Functionalized Graphitic Carbon Nitride Decorated with Palladium: an Efficient Heterogeneous Catalyst for Hydrogenation Reactions Using KHCO2 as a Mild and Noncorrosive Source of Hydrogen.

Functionalization of the widely known graphitic carbon nitride (GCN) material has been performed, and a novel heterogeneous catalyst is reported by incorporating palladium over the surface of functionalized GCN. GCN was functionalized using an optimized ratio of sulfuric acid, nitric acid, and hydrogen peroxide. The developed catalyst was characterized by powder X-ray diffraction, IR, scanning tunneling microscopy, tunneling electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, thermogravimetric analysis, and solid-state CP-NMR. The developed material containing ≤1% Pd exhibits superior catalytic activity in comparison to other carbon support materials (such as 5% Pd/C) for various hydrogenation reactions under mild conditions. Potassium formate has been chosen as the best hydrogen source among other alkali metal formates. The developed catalyst was also able to catalyze a one-pot three-step reaction for the synthesis of N-benzylaniline which is a precursor of various antihistamine and anticholargenic drugs. Moreover, the catalyst could be recycled multiple times and consistent activity was reported.

Introduction

Overdependence on conventional energy sources has contributed to diminishing resources in near future. In addition, the use of conventional fossil fuels is continuously challenging the ecosystem and changing the global carbon cycle.[1] Growing demand of the energy consumptions and climate change has envisioned the need of clean and sustainable energy.[2] It has become essential to mitigate the adverse effect of fossil fuels on climate; thus, there is an urgent need of sustainable fuel to fulfill future energy requirements. Among other clean energy vehicles, hydrogen is an emission-free fuel that generates water and energy when burned in the presence of oxygen. Hence, hydrogen is considered as a clean energy vehicle and a sustainable fuel for the next generation.[3,4] Hydrogen and fuel cells have the potential to overcome the issue of environmental pollution.[5,6] It is used as a fuel in spacecrafts, electric vehicles, fuel cells, and so forth.[6−8] In general, it requires a large amount of primary energy for the industrial scale production of hydrogen. The most common industrial methods of hydrogen production include steam methane reforming and electrolysis.[9,10] However, the reforming method generates syngas which contains carbon monoxide along with hydrogen.

Hydrogen has been traditionally used for various chemical synthesis processes; however, the safety issues always remained a concern. Although chemical sources of hydrogen have also been in practice, these processes resulted in the issue of pollution and were not considered sustainable. The clean and renewable ways of hydrogen production are water splitting, biocatalyzed electrolysis, fermentative hydrogen production, biomass gasification, and so forth.[11−13] Although hydrogen is a clean energy source, the safety issues pertaining to the storage and transport limit its barrier-free usage. Chemical storage methods have been found to be suitable alternatives to the physical storage methods.[14−16] The conventional chemical hydrogen storage materials are amine-borane, metal hydrides, carbohydrates, formic acid, and so forth.[17−19] Alkali metal formates, especially potassium formate (KHCO2), have emerged as a mild and noncorrosive hydrogen source in comparison to formic acid at high concentrations.[20−23] Alkali formates release H2 on decomposition in the presence of suitable catalysts and convert to bicarbonates, which can be regenerated further to store hydrogen again (Figure ). The released hydrogen can be used for various hydrogenation reactions under ambient conditions. Efforts on decomposition of alkali metal formates to release hydrogen gas have been carried out using various metal-based homogeneous and heterogeneous catalysts.[24−26] In the case of heterogeneous catalysts, metals are generally supported over various materials such as activated carbon, graphene, graphene oxide, graphitic carbon nitride (GCN), and so forth.[27−29] These supports provide dispersion of active catalysts onto the surfaces and also add to the stability.

Figure 1

Schematic representation of the formate–bicarbonate cycle.

