29Si CPMAS NMR and near-IR study of sol–gel microporous silica with tunable surface area

High-resolution solid-state NMR and IR techniques were used to investigate the surface structure of microporous silica with tunable surface areas, obtained by a sol gel method acting on a variation of gelation time. ²⁹Si MAS NMR and Med-IR evidenced a similar number of silanol groups in all the silicas. Near-IR results revealed the existence of a different ratio of hydrogen bonded silanols and free silanols as a function of surface area. ²⁹Si CP-MAS experiments, including CP spin dynamics studies, were employed to probe hydrogen bonding and local structural environments of various hydroxyl groups of silica surface. These studies showed that for high surface area silicas both free and hydrogen bonded silanols are present at the surface of the samples and the amount of free silanol groups increases with the increasing of the surface area of the samples.     

Structure of hybrid (organic/inorganic) TiO2–SiO2 xerogels II: thermal behavior as monitored by temperature-programmed techniques and spectroscopy

Titanium-containing xerogels made by acid pre-hydrolysis of tetraethylorthosilicate (TEOS) and dimethyl-dimethoxysilane (SiMe₂(OMe)₂), followed by reaction with dichloro-diisopropyl titanium (TiCl₂(i-OPr)₂) were characterized by means of several techniques. The decomposition patterns of these xerogels, both in the presence and in the absence of oxygen are discussed in terms of the gaseous products' evolution under temperature-programmed (TP) reaction conditions, and diffuse reflectance infrared spectroscopy (DRIFTS). The primary decomposition routes of residual alkoxide groups are: (i) auto-redox (CO, CO₂) and dehydration (to olefins) under pyrolysis conditions; (ii) oxidative (CO, CO₂) under flowing air. The decomposition of the Si–C bond upon heating, as monitored by all three techniques, follows a pattern independent of that of (CH) species from −OR groups. The ²⁹Si MAS/NMR of these materials is dominated by the so-called D² (Me₂Si(OX)₂), Q³ ((OR)Si(OX)₃) and Q⁴ (Si(OX)₄) resonances. Fairly homogeneous proton envelopes around Si centers are evident from the NMR spectra of these materials in the proton cross-polarization mode.     

Solid-state NMR and XANES studies of lithium and silver silicate gels synthesized by the sol-gel route

The objective of this study is to understand the effect of low temperature sol-gel synthesis on the microstructural properties of lithium [xLi₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] and silver [xAg₂O-(1−x)SiO₂; x=0.1-0.8 in steps of 0.1] silicate xerogels via solid state nuclear magnetic resonance (NMR) and X-ray absorption near edge structure (XANES) techniques. The Li silicate xerogels were analyzed with solid-state ⁷Li and ²⁹Si NMR and the Ag silicate xerogels were studied with Ag XANES. At high Li loading, ⁷Li NMR shows quadrupolar satellite transitions attributed to LiNO₃, a phase also found with X-ray diffraction (XRD). At low Li loading, both NMR and XRD results show an amorphous xerogel. The silicate network is monitored with ²⁹Si NMR and shows evidence of Li incorporation. For the Ag silicate xerogels, Ag-L-III XANES spectral studies show a local environment similar to AgNO₃ for low Ag loading levels, and an increased Ag oxidation for higher Ag loading levels. Si K edge spectra show only an amorphous phase, with no evidence of a crystalline quartz phase. The electrical conductivity of the lithium silicates was estimated from impedance data and the highest conductivity is exhibited by the 0.3Li₂O-0.7SiO₂ composition xerogel. The conductivity dependence on loading level strongly suggests that the observed conductivity is due to Li⁺ mobility. However, further experimental studies are needed to rule out the possibility that the conductivity is, at least in part, due to H⁺ mobility. Variation in conductivity is explained qualitatively using existing theoretical models.     

