Ethylene Gas Mixtures Using Wet ZIF-8

Article pubs.acs.org/IECR

Separation of Methane/Ethylene Gas Mixtures Using Wet ZIF‑8 Xiao-Xin Zhang, Peng Xiao, Chang-Hua Zhan, Bei Liu, Rui-Qin Zhong,* Lan-Ying Yang, Chang-Yu Sun,* Huang Liu, Yong Pan, Guang-Jin Chen, and Nan Li State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P. R. China ABSTRACT: The separation of methane/ethylene gas mixtures by the adsorption−hydration method was investigated by using a wet zeolitic imidazolate framework (ZIF) microporous material, ZIF-8. The influences of the initial gas−solid ratio (the volume ratio of gas mixture to ZIF-8) and water content of wet ZIF-8 on the separation performance were studied systematically at 274.15 K. The experimental results show that gas−solid ratio of ca. 310 and water content of ca. 38.50 wt % are the suitable conditions for the separation of methane/ethylene mixture with wet ZIF-8 on the premise of hydrate formation. When water content of wet ZIF-8 is 38.50 wt % and the gas−solid ratio is 297, the mole fraction of ethylene in gas phase decreases from 30.91% to 14.44%, the selectivity coefficient reaches 5.57, and the adsorption quantity of ethylene reaches 3.37 mmol/g after one stage of the adsorption−hydration process. Compared with a single adsorption method or hydration method, methane/ethylene gas mixtures can be further separated with the synergistic effect of gas adsorption and hydrate formation. The morphology and crystal structure of ZIF-8 remains after a series of processes of saturation with water, adsorption−hydration, hydrate decomposition, and drying.

1. INTRODUCTION The unsaturated hydrocarbons such as ethylene and propylene, which are the important building blocks in the petrochemical industry,1−3 are always produced by naphtha cracking or dehydrogenation of the corresponding paraffins under the conditions of cryogenic distillation. Although the process of cryogenic distillation based on the different boiling points or vapor pressures through fractional distillation is energyconsuming,4,5 it has been used to separate paraffins or purify olefins during the past 70 years.6−8 In view of its high energy consumption, new methods to separate the olefins from the paraffins are urgently required. In recent years, many studies concern the replacement of cryogenic distillation with adsorptive separation. Compared with cryogenic distillation, adsorptive separation is a good choice to separate olefins from paraffins for energy efficiency and cost savings.6 Referring to the adsorbent, zeolitic imidazolate frameworks (ZIFs) are a good option due to their high stability and flexible topological structures. ZIFs have been used in gas and chemical storage,9−11 separation,12−16 selective catalysis,17−19 ion exchange20 and so on. However, there are few reports on separating olefins from paraffins by ZIFs. Wang et al.21 developed an improved synthesis process of Cu-BTC ([Cu3(BTC)2, BTC = benzene-1,3,5-tricarboxylate), and the ethylene/ethane sorption ratio using it was 1.2 at 295 K and 80 kPa. Das et al.22 explored a new approach to construct the activated UTSA-36a, and it exhibited high adsorption selectivity of C2H6, C2H4, and C2H2 over CH4 with the separation coefficient from 11 to 25 when the temperature ranged from 273 to 296 K. He et al.23,24 tried to tune the pore and cavity sizes within several metal−organic frameworks (MOFs) and explored two new porous MOFs, UTSA-33 and UTSA-34, which exhibited high adsorption capacity and separation selectivity of C2 hydrocarbons over methane at room temperature. Horike et al.25 used a solid solution, which was formed by integrating porous coordination polymers (PCP) © XXXX American Chemical Society

with MOFs, to improve separation performance for CH4/C2H6 gas mixture. It is known that when a gas mixture forms a hydrate partially, the relative concentration of each component in the hydrate phase and in the residual vapor phase might be different. Since the component that can form the hydrate more easily might be enriched in the hydrate phase, the gas mixture can then be separated through hydrate formation.26−32 In a porous media system with the presence of water, a gas mixture can be separated by the synergistic effect of gas adsorption and hydrate formation, which is called the hybrid separation method of adsorption−hydration. We have adopted this method to examine the separation performance of active carbon for CO2/CH4 mixture.33 ZIF-8 has good water-stability and can maintain its structure for 7 days in boiling water.34 In this work, the first example of a microporous MOF applied for the separation of CH4/C2H4 gas mixture by the adsorption− hydration method was performed. The influence of the initial gas−solid ratio (the volume ratio of the gas mixture (CH4 + C2H4) into the sapphire cell and ZIF-8) and water content of wet ZIF-8 on the separation performance was investigated.

