Many metal-organic framework (MOF) compounds display bendable porosity—i.e., their lattices can abide ample deformation. Knebel et al. formed film layers of the MOF ZIF-8 and begin that it was adapted into a arctic polymorph with a stiffer filigree in acknowledgment to an activated electric acreage (see the Angle by Gascon). This change bargain gas carriage but, for assertive gas mixtures such as propane and propene, additionally bigger their break factor.
Science, this affair p. 347; see additionally p. 303
Gas carriage through metal-organic framework membranes (MOFs) was switched in situ by applying an alien electric acreage (E-field). The switching of gas alteration aloft E-field animosity could be explained by the structural transformation of the zeolitic imidazolate framework ZIF-8 into polymorphs with added adamant lattices. Alteration abstracts beneath a direct-current E-field poling of 500 volts per millimeter showed reversibly controlled switching of the ZIF-8 into arctic polymorphs, which was accepted by x-ray diffraction and ab initio calculations. The stiffening of the filigree causes a abridgement in gas carriage through the film and sharpens the atomic sieving capability. Dielectric spectroscopy, polarization, and deuterium nuclear alluring resonance studies appear low-frequency resonances of ZIF-8 that we aspect to filigree adaptability and linker movement. Aloft E-field polarization, we empiric a defibrillation of the altered filigree motions.
Metal-organic frameworks (MOFs) are an arising chic of abstracts for gas separation, storage, and ablution (1–3). However, MOFs abridgement the abeyant for aciculate atomic sieving (4, 5). For example, the MOF ZIF-8, with a pore bore of 3.4 Å, allows the circulation of molecules with beyond active diameters, such as n-hexane, benzene, and mesitylene (6), because of filigree flexibility. Indeed, MOFs are currently referred to as “soft absorptive crystals” (7). Recently, Bennett et al. presented a angle on the coaction of altered defects and disorders, with examples to prove that actual birthmark engineering enhances break in MOFs (8). Many studies accept been performed to acknowledge the adaptability mechanisms (9), but alone one abstract abstraction has explored how to accomplishment the adaptability by agency of electric fields (E-fields) (10).
Soft porosity is generally accompanying to breath effects, microburst deformation, and bendable modes (4, 11). Thermally triggered filigree motions accept been affiliated to bedfellow transport; for instance, in ZIF-8, the appropriate activity for filigree movement is low, appropriately acceptance advancement throughout the framework (12). Nonetheless, adamant MOFs additionally exist, such as HKUST-1, which contains anchored tridentate linker molecules in which alone thermal filigree accordance are empiric (13). Terahertz spectroscopy of ZIF-8 appear that linker circling (14), microburst deformation, and breath and bendable modes are thermally activated aural the ZIF-8 filigree (11) and are all-knowing at allowance temperature. In antecedent studies, alien E-fields afflicted solutions (15) and dispersions (16) through ionic, dipolar, and cyberbanking interactions.
We appearance that the stiffening of the ZIF-8 filigree through E-field animosity can abate atomic carriage and access atomic sieving capability. The accident of filigree distortions is able-bodied known, but our compassionate up to now is bound to ferro- or piezoelectric MOFs (17, 18) or abstracts in general. In addition, appearance changes in conductive allocation polymers beneath electric currents accept been empiric (19). However, we could not acquisition any affirmation via animosity abstracts for the ferroelectric behavior of ZIF-8. We adapt the empiric furnishings of ZIF-8 in an E-field as capricious appearance transformations amid the accepted cubic acceding and polarized monoclinic or triclinic amplitude groups.
We empiric signals for gating furnishings in ZIF-8; Devautour-Vinot et al. (20) and Frunza et al. (21) showed these furnishings in samples of UiO-66(Zr) and MOF-5, respectively, appliance dielectric spectroscopy and nuclear alluring resonance (NMR) (22, 23). The use of 2H NMR enabled altitude of actual low frequencies of MOFs, apprehension appropriate bands attributed to filigree and linker motions. Because bendable modes and microburst anamorphosis are acoustic phenomena at allowance temperature in ZIF-8 (11), we assured that the filigree was stiffened through E-field polarization.
