Efficient solvent-free amide synthesis via Ritter reaction catalyzed by a reusable Fe₃O₄/g-C₃N₄/ NTMPA nanocomposite

Abstract

Amide formation through the Ritter reaction remains a valuable transformation in pharmaceutical and materials chemistry, yet conventional protocols rely heavily on corrosive homogeneous acids and non-green conditions. In this study, a magnetite nanoparticles/graphitic carbon nitride/ nitrilotri(methylphosphonic acid (Fe₃O₄/g-C₃N₄/NTMPA) magnetic nanocomposite (MNC) was synthesized via a co-precipitation method and employed as an efficient and recyclable solid acid catalyst for solvent-free Ritter reactions at 80 °C. Structural and morphological analyses using FT-IR, XRD, FE-SEM, TEM, DLS, and EDX confirmed successful incorporation of NTMPA and uniform distribution of Fe, C, N, O, and P throughout the composite. BET analysis showed a surface area of 11.421 m²/g, pore volume of 0.0588 cm³/g, and mean pore diameter of 20.593 nm, indicating a mesoporous structure conducive to catalytic accessibility. TGA revealed a major decomposition step of 23.88% between 500 and 600 °C corresponding to g-C₃N₄ degradation, confirming appropriate thermal stability. The catalyst demonstrated broad substrate applicability, converting tertiary alcohols and benzylic alcohols to the corresponding amides in high to excellent yields in 1.25–6 h. Aromatic nitriles consistently delivered yields above 90% within 5–6 h, with electron-withdrawing substituents further enhancing reactivity and aliphatic nitriles provide the related amides in 87–98% yields within 1.5–5.5 h. The catalyst retained over 90% activity after six cycles, with post-reaction analyses confirming structural integrity. These results highlight Fe₃O₄/g-C₃N₄/NTMPA as a robust, magnetically recoverable, and environmentally compatible catalyst for green amide synthesis.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-35371-2.

Introduction

Amides are fundamental organic compounds that hold a central place in both natural and synthetic chemistry due to their structural and chemical versatility. Characterized by a carbonyl group (C = O) linked to a nitrogen atom, they are essential building blocks in proteins, forming peptide bonds that link amino acids and serve as the structural units of living organisms¹. Beyond their biological significance, amides are critical intermediates in the synthesis of pharmaceuticals, agrochemicals, and polymers, featuring prominently in bioactive molecules from small-molecule drugs to complex peptide-based therapeutics². Amide bonds are integral to the backbone of many therapeutic agents, including antibiotics, anticancer drugs, and pain relievers². In materials science, amides contribute to the development of advanced materials like nylons and aramids, valued for their chemical stability, mechanical properties, and resistance to hydrolysis³. Furthermore, amides play a significant role in various industrial applications, such as in the production of plastics, lubricants, dyes, cosmetics, and surfactants. This broad utility across fields highlights the importance of developing efficient and sustainable methods for amide synthesis, making it a priority in both organic synthesis and industrial chemistry⁴.

Amides have been synthesized through various methods, Fig. 1, including direct acylation of amines (Fig. 1A)⁵, the Schmidt reaction (Fig. 1B)⁶, transamidation (Fig. 1C)⁷, Ritter reaction (Fig. 1D)⁸, aminocarbonylation (Fig. 1E)⁹, oxidative amidation (Fig. 1F)¹⁰, hydrolysis of nitriles (Fig. G)¹¹, reductive amination (Fig. 1H)¹², [1,3]-acyl shift of n-carboxyanhydrides which is the reaction of carboxylic acid with isocyanate followed by a [1,3]-acyl shift (Fig. 1I)¹³, and biocatalytic amide bond formation (Fig. 1J)¹⁴. Among these, transamidation, oxidative amidation, and aminocarbonylation represent more modern approaches. However, these methods often require harsh reaction conditions or toxic reagents, making them less favorable in terms of green chemistry.

Fig. 1.

Different routes for the synthesis of amides.

The Ritter reaction is one of the most powerful and versatile methods for synthesizing amides from nitriles and alcohols or alkenes, often under mild conditions⁸. This reaction proceeds through the generation of a carbocation intermediate, which reacts with the nitrile to form a nitrilium ion, eventually yielding an amide upon hydrolysis. The major advantages of the Ritter reaction are its broad substrate scope and its ability to form amides in a single step without requiring pre-functionalization of the starting materials. It also tolerates a wide variety of functional groups, making it a highly adaptable method for both academic and industrial applications¹⁵. Despite its benefits, the traditional Ritter reaction often relies on strong acids, such as sulfuric acid or hydrochloric acid, posing environmental and operational challenges, particularly in catalyst recovery and reuse. Thus, the exploration of more sustainable and efficient catalytic systems within the framework of the Ritter reaction remains a significant area of research in the pursuit of greener methodologies for amide synthesis.

Recent advancements in the Ritter reaction have shifted focus toward more sustainable acidic catalytic systems, moving away from traditional strong acids such as sulfuric acid and hydrochloric acid¹⁶. Various Lewis and Brønsted acid catalysts have been explored, including sulfated tungstate¹⁷, sulfonic acid-functionalized silica¹⁸,and ionic liquids like [BSmim]CuPW₁₂O₄₀¹⁹. Notable examples also include zeolite Y nanoparticles (NPs)²⁰, sulfonic acid-functionalized (bio)materials²¹, and sulfated polyborate catalysts²². Additionally, nafion/layered double hydroxide nanohybrids²³ and m-phenolsulfonic acid-formaldehyde resin²⁴, and amberlyst15(H)²⁵ have shown promise as effective catalysts. Moreover, metal-organic frameworks (MOFs) like copper-MOF¹⁰, and magnetic CoFe₂O₄ nanoparticle (NP) immobilized N-propyl diethylenetriamine sulfamic acid²⁶ have been employed for enhanced catalytic performance. Finally, TFA/TfO²⁷ represent additional catalytic systems investigated for their effectiveness in the Ritter reaction. These developments reflect the ongoing efforts to refine catalytic systems in the Ritter reaction, aiming to achieve greater sustainability and operational convenience in amide synthesis²⁸.

