Insight into the mechanism of gold-catalyzed reduction of nitroarenes based on the substituent effect and: In situ IR

Article abstract of DOI:10.1039/c7nj00157f

A study is presented here on the mechanistic pathway of catalytic reduction of nitroarenes with diverse substituent groups in the presence of ultra-small gold nanoparticles and NaBH₄. The kinetic data based on the substituent effect and the Hammett plot, together with analyses of intermediates, revealed that this reaction followed a direct route and that all of the reaction steps were rapid without accumulation of intermediates. In particular, in situ IR was found to be an effective method for the real-time monitoring of this model reduction in our catalytic system and confirmed the presence of a hydroxylamine intermediate. These results will provide a new insight into the mechanism of gold-catalyzed reduction of nitroarenes and offer a reference for future mechanism tests of heterogeneous nanocatalysis.

Full text of DOI:10.1039/c7nj00157f

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Wang, W. Zou,
J. Wang, Y. Ge, R. Lu and S. Zhang, New J. Chem., 2017, DOI: 10.1039/C7NJ00157F.
This is an Accepted Manuscript, which has been through the
Volume 40 Number
1 January 2016 Pages 1–846
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
Accepted Manuscripts are published online shortly after
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Page 1 of 16
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00157F
Insight into the Mechanism of Gold-Catalyzed Reduction of
Nitroarenes Based on Substituent Effect and in-situ IR
Cui Wang, Wei Zou, Jiasheng Wang, Yuzhen Ge, Rongwen Lu*, Shufen Zhang
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian,
People’s Republic of China.
Corresponding Author: * Rongwen Lu
Eꢀmail: lurw@dlut.edu.cn
Abstract
A study is presented here on the mechanistic pathway of catalytic reduction of
nitroarenes which have diverse substituent groups in the presence of ultra small gold
nanoparticles and NaBH4. The kinetic data based on substituent effect and Hammett
plot, together with the analyses of intermediates, revealed that this reaction followed a
direct route and all the reaction steps were rapid without accumulation of
intermediates. Especially, inꢀsitu IR was found to be an effective method for realꢀtime
monitoring this model reduction in our catalytic system and confirmed the presence of
intermediate of hydroxylamine. These results will provide a new insight into the
mechanism of goldꢀcatalyzed reduction of nitroarenes and offer a reference for future
mechanism test of heterogeneous nanocatalysis.
1 Introduction
The catalytic reduction of nitroarenes with sodium borohydride to the corresponding
amino compounds has been considered as one of the most trustful model reactions to
test the activity of a given nanocatalyst.1ꢀ6 However, for respective catalyst, this
reaction remains controversial to the mechanistic pathways of reduction process. One
proposed route is that the nitro group in nitroarenes is firstly reduced into nitroso and
then hydroxylamine, finally aniline compound.7 Another route is a condensation

New Journal of Chemistry
Page 2 of 16
View Article Online
DOI: 10.1039/C7NJ00157F
process, azoxy compound is firstly generated from nitroso and hydroxylamine
compounds then reduced into a series of consecutive products, like azo, hydrazo, and
aniline compounds.8, 9 Thus, the number of papers related to the mechanism study of
the reduction of nitroarenes with a given nanocatalyst is steadily increased.10ꢀ13
A variety of methods have been applied to take insight into the mechanistic
pathway due to the controversy of reaction process. A. Noschese and coꢀworkers14
monitored the reaction profiles by GCꢀMS and proved the presence of azo compound.
Likewise, S. Kundu15 and K. Layek16 directly used the possible intermediates (nitroso
and azo compound) as reactants instead of the parent substrates and proved
mechanistic information for the reduction. Besides, S. Wunder and coꢀworkers17, 18
presented a systematic kinetic analysis to study the true kinetic constant of the
reaction along with the thermodynamic adsorption constants for both pꢀnitrophenol
ꢀ
(pꢀNip) and BH4 on gold nanoparticles. By using kinetic isotope effect, S.
Fountoulaki19 suggested that an in situꢀformed gold hydride ([Au]ꢀH) is responsible
for the reduction of nitroarenes. Moreover, density functional theory method was also
extended to study the reaction mechanism.20, 21 However, beside all these detailed
studies, very little attention had been paid on the substituent effect in this
heterogeneous catalysis which was always used as a delicate probe into the electronic
demand of the reaction in physical organic chemistry22. Therefore, the study of
substituent effect to understand the correlation of molecule structure with reactivity of
substrates and explore the catalytic mechanism on nanocatalyst is full of interest.
Herein, we reported a mechanism pathway for the reduction of nitroarenes by
NaBH4 based on the substituent effect. Ultra small gold nanoparticles supported on
silica (AuꢀSiO2) were synthesized and used as the heterogeneous catalyst. It was
found that the substituent group in nitroarenes had a profound effect on the AuꢀSiO2
catalyzed reduction reactivity. By combining the analysis of substituent effect and
Hammett plot, this finding deepened our understanding of the goldꢀcatalyzed
reduction of nitroarenes by NaBH4. Besides, inꢀsitu IR is found to be a suitable and
direct analysis method for realꢀtime tracing this AuꢀSiO2 catalyzed reduction process
which is beneficial to verify the mechanism pathway.

