Research Article
www.acsami.org
Cite This: ACS Appl. Mater. Interfaces 2017, 9, 37848-37855
Improving Water-Treatment Performance of Zirconium MetalOrganic Framework Membranes by Postsynthetic Defect Healing
Xuerui Wang,†,§ Linzhi Zhai,† Yuxiang Wang,† Ruitong Li,† Xuehong Gu,‡ Yi Di Yuan,† Yuhong Qian,†
Zhigang Hu,†,∥ and Dan Zhao*,†
†
Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore
College of Chemistry and Chemical Engineering, Nanjing Tech University, No. 5 Xinmofan Road, Nanjing 210009, P. R. China
‡
Downloaded via BEIJING NORMAL UNIV on March 7, 2023 at 02:54:03 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information
*
ABSTRACT: Microporous metal-organic frameworks (MOFs) as
building materials for molecular sieving membranes offer unique
opportunities to tuning the pore size and chemical property. The
recently reported polycrystalline Zr-MOF membranes have greatly
expanded their applications from gas separation to water treatment.
However, Zr-MOFs are notoriously known for their intrinsic defects
caused by ligand/cluster missing, which may greatly affect the
molecular sieving property of Zr-MOF membranes. Herein, we
present the mitigation of ligand-missing defects in polycrystalline
UiO-66(Zr)-(OH)2 membranes by postsynthetic defect healing
(PSDH), which can help in increasing the membranes’ Na+ rejection
rate by 74.9%. Intriguingly, the membranes also exhibit excellent
hydrothermal stability in aqueous solutions (>600 h). Our study
proves the feasibility of PSDH in improving the performance of
polycrystalline Zr-MOF membranes for water-treatment applications.
KEYWORDS: metal-organic frameworks, polycrystalline membranes, hollow fibers, UiO-66, postsynthetic defect healing,
water treatment
■
INTRODUCTION
The hydrothermal stability of MOFs is especially dominated
by the strength of metal−ligand bonds.16,17 Thus, researchers
are looking for more robust coordination bonds such as Hf−O
(802 kJ mol−1) or Zr−O (776 kJ mol−1) to construct MOFbased membranes.18 Recently, Li and co-workers pioneered the
synthesis of polycrystalline UiO-66(Zr) membranes by an in
situ solvothermal approach and explored their applications in
seawater desalination and pervaporation dehydration.19,20
Subsequently, hydrophobic UiO-66(Zr)-(CH3)2 membranes
were developed for CO2/N2 separation.21 The aperture size of
polycrystalline UiO-67(Zr) membranes was tailored by loading
azobenzene molecules to afford a H2/CO2 separation factor of
14.7 by Knebel et al.22 Just recently, the hydrothermal stability
of UiO-66(Zr)-NH2 membranes was experimentally demonstrated with seawater desalination by pervaporation at 75 °C.23
Despite their superior water stability, Zr-MOFs are
increasingly found to have intrinsic defects caused by ligand
missing, leading to dramatically enhanced porosity and aperture
size that are unfavorable for molecular sieving separation.24−26
Zhou et al., on the basis of neutron powder diffraction study,
confirmed that, on average, 1 out of 12 ligands is missing from a
Metal-organic frameworks (MOFs), a type of porous and
crystalline materials, are constructed with metal ions/clusters
and organic ligands via coordination bonds.1,2 MOFs feature
well-defined and highly tunable porous structures because of
the vast types of secondary building unit, ligand topology,
connectivity, and chemical functionality.3 Therefore, MOFs are
promising building materials for molecular sieving membranes
that can separate mixtures based on the molecular size and
shape of individual components.4,5 Over the past decade,
intensive efforts have been made to fabricate MOF-based
polycrystalline or composite membranes for gas separation.6−9
However, the practical application of MOF-based membranes
for liquid separation, especially those involving water, is
severely suppressed by the poor hydrothermal stability of
MOFs.10 For example, polycrystalline MOF-5 membranes are
stable in organic solvents,11 but MOF-5 crystals would degrade
upon exposure to moist ambient air.12 Polycrystalline ZIF-8
membranes have been demonstrated to be competent to reject
ions in seawater through pervaporation.13 However, a recent
study unveils the dynamic degradation of ZIF-8 in aqueous
solutions through continuous Zn2+ leaching, indicating the
challenge of using ZIF-8 membranes for separations involving
water.14,15
© 2017 American Chemical Society
Received: August 24, 2017
Accepted: October 10, 2017
Published: October 10, 2017
37848
DOI: 10.1021/acsami.7b12750
ACS Appl. Mater. Interfaces 2017, 9, 37848−37855
Research Article
ACS Applied Materials & Interfaces
UiO-66(Zr) framework.24 The defective structure was subsequently identified in molecular-level precision by Yaghi et
al.25 The 8% coordination vacancies generated by the missing
bridging ligands from Zr6O4(OH)4 clusters are mainly chargecompensated by water molecules, evidenced by X-ray
diffraction of the in situ activated single crystal.25 A similar
defective structure has also been discovered in the isoreticular
counterpart UiO-67(Zr) by Farha et al.26 Such intrinsic defects
in Zr-MOFs will, unfortunately, be inherited by the polycrystalline Zr-MOF membranes. For example, the rejection rate of
monovalent ions (e.g., Na+ with a hydrated diameter of 7.16 Å)
in the UiO-66(Zr) membrane is relatively lower than the
molecular sieving selectivity determined on the basis of the
theoretical 6.0 Å aperture of the UiO-66(Zr) framework,
indicating a defective structure of the membrane.19,27,28 In
addition, the CO2/N2 selectivity is merely 2.2 for the
polycrystalline UiO-66(Zr)-(CH3)2 membrane, which is much
lower than the expected result assuming a perfect crystalline
membrane layer.21
Theoretically, the nonbridging groups, such as water
molecules in UiO-66(Zr),25 can be fully displaced by fitting
dicarboxylates, which can seal the framework defects by
bridging two adjacent clusters together.29−31 Denny Jr. and
Cohen first demonstrated the feasibility of in situ ligand
exchange for MOFs incorporated in mixed-matrix membranes.30 Recently, Yaghi et al. achieved mechanically robust
MOF-520-BPDC by precisely replacing the formate ligand with
4,4′-biphenyldicarboxylate (BPDC) as a “girder” in mechanically unstable MOF-520.31 Herein, we report the first study of
mitigating the intrinsic defects of polycrystalline UiO-66(Zr)(OH)2 membranes by postsynthetic defect healing to afford
highly selective membranes for water treatment.
