As an emerging nanomaterial, graphene oxide (GO) has gained a special interest in the field of nanocomposite membranes for gas separation. However, its agglomeration within the polymer matrix inhibits its beneficial effects. In the present study, the effect of increased oxygenated groups of GO on its agglomeration tendency and the gas separation properties of the resultant mixed matrix membrane (MMM) was investigated for the first time. GO and carboxylated GO (GO-COOH) were incorporated into polysulfone (PSU), and the two types of membranes were compared in terms of nanofiller dispersion, chemical and crystalline structure, tensile and thermal properties, and CO2/CH4 separation. The homogenous dispersion of GO-COOH compared to GO led to the higher tensile strength, elastic modulus, and thermal stability of the MMM. The PSU/GO-COOH membrane at an optimal nanofiller concentration exhibited significantly higher CO2 permeability and CO2/CH4 selectivity in both pure and mixed gas experiments. Using CO2/CH4 mixed gas with a 40/60 volume ratio, the PSU/0.2% GO-COOH membrane provided a permeability of 55.20 barrer and a CO 2 selectivity of 280.20, which were almost 18 and 29 times, respectively, higher than those of the neat PSU membrane. The performance of the mentioned membrane was significantly superior to those of the previous literature and surpassed the 2019-updated Robeson upper bound.

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