Thermodynamically Guided Synthesis of Mixed-Linker Zr-MOFs with Enhanced Tunability

Article abstract of DOI:10.1021/jacs.6b03263

Guided by thermodynamics, we have synthesized two mixed-linker zirconium-based metal-organic frameworks (Zr-MOFs), namely, PCN-133 and PCN-134. Both of them possess a layer-pillar structure, in which the connection between Zr₆ clusters and primary BTB linkers form a (3,6)-connected kdg layer that is further extended into 3D frameworks by auxiliary DCDPS/TCPP linkers (BTB = benzene tribenzoate, DCDPS = 4,4′-dicarboxydiphenyl sulfone, TCPP = tetrakis(4-carboxyphenyl)porphyrin). PCN-134 demonstrates high porosity (N₂ uptake of 717 cm³·g^-1 and BET surface area of 1946 cm²·g^-1) and excellent chemical stability in aqueous solutions with pH values ranging from 0 to 13. More importantly, PCN-134 tolerates the partial absence of auxiliary linkers leading to structural defects during the assembly process while preserving its framework integrity. Furthermore, the defect density can be systematically controlled by tuning the occupancy of the auxiliary linker, which in turn affects the MOF properties. For instance, the dichromate uptake of PCN-134 is tuned by adjusting the BTB/TCPP ratios, which gives rise to an efficient dichromate absorbent when the TCPP molar ratio in linkers is set as 22%. In addition, the photocatalytic reduction of Cr(VI) in aqueous solution was also performed by PCN-134-22%TCPP which exhibits excellent catalytic activity. This work not only opens up a new synthetic route toward mixed-linker MOFs, but also provides tunable control of MOF defects and, in turn, the properties.

Full text of DOI:10.1021/jacs.6b03263

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Article
Thermodynamically Guided Synthesis of Mixed-
Linker Zr-MOFs with Enhanced Tunability
Shuai Yuan, Jun-Sheng Qin, Lanfang Zou, Ying-Pin Chen, Xuan Wang, and Hong-Cai Zhou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b03263 • Publication Date (Web): 06 May 2016
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Thermodynamically Guided Synthesis of Mixed-Linker Zr-
MOFs with Enhanced Tunability
Shuai Yuan,†,‡ JunꢀSheng Qin,†,‡ Lanfang Zou,† YingꢀPin Chen,†,§ Xuan Wang,† Qiang Zhang,† and HongꢀCai
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Zhou*,†,§
† Department of Chemistry, Texas A&M University, College Station, Texas 77843ꢀ3255, United States
§ Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842, United States
ABSTRACT: Guided by thermodynamics, we have synthesized two mixedꢀlinker zirconiumꢀbased metal–organic frameworks (Zrꢀ
MOFs), namely PCNꢀ133 and PCNꢀ134. Both of them possess a layerꢀpillar structure, in which the connection between Zr6 clusters
and primary BTB linkers form a (3,6)ꢀconnected kdg layer that is further extended into 3D frameworks by auxiliary DCDPS/TCPP
linkers (BTB = benzene tribenzoate, DCDPS = 4,4'ꢀdicarboxydiphenyl sulfone, TCPP = tetrakis(4ꢀcarboxyphenyl)porphyrin).
PCNꢀ134 demonstrates high porosity (N2 uptake of 717 cm3·g–1 and BET surface area of 1946 cm2·g–1) and excellent chemical staꢀ
bility in aqueous solutions with pH values ranging from 0 to 13. More importantly, PCNꢀ134 tolerates the partial absence of auxilꢀ
iary linkers leading to structural defects during the assembly process while preserving its framework integrity. Furthermore, the
defect density can be systematically controlled by tuning the occupancy of the auxiliary linker, which in turn affects the MOF propꢀ
erties. For instance, the dichromate uptake of PCNꢀ134 is tuned by adjusting the BTB/TCPP ratios, which gives rise to an efficient
dichromate absorbent when the TCPP molar ratio in linkers is set as 22%. In addition, the photocatalytic reduction of Cr(VI) in
aqueous solution was also performed by PCNꢀ134ꢀ22%TCPP which exhibits excellent catalytic activity. This work not only opens
up a new synthetic route toward mixedꢀlinker MOFs, but also provides tunable control of MOF defects and in turn, the properties.
located in predetermined positions in the crystalline lattice.
