Ion-Exchanged Zeolites Y for Selective Adsorption of Methyl

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Ion-exchanged zeolites Y for selective adsorption of methyl mercaptan from natural gas: experimental performance evaluation and computational mechanism explorations Xi Chen, Ben-xian Shen, Hui Sun, and Guo-xiong Zhan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01982 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

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Industrial & Engineering Chemistry Research

Ion-exchanged zeolites Y for selective adsorption of methyl mercaptan from natural gas: experimental performance evaluation and computational mechanism explorations Xi Chen, Ben-xian Shen*, Hui Sun*, and Guo-xiong Zhan Address: State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Email: [email protected], [email protected] Abstract: Zeolites Y were modified by ion-exchange method and their structural properties were examined using N2 adsorption, FTIR, XRD, XPS and chemical composition analysis. The dynamic adsorption of methyl mercaptan (CH3SH) on different ion-exchanged zeolites Y was conducted at a fixed-bed adsorption column. The effects of gas hourly space velocity (GHSV), operation temperature and composition of the feed gas on the performance of CH3SH adsorption on ion-exchanged zeolites Y were studied carefully. Furthermore, the adsorption mechanism for CH3SH and CO2 adsorption on ion-exchanged zeolites were revealed by using density functional theory (DFT) calculation method. Among all the ion-exchanged samples, Cu-Y holds the highest CH3SH breakthrough adsorption capacity, q, of up to 70 mg/g. When using natural gas containing 4% CO2 as the feed, q of Cu-Y was slightly reduced to 64 mg/g. The DFT calculation results indicate that the S-M bond is formed between CH3SH and Cu2+ during CH3SH adsorption, which benefits the adsorption of CH3SH on Cu-Y zeolite. 1

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Moreover, the DFT calculation suggests the weak Cu-O bonding interaction formed in the adsorption of CO2 on Cu-Y, which interaction releases much smaller energy compared with CH3SH adsorption. The CH3SH-saturated Cu-Y sample can be regenerated by thermal treatment under air atmosphere at 350

o

C. After six

adsorption-regeneration cycles, the regenerated Cu-Y shows q of 55 mg/g, which is 21.4 % lower than that of the fresh sample. Keywords: ion-exchanged zeolites Y; methyl mercaptan; selective adsorption desulfurization; S-M interaction

1. Introduction As a major energy resource, natural gas usually contains various impurities including carbon dioxide (CO2), hydrogen sulfide (H2S), and trace of organosulfurs such as carbonyl sulfide (COS) and mercaptans.1 In order to meet the stringent emission standards, the harmful sulfur-containing compounds in natural gas should be removed efficiently. Nowadays, the natural gas purification process is dominated by chemical absorption

method

N-methyldiethanolamine

using

a

(MDEA),

variety

of

alkanolamine

diethanolamine

(DEA),

solvents

(e.g.

monoethanolamine

(MEA)).2-4 These solvents show excellent performance for the removal of H2S and CO2 from natural gas. However, they have been proven to be less effective for removing organosulfurs from natural gas.5 An available technique for the removal of COS is based on catalytic hydrolysis, which can convert COS to H2S and CO2.6-9 Hence, catalytic 2


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hydrolysis coupled with amine treating can easily remove COS from natural gas. As a result, the desulfurization performance for natural gas can be largely determined by the removal efficiency of mercaptans. So the removal of mercaptan compounds from amine-treated natural gas is receiving the wide research interests.6,15 At present, the efficient removal of mercaptans is generally based on fixed bed adsorption and chemical conversion. Active carbon10-11 and metal-promoted zeolites12-14 have been reported as the efficient adsorbents for mercaptans removal. In most cases, active carbons show lager capacities and lower costs than zeolites, however, they also show the disadvantages of poor regenerability and inflammability, which is not a problem for zeolites.15 Transition metal ion-exchanged zeolites Y have been proved promising for desulfurization in many processes.16-18 Furthermore, Song19-20 and Yang21-22 respectively proposed two distinct adsorption desulfurization mechanisms: S-M interaction and π-complexation. Density functional theory (DFT) calculations have been widely used to study the adsorption of different molecules on solid materials,23,24 and can help us understand the adsorption mechanism (e.g. thiophene, benzothiophene and DMDS adsorption on zeolites) better.25,26 The metal modified zeolites can be, therefore, expected to display high efficiency for the removal of mercaptans from natural gas. However, low concentration of CO2 still exists in the amine-treated natural gas, so the competitive adsorption between CO2 and mercaptans should be considered during the development of adsorbents. Unfortunately, few publications report the competitive adsorption behavior 3