■
methyl blue were tested using deionized (DI) water, 2000 ppm
aqueous salt solutions (i.e., FeCl3, CrCl3, ZnCl2, MgCl2, NaCl, and
H3BO3), and 100 ppm aqueous methyl blue solution. To attain stable
separation performance, each test was initially stabilized for 12 h to
eliminate the adsorption effects and then the feed and permeate
samples were collected every 12 h for three times. Meanwhile, the feed
solution was renewed every 12 h by the needle valve connected with
the membrane module. Water flux (F, kg m−2 h−1), water permeance
(P, kg m−2 h−1 bar−1), and rejection (R, %) were calculated as follows
w
F=
(1)
AΔt
P=
R=
F
pf − pp − Δπ
Cf − C p
Cf
× 100%
(2)
(3)
where w is the weight of water collected from the permeate side, kg; A
is the effective membrane layer, m2; Δt is the collecting time, h; Pf and
Pp are the absolute pressures in the feed and the permeate, bar; Δπ is
the difference of osmosis pressures between the feed and permeate
solutions, bar; Cf and Cp are the ion/methyl blue concentrations in the
feed and the permeate, respectively, which are determined by
inductively coupled plasma optical emission spectrometry or UV−vis
spectrometry. The water flux and rejection were obtained from the
averaged value of three data points. Regarding the acid resistance test,
the polycrystalline UiO-66(Zr)-(OH)2 membrane was soaked in
aqueous methyl blue solution (50 mL) under acidic conditions (pH 1)
for 7 days and washed with DI water for further characterization.
Postsynthetic Defect Healing. The as-synthesized hollow fiber
polycrystalline UiO-66(Zr)-(OH)2 membrane was soaked in a mother
solution with a molar composition of 2 DOBDC/500 DMF.
Postsynthetic defect healing was performed at 120 °C for 1 day in a
Teflon-lined stainless steel autoclave. The UiO-66(Zr)-(OH)2 crystal
particles were also solvothermally treated under the same conditions to
prepare the sample for comparison. All of the membranes and crystal
particles were completely activated as mentioned above.
Characterization. The morphology of the Al2O3 hollow fiber,
seeded Al2O3 hollow fiber, and the as-synthesized polycrystalline UiO66(Zr)-(OH)2 membrane was observed by field emission scanning
electron microscopy (FESEM, JSM-7610F, JEOL). The crystal phases
were measured by a powder X-ray diffraction (PXRD, MiniFlex 600,
Rigaku) system equipped with a Cu sealed tube (λ = 0.154178 nm)
with a scan rate of 0.04 deg s−1 in the 2θ range of 5−50°. The sample
(50 mg) with the strongest peak intensity at 7.4° was denoted 100%
crystallinity, and the relative crystallinity was obtained by calculating
the ratio of the peak intensity according to the previous report.33 X-ray
photoelectron spectroscopy (XPS) measurements were performed
using a Kratos AXIS Ultra XPS system (Kratos Analytical Ltd) with
monochromated Al Kα radiation (hλ = 1486.6 eV). Fourier transform
infrared spectroscopy (FTIR) spectra were obtained with a Nicolet
6700 FTIR spectrometer. Thermogravimetric analysis (TGA) was
performed using Shimadzu DTG-60AH. UiO-66-(OH)2 samples (ca. 8
mg) were added to platinum crucibles before the measurement. Each
TGA run was made in two different heating stages under a
simultaneous feed of air (20 mL min−1). In the first step, the samples
were heated in the temperature range of 20−100 °C and kept for 30
min; in the second step, the samples continued to be heated to 950 °C
at a rate of 5 °C min−1.
EXPERIMENTAL SECTION
Preparation of Polycrystalline UiO-66(Zr)-(OH)2 Membranes.
The polycrystalline UiO-66(Zr)-(OH)2 membranes were fabricated on
the outer surface of porous Al2O3 hollow fibers (O.D.: 1.96 mm,
length: 50 mm, porosity: ca. 40%, average pore size: 1.6 μm) by the
secondary growth method. The hollow fibers were fabricated
according to the previous report.32 The supports were first seeded
with UiO-66(Zr)-(OH)2 crystals by in situ solvothermal synthesis in
the hydrous mother solution with a molar composition of 1 ZrCl4/1
DOBDC/1 H2O/500 dimethylformamide (DMF)/100 formic acid.
The crystallization was conducted at 120 °C for 1 day in a Teflon-lined
stainless steel autoclave. Prior to in situ seeding, both ends of the
Al2O3 hollow fibers were capped with Teflon tape to ensure no seed
formation on the inner surface. After cooling to room temperature, the
seeded Al2O3 hollow fibers were intensively washed with DMF and
ethanol and then dried at room temperature. The crystals within the
membrane layer were further fused together to form a continuous and
well-intergrown polycrystalline UiO-66(Zr)-(OH)2 membrane by
secondary growth in another anhydrous mother solution with a
molar composition of 1 ZrCl4/1 DOBDC/500 DMF/100 formic acid.
