ZSM-5 zeolite plays a vital role in petroleum cracking and refining of petrochemical products due to its microporous structure (0.51-0.56 nm) and tunable solid acid characteristics. Since zeolite micropores limit the mass transfer of reactants, the research focus has been to improve the accessibility of reactants to the acid sites of zeolites to improve their catalytic activity over the past decades.
Usually, alkaline solution post-treatment is used to selectively desiliconize and etch out the intracrystalline mesopores to improve the accessibility of the acid sites of the zeolite. However, because the four-coordinated aluminum in the pristine zeolite inhibits desiliconization, this method is only suitable for the synthesis of ZSM-5 zeolites with high silicon to aluminum ratio. The synthesis of low silicon to aluminum ratio ZSM-5 with good accessibility remains challenging.
Recently, the researchers from the Institute of Metal Research (IMR) of the Chinese Academy of Sciences, collaborating with the researchers from the University of Manchester, developed an effective method to prepare hollow ZSM-5 zeolite catalyst with low silicon to aluminum ratio. The study was published online in Angewandte Chemie International Edtion.
The researchers first prepared a pristine zeolite containing extra framework aluminum from a high-temperature rapid aging zeolite synthesis mother liquor. Then, they reinserted the extra framework aluminum into the frame aluminum by dissolution and recrystallization of tetrapropylammonium hydroxide. A hollow structure was formed at the same time.
The novelty of this method lies in the use of pristine zeolite containing extra framework aluminum as the mother phase, which breaks the restriction of zeolite dissolution and recrystallization by the four-coordinated framework aluminum, thereby hollow ZSM-5 zeolite with silicon to aluminum ratio as low as 16 was prepared.
Hollow ZSM-5 zeolite with low silicon to aluminum ratio shows excellent catalytic performance in hydrocarbon cracking, especially in the cracking of bulky molecules, with high activity, high stability and high propylene selectivity. The excellent catalytic performance of low silicon to aluminum ratio hollow ZSM-5 zeolites originated from good accessible acid sites.
This method is suitable for the synthesis of low silicon to aluminum hollow ZSM-5 zeolites, and it provides a feasible technical route for other hollow heteroatom-doped zeolites.
Schematic illustration of the preparation of the mesoporous ZSM-5-P nanoboxes via the rapid ageing (of the precursor sol gel mixture) and postsynthetic TPAOH treatment. (Image by IMR)
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