Commensurate CO2 Capture, and Shape Selectivity for HCCH over

Jun 3, 2016 - ... Nanotechnology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand ...... 2011, 255, 1791– 1823 DOI: 10.1016/j.ccr.2011.0...
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Commensurate CO2 Capture, and Shape Selectivity for HCCH over H2CCH2, in Zigzag Channels of a Robust CuI(CN)(L) Metal−Organic Framework Reece G. Miller,† Peter D. Southon,‡ Cameron J. Kepert,*,‡ and Sally Brooker*,† †

Department of Chemistry and MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand ‡ School of Chemistry, The University of Sydney, Camperdown, New South Wales 2006, Australia S Supporting Information *

ABSTRACT: A novel copper(I) metal−organic framework (MOF), {[CuI2(py-pzpypz)2(μ-CN)2]·MeCN}n (1·MeCN), with an unusual topology is shown to be robust, retaining crystallinity during desolvation to give 1, which has also been structurally characterized [py-pzpypz is 4-(4-pyridyl)-2,5-dipyrazylpyridine)]. Zigzag-shaped channels, which in 1·MeCN were occupied by disordered MeCN molecules, run along the c axis of 1, resulting in a significant solventaccessible void space (9.6% of the unit cell volume). These tight zigzags, bordered by (CuICN)n chains, make 1 an ideal candidate for investigations into shape-based selectivity. MOF 1 shows a moderate enthalpy of adsorption for binding CO2 (−32 kJ mol−1 at moderate loadings), which results in a good selectivity for CO2 over N2 of 4.8:1 under real-world operating conditions of a 15:85 CO2/N2 mixture at 1 bar. Furthermore, 1 was investigated for shape-based selectivity of small hydrocarbons, revealing preferential uptake of linear acetylene gas over ethylene and methane, partially due to kinetic trapping of the guests with larger kinetic diameters.

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or krypton.35 An important advantage of using shape-based selectivity, by MOFs with novel channel topologies, is an intrinsic lowering of the enthalpic penalty that must be paid in order to remove the guest and regenerate the adsorbent. Such MOFs, with highly constrained internal structural features, are also of interest because of the potential for kinetic, rather than thermodynamic, selectivity. Kinetic trapping of guests, much like that seen for membrane separations,36 is a very interesting means of separation because it facilitates high guest selectivity with a low enthalpy of adsorption. This, in turn, allows for a substantial reduction, relative to thermodynamic separations, in the energetic cost of regeneration.30−33 Herein we describe a robust new cyano-bridged copper MOF, {[Cu I 2 (py-pzpypz) 2 (μ-CN) 2 ]·MeCN} n (1·MeCN, where MeCN = acetonitrile and CN = cyanide; Figure 1), with zigzag channels that (after desolvation to 1) facilitate commensurate CO2 binding as well as shape selectivity for linear acetylene over ethylene and methane.

INTRODUCTION The synthesis of metal−organic frameworks (MOFs) has been a hot topic in recent years because of a variety of potential applications for this rapidly evolving class of materials.1−9 Potential MOF applications include catalysis,2,10−13 sensing,6,9,14−17 gas storage,8,18−20 and separations of chiral enantiomers21,22 or mixtures of gases.5,7,23−28 The last of these is probably the most prominent because it is driven by environmental concerns over increasing levels of atmospheric carbon dioxide (CO2).23,26,29 A key advantage that MOFs have over other materials, such as activated carbon and zeolites, is that a wide range of topologies can be accessed by variation of the ligand design and metal-ion geometry.4,23 Despite this, to date, the foremost strategy for developing selectivity for one gas over another has been through functionalization of the internal surface of the MOF with groups that interact with the guest.23,26 Nevertheless, MOFs with novel topologies continue to be of interest because they may be able to carry out particularly difficult separations, preferably without strongly interacting with the guest molecules.24,30−33 This has been clearly demonstrated in some recent high-profile examples, including the separation of hexanes by MOF [FeIII2(BDP)3]n (where BDP = benzene-1,4dipyrazolate), which has triangle-shaped channels running through the lattice,34 and noble-gas separations by MOF pores, which are fully commensurate with xenon but not argon © XXXX American Chemical Society

