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

https://red.mnstate.edu/bulletins/1064/thumbnail.jp

Adsorption of Acetone Vapor by Cu-BTC: An Experimental and Computational Study

We report an experimental and theoretical study of acetone adsorption in the metal–organic framework (MOF) compound Cu-BTC. The isosteric heat of adsorption could be derived experimentally and was found to be −60 kJ mol^–1. This value matches the theoretical data obtained by DFT-based methods at zero coverage. In situ DRIFT measurements allowed us to precisely describe the adsorption steps from zero coverage to saturation. Two main adsorption sites were determined for the adsorption of acetone. The small cavities were found to interact through van der Waals interaction with acetone, while the Cu­(II) site was found to interact with the carbonyl function of acetone. On the basis of the in situ infrared experiments, it was demonstrated that the small cavities were first in interaction with acetone. DFT proved consistent with these findings by giving the energy of interaction in the different sites explored but also by providing calculated infrared spectra of adsorbed acetone in Cu-BTC. Using acetone as a probe allowed showing that dispersive interactions with the pore sites of the Cu-BTC can be dominant among all other interactions. Additionally, the adsorption of acetone in Cu-BTC proved not fully reversible unless exposed to atmospheric moisture

Hosting Ability of Mesoporous Micelle-Templated Silicas toward Organic Molecules of Different Polarity

Spin probe water solutions were adsorbed onto differently treated micelle-templated silicas (MTS) of different pore sizes to analyze the hosting ability of the MTS surface toward different organic molecules. The MTS synthesis was performed at 388 K by self-assembly of inorganic silica and micelles of cetyltrimethylammonium bromide (CTAB) to which different amounts of 1,3,5 trimethylbenzene (TMB) were added at different TMB/CTAB ratios to modify the pore size:  40, 65, and 80 Å pore diameter were obtained for TMB/CTAB ratio = 0, 2.7, and 13, respectively. As-synthesized MTS, calcined MTS, and octyldimethyl(C8) grafted MTS were used. These MTS were characterized by means of nitrogen sorption isotherms and TEM as homoporous silica with regular and reproducible structure. Different spin probes (nitroxides) were taken as models for different types of organic molecules, namely, neutral and charged molecules and surfactants. The computer aided analysis of the electron paramagnetic resonance (EPR) spectra of these probes provided information on the hosting ability of the differently treated solid surface in respect of the different structure and hydrophilicity of the probes. The spectral analysis allowed the depiction of the probable distribution and location of the different probes at the differently treated silica surfaces. For the as-synthesized MTS, void space became available for the probe adsorption in vicinity of the surface when TMB was used in the synthesis and then evaporated. For the calcined MTSs, the hydrophobic sites at the solid surface, namely, siloxanes, increased by increasing the TMB content in the synthesis mixture. The binding of the EPR probe with the surface of these MTS is favored when both hydrophilic and hydrophobic interactions occur, as found with surfactant probes bearing both a hydrophilic and a hydrophobic moiety. For the C8-grafted MTSs, the results provided a proof of the quality of grafting:  the surface is largely hydrophobic and favors self-aggregation of the surfactant probes, led by chain−chain interactions

Experiment and Theory of Low-Pressure Nitrogen Adsorption in Organic Layers Supported or Grafted on Inorganic Adsorbents: Toward a Tool To Characterize Surfaces of Hybrid Organic/Inorganic Systems

We report experimental nitrogen adsorption isotherms of organics-coated silicas, which exhibit a low-pressure desorption branch that does not meet the adsorption branch upon emptying of the pores. To address the physical origin of such a hysteresis loop, we propose an equilibrium thermodynamic model that enables one to explain this phenomenon. The present model assumes that, upon adsorption, a small amount of nitrogen molecules penetrate within the organic layer and reach adsorption sites that are located on the inorganic surface, between the grafted or adsorbed organic molecules. The number of accessible adsorption sites thus varies with the increasing gas pressure, and then we assume that it stays constant upon desorption. Comparison with experimental data shows that our model captures the features of nitrogen adsorption on such hybrid organic/inorganic materials. In particular, in addition to predicting the shape of the adsorption isotherm, the model is able to estimate, with a reasonable number of adjustable parameters, the height of the low-pressure hysteresis loop and to assess in a qualitative fashion the local density of the organic chains at the surface of the material

Rietveld Structure Refinement of Zeolite ECR-1

In this work, we present the structure refinement of ECR-1 to give the first direct evidence of the proposed structure of this synthetic zeolite. In fact, a model of the structure of ECR-1 was proposed on the basis of high-resolution transmission electron microscopy (HRTEM) evidence and the structure solution of the synthetic gallo-silicate TNU-7, but it has not been refined to date. The proposed model consists of structure layers of mordenite (MOR) and mazzite (MAZ) connected in a regular 1:1 stacking sequence and framework topology EON. Because single crystals of ECR-1 cannot be synthesized, the structure was refined using the Rietveld method. High-resolution synchrotron powder diffraction data were collected on both the synthetic Na-ECR-1 and NH4-ECR-1 samples at ESRF. Na atoms located on the axis of the eight-member ring channels in mordenite and zeolite omega are not present in Na-ECR-1. In Na-ECR-1, the equivalent sites lay near the walls of the eight-membered-ring channels. This difference is presumably at the basis of the formation of ECR-1 because, during growth, the local symmetry deformation of the eight-membered-ring channel prevents the formation of the MOR or MAZ structures and justify the periodical shift from one structure to the other. A quantitative explanation of the anisotropic peak broadening observed in the powder patterns is also given

