Reaction mechanisms and kinetics in the n-hexane cracking over zeolites

Abstract

The reaction mechanisms of the cracking of n-hexane over MFI zeolites ( Si Al =10–75 ) have been studied at 400°C at low conversions in a microflow reactor. The reaction was found of first order in the partial pressure of n-hexane and second order in the aluminium content of the zeolites. The analysis of the kinetic parameters and selectivities showed that the reaction predominantly proceeded through the protolytic monomolecular mechanism involving the formation then the decomposition of an adsorbed carbonium ion-like species. This decomposition leads to n-alkanes and adsorbed carbenium ion-like species or, most probably, to n-olefins. The linear olefins (mainly butenes) should be the primary products of the cracking of n-hexane and, as such, should be considered as the main source of the subsequent reactions, notably the hydride transfer reactions. The latter reaction was favoured at high conversions, long contact times and high aluminium contents. It is suggested that, in the n-hexane cracking, the acid as well as the conjugated base sites of the zeolite play an important role in the protolysis process and explain the kinetic orders reported here and in the literature data. Two reaction paths have been envisaged: (i) the second order/high temperature (>450°C) mechanism which involved one n-hexane molecule and one zeolite acid site in the rate determining step and; (ii) the third order/low temperature mechanism which involved one n-hexane molecule and two zeolite acid sites in the rate determining step. In the latter case the protolysis step needed the anchimeric assistance of a second active site. In the proposed mechanistic schemes, the gaseous n-alkane molecule was adsorbed on Brönsted acid sites then decomposed with the help of the conjugated base sites.