Luis R. Pizzio

Acid catalysts prepared by impregnation of tungstophosphoric acid solutions on different supports

Abstract Tungstophosphoric acid (TPA) in 50% v/v ethanol–water solutions was used to impregnate carbon, TiO2, SiO2 and γ-Al2O3 (Spheralite and Akzo) at 20°C. The solutions were characterized by UV-visible and nuclear magnetic resonance spectroscopies and the results indicated that the main species present was the undegraded anion of the acid both before and after the contact with carbon, TiO2, SiO2 and γ-Al2O3 Akzo. However, the nuclear magnetic resonance spectrum of the solution in contact with γ-Al2O3 Spheralite allowed to observe a tungstodiphosphate anion. X-ray diffraction of the impregnated solids dried at 70°C showed the same diffraction patterns as the supports, possibly owing to a high dispersion of non-crystalline species. The Fourier transform infrared and nuclear magnetic resonance spectroscopies of carbon, TiO2 and SiO2 impregnated with TPA showed that the species present was the undegraded [PW12O40]3− anion. When using carbon and TiO2, the supported acid was thermally stable up to 425°C, similarly to the bulk acid, whereas on SiO2 support, the anion underwent partial degradation from 365°C onwards. In TPA-impregnated γ-Al2O3 samples dried at 70°C, diffuse reflectance and nuclear magnetic resonance spectroscopies allowed us to observe a partial degradation of the anion, the extent of which was not appreciably increased by the subsequent calcination. The specific activity of the catalysts in the isopropanol dehydration reaction showed that TPA supported on carbon, TiO2 and SiO2 present higher acidity than the bulk acid, unlike in alumina where the partially degraded species gave a lower acidity.

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.

Tungstophosphoric and tungstosilicic acids on carbon as acidic catalysts

Abstract Tungstophosphoric (TPA) and tungstosilicic acid (TSA) catalysts supported on carbon were studied. They were prepared by equilibrium and incipient wetness impregnation techniques. Solutions of TPA and TSA in 50% v/v ethanol–water were used to impregnate carbon at 20°C. The Fourier transform infrared and 31 P nuclear magnetic resonance spectroscopies of TPA and TSA supported catalysts dried at 70°C showed that the species present were the undegraded [PW 12 O 40 ] 3− and [SiW 12 O 40 ] 4− anion, respectively. X-ray diffraction of the impregnated solids showed the same diffraction patterns as the supports, possibly owing to a high dispersion of non-crystalline species. The scanning electron microscopy with energy dispersive X-ray analysis of the supported acids showed a uniform distribution of TPA and TSA in the carbon particles with low heteropolyacid content. In the samples with high content or prepared by incipient wetness method small grains of TPA and TSA were found. These grains showed to be nearly inactive for isopropanol dehydration. The specific conversion of the catalysts for this reaction increases regularly with the increase of the adsorbed heteropolyacid content.

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.

Isoamyl acetate production catalyzed by H3PW12O40 on their partially substituted Cs or K salts

Abstract The Cs2.9H0.1PW12O40, Cs2.6H0.4PW12O40, K2.9H0.1PW12O40 and K2.5H0.5PW12O40 salts were synthesized and impregnated by means of the incipient wetness method with a H3PW12O40 solution in ethanol–water. The solids were characterized by N2 adsorption–desorption isotherms, FT-IR, XRD, 31 P MAS–NMR, thermogravimetric and differential thermal analysis, and the acidic behavior was studied by potentiometric titration with n-butylamine and isopropanol dehydration. The proton content evaluated from the thermogravimetric weight loss in the temperature zone ascribed to the deprotonation step showed a good agreement with the values drawn from the synthesis stoichiometry. The physical–chemical characterization led to establish the presence of the Keggin salt structure and well dispersed acid impregnated on the partially substituted salts. The mainly microporous salts as well as the impregnated salts possessed higher acidity than the bulk acid. Their catalytic activity in the esterification of acetic acid and isoamyl alcohol to obtain isoamyl acetate showed a straight correlation with the acidic characteristics of the studied solids. Both the high conversion and selectivity obtained are important results for the use of these type of heteropolycompounds mainly in the flavor and fragrances industries.

