Coke formation and coke profiles during the transformation of various reactants at 450°C over a USHY zeolite Original Research Article

Coking of a USHY was studied in a fixed-bed reactor at 450°C. Different model compounds were used: an alkane (n-heptane), an alkene (propene), a naphthenic (methylcyclohexane) and aromatics (m-xylene, 1-methyl-naphthalene). Those reactants present very different coking rates in the order: 1-methyl-naphthalene>propene⪢m-xylene>n-heptane>methylcyclohexane. The coke profile along the reactor is a function of the coke precursors. By using a triple-bed reactor, it is shown that the coking process proceeds primarily with olefins and aromatic reactants and secondarily with alkanes and naphthenes. In the first case, coke is preferentially deposited on the first part of the bed, whereas in the second case, the coke deposit 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 six aromatic rings located in the zeolite pores. Also, highly polyaromatic compounds located either inside the pores or on the external surface of zeolite crystallite were observed.