Adsorption and Desorption Performance of Dichloromethane over

Aug 7, 2013 - Nowadays, the most widely used adsorbent among the various adsorbents for VOCs adsorption is activated carbon because of its low cost, h...
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Adsorption and Desorption Performance of Dichloromethane over Activated Carbons Modified by Metal Ions Hongyan Pan, Min Tian, Hui Zhang, Yu Zhang, and Qian Lin* School of Chemical Engineering, Guizhou University, Guiyang 550025, China ABSTRACT: Activated carbons (ACs) were modified separately by impregnation with different types of metal ions, Al3+, Mg2+, Li+, Fe3+, and Ag+ in this work. Adsorption breakthrough experiments of dichloromethane (DCM) were carried out for measuring working adsorption capacities (Qm) of modified ACs toward DCM, and temperature programmed desorption (TPD) experiments were conducted for estimating desorption activation energy (Ed) of DCM on these ACs. The effects of metal ions on the adsorption/ desorption of DCM on the modified ACs were discussed with the help of hard and soft acid and base (HSAB) theory. Results showed that Qm of DCM on these ACs followed the order: QAl(III)/AC > QLi(I)/AC > QMg(II)/AC > QFe(III)/AC > QAC > QAg(I)/AC, and the Ed of DCM followed the same order. As compared to that of the original AC, the surface local acid hardness of the modified ACs was enhanced with the doping of hard acid metal ions such as Al3+, Li+, Mg2+, and Fe3+, and thus the interactions between DCM and Al(III)/AC, Li(I)/AC, Mg(II)/AC, and Fe(III)/AC surfaces were also enhanced due to the hard base characteristic of DCM. However, when the AC was doped with soft acid Ag+, its surface local acid hardness was decreased. As a result, the interaction between DCM and Ag(I)/AC was weakened.

1. INTRODUCTION Dichloromethane (DCM) is an important industrial chemical and widely used as organic solvent in various chemical and polymer syntheses and vesicants. It has vapor pressures greater than 133.3 Pa at room temperature and is easily vaporized at ambient temperature and pressure. It is harmful to humans and the environment because of its widespread release through industries. It has been pointed out that the long-term exposure to DCM as well as other volatile organic compounds (VOCs), even with very low concentrations, can cause adverse effects or damage for human beings.1,2 Given the increased concern of the health hazard associated with VOCs, the research on the abatement of VOCs has been attracting unabated attention. Adsorption technology is one of the major methods to remove DCM, especially for the adsorption of DCM at low concentration. Nowadays, the most widely used adsorbent among the various adsorbents for VOCs adsorption is activated carbon because of its low cost, high surface area, and unique microporosity.3 In our previous study, ACs with micropores showed high adsorption performance for styrene.4 Many researchers have reported that transitional metal ionloaded ACs possess high adsorption capacity of VOCs since the loaded transitional metal ion is an active adsorption site or a catalyst initiator during the reactive adsorption process. Bikshapathi et al. prepared Fe(III)-impregnated activated carbon fiber (ACF) and carbon nanofiber (CNF) for adsorption of CCl4, and reported that the CCl4 adsorption capacities of Fe(III)/ACF and Fe(III)/CNF were higher than those of virgin materials because of the uniform dispersion of active sites such as Fe nanoparticles over the surfaces of the ACFs and CNF.5 Xiao et al. reported that a Na-doped activated carbon (IAC) demonstrated higher adsorption capacity of H2S compared to © XXXX American Chemical Society

the virgin AC. They also found that the ACs loaded separately with copper, cobalt, and silver had high dichloroethylene uptakes.6 Although the application of some metal ions to modify the ACs can enhance their adsorption of VOCs, its mechanisms are complex and are not yet fully understood. Therefore, it is necessary to know further the effects of metal ions loaded on activated carbons on the adsorption/desorption of VOCs. Understanding the interaction mechanism between VOCs molecules and the activated carbon surfaces loaded with different metal ions will be helpful in the design of new adsorbents for effective adsorption of VOCs. The purpose of this work is to investigate the effects of loading different metal ions on ACs on the adsorption/desorption performance of DCM. We use the impregnation method to modify ACs separately with different metal salt solutions. Textural structures of the modified ACs are characterized. The adsorption and desorption characteristics of DCM on the metal ionsimpregnated ACs are studied by using a fixed-bed adsorption and temperature-programmed desorption (TPD) experiment, separately. With the help of hard and soft acids and bases (HSAB) theory, the effects of metal-ions-modified AC on its adsorption ability of DCM are discussed and reported here.7

