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Adsorption and Its Mechanism of CS2 on Ion-Exchanged Zeolites Y Xi Chen, Ben-xian Shen,* Hui Sun,* Guo-xiong Zhan, and Zhen-zhen Huo State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: A series of ion-exchanged zeolites Y were prepared by postsynthesis method and characterized using multiple techniques including XRD, N2 adsorption, XPS, FTIR, and chemical composition analysis. The adsorption of carbon disulfide (CS2) on ion-exchanged zeolites Y was investigated by dynamic adsorption tests carried out at a fixed-bed adsorption unit coupled with density functional theory (DFT) computation. The effects of metal ions, gas hourly space velocity (GHSV), and operation temperature on the adsorption capacities of CS2 on ion-exchanged samples were studied carefully. In addition, the mechanism for CS2 adsorption on zeolite Y was discussed. Among all the ion-exchanged samples, Ag−Y zeolite holds the highest CS2 breakthrough adsorption capacity up to 44.8 mg/g. The XPS analysis indicates that Ag+ was introduced into the zeolite Y structure. The DFT study indicates that the S−M (σ) bond involving CS2 and Ag+ was formed, which strengthens the S−Ag interaction and, therefore, benefits the adsorption of CS2 on Ag−Y zeolite. The used Ag−Y zeolite can be regenerated by thermal treatment at 400 °C under an air atmosphere. After seven adsorption−regeneration cycles, the used Ag−Y maintains the adsorption capacity of 37.6 mg/g. in adsorption desulfurization processes.6−12 Specifically, Na−Y zeolites are found efficient in the selective adsorption of H2S from tail gas.13 Besides, transition metal ion-exchanged zeolites Y have been reported as promising sorbents for adsorption desulfurization in many processes.14−16 As for adsorption of thiophene, benzothiophene, and 2-methyl benzothiophene on ion-exchanged zeolites Y, Yang et al.17−20 and Song et al.,21−23 respectively, proposed two distinct mechanisms: π-complexation and sulfur−metal (S−M) bond formation. Different from the removal of sour sulfides including H2S and mercaptans, few researches have studied the adsorption removal of CS2 based on modified zeolites Y. Moreover, the adsorption mechanism of CS2 on modified Y zeolites is still not clear. Nowadays, density functional theory (DFT) is widely used to study adsorption behaviors of small molecules on zeolites24−26 and give a better understanding of adsorption mechanisms of sulfur species such as thiophene, benzothiophene, and dibenzothiophene on ionexchanged zeolites.27,28 Therefore, this study aims to screen an

1. INTRODUCTION As a hazardous compound, carbon disulfide (CS2) is proven to trigger severe diseases such as coronary artery disease and optic atrophy.1 However, carbon disulfide widely exists in various fossil energy resources (e.g., coal, petroleum, petroleum gas, and natural gas), leading to its increasing emission from industrial tail gases because of the enhanced energy production.2 Moreover, carbon disulfide is involved in the production of rayon, cellophane, and cellulose xanthate, causing a large amount of carbon disulfide emission into the workspace.3 Due to harmful properties, direct emission of carbon disulfide to the atmosphere, hence, is prohibited. Moreover, even trace amounts of carbon disulfide can lead to significant poisoning deactivation of catalysts in numerous industrial catalytic processes.4,5 Therefore, reducing emission of CS2 is becoming an increasingly crucial issue, which greatly boosts the research on the removal of CS2 from various industrial gas mixtures. Among all potential methods, the adsorption process attracts increasing interests in academic and industrial research because of its extremely high removing selectivity as well as attractive economic advantage. Various adsorbents including modified composite oxides, activated carbon, and zeolites have been used © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

January 18, 2017 May 4, 2017 May 12, 2017 May 12, 2017 DOI: 10.1021/acs.iecr.7b00245 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

