NaY Structure for the Adsorption Desulfurization

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Materials and Interfaces

Cu2+ modified silicalite-1/NaY structure for the adsorption desulfurization of dimethyl disulfide from methyl tert-butyl ether Chao Yang, Xuan Meng, Dezhi Yi, Zhiming Ma, Naiwang Liu, and Li Shi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01211 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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Industrial & Engineering Chemistry Research

Cu2+ modified silicalite-1/NaY structure for the adsorption desulfurization of dimethyl disulfide from methyl tert-butyl ether Chao Yang, Xuan Meng, Dezhi Yi, Zhiming Ma, Naiwang Liu, Li Shi* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China E-mail: [email protected]

Abstract: Selective adsorption desulfurization of dimethyl disulfide (DMDS) from methyl tert-butyl ether (MTBE) has been studied on the silicalite-1/CuY core-shell composites. Different copper ion source (CuCl2, Cu(NO3)2 and CuSO4) were investigated to form CuY as the core by Cu2+ ion-exchange on NaY zeolite. These silicalite-1/CuY core-shell composites were synthesized at the mass ratio of tetraethyl orthosilicate (TEOS)/ tetrapropylammonium hydroxide (TPAOH)/ ethanol/ H2O/ CuY=20 g:19 g:17 g:87 g:5 g. Results showed that the core-shell Y-CuCl2 displayed the best performance in desulfurization of DMDS with a sulfur adsorption capacity to be 32.882 mgs/gadsorbent, owning to its significant mass gain and compact coatings after being coated by silicalite-1 on Y-CuCl2. Also, the preparation process of CuY and the shape selective adsorption mechanism of desulfurizing DMDS from MTBE on the silicalite-1/CuY core-shell composites were expounded. Key words: Silicalite-1/CuY core-shell composites; Methyl tert-butyl ether; Dimethyl disulfide; selective adsorptive desulfurization 1. Introduction Dimethyl disulfide (DMDS) is a volatile organic sulfur compound which is produced mainly from petroleum refining processes, and enemy-related activities.1 DMDS will cause toxic effects when it is inhaled or absorbed by skin. Also, when fuel with DMDS is burned, the sulfur combines will create oxygen (SOx) emissions which have negative environmental and health effects,2-5 cause acid rain and decrease the lifetime of any operating system and units.6,7 In recent years, the crude oil is getting much worse. The content of DMDS,

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which exists in the liquid hydrocarbon streams especially in low hydrocarbons is getting much higher.8 Methyl tert-butyl ether (MTBE) is produced by the reaction of isobutene (contained in C4-fractions) and methanol, which is good at enhancing octane property when used as a gasoline additive agent.9 Due to the source of C4 materials, MTBE contains various kinds of sulfur compounds including mercaptane, thiophene, DMDS and so forth; among which the content of DMDS reaches more than 45%.1,10 Considering that the sulfur content of gasoline must be less than 10 ppmw since 2009,11-13 the removal of sulfur for ultra-clean transportation fuels has been mandated by governments all over the world.8, 14, 15 Nowadays, the industrial desulfurization methods for MTBE products are mainly hydrodesulfurization, distillation, and adsorption.16-21 As a conventional method in removal of sulfur compounds, hydrodesulfurization (HDS) processes are carried out with Co-Mo/A12O3 or Ni-Mo/A12O3 as the catalysts at high temperatures (300-340 °C) and high pressures (20-100atm of H2),16-17 which is not only unsatisfied with the current sulfur level requirements but is ineluctable to reduce the octane number of MTBE. meanwhile, the distillation needs huge energy consumption and is always accompanied with the loss of MTBE. Whereas, adsorption desulfurization can be carried out at atmospheric temperature and low pressure with a simple operation and low energy consumption. Generally, adsorption desulfurization could remove the sulfur compounds from fuels via π-complexation,18-24 van der Waals’ or electrostatic interactions,25,26 and reactive adsorption by chemisorption at elevated temperatures.20,27 However, when desulfurizing DMDS from MTBE by adsorption, a strong competitive adsorption between MTBE and DMDS on π complex adsorbents would occur, owing to the fact that the oxygen atom in MTBE is more electronegative than sulfur atom in DMDS. The key challenge of this approach is to explore an adsorbent that can selectively adsorb sulfur compounds but leave other compounds unchanged. Desulfurization on various adsorbents such as the activated carbons (ACs), modified composite oxide,


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and zeolite,,28-30,7 have already been studied indepth. Y zeolites have been found to be promising materials and used in adsorptive desulfurization extensively.5, 31, 32 Lidan Lv et al.33 studied the adsorptive separation of dimethyl disulfide from liquefied petroleum gas by different zeolites, and concluded that 5 wt% Ag2O/NaY showed the highest breakthrough sulfur capacity reaching up to 87.86 mgs/gadsorbents, and the direct S-Ag(I) interaction played an important role in the evidently improved adsorption ability and selectivity. Wakita et al.32 investigated the removal of dimethyl sulfide and t-butylmercaptan in the city gas by using NaY, NaX, Hβ zeolite. They found that the adsorption site of NaY was the Na+ in the supercage with a maximum sulfur capacity to be 1.1 mmols/gadsorbents of DMS on NaY. Lee et al.8 used ion-exchanged zeolites to remove DMDS from C4 hydrocarbon mixture and claimed that ion-exchanged zeolites were favorable adsorbents with high capacity in removal of DMDS from gas hydrocarbon mixture at ambient conditions. Results showed that the best sulfur capacity was 8.70 mgs/gadsorbents, when β zeolite with a SiO2/Al2O3 ratio to be 25 contented 10 wt% of Cu(I). Yawei Zhao et al.34 aimed at the removal of DMDS from MTBE by using ZSM-5 zeolite. Investigation showed that both the pore structure and acidity played important roles in determining the sulfide adsorption process and CLD-modified ZSM-5 zeolite exhibited an optimal DMDS adsorption capacity to be 8.24 mgs/gadsorbents. Our research group also tried many efforts in desulfurizing DMDS. Dezhi Yi et al.1,10 used silver-modified bentonite and liquid-phase ion exchanged NaY with Cu2+, Ni2+, Co2+ and Ce3+ to remove DMDS from n-octane solution and concluded that multilayer intermolecular forces and S-M (σ) bonds played important roles in desulfurization process. In this research, we tried to synthesize a functional material to achieve desulfurizing DMDS from MTBE. Considering the structure of DMDS (

