Helge Reinsch

Formation and characterisation of Mn-MIL-100

The synthesis and characterisation of the Mn-based analogue of the MIL-100 framework is reported. The title compound is synthesised from a methanolic solution of trimesic acid and manganese(II) nitrate in very short reaction times. During the reaction, the Mn2+ ions are oxidized in situ to Mn3+ ions. The obtained MOF was characterised by XRPD measurements, IR spectroscopy, thermogravimetric measurements and sorption experiments. To have a closer look at the crystallisation occurring after the in situ metal oxidation reaction that results in the formation of the title compound, in situ EDXRPD experiments under solvothermal conditions were carried out. These time-resolved measurements could be evaluated by two different kinetic models (by Avrami and Gualtieri) for crystallisation. The results indicate a two stage reaction process, which is dominated by different reaction mechanisms.

A new Al-MOF based on a unique column-shaped inorganic building unit exhibiting strongly hydrophilic sorption behaviour

The new Al-based metal-organic framework [Al(13)(OH)(27)(H(2)O)(6)(BDC-NH(2))(3)Cl(6)(C(3)H(7)OH)(6)] denoted CAU-6 (CAU = Christian-Albrechts-Universität) was solvothermally synthesized in 2-propanol and was thoroughly characterized. The framework structure exhibits a unique column-shaped inorganic building unit, which is based on stacked, corner-sharing Al(13)-clusters. The compound exhibits unprecedented hydrophilicity for metal-organic frameworks.

CAU-3: A new family of porous MOFs with a novel Al-based brick: [Al2(OCH3)4(O2C-X-CO2)] (X = aryl)

A new family of Al-based MOFs denoted as CAU-3 (CAU = Christian-Albrechts-Universität) was discovered in the solvothermal system Al(3+)/aryldicarboxylic acid/NaOH/methanol by applying high-throughput-methods. The three compounds reported in this article [Al(2)(OCH(3))(4)BDC], [Al(2)(OCH(3))(4)BDC-NH(2)] and[Al(2)(OCH(3))(4)NDC] (BDC = 1,4-benzenedicarboxylate; NDC = 2,6-naphtalenedicarboxylate) are all based on the same unprecedented inorganic building unit [Al(12)(OCH(3))(24)](12+), which is a dodecameric cyclic aluminium-methanolate-cluster. The material CAU-3-NDC was found to exhibit the highest surface area as well as the highest micropore volume of all Al-based MOFs reported until now.

Water adsorption behaviour of CAU-10-H: a thorough investigation of its structure–property relationships

Aluminium isophthalate CAU-10-H [Al(OH)(benzene-1,3-dicarboxylate)]·nH2O exhibits water adsorption characteristics which make it a promising adsorbent for application in heat-exchange processes. Herein we prepared a stable coating of this MOF and evaluated its long-term stability under closed-cycle conditions for 10 000 water adsorption and desorption cycles, which are typical lifetimes for adsorption heat storage (AHS) applications. No degradation of the adsorption capacity could be observed which makes CAU-10-H the most stable MOF under these humid cycling conditions reported until now. Moreover, thermophysical properties like thermal conductivity and heat of adsorption were directly measured. In order to identify the structural features associated with the adsorption behaviour, the structural differences between the dry and the water loaded CAU-10-H were studied by Rietveld refinements and second harmonic generation (SHG) microscopy. The observed transition of space group symmetry from I41 to I41/amd between the humid and dry forms is induced by the adsorption/desorption of water into/out of the MOF channels. This originates from a torsional motion around the C–C bond between the carboxylate groups and the aromatic ring in half of the linker molecules. These observations are in excellent agreement with molecular dynamics simulations which confirm the energetic benefit of this transition.

Green synthesis of zirconium-MOFs

The synthesis of Zr-MOFs under green, industrially feasible conditions was investigated. Two new compounds with bcu-topology and the fluorinated analogue of UiO-66 exhibiting fcu-topology were obtained and characterised. All products exhibit permanent porosity. In the bcu-frameworks the interaction with sulfate anions apparently induces an unusual eightfold connectivity of the Zr cluster.

A Flexible Photoactive Titanium Metal–Organic Framework Based on a [TiIV3(μ3-O)(O)2(COO)6] Cluster

The synthesis of titanium-carboxylate metal-organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium-based MOFs by the use of titanocene dichloride to synthesize COK-69, the first breathing Ti MOF, which is built up from trans-1,4-cyclohexanedicarboxylate linkers and an unprecedented [Ti(IV)3(μ3-O)(O)2(COO)6] cluster. The photoactive properties of COK-69 were investigated in depth by proton-coupled electron-transfer experiments, which revealed that up to one Ti(IV) center per cluster can be photoreduced to Ti(III) while preserving the structural integrity of the framework. The electronic structure of COK-69 was determined by molecular modeling, and a band gap of 3.77 eV was found.

