Oxygen and structural effects on silicalite 29Si spin-lattice relaxation studied by high-resolution 29Si solid-state NMR

Monodisperse silicalite crystals (170 mu m x 40 mu m x 40 mu m) have been synthesized and the Si-29 spin-lattice relaxation rates have been measured by high-resolution solid-state NMR. Loading of 5 wt % p-xylene into the silicalite! channel system reduces the 29Si spin-lattice relaxation time compared to that of empty silicalite. The relaxation time is unaffected when p-xylene was replaced by p-xylene-d(10) of the same loading. This indicates that dipole-dipole coupling between protons of guest molecules and silicon atoms does not contribute significantly to the Si-29 spin-relaxation time. A novel method employing aromatic endoperoxide is demonstrated for controlled addition of molecular oxygen into silicalite samples. The results showed that interaction with a paramagnetic oxygen molecule is not the only relaxation mechanism for Si-29 nuclei, and that zeolite structural changes caused by loaded organic guest molecules also play an important role ia determining the Si-29 spin-lattice relaxation time.