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Ma, L., Li, J., and Ma, X. (2019). "Preparation and adsorption of CO2 and H2 by activated carbon hollow fibers from rubber wood (Hevea brasiliensis)," BioRes. 14(4), 9755-9765.

Abstract

A series of activated carbon hollow fibers doped with charcoal powder (WC-ACHFs) were prepared from wood waste with great potential for application in the gas adsorption of CO2 and H2. The hydrogen storage of WC-ACHF-1.0% increased approximately 10.5% more than that of activated carbon hollow fibers (ACHF), and the highest hydrogen uptake reached 4.51 wt% at 77 K and 100 bar. Regarding the CO2 adsorption, the highest adsorption amount reached 7.13 mmol g-1 and the mass content was 31.35 at 273 K, which was 49.8% higher than the sample without doping. In addition, the multiple heteroatoms (N, P) from wood waste liquefaction had a synergistic effect on the gas adsorption properties of WC-ACHFs. These results showed that a facile method was promising for the preparation of wood-derived activated carbon hollow fibers from forestry and agricultural residues in the application of gas adsorption.


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Preparation and Adsorption of CO2 and H2 by Activated Carbon Hollow Fibers from Rubber Wood (Hevea brasiliensis)

Liyan Ma,a,b Jianing Li,*,a and Xiaojun Ma *

A series of activated carbon hollow fibers doped with charcoal powder (WC-ACHFs) were prepared from wood waste with great potential for application in the gas adsorption of CO2 and H2. The hydrogen storage of WC-ACHF-1.0% increased approximately 10.5% more than that of activated carbon hollow fibers (ACHF), and the highest hydrogen uptake reached 4.51 wt% at 77 K and 100 bar. Regarding the CO2 adsorption, the highest adsorption amount reached 7.13 mmol g-1 and the mass content was 31.35 at 273 K, which was 49.8% higher than the sample without doping. In addition, the multiple heteroatoms (N, P) from wood waste liquefaction had a synergistic effect on the gas adsorption properties of WC-ACHFs. These results showed that a facile method was promising for the preparation of wood-derived activated carbon hollow fibers from forestry and agricultural residues in the application of gas adsorption.

Keywords: Wood waste; Activated carbon fibers; Hydrogen storage; CO2 adsorption

Contact information: a: Ministry of Agriculture Key Laboratory of Biology and Genetic Resource Utilization of Rubber Tree/State Key Laboratory Breeding Base of Cultivation & Physiology for Tropical Crops, Rubber Research Institute, Chinese Academy of Tropical Agricultural Sciences, Danzhou 571737, China; b: Department of Wood Science and Technology, Tianjin University of Science & Technology, Tianjin 300222, China; *Corresponding author: ljn206@163.com;mxj75@tust.edu.cn

INTRODUCTION

With the continuous consumption of fossil energy, global carbon emissions have dramatically increased, resulting in the greenhouse effect, haze, and other environmental hazards. The development of carbon capture and storage (CCS) technologies, as well as sustainable, efficient, and clean energy sources and carriers, including supercapacitors, hydrogen storage and bioenergy, has become extremely urgent (Chen et al. 2019).

Porous carbon materials have been proposed for a long time as promising CO2 and H2 adsorbents, which is due to their light weight, high specific surface area, complete reversibility, low cost, and availability (Ramesh et al. 2015; Sangeetha and Selvakumar 2018). For CO2 adsorption, the increase of specific surface area, the highly porous structure, and the enhancement of the interaction between the gas and carbon material by doping heteroatom are of great importance for the adsorption of carbon materials. However, regarding the hydrogen storage, under weak van der Waals forces, narrower pores result in stronger hydrogen molecules-to-surface interactions, and higher heat of the adsorption and the adsorption capacity. Both theoretical studies and experimental research have suggested that 0.6-nm-wide pore size is ideal for hydrogen storage at 77 K due to the overlap of potential fields from the pore walls on both sides (Ramesh et al. 2015; Sethia and Sayari 2016). Hence, ultramicropore carbon materials have important significance in hydrogen storage due to their enhanced adsorption heat (Huang et al. 2018a).

