Visible-Light-Driven Chemoselective Hydrogenation of Nitroarenes to Anilines in Water through Graphitic Carbon Nitride Metal-Free Photocatalysis

Article abstract of DOI:10.1002/asia.201800515

Green and efficient procedures are essential for the chemoselective hydrogenation of functionalized nitroarenes to form industrially important anilines. Herein, it is shown that visible-light-driven, chemoselective hydrogenation of functionalized nitroarenes with groups sensitive to forming anilines can be achieved in good to excellent yields (82–100 %) in water under relatively mild conditions and catalyzed by low-cost and recyclable graphitic carbon nitride. The process is also applicable to gram-scale reaction, with a yield of aniline of 86 %. A study of the mechanism reveals that visible-light-induced electrons are responsible for the hydrogenation reactions, and thermal energy can also promote the photocatalytic activity. A study of the kinetics shows that this reaction possibly occurs through one-step hydrogenation or stepwise condensation routes. A wide range of applications can be expected for this green, efficient, and highly selective photocatalysis system in reduction reactions for the synthesis of fine chemicals.

Full text of DOI:10.1002/asia.201800515

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Visible-light-driven chemoselective hydrogenation of nitroarenes
to anilines in water via graphitic carbon nitride metal-free
photocatalysis
Authors: Gang Xiao, Peifeng Li, Yilin Zhao, Shengnan Xu, and Haijia
Su
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10.1002/asia.201800515
Chemistry - An Asian Journal
FULL PAPER
Visible-light-driven chemoselective hydrogenation of nitroarenes
to anilines in water via graphitic carbon nitride metal-free
photocatalysis
Gang Xiao, Peifeng Li, Yilin Zhao, Shengnan Xu, Haijia Su*
Abstract: Green and efficient procedures are highly required for the
chemoselective hydrogenation of functionalized nitroarenes to
industrially important anilines. Here, we show that visible-light-driven,
chemoselective hydrogenation of functionalized nitroarenes bearing
the sensitive groups to anilines can be achieved in good to excellent
yields (82-100%) in water under relatively mild conditions, catalyzed
by low-cost and recyclable graphitic carbon nitride. It is also applicable
in gram-scale reaction with 86% yield of aniline. Mechanism study
reveals that visible light induced electrons are responsible for the
hydrogenation reactions and thermal energy can also promote the
photocatalytic activity. Kinetics study shows that this reaction possibly
occurs via one-step hydrogenation or stepwise condensation route.
Wide applications can be expected using this green, efficient, and
highly selective photocatalysis system in reduction reactions for fine
chemical synthesis.
with other easily removed and reducible groups to produce
several industrially important anilines remains challenging with
limited success achieved.[1,2,6] It is highly desirable to develop a
green procedure for the efficient and chemoselective
hydrogenation of functionalized nitroarenes.
