Silica-Coated Magnetic Nano-Particles: Application in Catalysis - ACS

Chapter 1

Silica-Coated Magnetic Nano-Particles: Application in Catalysis Downloaded by 80.82.78.170 on December 24, 2016 | http://pubs.acs.org Publication Date (Web): December 19, 2016 | doi: 10.1021/bk-2016-1238.ch001

Rakesh K. Sharma,*,1 Manavi Yadav,1 and Manoj B. Gawande*,2 1Green

Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi 110007, India 2Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, Šlechtitelů 27, 783 71, Olomouc, Czech Republic *E-mail: [email protected]

The field of nanoscience and catalysis has been inseparably associated to each other for quite a long time. Since decades scientists and researchers have focused on the use of nanomaterials as a vehicle for supporting other catalytic systems to facilitate recovery. Recently, magnetic nano-particles have been extensively considered for serving the dual role of a catalyst/catalyst support and a magnetically recoverable entity. The aim of this chapter is to discuss the role of silica-coated magnetic nano-particles (SMNPs) in catalysis. Special attention is given to recent developments and advances in various organic transformations including coupling, oxidation, reduction, and multi-component reactions using these SMNPs as catalytic supports. Easy accessibility, effortless magnetic recoverability and recyclability are some of the main features that make the use of these silica-coated magnetic nano-particles green and sustainable.

Introduction The modern era of chemistry is moving towards the goal of sustainability (1). Due to progressively stringent environmental norms and economic pressure, significant attention has been directed towards the use of sustainable catalysts to accomplish low preparation cost, high activity, excellent selectivity, waste © 2016 American Chemical Society Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by 80.82.78.170 on December 24, 2016 | http://pubs.acs.org Publication Date (Web): December 19, 2016 | doi: 10.1021/bk-2016-1238.ch001

reduction, high stability, efficient recovery, good recyclability and simplified product purification, without affecting the yield and quality of reaction. Due to these special features, the design and development of a “benign” catalyst has been one of the biggest challenges for chemists since many years (2). Conventionally, catalysts have been categorized as homogeneous and heterogeneous. Homogenous catalysis hold advantages such as a great activity and selectivity. This is due to their good solubility in reaction media, which enhances the accessibility of the catalytic site for the substrate. However, its major drawback is the difficulty in separation from the reaction medium. Heterogeneous catalysis eliminates the aforementioned limitations as active molecules are immobilized onto a solid support making the isolation and separation of the catalysts a simple procedure (3). But, due to decreasing interaction of reagents with the active catalytic surface and the tendency of metal leaching from solid supports, heterogeneous catalysts are often considered less efficient in contrast to the homogeneous ones. Recently, nano-particles (NPs) have emerged as excellent sustainable alternatives to conventional materials for connecting the gap between homogeneous and heterogeneous catalysis. They possess unique properties that vary drastically from bulk materials. While employing them as supports, substantial enhancements were observed in loading, catalytic activity, selectivity, and stability, which is accredited to their large surface-to-volume ratio. This is due to the increase in the exposed surface area of the active component of the catalyst that enhances the contact between the reagents and the catalytic site. In spite of the several advantages associated with nano-catalysts, the inconvenience and inefficiency of the tedious separation methods like centrifugation and filtration, hamper the sustainability and economy of the nano-catalytic strategy (4, 5). To overcome these issues, magnetic nano-particles (MNPs) appeared to be the most logical solution as ideal supports (6–8). They not only combine the best attributes of NPs but also own additional advantages such as convenient magnetic recovery, low toxicity and cost effectiveness. Unlike the cumbersome separation procedures, the magnetic approach eliminates the use of auxiliary substances (solvents, filters, etc.) and prevents catalyst oxidation and loss of catalyst, making the process cleaner, environmentally safe and fast. The SMNPs are characterized using various standard physicochemical techniques such as powder X-ray diffraction analysis (XRD), Fourier transform Infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM) and X-ray energy dispersive spectroscopy (EDS). Figure 2 depicts the various characterization techniques deployed for examining morphology, crystallinity, functionality, magnetic property and chemical composition of these nano-particles. However, it has been found that MNPs have a tendency to agglomerate into a large cluster due to anisotropic attraction which can restrict their use in various applications. This drawback can be eliminated by coating their surface with suitable protecting agents. Several encapsulation procedures have been proposed amongst which silica has gained maximum attention. There are many key advantages of silica coating (Figure 1) due to which silica-coated 2 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


magnetic nano-particles (SMNPs) emerged as an important catalytic support in heterogeneous catalysis (9, 10).