A novel sol-gel method for the synthesis of γ-aluminium oxide: development of the sol-gel transformation and characterization of the xerogel

A novel method for the synthesis of aluminium oxide gel has been developed, whereby the sol-gel transformation was investigated. Aluminium tri-sec-butoxide was used as precursor while acetone was chosen as solvent. The synthesis was carried out in a special reactor, which allowed the dosing of steam. ²⁷Al NMR spectroscopy showed that during the sol-gel process the signal at δ∼3 ppm increases strongly corresponding to the formation of hexacoordinated aluminium species. Beside hydrolysis and condensation reactions, the coordination of acetone to a strong Lewis acid aluminium site occurs, which was shown by FTIR and ²⁷Al NMR spectroscopy. Viscosimetric analysis showed that at the beginning of the sol-gel process short polymers are observed while before the gelation a three-dimensional polymer network is formed. After pyrolyzing the gel a high surface area γ-aluminium oxide xerogel was formed. The effect of heating on the morphology and structure was examined by nitrogen physisorption (BET and pore size distribution), XRD and ²⁷Al MAS NMR spectroscopy.     

Microstructural evolution of amorphous silica following alkali–silica reaction

Combining X-ray diffraction, NMR (CP-MAS and ²⁹Si NMR-MAS) and diffuse reflectance infra-red Fourier transformed spectroscopy (DRIFTS), the local structural changes induced during the reactivity of amorphous (a-SiO₂) under alkali–silica reaction (ASR) have been investigated. XRD patterns provide useful and new qualitative information concerning the reactivity of a-SiO₂. A shift of the maximum of the halo towards the high values of 2θ is observed when the reaction progresses. NMR shows that ASR is accompanied in a-SiO₂ by the diminution of Q₄ species and by the formation of Q₃ (and to a less extend to Q₂) species. This importance played by Q₃ species confirms the results obtained with natural alkali–silica gels. The formation of OH bonds observed by DRIFTS can be followed at ca. 950 cm^?1 and confirms NMR results. The microstructural state of silica (here represented by the medium- and short-range order (MRO) of amorphous silica) plays a crucial role in governing the kinetics of the ASR.     

Multinuclear NMR study of phosphosilicate gels derived from POCl3 and Si(OC2H5)4

Solid state ¹H, ²⁹Si and ³¹P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P₂O₅ in P₂O₅-SiO₂ were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P₂O₅ gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix.     

Investigation of inorganic network formation in photopatternable hybrid sol-gel films by Si liquid NMR

Liquid ²⁹Si NMR spectroscopy was used to investigate the sol-gel process of methacryloxypropyltrimethoxysilane. This hybrid precursor was involved in the realization of optical elements in laminated crack-free thick films (up to 100 μm), through spatially controlled photopolymerization. Understanding the formation of the inorganic network was of first importance to insure the creation of crack-free photopatterned thick films in a laminated configuration. Material preparation required evaporation of the volatile solvents released during the sol-gel process and limitation of the condensation degree. Both conditions were achieved by a drying process at room temperature. The structure and the composition of the sols were investigated and compared to non-dried sols. Evolution of inorganic species distribution was also studied under increasing aging time or storage temperature. NMR peak fitting of T1 species gave fruitful information on the sol structure evolution during the sol-gel process. It pointed out the presence of a large variety of oligomers in the sol. The study also allowed the identification of more constraint cyclic species in dried sols stored at room temperature. Their amount significantly decreased when increasing the storage temperature and was attributed to ring opening of cyclic species. Consequently, the structure of the dried sol will depend both of the aging time and of the storage temperature. All these results have to be taken into account when the degree of condensation has to be limited to achieved photopatternable hybrid layers for specific optical applications.     

Irradiation effects on the OH-related infrared absorption band in synthetic wet silica

The effects of β-irradiation on the OH-related infrared (IR) absorption band in synthetic wet silica samples have been investigated by Fourier transform infrared spectroscopy. Depending on the accumulated doses, β-irradiation affects different zones of the IR composite band at about 3670 cm⁻¹, assigned to the OH stretching modes of silanol groups. These modifications are independent of the original OH content. The results are discussed considering possible radiation-induced changes of the silanol bonding configuration and of the glass network. These are monitored by revealing the IR band a 2260 cm⁻¹, which is related to the distribution of Si–O–Si bond angle. We have identified the existence of two regimes as a function of radiation dose; low radiation doses produce the reduction of the Si-OH hydrogen-bonded states in favor of the isolated ones, while the opposite effect is observed at higher doses, where the formation of H-bonded pairs seems to be favored.     

