A Contoured Aliphatic Pore Environment in a Cubic Frame-work with

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CUB-5: A Contoured Aliphatic Pore Environment in a Cubic Frame-work with Potential for Benzene Separation Applications. Lauren K. Macreadie, Emily J. Mensforth, Ravichandar Babarao, Kristina Konstas, Shane G. Telfer, Cara M. Doherty, John Tsanaktsidis, Stuart R. Batten, and Matthew R Hill J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13639 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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Figure 1. (a) Illustration of 1,4-cdc and 1,4-bdc. Cubic cavity of (b) CUB-5 and (c) MOF-5. Cross section of (d) CUB-5 and (e) MOF-5 demonstrating the proton location of the ligand relative to the void spaces, modelled at room temperature. 89x120mm (96 x 96 DPI)


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Figure 2. (a) N2 isotherms at 77 K for CUB-5 and MOF-5 demonstrate, unambiguously, the surface area differences between the MOFs. Inset shows the pore size distribution of CUB-5 derived from the N2 adsorption isotherm. The smaller pore aperture at 6.5 Å being confirmed with simulations. (b) CO2 and CH4 adsorption isotherms of CUB-5 recorded at 273 K and 298 K. 89x129mm (96 x 96 DPI)



Normalized hydrocarbon capacity uptake of CUB-5 and MOF-5 at 0.1 kPa. Adsorption isotherms on CUB-5 and MOF-5 for b) linear and branched C6 hydrocarbon isomers and c) benzene and n-hexane at 298 K. Filled and open symbols represent CUB-5 and MOF-5 respectively. 100x189mm (96 x 96 DPI)


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CUB-5: A Contoured Aliphatic Pore Environment in a Cubic Framework with Potential for Benzene Separation Applications. Lauren K. Macreadie,*1 Emily J. Mensforth,1,2 Ravichandar Babarao,1,3 Kristina Konstas,1 Shane G. Telfer,4 Cara M. Doherty,1 John Tsanaktsidis,1 Stuart R. Batten*2 and Matthew R. Hill*1,5 1. CSIRO, Normanby Road, Clayton 3168, Victoria, Australia 2. School of Chemistry, Monash University, Clayton 3800, Victoria, Australia 3. School of Science, RMIT University, Melbourne 3001, Victoria, Australia 4. MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Palmerston North 4442, New Zealand 5. Department of Chemical Engineering, Monash University, Clayton 3800, Victoria, Australia Supporting Information Placeholder ABSTRACT: One prominent aspect of metal organic frameworks (MOFs) is the ability to tune the size, shape and chemical characteristics of their pores. MOF-5, with its open cubic connectivity of Zn4O clusters joined by two dimensional, terephthalate linkers, is the archetypal example: both functionalized and elongated linkers produce isoreticular frameworks that define pores with new shapes and chemical environments. The recent scalable synthesis of cubane-1,4dicarboxylic acid (1,4-H2cdc) allows the first opportunity to explore new applications for the most prospective reticular architectures. Herein we describe the use of 1,4-H2cdc to construct [Zn4O(1,4-cdc)3], referred to as CUB-5. Isoreticular with MOF-5, CUB-5 adopts a cubic architecture but features aliphatic, rather than aromatic, pore surfaces. Methine units point directly into the pores, delivering new and unconventional adsorption locations. Our results show that CUB-5 is capable of selectively adsorbing high amounts of benzene at low partial pressures, promising for future investigations into the industrial separation of benzene from gasoline using aliphatic MOF materials. These results present an effective design strategy for the generation of new MOF materials with aliphatic pore environments and properties previously unattainable in conventional frameworks.

The ability to develop crystalline, porous materials has inspired the thriving field of metal organic framework (MOF) chemistry.1,2 The archetypal MOF-5 with its Zn4O secondary building units (SBUs) connected together by benzene-1,4-dicarboxylic acid (1,4H2bdc) launched the reality of this concept.3 The numerous coordination modes available from the carboxylate functionality, and the ease of modulating the phenyl ring, created a rich foundation for the development of new MOFs.4 Conversely, restriction to solely phenyl interactions within adsorbates represents a possible limitation and reduced variation in the pore chemical environment of the materials.

