Au@zirconium-phosphonate nanoparticles as an effective catalytic system for the chemoselective and switchable reduction of nitroarenes

Article abstract of DOI:10.1039/c8gc03513j

In the present paper, a novel inorganic-organic layered material, a zirconium phosphate aminoethyl phosphonate, ZP(AEP), bearing aminoethyl groups on the layer surface, was used to immobilize AuNPs by a two-step procedure. The gold-based catalyst, Au1@ZP(AEP), containing 1 wt% Au, was characterized in terms of physico-chemical properties and TEM analysis revealed that the AuNPs have a spherical shape and an average size of 7.8 (±2.4) nm. Au1@ZP(AEP) proved its high efficiency for the chemoselective reduction of nitroarenes under mild conditions. Both batch and flow condition protocols have been defined. The catalytic system has been proven to be able to easily switch chemoselectivity allowing the control of the reduction of a series of nitroaromatics towards their corresponding azoxyarenes (2a-k) or anilines (2a-l) in 96% EtOH or abs EtOH, respectively, by using NaBH₄ as a reducing agent, in good to excellent yields. Recovery and reuse of the catalytic system has been investigated proving the benefits of the flow approach.

Full text of DOI:10.1039/c8gc03513j

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Au@Zirconium‐Phosphonate Nanoparticles as Effective Catalytic
System for the Chemoselective and Switchable Reduction of
Nitroarenes
Received 00th January 20xx,
Accepted 00th January 20xx
Francesco Ferlin,a Matteo Cappelletti,a Riccardo Vivani,b Monica Pica,*b Oriana Piermatti,*a and
Luigi Vaccaro*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
In the present paper, a novel inorgano‐organic layered material, a zirconium phosphate aminoethyl phosphonate, ZP(AEP),
bearing aminoethyl groups on the layer surface, was used to immobilize AuNPs by a two step procedure. The gold‐based
catalyst, Au1@ZP(AEP), containing 1 wt% Au, was characterized in terms of physico‐chemical properties and TEM analysis
revealed that the AuNPs have a spherical shape and an average size of 7.8 (± 2.4) nm. Au1@ZP(AEP) proved its high efficiency
for the chemoselective reduction of nitroarenes under mild conditions. Both batch and flow conditions protocols have been
defined. The catalytic system has been proven to be able to easily switch chemoselectivity allowing to control the reduction
of a series of nitroaromatics towards their corresponding azoxyarenes (2a‐k) or anilines (2a‐l) in EtOH 96% or EtOH abs,
respectively, by using NaBH4 as reducing agent, in good to excellent yield. Recovery and reuse of the catalytic system has
been investigated proving the benefits of the flow approach.
Noble metals are well known and widely studied heterogeneous
catalysts in several fields. Due to their expensiveness, they are
generally used in low amounts and in the form of nanoparticles,
which assure a large fraction of metal atoms exposed on the
surface.6 Moreover, their use in catalyzed sustainable industrial
processes needs that they can be easily handled and recovered.
In order to prevent the metal nanoparticle (MNP) aggregation
and enabling the catalyst recovering and reuse, MNPs can be
dispersed on solid supports.7
Supported gold NPs (AuNPs) are largely used as catalytic
systems for promoting a plethora of transformations, including
oxidations, hydrogenations and C–C bond forming processes.8
Among these, the selective reduction of nitroaromatics into
aniline, azo‐, and azoxy‐compound is an important process
because these moieties are often endowed in active natural
products, dyes, pigments, agrochemicals and pharmaceuticals.9
A large number of procedures have been developed for the
reductive transformation of nitroarenes.10 Very few examples
exist in the literature where a single reducing system is able to
chemoselectively reduce nitroaromatic compounds into only
one of the possible reductive products (e.g, aniline, or azo‐, or
azoxy‐compounds) and that it is also capable of switching its
selectivity to access a different prouct by varying the reaction
conditions.11
Introduction
Modern and efficient synthetic methodologies rely on the
definition and use of efficient catalytic systems able to permit
the access to the desired product with high chemical and
environmental efficiency.1
Although industrial production may still prefers the use of well‐
established homogeneous methods,2 heterogeneous catalytic
systems feature evident advantages in terms of easier recovery
provided that the catalyst does not lose activity during the
reuse. Heterogeneous catalytic systems may also offer the
additional advantage to control a change in selectivity and
possibily increase the catalytic stability/efficiency. They should
be appealing from both the chemical and sustainability point of
views.3
Our research is aimed at the definition of green and sustainable
protocols for organic transformations based on the use of
safer/sustainable reaction media,4 and on the preparation and
use of recoverable catalytic systems adequate for being used in
continous‐flow reactors and featuring a minimal leaching.5
These aspects are of key interest in the context of green
chemistry, where chemical efficiency has to be combined with
economic and environmental needs.1‐3
Immobilized AuNPs have been widely used for the selective
reduction of nitroarenes to anilines by using NaBH4,12
silane,12a,13 formic acid14 as reducing agents, while only few
examples report the preparation of azo15 or azoxy compounds16
by using 2‐propanol as reducing agent by light activation. The
chemoselective preparation of all three compounds were
reported by Liu at al. by using supported AuNPs on meso‐CeO2
a. Laboratory of Green Synthetic Organic Chemistry – Dipartimento di Chimica,
Biologia e Biotecnologie, Università di Perugia Via Elce di Sotto, 8 ‐ 06123
Perugia, Italy. E‐mail: luigi.vaccaro@unipg.it; Web: www.dcbb.unipg.it/greensoc
b. Dipartimento di Scienze Farmaceutiche, Università di Perugia, Via del Liceo, 1,
06123 Perugia, Italy
Electronic Supplementary Information (ESI) available: [experimental procedures
and calculation, copies of 1H and 13C NMR spectra. See DOI: 10.1039/x0xx00000x
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phosphonate groups more easily than the tridentate HPO4 groups
and 2‐propanol/water combination as reducing agent at 30‐80
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DOI: 10.1039/C8GC03513J
°C to obtain azoxy‐, azo‐, or aniline compounds with high typical of microcrystalline ‐ZP.
yield.11c
In the present paper, a gel of nanosized ‐ZP in propanol reacted
The development of a mild and chemoselective catalytic system
which allows to obtain more than one target product by easily
controlled switchable reduction conditions is still of great
importance.
with different amounts of aminoethylphosphonic acid (H2AEP) to
obtain zirconium phosphate aminoethylphosphonates, ZP(AEP)x,
with different amounts of aminoethyl groups. Due to the low
solubility of H2AEP in propanol, the reaction was carried out in a 1:1
v/v PrOH:H2O solution, at 80 °C. The Ptot/Zr molar ratio of the
ZP(AEP)x obtained samples, determined by ICP‐OES analysis, was 2 in
all cases, so that the following general composition can be assigned:
It is known that the catalytic activity of AuNPs significantly
decreases for particle size above 10 nm.17 In light of this, the key
point in the design of supported AuNPs catalysts is the choice of
suitable supports, assuring the NPs retention and preventing
their aggregation or coalescence. Besides, thiols or amino
groups are also known to stabilize AuNPs.18 In this context,
layered zirconium phosphonates and phosphates with high
surface area can offer attractive supports for metal
nanoparticles19 because of their chemical and thermal stability
together with a facile tunability of their structural features.19,20
Zr(O3PC2H4NH2)x(O3POH)2‐x
.
Then, the
x
values have been calculated from the
phosphonate/phosphate molar ratios (hereafter AEP/P) obtained by
31P liquid NMR; the corresponding NMR spectra were reported in
Figure S1, while the calculated x values are shown in Table 1.
