Oxidized graphitic carbon nitride as a sustainable metal-free catalyst for hydrogen transfer reactions under mild conditions

Article abstract of DOI:10.1039/d0gc01123a

The development of green and sustainable transfer hydrogenation protocols without the use of expensive noble metals and toxic solvents is a challenging task. Herein, a highly stable, low-cost, metal-free heterogeneous catalyst, oxidized graphitic carbon nitride (O-GCN), has been developed, which exhibits efficient catalytic hydrogen transfer reactions of carbonyl compounds to corresponding alcohols under mild reaction conditions. The heterogeneous catalyst was synthesized by the chemical oxidation of graphitic carbon nitride (GCN) nanosheets, which results in the generation of carboxyl, hydroxyl and ketone groups over the GCN surface. These hydrophilic groups functionalized on the surface of O-GCN nanosheets act as catalytically active sites for the hydrogen transfer reactions of carbonyl compounds. A wide range of substrates was investigated for the hydrogen transfer reactions using 2-propanol both as a hydrogen donor and a solvent. The O-GCN nanosheets resulted in high yields and high turnover numbers (TON) demonstrating the versatile catalytic potential of the as-synthesized catalyst. The detailed optimization of the reaction parameters (temperature, time and catalyst amount) was performed, in addition to the calculation of green metric parameters. Moreover, the catalyst could be easily recovered and was used for five runs without any significant loss in catalytic activity. This study provides a green, sustainable, attractive, and useful methodology for the hydrogen transfer reactions of a wide range of carbonyl compounds.

Full text of DOI:10.1039/d0gc01123a

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Oxidized graphitic carbon nitride as a sustainable
metal-free catalyst for hydrogen transfer reactions
under mild conditions†
Cite this: DOI: 10.1039/d0gc01123a
a
Priyanka Choudhary,a Ashish Bahuguna, ‡a Ajay Kumar,
Sandeep Singh Dhankhar, b C. M. Nagaraja b and Venkata Krishnan
*
a
The development of green and sustainable transfer hydrogenation protocols without the use of expensive
noble metals and toxic solvents is a challenging task. Herein, a highly stable, low-cost, metal-free hetero-
geneous catalyst, oxidized graphitic carbon nitride (O-GCN), has been developed, which exhibits efficient
catalytic hydrogen transfer reactions of carbonyl compounds to corresponding alcohols under mild reac-
tion conditions. The heterogeneous catalyst was synthesized by the chemical oxidation of graphitic
carbon nitride (GCN) nanosheets, which results in the generation of carboxyl, hydroxyl and ketone groups
over the GCN surface. These hydrophilic groups functionalized on the surface of O-GCN nanosheets act
as catalytically active sites for the hydrogen transfer reactions of carbonyl compounds. A wide range of
substrates was investigated for the hydrogen transfer reactions using 2-propanol both as a hydrogen
donor and a solvent. The O-GCN nanosheets resulted in high yields and high turnover numbers (TON)
demonstrating the versatile catalytic potential of the as-synthesized catalyst. The detailed optimization of
the reaction parameters (temperature, time and catalyst amount) was performed, in addition to the calcu-
lation of green metric parameters. Moreover, the catalyst could be easily recovered and was used for five
runs without any significant loss in catalytic activity. This study provides a green, sustainable, attractive,
and useful methodology for the hydrogen transfer reactions of a wide range of carbonyl compounds.
Received 30th March 2020,
Accepted 30th June 2020
DOI: 10.1039/d0gc01123a
rsc.li/greenchem
involves two types of reactions, i.e., oxidation and reduction.9
The classical examples of oxidative hydrogen transfer reactions
1. Introduction
The reduction of carbonyl compounds to their corresponding include the oxidation of alcohols, alkane, sulfides, etc.10 The
alcohols is an important reaction in organic synthesis. reductive hydrogen transfer reaction is observed in the
Aromatic alcohols are functional groups present in most of the reduction of carbonyl compounds or nitro compounds using
perfumes, bioactive molecules, agrochemicals, and fine hydrogen donors, such as alcohols, formates and NH3BH3.11
chemicals.1–4 The direct synthesis of these compounds can be The catalytic transfer hydrogenation provides various advan-
done in a simple manner via the hydrogenation of their corres- tages over other reduction methods due to the following
ponding carbonyl compounds.5 Hydrogenation using direct reasons: (i) the hydrogen source used is easy to handle (i.e. no
hydrogen is a dangerous and a difficult process to handle, requirement of high-pressure vessels), (ii) possible causalities
which requires very high temperature and pressure.6–8 Hence are minimized, (iii) use of mild reaction conditions which can
alternative approaches for the hydrogen transfer reactions result in enhanced selectivity, and (iv) use of chiral ligands
have attracted much attention. In general, hydrogen transfer facilitates the asymmetric transfer hydrogenation. To date, a
major proportion of the work done on the catalytic transfer
hydrogenation of carbonyl compounds has been performed
using transition metals, such as Rh, Ir, Ni, Pt and Ru.11–16
These transition metal-based catalytic systems have proved to
aSchool of Basic Sciences and Advanced Materials Research Center, Indian Institute
of Technology Mandi, Mandi 175075, Himachal Pradesh, India.
be very effective, but from the view point of sustainability, they
have certain disadvantages, such as high cost, inadequate
accessibility, and environmental toxicity. Although, Ru and Ir
are found to be most active in a homogeneous environment,
their reusability and recyclability remain
Hence, the need for the design and development of a green,
E-mail: vkn@iitmandi.ac.in
bDepartment of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001,
India
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc01123a
a
concern.17,18
‡Current address: Casali Center of Applied Chemistry, Institute of Chemistry,
The Hebrew University of Jerusalem, Jerusalem 9190401, Israel.
