Preparation of mesoporous solid superbases by using metal oxide interlayers

Mesoporous solid superbases have high potentials for applications as environmentally friendly catalysts in diverse reactions. However, the generation of strong basicity on mesoporous silica is quite difficult because of the weak guest-host (precursor-silica) interaction and the poor alkali-resistant capacity of host. In this paper, a method was developed to generate strong basicity on mesoporous silica SBA-15 by precoating a metal oxide layer prior to modification by base precursor KNO₃. The results show that the guest-host interaction is enhanced by the introduction of metal oxides including Al₂O₃, MgO, ZrO₂, and CeO₂, which can be quantitatively characterized by hydrophilicity and electronegativity of the surface of host. As a result, the decomposition of KNO₃ can proceed at much lower temperatures as compared with KNO₃ supported on pure SBA-15. The alkali-resistance of mesoporous silica can also be tailored by the introduction of metal oxides, while mesostructure and basicity of resultant materials are strongly dependent on the dispersion state of metal oxides. Al₂O₃ and MgO can form smooth layers on the surface of SBA-15, which prevents the host from corroding by resultant strongly basic species. However, ZrO₂ and CeO₂ tend to aggregate, and thus the reaction of basic species with siliceous frameworks is still inevitable. We demonstrate that in the presence of an Al₂O₃ interlayer, materials possessing both ordered mesostructure and superbasicity (H_- = 27.0) are successfully fabricated, which is impossible to realize on pure mesoporous silica. The present method may open a new way for the design and synthesis of functional materials with strong basicity.