Poly(amic acid) salt-stabilized silver nanoparticles as efficient and recyclable quasi-homogeneous catalysts for the aqueous hydration of nitriles to amides

Article abstract of DOI:10.1039/c5nj02497h

Water-soluble silver nanoparticles stabilized by poly(amic acid) salt (Ag-PAAS) were synthesized by a one-pot method and were characterized by using UV-vis absorption, transmission electron microscopy, powder X-ray diffraction, and ζ potential measurements. The Ag-PAAS catalyzed selective hydration of nitriles to amides in water allowed for not only highly efficient quasi-homogeneous catalytic reactions under mild conditions, but also an easy recovery and reuse of the catalyst attributed to the pH response of the PAAS.

Full text of DOI:10.1039/c5nj02497h

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Poly(amic acid) salt-stabilized silver nanoparticles
as efficient and recyclable quasi-homogeneous
catalysts for the aqueous hydration of nitriles to
amides†
Cite this:DOI: 10.1039/c5nj02497h
Jun Li, Guannan Tang, Yuchen Wang, Ya Wang, Zixi Li and Hengfeng Li*
Water-soluble silver nanoparticles stabilized by poly(amic acid) salt (Ag–PAAS) were synthesized by a
one-pot method and were characterized by using UV-vis absorption, transmission electron microscopy,
powder X-ray diffraction, and z potential measurements. The Ag–PAAS catalyzed selective hydration of
nitriles to amides in water allowed for not only highly efficient quasi-homogeneous catalytic reactions
under mild conditions, but also an easy recovery and reuse of the catalyst attributed to the pH response
of the PAAS.
Received (in Montpellier, France)
16th September 2015,
Accepted 23rd October 2015
DOI: 10.1039/c5nj02497h
www.rsc.org/njc
and have exhibited noticeable catalytic activity in the hydration
of nitriles.18–22
1 Introduction
The significance of the hydration of nitriles to the corresponding
Owing to their relatively low cost and easy synthesis, Ag
amides under ecologically friendly conditions is undoubted in nanoparticles (Ag NPs) have been widely used as efficient cata-
our current social environment.1 Nitrile hydration reactions are lysts for the selective oxidation of olefins and in the reduction of
conventionally catalysed by concentrated acids or bases, which many organic dyes and nitroaromatic compounds.13,23–26 Since
usually cause over-hydrolysis of amides to undesirable carboxylic the first report on the hydration of nitriles to amides catalyzed
acids and the formation of a massive amount of salts after the by Ag NPs was published by Mitsudome et al.,2 Ag NPs have
neutralization of the catalysts.1,2 Metal-mediated hydration of attracted more and more attention in the catalytic hydration of
nitriles is a more promising alternative, especially owing to its nitriles. Unfortunately, most of the reported Ag-based catalysts
better selectivity toward amides.2,3 A number of homogeneous required high reaction temperature (4140 1C) and even an inert
transition-metal complexes have been reported for activating the reaction atmosphere to function.2,11,13,14,27–29 Sherbow et al.
