Aqueous Flow Reactor and Vapour-Assisted Synthesis of Aluminium Dicarboxylate Metal-Organic Frameworks with Tuneable Water Sorption Properties.

Energy-efficient indoors temperature and humidity control can be realised by using the reversible adsorption and desorption of water in porous materials. Stable microporous aluminium-based metal-organic frameworks (MOFs) present promising water sorption properties for this goal. The development of synthesis routes that make use of available and affordable building blocks and avoid the use of organic solvents is crucial to advance this field. In this work, two scalable synthesis routes under mild reaction conditions were developed for aluminium-based MOFs: (1) in aqueous solutions using a continuous-flow reactor and (2) through the vapour-assisted conversion of solid precursors. Fumaric acid, its methylated analogue mesaconic acid, as well as mixtures of the two were used as linkers to obtain polymorph materials with tuneable water sorption properties. The synthesis conditions determine the crystal structure and either the MIL-53 or MIL-68 type structure with square-grid or kagome-grid topology, respectively, is formed. Fine-tuning resulted in new MOF materials thus far inaccessible through conventional synthesis routes. Furthermore, by varying the linker ratio, the water sorption properties can be continuously adjusted while retaining the sigmoidal isotherm shape advantageous for heat transformation and room climatisation applications.

Control of Metal-Organic Framework Crystallization by Metastable Intermediate Pre-equilibrium Species.

There is an increasing amount of interest in metal-organic frameworks (MOFs) for a variety of applications, from gas sensing and separations to electronics and catalysis. However, the mechanisms by which they crystallize remain poorly understood. Herein, an important new insight into MOF formation is reported. It is shown that, prior to network assembly, crystallization intermediates in the canonical ZIF-8 system exist in a dynamic pre-equilibrium, which depends on the reactant concentrations and the progress of reaction. Concentration can, therefore, be used as a synthetic handle to directly control particle size, with potential implications for industrial scale-up and gas sorption applications. These findings enable the rationalization of apparent contradictions between previous studies of ZIF-8 and opens up new opportunities for the control of crystallization in network solids more generally.

Probing the Evolution of Palladium Species in Pd@MOF Catalysts during the Heck Coupling Reaction: An Operando X-ray Absorption Spectroscopy Study.

The mechanism of the Heck C-C coupling reaction catalyzed by Pd@MOFs has been investigated using operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) combined with transmission electron microscopy (TEM) analysis and nuclear magnetic resonance (1H NMR) kinetic studies. A custom-made reaction cell was used, allowing operando PXRD and XAS data collection using high-energy synchrotron radiation. By analyzing the XAS data in combination with ex situ studies, the evolution of the palladium species is followed from the as-synthesized to its deactivated form. An adaptive reaction mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction, which then gradually transform into Pd nanoclusters with 13-20 Pd atoms on average in later catalytic turnovers. Consumption of available reagent and substrate leads to coordination of Cl- ions to their surfaces, which causes the poisoning of the active sites. By understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.

Green synthesis of a new layered aluminium citraconate: crystal structures, intercalation behaviour towards H2O and in situ PXRD studies of its crystallisation.

A new Al-based layered MOF [Al2(OH)4(O2C-C3H4-CO2)]·nH2O denoted as CAU-15-Cit was synthesised under mild aqueous conditions. It exhibits a layered structure incorporating infinite chains of edge-sharing AlO6 polyhedra being interconnected by citraconate anions to arrange into layers, which are stacked in an AAA fashion (citraconic acid = methylmaleic acid = H2Cit, HO2C-C3H4-CO2H). The crystal structures of the hydrated and dehydrated MOF were determined ab initio from powder X-ray diffraction (PXRD) data. The hydrated form of this compound (n≈ 3) crystallises in the space group C2/c (a = 7.4074(8), b = 23.006(3), c = 7.0890(4) Å, ß = 85.024(7)°) and is converted to a triclinic anhydrous form (n = 0) upon dehydration (a = 7.0010(3), b = 7.5062(8), c = 9.2212(8) Å, α = 72.143(7), ß = 88.617(9), γ = 85.242(8)°, space group P1[combining macron]) in which the layers are interdigitated with a decreased interlayer distance. Physisorption measurements of the anhydrous form indicated no porosity towards nitrogen but an uptake of water vapour was measured showing a sigmoidal adsorption curve and a capacity of ≈24 wt%. This is in good agreement with the theoretical capacity for complete intercalation (≈25 wt%). Based on the temperature dependent PXRD data of the hydrated form, the intercalated water is removed at around 100 °C and the framework decomposes above 350 °C. The dehydration process was further compared to the behaviour of the structurally related aromatic variant of the MOF based on phthalic acid, denoted as CAU-15 [Al2(OH)4(O2C-C6H4-CO2)]·nH2O. The crystallisation of CAU-15-Cit was investigated by means of in situ PXRD measurements during the synthesis using synchrotron radiation at temperatures between 90 and 130 °C. Evaluation of the kinetics using the Sharp-Hancock method clearly indicated different kinetic regimes for the reactions, regardless of the synthesis temperature. At lower temperatures the rate limiting step during the initial period is nucleation, while this initial reaction stage is kinetically limited by diffusion at higher temperatures. The second reaction stage at all temperatures is approximately of the first order.