Structure of organic-inorganic hybrid low-melting glasses from Si NMR and ab initio molecular orbital calculations

The structure of organic-inorganic hybrid glass precursor Me₂Si(OPO(OH)₂)₂ and low-melting glasses SnO-Me₂SiO-P₂O₅ has been examined by ²⁹Si static and MAS NMR, respectively. The ²⁹Si MAS NMR spectra of SnO-Me₂SiO-P₂O₅ were deconvoluted into three Gaussian peaks, whose chemical shifts were located at −3.1, −10.2 and −18.1 ppm. Ab initio molecular orbital calculations have also been carried out for the clusters modeling the structure of precursor and glasses. From the calculations of ²⁹Si NMR shielding constants, the experimental chemical shifts at −10.2, −3.1 and −18.1 ppm were assigned to ²⁹Si atoms in P-O-Si(Me)₂-O-P network linkage, in P-O-Si(Me)₂-OH terminating structure and in P-O-Si(Me)₂-O-Si network linkage, respectively. Based on the area ratio of the assigned three peaks, the network structures around Si constructing SnO-Me₂SiO-P₂O₅ low-melting glasses were discussed in detail.     

Synthesis and characterization of NH2-porphyrins covalently immobilized on modified-SBA-15

The immobilization of two different succinic-modified carboxy-imide-porphyrins on the surface of amino-modified SBA-15 is reported. The SBA-15 was synthesized and modified by reacting its hydroxyls groups with the OH and/or OR groups of 3-amino propyl triethoxy silane, anchoring the amino group on its surface. Separately, two different derivatives amino-porphyrins (NH₂-5-m-Etio-III-Ni²⁺ and NH₂-2βpyrrolic-m-5,10,15,20-TPP-Ni²⁺) were modified using succinic anhydride to provides these with the carboxyl group appropriate to react covalently with the amino groups previously anchored on the SBA-15; these new hybrid materials have the appropriated textural and chemical characteristics to be used in heterogeneous catalysis. The samples were characterized by XRD, N₂ adsorption isotherms, HR-TEM, TGA/DTA, FTIR, UV/VIS, ¹³C and ²⁹Si NMR-CP/MAS.     

Vibrational spectroscopic studies of sol-gel derived physical and chemical bonded ORMOSILs

Fourier transform infrared (FT-IR) and FT-Raman Spectroscopy have been utilized to characterize the structure of physical and chemical bonded ORMOSILs (organically modified silicates) and to compare with the physical properties, reported earlier. The classic and sono ORMOSILs were synthesized by sol-gel method via traditional- and sono-catalysis routes, respectively. The physical and chemical bonding is established by Poly(ethylene glycol) (PEG) and Poly(dimethoxysilane) (PDMS), respectively. Existence of band at ∼560 cm⁻¹ (FT-IR) and ∼490 cm⁻¹ (FT-Raman) for the sono and classic gels with 5-10 wt% of PEG indicates the presence of structural defect due to the four-membered siloxane rings. Application of long molecular PEG chain and PDMS increases this defect. Modification in the intensity and peak position of siloxane (Si-O-Si) bands is found correlative with the physical properties. The xerogels with 5-10 wt% of PEG possess a large number of residual surface silanol groups than the xerogels with long molecular PEG chains and PDMS and is found related with the ν(OH) band at ∼3470 cm⁻¹. These results are attributed to the way of hydrolysis-condensation reactions and discussed with the help of Percolation and DLVO models.     

