Paraffin Selectivity in Azo Compound and Its

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 27521−27530

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Observation of Olefin/Paraffin Selectivity in Azo Compound and Its Application into a Metal−Organic Framework Seo-Yul Kim,† Tae-Ung Yoon,† Jo Hong Kang,† Ah-Reum Kim,† Tea-Hoon Kim,† Seung-Ik Kim,† Wanje Park,† Ki Chul Kim,*,‡ and Youn-Sang Bae*,† †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea Department of Chemical Engineering, Konkuk University, Seoul 05029, Republic of Korea

‡

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S Supporting Information *

ABSTRACT: Olefin/paraffin separation is an important and challenging issue because the two molecules have similar physicochemical properties. Although a couple of olefin adsorbents have been developed by introducing inorganic nanoparticles into metal−organic frameworks (MOFs), there has been no study on the development of an olefin adsorbent by introducing a certain organic functional group into a MOF. In this study, we posited that azo compounds could offer olefin/paraffin selectivity. We have revealed using firstprinciples calculations that the simplest aromatic azo compound (azobenzene, Azob) has an unusual propylene/ propane selectivity due to special electrostatic interactions between Azob and propylene molecules. On the basis of this interesting discovery, we have synthesized a novel propylene adsorbent, MIL-101(Cr)_DAA, by grafting 4,4′-diaminoazobenzene (DAA) into open metal sites in a mesoporous MIL101(Cr). Remarkably, MIL-101(Cr)_DAA exhibited enhanced propylene/propane selectivity as well as considerably higher propylene heat of adsorption compared to pristine MIL-101(Cr) while maintaining the high working capacity of MIL-101(Cr). This clearly indicates that azo compounds when introduced into MOFs can provide propylene selectivity. Moreover, MIL101(Cr)_DAA showed good C3H6/C3H8 separation and easy regeneration performances from packed-bed breakthrough experiments and retained its propylene adsorption capacity even after exposure to air for 12 h. As far as we know, this is the first study that improves the olefin selectivity of MOF by postsynthetically introducing an organic functional group. KEYWORDS: adsorption, propylene, propane, gas separation, metal−organic frameworks, olefin−paraffin separation, postsynthetic modification, surface modification

1. INTRODUCTION

Mn, and Fe) and HKUST-1, show noticeable olefin/paraffin selectivities, they have low working capacities in the pressure range of typical pressure swing adsorption (PSA) processes.4,10−12 Their dreadful working capacities are due to small pore sizes and excessively strong interactions between olefin molecules and OMSs. Therefore, strategies for developing olefin selective MOFs with moderate adsorption strengths and proper pore sizes need to be established. An attractive feature of MOFs is the postsynthetic modification (PSM), which additionally manipulates the structure of already synthesized MOFs.13 Among various PSM methods, the loading of Cu(I) ions within the cavities of MOFs has been recently considered for olefin/paraffin separation due to their strong π-complexations with the π-orbital of olefin molecules.14,15 Except for this method, however, no other PSM

One of the most substantive subjects in the petrochemical industry is olefin/paraffin separation, since their similar physical properties require enormously energy-intensive processes when performed by currently used cryogenic distillations.1−3 Energysaving adsorptive separation is regarded as the most promising process, but its success greatly depends upon the efficiency of an adsorbent.4,5 For now, few of the materials developed to date fill the needed properties, including sufficient olefin adsorption capacity, high olefin/paraffin selectivity, and cyclic adsorption and desorption. In recent years, metal−organic frameworks (MOFs) have been extensively studied as promising materials for various adsorptive separations.6−9 This is attributed to their high porosity (∼8000 m2/g) and unique characteristics that geometry and surface properties of pores are tailorable. Nevertheless, studies on the MOFs for olefin/paraffin separation are still in the early stage. Even though some of the MOFs with open metal sites (OMSs), such as M/DOBDC (M = Co, Mg, © 2018 American Chemical Society