Nanomaterial catalysts form a bridge between homogeneous and heterogeneous catalysts, therefore playing an important role in green chemistry and sustainability.[30,31] Several efforts on developing the solid supports for metal nanoparticles have been carried out and led to exploration of various carbon-based supports for catalytic applications. GCN is one of the recently emerged carbon materials used for various applications such as photocatalysis (because of a low band gap of 2.7 eV), heterogeneous organocatalysis (because of the presence of Lewis and Brønsted basic sites), and support material for various metals owing to stability and adsorption capability.[32−35] A review article by Gong et al. critically describes the applications of GCN as a potential catalyst or catalyst support for various reactions in detail.[36] Functionalization of GCN for various applications is also an interesting area to explore, and continuous efforts are being made in this direction. Sulfonic acid functionalization of graphitic carbon nitride (sg-CN) has been successfully carried out by Varma et al.[37,38] Wang’s group modified GCN by grafting the organic moiety onto it and used it for C–H bond activation reaction.[39] Lithium ion-functionalized GCN has been used for high-capacity hydrogen storage by Jena et al.[40] Jiang’s group synthesized polyethylamine-functionalized GCN and claimed it as an effective material for CO2 capture.[41] Krishnan et al. developed functionalized GCN as a catalyst for Knoevenagel condensation, biomass conversion, and synthesis of functionalized indoles.[42−45] Markus Antonietti’s group developed Pd-GCN-catalyzed selective hydrogenation of phenol and its derivatives.[46] Wang’s group also used Pd over the mesoporous GCN surface and for the hydrogenation of quinolone and phenol and semihydrogenation of phenylacetylene.[47−49] The same group also explored the effect of the alkaline metal in selective hydrogenation of phenol.[50] Recently, our group has developed a Ru-gC3N4 catalyst to perform photocatalytic transfer hydrogenation of aldehydes and ketones.[51] We also ventured a Pd-gC3N4 catalytic system for in situ hydrogenation of alkenes and nitro functionalities in the presence of magnesium and water.[52]

To continue our efforts toward sustainable chemistry, herein, we report oxidative functionalization of GCN using sulfuric acid, nitric acid, and hydrogen peroxide mixture. The functionalized GCN (to be used OGCN hereafter) has been used as a support for Pd metal to fabricate a nanocomposite (Pd-OGCN) where the oxidized carbon nitride backbone is decorated with palladium. The synthesized nanocomposite has been found to be an efficient heterogeneous catalyst for in situ hydrogenation of nitro, imine, and alkene functionalities. The Pd-OGCN nanocomposite exhibited better catalytic activity than its counterpart Pd-GCN material. The reason for good catalytic performance can be contemplated because of oxidative surface modification of GCN to OGCN. The extensive functionalization of the GCN surface has been supported by solid-state CP-NMR data also. We assume that the functionalized surface holds the Pd metal strongly and also helps the reactants to interact with the catalytic surface more efficiently. Alkali metal formates are noncorrosive sources of hydrogen gas. Hence, potassium formate has been used as a mild and noncorrosive source of hydrogen in this study. A typical formate–bicarbonate cycle for release and storage of hydrogen gas has been presented in Figure . Formates after releasing H2 in the reaction get converted to bicarbonates. The generated bicarbonates can be recycled to formates by reducing them with hydrogen gas under controlled pressure. Hence, the catalytic cycle proposes the sustainability of the protocol where potassium formate has been used as a noncorrosive source of hydrogen for various hydrogenation reactions.

Results and Discussion

Synthesis of GCN, oxidized graphitic carbon nitride (OGCN), and the Pd-OGCN nanocomposite is presented in Scheme . The obtained materials such as GCN, OGCN, and Pd-OGCN have been characterized by various macroscopic, microscopic, and spectral techniques.

Scheme 1

Schematic Diagram for the Synthesis of GCN, OGCN, and Pd-OGCN

The powder X-ray diffraction (PXRD) patterns of GCN, OGCN, and Pd-OGCN are shown in Figure . The calcination of urea at 550 °C for 3 h resulted in two distinguished peaks at 12.9 and 27.4° because of two different diffraction planes. The intense peak at 27.4° corresponds to the (002) diffraction plane and confirms the π–π stacking and interplaner d-spacing of the aromatic systems, while the weak diffraction peak at 12.9 corresponding to the (100) plane resulted because of interplaner separation. When GCN was oxidized to OGCN, the peak corresponding to the (002) plane was shifted to a slightly lower value of 27.1°, which confirms the change in d-spacing after functionalization of GCN to OGCN. In addition, the 12.9° peak (present in GCN) was also found to shift toward a lower value (12.2°) with a sharp increase in intensity of the peak in the case of OGCN. The sharp and intense peak at 12.2° resembles the pattern of a similar peak in graphene oxide, and this assumption could be correlated with the transformation of GCN to OGCN because of oxidation of the surface. The PXRD patterns of the Pd-OGCN composite show peaks corresponding to the OGCN material but the peak for Pd was not visible because of low loading of the metal and confirmed the successful synthesis of the material. The peak at 27.2° slightly shifts (from 27.1 in OGCN) because of metal incorporation but appears same as in OGCN which predicts no change in interplaner spacing after composite fabrication of Pd with OGCN.