The linker motion in ZIF-8 was nonetheless afflicted by polarization; however, the activation barrier for complete alignment with the electric fields was actual high, with a amount of 49 to 60 kJ/mol. The accomplished accession to the permittivity change originated from the ions (24). As the admeasurement selectivity and gas carriage were switched by polarizing the ZIF-8 membrane, the selectivity for a propene-propane admixture increased. Thus, induced animosity is a nondestructive way to ascendancy gas carriage through polycrystalline MOF layers. We appearance that capricious distortions of apparent ZIF-8 membranes with E-fields can enhance atomic sieving. The circuitous apparatus basal this aftereffect was advised by DFT calculation, 2H NMR, dielectric spectroscopy and polarization, and gas alteration experiments; all techniques gave carefully commensurable results.
We acclimated a polytetrafluoroethylene (PTFE) Wicke-Kallenbach alteration corpuscle (Fig. 1, A and B) to accomplish E-field–assisted gas alteration abstracts in situ on ZIF-8 membranes. [See (25) for accurate altitude protocols.] Scanning electron microscopy (SEM) of cantankerous sections of the Al2O3- and TiO2-supported ZIF-8 film layers and energy-dispersive x-ray spectroscopy (EDXS) were performed (Fig. 1, C and D) to actuate the advance of the ZIF-8 layers, which were 1 μm and 20 μm thick, respectively. The MOF layers were able-bodied intergrown and did not access the supports.
(A and B) The alteration corpuscle for in situ E-field–controlled gas alteration measurements. (C and D) Cross-sectional SEM images of the ZIF-8 band on a acknowledging ceramic. In (C), ZIF-8 of 1-μm array is developed on top of an Al2O3 acknowledging layer; in (D), ZIF-8 of 20-μm array is developed on top of a TiO2 support. Calibration bars, 2 μm (C), 10 μm (D). Color cipher of EDXS: blue, Zn L-α1,2; red, Ti K-α1; green, Al K-α1.
Gas carriage through the ZIF-8 film layers was measured, and the permeances (moles of gas per delayed time, film area, and burden difference) were in acceptable accordance with abstract ethics (Fig. 2, A to C) (26–28). The single-gas permeances of H2, CO2, and CH4 on the 20-μm ZIF-8 film band were abstinent with and afterwards an in situ–applied E-field of 500 V/mm erect to the MOF band (Fig. 2A). Cycles of switching amid the polymorphs were found—for example, for CO2 appliance both the 20-μm and 1-μm ZIF-8 membranes (Fig. 2, C and E). ZIF-8 is accepted as one of the best film break abstracts for propene and propane (C3H6/C3H8) (27, 28). We afar such a admixture appliance the 1-μm ZIF-8 film (Fig. 2F). The break agency for the C3H6/C3H8 admixture added from 6 to 8 afterwards applying the E-field.
In (A), the in situ abstinent single-gas permeances for H2, CO2, and CH4 are apparent afore applying an E-field, aloft appliance of an E-field, and afterwards relaxation. For all gases, the permeances abatement back an E-field of 500 V/mm is applied. Back the E-field is switched off, afterwards a alleviation time of ~100 min, the permeance anon increases. (B) The ideal selectivities α (quotient of the ideal permeances P1/P2) were affected from these single-gas permeances. The admeasurement selectivity decreases for abate gases afterwards polarization. (C and E) A affirmation of the in situ switching of CO2 is apparent in a artifice of permeance against time for the 20-μm film (C) and the 1-μm film (E). The CO2 permeance decreases back the E-field is applied, charcoal in this accompaniment for about 100 min afterwards abeyance of the E-field, and again recovers; the permeance of alone the attenuate film recovers completely. (D) The single-gas permeances through the HKUST-1 film appearance no acknowledgment to the E-field. (F) For propene-propane mixtures in ZIF-8, the complete break agency α (quotient of the complete permeances Ppropene/Ppropane) increases aloft E-field poling.
Polarity changeabout of the E-field yielded the aforementioned aftereffect on gas transport. Because ZIF-8, as a adjustable MOF, allows aboideau opening, microburst deformation, and bendable approach appearance changes (11), HKUST-1, as a adamant MOF, was advised for comparison. As expected, no E-field access on permeance through the HKUST-1 film could be empiric (Fig. 2D), as no microburst anamorphosis or aboideau aperture is accessible (13). The permeances of the delving gases H2, CO2, and CH4 through the HKUST-1 band were in acceptable acceding with appear ethics (29).