2. EXPERIMENTAL SECTION 2.1. Adsorbent Preparation. Pretreatment of ACs: First, the ACs [(40−60) mesh] obtained from Zhaoyang Senyuan Activated Carbon Company, China, were washed separately by hydrochloric acid (HCl, 99 %), sodium hydroxide (NaOH, 99 %), Received: March 22, 2013 Accepted: July 22, 2013

A

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ASAP 2010 M instrument equipped with commercial software for calculation and analysis.12 BET surface area (SBET) and micropore volume (VMicropore) were calculated using standard Brunauer−Emmett−Teller (BET) and t-plot method, respectively, and the total pore volume (VTotal) was estimated on the basis of the volume adsorbed when p/p0 = 0.99. The element content of Al3+, Mg2+, Li+, Fe3+ and Ag+ doped on the modified ACs was analyzed by X-ray photoelectron spectroscopy (XPS) on Perkin-Elmer SSX 100/206 photoelectron. XPS characterizations of samples Al(III)/AC, Li(I)/AC, Mg(II)/ AC, Fe(III)/AC, and Ag(I)/AC: A PHI5300 X-ray photoelectron spectrometer with a nonmonochromatized Al K radiation (1486.6 eV) was used for this analysis. The source was operated at 15 kV and 34 mA. Prior to examination, the samples were dried at 373 K under vacuum for 2 h to remove adsorbed contaminants. XPS was run in retarding mode using a survey pass energy of 89.45 eV and multiplex pass energy of 35.75 eV. Atomic ratios were calculated from the XPS survey spectra after the relative peak areas were corrected by sensitivity factors based on the transmission characteristics of the Physical Electronics XPS apparatus.13 A combustion experiment was carried out to determine the contents of metal ions loaded on the tested ACs. This method is described in the previous reports.14

and deionized water in turn for several times, to remove the dissoluble inorganic or organic impurities. Then, the ACs were dried at 393 K in vacuum for 24 h. This original activated carbon was denoted as AC. Modification of ACs with different metal ions: First, 10 g of ACs were separately weighed, and immersed separately into 100 mL of aluminum nitrate (Al(NO3)3, 99 %), lithium nitrate (LiNO3, 98.5 %), magnesium nitrate (Mg(NO3)2, 99 %), ferric nitrate (Fe(NO3)3, 98.5 %), and silver nitrate (AgNO3, 99.8 %) in aqueous solution (0.01 mol·kg−1) with pH = 5; these chemical samples were supplied by Guangzhou Chemical reagent factory (China) and no additional purification was done. Second, the slurries were stirred for 24 h at 303 K, and after that the samples were filtered and then dried at 393 K for 24 h. The obtained ACs modified with different metals were denoted as Al(III)/AC, Li(I)/ AC, Mg(II)/AC, Fe(III)/AC, and Ag(I)/AC, respectively. 2.2. Adsorption Experiments. For testing six samples, Al(III)/AC, Li(I)/AC, Mg(II)/AC, Fe(III)/AC, Ag(I)/AC, and AC, the fixed-bed adsorption experiments were performed at 298 K under ambient pressure for determination of working adsorption capacities (Qm) toward DCM. Details of measurement procedure are described elsewhere.8,9 The gas mixture of DCM vapor and pure nitrogen gas (N2, 0.99999) was delivered to the fixed bed packed with modified AC whose temperature was 298 K, and the u(T) was 0.5 K. The flow-rate of gas mixture through the fixed bed was controlled by mass flow controllers (BJQXHC Electron Company, China) and the relative standard uncertainty of the controllers was to be 1 %. In this work, the flow-rate of the gas mixture was 0.667 mL·s−1, and the concentration of DCM was 8.825 g·m−3. The composition of exit gas stream from the fixed bed was determined by online gas chromatography ((GC9160, Shanghai Hua’ai Chromatograph Company, China) equipped with a hydrogen flame ionization detector (FID) and with workstation HW-2000 (NanJing QianPu company, China). In this study, the breakthrough time was defined as the time when the effluent concentration of DCM at the outlet reached 5 % of feed concentration. When the outlet concentration of DCM was equal to its inlet concentration, the adsorption experiment would be stopped. According to adsorption breakthrough curves obtained experimentally, the Qm can be calculated from the following equation:

3. RESULT AND DISCUSSION 3.1. Characterization of the Modified ACs. 3.1.1. Textural Structures of the Modified ACs. Figure 1 presents the N2

tB

Qm =

VC0t B − ∫ VC(t ) dt 0 M

(1) Figure 1. Nitrogen adsorption isotherms on the ACs loaded with metal ions: ▲, Al(III)/AC; Δ, Li (I)/AC; ●, Mg(II)/AC; ○, Fe(III)/ AC; ■, AC; □, Ag(I)/AC.

where C0 is the inlet concentration of DCM, C(t) is the effluent concentration of DCM, V is the volumetric flow rate of gas mixture, M is the amount of ACs, and tB is the breakthrough time (C(t)/C0 = 0.05).10 2.3. TPD Experiments. The TPD experiments were separately conducted on Autochem 2920 instrument (Micromeritics, USA) with a thermal conductivity detector, at different heating rates [(0.05 to 0.117) K·s−1].11 First, the tested sample that had adsorbed DMC vapor was put in a fixed-bed quartz flow reactor. Subsequently the fixed-bed reactor was placed in a reaction furnace and then heated in the N2 (99.999 %) flow at a constant rate of 0.667 mL·s−1. The DCM desorbed gradually from the sample with the increasing temperature; it was analyzed by thermal conductivity detector equipped in Autochem 2920 instrument, and thus the effluent curves of the desorbed DCM was recorded, which were called the TPD curves. 2.4. Characterization of Adsorbents. The specific surface area and pore volume of original AC and modified ACs were measured at 77 K on a Micrometrics gas adsorption analyzer

adsorption isotherms of the modified ACs. It is observed that at lower relative pressure (P/P0 < 0.1), all isotherms showed a steep increase of adsorbed amount, suggesting the existence of a large quantity of micropores in these ACs, and at higher relative pressure, these isotherms showed a slow increase in the amount of adsorbed nitrogen.15 N2 isotherms of the modified ACs were slightly lower compared to that of the original AC, indicating that the surface area of the modified ACs became somewhat smaller in comparison with the original AC. Table 1 lists the parameters of pore structure for the modified ACs. It can be seen that after modification, both the surface areas (SBET and SMicropore) and pore volumes (VTotal and VMicropore) of the modified ACs decrease slightly, which is mainly attributed to the introduction of metal on carbon surfaces. 3.1.2. Metal Content on the Modified ACs. Table 2 lists the elemental analysis by XPS of the modified ACs. It indicates that B

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Table 1. Parameters of Pore Structure for ACs activated carbons

SBET/m2·g−1

VTotal/cm3·g−1

VMicropore/cm3·g−1

SMicropore/m2·g−1

AC Li(I)/AC Ag(I)/AC Al(III)/AC Fe(III)/AC Mg(II)/AC

898 844 837 828 813 808

0.54 0.52 0.52 0.52 0.51 0.51

0.36 0.35 0.34 0.34 0.34 0.33

621 5901 5801 596 562 561

Table 2. Elemental Analysis of the Modified ACs by XPS elemental distribution (%)

a

activated carbons

C

N

O

Ma

AC Li(I)/AC Ag(I)/AC Al(III)/AC Fe(III)/AC Mg(II)/AC

89.37 91.67 91.40 87.00 90.67 92.11

0.82 0.30 0.55 0.55 0.53 0.31

9.80 7.74 7.77 10.6 8.46 7.35

0.29 0.28 0.29 0.34 0.23

Percentage of metal elements on the surface of modified ACs.