Figure 1. Advantages of silica coating on MNPs

Figure 2. Various characterization techniques employed for the investigation of SMNPs 3 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

SMNPs have established excellent catalytic activities in several research works including a wide range of organic reactions such as carbon-carbon coupling (Suzuki, Heck, Sonogashira, Stille, Hiyama), carbon-heteroatom coupling, acetylation, oxidation, hydrogenation, olefin metathesis, asymmetric synthesis, and photocatalysis (6–11). In this chapter, we briefly summarize the synthetic strategies of SMNPs. Then we highlight the breakthroughs in the applications of SMNPs in catalysis that have most recently appeared.


Synthesis of SMNPs For the utilization of iron oxides in diverse areas, their synthesis in nano dimension has been an active and challenging area of research during the past few decades. The processes comprises of careful selection of concentration of the reactants, temperature, method of mixing, pH and rate of oxidation (12). Several processes are responsible for the morphology of the magnetic nano-particles that includes nucleation, growth, aggregation and adsorption of impurities (13). Due to the sensitivity of the synthetic procedures, observed during both the reproducibility and scale-up processes, it is difficult to synthesize specific MNPs with desired shape and size. Various chemical and physical synthetic methods have been developed to produce magnetic nano-particles. This includes the most commonly used co-precipitation method, micro-emulsion, hydrothermal, solvothermal, thermal decomposition, electrochemical, ball-mill method, gas-phase deposition, electron beam lithography. Magnetic nano-particles can also be fabricated using biological microorganisms and green method using renewable resources. Further, in several preparative methodologies, agglomeration of the nano-oxide takes place which is prevented by employing surfactants or by capping with organic acids, or by coating with silica. The following table briefly discusses the various techniques for the synthesis of silica supported magnetic nano-particles (Table 1).

Applications of SMNPs in Catalysis Owing to the remarkably unique properties of magnetically retrievable SMNPs, such as ease in control over size, shape and morphology, they offer many advantages towards clean and sustainable chemistry. In fact, the use of SMNPs has brought a revolution in several areas including catalysis, medicine, biology, environmental remediation and many more (42). In recent times, the design, synthesis and catalytic activities of supported magnetic nanocomposites have received tremendous attention as they provide environmentally benign alternatives to current catalytic techniques. Due to their unique physico-chemical properties, they are considered to be a fascinating choice in heterogeneous catalysis (Figure 3).

4 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

5


Table 1. Methods used for the synthesis of silica-coated magnetic nano-catalysts S. No.

Nature of Magnetic Nano-particles

Method

Approach

Techniques Used

Basic reagents

Conditions

References

1

Magnetic Nanoparticles

Physical method

Gas phase Deposition

Laser vaporization, thermal vaporization, arc discharge, plasma vaporization, and solar energy-induced evaporation

Iron precursors, solvents, etc.

--

(14)

Electron beam lithography

Electron beam lithograph

--

--

(15)

Ball milling method

Ball mill

--

--

(16)

Co-precipitation

Reduction

Metal salts, base

20-90 °C, pHCu(II)>Ni(II)>Mn(II)>Cd(II)>Hg(II), which indicates that the performance of Fe3O4@SiO2/Schiff base complex of Co(II) is best to efficiently catalyze the reaction between phenylene-1,2-diamines and1,2-diketones in aqueous media at room temperature (Scheme 11). This eco-friendly method provides several advantages such as mild reaction conditions, shorter reaction time, green media, good to excellent yields, simple work-up, and nano-catalyst stability.

Scheme 11. Synthesis of quinoxaline derivatives catalyzed by Fe3O4@SiO2/Schiff base complex of Co2+ ion

C-O Coupling Reaction Zolfigol et al. developed an efficient water tolerant Pd-containing phosphorus silica magnetite [Fe3O4@SiO2@PPh2@Pd(0)] which proved itself as a highly active and stable nano-catalyst for aqueous phase coupling reactions, namely the O-arylation of phenols with aryl halides [ArX (X = Cl, Br and I)] and the Sonogashira coupling reaction under mild conditions (Scheme 12) (75). Besides the easy separation and recyclability of the catalyst via an external magnet, the salient features of this protocol include high efficiency, usage of NaOH as base in water, excellent activities, and a cleaner reaction profile.



