Sustainable visible light assisted in situ hydrogenation via a magnesium-water system catalyzed by a Pd-g-C3N4 photocatalyst

Article abstract of DOI:10.1039/c8gc02221f

A non-hazardous and relatively mild protocol was formulated for an effectual hydrogen generation process via a "magnesium-activated water" system with a Pd-g-C₃N₄ photocatalyst under visible light at room temperature. Water functions photochemically as a hydrogen donor without any external source with the Pd-g-C₃N₄ photocatalyst. The synthesized Pd-g-C₃N₄ photocatalyst is highly efficient under visible light for the selective reduction of a wide range of unsaturated derivatives and nitro compounds to afford excellent yields (>99%). The photocatalyst Pd-g-C₃N₄ could be easily recovered and reused for several runs without any deactivation during the photochemical hydrogen transfer reaction process.

Full text of DOI:10.1039/c8gc02221f

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Sustainable visible light assisted in situ
hydrogenation via a magnesium–water system
catalyzed by a Pd-g-C N photocatalyst†
Cite this: DOI: 10.1039/c8gc02221f
3
4
Priti Sharma * and Yoel Sasson
A non-hazardous and relatively mild protocol was formulated for an effectual hydrogen generation
process via a “magnesium-activated water” system with a Pd-g-C photocatalyst under visible light at
3 4
N
room temperature. Water functions photochemically as a hydrogen donor without any external source
Received 17th July 2018,
Accepted 24th October 2018
with the Pd-g-C N4 photocatalyst. The synthesized Pd-g-C N photocatalyst is highly efficient under
3
3 4
visible light for the selective reduction of a wide range of unsaturated derivatives and nitro compounds to
afford excellent yields (>99%). The photocatalyst Pd-g-C could be easily recovered and reused for
DOI: 10.1039/c8gc02221f
3 4
N
rsc.li/greenchem
several runs without any deactivation during the photochemical hydrogen transfer reaction process.
Nevertheless these reported processes call for laborious syn-
Introduction
thesis procedures with the use of costly ligands and noble
9
The hydrogenations of unsaturated hydrocarbons and nitro metals. Recently, a g-C
3
N
4
semiconductor exhibited interest-
groups are vital reactions in organic synthesis; both processes ing physicochemical attributes namely visible light activity,
are extensively used in the production of fine chemicals, high stability, nontoxicity, metal free organocatalysis, simple
1
10
fragrances, pharmaceuticals and agrochemicals. Over the past fabrication and material inertness which make it the best fit
few decades, a considerable amount of effort has been devoted for unique photocatalytic activity from the environmental and
1
1
to the area of catalytic hydrogenation using hydrogen (H ) economic perspectives. In the last few years, our research
2
2
gas. However, hydrogen generation, storage, and transpor- group demonstrated water activation through reducing metals
tation are highly hazardous and costly, and require special (Zn, Mg and Fe) provides a “green” source of H
facilities. An efficient utilization of solar energy could miti- transfer reactions, however the drawback of the reaction is the
2
for hydrogen
3
1
2
gate many energy and environmental issues via direct conver- use of a high temperature and time-consuming protocol. Out
4
2
sion of light energy into chemical energy. In the same of the above list, Mg–H O is the preferred reagent due to its
context, photocatalytic hydrogen generation from water split- safety, versatility, environmental benignancy and ease of
ting using semiconductors has attracted worldwide attention handling.
5
for promising green production of hydrogen gas. In a notable
Herein, we describe for the first time a pressure free, mild
work, J. Song and B. Han stated that nitrobenzenes could be photochemical protocol for selective reduction of olefin and
reduced to the corresponding anilines with high selectivity by nitro compounds using a non-hazardous, abundant, and
the H
2
from water splitting and glucose reforming via Pd/TiO
2
2 2
eco-friendly “H O–Mg” pair as a H donor in the presence of a
6
under UV irradiation (350 nm). Recently, in another notable Pd-g-C N photocatalyst at room temperature (Scheme 1). The
3
4
work, B. Török et al. reported chemoselective reduction of a new reaction system is a more expedient and greener protocol
1
2c,13
broad variety of functional groups by a Pd/C–Al–H
2
O system compared to earlier studies.