Experimental Screening of Porous Materials for High Pressure Gas Adsorption and Evaluation in Gas Separations: Application to MOFs (MIL-100 and CAU-10)

A high-throughput gas adsorption apparatus is presented for the evaluation of adsorbents of interest in gas storage and separation applications. This instrument is capable of measuring complete adsorption isotherms up to 40 bar on six samples in parallel using as little as 60 mg of material. Multiple adsorption cycles can be carried out and four gases can be used sequentially, giving as many as 24 adsorption isotherms in 24 h. The apparatus has been used to investigate the effect of metal center (MIL-100) and functional groups (CAU-10) on the adsorption of N(2), CO(2), and light hydrocarbons on MOFs. This demonstrates how it can serve to evaluate sample quality and adsorption reversibility, to determine optimum activation conditions and to estimate separation properties. As such it is a useful tool for the screening of novel adsorbents for different applications in gas separation, providing significant time savings in identifying potentially interesting materials.

First examples of aliphatic zirconium MOFs and the influence of inorganic anions on their crystal structures

Utilizing the aliphatic linker molecule adipic acid (1,6-hexanedioic acid, HO2C–C4H8–CO2H) or 3-methyladipic acid (racemic mixture, HO2C–C4H7CH3–CO2H), the first crystalline zirconium adipates were synthesized under aqueous conditions. Their structures were deduced from powder X-ray diffraction data and were confirmed by Rietveld refinements. For all three compounds, the inorganic nodes are related to the well-known Zr6O4(OH)4 cluster frequently observed in aromatic zirconium MOFs. Employing ZrOCl2·8H2O and 3-methyladipic acid, a framework with bcu topology was obtained. Starting from adipic acid and Zr(SO4)2·4H2O, we observed the incorporation of sulfate into the crystal structure. Four sulfate anions are coordinated to each Zr–oxo cluster in a bidentate fashion. In this complex structure, square grids formed by Zr–oxo clusters and adipate anions and furthermore a hydrogen-bonded inorganic dia net can be observed. The third compound presented here is structurally related to the zirconium methyladipate. Using adipic acid and adding CrO42− under strongly acidic conditions leads to the incorporation of Cr2O72− into the bcu net. The dichromate anions are coordinated twofold to two different Zr–oxo clusters in a monodentate fashion and thus serve as inorganic connectors between the frameworks nodes.

Dihydroxybenzoquinone as Linker for the Synthesis of Permanently Porous Aluminum Metal–Organic Frameworks

Two new dihydroxybenzoquinone-based metal-organic frameworks, ((CH3)2NH2)3[Al4(L1)3(L1(•))3]·3DMF (1, denoted CAU-20) and ((CH3)2NH2)3[Al4(L2)3(L2(•))3]·9DMF (2, denoted CAU-20-Cl2), were synthesized at 120 °C in DMF using 2,5-dihydroxy-p-benzoquinone ((C6H2(OH)2(O)2), H2L1) and 2,5-dichloro-3,6-dihydroxy-p-benzoquinone ((C6Cl2(OH)2(O)2), H2L2), respectively. Compared to other Al-MOFs, which contain carboxylate or phosphonate groups that connect the metal sites, in 1 and 2 the Al(3+) are coordinated by oxido groups. The metal ions are octahedrally surrounded by oxygen atoms of the deprotonated linker molecules to generate honeycomb layers with a metal to linker ratio of Al: L1/L2 = 2:3. The layers contain L1(2-) and L2(2-) ions as well as linker radical ions L1(•3-) and L2(•3-) in a molar ratio of 1 to 1. The presence of radical ions was confirmed by EPR and UV-vis-spectroscopic measurements, and the composition was determined from a combination of PXRD, (1)H NMR, TG, and elemental analyses. Charge balance is accomplished through intercalation of (CH3)2NH2(+) ions which are formed by partial hydrolysis of DMF. In the structures of 1 and 2 the eclipsed layers are AA and ABAB stacked, respectively, and one-dimensional hexagonal channels with diameters of ca. 9 and 6 Å are formed. Both compounds exhibit permanent porosity and have specific surface areas of 1440 and 1430 m(2) g(-1), respectively.

Synthesis and structure of Zr(IV)- and Ce(IV)-based CAU-24 with 1,2,4,5-tetrakis(4-carboxyphenyl)benzene

Two new MOFs denoted as M-CAU-24 (M = Zr, Ce) based on 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (H4TCPB) were obtained under mild reaction conditions within 15 min. The MOFs with composition [M6(μ3-O)4(μ3-OH)4(OH)4(H2O)4(TCPB)2] crystallise in the scu topology, a connectivity hitherto unreported for Zr-MOFs with tetracarboxylate linker molecules. Zr-CAU-24 exhibits UV/blue ligand-based luminescence.