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safe, and low-cost metal-free catalytic system capable of per- variety of carbonyl compounds extending its potential for com-
forming transfer hydrogenation reactions is greatly felt among mercial use. Among various hydrogen donors, 2-propanol is
researchers.
very popular since it is inexpensive, volatile, and non-toxic. In
Recently, carbon-based materials have emerged as an addition, it possesses good solvent properties, and can be con-
effective catalyst for a large number of applications, such as in verted into acetone, which is easy to remove from the reaction
industrial catalysis, organic catalysis, supercapacitors, electro- medium.11 Hence, a highly efficient and sustainable method
catalysis, and many more.19–22 Carbon-based materials exhibit for the transfer hydrogenation of carbonyl compounds under
significantly high stability and an eco-friendly performance. In neat conditions has been developed using the metal-free
addition, ease of availability and low cost make them a promis- heterogeneous O-GCN catalyst.
ing choice as heterogeneous catalysts. Various carbon
materials, such as amorphous carbon, graphene, graphitic
carbon nitride, activated carbon and carbon nanotubes, have
2. Results and discussion
2.1. Synthesis and structural studies
been widely employed in many catalytic organic
transformations.23–25 Among others, lately, graphitic carbon
nitride (GCN) due to its interesting chemical and physical fea- The chemical oxidation method was used for the synthesis of
tures has been considered suitable to act as a metal-free cata- highly dispersible hydrophilic O-GCN nanosheets. Acid
lyst or catalyst support for industrially indispensable hetero- etching by H2SO4 and HNO3 followed by oxidation with H2O2
geneous catalytic hydrogenation.26
led to the generation of various hydrophilic functional groups
GCN provides a large number of catalytic reaction sites (i.e. –OH, –COOH and –CvO) at the defect sites and at the
owing to its high surface area and sheet-like morphology. open ends of the GCN structure. The mechanism of oxidation
There have been several reports on GCN being used as a cata- can be summarized as below. The concentrated H2SO4 and
lyst support for noble metals, such as Ru and Pd for catalytic HNO3 solutions effectively etch the surface of GCN and this
hydrogenation reactions.27,28 GCN as an active support exhibits results in the formation of hydroxyl groups. In the presence of
excellent catalytic performance. Due to its unique tri-s-triazine a strong oxidative environment created by H2O2, these hydroxyl
structure, it can be easily functionalized and its chemical pro- groups are further converted into quinone groups or can break
perties can be tuned accordingly.22,29 Earlier, Yang and co- into OvC–OH groups.33 The slow oxidative etching process
workers reported that CvO functional groups on the carbon- improves the formation of the porous structure; meanwhile,
based catalysts act as catalytically active sites for the reductive the two-dimensional (2D) nanosheet-like structure of GCN is
hydrogen transfer reaction. The oxygen functionalization over also well-preserved. The synthesis scheme for O-GCN
GCN can be achieved through a chemical oxidation method, nanosheets is shown in Scheme 1.
which involves the treatment of GCN with strong oxidizing
The crystalline nature and chemical properties of GCN and
agents, such as KMnO4, K2Cr2O7 and H2O2, in an acidic O-GCN are studied by powder X-ray diffraction (PXRD), Raman
medium. This leads to the functionalization of hydroxyl, car- spectroscopy, Fourier-transform infrared (FTIR) spectroscopy,
bonyl, and carboxylic groups on the surface of GCN and thermogravimetric (TGA) analysis. The PXRD patterns of
nanosheets. As a result of this surface functionalization, GCN GCN and O-GCN nanosheets are shown in Fig. 1(a). The PXRD
nanosheets initially having a Brønsted basic character due to pattern for GCN shows a low-angle reflection peak (100) at
the presence of N atoms, acquire an additional Brønsted acidic 13.17°, corresponding to the d-spacing of 0.693 nm, which
character. The catalytic potential of this oxidized GCN has originates due to the interplanar structural packing of the
been explored previously for hydrogen evolution and environ- motif and a strong peak (002) at 27.62°, corresponding to the
mental remediation.30–32 However, to the best of our knowl- d-spacing of 0.324 nm, which stems from the characteristic
edge, to date, there has been no report exploring the catalytic carbon nitride interlayer stacking reflections.34,35 The XRD
hydrogenation potential of oxidized GCN owing to its carbonyl pattern of O-GCN shows slight variation, and the (002) peak of
functional groups.
