchyma cells, and the main components were free and esterified fatty acids.

Figure 2 shows the material balance of two poplar raw materials (P, P_E) in the process of alkali treatment at room temperature, reflecting the change in content and removal rate of α-cellulose, hemicellulose, and Klason-lignin in P and PE after alkali treatment (alkali extraction conditions are shown in Fig. 2). It can be seen from Fig. 2 that after 100 g of P was extracted with alkali at room temperature, 76.51 g of HPR (PK_20-3) could be obtained, and similarly, 76.86 g of P_EK_20-3 could be obtained from 100 g of P_E. Whether the raw materials could be pre-extracted with an organic solvent or not, the hemicellulose removal rate reached up to 65% and the lignin retention rate reached 91% after extracting for 3 h with 20 wt% KOH at 30 ℃. The results show that alkali pre-extraction can remove a large amount of hemicellulose and only a small amount of lignin, which can be attributed to the strong ability of alkali liquor to dissolve xylose (main components of hemicellulose in poplar). At the same time, the removal rate of hemicellulose by alkali extraction was not noticeably improved by organic solvent extraction.

Table 1. Composition Analysis of Carbon Precursors

(a)

(b)

Fig. 2. Material balance of poplar in the alkali extraction process (a) Sample was not extracted; (b) Extracted sample

Activated carbons were prepared from the above four carbon precursors under the same conditions. Table 2 shows that the yield of activated carbons was not noticeably changed whether it was extracted with an organic solvent or not. Figure 3a shows the N₂ adsorption/desorption isotherms of poplar-based activated carbons at 77 K. The N₂ adsorption/desorption isotherms of poplar-based activated carbons can be classified as type I. The adsorption capacity of N₂ increased remarkably at lower relative pressure (P/P0 < 0.05). All isotherms show narrow elbows and horizontal platforms, and almost no hysteresis loops, which indicates that the pores of activated carbons were mainly microporous. Figure 3b shows that the pore sizes of poplar-based activated carbons were mainly distributed in the range of less than 2 nm, and the pore structures of the activated carbons prepared after pretreatment were better than that of the activated carbon prepared without pretreatment (PAC). Surprisingly, the pore volume of PAC was found to be higher than that of other activated carbons when the pore size was less than 0.6 nm, while the pore volume of other activated carbons was higher than that of PAC when the pore size was greater than 0.6 nm. This may indicate that PAC is more advantageous for ultramicropores (< 0.6 nm).

The main pore structure parameters of poplar-based activated carbons calculated in Fig. 3a are listed in Table 2 to further understand the pore structures of activated carbons. Table 2 shows that the specific surface area and pore volume of P_EAC were remarkably increased compared with PAC. The S_BET value increased from 1401 to 2584 m²·g^-1 (approximately 84.4% increase), and V_total increased from 0.59 to 1.13 cm³·g^-1 (approximately 91.5% increase), indicating that the pre-extraction of organic solvent optimized the pore structures of activated carbons prepared from poplar (no pretreatment). However, the pore structure of P_EK_20-3AC was not better than that of PK_20-3AC, indicating that the pre-extraction of organic solvent did not promote the preparation of activated carbons from poplar wood after alkali extraction. It has been shown before that the composition of poplar and the removal rate of hemicellulose will not be noticeably changed by the pre-extraction of organic solvent, so it is speculated that part of the extracts in the parenchyma cells is removed by organic solvent, many small channels are formed on the cell wall, and fibers are moistened, which leads to changes in the pore structures of activated carbons. However, a large amount of hemicellulose in poplar is removed by alkali extraction, which will also lead to fiber swelling and composition change. When poplar fiber is treated with alkali, its morphology and composition change more obviously. Therefore, the effect of organic solvents on the pore structure of activated carbon is masked to a certain extent under the strong action of an alkaline solution.

The micromorphology of activated carbons was characterized by SEM. It can be seen from Fig. 4a through 4h that activated carbons still maintained the structure of plant fibers, the pore structure of poplar fibers provided a reaction site for the activation of KOH, and the external surface of raw materials became rough under the erosion of activators, which shows the role of activators in erosion and pore formation. The organic solvent forms fine channels on the plant fiber, and these channels provide more activation sites for the activator, resulting in more pore structures of the activated carbons. Similarly, the process of hemicellulose removal will leave more pores, which are developed based on the original pores and may explain the covering effect of alkali liquor on organic solvents.