hydration of nitriles.4–10 However, they usually suffered from found that quasi-homogeneous Ag NPs stabilized by the 1,3,5-
various drawbacks, especially difficulties in the recovery and triaza-7-phosphaadamantane were active for the hydration of
reuse of the catalyst, as well as the high cost of ligands and the nitriles at 90 1C, but the extremely low Ag loading and long
special handling of sensitive complexes.4,5,7,9,10 In contrast, reaction time (hundreds of hours) severely restrict their further
heterogeneous transition-metal nanocatalysts display many development.27 Ag NP-grafted imine network materials showed
advantages such as high activity, and easy handling and high performance for the hydration of nitriles at 90–100 1C,
recovery.2,11–14 To date, many heterogeneous catalytic systems though the materials preparation was somewhat tedious and
have been found to be effective in the selective hydration of time-consuming.30,31
nitriles; however, they mainly comprised expensive noble metals
Nowadays, a novel protocol based on ‘‘smart’’ nanocatalysts has
(Ru, Pd, Au, etc.).12,15–18 From an economic point of view, been receiving significant attention, where quasi-homogeneous
developing efficient and inexpensive metal catalysts is still NPs are loaded on a stimuli-responsive polymer that endows the
inciting chemists and materials scientists. Recently an increas- NPs with facile recovery and reusability by simple environmental
ing number of inexpensive and heterogeneous catalysts, such changes, such as pH or temperature.32,33 The combination of
as MnO2, CeO2, Co3O4 and Ni nanoparticles, have been developed the efficiency of a homogeneous catalyst and the durability of a
heterogeneous catalyst might be one of the most exciting
challenges in the search for more efficient catalytic systems. In
this paper, we present a one-pot synthesis of Ag NPs stabilized by
the pH-responsive poly(amic acid) salt (Ag–PAAS). The Ag–PAAS
School of Materials Science and Engineering, Central South University, Changsha,
410083 China. E-mail: lihf@csu.edu.cn; Fax: +86-731-88877692;
Tel: +86-731-88877793
was found to be highly active for the catalytic hydration of
various nitriles in an aqueous medium and was easily recycled
† Electronic supplementary information (ESI) available: TEM image, XPS data of the
Ag NPs and the NMR data of the reaction products. See DOI: 10.1039/c5nj02497h
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after the reaction by its pH response. To the best of our vigorously stirred at the desired temperature for a certain period
knowledge, this is the first report on the use of recyclable and of time under air. The course of the reaction was monitored by
quasi-homogeneous Ag NPs for the catalytic hydration of nitriles regularly taking samples of B30 mL, which, after extraction with
in water.
ethyl acetate (3 ꢁ 2 mL), were analyzed by GC. In most cases,
after the completion of the reaction, clean and extensive crystals
of the corresponding amide were obtained by cooling the
mixture at low temperature, which were separated by direct
decantation and analyzed by 1H NMR. For those amides that
did not precipitate, the product was extracted with ethyl acetate
(3 ꢁ 2 mL) and dried over anhydrous MgSO4. The combined
organic layers were then concentrated by rotary evaporation
and the resulting residue was analyzed by 1H NMR. In some
cases additional purification by column chromatography was
required (silica gel with CH2Cl2 as the eluent).
2 Experimental
2.1 Materials and methods
Silver acetate (AgAc) was purchased from Aldrich with a purity
of 99.99%. Nitrile compounds (from J&K Scientific) were used
without further purification. Sodium borohydride (NaBH4), tri-
ethylamine (TEA), N,N-dimethylformamide (DMF), 4,40-oxydianiline
(ODA), 3,30,4,40-benzophenonetetracarboxylic dianhydride (BTDA)
and any other reagents were obtained from Sinopharm Chemical
Reagent Co., Ltd. All reagents were of analytical grade or better.
UV-vis spectra were recorded on a Shimadzu UV-vis spectro-
photometer (model UV-2450) at a scanning speed of 1 nm sꢀ1. The
concentration of the Ag–PAAS solution used for UV-vis absorption
measurements was 0.3 mM. The morphology and size of the silver
nanoparticles were characterized on a TITAN transmission electron
microscope (TEM, model G2 60-300). The test samples were made
by placing drops of Ag–PAAS solution on a carbon-coated copper
grid and drying them at room temperature. Powder X-ray diffraction
(XRD) patterns were collected using a Japan Rigaku X-Ray diffracto-
meter (D/max 2500/PC). The XRD sample (denoted as Ag-PAA) was
prepared by precipitating the Ag NPs in an acidic solution (pH B 2),
then washing the precipitate with acetone and acidic water several
times, and finally drying it in a vacuum at room temperature.
z potentials of the Ag NPs were measured on a Malvern Zetasizer
nano S. The concentration of the Ag–PAAS solution used for
z potential measurements was identical to that used in the UV-vis
absorption measurements. Gas chromatography (GC-FID) was
carried out on a Shimadzu gas chromatograph (GC-2010) equipped
2.4 Recycling of the Ag–PAAS catalyst
In a typical experiment, pyrazinecarbonitrile (1 mmol) and the
Ag–PAAS solution (3 mL, 10 mM) were introduced into a Teflon-
sealed tube, and the reaction mixture was vigorously stirred at
90 1C for 1 h. After cooling at room temperature, drops of 1 M
acetic acid were added until the solution reached pH B 2,
causing the Ag–PAAS catalyst to precipitate. The precipitate was
then separated by filtration and washed with ethyl acetate prior
to reuse. After that, the solid catalyst was mixed with fresh water
(3 mL), and the pH of the mixture was adjusted to 10.0 with
triethylamine, resulting in complete redispersion of the Ag–PAAS
precipitate. Finally, the fresh nitrile was added and the tube was
heated to 90 1C for the next hydration cycle.
