I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[19670]
Abstract: Flexible metal–organic frameworks (MOFs) hold great promise as smart materials for specific applications such as gas separation. These materials undergo interesting structural changes in response to guest molecules, which is often associated with unique adsorption behavior not possible for rigid MOFs. Understanding the dynamic behavior of flexible MOFs is crucial yet challenging as it involves weak host–guest interactions and subtle structural transformation not only at the atomic/molecular level but also in a nonsteady state. We report here an in-depth study on the adsorbate- and temperature-dependent adsorption in a flexible MOF by crystallizing atomic gases into its pores. Mn(ina)2 shows an interesting temperature-dependent response toward noble gases. Its nonmonotonic, temperature-dependent adsorption profile results in an uptake maximum at a temperature threshold, a phenomenon that is unusual. Full characterization of Xe-loaded MOF structures is performed by in situ single-crystal and synchrotron X-ray diffraction, IR spectroscopy, and molecular modeling. The X-ray diffraction analysis offers a detailed explanation into the dynamic structural transformation and provides a convincing rationalization of the unique adsorption behavior at the molecular scale. The guest and temperature dependence of the structural breathing gives rise to an intriguing reverse of Xe/Kr adsorption selectivity as a function of temperature. The presented work may provide further understanding of the adsorption behavior of noble gases in flexible MOF structures.
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Nov 2020
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[21497]
Abstract: An organic subcomponent was designed with 2-formyl-8-aminoquinoline and triazole-pyridine ends. The relative orientations and geometries of these two ends enabled this subcomponent to assemble together with ZnII and LaIII cations to generate a heterobimetallic tetrahedral capsule. The LaIII cations each template three imine bonds that hold together a 3-fold-symmetric metallo-ligand, defining the center of each tetrahedron face. The ZnII cations occupy the other ends of these C3 axes, defining the vertices of the tetrahedron. This is the first example where subcomponent self-assembly brought into being the faces of a polyhedron, as opposed to the vertices. Host–guest studies show positively cooperative binding toward ReO4–, the encapsulation of which also resulted in the quenching of capsule fluorescence.
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Nov 2020
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B18-Core EXAFS
I15-Extreme Conditions
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Diamond Proposal Number(s):
[22856, 22930]
Abstract: For magnesium ion batteries (MIBs) to be used commercially, new cathodes must be developed that show stable reversible Mg intercalation. VS4 is one such promising material, with vanadium and disulfide anions [S2]2– forming one-dimensional linear chains, with a large interchain spacing (5.83 Å) enabling reversible Mg insertion. However, little is known about the details of the redox processes and structural transformations that occur upon Mg intercalation and deintercalation. Here, employing a suite of local structure characterization methods including X-ray photoelectron spectroscopy (XPS), V and S X-ray absorption near-edge spectroscopy (XANES), and 51V Hahn echo and magic-angle turning with phase-adjusted sideband separation (MATPASS) NMR, we show that the reaction proceeds via internal electron transfer from V4+ to [S2]2–, resulting in the simultaneous and coupled oxidation of V4+ to V5+ and reduction of [S2]2– to S2–. We report the formation of a previously unknown intermediate in the Mg–V–S compositional space, Mg3V2S8, comprising [VS4]3– tetrahedral units, identified by using density functional theory coupled with an evolutionary structure-predicting algorithm. The structure is verified experimentally via X-ray pair distribution function analysis. The voltage associated with the competing conversion reaction to form MgS plus V metal directly is similar to that of intermediate formation, resulting in two competing reaction pathways. Partial reversibility is seen to re-form the V5+ and S2– containing intermediate on charging instead of VS4. This work showcases the possibility of developing a family of transition metal polychalcogenides functioning via coupled cationic–anionic redox processes as a potential way of achieving higher capacities for MIBs.
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Oct 2020
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B22-Multimode InfraRed imaging And Microspectroscopy
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Jiangnan
Li
,
Zhengyang
Zhou
,
Xue
Han
,
Xinran
Zhang
,
Yong
Yan
,
Weiyao
Li
,
Gemma L.
Smith
,
Yongqiang
Cheng
,
Laura J.
Mcormick Mpherson
,
Simon J.
Teat
,
Mark D.
Frogley
,
Svemir
Rudic
,
Anibal J.
Ramirez-cuesta
,
Alexander J.
