Synthesis of nitrogen-doped graphene coupled acid fuchsin photocatalyst turns aryl-vinyl into aromatic aldehydes under visible light

Abstract

A schematic strategy is presented to overcome the problem of low photocatalytic performance of graphene. Herein, we synthesized nitrogen-doped graphene (NDG)-coupled acid fuchsin (AF) photocatalyst, i.e.; NDGCAF photocatalyst. The NDGCAF photocatalyst has excellent solar light harvesting ability, band gap suitability, and high molar extinction coefficient than the NDG photocatalyst. Due to these properties, the NDGCAF photocatalyst has the ability to oxidize aryl-vinyl into aryl-vinyl-aldehyde under the irradiation of visible light. In this context, it exhibited the utmost conversion efficiency of aryl-vinyl to aryl-vinyl-aldehyde with a good yield of 98.15%. Current research highlights the significant application of NDGCAF light-harvesting photocatalysts in the research field of organic transformations.

Keywords

Nitrogen-doped graphene (NDG)NDGCAF photocatalyst aryl-vinyl

1 Introduction

Globally, energy is shifting towards green, sustainable, and renewable sources [1 –3], which is a result of efforts being made to solve these issues. A renewed interest in visible light irradiated photo-redox catalysis has been observed in both academic and industrial circles among the different paths investigated [4]. Several considerations, such as visible light’s natural abundance, renewability, ease of use, and eco-friendliness, justify the focus on using it for organic transformations. Because of these properties, visible light is a potential source of energy for accelerating chemical reactions, especially when it comes to organic transformations. Using visible light for photo-redox catalysis is consistent with the larger worldwide trend of industry implementing eco-friendly practices [5, 6]. These initiatives seek to develop a more resilient and sustainable energy environment by utilizing natural resources and reducing their negative effects on the environment. Green chemistry advances could result from ongoing study and development in this area, offering workable answers to the energy problems brought on by industrial and economic expansion [7]. Investigating visible light-mediated processes offers a viable path toward the creation of sustainable inexpensive catalysis and environmentally acceptable energy technologies, especially as the globe works through the challenges of the energy transition [8]. The expensive metal-based photoactive catalysts have the disadvantages of being toxic and having reusability problems, therefore removal of expensive metal-based photoactive catalysts is very difficult, especially in the medicinal industry [9]. A series of ways are available for this organic conversion, which may be extensively separated into enzymatic, chemical methods [10] and ozonolysis along with expensive metal-based photoactive variants [11]. Among these variants, the expensive transition metal is a photoactive catalyst in combination with radical acids along with other oxidizing reagents [12]. The drawback of these processes is safety concerns, expensive and toxic metals, or use of over stoichiometric amounts of oxidants, and would produce a large number of byproducts. In the field of oxidative cleavage of olefins, great progress has been made over the past decades [13]. A lot of efforts have been increased to disconcert the use of different expensive metal catalysts for the oxidative cleavage of C–C double bonds. Additionally, the oxidative cleavage of C–C double bonds is a valuable transformation in organic synthesis, particularly in the production of key intermediates for various industries, including pharmaceuticals and fine chemicals. Traditionally, expensive metal catalysts such as osmium tetroxide (OsO₄) and ruthenium tetroxide (RuO₄) have been employed for oxidative cleavage reactions. However, these catalysts can be costly and may pose environmental and safety concerns due to their toxicity [14 –16]. As far as we are aware, only a few photochemical reports have been received on the photo-oxidative cleavage of C–C double bonds of organic molecules [17].

Therefore, it is highly desirable to develop a safe, and inexpensive metal-free green route for the conversion of substrate into value-added products. Because of its special qualities and potential uses, nitrogen-doped graphene is currently at the center of many experimental and theoretical research projects [18, 19]. In fact, the characteristics result in exceptionally efficient charge and thermal carrier transfer. It should be highlighted that charge transfer and superior charge carrier mobility at interfaces are crucial for the fundamental characteristics of photocatalytic and photovoltaic systems in addition to physical, chemical, and biological systems. Additionally, the BET surface area of nitrogen-doped graphene (NDG) gives the photocatalyst a lot of surface-active sites and makes it easier to generate charge transport carriers, which increases efficiency [20 –22]. The application of NDG-based materials as photocatalysts is limited by their weak photoinduced electron/energy transfer characteristic, even with their advantageous BET surface area [23]. Up till now, NDG has had some degree of success with techniques like band gap engineering and p-/n-type doping [20 –22]. A few encouraging instances of covalent functionalization of NDG with chromophores, including chromophores, have been reported in recent years [23]. This successfully disrupts the gapless NDG p-conjugation, resulting in a significant energy band gap and photoinduced electron/energy transfer from the chromophores to graphene [24, 25]. However, to fully realize the promise of this technique, much research is needed. The effectiveness of NDG-chromophore coupled photocatalysts may be influenced by various aspects, such as photostability; however, the crucial element is the highly efficient light harvesting and energy transfer from chromophores to NDG. Therefore, when creating these photocatalysts, appropriate chromophore systems must be chosen and incorporated. Consequently, it is possible to create an effective NDG-based photocatalyst by utilizing low-cost starting materials to create a customized acid fuchsin (AF) system that is better than the documented chromophores.

