Synthesis Of Mesoporous Molecular Sieves With Zeolitic Units Pore Walls By Dissolving Microporous Zeolites In Na₂SiO₃Solution

The conventional aluminosilicate zeolites possess the strong acidity and hydrothermal stability. But the pore size is narrow. Mesoporous M41S have the properties of large pore size and high surface area, the weaker surface acidity and poorer hydrothermal stability limited their applications. To combine the advantages of zeolites and mesoporous materials, in this dissertation, a series of mesoporous aluminosilicates with strong acidity and high hydrothermal stability were synthesized.Zeolites (HZSM-5, HY and Hβ) were selectively desilicated with Na2SiO3solution as alkaline medium. Mesoporous aluminosilicate molecular sieves (ZM, YM and BM) were hydrothermally synthesized, using all of the obtained zeolitic structural units, the removed silica and Na2SiO3as silicon and aluminum source, cetyltrimethylammonium bromide (CTAB) as surfactant. The physico-chemical characterization results indicated the obtained three samples were all mesoporous aluminosilicates with corresponding mother zeolitic structural units in the pore walls. Compared with Al-MCM-41, they had less isolated silanol groups, larger pore wall thickness, and higher percentage of AlⅣ. The acidity sites density, the proportion of strong acid sites, the B/L ratio and the hydrothermal stability of the three samples were significantly improved than those of Al-MCM-41.Cumene Cracking was comparatively tested. The benzene yield over the three synthesized protonated mesoporous aluminosilicates was ca.3-4times of that over HAl-MCM-41. Their catalytic stabilities were also significantly improved than those of the mother zeolites. The catalytic performances in the probe reactions (including the alkylation of benzene with1-dodecene and the benzylation of benzene with benzyl alcohol) over protonated ZSM-5, ZM and Al-MCM-41were evaluated. The results showed the catalytic activities over HZM had greatly increased than those over HAl-MCM-41or HZSM-5. The three mesoporous aluminosilicates combined the dual advantages of strong acidity and large pore size.A series of mesoporous aluminosilicates ZM with various Si/Al were synthesized. The HDS performance of DBT over ZM-supported Pd and Pt catalysts were studied. Their HDS activities could be affected by the dispersion of active components, and the ratio of Bronsted to Lewis acid sites (B/L ratio). Therefore, ZM (60) supported Pd and Pt catalysts, which possessed high metal dispersion and B/L ratio, showed the best HDS performance among the catalysts studied.Theoretical analysis suggested Na2SiO3system provided OH" by the hydrolysis of SiO32-, OH-was consumed in the desilication of aluminosilicate zeolites. The resulting decrease in OH-concentration will induce a further hydrolysis of SiO32-until a new equilibrium has been established. In this manner, OH-is controllably released; the concentration of free OH-is steady. Theoretical calculation and experimental investigation showed the desilication result of HZSM-5in Na2Si03[C (OH-)=0.28M] is comparable to that in NaOH [C (OH-)=4.20M]. The discussions indicated the initial silicon species in Na2SiO3could accelerate the desilication through crosslinking the removed silicon from zeolites. Thus the desilication could be carried out quickly under mild alkalinity.