Sulfonic acid functionalized graphitic carbon nitride as solid acid–base bifunctional catalyst for Knoevenagel condensation and multicomponent tandem reactions

Priyanka Choudhary a, Arghya Sen a, Ajay Kumar a, Suman Dhingra b, C. M. Nagaraja b and Venkata Krishnan *a
aSchool of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Mandi 175075, Himachal Pradesh, India. E-mail: vkn@iitmandi.ac.in
bDepartment of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, India

Received 30th April 2021 , Accepted 14th July 2021

First published on 20th July 2021


Abstract

Rational design and development of acid–base bifunctional heterogeneous catalysts for organic synthesis is a tough process. A highly efficient, non-toxic, metal-free, low-cost, acid–base bifunctional sulfonated graphitic carbon nitride (S-g-C3N4) catalyst has been developed. The as-synthesized S-g-C3N4 catalyst exhibits high catalytic potential towards Knoevenagel condensation and sequential tandem reactions. The as-synthesized catalyst was developed by sulfonation of graphitic carbon nitride (g-C3N4). The sulfonation leads to surface functionalization of –SO3H groups onto the catalyst surface. These –SO3H groups impart acidic nature to the S-g-C3N4 catalyst along with the preexisting basic nature of the catalyst due to the presence of N-containing moieties. These dual acid–base functionalities behave as active sites for the sequential catalytic reactions to occur. The S-g-C3N4 catalyst exhibits high turnover numbers (TON) and high yields in shorter reaction time at optimum conditions of temperature which demonstrates the high catalytic activity of the S-g-C3N4 nanosheets. The corresponding green metrics parameters were also calculated, in addition to demonstrating the excellent catalyst recyclability and reusability. The as-synthesized S-g-C3N4 catalyst provides a metal-free, sustainable and green approach for utilizing acid–base bifunctional catalysts for sequential organic synthesis.


Introduction

The Knoevenagel condensation of aldehydes/ketones with active methylene compounds is an effective and versatile methodology for the synthesis of carbon–carbon double bonds.1 These carbon–carbon bonds have various applications in the synthesis of agrochemicals, cosmetics, pharmaceuticals, fine chemicals, biologically significant heterocyclic compounds and many more.2,3 Conventionally Knoevenagel condensation was carried out between aldehydes or ketones and active methylene compounds using homogenous base catalysts, such as primary and secondary amines, piperdine, pyridine, alkali hydroxides etc.4 These homogenous catalysts have several issues like time-consuming workups, undesirable side products, requirement of high temperature, difficult catalyst recovery and reusability.5 Also, the use of harmful solvents as reaction medium adds up to environmental toxicity. The need for the development of the synthetic strategies using heterogeneous catalysts for Knoevenagel condensation under mild conditions have gained much attention in the past few years. Various heterogeneous catalysts, such as zeolites,6 metal–organic frameworks,7 mesoporous silica,8 zirconia,9 graphene-based materials,10 multiwalled carbon nanotubes11 have been reported for Knoevenagel condensation. Still, several issues exist with their catalytic performance.

Tandem reactions are multistep reactions occurring in one pot. In such cases, consecutive multicomponent reactions take place one after another.12 These processes not only enable a higher atom economy, but also minimize the waste generation and time required for the completion of the reaction, in addition to optimizing the use of solvents and energy consumption.13 Acid–base bifunctional catalysts for Tandem reactions have attracted a lot of attention in recent years. The synergistic effect between the acidic and basic sites plays a crucial in the catalytic process as they can simultaneously activate both electrophiles and nucleophiles in multicomponent reactions.14 However, homogenous acid–base bifunctional catalysts cannot achieve high reaction efficiency due to the self-neutralization of active acidic and basic sites.15 These issues can be resolved by employing heterogeneous acid–base catalysts which will have higher catalytic efficiency and higher product yield. Such catalysts can be designed by functionalizing suitable acidic (e.g., –SO3H) and base sites (e.g., –NH2) onto the support materials, such as graphene, silica, boron nitride, etc. Rationally designed well-defined acidic and basic sites can lead to high performance and also help in understanding reaction mechanism.16

Heteroatom conjugated carbon materials are well-known materials because of their promising applications in organic catalysis, fuel cells, adsorption, separation, energy storage, sensing, etc.17–20 Heteroatom conjugation can alter the geometric and electronic properties of carbonaceous materials by improving the interactions within the carbon framework and increases the number of active sites.21 Among others, nitrogen atom conjugation is found to be more effective due to its comparable atomic size to carbon and its tendency to form strong covalent bonds.22 These nitrogen conjugated materials can serve higher active sites and extrinsic defects which can enhance the catalytic performance of these materials as compared to unconjugated carbon counterparts.23,24 In recent years, carbon–nitrogen conjugated material, graphitic carbon nitride (g-C3N4) has emerged as an excellent heteroatom carbon material for surface functionalization.25 The sheet-like morphology of g-C3N4 provides high surface area for the functionalization of catalytically active sites. g-C3N4 has been employed for various applications, such as organic catalysis, photocatalysis, supercapacitors, fuel cells, industrial catalysis and many more.26–28 g-C3N4 possesses excellent stability, high surface area and exhibits eco-friendly performance. Also, the ease of availability, low cost and metal-free nature make it a suitable candidate for heterogeneous catalysis.29,30 It consists of a strong covalent C–N network in a unique tri-s-triazine pattern, which makes it easier to tune the chemical properties.31 The N and H atoms present g-C3N4 imparts Lewis basic and Brønsted basic properties which can be modified with effective surface functionalization.32 g-C3N4 can be used utilized as an efficient acid–base bifunctional catalyst if along with preexisting basic sites suitable acidic sites are also introduced onto its surface. Sulfonation of g-C3N4 with chlorosulfonic acid (ClSO3H) can result in the functionalization of sulfonic groups onto its surface.33 These sulfonic groups endow Brønsted acid properties to g-C3N4 along with the inherently present Brønsted basic properties due to N atoms. Hence, sulfonated graphitic carbon nitride (S-g-C3N4) can be effectively utilized as a heterogeneous catalyst for acid–base sequential reactions.

