A highly porous metal-organic framework for large organic molecule capture and chromatographic separation

Article abstract of DOI:10.1039/c7cc01063j

A highly porous metal-organic framework (MOF) with large pores was successfully obtained via solvothermal assembly of a “click”-extended tricarboxylate ligand and Zn(ii) ions. The inherent feature of large-molecule accessible pores endows the MOF with a unique property for utilization toward large guest molecules.

Full text of DOI:10.1039/c7cc01063j

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C7CC01063J
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A Highly Porous Metal-Organic Framework for Large Organic
Molecule Capture and Chromatographic Separation
Pei-Zhou Li,a,† Jie Su,b,† Jie Liang,a,† Jia Liu,a Yuanyuan Zhang,a Hongzhong Chena and Yanli Zhao*a,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A highly porous metal-organic framework (MOF) with large pores developing MOFs with large-molecule accessible pores,
was successfully achieved via solvothermal assembly of a “click”- although other strategies have been also investigated such as
extended tricarboxylate ligand and Zn(II) ions. Inherent feature of mixed-ligand combination or template-directing syntheses.10.11
large-molecule accessible pores endows the MOF with unique Herein, we present the successful demonstration on the
construction of a MOF, [Zn6(C33H18N9O6)4(H2O)3]n (1),12 with
property for the utilization toward large guest molecules.
large-molecule accessible pores assembled from click-
Metal−organic frameworks (MOFs) as a class of promising
porous materials have attracted tremendous attention over
the past decade owing to their intriguing architectures and
high porosity as well as wide application potential.1,2 Distinct
from other inorganic porous materials such as zeolites, porous
silica and activated porous carbons, MOFs are typically built by
the self-assembly of polytopic bridging ligands with metal
ions/clusters under solvothermal conditions.1,2 Inherent
structural feature with organic moieties endows MOFs with
chemical tenability, such as adjustable composition, tunable
pore size and modifiable pore surface.3,4 As a unique group of
porous materials, the pores in constructed MOFs play a crucial
role toward the target applications.5,6 Abundant MOF
structures with different pore size and pore shape have been
fabricated by judicious combination of organic ligands and
metal ions/clusters, with which great efforts have been
dedicated to the exploitation for gas or small molecule capture
and storage.2 However, developing efficient MOFs with
suitable accessible pores for large-molecule-based applications
such as heterogeneous catalysis, bioimaging and drug delivery,
organic pollutant removal as well as organic molecule
separation is also highly desired.6-9 Extending the length of
organic backbones should be a simple and feasible strategy for
extended tricarboxylate ligand and Zn(II) ions, and its
remarkable capabilities for large molecule capture and
chromatographic separation probed by organic dye molecules.
Fig. 1 (a) Schematic illustration of click-extended tricarboxylate ligand,
H3L1; (b) L1 connected three Zn3 units; (c) 1D porous channels along c-axis
in 3D framework of crystal 1, and (d) 1D porous channels against (110) plane
in crystal 1.
Among versatile click chemistry, copper(I)-catalyzed azide-
alkyne cycloaddition is a typical reaction and could usually be
carried out under mild conditions with a high yield, which has
been intensively utilized in fabricating/modifying various
functional materials and also in the design and synthesis of
organic ligands for MOF constructions.13-15 Taking advantage of
click reaction, an tricarboxylate ligand, 4,4',4''-benzene-1,3,5-
tryl-tri(1H-1,2,3-triazol-1-yl)benzoic acid (H3L1, Fig. 1a), was
designed and successfully synthesized. Literature reports show
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that Zn-based MOF crystals usually are transparent and also
Journal Name
and its Brunauer-Emmett-Teller (BET) surface area was
1
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present various architectures with interesting pores.9f,16 Thus, calculated to be 2819 m2 g-1, which is DcOoIm: 1p0a.1r0a3b9l/eC7wCCit0h10o6u3Jr
Zn(II) ion was selected for the coordination with H3L1 in N,N’- previously reported Zn-based MOF having tricarboxylate
dimethylformamide (DMF) through solvothermal reaction, and components.19 The pore size distribution was further
subsequently block colorless crystals of MOF 1 were obtained calculated based on the N2 sorption isotherms at 77 K with the
(see the experimental section and Fig. S1 in ESI).