Given this difference, an ionic displacement and a stiffening of the arrangement were advised to annual for the switching of the gas carriage through the ZIF-8 layer. As apparent in Fig. 2, C and E, alleviation of the polarized ZIF-8 band afterwards abatement of the E-field appropriate ~1.5 hours in a connected gas flow. In addition, at atomic for the 20-μm layer, the permeance ethics did not absolutely acknowledgment to the aboriginal values, alike afterwards an added 104 min. It is cryptic why the 20-μm ZIF-8 band did not acknowledgment to the aboriginal break performance, although the 1-μm attenuate band did. Because we begin that the permeances of all gases through the ZIF-8 film abatement back the E-field is activated (Fig. 2A), this aftereffect seems not to be benign at aboriginal glance. The ideal and complete break factors—calculated as the arrangement of the single-gas or mixed-gas permeances—changed via the E-field–induced switching process. Figure 2A shows the single-gas alteration ethics for the baby delving molecules H2, CO2, and CH4; Fig. 2B shows the affected ideal break factors for the 20-μm band before, during, and afterwards applying an E-field of 500 V/mm. Back applying the E-field, the selectivities and permeances were lower for the abate molecules than for the beyond ones (from H2 and CO2 to CH4). A baby addition of pores in the stiffened ZIF-8 could explain the lower selectivity for abate gases and the college selectivity for propene-propane mixtures. Afterwards the membranes relaxed, the aboriginal admeasurement selectivity was absolutely recovered alone for the 1-μm ZIF-8 membrane.
As begin by in situ gas alteration measurements, the MOF film was captured in a “switched” accompaniment afterwards antecedent animosity for 1 to 2 hours, appropriately enabling the assay of the “switched” ZIF-8. Afore alleviation occurred, the ZIF-8 film was analyzed by x-ray diffraction (XRD) (fig. S1). The XRD assay of the polarized ZIF-8 film appear a compatible filigree strain. The XRD after-effects were aesthetic appliance TOPAS 4.1 with Rietveld refinement. The acuteness changes and reflex accouterment (fig. S1) are appropriate of compatible strain. [See (25) for filigree abstracts for the polarized R3m and Cm polymorphs of ZIF-8.] Filigree baloney (fig. S1) confused the reflexes to lower 2θ values, as depicted in a affected filigree bury in Fig. 3A.
(A) Filigree bury of the undisturbed ZIF-8 aboideau (blue) with the aforementioned filigree adulterated by an alien abeyant U of the electric acreage backbone (red), |U| = 0.02 eV Å−1 e−1 in the 〈111〉 direction. See (25) for added beginning and abstract XRD data. (B) Dielectric spectra of the abstract allotment of the permittivity from 473 to 373 K, in accomplish of 20 K, over the abundance ambit 100 to 105 Hz with a voltage amplitude of 0.1 V. ZIF-8 shows a moment that responds to an AC E-field, as advised by dielectric alleviation spectroscopy. (C) 2H NMR T1 (squares) and T2 (circles) alleviation curves of ZIF-8 as a action of temperature for the C-D accumulation of the deuterated 2-mIM linker in ZIF-8; a actual apathetic motion is empiric in the arena apparent in yellow. The after simulation after-effects are apparent as solid lines. (D) The aforementioned abstracts on an broadcast temperature scale. (E) The change of T1 and T2 alleviation times with temperature, assuming the attendance of three capital motions: (i) fast (τa ~ 1 ps) but about small-amplitude librations (ϕa = ±30° at ~500 K) of the linker about the ZIF-8 window even (22) and two abundant slower modes, (ii) and (iii). (F) A representation of the archaic beef of the ZIF-8 aboideau as a space-filling archetypal of the altered apish polymorphs. Through anamorphosis of the assemblage cell, the imidazolate linkers adjust (along with stiffening the network) and thereby abstain steric activity barriers of the angled assemblage cells.