3 Results and discussion
Ag NPs were in situ formed in an ice-cold PAAS aqueous solu-
tion by the reduction of silver acetate with sodium borohydride
(Scheme 1). First, Ag ions were loaded into poly(amic acid)
(PAA) chains in aqueous solution by an ion-exchange reaction
between the tertiary ammonium (TEA)+ and Ag ions to generate
self-assembled PAA–Ag(I) complexes.34 Silver polycarboxylate thus
formed was then converted into zero-valent Ag by fast reduction
using sodium borohydride, which then nucleated and grew
into Ag NPs. With the aid of powerful cation-complexing pro-
perties and a considerable steric hindrance effect of the PAA
1
with a 30 m capillary column using N2 as the carrier. H NMR
spectra were recorded on a Bruker nuclear magnetic resonance
spectrometer (Avance III, 400 MHz). Inductively coupled plasma
(ICP-OES) was recorded on a Thermo Fisher inductively coupled
plasma optical emission spectrometer (iCAP 6300).
2.2 Synthesis of Ag–PAAS
First, water-soluble BTDA/ODA-based PAAS was synthesized by
adding two molar equivalents of TEA to neutralize poly(amic acid)
(PAA) obtained through polycondensation of ODA and BTDA in
DMF (15 wt% solid, 1.65 dL gꢀ1 inherent viscosity at 25 1C).
Subsequently, an aqueous solution of AgAc (50 mM, 10 mL) was
mixed with PAAS solution (10 mM based on PAA monomer, 25 mL)
under 1000 rpm stirring in an ice-water bath. NaBH4 (50 mM,
15 mL) was then rapidly injected into the solution, affording a deep
brown solution. The mixture was stirred for another 30 min to
ensure the complete reduction of silver ions. The obtained Ag–PAAS
solution was stored in a refrigerator at 4 1C for further use.
2.3 General procedure for the catalytic reaction
In a typical experiment, the nitrile (1 mmol) and the Ag–PAAS
solution (3 mL, 10 mM) were added to a Teflon-sealed tube and Scheme 1 Schematic diagram for the synthesis of PAAS-stabilized Ag NPs.
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Fig. 1 (A) TEM image, (B) size distribution and (C) HRTEM image of PAAS-stabilized Ag NPs.
molecules,35,36 the ensuing Ag NPs are unexpectedly stable
without any precipitation even after six months’ storage under
ambient conditions. Moreover, the preparation method as
described above is simple and suitable for the scale-up produc-
tion up to hundreds of milliliters at a time.
3.1 Characterization of the Ag–PAAS material
Fig. 3 Images of (A) fresh Ag–PAAS nanocomposite solution, (B) after
addition of drops of 1 M acetic acid until the pH of solution decreased to
B2 and (C) redispersion of precipitates after adjusting the pH of solution
to B10 with triethylamine. (D) UV-vis spectra of the Ag–PAAS nano-
composite solution at different pH values.
The TEM image (Fig. 1) shows that the spherical Ag NPs are
highly dispersed in the PAAS solution with a diameter of 4.8 ꢂ
1.5 nm. The high-resolution TEM image shows that the Ag NPs
have well-defined crystalline planes with an interplanar d spacing
of 0.24 nm, corresponding to the d111 (0.2359 nm) plane for face-
centered cubic (fcc) silver.37 The broadening XRD pattern in Fig. 2
further evidences the existence of Ag nanocrystals, and the
average crystallite size of the Ag NPs was calculated to be 5.6 nm
using the Scherrer equation,37,38 which is in good agreement
with the TEM results. It also implies here that the Ag NPs were
well stabilized throughout the whole processing either by PAAS
in the alkaline solution-phase or by PAA in the acid solid-phase,
which is quite favorable for the following recyclable catalytic
applications.
