Enhancement of Ethane Selectivity in EthaneâEthylene Mixtures by

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Enhancement of Ethane Selectivity in Ethane−Ethylene Mixtures by Perfluoro Groups in Zr-Based Metal-Organic Frameworks João Pires,† Joana Fernandes,† Kevin Dedecker,‡,§ Jose ́ R. B. Gomes,∥ Germań Peŕ ez-Sań chez,∥ Farid Nouar,⊥ Christian Serre,⊥ and Moiseś L. Pinto*,# †

Centro de Química e Bioquímica and CQE, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal Institut Lavoisier de Versailles, UMR CNRS 8180, Université de Versailles St-Quentin-en-Yvelines, Université Paris-Saclay, 78035 Versailles Cedex, France § Centre de Recherche sur la Conservation, USR3224: CNRS-MNHN-MCC, Sorbonne Universités, 36 rue Geoffroy-Saint-Hilaire, 75005 Paris Cedex, France ∥ Department of Chemistry, CICECO - Aveiro Institute of Materials, University of Aveiro, Campus Universitário de Santiagoí, 3810-193 Aveiro, Portugal ⊥ Institut des Matériaux Poreux de Paris (IMAP), UMR CNRS 8004, Ecole Normale Supérieure de Paris, Ecole Supérieure de Physique et de Chimie Industrielles de Paris, PSL University, 75005 Paris, France # CERENA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, n° 1, 1049-001 Lisboa, Portugal

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‡

S Supporting Information *

ABSTRACT: A series of zirconium dicarboxylate-based metal-organic frameworks (Zr MOFs) of the UiO-66 (tetrahedral and octahedral cages) or MIL-140 (triangular channels) structure type were investigated for the separation of ethane/ethylene mixtures. The adsorption, investigated both experimentally and computationally, revealed that the size and type of pores have a more pronounced effect on the selectivity than the aromaticity of the linker. The increase in pore size when changing from benzene to naphthalene (NDC) dicarboxylate ligand makes UiO-NDC less selective (1.3−1.4) than UiO-66 (1.75−1.9) within the pressure range (100−1000 kPa), while the threedimensional (3D) pores of the UiOs favor the adsorption of ethane due to the interactions between ethane with more spacers than in the case of the 1D channels of MIL-140s. The impact of the functionalization revealed a very interesting increase of selectivity when two perfluoro groups are present on the aromatic ring (UiO-66-2CF3) (value of 2.5 up to 1000 kPa). Indeed, UiO-66-2CF3 revealed a unique combination of selectivity and working capacity at high pressures. This is due to a complex adsorption mechanism involving a different distribution of the guest molecules in the different cages associated with changes in the ligand/perfluoro orientation when the pressure increases, favoring the ethane adsorption at high pressures. KEYWORDS: gas separation, metal-organic frameworks, ethane/ethylene separation, gas adsorption, simulation

1. INTRODUCTION The main operation of the ethylene purification, an important basic raw material for the synthesis of several products (world production capacity will reach 200 million tons in 2020),1,2 is its separation from the ethane. The cyclic adsorption separation process can be an alternative to the cryogenic distillation, currently used in the industry with high energy consumption, if the adsorbent material used presents more adsorption affinity to ethane than to ethylene. Several adsorbents reported in the literature, for long time, show high adsorbed amounts and high selectivity for this separation. Nevertheless, as already mentioned, cryogenic separation remains the technique of choice, even though columns with more than 100 plates and very high reflux rates need to be used,3 mainly because the majority of reported adsorbents (zeolites and clay based materials for instance) show higher affinity to adsorb ethylene over ethane.4 So, although the © XXXX American Chemical Society

selectivity can be high, they are not attractive for this industrial application because the adsorbent selectivity will occur at the cost of ethylene being the more adsorbed species, with a concomitant low purity of ethylene in the stream that is obtained in the blowdown step (desorption) of the cyclic separation. Metal-organic frameworks (MOFs) are presently a wellknown recent class of ordered porous hybrid materials that display a large variety of structures and chemical compositions.5−7 This makes MOFs highly interesting to the design of porous materials with new properties and opens the possibility to investigate how the selectivity of the materials can be tuned for the desired order, i.e., selective for ethane rather than for Received: April 23, 2019 Accepted: July 2, 2019 Published: July 2, 2019 A

DOI: 10.1021/acsami.9b07115 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic representation of the pores of the Zr MOFs with benzene (BDC) and NDC linkers and two different structure topologies. The linkers that form the tetrahedral and octahedral cages are in red and green (top line), respectively, and the linkers that form the triangular 1D channels (bottom line) are in yellow. Available pore sizes are depicted by the spheres inside the pores, with the respective value. Zirconium and oxygen atoms are blue and red spheres, respectively, in the green polyhedra that represents the zirconium coordination in the discrete oxoclusters (top line) and infinite metal oxide rods (bottom line). Hydrogen atoms were not presented for the sake of simplicity.

adsorption.22 A review on the recent studies of MOFs for this separation can be found elsewere.23 Nevertheless, all of these cases have still significant drawbacks for a potential industrial application. A bulk ethane/ethylene separation with adsorbents will have to be performed with pressure swing cycles, in operations above atmospheric pressure, which are the most common conditions used in industry due to technical and economic reasons. In the pressurization step, the pressure is increased above atmospheric pressure (typically 5−10 times) and the blowdown/ regeneration is done until atmospheric pressure.24 Although the regeneration under moderate vacuum is also possible, it is very costly and is only used if no other separation alternative is available. The ZIF-7, ZIF-8, MAF-49, and Fe2(O2)(dobdc) are saturated below atmospheric pressure (in ZIF-7 and MAF-49, saturation occurs below 30 kPa), which makes it impossible to regenerate their adsorption capacities above atmospheric pressure. In PCN-250 and Ni(bdc)(ted)0.5, the adsorption curves are also approaching a plateau at atmospheric pressure for the ethane adsorbed amounts, which is also a problem for applications in standard pressure swing operations. In fact, most of the above-cited studies were only performed up to atmospheric pressure with adsorption isotherms or breakthrough curves, which are not sufficient for an assessment at industrial application conditions. In the present work, the adsorption isotherms of ethane and ethylene were determined for pressures up to 1000 kPa (10 times the atmospheric pressure) to clearly evaluate the relevance of the materials for possible application in separation processes. Another important issue for an industrial application of an adsorbent is the stability of the material, which is quite often the Achilles heel of the MOFs. For example, IRMOF-8 is unstable in the presence of small amounts of water, which turns it unsuitable for large-scale operations. Fe2(O2)(dobdc) is air/moisturesensitive and needs to be handled in a glovebox.21 In fact, the chemical stability of some of the Zn-, Ni-, and Cu-based MOFs cited above is still not well established. Thus, the quest for an