2.3. Adsorption Breakthrough Experiment. The dynamic adsorption performance of CS 2 on different adsorbents was studied on a laboratory-scale adsorption unit (Figure 1). A quartz fixed-bed adsorption column (having 100

efficient and renewable Y zeolite adsorbent modified by the ionexchange method for the removal of CS2 from tail gas and reveal the CS2 adsorption mechanisms by DFT calculation. In this work, we focused on the adsorption removal of CS2 from tail gas by applying a series of ion-exchanged zeolites Y. Co−Y, Zn−Y, Cu−Y, Ni−Y, and Ag−Y zeolites were synthesized via an ion-exchange method. They were characterized by X-ray diffraction (XRD), N2 adsorption, energy disperse spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculation was performed to study the CS2 adsorption mechanisms on Na− Y and Ag−Y. In addition, CS2 adsorption experiments were carried out in a fixed bed to investigate the dynamic adsorption characteristics of ion-exchanged zeolite samples. Furthermore, the multicycle adsorption and desorption of Ag−Y zeolite were also studied.

Figure 1. Experimental setup for dynamic adsorption tests.

2. EXPERIMENTAL SECTION 2.1. Preparation of Ion-Exchanged Zeolites Y. A series of samples were prepared by a liquid-phase ion-exchange process reported in previous publications.15,16,22 Using the commercial Na−Y zeolite (Nankai University Catalysts Co., Ltd., Tianjing, China) as the starting material. Ten grams of zeolite was immersed into 100 mL of 1 M solution of Cu2+, Co2+, Ni2+, and Zn2+ nitrates and 0.2 M AgNO3 solution and ion exchanged with stirring at 90 °C for 12 h. Then, the products were separated by filtering and washed with copious amount of deionized water. This filtering−washing process was repeated twice, and the solid then was dried at 120 °C overnight. Prior to the adsorption experiments, the samples were calcined in a muffle furnace under an air atmosphere at 400 °C for 5 h in order to ensure dehydration. 2.2. Characterization of Adsorbents. The crystal structure was characterized by using a powder X-ray diffraction (XRD) technique. XRD patterns were collected on a D8 Advance X-ray diffractometer (Bruker, Germany) using Nifiltered Cu Kα radiation operated at 40 kV and 100 mA. The data were collected with 2θ increasing from 10° to 80° at a step size of 0.02°. N2 adsorption was performed at −196 °C using an AS-6B physisorption analyzer (Qantachrom, USA) in order to characterize the pore structure of various ion-exchanged zeolites. All the samples were degassed at 200 °C for 2 h at a vacuum level of 10−2 Pa prior to the N2 adsorption. The surface area and volume of samples were calculated using the BET method and BJH method, respectively. In order to identify the chemical state of metal located on the zeolite, the X-ray photoelectron spectroscopy (XPS) analysis of the Ag-exchanged sample was conducted on an ESCALAB 250 Xi XPS spectrometer (ThermoFisher, USA) equipped with Mg anode. Fourier transform infrared spectroscopy (FTIR) analysis was carried out on a Nicolet 6700 Fourier transform spectrometer (ThermoFisher, USA) with an interval of 2 cm−1. Absorbance spectra in the wavenumber range of 400−4000 cm−1 were recorded. Chemical compositions of the parent Na−Y zeolite and ionexchanged samples were determined by applying energy disperse spectroscopy (EDS) analysis performed on a Falion 60 S energy dispersive spectrometer (EDAX, USA) operated at 20 kV and 20 nA.