The Ag–PAAS solution remained highly homogeneous in a
wide pH range (pH 4 3.5) and swiftly precipitated from the
solution at pH o 3 as shown in Fig. 3(A) and (B). The corre-
sponding surface plasmon bands of the PAAS-stabilized Ag NPs
are presented in Fig. 3(D). When the pH of the solution was
decreased, the UV-vis spectrum showed blue-shifts in the absorption
peak, from 406 nm (pH 8.5) to 409 nm (pH 4.7), 410 nm (pH 3.5),
and 412 nm (pH 2.9), probably as a result of the more compressive
wrapping of the PAA molecule layers onto Ag NPs, derived from
the incremental protonation of the carboxylic groups of PAA.
Notably, the red brown precipitate was redispersed smoothly in
water by increasing the pH of the solution to B10 (see Fig. 3(C))
and the same plasmon bands were recovered, implying that the
size and shape of the Ag NPs remained unaltered during
pH switching. The TEM image of the Ag NPs after redispersion
(Fig. S1 in the ESI†) shows no obvious changes with respect
to the dispersity and mean size of the NPs, further verifying
the favorable effect of the pH-mediated response of Ag NPs
by PAAS.
z potential measurements were used to study the stability of
the Ag–PAAS dispersion as shown in Fig. 4. The Ag NPs were
highly negatively charged at nearly neutral and high pH levels
(pH 6–11) with a maximum z potential of up to 60 mV, which
presumably originated from the coating of PAA chains with
huge numbers of negatively charged and extended carboxylate
groups, responsible for the extraordinary stability and homo-
geneity of the colloidal system owing to the electrostatic repul-
sion mechanism.32,39 A dramatic decrease in the charge density
occurred as the pH decreased in the range 2.5–4.0, because
of the increasing protonation of the PAA carboxylate moieties,
resulting in the concomitant collapse of the polymer chains
obscuring the solution gradually. At pH o 2.4, the z potential
reached the minimum, indicating that the majority of PAA
coatings on Ag NPs were protonated, leading to complete
salting-out of the NPs from the solution. The lack of electro-
static charges on the NP surface (|z| o 20) generally brings
about severe instability of NPs, but the pH-responsive disper-
sion of the Ag–PAAS here was highly reversible (see the UV-vis,
TEM and XRD analyses mentioned above), and thus it is reason-
able to assume that at lower pHs a denser wrapping of PAA on
Fig. 2 X-ray diffraction pattern of the Ag–PAA hybrid.
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Table 2 Hydration of diverse nitriles catalyzed by Ag–PAAS
Time
(h) Product
Yielda
(%)
TOFb
(hꢀ1
Entry Substrate
1
)
9
12
11
90
95
3.33
2.64
2.79
2
3
4
5
6
92 (88)
Fig. 4 z potential measurements of Ag–PAAS solution at different pH values.
8
10
12
1
92 (87)
90
3.96
3
the Ag NPs should instead prevent the Ag NPs from agglomera-
tion and the resulting Ag-PAA hybrid can be readily redissolved
in relatively high pH media.
3.2 Catalytic applications for the hydration of nitriles
77
2.14
Subsequently, the application of the Ag–PAAS was explored
for the catalytic hydration of nitriles under aerobic conditions.
First, the hydration of benzonitrile as a model substrate was
performed to optimize the reaction conditions (Table 1). The
reaction did not proceed at 90 1C even after 20 h in blank PAAS
and NaBH4–PAAS solutions (entries 1 and 2), ensuring that the
reaction was not catalyzed by PAAS and NaBH4. However, when
Ag NPs were loaded onto PAAS and used as a catalyst for the
hydration of benzonitrile, a dramatic conversion of nitrile to
amide occurred within 9 h (entry 3). Various mixed solvents
were used to optimize the mass transfer efficiency (entries 4–7).
