Graciela Baronetti

The state of the art on Wells–Dawson heteropoly-compounds: A review of their properties and applications

The scientific literature concerning the structure, hydrolytic stability in solution, thermal stability in the solid state, redox-acid properties and applications of heteropoly-compounds (HPCs) with Wells–Dawson structure is summarized in the present work. Wells–Dawson heteropoly-anions possess the formula [(X n+ )2M18O62] (16−2n)− where X n+ represents a central atom (phosphorous(V), arsenic(V), sulfur(VI), fluorine) surrounded by a cage of M addenda atoms, such as tungsten(VI), molybdenum(VI) or a mixture of elements, each of them composing MO6 (M-oxygen) octahedral units. The addenda atoms are partially substituted by other elements, such as vanadium, transition metals, lanthanides, halogens and inorganic radicals. The Wells–Dawson heteropoly-anion is associated with inorganic (H + , alkaline elements, etc.) or organic countercations forming hybrid compounds. Wells–Dawson acids (phospho-tungstic H6P2W18O62·24H2O, phospho-molybdic H6P2Mo18O62·nH2O and arsenicmolybdic H6As2Mo18O62·nH2O) possess super-acidity and a remarkably stability both in solution and in the solid state. These properties make them suitable catalytic materials in homogeneous and heterogeneous liquid-phase reactions replacing the conventional liquid acids (HF, HCl, H2SO4, etc.). Although, the application of the acids in heterogeneous gas-phase reactions is less developed, there is a patented method to oxidize alkanes to carboxylic acids on a supported Wells–Dawson catalyst that combines acid and redox properties. Wells–Dawson anions possess the ability to accept or release electrons through an external potential or upon exposure to visible and UV radiation (electro and photochemical reactions). Additionally, Wells–Dawson HPCs catalyze the oxidation of organic molecules with molecular oxygen, hydrogen peroxide and iodosylarenes; epoxidation and hydrogenation in homogeneous and heterogeneous liquid-phase conditions. The ability of transition metal substituted Wells–Dawson HPCs to be reduced and re-oxidized without degradation of the structure is promising in the application of those HPCs replacing metalloporphyrins catalysts in redox and electrochemical reactions.

Hydrogen production via catalytic gasification of ethanol. A mechanism proposal over copper–nickel catalysts

Ethanol gasification using Cu–Ni–K/γ-Al2O3 catalysts was studied. The reaction was carried out at a low temperature (300°C) and atmospheric pressure. The influence of the diffusional effects, the residence time and the water/ethanol molar ratio in the feed on the ethanol conversion and on the product distribution was analysed. Additional experiments were performed with monometallic catalysts, such as Cu–K/γ-Al2O3 and Ni–K/γ-Al2O3 catalysts. Ethanol gasification is favoured by a diminution of the diffusional resistances, high residence time and low water to ethanol feed ratio. A probable reaction mechanism is postulated, which is consistent with the experimental results and let identify the function of each metal (copper and nickel).

Solvent‐Free Approach to 3,4‐Dihydropyrimidin‐2(1H)‐(thio)ones: Biginelli Reaction Catalyzed by a Wells–Dawson Reusable Heteropolyacid

Abstract The Biginelli reaction is performed in an efficient, simple, solvent‐free procedure, using a small amount of H6P2W18O62 · 24H2O, a reusable heteropolyacid catalyst with Wells–Dawson structure. Both aromatic and aliphatic aldehydes, and different β‐dicarbonyl compounds and urea or thiourea, were used as starting materials. Seventeen examples of dihydropyrimidinones and dihydropyrimidinethiones were prepared by heating the reactants either in refluxing acetonitrile or in the absence of solvent. Operational simplicity, the use of a noncorrosive, reusable catalyst in solventless conditions, short reaction times, and very good to excellent yields in most of the selected examples are the main advantages of the method.

Efficient deprotection of phenol methoxymethyl ethers using a solid acid catalyst with Wells-Dawson structure

Deprotection of various phenols from their respective methoxymethyl ethers using an heteropolyacid catalyst was studied. The catalyst was the Wells-Dawson heteropolyacid, used both in bulk or supported on silica. Yields were high to quantitative after less than one hour reaction time and the catalyst was easily recoverable and reusable.

A Fast and Efficient Deprotection of Aldehydes from Acylals Using a Wells‐Dawson Heteropolyacid Catalyst (H6P2W18O62 · 24H2O)

Abstract A rapid and efficient method for the deprotection of aldehyde 1,1‐diacetates is described. The reaction was carried out using a Wells‐Dawson type catalyst supported on silica. The catalyst, used in 1% molar quantity, is easily recoverable and reusable and maintains the activity after its use in four consecutive reaction batches. The yield of the deprotection was 92–100% (15 examples including nitrobenzaldehyde 1,1‐diacetates). Reaction conditions involve short times and the use of toluene as the solvent; isolation is simple and the products are nearly pure.

Nickel-based doped ceria-supported catalysts for steam reforming of methane at mild conditions

ABSTRACT Hydrogen is widely considered a promising green energy vector. It may be produced by steam reforming of methane which may be obtained from renewable resources such as biogas or biomass gasification. Nickel catalysts supported in ceria oxides doped with La, Pr, or Zr were used. Catalysts were prepared by wet impregnation of supports previously obtained by coprecipitation using the urea method and subsequent calcination at 600ºC. Solids were then characterized by Brunauer–Emmett–Teller, X-Ray iffraction, and oxygen storage capacity/oxygen storage capacity complete measurements. Catalysts have shown both effectiveness and stability in methane steam reforming reaction at 600ºC with a steam/methane ratio close to the stoichiometric.