(M = Mg, Zn) by 2H Solid-State NMR Spectroscopy - ACS Publications

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Capturing Guest Dynamics in Metal−Organic Framework CPO-27‑M (M = Mg, Zn) by 2H Solid-State NMR Spectroscopy Jun Xu, Regina Sinelnikov, and Yining Huang* Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) are promising porous materials for gas separation and storage as well as sensing. In particular, a series of isostructural MOFs with coordinately unsaturated metal centers, namely, CPO-27-M or M-MOF-74 (M = Mg, Zn, Mn, Fe, Ni, Co, Cu), have shown exceptional adsorption capacity and selectivity compared to those of classical MOFs that contain only fully coordinated metal sites. Although it is widely accepted that the interaction between guest molecules and exposed metal centers is responsible for good selectivity and large maximum uptake, the investigation of such guest−metal interaction is very challenging because adsorbed molecules are usually disordered in the pores and undergo rapid thermal motions. 2H solid-state NMR (SSNMR) spectroscopy is one of the most extensively used techniques for capturing guest dynamics in porous materials. In this work, variable-temperature 2H wide-line SSNMR experiments were performed on CPO-27-M (M = Mg, Zn) loaded with four prototypical guest molecules: D2O, CD3CN, acetone-d6, and C6D6. The results indicate that different guest molecules possess distinct dynamic behaviors inside the channel of CPO-27-M. For a given guest molecule, its dynamic behavior also depends on the nature of the metal centers. The binding strength of guest molecules is discussed on the basis of the 2H SSNMR data.

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INTRODUCTION The discovery of metal−organic frameworks (MOFs) in recent years has opened up great opportunities for the development of gas separation and storage materials as well as sensors.1 As inorganic−organic hybrid porous materials, MOFs often possess very large surface areas that give rise to high gas storage capacity and channels/cavities that allow the selective inclusion of guest molecules on the basis of their size and polarity. In particular, MOFs with coordinately unsaturated metal sites such as CPO-27-M2−13 (M = Mg, Zn, Mn, Fe, Ni, Co, Cu), which are also referred to as M-MOF-744 or M/ DOBDC7 and DOBDC = 2,5-dioxido-1,4-benzenedicarboxylate, have shown exceptional adsorption capacity and selectivity, compared to those of classical MOFs that consist only of coordinately saturated metal sites.12−21 As Figure 1 illustrates, the structure of as-made CPO-27-M consists of one-dimensional honeycomb channels with a diameter of ∼11 Å, formed by the interconnecting 3-fold helical chains of edge-sharing MO6 units. In each MO6, five of the six oxygens belong to four DOBDC linkers, and the remaining oxygen is from a water molecule directly bound to the M2+ center. This water molecule can be removed upon dehydration. The coordination site vacated by activation is also known as an open metal site (OMS). An OMS is the preferential adsorption site for various guest molecules.12,13,20−26 The strong interaction between an OMS and guest species is responsible for good guest selectivity and large maximum uptake. Furthermore, the adsorption © XXXX American Chemical Society

Figure 1. (Left) Channels of activated CPO-27-M. (Right) M2+ local environment in as-made and activated CPO-27-M.

behavior of CPO-27-M strongly depends on the nature of the metal centers.5,12−28 For instance, the maximum CO2 uptake of CPO-27-Mg is more than 3 times the uptake of CPO-27-Zn. Thus, it is fundamentally important to obtain molecular-level knowledge on how guest molecules interact with OMSs. Received: March 3, 2016 Revised: April 17, 2016

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DOI: 10.1021/acs.langmuir.6b00851 Langmuir XXXX, XXX, XXX−XXX

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(VOCs) with several prototypical functional groups (cyano, carbonyl, and π electrons) that can potentially interact with OMSs.

