Modified CARDO-Based Copolyimides with Improved Sour Mixed-Gas Permeation Properties

Abstract

Chemical modification is an interesting tool to introduce functional groups into polymers’ backbones to tailor their properties in a desired fashion. Polymers used in membrane-based gas separation applications need to possess high gas permeability and selectivity during mixed-gas separation under harsh operational conditions of pressure and temperature. Herein, we report the modification of three copolyimides containing the moiety 9,9-bis­(4-aminophenyl)­fluorine (CARDO) through Friedel–Crafts alkylation to introduce bulky tert-butyl groups into their backbones. Membranes prepared from the modified polymers were evaluated using pure- and sweet mixed-gas under aggressive feed pressures. Based on the promising results obtained, a selected copolyimide was subjected to sour mixed-gas separation under feed pressures up to 500 psi. Mixed-gas separation studies, especially those containing hydrogen sulfide, are of great interest to develop membranes for use in sour natural gas reserve treatment. For example, the H2S and CO2 permeability coefficients of 6FDA-Durene/6FDA-CARDO­(t-Bu) at 500 psi were recorded as 456 and 238 Barrer, respectively, where the H2S/CH4 and CO2/CH4 are 20.4 and 10.6, respectively. The obtained results are very promising and even superior to many glassy polymers reported previously. This work represents an example of the benefit of chemical modification on polymers to tailor their structure–property relationship. Future works will be focused on the fabrication and testing of asymmetric or composite hollow fibers to imitate the membrane modules used industrially.

Enhanced High-Pressure Mixed-Gas Sieving Properties of Thermally Cross-Linked Polyimide Membranes

“…11,27−31 The CARDO(OH) monomer is a derivative of the CARDO monomer [9,9-bis(4-aminophenyl)fluorene], which was studied thoroughly by our group. 10,11,27,28,32 Interestingly, the chemical structure of CARDO(OH) includes two hydroxyl groups located at the 2 and 7 positions of its fluorene moiety. These hydroxyl groups can act as (1) bulky groups within the polymer backbone to enhance the diffusion of gas molecules [i.e., V W of CARDO = 321.6 Å 3 and V W of CARDO(OH) = 338.5 Å 3 ], (2) polar groups to increase the gas-polymer interaction (i.e., higher solubility), and (3) sites for thermal cross-linking.…”

“…Therefore, the permeation through the membrane depends strongly on the solubility and diffusivity of the gas molecules, and the overall membrane performance relies on the difference between the solubility and diffusivity coefficients of those molecules. Hence, introducing new functional groups into the polymer’s backbone can (1) enhance the excess free volume within the membrane structure by altering the interchain packing (i.e., faster diffusion) − or (2) increase the polymer-gas interaction to improve the gas molecules solubility. − …”

“…The block copolyimide 6FDA-Durene/CARDO­(OH) (3:1) was prepared from the following three monomers: 6FDA [4,4′-(hexafluoroisopropylidene)­diphthalic anhydride], Durene (2,3,5,6-tetramethylbenzene-1,4-diamine), and CARDO­(OH) [9,9-bis­(4-aminophenyl)-9 H -fluorene-2,7-diol]. Polymeric membranes prepared from 6FDA-based copolyimides and containing the Durene moiety have shown great performance for mixed-gas separation in challenging environments. ,− The CARDO­(OH) monomer is a derivative of the CARDO monomer [9,9-bis­(4-aminophenyl)­fluorene], which was studied thoroughly by our group. ,,,, Interestingly, the chemical structure of CARDO­(OH) includes two hydroxyl groups located at the 2 and 7 positions of its fluorene moiety. These hydroxyl groups can act as (1) bulky groups within the polymer backbone to enhance the diffusion of gas molecules [i.e., V W of CARDO = 321.6 Å 3 and V W of CARDO­(OH) = 338.5 Å 3 ], (2) polar groups to increase the gas-polymer interaction (i.e., higher solubility), and (3) sites for thermal cross-linking.…”

Copolyimide asymmetric hollow fiber membranes for high‐pressure natural gas purification

“…Most published data on membranes for CO 2 removal are done under close‐to ideal conditions, using pure‐gas permeation properties as performance indicators 16 . Several studies have demonstrated that the performance of membrane‐based CO 2 removal from mixed‐gas streams containing CH 4 , heavy hydrocarbons and inert gases deviates significantly from pure‐gas properties, due to a combination of competitive sorption, membrane plasticization, concentration polarization and convective mass transfer 17–23 . Additionally, most of the membranes reported in literature are characterized and tested using simple dense‐film geometry (i.e., planar geometry).…”

“…16 Several studies have demonstrated that the performance of membrane-based CO 2 removal from mixed-gas streams containing CH 4 , heavy hydrocarbons and inert gases deviates significantly from pure-gas properties, due to a combination of competitive sorption, membrane plasticization, concentration polarization and convective mass transfer. [17][18][19][20][21][22][23] Additionally, most of the membranes reported in literature are characterized and tested using simple dense-film geometry (i.e., planar geometry). For large-scale industrial membrane-based application of gas separation, in particular, asymmetric or composite hollow fiber (i.e., tubular geometry) is often preferred, owing to its high packing density, resistance to high transmembrane pressures, and the ability to create ultra-thin selective layers.…”