Lubomira Tosheva

Silicalite-1 containing microspheres prepared using shape-directing macro-templates

Silicalite-1 microspheres were prepared by a novel method based on the use of anion exchange resins as shape-directing macro-templates. In a first step, spherical beads of the ion exchange resin we ...

Zeolite beta spheres

Zeolite beta was crystallized from clear homogeneous solutions within the pores of anion exchange resin beads. The organic macrotemplate was then removed by combustion leaving stable macrospheres of zeolite beta. The zeolite beta crystallization within the resin beads was investigated by X-ray diffraction, Raman spectroscopy, SEM and nitrogen adsorption measurements. Crystallization within the macrotemplates was compared with the crystallization from bulk solution. The influence of the resin beads on the crystallization process was investigated by increasing the amount of ion exchanger present during synthesis and also by preparing bulk samples in the absence of macrotemplates.

Silicalite-1 macrostructures - preparation and structural features

Zeolite macrostructures in the form of silicalite-1 spheres were prepared using anion exchange resin beads as shape directing macrotemplates. The resin was removed after the synthesis by combustion leaving solid spherical particles identical in shape and size to the original resin beads. The formation of the silicalite-1 spheres was studied by Raman spectroscopy, X-ray diffraction, scanning electron microscopy and nitrogen adsorption measurements. Samples prepared for different times of treatment and at two different temperatures, 100°C and 165°C, were investigated. The spheres obtained for short treatment times consist of amorphous silica, which after longer hydrothermal treatment in the presence of TPA cations is partially or entirely transformed into an MFI structure. A single treatment at 100°C resulted in hard silicalite-1 spheres of low crystallinity, whereas fully crystalline spheres could be obtained by a treatment at the higher temperature. However, the spheres prepared at 165°C have inferior mechanical properties and can even loose their shape at certain conditions. A better control of both hardness and crystallinity was achieved by using a two-step synthesis procedure, where a treatment at 100°C was followed by a treatment at 165°C.

Spherical silica macrostructures containing vanadium and tungsten oxides assembled by the resin templating method

Abstract Silica-based spherical macrostructures containing vanadium and tungsten oxides were prepared using anion exchange resin beads as a shape-directing macrotemplate in a multi-step procedure. In a first step resin beads were treated with a clear and homogeneous TPA-silicate synthesis solution at 170 °C for three different times and resin-silicate composites were obtained. Short treatment times resulted in amorphous silica whereas the longest time yielded highly crystalline silicalite-1. In a second step anionic vanadium or tungsten species were introduced into the resin-silicate composites by utilization of the residual ion exchange capacity of the resin. In a final step the ion exchange resin was removed by calcination leaving behind vanadium- or tungsten-containing silica spheres. The influence of the concentration and the pH of the solutions used for vanadium and tungsten ion exchange on the properties of the final spheres was investigated. The product particles were extensively characterized by XRD, SEM, AAS, nitrogen adsorption measurements, Raman and UV–vis diffuse reflectance spectroscopy. Generally, the nature of the metal species formed within the spheres was isolated and highly dispersed monomers in tetrahedral coordination in amorphous samples and V2O5 or WO3 crystallites in crystalline ones.

New Insights into Structural Alteration of Enamel Apatite Induced by Citric Acid and Sodium Fluoride Solutions

Attenuated total reflectance infrared spectroscopy and complementary scanning electron microscopy were applied to analyze the surface structure of enamel apatite exposed to citric acid and to investigate the protective potential of fluorine-containing reagents against citric acid-induced erosion. Enamel and, for comparison, geological hydroxylapatite samples were treated with aqueous solutions of citric acid and sodium fluoride of different concentrations, ranging from 0.01 to 0.5 mol/L for citric acid solutions and from 0.5 to 2.0% for fluoride solutions. The two solutions were applied either simultaneously or consecutively. The citric acid-induced structural modification of apatite increases with the increase in the citric acid concentration and the number of treatments. The application of sodium fluoride alone does not suppress the atomic level changes in apatite exposed to acidic agents. The addition of sodium fluoride to citric acid solutions leads to formation of surface CaF2 and considerably reduces the changes in the apatite P-O-Ca framework. However, the CaF2 globules deposited on the enamel surface seem to be insufficient to prevent the alteration of the apatite structure upon further exposure to acidic agents. No evidence for fluorine-induced recovery of the apatite structure was found.

Micron- and nanosized FAU-type zeolites from fly ash for antibacterial applications

The development of procedures for the synthesis of nanosized zeolites from cheap sources in the absence of organic templates is of great industrial importance. In this work, the size of FAU-type zeolites was decreased from micron- to nano-dimensions by reducing the amount of distilled water added to fly ash after alkali fusion. The fly ash zeolites showed high antimicrobial activity after introduction of Cu and Ag, which was associated with the release of Cu/Ag into the medium. This activity of Cu-containing zeolites from fly ash was similar to the activity of Cu-containing zeolites prepared by conventional methods but lower than that of Ag-containing zeolites.

Porous materials viaegg-constituents templating

The possibility to prepare porous materials with controlled composition and porous structure using environmentally benign, readily available and cheap templates such as egg-derived scaffolds was explored. Different parts of the egg were used for the preparation of: (i) porous disks and beads of SiO2 and SiO2–Al2O3; (ii) micro-/macroporous interwoven hollow tubes; (iii) hybrid eggshell membrane/zeolite structures; and (iv) zeolite/calcite composite. The synthesis conditions and the properties of obtained materials are discussed. The materials were characterized by XRD, SEM, N2 adsorption measurements and chemical analysis.

