O. Novaro

Theoretical study on a reaction pathway of Ziegler–Natta‐type catalysis

Ab initio SCF–LCAO–MO all‐electron calculations have been performed for the catalyzed reaction, related to the Ziegler–Natta polymerization process TiCl4⋅Al(CH3)3+C2H4 →TiCl4⋅Al(CH3)2⋅C3H7. The energy of the Ti–Al–ethylene complex during the reaction pathway is associated to the following processes: First, the ethylene coordinates to the catalyst complex and subsequently it reacts with the methyl moiety producing an activation energy barrier mainly due to the breaking of the ethylene double bond. The computed barrier height (15 kcal/mole) is in reasonable agreement with experimental activation energies. The identification of the energy barrier is based on the analysis of the electron population, bond energies, and molecular orbitals of the system as they evolve during the reaction. A comparison with the qualitative picture resulting from the classical model of Cossee and with semiempirical calculations is presented. From these calculations a model is derived which leads to a tentative suggestion for the e...

Ab initio study of small gold clusters

Abstract The most stable geometries (lowest energy configurations) as well as the lowest-lying isomers of Aun, n=2–6, clusters are determined through ab initio calculations of the cluster total energy. It is found that Hartree–Fock plus second-order perturbation method of Moller–Plesset combined with a double-zeta basis set and an 11 valence electrons relativistic pseudopotential provide reliable results for a qualitative and quantitative analysis of the structures. Two-dimensional planar configurations are obtained as the lowest energy structures for all the gold cluster sizes investigated. Vibrational frequencies for all the investigated isomers were obtained.

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.

Preparation of sol-gel sulfated ZrO2SiO2 and characterization of its surface acidity

Abstract Zr Si mixed oxides containing 1.3 to 6.7 wt.-% Zr have been prepared by a sol-gel technique, using tetraethoxysilane and zirconium acetate as precursors. Zirconium is incorporated into the lattice of silica, but can easily be extracted either by self steaming or by acid treatment. The solids obtained in basic medium show a low surface area and low acidity. The samples prepared in acidic medium show large surface areas between 410 and 740 m 2 /g and can be converted to strong acids by contact with a solution of sulfuric acid. They retain pyridine up to 773 K and contain a total number of acid sites comparable to that of HY zeolites i.e. 1.7 meq/g. The catalytic activity for the bifunctional isomerization of n-hexane expressed per gram of zirconium is comparable for the sample containing 4wt.-% Zr and sulfated zirconia.

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

The importance of non-additive terms in the interaction energy of tetrahedral Be4 clusters

Abstract A study of three- and four-body interaction energies in the Be 3 (equilateral triangle) and Be 4 (tetrahedral) clusters at several internuclear distances is presented. The SCF LCAO MO method with a 9s2p gaussian basis set for each Be atom has been used. At this level of approximation it is concluded that the two- and four-body terms give a repulsive contribution to the interaction energy, whereas the three-body terms are attractive. Although the latter are not sufficiently strong to stabilize the Be 3 clusters they do imply a bound state for Be 4 in the region that is close to the nearest neighbour distance in solid beryllium.

A molecular orbital study of the hydration of ions. The role of nonadditive effects in the hydration shells around Mg2+ and Ca2+

The role of divalent cations in biology, in particular Ca2+ and Mg2+, has led to many studies of the coordination of these ions. The existence of empirical rules serves to characterize these properties, however, a detailed quantum mechanical analysis of the fundamental causes which are responsible for the nature of such properties is lacking. Here, we perform a molecular orbital study of the interactions involved in the hydration scheme of the ions looking particularly into the role of nonadditive effects. Notable differences are found for the cases of Mg2+ and Ca2+.

A comparative theoretical study of stable geometries and energetic properties of small silver clusters

Abstract The systematic quantum-mechanical investigation of the stable geometries and some energetic characteristics of neutral silver clusters up to the hexamer is performed by the all-electron spin density approach with non-local corrections included. We compare these results with experimental data and with previous calculations on anionic silver clusters. We find that for the hexamer the ground-state geometry of the neutral cluster differs from the geometry of the anionic. For the neutral silver clusters Ag n the transition from a 2D conformation to a 3D occurs at n > 6. The size effects for the atomic fragmentation energy yield opposite alternations for neutral and anionic clusters.

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.

Mechanism of oligomerization of α-olefins with Ziegler-Natta catalysts

Abstract A mechanism for the oligomerization of ethylene with homogeneous titanium-aluminum catalysts is proposed. The mechanism is established by a self-consistent all-valence electron molecular orbital calculation. It is shown that the original catalyst has a trigonal-bipyramidal structure that changes to an octahedral structure only after the monomer is coordinated. The reaction pathways are studied, showing that both the chain propagation and the β-transfer responsible for the α-alkene liberation are favored decisively by the Ti d-orbitals. This implies that chain growth can occur without any radical breaking and justifies the low activation energy of the process. Finally these results are related to experimental chemical data such as yields and rate constants and to physical measurements on the catalytic complex.