In summary, for poplar, the removal rate of hemicellulose and the yields of activated carbons under alkali extraction were less affected by the pre-extraction of organic solvent. Although organic solvent can optimize the pore structure of activated carbons prepared from untreated raw materials, it led to no noticeable improvement in the preparation of activated carbon from HPR. Therefore, the pre-extraction of organic solvent can be ignored when preparing activated carbon from HPR after alkali extraction.

Fig. 3. N₂ adsorption/desorption isotherms (a) and pore size distributions (b) of activated carbons prepared with P, PK_20-3, P_E, and P_EK_20-3 as carbon precursors

Fig. 4. SEM images of PAC (a, b), PK_20-3AC (c, d), P_EAC (e, f), and P_EK_20-3AC (g, h)

Table 2. Yield and Main Pore Structure Parameters of Activated Carbons Prepared with P, PK_20-3, P_E, and P_EK_20-3 as Carbon Precursors

Effect of Hemicellulose Pre-extraction on Preparation of Activated Carbons

Effect of alkali treatment degree on the dissolution rate of components, the removal rate of Klason-lignin and hemicellulose

The degree of hemicellulose removal was regulated by changing alkali concentration and alkali treatment time. Alkali pretreatment resulted in a quality loss of raw materials, mainly a large amount of hemicellulose and a small amount of lignin. Table 3 shows the dissolution rate, Klason-lignin removal rate, and hemicellulose removal rate under different alkali treatment conditions.

Table 3. Dissolution Rate, Klason-lignin Removal Rate, and Hemicellulose Removal Rate Under Different Alkali Treatment Conditions

Under the set conditions (alkali treatment time was 1 to 3 h, alkali concentration was 4 to 20 wt%), 14.34 to 23.49 wt% of the components were removed from the poplar, including 38.18 to 65.70 wt% of hemicellulose and 6.21 to 8.60 wt% of Klason-lignin. With increased alkali concentration and alkali treatment time, the component dissolution rate and hemicellulose removal rate showed an upward trend, and the growth rate gradually tended to be stable. A large amount of hemicellulose can be removed under 1 h, and more alkali treatment time cannot noticeably improve the dissolution rate (an average increase of 1.48% from 1 h to 3 h) and hemicellulose removal rate (an average increase of 4.03% from 1 h to 3 h). Similarly, the dissolution rate and hemicellulose removal rate are not noticeably improved under high alkali concentrations. For Klason-lignin, the removal rate did not change noticeably with the change of conditions. In conclusion, it is feasible to regulate the removal degree of hemicellulose by changing alkali concentration and alkali treatment time.

Effect of hemicellulose removal degree on the yield and pore structure of activated carbons

In this study, HPR with a hemicellulose removal rate of 38.18 to 65.70 wt% was prepared as carbon precursors to explore the influence of hemicellulose removal on the subsequent preparation of activated carbons. Table 4 shows the yield of poplar-based activated carbons prepared under different removal degrees of hemicellulose. The results show that the yield of PAC is 18.0%, and the yield of PK_x-yAC is 16.0 to 16.9%. Figure 5a and b show the relationship between dissolution rate/hemicellulose removal rate and activated carbon yield. Carbon yield is evaluated in two ways: by evaluating the yield of the carbon production process through comparing the quality of activated carbons with that of carbon precursors (HPR). The other way is to evaluate the yield of the whole process (pretreatment and carbon production) by comparing the quality of activated carbons with that of the original poplar. The results show that with increased dissolution rate, the yield (relative to carbon precursor) slightly increased, and the yield increased 1.4% for every 10% dissolution rate. The yield (relative to poplar) slightly decreased, and the yield decreased 0.6% for every 10% dissolution rate. Furthermore, the change of yield with hemicellulose removal rate is consistent with the dissolution rate because xylan is the main component of dissolution. When 38.18 to 65.70 wt% hemicellulose was selectively extracted from poplar, the yield of final activated carbons only decreased 1.1 to 2.0%, proving that hemicellulose was mainly separated as volatile during the preparation of activated carbons, and its contribution to the final carbon quality was inferior to that of other components. Benoît et al. (2008) also proved this in their research. They carbonized/activated hemicellulose, cellulose, and lignin respectively, then calculated the weight contribution of each component to the quality of chars and activated carbons according to the composition of raw materials. In their results, the weight contribution of hemicellulose to chars was 14.1% to 24.5%, and that of lignin was 55.0% to 79.0%; the weight contribution of hemicellulose to activated carbons was 13.6% to 23.6%, and that of lignin was 53.5% to 78.8% (Benoît et al. 2008). It is obvious that hemicellulose is not the main weight contributor of carbon.

