This document is the Accepted Manuscript version of a Published Work that appeared in final form in Crystal Growth and Design, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see: https://dx.doi.org/10.1021/acs.cgd.6b01054. The influence of the amide groups in the CO2/N2 selectivity of a series of isoreticular, interpenetrated metal-organic frameworks Vahid Safarifard,†,‡ Sabina Rodríguez-Hermida,† Vincent Guillerm,† Inhar Imaz,† Mina Bigdeli,‡ Alireza A. Tehrani,‡ Jordi Juanhuix,§ Ali Morsali, ‡* Mirian E. Casco,┴ Joaquín Silvestre-Albero,┴ Enrique V. Ramos-Fernandez┴ and Daniel Maspoch†,#* †Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, 08193 Barcelona, Spain. ‡ Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Islamic Republic of Iran. § ALBA Synchrotron, 08290 Cerdanyola del Vallès, Barcelona, Spain. ┴ Laboratorio de Materiales Avanzados, Departamento de Química Inorgánica-Instituto Universitario de Materiales, Universidad de Alicante, Ctra. San Vicente-Alicante s/n, 03690 San Vicente de Raspeig, Spain. # ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain. Metal-Organic Frameworks, Amide, Adsorption, Separation, Breakthrough ABSTRACT: Here we report the use of a pillaring strategy for the design and synthesis of three novel amide- functionalized metal-organic frameworks (MOFs), TMUs-22/-23/-24, isoreticular to the recently reported imine- functionalized TMU-6 and TMU-21 MOFs. An extensive study of their CO2 sorption properties and selectivity for CO2 over N2, from single gas sorption isotherms to breakthrough measurements, revealed that not only the incorporation of amide groups but also their accessibility is crucial to obtain enhanced CO 2 sorption and CO2/N2 selectivity. Therefore, the MOF with more accessible amide groups (TMU-24) shows a CO2/N2 selectivity value of ca. 10 (as revealed by breakthrough experiments), which is ca. 500 % and 700 % of the selectivity values observed for the other amide- containing (TMU-22 and TMU-23) and imine-containing (TMU-6 and TMU-21) MOFs. intermolecular interactions with CO2.21 Amide groups INTRODUCTION possess two types of donor-acceptor affinity sites: the -NH Microporous metal−organic frameworks (MOFs), moiety that can act as an electron acceptor (Lewis acid), constructed from metal-based nodes and polytopic and the -C=O group that can act as an electron donor organic ligands, are a promising class of porous materials (Lewis base); both allowing to establish NH···OCO and for carbon capture, separation and storage owing to their NC=O···CO2 interactions.22-24 In the last few years, the very high surface areas, tuneable pore sizes and shapes, study of some amide-functionalized MOFs has confirmed and adjustable pore surface functionality.1-3 For these their potential for selectively adsorb CO2.5, 22, 24-30 For specific applications, it is widely accepted that certain example, Chen, Wang et al. have reported that pores chemical functionalities located at the pore surfaces of decorated with acylamide groups in the Cu(II)-based MOFs can increase their CO2 uptake and selectivity.4-10 UTSA-48 MOF promote strong interactions with CO2 and Among these functionalities, _ENREF_6CO2 has been high selectivity for CO2 over CH4.5 Recently, Wang et al. found to bind at unsaturated metal centres via the reported a 3-fold interpenetrated acylamide-MOF that electron rich terminal oxygen atom11-13. Also, CO2 can shows higher selective adsorption of CO2 rather than interact with electron rich Lewis basic groups via electron other gases with a significant isosteric heat.31 Similarly, deficient carbon atom.14 For this purpose, most efforts Zaworotko et al. found that the decoration of a rht-type have been made on the direct and post-synthetic MOF also with acylamide functions can