Layered compounds with an ‐type structure consist of planes
of zirconium atoms bonded through monohydrogen phosphate
Table 1. Composition of the ZP(AEP)x compounds obtained
for different H2AEP/Zr molar ratios in the starting reacting
mixture.
groups, O3POH and/or phosphonate groups, O3PR.21
To our knowledge, no examples are reported in the literaure on
AuNPs supported on α‐zirconium phosphates or phosphonates.
With this aim, we decided to investigate the use of a new taylor‐
made inorgano‐organic material, bearing amino groups
covalently bonded to an inorganic backbone, as support for
AuNPs. Specifically, in the present paper the synthesis of a new
nanosized zirconium phosphate aminoethylphosphonate
(ZP(AEP)x) was reported. ZP(AEP)x was prepared by topotactic
anion exchange reaction of part of the phosphate groups of
H2AEP/Zra
Ptot/Zrb
AEP/Pc
x
1.0
2.0
3.0
1.0
2.0
2.0
2.0
2.0
0.56
0.64
0.85
0.56
0.72
0.78
0.92
0.72
aMolar ratio in solution; bMolar ratio in the solid, calculated by ICP
nanocrystalline
α‐zirconium
phosphate
(α‐ZP)
with
c
analysis; Molar ratio in the solid, Calculated by 31P liquid NMR
aminoethylphosphonate groups. Then, the particle surface of
the support was decorated with AuNPs through a two step
procedure, involving the adsorption of Au(III) species on the
solid surface, followed by reduction with NaBH4. The
Au@ZP(AEP) catalysts, containing 1 and 2.4 wt% AuNPs, were
then characterized by Transmission Electron Microscopy (TEM),
X‐Ray Powder Diffraction (XRPD) and UV–Vis Diffuse
Reflectance Spectroscopy (DRS) analysis. Finally, the catalyst
with 1 wt% Au loading was used for the selective reduction of
nitroarenes, under green and mild conditions, for the
preparation of anilines and azoxyarenes both under batch and
continuous flow conditions.
analysis.
Thermogravimetric analysis profiles of ZP(AEP)x were reported in
Figure S2. They show a first weight loss at ca. 100 °C, due to
intercalated/adsorbed water and/or solvent, and a second loss in the
range 200‐800 °C, due to the decomposition of the organic moieties
and to the loss of condensation water. From the second weight loss
it is generally possible to calculate the formula weight of the
compound. It is noteworthy that, despite the uncertainty of the TG
data, due to the partial overlapping of dehydration and
decomposition steps, the calculated formula weights of the materials
are in good agreement with those obtained by ICP‐OES and 31P liquid
NMR, with a maximum difference of 7%. Figure 1 shows the XRPD
patterns of the ZP(AEP)x samples, together with that of the
unmodified nanosized ZP, for comparison.
Results and Discussion
Catalyst synthesis and characterization. In previous works it has
been proven that, differently from microcrystalline ‐type ZP,
nanocrystalline ‐ZP is able to give topotactic anion exchange
reactions with phosphonic acids in relatively mild conditions,22
providing single phase zirconium phosphate‐phosphonate
compounds, where the phosphate and phosphonate groups are
randomly arranged in the structure. The higher reactivity of
nanosized ZP, with respect to the corresponding microcrystalline
compound, has been ascribed both to the larger amount of surface
exposed to the solution, and to the higher fraction of low
coordinated edge phosphate groups, which can be replaced by
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The interlayer distance, estimated according to the model of
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DOI: 10.1039/C8GC03513J
Figure 2, is in perfect agreement with the experimental value of 9 Å.
Figure 2. Ideal representation of an α‐Zr phosphate in which a half of
phosphates have been replaced by aminoethylphosphonate groups.
Figure 1. XRPD patterns of: nanosized ZP (a); ZP(AEP)0.72 (b),
ZP(AEP)0.78 (c), ZP(AEP)0.92 (d).
Synthesis and characterization of Au@ZP(AEP). The catalytic
activity of AuNPs strongly depends, among other factors, on the
particle size and it has been proved that they are highly active when
their size is below 10 nm. The use of AuNPs supported and dispersed
on a solid matrix is a quite attractive strategy6 in order to reduce and
stabilize the Au particle size, thus preventing them from
Differently from pristine nanosized ZP, the ZP(AEP)x samples are
almost amorphous and few broad peaks are observed in the
corresponding XRPD patterns. However, some structural
considerations can be attempted: 1) the peak of ZP at 33.8° 2, agglomeration. In light of this, the optimal catalyst loading in Au
associated with the Zr−Zr separaꢀon in the α‐type layer (the (020) supported systems is generally in the range 1‐3 wt%. In the present
reflection of α‐ZP), was still observed in the pattern of ZP(AEP)x. This paper ZP(AEP)0.72 was used as support for AuNPs: samples with 1.0
fact is in agreement with previous works, which proved that the and 2.4 wt% of Au loading, were prepared through a two‐step
replacement of phosphonate groups for monohydrogen phosphate procedure as reported in the Experimental Section. First, suitable
‐
groups maintained unaltered the structure of the α‐type layer to amounts of a AuCl4 solution were added to a suspension of
some extent.22 2) The patterns of the ZP(AEP)x samples show a broad ZP(AEP)0.72 in water and the mixture was left under stirring at 30 °C
for about one day. At first, the solution had a bright yellow color, due
to the presence of soluble Au(III) species; then, a progressive
decolorization of the solution was observed, while the solid gradually
acquired a pale yellow color. It is reasonable to suppose a
peak around 9.8° 2, that can be attributed to a phase with an
interlayer distance of  9 Å: this value lies between those of the pure
Zr aminoethylphosphonate Zr(O3PC2H4NH2)2 (d = 11.3 Å)23 and pure
monohydrate ‐ZP (d = 7.6 Å). Moreover, the peak associated to the
interlayer distance of the parent monohydrate ‐ZP at 11.65° was
never observed. These results are in agreement with previuos data
on the topotactic exchange reactions of nanocrystalline ZP with aryl‐
and alkylphosphonic acids22 and suggest that single phase mixed
compounds, containing randomly distributed phosphate and
aminoethylphosphonate groups on the surface of layers with an ‐
type structure, are formed. Moreover, the coexistence of free –
C2H4NH2 and –POH groups between adjacent layers let to suppose
that –NH3+ PO‐ interactions can occur to some extent.
‐
chemisorption of the AuCl4 ions through an ion exchange reaction
+
involving protonated O3PC2H4NH3 groups, reasonably located on
the layer surface of ZP(AEP)0.72 (due to the repeated washings of the
solid with acidic solution), according to the following scheme:
+
‐
+
‐
(PC2H4NH3 )(s) + AuCl4 (aq)  (PC2H4NH3 AuCl4 )(s)
An approximate prediction of the interlayer distance of a mixed
phosphate aminoethylphosphonate compound can be obtained by a
simple structural model, starting from the α‐type crystallographic
structure of Zr(PC2H4NH3Cl)2, solved from XRPD data in our
laboratory, and by replacing a half of aminoethyl groups with ‐OH
groups. Interlayer chlorine atoms, present in the starting
structure,were also eliminated. In this model, the stacking of
adjacent layers was set taking into account that the amino groups af
a layer can interact via hydrogen bonds with P‐OH groups (NO
distance  2.8 Å) of adjacent layers, and no other close contact is
present among groups in the interlayer region, as shown in Figure 2.
Figure 3. Pictures of Au1@ZP(AEP) (A) and Au2.4@ZP(AEP) (B)
After the addition of the reducting agent, NaBH4, the solution turned
to pink/plum, which is the typical color of AuNPs. Gold loadings of 1
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and 2.4 wt% were confirmed by ICP analysis. The dried samples are loading, being 7.8 (± 2.4) nm and 10.6 (± 5.7) nm forVAieuw1A@rticZlPe(OAnEliPne)
DOI: 10.1039/C8GC03513J
shown in Figure 3 and they will be hereafter labelled as Au1@ZP(AEP) and Au2.4@ZP(AEP), respectively.