mm long and inner diameter of 5 mm) was filled with 0.4 g of different ion-exchanged zeolites. The adsorption temperature was controlled using a thermostatic circulating bath. Prior to each adsorption run, pure nitrogen gas was introduced to the system while the bed was heated to each required adsorption temperature. Then, nitrogen gas containing about 400 mg/m3 (on weight of S basis) CS2 was fed from the top with a flow rate of 80 mL/min under atmospheric pressure. A gas chromatograph with flame photometric detector (FPD) was used to analyze the concentration of CS2 in the outlet gas. Furthermore, to study the effect of gas hourly space velocity (GHSV) on CS2 removal efficiency, N2 gas was first fed, and the adsorption column was maintained at 25 °C. Then, the CS2-containing N2 gas was fed with five flow rates: 40, 80, 160, 240, and 320 mL/min. Meanwhile, the dynamic adsorption experiments were conducted with a constant gas flow rate of 40 mL/min at different operation temperatures (25, 40, 60, and 80 °C) in order to examine the effect of temperature on CS2 adsorption on the modified zeolite bed. Once CS2 was detected in the outlet gas, the adsorption was considered to reach the breakthrough point. The CS2 removal rate is calculated using eq 1. η=

C0 − C × 100% C0

(1)

The CS2 breakthrough adsorption capacity is calculated using eq 2. ⎡ q = ⎢QC0 ⎢⎣

∫0

t

⎛ C⎞ ⎤ ⎜1 − ⎟dt ⎥/m C0 ⎠ ⎥⎦ ⎝

(2)

where η is the CS2 removal rate, C0 is the concentration of CS2 in the inlet gas (mg/m3), C is the concentration of CS2 in the outlet gas (mg/m3), t is the breakthrough time (min), Q is the inlet gas flow rate (m3/min), and m is the mass of sample (g). 2.4. Desorption Experiment. Desorption of CS2 from the Ag−Y zeolite was carried out in a fixed bed reactor (Figure 2). Water vapor, N2, or air was introduced to the fixed bed containing the CS2-saturated Ag−Y zeolite at the rate of 100 mL/min, while the temperature was raised from room temperature to different desorption temperatures and then maintained for 4 or 10 h. In addition, to test the effect of vacuum desorption, a vacuum pump was connected to the fixed B

DOI: 10.1021/acs.iecr.7b00245 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Pseudopots (DSPP) was applied. All self-consistent-field (SCF) tolerances were set to fine to improve the computational accuracy. The tolerance of energy and displacement was converged to 1.0e−5 and 0.005 Å, respectively. The adsorption energies were calculated using eq 3 ΔE = E(Z) + E(X ) − E(Z + X )

(3)

where E(Z + X) is the total energy of the optimized adsorption complex, E(Z) is the energy of the optimized 12T cluster, and E(X) is the energy of the isolated adsorbate molecule. Figure 2. Experimental setup for desorption experiment.

3. RESULTS AND DISCUSSION 3.1. Characterization of Zeolites Y. 3.1.1. Chemical Composition. The chemical compositions of all zeolite samples are summarized in Table 1. Using the ion-exchange method, various metal cations were introduced into the structures of zeolite Y successfully. Besides, the ion-exchanged samples show a similar exchange degree. In this case, the exchange degree can be controlled by the concentration of the ion solution. Moreover, a slight increase in the n(Si)/n(Al) ratio can be noticed after ion exchange. Such changes in the n(Si)/n(Al) ratio suggest that the evolution of the Na−Y structure happens during the ion-exchange process and causes the dealumination of zeolite framework.22 3.1.2. XRD. The XRD patterns corresponding to different ion-exchanged zeolite Y are presented in Figure 5. Compared to the standard pattern, the consistent XRD patterns of ionexchanged zeolites Y indicate that ion-exchanged samples keep the same framework structure to the parent Na−Y zeolite. A similar result has also been found elsewhere.20 However, the peak intensity of the Co, Ni, Cu, and Ag ion-exchanged samples exhibit slightly reduced peak intensity as compared to the original Na−Y sample, suggesting the crystallinity loss during the ion-exchange process or the following thermal treatment.14 3.1.3. Specific Surface Area and Porosity. The specific surface area and porosity of different zeolite samples are listed in Table 2. Compared with the parent Na−Y zeolite, the ionexchanged samples show both a smaller specific surface area (SBET and Smicro) and lower pore volume (Vtot and Vmicro). This result supports our previous conclusion that ion exchange has a slight effect on the Na−Y zeolite structure. 3.1.4. FTIR and XPS. The FTIR spectra of different ionexchanged zeolite samples are shown in Figure 6. Co, Cu, Zn, and Ni ion-exchange samples have the same spectra as those of the parent Na−Y zeolite. The main skeletal vibration peaks can be assigned to T−O bending vibration at 460 cm−1, double 6