Page 3 of 16
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00157F
2 Experimental Details
2.1 Materials
Polyoxyethylene (20) cetyl ether (Brij®58) was purchased from Acros Organics.
Chloroauric acid, sodium borohydride (NaBH4), tetraethyl orthosilicate (TEOS),
ammonium hydroxide (NH3·H2O, 25%–28%), cyclohexane, methanol, ethanol,
isopropanol (IPA), tetrahydrofuran (THF) and all of nitro compounds were purchased
from Sinopharm Chemical Reagent Co., Ltd.. All chemicals were analytical grade.
2.2 Preparation of Au-SiO2
The AuꢀSiO2 was synthesized as described previously.23 Typically, the synthesis
was carried out in a reverse microemulsion system consisted of 3.37 g of Brij®58 and
15 mL of cyclohexane at 52 °C. Aqueous chloroauric acid (0.1 M, 0.2 mL) was added
dropwise into the system under stirring. After 5 min, NaBH4 aqueous solution (0.14M,
0.7mL) was added and then maintained for 30 min. Finally, 0.5 mL NH3·H2O and 1.0
mL TEOS were introduced to prepare the silica support. The AuꢀSiO2 sample was
collected by centrifugation/washing cycle for three times. The product was dried at
100 °C for 10 h and calcined at 500 °C for 2 h in an air flow of 80 mL/min.
2.3 Catalytic Reaction
As a typical reaction, aqueous solution of nitroarenes (4 mmol/L, 50 mL),
NaBH4 powder (75 mg, 10ꢀfold for nitroarenes) and AuꢀSiO2 catalyst (20 mg, 1mol%
Au) were added in reactor at room temperature under air. After each run, the AuꢀSiO2
was collected by centrifugation/washing cycle three times. The product was detected
by an Agilent 1100 HPLC and 6130 MSD and a Bruker Avance III 500 1H NMR.
Kinetic experiments: all the reactions proceeded with 50 mL of the nitroarenes
solution (4 mmol/L in water), 75 mg of NaBH4 and 20 mg of AuꢀSiO2. Then the
reduction progress was monitored by Agilent 8453 UVꢀvis spectrometer for
pꢀnitrophenol, pꢀnitrobenzenethiol, mꢀ and pꢀphenylenediamine, and by Agilent 1100
HPLC for mꢀnitrophenol, mꢀ and pꢀnitrobenzoic acid
methods was done in supporting information. Each reaction was repeated at least
three times
. A comparison of these two
.

New Journal of Chemistry
Page 4 of 16
View Article Online
DOI: 10.1039/C7NJ00157F
In the inꢀsitu IR investigation，the concentration of pꢀNip was increased to
enhance the intensity of the absorption peak. Reaction conditions: 50 mL of the pꢀNip
solution (40 mmol/L in water), 750 mg of NaBH4, and 100 mg of AuꢀSiO2, at room
temperature.
3 Results and Discussion
3.1 Evaluation of the Reduction
The ultra small gold nanoparticles supported on silica in this study was generated
as described previously.23 The silica support was monodispersed with an average size
around 28 nm, and the dark points dispersed on the nanosphere are the gold particles
with an ultra small size. The amount of gold was obtained to be 2.02 wt% by
inductively coupled plasmaꢀoptical emission spectrometer, Perkin Elmer Optima 2000
(ICP).
Figure 1 TEM morphology of AuꢀSiO2 (inset showed HRTEM of AuꢀSiO2
)
At the initial stage of catalytic studies, the reduction of pꢀNip by NaBH4 was
selected to check the catalytic activity of ultra small gold nanoparticles as it had been
a model reaction for metal nanoparticles. The reaction was carried out using AuꢀSiO2
(1 mol% for pꢀNip) in water at room temperature under air. As shown in Figure 2a,
the progress can be easily monitored by UVꢀvis spectrometer. After the addition of

