Mixed matrix membranes for organic solvent nanofiltration

Polyimide (PI) (P84) based organic solvent nanofiltration (OSN) membranes are well documented in the literature. One drawback of P84 based OSN membranes is the decline in the flux performance over time and with pressure. This is attributed to the compaction of the membrane top layer. The present work introduces novel P84 based OSN membranes which overcome the problem of compaction by incorporating an organic-inorganic hybrid network within the membrane matrix. The inorganic network also acts as a crosslinking agent for the polyimide. 3-Aminopropyl trimethoxysilane (APTMS) was used as a crosslinking agent as well as an organosilicone precursor to generate an inorganic network. An inorganic organosiloxane network (Si-O-Si) was generated in the membrane by hydrolysis and condensation of methoxysilane in APTMS. The impact of APTMS on the resulting PI membranes was investigated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), water contact angle and energy diffracted X-ray (EDX). FTIR analysis confirmed the presence of the Si-O-Si network in the membrane and EDX analysis confirmed the homogeneous distribution of this network throughout the membrane including in the thin separation layer. Nanofiltration performance of these membranes was evaluated in solvents such as acetone, dimethylformamide (DMF) and dichloromethane (DCM). Flux profiles showed a great improvement in terms of resistance to compaction after treating with this organic-inorganic based crosslinker, although the inorganic network reduced the flux. After treatment with APTMS, membranes increased in rigidity and strength, and there was no swelling even in high swelling solvents such as DCM. Addition of maleic acid to the dope solution provided an improvement in the flux of the crosslinked membrane. The flux could be also manipulated by varying the crosslinking conditions. Nanofiltration (NF) performance of the mixed matrix membrane (MMM) was compared with the commercially available membrane Duramem™300 in different solvents, and was found to be more resistant to compaction but with lower flux as compared to Duramem™. The performance of MMM was also compared with Duramem™200 for a case study of active pharmaceutical ingredient (API) purification.