bed to reach the pressure level of 10−1 to 10−2 Pa, while the temperature was raised to 400 °C and then maintained for 4 or 10 h. After that, 0.4 g of desorption-treated sample was filled into the aforementioned fixed-bed adsorption column, and the dynamic adsorption experiment was carried out under the following conditions: inlet gas flow rate 160 mL/min and operation temperature 25 °C. Each adsorption−regeneration cycle was repeated for three times at least, and the error level can be controlled under 4%. 2.5. Cluster Models. DFT computation based on cluster models of the active site are performed to reveal the mechanism for CS2 adsorption on zeolite Y.24−29 As shown in Figure 3, a 12T cluster (Si12O33H18) model including a single six ring (S6R) around site II and a partial of a supercage was extracted from the Y zeolite framework. The Si atoms of dangling Si−O bonds at the edge of the cluster model were replaced by hydrogen atoms, and the length of the O−H bonds were set to 1.1 Å. The terminal O−H bonds were fixed, while all other atoms were allowed to relax during geometry optimizations. As shown in Figure 4, to stabilize the exchanged cation, one or two Si atoms of the S6R can be replaced by Al atoms to introduce the extra negative charge. The negative charges then can be balanced by Na+, Co2+, Zn2+, Ni2+, or Ag+ in ion-exchanged zeolites Y, respectively. 2.6. Density Functional Theory (DFT) Calculation. All the DFT calculations were carried out using the Dmol3 module in Materials Studio 7.0 package (Accelrys Ltd.). The Perdew− Burke−Ernzerhof (PBE) functional with general gradientcorrected (GGA) was used to treat electronic energy of exchange correlation. The system spin was set unrestricted. The Grimme method for density functional dispersion correction (DFT-D) was carried out to describe the long-range van der Waals interactions. The basis set of double numerical plus polarization (DNP) and core treatment of DFT Semicore

Figure 3. Structures of the FAU zeolite showing site II and a 12T zeolite cluster models showing site II. C


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Figure 4. Cluster model of M(I)X zeolites (a), where M could be an Na or Ag atom, and M(II)X zeolites (b), where M could be a Co, Zn, Cu, or Ni atom.

Table 1. Chemical Compositions of Na−Y and IonExchanged Zeolites samples

chemical composition

n(Si)/n(Al)

Na−Y Ag−Y Cu−Y Zn−Y Co−Y Ni−Y

Na0.292Al0.304Si0.866O2.334 Na0.105Ag0.181Al0.301Si0.868O2.321 Na0.101Cu0.098Al0.298Si0.867O2.329 Na0.111Zn0.091Al0.300Si0.863O2.322 Na0.098Co0.094Al0.295Si0.866O2.318 Na0.107Ni0.093Al0.298Si0.867O2.327

2.85 2.88 2.91 2.88 2.92 2.91

Figure 6. FTIR spectra of ion-exchanged zeolites Y.

