In situ production of silver nanoparticles on an aldehyde-equipped conjugated porous polymer and subsequent heterogeneous reduction of aromatic nitro groups at room temperature

Article abstract of DOI:10.1039/c5cc04476f

In a metal-free procedure, chelating thiol groups and an electrophile react to assemble a robust, conjugated porous polymer with pendant aldehyde functionalities. These groups are able to reduce Ag(I) ions to generate, in situ, Ag(0) nanoparticles evenly dispersed in the polymer matrix. The Ag(0)-polymer composite enables selective reduction of aromatic nitro compounds as a heterogeneous catalyst, and can be conveniently recycled multiple times.

Full text of DOI:10.1039/c5cc04476f

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In situ production of silver nanoparticles on an
aldehyde-equipped conjugated porous polymer
and subsequent heterogeneous reduction of
aromatic nitro groups at room temperature†
Cite this: Chem. Commun., 2015,
51, 12197
Received 1st June 2015,
Accepted 24th June 2015
Jie Liu,a Jieshun Cui,a Filipe Vilela,*b Jun He,c Matthias Zeller,d Allen D. Hunterd
DOI: 10.1039/c5cc04476f
and Zhengtao Xu*a
www.rsc.org/chemcomm
In a metal-free procedure, chelating thiol groups and an electro-
CPPs loaded with metal nanoparticles have also proven to be
phile react to assemble a robust, conjugated porous polymer with an efficient method in the development of novel heterogeneous
pendant aldehyde functionalities. These groups are able to reduce photocatalysts. A conjugated microporous polymer was reported
Ag(I) ions to generate, in situ, Ag(0) nanoparticles evenly dispersed to photocatalyse Suzuki–Miyaura cross-coupling reactions when
in the polymer matrix. The Ag(0)–polymer composite enables loaded with Pd nanoparticles.6 Li et al. also reported that
selective reduction of aromatic nitro compounds as a heteroge- mesoporous g-C3N4, can be used as photoactive anchoring
neous catalyst, and can be conveniently recycled multiple times.
support for Pd nanoparticles to perform the same chemical
transformations.7 These systems rely on the mixing of the CPP
Anthropogenic climate change as a result of our current energy with a solution of nanoparticles leading to a cumbersome and
demands is of great concern in today’s society. The need for wasteful process. By comparison, and to the best of our knowl-
new sources of clean and renewable energy alongside improved edge, there have been very few reports on nanoparticle synthesis
sustainable chemical processes has been the driving force in situ directly within a CPP network.8
behind the development of new materials. Conjugated porous
Herein, we report a novel aldehyde-equipped porous poly-
polymers, (CPPs) as a growing class of advanced organic mer that can be easily synthesized in the absence of any metal
materials have received much attention due to their facile catalyst, and can then be further post-modified to incorporate
synthesis, low cost, and flexible chemical functionality. As the Ag nanoparticles (Ag NPs) via the use of Tollens’ reagent. Our
synthetic principles and structural studies continue to be strategy allows for in situ formation of the Ag nanoparticles
extensively investigated, emphasis is being shifted to the pro- leading to a stable hybrid material that can be employed as a
perties and applications of these systems as novel organic heterogeneous catalyst for the reduction of aromatic nitro com-
zeolites. They have shown excellent performance in areas such pounds at room temperature. This application is particularly
as gas storage1 and separation,1d by virtue of their high surface relevant to the pharmaceutical and agrochemical industries
areas and porosity; metal-binding with high selectivities;2 and since synthesis of anilines or amines from the corresponding
heterogeneous catalysis3 including photocatalysis.4 Further- nitro compounds lead to versatile intermediates and precursors
more, the combination of metal-binding ability with hetero- in the preparation of dyes, pharmaceuticals, pigments, agro-
geneous catalysis has the potential to develop materials with chemicals and polymers.9
enhanced catalytic activity. Although underexplored, there are a
few examples of this strategy.5
CPPs are generally synthesized via metal catalyzed coupling
reactions. Typically these include Suzuki–Miyaura,10 Sonogashira–
Hagihara,11 Yamamoto and Glaser polycondensations.12 Other
strategies such as oxidative polymerizations also rely on metals
(typically FeCl3).13 Recently we reported a metal-free polymeriza-
tion method simply by mixing two monomers in the presence
of a mild base N,N0-diisopropylethylamine (DIPEA) that promoted
C–N bond formation to produce a porous polymer framework in
high yield.2
With regards to the synthesis of an aldehyde-equipped CPP,
DIPEA was again employed to promote C–S bond formation
via the use of a rigid monomer containing multiple chelating
sites, triphenylene-2,3,6,7,10,11-hexathiol (HTT) (Scheme 1).