Recently, this has been demonstrated by copolymerization of
INTRODCUTION
Metal–organic frameworks (MOFs), which are also known
multiple topologically distinct linkers to produce isoreticular
as porous coordination networks (PCNs), are an emerging
class of porous materials assembled by coordination bonds
between metal ions (or clusters) and organic linkers.1 Owing
to their structural and functional tunability, the study of MOFs
has become one of the fastest growing research topics in synꢀ
thetic chemistry and material science. One of the recent MOF
synthetic approaches focuses on the creation of “heterogeneity
within order,”2 which integrates multiple synergistic functionꢀ
alities into one ordered framework. Among them, a common
strategy is the utilization of linkers with similar length but
distinct functionalities in the construction of mixedꢀlinker
MOFs.3 One of the representative examples is the successful
synthesis of multivariate MOFs (MTVꢀMOFs), demonstrated
by Yaghi and coꢀworkers,3b in which up to eight linear linkers,
each bearing a different functional group, were introduced into
one framework. To monitor the distribution of functional
groups of MTVꢀMOFs, further efforts were devoted to eluciꢀ
date the apportionments of functional groups by solid state
NMR and molecular simulation. However, it is still tremenꢀ
dously challenging to determine the exact position of each
component due to the lack of control over the distribution of
the disordered functional groups.4
sets of MOFs with systematically modulated pore architecꢀ
tures.6 So far, this approach has only been shown with soft
Lewis acidic metal species (M2+).5,7 However, the resultant
materials usually exhibit relatively poor chemical stabilities,
which severely limits their applications.
Owing to its superior stability and tunable connectivity,8,9
the Zr6 cluster is considered to be an attractive building block
for the preparation of mixedꢀlinker MOFs. Furthermore, Zrꢀ
MOFs are known for their extraordinary tolerance of defects,10
which makes them ideal for the construction of mixedꢀlinker
MOFs with various linker ratios. For instance, the linker ratios
of these ZrꢀMOFs can be altered by partially replacing one
linker with defects. Likewise, the defect concentration can also
be tuned by judicious control of linker ratios. Nevertheless, the
oneꢀpot synthesis of ZrꢀMOFs using multiple linkers of differꢀ
ent lengths has seldom been explored.11 Efforts have been
devoted exclusively in our lab to producing mixedꢀlinker Zrꢀ
MOFs starting from a combination of linear linkers, but to no
avail. In the literature, 12ꢀconnected ZrꢀMOFs with UiO strucꢀ
tures are thermodynamically more favored, and thus UiO
structures are always generated instead of mixed linker phasꢀ
es.12
To address this issue, researchers have tried to control the
arrangement of functionalities by the introduction of two or
more topologically distinct linkers bearing different functional
groups.5 Multiple functional groups were consequently anꢀ
chored in predefined positions of the MOF pores through the
following steps: (i) the linker backbones account for a predeꢀ
termined topology of framework, and (ii) different linkers are
Our previous work demonstrated that mixed linker Zrꢀ
MOFs can be obtained by a kineticallyꢀcontrolled synthetic
strategy, namely Sequential Linker Installation (SLI).11 This
strategy utilizes a stable ZrꢀMOF with inherent coordinatively
unsaturated sites as a matrix, and postꢀsynthetically installs
linkers with different functionalities and lengths sequentially
through kinetic control. In this case, the terminal –OH–/H2O
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ligands capped on the coordinatively unsaturated sites of the layers (Figure 1c). However, the introduction of linear linkers
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Zr6 cluster were substituted by linear dicarboxylate linkers via
an acidꢀbase reaction. However, some potential limitations
still remain with this strategy. Firstly, SLI usually requires a
complicated procedure and specific control of reaction condiꢀ
tions as a postꢀsynthetic modification strategy. Moreover, it
requires extra effort to precisely control the linker ratio and
defect concentration since the subsequently installed linker
tends to react with all terminal –OH–/H2O ligands on the Zr6
cluster. In this regard, the facile oneꢀpot synthesis of mixedꢀ
linker ZrꢀMOFs is highly desired.
tends to generate 12ꢀconnected UiO structures largely driven
by thermodynamics.12,14 A close examination of the Zr–BTB
structure reveals that each pair of linkers can be extended with
three bent ditopic fragments. Bearing such structural rationaliꢀ
zation in mind, we selected a bent ditopic linker, DCDPS.