non-local density functional theory. The result reveals that the
Single-crystal structural analysis reveals that 1 has a porous pore size distribution of 1 shows a broad peak ranging from
network with a framework formula of [Zn6(L1)4(H2O)3]n (Fig. 1 ~10 to ~20 Å with a focused peak at 13.5 Å (Fig. 2b), which is
and S2). As illustrated in Fig. 1b, three neighbouring Zn(II) ions consistent well with the observation from its crystal structure.
are bridged by six distributed carboxylate groups from six Gas adsorption analyses further confirm that MOF 1 has a high
different L1 ligands, giving the formation of a linear Zn3 porosity with large pore size.
cluster.16 In the linear Zn3 cluster, the middle Zn(II) is
coordinated by six oxygen atoms forming a typical octahedral
geometry. The other two terminal Zn(II) ions showing the same
tetrahedral geometry are coordinated by four oxygen atoms,
three of which are from three carboxylates and one from the
capped water molecule. Two superimposed L1 ligands then
bridge three linear Zn3 clusters to form a (Zn3)3(L1)2 subunit
(Fig. 1b), which subsequently connects by each other to
construct an uninodal (10,3)-net with intrinsically chiral SrSi2
topology in a “srs” symbol.16b The same two networks sharing
a capped water molecule coordinated with terminal Zn(II) ions
finally give the formation of the porous framework of 1.
Having click-extended long organic backbone of the
tricarboxylate ligand, large pores with opening windows of
14.8 × 19.9 Å2 along a-axis and opening diameters of
approximate 10.2 Å perpendicular to the (110) plane are
successfully achieved in 1 (Fig. 1c,d). After removing the
discrete solvents in the pores of 1, the MOF structure was
Fig. 3 (a) Structural illustration of the dye molecules, (b) comparison of the
calculated using PLATON/VOID program.17 The total solvent-
crystal colors before and after the dye soaking and (c) color change of the
crystals after soaking in Dox/DMF solution, clearly showing that the dye and
Dox molecules were adsorbed by 1.
accessible volume of 1 was estimated to be 76.1% and the
density of the desolvated framework was calculated to be only
0.468 g cm-3, further indicating that 1 possesses a high
porosity. The structural analysis and calculations clearly reveal
that 1 has a highly porous framework with large pores.
Inspired by the high porosity and large pores of 1,
investigations on large molecule based applications were
subsequently carried out. Organic dyes, as a big group of
organic molecules frequently used as biological markers for
bioimaging, have variety of colors and sizes.20 Given visible
color feature of some organic dyes and transparent colorless
nature of the crystal 1, organic dyes should be ideal probes to
investigate large-molecule uptake. Therefore, several organic
dyes with different molecular sizes were selected for such
investigation. The activated crystals of 1 were soaked in the
DMF solution of methyl yellow (MeY, yellow), methylene blue
(MeB, blue), and rhodamine 6G (R6G, red), respectively (Fig.
3a). For all three samples, color changes of the crystals of 1
were clearly observed by naked eyes after soaking for about
two hours (Fig. 3b). The uniform distribution of dye colors
throughout the crystals of 1 suggests that the organic dye
molecules deeply penetrate into the large pores instead of
only adsorbing on the external surface of the crystals. The dye-
700
N2 adsorption
600
N2 desorption
500
400
300
Pore size distribution
200
100
1
2
3
4
5
6
7
8
Pore Width / nm
0.4 0.6
Relative Pressure P/P0
0
0.0
0.2
0.8
1.0
Fig. 2 N2 adsorption/desorption isotherms at 77 K and pore-size distribution
of activated 1.
Gas adsorption measurements were then performed to
further confirm its porosity. After the activation process (Fig.
