Trianglamine-Based Supramolecular Organic Framework with

Oct 8, 2018 - Here we introduce for the first time a metal-free trianglamine-based supramolecular organic framework, T-SOF-1, with permanent intrinsic...
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Trianglamine-Based Supramolecular Organic Framework with Permanent Intrinsic Porosity and Tunable Selectivity Arnaud Chaix, Georges Mouchaham, Aleksander Shkurenko, Phuong Hoang, Basem Moosa, Prashant M. Bhatt, Karim Adil, Khaled N. Salama, Mohamed Eddaoudi, and Niveen M Khashab J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08770 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Journal of the American Chemical Society

Trianglamine-Based Supramolecular Organic Framework with Permanent Intrinsic Porosity and Tunable Selectivity Arnaud Chaix,†# Georges Mouchaham,‡# Aleksander Shkurenko,‡ Phuong Hoang,† Basem Moosa,† Prashant M. Bhatt,‡ Karim Adil, ‡ Khaled N. Salama,§ Mohamed Eddaoudi,‡ and Niveen M. Khashab*† †

Smart Hybrid Materials (SHMs) Laboratory, Advanced Membranes and Porous Materials Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. ‡

Functional Materials Design, Discovery and Development Research Group (FMD3), Advanced Membranes and Porous Materials Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. §

Sensors Lab, Electrical Engineering Program, Computer, Electrical and Mathematical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.

ABSTRACT: Here we introduce for the first time a metal-free trianglamine-based supramolecular organic framework, T-SOF-1, with permanent intrinsic porosity and high affinity to CO2. The capability of tuning the pore aperture dimensions is also demonstrated by molecular guest encapsulation to afford excellent CO2/CH4 separation for natural gas upgrading.

porous network compared to other spontaneously selfassembled frameworks. Although the size and shape of the macrocycle offers the prospective to dictate generally limits the pore-aperture size and the overall guest selectivity, the controlled packing of these host molecules in the solid state remains of prime importance.

Porous organic materials have attracted significant attention due to their foreseen potential in various key applications 1-2 3 pertaining to selective gas separation, catalysis, and drug 4 delivery. Such systems include crystalline covalent organic 5,6 frameworks (COFs), amorphous porous organic polymers 7 8-10 (POPs), and supramolecular organic frameworks (SOFs). Nevertheless, attainment of crystalline SOFs with inherent permanent porosity remains a challenge, in contrast to extended porous networks, because conventionally molecules tend to closely pack and maximize attractive intermolecular contacts. Appropriately, there is an ongoing quest to prepare molecular crystals with defined lattice voids or open channels as this class of materials should have superior solution 11 processability compared to its extended counterpart. Macrocycle-based supramolecular organic frameworks (MSOFs) with intrinsic porosity, based on the assembly of ca12 13 14 lixarenes, bisurea, cucurbiturils, and more recently pil15-18 larenes, have emerged as an interesting class of crystalline porous materials. Principally, M-SOFs encompass macrocyclic building blocks held together by non-covalent interactions such as hydrogen bonding, π-π stacking, and van der 19 Waals interactions. Prominently, the unique structural features of the macrocyclic building blocks, namely their cyclic shape (intrinsic pore) associated with the ability for their decoration with multiple functional groups, offers great potential for the design and construction of SOFs with a 8 variety of architectures. They are also more prone to afford a

Figure 1. The crystal structure of T-SOF-1 showing the hexagonal arrangement of trianglamines and their tubular packing endowing triangular channels running along c-axis. H, C, N, O and Cl atoms are illustrated in white, grey, blue, red and green, respectively. H···Cl interactions are plotted as green dashed lines. Schiff base macrocycles, such as trianglimines, offer a modular library of building blocks with respect to size, geometry, 20-22 Nevertheless, these imine macrocycles and functionality. are based on dynamic covalent bond formation and thus suffer from poor stability. Efforts to preserve these structures focused mainly on reduction to amines, which increased the