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. The experimental apparatus used in this study has been described in our previous articles.16,30,31,33 The schematic diagram of the experimental apparatus is shown in Figure 1. The critical part of the experimental apparatus is a transparent sapphire cell and a steel-made blind cell. The effective volume of the sapphire cell and the blind cell plus the tubes connecting two cells are 34.14 and 110.00 cm3, respectively. A secondary platinum resistance Received: March 11, 2015 Revised: June 15, 2015 Accepted: July 29, 2015

A

DOI: 10.1021/acs.iecr.5b00941 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

ZIF-8 was weighed using a balance with a precision of 0.1 mg. In this study, the wet ZIF-8 with different water contents was prepared following the procedures below. First, a certain quantity of ZIF-8 sample was dried at 373.15 K until its mass remained constant. Then it was placed in a beaker and mixed with a certain amount water to form a kind of slurry. Thereafter, the beaker with the slurry of ZIF-8 was put into a vacuum drying oven and kept in vacuum state for more than 2 days to degas ZIF-8 and make sure that water was uniformly distributed. Finally, the slurry of ZIF-8 was dried at 353.15 K in an electric jacket for different time to obtain the wet ZIF-8 samples with different water contents. Water content, Xw, of wet ZIF-8 was calculated by the following equation: m − md Xw = w × 100% mw (1) where mw and md are the mass of wet and dry ZIF-8, respectively, which were weighed precisely using a balance with a precision of 0.1 mg. 2.3. Separation Procedures. The separation performance of wet ZIF-8 was examined according to the following procedures. First, the sapphire cell was dismounted from the apparatus, washed with distilled water, and dried. A certain mass of wet or fresh ZIF-8 was loaded into the sapphire cell. Then it was installed and purged through vacuuming. Afterward, the top valve of the sapphire cell was closed. Second, the blind cell and the tubes connecting two cells were purged with the feed gas 4−5 times to ensure the absence of air. The blind cell was then filled with a sufficient amount of the feed gas and the air-bath temperature was set to the desired value. When the temperature and pressure of the system kept constant, the pressure of the feed gas in the blind cell was recorded as the initial pressure. Third, the feed gas was injected into the sapphire cell through its top valve from the blind cell until the desired pressure of the sapphire cell was attained. The valve between the blind cell and the sapphire cell was turned off. The adsorption and hydration processes then started. When the pressure of the blind cell and the sapphire cell remained constant, both the adsorption and the hydration processes were deemed to be complete. The final pressures of the blind cell and the sapphire cell were recorded. The equilibrium gas mixture in the sapphire cell was sampled under constant pressure by pushing the piston using a hand pump and its composition was analyzed by the HP 7890 gas chromatograph. When investigating the effect of gas−solid ratio or water content of wet ZIF-8 on the separation performance for CH4/ C2H4 gas mixture, the fresh wet ZIF-8 first formed hydrate and dissociated; thereafter, the recycled wet ZIF-8 was used for each experimental run to eliminate the influence of induction period of hydrate nucleation. To examine the influence of gas−solid ratio, after one group of experiments at a certain gas−solid ratio, the sapphire cell with wet ZIF-8 was vacuumed and the temperature of the system was set to 298.15 K and kept for 2 h. Afterward, the temperature of the system was regulated to the specified value and the sapphire cell was vacuumed again. The separation experiment was processed with another specified gas−solid ratio. A series of separation experiments at different gas−solid ratios were then performed. To examine the effect of water content, after the separation experiment with a higher water content of wet ZIF-8 was finished, the sapphire cell with

Figure 1. Schematic diagram of the experimental apparatus. RTD, resistance thermocouple detector; DPT, differential pressure transducer; DAS, data acquisition system.