ring vibration at 580 cm−1, and tetrahedron stretching vibration at 1025, 780, and 725 cm−1.30,31 Moreover, the vibration band appearing at about 1630 cm−1 can be attributed to O−H bending or H2O deformation mode,32 while the broad peak near 3500 cm−1 can be assigned to O−H stretching vibration of hydration water.33 As for the Ag−Y zeolite, the absence of a tetrahedron stretching vibration at 720 cm−1 can be ascribed to the changes in the zeolite framework resulting from Ag exchange.30 The XPS pattern of Ag 3d spectra for the Ag−Y zeolite is presented in Figure 7. The XPS patterns of other ionexchanged samples are shown in the Supporting Information. The peak at a binding energy of 368.8 eV indicates that silver is present as Ag+ in the Ag−Y zeolite structure.14 3.2. Dynamic Adsorption Performance. Adsorption breakthrough curves of CS2 on parent Na−Y and ionexchanged zeolites are presented in Figure 8. Na−Y shows a very short breakthrough time (around 20 min), indicating that the unmodified Na−Y can effectively adsorb CS2 from tail gas but shows low adsorption capacity. All the ion-exchange modified zeolites show longer adsorption breakthrough times than the Na−Y sample, suggesting their higher CS2 adsorption capacities. Specifically, among these ion-exchanged samples, Ag−Y exhibits the best CS2 removal performance, which reaches an adsorption breakthrough time of 620 min. The breakthrough adsorption capacity for CS2 ranks in the order of Ag−Y > Cu−Y > Ni−Y/Zn−Y/Co−Y > Na−Y. In

Figure 5. X-ray diffraction patterns of Na−Y and ion-exchanged samples.

Table 2. Specific Surface Area and Porosity of Various Zeolites Y samples

SBET (m2/g)a

Vtot (cm3/g)b

Smicro(m2/g)c

Vmicro(cm3/g)d

Na−Y Ag−Y Cu−Y Zn−Y Co−Y Ni−Y

578 511 514 498 476 454

0.317 0.272 0.285 0.297 0.273 0.271

521 489 513 485 478 466

0.273 0.261 0.269 0.254 0.249 0.241

a

BET surface area. bTotal pore volume. cMicropore area. dMicropore volume.

D


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Figure 7. XPS curves of Na−Y and Ag 3d regions on the surface of Ag−Y.

Figure 9. Breakthrough curves for CS2 adsorption on Ag−Y zeolite bed with different GHSV at 25 °C.

Figure 8. Breakthrough curves for CS2 adsorption on different ionexchange zeolites with GHSV of 19,200 h−1 at 25 °C.

Figure 10. Breakthrough adsorption capacity for CS2 adsorption on Ag−Y zeolite bed with different GHSV at 25 °C.

space velocity, the diffusion time was shortened. As a result, the breakthrough adsorption capacity decreases. In addition, the influence of operation temperature on CS2 adsorption on Ag−Y zeolite was examined. The results of different GHSV tests indicate that the molecular diffusion effect can only be neglected compared to the adsorption process at GSHV lower than at least 9600 h−1. So, a GHSV of 4800 h−1 was chosen to eliminate the diffusion limitations and make sure the temperature effect was solely caused by the adsorption step. With a constant GSHV of 4800 h−1 , the adsorption breakthrough curves at different temperatures are shown in Figure 11. With temperature increasing from 25 to 80 °C, the adsorption breakthrough time was reduced from 1275 to 1010 min. Furthermore, the breakthrough adsorption capacity was found to decrease from 50.9 to 40.4 mg/g (Figure 12). By increasing the operation temperature, the molecules diffusion rate was increased. On one hand, the increment of molecules diffusion rate can shorten the mass transfer zone and, therefore, benefits the improvement the utilization efficiency of adsorption bed. On the other hand, the adsorption of CS2 on Ag−Y is an exothermic process. Increasing adsorption temperature could lead to a decreasing equilibrated adsorption capacity. In the case of CS2 adsorption on Ag−Y, the

