At the present time, magnetic catalysts attract enormous interest [1-5] and have an extensive variety of potential uses due to their high surface area, biocompatibility, and unique magnetic properties [6–10]. Furthermore, magnetic nanoparticles separated simply by applying the magnetic interaction using a magnetic field [11-14].
In the family of nanomagnetic materials, magnetite Fe3O4 exhibits excellent application such as magnetic bio-separations , drug delivery [16, 17], magnetic resonance imaging (MRI) , hyperthermia treatment of cancer cells [19, 20], and catalysts [21, 22]. Thus, we investigate the existence and prepration of some types of Fe3O4 nanoparticles and their hybrids as catalysts in this study.
- Various types of Fe3O4 nanohybrid catalysts
Most Schemes for iron-based nanoparticles recovery is the result of the magnetic particle (Fe, Fe3O4, Fe2O3, etc.) as a support which is announcer to a different catalytically active metal . Catalyst training is performed when the second metal [24-26] or organo catalyst [27, 28] is anchored via a linker to the nanoparticle directly or instead of a protective polymer [29, 30] or silica coating [31, 32]. As depicted in Figure 1, some clusters have devolved a series of magnetic nanoparticles as beneficial catalysts in organic transformations.
Figure 1. Various types of magnetically nanoparticles
- Nanohybrid Fe3O4 catalysts having silica as protective coating
Some types of nanohybrid Fe3O4 catalysts having silica as protective coating (1-7) are displayed in the Figure 2.
Figure 2. Typical nanohybrid Fe3O4 catalysts having silica coating
Fe3O4@SiO2 nanoparticles (1) were prepared according to Scheme 1 procedure by Hu et al.  using the chemical co-precipitation method. In this procedure, FeCl3·6H2O and FeCl2·4H2O were dissolved in aqueous HCl. Then, aqueous NaOH was added under vigorous stirring to produce black precipitate instantly .
Scheme 1. Synthetic steps of preparing Fe3O4@SiO2 nanoparticles (1) 
The synthetic method for preparing the Fe3O4/SiO2-PropylPip-SO3H magnetic nanoparticles (MNPs) is shown in Scheme 2 . In the procedure, silica was coated on Fe3O4 MNPs, then condensation of hydroxyl groups of MSNPs with (3-chloropropyl) trimethoxysilane (CPTMS) yielded Fe3O4/SiO2-Propyl-Cl MNPs. Subsequently, the reaction of the chloro groups of Fe3O4/SiO2-Propyl-Cl with the amine group of piperazine gained Fe3O4/SiO2-supported piperazine (Fe3O4/SiO2-Propyl-Pip). At the last step, Fe3O4/SiO2-Propyl-Pip was condensed with chlorosulfonic acid which produced N-propylpiperazine sulfonic acid-functionalized Fe3O4 magnetic nanoparticles (Fe3O4/SiO2-Propyl-Pip-SO3H).
Scheme 2. Preparation of Fe3O4/SiO2-Propyl-Pip-SO3H magnetic nanoparticles (2) 
In this protocol, for surface modification, magnetic nanoparticles were covered with APTES to achieve aminofunctionalized magnetic nanoparticles. Finally, reaction of amino groups and chlorosulphuric acid yielded SA–MNPs (sulphamic acid-functionalized magnetic Fe3O4 nanoparticles) (Scheme 3) .
Scheme 3. Preparation of SA-MNPs (sulphamic acid functionalized magnetic Fe3O4 nanoparticles) 
In addition, the prepration of some other magnetic nanoparticles was found to have silica as protective coating in the reported articles such as Fe3O4@SiO2-SO3H , Fe3O4 @SiO2@LCy , Fe3O4/SiO2-urea , MNPs-Guanidine , and etc.
3.1 Nanohybrid Fe3O4 catalysts supported heteropolyacids
Some types of nanohybrid Fe3O4 catalysts having silica supported heteropolyacids (8-10) are indicated in the Figure 3.
Figure 3. Typical nanohybrid Fe3O4 catalysts having silica supported heteropolyacids
Fe3O4@SiO2-imid-H3PMo12O40 nanoparticles (10) were produced from immobilization of phosphomolybdic acid nanoparticles on imidazole functionalized Fe3O4@SiO2 . In this procedure, Fe3O4@SiO2 was added to the solution of 3-chlorotriethoxypropylsilane and imidazole in p-xylene to prepare Fe3O4@SiO2-imid. Then, Fe3O4@SiO2-imid was added to an acetonitrile solution of PMAn . Likewise, the similar process was used for the synthesis of Fe3O4@SiO2-imid-PMAb (PMAn = nano H3PMo12O40, PMAb= H3PMo12O40) (Scheme 4).