ethylene. When strong local charges are present in the MOF structure like coordinatively unsaturated metal sites as it happens, for instance, in the activated HKUST-1 (copper benzene-1,3,5-tricarboxylate)8−10 and in Fe2(dobdc),11 ethylene interacts strongly with these sites and is preferentially adsorbed over ethane, thus not presenting any advantage over the classic adsorbents like zeolites, for example. Therefore, the absence of strong local charges at the structure of the materials arises as a necessary requisite for obtaining structures that preferentially interact with ethane. Previously, it was found that IRMOF-8, a Zn-based MOF, presents selectivity for ethane12 and that this is enhanced in the interpenetrated form of the material.13 More recently, this enhancement due to an interpenetration of the structure has also been observed in PCN-245.14 Imidazole MOFs, ZIF-7 and ZIF-8, were also reported to have selectivity for ethane.15−17 Although in all cases selectivity was not very high (2.8−1.6, 2.7, and 1.85 in the cases of IRMOF-8, ZIF-7, and ZIF-8, respectively), it helped to raise some fundamental questions about the characteristics needed in the porous MOFs to obtain the selectivity for ethane, since many other materials do not show such capability. Other structures (PCN-250 and Ni(dbc)(ted) 0.5) were then reported with ethane preferential adsorption with selectivity around 2.18,19 After, some MOFs were reported with higher selectivities. MAF-49 was found to have a selectivity of 9 because of the particular position of functional groups on the ultramicropores that interact preferentially with ethane through hydrogen bonding,20 although a lower selectivity of 2.7 was reported later for the same material by other authors.21,22 Another type of specific interaction was explored in the case of Fe2(O2)(dobdc).21 In this material and at low pressure, ethane forms hydrogen bonds with the preoxo groups bonded to the unsaturated metal sites at the surface of the pores. On the Cu(Qc)2 material, a selectivity of 3.4 was reported and attributed to an adaptive structure to the adsorbed molecules that favors ethane B


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materials, e.g., zeolites, can lead to inaccurate results because of the intrinsic structural features of MOFs.9,31 In this regard, the most common force fields from the literature used in GCMC simulations have been recently evaluated in the adsorption of methane by Zr-based MOFs, including the UiO66 family with BDC, NDC, and BPDC linkers.32 Despite the general qualitative agreement between the adsorbed amounts, quite significant quantitative differences were found in the methane isotherms calculated using distinct force fields. The adsorption isotherms followed the same trend, with the methane adsorbed amounts increasing with the pore volume (i.e., BDC < NDC < BPDC linkers), independently of the chosen force field. Furthermore, the same qualitative agreement of the force fields on the outcome of simulations with missing linkers was obtained. Overall, generic force fields, such as DREIDING or UFF for the framework and TraPPE for methane, were found to reproduce quite satisfactorily the experimental adsorption, but with slight overestimation in the methane uptake, especially at low pressure, where the host− guest interactions greatly control the adsorption. Ab initiobased force fields, e.g., SAPTFF or MEDFF, succeeded in reproducing single molecule adsorption energies because of a much accurate description of host−guest interactions compared with the generic force field, but were outperformed by the generic force field at high pressures (i.e., high methane loading) because of their apparent poor performance to describe the guest−guest interactions. Note that the TraPPE force field was fitted against vapor−liquid equilibrium data, which ensures that the gas−gas interactions are, in principle, accurately reproduced. The combination of the generic DREIDING (all framework atoms except Zr), UFF (framework Zr atoms), and TraPPE (adsorbate) force fields was also used in the study of the adsorption and diffusion of light alkanes in UiO-66,33 with good agreement between the experimentally measured adsorption enthalpies and those calculated by GCMC at zero coverage. The results from the simulations in these previous works clearly helped to understand how the gases interacted with the Zr-based MOFs. Herewith, the GCMC simulations are used to analyze the preferential adsorption locations upon pressure increase and to provide evidences of the effects of structural and chemical characteristics of MOFs on the selectivity of ethane/ ethylene separation. Experimental adsorption results are compared to data from computer simulations to understand, at molecular level, the topology and linker effects on the selectivity. The studies present in this work revealed that, from the several strategies, introducing bulky −CF3 groups in the ligands at the 3D cages is the most effective way to obtain Zr MOFs with a selectivity for ethane at high pressures. This also translates to a good working capacity, i.e., most of the adsorption capacity can be recovered by returning to the atmospheric pressure, which is not observed in many of the MOFs reported so far. Indeed, in this work, we unveil the marked effect of the linker’s position and orientation on the adsorption isotherms and selectivity of the materials, when bulky groups are present on the linker, which can be exploited to effectively separate ethane from ethylene.