The guest−metal interaction in CPO-27-M has been extensively studied by X-ray and neutron diffraction experiments.7,12,13,20−26 However, the disordering and dynamic behavior of guest molecules often prevents an accurate characterization of host−guest interaction by diffraction-based techniques even though suitable single crystals are available. Vibrational (IR and Raman) spectroscopy has also been utilized to confirm the binding of guest molecules, but the strong background signals from the MOF itself usually dominate the spectrum, making the analysis of weak adsorbate signals rather difficult.18,19,23,25 Thermodynamic data such as adsorption isotherms provide information on the binding strengths but not directly on the binding mechanisms.5,7−9,11−18,20−24,27,28 The results from theoretical modeling have to be validated experimentally.5,12,13,15,17−19,21,22 Solid-state nuclear magnetic resonance (SSNMR) spectroscopy is a method complementary to diffraction-based techniques because it is very sensitive to local structure and short-range ordering. A previous study has demonstrated that 25Mg SSNMR is a sensitive probe of adsorbing guest molecules on the Mg center in CPO-27-Mg.29 The dehydration of CPO-27-Mg induces a Mg2+ local structure change from a perfectly ordered to a disordered environment, although long-range ordering of the framework remains. The disordering of Mg2+ local structure persists if the sample is partially rehydrated or loaded with volatile organic compounds (VOCs). The local ordering can be completely restored only if enough water is added. Two other papers reported variabletemperature (VT) 13C NMR spectra, relaxometry measurements, and computer simulations of CO2 adsorbed inside CPO-27-Mg.30,31 One of the most striking observations in these reports is that they claimed that the CO2 dynamics above 150 K should be interpreted as rapid nonlocalized hopping between neighboring Mg centers31 rather than localized uniaxial rotation.30 Another recent paper involving the use of VT 17O SSNMR technique indicates that the dynamics of CO2 in CPO-27-Mg (or Zn) includes both hopping and wobbling.32 2 H SSNMR spectroscopy is one of the most extensively used NMR techniques to probe guest dynamics occurring inside porous materials.33−40 2H is a spin-1 nucleus whose NMR pattern is predominately broadened by the quadrupolar interaction between its nuclear quadrupole moment and the surrounding electric field gradient (EFG). Two parameters are typically used to describe the quadrupolar interaction: the quadrupolar coupling constant (CQ) and the asymmetry parameter (ηQ). In the case of 2H SSNMR, ηQ is always ∼0 for stationary C−D and O−D bonds because the EFGs are approximately axially symmetric. Meanwhile, CQ of 2H is characteristic of chemical bonds and is documented in the literature.41 The 2H SSNMR pattern of the stationary C−D or O−D bond is a well-defined Pake doublet, from which the CQ value can be readily derived. The line shape of the 2H SSNMR pattern is very sensitive to both the type and the rate of motion that the molecule experiences.42,43 Figure S1 (Supporting Information) provides several examples to demonstrate the effect of motion on the 2H line shape. In this work, 2H static SSNMR experiments were performed over a temperature range of 133 to 293 K to investigate the dynamics of four adsorbed guest molecules: D2O, CD3CN, acetone-d6, and C6D6 in CPO-27-Mg and CPO-27-Zn. D2O was chosen to understand the water effect because a trace amount of water can significantly influence the adsorption performance of other guest molecules.44,45 CH3CN, acetone, and benzene are all common volatile organic compounds

