Fine-tuning the membrane performance in solvent-resistant nanofiltration: a chemical approach

Solvent-resistant nanofiltration (SRNF), also referred to as organic solvent nanofiltration (OSN), has gained a lot of attention as it opens up the possibility to reduce energy consumption and solvent waste. To widen its application, extensive research has been done towards obtaining good performing membranes with a high solute retention in combination with a high solvent permeance. With this work, it aimed to obtain a better insight in how the molecular structure of the membrane polymer influences the membrane performance. In the first part, binaphthalene-based polymer membranes are prepared, which combine a PIM-based concept with a molecularly adjustable structure. The implementation of a more contorted and rigid structure results in an improved permeance, while maintaining a high retention. By using a binaphthalene-based acyl chloride, it is possible to adjust the alkyl linker in between the naphthalene units, resulting in a change in the dihedral angle. A smaller angle resulted in a more closed structure and hence a higher retention. By combining the binaphthalene-based acyl chloride with various amine monomers, membranes with a different polarity and thus a different solvent affinity are obtained as a consequence of the difference in reactivity of the amines. Moreover, the atropoisomerism of binaphthalenes opens up the possibility to study the influence of the enantiomeric excess on the membrane performance. It shows that when an optically pure binaphthalene-based acyl chloride is used, the membrane performance excels independent of which amine is used. This study adds a polymer's enantiomeric excess to the toolbox of parameters that can be used to tailor membrane properties. In the second part, the influence of a polymer's enantiomeric excess and diasterioisomerism is studied more in-depth by implementing a 1,2-disubstituted cyclohexane with two adjacent asymmetric centers. Since the 1,2-disubstituted cyclohexane can be introduced as acyl chloride or as amine monomer, it opens up the possibility to study how the positioning of the functional groups play's a role in the membrane preparation and thus the membrane performance. If 1,2-disubstitued cyclohexane is used as acyl chloride, higher monomer concentrations and a much longer reaction time are required to obtain a good membrane performance. This is in sharp contrast to the use of 1,2-disubtituted cyclohexane as amine monomer. In the latter case, the monomer concentrations can be decreased tremendously and a reaction time of only 1 min is already sufficient to obtain a good membrane. Cis-,trans- and R,R-CHDA are used to study the influence of stereochemistry. This simple change in isomerie results in a different molecular weight cut-off curve for the different membranes. This shows that the stereochemistry of a monomer can play an important role in obtaining a membrane with the desired membrane performance (e.g. MWCO). All this research clearly shows that a molecularly adjustable structure and stereochemistry are two important, additional parameters that can be used to optimize the membrane performance towards its application. Untill now, all membranes prepared in this research are polyamides. Finally, to be able to implement the membranes in more extreme conditions, such as higher tempertures or extreme pH, it is aimed to prepare polyamine membranes. Polyamines are much more robust than polyamides. However, due to the lower reactivity of the monomers, too many parameters influenced the membrane preparation, which made it impossible to prepare reproducable membranes.

Publication year:2022

Accessibility:Embargoed