| Summary: | 碩士 === 中國文化大學 === 材料科學與製造研究所 === 90 === A new methodology is reported for concurrent qualitative and quantitative characterization of internal and external acid sites of zeolitic catalysts. H-ZSM-5 zeolites with varied Si/Al ratios have been examined by solid-state 31P MAS NMR using different adsorbed probe molecules, namely trimethylphosphine oxides(TMPO) and tributylphosphine oxide(TBPO), in conjunction with elemental analysis. For H-ZSM-5 zeolites, up to seven distinct 31P resonance peaks at 86, 75, 67, 63, 53, 43, and 30 ppm were identified from the 31P NMR spectra of adsorbed TMPO. The newly observed resonance at 30 ppm may be ascribed due to mobile TMPO and the peak at 43 ppm is assigned to physisorbed TMPO. Whereas the peaks above 53 ppm are responsible for TMPOH+ complex to Brønsted sites; resonance occur at higher chemical shifts reflect acid sites having higher strengths.
Respective 31P NMR experiments performed with TMPO and TBPO adsorbed on a mesoporous MCM-41 sample provide further correlation of internal and external acid sites. Consequently, it is concluded that resonance at 75 and 53 ppm arise exclusively from the internal Brønsted sites, whereas the other characteristic peaks at 86, 67, and 63 ppm are associated with both internal and external acid sites. Moreover, upon increasing Si/Al ratio of HZSM-5, while the concertration and distribution of internal sites were found to vary toward higher acid strengths, it has nearly no effect on the strength of the external acid sites.
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