Through ab initio body anatomic approach (DFT) calculations, we begin that appearance transformation changes the pore attached bore from 3.4 Å for cubic ZIF-8 to 3.6 Å for the Cm phase, which is the best ascendant polarizable appearance (figs. S1 and S6 to S8). The decreased permeances are acquired by the stiffening in the ZIF-8 lattice, and we affected a aggregate modulus access from 29 to 61 (25). This stiffening additionally influences the linker motion; linker rotations (Fig. 3D) are primarily inhibited, as apparent in Fig. 3F. This stiffening originates from changes in the band angles and altered linker orientations for the alone polymorphs, apparent as blooming and red linkers in Fig. 3F. However, the film charcoal intact, as ahead accepted by gas alteration abstracts (Fig. 2, A to C), because aition of the film would appearance gas breakthrough.
When an E-field is applied, a polymorphic appearance transformation occurs (Fig. 3A and fig. S1). To added investigate these results, we performed dielectric spectroscopy assay afterward procedures agnate to those acclimated with MOFs UiO-66 and MOF-5 (20, 21). We activated dielectric spectroscopy to ZIF-8, as its linker 2-mIM is dipolar and rotatable; Fig. 3B shows the abstract allotment ε′′ of the permittivity ε advised against abundance disconnected by 2π. The dielectric acknowledgment of ZIF-8 is axiomatic by the temperature-dependent maxima in the permittivity, which confused with accretion temperature to college frequencies. The atomic resonances were aflame by an AC voltage amplitude of 0.1 V. The temperature access on the position of the ε′′ maxima was investigated. The maxima accord to a alleviation process, whose alleviation amount τ–1 is declared by the affiliation ωτ ≈ 1, area ω is the angular abundance of the AC voltage. The positions of the aiguille maxima confused to college frequencies as the temperature increased. This award credibility to a thermally activated process, which could be declared by an Arrhenius affiliation τ–1 = τ0–1 exp(–EA/kT), area EA is activation energy, k is the Boltzmann constant, and T is complete temperature (30). The after-effects were attributed to apathetic linker or filigree motions, which lie about in the abundance arena of 10 Hz to 10 kHz. The amount of EA for the motion was 49 ± 6 kJ/mol and the prefactor τ0–1 was 1.7 × 109 s–1. Our allegation are agnate to those for added MOFs (20, 21).
If our beginning allegation corresponded to dipolar linker motion, a gate-opening-and-closing resonance would be observed. From the dielectric spectroscopy results, it can be affected that a able changeless DC E-field can adjust the dipoles. However, aboideau aperture was affected by DFT, and the appropriate activity was too aerial for E-field alignment in our agreement (25). We performed 2H NMR to added investigate linker advancement (Fig. 3, C to E). The temperature dependences of the T1 and T2 relaxations for the deuterated C-D accumulation of 2-mIM accommodate advice about altered motions apparent by this linker. The T1 alleviation is best acute to fast motions with ante commensurable to ω0 (where ω0/2π = 6.143 × 107 Hz is the NMR abundance of the 2H nuclei), admitting the T2 alleviation can be actual acute to slower modes. If the motion is an activated process, τ = τ0 exp(EA/RT) (where τ is the alternation time of the motion), the T1 alleviation is characterized by a valley-like temperature assurance with a bright minimum (marked with an asterisk in Fig. 3D) agnate to the temperature area τ−1 ≈ ω0. The abruptness of the alleviation ambit alfresco the minimum is bent by the activation activity EA of this accurate motion.
If there are assorted motions, again the able alternation amount is formed by a beeline aggregate of the appropriate alternation ante of the accurate motions. As a result, if the appropriate alternation times and agnate activity barriers are essentially different, the alleviation time temperature assurance ambit contains assorted minima afar by regions of bounded maxima or ambit bends. In the present case, the beginning temperature assurance showed absolutely this situation. A actual apathetic motion is apparent in Fig. 3, C and D, in the arena apparent in yellow, characterized by a actual ample activation barrier of EA = 60 ± 5 kJ/mol and a pre-exponential agency of τ0 ≈ 10–10 s. We accept that this motion and that begin by dielectric spectroscopy are the same. Geometrically, this award can be interpreted as a large-amplitude ϕϕc = 80° (±10°) aberration of the linker about to the ZIF-8 window even at a amount of ~1 Hz to 5 × 104 Hz aural the temperature ambit of 300 to 600 K, which is the aforementioned abundance ambit as that in dielectric spectroscopy. An analogy of the accessible motions is apparent in Fig. 3E. The stiffening of the arrangement and the altered pore diameters are pictured in Fig. 3F; from experiments, we begin that Cm is the best ascendant phase, which has a pore bore of 3.6 Å with a abundant college aggregate modulus, appropriately stiffening and cutting atomic sieving at 3.6 Å, optimal for propane-propene separation. It should be accessible to added get ZIF-8 into the R3m appearance with abundant college fields, area the pore attached bore would be 3.1 Å (Fig. 3F) (25).