adsorbent that effectively adsorbs ethane over ethylene and can be usable at industries is still open. In the present work, we study a series of robust Zr dicarboxylate MOFs, derived from the UiO-66 and MIL-140 structure types, bearing different structures and chemical functionalities. UiOs are based on discrete Zr6 oxoclusters,25 while MIL-140s exhibit infinite metal oxide rods as building units.26 This leads to a three-dimensional (3D) pore system for UiOs against 1D triangular channels for the MIL-140s (cf. Figure 1). These MOFs are hydrolytically and thermally very stable and present, from this perspective, a realistic possibility for industrial application in large-scale processes. This is of a considerable advantage when comparing to most of the abovedescribed MOFs since many of them are not stable in the presence of moisture. This characteristic encouraged us to explore the Zr dicarboxylate MOFs for such separation. The aromaticity of the linker was found to have a strong effect in obtaining IRMOF-type structures displaying selective adsorption for ethane over ethylene, since the IRMOF based on naphthalene dicarboxylate linkers (IRMOF-8) was selective for ethane while that based on benzene dicarboxylate (IRMOF-1) was not.13 This prompted us to investigate the combined effect of topology and aromaticity of the linker in Zr dicarboxylate MOFs, as schematically illustrated in Figure 1. The UiO-66 and UiO-NDC porosity is composed of tetrahedral cages of about 7.5 and 10 Å, respectively, and octahedral cages of about 10 and 13 Å, respectively.25−27 The MIL-140A and MIL-140B pores are one-dimensional triangular channels with diameters of about 3.2 and 4 Å, respectively.26 Due to the slightly bigger size of the ethane molecule, it is expected that this species will interact more with the pore walls by nonspecific dispersion forces than ethylene (i.e., as a consequence of the higher polarizability of ethane than that of ethylene).28 To selectively increase the interactions with ethane, other two possibilities were also tested. First, the size of the pores in the UiO-66 topology was decreased by using a smaller fumarate linker (UiO-Fum) instead of the dicarboxylate in UiO-66 to understand if increasing the confinement effect would be beneficial for the separation. In the second strategy, bulky polarizable halogenbased groups like Br and CF3 (UiO-66-2CF3 and UiO-66-Br) were introduced in the benzene ring to probe the effect of such groups on ethane/ethylene selectivity. Indeed, as shown below, the presence of two bulk CF3 groups has proven to effectively improve the selectivity due to a change in orientation of the groups with pressure. The structure and functionalization of the linker in the UiO66 was previously addressed for light alkanes and their mixtures at low pressures.29 Single-component adsorption measurements and grand canonical Monte Carlo (GCMC) simulations demonstrated that the functionalization of the linker over other structure features plays a key role in the separation of similar alkanes as, for instance, in the case of ethane and ethylene mixtures. Despite GCMC calculations describe reasonably the adsorption and selectivity of light alkanes, two main aspects make the comparison with experiments a tough task. On the one hand, MOFs usually exhibit different structural defects such as heterogeneity, dislocation, and linker or metal vacancies, among others,30 which hamper the construction of realistic structural models to use in the GCMC simulations. On the other hand, the application of force fields in the GCMC calculations that were originally derived for studying gas adsorption in other

2. MATERIALS AND METHODS 2.1. Materials Synthesis. The synthesis of zirconium MOFs was carried out under solvothermal conditions in dimethylformamide (DMF) using ZrCl4 (for MIL-140A, MIL-140B, UiO-66, UiO-66-Br, C


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parameters were taken from the united atom TraPPE model.49 The Lorentz−Berthelot mixing rules were used to calculate the mixed LJ parameters. The partial atomic charges were taken from the DFT calculations as described above and can be found in the SI (Figure S4).

UiO-66-2CF3, and UiO-NDC) or ZrOCl2 (for UiO-Fum) as the zirconium source and the corresponding carboxylic acid. All experimental details are given in the Supporting Information (SI). 2.2. Materials Characterization. All synthesized materials were characterized with the powder X-ray diffraction (PXRD), thermogravimetric analyses (TGA), and low-temperature nitrogen adsorption, to ascertain the phase structure and purity. The experimental details are described in the SI, and the results are presented in Figures S1−S3 and Table S1. 2.3. High-Pressure Adsorption of Ethane and Ethylene. Pure gas isotherms with ethane (Air Liquide, 99.995%) and ethylene (Gasin, 99.99%) were measured on each material. A conventional stainless steel volumetric apparatus was used to carry out adsorption experiments up to 1000 kPa. The apparatus was equipped with a pressure transducer (Pfeiffer Vacuum, APR 266), connected to a vacuum system, which could maintain a vacuum better than 10−2 Pa. The adsorption temperatures of 25 or 45 °C were maintained with a stirred thermostatic water bath (Grant Instrument, GD-120). Before experiments, each sample was degassed for 2.5 h at 150 °C. In the calculation of the adsorbed amounts, the nonideality of the gas phase was considered by using the second virial coefficients. Excess adsorbed amounts determined experimentally were converted to absolute amounts using the gas density and porous volumes of the samples. The isotherm data were fitted with a virial equation of the

3. RESULTS AND DISCUSSION 3.1. Effects of the Type of Pore and of Linker’s Aromaticity on Adsorption. The pore structural arrangement and linker effects on the ethane/ethylene adsorption can be understood from Figures 1 and 2. The experimental and

nads

form p = K exp(C1nads + C 2nads 2 + C3nads 3), where p is the pressure, nads is the adsorbed amount, K is Henry’s constant, and C1, C2, and C3 are the virial coefficients. The selectivity and phase diagrams of binary C2H6/C2H4 mixtures were estimated by a method based on the ideal adsorbed solution theory (IAST), which has been described in detail in previous works.34,35 2.4. Computational Methods. The VASP code36−38 was used to optimize the structures of the MOFs considered in the present study. The molecular model of each MOF was obtained by repetition of a unit cell in the three spatial directions. These initial models were constructed from the crystallographic parameters published in the literature.25−27 The VASP calculations considered the PBE exchangecorrelation functional.39 Dispersion corrections were introduced in the calculations by the method of Grimme et al. using the rational damping form of Becke and Johnson.40 The projected augmentedwave (PAW) method41,42 was considered to take into account the effect of core electrons in the valence electron density. Valence electronic states were described using a set of plane waves with a cutoff set of 415 eV. A single k-point was considered for the numerical integration in the reciprocal space. All of the structures were optimized with the conjugate-gradient algorithm. The convergence criteria were 10−5 eV for the total energy change and 10−2 eV/Å for the forces acting on the atoms. Atomic charges within the densityderived electrostatic and chemical (DDEC6) formalism were calculated with the Chargemol program,43,44 using charge densities generated with the VASP code. Optimized structures are available in the SI. The pure component adsorption isotherms for ethane and ethylene, the adsorption selectivity of the binary ethane/ethylene mixture, and the isothermal−isobaric xy diagrams of the ethane/ ethylene mixture were obtained from grand canonical Monte Carlo45 (GCMC) simulations using the RASPA code.46 The chemical potential, temperature, and volume were fixed in the simulations. The simulations consisted of 20 000 steps for equilibration prior to 200 000 steps for production, considering translation, rotation, and regrowth in random positions moves. This approach ensured a reproducibility of four decimal places in the uptake. The LennardJones (LJ) potential was cut and shifted with a cutoff distance of 12.8 Å. The MIL-140A, MIL-140B, UiO-66, UiO-NDC, and UiO-66-2CF3 frameworks optimized by density functional theory (DFT) were considered rigid in the GCMC simulations. As in previous studies, the LJ parameters of the all framework atoms were taken from the DREIDING47 force field, while the zirconium atoms use the parameters from the UFF48 force field. The ethane and ethylene molecules were considered as uncharged, and the Lennard Jones

