Influence of Metal Substitution on the Pressure-Induced Phase

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Influence of Metal Substitution on the Pressure-Induced Phase Change in Flexible Zeolitic Imidazolate Frameworks C. Michael McGuirk,† Tomcě Runcě vski,†,‡ Julia Oktawiec,† Ari Turkiewicz,† Mercedes K. Taylor,†,‡ and Jeffrey R. Long*,†,‡,§ †

Department of Chemistry, University of California, Berkeley, California 94720, United States Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on November 7, 2018 at 20:15:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Metal−organic frameworks that display step-shaped adsorption profiles arising from discrete pressure-induced phase changes are promising materials for applications in both highcapacity gas storage and energy-efficient gas separations. The thorough investigation of such materials through chemical diversification, gas adsorption measurements, and in situ structural characterization is therefore crucial for broadening their utility. We examine a series of isoreticular, flexible zeolitic imidazolate frameworks (ZIFs) of the type M(bim)2 (SOD; M = Zn (ZIF-7), Co (ZIF-9), Cd (CdIF-13); bim− = benzimidazolate), and elucidate the effects of metal substitution on the pressure-responsive phase changes and the resulting CO2 and CH4 step positions, pre-step uptakes, and step capacities. Using ZIF-7 as a benchmark, we reexamine the poorly understood structural transition responsible for its adsorption steps and, through high-pressure adsorption measurements, verify that it displays a step in its CH4 adsorption isotherms. The ZIF-9 material is shown to undergo an analogous phase change, yielding adsorption steps for CO2 and CH4 with similar profiles and capacities to ZIF-7, but with shifted threshold pressures. Further, the Cd2+ analogue CdIF-13 is reported here for the first time, and shown to display adsorption behavior distinct from both ZIF-7 and ZIF-9, with negligible pre-step adsorption, a ∼50% increase in CO2 and CH4 capacity, and dramatically higher threshold adsorption pressures. Remarkably, a single-crystal-to-single-crystal phase change to a pore-gated phase is also achieved with CdIF-13, providing insight into the phase change that yields step-shaped adsorption in these flexible ZIFs. Finally, we show that the endothermic phase change of these frameworks provides intrinsic heat management during gas adsorption.

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INTRODUCTION Metal−organic frameworks are a structurally diverse class of porous and crystalline three-dimensional materials composed of inorganic nodes bridged by multitopic organic linkers, and have garnered extensive interest for applications such as gas storage and gas separations.1−7 While most metal−organic frameworks are structurally rigid and display Type I gas adsorption isotherms (Figure 1, red trace), some frameworks undergo pressure-induced phase changes and correspondingly exhibit “step-shaped” adsorption profiles characterized by minimal guest uptake below a temperature-dependent threshold pressure, followed by a sharp rise in adsorption (corresponding to a Type V adsorption isotherm; Figure 1, green trace).8−18 Because they exhibit gas uptake and release over a narrow, temperature-controlled pressure region, flexible frameworks can offer more efficient adsorption−desorption cycling and reduced thermal management requirements relative to their rigid counterparts, for example in applications such as CO2 and CH4 capture and storage.3,8,12,19−21 On the other hand, flexible metal−organic frameworks tend to display © XXXX American Chemical Society

poor stability and undergo large crystal volume changes that would necessitate extensive engineering considerations prior to incorporation into an adsorption process.8−11,22−24 Therefore, it remains of great interest to pursue robust frameworks that exhibit Type V adsorption behavior through pressureresponsive phase transitions while exhibiting minimal crystallographic volume changes. One interesting material in this regard is the framework Zn(bim)2 (ZIF-7; bim− = benzimidazolate, sodalite or SOD topology, Figure 2), which belongs to the subclass of highly chemically and thermally stable zeolitic imdazolate frameworks (ZIFs) that are most commonly composed of Zn2+ nodes bridged by imidazolate linkers in a zeolite topology.25−31 Although ZIFs tend to be structurally rigid, ZIF-7 displays prominent step-shaped adsorption due to guest pressureresponsive structural rearrangements, as first characterized in the adsorption of CO2 and short chain (C2−C3) olefins and Received: September 5, 2018

