A High Free-Volume Pentiptycene-Based Polyimide for CO2 Removal

The widespread use of gas-separation processes in the chemical and petrochemical industries has motivated many to search for gas-separation processes more efficient than conventional processes. Gas separation using polymeric membranes has emerged as a more cost effective, less energy demanding, and more environmentally friendly alternative [1]. Among polymers used in membrane fabrication, polyimides (PIs) exhibit good chemical, mechanical, and thermal stability as well as excellent processability and solubility in different solvents [2]. However, common polyimides have low free volume (just between 10-20%), as explained by Robeson’s upper bound and Freeman’s theory [3, 4]. The incorporation of a rigid moiety into the PI backbone was found to lead to highly permeable and selective PIs, known as polymers of intrinsic microporosity (PIM)-PI. Pentiptycene is a highly rigid moiety from iptycene family consisting of five benzene rings forming an H-shaped core block. The bulky structure of pentiptycene impedes chain packing and provides large molecular space between arene rings. Furthermore, pentiptycene’s open clefts endow pentiptycene-based membrane with high internal free volume equivalent to the kinetic diameter of light gases [5]. However, the presence of two etheric oxygens in the backbone of pentiptycene-based membranes causes chain packing, resulting in a decrease in regularity and free volume [6]. So far, all pentiptycene-based PI membranes synthesized from pentiptycene diamines have shown low permeability but high selectivity due to their lower free volume. A range of 0.171-0.197 has been reported for the fraction free volume of pentiptycene-based membranes.

In this paper, for the first time we report the synthesis and characterization of a pentiptycene-based dianhydride (PPDAn) and a pentiptycene imide-containing diamine (PPImDA). Two novel pentiptycene-based PIs [PPDAn-Durene and PPImDA-4, 4′-hexafluoroisopropylidene diphthalic anhydride (6FDA)] are synthesized using the high temperature one-step imidization reaction. PPImDA-6FDA shows high molecular weight and excellent solubility in common organic solvents, and has an extraordinarily high free volume of 0.233 (confirmed by XRD patterns) due to the presence of −C (CF₃)₂− units, the rigid durene, and the pentiptycene moiety in the polymer structure. Finally, gas transport properties of PPImDA-6FDA (in CO₂ removal from natural and flue gas streams) are investigated and compared to those of 6FDA-durene. The high free volume of PPImDA-6FDA points to the great potential of this polymer for CO₂removal.

References

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[2] Ghanem, B., Nasser Alaslai, N., Xiaohe Miao, M., Pinnau, I., Novel 6FDA-based polyimides derived from sterically hindered Tröger’s base diamines: Synthesis and gas permeation properties, Polymer, 2016, 96, 13–19.

[3] Robeson, L.M., The Upper Bound Revisited. J. Membr. Sci., 2008, 320, 390-400.

[4] Freeman, B.D., Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes, Macromolecules, 1999, 32 (2), 375–380.

[5] Luo , Sh., Liu , Q., Baohua Zhang , B., Wiegand , J.R., Freeman , B.D., Ruilan Guo, R., Pentiptycene-based polyimides with hierarchically controlled molecular cavity architecture for efficient membrane gas separation, J. Membr. Sci. 2015, 480, 20-30.

[6] Luo, Sh., Stevens, K.A., Park, J., Moon, J.D., Qiang Liu, Q., Freeman, B.D., Guo, R., Highly CO₂-selective gas separation membranes based on segmented copolymers of poly (ethylene oxide) reinforced with pentiptycene-containing polyimide hard segments. ACS Appl. Mater. Interfaces, 2016, 8 (3), 2306–2317.