After belief the circuitous filigree animosity of ZIF-8, we advised the ionic filigree acknowledgment in detail. A piezo- or ferroelectric acknowledgment of ZIF-8 was accepted from the I-43m amplitude group. From dielectric spectroscopy and NMR analysis, we apperceive that the Zn2 ions of ZIF-8 contribute, because the activation barrier for dipolar linker motion is too aerial for the activated acreage (24). Appliance a 200-μm ZIF-8 distinct clear as a capacitor (Fig. 4, A and B), we abstinent the abundance assurance of the electrical application (Fig. 4C) and begin a beeline correlation, with no ferro- or piezoelectric response. In the animosity abstracts (Fig. 4D), no ferroelectric hysteresis bend was observed, but “soft” filigree movement in the animosity administration is displayed as a ample curve, which is appropriate of bendable materials. Afterwards ferroelectric hysteresis, no affirmation for the classical ferro- and piezoelectric ionic acknowledgment of the filigree could be found. Thus, the filigree acknowledgment begin by XRD (fig. S1) can be affected to be neither a classical ferroelectric acknowledgment nor a piezoelectric response.
(A) Wiring diagram of the ZIF-8 single-crystal animosity measurement. Calibration bar, 200 μm. (B) SEM angel of the clear at 20 kV amid segments of nut adhering tape. The calibration bar and the behemothic ZIF-8 clear are 200 μm. (C) The electrical application in the ambit of 1 to 107 Hz. The after-effects appearance a about beeline curve, advertence the clear to be about an ideal insulator afterwards a piezo- or ferroelectric response. (D) The animosity ambit is abstinent at 20 kHz over the ambit of –775 to 775 V/mm. ZIF-8 shows a ample animosity ambit appropriate of soft-material deformation, but afterwards ferroelectric hysteresis.
The aing 2-methylimidazolate linker molecules in ZIF-8 comedy a analytical role in gas separation. Adsorption and circulation on MOFs are accompanying to linker movements. Commensurable to a defibrillation, we could abate the framework fibrillation aloft E-field animosity and anneal the network. Here, the ZIF-8 filigree was polarized and a transformation from cubic to monoclinic and triclinic polymorphs, which led to apparent changes in the gas alteration performance, occurred. However, with such low fields, the linkers were not absolutely accumbent by the E-field. An estimated 30% of the MOF film band was adapted into polymorphs with metastable ZIF-8 states as the filigree stiffened (fig. S1). This switching led to decreased break selectivity for baby molecules, such as H2, CO2, and CH4. In contrast, for propene-propane mixtures, the stiffening of ZIF-8 bigger atomic sieving, and the selectivity added from 6 to 8. In the Cm amplitude group, the attached pore bore of ZIF-8 afflicted from 3.4 to 3.6 Å in the polymorph. With college E-fields than acclimated in this paper, a abounding appearance transformation should be possible, appropriately added acceptable gas separation. Added bendable MOFs could be acclimated as acute abstracts through electrical polarization. Force-field calculations appearance that a complete linker alignment cannot be accomplished by our anemic E-fields, as the activation barrier lies amid 49 and 60 kJ/mol. However, as apparent through the animosity studies and impedance spectroscopy, a bendable approach or microburst anamorphosis can be found, but no classical ferro- or piezoelectric acknowledgment was empiric in the lattice.
P. Heitjans, J. Kärger, Eds., Circulation in Condensed Matter—Methods, Abstracts and Models (Springer, ed. 2, 2005).
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