Scheme 4. Synthetic steps of Fe3O4@SiO2-imid-PMAn
Furthermore, the prepration of some other magnetic nanoparticles was found to have silica supported heteropolyacids reported in articles such as Fe3O4@Si-Gu-Prs (8) , TSAMNP catalyst (9) , and etc.
- 2 Nanohybrid Fe3O4 catalysts supported ionic liquids
The synthetic procedure of acidic IL supported on magnetic nanoparticles ([HSO3PMIM]OTf-SiO2@MNPs) is demonstrated in Scheme 5. The coresponding ionic liquid [HSO3PMIM]OTf was produced by treatment of 1-methyl-3H-imidazole with 1,3-propanesultone, followed by treatment with HOTf. Finally, the silica coated magnetite nanoparticles was treated with [HSO3PMIM]OTf in dichloromethane by sonication to produce [HSO3PMIM]OTf-SiO2@MNPs .
Scheme 5. Preparation of [HSO3PMIM]OTf-SiO2@MNPs 
Imidazole functionalized magnetic Fe3O4 nanoparticles are reported by Nazari et al. . In the present study, the reaction of (3-chloropropyl)-trimethoxysilane with imidazole led to product 1 (Si-Im). Finally, MNPs were coated with Si-Im to reach the desired nanoparticles (Scheme 6).
Scheme 6. Preparation of imidazole functionalized magnetic nanoparticles 
After preparing Fe3O4-MNPs, 1-methyl-3-(3-trimethoxysilylpropyl) imidazolium chloride (IIL-Cl) was attained from the reaction of N-methyl imidazole with (3-chloropropyl) trimethoxysilane. Then, the immobilized chloride IL on MNPs (MNP–IIL–Cl) and an excess amount of NaOAc (or KHSO4) were added into the deionized water. As depicted in the Scheme 7, MNPs-IIL-OAc and MNPs-IIL-HSO4 were obtained as a powder. In this procedure, 1-methyl-3-(3-trimethoxysilylpropyl) imidazolium hydrogen sulfate (MNPs-IIL-HSO4), 1-methyl-3-(3-trimethoxysilylpropyl) imidazolium acetate (MNPs-IIL-OAc), and 1-methyl-3-(3-trimethoxysilylpropyl) imidazolium chloride (MNPs-IIL-Cl) were prepared .
Scheme 7. Preparation steps of producing MNPs–IILs 
The ionic liquid 1-methyl-3-(3-trimethoxysilylpropyl) imidazolium chloride immobilized functionalized on Fe3O4 nanoparticles (IL-MNPs) was further reported by Safari and Zarnegar. The synthetic steps are shown in the the Scheme 8. In this protocol, N-methyl imidazole reacted with (3-chloropropyl) trimethoxysilane to prepare 1-methyl-3-(3-trimethoxysilylpropyl) imidazolium chloride (IL). Then, IL was added to the external surface of MNPs to attain IL-MNPs .
Scheme 8. Preparation steps of IL-Fe3O4 nanoparticles 
As illustrated in the Scheme 9, IL-Ni(II)-functionalized magnetic Fe3O4 nanoparticles are prepared. The IL addition to an acetonitrile solution of NiCl2 led to IL-Ni(II). Then, IL-Ni(II) immobilized on the external surface of Fe3O4 nanoparticles to achieve IL-Ni(II)-MNPs as the desired product .
Scheme 9. IL-Ni(II)-MNPs preparation steps 
The Fe3O4@SiO2~FLU nanomagnetic catalyst was organized as depicted in the Scheme 10. Edvidently, iron oxide is coated by a silica layer to immobilize organic catalyst on the surface of inorganic support because of that organosilanes can act as a linker between the organic catalyst and the support. As reported, chloride in the linker was replaced by nitrogen of the triazole via an SN2 reaction .
Scheme 10. Fe3O4@SiO2~FLU nanomagnetic catalyst prepration 
- 3 Nanohybrid Fe3O4 catalysts supported metal bounds
Herein, synthesis steps of manganese (III) salen complex immobilized on Fe3O4 nanoparticles reported by S.M. Sadeghzadeh et al. is demonstrated in Scheme 11. For the synthesis of compound (A) MNP, Fe3O4\SiO2 MNPs were added to ethyl 3,4-diaminobenzoate. Then, for synthesis of Fe3O4\SiO2\Salen\Mn nanoparticles (C), compound (A) was reacted with the solution of salicylaldehyde and subsequently with Mn(OAc)2 .