metal elements had been detected on the modified ACs except for the original AC, which implied these metals were separately doped on the surface of ACs. Table 3 lists the metal content on the modified ACs through combustion experiments. The data in Table 3 further confirmed that these metals were successfully loaded on the surfaces of these ACs, and the doping amounts of the metal on the modified ACs were very close. 3.2. The Working Adsorption Capacity of DCM on the Modified ACs. Figure 2 shows the breakthrough curves of DCM in the fixed bed packed separately with AC and the modified ACs. The result indicates that in comparison with that of the original AC, the breakthrough curves of DCM in the fixed bed separately packed with the ACs such as Al(III)/AC, Li(I)/AC, Mg(II)/AC, and Fe(III)/AC shift slightly toward right, and their breakthrough times became longer, indicating that the DCM working adsorption capacities of these four ACs were improved and thus became higher. On the other hand, it is noticed that the breakthrough curve of DCM in the fixed bed packed with the Ag(I)/AC shifts slightly toward the left compared to that of the original AC, and its breakthrough time became shorter, indicating that the DCM working adsorption capacity of the Ag(I)/AC was weakened. These facts, worthy of study,indicate that loading different metal onto the surfaces of the ACs would result in different influences on the DCM adsorption performance of the modified ACs. Table 4 lists the working adsorption capacities of DCM on these ACs, which were calculated according to eq 1 and on the basis of experimental breakthrough curves. The adsorption capacities of DCM on these modified ACs followed the order: QAl(III)/AC > QLi(I)/AC > QMg(II)/AC > QFe(III)/AC > QAC > QAg(I)/AC. That is to say, the loading of Al3+, Li+, Mg2+, and Fe3+ onto the ACs can enhance the adsorption of the modified ACs toward DCM, while the loading of Ag+ onto the ACs would weaken

Figure 2. Breakthrough curves of DCM adsorbed on metal ions modified ACs: ▲, Al(III)/AC; Δ, Li (I)/AC; ●, Mg(II)/AC; ○, Fe(III)/AC; ■, AC; □, Ag(I)/AC.

the adsorption of the modified ACs toward DCM, which can be ascribed to the physical and chemical properties change of the ACs. These phenomena are further explained in the following sections. 3.3. The desorption Properties of DCM on the Modified ACs. Desorption performance of DCM over the modified ACs was investigated by TPD technique. The desorption activation energy, Ed, is usually used to evaluate the desorption performance of an adsorbate on an adsorbent. It can be estimated with the following equation: ⎛ β ⎞ ⎛E ⎞ ⎛ E ⎞ ln⎜ H2 ⎟ = −⎜ d ⎟ − ln⎜ d ⎟ ⎝ RTP ⎠ ⎝ k0 ⎠ ⎝ RTP ⎠

(2)

where Tp is the peak temperature of a TPD curve (K), βH is the heating rate (K·s−1), Ed is desorption activation energy (kJ·mol−1), R is gas constant (kJ·mol−1·K−1), and k0 is a constant that depends on the desorption kinetics.16,17 Figure 3 shows the TPD curves of the tested ACs without adsorption of DCM at βH = 0.117 K·s−1. It shows that almost no peak can be detected in each TPD curve in the whole measurement range, indicating there are almost no organic impurities on the tested ACs. Figure 4 panels a to f show the TPD curves of DCM desorption from the tested ACs at heating rates βH in the range from 0.05 to 0.117 K·s−1. It is clearly observed that each TPD curve shows a distinct peak due to the desorption of DCM

Table 3. Content of the Metal Element on the Surfaces of the ACs activated carbons metal ion impregnated Qmetal (mmol·g−1)a a

AC

Li(I)/AC

Ag(I)/AC

Al(III)/AC

+

+

3+

Li 0.498

Ag 0.489

Al 0.502

Fe(III)/AC 3+

Fe 0.505

Mg(II)/AC Mg2+ 0.486

Content of metal element loaded. C

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from the carbon surfaces, and the peak temperature (Tp) increased with the increasing heating rates (βH). When a series of peak temperatures Tp at different βH were obtained from Figure 4, the Ed of DCM on the modified ACs could be estimated according to eq 2. The plot of ln (RT2p/βH) against 1/Tp for the TPD of DCM on the modified ACs is shown in Figure 5. From the slopes of these lines, the Ed of DCM on the tested ACs can be obtained. Table 4 lists the Ed of DCM on the modified ACs. The data in Table 4 show that the Ed of DCM on the modified ACs followed the order: EAl(III)/AC > ELi(I)/AC > EMg(II)/AC > EFe(III)/AC > EAC > EAg(I)/AC. As compared to that of the original AC, the Ed of DCM on the Al(III)/AC, Li(I)/AC, Mg(II)/AC, and Fe(III)/AC became higher, while the Ed of DCM on the Ag(I)/AC was lower. The higher the Ed of DCM is, the more difficult the DCM desorbs from the surface of the ACs. In the other words, the higher Ed of DCM meant the

