Effect of Surface Chemistry and Textural Properties of Activated

Article pubs.acs.org/jced

Effect of Surface Chemistry and Textural Properties of Activated Carbons for CH4 Selective Adsorption through Low-Concentration Coal Bed Methane Hongyan Pan,† Yun Yi,‡ Qian Lin,*,‡ Guangyan Xiang,§ Yu Zhang,† and Fei Liu† †

Department of Chemical Engineering, Guizhou University, Guiyang 550025, China Key Laboratory of Guizhou Province for Green Chemical Industry and Clean Energy Technology, Guiyang 550025, China § Department of College of Pharmacy, Zunyi Medical College, Zunyi 563000, China ‡

ABSTRACT: A series of activated carbon were prepared, modified, and characterized by FTIR, Boehm titration, and N2 adsorption/ desorption isotherms. Adsorption breakthrough experiments of CH4 through low-concentration coal bed methane (CBM) were carried out for measuring adsorption capacities (Qm) of adsorbents toward CH4 at 293 K. Adsorption isotherms of CH4, N2, and O2 were measured between 25 mm Hg and 760 mm Hg at 293 K and fitted by Langmuir model to calculate separation coefficient of CH4 against N2 and O2 (αCH4/N2 and αCH4/O2). Results show that CH4 adsorption occurs mainly at the micropore of activate carbon, and the surface basic group of activate carbon can strengthen its adsorption ability toward CH4. Adsorption capacities of CH4 on modified AC are higher than that on original AC. The adsorbent KCl/AC has the largest micropore volume and more amount of basic group and this makes it the largest uptake of CH4 (7.89 mL/g) at 293 K and 1 atm; it is 38.9% higher than that of original AC (5.68 mL/g). Separation coefficient of αCH4/N2 on KCl/AC is 5.33, compared to 3.85 for AC; it is 38.4% higher than that of the original AC.

1. INTRODUCTION Coal bed methane (CBM) with methane (CH4) as the main ingredient is a kind of unconventional gas. The concentration of methane in CBM is usually lower than 30% (volume fraction, similarly hereinafter) in China, because a large amount of air is injected into the mines during the gas extraction in underground with the aim of coal mine safety.1 According to the Emission Standard of Coalbed Methane/Coal Mine Gas,2 only when the methane concentration in CBM is higher than 30% can they be banned to discharge into air, which leads to a large amount of low-concentration CBM emission into air. It reported that the CBM has been released into the atmosphere every year is as high as 19 billion m3 in China, which equals to about 200 million tons of standard coal.3 Furthermore, CH4 is also a kind of greenhouse gas, and its ozone destruction is regarded as seven times stronger than that of CO2 in the same volume.4 Therefore, CH4 should be separated and accumulated from the low-concentration CBM and regarded as one kind of potential energy rather than released into atmosphere, which is very important to improve the economic benefits and reduce the environmental pollution. Pressure swing adsorption is one of the main methods to separate and accumulate CH4 from CBM.5 Adsorbent is the core of adsorption; the common adsorbents are activated carbons (AC),6,7 metal−organic frameworks (MOFs) and activated carbon fibers (ACFs). Among them, AC is the most © XXXX American Chemical Society

widely used adsorbent and more suitable for CH4 separation and accumulation from low-concentration CBM in industry because of its low cost, stable property, and unique microporosity. In order to enhance CH4 adsorption capacity and CH4/N2 separation efficiency, previous researchers use different methods to modify adsorbents. Feng et. al8 used urea, NH3, and ammonia aqueous solution to modify carbon spheres (CS) for CH4 adsorption, and found that CH4 adsorption capacity of modified CS were lower than that of original CS because of the decreasing of Brunauer−Emmett−Teller (BET) SAA and total pore volume. Hao et. al 9 modified coal with H 2 O 2 , (NH4)2S2O8, and HNO3 and found that coal with a higher amount of oxygen surface group had lower CH4 adsorption capacity. Ma et. al10 used Ti to modify AC and found that the modified AC had higher CH4 adsorption capacity, however, it also had higher N2 adsorption capacity. Li et. al11 used ammonia to modify ACF and found that the ACF modified by 5 mol/L ammonia had the largest micropore volume and higher adsorption capacity of CH4, moreover, it also had the higher N2 adsorption capacity. Received: January 24, 2016 Accepted: April 26, 2016