Scheme 12. Magnetic Pd catalyzed (a) O-arylation of phenols and (b) Sonogashira cross-coupling reaction

C-S Coupling Reaction Due to the tendency of thiols to undergo oxidative coupling to disulfides along with the possibility of organosulfur binding to the metal that results in catalyst deactivation, C-S bond formation is less studied than C-C bond forming reactions (76). Considering the importance of the flexible macrocyclic and chelating effect of N-and O-containing ligand that might assist in stabilizing the reactive palladium intermediates, Movassagh and co-workers designed heat- and air-stable, silica-coated magnetic nanoparticle (MNP)-supported palladium(II)-cryptand 22 complex [Fe3O4@SiO2@C22–Pd(II)] catalyst for performing the Suzuki reaction and S-arylation of thiols (Scheme 13) (77). Similarly, another group synthesized magnetically retrievable nano-catalyst stabilized (mPANI/pFe3O4) with mesoporous polyaniline (PANI) and applied in the S-arylation of various aryl, alkyl and heterocyclic halides with thiophenol to obtain unsymmetrical diaryl sulfides in moderate to high yields (78). The catalyst was also used for the S-arylation of various aryl iodides with thiourea to obtain symmetrical diaryl sulfides selectively in water (Scheme 14). It was assumed that the mesoporosity of the polyaniline in the mPANI/pFe3O4 catalyst provides direct access to the Fe3O4 nano-particles, which reacts with aryl chloride to give an intermediate. This intermediate when attacked by thiol (nucleophile) in the presence of a base may result in the formation of intermediate, which further undergoes reductive elimination with the formation of product and regeneration of the catalyst. A control experiment was also conducted to confirm that the catalytic activity originated from the porous Fe3O4 and not from temporarily leached Fe3O4. Therefore, the mesoporosity of the polyaniline enhances both the efficiency and stability of the porous magnetic Fe3O4 nano-particles in both coupling reactions. 20 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 13. Use of palladium (II)-cryptand 22 complex catalyst in (a) the formation of aryl-sulphur bond and (b) Suzuki coupling reaction

Scheme 14. Formation of C-S bond in water using mPANI/pFe3O4



In Oxidation Reactions Oxidation reactions are of central importance in organic and biochemistry. They give rise to a number of compounds which are important structural motifs of several natural and synthetic molecules (79). With the increasing environmental concerns, efforts have been directed towards the development of oxidation systems using environmentally benign molecular oxygen as a sole oxidant (79). In this regard, Ruthenium catalyzed oxidation reaction has emerged as a popular synthetic tool for the synthesis of selective oxygenated products both under homogeneous and heterogeneous conversions using environmentally benign oxidants such as dioxygen, and hydrogenperoxide (80). Due to the remarkable behaviour exhibited by ruthenium, Podolean et al. reported the synthesis of an active, selective and easily recoverable catalyst, Ru(III)/functionalized silica-coated magnetic nanoparticles (Ru(III)-MNP), and applied it in the capitalization of renewable sources by carrying out the oxidation of levulinic acid to succinic acid (Scheme 15) (81). This is the first report in literature that deals with the catalytic oxidation of levulinic acid with molecular oxygen under mild experimental conditions, without the need of base and organic solvents, thereby making it an excellent example of a green catalytic oxidation with a stable nanomagnetic recyclable catalyst.

Scheme 15. Oxidation of levulinic acid to succinic acid using Ru(III)/functionalized SMNPs Inspired by the exceptional behavior and superparamagnetic properties exhibited by MNPs, suitable both for catalytic reactions and magnetic recovery, a new nano-catalyst was developed by immobilizing thiosemicarbazide ligand on the surface of silica -coated magnetite nanoparticles (SCMNPs), followed by complexation with MoO2(acac)2 (82). The prepared catalyst exhibited high catalytic activity towards the epoxidation of olefins and allyl alcohols with tertbutyl hydroperoxide (TBHP) and cumene hydroperoxide (CHP) under mild reaction conditions. The key asset of this system is relatively strong interaction of molybdenum complex grafted on the surface of MNPs which rule out the leaching of the catalyst throughout the reaction. Besides this, thiosemicarbazide being a good donor ligand intensify the catalytic activity of the prepared nanomaterial in the epoxidation of olefins.