After a large amount of hemicellulose was removed, the lignin content in the HPR increased, so the carbon yield of the residues increased to a certain extent (from 18.0% of PAC to 19.2 to 21.6% of PK_x-yAC). However, although the weight contribution of hemicellulose is lower than that of lignin, hemicellulose can also form part of activated carbon in the preparation process. Therefore, after a large amount of hemicellulose in poplar is extracted, the carbon yield of poplar inevitably decreases. Fortunately, the yield of activated carbons is not noticeably reduced compared with the amount of hemicellulose removed. When poplar is used to prepare activated carbons, a large amount of hemicellulose is converted into highly mixed non-condensable gases, such as CO, CO₂, and CH₄, which are difficult to capture and effectively use. If this unstable hemicellulose is separated, it can be used for many purposes after simple purification (Chen and Lee 2020; Zhao et al. 2020). At present, there is a large number of studies on the extraction and purification of hemicellulose from plant fibers (Scapini et al. 2021; Arzami et al. 2022). From a cost perspective, after hemicellulose is separated, the total amount of activated carbon prepared from raw materials that suffer weight loss does not decrease to a large extent, which has more economic value.

The pore structure of activated carbons has an important influence on the adsorption performance. Figure 6a and b show the N₂ adsorption/desorption isotherms of activated carbons prepared under different hemicellulose removal degrees at 77 K. These isotherms can be classified as Type I, which are the characteristics of microporous materials. It can be found that the N₂ adsorption capacity of PK_x-yAC is always higher than that of PAC. Figure 6c and 6d show that the pore size of activated carbons is mainly distributed in the range of less than 2 nm. The main pore structure parameters of poplar-based activated carbons calculated in Fig. 6a and b are listed in Table 4. Table 4 shows that the specific surface area and pore volume of PK_x-yAC are noticeably increased compared with PAC. S_BET increased from 1401 m²·g^-1 to 2154 to 3066 m²·g^-1 (about 53.7 to 118.8% increase), and Vtotal increased from 0.59 cm³·g^-1 to 0.87 to 1.32 cm³·g^-1 (approximately 47.5 to 123.7% increase). A total of 40.74 wt% hemicellulose in poplar was removed under 4 wt% KOH concentration and 3 h alkali treatment, the activated carbon prepared from HPR gave the highest BET surface area and pore volume, 3066 m²·g^-1 and 1.32 cm³·g^-1, respectively. It can be said that HPR after partial hemicellulose removal are more potential carbon precursors. Yagmur also proved this in his research, where under the self-hydrolysis at 130 to 170 ℃, the specific surface area and pore volume of the final carbon materials was increased by reducing the hemicellulose content of the hardwood materials. Additionally, the activated carbon obtained by self-hydrolysis at 150 ℃ had the highest specific surface area and pore volume, which were 2143 m²·g^-1 and 1.474 cc·g^-1, respectively (Yagmur et al. 2013).

The pores of PK_x-yAC prepared in this study were mainly microporous, but the mesoporous surface area, mesoporous volume, and average pore size of PK_4-1AC and PK_4-3AC were higher than those of other activated carbons. Because 14.34 to 15.46% of the components are dissolved under 4 wt% alkali concentration, and 18.11 to 23.49% of the components are dissolved under 7 to 20wt% alkali concentration, it is speculated that this slight change is caused by the difference in hemicellulose removal. The dissolution of a small number of components is conducive to the formation of mesopores, but with the increase of component dissolution rate, mesopores tend to decrease, and activated carbon is transformed into carbon materials with a higher micropore ratio. It can be said that the pore structures of activated carbons can be changed to a certain extent by changing the removal degree of hemicellulose, which is reflected in the small change of micropores and mesopores. Figure 6c and 6d also show this, the pore size distribution of the activated carbons prepared after 4 wt% KOH treatment is wider, with higher specific surface area, mesoporous surface area, and average pore size, while the pore size distribution of the activated carbons prepared after 7 wt% and 10 wt% KOH treatment is more concentrated, with the highest point concentrated at 0.8 to 1 nm, and the pores are more concentrated under the longer KOH treatment time.

In summary, after the selective pre-extraction of 38.18 to 65.70 wt% hemicellulose from poplar, the final yield of activated carbon only decreased 1.1 to 2.0%, but the pore structure of activated carbon was optimized. The activated carbons prepared from HPR have a high proportion of micropores, and PK_4-3AC has the highest specific surface area and pore volume (3066 m²·g^-1 and 1.32 cm³·g^-1). Additionally, the pore structures of activated carbons can be controlled to some extent by changing the removal degree of hemicellulose.

