J. Méndez-Vivar

Spectroscopic study on the early stages of the polymerization of hybrid TEOS–RSi (OR′)3 sols

Abstract A spectroscopic study of sols composed of tetraethyl orthosilicate and alkyl alkoxysilanes was performed. The molar ratio range studied was 95–60 for tetraethyl orthosilicate and 5–40 for the alkyl alkoxysilanes. The 29 Si Nuclear Magnetic Resonance high resolution study along the first 20 h of the polymerization allowed us to demonstrate that cross-condensation predominated over self-condensation in the tetraethyl orthosilicate–methyl triethoxysilane and tetraethyl orthosilicate–phenyl triethoxysilane sols, compared to tetraethyl orthosilicate–ethyl triethoxysilane in the conditions studied. These results were consistent with the small angle X-ray scattering results, where two types of growing olygomeric species were found: those with a fractal dimension value D f =2.3, corresponding to randomly branched olygomers, and the others with a fractal dimension value D f =1.8, corresponding to multiparticle diffusion-limited aggregates. Inspite of performing a complete Fourier transform infrared spectroscopy study from sols to gels, only slight differences were found in the spectra.

Control of the polymerization process of multicomponent (Si, Ti, Zr) sols using chelating agents

Abstract A new experimental strategy was developed to obtain SiTi Zr oxide transparent sols via the sol–gel process. A pre-hydrolyzed tetraethyl orthosilicate (TEOS) sol was used as the Si precursor. Five chelating agents were used to stabilize monomeric Ti and Zr precursors, avoiding precipitation. The sols were characterized by UV–Vis spectroscopy in order to characterize the Ti and Zr species obtained. The sols were also studied by 29 Si NMR, small angle X-ray scattering (SAXS), Fourier transform infrared spectroscopy (FTIR), thermal gravimetric (TGA) and differential thermal analyses (DTA). Based on the study of series of SiTi, SiZr and SiTiZr UV–Vis spectra obtained with every chelating agent, we assigned the bands. In some cases it was possible to assign the bands to the chelation of Ti and Zr specifically. The stability of the SiOTi and SiOZr bonds from sols to oxides was demonstrated by FTIR spectroscopy. A strong influence of the molecular structure of the chelating agents in the sols was observed by 29 Si NMR, and in the oxides by the TGA and DTA studies. The chelating agent that allowed to obtain a homogeneous pore size structure after thermal treatment at 873 K was isoeugenol.

Spectroscopic Study of SiO2-TiO2 Sols Prepared Using Stabilizing Agents

Two chemical routes were employed to prepare Si−Ti polymers, using acetylacetone (acacH) and acetic acid (HOAc) as modifying agents. The homogeneous and transparent sols were studied at different aging times. The characterization techniques were29Si NMR, FTIR, UV-Vis and TGA. When HOAc was used, the oligomeric species in the first stages of polymerization were assigned by29 Si NMR. The Ti−O−Si bond vibration was detected by FTIR in the 950–960 cm−1 region. UV-Vis spectroscopy showed bands of acac bonded to Ti at 300 nm and 365 nm, and OAc− bonded to Ti was found at 320 nm. The TGA study allowed the establishment of the xerogel formulas.

Homogeneous SiTi and SiTiZr polymeric systems obtained from monomeric precursors

Abstract A new chemical strategy for the preparation of a homogeneous SiTiZr sol was developed. The procedure is based on strategies developed for the SiTi and SiZr. Sols were characterized by Fourier transform infrared spectroscopy (FTIR). Assignments of the simultaneous formation of the SiOTi and SiOZr bonds were made. The xerogels and oxides were also analyzed by 29Si CP MAS NMR, X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron paramagnetic resonance. Zr inhibits condensation of the siloxane species in the xerogels. All the solid samples, including those calcined at 900°C were amorphous. EPR results indicate that the Ti(III) symmetry is octahedrally distorted.

The Role of 2-(Methacryloyloxy) Ethyl Acetoacetate in the Polymerization of Hybrid Multicomponent (Si, Ti, Zr) Sols

Abstract2-(Methacryloyloxy) ethylacetoacetate (MEEAH) was used as stabilyzing agent to control the chemical reactivity of Ti and Zr monomeric alkoxides in a Si—Ti—Zr sol. The organic and inorganic polymerizations were carried out simultaneously. The organic polymerization was performed using benzoyl peroxide as the initiator. The inorganic polymerization was done via the sol-gel process. The polymerization was studied by several spectroscopic techniques, including Fourier Transform Infrared (FTIR), UV-Vis, 29Si NMR, Small Angle X-ray Scattering (SAXS) and X-ray diffraction (XRD). Theoretical calculation and experimental results showed that MEEAH acted as a monodentate ligand, mainly. This led to a high condensation extent of the Si species in the sol. A short range order gel was obtained, according to the SAXS results. The Si—O—Ti and Si—O—Zr bonds were detected by FTIR. The bonds remained stable, because a sample calcined at 1173 K was amorphous according to the XRD results, indicating structural homogeneity.

