Development and application of an exchange model for anisotropic water diffusion in the microporous MOF aluminum fumarate.

Diffusion of water in aluminum fumarate was studied by means of pulsed field gradient (PFG) nuclear magnetic resonance (NMR). Due to water molecules exchanging between the intracrystalline anisotropic pore space and the isotropic intercrystalline void space the model of intracrystalline anisotropic diffusion fails to describe the experimental PFG NMR data at high observation times. Therefore, the two-site exchange model developed by Kärger is extended to the case of exchange between an anisotropic and an isotropic site. This extended exchange model is solved by numerical integration. It describes the experimental data very well and yields values for the intracrystalline diffusion coefficient and the mean residence times of the respective sites. Further PFG NMR studies were performed with coatings consisting of small aluminum fumarate crystals, which are used in adsorptive heat transformation applications. The diffusion coefficients of water in the small crystal coating are compared to the values expected from the extended two-site exchange model and from the model of long-range diffusion.

Ambient pressure synthesis of MIL-100(Fe) MOF from homogeneous solution using a redox pathway.

Micro- to mesoporous iron(iii) trimesate MIL-100(Fe) is a MOF of high interest for numerous applications. With regard to large-scale synthesis, e.g., by continuous flow or the in situ deposition of coatings, a replacement for the conventional, hydrothermal low-yield fluoride-containing synthesis is desirable. In this contribution, we present a method to synthesize crystalline fluoride-free MIL-100(Fe) from iron(iii) nitrate and trimesic acid in zeotropic DMSO/water solution at normal ambient pressure involving a DMSO-nitrate redox pathway. Yields of 72%, surface areas of SBET = 1791 m(2) g(-1) and pore volumes of Vpore = 0.82 cm(3) g(-1) were achieved.

High-yield, fluoride-free and large-scale synthesis of MIL-101(Cr).

MIL-101(Cr), one of the most important prototypical MOFs, is well investigated and widely used in many scientific fields. With regard to MOF synthesis in general, the addition of a modifier is commonly used to improve the properties of the products. The effect of inorganic (mineral) and organic acid modifiers was thoroughly investigated in the synthesis of MIL-101(Cr) and HNO3 could increase the yield to over 80% of a product with average SBET > 3200 m(2) g(-1) in repeated experiments (from an average of 50% in most published syntheses) in small-scale laboratory synthesis. The large-scale synthesis could use the finding of HNO3 addition and produce MIL-101(Cr) in >100 g quantities with yields near 70% and BET-surface areas near 4000 m(2) g(-1). The addition of acetic acid (CH3COOH) together with seeding could decrease the reaction temperature, the lowest being 160 °C (from typically 220 °C in published procedures), with still relatively good yield and BET surface area of the product. The use of other strong inorganic or weak carboxylic acids as modulators typically caused a decrease in yield and porosity.

Porous coordination polymers as novel sorption materials for heat transformation processes.

Porous coordination polymers (PCPs)/metal-organic frameworks (MOFs) are inorganic-organic hybrid materials with a permanent three-dimensional porous metal-ligand network. PCPs or MOFs are inorganic-organic analogs of zeolites in terms of porosity and reversible guest exchange properties. Microporous water-stable PCPs with high water uptake capacity are gaining attention for low temperature heat transformation applications in thermally driven adsorption chillers (TDCs) or adsorption heat pumps (AHPs). TDCs or AHPs are an alternative to traditional air conditioners or heat pumps operating on electricity or fossil fuels. By using solar or waste heat as the operating energy TDCs or AHPs can significantly help to minimize primary energy consumption and greenhouse gas emissions generated by industrial or domestic heating and cooling processes. TDCs and AHPs are based on the evaporation and consecutive adsorption of coolant liquids, preferably water, under specific conditions. The process is driven and controlled by the microporosity and hydrophilicity of the employed sorption material. Here we summarize the current investigations, developments and possibilities of PCPs/MOFs for use in low-temperature heat transformation applications as alternative materials for the traditional inorganic porous substances like silica gel, aluminophosphates or zeolites.

Programming MOFs for water sorption: amino-functionalized MIL-125 and UiO-66 for heat transformation and heat storage applications.

Sorption-based heat transformation and storage appliances are very promising for utilizing solar heat and waste heat in cooling or heating applications. The economic and ecological efficiency of sorption-based heat transformation depends on the availability of suitable hydrophilic and hydrothermally stable sorption materials. We investigated the feasibility of using the metal-organic frameworks UiO-66(Zr), UiO-67(Zr), H2N-UiO-66(Zr) and H2N-MIL-125(Ti) as sorption materials in heat transformations by means of volumetric water adsorption measurements, determination of the heat of adsorption and a 40-cycle ad/desorption stress test. The amino-modified compounds H2N-UiO-66 and H2N-MIL-125 feature high heat of adsorption (89.5 and 56.0 kJ mol(-1), respectively) and a very promising H2O adsorption isotherm due to their enhanced hydrophilicity. For H2N-MIL-125 the very steep rise of the H2O adsorption isotherm in the 0.1 < p/p0 < 0.2 region is especially beneficial for the intended heat pump application.

High performance metal-organic-framework coatings obtained via thermal gradient synthesis.

For many possible applications of metal-organic frameworks, a coating onto a metallic support capable of both superior heat and mass transfer is required. A heated substrate in contact with a chilled solution of metal salt and linker reproducibly yields polycrystalline, highly stable, thermally conductive MOF coatings at a growth rate of 50 µm h(-1), illustrated by the formation of Cu(3)(btc)(2) as an example.

MOFs as adsorbents for low temperature heating and cooling applications.

The 3D metal-organic framework (MOF) (3)(infinity){[Ni(3)(mu(3)-btc)(2)(mu(4)-btre)(2)(mu-H(2)O)(2)]. approximately 22H(2)O} is found to be a reversibly dehydratable-hydratable water-stable MOF material with a large loading spread of 210 g/kg as a candidate for solid adsorbents in heat transformation cycles for refrigeration, heat pumping, and heat storage.