A critical factor in enhancing the practical applicability of catalytic systems in the Ritter reaction is their ability to be easily recovered and reused, which aligns with the principles of green chemistry through reducing wastes and costs in large-scale applications²⁶. Catalysts that can be efficiently separated from the reaction mixture, especially those that avoid complex purification steps, are highly valued in the industry^29–31. Among the various systems, magnetic acidic catalysts have gained attention due to their facile recovery through magnetic separation, simplifying the catalyst handling and reuse process³². Several recent studies have reported successful applications of magnetic catalysts in organic reactions, including Cu-supported amine-functionalized Fe₃O₄ for coupling reaction³³, Fe₃O₄@gC₃N₄@Thiamine for the synthesis of spirooxindole-pyran derivatives and 2-amino-4H-pyrans³⁴. Additional reported systems are Fe₃O₄@Zein-copper(II) for the synthesis of novel fluorescent 1,4-disubstituted-1,2,3-triazoles³⁵, Fe₃O₄@EDA-SO₃H for the synthesis of synthesis of quinoxalines, benzothiazoles and benzoxazoles³⁶, Fe₃O₄@SiO₂-S-Bu-SO₃H for the synthesis of benzothiazole derivatives³⁷, Fe₃O₄@Glycerol‑Cu in the green synthesis of 2‑amino‑4H‑chromenes³⁸, Fe₃O₄@ZIF-8@Glycerol-Ni in the synthesis of dihydropyrimidinones³⁹, Fe₃O₄@CPTMS@Asp@Ni in the synthesis of substituted acridines⁴⁰, and Ni catalyst anchored on Fe₃O₄@MIL-101 has shown excellent efficiency in one-pot Suzuki–Miyaura coupling⁴¹ have demonstrated effective reusability and high catalytic performance in organic synthesis, underscoring the potential of these systems in sustainable organic synthesis.

NTMPA exhibits strong acidic properties due to the presence of three phosphonic acid groups that making it a strong Brønsted acid catalyst in various acid-catalyzed transformation in organic synthesis⁴². NTMPA is one of the most important family of aminopolyphosphonic acids as strong chelating agents toward metal ions, enabling stable binding to metal oxide surfaces such as Fe₃O₄, thereby reducing leaching and enhancing catalyst durability⁴³. Third, phosphonic acids show excellent anchoring affinity to oxide and graphitic supports, creating robust hybrid materials suitable for heterogeneous catalysis⁴⁴. Fourth, NTMPA is non-volatile, thermally stable, and widely used in environmentally benign applications (e.g., scale inhibition, water treatment), aligning well with green chemistry principles⁴⁵. Finally, despite its strong acidity and stability, NTMPA has not previously been reported as a catalytic component for the Ritter reaction, offering both novelty and functional advantages in designing recyclable, magnetically recoverable solid acid catalysts.

Coating Fe₃O₄ NPs with different materials serves several purposes, such as improving stability, enhancing functionality, preventing aggregation, and allowing specific surface modifications for various applications^46,47. Some commonly used coatings are silica (SiO₂) coating⁴⁶, polymer coatings such as polyethylene glycol (PEG)⁴⁸, carbon coatings like graphene⁴⁹, organic acid coatings like oleic acid⁵⁰, and g-C₃N₄⁵¹. The g-C₃N₄ coating provides a protective layer around Fe₃O₄ NPs, increasing their resistance to chemical degradation and oxidation⁵¹. Additionally, the magnetic nature of Fe₃O₄ enables easy recovery of the catalyst with an external magnetic field, which simplifies the recycling process and makes the material suitable for repeated use in heterogeneous catalysis³².

This study focuses initially on synthesizing a novel, heterogeneous, magnetically separable Fe₃O₄/g-C₃N₄/NTMPA MNC for the efficient catalysis of the Ritter reaction under mild and benign conditions (Fig. 2). By integrating the magnetic properties of Fe₃O₄ with the protective and conductive characteristics of g-C₃N₄ and the strong acidity of NTMPA to develop a sustainable and reusable catalytic system that aligns with green chemistry principles. Then it is employed in a green, solvent-free approach for amide formation via the Ritter reaction, while specifically addressing key limitations of earlier methods such as prolonged reaction times, harsh conditions, and difficult catalyst recovery by providing a more efficient, recoverable, and environmentally friendly alternative.

Fig. 2.

Synthesis of amide derivatives through Ritter reaction catalyzed by an efficient magnetically Fe₃O₄/g-C₃N₄/NTMPA MNC.

Experimental

Chemicals

All solvents and chemicals were purchased from Merck, and Sigma-Aldrich chemical companies and were applied with no further purification. The following chemicals were purchased from Sigma-Aldrich to synthesize Fe₃O₄/g-C₃N₄/NTMPA MNC: melamine, iron (III) chloride hexahydrate (FeCl₃.6H₂O), iron (II) chloride tetrahydrate (FeCl₂.4H₂O). Ammonium hydroxide (NH₄OH) and potassium bromide (KBr) were bought from Merck company. The solvents were used with no more purification.

Characterization instruments

The spectra of ¹H NMR at 400 MHz and ¹³C NMR at 100 MHz were reported by a Bruker Avance DPX spectrometer in pure deuterated chloroform (CDCl₃) and deuterated dimethyl sulfoxide (DMSO-d6) solutions. All ¹H and ¹³C NMR spectra were recorded with TMS as an internal standard. Chemical shifts (δ) are reported in ppm, coupling constants (J) in Hz, and splitting patterns are indicated as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplets), or br (broad). The FT-IR spectra of the materials were recorded on a Shimadzu FT-IR 8300 spectrometer using a potassium bromide pellet. The phase composition and the crystalline structure of MNPs were investigated by Bruker AXS D8-Advance X-ray diffractometer with Cu Kα radiation (λ = 1.5418). The magnetization of NPs was measured by using an instrument model BHV-55 vibrating sample magnetometer. The thermal stability of the catalyst was determined by thermal gravimetric analysis (TGA) using a TA Q600 device. The Field emission scanning electron microscopy (FE-SEM) image of the catalyst was performed by MIRA3 TESCAN and HITACHI S-4160, and the elemental characterization was conducted by energy-dispersive X-ray (EDX) spectroscopy accessory to FE-SEM. The Transmission Electron Microscopy (TEM) images were recorded using the Zeiss- EM10C and Philips EM208 transmission electron microscope operated at 100 kV accelerating voltage. The specific surface area of the nanocatalyst was determined through Brunauer Emmett Teller (BET) analysis measured on a Microtrac Bel Corp-Belsorp mini II device. The size distribution of the NPs was measured by the Dynamic light scatterings (DLS) technique using a HORIBA-LB 550 particle size analyzer. The loading amount of Cu in the nanocatalyst and the leaching amount were measured by the Varian Vista-pro ICP analyzer. The reaction progress and purity of the products were monitored by thin-layer chromatography (TLC) on Merck silica gels 60 F254 aluminum plates and visualized under UV light (254 nm). TLC solvent systems typically consisted of n-hexane/ethyl acetate mixtures in ratios ranging from 8:2 to 6:4 (v/v), depending on product polarity. Column chromatography was carried out on silica gel (230–400 mesh), using gradient mixtures of n-hexane/ethyl acetate as the mobile phase unless otherwise stated. Fractions were monitored via TLC, combined, and concentrated under reduced pressure. The UV-Vis absorption spectra were recorded using a PerkinElmer Lambda 25 UV-Vis spectrophotometer. The melting points of products were determined in open capillaries using an Electrothermal 9100 apparatus and were compared with literature values.