Series of reactions between DCDPS and ZrCl4 were carried
out under solvothermal conditions which didn’t give rise to a
crystalline product, possibly due to the incompatible conforꢀ
mation. The DCDPS as an auxiliary linker is expected to form
a layerꢀpillar structure when combined with the Zr–BTB layer.
As expected, the reaction between ZrCl4, BTB, and DCDPS
gave rise to single crystals of a mixedꢀlinker ZrꢀMOF with a
layerꢀpillar structure, denoted as PCNꢀ133 (Figures 2c and
S1a). With the successful construction of PCNꢀ133, we proꢀ
posed that tetratopic linkers can also work as pillars to support
Zr–BTB layers if they meet the size and topology compatibilꢀ
ity. Theoretically, TCPP can be regarded as a combination of
two mirrorꢀsymmetric DCDPS moieties (Figures 2b and S2).
Furthermore, TCPP tends to form porous frameworks with 6ꢀ
or 8ꢀconnected Zr6 clusters under a wide range of synthetic
conditions, which have been reported in previous studies.9 In a
word, TCPP satisfied all the criteria required to construct a
mixedꢀlinker ZrꢀMOF with BTB. To prove our hypothesis,
another mixedꢀlinker ZrꢀMOF, PCNꢀ134 (Figures 2d and
S1b), was successfully synthesized with BTB and TCPP as
precursors through judicious control of reaction conditions
that steered the system toward multicomponent MOFs and
away from competing phases.
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Scheme 1. The thermodynamically controlled strategy for
the synthesis of mixed-linker Zr-MOFs
In the light of existing literature and our previous work,8ꢀ11
we proposed a thermodynamically guided strategy for the synꢀ
thesis of mixedꢀlinker ZrꢀMOFs. The formation of the Zr–
carboxylate bond is an exothermic process, so MOFs with
highly connected Zr6 clusters are thermodynamically favored.
This explains why thermodynamically favored high connected
UiO structures always prevail in the synthesis of mixedꢀlinker
ZrꢀMOFs directly from linear linkers. To integrate two linkers
within a ZrꢀMOF structure, the linkers must be selected raꢀ
tionally (Scheme 1): with any single linker, it is difficult or
impossible to form 12ꢀconnected ZrꢀMOFs. However, a comꢀ
bination of two linkers should give rise to a highly connected
ZrꢀMOF as a thermodynamically favored product. To demonꢀ
strate the effectiveness of our proposed strategy, two mixedꢀ
linker ZrꢀMOFs, namely Zr6O4[OH]4[BTB]2[DCDPS]3 (PCNꢀ
Structural Description. Singleꢀcrystal Xꢀray analysis reꢀ
vealed that PCNꢀ133 crystallized in the hexagonal space group
P6/mmm (No. 191, Table S1). The Zr6 clusters and the BTB
linkers are twoꢀfold disordered in the solved crystal structure.
Note that the disordered Zr6 clusters have also been encounꢀ
tered in another ZrꢀMOF with hexagonal crystal system reꢀ
ported by our group.15 Each individual Zr6 cluster was conꢀ
nected by six carboxylate groups from six BTB linkers in the
abꢀplane, and each BTB linker was attached to three Zr6 cores
to lead to a 2D layer (Figure S3). The 2D layer was further
pillared by DCDPS fragments that linked each pair of Zr6 clusꢀ
ters from adjacent layers by replacing the terminal –OH–/H2O
ligands (Figures 2 and S4a). From the topological viewpoint,
each Zr–BTB layer can be simplified into a kgd net and then
extended into a (3,8)ꢀconnected tfz-d net by DCDPS moieties
(Figure S4b).16
133, BTB
dicarboxydiphenyl
Zr6O4[OH]6[H2O]2[BTB]2[TCPP]
tetrakis(4ꢀcarboxyphenyl)porphyrin),
=
benzene tribenzoate, DCDPS
sulfone)
=
4,4'ꢀ
and
=
(PCNꢀ134, TCPP
were
synthetically
achieved. They show increased porosity and stability comꢀ
pared to the parent MOFs constructed from a single linker.
