zeolite y is an important catalyst in fluidized catalytic cracking (FCC) and hydrocleavage processes, the two primary secondary processing steps of crude oil in petroleum refineries worldwide. Zeolite y is used to convert the high boiling, high molecular weight hydrocarbon components of crude oil into higher value gasoline and olefin gases. These reactions are typically conducted under harsh, hydrothermal conditions; therefore increasing the hydrothermal stability of zeolite y is one of the main goals for improving the performance of this important industrial catalyst.
Zeolite y is composed of a highly ordered, alkaline earth aluminum silicate (Al-Si) framework that has four Si-O-Al bonding sites. The high Si/Al ratio of zeolite y is due to its unique structural characteristics and its ability to host a large number of acid centers, and this attribute makes it very attractive as a catalyst for petrochemical reactions. However, the high level of Al atoms present in zeolite y is detrimental to its hydrothermal stability, which is why zeolite y needs post-treatment to achieve better stability.
Dealumination of zeolite y is an effective method for improving its hydrothermal stability. It is a process that involves removing some of the framework aluminum to reduce its concentration, and subsequently adjusting the ratio of the different acid centers. This will greatly enhance the catalytic cracking performance of zeolite y.
The most common treatment method for zeolite y is hydrothermal dealumination, which involves subjecting the zeolite to steam at high temperatures to remove the non-framework aluminum and move it to the pore site region where it is less active. The process also results in the formation of hydroxyl nests and silicon-rich regions, which significantly improves the pore structure properties of zeolite y.
Chemical reagents can be used to perform dealumination on zeolite, as they are able to interact with the aluminum ions and decompose them. These reagents can be used to remove a specific layer of the crystal structure or a specific group of atoms, and can thus control the amount of aluminum removed from the zeolite. The most commonly used reagents for Y dealumination are sodium hydroxide and acetylacetone.
Both hydrothermal and chemical methods for modifying zeolite y can result in substantial improvements in its hydrothermal stability, but they cannot fully address all the shortcomings of commercial Y zeolite. Therefore, other methods for modifying zeolite Y to increase its hydrothermal stability must be investigated and developed.
Surfactant-assisted oxidative leaching of rare-earth-containing USY can drastically improve the hydrothermal stability of the material, and it can also be used to adjust the acid density and strength of the mesoporous structure. This modification can be used to prepare hierarchical zeolite y with improved hydrothermal stability, which is an important step toward the development of a stable and efficient industrial FCC catalyst. Moreover, it is highly suitable for other reactions involving macromolecules, such as gas phase hydrogenation and hydrocarbon dehydrogenation. The improved hydrothermal stability and acidity of this new zeolite will enable it to compete with other types of catalysts, especially in the industrial aromatics catalytic deolefination reaction.
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