Photocatalytic C–H activation and oxidative esterification using Pd@g-C3N4

Article abstract of DOI:10.1016/j.cattod.2017.06.009

Graphitic carbon nitride supported palladium nanoparticles, Pd@g-C₃N₄, have been synthesized and utilized for the direct oxidative esterification of alcohols using atmospheric oxygen as a co-oxidant via photocatalytic C–H activation.

Full text of DOI:10.1016/j.cattod.2017.06.009

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Photocatalytic CeH activation and oxidative esterification using Pd@g-C3N4
Sanny Vermaa,1, R.B. Nasir Baiga,1, Mallikarjuna N. Nadagoudab, Rajender S. Varmab,⁎
a
Oak Ridge Institute for Science and Education, P. O. Box 117, Oak Ridge, TN 37831, USA
Water Systems Division, Water Resources Recovery Branch, National Risk Management Research Laboratory, U. S. Environmental Protection Agency, 26 West Martin
b
Luther King Drive, MS 443, Cincinnati, Ohio 45268, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
CeH activation
Visible light
Heterogeneous catalysis
Oxidative esterification
Alcohols
Graphitic carbon nitride supported palladium nanoparticles, Pd@g-C3N4, have been synthesized and utilized for
the direct oxidative esterification of alcohols using atmospheric oxygen as a co-oxidant via photocatalytic CeH
activation.
1. Introduction
development of sustainable and greener protocols in organic synthesis
[17–21], herein we describe a superior method which deploys air as an
Esters are important functional groups present in natural products
and pharmaceutical active scaffolds [1]. In many instances, they serve
as starting materials for the synthesis of a wide range of active phar-
maceutical ingredients and important polymeric compounds [2]. Me-
thyl esters are the simplest esters [3] and have been mainly utilized as
precursors in multistep organic synthesis [4]. In general, they are
synthesized by the reaction of activated carboxylic acid with methanol.
Methyl esters have also been accessed using acid catalyzed condensa-
tion of carboxylic acid with methanol. Although, these methods provide
high yield but they demand higher temperatures or require the acti-
vation of carboxylic acid which may interfere with the sensitive func-
tional groups present in the vicinity [5,6]. Direct synthesis of methyl
esters has also been reported using oxidants such as MnO2, Na2Cr2O7,
etc. [7,8]. The serious problem in these methods is the generation of a
large amount of toxic waste typical of stoichiometric reactions. The
growing environmental concerns in chemical research have prompted
researchers worldwide to seek greener pathways for all transformations
preferably deploying catalytic reactions. We focused on esterification
reactions using alcohols in the direct synthesis of methyl esters where
few general methods have been documented [9–14]. Lei et al. has re-
ported the synthesis of methyl esters using palladium (Pd) catalyst
under oxygen atmosphere (oxygen balloon) [15] with moderate success
due to compromising yields along with the loss of precious Pd complex.
Although the synthesis of esters using photoactive VO@g-C3N4 and
hydrogen peroxide is known [16] but we envisioned that the use of air
is more sustainable then hydrogen peroxide. Working towards the
oxidant. Accordingly, graphitic carbon nitride supported palladium
nanoparticles (Pd@g-C3N4) were synthesized in view of the inherent
photoactivity of graphitic carbon nitride (g-C3N4) and their application
in cascade oxidative CeH activation-esterification of alcohols is de-
monstrated. In Pd@g-C3N4, the nitrogenous framework of g-C3N4 pro-
vides electron rich environment for holding palladium nanoparticles
and absorbs visible light, which is further dispersed in reaction media
thus accomplishing aerial oxidative esterification.
2. Materials and methods
2.1. General methods
All the reactions were performed in an oven-dried apparatus in a
closed box using domestic bulb (20 W), with wave length in the range
of 400–700 nm and were stirred magnetically. 1H NMR spectra were
recorded at 300 MHz instrument. Chemical shifts are reported in parts
per million downfield from the internal reference, tetramethylsilane
(TMS).
2.2. Synthesis of materials
2.2.1. Synthesis of g-C3N4
The graphitic carbon nitride, g-C3N4, was synthesized by calcination
of urea at 500 °C for 2 h [16].
⁎
Corresponding author.
E-mail addresses: varma.rajender@epa.gov, rajvarma@hotmail.com (R.S. Varma).
Equal contribution.
1
http://dx.doi.org/10.1016/j.cattod.2017.06.009
Received 14 March 2017; Received in revised form 6 May 2017; Accepted 13 June 2017
0920-5861/PublishedbyElsevierB.V.
Pleasecitethisarticleas:Verma,S.,CatalysisToday(2017),http://dx.doi.org/10.1016/j.cattod.2017.06.009

