A Novel Chlorin-resistant Polyamide Containing Florine Reverse Osmosis Composite Membrane

Reverse osmosis is called as a water purification technology in the21th century, and its core is the high functional reverse osmosis (RO)membrane. However, the conventional polyamide RO membrane is easyto be chloridized, which greatly limits the development and application ofreverse osmosis technology. On the basis of chlorination mechanism, inthis study, a new functional monomer of2,2-difluoro-1,3-diaminopropane(DFPDA) was synthesized, and then DFPDA was reacted with acylchlorine to prepare the novel chlorine-resistant reverse osmosis compositemembrane via interfacial polymerization technology.The DFPDA was synthesized through three reactions includingfluorination, amidation and reduction.The diethyl malonate as substratewas first reacted with the fluorination reagent of selectfluor to get the firstintermediate2,2-difluoro-diethyl-malonate (DFDEM), and then theDFDEM was amidated by ammonia water to prepare the secondintermediate2,2-difluoro-malonamide (DFMA). Finally, the resultingDFMA was reduced via boron hydride (BH3) to obtain the final productDFPDA. The reaction conditions of fluorination, amidation and reductionwere further discussed. The results showed that the reactions in thepresence of selectfluor and ammonia water were safer and easier than the traditional fluorination and amidation processes. The total yield ofreactions reached53.2%.Then, the synthesized DFPDA was reacted with5-choroformyloxyisophaloyl chloride (CFIC) to prepare a novelpolyamide-urethane containing fluorine DFPDA-CFIC membrane viasingle-step interfacial polymerization. However, the separationperformance of the resulting DFPDA-CFIC membrane was poor.Therefore, the functional monomer of DFPDA was proposed to react withthe remaining acyl chlorine monomers or groups of the substratemembrane (MPD-TMC) to get a super-thin chlorine-resistant film viatwo-step interfacial polymerization method, and both separation andchlorine-resistant performances of the resulting MPD-TMC-DFPDAmembrane were better than that of DFPDA-CFIC membrane. The effectsof membrane formation technologies including reaction time, curingtemperature, curing time, monomer concentration and glycolconcentration on the membrane performance were investigated. Thesurface morphology and chemical structure were analyzed via ScanningElectron Microscope (SEM), atomic force microscope (AFM), X-rayphotoelectronic spectroscopy (XPS) and Attenuated Total ReflectanceInfared (ATR-IR) and compared with that of the conventional polyamideRO membrane MPD-TMC.Lastly, the chemical stabilities to chlorine of three membranes wereevaluated by immersing membrane into different concentration NaClOsolutions, and FTIR-ATR and EDX were used to analyze the chemicalstructure and surface morphology before and after chloridized. Theresults showed that the chlorine-resistant performance of RO membranewas closely related with the chemical structure of active layer of ROmembrane. In conclusion, the polyamide containing fluorine RO compositemembrane via grafting super-thin chlorine-resistant film was found withenhanced chlorine resistance performance. The explored membraneformation law of two-step interfacial polymerization can provide a goodbasis for the preparation of high performance RO composite membrane.