J.W. de Haan

Characterization of Modified Silica Powders by Fourier Transform Infrared Spectroscopy and Cross-Polarization Magic Angle Spinning NMR

Silica gel and Cab-O-Sil were chemically modified (silylated) with 3-aminopropyltriethoxysilane and 3-methacryloxypropyltrimethoxysilane under carefully controlled conditions. Subsequently the products were investigated by elemental analysis, Fourier transform IR spectroscopy, and 13C and 29Si cross-polarization magic angle spinning NMR (CP-MAS NMR). The influence of the reaction conditions of the silylation and the effect of subsequent heat treatment and water addition were studied. The resulting differences shed new light on the combined effects of reaction conditions and silica surface structures on the course of the reactions. Some assignments of 29Si NMR signals to specific structures were confirmed, while in one case a reassignment was proposed.

Silica gel dissolution in aqueous alkali metal hydroxides studied by 29SiNMR

The rate of silica gel dissolution in aqueous alkaline media was investigated using 29Si nuclear magnetic resonance spectroscopy. A profound difference of alkali metal hydroxides on the dissolution rate of amorphous silica gel was observed. The dissolution rate increases in order LiOH ≈ CsaOH) < (RbOH ≈ NaOH) < KOH, as was confirmed by the β = silicomolybdate complexation method. Silica gel dissolution involved formation of monomeric silicic acid, Q0. The monomeric anions oligomerise into dimer species, which in turn form cyclic and lineartrimer species. The structure of highly polymerised silicate species depends on the alkali metal cation, i.e. low pH silicate solutions have structurally different silicate species as a function of alkali metal hydroxide, as is shown by 29Si-NMR spectroscopy. Potassium cations gave rise to more polymerised silica species compared with other alkali metal cations studied. On the contrary, when lithium hydroxide and silica gel are mixed in a molar SiO2/Li2O = 1:1, a microcrystalline phase is formed which consists of lithium silicate crystals. Crystallisation of the lithium silicate proceeds via monomeric silica being in solution.

The structure directing effect of cations in aqueous silicate solutions. A 29Si-NMR study

Abstract Studies of the dissolution of amorphous silica gel in aqueous hydroxides by 29 Si-MAS-NMR, give information on the oligomerisation of dissolved monomeric silicic acid and on the interaction between cations and the silica gel surface. The dissolution rate increases in the order (LIOH ≈ CsOH)

Chromatographic and solid state nuclear magnetic resonance study of the changes in reversed-phase packings for high-performance liquid chromatography at different eluent compositions

A number of long and short chain reversed-phase packings for high-performance liquid chromatography from two different silica substrates, derivatized with mono- or trifunctional silane reagents, were studied under simulated routine conditions. The changes in the properties of the packings are described in terms of loss of silanes, gain in silanol content and rearrangements of the silica-to-silane bondings. Chromatographic techniques, solid-state NMR spectroscopy and elemental analyses were used to characterize and partially to quantify these changes. As expected, long chain phases are more stable than short chain phases with the same silane-to-silica attachment. More surprisingly, the reversed phases derived from monofunctional silanes are much more stable than phases with the same silanes chain length prepared from trifunctional silanes. An explanation of the changes is offered in terms of siloxane hydrolysis and rearrangement plus concomitant polymerization of multifunctional silanes. The impact on the chromatographic behaviour is discussed.

Characterization and stability of silanized and polymer-coated octadecyl reversed phases

Three different reversed phases for high-performance liquid chromatography obtained with monofunctional octadecylsilane or with cross-linked polymethyloctadecylsiloxane with and without trimethylsilane precapping of the substrate were synthesized on the same batch of Nucleosil silica. After extensive characterization, these phases were subjected to artificial ageing under simulated routine conditions. Changes in properties of the bonded-phase packings were quantified with chromatographic techniques, solid-state 29Si NMR and elemental analysis. These changes are correlated with lipophilic and polar selectivity, accessibility of silanol groups at the surface, loss of silanes/siloxanes, separation performance and silica degradation of the stationary phases. Improved surface shielding properties and stability towards eluents of high pH are considerable advantages of cross-linked polymer-coated and octadecyl stationary phases. Precapping of the substrate with trimethylsilanes prior to polymer coating reduces the concentration of silanol groups to less than 50% and yields stationary phases which exhibit further decreased silanol interactions because the residual silanol groups are shielded in a double manner by the trimethylsilyl groups and by the polymer layer on top of it. This also leads to an increase in stability. The non-precapped polymer-coated stationary phase showed an advanced cross-linking of the polymers after ageing experiments with high pH buffer solutions, resulting in improved silanol shielding and large shifts in selectivity. After precapping with trimethylsilane-enolate the polymeric methyloctadecylsiloxane coating proved to be more stable than the non-precapped phase. Also, the precapped phase showed hardly any chemical bonding with the silica surface; only Van der Waals and other dispersive interactions with the anchored trimethylsilanes at the surface exist. The high mobility of the polymer coating at the pore surface of the substrate is retained for the precapped stationary phase, also after simulated ageing experiments. This precapped polymer coated stationary phase showed an almost consistent separation performance after ageing experiments with high pH buffer solutions, but a small shift in selectivity was observed.

