Highly active Ru-g-C3N4 photocatalyst for visible light assisted selective hydrogen transfer reaction using hydrazine at room temperature

Article abstract of DOI:10.1016/j.catcom.2017.08.019

The present study describes the highly efficient heterogeneous photoactive catalyst Ru-g-C₃N₄ screened for selective transfer hydrogenation of nitroarenes and olefins. Photoactive catalyst Ru-g-C₃N₄ exhibits excellent reactivity in visible light under very mild reaction conditions via using hydrazine hydrate as a source of hydrogen with high turnover number. The easily separable heterogeneous photoactive Ru-g-C₃N₄ catalyst is straightforward to handle in visible light (LED lamp), non-toxic, environmentally friendly and at the same time eliminates the use of high pressure hydrogenation reactors without the need for external sources of energy.

Full text of DOI:10.1016/j.catcom.2017.08.019

CatalysisCommunications102(2017)48–52
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Highly active Ru-g-C3N4 photocatalyst for visible light assisted selective
hydrogen transfer reaction using hydrazine at room temperature
Priti Sharma⁎, Yoel Sasson
Casali Centre of Applied Chemistry, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords:
Visible light
G-C3N4
Photocatalysis
Hydrogen transfer
Heterogeneous
Semiconductor
The present study describes the highly efficient heterogeneous photoactive catalyst Ru-g-C3N4 screened for se-
lective transfer hydrogenation of nitroarenes and olefins. Photoactive catalyst Ru-g-C3N4 exhibits excellent re-
activity in visible light under very mild reaction conditions via using hydrazine hydrate as a source of hydrogen
with high turnover number. The easily separable heterogeneous photoactive Ru-g-C3N4 catalyst is straightfor-
ward to handle in visible light (LED lamp), non-toxic, environmentally friendly and at the same time eliminates
the use of high pressure hydrogenation reactors without the need for external sources of energy.
1. Introduction
increases its photo catalytic activity due to two dimensional (2D)
polymer consisting of interconnected tri-s-triazine units via tertiary
The reduction of nitroarenes to the corresponding amines is an
important key transformation in organic synthesis as well as in the
chemical industry for the preparation of various chemicals, medicinal,
printing dye and polymers based printing industry [1–3]. Numerous
catalysts were reported for the nitroarens hydrogenation, including
molecular hydrogen, homogenous catalyst, or supported catalysts on
different support [4–7]. Literature shows some of these catalysts are not
commercially efficient in the several cases due to harsh reaction con-
ditions, expensive catalysts, reagents, multi-step synthetic procedures,
and the need for special facilities for generation, storage &
transportation of hydrogen gas. Frequently hazardous wastes were
generated in these processes [8–10]. In view of the above basic points
the need to develop safe, greener, sustainable synthesis, low cost, non-
toxic, visible light-assisted protocol is highly desired.
Solar energy offers a great potential usage as a clean and economical
energy source for organic transformations due to high abundance, in-
expensive and thus rapidly growing worldwide during the last two
decades [11–13]. In the same context, recently metal free semi-
conductors polymeric carbon nitride exhibits significant attention in
recent years, due to unique photochemical properties like; visible light
energy storage, semiconductor properties (~2.7 eV band gap), high
stability, nontoxic, metal free organocatalyst, environmentally benign
photo catalysts and easy synthesis from cheap raw material without any
additional support [14,15]. Whereas at the same time with doping of
metals like Cu [16], Fe [17], Pd [18], Ag [19,20], Pt [21], V [22] and
single Atom (Pd/Pt) Supported [23] graphitic carbon nitride (g-C3N4)
amines. However the regular arrangement of nitrogen atoms in g-C3N4
creates mobile electrons and holes after visible light absorption; which
results in enhancement of its electronic and catalytic properties
[24,25]. Precisely, g-C3N4 material in photo catalysis has been ex-
tensively studied for various organic transformations in environmental
and economic perspectives [26,27].