Unfortunately, protic solvents ethanol and iso-propanol afforded
significantly lower conversions, and the addition of aprotic
solvents such as N,N-dimethylformamide and pyridine further
decreased the conversions. A prolonged period of time was
7
89
29.7
8
9
CH3CN
16
15
CH3CONH2
(73)
(88)
1.52
1.96
10
11
12
9
97 (90)
3.59
1
10c
499
97
33.3
3.23
1
499 (85) 33.3
1
499
499
33.3
1000
Table 1 Optimization of reaction conditions for the PAAS-stabilized Ag
13
1d
NPs-catalyzed hydration of benzonitrile
Reaction conditions: substrate (1 mmol), Ag catalyst (3 mol%), H2O
(3 mL), at 90 1C, in air. a Yields were determined by GC and confirmed by
1H NMR (isolated yields are given in parentheses). The selectivity is 499%.
b Turnover frequencies ((mol product/mol Ag)/time). Performed at 40 1C.
c
d Nitrile (10 mmol), Ag catalyst (0.1 mol%), H2O (5 mL).
Entry Catalyst
Solvent
Time (h) Yielda (%) TOFb (hꢀ1
)
1
2
PAAS
NaBH4–
PAAS
H2O
H2O
20
20
—
Trace
—
—
required to obtain a good-to-excellent yield for the cases of
lowering the catalyst dosage or reaction temperature, though
the turnover frequency (TOF) increased moderately as the Ag
loading was decreased (entries 8 and 9).
3
4
5
6
Ag–PAASc
Ag–PAAS
Ag–PAAS
Ag–PAAS
Ag–PAAS
Ag–PAAS
Ag–PAAS
H2O
9
22
90
21
20
5
4
83
94
3.33
0.32
0.42
0.076
0.067
2.31
5.22
EtOH/H2O (1 : 2)
iPrOH/H2O (1 : 2) 16
DMF/H2O (1 : 2)
Pyridine/H2O (1 : 2) 20
H2O
H2O
22
7
8d
9e
12
18
Under the optimized reaction conditions, the scope of the
Ag–PAAS catalyst was then examined with regard to various
types of nitriles (Table 2). The Ag–PAAS catalyst displayed high
Reaction conditions: benzonitrile (1 mmol), Ag catalyst (3 mol%), H2O
(3 mL), at 90 1C, in air. a Yields were determined by GC and confirmed activity for the hydration of activated, inactivated, aliphatic,
b
and heterocyclic nitriles in water. Benzonitrile derivatives bearing
c
by 1H NMR. The selectivity of all the reactions is B100%. Turnover
frequencies ((mol benzamide/mol Ag)/time). The original pH of the
d
e
electron-withdrawing groups (entries 4–7) seemed more active
than those bearing electron-donating groups (entries 2 and 3),
Ag–PAAS solution is 10.0. At 80 1C. Performed with 1 mol% Ag
catalyst.
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except in the case of 4-chlorobenzonitrile and 4-bromobenzo-
nitrile (the low solubility of halogen-substituted benzonitriles is
frequently a problem in aqueous hydration reactions). Further-
more, less reactive aliphatic substrates were also efficiently
hydrated to the corresponding amides in good to excellent yields
(entries 8–10). Acetonitrile and acrylonitrile showed relatively
low reactivity, due to the increased electron density of the nitrile
carbon compared with that of benzonitriles. To our surprise, the
hydration of cinnamonitrile smoothly afforded cinnamamide in
97% GC yield with an intact alkene bond in a short period of
time, indicating that the replacement of a hydrogen atom of
acrylonitrile by a phenyl group produced a dramatic increase in
the electrophilicity of the nitrile carbon atom (entry 10). The
metal-catalyzed hydration of heteroaromatic nitriles is usually
Fig. 5 Reuse of the Ag–PAAS catalyst in the hydration of pyrazinecarbo-
nitrile. Reaction conditions were identical to those indicated in Table 2 (1 h
reaction time in each cycle).