The synthesis was conducted at 120 °C for 3 days. The membranes
were intensively activated by soaking in fresh DMF for 12 h and
repeating five times. After that, the residual ligands and DMF were
completely exchanged with hot ethanol using Soxhlet extraction.
Before membrane performance tests, the membrane was dried at room
temperature overnight.
Water-Treatment Performance Test. The separation performance of UiO-66(Zr)-(OH)2 membranes was tested with aqueous
solutions at a pressure difference of 3 bar using a home-built
membrane filtration apparatus at room temperature (Figure S1). One
end of the membrane was sealed with silicone, and the other open end
was assembled in the module. The effective length of the membrane is
approximately 40 mm. The water flux and rejection of metal ions and
■
RESULTS AND DISCUSSION
Preparation and Characterization of UiO-66(Zr)-(OH)2
Membranes. UiO-66(Zr)-(OH)2 is isoreticular to UiO66(Zr) with ligand benzenedicarboxylate (BDC) being replaced
by 2,5-dihydroxy benzenedicarboxylate (DOBDC). The extra
hydroxy groups help to reduce the aperture size to around 4.0
Å in UiO-66(Zr)-(OH)2,34 which should be more beneficial
37849
DOI: 10.1021/acsami.7b12750
ACS Appl. Mater. Interfaces 2017, 9, 37848−37855
Research Article
ACS Applied Materials & Interfaces
Figure 1. Scanning electron microscopy (SEM) images of the porous alumina hollow fiber (a), seed layer (b, c), and membrane layer (d, e). The red
and green lines in (e) indicate the intensity of Zr and Al elements by energy-dispersive spectrometry line analyses, respectively. (f) Energy-dispersive
X-ray (EDX) elemental mapping of the cross-section shown in (e).
substrates with UiO-66(Zr)-(OH)2 crystal seeds using the
hydrous solution (kinetically favorable condition) and then
grow crack-free membranes by heating the seeded substrates in
the anhydrous solution (thermodynamically favorable condition, illustration of the process shown in Figure S6).
Monolayered irregular crystal seeds, determined by almost
identical crystal size (0.75 μm) and layer thickness (0.88 μm),
were obtained on the substrates after 1 day reaction in the
hydrous solution (Figures 1b,c, and S4k). These crystals were
proven to be UiO-66(Zr)-(OH)2 by powder X-ray diffraction
(PXRD, Figure S7c). The in situ synthesized seeds effectively
prevent the peeling of seed layer compared to that obtained by
external seeding techniques, such as dip-coating, rubbing, and
vacuum coating.38 The same conclusion was obtained by Jin et
al. as well, who pioneered a reactive seeding method wherein
the substrate acts as the precursor of metal ions for nucleation
and crystal growth.39 Intriguingly, well-intergrown and crackfree polycrystalline UiO-66(Zr)-(OH)2 membranes can be
obtained after 3 day secondary growth in the anhydrous
solution (Figure 1d,e, and PXRD shown in Figure S7d). The
crystal grain size increases to 2.15 ± 0.29 μm through the
epitaxial growth of the previously nucleated irregular seeds, and
the membrane thickness is 3.5 μm. Homogeneous distribution
of the Zr element is clearly demonstrated by EDX line and
mapping analyses (Figures 1e,f and S8). The relative round
crystal shape on the membrane surface, which is different from
the typically well-defined octahedral shape,19 further proves the
slow nucleation and growth in the anhydrous solution during
the second step to prevent the formation of cracks. The slow
growth could minimize generation of membrane defects inside
the membrane layer.21
Separation Performance. The integrity of the membrane
was initially evaluated by pure water permeation under a
pressure difference of 3 bar. The fresh UiO-66(Zr)-(OH)2
membranes prepared by in situ growth and secondary growth
exhibited water fluxes of 2.40 and 6.72 kg m−2 h−1, respectively,
which are much lower than those of the pristine and seeded
alumina hollow fiber substrates (Table 1). These results
indicate the formation of a continuous membrane layer on
toward the rejection of monovalent ions during seawater
desalination. The membranes were fabricated on porous
ceramic hollow fiber substrates (SEM shown in Figure 1a and
photo shown in Figure S2), which could endow high module
packing density and reduced capital investment for practical
applications.35 Formic acid was used as the modulator to
promote the nucleation (Figure S3a,b). Initially, we attempted
to prepare the membrane by in situ growth from the anhydrous
mother solution with a molar composition of 1 Zr/1 DOBDC/
500 DMF/100 formic acid at 120 °C for 3 days. Unfortunately,
we could not observe any crystals grown on the hollow fiber
surface (Figure S4b,g). However, the UiO-66(Zr)-(OH)2
powder collected from the bottom of the autoclave exhibits a
relative crystallinity of 26% (Figure S3c,d) and reasonable
Brunauer−Emmett−Teller surface area (564 m2 g−1, Figure
S5), suggesting that the UiO-66(Zr)-(OH)2 crystals prefer
homogeneous nucleation from solution rather than crystal
growth on substrates, possibly due to the limited heterogeneous
nucleation sites on substrates. Well-intergrown membranes
could be obtained after 9 day reaction in the anhydrous
solution (Figure S4g−j). Interestingly, we found that adding a
trace amount of water in the reaction media could dramatically
shorten the induction period to 1 day (Figure S4k). This is
probably due to the easier formation of Zr4+ oxo/hydroxo
clusters favored by water.36 Another possible reason is the
facilitated attachment of nucleus onto the substrate by water, as
previously demonstrated in a zeolite membrane.37 However,
visible cracks can be easily found from the membrane surface
(Figure S4l,m), which can be attributed to the shrinkage of
Zr6O4(OH)4 clusters once the charge-compensating water
molecules are removed by high-energy electron beam under the
high vacuum condition (1.91 × 10−4 Pa) during SEM
observation. Previously, Yaghi et al. announced a 0.55%
shrinkage of the unit cell of UiO-66(Zr) under vacuum.25 A
video visually demonstrates the process of crack formation in
the membranes under SEM observation (Video S1).
