Patricia Graciela Vazquez

Efficient Tetrahydropyranylation of Phenols and Alcohols Catalyzed by Supported Mo and W Keggin Heteropolyacids

Abstract The protection of phenols as tetrahydropyranyl acetals is studied using supported Keggin heteropolyacids as catalysts, specifically molybdophosphoric or tungstophosphoric acids supported on silica. Phenol tetrahydropyranyl acetals are obtained in short times at room temperature with excellent yields. Protection of alcohols is also performed under mild conditions using both supported heteropolyacids, with yields ranging between good and quantitative. The reactions are clean and their workup is very simple. The use of these solid catalysts allows replacing the usual soluble inorganic acids, contributing to waste reduction.

Role of vanadium and pyridine in heteropolycompounds for selective oxidation of alcohols with hydrogen peroxide

AbstractThis study describes the application of heteropolyacids H3PMo12O40, H4SiMo12O40, H4PMo11VO40, H5PMo10V2O40, H9PMo6V6O40, and a hybrid pyridine-modified heteropolyacid with Keggin structure for selective oxidation of alcohols to ketones or aldehydes using aqueous hydrogen peroxide and acetonitrile as solvent. Performance of these different catalysts in 1-phenylethanol oxidation was studied. Influence of reaction temperature, amount of catalyst and hydrogen peroxide and reaction time on the yield of acetophenone was investigated to obtain optimal reaction conditions. Oxidation ability of the catalyst depended on the number of vanadium atoms present in the Keggin ion and to a lesser extent on pyridine substitution in the Keggin secondary structure. In order to explore the applicability of the method for selective oxidation of alcohols to ketones or aldehydes, various alcohols were investigated according to the general procedure using hybrid pyridine-modified heteropolyacid.n Graphical AbstractThis study describes the application of heteropolyacids H3PMo12O40, H4SiMo12O40, H4PMo11VO40, H5PMo10V2O40, H9PMo6V6O40, and a hybrid pyridine-modified heteropolyacid with Keggin structure for selective oxidation of alcohols to ketones or aldehydes using aqueous hydrogen peroxide and acetonitrile as solvents.

New Vanadium Keggin Heteropolyacids Encapsulated in a Silica Framework: Recyclable Catalysts for the Synthesis of Highly Substituted Hexahydropyrimidines Under Suitable Conditions

Solid acid catalysts based on the direct incorporation of the vanadium Keggin heteropolyacid (PMo11V) structure during the synthesis of silica by the sol–gel technique, in acidic media using tetraethyl orthosilicate (PMo11VSiO21, PMo11VSiO22, PMo11VSiO23, and PMo11VSiO24), were prepared and characterized by 31P-NMR, FT-IR, XRD, and textural properties (SBET). The acidic characteristics of the catalysts were determined by potentiometric titration with n-butylamine. A series of highly substituted hexahydropyrimidines were synthesized using these new materials, encapsulated in a silica framework, as catalyst in solvent-free conditions. This methodology requires short reaction time (1.5xa0h), a temperature of 80xa0°C in solvent free-conditions to obtain good to excellent yields of trifluoromethyl-hexahydropyrimidine derivatives. The Keggin catalyst embedded in the silica matrix is insoluble in polar media, which allows easy removal of the reaction products without affecting their catalytic activity.Graphical Abstract

Simple and Friendly Sulfones Synthesis Using Aqueous Hydrogen Peroxide with a Reusable Keggin Molybdenum Heteropolyacid, Immobilized on Aminopropyl-Functionalized Silica

Keggin molybdenum heteropolyacid (H3PMo12O40), immobilized on aminopropyl-functionalized silica catalysts, were made using two immobilization methods: incipient wetness and equilibrium adsorption. The material prepared for the equilibrium adsorption technique was found to be a highly efficient, ecofriendly, and recyclable heterogeneous catalyst for the selective oxidation of sulfides to sulfones in excellent yields, under mild reaction condition using 35% w/v aqueous hydrogen peroxide as the oxidant.

Hydrotreating catalysts on alumina, titania or zirconia from ethanol/water solutions of heteropolyacids

Abstract Supported PNiMo(W) catalysts for hydrodesulfurization and hydrodenitrogenation were prepared. Heteropolyacids were used as precursors and supported on alumina, titania and zirconia. The tungstophosphoric and molybdophosphoric acids showed a stable Keggin-type structure when the supports were zirconia and titania. However, the heteropolyacids on alumina were degraded during the drying and/or calcination of the impregnated samples. The, catalysts prepared, on alumina had the highest activity, in the order of commercial ones. The catalysts on titania, were more active than the samples supported on zirconia.