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DOI: 10.1021/jacs.8b09631 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

from an ostensible linker-regulated pore-gating mechanism involving low (∼10%) crystal-volume changes.32−42 The robustness and flexibility of ZIF-7 have additionally proven advantageous in the design of membranes exhibiting selectivity for H2 or CO2 in capture and separation processes.43−48 Despite its extensive use in gas storage and separations studies, the ZIF-7 phase change remains poorly understood and consequently so does the structural impetus for the stepped adsorption behavior. While single-crystal X-ray diffraction data for the solvated “open” phase show the expected SOD topology,25,49 single crystals of the closed phase responsible for the pre-step region have proven inaccessible. Thus, the root cause of the stepped adsorption behavior remains largely speculative.32,33,37,41,42 A structure of the solvent-free closed phase obtained by refinement using powder X-ray diffraction data has been previously reported, suggesting that benzimidazolate rotation and topological distortion occur in concert to gate the pore apertures, with a counterintuitive decrease (∼10%) in framework density relative to the open phase.38 While this latter study provided key insights into the ZIF-7 phase change, as discussed below, we find this solution to be of uncertain reliability, as supported by previous simulations that suggest the activated phase (i.e., all solvent removed via heating under dynamic vacuum) to be of higher density.39 A firm comprehension of the phase change behavior thus remains lacking, and is of interest both from a fundamental perspective and toward the realization of structurally related, robust flexible frameworks. The benefits of Type V adsorption in ZIF-7 are only realized for gases and processes for which the isotherm shape, capacity and step position of the material are fortuitously in line. Therefore, in order to broaden the potential of this material, the realization of structurally related frameworks with distinct adsorption behavior is necessary. We have previously demonstrated that functionalization of the 1,4-benzenedipyrazolate ligand (bdp2−) in the flexible framework Co(bdp) gives rise to a family of isoreticular materials that exhibit tunable CH4-induced phase changes and corresponding step adsorption pressures.8,9 However, as ligand functionalization prevents structural flexibility in the SOD topology of ZIF-7,26,27,50−53 we sought instead to explore the effects of metal substitution on step-shaped adsorption for this framework type. Herein, we present the detailed investigation of a series of isoreticular, flexible zeolitic imidazolate frameworks (M(bim)2; M = Zn, Co, Cd; SOD topology) and the impact of changing the metal center on adsorption behavior, as characterized via single-crystal X-ray diffraction, gas adsorption, and in situ synchrotron powder X-ray diffraction using CO2 and CH4 as probe molecules. We also reassess the previously reported structure of activated ZIF-7,38 detail the first characterization of structural flexibility in ZIF-9 (Co(bim)2; SOD),25,34−36,54−58 and report the adsorption behavior of the novel flexible Cd-based analogue (CdIF-13; Cd(bim)2; distorted SOD), which deviates significantly from that of the zinc and cobalt congeners. Importantly, the first single-crystal structure of the “closed” pore-gated phase in this structure-type is discussed, providing fresh insight into the pressure-induced phase change. As the conditions of industrial gas storage and separation applications vary significantly, the ability to modify the step profile in these robust materials is particularly advantageous.

Figure 1. Adsorption behavior for microporous materials exhibiting classical Langmuir adsorption (Type I, red) and step-shaped adsorption (Type V, green). Maximum adsorption pressure (Pads) and minimum desorption pressure (Pdes) are indicated by vertical gray lines; relative usable capacities achieved with both types of porous materials are indicated by double-headed arrows in respective colors. Materials that exhibit step-shaped adsorption profiles can display greater usable capacities than materials exhibiting Type I behavior through comparable pressure swings.

Figure 2. Benzimidazole (Hbim) and portions of the single-crystal structure of as-synthesized ZIF-7 (Zn(bim)2, SOD),25 which contains two distinct six-membered ring apertures and one four-membered ring aperture. Gray, dark blue, and light blue spheres represent C, N, and Zn atoms, respectively; hydrogens and solvent are omitted for clarity.

paraffins below 1 bar at 25 °C.32−36 In these applications, the framework displays guest-specific threshold pressures resulting B


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Figure 3. (a) Synchrotron powder X-ray diffraction patterns (λ = 0.45241(4) Å) of ethanol-solvated (top) and fully activated (bottom) ZIF-7 collected at 25 °C. (b) CO2 adsorption isotherm for ZIF-7 at 25 °C; filled circles correspond to adsorption and open circles to desorption. (c) In situ variable CH4 pressure synchrotron powder X-ray diffraction patterns (λ = 0.45241(4) Å) of ZIF-7 collected at 30 °C, with CH4 pressure increasing from bottom to top. Red and blue patterns correspond to closed and open phases, respectively, with purple indicating patterns in which both phases are present. (d) High pressure CH4 adsorption isotherms for ZIF-7 at 30, 45, and 60 °C; total adsorption capacity is in units of v/v (cm3 STP cm−3).