Figure 3. TPD curves of the six different activated carbons without adsorption of DCM, experimental condition: βH = 0.117 K·s−1 (flow rate, 0.667 mL·s−1): ▲, Al(III)/AC; Δ, Li (I)/AC; ●, Mg(II)/AC; ○, Fe(III)/AC; ■, AC; □, Ag(I)/AC.

Figure 4. Effect of heating rates on TPD curve for the desorption of DMC from ACs (a−f). For the nitrogen gas, flow rate = 0.667 mL·s‑1: ■, 0.050 K·s‑1; □, 0.067 K·s−1; ▲, 0.083 K·s−1; Δ, 0.100 K·s−1; ★, 0.117 K·s−1. D

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Table 4. The Desorption Activation Energy of DCM on the Modified ACs Ed

Qm

activated carbons

0.050 K·s−1

0.067 K·s−1

peak temperature Tp with different heating rates (K) 0.083 K·s−1

0.100 K·s−1

0.117 K·s−1

kJ·mol−1

mg·g−1

Al(III)/AC Li (I)/AC Mg(II)/AC Fe(III)/AC AC Ag(I)/AC

404.15 384.35 399.65 410.15 403.35 380.65

407.55 392.95 414.15 419.35 409.35 389.35

415.85 400.45 421.05 423.15 419.15 401.85

421.45 406.75 423.15 432.15 428.65 404.75

432.15 410.75 429.15 443.55 435.15 411.75

35.42 34.87 34.22 31.8 29.76 27.9

321.1 301.9 291.2 253.7 229.7 217.4

detail in the reference.20 On the other hand, according to the classification of Pearson’s HSAB theory, the ions Al3+, Li+, Mg2+, and Fe3+ belong to hard acid, while Ag+ belongs to soft acid.21 When the ACs were doped separately with hard acid Al3+, + Li , Mg2+, and Fe3+, the local acid hardness of these AC surfaces would be enhanced. As a result, according to HSAB principle “hard acids prefer to be bonded to hard bases”, it can be predicted that the interaction between DCM and Al(III)/AC, Li(I)/AC, Mg(II)/AC, and Fe(III)/AC could be enhanced since DCM is a hard base. In addition, when the AC were doped with soft acid Ag(I), the local acid hardness of this AC surfaces could be weakened. As a result, it can be predicted that the interaction between DCM and Ag(I)/AC surfaces would be weakened. Interestingly, this prediction was confirmed by Qm and Ed of the DCM on the modified ACs and the original AC, as shown in Table 4. It can be seen again that after the hard acid ions Al3+, Li+, Mg2+, and Fe3+ were loaded onto the AC surfaces, the working adsorption capacity and desorption activation energy of DCM on the Al(III)/AC, Li(I)/AC, Mg(II)/AC, or Fe(III)/ AC became higher compared to that of the original AC, and after the hard acid ion Ag+ was loaded onto the AC surfaces, the working adsorption capacity and desorption activation energy of DCM on the Ag(I)/AC became lower compared to that of the original AC. In short, the prediction on the basis of Pearson’s HSAB theory was in good agreement with the working adsorption capacity and desorption activation energy of DCM on these ACs estimated separately from experimental adsorption breakthrough curves and TPD spectra.

