Patricia G. Belelli

Methanol adsorption on magnesium oxide surface with defects: a DFT study

Abstract The methanol adsorption on several defects of the magnesium oxide surface were studied. Structural and electronic study with geometrical optimization and natural bond orbital (NBO) analysis were performed using a density functional theory (DFT) method. Oxygen and magnesium with different coordination numbers have very different reactivity in this surface producing dissociated and non-dissociated species. These results are in agreement with infrared spectroscopy observations where CH 3 OH, OCH 3 and OH species were found in defective MgO surfaces.

Proton abstraction ability of MgO: a DFT cluster model study of the role of surface geometry

Methanol and isocyanic acid adsorptions on a defective MgO surface have been studied. Equilibrium geometries, adsorption energies, atomic and molecular charges and electronic densities were obtained using a density functional theory method. Oxide surface atoms with different coordination numbers show very different reactivity giving both molecular and dissociated adsorbed species. The methanol molecule requires lower coordination numbers of the active site than the isocyanic acid molecule for dissociative adsorption. The role of the acidic and basic sites has been considered and analyzed in terms of natural bond orbital charges. The main vibration frequencies have been compared with available infrared spectroscopic data.

Theoretical and Spectroscopic UV Study of the EtInd2ZrCl2/MAO, EtInd2ZrCl2/MAO-AlCl3 and EtInd2ZrCl2/MAO-Ethyl Benzoate Interaction

This work presents a theoretical and experimental study of the active sites formation in the system EtInd 2 ZrCl 2 /MAO. Results about the effect of additives as a Lewis acid (AlCl 3 ) or a Lewis base (EB) are also analyzed. The theoretical study takes into account all the known steps in the active site formation: alkylation of zirconocene, formation of a polarized complex metallocene: MAO, formation of a contact ion pair and finally a solvent separated ion pair. Other reactions of the zirconocene with other aluminium alyls (AlCl 3 and AlMe 3 ) are also presented and studied. UV-vis experimental results of the additive effects are reported and explained taking into account the theoretical results. A short review of our results and others is presented to complete and discuss the ideas. The most probable sites are loosened contact ion pairs, not the solvent separated ion pairs.

Addition of Lewis bases and acids. Effect on α-olefins polymerization with soluble metallocenes, 1. Ethylene

In this paper the soluble system rac-Etnd 2 -ZrCl 2 /methylaluminoxane (MAO) was studied in propylene polymerization with additives. Lewis acid and base such as AlCl 3 and ethyl benzoate (EB) were used to evaluate their effects on polymerization kinetics, catalyst activities and polymer properties. The results obtained show a trend to improve the performance when AlCl 3 is added. The activity is decreased with EB, but the effect it not as strong a for ethylene (see Part 1). To further elucidate some of the different aspects of metallocene activation and reactivation in the reaction media, several reactions were carried out using different preactivation steps, with and without AlCl 3 . The preactivations were carried out taking account of the different results obtained with both additives (AlCl 3 and EB). The pretreatment with EB was not considered because the activity is decreased in the polymerization. AlCl 3 improves the activity when it is present during the preactivation step, but the presence of EB in the reactor during the polymerization step has a poisoning effect. These results show the importance of the preactivation step carried out without olefin. The molecular weight of polypropylene obtained remains almost unchanged in all the evaluated conditions. A complete discussion of ethylene and propylene results is included, taking account of Part 1 and 2.

DFT studies of zirconocene/MAO interaction

Abstract A theoretical study of Cp 2 ZrCl 2 /MAO system is presented based on Density Functional Theory (DFT) calculations performed with gaussian basis-sets. The MAO cocatalyst was modeled as a small finite section of the most probably species in solution. The geometry of the cation–counterion forming the ionic-pair active site was fully optimized and its electronic structure was examined in terms of the Natural Bond Orbital (NBO) population analysis. In this way the ionic-pair interaction can be related to the electronic charge changes around the zirconium cation. The results indicate the formation of a stable ionic-pair and the redistribution of charge in the cation complex and counterion. The latter was concluded by mapping the charge density difference. The Laplacian of charge density confirm the ionic interaction of the metallocene/MAO system.

Silica-supported metallocene for ethylene polymerization

Abstract We analyzed the effect of the addition of a Lewis acid (AlCl3) or a Lewis base (EB-ethyl benzoate) to the supported system rac-EtInd2ZrCl2/MAO/SiO2. We studied the additive effect in two different ways: as external and internal component. The results indicate that the additives are acting in different manners depending on the method of addition and the type of additive. An important result was obtained when the Lewis acid was added as an internal component. The molar ratio [MAO]:[AlCl3] of the co-impregnation step would be the most important factor to obtain an active supported catalyst.

Effect of Co‐ and Non‐Copolymerizable Lewis Bases in Propylene Polymerization with EtInd2ZrCl2/MAO

This paper reports the effect of co- and non-copolymerizable Lewis bases in propylene polymerization with the soluble system EtInd 2 ZrCl 2 /MAO. With non-copolymerizable Lewis bases, the activity and the molar mass of poly(propylene) are diminished. This result is unusual because there is agreement about the cationic nature of the active site in Zirconocene/MAO systems and the effect of a Lewis base must be to decrease the termination constant versus the propagation constant, considering the diminished electrophilicity of Zr. In the case of copolymerizable Lewis bases, the negative effect on activity is higher. An explanation of the data is given, with a new model of Lewis bases/cationic. Zr interaction, taking account of recent reports about the structure of active sites.

A theoretical and experimental study of the possible phenytriethoxysilane species found on treated silica

Abstract A modified extended Huckel molecular orbital (EHMO) method, which includes core–core repulsion terms, was used to study the silane adsorption on SiO 2 surfaces. The SiO 2 treated at 300°C was chemically modified with an alcoxysilane, phenyltriethoxysilane (PTES). Therefore, we study the possible adsorption of PTES on two planes (with isolated and geminal silanols) and the possible species formed once the reaction with a SiO 2 surface is finished. Starting from this analysis, it is possible to evaluate the PTES selectivity to the different anchoring points on the SiO 2 surface.

A THEORETICAL STUDY OF UNSATURATED OLEFIN HYDROGENATION AND ISOMERIZATION ON Pd(111)

The addition of hydrogen to the carbon–carbon double bond of 2-butenes adsorbed on Pd(111) was studied within the density functional theory (DFT) and using a periodic slab model. For that purpose, the Horiuti–Polanyi mechanisms for both complete hydrogenation and isomerization were considered. The hydrogenation of cis and trans-2-butene to produce butane proceeds via the formation of eclipsed and staggered-2-butyl intermediates, respectively. In both cases, a relatively high energy barrier to produce the half-hydrogenated intermediate makes the first hydrogen addition the slowest step of the reaction. The competitive production of trans-2-butene from cis-2-butene requires the conversion from the eclipsed-2-butyl to the staggered-2-butyl isomer. As the corresponding energy barrier is relatively small and because the first of these isomers is less stable than the second, an easy conversion is predicted.

MO studies of propylene adsorption on EtInd2ZrCH3+‖SiO2*MAO−

Abstract The system EtInd 2 ZrCH 3 + ‖SiO 2 *MAO − was studied theoretically by means of an extended Huckel method (EHMO) for the propylene adsorption. The adsorption of the different forms of olefins was evaluated on the active site formed by metallocene and silica, in presence or not of methylaluminoxane (MAO). In this way, it is possible to appreciate the important function of MAO when the active site is modeled considering all components. According to these results the presence of MAO avoids the adsorption of some forms of propylene.