Promoting effect of alkali metal on the catalytic performance of hierarchical Pt/Beta in the aromatization of n-heptane

Abstract

Previous research showed that hierarchical Beta zeolite supported Pt was an effective catalyst for the aromatization of alkanes and the modification with K could further enhance its catalytic performance; however, the intrinsic effect of alkali metal on the alkanes aromatization over Pt/Beta was still rather obscure. In this work, a series of hierarchical Pt/Beta-Me catalysts were prepared by modifying the Beta zeolite with different alkali metal nitrates (MeNO3, Me = Li, Na, K and Rb) through ion-exchange and used in the aromatization of n-heptane; the effect of alkali metal on the catalytic performance of Pt/Beta-Me was then systematically investigated. The results indicate that the alkali metals used to modify the Pt/Beta-Me catalysts can not only passivate the strong Brønsted acid sites of Beta zeolite, but also enhance the Pt dispersion and induce an electron-rich chemical environment for the supported Pt species. Accordingly, the modification with alkali metal can effectively suppress the cracking reactions and carbonaceous deposition and meanwhile promote the dehydroaromatization of alkanes, in favor of enhancing the catalytic stability and aromatization selectivity of Pt/Beta-Me. The extent of promoting effect of various alkali metals follows the order of Rb > K > Na > Li. In particular, the Pt/Beta-Rb catalyst exhibits excellent performance in the aromatization of n-heptane, with a much higher selectivity to aromatics (78.2%) and larger turnover number of aromatic products (Pt-based TON, 19.4 × 103) than the unmodified Pt/Beta-H one (27.7% and 4.8 × 103, respectively). These results help to clarify the role of alkali metal played in the catalysis of Pt/Beta-Me for n-heptane aromatization and pave the way for designing more efficient catalysts in the aromatization of alkanes.