Magnetically Separable Nanocatalysts: Bridges between Homogeneous and Heterogeneous Catalysis

Core-shell bimetallic nanoparticles robustly fixed on the outermost surface of magnetic silica microspheres

The major challenges in practically utilising the immense potential benefits of nanomaterials are controlling aggregation, recycling the nanomaterials, and fabricating well-defined nanoparticulate materials using innovative methods. We present a novel innovative synthetic strategy for core–shell bimetallic nanoparticles that are well-defined, ligand-free, and robustly fixed on the outermost surface of recyclable magnetic silica microspheres. The strategy includes seeding, coalescing the seeds to cores, and then growing shells from the cores on aminopropyl-functionalised silica microspheres so that the cores and aminopropyl moieties are robustly embedded in the shell materials. The representative Au–Ag bimetallic nanoparticles fixed on the microsphere showed excellent catalytic performance that remained consistent during repeated catalytic cycles.

Magnetically recoverable ruthenium catalysts in organic synthesis

Magnetically recyclable catalysts with magnetic nanoparticles (MNPs) are becoming a major trend towards sustainable catalysts. In this area, recyclable supported ruthenium complexes and ruthenium nanoparticles occupy a key place and present great advantages compared to classic catalysts. In this micro-review, attention is focused on the fabrication of MNP-supported ruthenium catalysts and their catalytic applications in various organic syntheses.

Preparation of uniform magnetic recoverable catalyst microspheres with hierarchically mesoporous structure by using porous polymer microsphere template

Merging nanoparticles with different functions into a single microsphere can exhibit profound impact on various applications. However, retaining the unique properties of each component after integration has proven to be a significant challenge. Our previous research demonstrated a facile method to incorporate magnetic nanoparticles into porous silica microspheres. Here, we report the fabrication of porous silica microspheres embedded with magnetic and gold nanoparticles as magnetic recoverable catalysts. The as-prepared multifunctional composite microspheres exhibit excellent magnetic and catalytic properties and a well-defined structure such as uniform size, high surface area, and large pore volume. As a result, the very little composite microspheres show high performance in catalytic reduction of 4-nitrophenol, special convenient magnetic separability, long life, and good reusability. The unique nanostructure makes the microspheres a novel stable and highly efficient catalyst system for various catalytic industry processes.

Recent Advances in the Application of Magnetic Nanoparticles as a Support for Homogeneous Catalysts

Magnetic nanoparticles are a highly valuable substrate for the attachment of homogeneous inorganic and organic containing catalysts. This review deals with the very recent main advances in the development of various nanocatalytic systems by the immobilisation of homogeneous catalysts onto magnetic nanoparticles. We discuss magnetic core shell nanostructures (e.g., silica or polymer coated magnetic nanoparticles) as substrates for catalyst immobilisation. Then we consider magnetic nanoparticles bound to inorganic catalytic mesoporous structures as well as metal organic frameworks. Binding of catalytically active small organic molecules and polymers are also reviewed. After that we briefly deliberate on the binding of enzymes to magnetic nanocomposites and the corresponding enzymatic catalysis. Finally, we draw conclusions and present a future outlook for the further development of new catalytic systems which are immobilised onto magnetic nanoparticles.

Ultrasonic-assisted ultra-rapid synthesis of monodisperse meso-SiO2@Fe3O4 microspheres with enhanced mesoporous structure

A core-shell-type of meso-SiO2@Fe3O4 microsphere was synthesized via an ultrasonic-assisted surfactant-templating process using solvothermal synthesized Fe3O4 as core, tetraethoxysilane (TEOS) as silica source, and cetyltrimethyl ammonium bromide (CTAB) as templates. The samples were characterized by FT-IR, XRD, TEM, N2 adsorption-desorption technology, and vibrating sample magnetometer (VSM). The results show that as-prepared meso-SiO2@Fe3O4(E) and meso-SiO2@Fe3O4(C) microspheres, treated by acetone extraction and high temperature calcination, respectively, still maintain uniform core-shell structure with desirable mesoporous silica shell. Therein, the meso-SiO2@Fe3O4(E) microspheres possess a distinct pore size distribution in 1.8-3.0 nm with large specific surface area (468.6 m(2)/g) and pore volume (0.35 cm(3)/g). Noteworthily, the coating period of this ultrasonic-assisted method (40 min) is much shorter than that of the conventional method (12-24 h). The morphology of microspheres and the mesoporous structure of silica shell are significantly influenced by initial concentration of CTAB (CCTAB), ultrasonic irradiation power (P) and ultrasonic irradiation time (t). The acceleration roles of ultrasonic irradiation take effect during the whole coating process of mesoporous silica shell, including hydrolysis-condensation process of TEOS, co-assembly of hydrolyzed precursors and CTAB, and deposition of silica oligomers. In addition, the use of ultrasonic irradiation is favorable for improving the homogeneity of silica shell and the monodispersity of meso-SiO2@Fe3O4 microspheres.