In this work, we report a highly efficient, metal-free, heterogeneous acid–base bifunctional S-g-C3N4 catalyst for Knoevenagel condensation and sequential multicomponent tandem reactions under mild and environment friendly conditions. S-g-C3N4 catalyst was rationally designed and developed by effective sulfonation using ClSO3H. A series of S-g-C3N4 catalysts (S-g-C3N4 0.5, S-g-C3N4 1.0, S-g-C3N4 1.5 and S-g-C3N4 2.0) was synthesized by varying the degree of sulfonation. The S-g-C3N4 catalyst was thoroughly characterized by several techniques which confirm the successful synthesis of the catalysts. The series of S-g-C3N4 catalysts exhibit remarkable catalytic activity for Knoevenagel condensation and Tandem reactions. The detailed optimization studies conducted using S-g-C3N4 catalyst show high catalytic efficiency with high yield and high turnover numbers (TON). The optimized reactions were carried out under mild conditions in environmentally benign solvents and the developed protocol was examined for a wide range of substrates. The use of environmentally benign solvents and mild conditions can reduce the amount of waste generated and decrease the overall cost of the process.34 The S-g-C3N4 catalyst exhibited a high atom economy and lower E-factor which makes it a sustainable and environmental friendly catalyst. The use of inexpensive metal-free S-g-C3N4 heterogeneous catalyst for organic synthesis under mild conditions with high catalytic efficacy can pave a way for a greener and sustainable future.

Results and discussion

Synthesis and characterization

The S-g-C3N4 nanosheets were synthesized via successive calcination and sulfonation of dicyandiamide as shown in Scheme 1. The sulfonation of g-C3N4 nanosheets was done by using ClSO3H as sulfonating agent. Different S-g-C3N4 catalysts were synthesized by varying the amount of ClSO3H used which in turn varies the degree of sulfonation onto the catalyst. These catalysts were labeled as S-g-C3N4 0.5, S-g-C3N4 1.0 (S-g-C3N4), S-g-C3N4 1.5 and S-g-C3N4 2.0. The sulfonation of g-C3N4 nanosheets results in the functionalization of –SO3H groups onto its surface.
image file: d1qm00650a-s1.tif
Scheme 1 Schematic representation of the synthesis of g-C3N4 and S-g-C3N4 nanosheets.

Structural analysis of the as-synthesized g-C3N4 and S-g-C3N4 catalysts was done using powder X-ray diffraction (PXRD) and the obtained results are depicted in Fig. 1(a). g-C3N4 nanosheets show two pronounced diffraction peaks at 13.28° and 27.34°. The weak diffraction peak at 13.28° arises due to the interplanar structural packing of tri-s-triazine motif which is indexed to (100) plane having the interlayer distance of 0.675 nm.25 The strong diffraction peak at 27.34° is assigned to characteristic interlayer stacking of rings system which is indexed to (002) plane having the interlayer spacing of 0.326 nm.35 The high intensity of the peak indicates higher crystallinity of g-C3N4 with lesser bulk defects.36 The PXRD patterns for different S-g-C3N4 catalysts (0.5, 1.0, 1.5, 2.0) are similar to that of g-C3N4 as shown in Fig. S1 (ESI). It is observed that on sulfonation the intensity of (002) and (100) peaks decreases remarkably as the degree of sulfonation increases. The (002) peak broadens and shifts to a higher 2θ value showing strong interaction between the –SO3H groups and g-C3N4 nanosheets thereby decreasing the gallery distance having a denser stacking whereas the (100) peak weakens as sulfonation increases. The decrease in the intensity of diffraction peaks can be attributed to the structural changes in S-g-C3N4 nanosheets caused to sulfonation.33,37


image file: d1qm00650a-f1.tif
Fig. 1 (a) PXRD patterns, (b) Raman spectra, (c) FTIR spectra and (d) TGA plots of g-C3N4 and S-g-C3N4 nanosheets.

Raman spectra of the g-C3N4 and S-g-C3N4 catalysts were recorded using a 785 nm excitation source. The Raman bands between 400–1000 cm−1 arise because of the in-plane vibrations of the heterocyclic ring of tri-s-triazine motif.38,39 The broad absorption bands between 1400–1700 cm−1 can be ascribed to sp3 and sp2 hybridized carbons ring stretching vibrations.40 The sharp band at 1236 cm−1 in g-C3N4 is shifted to 1243 cm−1 in S-g-C3N4 which indicates effective sulfonation onto the g-C3N4 nanosheets.33 The FTIR analysis further confirms the –SO3H functionalization over g-C3N4 surface as shown in Fig. 1(c). Both the g-C3N4 and S-g-C3N4 spectra show the presence of characteristic broad peaks between 3000 cm−1 and 3500 cm−1 which are originated due to the N–H stretching vibrations in –NH2 or –NH groups and the characteristic bands between 1000–1200 cm−1 are ascribed to the C–N stretching vibrations of the heterocyclic ring structure.37 The sharp peaks at 803 cm−1 in g-C3N4 belongs to the breathing mode of tri-s-triazine units shifts to 805 cm−1 in S-g-C3N4 confirms the sulfonation of g-C3N4.33 The surface functionalization was further evidenced by the characteristic peaks at 1146 cm−1 and 1069 cm−1 which are ascribed to asymmetric and symmetric stretching of S[double bond, length as m-dash]O groups, respectively.41 This confirms the functionalization of sulfonic groups onto the g-C3N4 layers.

