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Studies on the Enhanced Acidic Sites in Zeolite HZSM-5

Wang Kaixuan;Guo Zhengya;Wang Jingzhong;Yu Lihua;Ding Datong (Nankai University)Deng Feng;Li Liyun;Du Youru (Wuhan Institute of Phys. the Chinese Academy ofScience)  
AbstractThe cnchanced catalytic activity of the mild hydrothemal dealuminated zeolite HZSM-5 was found in the condensation reaction of aldehyde. The NH3 temperature -programmed desorption experiments(by using a domestic omegatron mass spectroscopic equipment) showed that the NH3 desorption of the mild hydrothermal treated zeolite HZSM-5 was extended to the higher temperature in comparing with the parent material. The fact indicated the presence of the enchanced acidic sites in the hydrothermal treated sample.The framework Si/Al ratios of both parent and hydrothermal treated zeolite HZSM-5 samples were determined by 29Si MAS NMR observations in this way the dealumination degree of the sample with enchanced catalytic activity could be estimated quantitively.In comparing with the parent material, the 27Al MAS NMR spectra of the mild hydrothermal treated zeolite HZSM-5 samples appeared two additional NMR signals around 0ppm and 30ppm. The former was attributed to the sirfold-oxygen coordinahon non-framework aluminium created by the dealuminahon.The 30ppm signal was attributed to extra-lattice aluminium which was probably penta-coordinated. In the 27Al-IH CP / MAS NMR spectra. the 30pmm signal was enhanced.The fact indicated that the extra-lattice aluminium was strongly coupled in same way with protons in its local environment. Since the 30ppm signal didn't change much after impregnation the sample with a solution of acetylacetone(acac) in ethanol, and so this type of penta-coordinated extra-lattice aluminium was not as easy tO be removed by acac as the sixfold-oxygen coordinated non-framework aluminium.The mechanism of the enhanced acidity must be in same way related to those penta-coordinated extra-lattice Al species presenting in the mild hydrothermal treated zeolite HZSM-5 samples.
【CateGory Index】: O641.3
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