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and adsorption mechanisms of CO2 and mercaptans, especially based on ion-exchanged zeolites Y. In this work, we focused on the adsorption removal of CH3SH from natural gas by using a series of transition metal (Cu2+, Co2+, Zn2+, Ni2+) exchanged zeolites Y. They were characterized by energy disperse spectroscopy (EDS), N2 adsorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The competitive adsorption between CO2 and CH3SH on ion-exchanged samples was evaluated carefully and the selective adsorption mechanism was explored by using DFT calculation. In addition, the multiple adsorption-regeneration cycling process of fixed-bed filled with Cu-Y was also investigated.

2. Experimental 2.1 Preparation of ion-exchanged zeolites Y The commercial Na-Y in powder form (Nankai University Catalysts Co., Ltd., China) was used as the staring material. All ion-exchanged samples were prepared by liquid-phase ion-exchange method reported in the previous publications.16-19 10 g parent Na-Y was immersed in 200 ml 0.5 M solutions of Cu(NO3)2, Co(NO3)2, Ni(NO3)2 and Zn(NO3)2, respectively, and the mixtures were stirred at 80 oC for 24 h. Then the resulting products were separated by filtering, washed twice, hereafter dried at 120 oC overnight and finally calcined in a muffle furnace at 450 oC for 5 h.

4


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2.2 Characterizations Energy disperse spectroscopy (EDS) analysis was carried out on a Falion 60 S energy dispersive spectrometer (EDAX, USA). Five element point analyses were performed at random positions to obtain the average elemental composition of each sample. The surface area and pore structure of samples were analyzed on performed at an As-6B physical adsorption analyzer (Qantachrom, USA). Prior to the N2 adsorption test, the samples were heated to 200 oC and maintained for 2 hours under vacuum level of 10×10-2 Pa. The αs-plot method was used to calculate the surface area and pore volume of samples. X-ray diffraction (XRD) analysis was carried out on a D8 X-ray diffractometer (Bruker, Germany) equipped with Cu-Kα radiation, the 2θ scanning angle range was 3-50° with a step of 0.02°. Fourier transform infrared spectroscopy (FTIR) spectra were collected on a Nicolet 6700 Fourier Transform spectrometer (ThermoFisher, USA) with a resolution of 2 cm-1 in the range of 400-4000 cm-1. X-ray photoelectron spectroscopy (XPS) analysis of Cu-exchanged samples was conducted on a PHI-5300 ESCA spectrometer (PE, USA) equipped with Mg anode.

2.3 Adsorption breakthrough experiment A laboratory scale adsorption set-up (see Figure 1) was used to evaluate the dynamic 5



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adsorption performance of CH3SH on different zeolite samples. 0.2 g adsorbent sample was loaded in a quartz fixed-bed adsorption column (3 mm i.d and 100 mm length). The temperature of adsorption column was controlled by using a thermostatic circulating bath. When reaching the desired temperature, the adsorption column was first fed with nitrogen gas, and then methane gas having CH3SH concentration of 500 mg/m3 with a flow rate of 100 mL/min. A gas chromatograph with flame photometric detector (FPD) was used to analyze the concentration of CH3SH in the outlet gas. Moreover, in order to examine the influence of gas hourly space velocity (GHSV) on CH3SH removal, five different flow rates (i.e. 50, 100, 150, 200 and 250 mL/min) and a constant temperature of 25

o

C were used. Meanwhile, four different adsorption

temperatures (i.e. 25, 40, 60 and 80 oC) and a constant flow rate of 100 mL/min were used in order to study the effect of operation temperature on CH3SH adsorption on ion-exchanged zeolites. In addition, to figure out the competitive adsorption behavior between CO2 and CH3SH, the dynamic experiments were performed at gas flow rate of 100 mL/min and operation temperature of 25 oC using the natural gas with both CH3SH (concentration of 500 mg/m3) and CO2 (concentrations of 0.5, 1, 2 or 4 %). The concentration of CO2 in the outlet gas was monitored by using a Gastec tube detector (4H, Japan).

