Effect of non-ionic surface active agents on TEOS-derived sols,gels and materials

The sol-gel process, starting from tetraethylorthosilicate precursor, is a suitable technique for the preparation of silica thin films. The use of specific organic additives, like non ionic surface-active agents, drastically modifies the gelation process and allows the preparation of microporous materials with a high microporous volume. The effects of additives on the sol, gel and material characteristics have been investigated by several methods such as ²⁹Si NMR, QELS, SAXS (for sols and gels), and N₂ adsorption, FESEM (for fired materials). It appears that the interactions of surface active agents with TEOS derived species limit condensation reactions and particle growing. A brittle gel structure is generated which leads to highly porous microporous silica after the elimination of organic chains by thermal treatment at 450°C. The material porous texture (specific surface area, pore size distribution and porous volume) can be varied especially by varying the surface active agent chain length and quantity. This kind of sol-gel system is suitable to prepare microporous silica membranes candidate for gas separation or catalytic reactor applications.     

Synthesis of PDMS-Based Porous Materials for Biomedical Applications

Polydimethylsiloxane (PDMS) and tetraethoxysilane (TEOS)-based porous organically modified silicates (ORMOSILs) for biomedical applications were synthesized through a sol-gel process, using sucrose particles as templates. These materials were characterized by ²⁹Si CP-MAS NMR spectroscopy, thin film X-ray diffraction, and scanning electron microscopy. Their bioactivity was evaluated using a simulated body fluid (SBF) of Kokubo recipe. These materials had a bimodal porous structure with pores of 300–500 m and 10–50 m in diameter. NMR showed that the silanol groups of the PDMS chain cross-linked to silica derived from the hydrolysis and condensation of TEOS. The samples containing Ca(II) exhibited apatite deposition on the pore walls within 3 days in SBF.     

Identification and quantification of cis and trans isomers in aminophenyl double‐decker silsesquioxanes using 1H–29Si gHMBC NMR

Cis and trans isomers of a series of double‐decker silsesquioxanes (DDSQ) were characterized by two‐dimensional NMR techniques. The ¹H NMR spectra of these species have not previously been assigned to a degree that allows for quantification. Thus, ¹H–²⁹Si HMBC correlations were applied to facilitate ¹H spectral assignment and also to confirm previous ²⁹Si assignments for this class of silsesquioxanes. With the ability to identify all the pertinent resonances of the ¹H NMR spectrum, ²⁹Si NMR is no longer required for quantification and required only for characterization. This not only saves time and material but also provides a more accurate quantification, thus allowing for the ratio of cis and trans isomers present in each compound to be determined. A more accurate measure of the cis/trans ratio enables the investigation of its influence on the physical and chemical properties of DDSQ nanostructured materials. Copyright © 2013 John Wiley & Sons, Ltd.     

Hybrid materials of SBA-16 functionalized by rare earth (Eu, Tb) complexes of modified β-diketone (TTA and DBM): Covalently bonding assembly and photophysical properties

Novel mesoporous SBA-16 type of hybrids TTA-S16 and DBM-S16 were synthesized by co-condensation of modified β-diketone (TTA-Si and DBM-Si, DBM=1,3-diphenyl-1,3- propanepione, TTA=2-thenoyltrifluoroacetone) and tetraethoxysilane (TEOS) in the presence of Pluronic F127 as template, which were confirmed by FTIR, XRD, ²⁹Si CP-MAS NMR, and N₂ adsorption measurements. Novel organic-inorganic mesoporous luminescent hybrid containing RE³⁺ (Eu³⁺, Tb³⁺) complexes covalently attached to the functionalized ordered mesoporous SBA-16 (TTA-S16 and DBM-S16), which were designated as bpy-RE-TTA-S16 and bpy-RE-DBM-S16, were obtained by sol-gel process. The luminescence properties of these resulting materials were characterized in detail, and the results reveal that mesoporous hybrid material bpy-Eu-TTA-S16 present stronger luminescent intensities, longer lifetimes, and higher luminescent quantum efficiencies than the corresponding DBM-containing materials bpy-Eu-DBM-S16, while bpy-Tb-DBM-S16 exhibit the stronger characteristic emission of Tb³⁺ and longer lifetime than the corresponding TTA-containing materials bpy-Tb-TTA-S16.     