GCN is broadened and shifted from 27.62 to 27.91 due to the
In this work, we report highly efficient catalytic hydrogen reduction in the gallery distance between the O-GCN layers.
transfer reactions of carbonyl compounds to their corres- The O-GCN layers are planarized by the hydrogen bonding and
ponding alcohols using 2-propanol as the hydrogen source π–π stacking interactions, which results in a denser packing
and using a rationally designed oxidized graphitic carbon and hence reduce the gallery distance. The broadened (002)
nitride (O-GCN) as a heterogeneous metal-free catalyst. The peak and weakened (100) peak indicates the decreased crystal-
characterization results confirmed that the O-GCN catalyst was linity and weakened periodic structure of O-GCN after chemi-
successfully synthesized with remarkable catalytic properties. cal oxidation.36,37
Detailed optimization reactions carried out using the O-GCN
Raman spectra were measured to investigate the presence
catalyst displayed both high efficiency and specificity of the of oxygen species in the O-GCN nanosheets and the corres-
catalyst towards transfer hydrogenation of carbonyl com- ponding Raman spectra of GCN and O-GCN nanosheets are
pounds with high turnover numbers (TONs). In addition, the shown in Fig. 1(b). The intense peaks at 480 and 706 cm−1
catalyst exhibited excellent recyclability and versatility. corresponds to the ring breathing modes of the heterocyclic
Furthermore, the designed protocol can be used for a wide heptazine ring. The peaks at 754 and 978 cm−1 arise due to
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Scheme 1 Schematic presentation of the synthesis of GCN and O-GCN nanosheets.
Fig. 1 (a) PXRD patterns of GCN and O-GCN nanosheets and (b) Raman spectra of GCN and O-GCN nanosheets.
the in-plane vibrations of the C–N bond for both GCN and vibrations of aromatic rings.19 The peaks at 1635 and
O-GCN.38 The broad bands present in the region 1400 to 1624 cm−1 in GCN and O-GCN, respectively, can be attributed
1800 cm−1 are attributed to the ring stretching vibrations to the CvN stretching vibrations in GCN, whereas in O-GCN
arising due to sp3 and sp2 hybridized carbons present in both this peak can be ascribed to the stretching vibrations of both
GCN and O-GCN nanosheets. In addition, the peak at CvN and CvO bonds.20,43 As a result of acid oxidation, the
1248 cm−1 in GCN shifts to a broad band at 1256 cm−1 in adsorption bands in O-GCN become more sharp and intense
O-GCN. This shift in the peak can be attributed to OH in the because the H bond along the polymeric melon units that are
plane bending mode in the C–OH functional group.39
left after chemical oxidation have the better ordered packing.44
The FTIR spectra of GCN and O-GCN shows similar absorp- The sharp peaks at 1085 and 1452 cm−1 in the FTIR spectrum
tion bands, which are presented in Fig. 2(a). The broad absorp- of O-GCN have originated due to the presence of C–O–C and
tion bands between 3000 and 3500 cm−1 originate from the N– C–O–H groups, respectively.45 It has been reported earlier that
H stretching vibrations of peripheral amino groups and the O– the force constants for the skeletal stretching vibrations of C–
H
stretching vibrations of surface hydroxyl groups.40,41 N and C–O bonds occur in the same IR regions; hence, from
However, broader and sharp peaks are observed for O-GCN the FTIR spectra it is not feasible to distinguish between GCN
nanosheets, which can be accounted for the introduction of and O-GCN.46
oxygen functionalities. The sharp peaks at 807 and 880 cm−1
The thermal stability of the as-synthesized GCN and O-GCN
correspond to the breathing mode of the s-triazine ring in nanosheets was investigated using thermogravimetric analysis
GCN, whereas in O-GCN these peaks are slightly shifted to 794 (TGA) and their corresponding results are depicted in Fig. 2(b).
and 875 cm−1, respectively, due to the incorporation of oxygen From the results, it is found that GCN was thermally more
moieties.42 The peaks between 1230 and 1450 cm−1 in both stable than O-GCN. For the GCN nanosheets, the weight loss
GCN and O-GCN can be ascribed to the C–N stretching of 4.32% was observed up to 450 °C, whereas for the O-GCN
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Fig. 2 (a) FTIR spectra of GCN and O-GCN and (b) TGA plots of GCN and O-GCN nanosheets.
nanosheets about 20.68% weight loss was observed. The first groups (–CvO, –OH and –COOH).47 The high-resolution trans-
weight loss for GCN is observed up to 150 °C of about 2.68% mission electron microscopy (HRTEM) of O-GCN clearly shows
due to the adsorbed water molecules, whereas for O-GCN, a the presence of rough etching edges. Furthermore, the exist-
greater weight loss was observed up to 150 °C of about 5.71%, ence and distribution of the constituent elements (C, N and O)
which can be ascribed to the incorporation of more amount of in O-GCN were confirmed by energy dispersive X-ray analysis
water molecules due to the stronger H-bonding interactions (EDAX) and elemental mapping, as shown in Fig. 3( j–m).