(Au loading = 1 wt%) and Au2.4@ZP(AEP) (Au loading = 2.4 wt%).
Figure 7 shows the XRPD patterns of ZP(AEP)0.72, Au1@ZP(AEP) and
Figure 4 shows TEM images of the ZP(AEP)0.72 support at two
Au2.4@ZP(AEP). The pattern of ZP(AEP)0.72 did not undergo
significant changes after treatment with AuCl4 /NaBH4. Moreover,
the reflection at 38.3° 2, corresponding to the (111) crystallographic
different magnifications, while Figures 5 and 6 show representative
TEM images of the Au1@ZP(AEP) and Au2.4@ZP(AEP) samples,
respectively, together with the corresponding Au particle size
‐
planes of the face‐centered cubic Au structure (JCPDS No. 4‐784),
distribution, obtained by analyzing at least 150 nanoparticles.
was observed only at the highest Au loading (pattern (b)).
Figure 4. Representative TEM images of ZP(AEP)0.72
.
Figure 6. TEM image of Au2.4@ZP(AEP) and Au particle size
distribution.
Figure 5. TEM image of Au1@ZP(AEP) and Au particle size
distribution.
The support consisted of particle aggregates with irregular size and
shape, typical of quite amorphous materials and after treatment with
HAuCl4/NaBH4 the surface of ZP(AEP)0.72 aggregates was decorated
with AuNPs, exhibiting an almost spherical morphology (Figures 5
and 6). The average particle size increased with increasing the gold
Figure 7. XRPD patterns of ZP(AEP)0.72 (a); Au2.4@ZP(AEP) (b);
Au1@ZP(AEP) (c).
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Figure 8 shows the absorption spectra, in Kubelka–Munk units,24 of
Au1@ZP(AEP) and Au2.4@ZP(AEP): K and S are the absorption and
scattering coefficients, respectively. An absorption band in the visible
region is observed, due to the surface plasmon resonance of the
AuNPs. The presence of just one absorption band indicates that the
AuNPs are nearly spherical particles,25 in agreement with TEM
analysis, while the red shift of the absorption maximum (from 527
nm to 530 nm for Au1@ZP(AEP) and Au2.4@ZP(AEP), respectively) is
in agreement with the increase of the Au particle size with increasing
the Au loading.26
View Article Online
DOI: 10.1039/C8GC03513J
Figure 9. FTIR spectra of (a) ZP(AEP) and (b) Au1@ZP(AEP) samples.
Representative SEM pictures of ZP(AEP) and Au1@ZP(AEP), collected
at different magnifications, with both secondary and backscattered
electrons, are shown in Figure 10. A quite different morphology was
observed for ZP(AEP) and Au1@ZP(AEP). Specifically, while the
particle surface of the unloaded support appears smooth (Figure
10A, C), that of Au1@ZP(AEP) exhibits a certain roughness (Figure
10D). Moreover, backscattered electron images of Au1@ZP(AEP)
show the presence of bright spots (Figure 10E, F), not observed in
ZP(AEP) (Figure 10B), due to AuNPs.
Figure 8. Absorption spectra in terms of the Kubelka–Munk
function (K/S) of Au1@ZP(AEP) (a) and Au2.4@ZP(AEP) (b).
FTIR spectrum of ZP(AEP) is shown in Figure 9a. According to the
literature, three main absorption regions can be identified.27 The
strong band in the 850‐1100 cm‐1 probably is the result of the
crowding of many absorption effects in that region: P‐C, P‐O, C‐N
stretching motions, and also the out‐of‐plane N‐H bending
absorption. Between 1280 and 1800 cm‐1 the absorptions of C‐NH2,
C‐NH3+ and CH2 can be observed. Specifically, the absorptions at 1519
and 1283 cm‐1 can be assigned to the NH3+ deformation bands, the
band at 1620 cm‐1 is characteristic of the N‐H bending, while the
absorption at 1470 cm‐1 can be assigned to the CH2 bending. Above
2500 cm‐1 a quite wide band is present and it derives from the
+
stretching of hydrogen bonded NH2, NH3 , and that from the OH
between water and phosphate groups.
The FTIR spectrum of Au1@ZP(AEP) (Figure 9b) is quite similar to that
of ZP(AEP), with a general increase of the absorption intensities. This
change can be attributed to a variation of the vibrational structure,
due to some dipole interactions of the external goups with gold
nanoparticles.28
Figure 10. SEM pictures of ZP(AEP) (A, B, C) and Au1@ZP(AEP) (D, E,
F) collected by secondary (A, C, D) and backscattered (B, E, F)
electrons.
Energy dispersive X‐ray (EDX) analysis performed on both samples
provided interesting information about the elemental composition
of their surface. As shown in Figure 11, besides P, Zr, C, N and O, also
Cl was detected in ZP(AEP), confirming the presence of –NH3Cl
groups on the surface of the starting material. After treatment with
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HAuCl4 and NaBH4, the chlorine peak disappeared, while that of Au We also observed that when absolute EtOH (EtOHabs) was used as
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‐
appeared, proving the Cl‐/AuCl4 anion exchange occurred on the reaction medium, the catalyst was more efficient and in the presence
DOI: 10.1039/C8GC03513J
support surface.
of just 0.5 mol% of Au1@ZP(AEP) a complete conversion can be
obtained after 3 h when using 6 eq of NaBH4 (Table 2, Entries 5 and
6 vs. Entries 1 and 2) with a preferential formation of aniline 4a.
Using Au1@ZP(AEP) in 1 mol% in EtOHabs and in the presence of 6 eq
of NaBH4, complete conversion and selectivity towards 4‐
metoxyaniline (4a) was obtained after 2 h (Table 2, Entry 8).
Table 2. Optimization of the reaction conditions in the
reduction of 1a.
Figure 11. EDX spectra of (A) ZP(AEP) and (B) Au1@ZP(AEP), referred
to the areas indicated by the red cross in the corresponding SEM
pictures.
entrya
Au1@ZP(AEP)
(mol%)
NaBH4
(eq)
3
Medium
C
(%)b
2a:3a:4a
ratiob
98:2:0c
Specific surface area, calculated by the BET method, of ZP(AEP) is less
than 5 m2/g, probably due to the tendency of the particles to
aggregate during the drying process. After immobilization of gold
nanoparticles the surface area increases up to 11 m2/g, in agreement
with that observed in the SEM pictures of Au1@ZP(AEP), showing a
more pronounced roughness of the surface, with respect to ZP(AEP).
It is reasonable to attribute these modifications to the procedures
used for the synthesis of the AuNPs.
1
2
3
4
5
6
7
8
0.5
EtOH 96%
EtOH 96%
EtOH 96%
EtOH 96%
EtOHabs
42
0.5
1
6
3
6
3
6
3
6
40
83
99
98
99
99
>99
97:3:0
97:3:0d
96:4:0
1
0.5
0.5
1
87:6:7
EtOHabs
EtOHabs
EtOHabs
.
