NMR Studies On Solid Acids And Crystallization Of Molecular Sieves

In the chapter 1 of this dissertation, various solid-state NMR techniques and their applications to the structure and acidity characterization of zeolites as well as the synthesis mechanism of molecular sieves were briefly introduced. Chapter 2 focus on the synthesis of mesoporous solid acids and their NMR characterization. The characterization of crystallization process of two kinds of molecular sieves is the subject of the later two chapters.Mesoporous ZrO₂, MoO_x/ZrO₂ and WO_x/ZrO₂ materials were prepared and their solid acidity was thoroughly studied by solid-state NMR techniques and DFT calculations. Two distinct types of Bronsted acid sites with acid strength stronger than zeolite HZSM-5, comparable to sulfated zirconia, but still weaker than 100% H₂SO₄, were found to be present on the mesporous MoO_x/ZrO₂ and WO_x/ZrO₂ materials. With the help of theoretical calculation, the detailed structures of Bronsted sites formed on the surface of the mesoporous catalyst were revealed and the predicted acid strengths of these sites were in good agreement with experimental observations. Besides the weak acidic Zr-OH groups, Lewis acid sites (coordinatively unsaturated Zr⁴⁺ sites) are present on the surface of mesoporous ZrO₂. After the introduction of Mo or W species, the coordination of Mo-OH or W-OH to the unsaturated Zr⁴⁺ sites leads to the appearance of bridging Mo-OH-Zr (or W-OH-Zr) hydroxyl groups that act as Bronsted acid sites, and a remarkable decrease in the concentration of Lewis acid sites present on the surface of ZrO₂. The bridging Mo-OH-Zr or W-OH-Zr hydroxyl groups in the form of monomer and oligomer states are responsible for the strong Bronsted acidity of the MoO_x/ZrO₂ and WO_x/ZrO₂ materials. Based on our NMR experimental and theoretical calculation results, a possible mechanism was proposed for the formation of acid sites on these mesoporous materials.Two types of industrially important molecular sives AlPO₄-5 and MgAPO-36 were prepared under the hydrothermal condition. Multinuclear solid-state NMR techniques, combined with powder X-ray diffraction (PXRD), infrared (IR) and SEM spectroscopy, were employed to monitor their crystallization process. For AlPO₄-5, the crystallization process is characterized by the evolution of intermediate gels, in which the long-rang ordering arrangement is probed by PXRD, revealing the threshold of the crystallization around 120 min. The appearance of ³¹p signals at ca.-22 and -29 ppm due to the structural P-O-Al unit and ¹⁹F signal at -120 ppm due to the structural F-Al_pen-O-P unit in the NMR spectra of the series intermediate gel indicates that the crystalline framework is starting to form. The onset of the crystallization is also evidenced by the presence of the pentacoordinated Al in the structural F-Al_pen-O-P unit which is considered to be associated with the ordered framework. More information about the local ordering of the gels is obtained from two-dimensional ²⁷Al→³¹p HETCOR and ³¹p/²⁷Al double-resonance experiments. In combination with ¹H→³¹p CP/MAS experiments, two micro-domains can be identified in the 120 min heated gel. A possible evolution mechanism consisting of three successive stages is proposed for the crystallization process.For the magnesium substituted aluminophosphate MgAPO-36, the long-range ordering arrangement of the aged as well as heated intermediate gels were probed by PXRD, revealing that the crystallization of the framework begins at about 1.5 h at 423 K, when the structural Al-O-P and Mg-O-P units from the ATS framework were proved to be formed by ³¹p NMR. After the stick-like crystallites start to form in the semi-crystalline phase, they consequently aggregate into irregular sphere crystals. More information about the local ordering of the gels is obtained from two-dimensional ²⁷Al→³¹p HETCOR and ¹H→³¹p CP/MAS experiments. Different micro-domains can be identified with varied condensation degree of the P coordinating sphere, in which five type of P(nAl) (n=1-4) units were determined by ³¹p/²⁷Al double-resonance experiments. A possible evolution mechanism consisting of three stages is proposed for the crystallization process.