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Fig. 1. Synthesis of Pd@g-C3N4.
2.2.2. Synthesis of Pd@g-C3N4 catalyst
presence of palladium confirming the fact that the g-C3N4 holds the
palladium nanoparticles tightly which minimizes the Pd leaching and
supports efficient recycling.
100 mg of g-C3N4 was dispersed in water (500 mL) under sonica-
tion. The aqueous solution of palladium nitrate hydrate (10.82 mg of Pd
(NO3)2 in 5 mL water) was added and stirred for 4 h. The reaction
temperature was raised to 60 °C and the excess of sodium borohydride
was added, in portion, with constant stirring. The reaction mixture
becomes black after NaBH4 addition. The reaction was further stirred
for 4 h and cooled down to room temperature. The catalyst was cen-
trifuged, washed with methanol and dried under vacuum at 50 °C. The
Pd@g-C3N4 catalyst was isolated as off black powder (Fig. 1) and
characterized using SEM, TEM, XRD, XPS and ICP-AES analysis.
3. Result and discussion
The synthesized catalyst, Pd@g-C3N4, was characterized using
scanning electron microscopy (SEM), transmission electron microscopy
(TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy and
Inductive coupled plasma atomic emission spectroscopy (ICP-AES). The
SEM image (Fig. 3) obtained for g-C3N4 and Pd@g-C3N4 affirms the
immobilization of palladium nanoparticles with the visible difference in
their morphology before and after the impregnation of nanoparticles
which was further supported by TEM analysis (Fig. 4). The EDX spec-
trum of Pd@g-C3N4 (Fig. 4c) indicates the presence of palladium metal
in Pd@g-C3N4. The XRD analysis confirmed the formation of metallic
graphitic carbon nitride-Pd catalyst (Pd@g-C3N4), a peak at 27.4° is
assigned to graphitic carbon nitride g-C3N4 (Fig. 5). The peaks at 40.1°,
46.1°, 68.2° represent the metallic Pd nanoparticles in zero oxidation
state. Thus, the XRD pattern of Pd@g-C3N4 confirms the crystalline
structure of Pd-nanoparticles over g-C3N4 surface and the XRD analysis
was further supported by XPS analysis (Supporting Information; S1).
The weight percentage of Pd (4.59%) was calculated using ICP-AES
analysis.
2.2.3. General procedure for the oxidative esterification of alcohols
A 25 mL side-armed round bottomed flask equipped with a mag-
netic stirring bar and a balloon filled with air was charged with alcohol
(1 mmol), catalyst Pd@g-C3N4 (10 mg) and 5.0 mL of methanol. The
reaction mixture was exposed to visible light irradiation using 20-W
domestic bulb. The progress of the reaction was monitored by TLC.
After the completion of the reaction, Pd@g-C3N4 catalyst was separated
using centrifuge. The product was extracted using ethyl acetate, dried
over sodium sulfate, concentrated and characterized using NMR.
2.3. Recycling of Pd@g-C3N4 catalyst
The synthesized Pd@g-C3N4 catalyst was screened for the oxidative
esterification of alcohols using aerial flow under visible light irradia-
tion. We conjectured that the graphitic carbon nitride, being endowed
with nitrogenous electron rich photoactive chromophore, would be a
good candidate for this exploration. These conditions would help ac-
celerate the CeH activation via visible light adsorption thus providing
the required amount of activation energy for crossing transition barrier
in the esterification of alcohols. The inherent photo active nature of g-
C3N4 encouraged us to combine it with Pd metal which may offer sy-
nergic jump for the oxidative esterification. Palladium nanoparticles
were immobilized over the surface of graphitic carbon nitride and
screened for the oxidative esterification of benzyl alcohol with me-
thanol under visible light using air (Table 1); reactions were performed
with 25 mg of Pd@g-C3N4 bearing different Pd-loading in methanol
(Table 1, entries 1–6) which was imperative for its photo-active CeH
activation towards the oxidative esterification. The reaction of benzyl
alcohol with Pd@g-C3N4 bearing about 1% Pd required 12 h to afford
47% of methyl benzoate (Table 1, entry 1). The increase in Pd con-
centration to 2%, 3% and 4% resulted in better yields and delivered the
desired product in 67%, 73% and 87%, respectively (Table 1, entries
2–4). The Pd@g-C3N4 catalyst with 5% Pd-loading was most effective as
it offered methyl benzoate in near quantitative yield (96%, Table 1,
entry 5). Further increase in Pd concentration did not have significant
impact on the progress of reaction (Table 1, entry 6). After identifying
the effective Pd-loading for the catalyst it was essential to optimize the