7
successfully (24 h, high T). In a different approach, reagents
8
(MgH
2 4 3
, MgLiBH , CH OH, diborane) were also reported for
H
2
generation; but typically under harsh reaction conditions.
Results and discussion
g-C
Chen.
3
N
4
1
was fabricated via the known procedure of Wei
0a,14
Casali Center of Applied Chemistry, Institute of Chemistry, The Hebrew University of
Jerusalem, Jerusalem 91904, Israel. E-mail: priti.sharma@mail.huji.ac.il
3 4
The photocatalyst Pd-g-C N was prepared by
stirring PdCl with g-C N in water in an open round bottom
2
3 4
†
Electronic supplementary information (ESI) available: Physical appearance,
flask until dry for 24 h (ESI 1a and b) (Fig. S1†) (Scheme 1A).
The typical X-ray diffraction (XRD) patterns of (a) g-C
and (b) Pd-g-C N materials are shown in Fig. 1. For the pure
reaction setup, FT-IR spectra, nitrogen adsorption–desorption isotherm, XPS
spectra, TEM images, TEM-EDAX pattern analysis, deuterated GC-MS chart, and
water contact angle. See DOI: 10.1039/c8gc02221f
3 4
N
3
4
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g-C
3
N
4
, the two distinct diffraction peaks indexed at 13.4° and
1
5
2
7.2°, which are in good agreement with the reported values
that correspond to the (100) and (002) diffraction planes of the
graphitic g-C materials, respectively, as per JCPDS 87-1526
3 4
N
1
6
(
Fig. 1A).
The (100) plane diffraction peak at 13.4° corresponds to the
interplanar structural packing (hole-to-hole nitride pores) in
1
0a,16
g-C N .
The intense XRD peak at 27.2° corresponds to the
002) plane and is attributed to the stacking of the conjugated
aromatic system. The Pd-g-C material diffraction pattern
002) plane shows a low intensity compared to the g-C N
3
4
(
3 4
N
(
3
4
material, probably due to partial loading of Pd over the conju-
gated aromatic system unit of the g-C photoactive material
3 4
N
1
0a
(
Fig. 1A).
Fabrication of g-C
3 4 3 4
N and Pd-g-C N was well characterized
using FT-IR spectroscopy. The results are shown in Fig. S3† (a)
Scheme 1 (A) g-C
photocatalyzed hydrogenation.
3
N
4
and Pd-g-C
3
N
4
fabrication and (B) Pd-g-C
3
N
4
g-C N and (b) Pd-g-C N . In the FT-IR spectrum of g-C N , a
3
4
3
4
3 4
broad band at nearly 3020–3500 cm− corresponds to the N–H
1
Fig. 1 (A) XRD pattern: (a) g-C
(a) at 5 µm and (b) at 2 µm} and Pd-g-C
{(C’) at 1 µm and (D’) at 200 nm}.
3
N
4
and (b) Pd-g-C and (b) Pd-g-C
3
N
4
; (B) UV-Vis spectra; (C) UV-Tauc plots, (a) g-C
3
N
4
3 4 3 4
N ; (D) SEM images: g-C N
{
3
N
4
3 4
at {(c) 2 µm and (d) at 1 µm}; and (E) TEM images: g-C N at {(A’) 1 µm and (B’) at 0.2 µm} and Pd-g-
3 4
C N
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stretching vibrations of the incomplete condensed amine role in enhancing the performance of the catalyst for
−
1
−1
21
groups of C N and the peaks at 1250 cm and 1625 cm
hydrogenation.
Currently, an efficient photocatalytic generation of hydro-
the C–N heterocyclic stretching vibration modes, respectively gen from water under ambient conditions is one of the most
3
4
belong to the aromatic C–N stretching vibration modes and
1
7
8c
(
Fig. S3†). Additionally, the characteristic breathing mode of challenging transformations in synthetic chemistry. In the
the triazine repeating units at 810 cm , related to the s-tri- past few years, our research group has demonstrated that
azine ring absorption band vibrations, is well observed in the water can be activated by reducing metals (Zn, Mg, Fe) and can
fabricated g-C N pale yellow material and Pd-g-C N .
strong intense bands at 1625, 1550, 1410 and 1250 cm were gen.
