Studies on the Enhanced Acidic Sites in Zeolite HZSM-5

The 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-¹H 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.