Blake
,
Junliang
Sun
,
Martin
Schroeder
,
Sihai
Yang
Diamond Proposal Number(s):
[22137]
Abstract: Structural transitions of host systems in response to guest binding dominate many chemical processes. We report an unprecedented type of structural flexibility within a meta-rigid material, MFM-520, which exhibits a reversible periodic-to-aperiodic structural transition resulting from a drastic distortion of a [ZnO4N] node controlled by the specific host–guest interactions. The aperiodic crystal structure of MFM-520 has no three-dimensional (3D) lattice periodicity but shows translational symmetry in higher-dimensional (3 + 2)D space. We have directly visualized the aperiodic state which is induced by incommensurate modulation of the periodic framework of MFM-520·H2O upon dehydration to give MFM-520. Filling MFM-520 with CO2 and SO2 reveals that, while CO2 has a minimal structural influence, SO2 can further modulate the structure incommensurately. MFM-520 shows exceptional selectivity for SO2 under flue-gas desulfurization conditions, and the facile release of captured SO2 from MFM-520 enabled the conversion to valuable sulfonamide products. MFM-520 can thus be used as a highly efficient capture and delivery system for SO2.
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Oct 2020
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I11-High Resolution Powder Diffraction
I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[21726, 23666]
Abstract: Three-dimensional (3D) covalent organic frameworks (COFs) are rare because there is a limited choice of organic building blocks that offer multiple reactive sites in a polyhedral geometry. Here, we synthesized an organic cage molecule (cage-6-NH2), which was used as a triangular prism node to yield the first cage-based 3D COF, 3D-CageCOF-1. This COF adopts an unreported twofold interpenetrated acs topology and exhibits reversible dynamic behavior, switching between a small-pore (sp) structure and a large-pore (lp) structure. It also shows high CO2 uptake and captures water at low humidity (<40%). This demonstrates the potential for expanding the structural complexity of 3D COFs by using organic cages as the building units.
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Sep 2020
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[22214]
Abstract: Polyoxometalate Molybdenum blue (MBs) complexes typically exist as discrete multianionic clusters and are composed of repeating Mo building units. MB Wheels, such as {Mo176} and {Mo154} are made from pentagon-centered {Mo8} building blocks joined by equal number of {Mo1} units as loin, and {Mo2} dimer units as skirt along the ring edge and the ring sizes of the Mo-blue wheels are modulated by the {Mo2} units. Herein we report a new class of contracted lanthanide-doped MB structures that have replaced all the {Mo2} units with lanthanide ions on the ring edge with the general formula {Mo90+2nLn10-n} (n = 0, 1). We show three examples of this new decameric {Mo90Ln10} (Ln = La, Ce and Pr) framework synthesized by hydrothermal methods, and demonstrate that later Ln ions result in {Mo92Nd9}, conserving one {Mo2} linker unit in its structure, as a consequence of the lanthanide contraction. Remarkably the decameric {Mo90Ln10} compounds are the first examples of charge-neutral molybdenum wheels as confirmed by BVS and redox titrations. We detail our full synthetic optimization for the isolation of these clusters and complete characterization by X-ray, TGA, UV-vis and ICP studies. Finally we show that this fine-tuned self assembly process can be utilised to selectively enrich Ln-Mo Blue wheels for effective separation of lanthanide ions.
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Sep 2020
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B22-Multimode InfraRed imaging And Microspectroscopy
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Xinchen
Kang
,
Bin
Wang
,
Kui
Hu
,
Kai
Lyu
,
Xue
Han
,
Ben F.
Spencer
,
Mark D.
Frogley
,
Floriana
Tuna
,
Eric J. L.
Mcinnes
,
Robert A. W.
Dryfe
,
Buxing
Han
,
Sihai
Yang
,
Martin
Schroeder
Diamond Proposal Number(s):
[19171]
Open Access
Abstract: Efficient electro-reduction of CO2 over metal–organic framework (MOF) materials is hindered by the poor contact between thermally synthesized MOF particles and the electrode surface, which leads to low Faradaic efficiency for a given product and poor electrochemical stability of the catalyst. We report a MOF-based electrode prepared via electro-synthesis of MFM-300(In) on an indium foil, and its activity for the electrochemical reduction of CO2 is assessed. The resultant MFM-300(In)-e/In electrode shows a 1 order of magnitude improvement in conductivity compared with that for MFM-300(In)/carbon-paper electrodes. MFM-300(In)-e/In exhibits a current density of 46.1 mA cm–2 at an applied potential of −2.15 V vs Ag/Ag+ for the electro-reduction of CO2 in organic electrolyte, achieving an exceptional Faradaic efficiency of 99.1% for the formation of formic acid. The facile preparation of the MFM-300(In)-e/In electrode, coupled with its excellent electrochemical stability, provides a new pathway to develop efficient electro-catalysts for CO2 reduction.