Very recently, our research group reported an efficient, simple
protocol; which has been developed for the hydrogen transfer reaction
of the carbonyl functional group (> C]O) using a heterogeneous
photoactive catalyst Ru-g-C3N4, the reaction occurs efficiently at room
temperature in the presence of visible light using different alcohols as a
hydrogen source [28]. In continuation of our ongoing research of novel
photo catalysts, herein we report for the first time Ru-g-C3N4 as a high
performance photo catalyst for the reduction of nitroarenes to the
corresponding amines at room temperature under visible light irradia-
tion (Scheme 1) using hydrazine monohydrate as hydrogen source (SI
S5).
2. Experimental
2.1. Materials
The material used in the experiments such as Urea, Alcohols,
Ruthenium chloride, Deionized water, Nitro arenas, Olefins, Hydrazine
(hydrogen source) chemicals were purchased from commercial firms
(Sigma Aldrich and reliable resources) and used without further
⁎
Corresponding author.
E-mail address: priti.sharma@mail.huji.ac.il (P. Sharma).
http://dx.doi.org/10.1016/j.catcom.2017.08.019
Received 2 June 2017; Received in revised form 1 August 2017; Accepted 11 August 2017
Availableonline24August2017
1566-7367/©2017PublishedbyElsevierB.V.

P. Sharma, Y. Sasson
CatalysisCommunications102(2017)48–52
loading of ruthenium chloride (RuCl3) for 6 h in ethanol solution at
50 °C temperature followed by till dryness.
2.3. General protocol for the photocatalytic hydrogen transfer reaction via
Ru-g-C3N4 catalyst
The photocatalytic hydrogenation was carried out in a 25 mL oven
dried, R.B. (round bottom flask), under visible light (9 W LED domestic
ceiling lamp) with high magnetic stirring ~800 rpm at room tem-
perature. In catalysis run, substrate (1 mmol), photocatalyst Ru-g-C3N4
(30 mg), ethanol solvent (10 mL), Hydrazine (10 mmol) were allowed
to stir (800 rpm). The reaction samples were taken periodically time to
time with fixed intervals and analyzed by gas chromatography. The
samples were added with DCM (dichloromethane) solvent for dilution
and filtered by Whatman paper before injection into a gas chromato-
graph (Scheme 1) (SI S9).
Scheme 1. Photoactive Ru-g-C3N4 catalyzed transfer hydrogenation.
1200
1000
800
600
400
200
0
2.4. Characterization
XRD measurements were performed on
a D8 Advance dif-
fractometer (Bruker AXS, Karlsruhe, Germany) with a goniometer ra-
dius 217.5 mm, Göbel Mirror parallel-beam optics, 2° Sollers slits, and
0.2 mm receiving slit. A low background quartz sample holder was
carefully filled with the powder samples. XRD patterns from 5° to 85° 2θ
were recorded at room temperature using Cu Kα radiation
(λ = 0.15418 nm) with the following measurement conditions: tube
voltage of 40 kV, tube current of 40 mA, step scan mode with a step size
0.02° 2θ and counting time of 1 s per step for preliminary study and 12 s
per step for structural refinement. The instrumental broadening was
determined using LaB6 powder (NIST-660a). XPS analysis was con-
ducted using XPS Kratos AXIs Ultra (Kratos Analytical Ltd., UK) high
resolution photoelectron spectroscopy instrument. FTIR spectra were
collected by a Bruker (Alpha-T). Sample morphology was observed by
extra High-Resolution Scanning Electron Microscopy (Magellan™ 400
L). X-ray diffraction patterns were collected by using a Bruker AXS D8
Advance. UV/Vis absorption spectra were monitored by Carry 100 Bio
and Diffuse Reflectance analysis was carried out by an Integrating
Sphere (JASCOV-650 Series ISV-722). FTIR spectra were collected by a
Bruker (Alpha-T).
C3N4
Ru-C3N4
1.5
2.0
2.5
Energy (eV)
3.0
3.5
4.0
Fig. 1. Tauc plots, (a) g-C3N4, (b) Ru–g-C3N4.
Table 1
Reaction parameters optimization for photocatalytic hydrogenation reaction.