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RESULTS AND DISCUSSION Synthesis of 1·MeCN. Surprisingly, the 1:1 solvothermal reaction of copper(II) acetate with py-pzpypz in MeCN/H2O resulted in an acetate-free copper(I) complex. Orange needleReceived: April 3, 2016

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DOI: 10.1021/acs.inorgchem.6b00813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Supporting Information for full discussion) cyanide ion. Cu(1) is further coordinated to pyrazine [N(2) in Figure 1], whereas Cu(2) is further coordinated to pyridine [N(7B)]. The application of both 2-fold axes completes the approximately tetrahedral CN3-donor coordination of both copper(I) centers, with Cu(1) bound by two NPz, one CN, and one NC, whereas Cu(2) is bound by two NPy, one CN, and one NC. This also leads to the formation of 1D chains of cyanidebridged copper(I) centers, which zigzag down the crystallographic c axis (Figures 1, 2, and S1). Each copper(I) center makes a short Cu−cyanide bond (average 1.923 Å) and a longer Cu−NAz bond (average 2.114 Å). Figure 1. Solvothermal synthesis of MOF 1·MeCN from py-pzpypz, copper(II) acetate, and KCN in MeCN/H2O [the Cu(CN)n chain is highlighted in pale blue].

shaped crystals of the copper(I) cyanide MOF 1·MeCN were obtained, in up to 34% yield. The 1,2-bridging cyanide ions presumably result from copper-catalyzed cleavage of the C−C bond in MeCN, coupled to a reduction from copper(II) to copper(I), as has been documented in other systems37,38 including, recently, another 3D framework.39 The yields of 1·MeCN were both low and highly variable (5−34%). The low solubility of the free ligand in MeCN led to it also precipitating on cooling (identified by NMR spectroscopy), so it had to be removed from 1·MeCN by washing with chloroform. Increasing the reaction time, to as long as 7 days, made little difference to the yield. Hence, given the highly interesting topology of 1·MeCN (see the next section), a more reliable and higher-yielding method of preparation was sought. Not surprisingly, in order to get a significant improvement in yield, it was necessary to add potassium cyanide (KCN; Caution! Highly toxic!) to the mixture. The best yields (78%) were obtained by the reaction of a 1:1:1 mixture of copper(II) tetrafluoroborate, KCN, and py-pzpypz in 20:1 MeCN/H2O under solvothermal conditions (160 °C) for 5 days. Given the clear risks associated with the use of KCN in wet solvents, extreme care was taken in carrying out, and working up, the reaction. The stainless steel reaction vessel was loaded and sealed, and later opened, in a fumehood on “boost”, and the scale was never increased beyond 100 μmol. This required larger samples to be prepared in several batches, but as the synthesis itself is not labor intensive, the preparation of sufficient sample for analysis was readily achieved. Structures of MOFs 1·MeCN and 1. The 3D MOF 1· MeCN crystallizes in the orthorhombic space group Pbcn (Figure 1 and Table 1) with two half-occupancy copper(I) centers, one py-pzpypz ligand, one 1,2-bridging CN anion, and a half-occupancy MeCN in the asymmetric unit. The two independent, approximately tetrahedral, copper(I) centers lie on separate 2-fold rotation axes running parallel to the b axis and are bridged by the end-for-end disordered (see the

Figure 2. Two ways that the disordered, half-occupancy MeCN molecules (shown as space filling; in both cases packing head-to-tail) can pack in 1·MeCN, without making impossibly close contacts, within the zigzag channels formed along the c axis by the 1D chains of end-to-end, cyanide-bridged copper(I) centers. They completely fill the channels.

Surprisingly, the py-pzpypz ligand only coordinates via two of the six nitrogen donors available (Figures 1 and S1) and bridges two adjacent zigzagging copper(I) cyanide chains. Specifically, N(7) of one of the pyridine rings is bound to a copper(I) center, Cu(2), on one chain, and one of the nitrogen atoms of one of the pyrazine rings, N(2), is bound to the other copper(I) center, Cu(1D), on the adjacent chain (Figures 1, 2, and S1). Interestingly, this unusual coordination leaves the N(4) pyridine ring, the N(5)/N(6) pyrazine ring, and N(3) of the N(2)/N(3) pyrazine ring uncoordinated. The N(5)/N(6) pyrazine ring is free to rotate, but this terpyridine-like unit remains relatively planar (Figure S1). The observed conformation avoids energetically unfavorable nitrogen-lone-pair repulsion between the N(4) and N(5) nitrogen atoms, and upon coordination through N(2) rather than N(3), the resulting N(2)/N(3) pyrazine ring conformation also avoids potential nitrogen-lone-pair repulsion, between N(3) and N(4). Furthermore, a myriad of weak, nonclassical, hydrogen-bonding interactions occur between the four “spare” azine/diazine nitrogen atoms and the electron-deficient C−H protons on adjacent rings (Table S2). The zigzag-shaped channels, running down the crystallographic c axis of the structure in the as-synthesized material 1· MeCN, contain the disordered acetonitrile molecules, so the channels are most clearly observed using those as guides to the eye (Figures 2 and S2). The topology of the channels is defined by the zigzagging 1D chains of [CuI(CN)]n that are generated

Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Copper(I) Centers in 1·MeCN and 1

Cu−CN Cu−NAz CN−Cu−CN NAz−Cu−NAz Td distortion40

Cu(1)/Cu(2) in 1·MeCN

Cu(1)/Cu(2) in 1

1.922(2)/1.924(2) 2.122(2)/2.106(2) 122.89(13)/122.35(13) 98.32(10)/94.51(10) τ4 = 0.91/0.86

1.921(4)/1.917(4) 2.190(3)/2.150(3) 127.9(2)/125.6(2) 97.0(2)/92.7(2) τ4 = 0.88/0.85 B

DOI: 10.1021/acs.inorgchem.6b00813 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry by the two different 2-fold rotation axes, which are parallel to the b axis and on which the cyanide-bridged Cu(1) and Cu(2) centers lie. The bridging py-pzpypz ligands form the channel walls and separate them from one another (Figures 2 and S2). A single crystal of 1·MeCN was desolvated by heating it at 90 °C for 2 h in the dinitrogen (N2) stream of the diffractometer. The resulting crystal, of 1, was cooled to 60 °C and structurally characterized (Figure 3 and Table 1). No physical changes to

Figure 4. Comparison of the PXRD data, over the 2θ range of 5−30°, for the “as-prepared” sample of 1·2H2O (green), the sample of 1· 2H2O after gravimetric gas adsorption isotherms (black), and that calculated by Mercury 3.3 from the single-crystal X-ray diffraction structure of 1·MeCN (red). For the full PXRD pattern (5° ≤ 2θ ≤ 50°), see Figure S5.

Figure 3. Guest-accessible void volume in 1, as viewed down the crystallographic a axis. The external void surface is shown in gray and the internal in green. The void volume was calculated using Mercury 3.3.

the crystal were apparent with this treatment (Figure S3). Desolvated MOF 1 is isostructural with solvated 1·MeCN, with only slight changes to the cell axes and volume (Figure 3 and Table S3) but now has a significant guest-accessible void volume (9.6% of the unit cell volume). The zigzag channels are empty and remain completely isolated from one another (Figures 2 and 3), such that there is only a single means of guest access into the MOF interior, the channel openings. In addition, only guests that are capable of navigating the approximately 125° corners of the zigzag will be able to diffuse into the MOF. This led us to hypothesize that 1 might demonstrate shape-based selectivity. Stability of 1 to Temperature and Gas Exchange. The stability of 1 to high temperatures and to gas exchange was probed by powder X-ray diffraction (PXRD; Figure 4). There was no change in the PXRD pattern after the gravimetric isotherm measurements (see later), confirming that this MOF is robust to gas sorption/desorption. The thermal stability was investigated by variable-temperature PXRD. The sample was heated under vacuum from 30 to 100 °C, with a ramp rate of 5 °C min−1, and PXRD patterns were recorded every 20 °C (Figure S7). No structural changes were observed. However, upon further heating to 120 °C, a small amount of a second crystalline phase is observed, and by 140 °C, the entire sample had undergone this phase transition (Figure S6). Upon cooling 1 back to 85 °C, the hightemperature phase remains, indicating that the phase transition is irreversible (Figure S7). The high-temperature phase is stable from 140 °C until around 220 °C, at which point a loss of crystallinity is observed. Clearly, the high-temperature phase has good thermal stability because it does not decompose until T > 220 °C. Gas Exchange and Selectivity. The adsorption of CO2, N2 (Figure 5), and CH4 (Figure S9) into 1 was measured at 25 °C (Table 2). MOF 1 can reversibly take up CO2 and small

Figure 5. Gravimetric adsorption isotherms for CO2 (red) and N2 (blue) into 1, at 25 °C, in the range of 0−10 bar.