It has been well-documented that the nucleation and growth
of polycrystalline membranes can be well-balanced by the
secondary growth method.5,7 Here, we propose to first sow the
37850
DOI: 10.1021/acsami.7b12750
ACS Appl. Mater. Interfaces 2017, 9, 37848−37855
Research Article
ACS Applied Materials & Interfaces
the porous alumina hollow fibers, in line with the SEM image
(Figure 1e).
ions might metallate with the hydroxyl groups of DOBDC
during the separation process, leading to the reduced water flux
compared to the initial pure water flux. Previously, Cohen and
co-workers reported that 2,3-dihydroxyterephthalate-exchanged
UiO-66 films could metallate with Fe3+ by soaking in FeCl3
solution at room temperature for 24 h.45 To test this
hypothesis, UiO-66(Zr)-(OH)2 crystals were soaked in an
aqueous FeCl3 solution with the conditions identical to those of
the membrane tests. Interestingly, Fe could be clearly detected
by XPS even after thorough washing, as reported by Cohen and
co-workers (Figures 3a and S9).46 More interestingly, a small
cavity of 4.65 Å was identified by N2 sorption tests in UiO66(Zr)-(OH)2 crystals treated with FeCl3 solution (highlight in
cyan, Figure 3b,c). However, the Fourier transform infrared
spectroscopy (FTIR) spectra were identical in the pristine and
FeCl3-treated MOFs (Figure 3d). Therefore, we believe that
the Fe3+ ions are just physically adsorbed or trapped within the
defects of polycrystalline UiO-66(Zr)-(OH)2 membranes,
resulting in the reduced water permeance and the slight
increase of rejection ratio (Part 2 in Figure 2).
The separation performance of the as-synthesized UiO66(Zr)-(OH)2 membrane is quite stable during the 600 h
operation as mentioned above, indicating great potential for
practical applications. The acid resistance of the membrane was
further evaluated by soaking the membrane in 100 ppm methyl
blue solution with a pH value of 1. After 7 day soaking, the
octahedral UiO-66(Zr)-(OH)2 crystals could still be clearly
identified from the membrane (Figure S10), indicating its
superior water stability even under acidic conditions.
Separation Mechanism. The membrane selectivity is
largely dependent on the diameter of probe ions or molecules
(Figure 4a). The rejection rate is 26% for Na+ (hydrated
diameter: 7.16 Å), 42.5% for Zn2+ (8.60 Å), 54.7% for Fe3+
(9.14 Å), and 98.7% for methyl blue (21 Å × 13.6 Å × 8.1 Å).
Meanwhile, the water flux is in the order of pure water (0.81 kg
m−2 h−1) > methyl blue solution (0.77 kg m−2 h−1) > Na+
solution (0.73 kg m−2 h−1) > Fe3+ solution (0.56 kg m−2 h−1) >
Zn2+ solution (0.45 kg m−2 h−1). Thus, we speculate that the
effective pore size of the membrane is between 9.1 and 12.6 Å,
which is almost identical to the defective aperture size of UiO66(Zr)-(OH)2 crystals caused by ligand missing. A wider
micropore of 11.79 Å could be clearly observed from the pore
size distribution data calculated from N2 sorption isotherms
(Figure 3c). The pores with sizes ranging from 10 to 11 Å were
also detected from defective UiO-66(Zr).47 Therefore, hydrated
Na+ ions (7.16 Å) can diffuse through the porous membrane
layer without much resistance in this case, leading to a poor
Na+ rejection (26%). The relatively low water flux for aqueous
solutions containing multivalent metal cations (highlighted in
Figure 4a) can be attributed to the delayed transport of water
molecules by the slow diffusion of hydrated Mg2+, Zn2+, or Fe3+
ions because of the single-file diffusion within the microporous
channels (Figure 4b).48 Notably, the water permeance for the
Na+ solution is 0.286 kg m−2 h−1 bar−1, which is 2 times higher
than that of the reported polycrystalline UiO-66(Zr)
membranes (0.14 kg m−2 h−1 bar−1).19 This enhancement
can be attributed to the highly hydrophilic channels caused by
the hydroxyl groups of DOBDC ligands. In addition, the
defective pores of UiO-66(Zr)-(OH)2 membranes with a wider
aperture size may be another reason.
Postsynthetic Defect Healing. Compared with attaining a
higher water permeance, a more important target for the watertreatment membrane is to attain improved selectivity for water
Table 1. Pure Water Flux of Substrates and Membranes
sample
synthesis condition
fluxa (kg m−2 h−1)
substrate
seeded substrate
membraneb
membranec
NA
120 °C for 1 day
120 °C for 3 days
120 °C for 9 days
9700 ± 150
376 ± 72
6.72
2.40
a
Pressure difference of 3 bar. bSecond growth in the anhydrous
solution. cIn situ growth in the anhydrous solution.