Green Synthesis of a New Al-MOF Based on the Aliphatic Linker Mesaconic Acid: Structure, Properties and In Situ Crystallisation Studies of Al-MIL-68-Mes.

A new aluminium metal-organic framework (MOF), based on the short aliphatic linker molecule mesaconic acid (H2 Mes; methylfumaric acid) is reported. Al-MIL-68-Mes with composition [Al(OH)(O2 C-C3 H4 -CO2 )]⋅n H2 O is obtained after short reaction times of 45 minutes under mild, aqueous synthesis conditions (95 °C). It exhibits a kagome-like framework structure with large hexagonal, and small trigonal channels (diameters of ≈6 and ≈2 Å, respectively) and a specific surface area of SBET ≈1040 m2 g-1 (VMIC =0.42 cm3 g-1 ). A sigmoidal vapour sorption isotherm for water, and uptakes of water and methanol above 30 wt. % were observed. Al-MIL-68-Mes is stable against water ad-/desorption and its thermal stability is 350 °C in air. The proton conductivity for the hydrated MOF showed values up to 1.1×10-5  S cm at 130 °C and 100 % relative humidity, which exceeds the values observed for the non-hydrated compound by up to four orders of magnitude. Using synchrotron radiation the crystallisation of the MOF by in situ PXRD was also studied at temperatures from 80 to 100 °C. Kinetic evaluation revealed that the induction periods and crystallization times vary depending on the synthesis batch, but the rate limiting steps are consistently observed.

Combined in- and ex situ studies of pyrazine adsorption into the aliphatic MOF Al-CAU-13: structures, dynamics and correlations.

The intercalation of different pyrazines (pyrazine, methylpyrazine, 2,5-dimethylpyrazine, 2,3-dimethylpyrazine, trimethylpyrazine and tetramethylpyrazine) into the trans-1,4-cyclohexanedicarboxylate (CDC2-) based Al-MOF [Al(OH)(CDC)], denoted as CAU-13, was investigated. The adsorption of the guest molecules into the flexible MOF was carried out from an aqueous solution or via vapour phase adsorption, starting with the hydrated narrow-pore form of the framework [Al(OH)(O2C-C6H10-CO2)]·H2O (CAU-13-np). The obtained host-guest systems were characterised by thermogravimetry and vibrational spectroscopy and their crystal structures were elucidated using powder X-ray diffraction (PXRD) data. The crystal structures indicate that guest molecules forming hydrogen-bonds with the host framework (pyrazine, methylpyrazine and 2,5-dimethylpyrazine) induce a slight opening of the channels, resulting in a semi-open framework conformation (CAU-13-so). For the bulkier guests 2,3-dimethylpyrazine, trimethylpyrazine and tetramethylpyrazine, only van der Waals interactions can be observed between the host and the guest molecules and a large pore conformation is observed (CAU-13-lp). We carried out in situ PXRD studies using synchrotron radiation during the adsorption of the respective guest molecules from aqueous solutions with various concentrations and at different temperatures. In general, stronger host-guest interactions required milder adsorption conditions while harsher conditions nevertheless accelerated the conversion. The kinetic parameters for the intercalation of pyrazine indicate that the rate limiting step differs, depending on the intercalation temperature.