Characterizations of MOF NPs and pMOF MSs
The crystalline structures of the prepared MOF particles were studied by XRD. Both the characteristic peaks of the MOF NPs and pMOF MSs were consistent with simulated NH2-UiO-66 (Figure 2d), proving the successful formation of NH2-UiO-66 crystals after solvothermal reaction.54 The incorporation of polymer did not disturb the arrangement of frameworks. FTIR spectra of the MOF NPs and pMOF MSs displayed the strong peaks of O-Zr at 660 cm-1, -NH2 at 1258, 1652, 3335, and 3460 cm-1, and O=C=O at 1385 and 1566 cm-1, illustrating the homogeneous chemical structures of two MOFs (Figure S1).54-56 There was no obvious PSF characteristic peak in the FTIR spectrum of the pMOF MSs owing to the relatively small amount of embedded polymers. The peak about O=C=O bond of the pMOF MSs at approximately 1385 cm-1 showed red-shift (Figure 2e). This was ascribed to the dipole-dipole interaction between polar groups of MOFs (O=C=O) and PSF (-SO2-), implying the existence of polymer chains in frameworks. To further confirm the polymer embedding, XPS was employed to study the chemical bonding states of the MOF NPs and pMOF MSs (Figure 2f and Figure S2). High resolution XPS spectra of both MOF NPs and pMOF MSs displayed Zr 3d peaks, while the Zr atomic content decreased from 5.4% (MOF NPs) to 4.7% (pMOF MSs ) as no Zr element in PSF. The new S 2p peak of the pMOF MSs proved the successful PSF embedding as well. Based on 0.5% sulfur atomic content and the molecular formulas of NH2-UiO-66 cells (Zr24O120C192N24) and PSF repeat units (C27O4S), the PSF content of the pMOF surface could be roughly calculated at 2.0 units per cell.
The N2 adsorption-desorption isotherms of the MOF NPs and pMOF MSs were measured to investigate the influence of polymer embedding on the porosities of MOFs. Both MOF NPs and pMOF MSs exhibited type-I adsorption behavior (Figure 3a,b), proving their microporous features. The BET surface area of the MOF NPs was calculated to be 725 m2 g-1, which agreed with that of the NH2-UiO-66 particles reported in previous studies.57 For the pMOF MSs, the measured BET surface area (799  m2g-1) was slight larger than that of the MOF NPs (Table S1). The micropore size distributions of MOF NPs and pMOF MSs were calculated by Non-Local Density Functional Theory (NLDFT) and displayed in Figure 3a,b with two main peaks at 1.1 and 1.3 nm, which were in accordance with those of the typical NH2-UiO-66 particles prepared by solvothermal synthesis.58Compared with the MOF NPs, the larger and smaller pores of the pMOF MSs showed lower and higher peak intensities, respectively, revealing that the PSF chains facilitated the regular arrangements of crystals. The CO2, CH4, and N2 gas adsorption properties of the MOF NPs and pMOF MSs were measured at 25 °C (Figure 3c,d). The adsorption capacities of two MOF materials were ordered by the polarizabilities of CO2(26.3×10-25 cm-3), CH4 (26.0×10-25cm-3), and N2(17.6×10-25cm-3). The pMOF MSs showed slightly higher CO2 uptake of 36.7  mL g-1 in contrast with the MOF NPs (32.0  mL g-1) due to the larger BET surface area. For CH4 or N2, the MOF NPs and pMOF MSs exhibited similar adsorption capacities. The adsorption selectivity of the prepared MOF NPs and pMOF MSs was calculated by Henry’s law (Figure S3). After polymer embedding, the CO2/CH4 and CO2/N2 selectivities increased from 4.0 and 13.5 (MOF NPs) to 4.4 and 17.9 (pMOF MSs).