Zbigniew Olejniczak

Novel “ship-in-the-bottle” type catalyst: evidence for encapsulation of 12-tungstophosphoric acid in the supercage of synthetic faujasite

A new type of “ship-in-the-bottle” catalyst has been obtained by encapsulation of heteropolyacid H3PW12O40 in the supercage of faujasite. As shown by XRD and IR, the structure of zeolite Y after encapsulation is retained.31P MAS NMR reveals that the signal at −14.4 ppm, assigned to the Keggin unit, is in the case of the encapsulated catalyst much broader than the line observed for a sample containing the heteropolyacid adsorbed on the zeolite. The broadening is due to interaction between phosphorus and aluminium from zeolite. Highly active catalysts are formed by encapsulation, as exemplified here by strongly increased activity and modified selectivity in isomerization and disproportionation ofm-xylene.

Immobilization of dodecatungstophosphoric acid on dealuminated zeolite Y: a physicochemical study

Abstract 12-Tungstophosphoric acid (PW 12 ) has been supported on dealuminated zeolite Y. The catalysts were studied by XRD, DTA/DTG, IR, argon adsorption and MAS NMR spectroscopy. The structure of dealuminated faujasite is retained upon contact with a very strong PW 12 acid, which becomes thermally stabilized. Two types of Keggin anions exist at the surface: those strongly interacting with the OH groups of the support, prevailing at low coverages, and those weakly interacting with the zeolite, prevailing at high coverages. Transformations of m -xylene proceed along three pathways: isomerization, disproportionation and dealkylation. Pure zeolite shows high selectivity to isomerization. Deposition of PW 12 acid dramatically increases the selectivity of disproportionation to toluene and trimethylbenzenes, which rises with the growing acid loading.

Oxidative dehydrogenation of isobutane on MCM-41 mesoporous molecular sieves

Abstract MCM-41 mesoporous molecular sieves containing silicon and vanadium have been prepared by a direct hydrothermal route. Aluminium-containing [Si,Al]-MCM-41 and purely siliceous [Si]-MCM-41 solids were synthesised for comparison purposes. 51V static and magic-angle-spinning NMR studies of [Si,V]-MCM-41 showed one type of VO4 in the as-prepared samples (site I) and two types of monodispersed and distorted to various extent VO4 tetrahedra, chemically bound to the walls of MCM-41 (sites IVa and IVb), in the calcined and rehydrated samples. NMR parameters of vanadium units are given. The catalytic performance of MCM-41 materials was probed in the oxidative dehydrogenation (ODH) of isobutane. The highest selectivities to isobutene (48–59%) were observed for the [Si,V]-MCM-41 materials with Si/V=85 and 30, respectively. On the contrary, a sample containing V2O5 bulk-like species exhibited much lower selectivity to isobutene, especially at the isobutane conversion higher than 5%. The presence of isolated vanadium species, as found by 51V NMR, was responsible for the high olefin selectivity in the ODH of isobutane.

SiOC glasses produced from silsesquioxanes by the aerosol-assisted vapor synthesis method

In this paper, we describe a new method based on aerosol-assisted vapor synthesis for making glass materials by pyrolysis of readily available silsesquioxanes CH3Si(OCH3)3 and CH3Si(OC2H5)3. Combined powder X-ray diffraction (XRD) and spectroscopic studies in the far infrared region (FIR) showed that under applied conditions the method yielded amorphous materials. Subsequent structural studies with the application of the (29)Si and (13)C MAS NMR, Raman, and middle infrared (MIR) techniques led to the conclusion that the pyrolysis of the silsesquioxane precursors resulted in glass materials with a structure of amorphous silica v-SiO2. In the case of certain glasses prepared from CH3Si(OCH3)3, they were also shown to possess in the structure some Si-C bonds (black glasses), whereas those originated from CH3Si(OC2H5)3 were composites that in addition to the silica glass phase contained domains of free/unbound carbon.

XRD and 29Si MAS NMR spectroscopic studies of carbon materials obtained from pyrolyses of a coal tar pitch modified with various silicon-bearing additives

A typical coal tar pitch used in manufacturing of graphite electrodes was individually modified with various silicon-bearing additives yielding mixtures that were pyrolyzed (carbonized) stepwise at increased temperatures up to 1650 °C. Three commercially available additives such as the elemental Si, silica SiO2, and poly(carbosilane) {–CH2–SiH(CH3)–}n as well as reference silicon carbide SiC were used. The resulting intermediate and final materials were studied with the XRD and 29Si MAS NMR methods. For two additives, i.e., elemental Si and poly(carbosilane), the beginning of their conversion to SiC was detected at 1300 °C while for SiO2 higher temperatures were required under applied conditions.

Heterogenization of 12-tungstophosphoric acid on stabilized zeolite Y

Abstract12-tungstophosphoric acid (PW12) has been supported on dealuminated zeolite Y, containing a secondary pore system with the predominant pore radii of 15 Å. The interaction between the heteropolyacid and the zeolite matrix has been studied using 29Si, 27Al and 31P MAS NMR. Distribution of two types of the Keggin anions, strongly and weakly interacting with the support, has been estimated quantitatively. Increased acidic properties of the PW12/zeolite Y system were demonstrated by a change in the selectivity pattern of m-xylene transformations in the gas phase (significantly increased dealkylation and transalkylation paths at the expense of isomerization). Redox properties were evaluated in oxidation reactions with H2O2 in the liquid phase. It has been shown that the PW12/zeolite Y catalysts can selectively oxidize a number of organic compounds at relatively low temperatures.