Received: June 11, 2018 Accepted: July 24, 2018 Published: July 24, 2018 27521

DOI: 10.1021/acsami.8b09739 ACS Appl. Mater. Interfaces 2018, 10, 27521−27530

Research Article

ACS Applied Materials & Interfaces

immersed in hot ethanol (333 K) to remove unreacted H2BDC in MOF pores. Finally, it was dried at 353 K for 12 h. The MIL-101(Cr)_DAA samples were prepared by a reflux reaction. After activating MIL-101(Cr) (0.5 g) under vacuum at 423 K for 18 h, it was mixed with 0.75 mmol of DAA and 30 mL of anhydrous toluene in a reflux system. To remove any residual DAA in MOFs pores, the obtained powder was washed with ethanol several times until no color change in the solvent was detected. Finally, the powder product was dried at 353 K for 12 h. 2.4. Characterizations. X-ray powder diffraction analyses were performed using Ultima IV (Rigaku Co., Japan) in 3° < 2θ < 50° at a speed of 0.01°/s. The images of scanning transmission electron microscopy/energy dispersive spectrometry (STEM-EDS) were obtained by a JEM-ARM200F (JEOL Ltd., Japan). MIL-101(Cr) and MIL-101(Cr)_DAA powders were digested for nuclear magnetic resonance (NMR) spectroscopy measurements (Supporting Information). 1H NMR spectra of the solutions were obtained from a Bruker 400 MHz FT-NMR spectroscopy (Bruker, USA). The N2 adsorption/desorption isotherms at 77 K were recorded on a Micromeritics 3Flex surface characterization analyzer (Micromeritics Instruments, USA). The BET surface areas were calculated within the consistency criteria.31−33 The total pore volumes were measured using adsorption points at P/P0 > 0.99. Prior to each measurement, the powders were degassed under vacuum at 423 K for 12 h. 2.5. Adsorption Measurements. The ethane, ethylene, propane, and propylene adsorption isotherms at 303, 323, and 343 K were also recorded with the 3Flex analyzer (Micromeritics Instruments, USA). Prior to each measurement, the powders were degassed under vacuum at 423 K for 12 h. The propylene adsorption and desorption in MIL-101(Cr)_DAA for 20 cycles were recorded using a TGA 8000 (PerkinElmer, USA). Prior to the measurement, the powder was heated at 423 K for 12 h under nitrogen atmosphere. In each adsorption step, a 54 mL/min of propylene/nitrogen mixture (25/75, v/v) was introduced into the sample chamber for 15 min. In each desorption step, a 54 mL/min of nitrogen was flowed for 15 min. This adsorption and desorption cycle was performed at 303 K. 2.6. Breakthrough Experiments. The breakthrough measurements were performed in a custom-built equipment with pelleted samples (see the details in the Supporting Information). After degassing the pellets at 423 K for 12 h under vacuum, they were placed within a stainless-steel bed (15 cm × 0.44 cm). Then, the bed was degassed again using a 100 mL/min helium flow at 423 K for 1 h. At each run, a 100 mL/min helium flow was introduced into the bed for 20 min, followed by a 100 mL/min helium/propylene/propane mixture flow (helium/propylene/propane = 92/4/4).

strategy has been suggested for developing a MOF for olefin/ paraffin separation. Introducing azo compounds (−NN−) into MOFs has been recently gathered great attention because of their interesting photoswitchable property.16−24 Being exposed to UV beam, a trans-isomer of azo compounds switches into a cisisomer. When these azo compounds are included in the linkers of MOFs, gas adsorption behaviors can be controlled in situ through the changes in the pore size by the photoswitchable behavior of azo compounds. Nevertheless, the effect of the azogroup itself on gas adsorption has not gained much attention, although it could have special interactions with specific gas molecules due to electron configurations in the π-orbital. In this study, we posited that azo compounds could offer olefin/paraffin selectivity. We selected azobenzene (Azob), which is the simplest azo compound. Interestingly, from firstprinciple theory calculations, we found that a propylene molecule has a stronger interaction with Azob than does a propane molecule. On the basis of this result, we postsynthetically prepared an Azob-incorporated MIL-101(Cr) by grafting 4,4′-diaminoazobenzene (DAA) to coordinatively unsaturated Cr3+ sites in MIL-101(Cr) (MIL-101(Cr)_DAA). We selected MIL-101(Cr) as a pristine material since it has large surface area, big pore sizes (29 and 34 Å), high density of OMS, and good hydrothermal stability.25,26 We characterized the obtained MIL101(Cr)_DAA material by various methods, such as proton nuclear magnetic resonance (1H NMR), elemental analysis (EA), spherical aberration correction scanning transmission electron microscopy (STEM-EDS), and powder X-ray diffraction (PXRD). We also investigated the potential of MIL101(Cr)_DAA for ethylene/ethane and propylene/propane separations by measuring isosteric heats of adsorption and adsorption isotherms and by predicting selectivities from ideal adsorbed solution theory (IAST). Finally, we further evaluated the material as a potential olefin adsorbent by measuring breakthrough curves under dynamic mixture flow conditions, propylene isotherms after exposing it to the air, and cyclic adsorption−desorption profiles.

