Xim Bokhimi

Correlation between the magnetism of non-stoichiometric zinc ferrites and their catalytic activity for oxidative dehydrogenation of 1-butene

Abstract Non-stoichiometric zinc ferrites, which were used as catalysts for the oxidative dehydrogentation of 1-butene, were synthesized by using co-precipitation and hydrothermal methods. Their atom local order was analyzed with Mossbauer spectroscopy and the refinement of the crystalline structures; their macroscopic magnetization was measured by the vibrating sample method. Their magnetic and catalytic properties depended on the synthesis conditions; the analysis of the local atom distribution shows that these properties were related to the occupation of the tetrahedral sites with iron ions. This occupation was larger when the ferrites were prepared under hydrothermal conditions than by co-precipitation. The parallelism between the macroscopic magnetization of the ferrites and their capacity of transforming 1-butene into butadiene, CO 2 and 2-butene, by an oxidative dehydrogenation reaction, suggests that the “freezing” of the magnetic moments in the octahedral sites could cause this catalytic behavior.

Characterizations of the thermal decomposition of brucite prepared by sol–gel technique for synthesis of nanocrystalline MgO

Abstract Brucite was synthesized by the sol–gel technique using magnesium diethoxide in a homogeneous reaction medium. Its decomposition, including dehydroxylation, crystallization and phase transformation was studied by XRD, TGA–DTG, CTEM, HRTEM, SAED and FTIR techniques. It has been proposed that during the dehydroxylation and decomposition of the magnesium diethoxide gel, some vacancies are generated. The decomposition of brucite with hexagonal structure to form periclase with the cubic structure occurs in the temperature range from 473 to 773 K. In this case, nanocrystalline MgO is formed from the break down of the brucite crystalline structure.

Effects of structural defects and acid-basic properties on the activity and selectivity of isopropanol decomposition on nanocrystallite sol-gel alumina catalyst

The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transform infrared FTIR spectroscopy of pyridine adsorption and temperature-programmed desorption of CO and NH . The number of acid and 23 basic sites on the samples varied with the calcination temperatures of the samples. The populations of the three different aluminum ions—tetrahedral, pentacoordinated and octahedral, which were identified by the 27 Al MAS NMR, were strongly affected by the sample calcination temperature and the crystalline composition. In the reaction temperature range between 100 and 2508C, isopropanol decomposition on sol-gel alumina catalysts was carried out. It was found that isopropanol decomposition on alumina catalyst was a structural-defect sensitive reaction. The dehydrogenation selectivity to acetone depended on the surface basic sites and the concentration of aluminum vacancies in the crystalline structure of g-Al O. 23 Bimolecular reaction to isopropylether was largely governed by the pentacoordinated aluminum ions which were related to the coordinately unsaturated aluminum ions. A mechanism for the formation of isopropylether was proposed: oxygen . q vacancies were suggested to involve the adsorption step of isopropanol, an intermediate species, CH HC , reacted with 32

Local order in titania polymorphs

Abstract Crystalline structure local order of titania polymorphs, brookite, anatase and rutile is described with a representative octahedron for each phase. The detailed analysis of the atomic bond lengths forming these octahedra helps to understand the macroscopic properties observed in these materials. In rutile samples with an average crystal size between 9.77(9) and 149(2) nm , these lengths did not depend on the crystal size. This behavior is not necessarily similar for anatase and brookite, which have crystalline structures with lower symmetry. The dependence of anatase on rutile in synthesis conditions suggests that atom bond lengths in this titanium polymorph depend on crystal size. Brookite is stabilized with copper and chloride ions. Anatase open crystalline structure can dissolve cations and anions that stabilize it at high temperatures, or can be used as precursors for preparing titanates at low temperatures. Rutile, which is normally obtained after annealing brookite and anatase at high temperatures, can be synthesized at low temperatures in the presence of platinum or tin ions during synthesis or by using TiCl3 as precursor.

Synthesis, characterization and catalytic properties of Pt/CeO2–Al2O3 and Pt/La2O3–Al2O3 sol–gel derived catalysts

Platinum supported on La 2 O 3 -Al 2 O 3 and CeO 2 -Al 2 O 3 were prepared by the sol-gel method. High specific surface areas between 572-418 m 2 /g were obtained. FTIR-pyridine absorption band proved the formation of Lewis sites with comparable intensity in the supports. The stabilization of the γ-Al 2 O 3 in the La 2 O 3 -Al 2 O 3 support was observed by X-ray diffraction when the sample was calcined at 1100 C, while segregation of CeO 2 was observed at such temperature in the CeO 2 -Al 2 O 3 preparation. The formation of Al V and Al VI coordinated aluminum was observed by MAS-NMR 27 Al spectra in La doped-alumina. In the Ce doped-sample only Al VI coordination was identified. Propene and acetone were the products in the iso-propanol decomposition over CeO 2 -Al 2 O 3 , meanwhile, for the La 2 O 3 -Al 2 O 3 support only propene was obtained. The platinum (0.5 wt.%) particle size distribution as determined by electron microscopy showed very small particles <1.0 nm. On the other hand, in the n-heptane dehydrocyclization the activity and selectivity to toluene of the Pt/La 2 O 3 -Al 2 O 3 catalyst was found bigger than those of Pt/CeO 2 -Al 2 O 3 catalyst. It is proposed that doped-alumina prepared by the sol-gel method has structural defects which induce modification in the metal activity.