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EXPERIMENTAL SECTION

Sample Preparation. As-made CPO-27-M (M = Mg, Zn) samples were prepared following procedures modified from those reported in the literature.2,3 CPO-27-Mg ([Mg2(C8H2O6)(H2O)2]·8H2O): 0.75 mmol of 2,5dioxido-1,4-benzenedicarboxylic acid (H4DOBDC, Sigma-Aldrich, 98%) was dissolved in 10 mL of tetrahydrofuran (THF, reagent grade, Caledon) in a Teflon-lined inlet of an autoclave (23 mL). An aqueous sodium hydroxide solution (3 mL, 1 M) was added to this solution. 1.5 mmol of magnesium nitrate hexahydrate (Mg(NO3)2· 6H2O, Sigma-Aldrich, 99%) was first dissolved in 5 mL of deionized water and then added to the Teflon inlet while stirring, upon which a yellow precipitate formed. The autoclave was sealed, and the mixture reacting at 110 °C for 3 days. The resulting light-yellow powder was filtered, repeatedly washed with THF, and dried at room temperature (yield: 75% based on Mg). CPO-27-Zn ([Zn 2(C8H2 O6)(H2O)2 ]·8H2 O): 0.75 mmol of H4DOBDC was dissolved in 10 mL of THF in a Teflon-lined inlet of an autoclave (23 mL). An aqueous sodium hydroxide solution (3 mL, 1 M) was added to this solution while stirring. 1.65 mmol of zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma-Aldrich, 98%) was dissolved in 5 mL of deionized water and added to the Teflon inlet. After 1 h of vigorous stirring, the autoclave was sealed and the mixture reacted at 110 °C for 3 days. The resulting yellow-green powder was filtered, repeatedly washed with THF, and dried at room temperature (yield: 80% based on Zn). To prepare activated samples, as-made CPO-27-Mg was first preexchanged with fresh methanol several times. After that, ∼0.1 g of a methanol-exchanged sample was added to a glass tube, inserted into a Schlenk tube, and then activated under dynamic vacuum at 523 K for 6 h. The mass of dehydrated CPO-27-Mg was accurately determined, and then a precisely measured volume of deuterated guest compounds was injected into the glass tube containing dehydrated samples. The tube was flame-sealed and placed in an oven at 383 K overnight, allowing the guest molecules to disperse uniformly throughout the sample. Prior to 2H SSNMR experiments, guest-loaded samples were tightly packed into 5 mm o.d. glass tubes in a glovebox and sealed with multilayers of Teflon tape. The packed samples were under N2 protection during all NMR experiments. 2H SSNMR patterns did not show any significant change after storing the packed samples in a desiccator for 6 months. Guest-loaded CPO-27-Zn samples were obtained in a similar way but using a lower activation temperature (423 K) and without methanol pre-exchange. The theoretical loading amount was calculated according to the ratio of guest molecules to metal centers. The identity and crystallinity of CPO-27-M samples were examined by powder X-ray diffraction. PXRD patterns (Figure S2, Supporting Information) were recorded on a Rigaku diffractometer using Co Kα radiation (λ = 1.7902 Å). Samples were scanned at 5° ≤ 2θ ≤ 45° at a scan rate of 10°/min with a step size of 0.02°. The guest contents of CPO-27-M samples were checked by thermogravimetric analysis (TGA). The samples were heated under an N2 atmosphere on a Mettler Toledo TGA/DTA851e instrument from 25 to 500 °C at a constant heating rate of 10 °C/min. NMR Characterization. 2H static SSNMR spectra of a guestloaded CPO-27-M sample were recorded on a Varian Infinity Plus 400 WB spectrometer at 61.3 MHz in a magnetic field of 9.4 T using a horizontal 5 mm static probe and a quadrupole echo sequence.46 The 90° pulse of 2H was 3.6 μs, and the interpulse delay τ was set to 30 μs. The chemical shift of 2H was referenced to D2O at 4.8 ppm, relative to neat (CD3)4Si. The pulse delay of 2H was 2 s, and the number of scans was between 2048 and 8196. To capture dynamic features of guest species, variable-temperature (VT) 2H SSNMR experiments were performed between 133 and 293 K. Temperature was controlled by a Varian VT unit. The sample was B


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Figure 2. Experimental and simulated 2H static spectra of the 0.6 D2O/Mg sample as a function of temperature. Dynamic model for simulation: π flip-flop of D2O about its C2 axis.

effect. When the temperature decreases, the 2H line shape starts to evolve when the temperature is below 193 K: two horns with a separation of ∼150 kHz emerge at 173 K and become more evident at 153 K. The change in the 2H line shapes below 193 K can be simulated by assuming that D2O is undergoing a π flip-flop motion about its C2 axis in the intermediate regime. It is interesting that the 2H static spectrum of the 0.6 D2O/ Zn sample at 293 K (Figure 3) looks distinctly different from that of the 0.6 D2O/Mg sample (Figure 2). The spectrum consists of at least two components: one broad feature whose line width is comparable to that of the 0.6 D2O/Mg sample, indicating that these D2O molecules are tightly bound to Zn2+,

kept at any given temperature for at least 10 min to ensure that thermal equilibrium was reached before acquiring the NMR signal. The background of 2H SSNMR spectroscopy was introduced in the Supporting Information. 2H NMR patterns were simulated using the EXPRESS package47 and QuadFit software.48 Specifically, we varied the input of EXPRESS (i.e., CQ(2H) of stationary C−D and O−D bonds and dynamic models) to yield motionally averaged 2H EFG parameters as well as the 2H line shape in the intermediate regime that matches the experimentally observed 2H NMR spectrum. We used QuadFit to simulate the 2H line shape when different types of motions coexisted. The ηQ value of stationary C−D and O−D bonds was assumed to be zero.