between the adsorbed water molecules and the –CvO, –OH
To investigate the presence and composition of all the con-
and –COOH groups functionalized on the surface of O-GCN stituent elements along with their chemical states, XPS ana-
nanosheets. For GCN, the second weight loss was negligible, lysis of the GCN and O-GCN nanosheets was carried out and
whereas for O-GCN, a higher second weight loss of 5.81% was the results are presented in Fig. 4. The survey spectrum of
observed in the temperature range of 200 to 350 °C, which GCN given in Fig. 4(a) shows the presence of carbon and nitro-
accounts for the degradation of carboxylic groups. After gen. Fig. 4(b) shows three deconvoluted peaks of the C 1s spec-
500 °C, the aromatic C–N network of the heterocyclic ring trum of GCN at 288.72, 287.09 and 285.33 eV. The most
starts decomposing and up to 700 °C it gets completely decom- intense peak was found at 288.72 eV, which is ascribed to the
posed in both GCN and O-GCN nanosheets. The GCN and C–N–C sp2 hybridized carbon–nitrogen bonds of the s-triazine
O-GCN nanosheets are held together via the hydrogen bonding unit.36 The deconvoluted peak at 285.33 eV is relatively less
interactions between the NH/NH2 groups. The hydrogen intense, which corresponds to the graphene CvC bonds.
bonding interactions between the nanosheets are not very Furthermore, the weakest peak appears at 287.09 eV, which is
strong such that they can withstand high temperature; hence, due to CvN bond of the trigonal C–N network. Fig. 4(c) rep-
GCN and O-GCN sheets get completely disintegrated by resents the deconvoluted spectrum of N 1s of GCN in which
650 °C.1
the most intense peak at 399.21 eV corresponds to the sp2
hybridized nitrogen atom bonded to the carbon atoms in the
s-triazine ring. The less intense peaks at 400.63 and 401.75 eV
2.2. Morphological and compositional studies
The morphology of the GCN and O-GCN catalysts was exam- correspond to the bridging N atoms in the (C)3 moieties and
ined using scanning electron microscopy (SEM) and the C–N–H bond, respectively.41,48
results are presented in Fig. 3(a–d). The SEM images of GCN
The survey spectrum of O-GCN nanosheets provides strong
shows a sheet-like morphology, which is crumbled and aggre- evidence for the presence of oxygen species in O-GCN along
gated, wherein O-GCN also shows a sheet-like morphology with carbon and nitrogen. The C 1s spectrum of O-GCN shows
where the nanosheets are stacked upon one another. In an intense peak at 289.31 eV, which can be attributed to
addition, the nanoscale morphologies were investigated using –COOH species (Fig. 4d).29,48 In addition, the less intense
transmission electron microscopy (TEM), which are presented peaks at 287.65 and 285.68 eV are ascribed to CvO and C–O
in Fig. 3(e–h), wherein a sheet-like morphology was observed bonds, respectively.29 The peaks at 287.02 and 284.84 eV can
for both GCN and O-GCN catalysts. In GCN, the sheets were be attributed to CvN and CvC, respectively, as that of the
present in a compact form, while in O-GCN the sheets had s-triazine GCN framework.30 The N 1s spectrum shows three
agglomerated into non-uniform networks. Formation of these deconvoluted peaks at binding energies 399.92, 400.77 and
non-uniform networks can possibly be driven by the π–π stack- 402.33 eV corresponding to CvN–C, N–(C)3 and C–N–H,
ing and hydrogen bonding interactions between the layers, respectively, as shown in Fig. 4(e). The peaks in the C 1s and N
which are facilitated by the oxygen-containing functional 1s spectra of O-GCN corresponding to the s-triazine units of
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Fig. 3 SEM images of (a and b) GCN and (c and d) O-GCN, TEM images of (e and f) GCN and (g and h) O-GCN, (i) EDAX of O-GCN nanosheets and
( j–m) elemental mapping of O-GCN nanosheets.
GCN are slightly shifted as compared to GCN due to the incor- classification that slit-shaped mesopores are present in GCN
poration of various oxygen moieties. Fig. 4(f) shows the O 1s and O-GCN nanosheets, which can be attributed to the
spectrum having three core levels at 534.10, 532.98 and 531.87 stacking of the thin nanosheets. The surface area of GCN
eV attributable to the C–OH, CvO and OvC–OH groups, was found to be 22.791 m2
g
−1, whereas a relatively lower
respectively.37,49 The atomic percentage calculated for OvC– surface area was observed for O-GCN of about 17.596 m2
OH at 531.87 eV is 70.71%, for CvO at 532.98 eV is 25.12%
−1, which corresponds to the incorporation of oxygenous
g
and for C–OH at 534.10 eV is 4.17%. These binding energy groups onto the O-GCN nanosheets. In addition, the minor
signals reveal that the oxygen species are not simply present as decrease in the surface area for O-GCN nanosheets as com-
absorbed water but as various functional groups on the pared to that of GCN nanosheets indicates greater stability
surface, as a result of the chemical oxidation of GCN. Table S1 of GCN sheets against the oxidation-derived structural
(refer to the ESI†) shows the atomic composition of all the con- changes. Fig. 5(b) represents the surface area plots of GCN
stituent elements present in O-GCN obtained from XPS ana- and O-GCN nanosheets, which show the gradual increase in
lysis, which indicates that about 4.32% of oxygen is incorpor- the specific surface area with an increase in the relative
ated in the O-GCN nanosheets.
pressure.
