Direct conversion of ethene to propene on Ni/AlMCM-41 – study of the reaction mechanism

Introduction: Recently, it has been shown that the direct conversion of ethene to propene (ETP-reaction) is an attractive alternative to face the increasing demand of propene [1-3]. In this reaction, it is proposed that propene is producing through a sequence of three single chemical reactions [2]. This sequence consists in the dimerization of ethene to 1-butene in the first step. In the second one, 1-butene dimerizes to 2-butene that reacts with another molecule of ethene to produce propene through a metathesis reaction. More recently, it has been suggested that during the ETP-reaction, propene is participating in other type of reactions [4]. Therefore, in this work a detailed study of the reaction mechanism of the ETP-reaction has been carried out by investigating separately the isomerization of butene isomers, the metathesis of ethene and 2-butene and the retro-metathesis of propene. In situ DRIFTS characterization was also performed. Experimental: The catalysts synthesis procedure is described in [4]. The following experiments were performed; (a) Isomerization of 1-butene, cis-butene, trans-butene by feeding a mixture of 2 vol.% of 1- or cis- or trans-butene with a GHSV of 200 l h-1gcat-1; (b) Metathesis of ethene and 2-butene (trans- or cis-butene) by feeding a mixture of ethene and trans- or cis-butene with a feed concentration of 2.5 vol.% for each olefin, GHSV 1.4 h-1gcat-1; (c) Metathesis of propene was carried out by feeding a mixture of 5 vol.% propene in nitrogen, GHSV 1.4 h-1gcat-1. All experiments were performed in a fixed-bed reactor consisting of a quartz tube with an inner diameter of 0.6 cm at atmospheric pressure. The temperature was changed from 50 to 475 °C. The feed and reaction products were analyzed by on-line CG/MS. In situ DRIFTS was done for ETP-experiments by feeding to a DRIFTS cell a mixture of 2.5% of ethene in nitrogen. The DRIFTS spectra were collected at different temperature without interrupting the ethene feed. Results and short discussion No side reactions were observed in the isomerization of cis- and trans-butene. The contrary was observed in the isomerization of 1-butene. The occurrence of the isomerization of butene confirms the acidity of the catalysts. No catalytic cracking of butene was observed because of the modest acidity of the catalysts. Metathesis of ethene and butenes did not produce propene as main reaction product, indicating that Ni/AlMCM-41 catalysts are not efficient catalysts in metathesis reactions. This behavior was confirmed by feeding propene into the reactor (the retro-metathesis of propene), where the main reaction products were hexenes isomers. The in situ DRIFTS experiments confirmed the formation of oligomeric species on the surface of the catalyst during the ETP-reaction. This type of compounds might be precursor for different carbon species that were observed in the ETP-reaction [4]. The results obtained in this work strongly suggest that a conjunct polymerization (hydropolimerization) is taking place, where propene is one of the main reaction products at high temperature. This conjunct polymerization yields a product that is a complex mixture of saturated (alkanes and cycloalkanes) and unsaturated (alkanes, alkapolyenes, cycloalkenes, and cycloalkapolyenes) hydrocarbons, and occasionally even aromatic compounds [5]. No metathesis properties could be confirmed on Ni/AlMCM-41 catalysts. Detailed results will be discussed during the poster session. References [1] M. Iwamoto, Y. Kosugi, J. Phys. Chem. C., 111 (2006) 13. [2] M. Iwamoto, Catal. Surv. Asia, 12 (2008) 28. [3] T. Lehmann, T. Wolff, V.M. Zahn, P. Veit, C. Hamel, A. Seidel-Morgenstern, Catal. Commun., 12 (2011) 368. [4] L. Alvarado Perea, T. Wolff, P. Veit, L. Hilfert, F. T. Edelmann, C. Hamel, A. Seidel-Morgenstern, J. Catal. 305 (2013) 154. [5]. Olah, G.A.,Á. Molnár, Hydrocarbon Chemistry, 2003