Ethylene by An Ethane

showed that MIL-142A had remarkably higher C2H6 adsorption capacities ... 26.2-23.8 kJ/mol, which is a favourable property for the regeneration of MIL...
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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Highly Adsorptive Separation of Ethane/Ethylene by An EthaneSelective MOF MIL-142A Yongwei Chen, Houxiao Wu, Daofei Lv, Renfeng Shi, Yang Chen, Qibin Xia,* and Zhong Li School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: In the petrochemical industry, ethane/ethylene (C2H6/C2H4) separation is one of the most important processes. Herein, we reported an iron-based metal−organic framework MIL-142A for efficiently trapping C2H6 from C2H6/C2H4 mixture, exhibiting preferential adsorption of C2H6 over C2H4. After various characterization techniques to confirm the successful preparation of MIL-142A, the C2H6 and C2H4 adsorptive separation performance was systematically investigated. Results showed that MIL-142A had remarkably higher C2H6 adsorption capacities than that of C2H4 at 298, 288, and 278 K, such as C2H6 and C2H4 adsorption capacities of 3.8 and 2.9 mmol/g at 298 K and 100 kPa, respectively. In addition, the C2H6 and C2H4 adsorption heats were relatively low and in the corresponding ranges 27.3−25.1 and 26.2−23.8 kJ/mol, which is a favorable property for the regeneration of MIL142A with less energy penalty. Furthermore, it was inferred that the difference between the adsorption heat for C2H6 and C2H4 with the MIL-142A framework governed the selective adsorption of C2H6 over C2H4. The ideal adsorbed solution theory (IAST) selectivity of C2H6/C2H4 was high up to 5.8 in the initial low pressure and then decreased to 1.5 at 298 K and 100 kPa. Besides, breakthrough experiments also corroborated its efficient separation of C2H6/C2H4 mixture. Additionally, MIL-142A had excellent moisture stability, and its framework remained still intact after being exposed to humid conditions (80% RH) for 15 days. These comprehensive properties demonstrated that this unique material MIL-142A, as a high-performance C2H6-selective adsorbent, can be potentially used for highly adsorptive separation of the C2H6/C2H4 mixture.

1. INTRODUCTION The separation of olefin/paraffin mixture is a critical process for the petrochemical industry because they are important and fundamental feedstocks that can be further converted into essential and versatile products.1,2 Conventionally, industrialbased separation of olefin/paraffin mixture is mainly achieved by means of cryogenic distillation.3,4 However, this process requires extremely enormous energy consumption for obtaining high-grade products because of their similarities in physicochemical properties, such as kinetic diameter and boiling point.5,6 For example, C2H6/C2H4 separation is generally carried out at −25 °C and 23 bar with a distillation column consisting of over 100 trays.7,8 Thus, developing a cost-effective and low-energy separation technique is highly required to meet the industrial demand for olefin/paraffin separation. Among various technologies, adsorption separation using porous materials is found to be a promising energy-saving alternative, and the development of high-performance solid adsorbents is the key step.9 Although zeolites and carbon-based materials can be applied for such separation applications,10−13 these adsorbent materials generally cannot satisfy the criteria for olefin/paraffin separation, such as high adsorption capacity and selectivity and easy regeneration. Gratifyingly, newly reported porous materials have inspired researchers to © XXXX American Chemical Society

investigate the separation behavior of olefin and paraffin molecules with excellent separation properties.1,14 Particularly, the discovery of crystalline porous materials, metal−organic frameworks (MOFs),15 as an emerging class of porous materials, have attracted considerable attention owing to their unparalleled features, such as record-breaking porosity, welldefined pore structure, and diversified and tailorable functionality.16−21 As a result, increasing efforts have been devoted to study the adsorptive separation of olefin/paraffin mixture using MOFs.22−25 Generally speaking, MOFs using for olefin/paraffin separation is divided into two categories on the basis of their selectivities: alkene-selective MOFs and alkane-selective MOFs.26 For alkene-selective MOFs, the formation of πcomplexation within MOFs usually plays an important role in olefin/paraffin separation process.27 Currently, several MOFs intrinsically possessing unsaturated open metal sites, such as HKUST-128 and MOF-74,29 exhibit high olefin adsorption capacities as well as excellent olefin/paraffin separation Received: Revised: Accepted: Published: A

