Amine-based vapor-phase crosslinking processes of polyimide-based membranes for energy efficient separations

Abstract

Membrane processes are crucial in the industrial and engineering chemistry, for separation and purification of not only chemicals but also gases and wastewater treatment. In order to obtain high quality products from membrane processes, the membranes must be engineered to cater such performance, namely by chemical crosslinking process. One of the emerging crosslinking methods is the vapor phase crosslinking (VPC). The advantages of VPC for enhancing membranes are usage of fewer amounts of chemicals, with specific crosslinking of the top selective layer only (no unnecessary crosslinking of bulk layer). Therefore, membranes can boost their quality without sacrificing much of their productivity (quantity of flux). In this paper, several utilization of amine molecules for VPC of polyimide membranes for gas separations, pervaporation (gas-liquid separation), and organic solvent nanofiltration (OSN) are covered. The performance enhancements of the polymeric membranes for the aforementioned processes are summarized. Furthermore, the molecular mechanisms of the VPC of the amine-based vapors are reviewed, based on X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared characterizations. It was found that there are several possible scenarios of interaction between amines and polyimides, which are not only involving crosslinking (combination of several polymeric chains), but also grafting, and sometimes chain scissions. The investigation of molecular mechanism of the amine-based VPC process provides insights to the relationship between structural configurations towards the separation performance of various types of membrane processes, which paves way for the development of membranes of the future.