Figure 2. Experimental (closed symbols) and simulated (open symbols) ethane and ethylene adsorption isotherms, at 25 °C, on the Zr MOFs with BDC and NDC linkers and two different topologies of structures. The continuous lines represent the virial isotherm fit to the experimental data. The vertical axis scale is the same for better comparison of the adsorbed amounts of the gases in the four materials considered.

simulated adsorption results summarized in Figure 2 display a dependence of the adsorption behavior on the type of linker (BDC or NDC) and on the type of topology of the structure. The most striking observation is that if the MOF structure is arranged in 3D tetrahedral and octahedral cages (cf. UiO-66 and UiO-NDC in Figure 2) rather than in straight channels (cf. MIL-140A and MIL-140B in Figure 2), it adsorbs more ethane than ethylene. The comparison of the materials with BDC (single aromatic ring) or NDC (two aromatic rings) linkers shows that, as expected, the materials with the NDC linkers adsorb slightly more ethane than the MOFs with BDC linkers, with the effect of preferential adsorption being more relevant on going from UiO-66 to UiO-NDC than from MIL-140A to MIL-140B. Nevertheless, the adsorption isotherms for MIL140A and MIL-140B depicted in Figure 2 indicate that ethylene is more adsorbed in MIL-140A than ethane for all pressure range, while in MIL-140B, ethane is slightly more adsorbed than ethylene below 200 kPa. Therefore, these results show that the increased aromaticity in the materials with NDC linkers seems to lead to much more favorable interactions with ethane than with ethylene. Obviously, these effects are, to some extent, masked by the pore size increase (cf. Figure 1) caused by the linker size, which influences the overall adsorption behavior as well, namely, the total adsorbed amounts. Indeed, the effect of the pore size is translated into D


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ACS Applied Materials & Interfaces a significant increase in the surface areas and pore volumes of the materials (Table S1), which in turn lead to the increase of the adsorbed amounts for both gases (Figure 2). Despite some slight divergences in adsorbed amounts are noted when comparing experimental and simulation results in Figure 2 (this will be analyzed in detail below), both sets of results agree on the affinity of the structures to absorb more ethane or ethylene, i.e., computer simulations also suggest that the tetrahedral and octahedral cages are important to obtain material structures that can adsorb more ethane than ethylene. Some small differences between the calculated and experimental adsorbed amounts may be ascribed to the presence of some defects in the structure of the materials, as discussed below and in detail in the Supporting Information. From the comparison of the adsorbed amounts of each gas on the materials with a 3D pore structure (UiO-66 and UiONDC), it can be concluded that the material displaying higher pore volume and surface area (UiO-NDC, cf. Table S1) adsorbs larger amounts of ethane and ethylene. This is expected considering the large pore sizes of these materials comparing with the small molecular sizes for ethane and ethylene,50 i.e., no significant restriction to the packing of the molecules inside the pores is expected. On the contrary, no significant increase in adsorbed amounts is observed from the MIL-140A to the MIL-140B, since in these cases, changing the linker from BDC to NDC increases only slightly the pore size (about 0.8 Å, Figure 1) and the surface area (about 12%, Table S1). Considering that the pore size is very close to the molecular size of ethane and ethylene, the adsorption in these materials is likely very constrained due to packing restrictions inside the pores. This detail is further discussed in the context of the molecular simulation results. The increase in temperature to 45 °C decreases the adsorbed amounts of both gases in all materials (Figure S5) as anticipated for an exothermic process of physical adsorption. Again, the results agree on the materials that display preferential adsorption of ethane over ethylene presenting a somewhat similar situation to that observed at 25 °C (cf. Figures 2 and S5), which demonstrates a consistency of the results at both temperatures. The affinity of the materials at low pressures for the adsorption of each gas is best described by Henry’s constant (K) obtained from the experimental data by fitting of the virial equation (Table S2). Only for MIL-140A, the ethane K value is marginally lower than the ethylene K value. In all other cases, the ethane adsorption presents a higher value than that for ethylene, implying that a selective adsorption of ethane is expected at very low pressures. The ratio of Henry’s constants (Kethane/Kethylene) can be used as a first estimation of the selectivity, at 25 °C and at very low pressures, with values of 0.91, 2.05, 3.52, and 1.49 for MIL-140A, MIL-140B, UiO-66, and UiO-NDC, respectively. The selectivities of the separation of ethane/ethylene mixtures with pressure, obtained from the experimental data with IAST calculations and from the GCMC simulation of binary mixtures, confirm this trend, as depicted in Figure 3. Selectivity at 45 °C is presented in Figure S6. The material with the best selectivity, UiO-66, has tetrahedral and octahedral cages, which again confirms that this type of topology is central to obtain a high selectivity for ethane (Figure 3). Interestingly, the ethane/ethylene selectivity estimated for UiO-NDC is lower than that found for UiO66, showing that increasing the aromaticity of the linker (naphthalene dicarboxylate for UiO-NDC vs benzene

Figure 3. Selectivity of adsorption of the binary mixture ethane/ ethylene, with pressure, at 25 °C. Closed and open symbols represent selectivity calculated from the experimental adsorption data and from binary mixture adsorption simulation, respectively.