Silicon Carbide Modified Carbon Materials. Formation of Nanocrystalline SiC from Thermochemical Processes in the System Coal Tar Pitch/Poly(carbosilane)

Poly(carbosilane) or PCS, {–CH2–SiH(CH3)–}n, is used as a Si-bearing precursor in combination with a coal tar pitch to study thermally induced transformations toward SiC-modified carbon composites. Following mixing of the components in the molten pitch at 160°C, the mixture is heated under argon atmosphere at 500°C yielding a solid carbonizate that is further subjected to separate pyrolysis experiments at 1300°C or 1650°C. At temperatures up to 500°C, the PCS reacts with suitable pitch components as well as undergoing decomposition reactions. At higher temperatures, clusters of prevailingly nanocrystalline β-SiC are confirmed after the 1650°C pyrolysis step with indications that the formation of the compound starts at 1300°C. 29Si MAS NMR, XRD, FT-IR, XPS, and elemental analysis are used to characterize each pyrolysis step, especially, from the viewpoint of transformation of silicon species to silicon carbide in the carbon matrix evolved from the pitch.

Gallium oxide promoted zeolite catalysts for oxidehydrogenation of propane

Abstract Novel gallium-containing catalysts for oxidehydrogenation of propane, based on zeolite Beta, ZSM-5 and ferrierite, have been prepared and characterised by scanning electron microscopy, IR, MAS NMR and Raman spectroscopies. The catalytic properties of zeolitic matrixes with B, Al, and both ions at tetrahedral sites have been studied. Transformation of propane on pure zeolites and promoted with gallium (III) oxide depended on the structure of the matrix, its morphology and the type of cations occupying zeolite framework sites. Formation of new hydroxyl groups has been evidenced for some MFI zeolites promoted with Ga 2 O 3 .

Sol–gel synthesis of iron oxide–silica composite microstructures

Spherical silica particles doped with iron oxide have been synthesized via base-catalyzed one-pot sol–gel process using tetraethoxysilane (TEOS) and iron(III) ethoxide (ITE) as co-precursors. In the modified Stöber process adopted, depending on the concentration of ITE in the starting composition, materials of various morphologies were observed under a scanning electron microscope and an atomic force microscope. The presence of ITE significantly affected the formation process of particulate silica; the spherical particles were formed accompanied by the co-presence of irregular-shaped finer aggregates. The fraction of the aggregates with rough surfaces increased with an increase of the ITE content in the reaction mixture. Both the spherical particles and irregular-shaped aggregates contained iron hydroxide and they exhibited paramagnetic behavior. The chemical composition and physicochemical properties of the materials were determined using various complementary spectroscopic methods.

“Silicified” pyrochlore from nepheline syenite (mariupolite) of the Mariupol Massif, SE Ukraine: A new insight into the role of silicon in the pyrochlore structure

Abstract Pyrochlore-supergroup minerals containing relatively high Si concentration are quite common in various geochemical parageneses, e.g., carbonatites, alkaline syenites, pegmatites. However, the role of Si and the mechanism of its incorporation into the structure of these minerals, although widely discussed, have not been explained definitively. Our paper reports the results of comprehensive SEM, EPMA, XRD, TEM, and MAS-NMR studies performed for the first time on a natural pyrochlore, which is the late-magmatic to early hydrothermal accessory component of the nepheline syenite in the alkaline Mariupol massif in Ukraine. It represents partly metamict, patchy-zoned, A-cation depleted, REE-, U-, and Th-bearing fluornatropyrochlore, locally exceptionally rich in SiO2 (up to 13.01 wt%) that underwent both primary and secondary alterations, leading to kenopyrochlore or hydropyrochlore species. The primary alteration was induced by high-temperature, Ca2+- and Si4+-rich, and F- moderate fluids, which affected only some domains of the pyrochlore crystals and resulted in filling the A site vacancies mainly by Ca2+, but also Mn2+, Sr2+, and K+. The secondary alteration, induced by the exposure of the host rock to ground water driving fluid-mediated coupled dissolution-reprecipitation process, affected the whole pyrochlore crystals (both Si-enriched and Si-free domains) and caused, among others, the leaching of some A- and Y-site components. TEM investigations indicate that the selected-area electron diffraction patterns taken from Si-poor areas show strong and sharp diffraction spots related to well-crystalline pyrochlore, whereas the Si-rich areas show weaker spots with a diffuse diffraction halo that are typical of metamict material. Due to the fact that no intergrowth with other Si-bearing phases was observed in the TEM images even at very high magnification, it might be concluded that Si4+ can occupy severely a-decay damaged and chemically altered portions of this structure. The absence of Si in the sixfold-coordinated B site has been corroborated both by compositional relationships, and by the lack of any [6]Si4+ signal around -200 ppm in the MAS-NMR spectrum. A broad signal in the spectrum appearing at around -84 ppm, points to an amorphous species with tetrahedrally coordinated Si, close to Q(2) species defined as Si atom with two bridging O atoms, i.e., [Si(OSi)2(-)2], in the form of finite-length chain-like structures, located in the damaged A and B sites of the primary structure.