−
1
1
7c
The act as a cheap, green, abundant and universal source of hydro-
3
4
3 4
−
1
12c
(Schemes 2–4). We exhibited here a pressure free,
1
0a
in good agreement with the reported literature values of the photochemical protocol for the reduction of alkenes and nitro
typical stretching vibration modes of triazine derived repeating groups using a “water/Mg” pair as a H donor in the presence
2
3 4
units (partial & fully condensed). After systematic loading with of the Pd-g-C N photocatalyst. In a typical example, styrene
Pd as per the referred programme, the arrangement of all (10 mmol, 1.04 g), Mg powder (15 mmol, 0.364 g), NiCl
2
1
7a
g-C N molecular structures remains the same (Fig. S3†).
(1 mmol, 0.129 g) Pd-g-C N (30 mg, 0.0012 mmol 0.4%
3
4
3 4
The BET surface area (BET), total pore volume (V
mean pore size (radius) Dv(r) of g-C are found to be 12 cm height) were stirred in a closed glass tube and degassed
, 0.013 cc g , and 15.366 Å, respectively, under N gas for 10 minutes (Schemes 2–4) for 12 h (Fig. S2†).
p 2
) and loading) and H O (10 mL) under visible light (15 W LED lamp,
3 4
N
2
−1
−1
1
5.203 m
g
2
however for the Pd-g-C
3 4
N catalyst these values are found to be The final reaction mixture was found to contain 99% conver-
2
−1
−1
2
3.22 m
g , 0.018 cc g and 17.356 Å, respectively. The sion of styrene to ethyl benzene (based on the GC analysis).
8
a,b
increase in the surface area (14.16%), V (38.46%), and Dv(r)
Several studies have reported
the addition of a trace
generation
Taub et al. reported a
p
(
12.95%) in Pd-g-C
3
N
4
might be due to the transformation into amount of metal salts for the enhancement of H
2
1
0a
8a,b,13
monolithic sheets from bulk g-C
3
N
4
(Fig. S5†).
2
during the Mg–H O reaction.
In the SEM analysis of g-C N , the semiconductor seems to crucial step to add a trace amount of metal for the enhance-
3
4
be aggregated, whereas Pd-g-C
3
4
N shows a smaller unit of ment of hydrogen generation during the magnesium water
1
3
agglomeration with a denser morphology (Fig. 1D). The Pd-g- reaction. However, the phenomenological rate of dihydrogen
C N TEM-EDX images refer to Pd loading over g-C N (Fig. 1E formation is directly proportional to the total availability of
3
4
3 4
and S6, S7†).
magnesium sites, which will generally increase with higher
, the
The UV-vis diffuse reflectance spectra of the prepared surface area. On addition of a catalytic amount of NiCl
2
g-C N and Pd-g-C N materials were recorded and are shown Mg(OH) oxide/hydroxide passive layer is rapidly removed from
3
4
3
4
2
in Fig. 1B and C. The g-C
3
N
4
material spectrum indicates that Mg metal, and the Mg surface is readily available for further
1
2c,22,23
the visible light absorption is ascribed to the band gap tran- reaction.
sition, corresponding to 2.0 eV for pure g-C N . The Pd-g-C N (Mg, Fe, Ni, Zn) in terms of the amount and particle size. It
photocatalyst shows a similar absorption pattern and shape was concluded that NiCl is not responsible for the chemical
with an observed band gap of 1.8 eV and is expected to show nature of the intermediates, but alters the physical nature
an increased photocatalytic performance (Fig. 1B and C).
