Patricia Vázquez

Supported Keggin type heteropolycompounds for ecofriendly reactions

Abstract New supported Keggin heteropolyacid (HPA) catalysts were prepared in order to make a contribution to the field of catalyzed acid reactions by means of ecofriendly technologies. The esterification of isoamyl alcohol and acetic acid in gaseous phase was studied using tungstophosphoric acid (TPA) supported on different carriers, silica materials, carbon and alumina. These catalysts were characterized by FT-IR, DRS, TPR and their acidity was evaluated through the isopropanol dehydration test reaction. The higher acidic catalysts prepared with silica materials led to the greatest activities due to the higher proton availability which, in turn, was related to the lower interaction and dispersion of the active species. In order to perform this acid reaction in heterogeneous liquid phase and to avoid the tungstophosphoric or molybdophosphoric acid (MPA) solubilization during reaction, functionalized silica and SiMCM-41, and polyvinyl alcohol hydrogel–polyethylenglycol (PVA-PVG) beads were used as new supports. The physical–chemical characterization of the supported catalysts by FT-IR, XRD, 31 P MAS NMR and TG-DTA showed mainly undegraded primary Keggin structure. The solids exhibited low or negligible solubilization and their acidity, measured by potentiometric titration with n-butylamine, was high. The SiMCM-41 and polymeric bead-supported catalysts lead to obtain attractive activity in the isoamyl acetate synthesis through esterification reaction.

Silica-supported heteropolyacids as catalysts in alcohol dehydration reactions

Catalysts based on Keggin-type heteropolyacids supported on silica were prepared for their use in dehydration of alcohols in liquid phase. Species present in the catalysts were characterized by diverse physicochemical techniques, disclosing that both molybdophosphoric and tungstophosphoric acids are found with their primary Keggin structure intact. Their catalytic behavior was studied in the dehydration of alcohol of interest for the production of intermediary compounds for fine chemical industries. Specifically, the dehydration of 1,2-diphenylethanol, 1-(3,4-dimethoxyphenyl)-2-phenylethanol and cholesterol was studied. It was obtained a process for the direct dehydration of cholesterol. It was found that the catalysts are very active and selective, also allowing an easy separation from the reaction medium. The same catalysts were used several times without appreciable loss of catalytic activity and after the reaction they showed physicochemical properties similar to those of the original catalysts.

Catalysts based on supported 12-molybdophosphoric acid

Catalysts preparation by equilibrium adsorption of 12-molybdophosphoric acid in water–ethanol solutions onto silica, titania, carbon and alumina is studied. The solute/support interaction follows the decreasing order: alumina, carbon, silica and titania. It is observed that the molybdophosphoric acid keeps its Keggin-type structure after being supported at room temperature on all supports. This species is stable up to 500°C on silica, 400°C on titania or carbon and 250°C for alumina. The acidity of the supported heteropoly acid decreases in the following order: silica>titania>carbon>alumina.

Tungstophosphoric and Molybdophosphoric Acids Supported on Zirconia as Esterification Catalysts

The preparation and characterization of supported acid catalysts, based on tungstophosphoric (TPA) or molybdophosphoric (MPA) acids using ZrO2 as support, are presented. ZrO2 was prepared from zirconium oxychloride aqueous solutions by adding ammonium hydroxide solution. Two different techniques were used to obtain the catalysts, equilibrium adsorption and incipient wetness impregnation. The catalyst characterization allowed us to determine the species present on the carrier surface and their acidic properties. The catalytic activity for esterification of acetic acid with isoamyl alcohol of the zirconia-supported TPA catalysts was greater than that corresponding to those based on MPA. Moreover, in the preparative conditions used, the incipient wetness impregnation led to heteropolyacids supported on ZrO2 with better catalytic performance.