The as-synthesized g-C3N4 and S-g-C3N4 catalysts were subjected to thermogravimetric analysis (TGA) to examine their thermal stability and the obtained results are presented in Fig. 1(d). It can be observed that due to surface functionalization the stability of S-g-C3N4 nanosheets was decreased as compared to that of g-C3N4 nanosheets. g-C3N4 nanosheets were found to be quite stable up to 500 °C with a weight loss of 3.42%. The weight loss of 2.56% was observed up to 140 °C due to the loss of surface adsorbed water molecules. For S-g-C3N4 nanosheets, a greater weight loss of 16.35% was observed up to 500 °C. The first 11.68% weight loss up to 140 °C could be ascribed to a large number of water molecules incorporated onto the S-g-C3N4 surface due to stronger H-bonding between surface functionalized –SO3H groups and the adsorbed water molecules. Another weight loss of about 4.67% from 200–500 °C was observed due to degradation of surface functionalized sulfonic groups. Above 500 °C, the heteroatom C–N ring of the tris-triazine motif starts degrading and finally decomposes at 600 °C. This can be ascribed to the reason that the H-bonding interactions between the terminal amino groups of the tris-triazine ring are not very strong to withstand such a high temperature. Hence after 600 °C the g-C3N4 and S-g-C3N4 nanosheets are completely disintegrated.

The morphological investigations of the as-synthesized catalysts were done using scanning electron microscopic (SEM) analysis and the obtained images are presented in Fig. S2 (ESI) for both g-C3N4 and S-g-C3N4 catalysts. The SEM images of g-C3N4 reveal that it owns a sheet-like morphology wherein the sheets are aggregated. The S-g-C3N4 also possesses a nano-sized sheets-like morphology where sheets are crumbled and stacked. Further, the transmission electron microscopy (HRTEM) was used to examine the nanoscale morphologies of g-C3N4 and S-g-C3N4 catalysts and the obtained results are presented in Fig. 2. TEM images of g-C3N4 and S-g-C3N4 both show sheet-like morphology. The sheets in g-C3N4 catalyst, shows compact morphology, whereas sheets in S-g-C3N4 catalyst shows stacked morphology due to acid–base interactions within the catalyst surface. The energy dispersive analysis of the x-ray (EDAX) spectrum of S-g-C3N4 nanosheets is shown in Fig. 2(e) which confirms the presence of all the constituent elements (C, O, N and S) in the as-synthesized S-g-C3N4 catalyst. Fig. 2(f–j) shows the elemental mapping of S-g-C3N4 nanosheets which confirms the successful sulfur moiety functionalization of g-C3N4 nanosheets.


image file: d1qm00650a-f2.tif
Fig. 2 TEM images of (a and b) g-C3N4 nanosheets and (c and d) S-g-C3N4 nanosheets, (e) EDAX spectrum of S-g-C3N4 nanosheets and (f–j) elemental mapping of S-g-C3N4 nanosheets.

X-Ray photoelectron spectroscopy (XPS) measurements were done to examine the status of constituent elements in g-C3N4 and S-g-C3N4 catalysts and the obtained results have been presented in Fig. 3. The XPS survey spectra of g-C3N4 and S-g-C3N4 catalysts are depicted in Fig. S3 (ESI). The survey spectrum of g-C3N4 exhibits the presence of only C and N elements. Fig. 3(a) represents the C-1s XPS spectrum of g-C3N4, which is further deconvoluted into three distinct peaks. The presence of a characteristic peak at 285.58 eV is ascribed to the sp2 hybridized C[double bond, length as m-dash]C bonds.42 The peak at 287.55 eV is attributed to the C[double bond, length as m-dash]N bond of trigonal C–N network and the intense peak at 289.30 eV arises due to the carbon–nitrogen bond with sp2 hybridization in s-triazine unit.40,43 The N-1s XPS spectrum of bare g-C3N4 can be deconvoluted into multiple peaks as shown in Fig. 3(b). The strong peak at 399.51 eV can be ascribed to the sp2 hybridized nitrogen atom bonded to carbon atoms.25 The peak at 401.01 represents bridged N atoms in (C)3 moieties and the peak at 402.06 eV is assigned to C–N–H bond.44,45


image file: d1qm00650a-f3.tif
Fig. 3 (a and b) Deconvoluted C-1s and N-1s XPS spectra of g-C3N4 nanosheets, (c–f) deconvoluted C-1s, N-1s, O-1s and S-2p XPS spectra of S-g-C3N4 nanosheets.

The XPS survey spectrum of S-g-C3N4 nanosheets reveals the presence of sulfur and oxygen in addition to carbon and nitrogen which confirms the effective –SO3H functionalization on the g-C3N4 nanosheets. Fig. 3(c) represents C-1s XPS spectrum of S-g-C3N4 catalyst which shows the characteristic peaks of g-C3N4 in the as-synthesized catalyst. The additional peak at 285.91 eV corresponds to C–S[double bond, length as m-dash]O bonds formed after –SO3H functionalization.19Fig. 3(d) represents the N-1s XPS spectrum of S-g-C3N4 showing three deconvoluted peaks at 399.47 eV, 400.73 eV and 402.04 eV which can be attributed to sp2 hybridized N-atom in the triazine ring, nitrogen atom in N–(C)3 functionalities and bridging nitrogen atom C–N–H bonds, respectively. As a result of –SO3H functionalization, the peaks corresponding to s-triazine units in C-1s and N-1s XPS spectra of S-g-C3N4 nanosheets are slightly shifted as compared to g-C3N4 nanosheets.46,47Fig. 3(e) represents the O-1s XPS spectrum of S-g-C3N4 nanosheets, the peak at 532.54 eV is ascribed to S[double bond, length as m-dash]O bonds.19Fig. 3(f) represents S-2p XPS spectrum of S-g-C3N4. The deconvoluted peaks at 169.22 eV and 170.53 eV sulfonic (–SO3H) and sulfone (–SO2) groups, respectively.33 Table S1 (ESI) reveals the atomic percentage of all the constituent elements in different S-g-C3N4 catalysts (0.5, 1.0, 1.5, 2.0) obtained from XPS analysis and it was observed that with the increase in the content of chlorosulfonic acid the extent of sulfonation is enhanced, hence the amount of sulfur also increases from S-g-C3N4 0.5 to S-g-C3N4 2.0 catalyst.