Page 5 of 16
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00157F
catalyst, the absorption of pꢀNip ions at 400 nm decreased with reaction time. Finally,
the reactant pꢀNip was smoothly converted to product after 25 min with over 99%
conversion. The only product observed was pꢀaminophenol and no byꢀproduct was
detected by HPLCꢀMS and 1H NMR.
Figure 2 UV–Vis profiles of the reduction of pꢀNip by NaBH4 in water (a), methanol (b), ethanol
(c), and THF (d). Reaction conditions: 50 mL of pꢀNip solution (4 mmol/L), 1 mol% AuꢀSiO2, and
10ꢀfold NaBH4, at room temperature.
For metal nanoparticlesꢀcatalyzed reduction of pꢀNip by NaBH4, most of the
reporters chose water as the solvent without a clear reason, therefore solvent effect
was taken into account in this reaction. As is conjectured that water provides
protonation for the reduction, other solvents with different protonꢀdonating ability
(methanol, ethanol and THF) were investigated to compare with water and the results
are shown in Figure 2. It was found that solvent had a significant effect on the
performance of reaction rate in the same experimental condition. It took only 25
minutes to accomplish the conversion of pꢀNip in water, while over 2 hours was

New Journal of Chemistry
Page 6 of 16
View Article Online
DOI: 10.1039/C7NJ00157F
required in methanol, ethanol and THF. Meanwhile the reactivity decreased in
different solvents in the following order: water > methanol > ethanol > THF. This
result is in accordance with the previous conjecture that the protonation of solvent
plays an important role in the reaction. Another way to modulate the protonation of
solvent is changing the pH of the solution24. As shown in Table S1 and Figure S2 in
supporting information, it was found that the catalytic reduction proceeded much
slower at higher pH values throughout the pH range from 8.21 to 12.0 without kinetic
changes. These results proved again the great impact of protonation of solvent on our
reaction system. On the other hand, due to the formation of pꢀNip ions with the
addition of NaBH4, polar solvent is favorable to dissolve and stabilizes the charged
species. As the polarity of water is much higher than that of others (dielectric constant:
water 78.4, methanol 32.7, ethanol 24.5, THF 7.625), the reduction of pꢀNip in water is
the fastest. Therefore, water is not only an environmental solvent, but also plays an
important role in the reduction of pꢀNip.
3.2 Substituent effect and kinetic analysis
Organic compounds have a unique capacity that structural variation permits a
significant effect on electrophilic or nucleophilic character in the vicinity of reaction
center. The correlation of structure with reactivity could be used as a delicate probe
into the electronic demand of the reaction. The use of this information in deducing
mechanism is a highly developed art.22
Currently, for goldꢀcatalyzed reduction of nitroarenes by NaBH4, the most
widely reported was the reduction of pꢀNip. However, for other substituent aromatic
nitro compounds, very little is considered whether the nitroarenes with
electronꢀdonating or electronꢀwithdrawing group will react faster or slower in the
reduction, and the effect of substituent group has yet to be determined. So that
substituent effect was investigated here to understand the correlation of structure with
reactivity and the reduction mechanism.
In view of the substituent effect on the activity of AuꢀSiO2 as catalyst for the
reduction of nitroarenes, various types of nitro compounds were investigated in the
reactions. All the reactions were performed in aqueous solution at room temperature.

Page 7 of 16
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00157F
The results are listed in Table 1. Obviously, all the nitroarenes with diverse
substituent groups could be reduced into corresponding amino compounds with
excellent yields (most of the conversions were higher than 99%). However, there was
considerable diversity in the reaction time. The reduction of pꢀnitrobenzoic acid was
the fastest, which took only 5 minutes to reach quantitative conversion, while it
needed 50 minutes for pꢀnitrobenzenethiol to finish the reaction. These results
reflected that substituent group has a great effect on the reactivity of nitroarenes for
the reduction catalyzed by AuꢀSiO2.
Table 1 Reduction of various nitroarenes over AuꢀSiO2
Time
(min)
Conversion Selectivity
Substrate
Product
(%)
(%)
24
12
29
15
5
99.7
>99
99.0
99.9
99.3
99.2
98.8
>99
>99
>99
>99
>99
9
30
50
98.2
98.9
>99
>99
Reaction conditions: nitroarenes aqueous solution (4 mmol/L, 50mL), AuꢀSiO2 (20 mg, 1 mol%
Au), NaBH4 (75 mg, 10ꢀfold for nitroarenes), at room temperature under air. The products were
detected by HPLCꢀMS and 1H NMR.
To rationalize the reason for the effect of substituent group in the goldꢀcatalyzed