Visible-light-driven photocatalysis mediated by semiconductors
and metal nanoparticles has caused increasing interests in
utilizing abundant solar energy to perform diverse chemical
reactions.[7-11] It is a sustainable and green procedure for fine
chemicals synthesis.[12] It also provides new opportunities in fine
chemical synthesis by manipulating the selectivity of chemical
transformations and introducing novel reaction mechanisms.[13-15]
Considering the non-renewability and potential environmental
hazards of metals,[16] alternative photocatalysts based on metal-
free materials,[17] especially graphitic carbon nitride (g-
C3N4),[11,18,19] are under rapid development. This is because of
their low-cost and non-toxic nature, and the convenience in
modifying their composition, structure and properties using the
well-established organic protocols.[20]
Introduction
Graphitic carbon nitride is the most stable allotrope of various
carbon nitrides at ambient conditions and can be easily fabricated
by a one-step thermal polymerization of abundant nitrogen-rich
precursors such as cyanamide, dicyandiamide, melamine, urea,
and thiourea.[21,22] It was first recognized as a heterogeneous
catalyst in the pioneer works for the Friedel-Crafts acylation of
benzene reported in 2006.[23,24] In 2009, Wang et al. discovered
that g-C3N4 acted as a metal-free conjugated semiconductor
photocatalyst in visible-light-driven H2 evolution from water
splitting.[18] Since then, significant efforts have been made in g-
C3N4 mediated photocatalysis owing to its superior characteristics
containing appropriate visible-light response, thermal stability up
to 600 °C, chemical stability in acid, alkali, and organic solvents,
high surface area, low cost, and the convenience towards tuning
its properties.[18,19,25-27] It has hence found various applications in
energy and environmental fields, especially in hydrogen
evolution,[28] CO2 reduction,[29] water and gas purification,[30] and
organic synthesis.[15,31] Furthermore, as a heterogeneous catalyst,
g-C3N4 possesses multiple functionalities such as basic sites, H-
bonding motif, and electronic properties.[32] These favorable
features together with the proper band structure (+1.4 eV and -
1.3 eV versus the normal hydrogen electrode) suggest that g-
C3N4 will find broader applications in promoting organic redox
synthesis as a gentle metal-free photocatalyst.[33] Oxidation
reactions catalyzed by g-C3N4 have been well studied
recently.[11,33-35] However, very few research on reduction
reactions for fine chemical synthesis mediated by g-C3N4
photocatalysis has been reported.[36] This motivated us to carry
As important intermediates for the manufacture of fine chemicals
(agrochemicals, pharmaceuticals, dyes, and pigments) as well as
bulk chemicals (polymers), functionalized amines represent an
important type of feedstock in the chemical industry with over four
million tons produced per year mainly by hydrogenation of the
corresponding nitroarenes.[1-3] Non-catalytic methods using
stoichiometric reducing agents such as Fe and S compounds can
be used, but these processes will cause serious environmental
problems by creating large amounts of toxic waste.[4] Catalytic
hydrogenation of nitroarenes using noble metals (Pd, Pt, Rh, Ru,
and Au) or more abundant transition metals (Fe, Co, and Ni) is
the most widely used method for the production of anilines.[5,6]
However, the use of large amounts of metal catalysts, harsh
reaction conditions, organic medium, and expensive or harmful
hydride source make these procedures not green.[5,6] Furthermore,
the chemoselective hydrogenation of nitroarenes functionalized
[*]
Dr G. Xiao, P. Li, Y. Zhao, S. Xu, Prof. Dr. H. Su
State Key Laboratory of Chemical Resource Engineering, Beijing
Advanced Innovation Center for Soft Matter Science and
Engineering (BAIC-SM), College of Life Science and Technology
Beijing University of Chemical Technology
No. 15, North 3rd Ring Rd East, Chaoyang District, Beijing 100029,
P. R. China
E-mail: suhj@mail.buct.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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out the proof-of-concept study for g-C3N4 in catalyzing the
hydrogenation of functionalized nitroarenes.
transition metals, and metal-based photocatalysts (Table S3). For
noble metals and metal-based photocatalysts, although the
hydrogenation of nitrobenzene to aniline can be achieved under
very mild conditions, the use of precious or hazardous catalysts
restricts the applications. Low cost catalysts based on Co or Fe
have also been used for this reaction, but high H2 pressure was
needed. The present study demonstrate that an excellent aniline
yield can be achieved through a green and low-cost procedure at
very low reaction temperatures.
Herein, we demonstrate that g-C3N4 is an active, recyclable, low-
cost, metal-free photocatalyst for the chemoselective
hydrogenation of functionalized nitroarenes in water under
relatively mild conditions and visible light irradiation. Hydrazine
hydrate (N2H4·H 2O), a green and safe reducing reagent only
producing nitrogen gas as a byproduct,[37] was used as the hydride
source as well as the hole scavenger. This green, efficient, and
highly selective procedure is of high potential in the synthesis of
various chemicals through hydrogenation using abundant solar
energy.
