Unambiguously distinguishing Si[3Si,1Al] and Si[3Si,1OH] stuctural units in zeolite by 1H/29Si/27Al triple resonance solid state NMR spectroscopy

We present an experimentally feasible triple-resonance NMR method that establishes the correlation among three different nuclei, avoiding the difficulty to directly explore the weak coupling between two NMR nuclei, such as 29Si and 27Al. Using this method, we are able to give an unambiguous assignment to the various peaks in 29Si CP NMR spectrum of MCM-22 zeolite and discriminate 29Si signals from SiOHAl and SiOH groups. In addition, in combination with 1H/27Al double-resonance technique, the 1H/27Al/29Si triple-resonance experiment suggests the presence of two different kinds of Brönsted acid sites in H-MCM-22 zeolite.