Synthesis of methyl 3,5-diaminobenzoate to develop polyamide thin film composite membrane for investigating the impact of in-situ methyl hydrolysis on its permeate flux and sulfate rejection

The performance of the desalination membranes has been improved by exploiting the potential of interfacial polymerization by using different combinations of reacting monomers. However, for successful interfacial polymerization, the aqueous amine must diffuse to the organic phase resulting in polyamide growth. Owing to that stringent requirement, using an essential amine 3,5-diaminobenzoic acid (DABA) for membrane fabrication in the literature has not been successful. The 3,5-diaminobenzoic acid is an analog of meta-phenylenediamine with an additional carboxylic group on the benzene ring. The presence of an additional carboxylic group in the case of 3,5-diaminobenzoic acid makes it attractive to develop a membrane with additional carboxylic groups in the active layer of the membrane. However, this carboxylic group hinders the diffusion of the 3,5-diaminobenzoic acid during interfacial polymerization, hence no appreciable growth of polyamide active layer. The current study resolved this challenge by masking the carboxylic group through methyl esterification yielding a methyl-ester-containing version N-DABA. When used during interfacial polymerization, the N-DABA yielded a dense polyamide active layer. To take advantage of the additional carboxylic group of 3,5-diaminobenzoic acid, the methyl ester group was partially hydrolyzed using an in-situ basic hydrolysis approach. Hence, two variants of membranes were fabricated in the current study, N-DABA/TMC and m-N-DABA/TMC membrane, and several desalination experiments were performed. The desalination experiments revealed that the membranes possessed preferential rejection of SO₄²⁻ ions (Na₂SO₄) reaching around 96% for the N-DABA/TMC membrane with a permeate flux of around 25.7 L m⁻² h⁻¹ at 20 bar. In the case of m-N-DABA/TMC membrane, a rejection of around 94.7% was recorded for Na₂SO₄. However, a significant enhancement of nearly 2 folds was recorded in the permeate flux reaching around 51.4 L m⁻² h⁻¹ at 20 bar. Moreover, the membranes showed a significant enhancement in the antifouling properties by achieving normalized fluxes of 0.88 and 0.73 for the N-DABA/TMC and m-N-DABA/TMC membranes, respectively.