In the detailed XPS study, the displayed C 1s XPS spectrum of the organic substrate. With the extensive established role of
shows two distinct peaks corresponding to the (–C–N–) and NiCl in dihydrogen generation using the Mg/H O system, we
M. Chidambaram et al. used various metals
12c
3
4
3 4
2
1
8
(microenvironment) of the Mg layer, to facilitate the approach
2
2
(
–NvC–) at 284.9 and 288.3 eV, respectively, for the fabricated
1
9
3 4
polymeric g-C N material (Fig. S4C†). The N 1s spectrum of
g-C
3 4
N clearly fitted into three separate intense peaks at 398.9,
4
00.1 and 401.46 eV, respectively (Fig. S4B†). The peak at 398.9
2
eV is assigned to the presence of pyridine-N species [sp -hybri-
dized nitrogen (CvN–C)]. The peak at 400.1 eV corresponds
to nitrogen atoms trigonally bonded with the carbon (sp or
sp ) atoms of amino functional groups (C–N–H). Whereas, the
2
0
2
Scheme 2 Water activation via Mg metal.
3
2
peak at 401.46 eV describes the terminal amino groups (–NH )
and also attributed to charging effects or positive charge
localization in heterocyclic molecules (Fig. S4D†). The catalyst
shows Pd chemical states, Pd 3d5/2 at 335.16 and Pd 3d3/2 at
3
3
40.42 corrosponding to Pd(0). Interestingly, the peak at
35.16 eV refers to Pd(0) of Pd-g-C and is more negative
3 4
Scheme 3 Olefin hydrogenation via the Pd-g-C N photocatalyst.
3 4
N
than that of normal metallic palladium from the literature,
which was probably due to the electron transfer from the
1
9a
nitrogen atoms of g-C
suggested that Pd and the nature of the interaction between
Pd and N species in the g-C N might play a synergetic Scheme 4 Nitro group hydrogenation via the Pd-g-C
3
N
4
to Pd (Fig. S4E†).
These results
N
3 4
photocatalyst.
3
4
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proceed with the optimized NiCl
lyst (Scheme 2).
2
usage as a physical co-cata- Whereas in the presence of a light source, we found faster
styrene conversion (99% in 8 h) (Table 1, entry 15). In another
Herein, we performed a photochemical hydrogen gene- controlled reaction, we screened the reaction using MgO
ration reaction under visible light (15 W table LED lamp) at instead of Mg (under the optimized reaction conditions). The
room temperature with 12 cm height distance from the visible observed result with only 15% styrene conversion clearly
light source LED lamp to the reaction mixture in a pressure suggests that MgO does not function well in photochemical
glass tube to nullify the heating impact (Fig. SI-1b, S2†). The hydrogenation (Table 1, entry 16), which might be due to NiCl
variation of different reaction parameters (dark, light, heat) usage for Mg regeneration from MgO (Scheme 2).
2
demonstrated a considerable effect on the reaction over in situ
For further evaluation of the Pd-g-C
3 4
N photocatalytic
hydrogenation of styrene, and is summarized in Table 1.
activity, various substrates were screened (Table 2). Styrene
For validation of the photocatalyst Pd-g-C N , a control reac- and 4-bromostyrene were successfully hydrogenated with 95%
3
4
3 4
tion was carried out by using only g-C N (Table 1, entry 1), and 98% yield, respectively (Table 2, entries 1 and 2). All the
resulting in no product formation (24 h). Interestingly under screened substrates (olefin and nitro groups) have shown an
visible light, the heterogeneous photocatalyst Pd-g-C N deli- appreciable yield (>99%) without any ring hydrogenation.
3
4
vered excellent reactivity to deliver the corresponding product Furthermore, non-aromatic cyclohexanone and cyclooctadi-
under the optimized reaction conditions (using substrate nene were converted to the hydrogenated product easily with
styrene) at room temperature. The reagent Mg and cocatalyst 100% and 80% yield (Table 2, entries 3 and 4). Furthermore,
NiCl
were obtained, respectively (without PdCl
and 4). A control reaction under dark conditions resulted in
17% of the product in 24 h (Table 1 entry 9). This experiment
supports the fact that the reagent and co-catalyst alone are not
sufficient for the reaction progress. These observations clearly
indicate that a combination of the photocatalyst, reagent and
2
were separately used, but only 5% and 1% conversion the reaction of monosubstituted allyl benzene and
2
) (Table 1, entries 3
<
3 4
Table 2 Olefin reduction via the Pd-g-C N photocatalyst
3 4 2 2
cocatalyst (Pd-g-C N + Mg/H O + NiCl ) and visible light is
critical for hydrogenation using H O as the H source (Table 1,
2
2
Entry
1
Substrate
Product
Yield (%)
95
entries 1–13). For the further evalution of the photocatalysis
reaction, we screened the optimized reaction (using substrate
styrene) under a hydrogen atmosphere under dark conditions;
we observed that the reaction proceeds slowly with 41% (12 h)
styrene conversion into ethylbenzene at RT (Table 1, entry 14).
