Abstract
A series of defective ZSM-5 zeolites (~300 nm, SiO2/Al2O3 ratio of 55, 100, 480 and 950) were systematically studied by XRD, SEM, 29Si MAS NMR, argon physisorption, NH3-TPD and FT-IR technologies. The nature, the amount and the accessibility of the acid sites of defective ZSM-5 zeolites are greatly different from reported ZSM-5 zeolites with a perfect crystal structure. The Brønsted acid sites (Si(OH)Al) with strong acid strength and the Brønsted acid sites (hydroxyl nests) with weak acid strength co-existed over defective ZSM-5 zeolites, which leads to a unique catalytic function. Zn(C2H5)2 was grafted onto defective ZSM-5 zeolites through the chemical liquid deposition (CLD) method. Interestingly, FT-IR spectroscopic studies found that Zn(C2H5)2 was preferentially grafted on the hydroxyl nests with weak acid strength rather than the Si(OH)Al groups with strong acid strength over different defective ZSM-5 zeolites. In particular, home-built operando dual beam FTIR-MS was applied to study the catalytic performance of Zn species located in different sites of defective ZSM-5 zeolites under real n-hexane transformation conditions. Results show that Zn species grafted over hydroxyl nests obtain better dehydrogenative aromatization performance than Zn species over Si(OH)Al groups. This study provides guidance for the rational design of highly efficient alkane dehydrogenative aromatization catalysts.
Highlights
The formation of a “hydroxyl-nest defect” over silica-alumina zeolites, consisting of 4 silanols, a substitute for [AlO4 ] tetrahedron, was firstly proposed by Barrer and Makki [1]
The effluent of DB-FTIR cell reactor was monitored by mass spectrometer (Figure 11), Zn0.51/Z950 generates a higher amount of olefins and aromatics than Zn0.50/Z950R, which indicates that Zn species located in hydroxyl nests obtained better dehydrogenative aromatization performance than Zn species located in Si(OH)Al groups
Monitored by mass spectrometer (Figure 11), Zn0.51/Z950 generates a10 higher amount of olefins and aromatics than Zn0.50/Z950R, which indicates that Zn species located in hydroxyl nests obtained better dehydrogenative aromatization performance than Zn species located in Si(OH)Al groups
Summary
The formation of a “hydroxyl-nest defect” over silica-alumina zeolites, consisting of 4 silanols, a substitute for [AlO4 ] tetrahedron, was firstly proposed by Barrer and Makki [1]. The well-developed porous structure with a high surface area and strong acidity endows the extraordinary performance of HZSM-5 as a successful solid acid catalyst. The acid strength of HZSM-5 originating from the Si(OH)Al groups is strong and the attenuation by post-treatment methods such as dealumination and ion-exchange by base metals is widely studied [16,17]. Considering the weak acidity of hydroxyl nests, by regulating the ratio of hydroxyl nests to Si(OH)Al groups, the defective HZSM-5 zeolites with tunable acidity are expected to show novel catalytic properties. We find that Zn(C2 H5 ) was preferentially grafted on the hydroxyl nests with weak acidity rather than on the Si(OH)Al groups with strong acidity over different defective ZSM-5 zeolites. The Zn species grafted over hydroxyl nests obtain better dehydrogenation performance than Zn species over Si(OH)Al groups
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