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overall flexibility of the framework and consequently led to the loss of porosity. Trianglamines showing host-guest properties were isolated so far by “tieing” the amines followed by 23,24 crystallization with alkyl alcohol guests. Nonetheless, they can be readily prepared, purified, and scaled up at a relatively low cost. Herein, we report for the first time a SOF based on trianglamine, namely T-SOF-1, with permanent porosity and application in gas capture and selective separation (Figure 1). Moreover, a new strategy of molecular guest (iodine/iodide) encapsulation was investigated to tune the pore size of SOFs and consequently improve the selectivity of this platform. Trianglamine (T) was prepared by reacting (R,R)-1,2diaminocyclohexane (20 mmol) with terephthaladehyde (20 mmol) in methanol in the presence of trimethylamine (50 mmol) followed by reduction with sodium borohydride 1 13 (Scheme S1). H and C NMR confirm the structure and purity of the prepared trianglamine (Figure S1). Survey of the open literature revealed that present trianglamines crystalline structures are influenced in most cases by the deployed crystallizing solvent, affecting both the conformation and the 3D packing of the macrocycles. Distinctly in some cases, the solvent molecules are tightly entrapped between the macrocycles rendering their complete removal a delicate step for 25 the activation of the host solid. Thus, in our quest to explore the potential of trianglamines in M-SOFs, we sought to direct the supramolecular packing by transforming the organic macrocycle to a salt as a fully protonated trianglamine 25 maintains C3 molecular symmetry in the solid state. Crystallizing the trianglamine in the presence of HCl promoted tubular packing with chloride counter ions as directing26 agents. Concretely, crystals of T-SOF-1 were obtained, in a quantitative yield, by slow diffusion of acetone vapors (during 72 hours) into an aqueous solution of trianglamine acidified with excess of HCl (see ESI for detailed procedure). Single crystal X-Ray diffraction (SCXRD) study revealed that T-SOF-1 (formulated (H6T)Cl6·16.5(H2O) see ESI for elemental analysis) crystallizes in a trigonal chiral space group P321 with cell parameters a = b = 22.378(2) Å and c = 7.1487(7) Å (Figure 1). Close examination of the crystal structure revealed that trianglamines are packed in a hexagonal fashion parallel to the crystallographic plane (001) while they are perfectly aligned along the c axis, generating nanotubular channels delimitated by the intrinsic triangular cavity of the macrocycle (section of about 6.3 Å). The amino groups of trianglamines were all protonated and Cl ions are disposed on both sides of each macrocycle facing the corresponding + H-atom and interacting by means of N–H ···Cl ionic Hbonds which, together with and C–H···Cl , consolidates the nanotubular packing. Adjacent macrocycles are interacting with each other through weaker supramolecular interactions (i.e. C–H···Cl and π-π stacking). Moreover, the hexagonal packing of the nanotube afforded the structure of T-SOF-1 with smaller channels of ca. 4.5 Å. The resultant onedimensional channels are fully occupied by solvent molecules (i.e. water). The phase purity of the bulk as well as the stability of the supramolecular framework in air (after removing crystals form solvent) has been assessed by powder X-ray diffraction (PXRD) experiments (Figure S2). Prior assessing the permanent porosity of T-SOF-1, activation of the framework was done by first washing the crystals with acetonitrile (ACN) then soaking them in the same sol-

vent overnight. The N2 adsorption isotherm at 77 K was collected on the solvent-exchanged crystals of T-SOF-1 activated under dynamic vacuum at 323 K for 12 hours (Figure S3). It revealed a fully reversible isotherm characteristic of a porous material with permanent microporosity (Figure 2a). The apparent Brunauer-Emmett-Teller surface area (SBET) was 2 -1 estimated to be 170 m .g with a calculated micropore vol3 -1 ume at P/P0 ≈ 0.8 of ca. 0.11 cm ·g , positioning T-SOF-1 among the highest porous macrocycle-based frameworks 12, 14 previously reported.