2
3
98
100
Table 1 Photochemical hydrogenation reaction optimization under
various parameters
4
80
5
6
92
90
Yield Time
Entry Catalyst optimization
(%)
(h)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
g-C
Pd
Mg
3
N
4
00
<2
<5
<1
<5
<2
<15
<3
<17
>99
>99
<8
24
24
24
24
24
24
12
12
24
12
8
NiCl
Pd-g-C
Mg + NiCl
Pd-g-C
Pd-g-C
Pd-g-C
Pd-g-C
Pd-g-C
2
7
90
90
3
N
4
2
3
3
3
3
3
N
4
N
4
N
4
N
4
N
4
+ Mg
+ NiCl
+ Mg + NiCl
+ Mg + NiCl
+ Mg + NiCl
2
8
9
2
2
2
(in the dark)
(under visible light)
(heating at 80 °C)
0
1
2
3
4
Pd/C (4 wt%) + Mg + NiCl
Pd/C (4 wt%) + Mg + NiCl
Pd-g-C
in a hydrogen atmosphere
Pd-g-C + Mg + NiCl (under visible light) in
a hydrogen atmosphere
Pd-g-C + MgO + NiCl
2
(under visible light)
(heating at 80 °C)
12
8
85
99
2
>92
3
N
4
+ Mg + NiCl
2
(under dark conditions) 41
12
1
0
1
5
6
3
N
4
2
>99
>15
8
1
N
3 4
2
(under visible light)
24
Reaction conditions: Styrene (10 mmol), Mg powder (15 mmol, Reaction conditions: Substrate (10 mmol), Mg powder (15 mmol,
.364 g), NiCl (0.1 mmol, 0.129 g), under visible light (15 W LED 0.364 g), NiCl (0.1 mmol, 0.129 g), Pd-g-C , 30 mg (0.8 wt% Pd,
lamp, 12 cm height), water (10 mL), and photocatalyst Pd-g-C
0 mg (0.8 wt% Pd, 0.0022 mmol).
0
2
2
3 4
N
N
3 4
0.0022 mmol), and water (10 mL) solvent under visible light (15 W
LED lamp, 12 cm height).
3
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disubstituted 4-allyl anisole proceeds smoothly under visible
light with 92% and 90% yield without the ring hydrogenation
(Table 2, entries 5 and 6). After exploring olefin hydrogenation,
we proceed with the hydrogenation of the nitro group to the
corresponding amines. 4-Nitrotoluene and 4-nitro benzoic acid
were converted into amine substituted groups with 95% and
9
0% conversion, respectively (Table 3, entries 1 and 2).
Furthermore, the aryl halide substituted aromatic nitro group
viz 4-iodo nitrobenzene, 4-bromo nitro benzene, and 4-flouro
nitro benzene, shows appreciable reactivity (Table 3, entries
3
–5). Furthermore, the disubstituted rings also show good
reactivity under the optimized photochemical reaction
conditions (Table 3, entries 7 and 8).
As per the literature, a plausible mechanism is (previously
2
4
published reports ) shown in Scheme 5. After absorption of
visible light (>420 nm), excited electrons transfer to the hetero-
junction from the g-C N to the doped Pd (band gap
3
4
∼
2.0–∼2.5 eV) center enriching the electron density over the
2
4a,b,25
Pd centre to initiate the hydrogenation reaction steps.