Figure 2

PXRD patterns of GCN, OGCN, and the Pd-OGCN nanocomposite.

The synthesis of OGCN from GCN and Pd-loaded Pd-OGCN was further characterized by Fourier-transform infrared spectroscopy (Figure ). In the IR spectrum of GCN, the broad absorption band in the range of 3100–3300 cm–1 indicates the existence of N–H stretching vibrations, while the absorption bands in the ranges 1380–1400 and 1600–1630 cm–1 correspond to the C–N and C=N stretching vibrations of the aromatic triazine ring system, respectively.

Figure 3

IR spectra of GCN, OGCN, and Pd-OGCN nanocomposite materials.

In the IR spectrum of OGCN, there was a distinctive change in the absorption bands in the ranges of 3000–3300 and 1600–1620 cm–1. The strong absorption band at 1601 cm–1 was observed in OGCN, which confirms the change in C=N stretching modes of GCN after functionalization. Furthermore, the intensity of the band in the range 3000–3300 cm–1 was also enhanced after functionalization of GCN to OGCN, which confirms the OH stretching vibrations of carboxylic or hydroxyl groups in addition to N–H vibrations. In the case of Pd-OGCN, the IR absorption bands are similar to OGCN, which confirms the retention of chemical functionalities after Pd-OGCN composite synthesis.

To find out more information of surface functionalization, we characterized GCN and OGCN by solid-state CP-NMR (Figure ). For the bare GCN, only two peaks are observed at δ 164.0 and δ 155.89, respectively. The peak at δ 164 corresponds to the carbon in the aromatic system attached to the amine group outside of the ring (i.e., C–NH2). Moreover, the peak at δ 155.89 corresponds to the carbon atom attached to bridging nitrogen atoms of heptazine units. In the case of OGCN, the solid-state 13C NMR signal splits into multiple peaks which confirm the extensive surface functionalization of GCN. An increase in the number of signal also suggests the possibility of functionalization both on edges and the interior of the heptazine ring system.

Figure 4

Solid-state 13C NMR of (a) GCN and (b) OGCN materials.

Microscopic analyses of GCN, OGCN, and Pd-OGCN were carried out using scanning tunneling microscopy (SEM), as shown in Figure . It is clearly visible from the SEM images that GCN consists of wrinkled flower morphology (Figure a,b) and OGCN consists of porous bud morphology (Figure c,d). However, the morphology in the case of Pd-OGCN has changed into rod-shaped structures (Figure e,f) along with some porous bud morphology intact. Further morphological analysis was carried out using tunneling electron microscopy (TEM), and images are presented in Figure . TEM analysis of GCN (Figure a) shows wrinkled morphology similar to SEM analysis. The morphology of OGCN also confirmed the porous nature of the material shown (Figure b,c).In the case of Pd-OGCN, the Pd nanoparticles are uniformly distributed onto the OGCN surface (Figure d–f). The presence of Pd nanoparticles has also been confirmed by the high-resolution TEM image of Pd-OGCN (Figure f), and the average size of the nanoparticle has been found to be from 2.5 to 4.5 nm.

Figure 5

SEM images of GCN (a,b), OGCN (c,d), and Pd-OGCN (e,f) nanocomposite materials.

Figure 6

TEM images of GCN (a), OGCN (b,c), and Pd-OGCN (d–f) nanocomposite materials.