More importantly, the defects of PCNꢀ134 can be controlled
by adjusting the linker ratios, which in turn affects its behavꢀ
2–
iors on the uptake and conversion of Cr2O7 ions in aqueous
Reaction of ZrCl4, BTB and TCPP in DMF yields red crysꢀ
tals of PCNꢀ134. The substitution of TCPP by Ni(II)ꢀTCPP
under similar conditions further yielded PCNꢀ134(Ni), which
was suitable for Xꢀray diffraction. Similar to PCNꢀ133, PCNꢀ
134(Ni) also crystallized in the space group P6/mmm (Table
S1). Because of the symmetry of the hexagonal crystal system,
the Zr6 cluster and BTB fragment were also 2ꢀfold disordered
in the resolved structure. The Zr6 clusters were linked with 3ꢀ
connected BTB fragment into a (3, 6)ꢀconnected kdg net that
was further supported by Ni(II)ꢀTCPP linkers to form a 3D
layerꢀpillar structure (Figure 2). To better understand the
structure of PCNꢀ134(Ni), an ordered structure was simulated
in the orthogonal space group Imma. With increased unit cell
size and decreased symmetry, the disorder of Zr6 clusters,
BTB, and Ni(II)ꢀTCPP fragments are eliminated. Due to the
topological restriction, each cluster is 10ꢀconnected with six
carboxylates from BTB and four carboxylates from TCPP
(Figure S5), leaving two connection sites terminated by –OH–
solutions.
RESULTS AND DISCUSSION
Structural Design. Guided by the thermodynamically conꢀ
trolled synthetic strategy, we seek linkers that cannot form a
reasonable structure with 12ꢀconnected Zr6 clusters. To meet
such a challenge, BTB was adopted as a primary linker beꢀ
cause its geometry does not topologically match with 12ꢀ
connected Zr6 cluster.13 As shown in Figure 1a, the Zr6 cluster
reduces its connectivity to six with a hexagonal shape and is
extended by the triangular BTB linker, thus forming a 2D layꢀ
er with a (3,6)ꢀconnected kdg topology (Figure 1b). Besides
connection with six carboxylate groups within the layer, six
pairs of terminal –OH–/H2O ligands on the Zr6 cluster were left
above and below the layer, poised for carboxylate linkers.
Intuitively, the Zr–BTB layer can be extended into a 3D netꢀ
work by ditopic linkers, which act as pillars to support the
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/H2O ligands. The occupancy of Ni(II)ꢀTCPP is refined as 1/3,
connected net with a point symbol of {416.620.89}{43}2{44.62}
(Figure S6).17 The samples of PCNꢀ134 with metalꢀfree TCPP
linker was confirmed by powder Xꢀray diffraction (PXRD)
and used for the further studies except for defect analysis by
chemical titration.
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as each cluster is only coordinated with four carboxylates from
TCPP fragments and two pairs of terminal –OH–/H2O ligands.
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The linker ratio is consistent with the H NMR results of diꢀ
gested samples. From the topological viewpoint, the Zr–BTB
layer was extended by TCPP fragments into a (3,4,10)ꢀ
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Figure 1. Design of mixedꢀlinker ZrꢀMOFs: (a) the 2D Zr–BTB layer composed of Zr6 cluster and primary BTB linker; (b) (3,6)ꢀ
connected net originated from 2D Zr–BTB layer; and (c) the view of layerꢀpillar structure of (3,6)ꢀconnected net supported by another
auxiliary linker.
Figure 2. Construction of mixedꢀlinker ZrꢀMOFs: (a) the 2D Zr–BTB layer formed by 6ꢀconnected Zr6 cluster and BTB; (b) PCNꢀ224
formed by 6ꢀconnected Zr6 cluster and TCPP; (c) PCNꢀ133 formed by 12ꢀconnected Zr6 cluster, BTB, and DCDPS; and (d) PCNꢀ134
formed by 10ꢀconnected Zr6 cluster, BTB, and TCPP.
Enhanced Porosity and Stability. The purities of PCNꢀ133
and PCNꢀ134 were confirmed by the comparison of their exꢀ
perimental PXRD patterns and simulated patterns derived
from singleꢀcrystal Xꢀray diffraction (Figures 3a and S7).