The activated Pd-g-C N surface (via absorbed visible light) is Scheme 5 Probable mechanism for Pd-g-C N4 catalyzed photo-
3
4
3
catalytic hydrogenation reaction.
capable of activating absorbed H
2
O molecules (via–NH– and
–
NH active sites of g-C ) to Mg to form an activated Mg–
2
3 4
N
H O complex over Pd-g-C N (Scheme 5). Eventually, Mg
2
3 4
donates electrons to H
neously.
2
O to generate H
2
and Mg(OH)
2
simulta- help of a catalytic amount of NiCl
Furthermore, Mg(OH)
2
activated again with the (Scheme 5).
2
in the reaction mixture
1
2c
12c
To support the fact that water really acts as a hydrogen
source, we performed the hydrogenation reaction (see the
2 2
details in the ESI†) with D O solvent in place of H O solvent
3 4
Table 3 Nitro group reduction via the Pd-g-C N photocatalyst
with both the substrate olefin and nitro group hydrogenation
under the optimized reaction conditions (see ESI, Fig. S8 and
1
2a
S9†).
2
Experiments carried out with D O as the solvent
resulted in 60% and 55% yield of deuterated ethylbenzene and
1
2a
4
-amino toluene, respectively (confirmed via GC-MS analysis),
clearly suggesting that water is the only source of H generation
for the photo-hydrogenation described here (Fig. S9†).
The observed water contact angle for the Pd-g-C
Entry
1
Substrate
Product
Yield (%)
2
95
3 4
N catalyst
2
3
4
5
6
90
26
surface is 19.7°. As per the Cassie and Wenzen model, a
material with a water contact angle of more than 90° is classi-
fied as hydrophobic, whereas that showing a contact angle less
than 90° is categorized as hydrophilic. This supports the fact
that Pd-g-C N can absorb water molecules easily due to the
100
99
2
7
3
4
presence of –NH– and –NH
bonding and van der Waals forces over the g-C
Fig. S10†). The recycling study results of the photocatalyst Pd-
g-C for (A) olefin (styrene) (B) and nitro group (4-nitro
2
active sites along with hydrogen
99
3 4
N
surface
(
80
3 4
N
toluene) hydrogenation are shown in Fig. 2.
For a better understanding of the process, the reaction kine-
tics was measured as shown in Fig. 2. To evaluate the
optimum size for the Mg particle, we screened three different
sizes 0.08–0.1 µm, 0.1–0.5 µm and 0.5–1.5 µm (Fig. 2A). The
results suggest that the most active size is 0.1–0.5 µm. The
7
8
90
90
2
available surface area of Mg is a crucial factor for Mg–H O
complex formation. In another reaction kinetics profile, we
examined the reaction progress at three different stirring
speeds 600, 800, and 1000 rpm (Fig. 2B). Since the reaction
mixture is heterogeneous in nature (Pd-g-C N + Mg/H O +
Reaction conditions: Substrate (10 mmol), Mg powder (15 mmol,
0
0
.364 g), NiCl
2 3 4
(0.1 mmol, 0.129 g), Pd-g-C N , 30 mg (0.8 wt% Pd,
.0022 mmol), and water (10 mL) solvent, under visible light (15 W
LED lamp, 12 cm height).
3
4
2
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Fig. 3 Recycling study for the Pd-g-C
3
N
4
photocatalyst. (A) Styrene
hydrogenation, (B) Nitrobenzene hydrogenation (Under optimized reac-
tion conditions).
3 4
Fig. 2 Pd-g-C N photocatalyst reaction kinetics profile for the photo-
ferred into the pressure tube for further continuation of the
reaction without the photo-catalyst (fresh addition of the Mg
reagent and Ni catalyst). GC analysis of the filtrate confirmed
that the conversion was up to 42% of the corresponding pro-
ducts, respectively (styrene to methylbenzene). After a 2 h time
reaction; (A) Mg particle size (µm) impact, (B) stirring speed effects, (C)
2
co-catalyst NiCl amount impact, and (D) fresh and reused catalyst
comparison.