6


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1-CO2 cylinder; 2-CH3SH and CH4 cylinder; 3-N2 cylinder; 4, 5, 6, 7-mass flow controller; 8-temperature controller; 9-adsorption column; 10-gas chromatograph; 11-NaOH solution; 12-vent

Figure 1. Experimental set-up for dynamic adsorption tests.

The CH3SH breakthrough adsorption capacity, q, is calculated using Eq. (1). t  C  q = QC0 ∫ (1 − )dt  / m 0 C0  

(1)

where C0 and C are the concentrations of CH3SH in inlet and outlet gases, respectively, mg/m3; t is the CH3SH breakthrough time, min; Q is the inlet gas flow rate, m3/min; m is the mass of adsorbent sample, g.

2.4 Regeneration experiment A fix bed reactor was used to conduct the regeneration of the used Cu-Y samples. Air was introduced to the reactor loaded with CH3SH-saturated Cu-Y zeolite at a flow rate of 80 mL/min, while the reactor was heated to a desired temperature and maintained for 4 h. After that, 0.2 g treated sample was used to evaluate the adsorption performance through the dynamic adsorption experiment carried out under aforementioned conditions.

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2.5 Structure mode

Figure 2. Structures of the FAU zeolite showing site II (a) and a 12T zeolite cluster models showing site II (b).

Figure 2 (a) shows the structure of the zeolite Y framework (the structural properties are shown in Supporting Information). The active sites (site II) are located in the center of the six ring (S6R), which are the active center for catalysis reaction and adsorption.23-26 A 12T cluster (Si12O33H18) model which has a S6R around site II was extracted from the zeolite Y framework (see Figure 2 b) as the original cluster model in this work. The Si atoms at the edge of cluster model were replaced by H atoms, and the length of O-H bond was fixed at 1.1 Å. Meanwhile, other atoms in cluster model were allowed to relax during the geometry optimization process. One or two Si atoms were replaced by Al atoms to introduce the extra negative charges (see Figure 3) and to stabilize the exchanged cations. Three types of cluster models were established. In the first cluster, the negative charge was balanced by Na+ to 8


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represent Na-Y zeolite. There are two different Al distributions in the second and third ones. The negative charges were balanced by Cu2+ to represent Cu-Y zeolites. The second cluster is denoted as M(i)-Y and the third one is denoted as M(ii)-Y.

Figure 3. The cluster model (a) represents M(i)-Y zeolite and cluster models (b) and (c) with different Al distributions represent M(ii)-Y zeolites.

2.6 Computation method The Dmol3 module in Materials Studio 7.0 package (Accelrys Ltd.) was used to perform all DFT calculations in this work. The electronic energy calculations were performed using the Perdew-Burke-Ernzerhof (PBE) function with the Generalized Gradient Approximation (GGA). The system spin was set to unrestricted in this work. The Grimme method for DFT-D was performed to describe the van der Waals interactions. The basis set of polarization functions (DNP) and core treatment of DFT Semi-core Pseudopots were applied. In order to improve the computational accuracy, all SCF tolerances were set to fine. The adsorption energies were calculated as following: ∆E= E(Z+X)−E(Z)−E(X)

(2)

where E(Z+X) is the total energy of the adsorption complex, E(Z) is the energy of 9



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cluster and E(X) is the energy of the adsorbate.