difficult in homogeneous catalysis, because of the strong coordi- reaction medium (similar to the case of 2-cyanopyridine). As
nating ability of the heteroatoms to the metal center.9,40–43 While shown in Fig. 5, the Ag–PAAS could be reused eight repeated
in our experiments the reactions furnished a quantitative yield of times with loss of activity (o5%) being observed only after the
the corresponding amides within only 1 h (entries 11–13). The sixth cycle (cumulative TON after the 8 cycles = 263). The TEM
hydration of 2-thiophenecarbonitrile afforded 97% GC yield even image of the catalyst after eight batches showed that the Ag NPs
at 40 1C (entry 11). A large-scale experiment of the hydration of were still highly dispersed with slightly increased grain sizes
2-cyanopyridine (10 mmol) with a smaller amount of Ag–PAAS (5–10 nm) (see Fig. S2 in the ESI†). ICP-OES analysis was used to
catalyst (0.01 mmol, 0.1 mol%) showed a TOF of 1000 hꢀ1 (entry 13). determine the Ag content leaching into the product phase,
To the best of our knowledge, this value is unprecedented for the resulting in an average residual concentration of 15.8 ng mLꢀ1
Ag-mediated hydration of nitriles at such a low temperature to date (1.46 ꢁ 10ꢀ4 mM) per cycle, which is 0.00146% of the original
(see Table 3). Thus, the Ag–PAAS is catalytically active for the concentration of the catalytic solution (10 mM). Such an extremely
hydration of nitriles in air at relatively mild temperatures. In low loss of the catalyst during recycling combined with the high
contrast, previously reported Ag NP catalysts required an inert stability of NPs during catalysis due to the polymer stabilizer
atmosphere and high reaction temperatures (4140 1C) and might be responsible for the excellent catalytic performance for
only afforded low TOFs under mild conditions.
the hydration of pyrazinecarbonitrile.
Because the pH-induced precipitation–redispersion process
It is well accepted that the possible mechanism of the
of Ag–PAAS is highly reversible, the Ag–PAAS catalyst was easily hydration of nitriles involves the coordination of water (or OHꢀ)
separated by adjusting the pH of solution to B2 and then and aromatic nitrile on the ‘‘bifunctional’’ Ag surface.2,11,28 To
decanting the product phase. The dark precipitate of Ag–PAAS elucidate the critical pathway of the Ag-catalyzed transforma-
was collected and redissolved in 3 mL of water by increasing the tion in this study, the effect of the concentration of OHꢀ on the
pH to 10.0 with triethylamine. The activity of the recycled catalytic activity is shown in Table 4. Decreasing the pH from
catalysts was examined for the hydration of pyrazinecarbo- the original 10.0 to 7.6 led to a 10-fold decrease in the reaction
nitrile. The high water solubility of its amide product generally rate, indicating that a basic co-catalyst is essential for efficient
makes it difficult to obtain a high isolated yield using a direct catalysis. On the other hand, further increase in the pH up
decantation method and to separate the catalyst from the to 11.9 only slightly increased the TOF, indicating that an
Table 3 Comparison of catalytic activity of the Ag–PAAS catalyst in the hydration of 2-cyanopyridine with previously reported systems
Catalyst
Temp. (1C)
Time (h)
Catalyst loading (mol%)
Yield (%)
TOFa (hꢀ1
)
Ref.
Ag–SiO2
140
140
90
140
0.4
0.25
7
2
12
1
7
7
7
24
2
1
3
37
99
96
77
95
499
24
43
87
93
93
133
2.7
19.3
1.9
67
0.68
1.2
28
2
31
9
42
15
41
40
Ag–hydroxyapatite
mPMF–Ag0
B5b
2
Au–NHC
Nafion–Ru
Ru–chitosan
Ru complex
175
4.2
1.5
5
5
4.3
2
120 (microwave)
100
100
120
135
100
90
RuCl2(PTA)4
Fe3O4@SiO2@SePh@Ru(OH)x
Pd/C-500Hox
RhCl(COD){P(NMe2)3}
Ag–PAAS
2.9
1.9
9.9
43
16
10
5
0.1
99
499
1
1000
This work
a
Turnover frequencies ((mol benzamide/mol Ag)/time) were calculated from the yield and time data in the papers by taking into account the
total amount of metal used. The content of Ag was estimated from the thermogravimetric analysis of the mPMF–Ag0 material displayed in the
b
corresponding literature.
