Highly stable covalent organic framework-Au nanoparticles hybrids for enhanced activity for nitrophenol reduction

Article abstract of DOI:10.1039/c3cc49176e

Gold [Au(0)] nanoparticles immobilized into a stable covalent organic framework (COF) have been synthesized via the solution infiltration method. The as-synthesized Au(0)@TpPa-1 catalyst shows high recyclability and superior reactivity for nitrophenol red

Full text of DOI:10.1039/c3cc49176e

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Highly stable covalent organic framework–Au
nanoparticles hybrids for enhanced activity
for nitrophenol reduction†
Cite this: Chem. Commun., 2014,
50, 3169
Received 3rd December 2013,
Accepted 6th January 2014
Pradip Pachfule,a Sharath Kandambeth,a David Dıaz Dıazbc and Rahul Banerjee*a
´
´
DOI: 10.1039/c3cc49176e
www.rsc.org/chemcomm
Gold [Au(0)] nanoparticles immobilized into a stable covalent organic
Gold (Au) nanoparticles decorated on suitable supports hold
framework (COF) have been synthesized via the solution infiltration unique advantages as heterogeneous catalysts under mild conditions,
method. The as-synthesized Au(0)@TpPa-1 catalyst shows high even at ambient temperature with high reaction rates.7 Although
recyclability and superior reactivity for nitrophenol reduction reac- there are reports of host supported Au catalysts, which showed high
tion than HAuCl4Á3H2O.
catalytic activity for oxidations, hydrogenation, cyclization, rearrange-
ment, C–C coupling reactions, etc., the unstable supports holding
Covalent organic frameworks (COFs) are lightweight and porous these nanoparticles bring limitations to their uses.8 Most impor-
materials well known for their applications in gas storage, cata- tantly, the weak interactions between supports and nanoparticles
lytic supports, semiconductive and photoconductive devices.1 In usually result in the sintering and leaching of the nanoparticles.
addition to other porous materials, viz. charcoal, dendrimers, In these regards, Au catalysts supported on carbon, CeO2, Al2O3,
polymers, mesoporous silica, zeolites, MOFs, etc., metal doped Fe2O3, etc., have been used as catalysts for the hydrogenation of
COFs have also been practised as catalysts for organic reactions.2 environmental pollutant 4-nitrophenol (4-NPh), which is anthro-
Although the well ordered periodicity and high surface area of pogenic, toxic and inhibitory in nature, to yield industrially important
COFs facilitated the usage of these materials as catalyst supports, anilines like 4-aminophenol.2a,8c Since 4-aminophenol (4-APh) is
the stability of host COF supports in aqueous–acidic–alkaline important due to its usage as a developer in black and white films
reaction media still remains a crucial aspect for using the same and as an intermediate for the synthesis of drug paracetamol, for its
catalyst over a number of cycles.3 Despite the limited surface area synthesis a catalytic system with high associated product yield, selec-
of COFs, nanoparticles@COFs have been used as catalyst for tivity and recyclability is desirable. In order to simplify the modus
Suzuki coupling, C–H activation, Knoevenagel condensation, nitro operandi, herein for the first time, we report a simple synthetic route
reduction, glycerol oxidation, etc.4 Since the COF based supports to acquire a highly stable, porous and crystalline COF (TpPa-1)
used to decorate the nanoparticles are unstable in aqueous and supported Au nanoparticles hybrids via the solution infiltration
most of the organic solvents, tedious synthetic protocols are method.9 Herein, along with the synthesis of the robust and stable
necessary for the synthesis as well as usage of these catalysts in Au(0)@TpPa-1 catalyst, the catalytic activities of synthesized catalyst
the aforementioned reactions.5 Moreover, the concerns regarding towards 4-nitrophenol (4-NPh) reduction are illustrated.
sintering, leaching, stability and recyclability of supported nano-
As shown in Scheme 1, the Au(0)@TpPa-1 catalyst was synthesized
particles in diverse reaction conditions remain at the forefront.6 by mixing HAuCl4Á3H2O (1.5 mg, 0.004 mmol) with evacuated TpPa-1
Thus, in order to overcome these issues regarding the optimal (50 mg) in 5 ml methanol under vigorous stirring.9 From the obtained
interaction, we believe that the synthesis of new materials, having mixture, solvent was removed by evacuation and subsequently 3 ml
strong interactions between support and loaded nanoparticles methanol was added. To this solution, 2 ml of 0.5 mmol (20 mg)
with high stability in aqueous–acidic–alkaline media, is desired.
