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Subject: search.filters.subject.Brønsted acid

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    Generation of reductive Zn species over Zn/HZSM–5 catalysts for n–pentane aromatization
    (2016-09-05)
    Tamiyakul, Sikarin
    ;
    Sooknoi, Tawan
    ;
    Lobban, Lance L.
    ;
    Jongpatiwut, Siriporn
    The effects of hydrogen treatment on Zn/HZSM–5 catalysts to the evolution of zinc species and their activity in n-pentane aromatization were studied. By heating Zn/HZSM–5 catalysts under inert atmosphere, most of zinc species appeared as (ZnOH)<sup>+</sup>. The XPS and IPA–TPD results showed that using H<inf>2</inf> as a treatment gas, the hydrodehydroxylation of (ZnOH)<sup>+</sup> generated (ZnH)<sup>+</sup> species with the recovery of some Brønsted acid sites. It was found that under the hydrogen atmosphere, the H<inf>2</inf> molecule was dissociated on this Zn species and generated the H<inf>2</inf>-chemisorbed Zn species. Compared with HZSM-5 catalyst, the presence of (ZnOH)<sup>+</sup> species improved the aromatics selectivity from 11% to 22%. Nevertheless, after pre-treating Zn/HZSM–5 with H<inf>2</inf>, the formation of H<inf>2</inf>-chemisorbed Zn species remarkably increased the BTX aromatics selectivity to 31%. It was found that the H<inf>2</inf>-chemisorbed Zn species was not stable in the absence of H<inf>2</inf> however it could be regenerated upon H<inf>2</inf> treatment.
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    Additional Brønsted acid sites in [Ga]HZSM-5 formed by the presence of water
    (2009-06-20)
    Ausavasukhi, Artit
    ;
    Sooknoi, Tawan
    The incorporation of Ga(NO<inf>3</inf>)<inf>3</inf> into HZSM-5 zeolite results in additional Brønsted acid sites evidenced by <sup>1</sup>H MAS NMR and FTIR. Such acid sites are proposed to arise from the incorporated Ga extra-framework (GaO(OH)) and insertion of such species with defect hydroxyls into the zeolite framework (GaOHSi). These additional Brønsted acid sites are particularly formed under steaming and may well be responsible for the change in the catalytic behaviour of [Ga]HZSM-5 when water is present in the reaction stream. In the reaction using ethylene as a feed, higher hydrocarbons, namely aromatics, can be produced over "reduced Ga species". In contrast, the conversion of ethanol over [Ga]HZSM-5 showed an increased activity in a manner similar to that over a typical HZSM-5, but with higher acidity. The formation of the additional acid sites is readily reversible at high temperature (>450 °C) and hence the steam treatment can be used for re-dispersing the incorporated Ga species. © 2009 Elsevier B.V. All rights reserved.