Enhancing membrane permeance through structural and distributional manipulation of amine monomers in interfacial polymerization

Abstract

Thin film composite (TFC) membranes with an ultrathin polyamide layer synthesized by interfacial polymerization (IP) are commonly adopted for reverse osmosis (RO) due to their energy efficiency and scalability. Although several methods have been tried to improve membrane performance, the manipulation of amine monomers, which have a direct influence on membrane structure and performance, still lacks deep investigation. This thesis aims to improve membrane permeance through the manipulation of amine monomers in terms of structure and distribution. Covalent organic frameworks (COFs), as an example of a competitive emerging novel material for membrane technology, which can be synthesized at the interface, are also investigated.

Three aromatic diamine monomers (i.e., m-phenylenediamine (MPD), o-phenylenediamine, and p-phenylenediamine) were used for TFC RO membrane fabrication. The resultant TFC-M2 and TFC-P2 membranes had typical “ridge-and-valley” morphologies, while the formed TFC-O2 membrane had a sponge-like morphology, which can be attributed to the different properties of amine monomers. TFC-M2 membrane possessed the highest rejection towards salts and organic micropollutants due to its smallest pore size. In addition, 2% w/w is the optimal concentration for the three polyamide membranes, but the effect of amine concentrations on membrane performance differed.

On the other hand, the distribution of MPD aqueous solution within the substrate influences the morphology and separation performance of resultant membranes. Three classical approaches for MPD loading during the IP reaction, i.e., vacuum filtration (TFC-V), roller (TFC-R), and air gun (TFC-A), were systematically investigated. The vacuum-assisted approach can greatly enhance the availability of MPD monomers, which could, in turn, result in enhanced “ridge-and-valley” morphology of the polyamide rejection layer because of the enhanced nanofoaming effect. Furthermore, the TFC-V membrane demonstrated the highest water permeance of 2.8 ± 0.4 L m-2 h-1 bar-1 compared to TFC-R and TFC-A membranes of 2.1 ± 0.2 L m-2 h-1 bar-1 and 2.1 ± 0.4 L m-2 h-1 bar-1, respectively.

To improve the permeance of TpPa-COF membranes constructed of 2,4,6-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa) monomers, aniline (An) molecules were introduced as end-capping molecules during membrane synthesis. The membrane synthesized with 30 percent of An molecules (TpPaAn-30/HPAN membrane) maintained a continuous COF layer with a thickness of ~ 20 nm but lower density or more free channels compared to the control membrane. The optimized TpPaAn-30/HPAN membrane had improved permeance for various organic solvents (e.g., 11.9 L m-2 h-1 bar-1 for methanol, fourfold of the control membrane) and maintained rejection towards methyl blue (>90%). It also allowed the passage of rhodamine B but blocked methyl blue when filtrating a mixed-dye methanolic solution.

In short, this thesis explored the structural and distributional manipulation of amine monomers for the improvement of membrane performance. Both polyamide-based TFC membranes (the state-of-the-art desalination materials) and COF membranes (one kind of emerging novel materials) were investigated. The findings gained in this thesis can deepen our understanding of membrane synthesis–structure–performance relationships and promote further advancement of membrane technology.