Effect of guest uptake and high pressure on Zn- and Zr- metal-organic frameworks

Porous materials are essential to our everyday lives, for example as an effective catalyst in the cracking of crude oil, or as water softeners in washing powder. When developing novel functional porous materials, it is necessary to fully understand their structure-property relationships to maximise their ability to be used in industrially relevant settings. This thesis aims to understand the mechanical and adsorption properties of a class of porous solids metal-organic frameworks (or MOFs), which have many potential applications owing to their tuneable structures. Due to the inherent 3-D crystalline structure of MOFs, a wide range crystallographic techniques were used to determine structure-property relationships. To achieve maximum in-depth structural knowledge, both classical and quantum theoretical approaches were also applied to complement the understanding of both the energetics and structural details. Chapters One and Two begin with an overview of the state of the art studies carried out on MOFs, focusing on the use of high-pressure crystallography to understand their properties. In addition, there is emphasise on the importance of complementary computational methods that are used in the characterisation of MOFs. In Chapter Three, an isostructural series of MOFs (zeolitic imidazolate frameworks, or ZIFs) were studied for methanol adsorption by employing both experimental and molecular simulation techniques. These frameworks are gating materials, where the imidazole linker rotates upon adsorption of guest, and it was found that through ligand substitution the gate opening angle and onset pressure to gating could be tuned. By using high-pressure Xray crystallography the structure of the ZIFs were studied upon the uptake of guest and the degree of ring rotation quantified. In combination with periodic DFT and grand canonical Monte Carlo simulations the energy barrier to rotation and energies of adsorption could be calculated, respectively. Chapter Four focuses on one ZIF in particular, ZIF-8 ((Zn6(MeIm)12, MeIm = 2- methylimidazole) and details the adsorption of a selection of gases into the pores. The experimental method of cryogenic gas loading into a diamond anvil cell in this chapter is novel to MOFs. This method, in combination with molecular crystallography, is used to determine the structural response of the framework to guest-uptake as well as the crystallographic positions of the adsorption sites. In combination with in silico methods, the adsorption energies of guest-sites could be calculated, detailing which interactions drive the gating behaviour. The method of cryogenic loading highlighted how extreme conditions can be used to extract useful information about structural behaviour of MOFs on uptake of gas molecules into the pores, and when used in combination with computational methods, we have a powerful tool to analyse both positions and energies of adsorption sites. With this information, progress can be made in developing MOFs to maximize favourable interactions and lead to the development of MOFs with better selective gas storage properties. Chapter Five focuses on the synthesis and characterisation of the physical properties of a series of Zr-containing MOFs, called UiO-MOFs. The high valency of Zr(IV) and 12-fold coordination of the metal cluster in these materials, are associated with high shear and bulk moduli, which surpass those of other MOFs. A combination of single-crystal nano-indentation, high-pressure X-ray diffraction studies, density functional theory (DFT) calculations, and first-principles molecular dynamics (MD) simulations were used to determine the compressibility, elasticity and hardness of these materials, whose mechanical robustness was correlated to their different structural features, in-particular, how using non-linear linkers between the metal clusters stabilises the framework to compression. Chapter Six expands upon the series of Zr-MOFs in Chapter Five, and looks at how the mechanical properties of these MOFs are affected upon increasing the linker length. The experimentally determined elastics modulus of one of the frameworks, UiO-sdc (Zr6O4(OH)4(sdc)6 where sdc =4,4’-stillbene dicarboxylate), was found to lie above those of other highly porous MOFs. In addition, the elastic modulus was found to decrease linearly as a function of increasing the linker length, extending the structure-property relationships determined in Chapter Five

Perspectives on the Influence of Crystal Size and Morphology on the Properties of Porous Framework Materials

Miniaturization is a key aspect of materials science. Owing to the increase in quality experimental and computational tools available to researchers, it has become clear that the crystal size and morphology of porous framework materials, including metal-organic frameworks and covalent organic frameworks, play a vital role in defining the physicochemical behaviour of these materials. However, given the multiscale and multidisciplinary challenges associated with establishing how crystal size and morphology affect the structure and behaviour of a material–from local to global structural modifications and from static to dynamic effects–a comprehensive mechanistic understanding of size and morphology effects is missing. Herein, we provide our perspective on the current state-of-the-art of this topic, drawn from various complementary disciplines. From a fundamental point of view, we discuss how controlling the crystal size and morphology can alter the mechanical and adsorption properties of porous framework materials and how this can impact phase stability. Special attention is also given to the quest to develop new computational tools capable of modelling these multiscale effects. From a more applied point of view, given the recent progress in this research field, we highlight the importance of crystal size and morphology control in drug delivery. Moreover, we provide an outlook on how to advance each discussed field by size and morphology control, which would open new design opportunities for functional porous framework materials

Conformational isomerism controls collective flexibility in metal-organic framework DUT-8(Ni)

Metal-organic frameworks (MOFs) are coordination networks with organic ligands containing potential voids. Some MOFs show pronounced structural flexibility that may result in closing and re-opening these pores. Here, we show that collective flexibility in a MOF-DUT-8(Ni)-is controlled by conformational isomerism. DUT-8(Ni), a pillared-layer MOF with Ni 2 paddle-wheels, dabco pillars and naphthalene dicarboxylate (ndc) linkers, can crystallize in many conformational isomers that depend on the orientation of the non-linear ndc linkers with respect to each other. While the open form is compatible with several of these conformations, only one of them, with alternating linker orientations, is stable as the closed form. We show, by means of first principles calculations, that in the stable closed form, the appreciable lattice strain is compensated by London-dispersion forces between the ndc linkers that arrange with maximum overlap in a stacking order similar to the stacking in graphite. We substantiate these results by well-tempered metadynamics calculations on the DFT-based Born-Oppenheimer potential energy surface, by refined X-ray diffraction data and by nitrogen adsorption data obtained by experiment and grand-canonical Monte-Carlo simulations based on the DFT-optimized and PXRD-derived geometries. While the reported origin of flexibility cannot be generalized to all flexible MOFs, it offers a rational design concept of folding mechanisms in switchable MOFs by exploitation of the stabilization effect of linker stacking in the closed form. </p