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.

Direct modification with tungstophosphoric acid of mesoporous titania synthesized by urea-templated sol-gel reactions.

A series of mesoporous titania materials modified with tungstophosphoric acid (TPA) were successfully synthesized by using urea as low-cost template via sol-gel reactions, followed by removing the urea by extraction with water. They were characterized by FT-IR, 31P MAS-NMR, XRD, DTA-TGA, DRS, TEM and BET. The samples presented mesopores with a diameter higher than 3.0 nm. The S(BET) of the solids decreases with the increase of the TPA content and with the increase of the calcination temperature. According to FT-IR and 31P MAS-NMR studies the main species present in the samples is [PW12O40]3- anions, which was partially transformed into [P2W21O71]6- and [PW11O39]7- anion during the synthesis and drying step. The XRD patterns of the modified samples only exhibited the characteristic peaks of anatase phase. The presence of TPA retarded the crystallization of the anatase phase and its transformation into rutile phase. The point of zero charge decreased in parallel with the increment of tungstophosphoric acid in the samples and with the increase of the calcination temperature. The band gap energy decreased as a result of the introduction of TPA into the TiO2 matrix, but remained practically constant with the increase of the calcination temperature.

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.

Equilibrium adsorption of 11-tungstophosphate anion on different supports

Abstract The equilibrium adsorption on TiO2, SiO2 and γ-Al2O3 (Spheralite and Akzo) of [PW11O39]7− anion from solution in water was studied at 20°C. The solutions were characterized by UV–visible spectroscopy, both before and after the contact with the supports, and the results indicated that the main species present was the undegraded anion. The solids so obtained were dried at 70°C and then studied by X-ray diffraction, showing the same diffraction patterns as that of the supports. This may be due to a high dispersion of non-crystalline adsorbed species. By Fourier transform infrared and nuclear magnetic resonance spectroscopies of SiO2 or TiO2 impregnated with [PW11O39]7− anion, it was found that at 70°C the species present are this anion without transformation or this anion together with the dimeric species [P2W18O62]6− and [P2W21O71]6−, respectively. For the silica-supported sample, any important transformation of the [PW11O39]7− anion does not occur up to 425°C, meanwhile calcination until this temperature of titania-supported sample generates an increase of the [P2W21O71]6− species amount. In [PW11O39]7− on γ-Al2O3 samples dried at 70°C, diffuse reflectance and nuclear magnetic resonance spectroscopies allowed to observe a partial degradation of the anion only when alumina Spheralite was used. In the samples prepared on both aluminas, calcination at 425°C leads to total degradation of the present species. The results obtained were compared with those previously found when the same supports were impregnated with tungstophosphoric acid solution in ethanol–water.

Low-frequency ultrasound induces oxygen vacancies formation and visible light absorption in TiO2 P-25 nanoparticles

Low-frequency ultrasound (LFUS) irradiation induces morphological, optical and surface changes in the commercial nano-TiO(2)-based photocatalyst, Evonik-Degussa P-25. Low-temperature electron spin resonance (ESR) measurements performed on this material provided the first experimental evidence for the formation of oxygen vacancies (V(o)), which were also found responsible for the visible-light absorption. The V(o) surface defects might result from high-speed inter-particle collisions and shock waves generated by LFUS sonication impacting the TiO(2) particles. This is in contrast to a number of well-established technologies, where the formation of oxygen vacancies on the TiO(2) surface often requires harsh technological conditions and complicated procedures, such as annealing at high temperatures, radio-frequency-induced plasma or ion sputtering. Thus, this study reports for the first time the preparation of visible-light responsive TiO(2)-based photocatalysts by using a simple LFUS-based approach to induce oxygen vacancies at the nano-TiO(2) surface. These findings might open new avenues for synthesis of novel nano-TiO(2)-based photocatalysts capable of destroying water or airborne pollutants and microorganisms under visible light illumination.