Toward Better Understanding of the Catalytic Action of Acidic Zeolites: Investigation in Methane and Ethane Activation and Transformation

Studies of cracking reactions of alkanes with three or more carbon atoms have been central to the development of our understanding of the catalytic action of acidic zeolites, which are important catalysts in petrochemical and chemical industries. However, the mechanisms of the simplest cracking reactions, that is, the cracking of the C–H and C–C bonds in methane and ethane, have only been studied theoretically, not experimentally. Here we show that ethane is converted over a H-MFI zeolite at 510–550 °C with formation of such primary products as ethene, hydrogen, methane, and propane. To explain these results, we suggest and consider two catalytic cycles of the reaction. The first cycle involves protolytic cracking of the C–H bond with formation of hydrogen and ethoxide group, the latter decomposing into ethene and the zeolite acid site. We propose that the second cycle is initiated by the protolytic cracking of the C–C bond that results in formation of methane and a methoxide group as an intermediate. We theorize that this reacts with ethane molecules regenerating the zeolite acid site and producing (i) methane and ethene via hydrogen transfer and (ii) propane via a C–C bond formation reaction. Both suggested catalytic cycles are fully supported by the kinetic results of this study and are in a good agreement with recent theoretical work. We also demonstrate that the cracking of the C–H bond in methane (that could proceed via methoxide intermediate) does not occur over H-MFI zeolites up to 700 °C most likely because of a high activation energy. The proposed involvement of methoxide groups, as active intermediates, in ethane transformation provides a basis and excellent opportunity for theoretical studies of such interesting reactions as hydrogen transfer and C–C bond formation with participation of these surface species

Insights into Brønsted Acid Sites in the Zeolite Mordenite

The unique feature of the zeolite catalysts is the presence of catalytically active acidic hydroxyls, also known as Brønsted acid sites (BAS), in the zeolite micropores of molecular dimensions. The accessibility and catalytic properties of BAS depend on their local environment, and it is therefore important to know the exact locations of BAS and the number of BAS in these locations. This paper reports a detailed FT-IR investigation into BAS present in the acidic and partially Na-exchanged samples of industrially important mordenite (MOR) zeolite. Our results demonstrate the existence of (at least) six distinct BAS that can be visualized by six single bands in Fourier self-deconvolution traces of the IR spectra. The quantitative estimates for the amounts of these distinct BAS were obtained using the six-band deconvolution method developed in this work. These estimates show that in the purely acidic H-MOR sample about 25% of BAS are located in eight-membered ring (8-MR) channels (O1–H and O9–H hydroxyls), ∼13% of BAS are at the intersections between the side pockets and 12-MR channels (O5–H hydroxyls), and ∼62% of BAS are located in 12-MR channels (∼39% correspond to O2–H and/or O10–H hydroxyls and the remaining 23% to O3–H and O7–H hydroxyls). These quantitative data demonstrate that the acid sites are distributed quite evenly between oxygen atoms in different crystallographic positions, thus revealing the complexity of the experimental identification of distinct BAS in mordenites and explaining the variety of the earlier suggestions regarding their positions in these zeolites