Temperature-programmed desorption (TPD) measurements
were carried out to determine the acidic and basic strength of
2.3. Surface property studies
The surface area of the as-synthesized GCN and O-GCN the as-synthesized O-GCN nanosheets. The amount of acidity
nanosheets was investigated using Brunauer–Emmett–Teller was estimated by NH3-TPD analysis using ammonia as the
(BET) gas sorption measurements. The nitrogen adsorption– probe, while the amount of basicity was estimated by CO2-TPD
desorption isotherms for GCN and O-GCN nanosheets are analysis using carbon dioxide as the probe. Due to its basic
shown in Fig. 5(a). Both the isotherms for GCN and O-GCN nature, NH3 probe gets easily adsorbed over acidic sites,
represent typical type-IV physisorption and a H3 hysteresis whereas the CO2 probe due to its acidic nature is adsorbed
loop, in accordance with the Brunauer–Deming–Deming– over basic sites. The probes NH3 and CO2 were adsorbed at
Teller (BDDT) classification.50 It can be inferred from BDDT 50 °C over the catalyst surface and the gas was desorbed at
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Fig. 4 (a) Survey spectra of GCN and O-GCN, (b and c) deconvoluted spectra of C 1s and N 1s of GCN nanosheets, and (d–f) deconvoluted spectra
of C 1s, N 1s and O 1s of O-GCN nanosheets.
Fig. 5 (a) N2 adsorption–desorption isotherms for GCN and O-GCN nanosheets and (b) BET surface area plots for GCN and O-GCN nanosheets.
temperatures between 50 and 450 °C. It is quite well known in sesses higher acidic and basic sites as compared to GCN. The
the literature that bare GCN possesses lower acidic and basic NH3-TPD analysis of O-GCN nanosheets also showed two
sites in comparison with its functionalized counterparts.51 As maximas at 310 and 125 °C, ascribable to moderate and weak
reported in our previous article, the CO2-TPD analysis of pure acidic sites. The CO2-TPD analysis of O-GCN shows two
GCN shows three peaks having maxima at 120, 215 and maximas at 300 and 135 °C originating due to the moderate
460 °C, which corresponds to weak, moderate and strong basic and weak basic sites. According to the TPD results, the total
sites, respectively, having a total basicity of 0.0095 mmol g−1 22
.
acidic sites on O-GCN were determined to be 0.059 mmol g−1
.
The as-synthesized O-GCN catalyst was examined for both These acidic sites originate due to the incorporation of
acidic and basic sites using NH3-TPD and CO2-TPD analyses, hydroxyl (–OH) and carboxylic acid groups (–COOH) onto the
as shown in Fig. S1 (ESI†), and it was observed that it pos- O-GCN surface and the total Brønsted basic sites on O-GCN
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Table 1 Optimization of O-GCN catalyzed hydrogen transfer reactiona
were found to be 0.051 mmol g−1, which can be attributed to
the presence of nitrogen sites.
3. Catalytic activity studies
The catalytic potential of the as-synthesized heterogeneous
catalyst, O-GCN nanosheets, was investigated for the hydrogen
transfer reaction of the carbonyl compounds. Herein, we devel-
oped a simple protocol under ambient catalytic reaction con-
ditions for the transfer hydrogenation of carbonyl compounds
with a catalytic amount of base and a metal-free catalyst,
O-GCN in an optimal reaction time using 2-propanol as a
hydrogen source. The catalytic hydrogen transfer reaction was
performed using 4-chlorobenzaldehyde and 2-propanol as
model substrates under the optimized reaction conditions,
viz., 4-chlorobenzaldehyde (1 mmol), isopropanol (5 mL), KOH
(0.12 mmol) and O-GCN catalyst (20 mg) at 80 °C (Scheme 2).
The reaction progress was monitored using thin-layer chrom-
atography (TLC), and the pure product was obtained through
Amount of
base (mmol) (°C)
Temperature Time
Entry Catalyst
(h)
Yieldb (%)
1
2
3
4
5
6
7
8
O-GCN
O-GCN
O-GCN
O-GCN
O-GCN
O-GCN
O-GCN
O-GCN
GCN
0.12
0.25
0.12
0.12
0.25
0.25
0.12
No base
0.12
RT
RT
60
60
60
60
80
80
80
80
80
80
10
10
4
8
4
8
4
10
8
8
15
22
65
78
82
90
92
Trace
9
7
12
18
10
11
12
No catalyst 0.12
No catalyst 0.25
No catalyst No base
8
10
No reaction
a Reaction conditions: O-GCN (20 mg), 2-propanol (5 mL), carbonyl
column chromatography, which was further characterized by compound (1 mmol), base (KOH). b Isolated yields.
NMR spectroscopy. Furthermore, green metrics parameters
were also calculated for the model reaction and are given in
Table S2 (ESI†).
suited for the reaction conditions and further increase in the
catalyst amount does not bring any significant change in the
product yield. Several control reactions were also performed
under the optimized conditions to verify the catalytic potential
of the as-synthesized O-GCN catalyst (entries 8–12); the yields
obtained were relatively very low, which suggests that the sub-
sequent use of both the O-GCN catalyst and base were equally
important for the hydrogen transfer reaction to proceed and
result in higher product yields.
