(672f) High-Performance Hydroxyl-Functionalized Polyimides for Natural Gas Separation

Natural gas separation has grown to one of the largest scale industrial applications of membrane technology during the past three decades. Introducing membrane technology to the natural gas industry presents a major change in conventional gas processing plants with projected growth specifically for CO₂/CH₄ separation. The most commonly used commercial membrane material for CO₂ removal from natural gas is based on cellulose acetate (CA) which has pure-gas selectivity of about 32-35 but under high-pressure, mixed-gas conditions, the CO₂/CH₄ selectivity often drops to less than 15 coupled with moderate CO₂ permeability. The modest performance of CA membranes provides a great incentive to develop more advanced polymer membrane materials with higher selectivity and permeability. Introduction of pendant polar groups, such as -OH and -COOH has been used for the structural design of polyimides with enhanced CO₂/CH₄ selectivity and functionality for various cross-linking mechanisms [1].

Here, we discuss the effect of hydroxyl functionalization on the m-phenylene diamine moiety of 6FDA- and triptycene dianhydrides-based polyimides for gas separation applications [2, 3]. Pure-gas permeability coefficients of He, H₂, N₂, O₂, CH₄, and CO₂ were measured at 35 °C and 2 bar. The introduction of hydroxyl groups in the diamine moiety of 6FDA-diaminophenol (DAP) and 6FDA/TDA-diamino resorcinol (DAR) polyimides tightened the overall polymer structure due to hydrogen bonding and charge transfer complex formation compared to unfunctionalized 6FDA/TDA-m-phenylene diamine (mPDA). 6FDA-mPDA had a pure-gas CO₂ permeability of 14 Barrer and CO₂/CH₄ selectivity of 70. The hydroxyl-functionalized polyimides 6FDA-DAP and 6FDA-DAR exhibited very high pure-gas CO₂/CH₄ selectivities of 92 and 94, respectively, with a moderate CO₂ permeability of 11 and 8 Barrer. It was demonstrated that hydroxyl-containing polyimide membranes maintained very high CO₂/CH₄ selectivity (~ 75 at CO₂ partial pressure of 10 bar) due to CO₂ plasticization resistance when tested under high-pressure mixed-gas conditions.

TPDA-mPDA had a pure-gas CO₂ permeability of 349 Barrer and CO₂/CH₄ selectivity of 32. The dihydroxyl-functionalized TDA-DAR polyimide exhibited enhanced pure-gas CO₂/CH₄ selectivity of 46 with a moderate decrease in CO₂ permeability to 215 Barrer. The CO₂ permeability of TDA-DAR was ∼30-fold higher than that of a commercial cellulose triacetate membrane coupled with 39% higher pure-gas CO₂/CH₄ selectivity. The TDA-based dihydroxyl-containing polyimide showed good plasticization resistance and maintained high mixed-gas selectivity of 38 when tested at a typical CO₂ natural gas wellhead CO₂ partial pressure of 10 atm. Functionalization with hydroxyl groups may thus be a promising strategy towards attaining highly selective polyimides for economical membrane-based natural gas sweetening.

KEYWORDS

6FDA, triptycene polyimides, hydroxyl functionalization, mixed-gas permeation

 

REFERENCES

1. Stern, S. Alexander, Hiroyoshi Kawakami, Ajay Y. Houde, and Guangbin Zhou. "Material and process for separating carbon dioxide from methane." U.S. Patent 5,591,250, issued January 7, 1997.

2. Alaslai, Nasser, Bader Ghanem, Fahd Alghunaimi, Eric Litwiller, and Ingo Pinnau. "Pure-and mixed-gas permeation properties of highly selective and plasticization resistant hydroxyl-diamine-based 6FDA polyimides for CO ₂/CH ₄ separation." Journal of Membrane Science 505 (2016): 100-107.

3. Alaslai, Nasser, Bader Ghanem, Fahd Alghunaimi, and Ingo Pinnau. "High-performance intrinsically microporous dihydroxyl-functionalized triptycene-based polyimide for natural gas separation." Polymer 91 (2016): 128-135.