A

DOI: 10.1021/acs.jced.6b00066 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

In addition, our previous studies12 have demonstrated that the micropore volume of ACF was favorable to styrene adsorption. Gu et.al13 reported that larger surface area of GACs were benefit for CH4 enrichment. Mohamed14 indicated that the basic functional groups of AC, such as N−H, were likely to bond with nonpolar or weak polar substances. Although the application of different method to modify adsorbents can influence their adsorption of CH4, its mechanisms are complex and not yet fully understood. Especially, there are lots of oxygen and nitrogen presence in the CBM, which will increase the difficulty of separation and accumulation of CH4 from low-concentration of CBM. Therefore, it is necessary to know the relation between CH4 selective adsorption from low-concentration CBM and the pore structure and surface properties of activated carbons, which will be helpful in the design of new adsorbents for CH4 selective adsorption from CBM. The purpose of this work is to investigate the effect of surface chemical properties and pore structure of ACs on CH4 selective adsorption from low concentration CBM. A series of activated carbons with different pore structure and chemical properties are selected to test the adsorption ability toward CH4 in lowconcentration CBM. Furthermore, in order to raise the micropore volume and surface basic group amount of AC, basic chemical agents, such as ammonia solution, potassium hydroxide, and composite modification with potassium hydroxide and hydrochloric acid are used to modify AC; by comparison, the AC is also modified by hydrochloric acid. Activated carbons are characterized by FTIR, Boehm titration, and N2 adsorption. The adsorption characteristics of CH4 from low-concentration CBM on the ACs are studied by using a fixbed adsorption experiment, and adsorption isotherms of pure gas CH4, N2, and O2 on the modified ACs are also studied to measure the separation coefficient of CH4 against N2 and O2. The effect of pore structure and surface chemical properties of ACs on its adsorption ability toward CH4 will be discussed and reported here.

meritics Co., U.S.A.) at 77 K, and N2 was used as the adsorbate. The nitrogen adsorption−desorption isotherms were used to determine BET surface area (SBET), micropore volume (Vmic), and mesopore volume (Vmes). The surface functional groups of adsorbents were characterized using a Nicolet iS50 FTIR infrared spectrometer (Thermo Fisher Scientific, U.S.A.). The samples were mixed with 99.99% KBr at a ratio of 1:500, and the spectra were recorded in the range of 400−4000 cm−1. The amount of acidic/basic groups on adsorbents surface was investigated using Boehm titration. Details of measurement procedure are described in the reference.15 2.3. Adsorption Breakthrough Experiments of CH4 from Low-Concentration CBM. Breakthrough curves were obtained in a continuous fixed-bed adsorption device. Details of measurement procedure are described in elsewhere.16 The component of simulating coal-bed methane was CH4 20%, O2 16%, and N2 64%, and the gas flow was 20 mL/min, which was regulated by a mass flow controller (Beijing Seven Star ElectronicsCo., LTD). First, the feed stream was sent into the adsorption column (14 mm i.d., 130 mm height) filled with 6 g of adsorbent whose temperature was 298 ± 0.5 K. The effluent gas from the fix-bed was subsequently separated with a 5A zeolite chromatogram column and then determined by online gas chromatography (GC-9560, Shanghai Hua’ai Chromatograph Company, China) equipped with thermal conductivity detector (TCD). The experiments were stopped when the concentration of CH4, N2, and O2 in the effluent gas was equal to that of in the initial gas. In this work, the breakthrough time of CH4 was defined as the time when the CH4 concentration of the effluent gas increased to 5% of that in the initial gas. According to the breakthrough time, CH4 adsorption amount of the adsorbents can be calculated by eq 1 tB