Indirect Observation of Structured Incipient Zeolite Nanoparticles in Clear Precursor Solutions

The structure of the zeolite precursor particles present in clear synthesis solutions yielding colloidal silicalite-1 (purely siliceous zeolite with MFI-type structure) is one of the most controversial issues in modern zeolite science. Owing to the nature of the synthesis and the absence of Al, these systems are widely used as models to study the mechanism of zeolite crystallization in general. Ever since the “nanoslab” hypothesis was proposed byMartens et al. it has been the subject of a lot of criticism. The major disagreement is about the structure of the subcolloidal precursor particles present in the initial clear mixtures. Whereas Martens et al. suggest that these particles have MFI structural features, an amorphous nature of the particles has been proposed in numerous other studies. In some cases, the nanoparticles were extracted from the synthesis solutions, and concerns have been raised about possible changes in their structure or interference from the extraction procedure. 5] Other authors base their arguments against the crystalline nature of the precursor particles on the results of solid-state NMR analysis after Si enrichment. A recent NMR study on aqueous silicate solutions revealed the presence of structural units typical of the MFI structure. Other complementary techniques such as dynamic light scattering have also been used to further characterize the species present in the precursor solutions. Thus, despite the fact that a remarkable number of papers discussing the structure of the silicalite-1 precursor particles have already been published, the discussion is still ongoing and new publications regularly appear. Infrared spectroscopy has also extensively been used to study the structure of the silicalite-1 precursor particles. 7,12, 13] Based on the presence of a band near 550 cm , which is widely accepted to be characteristic of the MFI structure, either crystalline or amorphous character of the precursor particles has been suggested. These IR studies were performed on freeze-dried samples, extracted precursor particles, and original sols. However, the IR spectra published so far showed substantial differences in the 1000–1300 cm 1 spectral range which indicate that specific sampling affects the structural features of the nanoparticles. Recently, IR spectroscopy with synchrotron radiation was used for the first time to study in situ catalytic reactions in zeolite crystals. On the basis of synchrotron-based grazing-incidence reflection-absorption infrared (RAIR) spectra of ultrathin zeolite films on Au surfaces, we give unambiguous evidence for the existence of incipient zeolite nanoparticles in clear precursor solutions: these are subcolloidal particles having all the structural features of the zeolite framework but a size of only a few unit cells which is insufficient to generate a diffraction pattern. The novelty in our study is twofold: sample preparation and method of characterization. First, to separate the subnanoparticles from the solution, we used the Langmuir–Blodgett (LB) method to deposit them on Au substrates. The LB method allows concentration of the subcolloidal particles at the air/water interface as floating films and transfer of these films to the solid supports, which ensures preservation of the native particle structure. Second, we prepared the LB films on Au surfaces in order to characterize the samples by RAIR spectroscopy, which is the most efficient spectroscopic method for structural analysis of ultrathin films. Vibrational spectroscopy has great advantages over diffraction methods in detecting crystalline particles with a size of just 2–3 unit cells, because such particles are large enough to generate phonon modes of the corresponding crystal lattice but too small to give rise to detectible Bragg reflections even when synchrotron X-ray radiation is used. In addition, the vibrational spectra are easy to interpret and can unambiguously distinguish between amorphous and crystalline features, which is sometimes a problem for other surface-sensitive spectroscopic methods. Enhancement of the IR absorption due to the presence of a strongly reflecting noble metal surface allows the detection of even monomolecular layers when grazing-incident RAIR spectroscopy is [*] Dr. L. Tosheva, L. H. Wee, Dr. A. M. Doyle Division of Chemistry and Materials Manchester Metropolitan University Chester St., Manchester M15GD (UK) Fax: (+44)161-247-6357 E-mail: l.tosheva@mmu.ac.uk a.m.doyle@mmu.ac.uk

One-pot template extraction and alumination of BEC-type zeolite

The present work reports on the one-pot template extraction and isomorphous substitution of aluminum for germanium in a BEC-type framework. The obtained data show that a partial replacement of Ge with Al in the zeolite framework is possible by post-synthesis wet chemistry methods. Complementary techniques, such as NMR, XRD, TG/DTA, SEM and X-ray fluorescence analyses were employed in order to fully characterize the post-synthesis modified BEC-type samples. Further development of this approach is expected to stabilize Ge-rich zeolite frameworks.

Modified colloidal silicalite-1 crystals and their use for preparation of Langmuir-Blodgett films

Purified colloidal Silicalite-1 crystals with a size of 100 nm were modified by centrifugation and re-dispersion in methanol, ethanol, 2-propanol or 1-butanol followed by sonication for two hours. After centrifugation and re-dispersion in fresh solvents, the alcohol suspensions of modified hydrophobic Silicalite-1 were spread at the air-water interface in a Langmuir-Blodgett trough and pressure-area isotherms were recorded. The floating monolayers were successfully transferred to Si wafers. The interaction between Silicalite-1 and methanol was studied in detail by collecting ATR-IR spectra of a Silicalite-1 suspension in methanol upon in situ methanol evaporation and during exposure to air. The increased hydrophobicity of the methanol modified Silicalite-1 was attributed to esterification of the surface of the zeolite nanocrystals. The modification procedure was also applied to micron-sized Silicalite-1 and corresponding Langmuir-Blodgett films were prepared.