The reaction conditions for the transfer hydrogenation reac-
tions were optimized using 4-chlorobenzaldehyde and 2-propa-
nol as the model reaction by varying various parameters as
shown in Table 1. Initially, the reaction was performed at an
ambient temperature using 0.12 and 0.25 mmol of base (KOH)
(entries 1 and 2), which led to the very low yields and longer
reaction time. In order to increase the product yield and to
reduce the reaction time period, the reaction temperature was
increased to 60 °C (entries 3–6). It was found that upon
increasing the temperature the yield of the product obtained
also increased drastically. The optimized conditions at entry 6,
with 0.25 mmol of base for 8 h shows the highest yield at
60 °C. Further to reduce the amount of base to a catalytic
amount, the reaction temperature was increased to 80 °C
(entry 7) and it was observed that the reaction was completed
within 4 h with the product yield up to 92%. Further decrease
in the amount of base (entry 8) resulted in lower product yield.
Hence, the optimized reaction condition at entry 7, i.e., using
a catalytic amount of base for an optimized reaction time with
high product yields, was considered as the optimized reaction
condition for performing further reactions. The amount of
catalyst used was also optimized as shown in Table S3 (ESI†)
and it was observed that 20 mg of the O-GCN catalyst was most
In order to understand the scope and generality of the
developed catalytic system, O-GCN nanosheets were further
investigated for a wide range of substrates to explore the
effects of steric and electronic variations on the backbone of a
carbonyl substrates. A wide variety of carbonyl compounds
were reduced to their corresponding alcohols successfully with
a high yield and high TON, as shown in Table 2. The detailed
calculations of TON have been presented in the ESI (S3†). It
was observed that the aromatic aldehydes reacted more readily
as compared to the aliphatic aldehydes. In addition, the reac-
tion rates were influenced by the electronic effects of the
different substituents on the carbonyl substrates. Substrates
with electron-withdrawing groups were found to be less reac-
tive (entries 2 and 3) as compared to substrates with electron-
donating groups (entries 4–6). The position of substituents at
the ortho and meta positions (entries 7–10) of the aryl ring
hampered the reactivity and product yields, which can be
attributed to steric effects. The aldehydes containing hetero-
cycle-based substituents, such as indole and thiophene
(entries 12 and 13) were also subjected to transfer hydrogen-
ation; they resulted in good yields, demonstrating the
efficiency of this protocol for the synthesis of various bioactive
molecules. Aliphatic carbonyl substrates (entry 14) resulted in
the low yield of the corresponding alcohol due to the competi-
Scheme 2 Hydrogen transfer reaction between 4-chlorobenzaldehyde
and 2-propanol using the O-GCN catalyst.
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Table 2 (Contd.)
Table 2 Substrate scope for transfer hydrogenation using O-GCN
nanosheets as the catalyst
Sl.
no.
Carbonyl
substrates
Time
(h)
Yielda
(%)
Products
TON
Sl.
no.
Carbonyl
substrates
Time
(h)
Yielda
(%)
Products
TON
10
4
84
71 186
1
4
88
74 576
11
12
4
4
90
80
76 271
67 796
2
4
92
77 996
74 576
3
4
5
4
4
4
88
92
94
13
4
75
63 559
14
15
4
4
62
87
52 542
73 728
77 996
79 661
Reaction conditions: O-GCN (20 mg), 2-propanol (5 mL), carbonyl
compound (>CvO) (1 mmol), KOH (0.12 mmol). a Isolated yields.
tive aldol reaction. Furthermore, reaction with benzophenone
(entry 15) also resulted in high yields, which suggests that the
developed protocol can be effectively utilized for a wide variety
of carbonyl compounds. In addition, the substrates with
reduction-susceptible functional groups, such as nitro,
remained unaffected during the reaction, proving the high
chemoselectivity of the developed protocol.
6
7
4
4
88
82
74 576
69 441
3.1. Mechanism of catalytic hydrogen transfer reactions
A plausible mechanism for transfer hydrogenation is pre-
sented in Scheme 3. In brief, the –CvO species functionalized
on the surface of O-GCN act as catalytically active sites in the
catalyst. The first step involves the deprotonation of 2-propanol
by KOH. Subsequently, potassium can also coordinate to the
reactive –CvO species on the O-GCN surface. Furthermore,
the deprotonated isopropoxide gets attached to the co-
ordinated potassium forming a potassium salt on the O-GCN
surface resulting in the formation of (1). The carbonyl sub-
strate is then activated via (1) through coordination to the pot-
assium forming intermediate (2). At this step, the concerted
8
9
4
4
80
88
67 796
74 576
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Scheme 3 Plausible mechanism of catalytic hydrogen transfer reaction using O-GCN nanosheets as the catalyst.
hydrogen transfer takes place between 2-propanol and carbo- base, the reaction resulted in a corresponding alcohol product,
nyl substrate to yield intermediate (3), accompanied by a sub- but the observed yield was less, which suggests that the hydro-
sequent release of acetone. The intermediate (3) further under- gen for the transfer hydrogenation process comes from 2-pro-
goes alkoxide exchange with a 2-propanol moiety and the panol. The proposed mechanism is quite similar to previously
resulting alcohol is eliminated. The same cycle continues reported mechanisms based on hydride transfer in the
thereafter. In order to confirm the hydrogen transfer process, Meerwein–Ponndorf–Verley reduction of ketones.14,52,53
control reactions with a non-hydrogen donor alcohol (tert-
butyl alcohol) and a non-protic base (K2CO3) were carried out,
as shown in the ESI (S6†). It was observed that no alcohol
product was formed with tert-butyl alcohol, which confirms
4. Recyclability studies
that there must be a hydrogen source for transfer hydrogen-
ation to take place. Also for the reaction with a non-protic
Recyclability, reusability, and long-term stability of a hetero-
geneous catalyst are important characteristic features. In this
Fig. 6 (a) Recycling study of the heterogeneous O-GCN catalyst and (b) PXRD patterns of fresh and recycled O-GCN catalyst.