Surface area measurements of the g-C3N4 and S-g-C3N4 catalysts were done by using Brunauer–Emmett–Teller (BET) analysis and the obtained plots are presented in Fig. 4. Nitrogen adsorption–desorption isotherms for g-C3N4 and S-g-C3N4 nanosheets are shown in Fig. 4(a and d) which represent typical Type-IV physisorption and a H3 hysteresis loop. According to Brunauer–Deming–Deming–Teller (BDDT) classification both g-C3N4 and S-g-C3N4 catalysts possess slit-shaped mesopores which are caused due to the stacking of thin nanosheets.48 The surface area of g-C3N4 and S-g-C3N4 nanosheets was calculated to be 115.211 m2 g−1 and 88.146 m2 g−1, respectively. The relatively lower surface area of S-g-C3N4 nanosheets can be attributed to the functionalization of sulfonic groups on the g-C3N4 nanosheets which results in acid–base interactions within the catalyst surface which result in agglomeration of sheets.49 Surface area plots of g-C3N4 and S-g-C3N4 catalysts have been depicted in Fig. 4(b and e) respectively, the plots indicate that with the gradual increase in the relative pressure, the specific surface area of catalysts increases.50Fig. 4(c and f) represents the pore size distribution plots of g-C3N4 and S-g-C3N4 catalysts. The obtained mean pore size for g-C3N4 and S-g-C3N4 catalysts were found to be 2.61 nm and 2.63 nm, whereas mean pore volume was found to be 0.113[thin space (1/6-em)]cm3 g−1 and 0.090 cm3 g−1, respectively.


image file: d1qm00650a-f4.tif
Fig. 4 BET plots of (a and d) N2 adsorption–desorption isotherms, (b and e) surface area plots (c and f) pore volume plots of g-C3N4 and S-g-C3N4 nanosheets.

The amount of acidic and basic sites present onto the S-g-C3N4 nanosheets was examined using temperature-programmed desorption (TPD) measurements. Ammonia probe was used for determining the acidity because of its basic nature it can be easily adsorbed on acidic sites by NH3–TPD. Similarly, carbon dioxide probe due to its acidic nature was used for determining basicity by CO2–TPD. The NH3 and CO2 probes were adsorbed on the S-g-C3N4 catalyst at 50 °C and were detached between the temperatures ranging from 50 °C to 400 °C. The NH3–TPD analysis of S-g-C3N4 nanosheets exhibits two distinct peaks at 175 °C and 317 °C ascribable to moderate and strong acidic sites (Fig. 5(a)). The total acidity of S-g-C3N4 was found to be 0.147 mmol g−1. The CO2–TPD analysis shows peaks at 129 °C, 220 °C and 311 °C attributable to weak, moderate and strong (Fig. 5(b)). The total basicity of S-g-C3N4 catalyst was determined to be 0.097 mmol g−1. The acidic sites originate due to surface functionalized –SO3H groups and basic sites are due to the presence of N moieties on the S-g-C3N4 surface.


image file: d1qm00650a-f5.tif
Fig. 5 (a) NH3 TPD profile and (b) CO2 TPD profile of S-g-C3N4 nanosheets.

Catalytic activity

The catalytic potential of the prepared bifunctional S-g-C3N4 catalysts (0.5, 1.0, 1.5, 2.0) was investigated for Knoevenagel condensation and multicomponent tandem reactions. Suitable protocols under ambient reaction conditions using environmentally benign solvents were developed using bifunctional S-g-C3N4 catalyst in an optimal time. The Knoevenagel condensation was conducted using 4-nitrobenzaldehyde and malononitrile as the model reactants under the specific reaction conditions wherein 1 mmol of 4-nitrobenzaldehyde, 1 mmol of malononitrile and 20 mg of S-g-C3N4 catalyst were allowed to react in ethanol (EtOH) at 50 °C as shown in Scheme 2. Further, thin-layer chromatography (TLC) was used to monitor the progress of the reaction. The obtained product was characterized by using nuclear magnetic resonance (NMR) spectroscopic analysis. Moreover, the green metrics parameters were calculated for reactions to examine the sustainability of the developed protocol (Table S2, ESI).
image file: d1qm00650a-s2.tif
Scheme 2 Knoevenagel condensation using 4-nitrobenzaldehyde and malononitrile.

The optimization of reaction conditions for the Knoevenagel condensation was performed using 4-nitrobenzaldehyde and malononitrile (Table 1). At first, the reaction was performed at room temperature (RT) and the reaction progress was monitored at different time intervals (entries 1–3). It was observed that within 30 min majority of reactants were converted into products resulting in high yields. No significant increase in the product yield was observed with increase in time. Furthermore, to increase the product yield the temperature was increased to 50 °C which resulted in high yields (entry 4). Furthermore, in addition to ethanol other environmentally benign solvents, such as water, isopropyl alcohol (IPA), Dimethyl carbonate (DMC) were also investigated for the model reaction (entries 5–7). It was found that the highest product yield was observed in ethanol than other solvents. In addition, the reaction was also carried out in non-green solvents, such as dimethyl sulfoxide (DMSO) and dichloromethane (DCM) and it was observed that reaction resulted in relatively lower yields (entries 8 and 9). Hence it was concluded that the optimized reaction condition with ethanol as solvent at 50 °C (entry 4) was used as a model reaction condition for further reactions. The reaction was carried with different S-g-C3N4 catalysts at the optimized condition as presented in Table S4 (ESI). It was found that the reaction resulted in the highest yield with S-g-C3N4 1.0, which can be attributed to the reason that an appropriate balance between acidic and basic sites is required to complete the desired reaction in optimal time. Hence, S-g-C3N4 1.0 was chosen as the best catalyst and the rest of the reactions were performed with this catalyst. The optimized catalyst amount was also determined as shown in Table S5 (ESI). The highest yield was found with 20 mg of catalyst and a further increase in the amount of catalyst used does not bring an increase in the reaction rate. Under the optimized reaction conditions, various control reactions were performed with bare g-C3N4 nanosheets and without any catalyst to examine their catalytic activity (entries 10 and 11), which resulted in lower and trace yields, respectively. Hence, it was ascertained that S-g-C3N4 nanosheets can efficiently catalyze the C–C double bond formation reaction with high product yields under the optimized conditions.

