COKING, AGING, AND REGENERATION OF ZEOLITES .17. COMPOSITION AND LOCATION OF CARBONACEOUS COMPOUNDS RESULTING FROM ISOBUTENE AND PROPENE TRANSFORMATION ON A 5A ZEOLITE

The formation of coke from propene and isobutene on a 5A zeolite and i ts effect on the adsorption capacity were investigated under the follo wing conditions: flow reactor, 623 K; Palkene = 1 bar; WHSV = 0.65 h(- 1) (propene) or 0.85 h(-1) (isobutene). Initially the formation of cok e was 1.5 times slower from isobutene than from propene, A plateau in the formation of coke was found after 30 h, the percentage of coke at this plateau being of 10 wt% from isobutene and of 13 wt% from propene . The composition and the location of coke were determined. At short t ime onstream coke was constituted of alkylbenzenic and naphtalenic com pounds trapped in the ex cages of the zeolite, The growth of these com pounds led to polyaromatic compounds that could overflow onto the oute r surface of the crystallites. A larger a mount of these polyaromatic compounds was found with isobutene. Coke formed from isobutene and fro m propene affected differently the adsorption capacity of the zeolite, To explain these differences it was proposed that because of diffusio n limitations, isobutene was transformed into coke only in the cages c lose to the outer surface of the zeolite crystallites. On the contrary , coke formed from propene was homogeneously distributed within the cr ystallites.