dicarboxylate for UiO-66) does not lead necessarily to an increase in selectivity for ethane. Simulation results agree with this finding (open points in Figure 3). This is in clear contrast to the behavior of IRMOF-type MOFs where it was found previously that the selective adsorption of ethane was enhanced by increasing aromaticity; the IRMOF-8 (naphthalene dicarboxylate linkers) material was considerably more selective for ethane than IRMOF-1 (benzene dicarboxylate linkers).13 Nevertheless, a more significant enhancement was found for the interpenetration effect in IRMOF-8 suggesting that other structural effects may play a significant role.13 In the case of the UiO-type structure, the effect of the aromaticity is not so relevant, which may be also due to the pore size increase effect upon changing the linker from benzene to naphthalene dicarboxylate. This prompted us to explore computationally the effect of increasing even further the aromaticity of the linker. There are not many reported examples of structures with big aromatic systems in the linkers. Two of the most characterized ones are Zn MOFs based on pyrene dicarboxylic acid, in interpenetrated, IRMOF-13, and noninterpenetrated, IRMOF-14, forms.51 The results (Figures S7−S9) do not show an increase relatively to the corresponding interpenetrated and noninterpenetrated IRMOF-8 counterparts, in agreement with the results of Lahoz-Martiń et al.,52 indicating that, in fact, the increase in aromaticity is not sufficient to increase selectivity for ethane if the pore size increases too significantly. For the MIL-140A, selectivity is always below 1 (cf. Figures 3 and S6), meaning that ethylene is more adsorbed than ethane in all conditions studied. The MIL-140B selectivity is slightly higher, since it starts at about 1.5 and decreases to values below 1 with increasing pressure. Additional demonstration of the difference in performance of the materials for ethane/ethylene separation is obtained from the phase diagrams at constant pressure. The example depicted in Figure 4 clearly demonstrates an inversion of the affinity of the materials when the type of structure changes. The MIL-140A and MIL-140B cases cannot be used for mixture separation since they exhibit low selectivity (values close to the diagonal line of the diagram). The behavior is worse in the case of MIL-140A since ethylene is more adsorbed than ethane. Also, although MIL-140B is ethaneselective at low pressures, the increase of pressure favors the adsorption of ethylene, probably due to its smaller size and more efficient packing in the narrow pores leading to a reversion of selectivity (Figure 3). Conversely, the UiO-66 E


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Figure 4. Isothermal (25 °C), isobaric (500 kPa) xy diagrams of ethane/ethylene mixture adsorption on MIL-140A, MIL-140B, UiO66, and UiO-NDC. y is the molar fraction of ethylene on the gas phase, and x is the molar fraction of ethylene in the adsorbed phase. Figure 5. Density maps for ethane adsorption at 100 kPa, for the UiO-66 (top left), UiO-NDC (top right), MIL-140A (bottom left), and MIL-140B (bottom right). In the case of MIL-140A (bottom left), some missing linkers (on purpose) of the structure can be noted (see the SI for additional discussion).

presents better performance for a possible application in ethane/ethylene separation because the values are more departed from the diagonal line (i.e., x = y shown as a dashed black line in Figure 4). For example, an equimolar gas mixture (y = 0.5) corresponds to an equilibrium adsorbed phase with a molar fraction of about 0.35 in ethylene, meaning that most of the adsorbed phase is composed of ethane (0.65 molar fraction). The results presented in Figure 4 are obtained at 500 kPa (≈5 atm) and are thus relevant for the assessment of the materials for applications in cyclic processes like pressure swing adsorption (PSA). If a PSA regeneration step is considered at near-ambient pressure (≈100 kPa), the working capacity (defined by the difference in amounts adsorbed at the working pressure and regeneration pressures) is clearly bigger for the UiO-NDC case (about 4.5 mmol g−1 at 25 °C) than for the other materials, due to the shape of the isotherms (Figure 2). In fact, the UiO-NDC displays adsorption isotherms with an open curvature with a significant increase of adsorbed amounts above 100 kPa, which is not the case for the other studied MOFs. However, the selectivity is not very attractive. The enhanced interaction of the ethane with some of the adsorbent structures can be quantified energetically by the isosteric heats of adsorption (Figure S10). Results indicate that the interaction is more energetic with ethane than with ethylene, in the full range of adsorbed amounts, for the UiONDC structure, contrary to that found for the other MOFs considered in this work. This indicates again that the increase of the aromaticity of the linker on going from BDC to NDC can enhance the interaction with ethane, but, as discussed above, this enhancement is highly dependent on the pore type and pore size. It must be noted that some details of the model structures chosen to describe the experimental results can affect the proper description of systems. Some preliminary effort was done to understand the effect of the presence of OH groups in the zirconium oxide clusters or rods and possible missing linkers, particularly for MIL-140A. A more complete discussion on the details of the materials structure and their influence in the simulation results is presented in the SI. The results from the computer simulations are of paramount importance to understand in detail the effect of the structure on the adsorption of both gases and on the separation selectivity. The occupancy density at low pressures was investigated for attaining an atomistic picture of the differences observed in the adsorption behavior of the gases in the materials. Figures 5 and

6 show the density maps of ethane and ethylene, respectively, on the structures with straight channels or with 3D cages

Figure 6. Density maps for ethylene adsorption at 100 kPa, for the UiO-66 (top left), UiO-NDC (top right), MIL-140A (bottom left), and MIL-140B (bottom right). In the case of MIL-140A (bottom left), some missing linkers (on purpose) of the structure can be noted (see the SI for additional discussion).

(figures of the density without the material’s framework are presented in Figures S11 and S12). The results show that the preferential adsorption sites in the UiO-66 and UiO-NDC are the tetrahedral cages, more specifically, in the narrow space between the three linkers and the metal cluster. These locations provide an enhanced interaction of the molecules with the surrounding linkers and the metal cluster, i.e., the molecules are in close contact with the linkers that are facing the interior of the pore. Besides originating materials with larger pore sizes, the NDC linker based on two fused aromatic rings holds a bigger linker surface which is facing the interior of the pores. The NDC linker interacts more favorably, compared to the BDC linker (UiO-66), and allows the UiO-NDC to interact more strongly with ethane than with ethylene in a wide range. At 10 kPa (Figure S11), the density inside the UiOF


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NDC, does not have enough effect on the desired ethane selectivity increase. Also, it was concluded that the UiO-type topology is more effective to increase the ethane selectivity. Thus, based on this structure, other possibilities were explored for this purpose. The first was to decrease the pore size by using the shorter fumarate linker (Figure S14) to obtain the UiO-Fum (Figure S15). The second possibility was to introduce bulky halogen-based group substitutions in the aromatic ring, like Br and CF3, by using BDC-Br and BDC2CF3 linkers (Figure S14) during the synthesis, to obtain the UiO-66-Br and UiO-66-2CF3 materials, respectively. Overall, the UiO-Fum and UiO-66-2CF3 present lower ethane and ethylene uptakes compared to the parent UiO-66, while UiO66-Br presents similar adsorbed amounts (cf. Figures 2 and 7).

