Core/shell-structured Al-MWW@B-MWW zeolites for shape-selective toluene disproportionation to para-xylene

A shape-selective core/shell-structured Al-MWW@B-MWW composite catalyst has been hydrothermally synthesized through isomorphically overgrowing borosilicate on premade MCM-22 aluminosilicate. The secondary growth of borosilicate enlarged obviously the thickness of the platelet crystallites of MCM-22 and increased the surface Si/Al ratio from 16 to 222. The Fourier transform infrared (FTIR) spectra of adsorbed 2,6-di-tert-butylpyridine indicated that the Brønsted acid sites located on the external surface were virtually covered completely by the generated B-MWW layer, whereas those acid sites within channels were still accessible and detectable by using pyridine or ammonia as probing molecules. When applied to the disproportionation of toluene on a fixed-bed reactor, the Al-MWW@B-MWW composite catalysts exhibited significantly enhanced para-xylene selectivity in comparison with normal MCM-22 and its physical mixture with B-MWW. Al-MWW@B-MWW’s unique catalytic behaviors were ascribed to an effective suppression of para-xylene isomerization as a result of removal of non-shape-selective acid sites on the external surface.

Core/shell-structured MWW-type zeolites prepared by isomorphic overgrowth of borosilicate on premade MCM-22 crystallites are highly shape-selective materials for the disproportionation of toluene as a result of suppressing the isomerization of para-xylene on the external surface of crystallites.Figure optionsDownload high-quality image (189 K)Download as PowerPoint slideHighlights
► Isomorphic overgrowth of borosilicate on MCM-22 leads to a core/shell-structured zeolite.
► Secondary growth of borosilicate removes the strong acid sites on the external surface.
► Al-MWW@B-MWW is highly shape selective to toluene disproportionation.
► The removal of surface acidity suppresses the isomerization of para-xylene on the crystallite surface.