Abstract
Monodisperse silicalite crystals (170 μm × 40 μm × 40 μm) have been synthesized and the 29Si 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-d10 of the same loading. This indicates that dipole−dipole coupling between protons of guest molecules and silicon atoms does not contribute significantly to the 29Si 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 29Si nuclei, and that zeolite structural changes caused by loaded organic guest molecules also play an important role in determining the 29Si spin−lattice relaxation time.
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