December 20, 2017 March 1, 2018 March 5, 2018 March 5, 2018 DOI: 10.1021/acs.iecr.7b05260 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

(terephthalic acid and 1,3,5-tris(4-carboxyphenyl)benzene)40 and investigated its gas separation performance of C2H6/C2H4 mixture. Interestingly, MIL-142A exhibited preferential adsorption of C2H6 over C2H4, high C2H6 adsorption capacity as well as excellent separation performance. Besides, MIL-142A had excellent moisture stability, which its framework could be well retained after humid exposure. Considering practical industrial separation demand, MIL-142A holds great promise as a highperformance C2H6-selective adsorbent for real-world application of C2H6/C2H4 separation.

efficiency. Another effective strategy to create olefin-selective adsorption sites is that the introduction of transition metal ions inside MOFs pores, such as Ag(I) and Cu(I), has been recently reported for olefin/paraffin separation applications,30−32 benefiting from the fact that transition metal ions can generally form π-complexation with the olefin molecules.4 Although the loading of Ag (I) and Cu(I) ions into the frameworks can improve their olefin/paraffin separation performance in comparison with their original materials, this also generally results in the decreasing porosities and specific surface areas of the MOFs and thus impairs their adsorption capacities. Because π-complexation is a weak chemical bond, the following desorption of C2H4 would consume more energy than the physisorption process.22 For instance, the adsorption heat for C2H4 in (Cr)-MIL-101-SO3Ag is high, up to 120 kJ/mol, at initial loading due to the enhanced affinity toward olefin molecules.31 However, studies of alkane-selective MOFs are still very limited,22,33,34 because it is challenging to judiciously construct MOFs that paraffin molecules are preferentially adsorbed within their pores so that paraffins have stronger interactions with the frameworks than with olefins.26 For alkane-selective MOFs, Gücüyener et al.35 first found that ZIF-7 exhibited preferential adsorption of C2H6 over C2H4 by gate-opening effect, in which C2H6 could trigger the structure opened at lower pressure than C2H4. However, the uptakes of C2H6 and C2H4 were below 2 mmol/g at 298 K and 100 kPa. Then, Zhang and co-workers reported that separating C2H4 from C2H6 could be achieved by MAF-49 showing preferential adsorption of C2H6, as a C2H6-selective adsorbent.36 Similarly, C2H6 adsorption capacity remained comparatively low. Recently, our group reported two C2H6-selective MOFs, Ni(bdc)(ted)0.537 and PCN-250,22 both having high C2H6 adsorption capacities exceeding 5.0 mmol/g at 298 K and 100 kPa. From an industrial viewpoint, alkane-selective MOFs would be more favorable and economical for olefin/paraffin separations. The reasons are as follows: (i) As is well-known, π-complexation is a kind of weak chemical bond, which would need more energy penalty to desorb olefins than the physisorption counterparts of preferential adsorption paraffins.22 (ii) If olefins are preferentially adsorbed, an additional desorption step is necessarily required to obtain high-purity olefins, making its implementation difficult and complicated; in contrast, polymer grade olefins (e.g., 99.95% for C2H4) can be directly obtained only after a single adsorption step, simplifying the whole separation scheme.38 (iii) In the case of typical cracked gas mixture (C2H6/C2H4 1:15),36 using C2H6-selective MOFs to trap C2H6 at a low concentration, rather than predominantly containing C2H4, is undoubtedly a more preferred choice for the sake of saving cost and requiring fewer adsorbents to be packed into the adsorbent bed. Consequently, although it remains a formidable challenge, choosing alkane-selective MOFs for olefin/paraffin separation is an economical and energy-efficient strategy for directly obtaining high-purity olefins to meet industrial demand. Recently, iron-based MOFs have been extensively studied owing to their robust frameworks, such as high thermal and chemical stability, nontoxicity, and natural abundance.39 However, iron-based MOFs have rarely been reported as C2H6-selective adsorbents for olefin/paraffin separation.22 In this work, we reported a microporous interpenetrated structure, MIL-142A comprising Fe cations and double ligands