a Department of Biology and Chemistry, City University of Hong Kong,
83 Tat Chee Avenue, Kowloon, Hong Kong, China. E-mail: zhengtao@cityu.edu.hk
b School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh,
EH14 4AS, UK. E-mail: f.vilela@hw.ac.uk
c School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, Guangzhou, China
d Youngstown State University, One University Plaza, Youngstown, Ohio 44555, USA
† Electronic supplementary information (ESI) available: Experimental proce-
dures; network synthesis; elemental analysis, N2 and CO2 sorption, TGA, IR,
NMR, and PXRD data. CCDC 1401125. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c5cc04476f
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Scheme 1 Synthesis of the aldehyde-equipped CPP PHTT_CHO. Conditions:
dimethylacetamide (DMA), DIPEA, N2, 90 1C, 3 hours, then 140 1C, 12 hours.
Fig. 1 N2 sorption isotherms at 77 K for the activated solid samples of
PHTT_CHO (A) and PHTT_Ag (B).
This methodology offers great advantages: the most important
being the absence of any metal catalysts. In general, thiols can
easily undergo aromatic nucleophilic substitution without
deactivating the substrate, thus facilitating the formation of the
polymer framework. Utilizing this efficient aromatic nucleophilic
substitution of thiol molecules we chose 2,3,5,6-tetrafluoro-
terephthaldehyde (TFTA), as a linear bridge to connect the HTT
moieties into a 3D-connected CPP, with TFTA being the source of
aldehydes.
The assembly of the polymer simply involves heating a
mixture of DMA, TFTA, HTT (which was freshly prepared by
the deprotection of HBuTT;14 see ESI†), together with the mild
base DIPEA (see Scheme S1, ESI†) to produce a dark red solid in
high yield (96.8%). To further demonstrate the simplicity of this
polymerization, a model reaction between 1,2-benzenedithiol
and TFTA is conducted under similar reaction conditions with
high yield (88%). A single crystal structure is presented for this
molecular system (Fig. S9, ESI†).
oxidation to form carboxyl groups, providing a convenient entry
into diverse ranges of functional derivatives. An interesting
post-modification strategy, known as the silver mirror reaction,
relies on Tollens’ reagent, which is generally used to qualita-
tively detect the presence of aldehydes.
Upon stirring with a freshly prepared Tollens’ reagent (see
ESI†) at 50 1C for 2 hours, the PHTT_CHO powder turned from
dark red to black to afford the PHTT_Ag sample (Scheme 2). The
strong PXRD peaks (Fig. S10, ESI†) of the silver loaded polymer
sample indicate that the Ag NPs are evenly dispersed on the polymer
support. The average particle size of Ag NPs is about 10 nm in
diameter calculated based on the X-ray diffraction patterns.16
Transmission electron microscopy (TEM) measurements were con-
ducted on both PHTT_CHO and PHTT_Ag samples. Ag NPs can be
readily seen in Ag NPs loaded sample (PHTT_Ag, Fig. 2B) compared
to the original dark red PHTT_CHO polymer (Fig. 2A). Scanning
electron microscope (SEM) (Fig. 2C) together with the corres-
ponding elemental map of Ag atoms (Fig. 2D) obtained by energy
dispersive X-ray (EDX) technique on PHTT_Ag, supported the notion
of evenly distributed Ag throughout the polymer matrix. The
amount of Ag NPs was quantified by ICP elemental analysis to be
33.5 wt%. As is consistent with previous studies, where Ag NPs and
their aggregates was shown to block gas access to the pores,2
the porosity of activated PHTT_Ag is markedly suppressed, with
N2 sorption revealing a decrease in surface area to 126 m2 gꢀ1
(686 m2 gꢀ1 before Ag NPs generation; Fig. 1). Additionally, the
PHTT_Ag powder is more conductive (6.3 ꢁ 10ꢀ5 S mꢀ1; two-
probe measurement) than PHTT_CHO (4.9 ꢁ 10ꢀ6 S mꢀ1); the
enhanced conductivity can be partly attributed to the formation
of the Ag NPs, even though the polymer backbone also under-
goes changes in electronic structure/properties with the pendant
aldehyde groups oxidized into carboxylate units. Efforts to remove
Elemental analysis of PHTT_CHO is consistent with the
calculated values for C, H, N (see ESI†). Thermogravimetric
analysis (TGA, Fig. S1, ESI†) reveals a stable weight region up to
