Highly active g-C3N4 as a solid base catalyst for knoevenagel condensation reaction under phase transfer conditions

Article abstract of DOI:10.1039/c7ra03051g

In a promising approach, heterogeneous g-C₃N₄ as a solid base catalyst exhibits appreciable activity in Knoevenagel condensations at room temperature for the synthesis of substituted stilbene in presence of a crown-ether phase transfer catalyst. High yield of the product substituted stilbene were isolated in a very short reaction time period (～30 min). The solid base g-C₃N₄ catalyst was proven to be recyclable for several runs. Various aromatic substrates were screened using heterogeneous base g-C₃N₄ catalyst, exhibiting appreciable corresponding product yield (～99%) at room temperature.

Full text of DOI:10.1039/c7ra03051g

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Highly active g-C3N4 as a solid base catalyst for
knoevenagel condensation reaction under phase
transfer conditions†
Cite this: RSC Adv., 2017, 7, 25589
*
Priti Sharma
and Yoel Sasson
In a promising approach, heterogeneous g-C3N4 as a solid base catalyst exhibits appreciable activity in
Knoevenagel condensations at room temperature for the synthesis of substituted stilbene in presence of
a crown-ether phase transfer catalyst. High yield of the product substituted stilbene were isolated in
a very short reaction time period (ꢀ30 min). The solid base g-C3N4 catalyst was proven to be recyclable
for several runs. Various aromatic substrates were screened using heterogeneous base g-C3N4 catalyst,
exhibiting appreciable corresponding product yield (ꢀ99%) at room temperature.
Received 14th March 2017
Accepted 5th May 2017
DOI: 10.1039/c7ra03051g
rsc.li/rsc-advances
claims synthesis of g-C3N4 carbon nitride mesoporous nano-
particles with high nitrogen content, exhibited appreciable
Introduction
catalytic transesterication of b-keto esters with different alco-
hols at temperature of 110 ꢁC.26 In another work Bitter et al.
reported basic nitrogen containing carbon nanotubes synthesis
(various amount of pyridine nitrogen incorporation) and its
catalytic activity in Knoevenagel condensations at 80 ꢁC.27
Recently, Park and coworkers demonstrated mesoporous
carbon nitride (MCN) as base catalyst for Knoevenagel
condensation of aromatic aldehydes with ethylcyanoacetate by
using microwave irradiation.28
The base catalyzed organic reactions are of great importance in
the chemical process industry. Typical examples are aldol
condensations, Knoevenagel condensations and trans-
esterication reactions.1–4 However, the above mentioned name
reactions typically require stoichiometric amounts of base
along with several reaction conditions such as high reaction
temperature, long reaction time and lack of recyclability.5 Over
the decades researchers have sought solid base catalysts such as
metal oxides6–8 organic materials9 or alkali doped meso-
porous,10 zeolites11 or other solid supported materials.12,13
Nevertheless such materials are oen instable, and suffer from
leaching and deactivation partial decomposition under harsh
reaction condition and low turnover numbers are obtained.14
Recently organocatalysis has emerged as a promising tech-
nology for application in green catalysis applications.15–17 In the
same context, g-C3N4, metal free organocatalyst semiconductors
was recently reported as an efficient and environmentally
benign photo catalyst with working capacity under visible
light.18–22 However, the surface basic nature of g-C3N4 is rela-
tively less explored in organocatalysis application.23 The fabri-
cation of g-C3N4 is based on direct thermal condensation of
high nitrogen content precursor example, urea, cyanamide or
dicyandiamide.24 Consequently, g-C3N4 opens the doors of
Lewis-base characters for coordination chemistry and base-
catalyzed organic reactions.25 In a notable work Vinu et al.