2. EXPERIMENTAL SECTION 2.1. Materials. Iron(III) chloride hexahydrate (FeCl3· 6H2O, 99%) was purchased from Aladdin. Terephthalic acid (H2bdc, 99%) was obtained from Tokyo Chemical Industry Co. Ltd. (TCI). 1,3,5-Tris(4-carboxyphenyl)benzene (H3btb, 98%) was from Beijing HWRK Chem Co. Ltd. N,N-Dimethylformamide (DMF, 99.5%) and methanol (MeOH, 99.5%) were provided by Guangdong Guanghua Sci-Tech Co. Ltd. 2.2. Preparation of MIL-142A. MIL-142A was prepared according to the previous procedures reported by Chevreau and co-workers.40 In a typical synthesis, FeCl3·6H2O (1.5 mmol, 405 mg), H2bdc (0.5 mmol, 83 mg), and (H3btb) (0.66 mmol, 292 mg) in 3 mL of DMF were sonicated for 10 min in a Pyrex vial (20 mL). Then the Pyrex vial was heated in an oven, where the temperature was gradually increased to 150 °C within 1 h and then kept at the same temperature for 20 h. After natural cooling, the mixture was filtered to obtain the as-synthesized MIL-142A and then successively immersed in DMF for 24 h and in MeOH for 24 h, during which DMF and MeOH were decanted and refreshed every 12 h, respectively. The resultant activated MIL-142A was collected by centrifugation and dried at 150 °C under vacuum for 8 h. 2.3. Characterization. Powder X-ray diffraction (PXRD) of MIL-142A was collected on a Bruker D8 Advance X-ray diffractometer at room temperature, using Cu Kα radiation (40 kV and 40 mA) in the range 4−40°. The morphology of MIL142A was characterized by scanning electron microscopy (SEM, Hitachi S-4800). Prior to the observation, the MIL142A sample was coated with a thin layer of gold in vacuum to enhance its conductivity. Thermogravimetric analysis (TGA) was performed on a TGA Q500 instrument by heating the sample at 5 °C/min from room temperature to 600 °C in a nitrogen atmosphere. N2 adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2460. The activated MIL-142A sample was outgassed at 423 K for 6 h before the measurement. The pore textural properties were calculated by the Brunauer−Emmett−Teller (BET) equation and the pore size distribution curve was obtained using density functional theory (DFT) method. 2.4. Gas Adsorption Measurements. C2H6 and C2H4 adsorption isotherms were collected on a 3Flex Surface Characterization Analyzer (Micromeritics, USA) at 278, 288, and 298 K. The adsorption temperature was well controlled by a water bath in which the sample tube was immersed. Prior to each measurement, approximately 100 mg of activated MIL142A sample was degassed at 423 K for 6 h. Ultrahigh purity grade C2H6 (99.99%) and C2H4 (99.99%) were used for the gas adsorption measurements. 2.5. Breakthrough Tests. The breakthrough experiments of mixed gas mixture C2H6/C2H4 (1:1, v/v) for MIL-142A were conducted in a homemade experimental apparatus at 298 K and under atmospheric pressure, as shown in Figure S1. B

DOI: 10.1021/acs.iecr.7b05260 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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ascribed to highly crystalline structure during the formation process, consistent with the PXRD results. The permanent porosity of MIL-142A is determined by N2 adsorption measurement at 77 K. As illustrated in Figure 3, the

