Enabling mesopore engineering inside Al-rich MOR zeolites via a sequential fluorination-acid leaching-alkaline treatment strategy

Abstract

A sequential fluorination-acid leaching-alkaline treatment strategy was used to construct mesoporosity in a commercially available Al-rich mordenite (MOR) zeolite (Si/Al = 7.9). A well-defined intracrystalline mesopore was created using this consecutive strategy. In contrast, single or binary combination of fluorination, acid leaching (treated with 1 M HNO₃ at 353 K for 1 h) and alkaline treatment (treated with 0.2 M NaOH at 333 K for 0.5 h) hardly constructed the intracrystalline mesopore inside the Al-rich (MOR) zeolite. The rational interplay between fluorination and acid-leaching enabled tailoring of mesoporosity. The low-level fluorination (treated with 0.05 M NH₄F) triggered the transformation of less framework Al sites to Al–F complexation and led to the removal of fewer Al sites by acid leaching. As a result, the high density of Al sites caused an excess shielding effect and resulted in poor mesoporosity of the zeolite upon alkaline treatment. In the case of medium-level fluorination (treated with 0.1–0.3 M NH₄F), the transformation of moderate framework Al sites into Al–F complexation and subsequent extraction of these Al species by acid leaching alleviated the excess shielding effect and triggered the controlled dissolution of zeolitic matrix in the subsequent alkaline treatment step, leading to the creation of an intrazeolite mesopore centered at ca. 14 nm for AT-Ac-0.1F/HM and 20 nm for AT-Ac-0.3F/HM. The high-level fluorination (treated with 0.5 M NH₄F) led to severe dislodgement of framework Al sites along with the creation of NMR-invisible and acid-free non-framework Al species. Upon acid leaching, these created Al species remained on the zeolite matrix and passivated the mesoporosity development in the subsequent alkaline treatment. The combination of high-level fluorination and low-level acid leaching (treated in 0.3 M HNO₃ at 353 K for 1 h) triggered the suitable transformation and removal of Al sites and enabled the construction of an intracrystalline mesopore centered at ca. 10 nm upon alkaline treatment. This consecutive post-synthesis protocol enriches the toolbox for the mesopore tailoring of Al-rich zeolites and provides new insights into the negative role of non-framework Al sites in the mesopore engineering.