In addition W. Fan et al. described the controlled synthesis
of g-C3N4 with high nitrogen content (modied) and claims
higher product yield (40 ꢁC).29 Further, in a remarkable series of
work Markus Antonietti exhibited the lewis base character of
mpg–C3N4 in the various applications viz.; aliphatic C–H bond
oxidation,30 aerobic oxidative coupling of amines,31 selective
oxidation of alcohols using visible light.32 Recently Markus
Antonietti, extend the application of deprotonated (treated with
aq. tBuOK, KOH or K2CO3) mesoporous graphitic carbon nitride
(mpg–C3N4) catalysts screened for Knoevenagel and trans-
esterication reactions at 110 ꢁC.33 However, the reported
procedure for knoevenagel condensation reaction faces the
drawback of high temperature, long reaction time period and
most of these catalysts are not completely recyclable. In addi-
tion the above fabrication methods are not efficient and not
suitable for large scale applications. Stilbenes and its deriva-
tives are important intermediates in the agrochemical34–36
polymer,37,38 food39 pharmaceutical industries40–42 Moreover
stilbene synthesis reported procedures suffer from long reac-
tion time, high metal content, high base usage, non recyclable
and accidental prone at higher temperature.43,44–46 On the other
hand phase transfer catalysis (PTC) offer, efficient, mild and
environment friendly reaction conditions, with the replacement
of hazardous and sensitive bases or other chemicals.47–52
Casali Center of Applied Chemistry, Institute of Chemistry, The Hebrew University of
Jerusalem, Jerusalem 91904, Israel. E-mail: priti.sharma@mail.huji.ac.il
† Electronic supplementary information (ESI) available: Physical appearance of
g-C3N4, SEM-EDX images g-C3N4, TEM-EDX images g-C3N4, TEM images of
g-C3N4 with different magnication
&
SAED pattern, characterization,
experimental, heterogeneous study of catalyst g-C3N4. See DOI:
10.1039/c7ra03051g
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The characteristics absorption bands of urea (1456, 1578, 3435,
3341 cmÀ1) were disappeared aer transformation into solid
basic g-C3N4 via controlled calcinations (urea at 550 ꢁC for 3 h)
(Fig. S2a and b†). Additionally, the characteristic breathing
mode of the triazine repeating units at 810 cmÀ1, related to the
s-triazine ring absorption band vibrations; which are well
observed in synthesized g-C3N4 pale yellow material (Fig. S2b†).
The strong intense bands at 1625, 1550, 1410 and 1250 cmÀ1
were in good agreement to the reported values of typical
stretching vibration modes of triazine derived repeating units
(Fig. S2b†).56
Scheme 1 g-C3N4 synthesis from urea.
In the present research study we combined the solid base g-
C3N4 with a phase transfer agent to catalyze the Knovenagel
condensation under ambient conditions, to the best of our
knowledge this combination is novel and was not reported
previously. We have examined various governing factors such
as, various phase transfer catalysts (PTC), solvent, temperature
effect and different substrates. The optimized procedure yiel-
ded over 90% yield of several stilbene derivatives (Scheme 1).
The optical properties of synthesized material, solid basic g-
C3N4 were analyzed by UV-vis spectroscopy. Synthesized solid
basic g-C3N4 show maximum absorption below 550 nm (visible
range), which attributes valence band (VB) to the conduction
band (CB) transition (Fig. 2).57
A detail XPS study was carried out for molecular arrange-
ment with chemical properties with systematic formation
analysis of heterogeneous basic g-C3N4 catalyst, (Fig. S3A–C†).
In the displayed C1s spectrum shows two characteristic peaks
corresponding the (–C–N–) and (–N]C–) at 284.8 and 288 eV;
respectively in the fabricated g-C3N4 material (Fig. S3C†). High
resolution N1s spectrum of g-C3N4 the N1s spectrum clearly
deconvoluted into three intense peaks at 399.9, 401.3 and
405.6 eV, respectively (Fig. S3B†). The peak at 399.9 eV is
assigned to the presence of pyridine-N species [sp2-hybridized
nitrogen (C]N–C)], where each nitrogen (N) atom is bonded to
two carbon atoms within aromatic ring located at edges of the
graphite planes of g-C3N4 material (Fig. S3B†). The peak at
401.3 eV corresponds to nitrogen atoms trigonally bonded with
C carbon (sp2 or sp3) atoms of amino functional groups (C–N–
H). Whereas, the peak at 405.6 eV describes the terminal amino
groups (–NH2), which is considered to be strong reason for the
enhancing basic nature to the solid g-C3N4 material (Fig. S3B†).