consideration of the higher CS2 removal efficiency, this research was, therefore, focused on Ag−Y zeolite. Furthermore, dynamic adsorption experiments were performed with the gas hourly space velocity (GHSV) increasing from 4800 to 38,400 h−1 in order to evaluate the effect of GHSV on the adsorption performance of the Ag−Y zeolite bed. The adsorption breakthrough curves corresponding to different GHSV are depicted in Figure 9. Increasing GHSV leads to shortening breakthrough time and declining breakthrough adsorption capacities (Figure 10). In our experiments, the bed adsorbent height and packing density of zeolite were fixed. As the space velocity increases from 4800 to 38,400 h−1, the breakthrough adsorption capacity, q, decreases from 50.9 to 35.1 mg/g. However, the breakthrough adsorption capacities under high GHSV and low GHSV have different changing trends. When the space velocity was increased from 4800 to 9600 h−1, the breakthrough adsorption capacity is decreased by only 0.6 mg/g. It confirms that the molecule diffusion is not a major limiting step under low GHSV. However, when the space velocity increases from 9600 to 38,400 h−1, the breakthrough adsorption capacity is reduced by 14.5 mg/g, suggesting the significant impact of molecule diffusion on dynamic adsorption performance of CS2 on the Ag−Y zeolite bed. By increasing the E


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models before and after adsorption and the adsorption energies are listed in Table 3. The computational energies for CS2 adsorption on Na−Y and Ag−Y are 43.121 and 78.481 kJ/mol, respectively. It can be concluded that Ag−Y has a much stronger interaction with CS2 as compared to Na−Y, which is in good agreement with the result of dynamic adsorption experiments. For CS2 adsorption on Na−Y, neither Mayer bond order between Na and S nor significant change in CS bond length was found, indicating no direct interaction (S−M interaction) formed between Na and S. However, the Mayer bond orders for CS1 and CS2 increase from 1.870 for bulk CS2 to 1.946 and 1.910 for adsorbed CS2, while the bond angle of CS2 decreases from 180° to 176.22°. It means that the CS bond can be strengthened after adsorption. Furthermore, the Mulliken population analysis (Table 4) can help us understand the adsorption mechanism better. After adsorbing CS2, the Na 3s orbital population is decreased from 0.091 to 0.077, while the Na 3p and 3d orbital population was increased from 0.141 and 0.057 to 0.164 and 0.071, respectively. Here, S1 and S2 exhibit the consistent varying trend. Specifically, the S1 3s orbital population was reduced, while both S1 3p and 3d orbital population is increased. Meanwhile the C 2s and 2p orbital population was increased from 1.049 and 2.238 to 1.097 and 2.433, respectively. Such computational results indicate that the π-complexation may be involved during the adsorption process. In comparison to Na−Y, Ag−Y shows quite different interaction with CS2. For CS2-equilibriated Ag−Y, the bond length and Mayer bond order of Ag−S1 are calculated to be 2.469 Å and 0.567, respectively. The bond length and Mayer bond order of CS1 change from 1.571 Å and 1.870 for bulk CS2 to 1.597 Å and 1.667 for the adsorbed CS2. Meanwhile, the bond length and Mayer bond order of CS2 change from 1.571 Å and 1.870 to 1.555 Å and 2.056, respectively. It is indicated that the CS1 bonding is weakened, while the CS2 bonding is strengthened. Moreover, the Mulliken population analysis (Table 4) shows the orbital populations of Ag 5s, Ag 5p, S1 3d, and S2 increase, while the orbital populations of S1 3s and S1 3p are reduced. However, the C 2s, 2p and 3d orbital population has no significant change during adsorption. All the computational results indicate that the S−M (σ) bond between S1 and Ag rather than π-complexation between Ag and both CS double bonds plays the crucial role in determining the adsorption of CS2 on Ag−Y. Zeolite Ag−Y hosts CS2 through both chemical (S−M interaction) and physical interactions. Furthermore, the strong S−M interaction between CS2 and Ag+ enhances the adsorption of CS2 on Ag−Y. 3.4. Desorption and Regeneration of CS2-Saturated Ag−Y Zeolite. Considering that some of the breakthrough times exceed 10 h and in many cases desorption will be slower than adsorption, desorption experiments were run for 4 and 10

Figure 11. Breakthrough curves for CS2 adsorption on Ag−Y zeolite bed with the GHSV of 4800 h−1 at different operation temperatures.