FTIR spectroscopic and 'H MAS NMR studies of the influence of lattice chemistry and structure on Bronsted acidity in zeolites

The influence of the Si/Al ratio, of the nature of the T-atom and of the pore size on the acidic strength of Brønsted sites in zeolites has been investigated using changes of the vibrational properties of Brønsted OH(OD) groups and a shift change of Brønsted protons in nuclear magnetic resonance upon adsorption of weak bases. Deuterated acetonitrile and trichloro-acetonitrile have been chosen to probe the acidic strengths of ZSM-5, FeZSM-5, mordenite and zeolite Y, which are often used as catalysts. From the results of the FTIR and 1 H MAS NMR studies it can be concluded that the chemical composition of the lattice dominates the acidic strength of the Brønsted sites in zeolites. Differences in structure or pore size play a much smaller role.

Particle size dependence of the Young's modulus of filled polymers: 2. Annealing and solid-state nuclear magnetic resonance experiments

Experimental results are reported from which it appears that in the case of polymer filled with silane-treated glass beads the Youngs modulus is, in accordance with present theory, independent of the particle size of the filler. However, if pure glass beads are used as filler, the Youngs modulus appears to be significantly dependent on the particle size. In a previous report a hypothetical explanation was given for this behaviour. It was assumed that, owing to the free filler surface offered to the polymer in the case of the untreated glass beads, the morphology of the polymer matrix adjacent to the filler particles is different from that of the bulk of the material. The results of annealing and solid-state nuclear magnetic resonance experiments reported here appear to support this hypothesis.

Study of the changes in mono-, di- and trifunctional octadecyl-modified packings for reversed-phase high-performance liquid chromatography with different eluent compositions

Two different types of silica substrates uniformly modified with reversed-phase high-performance liquid chromatographic (RP-HPLC) phases obtained with mono-, di- and trifunctional octadecylsilanes were subjected to artificial ageing under simulated routine conditions and were subsequently analysed and evaluated. Changes in the properties of the bonded phase packings were characterized and quantified with chromatographic techniques, solid-state NMR and elemental analysis. These changes were correlated with selectivity, loss of silanes, gain in silanol content and rearrangement of the silica to silane bonding. When eluents of extremely high pH were used, the multifunctional octadecylsilane stationary phases showed a higher resistance towards ligand stripping. The substrate shielding of difunctional octadecylsilanes showed superior characteristics due to direct surface attachments. The rigidity of the silica substrate used for modification influences the stability of the RP-HPLC phases, especially for the monofunctionally modified octadecylsilane stationary phase. With the use of multifunctional C18 silanes for modification, the influence of the characteristics of the substrate on the stability of the RP-HPLC phases was reduced compared with monofunctional C18. In contrast to monofunctionally modified phases, the polar selectivity of the multifunctionally modified phases remained more or less constant after intensive use with aggressive eluents. A chromatographic characterization method for obtaining information about the stability, capacity, selectivity and silica degradation of different RP-HPLC stationary phases is also presented.

Chromatographic activity of residual silanols of alkylsilane derivatized silica surfaces

Residual silanols on n-octadecylsilane derivatized silica surfaces have a pronounced effect on the chromatographic performance of reversed-phase high-performance liquid chromatography (RP-HPLC) materials. The present study describes how the results of solid-state NMR investigations on two commercially available reversed-phases for HPLC are verified chromatographically. One phase is a dimethyl-n-octadecylsilane derivatized silica substrate (Rx-C18), while the other is the same substrate derivatized with di-isobutyl-n-octadecylsilane (“stable bond”, SB-C18). Four column tests, taken from the literature, are performed in order to assess silanol activity and column hyprophobicity. It is concluded that on the SB-C18 phase, more residual silanols are analyte accessible and that the isobutyl groups contribute significantly to hydrophobicity. Generally, the results of the solid-state NMR method for determining shielded and accessible residual surface silanols on RP-HPLC stationary phases could be confirmed.

A 69Ga and 71Ga MAS NMR study of the gallium analogue zeolite ZSM-5

69Ga and 71Ga magic-angle-spinning (MAS) NMR spectra of a series of gallium analogue zeolites H—(Ga)ZSM-5, with varying Si/Ga ratios, were obtained at 14.1 and 7.1 T. The quadrupole coupling constant e2qQ/h was deduced from the shift difference of the centre of gravity of the 69Ga and 71Ga resonance. An analysis of the linewidth as a function of the magnetic field shows that the width is not only determined by the second-order quadrupolar broadening, but also by a distribution of chemical shifts. The values obtained for the quadrupole coupling constant and chemical shift distribution are compared to those found for 27Al in H—(Al)ZSM-5. It is argued that the observed differences are related to the different NMR properties of the isotopes, and do not point to any structural changes of the ZSM-5 lattice.