Perfluorinated nafion-modified SBA-15 materials for catalytic acylation of anisole
Mesoporous SBA-15 silica materials have been functionalized with perfluorosulfonic acidic Nafion resin using a post-synthetic impregnation method. The remarkable physicochemical properties of hexagonally ordered SBA-15 silicas (high surface area, large and narrow pore size distribution and thermal stability) make them particularly attractive for the immobilization of strongly acidic perfluorosulfonic sites. The loading of Nafion resin was varied between 10 and 20 wt%. The synthesized Nafion/SBA-15 hybrid materials were evaluated in the Friedel Crafts acylation of anisole. The optimum catalytic performance of Nafion/SBA-15 hybrid material with a resin loading around 13 wt% was compared with other perfluorosulfonic acid-based catalysts either commercially available or prepared following recipes reported in the literature. The influence of several variables on the acylation of anisole, such as the reaction temperature and the anisole/acylating agent ratio, has also been assessed by means of a factorial design of experiments. The catalytic activity of Nafion-modified SBA-15 materials is strongly affected by the reaction temperature, whereas a lesser effect was evidenced for the reaction mixture composition in the range under study. The optimal operation conditions in terms of anisole conversion were achieved at 150ºC and equimolar anisole/acetic anhydride mixture. The deposition of poly-acetylated by-products on the catalytic acid sites is mainly responsible for the catalyst deactivation, whereas the leaching of sulfonic groups and their contribution in the anisole acylation via homogeneous catalysis has been ruled out. Finally, the recovery of the catalytic activity by regeneration of the spent catalyst by refluxing in nitric acid solution was demonstrated. The characterization of the regenerated catalyst indicates that the regained activity is likely related to the displacement of the deactivating by-products out of the perflurorosulfonic acid sites during the nitric acid regeneration.
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