(a)

(b)

(c)

(d)

(e)

Figure 1. (a) Illustration of 1,4-H2cdc and 1,4-H2bdc. Cubic cavity of (b) CUB-5 and (d) MOF-5. Cross section of (c) CUB5 and (e) MOF-5 demonstrating the proton location of the ligand relative to the void spaces, modelled at room temperature. 1



Separation of hydrocarbons using low energy processes is a key area from an industrial standpoint where the strategic design of the MOF pore chemical environment can avoid energy expensive separations based on changes of phase.5-6 Major areas of interest involve the separation of hydrocarbons from crude oil, paraffin from olefins and derivatives of benzene from one another.7 Naturally, the similar physical properties and molecular dimensions of such compounds creates a challenge in this endeavor.8 Of critical importance is the separation of benzene from gasoline, where removal of this low concentration impurity could potentially be performed by introduction of a highly selective adsorbent.9-10 Employing MOFs for the separation of industrially important hydrocarbon mixtures has been experimentally investigated through purposeful pore functionalization or generation of pore flexibility within a framework.5,11,6, 12-14 Atypical chemical environments in MOF pores has been achieved to a limited extent by substitution of the aromatic linker from the parent MOF with an aliphatic linker.15-21 Investigations pertaining to the replacement of 1,4-H2bdc with flexible aliphatic linkers, such as adipic acid or cyclohexane-1,4-dicarboxylate, in UiO-66 and the MIL-53 series resulted in a significant change in structural properties within the MOF, despite retention of the original topology, or destruction of topology and production of an entirely new MOF.15-16, 22 Employment of the rigid, aliphatic linker bicyclo[2.2.2]octane-1,4-dicarboxylate to form an aliphatic MOF material, analogous to MOF-5, also resulted in a topological different framework.23-24 Cubane-1,4-dicarboxylic acid (1,4H2cdc) is a rigid, aliphatic linker that contains eight carbon atoms arranged in a near perfect cubic arrangement, with six hydrogen atoms and two carboxylate groups at the vertices (Figure 1a).25 Previously believed to be unstable, the molecule possesses significant kinetic stability and can withstand high temperatures, due to the non-concerted thermolysis pathway needed to initiate molecular decomposition.26 Importantly, the similar lengths of 1,4-H2cdc and 1,4-H2bdc, in terms of the metrical spacing of metal clusters, provides significant scope for use of 1,4-H2cdc in MOF synthesis (Figure 1a). The capacity for 1,4-H2cdc to form coordination polymers is shown in the 2D sheet [Zn(1,4cdc)2(dmf)]n however a MOF employing 1,4-H2cdc as the linker is yet to be reported.27 Furthermore, the non-planar nature and higher steric bulk of 1,4-H2cdc may be exploited to avoid interpenetration in MOFs with smaller pore diameters. This communication investigates the use of 1,4-H2cdc, a molecule recently accessible at requisite scale,25 to create an aliphatic MOF analogous to MOF-5 by employing the reticular synthetic route.28 Here we explore the synthesis, structural properties and vapor sorption of the compound [Zn4O(1,4-cdc)3]n, herein referred to as CUB-5 (Figure 1b). The solvothermal reaction of Zn(NO3)2⸱6H2O and 1,4-H2cdc in DMF at 100 °C afforded large colorless, cubic crystals of [Zn4O(1,4-cdc)3]n (CUB-5) in 72 % yield. Interestingly, noninterpenetrated CUB-5 formed under atmospheric conditions, alternate to the moisture-free synthetic conditions required to synthesize non-interpenetrated MOF-5.29-31 CUB-5 crystallizes in the space group Pm-3m and possesses the same primitive cubic topology as MOF-5 (Figure 1b and 1c and Section S1 for further crystallographic discussion). The distance between the oxygen atoms of adjacent Zn4O clusters in CUB-5 is 12.7 Å, slightly shorter than the corresponding distance in MOF-5 (12.9 Å). This variance is consistent with the difference in length of 1,4-H2cdc and 1,4-H2bdc, approximately 5.68 Å and 5.73 Å, respectively. The thermal stability of CUB-5 was analyzed using thermogravimetric analysis (Figure S3) and moisture stability studied through exposing the MOF to either 33 % or 75 % humidity, and subsequent structural analysis using powder X-ray