thermometer (type-pt100) is used to measure the temperature of the air bath, and a calibrated Heise pressure gauge and two differential pressure transducers are used to measure the system pressure. The variation of system temperature and pressure with time are recorded and displayed by a computer. The accuracy of temperature and pressure measurement is ±0.20 K and ±0.025 MPa, respectively. 2.2. Experimental Material. Methane (99.99%) and ethylene (99.99%) were purchased from Beijing AP Beifen Gases Industry Company Limited. The gas mixture containing 69.09 mol % CH4 and 30.91 mol % C2H4 was prepared in our laboratory. The composition of the feed gas and the equilibrium gas mixtures was analyzed by a Hewlett-Packard 7890 gas chromatograph. Distilled water was adopted with the conductivity less than 10−4 S·m−1. ZIF-8 was purchased from Sigma-Aldrich. The nitrogen gas adsorption isotherm of ZIF-8 was measured by ASAP 2020 at 77.26 K (Figure 2), which exhibits a type-I sorption behavior with Brunauer−Emmett− Teller (BET) surface area of 1273.15 m2/g and pore volume of 0.61 cm3/g determined by t-plot method.35,36 The skeletal density of ZIF-8 (ρs) is 0.92 g/cm3.11 The mass of water and

Figure 2. Nitrogen gas adsorption isotherm at 77.26 K for ZIF-8. P/P0 is the ratio of gas pressure (P) to saturation pressure (P0), with P0 = 740 Torr. B


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where zi and yi (i = 1,2) are the molar fraction of CH4 and C2H4 in the feed gas and in the equilibrium gas phase of the sapphire cell, respectively. The corresponding molar fraction of CH4 and C2H4 in the solid phase of material is defined as n1 x1 = n1 + n2 (10)

the wet ZIF-8 was vacuumed, and the temperature of the air bath was set to 298.15 K for 2 h. Then the sapphire cell was demounted from the equipment and the recycled wet ZIF-8 was put into a beaker and dried to a certain mass with a certain lower water content. The separation experiments for wet ZIF-8 with different water contents at a certain gas−solid ratio were therefore performed. 2.4. Data Processing. According to eqs 2−4, the adsorption capacity of pure methane and ethylene in dry ZIF-8 was calculated. The net mole number, np, of methane or ethylene stored in the pores of ZIF-8 is determined by np =

P0V0 PV − 1 2 Z0RT Z1RT

S= (2)

(3)

β=

R2 =

x 2/x1 y2 /y1

(13)

n2 ntz 2

(14)

M = nt − nE

(15)

3. RESULTS AND DISCUSSION 3.1. Blank Experiment. The validity of the experimental method has been verified by the comparison between the adsorption isotherm of pure CH4 in dry ZIF-8 at 300.15 K11 and the other literature data.37 For verifying the feasibility for separation of CH4/C2H4 gas mixture, the adsorption isotherms of pure C2H4 and CH4 in dry ZIF-8 at 274.15 K were measured. As shown in Figure 3, the adsorption amount of C2H4 is much higher than that of CH4 under the same conditions. The difference between the adsorption amounts of C2H4 and CH4 shows great promise for their separation. In addition, Mu et al.11 measured the adsorption and hydration behavior of CH4 in wet ZIF-8 with five different water contents under hydrate formation conditions, and the overall storage

(5)

where P1 represents the residual pressure of the blind cell after injecting gas into the sapphire cell from it. The compressibility factors Z0 and Z2 are also calculated using the Benedict− Webb−Rubin−Startling equation of state. The total mole number (nE) of the equilibrium gas phase in sapphire cell is calculated by

nE =

(12)

The adsorption quantity of CH4 and C2H4, M, is calculated as follows:

where ρs is the skeletal density of ZIF-8 and md is the mass of dry ZIF-8. According to the following formula, the separation performance of wet ZIF-8 for methane/ethylene gas mixtures was calculated. The total mole number of the gas mixture (nt) that is discharged into the sapphire cell is calculated: P0V0 PV − 1 0 Z0RT Z 2RT