Qm =

2. MATERIALS AND METHODS 2.1. Adsorbents Preparation. Pretreatment of ACs. AC, AC1, AC2, AC3, AC4, AC5, and AC6, purchased from TangshanTanye Activated carbon company, China, were washed with acid, alkaline, and hot deionized water in turn for several times in order to remove the dissoluble impurities. Then the ACs were dried at 393 K for 12 h, crushed, and sieved to a particle size in the range of 0.25−0.38 mm. Modification of ACs. First, 40 g of AC was immersed separately into 200 mL of 50% ammonia solution, 2% potassium hydroxide solution, and 10% hydrochloric acid; these chemical agents were purchased from Guangzhou Chemical reagent factory (China) and no addition purification was done. Second, slurries were stirred for 24 h at 298 K and then filtered and washed by hot distilled water until the pH of the solution was 7. After that, the samples were dried at 80 °C for 12 h. The obtained ACs modified by different acidic/basic agents were denoted as N/AC, K/AC, and Cl/AC, respectively. In addition, 20 g of Cl/AC was immersed into 100 mL of 2% potassium hydroxide solution to prepare the sample KCl/AC according to the above preparation process. 2.2. Characterization of Adsorbents. The surface areas and pore structure of those adsorbents were measured using automatic surface area and pore size analyzer Nove 1000e (Quantachrome Co., U.S.A.) and ASAP2010 analyzer (Micro-

VC0t B − ∫ VC(t ) dt 0 M

(1)

where Qm is the adsorption capacity of CH4 per unit adsorbent, C0 is CH4 concentration of feed gas, C(t) is CH4 concentration of exhaust gas, tB is breakthrough time C(t)/C0 = 0.05, and M is the quality of adsorbent.16 2.4. Adsorption Isotherms of Pure Gas CH4, N2, and O2 on Modified AC. Adsorption isotherms of pure gas CH4, N2, and O2 on modified AC were measured using ASAP 2020 M adsorption Analyzer at 298 K. First, the samples were pretreated by evacuation at 473 K for 7 h to remove water vapor and other impure gases. Then adsorption isotherms of CH4 and N2 on the modified AC were measured at 298 K and the adsorption pressure was 25−760 mm Hg. All the adsorption isotherms of CH4, N2, and O2 have been calculated based on the Langmuir model q=

qmbP 1 + bP

(2)

where q is the gas uptake (mmol/g), qm is the maximum gas uptake (mmol/g), b is fitting constants, and p is adsorption pressure. According to the fitting parameters of qm and b, the separation coefficient of αCH4/N2(O2) is calculated by using eq 3.11 B



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Figure 1. N2 adsorption−desorption isotherms (a) and pore size distribution of adsorbents (b,c).

Table 1. BET Surface Area and Pore Structure Parameters of Adsorbents Deduced from N2 Adsorption Data cumulative pore volume (cm3/g) 3

2

adsorbents

VMic/(cm /g)

SBET/(m /g)

0.5−0.764 nm

0.764−1.528 nm

1.528−2 nm

2−50 nm

AC AC1 AC2 AC3 AC4 AC5 AC6

0.51 0.45 0.39 0.19 0.19 0.14 0.13

1374.2 1109.5 1054.3 577.0 516.2 413.3 372.3

0.26 0.166 0.108 0.090 0.1177 0.0931 0.0767

0.249 0.152 0.152 0.0615 0.0389 0.0226 0.0337

0.086 0.057 0.091 0.0063 0.0054 0.0041 0.0042

0.09 0.04 0.11 0.10 0.06 0.06 0.08

xCH4

αCH4 /N2(O2) =

yCH

4

(1 − xCH4) (1 − yCH ) 4

=

the pore size less than 2 nm, indicating the presence of larger micropores and less mesopores in those samples. Table 1 lists the BET surface area and pore structure parameters of the adsorbents. It shows that all samples contain larger amount of micropores volume and its micropore volume follows the order AC > AC1 > AC2 > AC3 = AC4 > AC5 > AC6. It also lists the pore volume with pore size distribution of 0.5−0.764, 0.764−1.528, 1.528−2, and larger than 2 nm, which corresponds to different times of CH4 diameter separately. As shown in Table 1, when the micropore diameter is less than 2 nm, the pore volume of all samples is mainly distributing in pores whose size is between 0.5 and 1.528 nm, which is 1.3−4 times that of CH4 diameter. Among them, AC has the largest micropore volume. Gu et. al13 indicated that the effective pore size for CH4 enrichment is in the range of 0.5−1.3 nm. Therefore, it can be concluded that the micropore diameter of the selected activated carbon is suitable for CH4 adsorption. Figure 2 shows the FTIR spectra of the adsorbents. It can be seen that all samples show similar shapes of the spectra. The band at 3436 cm−1 is attributed to the vibration of −OH. The band at 2068 cm−1 is attributed to stretching vibration of −CH,

qmCH bCH4 4

qmN (O )bN2(O2) 2

2

(3)

