Brnsted/Lewis Acid Synergy in Dealuminated HY Zeolite: A Combined Solid-State NMR and Theoretical Calculation Study

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• 作者：Shenhui Li ; Anmin Zheng ; Yongchao Su ; Hailu Zhang ; Lei Chen ; Jun Yang ; Chaohui Ye ; Feng Deng
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：September 12, 2007
• 年：2007
• 卷：129
• 期：36
• 页码：11161 - 11171
• 全文大小：506K
• 年卷期：v.129,no.36(September 12, 2007)
• ISSN：1520-5126

文摘

The Brnsted/Lewis acid synergy in dealuminated HY zeolite has been studied using solid-state NMR and density function theory (DFT) calculation. The ¹H double quantum magic-angle spinning(DQ-MAS) NMR results have revealed, for the first time, the detailed spatial proximities of Lewis andBrnsted acid sites. The results from ¹³C NMR of adsorbed acetone as well as DFT calculation demonstratedthat the Brnsted/Lewis acid synergy considerably enhanced the Brnsted acid strength of dealuminatedHY zeolite. Two types of Brnsted acid sites (with enhanced acidity) in close proximity to extra-frameworkaluminum (EFAL) species were identified in the dealuminated HY zeolite. The NMR and DFT calculationresults further revealed the detailed structures of EFAL species and the mechanism of Brnsted/Lewisacid synergy. Extra-framework Al(OH)₃ and Al(OH)²⁺ species in the supercage cage and Al(OH)²⁺ speciesin the sodalite cage are the preferred Lewis acid sites. Moreover, it is the coordination of the EFAL speciesto the oxygen atom nearest the framework aluminum that leads to the enhanced acidity of dealuminatedHY zeolite though there is no direct interaction (such as the hydrogen-bonding) between the EFAL speciesand the Brnsted acid sites. All these findings are expected to be important in understanding the roles ofLewis acid and its synergy with the Brnsted acid in numerous zeolite-mediated hydrocarbon reactions.