Preparing high surface area porous carbon from biomass by carbonization in a molten salt medium

Abstract

The preparation of porous carbon with a high surface area from biomass is important for its practical application. In the present study, peanut shells were transformed into porous carbon through a simple ZnCl₂-molten salt synthesis (MSS) process. The carbonization and two activation processes could be completed together in one step, and carbonization time and temperature were reduced significantly because of the favorable flux environment for carbonization provided by the molten ZnCl₂ medium. The properties of peanut shell-activated carbon (PAC) were characterized by XRD, TG-DSC, SEM, TEM, FT-IR spectra and BET isotherms. The results showed that the as-prepared PAC was amorphous and had a hierarchical porous structure with a high surface area of 1642 m² g⁻¹. Some functional groups were retained on the surface of the PAC and provided more adsorption sites. When the prepared PAC was used as an adsorbent to remove methylene blue (MB) dye from an aqueous solution, it exhibited superior adsorption capacity as high as 876 mg g⁻¹, which indicates that the PAC from peanut shells can be used as a low-cost and effective adsorbent for water purification.

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1. Introduction

The consumption of activated carbon (AC) is continuously increasing with rapid economic development because activated carbon is extensively used in various fields such as wastewater treatment, air purification, hydrogen storage, gas separation, heterogeneous catalysis, and composite materials for battery electrodes.^1–4 For the increasingly serious water resource pollution problems, activated carbon has been considered as one of the most effective absorbents for purifying water, due to its large specific surface area and highly developed porous structure.^5–8 However, its large scale usage is restricted due to the relatively high cost of generation,^9,10 and it is important to find a suitable method to produce AC efficiently and economically.

To reduce the cost of AC, cheap and readily available precursors, such as agricultural and biomass by-products or waste, have been tried sequentially.¹¹ Many low-cost carbon precursors have been reported in the literature, for example waste litchi shells, bamboo waste, macadamia nut endocarp, corn cobs, waste wood shavings, grape stalks, peanut shells, coffee waste residue and rice husks.^12–19 Most of the activated carbons from biomass are produced by two procedures, carbonization and activation. Basically, the activations have two different processes: physical activation and chemical activation. The physical activation is a two-step process. In the first step, carbonaceous material undergoes carbonization below 800 °C in an inert atmosphere. Then, the resulting char is activated at a higher temperature (800–1000 °C) in the presence of suitable gases such as carbon dioxide, steam, air or their mixtures.²⁰ In contrast, the chemical activation is a one-step process, involving the impregnation of a carbonaceous material with an activation agent and heat treatment at a temperature of 450–900 °C under an inert atmosphere.²¹ Chemical activation is better than physical activation due to its simpler production process and higher yields.²² However, the procedure of chemical activation is very complicated and the adsorption capacity of the prepared AC is relatively low.²³ Consequently, searching for a simple method to prepare low-cost and high adsorption capacity AC becomes more important.

The molten salt synthesis (MSS) method is based on the use of a salt or multi-salts with a low melting point as the molten medium for the desired synthesis. The most common salts are typically chlorides, sulfates, carbonates, and hydroxides owing to their availability and their low costs. In the molten salt process, the molten salt provides a favorable flux environment (from 100 °C to over 1000 °C) with the solubility and diffusivity required for solid-phase reactions, which can greatly reduce the synthesis temperature. This MSS process has indeed already succeeded in the synthesis of various nano-powders such as ferrites, titanates, niobates, mullite, aluminum borate, and wollastonite.²⁴ Porous carbon materials with high specific surface areas were prepared by similar MSS processes using glucose or ionic liquids as source materials.^25,26 To the best of our knowledge, little attention has been paid to the synthesis of activated carbon from biomass by the MSS method.

