Retained and desorbed products from reaction of 1-hexene over H-ZSM5 zeolite: Routes to coke precursors

Abstract

Retained and desorbed products have been studied from the reaction of 1-hexene over H-ZSM5 zeolite at 353–593 K, with supplementary information using cyclopentadiene, cyclopentadiene dimer, and n-hexane. At 353–393 K the retained products from 1-hexene were paraffinic, and the associated dehydrogenation products (alkylbenzenes) were desorbed. The retained paraffins contained some chain branching and a distribution in Cn(C 8C 15, C 12 max) resulting from facile olefin oligomerization, cracking and isomerization. At > 393 K unsaturated cyclic components were increasingly important in the retained products, with the evolutionary sequence, (alkyl)cyclopentadienes, (alkyl)indanes/indenes/tetralins, (alkyl)dihydronaphthalenes, leading to higher fused ring aromatics. Alkylbenzenes were by-products to this main sequence. Except for the (difficult) first steps, the residue-forming reactions from n-hexane followed those of 1-hexene. The reactions of cyclopentadiene and cyclopentadiene dimer indicated two distinct reaction paths leading to tetralin and methylindanes/indenes. From the dependence of residue weight and accessible pore volume on 1-hexene treatment conditions, it is concluded that the residue formed at 393–593 K was mainly within the pores: heating to ≥ 593 K caused this residue to migrate to the external zeolite surface where, free from size/shape selectivity restrictions, conversion into relatively involatile, higher molecular weight, products was possible. On H-ZSM5 which had been SiCl 4-treated to deacidify the external surface, the 1-hexene reaction at 513 K was the same as without SiCl 4 treatment. At 593 K, surface deacidification caused a large decrease in the proportion of higher aromatics, confirming that the latter are mainly formed at the external surface.