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• W2044974329 abstract "Catalytic acidity of aluminosilicate zeolites is attributed to acidic aluminum sites in both framework structure and extraframework debris. High resolution 27Al n.m.r. has been used extensively to characterize these active centers, but only limited information is obtained. Typically, three Al signals are observed: a 60 ppm signal due to tetrahedral framework Al (zeolitic); a 30 ppm penta-coordinated extraframework signal; and a 0 ppm octahedral extraframework signal. In this work we report the observation of two additional tetrahedral Al signals in ultrastable Y zeolites. The new observation is made possible by treating samples with NH3 gas and by applying a long excitation pulse. The new n.m.r. signals are located at about 66 and 72 ppm relative to an Al(NO3)3 aqueous solution in addition to the three conventionally assigned peaks. Both new tetrahedral 27Al n.m.r. peaks are observed in a USY that was extensively hydrothermally treated, whereas only the 72 ppm peak was observed for a sample less extensively treated. The structural nature of the two signals has not been identified rigorously. However, based on the behaviors of the two peaks and the data of other “model” materials containing octahedral and tetrahedral Al sites, we propose that these new peaks are attributed to distorted tetrahedral Al atoms that are n.m.r. “invisible” under other experimental conditions. Kinetic effects of NH3 adsorption and 15N n.m.r. indicate that the Al sites associated with the two new peaks are more acidic than the Al sites represented by the 60 ppm peak. We hope that these results can be used for a more quantitative characterization of Al sites in zeolites and for new insights into adsorbent solvation effects on the structures of microporous materials." @default.
• W2044974329 created "2016-06-24" @default.
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• W2044974329 date "1996-04-01" @default.
• W2044974329 modified "2023-09-25" @default.
• W2044974329 title "Observation and study of new tetrahedral Al sites in NH3-treated, steamed zeolites using MAS 27Al and 15N n.m.r." @default.
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• W2044974329 doi "https://doi.org/10.1016/0144-2449(95)00153-0" @default.
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