3. RESULTS AND DISCUSSION 3.1. Pore Structures and Surface Properties of Adsorbents. Figure 1a shows N2 adsorption−desorption isotherms of seven kinds of adsorbents at 77 K. It can be seen that the N2 adsorption capacity of those adsorbents increases sharply when relative pressure (P/P0) is very low and it reaches more than 90% of the saturation adsorption amounts when P/P0 is less than 0.1, indicating that large amounts of micropores exist in those adsorbents. Those samples also show little change of N2 uptake when relative pressure increase from 0.1 to 1, suggesting the existence of less mesopores. Among those samples, AC has the highest adsorption isotherm, indicating that the surface area and micropore volume of AC is the largest. Figure 1b,c shows the pore size distribution of seven adsorbents. The pore of all samples are mainly distributed in C



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with AC at 298 K and 1 atm. It can be seen that before the initial breakthrough time of CH4, the volume fraction of CH4, N2, and O2 in the effluent gas is about 0%, 80%, and 20%, respectively, which means the AC can adsorb CH4 but not adsorb N2 and O2. This is because the polarizability of CH4 (26 × 10−25 cm−3) molecules is higher than that of N2 molecules (17.6 × 10−25 cm−3) and O2 molecules (15.8 × 10−25 cm−3) and it is more polarizable,19 which make the interaction between CH4 molecules and AC is higher than that of N2 (or O2) and AC, and CH4 molecules to be adsorbed prefer. Before the initial breakthrough time of CH4, the volume fraction of CH4, N2 and O2 in the effluent gas on the adsorbents AC1, AC2, AC3, AC4, AC5, and AC6 is also about 0%, 80% and 20%, which indicates those activated carbon can also adsorb CH4 but not adsorb N2 and O2. Therefore, this paper only lists the breakthrough curves of CH4 on various adsorbents, and the data are shown in Figure 4.

Figure 2. FTIR spectra of the adsorbents.

The bands at 1635 and 1448 cm−1 are attributed to stretching vibration of −NH2, and the band around 1086 cm−1 is attributed to the vibration of acidic carboxylic anhydride.17,18 The bands in the region from 900 to 700 cm−1 are ascribed to the vibration of the aromatic structure.13 Thus, it can be seen that there are a certain amount of acidic oxygen-containing groups and basic nitrogen-containing groups on the surfaces of the activated carbons. Table 2 lists the results of Boehm titration for activated carbons. It can be seen that those adsorbents contain a certain Table 2. Boehm Titration Results of Adsorbents adsorbents acidic group amount (mmol/g) basic group amount (mmol/g)

AC

AC1

AC2

AC3

AC4

AC5

AC6

1.24

1.24

1.12

1.25

1.07

1.15

1.13

0.65

0.66

0.53

0.73

0.58

0.54

0.51

Figure 4. Breakthrough curves of CH4 in CBM on seven activated carbons.

Figure 4 shows adsorption breakthrough curves of CH4 in simulated coal-bed methane on seven kinds of activated carbons. It can be seen that the sample AC has the longest breakthrough time, and the sample AC6 has the shortest breakthrough time; longer breakthrough time means the larger adsorption capacities, which means the AC has the largest CH4 adsorption capacity and AC6 has the smallest adsorption capacity. On the basis of the experimental breakthrough curves of Figure 4, working uptake of CH4 on activated carbons are calculated according to eq 1, and the results are shown in Table 3. It can be seen that the adsorption amount of CH4 on the adsorbents are in order of QAC > QAC1 > QAC2 > QAC3 > QAC4 > QAC5 > QAC6, which is nearly coincident with the order of the micropore volume and the BET surface area of activated carbons as shown in Table 1. It indicates that the micropore volume and surface area of adsorbents have a significant impact on the selective adsorption ability of CH4. AC has the highest micropore volume and shows the largest uptake of CH4. In order to investigate the effect of pore volume in different pore size of adsorbents on CH4 uptake, CH4 adsorption amount on seven activated carbons as a function of its pore volume in the pore size distributed in 0.5−0.764 nm (a), 0.764−1.528 nm (b), 1.528−2 nm (c), and larger than 2−50 nm (d) are shown in Figure 5 and the data are linearly fitted. It shows that a positive correlation between micropore volume with its pore size in the range of 0.5−0.764 nm (a), 0.764−