New Journal of Chemistry
Page 8 of 16
View Article Online
DOI: 10.1039/C7NJ00157F
reduction, detailed kinetic analyses were conducted during the reduction using NaBH4
as reducing agents. And a powerful tool for providing valuable information about the
substituent effect, Hammett equation22, was used for the interpretation of mechanism.
The kinetic analyses were determined by monitoring the reduction process of the
substituted nitroarenes (Table 1) using HPLC. For an accurate comparison, all the
kinetic analysis began with 50 mL of 4 mmol/L nitroarenes aqueous solution, 1 mol%
AuꢀSiO2, and 10ꢀfold NaBH4. Then the reduction progress was monitored along with
reaction time. Each detection was repeated for at least three times, and the average
values are depicted in Figure 3. All the plots of lnCt versus reaction time yield straight
lines, which indicated that the reduction is a pseudoꢀfirstꢀorder reaction for
nitroarenes in the presence of AuꢀSiO2 using NaBH4 as reducing agent26. Considering
that the excess enough of hydride species and the pseudoꢀfirstꢀorder reaction on the
nitroarenes, eq 1 can be applied:
lnCt=ꢀkxt+lnC0
(1)
where k is the rate constant, x is the substituted nitroarenes, Ct and C0 represent the
concentration of nitroarenes at reaction time t and the beginning, respectively.
Therefore, the slope of the plots in Figure 3 is the rate constant kx according to eq 1. It
is worth noting that the kinetic activity of nitroarenes is significantly affected by the
substituent group.
Figure 3 Evolution of the concentration of the nitroarenes with diverse substituent groups.
Reaction conditions: 50 mL of 4 mmol/L nitroarenes aqueous solution, 20 mg AuꢀSiO2, and 75 mg

Page 9 of 16
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00157F
NaBH4, at room temperature.
On the basis of this kinetic analysis, there is a Hammettꢀtyped correlation between
the reaction rate constant of nitroarenes and the electronꢀdonating ability of the
substituent group. The Hammett equation22, 27 is written as
lg (kx/ kH)=ρσx
(2)
where k is the rate constant, σ is the substituent constant, and ρ is reaction constant.
As strong resonance interaction occurs between reaction center and some substituents,
the revised substituent constants are widely used for such situations instead of original
σ.27, 28 The Hammett plot here shows a good correlation between the revised σ values
and the rate constant kx (Figure 4). The correlation gives positive slopes: ρ=0.54. This
result revealed that the electronꢀdonating group is not conducive to form and stabilize
the transition state19, 22, so that the reduction rate was associated with the
electronꢀdonating capacity of substituent groups. The result also suggested that there
was a development of negative charge in the transition state, or there was a hydride
transfer occurs in the reaction process.19, 22
Figure 4 Hammett plot of the rate against revised substituent constants.
3.3 Mechanism
To deep understand the mechanism of the nitroarenes reduction, it is desirable to
confirm the reaction route involved in AuꢀSiO2. In general, there are two different

