The impact of indium and manganese modifiers on the catalytic properties of zinc-alumina propane dehydrogenation catalysts

Abstract

Supported zinc-containing catalysts are considered as a viable alternative to the currently-deployed Cr-based (toxic) and Pt-based (costly) catalysts of alkane dehydrogenation, as reflected also in the increased number of reports related to the Zn-based catalysts in recent years. Here, we present a study of alumina-supported Zn-In and Zn-Mn propane dehydrogenation (PDH) catalysts prepared by the tailored methods. Specifically, the introduction of zinc relied on the chemical vapor deposition (CVD, using Zn vapor) onto alumina or alumina-supported indium or manganese materials, the latter two prepared by the impregnation of alumina with aqueous solutions of the respective metal nitrates followed by the calcination and hydrogen treatment. Alternatively, both zinc and indium were introduced by the co-impregnation of alumina with aqueous zinc and indium nitrates. The PDH tests revealed that the “dilution” of the active Zn2+ surface sites with In+ cations lead to a significant improvement in the catalytic performance, i.e., the introduction of In+ increased propene selectivity and attenuated the undesirable cracking and hydrogenolysis side reactions as well as decreased coke formation. In contrast, Mn2+ cations improved the catalytic properties of Zn2+ surface sites to a lesser extent than In+ cations. All prepared catalysts were characterized by DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy of the adsorbed CO and H2 probe molecules. It was found that surface In+ and Mn2+ cations affect Zn2+ sites (electronic and geometric effect), which can be assessed from the position of the IR bands of zinc hydridespecies, formed due to the heterolytic hydrogen dissociation on the Zn2+ active sites. The effect of In and Mn dopants also was confirmed by the molecular DFT simulation.