A soluble porous organic polymer for highly efficient organic-aqueous biphasic catalysis and convenient reuse of catalysts

Article abstract of DOI:10.1039/c9ta04594e

Functional porous organic polymers (POPs) such as PIM-1 with good solubility have been used for the homogenization of heterogeneous catalysts very recently, in addition to the traditional gas/liquid adsorption and separation. As emerging functional POPs, Azo-POPs have been studied broadly, although they are usually insoluble. This paper reports a novel soluble Azo-POP (named Azo-POP-7) with a fluorene-derived β-hydroxy-azobenzene subunit in the polymer main chain for the first time via a facile azo-coupling polymerization of contorted monomers. The resultant polymers exhibited a Brunauer-Emmett-Teller (BET) specific surface area of 364 m² g^-1 and good solubility in most organic solvents. Moreover, the soluble catalyst Pd@Azo-POP-7 showed the highest efficiency (k_app = 4.95 × 10^-2 s^-1) for the organic-aqueous biphasic reduction reaction of 4-nitrophenol with good recyclability (12 cycles). This work introduces a novel strategy for designing novel functional POPs for organic/aqueous biphasic catalysis.

Full text of DOI:10.1039/c9ta04594e

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DOI: 10.1039/C9TA04594E
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A Soluble Porous Organic Polymer for Highly Efficient Organic-Aqueous Biphasic Catalysis
and Convenient Reuse of Catalysts
This paper reported
a
novel soluble Azo-POP (named Azo-POP-7) with
fluorene-dirived -hyroxy-azobenzene subunit in the polymer main chain for the first time, via
facile azo-coupling polymerization on contorted monomers. The resultant polymers have
Brunauer-Emmett-Teller (BET) specific surface areas of 364 m2.g-1 and a good solubility in most
organic solvents. Moreover, the soluble catalyst of Pd@Azo-POP-7 was found highest efficiency
(kapp = 4.95×10-2 s-1) for the organic-aqueous biphasic reduction reaction on 4-nitrophenol with
good recyclability (12 cycles).

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DOI: 10.1039/C9TA04594E
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A Soluble Porous Organic Polymer for Highly Efficient Organic-
Aqueous Biphasic Catalysis and Convenient Reuse of Catalysts
Hao-Xi Fu, [a] Zhi-Hao Zhang, [a] Wenhao Fan,[c] Shuifeng Wang[d], Yan Liu[b] and Mu-Hua Huang*[a,b]
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Functional Porous Organic Polymers (POPs) such as PIM-1 with heterogeneous catalysts showed lower activity compared with
homogeneous catalysts due to their poorer solubility. The
homogenization of heterogeneous MNPs catalysts is an alternative
strategy to circumvent this drawback that combines the merits of
homogeneous and heterogeneous catalysis to retain high activity
and allow the recovery and reuse of the catalysts. Cao group
reported soluble PIMs-based porous coordination polymers as a
platform to homogenize heterogeneous PdNPs for C-H activation
reactions9. Xu group developed the porous organic cage (CC3-R) as
support to load Rh to prepare soluble Rh/CC3-R-homo for
methanolysis of ammonium borane (AB). Another challenge in
applying metal NPs as catalysts was the difficulty to separate them
from reaction mixtures due to their small sizes. And organic/aqueous
biphasic reaction has been designed as a simple and efficient
alternative to separate catalysts from the reaction mixture11, 12. For
example, organic-soluble catalysts will help a lot in the catalytic
reduction of 4-nitrophenol by NaBH4 in water, where all the
substrates and products were in water phase, while catalysts in
organic phase. As far as recycling catalysts and reducing the metal
leaching are concerned, functional POPs possess the advantage of
coordination sites as well as catching metal nanoparticles tightly.
good solubility have been used for homogenization of
heterogeneous catalysts very recently, in addition to the traditional
gas/liquid adsorption and separation. As emerging functional POPs,
Azo-POPs have been studied broadly, though they were usually
insoluble. This paper reported a novel soluble Azo-POP (named
Azo-POP-7) with fluorene-dirived -hyroxy-azobenzene subunit in
the polymer main chain for the first time, via facile azo-coupling
polymerization on contorted monomers. The resultant polymers
have Brunauer-Emmett-Teller (BET) specific surface areas of 364
m2.g-1 and a good solubility in most organic solvents. Moreover, the
soluble catalyst of Pd@Azo-POP-7 was found highest efficiency
(kapp = 4.95×10-2 s-1) for the organic-aqueous biphasic reduction
reaction on 4-nitrophenol with good recyclability (12 cycles). This
work open up a novel strategy for designing novel functional POPs
towards organic/aqueous biphasic catalysis.
