Mercedes Boronat

Oxygen activation on gold nanoparticles: separating the influence of particle size, particle shape and support interaction

The adsorption and dissociation of molecular O(2) on extended gold surfaces, isolated gold nanoparticles of different size and shape, and small gold clusters supported on TiO(2) have been investigated by means of DFT calculations. The influence of particle size and morphology on the mode of adsorption and activation of O(2) have been analyzed separately, and it has been found that O(2) dissociation is very sensitive to the arrangement of the gold surface atoms. The control of this variable through catalyst preparation opens one way to improve the activity of gold catalysts.

Bifunctional Acid–Base Ionic Liquid Organocatalysts with a Controlled Distance Between Acid and Base Sites

Bifunctional acid-base ionic liquid organocatalysts with different distances between the two sites have been synthesised, and their activity for the Knoevenagel condensation has been tested. As has been found to be the case with enzymes, the distance between the acidic and basic sites determines the activity of the bifunctional organocatalyst, and at the optimal distance the reaction rate increases by two orders of magnitude with respect to the purely acidic or basic counterpart organocatalysts. The experimental results have been rationalised through the study of the reaction mechanism of the Knoevenagel condensation between malononitrile and benzaldehyde by means of DFT calculations. It has been found that it consists of two consecutive steps. First, deprotonation of malononitrile on the basic site to obtain a methylene carbanion intermediate takes place, and second, co-adsorption and activation of benzaldehyde on the acid centre of this intermediate followed by the C-C bond-formation reaction. The calculations and the kinetic study indicate that there is an inversion of the rate-controlling step when the distance between the acidic and the basic sites is modified, with a direct implication on the reaction rate.

On the activation of molecular hydrogen by gold: a theoretical approximation to the nature of potential active sites

The study of adsorption and dissociation of molecular hydrogen on single crystal Au(111) and Au(001) surfaces, monoatomic rows in an extended line defect and different Au nanoparticles by means of DF calculations allows us to firmly conclude that the necessary and sufficient condition for H2 dissociation is the existence of low coordinated Au atoms, regardless if they are in nanoparticles or at extended line defects.

The skeletal isomerization of but-1-ene catalyzed by theta-1 zeolite

The complete mechanism for the skeletal isomerization of but-1-ene catalyzed by theta-1 zeolite was investigated theoretically by means of DFT calculations, and the influence of the local geometry of the active site and of electrostatic effects on the geometries of the species involved and on activation and reaction energies were analyzed. It was found that whereas the geometry and stability of alkoxide intermediates are strongly influenced by the local geometry of the active site, the ionic transition states are mainly stabilized by electrostatic n effects. Both aspects result in a lowering of the activation energies of the elementary steps that form the mechanism and therefore in enhanced n reactivity compared with previous studies that used a general H3Si–OH–AlH2–O–SiH3 cluster.

Investigation on the Beckmann rearrangement reaction catalyzed by porous solids: MAS NMR and theoretical calculations.

In the last years, in situ solid-state NMR has been applied to investigate the Beckmann rearrangement of oximes into amides using zeolites and mesoporous materials of different structure containing Brønsted acids or silanol groups as active sites. DFT methods have been applied to model the geometry of the complexes resulting from adsorption of reactants, reaction intermediates and products on clusters representing the zeolite centers, and their (15)N and (13)C NMR chemical shift calculated theoretically. This article reviews the results reported in the bibliography on the Beckmann rearrangement of various oximes (acetophenone oxime, cyclohexanone oxime and cyclododecanone oxime) mainly using in situ(15)N NMR spectroscopy and theoretical calculations, and are compared with those obtained by in situ infrared spectroscopy. The combination of experiment and theory has been shown to be very useful for the interpretation of the NMR spectra and the identification of the species present at different reaction temperatures.

Cluster and periodic abinitio study of the ethane-ethene hydride transfer reaction catalyzed by acid chabazite. Is the cluster model able to describe accurately the host–guest interactions?

The ethane–ethene hydride transfer reaction on acid chabazite is used to test the performance of cluster calculations with respect to periodic ones for the study of bimolecular hydrocarbon reactions on acid zeolites. The following computational levels are considered: Hartree–Fock, MP2, aposteriori density functional theory (DFT) correlation using the PW91 functional, and several DFT hybrid B3PW91 procedures with different percentages of Hartree–Fock exchange. In all cases, a polarized basis set of double-zeta quality is used. Three periodic models with different numbers of acid sites and adsorbed substrates are explored to account for the effect of Si:Al ratio and coverage. Cluster and periodic calculations at different computational levels have been performed using the local geometries obtained from cluster constrained optimizations at the B3PW91 level. Our results indicate that the electrostatic field generated by the crystalline framework has only a relative influence on the stability of charged intermediates, that depends on the coverage and composition of the model, and also on the computational method used. It is noted in particular that the additional stability provided by the crystalline Madelung potential on the charged intermediate increases with the percentage of Hartree–Fock exchange in the DFT hybrid B3 scheme.