Silica-supported oxides, pyridine adsorption

Fig. 8. Diflierential heat of adsorption for pyridine adsorbed on silica-supported oxides that showed only Lewis acidity. (Adapted from Ref. 104.)...

Dumesic and co-workers studied the activity of isopropanol dehydration (247) on a series of silica-supported oxide catalysts as well as the acidic properties of these materials using IR spectroscopy and TGA of adsorbed pyridine (59) and adsorption microcalorimetry of pyridine at 473 K (18,104). Samples that showed only Lewis acidity were at least one to two orders of magnitude less active than the samples that displayed Brpnsted acidity. The activity of the latter samples increased in the order Sc < Ga < Al + This is the same order found for differential heats of pyridine adsorption on the Brpnsted acid sites, and a good correlation between the heats and the activity was found. No correlation was found with the initial heats or for the samples that had only Lewis acidity. 

Bulk Metal Oxides. Extensive Raman chemisorption studies on high surface area alumina and silica supports have been performed because of the industrial importance of these oxides and their weak background Raman vibrations. The most informative studies resulted from the adsorption of pyridine since this probe molecule is very sensitive to the type of acid sites (Brpnsted and Lewis) present on the silica and alumina surfaces. On the alumia support, Lewis pyridine was predominately observed and on the silica support both Lewis and Brpnsted pyridine were observed. " The surface concentrations of pyridine on the silica surface were very small in comparison to the pyridine coverages on the alumina surface. The signal intensities were dramatically enhanced by the... 

Silica-alumina has been used to support the CuO coupled with Ga203, and Sn02 dispersed phases to enhance the catalytic properties of CuO-based catalysts in reactions of environmental importance (hydrocarbon combustion, NO and N2O decomposition and reduction [88]). The acidic properties of such oxide systems were studied from a qualitative (nature of the acid sites) and quantitative (number, acid strength, and strength distribution of acid sites) points of view through the adsorption and desorption of two basic probes (ammonia and pyridine) by coupled volumetric-calorimetric technique and XPS and FT-IR spectroscopy. 

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