Mechanochemical synthesis has recently emerged as a scalable “green” approach for the preparation of MOFs, but current understanding of the underlying reaction mechanisms is limited. In this work, an ...investigation of the reaction pathway of the mechanochemical synthesis of MOF-74 from ZnO and 2,5-dihydroxyterephthalic acid (H4HDTA), using DMF as a liquid additive, is presented. The complex reaction pathway involves the formation of four short-lived intermediate phases, prior to the crystallization of MOF-74. The crystal structures of three of these intermediates have been determined using a combination of single-crystal and powder X-ray diffraction methods and are described here. The initial stages of the reaction are very fast, with a DMF solvate of H4HDTA forming after only 2 min of milling. This is followed by crystallization, after only 4 min of milling, of a triclinic one-dimensional coordination polymer, Zn(H2DHTA)(DMF)2(H2O)2, which converts into a monoclinic polymorph on additional milling. Highly crystalline MOF-74 appears after prolonged milling, for at least 70 min.
Thermally-densified hafnium terephthalate UiO-66(Hf) is shown to exhibit the strongest isotropic negative thermal expansion (NTE) effect yet reported for a metal-organic framework (MOF). ...Incorporation of correlated vacancy defects within the framework affects both the extent of thermal densification and the magnitude of NTE observed in the densified product. We thus demonstrate that defect inclusion can be used to tune systematically the physical behaviour of a MOF.
Understanding the molecular mechanism of proton conduction is crucial for the design of new materials with improved conductivity. Quasi-elastic neutron scattering (QENS) has been used to probe the ...mechanism of proton diffusion within a new phosphonate-based metal–organic framework (MOF) material, MFM-500(Ni). QENS suggests that the proton conductivity (4.5 × 10–4 S/cm at 98% relative humidity and 25 °C) of MFM-500(Ni) is mediated by intrinsic “free diffusion inside a sphere”, representing the first example of such a mechanism observed in MOFs.
Elementary women Murray, Claire A
Nature chemistry,
07/2019, Volume:
11, Issue:
7
Journal Article
Peer reviewed
The contributions of women to the development of the periodic table have long been overlooked. Claire Murray relates how the recent ‘Setting their table’ conference set out to highlight their ...prominent role in element discovery and use.
MFM‐300(Al) shows reversible uptake of NH3 (15.7 mmol g−1 at 273 K and 1.0 bar) over 50 cycles with an exceptional packing density of 0.62 g cm−3 at 293 K. In situ neutron powder diffraction and ...synchrotron FTIR micro‐spectroscopy on ND3@MFM‐300(Al) confirms reversible H/D site exchange between the adsorbent and adsorbate, representing a new type of adsorption interaction.
High storage capacity: MFM‐300(Al) shows reversible uptake of NH3 over 50 cycles with an exceptional packing density of 0.62 g cm−3 at 293 K. Neutron diffraction and FTIR micro‐spectroscopy confirm reversible H/D site exchange between the adsorbent and adsorbate, representing a new type of adsorption interaction.
The demand for xylenes is projected to increase over the coming decades. The separation of xylene isomers, particularly p- and m-xylenes, is vital for the production of numerous polymers and ...materials. However, current state-of-the-art separation is based upon fractional crystallisation at 220 K which is highly energy intensive. Here, we report the discrimination of xylene isomers via refinement of the pore size in a series of porous metal-organic frameworks, MFM-300, at sub-angstrom precision leading to the optimal kinetic separation of all three xylene isomers at room temperature. The exceptional performance of MFM-300 for xylene separation is confirmed by dynamic ternary breakthrough experiments. In-depth structural and vibrational investigations using synchrotron X-ray diffraction and terahertz spectroscopy define the underlying host-guest interactions that give rise to the observed selectivity (p-xylene < o-xylene < m-xylene) and separation factors of 4.6-18 for p- and m-xylenes.
Magnesium potassium phosphate cements (MKPC) were investigated to determine their efficacy towards retardation of reactive uranium metal corrosion. Optimised low-water content, fly ash (FA) and blast ...furnace slag (BFS) blended MKPC formulations were developed and their fluidity, hydration behaviour, strength and phase assemblage investigated. In-situ time resolved synchrotron powder X-ray diffraction was used to detail the early age (~60 h) phase assemblage development and hydration kinetics, where the inclusion of BFS was observed to delay the formation of struvite-K by ~14 h compared to FA addition (~2 h). All samples set within this period, suggesting the possible formation of a poorly crystalline binding phase prior to struvite-K crystallisation. Long-term corrosion trials using metallic uranium indicated that MKPC systems are capable of limiting uranium corrosion rates (reduced by half), when compared to a UK nuclear industry grout, which highlights their potential application radioactive waste immobilisation.