The morphologies of g-C3N4 material visualized by SEM
spectroscopy with different magnication at 5 mm, 2 mm and 1
Results and discussion
Solid basic g-C3N4 catalyst is formulated via using urea and
following the known reported procedure by Wei Chen et al.53
The obtained solid in pale yellow color g-C3N4 was thoroughly
analyzed by physicochemical characterization techniques
(Scheme 1) (Fig. S1–S8†).
The typical X-ray diffraction (XRD) pattern of g-C3N4 material
is depicted in Fig. 1. The formation of g-C3N4 is clearly observed
in the XRD pattern where aer calcination at 550 ꢁC for 3 h,
intense typical peaks at 13.20ꢁ and 27.20ꢁ appeared (Fig. 1).
These peaks are in good agreement with reported values which
attributes to (100) and (002) diffraction plane, respectively
(Fig. 1).54 The intense XRD peak of solid basic g-C3N4 at 27.2ꢁ
corresponds to the (002) plane and is characteristic for stacking
of the conjugated aromatic system (Fig. 1).55
Fabrication of g-C3N4 from urea is well characterized also by
FTIR spectroscopy and visualized in Fig S2† (a) urea, (b) g-C3N4.
Fig. 1 XRD pattern of g-C3N4.
Fig. 2 g-C3N4 UV-vis spectra.
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mm respectively (Fig. 3A–C). The monomer unit of g-C3N4 and desorption analysis. The calculated BET surface area is
appeared to aggregation of uniform sheet structure, each 15.00 m2 gÀ1, total pore volume (Vp) 0.013 cm3 gÀ1, and mean
particle agglomeration of many small layers in g-C3N4.58 More- pore size (radius) Dv(r) of g-C3N4 is found to be 15.3 nm,
over, the EDX pattern of g-C3N4 indicates the presence of respectively (Fig. S4†).56
elementary carbon and nitrogen in the synthesized solid basic
A classical Knoevenagel condensation reaction, proceeds
g-C3N4 material (Fig. S5†). The morphological representative with stoichiometric amount of base at higher temperature with
TEM images of g-C3N4 material at 1 mm, 100 nm and 10 nm standard substrates.60,61 Here we tried to develop a protocol
scale show crumbled sheets, multilayer with a graphite like under mild reaction condition (precisely at room temperature
structure (Fig. 4A–C).57,59 Furthermore, the SAED pattern shows and short reaction time period) using phase transfer catalyst
a broad ring with lower intensity, supports amorphous nature of (PTC) with heterogeneous g-C3N4 base in nature catalyst to
the synthesized g-C3N4 heterogeneous basic materials (Fig. S6 activate the substrate at room temperature and with minimum
and S7†).
time period to afford the appreciable corresponding yield
The surface textural properties of the synthesized samples selectively.
are determined with the help of the BET multilayer adsorption
In the present study, we exclusively used g-C3N4 basic in
nature as the heterogeneous catalyst with phase transfer cata-
lyst for the Knoevenagel condensation reaction (Schemes 2 and
3). For the efficiency measurement of g-C3N4 with phase transfer
catalyst a model reaction was chosen between 4-nitrotoluene (1
mmol), benzaldehyde (1 mmol), at room temperature (25 ꢁC)
with 18Crown ether 6 (0.1 mmol) (PTC), solvent toluene. (5 mL),
g-C3N4 catalyst (30 mg) (Schemes 2 and 3). Within a short
reaction time period (30 min) the product formation of 4-nitro
trans-stilbene is conrmed with GC in excellent yield (99%).
Interestingly, Knoevenagel condensation reaction progresses
smoothly in presence of basic g-C3N4 heterogeneous catalyst
with PTC in a quite short reaction time period (30 min) at room
temperature (25 ꢁC) to afford the corresponding product yield.