Activated carbon hollow fibers (ACHF) are a kind of non-crystalline porous carbon obtained by the pyrolysis of carbon materials. Carbon generates more pores and changes its volume, shape, and size through activation steps (Wróbel-Iwaniec et al. 2015). Oxidizing gases react preferentially with amorphous carbon and carbon at lattice defects in the carbide during physical activation. Carbon is burned out, volatilized, and etched continuously, and the pores are thus formed, thereby increasing the pore structure of the activated carbon (Huang and Zhao 2016; Bader and Ouederni 2017). Therefore, some researchers have studied one-dimensional wood-based carbon fibers with controllable carbonaceous architecture and pore size distribution as gas adsorption materials.

In this work, the authors studied a facile approach to fabricate activated carbon hollow fibers doped with charcoal powder (WC-ACHFs) from rubber wood waste via liquefaction, melt spinning, and steam activation methods. The effects of wood charcoal on the morphology, the specific surface area, the structure, and the composition of ACHF were investigated. This work exhibits a pathway to design economic and sustainable gas adsorption materials from forestry and agricultural residues.

EXPERIMENTAL

Materials

Rubber wood (Hevea brasiliensis) waste was supplied by a wood furniture factory (Tianjin Dongsheng Furniture Co., Ltd, Tianjin, China). Commercial wood charcoal powder (WC) was purchased from Donghai Carbon Corporation, Tianjin, China. Phenol, hexamethylenetetramine, hydrochloric acid (HCl, 37.9 wt% in water), formaldehyde (CH2O, 37.9 wt% in water), methanol (CH3OH, 99.9 wt% in water) and phosphoric acid (H3PO4, 37.9 wt% in water) were purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd. (Tianjin, China). All chemicals were supplied as analytical grade and were used without further purification.

Preparation of activated carbon fibers with hollow structure by doping with charcoal powder

The ACHF was prepared from rubber wood. First, 20 g of wood (0.2 < diameter of particle < 0.8 mm), 120 g of phenol, as well as 9.6 g of H3PO4 were mixed in a certain proportion and heated with stirring from room temperature to 160 °C to prepare wood liquefaction through oil bath reaction (at 160 ℃ for 2 h). Then, the different proportions of charcoal powder (0.5 wt%, 1 wt%, 2 wt%, and 3wt%, based on wood liquefaction) were added to the mixed solution of wood liquefaction and 10 wt% hexamethylenetetramine (on the weight of liquefaction solution) to prepare the spinning solution. The primary fibers were further obtained through melt spinning. The primary fibers were semi-cured in a mixture solution of 18.95% HCl and 18.95% CH2O. Then, the semi-cured fibers were dipped into 18.95% methanol solution at 50 °C for 1 h. Subsequently, the semi-cured fibers were dipped into the same solution described in the first step to obtain the hollow fibers. The five groups of hollow fibers were placed in a tube furnace. Under the protection of the flow rate of 100 mL/min N2, the heating rate at 5 ℃/min was uniformly heated to the set activation temperature (800 °C), held isothermally for 1 h at a steam flow of 10 g/min, and after that cooled to 25 ℃ to obtain the activated carbon hollow fibers doped with charcoal powder (Scheme 1).

Scheme 1. Schematic illustration of WC-ACHFs fabrication

Methods

The morphology and energy dispersive spectroscopy (EDS) mapping were obtained via scanning electron microscopy (SEM, model SU1510; Hitachi, Tokyo, Japan).

The surface analysis was achieved through a Kratos Axis UltraDLD multi-technique X-ray photoelectron spectrometer (XPS) (ESCALAB 250Xi; Thermo Fisher Scientific Inc., Waltham, MA, USA).

The N2 adsorption–desorption isotherms, the surface area, and the porosity of the samples were obtained at 77 K using an ASAP 2020 automatic apparatus (Micromeritics Instrument Co., Norcross, GA, USA). The micropore area (Smicro) and micropore volume (Vmicro) were obtained by the t-plot method. The mesopore area (Smeso) and mesopore volume (Vmeso) were calculated by the Barrett–Joyner–Halenda method.

