Vicente Fornés

Delaminated zeolite precursors as selective acidic catalysts

Heterogeneous catalysis is important in fine-chemical and pharmaceutical manufacture and in petroleum refining. Many of the catalysts used by these industries are based on aluminosilicates, which combine high stability with excellent activity in acid-mediated reactions. Within this class of material, zeolites — microporous crystalline aluminosilicates with three-dimensional framework structures — have attracted particular attention: they are significantly more active than the layered structures (clays) and mesoporous structures (whose walls are amorphous),, and they impart shape selectivity on the reaction products. The selectivity arises from the fact that the catalytically active acid sites have to be accessed through uniformly sized pores and voids, imposing size constraints on the accessibility to reactants and the nature of the intermediates and products. Providing access for larger molecules to the catalytic sites would expand the range of reactions that zeolites can catalyse. But attempts to increase the pore size of zeolites have met with only limited success. Here we describe an approach in which a layered zeolite precursor, is delaminated, in much the same way as the layered structure of a clay may be unbound. The result is an aluminosilicate whose zeolite-type catalytic sites are contained within thin, readily accessible sheets. Performance tests show that the delamination process improves the accessibility of the catalytic sites without affecting their activity. We expect that our approach could be adapted to other layered zeolite precursors,, thus paving the way to exfoliated zeolite precursors as a new class of catalysts.

Layered Double Hydroxides as Highly Efficient Photocatalysts for Visible Light Oxygen Generation from Water

Oxygen generation through photocatalytic water splitting under visible light irradiation is a challenging process. In this work we have synthesized a series of Zn/Ti, Zn/Ce, and Zn/Cr layered double hydroxides (LDH) at different Zn/metal atomic ratio (from 4:2 to 4:0.25) and tested them for the visible light photocatalytic oxygen generation. The most active material was found to be (Zn/Cr)LDH with an atomic ratio of 4:2 that exhibits two absorption bands in the visible region at lambda(max) of 410 and 570 nm. It was found that the efficiency of these chromium layered double oxides for oxygen generation increases asymptotically with the Cr content. Using iron oxalate as chemical actinometer we have determined that the apparent quantum yields for oxygen generation (Phi apparent = 4 x mol oxygen/mol incident photons) are of 60.9% and 12.2% at 410 and 570 nm, respectively. These quantum yields are among the highest values ever determined with visible light for solid materials in the absence of light harvesting dye. The overall efficiency of (Zn/Cr)LDH for visible light oxygen generation was found to be 1.6 times higher than that of WO(3) under the same conditions.

Zeolites as base catalysts: Condensation of aldehydes with derivatives of malonic esters

Abstract The condensations of benzaldehyde with ethyl cyanoacetate, ethyl malonate, and ethyl acetoacetate were carried out with high activity and selectivity on lithium-, sodium-, potassium- and caesium-exchanged X and Y zeolites. The activity of the zeolites increased with decrease in the framework silicon-to-aluminium ratio of the zeolite and increase in the radius of the counter cation. Under reaction conditions, it was found that most of the basic sites in alkaline X and Y zeolites have pKb⩽10.3, and sites with pKb⩽13 were present only in the CsX sample. This catalyst is more active than pyridine, and less active but more selective than piperidine. It is shown that, on these catalysts, in the condensation reactions studied, the controlling step is not proton abstraction but attack of the carbonyl group by the carbanion.

Determination of base properties of hydrotalcites: Condensation of benzaldehyde with ethyl acetoacetate

By carrying out the condensation of benzaldehyde with activated methylenic groups with different pK values, in the presence of a MgAl hydrotalcite catalyst, it has been found that this material shows basic sites with pK values up to 16.5. However, most of the basic sites show pK values in the range 10.7–13.3. In the use of zeolites as catalysts, the only reaction observed was Knoevenagel condensation, while on calcined hydrotalcite other reactions, the Michael-type addition and the Claisen condensation, also occur. These reactions require stronger basicities than the Knoevenagel condensation to occur. By carrying out the Knoevenagel condensation, using methylenic groups of different pK values, in the presence of increasing amounts of benzoic acid, it is possible to measure the total amount of basic sites and their strength distribution as determined under reaction conditions. By increasing the Mg/Al ratio in the hydrotalcite, the number of basic sites with 9.0 ≤ pK ≤ 13.3 increases, whereas the amount of basic sites within 13.3 ≤ pK ≤ 16.5 decreases.

