Articles written in Bulletin of Materials Science

    • Negative thermal expansion and cationic migration in zeolite Y used in FCC catalysts


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      The influence of extra-framework cations on the intrinsic negative thermal expansion (NTE) of calcined ultrastable and lanthanum-exchanged zeolites Y was analysed. High-temperature thermal behaviour, cationic migration and dealumination were examined by in situ high temperature X-ray powder diffraction and thermogravimetric (TG) analyses. We analysed protonated ultrastable zeolite Y and zeolite Y containing 13% La$_2$O$_3$ (LaY), and investigated howthe presence of La$^{3+}$ influences thermal behaviour of zeolite Y. Uncalcined LaY was also analysed to observe the La$^{3+}$ migration before calcination. Mass loss up to 990$^{\circ}$C was accompanied by TG analyses. X-ray powder diffraction data were analysed by Rietveld method. Our data show that lanthanum migration from supercages to sodalitic cavities is not reversible. However, some La$^{3+}$ migration occurs above 400$^{\circ}$C and we observe emptying of site I$^{\prime}$, migration to site I and to some extent to site II$^{\prime}$. Furthermore, we examined the dealumination process as well. Both samples showtwo distinct thermal behaviours, positive or near-zero thermal expansion up to 500$^{\circ}$C and NTE above this temperature. Together with intrinsic NTE in faujasite type zeolites, the dealumination process is largely responsible for NTE at high temperatures.

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