Synthesis and characterization of a new type of chemically bonded liquid crystal stationary phase for HPLC

Summary Two commercially available liquid crystals, 4-cyano-4′-n-pentyl-1,1′-bipheny and 4-cyano-4′-n-pentoxy-1,1′-bipheny, are bonded to a silica hydride surface via hydrosilation in the presence of a free radical iniator, t-butyl peroxide. Elemental analysis, diffuse reflectance Fourier trans-form infrared spectroscopy, and¹³C and²⁹Si CP-MAS NMR spectroscopy are used to confirm the success of the bonding reaction. The¹³C CP-MAS spectra suggest a difference in the bonded phase morphology of the two materials. Static hydrolytic stability tests indicate these materials do not degrade significantly in both acidic and basic solutions. Chromatographic tests confirm that these two bonded phase behave differently with respect to their retention of PAHs, alkyl-substituted benzenes and benzodiazepines.     

Characterization of the hydrolysis and polymerization processes of methacryloxypropyltrimethoxysilane

The sol-gel synthesis of hybrid methacrylate-silica materials using methacryloxypropyltrimethoxysilane (MPS) as precursor, has been analyzed using ¹H and ²⁹Si liquid NMR as well as ²⁹Si and ¹³C MAS-NMR. Under the present experimental conditions (H₂O/MPS=3; pH=2), hydrolysis of methoxy groups is fast. However, 5% of unreacted alkoxy groups are still present in the sol after 14 days aging. Condensation reactions lead mainly to cyclic or short linear species. The number of cross-linking points never exceeds 20% of the Si units, which seems to prevent the formation of a gel. Polymerization of methacrylate groups occur easily under short times of U.V. irradiation. A quantitative analysis shows about 20% of residual unreacted groups in the polymerized materials. Addition of free methylmethacrylate in the hydrolyzed sols helps in getting a fully polymerized organic network.     

Time‐Resolved In Situ MAS NMR Monitoring of the Nucleation and Growth of Zeolite BEA Catalysts under Hydrothermal Conditions

Time‐resolved ¹³C, ²³Na, ²⁷Al, and ²⁹Si MAS NMR has been applied in situ for monitoring the hydrothermal synthesis of zeolite BEA. Isotopic labelling with ²⁹Si and ¹³C isotopes has been used to follow the fate of siliceous species and structure directing agent ((¹³CH₃−CH₂)₄NOH). Two mechanistic pathways, namely solution‐mediated and solid–solid hydrogel rearrangement have been distinguished for two synthesis procedures studied. The mechanisms of structure‐directing behavior of TEA⁺ cations in two reaction pathways have been elucidated. The results show that multinuclear MAS NMR can serve as a superior tool for monitoring hydrothermal synthesis of various solids including zeolites, zeotypes, mesoporous materials, metal–organic frameworks and so on and for the design of novel outstanding materials for different applications.     

Fast and Selective Aqueous-Phase Oxidation of Styrene to Acetophenone Using a Mesoporous Janus-Type Palladium Catalyst

A heterogeneous Janus-type palladium interphase catalyst was obtained by selective surface modification of a hollow mesoporous silica material. The catalyst comprises hydrophobic octyl groups on one side of the silica nanosheets and single-site bis-imidazoline dichlorido palladium(II) complexes on the other. The structure of this composite material has been analyzed by means of elemental analysis, atomic absorption spectroscopy, BET surface analysis, TGA, SEM and solid-state CP-MAS ¹³C and ²⁹Si NMR spectroscopy. The catalyst showed extraordinary activity for the aqueous-phase oxidation of styrene to acetophenone using 30% hydrogen peroxide as the oxidant. An 88% yield of acetophenone could be achieved after 60 min.     

γ radiolyzed amorphous silica: A study with 29Si CP-MAS NMR spectroscopy

Precipitated amorphous silica has been γ radiolyzed at 333, 666 and 1000 kGy in air. The structural changes undergone during irradiation have been studied by ²⁹Si CP-MAS NMR spectroscopy. The spectra suggest that silica looses the hydrogen atoms attached to the silanol groups and the resulting silanol radicals undergo a crosslinking reaction by forming peroxysiloxane groups or by reacting with E′ centres yielding siloxane groups.     

Porous Siloxane-Silica Hybrid Materials by Sol-Gel Processing

Porous hybrid materials have been fabricated by sol-gel processing of tetraethoxysilane (TEOS) and 1,3,5,7-tetramethyl-tetrakis(ethyltriethoxysilane)-cyclotetrasiloxane (1) in the presence of the cationic surfactant, cetyltrimethylammonium bromide (CTAB). The chemical and physical properties of these materials have been analysed by FT-IR spectroscopy, solid state ²⁹Si NMR spectroscopy, powder X-ray diffraction and nitrogen adsorption-desorption studies. FT-IR spectroscopy established that the CTAB surfactant can be extracted from a crushed gel using ethanol as a solvent. Solid state ²⁹Si NMR spectroscopy showed the presence of D, T and Q species as expected from the structure of the precursors. Broad bands observed for the D units at –18 ppm and the T units at –63 ppm suggested that the cyclotetrasiloxane was held in a rigid environment and bound to the Q species of the silica matrix derived from the TEOS. NMR spectroscopy confirmed that solvent extraction resulted in further condensation of the silica matrix. Powder X-ray diffraction indicated that the materials possess short-range order and small domain sizes, as shown by broad diffraction peaks. The condensation induced by solvent extraction led to a decrease in the lattice and domain size of the samples, generally resulting in a less ordered material. Nitrogen adsorption-desorption isotherms were typical of microporous materials with pore diameters of 18 Å and a narrow size distribution.     