300 1C, indicative of the non-volatile, and polymeric nature
of the product. As expected, the peak at 1681 cmꢀ1 in the IR
(Fig. S2, ESI†) confirms the presence of the aldehyde moiety. In
addition, solid state 13C NMR of a PHTT_CHO sample confirms
the polymer structure as shown in Scheme 1. As seen in Fig. S3
(ESI†), the aromatic region of the solid state 13C NMR spectrum
of PHTT_CHO is dominated by 3 distinct peaks (135.8, 129.2
and 123.8 ppm) corresponding to the aromatic carbon signals
of the polymer backbone, the peak at 188.8 ppm is assigned to
the aldehyde moieties in the PHTT_CHO framework. Signifi-
cant porous character of the activated sample of PHTT_CHO
was revealed in both N2 sorption (at 77 K) and CO2 sorption
(at 273 K, see Fig. S4, ESI†) experiments. A typical type-I N2 gas
adsorption isotherm (Fig. 1A) was observed, with a Brunauer–
Emmett–Teller (BET) surface area of 686 m2
g
ꢀ1. QSDFT
analysis on pore size distribution and pore volume indicated an
average pore width of 1.03 nm and a pore volume of 0.374 cm3 gꢀ1
(Fig. S6, ESI†).
Aldehyde functionalized CPPs are rare.15 The advantages of
affixing the aldehyde functionality (e.g., –CHO) onto a CPP are
obvious, since this moiety offers great versatility for facile and
efficient post-modification reactions even in heterogeneous
conditions. Aldehydes can readily undergo both reductions
Scheme 2 The preparation of PHTT_Ag from PHTT_CHO with the in situ
(e.g., to give alcohol –CH2OH or halide CH2X functions) and production of silver nanoparticles.
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Table 1 Efficiencies of the PHTT_Ag catalyst in the reduction of various
nitroaromatics into aminesa
Time
(h)
Conversionb
(%)
Entry
1
Substrate
Product
6
6
96
80
2
Fig. 2 TEM images of (A) PHTT_CHO, (B) PHTT_Ag, (C) SEM image of
PHTT_Ag, and (D) the corresponding EDX mapping for Ag atom (purple
indicates presence of Ag).
3
4
6
6
99
99
the Ag NPs from the polymer matrix are ongoing, so as to
facilitate the characterization of such changes on the polymer
backbone.
5
6
12
6
84
97
Nanoparticles of noble metals such as silver are very impor-
tant due to their applications in catalysis.17 Polymeric materials
that support silver nanoparticles whilst retaining catalytic
activity present distinct advantages over homogeneous systems,
with regards ease of separation, recovery and reusability.
The catalytic activity of the PHTT_Ag polymer was examined
under especially mild conditions for the reduction of aromatic
nitro groups, by simply stirring the substrates with NaBH4
(an inexpensive, bench stable, stoichiometric reductant) and
the polymer (Ag/substrate molar ratio: about 5%) in ethanol at
room temperature and under air (see details in ESI†). We tested
the feasibility of PHTT_Ag as a catalyst for the reduction
of various nitroarenes, keeping all other reaction conditions
unaltered. As shown in Table 1, the catalyst PHTT_Ag was
found to be very effective toward the reduction of nitro com-
pounds to their corresponding anilines, regardless of the types
and position of the substituents. Two control experiments were
7
12
12
93
96
8
Cycle1
9
Cycle2c
12
12
95
96
10
Cycle3c
a
Reaction conditions: substrate (0.084 mmol), NaBH4 (0.672 mmol)
and PHTT_Ag (1.4 mg, Ag/substrate molar ratio: 5%) in 0.5 mL ethanol
performed: (i) in the absence of PHTT_Ag there was no conver- at RT. Conversion is determined via 1H NMR using 1,2,4,5-
sion of nitroaromatic compounds into amines; (ii) a freshly
b
c
tetramethylbenzene as the internal standard. See ESI for the cycling
procedure.
prepared silver mirror with benzaldehyde showed reduced
activity in the reduction of nitrobenzene when compared to
PHTT_Ag using similar conditions (followed by TLC). In parti- that PHTT_Ag does not leach Ag NPs under the conditions
cular, this reaction also showed high efficiency for the selective tested. This was also confirmed via ICP measurements where
reduction of nitro groups in the presence of other reducible the amount of Ag within PHTT_Ag was found to be 33.4 wt%
functional groups (e.g. alkenes, entry 6–8). The recyclability of after the catalytic reactions, very close to the 33.5 wt% measured
the PHTT_Ag solid was also investigated.
