Preparation of MOF NPs and pMOF MSs
To illuminate the concept of improving separation performance of MMMs by incorporating pMOF MSs, we employed NH2-UiO-66 as filler and PSF as matrix. As one of the most studied MOFs materials, NH2-UiO-66,47,48 constructed by the coordination between Zr6O4(OH)4 centers and NH2-BDC linkers, has excellent chemical stability and good adsorption capacity for CO2.49PSF with cost-effective and thermally stable features possesses great potential in fabrication of high-performance membranes.50,51 Figure 1 showed the synthesis procedures of the MOF NPs and MSs. The NH2-UiO-66 MSs were obtained by the simple one-pot synthesis via introducing PSF in precursor solutions, while the NH2-UiO-66 NPs were prepared without polymer additives. On account of solubility, the polymer was added with concentration of 6.0 wt/v% in precursor solutions, because the dissolutions of polymers and precursors influenced each other. As shown in SEM images (Figure 2a), the NH2-UiO-66 NPs without polymer additives possessed well-defined octahedral structure, with the size of 150˗200 nm. For the polymer-embedded pMOF MSs, the obvious different spherical structure appeared (Figure 2b,c). The size of pMOF microspheres was about 1˗2 μm, which was much larger than that of the MOF NPs. During the crystallization process of MOFs, the added polymer chains in precursor solution prompted the assemblage of the nuclei and the intergrowth of crystals, thereby leading to the formation of large polycrystalline spherical structures.52,53 Moreover, it was noticeable that the prepared pMOF MSs possessed relatively rough surface, which might be beneficial to improve the interfacial interaction between MSs and matrixes due to the enlarged contacting area.