) and MTBE (

).

it could be easily found that the molecular

scale of DMDS was smaller than that of MTBE. Therefore, we envisaged a kind of core-shell material which was compound by two functional part: one as the shell to



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prevent MTBE from migrating into the core structure, permitting only DMDS to pass by; the other as the core to adsorb DMDS, holding back DMDS from migrating out of the core-shell structure. We coated the Cu2+ ion-exchanged NaY (CuY) with an silicalite-1 shell (pure silica crystal) to form a silicalite-1/CuY core-shell composite to achieve this goal. Many literatures had reported coating silicalite-1 on ZSM-5 or β zeolite,35-37 whereas, coating silicalite-1 on CuY was rarely investigated, especially applying the silicalite-1/CuY core-shell composites to adsorb DMDS from MTBE. In this paper, Different copper ion source (CuCl2, Cu(NO3)2 and CuSO4) were used to synthesize silicalite-1/CuY core-shell composites. Moreover, these sorbents were characterized by Scanning electron microscopy (SEM), Transmission electron microscope (TEM), X-ray diffraction (XRD), ICP-OES measurements, Raman Spectra, Fourier transform infrared spectra (FT-IR), and N2 adsorption-desorption experiment. Selective adsorption mechanism of silicalite-1/CuY core-shell composites was also discussed based on these experimental data and characterization analysis. 2. Experimental 2.1. Chemicals Analytically pure copper chloride (CuCl2), copper nitrate (Cu(NO3)2), copper sulfate

(CuSO4),

ethanol

(Et),

tetraethyl

orthosilicate

(TEOS)

and

25%

tetrapropylammonium hydroxide (TPAOH) solution were obtained from Shanghai Tansoole Company. NaY of industrial grade was provided by Wenzhou Catalyst Factory. Deionized water was used in all experiments. 2.2. Synthesis of CuY, CuY core-shell composite molecular sieves and silicalite-1 2.2.1. Preparation of CuY zeolites by liquid phase ion-exchange method A series of NaY zeolites modified by different Cu2+ (CuCl2, Cu(NO3)2, CuSO4) were prepared by liquid phase ion-exchange method10, 34, 38: Certain amount of NaY zeolites were added into 0.5 mol/L Cu2+ solution with a solid to liquid ratio to be 1 g: 20 ml; and after a 24 h-stirring in the water bath at 90 °C, the mixture was filtered by filter paper (the maximum pore size of filter paper to be 15-20 µm); then the


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residue was washed with enough water, dried at 120 °C overnight, and finally calcined in air at 450 °C for 6 h. And the modified NaY zeolites was recorded as Y-CuCl2, Y-Cu(NO3)2 and Y-CuSO4, respectively. 2.2.2. Synthesis of CuY core-shell composite molecular sieves In order to synthesize the silicalite-1/CuY core-shell structured composite molecular sieves, a sol-gel coating process with TPAOH as the template was conducted as follows39: In this procedure, the silicalite-1 coatings was synthesized from silicalite-1 shell precursor solution which consists of TEOS (as the silica source), TPAOH (as template or structure-directing agent) and ethanol, as well as deionized water with a mass ratio of TEOS/ TPAOH/ ethanol/ H2O=20 g:19 g:17 g:87 g. In order to hydrolyze the TEOS sufficiently in deionized water, TEOS was added dropwise and stirred for 2 h at room temperature, followed by adding 5 g CuY. After being intensively mixed, the mother liquor was transferred into an autoclave, then a hydrothermal synthesis was carried out at 180 °C for 24 h. The as-made silicalite-1/CuY core-shell structured composites were collected by filtration, washed with enough deionized water, dried at 120 °C overnight, and finally calcined in air at 550 °C for 6 h. 2.2.3 Preparation of silicalite-1 Silicalite-1 was prepared at the same mass ratio of TEOS/ TPAOH/ ethanol/ H2O =20 g:19 g:17 g:87 g with the same synthetic method only without adding CuY. 2.3. DMDS adsorption experiments 2.3.1. Static tests Sulfur adsorption capacity of the adsorbents were obtained from static tests, which were carried out by adding 0.2 g adsorbent into a 20 mL MTBE solvent mixed with DMDS as the solute (sulfur content: 682.34 mg/L) within an 30 ml airtight container standing at room temperature for 24 h. A TS-3000 fluorescence sulfur tester was used to analyze the concentration of sulfur before and after the static tests.1, 10, 40 The sulfur adsorption capacity of the adsorbent was calculated as follow:



Sulfur adsorption capacity(mgs/gadsorbent) = (682.34- Ct)×20/0.2 where the Ct (mg/L) is the sulfur concentration of MTBE solution after static tests. 2.3.2. Dynamic Tests In order to acquire the dynamic active data of the adsorbents, these adsorbents was in turn filled into a fixed bed flow reactor at atmosphere pressure (0.1 MPa) and room temperature (25 °C), with a weight hourly space velocity (WHSV) of 5 h-1 to evaluate their adsorption desulfurization as time went on. About 0.89 g adsorbent samples (20-40 mesh) were fixed in the middle of a quartz column (length: 250 mm; internal diameter: 6 mm); the spare spaces up and down were filled with quartz sand (20-40 mesh). Then MTBE solution was pumped into the fixed-bed flow reactor at a flow rate of 6 mL/h (sulfur content: 245.06 mg/L). The export sulfur content was analyzed periodically by a TS-3000 fluorescence sulfur tester every 30 min. 2.4. Characterization 2.4.1. Scanning electron microscopy and transmission electron microscope Scanning electron microscopy (SEM) images were recorded on a Hitachi S-3400 microscope working at 15 KV acceleration voltage with a magnification of 1-20 K. Transmission electron microscope (TEM) images were conducted on a JEM-2100 working at 200 KV with a magnification of 2-1500 K. 2.4.2. X-ray diffraction A X-ray Diffraction (XRD) was applied to analyze the morphology of this silicalite-1/NaY core-shell structure in the powder state. XRD was performed by a D8 Advance polycrystalline diffractometer equiped with Cu Kα radiation (40 KV, 100 mA) over the range from 10° to 75° in a step of 0.02°. 2.4.3. In situ FTIR measurements Fourier transform infrared spectra (FT-IR) was collected with a Magna-IR550 spectrometer (Nicolet Company) by mixing the adsorbent sample powder with the dried KBr in an appropriate ratio (1:100 in mass). And the finely ground adsorbents were pressed into a self-supporting wafer with a diameter of about 10 mm as the


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scanning sample. 2.4.4. Raman spectra. To obtain the Raman spectra, a Renishaw System 100 Raman spectrometer was used with a 514 nm red excitation from an Ar laser (power of 3 mW). The Raman scattered light was detected perpendicular to the laser beam with a Peltier-cooled CCD detector, and the spectral resolution was maintained at 1 cm-1. 2.4.5. Molar Ratio of Copper and Sulfur as well as Cu2+ loading amount. In order to verify the molar ratio of copper and sulfur on the deposit, as well as the actual Cu2+ loading amount, ICP-OES measurements (total copper in solid sample) were conducted in the inductively coupled plasma-atomic emission spectrometer via a Agilent 725 ICP-OES. 2.4.6. N2 adsorption-desorption In order to obtain the surface area and pore volume of the adsorbents, a JW-BK200C instrument was used by adsorbing nitrogen at -196 °C on 150 mg of sample, which must be previously degassed at 300 °C for at lest 2 h under high vacuum atmosphere. The BET surface area (St), total pore volume (Vt), micropore volume (Vmic) could be calculated by nitrogen isotherms. 3. Results and discussion 3.1. Desulfurization performances of the core-shell CuY zeolites. 3.1.1. Static active data analysis


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mgs/gadsorbent

mg/L 32.882 Raw material sulfur concentration: 682.34

722.57

776.74

23.553 26.58

759.56

677.07

717.74

416.54

446.81

353.52 0.527

-9.44

-7.722

b

c

d

-4.023

a

-3.54

e

f

g

Sulfur adsorbtion capacity (mgs/gadsorbent)

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

Sulfur content after adsorption (mg/L)


h

Fig. 1. Sulfur content after adsorption and sulfur adsorption capacity of DMDS in MTBE solution on different absorbents: a. NaY, b. silicalite-1, c. Y-CuCl2, d. Y-Cu(NO3)2, e. Y-CuSO4, f. core-shell Y-CuCl2, g. core-shell Y-Cu(NO3)2, h. core-shell Y-CuSO4

Sulfur adsorption capacity of DMDS in MTBE solution was obtained by static adsorption activity tests which were actualized by adding moderate amount of adsorbents into MTBE solution containing DMDS within an airtight container standing at room temperature for 24 h. Fig. 1 illustrated the sulfur content after adsorption and sulfur adsorption capacity of DMDS in MTBE solution on different absorbents. Results showed that compared with the raw material sulfur concentration 682.34 mg/mL, the sulfur content after adsorption of NaY, Y-CuCl2, Y-Cu(NO3)2 and Y-CuSO4 was increased, resulting in corresponding negative sulfur adsorption capacities to be -4.023, -9.44, -7.722 and -3.54 mgs/gadsorbents, which suggested that MTBE could be adsorbed on these absorbents more easily than DMDS. As a matter of fact, DMDS and MTBE were adsorbed on these absorbents by π-complexation of Cu-S and Cu-O.19,41,42 Combined the structure of DMDS ( (

) and MTBE

), it could be found that the methyl and tert butyl are both electron donating

groups, and the electron donating ability of tert butyl is better than that of methyl. By comparison, the electronegativity of O in MTBE is weakened more than S in DMDS, leading to a smaller electronegativity gap between S and O. When they formed Cu-S and Cu-O, It didn't make much difference. In MTBE solution, owing to that the


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MTBE was much more than DMDS, the contact probability between adsorbents and MTBE was absolutely much more than that of DMDS. Thus, for DMDS, MTBE came into being a strong competitive adsorption on these π-complexation adsorbents.43 Among these adsorbents, Y-CuCl2 performed the best absorption ability, followed by Y-Cu(NO3)2, and Y-CuSO4, making Y-CuCl2 an opportunity in the storage of sulfur if it could be endowed with selectivity between DMDS and MTBE. To accomplish this conceive, we assumed a substance as a coating to selectively transmit DMDS from MTBE and the CuY as the core to store sulfur. Combined with the adsorption data of silicalite-1, which is lower than the raw material sulfur concentration, indicating that silicalite-1 could selectively adsorb DMDS from MTBE, silicalite-1 was determined as the coating. when the CuY zeolites were coated by silicalite-1 to form the core-shell structures, the results were obviously improved. Fig. 1 depicted that after being coated, the sulfur adsorption capacity of Y-CuCl2 raised from -9.44 to 32.882 mgs/gadsorbents; Y-Cu(NO3)2 and Y-CuSO4 also showed a substantial upgrade. That was because when coating CuY zeolites with silicalite-1, the silicalite-1 coating could prevent MTBE from migrating into the core structure, permitting only DMDS to pass by and the core CuY zeolites would adsorb DMDS, holding back DMDS from migrating out. Overall, the core-shell zeolites could notabely improve the sulfur adsorption capacity of both CuY and silicalite-1. 3.1.2. Dynamic active data analysis