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structureless Pawley fitting64 using the published unit cell parameters, to test for their validity. Although the fitting gave a satisfactory difference curve and figures-of-merit (Supporting Information for details), we conclude that the analysis is not convincing evidence for the given unit cell parameters, owing to an apparent overparameterization of the fit stemming from a large unit cell volume, low symmetry (P1̅), and diffraction peak broadness. We attempted ab inito unit cell indexing using the singular value decomposition method,65 as implemented in TOPAS.66 However, due to the aforementioned features (large unit cell, low symmetry, and broad diffraction peaks), the indexing failed to unambiguously indicate sets of unit cell parameters. Therefore, we contend that the structure of the closed phase of ZIF-7 remains unknown and remains an important target, but emphasize that previous works using diffraction and simulation provide valuable insights into the ZIF-7 phase change.38,39,67 This persistent difficulty in determining the exact structure of the closed phase of ZIF-7 from synchrotron powder X-ray diffraction data indicates the importance of obtaining single-crystal X-ray diffraction quality crystals and data for a member of this structure type in the closed phase, in order to improve understanding of the pressure-induced phase change. Toward such an improved understanding of the phase change and adsorption behavior of ZIF-7, we carried out high-

RESULTS AND DISCUSSION ZIF-7 Activated Structure and High-Pressure Gas Adsorption. In light of the ZnO impurity present in samples used in previous ZIF-7 structural and gas adsorption studies,25,59 we deemed it prudent to reevaluate the activated structure and gas adsorption measurements using phase-pure material. Phase-pure ZIF-7 was synthesized at room temperature in an ethanol/water mixture in the presence of ammonium hydroxide (Figure 3a),60 rather than via the prototypical solvothermal route in DMF, which produces variable amounts of ZnO impurity (for location of ZnO reflections see Figure S2). Impurity-free ZIF-7 can also be synthesized in acetonitrile with triethylamine.39,61,62 After washing with ethanol and subsequent activation at 100 °C, the ZnO-free closed phase was isolated as a polycrystalline powder (Figure 3a) that was confirmed to be solvent-free by 1 H NMR spectroscopy (Figure S3). When synthesized in this manner, ZIF-7 shows the expected step-shaped CO 2 adsorption profile at 25 °C (Figure 3b). With this phase pure material in hand, we attempted a Rietveld refinement63 of the published structure38 against synchrotron powder X-ray diffraction data collected at the Advanced Photon Source. The analysis indicated that the published crystal structure does not represent the structure of the sample (see Figure S42 for details). Next, we attempted a C


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Figure 4. (a) Synchrotron powder X-ray diffraction patterns (λ = 0.45241(4) Å) of DMF-solvated (top) and fully activated (bottom) ZIF-9 collected at 30 °C. (b) CO2 adsorption isotherm for ZIF-9 at 25 °C; filled circles correspond to adsorption and open circles to desorption. (c) In situ variable CH4 pressure synchrotron powder X-ray diffraction patterns (λ = 0.45241(4) Å) of ZIF-9 collected at 30 °C, with CH4 pressure increasing from bottom to top. Red and blue patterns correspond to closed and open phases, respectively, with purple indicating patterns in which both phases are present. (d) High pressure CH4 adsorption isotherms for ZIF-9 collected at 30, 45, and 60 °C; total adsorption capacity is in units of v/v (cm3 STP cm−3).

a loading of 42 v/v (1.5 mmol/g, i.e., corresponding to the step region) was calculated to be −14.06 ± 0.07 kJ/mol. In situ powder X-ray diffraction data at variable CH4 pressures show a clean transition from the closed phase under vacuum to the SOD-like open phase above the threshold pressure (see Table S3 for unit cell parameters), with no observable intermediate crystalline structure in the step region, in line with an analogous study using CO2 as a probe at pressures below 1 bar (Figure 3c).32 Although the structural transition is not fully understood at this stage, these data provide additional confirmation that a phase change is responsible for the observed stepped adsorption. Phase Change and Gas Adsorption Behavior of ZIF-9. In order to investigate the effect of metal substitution on the ZIF-7 phase transition and adsorption behavior, we turned to Co(bim)2 (ZIF-9).25 The single-crystal structure of solvated ZIF-9 was first reported soon after ZIF-7,25 and the two frameworks exhibit very similar metal−ligand bond lengths, porosity, density, and ligand orientations.25,49 Yet, studies of ZIF-9 have primarily focused on the catalytic properties of the framework,55,56,73,74 despite observations of Type V adsorption.34−36,57,74 Consequently, it was previously unknown whether ZIF-9 undergoes a reversible phase change concomitant with its step-shaped adsorption and, if so, how this structural transition and adsorption behavior compares to ZIF7. For this study, phase-pure ZIF-9 was obtained via a modified