Figure 12. Breakthrough adsorption capacity for CS2 adsorption on Ag−Y zeolite bed with the GHSV of 4800 h−1 at different operation temperatures.

adsorption temperature shows an overwhelming effect on equilibrated adsorption capacity over diffusion rate. Therefore, we can believe that lower temperature is in favor of CS2 adsorption on a fixed bed of Ag−Y zeolite. 3.3. Mechanism for CS2 Adsorption on Na−Y and Ag− Y Zeolites. The adsorption configurations of CS2 on Na−Y and Ag−Y cluster models are displayed in Figure 13. The computational calculation results for other ion-exchanged samples are given in the Supporting Information (Figures S5 and S6). The bond length and Mayer bond order of cluster

Figure 13. Adsorption configurations of CS2 on Na−Y(a) and Ag−Y(b) cluster models. F


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Industrial & Engineering Chemistry Research Table 3. Adsorption Energy, Bond Length, and Mayer Bond Orders of Cluster Models before adsorption

after adsorption

bond

bond length (Å)

Mayer bond order

bond length (Å)

Mayer bond order

adsorption energy(kJ/mol)

CS2/Na−Y

Na−S CS1 CS2

− 1.571 1.571

− 1.870 1.870

3.393 1.576 1.571

− 1.946 1.910

43.121 43.121 43.121

CS2/Ag−Y

Ag−S1 CS1 CS2

− 1.571 1.571

− 1.870 1.870

2.469 1.597 1.555

0.567 1.667 2.056

78.481 78.481 78.481

water vapor or nitrogen flow provide very low desorption efficiency. After desorption under vacuum, water vapor, and nitrogen sweeping, the revived Ag−Y shows the low adsorption capacity of 3.6, 11.4, and 12 mg/g, accounting for around 8%, 25%, and 27% of the capacity of fresh sample (44.8 mg/g). As we have mentioned before, chemical adsorption involving Ag− S interaction is responsible for the strong binding energy for CS2 adsorption on the Ag−Y zeolite. As a result, the adsorbed CS2 was hardly released from adsorption sites. However, the Ag−Y zeolite undergoing thermal treatment under an air atmosphere shows the very close adsorption capacity of 41.2 mg/g to the fresh Ag−Y zeolite. Different from the treatment under vacuum, water vapor, and nitrogen sweeping, the chemisorbed CS2 can be oxidized into SOx during thermal treating under an air atmosphere, so that the CS2-occupied adsorption active sites can be released and to host other guest molecules.34 In addition, the influence of operation temperatures on regeneration of the Ag−Y zeolite under an air atmosphere is examined (Figure 15). With an increasing temperature, the

Table 4. Mulliken Population Analysis for CS2 Adsorption on Na−Y and Ag−Y system CS2/Na−Y

atom Na

S1

C

S2

CS2/Ag−Y

Ag S1

C

S2

before adsorption

after adsorption

3s 3p 3d 3s 3p 3d 2s 2p 3d 3s 3p 3d

0.091 0.141 0.057 1.950 4.198 0.152 1.049 2.238 0.067 1.950 4.198 0.152

0.077 0.164 0.071 1.912 4.095 0.166 1.097 2.433 0.067 1.918 4.117 0.166

5s 5p 3s 3p 3d 2s 2p 3d 3s 3p 3d

0.244 0.192 1.950 4.198 0.152 1.049 2.238 0.067 1.950 4.198 0.152

0.415 0.259 1.899 4.090 0.209 1.098 2.352 0.065 1.916 4.015 0.175

h, respectively. However, there is no significant increase in breakthrough capacity of regenerated Ag−Y found as desorption time is extended (Figure 14). Therefore, desorption time is fixed at 4 h. Both vacuum operation and sweeping with

