Hydrocarbon Pool Mechanism of the Zeolite-Catalyzed Conversion of Ethene to Propene

In order to detail the hydrocarbon pool mechanism of the zeolite-catalyzed ethene to propene (ETP) conversion, ethene conversion was conducted over H-UZM-35 zeolite in the presence of a small concentration (2 mol % of ethene) of 10 different aromatic hydrocarbons. Among the cofeeds employed, only C-2 substituted bicyclic aromatic species (i.e., 2-methynaphthalene, 2-ethylnaphthalene, and 2-isopropylnaphthalene) were found to have beneficial effects on the selective formation of propene. The overall experimental results of this study led us to propose a bicyclic aromatic-based mechanism of the zeolite-catalyzed ETP reaction where naphthalene, C-2 substituted bicyclic aromatics, and 2-isopropyl-7-methynaphthalene serve as hydrocarbon pool species. The theoretical results demonstrate that the Gibbs free energy barriers of the transition states for the ethylation of key bicyclic hydrocarbon pool species (naphthalene, 2-methylnaphthalene, and 2-ethylnaphthalene) are significantly lower compared with the transition states for the same elementary step of the corresponding monocyclic ones (benzene, toluene, and ethylbenzene). Intuitively, this can be understood given that the transition state for the ethylation of each of the former aromatic species has a larger number (5 vs 3) of resonance structures than that of each of the latter ones and is thus thermodynamically more stable.