2. EXPERIMENTAL SECTION 2.1. First-Principles Calculations. All calculations on structural models of interest, namely, Azob, propane, propylene, (Azob + propane), and (Azob + propylene), were performed using the Gaussian09 software package.27 Their electronic structures were sophisticatedly optimized by employing the M06 method with the 631+G(d,p) followed by the more accurate MP2 method with the 6311++G(d,p). The binding energies of propylene and propane with Azob defined by Eb = EAzob + Emolecule − E(azob+molecule)11,28 were calculated at the MP2 method with the 6-311++G(d,p). Electronic density of states (DOS) were further examined at the MP2 method with the 6-311G(d,p) and analyzed using the multiwfn program.29 2.2. Materials. 1,4-Benzenedicarboxylic acid (H2BDC, 98%), Cr(NO3)3·9H2O (98.5%), and 4,4′-diaminoazobenzene (DAA, 95%) were purchased from Alfa Aesar. Ammonium fluoride (NH4F, ≥98%), hydrofluoric acid (HF, 48−51%) and ethanol (EtOH, 99.5%) were obtained from Sigma-Aldrich, J. T. Baker, and Daejung Chemical Co., respectively. 2.3. Syntheses of MIL-101(Cr) and MIL-101(Cr)_DAA. MIL101(Cr) was synthesized based on the literature.30 A reactant mixture of H2BDC/Cr(NO3)3·9H2O/HF/H2O in a molar ratio of 1/1/1/265 was transferred to a Teflon-lined stainless-steel reactor and then placed in an oven at 493 K for 8 h. After that, the temperature of the reactor was slowly lowered to ambient temperature. The resulted green powder was washed with deionized water, soaked in hot water (353 K) and

3. RESULTS AND DISCUSSION 3.1. First-Principles Calculations. We chose azobenzene (Azob), the simplest aromatic azo compound, as a reference fragment to assess the potential of azo compounds for the selective adsorption of olefin molecules over paraffin molecules. First-principles calculations27 were performed to predict the binding energies (Eb) of Azob with propylene and propane molecules. Table 1 summarizes the obtained binding energies and the shortest distances between nitrogen atoms in Azob and carbon atoms in guest molecules, namely, D[N−C]. Remarkably, the binding energy of propylene (32.7 kJ/mol) was approximately 25% higher than that of propane (26.3 kJ/mol). Moreover, D[N−C] for propylene was considerably shorter (3.298 Å) than that for propane (3.538 Å) (Figure 1). The thermodynamic and structural analyses highlighted that propylene would have a stronger interaction with Azob than propane. Such a preferential interaction between Azob and propylene can be discussed from the electronic and structural points of 27522


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respectively, were calculated for atoms in Azob and guest molecules (Figure 2a). Generally, the PDOS of Azob overlaps

Table 1. Binding Energies and Parameters from FirstPrinciples Calculations pristine azobenzene binding energy [kJ/mol]a D(N−C) [Å]b Average charge of N atomsc

+0.146

azobenzene− propylene

azobenzene− propane

32.731

26.290

3.298 +0.191

3.538 +0.148

a

Calculated from structures geometrically optimized at the MP2 with 6-311++G(d,p). bShortest distance between N atom in azobenzene and C atom in guest molecule. cAverage Mulliken charge of two N atoms in azobenzene.