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RESULTS AND DISCUSSION D2O in CPO-27-M. As an ever-present component in many gas mixtures, the presence of water vapor affects the adsorption of other guest molecules. It is reported that a significant fraction of the adsorption capacity is lost when activated CPO-27-M is exposed to a humid atmosphere (namely, the water effect).44,45 The strong water−metal interaction has been suggested to be responsible for such a water effect. However, it is not straightforward to study this interaction, in particular at low water coverage because water molecules are usually disordered and undergo rapid thermal motions. 2H SSNMR spectroscopy is therefore very suitable for providing molecular-level information about adsorbed water in this type of MOF. 2 H static spectra of D2O in the 0.6 D2O/Mg sample as a function of temperature are shown in Figure 2. The spectrum at 293 K exhibits a single, broad pattern with a CQ of 97.5 kHz and a nonzero ηQ of 0.75, which is typical of the π flip-flop of D2O about its C2 symmetry axis in the fast-limit regime. If a D− O−D angle of 104.5° is used,49 then we are able to reproduce the observed 2H pattern using a CQ of 195 kHz for the stationary O−D bond, and this value agrees well with those found in various hydrates of inorganic compounds.41 The absence of other motions indicates that D2O is bound to Mg2+ so tightly at room temperature that it is difficult for it to jump between neighboring Mg2+ sites. The Mg2+ ions covered by D2O are therefore deactivated and are not accessible to other guest molecules, giving rise to the above-mentioned water

Figure 3. Experimental 2H static spectra of the 0.6 D2O/Zn sample as a function of temperature. C


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Figure 4. Experimental 2H static spectra of the 0.6 CD3CN/Mg sample as a function of temperature. Dynamic model for simulation: rotation of methyl C−D about its C3 axis followed by nonlocalized six-site (or two-site) hopping motions.

spectra of the 0.6 D2O/Zn sample always exhibit both broad and narrow components. When the temperature drops, it seems that the weakly bound D2O molecules (i.e., the narrow component) are gradually converted to the tightly bound D2O molecules (i.e., the broad component), but such a transition is never completed even at the lowest temperature measured (153 K), revealing a weak interaction between D2O and Zn2+. Although the observed line shape is difficult to simulate, the fwhh of the central narrow component indeed increases with decreasing temperature, suggesting that the motion of weakly bound D2O molecules becomes more constrained upon cooling. CD3CN in CPO-27-M. CPO-27-M has been considered to be a promising candidate for the removal of toxic gases.16,27 Many toxic volatile organic compounds (VOCs) such as pesticides contain cyano groups, thus CH3CN serves as an interesting model of these VOCs. Furthermore, CH3CN is a Lewis base, which potentially can interact with OMSs (Lewis acids) of CPO-27-M. The 2H static spectrum of the sample with a loading of 0.6 CD3CN/Mg at 293 K is shown in Figure 4. Unlike the 2H spectrum of the 0.6 D2O/Mg sample, two components can be unambiguously identified in this case, including a broad component with a nonzero ηQ and a center narrow component with a zero ηQ (Pake doublet). Simulation of the observed line shape uncovers that the broad component has a CQ of 35 kHz and an ηQ of 0.45, whereas the narrow component has a CQ of 12 kHz and an ηQ of 0. It is worth noting that the Pake doublet observed is much narrower than that arising from the rapid C3 rotation of the −CD3 methyl group in CD3CN (CQ ≈ 50 kHz),41,51,52 implying that there must be an additional reorientation of the whole CD3CN molecule. Moreover, this additional motion must possess C3 or higher rotational symmetry to yield a Pake doublet. As

and a narrow center component exhibiting a Lorentzian line shape with a full width at half-height (fwhh) of ∼12 kHz, which is likely due to the disordered D2O that only weakly interact with Zn2+ ions. It is worth mentioning that the 2H spectrum of the 0.6 D2O/Zn sample is similar to that of the 1.0 D2O/Mg sample (Figure S3, Supporting Information). In the case of CPO-27-Mg, the water initially coordinated to Mg2+ ions (i.e., the broad component) interacts with the water molecules subsequently entering the channels (i.e., the narrow component), probably via hydrogen bonding. As a previous 25Mg SSNMR study shows,29 such heterogeneous distribution of water in channels leaves a significant amount of fivecoordinated Mg unaffected even though in principle every Mg2+ should adsorb one water molecule, resulting in an obvious loss of observed 25Mg intensity due to five-coordinated Mg in partially rehydrated samples compared to the as-made sample. Several other reports also illustrate the coexistence of two types of adsorbed molecules in CPO-27-M: one is in close proximity to exposed M2+ ions, and the second one is weakly bound.12,21,26 The observation of a large amount of mobile D2O in the 0.6 D2O/Zn sample unambiguously indicates that five-coordinated Zn2+ of activated CPO-27-Zn has a lower affinity for H2O than does five-coordinated Mg2+ of activated CPO-27-Mg, agreeing well with the known thermodynamic data and theoretical calculations. According to TGA measurements, the water molecules in CPO-27-Zn desorb at a significantly lower temperature than those in CPO-27-Mg.2,3 In addition, the calculated hydration energy of five-coordinated Zn2+ to six-coordinated Zn2+ (−91.2 kJ·mol−1) is lower than the energy for Mg2+ (−102.6 kJ·mol−1).50 2 H static spectra of the 0.6 D2O/Zn sample (Figure 3) also indicate that D2O exhibits different dynamic behavior at low temperature compared to that of the 0.6 D2O/Mg sample. 2H D


