Aluminum incorporation in MCM-41 mesoporous molecular sieves

Twelve MCM-41 samples with a Si/Al ratio varying from 12 to 200 were prepared by using sodium aluminate in a tetramethylammonium hydroxide solution as aluminum source and an unexchanged CTMAC1 template solution. The thermogravimetric analysis (TGA) weight losses caused by thermal removal of the template cations, which are associated with the siloxy groups on the one hand and the aluminum sites on the other hand, correspond with these different Al contents. X-Ray diffraction (XRD) revealed that at Si/Al=5, in addition to MCM-41, a lamellar phase and a dense crystalline tridymite structure are formed. Al-27 Magic-angle spinning nuclear magnetic resonance (MAS NMR) indicates that upon calcination the tetrahedral aluminum sites are altered. Fourier transform infrared (FTIR) analysis of proton-exchanged MCM-41 with acetonitrile as probe molecule showed a very weak Bronsted and a rather strong Lewis acidity. The latter may be the result of the distortion of the aluminum sites by thermal treatment.