Control of the Photonic Crystal Properties of fcc-Packed Submicrometer SiO2 Spheres by Sintering

We acknowledge M. Planes for his help during SEM characterization. This work was partially financed by the Spanish CICyT project No. MAT97-0698-C04 and the Fundacion Ramon Areces

Photonic crystal properties of packed submicrometric SiO2 spheres

In this letter, we investigate the optical properties of packed monodisperse silica submicron spheres by means of optical transmission measurements. The results are compatible with a three dimensional face centered cubic order in these solid structures. The lattice parameter of these structures, and therefore their optical properties, can be easily tuned through the sphere size (between 200 and 700 nm) thus covering the whole visible and near infrared spectrum.

Thermal decomposition of hydrotalcites. An infrared and nuclear magnetic resonance spectroscopic study

Calcined hydrotalcites have been used extensively as catalysts for base-catalysed reactions. The calcination procedure in critical in determining the behaviour of the final material. The characteristics of the hydrotalcite, following the different stages of calcination, have been studied by means of thermogravimetric (TG), X-ray diffraction (XRD), IR, 1H and 27Al NMR techniques. We have shown that dehydroxylation begins within layers and then in a second stage occurs between adjacent layers, causing collapse of the structure. This process is accompanied by a change from octahedral to tetrahedral coordination of the aluminium. Decarbonation leads to the formation of micropores of radius <1.75 nm. Dehydroxylation and decarbonation are reversible processes, and their rates depend on the calcination temperature.

Structural considerations about SiO2 glasses prepared by sol-gel

Abstract Silica gels were prepared from alcoholic solutions of tetraethylorthosilicate (TEOS) with different H2O/TEOS molar ratios. In the present work the transformation with temperature of these gels has been studied using IR spectroscopy together with measurements of density and specific surface area, thermal analysis and SEM. The IR spectra of these gels, measured as a function of temperature and H2O/TEOS ratio, confirm that the relative concentration of OH groups is temperature dependent. The total OH content, also depends on the H2O/TEOS ratio in the gel. The combined results of IR, density and specific surface area measurements allow some structural considerations to be advanced about the final SiO2 glasses obtained.

Selective and Shape-Selective Baeyer–Villiger Oxidations of Aromatic Aldehydes and Cyclic Ketones with Sn-Beta Zeolites and H2O2

Sn-Beta is used as a heterogeneous catalyst for the Baeyer-Villiger reaction with hydrogen peroxide. Cyclic ketones are transformed into the corresponding lactones, while unsaturated ketones are oxidized to the corresponding unsaturated lactones with very high chemoselectivity. The catalyst is also selective for the oxidation of aromatic aldehydes with H2O2, producing the formate ester or the corresponding hydrolyzed product, that is the alcohol. Shape-selective oxidations are observed for isomeric reactants with different molecular shapes. The catalytic Sn sites have been characterized by 119Sn MAS-NMR spectroscopy, and tetrahedral incorporation into the zeolite framework has been demonstrated. In situ IR spectroscopy and 18O labeling experiments have shown that the oxidation mechanism involves an intermediate of the Criegee type.

Influence of preparation conditions on the structure and catalytic properties of SO42−/ZrO2 superacid catalysts

Abstract SO42−/ZrO2 samples were prepared by impregnation of hydrous zirconium oxides with 0.5M H2SO4 solutions. It was found that the pH of precipitation of the precursor Zr(OH)4 has a strong influence on the structure of crystalline ZrO2 and on the catalytic properties of the SO42−/ZrO2 for the isomerization of n-butane. The nature and strength of acid sites in SO42−/ZrO2; samples were studied by infrared spectra of absorbed pyridine and by temperature-programmed desorption (TPD) of ammonia. Acidity and catalytic results have been compared with those obtained on an acid zeolite as H-Mordenite. It has been found that infrared spectra of pyridine can not be used for the determination of the superacid character of SO42−/ZrO2. However, the NH3-TPD technique shown that in the case of SO42−/ZrO2 samples there are superacid sites in which the NH3-desorption peak appears at about 815 K. A correlation between the number of those superacid sites and the activity for the isomerization of n-butane has been observed.