Coke formation and coke profiles during the transformation of various reactants at 450 degrees C over a USHY zeolite

Coking of a USHY was studied in a fixed-bed reactor at 450 degreesC. Differ ent model compounds were used: an alkane (n-heptane). an alkene (propene), a naphthenic (methylcyclohexane) and aromatics (m-xylene, 1-methyl-naphthal ene). Those reactants present very different coking rates in the order: 1-m ethyl-naphthalene > propene much greater than m-xylene > n-heptane > methyl cyclohexane. The coke profile along the reactor is a function of the coke p recursors. By using a triple-bed reactor, it is shown that the coking proce ss proceeds primarily with olefins and aromatic reactants and secondarily w ith alkanes and naphthenes. In the first case, coke is preferentially depos ited on the first part of the bed. whereas in the second case, the coke dep osit is rather concentrated on the last part of the bed (after formation of coke maker molecules). GC-MS analysis has shown that whatever the reactant , coke is mainly constituted by small polyaromatic compounds with four to s ix aromatic rings Located in the zeolite pores. Also. highly polyaromatic c ompounds located either inside the pores or on the external surface of zeol ite crystallite were observed. (C) 2001 Elsevier Science B.V. All rights re served.