22400nt Vc

The selectivity coefficient for C2H4, β, between solid phase and gas phase as well as the recovery ratio, R2, of C2H4 in the adsorption phase are selected to characterize the separation ability of the method coupling adsorption and hydration:

T is the system temperature; P0 is the initial pressure of the blind cell; P1 is the equilibrium pressure of the system; R represents the universal gas constant. Compressibility factors Z0 and Z1 are calculated using the Benedict−Webb−Rubin− Startling equation of state. V0 is the total volume of the blind cell plus the tubes connecting two cells; Vs is the effective volume of sapphire cell; Vc is the skeleton volume of ZIF-8, which is defined as m Vc = d ρs (4)

nt =

(11)

The initial gas−solid ratio is calculated as follows:

where V2 = V0 + Vs − Vc

n2 n1 + n2

x2 =

PEVg Z ERT

(6)

where PE represents the equilibrium pressure of the sapphire cell and ZE represents the compressibility factor corresponding to T, PE, and gas phase composition in the sapphire cell. Vg represents the volume of equilibrium gas in the sapphire cell, which is determined by Vg = Vs − Vc − Vw

(7)

where Vw is the volume of the water in the wet ZIF-8. The total adsorbed mole number of CH4 (n1) and C2H4 (n2) in the wet ZIF-8 are calculated as follows: n1 = ntz1 − nEy1 (8) n2 = ntz 2 − nEy2

Figure 3. Comparison of pure C2H4 and CH4 adsorption isotherms in dry ZIF-8 at 274.15 K in this work compared with literature values (C2H4 adsorption isotherm at 293 K38 and CH4 adsorption isotherm at 300 K39).

(9) C


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Table 1. Separation Results of the Wet ZIF-8 at Different Initial Gas-Solid Ratios When under Water Content of 29.99 wt % and 274.15 K for the Feed Gas Mixture 69.09 mol % CH4 + 30.91 mol % C2H4 run

P0, MPa

PE, MPa

S (v/v)

y2, %

x2, %

R2, %

β

M (mmol/g)

1 2 3 4 5 6

4.67 4.29 3.82 3.34 2.82 2.29

3.32 2.96 2.40 2.02 1.74 1.49

459 414 360 308 253 200

20.08 18.63 16.97 15.88 16.25 17.12

51.13 52.24 50.06 50.24 51.31 54.26

57.68 61.74 68.22 71.09 69.41 65.18

4.16 4.78 4.90 5.35 5.43 5.75

7.74 7.31 7.32 6.50 5.11 3.59

group of gas−solid ratio. In comparison, for the system with gas−solid ratio of 200, only one stage of pressure drop was observed. The time to attain equilibrium for the systems with gas−solid ratio 253 and 200 are 240 and 20 min, respectively, and the pressure drop in the system with gas−solid ratio 253 is much larger, implying that the system with gas−solid ratio of 200 only possesses the adsorption stage and no hydrate is formed at the initial pressure of only 2.29 MPa in the sapphire cell. For the systems with gas−solid ratio varying from 308 to 459, with corresponding equilibrium pressure from 2.02 to 3.32 MPa, no induction period was observed, and the process of adsorption and hydration took place simultaneously. The time to reach equilibrium in general increases as the gas−solid ratio decreases. As can be seen from Table 1, with the increase of the initial gas−solid ratio S, the fraction of C2H4 in the gas phase (y2) decreases first and then increases, while the recovery rate of C2H4 (R2) shows the inverse trend. The selectivity coefficient of C2H4 (β) continuously decreases, and the adsorption capacity (M) continuously increases. Since the values of the surface area and the water amount that could turn into hydrate were fixed, there exists a optimum gas−solid ratio for wet ZIF-8 systems with fixed water content. When S = 308 (PE = 2.02 MPa), y2 reaches the minimum value of 15.88%, and R2 reaches the maximum value of 71.09%, which is suitable separation performance for the ZIF-8 system with the fixed water content of 29.99%. When S decreases to 200, in which hydrate will not form, y2 reaches 17.12%, R2 reaches 65.18%, and M reaches 3.59 mmol/g. The corresponding parameters are worse than those of the other systems with higher gas−solid ratio such as S = 253 (y2 = 17.12%, R2 = 69.41%, M = 5.11 mmol/g), indicating the advantage of the adsorption−hydration process compared with the single-adsorption process. 3.3. Effect of Water Content. As mentioned above, when S is 308 (P0 = 3.34 MPa), better separation performance has been obtained for the ZIF-8 system with water content of 29.99 wt %. Therefore, a series of separation experiments were performed for the separation of gas mixture 69.09 mol % CH4 + 30.91 mol % C2H4 using wet ZIF-8 with different water contents under a similar initial gas−solid ratio (S is around 310) and the initial pressure of the sapphire cell (P0 was around 3.34 MPa). The experimental temperature was kept at 274.15 K and water contents of wet ZIF-8 are 42.41, 38.50, 34.16, 29.99, 25.31, 19.90, 13.79, and 6.54 wt %, respectively. In each group of separation experiments with different water contents, the recycled wet ZIF-8 was used in which hydrate formed at a higher pressure and then dissociated. The corresponding experimental conditions and results of runs 7 to 14 are listed in Table 2. The separation experimental results of dry ZIF-8 (run 15) are also listed in Table 2 for comparison. The kinetic curves of this series separation experiments are depicted in Figure 5.