NaBH4 solution in methanol was added wile stirring, and the
obtained mixture was dried under vacuum after washing to obtain
the Au(0)@TpPa-1 catalyst (see the ESI,† Section S3). The consequen-
tial feature of the reported Au(0)@TpPa-1 catalyst having high stability
in aqueous and common organic solvents is associated with the
proper selection of the support chosen as a stable COF (TpPa-1)
having a well ordered porous architecture enriched with oxygen and
nitrogen containing a crystalline framework, which provides extra
strength to the nanoparticles. Here, the stability of synthesized
a Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune 411008, India. E-mail: r.banerjee@ncl.res.in;
Tel: +91 2025902535
b
¨
¨
¨
Institut fur Organische Chemie, Universitat Regensburg, Universitatsstr. 31,
93053 Regensburg, Germany
c IQAC-CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain
† Electronic supplementary information (ESI) available: Experimental procedures,
IR, gas adsorption and additional supporting data. See DOI: 10.1039/c3cc49176e
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composition within the COF framework remained intact (Fig. 1b).
After incorporation of the Au(0) nanoparticles inside the TpPa-1
template, the appearance of strong representative peaks for CQC
(B1578 cmÀ1) and the C–N (B1258 cmÀ1) stretching of TpPa-1,
confirmed the metal loading without disturbing the basic TpPa-1
architecture. The complete reduction of Au(III) to Au(0) in presence of
excess NaBH4 was confirmed from the X-ray photoelectron spectro-
scopy (XPS) analysis (see the ESI,† Fig. S9). The extent of Au nano-
particle loading on TpPa-1 was further confirmed by the decreased N2
adsorption and BET surface area of Au(0)@TpPa-1 (339 m2 gÀ1) in
comparison with the as synthesized TpPa-1 (484 m2 gÀ1). As the pore
surface (partially or fully) and interlayer spacings in the host COF
framework are occupied by finely dispersed Au(0) nanoparticles
located at the surface as well as inside the COF matrix, the observed
decrease in BET surface area and pore size is justified (see the ESI,†
Fig. S7 and S8). From the EDAX, ICP and TGA analysis performed for
Au(0)@TpPa-1, the loading of B1.2 wt% Au nanoparticles on TpPa-1
was confirmed (see the ESI,† Fig. S6 and S19).
Scheme 1 Synthesis of the Au(0)@TpPa-1 catalyst using the solution
infiltration method for nitrophenol reduction reaction. Inset image: the optical
images of the color change observed for the conversion of 4-nitrophenol to
4-aminophenol after the addition of Au(0)@TpPa-1.
As the morphological factors of the supported nanoparticles
contribute to the actual reactivity and recyclability of the catalysts,
we have performed SEM and TEM analyses on the synthesized
catalyst.10 The flower like morphology of the pristine TpPa-1 was
catalyst was assumed to be optimal since the TpPa-1 is stable in observed to be maintained to some extent even after the loading of
aqueous, acidic as well as alkaline media. Au(0) nanoparticles (see the ESI,† Fig. S2 and S4). The micrometer
The well maintained PXRD patterns of Au(0)@TpPa-1 with addi- sized petals of TpPa-1 observed in the SEM analysis were found to
tional peaks for (111) and (200) planes appearing at B391 (2y) and be the aggregation of the few sheet-like structures from the TEM
B441 (2y) compared to TpPa-1, demonstrated the successful loading analysis, which serves as host for the doped nanoparticles (Fig. 1d
of Au nanoparticles with retention of the COF integrity and minimal of the ESI,† Fig. S3). The loading of finely distributed 5 Æ 3 nm
loss of crystallinity, as shown in Fig. 1a. The presence of all the charac- sized Au(0) nanoparticles was clearly visible in the TEM analyses
teristics peaks of TpPa-1 in the FT-IR spectrum of Au(0)@TpPa-1 performed for Au(0)@TpPa-1 (Fig. 1c), in concurrence with the
confirmed that, even after strong treatment with NaBH4, the chemical particle size calculated from the observed PXRD patterns using the
Scherrer equation. The 3D loading of these nanoparticles through-
out the matrix of TpPa-1 was clearly perceptible from the dark field
TEM imaging of the Au(0)@TpPa-1 (Fig. S5, ESI†). The interplanar
d-spacing (0.231 nm) in the lattice fringes indicates the formation
of Au(0) nanoclusters oriented in the (111) plane (Fig. 1d; inset i).
The SAED pattern reveals the growth of nanocrystals with (200) and
(220) orientations, with the presence of nanocrystalline texture in
agreement with the observed XRD patterns (Fig. 1d; inset ii).
