B18-Core EXAFS
|
Jamie W.
Gittins
,
Chloe J.
Balhatchet
,
Yuan
Chen
,
Cheng
Liu
,
David G.
Madden
,
Sylvia
Britto
,
Matthias J.
Golomb
,
Aron
Walsh
,
David
Fairen-Jimenez
,
Sian E.
Dutton
,
Alexander C.
Forse
Diamond Proposal Number(s):
[14239]
Open Access
Abstract: Two-dimensional electrically conductive metal–organic frameworks (MOFs) have emerged as promising model electrodes for use in electric double-layer capacitors (EDLCs). However, a number of fundamental questions about the behaviour of this class of materials in EDLCs remain unanswered, including the effect of the identity of the metal node and organic linker molecule on capacitive performance, and the limitations of current conductive MOFs in these devices relative to traditional activated carbon electrode materials. Herein, we address both these questions via a detailed study of the capacitive performance of the framework Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with an acetonitrile-based electrolyte, finding a specific capacitance of 110–114 F g−1 at current densities of 0.04–0.05 A g−1 and a modest rate capability. By directly comparing its performance with the previously reported analogue, Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), we illustrate that capacitive performance is largely independent of the identity of the metal node and organic linker molecule in these nearly isostructural MOFs. Importantly, this result suggests that EDLC performance in general is uniquely defined by the 3D structure of the electrodes and the electrolyte, a significant finding not demonstrated using traditional electrode materials. Finally, we probe the limitations of Cu3(HHTP)2 in EDLCs, finding a limited stable double-layer voltage window of 1 V and only a modest capacitance retention of 81% over 30 000 cycles, both significantly lower than state-of-the-art porous carbons. These important insights will aid the design of future conductive MOFs with greater EDLC performances.
|
Jun 2021
|
|
I11-High Resolution Powder Diffraction
|
Open Access
Abstract: Stimuli-responsive behaviors of flexible metal–organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO2-adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO2 migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility.
|
Mar 2019
|
|
I19-Small Molecule Single Crystal Diffraction
|
Abstract: Many zeolitic imidazolate frameworks (ZIFs) are promising candidates for use in separation technologies. Comprising large cavities interconnected by small windows they can be used, at least in principle, as molecular sieves where molecules smaller than the window size are able to diffuse into the material while larger are rejected. However, “swing effect” or “gate opening” phenomena resulting in an enlargement of the windows have proven to be detrimental. Here, we present the first systematic experimental and computational study of the effect of chemical functionalisation of the imidazole linker on the framework dynamics. Using high-pressure (HP) single-crystal X-ray diffraction, density functional theory, and grand canonical Monte Carlo simulations, we show that in the isostructural ZIF-8, ZIF-90 and ZIF-65 functional groups of increasing polarity (-CH3,-CHO, -NO2) on the imidazole linkers provide control over the degree of rotation and thus the critical window diameter. On application of pressure, the substituted imidazolate rings rotate resulting in an increase in both pore volume and content. Our results show that the interplay between the guest molecules and the chemical function of the imidazole linker is essential for directing the swing effect in ZIF frameworks and therefore the adsorption performance.
|
Dec 2017
|
|
I11-High Resolution Powder Diffraction
I12-JEEP: Joint Engineering, Environmental and Processing
I15-Extreme Conditions
|
Matthew J.
Cliffe
,
Elizabeth
Castillo-martinez
,
Yue
Wu
,
Jeongjae
Lee
,
Alexander C.
Forse
,
Francesca C. N.
Firth
,
Peyman Z.
Moghadam
,
David
Fairen-jimenez
,
Michael W.
Gaultois
,
Joshua A.
Hill
,
Oxana V.
Magdysyuk
,
Ben
Slater
,
Andrew L.
Goodwin
,
Clare P.
Grey
Diamond Proposal Number(s):
[9940, 15118, 12554, 13681, 13843]
Abstract: We report a hafnium containing MOF, hcp UiO-67(Hf), which is a defective layered analogue of the face-centered cubic fcu UiO-67(Hf). hcp UiO-67 accommodates its low- ered ligand:metal ratio compared to fcu UiO-67 through a new structural mechanism: the formation of a condensed ‘double cluster’, analogous to the condensation of coor- dination polyhedra in oxide frameworks. In oxide frameworks variable stoichiometry can lead to more complex defect structures, e.g. crystallographic shear planes or mod- ules with differing compositions, which can be the source of further chemical reactivity; likewise the layered hcp UiO-67 can react further to form two-dimensional nanosheets. Delamination of hcp UiO-67 occurs through the cleavage of strong hafnium-carboxylate bonds and is effected under mild conditions suggesting that defect ordered MOFs could be a productive route to porous two-dimensional materials.
|
Mar 2017
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[9750]
Abstract: The flexibility and structure transition behaviour of ZIF-8 in a series of samples with different particle size has been studied using a combination of high-resolution N2 gas adsorption isotherms and, for the first time, a broad in situ PXRD and Rietveld analysis. During the stepped adsorption process, large particles showed a narrow adsorption/desorption pressure range with a shorter equilibrium time due to lower kinetic hindrance, deriving from higher amount of active sites. In situ PXRD showed that both the rotation of imidazole ring and a bend in the methyl group led to the gate opening of ZIF-8
|
Feb 2016
|
|
I11-High Resolution Powder Diffraction
|
Abstract: Hungry hungry micropores: Reaction of [Zn4(μ3-OH)2(L)4(tBu)2] (LH=8-hydroxyquinoline) with CO2 forms a 3D microporous material with spherical zinc carbonate nanoclusters packed in a diamondoid manner in the solid state. This compound exhibits one of the highest Brunauer—Emmett—Teller surface areas for a noncovalent porous material, with high H2, CO2, and CH4 uptake.