significantly functionalization of MOFs with amine groups.15-18 More enhance the CO2 uptake and the selective adsorption of recently, another approach based on the combined CO2 over N2 in comparison to PCN-61; an isoreticular optimization of pore size and high charge density has analogue with acetylene moieties instead of amide anticipated very promising results.4, 19, 20 groups.24 Remarkably, comparable trends of selectivity have also been observed in other MOFs decorated with Besides the use of amine groups,_ENREF_20 the acylamide25-28 and oxamide22 groups as well as in some intrinsic characteristics of the amide groups make them also very attractive to establish cooperative Figure 1. 2D layers formed by the association between Zn(II) ions and oba linkers are further pillared by amide/imine- functionalized dipyridyl-based ligands yielding to 3D-interpenetrated porous pcu-MOFs. recent porous covalent organic frameworks also showing pores functionalized with imine groups [called containing acylamide groups.29, 30 TMU-6 and TMU-21 (TMU = Tarbiat Modares Recent studies reported by us have shown that a series University)].33, 34 Herein, inspired by this approach and of porous interpenetrated MOFs can be designed by the potential of the amide groups for enhancing the pillaring 2D layers, comprising Zn(II) ions and the V- selectivity towards CO2, we report the synthesis of three shaped dicarboxylate ligand 4,4ʹ -oxybisbenzoic acid newly acylamide-containing MOFs. The selected pillar (H2oba), with linear dipyridyl-based ligands (Fig. 1).32-34 ligands are N,Nʹ -bis(4-pyridinyl)-terephthalamide (bpta), Following this approach, we found that the pores of the N,Nʹ -(1,4-phenylene)diisonicotinamide (bpfb), and N,Nʹ - synthesized MOFs can be easily functionalized with the bis-(4-pyridylformamide)-1,5-naphthalenediamine (bpfn), two groups incorporated in the pillar ligands since the Figures 1 and S1, which lead to the assembly of: pores of these MOFs, which run perpendicular to the [Zn2(oba)2(bpta)]·(DMF)3 (TMU-22; where DMF is N,N- {Zn(oba)} layers, are also delimited by these ligands.29, 30, 35 dimethylformamide); [Zn2(oba)2(bpfb)]·(DMF)5 (TMU- For example, the use of the pillar ligands N,Nʹ -bis-(4- 23), in which the acylamide group is inverted in pyridylmethylene)-1,4-benzenediamine (bpmb) and its comparison to TMU-22; and [Zn2(oba)2(bpfn)]·(DMF)2 naphthalene analogue N,Nʹ -bis-(4-pyridylmethylene)-1,5- (TMU-24), in which the use of the naphthalene analogue naphthalenediamine (bpmn) recently allowed the also induces that the acylamide groups are more directed formation of two isoreticular 3-fold interpenetrated MOFs towards the pores than in TMU-22 and TMU-23 (Figure 2 2). Importantly, the resulting 3-fold interpenetrated Crystallography TMUs-22/-23/-24 are isoreticular to TMU-6 and TMU-21. Crystallographic data for TMU-22 and TMU-23 were The similarities (structure and stability) and differences collected at 100 K at XALOC beamline at ALBA (functional group and accessibility) of these MOFs allow synchrotron36 ( = 0.79474 Å and 0.82653 Å, respectively). studying the influence of the amide and the imine groups Data were indexed, integrated and scaled using the XDS37 on their N2 and CO2 sorption properties as well as on their and IMOSFLM38 programs. Absorption correction was not selective sorption of CO2 over N2. More specifically, this applied. SCXRD for TMU-24 was collected at 293 K on a latter selectivity was evaluated by performing kinetics and Bruker AXS SMART Apex diffractometer using graphite breakthrough experiments for a CO2/N2 gas mixture. monochromated Mo-K radiation ( = 0.71073 Å) and were corrected for Lorentz and polarization effects. The EXPERIMENTAL SECTION frames were integrated with the Bruker SAINT39 software