Entry
Reaction condition
Time (h)
Yield (%)
1.
2.
3.
4.
5.
6.
7.
RuCl3 (no light)
RuCl3 (visible light)
g-C3N4 (no light)
g-C3N4 (visible light)
Ru-g-C3N4 (no light)
Ru-g-C3N4 (visible light)
No catalyst (blank reaction)
12
12
12
12
24
10
24
< 5
< 5
0
0
< 9
99
3. Results and discussion
Graphitic semiconductor support g-C3N4 and Ru-g-C3N4 photo-
catalyst were fabricated via our recently reported procedure [28].
Heterogeneous photoactive catalyst Ru-g-C3N4 was fabricated by
simple stirring ruthenium chloride (RuCl3) salt with graphitic g-C3N4 in
ethanol solvent in round bottom flask at 50 °C till dry in 6 h. The syn-
thesized photo catalyst Ru-g-C3N4 is well characterized by various
characterization techniques.
00
Reaction conditions: Ru-g-C3N4 (30 mg), nitroarene (1 mmol), NH2NH2·H2O (10 mmol),
Isolated yields, visible light irradiation > 420 nm (9 W LED domestic ceiling lamp),
room temperature, Conversion based on GC analysis.
Transformation of urea into g-C3N4 after calcinations at 550 °C for
3 h exhibits clearly with intense emergence peak at 27.20 in the typical
X-ray diffraction (XRD) pattern. In case of g-C3N4, and Ru-g-C3N4 the
two distinct diffraction peaks at 13.20 and 27.20 do match well with
the reported procedure that corresponding to (100) and (002) diffrac-
tion plane of the graphitic g-C3N4 materials, respectively with the dis-
appearance of urea characteristics background peaks (Fig. S1) [29].
Most apparently due to partial loading of ruthenium salt, Ru-g-C3N4
diffraction of (002) plane shows less intense comparatively to the g-
C3N4 material (Fig. S1).
purification. LED lamp (9 W domestic ceiling down light Epistar LED
ceiling lamp) used as a visible light source. GC analyses were performed
using Focus GC from Thermo Electron Corporation, equipped with low
polarity ZB-5 column. GC analyses were performed using Trace 1300
Gas Chromatograph model from Thermo Scientific, equipped with the
Rxi-1ms (cross bond 100% dimethyl polysiloxane) column. Conversion
based on GC area. The ruthenium (Ru) content of catalyst was de-
termined by an Inductively Coupled Plasma Mass Spectrometry
(ICPMS) spectrometer (Agilent 7500 cx).
Conducted FT-IR characterization analysis confirms synthesis of g-
C3N4 from urea and ruthenium loaded Ru-g-C3N4 fabrication (Fig. S2)
systematically. The absorption bands at 3430, 3345, 1454, 1582 cm−1
are the typical absorption bands of urea (raw material) and vanished
after transformation into g-C3N4 in calcinations process. The sharp
absorption at 1248 cm−1 and 1624 cm−1 shows the CeN and C]N
stretching vibrations, respectively [30,31]. Furthermore, the
2.2. Semiconductor graphitic carbon nitride (g-C3N4), Photocatalyst Ru-g-
C3N4 synthesis
Graphitic carbon nitride (g-C3N4) and Ru-g-C3N4 was fabricated by
following our group earlier reported procedure [28]. Photocatalyst Ru-
g-C3N4 is prepared by stirring with 1 g of g-C3N4 with 8% by weight
49

P. Sharma, Y. Sasson
CatalysisCommunications102(2017)48–52
Table 2
Photoactive catalyst Ru-g-C3N4 screening for nitro group hydrogenation using hydrazine as hydrogen source.
Entry
1.
Substrate
Product
Time (h)
10
Yield (%)
85
TON
3728
H
NH2
COO
I
NO2
3C
H
3C
2.
3.
12
10
80
95
3508
4170
O2
N
H
COO
H2N
H2N
H
O2
N
I
4.
5.