3. Results and discussion 3.1 Characterization of zeolites Y Chemical composition The chemical compositions of parent Na-Y and ion-exchanged zeolites are shown in Table 1. Via ion-exchange, metal ions (Cu2+, Zn2+, Co2+ and Ni2+) were successfully introduced into the structure of zeolite Y. Furthermore, Cu-Y, Co-Y and Ni-Y show very similar exchange degree, indicating that zeolite Y has similar exchange capacities for Cu2+, Co2+ and Ni2+ ions under the aforementioned preparation condition. Zn-Y shows the highest exchange degree, suggesting its higher selectivity for Zn2+ as compared to Cu2+, Co2+ and Ni2+ ions.22 Furthermore, the n(Si)/n(Al) ratios of exchanged zeolites Y are slightly increased, resulting from the dealumination effect of exchange on the framework.27 Table 1. Chemical compositions of ion-exchanged samples Samples

Chemical composition

n(Si)/n(Al)

Na-Y

Na0.265Al0.301Si0.897O2.378

2.98

Cu-Y

Na0.026Cu0.118Al0.288Si0.889O2.341

3.08

Zn-Y

Na0.004Zn0.130Al0.291Si0.879O2.327

3.02

Co-Y

Na0.029Co0.112Al0.286Si0.873O2.302

3.05

Ni-Y

Na0.021Ni0.121Al0.287Si0.880O2.325

3.06

XRD The XRD patterns of parent Na-Y and ion-exchanged zeolites are provided in Figure 4. No significant change can be found for the ion-exchanged zeolites, indicating the 10


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well-retained framework of zeolite Y after ion exchange process.27 However, the peak intensities of ion-exchanged samples are reduced slightly as compared to that of the parent Na-Y. Such crystallinity loss can be attributed to ion-exchange and following calcination treatment.28

Figure 4. XRD patterns of parent Na-Y and ion-exchanged zeolites

N2 adsorption The pore structures of parent Na-Y and ion-exchanged zeolites are shown in Table 2. All the ion-exchanged zeolites show reduced surface areas and pore volumes, which is consistent with the result of XRD analysis. Table 2. Structural properties of various zeolites Y St (m2/g)a

Vtot(cm3/g)b

Smi (m2/g)c

Vmi (cm3/g)d

Na-Y

588

0.322

516

0.271

Cu-Y

504

0.306

475

0.255

Zn-Y

496

0.289

486

0.241

Co-Y

508

0.291

448

0.235

Ni-Y

488

0.285

453

0.237

Sample

a-Surface area b-Total pore volume 11



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c- Micropore area d- Micropore volume

FTIR The FTIR spectra of parent Na-Y and ion-exchanged zeolites are illustrated in Figure 5. All ion-exchanged samples show the same FTIR spectra to that of Na-Y, and agree with the previous publications.29 The vibrations at 460, 720 and 1040 cm-1 are assigned to T-O bend, which can be designated as TO4 in the zeolite Y framework.30 The peaks at 790, 580 and 1160 cm-1 are assigned to the external linkages between tetrahedrons (double 6 ring vibration).30 Moreover, the bands at 3450 and 1630 cm-1 can be assigned to hydroxyl stretching vibration.31,32

Figure 5. FTIR spectra of fresh Na-Y and ion-exchanged zeolites

XPS The XPS patterns of Cu 2p spectra for Cu-Y samples before and after adsorption are presented in Figure 6 (a). Both samples show main peaks at 933 and 953 eV, which are assigned to Cu 2p3/2 and 2p1/2, respectively.33 Moreover, the XPS pattern of S 2p for 12


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CH3SH-saturated Cu-Y is shown in Figure 6 (b). The peak at 163.6 eV corresponds to CH3SH molecule adsorbed on Cu-Y zeolite.34 These XPS results can help us set up the adsorption models for DFT simulation.

Figure 6. XPS spectra of Cu-Y zeolite. (a) Cu 2p for sample before and after adsorption, and the S 2p XPS spectra of Cu-Y after adsorption (b).

3.2 Dynamic adsorption performance Figure 7 presents the adsorption breakthrough curves for CH3SH on different ion-exchanged zeolites Y. The parent Na-Y zeolite shows a short breakthrough time of around 60 min. In comparison, all modified samples except Ni-Y display longer 13



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adsorption breakthrough times, suggesting their higher CH3SH adsorption capacities. Among the ion-exchanged samples, Cu-Y exhibits the best CH3SH removal performance, which reaches an adsorption breakthrough time of 280 min.