14:0:86e
42:1:57
0:0:100f
Catalytic Activity. Due to its smaller Au particle size, Au1@ZP(AEP)
was selected for being used in the catalytic tests on our targeted
process, the chemoselective reduction of nitroaromatics. We started
our study testing the catalytic activity of Au1@ZP(AEP) in the
reduction of 4‐nitroanisole (1a) in EtOH 96% or EtOH absolute as a
representative safe and effective medium for such a process. We
have used NaBH4 as reducing agent and after a reaction time of 3 h
at 30 °C, we evaluated the different conditions to access the selective
preparation of the corresponding azoxy derivative 2a and the 4‐
metoxyaniline (4a). The reactions were performed in a closed vial
1
.
aReaction conditions: 1a (0.1 mmol), Au1@ZP(AEP) (0.5 or 1 mol%), NaBH4
(3 or 6 eq), medium (1.8 mL), at 30 °C for 3h. b Conversion and product ratios
determined by GLC and 1H‐NMR analyses. c Reaction temperature 50 °C: C =
47%, 2a:3a:4a = 81:18:1 . dCatalyst: Au2.4@ZP(AEP) (1 mol%): C = 42%,
2a:3a:4a = 97:3:0. eReaction time 4 h: C: >99%, 2a:3a:4a = 0:1:99. f Reaction
time 2 h.
paying attention to select the size and the scale of the reactants in Table 3. Substrate scope for the preparation of
azoxyderivatives 2a-k.
order to fill‐up the reactor volume with reaction medium (ca. 0.05M).
When the reaction was performed in EtOH 96% in presence of 0.5
mol% of Au1@ZP(AEP) and 3 eq of NaBH4 the azoxy derivative 2a was
obtained with high selectivity, but the conversion was low (42%,
Table 2, Entry 1). Increasing the reaction temperature to 50 °C, the
conversion slightly increases but a lower selectivity was observed (C
47%, 2a:3a:4a = 81:18:1, Table 2, Entry 1, note c). Increasing the
amount of NaBH4 to 6 eq the reactivity and selectivity was
unchanged (Table 2, Entry 2). In the presence of 1 mol% of
Au1@ZP(AEP) and 3 eq of NaBH4 a conversion 83% was obtained still
with a high selectivity of product 2a (Table 2, Entry 4). A complete
conversion with a good selectivity (96%) was obtained in presence of
6 eq. of NaBH4 (Table 2, Entry 4). When the reaction was performed
in presence of 1 mol% of Au2.4@ZP(AEP) the conversion was only
42% (Table 2, Entry 4, note d).
entrya
R
t
C
(%)b
99
2:3:4
ratiob
96:4:0
Y of 2
(%)c
90
(h)
3
1
2
3
4
5
6
7
8
9
a: 4‐OMe
b: H
2
1
1
1
3
3
6
3
99
99
>99
93
93
90
99
89
95:5:0
89:11:0
85:8:0:7d
50:7:0:43e
98:2:0
90
86
83
44
88
78
92
74
c: 4‐Cl
d: 4‐Br
e: 4‐I
f: 4‐Me
g: 3‐Me
h: 2‐Me
i: 3‐CH2=CH2
90:10:0
95:5:0
87:13:0
6 | J. Name., 2012, 00, 1‐3
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10
11
j: 3‐COMe
2
2
95
99
88:12:0f
82:6:0:12f
80
76
8
h: 2‐Me
i: 3‐CH2=CH2
j: 3‐COMe
6
6
5
4
4
>99
96
View Article Online
DOI: 10.1039/C8GC03513J
k: 4‐CO2Me
9
>99
>99
>99
>99
97
98
96
94
10
11
12
aReaction condition: 1 (0.1 mmol), Au1@ZP(AEP) (1 mol%), NaBH4 (6 eq),
k: 4‐CO2Me
b
reaction medium: EtOH 96% (1.8 mL), at 30 °C. Conversion and product
l: 6‐nitroquinoline
ratios determined by GLC and 1H‐NMR analyses. cYield of the isolated
products 2 after column chromatography. d 7% of de‐halogenated product
aReaction condition: 1 (0.1 mmol), Au1@ZP(AEP) (1 mol%), NaBH4 (6 eq),
e
f
was observed. 43% of de‐halogenated product was observed. The
corresponding hydroxy aryl compound was observed. 12% of trans‐
esterification ethyl ester product was observed.
b
g
reaction medium: EtOHabs (1.8 mL), at 30 °C. Conversion and product
ratios determined by GLC and 1H‐NMR analyses. Yield of the isolated
c
products 4. d 75% of de‐halogenated product was observed.
The recovery and the reuse of the solid catalyst has been performed
both in EtOH 96% and EtOHabs in the representative reduction of 4‐
nitroanisole (1a) (Table 5). For each reuse the catalyst was separated
by centrifugation, washed with EtOH, aqueous HCl 10‐2 M to
eliminate the borate, water and eventually with EtOH, dried at 130
°C for 15 h and reused in the next run. The solid catalyst was used
five consecutive times without loss of activity and selectivity in
EtOHabs, and only a small reduction in the catalytic activity was
observed in EtOH 96% but most importantly the chemoselectivity did
not change (Table 5). The ICP‐OES analysis of crude products
obtained during the five consecutive run by usual work‐up procedure
(filtration, distillation of EtOH, extraction with ethyl acetate/H2O to
eliminate the inorganic reagents, distillation of solvent under
reduced pressure), showed a very low content of gold (2.7 ppm). As
confirmation of this data, the Au content on the recovered catalyst
was determined by ICP‐OES analysis showing that the content of gold
remained almost unchanged (0.96‐1.05 wt% vs. 1.0 wt % of pristine
catalyst). These results are of additional support to show that the
aminoethyl‐ and the phosphate groups on the surface of support are
very effective to stabilize the gold nanoparticles.
The preliminary results obtained do not only show the high efficiency
of our solid Au‐catalytic system, but they also prove that with this
catalyst a route towards the chemoselective switchable preparation
of 2a or 4a is also paved. To evaluate the applicability of these
preliminary results, the reaction conditions were extended obtained
to a series of nitroarenes 1a‐l with the aim of preparing
azoxyderivatives 2a‐k and anilines 4a‐l. Azoxyderivatives 2a‐k have
been prepared with very good yields and selectivities performing the
reaction in EtOH 96%, by suing 1 mol% of Au1@ZP(AEP) and in the
presence of 6 eq of NaBH4 (Table 3). For 4‐bromo (1d) and 4‐
iodonitrobenzene (1e) an additonal 7% and 43% of de‐halogenated
products were observed, respectively (Table 3, Entries 4 and 5). In
the case of 3‐nitroacetophenone (1j), the corresponding hydroxyl
aryl compound deriving from the reduction of the carbonyl group
was isolated (Table 3, Entry 10), while for methyl 4‐nitrobenzoate
(1k) a 12% of trans‐esterification product, the ethyl ester derivatives,
was isolated (Table 3, Entry 11). The substrate scope for the
alternative chemoselective protocol for the preparation of anilines 4
was also investigated. By using EtOHabs as reaction medium, 1 mol%
of Au1@ZP(AEP) in the presence of 6 eq of NaBH4, a series of anilines
4 could be prepared with high yields and selectivities. The results are
reported in Table 4. For 4‐iodonitrobenzene (1e) a 75% of de‐
halogenated product was observed (Table 4, Entry 5) while 4‐bromo
(1d) did not undergo the de‐halogenation. The reduction of 3‐
nitroacetophenone (1j) gave rise to the corresponding hydroxy
aniline 4j (Table 4, Entry 10), while methyl 4‐nitrobenzoate (1k) did
not lead to the trans‐esterification product (Table 4, Entry 11).
Table 5. Recovery and reuse of catalyst in the preparation 4-
metoxyaniline (4a) and azoxy derivative (2a).