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regard, recyclability experiments were carried out under the dicyandiamide was heated at 550 °C at a ramp rate of 3 °C
optimized reaction conditions for transfer hydrogenation min−1 in a muffle furnace for 4 h. The sample was allowed to
using 4-chlorobenzaldehyde (1 mmol) and 2-propanol (5 mL) cool to room temperature and then crushed to acquire fine
as model substrates, as shown in Fig. 6(a). After the reaction, powder. The sample was recalcinated at 500 °C with a ramp
the heterogeneous O-GCN catalyst was recovered by centrifu- rate of 5 °C min−1 for 2 h. A yellow colored product, GCN
gation and further washed with DI water and ethanol. The nanosheets, were obtained, which were used directly without
recovered catalyst was then dried in an oven at 80 °C for 12 h any additional purification.
and was further used for five runs. No significant decrease was
6.2.2. Synthesis of oxidized graphitic carbon nitride
observed in the activity of the recycled O-GCN catalyst; (O-GCN). 1 g of the as-synthesized GCN powder was added
however, a slight decrease in the yield was observed after every into a 20 mL mixture solution of concentrated H2SO4 (98%)
cycle, which can be attributed to the loss of the catalyst during and HNO3 (69%) in a 1 : 1 ratio. The resulting mixture was
recovery. The PXRD measurements of the recycled catalyst heated at 40 °C under sonication for 2 h and then 5 mL of
were done and the results are presented in Fig. 6(b). The XRD H2O2 (6%) was added dropwise and the reaction mixture was
patterns of the recycled O-GCN catalyst show a good match further sonicated for 3 h. A whitish product was obtained. The
with that of the fresh catalyst, hence confirming the structural reaction mixture was further poured into 100 mL of ice-cold
integrity and stability of the as-synthesized catalyst.
deionized water. The dilute suspension of the oxidized graphi-
tic carbon nitride (O-GCN) was then filtered and washed with
deionized water and acetone alternatively, and then dried in
an oven at 80 °C for 12 h. Finally, the white-colored O-GCN
nanosheets were obtained.
5. Conclusion
In conclusion, a highly efficient and sustainable protocol
has been developed for the hydrogen transfer reaction of car-
bonyl compounds using a heterogeneous catalyst, O-GCN
nanosheets. A high catalytic performance was observed with
high product yields in a shorter reaction time using 2-propanol
as a hydrogen source. The detailed optimization of reaction
parameters suggests that the use of a catalytic amount of base
along with the O-GCN nanosheets accelerated the reaction rate
and resulted in high product yields, whereas base alone was
not able to sustain the reaction in the forward direction. The
developed O-GCN catalyst shows a high TON, which indicates
its promising potential for large-scale applications. The
O-GCN catalyst remains active under thermal conditions even
after several reaction cycles, suggesting high stability and
6.3. Catalytic activity studies
The catalytic activity of the O-GCN catalyst was studied for
transfer hydrogenation reactions. The reaction was carried out
in a 25 mL sealed tube at 80 °C with magnetic stirring at 500
rpm. For a typical reaction, carbonyl compound (1 mmol),
2-propanol (5 mL), base (0.125 mmol) and O-GCN catalyst
(20 mg) were allowed to react while stirring. The reaction pro-
gress was monitored using thin-layer chromatography (TLC).
After the completion of the reaction, the catalyst was removed
using centrifugation and the reaction mixture was concen-
trated via evaporation under reduced pressure. Then, the reac-
tion workup was done in ethyl acetate–water using a separating
funnel. The organic layer was dried over sodium sulfate.
Finally, the pure product was obtained via column chromato-
graphy using 100–200 mesh silica gel in 10% ethyl acetate/
hexane.
recyclability. This newly developed protocol provides
a
novel, green and sustainable approach for hydrogen transfer
reactions.
6.4. Compound characterization
Phenylmethanol (3a).55 Colorless liquid, 88%; 1H NMR
(500 MHz, DMSO-d6) δ (ppm) 7.42–7.36 (m, 4H), 7.28 (t, 1H,
J = 6.85 Hz), 5.33 (t, 1H, J = 6.15 Hz), 4.62 (t, 2H, J = 6.15 Hz).
13C NMR (125 MHz, DMSO-d6) δ (ppm) 142.7, 128.2, 126.8,
126.6, 63.2.
6. Experimental section
6.1. Chemicals
Nitric acid (69%, Emplura), sulphuric acid (98%, Fisher
Scientific), isopropyl alcohol (99.5%, Sisco Research), hydro-
gen peroxide (6%, Merck), dicyandiamide (99% Sigma-
Aldrich), 99.9% Cambridge isotope dimethyl sulfoxide – d6
(DMSO-d6, 99.8% atom D%, Euriso-top), hexane (Fisher
Scientific) and ethyl acetate (Merck) were bought and used
without any further purification. Deionized water (18.2 MΩ
cm) was obtained from a double-stage water purifier (ELGA
PURELAB Option-R7).