capacities of ZIF-8 framework could be increased remarkably by coupling the adsorption and hydration methods. In this work, the C2H4 adsorption isotherm was measured at a pressure of more than 0.1 MPa. As far as we know, no C2H4 adsorption isotherm measured at pressure higher than 0.1 MPa was reported. Although one measured at the pressure lower than 0.1 MPa38 was also added to Figure 3, it can hardly be compared with our data. The CH4 adsorption isotherms in this work and in the literature39 show similar variation tendencies as the pressure increases, and the adsorption capacity of ZIF-8 decreases as the temperature increases. According to the phase equilibrium condition curves of CH4 hydrate and C2H4 hydrate in a pure water/ice system,40,41 the formation pressure of CH4 hydrate is above that of C2H4 hydrate under the same conditions. That is, C2H4 forms hydrate much more easily than CH4, and C2H4 prefers to form a clathrate in solid phase more than CH4. Therefore, the separation performance can be strengthened by coupling the adsorption and hydration processes. 3.2. Effect of Gas−Solid Ratio. To examine the effect of the gas−solid ratio on the separation performance of wet ZIF-8, six experimental runs (runs 1 to 6) were performed at different gas−solid ratios. The experimental conditions and results of separation of gas mixture (69.09 mol % CH4 + 30.91 mol % C2H4) are listed in Table 1. In these experimental runs, water content of the wet ZIF-8 was 29.99 wt %, and the experimental temperature was specified at 274.15 K. The recycled wet ZIF-8 was used to ensure the formation of hydrate in the systems. The kinetic curves for wet ZIF-8 at different initial gas−solid ratios are depicted in Figure 4. As observed, for the system with gas−solid ratio of 253, there exist two different stages of pressure drop: the adsorption stage and the hydrate formation stage. There exists a certain time of induction period between the adsorption stage and the hydrate formation stage for this

Figure 4. Kinetic curves of the separation experiments for wet ZIF-8 at different gas−solid ratios under water content of 29.99 wt % at 274.15 K. D


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Table 2. Separation Results for the Feed Gas Mixture 69.09 mol % CH4 + 30.91 mol % C2H4 Using Wet ZIF-8 at Different Water Contents When under Similar Initial Gas−Solid Ratio at 274.15 K run

XW, %

P0, MPa

PE, MPa

S (v/v)

y2, %

x2, %

R2 , %

β

M (mmol/g)

7 8 9 10 11 12 13 14 15

42.41 38.50 34.16 29.99 25.31 19.90 13.79 6.54 0.00

3.45 3.40 3.36 3.34 3.44 3.34 3.43 3.30 3.44

1.84 1.88 1.99 2.02 2.09 2.19 2.27 2.39 2.43

308 297 303 308 309 314 324 321 332

15.33 14.44 15.72 15.88 15.69 16.75 17.41 19.22 18.10

45.90 48.44 49.66 50.24 50.75 53.54 52.96 56.53 56.55

75.68 75.90 71.90 71.09 71.26 66.66 65.06 57.27 60.93

4.69 5.57 5.29 5.35 5.54 5.73 5.34 5.46 5.89

7.59 6.95 6.54 6.50 6.47 5.83 5.94 4.85 5.34

Figure 5. Variation of pressure with elapsed time for wet ZIF-8 systems with different water contents when under similar initial gas− solid ratio at 274.15 K.