Before the separation experiment, the activated MIL-142A (200 mg) sample was heated at 423 K under vacuum for 60 min and then the adsorbent was packed into a stainless steel column (4.0 mm inner diameter and 280.0 mm length), where the two ends of the column were filled with glass wool. The column was placed in an oven and held at 298 K. Before the breakthrough experiments, He gas was fed into the column filled with adsorbent at a flow rate of 10 mL/min for purging 60 min. Then, the mixed gas C2H6/C2H4 mixture was introduced at the inlet and controlled by the mass flow controller as 2 mL/min. The composition of outlet gas from the column was monitored by a gas chromatography (GC-9560, Shanghai Wuhao, China) with a flame ionization detector (FID). Subsequently, dynamic separation performance of MIL-142A was confirmed by the experimental breakthrough curves of C2H6/C2H4 mixture.

3. RESULTS AND DISCUSSION 3.1. Characterization of MIL-142A. The purity and crystallinity of the synthesized MIL-142A are characterized by PXRD. As shown in Figure 1, both the as-synthesized and

Figure 3. N2 adsorption/desorption isotherms of MIL-142A at 77 K. The inset illustrates the pore size distribution.

reversible isotherm is classified as type I, which is a typical indication of microporous nature. The Brunauer−Emmett− Teller (BET) and Langmuir surface areas of MIL-142A were calculated to be 1555 and 1773 m2/g, respectively. The measured total pore volume was 0.686 cm3/g, in which a particularly large percentage was micropore region and the micropore volume was high up to 0.531 cm3/g. The BET surface area and total pore volume were close to previously reported values (1580 m2/g and 0.70 cm3/g),40 indicating that almost completed activation was achieved. Furthermore, the pore size distribution was distributed in the range of 5−14 Å and mainly peaked at 10 Å. The thermal stability of MIL-142A is estimated by TGA, depicted in Figure S2. According to the TGA result, no structural decomposition of MIL-142A was observed until 300 °C. The TGA curve showed a weight loss of 12.6% from 30 to 100 °C associated with the removal of surface adsorbed guest molecules and then followed a plateau within the temperature range 100−300 °C, indicating that no residual solvent molecules remained in the pores and the framework remained unaffected before reaching to 300 °C. Further increasing temperature underwent an abrupt weight loss because of its structural decomposition. 3.2. C2H6 and C2H4 Adsorption of MIL-142A. C2H6 and C2H4 isotherms were measured on MIL-142A at 298, 288, and 278 K, as shown in Figure 4a−c. At any given temperature, similar isotherms of MIL-142A for C2H6 and C2H4 were observed under idential conditions. It was observed that the adsorption capacities of both C2H6 and C2H4 decreased with an increase in temperature, reflecting that the process was thermodynamically controlled nature of physisorption. Evidently, MIL-142A had a higher adsorption capacity of C2H6 than that of C2H4, which exhibited preferential adsorption of C2H6 over C2H4. For example, the adsorption capacities of C2H6 and C2H4 were 3.8 and 2.9 mmol/g at 298 K and 100 kPa, respectively. By careful comparison, we also found that the adsorption capacity of C2H6 for MIL-142A was superior to some reported MOFs with respect to preferential adsorption of C2H6 listed in Table S1, such as IRMOF-8 (3.6 mmol/g),34 ZIF-4 (2.2 mmol/g),41 ZIF-7 (2.0 mmol/g),33 and MAF-49 (1.7 mmol/g),36 but lower than those of several MOF

Figure 1. PXRD patterns of the as-synthesized and activated MIL142A samples.