Influence of isoeugenol as a chelating agent on the structure of Si-Ti polymeric systems obtained from alkoxides

Abstract Clear sols containing Si and Ti were prepared by the sol–gel process. The Si and Ti precursors were tetraethoxysilane (TEOS) and titanium isopropoxide (Ti(OPri)4), respectively. In one case the Ti precursor was used as received (Si–Ti sol); in the second case Ti was stabilized using isoeugenol (2-methoxy-4-propenylphenol, ISOH) as a chelating agent (Si–Ti(ISO) sol). In both cases H2O was produced by in situ reaction of acetic acid (HOAc) and the solvent, ethanol (EtOH), for the hydrolysis step. The sols were studied during gelation by 29Si NMR, SAXS, XRD, FTIR and UV–Vis. spectroscopies. According to the 29Si NMR results on the Si–Ti sol, a larger concentration of Q4 species was obtained in the Si–Ti sol compared to Si–Ti(ISO). It was possible to differentiate between the UV–Vis. bands for the mono- and bi-dentate OAc− and ISO− species bonded to Ti3+ in octahedral TiO6 chromophore groups. The FTIR results showed the formation of the Ti–O–Si bonds at 965–968 cm−1. The steric hindrance of ISO− bonded to Ti in Si–Ti(ISO) led to a more stable sol compared to Si–Ti.

Porous SiO2–TiO2–ZrO2 Obtained from Polymeric Systems Prepared by the Sol–Gel Process

An experimental strategy was developed to obtain Si–Ti–Zr transparent sols via the sol-gel process. The chelating agents isoeugenol (2-methoxy-4-propenylphenol, isoH), salicylaldehyde (2-hydroxybenzaldehyde, salH), and itaconic anhydride (2-methylenesuccinic anhydride, anhH) were employed separately to stabilize monomeric Ti and Zr precursors, in order to control their chemical reactivity, avoiding precipitation. In all cases a prehydrolyzed tetraethyl orthosilicate (TEOS) sol was the Si source. The sols were polymerized at room temperature (293 K) to obtain gels and these were dried and calcined at 873 K in air. The radial distribution functions (RDF) of the gels were obtained at room temperature. The solids were studied by scanning electron microscopy (SEM). The porosity and surface area of solids were determined by N2 adsorption. The surface area results obtained range between 83–198 m2/g. The average pore diameters are 1.44–1.61 nm.

SYNTHESIS OF MOLYBDENUM OXIDES BY THE SOL-GEL METHOD

Abstract Molybdenum oxides were synthesized by the sol-gel method, from a precursor prepared with MoOCl 4 and nPrOH. The precursor was hydrolyzed, using different catalysts. The process was studied by infrared spectroscopy (IR) and diffuse reflectance. It was found that the hydrolysis produced new species. The catalysts which were used did not significantly modify the spectroscopic properties of the products. A thermal treatment transformed the initial oxides into MoO 3 at 600 °C, and upon heating at 900 °C transformed them into Mo 8 O 23

Correlation between fractal dimension and surface characterization by small angle X-ray scattering in marble.

Among several analysis techniques applied to the study of surface passivation using dicarboxylic acids, small angle X-ray scattering (SAXS) has proved to be relevant in the physicochemical interpretation of the surface association resulting between calcium carbonate and the molecular structure of malonic acid. It is possible to establish chemical affinity principles through bidimensional geometric analysis in terms of the fractal dimension obtained experimentally by SAXS. In this Article, we present results about the adsorption of malonic acid on calcite, using theoretical and mathematical principles of the fractal dimension.

Experimental Design of Si-Ti-Zr Polymers Containing Hybrid Si Species, Spectroscopic Study and Thermal Evolution

A Si-Ti-Zr polymeric system containing hybrid Si species was prepared, using Tetraethyl orthosilicate (TEOS) and Methyl triethoxysilane (MTES) as the Si sources. The TEOS-MTES molar ratio was 60-40, respectively. Monomeric Ti and Zr alkoxides were used. The final molar ratios Si-Ti-Zr were 82-7-11, respectively. The samples were characterized by 29Si NMR, SAXS, FTIR and UV-Vis spectroscopies. The NMR results showed that the hybrid Si species were produced to a very low extent and that the kinetics of the polymerization was controlled by Ti and Zr. According to the SAXS results, linear oligomers were obtained. The Si—O—Ti and Si—O—Zr bonds were detected by FTIR. In the latter case, the bonds are stable, as the corresponding vibrations were detected in the oxide calcined at 773 K. A microporous structure was obtained, where the average pore diameter was 1.4 nm.