Biphasic catalysis using amphiphilic polyphenols-chelated noble metals as highly active and selective catalysts

In the field of catalysis, it is highly desired to develop novel catalysts that combine the advantages of both homogeneous and heterogeneous catalysts. Here we disclose that the use of plant pholyphenol as amphiphilic large molecule ligand/stabilizer allows for the preparation of noble metal complex and noble metal nanoparticle catalysts. These catalysts are found to be highly selective and active in aqueous-organic biphasic catalysis of cinnamaldehyde and quinoline, and can be reused at least 3 times without significant loss of activity. Moreover, the catalytic activity and reusability of the catalysts can be rationally controlled by simply adjusting the content of polyphenols in the catalysts. Our strategy may be extended to design a wide range of aqueous-organic biphasic catalysis system.

Pickering-emulsion inversion strategy for separating and recycling nanoparticle catalysts

With the recent advances in nanoscience and nanotechnology, more and more nanoparticle catalysts featuring high accessibility of active sites and high surface area have been explored for their use in various chemical transformations, and their rise in popularity among the catalysis community has been unprecedented. The industrial applications of these newly discovered catalysts, however, are hampered because the existing methods for separation and recycling, such as filtration and centrifugation, are generally unsuccessful. These limitations have prompted development of new methods that facilitate separation and recycling of nanoparticle catalysts, so as to meet the burgeoning demands of green and sustainable chemistry. Recently, we have found that Pickering-emulsion inversion is an appealing strategy with which to realize in situ separation and recycling of nanoparticle catalysts and thereby to establish sustainable catalytic processes. We feel that at such an early stage, this strategy, as an alternative to conventional methods, is conceptually new for readers but that it has potential to become a popular method for green catalysis. This Concept article aims to provide a timely link between previous efforts and both current and future research on nanoparticle catalysts, and is expected to facilitate further investigation into this strategy.

Polymer- and dendrimer-coated magnetic nanoparticles as versatile supports for catalysts, scavengers, and reagents

The work-up of chemical reactions by standard techniques is often time consuming and energy demanding, especially when chemists have to guarantee low levels of metal contamination in the products. Therefore, scientists need new ideas to rapidly purify reaction mixtures that are both economically and environmentally benign. One intriguing approach is to tether functionalities that are required to perform organic reactions to magnetic nanoparticles, for example, catalysts, reagents, scavengers, or chelators. This strategy allows researchers to quickly separate active agents from reaction mixtures by exploiting the magnetic properties of the support. In this Account, we discuss the main attributes of magnetic supports and describe how we can make the different nanomagnets accessible by surface functionalization. Arguably the most prominent magnetic nanoparticles are superparamagnetic iron oxide nanoparticles (SPIONs) due to their biologically well-accepted constituents, their established size-selective synthesis methods, and their diminished agglomeration (no residual magnetic attraction in the absence of an external magnetic field). However, nanoparticles made of pure metal have a considerably higher magnetization level that is useful in applications where high loadings are needed. A few layers of carbon can efficiently shield such highly reactive metal nanoparticles and, equally important, enable facile covalent functionalization via diazonium chemistry or non-covalent functionalization through π-π interactions. We highlight carbon-coated cobalt (Co/C) and iron (Fe/C) nanoparticles in this Account and compare them to SPIONs stabilized with surfactants or silica shells. The graphene-like coating of these nanoparticles offers only low loadings with functional groups via direct surface modification, and the resulting nanomagnets are prone to agglomeration without effective steric stabilization. To overcome these restrictions and to tune the dispersibility of the magnetic supports in different solvents, we can introduce dendrimers and polymers on Co/C and Fe/C platforms by various synthetic strategies. While dendrimers have the advantage of being able to array all functional groups on the surface, polymers need fewer synthetic steps and higher molecular weight analogues are easily accessible. We present the application of these promising hybrid materials for the extraction of analytes or contaminates from complex aqueous solutions (e.g. waste water treatments or blood analytics), for metal-, organo-, and biocatalysis, and in organic synthesis. In addition, we describe advanced concepts like magnetic protecting groups, a multistep synthesis solely applying magnetic reagents and scavengers, and thermoresponsive self-separating magnetic catalysts. We also discuss the first examples of the use of magnetic scaffolds manipulated by external magnetic fields in flow reactors on the laboratory scale. These hold promise for future applications of magnetic hybrid materials in continuous flow or highly parallelized syntheses with rapid magnetic separation of the applied resins.

