Microstructure of silica monolayer solid acid catalysts determined by 29Si NMR spectroscopy

The nuclear magnetic resonance (NMR) of ²⁹Si showed that the silica monolayer, which was prepared by chemical vapor deposition (CVD) of Si(OCH₃)₄ at 593 K on alumina, mainly consisted of Si(OAl)₁(OSi)₃ and Si(OAl)₁(OSi)₂(OH)₁ species, in agreement with the mechanism of formation of monolayer. The Brønsted acid site is suggested to be the latter species. On the other hand, such an isolated species as Si(OAl)₃(OH)₁ was formed from Si(OCH₃)(CH₃)₃. Lack of acidity on this species indicates that the acidity requires the siloxane network.     

Structural study of arylsilanes using 29Si NMR

²⁹Si NMR data (σ and ¹H, ²⁹Si coupling constants) of a series of arylsilanes were studied in relation to their molecular structure. It is shown that silicon chemical shift variation is too weak to allow empirical attribution. In contrast, ²⁹Si, ¹H coupling constants across the aromatic ring, especially the ³J, allow structural assignment. Generally, SPT experiments are sufficient to obtain these values. However, full attribution of polysilylated derivatives sometimes needs other experiments to identify all the silicons. A 2D J-resolved INEPT experiment is described and has proven to be an efficient method for ²⁹Si NMR signal identification. The usefulness of this approach is demonstrated by the complete analysis of all the ²⁹Si lines in a trisilyl derivative. An easy-to-use INEPT decoupled-type sequence is also proposed for a rapid characterization of polysilylated derivatives or mixture of isomers.     

29Si NMR study of configurations of cyclo-linear polymethylsiloxanes

A technique based on the²⁹Si NMR spectra for analyzing the spatial structure of cyclolinear polymethylsiloxanes has been developed. The role of certain factors influencing the formation of the spatial structure of the chains during the preparation of these polymers has been studied.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1460–1466, August, 1993.     

A study of the adsorption activities of silanol surface structures on a fused silica model substrate by combining 29Si CP MAS NMR and inverse gas chromatographic data

The possibilities of inverse gas-solid chromatography (IGC) in obtaining chromatographic data on fumed silica were examined. Aerosil A-200, a fused silica model substrate in ²⁹Si nuclear magnetic resonance analysis, was trimethylsilylated to different degrees. IGC was used to very reproducibly determine the free specific energies of adsorption of several functionalized probe solutes. Hydrogen bonding solutes have a free specific energy of adsorption that is at least about 50% higher than that of non-hydrogen bonding probe solutes. NMR was used in combination with elemental analysis to calculate surface concentrations of the different chemical surface structures. IGC data and surface concentrations were combined in order to determine the contribution of each type of surface structure to the total free specific adsorption energy. It could be concluded that residual silanols from the reaction of dihydroxydi-siloxysiloxane (Q₂ groups) with trimethylchlorosilane possess a higher adsorption activity than the silanols initially present.     

Dehydration of the surface of pyrogenic silica by 1H nuclear magnetic resonance

The proton magnetic resonance method has been used at a frequency of 500 MHz in the temperature ranee 290–870 deg K to investigate dehydration of Aerosil surface. Conclusions have been drawn in relation to the nonhydroxylic nature of the primary water adsorption centers.Translated from Teoreticheskayi i Eksperimental'naya Khimiya, Vol. 24, No. 2, pp. 235–238, March–April 1988.     

29Si NMR spectroscopy of cyclosiloxanes

Journal of Structural Chemistry -     

New bonding modes of gas-phase deposited gamma-aminopropyltriethoxysilane on silica studied by 29Si CP/MAS NMR

Besides the well-known reaction between the ethoxy groups of the silane end of the gamma-aminopropyltriethoxysilane (APTS) molecule and the silanols of silica, the amino ends of APTS molecules were observed to react in the gas phase with ethoxy groups of other APTS molecules and silanols of silica at elevated temperatures on the silica surface, dehydroxylated at 600 degrees C, forming Si-N linkages, as established by 29Si CP/MAS NMR.     