100

Desulfurization rate of DMDS(%)

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

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NaY Silicalite-1 Y- CuCl2 Y- Cu(NO3)2 Y- CuSO4 core-shell Y- CuCl2 core-shell Y- Cu(NO3)2 core-shell Y- CuSO4

80 60 40 20 0 -20 0

1

2

3

4

Time (h) Fig. 2. Breakthrough curves of NaY, silicalite-1, CuY and silicalite-1/CuY core-shell structured composites adsorbing DMDS in MTBE solution

In order to further evaluate the desulfurization performances of these as-made adsorbents, they were tableted, screened (20-40 mesh), and then fixed into a quartz column reactor. Every 30 min the export sulfur concentration was collected and analyzed. The breakthrough curves of different adsorbents were shown in Fig. 2. With the elapse of time, the desulfurization performances of the adsorbents overall showed a downward trend. Unsurprisingly, for the sake of the competitive adsorption, NaY and CuY zeolites exhibited a poor performance in desulfurization. Strictly speaking, none desulfurization capacity were observed on NaY and CuY zeolites; throughout the adsorption tests, the desulfurization rate of NaY and CuY were all along negative. The breakthrough curve of silicalite-1 depicted in Fig. 2 suggested that silicalite-1 could adsorb DMDS from MTBE to a certain extent, supporting that silicalite-1 owned some selective adsorption characteristic. When the CuY zeolites were coated with silicalite-1, the desulfurization rate were remarkably improved. As shown in Fig. 2, within 1.5 h all the core-shell structured CuY zeolites could remove DMDS from MTBE solution for 100%. Among them, the core-shell Y-CuCl2 displayed the best performance, for which more than 90% desulfurization rate could be achieved and kept for about 3.5 hours. After that, the desulfurization rate of core-shell Y-CuCl2 declined sharply, suggesting that the zeolite came into the end of its active life.


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3.2. SEM and TEM images of different adsorbents (a)

(b)

(c)

(d)

(e)

(f )

Fig. 3. SEM images of NaY zeolite, silicalite-1 and silicalite-1/CuY composites:(a) NaY×16K, (b) Y-CuCl2 ×16K, (c) silicalite-1×16K, (d) core-shell Y- CuCl2 ×16K, (e) core-shell Y- Cu(NO3)2×16K, (f) core-shell Y- CuSO4 ×16K

(a)

(b)

(c)

(d)

(e)

(f )

Fig. 4. TEM images of NaY (a), (b), (c); and core-shell Y-CuCl2 (d), (e), (f).

The SEM images in Fig. 3 (a) and (b) showed that after being ion-exchanged by different Cu2+, there were no obvious changes on NaY zeolites. In this study, the Cu2+ of these core-shell structures came from CuCl2, Cu(NO3)2 and CuSO4. Results showed that after being coated by silicalite-1, the Y-CuCl2 formed an significant coating, better than Y-Cu(NO3)2 and Y-CuSO4. From TEM images (Fig. 4 a, b, c), the significant silicalite-1 coating could be obviously observed against with the smooth surface of NaY (Fig. 4 d, e, f ), which was consistent with the SEM images and adsorption activity data. 3.3. Mass gain of CuY zeolites after coating silicalite-1.



14 12

in theory after filtration after centrifugation

mass gain (g)


160 140

mass gain (%)

120

10

100

8

80

6

60

4

40

2

20

0

Mass gain (%)

16

Mass gain (g)

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

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0 core-shell Y- CuCl2 core-shell Y- Cu(NO3)2 core-shell Y- CuSO4

Fig. 5 Mass gain of CuY zeolites after coating silicalite-1

In order to investigate the actual yield of CuY zeolites after coating silicalite-1, two different methods were conducted to collect the silicalite-1/CuY core-shell structured composites by filtration (maximum pore size of filter paper to be 15-20 µm) and centrifugation (6000 r/min for 30 min), respectively. The mass gain of CuY zeolites after coating silicalite-1was shown in Fig. 5. In this study, the CuY zeolites were fixed at 5g before hydrothermal reaction. According to the mass ratio (TEOS/TPAOH/ethanol/H2O/CuY = 20 g:19 g:17 g:87 g:5 g), the mass gain of CuY zeolite must be a straight line in theory as shown on the blue area or line. After centrifugation, the mass gain of CuY zeolites were similar and close to that in theory. However, the red area or line showed that after filtration, the mass gain had decreased a lot. Combined with SEM patterns (Fig. 3), we assumed that quite a number of silicalite-1 crystals did not grow on the external surface of CuY zeolite but formed a homogeneous nucleation in the solution, and these silicalite-1 crystals made no contribution to the external surface modification of the CuY zeolite;36 Meanwhile, the mass gain showed on the the red area or line suggested that some silicalite-1 crystals had been coated on CuY surface, providing the silicalite-1/CuY core-shell structured composites a significant shape selective adsorption capacity compared with the uncoated ones. In this research, the core-shell composites by filtration were studied.