10
10
99
4345
4389
Br
O2N
O2N
Br
H2N
100
F
H2N
F
6.
12
90
3950
NO2
NH2
7.
8.
10
10
99
99
4345
4345
NO2
NO2
NH2
NH2
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Reaction conditions: Ru-g-C3N4 (30 mg), Nitro group substrate (1 mmol), NH2NH2·H2O (10 mmol), methanol solvent (10 mL) Isolated yields, visible light irradiation > 420 nm (9 W
LED domestic ceiling lamp), room temperature, Conversion and product yield based on GC analysis.
characteristic breathing mode of the triazine units at 810 cm−1 (s-
triazine ring) were well observed in fabricated g-C3N4 (Fig. S2.a, b) and
Ru-g-C3N4 photocatalyst respectively. The intense bands at 1625, 1554,
1408 and 1249 cm−1 were assigned to typical stretching vibration
modes of triazine derived repeating units (Fig. S2. a, b).
by SEM & TEM analysis (Fig. S4, A–D). The monomer unit of g-C3N4
with long sheet alike structure, further exhibits agglomeration of many
small unit of g-C3N4 in Ru-g-C3N4 and dense morphology [35,36].
3.1. Photo catalytic activity
Tauc plots exhibits the band gap for g-C3N4 support and Ru-g-C3N4
photo catalyst is observed at ~2.84 eV, ~3.00 eV respectively, which
are in good agreement with literature value (Fig. 1, Figs. SI–S3) [32].
In XPS study, C1s XPS plot show peak of CeC and N]CeN at 284.8
and 288 eV; respectively in both fabricated g-C3N4 and Ru-C3N4 ma-
terial. The N1s spectrum of g-C3N4 and photo catalyst Ru-g-C3N4 ma-
terial can be deconvoluted into three peaks 399.9, 401.3 and 405.6 eV.
However, the intense increase in peak at 405.6, belonging to the
terminal amino groups (eNH2) (Fig. S5) [33]. This surprising phe-
nomenon may be due to the transformation of long sheet of g-C3N4 into
simpler and smaller flakes unit consequently creates more terminal
amino groups (eNH2) during metal loading process (Fig. S5). The main
peak of the Ru-g-C3N4; XPS spectrum can be deconvoluted into two
doublets of ruthenium 3p3/2 and Ru 3p1/2, which leads to two binding
energies at 464.5 and 485.3 eV, corresponds to ruthenium oxide species
(Fig. S5, C) [17,34].
Generally, hydrogenation requires high temperature with high hy-
drogen gas pressure with long reaction time period. In the present study
we tried to exhibit here an easy photochemical protocol under mild
reaction conditions viz; without high pressure, nontoxic, room tem-
perature, without using accidental prone hydrogenating agent, minimal
reaction time period for nitroarene conversion to corresponding amine
in the presence of hydrazine as hydrogen donor agent.
The fabricated Ru-g-C3N4 photo catalyst was investigated for photo
catalytic hydrogenation using 4-nitrotoluene as a standard substrates
under optimized reaction conditions viz; Ru-g-C3N4 (30 mg),
Nitroarene substrate (5 mL), NH2NH2·H2O (10 mmol), methanol sol-
vent (5 mL) at room temperature under visible light [(9 W domestic
LED ceiling lamp) visible light source] with magnetic stirring
(~800 rpm). The conversions of 4-nitrotoluene to corresponding amine
was found to be 85% after 10 h of reaction time period high turnover
number 3728 (SI, S9).
The morphologies of g-C3N4 and Ru-g-C3N4 material are visualized
50

P. Sharma, Y. Sasson
CatalysisCommunications102(2017)48–52
Table 3
Photoactive catalyst Ru-g-C3N4 screening for olefin hydrogenation using hydrazine as hydrogen source.
Entry
1.
Substrate
Product
Time (h)
10
Yield (%)
100
TON
4389
O
O
2.
3.
4.
12
10
10
80
95
99
3508
4170
4345
Br
Br
5.
6.
12
10
85
99
3728
4345
H3CO
H3CO
7.
8.