Figure 6. Variation of C2H4 composition in equilibrium gas phase (y2) or adsorption quantity of CH4 and C2H4 (M) with water content (Xw) for wet ZIF-8 systems.

When water contents in ZIF-8 are 0 (run 15) and 6.54 wt % (run 14), the pressure drops until equilibrium is reached at 1.01 and 0.91 MPa, respectively (Figure 5). In comparison, the pressure drop for the systems with water content equal to or higher than 13.79% (runs 7−13) is much greater than those with water content less than 13.79% (runs 14 and 15). There is a distinguishable time period between the adsorption stage and the hydrate formation stage in the pressure curves when water content is 13.79%, implying the induction process of hydrate formation. From the pressure drop data and the trend of pressure variation with elapsed time, it could be concluded that hydrate has not formed in the systems with water content less than 13.79% when S is around 310 and the initial pressure of the sapphire cell (P0) is around 3.34 MPa. Besides, the time to reach equilibrium for the wet ZIF-8 system with water content of 13.79% is longer than that with water content of 19.90%. An induction period exists for the system with water content of 13.79%; however, with the increase of water content, the induction period for hydrate formation disappears. Figure 6 shows the values of y2 and M with the variation of water contents in the pores of ZIF-8. For the system with water content of 6.54%, y2 reaches the highest value and M reaches the lowest value, while for the system with water content of 38.50%, y2 reaches the lowest value. There is a sharp rise for y2 and a rapid decline of M when water content changes from 0 to 6.54%, which is due to the occupation of part of the adsorption sites by water molecules and no hydrate formation; meanwhile, the solubility of CH4 and C2H4 in water is too low to make up for the amount of occupied adsorption sites by water. When water content increased from 6.54% to 13.79%, a sharp decline of y2 was found as shown in Figure 6. Hydrate formed in the system with water content of 13.79%, and C2H4 forms hydrate

much more easily than CH4, which could be judged by the hydrate formation conditions.40,41 The values of y2 in the systems with the water content equal to or more than 13.79% are also smaller than those with water content less than 13.79%. It could therefore be concluded that there exists an obvious advantage of wet ZIF-8 with hydrate formed over dry ZIF-8 or wet ZIF-8 without hydrate formation in the separation of CH4/ C2H4 gas mixture. With the increase of water content, more hydrates form inside of the pores or on the surface of ZIF-8, among which more C2H4 hydrate forms accordingly. As a result, y2 decreases and reaches the lowest value of 14.44% when water content is 38.50%. Meanwhile, R2 reaches the maximum value of 75.90% and β reaches 5.57. Therefore, water content of 38.50% could be assumed as the best value to separate CH4/C2H4 for wet ZIF-8 and was adopted to further investigate the effect of the feed gas composition on the separation performance. The separation performance for another group of feed gas with a composition of 84.98 mol % CH4 + 15.02 mol % C2H4 was explored in run16, and the results are listed in Table 3. The experimental results for run 8 with feed gas composition of 69.09 mol % CH4 + 30.91 mol % C2H4 under similar conditions are also listed in Table 3, and the kinetic curves of runs 16 and 8 are drawn in Figure 7 for comparison. Taking the equilibrium time and the drop of pressure into account, we thought the hydrate had not formed in the separation process of wet ZIF-8 with water content of 39.85% for run 16. Since the feed gas composition in run 16 is close to the equilibrium gas composition in run 8, the molar fraction of C2H4 in the equilibrium gas mixture could be decreased from 30.91% to E


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Table 3. Separation Results for Different Compositions of the Feed Gas Mixture Using Wet ZIF-8 under Similar Initial Gas− Solid Ratio at 274.15 K run

XW, %

P0, MPa

PE, MPa

S (v/v)