NDC pores is clearly bigger in the corners of the tetrahedral cages in the ethane case compared to that of ethylene. However, at higher pressures, this difference vanishes (Figure S12). In fact, the van der Waals interactions are expected to be stronger in ethane than in ethylene (the polarizability can be used to estimate this type of interaction,53 which is higher in the case of ethane than of ethylene28). The Lennard-Jones parameter ε used to simulate the dispersion interaction of the gases is higher for ethane than for ethylene. This means that, in the absence of other types of interactions, ethane will interact more strongly with the material’s framework than ethylene. Adding one additional aromatic ring to the linker (changing BDC by NDC) increases significantly the surface of the porous structure that can interact with the adsorbed molecules only by dispersion forces, i.e., the structure with NDC linkers are potentiating the dispersion interaction with the adsorbed molecules. However, since the pore size also increases, this interaction difference is not translated to an effective difference between the adsorption amounts of ethane and ethylene and, thus, to a higher selectivity. This effect can be observed in Figure S11 by comparing the ethane and ethylene densities in both UiO-66 and UiO-NDC, i.e., at 10 kPa, the pores are more homogeneously filled in the UiO-66 cases due to the higher adsorption potential at the center of the pores. In the structures with straight pores (MIL-140A and MIL140B), the interactions with ethane look similar when the BDC is replaced by NDC, which demonstrates that the effect of the linker on the adsorption is also dependent on the topology of the structure (cf. Figures 5 and 6). In fact, as discussed above, the enhancement in the interaction for ethane is minimal when comparing the results (experimental and simulation) of MIL140A and MIL-140B (Figure 2). On these structures, several differences explain the less effective interaction of the linkers with the adsorbed molecules. One difference is that, in the MIL-140-type structures, only half of the linkers are arranged with the aromatic rings facing the interior of the pores, while the other half is staking between two metal oxide rods (Figure 1). In UiO-type structures, all aromatic rings are facing the interior of the tetrahedral cages (Figure 1). Another difference is that the MIL-140-type structures are composed of onedimensional pore channels, which means that the adsorbed molecules never interact with linkers in the direction along the symmetry axis of the pores, contrary to what happens in the 3D cages of UiO-type structures that allow the adsorbed molecules to interact with the linkers in all dimensions. Moreover, the linkers that are facing the interior of the pores in the MIL-140-type structures are alternating at the top and at the bottom of the pore wall along the zirconium oxide chains. Therefore, adsorbed molecules that are close to the pore walls can only interact closely with one linker since the other linker is misaligned (cf. Figure S13). In the tetrahedral cages of the UiO-type structures, the adsorbed molecules can interact with three linkers near the corners of the tetrahedra (short videos are available in the SI to illustrate these MIL-140-type and UiO-type structural features). These features, which can be confirmed from the CIF files in the SI, make the UiO-type structures more effective to potentiate the interactions of the adsorbed molecules with the linkers than the MIL-140-type structures. 3.2. Effects of the Pore Size and of Linker’s Functionalization on Adsorption. From the results and discussion above, it can be concluded that increasing the size of the aromatic system of the linkers, i.e., going from BDC to

Figure 7. Ethane and ethylene adsorption isotherms, at 25 °C, on UiO-Fum, UiO-66-Br, and UiO-66-2CF3. The lines represent the virial isotherm fits to the experimental data.

For the UiO-Fum and UiO-66-Br, the isotherms of ethane and ethylene are very similar, but the isotherms observed for UiO66-2CF3 are different, with ethane displaying higher adsorbed amounts than ethylene (Figure 7). This difference of behavior among UiO-Fum, UiO-66-Br, and UiO-66-2CF3 can also be evaluated by comparing Henry’s constants obtained from the G


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reasonable operating pressure window below saturation of the pores, i.e., the curvature of the isotherm should not be too steep. This is precisely the case of the isotherms of UiO-662CF3. In fact, considering the working capacity of an adsorbent as the difference between adsorbed amounts at the highest and lowest pressures in a cyclic process, an interesting value of about 1 mmol g−1 is obtained between the atmospheric pressure (100 kPa) and 1000 kPa. We confirmed that the adsorbed gases can be easily removed by vacuum, as expected for a physisorption process. Moreover, as found in a recent work, the highly hydrophobic UiO-66-2CF3 material maintained the adsorption properties after being exposed to water vapor and low concentrations of acetic acid, which underlines its stability.54 Thus, this material presents an excellent combination of features that makes it suitable for application in bulk separations with pressure swing that were not observed for other MOFs so far. Further studies need to be conducted before a possible application of this model material, relying on a not yet commercially available ligand, mainly to establish the dynamics of adsorption in columns. However, these studies only make sense for the material already shaped in big particles that will be after used in columns (i.e., spheres, pellets, or similar), since these properties depend significantly on the particle sizes, on the method of shaping, and on the binder used. These issues are out of the scope of the present work and a careful investigation of them will be addressed in the future. Nevertheless, the unique properties of adsorption capacity and selectivity of UiO-66-2CF3 are clearly established by the results presented here. To rationalize these very interesting experimental results for UiO-66-2CF3, several models for the structure were considered, since, as we will show, the bulky −CF3 groups may introduce a significant influence on the orientation of the linker in the structure and on the available porosity for adsorption. Indeed, the presence of groups on the benzene ring of the linker introduces a considerable complexity in structure because several positions for the groups and different orientations inside the pores are possible. Thus, several possibilities for the structure need to be considered for a proper assessment of this complexity of the system with simulation methods. In this case, the −CF3 groups need to be in alternating positions in the aromatic ring in lateral neighboring linkers in the same metal cluster node due to steric hindrance. This substitution in the UiO-66 structure (Figure 1) will give a substituted structure that, after a full periodic DFT optimization, maintains the linker orientation, i.e., the dihedral angle between the carboxylate group and the aromatic ring of the linkers is close to 0°, corresponding to a

isotherms (Table S3), with the selectivities being 1.4, 1.0, and 2.5, respectively. Indeed, the UiO-Fum and UiO-66-Br results clearly indicate that reducing the pore size or introducing Br groups do not lead to an improvement in selectivity compared to the parent UiO-66, as can be more easily seen in Figure 8.