More importantly, PCNꢀ134 exhibits an excellent chemical
stability in aqueous solution within a wide range of pH values
for 24 h. The chemical stability was evaluated by suspending
the asꢀsynthesized samples in HCl or NaOH aqueous solutions
at room temperature for 24 h. After the stability test, the samꢀ
ples were thoroughly washed with acetone and dried before
characterization. As shown in Figure 3a, the PXRD patterns
show the framework of PCNꢀ134 remains intact upon immerꢀ
sion in aqueous solutions ranging from pH = 0 to 13, although
slight differences of the peak strength can be observed in the
PXRD patterns. Moreover, the N2 sorption isotherms of samꢀ
ples after similar treatment in aqueous solutions confirmed that
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PCNꢀ134 can survive under these conditions (Figure 3b). Theꢀ
ratio = 25%) reaches 717 cm3·g–1 at 77 K (Figure 3b), which is
almost four times as much as the prototypical MOF (179
cm3·g–1).13a
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se results suggest that no framework collapse or phase transiꢀ
tion happens during the stability test. The stability of PCNꢀ133
is relatively lower than PCNꢀ134, possibly due to the hydroꢀ
philic –SO2 group and the flexibility of DCDPS linker (Figꢀ
ures S7 and S8).
Control of Linker Ratio. Different from other mixedꢀlinker
MOFs, PCNꢀ134 takes advantage of the nature of ZrꢀMOFs,
possessing high porosity and stability, which allows a high
defect density. In this way, the linker ratio can be tuned if auxꢀ
iliary linker is partially absence. Herein, we explored the posꢀ
sible linker ratios that can be realized within PCNꢀ134 system.
Compared to the reported ZrꢀMOFs with 6ꢀ or 8ꢀconnected
Zr6 clusters (for instance, PCNꢀ222, PCNꢀ224, PCNꢀ225, and
PCNꢀ777),9,18 PCNꢀ134 shows dramatic increased stability,
especially in basic media. This is presumably attributed to the
highly connected metallic clusters that are highly resistant to
the attack of water, acid, and base.15,19,20 The low connected Zr
SBU, with more terminal –H2O and –OH– groups on the clusꢀ
ter, is prone to be attacked by acid or base, which is expected
to reduce the MOF stability in acidic or basic media. In addiꢀ
tion, PCNꢀ134 also exhibits a much improved stability comꢀ
pared to the 2D Zr–BTB MOF, which can be attributed to the
fact that the 2D layered structure reduces the rigidity of the
whole framework and further influences the stability of the
material.13a Overall, the increased number of connections and
rigid framework accounts for the high stability of PCNꢀ134,
which is a prerequisite for practical applications.
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As the linker ratio of reactants would strongly affect the
phase purity of the product, we must first optimize the synthetꢀ
ic condition of PCNꢀ134 with different linker ratios. Highꢀ
throughput methods were employed to study the influence of
reaction parameters on the products. The effects of linker ratio
and modulating reagent were investigated (Table S2) and
summarized into a phase diagram (Figure 4a). The product of
each trial was recorded by PXRD to determine the phase puriꢀ
ty. At a fixed concentration of modulating reagent (1.7 M), the
molar ratio of BTB in the starting material determines the
phase purity of the product. When the TCPP is in majority,
only a Zr–TCPPꢀMOF, namely PCNꢀ224, will be formed.9b
With an increased BTB ratio, PCNꢀ134 starts to form, while
PCNꢀ224 still exists as impurity (Figure S9). When the molar
ratio of TCPP decreases to 30%, pure PCNꢀ134 phase is obꢀ
tained. A further decrease of TCPP ratio does not affect the
framework of the PCNꢀ134 phase, which is confirmed by
PXRD patterns (Figures 4b and S10). However, it will affect
the linker ratios in the final products. The linker ratio of BTB
and TCPP in defect free PCNꢀ134 is 2:1, so a pure PCNꢀ134
phase only appears at high ratios of BTB. As shown in Figure
S11, the linker ratio in the starting material is proportional to
the linker ratio in the PCNꢀ134 as proved by 1H NMR results.
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Figure 3. Stability test of PCNꢀ134 (TCPP ratio = 25%): (a) the
PXRD patterns and (b) the N2 isotherms after immersion in aqueꢀ
ous solutions with different pH values at room temperature for 24
h.
Figure 4. (a) Phase diagram showing the products under different
synthetic conditions, and (b) the photos of PCNꢀ134 with different
TCPP ratios, except for the last one, which is PCNꢀ224.
