1964 Volume 37 Issue 12 Pages 1797-1809
This study has aimed to secure an optimum means for the preparation of an alumina-boria catalyst by a systematic examination of the relationship between the physical characteristics of the catalyst and its activity, and to clarify the mechanism of the development of activity on the basis of the experimental evidence about the chemical interaction of alumina with boric acid and about the chemical structure of the catalyst. The applicability of the catalyst has been studied kinetically for some specific reactions relating to the transalkylation of aromatic compounds. It has been found that alumina-boria shows a higher activity for these reactions than does silica-alumina.
The results and conclusion may be summarized as follows:
I) Boric acid is dehydrated to produce metaboric acid as a probable intermediate and reacts with an active alumina at about 130°C with little disturbance of the crystal structure of alumina, and effective acidic centers are produced. Most of the acid centers are of the Brönsted type. Alumina-boria does not show any definite chemical structure characteristic of aluminum borates, such as 9Al2O3·B2O3, 2Al2O3·B2O3 or Al2O3·B2O3. It is suggested that such a bond as Al-O-B-O-Al may be formed between surfaces of the crystal structure of alumina and may result in acidic centers.
II) The catalytic activity of toluene disproportionation shows a good correlation with the Bronsted acidity. This is well compatible with the likely theory that the reaction proceeds via a carbonium ion mechanism.
III) The activity of the disproportionation is considerably affected by the structure of the alumina used as the catalyst base as well as by the boria content of the catalyst. The catalyst prepared from η-alumina shows a higher activity than that from χ-alumina, while α-alumina does not give an active catalyst because of its poor reactivity with boric acid. A rise in the crystallinity of alumina lowers the reactivity. Excessive boria reduces the catalytic activity. An effective catalyst for the transalkylation of aromatic compounds can be obtained when alumina-boria is calcined above 400°C.
IV) The results of such reactions as the disproportionation of toluene, the isomerization of xylene and the transethylation between benzene and diethylbenzene can be well interpreted kinetically by assuming that the rates of these reactions are controlled by surface reactions between adsorbed molecules of the reactants.
V) In the disproportionation of toluene, a decrease in the activity of the catalyst with the length of the run was observed. This phenomenon can be explained analytically by taking into account the fact that some contaminant such as the carbonaceous material, which is produced by side reactions (cracking of toluene and xylene), covers effective sites of the catalyst.
VI) It seems to be due essentially to the higher concentration of active sites in alumina-boria that its activity of the transalkylation of aromatic compounds is higher than that of silica-alumina.
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