Artur Ferreira

Interactions between calcium, phosphate, and albumin on the surface of titanium

The deposition of calcium phosphate on chemically polished commercially pure titanium immersed in Hanks balanced salt solution (HBSS) with bovine serum albumin (BSA) (concentrations 0 and 4 mg/mL) has been investigated. Electrochemical techniques, 125I labeling of albumin, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were used. A tricalcium phosphate layer with a thickness of ca. 1 microm was formed for periods of immersion in HBSS ranging between 1 and 2 weeks. A concentration of 4 mg/mL of BSA prevented its formation, even for periods as long as 1 month. In the absence of BSA, the electrochemical behavior of titanium specimens was significantly affected by the length of immersion time, reflecting the changes that slowly occur on their surface. In the presence of BSA, the surfaces maintained most of their original electrochemical activity. Surface studies have shown that calcium and phosphate become incorporated in the surface at very early stages of immersion. Albumin, which was rapidly adsorbed on titanium, was slowly desorbed when titanium was placed in HBSS. Protein and phosphate may coexist on the same surface, but initially adsorbed albumin molecules prevent the precipitation of a thick layer of tricalcium phosphate.

Isomorphous substitution in the microporous titanosilicate ETS-10

Abstract Silicon has been isomorphously substituted by aluminium and gallium in the microporous titanosilicate ETS-10 to produce ETAS-10 and ETGS-10 respectively. For aluminium substitution a series of samples have been prepared with Al/Ti ratios ranging from 0.10 to 0.48. Both aluminium and gallium substitute exclusively in tetrahedral silicon sites in a manner which avoids TiOAl and TiOGa linkages. Based upon detailed analysis of solid-state NMR spectra, models for the ordering of aluminium in ETAS-10 are discussed.

Evidence for the Aging of Wax Deposits in Crude Oils by Ostwald Ripening

Abstract The ageing of wax deposits is known to occur in paraffinic waxes forming in pipelines and storage tanks. It is characterized by a hardening of the deposit and an increase in paraffin content. A mechanism for the ageing of deposits, based on diffusion caused by temperature-concentration gradients within the wax, has been previously proposed. This work presents evidence indicating Ostwald Ripening to be another ageing mechanism of wax deposits. Rheology of paraffinic crudes, kept isothermally at temperatures in the neighborhood of the pour point, shows a kinetic of hardening of the oil samples. The X-ray diffraction and Cross Polar Microscopy indicate this phenomenon to be caused by an increase of the crystallites size with time. The DSC measurements support the idea that recrystallization takes place in the wax sample. The evidence gathered shows another ageing mechanism based on the recryztallization of the paraffins, such as Ostwald Ripening, to be responsible for the ageing of wax deposits besides the diffusion mechanism previously proposed

Synthesis and structural characterization of novel tin and titanium potassium silicates K4M2Si6O18

The synthesis of a new potassium titanosilicate, K4Ti2Si6O18 (Ti–AV-11), possessing the crystal structure of potassium stannosilicate AV-11, has been reported. The unit cell of this material is trigonal, space group R3 (no. 146), Z ¼ 3; a ¼ 10:012; c=14.8413 A ˚ , g ¼ 120 � ; V ¼ 1289 A ˚ 3 . The structure of AV-11 is built up of MO6 (M=Sn, Ti) octahedra and SiO4 tetrahedra by sharing corners. The SiO4 tetrahedra form helix chains, periodically repeating every six tetrahedra. These chains extend along the [001] direction and are linked by isolated MO6 octahedra, thus producing a mixed octahedral–tetrahedral oxide framework. AV-11 materials have been further characterized by bulk chemical analysis, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), 29 Si and 119 Sn magic-angle spinning (MAS) NMR spectroscopy.

Polymorphism in tetra-butylammonium salts of Keggin-type polyoxotungstates

Abstract Some tetra-n-butylammonium salts of Keggin-type polyoxotungstates, namely α-[PW11O39]7−, α-[PW11M(H2O)O39]5−, MII=Co, Ni, α-[PW11CuO39]5−, α-[PW11Fe(H2O)O39]4− and α-[PW11Fe(OH)O39]5−, with the general formula [(C4H9)4N]4Hx[anion]·nH2O, were studied by powder X-ray diffraction. The salts α-[(C4H9)4N]4H3[PW11O39] and α-[(C4H9)4N]4H[PW11CuO39] were obtained in two crystalline phases, a behavior that has been scarcely documented in compounds with Keggin polyoxometalates. One of the phases corresponds to a body-centered cubic unit cell (Im 3 symmetry) and the other has a body-centered tetragonal unit cell (I41 or I4122 symmetry). For the compounds with Cu, the cell parameters of the cubic and tetragonal structures were a=17.675(2) and a=18.775(2), c=14.646(2) A, respectively. The cubic phase was obtained by precipitation from aqueous solution. The tetragonal phase was isolated by crystallization from acetonitrile. The remaining compounds were found to crystallize with one of these two crystal structures. α-[(C4H9)4N]4Hx[PW11M(H2O)O39]·nH2O, with M=Co, Ni, x=1, and M=Fe, x=0, n=1–2, were found to crystallize with the body-centered cubic structure. The method of synthesis used to prepare α-[(C4H9)4N]4H[PW11Fe(OH)O39] afforded the tetragonal phase. The conversion of the cubic into the tetragonal phase and the stability of each one are discussed.

