Experimental Study of Silver-Loaded Mesoporous Silica for the

Article pubs.acs.org/jced

Experimental Study of Silver-Loaded Mesoporous Silica for the Separation of Ethylene and Ethane Lei Yu,† Wei Chu,†,‡ Shizhong Luo,*,† Jiandong Xing,† and Fangli Jing*,†,‡ †

Department of Chemical Engineering, Sichuan University, Chengdu 610065, People’s Republic of China Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610207, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Three kinds of mesoporous silica materials including SYSG (commercial silica gel), SBA-15, and KIT-6 are used as the carrier for supporting silver nitrate via incipientwetness impregnation method. The resultant materials Ag/SYSG, Ag/SBA-15, Ag/KIT-6 are characterized by low-temperature nitrogen adsorption/desorption, X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy and also are used as adsorbents for ethylene and ethane separation. It is found that both the surface area and the dispersion of Ag cations have significant influences on the separation performances. The surface modification by AgNO3 has the adsorption mechanism changed through π-complexation, which is in contrast to the physical adsorption through van der Waals force that is usually dominated by the surface area. The loss of surface area weakens the physical adsorption of ethane, whereas the well-dispersed Ag cations could promote the adsorption of ethylene; therefore, the selectivity is improved. As a result, Ag/SBA-15 shows the best separation performances among the tested materials.

1. INTRODUCTION In modern chemical and petroleum industry, olefins are usually applied as raw materials rather than as alkanes with same number of carbon atom. A typical example is ethylene, as it is raw material of a series of industrial products such as polyethylene, ethylene oxide, vinyl acetate, synthetic resin, polyester acetaldehyde, alcohols, and so forth. Petroleum cracking gas is rich in lower carbon hydrocarbons such as ethylene and ethane,1−5 therefore, separating ethylene from the cracking gas is of significant value. Currently, the main technology to separate ethylene and ethane is the cryogenic distillation method at −30 °C and 300 kPa. However, the similar properties of two molecules in molecular size and boiling points cause that the separation process require more than 100 theoretical plates and large reflux ratios, leading to huge energy consumption and operation cost.6−8 Various methods including membrane separation,9 extraction,10 adsorption,11−22 absorption,23 and so forth thus have been developed. The adsorption approach is rather attractive among these processes, because it can be simply operated at ambient and hence be energy effective.24 In order to improve the olefins/paraffins separation factor, incorporating Ag+ or Cu+ to adsorbents is favorable for olefins’ π-complexion between metal cation and a double CC bond of olefins.8,24 For example, Yang et al.24−26 have prepared the Ag+-exchanged resins, monolayered CuCl/γ-alumina, monolayered CuCl/pillared clays, and monolayered AgNO3/SiO2 adsorbents for selective adsorption of light olefin over corresponding paraffin via π-complexation, ethylene/ethane separation factor could reach 2−5. Silver ion exchanged molecular sieves prepared by David et al.27 showed amazing inert to ethane under 0.1 MPa, leading to infinite ethylene/ © XXXX American Chemical Society

ethane separation factor. The low adsorption capacity was another drawback for this type of adsorbents. Among the commercial adsorbents involving π-complexation mechanism, such as activated carbon,28−30 Y molecular sieve,31 13X molecular sieve32 and 4A molecular sieve,33,34 carbon molecular sieve35,36 and silica gel, mesoporous silica materials appears to be the promising candidates for olefins and paraffins separation due to its features of large specific surface areas, narrow pore size distribution, adaptable pore size, and facile functionalization.37−49 As an example, mesoporous adsorbent Ag-MCM-41 was successfully applied to the separation of propane and propylene by forming CC complexation with olefins.50 To best of our knowledge, few works were focused on the effect of the morphology and microstructure of mesoporous silica on ethylene/ethane adsorption behavior. Herein, three kinds of mesoporous silica, SBA-15, KIT-6, together with commercial silica gel SYSG, were selected for the ethylene/ ethane separation process.