The Knoevenagel reaction has been studied for solvent
impact using different solvents namely the highly polar DMSO,
DMF and non-polar toluene. Reaction was carried out in highly
polar solvent with DMSO and DMF as solvent under optimized
reaction condition [4-nitrotoluene (1 mmol), benzaldehyde (1
mmol), room temperature (25 ꢁC), 18Crown ether 6 (0.1 mmol)
(PTC), solvent (5 mL), catalyst g-C3N4 (30 mg)] resulting in 10%
and 15% yield of corresponding product respectively in 5 h of
reaction time period. Surprisingly with non polar solvent
toluene with g-C3N4 basic heterogeneous catalyst reaction
progress selectively to yield 4-nitrostilbene product with 99%
yield under same optimized reaction conditions within 30 min
Fig. 3 SEM images of g-C3N4 at different magnifications: (A) at 5 mm
g-C3N4, (B) at 2 mm g-C3N4, (C) at 1 mm g-C3N4.
Fig. 4 TEM images of g-C3N4 at different magnifications: (A) at 1 mm
g-C3N4, (B) at 100 nm g-C3N4, (C) at 10 nm g-C3N4.
Scheme 2 g-C3N4 catalyzed Knoevenagel condensation with 18Crown ether using benzaldehyde and 4-nitrotoluene.
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methoxyethoxy)ethane were tested as PTC for the Knoevenagel
reaction under optimized reaction conditions with standard
reactants. The obtained results exhibited ꢀ10% conversion of
substituted E-stilbene. From Table 1 clears the open chain
crown ethers are not much efficient to initiate the reaction in
comparison to closed chain crown ether as PTC (Table 1 entries
7, 8).
Scheme 3 Knoevenagel condensation reaction using g-C3N4 with
To designate the basic role of g-C3N4 heterogeneous catalyst
in a Knoevenagel condensation reaction under optimized
reaction via model reaction using 4-nitrotoluene (1 mmol) and
benzaldehyde (1 mmol), at room temperature (25 ꢁC) with PTC
(0.1 mmol), solvent toluene (5 mL) and g-C3N4 (30 mg) catalyst
is performed. For the reaction progress crosscheck, samples
were withdrawn periodically (15 min) from the reaction mixture
and analyzed by GC. In a controlled experiment without g-C3N4
catalyst reaction is performed under optimized reaction
condition (Table 2, entry 1). The result of controlled experiment
shows no progress of reaction in absence of g-C3N4 catalyst.
Further, another set of controlled experiments were performed
at higher temperature (at 50 ꢁC, 60 ꢁC, 70 ꢁC) without the use of
g-C3N4 using model reaction substrates. The result do not
shows any appreciable progress of reaction (10%). The result
signies that only higher temperature is not sufficient to initiate
reaction, base usage also plays a crucial role in Knoevenagel
condensation reaction (Table 2, entry 2).
A new set of reaction with inorganic base (KOH, K2CO3) were
screened under optimized reaction condition 4-nitrotoluene (1
mmol) and benzaldehyde (1 mmol), at room temperature (25
ꢁC) with 18Crown ether 6 (PTC) (0.1 mmol) and solvent toluene
(5 mL). M. Chidambaram et al. reported64 inorganic base KOH
and K2CO3 reactivity with 18Crown ether 6 (PTC), shows 98%
yield at room temperature, which we also reproduced with 99%
conversion of corresponding product (Table 2, entry 3, 4). But
unfortunately this procedure (use of KOH and K2CO3) does not
provide the recyclability of base which is essential criteria for
environment friendly and industrial process for continuous
production.
PTC at room temperature.
of reaction time period. This might be due to nonpolar solvent
toluene might have easily provided the accessibility of 18Crown
ether 6 (PTC) to g-C3N4 due to nonpolar nature. Therefore,
toluene as solvent is chosen for the rest of the reaction opti-
mization procedures. However without 18Crown ether 6 (PTC)
no product formation was detected under optimized reaction
condition with toluene solvent even aer 5 h.