According to the mechanism of MSS, we selected ZnCl₂ (melting point 290 °C) as a molten salt system to convert peanut shells to a porous carbon material by a molten salt carbonization process at 480 °C under a nitrogen flow. The common activating agents during the carbonization process are ZnCl₂, H₃PO₄, KOH and K₂CO₃. ZnCl₂ is one of the most widely used agents. ZnCl₂ can act as a dehydrating agent for promoting the decomposition of carbonaceous material during the pyrolysis process, restrict the formation of tar, and increase the carbon yield.⁴ Furthermore, the molten salt of ZnCl₂ with its low melting point of 290 °C can provide a favorable flux environment at a low temperature with the solubility and diffusivity required for solid-phase reactions, which can greatly reduce the carbonization temperature.²⁷ Compared with the conventional processes, MSS can complete the carbonization and two activation processes within a single step; thus, the complexity of the process and the reaction time for the preparation of peanut shell activated carbon (PAC) can be significantly reduced. The as-prepared PAC is amorphous and has a porous structure with a high surface area of 1642 m² g⁻¹. When the PAC was used as an adsorbent to remove methylene blue from an aqueous solution, it exhibited an excellent adsorption capacity of 876 mg g⁻¹.

2. Materials and methods

2.1 Materials

Peanut shells, low-value agricultural by-products, were collected from a local market in Kaifeng (Henan Province, China) and used as the precursor materials for the production of activated carbon. The adsorbate of methylene blue (MB) was purchased from Kermel chemical reagent Co., Ltd (Tianjin, China). Stock solutions of MB were prepared by dissolving accurately weighed samples of dye in distilled water to obtain a concentration of 1000 mg L⁻¹; the required concentrations of solutions in subsequent experiments were obtained by successive dilutions. Zinc chloride (≥98.0%) was acquired from Tianjin Dengke chemical reagent Co., Ltd (Tianjin, China). Ethyl alcohol (≥99.7% purity) was acquired from Fengchuan Chemical Reagent Co., Ltd (Tianjin, China) and silver nitrate (≥99.8%) was acquired from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the chemicals were used without further purification. Water used in the experiments was deionized water.

2.2 Preparation

Peanut shells were washed in running tap water to remove dirt particles adhering on their surface and then washed with distilled water several times. The washed peanut shells were transferred to an oven, dried at 110 °C for 24 h, crushed and sieved to pass through an 80-mesh screen (US standard) and retained on a 100-mesh screen. The crushed peanut shells were mixed with ZnCl₂ in a ratio of 1 : 5 by mass at room temperature in air. The molten salt medium with the ratio of 1 : 5 for biomass to ZnCl₂ could provide a sufficient melting liquid phase and embed the biomass completely. Compared with multi-phase molten salt systems used in the literature,^25,26,28 the single-phase molten salt of ZnCl₂ is more convenient in operation. Then, the resulting materials were carbonized in a horizontal tube furnace (NBD OTF-1200X). The carbonization procedure is as follows: samples (6 g) were placed into the reactor and heated from room temperature to 200 °C (±5) for 30 min and then at 480 °C (±5 °C) for 90 min under nitrogen flow. The activated samples were cooled inside the tube furnace in the presence of nitrogen flow. The samples were washed with deionized water several times to make sure no Cl⁻ could be detected in the filtrate. The method to detect existence of Cl⁻ in the filtrate was based on the reaction between Cl⁻ and Ag⁺. There was no white precipitate after a few drops of AgNO₃ (0.1 mol L⁻¹) were dropped into the filtrate, which proved that no Cl⁻ was present in the filtrate. Finally, the samples were dried at 110 °C for 24 h.

2.3 Characterization

The morphologies and microstructures of the samples were characterized by transmission electron microscopy (Tecnai G2 20, FEI) and scanning electron microscopy (JSM 7500F, JEOL). The elements in the sample were analyzed by energy dispersive X-ray spectroscopy (EDS, Bruker, XFlash Detector 5030). The specific surface area of the PAC was measured based on N₂ adsorption isotherms at 77 K using a surface area analyzer (NOVA4200e, Quantachrome). The pore size distribution was obtained by the DFT method adopting a slit pore model. The pyrolysis process was investigated by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) (Netzsch STA409PC). X-ray diffraction patterns were recorded using an X-ray diffractometer (D8 ADVANCE, Bruker) with Cu Kα radiation. The functional groups of the PAC were characterized by Fourier-transform infrared spectroscopy (IR300, Thermo Nicolet).