Scheme 11. Synthesis steps of Fe3O4\SiO2\Salen\Mn nanoparticles 
Another Fe3O4 supported gold dipyridine complex nanoparticles was formed via sodium tetrachloroaurate(III) hydrate attaching to a dipyridine ligand (Scheme 12) .
Scheme 12. Fe3O4 supported gold dipyridine complex nanoparticles synthesis 
Cu(II)/2-Aminobenzenthiol complex immobilized on Fe3O4/SiO2 NPs is reported to catalyze synthesis of 1,2,3-triazoles. As depicted in Scheme 13 and Scheme 14, Fe3O4@SiO2-(3-chloropropyl) trimethoxysilane was prepared to react with 2-aminobenzenethiol and subsequently, copper acetate was added to the mixture in order to prepare Fe3O4@SiO2-ABT/Cu(OAc)2 .
Scheme 13. Cu(OAc)2/2-aminobenzenthiol complex coated on Fe3O4/SiO2NPs formation 
Cu(acac)2/NH2-T/SiO2@Fe3O4NPs was prepared through immobilization of copper(II) acetylacetonate on the external surface of amine-terminated Fe3O4/SiO2 NPs (Scheme 15). In this protocol, the synthesized NH2-T/SiO2@Fe3O4NPs was added to a solution of [Cu(acac)2] .
Scheme 15. Stepwise synthesis of Cu(acac)2/NH2-T/SiO2@Fe3O4NPs 
Fe3O4@SiO2@SiO2(CH2)3@AVOPc was formed by covalent binding of an amino vanadium oxo phthalocyanine, on the surface of Fe3O4/SiO2 NPs . Fe3O4@SiO2@SiO2(CH2)3Cl (ClPMNPs) were prepared as depicted in Scheme 16. In the next step for covalently linking the ClPMNPs to AVOPc, ClPMNPs was sonicated and subsequently, they were added to the AVOPc in order to produce Fe3O4@SiO2@SiO2(CH2)3@AVOPc (AVOPc-MNPs) (Scheme 17) .
Scheme 16. Chloro functionalized magnetic nano particles (II) formation 
Scheme 17. Synthesis of AVOPc-MNPs 
Another type of magnetic nano particles based on a biimidazole Cu(I) complex was produced by covalent binding of biimidazole on chloride-functionalized Fe3O4/SiO2 NPs, followed by reacting with CuI .
3.4. Nanohybrid Fe3O4 catalysts supported an additional bound
An organic-inorganic type of silica with ionic liquid basis was produced as indicated in Scheme 18. In this protocol, diazoniabicyclo [2.2.2] octane dichloride groups, [(MeO)3Si(CH2)3N+(CH2CH2)3N+(CH2)3 Si(OME)3]Cl2, were produced as a precursor reagent to achieve the desired nano particles .
Scheme 18. Prepration steps of Fe3O4@SiO2/DABCO 
An inorganic-organic type of MNPs H6P2W18O62/pyridino- Fe3O4 (HPA/TPI-Fe3O4) produced by Tayebee et al. (Scheme 19). The Wells-Dawson heteropolyacid H6P2W18O62 immoblized on the external surface of Fe3O4 nanoparticles with N-[3-(triethoxysilyl)propyl] isonicotinamide (TPI) linker .
Scheme 19. HPA/TPI-Fe3O4 nano particles as catalyst 
The N-propylcarbamothioyl benzamide complex of Bi(III) fixed on Fe3O4/SiO2 NPs has been reported by Mobinikhaledi et al. . In this report, Fe3O4/SiO2 reacted with (3-aminopropyl) triethoxysilane (APTES) to yield Fe3O4/SiO2-NH2 MNPs. Then, the condensation reaction of amino groups of Fe3O4/SiO2-NH2 with benzoyl isothiocyanate led to Fe3O4/SiO2-supported carbamothioyl benzamide (Fe3O4/SiO2-NH-ligand) formation. Lastly, the reaction of Fe3O4/SiO2-supported carbamothioyl benzamide with Bi(NO3)3·5H2O led to Bi(III) complex of Fe3O4/SiO2-NH-ligand magnetic nanoparticles (MNPs) (Scheme 20).
Scheme 20. Preparation of Fe3O4/SiO2-propyl-NH-ligand-Bi(III) MNPs 
- Nanohybrid Fe3O4 catalysts directly bounded in the absence of silica
Multi-walled carbon nanotubes supported Fe3O4 nano particles is prepared according to the synthetic procedure which was depicted in Scheme 21. In this study, carboxylic acid functionalized MWCNTs used as starting materials. Finally, the solution of NH3 was added to produce black magnetic Fe3O4NPs/MWCNTs .
