Porous Organic Cage Membranes for Versatile Molecular Separations

Abstract

Membranes with excellent permeance and high selectivity offer an attractive route to molecular separations because technologies such as distillation and chromatography are energy-intensive. However, it remains challenging to fine-tune the structure and porosity in membranes so that they can effectively separate similar-sized molecules. In this thesis, a series of porous organic cage (POC) based membranes have been fabricated via different approaches to investigate the possibility of making membranes with excellent performance using POC as building blocks. In Chapter 3, the fabrication of composite membranes that comprise crystalline POC films have been fabricated by a novel interfacial synthesis approach on a polyacrylonitrile support (CC3-PAN). These membranes are continuous, crystalline, and exhibited ultrafast solvent permeance (including 177.3 L·m-2·h-1·bar-1 for acetone, 147.5 L·m-2·h-1·bar-1 for acetonitrile, 136.9 L·m-2·h-1·bar-1 for hexane, 55.9 L·m-2·h-1·bar-1 for toluene, and 42.9 L·m-2·h-1·bar-1 for water), and high rejection of organic dyes with molecular weights over 585 g∙mol-1. Other synthetic strategies were used to prepare the CC3 membranes, including spin-coating, casting, in-situ synthesis, and sonochemistry, and the obtained membranes showed different crystallinity and separation performance. Unlike the crystalline CC3 membranes, amorphous CC3 membranes fabricated via spin-coating have demonstrated the molecular weight cut-off (MWCO) shifts to ~400 g·mol-1. The crystalline CC3-PAN membrane discussed in Chapter 3 is dynamic, and its non-covalent structure could be switched using a solvent. The results presented in Chapter 4 demonstrate that the pore aperture of CC3-PAN can be switched in methanol to generate larger pores that provide increased methanol permeance and higher MWCO (1400 g∙mol-1). By varying the water/methanol ratio, the membrane can be switched between two phases that have different selectivities, such that a single, ‘smart’ crystalline membrane can perform graded molecular sieving. This effect has been exemplified by separating three organic dyes in a single-stage, single-membrane process. In Chapter 5, a series of thin-film composite membranes comprising a cage nanofilm separating layer have been fabricated at an aqueous–organic interface, using a series of reduced amine functionalised POCs cages. The obtained nanofilms were continuous and robust with an ultrathin thickness.