Materials and Characterization package. Absorption corrections were applied using the The commercially available reagents were used without program SADABS40 giving max./min. transmission factors further purification. Ligands, TMU-6 and TMU-21, were of 1.000/0.270. The structures were solved by direct synthesized following reported procedures.33,34 See methods and subsequently refined by correction of F2 Supporting Information and Figure S1 for synthetic against all reflections, using SHELXS201341 and details. Elemental analyses were collected on a CHNS SHELXL201342 within the WinGX package.43 All non- Thermo Scientific Flash 2000 elemental analyser. Powder hydrogen atoms were refined with anisotropic thermal X-ray diffraction (PXRD) measurements were collected on parameters by full-matrix least-squares calculations on F2 a Philips X’pert diffractometer with monochromatic Cu- using the program SHELXL2013. Hydrogen atoms were Kα radiation (λCu = 1.5406 Å). Thermogravimetric analysis inserted at calculated positions and constrained with (TGA) were carried out in a Perkin Elmer Pyris 1 under N 2 isotropic thermal parameters. In TMU-24, the carbonyl atmosphere and heating rate of 10 °C/min. Fourier group of the N,N-ligand is distorted in two positions with transform infra-red (FT-IR) spectra were recorded on a 55 % and 45 % of occupancy. The structures contain some Bruker Tensor 27FTIR spectrometer equipped with a disorder molecules. Attempts to adequately model of Golden Gate diamond attenuated total reflection (ATR) disordered DMF molecules (40 electrons) were cell, in absorption mode at room temperature. unsatisfactory; therefore the PLATON/SQUEEZE44 routine has been applied to mask out the disordered electron density. The residual electron density was General synthesis of TMU-22, TMU-23 and TMU-24 assigned to 3, 5 and 1 molecules of DMF in TMU-22 (509 A mixture of Zn(NO3)2.6H2O (0.297 g, 1 mmol), H2oba electrons per unit cell), TMU-23 (876 electrons located (0.258 g, 1 mmol), and the corresponding amide ligand per unit cell) and in TMU-24 (384 electrons per unit cell). (0.5 mmol) and DMF (50 mL) was sonicated until all Crystallographic and refinement data, and the main bond solids were uniformly dispersed (≈ 3 minutes) and divided distances and angles are listed in Table 1 and Tables S1-3. into 7 glass vials. The vials were heated at 120 °C for 3 days and then cooled to room temperature. Colourless (TMU- 22) and red-brown (TMU-23 and TMU-24) crystals were Table 1. Crystal and structure refinement data. obtained as pure phases, washed by DMF, and dried at room temperature. Compound TMU-22 TMU-23 TMU-24 Data for TMU-22. Yield: 44 %. FT-IR (cm−1): 1691 (m), Emp. C55H51N7O15Z C61H65N9O17Z C28H23N3O7 1595 (vs), 1504 (s), 1395 (vs), 1331 (m), 1298 (m), 1238 (vs), formula n2 n2 Zn 1160 (vs), 1098 (m), 777 (m), 659 (m), 524 (m). EA on Formula 1180.76 1326.96 578.86 solvent free sample: calcd. (%) for C46H30N4O12Zn2: C, weight 57.46; H, 3.15; N, 5.83; found: C, 57.50; H, 3.11; N, 5.74. Crystal Monoclinic Monoclinic Monoclinic Data for TMU-23. Yield: 36 %. FT-IR (cm−1): 1673 (vs), system 1597 (vs), 1540 (m), 1510 (m), 1399 (vs), 1309 (m), 1220 (vs), 1159 (s), 1088 (m), 1067 (m), 1014 (m), 801 (m), 658 (m), Space P21/n P21/c C2/c 524 (m). EA on solvent free sample: calcd. (%) for group C46H30N4O12Zn2: C, 57.46; H, 3.15; N, 5.83; found: C, 57.00; CCDC ref. 1480227 1480228 1480229 H, 3.10; N, 5.65. Unit cell dimensions Data for TMU-24. Yield: 38 %. IR (cm−1): 1667 (vs), 1595 a (Å) 14.680(4) 15.110(3) 14.881(4) (vs), 1570 (m), 1505 (s), 1386 (vs), 1235(s), 1158 (vs), 1089 (m), 1065 (m), 1015 (m), 878 (m), 659 (m), 522 (m). EA on b (Å) 24.780(5) 24.110(5) 24.993(6) solvent free sample: calcd. (%) for C50H32N4O12Zn2: C, c (Å) 16.270(5) 15.910(5) 16.013(4) 59.37; H, 3.19; N, 5.54; found: C, 57.10; H, 3.12; N, 5.01.  (deg) 113.27(5) 112.10(6) 111.422(5) 3 V (Å3) 5437.0(2) 5370.2(2) 5544.0(2) The CO2/N2 selectivities (σ) of samples were evaluated by the following equation: Z 4 4 8 F (000) 2440 2760 2384  qCO2    θ range (°) 1.850-33.822 1.626-33.681 1.630-  pCO  28.396   2   q N2  Ind. Refln. 11071 (0.1528) 12947 (0.1164) 6462   (Rint) (0.0637)  pN  Goodness- 1.331 1.090 1.018  2 of-fit on F2 where qi is the adsorption capacity of i component, pi is Final R R1 = 0.1402 R1 = 0.0939 R1 = 0.0717 the partial pressure of i component. indices wR2 = 0.3805 wR2 = 0.2444 wR2 = The amount adsorbed for each gas was calculated by [I > 2σ(I)] 0.1819 the integration of the breakthrough curves. For this purpose, breakthrough experiments with H2 (not adsorbed in these materials), have been performed using the same bed and the same flows. Then, the H2 Low-Pressure Sorption Measurements breakthrough curve has been used to calculate the time Volumetric N2 and CO2 sorption isotherms were zero and more importantly to consider the shape of the collected at 77 K (N2) and 203 K, 258 K, 273 K, 288 K and breakthrough in the integration. The amount of gas 298 K (CO2) using an ASAP 2020 HD (Micromeritics). adsorbed was the different between the shape of the CO2 Temperature was controlled by using a liquid nitrogen or N2 breakthrough and the H2 breakthrough. Thus, the bath (77 K) or a Lauda Proline RP 890 chiller (203 K – 298 dispersion and the very small pressure drop can be K). The total pore volumes (Vt) were calculated at P/P0 = considered. 0.95 (N2) and at 760 torr (CO2). Kinetics of adsorption Kinetic evaluation of the sorbents was performed at 298 K in a home-made glass manometric adsorption equipment using pure gas components (CO2 and N2). This equipment is constituted by a manifold of a well-known volume (to define the initial gas dose), a sample holder containing the adsorbent and a pressure transducer. Prior to the adsorption experiment, the samples were degassed under vacuum at the activation temperature used for single gas sorption experiments. The initial pressure in the manifold was defined at 500 mbar for all adsorbates. Once equilibrated, the manifold was expanded to the sample holder, pressure readings being recorded every second and lasting for ten minutes. Moreover, the sample holder was immersed in a temperature-controlled water bath at 298 K to minimize any heat effect due to the adsorption process itself. Breakthrough measurements Breakthrough curve experiments for different mixture of gases were carried out using a column. The samples powders were packed in the middle part of the column. Here, the samples masses used were 0.30 g. The flow rates of all gases were controlled by mass flow controllers. Before the measurement, the samples were activated at the desired temperature for 2 h using a He total flow of 50 mL.min-1. The gas stream from the outlet of the column was analysed online with a mass spectrometer. 4 Figure 2. View of the pore channels of a) TMU-6, b) TMU-22, c) TMU-23, d) TMU-21 and e) TMU-24, highlighting the acylamide groups. Colour code: N (green), O (purple), the more accessible O atoms in TMU-24 (lavender). Results and discussion Here it is very important to note that, even though Synthesis and crystal structures TMUs-22/-23/-24 are isoreticular, their acylamide groups do not show the same orientation on the pore surfaces TMUs-22/-23/-24 were synthesized following the same (Figure 2b,c,e) as well as do not participate in the same pillaring strategy reported for TMU-6 and TMU-21.33,34 number of supramolecular interactions between the Solvothermal reactions between Zn(NO3)2.6H2O, H2oba interpenetrated networks (Tables S4-6). In TMU-23, all