Polybenzimidazole membranes for organic solvent nanofiltration: formation parameters and applications (2015)

Recently, organic solvent nanofiltration (OSN) became an attractive tool for molecular separation in organic feed streams. Although commercial polymeric membranes are available and easily accessible, they still face several issues in OSN conditions. For instance, commercial membranes usually have low discrimination between the target molecules and waste reagents, poor stability in organic solvents and acidic/basic conditions and inconsistent performance over service life time. The main objective of this research was to study the fabrication and function of polybenzimidazole (PBI) membranes for OSN in order to obtain a more versatile membrane than the available commercial ones. The stability of PBI membranes in harsh solvents and corrosive environments was achieved by chemically crosslinking the polymeric backbone. The chemical resistance of crosslinked PBI membranes was demonstrated by exposing the membranes to realistic conditions used in pharmaceutical and chemical separation processes, such as organic solvents containing acids or bases. Further, the crosslinking reaction itself was studied thoroughly by analysing the reaction mechanism and the effect of reaction parameters on membrane stability and performance. A better understanding of these parameters is crucial to the development of robust, reproducible and scalable membranes. The analysis was carried out using a statistical approach based on Design of Experiments (DoE) methodology, which enabled reduction of the number of experiments, to study more than one factor at a time and to identify the relevant parameters and their interactions. A phenomenological interpretation of the statistical models was also attempted. The investigation of the reaction mechanism revealed the formation of charged complexes in the PBI backbone that can be used to facilitate the transport of solutes carrying a charge opposite to that of the membrane’s surface. This was studied by filtration experiments through PBI membranes using charged and neutral solutes with similar molecular weights. The charge of crosslinked and uncrosslinked membranes was modified by applying different treatments to the membranes. It was found that the transport of solutes through PBI membranes was based on both size exclusion and ion exchange. Finally, the effect of the choice of co-solvent or non-solvent additive in the fabrication of PBI membranes was studied and it was found that different additives result in membranes with different properties. However, no correlation was found between the properties of the solvents and the performance of the membranes. In conclusion, the availability of such membranes could lead to a wider implementation of membrane units in many industries which require separation of molecules from organic solvents. This was demonstrated by applying crosslinked PBI membranes to OSN membrane cascades in which the problem of insufficient separation capability was addressed. Also, uncrosslinked PBI membranes were successfully molecularly imprinted and have been shown to have potential as size exclusion barriers and shape specific absorbents.