The most common membranes used for organic solvent nanofiltration (OSN) are integrally skinned asymmetric membranes formed via the immersion precipitation phase inversion process. These membranes are flexible, durable and easy to produce. However control of the porous properties of these membranes is not possible on a molecular level. Integrally skinned asymmetric membranes always exhibit signs of a pore size distribution, which leads to rejections not being optimum. Metal organic frameworks (MOFs) were added to polymeric OSN membranes in an attempt to improve the control over porous properties and separation properties above those achievable using integrally skinned asymmetric polymer membranes.
MOFs are crystalline materials with regular porous structures. MOFs have been used for gas separation purposes. This thesis describes the fabrication of hybrid polymer/MOF membranes for OSN applications. MMM fabrication is the tradition approach to create a hybrid polymer/MOF membrane, and contains discrete particles of MOF in a continuous polymer phase. The advantage of MMMs is that they are easy to produce; however, due to the discrete nature of the MOF in the membrane; the permeation of molecules through the membrane is not completely controlled by the MOF. MMMs were shown to have little difference in performance to polymer nanofiltration membranes for OSN.
In order for the MOF to exhibit more control over the permeation properties of membranes, in-situ growth (ISG) membranes were developed, whereby MOF material is grown within the pre-existing pores of polymer membranes. Using the MOF HKUST-1 and ultrafiltration polyimide supports ISG membranes were produced and were shown to outperform MMMs in terms of both solute retention and flux decline. Energy-dispersive X-ray spectroscopy (EDX) was used to reveal the distribution of HKUST-1 throughout ISG membranes, which was found to be even across the surface and throughout the cross-section, showing that a continuous phase of MOF had been grown.