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Table 4 Hydration of benzonitrile to benzamide by Ag–PAAS at different
pH values
7 P. Crochet and V. Cadierno, Dalton Trans., 2014, 43,
12447–12462.
8 M. L. Buil, V. Cadierno, M. A. Esteruelas, J. Gimeno, J. Herrero,
Entry
pHa
TOFb (hꢀ1
)
˜
S. Izquierdo and E. Onate, Organometallics, 2012, 31,
1
2
3
4
11.9
10.0
9.0
3.96
3.33
1.30
0.27
6861–6867.
´
9 R. S. Ramon, N. Marion and S. P. Nolan, Chem. – Eur. J.,
7.6
2009, 15, 8695–8697.
a
´
´
´
10 E. Tomas-Mendivil, R. Garcıa-Alvarez, C. Vidal, P. Crochet
and V. Cadierno, ACS Catal., 2014, 4, 1901–1910.
11 E. L. Downs and D. R. Tyler, Coord. Chem. Rev., 2014, 280,
28–37.
The pHs of reaction solutions were adjusted with triethylamine or
b
acetic acid. Turnover frequencies ((mol benzamide/mol Ag)/time)
were calculated at 70% conversion.
acceleration effect from the OHꢀ reached saturation and some
other unknown rate-limiting factors were present. Studies to
elucidate the detailed mechanism of the catalytic action of the
Ag–PAAS system are still in progress in our lab.
12 S. Kumar and P. Das, New J. Chem., 2013, 37, 2987–2990.
13 H. Woo, K. Lee, S. Park and K. Park, Molecules, 2014, 19,
699–712.
14 A. Y. Kim, H. Bae, S. Park, S. Park and K. Park, Catal. Lett.,
2011, 141, 685–690.
15 R. B. N. Baig, M. N. Nadagouda and R. S. Varma, Green
Chem., 2014, 16, 2122–2127.
4 Conclusions
16 K.-i. Shimizu, T. Kubo, A. Satsuma, T. Kamachi and
K. Yoshizawa, ACS Catal., 2012, 2, 2467–2474.
17 Y.-M. Liu, L. He, M.-M. Wang, Y. Cao, H.-Y. He and K.-N. Fan,
ChemSusChem, 2012, 5, 1392–1396.
18 T. Subramanian and K. Pitchumani, Catal. Commun., 2012,
29, 109–113.
19 C. Battilocchio, J. M. Hawkins and S. V. Ley, Org. Lett., 2014,
16, 1060–1063.
20 M. Tamura, A. Satsuma and K.-i. Shimizu, Catal. Sci. Technol.,
2013, 3, 1386–1393.
21 Y. Gangarajula and B. Gopal, Chem. Lett., 2012, 41, 101–103.
22 K. Yamaguchi, Y. Wang, H. Kobayashi and N. Mizuno,
Chem. Lett., 2012, 41, 574–576.
23 P. Christopher and S. Linic, ChemCatChem, 2010, 2, 78–83.
24 Z.-J. Jiang, C.-Y. Liu and L.-W. Sun, J. Phys. Chem. B, 2005,
109, 1730–1735.
In summary, a novel quasi-homogeneous Ag NP-based catalyst
was developed using PAAS as a stabilizer. The Ag–PAAS catalyst
displayed high activity for the selective hydration of nitriles to
amides in an aqueous medium under relatively mild condi-
tions, especially for the hydration of heteroaromatic nitriles
(a maximum turnover frequency of 1000 hꢀ1 was obtained for
the hydration of 2-cyanopyridine). Moreover, the pH response
of PAAS afforded a reversible precipitation–redispersion switch of
Ag NPs, allowing efficient recovery and recycling of the catalyst.
The catalyst could be reused for eight runs without significant loss
(o5%) in its initial catalytic activity, due to the extremely low Ag
leaching during recovery handling and the high stability of Ag NPs
during catalysis. The Ag–PAAS catalyst here combines the advan-
tages of the high efficiency of a quasi-homogeneous catalyst and
the easy recovery and reuse of a heterogeneous catalyst.
25 R. Xu, D. Wang, J. Zhang and Y. Li, Chem. – Asian J., 2006, 1,
888–893.
