Carbon molecular sieve membrane from a microporous spirobisindane-based polyimide precursor with enhanced ethylene/ethane mixed-gas selectivity

Abstract

Ethylene is typically produced by steam cracking of various hydrocarbon feedstocks. The gaseous products are then separated in a demethanizer followed by a deethanizer unit and finally sent to a C₂ splitter for the final purification step. Cryogenic distillation of ethylene from ethane is the most energy-intensive unit operation process in the chemical industry. Therefore, the development of more energy-efficient processes for ethylene purification is highly desirable. Membrane-based separation has been proposed as an alternative option for replacement or debottlenecking of C₂ splitters but current polymer membrane materials exhibit insufficient mixed-gas C₂H₄/C₂H₆ selectivity (<7) to be technically and economically attractive. In this work, a highly selective carbon molecular sieve (CMS) membrane derived from a novel spirobisindane-based polyimide of intrinsic microporosity (PIM-6FDA) was developed and characterized. PIM-6FDA showed a single-stage degradation process under an inert nitrogen atmosphere which commenced at ∼480 °C. The CMS formed by pyrolysis at 800 °C had a diffusion/size-sieving-controlled morphology with a mixed-gas (50% C₂H₄/50% C₂H₆) ethylene/ethane selectivity of 15.6 at 20 bar feed pressure at 35 °C. The mixed-gas ethylene/ethane selectivity is the highest reported value for CMS-type membranes to date.