Flexible thermally treated 3D PIM-CD molecular sieve membranes exceeding the upper bound line for propylene/propane separation

Abstract

Polymers of intrinsic microporosity (PIM) incorporated with beta-cyclodextrin (β-CD) (referred to as PIM-CD) are prepared via nucleophilic substitution copolymerization and then thermally treated at elevated temperatures from 300 to 600 °C. After the decomposition of thermally labile CD, the spaces originally occupied by CD convert to micro-pores and cross-linking points in the polymer matrix. The thermally induced three-dimensional (3D) cross-linking network not only possesses ultra-fine micro-pores and interconnected microvoids but also shows a superior molecular sieve ability for propylene/propane separation. The C₃H₆/C₃H₈ selectivity of the thermally treated membranes increases remarkably, about 3 times compared to that of the original untreated membranes. In particular, the PIM-CD membranes thermally treated at 300 or 400 °C are flexible, and they have C₃H₆/C₃H₈ separation performance exceeding the upper bound line in both pure and mixed gas tests. As observed from the gas sorption isotherms, membranes with a higher content of CD units have a higher gas sorption capacity. Additionally, the diffusivity selectivity of PIM-CD membranes after thermal treatment increases more quickly than that of the PIM membrane and contributes more to the permeability selectivity.