2.5 Nanofiltration performance of hierarchical MOF lamellar
membranes
Nanofiltration performance was evaluated after immersing hierarchical
MOF lamellar membrane in corresponding solution for 3 h to reach a fully
equilibrium state. Based on the home-made device, solvent permeance and
dye rejection were tested. Dyes dissolved in methanol (10 mg
L-1) were used for rejection measurement and analyzed
by UV-vis spectrophotometer. The solvent permeance P (L
m-2 h-1 bar-1) was
calculated by using the following equation:
\(P=\frac{V}{P\times A\times t}\) (1)
where V , ΔP , A and t , represent the permeate
volume (L), operating pressure (bar), effective membrane area
(m2), and testing time (h), respectively. The
following expression was obtained for the calculation of dye rejection
(R , %):
\(R=\left(1-\frac{C_{p}}{C_{f}}\right)\times 100\) (2)
where C f and C p are the
concentration of feed solution and permeate solution, respectively. The
obtained data were average of three
parallel tests.
3 RESULTS AND DISCUSSION
3.1 Preparation and characterization of hierarchical MOF lamellar
membranes
MOF nanosheets were solvothermally synthesized from a mixed solution of
metal ions (Ni2+, Co2+) and organic
linkers under continuous ultrasonication.[37]Here, three kinds of organic linkers with distinct functional groups
(–CH3,
–NH2, and
no extra groups) were selected
(Figure S1), which are designed to construct MOF nanosheets bearing
intrinsic pores with close size but different groups. These nanosheets
were denoted as MOF-CH3, MOF-NH2, and
MOF-BDC, corresponding to the constructed organic ligands, respectively.
AFM and SEM images in Figure 1a and Figures S2, S3 show that the
synthesized MOF samples display typical sheet morphology, with uniform
transverse size of ~ 1.5 μm and thickness of
~ 3.7 nm. And the XRD
spectra (Figure 1b) show that three
MOF nanosheets possess the same lattice structure, where the typical
(200) peak at 2θ = 8.7°
reflects the intrinsic pores.[37] This agrees with
the HRTEM images in Figure S4, which visually show the lattice spacing
of ~ 1.05 nm for these
nanosheets.[38,39] Furthermore, N2sorption/desorption spectra, Figures 1c and S5, reveal that the average
pore sizes for MOF-CH3, MOF-NH2 and
MOF-BDC nanosheets are 1.12 nm, 1.05 nm, and 1.29 nm, respectively. It
should be noted that the pore size of MOF-CH3 and
MOF-NH2 nanosheets is smaller than that of MOF-BDC
nanosheet, which is originated from the steric effect of
–CH3 and –NH2 groups on the
pores.[40,41]
FTIR result (Figure S6) shows that, compared with MOF-BDC nanosheet, the
typical C=O peak gives a blue shift for
MOF-CH3 and
MOF-NH2 nanosheets at 1360 ~ 1380
cm-1, owing to the presence of
–CH3/–NH2 groups adjacent to C=O
group. Meanwhile, the grafting of –NH2 groups bring a
new strong peak at 1253 cm-1, which assigned to
Ar–N on FTIR and a typical
characteristic peak of N element at 400 eV in XPS spectra of
MOF-NH2 (Figure S7).[42] While the
decorating of –CH3groups is supported by the higher C element content for
MOF-CH3 relative to MOF-BDC. Specifically, the content
of N element on
MOF-NH2 nanosheet is
tested to be 2.95%, matching well with the theoretical value calculated
from lattice structure. This also implies that there is one
–NH2 group in a unit cell,[41]and the condition is identical for MOF-CH3 nanosheet
(Figures 1g and i), this implies the topological structure of MOF is
explicitly constructed during synthetic process. Note that there is no
extra group in the pore of MOF-BDC nanosheet, which is designed as a
comparison.