Role of crystal size on swing-effect and adsorption induced structure transition of ZIF-8.

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.This work was funded by the EPSRC IAA Partnership Development Award (RG/75759). D.F.-J. thanks the Royal Society for funding through a University Research Fellowship. We thank Diamond Light Source for beamtime at beamline I11 (visit EE9750).This is the author accepted manuscript. The final version is available from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C6DT00565

Role of crystal size on swing-effect and adsorption induced structure transition of ZIF-8

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.This work was funded by the EPSRC IAA Partnership Development Award (RG/75759). D. F.-J. thanks the Royal Society for funding through a University Research Fellowship. We thank Diamond Light Source for beamtime at beamline I11 (visit EE9750).Peer reviewe

Phase Transitions in Zeolitic Imidazolate Framework 7: The Importance of Framework Flexibility and Guest-Induced Instability

A study of the phase transitions in ZIF-7 (zeolitic imidazolate frameworks- (Zn(PhIm)2, PhIm = benzimidazolate)) as a function of guest occupancy and temperature was reported. Raman spectra of an as-synthesized sample were collected in air between 297 and 421 K. The major contributions of the spectra come from the vibrational modes of the benzimidazolate ligand. Upon heating, most of the Raman bands remain similar and keep the same frequencies until 357 K, indicating that the structure of ZIF-7 seems to be stable in this temperature range. Above 357 K, strong modifications are observed in the regions corresponding to the lattice modes. The formation of ZIF-7-II is attributed to the loss of dimethylformamide (DMF) solvent molecules from the ZIF-7-I framework. This can be confirmed by the differential scanning calorimetry and thermogravimetric analysis traces of ZIF-7-I. The highly-distorted and locally-strained nature of ZIF-7-II leads to its poor crystallinity, reflected by X-ray powder diffraction and scanning electron microscope

A breathing zirconium metal-organic framework with reversible loss of crystallinity by correlated nanodomain formation

The isoreticular analogue of the metal-organic framework UiO-66(Zr), synthesized with the flexible trans-1,4-cyclohexanedicarboxylic acid as linker, shows a peculiar breathing behavior by reversibly losing long-range crystalline order upon evacuation. The underlying flexibility is attributed to a concerted conformational contraction of up to two thirds of the linkers, which breaks the local lattice symmetry. X-ray scattering data are described well by a nanodomain model in which differently oriented tetragonal-type distortions propagate over about 7-10 unit cells

Pt accelerated coarsening of A15 precipitates in Cr-Si alloys

The effect of alloying Cr-rich Cr-Si alloys with Pt was investigated by a combination of complementary experimental methods and atomic scale modelling. The investigated Cr-Si and Cr-Si-Pt (Cr ⩾86 at.%) alloys developed a two-phase microstructure consisting of Cr solid solution (Crss) matrix and strengthened by A15 precipitates during annealing at 1200\ub0C. It was found that additions of 2 at.% Pt increase the coarsening rate by almost five times considering annealing times up to 522 h. Pt was found to change the precipitate matrix orientation relationship, despite its low influence on the Crss matrix/A15 precipitate misfit. Through this experimental and modelling approach new insight has been gained into mechanisms of enhanced coarsening by Pt addition. The increased coarsening is principally attributed to a change in interface composition and structure resulting in different thermodynamic stabilities: Pt-containing A15 phase was found to have a broader compositional range if both elements, Pt and Si, are present compared to only Si. Additionally, the Crss phase was found to have a higher solubility of Pt and Si over just Si. Both factors additionally facilitated Ostwald ripening

Synthesis of porous metal phosphonate frameworks for applications in gas separation and storage