Multifunctional two-photon active silica-coated Au@MnO Janus particles for selective dual functionalization and imaging

Monodisperse multifunctional and nontoxic Au@MnO Janus particles with different sizes and morphologies were prepared by a seed-mediated nucleation and growth technique with precise control over domain sizes, surface functionalization, and dye labeling. The metal oxide domain could be coated selectively with a thin silica layer, leaving the metal domain untouched. In particular, size and morphology of the individual (metal and metal oxide) domains could be controlled by adjustment of the synthetic parameters. The SiO2 coating of the oxide domain allows biomolecule conjugation (e.g., antibodies, proteins) in a single step for converting the photoluminescent and superparamagnetic Janus nanoparticles into multifunctional efficient vehicles for theranostics. The Au@MnO@SiO2 Janus particles were characterized using high-resolution transmission electron microscopy (HR-)TEM, powder X-ray diffraction (PXRD), optical (UV-vis) spectroscopy, confocal laser fluorescence scanning microscopy (CLSM), and dynamic light scattering (DLS). The functionalized nanoparticles were stable in buffer solution or serum, showing no indication of aggregation. Biocompatibility and potential biomedical applications of the Au@MnO@SiO2 Janus particles were assayed by a cell viability analysis by coincubating the Au@MnO@SiO2 Janus particles with Caki 1 and HeLa cells. Time-resolved fluorescence spectroscopy in combination with CLSM revealed the silica-coated Au@MnO@SiO2 Janus particles to be highly two-photon active; no indication for an electronic interaction between the dye molecules incorporated in the silica shell surrounding the MnO domains and the attached Au domains was found; fluorescence quenching was observed when dye molecules were bound directly to the Au domains.

Pd/C as a catalyst for completely regioselective C-H functionalization of thiophenes under mild conditions

The completely C3-selective arylation of thiophenes and benzo[b]thiophenes was achieved by using Pd/C as a heterogeneous catalyst without ligands or additives under mild reaction conditions. The practicability of this transformation is demonstrated by notable functional group tolerance and the insensitivity of the reaction to H2 O and air. This method is also applicable to nitrogen- and oxygen-containing heterocycles, yielding the corresponding C2-arylated products. Three-phase tests along with Hg-poisoning and hot-filtration tests suggest that the catalytically active species is heterogeneous in nature.

Magnetically separable core-shell structural γ-Fe2O3@Cu/Al-MCM-41 nanocomposite and its performance in heterogeneous Fenton catalysis

To target the low catalytic activity and the inconvenient separation of copper loading nanocatalysts in heterogeneous Fenton-like reaction, a core-shell structural magnetically separable catalyst, with γ-Fe2O3 nanoparticles as the core layer and the copper and aluminum containing MCM-41 as the shell layer, has been fabricated. The role of aluminum has been discussed by comparing the copper containing mesoporous silica with various Cu contents. Their physiochemical properties have been characterized by XRD, UV-vis, FT-IR, TEM, nitrogen physisorption and magnetite susceptibility measurements. Double content Cu incorporation results in an improved catalytic activity for phenol degradation at the given condition (40°C, initial pH=4), but leads to a declined BET surface area and less ordered mesophase structure. Aluminum incorporation helps to retain the high BET surface area (785.2m(2)/g) and the regular hexagonal mesoporous structure of MCM-41, which make the catalyst possess a lower copper content and even a higher catalytic activity than that with the double copper content in the absence of aluminum. The catalysts can be facilely separated by an external magnetic field for recycle usage.

A core-shell-satellite structured Fe3O4@MS-NH2@Pd nanocomposite: a magnetically recyclable multifunctional catalyst for one-pot multistep cascade reaction sequences

A hierarchical core-shell-satellite structured composite system Fe3O4@MS-NH2@Pd, which was composed of Pd nanoparticles well-dispersed on an amino group functionalized mesoporous silica (MS-NH2) nanosphere, and superparamagnetic Fe3O4 nanoparticles scattered inside the silica sphere, was prepared by using a facile procedure. The composite combined the catalytic properties of amino groups and Pd nanoparticles with superparamagnetic properties of magnetite into a single platform. This integrated nanosystem acted as an efficient magnetically recyclable noble metal-base multifunctional nanocatalyst and showed excellent catalytic activity, selectivity and stability for the direct synthesis of α-alkylated nitriles under mild conditions through facile one-pot multistep cascade reaction sequences.

