Exceptional CO₂ and H₂S adsorption by tuning micro/mesopore ratios with embedded graphene oxide/N-doped carbon quantum dots in MIL-101(Cr): Experimental and computational insights

Herein, a novel nanocomposite was developed to adjust the textural properties of metal-organic frameworks (MOFs) for adsorptive applications. To this end, nitrogen-doped carbon quantum dots/reduced graphene oxide nanocomposite (RC) was embedded into MIL-101(Cr) crystals, named RC-ML-x nanocomposites. The prepared nanoadsorbents were thoroughly characterized by different techniques. Results revealed that the main drawback of microporous MOFs, lack of mesopores, could be solved by embedding RC nanoparticles into MOFs, decreasing the micropore/mesopore volume ratio from 7.71 to 1.15. Optimizing the mesopore volume in RC-ML-1 dramatically improved the surface area and total pore volume by 40 % compared to pristine MIL-101(Cr). Adsorption experiments indicated that the sample containing 1 wt% had outstanding CO₂ and H₂S adsorption capacity of 25.79 and 34.15 mmol g^-1 at 35 and 15 bar in 25 °C, respectively, elevated up to 15.80 % and 19.26 % compared to pristine MIL-101(Cr). This may be attributable to the cumulative effect of suitable micropore/mesopore volume ratio and the creation of the unsaturated metal sites and nitrogen functional groups by RC loading. In addition, the adsorption selectivity in different gas mixtures of CO₂/CH₄, H₂S/CH₄, CO₂/N₂, and H₂S/N₂ was analyzed by IAST. It was found that the samples containing 10 and 5 wt% had the highest selectivity toward CO₂ and H₂S, respectively, over N₂ and CH₄. Considering the simple approach adopted to tune the structure of microporous MOFs to achieve impressive gas adsorption and great cyclic capacity, the proposed RC-ML-x nanocomposites can be potential candidates for the adsorption and separation of CO₂ and H₂S.