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3.4. Textural properties of adsorbents. Table. 1. Structural properties of NaY, silicalite-1, CuY and silicalite-1/CuY core-shell structures *

Adsorbents

St (m2/g)

*

Vt (cm3/g)

*

*

Vmic (cm3/g)

Da (nm)

*

Dmic (nm)

NaY

782.879

0.362

1.849

0.28

0.8395

Silicalite-1

483.074

0.347

2.874

0.188

0.6570

Y-CuCl2

587.642

0.358

2.018

0.2611

0.8363

Y-Cu(NO3)2

531.025

0.275

2.365

0.2324

0.8426

Y-CuSO4

416.589

0.229

2.998

0.155

0.8856

CSY-CuCl2

562.882

0.315

2.240

0.2202

0.6695

CSY-Cu(NO3)2

458.477

0.295

2.434

0.1809

0.8409

CSY-CuSO4

371.467

0.279

3.179

0.1477

*

*

*

0.8497

*

*

CSY-CuCl2: core-shell Y-CuCl2; St: total surface area; Vt: total pore volume; Da: average pore

size; *Vmic: micropore pore volume; *Dmic: micropore pore size. 0.8

0.035


0.030

3 -1 -1 dV/dD (cm g nm )

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


0.025

0.6

0.020 0.015

0.4 0.010 0.005

0.2

0.0

0.000 2

0

1

2

3

3

4

4

5

5

6

7

8

6

9

10

7

11

12

8

13

14

9

15

10

Pore width (nm) Fig. 6. BJH pore size distributions of different adsorbents

The structural properties of NaY, silicalite-1, CuY and core-shell structures were showed in Table.1. NaY zeolite owned the largest total surface area (St) to be 782.879 m2/g among silicalite-1, CuY, and their core-shell structures (Table. 1). Besides, NaY also provided a larger total pore volume (Vt) and microporous pore volume (Vmic). As the core, NaY could provide the core-shell structure more active adsorption centers. After being modified by Cu2+, though its Vt and Vmic were correspondingly becoming smaller, the generation of Cu-S bonds made CuY a stronger adsorption capacity for DMDS, which could be certified by the static and dynamic active data in Fig. 1 and Fig. 2. Table. 1 and Fig. 6 also illustrated the reason for choosing silicalite-1 as the



Page 14 of 31

coating. From Table.1, we could obviously find that silicalite-1 possessed a smaller microporous channel with a Dmic to be 0.6570 nm, which could hold back the adsorbates with macromolecules from spreading into the core zeolite. And this constituted the foundation of silicalite-1/CuY core-shell composites for selective adsorption of DMDS from MTBE. In general, when NaY was modified by Cu2+, the St, Vt and Vmic were slightly decreased; and after being coated they would be further decreased. On the contrary, the Da and Dmic increased to a certain. After NaY being modified by CuCl2 and then coated, the St, Vt and Vmic of core-shell structures were maintained at a high value, ensuring the core-shell Y-CuCl2 a good adsorption and accumulating storage capacity for DMDS; in addition, the Dmic changed only a little from 0.6570 nm to 0.6695 nm, making this core-shell structured composites maintaining a high selectivity of DMDS form MTBE solution. NaY Silicalite-1 core-shell Y- CuCl2 core-shell Y- Cu(NO3)2 core-shell Y- CuSO4

0.0

0.2

Y- CuCl2 Y- Cu(NO3)2 Y- CuSO4

0.4

0.6

0.8

1.0

Relative Pressure (P/P0) Fig. 7. N2 adsorption-desorption isotherms of NaY, CuY, silicalite-1 and core-shell structures.

Owing to the fact that no obvious hysteresis loop and notable plateau at high relative pressure were observed, as shown in Fig. 7, the N2 adsorption-desorption isotherms of NaY, CuY, and core-shell structures were considered belonging to type-I, suggesting a microporous property of these adsorbents.44 Differently, the N2 adsorption-desorption isotherms of silicalite-1 was situated between type-I and type-Ⅳ, indicating that silicalite-1 showed both microporous and mesoporous


Page 15 of 31

properties. Due to the presence of mesopores, the average pore size (Da) of silicalite-1 was obviously larger than that of NaY, which clarified the result displayed in Table.1 that the Da of silicalite-1 was larger than that of NaY zeolite. As a matter of fact, the selective adsorption of zeolite is only based on the micropores not the mesopores; to some certain, the smaller the micropore size was, the better the shape selecting function would be. Combined with the active adsorption data of NaY and silicalite-1, the latter has remarkable advantages in shape selection of DMDS from MTBE. As a result, after the silicalite-1 was coated on CuY zeolite, the core-shell structure would insure a lager sulfur adsorption capacity, as well as a good shape selectivity, making it a appreciable adsorbent for removing DMDS from MTBE. 3.5. XRD patterns of adsorbents.

(8) (7)

Intensity (a.u.)

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


(6) (5) (4) (3) (2) (1)

10

20

30

40

50

60

70

2-Theta (degrees) Fig. 8. X-ray diffraction patterns of different absorbents: (1) NaY, (2) silicalite-1, (3) Y-CuCl2, (4) Y-Cu(NO3)2, (5) Y-CuSO4, (6) core-shell Y-CuCl2, (7) core-shell Y-Cu(NO3)2, (8) core-shell Y-CuSO4

The mineralogical of the NaY, silicalite-1, CuY and their core-shell structures were depicted in Fig. 8. From the X-ray diffraction patterns, the diffraction peaks at 2θ=15.640 and 23.640 were considered the feature peaks of NaY zeolite.10 And after being modified by Cu2+, these feature peaks still exited, with only a decreased intensity. That was because the Cu2+-modifying process caused the framework defects.



Page 16 of 31

After being coated with silicalite-1, the XRD patterns of the silicalite-1/CuY core-shell structures showed the same diffraction peaks, suggesting that the crystalline zeolite framework was retained during the coating process;45 and the intensity of the feature peaks further decreased, which may mostly ascribe to the coating process of diluted zeolites as the mesoporous shell and the weak shielding effects of mesoporous shells on X-rays.45 Compared with the X-ray diffraction patterns of NaY and silicalite-1 crystal, a great distinction could be observed in the XRD patterns of core-shell CuY, but the same feature peaks was preserved. The most intense peaks of MFI-type material, especially between 2θ=22-25 indicated that the silicalite-1 shell was successfully coated on the surface of CuY.46 3.6. In situ FTIR spectra of adsorbents. (1)

