期刊
ACS CATALYSIS
卷 8, 期 4, 页码 3076-+出版社
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.7b03533
关键词
zeolites; Lewis acid; phosphine oxides; probe molecule adsorption; Sn-Beta; titration; glucose isomerization; aldol condensation
资金
- U.S. Department of Energy, Office of Basic Energy Sciences [DE-SC0016214]
- University of Alberta
- Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Grant Program
- NSERC CREATE
- Deutsche Forschungsgemeinschaft DFG [IRTG2022]
- National Science Foundation Graduate Research Fellowship [122374]
- Queen Elizabeth II Fellowship
The identity of the active sites of hydrophobic Lewis acid zeolites was elucidated by solid-state P-31 nuclear magnetic resonance (NMR) spectroscopy following adsorption of trimethylphosphine oxide (TMPO) probe molecules. The adsorption of TMPO on these materials resulted in distinct P-31 NMR resonances between delta(iso) = 59.9 and 54.9 ppm that correspond to unique chemical environments of the Lewis acidic heteroatoms in the Beta zeolite framework. The NMR resonances were assigned to sites in Sn-Beta zeolites by correlating the variation of P-31 NMR spectra during TMPO titration experiments with the corresponding changes in the Sn-119 NMR spectra. This method allowed us to establish quantitative relationships between the assignments for each site and the catalytic activity for the glucose isomerization and aldol condensation reactions. The rate of glucose isomerization directly correlated with the combined integrated intensities of P-31 MAS NMR resonances at delta(iso) = 55.8 and 54.9 ppm, which amounted to 12-33% of total Sn sites. In contrast, the integrated peak area of a different site at delta(iso) = 58.6 ppm was shown to correlate with aldol condensation activity. The probing method used to identify and quantify distinct active sites within the framework of low-defect Lewis acid zeolites developed in this work is applicable to a wide range of microporous materials, regardless of heteroatom identity.
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