The alumination mechanism of porous silica materials and properties of derived ion exchangers and acid catalysts

Abstract

We herein investigate the alumination mechanism of siliceous micro- and mesoporous materials (SBA-15, SBA-16, and dealuminated Y-zeolite) with NaAlO₂ to synthesize new ion exchangers and acid catalysts. We show that in aqueous alkaline solution, the materials’ surface is partially dissolved to form Si(OH)_x groups (x = 1, 2, 3) that react with tetrahedral aluminum sites. The amount of introduced aluminum depends on the aqueous treatment temperature, pore diameters and structure of siliceous parents. The incorporation works best for hexagonal SBA-15 mesopores at the cost of micropores. Zeolite Y micropores remain accessible upon alumination. All Na-forms can quantitatively be ion exchanged. They are thus potential carriers for catalytically active metal ions. The presence of different Al(OH) groups is proven by ¹H{²⁷Al} TRAPDOR. The ²⁷Al MQMAS NMR spectra indicate the formation of octahedral and pentahedral aluminum upon transformation into H-forms. On [Al]SBA-15 up to 18% of ion exchange sites can be transformed into Brønsted acid sites. The acid properties were investigated using the probe molecules NH₃, acetonitrile-d₃, and TMPO. On mesoporous SBA-15 and SBA-16, Brønsted acid sites show a flexible coordination between Si(OH) and tetrahedral aluminum. The sites are weak and form exclusively in the presence of a strong base or counter ion. Zeolite acid site strength was found for re-aluminated Y. TMPO loading combined with ²⁷Al and ³¹P MAS NMR spectroscopy indicates the presence of Lewis acidic framework aluminum and the presence of several distinct Brønsted and Lewis acid sites on H-forms.