Another Schiff base complex coated magnetic nano-catalyst was designed by Chen et al. by covalent binding of a tetradentate ligand, N,N′-bis(3-salicylidenaminopropyl)amine (salpr), on the surface of silica-coated magnetic nanoparticles (Fe3O4/SiO2) followed by complexation with Cu(OAc)2 (83). The prepared Fe3O4/SiO2/Cu(II)salpr catalyst presented high activity for selective oxidation of various alkyl aromatics with tert-butyl hydroperoxide (TBHP) as oxidant (Scheme 16).

Scheme 16. Catalytic oxidation of alkyl aromatics in the presence of Fe3O4/SiO2/Cu(II)salpr

Another important feature in designing a catalyst is the immobilization strategy used for linking metal complexes to supports, which should not only be mild but also preserve the chemical functional activity of the complex besides giving quantitative conversion (84). Click reactions represent a marvellous approach for ligation, which prevents the leaching of the complexes from the support during the reaction. This is due to the strong binding between the support and nano-particles that withstand the harsh conditions of the reaction (85). Recently, a new catalyst was developed using the metal (Co and Ni) complex successfully immobilized on MNPs via click reaction. For the synthesis, the MNPs were first modified and functionalized with 3-azidopropyltrimethoxysilane and then the clicked Co and Ni metal complexes were immobilized on the MNPs (86). The nano-catalyst efficiently oxidized both primary and secondary alcohols to carbonyl with improved yield in a solvent less system rendering a greener approach (Scheme 17).



Scheme 17. Use of MNP-immobilized clicked metal complexes for the oxidation of alcohols Due to the detrimental and corrosive nature of mercaptans (RSH) found in petroleum products like LPG, naphtha, gasoline, kerosene, and ATF, it is essential to convert them into a less deleterious form before end use (87). In this regard, Singh and co-workers reported a benign protocol for the preparation of magnetic silica beads functionalized cobalt phthalocyanine catalyzed by immobilizing tetrasulfonated Co(II) phthalocyanine (CoPcS) on the amino functionalized silica-coated magnetic nano-particles (Fe3O4@SiO2, SMNP) via a sulfonamide linkage (88). The synthesized catalyst was found to be efficient in the successful oxidation of mercaptans to disulfides in an aqueous medium by using molecular oxygen as oxidant under alkali free conditions (Scheme 18), thereby making it a clean and economically feasible route for the oxidation of mercaptants to disulfides. Although the majority of catalyst advancements relies on utilizing noble metals, the current challenge dwell in searching for more earth-abundant and non-toxic metals (89). In this perspective, manganese proved to be competitive and superior candidate than conventional catalysts in terms of activity, selectivity, and functional group tolerance since it exhibits remarkable versatile redox chemistry. Likewise, while conceiving a system for eventual scale-up and industrial use, manganese was found to be a lot more appealing than other transition metal-based catalysts because of cost and environmental ramifications. Very recently, Sharma and co-workers developed a magnetic nano-catalyst via covalent grafting of manganese acetylacetonate complex on amine functionalized SMNPs (90). The obtained nano-catalytic system was found to be effective for the oxidation of organic halides and alcohols to carbonyl compounds with excellent yields (Scheme 19). The key features of this work include effortless magnetic recovery, employment of H2O2 as the sole and green oxidant, and solvent-free (or use of nontoxic ethanol as solvent) mild reaction conditions, high activity, and shorter reaction time. 24 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 18. Oxidation of mercaptans to disulfides using CoPcS@ASMNP

Scheme 19. Oxidation of organic halides and alcohols using SMNP-based manganese nano-catalyst In Reduction Reactions The catalytic reduction of various organic compounds has always gained considerable attention. One such example include the catalytic reduction of aromatic nitro compounds to its corresponding amines due to its enormous commercial applications in dye stuffs, additives, agriculture, pharmaceuticals and in other chemical industries (91). Although, numerous catalytic systems 25 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


have been developed for this reaction with different metals such as, Ru, Pd, Pt, Bi, Pt/Ni, Pt/Pd and V. But, due to their limited availability and expensive nature, their utilization led to increase in the overall expenditure (92). This encouraged the researchers to search for more economical and environmentally acceptable alternatives. The copper (II) acetylacetonate complex decorated on amine functionalized SMNPs (Cu(II)-acac@NH2-Si-Fe3O4)and its catalytic activity was evaluated for the reduction of nitroarenes in aqueous medium at room temperature using sodium borohydride (Scheme 20) (93). This catalyst selectively reduced the nitro group even in the presence of other active functional groups. In addition, mild reaction conditions, simple work-up procedure and use of green solvent made the protocol more fascinating.