Figure 2. (a) N2 isotherms at 77 K for CUB-5 and MOF-5 demonstrate, unambiguously, the surface area differences. Inset shows the pore size distribution of CUB-5 derived from the N2 adsorption isotherm. The smaller pore aperture at 6.5 Å being confirmed with simulations. (b) CO2 and CH4 adsorption isotherms of CUB-5 recorded at 273 K and 298 K. diffraction (PXRD) (Figure S2). The PXRD analysis revealed moisture sensitivity of CUB-5 is similar to MOF-5, showing degradation of the MOF with exposure to moisture (Figure S2). To analyze the properties of CUB-5 and develop a decent comparison to non-interpenetrated MOF-5, gas sorption analysis, theoretical simulations and Positron Annihilation Lifetime Spectroscopy (PALS) were employed. Adsorption isotherms at 77 K revealed a Brunauer–Emmett–Teller (BET) and Langmuir surface areas of 2614 m2 g-1 and 3007 m2 g-1 respectively. H2 adsorption of 200 cm3g-1 was reached at an equivalent temperature and pressure (Figure S4 and S6). Interestingly, the zero-coverage enthalpy of adsorption of H2 on CUB-5 calculated from the adsorption isotherms was found to be 7.5 kJ mol-1, higher than the literature reports of MOF-5 of 3.8 to 5.3 kJ mol-1. The higher enthalpy of adsorption demonstrated in CUB-5 could be attributed to the presence of smaller pores in the MOF when compared with MOF-5 and the change in the chemical make-up of the pore enviroment.32 The surface area of CUB-5 is at the lower range of the reported BET values of non-interpenetrated MOF-5, 2296 – 3800 m2 g-1 in line with expectations.31, 33-35 The total pore volumes of CUB-5 and MOF-5 are 0.92 cm3 g-1 and 1.20 cm3 g-1, respectively. To further compare the textural properties of MOF-5 and CUB-5, theoretical calculations were employed. Theoretical N2 adsorption isotherms lead to a BET


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Journal of the American Chemical Society surface area for CUB-5 and MOF-5 of 3107 m2 g-1 and 3625 m2 g1 respectively (Figure 2a). Interestingly, further calculations on CUB-5 illustrate that although interpenetration is possible within CUB-5 it is highly improbable and leads to a non-porous material due to the steric bulk associated with 1,4-cdc. The pore size distribution (PSD) of CUB-5, calculated from N2 adsorption data at 77 K indicates two pore sizes, of ca. 0.7 nm and 1.2 nm compared to MOF-5 at 1.4 nm (Figure 2a).3 Theoretical calculations, using the CUB-5 model, further support the bimodal distribution with pores of 0.7 nm and 1.15 nm in size reported which can be attributed to the tight corners formed between the linkers and the Zn4O SBUs (Figure S10). Furthermore, PALS (a)

(b)

(c)

Figure 3. a) Normalized hydrocarbon capacity uptake of CUB-5 and MOF-5 at 0.1 kPa. Adsorption isotherms on CUB-5 and MOF-5 for b) linear and branched C6 hydrocarbon isomers and c) benzene at 298 K.