Silica and alumina impregnated with dimethylformamide solutions of molybdophosphoric or tungstophosphoric acids for hydrotreatment reactions

Heteropolyacid (HPA) based catalysts were prepared by impregnation of silica and alumina with dimethylformamide solutions of molybdophosphoric and tungstophosphoric acids. This solvent allows the preservation of the Keggin unit during the impregnation independently of the nature of the support. Ni-promoted catalysts were prepared by impregnation with nickel nitrate solutions of the supported HPA. The stronger interaction of the HPA with alumina than with silica allows a better dispersion of the polyoxometallate species on the former one whereas the formation of bulk oxides is observed on silica. The performance of these HPA–Ni/support catalysts for hydrodenitrogenation and hydrodesulfurization reactions is related to the precursor–support interaction.

Influence of the Alcohol Molecular Size in the Dehydration Reaction Catalyzed by Carbon-Supported Heteropolyacids

The catalytic performance of tungstophosphoric and molybdophosphoric acids supported on two commercial carbons, for dehydration reactions of several alcohols, was studied. The alcohol conversion depended on their molecular size, while selectivity to alkenes was also related to cyclodimer molecular size formed by certain alkenes in acidic medium. Acidity and mean pore diameter of the catalysts showed an influence on the conversion degree of the alcohols into alkenes and also into alkene cyclodimerization.

NiMo(W)-based hydrotreatment catalysts supported on peach stones activated carbon

Abstract The NiMo(W) and PNiMo(W) catalysts supported on carbon obtained from peach stones were studied. The catalysts were prepared using two different impregnation techniques: pore filling and excess of solution. Impregnating solutions of molybdenum or tungsten iso- and heteropolyanions were used as precursors of the active components. The textural properties of the catalysts were determined, and also they were characterized by Fourier transform infrared spectroscopy, temperature programmed reduction and scanning electron microscopy. Their surface acidity and their hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities were measured. The positive influence of phosphorus on HDS and HDN activity of both molybdenum and tungsten-based catalysts was confirmed. It was found that the HDN activity depends strongly on the preparation method and on the W and Mo precursors used in the impregnating solution. The PNiMo and PNiW catalysts on carbon prepared by impregnation with heteropolyanions solutions were as or more active for HDN reaction than those obtained from the classical isopolyanions.

SILICA-SUPPORTED HETEROPOLYACIDS READILY INDUCE CYCLODIMERIZATION OF STYRENES AND STILBENES

ABSTRACT Heteropolyacids such as molybdophosphoric and tungstophosphoric acids, supported over silica, readily induce cyclodimerisation reactions of styrenes and stilbenes affording a mixture of indane and/or tetraline derivatives with remarkably high efficiency and in reduced reaction time.

Adsorption studies and species characterization in catalysts obtained from aqueous solutions or phosphomolybdic acid and γ-alumina

The equilibrium adsorption on alumina of phosphomolybdic acid (H3PMo12O40), with a Keggin phase structure, has been studied from its solutions in water, 0.1 mol dm–3 HCl and ethanol–water. The molybdenum adsorption isotherms showed only one plateau, in all cases, indicating the adsorption of only one main species. NMR spectra of the initial solutions showed that the PMo12O403– depolymerizes to PMo11O397– in water, whereas it remains as PMo12O403– in solutions acidified with HCl or in ethanol–water. Nevertheless, the species present in the solutions obtained after the adsorption was PMo11O397– in the two first solvents, and PMo12O403– for ethanol–water. Diffuse reflectance spectra of representative solid samples of the adsorption tests from solutions in water and in 0.1 mol dm–3 HCl showed that the adsorbed species was PMo11O397– for high-concentration wet samples and that decomposition of the lacunar phase starts at low temperatures. For the samples obtained by adsorption from solutions of H3PMo12O40 in ethanol–water, it can be concluded that the adsorbed species is PMo12O403–, which is not degraded below 230 °C. For low-concentration samples, total decomposition of phosphomolybdic anions was observed, regardless of the solvent used. By X-ray diffraction of wet, dried or calcined samples, it was observed that the adsorbed species are highly dispersed on the alumina surface.

Green chemistry in Brazil

The philosophy of green chemistry has been very well received in Latin America’s research and development programs. In this review we describe the green chemistry contributions of Brazilian research groups over the last three years.