amount of acidic group and basic group, which is in consistent with the results of Figure 2. In addition, the total acidic group amounts are higher than basic group amounts on the activated carbons. 3.2. Dynamical Adsorption of CH4 from Low-Concentration CBM on Adsorbents. Figure 3 shows the adsorption breakthrough curves of CH4, N2, and O2 upon separation of CH4/N2/O2 (20%/64%/16%) mixture in the fixed-bed packed

Figure 3. Breakthrough curves of CH4/N2/O2 (20%/64%/16%) on activated carbon AC at flow rates of 20 mL/min at 298 K and 1 atm. D



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Table 3. Working Adsorption Capacity of CH4 from CBM on Seven Kinds of Activated Carbons activated carbons

AC

AC1

AC2

AC3

AC4

AC5

AC6

breakthrough time (s) adsorption amount Qm (mL/g)

538 5.68

470 5.0

369 3.91

309 3.26

276 2.91

184 1.94

105 1.11

Figure 5. CH4 uptake on activated carbons vs its micropore volume in different pore size distribution, micropore volume (0.5−0.764 nm) (a), micropore volume (0.764−1.528 nm) (b), micropore volume (1.528−2 nm) (c), and mesopore volume (2−50 nm) (d).

1.528 nm (b), and 1.528−2nm (c) and CH4 adsorption amounts, and the linear fitting parameters R are 67.5%, 79.7%, and 57.1%, respectively. However, a negative correlation between mesopore volume with its pore size in 2−50 nm (d) and CH4 adsorption amounts and its linear fitting parameters R is −19.8%. If the linear fitting parameter R is larger, the effect of pore volume in this pore size on CH4 adsorption amounts will be higher, which indicates that the micropore volume with its pore size between 0.5 and 2 nm of all samples is suitable for CH4 adsorption, especially the pore size in the range of 0.764− 1.528 nm; however, the pore size in the range of 2−50 is not suitable for CH4 adsorption. From the above results, it can be seen that the micropore with pore size less than 2 nm is suitable for CH4 adsorption. In order to further study the relationship between micropore volume and basic group amount of activated carbons and its selective adsorption ability toward CH4 in low-concentration CBM, the adsorption capacity of CH4 on seven activated carbons as a function of its micropore volume and basic group amount are shown in Figure 6. It can be seen that the adsorption capacity of CH4 increase with the increasing of the micropore volume of activated carbons, but not in a linear relationship. For instance, the micropore volume of AC3 (0.19 cm3/g) and AC4 (0.19 cm3/g) are the same, but CH4 uptake on AC3 is larger than that on AC4. This is because the different amount of basic group on activated carbons surfaces. Mohamed

Figure 6. CH4 uptake on activated carbons versus its micropore volume and basic group amount.

et. al14 indicated that basic functional groups, such as N−H, on the activated carbon surface were likely to bond with nonpolar or weak polar substances. Feng et. al indicated that nitrogen content of samples provide a positive adsorption sites for CH4 adsorption. Thus, we can speculate that AC3 has higher adsorption capacity of CH4 due to the more amount of basic group on its surface; the basic group amounts on AC3 (0.73 mmol/g) is higher than that on AC4 (0.58 mmol/g). Meanwhile, AC5 and AC6 also have a similar phenomenon. Although the micropore volume of AC5 (0.14 cm3/g) is 7.7% E