Structural analysis of bioactive glasses

In this study four series of single and mixed alkali glass systems were made and investigated using MAS NMR. Additionally the densities of the glasses were measured experimentally, as well as calculated theoretically using Doweidar’s model. MAS NMR was used to obtain a quantitative structural understanding of glasses by calculating the concentrations of bridging and non-bridging oxygens per silicon oxygen tetrahedron as a function of the alkali oxide concentration expressed as Qⁿ. ²⁹Si MAS NMR spectra exhibited a single resonance corresponding closely with Si in a Q² state. The chemical shift of the ³¹P MAS NMR peak was attributed to phosphate in an orthophosphate environment. The ²⁹Si NMR spectra are in agreement with the density data. Using Doweidar’s model the proportions of Q² and Q³ were calculated, showing that all glasses studied are predominantly Q² in structure, i.e. silica chains which readily dissolve. The changes in the chemical shifts of the Q² and Q³ species with composition have been interpreted as resulting from the preferential association of Na⁺ with Q³ and Ca²⁺ with Q².     

Effect of minor additions on structure and volatilization loss in simulated nuclear borosilicate glasses

Structural and thermal properties are reported for a range of caesium oxide-containing alkali borosilicate glasses, of the form xCs₂O(100 − x)ZMW (0 < x < 10), where ZMW represents a variety of simulated base-glasses. Glass densities increase and glass transition temperatures decrease with increase in caesium oxide concentration. Mass-loss from the melt is found to depend on composition in the same manner as the fraction of silicon Q³ units, resolved from ²⁹Si MAS NMR, and is related to the presence of danburite medium-range order units, resolved from ¹¹B MAS NMR. Volatilization is shown to occur even in the absence of caesium oxide and the mixed alkali borosilicate composition of the volatile species, evolved from the melt at high temperature, is independent of the starting composition of the glass.     

Quantitative measurement of Q species in silicate and borosilicate glasses using Raman spectroscopy

Raman spectroscopy has been used to measure the fraction of tetrahedral silicate units connected at three corners into the network (Q³) in binary lithium silicate glasses and also in the more complex borosilicate glasses used for waste immobilization. Agreement within experimental error was obtained with ²⁹Si MAS NMR measurements of the same samples. Raman provides an alternative method of structural determination for silicon-containing glasses with a high content of paramagnetic species where NMR loses resolution. Analysis was performed on borosilicate glasses containing up to 11.98 mol% Fe₂O₃ and the Q³ values obtained by Raman spectroscopy agree within error with the published ²⁹Si NMR results from borosilicate glasses containing the equivalent quantity of Al₂O₃.     

Speciation of hydrogen in silica glass by 1H MAS NMR

The H-bearing species in silica glass melted quartz powder in hydrogen atmosphere have been investigated using high-resolution ¹H magicangle spinning nuclear magnetic resonance (¹H MAS NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and quadrupole mass spectrometry. The results show that molecular H₂, hydroxyl (OH) groups and hydride (SiH) groups are present in the glass. The physically dissolved molecular H₂ is represented by a broad NMR peak with the effects of strong H–H magnetic dipolar interactions. Isolated OH groups and hydrogen bonding OH groups in silica glass have been distinguished by the chemical shift of the ¹H NMR spectrum. The influence of E’ center on the ¹H MAS NMR spectrum are also analyzed.     

Influence of additives and post-synthesis treatment on the structural properties of sol-gel prepared alumina-doped zirconia studied by EXAFS-spectroscopy and X-ray diffraction

Alumina- (5 and 10 wt%) doped zirconia samples were prepared by sol-gel processing of aluminum sec-butoxide and zirconium butoxide in the presence of acids (acetic, sulfuric or nitric acid) or organic additives (Pluronic P123). X-ray absorption spectroscopy, X-ray diffraction and nitrogen adsorption experiments showed that the structure of the synthesized mixed oxides is strongly influenced by the chemical synthesis parameters.     