Table 1 Optimization of S-g-C3N4 catalyzed Knoevenagel condensation reactiona

image file: d1qm00650a-u1.tif

Entry Catalyst Temperature (°C) Solvent Time (min) Yieldb (%)
a Reaction conditions: S-g-C3N4 (20 mg), carbonyl compound (1 mmol), active methylene compound (1 mmol). b Isolated yields.
1 S-g-C3N4 RT EtOH 10 36
2 S-g-C3N4 RT EtOH 20 65
3 S-g-C3N4 RT EtOH 30 87
4 S-g-C3N4 50 EtOH 30 94
5 S-g-C3N4 50 H2O 30 85
6 S-g-C3N4 50 IPA 30 55
7 S-g-C3N4 50 DMC 30 60
8 S-g-C3N4 50 DMSO 30 46
9 S-g-C3N4 50 DCM 30 65
10 g-C3N4 50 EtOH 90 27
11 No Catalyst 50 EtOH 120 Trace


The scope and generality of the optimized S-g-C3N4 catalytic system were examined using a wide range of substrates. Different substituents were varied on the carbonyl and active methylene compounds to investigate the electronic and steric effects. A wide range of arylidene derivatives of active methylene compounds were synthesized using S-g-C3N4 nanosheets with high yields, as presented in Table 2. The TON corresponding to each derivative was also calculated and the detailed calculations have been reported in Section S3 (ESI). It was observed that the rate of reaction was influenced by different electronic and steric variations on the carbonyl substrates. The carbonyl derivatives with electron-withdrawing groups (EWG) (entries 2 and 3) were found to be more reactive and resulted in higher yields in comparison to those with electron-donating groups (EDG) (entries 4–6). The steric effects was examined by varying substituents at ortho and meta positions which resulted in lower yields as compared to para substituents (entries 7–10). The as-developed protocol was also subjected to heterocyclic compounds (entry 11), the reaction resulted in good product yield which is of profound importance from biological perspective for the synthesis of bioactive products.

Table 2 Substrate scope for the synthesis of arylidene derivatives of active methylene compoundsa

image file: d1qm00650a-u2.tif

Sl. No. Carbonyl compound Active methylene compound Product Time (min) Yieldb (%) TON
a Reaction conditions: S-g-C3N4 (20 mg), carbonyl compound (>C[double bond, length as m-dash]O) (1 mmol), EtOH (5 mL), active methylene compound (1 mmol). b Isolated yields.
1 image file: d1qm00650a-u3.tif image file: d1qm00650a-u4.tif image file: d1qm00650a-u5.tif 30 90 3688
2 image file: d1qm00650a-u6.tif image file: d1qm00650a-u7.tif image file: d1qm00650a-u8.tif 30 92 3770
3 image file: d1qm00650a-u9.tif image file: d1qm00650a-u10.tif image file: d1qm00650a-u11.tif 30 94 3852
4 image file: d1qm00650a-u12.tif image file: d1qm00650a-u13.tif image file: d1qm00650a-u14.tif 30 87 3565
5 image file: d1qm00650a-u15.tif image file: d1qm00650a-u16.tif image file: d1qm00650a-u17.tif 30 85 3483
6 image file: d1qm00650a-u18.tif image file: d1qm00650a-u19.tif image file: d1qm00650a-u20.tif 30 84 3442
7 image file: d1qm00650a-u21.tif image file: d1qm00650a-u22.tif image file: d1qm00650a-u23.tif 30 82 3360
8 image file: d1qm00650a-u24.tif image file: d1qm00650a-u25.tif image file: d1qm00650a-u26.tif 30 84 3442
9 image file: d1qm00650a-u27.tif image file: d1qm00650a-u28.tif image file: d1qm00650a-u29.tif 30 88 3606
10 image file: d1qm00650a-u30.tif image file: d1qm00650a-u31.tif image file: d1qm00650a-u32.tif 30 86 3524
11 image file: d1qm00650a-u33.tif image file: d1qm00650a-u34.tif image file: d1qm00650a-u35.tif 30 75 3084


Furthermore, the catalytic potential of acid–base bifunctional S-g-C3N4 catalyst was also examined for sequential multicomponent tandem reactions. The reactions were performed using 4-nitrobenzaldehyde (1 mmol), malononitrile (1 mmol), 2-napthol (1 mmol) and S-g-C3N4 catalyst (30 mg) in EtOH at 50 °C (Scheme 3). The corresponding green metrics parameters were also calculated as shown in Table S3 (ESI).


image file: d1qm00650a-s3.tif
Scheme 3 Multicomponent tandem reactions using 4-nitrobenzaldehyde, malononitrile and 2-napthol.

The optimized reaction conditions for the multicomponent tandem reaction were determined using 4-nitrobenzaldehyde and malononitrile and 2-napthol by varying reaction parameters (Table 3). The reaction was conducted at RT and the reaction progress was monitored at different time intervals (entries 1 and 2). It was found that after 60 minutes only 56% yield was obtained. To complete the reaction in an optimal time, the temperature of the reaction mixture was increased to 50 °C and after 60 minutes about 72% product yield was obtained (entry 3). In order to further increase the yield, reaction time was increased to 120 minutes which resulted in 92% product yield (entry 4). Further increase in reaction time resulted in the formation of undesirable products and decreased product yield. In addition to ethanol, other green solvents, such as water, isopropyl alcohol (IPA), dimethyl carbonate (DMC) were also examined for the model reaction (entries 5–7) which resulted in lower yields as compared to ethanol. The reaction was also carried out in non-green solvents, such as dimethyl sulfoxide (DMSO) and dichloromethane (DCM) and it was observed that reaction resulted in relatively lower yields (entries 8 and 9). Hence it was inferred that the optimized reaction condition with ethanol as solvent at 50 °C (entry 4) could be used as model reaction condition for further reactions. The control reaction with g-C3N4 as catalyst and no catalyst were also done under the optimized conditions (entries 10 and 11) which resulted in trace yields and no reaction, respectively. Table S6 (ESI) shows the optimization of catalyst amount for multicomponent tandem reactions.