2.4 Adsorption experiments

In equilibrium adsorption experiments, the effects of adsorbent dose, contact time, initial MB concentration and temperature were investigated. For each experiment, the given dose of PAC and 50 mL of MB solution at a certain concentration were placed into a 50 mL conical flask and sealed. Then, the samples were agitated on a thermostatic shaker with a shaking of 140 rpm at a given temperature. After adsorption, the MB solution was centrifuged for 5 min at a rotation speed of 8000 rpm to obtain a supernatant liquid and its absorbance was determined with a UV/visible spectrophotometer (Shimadzu, UV-2450) at the maximum wavelength (λ = 663.5 nm). Subsequently, the concentration of dye was calculated from a calibration curve. All the experiments were carried out in bipartite. The removal efficiency (%) and the amount of MB adsorbed at time t (q_t, mg g⁻¹) and that at equilibrium (q_e, mg g⁻¹) were calculated by using the following equations, respectively:

Removal (%) = (C₀ − C_e)/C₀ × 100 (1)

q_t = V × (C₀ − C_t)/M (2)

q_e = V × (C₀ − C_e)/M (3)

where C₀, C_t and C_e (mg L⁻¹) are the initial, t-time and equilibrium concentrations of MB, respectively; V (L) is the volume of solution and M (g) is the weight of PAC.

3. Results and discussion

3.1 Formation mechanism of PAC

The preparation of PAC involves three processes, as shown in Scheme 1: crushing of peanut shells, carbonization in a molten salt medium and removal of molten salt. To elucidate formation mechanism of the PAC, the pyrolysis behaviors of peanut shells and MSS precursor were investigated by TG-DSC measurements in a N₂ atmosphere, as shown in Fig. 1. For the peanut shells in Fig. 1a, the graph shows that the initial weight loss of the process occurred at 96 °C, which is due to the loss of free and bound water present in the peanut shell matrix. The maximum weight loss occurred in the range of 220–360 °C and corresponds to the rapid decomposition of hemicellulose to volatile matters and tars. The final weight loss occurred in the range of 370–470 °C, which is attributed to the carbonization process of cellulose and lignin. These results are consistent with experimental data in the literature.²⁹

Scheme 1 Schematic illustration of PAC formation mechanism.

Fig. 1 TG-DSC curves of the peanut shell (a) and the peanut shell mixed with MSS (b).

For the MSS precursor in Fig. 1b, the weight loss and decomposition peaks (300–450 °C) of hemicellulose, cellulose and lignin still exist during the decomposition process; however, the weight loss and exothermic peaks become weakened in comparison with those of the peanut shell, which can be attributed to the high content of ZnCl₂ (1 : 5 ratio for ZnCl₂ and peanut shells) in the precursor. Moreover, it can be noted that there is an additional endothermic peak at around 290 °C, which indicates that ZnCl₂ melted at this temperature and the solid precursor transformed into the molten salt system.³⁰ When the temperature was around 500 °C, ZnCl₂ began to evaporate from the molten salt system and the evaporation of ZnCl₂ caused the major mass loss (about 63%) in this process. In the carbonization process of the peanut shell, the molten salt plays an important role for PAC formation. The use of ZnCl₂ could promote the extraction of water molecules from lignocellulosic structures of the precursors at a high temperature. When hemicellulose, cellulose and lignin decompose at 300–450 °C, the low-molecular-weight volatile compounds escape and leave space or cavities in the matrix; in the meantime, ZnCl₂ melting above 300 °C diffuses to occupy the cavities. The porous structure in the PAC can be achieved by water rinsing to remove ZnCl₂ in the cavities.³¹

3.2 Characterization of PAC

Fig. 2 shows the X-ray diffraction pattern of the as-synthesized porous carbon sample prepared using a molten salt. The characteristic peaks of activated carbon positioned at 2θ = 24° and 42° correspond to the reflections of the (002) and (100) planes, respectively. The XRD pattern shows that the peanut shell activated carbon (PAC) is in an amorphous state.³²

Fig. 2 XRD pattern of the porous carbon sample prepared by the molten salt method.

