Dehydrocyclization of paraffins. Influence of chlorine on cyclization pathway over Pt-Al 2O 3 catalysts

Abstract

Platinum-alumina catalysts are usually prepared by impregnation with chloroplatinic acid (1). McHenry et al. (b), have proposed that a fraction of the chloroplatinum complex survived the catalyst pretreatment; this complex-but not metallic platinum-was soluble in aqueous HF. They found that dehydrocyclization was catalyzed only by the soluble chloroplatinum fraction; isomerization and cracking were catalyzed by the metallic platinum. Other laboratories subsequently reported results which supported this proposal (S-5). Bursian et al. (6’) claimed that the soluble Pt-Cl-A1,03 complex was even more specific and catalyzed only cyclizations to naphthenes and that all dehydrogenation took place on the metallic platinum. In contrast, Kluksdahl and Houston (7) and Johnson and Keith (8) found that the soluble platinum was present only after the reduced catalyst was exposed to oxygen. Thus, the soluble form could not be the active dehydrocyclization catalyst but may still be the source of the Pt active for dehydrocyclization. The aromatic isomer distribution may be influenced by the various catalytic materials (9) ; hence, it could be expected that a different isomer distribution may be obtained when chlorine is present than when it has been removed. Thus, we have determined the aromatic isomer distribution for the dehydrocyclization of 3-methylheptane over catalysts with and without chlorine.