Effect of pendent bulky groups on pure- and sour mixed-gas permeation properties of triphenylamine-based polyimides

Abstract

Three diamine triphenylamine-based monomers, 4,4′-diaminotriphenylamine (TPA), 4,4′-diamino-4″-(dimethylamino)triphenylamine (TPA(NMe2)), and 4,4′-diamino-4″-(tert-butyl)triphenylamine (TPA(t-Bu)) were reproduced in preparation for their 6FDA-based homopolyimides: 6FDA-TPA, 6FDA-TPA(NMe2) and 6FDA-TPA(t-Bu). The bulkiness effect of two pendent groups (dimethylamino and tert-butyl) on the gas transport properties of the polymers was evaluated. Both bulky groups enhanced the fractional free volume (FFV) within the membranes matrices, thereby increasing gas diffusivity, which resulted in greater CO2 permeability of 6FDA-TPA(NMe2) by almost 32% and 6FDA-TPA(t-Bu) by 3 folds compared to their nonfunctionalized analog 6FDA-TPA. Moreover, introducing bulky groups had minimal effect on gas solubility, indicating that separation through these materials is a diffusion-driven process. Afterwards, sweet mixed-gas permeation properties where measured for 6FDA-TPA and 6FDA-TPA(t-Bu) membranes using a multicomponent gas mixture (N2, CH4, C2H6 and CO2) at various feed pressures up to 500 psi and a fixed temperature of 22 °C. Once again, the introduction of the bulky group (t-Bu) resulted in an increase in the mixed-gas CO2 permeability coefficient by 3 folds, while the CO2/CH4 selectivity coefficient dropped by around 33%. Moreover, the sour mixed-gas permeation properties using a high H2S-content multicomponent gas mixture (20 vol% H2S) were measured for 6FDA-TPA and 6FDA-TPA(t-Bu) membranes at feed pressures up to 500 psi and 22 °C. The membranes showed nonideal behavior in the presence of hydrogen sulfide, where both membranes performed similarly and the effect of the bulky group (t-Bu) was negligible.