Article pubs.acs.org/jced
Adsorption Properties of C2H4 and C3H6 on 11 Adsorbents Wei Su,† Ai Zhang,† Yan Sun,‡ Meng Ran,† and Xiaojing Wang*,† †
Tianjin Key Laboratory of Membrane and Desalination Technology, School of Chemical Engineering and Technology, Tianjin University, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300350, P. R. China ‡ Department of Chemistry, School of Science, Tianjin University, Tianjin 300350, P. R. China S Supporting Information *
ABSTRACT: Adsorption isotherms of pure C2H4 and C3H6 were measured on 11 adsorbents and at pressures up to 0.8 MPa using a volumetric method. The Brunauer−Emmett−Teller (BET) surface area of the adsorbents had a significant effect on adsorption capacities of both C2H4 and C3H6. The metal organic framework MIL-101 had the highest BET surface area and the highest adsorption capacity. For zeolite molecular sieves 5A, the C2H4 adsorption capacity was higher than that of C3H6, which is opposite to the results for most adsorbents. The isosteric heats of C3H6 on AC-1, 5A, MIL-101, ZIF-8, and SG1 were higher than those of C2H4. The C3H6/C2H4 separation selectivities were calculated using the ideal solution theory, and the activated carbon AC-1 had the highest separation selectivity of 8.8.
1. INTRODUCTION Ethylene (C2H4) is a versatile simple organic chemical which is usually produced by the petrochemical industry.1 One important use of C2H4 is in the production of ethylbenzene.2 To inhibit side reactions in these reactions, the volume fraction of C3H6 is generally required to be less than 0.15%.3 Adsorption separation technology is a promising method to separate C3H6 from C2H4. The adsorption equilibrium of C2H4 and C3H6 on various adsorbents has been reported. Costa et al.4 studied the adsorption equilibrium of ethylene, propane, propylene, carbon dioxide, and their mixtures on 13X zeolite. Bao et al.5 measured the adsorption of C2H6, C2H4, C3H8, and C3H6 on Mg-MOF-74, which presented a high separation selectivity of C2H4/C2H6 (13.9), C3H6/C3H8 (8.9), and C3H6/ C2H4 (4.7) at 298 K in a vacuum swing adsorption process. Selective adsorption of C2H4/C2H6 and C3H6/C3H8 were studied in various MOF adsorbents such as CPO-27,6 ZIF-8,6 M2(dobdc),7 ZIF-4,8 and so on. However, there has been a little research focusing on the separation of C2H4/C3H8. Ji et al.9 investigated C2H4/C3H6 dynamic adsorption−desorption properties on activated carbons. Ye et al.10 measured the adsorption isotherms of C2H4 and C3H6 on 15 commercial activated carbons and studied the effects of the activated carbon BET surface area and microporous volume on the separation of propylene from dry gases. Peng et al.11 studied the binary adsorption of C2H4 and C3H6 on silicalite-1. MOFs with high BET surface areas and large pore volumes are promising adsorbents for gas separation C2H4/C3H6. In this work, adsorption isotherms of C2H4 and C3H6 on 11 adsorbents were determined. The adsorbents include three MOFs (MIL101, Cu-BTC, and ZIF-8), two ordered mesoporous adsorbents (SBA-15 and CMK-3), one activated carbon (AC-1), three © XXXX American Chemical Society
zeolite molecular sieves (5A, Y, and 13X) and two silicon gels (SG-1 and SG-2). In addition, adsorption capacities and separation selectivities are presented and discussed.