Porous organic polymers (POPs) have been emerging porous
materials by showing broad potential applications1-3, although
limited solubility remained as a challenge. Soluble POPs received lots
of attention owing to providing an opportunity to address the issue
of processability. E.g., McKeown et. al. developed polymers of
intrinsic microporosity (PIMs), Cooper et. al. reported soluble
conjugated microporous polymers (SCMPs)6, Hu et. al. developed
soluble conjugated polymeric nanoparticles (SCPNs) by the aid of
confined nanoreactors, Tan group reported soluble hyperbranched
porous organic polymers (SPOPs)8 for drug release very recently. As
far as catalysis is concerned, loading metal nanoparticles (NPs) on
POPs has proved to be a very successful strategy regarding the
separation and reuse issues in industrial applications. However,
a. School of Materials Science and Engineering, Beijing Institute of Technology, No.
5, Zhongguancun South Street, Beijing 100081, China
E-mail: mhhuang@bit.edu.cn
b. Experimental Center for Advanced Materials, Beijing Institute of Technology, No.
5, Zhongguancun South Street, Beijing 100081, China
c. Beijing Center for Physical & Chemical Analysis, Beijing, 100089, China
d. Analytical and Testing Center, Beijing Normal University, Beijing, 100081, China
†Electronic Supplementary Information (ESI) of the experimental details of Azo-
POP-7 and the characterizations of Azo-POP-7 are available. See
DOI: 10.1039/x0xx00000xx
Figure 1 Chemical structure of insoluble and soluble Azo-POPs
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Although azo-containing POPs have been studied very broadly (e.g. To improve the solubility of diazonium reagent, acetonitrile and THF
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Azo-COP-113, ALP-214, Azo-POF-115, Azo-MOP-116, Azo-POP-116 and were mixed with water at 4:1, respectivelyD, OunI:f1o0r.t1u0n39a/tCe9lyT,At0h4e59a4sE-
HAzo-POP-218 in Figure 1), all of them were insoluble owing to their prepared Azo-POP-7-b and Azo-POP-7-c had even lower BET surface
cross-linked structure. It is worth mentioning that Han and Song et. area of 28 and 2 m2g-1, which revealed water was more suitable for
al. reported the GA-azo-POPs of linear polymer chain, but still azo-coupling polymerization than other organic solvents (entry 2, 3
insoluble in organic solvents. Actually, contorted monomers have in Table S1). To our delight, when HCl was used instead of HBF4, the
been proved to help increase the solubility of polyimide materials20, obtained Azo-POP-7-d had a BET SA of 260 m2g-1 owing to the good
21, and enhance the performance of porous polymers such as PIMs22, solubility of diazonium chloride salts in water. In addition, potassium
23
and nanofilms24. In this paper, molecular design was carried out phenolate of the bisphenol B1 was found to have better solubility
to prepare soluble Azo-POP-7 (Figure 1) for the organic/aqueous than sodium phenolate in water. It helped the azo-coupling
biphasic reduction of 4-nitrophenol. The characterization of the as- polymerization to afford Azo-POP-7-e in 88% yield with BET SA as
synthesized Azo-POP-7, as well as soluble catalyst of Pd@Azo-POP-7 high as 364 m2g-1, when HCl/KOH were used as acid-base regulators.
HO
OH
for highly-efficient reduction on 4-nitrophenol in organic/aqueous
biphasic system will be addressed in detail.
H2N
NH2
N2
N2
1, HCl
2, NaNO2
HO
N N
OH
N
In order to have soluble porous organic polymers with -hydroxyl-
azobenzene subunit, we chose the contorted monomer of fluorene-
based bisphenol B1 and dual diazonium reagent C to make the
soluble Azo-POP-7 (Scheme 1) via azo-coupling polymerization in
aqueous solution. The optimization of the polymerization was
carried out (Table S1) by looking at BET surface area of the resulting
Azo-POP-7 sample. Initially, HBF4/NaOH were used as acid-base
regulators in the preparation of Azo-POP-7-a, and BET specific
surface area as low as 132 m2g-1 (entry 1 in Table S1) was obtained ,
due to the poor solubility of fluoroborate diazonium salts in water.