detected the bimodal distribution in the pore architecture (Table S3). The relative number of pores, as indicated by the associated intensity values, also correlate with the N2 distribution with 6 % and 27 % respectively with the larger pore being an average size due to positrons sampling the cubic cell, the windows and channels.36 Additional gas adsorption measurements were performed using CO2 and CH4 (Figure 2b) and their respective enthalpy of adsorption values calculated using the Van't Hoff method (Figure S8 and S9). CUB-5 adsorbs 16 cm3 g-1 of CH4 at 273 K, with a zero-coverage enthalpy of adsorption of 12.7 kJ mol-1. For CO2 adsorption in CUB-5, 62 cm3g-1 is adsorbed at 273 K with zerocoverage enthalpy of adsorption of 19.3 kJ mol-1, much higher than MOF-5.37-38 Owing to its contoured pore surface and high affinity sites, CUB-5 outperforms MOF-5 at low pressure, despite its lower surface area, while the higher pore volume dominates the adsorption process at higher pressures. The adsorption properties of CUB-5 and MOF-5 were explored, due to their analogous topology, in the presence of varying hydrocarbons using single component gas and vapor sorption experiments measured at 298 K (Figures 3, S7 and S11). A broad range of hydrocarbons were investigated including short chain aliphatic molecules (ethane and ethylene), cyclic molecules (benzene and cyclohexane) and C6 hydrocarbons (n-hexane, 2,3dimethylbutane and 2-methylpentane) to provide a thorough understanding of effects of the new pore shape and surface chemistry within CUB-5. The notable differences in adsorption behavior between CUB-5 and MOF-5 occur at low partial pressures, where a steep incline of adsorption occurs. Interestingly, the adsorption isotherms for hydrocarbons are Sshaped: a short induction period is followed by a sharp increase in adsorbed vapor due to the cooperative effect between guest species. To effectively compare the vapor adsorption by CUB-5 and MOF5, the adsorption capacities were normalized by surface area of the frameworks at pressures of 0.1 kPa (Figure 3a) and 1.0 kPa (Figure S12, Table S1 and S2). Of immediate interest is the rapid adsorption of benzene within CUB-5 at low pressures compared with MOF-5 (Figure 3b and Figure 3c). DFT-D3 calculations show binding energies of −49.5 kJ mol⁻1 and −42.5 kJ mol⁻1 of benzene in CUB-5 and MOF-5 respectively, revealing a substantial difference in the binding energy of approximately 6 – 7 kJ mol⁻1. The strong and numerous interactions of benzene with CUB-5 that lead to the stark change in bond enthalpies were revealed by simulations, as shown in Figure S13, with close interactions below 3.3 Å highlighted between the π and CH components of benzene and the methine moieties of cubane. The high preference for benzene adsorption within CUB-5 at these low partial pressures can be exploited in the aforementioned need to separate benzene from gasoline. MOFs possessing this new pore shape and chemistry could potentially be implemented in this industry for the separation at the gaseous stage of production. Aliphatic hydrocarbon vapors also adsorb at a lower pressure within CUB-5 compared with MOF-5. Interestingly, both frameworks show a preference for adsorption of n-hexane at low pressures over branched C6 isomers. DFT-D3 calculations show a binding energy of n-hexane in CUB-5 and MOF-5 to be −59.9 kJ mol⁻1 and −53.5 kJ mol⁻1, respectively, again revealing a large difference in binding energies. This is not altogether unexpected, as the flexibility of n-hexane and isomers of low steric bulk allow the molecule to maximize its interactions with the pore internal surface.39-40 The adsorption isotherms of ethane and ethylene also show a high gas uptake within CUB-5 at low pressures. Interestingly, the ethane adsorption isotherm has a convex shape,



highlighting the importance of adsorbate-adsorbate interactions during vapor sorption experiments (Figure S7). The stark difference in vapor adsorption between the topologically analogous frameworks, CUB-5 and MOF-5, highlights the importance of pore shape during the adsorption process and further investigations are underway to effectively understand the separation capabilities of CUB-5 in reference to other materials selective for benzene adsorption41-43. Additionally, the interesting differences in vapor sorption capabilities at low pressures between linear and branched hydrocarbons provides a foundation for further separation investigations.7, 44-45 The physical structure of 1,4-cdc allows for greater protrusion of methine into the void space of CUB-5, allowing for increased interactions with the vapor guests, thus inducing strong interactions at low pressures. Relative to the surface area, CUB-5 can adsorb these hydrocarbons in far greater quantities that its aromatic counterpart MOF-5. In summary, CUB-5, isoreticular with MOF-5, is formed through an isoreticular synthetic route employing 1,4-cdc, an aliphatic linker unique due to its inherit rigidity and capability to emulate 1,4-bdc. Of significant interest is the effect the CUB-5 pore shape has on hydrocarbon adsorption. The higher number of methine moieties protruding into the CUB-5 pores generate increased locations for interactions between hydrocarbon guests and the pore surface, leading to increased adsorption at lower pressures compared with MOF-5. The adsorption of benzene at low partial pressures provides a promising landscape for future investigations into the separation of benzene from gasoline using aliphatic MOF materials. This work lays the foundation for exciting advances in the chemistry and application of porous materials. This isoreticular method of adopting a 3D ligand could open up new fields of porous materials to complement those of existing frameworks.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Supporting Information includes detailed experimental procedures, single crystal X-ray diffraction data, powder X-ray diffraction data, thermogravimetric data and gas adsorption isotherms (PDF).

AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected] *[email protected] *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT MRH Acknowledges FT 1301000345. This project was supported by a Catalyst Fund grant from the Ministry of Business, Science and Innovation of New Zealand (MAUX1609).

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A Contoured Aliphatic Pore Environment in a Cubic Frame-work with

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