Dehydrocyclodimerization of conjugated dienes catalyzed by solid bases

Abstract

1,3-Butadiene and 2-methyl-1,3-butadiene undergo dehydrocyclodimerization to form aromatics over solid base catalysts such as ZrO 2, MgO, CaO, SrO, La 2O 3, and Nd 2O 3 below 473 K. Among the catalysts examined, ZrO 2 exhibited the highest activity. In the reaction of 1,3-butadiene, the main products were ethylbenzene, o- and p-xylenes. Over MgO catalyst, o-xylene was most selectively formed, while over the other catalysts ethylbenzene was selectively formed. In the reaction of 2-methyl-1,3-butadiene, which is more reactive than 1,3-butadiene, the main products were m- and p-cymenes. Over MgO and CaO catalysts, m-cytnene was predominant, while over ZrO 2 catalyst, p-cymene was primarily formed. On the basis of product distributions and poisoning experiments with carbon dioxide, water, pyridine and ammonia, it is suggested that basic sites on the catalysts participate in the reaction, and the reaction mechanisms are discussed. Over MgO, it is proposed that the main reaction involves anionic intermediates for dehydrocyclodimerization of both 1,3-butadiene and 2-methyl-1,3-butadiene. Over ZrO 2, base-catalyzed Diels-Alder reaction followed by double bond isomerization and dehydrogenation are proposed.