2. EXPERIMENTAL SECTION 2.1. Adsorbents. Eleven adsorbents were studied in this work. The activated carbon AC-1 was prepared with steam activation of coconut shells in our lab.12 The three zeolite molecular sieves 5A (Energy Chemical), 13X (Nankai catalyst plant), and Y (Nankai catalyst plant) and the two silica gels SG1 (Qingdao Haiyang) and SG-2 (Aladdin) were purchased commercially from the indicated company. The three MOFs MIL-101, Cu-BTC, and ZIF-8 were synthesized using hydrothermal methods.13−15 The ordered mesoporous materials SBA-15 and CMK-3 were synthesized according to the procedure of Zhou et al.16 AC-1, SG-1, and SG-2 were granule, zeolites 13X, 5A, and Y were pellet, and the other five adsorbents were powder. The BET surface areas, pore volumes, and average pore sizes of the adsorbents are shown in Table 1. The BET surface areas of the synthesized adsorbents were close to previously reported values.13−16 The average pore sizes of CMK-3, SBA-15, and SG-2 are all larger than 2 nm, which indicates that they are all mesoporous materials. The pore size distribution for MIL-101 ranges from 1 to 3 nm, so this adsorbent has a microporous structure with some smaller mesoporous structures.13 On the basis of the average pore sizes, the other adsorbents are all microporous adsorbents. Received: August 22, 2016 Accepted: November 25, 2016
A
DOI: 10.1021/acs.jced.6b00749 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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3. RESULTS AND DISCUSSION 3.1. Adsorption of C2H4 and C3H6. The adsorption isotherms of C2H4 and C3H6 on the 11 adsorbents at 293 K and at pressures up to 0.8 MPa are shown in Figure 1.The adsorptions of C3H6 are all type I on all the microporous absorbents (Figure 1a−h). The isotherms of C2H4 on 5A is also type I (Figure 1c). However, the C2H4 isotherms on SG-1 (Figure 1b) and MIL-101 (Figure 1f) are nearly linear, and the curves for the other five microporous adsorbents (Figure 1a,d,e,g, and h) are between type I and linear. The Langmuir−Freundlich (L-F)19 equation has been widely applied to adsorption isotherms, and the equation has been found to fit well with type I isotherms with three parameters. The equation can be expressed as
Table 1. Textural Properties of the Adsorbents AC-1 CMK-3 MIL-101 Cu-BTC ZIF-8 5A Y 13X SBA-15 SG-1 SG-2
SBET (m2·g−1)
pore volume (mL·g−1)
average pore size (nm)
1791 1203 2794 1231 1048 325 311 277 967 577 339
0.85 1.10 1.43 0.63 0.63 0.28 0.24 0.19 1.51 0.32 0.79
1.08 2.47 1.88 1.32 1.09 0.49 1.03 1.01 9.52 1.24 9.07
⎛n⎞ B ·pq θ=⎜ ⎟= 1 + B ·pq ⎝ nm ⎠
2.2. Adsorption Isotherm Measurements. The adsorption isotherms of C2H4 and C3H6 on the 11 adsorbents were collected using a volumetric experimental apparatus. The measurements were based on volumetric principles, and the device has been described in our previous studies.17,18 The pressure transmitter (M3 in the previous study: 0.0−1.6 MPa) used in this work is the same as that used in the work by Zhang et al.13 The adsorption isotherms were measured at 293 K, and the highest pressure was about 0.8 MPa. The helium (99.9995%), C2H4 (99.995%), and C3H6 (99.995%) used for the measurements were obtained from Tianjin Liufang Industrial Gases Co., Ltd.
(1)
where θ is the percent surface coverage, n is the adsorption amount, nm is the monolayer adsorption saturation capacity, B is the Langmuir constant, p is the equilibrium pressure, and q is the adsorbent surface inhomogeneity parameter. The L-F equation was used to fit the isotherms of C2H4 and C3H6 on the microporous materials and the fitted curves are shown as the solid lines in Figure 1a−h. The adsorption isotherms of C3H6 on the three mesoporous adsorbents CMK-3, SBA-15, and SG-2 are all type IV (Figure 1i, j, and k), whereas those of C2H4 are close to linear. The temperature (293 K) of adsorption isotherms measured is above the Tc of C2H4 (282.4 K) and below the Tc of C3H6
Figure 1. Adsorption isotherms on 11 adsorbents at 293 K: a, AC-1; b, SG-1; c, 5A; d, Y; e, 13X; f, MIL-101; g, Cu-BTC; h, ZIF-8; i, CMK-3; j, SBA15; k, SG-2. ■, C2H4 and □, C3H6. Solid lines (a−h), L-F model; solid lines (i−k), quartic equation. B
DOI: 10.1021/acs.jced.6b00749 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 2. Dependence of adsorption capacity at 0.5 MPa on BET surface area.
