On the enhanced para-selectivity of HZSM-5 modified by antimony oxide

HZSM-5 was modified with antimony oxide by solid-state reaction at 773 K to enhance diffusional constraints. Crystalline phases of antimony oxide were not detected at a Sb2O3 loading up to 10.2 wt%. Antimony oxide was found to strongly interact with the hydroxyl groups of the zeolite, i.e., silanol and bridging hydroxyl groups, and with extra-framework aluminum oxide. Only a small amount of Sb2O3 penetrated the pores of the zeolite, while the main fraction of Sb2O3 was deposited on the external surface of the zeolite crystals. With increasing antimony oxide loading a pronounced pore narrowing of the zeolite was observed, which suppressed the further penetration of antimony oxide into the pores of the zeolite. The dispersion of antimony oxide on the surface of the zeolite led to complete removal of unselective Brønsted acid sites and to a narrowing/blocking of the pore mouth openings of the zeolite. Both effects induced enhanced para-selectivity for toluene disproportionation.