Figure 7. Adsorption breakthrough curves for CH3SH on different ion-exchanged samples at operation temperature of 25 oC

Moreover, to evaluate the effect of gas hourly space velocity (GHSV) on adsorption performance of Cu-Y zeolite bed, the dynamic tests were carried out at five different GHSV: 12000, 24000, 36000, 48000 and 60000 h-1, respectively. The adsorption breakthrough curves at different GHSV are depicted in Figure 8. By increasing the GHSV, the breakthrough time was shortened and adsorption capacity was declined. In the dynamic tests, the packing density and height of Cu-Y bed were fixed. As the GHSV increases from 12000 to 60000 h-1, the breakthrough adsorption capacity, q, decreases from 70 to 53 mg/g. However, q exhibits distinct decreasing trends during different GHSV ranges. As shown in Figure 9, when the GHSV was increased from 12000 to 36000 h-1, q of Cu-Y is reduced by only 2 mg/g, indicating the slight influence on 14


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molecule diffusion under low GHSV. With the GHSV increasing from 36000 to 60000 h-1, q is remarkably reduced by 24 mg/g. It confirms that the molecule diffusion can become a major controlling factor at high GHSV. The adsorption time is shortened by increasing the GHSV, as a result, the breakthrough adsorption capacity of CH3SH on Cu-Y zeolite bed decreases largely.

Figure 8. Adsorption breakthrough curves for CH3SH on Cu-Y bed with different GHSV at operation temperature of 25 oC

Figure 9. Breakthrough adsorption capacity for CH3SH on Cu-Y bed with different GHSV at operation temperature of 25 oC

Furthermore, the effect of operation temperature on CH3SH adsorption on Cu-Y 15



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zeolite bed was tested (see Figure 10). The results of adsorption experiments performed at different GHSV indicate that the diffusion effect can only be neglected at GSHV lower than 36000 h-1. Therefore, we carried out the adsorption tests with a GHSV of 24000 h-1 to examine the effect of operation temperatures. As shown in Figure 10, with operation temperature increasing from 25 to 80 oC, the breakthrough time was shortened from 280 to 225 min and the breakthrough adsorption capacity of Cu-Y zeolite bed was reduced from 70 to 56 mg/g (see Figure 11). An enhanced adsorption temperature can benefit the molecular diffusion and, thus, can shorten the mass transfer zone and improve the utilization efficiency of Cu-Y zeolite bed. However, the adsorption of CH3SH on Cu-Y is an exothermic process, so the equilibrium adsorption capacity could be reduced at a high operation temperature. In the case of CH3SH adsorption on Cu-Y zeolite, operation temperature exhibits an overwhelming influence on equilibrium adsorption capacity over diffusion rate.

Figure 10. Adsorption breakthrough curves for CH3SH on Cu-Y zeolite bed. Conditions: GHSV of 24000 h-1, operation temperatures of 25, 40, 60 and 80 oC. 16


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Figure 11. Breakthrough adsorption capacity for CH3SH on Cu-Y zeolite bed. Conditions: GHSV of 24000 h-1, operation temperatures of 25, 40, 60 and 80 oC.

3.3 The competitive adsorption behavior between CO2 and CH3SH Organosulfur compounds are considered to be more harmful to environment as well as the health of human beings and more intractable to remove in a general natural gas absorption purification process. Therefore, the selective desulfurization based on Cu-Y is significantly crucial to the adsorption removal of CH3SH, which can benefit the natural gas purification. Here, we carried out several dynamic adsorption tests using the natural gas containing both CH3SH and CO2 to examine the selective desulfurization performance of Cu-Y. Considering the typical concentration of CO2 in amine-treated natural gas, the CO2 concentrations of no more than 4 % were used. The adsorption breakthrough curves for CO2 and CH3SH on Cu-Y are depicted in Figure 12 (the breakthrough adsorption capacities for CO2 and CH3SH on other ion-exchanged samples are shown in Figure S4). With CO2 concentration in feed gases increasing from 0.5 to 4 %, no significant change on CH3SH breakthrough time can be 17



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observed (displaying a constant value of 260 min). Meanwhile, CO2 seems to reach the breakthrough point in the beginning of adsorption. Furthermore, as shown in Figure 13, Cu-Y shows a CH3SH breakthrough adsorption capacity of 64 mg/g (with 4 % CO2 in the feed), bearing 8.6 % lower adsorption capacity. It means that CO2 could only lead to slight decrease on CH3SH adsorption capacity. Therefore, we can believe that Cu-Y zeolite has an excellent selective adsorption performance for CH3SH from the real natural gas mixtures.

