Unravelling surface and interfacial structures of a metal–organic framework by transmission electron microscopy

Metal-organic frameworks (MOFs) are crystalline porous materials with designable topology, porosity and functionality, having promising applications in gas storage and separation, ion conduction and catalysis. It is challenging to observe MOFs with transmission electron microscopy (TEM) due to the extreme instability of MOFs upon electron beam irradiation. Here, we use a direct-detection electron-counting camera to acquire TEM images of the MOF ZIF-8 with an ultralow dose of 4.1 electrons per square ångström to retain the structural integrity. The obtained image involves structural information transferred up to 2.1 Å, allowing the resolution of individual atomic columns of Zn and organic linkers in the framework. Furthermore, TEM reveals important local structural features of ZIF-8 crystals that cannot be identified by diffraction techniques, including armchair-type surface terminations and coherent interfaces between assembled crystals. These observations allow us to understand how ZIF-8 crystals self-assemble and the subsequent influence of interfacial cavities on mass transport of guest molecules

Systematic and In Situ Energy Dispersive X-ray Diffraction Investigations on the Formation of Lanthanide Phosphonatobutanesulfonates: Ln(O3P−C4H8−SO3)(H2O)\mathrm{Ln(O_3P-C_4H_8-SO_3)(H_2O)} (Ln = La-Gd)

Using the flexible linker H(2)O(3)P-C(4)H(8)-SO(3)H (H(3)L) and rare earth ions Ln(3+) (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) we were able to synthesize the new isostructural inorganic organic hybrid compounds Ln(O(3)P-C(4)H(8)-SO(3))(H(2)O). High-throughput experiments were employed to study the influence of the molar ratios Ln:H(3)L and pH on the product formation. The crystal structure of the compounds Sm(O(3)P-C(4)H(8)-SO(3))(H(2)O) (1) and Pr(O(3)P-C(4)H(8)-SO(3))(H(2)O) (2) were determined by single crystal diffraction. The structures are built up from chains of edge-sharing LnO(8)-polyhedra that are connected by the phosphonate and sulfonate groups into layers. These layers are linked by the -(CH(2))(4)- group to form a three-dimensional framework. The synthesis of compound 1 was scaled up in a conventional oven as well as in a microwave reactor system. A modification of a microwave reactor system allowed its integration into the beamline F3 at HASYLAB, DESY, Hamburg. The crystallization was investigated in situ by means of energy dispersive X-ray diffraction using conventional as well as microwave heating methods applying temperatures varying from 110 to 150 °C. The formation of Sm(O(3)P-C(4)H(8)-SO(3))(H(2)O) takes place in two steps. In the first step a crystalline intermediate was observed, which transforms completely into compound 1. The method by Sharp and Hancock was used to determine the rate constants, reaction exponents, and the Arrhenius activation energy for both reaction steps. Comparing both heating methods, microwave heating leads to fully crystallized reaction product after shorter reaction times, but neither the temperature nor the heating method has significant influence on the induction time

Discovery of New Calcium Etidronates Employing Ultrasound Adapted High-Throughput Methods

The formation of calcium etidronates was investigated employing high-throughput ultrasonic synthesis. In the study of the system CaCl2/H2O3P-C(OH)(CH3)-PO3H2/H2O/KOH three new crystalline products were obtained in dependence of the deprotonation degree of the bisphosphonic acid HEDP (1-hydroxyethylidene-1,1-diphosphonic acid, H2O3P-C(OH)(CH3)-PO3H2) and the sonication time. In a large region of the parameter space and at short reaction times [Ca(HO3P-C(OH)(CH3)-PO3H)(H2O)]·2.5H2O (1) forms. At higher pH [KCa(HO3P-C(OH)(CH3)-PO3)(H2O)]·H2O (2) containing the asymmetrically deprotonated HO3P-C(OH)(CH3)-PO33– ion and [K2Ca(O3P-C(OH)(CH3)-PO3)(H2O)6] (3) with the fully deprotonated ligand are obtained. The crystal structures of 1 and 2 were solved and refined from X-ray powder diffraction data. The thermal decomposition of the compounds was investigated. If exposed to moderate temperatures 1.5 water molecules per formula unit are removed from the crystal structure of 1 and the pseudopolymorphic compound [Ca(HO3P-C(OH)(CH3)-PO3H)(H2O)2] is formed [Uchtman J. Phys. Chem. 1972, 76, 1304−1310]