In situ FTIR spectroscopic investigation of active sites and adsorbate interactions in mesoporous aluminosilicate SBA-15 molecular sieves

Siliceous mesoporous molecular sieve SBA-15 was synthesized, and incorporated with aluminum via post-synthesis procedures. The type and concentration of hydroxyl groups in siliceous and aluminosilicate SBA-15 as a function of the aluminum content were investigated by in situ Fourier-transform infrared spectroscopy (FTIR) following adsorption of various probe molecules, such as pyridine, 1-hexene, benzene, and crotonaldehyde. The data show that isolated silanol groups on the pore surface of siliceous SBA-15 are the primary sites for aluminum incorporation. While different acid sites were identified in aluminosilicate SBA-15, a strong dependence of acidity/hydrocarbon adsorption capacity on the aluminum content was not observed, primarily due to the fact that the amorphous SBA-15 mesoporous walls lack well-defined tetrahedral symmetry for aluminum as found in crystalline zeolite materials. However, the aluminosilicate SBA-15 still showed activity toward adsorbate adsorption and transformation. At elevated temperatures up to 450 °C, a structural transformation of crotonaldehyde to carbonaceous matter (coke) was observed.