Synthesis and structure of a novel microporous framework stannosilicate

The second example of a microporous framework stannosilicate with a known structure (AV-7) has been reported. The structure of AV-7, which is very similar to the structure of both the zirconosilicate mineral kostylevite and the previously reported microporous titanosilicate UND-1, has been studied by the Rietveld method and further characterised by bulk chemical analysis, powder X-ray diffraction, scanning electron microscopy, 23Na, 29Si and 119Sn magic-angle spinning NMR spectroscopy and thermogravimetry.

Ab initio structure determination of novel small-pore metal-silicates: knots-and-crosses structures

Abstract Sodium chloride stannosilicate AV-13 (Na2.26SnSi3O9Cl0.26·xH2O) and zirconium and hafnium analogues of this material have been prepared and their structures solved from powder X-ray diffraction data using direct methods, and 23Na, 29Si and 119Sn solid-state NMR. AV-13 materials are small-pore solids, probably more adequately described as tunnel structures. The AV-13 framework consists of corner-sharing MO6 (M=Sn, Zr, Hf) octahedra and SiO4 tetrahedra. The latter form six-membered [Si6O18]12− rings, which are interconnected by MO6 octahedra. The structure is better understood by considering a three-dimensional knots-and-crosses lattice. In a given layer, successive distorted-cube M8 cages contain [Na6−x(H2O)x](H2O,Cl−) octahedra (knots) and cyclohexasilicate (crosses) units. While the former are extra-framework species, the six-membered rings are, of course, part of the framework. The cages are accessed via seven-membered [M3Si4O27]26− windows, with free aperture ca. 2.3×3.2 A, one per each pseudo-cube face. Pilling up layers generates the structure, with knots-and-crosses alternating. The non-framework five-coordinated Na cations are disordered.

Synthesis of microporous titano-alumino-silicate ETAS-10 with different framework aluminum contents

Abstract Optimized synthesis conditions for obtaining highly crystalline microporous titano-alumino-silicate ETAS-10 materials with different framework aluminum contents (Al/Ti molar ratio 0.1–0.48) have been reported. Samples with low Al contents (Al/Ti ∼0.10) have been prepared using anatase and pseudoboehmite as, respectively, the Ti and Al sources. Good quality ETAS-10 materials with Al framework contents above 0.35 have been obtained using a modification of the synthesis procedure that involves drying and calcining the parent gel prior to the hydrothermal treatment. Changing the K and Na cation composition of the parent gel allows the fine-tuning of the framework Al content. It has been found that 29Si MAS NMR spectroscopy provides the best means of assessing the framework Al content of ETAS-10 materials.

Microporous titanosilicates Cu2+– and Co2+–ETS-4 for storage and slow release of therapeutic nitric oxide

Although nitric oxide (NO) is very toxic in low concentrations it plays a key role in the regulation of several biological systems of the human body. The potential of copper and cobalt ion-exchanged microporous titanosilicate ETS-4 for therapeutic storage and release of nitric oxide was assessed. Materials were characterized by powder X-ray diffraction, nitrogen adsorption at -196 °C and chemical analysis. Cation-exchanged materials showed an increased surface area and minor structural changes, relative to the parent Na-ETS-4. The NO storage and release kinetics was studied in the gas and liquid phases. NO loaded materials were also studied by diffuse reflectance infrared Fourier transform spectroscopy. The NO amounts released in the liquid phase were within the physiological range (nanomolar), while the release kinetics was slow (0.023 min-1 for 85% release). The NO loading and release from ETS-4 were affected by the presence of Cu2+ and Co2+ cations. Assays with HeLa cells indicated that the materials have low cytotoxicity, which depends on the ion-exchanged cation. The performance of ETS-4 improved when the extra-framework Na+ cations were exchanged by Cu2+ and Co2+.

Novel Microporous and Layered Luminescent Lanthanide Silicates

Recent developments in the field of luminescent lanthanide silicates have been reviewed. Combining in a single material microporosity and optical properties may lead to new types of sensing devices. Optically active centers (lanthanide ions or molecules) may be introduced in the micropores or/and embedded in the materials framework. Layered lanthanide silicates also offer the potential for host-guest chemistry, affording efficient luminescent materials and allowing finetuning of properties. Preliminary work indicates that these new materials may be prepared, by hydrothermal methods, in the form of powders and films.