and the correspondent pillar ligand in DMF at 120 oC for 3 donor –N-H and acceptor –C=O groups of the two days produced prismatic crystals of TMUs-22/-23/-24 acylamide groups per pillar ligand establish H-bonds and suitable for single-crystal X-ray diffraction analyses. The N-H···π interactions with the other two networks forming simulated (derived from the single crystal structures) and the 3-fold interpenetrated structure. In TMU-22, each experimental powder X-ray diffraction (PXRD) patterns donor –NH moiety of the two acylamide groups per pillar were consistent (Figure S2), confirming that all ligand establishes H-bonds with carboxylate groups of the synthesized MOFs can be obtained as pure phases; as also other networks, whereas the two acceptor -C=O groups corroborated by Fourier transformed infrared do not participate in H-bonds. In TMU-24, only one of spectroscopy and elemental analysis (performed on the the two acylamide groups forms H-bonds. Interestingly, activated samples, vide infra). the other acylamide group is pointing towards the pores TMU-22 and TMU-23 crystallize in the monoclinic so that it does not participate in direct H-bonds with the P21/n and P21/c space groups, respectively (Table 1). They other networks (Figure 2e). Altogether makes this are neutral 3-fold interpenetrated MOFs formed by acylamide group easily accessible to potential interactions {Zn(oba)} layers connected through the N,Nʹ -pillar with CO2 guest molecules. Contrariwise, even though all ligands. As in TMU-6, their basic unit is a dinuclear zinc the other H-bonded acylamide groups of TMUs-22/-23/- cluster, in which both Zn(II) centres are penta- 24 are located on the surfaces of the pores, they are not coordinated to four carboxylate O atoms from three fully directly pointing towards the channels, and therefore they deprotonated oba ligands and to one N atom from the are in principle less accessible to guest molecules. N,Nʹ -donor ligand (Figure 1). The geometry around the metal centres can be described as a distorted square pyramid, with Addison parameter45 τ values in the range Topology of 0.35-0.49. In these structures, each nonlinear Topological analysis performed with TOPOS47 revealed dicarboxylate oba ligand is coordinated to three Zn(II) that all the studied MOFs show the same 3-fold centres forming the {Zn(oba)} layers along the ab plane. interpenetrated, 6-connected net with pcu topology Because of the dinuclear nature of the metal cluster, these (Figure 1). The pcu topology is ubiquitous in MOFs, layers are formed by two sheets of parallel oba ligands including the iconic MOF-5.48 Here, it is worth to that form π-π stacking interactions between them (Figure mention that the pcu net is edge-transitive with cubic 1; Tables S4, S5). These {Zn(oba)} layers are then pillared symmetry and Pm-3m space group. However, in the by the linear acylamide-functionalized ligands, extending present case, the overall symmetry of TMUs-22/-23/-24 the structure in three dimensions. It is important to decreased to monoclinic (Table 1). This symmetry mention here that, due to the conformational flexibility of decrease is attributed to the fact that the points of both oba and bpta ligands, TMU-22 is a supramolecular extension of the Zn(II) dimer do not match with the conformational isomer of a reported MOF.31 Although vertices of the perfect octahedron required to produce both compounds are 3-fold interpenetrated, they still highly symmetrical pcu-MOFs (Figure 1). In addition, the possess apparent 1D pore channels (≈ 7.0 × 6.5 Å and ≈ 6.8 use of a pillaring strategy with mixed ligands (including × 6.4 Å -including van der Waals (vdW) radii- for TMU-22 the flexible oba and pillar N,Nʹ -ligands) also forces the and TMU-23, respectively) running along the <101> symmetry to differ from that of MOF-5. direction and noticeable free space (33 % for TMU-22 and 32 % for TMU-23), comparable to the values found for Thermal stability TMU-6 (≈ 8.1 × 7.4 Å; and 33 % free space)46 (Figures 1 and S3-5). The channels are filled with disordered guest The TGA diagrams of all as-synthesized materials solvent molecules, squeezed due to thermal disorder. revealed a first weight loss of 16.08 % (room temperature (r.t.)