Propane Transformation on In-Modified Zeolite BEA

In-modified zeolites possess promising catalytic properties for light alkane dehydrogenation and aromatization. However, the role of different indium species, which are present in zeolite pores after activation, remains unknown. Here, the transformation of propane on BEA zeolite containing either In+ or InO+ species in zeolite pores has been monitored with 13C MAS NMR at 298–773 K. It is inferred that In+/H-BEA zeolite with In+ sites is inactive for alkane conversion at T +/H-BEA zeolite occurs by two parallel routes: dehydrogenation of propane followed by the formed alkene aromatization to simple aromatic hydrocarbons and the alkane oxidation resulting in C2–C3 carboxylic acids. Propane activation by either C–H or C–C bond cleavage on InO+ sites and pathways of carboxylic acid formation from the products of the alkane dissociative adsorption are discussed

Different Efficiency of Zn²⁺ and ZnO Species for Methane Activation on Zn-Modified Zeolite

Understanding methane activation pathways on Zn-modified high-silica zeolites (ZSM-5, BEA) is of particular importance because of the possibility of methane involvement in coaromatization with higher alkanes on this type of zeolites. Herein, two samples of Zn-modified zeolite BEA containing exclusively either small zinc oxide clusters or isolated Zn²⁺ cations have been synthesized and thoroughly characterized by a range of spectroscopic methods (¹H MAS NMR, DRIFTS, XPS, EXAFS, HRTEM) to show that only one of the Zn-species, either Zn²⁺ cations or ZnO small clusters, exists in the void of zeolite pores. The ability of zinc sites of different nature to promote the activation of methane C–H bond with the zeolite Brønsted acid sites (BAS) has been examined in the reactions of methane H/D hydrogen exchange with BAS and the alkylation of benzene with methane. It has been found that both ZnO and Zn²⁺ species promote the reaction of H/D exchange of methane with BAS. The rate of H/D exchange is higher by 2 and 3 orders of magnitude for the zeolite loaded with ZnO or Zn²⁺ species, respectively, compared to pure acid-form zeolite H-BEA. So, the promoting effect of Zn²⁺ cations is more profound than that of ZnO species for H/D exchange reaction. This implies that the synergistic effect of Zn-sites and BAS for C–H bond activation in methane is significantly higher for Zn²⁺ cations compared to small ZnO clusters. It has been revealed, however, that only Zn²⁺ cations promote the alkylation of benzene with methane, whereas ZnO species do not. The isolated Zn²⁺ cations provide the formation of zinc-methyl species, which are further transformed to zinc-methoxy species. The latter is the key intermediate for the performance of the alkylation reaction. Hence, while both zinc oxide clusters and Zn²⁺ cationic species are able to provide a synergistic effect for the activation of C–H bonds of methane displayed by the dramatic acceleration of H/D exchange reaction, only the Zn²⁺ cationic species perform methane activation toward the alkylation of benzene with methane. This implies that only the Zn²⁺ cations in Zn-modified zeolite can activate methane for the reaction of methane coaromatization with higher alkanes