Michael T. Wharmby

Opening the gate: framework flexibility in ZIF-8 explored by experiments and simulations.

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 H(2) and CH(4). 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 a 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 a further impact on the design of advanced molecular sieving membrane materials for gas separation, adjusting the access of fluids to the porous network.

Flexibility and swing effect on the adsorption of energy-related gases on ZIF-8: combined experimental and simulation study

ZIF-8, a prototypical zeolitic porous coordination polymer, prepared via the self-assembly of tetrahedral atoms (e.g. Zn and Co) and organic imidazolate linkers, presents large cavities which are interconnected by narrow windows that allow, in principle, molecular sieving. However, ZIF-8 shows flexibility due to the swing of the imidazolate linkers, which results in the adsorption of molecules which are too large to fit through the narrow window. In this work, we assess the impact of this flexibility, previously only observed for nitrogen, and the level of agreement between the experimental and simulated isotherms of different energy-related gases on ZIF-8 (CO(2), CH(4) and alkanes). We combine experimental gas adsorption with GCMC simulations, using generic and adjusted force fields and DFT calculations with the Grimme dispersion correction. By solely adapting the UFF force field to reduce the Lennard-Jones parameter ε, we achieve excellent agreement between the simulated and experimental results not only for ZIF-8 but also for ZIF-20, where the transferability of the adapted force field is successfully tested. Regarding ZIF-8, we show that two different structural configurations are needed to properly describe the adsorption performance of this material, demonstrating that ZIF-8 is undergoing a structural change during gas adsorption. DFT calculations with the Grimme dispersion correction are consistent with the GCMC and experimental observations, illustrating the thermodynamics of the CH(4) adsorption sites and confirming the existence of a new adsorption site with a high binding energy within the 4-ring window of ZIF-8.

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), and coordinatively unsaturated sites, with concentrations up to 4.25 mmol g(-1).

Mechanical Tunability via Hydrogen Bonding in Metal-Organic Frameworks with the Perovskite Architecture

Two analogous metal-organic frameworks (MOFs) with the perovskite architecture, [C(NH2)3][Mn(HCOO)3] (1) and [(CH2)3NH2][Mn(HCOO)3] (2), exhibit significantly different mechanical properties. The marked difference is attributed to their distinct modes of hydrogen bonding between the A-site amine cation and the anionic framework. The stronger cross-linking hydrogen bonding in 1 gives rise to Youngs moduli and hardnesses that are up to twice those in 2, while the thermal expansion is substantially smaller. This study presents clear evidence that the mechanical properties of MOF materials can be substantially tuned via hydrogen-bonding interactions.

Aerobic Epoxidation of Olefins Catalyzed by the Cobalt‐Based Metal–Organic Framework STA‐12(Co)

A Co-based metal-organic framework (MOF) was investigated as a catalytic material in the aerobic epoxidation of olefins in DMF and exhibited, based on catalyst mass, a remarkably high catalytic activity compared with the Co-doped zeolite catalysts that are typically used in this reaction. The structure of STA-12(Co) is similar to that of STA-12(Ni), as shown by XRD Rietveld refinement and is stable up to 270 °C. For the epoxidation reaction, significantly different selectivities were obtained depending on the substrate. Although styrene was epoxidized with low selectivity due to oligomerization, (E)-stilbene was converted with high selectivities between 80 and 90 %. Leaching of Co was low and the reaction was found to proceed mainly heterogeneously. The catalyst was reusable with only a small loss of activity. The catalytic epoxidation of stilbene with the MOF featured an induction period, which was, interestingly, considerably reduced by styrene/stilbene co-epoxidation. This could be traced back to the formation of benzaldehyde promoting the reaction. Detailed parameter and catalytic studies, including in situ EPR and EXAFS spectroscopy, were performed to obtain an initial insight into the reaction mechanism.

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.

Mechanical Properties of a Calcium Dietary Supplement, Calcium Fumarate Trihydrate

The mechanical properties of calcium fumarate trihydrate, a 1D coordination polymer considered for use as a calcium source for food and beverage enrichment, have been determined via nanoindentation and high-pressure X-ray diffraction with single crystals. The nanoindentation studies reveal that the elastic modulus (16.7-33.4 GPa, depending on crystallographic orientation), hardness (1.05-1.36 GPa), yield stress (0.70-0.90 GPa), and creep behavior (0.8-5.8 nm/s) can be rationalized in view of the anisotropic crystal structure; factors include the directionality of the inorganic Ca-O-Ca chain and hydrogen bonding, as well as the orientation of the fumarate ligands. High-pressure single-crystal X-ray diffraction studies show a bulk modulus of ∼ 20 GPa, which is indicative of elastic recovery intermediate between small molecule drug crystals and inorganic pharmaceutical ingredients. The combined use of nanoindentation and high-pressure X-ray diffraction techniques provides a complementary experimental approach for probing the critical mechanical properties related to tableting of these dietary supplements.

Re-Determination of the Crystal Structure of MIL-91(Al)

The structure of one of the first permanently porous metal phosphonates, MIL‐91(Al) was re‐determined using high resolution synchrotron powder X‐ray diffraction data. The new model is in a lower symmetry space group, with no disordered ligands in the structure, whilst remaining otherwise consistent with the reported compound. New milder synthetic conditions were also developed.

Chapter 10:Open Framework and Microporous Metal Phosphonate MOFs with Piperazine-based Bisphosphonate Linkers

The aluminium methylphosphonates of Maeda opened up the field of fully ordered, microporous organic–inorganic hybrids. They possess remarkable adsorption properties, but subsequent efforts to prepare other microporous methylphosphonates met with relatively little success, and porous metal carboxylates have since dominated this field. More recently, though, there has been an upsurge in interest in metal phosphonate MOFs, where bisphosphonate or phosphonate-carboxylate linkers enable extended 3D frameworks to be prepared, some with permanent porosity. In particular, the N,N′-piperazinebis(methylenephosphonic acid) is a versatile framework-forming linker, in large part because of its ability to display variable coordinative behaviour through different numbers of O and N atoms, and of the methylenephosphonate groups to adopt either axial or equatorial configurations. The first frameworks prepared with this linker, with divalent cations in tetrahedral coordination, were small pore solids that showed reversible dehydration but no permanent porosity. Our further studies in this area have found a rich structural chemistry with both divalent and trivalent metal cations. For divalent cations that display octahedral coordination, the structure type STA-12 (M2L·nH2O, M = Mn, Fe, Co, Ni, n= 7 or 8) is found to be of particular importance, displaying large pores, a rich structural chemistry and catalytic activity. For trivalent cations, there is a particularly diverse range of metal phosphonate MOFs, where the phosphonate MOF to form depends on cation size, pH of crystallising medium and the presence or absence of methyl groups on the piperazine ring. The products include the MIL-91 (Al, Fe); lanthanide phosphonates La–Nd with the lanthanide in 7 or 8-fold coordination and lanthanide phosphonates Gd–Yb with the metal octahedral. These exhibit reversible dehydration, cation exchange, ‘breathing’ and permanent porosity.

The automated XPDF beamline at Diamond Light Source

Alongside optimised hardware, significant effort has been put into software to produce a user-friendly, automated beamline. Integrated data collection and analysis software has been developed which will perform automatic data processing to deliver real-time Bragg, total scattering and PDF data. 2-D scattering data from an area detector can be corrected, integrated to 1-D and processed to X-ray PDFs, all from within a single interface. The PDF processing pipeline and examples of typical experiments performed on the XPDF beamline will be presented. The new PDF processing software is freely available as part of the DAWN software package [1].