A water-stable metal-organic framework: serving as a chemical sensor of PO4 3– and a catalyst for CO2 conversion

Article abstract of DOI:10.1007/s11426-017-9103-3

A new 2D Eu-BTB framework (1) with stratified gridding structure of about 14.6 ?×16.9 ? was synthesized and characterized. Compound 1 displays excellent water stability with the pH 2–12. The luminescent investigations suggest that 1 could represent a chemical sensor of PO₄ ^3- with high sensitivity and selectivity. Importantly, 1 as a sensor of PO₄ ^3- can be reused at least five times. On the other hand, the catalytic investigations of 1 were carried out, indicating that 1 could be demonstrated as a recyclable catalyst for CO₂ conversion with epoxides.

Full text of DOI:10.1007/s11426-017-9103-3

SCIENCE CHINA
Chemistry
doi: 10.1007/s11426-017-9103-3
• ARTICLES •
A water-stable metal-organic framework: serving as a chemical
3–
sensor of PO4 and a catalyst for CO2 conversion
Bin Zhai1, Hang Xu2,3*, Zhong-Yi Li1, Chun-Shuai Cao2,3 & Bin Zhao2,3*
1College of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 476000, China
2Key Laboratory of Advanced Energy Material Chemistry, Department of Chemistry, Nankai University, Tianjin 300071, China
3Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China
Received May 16, 2017; accepted June 27, 2017; published online September 18, 2017
A new 2D Eu-BTB framework (1) with stratified gridding structure of about 14.6ꢀÅ×16.9ꢀÅ was synthesized and characterized.
Compound 1 displays excellent water stability with the pH 2–12. The luminescent investigations suggest that 1 could represent a
chemical sensor of PO43− with high sensitivity and selectivity. Importantly, 1 as a sensor of PO43− can be reused at least five times.
On the other hand, the catalytic investigations of 1 were carried out, indicating that 1 could be demonstrated as a recyclable catalyst
for CO2 conversion with epoxides.
MOFs, water and pH stability, luminescent detection, CO2 conversion
Citation:
Zhai B, Xu H, Li ZY, Cao CS, Zhao B. A water-stable metal-organic framework: serving as a chemical sensor of PO43– and a catalyst for CO2 conversion.
Sci China Chem, 2017, 60: doi: 10.1007/s11426-017-9103-3
1 Introduction
the investigations were mainly focused on detecting small
molecules/cations, but comparably, only a few sensors of
anions have been reported [23,24]. Remarkably, phosphorus
(PO43−), as a pollutant anion, would cause water eutrophi-
cation, accompanying with excessive algal growth and red
tide, etc. [25]. Generally, most of PO43− comes from indus-
trial/agricultural wastewater, and the effective detection of
PO43− would prevent it into aquatic ecosystems. Therefore,
it is urgent to explore novel chemical sensors for detecting
Metal-organic frameworks (MOFs), as an emerging in-
organic-organic hybrid material, have drawn tremendous
attention as its designable structures [1,2] and promising
applications in chemical sensing [3,4], gas storage/separa-
tion [5,6], information storage [7,8] and catalysis [9,10],
etc. [11–13]. Notably, lanthanide-based MOFs (Ln-MOFs)
could represent the suitable candidate of luminescent sensors
because of its excellent optical properties, such as long
luminescence lifetimes and high colour purity, which mainly
attribute from the f-f transitions via an ‘antenna effect’
[3,4]. Inspiring by the outstanding luminescent properties,
quite a lot of Ln-MOFs as chemical sensors have been
explored and reported [3,4,14–22]. Among those studies,
PO43−
.
On the other hand, Ln3+ ions, as Lewis acidic sites, could
represent an effective catalyst for carbon dioxide (CO2) con-
version [9,26,27], which would not only provide useful or-
ganic products, but also fix the greenhouse gas of CO2 to
weaken global warming [28–34]. Based on this point, Ln3+
in MOFs may display excellent catalytic effect as its adsorp-
tion capacity of substrates and CO2 [5,6], and some examples
have been reported [9,28a,35,36]. Among those studies, CO2
*Corresponding authors (email: xuhang@mail.nankai.edu.cn;
zhaobin@nankai.edu.cn.)
