PubMed Health⌕ Search

Biomedical subjects

A Corma

Publications and source records attributed to A Corma.

18 recordsLinked to original sources
PubMed2006

Single-site homogeneous and heterogeneized gold(III) hydrogenation catalysts: mechanistic implications.

Au(III)-Schiff base complexes are active hydrogenation catalysts, giving turnover frequencies similar to those of the corresponding complexes of Pd(II), which has the same d8 electronic structure as Au(III). The mechanism of the reaction has been studied in detail by a combination of kinetic experiments and theoretical calculations. It is predicted and tested that the nature of the solvent plays a critical role for the heterolytic cleavage of H2 (controlling step). Taking this into account, and by properly selecting the nature of solid supports (polarity and proton-donating ability), it was possible to strongly increase the activity of the homogeneous Au(III) and Pd(II) catalysts by grafting them onto the surface.

Journal Article↗
PubMed2005

Surface characterization and properties of ordered arrays of CeO2 nanoparticles embedded in thin layers of SiO2.

We demonstrated the surface composite character down to the nanometer scale of SiO(2)-CeO(2) composite high surface area materials, prepared using 5 nm colloidal CeO(2) nanoparticle building blocks. These materials are made of a homogeneous distribution of CeO(2) nanoparticles in thin layers of SiO(2), arranged in a hexagonal symmetry as shown by small-angle X-ray scattering and transmission electron microscopy. Since the preparation route of these composite materials was selected in order to produce SiO(2) wall thickness in the range of the CeO(2) nanoparticle diameter, these materials display surface nanorugosity as shown by inverse chromatography. Accessibility through the porous volume to the functional CeO(2) nanoparticle surfaces was evidenced through an organic acid chemisorption technique allowing quantitative determination of CeO(2) surface ratio. This surface composite nanostructure down to the nanometer scale does not affect the fundamental properties of the functional CeO(2) nanodomains, such as their oxygen storage capacity, but modifies the acid-base properties of the CeO(2) surface nanodomains as evidenced by Fourier transform IR technique. These arrays of accessible CeO(2) nanoparticles displaying high surface area and high thermal stability, along with the possibility of tuning their acid base properties, will exhibit potentialities for catalysis, sensors, etc.

Journal Article↗
PubMed2004

Reaction intermediates in acid catalysis by zeolites: prediction of the relative tendency to form alkoxides or carbocations as a function of hydrocarbon nature and active site structure.

The mechanism of protonation of ethene, propene, and isobutylene adsorbed on seven different Brønsted acid sites of mordenite has been studied at the ONIOM (B3PW91/6-31G(d,p):MNDO) theoretical level to assess the influence of olefin size and local geometry of the active site on the species and energies involved. The activation energies for olefin protonation are determined by short- and medium-range electrostatic effects and reflect the order of stability of primary, secondary, and tertiary carbenium ions. On the other hand, the stability of covalent alkoxides depends linearly on the AlO(b)Si angle value in the complex, which in turn is determined by the corresponding value in the deprotonated zeolite. It is also shown that the mechanism of protonation of isobutylene is different from that of ethene and propene and involves a free tert-butyl carbenium ion as a true reaction intermediate. Whether this carbenium ion is converted into a covalent alkoxide depends on the T position on which the Al is located. All these findings allow us to predict, on the basis of the position and local geometry of the Brønsted acid site, whether the reaction intermediates of olefin protonation will be covalent alkoxides or free carbenium ions.

Journal Article↗
PubMed2003

Synthesis of ITQ-21 in OH- media.

ITQ-21 has been synthesized from F- free gels; aluminum can be incorporated and this does not affect the rate of nucleation. The introduction of stoichiometric amounts of NH4F with respect to the number of double four rings (D4R) present in the structure leads to an increase in the nucleation and crystallization rate with a quantitative incorporation of fluoride ions into the solid.

Journal Article↗
PubMed2001

Characterization of germanium site distribution in zeolite ITQ-7 by photoluminescence.

Zeolite ITQ-7 containing germanium emits luminescence upon excitation at the wavelength of the absorption maxima; control experiments with amorphous GeO2 and all-silica zeolites indicate that the emission is attributable to Ge atoms occupying framework positions; the emission decays on the nanosecond time scale and it fits to variable proportions of three exponential kinetics, this being compatible with the presence of three families of Ge atoms in the solid.

Journal Article↗
PubMed2001

ITQ-16, a new zeolite family of the beta group with different proportions of polymorphs A, B and C.

ITQ-16 is a new zeolite family formed by polymorphs A, B and C of the beta intergrowth in which the proportion of polymorph C can be controlled by changing the organic structure directing agent and/or by changing the Ge content of the synthesis gel; ITQ-16 can be synthesised either in fluoride or fluoride-free medium and Al can be introduced in the framework giving a material with very strong Brönsted acidity.

Journal Article↗
PubMed2001

Sn-zeolite beta as a heterogeneous chemoselective catalyst for Baeyer-Villiger oxidations.

