Influence of Silanol Defects of ZSM-5 Zeolites on Trioxane Synthesis from Formaldehyde
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The silanol defects in ZSM-5 zeolite have been recognized as an important factor in catalytic activity. Here, ZSM-5 zeolites with different amounts of silanol defect sites were synthesized from synthetic gels containing fluoride medium and were applied as catalysts for trioxane synthesis. The results of XRD, SEM, NH3-TPD, Py-IR, OH-IR, 27Al MAS NMR, 1H MAS NMR, and TG indicated that all ZSM-5 zeolites showed similar crystal size, relative crystallinity, porosity, and the number of Brønsted acid sites. However, the silanol defects reduced obviously and the number of Lewis acid sites reduced correspondingly when a little NH4F was added in the synthesis gels and both of them decreased slightly with increase of F/Si ratio. Compared with ZSM-5 zeolite prepared in hydroxide medium, ZSM-5 zeolite prepared in fluoride medium displayed higher selectivity to trioxane and increased gradually with increase of F/Si ratio. Moreover, the lifetime of ZSM-5 zeolite prepared in fluoride medium was longer than that of prepared in hydroxide medium. Thus, ZSM-5 zeolite prepared in fluoride medium which contained the few silanol defects and low Lewis acid sites is an efficient catalyst for trioxane synthesis.
KeywordsZSM-5 zeolite Fluoride Silanol defects Lewis acid Trioxane synthesis
This work was supported by the National Key R&D Program of China (Grant Number 2018YFB0604902). The funding source has no role in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.
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Conflict of interest
The authors declare no conflict of interest.
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