-300 °C) for TMU-6, 13.71 % (80-325 °C) for TMU-21, TMU-24 crystallizes in the monoclinic C2/c space group 9.93 % (100-240 °C) for TMU-22, 15.16 % (r.t.-300 °C) for (Table 1), showing a crystal structure isoreticular to those TMU-23, and 15.46 % (r.t.-325 °C) for TMU-24. In all of TMUs-6/-21/-22/-23. Thus, it also consists on a 3-fold cases, this weight loss was attributed to the removal of all interpenetrated network that exhibits 1D pore channels of guest solvent molecules. Decomposition of all materials ≈ 6.7 × 5.5 Å (free space = 30 %) functionalized with starts at around 400 oC, confirming their high thermal acylamide groups. These values are similar to those stability (Figure S8). exhibited by its imine-functionalized TMU-21 analogue, which shows a pore size of ≈ 8.2 × 7.0 Å and a free space of 30 %.46 (Figures S6-7). 5 Activation process TMU-23 ≈0 ≈ 0 / 0.27 0.27 7.2 All compounds were activated using a very similar TMU-24 ≈0 ≈ 0 / 0.22 0.25 6.3 protocol that started with a solvent-exchange step. acalculated at P/P0 = 0.95. Calculated at P = 760 torr. c b TMUs-6/-21 and TMUs-22/-23/-24 were immersed in Calculated with PLATON. acetonitrile and dichloromethane, respectively, for 3 days, during which the solvent was exchanged once daily. Note here that previous stability tests showed that all CO2 sorption at various temperatures and heats of synthesized MOFs were stable in these solvents (Figure adsorption S9) for at least 24 h at r.t. Then, each MOF was evacuated Contrariwise to the adsorption of N2, the CO2 sorption for 12 h under vacuum at 150 °C prior to gas sorption isotherms at various temperatures revealed that all of measurements. It is noteworthy that PXRD diagrams them are porous (Figures 3a and S12a-16a). CO2 uptake at collected after the sorption studies confirm that the 203 K were calculated to be 9.0 mmol.g-1, 4.5 mmol.g-1, 7.2 frameworks retain their crystallinity and remained mmol.g-1, 7.2 mmol.g-1 and 6.3 mmol.g-1 for TMU-6, TMU- unaltered after both processes (Figure S10). These 21, TMU-22, TMU-23 and TMU-24, respectively (Table 2). observations were further corroborated by elemental The observed pore volumes (at 760 torr) were: 0.32 cm3.g-1 analysis and TGA, in which only the weight loses around (TMU-6), 0.16 cm3.g-1 (TMU-21), 0.25 cm3 g-1 (TMU-22), 400 oC corresponding to their decomposition were 0.27 cm3.g-1 (TMU-23) and 0.22 cm3.g-1 (TMU-24). These observed (Figure S8). pore volumes calculated from the CO2 adsorption data are in agreement with the theoretical values extracted from Nitrogen sorption at 77 K the crystal structures. The variations observed can be attributed to small structural changes widely observed in N2 isotherm at 77 K on TMU-22 showed a characteristic such interpenetrated sorbents.35, 55-57 type I behaviour of microporous materials, as expected from its crystal structure (BET area (ABET) = 680 m2.g-1; pore volume (Vt) = 0.28 cm3.g-1). Contrariwise, TMU-21, a 30 TMU-23 and TMU-24 were found to be non-porous materials (negligible N2 uptake). In the case of TMU-6, it CO2 adsorption Qst (kJ.mol ) 25 -1 showed much lower porosity than that expected from its crystal structure (ABET = 150 m2.g-1; Vt = 0.08 cm3.g-1) 20 (Table 2, Figure S11). Thus, all activated compounds, except TMU-22, reveal a “N2-phobic” behavior.49-52 We 15 