Figure 2. (a) N2 sorption at 77 K for T-SOF-1 (blue) and TPowder (black). (b) Comparison of the N2 (squares) and CO2 (circles) sorption at 298 K for T-SOF-1 (in blue) and TPowder (in black). Filled and empty marks refer to adsorption and desorption cycles, respectively. Due to the high ratio of secondary ammonium groups (i.e. + 6 –NH2 groups per macrocycle) that are decorating the accessible inner pore of the nanotubular channels and confined in a relatively tight circumference (aperture size of about 6 Å), T-SOF-1 offers a highly favorable environment for CO2 capture. In this regard, CO2 gas adsorption studies were performed and revealed that T-SOF-1 is capable of 3 -1 adsorbing up to ca. 23 cm .g of CO2 at 298K and 760 Torr (Figure 2b). More interestingly, the CO2 adsorption isotherm is relatively steep at relatively low pressures with nearly half

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Journal of the American Chemical Society of the uptake at 760 Torr achieved at only 100 Torr, suggesting the high affinity of this supramolecular framework towards CO2 at ambient conditions. The calculated isosteric heat of adsorption (Qst), using multiple CO2 adsorption isotherms at 268, 278, 288, and 298 K (Figure S4), is relatively -1 constant around 30 kJ·mol and comparable to values ob27 tained for porous materials containing ammonium groups. It is to be noted that performing gas experiments studies on the trianglamine T in its amorphous state, generated in the absence of HCl, showed a nominal N2 adsorption at 77 K and 3 -1 relatively low CO2 uptake at 298 K (ca. 5 cm .g at 760 Torr, Figure 2).

(Figure 3). Although the quality of the collected data was not sufficient to determine the nature of the iodo-species entrapped within the channels, Raman studies revealed the presence of polyiodide (I3 and I5 ) rather than iodine (Figure S7). Moreover, X-Ray Photoelectron Spectroscopy (XPS) was performed and the results were in agreement with the Raman data pertaining to the presence of I3 and I5 species (Figure S8). The ratio of I3 to I5 could not be determined accurately since it was observed that I5 tends to transform to I3 under laser irradiation (Figure S9). FT-IR and TGA spectra showed no clear difference between T-SOF-1 and I@T-SOF-1 (Figure S10). Moreover, the loading amount of iodine was -1 estimated to be ca. 163 mg·g based on UV-Vis absorption study on iodine solutions before and after soaking with TSOF-1 for 3 hours (Figure S11). Energy-Dispersive X-Ray Spectroscopy (EDX) systematically showed the presence of Cl and I elements with Cl/I atomic ratio of ca. 70/30 (Figure S12). Furthermore, elemental analysis for C, H, N and Cl elements was carried out on I@T-SOF-1, degassed under vacuum at 50 °C during 12 hours, proposing the following chemical formula [(H6T)Cl6(I)1.5]·0.5H2O.This chemical formula is also in good agreement with the amount of loaded iodine/iodide estimated from UV-Vis dosing. This suggests that the charge balance is maintained by the formation of a charge transfer complex between the trianglamines and the polyiodides similarly to 29 what has been already observed in former studies.

Figure 3. (a) Preparation of I@T-SOF-1. (b) Crystal packing along c-axis of T-SOF-1 (left) and I@T-SOF-1 (right). Color code: C, grey; H, white; N, blue; O, red; Cl, green and I, orange. Guest encapsulation in macrocycle-based organic crystals have been recently reported where iodine can be readily 28 adsorbed by pillar[6]arene crystals. Exploring host-guest chemistry in frameworks with permanent porosity is an appealing approach toward adjusting the pore size and in turn control/improve the selectivity in separation applications. To this end and as a proof of concept, iodine was selected as a guest molecule due to its relatively smaller size (< 4.5 Å), in contrast to the tubular channels of T-SOF-1 (ca. 4.5 and 6.3 Å), and it’s prospective not to completely hinder the access to the triangular channels (Figure S5a). Appropriately, fresh T-SOF-1 crystals were soaked in a solution of iodine in -1 ACN (1 mg·mL ) for 3 hours, yielding I@T-SOF-1. Over the course of the soaking process, colorless crystals turned swiftly (in less than 1 minute) to brownish-orange before changing to dark brown (Figure S5b and S5c). Although the macroscopic shape of the crystals was well preserved after iodine loading, microscopically they showed many cracks/defects (Figure S6) which, together with their high sensitivity to the X-ray beam, made SCXRD studies and structure elucidation very challenging. Pertinently, we were able to solve the crystal structure of I@T-SOF-1 revealing, as expected, similar packing to T-SOF-1 with large electronic charge density, associated to I atoms, in both small and bigger channels

