MCM-41 with Improved Hydrothermal Stability: Formation and Prevention of Al Content Dependent Structural Defects

Abstract

The effects of Si/Al ratio of Al-containing MCM-41 materials prepared by either aluminum incorporation or postsynthesis alumination on the formation or prevention of structural defects and improvement of hydrothermal stability of MCM-41 have been investigated in detail. The states of Al and Si in the Al-containing materials are analyzed by MAS NMR, and the detailed properties of the materials subjected to hydrothermal treatment under different conditions are characterized by ICP, XRD, N2 adsorption, and FTIR. XRD and N2 adsorption results indicate that the amount of structural defects formed during calcination increases with the amount of aluminum in Si−Al−MCM-41. After calcination, Si−MCM-41 and Si−Al−MCM-41 having high Si/Al ratio (≥50) still show uniform pore structures, but structural defects are formed on the calcined Al-rich MCM-41 (Si/Al = 25 or 10). Structural defects are formed on all Si−Al−MCM-41 materials during hydrothermal treatment in boiling water. Although the mesoporous framework of Si−Al−MCM-41 is still found to be somewhat preserved after treatment in boiling water for 1 month, serious blockage of the pore channel is observed. To prevent the formation of structural defects, postsynthesis alumination has been found to be an effective method to prepare Al-containing MCM-41 with uniform pore structure. Moreover, the resulting uniform mesoporous structure of alumina-modified MCM-41 could be maintained, without the formation of structural defects, upon hydrothermal treatment in boiling water for 1 week.