Correlating Pressure‐Induced Emission Modulation with Linker Rotation in a Photoluminescent MOF

Conformational changes of linker units in metal‐organic frameworks (MOFs) are often responsible for gate‐opening phenomena in selective gas adsorption and stimuli‐responsive optical and electrical sensing behaviour. Herein, we show that pressure‐induced bathochromic shifts in both fluorescence emission and UV‐Vis absorption spectra of a two‐fold interpenetrated Hf MOF, linked by 1,4‐phenylene‐bis(4‐ethynylbenzoate) ligands ( Hf‐peb ), are induced by rotation of the central phenyl ring of the linker, from a coplanar arrangement to a twisted, previously unseen conformer. Single‐crystal X‐ray diffraction, alongside in situ fluorescence and UV‐Vis absorption spectroscopies, measured up to 2.1 GPa in a diamond anvil cell on single crystals, are in excellent agreement, correlating linker rotation with modulation of emission. Topologically isolating the 1,4‐phenylene‐bis(4‐ethynylbenzoate) units within a MOF facilitates concurrent structural and spectroscopic study in the absence of intermolecular perturbation, allowing characterisation of the luminescence properties of a high‐energy, twisted conformation of the previously well‐studied chromophore. We expect the unique environment provided by network solids, and the capability of combining crystallographic and spectroscopic analysis, will greatly enhance understanding of luminescent molecules and lead to the development of novel sensors and adsorbents

Tuning the high-pressure phase behaviour of highly compressible zeolitic imidazolate frameworks: from discontinuous to continuous pore closure by linker substitution

The high-pressure behaviour of flexible zeolitic imidazolate frameworks (ZIFs) of the ZIF-62 family with the chemical composition M(im)2−x(bim)x is presented (M2+=Zn2+, Co2+; im−=imidazolate; bim−=benzimidazolate, 0.02≤x≤0.37). High-pressure powder X-ray diffraction shows that the materials contract reversibly from an open pore (op) to a closed pore (cp) phase under a hydrostatic pressure of up to 4000 bar. Sequentially increasing the bim− fraction (x) reinforces the framework, leading to an increased threshold pressure for the op-to-cp phase transition, while the total volume contraction across the transition decreases. Most importantly, the typical discontinuous op-to-cp transition (first order) changes to an unusual continuous transition (second order) for x≥0.35. This allows finetuning of the void volume and the pore size of the material continuously by adjusting the pressure, thus opening new possibilities for MOFs in pressure-switchable devices, membranes, and actuators

Tuning the High-Pressure Phase Behaviour of Highly Compressible Zeolitic Imidazolate Frameworks: From Discontinuous to Continuous Pore Closure by Linker Substitution

The high‐pressure behaviour of flexible zeolitic imidazolate frameworks (ZIFs) of the ZIF‐62 family with the chemical composition M(im)(2−x )(bim)(x) is presented (M(2+)=Zn(2+), Co(2+); im(−)=imidazolate; bim(−)=benzimidazolate, 0.02≤x≤0.37). High‐pressure powder X‐ray diffraction shows that the materials contract reversibly from an open pore ( op ) to a closed pore ( cp ) phase under a hydrostatic pressure of up to 4000 bar. Sequentially increasing the bim(−) fraction (x) reinforces the framework, leading to an increased threshold pressure for the op ‐to‐ cp phase transition, while the total volume contraction across the transition decreases. Most importantly, the typical discontinuous op ‐to‐ cp transition (first order) changes to an unusual continuous transition (second order) for x≥0.35. This allows finetuning of the void volume and the pore size of the material continuously by adjusting the pressure, thus opening new possibilities for MOFs in pressure‐switchable devices, membranes, and actuators

Role of particle size and surface functionalisation on the flexibility behaviour of switchable metal-organic framework DUT-8(Ni)

Flexible MOF nanoparticles, i.e. MOF nanoparticles that change their structure upon external stimuli such as guest uptake, are promising for numerous applications including advanced gas adsorption, drug delivery and sensory devices. However, the properties of MOFs are typically characterised based on the bulk material with no consideration of how the particle size and external surface influences their performance. This combined computational and experimental contribution investigates the influence of the particle size and surface functionalisation on the flexibility of DUT-8(Ni) (Ni2 (2,6-ndc)2 dabco, ndc = naphthalene dicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane, DUT=Dresden University of Technology). DUT-8 nanoparticles remain rigid in their open pore form while microparticles, synthesised under slightly different conditions, undergo gate opening upon nitrogen adsorption suggesting that the particle size has an important role to play in the flexibility of DUT-8. While the adsorption environment at the surface capped with modulators smaller than the 2,6-ndc ligand is very different compared to the bulk of the crystal with considerably weaker guest-framework interaction, simulations reveal that the nanoparticles should close. We conclude that the size of the nanoparticles is not the major contributor for keeping DUT-8 nanoparticles open but that it is more likely that defects or nucleation barriers dominate. Moreover, our work reveals for the first time that functionalising the external surface of nanoparticles with different modulators or capping groups offers the opportunity to manipulate the gate opening / closing pressure. This principle is generally applicable and could be exploited to tune the gate openig / closing pressure for the application of interest