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).