Figure 8. (a) Selectivity of adsorption of the binary mixture ethane/ ethylene, with pressure, at 25 °C, for UiO-66, UiO-Fum, UiO-66-Br, and UiO-66-2CF3. (b) Isothermal (25 °C), isobaric (500 kPa) xy diagrams of ethane/ethylene mixture adsorption on UiO-66, UiOFum, UiO-66-Br, and UiO-66-2CF3. y is the molar fraction of ethylene on the gas phase, and x is the molar fraction of ethylene in the adsorbed phase.

Contrarily, the presence of CF3 groups in the structure leads to a remarkable improvement, since the UiO-66-2CF3 presents a slightly better selectivity over a larger pressure range than the parent UiO-66 (Figure 8). This is a clear advantage over some reported MOFs with ethane selectivity that only presents separation capabilities at very low pressures (below atmospheric pressure).14−16,19−22 In fact, for application in bulk separations, it is highly desirable to operate with pressure cycles above atmospheric pressure (100 kPa) and with a

Table 1. Characteristics of the Structure Models for the UiO-66-2CF3 structurea c

initial (no rotation) first attempt second attempt third attempt fourth attemptd fifth attemptd

total energy

accessible volumeb

pore limiting diameterb

maximum pore diameterb

(eV)

(cm3 g−1)

(Å)

(Å)

−4277.49 −4279.63 −4279.14 −4279.62 −4275.65 −4264.04

0.221 0.217 0.218 0.217 0.221 0.220

1.30 1.92 2.04 1.92 1.45 2.20

7.34 4.85 4.72 4.85 6.70 5.08

pore systemb 3D 3D 2D (slits) 3D 2D (slits) 2D (slits)

a

The procedure to obtain these structures is described in the text. bCalculated with Poreblazer55 using helium as a probe molecule. cStructure obtained by reoptimization with the DFT method employed in the present work of a structure from a previous work.56 dOnly fluorine atomic positions were optimized. H


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ACS Applied Materials & Interfaces local energy minimum (Figure S16). However, this structure does not give a proper description of the ethane and ethylene adsorption (results are discussed below) and we investigated other possible structural arrangements of the linkers, namely, with a different dihedral angle (Figure S17). Three new structures were considered starting with a rotation (initial angle of 25° before optimization) of only the linkers at the vertical planes in Figure 1 (first attempt), rotation of all linkers (second attempt), and rotation of only the linkers at the horizontal planes in Figure 1 (third attempt), which was followed by full DFT optimization of the atomic positions (i.e., cell parameters were kept fixed for avoiding another variable) to obtain new local minima. At the end of the optimization, the structures obtained in the first and third attempts are very similar with only one plane of linkers with dihedral angles close to 0° and the other two planes with dihedral angles between about 40 and 50° (Figure S18). These also correspond to the most stable structures under vacuum, according to the calculations (Table 1). Two other structures were considered by imposing dihedral angles of 10 and 25° in all linkers and optimizing only the −CF3 groups (fourth and fifth attempts, respectively), to better highlight the influence of the rotation of the linkers in the structure properties. All of these structures are included in the SI. As can be observed from Table 1, the orientation of the linkers in the structure leads to changes not only in the exposed atoms but also in the volume available for adsorption, i.e., the maximum pore size and the type of pore in the system are altered. It is important to note that the rotation of the linkers decreases the maximum pore size (this pore corresponds to the tetrahedral cages because the −CF3 are pointing to the octahedral cages when the rotation is close to 0°; cf. Figure S16), but increases the pore limiting diameter (Table 1), as can be better seen by comparing values of the initial structure (about 0° angle) to those of the four (10° angle) and fifth (25° angle) attempts. The most important point is to observe that these possible structural variations due to the linker rotation have a considerable influence on the predicted ethane and ethylene adsorption isotherms by GCMC (Figure 9). If no rotation is considered on the linkers (initial model), then the adsorption amount is clearly overestimated, particularly at lower pressures. On the contrary, the estimated amounts from the first and third attempt model structures are predicting the adsorbed amounts at low pressures, but significantly failing below at higher pressures. According to the DFT calculations, these last model structures are the most probable ones in vacuum and, from this perspective, it is expected that they describe better the UiO-66-2CF3 structure at low pressures. Indeed, the computed pore volume from the first and third attempt model structures (Table 1) are closer to the experimental pore volume from the nitrogen adsorption at −196 °C (Table S1). When a systematic rotation of the linkers is imposed in the model structures, the effect of the angle of rotation on the ethane and ethylene adsorption may be better evaluated. The rotation of 10° in the linkers (fourth attempt model) corresponds to an increase in the adsorption amounts relatively to the structure with no rotation. However, in the case of ethane, this increase is significantly higher than in the case of ethylene. When rotation continues to increase, the adsorbed amounts decrease, as can be ascertained by the results calculated with a linker rotation angle of 25° (fifth attempt). These results indicate that the variation of the amount adsorbed is not proportional to the rotation angle of the linkers

Figure 9. Experimental and simulated adsorption isotherms, at 25 °C, of (a) ethane and (b) ethylene, on UiO-66-2CF3. Simulated adsorption isotherms were obtained using the model structures with different dihedral angles on the linkers (see text for description).

