MOBILITY OF THE ACIDIC PROTON IN BRONSTED SITES OF H-Y, H-MORDENITE, AND H-ZSM-5 ZEOLITES, STUDIED BY HIGH-TEMPERATURE H-1 MAS NMR

mobility of the acidic proton of the Bronsted site in three zeolites h as been investigated by means of high temperature (up to 660 K) MAS NM R. At high temperatures, spinning sidebands (SSB), caused by the dipol ar interaction between the acidic proton and the neighboring aluminum nucleus, gradually disappear. This means that the acidic protons becom e mobile. At the same time protons from terminal SiOH groups remain st ationary over the time scale of the spinning speed. Fitting of the tem perature dependence of the second moment of the SSB pattern with known models enables the estimates for the activation energy of proton mobi lity to be given. These were found to be 45, 54, and 61 KJ/mol for HZS M-5, H-mordenite, and H-Y, respectively. In our interpretation the act ivation energy corresponds to the average proton affinity difference o f oxygen atoms in the first coordination sphere of the aluminum atom a t the Bronsted center.