activated MIL-142A exhibited well-defined crystal structure, in agreement with the reported pattern of MIL-142A, 40 confirming that this material was successfully prepared with high purity. Simultaneously, the patterns of as-synthesized and activated samples were almost identical, suggesting that the structural integrity was well retained after the solvent exchange treatment. To further investigate the morphology and size of MIL-142A, SEM images of activated samples are presented in Figure 2. It was observed that MIL-142A particles were found to be a hexagonal plate-like morphology with an average size around 3 μm and a thickness of about 0.3 μm. Besides, MIL-142A crystals exhibited uniform and regular geometry, which was

Figure 2. SEM images of MIL-142A. C

DOI: 10.1021/acs.iecr.7b05260 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. C2H6 and C2H4 adsorption isotherms for MIL-142A at (a) 298 K, (b) 288 K, and (c) 278 K. (d) Isosteric heats of C2H6 and C2H4 adsorption for MIL-142A.

materials, such as PCN-250 (5.2 mmol/g)22 and Ni(bdc)(ted)0.5 (5.0 mmol/g).37 It is well-known that the adsorption heat (Qst) is an intrinsic nature that reflects the adsorption interactions between gas molecules and the framework. In general, C2H6 has two hydrogen atoms more than C2H4, giving a remarkably stronger affinity of the pore surface toward C2H6. Thus, we speculated that it was the difference in adsorption heats that played an important role in determining selective adsorption C2H6 over C2H4. To confirm the stronger interaction between C2H6 and the framework, the Qst was estimated using the Clausius− Clapeyron equation based on C2H6 and C2H4 adsorption isotherms collected at 298, 288, and 278 K. As illustrated in Figure 4d, the values of both C2H6 and C2H4 were decreased with an increase of the adsorbed amount, whereas the value of C2H6 was always higher than that of C2H4, corroborating the stronger affinity of MIL-142A toward C2H6. Specifically, the initial value of C2H6 was 27.3 kJ/mol and then gradually decreased to 25.1 kJ/mol. Simultaneously, the initial value of C2H4 (25.1 kJ/mol) was lower than that of C2H6 and sharply decreased at the lower adsorbed amount (below 1 mmol/g) and then gradually decreased to 23.8 kJ/mol when the adsorbed amount exceeded 1 mmol/g. Additionally, the comparatively low adsorption heats of C2H6 and C2H4 on MIL-142A also confirmed the physisorption process. For practical C2H6/C2H4 separation application, the moderate heat of adsorption is an economic development for releasing adsorbed molecules during the desorption step. The difference of adsorption heats between C2H6 and C2H4 can be explained by the following reasons. (i) Considering the factor of kinetic diameter (C2H6 4.443 Å vs C2H4 4.163 Å),23 the larger molecule size generally induces stronger surface force toward surrounding walls for a given porous material.42 (ii) The adsorption affinity between adsorbed molecules and the MIL-

142A framwork should be mainly contributed by van der Waals (vdW) attractive interaction due to the lack of specific adsorptive sites. Meanwhile, the vdW interaction is related with the polarizability of the adsorbed molecule (C2H6 4.47 × 10−25 cm3 vs C2H4 4.252 × 10−25 cm3).38 Therefore, the vdW interaction of C2H6 within the MIL-142A is stronger than that of C2H4. (iii) Indeed, the Lennard-Jones parameters for C2H6 are somewhat higher than for C2H4, which indicates the higher C2H6 dispersion interactions because of the two additional hydrogen atoms.3843,44 As a consequence, the combinational factors trigger the higher adsorption heat of C2H6 than that of C2H4, thus inducing the preferential adsorption of C2H6 over C2H4. 3.3. C2H6/C2H4 Separation Behavior of MIL-142A. To investigate the C2H6/C2H4 separation performance for MIL142A, IAST was used to calculate the selectivity of C2H6/C2H4 binary mixture, which is an important tool for assisting in evaluation of gas mixture separations.45,46 The measured data for experimental single-component isotherms of C2H6 and C2H4 at 298 K were well fitted using the dual site Langmuir− Freundlich (DSLF) model: q = qm1

