The effect of hydroxyl functionalization on the m-phenylene diamine moiety of 6FDA dianhydride-based polyimides was investigated for gas separation applications. Pure-gas permeability coefficients of He, H 2 , N 2 , O 2 , CH 4 , and CO 2 were measured at 35 °C and 2 atm.The introduction of hydroxyl groups in the diamine moiety of 6FDA-diaminophenol (DAP) and 6FDA-diamino resorcinol (DAR) polyimides tightened the overall polymer structure due to increased charge transfer complex formation compared to unfunctionalized 6FDA-m-phenylene diamine (mPDA). The BET surface areas based on nitrogen adsorption of 6FDA-DAP (54 m 2 g -1 )and of 6FDA-DAR (45 m 2 g -1 ) were ~18% and 32% lower than that of 6FDA-mPDA (66 m 2 g -1 ).6FDA-mPDA had a pure-gas CO 2 permeability of 14 Barrer and CO 2 /CH 4 selectivity of 70. The hydroxyl-functionalized polyimides 6FDA-DAP and 6FDA-DAR exhibited very high pure-gas CO 2 /CH 4 selectivities of 92 and 94 with moderate CO 2 permeability of 11 and 8 Barrer, respectively. It was demonstrated that hydroxyl-containing polyimide membranes maintained very high CO 2 /CH 4 selectivity (~ 75 at CO 2 partial pressure of 10 atm) due to CO 2 plasticization resistance when tested under high-pressure mixed-gas conditions. Functionalization with hydroxyl groups may thus be a promising strategy towards attaining highly selective polyimides for economical membrane-based natural gas sweetening.

The synthesis and gas permeation properties of two 6FDA-dianhydride-based polyimides prepared from 2,6-diaminotriptycene (6FDA-DAT1) and its extended iptycene analog (6FDA-DAT2) are reported. The additional benzene ring on the extended triptycene moiety in 6FDA-DAT2 increases the free volume over 6FDA-DAT1 and reduces the chain packing efficiency. The BET surface area based on nitrogen adsorption in 6FDA-DAT2 (450 m 2 g -1 ) is ~40% greater than that of 6FDA-DAT1 (320 m 2 g -1 ). 6FDA-DAT1shows a CO 2 permeability of 120 Barrer and CO 2 /CH 4 selectivity of 38, whereas 6FDA-2 DAT2 exhibits a 75% increase in CO 2 permeability to 210 Barrer coupled with a moderate decrease in selectivity (CO 2 /CH 4 = 30). Interestingly, minimal physical aging was observed over 150 days for both polymers and attributed to the high internal free volume of the shape-persistent iptycene geometries. The aged polyimides maintained CO 2 /CH 4 selectivities of 25-35 along with high CO 2 permeabilities of 90-120 Barrer up to partial CO 2 pressures of 10 bar of an aggressive 50:50 CO 2 :CH 4 mixed-gas feed, suggesting potential application in membranes for natural gas sweetening.

Please cite this article as: Alaslai N, Ghanem B, Alghunaimi F, Pinnau I, High-performance intrinsically microporous dihydroxyl-functionalized triptycene-based polyimide for natural gas separation, Polymer (2016), doi: 10.1016/j.polymer.2016.03.063. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT AbstractA novel polyimide of intrinsic microporosity (PIM-PI) was synthesized from a 9,10-diisopropyltriptycene-based dianhydride (TPDA) and dihydroxyl-functionalized 4,6-diaminoresorcinol (DAR). The unfunctionalized TPDA-m-phenylenediamine (mPDA) polyimide derivative was made as a reference material to evaluate the effect of the OH group in TPDA-DAR on its gas transport properties. Pure-gas permeability coefficients of He, H 2 , N 2 , O 2 , CH 4 , and CO 2 were measured at 35 °C and 2 atm. The BET surface area based on nitrogen adsorption of dihydroxylfunctionalized TPDA-DAR (308 m 2 g -1 ) was 45% lower than that of TPDA-mPDA (565 m 2 g -1 ).TPDA-mPDA had a pure-gas CO 2 permeability of 349 Barrer and CO 2 /CH 4 selectivity of 32.The dihydroxyl-functionalized TPDA-DAR polyimide exhibited enhanced pure-gas CO 2 /CH 4 selectivity of 46 with a moderate decrease in CO 2 permeability to 215 Barrer. The CO 2 permeability of TPDA-DAR was ~30-fold higher than that of a commercial cellulose triacetate membrane coupled with 39% higher pure-gas CO 2 /CH 4 selectivity. The TPDA-based dihydroxyl-containing polyimide showed good plasticization resistance and maintained high mixed-gas selectivity of 38 when tested at a typical CO 2 natural gas wellhead CO 2 partial pressure of 10 atm.

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Triptycene dimethyl-bridgehead dianhydride-based intrinsically microporous hydroxyl-functionalized polyimide for natural gas u p g r a d i n g ,

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Two novel Tröger's base-based di-o-substituted diamine monomers were synthesized and used to prepare two intrinsically microporous 6FDA-based polyimides (PIM-PI-TB-1 and PIM-PI-TB-2) with high molecular weight, high thermal stability and excellent solubility in common organic solvents. Compared to previously reported methods for preparing TB-based diamines, which are based on reduction of dimerized nitro-substituted anilines or condensation of phenylenediamine derivatives with dianhydrides, the novel protocol can be used to prepare different functionalized TB-based diamine monomers from a wide variety of aniline derivatives. PIM-PI-TB-1 (made from 6FDA and dibromo-tetramethyl-substituted TB diamine) and PIM-PI-TB-2 (made from 6FDA and tetramethyl-substituted TB diamine) are intrinsically microporous polymers with high BET surface areas of 440 m 2 /g and 580 m 2 /g, respectively. Pure-gas permeability coefficients of He, H 2 , N 2 , O 2 , CH 4 , and CO 2 were measured at 35 °C and 2 bar for fresh and 180 days aged films. Both TB-based polyimides exhibited high gas permeability with moderate selectivity. The gas permeability dropped significantly coupled with a moderate increase in selectivity after long-term physical aging of 180 days.

An intrinsically microporous polyimide is synthesized in m‐cresol by a one‐pot high‐temperature condensation reaction of 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and newly designed 2,6 (7)‐dihydroxy‐3,7(6)‐diaminotriptycene (DAT1‐OH). The 6FDA‐DAT1‐OH polyimide is thermally stable up to 440 °C, shows excellent solubility in polar solvents, and has moderately high Brunauer‐Teller‐Emmett (BET) surface area of 160 m2 g−1, as determined by nitrogen adsorption at −196 °C. Hydroxyl functionalization applied to the rigid 3D triptycene‐based diamine building block results in a polyimide that exhibits moderate pure‐gas CO2 permeability of 70 Barrer combined with high CO2/CH4 selectivity of 50. Mixed‐gas permeation studies demonstrate excellent plasticization resistance of 6FDA‐DAT1‐OH with impressive performance as potential membrane material for natural gas sweetening with a CO2 permeability of 50 Barrer and CO2/CH4 selectivity of 40 at a typical natural gas well partial pressure of 10 atm.