The chemistry of coke deposits formed on a PtSn catalyst during dehydrogenation of n-alkanes to mono-olefins

Abstract

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 GC-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-Alder-type 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.