Since we have observed a uniform loading of 5 Æ 3 nm sized Au
nanoparticles on the TpPa-1 matrix with a fine dispersion, we
evaluated the kinetics as well as energetic parameters of the well
known nitrophenol reduction reaction using the Au(0)@TpPa-1
catalyst. In order to confirm the heterogeneity of the catalyst, we
carried out a standard leaching experiment using the Au(0)@TpPa-1
catalyst, which confirmed the stable loading of Au nanoparticles on
TpPa-1 in water as well as common organic solvents (see the ESI,†
Fig. S16). The strong interaction of Au nanoparticles with TpPa-1
proves that the heterogeneity of the catalyst remains intact in
standard catalytic conditions. The as-synthesised Au(0)@TpPa-1
Fig. 1 (a) PXRD profile of Au(0)@TpPa-1. Matching PXRD plots of TpPa-1, were tested for the catalytic reduction of 4-NPh in the presence of
Au(0)@TpPa-1 and Au(0)@TpPa-1 after 3rd and 5th catalytic cycle.
(b) Comparison of the FT-IR spectra of Au(0)@TpPa-1 with TpPa-1, catalyst
recovered after 3rd and 6th run. (c) Low resolution TEM image of
Au(0)@TpPa-1. Inset image: nanoparticles size distribution histogram.
excess NaBH4 in water as the solvent. The reduction kinetics were
monitored by UV-vis absorption spectroscopy of the reaction
mixture after the addition of the catalyst. As the reaction proceed,
the absorbance of 4-NPh at B400 nm started decreasing along
with a related increase in the B300 nm peak corresponding to
(d) High resolution TEM image of Au(0)@TpPa-1. Inset images: (i) single
Au nanoparticle showing d-spacing, (ii) the corresponding SAED pattern.
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Fig. 2 (a) Typical time-dependent evolution of UV-vis spectra showing the catalytic reduction of 4-NPh to 4-APh by Au(0)@TpPa-1. (b) Kinetics of the
reduction reaction of 4-NPh to 4-APh. Inset image: plots of the ln[4-NPh] absorbance of 4-nitrophenol at 400 nm obtained from (a) versus time for the
reduction of 4-nitrophenol catalyzed by Au(0)@TpPa-1, HAuCl4Á3H2O and TpPa-1. (c) Conversion of 4-NPh during 6 cycles of reaction by the Au@TpPa-1
catalyst. Inset image: morphological changes observed in the Au(0)@TpPa-1 catalyst with an increasing number of cycles traced by optical imaging and SEM.
4-aminophenol (4-APh). The reaction did not occur when using further confirms the stability of the prepared catalysts. Hence, it can
only TpPa-1 as the catalyst and proceeded comparatively slower be confirmed that the prominent catalytic activity of Au(0)@TpPa-1
with HAuCl4Á3H2O (conversion time: 20 min) and Au(0)@TpPa-1 might be assigned to the highly stable, two dimensional support
(2.20 wt%, conversion time: 18 min) as the catalysts (Section S4, (TpPa-1) which holds the loaded nanoparticles to a high extent.
ESI†). When 1.2 wt% Au loaded Au(0)@TpPa-1 catalyst was used as
In summary, for the first time, we have synthesized a COF-
the catalyst, a significant catalytic activity was observed and con- supported highly stable Au(0) based catalyst via solution infiltra-
version of 4-NPh to 4-APh was completed within 13 min. The tion methods, which shows a high activity towards nitrophenol
probable reason of the momentous activity shown by Au(0)@TpPa-1 reduction reaction. The synthesized Au(0)@TpPa-1 catalyst shows
(1.20 wt%) over Au(0)@TpPa-1 (2.20 wt%) may be the fine distribu- superior reactivity for nitrophenol reduction reaction than
tion of very tiny nanoparticles (5 Æ 3 nm) on the TpPa-1 matrix, HAuCl4Á3H2O, imparting the advantages of heterogeneous cata-
which lead to a very large surface area of the nanoparticles and lysts. The phenomenon of maintaining crystallinity over a number
high particle number per unit mass for the catalyst. The increased of cycles is very rare in the literature and herein we have demon-
fraction of the atoms at the surface in Au(0)@TpPa-1 leads to a strated the usefulness of stable and crystalline supports for an
significantly higher catalytic activity.
In view of the fact that the concentration of BH4 added in the unchanged reactivity for more than 6 cycles shown by the reported
system is in excess compared to the concentration of 4-NPh, it is catalyst is promising towards the heterogenization of Au catalysts
improved catalytic activity. The overall recyclability with almost
À
À
assumed that the concentration of BH4 remains constant during for commercially important transformation reactions.
the reaction. Under this circumstance, pseudo-first-order kinetics
RB and PP acknowledges CSIR (CSC0102 and CSC0122) for
have been used to evaluate the kinetic reaction rate of the catalytic funding.
reaction (Fig. 2b, inset). Herein, a linear correlation of ln[4-NPh]
versus time at any instant is obtained. Among all the tested catalysts
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