No abstract

The mobility of the acidic proton of the Bransted site in three zeolites has been investigated by means of high temperature (up to 660 K) MAS Nh4R. 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 Bransted center.

Recent developments in sample rotation technology have had a profound impact on magic-angle-spinning NMR. First, rotation frequencies approaching, and even exceeding, strong homonuclear spin interactions have made high-resolution solid-state (1)H spectroscopy much more accessible. Second, the new concept of fast rotation sweep spectroscopy has emerged. Third, high-resolution NMR at cryogenic temperatures has become feasible, offering an enormous sensitivity gain and the opportunity to study a wide range of physical phenomena.

We demonstrate that two-dimensional solid-state NMR chemical-shift correlation spectra can be recorded under low-power conditions. Except for the cross-polarization period, no rf-field amplitudes above 40 kHz are used. Such experiments require the use of fast (>50 kHz) magic-angle spinning (MAS). A comparison with the high-power version of the experiment shows no general line broadening but some changes in the polarization-transfer dynamics.