Figure 1. Geometrically optimized structures of azobenzene− propylene(propane) complexes after the binding of propylene (propane) to azobenzene. The closest distances between nitrogen atoms in azobenzene and carbon atoms in propylene and propane are presented in (a) and (b), respectively.

view. From the electronic point of view, this may result from strong electrostatic interactions between NN in Azob and CC in propylene molecules. To validate this hypothesis, we examined the charge distributions in the optimized structures of Azob, propylene, propane, Azob−propylene, and Azob− propane (Figures S1 and S2). As displayed in the figures, each Azob and propylene molecule in the Azob−propylene complex would experience an electronic localization. Specifically, the averaged charge of 0.146 a.u. for the two nitrogen atoms in Azob was predicted to largely increase to 0.191 as a propylene molecule was bound to Azob, indicating an electron transfer from the two nitrogen atoms to the two benzene rings. Simultaneously, electrons in the propylene molecule would be localized to one of the carbon atoms in CC, which would be closer to the two nitrogen atoms in Azob, exhibiting the decrease of the charge of the carbon atom from −0.475 to −0.653 a.u. after the binding. This charge localization would lead to the enlarged charge difference between nitrogen atoms in Azob and the carbon atom in propylene and thus could result in the strong electrostatic interaction between the molecules. In contrast, such a charge localization phenomenon was not clearly observed in the Azob−propane complex, suggesting that this would have been originated from unique interactions between propylene and Azob. To comprehensively understand the role of electrons in Azob for the preferred interaction with propylene, the total and partial electronic density of states (DOS), namely, TDOS and PDOS,

Figure 2. (a) Total and partial electronic density of states, namely, TDOS and PDOS, for Azob−propylene and Azob−propane complexes. (b) Contributions of s and p orbitals in Azob, propylene, and propane molecules to the DOS overlap between −10.5 and −8 eV.

with that of propylene in a similar way with the PDOS of Azob overlapping with that of propane, in the E−Ef range of −30 to −11 eV. A remarkable point is that both Azob and propylene significantly contribute to the electronic distribution in the E−Ef range of −11.5 to −8 eV, where the HOMO energy level of the Azob−propylene complex is positioned through the overlap of their PDOS. In contrast, the electronic distribution in the same E−Ef range for the Azob−propane complex is solely contributed by the Azob. This leads to the relatively strong binding strength of Azob with propylene compared with that of Azob with propane.34,35 In addition, the overlap of orbitals between the Azob and propylene is primarily due to the p orbitals of Azob and propylene, indicating that the interaction between pi electrons in Azob and propylene would play a critical role in the preferential interaction between Azob and propylene (Figure 2b). 3.2. Syntheses and Characterizations of MIL-101(Cr) and MIL-101(Cr)_DAA. To investigate whether this interesting preferential interaction between Azob and propylene could be experimentally implemented, we incorporated 4.4′-diami27523


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Figure 3. (a) Scheme for grafting of 4,4′-diaminoazobenzene (DAA) into Cr3+ CUS site in MIL-101(Cr). (b) Proposed structure for a largest pore of MIL-101(Cr)_DAA.

noazobenzene (DAA) within the cavities of MIL-101(Cr) through a postsynthetic grafting (Figure 3a).36−51 We selected DAA as a grafting ligand because amino groups in DAA could be coordinated to open metal sites (OMSs) in MIL-101(Cr). We selected MIL-101(Cr) as a host framework since MIL-101(Cr) has mesopores that can accommodate DAA ligands, and all of the OMSs in MIL-101(Cr) set their faces toward the center of pores.26 Therefore, we could efficiently utilize the space of the pores because the grafted DAA ligands would point the center of pores in MIL-101(Cr) (Figure 3b). Characterizations for prepared MIL-101(Cr)_DAA revealed successful incorporation of DAA within MIL-101(Cr). Figure S3 shows PXRD patterns for MIL-101(Cr) and MIL-101(Cr) _DAA samples. The diffraction patterns of both samples matched well with the reported one for MIL-101(Cr).52 This shows the successful synthesis of MIL-101(Cr) and the preserved crystal structure even after DAA grafting. To determine the existence of DAA within the MIL-101(Cr) _DAA sample, 1H NMR spectroscopy was measured after the powder was carefully washed, degassed, and digested. For comparisons, NMR spectra for digested MIL-101(Cr), pure DAA, and pure H2BDC were also measured. The appearance of the characteristic peaks of DAA (at 7.51, 6.62, and 5.76 ppm) as well as peaks of BDC linker clearly showed that MIL-101(Cr) _DAA had DAA ligands within its structure (Figure 4). The mole ratio of the DAA and BDC ligand was measured by comparing a characteristic peak of DAA (at 7.51 ppm; b in Figure 4) to a characteristic peak of H2BDC (at 8.04 ppm; a in Figure 4). The resulting molar ratio of DAA to BDC was 0.18:1, and since the number of BDC is the same as the number of Cr atoms in perfect MIL-101(Cr), the ratio of DAA to chromium site was predicted to be 0.18:1. Since an excess amount of DAA was injected into the pores of MIL-101(Cr), the observed low DAA/Cr ratio might have come from geometrical hindrance between bulky DAA molecules. The atomic ratio of nitrogen to carbon (N/C) in MIL-101(Cr)_DAA determined from NMR data and was comparable to the value from elemental analysis (Table S1). STEM-EDS images of a MIL-101(Cr)_DAA crystal clearly present that nitrogen atoms were homogeneously dispersed throughout the whole crystal (Figure 5). This indicated that DAA ligands were successfully incorporated within the structure

