ASPECTOS ENERGÉTICOS E ELETRÔNICOS DA ZEÓLITA H-ZSM-5 NA AÇÃO CATALÍTICA DA REAÇÃO DE DESIDRATAÇÃO DE ÁLCOOIS

Due to the growth of ecological concerns and the need to reduce dependence on fossil fuels, the dehydration of alcohols by acid catalysis has been used for the production of various hydrocarbons. Inside this theme, the H-ZSM-5 zeolite has been widely used as a catalyst for this reaction because its high efficiency. Thus, in order to understand the catalyzed reaction mechanism of the alcohol dehydration reaction, this work used the computational methodology ONIOM to study the catalytic behavior of the H-ZSM-5. It was modeled the dehydration reaction process for several alcohols (ethanol, propanol, isopropanol, butanol and 2-butanol) by modeling these alcohols within the zeolite cavity. The study was divided into 3 stages: the adsorption and protonation of alcohols by zeolite, the description of the hydroxyl outlet, and the formation of the double bond. The analysis of the results indicates that the first stage of the reaction occurs with the contact of alcohol with the zeolite cavity, where acid hydrogen promotes the protonation of alcohols, occurring differently for each alcohol. The dehydration process occurs, preferably, via E2 type elimination mechanisms. However, the profile of the energy curves indicates that for larger alcohols, the mechanism is intermediate between the elimination mechanisms E2 with some features of E1 (E2[E1]). Therefore, the zeolite converts alcohols to hydrocarbons in a specific way. Primary, lower-chain alcohols follow E2 mechanism, while secondary and longer-chain alcohols react by a slightly different mechanism, namely E2[E1].

Local structure change of luminescent Ag zeolite-A and -X studied by in situ XAFS and IR spectroscopy

The in situ X-ray absorption fine structure (XAFS) for the structural changes of Ag clusters produced in the cavity of luminescent zeolites by thermal treatment of Ag zeolite-A and Ag zeolite-X has been studied. The following procedures are compared: (i) samples are heated and cooled to room temperature under atmosphere (under air); (ii) samples are heated and cooled to room temperature in a vacuum and then exposed to air. It was confirmed that the Ag clusters were broken when the Ag zeolite was exposed to air for Ag zeolite-X, which complements our previous results for Ag12-A. It is suggested that the deformation of the Ag clusters plays an important role in the generation of a strong photoluminescence band, and Ag clusters may not be direct species producing the strong photoluminescence. The local structure of the Ag ions was found to be slightly different from that of the unheated species. The difference may originate from the formation and breakdown of Ag clusters in the zeolite cavity.