Constructing mesoporous solid superbases by a dualcoating strategy

Abstract

Great attention has been paid to the development of mesoporous solid superbases due to their versatile catalytic activity under mild conditions. In this paper, a dualcoating strategy was designed to generate superbasicity on mesoporous silica SBA-15 by precoating ZrO₂ prior to the loading of base precursor (KNO₃). The ZrO₂ interlayer performs two functions by enhancing the guest–host interactions to promote the decomposition of KNO₃ at low temperatures, and by improving the alkali-resistance of the siliceous host, which simultaneously overcomes two weaknesses of mesoporous silicas. As a result, new dualcoating composites possessing both an ordered mesostructure and superbasicity were successfully fabricated. However, direct modification of SBA-15 with KNO₃ results in a sample with only weak basicity and a collapsed mesostructure. Interestingly, both the amount and dispersion degree of ZrO₂ play an important role in the generation of strong basicity. An intact layer can be formed on SBA-15 provided that the content of ZrO₂ is higher than 30 wt% and the ammonia/water-induced hydrolysis (AIH) method is employed. The mesostructure cannot be preserved if the content of ZrO₂ is lower than 30 wt% or if conventional coating methods (i.e. impregnation and grinding) are used. The obtained materials containing ZrO₂ are active in heterogeneous synthesis of dimethyl carbonate (DMC) and the yield of DMC can reach 28.4%, which is obviously higher than that over the material without a ZrO₂ coating (9.8%). The present strategy may open a new way for the design and synthesis of functional materials with strong basicity.