Figure 12. Adsorption breakthrough curves for CO2 and CH3SH on Cu-Y zeolite bed (feed gas containing 500 mg/g CH3SH as well as (a) 0.5%, (b) 1, (c) 2 and (d) 4 % CO2, respectively. adsorption condition: GHSV of 24000 h-1, operation temperature of 25 oC)

18


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Figure 13. Effects of CO2 concentrations in feed gas on CH3SH breakthrough adsorption capacity (CH3SH content in feed gas: 500 mg/g; adsorption condition: GHSV of 24000 h-1, operation temperature of 25 oC)

3.4 The DFT calculation CH3SH adsorption on Na-Y and Cu-Y zeolite The adsorption configurations and energies of CH4 and CH3SH on Na-Y and Cu-Y clusters are shown in Figure 14 and 15, respectively. As shown in Figure 15, all clusters have much larger adsorption energies for CH3SH than CH4, indicating zeolites Y has remarkable adsorption selectivity for CH3SH over CH4. The adsorption energies, bond length and Mayer bond orders of cluster models are listed in Table 3. The computational energies for CH3SH adsorption on Na-Y, Cu(i)-Y and Cu(ii)-Y models are -81.2, -119.9 and -136.3 kJ/mol, respectively. The Cu-Y has much stronger interaction with CH3SH than the parent Na-Y zeolite, which is in good agreement with the dynamic test results.

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Figure 14. Adsorption configurations of (a) CH4/Na-Y, (b) CH4/ Cu(i)-Y, (c) CH4/Cu(ii)-Y, (d) CH3SH/Na-Y, (e) CH3SH/Cu(i)-Y, and (f) CH3SH/Cu(ii)-Y

Figure 15. Adsorption energies of CH4 and CH3SH on Na-Y, Cu(i)-Y and Cu(ii)-Y cluster models

For CH3SH adsorption on Na-Y, the bond length and Mayer bond order between Na and S are calculated to be 2.886 Å and 0.158, respectively. Furthermore, the Mulliken population analysis (see Table 4) shows the orbital populations of Na 3p is increased from 0.141 to 0.163, and S 3p and 3d are increased from 4.327 and 0.158 to 4.416 and 0.175, respectively. Such computational results indicate that the weak S-M bond between Na and CH3SH is formed as adsorption happens. 20


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Cu(i)-Y and Cu(ii)-Y models show the same CH3SH adsorption mechanism to Na-Y. In consideration of the higher CH3SH adsorption energy, the DFT calculation is, hence, focused on Cu(ii)-Y. The bond length and Mayer bond order of Cu-S are observed to be 2.273 Å and 0.842, respectively. From the Mulliken population analysis (see Table 4), the orbital populations of Cu 3d, 4s and 4p is increased from 9.497, 0.506 and 0.395 to 9.511, 0.566 and 0.486 when adsorbing CH3SH. Meanwhile, S 3s and 3p orbital populations are decreased from 1.853 and 4.327 to 1.807 and 4.114, respectively, while S 3d orbital population is increased from 0.158 to 0.228. All computational results confirm that the S-Cu bond shows a stronger interaction as compared to S-Na bond and plays the crucial role in determining the adsorption of CH3SH on Cu-Y. The strong S-M interaction related to Cu2+ and CH3SH, therefore, enhances the adsorption of CH3SH on Cu-exchanged zeolite Y largely. Table 3. The adsorption energies, bond length and Mayer bond orders of cluster models Before adsorption Bond

Bond length(Å)

CH3SH/Na-Y

CH3SH/Cu(i)-Y

CH3SH/Cu(ii)-Y

Mayer bond order

After adsorption Bond length(Å)