Sponge Mesoporous Silica Formation Using Disordered Phospholipid Bilayers as Template

Lecithin/dodecylamine/lactose mixtures in ethanol/aqueous media led to the formation of sponge mesoporous silica (SMS) materials by means of tetraethoxysilane (TEOS) as silica source. SMS materials show a “sponge-mesoporous” porosity with a pore diameter of about 5−6 nm, in accordance to the length of a lecithin bilayer. SMS synthesis was developed to create a new class of powerful biocatalysts able to efficiently encapsulate enzymes by adding a porosity control to the classical sol−gel synthesis and by using phospholipids and lactose as protecting agents for the enzymes. In the present study, the formation of SMS was investigated by using electron paramagnetic resonance (EPR) probes inserted inside phospholipid bilayers. The influence of progressive addition of each component (ethanol, dodecylamine, lactose, TEOS) on phospholipid bilayers was first examined; then, the time evolution of EPR spectra during SMS synthesis was studied. Parameters informative of mobility, structure, order, and polarity around the probes were extracted by computer analysis of the EPR line shape. The results were discussed on the basis of solids characterization by X-ray diffraction, nitrogen isotherm, transmission electron microscopy, and scanning electron microscopy. The results, together with the well-known ability of ethanol to promote membrane hemifusion, suggested that the templating structure is a bicontinuous phospholipid bilayer phase, shaped as a gyroid, resulting of multiple membrane hemifusions induced by the high alcohol content used in SMS synthesis. SMS synthesis was compared to hexagonal mesoporous silica (HMS) synthesis accomplished by adding TEOS to a dodecylamine/EtOH/water mixture. EPR evidenced the difference between HMS and SMS synthesis; the latter uses an already organized but slowly growing mesophase of phospholipids, never observed before, whereas the former shows a progressive elongation of micelles into wormlike structures. SMS-type materials represent a new class of biocompatible materials and open a bright perspective for biomolecule processing for pharmaceutical, biocatalysis, biosensors, or biofuel cell applications

Positronium Production in Engineered Porous Silica

Positronium (Ps) has been the subject of several experimental and theoretical investigations due to its many scientific applications. In this work high positronium yield was found in engineered porous silica. The studied materials were pellets of swollen MCM-41 and of commercial Davicat 1700, obtained by different compression pressures, with mesopores characterized by different structural and chemical features. The measurements were performed with a variable energy positron beam at room temperature. An estimation of the Ps mean diffusion length was obtained by measuring capped samples. A selected swollen MCM-41 sample (0.39 g/cm³) was measured also at cryogenic temperature (8 K). In this material both the Ps yield and the Ps diffusion length are found to be independent of temperature. The pore surface of the swollen MCM-41 samples is very interesting in comparison to commercial silica as it possesses hydrophobic patches to avoid ice formation at low temperature. Positron lifetime measurements show a high Ps survival time inside the mesoporous materials (∼110 ns), which promotes a high Ps mobility during cooling inside the pores favoring diffusion lengths up to 1 μm for swollen MCM-41 materials. Besides, it was possible to estimate the total Ps yield coming up outside the sample at high implantation energies and the time between the implantation of positrons and the Ps release

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 Cmc21 to a Pbn21 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 Pbn21 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

Insertion and Confinement of H₂O in Hydrophobic Siliceous Zeolites at High Pressure

The insertion of H2O in the siliceous zeolites TON (Theta-one) and MFI (Mobil Five) was studied at pressures up to 0.9 GPa by synchrotron X-ray diffraction, infrared spectroscopy, and Monte Carlo modeling. TON (orthorhombic, Cmc21) and MFI (monoclinic, P21/n) have 1D and 3D pore systems, respectively. H2O insertion was quantified by a combination of structure refinements and Monte Carlo modeling. Complete pore filling is observed at 0.9 GPa in the high-pressure forms of TON (orthorhombic, Pbn21) and MFI (orthorhombic, Pnma). This corresponds to more than twice as many H2O molecules per SiO2 unit in the 3D pore system of MFI than in the 1D pore system of TON. This results in a greater swelling of the MFI system as compared to the TON system upon insertion. In both cases, both experiments and modeling indicate that the density of water in the pores is close to that of bulk water at the same pressure. A greater degree of molecular disorder is observed in the 3D H2O network of MFI. Infrared spectroscopy indicates a weakening of the hydrogen bonds associated with geometrical constraints because of confinement. The majority of the H2O molecules are extruded on pressure release, indicating that this insertion is reversible to a great extent, which gives rise to the molecular spring properties of these materials

High-Pressure Phase Transition, Pore Collapse, and Amorphization in the Siliceous 1D Zeolite, TON

The siliceous zeolite TON with a 1-D pore system was studied at high pressure by X-ray diffraction, infrared spectroscopy, and DFT calculations. The behavior of this material was investigated using nonpenetrating pressure-transmitting media. Under these conditions, a phase transition from the Cmc21 to a Pbn21 structure occurs at close to 0.6 GPa with doubling of the primitive unit cell based on Rietveld refinements. The pores begin to collapse with a strong increase in their ellipticity. Upon decreasing the pressure below this value the initial structure was not recovered. DFT calculations indicate that the initial empty pore Cmc21 phase is dynamically unstable. Irreversible, progressive pressure-induced amorphization occurs upon further increases in pressure up to 21 GPa. These changes are confirmed in the mid- and far-infrared spectra by peak splitting at the Cmc21 to Pbn21 phase transition and strong peak broadening at high pressure due to amorphization