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KCl/AC > K/AC > N/AC ≈ Cl/AC > AC. The activated carbon KCl/AC has the largest micropore volume and BET surface area, and its micropore volume is 40.6% higher than that of AC. Table 5 lists the results of Boehm titration of the modified AC. It can be seen that the amount of acidic group and basic

and merely higher than that of AC6 (0.13 cm3/g), CH4 uptake on AC5 is 74.7% higher than that on AC6. This is because AC5 (0.54 mmol/g) has more amount of basic group than AC6 (0.51 mmol/g) as well as the slightly higher micropore volume. The above analysis indicates that the micropore volume and amount of surface basic group on activated carbons affect its selective adsorption ability toward CH4. CH4 adsorption takes place mainly at the micropore of activated carbons; the larger the micropore volume of activated carbons is, the higher adsorption amounts of CH4 will be. Likewise, the surface basic group amounts can strengthen the CH4 adsorption when the micropore volume of activated carbons is approximate. AC has the largest micropore volume and more basic group, which makes it has the largest amounts of CH4. In order to enhance the micropore volume or surface basic group amount of AC and investigate the effect of the pore structure and surface properties of modified ACs on CH4 selective adsorption from CBM, ammonia, potassium hydroxide, and the composition modification with potassium hydroxide and hydrochloric acid are used to modify AC; in comparison, hydrochloric is also used to modify AC, and the results are shown in Section 3.3. 3.3. Dynamical Adsorption of CH4 in Low-Concentration CBM on Modified AC. Figure 7 shows N 2

Table 5. Boehm Titration of the Modified AC adsorbents

AC

Cl/AC

N/AC

K/AC

KCl/AC

acids amount (mmol/g) bases amount(mmol/g)

1.24 0.65

1.5 0.53

0.51 1.63

0.48 1.51

0.61 1.28

group on the surface of modified AC are different. In comparison with the original AC, the acidic group amount on Cl/AC is enhanced but the basic group amount is decreased when the AC is modified by hydrochloric acid, whereas, the basic group amount on N/AC, K/AC, and KCl/AC is enhanced but the acidic group amount is decreased when the AC is modified by ammonia, potassium hydroxide, and potassium hydroxide and hydrochloric acid complex modification, respectively. This is determined by the properties of modified agents. Figure 8 shows the adsorption breakthrough curves of CH4 in simulating coal-bed methane on modified AC. It can be seen

Figure 7. N2 adsorption isotherms of the modified AC. Figure 8. Breakthrough curves of CH4 in simulating CBM on modified AC.

adsorption−desorption isotherms of modified AC at 77 K. It can be seen that the shape of adsorption isotherms of all activated carbons are similar and belong to type I according to IUPAC classification. In comparison with the original AC, N2 adsorption isotherms of the modified AC are higher, indicating that the surface area and micropore volume of modified AC are higher than that of the original AC. Table 4 lists the pore structure parameters of the modified AC. The data in Table 4 show that the micropore volume and BET surface area of the modified AC are larger than that of the AC. The order of the micropore volume of modified AC is

that the original AC has the shortest breakthrough time, the breakthrough curves of CH4 on modified AC shift slightly toward right, and their breakthrough times become longer, indicating that the CH4 uptake on modified AC are improved and thus become higher. Table 6 lists the working capacity of CH4 on modified AC. It can be seen that the adsorption capacity of CH4 on modified ACs are in the order of QKCl/AC > QK/AC > QN/AC > QCl/AC > QAC, indicating that the modified ACs adsorb more amount of CH4 than the original AC.

Table 4. BET Surface Area and Pore Structure Parameters of the Modified AC adsorbents

SBET/(m2/g)

Vmes/(cm3/g)

VMic/(cm3/g)

AC Cl/AC N/AC K/AC KCl/AC

1374.2 1464.8 1426.9 1665.0 1865.0

0.09 0.098 0.112 0.104 0.104

0.51 0.56 0.56 0.64 0.717

Table 6. Working Adsorption Capacity of CH4 from CBM on Modified AC

F

activated carbon

AC

Cl/AC

N/AC

K/AC

KCl/AC

breakthrough time (s) adsorption amount Qm (mL/g)

538 5.68

628.4 6.63

651.8 6.88

678.3 7.16

747.5 7.89



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Figure 9. Adsorption of CH4, N2, and O2 on adsorbents at 293 K. AC (a), Cl/AC (b), N/AC (c), K/AC (d), and KCl/AC (e).

