Mechanisms of cyclization of 1,3,5-hexatriene on oxide catalysts

Abstract

1. Cyclohexadiene, bensene, and methylcyclopentadienes are mainly formed from the hexatriene in the presence of silica gel and Al2O3 at 300–450°C. Increase in temperature (from 550 to 1100°C) and in the length of calcination of the silica gel (1–6 h) reduces the specific surface area and acidity of the catalyst and the degree of 1–5 and 1–6 cyclization of the triene. 2. In the presence of ZnO and Bi2O3, 1–6 cyclization of hexatriene to cyclohexadiene and benzene mostly occurs; methylcyclopentadienes are formed in insignificant amounts. 3. The 1–5 cyclization of hexatriene on silica gel and Al2O3 evidently takes place by a mechanism of acidic intramolecular alkylation. At 300–450°C, catalytic 1–6 cyclization of hexatriene is observed in addition to the thermal reaction. The idea was put forward that the catalytic 1–6 cyclization is facilitated by acceleration of the isomerization of the trans-triene to the cis isomer and by the effect of the catalyst surface on the conformation of the cis-hexatriene molecules.