|
Oct 2013
|
|
I11-High Resolution Powder Diffraction
|
Abstract: Ab initio molecular dynamics (AIMD) simulations have been used to predict structural transitions of the breathing metal–organic framework (MOF) MIL-53(Sc) in response to changes in temperature over the range 100–623 K and adsorption of CO2 at 0–0.9 bar at 196 K. The method has for the first time been shown to predict successfully both temperature-dependent structural changes and the structural response to variable sorbate uptake of a flexible MOF. AIMD employing dispersion-corrected density functional theory accurately simulated the experimentally observed closure of MIL-53(Sc) upon solvent removal and the transition of the empty MOF from the closed-pore phase to the very-narrow-pore phase (symmetry change from P21/c to C2/c) with increasing temperature, indicating that it can directly take into account entropic as well as enthalpic effects. We also used AIMD simulations to mimic the CO2 adsorption of MIL-53(Sc) in silico by allowing the MIL-53(Sc) framework to evolve freely in response to CO2 loadings corresponding to the two steps in the experimental adsorption isotherm. The resulting structures enabled the structure determination of the two CO2-containing intermediate and large-pore phases observed by experimental synchrotron X-ray diffraction studies with increasing CO2 pressure; this would not have been possible for the intermediate structure via conventional methods because of diffraction peak broadening. Furthermore, the strong and anisotropic peak broadening observed for the intermediate structure could be explained in terms of fluctuations of the framework predicted by the AIMD simulations. Fundamental insights from the molecular-level interactions further revealed the origin of the breathing of MIL-53(Sc) upon temperature variation and CO2 adsorption. These simulations illustrate the power of the AIMD method for the prediction and understanding of the behavior of flexible microporous solids.
|
Jun 2013
|
|
I11-High Resolution Powder Diffraction
|
Abstract: The scandium analogue of the flexible terephthalate MIL-53 yields a novel closed pore structure upon removal of guest molecules which has unusual thermal behaviour and stepwise opening during CO2 adsorption. By contrast, the nitro-functionalised MIL-53(Sc) cannot fully close and the structure possesses permanent porosity for CO2.
|
Jan 2012
|
|
I11-High Resolution Powder Diffraction
|
Abstract: The crystal structure of the small pore scandium terephthalate Sc2(O2CC6H4CO2)3 (hereafter Sc2BDC3, BDC = 1,4-benzenedicarboxylate) has been investigated as a function of temperature and of functionalization, and its performance as an adsorbent for CO2 has been examined. The structure of Sc2BDC3 has been followed in vacuo over the temperature range 140 to 523 K by high resolution synchrotron X-ray powder diffraction, revealing a phase change at 225 K from monoclinic C2/c (low temperature) to Fddd (high temperature). The orthorhombic form shows negative thermal expansivity of 2.4 × 10–5 K–1: Rietveld analysis shows that this results largely from a decrease in the c axis, which is caused by carboxylate group rotation. 2H wide-line and MAS NMR of deuterated Sc2BDC3 indicates reorientation of phenyl groups via π flips at temperatures above 298 K. The same framework solid has also been prepared using monofunctionalized terephthalate linkers containing -NH2 and -NO2 groups. The structure of Sc2(NH2-BDC)3 has been determined by Rietveld analysis of synchrotron powder diffraction at 100 and 298 K and found to be orthorhombic at both temperatures, whereas the structure of Sc2(NO2-BDC)3 has been determined by single crystal diffraction at 298 K and Rietveld analysis of synchrotron powder diffraction at 100, 298, 373, and 473 K and is found to be monoclinic at all temperatures. Partial ordering of functional groups is observed in each structure. CO2 adsorption at 196 and 273 K indicates that whereas Sc2BDC3 has the largest capacity, Sc2(NH2-BDC)3 shows the highest uptake at low partial pressure because of strong -NH2···CO2 interactions. Remarkably, Sc2(NO2-BDC)3 adsorbs 2.6 mmol CO2 g–1 at 196 K (P/P0 = 0.5), suggesting that the -NO2 groups are able to rotate to allow CO2 molecules to diffuse along the narrow channels.
|
Sep 2011
|
|
I11-High Resolution Powder Diffraction
|
Abstract: ZIF-8 is a zeolitic imidazole-based metal-organic framework with large cavities interconnected by narrow windows. Because the small size of the windows, it allows in principle for molecular sieving of gases such as H2 and CH4. However, the unexpected adsorption of large molecules on ZIF-8 suggests the existence of structural flexibility. ZIF-8 flexibility is explored in this work combining different experimental techniques with molecular simulation. We show that the ZIF-8 structure is modified by gas adsorption uptake in the same way as it is at very high pressure (i.e. 14,700 bar) due to a swing effect in the imidazolate linkers, giving access to the porosity. Tuning the flexibility, and so the opening of the small windows, has further impact on the design of advanced molecular sieving membrane materials for gas separation, adjusting the access of fluids to the porous network.
|
May 2011
|
|