In the present study, for the establishment the role of phase
transfer catalyst we proceeded with model reaction between 4-
nitrotoluene (1 mmol), benzaldehyde (1 mmol), at room
temperature (25 ꢁC) with PTC (0.1 mmol), solvent toluene (5 mL)
and g-C3N4 catalyst (30 mg). We screened the several phase
transfer catalysts under optimized reaction conditions, viz.;
TBAB (tetrabutyl ammonium bromide), THAB (tetrahexyl
ammonium bromide), THAB (tetrabutyl ammonium iodide),
THAB (tetrahexyl ammonium chloride), THAB (tetraethyl
ammonium chloride) and 18Crown ether 6. The screened
results were summarized in Table 1. From Table 1 result
describe itself clearly among all the screened phase transfer
catalysts only 18Crown ether 6 (PTC) is surprisingly working at
room temperature (25 ꢁC) and affords 99% yield of corre-
sponding product within a short reaction time period (1 h)
(Table 1 entry 6). The appreciable reactivity of 18Crown ether 6
(PTC) is might be due to the cyclic structure owning highly
exible, good binding affinity, and selective, which is more
appropriate to access the g-C3N4 active site, rather than long
aliphatic chain of other phase transfer catalysts (Table 1 entries
1–6). Further, Glym62,63 (open-chain analogues of crown ethers)
viz.; diglyme [bis(2 methoxyethyl)ether], triglyme [1,2-bis(2
From all the conducted experiments it is clear that a combi-
nation of heterogeneous basic catalyst g-C3N4, with 18Crown
Table 1 Phase transfer catalysts screening for Knoevenagel conden- Table
2 Knoevenagel condensation reaction optimization under
sation reactiona various parametersa
Time
(h)
Yield Entry
(%)
Reaction parameter
Time (h)
Yield (%)
Entry
PTC
1
2
18Crown ether 6 (PTC)
18Crown ether 6 (PTC) + heat
(50 ꢁC, 60 ꢁC, 70 ꢁC)
18Crown ether 6 (PTC) + KOH
18Crown ether 6 (PTC) + K2CO3
g-C3N4
g-C3N4 + heat
18Crown ether 6 (PTC) + g-C3N4
(Blank reaction)
12
12
5
1
2
3
4
5
6
7#
8#
Tetrabutyl ammonium bromide (TBAB)
Tetrahexyl ammonium bromide (THAB)
Tetrabutyl ammonium iodide (THAB)
Tetrahexyl ammonium chloride (THAB)
Tetraethyl ammonium chloride (THAB)
18Crown ether 6
12
12
12
12
12
30 min
12
10
5, 10, 10
ꢀ2
ꢀ5
ꢀ2
ꢀ1
99%
10
3
4
5
6
7
8
1
1
12
12
30 (min)
12
99
98
ꢀ1
88
99
00
Diglyme [bis(2-methoxyethyl)ether]
Triglyme [1,2-bis(2-methoxyethoxy)ethane]
12
10
a
a
Reaction conditions: 4-nitrotoluene (1 mmol), benzaldehyde (1
Reaction conditions: 4-nitrotoluene (1 mmol) and benzaldehyde (1
mmol), room temperature (25 ꢁC), PTC (0.1 mmol), solvent toluene (5 mmol), at with PTC (0.1 mmol), solvent toluene (5 mL), base & g-C3N4
mL) and g-C3N4 catalyst (30 mg). # glyme (3 mL).
catalyst (30 mg).
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Table 3 Screening of g-C3N4 catalyst using PTC for Knoevenagel condensation with different substratesa
Entry
1
Substrate 1
Substrate 2
Product
Time (min)
30
Yield (%)
99
2
30
99
3
4
5
6
7
30
30
30
30
30
99
99
85
88
80
8
30
85
a
Reaction conditions: substrate 1 (1 mmol) and substrate 2 (1 mmol), at room temperature (25 ꢁC), 18Crown ether 6 (PTC) (0.1 mmol), solvent
toluene (5 mL) and g-C3N4 basic catalyst (30 mg).
ether 6 (PTC) at room temperature (25 ꢁC) is necessary to
Aer optimizing the reaction protocol for Knoevenagel
improve towards more economic Knoevenagel condensation condensation model reaction and interesting exclusive results
rather than classical condensation reaction (Table 2). The per- within a short reaction time period, we proceeded to screen the
formed reaction establishes a peculiar role of phase transfer g-C3N4 heterogeneous basic catalyst with 18Crown ether 6 (PTC)
catalyst for Knoevenagel condensation reaction at room combinations with different substrates. Various reactants were
temperature (Tables 1 and 2).