© Science China Press and Springer-Verlag Berlin Heidelberg 2017
chem.scichina.com link.springer.com

2
Zhai et al. Sci China Chem
coupling with epoxides was found to be the priori reaction of
CO2 conversion due to its high atom efficiency and the impor-
tant product of cyclic carbonates [37–39]. Thus, it is neces-
sary to prepare Ln-MOFs as catalysts for CO2 cycloaddition
reaction with epoxides.
2.3 X-ray crystallography
Suitable crystal 1 was paced in a cooled N2 gas stream at
~126ꢀK for crystallographic data collection on a Super-
Nova Single Crystal Diffractometer equipped with graphite-
monochromatic Mo Kα radiation (λ=0.71073ꢀÅ). Data
reduction included absorption was performed by using the
SAINT program. The structures were solved by direct
methods and refined by full-matrix least squares on F2 with
SHELXS-97 and SHELXL-97 programs. The hydrogen
atoms were placed geometrically and refined using a riding
model. Detailed crystal data and structure refinement for 1
are shown in Table 1.
In this contribution, a new two-dimensional (2D) frame-
work [Eu(BTB)(phen)(H2O)2]n (1, H3BTB=1,3,5-benzen-
etribenzoate, phen=phenanthroline) was prepared, displaying
excellent water stability with pH range from 2 to 12. The
investigations on compound 1 as a luminescent sensor were
3−
carried out, revealing that 1 can selectively detect PO4
among various colourless anions, and the detection limit
could reach 4×10−5ꢀM. Importantly, compound 1 as the
3−
luminescent sensor of PO4 can be reused for five times,
showing the promising practical application. Furthermore,
the investigations on CO2 conversion reveal that 1 could
behave as a recyclable catalyst with high catalytic effect.
3 Results and discussion
3.1 Structural analyses and stability analyses
Solvothermal reactions of H3BTB, Eu(NO3)3 and phen in
N,N-diethylformamide for 3ꢀd at 130ꢀ°C yielded light yellow
single crystals. Structural analysis of crystal 1 reveals that
compound 1 crystallizes in the triclinic system with space
group P-1 (Table 1). As shown in Figure 1(a, b), each
nine-coordinated Eu3+ ion as a single node is bridged by
BTB3− into the 2D stratified structure, and the size of grid-
ding is about 14.6ꢀÅ×16.9ꢀÅ. Each 2D stratified structure
is stacking into the 3D supermolecule structure through
hydrogen-bond interaction (Figure 1(c)). The stability of
compound 1 was explored as below: first, samples 1 was
immersed into various solutions with pH range of 1–13 for
6ꢀh, and the powder X-ray diffraction (PXRD) of the treated
2 Experimental
2.1 Materials and method
All the chemical reagents were purchased from commercial
methods and used directly without any purification. The C,
H and N microanalyses were measured at the Institute of El-
emental Organic Chemistry, Nankai University. The fluores-
cent spectra were performed by an F-4500 FL Spectropho-
tometer (Hitachi, Japan) at room temperature. Powder X-ray
diffraction was tested on an Ultima IV X-ray diffractome-
ter (Rigaku, Japan) using Cu Kα radiation. Thermogravi-
metric analyses (TGA) were performed with a Netzsch TG
209 TG-DTA analyzer (Germany) under a nitrogen atmos-
phere. The metal content was measured by inductively cou-
pled plasma (ICP) atomic emission spectrometric analysis
(IRIS Advantage). CO2 sorption isotherm was examined on
Table 1 Crystal data and structure refinement for 1
Identification code
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
Compound 1
C39H27EuN2O8
803.58
1
the gas adsorption analyzer Autosorb-IQ2. H NMR spectra
126.15
were measured on a Bruker 300 or 400 spectrometer (Ger-
many) in CDCl3 and CDCl3 (7.26ꢀppm) was used as internal
reference.
triclinic
P-1
8.5197(4)
14.5430(9)
16.1239(9)
70.121(5)
83.041(4)
86.940(4)
2
2.2 Synthesis of [Eu(BTB)(phen)(H2O)2]n (1)
b (Å)
c (Å)
A mixture of H3BTB (0.1ꢀmmol, 43.8ꢀmg), 1,10-phen
(0.1ꢀmmol, 18.0ꢀmg) and Eu(NO3)3·6H2O (0.1ꢀmmol,
44.6ꢀmg) in H2O (3ꢀmL) and dimethyl formamide (DMF,
1ꢀmL) were sealed in a 15ꢀmL Teflon-lined tainless steel
autoclave. The autoclave was heated at 130ꢀ°C for 3ꢀd under
autogenous pressure and then cooled to room temperature at
a rate of 2ꢀ°C/h under ambient conditions. The light yellow
crystals were obtained and washed with H2O. Yield: 76%,
based on Eu(NO3)3·6H2O. Elemental analysis (%) Calcd. for
compound 1: C 58.24, H 3.36, N 3.48. Found: C 58.13, H
3.54, N 3.28.
