Synthesis Of Key Functional Monomers And Study On The Preparation And Performance Of Reverse Osmosis Membrane Based On In-situ Modification Via Interfacial Polymerization

Reverse osmosis (RO) is a separation technology with high efficiency and energy saving, and the RO membrane is the core of reverse osmosis technology. However, the current polyamide TFC reverse osmosis membrane is easy to be fouled and oxidated in the process of reverse osmosis. In this study, therefore, the antifouling polyamide-urea RO membrane and the chlorine-resistant polyimde-urethane membrane were modified respectively to obtain the antifouling as well as chlorine-resistant RO membrane according to the mechanisms of fouling and oxidation of polyamide membrane.Firstly, three key functional monomers, including 5-Isocyanato-isophthaloyl chloride (ICIC), N,N’-dimethyl-m-phenylenediamine (DMMPD) and 2,2-difluoro-1,3-diaminopropane (DFPDA), were be synthesized. ICIC was prepared by triphosgene (BTC) method via reverse adding mode of substrate, and this reaction process was safer and had higher reactivity compared with the traditional method by using phosgene or oxalyl chloride, and the yield reached 61.8%. DMMPD was synthesized by the reduction method with composite reductant NaBH4-I2, which had higher reduction efficiency and more pure product than the ordinary LiA1H4 method, and the top yield was 76.6%. The DFPDA was prepared by using the fluorination reagent of selectfluor and amidation reagent of ammonia water, which was safer and milder compared to the method by using fluorine F2 and ammonia NH3, and the top yield of 53.2% was obtained.Then, the synthesized DMMPD was reacted with 5-choroformyloxyisophaloyl chloride (CFIC) to prepare the polyimide-urethane DMMPD-CFIC membrane via single-step interfacial polymerization. The resulting DMMPD-CFIC membrane had good chlorine resistance performance, but its separation performance was relatively poor. And then, DMMPD was combined with MMPD to prepare the DMMPD/MMPD-CFIC and MMPD-CFIC-DMMPD membranes via respective sing-step and two-step interfacial polymerization, and the separation performances of both membranes were better than that of DMMPD-CFIC membrane. On the basis of this, the NaA zeolite and nano-Ag were doped in the two membranes respectively. The resulting polyimide-urethane membrane with doping NaA zeolite had high water flux, and its organic fouling resistance performance also was enhanced, whereas the membrane with doping nano-Ag had good biofouling resistance ability. Concurrently, the two types of membranes had few impacts on their chlorine-resistant performance.The polyamide-urea composite RO membrane had good antifouling performance, but it is susceptible to the active chlorine. In order to improve its chlorine resistance performance, the super-thin chlorine-resistant film of ICIC-DMMPD or ICIC-DFPDA was thus in-situ grafted on the initial antifouling polyamide-urea substrate membrane via two-step interfacial polymerization method, and at the same time the modified membrane was smoother and more hydrophilic, and its electrical property also was changed. The monomers of DMMPD and DFPDA had poor solubility in water, thus glycol was combined with water to prepare the aqueous phase solution. The 2 percent of DMMPD and DFPDA aqueous solutions could be prepared when the corresponding concentrations of glycol were 30% and 80% respectively, and as a result of this the obtained MMPD-ICIC-DMMPD and MMPD-DMMPD-DFPDA membranes had good separation performance as well as chlorine-resistant performance.Finally, the molecular dynamics simulation method was used to model the polyamide-urea membranes before and after grafting the super-thin chlorine-resistant film ICIC-DMMPD. According to the calculated results of interaction energy, it was proved that the modeling system of membrane grafted super-thin ICIC-DMMPD film was stable, and the super-thin chlorine-resistant film was attached to the initial substrate membrane via strong interaction forces of Van Der Waals force and electrostatic force. The variation of adsorption point of CIO" in modeling systems of membranes before and after grafting super-thin ICIC-DMMPD film was further simulated, and it was demonstrated microscopically that steric hindrance of-CH3 existed in imide functional bond (-CONCH3) was the key factor that impacted the chlorine resistance performance of the modified RO membrane. In addition, the diffusion coefficients of H2O, Na+ and C1 in the modeling systems of membrane were calculated. The results showed that all of the diffusion coefficients were decreased in the membrane grafted ICIC-DMMPD film. However, the diffusion coefficient of H2O was increased in the two-step polymerization modification membrane with doping NaA zeolite; on the contrary, the diffusion coefficients of Na+ and Cl" were decreased. Obviously, the graft of super-thin chlorine-resistant film could cause the decrease in water flux, but few impacts on the salt rejection, and the addition of NaA zeolite could improve the water flux of two-step polymerization modification membrane. From those, the influence mechanisms of super-thin chlorine-resistant film and NaA zeolite on the two-step polymerization modification RO membrane were ascertained.In conclusion, the antifouling polyamide-urea membrane with grafting super-thin chlorine-resistant film was found with enhanced chlorine resistance performance, and the chlorine-resistant polyimide-urethane membrane with doping nano-particle was discovered with improved antifouling ability. The novel membranes have large potential for industrial application.