pressure (>1 bar) gas adsorption studies using methanethe primary component of natural gasas a probe molecule. These conditions were chosen because materials exhibiting step-shaped isotherms have been proposed as optimal adsorbents for high-density CH4 storage,8,9 natural gas storage and separation applications largely occur at high pressures,7,68−70 a high CH4 pressure-induced phase transition has been previously observed,39 and because ZIF-7 shows negligible CH4 adsorption below 1 bar.34−37 We note that during the course of this work, the full step-shaped CH4 isothermal adsorption profile for ZIF-7 was reported;59 however, this study was performed with an impure sample (containing ∼20−25% ZnO impurity), as evidenced by the reported powder X-ray diffraction data and the low CH4 (and CO2) capacities. Using our ZnO-free material, the CH4 adsorption isotherm of ZIF-7 at 30 °C displays low uptake until a threshold pressure of ∼10 bar (similar to that reported for the impure sample) at which point step-shaped uptake occurs with a total adsorption capacity approaching 95 cm3 STP cm−3 (v/v) at 65 bar (Figure 3d).71 Notably, the phasepure material measured here shows a ∼50% higher adsorption capacity than the impure material at the highest measured pressure (45 bar).59 Using linear spline fits of adsorption isotherms at 30, 45, and 60 °C and the Clausius−Clapeyron relationship,72 the differential enthalpy of adsorption (Δhads) at D


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These results collectively demonstrate that upon substituting Zn2+ with Co2+, the flexibility and adsorption behavior of this structure type is retained, with decidedly similar adsorption capacities arising from the nearly identical open phase structures.25 Additionally, owing to the comparable pre-step capacity, step profile, and powder X-ray diffraction patterns, it may be inferred that ZIF-7 and ZIF-9 display structurally similar closed phases. Nonetheless, substitution of Zn2+ with Co2+ induces a substantial shift in the probe gas threshold pressures to higher values, thus broadening the potential application of step-shaped adsorption in ZIFs. Despite this advance, however, the adsorption profiles of these frameworks still show significant pre-step adsorption and relatively low capacities. To address these considerations we pursued a novel metal-substituted variant of ZIF-7 with similar stability and flexibility, but considerably altered adsorption properties. CdIF-13 Discovery and Characterization. In addition to expanding our fundamental understanding of the effects of metal-substitution in ZIF-7, we sought to realize an analogous material with considerably increased capacity by employing a metal center with a larger ionic radius and elongated metal− ligand bonds. Single-crystal X-ray diffraction data have revealed that cadmium(II) imidazolate frameworks (CdIFs) are isoreticular to their rigid Zn2+ and Co2+ counterparts, although they exhibit longer metal−nitrogen bonds, wider pore apertures, and more accessible pore volumes, all while retaining a high thermal stability.77 We therefore pursued the cadmium(II)-based analogue of ZIF-7, and ultimately synthesized the material as single crystals by heating a mixture of Cd(ClO4)2 and benzimidazole in DMF at 130 °C for 48 h. The single-crystal X-ray diffraction structure of DMFsolvated Cd(bim)2 (CdIF-13) reveals the anticipated threedimensional connectivity, with all metal−nitrogen bonds elongated by ∼0.2 Å, relative to the Zn and Co structures (Figure 5).25 Whereas previously reported CdIFs are close to topologically ideal,77 DMF-solvated CdIF-13 exists in a slightly distorted SOD topology (P21/n; SOD = R3̅) as reflected by additional low-angle reflections in its powder X-ray diffraction pattern, relative to the zinc and cobalt analogues (Figure 7a; for a comparison with the pattern generated from single-crystal X-ray diffraction data, see Figure S33). Nonetheless, CdIF-13 shows higher angle ligand orientations (∼55−75°, with respect to the plane of the six-membered ring; Figure 5b) and greater porosity than both ZIF-7 and ZIF-9 (For CdIF-13: Langmuir surface area = 615 m2/g, unit cell void space = 32.0%, pore volume = 0.24 cm3/g). Powder X-ray diffraction data (Figure 7a) confirm that CdIF-13 displays flexibility analogous to ZIF7 and ZIF-9 upon activation (see Supporting Information for activation conditions and Figure S10 for the 1H NMR spectrum of digested activated material), undergoing a structural transition to an ostensibly closed phase. Refinement of the fully activated Cd2+ phase structure against synchrotron powder X-ray diffraction data proved elusive; however, the apparent flexibility observed in the singlecrystal structure encouraged us to explore whether gradual exchange of the DMF solvent could induce a single-crystal-tosingle-crystal phase change to a closed phase. Indeed, slow exchange of DMF with dichloromethane over the course of a week yielded dichloromethane-solvated single crystals in a closed, pore-gated phase (Figure 6), the first single-crystal evidence of this phase. Compared to the DMF-solvated phase, the single-crystal structure of dichloromethane-solvated CdIF13 is characterized by a topological rearrangement, marked by