Furthermore, the concentration of the modulating reagent
also has an effect on the phase purity. When the ratio of TCPP
is 30%, the product varies from nonꢀcrystalline powders to
PCNꢀ134 phase with an increase of modulating reagent. Furꢀ
The pillar linker opens up the space between each Zr–BTB
layer, and therefore the porosity of PCNꢀ133 and PCNꢀ134 is
greatly improved compared to the prototypical MOF (Zr–BTB
structure).13a In particular, the N2 uptake of PCNꢀ134 (TCPP
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ther addition of modulating reagent gives rise to PCNꢀ224 Although the exact identity of defect sites in fully activated
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impurity, as confirmed by subsequent PXRD patterns (Figure
S12). The modulating reagent is known to create to the defect
sites during the MOF synthesis so that a MOF with low conꢀ
nected Zr6 clusters (PCNꢀ224) is favored compared to highly
connected ones (PCNꢀ134) at elevated modulating reagent
concentrations.
samples is unknown, the missing auxiliary linkers would most
likely be replaced with –OH2 and –OH groups to compensate
for the charge loss from the linkers. This is expected to proꢀ
vide extra active protons (Figure S18). Therefore, the active
protons can stoichiometrically react with strong base, and thus
the defect concentration can be estimated. Inspired by the
wellꢀestablished methods of surface hydroxyl moiety titration
in amorphous oxides (such as SiO2 and TiO2) as well as in
UiOꢀ67,22,23 we would like to quantitatively characterize the
proton concentration by chemical titration. The Ni(II)ꢀTCPP
was used to synthesize PCNꢀ134(Ni) in order to eliminate the
influence of protons from porphyrin center. Before titration,
PCNꢀ134(Ni) samples with different defect density were
washed thoroughly with water, DMF, and acetone, and then
dried under vacuum to remove the BA and water. The samples
were then titrated with an excess of CH3Li and the amount of
evolved methane was quantified by gas chromatographic analꢀ
ysis. The proton concentrations in PCNꢀ134 samples deꢀ
creased from 7.4 to 3.9 mmol·g–1 as the TCPP ratio increased
from 10% to 31%, which matched well with the theoretical
calculations (Figure S19). It should be noted that titration reꢀ
sults should be interpreted cautiously as the stability of samꢀ
ples will also affect the accessibility of active protons. For
example, PCNꢀ134ꢀ5%TCPP had a much lower proton conꢀ
centration than expected because of its high defect density and
low stability of pore architecture, which possibly collapses and
hinders the accessibility of defect sites. Indeed, PCNꢀ134ꢀ
5%TCPP exhibits low porosity as proven by nitrogen isoꢀ
therms.
Unlike the TCPP system, DCDPS can’t form crystalline
product with Zr6 clusters owing to its bent conformation, leadꢀ
ing to an easy control of phase purity. The reaction between
ZrCl4, BTB, and DCDPS gives a pure product, PCNꢀ133, at a
relatively wide range of DCDPS ratios from ~0.2 to ~0.8 (Figꢀ
ures S13 and S14).
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Control of Defects. A good control of pure PCNꢀ134 phase
with different linker ratios makes it possible to study the efꢀ
fects of linker ratios on the MOF properties. In fact, TCPP
ratio in the PCNꢀ134 framework can vary from 5% to 31%
without changing its structure (Figure S10). We proposed that
the TCPP fragment that acted as pillar is partially absent,
thereby creating defects. This was further proved by the subꢀ
sequent N2 uptake (Figure 5). The simulated N2 uptake of deꢀ
fect free PCNꢀ134 (i.e., PCNꢀ134ꢀ33%TCPP) is 657 cm3·g–1,
which is slightly higher than that of PCNꢀ134ꢀ31%TCPP (624
cm3·g–1). This suggests that PCNꢀ134ꢀ31%TCPP is almost
defect free. A decrease of TCPP ratio would increase the N2
uptake: with a TCPP ratio of 25%, PCNꢀ134 has a N2 uptake
of 717 cm3·g–1, which is even higher than the theoretical value.
The BET surface area of PCNꢀ134ꢀ25%TCPP was 1946
cm2·g–1, based on the nitrogen adsorption isotherm. This is
possibly caused by the defects between the Zr–BTB layers
resulting from missing TCPP fragments, which increases the
porosity and decreases the material density. Further decrease
of TCPP ratio would decrease the measured N2 absorbance,
possibly resulting from the excess defect that sacrifices the
MOF stability and crystallinity. However, as for the PCNꢀ133
system, the more DCDPS defect ratio in the framework, the
less nitrogen absorbance of the asꢀsynthesized material (Figure
S15). This result might be attributed to the fact that more
DCDPS defects in the framework would sacrifice the stability
and porosity of the asꢀsynthesized materials considering the
flexible nature of DCDPS.