Synthesis of Fe₃O₄/g-C₃N₄/NTMPA MNC

Synthesis of Fe₃O₄/g-C₃N₄

The g-C₃N₄ powder was synthesized by directly heating melamine at 520 ˚C for 4 h in a tube furnace under ambient air⁵². The Fe₃O₄@g-C₃N₄ MNC was subsequently prepared via chemical co-precipitation. In this process, 0.125 g of g-C₃N₄ was dispersed in a 500-mL beaker containing ethanol/water mixture (1:2) and subjected to ultrasonic treatment for 5 h at room temperature. Then, 1.83 g of FeCl₃·6 H₂O and 0.7 g of FeCl₂·4 H₂O were added to the g-C₃N₄ solution after being dissolved in 20 mL of distilled water separately. The mixture was stirred at 80 °C for 30 min before the gradual addition of ammonia solution (NH₄OH 25%) until the pH reached 10. The reaction proceeded under continuous stirring for an additional 30 min. Upon completion, the resulting product was cooled and subjected to successive washes with ethanol and distilled water. Finally, the product was dried at 80 °C⁵³.

Synthesis of Fe₃O₄/g-C₃N₄/NTMPA MNC

An amount of 0.25 g of Fe₃O₄/g-C₃N₄ was dispersed in 10 mL of distilled water. Meanwhile, NTMPA (0.23 g, in a 1:1 molar ratio relative to the initially used melamine) was dissolved in 5 mL of distilled water and subsequently introduced into the suspension. The mixture was stirred for 3 h at room temperature. The resulting brown precipitate was then separated using an external magnetic field, washed thoroughly with deionized water and ethanol, and dried at 60 °C for 5 h.

General procedure for the synthesis of amides via the Ritter reaction using Fe₃O₄/g-C₃N₄/NTMPA as a heterogeneous acid catalyst

To a mixture of nitrile (1.0 mmol) and alcohol (1.0 mmol), 0.06 g of the acid catalyst Fe₃O₄/g-C₃N₄/NTMPA was added, and the reaction mixture was stirred at 80 °C under solvent-free conditions for the appropriate time. The progress of the reaction was monitored by TLC using n-hexane/ethyl acetate as the eluent. Upon completion, ethanol was added to the reaction mixture to dissolve the precipitated amide product, leaving the catalyst behind as a brown, ethanol-insoluble solid. The catalyst was magnetically separated from this ethanolic solution and washed with ethanol. The two ethanolic solutions were then combined and evaporated under reduced pressure. The crude product, obtained as a solid, was purified either by recrystallization from ethanol or ethanol/water mixtures depending on product solubility or by column chromatography using silica gel and a gradient mixture of n-hexane/ethyl acetate (from 8:2 to 6:4). The structures of pure products were identified by comparing their melting point, FT-IR and ¹H- and C-NMR NMR spectra with those have been reported in the literature.

Characteristic data for N-(1-phenylethyl)benzamide (3a) (Table 3, entry 1): White solid; Yield: 95%; Mp:104–106 °C; FT-IR (KBr) ῡ: 3356.6, 3057.9, 2973.0, 2929.9, 1634.6, 1602.9, 1521.7, 1451.5, 1361.8, 1321.9, 1073.9, 760.6, 700.6 cm^− 1; ¹H NMR (400 MHz, CDCl₃) δ: 1.63 (d, J = 6.8 Hz, 3 H), 5.37 (m, 1H), 6.36 (s, 1H), 7.28–7.34 (m, 2 H), 7.41–7.36 (m, 3 H), 7.44–7.49(m, 2 H), 7.47–7.53 (m, 1H), 7.80 (d, J = 7.3 Hz, 2 H) ppm; ¹³C NMR (100 MHz, CDCl₃) δ: 21.8, 49.2, 126.4, 126.9, 127.5, 128.6, 128.8, 131.5, 134.5, 143.1, 166.6 ppm.

Table 3.

Synthesis of amides via Ritter reaction using Fe₃O₄/g-C₃N₄/NTMPA MNC.

Entry	Alcohol	Nitrile	Product	Time (h)	Yieldb (%)	TONc	TOFd	M.P.e/˚C (Ref.)
1				5.0	95	15.83	3.17	104–106 (104–106)56
2				4.5	93	15.50	3.44	153–155 (152–154)57
3				5.5	92	15.33	2.79	75–77 (75–77)58
4				5.5	90	15.00	2.73	70–72 (70–72)58
5				5.25	92	15.33	2.92	138–140 (136–139 )59
6				5.5	92	15.33	2.79	163–164 (162–165 )57
7				6.0	86	14.33	2.39	148–151 (156–157)57
8				4.5	94	15.67	3.48	134–137 (136–139)60
9				4.75	91	15.17	3.19	208–210 (208–210)61
10				4.0	93	15.50	3.87	148–150 (148–150)57
11				5.5	93	15.050	2.82	104–106 (106–108)57
12				5.5	91	15.17	2.76	108–110 (110–111)57
13				3.0	97	16.17	5.39	130–133 (131–133)62
14				4.0	95	15.83	3.96	137–139 (136–137)8
15				3.5	95	15.83	4.52	131–133 (131–133)8
16				5.0	82	13.67	2.74	103–106 (104–105)8
17				5.5	86	14.33	2.61	156–157 (156–157)63
18				6.0	90	15.00	2.50	108–110 (112 °C)57
19				6.5	86	14.33	2.20	120–123 (120–123)57
20				1.25	98	16.33	13.06	159–161 (158–163)64
21				1.50	97	16.17	10.78	148–150 (148–150)21
22				1.75	96	16.00	9.14	145–147 (144–145)8
23				2.0	87	14.50	7.25	119–122 (119–120)57
24				2.0	92	15.33	7.66	156–158 (156–158)8
25				2.0	85	14.17	7.08	123–125 (123–125)57

^aReaction conditions: Alcohol (1mmol), nitrile (1mmol), Fe₃O₄/g-C₃N₄/NTMPA MNC (0.06 g), 80 °C, solvent-free.

^bIsolated yield.

^cmmol product / g catalyst.

^d(mmol product / g catalyst)/ h.

^eThe melting point of the products was compared to those in the literature.

Results and discussion

As described in the experimental section, the synthesis of Fe₃O₄/g-C₃N₄/NTMPA was performed through three steps as illustrated in Fig. 3.

Fig. 3.

The preparation steps of Fe₃O₄/g-C₃N₄/NTMPA MNC.