Mayer

Adsorption

bond

energy(kJ/mol)

order

Na-S

-

-

2.886

0.158

C-S

1.837

1.065

1.844

1.062

H-S

1.356

0.958

1.359

0.897

Cu-S

-

-

2.307

0.782

C-S

1.837

1.065

1.829

1.052

H-S

1.356

0.958

1.361

0.885

Cu-S

-

-

2.273

0.842

C-S

1.837

1.065

1.838

1.043

H-S

1.356

0.958

1.390

0.7943

21


-81.2

-119.9

-136.3


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Table 4. Mulliken population analysis for CH3SH adsorption on Na-Y and Cu-Y System

Atom

Na CH3SH /Na-Y S

Cu CH3SH /Cu(i)-Y S

Cu CH3SH/Cu(ii)-Y S

Before adsorption

After adsorption

3s

0.091

0.100

3p

0.141

0.163

3d

0.057

0.060

3s

1.853

1.855

3p

4.327

4.416

3d

0.158

0.175

3d

9.482

9.522

4s

0.533

0.565

4p

0.402

0.515

3s

1.853

1.801

3p

4.327

4.061

3d

0.158

0.240

3d

9.497

9.511

4s

0.506

0.566

4p

0.395

0.486

3s

1.853

1.807

3p

4.327

4.114

3d

0.158

0.228

CO2 adsorption on Cu-Y zeolite The adsorption configurations of CO2 on Cu(i)-Y and Cu(ii)-Y cluster models are displayed in Figure 16. The calculated adsorption energies, Mayer bond order and bond length of cluster models are listed in Table 5. The energies for CO2 adsorption on Cu(i)-Y and Cu(ii)-Y models are -29.8 and -35.3 kJ/mol, respectively. As compared to the adsorption energies of CH3SH on Cu-Y, the smaller energy of adsorption for CO2 on Cu-Y indicates a weaker interaction between CO2 and Cu-Y. As a result, Cu-Y zeolite can selectively adsorb CH3SH from natural gas, which supports our previous conclusions 22


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from the dynamic adsorption experiments. In consideration of the stronger interaction, this study is still focused on Cu(ii)-Y model. The bond length and Mayer bond order of Cu-O1 are calculated to be 1.964 Å and 0.109, respectively. The Mayer bond order of C=O1 and C=O2 changes from 2.085 and 2.085 to 1.964 and 2.130. This computational result indicates a weak interaction between CO2 and Cu2+ (Cu-O) formed during the adsorption. As a result, the C=O1 bonding is weakened while the C=O2 bonding is strengthened as adsorption takes place. Furthermore, the Cu-O bond involves a weaker interaction as compared with the Cu-S bond.

Figure 16. Adsorption configurations of CO2 on Cu(i)-Y(a) and Cu(ii)-Y(b) cluster models Table 5. The adsorption energy, bond length and Mayer bond orders of cluster models Before adsorption Bond

Bond length(Å)

Mayer bond order

Cu−O1 CO2/Cu(i)-Y

CO2/Cu(ii)-Y

After adsorption Bond

Mayer

length(Å)

bond

2.458

0.109

1.177

2.085

1.180

1.958

C=O2

1.177

2.085

1.170

2.131

2.474

0.104

1.180

1.964

C=O1

1.177

2.085 23


energy(kJ/mol)

order

C=O1 Cu−O1

Adsorption

-29.8

-35.3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C=O2

1.177

2.085

1.170

2.130

3.5 Desorption and regeneration of CH3SH-saturated Cu-Y zeolite Desorption of CH3SH from Cu-Y zeolite was investigated by adopting thermal treatment at a fixed-bed reactor operated at various desorption temperatures under air atmosphere. The effect of treating temperatures on regeneration efficiency was examined. As shown in Figure 17, with an increasing operation temperature, the breakthrough adsorption capacity of regenerated Cu-Y zeolite is increased until reaching the maximum capacity of 58 mg/g (at 350 oC). Continuing to raise the temperature up to 500 oC, no significant change of adsorption capacity is observed. Once the temperature reaches 600 o