The separation performance of the membrane was further
evaluated by pressure-driven permeation of aqueous solutions
containing metal salts or methyl blue. To our surprise, the
water fluxes drastically decrease down to 1.05, 0.69, 0.72, and
1.06 kg m−2 h−1 for the aqueous salt solutions containing 2000
ppm FeCl3, CrCl3, ZnCl2, or NaCl, respectively (Part 1 in
Figure 2). A similar phenomenon was observed in the
Figure 2. Separation performance of the UiO-66(Zr)-(OH) 2
membrane for the removal of metal ions and methyl blue in aqueous
solutions for a continuous operation period of 600 h. Feed: pure water,
2000 ppm salt solutions (FeCl3, CrCl3, ZnCl2, MgCl2, NaCl, H3BO3),
or 100 ppm methyl blue solution; test condition: 3 bar at room
temperature. Insert: photo of feed solution (left) and the permeate
(right) for FeCl3 and methyl blue.
membrane synthesized by in situ solvothermal synthesis for 9
days (Table S2). Considering the difference of osmotic
pressure among pure water and the aqueous salt solutions,
we calculated the water permeance based on the normalized
driving force.40 The permeance of pure water is 2.24 kg m−2 h−1
bar−1, which is still much higher than that of salt solutions
(0.45, 0.28, 0.28, and 0.40 kg m−2 h−1 bar−1 for FeCl3, CrCl3,
ZnCl2, and NaCl solutions, respectively).
After 170 h of continuous operation, the membrane was
thoroughly washed with pure water to remove the adsorbed
metal ions. However, the water flux of washed membrane
stabilized at 1.26 kg m−2 h−1, which is only 18.75% of the
original value (Table 1 and Part 2 in Figure 2). Considering
that the Stokes diameter of hydrated Fe3+ can be as high as 8.12
Å,41 it would be difficult to permeate through the UiO-66(Zr)(OH)2 membrane layer, which theoretically should have an
aperture size of approximate 4.0 Å. Thus, we speculate that the
Fe3+ might be irreversibly trapped in the defects of UiO66(Zr)-(OH)2 crystals caused by ligand or cluster missing.24,42
This agrees well with the blocking effects of the MFI-type
zeolite membrane by Fe3+ cations, which causes a decrease in
water flux from 5.6 to 1.2 kg m−2 h−1 because of the rigid ion
blocking effect.43,44 However, it is worth noting that the metal
37851
DOI: 10.1021/acsami.7b12750
ACS Appl. Mater. Interfaces 2017, 9, 37848−37855
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) XPS Fe 2p spectra of FeCl3-treated UiO-66(Zr)-(OH)2 crystals. (b, c) N2 sorption isotherms at 77 K and pore size distribution based
on DFT calculation. (d) FTIR spectra of pristine and FeCl3-treated UiO-66(Zr)-(OH)2. Fresh UiO-66(Zr)-(OH)2 crystals were treated in 0.2 wt %
FeCl3 solution under 3 bar for 24 h.
We reason that the poor monovalent salt rejection rate of our
membrane is due to the defects caused by ligand missing.
Thermogravimetric analyses (TGAs) were conducted on UiO66(Zr)-(OH)2 crystals to quantify these defects: each
Zr6O4(OH)4 cluster is coordinated with 4.68 DOBDC ligands
on average, equivalent to 21.7% coordinative defects (Figure
5a). This is almost 2 times higher than the defects reported in
UiO-66(Zr) (∼10%),24,25 possibly due to the different ligand
reactivity and synthetic condition.
The coordinative defects are normally occupied by
monocoordinated modulators,24 water molecules,25 hydroxide
ions,26 chloride ions,50 and so forth. We hypothesize that by
replacing two monocoordinated moieties with one bridging
dicarboxylate ligand, a process we called postsynthetic defect
healing (PSDH, Figure 5b), the defective large aperture size can
be reduced, affording better separation performance of the
resultant membranes. Briefly, the as-synthesized membrane was
solvothermally treated in DOBDC solution at 120 °C for 1 day.
As expected, the coordinative defects can be mitigated after
PSDH: each Zr6O4(OH)4 cluster is now coordinated with five
ligands on average (Figure 5a), meaning 24% of the
coordinative defects have been mitigated by recoordinating
with DOBDC ligands. Meanwhile, this causes a slight decrease
in the surface area (from 883.2 to 703.2 m2/g) and pore
volume (from 0.4102 to 0.3487 cm3/g, Table S3). The result is
consistent with the previous investigation on defective UiO-66NH2.51 The improved separation performance of membranes
after PSDH was confirmed by water-treatment tests using
aqueous solutions containing Na+ or methyl blue (Figure 5c).
The rejection rate of Na+ is now 45% (74.9% increase
Figure 4. Separation performance of the polycrystalline UiO-66(Zr)(OH)2 membrane for the removal of metal ions and methyl blue in
aqueous solutions (a) and illustration of single-file diffusion of water
molecules and hydrated Mn+ ions (b). Test conditions: 2000 ppm salt
solutions and 100 ppm methyl blue solution at 3 bar and room
temperature.
over solutes. For example, NaCl rejection could be as high as
>99.5% for polymeric seawater reverse osmosis membranes.49
37852
DOI: 10.1021/acsami.7b12750
ACS Appl. Mater. Interfaces 2017, 9, 37848−37855
Research Article
ACS Applied Materials & Interfaces
Figure 5. (a) TGA curves of UiO-66(Zr)-(OH)2 crystals before and after PSDH; the weight was normalized with respect to the ZrO2 residue left
after heating up to 650 °C in air. (b) Scheme of PSDH by relinking two adjacent Zr6O4(OH)4 clusters (blue polyhedron) by one DOBDC ligand.
The yellow area indicates the effective aperture size for molecular sieving separation; the cyan area indicates the location of the recoordinated
DOBDC within the framework. (c) Separation performance of the membrane before and after PSDH.
compared to that in pristine membranes), whereas that of
methyl blue reaches as high as 99.8%, which is even higher than
that of the current polymeric membranes.52 Meanwhile, there is
only a minor decrease of the water flux after PSDH (from 0.77
to 0.69 kg m−2 h−1, 10.4% decrease). The water flux of the UiO66-(OH)2 membrane after PSDH is still higher than that of the
polycrystalline zeolite membranes (e.g., SOD and MFI, Table
S4).
