Synthesis of reaction-adapted zeolites as methanol-to-olefins catalysts with mimics of reaction intermediates as organic structure-directing agents

[EN] Catalysis with enzymes and zeolites have in common the presence of well-defined single active sites and pockets/cavities where the reaction transition states can be stabilized by longer-range interactions. We show here that for a complex reaction, such as the conversion of methanol-to-olefins (MTO), it is possible to synthesize reaction-adapted zeolites by using mimics of the key molecular species involved in the MTO mechanism. Effort has focused on the intermediates of the paring mechanism because the paring is less favoured energetically than the side-chain route. All the organic structure-directing agents based on intermediate mimics crystallize cage-based small-pore zeolitic materials, all of them capable of performing the MTO reaction. 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Intermediates in the methanol-to-hydrocarbons (MTH) reaction: a gas phase study of the unimolecular reactivity of multiply methylated benzenium cations

Svelle S, Bjorgen MA, Kolboe S, et al. Intermediates in the methanol-to-hydrocarbons (MTH) reaction: a gas phase study of the unimolecular reactivity of multiply methylated benzenium cations. CATALYSIS LETTERS. 2006;109(1-2):25-35.In order to reach a deeper insight into the reaction mechanism of the zeolite catalyzed methanol to hydrocarbons reaction (MTH), the proposed reaction intermediates, i.e., a series of multiply methyl-substituted benzenium ions has been generated in the gas phase by chemical ionization. The fragmentations of the corresponding long-lived (metastable) ions have been investigated. While expulsion of H-2 dominates for the lower homologues, elimination of methane dominates for the higher homologues, accompanied by increasing amounts CH3 center dot. Loss of larger fragments relevant to the MTH-reaction, in particular ethene, propene and even butene, is also observed in minor amounts. This latter finding is consistent with a proposed reaction cycle in the MTH reaction known as the paring mechanism, and the feasibility of this mechanism has thus been demonstrated. The metastable gas-phase ions studied here are considerably more energetic than those residing in a zeolite catalyst, but they were found to decompose with markedly higher selectivity towards alkenes as compared to those activated by collision-induced dissociation (CID)

Chemical deactivation of Cu-SSZ-13 ammonia selective catalytic reduction (NH3-SCR) systems

The chemical deactivation of Cu-SSZ-13 Ammonia Selective Catalytic Reduction (NH3-SCR) catalysts by Pt, Zn, Ca and P has been systematically investigated using a range of analytical techniques in order to study the influence on both the zeolitic framework and the active Cu2+ ions. The results obtained demonstrate a crucial impact of P, completely suppressing the catalytic activity as a result of different deactivation mechanisms (i.e. site blocking, disruption of the zeolite framework, CuO formation and else reduction in the number of isolated Cu2+ ions). A less pronounced drop in activity is found with Ca and Zn introduction, without an appreciable adverse effect on N2 selectivity, since the catalytic deactivation is mainly brought about through a pore blocking/filling mechanism. Additionally, a drop in the amount of Cu2+ ions with the formation of CuO species also takes place, observed to be most important for the Zn-deactivated materials. Deactivation by Pt strongly affects N2 selectivity, but without a significant influence on the active sites or the zeolitic structure, basically due to the high oxidation activity of the Pt species, which highly promote N2O and NO2 formation

Chemical deactivation of Cu-SSZ-13 ammonia selective catalytic reduction (NH3-SCR) systems

The chemical deactivation of Cu-SSZ-13 Ammonia Selective Catalytic Reduction (NH3-SCR) catalysts by Pt, Zn, Ca and P has been systematically investigated using a range of analytical techniques in order to study the influence on both the zeolitic framework and the active Cu2+ ions. The results obtained demonstrate a crucial impact of P, completely suppressing the catalytic activity as a result of different deactivation mechanisms (i.e. site blocking, disruption of the zeolite framework, CuO formation and else reduction in the number of isolated Cu2+ ions). A less pronounced drop in activity is found with Ca and Zn introduction, without an appreciable adverse effect on N2 selectivity, since the catalytic deactivation is mainly brought about through a pore blocking/filling mechanism. Additionally, a drop in the amount of Cu2+ ions with the formation of CuO species also takes place, observed to be most important for the Zn-deactivated materials. Deactivation by Pt strongly affects N2 selectivity, but without a significant influence on the active sites or the zeolitic structure, basically due to the high oxidation activity of the Pt species, which highly promote N2O and NO2 formation