The developed procedure for investigation of the weak acceptor sites was used for characterization of a series of alumina catalysts doped with chlorine or sulfate ions in addition to the earlier developed methods for determination of the concentrations of the electron donor sites and strong electron acceptor sites. The synthesized catalysts were also tested in ethanol dehydration reaction.
It was found that the modification of alumina with chloride and sulfate ions leads to a substantial increase of the catalytic activity in ethanol dehydration (Fig. 4). The addition of both ions is known to increase the alumina acidity. Chlorinated alumina is a widely used acidic catalyst whereas sulfated alumina is known to have very strong acid sites and was even claimed to be superacidic.
We used the ethylene formation rate at low ethanol conversion as the quantitative measure of the catalytic activity in the reaction leading to the desired product. Coking was negligible under these conditions. Modification with sulfates was found to be more efficient than modification with chlorides, producing more active catalysts. For the catalysts containing 2 and 4 wt.% of the ions the sulfated catalysts were approximately twice more active. Note that the molar weight of the sulfate group is almost 3 times higher than that of the chloride. Therefore, assuming that the sites of the same type are active on both types of the catalysts, it appears that alumina modification with the same number of sulfate groups generates approximately 6 times more sites active in the ethanol dehydration compared to the modification with chlorides. Most likely, this result indicates that the strength of the generated acid sites is still important as stronger acid sites are formed over sulfated alumina.
Figure 5 shows that the concentration of the weak acceptor sites tested using anthracene according to the procedure discussed in detail above linearly grows when the concentration of the introduced sulfates increases from 0 to 8 wt.%. After addition of even low concentration of the sulfates (2 wt.%) the measured concentration of the weak acceptor sites substantially increases. Meanwhile, the measured concentration of the strong acceptor sites depends on the sulfate concentration in a substantially different way. It does not increase much until the introduction of 4 wt.% SO42-. After introducing 8 wt.% SO42- the concentration of the strong acceptor sites almost doubles.
Meanwhile, the concentration of the electron donor sites gradually decreases with the introduction of sulfates. The donor sites are generally associated with the basic rather than acidic properties of solid catalysts. So, the decrease of their concentration when alumina is modified with sulfates reflects the trend of the basicity decrease. As the concentration of the donor sites and the catalyst activity in the ethanol dehydration reaction change in opposite directions on the introduction of sulfates, almost definitely, the donor sites are not related to the sites active in the ethanol dehydration.
Generally similar trends were observed for the sites of the chloride-doped catalysts (Fig. 6). The concentration of the weak acceptor sites substantially increases after introduction of only 1 wt.% Cl-. Then, their concentration slow grows between 1 and 4 wt.% Cl-. The relative increase in the concentration of the strong acceptor sites after introduction of 1 wt.% Cl- is not so big compared to that of the weak sites. Then, the growth in the concentration of the strong acceptor sites with the chloride concentration almost parallels that of the weak sites (Fig. 6). Meanwhile, the concentration of the donor sites almost linearly decreases with the addition of the chloride ions.
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