Mixed matrix membranes based on carbon quantum dots with enhanced CO2 capture performances

Abstract

Mixed matrix membranes (MMMs) can more readily surpass the Robeson upper bound by combining the complementary advantages of organic and inorganic membranes. However, these membranes inevitably suffer from nanoparticle agglomeration and the resulting non-selective interfacial defects, which significantly deteriorate their carbon dioxide (CO₂) separation performance. Furthermore, particle aggregation poses considerable challenges for fabricating thin-film nanocomposite (TFN) membranes. In this study, carbon quantum dots (CQDs) rich in amino and oxygen-containing functional groups were synthesized via a one-step method and used to fabricate MMMs for CO₂ separation. The small size of 0D CQDs enables their uniform dispersion in casting solutions, effectively preventing filler aggregation-induced degradation of CO₂ separation performance. Meanwhile, the addition of CQDs altered the disorder and hydrophilicity of Pebax 1657, significantly improving its CO₂ capture performance. The prepared membrane was characterized by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). With a feed pressure of 6 bar, the CO₂ permeability of the CQDs-Pebax 1657 membrane reached 248.3 Barrer, with a CO₂/N₂ selectivity of 78.1 (close to the 2019 Robeson upper bound). TFN membranes have also been developed, with a CO₂ permeance of 248.4 GPU and a CO₂/N₂ separation factor of 56.2. Under 100% relative humidity conditions, the CO₂ permeance was improved to 345.9 GPU, and the CO₂/N₂ separation factor was 64.2, outperforming most other Pebax-based membranes. The obtained membranes also exhibited good stability performances in a 72 h continuous test, denoting that CQDs can be considered an ideal nano-additive to effectively enhance membrane CO₂ capture permeances.