A29Si NMR study of natural and synthetic melanophlogites,the silica analogues of the clathrate hydrates of type I

²⁹Si MAS NMR spectra of natural melanophlogite as well as of synthetic melanophlogite indicate that the room temperature lattice structure has lower symmetry than expected from its proposed crystal structure. On heating the samples to temperatures above room temperature, the structure undergoes a reversible phase transition with a transition temperature characteristic of the locality. For the synthetic melanophlogite, the transition is over the range 298 K to 338 K. From low temperature NMR spectra it is concluded that the low temperature forms of synthetic melanophlogite and of melanophlogite from Sicily differ in their crystal lattice symmetries due to their characteristic guest species.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.On leave Mineralogisches Institut, CAU KIEL, FRG     

29Si NMR Spektroskopie von Cyclopentasilanen

The structure and ²⁹Si chemical shifts of the halodimethylsilylnonamethylcyclopentasilanes Si₅Me₉SiMe₂X (1–4) and the halononamethylcyclopentasilanes Si₅Me₉X (5–8) (X = F, Cl, Br, I) have been assigned using ¹J(SiSi) and ²J(SiSi) coupling constants derived from ²⁹Si-INADEQUATE and ²⁹Si-INEPT—INADEQUATE NMR spectra. The compounds exhibit good correlation between chemical shift, ¹J(SiSi) and Pauling electronegativities.     

29Si NMR study of structural ordering in aluminosilicate geopolymer gels

A systematic series of aluminosilicate geopolymer gels was synthesized and then analyzed using 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) in combination with Gaussian peak deconvolution to characterize the short-range ordering in terms of T-O-T bonds (where T is Al or Si). The effect of nominal Na2O/(Na2O + K2O) and Si/Al ratios on short-range network ordering was quantified by deconvolution of the 29Si MAS NMR spectra into individual Gaussian peaks representing different Q4(mAl) silicon centers. The deconvolution procedure developed in this work is applicable to other aluminosilicate gel systems. The short-range ordering observed here indicates that Loewenstein's Rule of perfect aluminum avoidance may not apply strictly to geopolymeric gels, although further analyses are required to quantify the degree of aluminum avoidance. Potassium geopolymers appeared to exhibit a more random Si/Al distribution compared to that of mixed-alkali and sodium systems. This work provides a quantitative account of the silicon and aluminum ordering in geopolymers, which is essential for extending our understanding of the mechanical strength, chemical and thermal stability, and fundamental structure of these systems.     

29Si NMR spectra of methylphenylcyclotri-and -tetrasilazanes

²⁹Si NMR spectra of various methylphenyl-substituted cyclotri- and -tetrasilazanes, composed of different combinations of the units d = Me₂SiNH, d^ph = MePhSiNH and d = Ph₂SiNH, have been investigated. For most of the compounds having more than one asymmetric centre, the spectra of both the individual isomers and their mixtures have been studied, and the signals of the individual isomers have been assigned. The effect of a different arrangement of phenyl groups in the rings, and the influence of the stereochemistry of the isomers on the ²⁹Si chemical shifts in cyclotri- and -tetrasilazanes have been studied. The ²⁹Si chemical shifts in cyclotrisilazanes are additive and can be represented as the sum of increments corresponding to the number and positions of the phenyl groups in the rings. In these compounds, the spatial structure of the isomers does not exert any noticeable effect on the spectra. On the contrary, chemical shifts are not simply additive in cyclotetrasilazanes: a pronounced stereochemical dependence is apparent.     

Revisiting structure of silica gels from water glass: an 1H and 29Si MAS and CP-MAS NMR study

The silica gels, derived from water glass solution with pH adjusted at 3.0 and 9.9, were revisited to investigate their constitution, although water glass has been studied for last tens of decades on gelation. Solid-state nuclear magnetic resonance spectroscopy was applied to the nuclei ¹H and ²⁹Si, by the use of magic angle spinning (MAS), ¹H → ²⁹Si CP-MAS (CP: cross-polarization), and modern techniques such as 2D HETCOR (two dimensional heteronuclear correlation), and variable-contact time CP techniques. Gelation time (t_gel) showed U-letter shape dependence on pH. All gels consisted of Qⁿ groups (n: 2, 3, and 4), where Qⁿ stands for a silicate unite [(O_1/2)_nSi (–O^?)_4?n] (n: 0–4). The analysis of the ¹H → ²⁹Si CP kinetics and ¹H-²⁹Si HETCOR spectra elucidated the presence of four kinds of ¹H nuclei, i.e., those giving a peak at 6.9 ppm in chemical shift δ: ¹H–OSi hydrogen bonded to H₂O molecules; one at 4.3 ppm: ¹H of adsorbed water molecules, hydrogen-bonded to the silanol groups; one at 1.7 ppm: ¹H–OSi confined in the gel lattice, including that forming aggregations like Si–O^H/_NaO–Si; and one at 4.2 ppm: ¹H of water molecules on the outermost hydration layer.     