version of the previously reported synthesis route, in which a cobalt(II) formate impurity is reliably removed, as confirmed by powder X-ray diffraction (Figure 4a).25,54,56 Upon complete activation (Figure S7), ZIF-9 indeed undergoes a crystalline phase change with a diffraction pattern resembling that of activated ZIF-7 (Figure 4a), although the large number of possible unit cell solutions for this phase precluded the identification of a structure solution. To the best of our knowledge, these data represent the first evidence of a structural transition in ZIF-9. Isothermal CO2 adsorption data further confirm that the observed structural flexibility of ZIF-9 coincides with the stepshaped adsorption behavior. At 25 °C, the threshold CO2 step occurs at ∼800 mbar, compared to a pressure of 400 mbar for ZIF-7 (Figure 4b). Beyond the observed shift in threshold pressure, the CO2 adsorption profiles of ZIF-7 and ZIF-9 are strikingly similar, with comparable pre-step adsorption, step steepness, hysteresis, and total capacities (0.72 and 0.73 mol/ mol, respectively, see Figure S17). Furthermore, for ZIF-9 the magnitude of Δhads at a capacity of 1.5 mmol/g was calculated to be −29 ± 2 kJ/mol, within error of the reported value of −28.2 kJ/mol for ZIF-7 determined from fitting adsorption measurements.39 This is also in strong agreement with the −27 to −29 kJ/mol value determined for ZIF-7 using differential microcalorimetry.39 High-pressure CH4 isotherms collected for ZIF-9 show stepshaped adsorption behavior analogous to that observed for ZIF-7, but the threshold pressure for ZIF-9 is ∼15 bar at 30 °C (Figure 4d), higher than the corresponding value of ∼10 bar for ZIF-7 (see Figure 3d). The overall adsorption profile for ZIF-9 is again very similar to that of ZIF-7the cobalt(II) variant exhibits a total volumetric capacity of 96 v/v at 60 bar and a calculated Δhads of −13.2 ± 0.1 kJ/mol at 42 v/v (1.5 mmol/g), compared to corresponding values of 95 v/v and −14.06 ± 0.07 kJ/mol for ZIF-7. Finally, in situ powder X-ray diffraction data collected at variable CH4 pressures show a clean transition from the closed phase to the SOD-like open phase with no intermediate crystalline structure (Figure 4c; see Table S3 for unit cell parameters). Given the similar ionic radii of Co2+ and Zn2+,75 the near identical unit cell parameters of solvated ZIF-9 and ZIF-7,25 and the visually similar activated PXRD patterns (Figures 4a,c and 3a,c), the parallels in adsorption profile and capacity are not surprising. Therefore, as these materials are very structurally similar, we hypothesize that the observed difference in pressure required to induce the phase transition from the closed to open phase is a result of the differences in metal−ligand bonding arising from the different electronic configurations of open-shell 3d7 Co2+ and closed-shell 3d10 Zn2+. The open-shell nature of Co2+ promotes ligand-to-metal π-bonding, thus increasing bond strength relative to the closedshell Zn2+ system, in which only σ-bonds should form. As the phase transition from the closed to open phase ostensibly includes rotation of the metal−ligand bonds, differences in bond strength should manifest in differences in the energy required to induce the phase change. Therefore, the apparently higher metal−ligand strength in ZIF-9 leads to a greater threshold gas pressure required for step-shaped adsorption to occur. Indeed, this rationale was first proposed to explain analogous differences in the mechanical pressure required to induce a phase transition in Co2+ and Zn2+ congeners of ZIF4,76 where greater pressure was required for the phase transition to occur in the Co2+-containing framework. E