b1p n b2 p m + q m2 1 + b1p n 1 + b2 p m

(1)

where q (mmol/g) is the adsorbed amount, qm1 and qm2 (mmol/g) are the saturation adsorption capacities, p (kPa) is the pressure of the gas at equilibrium, b1 and b2 (1/kPa) are the affinity coefficients of the sites, and n and m are the deviations from an ideal homogeneous surface. Both isotherms can be precisely described by this model over the whole measured pressure range in this work, giving the R2 values larger than 0.9999. The detailed fitted parameters are presented in Table S2. D

DOI: 10.1021/acs.iecr.7b05260 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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simultaneously occupied by C2H6.26 Remarkably, the difference in the C2H6 and C2H4 breakthrough times clearly demonstrated that MIL-142A could efficiently trap C2H6 from the C2H6/ C2H4 mixture under realistic conditions. Meanwhile, the C2H6 and C2H4 working adsorption capacities of MIL-142A from the dynamic breakthrough were calculated to be 3.3 and 1.8 mmol/ g, respectively, which were slightly lower than the static adsorption capacities obtained from single-component isotherms (3.8 mmol/g for C2H6 and 2.9 mmol/g for C2H6). Such reduction in C2H6 and C2H4 adsorption capacities can be attributed to the reasons that factors affecting gas coadsorption in the breakthrough experiments are more complex and underestimated by single-component gas measurement.47 Similar cases have also been reported in the literature.26,48 Then the separation selectivity based on the working adsorption capacities was calculated to be 1.8 for the equimolar C2H6/C2H4 mixture. Therefore, considering the theoretical IAST selectivity and breakthrough experiments, the combinational results verified that MIL-142A had excellent separation performance for discriminatively trapping C2H6 from C2H6/ C2H4 mixture. 3.4. Moisture Stability of MIL-142A. Except exceptional separation performance, the moisture stability of MIL-142A is also a crucial factor determining the potential of practical applications, because water vapor is omnipresent under various practical conditions. Considering the fact that most MOFs are moisture-sensitive and their frameworks will be collapsed upon being exposed to humid conditions,49 the moisture stability of MIL-142A is assessed by placing MIL-142A sample in wet air (relative humidity, RH 80% for 15 days at room temperature). As shown in Figure 7, it was observed that the structure of

Figure 5 shows the C2H6/C2H4 selectivity of MIL-142A predicted by IAST at 298 K. As depicted in Figure 5, the

Figure 5. C2H6/C2H4 (1:1, v/v) selectivity of MIL-142A predicted by IAST at 298 K.

selectivity of C2H6/C2H4 exhibited an abrupt decreasing trend from 5.8 to 1.5 in the range 0−10 kPa, particularly for extremely low pressure. When the pressure was above 10 kPa, no obvious fluctuation of selectivity was observed despite the increase of pressure. At 100 kPa, the selectivity of MIL-142A was slightly lower than those of most reported MOFs in Table S2, such as PCN-250 (1.9) and IRMOF-8 (1.8), but was comparable to that of ZIF-4 (1.5). The selective separation of C2H6 over C2H4 could be ascribed to the higher adsorption interaction of C2H6 molecules with the MIL-142A framework than that of C2H4, thus giving rise to the adsorptive separation of C2H6/C2H4. A similar case of PCN-250 has also been reported by our group.22 To further confirm the selective adsorption of C2H6 over C2H4 from real mixture, breakthrough experiments were preformed to test the separation performance of MIL-142A. We used the binary mixture containing C2H6/C2H4 (1:1, v/v) for the breakthrough experiments, where the gas mixture was introduced into this column at a flow rate of 2 mL/min. As displayed in Figure 6, C2H4 initially eluted through the column at 6.6 min and then reached saturation, whereas C2H6 was first detected until 8.4 min; the longer breakthrough time of C2H6 indicated stronger interactions between C2H6 molecules and the MIL-142A framework. In particular, the effluent concentration of C2H4 reached 1.2, which is called rollup that is ascribed to the desorption of initially adsorbed C2H4 and