2–
Elimination of trace Cr2O7 from water. Hexavalent
chromium is classified as a Group ‘A’ human carcinogen by
the U.S. Environment Protection Agency, because of its mutaꢀ
genicity, carcinogenicity, and teratogenicity for human
health.24 It is necessary and essential to completely eliminate
2–
Cr2O7 ions prior to wastewater discharge and drinking water
distribution. However, it is cumbersome with conventional
operation technologies owing to the low adsorption capacities,
slow kinetics, and inferior selectivity of traditional materials.25
Hence, an alternative approach and advanced materials are in
demand for the treatment of Cr2O72–contamination from waꢀ
ter.26 The missing linker sites in ZrꢀMOFs are reported to bind
strongly with anion such as arsenate and selenate.27 With conꢀ
trolled missing linker density, tunable porosity, and high
chemical stability, PCNꢀ134 is an ideal and attractive platform
for the study of Cr2O72– capture.
Defect Analysis. According to linker ratio control experiꢀ
ments, TCPP ratio in PCNꢀ134 can be systematically tuned
from 5% to 31% and the pillared TCPP linker would partially
miss creating defects. It’s likely that the defect positions in asꢀ
synthesized ZrꢀMOFs were capped by benzoates, owing to the
fact that PCNꢀ134 was synthesized using benzoic acid (BA) as
a modulating reagent (Figure S16). Consequently, the defect
concentration can be determined by analyzing the concentraꢀ
The performance of PCNꢀ134 in the removal of dichromate
from the aqueous solution was studied by placing the dried
adsorbent into a dichromate solution (50 ppm). The solution
2–
1
was stirred under ambient temperature, and the Cr2O7 conꢀ
tion of benzoate through H NMR technique. This method has
centration was determined by the UV/vis absorption spectrum
at 257 nm. The PCNꢀ134ꢀ22%TCPP showed the highest
been proved to be effective for determining the content of
terminal –OH2 and –OH groups in the 8ꢀconnected Zr6 cluster
of NUꢀ1000.21 The asꢀsynthesized samples were washed thorꢀ
oughly with DMF and then digested by D2SO4/DMSOꢀd6 for
1H NMR analysis. As described in Figure S17, the BA molar
ratio in all the samples increases as the TCPP molar ratio deꢀ
creases, indicating that the TCPP linkers were replaced by
terminal BA ligands to create defects. The experimental values
of BA concentration were in agreement with the theoretical
ones with slightly lower values within PCNꢀ134 system with
different TCPP molar ratios.
2–
2–
Cr2O7 uptake (Figure 6a): the concentration of Cr2O7 ions
decreased by 85% in 2 min and over 95% of dichromate can
be removed within 10 min. The terminal –H2O/OH groups on
missing linker defect sites are responsible for dichromate bindꢀ
ing. However, the high density of missing linker defects hamꢀ
pers the framework stability, which in turn affects the accessiꢀ
bility of active –H2O/OH groups on the Zr6 cluster. This exꢀ
plains the relatively low dichromate uptake in PCNꢀ134ꢀ
5%TCPP and PCNꢀ134ꢀ10%TCPP. PCNꢀ134ꢀ22%TCPP repꢀ
resents a delicate balance between stability, porosity, and deꢀ
2–
fects. After Cr2O7
adsorption measurements, the asꢀ
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synthesized samples were separated by centrifugation and
further digested for ICP measurements. The results suggest
that absorbed amounts of Cr2O72– measured by ICP were wellꢀ
matched with the UV data (Table S3 and Figure S21).
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Figure 5. Effects of TCPP defects of PCNꢀ134 system on N2 uptake measured at 77 K.
Figure 6. (a) The Cr2O72– adsorption capacities and (b) the photocatalytic activities for the reduction of Cr2O72– under visible light irradiaꢀ
tion of PCNꢀ134 system with different TCPP molar ratios.
Given the high adsorption capacity of PCNꢀ134ꢀ22%TCPP,
selective adsorption tests were performed for a series of aniꢀ
ons. As shown in Figure S22, the adsorption capacity of PCNꢀ
134ꢀ22%TCPP was maintained even when 2ꢀfold excess
that of MOFꢀ525, which is also composed of a TCPP linker
and a 12ꢀconnected Zr6 cluster, suggesting the positive contriꢀ
bution of defect sites to the catalytic activities. We inferred
that the binding of dichromate on the defect sites of Zr6 clusꢀ
ters facilitates the electron transfer process from photosensiꢀ
tizer to substrates and thus the reduction from Cr(VI) to Cr(III).