In this context, herein, we synthesized firstly nitrogen-doped graphene NDG photocatalyst via thermal method [9] for C–C coupling reaction in the presence of solar light, but NDG photocatalyst is less active for the same due to less solar light harvesting ability and molar extinction coefficient. For the improvement of the solar harvesting ability of NDG, we coupled the solar light active AF dye via condensation method [26], i.e.; NDGCAF photocatalyst (Scheme 1). This photocatalyst has excellent solar light harvesting ability, suitability of the band gap, reusability properties, and slow recombination charges [27]. Therefore, the newly designed NDGCAF photocatalyst is more applicable for the aerobic C–C double bond cleavage (Scheme 1). In photocatalysis, we speculated the use of UV visible light for the aerobic C–C double bond cleavage under much milder conditions. In this article, we demonstrated that we have achieved the strategy for the direct oxidative cleavage of the C–C double bond of Vinyl pyridine to the corresponding aldehyde using visible light irradiation and air as the source of oxidant (Scheme 1).

Scheme 1

Schematical representation of the conversion of Vinyl pyridine into Nicotinaldehyde by NDGCAF photocatalyst.

2 Experimental section

2.1 Materials

Glucose (G), melamine (M), triethylamine (TEA), ortho dichlorobenzene (ODCB), dimethylformamide (DMF), acid fuchsin (AF), NDGCAF photocatalyst was prepared by using the following materials.

2.2 Instruments and measurements

UV-visible analysis was done via Shimadzu Spectrophotometer (UV-visible 1900i), For the confirmation of different functional groups and the nature of chemical bonds, Fourier transform infrared spectroscopy (FTIR) was done by IR spirit Shimadzu FT-IR-8000. To investigate the X-ray diffraction (XRD) pattern a powder X-ray diffractometer (D8 Advance Eco made by Bruker, Germany) was used. The surface structure and extent of the element were obtained by Scanning electron microscope (SEM) images & electron dispersive X-ray spectroscopy (EDS) on JSM 6490 LV (made by JEOL, Japan). For the calculation of zeta-potential and particle size Nano-zeta sizer (NZS90) was used and Thermogravimetric analysis (TGA) was examined on Q 50 V20.13 Build 39 (made by the new castle, USA) BRUKER AVANCE NEO 500 MHz was used for 1H NMR.

2.3 Synthesis of nitrogen-doped graphene

Initially, a homogeneous mixture of glucose (G) and melamine (M) in a 1:1 ratio was prepared using a mortar and pestle. Subsequently, the dried mixture was introduced into a muffle furnace and covered with aluminum foil for the carbonization process. The muffle furnace was initially set from ambient temperature to 600°C at a ramping rate of 2.5°C/min under an inert atmosphere. After two hours, the temperature was further increased from 600°C to 800°C over one hour, with a ramping rate of 1.67°C/min in the inert atmosphere. The end result was the production of a gray powder, namely Nitrogen-doped graphene (NDG). (Scheme 2) [28].

Scheme 2

Synthesis of Nitrogen-doped Graphene.

2.4 Synthesis of NDGCAF photocatalyst by condensation method

In the synthesis process of the NDGCAF photocatalyst depicted in Scheme 3, the procedure unfolds as follows: To begin, a pre-dried round-bottom flask is utilized. In this setup, 200 mg of the pre-prepared ‘NDG’ is introduced as the base material, along with 325 mg of ‘AF,’ which functions as the light-absorbing chromophore. Following this, 1 mL of DMF (dimethylformamide) and 10 mL of ODCB are added to the combined powder, along with a catalytic measure of Triethylamine (TEA) (0.5 mL). This mixture undergoes reflux at a temperature of 183°C for a duration of 96 hours. In the subsequent step, the solution resulting from the reflux process undergoes filtration, followed by thorough rinsing with an aqueous solution. The remaining residue is then dried in an oven, resulting in the acquisition of the desired product, namely, the NDGCAF photocatalyst [25].