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Figure 5. (a) Brief schematic illustration of the nonlocalized multiple-site hopping on the (a, b) plane. (b) In the language of NMR, the hopping of a guest molecule between different M2+ ions in the (a, b) plane is equivalent to motions occurring at the base of a cone with a cone angle of θ. (c) Analytical simulations of 2H static powder patterns of multiple-site hopping in their fast-limit regime.

Figure 4. The center component gradually loses its Pake doubletlike feature at lower temperatures and eventually evolves into a Lorentzian peak (with approximately the same fwhh as for the center component at 293 K). Meanwhile, the broad component becomes more and more prominent, and two new edges with a separation of ∼78 kHz start to appear at 253 K, which are assigned to the CD3CN molecules that are not involved in the nonlocalized hopping (i.e., they are strongly bound to Mg2+ ions). With sufficient cooling (at 153 K), the center Lorentzian component disappears and only a Pake doublet with a CQ of 51 kHz is present. This CQ value is typical of a CD3CN molecule that undergoes only internal motion (rapid rotation of the C−D bond about the C3 axis of −CD3). With a low activation energy, the internal motion is still in fastlimit regime even at the lowest temperature measured (153 K). According to the geometry of −CD3, we were able to reproduce the motionally averaged CQ of 51 kHz by using a CQ of 153 kHz for the stationary C−D bond in −CD3. An accurate θ of the 0.6 CD3CN/Mg sample at 293 K is 65°. It should be mentioned that the observed featureless center component is not due to the six-site hopping in the intermediate regime (Figure S4). A possible explanation may be that other multiple-site hopping motions may coexist between 273 and 173 K (Figures S5 and S6). In addition, the θ values may also vary for different hopping motions. Simulating the spectra in the temperature range between 273 and 173 K did not generate unambiguous results. It appears that in the intermediate temperature range the dynamic behavior of the guest species is more complicated than in high- and lowtemperature regimes and that the existence of several motional species is likely. The 2H static spectrum of the 0.4 CD3CN/Zn sample at 293 K (Figure 6) exhibits only a typical Pake doublet with a CQ of 24 kHz. Simulation of the experimental line shape implies that

described in the literature, a certain guest molecule in CPO-27M can hop rapidly between neighboring metal ions.31,32 During such nonlocalized hopping, the angle between the Z axis of the intermediate frame and the crystallographic c axis, θ, remains constant. As illustrated in Figure 5a, there are six M2+ chains within a single channel and the M2+ ions in each chain are related to each other by a translation along the c axis. The hopping between different metal ions can thus be divided into two types: hopping between different M2+ chains and translation along the same M2+ chain. Only the first type causes a change in the 2H line shape because there is no reorientation of the EFG tensors in translation. Therefore, the hopping of guest molecules in three dimensions is equivalent to hopping among six M2+ ions in a single (a, b) plane. Because of a shorter path, the hopping between two neighboring M2+ ions is much more energetically favorable. Consequently, there are only five possible hopping motions: two-site, three-site, foursite, five-site, and six-site hopping. Six-site hopping can be conveniently visualized as a circular rotation of the whole molecule on a cone with cone angle θ, whereas the other four motions occur on an arc at angles of 60, 120, 180, and 240°, respectively (Figure 5b). The five hopping motions have different effects on the 2H line shape in the fast-limit regime (Figure 5c), among which only six-site hopping gives rise to the typical Pake doublet. Thus, the additional reorientation besides the rapid −CD3 group rotation is the six-site hopping of the whole CD3CN molecule. The weak, broad component is likely due to the rotation of the methyl C−D bond about its C3 axis plus a two-site hopping motion. We then performed VT 2H SSNMR experiments to determine θ, the angle between the Z axis of the intermediate frame (i.e., the C3 axis of CD3CN) and the crystallographic c axis. 2H static spectra of the 0.6 CD3CN/Mg sample over a broad temperature range from 293 to 153 K are shown in E


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Figure 6. Experimental and simulated 2H static spectra of the 0.4 CD3CN/Zn sample as a function of temperature. Dynamic model for simulation: rotation of methyl C−D about its C3 axis followed by nonlocalized six-site (or two-site) hopping motions.