Hydrogen adsorption and desorption studies were obtained in the pressure ranges 0 to 100 bar and at 77 K using a high-pressure Automated Sievert’s apparatus from Advanced Material Corporation, Norcross, GA, USA (Ramesh et al. 2015).

The CO2 adsorption analysis was measured at 273 K, and the adsorption isotherms were determined by dosing CO2 volumes of 0.2 standard temperature and pressure (STP) cmg-1 up to a P/P0 of 10-4, a P/P0 table was used to construct the CO2 adsorption isotherm up to a final P/P0 of 1.

RESULTS AND DISCUSSION

Morphology and Compositions

The morphology of the WC-ACHFs was observed using SEM, as shown in Fig. 1. Compared with the ACHF (Fig. 1a), the WC-ACHFs (Fig. 1b) exhibited abundant pore structure, and the surface of WC-ACHF-1.0% displayed a large amount of pores. This was due to the formation of a large number of agglomerated charcoal particles in the wall of ACHF, resulting in the formation of an ACHF surface that was not compact and the doping of charcoal powder with the main ingredient of carbon element provided more active sites for activation reaction as well as formed more pores during activation reaction involving the following several steps (1 through 3) (Fig. 1c). Therefore, the concentration range of charcoal powder used in this study had a remarkable effect on the surface morphology. In addition, the WC-ACHFs showed a rich porous structure on the inner and outer surface of the hollow fibers, which greatly increased its specific surface area. The EDX spectrum analysis indicated that some heteroatoms of O, N, and P were well-distributed in various samples, which may have been derived from chemical substances, such as hexamethylenetetramine and phosphoric acid, during wood liquefaction (Ma et al. 2014; Barman and Nanda 2018).

Fig. 1. (a) SEM image of ACHF; (b), (c), and (d) SEM images of WC-ACHF-1%; bottom row is the EDX spectra of fig.1d

The XPS of the prepared WC-ACHF samples, shown in Fig. 2a, clearly indicated the existence of C, N, O, and P in the sample. The C1s signals of all WC-ACHF samples exhibited an asymmetric tailing, which was the contribution of the intrinsic asymmetry of the graphite peaks (Fig. 2b) and an increase in the relative content of carbon bonded to oxygen-containing functions after the increasing of charcoal powder proportion (Table 1) (Kostoglou et al. 2017). The N1s spectrum (Fig. 2c) shows peaks at binding energies 396.41, 399.8, and 405.4 eV corresponding to nitride or aromatic amine, imide, and pyridine-nitrogen-oxide, respectively (Yang et al. 2019). The deconvolution of the O1s region spectrum (Fig. 2d) produced three groups of peaks at 530.8 eV, 532.1 eV, and 533.7 eV (Zhou et al. 2018). These peaks correspond to hydroxyl, carbonyl oxygen in amide, and carboxyl (Huang et al. 2018b; Guo et al. 2019). According to relevant reports that the multiple heteroatoms (N, O, and P) from wood liquefaction have a synergistic effect on hydrogen storage properties of ACHF, the existence of polar molecules is conducive to enhancing the chemisorption ability of nitrogen (He et al. 2018; Jeong et al. 2019). Nitrogen doping was beneficial to the adsorption of hydrogen at low pressure, but a higher nitrogen content at high pressure was not conducive to the adsorption of hydrogen. This was because nitrogen doping reduced the interaction between the hydrogen and pore structure of the samples, thereby affecting the overall nitrogen adsorption capacity of the material (Sethia and Sayari 2016). The hydrogen storage of WC-ACHF-3.0% with a nitrogen content of 1.04% (compared with WC-ACHF-0%, WC-ACHF-0.5%, WC-ACHF-1.0%, and WC-ACHF-2.0% with nitrogen contents of 0%, 0.77%, 1.06%, and 0.79%, respectively) at high pressure was affected by these factors.

