Propylene and Propane Separation Though Carbon Molecular Sieve Membranes Derived from a Tetraphenylethylene-Based Polymer of Intrinsic Microporosity (TPE-PIM)

Abstract

Efficient propylene and propane separation is a major challenge in the modern chemical industry. With current separation methods being highly energy-intensive, there is a pressing need to find alternative green technology. Membrane separation emerged as a promising candidate for propylene and propane separation. Their small footprint, low cost, reliability, and environmental friendliness give membrane separation systems a competitive edge in the race towards sustainable development. The continuous advancements in material science created avenues for new membrane materials such as carbon molecular sieve (CMS) membranes which exhibit exceptional gas separation performances for challenging applications due to their strong size-sieving capabilities. In this work, a carbon molecular sieve (CMS) membrane derived from a polymer of intrinsic microporosity (TPE-PIM) has been investigated for propylene/propane separation made by pyrolysis at 400, 450, 500, 550, 600, 650, and 700 ºC. TPE-PIM-derived CMS films showed excellent pure and mixed-gas permeability and selectivity, exceeding the upper bound limits for propylene and propane. Observed in this work was the presence of an optimal pyrolysis temperature at 600 ºC, where the film showed the best performance with a permeability of 41.6 Barrer and a selectivity of 197 based on pure-gas measurements but dropping to 34 Barrer and selectivity of 33 under equimolar mixed-gas conditions. Such performance could be attributed to the unique internal structural changes that occurred during the pyrolysis. In addition, propane permeability though the CMS films was slow and required long times to reach steady-state values. Such slow kinetics illustrates the molecular sieving capabilities of CMS membranes for bigger and more condensable gases. Several characterization techniques have been performed on the films to confirm CMS formation and showcase deeper molecular structure insights. X-ray diffraction of all TPE-PIM films showed a broad spectrum at each peak due to the material’s amorphous nature. Diffraction patterns also revealed a gradual peak shift for the (002) plane towards smaller values closer to that of pure graphite. Raman spectra showed the characteristic D and G peaks for carbon films prepared at 500 ºC and above. FTIR analysis was also performed to investigate the potential formation of triazine crosslinks in the thermally treated samples, but no conclusive results were obtained.