Highly acidic mesoporous aluminosilicates prepared from preformed HY zeolite in Na2SiO3 alkaline buffer system

摘要

Mesoporous aluminosilicates (YM) were hydrothermally synthesized by assembling zeolite subunits, which were generated by the hydrolysis of HY zeolite in Na2SiO3 aqueous solution, in the presence of cetyltrimethylammonium bromide (CTAB). The Na2SiO3 aqueous solution plays two roles in the synthesis. It provides a moderate alkaline medium with buffering effect for the desilication of HY zeolite and it acts as a supplementary silica source. The desilication of HY zeolite was optimized in terms of reaction temperature and time. Characterizations by XRD, nitrogen isotherms and TEM indicated that YM possessed a well-ordered mesostructure. FT-IR, U V-Raman, 27A1 MAS NMR, NH3-TPD, and pyridine-adsorbed IR showed that the characteristics of Y zeolite, such as double six-membered ring subunits, tetrahedral coordination aluminum and high ratio of Bronsted/Lewis acidity sites, were preserved in YM. SEM observation revealed YM possesed a different morphology from either mesoporous MCM-41 or Y zeolite. No separate zeolite crystals were found in the TEM observation. The hydrothermal stability of YM towards refluxed water (120 h) and steam (873 K, 6 h), and the activity in acid-catalyzed cumene cracking (623 K) were compared with MCM-41 prepared with conventional method or parent HY. Because of the uniform structure and surface properties, YM was more hydrothermally stable than MCM-41; it also exhibited more efficient catalytic activitation than MCM-41 and HY in cumene cracking.