Characterization of the Fe₃O₄/g-C₃N₄/NTMPA MNC

FT-IR analysis

The FT-IR spectra of (a) NTMPA, (b) g-C₃N₄ powder, (c) Fe₃O₄/g-C₃N₄, and (d) the Fe₃O₄/g-C₃N₄/NTMPA MNC are depicted in Fig. 4. The IR spectrum of NTMPA shows sharp band at 900–1000 cm^{− 1} that belongs to asymmetric stretching vibrations of the P-OH group. Peak at 1150 cm^{− 1} corresponds to P = O stretching, and a weak peak around 3500 cm^{− 1} is attributed to O-H stretching (Fig. 4a)⁴². In the FT-IR spectrum of g-C₃N₄ (Fig. 4b), a broad absorption band observed in the range of 3000–3400 cm^{− 1} is indicative of the stretching vibrations associated with NH and NH₂ functional groups present in the g-C₃N₄ framework. Furthermore, the bands appearing within the range of 1230–1640 cm^{− 1} are attributed to the stretching modes of carbon-nitrogen (C-N) bonds, encompassing both single and double bonds in the g-C₃N₄ structure⁵¹. A distinctive feature for the identification of graphitic carbon nitride is the sharp peak at 815 cm^{− 1}, which can be assigned to the vibrational modes of the s-triazine ring⁵¹. In the spectrum of Fe₃O₄/g-C₃N₄ (Fig. 4c), a pronounced peak at 570 cm^{− 1} corresponds to the Fe-O stretching vibrations characteristic of Fe₃O₄. The FT-IR spectrum of the Fe₃O₄/g-C₃N₄/NTMPA MNC (Fig. 4d) exhibits characteristic peaks consistent with those observed for Fe₃O₄/g-C₃N₄, exhibiting a slight shift due to interactions. Notably, peaks around 950 and 1150 cm^{− 1} are associated with P-OH and P = O stretching vibrations, respectively. Additionally, a broad band at 3500 cm^{− 1} is detected, which is indicative of O-H stretching vibrations³⁴.

Fig. 4.

FT-IR spectra of (a) NTMPA, (b) g-C₃N₄, (c) Fe₃O₄/g-C₃N₄, and (d) Fe₃O₄/g-C₃N₄/NTMPA, and XRD patterns of (e) Fe₃O₄ (f) Fe₃O₄/g-C₃N₄, and (g) Fe₃O₄/g-C₃N₄/NTMPA.

XRD analysis

Figure 4e-g illustrate the XRD patterns for (a) Fe₃O₄ (Fig. 4e), (b) Fe₃O₄/g-C₃N₄ (Fig. 4f), and (c) Fe₃O₄/g-C₃N₄/NTMPA (Fig. 4g). All three samples exhibit distinct diffraction peaks at 2θ values of 30.4°, 35.5°, 43.6°, 53.4°, 57.0°, and 62.8°, corresponding to the (220), (311), (400), (422), (511), and (440) indices, respectively. These peaks confirm the presence of the cubic spinel structure of Fe₃O₄, consistent with standard magnetite XRD patterns reported in the literature (Fig. 4e)³⁴. The XRD patterns of Fe₃O₄/g-C₃N₄ (Fig. 4f) and Fe₃O₄/g-C₃N₄/ NTMPA (Fig. 4g) exhibit a broad, featureless peak at low diffraction angles (2θ = 10–20°), indicative of the amorphous nature of g-C₃N₄³⁴. These findings indicate that the surface functionalization of Fe₃O₄ NPs does not induce any phase transformation. The reduced intensity of the diffraction peaks in the composite samples compared to the bare Fe₃O₄ is likely due to the g-C₃N₄ coating.

FE-SEM, TEM and DLS analyses

The SEM image of bulk g-C₃N₄ illustrates the microstructure and surface morphology of this layered compound (Fig. 5a). The g-C₃N₄ exhibits a solid agglomerate form with particle sizes ranging from hundreds of nanometers to several micrometers, characterized by a crumpled sheet-like structure (Fig. 5a). The FE-SEM images of Fe₃O₄/g-C₃N₄ and Fe₃O₄/g-C₃N₄/NTMPA are depicted in Fig. 5b,c, respectively. The surface characteristics and morphology of these MNCs were assessed using FE-SEM micrographs, which reveal a two-dimensional (2D) lamellar structure with well-dispersed spherical particles. These micrographs indicate uniformity in the Fe₃O₄/g-C₃N₄ and Fe₃O₄/g-C₃N₄/NTMPA MNCs with average particle sizes of approximately 10–12 nm and 17–22 nm, respectively (Fig. 5b,c).

Fig. 5.

SEM image of (a) Bulk-g-C₃N₄, and FE-SEM images (b) Fe₃O₄/g-C₃N₄, and (c) Fe₃O₄/g-C₃N₄/NTMPA.

The evaluation of TEM micrographs for the Fe₃O₄/g-C₃N₄/NTMPA MNC clearly shows the presence of attached NTMPA molecules and the deposition of dark Fe₃O₄ NPs on the surfaces of g-C₃N₄ (Fig. 6a,b). Furthermore, the transparent, thin, sheet-like structure of g-C₃N₄ is observable in the TEM images confirming the 2D morphology observed in FE-SEM.

Fig. 6.

TEM images of (a,b) Fe₃O₄/g-C₃N₄/NTMPA MNC, and (c) the size distribution of Fe₃O₄/g- C₃N₄/NTMPA.

DLS technique was also used to determine the hydrodynamic diameter of particle size. The size distribution of Fe₃O₄/g-C₃N₄/NTMPA MNC is centered at a value of 34 nm (Fig. 6c). The DLS result further confirms the presence of nanoscale particles, which aligns with the observations obtained from FE-SEM and TEM analyses, indicating the successful synthesis of the MNC and its uniform dispersion in the solvent. This is critical for evaluating the material’s potential applications in catalysis and nanotechnology⁵⁴.

EDX analysis

The purity of the Fe₃O₄/g-C₃N₄/NTMPA MNC was confirmed through the EDX spectrum with weight% and atom% values of the synthesized MNC. As illustrated in Fig. 7a, the distinct peaks indicate the presence of the expected elements. The spectrum for the Fe₃O₄/g-C₃N₄/NTMPA MNC exhibits characteristic peaks for nitrogen (N), oxygen (O), phosphorus (P), carbon (C), and iron (Fe) (Fig. 7a). These results confirm the successful integration of Fe₃O₄ and NTMPA into the graphitic carbon nitride framework. Moreover, elemental mapping performed via EDX analysis provides insight into the spatial distribution of these elements within the MNC. The mapping results demonstrate a homogeneous distribution of Fe, C, N, O, and P throughout the sample, indicating a well-dispersed composite structure (Fig. 7b). This uniform distribution is critical for enhancing the catalytic properties of the MNC, as it ensures optimal interaction between the components during chemical reactions.

Fig. 7.

(a) EDX spectrum of Fe₃O₄/g-C₃N₄/NTMPA MNC and (b) the element mapping of Fe₃O₄/g-C₃N₄/NMPA MNC.

