THE CHEMISTRY OF COKE DEPOSITS FORMED ON A PT-SN CATALYST DURING DEHYDROGENATION OF N-ALKANES TO MONO-OLEFINS

This work presents a detailed study of coke deposition on a Pt-Sn catalyst under industrial conditions, The spent catalyst was submitted to Soxhlet extraction and the ''soluble coke'' was characterized by means of IR, UV and CC-MS. The catalyst before and after coke extraction was characterized by means of IR, carbon content and temperature-programmed oxidation (TPO). Coke formation on the catalyst involves several processes: (i) successive dehydrogenation/cyclization of alkyl chains (polyene route); (ii) n-alkane oligomerization; (iii) Diels-Aldertype reactions. The structure of insoluble coke is quite different from that of soluble coke: the elimination of the latter one causes disappearance of the low-temperature oxidation zone (TPO). Soluble coke contains dominant proportion of aliphatic groups whereas polyaromatic structures predominate in insoluble coke.