2. EXPERIMENTAL SECTION 2.1. Materials. SYSG (20−40 mesh, Qingdao Haiyang Silica Co., Ltd., China, AR) was commercially obtained and dried at 150 °C for 6 h prior to use. The mesoporous SBA-15 was prepared as the literature reported by Zhao et al.51 and the mesoporous silica KIT-6 was prepared according to the procedure reported by Ryoo et al.52 AgNO3 (10 wt %) Special Issue: Memorial Issue in Honor of Ken Marsh Received: January 22, 2017 Accepted: April 28, 2017

A

DOI: 10.1021/acs.jced.7b00069 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Static adsorption system for ethane and ethylene separation, where V1−V5 represent the valves, and PLC represents programmable logic controller transforming pressure signal to electric signal and recording signal data.

because the better fitting performance under low pressure for gas adsorption.54−57 2.4. Characterizations. X-ray diffraction (XRD) patterns of the materials were recorded using a DX2000 diffractometer with Cu Kα radiation in the 2θ range from 0.5 to 5° and 10 to 80° at 40 kV and 30 mA. The N2 adsorption/desorption isotherms were measured by Quantachrome NOVA 1000e at −196 °C. The samples were degassed at 200 °C for 2 h prior to analysis. The Brunauer−Emmett−Teller (BET) surface area was determined using the adsorption data in a relative pressure ranging from 0.04 to 0.20. The total pore volume was calculated from the amount adsorbed at a relative pressure of about 0.99.58 The pore diameter was calculated from desorption branch by using the Barrett−Joyner−Halenda (BJH) method. Fourier transform infrared (FT-IR) measurements were conducted on a Nicolet 6700 spectrometer by means of the KBr pellet technique. Transmission electron microscopy (TEM) analysis was performed on a FEI Tecnai G2 F20 S-TWIN electron microscope operated at 120 kV. Samples for the TEM test were suspended in ethanol and supported on a carbon-coated copper grid.

(Chengdu Kelong Chemical) was assembled on these base materials by incipient wetness impregnation at ambient conditions for 4 h, then the solids were further dried at 100 °C for 12 h according to Delannoy et al.53 The preparation is detailed in Supporting Information. 2.2. Adsorption Performance Measurements. Ethylene and ethane equilibrium adsorption tests were conducted in the static adsorption system (Figure 1). It was first stripped under vacuum for 30 min and the purged with the high purity helium before each test. In this paper, the adsorption capacity of ethylene and ethane were measured at 303 K that was sustained by a constant temperature water bath and under a variable pressure with the maximum value of 0.7 MPa. The He (99.999%), C2H4 (99.99%), and C2H6 (99.99%) were obtained from Chengdu Dongfeng Industrial Gases Co., Ltd. One gram of sample was pretreated at 110 °C for 8 h in a vacuum oven prior to data collection and then loaded in the adsorption cell. The volume of adsorption cell and the reference cell was known, while the dead volume and gas adsorption volume were calculated according to the RK equation. Adsorption capacity was calculated in standard conditions. Each test was carried out twice to ensure the veracity of the experimental data. The used adsorbent was regenerated by vacuum. RK equation is in eq 1, which could turn to eq 2 p=

RT a − 0.5 V−b T V (V + b)

Vi + 1 =

a(V − b) RT + b − 0.5 i p T pVi (Vi + b)

3. RESULTS AND DISCUSSION 3.1. Microstructure. Figure 2 shows the low-angle XRD patterns of mesoporous silica SYSG, SBA-15, and KIT-6 before and after the introduction of Ag species. No evident diffraction peak is found for SYSG and Ag/SYSG, meaning that SYSG and Ag/SYSG possess nonordered mesoporous structure. Both SBA-15 and Ag/SBA-15 possess an intense diffraction peak accompanied by two weak ones, which can be respectively indexed as (100), (110), and (200) reflections of a 2D hexagonal pore symmetry (P6mm).59 KIT-6 and Ag/KIT-6 also show the visible diffraction peaks that are indexed as (211), (220), and (311) reflections of high structural order with Ia3d group.60−62 It means that the introduction of Ag does not change the crystalline characteristics of the parent mesoporous materials. However, the loss of diffraction intensity for samples Ag/SBA-15 and Ag/KIT-6 results from the decreased scatter contrast between pore walls and pore space after incorporation of Ag species into the mesopores. N2 adsorption/desorption isotherms and pore size distributions of mesoporous silica are demonstrated in Figure 3. All the

(1)

(2)

