Influence of Thermal and Mechanical Stimuli on the Behavior of Al-CAU-13 Metal-Organic Framework.

The response of the metal-organic framework aluminum-1,4-cyclohexanedicarboxylate or Al-CAU-13 (CAU: Christian Albrecht University) to the application of thermal and mechanical stimuli was investigated using synchrotron powder X-ray diffraction (SPXRD). Variable temperature in situ SPXRD data, over the range 80-500 K, revealed a complex evolution of the structure of the water guest containing Al-CAU-13H2O, the dehydration process from ca. 310 to 370 K, and also the evolution of the guest free Al-CAU-13 structure between ca. 370 and 500 K. Rietveld refinement allowed this complexity to be rationalized in the different regions of heating. The Berman thermal Equation of State was determined for the two structures (Al-CAU-13H2O and Al-CAU-13). Diamond anvil cell studies at elevated pressure (from ambient to up to ca. 11 GPa) revealed similarities in the structural responses on application of pressure and temperature. The ability of the pressure medium to penetrate the framework was also found to be important: non-penetrating silicone oil caused pressure induced amorphization, whereas penetrating helium showed no plastic deformation of the structure. Third-order Vinet equations of state were calculated and show Al-CAU-13H2O is a hard compound for a metal-organic framework material. The mechanical response of Al-CAU-13, with tetramethylpyrazine guests replacing water, was also investigated. Although the connectivity of the structure is the same, all the linkers have a linear e,e-conformation and the structure adopts a more open, wine-rack-like arrangement, which demonstrates negative linear compressibility (NLC) similar to Al-MIL-53 and a significantly softer mechanical response. The origin of this variation in behavior is attributed to the different linker conformation, demonstrating the influence of the S-shaped a,a-conformation on the response of the framework to external stimuli.

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.

Single-site metal-organic framework catalysts for the oxidative coupling of arenes via C-H/C-H activation.

C-H activation reactions are generally associated with relatively low turnover numbers (TONs) and high catalyst concentrations due to a combination of low catalyst stability and activity, highlighting the need for recyclable heterogeneous catalysts with stable single-atom active sites. In this work, several palladium loaded metal-organic frameworks (MOFs) were tested as single-site catalysts for the oxidative coupling of arenes (e.g. o-xylene) via C-H/C-H activation. Isolation of the palladium active sites on the MOF supports reduced Pd(0) aggregate formation and thus catalyst deactivation, resulting in higher turnover numbers (TONs) compared to the homogeneous benchmark reaction. Notably, a threefold higher TON could be achieved for palladium loaded MOF-808 due to increased catalyst stability and the heterogeneous catalyst could efficiently be reused, resulting in a cumulative TON of 1218 after three runs. Additionally, the palladium single-atom active sites on MOF-808 were successfully identified by Fourier transform infrared (FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopy.

Nanoscale Synthesis of Two Porphyrin-Based MOFs with Gallium and Indium.

Two porphyrin-based metal-organic frameworks (MOFs) containing gallium or indium, [Ga2(OH)2(H2TCPP)]·3DMF·3H2O (Ga-PMOF) and [In2(OH)2(H2TCPP)]·3DMF·4H2O (In-PMOF) (H6TCPP = 4-tetracarboxyphenylporphyrin), were discovered using high-throughput methods. The structure was refined by the Rietveld-method starting from the structure model of Al-PMOF, [Al2(OH)2(H2TCPP)]. The new PMOFs exhibit BET surface areas between 1150 and 1400 m(2) g(-1) and are also porous toward CO2 (Ga-PMOF, 15.2 wt %; In-PMOF, 12.9 wt %). They are thermally stable in air up to 330 °C, but show limited chemical stabilities toward acids and bases. In order to achieve size control, different synthesis routes were investigated, i.e., batch synthesis at different temperatures (yield: In-PMOF-bs-th 96%, Ga-PMOF-bs-th 87%), ultrasound-assisted synthesis (yield: In-PMOF-bs-us 85%), and continuous-flow synthesis (yield: Ga-PMOF-cf 71%). By using these different methods we could control the nucleation rate and the crystal size. The crystal sizes were found to vary about 60 to 160 nm and 70 to 130 nm for Ga- and In-PMOF, respectively, which was proven by dynamic light scattering (DLS), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements.

Flow-synthesis of carboxylate and phosphonate based metal-organic frameworks under non-solvothermal reaction conditions.

A continuous flow reactor was developed for the synthesis of porous metal-organic frameworks (MOFs) under mild reaction conditions. Commodity hardware was used to assemble the device, giving it a great degree of flexibility in its configuration. The use of paraffin to encapsulate reactions and also ultrasonic treatment were employed to prevent clogging of the reactor. Reactor design was optimised through studies of the synthesis of zirconium carboxylate framework UiO-66. Synthesis of the aluminium carboxylate CAU-13 was also performed, to demonstrate the versatility of the device. Finally the reactor was used to synthesise a new cadmium phosphonate framework, bearing the STA-12 network.

Mixed-linker MOFs with CAU-10 structure: synthesis and gas sorption characteristics.

The metal­organic framework compound [Al(OH)(BDC-Br)] (1) (BDC-Br2− = 5-bromo-1,3-benzenedicarboxylate) denoted CAU-10-Br was synthesised under solvothermal reaction conditions. Its structure was successfully refined by Rietveld methods. The framework is based on the connection of infinite helical chains of cis-corner-sharing AlO6-polyhedra via BDC-Br2− ions. Thus non-intersecting parallel channels are formed, each periodically varying in diameter between 1.1 and 6.6 Å. Nevertheless 1 adsorbs CO2 at 298 K, while it is non-porous towards H2 and N2 at 77 K. Employing high-throughput (HT) methods we identified synthesis conditions that lead to the formation of mixed-linker MOFs with CAU-10 topology. Starting with a molar ratio H2BDC:H2BDC-Br = 3:1 we established a synthesis procedure for the partially bromo-functionalised mixed-linker-MOF [Al(OH)(BDC)0.8(BDC-Br)0.2] (2) denoted as CAU-10-H/Br. Starting with a molar ratio H2BDC-NH2:H2BDC-NO2 = 1:1 the partially NO2- and NH2-functionalised mixed-linker MOF [Al(OH)(BDC-NO2)0.55(BDC-NH2)0.23(BDC-NHCHO)0.22] (3) denoted as CAU-10-NO2/NH2 was synthesised, in which the NH2-groups were found to be partially formylated. The partial bromo-functionalisation in CAU-10-H/Br (2) leads to a lower sorption capacity in comparison with the parent structure CAU-10-H, while the pore accessibility is comparable. The incorporation of NO2-, NH2- and NHCHO-groups in CAU-10-NO2/NH2 (3) does not only affect the accessibility of the pores for N2, but results also in an increased capacity for H2 and CO2 in comparison with the parent structure CAU-10-NO2. The reproducibility of the synthesis procedures was tested regarding the composition of the MOFs and the resulting gas sorption properties. In 2 a molar fraction of (BDC)2− = 80 ± 2% is found, while in 3 a molar fraction of (BDC-NO2)2− = 55 ± 2% is observed. Starting from the molar ratio H2BDC:H2BDC-CH3 = 1:1 mixed linker MOFs denoted as CAU-10-H/CH3 (4) were also obtained. In contrast to 2 and 3 the molar ratios of BDC2−/BDC-CH32− and especially the sorption properties differ substantially for every batch. The compounds were further characterised by X-ray powder diffraction, thermogravimetric/elemental analysis, NMR/Vis/IR-spectroscopy, and gas sorption measurements.