Table 3 Optimization of S-g-C3N4 catalyzed multicomponent tandem reactionsa

image file: d1qm00650a-u36.tif

Entry Catalyst Temperature (°C) Solvent Time (min) Yieldb (%)
a Reaction conditions: S-g-C3N4 (30 mg), carbonyl compound (1 mmol), malononitrile (1 mmol), 2-napthol (1 mmol). b Isolated yields.
1 S-g-C3N4 RT EtOH 30 30
2 S-g-C3N4 RT EtOH 60 56
3 S-g-C3N4 50 EtOH 60 72
4 S-g-C3N4 50 EtOH 120 92
5 S-g-C3N4 50 H2O 120 68
6 S-g-C3N4 50 IPA 120 52
7 S-g-C3N4 50 DMC 120 57
8 S-g-C3N4 50 DMSO 120 60
9 S-g-C3N4 50 DCM 120 55
10 g-C3N4 50 EtOH 120 22
11 No Catalyst 50 EtOH 120 Trace


Substrate scope for the multicomponent tandem reactions was also examined by varying different EWG and EDG over carbonyl substrates as shown in Table 4. The reactions resulted in high TON and product yields. It was observed that the different EWG and EDG influenced the rate of reactions and product yields. It was also observed that the carbonyl compounds with EWG (entries 2 and 3) are more reactive than EDG (entries 4 and 5) which demonstrates the effect of steric and electronic variations. The catalytic activity of the as-synthesized catalyst was compared with the previously reported literature reports as shown in Tables S7 and S8 (ESI). This demonstrates the better catalytic potential of our catalyst.

Table 4 Substrate scope for the synthesis of aryl substituted napthopyran derivativesa

image file: d1qm00650a-u37.tif

Sl. No. Carbonyl compound Product Time (min) Yieldb (%) TON
a Reaction conditions: S-g-C3N4 (30 mg), carbonyl compound (>C[double bond, length as m-dash]O) (1 mmol), EtOH (5 mL), malononitrile (1 mmol), 2-napthol (1 mmol). b Isolated yields.
1 image file: d1qm00650a-u38.tif image file: d1qm00650a-u39.tif 120 87 3625
2 image file: d1qm00650a-u40.tif image file: d1qm00650a-u41.tif 120 90 3750
3 image file: d1qm00650a-u42.tif image file: d1qm00650a-u43.tif 120 92 3833
4 image file: d1qm00650a-u44.tif image file: d1qm00650a-u45.tif 120 84 3500
5 image file: d1qm00650a-u46.tif image file: d1qm00650a-u47.tif 120 82 3416


Mechanism of catalytic action of S-g-C3N4 catalyst

A plausible mechanism for the catalytic action of S-g-C3N4 nanosheets is depicted in Scheme 4. The dual acid–base functionalities over the S-g-C3N4 surface acts as the active sites for the catalytic reaction. The acidic –SO3H moieties and basic –N moieties on the S-g-C3N4 surface can activate the nucleophiles and electrophiles, respectively. In the first step, the carbonyl substrates get activated by the basic sites of the S-g-C3N4 catalyst through proton abstraction and is stabilized by the acidic sites (i). Simultaneously, the active methylene compound is also activated by S-g-C3N4 catalyst which attacks the carbonyl carbon (ii) and forms intermediate (iii), which further eliminates OH ion and Knoevenagel condensation product is obtained. In the case of tandem reactions, the cycle further continues without the elimination of the Knoevenagel product (iv). In the meantime, 2-napthol undergoes keto-enol tautomerism and attacks on intermediate (iv) to form intermediate (v). The intermediate (v) further undergoes intramolecular cyclization and gets stabilized to form (vi), which further undergoes hydride shift to form the final product (vii). The final product then gets dissociated and the cycle continues with more carbonyl substrates. The proposed mechanism is found to be in good agreement with previously reported acid–base bifunctional catalysts.51–53
image file: d1qm00650a-s4.tif
Scheme 4 Proposed reaction mechanism for S-g-C3N4 catalyzed Knoevenagel condensation and multicomponent tandem reactions.

Recyclability studies

The recovery and reusability of the developed catalyst is a very important aspect in heterogeneous catalysis. S-g-C3N4 catalyst was subjected to recyclability studies, using the optimized reaction conditions for five cycles as presented in Fig. 6(a). After each cycle, the catalyst was recovered by centrifugation and then washed with ethanol and deionized water several times. The washed catalyst was then dried at 75 °C overnight and was further reused for the next cycle. After each cycle, a slight decrease in the catalytic activity of the recycled S-g-C3N4 catalyst was observed. This decrease can be attributed to the leaching of active sites. In order to analyze the leaching effect and the amount of active sites retained after subsequent cycles, XPS studies were performed on the S-g-C3N4 catalyst recovered after five cycles. The obtained results are presented in Fig. S4 and Table S9 (ESI), which show that after five cycles the amount of sulfur present in S-g-C3N4 catalyst was decreased to 0.57% as compared to the fresh S-g-C3N4 catalyst (0.74%). This suggests that after each cycle, a small amount of the sulfur (–SO3H moiety) got leached during the reaction process.54,55 Hence the observed difference in the product yields obtained in the first cycle (92%) and fifth cycle (79%) could be attributed to the leaching of the small amount of acidic sites. Despite the fact that there was a small amount of leaching of acidic sites, the catalyst showed reasonably good recyclability. The stability of the recovered catalyst was examined using PXRD measurements as shown in Fig. 6(b). It was observed that the XRD patterns of freshly synthesized and recovered S-g-C3N4 catalysts were in a good match with one another which confirms the structural stability of the S-g-C3N4 catalyst.
image file: d1qm00650a-f6.tif
Fig. 6 (a) Recyclability of S-g-C3N4 catalyst for multicomponent tandem reaction, (b) PXRD patterns of fresh and recovered S-g-C3N4 catalyst after five cycles.