Figure 7. Comparison of PXRD patterns of freshly made MIL-142A and MIL-142A after moisture stability test at 80% RH for 15 days.

humid treated MIL-142A remained intact by the comparison of PXRD patterns. It indicated that no framework collapse occurred during humid treatment. Besides, the N2 isotherm of MIL-142A after humid treatment is also measured to further thoroughly confirm its moisture stability. As depicted in Figure S3, only slight reduction in N2 uptake was observed compared with the case of freshly made MIL-142A, suggesting that most porosity was well preserved. Such excellent moisture stability can be associated with the factor that trivalent metal Fe(III) can form extremely strong coordination bonds in an MOF framework.50

Figure 6. Breakthrough curves for equimolar C2H6/C2H4 mixture in MIL-142A at 298 K. E

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(5) Adil, K.; Belmabkhout, Y.; Pillai, R. S.; Cadiau, A.; Bhatt, P. M.; Assen, A. H.; Maurin, G.; Eddaoudi, M. Gas/vapour separation using ultra-microporous metal-organic frameworks: insights into the structure/separation relationship. Chem. Soc. Rev. 2017, 46, 3402− 3430. (6) Chen, Y.; Qiao, Z.; Lv, D.; Duan, C.; Sun, X.; Wu, H.; Shi, R.; Xia, Q.; Li, Z. Efficient adsorptive separation of C3H6 over C3H8 on flexible and thermoresponsive CPL-1. Chem. Eng. J. 2017, 328, 360− 367. (7) Bao, Z.; Alnemrat, S.; Yu, L.; Vasiliev, I.; Ren, Q.; Lu, X.; Deng, S. Adsorption of ethane, ethylene, propane, and propylene on a magnesium-based metal-organic framework. Langmuir 2011, 27, 13554−13562. (8) Li, B.; Zhang, Y.; Krishna, R.; Yao, K.; Han, Y.; Wu, Z.; Ma, D.; Shi, Z.; Pham, T.; Space, B.; Liu, J.; Thallapally, P. K.; Liu, J.; Chrzanowski, M.; Ma, S. Introduction of π-complexation into porous aromatic framework for highly selective adsorption of ethylene over ethane. J. Am. Chem. Soc. 2014, 136, 8654−60. (9) Chen, Y.; Qiao, Z.; Lv, D.; Wu, H.; Shi, R.; Xia, Q.; Wang, H.; Zhou, J.; Li, Z. Selective adsorption of light alkanes on a highly robust indium based metal-organic framework. Ind. Eng. Chem. Res. 2017, 56, 4488−4495. (10) Birkmann, F.; Pasel, C.; Luckas, M.; Bathen, D. Trace adsorption of ethane, propane, and n-butane on microporous activated carbon and zeolite 13X at low temperatures. J. Chem. Eng. Data 2017, 62, 1973−1982. (11) Ma, C.; Wang, X.; Wang, X.; Yuan, B.; Wu, Y.; Li, Z. Novel glucose-based adsorbents (Glc-As) with preferential adsorption of ethane over ethylene and high capacity. Chem. Eng. Sci. 2017, 172, 612−621. (12) Luo, L.; Guo, Y.; Zhu, T.; Zheng, Y. Adsorption species distribution and multicomponent adsorption mechanism of SO2, NO, and CO2 on commercial adsorbents. Energy Fuels 2017, 31, 11026− 11033. (13) Liang, W.; Xiao, H.; Lv, D.; Xiao, J.; Li, Z. Novel asphalt-based carbon adsorbents with super-high adsorption capacity and excellent selectivity for separation for light hydrocarbons. Sep. Purif. Technol. 2018, 190, 60−67. (14) Herm, Z. R.; Bloch, E. D.; Long, J. R. Hydrocarbon separations in metal-organic frameworks. Chem. Mater. 2014, 26, 323−338. (15) Yaghi, O. M.; Li, G.; Li, H. Nature 1995, 378, 703−706. (16) Julien, P. A.; Mottillo, C.; Frišcǐ ć, T. Metal-organic frameworks meet scalable and sustainable synthesis. Green Chem. 2017, 19, 2729− 2747. (17) Moghadam, P. Z.; Li, A.; Wiggin, S. B.; Tao, A.; Maloney, A. G. P.; Wood, P. A.; Ward, S. C.; Fairen-Jimenez, D. Development of a Cambridge Structural Database subset: a collection of metal-organic frameworks for past, present, and future. Chem. Mater. 2017, 29, 2618−2625. (18) Cohen, S. M. The postsynthetic renaissance in porous solids. J. Am. Chem. Soc. 2017, 139, 2855−2863. (19) Bosch, M.; Yuan, S.; Rutledge, W.; Zhou, H. C. Stepwise synthesis of metal-organic frameworks. Acc. Chem. Res. 2017, 50, 857− 865. (20) Zhang, C.; Sun, L.; Zhang, C.; Wan, S.; Liang, Z.; Li, J. Novel photo-and/or thermochromic MOFs derived from bipyridinium carboxylate ligands. Inorg. Chem. Front. 2016, 3, 814−820. (21) Zhang, C.; Sun, L.; Yan, Y.; Liu, Y.; Liang, Z.; Liu, Y.; Li, J. Metal-organic frameworks based on bipyridinium carboxylate: photochromism and selective vapochromism. J. Mater. Chem. C 2017, 5, 2084−2089. (22) Chen, Y.; Qiao, Z.; Wu, H.; Lv, D.; Shi, R.; Xia, Q.; Zhou, J.; Li, Z. An ethane-trapping MOF PCN-250 for highly selective adsorption of ethane over ethylene. Chem. Eng. Sci. 2018, 175, 110−117. (23) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Selective gas adsorption and separation in metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1477−504.