(B1)
N
N
N
H2O, 0-5 o
C
KOH, 0-5 o
88%
C
Azo-POP-7
(C)
(A1)
Scheme 1 The synthesis of Azo-POP-7 by Azo-coupling of A1-derived
dual diazonium reagent C with bisphenol B1
The chemical structure of the as-synthesized Azo-POP-7 was
confirmed by the solution H-NMR and Fourier-transform infrared
1
(FT-IR) spectra.
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Figure 2 Characterization of Azo-POP-7 by 1H-NMR in CDCl3 (a), CP-MS-13C-NMR, asterisk marks mean spinning side bands (b), FT-IR (c) and NV2 iieswotAhrteicrlme Oantli7n7e
K (d).
DOI: 10.1039/C9TA04594E
The signals in the range from δ =6.63 to 7.91 ppm were observed in NaBH4 (Figure S9). Catalysts Pd@Azo-POP-7-2.5 and Pd@Azo-POP-7-
1
the solution H-NMR spectra, ascribing to the aromatic C-Hs in the
polymers (Figure 2a). The signals at 11.31 ppm were ascribed to the
OH of hydroxyazobenzene structure, owing to the intramolecular H-
bonding between OH and ortho-azo bond. As a sharp comparison,
the signal at 9.34 ppm was ascribed to the OH of end group in the
polymer chain, without H-bonding. In the solid-state cross-
polarization magic angle spinning (CP/MAS) 13C-NMR, signals in the
range from δ =119 to 150 ppm were observed ascribing to the
aromatic carbons, as well as 64 ppm for quaternary carbon in the
polymer backbone (Figure 2b). In the FT-IR spectrum of Azo-POP-7
sample, the peaks around 1449 and 1404 cm-1 were attributed to the
asymmetric vibration of the -N=N- bond (Figure 2c). The peaks
around 3350 cm-1 were assigned to the Ar-OH groups. The elemental
composition of Azo-POP-7 was confirmed by the found N% content
of 7.41% in elemental analysis (theoretically 7.47%), as well as C/N
ratio of 10.82 (theoretically 11.12%) (Table S2). The UV-Vis spectrum
of Azo-POP-7 showed two strong and broad adsorption peaks at 345
and 237 nm (Figure S7), which attributed to the n-π* transition and
π-π* transition of azobenzene chromophores, respectively. The
molecular weight (Mw) of Azo-POP-7 were determined to be 7639
g.mol-1 using 2D 1H DOSY-NMR (Figure S5), which was consistent with
the Mw of 7681 g.mol-1 (PDI = 2.1) by GPC measurement (Figure S4).
Porosity parameters of Azo-POP-7-e were studied by nitrogen
adsorption-desorption isotherm measurement at 77K (Figure 2d),
which exhibited combination of type I and IV isotherms according to
IUPAC classification. The reversible and rapid uptake at low
temperature (0.01-0.1 bar) revealed a permanent microporous
nature. The gradual increase in N2 uptake at relatively higher
pressures (P/P0>0.7) indicated that the polymers mainly possessed
mesopores formed by interstitial voids. The hierarchically
mesoporous property was further proved by pore size distributions
(Figure 2d) from the adsorption branch of the N2 isotherm by
nonlocal density functional theory (NLDFT). It showed mean pore
diameter of 17 nm, and pore volume of 1.76 cm3.g-1 (Table S3), which
could be an advantage for performing catalysis owing to the efficient
mass transport in mesoporous carrier.
1.0 were prepared by using 2.5wt% and 1wt% Pd(OAc)2 of Azo-POP-
7’s weight respectively. Transmission electron microscopy (TEM) and
high-resolution TEM (HRTEM) of the above samples were recorded
(Figure 3 and S12-13).
Figure 3 TEM images and size distribution of Pd NPs of Pd@AZO-POP-7-2.5
(a,b) and Pd@AZO-POP-7-1.0 (c,d).
