Temperature Effect on 1H Chemical Shift of Hydroxyl Groups in Zeolites and Their Catalytic Activities as Solid Acids

The influence of temperature on the H-1 chemical shift due to hydroxyl groups in H+-exchanged any kind of zeolites (ZSM-5 type zeolites including silicalite, mordenite and Y zeolites) was examined by variable temperature H-1 magic-angle spinning nuclear magnetic resonance (H-1 MAS NMR) spectroscopy measured from 298 to 673 K. The H-1 chemical shifts due to bridging hydroxyl groups (Si-(OH)-Al, Bronsted acid sites), which have a lower activation energy for proton jumping, terminal silanol groups, and nest silanol groups increased with temperature. The temperature dependence of the H-1 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 H-1 chemical shift and the acid strength of the hydroxyl groups in zeolites was discussed on the basis of the variable temperature H-1 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. It was found that the nest silanol groups of silicalite functioned as Bronsted acid sites and could catalyze the conversion of ethanol and 1-hexene with the same activity as the bridging hydroxyl groups in B-ZSM-5 at 673 K.