it is due to rapid rotation of C−D about the C3 axis of CD3CN combined with six-site hopping of the whole molecule. However, the change in the 2H SSNMR spectra of the CD3CN adsorbed on Zn2+ is distinctly different from what was observed for CD3CN adsorbed on Mg2+ upon cooling. The Pake doublet eventually becomes a pattern with a CQ of ∼36 kHz and an ηQ of ∼0.4 at 238 K. Spectral simulation reveals that it corresponds to nonlocalized two-site hopping with a θ of 62°, which is significantly smaller than the θ of 83° at 293 K. Thus, it seems that CD3CN is attracted to the walls at low temperature, resulting in a smaller angle (θ) between the C3 axis and the crystallographic c axis. Below 238 K, the observed 2 H SSNMR line shape is composed of a Pake doublet with a CQ of 51 kHz, which can be assigned to the methyl C−D rotation about its C3 axis and a squarelike center component that is difficult to simulate. The amount of CD3CN that only undergoes internal rotation of −CD3 increases with decreasing temperature, and the hopping motion finally freezes at 193 K. It is not straightforward to compare the relative binding strengths of CD3CN on Mg2+ and Zn2+ on the basis of 2H SSNMR data. There is an observable amount of CD3CN hopping between two adjacent sites at 293 K for CD3CN in CPO-27-Mg, which is a hint of stronger binding. However, the conversion from nonlocalized to localized CD3CN in CPO-27Mg is complete at a significantly lower temperature than for CPO-27-Zn (153 K vs 193 K). We will perform more

experiments in the future to thoroughly understand the binding of CD3CN in CPO-27-M. Acetone-d6 in CPO-27-M. Acetone is an important probe molecule for the evaluation of the acidity of solid acid catalysts.53 Moreover, acetone is also a model compound for a large group of carbonyl-containing compounds. The carbonyl group of acetone, also acting as a Lewis base, can coordinate with the exposed metal centers of CPO-27-M. Figure 7 illustrates the 2H static spectrum of 0.6 acetone-d6 ((CD3)2CO) per Mg2+ at 293 K. Similar to the 2H spectrum of the 0.6 CD3CN/Mg sample, two components are present: a relatively broad component (CQ = 34 kHz, ηQ = 0.51) and a narrow Pake doublet (CQ = 18 kHz) in the center. Both patterns are narrower than the spectrum solely because of the internal rotation of the −CD3 group in (CD3)2CO (which would give rise to a Pake doublet with a CQ of 51 kHz), suggesting that in addition to methyl rotation another motion must exist. Although the rapid π flip-flop of acetone about its CO bond (C2 axis) yields a nonzero ηQ, the ηQ value (∼0.7) associated with the π flip-flop is very different from the one extracted from the experimental data (0.51). Thus, the additional motion likely involves the reorientation of entire acetone molecule. We suggest that a fraction of the adsorbed acetone molecules undergo a hopping motion between two neighboring Mg2+ sites (i.e., two-site hopping), leading to the observed broad component with EFG parameters of CQ = 34 kHz and ηQ = 0.51. The remaining acetone molecules undergo F


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Figure 7. Experimental and simulated 2H static spectra of the 0.6 (CD3)2CO/Mg sample as a function of temperature. Dynamic model for simulation: rotation of methyl C−D about its C3 axis followed by nonlocalized six-site (or two-site) hopping motions.

environments in CPO-27-Mg upon loading of acetone.29 The present work clearly shows that the disordered Mg environment results from a weak interaction between acetone and Mg2+, leading to a distribution of the Mg−OC(CH3)2 distance. The 2H static spectrum of the CPO-27-Zn sample loaded with 0.2 acetone-d6 per Zn2+ at 293 K (Figure 8) looks different from that of the 0.6 (CD3)2CO/Mg sample. It contains only a Pake doublet with a CQ of 14 kHz because of a combination of rapid local methyl rotation and a six-site molecular hopping motion. The C2 axis of acetone is also in the (a, b) plane, and the angle (θ) between the C3 axis of the methyl group and the crystallographic c axis along the channel direction is 67°. The absence of hopping motions with shorter paths (e.g., two-site hopping) suggests that the acetone in CPO-27-Zn is much more mobile than the acetone in CPO-27-Mg, which is consistent with the fact that thermodynamic data indicate that more energy is required to break the Mg2+−OC(CH3)2 bond (93.3 kJ·mol−1) than the Zn2+−OC(CH3)2 bond (72.8 kJ· mol−1).58 Figure 8 also illustrates the 2H SSNMR spectra of the 0.2 (CD3)2CO/Zn sample as a function of temperature. The central Pake doublet is eventually converted to a pattern with a nonzero ηQ at 183 K. The values of CQ (25 kHz) and ηQ (0.44) suggested that the pattern is associated with the acetone that undergoes rapid methyl rotation and three-site molecular hopping with a θ of 67°. The observation of three-site hopping in the 0.2 (CD3)2CO/Zn sample instead of two-site hopping in

