Silica-Supported Perfluorobutylsulfonylimide And Catalysis Applications

Heterogeneous catalysis research has developed very rapidly over the past 20 years. In particular, solid acids have been concerned much on as"green"catalyst. However, most studies focused on oxygen acid, e. g. RSO₃H, R_fSO₃H, and less on nitrogen acid or carbon acid.The jargon"nitrogen acid"represents a kind of highly effective catalysts, bis[(perfluoroalkyl)sulfonyl]imides, whose anion exhibit a"weakly coordinating"characteristic since negative charge of nitrogen atom can be highly delocalized onto the whole S-O-N framework due to strongly electron-drawing and steric effects from the perfluorinated alkyl chains. The sulfonimides are a particularly versatile class of acids. They can be prepared in a large variety and are easily incorporated into polymeric systems.Immobilizing homogeneous catalysts onto inorganic materials is an important approach to prepare environmentally friendly and recyclable"green"clean catalysts.In this work, several functional polymerisable sulfonamide monomers were synthesized. Among them, sodium 4-styrenesulfonyl(per?uorobutylsulfonyl)imide (3) was selected as candidate for further study on polymerization and supporting.An acidic homopolymer, poly(4-styrenesulfonyl(perfluorobutylsulfonyl)- imide) (PSFSI) could be synthesized from 3. By means of immobilizing PSFSI onto SBA-15 silica, a new type of strongly acidic composite catalyst, PSFSI/SBA-15, was developed. The properties of the catalyst were characterized by NMR, FT-IR, XRD, SEM, TEM, XPS, TG, GPC, and N₂ adsorption-desorption technology. PSFSI/SBA-15 can be employed safely below 210 oC, with maximum content of 0.87 mmol/g and acid function between -8 to -11. The solid acid catalyst was highly effective and could be recycled five times without obvious loss of activity for the condensation of indole with structurally diverse aldehydes, which yielded diindolylmethane derivatives (DIMs). But catalyst loss was obvious in the condensation of o-phenylenediamine with aldehydes, which yielded 1, 2-disubstituted benzimidazoles.A solution to catalyst loss was resorted to PSFSI-MSMA/SiO₂, which was synthesized by a two-step sol-gel strategy. Functional polymer precursor PSFSI-MSMA was used to polymerize with inorganic precursor TEOS, and resulted to solid acid. The aforesaid methods and technologies were employed to characterize the hybrid material. The exchange capacity is 0.66 mmol/g.Esterification was selected as a model reaction to assess the catalytic activities of the PSFSI-MSMA/SiO₂. The catalyst could be recycled seven times without obvious loss of activity, and acted as water-tolerant solid acid.In summary, porous SiO₂-supported poly(4-styrenesulfonyl (perfluorobutylsulfonyl) imide) (PSFSI) are highly effective and environmentally friendly, and PSFSI-MSMA/ SiO₂ is a kind of recycled, green, and water-tolerant catalyst. Their further potential application in organic synthesis can be optimistically expected.