Stimuli-induced structural switchability in the pillared-layer metal-organic framework DUT-8

Metal-Organic Frameworks (MOFs) are highly porous materials built from inorganic nodes joined by organic linkers forming extended crystalline networks. One of the distinguishing features of metal-organic frameworks is the ability to adaptively change their crystal structure in response to external stimuli with significant porosity switching. Such structural switchability of MOFs offers new opportunities in gas separation, selective recognition, sensing, and energy storage. However, there are still open questions in understanding factors affecting switchability. The electronic structure of the metal in the building blocks, host-guest interactions, but also particle size, morphology, surface, desolvation conditions are involved into the responsiveness of the system. One of the representative of switchable metal-organic frameworks is pillared-layer DUT-8 (M2(2,6-ndc)2(dabco), M = Ni, Co, Cu, Zn, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). Depending on the metal node and particle size, it is possible to synthesize either switchable or rigid materials differing in physisorption isotherm profiles. In order to understand switching behaviour of DUT-8, the important parameters influencing structural switchability are addressed in my work. For this purpose, the impact of crystal size and morphology, as well as crystal surface on adsorption-induced structural transformations of DUT-8(Ni) were investigated. DUT-8(Ni) shows reversible structural transition between open (op) and closed pore phase (cp) upon adsorption/removal of guest molecules. To understand which particular crystal surfaces dominate the phenomena observed, crystals similar in size and differing in morphology were involved in a systematic study. The analysis of the data shows that the width of the rods (corresponding to the crystallographic directions along the layer) represents a critical parameter governing the dynamic properties upon adsorption of nitrogen at 77 K. This observation is related to the anisotropy of the channel-like pore system and the nucleation mechanism of the solid-solid phase transition triggered by gas adsorption. To investigate the influence of external surface on adsorption-induced switchability, DUT-8(Ni) samples were exposed to different treatment techniques. By means of analytical methods, it was revealed that the surface of samples was modified leading to a significant increase of the gate-opening pressure, reflecting the increase of activation barrier for phase switching form cp to op upon adsorption of nitrogen at 77 K. Furthermore, the properties of DUT-8(Zn) were studied precisely, focusing on the variation of particle size and morphology, host-guest interactions, desolvation conditions, selectivity and thermoresponsivity. Depending on the synthesis conditions, DUT-8(Zn) can be synthesised in macro-sized regime (150 µm) and micron-sized regime (0.5 µm). The solvent removal process (pore desolvation stress contracting the framework) significantly controls the cp/op ratio after desolvation and, subsequently, the adsorption induced switchability characteristics of the system. Among the applied desolvation techniques, the solvent exchange with subsequent heating causes phase transition from open (op) to closed pore phase (cp). After desolvation, the dense cp phase of DUT-8(Zn) shows no adsorption-induced reopening and therefore is non-porous for N2 at 77 K and CO2 at 195 K. However, polar molecules with a higher adsorption enthalpy, such as chloromethane at 249 K and dichloromethane (DCM) at 298 K can reopen the macro-sized crystals upon adsorption, while micron-sized crystals retain the cp phase. For macro-sized particles (160 µm), the outer surface energy is negligible and only the type of metal (Zn, Co, Ni) controls the DCM-induced gate opening pressure. The node hinge stiffness increases from Zn to Ni as confirmed by DFT calculations, X-ray crystal structural analysis, and low frequency Raman spectroscopy. This softer Zn-based node hinges and overall increased stabilization of cp vs. op phase shift the critical particle size at which switchability starts to become suppressed to even lower values. Hence, the three factors affecting switchability (energetics of the empty host, (Eop–Ecp) (i), particle size (ii), and desolvation stress (iii)) appear to be of the same order of magnitude and should be considered collectively, not individually. Crystal downsizing (0.5 µm) facilitates the responsivity of DUT-8(Zn) towards different guest molecules, not opening for macro-sized crystals. Among investigated adsorptives, the alcohols are in the center of attention due to ability to induce so called shape-memory effect in micron-sized crystals. The adsorption of alcohols stimulates the change of initial shape of pores (cp) into a temporary shape (op) which is maintained even after desorption. To brighten the crystal size range and to study the dependence of gate opening pressure from crystal size and morphology, differently shaped crystals in micron-sized regime were produced by face-selective coordination modulation. Morphology modification allowed to determine the critical parameter controlling switchable transformations in DUT-8(Zn). Thus, the crystal size engineering and morphology modification provide an opportunity not only to control the structural dynamics of MOFs, but also to tailor responsivity towards guest molecules, influencing the selective adsorption behaviour