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Sep 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[17377, 19072]
Abstract: Understanding and tuning the catalytic properties of metals atomically dispersed on oxides are major stepping-stones towards a rational development of single-atom catalysts (SACs). Beyond individual showcase studies, the design and synthesis of structurally regular series of SACs opens the door to systematic experimental investigations of performance as a function of metal identity. Herein, a series of single-atom catalysts based on various 4d (Ru, Rh, Pd) and 5d (Ir, Pt) transition metals has been synthesized on a common MgO carrier. Complementary experimental (X-ray absorption spectroscopy) and theoretical (Density Functional Theory) studies reveal that, regardless of the metal identity, metal cations occupy preferably octahedral coordination MgO lattice positions under step-edges, hence highly confined by the oxide support. Upon exposure to O2-lean CO oxidation conditions, FTIR spectroscopy indicates the partial de-confinement of the monoatomic metal centers driven by CO at pre-catalysis temperatures, followed by the development of surface carbonate species under steady-state conditions. These findings are supported by DFT calculations, which show the driving force and final structure for the surface metal protrusion to be metal-dependent, but point to an equivalent octahedral-coordinated M4+ carbonate species as the resting state in all cases. Experimentally, apparent reaction activation energies in the range of 96±19 kJ/mol are determined, with Pt leading to the lowest energy barrier. The results indicate that, for monoatomic sites in SACs, differences in CO oxidation reactivity enforceable via metal selection are of lower magnitude than those evidenced previously through the mechanistic involvement of adjacent redox centers on the oxide carrier, suggesting that tuning of the oxide surface chemistry is as relevant as the selection of the supported metal.
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Aug 2020
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I19-Small Molecule Single Crystal Diffraction
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Diamond Proposal Number(s):
[17379]
Abstract: The synthesis, structures and properties of [4]- and [3]-rotaxane complexes are reported where [2]rotaxanes, formed from het-erometallic {Cr7Ni} rings, are bound to a fluoride-centered {CrNi2} triangle. The compounds have been characterized by sin-gle crystal X-ray diffraction and have the formulae [CrNi2(F)(O2CtBu)6]{(BH)[Cr7NiF8(O2CtBu)16]}3 (3) and [CrNi2(F)(O2CtBu)6(THF)]{(BH)[Cr7NiF8(O2CtBu)16]}2 (4); where B = py-CH2CH2NHCH2C6H4SCH3. The [4]rotaxane 3 is an isosceles triangle of three [2]rotaxanes bound to the central triangle while the [3]rotaxane 4 contains only two [2]rotaxanes bound to the central triangle. Studies of the behavior of 3 and 4 in solution by small angle X-ray scattering (SAXS) and atomistic molecular dynamic simulations (AMDS) show that the structure of 3 is similar to that found in the crystal but that 4 has a different conformation to the crystal. C.w. and pulsed EPR spectroscopy were used to study the structures present and demonstrate that in frozen solutions (at 5 K) 4 forms more extended molecules than 3 and with a wider range of confor-mations.
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Aug 2020
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E02-JEM ARM 300CF
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Diamond Proposal Number(s):
[20195, 21979, 20198]
Abstract: Defect engineering can enhance key properties of metal-organic frameworks (MOFs). Tailoring the distribution of de-fects, for example in correlated nanodomains, requires characterization across length scales. However, a critical na-noscale characterization gap has emerged between the bulk diffraction techniques used to detect defect nanodomains and the sub-nanometer imaging used to observe individual defects. Here, we demonstrate that the emerging technique of scanning electron diffraction (SED) can bridge this gap uniquely enabling both nanoscale crystallographic analysis and the low-dose formation of multiple diffraction contrast images for defect analysis in MOFs. We directly image defect nanodomains in the MOF UiO-66(Hf) over an area of ca. 1 000 nm and with a spatial resolution ca. 5 nm to reveal domain morphology and distribution. Based on these observations, we suggest possible crystal growth processes underpinning synthetic control of defect nanodomains. We also identify likely dislocations and small angle grain boundaries, illustrating that SED could be a key technique in developing the potential for engineering the distribution of defects, or “microstruc-ture”, in functional MOF design.
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Jul 2020
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