UV Raman Spectroscopic Study On The Synthesis Mechanism Of Meso-microporous Y/MCM-41Molecular Sieve

Meso-microporous molecular sieves have potential applications in the catalytic conversion of large molecules by combining the performances of both microporous and mesoporous molecular sieves. The current research mainly focused on the synthesis and applications of the meso-microporous molecular sieves. However, there are few studies focusing on the synthesis mechanism. In this thesis, the formation mechanism of Y/MCM-41has been investigated by using UV Raman spectroscopy, X-ray diffraction (XRD), infrared spectroscopy (IR), and electron microscopies (SEM and TEM). The main contents can be summarized as follows.1. The formation mechanism of zeolite Y as the precursor for Y/MCM-41has been studied. The solid phase releases oligomeric silicate species to the liquid phase by depolymerization. The liquid phase promotes the depolymerization of the gel by dissolving the oligomeric silicon species, and delivers the active species during the crystallization. The4-membered ring species are formed in the solid phase of the gel at the early stages of crystallization, and then they connect with each other to form the zeolite framework. The higher the alkalinity, the lower polymerization of the aluminosilicate species, the faster the speed of the crystallization and the lower the framework Si/Al ratio are.2. The mesopore formation of Y/MCM-41has been investigated. The4-membered ring species in the zeolite Y synthsis gel promotes the formation of a metastable mesopore structure just after mixing the zeolite precursors with CTAB. This has been ascribed to its high anionic charge density as well as the appropriate multidentate coordination. In contrast, the low-polymerized aluminosilicates and well-crystallized zeolite crystals cannot assemble with CTAB at this stage. Lowering down the pH value to9.3, the oligomeric silicon species polymerize with zeolite Y building units to constitute the highly ordered hexagonal mesoporous phase of Y/MCM-41.3. The precursor structures and the pH adjustment during the formation process affect the final structure of Y/MCM-41. Lowering the pH value down to9.3in the second step has a twofold effect:1. Enhance the polymerization of the amorphous silicate species to form the mesoporous phase;2. Break down the zeolite Y crystal to zeolite building units by dealumination, which leads to the structural change of the precursors. For the first time, it was confirmed that pure mesophase containing zeolite building units can be prepared by using the precursor containing zeolite crystals. Combining the effects of both the precursor structure and the pH adjustment, pure Y/MCM-41mesoporous phase containing4-membered ring building units, pure Y/MCM-41mesoporous phase containing4-membered ring and double6-membered ring building units, and the mixture of zeolite Y and mesoporous phase can be synthesized.4. Compared with Al-MCM-41, Y/MCM-41shows higher hydrothermal/thermal stability and stronger acidity. The catalytic activity of Y/MCM-41was studied by1,3,5-triisopropylbenzene craking reaction. Y/MCM-41with different structure and acidities show different activities and product distributions. Therefore, the catalytic performances can be controlled by using different Y/MCM-41as the catalyst.