10
10
92
90
4038
3950
H
3CO
H
3CO
9.
10
10
90
90
3950
3950
O
O
H
O
H
O
10.
O
O
H
C
H
C
3
3
Reaction conditions: Photocatalyst Ru-g-C3N4 (30 mg), Olefin substrate (1 mmol), NH2NH2·H2O (10 mmol), methanol solvent (10 mL) Isolated yields, visible light irradiation > 420 nm
(9 W LED domestic ceiling lamp), room temperature, Conversion and product yield based on GC analysis.
Herein, we performed photo catalytic hydrogenation optimization
under various controlled experiments. Further the optimization results
are summarized in Table 1. For the clear evaluation of Ru-g-C3N4 cat-
alytic activity a blank reaction was performed without Ru-g-C3N4 photo
catalyst usage which results into no product formation even after 24 h
with visible light irradiation using 4-nitrotoluene as a standard sub-
strate under optimized reaction condition (Table 1 entry 7). At the same
time to show necessity of visible light in the hydrogenation reaction a
controlled experiment was performed under dark using similar opti-
mized reaction conditions in presence of RuCl3, g-C3N4 and Ru-g-C3N4
materials respectively. The result of controlled experiment shows quite
poor result with 5%, 0%, and < 9% corresponding hydrogenated pro-
duct formation even after 24 reaction time period (Table 1 entries 1, 3,
5). Whereas in presence of visible light Ru-g-C3N4 photo catalyst ex-
hibits remarkable reactivity to give corresponding hydrogenated pro-
duct under optimized reaction condition (substrate 4-nitrotoluene).
However the alone RuCl3 and g-C3N4 were not able to initiate the
photochemical hydrogenation reaction in terms of corresponding pro-
ducts yield in dark/visible light irradiation under optimized reaction
condition (Table 1 entries 2, 4).
51

P. Sharma, Y. Sasson
CatalysisCommunications102(2017)48–52
In conclusion, for the selective hydrogenation of nitro compounds to
the corresponding amines, and olefins a highly efficient straightforward
mild, protocol has been established under visible light using a hetero-
geneous photoactive catalyst Ru-g-C3N4 in the presence of hydrazine
monohydrate (proton donor). The photoactive catalyst Ru-g-C3N4 ex-
hibits excellent reactivity by producing appreciable yield of corre-
sponding products within 10 h. The salient added feature of the Ru-g-
C3N4 photo catalyst viz.; heterogeneous, photoactive, easy fabrication
makes Ru-g-C3N4 catalyst handy, recoverable and recyclable for sub-
sequent runs under photochemical conditions even over several reac-
tion cycles.
Acknowledgements
Author greatly acknowledges Hani Gnayem & Ofer Lahad for their
help to accomplish the present research work.
Scheme 2. Proposed mechanism of photocatalytic hydrogen transfer reaction.
Appendix A. Supplementary data
The synthesized photo catalyst Ru-g-C3N4 was screened with dif-
ferent nitroarenes evaluated in visible light under optimized reaction
conditions to explore the photo catalytic activity and results are sum-
marized in (Table 2 entries 1–8). Various substituted (mono and di)
shows good reactivity under optimized reaction conditions to give the
give the excellent product yield. However from the Table 2 it is clear
that, all the obtained results shows substituent effect did not play much
of a role in reactivity of substrate (Table 2 entries 1–8). The observed
results clearly show that Ru-g-C3N4 under visible light is the best
combination to hydrogenate the nitroarene into hydrogenated products
under optimized reaction conditions. It is noteworthy to mention here
that aromatic C]C double bond was not affected by hydrazine and
aromaticity remains intact. Further after observing the excellent re-
activity towards nitro group conversion into amines, we focused to
hydrogenate olefin group under similar optimized reaction condition.
Various aromatic and nonromantic groups were hydrogenated and re-
sults were summarized in Table 3. In the present study it is clear that
olefin group were successfully transformed into hydrogenated group
without disturbing the aromatic group (Table 3, entries 1–10) with high
turnover number.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2017.08.019.
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