Shape-selective catalysis in cavity-type molecular sieves: cavity-controlled catalytic principle

Area of Science:

• Catalysis and Materials Science
• Chemical Engineering
• Physical Chemistry

Background:

• The methanol-to-olefins (MTO) process is a key technology in C1 chemistry, converting non-petroleum feedstocks into valuable olefins using zeolite or molecular sieve catalysts.
• Molecular sieves offer shape selectivity and unique confinement effects crucial for catalytic performance, driven by their acidic properties and internal structure.
• Eight-membered ring (8-MR) and cavity-type molecular sieves are particularly important due to their large cages and restricted openings, influencing host-guest interactions.

Purpose of the Study:

• This review focuses on summarizing the adsorption characteristics, diffusion behavior, and reaction mechanisms of the MTO process within cavity-type molecular sieves.
• It aims to elucidate the principle of cavity-controlled methanol conversion, encompassing molecular adsorption, diffusion, intermediate formation, reaction pathways, and catalyst deactivation.
• The study seeks to provide insights into modifying molecular sieve catalysts and optimizing the MTO process.

Main Methods:

• The review synthesizes existing research on molecular adsorption and diffusion within various cavity-type molecular sieves.
• It analyzes the mechanisms governing MTO reactions and catalyst deactivation, emphasizing the role of the molecular sieve's internal cavity structure.
• Discussions include the impact of different cavity structures on guest molecule adsorption sites and diffusion.

Main Results:

• Different cavity structures within molecular sieves significantly alter preferential adsorption sites and diffusion behavior of guest molecules.
• The generation of critical intermediates in the MTO reaction is strongly governed by the specific cavity structure of the catalyst.
• Non-uniform distribution of coke species, a cause of catalyst deactivation, is also influenced by the catalyst's internal architecture.

Conclusions:

• The cavity-controlled catalytic principle in 8-MR and cavity-type molecular sieves is fundamental to understanding and optimizing the MTO reaction.
• Insights gained from studying these cavity-type sieves are valuable for designing improved molecular sieve catalysts.
• This understanding can lead to enhanced MTO process efficiency and broader applications of these catalysts in other C1 chemical transformations.