Solvent-processable 0D covalent organic framework quantum dot engineered composite membranes for biogas upgrading

Abstract

COF (covalent organic framework)-based composite membranes offer the opportunity for breakthroughs in separation performance through endowing COFs with easy film-forming properties. However, tough challenges of poor filler dispersity, limited functionality and interfacial compatibility severely restrict the development of COF-based composite membranes. Therefore, the careful design of physical and chemical structures for COFs is urgently needed. Here, a geometry transformation strategy, i.e. converting FCTF-1 (fluorinated covalent triazine frameworks) from 2D nanosheets to 0D quantum dots (QDs), is implemented to overcome some of the challenges. The greatly decreased filler size increases the solvent dispersibility and processability of FCTF-1 quantum dots (QD-FCTF-1), giving rise to thorough mixing between the polymer matrix and quantum dot filler. Apart from the changes in physical characteristics of filler, the geometry transformation using piranha solution introduces chemical functionality at the quantum dot edges, further strengthens the interfacial compatibility through the hydrogen bond interactions between polar hydroxy and amidine groups of QD-FCTF-1 and nitrile groups of PIM-1 (polymer of intrinsic microporosity). Moreover, these edge polar groups intensify favorable gas sorption due to their high affinity to CO2 molecules and thus significantly improve membrane gas pair selectivity. This work broadens the scope of framework-based materials and offers a new avenue for molecular separation.