Synthesis And Catalytic Application Of Hydrophobic Perfluoroalkylsulfonylimide Solid Acid

As is well-known, the catalysis application of solid acid is one of research issues of green chemistry. In this study, a new style of strong acid copolymer with excellent hydrophobicity was designed and prepared, and further used to catalyze esterification and Pechmann reaction.Firstly, we have successfully synthesized vinyl-style perfluoroalkylsulfonylimide monomer,p-CH2=CH-C6H4SO2NNaSO2CmF2m+1(m=1,4, designated as SSFAI), with sodium p-styrene sulfonate and perfluoroalkylsulfonyl amine. The structures of SSFAI were confirmed by FT-IR,1H NMR,19F NMR, MS technology.Perfluoroalkylsulfonylimide-functionalized polydivinylbenzene-based solid acid were synthesized by copolymerization of SSFAI with divinylbenzene (DVB) under solvothermal conditions, noted as H-PDVB-x-SSFAI (where x stand for the molar ratio of SSFAI and DVB). The optimal conpolymerization condition is: SSFAI:DVB (mol/mol)=0.3, CH3COOEt: H2O (v/v)=10:2as solvent, nano-CaCO3as porogen,0.3mmol%AIBN as initiator, strring at100℃for24h in a sealed autoclave. The resulted non-transparent gel was dried and ion-exchanged with6mol/L HCl, giving fine powder. The structure was characterized by FT-IR, XPS, TEM and N2adsorption-desorption technology, indicating successful introduction of perfluoroalkylsulfonylimide groups onto the network of polydivinylbenzene. The surface area of H-PDVB-x-SSFAI was346-385m2/g, and the most probable pore size distribution was3.8nm. Solid state13C MAS NMR technique was used to identify the acid strength of H-PDVB-x-SSFAI,2-13C-acetone as a probe molecule, and the13C chemical shift of2-13C-acetone were higher than220ppm, showing that H-PDVB-x-SSFAI was strong acidic. H-PDVB-x-SSFAI exhibited excellent hydrophobicity with static contact angle of higher than120°for water. The copolymer can be used safely below208℃according to TG analysis.The esterification of lauric acid and ethanol (1:5) is selected as model reaction. Under80℃, the93%-95%conversion of lauric acid was achieved with H-PDVB-0.3-SSFAI as catalyst for6h, and the highest Turnover Frequency (TOF) was up to30.5h-1(v.s.71.7%and31.9h-1, respectively with H2SCO4as catalyst). More importantly, the conversion of Athe hydrophobicity of H-PDVB-0.3-SSFAI repeled water far away from acid sites, and thus, smoothly promoted the reaction towards ester-producing direction.Furthermore, in the catalytic esterification of acetic/hexanoic acid with cyclohexanol, the conversion of acid reached at87-92%. However, in the reaction of tert butyl alcohol, the conversion was only27-39%. The results indicated that esterification of secondary alcohol, which has larger steric hindrance, was carried out smoothly due to the excellent acidity of H-PDVB-0.3-SSFAI. Additionally, the reaction of tertiary alcohol was still unsatisfactory.The recycle test was performed by esterification of acetic acid and amyl alcohol under110℃, giving89-93%of yield. After seven runs, the recovered catalyst still provided85%of yield, indicating no obvious loss of catalysis activity. H-PDVB-0.3-SSFAI has good reusability.Pechmann condensation was selected as another model reaction. In the condensation of resorcinol and ethyl acetoacetate under140℃,7-hydroxy-4-methyl-coumarin reached its maximum yield of88-91%. After four runs, the recovered catalyst provided slightly lower74%of yield, showing t at H-’ DVP-0.3-SSFAI could be reused to some degree under the higher temperature.