It revealed that the small Pd NPs were uniformly distributed on Azo-
POP-7, although a narrower size distribution of 1.88 ± 0.58 nm for
Pd@Azo-POP-7-1.0 (Figures 3c and 3d) than Pd@Azo-POP-7-2.5 with
3.36 ± 1.09 nm (Figures 3a and 3b). The smaller particle size meant
the aggregates of the ultrasmall PdNPs could be avoided when less
of Pd was used, which could lead to higher catalytic reactivity, similar
to the cases of single-atom catalysis25-27. So, Pd@Azo-POP-7-1.0 was
mainly used for the further application in catalysis in this work.
4-nitrophenol was regarded to be one of the most widespread
organic pollutants in waste water, which triggered the study on the
adsorption and transformation of 4-nitrophenol. The catalytic
reduction of 4-nitrophenol into 4-aminophenol by NaBH4 was initially
reported by Pal in 2002 using silver nanoparticles as a catalyst28. The
reduction reaction was a commonly used reaction to test the
catalytic activity of an aqueous solution under mild conditions. Many
catalysts have been designed to address this issue, however the
reuse of catalysts remained as a problem. Herein, we would like to
use Pd@Azo-POP-7-1.0 as a catalyst to address this issue, by having
high catalytic reactivity as well as convenient reuse of catalysts
(Figure 4).
The good thermal stability of Azo-POP-7 was confirmed by thermo-
gravimetric analysis (TGA), which showed no obvious decomposition
until 373 °C under N2 atmosphere (Figure S6). The morphology of
solid Azo-POP-7 was investigated by SEM and HR-TEM (Figure S8).
From the typical images of Azo-POP-7, it is obvious that the as-
prepared Azo-POP-7 were composed of fluffy particles (<100 nm)
and had massive irregular holes among the particles.
The sample of Azo-POP-7 showed quite good solubility in many
organic solvents such as DMF, DMSO, THF, CHCl3, 3-Pentanone and
dioxane (Table S4). This might be ascribed to their unique chemical
structures with contorted conformation in the polymer chain, which
prevented strong interaction of polymer chains. It was reported that
the Azo-POPs could stabilize metal ions18, such as Zn2+, Cu2+, Ag+,
which could then be converted into nanoparticles (NPs) through
9
reduction by NaBH4 . Herein, Pd NPs-decorated Azo-POP-7 (Pd@Azo-
POP-7) was prepared by the addition of specific amount of Pd(OAc)2
to a solution of Azo-POP-7 in chloroform, followed by reduction by
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Figure 4 Pd@Azo-POP-7-1.0 in the catalytic reduction of 4-nitrophenol (4-NP)
As a control, the phase separation of a solution of Azo-POP-7 in
chloroform and 4-nitrophenol in water was tested, which showed
easy phase separation after stirring/standing. And then a solution of
NaBH4 was added to the above mixture, no reduction reaction
occurred based on UV-Vis spectra monitoring of the water phase
(Table S5and Figure S15). And then a solution of NaBH4 was added to
the above mixture, no reduction reaction occurred based on UV-Vis
spectra monitoring of the water phase (Figure 5a). The maximum
absorption at 400 nm in UV-Vis spectra with the addition of alkaline
NaBH4 solution was observed, due to the formation of 4-
nitrophenolate ions under alkaline conditions. Further reduction of
these 4-nitrophenolate ions to the desired 4-aminophenol (4-AP) did
not occur without Pd@Azo-POP-7. In sharp contrast, after the
addition of Pd@Azo-POP-7-1.0 to the 4-NP and NaBH4 solution, the
colour of the solution changed from yellow to colourless within 1.7
min, indicating the complete reduction of 4-NP. Meanwhile, the
absorption peak at 400 nm related to 4-nitrophenolate ions gradually
decreased in intensity along with the increase of a new absorption
peak at about 297 nm for 4-aminophenolate ions. The ultrapure
Figure 5 UV−Vis spectra of the reduction on 4-NP (reaction mixture) with
NaBH4 catalyzed by Pd@Azo-POP-7-1.0 (a), and relationship between ln(C/C0)
and reaction time with R2 of 0.93. (b). Reaction conditions: [4-NP] = 2.4 × 10−2
M, [NaBH4] = 1.8 M, [Pd] = 9.5 × 10−5 M, and VCHCl3:VH2O = 2:4.