1105

(a) (b) (c) (d) (e) (f) (g) (h) 1622

2000

1750

1226

1500

1250

1011

1000

750

500

-1

wavenumbers( cm ) Fig. 9. In situ FTIR spectras recorded after the adsorption of DMDS in MTBE on different absorbents: (a) NaY, (b) silicalite-1, (c) Y-CuCl2, (d) Y-Cu(NO3)2, (e) Y-CuSO4, (f) core-shell Y-CuCl2, (g) core-shell Y-Cu(NO3)2, (h) core-shell Y-CuSO4

Fig. 9 displayed the in situ FTIR spectra recorded after the adsorption of DMDS in MTBE on different zeolites. As can be seen, there was no bands on NaY and CuY zeolites locating at 1226 cm-1, which was determined as the stretching vibration of C-S, indicating that owing to the fierce competitive adsorption between MTBE and DMDS, almost no DMDS was detected on NaY and CuY zeolite. While via physical intermolecular force and other interactions, the stretching vibration of C-S was


Page 17 of 31 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


catched on (b) silicalite-1 crystal and (f)-(h) core-shell zeolites, suggesting that DMDS was actually absorbed on these zeolites and that silicalite-1 crystal or silicalite-1 coating owned a shape-selection characteristic of molecular scale between DMDS and MTBE. The bounds observed at 1011 and 1105 cm-1 belonged to the stretching vibration of C-O-C which located at 1250~1000 cm-1 in theory, indicating that the MTBE was also absorbed on zeolite. And the bands at 1622 cm-1 could be assigned to the hydroxide radical vibrations of water adsorbed in the zeolites.36 3.7. Raman spectra of CuCl2 and the Cu-S compound as well as Cu2+ loading amount CuCl2 pure substance CuCl2 after adsorption

ν(Cu-Cl)

200

400

of DMDS

ν(C-S)

ν(Cu-S)

600

800

1000

1200

1400

1600

Raman shift (cm-1)

Fig. 10. Raman spectra of CuCl2 and the Cu-S compound Table. 2. Cu2+ source and their loading amounts in NaY zeolite Cu2+ source CuCl2

Cu2+ concentration

Cu2+ loading capacity in

Cu2+ loading amount

(mol/L)

theory (mg/g NaY)

in fact (mg/g NaY)

0.5

82.29

4.4

Cu(NO3)2

0.5

82.29

3.1

CuSO4

0.5

82.29

1.5

Fig. 10 presented the Raman spectrum of CuCl2 and the Cu-S compound after adsorbing DMDS. The features at 310 cm-1 are not observed in the spectrum of CuCl2 zeolite, but appeared in the spectrum of the CuCl2 after adsorption of DMDS. This peak was considered the characteristic of the Cu-S stretching vibrations,47 suggesting that DMDS could bind to Cu2+ with its sulfur atom. The peak at 690 cm-1 showed the v(C-S) vibration of DMDS adsorbed at Cu2+.48 and 546 cm-1 was related to the S-S stretching vibrations, characteristic of the disulphide bond. Results depicted that



DMDS was adsorbed on Cu2+ without cleavage of its C-S and S-S band. In fact, the electronegativity of sulfur atom was not that high, on the contrary, it was easy to lose the lone electron pairs; atoms of Cu2+ (1s22s22p63s23p63d94s0) could form the σ bonds with their empty s-orbitals and p-orbitals. Mixing one s orbital and three p orbitals gave four sp3 hybrid orbitals, which were directed toward a tetrahedral structure. The sulfur atoms on DMDS provided lone pair electrons to Cu2+ and formed the usual S-M (Q) bonds. In order to verify the molar ratio of Cu and S on the deposit, ICP-OES measurements (total copper in solid sample) were conducted in the inductively coupled plasma-atomic emission spectrometer via a Agilent 725 ICP-OES. The copper content of the deposit was 28.3 wt%, and the sulfur was 13.9 wt%. The molar ratio Cu/S=1 was calculated. Complexation reaction was presented as follow:

Agilent 725 ICP-OES was also applied to identify the actual Cu2+ loading amount, and results were listed in Table. 2. Cu2+ loading amount of CuCl2 on NaY zeolite reached the maximum quantity, providing more active centers for adsorption of DMDS, which was also consistent with the adsorption activity data. 3.8. Mechanism analysis


Page 18 of 31

Page 19 of 31

Na+

NaY

2+

NaY

Cu

CuY

iron exchange

2+

Cu solution

90 Ⅳ, 24 h hydrothermal reaction

silicalite-1 coating

MTBE+DMDS solution

180 Ⅳ, 24 h

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


monolayer hydrothermal reactor multilayer


MTBE

DMDS

MTBE+DMDS

MTBE

DMDS

solution

Fig. 11. The preparation process of CuY and the geometrical structures of silicalite-1/CuY core-shell structured composite, as well as the DMDS adsorption mechanism of core-shell silicalite-1/CuY in MTBE.

Fig. 11 displayed the preparation process of CuY and the geometrical structures of silicalite-1/CuY core-shell structured composite. As was known that the unit cell of Y zeolite consisted of 18 four-membered rings, 4 six-membered rings and 4 twelve-membered rings. The diameter of the main channel entrance with twelve-membered ring was 0.8-0.9 nm. The maximum diameter of the main hole was about 1.25 nm with a volume of 0.850 nm3, which provided Y zeolites a possibility for the storage of DMDS. And the Na+ ions were embedded in the pore gap or on crystal skeleton. Silicalite-1 was made up of ten-membered rings and eight