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an increase in framework density by 9.5% (excluding solvent), and significant linker rotation. In the primary six-membered ring aperture (Figure 6a,b), two benzimidazolate linkers lie at an angle of 20° relative to the plane of the six-membered ring with a shortest cross-pore C···C separation of 3.42(2) Å (Figure 6b), thereby gating the pore window (see Figures S38−S40 for space-filling models). The structure clearly shows that linker rotation and framework distortion can occur to block the pore windows, supporting the prevailing hypothesis that step-shaped adsorption arises from a pore-gating mechanism. The observed increase in crystal density is also in agreement with previous thermodynamic predictions for the phase change in ZIF-7.39 Of note, the two phases of CdIF-13 do not exhibit significant differences in coordination geometry at the nodes. Attempts to remove dichloromethane from the CdIF-13 single crystals to obtain a fully desolvated structure resulted only in a polycrystalline material. However, given the evident obstacles to refining fully activated structures from powder X-ray diffraction data for these materials, this structure provides the greatest insight to date into the phase change behavior of these flexible ZIFs. Due to its elongated metal−nitrogen bonds, CdIF-13 displays appreciable structural differences from ZIF-7 and ZIF-9, suggesting that this material may also display different adsorption behavior. Indeed, whereas ZIF-7 and ZIF-9 show stepped CO2 adsorption below 1 bar at 25 °C, activated CdIF13 shows negligible CO2 uptake in this pressure region. Instead, CdIF-13 displays step-shaped adsorption with a considerably shifted threshold pressure of ∼11 bar and negligible pre-step adsorption (Figure 7b), an apparent consequence of the ligand orientation observed in the poregated single-crystal structure. Additionally, the increased pore volume results in an approximately 50% increase in CO2 capacity. In situ powder X-ray diffraction data collected at 30 °C and under variable CO2 pressures show (Figure 7c) a transition from the closed phase to an open phase that was found via Pawley fitting64 to have a distorted SOD topology, similar to that observed in the DMF-solvated single-crystal structure (space group C2; a = 17.93(2) Å; b = 23.99(3) Å; c = 13.74(2) Å, β = 63.33(6)°; V = 5282(1) Å3; Figure S45, Table S3). Intriguingly, in contrast to the clean two-state phase change observed for the analogous experiment with ZIF-7,32 reflections from an apparent intermediate phase are observed at pressures of 11.8 and 12.7 bar within the step region, with reflections at 2.05 and 2.36 2θ being the most prominent (Figures 7c and 7d). The existence of an intermediate state is further supported by CO2 adsorption isotherms at temperatures at or above 30 °C, where CO2 uptake is separated into two distinct steps (Figure 7b), a behavior not observed in ZIF7 or ZIF-9 over a large range of temperatures.39 Moreover, previously collected in situ powder X-ray diffraction data for ZIF-7 show that no similar intermediate phase is observed under variable CO2 pressures.32 We currently ascribe this presence of a double-stepping phenomenon in CdIF-13, but not ZIF-7 and ZIF-9, to the apparent greater flexibility of this framework arising from the considerably longer metal−ligand bonds. Thus, CdIF-13 has greater degrees of freedom to access meta-stable structural states. Attempts to collect in situ powder X-ray diffraction data at elevated temperatures that induce greater separation of the two transitions have not yet proven successful, but the transient reflections in the variable-pressure powder X-ray diffraction data at 30 °C suggest that the

Figure 5. Portions of the single-crystal structure of DMF-solvated CdIF-13 as determined from data collected at 100 K. (a), (b) Topdown and side-on views, respectively, of a six-membered ring aperture. (c) Top-down view of the four-membered ring aperture. (d) Top-down view of a second six-membered ring aperture type. Gray, blue, and beige spheres represent C, N, and Cd atoms, respectively; hydrogens and solvent are omitted for clarity.

Figure 6. Portions of the single-crystal structure of dichloromethanesolvated CdIF-13 as determined from data collected at 100 K. (a), (b) Top-down and side-on views, respectively, of a six-membered ring aperture. (c) Top-down view of the four-membered ring aperture. (d) Top-down view of a second six-membered ring aperture type. Gray, dark blue, and beige spheres represent C, N, and Cd atoms, respectively; hydrogens and solvent are omitted for clarity.