Light is a key factor influencing the photocatalytic reactions.
The study on the effect of light intensity and wavelength on the
photocatalytic activity will provide guidelines for manipulating the
reaction process alternatively by controlling the incident
light.[10,39,40] More importantly, it will offer insights into the light
induced mechanism.[10,39,40]
A positive dependence of the
Results and Discussion
photocatalytic activity on the incident light intensity has been
observed for both semiconductors and metal nanoparticles
photocatalytic processes and is considered as an experimental
signature of light induced processes mediated by energetic
charge carriers (hot electrons and holes).[13,39-41] This relationship
was also observed for the g-C3N4 photocatalytic hydrogenation of
nitrobenzene to aniline with an almost linear dependence (R2 =
0.991, Figure 1). When the incident photons are absorbed by g-
C3N4, transient electron/hole pairs are generated on its surface.
The resulting hot electrons are transferred to the adsorbed
nitrobenzene molecules on the basic sites of g-C3N4. The hot
holes can be effectively scavenged by hydrazine which is a strong
reducing agent, thus strengthening the separation of the charge
carriers and the following reactions. Under a higher light intensity,
more photons are introduced into the photocatalyst to excite more
energetic charge carriers which will facilitate the hot electron
induced hydrogenation reaction of nitrobenzene to aniline.
Urea was selected as the precursor to synthesize the polymeric
g-C3N4 photocatalyst through the facile thermal condensation
procedure. Urea has been recognized as an ideal precursor for
the preparation of g-C3N4 because it is a low-cost, large-scale
industrial product and the gases generated during the
condensation of urea will act as soft templates to help the
formation of porous micro-structures.[38] TEM images (Figure S1)
demonstrate the formation of the characteristic layered platelet-
like morphology of g-C3N4 with loose and porous structures. The
Brunauer-Emmett-Teller (BET) specific surface area of 74.2 m2/g
was determined by the N2 adsorption/desorption analysis (Figure
S2). The XRD pattern (Figure S3) reveals a dominant (002) peak
at 27.3° and a minor (100) peak at 13.0°, arising from the graphitic
stacking of the conjugated aromatic C3N4 units and the in-plane
intervals of the periodic tri-s-triazine units.[18,31] XPS
measurements (Figure S4) further prove the formation of sp2
hybridized C-N coordination in g-C3N4 and the elemental ratio of
C:N:O is determined to be 0.77:1:0.17. The UV-vis-diffuse
reflectance spectrum (Figure S5) exhibits the ability of the
synthesized g-C3N4 photocatalyst for utilizing solar energy in the
visible light region with band gap estimated to be 2.80 eV.
Thermogravimetric analysis (TGA, Figure S6) proves the thermal
stability of g-C3N4 in air up to 460 °C. ICP-OES analysis together
with the two control experiments ruled out the possibility of the
trace metal impurities affecting the instinct activity of g-C3N4
photocatalyst (Table S1 and S2). The above characterization
results prove the successful synthesis of g-C3N4 photocatalyst
with porous structures, visible light utilization ability, and excellent
thermal stability.
Another possible mechanism for the observed higher activity at
higher light intensity is the photothermal effect, where the
wavelength of the incident light has a negligible effect on the
photocatalytic activity when the intensity remains identical.[42] The
The visible-light-driven hydrogenation of nitrobenzene to aniline
was chosen as the representative reaction for the proof-of-
concept experiments. The reaction was conducted in water under
relatively mild conditions (from room temperature to 100 °C,
visible light) using the synthesized g-C3N4 as the metal-free
photocatalyst and hydrazine hydrate as a green hydride source
and hole scavenger. After 20 hours (80 °C), we observed the
formation of aniline with excellent yield (> 99%) as expected. Dark
reaction yielded less than 5% of aniline and the control
experiment without the addition of g-C3N4 yielded no aniline.