29Si NMR investigations on oligosilane dendrimers

Starting with the high functionalized trisilane SiClMe(SiCl₂Me)₂ and tetrasilane SiMe(SiCl₂Me)₃ several octa- and decasilane dendrimers containing directly neighboured branchings were prepared. In these compounds the ²⁹Si NMR chemical shifts of the different silyl groups are shifted towards lower field compared with those of analogous groups in tetra- or hexasilanes. This observation is a helpful tool for the characterization of further dendritic oligomers by ²⁹Si NMR. Received: 3 June 1996 / Revised: 3 July 1996 / Accepted: 9 July 1996     

29Si NMR investigations on oligosilane dendrimers

Starting with the high functionalized trisilane SiClMe(SiCl₂Me)₂ and tetrasilane SiMe(SiCl₂Me)₃ several octa- and decasilane dendrimers containing directly neighboured branchings were prepared. In these compounds the ²⁹Si NMR chemical shifts of the different silyl groups are shifted towards lower field compared with those of analogous groups in tetra- or hexasilanes. This observation is a helpful tool for the characterization of further dendritic oligomers by ²⁹Si NMR. Received: 3 June 1996 / Revised: 3 July 1996 / Accepted: 9 July 1996     

Thermogravimetric study of silica with a chemically modified surface

Thermal methods were used to determine the thermal stability of organic groups introduced into the surface of silica precipitated following its reactions with dimethyl-dichlorosilane (DDS), methyltrichlorosilane (MTS) and methyltriethoxysilane (MTES).It was found that modification with MTS or DDS leads to a permanent hydrophobic film on the silica surface.

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Zusammenfassung Thermische Methoden wurden benutzt, um die thermische Stabilität von organischen Gruppen zu bestimmen, die auf die Oberfläche von ausgefällter Kieselsäure durch die Reaktion mit Dimethyldichlorsilan (DDS), Methyltrichlorsilan (MTS) und Methyltriäthoxysilan (MTES) eingebracht wurden. Die Modifizierung mit MTS oder DDS verursacht auf der Oberfläche der Kieselsäure eine permanente hydrophobe Schicht.

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Influence of pyrogenic silica hydration on water diffusion in surface layers

The surface hydration of pyrogenic silica (aerosil) has been studied by slow neutron scattering. It was shown that the rate of diffusion changes with the thickness of the layer monotonically. The mean square displacement of water molecules from the equilibrium position in aerosil hydration shell is smaller than in bulk water, but twice larger than in ice.     

Kinetics of interaction of 3-aminopropyltriethoxysilane with silica gel using elemental analysis and 29Si NMR spectra

 Three silica gel sample systems, modified with 3-amino-propyltriethoxy silane (APTS), were prepared by sequentially sampling the reaction mixture at various time intervals. The concentrations of 3-aminopropylsilyl groups (APS) bound on the silica surface were determined by elemental analysis. For the same sample systems, ²⁹Si NMR intensities of an (–O)₄Si species belonging only to the silica gel particles and corrected by a cross-polarization correction factor were also measured. Both the APS-concentrations and the correc-ted ²⁹Si NMR intensities depended upon reaction time, reflecting the rate of the APTS–silica gel reaction. Kinetic analysis of these data was made by use of the Gauss–Newton method, and the overall reaction was found to consist of three reaction processes (an initial fast reaction, a slower second reaction and a much slower third reaction). In particular, the conversion of (–O)₃SiOH to (–O)₄Si is predominant in the second reaction process and the pore size of a silica gel particle affects the reaction mechanism. Received: 1 November 1996 Accepted: 24 January 1997