Michael T. Janicke

Cooperative Formation of Inorganic-Organic Interfaces in the Synthesis of Silicate Mesostructures

A model is presented to explain the formation and morphologies of surfactant-silicate mesostructures. Three processes are identified: multidentate binding of silicate oligomers to the cationic surfactant, preferential silicate polymerization in the interface region, and charge density matching between the surfactant and the silicate. The model explains present experimental data, including the transformation between lamellar and hexagonal mesophases, and provides a guide for predicting conditions that favor the formation of lamellar, hexagonal, or cubic mesostructures. Model Q230 proposed by Mariani and his co-workers satisfactorily fits the x-ray data collected on the cubic mesostructure material. This model suggests that the silicate polymer forms a unique infinite silicate sheet sitting on the gyroid minimal surface and separating the surfactant molecules into two disconnected volumes.

Molecular and Atomic Arrays in Nano- and Mesoporous Materials Synthesis

Abstract A model is presented to explain the formation and morphologies of 3-d periodic surfactant-silicate mesostructures. The structures of lamellar, hexagonal tubular and a minimal surface cubic liquid crystal/silicate phase are described.

Short-and Intermediate-Range Structural Ordering in Glassy Boron Oxide

Ordering at short-length scales is a universal feature of the glassy state. Experiments on boron oxide and other materials indicate that ordering on mesoscopic-length scales may also be universal. The high-resolution nuclear magnetic resonance (NMR) measurements of oxygen in boron oxide glass presented here provide evidence for structural units responsible for ordering on short- and intermediate-length scales. At the molecular level, planar BO3/2 units accounted for the local ordering. Oxygen-17 NMR spectra resolved detailed features of the inclusion of these units in boroxol rings, oxygen bridging two rings, and oxygen shared between two nonring BO3/2 units. On the basis of these and corroborative boron-11 NMR and scattering results, boron oxide glass consists of domains that are rich or poor in boroxol rings; these domains are proposed to be the structural basis of intermediate-range order in glassy boron oxide.

Aluminum Incorporation in Mesoporous Molecular Sieves

Aluminosilicate mesoporous materials synthesized with various amounts of aluminum have been characterized by solid-state 27 Al NMR, powder X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and elemental analysis to determine the extent of aluminum incorporation and its location in the final product. The presence of aluminum in the framework is observed to influence the stability of the structure to calcination. These studies, in combination, establish that four-coordinate aluminum can be incorporated into the inorganic mesoporous framework, provided a monomeric alumina precursor is used in the synthesis. When the aluminum source is colloidal, residual octahedrally coordinated extraframework aluminum occurs in phase-separated amorphous particles. These results support the synthesis mechanism for mesoporous molecular sieves proposed by Monnier et al. and provide new insight towards isomorphous substitution into mesoporous silicate structures.

NMR Studies on Hydrolysis and Condensation Reactions of Alkoxysilanes Containing Si—H Bonds

Hydrolysis and condensation reactions of four organically modified alkoxides, used for the preparation of silicon oxycarbide gel precursors, have been followed by 29Si NMR. Triethoxysilane (HSi(OEt)3) and methyldiethoxysilane (MeHSi(OEt)2) react extremely fast compared to methyltriethoxysilane (MeSi(OEt)3) and tetraethoxysilane (Si(OEt)4). Co-hydrolysis reactions between different pairs of precursors—MeSi(OEt)3/Si(OEt)4; MeSi(OEt)3/HSi(OEt)3; MeHSi(OEt)2/Si(OEt)4; and MeHSi(OEt)2/HSi(OEt)3—were investigated by solution state 29Si and 17O NMR. Despite significantly different reactivities between precursors, evidence for co-condensation reactions has been found for each system. Finally, two-dimensional 29Si-1H heteronuclear correlation MAS-NMR spectroscopy was used to probe the local environments of the various Si sites in the product hybrid networks.

Newly Applied Two-Dimensional Solid-State NMR Correlation Techniques for the Characterization of Organically Modified Silicates

This paper highlights the use of two-dimensional (2D) solid-state NMR correlation techniques to probe the chemical homogeneity of organically modified silicate networks. Specifically, 29Si{1H} heteronuclear correlation (HETCOR) NMR experiments have revealed the spatial proximity of the two types of Si units present in a gel obtained from co-hydrolysis of methyldiethoxysilane and triethoxysilane. Similar information has also been obtained by using 2D 1H homonuclear correlation NMR spectroscopy. Such experiments were only possible by combining the use of high magnetic field (14.10 T) with fast MAS spinning rate (30 kHz).

An In-situ X-ray and NMR Study of the Formation of Layered Mesophase Materials

Abstract The formation of surfactant-silicate lamellar mesophases has been studied by quenching the reaction at various intervals using freeze-drying techniques and applying X-ray diffraction and 29 Si NMR spectroscopy to characterize the products. Results indicate that precipitation of a silica/surfactant material occurs rapidly within the earliest stages of the reaction. Longer range order is observed after several hours, in conjuction with slower polymerization of the silicate units at the surfactant-silicate interface.

Two-Dimensional 29Xe Exchange NMR Measurements of Xenon Dynamics in Na-A Zeolite

Two-dimensional (2D) exchange NMR is a powerful tool for measuring the dynamics and energetics of adsorbed xenon atoms undergoing slow exchange between the alpha-cages of Na-A zeolite. In this proceedings, we present recently published results, as well as additional discussion and data obtained, using 2D Xe-129 NMR to determine rate coefficients for intercage xenon hopping and to correlate them with adsorption and activation energies. Variable-temperature experiments establish the activation energy for hopping between alpha-cages to be 60+/-10 kJ/mol. The dependence of these kinetic and thermodynamic quantities on xenon occupancy of the alpha-cages reflects the importance of intracage interactions on the behavior of the adsorbed guest molecules.

Aluminum-27 double-rotation NMR investigations of SAPO-5 with variable silicon content

Summary Double-rotation NMR (DOR) studies of 27 Al species in SAPO-5, a silicon aluminophosphate molecular sieve with a one dimensional channel system, have revealed a minimum of three aluminum sites resulting from the synthesis. The DOR technique was used specifically to increase the spectral resolution by removing the broadening influences from second-order quadrupolar interactions associated with the spin 5/2 27 Al nuclei. The DOR investigations of SAPO-5 crystals with variable Si/Al ratios resulted in the identification of three aluminum species, two consistent with the reported isotropic shift values for AlPO 4 -5; however, these two resonances are only observable when small quantities of silicon are added to the synthesis. Increased substitution of silicon into the AFI framework caused the two peaks in the NMR spectra to coalesce into one resolvable resonance. The third aluminum species, observable in syntheses with only small amounts of silicon, corresponds to a condensed aluminophosphate phase similar to cristobalite.