Preparation and Characterization of Activated Carbons from Bamboo

Nov 14, 2016 - A series of activated carbons were prepared by H3PO4-mechanical force activation and characterized by N2 adsorption/desorption isotherm...
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Preparation and Characterization of Activated Carbons from Bamboo Sawdust and Its Application for CH4 Selectivity Adsorption from a CH4/N2 System Hongyan Pan,† Jingyun Zhao,† Qian Lin,*,† Jianxin Cao,*,‡ Fei Liu,† and Beilei Zheng† †

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

‡

ABSTRACT: A series of activated carbons were prepared by H3PO4-mechanical force activation and characterized by N2 adsorption/desorption isotherms, SEM, FTIR, CO/CO2-TPD, and XPS. Adsorption isotherms of CH4 and N2 were measured in the range of 25−760 mmHg at 20 °C and fitted by Langmuir model to calculate separation coefficient of CH4 against N2 (αCH4/N2). Results showed that H3PO4-mechanical force activation had significantly enhanced the micropore volume of activated carbons (ACs), in which the highest micropore volume (0.8 cm3/g) and BET specific area (1966 m2/g) were obtained under the optimum condition of phosphoric acid impregnation ratio of 1:1, activation temperature of 400 °C, and activation time of 60 min. The micropore volume and surface properties of ACs affected its selectivity adsorption ability toward CH4. The ACs with higher micropore volume had a larger adsorption amount of CH4. And the larger the O/C ratio of the ACs was, the larger the oxygen-containing groups on the ACs would be, which can improve the surface polarity of ACs and enhance the adsorption ability of N2 with certain polarity, and thus make the ACs having the smaller separation factor of αCH4/N2. Among all of the samples, AC-1-400 with the largest micropore volume and O/C ratio had the highest CH4 adsorption amount but the smallest αCH4/N2.

1. INTRODUCTION Activated carbons (ACs) are widely used as adsorbents and catalysts in separation, pollutants treatment, and gas storage due to their large surface area, well-developed micropore volume, and stable chemical properties. Nowadays, raw materials for the preparation of ACs usually are shell, wood, and coal, which are limitative, or expensive, or hard to renew, which lead to the limitation of their applications. Apart from the above materials, some renewable solid byproducts of agricultural wastes with high content of carbon and low content of ash could be used as raw materials for the preparation of ACs, such as rice hull, olive cake, corncob, and bamboo sawdust (BS) etc.1 In China, there are lots of BS produced every year;2 usually, they are burnt as wastes, resulting in environmental pollution and wasting of resources. Besides, the reserve of coal bed methane (CBM) in China is very large, but the concentration of CH4 in CBM is relative low, so directly using low-concentration CBM is very hard. Thus, a large amount of low-concentration CBM is emitted directly into the air every year. It has been reported that the released CBM every year is as high as 19 billion m3 in China,3 which not only leads to the wasting of resources but also damages the environment. CH4, N2, and O2 are the main ingredients of CBM, in which the high separation of CH4 against N2 is the key point for industrial CBM utilization. In our previous reports, it has been reported that the micropore volume is crucial for CH4 adsorption.3 Therefore, in this work, we use BS as a raw material to prepare AC with a high microporous volume and BET surface area. There are two methods to prepare ACs: physical activation and chemical activation. In comparison with physical activation, chemical activation usually takes place at a lower temperature and © XXXX American Chemical Society

the yield of ACs is higher. During the process of chemical activation, ZnCl2, KOH, and H3PO4 are usually used as activating agents to prepare ACs. And the ACs activated by ZnCl2 and KOH have higher micropore volume and BET surface; however, the activation temperature is usually higher than 700 °C, the activating agent consumption is larger, and moreover, those activation agents have a stronger corrosion to the equipment and severe contamination to the environment. Compared with ZnCl2 and KOH activation, the H3PO4 activation temperature is relatively low ( AC-0.5-400 > AC-1.5-400, which indicates the sample AC-1-400 has the largest amount of oxygencontaining groups. 3.3. Adsorption of CH4 and N2 on Adsorbents. Figure 8 shows the adsorption isotherms of CH4 and N2 on the three ACs at 293 K. It can be seen that the adsorption amount of CH4 is G

