Mesopore engineering in Al-rich mordenite zeolites by a tandem top-down protocol: a comparative study between fluoride leaching and fluorination

Abstract

Two protocols of sequential “fluoride leaching-alkaline treatment” and “fluorination-alkaline treatment” are used to introduce mesoporosity in a commercially available Al-rich mordenite (MOR) zeolite (Si/Al = 7.9). Intracrystalline mesopores centered around 8 nm have been constructed successfully by the sequential fluoride leaching-alkaline treatment, but not by the tandem fluorination-alkaline treatment. The different impact of fluorination and fluoride leaching on the mesoporosity development is closely associated with the distinct alteration to the state of Al sites. The extraction of dominant framework Al sites with the Si(1Al) configuration and the parallel displacement of Si species by the fluoride leaching with (NH₄)₂SiF₆ alleviate the excess shielding effect and trigger the controlled dissolution of the zeolitic matrix in the subsequent alkaline treatment step, leading to the creation of intrazeolite mesopores. In contrast, the fluorination results in the severe dislodgement of framework Al species, but these extracted Al species still remain on the zeolite in the form of AlF₃ phase. Upon exposure to the alkaline medium, the AlF₃ species are hydrolyzed to aluminum hydroxyfluoride complexes (Al(OH)_xF_y⁻) and further evolve into aluminum hydroxides. These non-framework Al species remaining on the zeolite will impose electronic repulsion to the OH⁻, which prevents the generation of hierarchical porosity.