Scheme 3

Synthesis of NDGCAF photocatalyst.

2.5 Photocatalytic reaction for the conversion of vinyl pyridine to Nicotinaldehyde

A model reaction was run to verify the feasibility of aerobic C–C double bond cleavage presuming visible light photo-redox catalysis by stirring vinyl pyridine and NDGCAF photocatalyst in different solvents and without solvent under visible light irradiation (blue LED light 18 W) at ambient temperature for overnight in a cylindrical flask. The equivalent vinyl pyridine was acquired (Table 1, entry 8) for the creation of the aldehyde. We concentrated our efforts on determining the shortest amount of time needed to complete the reaction, which was discovered to be 9 h. Moreover, we performed the photocatalytic reaction in the different solvents (DMSO, DMF, CH₃CN, and THF), and we got greater yield in the absence of the solvent. It is clear from the result of our first reaction that our speculation regarding the photo-redox catalysis to aerobic C–C double bond cleavage, was enhancing and reinforced. Encouraged by the outcome, we carried out some control tests to verify the crucial reactional variables, including the catalyst, air, and visible light. Traces (negligible) amounts occurred when photocatalyst and solar light were absent (Table 1 entry 6 and 7) respectively [29, 30].

Table 1
Verifications of the feasibility of C–C double bond cleavage in different reaction conditions

Entry Photocatalyst Light Aerobic Condition Solvent Yield (%)

1. NDGCAF On Present DMSO 88

2. NDGCAF On Inert atmosphere DMSO 10

3. NDGCAF On Present DMF 80

4. NDGCAF On Present CH₃CN 78

5. NDGCAF On Present THF 58

6. No Photocatalyst On Present DMSO 05

7. NDGCAF Off Present No solvent 05

8. NDGCAF On Present No solvent 98.15


1.	NDGCAF	On	Present	DMSO	88
2.	NDGCAF	On	Inert atmosphere	DMSO	10
3.	NDGCAF	On	Present	DMF	80
4.	NDGCAF	On	Present	CH₃CN	78
5.	NDGCAF	On	Present	THF	58
6.	No Photocatalyst	On	Present	DMSO	05
7.	NDGCAF	Off	Present	No solvent	05
8.	NDGCAF	On	Present	No solvent	98.15

Hereby, the presence of a photocatalyst and visible light are necessary requirements for the reaction. The reaction was inhibited when performed under the atmosphere of N₂ (Table 1 entry 2). The mandatory condition for completion of the reaction is visible light, air, and photocatalyst (NDGCAF) and establishing the photocatalytic model of the reaction. As we can see the DMSO is the best solvent, and the quantitative improvement of the catalyst was carried out. The obtained yield of aldehyde was found 88% and it was best with 1 mole % of catalyst (Table 1, entry1). But the special fact of our research paper that makes photooxidative cleavage of C–C double bond cleavage even more interesting is the neat (no use of solvent) condition. It was found that the conversion (Scheme 1) takes place in a good yield of 98.15% (Table 1, entry 8) in the same situation but without the use of solvent [29]. Additionally, we added (Fig. 1) the ¹H-NMR spectra of the photocatalytic products (Nicotinaldehyde) 1H NMR (500 MHz, Acetone) δ 10.19(s,1H), 8.86(s,1H), 8.85(s,1H), 8.24(d,1H), 7.60(d,1H).

Fig. 1

¹HNMR spectrum of Nicotinaldehyde.

2.6 Mechanistic route for the formation of Nicotinaldehyde via NDGCAF photocatalyst

From the literature lead, and on the basis of our experimental result a reasonable mechanism for the aerobic C–C double bond cleavage is depicted in Scheme 4. A singlet oxygen reaction pathway can be excluded; besides we know that photo-oxidative cleavage of aryl-substituted ethylene does not involve singlet oxygen. NDGCAF is photoexcited through visible light, and the obtained excited state NDGCAF* takes part in one-electron oxidation of 2-vinyl pyridine 1 to convert into alkene radical cation (1.⁺) additionally, the radical anion of NDGCAF transfer an electron to the reaction’s oxygen to create superoxide radical anion. The obtained alkene radical cation (1.⁺) goes through [2 + 2] cycloaddition with superoxide radical anion to provide a dioxetane 3, which eventually goes through oxidative cleavage to produce the aldehyde (3) [31].

Scheme 4

Mechanism of photo-oxidative cleavage of C–C double bond.