DOI: 10.1021/acs.energyfuels.6b02232 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

then decreases. Among all of the prepared samples, sample AC-1400 prepared at the condition of phosphoric acid impregnation ratio of 1:1, activation temperature of 400 °C, and activation time of 60 min has the largest micropore volume (0.8 cm3/g) and BET specific area (1966 m2/g). Moreover, FTIR, TPD, and XPS results show that only oxygen-containing groups and phosphorus-containing groups can be detected on the ACs, and the O/C ratio of ACs prepared at different impregnation ratios is in the order of AC-1-400 > AC-0.5-400 > AC-1.5-400. The adsorption amount of CH4 on the three ACs is in the following order: AC-1-400 > AC-1.5-400 > AC-0.5-400, which is consistent with the micropore volume of ACs, indicating the micropore volume of ACs has a significant impact on the adsorption ability of CH4. Additionally, the separation factor of αCH4/N2 on the three samples is in the following order: AC-1.5400 > AC-0.5-400 > AC-1-400, which is consistent with the O/C ratio of the ACs. The larger the O/C ratio, the larger the oxygencontaining groups on the ACs will be, which can improve the surface polarity of ACs and then enhance the adsorption ability of N2 with certain polarity. This is why the sample AC-1-400 has the largest N2 uptake and smallest separation factor of αCH4/N2 though it has the largest adsorption amount of CH4.

larger than that of N2 for the three ACs. The previous reports showed that the larger the polarizability of molecules was, the stronger the interaction between molecules and AC would be.27 CH4 polarizability is 26 × 10−25 cm−3, which is higher than that of N2 (17.6 × 10−25 cm−3). Thus, CH4 molecules can be adsorbed preferentially on ACs, and its adsorption amount will be higher than that of N2. The adsorption amounts of CH4 and N2 at 750 and 760 mmHg, respectively, are listed in Table 4. And the separation Table 4. Separation Coefficient of CH4/N2 of ACs adsorbent

CH4a/ (mmol/g)

N2b/ (mmol/g)

αCH4/N2

Vmic/ (cm3/g)

O/C ratio

AC-0.5-400 AC-1-400 AC-1.5-400

0.58 0.87 0.60

0.17 0.28 0.16

4.17 3.38 5.2

0.5 0.82 0.56

0.25 0.34 0.22

a

The adsorption amount of CH4 at 750 mmHg; bThe adsorption amount of N2 at 760 mmHg.

factor of αCH4/N2 calculated according to eq 2 is also listed in it. The data in Table 4 show that the adsorption amount of CH4 on the three ACs is in the order of AC-1-400 ≫ AC-1.5-400 > AC0.5-400, which is in accordance with the micropore volume of ACs. This consequence indicates the micropore volume of ACs is crucial for CH4 adsorption. However, the sample AC-1-400 has the smallest separation factor of αCH4/N2 though it has the largest adsorption amount of CH4. This is because the sample AC-1-400 has the largest O/C ratio (0.34), indicating it has the largest oxygen-containing groups. The larger the amount is of oxygen-containing groups on the ACs surface, the stronger the surface polarity of the ACs will be, which can enhance the adsorption ability of ACs toward the polar molecular.28,29 By comparing CH4 and N2, although both of them are nonpolar molecularly,30 N2 has a stronger quadrupole moment than CH4, which makes N2 have more polarity. Thus, the sample AC-1-400 with the most oxygencontaining groups has the largest adsorption amount of N2, which means it has the lowest separation factor of αCH4/N2. Sample AC-1.5-400 has the largest separation factor of αCH4/N2 though it has a smaller adsorption amount of CH4. This is because sample AC-1.5-400 has the least O/C ratio (0.22). The less the O/C ratio on ACs surface is, the less the amount of oxygen-containing groups and the weaker the surface polarity will be, which can reduce the adsorption ability of ACs toward N2 with certain polarity. Thus, sample AC-1.5-400 has a relatively higher separation factor of αCH4/N2. The above results show that the micropore volume of ACs has a significant impact on the adsorption ability of CH4. The larger the micropore volume of ACs is, the larger the adsorption amount of CH4 will be. In addition, the content of oxygencontaining groups on ACs surface affects its separation factor of αCH4/N2. The larger the O/C ratio of ACs is, the lower separation factor of αCH4/N2 will be.

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AUTHOR INFORMATION

Corresponding Authors

*(Q.L.) Tel.: +86-851-83604936. Fax: +86-851-3625867. Email: [email protected]. *(J.C.) E-mail: [email protected]. ORCID

Hongyan Pan: 0000-0002-0958-8985 Funding

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

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION The activated carbons with larger micropore volume and less mesopore volume were obtained by phosphoric acid activation of bamboo sawdust under the help of mechanical force. With the increase of the impregnation ratio, activation temperature, and activation time, the micropore volume of ACs first increases and H

DOI: 10.1021/acs.energyfuels.6b02232 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b02232 Energy Fuels XXXX, XXX, XXX−XXX