Scheme 20. Reduction of nitroarenes in aqueous medium at RT using magnetically separable copper nano-catalyst

In addition to the different synthetic strategies reported, a more facile access to fabricate core–satellite structured Au/Pdop/SiO2/Fe3O4 with controllable properties and activities were reported via a method of in situ reduction of polydopamine (94). Firstly, polydopamine-coated silica/magnetite nano-particles (Pdop/SiO2/Fe3O4 composites) were synthesized by the combination of a sol-gel process and an in situ polymerization method, in which TEOS as well as dopamine acted as the precursor for silica and polydopamine (Pdop), respectively. The Pdop/ SiO2/Fe3O4 composites revealed a multilayer core–shell structure, where Pdop is the outer shell of the composite. Then, numerous “satellites” of gold nano-particles were assembled on the surface of Pdop/ SiO2/Fe3O4 via the reduction of Au by the Pdop/SiO2/Fe3O4 composite itself. The resulting Au/Pdop/SiO2/Fe3O4 composite showed high catalytic performance in the reduction of methylene blue (MB), at ambient temperature using NaBH4. The salient feature of this protocol is the dual role played by polydopamine, since it can act as both carrier and reductant for the Au nano-particles and can also prevent the silica from etching in an excessive NaBH4 solution.



In Multicomponent Reactions This branch of combinatorial chemistry is gaining much attention due to high atom economy, fewer reaction steps and one-pot operation procedure. Researchers are continuously developing new nano-catalysts for their efficient synthesis (95). Naeimi et al. reported the preparation of sulfonic acid-functionalized silica-coated magnetic nano-particles (Fe3O4@silica sulfonic acid) (96). The catalyst was further used for one-pot synthesis of 1-substituted 1H-tetrazoles from an amine, triethyl orthoformate and sodium azide (Scheme 21). Good yields, short reaction times, solvent-free conditions, non-toxicity and recyclability with very easy operation are the most important advantages of this catalyst. The catalyst can be easily recovered from the reaction system by an external magnet and reused up to 6 times without noticeable deterioration in catalytic activity.

Scheme 21. Synthesis of 1-substituted 1H-tetrazoles under solvent free conditions using Fe3O4@silica sulfonic acid

In recent times, ionic liquids have become a fascinating choice since their rational design allows creating additional functionalities that influences its properties. When this is combined with the superb features of magnetic nano-support, the resulting entity provides suitable heterogeneous systems where different bond cleavage and formation processes can be induced by IL functionality (97). This approach was utilized by Zolfigol et al. to synthesise a novel, green and recoverable heterogeneous catalyst by immobilizing ionic liquid on silica-coated Fe3O4 magnetic nano-particles {Fe3O4@SiO2@(CH2)3Im}C(CN)3 (98). It was found that ILs with desirable structural diversity and special properties could be attained through the design and synthesis of novel cationic cores with suitable anionic counterparts. The applicability of the catalyst was tested for the synthesis of hexahydroquinoline derivatives by the condensation of dimedone, ethyl acetoacetate, ammonium acetate as a source of nitrogen and a good range of arylaldehydes under benign, green and solvent-free conditions (Scheme 22). The work exhibited environmentally mild reaction conditions, reusability of the catalyst, short reaction time, high yield and easy work-up.



Scheme 22. Synthesis of polyhydroquinolines using ionic liquid modified SMNPs Recently, Nezhad and co-workers immobilized tungstic acid (TA), a known solid acid catalyst in organic synthesis, onto solid supports to make it even more benign using the reaction of sodium tungstate with the pre-prepared 3-chloropropyl magnetic nano-particles (99). The catalyst was successfully used for the one-pot synthesis of spirooxindoles via the multicomponent reaction of isatins, 5-amino1,3-dimethyluracil and 2-cyanoacetates in water, a green solvent (Scheme 23). The results revealed that this new catalyst showed high catalytic activity, short reaction times and that it can be reused at least 5 times without any change in its catalytic activity. Moreover, the MNP-TA catalyst provides great promise towards further useful applications in other acid-catalyzed transformations in future.