3 Results and discussion

3.1 UV-visible spectra and fourier transform infrared spectroscopy

The UV-visible spectra of NDG, and NDGCAF photocatalyst, along with that of AF for comparison are shown in Fig. 2(a). ‘NDG’ has negligible absorbance in the visible range of 400 nm to 800 nm. On the other hand, the Q-bands of AF have (556 nm) broader peaks, after the coupling of NDG with AF the absorbance increases with redshift in the NDGCAF (567 nm) at the same concentration. NDGCAF photocatalyst has an excellent optical band gap (2.18 eV), determined by the Scherrer equation (1240/rightthreetimes). The NDG-coupled AF photocatalyst generates a donor-accepter type chromophore that enhances both the photocatalytic reaction and solar light conversion [25, 32]. The Fourier transform spectra (FTIR) depicted in Fig. 2(b) were employed to elucidate the linkage and functionality of AF, NDG, and the NDGCAF photocatalyst. In the spectra, the peak at 800 cm⁻¹ signifies the vibration mode (breathing mode) of the C–N bond within the triazine ring [28]. Various peaks emerge at 1566 cm⁻¹, 1640 cm⁻¹, 2060 cm⁻¹, and 2918 cm⁻¹, corresponding to C = C and C = O, N–H, and C–H stretching and vibration in the NDGCAF photocatalyst [33]. All vibrational peaks observed in the NDGCAF photocatalyst are analogous to those in NDG. Conversely, the peaks at 1578 cm⁻¹ and 1620 cm⁻¹ in AF denote the symmetric vibration of carboxylic and phenolic groups. Notably, the carboxylic group stretching band is entirely absent in the NDGCAF” photocatalyst [34].

Fig. 2

(a) The absorption UV-visible spectra NDG (black color Line), AF (red color line) and, NDGCAF photocatalyst (blue color line) and, (b) The transmittance infrared spectroscopy of AF (red color line), NDGCAF (blue color line) and, NDG (black color line) respectively.

3.2 Morphological studies and nature of the NDGCAF photocatalyst

The FESEM images of NDG and NDGCAF photocatalysts allowed for the observation of morphological changes. In Fig. 3(a), NDG exhibits a sheet-like structure. Upon coupling with AF dye, soft, floppy-type materials adhere to the sheet-like structure of Nitrogen-doped graphene (NDG), resulting in the formation of the NDGCAF 3(e)photocatalyst. Furthermore, we perform elemental mapping for the elemental composition detection. Here, in Fig. 3 (b–c) we obtained three different color mapping for Carbon(C), oxygen(O), and nitrogen(N) in the NDG photocatalyst. After the coupling of ‘AF’ to the ‘NDG’ we observed an additional color mapping sulfur (teal color) was observed which confirm the light harvesting chromophore coupled with to the NDG photocatalyst.

Fig. 3

(a) SEM image of NDG, (b) Carbon (red color) of NDG, (c) Oxygen (gold color) of NDG, Nitrogen (green color) of NDG and, (e) SEM image of NDGCAF photocatalyst, (f) Carbon (red color) of NDGCAF, (g) Oxygen (gold color) of NDGAF, (h) Sulfur (teal color) of NDGCAF and (i) Nitrogen (green color) of NDGCAF.

The nature of the catalyst was revealed through the X-ray diffraction pattern (XRD) of the photocatalyst. In Fig. 4(a), the crystalline nature of the NDGCAF photocatalyst was confirmed by the presence of a sharp peak. Within the NDGCAF photocatalyst, two distinct peaks were observed at 2θ values of 18° and 28°, corresponding to the (002) plane. The interlayer d-spacing of these peaks was measured at 4.04 nm and 1.64 nm, respectively. The broad peak at a 2θ value of 14° indicated a relatively low crystalline degree and a few-layer structure of the NDG with AF. Conversely, in the case of NDG, sharp peaks at 2θ values of 27° were identified, associated with the (002) graphitic plane [28]. Moreover, we conducted Raman spectra analysis to validate the binding of ‘AF’ to the ‘NDG’ sheet in Fig. 4b. In accordance with a previously reported article [28], nitrogen-doped graphene (NDG) derived from ‘M’ and ‘G’ exhibits two bands (D and G) at 1346 cm⁻¹ and 1580 cm⁻¹, respectively. Subsequent to the integration of ‘AF’ with the ‘NDG’ sheets, these bands shifted to 1360 cm⁻¹ and 1606 cm⁻¹. Specifically, the D-band experienced a shift of 14 cm⁻¹, while the G-band shifted by 26 cm⁻¹. This observed band shift serves as a confirmation of the covalent attachment of ‘AF’ to ‘NDG’. It is essential to note that the up-shifts in the D and G bands are indicative of the formation of newly created carbonyl amide groups [28].