five-membered rings with the maximum aperture to be less than 0.6 nm, which provided silicalite-1 a shape access to DMDS. When CuY was coated by silicalite-1, molecules with a size over 0.6 nm would be held back outside the channel and DMDS could form a complex compound with Cu2+. Accordingly, the silicalite-1/CuY core-shell structured composite reflected good shape selectivity and appreciable sulfur adsorption capacity. As a matter of fact, DMDS molecules were much smaller than that of MTBE, because of no branched chains and large-size atoms. As shown in Fig. 11, the maximum molecule size of MTBE was 0.74 nm and DMDS was 0.37 nm.49,50 DMDS could diffuse rapidly in the microporous channels of silicalite-1 coating; however, owing to the larger size and the branched structure, the diffusion of MTBE would be inevitably limited, leading to the enrichment of DMDS on the core CuY zeolite and the decreased concentration of DMDS in MTBE solution. 4. Conclusion After being ion exchanged by Cu2+, CuY zeolite was still made up with numerous super cages, which provided it itself a favorable space to store organic sulfur (DMDS), especially the active adsorption centers (Cu2+: via multilayer intermolecular forces and S-M (σ) bonds between DMDS and copper ions, the adsorption bond energy of DMDS on the adsorbents was increased); being coated with silicalite-1 made this core-shell structure preventing MTBE from going in, due to the aperture of silicalite-1 being less than 0.6 nm. By shape selective adsorption, the silicalite-1/CuY core-shell structured composite had performed well in desulfurizing DMDS from MTBE. Among three kinds of copper source, the silicalite-1 coating could be dispersed and covered on the surface of Y-CuCl2 well, reaching the best sulfur adsorption capacity to be 32.882 mgs/gadsorbent; and more than 90% desulfurization rate could be achieved and kept for about 3.5 hours. Acknowledgments Project financially supported by the National Science Foundation for Young Scientists of China (No. 21706065); the Open Project of State Key Laboratory of


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Chemical Engineering (SKL-ChE-18C02); China Postdoctoral Science Foundation (NO.2017M621389), Shanghai Sailing Program (NO.18YF1406300), and Explore and Research Foundation for Youth Scholars of Ministry of Education of China (NO. 222201814011). Reference [1] Dezhi Yi, Huan Huang, Xuan Meng, Li Shi. Desulfurization of Liquid Hydrocarbon Streams via Adsorption Reactions by Silver-Modified Bentonite. Ind. Eng. Chem. Res. 2013, 52, 6112-6118. [2] Xiong, J., Zhu, W., Li, H., Ding, W., Chao, Y., Wu, P.. Few-layered graphene-like boron nitride induced a remarkable adsorption capacity for dibenzothiophene in fuels. Green Chem. 2015, 17, 1647-56. [3] K. X. Lee, J. A. Valla. Investigation of metal-exchanged mesoporous Y zeolites for the adsorptive desulfurization of liquid fuels, Appl. Catal. B. 2017, 201, 359-369. [4] K. Leng, Y. Sun, X. Zhang, M. Yu, W. Xu. Ti-modified hierarchical mordenite as highly active catalyst for oxidative desulfurization of dibenzothiophene. Fuel. 2016, 174, 9-16. [5] Huang Huan, Salissou M. Nour, Yi Dezhi, Meng Xuan, Shi Li. Study on Reactive Adsorption Desulfurization of Model Gasoline on Ni/ZnO-HY Adsorbent. China Petroleum Processing and Petrochemical Technology. 2013, 15, 57-64. [6] Babish I V, Moulijn J A. Science and technology of novel processes for deep desulfurization of oil refinery streams: A review. Fuel. 2003, 82, 607-631. [7] Sentorun-Shalaby C., Saha S. K., Ma X. L., C Song. Meso-porous-molecular- sieve-supported nickel sorbents foradsorptive desulfurization of commercial ultra-low-sulfur diesel fuel. Appl Catal B: Environmental. 2011, 101, 718-726. [8] Lee, J., Beum, H. T., Ko, C. H., Park, S. Y., Park, J. H., Kim, J. N., Chun, B. H., Kim, S. Y. Adsorptive Removal of Dimethyl Disulfide in Olefin Rich C4 with Ion-Exchanged Zeolites. Ind. Eng. Chem. Res. 2011, 50, 6382-6390. [9] Armor, J. N.. Environmental catalysis. Applied Catalysis B: Environmental. 1992, 1, 221-256. [10] Dezhi Yi, Huan Huang, Xuan Meng, Li Shi. Adsorption-desorption behavior and mechanism of dimethyl disulfide in liquid hydrocarbon streams on modified Y zeolites. Applied Catalysis B: Environmental. 2014, 148, 377-386. [11] Li, D. Crucial Technologies Supporting Future Development of Petroleum Refining Industry. Chin. J. Catal. 2013, 34, 48-60. [12] M. Tang, L. Zhou, M. Du, Z. Lyu, X. D. Wen, X. Li, H. Ge. A novel reactive adsorption desulfurization Ni/MnO adsorbent and its hydrodesulfurization ability compared with Ni/ZnO, Catal. Commun. 2015, 61, 37-40. [13] L. Wang, L. Zhao, C. Xu, Y. Wang, J. Gao. Screening of active metals for reactive adsorption desulfurization adsorbent using density functional theory. Applied Surface Science. 2016, 399, 440-450. [14] Hernandez-Maldonado A. J., Yang R. T.. New sorbents for desulfurization of diesel fuels via π-complexation. AIChE Journal. 2004, 50, 791-801.



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Page 24 of 31

Page 25 of 31

TOC graphic Na

+

NaY

2+

NaY

Cu

CuY

iron exchange

2+

Cu solution

90 Ⅳ, 24 h hydrothermal reaction


MTBE+DMDS solution

180 Ⅳ, 24 h

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


monolayer hydrothermal reactor multilayer


MTBE

DMDS

MTBE+DMDS solution


MTBE

DMDS

722.57

776.74

759.56

677.07

mgs/gadsorbent

32.882

Raw material sulfur concentration：682.34

26.58

a

0.527

-9.44

-7.722

b

c

d

23.553

717.74

353.52 -4.023

Page 26 of 31

416.54

446.81

-3.54

e

f

g

Sulfur adsorbtion capacity (mgs/gadsorbent)

mg/L

h

Fig. 1. Sulfur content after adsorption and sulfur adsorption capacity of DMDS in MTBE solution on different absorbents: a. NaY, b. silicalite-1, c. Y-CuCl2, d. Y-Cu(NO3)2, e. Y-CuSO4, f. core-shell Y-CuCl2, g. core-shell Y-Cu(NO3)2, h. core-shell Y-CuSO4 100

