Solid-State NMR Studies Of Base-Induced Br(?)nsted Acid Sites In Mesoporous And Microporous Zeolites

In this dissertation, the nature of base-induced Bro|¨nsted acid sites in SBA-15 and HY zeolite are investigated by solid-state NMR techniques, especially probe molecule techniques. The interaction between CeO₂ andγ-Al₂O₃ is also studied and some results are obtained.SBA-15 is a new type mesoporous zeolite. It has larger pore size than microporous zeolite, and stronger hydrothermal stability than the other mesoporous zeolite such as MCM-41, which provides a potential for the material as versatile catalysts for conversion of large molecules. Since pure siliceous SBA-15 materials lack acidity, we introduce Al atoms into the framework of SBA-15 without destroying the order of the structure. We find that no signals from the bridging hydroxyl groups associated with Bro|¨nsted acid sites are directly observed in either ¹H MAS or ¹H/²⁷Al TRAPDOR NMR spectra of dehydrated Al-SBA-15 materials. But after the adsorption of basic molecules, such as TMP, TMPO and acetone, signals of these probe molecules adsorbed on Bro|¨nsted acid sites can be observed by ¹³C and ³¹P NMR spectra. We thus believe that Bro|¨nsted acid sites in dehydrated mesoporous Al-SBA-15 materials are not in the form of bridging hydroxyl groups, but originate from terminal silanol groups in the vicinity of aluminum atoms that can be induced to generate the bridging hydroxyl groups by the adsorption of basic molecules. It is interesting that the acid strength detected by different probe molecules is different.Using trimethylphosphine (TMP) and d₅-pyridine as basic probe molecules, the concentration of Bro|¨nsted acid sites on both HY zeolite and dealuminated HY zeolite...