Table 4. Substrate scope for the preparation of anilines 4a-l.
entrya
Run
Medium
t
C
2:3:4
(h)
(%)b
ratiob
1
2
3
4
5
6
7
8
9
10
Run 1
Run 2
Run 3
Run 4
Run 5
Run 1
Run 2
Run 3
Run 4
Run 5
EtOH 96%
EtOH 96%
EtOH 96%
EtOH 96%
EtOH 96%
EtOHabs
3
98
96:4:0
3
3
96
94
97:3:0
97:3:0
3
93
96:4:0
3
87
98:2:0
entrya
R
t
C
(%)b
>99
Yield
(%)c
96
2
>99
>99
>99
>99
>99
0:0:100
0:0:100
0:0:100
0:0:100
0:0:100
(h)
2
EtOHabs
4
1
2
3
4
5
6
7
a: 4‐OMe
b: H
EtOHabs
6
2
2
2
1
3
3
>99
>99
>99
>99
>99
>99
92
94
95
EtOHabs
15
15
c: 4‐Cl
d: 4‐Br
e: 4‐I
EtOHabs
[d]
‐
aReaction condition: 1 (0.1 mmol), Au1@ZP(AEP) (1 mol%), NaBH4 (6 eq),
reaction medium: EtOH 96% (1.8 mL), at 30 °C. b Conversion and product
ratios determined by GLC and 1H‐NMR analyses.
f: 4‐Me
g: 3‐Me
98
98
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Journal Name
96% (0.25M), at 30 °C. cConversion and product ratios deteVrmiewinAerdticbleyOGnLlCine
It is well known that the catalytic activity of gold nanoparticles is
strongly dependent on the particle size range.8b The slight reduction
of catalytic activity after recovery and reuse can be explained by
comparing the TEM images of the catalyst recovered after five runs
in EtOH 96 % or EtOHabs (Table 5). As shown in Figure S3, the AuNPs
still have a spherical morphology with good monodispersity and a
slight increase of the avearge particle size ranging from 7.8 (± 2.4)
nm to 11.4 (± 5.7) and 10.7 (± 6.2) nm, respectively.
and 1H‐NMR analyses.
DOI: 10.1039/C8GC03513J
With the optimal conditions we were able to set‐up an efficient
continuous flow production of azoxy derivative 2a and aniline 4a
starting from simple 4‐nitroanisole (1a). For both protocols we
observed a strong increase in TON and TOF value (Table 7). These
features were all related to the fact that switching from batch to flow
condition there is an increase in efficiency and stability of the catalyst
that derived from his stable packing inside the reactor.
For a better evaluation of reusability of Au catalyst the recycle test
has been also performed with moderate conversion in EtOHabs at 30
°C for a reaction time of 40 min (see Supporting information, Table
1S). The conversion remained almost constant for five run, only a
small reduction in the percentage of the expected 4‐methoxy aniline
(4a) was observed showing a moderate reduction of catalytic activity,
likely determined by the moderate increase of Au particles size.
This evaluation was also supported by the fact that a careful
measure of the gold species released (leached) in solution reveals
that just a minimal part of the gold catalyst was lost during the
reaction. Indeed, the ICP‐OES analysis showed that the leaching
of gold in solution is constant during the time of the experiments
in flow and is below 0.1 ppm (Fig. 12).
The two reaction pathways can be effectively switched by simply
tuning the solvent properties. We also decided to investigate the use
of Au1@ZP(AEP) catalyst under continuous flow conditions with the
aim of evaluating itsdurability and of definining a simple and greener
protocol to achieve the two possible products 2 or 4. Catalyst
Au1@ZP(AEP) was mixed with glass beeds (1mm of diameter), and
charged into a stainless steel column and the system was sealed with
appropriate screw caps and with connection tubes. After an initial
and necessary additional optimization of the flow conditions, we
found that benefits arise using the flow approach. In fact, the amount
of NaBH4 could be reduced to 3 eq (Table 6, Entry 6) without any loss
in efficiency of the reaction. Further optimization regarding the
identification of the appropriate residence time to achieve complete
conversions, in continuous flow conditions the reduction process
could be completed in only 50 min.
Table 7. Comparison of TON and TOF values from batch to
flow in the preparation of 4-metoxyaniline (4a) and azoxy
derivatives 2a with Au1@ZP(AEP) as catalyst.
TON
TOF (h‐1)
4a: 500
4a: 50
BATCH
FLOW
2a: 468
2a: 31
4a: 5000
2a: 7500
4a: 600
2a: 600
aReaction condition: 1a, Au1@ZP(AEP) (0.2 mol%), NaBH4 (3‐6 eq), EtOHabs
b
(0.25M), at 30 °C. 1a, Au1@ZP(AEP) (0.2 mol%), NaBH4 (3‐6 eq), EtOH
96% (0.25M), at 30 °C. cConversion and product ratios determined by GLC
and 1H‐NMR analyses.
Table 6. Optimization of the flow in the preparation of 4-
metoxyaniline 4a and azoxy derivatives 2a with
Au1@ZP(AEP) as catalyst.
entrya
NaBH4
Flow rate
C to 4a C to 2a
2a:3a
(eq)
(mL∙min‐1)
(%)a
(%)b
Ratioc
1
2
3
4
5
6
7
8
6
0.5
87
85
84:16
6
6
3
3
3
2
2
0.25
0.1
95
>99
73
92
>99
70
90:10
92:8
Figure 12. Leaching for the flow procedure in the preparation of 4‐
metoxyaniline (4a) and azoxy derivatives 2a with Au1@ZP(AEP) as
catalyst, in EtOH 96 % (red line), and EtOHabs (green line).
0.5
87:13
90:10
93:7
0.25
0.1
92
90
>99
84
>99
80
Exploiting the continuous flow reactors, we were indeed able to
selectively convert 4‐nitroanisole (1a) to the corresponding aniline in
almost quantitative yields. Equally good results were obtained for
the production of azoxy derivatives 2a where the selectivity was
constantly excellent (Figure 13). Considering that the flow reactor
0.25
0.1
89:11
90:10
95
95
aReaction condition: 1a, Au1@ZP(AEP) (0.2 mol%), NaBH4 (3‐6 eq), EtOHabs
(0.25M), at 30 °C. 1a, Au1@ZP(AEP) (0.2 mol%), NaBH4 (3‐6 eq), EtOH
b
8 | J. Name., 2012, 00, 1‐3
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ARTICLE
allows to avoid all of the usual steps consisting in the filtration and
washing of the catalyst, we have also achieved a significant a
decrease in the waste produced resulting in low E‐factor values
associated to the process (See experimental section).
suggest that in‐situ formed gold hydride [Au‐H] is responsible for
View Article Online
the reduction of nitroarenes.12a,12e To rationalize a possible
DOI: 10.1039/C8GC03513J
mechanistic pathway that explained the observed selectivity for
the reduction in EtOH 96% and EtOHabs, we perform the reduction
process in CD3OD. In this experiment, 0.1 mmol of 4‐nitroanisole
(1a), 0.5 mol% of Au1@ZP(AEP), 0.3 mmol of NaBH4 in 1.8 mL of
CD3OD were stirred in a closed vials for 20 min. The evolution of
reaction was monitored by NMR spectroscopy directly after
quickly filtration on celite. The NMR spectrum performed
immediately after filtration showed a conversion >99%, the 4‐
metoxyaniline (4a) and the corresponding hydroxylamine 5a
(4a:5a = 55:45) were present together with small a amount of the
corresponding azoxy‐derivative 2a. No significant amount of
nitroso‐, azo‐ or hydrazo‐derivatives were detected (Figure S4a).
After 1 h the mixture turned yellow and the NMR spectrum
showed the disappearance of hydroxylamine 5a with the
formation of the azoxy‐derivative 2a (4a:2a = 56:44) (Figure S4b).
Performing the reaction in CD3OD/D2O (96:4) the conversion was
94% and the NMR spectrum performed immediately after
filtration, showed the 4‐metoxyaniline (4a), the hydroxylamine
5a and the azoxy‐derivatives 2a (9:76:15) (Figure S5a) with
additional traces of azo‐derivative 3a. After 1 h, almost all the
hydroxylamine 5a was converted in azoxy‐derivative 2a (4a:2a =
10:90) (Figure S5b). According to these results, the presence of
water plays a crucial role on the catalytic activity of gold
nanoparticles and consequently on the mechanistic pathways.