α (°)
β (°)
γ (°)
Z
F(000)
804.0
Goodness-of-fit on F2
1.013
R1=0.0716,
wR2=0.1731
Final R indexes [I>=2σ(I)]
Final R indexes [all data]
R1=0.1061,
wR2=0.2032

Zhai et al. Sci China Chem
3
characteristic emission peaks were observed at 591 and
5
617ꢀnm (Figure S3), belonging to D0→7FJ (J=1 and 2)
transitions of Eu3+ ions, respectively [3,4]. On the basis of
the transition rules of Eu3+, when Eu3+ ions occupy inversion
or non-inversion center sites, the magnetic dipole transitions
(5D0→7F1) or the electric dipole transitions (5D0→7F2) are
dominated, respectively. In terms of compound 1, the inten-
sity of 5D0→7F1 transitions (591ꢀnm) is strongerthan 5D0→7F2
transitions (617ꢀnm) of 1, indicating that Eu3+ ions locate
in non-inversion centre, which accords with the structural
analyses [3,4].
The investigations of sensing anions were carried out
as following: 4ꢀmg samples 1 was grinded and dis-
persed in 3.76ꢀmL aqueous solution into suspension
through ultrasound process, and a series of NaX solutions
−
(0.24ꢀmL, 0.1ꢀM) (X=PO43−, F−, Cl−, Br−, I−, SCN−, ClO4 ,
OAc−, IO3 , SO32−, SO42−, NO3 , BF4 , C2O42−, CO32−, IO4 )
were slowly added to form 1-X suspension (6×10−3ꢀM). Then,
the resultant suspensions were examined by fluorescence
pectrophotometer under the perturbation of various anions,
and the corresponding emissions were recorded. As shown
in Figure 3, most of anions exhibit no obvious influence
on the luminescence or slightly decrease the emission, but
comparably, PO43− was found to quench the emission almost
completely. The results clearly reveal that samples 1 could
−
−
−
−
Figure 1 (a) The coordinated environments of Eu3+; (b) 2D framework of
1 along a direction; (c) 3D stacking structure of 1 (color online).
samples were recorded in Figure 2, suggesting that frame-
work of 1 could keep intact with the values of pH from 2
to 12. Second, to study the stability of compound 1, ther-
mogravimetric analyses (TGA) were performed under N2 at-
mosphere. As shown in Figure S2 (Supporting Information
online), from room temperature to 315ꢀ°C, a weight loss of
4.51% can be observed, attributing to the loss of coordinated
H2O molecules (Calcd: 4.48%). Subsequently, TGA curves
display a complicated weight from 315 to 495ꢀ°C, and start to
decompose sharply above 495ꢀ°C. Thus, the stable structure
supplies the foundation for compound 1 as a chemical sensor.
3−
represent a promising luminescent sensor of PO4 among
various colourless anions.
On the other hand, waste water always contains several
types of pollutant anions, therefore, the influence of mixed
anions on the emission should be explored. The suspension
3–
1 with mixed anions, containing PO4 and another anion
(6×10−3ꢀM), were tested by luminescent measurements, and
recorded in Figure S4. The results suggest that the quenching
effect of PO43– on the emission is not changed after introduc-
3−
ing other anions, indicating that 1 can selectively sense PO4
among the above anions.
Subsequently, the detection limit of 1 as a chemical sen-
sor of PO4 was explored, and a series of suspensions with
1-PO43− (4×10−5 to 6×10−3ꢀM) were prepared. As shown in
3.2 Luminescent behaviours and sensing properties
3−
Under excitation at 300ꢀnm, the solid-state photolumines-
cence spectra of compound 1 was measured, and the main
Figure 2 The powder XRD patterns of 1 after exposure to various aqueous
solutions with pH from 1.0 to 13.0 for 6ꢀh (color online).