Conclusion

In conclusion, a highly versatile and efficient heterogeneous catalyst, S-g-C3N4, was synthesized by surface functionalization of –SO3H groups on g-C3N4 nanosheets. This surface functionalization imparts acid–base dual nature to the as-synthesized S-g-C3N4 catalyst. The bifunctional nature of the S-g-C3N4 catalyst was examined for Knoevenagel condensation and sequential tandem reactions. The S-g-C3N4 catalyst resulted in high yield of desirable products and high TON were observed. The reactions were carried out at optimal time, temperature and in environmentally benign solvents, which makes the developed protocol more sustainable and greener process. The dual acid–base sites on the catalyst surface play an important role in the reaction process, activating both electrophiles and nucleophiles simultaneously resulting in lower reaction times. Also, the S-g-C3N4 catalyst was highly stable and can be reused up to 5 cycles with minor loss in catalytic activity. Hence, S-g-C3N4 nanosheets can be effectively utilized for acid–base sequential reactions under mild conditions and the developed protocol provides metal-free, sustainable and green approach.

Experimental section

Materials preparation

Synthesis of g-C3N4 nanosheets. g-C3N4 nanosheets were synthesized using a previously reported method.56 As a whole, dicyandiamide (2 g) was calcinated at 550 °C for 4 h (ramp rate 3 °C min−1). The obtained bulk g-C3N4 was crushed into a fine powder. This bulk g-C3N4 was further recalcinated at 550 °C for 2 h (ramp rate 5 °C min−1). The recalcination process results in the synthesis of highly exfoliated, light yellow g-C3N4 nanosheets. The yield of the obtained catalyst was 0.6 g.
Synthesis of S-g-C3N4 nanosheets. S-g-C3N4 nanosheets were also synthesized by using chlorosulfonic acid as the sulfonating agent. In brief, 1 g of g-C3N4 nanosheets were added to round-bottomed flask (RBF) along with 50 mL dichloromethane and allowed to stir for few minutes. The chlorosulfonic acid was added drop-wise to the reaction mixture slowly with continuous stirring. The amount of chlorosulfonic acid used varied for different S-g-C3N4 catalysts i.e., for S-g-C3N4 0.5, S-g-C3N4 1.0, S-g-C3N4 1.5, S-g-C3N4 2.0, the amount of chlorosulfonic acid used was 0.5, 1.0, 1.5 and 2.0 mL, respectively. After the addition of sulfonating agent, the mixture was allowed to react for 10 h at room temperature. Finally, the obtained product was washed with deionized water and methanol 5 times. The washed product was dried in an oven at 75 °C for 12 h. The S-g-C3N4 nanosheets were obtained as a light yellow powder.

Catalytic activity studies

Knoevenagel condensation. The Knoevenagel condensation reaction was carried with a carbonyl compound (1 mmol), active methylene compound (1 mmol), EtOH (5 mL) and S-g-C3N4 catalyst (20 mg) in 10 mL RBF at 50 °C with magnetic stirring of 400 rpm. In the end, the catalyst was recovered by centrifugation and the reaction mixture was concentrated by using a rotary evaporator. The final product was purified through column chromatography in an ethyl acetate-hexane mixture.

Tandem reactions

The Tandem reactions were carried with carbonyl compound (1 mmol), active methylene compound (1 mmol), 2-napthol (1 mmol), ethanol (5 mL) and S-g-C3N4 catalyst (30 mg) in 10 mL RBF at 50 °C with magnetic stirring of 400 rpm. In the end, the catalyst was recovered by centrifugation and the reaction mixture was concentrated by using a rotatory evaporator. The final product was purified through column chromatography in an ethyl acetate-hexane mixture.

