Solid State NMR Studies Of The Acidity Of Microporous And Mesoporous Materials

In this dissertation, aluminum species and the acid sites in several aluminosilicate materials have been studied using various solid-state NMR techniques. A new triple-resonance NMR method named TRAPDOR-CP has been developed and used as a powerful tool for spectral editing.The "NMR invisible Al" in dehydrated MCM-22 zeolite was studied by ¹H/²⁷A1 TRAPDOR method. The quadrupole coupling constants (Q_ccs) of different aluminum species (including Bronsted, Lewis site Al and extra-framework Al) were obtained, indicating that all these aluminum species contribute to the "NMR invisible Al".A new triple-resonance pulse sequence named TRAPDOR-CP was proposed, which is a combination of TRAPDOR and CP experiments and is able to explore correlation between two nuclei with relatively low NMR sensitivity and the same neighboring proton. Using this new technique, we can unambiguously discriminate ²⁹Si NMR signals from the Si[10H,3Si] and Si[lAl, 3Si] structural units in MCM-22 zeolite. In combination with the ¹H/²⁷A1 TRAPDOR results, we find that at least two different kinds of Bronsted acid sites (or SiOHAl groups) are present in MCM-22.Using trimethylphosphine (TMP) as a probe molecule, acid sites in a series of Al-MCM-41 materials with Si/Al ratios ranging from 16 to 80 were investigated. By means of ¹H/²⁷A1 TRAPDOR technique that has the ability to establish connectivity between ¹H and²⁷A1, the signal of bridging hydroxyl (SiOHAl) was firstly been observed in ¹H spectra. The concentration of the Bronsted sites determined from ³¹P MAS NMR is about two orders of magnitude lower than that of total SiOH groups, which renders their direct observation by ¹H MAS NMR spectroscopy difficult. However, these sites can be well studied by ³¹P→¹H CP/MAS and various triple-resonance TRAPDOR NMR, which provides a more detailed description about the interaction between TMP and the different hydroxyl groups in the Al-MCM-41 materials. Although the acid strength of the Bronsted sites in the Al-MCM-41 materials is lower than that of microporous zeolites, such as HY, these sites can protonate TMP molecules. No Lewis acid sites were found in our samples though the...