Organic–inorganic hybrid porous aerogel: efficient catalyst in transesterification reactions

Hybrid silica-based porous aerogels containing propylsulfonic acid and methyl functionalities were prepared by co-condensation route from the corresponding mercaptopropyltriethoxysilane and methyltriethoxysilane precursors using aqueous ammonia as hydrolyzing agent in the absence of any organic templates. The hybrid aerogel was dried under supercritical CO₂ and characterized by SEM, TEM, ¹³C CP-MAS NMR, ²⁹Si MAS NMR, TG/DTA, and nitrogen adsorption/desorption analyses. The hybrid-silica aerogel containing propylsulfonic acid serves as an efficient heterogeneous acid catalyst in transesterification of fatty acid esters with methanol in a batch reactor.     

29Si{1H} CP-MAS NMR comparison and ATR-FTIR spectroscopic analysis of the diatoms Chaetoceros muelleri and Thalassiosira pseudonana grown at different salinities

Diatoms are key indicators of marine environmental health. To further understand how diatoms respond to varying degrees of salinity, either due to climate change or brine waste discharge into marine environments, two different diatom species were studied. Thalassiosira pseudonana and Chaetoceros muelleri were cultured at three different salinities namely, 26 practical salinity units (PSU or parts per thousand), 36 PSU (standard salinity for culturing of seawater species) and 46 PSU. Changes in silica and organic content within the cultured diatoms were analysed using solid-state ²⁹Si{¹H} cross-polarization–magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) and attenuated total reflection–Fourier transform infrared (ATR-FTIR) spectroscopies coupled with analysis of variance. ²⁹Si CP-MAS NMR showed that qualitatively the Q4:Q3 area ratios of C. muelleri, grown away from standard salinities, increased in response to the formation of more condensed (2 ≡SiOH?→?≡Si–O–Si≡?+?H₂O) and/or an increase in closely associated organic matter to the Q4 component of the diatoms. This was not observed for T. pseudonana. However, both species showed the appearance of a new peak centered at 1575–1580 cm^?1 in the ATR-FTIR spectra, designated as the C═N band of nitrogenous purine-type compounds. Further, the C. muelleri species was shown to produce more extracellular polymeric substances at non-standard salinities. On this basis, results suggest that there is a strong relationship between diatom composition and salinity and that C. muelleri is more sensitive to its environment than T. pseudonana.     

Ageing processes of alkyl bonded phases in HPLC; a chromatographic and spectroscopic approach

Summary Laboratory use of HPLC columns packed with C₈ and C₁₈ bonded phases leads to changes in selectivities and retention volumes. FTIR,¹H NMR of hydrolysed bonded phases and solid state¹³C- and²⁹Si NMR were applied to characterize the materials. The results of the various techniques are in fair agreement except solid state NMR. Loss of silane and hydrolysis of surface siloxane groups have been observed for the C₈ bondes phase, while for the C₁₈ material the latter process seems to dominate. The solid state NMR results have been tentatively explained in terms of changing chain arrangements and mobilities.Dedicated to Professor J. F. K. Huber on the occasion of his 60th birthday.     

Raman and 29Si MAS NMR spectroscopy of framework materials containing three-membered rings

Raman and ²⁹Si MAS NMR spectroscopies are evaluated for the identification of three-membered rings (3MR) in framework oxide materials. Raman and ²⁹Si MAS NMR spectra from the 3MR-containing materials euclase, phenakite, clinohedrite, willemite, lovdarite, VPI-7, ZSM-18 and dipotassium zinc tetrasilicate are presented. The Raman spectra from these materials do not exhibit common bands representing vibrational modes assignable to individual 3MR. The dense beryllosilicate and zincosilicate minerals exhibit ²⁹Si MAS NMR resonances indicative of silicon positioned in 3MR while the molecular sieves lovdarite and VPI-7 give ²⁹Si MAS NMR resonances that can be assigned to silicons located at the center of “spiro-5” units that are constructed from two 3MR. Silicon atoms located in isolated 3MR in the molecular sieves ZSM-18 and dipotassium zinc tetrasilicate do not exhibit ²⁹Si MAS NMR resonances that can be distinguished from those assigned to silicons residing in 4MR and larger.The ²⁹Si MAS NMR spectra from the new materials VPI-8, VPI-9 and VPI-10 do not exhibit ²⁹Si MAS NMR resonances indicative of “spiro-5” units. The presence of isolated 3MR in these materials cannot be ruled out from the ²⁹Si MAS NMR spectroscopic results.     