analyzed and results are summarized in Table 3. In the referred
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Knoevenagel condensation reaction under optimized condi- However recently Junjie Guo et al. and co workers described the
tions monosubstituted benzaldehyde reacted with 4-nitro- exibility of graphene with 18Crown ether 6 and claims crown
toluene, benzylcynide and 4-nitrobenzylcynide with 99% yield ethers (highly exible, good binding affinity, and selective)
each respectively in a 30 min reaction time period (Table 3, embedded in graphene structure.65 With the good agreement to
entries 1, 2, 3). At the same time when disubstituted benzal- the literature that graphene is analogous to g-C3N4 struc-
dehyde reacts with monosubstituted another substrate viz.; ture;66–69 therefore in the present study 18Crown ether 6 might
benzylcynide the productivity of reaction is not affected and have coordinated with g-C3N4 structure via owning rich exi-
shows 85% of corresponding yield (Table 3, entry 5) of corre- bility and binding affinity for the easy availability of basic site
sponding product (Table 3, entry 5, 6, 8). Since –NO2 substituted (BS+) in organic medium at room temperature. Further Lewis
electron rich substrates are considered to be strong candidate basic site of g-C3N4 deprotonate the acidic (H+) from 4-nitro-
for enhancement of reaction progress (Table 3, entry 1–8). toluene followed by attack of the carbanion on the electrophilic
Further, disubstituted benzaldehyde reacted with disubstituted site of the benzaldehyde to yield the alcohol intermediate
another substrate and shows excellent yield of (18Crown ether resulting subsequently to dehydration to form E-stilbene.
6) exhibit excellent reactivity and not much affected by mono-
Nearly constant product yield was observed (>84%) even aer
substituted and disubstituted substrates (Table 3). It is clearly ve recycling experiments, conrmed that the catalyst is stable
visualized from the Table 3 the synthesized g-C3N4 with phase and the basic nature of the synthesized g-C3N4 heterogeneous
transfer catalyst PTC (18Crown ether 6) efficiently affords catalyst remains active (ESI: S10†) (Fig. 5). For further catalyst
moderate to excellent yields of the corresponding products for stability conrmation, FT-IR analyses of fresh g-C3N4 and spent
Knoevenagel condensation reaction.
g-C3N4 catalyst (ve recycles) was carried out to probe the
Aer systematic protocol assigning for Knoevenagel change of g-C3N4 at molecular level. The Fig. S8† shows the
condensation, we tried to explore a suitable mechanism major peaks of g-C3N4 is well preserved even aer the multiple
involved in the system. According to the earlier reported reuses (1625, 1550, 1410 and 1250, 810, 3100 cmÀ1). Further
mechanism, Knoevenagel condensation initiated with the BET analysis result of spent g-C3N4 catalyst are as follows; BET
deprotonation (either at higher temperature or with PTC) step surface area (SBET), total pore volume (Vp) and mean pore size
from acidic site of substrate (p-nitro toluene) with the help of (radius) Dv(r) is, 23.22 m2 gÀ1, 0.018 cm3 gÀ1 17.356 nm
base.50,51,64 In the present study g-C3N4 catalyst with 18Crown respectively. The observed increase in the surface area and pore
ether 6 (PTC) is rst time studied for the Knoevenagel size in spent catalyst, might be due to the transformation into
condensation. The described reaction most probably might monolithic sheets from bulk g-C3N4 via continues reuse
have followed the mechanism as shown in the Scheme 4. (Fig. ESI 9†). Both the performed characterization suggesting
Scheme 4 Most probable scheme for Knoevenagel condensation by g-C3N4 by using PTC.
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at the measured time intervals and analyzed by gas chroma-
tography. The samples were diluted and ltered with whatman
paper before injecting into a gas chromatograph.
Acknowledgements
Author gratefully acknowledges Hani Gnayem & Ofer for their
help to accomplish the present research work.
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the use of g-C3N4 owning the basic and heterogeneous nature,
with phase transfer catalyst for Knoevenagel condensation
reaction. The catalytic performance is surprisingly enhanced
due to use of phase transfer catalyst precisely 18Crown ether 6
(PTC) upto 99% yield within very short reaction time period (30
min) at room temperature with standard substrate. The newly
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