A study of active sites on fibrillar alumina catalysts
Hightower, Joe W.
Master of Science
Deuterium exchange with benzene and isomerization/ deuterium exchange of n-butenes were used as test reactions to investigate both the concentration and chemical nature of active sites on two kinds of fibrous alumina catalysts. The results of this investigation are comparable to results reported in the literature describing similar reactions studied over more conventional forms of gamma alumina. However, there were a few notable exceptions which make it difficult to draw general conclusions about the catalytic activity of such materials. It has been shown that the surface of alumina is quite complex and consists of several types of independent active sites. Three of these catalytic sites have been isolated in this work, and previous work has pin-pointed at least two more independent strong adsorption sites. Titration of the catalytic activity with chemical poisons that selectively deactivate only one type of site is the technique that was used to discriminate among the various sites. CC2 selectively poisons the deuterium exchange sites, and titration curves indicate that 6 x 1 12 molecules/cm2 is the "lethal dose" required to stop this reaction completely for both benzene and the n-butenes. Actually, the lethal dose increases with increasing reaction temperature, and this is the value at 75°C. If one adsorbed CC molecule poisons one active site, then the E-sites have a concentration of about 6 x 1 /cm , or in other words, they cover less than 1 percent of the catalyst surface. Since both the catalytic activity and the active site concentration differed by a factor of 1 between the two fibrillar catalysts, the activity/site was about the same for the two materials. The sites were thus assumed to be the same. Butene isomerization had to be divided into two types of reactions, one involving double-bond migration (1-butene to 2-butene) and the other involving cis-trans rotation (cis-2-butene to trans-2-butene). is an effective poison for the former (the I-sites), and by titration the site density was estimated to be about 15 x 1 cm. However, does not appear to be an effective poison for the cis-trans rotation reactions which must occur on separate I-sites. The surface intermediates in the double-bond migration reaction may possibly be the anti- and syn-1-methylirallyl species for conversion of 1-butene into cis- or trans-2-butene, respectively. These species can either be formed on the I'-sites. However, a radical ion CHg -A1 species was postulated to account for the intramolecular cis-trans rotation reaction, this species being found through reaction with a surface A1 Lewis acid site. Some minor complications seem to have made these findings somewhat less certain than would be desired. One of these is the fact that the added poisons may undergo slow surface chemical reactions that can alter their poisoning characteristics. Thus, the length of time of the poison has been in contact with the surface, as well as the temperature, can affect the results. Nevertheless, the overall conclusions are clear, and these results have added new insight into the chemical nature and catalytic activity of alumina surfaces.