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•Magnesium potassium phosphate cements were studied for nuclear waste encapsulation.•The optimised formulation in this study was determined to be 0.24 w/s ratio.•Different reaction kinetics were discovered for the FA/MKPC and BFS/MKPC binders.•Struvite-K was the only crystalline reaction product observed via in-situ SXPD.•Uranium trials revealed MKPC corrosion was reduced compared to a UK nuclear grout.
Li-N-H materials, particularly lithium amide and lithium imide, have been explored for use in a variety of energy storage applications in recent years. Compositional variation within the parent ...lithium imide, anti-fluorite crystal structure has been related to both its facile storage of hydrogen and impressive catalytic performance for the decomposition of ammonia. Here, we explore the controlled solid-state synthesis of Li-N-H solid-solution anti-fluorite structures ranging from amide-dominated (Li
4/3
(NH
2
)
2/3
(NH)
1/3
or Li
1.333
NH
1.667
) through lithium imide to majority incorporation of lithium nitride-hydride (Li
3.167
(NH)
0.416
N
0.584
H
0.584
or Li
3.167
NH). Formation of these solid solutions is demonstrated to cause significant changes to the thermal stability and ammonia reactivity of the samples, highlighting the potential use of compositional variation to control the properties of the material in gas storage and catalytic applications.
A wide solid solution based on the lithium imide anti-fluorite structure is demonstrated and related to its energy storage functions.
Modulation of pore environment is an effective strategy to optimize guest binding in porous materials. We report the post-synthetic modification of the charge distribution in a charged metal-organic ...framework, MFM-305-CH
, Al(OH)(L)Cl, (H
L)Cl = 3,5-dicarboxy-1-methylpyridinium chloride and its effect on guest binding. MFM-305-CH
shows a distribution of cationic (methylpyridinium) and anionic (chloride) centers and can be modified to release free pyridyl N-centres by thermal demethylation of the 1-methylpyridinium moiety to give the neutral isostructural MFM-305. This leads simultaneously to enhanced adsorption capacities and selectivities (two parameters that often change in opposite directions) for CO
and SO
in MFM-305. The host-guest binding has been comprehensively investigated by
synchrotron X-ray and neutron powder diffraction, inelastic neutron scattering, synchrotron infrared and
H NMR spectroscopy and theoretical modelling to reveal the binding domains of CO
and SO
in these materials. CO
and SO
binding in MFM-305-CH
is shown to occur
hydrogen bonding to the methyl and aromatic-CH groups, with a long range interaction to chloride for CO
. In MFM-305 the hydroxyl, pyridyl and aromatic C-H groups bind CO
and SO
more effectively
hydrogen bonds and dipole interactions. Post-synthetic modification
dealkylation of the as-synthesised metal-organic framework is a powerful route to the synthesis of materials incorporating active polar groups that cannot be prepared directly.
By tuning the tolerance factor, t, of the Ruddlesden–Popper oxide Ca2MnO4 through isovalent substitutions, we show that the uniaxial coefficient of linear thermal expansion (CLTE) of these systems ...can be systematically changed through large negative to positive values. High-resolution X-ray diffraction measurements show that the magnitude of uniaxial negative thermal expansion (NTE) increases as t decreases across the stability window of the NTE phase. Transitions to phases with positive thermal expansion (PTE) are found to occur at both the high-t and low-t limits of stability. First-principles calculations demonstrate that reducing t enhances the contribution to thermal expansion from the lowest frequency phonons, which have the characteristics of octahedral tilts and have negative mode Grüneisen parameter components along the NTE axis. By tuning t to the lower edge of the NTE phase stability window, we are hence able to maximize the amplitudes of these vibrations and thereby maximize NTE with a CLTE of −8.1 ppm/K at 125 K. We also illustrate, at the other end of the phase diagram, that an enhancement in compliance of these materials associated with the rotational instability provides another mechanism by which NTE could be yet further enhanced in this and related systems.
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