a jumping motion among all six Mg2+ sites (six-site hopping), resulting in the narrow center component with zero ηQ. The spectral simulations further reveal that the angle (θ) between the C3 axis of the methyl group and the crystallographic c axis (about which jumping occurs) is 72° for both two-site and sixsite hopping (Figure S7). 2 H static SSNMR spectra of the 0.6 (CD3)2CO/Mg sample over the temperature range from 293 to 143 K are shown in Figure 7. The shoulders of the broad component become more evident at lower temperature. At 233 K, the six-site hopping motion disappears and only the two-site hopping motion persists. The reason that the population of acetone molecules undergoing six-site hopping decreases upon cooling is that the path for six-site hopping is much longer than that for two-site hopping. When the temperature is further decreased, 2H spectra exhibit two noticeable changes: one with two edges emerges at ∼±40 kHz, corresponding to the stationary acetone molecules solely undergoing C3 methyl rotation.54−57 The other change is that the center component starts to display the line shape characteristic of two-site hopping in the intermediate regime. The rate constant of the two-site hopping is determined by simulating the line shape of the center component. Fitting the Arrhenius equation using these rate constants (Figure S7) yields an activation energy of E = 6.5 ± 0.6 kJ·mol−1. This activation energy is very low, implying that acetone only weakly interacts with Mg2+. This result is consistent with a previous 25 Mg SSNMR study, showing a distribution of local Mg G


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Figure 8. Experimental and simulated 2H static spectra of the 0.2 (CD3)2CO/Zn sample as a function of temperature. Dynamic model for simulation: rotation of methyl C−2H about its C3 axis followed by nonlocalized six-site (or three-site) hopping motions.

20 kHz, which is much smaller than the typical CQ value (∼90 kHz) for the in-plane rotation of benzene about its C6 axis,33−35 implying that the benzene molecule is involved in an additional motion. The observed line shape can be simulated on the basis of the six-site hopping model. Upon cooling, the Pake doublet gradually evolved into a Gaussian pattern with a fwhh of ∼16 kHz that emerges at 213 K. Meanwhile, two new horns are emerging at ∼±35 kHz at 203 K and become more prominent at lower temperatures. This new pattern can be unambiguously assigned to the in-plane rotation of benzene. The center Gaussian-type pattern eventually diminishes at 143 K. The CQ of the C−D bond in benzene, averaged by rapid in-plane rotation about its C6 axis, is therefore 92 kHz, agreeing well with the literature value.33−35 Using this value, we are able to determine that the angle between the C6 axis of benzene and the crystallographic c axis of CPO-27-Mg is 64° for six-site hopping at 293 K. The VT data clearly indicate that at room temperature the dynamics of adsorbed benzene molecules involves the combination of benzene in plane rotation and hopping among six Mg sites. At 143 K, the hopping motion ceases and only the in-plane rotation remains. It is noteworthy that the observed change in the 2H line shape of the 0.2 C6D6/ Mg sample from the Pake doublet to the featureless center component and then to a broader Pake doublet is similar to that of the 0.6 CD3CN/Mg sample, revealing similar origins. As mentioned earlier, a possible reason is that other multiple-site hopping motions may coexist between 263 and 153 K (Figures

the 0.6 (CD3)2CO/Mg sample again indicates the higher mobility of acetone in CPO-27-Zn than in CPO-27-Mg. The new spectral feature emerging when temperature is lowered below 173 K is the two new edges at ∼±40 kHz, and they become very apparent at 133 K. These two new edges belong to the pattern that is due solely to the internal rotation of the methyl group (Figure 8). Observing this pattern suggests that at 133 K the hopping motion ceases for a small fraction of the adsorbed acetone molecules, although for the vast majority of the acetone molecules the methyl rotation and three-site hopping still coexist. However, unlike the hopping in the 0.6 (CD3)2CO/Mg sample, the three-site hopping is still in the fast-limit regime at the lowest temperature measured (133 K), once again implying that the interaction between acetone and Zn2+ is weaker than that between acetone and Mg2+. C6D6 in CPO-27-M. MOFs with CPO-27 topology are promising candidates for the selective separation of alkenes from alkanes.17,18,20 The relatively strong interaction between an OMS and π system of an alkene is essential for this type of separation. In CPO-27-M (M = Mg and Zn), benzene can interact with exposed M2+ ions in such a way: the six Cδ−−Hδ+ bond dipoles of benzene combine to produce a region with negative electrostatic potential on the face of the π system. Electrostatic forces thus facilitate a natural attraction of cations to the surface of the π system, which is called the cation−π interaction.59 As Figure 9 illustrates, the 2H static spectrum of the 0.2 C6D6/Mg sample at 293 K exhibits a Pake doublet with a CQ of H