Synthesis and characterization of chemically cross-linked polyimide-siloxane hybrid films

Chemically cross-linked polyimide and silica hybrid films were prepared through the sol-gel processing. PI matrix was prepared by the reaction of pyromellitic dianhydride with a mixture of diamines e.g., oxydianiline and 2,5-diaminohydroquinon (2,5-DAHQ) to include pendant hydroxyl groups on the chain. These groups were reacted further with isocyanatopropyltriethoxysilane. An appropriate amount of tetraethoxysilane was then added and the sol-gel process was carried out to condense ethoxy groups from both types of silanes thus producing chemically bonded composite films. The films with different silica contents were evaluated by a variety of techniques including FTIR, ²⁹Si NMR, SEM, tensile, thermal, mechanical and thermogravimetric analyses. The chemical interaction between the phases brought about an intimate dispersion of the two phases, which resulted in the formation of nano-sized co-continuous domains. The tensile modulus of such films was higher and thermal expansion coefficient was much lower than those with similar silica contents without inter-phase bonding.     

Nitrogen-rich La-Si-Al-O-N oxynitride glass structures probed by solid state NMR

We investigate the local structures of oxynitride La-Si-(Al)-O-N glasses by ²⁹Si and ²⁷Al magic-angle spinning (MAS) solid state Nuclear Magnetic Resonance (NMR). The glasses studied span an unprecedented range of compositions, up to >50 at.% lanthanum and nitrogen out of the cations and anions, respectively, and achievable through a recently introduced glass preparation route. Transmission as well as scanning electron microscopy verified homogeneous samples over length-scales down to 20 nm. As the nitrogen content of the glasses increased, ²⁹Si NMR evidenced a progressive formation of Si-N bonds, with SiO₂N₂ tetrahedra dominating in the nitrogen-rich glass networks. In the oxygen-rich end of the series, aluminum is predominantly present in tetrahedral coordination as AlO₄, whereas the glasses with highest nitrogen contents have a major fraction of AlO₃N structural units. Trends in isotropic ²⁹Si and ²⁷Al chemical shifts and ²⁷Al quadrupolar couplings are compared with results of La-Si-Al-O glasses and are discussed in relation to the glass compositions and their proposed structures.     

1H, 29Si and 27Al NMR study of the destabilization process of a paracrystalline opal from Mexico

This is the first NMR study of the destabilization through whitening of paracrystalline opal-CT (SiO₂, nH₂O). The aim of this study is to understand the process leading to the degradation, particularly any changes in the structure and bonding of this material. ²⁹Si and ²⁷Al NMR signals are not significantly modified. In contrast, using for the first time ¹H as a probe in opal, we show that a comprehensive understanding can be obtained on both the local structure of the opal-CT and the destabilization process. Thus, we propose the latter is related to some changes in ¹H speciation (loss of strongly ‘bound’ water and appearance of new species) which induce structural reorganizations on the surface and in the rim of the silica nanograins constituting opal-CT.     

Increased Si NMR sensitivity in glasses with a Carr-Purcell-Meiboom-Gill echotrain

The use of a Carr-Purcell-Meiboom-Gill (CPMG) echotrain to increase the ²⁹Si NMR sensitivity in glasses was investigated. The echo intensity decay follows a stretched exponential behavior M(t) = M₀ exp[−(t/t2)^β] with values for the exponent β in the range of 0.41-0.65. The signal to noise in the spectra can be increased by a factor of up to 4 by taking the weighted sum of the spectra obtained from the individual echoes. However, differential T2 relaxation for the different Qⁿ species is observed, with a shorter relaxation time for Q⁴ than Q³. Thus, summing of the echoes leads to distorted spectral intensities and quantitative information can no longer be obtained from the spectra. To circumvent the problem with differential T2 relaxation, an alternative approach is developed in which the spectral intensity for each chemical shift value is determined from the stretched exponential fit to its echo decay. With this approach, the sensitivity can be increased by a factor of up to 2.4, while quantitative information can still be obtained from the spectra. The increased sensitivity permitted the detection of five-coordinated silicon in a potassium silicate glass with natural ²⁹Si abundance at ambient pressure.