BET and BJH analyses

The data obtained from the BET and BJH analyses of the synthesized Fe₃O₄/g-C₃N₄/NTMPA MNC are presented in Table 1. The BET surface area was measured at 11.421 m²/g, indicating a significant potential for adsorption applications. The total pore volume at a relative pressure (p/p₀) of 0.990 was determined to be 0.0588 cm³/g, reflecting the material’s capacity to accommodate various substances within its pore structure. Additionally, the mean pore diameter was calculated to be 20.593 nm, suggesting a mesoporous structure conducive to enhanced diffusion and accessibility for reactants.

Table 1.

Surface area and pore size measurements of Fe₃O₄/g-C₃N₄/NTMPA MNC.

BET plot
V m	2.6241	[cm3(STP) g−1]
as, BET	11.421	[m2 g−1]
Total pore volume(p/p0 = 0.990)	0.0588	[cm3 g−1]
Mean pore diameter	20.593
BJH plot
Vp	0.059142	[cm3 g−1]
rp, peak(Area)	1.21
ap	13.543	[m2 g−1]

In conjunction with the BJH analysis, the total pore volume was found to be 0.059142 cm³/g, with the peak pore radius at 1.21 nm. The BJH adsorption surface area was calculated to be 13.543 m²/g, which highlights the effectiveness of the synthesized MNC in providing ample active sites for chemical interactions. The combination of a high surface area and optimal pore structure suggests that the Fe₃O₄/g-C₃N₄/NTMPA MNC exhibits promising characteristics for various applications, including catalysis, adsorption, and drug delivery.

TGA and VSM analyses

The thermal properties of the Fe₃O₄/g-C₃N₄/ NTMPA MNC was investigated using TGA and DSC as shown in Fig. 8. TGA is an analytical technique commonly employed to assess a material’s thermal stability by monitoring changes in weight as the MNC is heated at a controlled rate. In this study, the thermal stability of the Fe₃O₄/g-C₃N₄/NTMPA nanocomposite was evaluated by heating the sample from 0 to 800 °C at a rate of 20 °C/min.

Fig. 8.

(a) Thermogravimetric analysis of Fe₃O₄/g-C₃N₄/NTMPA, (b) DSC of Fe₃O₄/g-C₃N₄/NTMPA, magnetization curves of (c) Fe₃O₄/g- C₃N₄, (d) Fe₃O₄/g- C₃N₄/NTMPA MNC, and (e) magnetically separation property of Fe₃O₄/g- C₃N₄/NTMPA MNC.

The TGA curve (Fig. 8a) exhibited a two-step thermal decomposition process. The initial weight loss observed in the range of 100–300 °C, which be attributed to the evaporation of physically adsorbed solvents, the desorption of moisture, and the removal of surface hydroxyl groups along with potential intermolecular condensation of phosphonic acid units⁵⁵. The second major weight loss, 23.88%, occurs between 500 and 600 °C, which corresponds to the decomposition of g-C₃N₄ sheets. Notably, the loading of Fe₃O₄ and NTMPA onto the g-C₃N₄ sheets leads to a decrease in thermal stability compared to pristine g-C₃N₄, which is happened from 609 to 761 °C. This reduction in thermal stability may result from the combination of NTMPA, the iron oxide, and the g-C₃N₄ matrix, which alters the decomposition pathway.

The DSC analysis, shown in Fig. 8b, provides crucial information about the thermal behavior of Fe₃O₄/g-C₃N₄/NTMPA MNCs. The DSC curve reveals multiple thermal transitions, indicating the material’s thermal stability and composition. The first endothermic peak observed at 125.88 °C is attributed to the removal of adsorbed water or other volatile substances from the composite, a typical behavior for materials containing organic components. The subsequent endothermic transition at 261.97 °C is associated with the intermolecular condensation of phosphonic acid units, which begins to decompose at this temperature. A more prominent endothermic peak at 331.07 °C corresponds to further decomposition, likely of the NTMPA component, as it breaks down at higher temperatures. These thermal events provide valuable insight into the material’s stability under thermal stress, confirming that the Fe₃O₄/g-C₃N₄/NTMPA MNC can withstand elevated temperatures before significant degradation occurs. This characteristic is essential for its potential use in high-temperature catalytic reactions, as it indicates that the catalyst will maintain structural integrity and performance during prolonged exposure to heat.

The VSM analysis presented in Fig. 8c,d reveals the magnetic properties of the Fe₃O₄/g-C₃N₄ and Fe₃O₄/g-C₃N₄/NTMPA MNCs. Both composites exhibit typical superparamagnetic behavior, with the Fe₃O₄/g-C₃N₄/NTMPA MNC (Fig. 8d) showing a slightly enhanced magnetization relative to Fe₃O₄/g-C₃N₄ (Fig. 8c). The saturation magnetization values for the Fe₃O₄/g-C₃N₄ and Fe₃O₄/g-C₃N₄/NTMPA MNCs are approximately 40 emu/g and 45 emu/g, respectively. This indicates that the Fe₃O₄/g-C₃N₄/NTMPA MNC possesses a marginally higher magnetization, which is beneficial for magnetic separation applications. Such properties are advantageous for catalytic applications, particularly in enabling the easy recovery of the catalyst from reaction mixtures, as demonstrated by the visual evidence in Fig. 8e. The VSM analysis confirms that the Fe₃O₄/g-C₃N₄/NTMPA MNC retains adequate magnetic properties for effective recovery, making it a promising candidate for recyclable catalysis in industrial processes.

Catalytic performance of Fe₃O₄/g-C₃N₄/NTMPA MNC for amide synthesis via the Ritter reaction

The Ritter reaction is a pivotal method for the formation of carbon-nitrogen bonds and is widely applied in the synthesis of amides, which serve as key intermediates in pharmaceutical and medicinal chemistry¹⁶. Traditionally, the reaction involves the interaction between nitriles and benzylic or tertiary alcohols, facilitated by concentrated sulfuric acid, a process known as the classical Ritter reaction. Due to the harsh conditions and corrosive nature of this method, numerous alternative approaches have been explored to develop milder and more environmentally friendly reaction conditions¹⁶. Consequently, there is a strong demand for efficient, sustainable, and versatile catalysts that can perform under milder conditions while maintaining high yields and selectivity.

Table 2 presents the optimization of reaction conditions for the Ritter reaction, highlighting the effect of various factors such as catalyst amount, solvent choice, reaction time, and temperature on the yield of amides.

Table 2.

Optimization of reaction conditions for the model reaction.