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Figure 9. Experimental and simulated 2H static spectra of the 0.2 C6D6/Mg sample as a function of temperature. Dynamic model for simulation: inplane rotation of benzene about its C6 axis followed by nonlocalized six-site hopping.

S5 and S6). In addition, the θ values may also vary for different hopping motions. 2 H static spectra of the 0.2 C6D6/Zn sample as a function of temperature are shown in Figure 10. The room-temperature (293 K) spectrum displays a typical Pake doublet with a horn splitting of ∼25 kHz (CQ = 33 kHz), corresponding to the benzene molecules that are undergoing rapid six-site molecular hopping and in-plane rotation. The angle (θ) between the C6 axis of benzene and the crystallographic c axis is 72°. The 2H static spectrum at 273 K consists of a new feature in the center. This feature then becomes broader with decreasing temperature, filling the gap between the two horns of the Pake doublet upon cooling. There is almost no change in the resulting flat top over a temperature range from 273 to 153 K. At 153 K, two shoulders at ∼±35 kHz and two edges at ∼±70 kHz become evident, which are the typical features of benzene that rotates in plane about its C6 axis, suggesting that these benzene molecules are strongly attached to Zn2+ ions. According to VT 2H SSNMR data, the binding strength of benzene on exposed Mg2+ is significantly stronger than that on Zn2+ because most of the benzene molecules in CPO-27-Mg have been transformed to bound species at 153 K. The cation−π interaction increases when the charge density of the ion increases. Mg2+ and Zn2+ have the same charge and very

similar ionic radii. However, the small electronegativity of Mg makes the Mg−O mainly ionic in nature, whereas there is a higher degree of covalency in the Zn−O bonding (i.e., more charge delocalization). Because Mg2+ has a higher charge density, it must possess a stronger cation−π interaction.

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CONCLUSIONS In this work, we examined the dynamics of four prototypical guest molecules (D2O, CD3CN, acetone-d6, and C6D6) in isostructural CPO-27-M (M = Mg and Zn) MOFs using variable-temperature 2H SSNMR spectroscopy, from which detailed dynamic information was obtained. The internal rotations of guest molecules, including the π flip-flop of D2O about the molecular C2 axis, internal rotation of the methyl group about its C3 axis, and in-plane rotation of C6D6 about its C6 axis, were identified. Additional reorientation of the whole molecule is essential to the simulation of 2H spectra of CD3CN, acetone-d6, and C6D6 in CPO-27-M, and this motion is the hopping of the guest molecule among neighboring M2+ ions. Different guest molecules exhibit different dynamic behavior upon adsorption in the same MOF. For a given guest molecule, its dynamics also depends on the nature of the metal ions. The relative binding strengths of the same guest molecule adsorbed on different metal ions were evaluated. It should be mentioned I


Article

Langmuir

Figure 10. Experimental and simulated 2H static spectra of the 0.2 C6D6/Zn sample as a function of temperature. Dynamic model for simulation: inplane rotation of benzene about its C6 axis followed by nonlocalized six-site hopping.

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that our results directly confirm the disordered local Mg environments in CPO-27-Mg samples probed previously in a 25 Mg SSNMR study. Detailed knowledge of the difference in dynamics of guest species in Mg- and Zn-CPO-27 available from 2H SSNMR spectroscopy will help to establish clear links between guest adsorption and the nature of the metal ions in porous MOFs, with clear applications in adsorption. The results presented in this work provide new physical insight into the behavior of prototypal guest species in CPO-27-M, a representative MOF with OMSs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00851.

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REFERENCES

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Additional experimental and simulated 2H SSNMR spectra and powder XRD patterns (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (519)661-2111, ext. 86384. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.H. thanks the Natural Science and Engineering Research Council (NSERC) of Canada for a Discovery Grant and a Discovery Accelerator Supplements Award. We thank the University of Western Ontario for a small ADF grant. J


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(M = Mg, Zn) by 2H Solid-State NMR Spectroscopy - ACS Publications

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