2.5.4. Permselectivity measurements
The details of the ED performance of the membranes are described in our
previous paper. 22 Briefly, ED experiments were
performed at a current density of 10 mA cm−2. The
selective layer faced the diluted chamber. First, 100 mL of 0.3 M
Na2SO4 solution, 100 mL of a solution
that contained 0.1 M NaCl (or 0.1 M LiCl) and 0.1 M
MgCl2, and 200 mL of a 0.01 M KCl solution were added to
the electrode compartments, diluted chamber, and concentrated chamber,
respectively. All solutions were circulated at flow rates of 86 mL
min−1 using peristaltic pumps. The ED tests were
performed for 1 h. Samples were collected from the concentrated chamber
for further analysis, and their ion concentrations were measured using
ICP-OES.
Ion permeation through membranes (\(J_{N^{n+}}\), mol
cm−2 s−1), which measures the
changes in the concentration of ions in the concentrated chamber, was
calculated as follows:
\(J_{N^{n+}}=\frac{\left(C_{t}\ -\ C_{0}\right)\ \bullet\ V}{A_{m}\ \bullet\ t}\),
(1)
where \(C_{0}\ \)and \(C_{t}\) are the molar concentrations (M) of the
ions (Nn+) in the concentrated chamber at the
beginning (t = 0 min) and end (t = 60 min), respectively, of the ED
test, \(V\) is the volume of solution in the concentrated chamber (200
mL), and \(A_{m}\) is the effective surface area of the membrane (7.07
cm2).
The permselectivity was calculated using the following equation:
\(P_{M^{+}{/D}^{2+}}=\frac{J_{M^{+}\ }\bullet\ C_{D^{2+}}}{J_{D^{2+\ }}{\bullet\ C}_{M^{+}}}\),
(2)
where \(J_{M^{+}}\) and \(J_{D^{2+}}\) (mol cm−2s−1) are the permeations of monovalent and divalent
cations, respectively, through the membrane after 60 min of testing and\(C_{M^{+}}\) and \(C_{D^{2+}}\ \)(M) are the average concentrations of
monovalent (Li+ or Na+) and divalent
(Mg2+) cations in the diluted chamber, respectively.
In this study, three sets of membranes were tested, and their separation
performances were calculated as arithmetic averages. The testing device
was thoroughly washed with DI water for 30 min after each test.
3. Results and discussion
3.1. Characterization of UiO-66(Zr)-NH2 andUiO-66(Zr/Ti)-NH2
The water stability of UiO-66(Zr)-NH2 was tested by
soaking its powder in water, and the XRD results confirmed the excellent
stability of UiO-66(Zr)-NH2 (Supporting Information,
Figure S1). UiO-66(Zr/Ti)-NH2 nanoparticles were
produced via the post-synthetic exchange of
Zr4+ ions with Ti3+ ions using a
TiCl3 solution. The SEM micrographs of
UiO-66(Zr)-NH2 and UiO-66(Zr/Ti)-NH2(Figures 1a and b, respectively) revealed that the size of their
particles was approximately 60 nm. However, the
post-synthetic ion exchange
process rendered the surface of the UiO-66(Zr/Ti)-NH2particles relatively rough. The XRD
patterns of UiO-66(Zr)-NH2 and
UiO-66(Zr/Ti)-NH2 (Figure 1c) presented sharp and
intense diffraction peaks without visible peak shifting, which confirmed
the highly crystalline structure of the UiO-66(Zr)-NH2and UiO-66(Zr/Ti)-NH2 nanoparticles. However, the
intensities of the XRD peaks of UiO-66(Zr/Ti)-NH2 were
lower than those of UiO-66(Zr)-NH2, which indicated the
formation of defects during the post-synthetic
process.30 FTIR spectra were used to analyze the
structure of the
UiO-66(Zr)-NH2 and
UiO-66(Zr/Ti)-NH2nanoparticles. The negligible shifts of the characteristic FTIR peaks
indicated that the post-synthetic process did not change the chemical
structure of UiO-66(Zr)-NH2 (Supporting Information,
Figure S2). The N2 adsorption–desorption isotherms
(Figure 1d) indicated that the specific area of the nanoparticles
increased from 764 m2 g−1 for
UiO-66(Zr)-NH2 to 1168 m2g−1 for UiO-66(Zr/Ti)-NH2, which
further confirmed the changes in the properties of the MOF nanoparticles
after the post-synthetic process. The pore size distributions of the
UiO-66(Zr)-NH2 and UiO-66(Zr/Ti)-NH2nanoparticles were calculated from the N2adsorption–desorption isotherms using the Saito–Foley (SF) and DFT
models, and the results revealed the presence of inherent pores with
sizes in the rage of 7-11 Å in the
MOF nanoparticles (Supporting Information, Figures S3 and S4,
respectively). This pore size distribution enabled the facile permeation
and sieving of the Na+, Li+, and
Mg2+ ions, which presented hydrated diameters of 7.2,
7.6, and 8.6 Å, respectively.29 The analysis of the
SRPES profiles of the samples provided critical details on the
post-synthetic process of the UiO-66(Zr)-NH2nanoparticles, such as the suppression of the Zr peak and emergence of
the Ti peak (Figure 1e), which implied the successful replacement of
Zr4+ ions with Ti3+ ions in
UiO-66(Zr/Ti)-NH2.
To analyze the XPS profiles of the
nanoparticles, we used Ti4+ ions to exchange a
fraction of the Zr4+ ions in
UiO-66(Zr)-NH2 using the same post-synthesis process
(Supporting Information, Figure S5). The 458.5 and 464.3 eV peaks in the
Ti 2p XPS profile of
UiO-66(Zr/Ti)-NH2-Ti4+ were ascribed
to Ti4+ 2p3/2 and
Ti4+ 2p3/2, respectively, (Supporting
Information, Figure S5b). The SRPES profiles of
UiO-66(Zr/Ti)-NH2 in the Ti 2p region shows two new
peaks at 457.8 eV and 463.6 eV, corresponding to Ti3+2p3/2 and Ti3+ 2p3/2,
respectively, revealing the coexistence of the Ti3+and
Ti4+ions in the structure of UiO-66(Zr/Ti)-NH2, and that was
ascribed to the oxidation of TiCl3 to
TiCl4 in air (Figure 1f). These results indicated the
successful replacement of Zr4+ ions with
Ti3+ ions in UiO-66(Zr/Ti)-NH2.
Furthermore, the Ti3+/Ti4+ molar
ratio of UiO-66(Zr/Ti)-NH2 was determined to be
approximately 6:5. In addition, the post-synthetic process was monitored
using ICP-OES,31 and the results indicated that the
Ti/Zr molar ratio of UiO-66(Zr/Ti)-NH2 was 9:4.
Moreover, the zeta potentials of the UiO-66(Zr)-NH2 and
UiO-66(Zr/Ti)-NH2 nanoparticles were tested (Supporting
Information, Table S1). The decrease in zeta potential from 42.5 for
UiO-66(Zr)-NH2 to 31.5 for
UiO-66(Zr/Ti)-NH2 confirmed the exchange of
Zr4+ ions with Ti3+ ions and
successful conversion of UiO-66(Zr)-NH2 into
UiO-66(Zr/Ti)-NH2.