for PHTT_Ag prior to catalysis. Further cycling of the PHTT_Ag
After the hydrogenation reaction, the catalyst can be con- also showed no loss in activity (followed via TLC). However, PXRD
veniently recovered from the reaction mixture by centrifugation experiments (Fig. S15, ESI†) at the 5th catalytic cycle started to
and subsequent washing with water and absolute ethanol. The reveal peaks of AgCl, which could have arisen from the Ag
reusability of the hybrid material was then tested after recovery. nanoparticles reacting with the residual NaCl trapped within
We observed no activity loss for the conversion of 1-allyloxy-3- the porous CPP during the earlier steps of synthesis (Scheme S1,
nitrobenzene into 3-allyloxyaniline (entry 8 in Table 1). In fact, ESI†). Apart from the ease of recovery generally associated
the conversion remained almost the same after 3 catalytic with polymer supports, catalyst recycling in the present case
cycles, Fig. S13 (ESI†). Furthermore, the isolated supernatant can also be attributed to the anionic form of the carboxylate
exhibited no catalytic activity under similar reaction conditions units in the PHTT_Ag composite (as a result of the alkaline
(as verified by the NMR data in Fig. S14, ESI†), which indicates conditions of aldehyde oxidation) – i.e., the carboxylate
that Ag NPs were not present in the reaction mixture proving anion minimizes the acid–base interaction with the (alkaline)
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4 (a) H. Urakami, K. Zhang and F. Vilela, Chem. Commun., 2013,
49, 2353; (b) K. Zhang, D. Kopetzki, P. H. Seeberger, M. Antonietti
and F. Vilela, Angew. Chem., Int. Ed., 2013, 52, 1432; (c) K. Zhang,
Z. Vobecka, K. Tauer, M. Antonietti and F. Vilela, Chem. Commun.,
2013, 49, 11158; (d) D. Zheng, C. Pang, Y. Liu and X. Wang, Chem.
Commun., 2015, 51, 9706; (e) J. Zhang, M. Zhang, C. Yang and
X. Wang, Adv. Mater., 2014, 26, 4121; ( f ) G. Zhang, M. Zhang, X. Ye,
X. Qiu, S. Lin and X. Wang, Adv. Mater., 2014, 26, 805; (g) Y. Cui,
Z. Ding, X. Fu and X. Wang, Angew. Chem., Int. Ed., 2012, 51, 11814;
(h) Z. Lin and X. Wang, Angew. Chem., Int. Ed., 2013, 52, 1735;
(i) X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M.
Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76;
( j) J. Sun, J. Zhang, M. Zhang, M. Antonietti, X. Fu and X. Wang,
Nat. Commun., 2012, 1139.
5 (a) R. Palkovits, M. Antonietti, P. Kuhn, A. Thomas and F. Schu¨th,
Angew. Chem., Int. Ed., 2009, 48, 6909; (b) P. M. Budd, B. Ghanem,
K. Msayib, N. B. McKeown and C. Tattershall, J. Mater. Chem., 2003,
13, 2721.
6 Z. J. Wang, S. Ghasimi, K. Landfester and K. A. I. Zhang, Chem.
Mater., 2015, 27, 1921.
7 (a) X.-H. Li, M. Baar, S. Blechert and M. Antonietti, Sci. Rep., 2013,
3, 1743; (b) X.-H. Li, X. Wang and M. Antonietti, Chem. Sci., 2012, 3, 2170.
8 S. Rifai, C. A. Breen, D. J. Solis and T. M. Swager, Chem. Mater., 2006,
18, 21.
9 (a) H.-U. Blaser, H. Steiner and M. Studer, ChemCatChem, 2009,
1, 210; (b) J. P. Adams and J. R. Paterson, J. Chem. Soc., Perkin
Trans. 1, 2000, 3695.
10 L. Chen, Y. Honsho, S. Seki and D. Jiang, J. Am. Chem. Soc., 2010,
132, 6742.