We moreover compared the present procedure with previously
reported results on heterogeneous catalysis using noble metals,
Figure 1. Dependence of aniline yield on the incident light intensity. See Table
S4 for detailed conditions and results for the reactions under different light
intensities.
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Figure 2. Action spectrum for the photocatalytic hydrogenation of nitrobenzene
to aniline using the g-C3N4 photocatalyst. See Table S5 for detailed conditions
and results for the reactions under different light wavelengths.
Figure 3. Dependence of aniline yield on the reaction temperature. See Table
S6 for detailed conditions and results for the reactions under different reaction
temperatures.
photocatalytic action spectrum presents one-to-one mapping
between the wavelength dependent apparent quantum yield
(AQY) and the light absorption spectrum of the photocatalyst. It is
a useful tool to distinguish whether the observed chemical
reaction occurs via a photo-induced process.[43] As shown in
Figure 2, higher AQY can be observed at shorter wavelength
where the g-C3N4 photocatalyst absorbs more light. The good
match between AQY and the photocatalyst’s light absorption
spectrum suggests that this reaction proceeds via a light induced
process.[44]
As demonstrated above, the hydrogenation of nitrobenzene to
aniline catalyzed by g-C3N4 is proven to be driven by a light
induced mechanism (Scheme 1). The population of hot electrons
generated via light excited charge separation is the key factor
influencing the observed photocatalytic activity. This helps us to
better understand the light induced mechanism for photocatalytic
reactions mediated by g-C3N4 as well as other semiconductors.
Furthermore, it provides us an extra tool to tune the chemical
reactions by appropriately adapting the incident light.
The photocatalytic reaction catalyzed by g-C3N4 can also utilize
thermal energy to promote the photocatalytic activity, supported
by the fact that higher aniline yields were obtained at elevated
reaction temperatures (Figure 3). This conclusion can also be
supported by several reported works, although none have
recognized this important phenomenon at that time.[11,33,34] One
reason responsible for this is that the relative population of
adsorbate excited vibrational states increases at higher reaction
temperatures according to the Bose-Einstein distribution, so less
energy from the hot electrons is required for the reactant
molecules to overcome the activation barrier.[39,40,42]
Although a similar phenomenon has also been observed for
plasmonic metal nanostructures mediated photocatalysis, the
underlying mechanism should be different because of the
different band structures. Recent studies have demonstrated a
signature of plasmonic metal nanostructures distinguished with
semiconductor photocatalysts that they can effectively couple
thermal and light to drive chemical transformations due to the
continuum of metal electron energy levels.[39,40,42] Semiconductors
possess discontinuous energy levels and their photocatalytic
activities are always lower at higher temperatures because the
charge carriers can more readily recombine.[39] However, in the
present reaction system, a strong reducing agent, hydrazine was
used as an effective hole scavenger, thus the recombination of
the charge carriers could be highly restricted. As the activation of
the adsorbed reactant molecules is much easier at higher
temperatures, the light induced hot electrons are expected to
activate more nitrobenzene molecules, thus higher aniline yields
will be observed.
Another interesting feature observed for the reactions at
different temperatures is that under dark condition, the
hydrogenation of nitrobenzene to aniline did not happen below
90 °C. Only 5 % of aniline yield was obtained when the reaction
temperature was as high as 100 °C under dark (Figure 3). This
result proves that the photothermal effect dose not play an
important role in the present light-induced reaction which is
consistent with the conclusion obtained from the study on light
intensity and wavelength.
Scheme 1. Light induced mechanism for the photocatalytic hydrogenation of
nitrobenzene to aniline catalyzed by g-C3N4.
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Table 1. Substrate scope of the visible-light-driven hydrogenation of
nitroarenes.[a]
Entry
1[b]
Substrate
Product
Yield (%)
82
16
5
6
93
92
2[c]
Figure 4. Kinetic profile of the visible-light-driven hydrogenation of nitrobenzene
via g-C3N4 metal-free photocatalysis. Reaction conditions: g-C3N4 (20 mg),
nitrobenzene (0.2 mmol), hydrazine hydrate (1 mmol, 5 equiv.), H2O as solvent
(2 mL), I = 0.5 W·cm-2, T = 80 °C.