Fig. 2. XPS spectra of WC-ACHF: (a) XPS analysis on all elements; (b) XPS analysis on C1s of WC-ACHFs; (c) XPS analysis on N1s; and (d) XPS analysis on O1s

Table 1. Results of the C1s Regions, Values Given in % of Total Intensity

All of the WC-ACHF samples possessed the ultramicroporosity (< 0.7 nm), and the maximum pore volume was distributed from 0.5 to 1 nm (Sawant et al. 2017). Clearly, WC-ACHF-1.0% with uniform ultramicroporosity possessed a higher pore volume compared with the others. The specific surface area and total pore volume of the WC-ACHF-1.0% reached a maximum at 1902 m2 g-1 and 0.860 cm3 g-1, which was attributed to the agglomeration of charcoal powder in the spinning solution and the less compact surface of ACHF produced by the spinning step, which provided more reaction sites for activation reaction and formed an abundant amount of pores on the wall of the ACHF. Nevertheless, the excessive increase of the content of charcoal powder and the decreased specific surface area (Fig. 3) was due to excessive charcoal powder adsorbing free +CH2OH, preventing the reaction of liquefied products with it from forming a network structure. This caused the primary fibers to become more compact and to exhibit pore-forming difficulty during activation reaction.

Fig. 3. Pore size distribution curves of all series (inset: Pore structure parameters of activated carbon hollow fibers)

Hydrogen Adsorption Performance

Figure 4a shows the hydrogen adsorption isotherms of all samples at 77 K and 1 bar pressure, which showed that the hydrogen adsorption was entirely reversible. The inset figure summarizes the H2 storage capacities of all sample materials, which shows the H2 storage ranging from 2.06 to 2.24 wt% at 77 K and 1 bar. These results were competitive with those of the recently reported hydrogen adsorbents fabricated on activated carbon (1.4 to 1.97wt% at 77 K and 1 bar) (Arshad et al. 2016; Rambau et al. 2018). Figure 4b exhibits the adsorption storage of hydrogen of all samples at 77 K and pressures from 0 to 100 bar with various charcoal powder proportions. The inset figure exhibits the H2 adsorption storage ranging from 4.08 to 4.51 wt% at 77 K and 100 bar. With an increased charcoal powder content, the hydrogen storage first increased and then decreased. The hydrogen storage capacity of WC-ACHF-1.0% was the highest, which was roughly the same as that of Fig. 4a. In addition, the hydrogen storage capacities at 77 K and 100 bar were investigated with respect to several textural properties. The obtained ACHF data indicated that the micropore volume, specific surface area, and total pore volume had no remarkable effect on the hydrogen absorption at high and low pressure (Fig. 4c through 4e). Even at high pressure the hydrogen storage capacity was less than WA-0.5. As reported in a previous study (Lee et al. 2017), this revealed that the hydrogen adsorption storage was not completely based on the pore volume and specific surface area. This indicated that pore size distribution played an important role in determining the hydrogen adsorption performance of the carbon fibers. The porosity of WC-ACHFs was mainly composed of two sets of ultramicropores with diameters of 0.50 and 0.56 nm (Fig. 3), and the porosity of ACHF was mainly 0.68 nm ultramicropores. In the samples prepared, WA-1.0 had the highest hydrogen storage capacity, which was attributed to the large ultramicropore volume from the pore size centered at 0.56 nm. Both theoretical studies and experimental results indicated that the pore size width of 0.56 nm was suitable for hydrogen adsorption storage (Sethia and Sayari 2016).

Fig. 4. (a) 77 K/H2 adsorption isotherms at 1 bar; (b) 77 K/H2 adsorption isotherms at 100 bar; (c) through (e) The fitting curves of micropore volume, specific surface area, total pore volume, and hydrogen storage of WC-ACHFs at 77 K and 100 bar

Under weak van der Waals forces, narrower pores resulted in stronger hydrogen molecules-to-surface interactions, higher heat of the adsorption and the adsorption capacity, and hence shows a great remarkable influence on the hydrogen adsorption storage of the carbon fibers (Sevilla and Mokaya 2014; Sethia and Sayari 2016). With the enlargement of the pore size, the interaction energy and hydrogen uptake decreased.

CO2 Adsorption Performance