Previous reports indicate that adsorbent is suitable to utilize in separating and purifying gases in industry when the gas separation factor is bigger than 2. Therefore, KCl/AC can be applied in industry to separate and purify the CH4 in lowconcentration CBM.

This is because the modified activated carbons have the larger micropore volume and surface basic group amount as shown in Tables 4 and 5. The adsorbent KCl/AC has the largest micropore volume and more basic group amount and shows the largest uptake of CH4 (7.89 mL/g); it is 38.9% higher than that of original AC (5.68 mL/g). Although the micropore volume of N/AC and Cl/AC are the same, N/AC has the higher amount of basic group and thus enhances its adsorption toward CH4. 3.4. Pure Gas Adsorption Isotherms. Figure 9 shows the adsorption isotherms of pure gas CH4, N2, and O2 on adsorbent AC (a), Cl/AC (b), N/AC (c), K/AC (d), and KCl/AC (e) at 293 K between 25 mm Hg and 760 mm Hg. It can be seen that the shapes of all isotherms belong to type I according to the IUPAC classification, therefore, all of the adsorption data in Figure 9 are fitted by using of Langmuir model. The parameters of the fitting results are given in Table 7, and the separation factors calculated by eq 3 are also listed in

4. CONCLUSION (1) A series of activate carbon were used to separate and accumulate CH4 from CBM by fixed-bed adsorption experiment at 298 K and 1 atm. FTIR and Boehm titration results illustrate that an amount of acidic group and basic group exists on activated carbon surface, and AC3 has the largest basic group contents. N2 adsorption results indicated that the micropore volume of activated carbon follows the following order: AC > AC1 > AC2 > AC3 ≈ AC4 > AC5 > AC6. The working adsorption capacities of CH4 from low-concentration CBM are in the order of QAC > QAC1 > QAC2 > QAC3 ≈ QAC4 > QAC5 > QAC6, which is almost consistent with the order of the micropore volume of activated carbons. It is concluded that CH4 adsorption take place mainly at the micropore of activate carbon, and the basic group amount on activate carbon surface can strengthen its adsorption ability toward CH4. AC has the largest micropore volume and more basic group contents, which make it has the largest CH4 adsorption capacity. (2) Adsorption capacity of CH4 on modified AC are higher than that on original AC. Adsorbent KCl/AC shows the largest micropore volume and more amount of basic group, which makes it have the largest uptake of CH4 (7.89 mL/g), and it is 38.9% higher than that of original AC (5.68 mL/g). αCH4/N2 on KCl/AC is 5.33, compared to the 3.85 for AC, and it is 38.4% higher than that of the original AC.

Table 7. Separation Coefficient of CH4/N2 and CH4/O2 of Different Adsorbent qm (mmol/g) adsorbent

CH4

N2

O2

αCH4/N2

αCH4/O2

AC Cl/AC N/AC K/AC KCl/AC

1.959 2.183 2.408 2.654 2.849

0.513 0.657 0.577 0.714 0.612

0.652 0.605 0.399 0.592 1.292

3.85 3.32 4.7 3.84 5.33

5.49 4.13 5.88 5.17 7.9

■

Table 7. The result shows that the separation factor of αCH4/N2 and αCH4/O2 on the samples followed the order KCl/AC > N/ AC > AC > K/AC > Cl/AC. In comparison with AC, the αCH4/N2 and αCH4/O2 on the activated carbons KCl/AC and N/ AC are higher, while the αCH4/N2 and αCH4/O2 on K/AC and Cl/ AC are lower. Among them, the sample KCl/AC has the largest separation factor of αCH4/N2 (5.33) and αCH4/O2 (7.9). Compared with AC, whose separation factor of αCH4/N2 and αCH4/O2 is 3.85 and 5.49, respectively, it is 38.4% and 43.9% higher, respectively, than that of the original AC.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-851-83604936. Fax: +86-851-3625867. Funding

This work was supported by the Natural Science Foundation of China (No. 21366008), Foundation of Guizhou Provincial ministry of education (No. (2014)267) and Science & Technology Foundation of Guizhou Province (No. (2014)2008). G



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Notes

The authors declare no competing financial interest.

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