amount of the catalyst required for the oxidative esterification. Thus,
To establish the recyclability of the Pd@g-C3N4 catalyst for oxida-
tive esterification of alcohol, a set of experiments were performed using
benzyl alcohol as a substrate in methanol. After the completion of each
reaction the Pd@g-C3N4 catalyst was recovered using a centrifuge,
washed with methanol and reused for the oxidative esterification of
benzyl alcohol using fresh reagents. The Pd@g-C3N4 catalyst could be
recycled and reused up to five times without losing its activity (Fig. 2).
The metal leaching of Pd@g-C3N4 catalyst was studied by ICP-AES
analysis before and after completion of the reaction. The Pd con-
centrations were found to be 4.59% before the reaction and 4.58% after
the 5th cycle. The ICP-AES of the mother liquor did not show the
Fig. 2. Histogram for recycling experiments.
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Fig. 3. SEM image of g-C3N4 (a, b); SEM image of
Pd@g-C3N4 (c, d).
Fig. 4. a) TEM images of g-C3N4; b) TEM images of g-
C3N4; c) EDX spectra of Pd@g-C3N4.
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Table 2
Oxidative esterification via CeH activation.a
Entry
1
Substrate
Product
Yieldb
96%
2
93%
3
4
95%
95%
Fig. 5. XRD pattern of (a) g-C3N4; (b) Pd@g-C3N4.
Table 1
Reaction optimization for oxidative esterification.a
5
91%
6
7
93%
93%
Entry
Catalyst
Yieldb
1
2
3
4
5
6
7
8
Pd@g-C3N4 (1% Pd, 25 mg)
Pd@g-C3N4 (2% Pd, 25 mg)
Pd@g-C3N4 (3% Pd, 25 mg)
Pd@g-C3N4 (4% Pd, 25 mg)
Pd@g-C3N4 (5% Pd, 25 mg)
Pd@g-C3N4 (10% Pd, 25 mg)
Pd@g-C3N4 (5% Pd, 20 mg)
Pd@g-C3N4 (5% Pd, 15 mg)
Pd@g-C3N4 (5% Pd, 10 mg)
Pd@g-C3N4 (5% Pd, 5 mg)
g-C3N4 (10 mg)
47%
67%
73%
87%
96%
96%
96%
96%
96%
78%
–
Reaction condition: a) Alcohol (1 mmol), methanol (5 ml), 20 W domestic bulb, room
temperature, air, 12 h; b) Isolated yield.
feature of this reaction is the use of aerial oxygen as a co-oxidant which
is trapped by Pd@g-C3N4 and used in the CeH activation and sub-
sequent oxidation esterification.
9
10
11
12
13c
Pd(NO3)2 (10 mg)
Pd@g-C3N4 (5% Pd, 10 mg)
5%
–
Notes
Reaction condition: a) Benzyl alcohol (1 mmol), methanol (5 ml), 20 W domestic bulb,
room temperature, 12 h; b) Isolated yield; c) Reaction was performed in dark.
The authors declare no competing financial interest.
Disclaimer
the reaction was performed using 20 mg, 15 mg, 10 mg and 5 mg of
Pd@g-C3N4 under visible light using air-flow (Table 1, entries 7–10);
quantitative conversion was observed when 20 mg, 15 mg, and 10 mg
of catalyst was employed. However, with 5 mg of the catalyst there was
a substantial decrease in the product yield (Table 1, entry 10). Control
experiments performed using g-C3N4 (Table 1, entry 11) and palladium
nitrate (Table 1, entry 12) did not offer more than 5% of desired pro-
duct as most of the starting material remained intact. The scope and the
reactivity of the reaction were next evaluated using these optimized
reaction conditions.
Accordingly, a wide range of aromatic alcohols were subjected to
the oxidative esterification (Table 2, entries 1–7). The insight analysis
of these reactions using a variety of alcohols bearing both, electron
withdrawing (Table 2; entry 2) and electron donating substituents
(Table 2; entries 3–5) did not show any significant impact in terms of
reactivity and the product outcome (Table 2; entries 2–5). Five mem-
bered heterocyclic compounds such as furfuryl alchol and 2-thiophe-
nemethanol were efficiently converted into corresponding methyl esters
without the formation of any side product (Table 2; entries 5 and 6).
The views expressed in this article are those of the authors and do
not necessarily represent the views or policies of the U.S.
Environmental Protection Agency. Any mention of trade names or
commercial products does not constitute endorsement or re-
commendation for use.
Acknowledgements
SV and RBNB were supported by the Postgraduate Research
Program at the National Risk Management Research Laboratory ad-
ministered by the Oak Ridge Institute for Science and Education
through an interagency agreement between the U.S. Department of
Energy and the U.S. Environmental Protection Agency.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.cattod.2017.06.009.
4. Conclusions
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In conclusion, a simple and highly efficient method for the synthesis
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proceeds via CeH activation using photoactive Pd@g-C3N4. The salient
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