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

The influence of temperature on the 1 H 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 1 H magic-angle spinning nuclear magnetic resonance ( 1 H MAS NMR) spectroscopy measured from 298 to 673 K. The 1 H chemical shifts due to bridging hydroxyl groups (Si–(OH)–Al, Brönsted 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 1 H 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 1 H chemical shift and the acid strength of the hydroxyl groups in zeolites was discussed on the basis of the variable temperature 1 H MAS NMR measurements of zeolites, previously reported thermal analysis data, such as the heat of NH 3 adsorption measured by NH 3 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 Brönsted 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.