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Figure 7. (a) Synchrotron powder X-ray diffraction patterns (λ = 0.45241(4) Å) of DMF-solvated (top) and fully activated (bottom) CdIF-13 collected at 30 °C. (b) CO2 adsorption isotherms for CdIF-13 at −5, 10, 25, 30, 40, and 50 °C. (c) In situ variable CO2 pressure synchrotron powder X-ray diffraction patterns (λ = 0.45241(4) Å) of CdIF-13 collected at 30 °C, with CO2 pressure increasing from bottom to top. Red and blue patterns correspond to closed and open phases, respectively, with purple indicating patterns in which both phases are present. (d) Expanded view of the low angle region of three representative powder X-ray diffraction patterns from variable CO2 pressure experiment at 30 °C (panel c). Gray lines highlight reflections from apparent intermediate phase of CdIF-13 during CO2 adsorption. (e) High pressure CH4 adsorption isotherms for CdIF-13 at −30, −20, −10, and 0 °C. Total adsorption capacity is in units of v/v (cm3 STP cm−3). (f) In situ variable CH4 pressure synchrotron powder X-ray diffraction patterns (λ = 0.45241(4) Å) of CdIF-13 collected at −30 °C, with CH4 pressure increasing from bottom to top. Red and blue patterns correspond to closed and open phases, respectively, with purple indicating patterns in which both phases are present. No transient intermediary phase is observed.

phase change for CO2, but not CH4, has been previously observed in other flexible metal−organic frameworks.9 This has been ascribed to the stronger adsorbate−framework interactions with CO2 stabilizing the meta-stable phases. We previously noted that the increased requisite threshold pressure for ZIF-9 over ZIF-7 results from stronger metal−ligand bonds. Yet, this was for a case where the activated phase of the two frameworks was ostensibly very similar. As we can infer from the activated PXRD patterns and the negligible pre-step adsorption profile of CdIF-13, the activated phase of CdIF-13 is slightly different from its predecessors; thus, this explanation is no longer relevant. Rather, we propose that in CdIF-13 the considerably longer metal−ligand bonds result in greater degrees of flexibility in regard to linker rotation and the framework architecture, allowing the material to access a more stable activated phase, which is characterized by considerably stronger intra-framework interactions. This stabilization of the closed phase, relative to the activated phases of ZIF-7 and ZIF-9, through ligand−ligand interactions necessitates considerably greater energetic input to open the framework, manifesting as the observed dramatically increased threshold pressure. Therefore, it may be possible to desirably tune the threshold pressure of CdIF-13 through ligand substitutions which modulate the strength of these inter-ligand interactions. An emergent application for porous materials with stepshaped CH4 adsorption is on-board natural gas storage for use

existence of two distinct phase transitions is indeed responsible for the observed adsorption behavior. The two observed phase transitions could potentially be a separation of ligand rotation and framework reorganization, or two discrete phase changes that both include linker rotation and framework reorganization, in either case passing through a meta-stable phase. Using isotherms collected at 25, 40, and 50 °C (Figure 7b), Δhads was calculated to be −23.73 ± 0.05 and −29.8 ± 0.4 kJ/mol for the first and second step regions, respectively. Similar to the dramatically shifted adsorption threshold in the presence of CO2, CdIF-13 adsorbs negligible CH4 below 80 bar at 30 °C. However, upon cooling to 0 °C the material exhibits step-shaped CH4 adsorption with a threshold pressure of ∼45 bar, negligible pre-step adsorption, and a high total capacity of 138 v/v at 65 bar (Figure 7e). A value of Δhads of −15.9 ± 0.7 kJ/mol was calculated in the step pressure regime, slightly higher than that observed for ZIF-7 and ZIF-9. In situ variable-pressure powder X-ray diffraction data (Figure 7f) show that CH4 adsorption results in a closed-to-open phase transition, wherein the open structure is similar to the DMFsolvated CdIF-13 single-crystal structure, as determined through Pawley fitting64 (space group = P21/n; a = 14.60(2) Å; b = 22.67(6) Å; c = 17.30(3) Å, β = 114.0(1)°; V = 5229(1) Å3 ; Figure S48, Table S3). Of note, no perceptible intermediate phase was observed by diffraction during CH4 adsorption under these conditions. The observation of intermediate framework structures during a pressure-induced G


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step-shaped adsorption profile, such as pre-step uptake, threshold pressure, and total capacity. Given that the conditions of industrial gas storage and separation vary significantly, the ability to modify the step profile in these robust materials is particularly advantageous, potentially indicating a path to new applications. In light of the ongoing difficulty in determining the structure of the fully activated phase of ZIF-7, ZIF-9, and CdIF-13, the solvent-induced single-crystal-to-single-crystal phase transition achieved with CdIF-13 is another significant breakthrough, providing the first empirical structural characterization of pore-gating in this flexible structure-type. Moving forward, this newly characterized phase will aid in identifying structural factors that control the pressure-induced step, thus allowing chemists to more predictably and precisely tune the phase-change pressure and expand the potential of zeolitic imidazolate frameworks as phase-change adsorbents. In particular, pursuing modifications that destabilize intra-framework contacts in the closed phase and increase pore size in the open phase, such as ligand substitution and extension, respectively, may help achieve threshold pressures and capacities relevant to emerging applications, including on-board natural gas storage.