Figure 4. Isotherms measured for T-SOF-1 (in blue) and I@T-SOF-1 (in orange): CO2 (circles), N2 (squares) and CH4 (triangles) at 298 K. Filled and empty marks refer to adsorption and desorption cycles, respectively. I@T-SOF-1 was subjected to gas sorption studies revealing that it is not porous to N2 at 77 K and 298 K (Figure S13). However, I@T-SOF-1 showed almost the same total uptake 3 -1 (ca. 20 cm ⋅g ) of CO2 at 298 K and 760 Torr, as compared to T-SOF-1 (Figure S13). This can be explained by the smaller kinetic diameter of CO2 (3.30 Å) compared to N2 (d = 3.64 Å), allowing its diffusion in the relatively narrowed poreapertures of I@T-SOF-1. The iodide content did not change during the multiple drying cycles as verified by CO2 isotherms measured at different temperatures (Figure S14). Similar to T-SOF-1, half of the uptake for I@T-SOF-1 is ob-

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served at relatively low pressure (ca. 100 Torr). The Qst calculated, using CO2 adsorption isotherms at 268, 278, 288, and 298 K (Figure S15), showed slightly higher values compared -1 to that of T-SOF-1, starting at 33 kJ·mol at low loading. The potential of I@T-SOF-1 for CO2/N2 separation was corroborated by column adsorption breakthrough experiment (Figure S16) carried out with a diluted gas mixture CO2/N2 10/90 3 -1 under a flow rate of 10 cm ⋅min , attesting to the selectivity of the adsorbent for CO2 with a retention time for CO2 of 6 minutes per gram (Figure S17). It is to be noted that the CO2 3 -1 uptake (ca. 6 cm ⋅g ) from breakthrough experiment is in good agreement with the uptake form the CO2 isotherm at 76 Torr (0.1 bar). T-SOF-1 and I@T-SOF-1 were then tested for natural gas upgrading (i.e. separation of CO2 from CH4). Compared to N2, CH4 shows higher condensability although it has a slightly bigger kinetic diameter (3.8 Å). Indeed, single gas sorption experiments conducted at 298 K on T-SOF-1 and I@T-SOF-1 (activated at 323 K under dynamic vacuum) revealed that CH4 is adsorbed only in the case of T-SOF-1 (up to ca. 11 3 -1 cm ·g at 760 Torr), while only negligible uptake was recorded for I@T-SOF-1 (Figure 4). This attests to the prospective of I@T-SOF-1 for the natural gas upgrading, CO2/CH4 separation, but most importantly to the great potential offered by our strategy in selectively tuning the sorption properties at a molecular level. In summary, we present a new class of macrocycle-based supramolecular organic frameworks by metal-free assembly of trianglamines, which showed both permanent porosity and notable CO2 adsorption capabilities. A molecular guest encapsulation strategy is also investigated for tuning the pore size and the overall selectivity of the system. This proved highly applicable for CO2/CH4 separation were only CO2 is adsorbed at ambient conditions. The presented results will pave the way to the design and construction of SOF materials with intrinsic porosity by carefully choosing macrocyclic building blocks with the right structural features and functionalities. Future work will investigate the potential of this material for the separation of hydrocarbon isomers with close physical properties such as branched versus linear paraffins.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed synthesis and characterization of T, T-SOF-1 and I@T-SOF-1 including NMR, MS, FT-IR UV-Vis, Raman spectroscopy, XPS, TGA, PXRD, SCXRD and gas sorption analysis.

AUTHOR INFORMATION Corresponding Author * [email protected] ORCID Niveen M. Khashab: 0000-0003-2728-0666 Georges Mouchaham: 0000-0001-8696-9733

Author Contributions #

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT Authors acknowledge Dr. Y. Belmabkhout for the helpful discussions and Dr. O. El-Tall for technical assistance.

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