Compounds characterization

2-Benzylidenemalononitrile (3a)57. White solid, 90%; 1H NMR (500 MHz, CDCl3) δ (ppm) (d, J = 7.55 Hz, 2H), 7.79 (s, 1H), 7.64 (t, J = 7.6 Hz, 1H), 7.55 (t, J = 7.55 Hz, 2H). 13C NMR (125 MHz, CDCl3): 159.9, 134.6, 130.9, 130.7, 129.6, 113.7, 112.5, 82.7.
2-(4-Chlorobenzylidene)malononitrile (3b)57. Light orange solid, 92%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.86 (d, J = 8.95 Hz, 2H), 7.85(s, 1H), 7.51 (d, J = 8.55 Hz, 2H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.3, 141.1, 131.8, 130.1, 129.2, 113.4, 112.3, 83.3.
2-(4-Nitrobenzylidene)malononitrile (3c)58. Off-white, 94%; 1H NMR (500 MHz, CDCl3) δ (ppm) 8.39 (d, J = 8.95 Hz, 2H), 8.08 (d, J = 8.55 Hz, 2H), 7.88 (s, 1H). 13C NMR (125 MHz, CDCl3) δ (ppm) 156.9, 150.3, 135.8, 131.3, 128.0, 124.6, 124.3, 112.6, 111.6, 87.5.
2-(4-Methylbenzylidene)malononitrile (3d)59. Light yellow solid, 87%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.91 (d, J = 8.95 Hz, 2H), 7.65 (s, 1H), 7.01 (d, J = 8.9 Hz, 2H), 3.91 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 164.8, 158.9, 133.4, 123.9, 115.1, 114.4, 113.3, 55.8.
2-(4-(Dimethylamino)benzylidene)malononitrile (3e)57. Bright orange solid, 85%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.82 (d, J = 8.95 Hz, 2H), 7.47 (s, 1H), 6.69 (d, J = 8.9 Hz, 2H), 3.15 (s, 6H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.1, 154.2, 133.8, 119.3, 115.9, 114.9, 111.6, 40.1.
2-(4-Methoxybenzylidene)malononitrile (3f)57. Light yellow solid, 84%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.91 (d, J = 8.9 Hz, 2H), 7.65 (s, 1H), 7.02 (d, J = 8.95 Hz, 2H), 3.92 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 164.8, 158.9, 133.4, 123.9, 115.1, 114.4, 113.3, 55.8.
2-(2-Methoxybenzylidene)malononitrile (3g)60. Off-white solid, 82%; 1H NMR (500 MHz, CDCl3) δ (ppm) 8.32 (s, 1H), 8.19 (dd, J = 7.55 Hz, J = 1.35 Hz, 1H), 7.59 (dt, J = 6.9 Hz, J = 1.4 Hz, 1H), 7.08 (t, J = 7.6 Hz, 1H), 6.98 (d, J = 8.95 Hz, 1H), 3.93 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.9, 154.5, 136.5, 128.9, 121.2, 120.2, 114.3, 112.9, 111.4, 55.9.
2-(2-Nitrobenzylidene)malononitrile (3h)61. Light yellow solid, 84%; 1H NMR (500 MHz, CDCl3) δ (ppm) 8.45 (s, 1H), 8.36 (d, J = 7.55 Hz, 1H), 7.88 (t, J = 7.55 Hz, 1H), 7.81 (t, J = 6.85 Hz, 2H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.8, 146.7, 134.9, 133.4, 130.4, 126.7, 128.8, 112.2, 110.9, 88.5.
2-(3,5-Dibromobenzylidene)malononitrile (3i)61. Light yellow solid, 88%; 1H NMR (500 MHz, CDCl3) δ (ppm) 8.45 (d, J = 1.4 Hz, 2H), 7.91 (t, J = 2.05 Hz, 1H), 7.65 (s, 1H). 13C NMR (125 MHz, CDCl3) δ (ppm) 156.4, 139.4, 135.6, 131.5, 124.2, 114.2, 112.7, 111.5, 86.2.
Ethyl 2-cyano-3-(4-nitrophenyl)acrylate (3j)58. White solid, 86%; 1H NMR (500 MHz, CDCl3) δ (ppm) 8.35 (d, J = 8.95 Hz, 2H), 8.30 (s, 1H), 8.13 (d, J = 8.95 Hz, 2H), 4.42 (q, J = 6.85 Hz, 2H), 1.42 (t, J = 7.55 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 161.4, 151.7, 149.7, 136.9, 131.5, 124.3, 114.5, 107.3, 63.4, 14.09.
2-(Thiophen-2-ylmethylene)malononitrile (3k)62. White solid, 75%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.89 (d, J = 5.5 Hz, 2H), 7.81 (d, J = 3.45 Hz, 1H), 7.27 (q, J = 4.8 Hz, 1H). 13C NMR (125 MHz, CDCl3) δ (ppm) 161.4, 151.7, 149.7, 136.9, 131.5, 124.3, 114.5, 107.3, 63.4.
3-Amino-1-phenyl-1H-benzo[f]chromene-2-carbonitrile (5a)63. White solid, 87%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.81–7.79 (m, 2H), 7.69–7.67 (m, 1H), 7.40–7.38 (m, 3H), 7.27–7.24 (m, 3H), 7.18 (d, J = 7.55 Hz, 2H), 5.24 (s, 1H), 4.57 (s, 2H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.5, 147.1, 144.3, 131.4, 130.7, 129.6, 128.9, 128.5, 127.2, 127.1, 127.0, 125.1, 123.8, 119.8, 116.6, 114.9, 62.4, 38.8.
3-Amino-1-(4-chlorophenyl)-1H-benzo[f]chromene-2-carbonitrile (5b)64. Light yellow solid, 90%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.82–7.79 (m, 2H), 7.61–7.59 (m, 1H), 7.41–7.39 (m, 3H), 7.25–7.20 (m, 2H), 7.11 (d, J = 8.95 Hz, 2H), 5.21 (s, 1H), 4.64 (s, 2H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.6, 147.1, 142.8, 132.8, 131.4, 130.6, 129.1, 128.5, 127.4, 125.2, 123.6, 119.7, 116.6, 114.8, 61.8, 38.3.
3-Amino-1-(4-nitrophenyl)-1H-benzo[f]chromene-2-carbonitrile (5c)63. Mustard yellow solid, 92%; 1H NMR (500 MHz, CDCl3) δ (ppm) 8.13 (d, J = 8.55 Hz, 2H), 7.87–7.82 (m, 2H), 7.53 (d, J = 8.95 Hz, 1H), 7.44–7.40 (m, 2H), 7.35–7.40 (m, 3H), 5.35 (s, 1H), 4.76 (s, 2H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.9, 151.2, 147.1, 146.9, 131.5, 130.4, 130.3, 128.8, 128.0, 127.6, 125.5, 124.4, 123.2, 116.7, 113.4, 60.6, 38.7.
3-Amino-1-(p-tolyl)-1H-benzo[f]chromene-2-carbonitrile (5d)64. Mustard yellow solid, 84%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.79 (t, J = 5.5 Hz, 2H), 7.69 (t, J = 4.1 Hz, 1H) 7.39–7.37 (m, 2H), 7.24 (t, J = 2.75 Hz, 1H), 7.06 (m, 2H), 5.20 (s, 1H), 4.57 (s, 2H), 2.25 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.5, 147.1, 141.4, 136.6, 131.4, 130.8, 129.6, 129.5, 128.4, 127.2, 127.0, 125.1, 123.8, 119.9, 116.6, 115.2, 62.58, 38.42, 21.0.
3-Amino-1-(4-methoxyphenyl)-1H-benzo[f]chromene-2-carbonitrile (5e)63. Mustard yellow solid, 82%; 1H NMR (500 MHz, CDCl3) δ (ppm) 7.79 (d, J = 5.5 Hz, 2H), 7.69 (t, J = 9.6 Hz, 1H) 7.39–7.29 (m, 2H), 7.24 (t, J = 4.8 Hz, 1H), 7.10 (d, J = 8.95 Hz, 2H), 6.78 (d, J = 8.95 Hz, 2H), 5.20 (s, 1H), 4.57 (s, 2H), 3.73 (s, 3H). 13C NMR (125 MHz, CDCl3) δ (ppm) 158.4, 146.9, 136.7, 131.4, 130.7, 129.5, 128.4, 128.2, 127.2, 125.1, 123.8, 119.9, 116.6, 115.2, 114.2, 62.6, 55.2, 38.0.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are thankful to the Advanced Materials Research Centre (AMRC), IIT Mandi for laboratory and characterization facilities. Ajay Kumar acknowledges the scholarship from Ministry of Education (MoE), Government of India.

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00650a
Current address: Department of Chemistry, Indian Institute of Technology Kanpur, Kalyanpur, Kanpur 208016, India.

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