Furthermore, the elemental composition of the constituent element present in OGCN and Pd has also been determined using energy-dispersive spectroscopy analysis, as presented in Figures S1 and S2 (refer the Supporting Information). Elemental mapping of the Pd-OGCN material, as shown in Figure S3, confirms the presence and distribution of carbon, nitrogen, oxygen, and palladium in the composite material. The amount of Pd in the material was found to be approximately ≤1% in the EDAX data. The EDAX data were also supported by X-ray photoelectron spectroscopy (XPS) analysis, which also confirmed the amount of Pd to be 0.98% which is almost similar to EDAX data.

To analyze the surface area of the catalytic materials, Brunauer–Emmett–Teller (BET) analysis was performed and data have been presented in Figure . By comparing the adsorption–desorption isotherms of all the partner materials, it can be observed that OGCN has higher adsorption–desorption capability in comparison to GCN. The reason for the higher surface area of OGCN could be functionalization of the GCN surface. The surface areas of GCN and OGCN were found to be 13.124 and 34.148 m2/g, respectively, which clearly supports the abovementioned statement (refer Figure S4). The surface area of Pd-OGCN was found to be 33.691 m2/g, which is almost similar to the surface area of the OGCN material. Moreover, the pore radius and pore volume of GCN, OGCN, and Pd-OGCN were calculated to be 21.4, 16.9, and 16.9 Å and 0.013, 0.33, and 0.27 cc/g, respectively. Values of both the pore radius and pore volume correspond to the surface areas of the materials.

Figure 7

Adsorption–desorption isotherms and pore area analysis of materials.

The XPS technique was employed to know the electronic state of the elements present in the precursor and catalytic materials (Figure ). XPS analyses were performed on bare OGCN and Pd-OGCN materials to see the incorporation and change in electronic environment of elements onto the surface of OGCN after Pd incorporation. Survey spectra (refer Figure S5) of OGCN and Pd-OGCN show the presence of all the constituent elements present in the material. Atomic composition of elements in the materials has been provided in Table S1. Deconvoluted spectra of constituent elements have been compiled and presented in Figure . In the case of OGCN, the deconvoluted spectra of C 1s show two characteristic peaks at 288.5 and 289.1 eV owing to N–C=N and C–N–C components of the material, respectively. The other peaks at 284.9 and 286.3 eV correspond to the C–C and C–O bond systems, respectively, in the material which confirms the oxidation of GCN to form the OGCN material. The deconvoluted spectra of N 1s show peaks at 399.1 and 400.5 eV representing the C=N–C and N(C)3 types of arrangement of nitrogen atoms in the material, respectively. In addition, the low intensity peaks at 404.9 and 406.6 eV correspond to N–O and N=O types of arrangements, respectively. The peaks at 531.6 and 532.2 eV in the spectra of O 1s correspond to C–O and C=O types of linkage, respectively, in the material contemplating the introduction of oxygen functionality (in the form of carboxyl or hydroxyl group) over the surface GCN material. This statement could also be verified by increased oxygen content (10.12%) in the material. Moreover, the presence of two distinct peaks at 165.8 and 169.6 in the deconvoluted spectra of S 2p confirms the presence of −SO2 functionality in the material which could arise because of H2SO4 of the oxidizing mixture (i.e., HNO3, H2SO4, and H2O2) used for the synthesis of OGCN.

Figure 8

XPS data of different elements in OGCN (a–d) and Pd-OGCN (e–h) materials.

The peak positions of elements in Pd-OGCN have been found in the range of 285–290 eV. Deconvoluted C 1s spectra show an intense peak at 288.2 which corresponds to C–N bonds. This peak has observed a shift of 0.3 eV from its peak position in OGCN, which confirmed the change in electronic environment because of the incorporation of Pd metal onto the surface. Another low intensity peak at 288.9 eV corresponds to a typical C–H bond which could be generated because of reduction of COOH groups of the OGCN during Pd-OGCN synthesis. The low intensity peaks at 286.2 eV also observed a slight shift (0.1 eV) from their original position in OGCN, confirming the change in C–O bond energy after the formation of the composite with the Pd metal. The deconvoluted N 1s spectra of Pd-OGCN witnessed only two prominent peaks at 398.8 and 400.2 eV, respectively, corresponding to C=N–C and N(C)3 types of bonding. These peak positions also observed a significant shift (0.3 eV) from their original positions in OGCN spectra (i.e., 399.1 and 400.5 eV). It was interesting to note that the peak for S 2p could not be observed in the case of Pd-OGCN unlike OGCN, where it is approximately 0.53%. This could be due to the removal of the sulfonic group by the action of KHCO2, which was used as a reducing agent in the Pd-OGCN synthesis. The deconvoluted spectra of O 1s in Pd-OGCN also witnessed a shift in the peak positions (531.6–531.3 and 532.2–532.7), which also confirms the change in electronic environment of oxygen atoms because of the incorporation of Pd metal onto OGCN. The Pd 3d spectra confirm the presence of Pd metal on the surface of OGCN, and the loading of the metal was found to be approximately 0.98 wt %. Peak positions of Pd in Pd-OGCN have been found at 335.5 and 340.8 eV, respectively, and representing Pd 5/2 and Pd 3/2 in the zero oxidation state on the material surface. Deconvoluted spectra of Pd-OGCN also confirmed the presence of some amount of Pd(II) in the material contemplating the bonding of Pd with hydroxyl or carbonyl functionalities present on the material surface.

The thermogravimetric analysis (TGA) of GCN, OGCN, and PGCN materials has been presented in Figure . GCN shows slight initial weight loss below 120 °C because of the loss of water molecules in the form of moisture. After that, GCN remains stable until 540 °C and then starts decreasing its weight and decomposes to almost below 5 wt % at 750 °C. In the case of OGCN, the material observes approximately 15% loss in weight until 300 °C and decreases sharply after that and complete decomposition occurs at 750 °C. It can be clearly seen from the TGA graph that OGCN attains stability after making a composite with Pd. The stability of the Pd-OGCN nanocomposite remains intermediate ranging between the stability of GCN and OGCN materials. It is visible that Pd-OGCN also loses an initial weight of 15% because of the loss of adsorbed moisture/water molecules until 150 °C. After that, it remains relatively stable up to 400 °C and loses only 15–20% weight. Thereafter, it decomposes quickly nearby 450 °C and loses approximately 97% weight until 600 °C, suggesting that the remaining amount is pure carbon and metal % in the material.

Figure 9

TGA of GCN, OGCN, and Pd-OGCN materials.

Catalytic Activity Studies

Hydrogenation of Chemical Functionalities

The Pd-OGCN catalyst has been utilized for the formate decomposition cum in situ hydrogenation of chemical functionalities such as nitro to amines, alkenes to alkanes, imine to amine, and so forth under mild reaction conditions. To verify the potential of our developed catalyst for the conversion of nitro functionality to amines, a model reaction of hydrogenation of nitrobenzene to aniline was studied (Table ). Various reaction parameters were optimized to see the best reaction conditions for the developed protocol. The developed Pd-OGCN nanocomposite catalyst showed the best activity among various other carbon support-based catalysts used for hydrogenation of the nitro group to amines (Table , S. nos. 1–12). It was interesting to note that Pd-OGCN with ≤1 wt % Pd showed excellent activity in comparison to similar carbon support-based catalysts such as Pd–C3N4 and Pd/C (i.e., charcoal or activated carbon) catalysts with higher metal contents. Pd-OGCN catalysts with 1% Pd showed better catalytic activity in comparison to Pd-GCN (2% Pd) and Pd/C (5% Pd) catalysts under similar reaction conditions.

Table 1

Optimization of Reaction Conditions for Hydrogenation of the Nitro Group

S. no.	catalyst optimization	metal %	temp.	time (h)	% yield
1	gC3N4 (GCN)	NA	RT	24	0
2	OGCN	NA	RT	24	0
3	Pd-GCN	2	RT	6	81
4	Pd-GCN	2	60 °C	2	92
5	Pd-OGCN	1	RT	6	97
6	Pd-OGCN	1	50 °C	2	95
7	Pd-OGCN	1	60 °C	1	96
8	Pd-OGCN	1	70 °C	1	94
9	Pd-OGCN	0.5	RT	6	58
10	Pd-OGCN	2	RT	3	98
11	Pd-OGCN	5	RT	1	98
11	Pd/C	5	RT	6	80
12	Pd/C	5	60 °C	2	88
13	Pd/charcoal	5	RT	6	73
14	Pd/charcoal	5	60 °C	2	85

Various aromatic nitro compounds could be hydrogenated using this protocol (Table , S. nos. 1–8), and excellent yields of the products were obtained. A majority of substituted nitrobenzenes were hydrogenated to the corresponding substituted anilines in good yields. It was interesting to note that bromo-substituted nitrobenzene led to dehalogenation in addition to hydrogenation, while 4-fluoronitrobenzene resulted in 4-fluoroaniline (Table , S. nos. 2 and 3). This could be correlated to the bond strength of the carbon halogen bond in different halonitrobenzenes. In addition to 4-methoxy and 4-methyl nitrobenzenes, this protocol has been found suitable for hydrogenation of 2-hydroxy and 2-amino nitrobenzene moieties also (Table , S. nos. 7 and 8).

Table 2

Substrate Scope for Hydrogenation of Nitro Functionality by KHCO2 in the Presence of the Pd-OGCN Catalyst at Room Temperature

The Pd-OGCN catalyst also showed excellent activity for hydrogenation of imine and alkenes (Scheme ). The reactions were very quick (i.e., 2–4 h reaction time) unlike hydrogenation of nitrobenzene and were performed under ambient conditions.

Scheme 2

Pd-OGCN-Catalyzed Hydrogenation of Alkene and Imine under Ambient Conditions

Various alkenes were hydrogenated by the Pd-OGCN catalyst using in situ-generated hydrogen to form the corresponding alkanes (Table , S. nos. 1–5). Good yields of ethyl benzene and 1,2 diphenylethane or bibenzyl were reported (Table , S. nos. 1–3) from the hydrogenation of styrene and stilbene, respectively. Both cis and trans stilbene were found to be hydrogenated to bibenzyl. Although allylbenzene could not be hydrogenated to the corresponding alkane under similar conditions, all other conjugated alkenes were hydrogenated to the corresponding alkanes. In addition, the developed protocol could also be extended for the hydrogenation of imines to amines (Table , S. nos. 6–10) in a very short reaction time with excellent yields.

Table 3

Pd-OGCN-Catalyzed Hydrogenation of Alkene and Imine and by KHCO2 at Room Temperature

After getting insights from the hydrogenation of nitrobenzene and imines, we designed a model one-pot reaction for the hydrogenation of nitrobenzene to N-benzylaniline via an imine intermediate (Scheme ). In brief, nitrobenzene was reduced to aniline in the first step followed by in situ reaction with benzaldehyde to form an imine. The in situ-formed imine intermediate in the same reaction mixture was reduced to N-benzylaniline. Hence, the Pd-OGCN catalyst was successfully utilized for a three-step reaction in one pot. The above-obtained N-benzylaniline or its derivatives can be used as a precursor to several drug molecules such as antazole, chloropyramine, tripelennamine, and so forth which are potent antihismaines in nature.

Scheme 3

Pd-OGCN-Catalyzed One-Pot Synthesis of N-Benzylaniline from Nitrobenzene and Benzaldehyde

Finally, recyclability study (Figure ) was carried out for the activity of the Pd-OGCN catalyst and there was no significant loss of activity even after multiple cycles. Recyclability study has been conducted on an optimized model reaction discussed above using 100 mg of the catalyst and 4 mmol of nitrobenzene. Moreover, it can be seen clearly that the catalyst is quite efficient even after multiple cycles and no loss of activity was observed. The apparent loss in the catalyst amount was observed during centrifugation, washing, and recollection from centrifugation tubes after the drying process. The ICP-OES data reconfirmed that there is negligible leaching (0.0004% or 4 ppm) of the metal after five cycles. Furthermore, the TGA and PXRD data of the recycled catalyst also support that there was no apparent change in material after the recycling process and has been presented in Figure S6. Moreover, EDAX spectra of recycled Pd-OGCN (Figure S7) also support the stability of the material, which showed no significant change after recycling and quantification.

Figure 10

Recyclability data of the Pd-OGCN nanocomposite for a model hydrogenation reaction.