Desulfurization rate of DMDS(%)

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

Sulfur content after adsorbtion (mg/L)



80 60 40 20 0 -20 0

1

2

3

Time (h)

4

Fig. 2. Breakthrough curves of NaY, silicalite-1, CuY and silicalite-1/CuY core-shell structured composites adsorbing DMDS in MTBE solution (a)

(b)

(c)

(d)

(e)

(f )


Page 27 of 31

Fig. 3. SEM images of NaY zeolite, silicalite-1 and silicalite-1/CuY composites:(a) NaY×16K, (b) Y-CuCl2 ×16K, (c) silicalite-1×16K, (d) core-shell Y- CuCl2 ×16K, (e) core-shell Y- Cu(NO3)2×16K, (f) core-shell Y- CuSO4 ×16K (a)

(b)

(c)

(d)

(e)

(f )

Fig. 4. TEM images of NaY (a), (b), (c); and core-shell Y-CuCl2 (d), (e), (f).

14

Mass gain (g)

140

mass gain (%)

mass gain (g)

12

160



120

Mass gain (%)

16

10

100

8

80

6

60

4

40

2

20

0

0

core-shell Y- CuCl2 core-shell Y- Cu(NO3)2 core-shell Y- CuSO4

Fig. 5 Mass gain of CuY zeolites after coating silicalite-1 0.8

0.035


0.030

3 -1 -1 dV/dD (cm g nm )

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


0.025

0.6

0.020 0.015

0.4

0.010 0.005

0.2

0.0

0.000

0

1

2

3

2

3

4

4

5

5

6

7

8

9

6

Pore width (nm)

10

7

11

12

13

8

Fig. 6 BJH pore size distributions of different adsorbents


14

9

15

10


NaY Silicalite-1 core-shell Y- CuCl2 core-shell Y- Cu(NO3)2 core-shell Y- CuSO4

0.0

0.2

Y- CuCl2 Y- Cu(NO3)2 Y- CuSO4

0.4

0.6

Relative Pressure (P/P0)

0.8

1.0

Fig. 7. N2 adsorption-desorption isotherms of NaY, CuY, silicalite-1 and core-shell structures. (8) (7) (6)

Intensity (a.u.)

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

Page 28 of 31

(5) (4) (3) (2) (1)

10

20

30

40

50

2-Theta (degrees)

60

70

Fig. 8. X-ray diffraction patterns of different absorbents: (1) NaY, (2) silicalite-1, (3) Y-CuCl2, (4) Y-Cu(NO3)2, (5) Y-CuSO4, (6) core-shell Y-CuCl2, (7) core-shell Y-Cu(NO3)2, (8) core-shell Y-CuSO4


Page 29 of 31 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


(1)

(a)

1105

(b) (c) (d) (e) (f) (g) (h) 1622

2000

1226

1750

1500

1011

1250

1000 -1

750

500

wavenumbers（cm ） Fig. 9. In situ FTIR spectras recorded after the adsorption of DMDS in MTBE on different absorbents: (a) NaY, (b) silicalite-1, (c) Y-CuCl2, (d) Y-Cu(NO3)2, (e) Y-CuSO4, (f) core-shell Y-CuCl2, (g) core-shell Y-Cu(NO3)2, (h) core-shell Y-CuSO4 CuCl2 pure substance CuCl2 after adsorption

ν(Cu-Cl)

200

400

of DMDS

ν(C-S)

ν(Cu-S)

600

800

1000

1200

Raman shift (cm-1)

1400

1600

Fig. 10. Raman spectra of CuCl2 and the complex compound



Na+

NaY

NaY

Cu2+ CuY

iron exchange

Cu2+ solution

90 ℃, 24 h hydrothermal reaction


MTBE+DMDS solution

180 ℃, 24 h

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

Page 30 of 31

hydrothermal reactor


MTBE

DMDS

MTBE

DMDS

MTBE+DMDS solution

Fig. 11. The preparation process of CuY and the geometrical structures of silicalite-1/CuY core-shell structured composite, as well as the DMDS adsorption mechanism of core-shell silicalite-1/CuY in MTBE. Table. 1. Structural properties of NaY, silicalite-1, CuY and silicalite-1/CuY core-shell structures Adsorbents

*S (m2/g) t

NaY Silicalite-1 Y-CuCl2 Y-Cu(NO3)2 Y-CuSO4 *CSY-CuCl 2 CSY-Cu(NO3)2 CSY-CuSO4

782.879 483.074 587.642 531.025 416.589 562.882 458.477 371.467

*V

t (cm

3/g)

0.362 0.347 0.358 0.275 0.229 0.315 0.295 0.279

*D

a

(nm)

1.849 2.874 2.018 2.365 2.998 2.240 2.434 3.179

*V

mic

(cm3/g)

0.28 0.188 0.2611 0.2324 0.155 0.2202 0.1809 0.1477

*D

mic

(nm)

0.8395 0.6570 0.8363 0.8426 0.8856 0.6695 0.8409 0.8497

CSY-CuCl2: core-shell Y-CuCl2; *St: total surface area; *Vt: total pore volume; *Da: average pore size; *Vmic: micropore pore volume; *Dmic: micropore pore size. *


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Table. 2. Cu2+ source and their loading amounts in NaY zeolite Cu2+ source

Cu2+ concentration (mol/L)

Cu2+ loading capacity in theory (mg/g NaY)

Cu2+ loading amount in fact (mg/g NaY)

CuCl2 Cu(NO3)2 CuSO4

0.5 0.5 0.5

82.29 82.29 82.29

4.4 3.1 1.5


NaY Structure for the Adsorption Desulfurization

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