4. CONCLUSIONS In this study, a comprehensive investigation of an iron-based MOF, MIL-142A, was reported for selective adsorption of C2H6 over C2H4, which is an important method to directly produce high-purity C2H4 from C2H6/C2H4 mixture by discriminatively trapping C2H6 molecules. Particularly, the C2H6 adsorption capacity of MIL-142A reached 3.8 mmol/g at 298 K and 100 kPa. Another favorable property was that the adsorption heats of C2H6 and C2H4 on MIL-142A were relatively low, which would largely save energy for regeneration. By analyzing the factors affecting preferential adsorption of C2H6 over C2H4, we concluded that the difference in adsorption heats of C2H6 and C2H4 with the framework determined the separation of C2H6/C2H4. Meantime, both the theoretical IAST selectivity and breakthrough experiments confirmed its efficient separation of C2H6 over C2H4. Moreover, MIL-142A showed excellent moisture stability. On the basis of the foregoing discussion, we concluded that MIL142A with comprehensive and superior properties can be a promising material for the field of efficient separation C2H6/ C2H4 mixture in industrial practice at ambient conditions.

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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.iecr.7b05260. Apparatus used for breakthrough experiments; TGA plot for MIL-142A; comparison of adsorption uptake and selectivities of C2H6/C2H4; fitting parameters of the dual site Langmuir−Freundlich model; calculation of C2H6/ C2H4 selectivity; calculation of working adsorption capacity and definition of adsorption selectivity; N2 adsorption/desorption isotherms at 77 K after moisture stability test (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Q. Xia. E-mail addresses: [email protected]. ORCID

Qibin Xia: 0000-0002-8563-6715 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21576092, 21436005 and U1662136).

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DOI: 10.1021/acs.iecr.7b05260 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX