Ferrite Nanoparticles as Catalysts in Organic Reactions: A Mini Review

Abstract

Ferrites have excellent magnetic, electric, and optical properties that make them an indispensable choice of material for a plethora of applications, such as in various biomedical fields, magneto–optical displays, rechargeable lithium batteries, microwave devices, internet technology, transformer cores, humidity sensors, high-frequency media, magnetic recordings, solar energy devices, and magnetic fluids. Recently, magnetically recoverable nanocatalysts are one of the most prominent fields of research as they can act both as homogeneous and heterogenous catalysts. Nano-ferrites provide a large surface area for organic groups to anchor, increase the product and decrease reaction time, providing a cost-effective method of transformation. Various organic reactions were reported, such as the photocatalytic decomposition of a different dye, alkylation, dehydrogenation, oxidation, C–C coupling, etc., with nano-ferrites as a catalyst. Metal-doped ferrites with Co, Ni, Mn, Cu, and Zn, along with the metal ferrites doped with Mn, Cr, Cd, Ag, Au, Pt, Pd, or lanthanides and surface modified with silica and titania, are used as catalysts in various organic reactions. Metal ferrites (MFe₂O₄) act as a Lewis acid and increase the electrophilicity of specific groups of the reactants by accepting electrons in order to form covalent bonds. Ferrite nanocatalysts are easily recoverable by applying an external magnetic field for their reuse without significantly losing their catalytic activities. The use of different metal ferrites in different organic transformations reduces the catalyst overloading and, at the same time, reduces the use of harmful solvents and the production of poisonous byproducts, hence, serving as a green method of chemical synthesis. This review provides insight into the application of different ferrites as magnetically recoverable nanocatalysts in different organic reactions and transformations.

1. Introduction

Ferrite comes from the Latin word “ferrum”, which means iron. Ferrites are a group of non-metallic ceramic compounds made by consolidating and heating significant measures of iron (III) oxide with at least one metal, such as Cu, Sr, Ba, Co, Mn, Ni, Zn, etc. Ferrites are generally ferrimagnetic in nature, which means they are attracted to magnets and, thus, can easily be magnetized by an externally applied magnetic field. The magnetic dipole moment associated with the electron spin determines the magnetic behavior of the ferrites [1]. Ferrite materials exhibit ferromagnetism, which shows magnetism at a temperature lower than the Curie temperature (T_C), thus, becoming paramagnetic at a temperature higher than T_C. Ferrites do not conduct electricity as ferromagnetic materials do but rather show extremely high electrical resistivity with specific resistivities 10¹⁴ times more than that of metals with dielectric constants around 10 to 16 or greater, making them potential candidates for transformers as a magnetic core [2]. A ferrite is usually described by the formula MFe₂O₄, where M is any bivalent metal (usually transition metals) such as Cu, Zn, Ni, Co, Mg, etc. [3]. According to the crystalline structure, ferrites can be categorized as spinel ferrite (MFe₂O₄), hexagonal ferrite (MFe₁₂O₁₉), and garnet ferrite (M₃Fe₅O₁₂), where M is a bivalent transition metal, such as Fe, Cu, Zn, Ni, Co, Mn, etc., and ortho ferrite (MFeO₃), where M is a trivalent metallic ion, such as a rare-earth ion [4,5].

Ferrites can be divided further, on the basis of magnetic coercivity, as soft and hard ferrites [6]. The principal ferrite compounds were first discovered in 1930 by Yogoro Kato, along with Takeshi Takei from the Tokyo Organization of Innovation [7]. They were granted a patent for discovering magnetic core oxide materials in the year 1932 in Japan (Japan PAT 98844). A Cu-Zn ferrite was newly invented commercially by the TDK corporation in 1935. During the First World War, ferrites were mainly used for radio communication equipment. Around 1950, the research on ferrites accelerated to meet the demand in the television industry. In 1952, a Ni-Zn/Mn-Zn ferrite was used by a researcher from the “Philips” company as a television component [8]. The ferrites exhibit good optical and mechanical properties, excellent electrical resistance, permeability, and magnetic properties, as well as good chemical/thermal stability [9,10]. Since the discovery of ferrite, it has become the best choice for different electronic and electrical industries (such as microwave devices, magnetic recordings, radio, television, and magneto–optical displays, as a core material for transformers, internet and sensing applications, solar cells, transducers, and supercapacitors) [11,12,13]. Ferrites are extensively used in biomedical applications [14], for instance, in disease diagnosis, drug delivery [15,16,17,18], MRI [17], and cancer treatment with magnetic hyperthermia [19,20]. The role of ferrites as a nanocatalyst is notable in various organic reactions (such as the photocatalytic decomposition of various dyes, alkylation, dehydrogenation, oxidation, C–C coupling, etc.). The smaller size of the nanoparticles with a greater surface area, selectivity, and easy recovery, retaining their catalytic activity for many cycles, make nanoparticles excellent catalysts [21]. Ferrite, when used in organic reactions as a catalyst, provides a green synthesis method with a more environment-friendly pathway of chemical synthesis by reducing reaction time and avoiding harmful byproducts, and providing the benefit of stability in reuse [22,23]. Ferrite nanoparticles can be extracted conveniently from reaction mixtures by applying an external magnet (Figure 1), facilitating the easy recovery and recyclability of the catalyst, thus, avoiding the catalyst loss that occurs with the centrifugation and filtration methods [24].

Research on catalysts is one of the major topics in organic synthesis. The primary function of a catalyst is to accelerate chemical reactions. With higher catalyst activity, the reactor size can be reduced, as well as the operating conditions such as temperature and pressure, leading to a decrease in operating costs [25,26]. Magnetic nanoparticles (MNPs) have garnered attention for their catalytic applications due to their unique properties [27,28]. Therefore, the primary goal of researchers is to develop nanocatalysts with high activity, excellent yield, reduced reaction time, and good selectivity [29,30]. Ferrite nanoparticles (FNPs) and their composites have a promising future as heterogeneous catalysts due to their easy separation and reusability through the application of an external magnetic field [31,32]. FNPs eliminate the use of harmful chemicals and solvents, making them environmentally friendly [33]. FNPs with a spinel structure exhibit high thermal and chemical stability, a larger surface area, moderate magnetization, and mechanical hardness, which make them predominantly used as catalysts in organic reactions (Figure 2) [34]. This mini-review provides insights into the catalytic applications of ferrite nanoparticles in various organic reactions. The focus of this study will primarily be on five spinel ferrites (CuFe₂O₄, ZnFe₂O₄, NiFe₂O₄, CoFe₂O₄, and MnFe₂O₄) that have been extensively employed as reusable heterogeneous catalysts in various organic reactions.

2. Synthesis

2.1. Synthesis of Nano-Ferrites

The methods for the preparation of ferrite nanocatalysts (Figure 3) include wet-chemical, sono-chemical technique [35], sol–gel microemulsion [36,37], co-precipitation method [38,39,40], microwave heating [41], hydrothermal routes [42], thermal decomposition [43,44], and mechanical or high-energy ball milling [45]. The properties of ferrites are significantly influenced by the chosen preparation method [46].

Among the various methods, co-precipitation is the most commonly used method for preparing ferrite nanoparticles. This method requires proper pH adjustment, temperature control, and appropriate ionic strength, along with a divalent metal to trivalent ion mole ratio of 1:2, to achieve superior quality nanoparticles. Reducing agents such as NaOH, NH₃, TMAOH (tetramethylammonium hydroxide), etc., are used to reduce iron precursors and produce monodispersed Fe₃O₄ nanoparticles with a wide range of particle size distributions, either in the presence or absence of surfactants [47]. Highly monodisperse nanoparticles with a narrow particle size distribution and uniform morphology can be obtained through the thermal decomposition method using various salts such as Fe(CO)₅, Fe(Cup)₃, Fe(acac)₃, etc. [48,49]. The microemulsion technique is employed to achieve nanoparticles with controlled size and shape [50,51], but this method involves high temperatures and complex reactions. The hydrothermal method is an alternative approach that avoids the requirements of the microemulsion process [52]. Another method for preparing Fe₃O₄ nanoparticles is sono-chemical, in which salts such as Fe(C₂H₃O₂)₂ and Fe(acac)₃ are sonicated [53,54,55]. A cost-effective, simple, and facile method for obtaining a homogeneous product with high purity is the sol–gel method. The sol–gel process involves the hydrolysis and condensation of precursor materials in a solution medium. Conventionally preferred precursor materials are metal alkoxides or inorganic salts. In the sol–gel method, citric acid forms a gel with a homogeneous distribution of precursor metal ions [56,57,58]. Each method has its own advantages and disadvantages (Table 1). However, green synthesis, which avoids the use of harmful chemicals and utilizes biological methods such as long-term mineralization with bacteria, is an emerging area of research [59,60,61]. Natural extracts from plants containing secondary metabolites, such as flavonoids, phenols, tannins, alkaloids, etc., are used to produce spinel nano-ferrites in an eco-friendly manner [62].

2.2. Synthesis of Metal-Doped Ferrites

Spinel ferrites have been extensively studied in recent years due to their excellent thermal resistance, as well as their notable magnetic and catalytic activity. Spinel ferrite is represented as AB₂O₄, where A represents a divalent metal cation, and B represents a trivalent metal cation. In the case of normal spinel, 8 out of 64 available tetrahedral sites are occupied by A cations, while 16 out of 32 available octahedral sites are occupied by B cations (Figure 4). On the other hand, if all tetrahedral sites are exclusively occupied by B cations, with random occupancy of both A and B cations in the octahedral sites, it is referred to as an inverse spinel. When both the divalent metal ions M²⁺ and the trivalent Fe³⁺ ions occupy both the tetrahedral (A) and octahedral (B) sites, it is classified as a mixed spinel (Figure 5).

Transition metals such as Cu, Zn, Co, Ni, and Mn, when incorporated into spinel ferrites, modify the redox activity of the ferrites [63,64]. Substituting Fe³⁺ with other metals in the octahedral sites affects the strength of the metal–oxygen bonds, enhancing the stability and redox properties of the ferrite [65,66]. Various metals such as Au, Ag, Pt, Pd, Cr, lanthanides, etc., as well as surface modifiers such as SiO₂, ZrO₂, TiO₂, etc., are incorporated into the ferrites to enhance their catalytic activity (Figure 6). The catalytic activity of metal-coated ferrites has been reported in various organic synthesis processes. For example, one-pot synthesis of magnetite nanoparticles coated with Pt has been reported for their magneto-sensitive catalytic applications [67]. Vaddula et al. synthesized the Fe₃O₄–Dopa–Pd nanocatalyst and used it as a catalyst in Heck reactions. They sonicated an aqueous mixture of Fe₃O₄ with dopamine and then reacted it with a methanolic solution of Pd to obtain the catalyst [68]. The photocatalytic properties of FNPs were enhanced by doping them with Ce⁴⁺, Mn²⁺, CO²⁺, and Ni²⁺ metal ions, which were synthesized using the co-precipitation method [69]. Fe₃O₄ modified with Ag was prepared through a thermal decomposition process by reacting Fe(NO₃)₃.9H₂O and AgNO₃ at high temperatures [70]. Fe₃O₄/Au composites were prepared using the seed deposition technique for the catalytic reduction of 4-nitrophenols [71]. Nanoparticles of Al-doped zinc ferrite were synthesized by a thermal treatment method in the presence of polyvinylpyrrolidone (PVP) as a capping agent [72]. Mn-doped La-Ce FNPs were synthesized using the hydrothermal route with an ionic liquid surfactant (ILS), and the doped samples exhibited a transition from ferromagnetism to paramagnetism [73].

Table 1. Reaction conditions with advantages and disadvantages of different methods of synthesis of metal ferrites.

Methods	Example	Reaction Temp (°C)	pH	Particle Size (nm)	Advantages	Disadvantages	Reference
Co-precipitation Method	NiFe2O4	80	12	~8	(1) Simple and easy method (2) The grain size and the homogeneity can be controlled	(1) High calcination temperature for better crystallinity (2) pH control for avoiding agglomeration (3) Broadly size distributed crystals obtained (4) Long reaction time	[74,75,76]
	MnFe2O4	70	11	200–290
Hydrothermal Method	NiFe2O44	150	10	53	(1) Easily accessible method (2) Particles with enhanced morphology (3) Better control of composition	(1) pH needs to be maintained around 9–12 (2) Stabilizing agents such as glycerol, sodium dodecyl sulfate, etc. required (3) Special equipment and high temperature required	[77,78]
	CoFe2O	180	9	34
Sono-chemical method	CuFe2O4	25	6	60	(1) Controlled reaction conditions (2) MNPs with high crystallinity (3) Low working temperature	(1) Less controllable shapes and dispersity (2) Medium yield	[79]
Sol–gel microemulsion	CoFe2O4	110	7	77	(1) Low temperature (2) Particles with controlled shape and size (3) Cheap (4) Homogenous and pure product	(1) Time-consuming (2) Large quantity of solvent required	[56,80]
	CoMn0.2Fe1.8O4	60–70	-	20
Thermal decomposition	MnFe2O4	270	-	18.9	(1) Monodispersed particles with smaller size (2) Better crystallinity (3) Suitable for large-scale production	(1) High temperature required (2) Toxic organic solvents required (3) Costly	[81]
Ball milling	MgFe2O4	-	-	12.3	(1) Simple and convenient (2) Less impurities	(1) High sintering temperature (2) Long-time milling (3) Less control on unwanted phase formation	[82]
Microwave heating	CoFe2O4	100–200	-	-	(1) Short period of time (2) High-quality yield with narrow size distribution (3) Better reproducibility at reasonable cost (4) Efficient heating	(1) Organic solvent (2) Medium yield	[83]

Table 1. Reaction conditions with advantages and disadvantages of different methods of synthesis of metal ferrites.

Methods	Example	Reaction Temp (°C)	pH	Particle Size (nm)	Advantages	Disadvantages	Reference
Co-precipitation Method	NiFe2O4	80	12	~8	(1) Simple and easy method (2) The grain size and the homogeneity can be controlled	(1) High calcination temperature for better crystallinity (2) pH control for avoiding agglomeration (3) Broadly size distributed crystals obtained (4) Long reaction time	[74,75,76]
	MnFe2O4	70	11	200–290
Hydrothermal Method	NiFe2O44	150	10	53	(1) Easily accessible method (2) Particles with enhanced morphology (3) Better control of composition	(1) pH needs to be maintained around 9–12 (2) Stabilizing agents such as glycerol, sodium dodecyl sulfate, etc. required (3) Special equipment and high temperature required	[77,78]
	CoFe2O	180	9	34
Sono-chemical method	CuFe2O4	25	6	60	(1) Controlled reaction conditions (2) MNPs with high crystallinity (3) Low working temperature	(1) Less controllable shapes and dispersity (2) Medium yield	[79]
Sol–gel microemulsion	CoFe2O4	110	7	77	(1) Low temperature (2) Particles with controlled shape and size (3) Cheap (4) Homogenous and pure product	(1) Time-consuming (2) Large quantity of solvent required	[56,80]
	CoMn0.2Fe1.8O4	60–70	-	20
Thermal decomposition	MnFe2O4	270	-	18.9	(1) Monodispersed particles with smaller size (2) Better crystallinity (3) Suitable for large-scale production	(1) High temperature required (2) Toxic organic solvents required (3) Costly	[81]
Ball milling	MgFe2O4	-	-	12.3	(1) Simple and convenient (2) Less impurities	(1) High sintering temperature (2) Long-time milling (3) Less control on unwanted phase formation	[82]
Microwave heating	CoFe2O4	100–200	-	-	(1) Short period of time (2) High-quality yield with narrow size distribution (3) Better reproducibility at reasonable cost (4) Efficient heating	(1) Organic solvent (2) Medium yield	[83]

3. Catalytic Activities of Spinel Ferrites

3.1. Heterogenous Catalysis of Ferrites in Organic Reactions

Simple ferrites, as well as mixed metal ferrites, are gaining importance as catalysts in organic synthesis (Figure 7) [84,85]. The octahedral site of the spinel ferrite exhibits higher catalytic activity compared to the tetrahedral site because of its larger size, which allows for better accommodation of reactant species. Therefore, understanding the preferred site for the metal cation is crucial for exploring the catalytic activity of the ferrite. Ferrites, as heterogeneous catalysts, play a significant role in various reactions. Heterogeneous catalysts are easier to handle, require less workup, and prevent metal contamination. The advantages of ferrites as heterogeneous catalysts include high yields under moderate reaction conditions, the ability to accommodate multifunctional groups, and easy catalyst recovery and recycling. In recent years, the utilization of magnetic nanoparticles (MNPs) as effective catalysts in organic synthesis has significantly increased. The high surface area, fewer coordination sites, and responsive morphologies of ferrite nanocatalysts at the nanoscale enable more efficient processes, offering advantages over conventional methods in organic reactions. Consequently, the reaction rate is maximized with minimal catalyst consumption.

3.1.1. Copper Ferrite

Among spinel ferrites, copper nano-ferrites (CuFe₂O₄) have emerged as one of the most widely used catalysts in various novel reactions. One important catalytic behavior of CuFe₂O₄ is its role in methanol decomposition to CO and H₂ [86]. Another significant application of CuFe₂O₄ is in the conversion of CO to CO₂ [87]. Murthy et al. reported the first synthesis of β, γ-unsaturated ketones and allylation of acid chlorides using CuFe₂O₄ nanomaterial as a recyclable heterogeneous initiator, without any additives, ligands, or co-catalysts [88]. The 20 nm nanocatalyst was prepared by the sol–gel method at room temperature using citrate as a precursor and tetrahydrofuran (THF) as a solvent, which yielded promising results for three consecutive cycles and significantly reduced the reaction time. A wide range of acid chlorides was reacted with cinnamyl chloride as well as allyl bromide under the mentioned conditions, although the reaction with long-chain aliphatic acid chloride did not yield allyl ketone (Scheme 1). The acylation of allyl halides by CuFe₂O₄ nanocatalyst was not affected by electronic effects or steric effects. It was reported that all acylation reactions were followed by allylic rearrangement [89,90] to eliminate the double bond from the conjugation. The mechanism involves the electrophilic attack of CuFe₂O₄ nanocatalyst at the γ carbon atom of the allylic group without any prototropic rearrangement, which then reacts with the acyl halides, resulting in the formation of stable allyl ketone, as confirmed by IR and NMR spectral studies.

In another study, CuFe₂O₄ nanoparticles were employed for the C–O coupling in the Ullmann reaction, which involved various aryl halides and phenols, as well as amino, methoxy, ketone, methyl, cyano, and halide derivatives (Scheme 2). The catalyst exhibited high yields without requiring any form of protection for sensitive functional groups such as MeCO, CN, and NH₂ [91].

In CuFe₂O₄, Cu(II) is a more active species compared to Cu(I), which enables easier electron transfer for the oxidative coupling reaction with an aryl halide. Utilizing 10 mol% of catalysts, a yield of 92% was achieved in the sixth run. This was further confirmed by the X-ray diffraction (XRD) patterns, which indicated that the crystal structure of the CuFe₂O₄ nanoparticles remained intact after six runs. These results explain the excellent recyclability and high stability of the magnetic catalyst.

A one-pot synthesis of α-aminonitrile derivatives was reported, involving the condensation of different aldehydes with amines and trimethylsilyl cyanides in the presence of CuFe₂O₄ nanocatalyst. Water was used as the solvent, and AcOH served as the promoter at room temperature [92]. The enhanced catalytic activity was attributed to the availability of active sites. The proposed mechanism involved the attack of the acidic proton of CuFe₂O₄ nanocatalyst in AcOH on the aldehydes, followed by a simultaneous reaction with the amines to form a major intermediate (Scheme 3). The catalyst exhibited recyclability and reusability for four cycles with no significant loss in activity. It was observed that benzaldehyde derivatives provided better yields compared to ketone derivatives, regardless of the substitution on the aromatic ring. Additionally, substituted benzaldehydes and substituted aryl amines yielded improved results.

A ligand-free, simple, efficient, and magnetically recoverable method for synthesizing aldehydes and ketones via the oxidative decarboxylation of phenylacetic acid has been reported using CuFe₂O₄ nanocatalyst [93]. The proposed mechanism involves the decarboxylation of phenylacetic acid, leading to the generation of active Cu species. These active Cu species further undergo oxidation through peroxy cuprate, resulting in the formation of the corresponding aldehyde (Scheme 4). The yield of the desired product was found to be higher when an electron-donating group was attached to the aromatic ring, as compared to an electron-withdrawing group. Furthermore, the position of the substituent on the aromatic ring was observed to impact the product yield, with ortho substituents exhibiting lower yields compared to meta and para substituents. Steric hindrance was identified as a contributing factor to the reduced yield in ortho-substituted compounds.

In a novel study, Cu nanoparticles synthesized through a green method using neem leaf extract were employed for the oxidation of various toluene derivatives under mild reaction conditions without the use of any solvent, resulting in an excellent yield of benzoic acid (Scheme 5). The catalyst exhibited recyclability for up to five cycles without any loss of activity [94]. Derivatives with electron-donating groups exhibited higher yields compared to derivatives with electron-withdrawing groups, suggesting that toluene derivatives with electron-donating groups were more reactive in the oxidative reaction. Notably, the oxidation of toluene derivatives with electron-withdrawing groups, such as 4-nitrotoluene, required harsh reaction conditions for a successful conversion. Additionally, it was observed that the electron-rich 2-hydroxytoluene provided an excellent yield. Interestingly, substituted toluene derivatives underwent oxidation efficiently, irrespective of the substituent group.

The catalyst was washed with ethanol and then reused for up to four cycles without any loss of activity in the oxidation of toluene. However, after the fifth cycle, the reactivity decreased due to a change in the morphology of the nanoparticles. CuFe₂O₄ catalysts were also employed for the synthesis of biaryls from aryl-boronic acid (Scheme 6) using various solvents such as methanol, THF, ethanol, etc. Additionally, the catalyst was used for catalyzing the oxidation of various aryl and aromatic alcohols [95]. The maximum catalyst recovery, shortest reaction time, and highest yield (83–96%) were achieved when using methanol as the solvent.

CuFe₂O₄ exhibited remarkable catalytic activity in the dehydrogenation of ethanol to acetaldehyde (97%) and hydrogen at 300 °C, with selectivity shifting towards acetone, CO₂, and hydrogen at higher temperatures of 400–500 °C [96]. The reversible transition of Cu²⁺ to Cu¹⁺ in the crystal structure of the ferrite is responsible for the high activity of CuFe₂O₄ in the oxidation-reduction–dehydrogenation process leading to the formation of acetaldehyde. The dissociation of the O-H bond of the ethanol molecule occurred on the [Cu-O] pair. The resulting ethoxy species bonded to the cupric ion, while the hydrogen atom bonded to the oxygen ion on the catalyst surface. The second hydrogen atom was released through the desorption of acetaldehyde, followed by the formation of a hydrogen molecule through the recombination of hydrogen atoms.

CuFe₂O₄ doped with molybdenum was studied as an active catalyst for the gas-phase oxidative coupling reaction of methanol and ethanol, resulting in the formation of hydroxy acetone, methyl acetate, ethyl acetate, ethyl–methyl–ether, with small amounts of dimethyl–ether, diethyl–ether, formaldehyde, and acetaldehyde [97]. The conversion of ethanol increased with temperature and molybdenum concentration. Higher catalytic activity was observed for a greater Cu¹⁺/Cu²⁺ ratio as well as Mo⁵⁺/Mo⁶⁺ ratios, while a higher Fe²⁺/Fe³⁺ ratio resulted in selectivity towards hydroxy acetone. The gain of electrons from interstitial oxygen led to the reduction of Cu²⁺ to Cu¹⁺, Fe³⁺ to Fe²⁺, and Mo⁶⁺ to Mo⁵⁺ or Mo⁴⁺, respectively. The unstable interstitial oxygen, upon losing electrons, became highly mobile and escaped from the lattice, transforming into active oxygen (O−) responsible for the observed transformation reaction.

A similar study was conducted on the aerobic oxidation of aqueous bioethanol in a liquid phase to acetic acid using a mild hydrothermal environment, employing Au-supported CuFe₂O₄ nanoparticles [98]. The enhanced activity of ethanol and oxygen was attributed to the oxidation-reduction activity of Fe²⁺ ions in conjunction with negatively charged Au^δ−. Efficient and environmentally friendly CuFe₂O₄-catalyzed oxidation of aryl alcohols to aldehydes was investigated using water as the solvent in the presence of dioxygen [99]. A novel magnetic nanocomposite, CuFe₂O₄ with carbon quantum dots (CQDs) derived from gelatin, was successfully synthesized for the reduction of ortho and para nitroaniline (ONA, PNA) using NaBH₄ as a reducing agent in an aqueous medium at 25 °C (Scheme 7) [100]. The nanocatalyst exhibited excellent reduction performance for nitroanilines due to the easy electron transfer from the negative BH₄⁻ ion to ortho and para nitroanilines (PNA and ONA) (Scheme 8). The Fermi level shift of CuFe₂O₄ nanocomposites facilitated electron acceptance. The recyclability study demonstrated that the catalytic activity of the reusable catalyst remained high, changing insignificantly from 98.8% to 96% for PNA reduction and from 96.16% to 90% for ONA after six runs.

CuFe₂O₄ is also utilized as a heterogeneous catalyst in the photosensitive Fenton process due to its narrow band gap, excellent response to visible light, low cost, and high photochemical stability [101]. Shi et al. developed soft magnetic CuFe₂O₄ with SiO₂ nanofibrous membranes, which exhibited efficient Fenton catalytic activity for organic pollutants (Scheme 9). The degradation rate reached 96% within 20 min, and the catalyst demonstrated exceptional recyclability (Figure 8) [102].

Copper ferrite (CuFe₂O₄) nanoparticles were synthesized using a microwave ignition reaction and coated with extracted chlorophyll obtained from mulberry green leaves to enhance the photocatalytic degradation of methylene blue dye (MB) [103]. In another study, a novel catalyst consisting of copper–nickel ferrite (CuNiFe₂O₄) loaded onto multi-walled carbon nanotubes (MWCNTs) was prepared through a coprecipitation method. This catalyst was employed in the sonophotocatalytic degradation process of Acid Blue 113 (AB113) dye (Figure 9) [104]. The catalyst exhibited remarkable efficiency in terms of dye removal (100%), total organic carbon (93%), and chemical oxygen demand (95%) under the following conditions: pH of the dye solution = 5, MWCNT-CuNiFe₂O₄ dosage = 0.6 g/L, AB113 dye concentration = 50 mg/L, UV light intensity = 36 W, ultrasonic wave frequency = 35 kHz, and treatment time = 30 min. The main reactive species responsible for the degradation of AB113 dye were holes (h⁺) and hydroxyl radicals (•OH).

3.1.2. Zinc Ferrite

Zinc nano ferrite has been utilized as a catalyst in the oxidation of organic reactions. Neodymium-doped ZnFe₂O₄ was investigated as a catalyst in the oxidative conversion of methane and coupling reactions [105]. There was a strong relationship between the catalytic effect and the structure of the oxide, along with the defects created as a result of substitution. The catalytic activities toward the coupling reaction followed the order:

ZnFe_1.5Nd_0.5O₄ < ZnFeNdO₄ < ZnFe_1.75Nd_0.25O₄ < ZnFe₂O₄ < ZnNd₂O₄

The important aspect was the redox nature of the reaction mixture, which played a crucial role in creating definite lattice defects through the removal of lattice oxygen associated with Fe³⁺ reduction. The positive charge resulting from the reduction of Fe³⁺ to Fe²⁺ was compensated by the presence of oxygen defects. The introduction of octahedrally coordinated Nd³⁺ ions generated new types of defects that facilitated methane combustion. ZnFe₂O₄ nanocatalysts exhibited the conversion of methanol, a substitute green fuel used in fuel cells, to CO and H₂ at relatively lower temperatures [106]. Catalytic tests demonstrated a significant transformation of the ferrite phase due to the influence of the reaction medium, as confirmed by Mössbauer spectra. Pd-substituted ZnFe₂O₄ was investigated for Heck and Suzuki cross-coupling reactions under aerobic conditions without the need for any ligands [107]. The substituted ZnFe₂O₄ exhibited excellent catalytic activity. A wide range of aryl halides and boronic acids were studied for the Suzuki reaction under optimized conditions (Scheme 10). The catalyst loading was low, and it could be easily recovered and reused (

Table 2. Activity of Pd-ZnFe₂O₄ nanocatalyst for Heck and Suzuki reaction (adapted from with permission from Ref. [107], 2013, Catalysis Communications).

	Recycle Runs	Conversion (%)	TOF (h−1)
Suzuki reaction	1st	100	542
	2nd	100	542
	3rd	99	528
	4th	99	528
	5th	98	516
Heck reaction	1st	100	1084
	2nd	100	1084
	3rd	99	1073
	4th	98	1063
	5th	97	1051

). Similarly, for the Heck reaction, various aryl halides with different alkenes were considered, except for aryl chloride, which was not catalyzed by the substituted ZnFe₂O₄ [22].

A magnetic-recyclable catalyst, Pd-doped ZnFe₂O₄ (ZnFe₂O₄@PDA/COF@Pd), was synthesized using a covalent organic framework (COF) as a carrier and polydopamine as a crosslinking agent. The catalyst demonstrated excellent catalytic activity in Suzuki coupling and the reduction of p-nitrophenol [108]. The COF prevented the aggregation and leaching of Pd nanoparticles, thereby enhancing the activity and stability of the catalyst. In another study, superparamagnetic Pd-ZnFe₂O₄ nanoparticles were effectively employed as a heterogeneous catalyst in carbon–carbon and carbon–oxygen cross-coupling reactions, including the Heck–Matsuda reaction for diazonium salt in an aqueous medium (Scheme 11), Sonogashira coupling reaction with ethanol, cyanation of aromatic halides without cyanide (Scheme 12), and Ullmann coupling reaction with phenols to obtain the corresponding coupled products (Scheme 13) [109]. These C–C coupling reactions failed to proceed without the catalyst, highlighting the significance of the catalyst in the reactions.

The catalytic activity of ZnFe₂O₄, doped with nitrogen and sulfur and synthesized through co-precipitation and wet impregnation processes, was investigated for the complete degradation of 4-chlorophenol [110]. Non-metal doping had an adverse impact on the structure, enhancing both the stability and activity of the catalyst. Nitrogen and sulfur doping increased the chemical oxygen demand (COD) removal efficiency to 98%. No leaching of Fe was observed, indicating a heterogeneous reaction mechanism. The magnetically separable catalyst exhibited stability, as demonstrated by phase analysis using X-ray diffraction (XRD), and could be reused for five consecutive cycles (

Table 3. Stability of the catalyst during the persulfate-activated oxidation of 4-CP (adapted from with permission from Ref. [110], 2022, Journal of Physics and Chemistry of Solids).

Number of Cycles	4-CP Conversion (%)	Reduction of COD
1	100	98.2
2	100	97.6
3	98.2	92.9
4	92.4	90.5
5	85.8	76.4

Reaction conditions: Temp = 30 °C, 4-CP: persulfate ratio= 1:3, Catalyst dosage = 0.1 g, 4-CP Concentration = 1 g/L, Time 60 min.

). The radicals formed through electron transfer from the metal ion in the catalyst to persulfate were responsible for the reaction, as shown in Equations (1)–(3).

Fe³⁺ + HSO₅⁻ → Fe²⁺ + SO₅²⁻ + H⁺

(1)

Fe²⁺ + HSO₄⁻ → Fe³⁺ + SO₄²⁻ + OH⁻

(2)

Fe³⁺ + PMS → Fe²⁺ + SO₄²⁻

(3)

The spinel structure of zinc ferrite crystals enhances the stability of the ferrite, resulting in improved efficiency. The photocatalytic decomposition of organic contaminants in water using zinc ferrites has been extensively studied. For example, ZnFe₂O₄ exhibits photocatalytic activity, but its efficiency can be further enhanced by coupling it with another semiconductor or by coupling it with carbon nanotubes and graphene (Figure 10). A magnetically separable graphene oxide nanocomposite, ZnFe₂O₄, was synthesized as an efficient and stable catalyst for the degradation of MB dye in aqueous solutions (Figure 11) under solar radiation [111]. The catalyst effectively degraded the MB dye through the formation of free radicals, as shown in Equations (4)–(8).

$${\mathrm{ZnFe}}_2{\mathrm{O}}_4 \stackrel{hv}{\to }({e^{-}}_{\mathrm{CB}} + {h^{+}}_{\mathrm{VB}}) {\mathrm{ZnFe}}_2{\mathrm{O}}_4$$

(4)

$$\mathrm{Graphene}+{e^{–}}_{\mathrm{CB}}({\mathrm{ZnFe}}_2{\mathrm{O}}_4) \stackrel{}{\to }{e}^{-•}(\mathrm{Graphene})$$

(5)

$${\mathrm{O}}_2+e^{–} (\mathrm{Graphene}) \stackrel{}{\to }{{\mathrm{O}}_2}^{•-}$$

(6)

$${{\mathrm{O}}_2}^{•-}+\mathrm{MB}\stackrel{}{\to }\mathrm{Degradation} \mathrm{product}$$

(7)

$${h^{+}}_{\mathrm{VB}}{\mathrm{ZnFe}}_2{\mathrm{O}}_4+\mathrm{MB} \stackrel{}{\to }\mathrm{Degradation} \mathrm{product}$$

(8)

In recent studies, a composite of spinel zinc ferrite (ZnFe₂O₄) and cellulose was successfully fabricated as an effective photocatalyst. The composite efficiently degraded methylene blue (MB) by 100% after 180 min at pH 6.5 and demonstrated reusability for up to four regeneration cycles [112]. SDS (sodium dodecyl sulfate)-coated Zn ferrites were utilized as excellent magnetic adsorbents for the removal of violet dye from wastewater across a wide range of dye concentrations [113]. Biocomposite-based materials such as zinc ferrite-chitosan (ZFN-CS) were capable of removing crystal violet and brilliant green dyes under ambient conditions in both simple and binary systems [114]. A ZnFe₂O₄ photocatalyst was synthesized using a facile reduction-oxidation method to investigate its catalytic activity for the decolorization of Orange II dye under Vis/ZnFe₂O₄/PS (persulfate, S₂O₈²⁻) conditions [115]. The reaction was initiated by SO₄•⁻ through electron transfer attack, while •OH contributed via electrophilic addition or hydrogen abstraction reactions on the N=N azo form of Orange II, resulting in the formation of 4-aminobenzenesulfonic acid and 1-amino-2-naphthol (Figure 12). Simultaneously, electrophilic addition of •OH occurred on the C=N hydrozone form of Orange II, leading to the production of 2-naphthol and 4-hydrazinebenzenesulfonic acid. The ZnFe₂O₄ photocatalyst exhibited high activity and stability with minimal iron and zinc leaching throughout the various experimental cycles, as shown in Equations (9)–(15).

$${\mathrm{S}}_2{\mathrm{O}}_8{2}^{-}+\equiv \mathrm{Fe}(\mathrm{II}) \stackrel{}{\to }{{\mathrm{SO}}_4}^{•-} + {{\mathrm{SO}}_4}^{2-} + \equiv \mathrm{Fe}(\mathrm{III})$$

(9)

$${{\mathrm{SO}}_4}^{•-}+{\mathrm{H}}_2\mathrm{O} \stackrel{}{\to }{{\mathrm{SO}}_4}^{2-} + •\mathrm{OH} + {\mathrm{H}}^{+} \mathrm{k} = 8.3·{\mathrm{M}}^{-1}·{\mathrm{S}}^{-1}$$

(10)

$${\mathrm{ZnFe}}_2{\mathrm{O}}_4+\mathrm{h}\mathsf{ὐ} \stackrel{}{\to }{\mathrm{ZnFe}}_2{\mathrm{O}}_4 ({e^{-}}_{\mathrm{CB}} + {h^{+}}_{\mathrm{VB}})$$

(11)

$${h^{+}}_{\mathrm{VB}}+{\mathrm{H}}_2\mathrm{O}\stackrel{}{\to } {\mathrm{H}}^{+} + •\mathrm{OH}$$

(12)

$${h^{+}}_{\mathrm{VB}}+{\mathrm{OH}}^{-} \stackrel{}{\to } •\mathrm{OH}$$

(13)

$$\mathrm{Fe}(\mathrm{III})+{e^{-}}_{\mathrm{CB}} \stackrel{}{\to } \mathrm{Fe}(\mathrm{II})$$

(14)

$${{\mathrm{S}}_2{\mathrm{O}}_8}^{2-}+{e^{-}}_{\mathrm{CB}} \stackrel{}{\to }{{\mathrm{SO}}_4}^{•-} + {{\mathrm{SO}}_4}^{2-} \mathrm{k} = 1.1 \times {10}^{10}·{\mathrm{M}}^{-1}·{\mathrm{S}}^{-1}$$

(15)

3.1.3. Nickel Ferrite

Nanosized nickel ferrite, whether in pure or doped form, finds application in various catalytic reactions. For instance, NiFe₂O₄ has been utilized as an effective catalyst in the water–gas shift (WGS) reaction within the industry [116]. Comparatively, the activity of NiFe₂O₄ (111) surfaces was found to be significantly higher than that of Fe₃O₄ (111) surfaces, as demonstrated by thermodynamic and kinetic studies on water adsorption and dissociation using the DFT + U approach. This disparity had a direct impact on catalytic activity. Regarding the dissociation of a single H₂O molecule, the activation barrier was low (0.18 eV) for the 0.25 ML Fe_tet1 termination, while there was no barrier for the 0.5 ML Fe_oct2–tet1 termination (Figure 13). This highlights the significance of water dissociation as a major controlling factor in the WGS reaction. The surface reactivity stemmed from the interaction between the 3σ orbitals of OH and the d orbitals of the surface Fe atom, resulting in the formation of new bonding orbitals that shifted the energy axis deeper, thereby leading to a greater energy gain and stronger water adsorption for the 0.5 ML Fe_oct2–tet1 termination.

CoFe₂O₄/NiFe₂O₄ was employed as a catalyst for the chemical looping reaction involving methanol and ethanol–water mixtures, which were subsequently deoxidized with CO₂ to produce CO [117]. Ni ferrospinels were tested as catalysts for the total oxidation of propane to CO₂ within the temperature range of 250–600 °C [118]. Nickel ferrite is also utilized in various coupling reactions. NiFe₂O₄-supported Pd nanoparticles were investigated as highly active and reusable catalysts in Sonogashira cross-coupling reactions, demonstrating high conversion rates even after the tenth run [119]. The redox behavior of NiFe₂O₄ imparted high activity and stability to the catalyst by creating oxygen defects that facilitated electron transfer between the metal and support. NiFe₂O₄ was studied as a catalyst without the use of copper for the Sonogashira reaction in a green solvent (water) (Scheme 14). Several aromatic and alkyl halides were effectively coupled with phenylacetylene under optimal reaction conditions (Scheme 15), resulting in excellent yields (Figure 14) in a short period of time [120]. A similar study was conducted using Fe₃O₄@Ni nanoparticles by using Euphorbia aculate extract that acted as a stabilizing agent and also served the purpose of a reducing agent and showed amazing catalytic activity in Sonogashira coupling and A³ coupling reactions [121].

Ferrite-nickel nanoparticles (Fe₃O₄-Ni MNPs) were investigated as heterogeneous catalysis for hydrogen-transfer reactions in the reduction of nitro and carbonyl compounds (Scheme 16) using glycerol as a hydrogen donor as well as solvent [122].

A robust magnetic nanocatalyst of NiFe₂O₄ was synthesized through a hydrothermal route for investigating the Claisen–Schmidt condensation between various aldehydes and acetylferrocene, including both aryl and heterocyclic compounds (Scheme 17). The catalyst demonstrated a high yield of acetylferrocene chalcones. The effect of different groups on the reaction rate and yield was examined [123]. The presence of more electronegative and conjugated phenyl groups activated the reaction more effectively, resulting in improved yield and reaction time.

In another study, the catalytic performance of Cu-incorporated NiFe₂O₄ nanoparticles was investigated for the reduction of nitroarenes to arylamines in the presence of NaBH₄ [124]. Similarly, Ni-doped magnetic reusable nanoparticles of CoFe₂O₄ were successfully employed as catalysts for the first time in the C–O coupling reaction between phenol derivatives and various aromatic halides [125]. A microwave irradiation method using NiFe₂O₄ MNPs as catalysts (Scheme 18) was reported for the facile synthesis of polyhydroquinoline derivatives. The catalyst exhibited efficient and effective activity (

Table 4. Comparison of the catalytic efficiency of NiFe₂O₄ MNPs with known catalysts.

Catalyst	Time (min)	Yield%	Reference
Nanocrystalline copper(II) oxide	35	80	[127]
ZnFe0.2Al1.8O4	32	86	[128]
Yb(Otf)3	480	90	[129]
Scolecite	60	91	[130]
ZnO Nps	20	86	[131]
NiFe2O4 MNPs	4	94	[126]

) compared to other known catalysts [126].

The reactions were carried out with 4-hydroxybenzaldehyde (1 mmol), dimedone (1 mmol), ammonium acetate (1.5 mmol), ethyl acetoacetate (1 mmol), and 23.4 mg of NiFe₂O₄ MNPs (10 mol%).

Various aromatic aldehydes with different electron-withdrawing or electron-donating groups at various positions (ortho, meta, and para) on the aromatic ring, as well as heterocyclic aldehydes, were examined in this study. It was observed that aldehydes with electron-donating groups required a longer reaction time. In a separate catalytic investigation using nickel ferrite, p-nitrophenol (P-NP) was reduced with NaBH₄ in the presence of Ag nanoparticles supported on magnetic NiFe₂O₄/oxidized graphite, and the catalyst exhibited remarkable activity [132].

NiFe₂O₄ has demonstrated significant photocatalytic activity. Qu et al. conducted a study using a novel magnetic catalyst composed of silica-supported spinel NiFe₂O₄ for heterogeneous Fenton, such as oxidation of rhodamine B [133]. The results are illustrated in Figure 15.

NiFe₂O₄ nanoparticles were synthesized using urea and glycine as a mixed fuel and were studied for the degradation of MB as a water contaminant under sunlight (Figure 16) [134]. The nanocatalyst exhibited good photocatalytic activity against dye degradation and demonstrated reusability up to four cycles.

Binary composites based on nickel ferrite, such as Ni_0.8Zn_0.2Fe₂O₄ nanoparticles, showed a high degradation efficiency of the order of 98.48% compared to other samples, which demonstrated good photocatalytic activity and reusability against dye degradation. NiFe₂O₄–based nanocomposites, including binary and ternary composites, have exhibited excellent performance in water treatment as photocatalysts for the removal of various organic pollutants such as MO, MB, RB, CR, phenol, and antibiotics. These nanocomposites demonstrated maximum efficiency in eliminating the contaminants [135].

3.1.4. Cobalt Ferrite

The strong magnetic anisotropy, good magnetization, and high coercivity of cobalt ferrite (CoFe₂O₄) at mild temperatures make it a commercially significant ferrite. CoFe₂O₄ nanoparticles with a size range of 40–50 nm were synthesized using the co-precipitation method. These nanoparticles were employed to investigate the aldol condensation of various aromatic aldehydes and acetophenone derivatives in an ethanolic solution (Scheme 19) [136].

A good yield of α, β-unsaturated ketones was obtained, and the catalyst was efficiently recovered using an external magnet. CoFe₂O₄, synthesized using the wet chemical method, was employed as a novel catalyst for the synthesis of methanol from CO₂ through hydrogenation [137]. The catalytic activity was found to increase with higher calcination temperatures. At elevated temperatures, there was a higher distribution of cations at octahedral sites, resulting in improved catalytic activity for methanol formation. The oxidation of alcohol is a crucial aspect of the modern chemical industry. Magnetically recoverable CoFe₂O₄ served as a catalyst for the oxidation of primary and secondary aliphatic alcohols, as well as benzylic alcohols, to their respective carbonyl products in the presence of water using oxone (potassium hydrogen monopersulfate) as the oxidizing agent (Scheme 20) [138]. The oxidation of various benzylic alcohols exhibited high yields of carbonyl compounds in short reaction times (

Table 5. Recycling of CoFe₂O₄ catalyst for the oxidative transformation of 2- chlorobenzyl alcohol to 2-chlorobenzaldehyde.

Entry	Time (min)	Yield%
1	30	90
2	30	88
3	30	89
4	30	90
5	30	90
6	30	88

). The presence of electron-withdrawing groups significantly reduced the reaction rate, while electron-donating groups on the benzene ring accelerated the reaction. It was observed that aliphatic alcohols underwent slow oxidation, and there were no reports of overoxidation of aldehydes to corresponding carboxylic acids. The catalyst could be easily separated and reused for more than six cycles without a significant decrease in activity.

The oxidation of alcohols using cobalt ferrite was investigated by Bhat et al. [139], who utilized active Ni(OH)₂ coated with CoFe₂O₄. Additionally, Dhar et al. studied the production of corresponding aldehydes [140] using CoFe₂O₄ nanoparticles doped with Cr and Zn, supported by graphene oxide nanosheets grafted with guanidine. These catalysts were examined for the aerobic oxidative reaction of alcohols and alkyl arenes, resulting in the formation of respective aldehydes and ketones. This was also investigated for the oxidative one-pot synthesis of 2-phenylbenzo [d]oxazole derivatives (Scheme 21) [141].

CoFe₂O₄ was also utilized in the oxidation of alkenes. Silica-coated CoFe₂O₄ nanoparticles, modified with a Schiff base molybdenum complex, were effectively employed as heterogeneous catalysts in the oxidative reaction of several alkenes using t-BuOOH as the oxidant (Scheme 22) [142]. The study demonstrated that CoFe₂O₄ nanoparticles efficiently catalyzed the conversion of alkenes into their corresponding aldehydes or epoxides, providing excellent yields (Scheme 23). The catalyst exhibited outstanding efficiency and recyclability, showcasing its potential as an alternative to numerous heterogeneous molybdenum Schiff base catalysts reported to date.

Considering carbon–carbon cross-coupling reaction, the Pd-supported CoFe₂O₄ nanocatalyst was inspected for Suzuki coupling reaction (Scheme 24) in presence of ethanol without any ligand [143]. The reaction required less amount of catalyst loading (1.6 mol%) and showed good catalytic recovery and reusability for multiple cycles.

CoFe₂O₄ nanoparticles were also employed in C–O bond formation to catalyze the reaction between various types of aryl halides and phenol derivatives [125]. The comparison of catalytic activity for the same reaction with nickel ferrite and cobalt ferrite, under similar conditions for all types of aryl halides, showed that NiFe₂O₄ completed the reaction in a shorter time, suggesting its better activity compared to CoFe₂O₄ (Figure 17).

A modified spinel CoFe₂O₄, incorporating varying amounts of Hf (IV), was synthesized using the sol–gel method with the gelating agent propylene oxide. The catalyst was investigated for its effectiveness in the conversion of ethyl acetate to CO₂ [144]. The activity of the Hf (IV) modified CoFe₂O₄ varied as:

CFO-Hf_0.1 < CFO-Hf_0.01 < CFO-Hf_0.5 < CFO-Hf

A strong correlation between the activity and specific surface area was reported. The CFO-Hf₁, with a specific surface area of 16 m²/g, was the most active, while CFO-Hf_0.1 having a surface area of 3 m²/g, was the least active. Hf (IV) was introduced to replace Fe in the CoFe₂O₄ structure, resulting in a slight increase in lattice parameters and micro-strain within the crystal lattice, along with a reduction in the average crystallite size. CoFe₂O₄, modified with diamine-N-sulfamic acid, was investigated as a heterogeneous catalyst for the production of amides through the Ritter reaction (Scheme 25) without the need for any solvent [145]. The catalyst exhibited recyclability and maintained its catalytic activity significantly over multiple cycles.

A similar nanocomposite consisting of silica and phosphotungstic acid (PTA)-coated CoFe₂O₄@SiO₂-PTA was examined for its catalytic properties in the N-formylation reaction (Scheme 26) involving various amines. The reaction was conducted without the use of any solvent and resulted in the production of formamides with high yields and in a significantly reduced reaction time [146].

At first, the carbonyl group present in formic acid coordinated with the PTA of the catalyst and got activated to form an intermediate. The carbonyl moiety of the intermediate then interacted with aniline via the NH₂ group forming another intermediate that produced the respective formamide by transferring the proton to the catalyst. In another mechanism, PTA was coordinated to formic acid as well as aniline for the formation of the intermediate, which rearranged and transferred the proton to give the product (Scheme 27).

In recent studies, a robust nanocatalyst of CoFe₂O₄ was prepared using the sol–gel technique by reacting o-phenylenediamine with aldehydes and was investigated as an effective catalyst in the formation of derivatives of 2-aryl benzimidazole (Scheme 28) in the absence of any solvent [147].

Studies have shown a rapid reaction within a short time frame when dealing with aldehydes that have electron-withdrawing substituents. This is due to the enhanced electrophilic nature of aryl aldehydes. Researchers explored the use of a CeZrO₂ composite consisting of CoFe₂O₄/NiFe₂O₄ with varying weight percentages (x = 20–100%) for the chemical looping reduction of methanol and a mixture of ethanol and water (with a molar ratio of 1:1) separately, aiming to produce CO with the assistance of the catalyst [117]. The highest production of CO was achieved using 20 wt% CoFe₂O₄ in the chemical looping process with methanol. In the case of ethanol, it was decomposed into CO, along with CH₄ and H₂, but the conversion of CH₄ was not facilitated without the presence of a configured pre-catalyst bed. The oxidation of carbon resulted in a high yield of CO when reacting with methanol. However, when reacting with a mixture of ethanol and water, the yield of CO remained low, at approximately 13 mol CO per kg CoFe₂O₄, due to the unconverted presence of CH₄ and H₂O (Scheme 29).

The photocatalytic performance of CoFe₂O₄ nanostructures was investigated, demonstrating the ability to degrade 91% of Congo red dye within 90 min at pH 9 [148]. Additionally, cobalt ferrite nanospheres were synthesized and tested for their photocatalytic degradation efficiency using Alizarin Red S (ARS) as a model pollutant. It was observed that after 1.5 h, the degradation rates were 82% for bare nanospheres, 87% for annealed nanospheres, and 91% for surface-treated nanospheres of cobalt ferrite [149].

Cobalt ferrite nanoparticles were prepared by the sol–gel method, and the photocatalytic degradation of MB and Evans Blue (EB) under visible light irradiation was evaluated (Figure 18) [150].

$${\mathrm{CoFe}}_2{\mathrm{O}}_4 \stackrel{hv}{\to }({e^{-}}_{\mathrm{CB}} + {h^{+}}_{\mathrm{VB}}){\mathrm{CoFe}}_2{\mathrm{O}}_4$$

(16)

CoFe₂O₄ (e⁻_CB) + O₂ →O₂^•−

(17)

H₂O → H⁺ + OH⁻

(18)

h⁺_VB + OH⁻ → •OH

(19)

O₂^•− + H⁼ →•OOH

(20)

•OOH + e⁻ → HOO^–

(21)

HOO⁻ + H⁺ → H₂O₂

(22)

H₂O₂ + e⁻ → •OH + OH⁻

(23)

CoFe₂O₄ + (h⁺_VB) + H₂O →•OH + H⁺

(24)

Dye (MB and EB) + •OH → CO₂ + H₂O (Byproduct)

(25)

Dye (MB and EB) + CoFe₂O₄ {h⁺_VB} → Oxidation product

(26)

Dye (MB and EB) + CoFe₂O₄ {e^–_CB} → Reduction product

(27)

The photocatalytic activity of the CoFe₂O₄ photocatalyst is initiated by visible light, which leads to the excitation of electrons from the valence band (VB) to the conduction band (CB), creating a hole in the VB (Equation (16)). The produced holes reacted with the surface H₂O or OH ion, forming hydroxyl radicals (^●OH), and the electrons of the CB reacted with the dissolved O₂ molecule forming superoxide radicals (^●O²⁻) (Equations (17)–(19)). The e⁻/h⁺ pairs recombination was inhibited by ^●O²⁻. The protonation of hydroperoxyl radicals produced hydrogen peroxide (H₂O₂), which further dissociates to give ^●OH (Equations (20)–(24)). The strong oxidizing species generated on the surface of CoFe₂O₄ react with the organic dye, resulting in the formation of H₂O and CO₂ (Equations (25)–(27)). To investigate the photolytic effects in more detail, cobalt zinc ferrites (CZF) were synthesized with the composition CoxZn_1−xFe₂O₄ using the citrate precursor method [151]. Among the different compositions tested, Co_0.5Zn_0.5Fe₂O₄ exhibited the highest degradation rate of 77% within 1 h under visible light for the degradation of MB dye, while also having no adverse impact on the environment.

3.1.5. Manganese Ferrite

Manganese ferrite magnetic nanoparticles show good catalytic activity. Several studies have been reported on using MnFe₂O₄ as catalysts in many organic reactions. Magnetically separable Pd doped MnFe₂O₄ nanoparticles were employed as a catalyst for the decarboxylative Sonogashira reaction, resulting in the formation of diarylacetylenes, which involves the reaction of phenyl propiolic acid and arene diazonium salts [152]. The catalyst was prepared using a one-pot co-precipitation method, aided by ultrasound, without the presence of any ligand. Synthesis of symmetrical and unsymmetrical diaryl acetylenes was done without any ligand and co-catalyst (Scheme 30). Arene diazonium salts, both electron-rich and electron-deficient, produced excellent results. There was a slight steric effect observed at the ortho position of the arenediazonium salt. The proposed mechanism involved the in situ formation of iodobenzamide, which subsequently underwent oxidative addition to the nanocatalyst in the presence of an alkynyl carboxylic acid (transmetallation). Finally, through decarboxylation followed by reductive elimination, diaryl acetylene was produced.

The catalytic properties of a core-shell structured manganese oxide supported by MnFe₂O₄ were investigated in the oxidation reaction of benzyl alcohol (BzOH) using air as the oxidant and hexane as the solvent within a temperature range of 60–90 °C [153]. The magnetic nanocatalyst displayed moderate activity but exhibited 100% selectivity towards benzaldehyde (BzH) under ambient conditions (Scheme 31). It was observed that solvents with nucleophilic properties hindered the interaction between the alcohol and the activated Mn sites. The formation of benzaldehyde increased with longer reaction times and higher temperatures; however, the selectivity towards dibenzyl ether decreased.

Single-domain MnFe₂O₄ was investigated for the catalytic conversion of cyclohexene in a flow system at temperatures ranging from 200 to 400 °C [154]. The reaction was examined using different catalysts, which facilitated dehydrogenation and hydrogenation reactions, leading to the production of benzene (as the main product), along with cyclohexane and methyl cyclopropane as byproducts. The catalytic activity was found to increase with an increase in the ratio of glycine to nitrates (G/N) and a decrease in both the BET surface area and the activation energy of the catalyzed reaction. The improved activity was attributed to the enhanced formation of MnFe₂O₄ crystallites, which acted as an active dehydrogenation catalyst.

Solid nanospheres (MSN) of MnFe₂O₄, prepared using the solvothermal technique, were employed as catalysts for the generation of hydroxyl radicals (HO•) and sulfate radicals (SO4•⁻) from potassium peroxymonosulfate (PMS). These radicals were utilized for the degradation and mineralization of para-nitrophenol (PNP) present in water [155]. PMS served the role of oxidant that was activated by the catalyst to generate SO₄^•− and HO^• radicals, whose formation in return was enabled by redox cycles between the oxidation states (II) and (III) of Fe and Mn (Scheme 32). The catalyst was able to remove 100% of PNP in 7 h using 4 mM of PMS and 0.5 g L⁻¹ of MSN at 40 °C and pH 5.8.

In a recent development, MnFe₂O₄-supported magnetically separable palladium catalysts were reported for the hydrogenation of nitrobenzene to aniline (Scheme 33), showing excellent catalytic activity with no by-products and above 96% aniline yield [156].

The catalytic activity of the Pd/MnFe₂O₄ nanocatalyst was unaffected by the hydrogenation temperature and resulted in a 96% yield of aniline. Furthermore, the catalyst demonstrated reusability for at least four cycles without any loss in catalytic performance.

Catalytic tests were conducted on Pd-decorated MnFe₂O₄ nanoparticles for the hydrogenation of nitrobenzene at four different reaction temperatures (283 K, 293 K, 303 K, and 323 K) and a constant hydrogen pressure of 20 bar (Figure 19). All three Pd-decorated catalysts achieved a nitrobenzene conversion of 99.9%, although the reaction times varied significantly. The temperature variation, however, had an impact on the aniline yield (YAN), with the highest yield of 96.8 n/n% observed for the Pd/MnFe₂O₄ (623 K) catalyst after 240 min of hydrogenation at 283 K. Due to its consistent activity regardless of the hydrogenation temperature, the 573 K catalyst was tested for reuse without regeneration. The Pd/MnFe₂O₄ magnetic catalyst demonstrated easy preparation, high conversion rates, yields, and selectivity, as well as the ability to be reused, making it successful for the industrial hydrogenation of nitro compounds (Figure 20).

A synergistic process of photocatalysis and adsorption was investigated for the degradation of Reactive Red 4 (RR4) dye solution by the immobilization of the TiO₂/chitosan layer on glass support (TiO₂/CS/glass) [157]. The degradation rate of RR4 by TiO₂/CS/glass was 32 times faster compared to the single layer of TiO₂, but it exhibited high dependence on the TiO₂ loading and the initial pH of the solution. The h⁺/OH species that diffused from the TiO₂ layer into the TiO₂/CS interface oxidized the chemisorbed RR4 anions at the interface; at the same time, the electrons so generated were transferred to the conduction band of TiO₂ (Figure 21). The surplus electrons in the conduction band of TiO₂ enhanced the number of superoxide ions produced, improving the photocatalytic degradation of RR4. In addition to its high catalytic efficiency for RR4 degradation, the immobilized TiO₂/CS demonstrated easy recovery from the reaction mixture, maintaining its high catalytic activity over multiple cycles of extended usage without any loss in efficiency.

Chitosan extracted from shrimp shells (Penaeus merguiensis) was investigated for its potential to remove Reactive Orange 107 dye (RO 107) from wastewater, showing a removal efficiency of 96.20% [158]. In recent studies, manganese ferrite nanoparticles were synthesized using ginger root/cardamom extract as fuel through the self-combustion method [159]. These MnFe₂O₄ nanoparticles, mediated by plant extracts, proved useful for the photocatalytic degradation of MB dye under visible light with different H₂O₂ concentrations. To enhance the photocatalytic performance of MnFe₂O₄ nano-ferrites, Zn²⁺ and La³⁺ ions were doped using a sol–gel auto-combustion technique for the degradation of the organic textile Malachite Green Dye (MGD) under natural solar radiation [160]. Substitution of zinc and lanthanum improved the photocatalytic performance of nano-ferrites Mn_1−xZn_xLa_yFe_2−yO₄ (x = 0.0, 0.01, 0.03; y = 0.0, 0.02, 0.04). The photocatalytic behavior of MnFe₂O₄ was found to increase with dopant concentration; about 96% of MGD was degraded by Mn_0.97Zn_0.03La_0.04Fe_1.96O₄ after 60 min of irradiation. The effect of doping with different compositions of cerium (Ce) was studied under visible light irradiation for the degradation of an aqueous solution containing Reactive Orange 16 (RO 16) and Congo Red (CR) 22. The degradation of MGD followed: Mn_0.97Zn_0.03La_0.04Fe_1.96O₄ > Mn_0.99Zn_0.01La_0.02Fe_1.98O₄ > MnFe₂O₄ with 96.1, 92.4, and 88.3% degradation, respectively.

The valence electrons of MnFe₂O₄ were excited to the conduction band by sunlight creating equal number of holes in the valence band. The excited electrons reduced the atmospheric oxygen to produce superoxide anion (O₂⁻). Simultaneously, the holes of the valence band reacted with the H₂O molecules to form hydroxyl radicals (Figure 22). The active radical species (O₂⁻ and ^●OH) degraded the dye molecules producing mineral acids, CO₂ and H₂O, thus, decolorizing the solution. Mn_0.5Zn_0.5Ce_0.3Fe_1.7O₄ NPs showed the highest percentage degradation efficiency, namely 97.25% (CD) and 98.42% (RO 16).

The recent catalytic uses of ferrite nanoparticles, especially metal ferrites of Cu, Zn, Ni, Co and Mn, in a wide spectrum of organic reactions such as C–C coupling, aldol condensation, polymerization, hydrogenation, as well as in oxidation reactions have been highlighted in this mini-review (

Table 6. Catalytic activities of various metal ferrites.

Catalytic Composition	Process Description	Catalytic Activities	References
Copper ferrite CuFe2O4	First β, γ -unsaturated ketones synthesis	Promising yield with reduced reaction time Reusable for three cycles	[88]
	C–O coupling in the Ullmann reaction	High yields No protection to the sensitive functional groups (i.e., MeCO, CN, and NH2)	[91]
	One-pot synthesis of 𝛼-aminonitrile derivatives	Catalyst reusable and recyclable for four cycles. Benzaldehyde derivatives gave better yields compared to the ketone derivative	[92]
	Oxidative decarboxylation of phenylacetic acid	Simple, effective, and ligand-free production of aldehydes and ketones	[93]
	Oxidation of toluene to benzoic acid	Excellent yield	[94]
	Synthesis of biaryls from arylboronic acid	Catalyst recovered and reused up to five cycles 83–96% yield	[95]
	Degradation of ethanol to acetaldehyde	Excellent yield (97%) Higher temperature (400–500 °C) facilitates acetone production	[96]
	Oxidative coupling reaction of methanol and ethanol	Yield increased with temperature and molybdenum concentration	[97]
	Reduction of ortho and para nitroaniline	Showed excellent reduction profile for nitroanilines	[100]
	Photosensitive Fenton process	Degradation rate of 96% in 20 min with outstanding recyclability	[102]
	Sonophotocatalytic degradation process of the Acid Blue 113 (AB113) dye	Catalyst showed exceptional efficiency of dye removal (100%), total organic carbon (93%), and chemical oxygen demand (95%)	[104]
Zinc ferrite ZnFe2O4	Conversion of methanol to CO and H2	Reaction temperature decreased Transformation of ferrite phase	[106]
	Suzuki cross-coupling reactions	Doping with Pd showed excellent catalytic activity No ligand needed in aerobic condition	[22,107]
	Carbon–oxygen cross-coupling reactions such as Heck–Matsuda reaction	Enhanced catalytic activity	[108]
	Sonogashira coupling reaction	Without the catalyst reaction not possible	[109]
	Photodegradation of MB dye in aqueous solution	Catalyst efficiently degraded the MD dye	[111]
	Removed crystal violet and brilliant green dyes	Excellent catalytic activity under ambient condition	[114]
	Decolorization of Orange II dye	Showed high activity and stability with very low iron and zinc leaching throughout the various cycles of the experiments	[115]
Nickel Ferrite NiFe2O4	Water gas shift reaction	Showed excellent output	[116]
	Chemical looping reaction with methanol	The catalyst further deoxidized CO2 to CO	[117]
	Sonogashira cross-coupling reactions	Catalyst highly active and reusable up to tenth run Reduced reaction time	[118,119,120,121]
	Hydrogen-transfer reactions in the reduction of nitro and carbonyl compounds	Effective reduction of nitro compounds to aniline	[122]
	Claisen–Schmidt condensation	Excellent yield of acetylferrocene chalcones More electronegative and conjugated phenyl groups yielded better products with reduced time	[123]
	Reducing nitroarenes to arylamines	Cu incorporation enhanced the activity	[124]
	C–O coupling reaction	Coupling of phenol derivatives and several aromatic halides successfully conducted	[125]
	Producing polyhydroquinoline	Easy method with microwave irradiation Electron-donating groups in the ring increased the reaction time Excellent yield and reusability	[126]
	Heterogeneous Fenton, such as oxidation of rhodamine B	Degraded the dye effectively	[133]
	Degradation of MB as a water contaminant	Good photocatalytic activity against dye degradation and showed reusability up to four cycles	[134]
	Water treatment as a photocatalyst with organic pollutants (MO, MB, RB, CR, phenol, antibiotics	Maximum removal of dyes	[135]
Cobalt ferrite CoFe2O4	Aldol condensation	A good yield of α, β-unsaturated ketone obtained	[136]
	Hydrogenation of CO2 to methanol	The catalyst effectively recovered and reused	[137]
	Oxidation of primary as well as secondary aliphatic alcohols and benzylic alcohols	Enhanced activity with increase in calcination temperature	[138,139,140]
	One photosynthesis of 2-phenylbenzo [d]oxazole derivatives	Easy method with great yield	[141]
	Oxidation of alkenes	Excellent yield of alkenes to their related aldehydes or epoxides	[142]
	Suzuki coupling reaction	Good catalytic recovery and reusability for multiple cycles Less catalyst loading	[143]
	Conversion of ethyl acetate to CO2	Activity increased with specific surface area	[144]
	Production of amides by Ritter reaction	No solvent required Catalyst recycled for several cycles retaining the catalytic activity	[145]
	N-formylation reaction	Good percentage yield Less reaction time	[146]
	Formation of derivatives of 2-aryl benzimidazole	Rapid reaction in short span of time for aldehydes bearing electron-withdrawing substituents	[147]
	Chemical looping reduction of methanol	Excellent catalytic activity	[117]
	Photocatalytic degradation Congo red dye and Alizarin Red S	A 91% of degradation of dyes took place, and activity increased with annealing temperature	[148,149]
	Photocatalytic degradation of MB dye and Evans Blue (EB) dye under visible light irradiation	Excellent degradation of the dyes without any hazardous effect on the environmental	[150]
Manganese Ferrite MnFe2O4	Decarboxylative Sonogashira reaction	Symmetrical and unsymmetrical diaryl acetylenes were synthesized without any ligands	[152]
	Oxidation reaction of benzyl alcohol	Nanocatalyst activity moderate but showed 100% selectivity towards benzaldehyde A 100% removal of PNP in 7 h	[153]
	Catalyse potassium peroxymonosulfate(PNP)	Excellent catalytic activity with 96% aniline yield and no by-products	[155]
	Hydrogenation of nitrobenzene to aniline	Catalyst reusable up to four cycles without loss in catalytic performance	[156]
	Synergistic process of photocatalysis and adsorption for the degradation of Reactive Red 4 (RR4) dye	High catalytic efficiencies for the degradation of RR4 with easy recovery from the reaction mixture maintaining high catalytic activity over many cycles of extended usage without losing efficiency	[157]
	Removal of Reactive Orange 107 dye (RO 107) and MB dye from wastewater	Removed more than 90% of the pollutant from the wastewater under visible light	[158,159]
	Degradation of organic textile Malachite Green Dye (MGD) under natural solar radiation	A total of 96% of MGD degraded after 60 min of irradiation Activity increased with doping concentration	[160]

).

4. Magnetic Recovery and Reusability of MFe₂O₄ as an Environment-Friendly Green Catalyst

Catalysts are widely used in organic reactions, and it is important for them to be environmentally friendly in order to prevent pollution and manage waste. The use of heterogeneous catalysts instead of homogeneous catalysts allows for easy recyclability and reuse, thus minimizing waste generation [161]. Separating homogeneous catalysts from reaction mixtures is often challenging due to their decomposition during the reaction and contamination issues, which hinders their reuse and recycling. In contrast, magnetic nanoparticles (MNPs) serve as novel heterogeneous catalysts in organic reactions due to their small size, high surface area, excellent loading capacity, and high catalytic activity. They can be easily separated using an external magnet, eliminating the need for centrifugation or complex filtration processes [162]. MNPs exhibit high stability, enabling repeated use and providing a high yield [163]. There is only a minimal decrease in product yield (1–5%) with the catalyst until the fifth to tenth run, which is significant from an economic perspective. The decline in yield is attributed to losses during washing and magnetic separation, as well as chemisorption blocking active sites and forming organic intermediates [164]. The advantages offered by MNPs make them the preferred choice of catalysts in organic reactions. This review focuses on the current developments in the field of magnetically supported nanocatalysts for various organic reactions.

5. Conclusions

The application of spinel ferrites (CuFe₂O₄, ZnFe₂O₄, NiFe₂O₄, CoFe₂O₄, and MnFe₂O₄) as heterogeneous catalysts in organic transformations has achieved significant advancements. This review provides a brief overview of the catalytic performance of these magnetically retrievable catalysts in various organic reactions. It is observed that these nanocatalysts enhance product yield and reduce reaction time under environmentally friendly conditions. The excellent magnetic properties of ferrite nanocatalysts enable easy recovery from reaction systems and reusability with minimal loss of activity. MFe₂O₄ has been successfully employed as a catalyst in several reactions, including condensation reactions, cyclization, oxidation of various alkenes, alkylation, C–C coupling, dehydrogenation reactions, and synthesis of organic compounds such as arylamines and acetylferrocene chalcones, among others. The size of ferrite nanoparticles depends on the type of transition metal and the synthetic methodology used. The catalytic activity of ferrites is greatly influenced by the nanoparticles’ morphology, doping atom ratios, impurities, and specific surface area. Transition metal ferrites increase electrophilicity by acting as Lewis acids, forming covalent bonds, and playing a crucial role in activating specific groups of reactants. However, in several reactions, the mechanisms are still uncertain, and further studies are needed to explore and explain the plausible mechanisms and unexpected results. Catalyst leaching leading to the loss of catalyst during repeated reuse is also an important aspect that requires investigation in future studies and research. Currently, limited research has been reported on rare earth-doped metal ferrites as heterogeneous catalysts in organic reactions. Further research can be conducted to explore the catalytic activities of ferrites, particularly rare earth metal ferrites, for a wider range of chemical transformations and organic synthesis. It is hoped that this review will provide guidance for future research in the field of ferrite catalytic activities.