Therefore, when the reduction of nitroarenes is performed in
EtOHabs the catalytic activity of gold nanoparticles is higher, there
was no accumulation of intermediate 5a that quickly undergoes
the successive reduction to 4‐metoxyaniline 4a (Scheme 1,
pathway a).12a,12e When the reduction of nitroarenes is performed
in EtOH 96%, the presence of water inhibits the catalytic activity
of gold nanoparticles, there is accumulation of the hydroxylamine
5a that in the presence of air turns to azoxy‐derivative 2a
(Scheme 1, pathway b).
Figure 13. Features of the flow procedure in the preparation of 4‐
metoxyaniline (4a) and azoxy derivatives 2a with Au1@ZP(AEP) as
catalyst.
The ICP‐OES analysis performed on solid catalyst recovered after
flow procedure for the production of 4‐metoxyaniline 4a and azoxy
derivatives 2a show a Au content of 1.03 wt% and 0.94 wt%,
respectively, very still to pristine catalyst value (1.0 wt%). TEM
images of catalyst recovered after flow procedure in EtOH 96 % and
EtOH abs. (Figure 14) show that the AuNPs still have a nearly
spherical morphology with good monodispersity but also that the
particle size range are kept small, very close to the starting material
(7.9 (± 2.7) and 9.4 (± 3.9) nm, respectively vs 7.8 (± 2.4) nm for the
pristine material). The flow procedure improves the performance of
the catalyst with respect to the batch procedure since the absence
of mechanical stirring preserves the characteristics of the catalyst,
improving recovery and reuse.
a)
b)
Figure 11. Representative TEM image of Au1@ZP(AEP) after 15 runs
in EtOH 96 % (a) and after 10 runs in EtOHabs (b) under fow conditions
Mechanistic Pathway. We observed that, both in EtOH 96% and
EtOHabs, the mixture during the reaction performed in a closed
vial was uncolored but when the vials was opened, under air, the
reaction mixture obtained in EtOH 96% quickly turned to yellow
while that obtained in EtOHabs remained colorless. Recent papers
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NHOH
NH2
NO2
414/3 A controller at a heating rate of 10 °C/min, with an air
flow of about 30 mL/min.
View Article Online
DOI: 10.1039/C8GC03513J
Au1@ZP(AEP)
a)
Quantitative solution phase 31P NMR spectra were measured on
a Bruker Advance III HD 400 spectrometer equipped with a
smartprobe using the standard inverse‐gated decoupling pulse
sequence available on the Bruker TopSpin 3.2 library. The
relaxation delay was set to 30 s and 16 to 32 scans were
collected for each spectrum. Referencing is relative to external
85% D3PO4 in D2O. Each NMR sample was prepared by
dissolving about 10‐15 mg of the solid in 3 M HF (≈0.1 mL) and
adding about 0.5 mL of D2O.
NaBH4, EtOH abs.
OMe
OMe
OMe
1a
5a
4a
NHOH
NH2
NO2
Au1@ZP(AEP)
b)
NaBH4, EtOH 96%
OMe
OMe
OMe
1a
5a
4a
1H‐NMR, 13C‐NMR spectra were recorded at 400 MHz and 100.6
MHz, respectively, on a Bruker DRX‐ADVANCE 400 MHz.
Zr, P and Au analyses were performed on an ICP Varian Liberty
inductively‐coupled plasma‐optical emission spectrometry (ICP‐
OES) with axial injection. Gold‐free solid samples were dissolved
with 3 M HF, while the gold‐based solid samples were dissolved
by treatment with 3 M HF, followed by treatment with a
suitable volume of aqua regia.
under air
OMe
N
N
O
MeO
2a
Transmission electron microscopy (TEM) analysis was carried
out by a Philips 208 transmission electron microscope,
operating at an accelerating voltage of 100 kV. Powders were
rapidly diluted in water, then supported on copper grids (200
mesh) precoated with Formvar carbon films and quickly dried.
Diffusive reflectance measurements were carried out by using a
portable spectrophotometer consisting of Avantes parts and a
deuterium‐halogen lamp (AvaLight‐DH‐2000‐FHS). The
reflectance signals were colllceted by an integrating sphere with
a 6 mm diameter viewing aperture and 88 irradiation angle (ISP‐
30‐6) and transferred to an AvaSpec–2048 charge‐coupled
device (CCD) detector via a quartz fiber‐optic system (diameter
Scheme 1. A plausible reaction pathways for reduction of
nitrobenzene in EtOHabs (a) and EtOH 96% (b).
Conclusion
In conclusion, we have reported the novel and efficient
preparation of AuNPs immobilized on a novel inorgano‐organic
layered material,
a
zirconium phosphate aminoethyl
phosphonate, ZP(AEP), bearing aminoethyl groups on the layer
surface. The gold‐based catalyst, Au1@ZP(AEP), containing 1
wt% Au, was fully characterized. Au1@ZP(AEP) proved to be
highly efficient for the chemoselective reduction of nitroarenes
under mild conditions. Both batch and flow condition protocols
have been defined proving the benefits of the flow approach
both in terms of chemical and environmental efficiency. We
have been able to control the reduction of a series of
nitroaromatics and switch the chemoselectivity towards
600 m). The acquisition of the spectra in the range 200 – 1100
nm was controlled by the AvaSoft software. The experimental
values of the reflectance, R, were normalized and used to
calculate the K (absorption)/S (scattering) coeffficient ratios
according to the following algorithm: K/S = (1‐R)2/2R, in
agreement with the Kubelka‐Munk theory.24
azoxyarenes (2a‐k) or anilines (4a‐l) using EtOH 96% or EtOHabs
,
respectively. Recovery and reuse of the catalytic system has
been investigated proving the benefits of the flow approach.
Infrared measurements were performed in the 300‐5000 cm‐1
range at 2 cm‐1 resolution by ALPHA‐FTIR spectrometer of
Bruker‐Optics, and ATR sampling on a diamond crystal by
Platinum‐ATR module.
SEM analysis was performed by a Zeiss LEO 1525 FE SEM,
equipped with a Angle selective Backscattered detector (AsB)
for high energy backscattered electrons imaging, at the LUNA
Laboratory of the Department of Physics and Geology of Perugia
University. The powder samples were put on an aluminum stub
precoated with a double sided adhesive conductive carbon
tape.
Nitrogen adsorption/desorption isotherms at 77ꢁK were
measured using a computer‐controlled Micrometrics ASAP
2010 adsorption analyzer. The preparation of samples was
made by degassing the materials at room temperature,
overnight. The specific surface area was calculated by the
Brunauer, Emmett and Teller (BET) theory.
Experimental section
General
Aminoethylphosphonic acid (H2O3PC2H4NH2, hereafter H2AEP)
was purchased from Epsilon Chimie, France. Hydrogen
tetrachloroaurate (III) were purchased from Alfa Aesar. Zirconyl
propionate (ZrO1.27(C2H5COO)1.46, MW = 218 Da) was supplied
by MEL Chemicals, England. All other reagents were supplied by
Aldrich and were used as received without further purification.
XRPD patterns were collected with a Philips X'Pert PRO MPD
diffractometer operating at 40kV and 40mA, with a step size
0.03341 and step scan 40 s, using CuKα radiation and an
X'Celerator detector. To minimize preferred orientations, the
powder samples were carefully side‐loaded onto a glass sample
holder.