The Baeyer-Villiger oxidation, first reported more than 100 years ago, has evolved into a versatile reaction widely used to convert ketones-readily available building blocks in organic chemistry-into more complex and valuable esters and lactones. Catalytic versions of the Baeyer-Villiger oxidation are particularly attractive for practical applications, because catalytic transformations simplify processing conditions while minimizing reactant use as well as waste production. Further benefits are expected from replacing peracids, the traditionally used oxidant, by cheaper and less polluting hydrogen peroxide. Dissolved platinum complexes and solid acids, such as zeolites or sulphonated resins, efficiently activate ketone oxidation by hydrogen peroxide. But these catalysts lack sufficient selectivity for the desired product if the starting material contains functional groups other than the ketone group; they perform especially poorly in the presence of carbon-carbon double bonds. Here we show that upon incorporation of 1.6 weight per cent tin into its framework, zeolite beta acts as an efficient and stable heterogeneous catalyst for the Baeyer-Villiger oxidation of saturated as well as unsaturated ketones by hydrogen peroxide, with the desired lactones forming more than 98% of the reaction products. We ascribe this high selectivity to direct activation of the ketone group, whereas other catalysts first activate hydrogen peroxide, which can then interact with the ketone group as well as other functional groups.

Adamantane↗
PubMed2001

Cluster and periodic calculations of the ethene protonation reaction catalyzed by theta-1 zeolite: influence of method, model size, and structural constraints.

The protonation of ethene by three different acid sites of theta-1 zeolite was theoretically studied to analyze the extent and relevance of the following aspects of heterogeneous catalysis: the local geometry of the Brønsted acid site in a particular zeolite, the size of the cluster used to model the catalyst, the degree of geometry relaxation around the active site, and the effects related to medium- and long-range interactions between the reaction site and its environment. It has been found that while the reaction energy is very sensitive to the local geometry of the site, the activation energy is mainly affected by the methodology used and by electrostatic effects on account of the carbocationic nature of the transition state.

Journal Article↗
PubMed2001

Zeolites as pheromone dispensers.

Modification of the chemical and structural properties of zeolites has led to the preparation of an optimized zeolitic dispenser for insect attractants such as n-decanol and trimedlure. The impact of zeolite variables such as the framework Si/Al ratio, nature of compensating cation, nature and strength of acid groups, and pore dimensions on the kinetics of emission has been studied, and the results are as follows. Zeolite pore dimensions and the presence or absence of acid sites have the greatest effect on the rate of release, decreasing with decreasing pore diameter and increasing acidity. Further tuning of the release characteristics is achieved by controlling the polarity and the polarizability of the framework by increasing the Si/Al ratio and nature of the compensating cations, i.e., the higher the polarizability and the lower the polarity, the slower the release of attractants.

Adsorption↗
PubMed1996

2H-->1H cross-polarization in deuterated glycine.

1H spectral details and CP kinetics are discussed. We recommend perdeuterated polycrystalline glycine as a standard for setting 2H-->1H cross-polarization (CP), for 1H chemical shift referencing and for checking the setting of the magic angle and the shims.

Deuterium↗
PubMed1994

27Al-->1H cross-polarization in aluminosilicates.

Solid-state nuclear magnetic resonance (NMR) cross-polarization (CP) from 27Al to 1H was set on kaolinite, verified by a variable-contact time experiment and applied to ultrastable zeolite Y. The technique is useful for the selective NMR observation of AlOH sites in aluminosilicates, especially those from extraframework species in zeolites.

Aluminum↗
PubMed1993

Solid-state NMR study of ordered mesoporous aluminosilicate MCM-41 synthesized on a liquid-crystal template.

A novel mesoporous aluminosilicate MCM-41, synthesized in a liquid-crystal system, was characterized by magic angle spinning NMR. Conventional Bloch decay and cross-polarization spectra were compared. The 27Al spectra showed that the calcined material was highly siliceous, while 29Si NMR proved that it was rich in single and geminal silanol groups. Two kinds of the silanol groups were detected: exchanging and non-exchanging protons with water on the timescale of the 1H experiment. The 13C spectra of the uncalcined material were consistent with the concept of the micellar arrangement of the cetyltrimethylammonium template inside MCM-41.

Aluminum Silicates↗
PubMed1993

Solid-state NMR study of ultrastable zeolite Y modified with orthophosphoric acid.

29Si, 27Al and 31P solid-state NMR under magic-angle spinning was used for the study of ultrastable zeolite Y modified with H3PO4. The material is a potential candidate for a new generation of oil-cracking catalysts. Bloch decay and cross-polarization spectra were compared. It was found that amorphous aluminium phosphate is formed during the P-impregnation and the following activation, and that it occupies the zeolite surface and internal voids. For a high P-content under steaming conditions, aluminium phosphate is converted into a crystalline form. No evidence has been found for incorporation of P atoms into the zeolite framework.

Aluminum↗
PubMedDate not supplied

Support vector machines for predictive modeling in heterogeneous catalysis: a comprehensive introduction and overfitting investigation based on two real applications.

This works provides an introduction to support vector machines (SVMs) for predictive modeling in heterogeneous catalysis, describing step by step the methodology with a highlighting of the points which make such technique an attractive approach. We first investigate linear SVMs, working in detail through a simple example based on experimental data derived from a study aiming at optimizing olefin epoxidation catalysts applying high-throughput experimentation. This case study has been chosen to underline SVM features in a visual manner because of the few catalytic variables investigated. It is shown how SVMs transform original data into another representation space of higher dimensionality. The concepts of Vapnik-Chervonenkis dimension and structural risk minimization are introduced. The SVM methodology is evaluated with a second catalytic application, that is, light paraffin isomerization. Finally, we discuss why SVMs is a strategic method, as compared to other machine learning techniques, such as neural networks or induction trees, and why emphasis is put on the problem of overfitting.

Alkenes↗