Scheme 23. One-pot synthesis of spirooxindoles in water using MNP supported tungstic acid 28 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The same group reported a new and clean procedure for chemical modification of MNPs with SH functionalized compounds such as L-cysteine and employed for catalyzing a multicomponent reaction (100). Vinyl functionalized magnetic nano-particles (VMNP) was synthesized by reacting SMNPs with trimethoxy(vinyl)silane. Reaction of VMNP substrate with L-cysteine in presence of azobisisobutyronitrile (AIBN) produced L-cysteine functionalized magnetic nano-particles (LCMNP). This LCMNP was utilized in a three-component coupling reaction between indole, salicylaldehyde and malononitrile as a catalyst for one-pot synthesis of 2-amino-4-(1H-indol-3-yl)-4H-chromene-3-carbonitrile (Scheme 24). The catalyst showed high activity and was reused with no appreciable loss in activity.

Scheme 24. One-pot synthesis of 2-amino-4H-chromene-3-carbonitriles in water using L-cysteine functionalized MNP

Miscellaneous Catalytic Reactions In addition to the above mentioned reactions, SMNPs are being used in various other organic transformations. Fan and co-workers developed an effective and recoverable catalyst by supporting ZnBr2 on MNPs coated by SiO2 for the synthesis of diphenyl carbonate from CO2 and phenol in the presence of CCl4 (101). This catalyst can be reused without significant loss in activity for 4 runs. Esmaeilpour et al. reported the synthesis of a Schiff base complex of metal ions-functionalized Fe3O4@SiO2 superparamagnetic nano-catalyst (Fe3O4@SiO2/Schiff base complex of metal ions) (102). The aforementioned catalyst was then employed for the conversion of aliphatic and aromatic aldehydes to their corresponding 1,1-diacetates compounds under solvent-free conditions at room temperature (Scheme 25). The advantages of the method (especially when the metal ion was Cr(III)), include easy and simple work up, short reaction times, mild reaction conditions, excellent chemoselectivity and excellent yields. 29 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 25. Synthesis of 1,1-diacetates from aldehydes under solvent free conditions using Fe3O4@SiO2/Schiff base complex of Cr(III) ion Sharma and co-workers have synthesized a copper based magnetic nano-catalyst by covalent grafting of quinoline-2-carboxaldehyde on amine functionalized SMNPs followed by immobilization of copper acetate (Cu-2QC@Am-SiO2@Fe3O4) (103). The resulting nano-catalyst showed excellent catalytic efficiency in the synthesis of carbamates via C–H activation of formamides under solventless conditions (Scheme 26). Additionally, broad substrate scope, use of green oxidant, less reaction time and easy recoverability made the protocol sustainable and environmentally benign.

Scheme 26. Synthesis of carbamates via C-H activation of formamides using Cu-2QC@Am-SiO2@Fe3O4

Conclusion In this chapter, we have provided an overview of the silica-encapsulated magnetic nano-particles and their major applications in the field of catalysis. Magnetic nanocomposites supported catalysts exhibit intrinsically high surface area, easy dispersion in reaction media and, at the same time, enable the trouble-free separation of the catalyst from the reaction mixture by simply 30 Sharma et al.; Ferrites and Ferrates: Chemistry and Applications in Sustainable Energy and Environmental Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


applying an external magnet. Silica coating strongly influences the physical and chemical properties of the magnetic core by offering numerous advantages such as prevention of agglomeration, chemical stability and easy mode of attachment. Moreover, due to the presence of Si-OH groups on the surface, it provides a magnificent platform for further versatile modifications. Thus, the silica modified superparamagnetic nanocomposites not only eliminate the use of tedious separation techniques, but also, make the protocol simple, economical and promising for industrial applications. Till now, magnetic nano-particles have opened the door for a wide range of research including, C-C, C-N, C-S, C-O cross-coupling reaction, oxidation, reduction and many other name reactions. Therefore, it is considered as one of the major growing areas in the catalytic domain. However, the main challenge is to exploit these magnetic materials in different emerging fields such as petrochemical industries, continuous flow reactor, microreactors mediated synthesis, etc. Due to the highly stable nature of SMNPs, a number of high-pressure and high-temperature organic transformations can be conducted. Besides this, minimal leaching is observed with these nano-catalysts due to the sturdy interaction between magnetic support and metal immobilized on it. The utility of such highly efficient matrices can be further extended in the accomplishing vapor phase reactions involving gas phase reactors. These magnetically recoverable nano-catalysts, therefore, provides a strong platform for future developments in catalytic field.

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