Thermogravimetric analysis was carried out by a NETZSCH
STA449 Jupiter thermal analyzer connected to a NETZSCH TASC
GLC analyses were performed by using the Hewlett‐Packard HP
5890A equipped with a capillary column DB‐35MS (30 m, 0.53
mm), an FID detector and hydrogen as the gas carrier. Gas
10 | J. Name., 2012, 00, 1‐3
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Chromatography with Electron Impact Mass Spectrometry (GC‐ [a] Calculated by ICP analysis. In the text the samplVeieswhAartvicelebOenleinne
EIMS) analyses were carried out by using a Hewlett‐Packard HP labelled as Au1@ZP(AEP) (Au loading
DOI: 10.1039/wC8tG%C)035a1n3dJ
=
1
6890N Network GC system/5975 Mass Selective Detector Au2.4@ZP(AEP) (Au loading = 2.4 wt%).
equipped with an electron impact ionizer at 70 eV. Thin layer
chromatography analyses were performed with silica gel on General procedure for catalytic reduction of nitroarenes
aluminum plates (silica gel 60 F254, Fluka). Column In a vial of 2 mL, Au1@ZP(AEP) catalyst (20 mg, 0.05 mmol of Au
chromatography purification was performed with silica gel per g, 1 mol%), nitroarene (
1, 0.1 mmol), NaBH4 (22.8 mg, 6 eq)
(230–400 mesh) eluting with hexane/ethyl acetate (98:2– and EtOHabs or EtOH 96% (1.8 mL) were placed. The reaction
70:30). Melting points were measured on a Büchi 510.
Anilines 4a
l 11c, 12b, 14a, 29, 30 and azoxy‐derivatives 2a
mixture was stirred at 30 °C for a selected time (Table 2 and 3);
k 11, 31, 32 the conversion of nitroarenes was monitored by GLC analyses.
–
–
are known compounds, azoxy‐derivative 2j is a new compound. After completion, the solid catalyst was separated by
Preparation of layered zirconium phosphate‐aminoethyl centrifugation and then washed with EtOH (2 x 1 mL). The
phosphonates (ZP(AEP)x)
ethanol solution, that contained the products and the borate
A gel in propanol (PrOH) of nanosized zirconium phosphate derived from NaBH4, was distilled at reduced pressure, the
(hereafter ZP) was used as starting material for the synthesis of residue was diluted with ethyl acetate (5 mL) and washed with
ZP(AEP)x and it was prepared according to Pica et al. method.33 water (2 x 2 mL). The organic layers were dried with sodium
The gel contained about 8 wt% of ZP.
sulfate and evaporated under reduced pressure. The solid
ZP(AEP)x compounds have been prepared by topotactic anion catalyst was washed with aqueous HCl 10‐2 M to eliminate the
exchange reaction of the phosphate groups of nanosized ZP borate, water and finally with EtOH, dried at 130 °C for 15 h and
with AEP groups. Specifically, a suitable volume of aqueous reused in the next cycle.
solutions of H2AEP 0.3 M, 0.6 M or 0.8 M was added to 1 g of ZP
in the form of gel in propanol (14 mL PrOH/1 g ZP), so that the 3,3’‐di‐(1‐hydroxyethyl)azoxybenzene (2j). Purification by
H2AEP/Zr molar ratio was 1, 2, 3, respectively, while the column chromatography on silica gel eluting with hexane/ethyl
(PrOH/H2O) volume ratio was 1 in all cases. The stirred mixture acetate (6:4). Obtained 11.4 mg of a pale yellow oil (80%).
was then heated at 80°C for three days. The ZP(AEP)x samples 1HNMR (400 MHz) δ: 1.50 (s, 3H), 1.54 (s, 3H), 2.52 (s, 2H), 4.90‐
were washed with a 10‐3 M HCl solution in H2O/PrOH (1:1 v/v) 4.99 (m, 2H), 7.38‐7.48 (m, 3H), 7.52‐7.54 (d, 1H, J = 7.6 Hz),
and then dried at 60°C for 15 hours. The samples were again 8.04‐8.06 (d, 1H, J = 7.8 Hz), 8.08 (s, 1H), 8.12‐8.14 (d, 1H, J = 8
washed with a 10‐3 M HCl solution and finally dried at 60°C for Hz), 8.25 (s, 1H).
15 hours.
13CNMR (100.6 MHz) δ: 25.120, 25.26, 69.69, 70.03, 119.26,
The AEP/Zr molar ratios (x) in ZP(AEP)x were determined by ICP 121.18, 122.66, 124.27, 126.73, 128.63, 128.80, 128.92, 143.87,
and 31P liquid NMR and the obtained values were reported in 146.48, 147.13, 148.24.
Table 1.
GC‐MS (m/z): 286 (41), 242 (5), 195 (5), 181 (15), 169 (8), 149
(8), 121 (88), 103 (100), 93 (30), 91 (24), 77 (56), 65 (20), 51 (11).
2. Preparation of gold nanoparticles supported on ZP(AEP)0.72 Anal. Calcd for C16H18N2O3 (286.13): C, 67.12; H, 6.34; N, 9.78;
ZP(AEP)0.72 was selected as support for the synthesis and Found C, 66.93; H, 6.42; N, 9.59.
immobilization of AuNPs. In a typical experiment, 150 mg of
ZP(AEP)0.72 were suspended in 6.0 mL (or 15 mL) of water, then, Typical procedure for
0.26 mL (or 0.62 mL) of a 0.03 M HAuCl4 solution was added, so preparation of 4a
a sustainable protocol for the
as to obtain materials with 1 and 2.4 wt% of Au loading. The In a vial of 10 mL, Au1@ZP(AEP) catalyst (100 mg, 0.05 mmol of
mixtures were left under stirring at room temperature for about Au per g, 1 mol %), 4‐nitroanisole 1a (76.5 mg, 0.5 mmol), NaBH4
one day. Then, 0.39 mL (or 0.94 mL) of a 0.4 M freshly prepared (114 mg, 6 eq), and EtOHabs (9 mL) were placed. The reaction
NaBH4 solution was added: the NaBH4/Au(III) molar ratio was mixture was stirred at 30 °C for 2 h. After removal and recovery
around 20 in both experiments. The solution color immediately of the ethanol via distillation (90% of solvent recovered) the
turned from pale yellow to pink/plum. The experimental product was separated simple by washing the residue with
conditions used for the preparation of the catalysts are cyclopenthyl methyl ether (3 x 2 mL). The solvent was
summarized in Table 6. The color of the mixture immediately evaporated by distillation (95% of solvent recovered) obtaining
turned from pale yellow to pink/plum. The solids were the pure product (yield of 4a 96%). The solid catalyst was
recovered by centrifugation, washed with water and dried at washed with aqueous HCl 10‐2 M to eliminate the borate, water
room temperature over P2O5.
and finally with EtOH, dried at 130 °C for 15 h and reused in the
next cycle.
Table 8. Experimental conditions for the synthesis and E‐factor calculation for product 4a (yield 96%): E‐factor =
composition data of Au@ZP(AEP).
HAuCl4 0.03 NaBH4 0.4 M
[0.0765 g (4‐nitroanisole, 1a) + 0.114 g (NaBH4) + 0.100 g
(Au1@ZP(AEP)) + 10.4 g (EtOH) – 0.059 g (4‐methoxyaniline,
4a)–0.095 g (recovered Au1@ZP(AEP)) – 9.5 g (recovered
EtOH)/ 0.059 g (4‐methoxyaniline, 4a) = 17.6.
Without recover of reaction medium the E‐factor value was
178.6.
