Characterization of the active sites on the surface of alumina ethanol dehydration catalysts by EPR using spin probes, страница 8

The nature of the surface sites accounting for the formation of radical cations upon adsorption of aromatic substances on solid catalysts largely remains a mystery despite being known for more than 50 years. It is well known that acceptor sites are observed either in the presence of some transition metals or on acid catalysts [20]. The possibility of ionizing various organic molecules after adsorption on acid catalysts seems to be one of the most intriguing properties of such materials. For instance, benzene with ionization potential 9.2 eV can be ionized after adsorption on high-silica zeolites or sulfated zirconia. No less surprising is the formation of radical cations in substantial concentrations as high as 1020 g-1 after adsorption of aromatic compounds with low ionization potentials about 7 eV on various zeolites and amorphous alumosilicates [20].

In the literature the electron acceptor sites have been attributed to Lewis acids sites accepting in this case only one electron instead of an electron pair [29], Bronsted acid sites [30], oxygen radicals formed during the catalyst thermal treatment [31], singlet oxygen [32] and transition metal impurities [33]. However, none of these models has been proven experimentally or theoretically. In fact, no model of surface active sites with the electron affinity approaching the experimental values has been suggested.

In our opinion the existing close links between the acidity and the electron acceptor ability of acid catalysts seem to indicate that some these two generally very different properties are interrelated. So, the observed good correlation between the concentration of the weak acceptor sites and the catalytic activity in ethanol dehydration can help us to understand better the nature of the surface sites accounting for both processes.

Alcohol dehydration in solution can be catalyzed both by acids and bases. Meanwhile, on solid catalysts this process is much more efficient on acid catalysts. Apparently, this means that the acid sites of desired strength are much more abundant on the surface of solid catalysts than the corresponding basic sites. This observation seems to indicate that the acid sites have to be rather strong to be active in alcohol dehydration, although it has been claimed that the concentration of acid sites is more important than their strength for this reaction as it takes place on various acid catalysts. It has been reported that the presence of strong acid sites leads to an increase of the catalytic activity in alcohol dehydration. In any case, we have no reason to believe that this reaction can be initiated by electron transfer on the electron acceptor sites.

The highest dehydration rate was observed when the ratio of Bronsted to Lewis acid sites was as high as possible [34]. H-ZSM-5 zeolite known to possess high concentration of strong Bronsted acid sites was reported to have very high dehydration activity substantially exceeding that of Al2O3, experiencing fast deactivation due to coking [35]. Overall, the analysis of the literature data suggests the sites active in alcohol dehydration are most likely related to sufficiently strong Bronsted acid sites.

Accurate determination of the active site concentration is a problem for most catalytic processes. The reported concentration of sites active in butanol dehydration on Al2O3 determined in poisoning experiments is about 1013 cm-2 that is about 1% of the monolayer [36]. Obviously, such poisoning experiments may affect some of the inactive sites in addition to the active ones and can serve as the upper limit of the active sites concentration. As this concentration is substantially lower that typical concentrations of the acid sites determined by FTIR, apparently, only the some of them with sufficient strength can initiate this reaction.