Use of new silica fillers as additives for polymers used in packaging of fruit

The objective of this work was to synthesize nanosilicas with different degree of hydrophobicity by the sol-gel method, using tetraethyl orthosilicate as a precursor. For this purpose, 3-aminopropyl triethoxysilane (APS) and 1,1,1,3,3,3 - hexamethyldisilazane (HMDS), were added during synthesis as modifiers. A commercial biopolymer (Hexamoll Dinch, BASF) intended for packaging of apples, was added to the new nanosilicas. The materials obtained were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, potentiometric titration, porosity, specific surface area and hydrophobicity/hydrophilicity by wetting test. Colorimetry was used to evaluate change in apple pulp color after contact with the different silicas.

Improved antimicrobial activity of silica–Cu using a heteropolyacid and different precursors by sol–gel: synthesis and characterization

Antimicrobial coatings are used to avoid the proliferation of fungi and bacteria inside urban buildings by incorporating low concentrations of antimicrobial agents (biocides). This study is focused on evaluating the antifungal activity of copper (II) supported on a silica matrix, and as counter-cation of a heteropolyacid, obtained from the sol–gel method using various catalysts, against two fungal isolates. The fungi used in this evaluation were Alternaria alternata and Chaetomium globosum isolated by routine microbiological techniques from biodeteriorated paints films. In all cases, fungi were selected due to their ability to grow on the paint films and their negative impact on human health. First, silicas were prepared using tetraethylorthosilicate and methyl trimethoxysilane as precursors and acetic acid, hydrochloric acid as catalysts of the hydrolysis reaction of the alkoxide. The effect of addition of copper (II) in the form of copper nitrate was studied. With respect to the morphology of the silica, most of them are similar, between pure and modified silica, in all cases. Once the stage of the synthesis and characterization of the prepared materials was performed, six of them were selected to evaluate their antifungal activity by agar plate inhibition test against both fungi of interest (A. alternata and C. globosum), and the percent inhibition was determined in each case. Tested Cu-based solids were shown to have a higher antifungal activity because they completely inhibit the growth of both fungi with lower concentrations relative to its control. This work was performed as a preliminary study, in order to guide the selection of a suitable organic biocide from a list of possible antifungal agents.Graphical Abstract

Selective Oxidation of Sulfides to Sulfoxides Using Modified Keggin Heteropolyacids as Catalyst

GRAPHICAL ABSTRACT Abstract The use of modified Keggin phosphomolybdic heteropolyacids (HPA) (V and Al) as catalysts in the selective oxidation of sulfides to the corresponding sulfoxides is reported. Excellent yields were obtained (13 examples: 79%–95%), using 35% (w/v) aqueous hydrogen peroxide as oxidant, and acetonitrile as solvent at 25°C. The relationship between the electron densities of the sulfur atoms, which was estimated by molecular theoretical calculations, and the oxidative relativities of the sulfur-containing compounds was also investigated. The results indicate that the time for 100% conversion decreases with an increase in electron density.

Ecofriendly catalysts based on mixed xerogels for liquid phase oxidations by hydrogen peroxide

Abstract In this study TiO 2 -CeO 2 and TiO 2 -V 2 O 5 mixed oxide were synthesized by sol-gel technique. The mixed xerogels were prepared with different load of TiO 2 and CeO 2 or V 2 O 5 , using titanium isopropoxide and hexahydrated cerium nitrate or vanadyl acetylacetonate as precursors, and ethanol (99.8%) as solvent. The gels were dried in air at 50xa0°C for 24xa0h and calcined in air at temperatures in the range 200–800xa0°C for 4xa0h. Moreover, a sample without cerium, 100% (w/w) TiO 2 , was prepared to be kept as reference material. The prepared solids were characterized by means of XRD, FT-IR and TEM. Their textural properties were determined by adsorption-desorption isotherms of N 2 at 77xa0K. The xerogels were tested as catalysts in the liquid phase oxidation of 2,6- dimethylphenol to 2,6-dimethyl- p -benzoquinone at 20xa0°C, using ethanol as solvent and aqueous hydrogen peroxide as a clean oxidizing agent.

Clean transesterification of β-ketoesters catalyzed by hybrid silica sol-gel

A mild, chemoselective and sustainable transesterification of β-ketoesters has been developed employing functionalized silica sol-gel as heterogeneous catalyst, which can be reused several times without appreciable loss of reactivity. Simplicity of operation as well as mild and environmentally benign nature of the reaction would enable applications to a wide variety of substrates.