In a typical run, the adsorbent is placed in the adsorption chamber and degassed under vacuum conditions, then the system is purged with He at the highest test pressure 0.7 MPa. When the adsorption experiment is performed, a certain pressure (0.1−0.7 MPa with interval of 0.1 MPa) gas of ethane/ ethylene is released and afterward adsorbed by the loaded materials, which is well controlled by the switching the valves V2/V3 and V4. The pressures before and after adsorption were recorded by programmable logic controller (PLC) devices. 2.3. Adsorption Model. Langmuir−Freundlich model was applied to fit the isothermal adsorption of ethylene and ethane B



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Table 1. Textural Properties of the Different Samples

a

sample

SBET (m2/g)

SYSG Ag/SYSG SBA-15 Ag/SBA-15 KIT-6 Ag/KIT-6

373.8 306.1 613.5 529.5 691.7 550.4

ΔSBET (%)a 18 20 14

VP (cm3/g)

Dp (nm)

1.03 0.89 0.96 0.79 0.99 0.82

11.0 11.6 6.3 5.9 5.8 5.9

ΔSBET = (Sbase silica − SAg‑loaded silica) × 100/Sbase silica

exhibit higher surface area of 613.5 and 691.7 m2/g, respectively, comparing with that of base SYSG (373.8 m2/ g). After the introduction of AgNO3, the surface area of the corresponding adsorbents decrease by 14%, 20%, and 18% to 529.5, 550.4, and 306.1 m2/g, respectively. Assuming that only 10% of Ag species was loaded on the base materials, the change in surface area is probably caused by the pore block. It is interesting to note that the average pore size remains almost unchanged while the total pore volume decreases by around 17% for all the three adsorbents. These results indicate that the mesoporous feature of the materials is maintained after the AgNO3 is loaded. However, a small amount of pores may be blocked, therefore resulting in the loss of surface area of the total pore volume. The unchanged average pore diameter means there is uniform dispersion of Ag species in the pores. The morphology study is performed on the selected samples KIT-6 and Ag/KIT-6 by TEM characterization, and the images are shown as Figure 4. The well-organized ordered cubic 3D mesoporous channels could be observed for the both samples. Even for the Ag species loaded sample, the aggregation of Ag species to form large particle is invisible, which can be assigned to the well-dispersed internal and external the pores.53 The result on the crystalline structure is consistent with the lowangle XRD patterns. Furthermore, the channels in the network have a regular diameter of about 5 nm according to the TEM, which is in good agreement with the pore size distribution data of N2 adsorption/desorption. 3.2. Dispersion. EDX surface scanning images of Ag, Si, and O elements for the Ag/SYSG, Ag/SBA-15, and Ag/KIT-6 adsorbents are shown in Figure 5. It can be seen that Si and O elements that represents the carrier dispersion are scattered in

Figure 2. Low-angle XRD patterns of mesoporous silica materials (a, SYSG; b, Ag/SYSG; c, SBA-15; d, Ag/SBA-15; e, KIT-6; and f, Ag/ KIT-6).

base samples (a,c,e) show a typical type IV isotherm according to the IUPAC classification, which indicates the mesoporous characteristic of the materials. On the other hand, the capillary condensation occurred at high relative pressure and the existence of H2 type hysteresis loop reveal the presence of cylindrical-like pores with a narrow distribution of pore size.63 When AgNO3 was loaded on these materials, the isotherms of the samples (b,d,f) show a similar shape to those of the base materials, implying the unchanged porosity. As far as the pore size distribution is concerned, no obvious change could be seen even after the introduction of the Ag species, centered at about 5 nm for KIT-6 based samples, about 6 nm for SBA-15 based samples and about 9 nm for SYSG based samples, although the SYSG based samples exhibit a little bit wider distribution compared to the rest two with ordered mesopores. The quantitative results of the textural properties like specific surface area (SBET), pore volume (Vp), and average pore size (Dp) are summarized in Table 1. The base SBA-15 and KIT-6

Figure 3. N2 adsorption/desorption isotherms and pore diameter distribution of mesoporous silica materials (a, SYSG; b. Ag/SYSG; c, SBA-15; d, Ag/SBA-15; e, KIT-6; and f, Ag/KIT-6). C



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Figure 4. TEM images of KIT-6 and Ag/KIT-6.