SEM was used to investigate the structure and morphology of the product, as shown in Fig. 3a and b. Fig. 3a exhibits the general morphology of the sample, showing that the sample is composed of porous particles in various sizes. As can be seen in the magnified image in Fig. 3b, surface of the PAC was full of irregular cavities, indicating that porosity was developed by the activation agent. The cavities can serve as the main channels to connect the inner surface of the PAC through the micropores.

Fig. 3 SEM and TEM images of the as-prepared PAC. (a) Low-magnification SEM, (b) high-magnification SEM, (c) low-magnification TEM and (d) high-magnification TEM.

The structural characterization in further detail was achieved using TEM, as shown in Fig. 3c and d. According to the TEM images shown in Fig. 3c, the produced activated carbon is made of several overlapping layers. The edge of the carbon sheet is uneven, indicating a porous structure for the PAC. No lattice fringe is found in the magnified image of the edge (Fig. 3d), agreeing well with the XRD result that the carbon is amorphous. Furthermore, the EDS analysis verified the absence of both Zn and Cl elements in the porous carbon, which indicated that ZnCl₂ had been removed from the sample (ESI).

Porosity of the as-prepared products was further demonstrated by BET adsorption isotherm measurements performed at 77 K using liquid nitrogen. Fig. 4a shows the nitrogen adsorption–desorption isotherms of the obtained samples. The samples prepared in this study exhibit Type IV isotherms, evidenced by a hysteresis loop that is a characteristic of mesoporosity. The BET specific surface area and the pore volume of the PAC were calculated to be 1642.72 m² g⁻¹ and 0.42 cm³ g⁻¹, respectively. In contrast to other AC synthesized by related techniques, the PAC has a higher specific surface area. For example, the AC from macadamia nut endocarp using microwave-induced phosphoric acid and zinc chloride activation agents showed a S_BET of 290.6 m² g⁻¹ and 377.1 m² g⁻¹, respectively.³³ The AC prepared from the peanut shells under conventional pyrolysis had a S_BET value of 95.51 m² g⁻¹.³⁴ Fig. 4b shows the DFT pore distribution curve. It can be noted that there are three distinct distributions (plateaus), which can be attributed to the presence of mesopores with pore radii of 9 nm, 13 nm and 22 nm, respectively.³⁵

Fig. 4 (a) Nitrogen adsorption–desorption isotherms of PAC. (b) The corresponding pore size distribution of PAC using DFT method.

Fig. 5 shows the FT-IR spectra of PAC prepared at pyrolysis temperatures of 230 °C and 480 °C. Compared with the PAC pyrolyzed at 230 °C, most of the functional groups remained in the PAC pyrolyzed at 480 °C. For the PAC sample at 480 °C, the presence of the bands at 3430 and 1616 cm⁻¹ can be attributed to N–H stretching vibration and N–H in-plane bending vibration, respectively. The peak observed at 2925 cm⁻¹ is assigned to the asymmetric C–H stretching vibration of CH₂. Absorption at the wavenumber of 2380 cm⁻¹ can be attributed to CO₂ asymmetric stretching vibration. The band observed at 1516 cm⁻¹ corresponds to the asymmetric stretching vibration of N=O. The two peaks centered at 1460 and 1386 cm⁻¹ are due to CH₂ bending vibration and CH₃ bending vibrations. Moreover, the band at 1251 and 1190 cm⁻¹ are attributed to C–C skeletal vibration, and the signal at 1083 cm⁻¹ is assigned to C–N stretching vibration. Finally, the band at 820 cm⁻¹ is assigned to N–H out-of-plane bending vibration. The FTIR spectra suggest that MB might be adsorbed on PAC through an electrostatic interaction between the cationic MB and the electron lone pairs on the amino groups in the PAC.³⁶

Fig. 5 FT-IR spectra of PAC.

