Equilibrium adsorption of 11-tungstophosphate anion on different supports

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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.

Introduction

The catalytic properties of heteropoly compounds have drawn wide attention in the last two decades owing to the versatility of these compounds as catalysts, which has been demonstrated both by successful large-scale applications and by promising laboratory results.

Heteropolyacids (HPA) have special properties which are of great value for catalysis, such as strong Brönsted acidity, ability to catalyze reversible redox reactions under mild conditions, high solubility in water and oxygenated solvents and fairly high stability in the solid state [1]. They may be used as homogeneous catalysts, as well as in phase-transfer catalysis and in liquid–solid or gas–solid heterogeneous reactions [2].

Acid and oxidation reactions catalyzed by solid heteropoly compounds (gas–solid and liquid–solid systems) can proceed via three main ways of reaction named surface, bulk type I (pseudoliquid) and bulk type II catalysis 2, 3.

In surface type catalysis, the reactions take place on the external and internal pore surfaces of solid catalysts. The reaction rate is proportional to the catalyst surface area.

Supported HPA catalysts are important because bulk compounds have a low specific surface area [4]. It is necessary to pay attention to the changes in the acid strength, the structures of aggregates and the possibility of decomposition.

The catalytic activity of supported HPA is related to the type of carrier, the HPA loading and conditions of pretreatment [5]. Basic solids such as Al2O3 and MgO tend to decompose HPA 6, 7, 8, 9. Acidic or neutral substances such as SiO2 [10], active carbon 11, 12or acidic ion-exchange resin [13]are suitable as supports.

The nature of the supported HPA species depends on the concentration of the impregnating solutions, the solvent used and the pH of the solutions, among other variables. Kozhevnikov et al. [14]have found that H3PW12O40 and H4SiW12O40 supported from concentrated aqueous solutions on activated carbon retain their Keggin-type structure. However, at low HPA contents, a partial decomposition of the H3PW12O40 was found.

In a previous work [15], the stabilizing effect on the Keggin structure of H3PW12O40 and H4SiW12O40 in solution caused by the addition of an organic solvent was verified by UV–visible spectroscopy. The structure remained intact, while the solutions of HPA in water showed different transformations depending on the heteroatom involved.

Then, the decomposition of HPA could take place both in the solution during the impregnation and/or on the solid as a consequence of the interaction with the support.

The partial hydrolysis of [PW12O40]3− anion can be produced during the impregnation step leading to the lacunar [PW11O39]7− species, due to the narrow pH range in which 12-tungstophosphate anion is stable. In general, strong interaction of heteropolyacids with supports is observed at low loading. The intrinsic properties of HPA prevail at high loading.

Due to the above-mentioned importance of the species present in the solid for the catalytic activity, the objective of this paper is to study the behavior of [PW11O39]7− anion in the solutions when these are put in contact with different supports (SiO2, TiO2 and Al2O3) during an adequate time to reach equilibrium. Also, the nature and thermal stability of the species resulting in the solids, after the equilibrium adsorption impregnation, are studied. Besides, these results are compared with those previously obtained using solutions initially containing the [PW12O40]3− anion.

Section snippets

Materials

The [PW11O39]7− (PW11) solutions were prepared from H3PW12O40·nH2O (Fluka p.a.) dissolved in distilled water and alkalinized, taking into account standard techniques previously reported [16].

The supports used were: TiO2 (Riedel-de Haën, surface area 9.8 m2/g); SiO2 (Grace, surface area 311 m2/g, mean pore diameter between 5 and 10 nm) and two different types of γ-Al2O3 (Akzo Chemie, surface area 280 m2/g, mean pore diameter 3.4 nm and Spheralite, surface area 282 m2/g, mean pore diameter 4.2

Characterization of the solutions by UV–visible spectroscopy

The UV–visible spectrum of PW11 solution before the contact with the supports (Fig. 1, spectrum a) shows a band with maximum at 245 nm, which is the characteristic charge transfer band of [PW11O39]7− heteropolyanion [17].

The spectra of the PW11 solutions after the contact during 72 h with SiO2, TiO2 and γ-Al2O3 (Akzo and Spheralite) are shown in Fig. 1, spectra b to e, respectively. In these spectra, only the characteristic band of the [PW11O39]7− species is observed. With regard to the

Species in solution

The aqueous solutions of [PW11O39]7− are stable at pH values in the range 2–6. Kyle [18]found that in weakly alkaline solutions (pH 7–9) the [PW11O39]7− degradation into [PW9O34]9− anion took some seconds, and that the period for the total decomposition varied from minutes to hours, depending on temperature and pH.

Special attention has been paid in this study to the behavior of the PW11 solutions. UV–visible spectroscopy showed that the main species in solution, after the contact with the

Conclusion

The preparation of supported catalysts, an important issue for a large number of industrial reactions, requires knowledge of different basic points, as which are the species present in the impregnating solutions and on the support, the support characteristics, the solute-support interaction and the effect of drying and calcination conditions.

Taking into account all characterizations performed, it can be assumed a weak adsorption of [PW11O39]7− ion on SiO2. This anion remains unaltered in

Acknowledgements

The authors thank Dr. Esther Ponzi and Lic. Marcelo E. Chimienti for their collaboration in the experimental measures.

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