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: chezhao@nus.edu.sg.
■
ORCID
Xuerui Wang: 0000-0003-2220-7531
Dan Zhao: 0000-0002-4427-2150
CONCLUSIONS
In summary, we present an efficient approach to healing the
intrinsic defects in water-stable polycrystalline UiO-66(Zr)(OH)2 membranes by postsynthetic defect healing. The healed
membranes exhibit NaCl rejection of 45% with water
permeance of 0.285 kg m−2 h−1 bar−1 and methyl blue rejection
of 99.8% with water permeance of 0.23 kg m−2 h−1 bar−1. Our
approach is based on the dynamic feature of coordination
bonds in MOFs and is reminiscent of the solvent-assisted linker
exchange reactions in bulk MOF crystals.29 This approach
offers a novel platform for further optimization and
functionalization of polycrystalline MOF membranes from
molecular levels. In the future, intensive research work would
be highly desired to optimize the membrane structure as well as
to expand the applications in niche markets.
■
Process of crack formation in the membranes under SEM
observation (AVI)
Present Addresses
§
Catalysis Engineering, Chemical Engineering Department,
Delft University of Technology, Van der Maasweg 9, 2629 HZ
Delft, The Netherlands (X.W.).
∥
Department of Chemical Engineering and Biotechnology,
University of Cambridge, West Cambridge Site, Philippa
Fawcett Drive, Cambridge CB3 0AS, U.K. (Z.H.).
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
■
ASSOCIATED CONTENT
ACKNOWLEDGMENTS
This work is supported by the National University of Singapore
(CENGas R-261-508-001-646), Ministry of Education Singapore (MOE AcRF Tier 1 R-279-000-472-112), and
Agency for Science, Technology and Research (PSF R-279000-475-305, IRG R-279-000-510-305).
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.7b12750.
XPS, PXRD, gas sorption isotherms, and SEM image
(PDF)
37853
DOI: 10.1021/acsami.7b12750
ACS Appl. Mater. Interfaces 2017, 9, 37848−37855
Research Article
ACS Applied Materials & Interfaces
■
(22) Knebel, A.; Sundermann, L.; Mohmeyer, A.; Strauß, I.; Friebe,
S.; Behrens, P.; Caro, J. Azobenzene Guest Molecules as LightSwitchable CO2 Valves in an Ultrathin UiO-67 Membrane. Chem.
Mater. 2017, 29, 3111−3117.
(23) Wan, L.; Zhou, C.; Xu, K.; Feng, B.; Huang, A. Synthesis of
Highly Stable UiO-66-NH2 Membranes with High Ions Rejection for
Seawater Desalination. Microporous Mesoporous Mater. 2017, 252,
207−213.
(24) Wu, H.; Chua, Y. S.; Krungleviciute, V.; Tyagi, M.; Chen, P.;
Yildirim, T.; Zhou, W. Unusual and Highly Tunable Missing-Linker
Defects in Zirconium Metal−Organic Framework Uio-66 and Their
Important Effects on Gas Adsorption. J. Am. Chem. Soc. 2013, 135,
10525−10532.
(25) Trickett, C. A.; Gagnon, K. J.; Lee, S.; Gándara, F.; Bürgi, H. B.;
Yaghi, O. M. Definitive Molecular Level Characterization of Defects in
UiO-66 Crystals. Angew. Chem., Int. Ed. 2015, 54, 11162−11167.
(26) Katz, M. J.; Brown, Z. J.; Colón, Y. J.; Siu, P. W.; Scheidt, K. A.;
Snurr, R. Q.; Hupp, J. T.; Farha, O. K. A Facile Synthesis of UiO-66,
UiO-67 and Their Derivatives. Chem. Commun. 2013, 49, 9449−9451.
(27) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.;
Bordiga, S.; Lillerud, K. P. A New Zirconium Inorganic Building Brick
Forming Metal Organic Frameworks with Exceptional Stability. J. Am.
Chem. Soc. 2008, 130, 13850−13851.
(28) Wu, D.; Maurin, G.; Yang, Q.; Serre, C.; Jobic, H.; Zhong, C.
Computational Exploration of a Zr-Carboxylate Based Metal-Organic
Framework as a Membrane Material for CO2 Capture. J. Mater. Chem.
A 2014, 2, 1657−1661.
(29) Deria, P.; Mondloch, J. E.; Karagiaridi, O.; Bury, W.; Hupp, J.
T.; Farha, O. K. Beyond Post-Synthesis Modification: Evolution of
Metal-Organic Frameworks via Building Block Replacement. Chem.
Soc. Rev. 2014, 43, 5896−5912.
(30) Denny, M. S., Jr.; Cohen, S. M. In Situ Modification of Metal−
Organic Frameworks in Mixed−Matrix Membranes. Angew. Chem., Int.
Ed. 2015, 54, 9029−9032.
(31) Kapustin, E. A.; Lee, S.; Alshammari, A. S.; Yaghi, O. M.
Molecular Retrofitting Adapts a Metal−Organic Framework to
Extreme Pressure. ACS Cent. Sci. 2017, 3, 662−667.
(32) Shi, Z.; Zhang, Y.; Cai, C.; Zhang, C.; Gu, X. Preparation and
Characterization of α-Al2O3 Hollow Fiber Membranes with FourChannel Configuration. Ceram. Int. 2015, 41, 1333−1339.
(33) Wang, X.; Yang, Z.; Yu, C.; Yin, L.; Zhang, C.; Gu, X.
Preparation of T-Type Zeolite Membranes Using a Dip-Coating
Seeding Suspension Containing Colloidal SiO2. Microporous Mesoporous Mater. 2014, 197, 17−25.