Table 2. Gravimetric Adsorption Data for CO2 and N2 into MOF 1 at 25 °C, at 1 and 10 bar, and the Calculated Uptake for a Standard 15:85 “Flue Gas” Mixture CO2 (mmol g−1) N2 (mmol g−1) CO2 (mol/mol)a N2 (mol/mol)a selectivity43 a

1 bar

10 bar

15:85 CO2/N2, 1 bar

0.82 0.07 0.33 0.03 11.7

1.09 0.35 0.44 0.14 3.1

0.29 0.06 0.12 0.02 4.8

Moles of guest per moles of copper(I).

amounts of N2. Adsorption is rapid, with full equilibration within 5−10 min per point and no hysteresis in the isotherm (Figure 5). On the other hand, the uptake of CH4 was much slower, with equilibration requiring 1−2 h, resulting in a small hysteresis (Figure S9). This is likely due to the slightly larger diameter of CH4 (calculated quantum-mechanical diameter: 4.046 Å) relative to CO2 (3.469 Å) and N2 (3.578 Å).41,42 C

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trapping of ethylene, like that seen for methane, was also anticipated to contribute to the selectivity for acetylene over ethylene. Note that recent quantum-mechanical calculations have shown that the kinetic diameters of these gases are a reasonable approximation of a pore size into which they can fit.42 As for the CO2 and N2 measurements, the adsorption of C2H2 and C2H4 into 1 was measured at 25 °C (Figure 7). The

These observations are consistent with intuitive expectations based on the diameters of the channels. The distance between hydrogen atoms at the narrowest point in the channel [H(12)− H(14)] is 6.0 Å, giving a void diameter of approximately 3.8 Å. The adsorption isotherm of CO2 into 1 reaches saturation at relatively low pressures, consistent with favorable host−guest behavior. On the other hand, the uptake of N2 is almost linear (Figure 5). The concentration of CO2 at saturation is 0.44 mol/mol copper(I), which is almost the same as the 0.50 mol/mol copper(I) concentration of MeCN in the as-synthesized 1· MeCN, which, in turn, equates to one MeCN molecule for every “zig” of each zigzag and, hence, full guest occupancy of these channels. This is consistent with our expectations, based on the simple inspection of this structure by eye, that the length of each zig in the c-axis zigzag channels does appear to be almost perfectly suited to the length and shape of CO2, just as it is for the CH3CN molecules in the as-synthesized material (Figures 2 and 3). It is unlikely that a higher occupancy of CO2 in the channels can be achieved because at 10 bar they already fully occupy the available channel space (Figure 5). The adsorption of CO2 into 1 was also measured at different temperatures, from 15 to 40 °C (Figure S8). Interestingly, the uptake at 6−10 bar is almost completely independent of the temperature. In contrast, at low pressures, the usual temperature dependence41,44 is observed, with lower temperatures enhancing uptake. The isosteric heats of adsorption were calculated, in the range of 0−10 bar, by simple interpolation of the Clausius− Clapeyron equation44 (Figure 6). At a very low surface

Figure 7. Volumetric adsorption isotherms for ethylene (brown) and acetylene (black) uptake in 1 at 25 °C.

Figure 6. Enthalpy of adsorption of CO2 into 1 as a function of uptake. The line joins the dots, as a guide to the eye.

adsorption of acetylene was slightly higher than that observed for CO2 (0.975 mmol g−1 at 1 bar; increasing pressure mode). The uptake at 1200 mbar, of 1.08 mmol g−1, equates to 0.43 mol/mol copper(I) and is again close to saturating the pore space, assuming a maximum possible adsorption of one C2H2 molecule per “zig” or “zag”, as was seen for MeCN. As predicted, the adsorption of C2H4 was substantially less (0.563 mmol g−1 at 1 bar; increasing pressure mode). Hence, MOF 1 shows a selectivity of 1.7:1 for C2H2 over C2H4 at 1 bar. The adsorption of both guests was hysteretic, despite a relatively tight equilibration criterion. This was particularly pronounced for C2H4, which is consistent with enhanced kinetic trapping of C2H4 relative to C2H2. The adsorption of C2H4 is also near-linear, implying that it is not nearing saturation even at the highest pressures studied (1200 mbar). CH4 (Figure S9) is less readily adsorbed than CO2, C2H2, or C2H4, as is expected from its lower polarizability.

