Temperature Effect on 1H Chemical Shift of Hydroxyl Groups in Zeolites

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The influence of temperature on 1H chemical shift due to hydroxyl groups in H+-exchanged any kinds of zeolites (ZSM-5 type zeolites including silicalite, mordenite and Y zeolites) was examined by variable temperatures 1H magic-angle spinning nuclear magnetic resonance (1H MAS NMR) spectroscopy measured from 298 to 673 K. The temperature dependence of the 1H chemical shift due to the bridging hydroxyl groups was more pronounced than that due to the terminal silanol groups and also the nest silanol groups. The relationship between the 1H chemical shift and the acid strength of the hydroxyl groups in zeolites was discussed on the basis of the variable temperature 1H MAS NMR measurements of zeolites, previously reported thermal analysis data, such as the heat of NH3 adsorption measured by NH3 temperature-programmed desorption, and also the catalytic activity for the conversion of ethanol and 1-hexene.