(4-Chlorophenyl)methanol (3b).55 White solid, 92%; 1H
NMR (500 MHz, DMSO-d6) δ (ppm) 7.38–7.36 (m, 2H),
7.33–7.31 (m, 2H), 5.27 (t, 1H, J = 6.2 Hz), 4.47 (d, 2H, J =
6.2 Hz). 13C NMR (125 MHz, DMSO-d6) δ (ppm) 141.6, 131.0,
128.2, 128.0, 62.1.
(4-Nitrophenyl)methanol (3c).56 Mustard yellow solid, 88%;
1H NMR (500 MHz, DMSO-d6) δ (ppm) 8.19 (d, 2H, J = 8.9 Hz),
7.58 (d, 2H, J = 8.95 Hz), 5.54 (t, 1H, J = 5.5 Hz), 4.63 (d, 2H, J =
5.5 Hz). 13C NMR (125 MHz, DMSO-d6) δ (ppm) 151.3, 146.9,
6.2. Materials preparation
6.2.1. Synthesis of graphitic carbon nitride (GCN). A pre- 127.5, 123.9, 62.5.
viously reported procedure was adopted for the synthesis of
p-Tolylmethanol (3d).56 White solid, 92%; 1H NMR
graphitic carbon nitride (GCN) nanosheets.54 In general, 2 g of (500 MHz, DMSO-d6) δ (ppm) 7.18 (d, 2H, J = 8.25 Hz), 7.11 (d,
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2H, J = 7.55 Hz), 5.10 (t, 1H, J = 5.5 Hz), 4.43 (d, 2H, J = 5.5
Diphenylmethanol (3o).29 White solid, 85%; 1H NMR
Hz), 2.27 (s, 3H). 13C NMR (125 MHz, DMSO-d6) δ (ppm) (500 MHz, DMSO-d6) δ (ppm) 7.37 (d, 4H, J = 7.55 Hz), 7.29 (t,
139.5, 135.6, 128.6, 126.5, 62.7, 20.7.
4H, J = 6.9 Hz), 7.26–7.17 (m,2H), 5.88 (d, 1H, J = 4.15 Hz), 5.69
(4-(Dimethylamino)phenyl)methanol (3e).57 White solid, (d, 1H, J = 3.45 Hz). 13C NMR (125 MHz, DMSO-d6) δ (ppm)
94%; 1H NMR (500 MHz, DMSO-d6) δ (ppm) 7.15 (d, 2H, J = 60.8, 32.6, 31.3, 25.3, 22.2, 13.8.
8.95 Hz), 6.69 (d, 2H, J = 8.95 Hz), 4.92 (t, 1H, J = 5.5 Hz), 4.39
(d, 2H, J = 5.5 Hz), 2.86 (s, 6H). 13C NMR (125 MHz, DMSO-d6)
δ (ppm) 149.6, 130.2, 127.8, 112.2, 62.9, 40.3.
Conflicts of interest
(4-Methoxyphenyl)methanol (3f).55 Colorless liquid, 88%;
1H NMR (500 MHz, DMSO-d6) δ (ppm) 7.25 (d, 2H, J = 8.25 There are no conflicts to declare.
Hz), 6.89 (d, 2H, J = 8.25 Hz), 5.12 (t, 1H, J = 5.5 Hz), 4.44 (d,
2H, J = 5.5 Hz), 3.72 (s, 3H). 13C NMR (125 MHz, DMSO-d6) δ
(ppm) 158.7, 135.0, 128.5, 114.0, 63.1, 55.5.
Acknowledgements
(2-Methoxyphenyl)methanol (3g).56 Light yellow liquid, 82%;
1H NMR (500 MHz, DMSO-d6) δ (ppm) 7.36 (d, 1H, J = 7.6 Hz), We are thankful to the Advanced Materials Research Centre
7.22–7.19 (m, 1H), 6.94–6.91 (m, 2H), 4.98 (t, 1H, J = 5.5 Hz), (AMRC), IIT Mandi, for laboratory and characterization facili-
4.48 (d, 2H, J = 4.8 Hz), 3.76 (s, 3H). 13C NMR (125 MHz, ties. Priyanka Choudhary acknowledges the research fellow-
DMSO-d6) δ (ppm) 156.5, 130.8, 128.1, 127.4, 120.6, 110.5, ship from the Department of Atomic Energy (DAE), India. Ajay
58.4, 55.6.
Kumar acknowledges the doctoral scholarship from the
1
(2-Nitrophenyl)methanol (3h).58 White solid, 80%; H NMR Ministry of Human Resource Development (MHRD), India.
(500 MHz, DMSO-d6) δ (ppm) 8.03 (d, 1H, J = 8.25 Hz), 7.82 (d,
1H, J = 6.85 Hz), 7.76 (t, 1H, J = 7.55 Hz), 7.52 (t, 1H, J = 7.55
Hz), 5.55 (t, 1H, J = 5.5 Hz), 4.81 (d, 2H, J = 4.8 Hz). 13C NMR
(125 MHz, DMSO-d6) δ (ppm) 146.9, 138.2, 133.8, 128.4, 127.9,
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