Porous metal phosphonate framework materials were synthesised by solvothermal reaction of bis(α-aminomethylenephosphonic acid) ligands with divalent and trivalent metal cations. The syntheses and characterisation by NMR and, where possible, single crystal X-ray diffraction of seven bisphosphonic acid ligands, including N,N′-piperazinebis(methylenephosphonic acid) (H₄L), its racemic and enantiopure (R) 2-methyl (H₄L′ and R-H₄L′) and 2,5-dimethyl (H₄L′′) derivatives, and N,N′-4,4′-bipiperidinebis(methylenephosphonic acid) (H₄LL) are reported. Syntheses of the known phase Y₂(LH₂)₃·5H₂O and the new phases, STA-13(Y) (St Andrews microporous material No. 13) and Y₂(R-L′H₂)₃·4H₂O, from reactions of Y(AcO)₃ with H₄L, H₄L′ and R-H₄L′ respectively are reported. The as-prepared and dehydrated structures of each phase have been determined from either laboratory or synchrotron powder X-ray diffraction data. Reaction of Y(AcO)₃ and H₄L′′ is shown to form a phase with a different structure. The features determining which structure crystallises are discussed. Syntheses of other rare-earth forms of STA-13 (Sc³⁺, Gd³⁺–Yb³⁺) and the porosity of each phase to N₂ are reported. STA-13(Y) is the most porous form with loadings of ∼3 mmol g⁻¹ and ∼4 mmol g⁻¹ for N₂ and CO₂ respectively. MIL-91(Fe) was synthesised for the first time from reactions of Fe³⁺ cations with H₄L. Its structure was confirmed by Rietveld refinement, but it was not porous. The first syntheses of [Fe₄L₁.₅(AcO)₁.₅(OH,H₂O)₃]·0.5NH₄5.5H₂O (L= L or L′) are reported, from reactions of H₄L or H₄L′ in the presence of an excess of Fe³⁺ cations. The phase is related to a previously reported Co phase. The synthesis of divalent metal bisphosphonate STA-12(Mg) (Mg₂(H₂O)₂L·5.6H₂O) was reported for the first time and its structure determined from single crystal X-ray diffraction. The dehydration behaviour of this material was compared with the known forms of STA-12. STA-12(Mg) is porous to both N₂ (∼5.5 mmol g⁻¹) and CO₂ (~ 8.5 mmol g⁻¹). Reaction of H₄LL with Co²⁺ and Ni²⁺ gave two materials isoreticular with STA-12, labelled STA-16(Co) and STA-16(Ni). The structures of both materials were solved from synchrotron powder X-ray diffraction data. On dehydration, STA-16(Co) undergoes a reversible structural transition to an unknown structure. By contrast, STA-16(Ni) retains the same symmetry in the dehydrated form and its structure was determined from synchrotron powder X-ray diffraction data. Both materials are porous to N₂, with an uptake of up to 22.2 mmol g⁻¹, and CO₂ with maximum loading of 21.7 mmol g⁻¹. NLDFT analysis of N₂ adsorption data confirm the crystallographically determined pore radii. Syntheses of other frameworks with divalent cations and initial reactions of H₄LL with trivalent cations are also reported

The Influence of Isomerism on Crystallization in Aluminum Pyridinedicarboxylate Coordination Compounds

The reactions of three isomeric pyridinedicarboxylic acid linkers - 2,4-, 2,6-, and 3,5-pyridinedicarboxylic acid - with various aluminum salts were investigated by high-throughput solvothermal synthesis. Two coordination compounds were obtained from the symmetrical linkers (2,6- and 3,5-pyridinedicarboxylic acid). The former is based on molecular Al-O dimers, similar to those reported in the coordination polymer CAU-16. The latter forms a porous extended framework and is a member of the CAU-10 family of compounds. Full characterization of both compounds is reported. Crystal structures were determined from high-resolution synchrotron powder diffraction data. The number of guest water molecules determined crystallographically is in excellent agreement with the number found by thermogravimetric and elemental analyses. It was found that the CAU-10 analogue undergoes a reversible, displacive phase transition from a non-centrosymmetric to a centrosymmetric space group on loss of guest water molecules, due to guest-framework interactions

Quasi-hydrostatic equation of state of silicon up to 1 megabar at ambient temperature

The isothermal equation of state of silicon has been determined by synchrotron x-ray diffractionexperiments up to 105.2 GPa at room temperature using diamond anvil cells. A He-pressuremedium was used to minimize the effect of uniaxial stress on the sample volume and ruby, gold andtungsten pressure gauges were used. Seven different phases of silicon have been observed along theexperimental conditions covered in the present study

Extending the Pore Size of Crystalline Metal Phosphonates toward the Mesoporous Regime by Isoreticular Synthesis

Crystalline microporous cobalt and nickel bisphosphonates with a hexagonal array of one-dimensional channels 1.8 nm in diameter have been prepared hydrothermally and provide the first example of the use of isoreticular chemistry in the synthesis of phosphonate metal-organic frameworks. The materials contain both physisorbed and coordinating water molecules in the as-prepared form, but these can be removed to give permanent extra-large microporosity, with pore volumes of up to 0.68 cm(3) g(-1), coordinatively unsaturated sites, with concentrations up to 4.25 mmol g(-1).</p