New Journal of Chemistry
Page 10 of 16
View Article Online
DOI: 10.1039/C7NJ00157F
routes proposed at the reduction step. In the direct route, the nitro compound was
reduced into the corresponding nitroso, hydroxylamine, and aniline compound
consecutively. The second route was a condensation route, the forming nitroso and
hydroxylamine reacted to give an azoxy compound and continued to be reduced in a
series of steps to the azo, hydrazo, and aniline compound.8, 9, 15
Comparing to the direct route, there is an extra azoxy compound generated in
condensation route. So, this extra intermediate was subjected to the reduction process
to check the reaction followed. It was found that when 4,4’ꢀdihydroxyazobenzene was
subjected to the reaction system instead of pꢀNip under the same conditions, the
conversion was only 39.2% in 2 hours. This reduction rate is far slower than that of
pꢀNip. Hence, the reduction of 4,4’ꢀdihydroxyazobenzene must be the
rateꢀdetermining step in the whole reduction process if the reduction pathway go
through the condensation route, and the condensation intermediates should be
accumulated. To verify this observation, HPLCꢀMS was used to monitor the whole
reduction of pꢀNip to check the intermediates. However, no traces of the
corresponding azoxyꢀ, azoꢀ, or hydrazo benzenes was found in the reaction mixture.
In addition, pꢀnitrobenzenethiol was also used as the substrate to check the the
presence of intermediates during the reaction by HPLCꢀMS, which had been
discovered the azobenzene intermediate in reports29, 30. However, it is failed to find
the trace of azobenzene in our reaction system. These results suggested that the
condensation route did not occur in the reduction of nitroarenes involved in AuꢀSiO2,
and confirmed that the reduction followed a direct route. As only parent reactant and
final product were found when using HPLCꢀMS to monitor the whole reduction, the
reduction must proceed rapidly without accumulative intermediate. The absence of
nitroso and hydroxylamine can attribute to their strong adsorption on
hydrideꢀmodified gold surface and a very fast reduction before desorption.16 This also
explains the reason that no formation of corresponding azoxy benzenes which
generated based on the accumulation of nitroso and hydroxylamine. Therefore, the
reduction of nitroarenes catalyzed by AuꢀSiO2 followed direct pathway, and there was

Page 11 of 16
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00157F
no accumulation of intermediates due to the high catalytic activity of gold
nanopaticles.
Scheme 1 Proposed mechanistic pathway of the goldꢀcatalyzed reduction of nitroarenes by NaBH4
in aqueous solution. The reaction proceeded very fast following direct route without intermediate
accumulation.
According to the analyses above, a reasonable mechanistic pathway was
proposed for this AuꢀSiO2 catalyzed reduction of nitroarenes by NaBH4 in aqueous
solution, that is, a direct route as shown in Scheme 1. Based on the substituent effect
and Hammett plot, it is reasonable to say that the hydride transfer occured in the
transition state. The reduction process on the surface of gold nanoparticles was that
the activated hydride attack the reaction center in nitro group consecutively,
meanwhile the solvent water provided protonation for the reduction. Hence the nitro
compound was reduced into the corresponding nitroso, hydroxylamine, and amino
compound consecutively. Due to the high catalytic activity of ultra small gold
nanoparticles synthesized, the reduction proceeded rapidly without intermediate
accumulation. In the meantime, the substituent group exhibited the significant impact
on the reactivity, which attributed to that the presence of electronꢀwithdrawing group
which is beneficial to the attack of hydride to reduce nitro group into amino group.

New Journal of Chemistry
Page 12 of 16
View Article Online
DOI: 10.1039/C7NJ00157F
These results deepen our understanding of the mechanism of AuꢀSiO2 catalyzed
reduction of nitroarenes by NaBH4. Comparing with the early reports, the mechanism
of this model reaction in our reaction system is similar with the catalyst that gold
nanoparticles supported on other carriers16, 19, 31, 32, although the interactions between
gold and carriers are different33, 34. This demonstrated that the catalytic mechanism
depends on the gold nanoparticles for this model reaction15, 29, 30. Furthermore, the
effect of protonation of water is proved for the mechanism in our reaction system, and
the catalytic activities can be greatly modulated by modifying the molecular structure
of substituent group due to the substituent effect, which provides insight into the
future design of reactants with substituent group to improve reaction efficiencies in
other reactions.
3.4 Investigation by in-situ IR
Comparing with the analytical techniques mentioned for this reaction mechanism
which require a considerable period of time to provide analytical responses19ꢀ21, 35
,
inꢀsitu IR is an onꢀline analytical technique which provides immediate analytical
result, however, requires strict conditions.36, 37 It demands the IR absorption peaks of
each component in reaction mixture do not overlap with each other. For this AuꢀSiO2
catalyzed reduction of nitroarenes, the catalyst AuꢀSiO2 will not interfere with
spectrum evolution of the single substrate due to the chemical inertness and clean IR
spectrum of silica support. Therefore, it is a good opportunity to monitor this catalytic
reaction by inꢀsitu IR.
The inꢀsitu IR spectrum of AuꢀSiO2 catalyzed reduction of pꢀNip is shown in
Figure 5. It is clear to see the peak changes of each component in the reaction mixture.
The intensity of the peak at 1520 cmꢀ1, which corresponds to CꢀH bending absorption
of aromatic amine, increased with the reaction time. While the peak at 1500 cmꢀ1,
corresponding to the CꢀH bending absorption of hydroxylamine compound, appeared
at the very beginning of the reduction and its intensity kept the original form,
reflecting the constant concentration of this intermediate at this test condition. As
expected, intensity of the peak at 1290 cmꢀ1, which corresponds to the NO2 symmetric
stretch absorption, decreased in reduction process.38 These results directly proved the