11 (a) J.-X. Jiang, F. Su, A. Trewin, C. D. Wood, H. Niu, J. T. A. Jones,
Y. Z. Khimyak and A. I. Cooper, J. Am. Chem. Soc., 2008, 130, 7710;
(b) J.-X. Jiang, F. Su, A. Trewin, C. D. Wood, N. L. Campbell, H. Niu,
C. Dickinson, A. Y. Ganin, M. J. Rosseinsky, Y. Z. Khimyak and
A. I. Cooper, Angew. Chem., Int. Ed., 2007, 46, 8574.
amine products, and therefore facilitates their release from the
polymer surface.
To sum up, several major advantages of stability and func-
tionality have been demonstrated for this aldehyde-equipped
CPP system that is conveniently produced using a metal-free
strategy under mild reaction conditions. The stability is high-
lighted by the aqueous, alkaline and oxidative conditions in the
silver mirror reaction procedure, conditions that would readily
destroy MOFs, COFs and other less robust porous polymers.
The Ag nanoparticles produced in situ were found to be evenly
blended with the polymer matrix, offering highly efficient
heterogeneous catalysis for the reduction of aromatic nitro
compounds at room temperature. The selectivity and reusabi-
lity of the composite catalyst also further illustrate the in situ
generation of metal particles as a potentially wide-scope, effec-
tive method for enhancing the materials properties of polymer/
metal nanoparticle hybrids.
This work was supported by City University of Hong Kong
(Project 9667092), the Research Grants Council of HKSAR (GRF
Project 103413) and the National Natural Science Foundation of
China (21471037, 21201042). The X-ray diffractometer at YSU
was funded by NSF Grant 1337296. We thank Mr Yan Lung
Wong for drawing Fig. S9 (ESI†).
Notes and references
1 (a) F. Su, C. L. Bray, B. Tan and A. I. Cooper, Adv. Mater., 2008, 12 (a) J. Schmidt, M. Werner and A. Thomas, Macromolecules, 2009,
20, 2663; (b) J.-Y. Lee, C. D. Wood, D. Bradshaw, M. J. Rosseinsky
and A. I. Cooper, Chem. Commun., 2006, 2670; (c) C. D. Wood, B. Tan,
42, 4426; (b) J.-X. Jiang, F. Su, H. Niu, C. D. Wood, N. L. Campbell,
Y. Z. Khimyak and A. I. Cooper, Chem. Commun., 2008, 486.
A. Trewin, H. Niu, D. Bradshaw, M. J. Rosseinsky, Y. Z. Khimyak, 13 J. Schmidt, J. Weber, J. D. Epping, M. Antonietti and A. Thomas,
¨
N. L. Campbell, R. Kirk, E. Stockel and A. I. Cooper, Chem. Mater., 2007,
Adv. Mater., 2009, 21, 702.
19, 2034; (d) N. B. McKeown, P. M. Budd, K. J. Msayib, B. S. Ghanem, 14 J. Cui and Z. Xu, Chem. Commun., 2014, 50, 3986.
H. J. Kingston, C. E. Tattershall, S. Makhseed, K. J. Reynolds and 15 V. Guillerm, L. J. Weselinski, M. Alkordi, M. I. H. Mohideen,
D. Fritsch, Chem. – Eur. J., 2005, 11, 2610; (e) C. D. Wood, B. Tan,
A. Trewin, F. Su, M. J. Rosseinsky, D. Bradshaw, Y. Sun, L. Zhou and
Y. Belmabkhout, A. J. Cairns and M. Eddaoudi, Chem. Commun.,
2014, 50, 1937.
A. I. Cooper, Adv. Mater., 2008, 20, 1916; ( f ) J. Germain, F. Svec and 16 H. Borchert, E. V. Shevchenko, A. Robert, I. Mekis, A. Kornowski,
´
J. M. J. Frechet, Chem. Mater., 2008, 20, 7069.
2 J. Liu, K.-K. Yee, K. K.-W. Lo, K. Y. Zhang, W.-P. To, C.-M. Che and 17 (a) H. Cong, C. F. Becker, S. J. Elliott, M. W. Grinstaff and J. A. Porco,
G. Gru¨bel and H. Weller, Langmuir, 2005, 21, 1931.
Z. Xu, J. Am. Chem. Soc., 2014, 136, 2818.
3 F. Goettmann, A. Fischer, M. Antonietti and A. Thomas, Angew.
Chem., Int. Ed., 2006, 45, 4467.
J. Am. Chem. Soc., 2010, 132, 7514; (b) N. Pradhan, A. Pal and T. Pal,
Colloids Surf., A, 2002, 196, 247; (c) B. Baruah, G. J. Gabriel, M. J.
Akbashev and M. E. Booher, Langmuir, 2013, 29, 4225.
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