3.3 Adsorption properties

To elucidate the adsorption properties of PAC, the adsorption behavior of methylene blue (MB) from an aqueous solution onto PAC was investigated under different conditions such as varying adsorbent dose, contact time, temperature and initial concentration.

Adsorbent dose is an important parameter for adsorption process. The effect of adsorbent dose was investigated using different amounts of adsorbent (from 0.01 to 0.06 g) in 50 mL of 100 mg L⁻¹ MB aqueous solution at 298 K for 5 h. The result shown in Fig. 6 reveals that the removal efficiency increases from 63.98% to 96.97% when the adsorbent dose increases from 0.01 to 0.06 g, which is attributed to the fact that more adsorbents can provide more surface area and adsorption sites. However, the adsorption capacity decreases from 458.39 to 120.67 mg g⁻¹ with the adsorbent dose increasing from 0.01 to 0.06 g. Because of the dependence of the total treatment cost on the cost of the adsorbent, the compromise between removal efficiency and amount of adsorbent should be optimized for the treatment. Consequently, the adsorbent dose of 0.02 g was selected in the subsequent experiments, which was considered to be sufficient for the removal of MB.

Fig. 6 Effect of adsorbent dose on adsorption of MB by PAC.

The adsorption of MB on PAC as a function of contact time is presented in Fig. 7. It was investigated with 0.02 g of adsorbent in 50 mL of 100 mg L⁻¹ MB at 298 K. As seen, the amount of adsorbed MB increases with increasing contact time. The MB adsorption on PAC exhibits an initial rapid adsorption in the first 70 min, followed by a slow adsorption that gradually reaches equilibrium. The adsorption equilibrium is reached within 150 min, and the equilibrium adsorption capacity of PAC is 353.1 mg g⁻¹. The fast adsorption of MB at the initial stages might be due to more availability of the uncovered surface and active sites on the adsorbent surface. When these adsorption sites are occupied gradually, the rate of adsorption decreases.

Fig. 7 Effect of contact time on the adsorption of MB by PAC.

The effect of initial MB concentration and temperature on the adsorption of MB onto the PAC were studied by adding 0.02 g of PAC to 50 mL of various initial MB concentrations (50, 100, 150, 200, 250 and 300 mg L⁻¹) and at various temperatures (298 K, 308 K, 318 K) for 5 h. The results are shown in Fig. 8. It is observed that the adsorption capacity of PAC increases with increasing initial MB concentration and temperature. The adsorption capacity significantly increases from 180.68 mg g⁻¹ to 876.35 mg g⁻¹, when the initial MB concentration increases from 50 mg L⁻¹ to 300 mg L⁻¹ at 318 K. The higher initial MB concentration provides a greater driving force, resulting from a greater concentration gradient between MB in solution and MB on the adsorbent surface. The equilibrium adsorption of MB onto PAC is also affected by temperature. It is observed that the equilibrium adsorption capacity of MB increases from 798.06 to 876.35 mg g⁻¹ as temperature increases from 298 K to 318 K. An increase in temperature can increase the ability for MB to diffuse from the solution to the PAC surface. An increase in temperature is beneficial for the adsorption, which indicates that the adsorption process is an endothermic process.

Fig. 8 Effect of temperature and initial MB concentration on the adsorption of MB by PAC.

4. Conclusions

The peanut shells can be transformed into a porous carbon material through a simple ZnCl₂-molten salt method. The PAC is an amorphous and porous carbon material with a high surface area (S_BET) of 1642.72 m² g⁻¹ and contains some functional groups on the surface. When the prepared PAC was used as an adsorbent to remove methylene blue (MB) dye from an aqueous solution, it exhibited an excellent adsorption capacity of 876.35 mg g⁻¹, which indicates that the PAC from peanut shells can be used as a low-cost and effective adsorbent for water purification.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 20871105 and 21576247).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12406a

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