Combined experimental and kinetic modelling studies of the pathways of propane and [formula omitted]-butane aromatization over H-ZSM-5 catalyst

Abstract

Kinetic studies of propane and n -butane aromatization were performed over H-ZSM-5 at 500 ∘ C at the conversions between 1% and 88%. Based on the results obtained, a kinetic model for aromatization of both alkanes has been developed. The model contains 38 rate constants, two adsorption constants, and 38 reaction steps to describe transformation of 13 components involved in these two reactions. Analysis of the experimental and kinetic modelling data has confirmed that propane and n -butane transformation over H-ZSM-5 occurs via two routes: (i) protolytic cracking (PC) of C–C and C–H bonds in alkane molecules and (ii) hydrogen transfer (HT) between the feed alkane and product alkenes adsorbed on acid sites. Cracking of C–C bonds is favoured over cracking of C–H bonds for both alkanes; and the n -butane reactivity in the PC steps is 5 times higher than that of propane. The contribution of the HT route into propane and n -butane transformations was estimated quantitatively for the first time and was shown to increase with alkane conversion, being always higher for the n -butane reaction (e.g., at conversion of 10%, this contribution was around 10% and 25% for propane and n -butane reactions, respectively). As a result, an increase in the reaction rate was observed at low n -butane conversions, demonstrating for the first time the autocatalytic character of this reaction. However, in agreement with the lower contribution of the HT steps in conversion of propane, the autocatalysis was not observed in the propane reaction. Light alkenes formed in the initial reaction steps give rise to fast alkene oligomerization/cracking steps that control alkene distribution during alkane aromatization. The rate constants of the alkene oligomerization/cracking steps are about 10 times higher than those of the aromatization steps, which, in turn, are about 1000 times higher than the sum of the rate constants of the propane PC steps. Direct comparison of the aromatics formation in propane and n -butane reactions shows that the aromatics concentration, when determined at the same conversions of two alkanes, is always higher for propane reaction. This finding is explained by the difference in the propane and n -butane reactivity, and allows us to predict higher aromatics concentration/selectivity in conversion of ethane as compared to reaction of propane.