M.E. Llanos

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

Effect of sulfation methods on TiO2–SiO2 sol–gel catalyst acidity

A series of x TiO2‐SiO2 (xD1.7, 4.0 and 6.7 wt.% TiO2) binary mixed oxides was prepared by a sol‐gel method in acidic and basic conditions. In order to upgrade the acidic properties of the obtained solids, their sulfation was carried out in three different ways: (1) by in situ (H2SO4 was admixed with the gel), (2) impregnating the dried solids with aqueous solutions of H2SO4, and (3) by impregnating as in (2) but with (NH4)2SO4. FTIR-pyridine adsorption and benzene deuteroexchange showed that the acid site strength depends on the sulfation method. The highest values of total acidity, determined by ammonia thermodesorption, correspond to the in situ sulfated samples. The obtained pore size distribution (4.0‐5.0 nm) was narrow and the specific BET areas (250‐350 m 2 /g) were large. X-Ray diffraction, RDF, XPS spectroscopy and catalytic activity in the 2-butanol dehydration show that one step sulfated in situ TiO2‐SiO2 sol‐gel preparations resulted in catalysts with homogeneous and strong acidity.

Acidic–base properties of silica–magnesia sol–gel mixed oxides: use of 2 butanol as test reaction

Abstract 2-butanol dehydration was performed as a test reaction for acid–base characterization of a series of unsulfated and sulfated silica–magnesia sol–gel mixed oxides covering the 1.3–6.7% MgO composition range. The dehydration of 2-butanol over non-sulfated silica–magnesia yields a mixture of butenes in which the fraction of 2-butene (Saytzeff orientation) is 33%. High selectivity for 1-butene (Hofmann orientation) is indicative of the E1cB mechanism over sulfated silica–magnesia oxides. The acid–base properties were also investigated by ammonia and carbon dioxide TPD and FTIR spectroscopy of adsorbed pyridine. Acid–base properties are discussed as a function of magnesium content (nature, strength and amount of sites). A very good correlation between the results of test reactions and acid–base characterization methods is observed.

Preparation of a complete series of single phase homogeneous sol-gels of Al2O3 and MgO for basic catalysts

Abstract Homogeneous single phase sol-gels of magnesia and alumina were prepared using magnesium and aluminum alkoxides as precursors. The gels were formed through a hydrolysis reactions followed by condensation. These materials behave as basic catalysts and, therefore, are superb anionic interchangers. The solids obtained were characterized by infrared spectroscopy, thermogravimetric analysis and anionic interchange tests. No evidence for the presence of the individual binary oxides was found. An important effect on the dehydroxylation of the surface and on the anionic exchange was found as a function of alumina content in the magnesia matrix. The best sample was the one containing 50% MgO-50% Al2O3.

On the surface basic properties of sulfated magnesia–silica sol–gel mixed oxides

Abstract The morphology and surface base properties of a family of sulfated MgO–SiO 2 mixed oxides prepared by the sol–gel method were studied. Morphological features of the sulfated system are found to vary with preparative conditions. Electron dispersive spectroscopy (EDS) analysis of the samples show a very high homogeneous distribution of sulfur and magnesium on the surface. It was found by CO 2 -Thermal programmed desorption (CO 2 -TPD) study that the basic site number, strength and density depend on the relative concentration of magnesium and sulfation. Fourier Transform Infrared (FTIR) spectroscopy of CDCl 3 yield complementary information on the nature and heterogeneity of surface basic centers. Aldolic condensation of acetone was performed as a test reaction for basicity characterization.

Sol–gel copper–magnesia–silica mixed oxides

Sol-gel magnesia-silica mixed oxides were prepared with tetraethoxysilane TEOS and magnesium ethoxide as molecular precursors. While still at the fluid phase before gelation, a solution of copper acetylacetonate was added. After gelation, the solids thus obtained were sulfated in two ways: by conventional impregnation and by an in situ impregnation . with H SO . The solids were characterized by TPD NH and CO . The solids impregnated in situ, containing 4.0 and 6.7 24 3 2 mol% of MgO show a high acidity. The mixed oxides showed high specific surface area values when the gelation is carried out at pH 3; however, at pH 9, areas three times smaller were obtained. Their catalytic activity was studied using the decomposition of 2-propanol as a test reaction. q 2000 Elsevier Science B.V. All rights reserved. ¨ 22 sites on mixed oxides; 2-Propanol decomposition