Entry	Catalyst	Amount (g)	Solvent	Temp. (°C)	Time (h)	Yield (%)b
1	Fe3O4/g-C3N4/NTMPA	0.06	n-Hexane	Reflux	24	10
2	Fe3O4/g-C3N4/NTMPA	0.06	CH2Cl2	Reflux	24	20
3	Fe3O4/g-C3N4/NTMPA	0.06	H2O	Reflux	8	72
4	Fe3O4/g-C3N4/NTMPA	0.06	Toluene	80	8	78
5	Fe3O4/g-C3N4/NTMPA	0.06	THF	Reflux	24	15
6	Fe3O4/g-C3N4/NTMPA	0.06	DMF	80	24	Trace
7	Fe3O4/g-C3N4/NTMPA	0.06	CHCl3	Reflux	24	18
8	Fe 3 O 4 /g-C 3 N 4 /NTMPA	0.06	Solvent-free	80	5	95
9	Fe3O4/g-C3N4/NTMPA	-----	Solvent-free	80	24	Trace
10	NTMPA	0.035	Solvent-free	80	5	92
11	Fe3O4/g-C3N4/NTMPA	0.03	Solvent-free	80	5	55
12	Fe3O4/g-C3N4/NTMPA	0.09	Solvent-free	80	5	95
13	Fe3O4/g-C3N4/NTMPA	0.12	Solvent-free	80	5	88
14	Fe3O4/g-C3N4/NTMPA	0.06	Solvent-free	R.T	12	50
15	Fe3O4/g-C3N4/NTMPA	0.06	Solvent-free	110	6	78
16	g-C3N4	0.01	Solvent-free	80	5	---
17	Fe3O4	0.01	Solvent-free	80	5	---
18	Fe3O4@g-C3N4	0.02	Solvent-free	80	5	---

^aReaction conditions: 1-Phenylethanol (1a) (1.0 mmol, 0.12 mL), benzonitrile (2a) (1.0 mmol, 0.1 mL), Fe₃O₄/g-C₃N₄/NTMPA MNC, and solvent (2.0 mL).

^bIsolated yield.

Optimization was performed using 1-phenylethanol (1a) and benzonitrile (2a) as a model system. The first set of experiments was conducted using a fixed catalyst amount (0.06 g) with different solvents. Solvent choice significantly influenced the reaction efficiency. N-hexane (Table 2, entry 1) and CH₂Cl₂ (Table 2, entry 2) under reflux conditions resulted in low yields of 10% and 20%, respectively, even after 24 h of reaction time. Using water as a solvent (Table 2, entry 3) improved the yield to 72% with a reduced reaction time of 8 h. This suggests that the polarity of water aids in better reaction efficiency. Toluene (Table 2, entry 4) at 80 °C further increased the yield to 78%, indicating that higher temperatures and moderate polarity solvents enhance performance. THF (Table 2, entry 5) and CHCl₃ (Table 2, entry 7) showed similarly low yields (15 and 18%, respectively) as non-polar solvents, emphasizing the importance of solvent selection for optimal reaction conditions. DMF (Table 1, entry 6) resulted in only trace product formation, likely due to its high boiling point, which could be detrimental to this reaction under the given conditions. The solvent-free reaction at 80 °C (Table 2, entry 8) produced a highly significant yield of 95% within just 5 h, demonstrating that eliminating the solvent improves both reaction efficiency and yield. This condition was selected as the best option, which avoids the use of harmful solvents, making the process greener and more cost-effective. In the next step of optimization, the role of the catalyst under solvent-free conditions at 80 °C was explored. Running the reaction without any catalyst (Table 2, entry 9) resulted in only trace amounts of product, confirming the critical role of the Fe₃O₄/g-C₃N₄/NTMPA MNC catalyst in the reaction. A slight reduction in the catalyst amount (0.03 g) (Table 2, entry 11) decreased the yield to 55% while increasing the amount to 0.09 g (Table 2, entry 12) resulted in the same maximum yield of 95% as 0.06 g (Table 2, entry 8). A further increase to 0.12 g (Table 2, entry 13) led to a slight drop in yield (88%), indicating that increasing the catalyst beyond 0.09 g may not significantly enhance the reaction but instead decrease the efficiency, possibly due to agglomeration or other factors. Also, using NTMPA alone as a homogeneous acid catalyst provided excellent yields (Table 2, entry 10) but lacked the recyclability of the MNC, which is a key feature aligned with green chemistry principles. The recyclability of the Fe₃O₄/g-C₃N₄/NTMPA MNC makes it a valuable catalyst in sustainable chemistry. So, 0.06 g was chosen as the best optimum amount of Fe₃O₄/g-C₃N₄/NTMPA for the reaction.

Temperature optimization revealed that 80 °C was ideal for maximizing conversion, producing a 95% yield within 5 h (Table 2, entry 8). Also, performing the reaction at room temperature (R.T) (Table 2, entry 14) for 12 h yielded only 50%, indicating that heat plays a crucial role in driving the reaction forward. Increasing the temperature to 110 °C (Table 2, entry 15) resulted in a 78% yield after 6 h, suggesting that while higher temperatures accelerate the reaction, they may not necessarily produce a better yield compared to the optimized condition at 80 °C (Table 2, entry 8). The reaction was also repeated at 80 °C for 5 h in the presence of g-C₃N₄, Fe₃O₄, and Fe₃O₄@C₃N₄ separately, which are the precursor components of the catalyst, in order to evaluate their individual contributions to the catalytic activity. TLC monitoring of these reactions revealed that all starting materials remained intact (Table 2, entries 16–18).

The optimal conditions for the Ritter reaction were identified as follows: 0.06 g of Fe₃O₄/g-C₃N₄/NTMPA, solvent-free at 80 °C, and a reaction time of 5 h. These conditions deliver excellent yields while minimizing environmental impact through the use of a recyclable catalyst and eliminating the need for solvents. The MNC’s performance under these conditions highlights its potential for scalable and sustainable amide synthesis.

After determining the optimized conditions, the capability of the catalyst in preparing a range of amide derivatives was explored. Table 3 provides the results for the synthesis of a diverse array of amide derivatives through the Ritter reaction under solvent-free conditions, catalyzed by Fe₃O₄/g-C₃N₄/NMPA at 80 °C. The study examines various alcohol substrates, including benzylic, tertiary, primary, allylic, and diphenyl alcohols (1a–1 h) and nitriles (2a–2i) as starting substrates to produce their corresponding amide products (Table 3, entries 1–25) with the yields predominantly exceeding 80%, indicating the efficacy of the catalyst in promoting the formation of amides. The catalytic activity was further quantified by determining the turnover number (TON) and turnover frequency (TOF) under optimized conditions, with the calculated values confirming the high intrinsic efficiency of the Fe₃O₄/g-C₃N₄/NTMPA system in promoting the Ritter reaction.