NiCl
2
), the reaction stirring speed plays a highly crucial role in span, the conversion is 42% and after removal of the catalyst it
determining the reaction rate. We found that fast stirring was reaches up to 45% conversion (which might be due to fresh
essential for the reaction rate (1000 rpm for 12 h, 99% yield) addition of the Mg reagent and Ni co-catalyst). But proceeding
(
Fig. 2B). In the next set of experiments, we modified the further with the unavailability of the Pd-g-C
N catalyst makes
3 4
amount of NiCl added to the reaction, 1 mmol, 2 mmol, and the progress of the reaction very slow. As confirmed by GC ana-
2
0
.1 mmol were screened (Fig. 2C). Since the amount of NiCl
plays a crucial role here in the Mg regeneration from MgO the hydrogen transferred product (styrene to ethylbenzene),
Scheme 2), the reaction progress exhibits strong dependence even with prolongation of the hot filtrate reaction by an
on the Ni amount. In the next screening, we used a fresh and additional 12 h under visible light (see ESI, Fig. S11†).
2
lysis, there was no improvement in the yield (<45% yield) of
(
reused catalyst, to realize that the photocatalyst is highly active
even after the 5 recycle (Fig. 2D).
For the molecular level analysis of the recycled Pd-g-C
photo-catalyst, XRD characterization was carried out. The per-
photocatalyst several formed characterization suggests that the five times recycled
times which does not show any major change in its activity Pd-g-C photocatalyst remains unchanged at the molecular
100–80% conversion) in both photo-reactions (Fig. 3). After level with the persistence of diffraction peaks indexed at the
the photochemical reaction was complete the solid photo- (100) plane at 13.4° and at the (002) plane at 27.2° even after
catalyst Pd-g-C
was separated via filtration with Whatman multiple reuses (see ESI, Fig. S12†). BET characterization after
filter paper, consequently washed with methanol and acetone 5 times recycled Pd-g-C shows a surface area (BET) of
N
3 4
th
3 4
We recycled and reused the Pd-g-C N
N
3 4
2
8
(
3 4
N
3 4
N
2
−1
−1
alternately five times and further oven dried (100 °C, over- 6.975 m g , a total pore volume (V
) of 0.012 cc g and a
p
night). From Fig. 3, it is clear that the Pd-g-C
3 4
N
photocatalyst mean pore size (radius) Dv(r) of 15.368 Å. The observed
could be reused many times without any change in its activity decrease in surface area, pore volume and pore size might be
and the product yield remained good for both the hydrogen- due to the continuous use of the reagent and co-catalyst (see
ation reactions Fig. 3A and B (ESI, 1c†) (olefin and nitro group ESI, Fig. S13A and B†). XPS analysis exhibits the persistence of
conversion).
the Pd(0) oxidation state with Pd 3d5/2 and Pd 3d3/2 at 335.0 eV
We performed Sheldon’s hot filtration test to test the and at 340.2 eV, respectively (see ESI Fig. S14A†). Pyridine-N
2
heterogeneous nature and stability performance of the syn- species [sp -hybridized nitrogen (CvN–C) at 398.9, amino
thesized Pd-g-C N photocatalyst (see ESI, Fig. S11†). The functional groups (C–N–H) at 400.1 and terminal amino
3
4
observed results have shown nearly negligible Pd leaching groups (–NH) group at 401.46 eV] well observed in the 5 times
into the reaction solution during the course of the process. To recycled Pd-g-C photocatalyst confirm well the N character-
perform Sheldon’s hot filtration test for the optimized photo- istic binding energy (see ESI, S14B†). In addition, SEM analysis
chemical hydrogenation reaction conditions, the photo-catalyst of the 5 times recycled Pd-g-C photocatalyst shows a well
Pd-g-C
was filtered out from the reaction mixture after 2 h maintained unchanged smaller unit of agglomeration with
of the photo-reaction. The reaction filtrate was again trans- denser morphology (see ESI, S15†). FT-IR analysis of the
3 4
N
N
3 4
3 4
N
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reused Pd-g-C
3 4
N catalyst (five recycles) was carried out to find
Notes and references
out the change of Pd-g-C N at the molecular level. Fig. S16†
3
4
−1
shows that the triazine repeating units at 810 cm vibration
modes are well preserved even after multiple reuses with other
characteristic IR absorption bands. (see ESI, S16†).
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