Scheme 21. Simplified schematic exhibition of the synthesis of Fe3O4 NPs/MWCNTs 
As the synthetic steps are illustrated in the Scheme 22, chitosan-coated Fe3O4 (Fe3O4@CS) synthesized via in situ co-precipitation of Fe2+ and Fe3+ ions with an aqueous solution of chitosan .
Scheme 22. Preparation steps for fabricating heterogeneous Fe3O4@CS nanoparticles 
Herein, Fe3O4@KCAR was synthesized in the presence of natural κ-carrageenan (KCAR) biopolymer (Scheme 23). KCAR dissolved in water is added to FeCl3.6H2O and FeCl2.4H2O, and then aqueous ammonia was added to the solution in order to produce the desired product .
Scheme 23. A schematic pathway for synthesis of Fe3O4@KCAR and rhodanine 
A dehydroascorbic acid covered Fe3O4 NPs (DHAA-Fe3O4) is also synthesis as the structure of the catalyst is illustrated in the Figure 4. For preparing DHAA-capped magnetite nanoparticles (DHAA-Fe3O4) Fe(OH)3 solution was prepared by addition of FeCl3.6H2O aqueous solution to NaHCO3 solution. Subsequently, a solution of vitamin C was reacted gradually to Fe3+ .
Figure 4. DHAA-capped magnetite nanoparticles 
In addition, Fe3O4-proline MNPs was produced with no additional linkers as shown in the Figure 5. In this process FeCl3·6H2O and FeCl2·4H2O salts were dissolved in deionized water and subsequently, proline and NH4OH solution was added to produce the desired Fe3O4-proline nanoparticles .
Figure 5. Preparation of Fe3O4-proline MNPs 
Moreover, Pd@agarose-Fe3O4 nanoparticles were synthesized. In this protocol, palladium nanoparticles supported on an agarose hydrogel which is attached to magnetic Fe3O4 nanoparticles .
The Fe3O4/ZIF-8 nanoparticles were prepared via the reaction between 2-methylimidazolate (MeIM) and zinc nitrate in the presence of Fe3O4 NPs as displayed in the Figure 6 [65, 66].
Figure 6. Fe3O4/ZIF-8 particles prepration. PAA=polyacrylic acid [65, 66]
A magnetic organic-inorganic Fe3O4 NP was prepared and called (Fe3O4 /PAA-SO3H) sulfonated-phenylacetic acid immoblized on Fe3O4 NPs. Phenylacetic acid immoblized initially on manufactured Fe3O4 NPs. Then, the Fe3O4/PAA was sulfonated by numerous amounts of chlorosulfonic acid to give Fe3O4/PAA-SO3H .
The Fe3O4@Nb2O5 nanocatalyst was prepared by coating magnetite nanoparticles with niobium oxide by using a simple wet impregnation method (Scheme 24). The Fe3O4@Nb2O5 nanocatalyst was formed using ammonium niobate oxalate hydrate (C4H4NNbO9·3H2O) as the niobium source. The hydrolysis of the niobium precursor in alkaline medium led to aggregation of niobium hydroxides over the surface of Fe3O4 nanoparticles .
Scheme 24. Synthesis of the niobium nanocatalyst 
Titanium dioxide Fe3O4 NPs (Fe3O4@SiO2@TiO2) were organized via the reaction of Fe3O4 NPs with tetraethyl orthosilicate and tetrabutyl titanate, respectively, as depicted in the Scheme 25 [69, 70].
Scheme 25. Preparation of Fe3O4@SiO2@TiO2 [69, 70]
TiO2-coated Fe3O4 NPs-supported sulfonic acid (Fe3O4-TiO2-SO3H (n-FTSA)) was achieved by immobilizing-SO3H groups on the surface of nano-Fe3O4-TiO2 (Scheme 26) .
Scheme 26. Fe3O4/TiO2-supported sulfonic acid nanoparticles formation 
Polyethylene glycol-Cu nanocomposite (Fe3O4-PEG-Cu) synthetic steps is depicted in Scheme 27. First, cyanuric chloride reacted with PEG-(OH)2 to prepare PEG-Cl4 . Second, PEG-Cl4 was linked with Fe3O4 NPs via covalent bindings (Fe3O4-PEG). Finally, the Fe3O4-PEG-Cu catalyst was produced by reducing copper ammonia complexes using hydrazine hydrate on the surface of Fe3O4-PEG nanocomposite .
