Silica-supported silicotungstic acid: A study by X-ray photoelectron spectroscopy
Introduction
Heteropoly acids (HPA), which belong to a large family of metal–oxygen cluster compounds known as polyoxometalates, have high Brønsted acidity and tuneable redox properties that make them attractive as acid and redox catalysts [1], [2], [3], [4]. HPAs with inorganic cage-type anions of the form [XM12O40]n− adopt a Keggin structure in which the hetero atom X (for example, phosphorus or silicon) is located within a cavity formed by linked MO6 octahedra, where the addenda atoms M are typically molybdenum or tungsten. The low surface areas of heteropoly acids (1–5 m2 g−1) require them to be supported on porous materials when used as catalysts [2]. Silica, which is neutral or slightly acidic with well-defined surface area and porosity, has frequently been used for this purpose.
Silica-supported phosphotungstic acid (H3PW12O40) has often been used as a model system for fundamental investigations of these materials and recent studies by X-ray photoelectron spectroscopy (XPS) have described the formation of a distinctive heteropolytungstate species at the interface with the silica surface. It is proposed that this species is in direct contact with the silica surface via an interaction with terminal WO groups of the Keggin anion [5], [6]. This proposal supports the view, as suggested in other work [7], [8], [9], that there is a continuous two-dimensional growth of a homogeneous monolayer of intact [PW12O40]3− Keggin anions across the surface of the silica support with increased loading of the catalyst. The interaction between [PW12O40]3− Keggin anions and silica surfaces has also been observed in 31P MAS NMR studies [8], [10] and it has been suggested that there is interaction with silanol groups to give ion-pairs of the form (≡SiOH2+)(H2PW12O40−).
We are currently investigating the nature of silica-supported silicotungstic acid (H4SiW12O40) and report here on our examination by W 4f and O 1s XPS of the pure acid and its supported form. We compare results for a typical catalytic sample with a loading of 25.5 wt.% with those for a sample with a considerably reduced loading of 3.2 wt.%. We estimate for our silica-supported samples that a saturated monolayer coverage of [SiW12O40]4− anions would correspond to a loading in the region of 50 wt.%. In order to consider the influence of silanol group content, two different silica support materials were used with an average of 3.4 and 1.5 silanol groups per 100 Å2, respectively, of silica surface.
Section snippets
Experimental procedure
Silicotungstic acid in the form of the hydrate (≈22 H2O) was supplied by Nippon Inorganic Colour and Chemical Co. Ltd. Silica gel (A732, a.k.a. G57, surface area = 300 m2 g−1, silanol content = 1.7 mmol g−1) was supplied by W. R. Grace and extruded fumed silica with a reduced silanol group content (Aerolyst 3040, surface area = 245 m2 g−1, silanol content = 0.6 mmol g−1) by Degussa AC.
Two supported silicotungstic acid catalysts were prepared using silica gel with loadings of 25.5 wt.% (sample A) and 3.2 wt.%
Results and discussion
The W 4f XP spectrum recorded from pure silicotungstic acid (Fig. 1) is composed of a well-resolved spin–orbit doublet with binding energies for the W 4f7/2 and W 4f5/2 core levels of 35.7 and 37.7 eV, respectively. The values are typical [11] of the presence of W(VI). The spectrum also shows a broad component on the high binding energy side (39.7 eV) which might be associated with electron energy losses and was not observed in the spectra recorded from the other samples (vide infra). Pure
Conclusion
The O 1s spectrum of pure silicotungstic acid contains distinct contributions due to the presence of W–O–W and W–O–Si bonds and the W 4f XP spectrum is a well-resolved spin–orbit doublet consistent with the presence of W(VI). This spin–orbit doublet is also present when silicotungstic acid is supported on silica but, in addition, there is a further doublet that can be assigned to perturbed W(VI) environments resulting from interactions of terminal WO bonds of individual Keggin anions, [SiW12O40]
Acknowledgements
We thank BP Chemicals and EPSRC for an award of a studentship to GRD.
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