Figure 3. Adsorption selectivities for C3H6/C2H4 at different pressures (note 5A is SEthylene/Propylene).
where x1 and y1 are the equilibrium mole fractions of component 1 in the adsorbed and fluid phases, respectively, and x2 and y2 are those same mole fractions for component 2. The ideal adsorbed solution theory (IAST)25,26 has been widely used to predict multicomponent adsorption equilibria based on pure component isotherms. The results for mixtures predicted from IAST calculations are consistent with the Grand Canonical Monte Carlo simulations of binary mixture adsorption.27,28 Therefore, IAST was used to evaluate the separation selectivity of C2H4/C3H6 on the 11 adsorbents and the result are shown in Figure 3. As shown in Figure 3, the maximum adsorption selectivity of AC-1 is much higher than those for the other adsorbents. With the exception of Y and 13X, the adsorption selectivities of the other microporous adsorbents are all larger than 4.0. Y and 13X had poor separating performance, and their separation selectivities were less than 2.0. The separation selectivity of the mesoporous adsorbents CMK-3, SBA-15, and SG-2 all decreased significantly with adsorption pressure. For example, the separation selectivity of CMK-3 went from 4.3 to 1.3 as the pressure increased from 0.4 to 1.8 MPa. The separation selectivity of C2H4/C3H6 for the absorbents at 293 K and 1.0 MPa are shown in Table 2. The separation selectivity decreases in the order: AC-1 > Cu-BTC > ZIF-8 > 5A > MIL-101 > SG-1 > SG-2 > SBA-15 > CMK-3 > 13X > Y. The separation selectivity of AC-1 under these condition was 8.7, which is higher than previous reported results for AC (8.2 at 298 K).9 This may be due to the smaller average pore size (1.08 nm) of AC-1 compared with that of AC (1.94 nm).9 3.3. Isosteric Heats of Adsorption. Isosteric heat of adsorption could provide useful information about the nature of the surface and the adsorbed phase and it varies with coverage.29 Combined with the Gibbs−Helmholtz equation, the isotheric heat of adsorption Qst was calculated using
(364.8 K). Therefore the C 2H4 adsorptions on three mesoporous adsorbents are monolayer adsorptions resulting in the linear adsorption isotherms.20,21 On the contrary, the C3H6 adsorptions are multilayer adsorptions, and the highest adsorption pressure for C3H6 was about 0.8 MPa which is little lower than the saturated adsorption pressure (1.011 MPa) of it. Those facts lead to type IV adsorption isotherms for C3H6 on mesoporous adsorbents. Figure 1 panels a−k show that for all the absorbents with the exception of 5A, Y, and 13X, the adsorption capacities of C3H6 are larger than those of C2H4. The adsorption capacities of C3H6 on the zeolites Y and 13X are almost the same as those of C2H4. However, the adsorption capacity of C3H6 on 5A is much lower than that of C2H4. The kinetic diameters of C2H4 and C3H6 are 4.163 and 4.678 Å, respectively. Possibly, molecular propylene is difficult to diffuse inside the pores of 5A, which results in a lower adsorption capacity. A similar result was reported by Derrah et al.22 The effect of BET surface area on the adsorption capacity of C2H4 and C3H6 at 0.5 MPa is presented in Figure 2. There is a linear relationship between the adsorption capacity and the BET surface area. MIL-101 had the largest BET surface area, 2794 m2·g−1, and as a result, MIL-101 also had the largest adsorption capacity for C2H4 (6.054 mmol·g−1) and C3H6 (12.294 mmol·g−1). Similar results were found by Chakraborty et al.23 They found that adsorbents with high BET surface areas possessed lower heats of adsorption (ΔH0) in the Henry regime which resulted in larger amounts of adsorption.23 3.2. Separation Selectivity. To compare the adsorption and separation performances of the different adsorbents, the separation selectivities for C2H4/C3H6, S1,2 were calculated using24 S1,2 =
(x1/y1) (x 2/y2 )
Q st = −ΔH = −R
(2) C
d ln f d(1/T )
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C2H4. A linear relationship was observed between the adsorption capacity and the BET surface area of the absorbents. The isosteric heats of C3H6 on AC-1, 5A, MIL-101, ZIF-8, and SG-1 are higher than those of C2H4. MIL-101 had the largest adsorption capacity for C2H4 and C3H6 which was due to its largest BET surface area. AC-1 had the highest separation selectivity for its high BET surface area and low average pore size.