Based on the experimental data, a tentative mechanism proꢀ
posed for the photoreduction of Cr(VI) by PCNꢀ134 is deꢀ
scribed in Figure S24. The porphyrin moieties from the linker
were excited by visible light to inject electrons into Zr6 clusꢀ
ters, yielding [TCPP]+. The binding dichromates on Zr6 clusꢀ
ters were then reduced by the photoelectron into Cr(III), and
the [TCPP]+ was reduced back to TCPP by ethanol. Additionꢀ
ally, the subsequent XRPD pattern suggests that the frameꢀ
–
moles of disturbing ions (F–, Cl–, Br–, NO3 , and SO42–) were
2–
added, compared with the Cr2O7 adsorption capacity in the
absence of disturbing ions. Meanwhile, the XRPD pattern
indicates that the network of the best performing material,
2–
PCNꢀ134ꢀ22%TCPP, was maintained after Cr2O7 adsorption
(Figure S23).
Moreover, the photocatalytic reduction of Cr(VI) to Cr(III)
was also conducted in aqueous solution with a pH value close
to 6.28 After being stirred for 1 h to reach an adsorptionꢀ
desorption equilibrium in a dark environment, the suspensions
were irradiated by a 300 W Xe lamp with 420 nm cutꢀoff filter.
The dichromate concentration was calculated by comparing
the UV/vis absorbance (at λ = 257 nm). As shown in Figure 6b
and Table S4, PCNꢀ134 with 22% TCPP molar ratio exhibited
the highest photocatalytic activity, which is higher than wellꢀ
known photocatalysts such as TiO2 and MILꢀ125ꢀNH2. It’s
worth mentioning that the performance of PCNꢀ134 exceeded
2–
work of PCNꢀ134 was still retained after Cr2O7 reduction
(Figure S25).
CONCLUSION
In conclusion, we have developed a thermodynamically guidꢀ
ed strategy for the synthesis of two mixedꢀlinker ZrꢀMOFs:
PCNꢀ133 and PCNꢀ134. Their 3D layerꢀpillar structures were
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(d) Wang, C.; Xie, Z.; deKrafft, K. E.; Lin, W. J. Am. Chem. Soc.
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5
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BTBs as the primary linkers, supported by auxiliary DCDPS
or TCPP linkers. PCNꢀ134 exhibits high porosity and excelꢀ
lent chemical stability in aqueous solutions with pH scale
ranges from 0 to 13. More importantly, the mixedꢀlinker Zrꢀ
MOF, PCNꢀ134, has significant tolerance of partial auxiliaryꢀ
linker absence during the assembly process, leading to a high
density of structural defects while preserving the overall strucꢀ
ture. In addition, the defect density can be systematically
tuned by adjusting the linker ratios, which in turn alters the
properties of PCNꢀ134. For instance, PCNꢀ134ꢀ25%TCPP
exhibits the highest nitrogen uptake and PCNꢀ134ꢀ22%TCPP
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exhibits the highest Cr2O7 absorbance compared with those
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of other TCPP molar ratios in PCNꢀ134 system. Furthermore,
PCNꢀ134ꢀ22%TCPP exhibits excellent photocatalytic activity
toward the reduction of dichromate in aqueous solution under
visible light irradiation. Apart from providing two new mixedꢀ
linker ZrꢀMOFs, the strategy developed herein shall lead to a
facile synthesis of a stable ZrꢀMOF with a controllable defect
density, which should extend the application of MOF materiꢀ
als in the near future.
ASSOCIATED CONTENT
Supporting Information. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*zhou@chem.tamu.edu
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The gas adsorptionꢀdesorption studies of this research was supꢀ
ported by the Center for Gas Separations Relevant to Clean Enerꢀ
gy Technologies, an Energy Frontier Research Center funded by
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DEꢀSC0001015. Structurꢀ
al analyses were supported as part of the Hydrogen and Fuel Cell
Program under Award Number DEꢀEEꢀ0007049. S. Yuan also
acknowledges the Texas A&M Energy Institute Graduate Fellowꢀ
ship Funded by ConocoPhillips. S. Yuan would also like to thank
Ms. Shanshan Liu for proofreading and helpful discussion.
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