reasoned that this behaviour could be due to the existence of different structural rearrangements 10 TMU-6 depending on the MOF during sorption or when exposed TMU-21 to cryogenic temperatures and/or under vacuum, 5 TMU-22 TMU-23 reducing/preventing access to the porosity. Indeed, TMU-24 similar temperature dependent behaviours are already 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 well-known in soft materials such as in MIL-53 (Al).53 -1 However, other factors cannot be fully excluded, such as CO2 Loading (mmol.g ) the existence of strong interactions between N2 and the b channel walls at 77 K that hinder diffusion into the 2.0 Ads. material.54 TMU-6 TMU-21 TMU-22 TMU-23 1.5 TMU-24 CO2 Uptake (mmol.g ) -1 Table 2. Summary of BET areas, total pore volumes and CO2 uptakes observed for TMUs-6/-21-24. 1.0 MOF ABET Total Vt Theo CO2 (m2.g- (cm3.g-1) Vt uptakeb 0.5 1) (cm3.g- (N2a / (mmol.g-1) 1 )c (N2, 77 CO2b) (203 K) 0.0 K) 0 100 200 300 400 500 600 700 800 TMU-6 150 0.08 / 0.32 0.22 9.0 Pressure (Torr) TMU-21 ≈0 ≈ 0 / 0.16 0.25 4.5 TMU-22 680 0.28 / 0.25 0.25 7.2 Figure 3. (a) Heats of adsorption of CO2 and (b) CO2 sorption isotherms at 298 K for TMUs-6/-21-24. 6 Isosteric heats of adsorption of CO2 (Qst) were TMU-6 17-14 8.2 1.8 calculated from the isotherms collected at different TMU-21 18-20 8.2 1.4 temperatures (Table 2 and Figures S12-16). The two acylamide-functionalized TMU-24 and TMU-22 exhibited TMU-22 23-24 8.6 2.0 the highest Qst for CO2, from 24 to 26 kJ·mol-1 and 23 to 24 TMU-23 12-18 8.2 1.7 kJ·mol-1, respectively. The different Qst for CO2 between TMU-24 24-26 14.1 10.0 TMU-24, TMU-22, and TMU-23 can be attributable to the different accessibility of their acylamide groups and to the different number of supramolecular interactions in which they are involved (vide supra). 0.8 CO2 0.7 N2 CO2/N2 separation properties Adsorbed amount (mmol.g ) -1 In light of the porosity of the materials for CO2 and 0.6 their apparent low affinity for N2, we further investigated 0.5 their potential for CO2/N2 separation by performing 0.4 kinetics experiments (Figures 4 and S17). As expected, the highest selectivity (σ) for CO2 over N2 was observed for 0.3 TMU-24 (σ = 14.1, Table 3); which is the material 0.2 exhibiting the highest Qst of CO2 and with the most 0.1 accessible acylamide groups pointing towards the porous channels. Overall, the CO2/N2 selectivity of TMU-24 0.0 estimated from kinetics experiment is ca. 70 % higher 0 100 200 300 400 than for the other materials of the family. time (s) These results were further confirmed by breakthrough experiments with CO2/N2 gas mixture (Figures 5 and S18). Figure 4. Adsorption kinetics of CO2 and N2 for TMU-24 at Once again, TMU-24 performed much better than its 298 K (initial manifold pressure 500 mbar). isoreticular variants, with a CO2/N2 selectivity of 10.0 at r.t. for an equimolar CO2/N2 gas mixture (Figure 5). This value represents 700 % of the selectivity found in similar conditions for its imine analogue TMU-21 (σ = 1.4). It also performs better than TMU-22 and TMU-23, the other acylamide containing MOFs with less accessible functional groups. Indeed, the selectivity of TMU-24 is 500 % and 580 % the selectivity of TMU-22 and TMU-23, respectively. Therefore, not only the incorporation of amide functional groups in the frameworks but also their accessibility for the CO2 guest molecules is crucial to enhance the CO2/N2 selectivity. This observation was further confirmed in more realistic gas mixture conditions (CO2:N2 = 1:9), in which TMU-24 was still found to perform well with a selectivity of 4.6 (Figure 5). It is important to remark that the CO2 adsorption capacity found in the breakthrough experiment at r.t. (≈ 0.65 mmol.g-1) for TMU-24 is slightly lower than the