8

38.50

3.40

1.88

297

16

39.85

3.41

2.42

294

y2, % z2 = 30.91% 14.44 z2 = 15.02% 8.98

8.98% through two separation stages: adsorption−hydration stage (run 8) and adsorption stage (run 16). In order to investigate the difference between hydration method and adsorption−hydration method during the separation process, the separation results with pure water42 and the wet ZIF-8 were compared and tabulated in the Table 4. Table 4. Comparison of the Separation Results for the Feed Gas (CH4 + C2H4) Mixtures Using the Hydration Method or the Adsorption−Hydration Method P0, MPa

PE, MPa

S (v/v)

y2, %

x2, %

β

42

T = 274.15 K, z2 = 19.86%, CH4 + C2H4 + pure water 3.5 2.75 113 14.77 35.12 T = 274.15 K, z2 = 30.91%, CH4 + C2H4 + wet ZIF-8 38.50 3.40 1.88 297 14.44 48.44

100

R2 , %

β

M (mmol/g)

48.44

75.90

5.57

6.95

28.45

58.75

4.03

4.41

through the study of the water physisorption properties and the water stability of metal−organic frameworks including HKUST-1, ZIF-8, MIL-100, and DUT-4. The study of Ortiz et al.44 showed that the pressure of water intrusion for ZIF-8 was 27 MPa by the study of the energetic performances of MOF materials through the water intrusion−extrusion experiments. Water molecules will cover the surface of ZIF-8 and decrease the bed voidage under the separation experimental condition. The schematic diagram for the mechanism of the adsorption−hydration method for the separation of methane/ ethylene gas mixtures using wet ZIF-8 is shown in Figure 8. It is known that ethylene gas has the priority over methane gas to be adsorbed on the adsorption sites of ZIF-8. As shown in Figure 8, ethylene or methane adsorbed may form hydrates in the pores or on the surface of ZIF-8 under suitable conditions. When the experimental conditions have not reached the point to form hydrates within wet ZIF-8, the molar faction of C2H4 would be higher than that of dry ZIF-8, and the adsorption capacity of C2H4 and CH4 would be less than those of dry ZIF8. The difference of C2H4 composition in equilibrium gas phase or adsorption capacity between wet ZIF-8 without hydrate formation inside and dry ZIF-8 is because water molecules occupying the adsorption sites prevent C2H4 molecules from being adsorbed within ZIF-8. If the hydrate forms in the pores or on the surface of ZIF-8, C2H4 molecules have the priority to be adsorbed on the adsorption sites of ZIF-8 and to be stored in the hydrate structure. The adsorption−hydration method not only utilizes the preferential adsorption of ZIF-8 for C2H4 but also uses the priority of C2H4 hydrate formation. Thus, the better separation results could be achieved by using wet ZIF-8 with proper water content. 3.5. Morphology and Crystal Structure of ZIF-8 and Recycled ZIF-8. In order to investigate the influence of separation operation with the adsorption−hydration method on the morphology and crystal structure of ZIF-8, SEM images and X-ray diffraction (XRD) analysis of fresh and recycled ZIF8 (dried after eight groups of separation experiment) are shown in Figures 9 and 10, respectively. Powder XRD patterns were collected using a SIMADU XRD 6000 diffractometer and Cu Kα radiation (0.15 nm, 40 kV and 40 mA), scanned at a rate of 2°/min in the range of 5−40°. There is no remarkable difference between SEM images of fresh (Figure 9a,b) and recycled ZIF-8 (Figure 9c,d). There is also no remarkable difference between the peak position of fresh and recycled ZIF8. The morphology of ZIF-8 particles remained after a series of processes of saturation with water, adsorption−hydration, hydrate decomposition, and drying. In addition, thermal gravimetric analysis (TGA) of the dry and the recycled wet ZIF-8 samples with water content of 39.85% were carried out on NETZSCH STA 409 PC/PG instrument from 298 to 1073 K with a heating speed of 10 K/ min in an argon flow. It can be seen from TGA curves shown in Figure 11 that there exists weight loss for the recycled wet ZIF8 before 373.2 K because of evaporation of water and the

Figure 7. Kinetic curves of the separation experiments for wet ZIF-8 at 274.15 K: (a) run 16, Xw = 39.85%, the feed gas mixture of 84.98 mol % CH4 + 15.02 mol % C2H4; (b) run 8, Xw = 38.50%, the feed gas mixture of 69.09 mol % CH4 + 30.91 mol % C2H4.