Figure 15. Breakthrough adsorption capacity for CS2 adsorption on Ag−Y regenerated under air atmosphere at different temperatures (adsorption condition: GHSV of 19,200 h−1, operation temperature of 25 °C).

regenerated zeolite shows increasing adsorption capacities until reaching the maximum of 41.2 mg/g. To continue to raise temperature, a reduced adsorption capacity of 40.4 mg/g is observed at 500 °C, which can result from the negative impact of thermal treatment at high temperature on a zeolite structure. A temperature of 400 °C, hence, can be considered a suitable regeneration condition. Multiple adsorption−regeneration cycles were also tested. The used Ag−Y zeolite was desorbed under an air atmosphere at 400 °C and filled in the fixed bed for the repeated breakthrough experiments with GHSV of 19,200 h−1 and

Figure 14. Breakthrough adsorption capacity for CS2 adsorption on Ag−Y regenerated at 400 °C (adsorption condition: GHSV of 19,200 h−1, operation temperature of 25 °C). G



■

adsorption temperature of 25 °C. Figure 16 displays the changes in breakthrough capacity for CS2 adsorption on a Ag−

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 86-21-64252851(Ben-xian Shen). *E-mail: [email protected]. Tel: 86-21-64252916 (Hui Sun). ORCID

Xi Chen: 0000-0003-4274-7578 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Training Program of the Major Research Plan of the National Natural Science Foundation of China (Grant 91634112), Natural Science Foundation of Shanghai (Grant 16ZR1408100), Fundamental Research Funds for the Central Universities of China (Grant 22A201514010), and Open Project of State Key Laboratory of Chemical Engineering (SKL-ChE-16C01).

Figure 16. Breakthrough adsorption capacity for CS2 adsorption on Ag−Y regenerated under air atmosphere at 400 °C (adsorption condition: GHSV of 19,200 h−1, operation temperature of 25 °C).

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Y zeolite during multiple adsorption−regeneration cycles. After three adsorption−regeneration cycles, the used Ag−Y has a breakthrough adsorption capacity of 38.2 mg/g, showing a 14.7% reduction of adsorption capacity compared to the fresh sample and keeping a small decrease after seven cycles. This indicates that the structure and active sites of Ag−Y zeolites have good thermal stability,34 which is very useful for practical applications.

4. CONCLUSIONS Ion-exchanged zeolites Y were postsynthesized as the adsorbents for the removal of CS2 from tail gas. Among all the adsorbent samples, the Ag−Y zeolite shows the highest CS2 breakthrough adsorption capacity up to 44.8 mg/g with the GHSV of 19,200 h−1 and operation temperature of 25 °C. High GHSV or operation temperature can reduce the CS2 adsorption capacity on the Ag−Y bed. Both XPS and chemical composition analysis confirm the introduction of Ag+ into the zeolite Y structure. The DFT computation indicates the formation of the Ag−S (σ) bond. The large adsorption energy resulting from the S−M (σ) interaction, therefore, benefits the adsorption of CS2 on the Ag−Y zeolite. The best regeneration performance of used Ag−Y zeolite was observed through thermal treatment at 400 °C under an air atmosphere. After seven adsorption−regeneration cycles, the used Ag−Y has the adsorption capacity of 37.6 mg/g.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00245. XPS pattern of Cu 2p spectra for Cu−Y zeolite; XPS pattern of Co 2p spectra for Co−Y zeolite; XPS pattern of Zn 2p spectra for Zn−Y zeolite; XPS pattern of Ni 2p spectra for Ni−Y zeolite; adsorption configurations of CS2 on Cu−Y, Co−Y, Zn−Y, and Ni−Y cluster models; adsorption energies of CS2 on Cu−Y, Co−Y, Zn−Y, and Ni−Y cluster models. (PDF) H


Article

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Adsorption and Its Mechanism of CS2 on Ion ... - ACS Publications

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