Acknowledgements
26 J. Davarpanah and A. R. Kiasat, Catal. Commun., 2013, 41, 6–11.
27 T. J. Sherbow, E. L. Downs, R. I. Sayler, J. J. Razink,
J. J. Juliette and D. R. Tyler, ACS Catal., 2014, 4, 3096–3104.
28 K.-i. Shimizu, N. Imaiida, K. Sawabe and A. Satsuma, Appl.
Catal., A, 2012, 421–422, 114–120.
This work was supported by the National Natural Science Founda-
tion of China (No. 51573209), the Hunan Provincial Innovation
Foundation for Graduates (CX2014B049) and the Free Exploration
Project for Undergraduates (ZY2015613).
29 K. Kawai, H. Kawakami, T. Narushima and T. Yonezawa,
J. Nanopart. Res., 2015, 17, 1–9, DOI: 10.1007/s11051-014-
2816-1.
Notes and references
1 R. Garcia-Alvarez, P. Crochet and V. Cadierno, Green Chem., 30 N. Salam, S. K. Kundu, R. A. Molla, P. Mondal, A. Bhaumik
2013, 15, 46–66.
and S. M. Islam, RSC Adv., 2014, 4, 47593–47604.
2 T. Mitsudome, Y. Mikami, H. Mori, S. Arita, T. Mizugaki, 31 K. Ghosh, M. A. Iqubal, R. A. Molla, A. Mishra, Kamaluddin
K. Jitsukawa and K. Kaneda, Chem. Commun., 2009, 3258–3260.
and S. M. Islam, Catal. Sci. Technol., 2015, 5, 1606–1622.
3 N. K. Thallaj, P.-Y. Orain, A. Thibon, M. Sandroni, R. Welter 32 Y. Yuan, N. Yan and P. J. Dyson, Inorg. Chem., 2011, 50,
and D. Mandon, Inorg. Chem., 2014, 53, 7824–7836.
11069–11074.
4 I. Ferrer, J. Rich, X. Fontrodona, M. Rodriguez and I. Romero, 33 X. Wu, X. Chen, H. Guan, X. Wang and L. Chen, Catal.
Dalton Trans., 2013, 42, 13461–13469. Commun., 2014, 51, 29–32.
5 E. Tomas-Mendivil, F. J. Suarez, J. Diez and V. Cadierno, 34 S. Qi, X. Shen, Z. Lin, G. Tian, D. Wu and R. Jin, Nanoscale,
Chem. Commun., 2014, 50, 9661–9664. 2013, 5, 12132–12135.
6 D. Kumar, C. A. Masitas, T. N. Nguyen and C. A. Grapperhaus, 35 J. Li, Y.-c. Fang, G.-w. He and H.-f. Li, J. Cent. South Univ.,
Chem. Commun., 2013, 49, 294–296.
2013, 20, 1475–1481.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015

View Article Online
NJC
Paper
36 N. Du, C. Wong, M. Feurstein, O. A. Sadik, C. Umbach and 40 W.-C. Lee and B. J. Frost, Green Chem., 2012, 14, 62–66.
B. Sammakia, Langmuir, 2010, 26, 14194–14202.
37 C. F. Bohren and D. R. Huffman, Absorption and Scattering of
Light by Small Particles, John Wiley & Sons, Inc., 1983.
38 R. Jenkins and R. L. Snyder, Introduction to X-ray powder
diffractometry, John Wiley & Sons, Inc., 1996.
41 W.-C. Lee, J. M. Sears, R. A. Enow, K. Eads, D. A. Krogstad
and B. J. Frost, Inorg. Chem., 2013, 52, 1737–1746.
42 G. K. S. Prakash, S. B. Munoz, A. Papp, K. Masood,
I. Bychinskaya, T. Mathew and G. A. Olah, Asian J. Org.
Chem., 2012, 1, 146–149.
39 D. Roy, J. N. Cambre and B. S. Sumerlin, Prog. Polym. Sci., 43 H. Joshi, K. N. Sharma, A. K. Sharma, O. Prakash, A. Kumar
2010, 35, 278–301. and A. K. Singh, Dalton Trans., 2014, 43, 12365–12372.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015
New J. Chem.