Conclusions

In conclusion, we state that the developed Pd-OGCN catalyst has been found to act as an efficient catalyst which leads to decomposition of formates to release hydrogen and results in hydrogenation of various chemical functionalities. The activity of this catalyst for the hydrogenation of nitro, imine, and aromatic olefins confirmed its wide substrate scope and applicability. The Pd-OGCN catalyst has potential to catalyze various other in situ hydrogenation reactions also. Further studies on application of Pd-OGCN for various chemical transformations are currently ongoing in our laboratory.

Experimental Section

GCN Synthesis

Graphitic carbon nitride was synthesized from urea by a conventional method.[53] In brief, 10 g of urea was placed in an alumina crucible and heated up to 550 °C in a furnace for 3 h. After 3 h, the furnace was allowed to cool down to room temperature (RT), and the obtained product (0.45 g) could be used without further purification.

Functionalized Graphitic Carbon Nitride (OGCN) Synthesis

GCN was functionalized with an optimized ratio of sulfuric acid, nitric acid, and hydrogen peroxide using a reported method with some modifications.[54] On account of significant modifications in the synthesis strategy, the material prepared by this method shows novel patterns in powder X-ray. In brief, 1 g of GCN was taken in a 250 mL conical flask, and 20 mL of 1:1 mixture of sulfuric acid and nitric acid was added to it. The mixture was then stirred for 1 h at RT followed by dropwise addition of 6 mL of H2O2 (30% w/w in H2O). The addition of H2O2 witnessed brown color fumes until the addition completed under stirring at RT. After the complete addition of H2O2, the mixture was allowed to stir at RT for another 30 min or until the solution becomes colorless. Finally, 150 mL of deionized water was added to the abovementioned mixture which led to the precipitation of a white color material. The mixture was centrifuged and washed thoroughly multiple times alternatively with water and acetone. The material was dried in an oven at 70 °C overnight and weighed 745 mg in yield.

Palladium-Decorated Functionalized Graphitic Carbon Nitride (Pd-OGCN) Synthesis

In a round-bottomed flask, 1 g of OGCN was taken, and 220 mL of deionized water was added to it. The mixture was stirred for 4–6 h at RT followed by the addition of 12 mL of aqueous KHCO2 solution (2 M) and different amounts (i.e., 11.1, 22.2, 44.5, and 89 mg) of PdCl2 dissolved in 20 mL of H2O. The abovementioned mixture was further stirred at RT for 24 h. After 24 h, the mixture was centrifuged and washed thoroughly with ethanol and deionized water. Finally, the obtained gray-colored material was kept for drying in an oven at 70 °C overnight to yield 700–720 mg of the nanocomposite material. These materials were given the names Pd-OGCN (11), Pd-OGCN (22), Pd-OGCN (44), and Pd-OGCN (89), depending upon the amount (in mg) of PdCl2 used.

Hydrogenation of Chemical Functionalities

In a sealed reaction tube, 1.0 mmol of the substrate (i.e., nitro, alkene, or imine) was placed and dissolved in 5 mL of ethanol. To this, 25 mg of the Pd-OGCN catalyst and 300 mg of KHCO2 were added, and the reactants were stirred at RT-60 °C until the completion of reaction monitored by thin layer chromatography. After completion of the reaction, all the products were extracted with dichloromethane or ethyl acetate and the catalyst was washed multiple times with water and ethanol followed by heating at 70 °C overnight to use in the next cycle.

One-Pot Synthesis of N-Benzylaniline

In a sealed reaction tube, 1.1 mmol of nitrobenzene and 1.0 mmol of benzaldehyde were dissolved in 10 mL of ethanol. To this, 40 mg of the Pd-OGCN catalyst and 500 mg of KHCO2 were added, and the reactants were stirred for 3 h at RT. After 3 h, 4 mL of dichloromethane (DCM) was added to the reaction mixture and stirred at RT for another 2–3 h until reaction completes. The progress of reaction was monitored by thin layered chromatography. After completion of the reaction, the catalyst was filtered off and the product was extracted with DCM or ethyl acetate and dried using a rotary evaporator. The product was confirmed by 1H and 13C NMR spectroscopy.

Note: The abovementioned reaction forms a viscous gel kind of mixture after the reduction of nitrobenzene to aniline. This gel matrix seizes the stirring of the magnetic stir bar and interferes in the reaction to proceed further. Hence, DCM was added to the abovementioned reaction after 3 h.