Crystallography and crystallite morphology of rutile synthesized at low temperature

Abstract Rutile was prepared at low temperature with titanium trichloride in an acid- and oxygen-rich environment. Samples were characterized with X-ray powder diffraction, transmission electron microscopy, and thermogravimetry; rutiles crystalline structure was refined with the Rietveld method. Crystallites were needles with their largest dimension along c lattice parameter; they grew in bundles parallel to their length axis, and contained hydroxyls in their lattice and on their surface. When samples were annealed, dehydroxylation generated lattice defects that produced microstrains and created conditions for crystallite sintering at temperatures above 300°C. Titanium–oxygen interaction in rutile depended on the annealing temperature, which gave rise to changes in the symmetry of the representative rutile titanium–oxygen octahedron. The atomic bond length between the oxygen atoms shared by adjacent octahedra decreased with the temperature, contracting the lattice. All this was caused by the variation of the number of hydroxyls in the lattice.

Effects of the surface structure and experimental parameters on the isopropanol decomposition catalyzed with sol–gel MgO

Abstract Isopropanol decomposition was used as a probe reaction for characterizing the surface properties of sol–gel MgO catalysts. The specific surface area and the pore size distribution of the samples, calcined at different temperatures, were measured with the BET method. Acidity and basicity of the catalyst were determined by using TPD-NH3 and TPD-CO2 techniques. The total conversion and acetone selectivity depended on the experimental conditions. Increasing reaction temperature significantly improved the total conversion, but reduced acetone selectivity. The activity and acetone selectivity obtained at the initial reaction were higher than that obtained after 15 min and 30 min of reaction. When water was added in the reaction stream, hydroxyl groups were formed on the MgO surface, a remarkable enhancement of the conversion and of the acetone selectivity was observed. The structural defects and the specific surface area of the catalyst are also related to the activity and selectivity of the isopropanol decomposition. Two mechanisms were suggested for the explanation of acetone formation: in the case of water absent in the fed stream, the acetone was produced by the dehydrogenation route; in the case of water present in the reaction mixture, acetone was produced by the pathway of dehydrogenation accompanying the dehydration.

Effect of pH on the incorporation of platinum into the lattice of sol–gel titania phases

Abstract Titania and platinum/titania catalysts were prepared by co-gelling titanium ethoxide and platinum acetylacetonate in acid or basic media. For the acid conditions, platinum promoted the formation of rutile; but, when the medium was basic, the samples were a mixture of anatase and rutile. Platinum also affected the titania energy band-gap. The reactivity, for methylcyclohexane dehydrogenation, of the samples prepared in a basic environment, was larger than that for those prepared in an acid medium. The highest effect and the most important modification of the TiO2 energy band-gap occurred for pH 3, which suggests a large incorporation of platinum into the lattice of titania phases.

Dehydroxylation and the Crystalline Phases in Sol-Gel Zirconia

The formation and evolution with temperature of the crystalline phases in sol-gel ZrO2 was analyzed by using X-ray powder diffraction, refinement of the crystalline structures, ESR, and UV-Vis spectroscopy. The precursor phase of crystalline zirconia was amorphous Zr(OH)4 with the same local order as the tetragonal crystalline phase. This amorphous phase dehydroxylated with temperature, generating nanocrystalline tetragonal zirconia, and producing point defects that stabilized the tetragonal structure, generated a paramagetic ESR signal with g values like the free electron, and had a light absorption band at 310 nm. When the sample was annealed at higher temperatures, it continued dehydroxilating, and the point defects disappeared, causing the transformation of the nanocrystalline tetragonal phase into nanocrystalline monoclinic zirconia. The two crystalline nanophases coexisted since the beginning of crystallization.

Cortisol controlled release by mesoporous silica

In this study four types of SBAs were synthesized and then impregnated with hydrocortisone. One is a straight SBA-15, obtained using Pluronics P123 as structuring agent; two others were modified using 1,3,5-trimethylbenzene as additive, and the fourth one was prepared using sodium iodide as additive. Three of these have in common a p6 mm symmetry with nanotubes packed hexagonally, yet they differ in their functional groups. The fourth sample is basically disordered. The drug release kinetics showed two stages: a fast-rate early stage dominated by the controlled release of the hydrocortisone adsorbed in the macropores of the reservoir, followed by a slow-rate delivery that we assume is controlled by the hydrocortisone diffusion through the nanopores. It is shown that the release kinetics can be strongly influenced by using different co-additives.