Annual Catalogue of the Minnesota State Normal School at Moorhead. Seventh Year. (1894-1895)

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Adsorption and Dynamics in Hierarchical Metal–Organic Frameworks

Adsorption and dynamics in hierarchical metal–organic frameworks are investigated by means of molecular simulation. The models of hierarchical porous solids are obtained by carving mesopores of different diameters out of a crystal of Cu-BTC (model A) or by inserting a microporous particle of Cu-BTC in an amorphous silica mesopore (model B). We show that the nitrogen adsorption isotherms at 77 K for the solids corresponding to model A can be described as a linear combination of reference adsorption isotherms for pure microporous and mesoporous solids. In contrast, the adsorption isotherms for model B cannot be described accurately as a sum of reference microporous and mesoporous adsorption isotherms. The inserted particle acts as a constriction which helps nucleate the liquid phase within the mesopore so that no capillary condensation hysteresis is observed. The dynamics of nitrogen adsorbed at 77 K inside the porosity of the hierarchical solids is also investigated. The Fickian regime is reached at long times which are not attainable with molecular dynamics simulations. At higher temperature, the faster self-diffusion makes it possible to obtain the diffusivity of the adsorbate. Nitrogen adsorbed in the microporosity of the hierarchical porous solids has a self-diffusion coefficient close to that of nitrogen adsorbed in pure Cu-BTC. In contrast, diffusion in the mesoporosity is faster than in the microporosity so that the overall diffusivity is faster than in pure Cu-BTC

Mechanism of H₂O Insertion and Chemical Bond Formation in AlPO₄‑54·<i>x</i>H₂O at High Pressure

The insertion of H₂O in AlPO₄-54·<i>x</i>H₂O at high pressure was investigated by single-crystal X-ray diffraction and Monte Carlo molecular simulation. H₂O molecules are concentrated, in particular, near the pore walls. Upon insertion, the additional water is highly disordered. Insertion of H₂O (superhydration) is found to impede pore collapse in the material, thereby strongly modifying its mechanical behavior. However, instead of stabilizing the structure with respect to amorphization, the results provide evidence for the early stages of chemical bond formation between H₂O molecules and tetrahedrally coordinated aluminum, which is at the origin of the amorphization/reaction process

Saturation of the Siliceous Zeolite TON with Neon at High Pressure

The insertion of neon and argon in the 1-D pore system of the zeolite TON was studied at high pressure by X-ray diffraction and by Monte Carlo (MC) molecular modeling. Rietveld refinements of the crystal structure of TON and the MC results indicate that 12 Ne atoms enter the unit cell of TON, completely filling the pores. This is much greater than the degree of filling observed for argon, which due to size considerations lies in a vertical plane in the pores. A phase transition from the <i>Cmc</i>2₁ to a <i>Pbn</i>2₁ structure occurs at 0.6 GPa with cell doubling. The compressibility and structural distortions, such as pore ellipticity, are considerably reduced as compared to the argon-filled or the empty-pore material. In addition, the crystalline form persists to pressures of the order of 20 GPa, and the <i>Pbn</i>2₁ phase is recovered after decompression. The results show the very strong and different effects of pore filling by noble gases on the structural stability and mechanical properties of this prototypical 1-D zeolite-type material