Figure
(617ꢀnm) of 1-X after treating with 6×10−3ꢀM anions (color online).
3
The luminescence intensity of the 5D0→7F2 transitions

4
Zhai et al. Sci China Chem
Figure 4, the gradually decreased emission of 1 can be ob-
served as the increasing concentration of PO43−, and the emis-
sion is almost quenched completely when the concentration
3−
of PO4 reaches 6×10−3ꢀM. Notably, the reduced emission
3−
can be clearly distinguished with 4×10−5ꢀM PO4 solution,
revealing that the detection limit of 1 as a chemical sensor
3−
of PO4 could reach 4×10−5ꢀM. Then, the relationship be-
tween the quenching effect and PO43− concentration was fur-
ther investigated through Stern-Volmer equation, and the cor-
responding results were recorded in Figure S6, which can be
linear fitted as I0/I=0.883+Ksv[PO43−] (I0 and I belong to the
3−
luminescent intensity of 1 before and after dropping PO4
respectively; Ksv represents quenching rate constant; [PO43−
,
Figure 5 The five cycling performance of compound 1 as a luminescent
sensor of PO43− (color online).
]
represents the concentration of PO43−). The value of Ksv is
found to be 3.97×103ꢀL/mol. Up to now, several MOF-based
(Figure S5), which could be a further evidence to support the
3−
recycle performance of compound 1. Thus, the results reveal
sensors of PO4 have been reported, and the corresponding
3−
that 1 could be considered as a recoverable sensor of PO4
,
detection limit has been summarized in Table S1 (Support-
ing Information online). Comparably, the detection limit of 1
is only inferior to our previous work, and better than others.
which is closer to the practical application.
The possible mechanism of quenching effect is further ex-
plored and discussed. First, the PXRD of 1-PO43− was tested
(Figure S7), and the well accordant PXRD shows that the
framework 1 is still intact, and the luminescence quenching is
not caused by the collapse of the framework. Second, the al-
kalescence caused by PO43− may influence the luminescence.
Then, a series of solutions with pH from 1.0 to 12.0 were pre-
pared, and the samples 1 were immersed into the above solu-
tions to explore for their luminescent properties. As shown in
Figure S8, the luminescence intensity of 1 was found to be no
obviously changed with pH from 2.0 to 11.0. Thus, the pertur-
bation of pH could be excluded for the emission quenching.
Third, BTB ligand in compound 1 can absorb photon from
the UV spectrum and transfer energy to Eu3+ to emit char-
acteristic luminescence, but PO43− has no obvious ultravio-
let absorption at 300ꢀnm (excitation wavelength), revealing
that the emission quenching is not caused by the competitive
absorption of photon energy. Last, the collision interaction
between PO43− and framework, namely dynamic mechanism,
would expend partial energy to quench the emission, which is
according with linear Stern-Volmer equation. Therefore, we
speculate that the collision interaction may lead to this emis-
sion quenching of compound 1.
3−
Thus, compound 1 can be considered as the sensor of PO4
with high sensitivity.
Up to now, the MOF-based chemical sensors are still not
employed in the practical applications as their high cost and
non-recyclable performance. Therefore, the recyclable sen-
sor could represent a promising approach to access to the
practical application. Then, 4ꢀmg sample 1 was immersed in
an aqueous solution of 6×10−3ꢀM PO43− for 1ꢀmin into 1-PO43−
suspension, and 1-PO43− was recovered by washing water for
several times. The emission could recover to the original
level, and the PXRD of the washed sample 1 remain well
accord with the simulated one from 1, and the repeated ex-
amin-ations were tested five times (Figure 5 and Figure S7).
Additionally, the luminescent responses of recycled 1 for var-
ious anions have been performed, and the results is similar to
the luminescent intensity of original 1 for the above anions
3.3 Catalytic properties
The catalytic reaction of CO2 and epoxides was explored
under 0.1ꢀMPa CO2, and styrene oxide as a model sub-
strate was initially employed in this reaction. As shown
in Table 2 (entries 1 and 2), 68.6% and 83.4% yield were
obtained with 2.5ꢀmol% 1 and 5ꢀmol% tetrabutylammonium
bromide (TBAB) at 60 and 70ꢀ°C, respectively. Compa-
rably, only 46.3% yield was obtained without catalyst 1
at 70ꢀ°C, indicating that compound 1 could dramatically
enhance the catalytic efficiency. Therefore, the appropriate
Figure 4 The liquid luminescent spectra of compound 1 with different con-
centrations of PO43− aqueous (color online).