Table 2. Separation Selectivity of C3H6/C2H4 on the Porous Adsorbents at 293 K and 1.0 MPa adsorbent
S1,2
AC-1 CMK-3 SG-1 SG-2 5A Y 13X SBA-15 MIL-101 Cu-BTC ZIF-8 AC4
8.8 2.0 4.4 2.7 5.0 (C2H4/C3H6) 1.2 1.5 2.5 4.8 5.8 5.0 8.2 (298 K)
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00749. Adsorption data and isosters of adsorption (PDF)
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Equation 3 can be integrated to give
AUTHOR INFORMATION
Corresponding Author
ΔH (4) RT where f is fugacity of the pure gas, ΔH is the isosteric enthalpy of adsorption, n is the amount adsorbed, and a is a constant. In this work, the adsorption isotherms of C2H4 and C3H6 on 5A, MIL-101, AC-1, SG-1, and ZIF-8 at 273, 293 and 313 K and at pressures up to 0.8 MPa were collected respectively and are shown in Figure S1. The plots of ln f versus 1/T were obtained for each adsorption amount and were expected to be linear according to eq 4 (Figure S2). Isosteric heats of adsorption are determined by the slope of these curves. As shown in Figure 4a, the isosteric heat of C2H4 for SG-1 decreased from 20 to 17 kJ/mol, as adsorption capacity is increased from 0.2 to 1.4 kJ/mol, while the isosteric heat of C2H4 for MIL-101, ZIF-8, and AC-1 remains constant at about 15.0, 15.0, and 10.5 kJ/mol, respectively. It is shown in Figure 4b that the isosteric heat of C3H6 on the adsorbents is higher than that of C2H4. The isosteric heat of C3H6 on MIL-101 and AC-1 increased with the adsorption capacity, which is opposite to that of C3H6.
*E-mail:
[email protected]. Tel.:+86-022-27406301. Fax: +86-022-27406301.
ln f = a −
Funding
Financial support from the National Natural Science Foundation of China (No 21206108; 21406004) and the Tianjin Municipal Science and Technology Commission (No 14JCYBJC21200) is greatly appreciated. Notes
The authors declare no competing financial interest.
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4. CONCLUSIONS The adsorption isotherms of C2H4 and C3H6 on 11 adsorbents were measured at 293 K and at pressures up to 0.8 MPa. The adsorption capacity of C3H6 was higher than that of C2H4 for most of the adsorbents. For zeolites Y and 13X the adsorption capacities of C3H6 and C2H4 are nearly the same, while the adsorption capacity of C3H6 on 5A is much lower than that of
Figure 4. Isosteric heats of a, C2H4 and b, C3H6 adsorption on MIL-101, SG-1, ZIF-8, 5A, AC-1. D
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DOI: 10.1021/acs.jced.6b00749 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