C, a reduced breakthrough adsorption capacity of 52 mg/g can be obtained, resulting

from the negative effect of thermal treating on the structure of Cu-Y zeolite. The temperature of 350 °C, therefore, can be considered as the best regeneration condition. Based on our DFT calculation results, chemisorption involving S-M interaction (Cu-S bond) is responsible for the strong binding energy for CH3SH adsorption on Cu-Y. As a result, the adsorbed CH3SH can hardly be released from the active sites. However, when CH3SH-saturated Cu-Y was regeneration in an air atmosphere at 350oC, the chemisorbed CH3SH was oxidized into SOx during the regeneration process. As a result, the CH3SH-occupied adsorption active sites can be released and host guest molecules again.35

24


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Figure 17. Breakthrough adsorption capacity for CH3SH adsorption on Cu-Y regeneration under different temperatures (adsorption condition: GHSV of 19200 h-1, temperature 25 oC)

In addition, the competitive adsorption behavior between CO2 and CH3SH on the regenerated Cu-Y was also evaluated (see Figure 18). Using the feed gas containing 500 mg/g CH3SH and 4 % CO2, the regenerated Cu-Y shows a CH3SH breakthrough adsorption capacity of 53 mg/g, bearing 8.6 % lower adsorption capacity. It indicates that the regenerated Cu-Y zeolite still has an excellent selective adsorption performance for CH3SH from the real natural gas mixtures.

Figure 18. Breakthrough adsorption capacity for CH3SH adsorption on Cu-Y regeneration under air atmosphere at 350 oC (feed gas containing 500 mg/g CH3SH and 4 % CO2, respectively; 25



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adsorption condition: GHSV of 24000 h-1, operation temperature of 25 oC)

The relationship of the adsorption-regeneration cycling times versus the breakthrough adsorption capacity of Cu-Y was also displayed in Figure 19. The regenerated Cu-Y shows a breakthrough adsorption capacity of 55 mg/g after undergoing two adsorption-regeneration cycles, bearing 21.4% lower value than that of the fresh Cu-Y. After six successive adsorption-regeneration cycles, the breakthrough adsorption capacity keeps constant.

Figure 19. Breakthrough adsorption capacity for CH3SH adsorption on Cu-Y regeneration under air atmosphere at 350 oC (adsorption condition: GHSV of 19200 h-1, temperature 25 oC)

4. Conclusions Metal modified zeolites Y were prepared by ion-exchange method and characterized using N2 adsorption, FTIR, XRD, XPS and chemical composition analysis. Their adsorption performances for removing CH3SH from natural gas were evaluated by using a fixed-bed adsorption unit. Among all the samples, Cu-Y zeolite shows the largest CH3SH breakthrough adsorption capacity of up to 70 mg/g with GHSV of 24000 h-1 and adsorption temperature of 25 oC. The breakthrough adsorption capacity can be reduced 26


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when using higher GHSV, higher operation temperature, or feed gas containing CO2. The DFT calculation results indicate the S-M bond forms during the adsorption process, which strengthens the interaction and, therefore, benefits the adsorption of CH3SH on Cu-Y zeolite. The adsorption energy of CO2 on Cu-Y zeolite is much smaller than that of CH3SH on Cu-Y. The CH3SH-saturated Cu-Y can be regenerated by thermal treatment under air atmosphere at 350 oC. After six adsorption-regeneration cycles, the regenerated Cu-Y has the breakthrough adsorption capacity of 55 mg/g.

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

Author information Corresponding Authors: Email: [email protected]. Tel: 86-21-64252851(Ben-xian Shen); Email: [email protected]. Tel: 86-21-64252916 (Hui Sun).

Supporting Information In Supporting Information we provide the structural properties of FAU zeolite; XPS 27



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pattern of Co 2p spectra for Co-Y zeolite; the XPS pattern of Ni 2p spectra for Ni-Y zeolite; the XPS pattern of Zn 2p spectra for Zn-Y zeolite; the XPS pattern of Ni 2p spectra for Ni-Y zeolite; the competitive adsorption behavior between CO2 and CH3SH on Co-Y, Zn-Y and Ni-Y zeolite; the DFT calculation results for CH4 adsorption on Na-Y and Cu-Y cluster models; adsorption energies of CH3SH on Co-Y, Zn-Y and Ni-Y cluster models.

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TOC

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Ion-Exchanged Zeolites Y for Selective Adsorption of Methyl

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