Regarding alcohols, 1-phenylethanol (1a) yielded amides (3a-3 g) with excellent yields, with most reactions completing within 4.5–6 h (Table 3, entries 1–7). (4-Chlorophenyl) methanol (1b) incorporates an electron resonance-donating chlorine substituent at the para position, yielding similar efficiency with amide products (3 h-3 L) and yields ranging from 91 to 94% within 4–5.5 h (Table 3, entries 8–12). 2-Methylpropan-2-ol (1c) exhibited exceptionally high yields (up to 97%) with shorter reaction times (3–4 h) across several nitrile substrates to produce related amides (3 m-3o). The stability of the tertiary carbocation intermediates contributes to the enhanced reactivity (Table 3, entries 13–15). Phenylmethanol (1d), a primary alcohol, resulted in an amide product (3p) with a respectable yield of 82% over a reaction time of 5 h. The absence of electron-withdrawing or donating groups on the phenyl ring suggests that this substrate provides a baseline reactivity (Table 3, entry 16). The presence of the electron-withdrawing nitro group on the para position of the phenyl ring significantly decreases the reactivity of (4-nitrophenyl) methanol (1e). This resulted in a yield of 86% (3q) over a slightly higher reaction time of 5.5 h. The nitro group destabilizes the carbocation, decreasing the tendency of nitrile attack (Table 3, entry 17). The o-tolyl methanol substrate (1f), which contains a methyl group in the ortho position relative to the hydroxyl group, produced an amide (3r) with a yield of 90% after 6 h. The presence of the methyl group, an electron-donating substituent, seems to affect the reaction time, likely through stabilization of the carbocation intermediate. The yield reflects the balance between steric hindrance from the ortho substituent and the overall electronic effects, showcasing that even with substituents that could hinder reactivity, the reaction remains efficient (Table 3, entry 18). The allylic alcohol, prop-2-en-1-ol (1 g), resulted in a yield of 86% for amide product (3s) after 6.5 h. The reaction may proceed through the formation of a more stable carbocation due to the allylic nature of the alcohol. While the yield is slightly lower than some of the other entries, it still demonstrates the high potential of this substrate in the Ritter reaction (Table 3, entry 19). Diphenylmethanol (1 h) was particularly effective, producing high yields of products (3t-3y) ranging from 85 to 98% across a variety of nitriles with remarkably short reaction times (1.25–2 h). The structure of diphenylmethanol, with its two phenyl rings, increases the stability of the carbocation intermediate formed during the Ritter reaction, leading to improved reaction efficiency (Table 3, entries 20–25).

In respect of nitriles, most reactions with aromatic nitriles were completed within 5–6 h and yielded high percentages (generally above 90%). This trend underscores the effectiveness of aromatic nitriles in the Ritter reaction. Benzonitrile, as a baseline nitrile, produced good yields across various alcohol substrates. Nitriles like 4-(trifluoromethyl)benzonitrile exhibited excellent yields, demonstrating the positive influence of electron-withdrawing groups on reactivity (Table 3, entry 2). Nitriles bearing electron-donating groups have less electrophilicity, resulting in a slightly longer reaction time (Table 3, entries 4–7). Entries featuring aliphatic nitriles (like acetonitrile and butyronitrile) showed that while they are less effective compared to their aromatic counterparts, they still provided acceptable yields when reacted with appropriate alcohols (Table 3, entries 3, 4, 11, 12, 21, 22, 23 and 25).

As a result, the Fe₃O₄/g-C₃N₄/NTMPA catalyst exhibited remarkable efficiency in promoting the Ritter reaction, providing high yields across a wide range of alcohol and nitrile substrates. Its ability to facilitate reactions under mild, solvent-free conditions further highlights its potential as a versatile and environmentally friendly catalyst for organic transformations.

The plausible mechanism of the Ritter reaction for the synthesis of amides from alcohols and nitriles catalyzed by the Fe₃O₄/g-C₃N₄/NTMPA MNC involves several key steps as follows (Fig. 9). In the first step, the Fe₃O₄/g-C₃N₄/NTMPA catalyst protonates and helps subsequent dehydration of the alcohol I, forming a highly reactive carbocation II. This step is particularly favorable due to the catalyst’s acidic sites, which enhance the formation of the carbocation intermediate. The nitrile undergoes nucleophilic addition to the carbocation, forming an intermediate nitrilium ion III, which is converted to the amide by reacting with the produced water in situ via the intermediate IV and regenerates the catalyst^62,64.

Fig. 9.

A plausible mechanism of amid formation via the Ritter reaction catalyzed by Fe₃O₄/g-C₃N₄/NTMPA MNC.

The MNC’s ability to be recycled and reused is crucial in light of both environmental and economic concerns. We assessed the magnetically heterogeneous Fe₃O₄/g-C₃N₄/NTMPA MNC performance, which has been effectively applied in subsequent reactions. Therefore, under optimum model reaction conditions, the MNC’s recyclability was investigated. Under solvent-free conditions at 80 °C, the MNC catalyst (0.06 g) catalyzed the Ritter reaction between 1-phenylethanol (1a) (1.0 mmol), and benzonitrile (2a) (1.0 mmol). After the reaction was finished, the MNC was separated from the reaction mixture using an external magnetic field, demonstrating both the MNC’s magnetic behavior and its ease of retrieval. After being thoroughly washed with ethanol and deionized water, the MNC was dried for 5 h at 60 °C before being used for the subsequent reactions under optimal conditions. The recovered MNC demonstrated steady activity in the reaction during seven runs, as seen in Fig. 10a. Fe₃O₄/g-C₃N₄/NTMPA has remarkable stability and recyclability up to six cycles, as indicated by the favorable yields of the isolated product.

Fig. 10.

(a) Recyclability of Fe₃O₄/g-C₃N₄/NTMPA MNC (Reaction conditions: 1-phenylethanol (1a) (1.0 mmol), benzonitrile (2a) (1.0 mmol), 0.06 g Fe₃O₄/g-C₃N₄/NTMPA NMC, solvent-free conditions at 80 °C); (b) TEM and (c) FE-SEM images of the recovered Fe₃O₄/g-C₃N₄/NTMPA MNC after 6 cycles of reaction.

The TEM and FE-SEM images were recorded after seven runs of the reaction to demonstrate the size and morphology of the Fe₃O₄/g-C₃N₄/NTMPA nanocomposite (Fig. 10b and c). The analysis of TEM micrographs for the Fe₃O₄/g-C₃N₄/NTMPA demonstrates the deposition of dark Fe₃O₄ NPs on the surfaces of g-C₃N₄ as well as the presence of NTMPA molecules (Fig. 10b). The SEM image depicts that the surface morphology of the Fe₃O₄/g-C₃N₄/NTMPA did not significantly alter after being used in six cycles of reaction, demonstrating the nanocatalyst’s stability and identity (Fig. 10c).

