Diamino-organosilicone APTMDS: A new cross-linking agent for polyimides membranes

Homo-polyimides and co-polyimides (PI) were synthesized from dianhydride of 6FDA (4,4′-(Hexafluoroisopropylidene) diphthalic anhydride) and diamine of ODA (4,4′-Oxydianiline) or with ODA and TeMPD (2,3,5,6-tetramethyl-1,4-phenylenediamine) by chemical imidization in a two-step procedure. In this work, APTMDS (bis(3-aminopropyl)-tetramethyldisiloxane) is a diamino organosilicone proposed as a new cross-linking agent to produce polyimides membranes for CO2/CH4 separation. Two approaches were studied: direct mixing of APTMDS in the polyimide solution or immersing a PI membrane in a APTMDS/methanol solution. The modified and original polyimides were then fully characterized. The gas transport properties of CO2, CH4 and their blends were investigated at 35 °C and 150 psi. For neat gases, CO2 and CH4 permeability decreased and ideal selectivity increased with increasing immersion time and APTMDS concentration. The performance of APTMDS modified 6FDA–ODA membrane are above the Robeson [7] upper bound curve. For gas mixtures, the permeabilities and separation factors were studied as a function of CO2 molar fraction (35 °C, 150 psi) and feed pressure (50/50 CO2/CH4 at 35 °C). It was found that CO2 and CH4 permeabilities decreased while separation factors increased with immersion time and concentration. The modified membranes can support pressures as high as 600 psi (45 atm) giving them a wider range of operation conditions. Finally, the plasticization of polyimides can be practically eliminated by APSMDS cross-linking with either membrane immersion in methanol solution or initial mixing in the polyimide at the reaction step.

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► A new crosslinking agent is proposed for polyimide membrane.
► The effect of concentration and method of introduction is presented.
► Permeability, diffusivity and selectivity are determined.
► The optimum composition is used for the separation of CO2, CH4 and their blends.