Page 13 of 16
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00157F
presence of nitro, amino, and hydroxylamine compounds during the reduction, and
definitely confirmed the direct mechanistic pathway for the AuꢀSiO2 catalyzed
reduction of pꢀNip. Comparing with the common analysis method, such as GCꢀMS of
reaction profiles16, kinetic isotope effect19, and substituent effect aforementioned, the
inꢀsitu IR exhibits a realꢀtime evolution of this AuꢀSiO2 catalyzed reaction system,
and obtained a direct information specifically related to substrate, product, and
intermediate during the reduction. Thus, the inꢀsitu IR is a suitable and direct method
for onꢀline tracing the model reaction in our system, which offer a reference for future
mechanism test of heterogeneous nanocatalysis.
Figure 5 Inꢀsitu IR spectrum recorded during the reduction progress.
Reaction conditions: 50 mL of the pꢀNip aqueous solution (40 mmol/L), 50mg AuꢀSiO2, and 750
mg NaBH4, at room temperature.
Reusability of AuꢀSiO2 was tested by performing a series of five consecutive
runs of the reduction of pꢀNip. The recycling experiments proceeded with the same
samples recovered by filtration/washing cycle for three times. It was observed that all
the runs could be finished in 25 minutes with almost 99% conversion and 99%
selectivity (Table S 2). This indicated the cycles proceed with neither deactivation of
catalyst nor selectivity. TEM images of the five times used AuꢀSiO2 did not show
alteration of the catalyst morphology and no aggregation of gold nanoparticles (Figure
S 3). Moreover, the content of gold ion in the solution was only 0.031 mg/L after the

New Journal of Chemistry
Page 14 of 16
View Article Online
DOI: 10.1039/C7NJ00157F
reaction, which illustrated there was no outflow of gold (7.5 mg/L addition). Thus, at
present, the AuꢀSiO2 catalyst reflected a good reusability.
4 Conclusions
In conclusion, a kinetic analysis based on substituent effect and Hammett plot was
present to confirm the mechanism of the reduction of nitroarenes. The reduction of
nitroarenes by NaBH4 was carried out using AuꢀSiO2 as the catalyst in water medium.
It is remarkable that the substituent group in nitroarenes has a significant effect on the
reaction efficiency that the nitroarenes substituted with electronꢀwithdrawing groups
are reduced much faster than the corresponding nitroarenes with electronꢀdonating
groups. The kinetic analysis based on this finding, together with analyses of
intermediates, brings a conclusion that the reduction of nitroarenes catalyzed by
AuꢀSiO2 follows a direct route. It is also proved that the protonation of water gives a
great impact on the reaction mechanism. Moreover, inꢀsitu IR is proved to be a
suitable and direct method to realꢀtime record the reaction evolution of this AuꢀSiO2
catalyzed reduction of nitroarenes and confirms the presence of hydroxylamine,
which is consistent with the direct route previously.
Acknowledgement
The authors are indebted to the Joint Funds of the National Natural Science
Foundation of China (U1608223), National Natural Science Foundation of China
(21576044 and 21536002), the Foundation for Innovative Research Groups of the
National Natural Science Foundation of China (21421005), and Dalian University of
Technology Innovation Team (DUT2016TB12).
References
1.
2.
3.
4.
5.
6.
7.
S. Xiong, Y. Wang, J. Zhu, et al., Langmuir, 2015, 31, 5504ꢀ5512.
A. Corma and P. Serna, Science, 2006, 313, 332ꢀ334.
M. V. Parmekar and A. V. Salker, RSC Adv., 2016, 6, 108458ꢀ108467.
B. Paul, K. Chakrabarti, S. Shee, et al., RSC Adv., 2016, 6, 100532ꢀ100545.
S. M. ElꢀSheikh, A. A. Ismail and J. F. AlꢀSharab, New J. Chem., 2013, 37, 2399.
H. Göksu, New J. Chem., 2015, 39, 8498ꢀ8504.
O. Mazaheri and R. J. Kalbasi, RSC Adv., 2015, 5, 34398ꢀ34414.