Figure 4. NMR spectra for digested MIL-101(Cr) and MIL-101(Cr) _DAA. For comparisons, reference spectra of DAA and H2BDC are also displayed.

of MIL-101(Cr)_DAA. It should be noted that almost no nitrogen atoms were observed in the STEM-EDS images of a pristine MIL-101(Cr) crystal (Figure S4). N2 adsorption isotherms were measured to evaluate the surface area of MIL-101(Cr)_DAA (Figure S5). The determined BET surface area of MIL-101(Cr)_DAA was 3501.6 m2/g, which was larger than that of MIL-101(Cr) (3179.7 m2/g). This seemed to be unusual since, in most previous studies, filling pores of a MOF resulted in reduced surface area.14,51,53−57 To explain such an unusual behavior, a computational structural model for MIL-101(Cr)_DAA was built by adding grafted DAAs into the crystal structure of MIL101(Cr).58 The number of grafted DAAs was assumed to be 37 per primitive unit cell of MIL-101(Cr) containing 204 Cr atoms based on the NMR results. Interestingly, the geometric surface area of the hypothetic MIL-101(Cr)_DAA model was 3717.0 m2/g, which was much higher than that of MIL-101(Cr) (3034.4 m2/g) (Table 2), which agreed well with our experimental observations. This interesting behavior might be 27524