Fig. 5. The relationship between dissolution rate / hemicellulose removal rate and activated carbon yield

Fig. 6. N₂ adsorption/desorption isotherms (a, b) and pore size distributions (c, d) of activated carbons prepared with PK_x-y as carbon precursors

Table 4. Yield and Main Pore Structure Parameters of Activated Carbons Prepared with Pk_x-Y as Carbon Precursors

Adsorption Performance of Activated Carbons for VOCs

The VOCs adsorption capacity of activated carbon was affected by both the pore structure of activated carbon and the molecular properties of VOCs. Figure 7a through 7c shows the breakthrough curves of toluene adsorption, and Fig. 7d through 7f shows the breakthrough curves of dichloromethane adsorption. The adsorption capacity of toluene and dichloromethane calculated according to Fig. 7 is listed in Table 5. The results show that the adsorption capacity of toluene on the activated carbons prepared after pretreatment (organic solvent extraction, alkali extraction, or both) is greatly increased. The saturated adsorption capacity of PEAC for toluene was increased from 378 m²·g^-1 to 573 m²·g^-1 (about 51.6% increase), compared with PAC, indicating that the extraction of organic solvent improved the adsorption capacity of toluene. In addition, the adsorption capacity of P_EK_20-3AC (521 m²·g^-1) for toluene is only higher than that of PAC (378 m²·g^-1) and lower than that of PK_x-yAC (649 through 733m²·g^-1), which is consistent with the change of specific surface area. The saturated adsorption capacity of PK_x-yAC for toluene was increased from 378m²·g^-1to 649 through 733 m²·g^-1 (about 71.7 through 93.9% increase), compared with PAC, indicating that the adsorption capacity of activated carbons for toluene was greatly improved after the selective dissolution of hemicellulose in poplar wood. It is well known that the adsorption capacity of porous carbon materials to VOCs is mainly determined by micropores (Zhang et al. 2017)， so the excellent adsorption capacity of PK_x-yAC to toluene is mainly caused by its developed microporous structure. This view has also been proved in some studies. It has been reported that micropores provide the main adsorption sites, while mesopores enhance intra-particle diffusion and shorten the adsorption time. Therefore, different pore sizes have different effects on the VOCs adsorption of carbon materials. It is even reported that micropores, especially narrow micropores, play a leading role in the VOCs adsorption of carbon materials. However, the increase of diffusion resistance in narrow pores may lead to a low adsorption rate.

Surprisingly, Fig. 7d through 7f and Table 5 show that the adsorption performance of poplar-based activated carbons for dichloromethane is different from that of toluene or even the opposite. Among all samples, PK_4-3AC has the highest specific surface area, pore volume, and toluene adsorption capacity, but its saturated adsorption capacity for dichloromethane (105 mg·g^-1) is the lowest; PAC has the lowest specific surface area, pore volume, and toluene adsorption capacity, but its saturated adsorption capacity for dichloromethane (184 mg·g^-1) is the highest, and the adsorption capacity of all activated carbons for toluene is noticeably higher than that of dichloromethane. To study the relationship between the adsorption capacity of toluene/dichloromethane and the pore structures of activated carbons, this study tried to explain this phenomenon from the perspective of the pore distribution of activated carbon and molecular dynamics diameter of VOCs. It is well known that the molecular dynamic diameters of toluene and dichloromethane are 0.67 nm and 0.33 nm, respectively. The relationship between pore structure parameters (S_BET, S_micro, S_ultra) and adsorption capacity of toluene / dichloromethane was plotted in Fig. 8 by linear fitting. The results show that the adsorption capacity of toluene has a good correlation with Smicro and Vmicro (R²= 0.9547, R²= 0.9595), and the adsorption capacity of dichloromethane is more closely related to Sultra and Vultra (R²= 0.4804, R²= 0.5026), which indicates that the adsorption of toluene is mainly determined by micropores, and the adsorption of dichloromethane is more closely related to micropores in these samples. It should be pointed out here that because these samples are activated carbon with a high proportion of micropores, the adsorption amount of toluene is also closely related to the total specific surface area and total pore volume. However, it is undeniable that there are still many other factors affecting adsorption in the actual adsorption process (Zhu et al. 2018). It can be concluded that the reason why PAC has a higher adsorption capacity of dichloromethane is that it has a superior ultramicroporous structure. Moreover, the adsorption capacity of activated carbons for dichloromethane is less than that of toluene because the volume of the ultramicropore is less than that of the micropore. This is consistent with previous studies, that is, when the main pore diameter of the porous adsorbent is 1 to 3 times the molecular dynamics diameter of the target VOCs, the adsorption capacity increases noticeably (Lozano-Castello et al. 2002; Zhang et al. 2019a). In this study, the selective extraction of hemicellulose has brought a more developed microporous structure to activated carbons, and the pores changed from ultramicropores (less than 0.6 nm) to larger micropores (0.6 through 2 nm). In addition, different pore distribution leads to the different adsorption capacity of activated carbons for VOCs with different molecular diameters. The best adsorption occurs at the place where the pore size matches the size of the adsorbate. Therefore, ultra-micropores are more suitable for the adsorption of dichloromethane with smaller molecular diameters.

It is undeniable that the poplar-based activated carbons prepared in this study has a long penetration time and satisfactory adsorption capacity for toluene and dichloromethane. Among them, the adsorption capacity of PK_4-3AC for toluene is up to 733 mg·g^-1, higher than that of commercial activated carbons and other carbon materials based on the traditional carbonization /activation process, and the adsorption capacity of PAC for dichloromethane is up to 184 mg·g^-1. Table 6 shows the adsorption capacity of other carbon materials for toluene and dichloromethane, and compares it with the activated carbons obtained in this study.

The Yoon-Nelson model is used to fit the adsorption data, and the fitting parameters are shown in Table 7. The results show that the Yoon-Nelson model has a good fitting effect on the toluene/dichloromethane breakthrough curves of poplar-based activated carbons, indicating that the Yoon-Nelson model can well predict the VOCs adsorption of activated carbons. In contrast, the KYN of dichloromethane is noticeably higher than that of toluene, indicating that the mass transfer resistance of dichloromethane in the activated carbons is lower and it can penetrate the adsorbents faster, which is related to the smaller molecular dynamics diameter of dichloromethane.

In conclusion, the adsorption capacities of toluene and dichloromethane are closely related to micropores (< 2 nm) and ultramicropores (< 0.6 nm) of activated carbons, respectively. PK_x-yAC is a toluene adsorbent with excellent performance because of its high proportion of microporous structure, and PAC has more advantages in the adsorption of dichloromethane because of its more abundant ultramicropores. Furthermore, the pre-extraction of hemicellulose greatly improves the VOCs adsorption capacity of activated carbons by increasing the micropores.

Fig. 7. Breakthrough curves of activated carbons for toluene (a through c) / dichloromethane (d through f)

Table 5. Adsorption Capacity of VOCs on Activated Carbons

Table 6. Adsorption Capacity of VOCs on Different Carbon Materials

Table 7. Y-N Model Fitting Parameters of Activated Carbons

Fig. 8. The relationship between pore structure parameters and adsorption capacity of toluene / dichloromethane

CONCLUSIONS

The pre-extraction of hemicellulose could noticeably increase the microporous surface area and microporous pore volume of poplar-based activated carbons, thus greatly enhancing the adsorption capacity of VOCs.
The removal rate of hemicellulose and the yields of activated carbons under alkali extraction were less affected by the pre-extraction of organic solvent.
After the selective pre-extraction of 38.18 to 65.70 wt% hemicellulose from poplar, the final yield of activated carbons only decreased 1.1 to 2.0%, but the pore structure of activated carbons was greatly improved. A total of 40.74 wt% hemicellulose in poplar was removed under 4 wt% alkali concentration and 3 h KOH treatment, the activated carbon prepared from HPR gave the highest BET surface area and pore volume; 3066 m²·g^-1 and 1.32 cm³·g^-1, respectively. The pore structures of activated carbons can be controlled to some extent by changing the removal degree of hemicellulose.
The adsorption results of VOCs show that because of the rich microporous and ultramicroporous structures, the adsorption capacity of PK_4-3AC for toluene is 733 mg·g^-1, and that of PAC for dichloromethane is 184 mg·g^-1, which are better than the previously reported adsorbents. Furthermore, the adsorption capacities of toluene and dichloromethane are closely related to micropores (< 2 nm) and ultramicropores (< 0.6 nm) of activated carbons, respectively.

ACKNOWLEDGMENTS

The authors greatly acknowledge the support of the Natural Sciences Foundation of China (No. 32271807).

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Article submitted: December 6, 2022; Peer review completed: January 21, 2023; Revised version received; February 13, 2023; Accepted: February 16, 2023; Published: February 27, 2023.

DOI: 10.15376/biores.18.2.2874-2896