17
3[d]
Kinetics study was carried out by recording the conversion of
nitrobenzene and formation of the products during the reaction.
The kinetic profile of the reaction (Figure 4) demonstrates that
nitrobenzene was smoothly converted into aniline with an
unobvious formation of other products. And during the initial four
hours, aniline formed as the sole product. This clearly illustrates
that a highly selective way of direct hydrogenation of nitrobenzene
to aniline should exist in the visible-light-driven hydrogenation of
nitrobenzene.[45] According to the previous studies, two stepwise
routes are generally accepted to be responsible for the
hydrogenation of nitrobenzene to aniline: one is the direct route
which proceeds via nitrosobenzene and N-phenylhydroxylamine
intermediates, and the other one is the condensation route with
azoxybenzene, azobenzene, and hydrazobenzene as the
intermediates.[4,46] Small amount of azoxybenzene and
azobenzene were detected during the reaction, suggesting that
the condensation route also exists in this reaction.
7
8
9
18
88
4[e]
5[f]
6[g]
19
20
86
100
21
10
100
7[h]
Having set the novel procedure for the hydrogenation of
nitrobenzene, the substrate scope, which is an important criterion
to evaluate its application potential,[47] was further studied using
several functionalized nitroarenes (Table 1). For all the substrates,
the expected functionalized anilines were obtained in good to
excellent yields (82-100%) under visible light irradiation and
optimized conditions (no reaction observed under dark). For
example, both electron-rich (methyl-, Table 1, Entry 1) and
electron-deficient (trifluoromethyl-, Table 1, Entry 2) substituents
were not interfering. For halogen bearing nitroarenes,
22
11
[a] Reaction conditions: g-C3N4 (20 mg), substituted nitroarenes (0.2 mmol),
hydrazine hydrate (1 mmol, 5 equiv.), H2O as solvent (2 mL), I = 0.5 W·cm-
2. [b] T = 90 °C, t = 32 h. [c] T = 70 °C, t = 24 h. [d] T = 70 °C, t = 32 h. [e] T
= 90 °C, t = 18 h. [f] T = 90 °C, t = 32 h. [g] T = 70 °C, t = 36 h. [h] T = 70 °C,
t = 24 h.
dehalogenation is always
a competitive process of the
The nitroarenes bearing other easily reducible moieties are the
most challenging substrates to be chemoselectively
hydrogenated.[2,45] In this respect, we were pleased to find that 3-
nitrobenzoic acid could be hydrogenated into 3-aminobenzoic
acid quantitatively and selectively without affecting the easily
reducible carboxyl group (Table 1, Entry 6). However, nitrile,
isonitrile, and aldehyde groups cannot be preserved in this
reaction (see Table S7 and detailed discussion in Supporting
hydrogenation reaction, leading to the formation of undesired
aniline.[45] However, using the present visible-light-driven
procedure, the dehalogenation was well suppressed with halogen
containing anilines obtained in good yields (Table 1, Entry 3 & 4).
Furthermore, dinitrobenzene which bears two nitro groups could
also be hydrogenated to form phenylenediamine with good yield
although higher temperature and longer time were needed (Table
1, Entry 5).
Information). Moreover,
a
nitroheteroarene substrate, 8-
nitroquinoline, was chemoselectively and quantitatively converted
to 8-aminoquinoline which is one of the industrially valuable
heteroaromatic amines, while its heterocyclic rings were kept
unaffected (Table 1, Entry 7).