in transportation, where driving range is determined by the difference in CH4 adsorbed at 35 bar and that which remains upon desorption down to 5.8 bar (Figure 1).8 One of the highest reported usable capacities under these conditions is 155 v/v, shown by Co(bdp), an air-sensitive compound that displays a step-shaped adsorption−desorption profile corresponding with an 100% change in crystal volume.8 In comparison, the usable capacities of ZIF-7 and ZIF-9 at 30 °C are 70 and 76 v/v, respectively. Although CdIF-13 shows negligible adsorption at this temperature, at −20 °C CdIF-13 has a usable capacity of 134 v/v (see Figure S23 for CdIF-13 desorption profile). As CdIF-13 is a benchtop-stable framework and only undergoes an approximately 10% volume change, the pursuit of synthetic methods for shifting the 30 °C threshold pressure between 5.8−35 bar and further increasing the capacity will be an important focus moving forward. If indeed the threshold pressure could be shifted to the relevant range, given the observed trend in CH4 adsorption capacity from −30 to 0 °C, we can conservatively approximate a usable capacity of 125 v/v at 30 °C. In summary, the realization of a second-row transition metal analogue for ZIF-7 yields a topologically analogous material with increased structural flexibility, considerably shifted step-threshold pressures, negligible pre-step adsorption, and increased adsorption capacities for CO2 and CH4. Intrinsic Heat Management. The management of exothermic enthalpies of adsorption and endothermic enthalpies of desorption is a substantialyet often overlooked barrier to the realization of porous materials for gas storage applications. Specifically, large changes in the local temperature of the adsorbent can occur upon cycling, dramatically reducing the usable capacity.19,78,79 In contrast to rigid frameworks, pressure-responsive materials offer an intrinsic tool for addressing this challenge, as the phase-change enthalpy can partially offset the heats of adsorption and desorption.8 Using a previously reported approach for assessing the enthalpy of adsorption in the presence of a phase change,8 it was found that all three frameworks have an endothermic closed-to-open phase change enthalpy (ΔHframework) between 2.5 and 2.7 kJ/ molframework, which reduces the overall heat released compared to adsorption in a corresponding rigid framework (for reference, the metal−organic framework Co(bdp) undergoes a near 100% change in volume during step-shaped adsorption of CH4 and has a ΔHframework of 7.0 kJ/molframework8). For example, during CO2 adsorption in CdIF-13, the −Δhads value is lowered by as much as 2.7 kJ/molCO2 in the step region, resulting in a 7% reduction in heat released over the full adsorption profile. On the opposite extreme, the heat released upon CH4 adsorption in ZIF-7 and ZIF-9 is offset by 18−20%, owing to the lower adsorption capacities of these frameworks and the lower values of −Δhads for methane. At first pass, the similar values of ΔHframework for the three frameworks are somewhat surprising, given the differences in threshold adsorption pressures. However, ΔHframework is representative of the energetic difference between the respective end states of each framework, and thus this metric offers additional evidence that a similar overall phase change occurs in these materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b09631. Full synthesis and characterization details of the frameworks and descriptions of crystallographic and gas adsorption data analysis (PDF) DMF-solvated CdIF-13 (CIF) dichloromethane-solvated CdIF-13 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

C. Michael McGuirk: 0000-0002-7420-1169 Mercedes K. Taylor: 0000-0002-0945-766X Jeffrey R. Long: 0000-0002-5324-1321 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The initial synthesis and characterization of the gas adsorption properties of ZIF-7 was supported by ExxonMobil Research and Engineering Company as part of a natural gas storage research project. All other aspects of this research were supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Contract No. DE-AC02-05CH11231. We thank the Philomathia Foundation and Berkeley Energy and Climate Institute for support of C.M.M. through a postdoctoral fellowship. Single-crystal X-ray diffraction data were collected at Advanced Light Source beamlines 11.3.1 and 12.2.1 at Lawrence Berkeley National Laboratory, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DEAC02-05CH11231. Powder X-ray diffraction data were collected at the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated by Argonne

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CONCLUSIONS Varying the metal center in flexible materials can induce dramatic changes in gas adsorption behavior. We have demonstrated that in the case of the flexible material ZIF-7, metal substitution alters fundamental characteristics of the H


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National Laboratory under Contract No. DE-AC0206CH11357. We thank Dr. Hiroyasu Furukawa for computational support and helpful discussions, and Dr. Katie R. Meihaus for editorial assistance.

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