Page 15 of 16
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00157F
8.
P. Veerakumar, M. Velayudham, K.ꢀL. Lu, et al., Appl. Catal., A, 2012, 439-440, 197ꢀ205.
A. Corma, P. Concepcion and P. Serna, Angew Chem Int Ed Engl, 2007, 46, 7266ꢀ7269.
M. Bano, D. Ahirwar, M. Thomas, et al., New J. Chem., 2016, 40, 6787ꢀ6795.
P. Herves, M. PerezꢀLorenzo, L. M. LizꢀMarzan, et al., Chem. Soc. Rev., 2012, 41,
5577ꢀ5587.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
F. Yang, S. Dong, C. Wang, et al., RSC Adv., 2016, 6, 52620ꢀ52626.
E. Blanco, I. EsteveꢀAdell, P. Atienzar, et al., RSC Adv., 2016, 6, 86309ꢀ86315.
A. Noschese, A. Buonerba, P. Canton, et al., J. Catal., 2016, 340, 30ꢀ40.
S. Kundu, S. Lau and H. Liang, J. Phys. Chem. C, 2009, 113, 5150ꢀ5156.
K. Layek, M. L. Kantam, M. Shirai, et al., Green Chem., 2012, 14, 3164ꢀ3174.
S. Wunder, F. Polzer, Y. Lu, et al., J. Phys. Chem. C, 2010, 114, 8814ꢀ8820.
S. Wunder, Y. Lu, M. Albrecht, et al., ACS Catal., 2011, 1, 908ꢀ916.
S. Fountoulaki, V. Daikopoulou, P. L. Gkizis, et al., ACS Catal., 2014, 4, 3504ꢀ3511.
L. B. Zhao, Y. F. Huang, X. M. Liu, et al., Phys. Chem. Chem. Phys., 2012, 14,
12919ꢀ12929.
21.
22.
A. Mahata, R. K. Rai, I. Choudhuri, et al., Phys. Chem. Chem. Phys., 2014, 16,
26365ꢀ26374.
N. S. Isaacs, Physical Organic Chemistry 2th, Addison Wesley Longman Limited, London,
1995, 146ꢀ150.
23.
24.
25.
C. Wang, X. Lin, Y. Ge, et al., RSC Adv., 2016, 6, 102102ꢀ102108.
L. Sun, Q. Zhang, G. G. Li, et al., ACS nano, 2017, 11, 3213–3228.
N. S. Isaacs, Physical Organic Chemistry 2th, Addison Wesley Longman Limited, London,
1995, 199ꢀ207.
26.
27.
Peter Atkins, Atkins' Physical Chemistry, Oxford University Press, Oxford, 2006, 797.
J. A. Dean, Lange’s handbook of chemistry 15th, McGrawꢀHill, New York, 1999,
934ꢀ940.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
C. Hansch, A. Leo and R. W. Taft, Chem. Rev., 1991, 91, 165ꢀ195.
Q. Zhang, D. A. Blom and H. Wang, Chem. Mater., 2014, 26, 5131ꢀ5142.
Q. Zhang and H. Wang, ACS Catal., 2014, 4, 4027ꢀ4033.
Y. Que, C. Feng, S. Zhang, et al., J. Phys. Chem. C, 2015, 119, 1960ꢀ1970.
H. Liu, J. Wang, Z. Feng, et al., Small, 2015, 11, 5059ꢀ5064.
H. Tang, J. Wei, F. Liu, et al., J. Am. Chem. Soc., 2016, 138, 56ꢀ59.
M. Turner, V. B. Golovko, O. P. Vaughan, et al., Nature, 2008, 454, 981ꢀ983.
S. Gu, Y. Lu, J. Kaiser, et al., Phys. Chem. Chem. Phys., 2015, 17, 28137ꢀ28143.
I. Marziano, D. C. A. Sharp, P. J. Dunn, et al., Org. Process Res. Dev., 2000, 4, 357ꢀ361.
C. Sun and D. Xue, CrystEngComm, 2015, 17, 2728ꢀ2736.
F. Visentin, G. Puxty, O. M. Kut, et al., Ind. Eng. Chem. Res., 2006, 45, 4544ꢀ4553.

New Journal of Chemistry
Page 16 of 16
View Article Online
DOI: 10.1039/C7NJ00157F
A new insight into the Au-SiO2-catalyzed reduction of nitroarenes based on
substituent effect and in-situ IR.