Xw, %

x2, %

3.12 5.57

For the pure water system at 274.15 K and the initial pressure of 3.5 MPa, y2 decreases from 19.86% to 14.77% and x2 reaches 35.12% using the hydration method. In comparison, for the system with wet ZIF-8 (38.50 wt %) at 274.15 K and the initial pressure of 3.40 MPa, y2 decreases from 30.91% to 14.44% and x2 reaches 48.44% using the adsorption−hydration method. β reaches 5.57, which is much higher than that using hydration method (β = 3.12). Obviously, the existence of ZIF-8 improved the separation performance for gas mixture of CH4/C2H4, which could be attributed to its selective adsorption of C2H4 and the large surface area beneficial for increasing the possibility of C2H4 hydrate formation. 3.4. Mechanism of Adsorption−Hydration Method. The adsorption−hydration method has been used to improve the methane gas storage capacity of ZIF-8.11 It was deduced that hydrate could form in the pores of ZIF-8 and increase the overall gas storage capacities remarkably.11 However, ZIF-8 was also deemed as hydrophobic by some researchers. Küsgens et al.43 found that ZIF-8 was highly inert and hydrophobic F


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Figure 8. Mechanism of adsorption−hydration method for the separation of methane/ethylene gas mixtures using wet ZIF-8.

Figure 9. SEM images of fresh (a, b) and recycled ZIF-8 (c, d).

4. CONCLUSIONS

weight of the recycled wet ZIF-8 samples decreases from 100% to approximately 69%. Besides, the weight of dry or recycled ZIF-8 only decreases by 5% when the temperature was increased to 873 K.

ZIF-8 was selected to separate CH4/C2H4 gas mixtures using the adsorption−hydration method due to its large surface area and high porosity. The separation performance of wet ZIF-8 G



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Article

AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 10 89733156. E-mail: [email protected] (C.Y. Sun). *E-mail: [email protected] (R.Q. Zhong). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support received from the National 973 Project of China (Grant No. 2012CB215005), the National Natural Science Foundation of China (Grant Nos. U1162205, 21276272, 21203249), and the Program for New Century Excellent Talents from Ministry of Education (Grant NCET12-0968) are gratefully acknowledged.

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REFERENCES

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Figure 10. Powder X-ray diffraction patterns for fresh ZIF-8 (a) and recycled ZIF-8 (b).

Figure 11. TGA curves for dry ZIF-8 (a) and recycled wet ZIF-8 with Xw = 39.85% (b).

with different water content and initial gas−solid ratios was investigated systematically. The results show that for the wet ZIF-8 system with water content of 29.99 wt %, the better separation performance was achieved when S was 308, in which the mole fraction of C2H4 decreased from 30.91% to 15.88%, the selectivity coefficient reaches 5.35, and the adsorption quantity of C2H4 reaches 3.27 mmol/g. For the influence of water content of wet ZIF-8, if water content is too low, for example, lower than 6.54 wt %, to form hydrate, water molecules will occupy the adsorption sites and prevent C2H4 molecules from being adsorbed within ZIF-8. In contrast, if water content is high enough to form hydrate in the pores and on the surface of ZIF-8, C2H4 molecules could be adsorbed on the adsorption sites of ZIF-8 and be stored in the hydrate structures as well, resulting in the high selective separation of CH4/C2H4 mixture. The appropriate water content is 38.50 wt % for wet ZIF-8 to separate CH4/C2H4 mixture using the adsorption−hydration method. The particle morphology of ZIF-8 remains after a series of processes of saturation with water, adsorption−hydration, hydrate decomposition, and drying. To form hydrate more quickly, the recycled wet ZIF-8 is recommended for the separation of CH4/C2H4 mixture using the adsorption− hydration method. H


Article

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Ethylene Gas Mixtures Using Wet ZIF-8

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