(34) Hu, Z.; Wang, Y.; Farooq, S.; Zhao, D. A Highly Stable MetalOrganic Framework with Optimum Aperture Size for CO2 Capture.
AIChE J. 2017, 63, 4103−4114.
(35) Wang, X.; Jiang, J.; Liu, D.; Xue, Y.; Zhang, C.; Gu, X.
Evaluation of Hollow Fiber T-Type Zeolite Membrane Modules for
Ethanol Dehydration. Chin. J. Chem. Eng. 2017, 25, 581−586.
(36) Ragon, F.; Horcajada, P.; Chevreau, H.; Hwang, Y. K.; Lee, U.H.; Miller, S. R.; Devic, T.; Chang, J.-S.; Serre, C. In Situ EnergyDispersive X-Ray Diffraction for the Synthesis Optimization and Scaleup of the Porous Zirconium Terephthalate UiO-66. Inorg. Chem. 2014,
53, 2491−2500.
(37) Di, J.; Zhang, C.; Yan, W.; Wang, X.; Yu, J.; Xu, R. Direct In Situ
Crystallization of Highly Oriented Silicalite-1 Thin Films on a Surface
Sol−Gel Process Modified Substrate. Microporous Mesoporous Mater.
2011, 145, 104−107.
(38) Wang, X.; Chen, Y.; Zhang, C.; Gu, X.; Xu, N. Preparation and
Characterization of High-Flux T-Type Zeolite Membranes Supported
on YSZ Hollow Fibers. J. Membr. Sci. 2014, 455, 294−304.
(39) Hu, Y.; Dong, X.; Nan, J.; Jin, W.; Ren, X.; Xu, N.; Lee, Y. M.
Metal−Organic Framework Membranes Fabricated via Reactive
Seeding. Chem. Commun. 2011, 47, 737−739.
(40) Lia, L.; Dong, J.; Nenoff, T. M.; Lee, R. Reverse Osmosis of
Ionic Aqueous Solutions on a MFI Zeolite Membrane. Desalination
2004, 170, 309−316.
REFERENCES
(1) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The
Chemistry and Applications of Metal-Organic Frameworks. Science
2013, 341, No. 1230444.
(2) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Selective Gas Adsorption and
Separation in Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38,
1477−1504.
(3) Adil, K.; Belmabkhout, Y.; Pillai, R. S.; Cadiau, A.; Bhatt, P. M.;
Assen, A. H.; Maurin, G.; Eddaoudi, M. Gas/Vapour Separation Using
Ultra-Microporous Metal−Organic Frameworks: Insights into the
Structure/Separation Relationship. Chem. Soc. Rev. 2017, 46, 3402−
3430.
(4) Gascon, J.; Kapteijn, F. Metal-Organic Framework Membranes
High Potential, Bright Future? Angew. Chem., Int. Ed. 2010, 49, 1530−
1532.
(5) Rangnekar, N.; Mittal, N.; Elyassi, B.; Caro, J.; Tsapatsis, M.
Zeolite MembranesA Review and Comparison with MOFs. Chem.
Soc. Rev. 2015, 44, 7128−7154.
(6) Denny, M. S., Jr.; Moreton, J. C.; Benz, L.; Cohen, S. M. Metal−
Organic Frameworks for Membrane-Based Separations. Nat. Rev.
Mater. 2016, 1, No. 16078.
(7) Qiu, S.; Xue, M.; Zhu, G. Metal-Organic Framework Membranes:
From Synthesis to Separation Application. Chem. Soc. Rev. 2014, 43,
6116−6140.
(8) Dechnik, J.; Gascon, J.; Doonan, C. J.; Janiak, C.; Sumby, C. J.
Mixed-Matrix Membranes. Angew. Chem., Int. Ed. 2017, 56, 9292−
9310.
(9) Hu, Z.; Kang, Z.; Qian, Y.; Peng, Y.; Wang, X.; Chi, C.; Zhao, D.
Mixed Matrix Membranes Containing Uio-66(Hf)-(OH)2 Metal−
Organic Framework Nanoparticles for Efficient H2/CO2 Separation.
Ind. Eng. Chem. Res. 2016, 55, 7933−7940.
(10) Wang, C.; Liu, X.; Demir, N. K.; Chen, J. P.; Li, K. Applications
of Water Stable Metal−Organic Frameworks. Chem. Soc. Rev. 2016, 45,
5107−5134.
(11) Kasik, A.; Lin, Y. S. Organic Solvent Pervaporation Properties of
MOF-5 Membranes. Sep. Purif. Technol. 2014, 121, 38−45.
(12) Yang, J.; Grzech, A.; Mulder, F. M.; Dingemans, T. J. Methyl
Modified MOF-5: A Water Stable Hydrogen Storage Material. Chem.
Commun. 2011, 47, 5244−5246.
(13) Zhu, Y.; Gupta, K. M.; Liu, Q.; Jiang, J.; Caro, J.; Huang, A.
Synthesis and Seawater Desalination of Molecular Sieving Zeolitic
Imidazolate Framework Membranes. Desalination 2016, 385, 75−82.
(14) Liu, X.; Li, Y.; Ban, Y.; Peng, Y.; Jin, H.; Bux, H.; Xu, L.; Caro, J.;
Yang, W. Improvement of Hydrothermal Stability of Zeolitic
Imidazolate Frameworks. Chem. Commun. 2013, 49, 9140−9142.
(15) Zhang, H.; Liu, D.; Yao, Y.; Zhang, B.; Lin, Y. S. Stability of ZIF8 Membranes and Crystalline Powders in Water at Room Temperature. J. Membr. Sci. 2015, 485, 103−111.