There are distinctive peaks for N, O, P, C, and Fe along with weight% and atom% values in the EDX spectrum for the recovered Fe₃O₄/g-C₃N₄/NTMPA NMC, which indicates the maintained elemental composition of the MNC after the recovery process (Fig. 11a). In comparison to the fresh MNC, which has an average size of 34 nm (Fig. 6c), the DLS analysis results show a little increase in the size of the reused MNC, with an average size of 36 nm, which suggests a small amount of NP aggregation during the recycling processes (Fig. 11b). The excellent deposition of Fe₃O₄ NPs and preservation of the crystalline structure during the reusing process are confirmed by the similarity of the XRD characteristic peaks in the reused MNC (Fig. 11c) to that in the fresh MNC (Fig. 4f). The recovered MNC’s FT-IR spectrum also shows the primary vibration bands associated with the Fe-O, P-O, C-N, and O-H bonds at 570, 1250, 1630, and 3400 cm^− 1, respectively (Fig. 11d). The preserved structure of the MNC is confirmed by the similar bands in the recovered (Fig. 11d) and fresh MNC (Fig. 4c).

Fig. 11.

(a) EDX image; (b) DLS image; (c) XRD pattern; and (d) FT-IR spectrum of Fe₃O₄/g-C₃N₄/NTMPA MNC after six reaction cycles.

A comparative study of catalysts for the preparation of amides through the Ritter reaction was conducted, highlighting the efficiency, reaction conditions, yield, and reusability of six catalysts (Table 4). Al(HSO₄) ₃ (0.16 g) produced a moderate yield of 85% under reflux with CH₃NO₂ in 2.5 h but lacked reusability (Table 4, entry 1). Multiwalled carbon nanotube bearing a p-toluenesulfonyl group (MWCNT-CSP) (0.05 g) demonstrated a low yield of 50% at 100 °C, requiring a prolonged reaction time of 24 h, indicating limited efficiency (Table 4, entry 2). Fe(NO₃) ₃·9 H₂O (0.2 g), under milder conditions at 80 °C, delivered a high yield of 96% in 3 h but was not reusable (Table 4, entry 3). Pentafluorophenylammonium triflate (PFPAT) (0.028 g), notable for its low catalyst loading and solvent-free conditions at 90 °C, achieved a good yield of 92% in 3 h (Table 4, entry 4). The γ-Fe₂O₃@SiO₂-HClO₄ (0.05 g) stood out with a yield of 97% under neat conditions at room temperature in 3 h and demonstrated reusability for five cycles, highlighting its stability and energy efficiency (Table 4, entry 5). Finally, Fe₃O₄/g-C₃N₄/NTMPA MNC (0.12 g) showed the best overall performance, achieving the highest yield of 98% in the reaction time of 5 h under solvent-free conditions at 80 °C. Moreover, it demonstrated excellent reusability over six cycles (Table 4, entry 6). This comparison underscores Fe₃O₄/g-C₃N₄/NMPA’s superiority in efficiency, sustainability, and practicality, making it a promising catalyst for the Ritter reaction.

Table 4.

Brief report of the efficiency of catalysts published in the literature and Fe₃O₄/g-C₃N₄/NTMPA MNC for the Preparation of amide 3a through the Ritter reaction.

Entry	Catalyst	Reaction Conditions	Time (h)	Yielda (%)	Cycleb	Refs.
1	Al(HSO4)3 (0.16 g)	CH3NO2/reflux	2.5	85	0	62
2	MWCNT-CS (0.05 g)	100 °C	24	50	0	21
3	Fe(NO3)3.9H2O (0.2 g)c	80 °C	3	96	0	58
4	PFPAT (0.028 g)	Solvent free/90 °C	3	92	0	65
5	γ-Fe2O3@SiO2-HClO4 (0.05 g)	Neat condition/R.T	3	97	5	66
6	Fe3O4/g-C3N4/NTMPA (0.12 g)	Solvent free/80 °C	5	98	6	This work

1-Phenylethanol (1a) (1.0 mmol) and benzonitrile (2a) (1.0 mmol) were used as substrates unless otherwise stated.

^aIsolated yield.

^bNumber of cycles reusing the catalyst.

^c1-Phenylethanol (0.5 mmol) and benzonitrile (30 mmol) were used.

Conclusion

The Fe₃O₄/g-C₃N₄/NTMPA magnetic nanocomposite developed in this work demonstrates strong potential as a heterogeneous solid acid catalyst for green amide synthesis via the Ritter reaction. Comprehensive structural analyses (FT-IR, XRD, FE-SEM, TEM, DLS, BET, TGA, and EDX) confirmed the successful assembly of a thermally stable, uniformly functionalized composite containing Fe, C, N, O, and P. Catalytic evaluations revealed that the MNC efficiently promotes carbocation formation even under solvent-free and mild conditions, enabling high conversions for a wide variety of alcohols and nitriles. Notably, tertiary alcohols afforded yields up to 97%, while benzylic and diarylmethyl alcohols produced amides in 82–98% yields across relatively short reaction times. This performance highlights the catalyst’s ability to maintain homoselectivity—that is, selective activation of the alcohol substrate without promoting undesirable side reactions such as rearrangements, over-alkylation, or hydrolysis—due to the controlled acidity provided by NTMPA and the stable microenvironment created by the g-C₃N₄ matrix. The catalyst further exhibited excellent operational stability, preserving more than 90% of its activity over six consecutive cycles with negligible structural degradation. This behavior confirms that the reaction proceeds via a true heterogeneous pathway, supported by post-reaction FT-IR, XRD, DLS, and EDX analyses showing no detectable leaching of active species. These attributes—high efficiency, substrate compatibility, structural robustness, and magnetic recoverability—position Fe₃O₄/g-C₃N₄/NTMPA as a compelling alternative to conventional homogeneous acids in sustainable amide synthesis.

This NTMPA-functionalized magnetic nanocomposite can be explored in future studies, for broader catalytic applications beyond the Ritter reaction, particularly in other acid-catalyzed organic transformations such as esterification, dehydration, and multicomponent reactions. Detailed kinetic and mechanistic investigations, including computational studies, could provide deeper insight into the role of NTMPA and the Fe₃O₄/g-C₃N₄ interface in facilitating carbocation formation. Furthermore, scaling up the process and evaluating catalyst performance in continuous-flow or industrially relevant conditions would enhance its practical applicability. The structural flexibility of NTMPA also opens opportunities to design next-generation phosphonic acid-based magnetic nanocatalysts with tunable acidity and improved recyclability.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

H.K.: Resources, Validation, Data Curation, Visualization. Methodology, Formal analysis, Investigation, Writing-original draft. A. R. S.: Resources, Methodology, Conceptualization, Supervision, Investigation, Writing − review & editing.

Data availability

Data is provided within the supplementary information file.

Declarations

Competing interests

The authors declare no competing interests.