Figure 5. Linear dependence between ln(RT2P/βH) and 1/Tp for TPD of DCM on modified ACs: ▲, Al(III)/AC; Δ, Li (I)/AC; ●, Mg(II)/AC; ○, Fe(III)/AC; ■, AC; □, Ag(I)/AC (dotted lines fitted using eq 2).

stronger interaction between DCM and the surfaces of the modified ACs. The results above indicate that the adsorption of DCM on the Al(III)/AC, Li(I)/AC, Mg(II)/AC, and Fe(III)/AC is stronger, while the adsorption of DCM on the Ag(I)/AC is weaker as compared to the the adsorption of DCM on the virgin AC. 3.4. Effect of Local Hardness of AC Surfaces on Adsorption/Desorption Properties of DCM. Generally speaking, the adsorption property of an adsorbent is not only determined by its porous microtexture, but also strongly influenced by the chemical property of its surface. As shown in Table 4, the loading of different transition metals ions like Al3+, Li+, Mg2+, Fe3+, and Ag+ on the surfaces of the ACs leads to different adsorption/desorption properties for DCM, which can be attributed to the changes of the surface chemical properties of the modified ACs. In this work, the HSAB principle would be applied to explain the effect of local hardness of carbon surfaces on the adsorption/desorption properties toward DCM of the ACs modified by different metal ions. The HSAB principle was proposed by Pearson in 1963 and has been one of the bases of modern chemistry. It can be expressed as “the hard acids prefer to be bonded to hard bases, and the soft acids prefer to be bonded to soft bases”.18,19 According to the classification of the Pearson HSAB theory, when the electronegativity (χ) of a species is above 3, it is regarded as a hard base; when χ of a species is in the range of 2.8 < χ < 3, it is regarded as borderline base, and when χ of a species is below 2.8, it is regarded as soft base. DCM was regarded as a hard base specie since its χ is 3.381, being above 3, and its hardness is 8.273, which can be calculated on the basis of density functional theory (DFT). The method of calculating electronegativity and hardness of a molecule had been stated in

4. CONCLUSION (1) The ACs can be modified by impregnation with different metal salt solutions. The working adsorption capacities of DCM on various ACs are in the order of QAl(III)/AC > QLi(I)/AC > QMg(II)/AC > QFe(III)/AC > QAC > QAg(I)/AC. The desorption activation energy for DCM on the modified ACs follows the order of EAl(III)/AC > ELi(I)/AC > EMg(II)/AC > EFe(III)/AC > EAC > EAg(I)/AC, which is in accordance with the order of adsorption capacities of DCM on various ACs. (2) The doping of Al3+, Mg2+, Li+, and Fe3+ on the AC improved the interaction between DCM and the Al(III)/ AC, Li(I)/AC, Mg(II)/AC, Fe(III)/AC surface toward DCM because of the hardness similarity of the metal and DCM according to HSAB theory, whereas the doping of Ag+ weakened the interaction between DCM and the Ag(I)/AC surface toward DCM because of the hardness dissimilarity between DMC and Ag(I).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-851-3635032. Fax: +86-851-3625867. E-mail: [email protected]. E

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Notes

(16) Zhang, Z. J.; Xian, S. K.; Xi, H. X.; Wang, H. H.; Li, Z. Improvement of CO2 adsorption on ZIF-8 crystals modified by enhancing basicity of surface. Chem. Eng. Sci. 2011, 66, 4878−4888. (17) Li, X.; Chen, X.; Li, Z. Adsorption equilibrium and desorption activation energy of water vapor on activated carbon modified by an oxidation and reduction treatment. J. Chem. Eng. Data 2010, 55, 3164−3169. (18) Pearson, R. G. The HSAB principleMore quantitative aspects. Inorg. Chim. Acta 1995, 240, 93−98. (19) Pearson, R. G. Soft and hard acids and bases. Chem. Eng. 1963, 43, 90−104. (20) Xia, Q. B.; Li, Z.; Xi, H. X.; Xu, K. F. Dibenzofuran desorption from Fe3+/TiO2 and Ce3+/TiO2 photocatalysts coated onto glass fibres. Adsorpt. Sci. Technol. 2005, 23, 357−366. (21) Alfarra, A.; Frackowiak, E.; Béguin, F. The HSAB concept as a means to interpret the adsorption of metal ions onto activated carbons. Appl. Surf. Sci. 2004, 228, 84−92.

The authors declare no competing financial interest. Funding

The authors thank the Guizhou Provincial Governor 339 Foundation (Grant No. (2011)32) and the Science & 340 Technology Foundation of Guizhou Province (Grant No. 341 (2012)2152) for financial support.

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ACKNOWLEDGMENTS Very special thanks to Prof. Zhong Li for his useful suggestion devoted to improve our work.

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REFERENCES

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