1
product of 4-AP was then characterized by its H-NMR (Figure S10)
and 13C-NMR spectra (Figure S11).
To quantitatively evaluate their catalytic reactivity of Pd@Azo-POP-
7-1.0 in CHCl3, the apparent rate constant (kapp) of the reduction of
4-NP to 4-AP was chosen as an important parameter. The linear
correlation of ln(C/C0) versus time (At and A0 represented the
absorbance at the intervals and the initial stage of 4-NP ion,
respectively) was obtained (Figure 5b), and the pseudo-first-order
rate constant (kapp) was estimated to be 4.95× 10−2 s−1 based on the
following eq. 1.
Next, the reusability of the Pd@Azo-POP-7 catalyst was evaluated.
Interestingly, Pd(OAc)2 replacing Pd@Azo-POP-7 on the same Pd
molar content was found work for the same reduction reaction,
however, it became ineffective after 3 recycles (Figure 6a), owing to
the aggregation of ultrafine PdNPs without supporting porous
carriers with functionality. To our delight, Pd@Azo-POP-7-1.0
showed very high reactivity on this reduction reaction even after 12
recycles, by simply separating down layer for the next run of catalytic
reaction. The excellent stability of Pd@Azo-POP-7 was observed
even after solvents was removed (named Pd@Azo-POP-7-dry). It was
dissolved in chloroform again and used to catalyze the reduction of
4-NP (Figure 6b) with high efficiency after 6 cycles. However, the
catalytic reactivity was little bit lower than Pd@Azo-POP-7-1.0
catalyst stored in chloroform, by showing longer time for the
completion of the same reduction reaction on 4-nitrophenol (Figure
6b).
ln (At/A0) = ln (Ct/C0) = -kapp
t
(1)
Such catalytic reactivity of Pd@Azo-POP-7 remained highest level for
this reduction reaction11, 12, compared with other known catalysts
(Table S6). Also, it was found that Pd@Azo-POP-7-2.5 in CHCl3
showed quite high efficiency (kapp=3.55× 10 − 2 − 1) for the same
s
reaction, although lower than Pd@Azo-POP-7-1.0 in CHCl3 (Figure
S16-17). This is consistent with the TEM image of both catalysts.
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Figure 6 Plots of the time required to get the 100% conversion of the
reduction on 4-NP by Pd@Azo-POP-7-1.0 and Pd(OAc)2 (a), and by Pd@Azo-
POP-7-1.0-CHCl3 and Pd@Azo-POP-7-dry. Reaction conditions: [4-NP] = 2.4 ×
10−2 M, [NaBH4] = 1.8 M, [Pd] = 9.5 × 10−5 M, and VCHCl3:VH2O = 2:4.
Furthermore, the ICP-MS detection on the aqueous phase showed
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cat after catalytic reduction reactions could be ignored. This might
be owing to the ortho-hyroxy-azobenzene functionality helped catch
PNPs.
In summary, a novel soluble porous organic polymer (Azo-POP-7)
with -hydroxy-azobenzene functionality was developed to stabilize
Pd nanoparticles in organic solvent, which was used in
organic/aqueous biphasic reduction reaction of 4-nitrophenol to give
4-aminophenol in high yield. It was found that Azo-POP-7 reduced
the particle sizes of PdNPs and improved their organic solubility,
accounting for their good activity and stability in the
organic/aqueous biphasic solution. A organic-soluble Pd@Azo-POP-
7-1.0 catalyst showed very high catalytic performance in the
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reduction of 4-nitrophenol in water phase with kapp=4.95× 10−2 s−1
,
and it could be easily separated from the biphasic solution and
remained high reactivity even after 12 recycles. The development of
more diverse soluble Azo-POPs with this strategy and more
applications of soluble Azo-POPs such as three-phase catalysis32 are
undergoing in our lab, which will be reported in due course.
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Conflicts of Interest
There are no conflicts of interest to declare.
Acknowledgements
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The authors thank the National Natural Science Foundation of China
(No. 21772013), Natural Science Foundation of Beijing Municipality
(No. 2162039) and the Fundamental Research Funds for the Central
Universities for generous support.
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