the whole plane, while O element maps are a little different for the three adsorbents caused by microstructure difference. Also, the Ag element of Ag/SYSG, Ag/SBA-15, Ag/KIT-6 adsorbents homogeneously is distributed in the area where Si and O elements are occupied, meanings that AgNO3 particles are successfully introduced to the surface or internal pores of mesoporous silica materials. 3.3. Functional Group Measured by FT-IR. Figure 6 gives IR spectra of the mesoporous silica and Ag-containing mesoporous silica. For mesoporous silica, the band at 960 cm−1 can be assigned to the stretching vibrations of silanol groups (Si−OH),64,65 and the bands at 1090 and 805 cm−1 are caused by the asymmetric and symmetric stretching vibrations of Si− O−Si frameworks, respectively.65 As for Ag-containing samples,

Figure 6. FT-IR spectra of mesoporous silica materials (a, SYSG; b, Ag/SYSG; c, SBA-15; d, Ag/SBA-15; e, KIT-6; and f, Ag/KIT-6).

the vibration bands at 1383 cm−1 are derived from nitrate.65 Moreover, the band at 961 cm−1 decreases slightly, implying the consumption of silanol groups due to the interaction between AgNO3 and silanol groups on mesoporous silica. The decrease in peak intensity (3−5%) seems to be linked with the origination of the support materials. Noteworthy, the band at 961 cm−1 of Ag/SBA-15 is weaker than that of Ag/SYSG and

Figure 5. Elements mapping of the Ag-loaded samples by EDX. D



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Figure 7. Adsorption equilibrium isotherms of ethane and ethane on different samples at 303 K (a, SYSG; b, Ag/SYSG; c, SBA-15; d, Ag/SBA-15; e, KIT-6; and f, Ag/KIT-6).

mesoporous silica when silver nitrate is loaded, leading to the physical adsorption capacity being weakened. While the enhanced selective adsorption of ethylene could be attributed to the change of adsorption mechanism in the case of the presence of Ag cations. The increase of adsorbed ethylene relied on the complexation of Ag+ and CC on account of the consumption of Si−OH by interacting with AgNO3, which could be evidenced by the loss of Si−OH band intensity (about 960 cm−1) for FT-IR results (Figure 6). The less intense of that for the sample Ag/SBA-15 is probably caused by the positive interaction between Ag cations and surface silanol groups, promoting to form a homogeneous Ag-dispersed surface. On the other hand, the SBA-15 based adsorbent lose the least surface after loading 10 wt % AgNO3, which also implied that the Ag species have the best dispersion. As a result, the adsorption of ethylene is enhanced through π-complexation, while the physical adsorption for ethane is restricted; the separation performance is therefore improved. It can also explain the difference in performance for Ag/SBA-15 and Ag/ KIT-6 although both samples have the similar adsorption capacity for ethylene, showing the synergetic effects from the surface area and the Ag species dispersion. The C2H4/C2H6 separation factor of the different samples is depicted in Figure 8. It is obvious to see that the separation factor of all three base adsorbents keep almost constant in the range from 1.0 to 1.5 as pressure increases gradually, implying

Ag/KIT-6, meaning that AgNO3 interacts better with silanol groups than support SBA-15. 3.4. Adsorption Performances. The adsorption equilibrium isotherms of ethane and ethane on the different samples are gathered in Figure 7. It can be seen from Figure 7a that adsorption capacity of ethylene and ethane in SYSG is low. At 0.7 MPa, ethane adsorption capacity and ethylene adsorption capacity are 15.8 and 17.0 cm3/g, respectively. Whereas for KIT-6 and SBA-15 under the same condition, ethane adsorption capacity is 29.9 and 28.5 cm3/g and ethylene adsorption capacity is 34.3 and 33.4 cm3/g. Obviously, the adsorption capacity is proportional to specific surface area because the materials with higher surface area offers more physical adsorption sites, thus SBA-15 and KIT-6 exhibit better adsorption capacity by van der Waals force. After impregnating 10 wt % silver nitrate on samples, the adsorption behavior of the adsorbents show evident difference for ethane and ethylene. Ethane adsorption amount declines to 13.1 cm3/g, and ethylene adsorption raises to 24.7 cm3/g for Ag/SYSG under 0.7 MPa. Ethane adsorption quantity of Ag/ SBA-15 and Ag/KIT-6 greatly reduces to 18.6 and 21.4 cm3/g, and ethylene adsorption capacity shifts to 37.3 and 37.7 cm3/g. The selective adsorption of the mixture is enhanced although the total adsorption capacity decreased for Ag/KIT-6 and Ag/ SBA-15 adsorbents. The decrease in adsorption capacity is probable due to the decrease of specific surface area of E



