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

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)(2)SiF6 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)(x)F-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.

Automated summary

Two protocols, sequential fluoride leaching-alkaline treatment and fluorination-alkaline treatment, are used to introduce mesoporosity in commercially available Al-rich mordenite zeolite. The sequential fluoride leaching-alkaline treatment successfully constructs intracrystalline mesopores, while the tandem fluorination-alkaline treatment does not. The different impacts of fluorination and fluoride leaching on mesoporosity development are closely associated with the alteration to the state of Al sites.