(16) Li, N.; Xu, J.; Feng, R.; Hu, T.-L.; Bu, X.-H. Governing Metal−
Organic Frameworks Towards High Stability. Chem. Commun. 2016,
52, 8501−8513.
(17) Low, J. J.; Benin, A. I.; Jakubczak, P.; Abrahamian, J. F.; Faheem,
S. A.; Willis, R. R. Virtual High Throughput Screening Confirmed
Experimentally: Porous Coordination Polymer Hydration. J. Am.
Chem. Soc. 2009, 131, 15834−15842.
(18) Guan, K.; Zhao, D.; Zhang, M.; Shen, J.; Zhou, G.; Liu, G.; Jin,
W. 3D Nanoporous Crystals Enabled 2D Channels in Graphene
Membrane with Enhanced Water Purification Performance. J. Membr.
Sci. 2017, 542, 41−51.
(19) Liu, X.; Demir, N. K.; Wu, Z.; Li, K. Highly Water-Stable
Zirconium Metal−Organic Framework UiO-66 Membranes Supported
on Alumina Hollow Fibers for Desalination. J. Am. Chem. Soc. 2015,
137, 6999−7002.
(20) Liu, X.; Wang, C.; Wang, B.; Li, K. Novel Organic−Dehydration
Membranes Prepared from Zirconium Metal−Organic Frameworks.
Adv. Funct. Mater. 2017, 27, No. 1604311.
(21) Liu, J.; Canfield, N.; Liu, W. Preparation and Characterization of
a Hydrophobic Metal−Organic Framework Membrane Supported on
a Thin Porous Metal Sheet. Ind. Eng. Chem. Res. 2016, 55, 3823−3832.
37854
DOI: 10.1021/acsami.7b12750
ACS Appl. Mater. Interfaces 2017, 9, 37848−37855
Research Article
ACS Applied Materials & Interfaces
(41) Nightingale, E. R., Jr. Phenomenological Theory of Ion
Solvation. Effective Radii of Hydrated Ions. J. Phys. Chem. 1959, 63,
1381−1387.
(42) Cliffe, M. J.; Wan, W.; Zou, X.; Chater, P. A.; Kleppe, A. K.;
Tucker, M. G.; Wilhelm, H.; Funnell, N. P.; Coudert, F.-X.; Goodwin,
A. L. Correlated Defect Nanoregions in a Metal-Organic Framework.
Nat. Commun. 2014, 5, No. 4176.
(43) Zhu, B.; Hu, X.; Shin, J.-W.; Moon, I.-S.; Muraki, Y.; Morris, G.;
Gray, S.; Duke, M. A Method for Defect Repair of MFI-Type Zeolite
Membranes by Multivalent Ion Infiltration. Microporous Mesoporous
Mater. 2017, 237, 140−150.
(44) Zhu, B.; Hong, Z.; Milne, N.; Doherty, C. M.; Zou, L.; Lin, Y. S.;
Hill, A. J.; Gu, X.; Duke, M. Desalination of Seawater Ion Complexes
by MFI-Type Zeolite Membranes: Temperature and Long Term
Stability. J. Membr. Sci. 2014, 453, 126−135.
(45) Fei, H.; Pullen, S.; Wagner, A.; Ott, S.; Cohen, S. M.
Functionalization of Robust Zr(Iv)-Based Metal−Organic Framework
Films via a Postsynthetic Ligand Exchange. Chem. Commun. 2015, 51,
66−69.
(46) Kim, M.; Cahill, J. F.; Fei, H.; Prather, K. A.; Cohen, S. M.
Postsynthetic Ligand and Cation Exchange in Robust Metal−Organic
Frameworks. J. Am. Chem. Soc. 2012, 134, 18082−18088.
(47) Vermoortele, F.; Bueken, B.; Le Bars, Gl; Van de Voorde, B.;
Vandichel, M.; Houthoofd, K.; Vimont, A.; Daturi, M.; Waroquier, M.;
Van Speybroeck, V.; Kirschhock, C.; De Vos, D. E. Synthesis
Modulation as a Tool to Increase the Catalytic Activity of Metal−
Organic Frameworks: The Unique Case of UiO-66(Zr). J. Am. Chem.
Soc. 2013, 135, 11465−11468.
(48) Skoulidas, A. I.; Sholl, D. S. Self-Diffusion and Transport
Diffusion of Light Gases in Metal-Organic Framework Materials
Assessed Using Molecular Dynamics Simulations. J. Phys. Chem. B
2005, 109, 15760−15768.
(49) Werber, J. R.; Osuji, C. O.; Elimelech, M. Materials for NextGeneration Desalination and Water Purification Membranes. Nat. Rev.
Mater. 2016, 1, No. 16018.
(50) Shearer, G. C.; Chavan, S.; Ethiraj, J.; Vitillo, J. G.; Svelle, S.;
Olsbye, U.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. Tuned to
Perfection: Ironing Out the Defects in Metal−Organic Framework
UiO-66. Chem. Mater. 2014, 26, 4068−4071.
(51) Peterson, G.; Destefano, M.; Garibay, S.; Ploskonka, A.;
McEntee, M.; Hall, M.; Karwacki, C.; Farha, O.; Hupp, J. Optimizing
Toxic Chemical Removal through Defect-Induced UiO-66-NH2
Metal-Organic Framework Chem. Eur. J. 2017, 10.1002/
chem.201704525.
(52) Zhao, S.; Wang, Z. A Loose Nano-Filtration Membrane
Prepared by Coating HPAN UF Membrane with Modified PEI for
Dye Reuse and Desalination. J. Membr. Sci. 2017, 524, 214−224.
37855
DOI: 10.1021/acsami.7b12750
ACS Appl. Mater. Interfaces 2017, 9, 37848−37855