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isotherms in MIL-101(Cr) and MIL-101(Cr)_DAA were compared at three different temperatures (303, 323, and 343 K) (Figure 6a,c,e). Pristine MIL-101(Cr) shows some C3H6/ C3H8 selectivity due to the open chromium sites.60 Remarkably, MIL-101(Cr)_DAA exhibited enhanced selectivity as compared to parent MIL-101(Cr), especially at low pressures. The isotherms show that MIL-101(Cr)_DAA has a stronger C3H6 adsorption strength than MIL-101(Cr), while both MOFs have similar C3H8 adsorption strength. To quantify the gas−solid interactions, isosteric heats of adsorption (Qst) for C3H6 and C3H8 were calculated from adsorption isotherms at 303, 323, and 343 K based on Clausius− Clapeyron equation.61 As displayed in Figure 7, the heat of adsorption for C3H6 in MIL-101(Cr)_DAA was much higher (∼49.1 kJ/mol) than that in MIL-101(Cr) (∼36.9 kJ/mol), while the heats of adsorption for C3H8 in both MOFs were almost same (MIL-101(Cr)_DAA: ∼35.7 kJ/mol; MIL101(Cr): ∼36.3 kJ/mol). As a result, the difference in heats of adsorption between propylene and propane at 0.1 mmol/g in MIL-101(Cr)_DAA was 13.4 kJ/mol, which was approximately 22 times higher than that in MIL-101(Cr) (0.6 kJ/mol). This may have originated from preferential interactions between DAA and propylene. At high loadings, around 2.0 mmol/g and higher, the difference in the heats of adsorption between propylene and propane in MIL-101(Cr)_DAA becomes smaller, while that in MIL-101(Cr) is almost maintained. This may be because at high loadings the strong adsorption sites for propylene are almost saturated with propylene molecules. From the pure adsorption isotherms of C3H6 and C3H8, the C3H6/C3H8 selectivities for equimolar mixtures were calculated based on ideal adsorbed solution theory (IAST),62 which has been successfully applied for the prediction of the mixture adsorption behaviors of MOFs.63−67 As shown in Figure 6b,d,f, the IAST-predicted C3H6/C3H8 selectivities of MIL-101(Cr) _DAA were considerably higher than those of MIL-101(Cr) at all temperatures, primarily owing to the aforementioned difference in the heat of adsorption for propylene between the two MOFs. At 303 K, the selectivity of MIL-101(Cr)_DAA was considerably high (∼4.6) at low pressures, although it decreased to 2.2 at 100 kPa. To further confirm the effect of DAA grafting on olefin/ paraffin separation, ethylene and ethane isotherms in MIL101(Cr) and MIL-101(Cr)_DAA and IAST-predicted C2H4/ C2H6 selectivities were also compared at three temperatures (Figure S6). Clearly, C2H4/C2H6 selectivities of MIL-101(Cr) _DAA were higher than those of MIL-101(Cr) at all temperatures. The working capacity should also be considered for adsorptive separation processes and is defined as the difference between adsorbed amount at adsorption pressure (e.g., 1 bar) and the adsorbed amount at low pressure (e.g., 0.1 bar). MIL101(Cr)_DAA showed significantly large working capacities for propylene (5.33 mol/kg) and ethylene (2.22 mol/kg) between 1 and 0.1 bar. It should be noted that benchmark materials, including zeolite 13X, M-MOF-74, and HKUST-1, show considerably high olefin/paraffin selectivities but have low working capacities due to the presence of strong adsorption sites in small micropores (Table S2). These results suggest that DAA grafting is beneficial for both C3H6/C3H8 and C2H4/C2H6 separations. These results will provide some insight researchers who desire to develop adsorbents for olefin/paraffin separation. 3.4. Evaluations of MIL-101(Cr)_DAA under More Realistic Conditions. To confirm the efficient separation of

Figure 5. (a) STEM image for MIL-101(Cr)_DAA and (b) corresponding EDS mappings of C (cyan), N (light green), O (purple), and Cr (red).

Table 2. Comparison of Experimental BET Surface Areas and Geometric Surface Areas Calculated from GCMC Simulations for MIL-101(Cr) and MIL-101(Cr)_DAA MIL-101(Cr) MIL-101(Cr)_DAA

experiment (m2/g)

simulation (m2/g)

3179.7 3501.6

3034.4 3717.0

because the large mesopores of MIL-101(Cr) (29 and 34 Å) create considerable dead void spaces, which are unfavorable in respect of maximizing surface area. Grafted DAAs could utilize the unused space, providing additional surface that could interact with guest molecules.59 Moreover, the reasonable match between experimental and simulated surface areas could be regarded as another verification for successful grafting of DAA into MIL-101(Cr). 3.3. Effect of DAA Grafting on Olefin/Paraffin Selectivity. To understand the influence of DAA grafting on olefin/paraffin separation, propylene and propane adsorption 27525


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Figure 6. Single-component gas adsorption isotherms of MIL-101(Cr) and MIL-101(Cr)_DAA for propylene and propane at (a) 303 K, (c) 323 K, and (e) 343 K. IAST-predicted propylene/propane selectivities at (b) 303 K, (d) 323 K, and (f) 343 K.

propylene was strongly retained. This clearly showed that MIL-101(Cr)_DAA can effectively separate C 3 H 6 /C 3 H8 mixture through a packed-bed. A used adsorbent should be easily regenerated in an adsorptive separation process. After a breakthrough run, MIL101(Cr)_DAA column was regenerated by flowing a 100 mL/ min of helium flow for 20 min at the ambient temperature. Remarkably, almost identical breakthrough curves were

C3H6/C3H8 mixture using MIL-101(Cr)_DAA, we obtained breakthrough curves from a packed-bed filled with MIL101(Cr)_DAA. For comparisons, we did the same experiments for pristine MIL-101(Cr). As displayed in Figure 8a, the pristine MIL-101(Cr) did not show any difference in the C3H6 and C3H8 breakthrough times, which indicated almost no capability of separating the mixture. Using a column packed with MIL101(Cr)_DAA, however, propane eluted first, whereas 27526


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molecules were quickly desorbed by simply flowing pure N2 between each cycle.