Zhai et al. Sci China Chem
5
Table 2 Synthesis of various cyclic carbonates from CO2 and epoxides with
catalyst 1
ried out, and the corresponding yield still keeps in 81.3% after
five cycling experiments. Then, the PXRD of used samples
and ICP of filter liquor were tested, and the results show that
the framework of 1 is still stable and no detectable leakage
of Eu3+ is observed, suggesting that compound 1 as a catalyst
possesses high recyclable performance.
a)
b)
Entry
Epoxides
Products
Yield (%)
68.6
Next step, a possible catalytic mechanism for CO2 conver-
sion with epoxides was proposed according to previous re-
ports (Figure S10) [30]. First, compound 1 can enrich CO2
and epoxides in its structure. Second, O atom in epoxides
binds with the Lewis acidic site of Eu3+ through coordina-
tion interaction, and the epoxy ring has been activated in this
process. Third, the nucleophilic Br− from TBAB attacks car-
bon atom to unfold epoxy ring, and the intermediate products
of oxygen anion has been obtained, which could be stabilized
by the open site of Eu3+ in framework. Finally, CO2 acti-
vates the oxygen anion into an alkylcarbonate salt, and the
cyclic carbonate is formed through intramolecular ring clo-
sure. Through the above process, the corresponding cyclic
carbonate is successfully prepared from CO2 and epoxides
with a high yield.
c)
1
2
83.4
46.3
<1
d)
3
e)
4
f)
5
81.3
4 Conclusions
In conclusion, a new Eu-BTB framework with 2D gridding
structure of about 14.6ꢀÅ×16.9ꢀÅ was prepared and charac-
terized, exhibiting outstanding water stability with the pH of
2–12. Notably, the investigations on the luminescent detec-
tion reveal that compound 1 could represent a chemical sen-
sor for detecting PO43− among various anions with detection
limit of 4×10−5ꢀM. More importantly, the recycle experiment
suggests that 1 as a sensor of PO43− can be reused at least five
times, which provides the foundation for the practical appli-
cation. On the other hand, 1 can be demonstrated as an ef-
fective catalyst for CO2 conversion with epoxides, and the
cycling number can reach at least five times.
6
92.8
7
8
65.3
71.4
9
52.5
a) Reaction conditions:
epoxides (2.0ꢀmmol), solvent-free,
Acknowledgments This work was supported by the National Natural Sci-
ence Foundation of China (21571123, 21571107, 21421001), the Natural
Science Foundation of Tianjin (15JCZDJC37700), 111 Project (B12015) and
MOE Innovation Team (IRT13022, IRT-13R30) of China.
catalyst loading (based on metal centre, 2.5ꢀmol%), TBAB
1
(32.4ꢀmg, 0.1ꢀmmol, 5ꢀmol%), CO2 (0.1ꢀMPa), 70ꢀ°C, 12ꢀh; b) determined
by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
standard; c) the reaction temperature is 60ꢀ°C; d) the reaction without
catalyst 1; e) the reaction without TBAB; f) the yield of fifth cycling.
Conflict of interest
The authors declare that they have no conflict of
interest.
reaction conditions should be determined as 2.5ꢀmol% 1 and
5ꢀmol% TBAB at 70ꢀ°C. Subsequently, various typical epox-
ides were employed in this reaction under the above condition
to explore the catalytic generality of catalyst 1. As shown in
entries 6–9, the quality yields of corresponding cyclic carbon-
ates were obtained, suggesting that 1 could be demonstrated
as a general catalyst for CO2 conversion with epoxides.
On the other hand, compound 1, as a stable solid catalyst,
can be separated and reused via centrifugation and filtration.
Thus, the investigations of 1 as a recyclable catalyst were car-
Supporting information The supporting information is available online
at http://chem.scichina.com and http://link.springer.com/journal/11426.
The supporting materials are published as submitted, without typesetting
or editing. The responsibility for scientific accuracy and content remains
entirely with the authors.
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