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the reaction container. Typically, after being charged with g-C3N4
photocatalyst (20 mg), the substrate (0.2 mmol), hydrazine hydrate (1
mmol), the solvent (2 mL H2O), the tube was sealed with a polypropylene
screw cap with silicone gasket. Then the photocatalytic reactions were
carried out with magnetic stirring at a certain temperature (well controlled
by an oil bath) under visible light irradiation using a LED lamp as the light
source (spectral output in the range of 400-800 nm as shown in our
previous work[48]). After certain times, 0.5 mL of the reaction solution were
collected and then filtered through a Millipore filter (0.45 μm) to remove the
solid photocatalyst particulates. After mixed with 1 mL of absolute ethanol,
the products were analyzed quantitatively using an Thermo Trace 1300
gas chromatography (GC) with HP-5 column using external standard
method. A Shimadzu GCMS-QP2010 was used to identify the products.
All the reactions were performed at least twice to obtain the reproducible
results. Experimental details for dark reactions, action spectrum
experiments, photocatalyst recycling experiments, and gram scale
reaction are given in Supporting Information.
For further evaluate the application potential of the present
visible-light-driven procedure, the reusability of the synthesized
metal-free photocatalyst, g-C3N4, was tested for four successive
cycles. To our delight, although we did not use the complex
regeneration processes, this metal-free photocatalyst could
maintain its activity and selectivity for aniline yield during the four
successive cycles (Table S8).
Finally, we performed
a
gram-scale reaction for the
hydrogenation of nitrobenzene (10 mmol, 1.23 g) using a 300 W
Xenon lamp as the simulated sunlight resource. We were pleased
to find that a good aniline yield (86%) was achieved at 80 °C after
20 h, which further demonstrated the applicability of the present
procedure for the production of anilines.
Conclusions
In summary, we have developed a novel, green, efficient, and
Acknowledgements
visible-light-driven
procedure
for
the
chemoselective
hydrogenation of functionalized nitroarenes in water using low-
cost g-C3N4 as a metal-free photocatalyst, and hydrazine hydrate
as the green reducing agent as well as hole scavenger. Visible
light acts as a critical factor in activating this reaction by inducing
charge separation in the photocatalyst to generate hot electrons.
Thermal energy can also be utilized to promote the photocatalytic
activity which facilitates the full utilization of abundant solar
energy. Kinetics study reveals that both a highly selective route of
one-step hydrogenation and the stepwise condensation route are
possibly responsible for this reaction. Several functionalized
nitroarenes bearing the sensitive groups were converted
smoothly into the corresponding anilines in good to excellent
yields (82-100%). The multiple-cycle use of the photocatalyst and
the applicability of this procedure in gram-scale reaction (86%
yield of aniline) further demonstrate the application significance of
this procedure. The general scope of this procedure will be
elaborated in the near future to extend the results of the present
report on g-C3N4 metal-free photocatalysis for the reduction
reactions for fine chemical synthesis.
The authors express their thanks for the supports from the
National Natural Science Foundation of China (21525625), the
National Basic Research Program (973 Program) of China
(2014CB745100), the (863) High Technology Project of China
(2013AA020302), the Chinese Universities Scientific Fund
(JD1417), the project funded by China Postdoctoral Science
Foundation (2017M610038), and the Fundamental Research
Funds for the Central Universities (ZY1712, XK1701).
Keywords: g-C3N4 • photocatalysis • visible light • green
chemistry • nitroarenes
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10.1002/asia.201800515
Chemistry - An Asian Journal
FULL PAPER
FULL PAPER
Visible-light-driven chemoselective
hydrogenation of functionalized
nitroarenes to produce industrially
important anilines can be achieved in
water using g-C3N4 as low-cost and
recyclable metal-free photocatalyst.
Abundant solar energy provides light
and heat to promote the reactions.
Wide applications in reduction
reactions for fine chemical synthesis
are expected using this green,
efficient, and highly selective
Gang Xiao, Peifeng Li, Yilin Zhao,
Shengnan Xu, and Haijia Su*
Page No. – Page No.
Visible-light-driven chemoselective
hydrogenation of nitroarenes to
anilines in water via graphitic carbon
nitride metal-free photocatalysis
photocatalysis system.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.