NaBH4/Au
molar ratio
20
Au loading
(wt%)[a]
1.0
M (mL)
(mL)
0.39
0.94
0.26
0.62
20
2.4
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 11
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Green Chemistry
Page 12 of 13
ARTICLE
Journal Name
Typical procedure for
preparation of 4a under flow conditions
a sustainable protocol for the
Acknowledgements
View Article Online
DOI: 10.1039/C8GC03513J
The Università degli Studi di Perugia and MIUR is acknowledged
for financial support to the project AMIS, through the program
“Dipartimenti di Eccellenza ‐ 2018‐2022”. The authors would
like to thank Prof. Paola Sassi for the FTIR analysis, Dr Fabio
Marmottini for surface area measurements and Dr Alessandro
di Michele for SEM analysis.
A premixed mixture of 1a (1.53 g, 10 mmol), NaBH4 (1.13 g, 3
eq) and EtOHabs (40 mL)) was charged into a glass column
functioning as a reservoir. The equipment was connected, by
using the appropriate valves, to a pump and installed into a box
thermostated at 30 °C. The reaction mixture was continuously
pumped (flow rate: 0.1 mL∙min‐1; residence time: 50 min)
through the catalyst column (15 cm, stainless steel HPLC
column) equipped with a 120 psi back pressure regulator, and
the reaction was monitored by GC. Product was collected in
fractions and gold content in the EtOH solution was periodically
measured by ICP‐OES analysis. After completion 20 mL of EtOH
was used to wash the catalyst inside the reactor. After removal
and recovery of the solvent via distillation (90% of solvent
recovered) the product was separated simple by washing the
residue with cyclopenthyl methyl ether (3 x 15 mL). The solvent
was evaporated by distillation (95% of solvent recovered)
obtaining the pure product (yield of 4a 97%). The catalyst inside
the column was dried under N2 flow at 180 °C and reused in the
next cycle.
Notes and references
‡ Experimental Section and representative E‐factor calculation,
full characterization of compounds and copies of the H and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
13C
1
2
3
(a) R. A. Sheldon Green Chem. 2017, 19, 18‐43; (b) R. A.
Sheldon, I. Arends, U. Hanefeld, Green Chemistry and
Catalysis. Wiley‐VCH: Weinheim, 2007.
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,
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Chem. Soc. Rev. 2018, 47, 2722‐2771; (b) S. Santoro, S. I.
E‐factor calculation for product 4a (yield 97%): E‐factor = [1.5 g
(4‐nitroanisole, 1a) + 1.13 g (NaBH4) + 32 g (EtOH) + 39 g (CPME)
– 1.2 g (4‐methoxyaniline, 4a) – 29 g (recovered EtOH)] – 37 g
(recovered CPME)] / 1.2 g (4‐methoxyaniline, 4a) = 5.4
Kozhushkov, L. Ackermann, L. Vaccaro, Green Chem. 2016, 18
,
3471‐3493; (b) L. Liu, A. Corma, Chem. Rev. 2018, 118, 4981–
5079; (c) R. Van Santen in Modern Heterogeneous Catalysis.
An Introduction. Weinheim: Wiley‐VCH, 2017, (d)
Rothenberg, G. (2017). Catalysis: Concepts and Green
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Typical procedure for
preparation of 2a under flow conditions
a sustainable protocol for the
4
For selected examples see: (a) S. Santoro, A. Marrocchi, D.
Lanari, L. Ackermann, L. Vaccaro, Chem. Eur. J. 2018, 24
,
A premixed mixture of 1a (2,29 g, 15 mmol), NaBH4 (1.7 g, 3 eq)
and and EtOH 96 %. (60 mL)) was charged into a glass column
functioning as a reservoir. The equipment was connected, by
using the appropriate valves, to a pump and installed into a box
thermostated at 30 °C. The reaction mixture was continuously
pumped (flow rate: 0.1 mL∙min‐1; residence time: 50 min)
through the catalyst column (15 cm, stainless steel HPLC
column) equipped with a 120 psi back pressure regulator, and
the reaction was monitored by GC. Product was collected in
fractions and gold content in the EtOH solution was periodically
measured by ICP‐OES analysis. After completion 20 mL of EtOH
was used to wash the catalyst inside the reactor. After removal
and recovery of the solvent via distillation (90% of solvent
recovered) the product was separated simple by washing the
residue with cyclopenthyl methyl ether (3 x 18 mL). The solvent
was evaporated by distillation (95% of solvent recovered)
obtaining the pure product (yield of 2a 90%). The catalyst inside
the column was dried under N2 flow at 180 °C and reused in the
next cycle.
13383–13390; (b) F. Ferlin, V. Trombettoni, L. Luciani, S. Fusi,
O. Piermatti, S. Santoro, L. Vaccaro, Green Chem. 2018, 20
,
1634–1639; (c) L. Luciani, E. Goff, D. Lanari, S. Santoro, L.
Vaccaro, Green Chem. 2018, 20, 183–187; (d) S. Santoro, F.
Ferlin, L. Luciani, L. Ackermann, L. Vaccaro, Green Chem. 2017,
19, 1601–1612; (e) F. Fringuelli, D. Lanari, F. Pizzo and L.
Vaccaro, Green Chem., 2010, 12, 1301‐1305; (f) S. Bonollo, F.
Fringuelli, F. Pizzo and L. Vaccaro, Synlett, 2007, 2683–2686;
(g) L. Castrica, F. Fringuelli, L. Gregoli, F. Pizzo and L. Vaccaro,
J. Org. Chem., 2006, 71, 9536–9539; (h) F. Fringuelli, F. Pizzo
and L. Vaccaro, Synlett, 2000, 311–314.
(a) E. Petricci, C. Risi, F. Ferlin, D. Lanari, L. Vaccaro, Scientific
Rep. 2018, 8, 10571; (b) F. Ferlin, L. Luciani, S. Santoro, A.
Marrocchi, D. Lanari, A. Bechtoldt, L. Ackermann, L. Vaccaro.
Green Chem. 2018, 20, 2888–2893; (c) F. Ferlin, S. Santoro, L.
Ackermann and L. Vaccaro, Green Chem., 2017, 19, 2510–
2514; (b) D. Rasina, A. Kahler‐Quesada, S. Ziarelli, S. Warratz,
H. Cao, S. Santoro, L. Ackermann and L. Vaccaro, Green Chem.,
2016, 18, 5025–5030; (c) X. Tian, F. Yang, D. Rasina, M. Bauer,
S. Warratz, F. Ferlin, L. Vaccaro and L. Ackermann, Chem.
Commun., 2016, 52, 9777–9780; (d) G. Strappaveccia, E.
Ismalaj, C. Petrucci, D. Lanari, A. Marrocchi, M. Drees, A.
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G. Strappaveccia, L. Luciani, E. Bartollini, A. Marrocchi, F. Pizzo
and L. Vaccaro, Green Chem., 2015, 17, 1071–1076; (f) E.
Ismalaj, G. Strappaveccia, E. Ballerini, F. Elisei, O. Piermatti, D.
5
E‐factor calculation for product 2a (yield 90%): E‐factor = [2.3 g
(4‐nitroanisole, 1a) + 1.7 g (NaBH4) + 47 g (EtOH) + 46 g (CPME)
– 3.5 g (azoxyderivative, 2a) – 42 g (recovered EtOH)] – 44 g
Gelman and L. Vaccaro, ACS Sustainable Chem. Eng., 2014, 2,
2461–2464.
(a) B. C. Gates, Chem. Rev. 1995, 95, 511‐522; (b)
Nanomaterials in Catalysis, P. Serp, K. Philippot, Eds.; Wiley‐
(recovered CPME)] / 3.5 g (4‐methoxyaniline, 2a) = 2.1
6
Conflicts of interest
There are no conflicts of interest to declare.
VCH, RSC Catalysis series, 2013, No. 17
; (c) Metal
Nanoparticles for catalysis: Advances and Application, F. Tao,
Ed.; 2014, Royal Society of Chemistry.
12 | J. Name., 2012, 00, 1‐3
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