one extracted from the isotherm at 500 mbar and r.t. (≈ 0.85 mmol.g-1). This feature can be attributed to a small amount of N2 adsorbed in these conditions (298 K and gas mixture). Table 3 Summary of CO2 separation properties in TMUs-6/-21/-22/-23/-24. MOF Qst of CO2 Selectivity Selectivity (kJ.mol-1) (Kinetics) (Breakthrou gh, CO2:N2 1:1) 7 of the pore shape of TMU-24 using a bulky, central a naphthalene core. This resulted in better accessibility of 0.005 N2 the acylamide groups, leading into a 70 % improvement CO2:N2 1:1 CO2 of CO2/N2 selectivity according to kinetics studies (from 0.004 8.2 to 14.1 for TMU-21 and TMU-24, respectively). Moreover, a dramatic increase of the selectivity was mmol N2or CO2/s 0.003 obtained from breakthrough experiments. The CO2/N2 selectivity for TMU-24 corresponds to ca. 700 % of this 0.002 imine- containing analogue, TMU-21. Moreover, its selectivity is ca. 500 % and 580 % of TMU-22 and TMU- 23, two isoreticular, amide containing MOFs with less 0.001 accessible functional groups. 0.000 ASSOCIATED CONTENT 0 50 100 150 200 250 300 Supporting Information. Synthetic procedure for ligand time (s) synthesis, crystallographic data, PXRD diagrams, TGA plots, gas sorption and separation data. This material is available b 0.020 free of charge via the Internet at http://pubs.acs.org. N2 CO2 CO2:N2 1:9 AUTHOR INFORMATION 0.015 Corresponding Authors mmol N2 or CO2/s * E-mails: daniel.maspoch@icn2.cat, 0.010 morsali_a@modares.ac.ir Author Contributions The manuscript was written through contributions of all 0.005 authors. All authors have given approval to the final version of the manuscript. 0.000 Notes 0 30 60 90 120 150 The authors declare no competing financial interest. time (s) ACKNOWLEDGMENTS Figure 5. CO2/N2 Breakthrough separation curves at 298 K This work was supported by the MINECO-Spain through for TMU-24 with total flow of 5 mL.min-1 for CO2:N2 = 1:1 (a) projects PN MAT2015-65354-C2-1-R, 2014-SGR-80, MAT2013- and 25 mL.min-1 for 1:9 (b). 45008-P and EU FP7 ERC-Co 615954. I.I. and E.V.R.F. thank the MINECO for the RyC fellowship RYC-2010-06530 and RyC-2012-11427 and V.G. is grateful to the Generalitat de Catalunya for a Beatriu de Pinós Fellowship. ICN2 CONCLUSIONS acknowledges the support of the Spanish MINECO through We successfully synthesized three novel, 3-fold the Severo Ochoa Centers of Excellence Program, under Grant SEV-2013-0295. E.V.R.F. and J.S.A. acknowledge the interpenetrated pcu-MOFs containing acylamide Generalitat Valenciana for PROMETEOII/2014/004. Support functional groups (TMU-22, TMU-23 and TMU-24), of this investigation by Tarbiat Modares University is isoreticular to the recently reported imine-containing gratefully acknowledged. MOFs, TMU-6 and TMU-21. The study of their sorption properties revealed that the insertion of amide functional REFERENCES groups in MOFs does not necessarily induce a noticeable (1) Zhou, H. C.; Long, J. R.; Yaghi (Eds.), O. M., Themed issue: improvement of the CO2/N2 separation properties. Metal-Organic Frameworks, Chem. Rev., 2012, 112, 673-1268 Indeed, another aspect that needs to be addressed is the (2) Long, J. R.; Yaghi (Eds.), O. 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Soc. 2013, 135, 11684- 11687. 9 For Table of Contents Use Only The influence of the amide groups in the CO2/N2 selectivity of a series of isoreticular, interpenetrated metal-organic frameworks. Vahid Safarifard, Sabina Rodríguez-Hermida, Vincent Guillerm, Inhar Imaz, Mina Bigdeli, Alireza A. Tehrani, Jordi Juanhuix, Ali Morsali,* Mirian E. Casco, Joaquín Silvestre-Albero, Enrique V. Ramos-Fernandez and Daniel Maspoch* The accessibility of amide functional groups within MOF structures is crucial to obtain enhanced CO 2 sorption and CO2/N2 selectivity. 10