Characterization of the active sites on the surface of alumina ethanol dehydration catalysts by EPR using spin probes, страница 7

The obtained data prove that the concentration of the weak acceptor sites correlates with the catalytic activity of the modified alumina catalysts in the ethanol dehydration reaction. Such correlation is illustrated by Figure 7. It includes data for two commercial alumina samples AOK-62-22 and AOK-78-59 as well as the aluminum oxides modified with chlorides and sulfates. A remarkably good correlation 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.

The correlation between the catalytic activity and the concentration of strong acceptor sites (Fig. 8) was not nearly as good. The data for the samples containing 2 and 4 wt.% SO₄^2- obviously fall out. For these samples the concentration of the strong acceptor sites was not much higher than on the original Al₂O₃ sample, whereas the catalytic activity and the concentration of the weak acceptor sites were substantially higher. Also, the straight line obtained for the remaining samples does not pass through the origin of coordinates. Overall, it appears that the strong acceptor sites alone are unlikely to account for the catalytic activity of the alumina catalysts in ethanol dehydration.

Industrial alumina materials used as the supports, catalysts and sorbents for various processes are known to contain iron, silicon and sodium. The iron and silicon impurities in concentrations below 0.1 wt.% have practically no effect on the catalytic activity. Meanwhile, sodium cations present in concentrations as high as 0.7 wt.% in some industrial alumina samples are known to be poisons for many acid-base catalytic processes. For example, it was shown that the Al₂O₃ with Na concentration 10 ppm was 3-5 times for active in isopropanol dehydration than Al₂O₃ with 180 ppm Na concentration and approximately 180 times more active than the one with 3020 ppm Na concentration [27]. Generally similar results on the effect of sodium concentrations below 1% were also reported for ethanol dehydration [28]. To study the effect of sodium on the catalytic activity and concentration of the acceptor and donor sites we prepared a series of alumina samples with sodium concentrations varying from 0 to 0.9 wt.% using ultrapure aluminum hydroxide as the starting material.

The effect of sodium on the concentrations of the weak acceptors sites and catalytic activity in ethanol dehydration is illustrated in Figure 9. The obtained results indicate that the sodium addition results in a substantial decrease both in the concentration of the acceptor sites and the catalytic activity in ethanol dehydration. Even small amount of sodium about 0.3 wt.% decrease the catalytic activity by almost a factor of two. Meanwhile, no apparent effect of sodium on the concentration of donor sites was observed.

Note that the sodium impurities about 0.2-0.5 wt.% are present in most commercial alumina samples prepared by precipitation or hydration of the products of hydrargillite thermal activation. These results clearly indicate that alumina supports with as low as possible sodium concentrations should be used for synthesis of catalyst for ethanol dehydration.

Again a good correlation of between the concentrations of weak acceptor sites tested using anthracene and the dehydration activity of the sodium-doped catalysts was obtained. However, in this case the linear dependence does not start from the origin of coordinates. These results suggest that some weak acceptor sites are present on the surface of sodium-doped alumina samples with negligible catalytic activity.

Apparently, these materials do not have any sufficiently strong acid sites that are active in the ethanol dehydration as the active protons are substituted with the sodium ions. Meanwhile, sodium forms of zeolites are known to possess weak acceptor sites, although both their concentration and strength are lower than in the acidic forms of the same zeolites. Most likely, the sodium-doped alumina samples prepared by us also have some weak acceptor sites related to sodium cations present on the surface that are not active in the acid-catalyzed dehydration reaction.