S3), N2 sorption measurement for the activated 1 was carried
out at 77 K. As illustrated in Fig. 2a, 1 exhibits reversible type I
sorption isotherm starting from a quickly increased step at low
pressure and ending to a plateau at relatively high pressure,
demonstrating that it possesses a dominated microporous
feature.18 At 1 atm, 1 shows an overall N2 uptake of 655 cm3 g-
probed
visible
large-molecule
soaking
experiments
demonstrate that large-porous MOF 1 constructed via click-
extension of organic backbones can be used in large-molecule
uptake applications, such as dye-based bioimaging. Keeping
these positive results in mind, the investigations were then
extended not only to others dyes (toluylene Red and
2 | J. Name., 2012, 00, 1-3
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rhodamine B in Fig. S7), but also to a large drug molecule for phenomenon was observed after passing through fresh 1-
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DOI: 10.1039/C7CC01063J
checking the generality of the large-molecule uptake. An packed column (Fig. 4c,d). Moreover, the packed MOF crystals
anticancer drug, doxorubicin (Dox), with red color was selected could be regenerated after thoroughly soaking in fresh DMF
for intuitively visible demonstration. As shown in Fig. 3c, after solvent. The demonstration clearly shows that the constructed
immersing the activated crystals of 1 into the DMF solution of MOF is highly promising porous materials for large molecule
Dox, the color change of the crystals from colorless to red was capture and organic pollutant removal.
clearly observed, indicating that the Dox molecule penetrates
(a)
(b)
into the pores of 1 and this large-pore MOF could be promising
porous materials for drug uptake in potential drug delivery
application.
Before
After
(a)
(b)
Before
After
500 550 600 650 700 750
Wavelength (nm)
(d)
(c)
Before
After
500 550 600 650 700 750
Wavelength (nm)
Before
After
(c)
(d)
Before
After
450 500 550 600 650 700
Wavelength (nm)
Fig. 5 (a) Images and (b) UV-Vis spectra of the DMF solution of MeB/BBR
mixture before and after passing through 1-packed column; (c) Images and (d)
UV-Vis spectra of the DMF solution of R6G/BBR mixture before and after
passing through 1-packed column.
Super-large molecules with polarity are impossible or the
last ones to pass through the traditional silica gel column,
although in some cases they are the desired products. The high
performance of the dye capture experiments via passing
through 1-based column inspired us to further investigate the
possibility of reversed processes to obtain the super-large
molecules firstly rather than small ones by the MOF capture
effect. The dye soaking investigation showed that a super-large
dye, Brilliant Blue R (BBR, blue), was too large to be
encapsulated by the MOF pores (Fig. S7). Thus, a DMF solution
of the MeB/BBR mixture was prepared for the investigation on
molecule-size dependent separation. When the dye mixture
MeB/BBR passed naturally through 1-packed column under
the gravity, it was clearly observed that small-sized MeB
molecule was captured at top part of the column, while the
large BBR molecule passed through the column (Fig. 5a). The
comparison of the UV-Vis spectra before and after the
separation process also reveals that a pure phase of BBR
solution was obtained (Fig. 5b). This effective size-dependent
separation capability was further confirmed by the separation
process of another dye mixture R6G/BBR (Fig. 5c,d). The
results clearly show that larger molecules can firstly pass
through the column, while small ones can be captured by the
450
500
550
600
Before
After
Wavelength (nm)
Fig. 4 (a) Images and (b) UV-Vis spectra of the DMF solution of MeB before
and after passing through 1-packed column; (c) Images and (d) UV-Vis
spectra of the DMF solution of R6G before and after passing through 1-
packed column.
On the other hand, some organic dyes as organic
pollutants can lead to environmental problems even at low
concentrations.21 Thus, organic dyes are also suitable probes
for investigating organic pollutant removal. In this case,
homemade columns were packed by crystals of 1 to examine
its performance on such application (Fig. S8). As shown in Fig.
4a, when the DMF solution of MeB dye passed slowly through
the 1-packed column, a clarified effluent was observed, while
the crystals in top part of the column became blue, indicating
that the MeB molecules were captured by the crystals of 1. To
further detect the concentration of MeB in the effluent, UV-Vis
spectrum was measured, clearly showing that MeB molecules
were removed from the DMF solution and fully captured by
crystals of 1 during the passing-through process (Fig. 4b).
When changing to R6G dye instead of MeB, the same
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These successful experiments of dye-probed organic
molecule uptake, capture and separation demonstrate that
MOF 1 is a highly promising porous material for large-molecule
based applications. Since the performance of porous materials
on the large-molecule based applications is always influenced
by the pore size, the demonstration of dye-probed large
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This research is financially supported by the Singapore
Academic Research Fund (RG112/15 and RG19/16).
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Table of contents
A highly porous metal-organic framework with large
pores presents large molecule based applications
probed by organic dye molecules.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5