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 has been investigated by means of high temperature (up to 660 K) MAS NMR. At high temperatures, spinning sidebands (SSB), caused by the dipolar interaction between the acidic proton and the neighboring aluminum nucleus, gradually disappear. This means that the acidic protons become mobile. At the same time protons from terminal SiOH groups remain stationary over the time scale of the spinning speed. Fitting of the temperature dependence of the second moment of the SSB pattern with known models enables the estimates for the activation energy of proton mobility to be given. These were found to be 45, 54, and 61 KJ/mol for HZSM-5, H-mordenite, and H-Y, respectively. In our interpretation the activation energy corresponds to the average proton affinity difference of oxygen atoms in the first coordination sphere of the aluminum atom at the Bronsted center.