Transition metal exchanged β zeolites: Characterization of the metal state and catalytic application in the methanol conversion to hydrocarbons

• Divalent metal cations are mainly present at exchange sites in β zeolite.
• Fe3+ ions are preferentially found as oxides of nanometric size.
• Cr3+ cations are oxidized during calcination to Cr6+ species.
• Mn2+, Zn2+, Fe3+ and Cr6+ species act as promoters for the methanol conversion.
• Chromium-containing beta zeolite is highly active for the production of hydrocarbons.

Various first-row transition metal cations (Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+) have been introduced to zeolite beta using ion exchange procedures. Both aluminum and transition metal sites were studied by UV–Vis spectroscopy, XPS and 27Al NMR. Generally, ion exchange favored the incorporation of Al defects into the zeolite framework. The effect of the divalent and trivalent cations on the final zeolite beta structure was found to be considerably different. While divalent cations were mainly exchanged for Bronsted acid sites after calcination, trivalent cations such as Fe3+ were mostly transformed into oxide-like species whilst Cr3+ species were oxidized to Cr6+ species. Their surface properties were quite distinct since only divalent cations generated strong Lewis acid sites, even though the Brönsted acidity decreased in all cases. Their catalytic performance was evaluated in the transformation of methanol to hydrocarbons. Various metal species, i.e., Cr6+, Mn2+, Fe3+ and Zn2+ were found to act as promoters of polymerization or aromatization reactions. In particular, Cr species were found to enhance the catalytic activity in the conversion of methanol and dimethyl ether into higher order hydrocarbons.

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