aLocal Structure of Framework Aluminum in Zeolite H-ZSM-5 during Conversion of Methanol Investigated by In Situ NMR Spectroscopy

摘要

In situ MAS NMR spectroscopy under continuous-flow conditions and spin-echo NMR experiments have been applied to investigate the quadrupolar interactions of framework aluminum atoms in zeolite H-ZSM-5 at elevated temperatures and during conversion of methanol. The strength of the ~(27)Al quadrupolar interactions of framework aluminum atoms in zeolites depends on the electric field gradient caused by the charge distribution in the local structure of the aluminum sites. Adsorption of methanol on bridging OH groups at 295 K under flow conditions leads to weak ~(27)Al quadrupolar interactions, corresponding to a quadrupole coupling constant of QCC = 4.4 MHz. After the temperature was raised to 573 K during methanol conversion, a strong decrease in the ~(27)Al MAS NMR signal of framework aluminum at 54 ppm occurred due to a significant increase in the ~(27)Al quadrupolar interactions. Comparison of ~(27)Al echo NMR spectra of zeolite H-ZSM-5 loaded with methanol and dimethyl ether and data given in the literature showed that this increase in the ~(27)Al quadrupolar interactions is caused by the formation of dimethyl ether (QCC = 11.2 MHz) and methoxy groups (QCC = 16.2 MHz) at bridging OH groups. Experiments performed during purging zeolite H-ZSM-5 with dry nitrogen at 573 K indicated a high hydroxyl proton mobility which has, however, no influence on the local structure of framework aluminum atoms.