coverage of CO2, this was determined to be 37 kJ mol−1, but a steady-state value of 32 kJ mol−1 is rapidly reached and maintained over a wide range of uptake, providing further evidence that there is only one possible site for CO2 to be located (one CO2 per “zig”). Given the highly constrained pore geometry within 1, we also investigated whether this material could be used for shapebased separation of the small hydrocarbons acetylene and ethylene, a separation that is industrially useful.7,27,28,34 Because acetylene, like CO2, is completely linear and acetonitrile is approximately linear, it was anticipated that acetylene would be taken up preferentially over the slightly wider ethylene molecule. Indeed, given the larger kinetic diameter of ethylene over acetylene [C2H4 (3.9 Å) > C2H2 (3.3 Å)],45 kinetic

CONCLUSIONS In summary, we report the high-yielding synthesis of a novel copper(I) MOF, 1·MeCN, which features a highly constrained, zigzag-shaped pore geometry. Desolvated MOF 1 is robust and is selective for CO2 over N2 and CH4 (11.7:1 CO2:N2 at 1 bar), with significant kinetic trapping occurring in the case of CH4. MOF 1 is also shape-selective for C2H2 over C2H4 (1.7:1 at 1 bar), partly due to slow diffusion of the larger hydrocarbon C2H4 through the narrow channels. Despite the highly constrained zigzag pore geometry, this selectivity is still modest, and further structural optimization is clearly necessary. However, these results provide a promising lead for future studies aimed at using shape-based kinetic trapping of guests to provide new materials capable of industrially useful separations.

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DOI: 10.1021/acs.inorgchem.6b00813 Inorg. Chem. XXXX, XXX, XXX−XXX

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All chemicals, including pyrazine and pyridine-4-carbaldehyde, were of reagent-grade and were used as received, with the exception of acetonitrile, which was HPLC-grade. 2-Acetylpyrazine was prepared from pyrazine according to the literature procedure,46 and py-pzpypz was prepared according to our published procedure.47 Synthesis of {[CuI2(py-pzpypz)2(μ-CN)2]·2H2O}n (1·2H2O). Caution! Reaction involves highly toxic KCN. Exercise extreme care. With the f umehood on boost, to a Teflon-lined stainless steel reaction vessel (50 mL) was added copper(II) tetrafluoroborate hydrate (32.5 mg, 96 μmol), py-pzpypz (30 mg, 96 μmol), and KCN (6.2 mg, 96 μmol) in MeCN/H2O (20:1, 42 mL). The vessel was sealed and transferred to an oven, where it was heated at 160 °C for 120 h. After this time, the oven was turned off and left to slowly cool to ambient temperature. After 36 h, the vessel was removed from the oven and opened in a f umehood on boost. The resulting orange needle-shaped crystals (31.5 mg, 37 μmol, 78% based on py-pzpypz) were filtered, washed with MeCN, CHCl3, and MeCN again (40 mL each), and then air-dried. Upon air drying, the half-occupancy MeCN molecules were gradually replaced by H2O molecules, giving 1·2H2O. Elem anal. Calcd for C38H28N14O2Cu2 (839.8 g mol−1): C, 54.35; H, 3.36; N, 23.35. Found: C, 54.51; H, 3.18; N, 23.61. TGA (Δm). Calcd for 1·2H2O: 3.5%. Found: 3.8%.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00813. Supplementary synthetic details, full instrument details and characterization procedures, further X-ray crystal structure information, refinement details and diagrams, PXRD patterns, and additional experimental adsorption isotherms (PDF) Crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.J.K.). *E-mail: [email protected] (S.B.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Campbell Microanalytical Laboratory (University of Otago) for performing the microanalyses. This work was supported by the University of Otago, including the award of a Ph.D. scholarship and postgraduate publishing bursary to R.G.M., RSNZ Marsden Grant (2008−2013) funding, and, most recently, MacDiarmid Institute travel funding (to R.G.M.) and the Science and Industry Endowment Fund (Australia).

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DOI: 10.1021/acs.inorgchem.6b00813 Inorg. Chem. XXXX, XXX, XXX−XXX