Scheme 27. Fe3O4-PEG-Cu nanoparticles prepration 
Fe3O4/HAp/Au was prepared by magnetic metal oxides (Fe3O4), noble metals Au, and hydroxyapatite (HAp, hydroxyapatite). This catalyst is an efficient one for many responses such as photocatalysis, molecular imaging, and drug delivery. Synthesis of Fe3O4/HAp/Au composite NPs was performed in three steps as demonstrated in Figure 7 including syntheses of Fe3O4 NPs (A), magnetic hydroxyapatite (Fe3O4/HAp) NPs (B), and Fe3O4/HAp/Au (C). Finally, the prepared catalyst was utilized for efficient photocatalytic application .
Figure 7. Synthetic procedure (A-C) and photocatalytic reaction of Fe3O4/HAp/Au 
4.1 Hybrid Pd- Fe3O4 nanoparticles for various C-C coupling reactions
The Suzuki-Miyaura coupling, Mizoroki-Heck, and Sonogashira reactions are important reactions using Pd catalysts to produce C-C coupling reactions. Recently Pd-Fe3O4 NPs were applied in C-C coupling reactions [75-77].
FePd-Fe3O4 composites were synthesized in various Fe/Pd ratios as the synthetic procedure is illustrated in the Figure 8. Additionally, TEM image of the urchin-like FePd–Fe3O4 composite nanoparticles is depicted in the Figure 8 .
Figure 8. Synthetic procedure of urchin-like FePd-Fe3O4 
A simple route based on time-dependent growth was performed to synthesize nanospheres Pd/Fe3O4 (Figure 9) using FeCl3·6H2O as the single iron resource, polyvinylpyrrolidone (PVP) as the capping agent, and sodium acetate as the precipitation agent. To investigate the catalytic activity of the catalyst Heck reaction of iodobenzene and styrene was selected which is illustrated in Scheme 28 [79, 80].
Figure 9. Synthetic procedure of Pd/Fe3O4 nanospheres [79, 80]
Scheme 28. Heck reaction using Pd/Fe3O4 nanospheres [79, 80]
Fe3O4@C–Pd catalyst synthesis was proceed via Stöber method in the synthesis of resin spheres composed of resorcinol–formaldehyde (RF) which were transformed to carbon spheres (Figure 10). The catalytic efficiency of the synthetic nanocomposite was studied in the Suzuki coupling reactions of diverse aryl halides and aryl boronic acids [81-84].
Figure 10. Fe3O4@C–Pd Catalyst prepration
4.2 Flower-Like organic capping agent of hybrid Fe3O4 NPs
Palladium nanoparticles fabricated magnetic Fe3O4 nanocomposite over Fritillaria imperialis flower extract synthesized to many uses such as as using as an efficient recyclable catalyst for the reduction of nitroarenes. In this catalyst, hybrid magnetic nanocomposite Fe3O4 NPs are at core and Pd NPs are at outer shell. For the goal synthasis, Fe3O4 nanocomposite, Fe3O4@Fritillaria using the plant extract, and Fe3O4@Fritillaria/Pd NPs was prepared, respectively (Scheme 29) .
Scheme 29. Synthetic steps of Fe3O4@Fritillaria/Pd to reduce nitroarenes 
Fe3O4 magnetic nanoparticles and their hybrids display capable applications in heterogeneous catalysis because of their ease of separation and good reusability. Preparation techniques of these nanocomposites are undergoing rapid development. In this report, we highlight the preparation of support materials to develop the quality of magnetically recoverable catalysts. Typical advancement on preparation of surface-modified MNPs was illustrated to obtain different type of Fe3O4 magnetic nanomaterials. Progressive types of Fe3O4 nanoparticles and their Hybrids as catalysts will be prepared in the future which will impress other type of known and unknown organic reactions.
The authors would like to acknowledge the management and principal of University of Mazandaran, Damghan University, University of Baghdad and University of Mosul for their constant motivation and support.
Conflict of Interest
The author declares that there is no conflict of interest.
Parvin Hajiabbas Tabar Amiri
Raad Muslim Muhiebes
Yasser Fakri Mustafa
Citation: H. Alinezhad, P. Hajiabbasi*, S.M. Tavakkoli, R.M. Muhiebes, Y.F. Mustafa. Progressive Types of Fe3O4 Nanoparticles and Their Hybrids as Catalysts. J. Chem. Rev., 2022, 4(4), 288-312.