Logic and Symbolism of Switchable Porous Framework Materials

Metal-Organic Frameworks (MOFs) are highly porous materials composed of organic linkers connected by inorganic nodes. A unique subset of MOFs shows switchability, the ability to switch between at least two distinct structures differing significantly in porosity. These unique guest dependent pore opening and closing processes offer new opportunities in gas separation, selective recognition, sensing and energy storage. However, the factors affecting switchability are poorly understood. Network topology, micromechanics of building blocks and their hinges, but also particle size, defects, agglomeration, desolvation conditions etc. are convoluted into the responsiveness of the system. In essence all factors are a consequence of the materials history including synthesis procedure and desolvation but also all subsequent handling steps such as mechanical and adsorption stress leading to a complex interplay of factors which are difficult to express clearly by ordinary writing systems, chemical or mathematical symbols without loss of intuitive understanding. Here we propose a symbolic language for the rationalization of switchability emphasizing the history dependent responsivity of many dynamic frameworks and their stimuli induced 1st order phase transitions. Color representations of the guest and host offer an intuitive understanding of switchability phenomena even for non-experts. The system follows a bivalent logic inspired by Freges Begriffsschrift providing a fundamental logic structure for the rationalization of statements and representation of logic gates. </p

The importance of crystal size for breathing kinetics in MIL-53(Al)

Herein we analyze the switching kinetics of a breathing framework MIL-53(Al) with respect to different crystallite size regimes. Synchrotron time-resolved powder X-ray diffraction (PXRD) and adsorption rate analysis of n-butane physisorption at 298 K demonstrate the decisive role of crystal size affecting the time domain of breathing transitions in MIL-53(Al)

Tailoring Adsorption Induced Switchability of a Pillared Layer MOF by Crystal Size Engineering

The pillared layer framework DUT-8(Zn) (Zn2(2,6-ndc)2(dabco), 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane, DUT = Dresden University of Technology) is a prototypical switchable MOF, showing characteristic adsorption and desorption induced open phase (op) to closed phase (cp) transformation associated with huge changes in cell volume. We demonstrate switchability strongly depends on a framework-specific critical particle size (dcrit). The solvent removal process (pore desolvation stress contracting the framework) significantly controls the cp/op ratio after desolvation and, subsequently, the adsorption induced switchability characteristics of the system. After desolvation, the dense cp phase of DUT-8(Zn) shows no adsorption-induced reopening and therefore is non-porous for N2 at 77 K and CO2 at 195 K. However, polar molecules with a higher adsorption enthalpy, such as the polar molecules such as chloromethane at 249 K and dichloromethane (DCM) at 298 K can reopen the macro-sized crystals upon adsorption. For macro-sized particles, the outer surface energy is negligible and only the type of metal (Zn, Co, Ni) controls the DCM-induced gate opening pressure. The framework stiffness increases from Zn to Ni as confirmed by DFT calculations, X-ray crystal structural analyses, and low frequency Raman spectroscopy. The partial disintegration of the Zn based node hinges produces an overall increased stabilization of cp vs. op phase shifts the critical particle size at which switchability starts to become suppressed to even lower values (dcrit dcrit ≈ 500 nm). Hence, the three factors affecting switchability (energetics of the empty host, (Eop-Ecp) (I), particle size (II), and desolvation stress (III)) appear to be of the same order of magnitude and should be considered collectively, not individually

Temperature Driven Transformation of the Flexible Metal–Organic Framework DUT-8(Ni)

DUT-8(Ni) metal–organic framework (MOF) belongs to the family of flexible pillared layer materials. The desolvated framework can be obtained in the open pore form (op) or in the closed pore form (cp), depending on the crystal size regime. In the present work, we report on the behaviour of desolvated DUT-8(Ni) at elevated temperatures. For both, op and cp variants, heating causes a structural transition, leading to a new, crystalline compound, containing two interpenetrated networks. The state of the framework before transition (op vs. cp) influences the transition temperature: the small particles of the op phase transform at significantly lower temperature in comparison to the macroparticles of the cp phase, transforming close to the decomposition temperature. The new compound, confined closed pore phase (ccp), was characterized by powder X-ray diffraction and spectroscopic techniques, such as IR, EXAFS, and positron annihilation lifetime spectroscopy (PALS). Thermal effects of structural transitions were studied using differential scanning calorimetry (DSC), showing an overall exothermic effect of the process, involving bond breaking and reformation. Theoretical calculations reveal the energetics, driving the observed temperature induced phase transition