Preparation and characterization of copolymerized aminopropyl/phenylsilsesquioxane microparticles

Poly(aminopropyl/phenyl)silsesquioxane (PAPSQ) microparticles with approximately 350 nm diameter were conveniently synthesized from the hydrolytic co-condensation of γ-aminopropyltriethoxysilane (APS) and phenyltriethoxysilane (PTES) in the presence of ethanol and water (EtOH-H₂O) by using tetramethylammonium hydroxide (TMAOH) catalysts. Hybrid compositions of PAPSQ containing both aminopropyl and phenyl groups were confirmed from FT-IR, ²⁹Si CP-MAS NMR, TGA. XRD patterns indicated that a certain ordered structure existed in PAPSQ molecules. Contents of amino groups of PAPSQ were determined by elemental analysis and back titration. Specific surface areas were evaluated through BET method. PAPSQ offers the potential utility as building blocks for diverse and novel organic/inorganic materials.     

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.     

Analysis of silica hydride and surface hydrosilation reactions by solid-state NMR in the preparation of<Emphasis Type="Italic"> p</Emphasis>-chlorobenzamide bonded silica phase

²⁹Si and ¹³C CP-MAS NMR spectroscopy was used to follow the conversion of native silica to a p-chlorobenzamide bonded silica material. The benzamide bonded phase was prepared via a hydrosilation reaction of a hydride silica intermediate with p-chloro-N-allylbenzamide. Solid-state NMR was used to show the disappearance of reactive surface hydride species (M_H) and to identify newly formed bonded chemical species on the silica surface. DRIFT spectroscopy, elemental analysis, and specific surface-area determinations (BET) of the prepared phases are also reported.     

New Approach for the Organisation and the Shaping of Organo-Bridged Silicas: An Overview

A new and general route to synthesize shape-controlled bridged silsesquioxanes has been developed by the hydrolysis-condensation of molecularly designed precursors bearing urea groups. The auto-association of the bridging organic units, owing to the hydrogen bonds developing between the urea groups, has been exploited to create new multifunctional organo-bridged silicas with peculiar shapes from the nano- to the micro-scale. Helical fibers with controlled handedness, hollow tubes and spheres, and lamellar plates have been produced according to the main organic substructure and also depending on the reaction conditions (catalyst, solvent and temperature). Spectroscopic techniques (solid state ¹³C and ²⁹Si NMR, FTIR and X-ray diffraction) and electronic microscopic measurements (SEM and TEM) allowed the characterisation of these hybrid materials.     

Contribution of multi-nuclear solid state NMR to the characterization of the <Emphasis Type="Italic">Thalassiosira pseudonana</Emphasis> diatom cell wall

A major issue in the study of biosilicification processes is the harsh chemical conditions required for silica dissolution, which often lead to denaturation of the associated bio-organic matter. In order to demonstrate the potential of solid state NMR for investigating silicified materials of natural origin, this technique was applied to isotopically enriched Thalassiosira pseudonana diatom cells. ²⁹Si, ¹H,³¹P, ¹³C and ¹⁵N solid state NMR studies were performed on whole cells, SDS-extracted and H₂O₂-cleaned silica shells. Cross-polarization techniques were useful for identifying the presence of mobile and rigid molecules, allowing loosely bound and silica-entrapped species to be discriminated. Successive cleaning procedures efficiently eliminated weakly associated organic matter. The H₂O₂-cleaned silica shell still contained carbohydrates (mainly chitin) and proteins as well as lipids. This suggests that the role of lipids in diatom shell formation may have been underestimated so far, demonstrating the potential of solid state NMR for studying composite biomaterials. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Structure of Hybrid Gels by Drift and NMR Spectroscopies

Hybrid inorganic-organic gels have been prepared by the sol-gel process using tetraethoxysilane (TEOS) as precursor, mixed with a low concentration of polytetrahydrofuran (PTHF), under acid catalysis. The hybrid xerogels were characterized by DRIFTS and Solid State ¹H, ¹³C and ²⁹Si NMR. The DRIFT spectra indicate that the polymer is responsible for decreasing the number of free silanol groups in comparison to pure silica. Solid-state NMR spectra reveal the types of silicate structures formed and the conditions for establishing chemical bonds between the two phases, which are responsible for the silica network flexibility. We have concluded that it is possible to design a hybrid gel with tailored properties, even at very low polymer concentration, by selecting the appropriate preparation route.