The concentration of the weak acceptor sites obtained in our experiments is lower by more than an order of magnitude. Their concentration determined by the developed procedure can be used as the lower limit of the active site concentration.
The experimental results obtained in this study seem to support the hypothesis attributing the weak acceptor sites to strong Bronsted acid sites as well. However, only some of such sites are active. Typical concentrations of conventional Bronsted acid sites on acid-doped alumina catalysts are higher than those of the weak acceptor sites. Furthermore, according to the results of quantum chemical calculations, their electron affinity is very low. Obviously, additional experimental and theoretical studies are required to understand the real meaning of the observed correlations and the true nature of the electron acceptor sites.
A procedure for characterization of weak acceptor sites by EPR using anthracene as the spin probe has been developed. Still such procedure involving heating of the sample at 80°C for 12 hours appears to be a simple, reasonably fast and readily reproducible method for characterization of the weak acceptor sites. This method can be used for investigation of the acceptor properties of various catalysts, including thermally unstable ones and the ones obtained after heat treatment at low temperatures. In this study it was applied for characterization of a series of doped alumina catalysts used for ethanol dehydration.
Modification of Al2O3 with sulfate and chloride ions was found to result in the increase of the catalytic activity in ethanol dehydration accompanied by the growth in the concentrations of the weak and strong acceptor sites. Meanwhile, the concentration of the electron donor sites decreased, suggesting that the donor sites are not related to the sites active in the ethanol dehydration. A remarkably good correlation between the concentration of the weak donor sites and the catalytic activity of the acid-modified catalysts almost passing through the origin of coordinates was obtained for all the studied samples. Therefore, it appears that the weak acceptor sites tested using anthracene are related to the sites active in the ethanol dehydration reaction. Besides the studied alcohol dehydration reaction the developed method for characterization of the weak acceptor sites can be useful for investigation of the sites active in other catalytic reactions taking place on acid catalysts. We believe that the observed good correlation between the concentration of the weak acceptor sites and the catalytic activity in such typical acid-catalyzed reaction as ethanol dehydration will help to understand better the nature of the surface sites accounting for both processes.
Acknowledgment. Financial support by the Russian foundation for Basic Research (Grant 10-03-00691) is acknowledged with gratitude.
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