Fig. 4

a) XRD spectra of NDG (black color line) and NDGCAF (blue color line) and b) Raman spectra of NDGCAF photocatalyst (blue colour line).

3.3 Thermogravimetric analysis of the NDGCAF photocatalyst

The thermogravimetric analysis (Fig. 5) was employed to explore the thermal characteristics of NDGCAF within the temperature range of 20°C to 800°C. Up to 580°C, NDGCAF exhibits a gradual weight reduction with rising temperature. In the interval from 25°C to approximately 580°C, a consistent weight loss of up to 17% occurs, attributed to the elimination of water/solvent content. Subsequently, a substantial weight loss is observed between 580°C and 780°C. The NDGCAF photocatalyst demonstrates thermal stability until 580°C, undergoing complete decomposition around 800°C. In contrast, nitrogen-doped graphene experiences its initial weight loss at 300°C, with a major loss occurring at 551°C [35]. Consequently, we can readily infer that our synthesized AF-coupled NDG photocatalyst exhibits higher thermal stability compared to NDG. This stability renders it suitable for the oxidative cleavage of C–C double bonds, a valuable transformation in organic synthesis.

Fig. 5

Thermogravimetric analysis of Nitrogen-doped graphene-based acid fuchsin photocatalyst (NDGCAF).

3.4 Particle size and zeta potential

Zeta potential (ZP) is influenced by the composition of the NDGCAF photocatalyst. ZP analysis was conducted using dynamic light scattering (DLS) methods. DLS offers superior results to other approaches in terms of resolution and dependability. This parameter displays the repulsion between charged particles in the dispersion. Due to electrostatic repulsion, highly charged particles, which are associated with high ZP, prevent particle aggregation. When the ZP is low, attraction triumphs over repulsion, and the mixture is likely to coagulate [36]. A nano dispersion is thought to be best stabilized at a ZP value of −30 mV. Therefore, the higher negative value of zeta potential of NDGCAF photocatalyst with −31.8 mV (Fig. 6b) in comparison to NDG with −28.5 mV (Fig. 6a) was observed. This clearly indicates the coupling of NDG and AF and the formation of NDGCAF photocatalyst [37].

Fig. 6

Zeta potential (a and b) and particle size (c and d) of NDG and NDGCAF photocatalyst respectively.

For the investigation of the average particle size of NDGCAF photocatalyst dynamic light scattering techniques were used. The average size of the photocatalyst was 295.1 d.nm (Fig. 6d) which is much smaller than NDG. Whereas the average size of NDG was 367.4 d.nm (Fig. 6c). From the report [28], Due to charge carrier excitation and surface-active sites small-size photocatalyst have higher efficiency than bigger one towards photocatalytic activity. Consequently, the synthesized NDGCAF photocatalyst is highly efficient than NDG [38].

4 Conclusion

To sum up, we introduce here a newly designed acid fuchsin nitrogen-doped graphene-based photocatalyst (NDGCAF) for the photo-oxidative cleavage of C–C double bond. For this, we made a budget-friendly approach by the modification via using an organic dye (acid fuchsin, AF) as a dopant which is easily available and cheap. The photocatalytic performance of NDGCAF photocatalyst in visible light is higher than NDG because it exhibits extended conjugation of π bond and small band gap. Owing to the excellent thermal stability NDGCAF photocatalyst demonstrates high catalytic efficiency through numerous cycles of photocatalytic activity. Furthermore, the photo-oxidative process via O₂ with NDGCAF photocatalyst was demonstrated by several experiments (Table 1). Most importantly, the specialty of this new work, there is no solvent is used during the conversion of aryl-vinyl into aryl-aldehyde. Thus, in this paper, we successfully demonstrate an artificial photocatalytic system for organic transformation. Throughout these new possibilities arise for the application of nitrogen-doped graphene-based photosystem and setting a new benchmark in the field of organic transformation.

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Entry	Photocatalyst	Light	Aerobic Condition	Solvent	Yield (%)
1.	NDGCAF	On	Present	DMSO	88
2.	NDGCAF	On	Inert atmosphere	DMSO	10
3.	NDGCAF	On	Present	DMF	80
4.	NDGCAF	On	Present	CH₃CN	78
5.	NDGCAF	On	Present	THF	58
6.	No Photocatalyst	On	Present	DMSO	05
7.	NDGCAF	Off	Present	No solvent	05
8.	NDGCAF	On	Present	No solvent	98.15