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Figure 9. Cyclic adsorption performance of adsorbents at 303 K (a, Ag/SYSG; b, Ag/SBA-15; c, Ag/KIT-6).

symmetrical structure of SBA-15 facilitates to spread AgNO3 species on the silica surfaces along the ordered pores channels, which helps to form a homogeneous surface modified by Ag species. The Ia3d multidirection pores structure of KIT-6 and the chaotic structure of SYSG have the channels of both materials easily blocked during the impregnation process, resulting in the loss of surface area and active adsorption sites. The improvement of separation performances after dispersing the Ag species on the base silica solid was because the adsorption mechanism is changed due to the presence of Ag cations. The physical adsorption takes place on the pure silica adsorbents while the π-complexation mainly occurs on Agcontained surface. The introduction of Ag species has a negative effect on the surface area. On the other hand, the welldispersed Ag species on surface could enhance the adsorption of ethylene by π-complexation. At the same time, the dispersed Ag species occupy the sites for physical adsorption, leading to the loss of adsorption capacity for ethane. As a consequence, the Ag/SBA-15 shows the best separation performances.

Figure 8. Separation factor of ethane/ethane on mesoporous silica materials at 303 K (a, SBA-15; b, KIT-6; c, SYSG; d, Ag/KIT-6; e, Ag/ SYSG; and f, Ag/SBA-15).

that original silica cannot effectively separate ethylene and ethane. Such a situation is significantly changed when 10 wt % silver nitrate is loaded on silica adsorbent. Ethylene/ethane separation factor of Ag/SYSG is 4.27 at atmospheric pressure, and those of Ag/SBA-15 and Ag/KIT-6 are 5.27 and 3.95, respectively. Thus, the separation factor order is easily Ag/SBA15 > Ag/SYSG > Ag/KIT-6. At 0.7 MPa, the same separation factor order shows up, and the ethylene/ethane separation factor of Ag/SYSG, Ag/SBA-15, Ag/KIT-6 are 1.93, 2.2 and 1.76, respectively. The improvement in separation factor is considerably attributed to the dispersion on Ag species. As aforementioned in porosity and morphology part, the Ag/SBA-15 has better dispersion of Ag species compared with the other rest. Furthermore, P6 symmetrical structure of SBA-15 can provide larger external surface as the pores channel orderly located in one direction, which is favorable for Ag+ complexation. On the contrary, the multidirection pores of Ia3d of KIT-6 and chaotic structured of SYSG are much easier to block during the impregnation process. 3.5. Reproducibility of Adsorbents. The cyclic use of the adsorbents modified by AgNO3 was investigated and the results are depicted in Figure 9 where only the adsorption capacity for ethylene is shown just because the physical adsorption of ethane could be considered to keep unchanged like in Figure 7. It is clearly found that the adsorption capacity for ethylene keeps nearly constant for the three different adsorbents after 10 cycles although they have distinct separation performances. The constant adsorption capacity for ethylene indicated that the Ag species sites do not lose and the adsorbed ethylene could be entirely desorbed. Thus, a good stability could be achieved.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00069. Mesoporous materials synthesis, adsorption model and separation factor, and adsorption data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86-28-85403836 (F.J.). *E-mail: [email protected] (S.L.). ORCID

Fangli Jing: 0000-0001-6029-7481 Funding

The project was supported by the National Natural Science Foundation of China (21476145, 21603153) and the Science and Technology Department of Sichuan Province Planning Project (2015GZ0173).

4. CONCLUSIONS Three kinds of mesoporous silica materials were employed as base materials for developing new adsorbents for the ethylene and ethane separation. The microstructure plays an important role in determining the separation performances. The P6

Notes

The authors declare no competing financial interest. F



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ACKNOWLEDGMENTS The authors would like to thank Huan Zhang for their assistances on static adsorption measurement. We also thank Jiajie Wang, Shouqiang Li, and Jie Deng for their useful discussion and kind helps.

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