(it passes through a maximum) and that this dependence is not the same for both gases. The rotation of the linkers starting from the initial structure (no rotation) corresponds to a volume increase of the octahedral cage and to a volume decrease of the tetrahedral cage (the maximum pore diameter in Table 1 corresponds to the tetrahedral cavity). This means that when the linkers are not rotated, the adsorption on the octahedral cavity must be significantly reduced, which is confirmed by the GCMC snapshots (Figure S19). On the other hand, the model structures with high rotation of the linker (which correspond to more stable structures in vacuum) have a significantly less open volume on the tetrahedral cages and much more open volume on the octahedral cages (Table 1), which will result in adsorption essentially on the octahedral cages (Figure S19). The confirmation of such behavior can be observed by comparing the GCMC density snapshots on the models with 10 and 25° rotation (fourth and fifth attempt models, Figure S19). The overall comparison of the GCMC results with the experimental ones suggests that the ethane and ethylene adsorption on the UiO-66-2CF3 occurs with a variation on the rotation of the linkers with increasing pressure of the gases (from 0 up to 1000 kPa). At low pressures, the linkers have, in the present notation, high rotation angles, and, with pressure, the rotation decreases, opening the volume of the tetrahedral cages and closing the volume on the octahedral ones. Such rotation does not have the same effect on the amounts adsorbed of each gas and seems to favor more the ethane, justifying thus the selectivity for this gas. A simultaneous modeling of the adsorption and rotation of the linkers by the GCMC methodology is possible but requires the definition of the rotation potential force field, which is not available for this structure. Nevertheless, the several model structures and corresponding GCMC results presented are sufficient to disclose the complex physical behavior of this I


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Our work shows that the improvement of the selectivity for ethane in Zr-based MOFs can be achieved more effectively by introducing bulky CF3 groups in the UiO-66 structure than by increasing the aromaticity of the linker. Indeed, the presence of these groups seems to produce a structure with pores whose volume depends on the pressure, which makes this structure more selective to ethane than to ethylene.

system that explain the ethane selectivity. Results obtained at 45 °C confirm the preference of ethane adsorption on UiO-662CF3 (Figure S20a). The isosteric heats of adsorption (Figure S20b) demonstrate that the interaction of ethane with the UiO-66-2CF3 structure is considerably stronger than that displayed by ethylene. This significant difference is, most probably, due to the establishment of stronger interactions between the ethane molecule with the CF3 groups, combined with a different accessibility of this adsorbate to the tetrahedral and octahedral cages, as discussed above. Finally, we probed to see if this selectivity for the paraffinic compound exhibited by UiO-66-2CF3 was extended to another very relevant industrial separation, the propane/propylene separation. The adsorption of these two gases on this material (Figure S21a) showed very little difference in the adsorbed amounts. Only at very low pressures, a slight preference for the adsorption of propane results (inset in Figure S21a), which in the end translates to a nonselective adsorption in almost the entire pressure range (Figure S21b). This different behavior relatively to the ethane/ethylene adsorption may be due to the bigger size of the propane and propylene molecules and to the smaller relative differences between them. Another less straightforward reason may be related to the influence of the adsorption of these gases on the rotation of the linkers discussed above, which in the case of propane and propylene may not be so different among them.

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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/acsami.9b07115.

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4. CONCLUSIONS Our investigations with two possible topologies for the Zr MOFs, with three-dimensional tetrahedral and octahedral pore arrangements (UiO-66 type) and with one-dimensional triangular pore channels (MIL-140 type), demonstrate that the three-dimensional pores of the UiO-type structure are more interesting to give the desired ethane selectivity. Because of the considerable increase in the pore size of the UiO-type structure, when the naphthalene dicarboxylate and the benzene dicarboxylate linkers are compared, the effect of the aromaticity increase in the former linker is not effective in our conditions (high pressure) and, thus, UiO-66 turns to have a better selectivity than UiO-NDC. Decreasing the pore size of the UiO-66-type structure, by using fumarate linker (UiO-Fum MOF) or by introducing Br substitutions on the aromatic ring (UiO-66-Br MOF), does not reveal improvements in the selectivity for ethane. We have found that the best strategy is to introduce two perfluoro groups on the aromatic ring (UiO-66-2CF3 MOF), with a concomitant improvement of the selectivity over a wide pressure range, with selectivity values close to 2.5, which is significantly better than those demonstrated by the other UiOtype structures. The computational effort to explain the adsorption behavior revealed a complex adsorption mechanism, where the dihedral rotation of the linkers relatively to the metal nodes has a strong influence. This rotation seems to favor the ethane adsorption because it interacts more strongly with the linkers than ethylene. This behavior translates to a selectivity of about 2.5 at pressures up to 1000 kPa, complemented by a significant increase in adsorbed amounts above the atmospheric pressure, that was not observed with other reported ethane-selective MOFs. Thus, UiO-66-2CF3 has a unique combination of selectivity and working capacity (about 1 mmol g−1 between 100 and 1000 kPa) that makes it very interesting for cyclic separations with pressure swing.

Detailed description of the materials synthesis, powder X-ray diffraction patterns, thermogravimetric curves, nitrogen adsorption isotherms, and textural characterization of the MOFs; detailed description of the computational methods and results obtained for the atomic charges; ethane and ethylene adsorption isotherms at 45 °C; additional density maps and structural plots (PDF) Crystallographic data (CIF files) of the structure models optimized by DFT and used in the GCMC calculations (ZIP) Videos of the model structures of UiO-66, MIL-140A, and density maps of ethane. UiO-66-1, Mil140b-2-1, Mil140b-1 (MP4) (MP4) (MP4) (MPG)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

João Pires: 0000-0002-8374-558X José R. B. Gomes: 0000-0001-5993-1385 Christian Serre: 0000-0003-3040-2564 Moisés L. Pinto: 0000-0003-3061-9632 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financed by Fundaçaõ para a Ciência e a Tecnologia (FCT) through project IF/00993/2012/CP0172/ CT0013. This work was developed in the scope of the Projects POCI-01-0145-FEDER-007679 | UID/CTM/50011/2019 (CICECO), UID/MULTI/00612/2019 (CQB), UID/ECI/ 04028/2019 (CERENA), and Programa Investigador FCT, financed by national funds through the FCT/MEC and cofinanced by FEDER under the PT2020 Partnership Agreement. K.D. and C.S. also acknowledge the Investissement d’avenir Labex Patrima ANR-10-LABX-0094-01. Dr. Emmanuel Magnier (ILV, Versailles) is thanked for his assistance in the synthesis of organic linkers. Dr. Nathalie Steunou and Dr. Eddy Dumas (ILV, Versailles) are thanked for their assistance in the synthesis and characterization of some MOFs. M. Bordonhos (CERENA, IST, Univ. Lisboa) is thanked for her assistance in the measurement of propylene and propane adsorption.

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