Terminal and bridging methoxyls on zeolites detected by13C magic angle spinning NMR spectroscopy

Pure silicalites, synthesized without any heteroatoms and treated with [13C]methanol vapour, exhibit in the 13C magic angle spinning NMR spectrum signals at 48, 49 and 50 ppm with respect to tetramethylsilane (TMS) after desorption at 200 °C. The signals at 48 and 49 ppm were assigned to terminal methoxy groups at the defects of the lattice. Similar signals were detected also on modified Cs-exchanged faujasite by impregnation with caesium acetate after calcination in air and also on samples of hydrotalcite. The basicity (nucleophilicity) of the zeolitic lattice oxygen controls the carbon deshielding of chemisorbed methylium ions at bridging positions so that methoxy carbon signals increase linearly from 53 to 60 ppm (TMS) with increasing intermediate electronegativity of the lattice oxygen. On the other hand, terminal methoxy species exhibit signals from 48 to 51 ppm. Terminal and bridging methoxy species on silicalite, LSX, CsNaX/Cs2O and hydrotalcite are discussed.