Figure 7. Isosteric heats of adsorption (Qst) for propylene and propane in MIL-101(Cr) and MIL-101(Cr)_DAA.

Figure 9. (a) Cyclic adsorption−desorption isotherms of MIL-101(Cr) _DAA for 20 cycles at 303 K. A propylene/nitrogen mixture (25/75, v/ v) was introduced during each adsorption step (15 min), and pure nitrogen was introduced during each desorption step (15 min). (b) Comparison among propylene isotherms in MIL-101(Cr)_DAA at 303 K before and after being exposed to the atmospheric air.

Until now, most studies have utilized Cu(I) ions for providing C3H6/C3H8 selectivity. This is an efficient strategy for obtaining high C3H6/C3H8 selectivity but has a stability issue due to a possibility of partial oxidization of Cu(I) to Cu(II) during the long-term operation of a separation process. Compared to the method involving Cu(I) ions, the current strategy of grafting DAA into unsaturated metal sites may be beneficial in terms of stability. To investigate the stability of MIL-101(Cr)_DAA, C3H6 adsorption isotherms were measured after being exposed to ambient air for 3 and 12 h. The nearly identical isotherms displayed in Figure 9b confirm that MIL-101(Cr)_DAA has a superior stability. These results clearly show that grafting DAA to open metal sites of a MOF is an effective method for developing stable C3H6 selective adsorbents.

Figure 8. (a) Breakthrough curves for MIL-101(Cr) and MIL-101(Cr) _DAA with a step-input of a helium/propane/propylene mixture (He/ C3H8/C3H6 = 92/4/4, v/v/v) at 303 K and 1 bar. (b) Breakthrough curves through a MIL-101(Cr)_DAA bed for 3 cycles. Between each cycle, the bed was regenerated by simply flowing helium for 20 min.

produced for 3 cycles (Figure 8b). To further assess the regeneration performance of MIL-101(Cr)_DAA, a cyclic C3H6 adsorption/desorption profile was measured in a TGA. As shown in Figure 9a, MIL-101(Cr)_DAA maintained its C3H6 adsorbed amount for 20 cycles. Notably, the adsorbed C3H6

4. CONCLUSIONS To answer our initial question about whether an azo compound could give olefin selective behavior to adsorbents, we performed research using both computational and experimental methods. 27527


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ACS Applied Materials & Interfaces Initially, through first-principle calculations, we found that Azob binds preferably with propylene due to strong electrostatic interactions between the NN system in Azob and the CC bond in propylene. This discovery was applied to develop a novel adsorbent by grafting 4.4′-diaminoazobenzene (DAA) into MIL-101(Cr). The resulting MIL-101(Cr)_DAA showed apparently improved propylene heat of adsorption as well as enhanced propylene/propane selectivity compared to a pristine MIL-101(Cr). Remarkably, breakthrough experiments showed that MIL-101(Cr)_DAA can efficiently separate C3H6 from C3H8 through a packed bed. To the best of our knowledge, this is the first time revealing the effect of organic functional group for propylene separation. We confirmed the superior stability of MIL-101(Cr)_DAA in air, which is regarded as a relative advantage of an organic functional group compared to an inorganic one. MIL-101(Cr)_DAA is only an example to utilize an azo compound in olefin/paraffin separation, and we are certain that induction of an azo compound into an adsorbent could be a novel and promising strategy for developing olefin selective adsorbents.

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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/acsami.8b09739.

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Experimental details for NMR, breakthrough experiments, supporting data including elemental analysis, atomic charge distribution, PXRD, STEM image, N2 isotherms at 77 K and ethane and ethylene isotherms at 303, 323, and 343 K (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.C.K.). *E-mail: [email protected] (Y.-S.B.). ORCID

Youn-Sang Bae: 0000-0002-3447-4058 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by “Next Generation Carbon Upcycling Project” (Project No. 2017M1A2A2043449) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea. Additionally, we would like to acknowledge the National Research Foundation of Korea under Grant (NRF2016R1A2B4014256). This work was also conducted under the National Institute of Supercomputing and Network/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC-2017C1-0014).

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Paraffin Selectivity in Azo Compound and Its

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