Enabling the mesopore engineering inside Al-rich MOR zeolite by sequential fluorination-acid leaching-alkaline treatment strategy

Abstract

The sequential fluorination-acid leaching-alkaline treatment strategy is used to construct mesoporosity in a commercially available Al-rich mordenite (MOR) zeolite (Si/Al = 7.9). The well-defined intracrystalline mesopore has been created by such consecutive strategy. In contrast, single or binary combincation of fluorination, acid leaching (treated in 1 M HNO3 at 353 K for 1 h) and alkaline treatment (treated in 0.2 M NaOH at 333 K for 0.5 h) hardly construct the intracrystalline mesopore inside the Al-rich (MOR) zeolite. The rational interplay between fluorination and acid-leaching enables the tailoring of mesoporosity. The low level fluorination (treated in 0.05 M NH4F) triggers transformation of less framework Al sites to Al-F complexation and leads to the removal of less Al sites by acid leaching. As a result, the high density of Al sites brings the excess shielding effect and results in the poor mesoporosity by alkaline treatment. In the case of medium level fluorination (treated in 0.1-0.3 M NH4F), the transformation of moderate framework Al sites into the Al-F complexation and subseqnent extraction of these Al species by acid leaching alleviates the excess shielding effect and triggers the controlled dissolution of zeolitic matrix in the subsequent alkaline treatment step, leading to the creation of intrazeolite mesopore centered at ca. 14 nm for AT-Ac-0.1F/HM and 20 nm for AT-Ac-0.3F/HM, respectively. The high level fluorination (treated in 0.5 M NH4F) leads to the severe dislodgement of framework Al sites, in parallel with the creation of the amuont of NMR-invisible and acid free non-framework Al species. Upon the acid leaching, these created Al species remains on the zeolite matrix and passivates 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) trigger the suitable transformation and removal of Al sites, and enable the construction of intracrystalline mesopore centered at ca. 10 nm by alkaline treatment. This consecutive post-synthesis procotol enrichs the toolbox for the mesopore tailoring of Al-rich zeolites and provides new insight for the negative role of non-framework Al sites.