ACIDIC HYDROXYL-GROUPS IN ZEOLITES X AND Y - A CORRELATION BETWEEN INFRARED AND SOLID-STATE NMR-SPECTRA

Five signals from Si(nAl) units, where n=0-4, are found in Si-29 magic-ankle-spinning (MAS) NMR spectra of zeolites X and Y (synthetic faujasites) with different Si/Al ratios. Since only Si(4Al), Si(3Al), Si(2Al), and Si(1Al) units are associated with bridging OH groups, there can be four possible kinds of such groups: Al3Si-OH-AlSi3, Al2SiSi-OH-AlSi3, AlSi2Si-OH-AlSi3, and Si3Si-OH-AlSi3. Four kinds of hydroxyls of different acid strengths are indeed found by IR spectroscopy, and their relative populations depend on the Si/Al ratio as do the relative intensities of Si-29 MAS NMR signals. The four kinds of hydroxyls have been correlated with Si(nAl) signals as observed by NMR. The distribution of acid sites of different strengths can therefore be deduced from Si-29 MAS NMR spectra, which may be of considerable practical importance in heterogeneous catalysis.