Preparation and characterization of TiO2–Graphene@Fe3O4 magnetic composite and its application in the removal of trace amounts of microcystin-LR

A facile and feasible approach is presented to prepare a novel magnetic photocatalyst TiO₂–graphene@Fe₃O₄ composite exhibiting a superior performance in the removal of trace microcystin-LR in water samples.

Nanocatalysis: Academic Discipline and Industrial Realities

Nanotechnology plays a central role in both academic research and industrial applications. Nanoenabled products are not only found in consumer markets, but also importantly in business to business markets (B2B). One of the oldest application areas of nanotechnology is nanocatalysis—an excellent example for such a B2B market. Several existing reviews illustrate the scientific developments in the field of nanocatalysis. The goal of the present review is to provide an up-to-date picture of academic research and to extend this picture by an industrial and economic perspective. We therefore conducted an extensive search on several scientific databases and we further analyzed more than 1,500 nanocatalysis-related patents and numerous market studies. We found that scientists today are able to prepare nanocatalysts with superior characteristics regarding activity, selectivity, durability, and recoverability, which will contribute to solve current environmental, social, and industrial problems. In industry, the potential of nanocatalysis is recognized, clearly reflected by the increasing number of nanocatalysis-related patents and products on the market. The current nanocatalysis research in academic and industrial laboratories will therefore enable a wealth of future applications in the industry.

Cross-linked poly(dimethylaminoethyl acrylamide) coated magnetic nanoparticles: a high loaded, retrievable, and stable basic catalyst for the synthesis of benzopyranes in water

A high loaded basic magnetic catalyst was synthesized using a distillation–precipitation–polymerization method. The catalyst shows high activity in water for the synthesis of 4H-benzopyrans.

A highly efficient and recyclable cobalt ferrite chitosan sulfonic acid magnetic nanoparticle for one-pot, four-component synthesis of 2H-indazolo[2,1-b]phthalazine-triones

A novel magnetical cobalt ferrite chitosan sulfonic acid (CoFe₂O₄-CS-SO₃H) was prepared and identified as an efficient catalyst for synthesis of 2H-indazolo[2,1-b]phthalazine-triones by one-pot, four-component reaction.

A facile and efficient synthesis of functionalized 4-oxo-2-(phenylimino)thiazolidin-5-ylideneacetate derivatives via a CuFe2O4 magnetic nanoparticles catalyzed regioselective pathway

Nano-CuFe₂O₄ spinel has been found to exhibit strong catalytic activity in the cascade reaction involving 1,4-addition and intramolecular electrophilic cyclization in a perfectly regiocontrolled manner and a series of functionalized 4-oxo-2-(phenylimino)thiazolidin-5-ylideneacetate derivatives were generated successfully.

Stabilizing Pd on the surface of amine-functionalized hollow Fe3O4 spheres: a highly active and recyclable catalyst for Suzuki cross-coupling and hydrogenation reactions

Preparation of palladium nanoparticles supported on amine-functionalized hollow Fe₃O₄.

Immobilization of palladium catalyst on magnetically separable polyurea nanosupport

Polyurea nanoparticles doped with magnetic nanoparticles were readily prepared via oil-in-oil nanoemulsification technique and utilized for supporting palladium catalyst.

One-pot electrochemical synthesis of polydopamine coated magnetite nanoparticles

Herein a facile and versatile one step synthesis of magnetite nanoparticles coated with polydopamine is described.

Fe3O4@SiO2–imid–PMAnmagnetic porous nanospheres as recyclable catalysts for the one-pot synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]xanthenes and 1,8-dioxooctahydroxanthene derivatives under various conditions

Synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]xanthenes and 1,8-dioxooctahydroxanthene derivatives by Fe₃O₄@SiO₂–imid–PMA nanocatalysts.

Sulfanilic acid-functionalized silica-coated magnetite nanoparticles as an efficient, reusable and magnetically separable catalyst for the solvent-free synthesis of 1-amido- and 1-aminoalkyl-2-naphthols

Sulfanilic acid-functionalized silica-coated magnetite nanoparticles as an efficient, reusable and magnetically separable catalyst for the solvent-free synthesis of 1-amido- and 1-aminoalkyl-2-naphthols.

Fabrication of magnetic amino-functionalized nanoparticles for S-arylation of heterocyclic thiols

The aminosilane coupling agent and polyethylene imine-600 were loaded onto magnetic nanoparticles to obtain magnetic nanoligands (MNLs) A, B, C and D.

Facile cyclization in the synthesis of highly fused diaza cyclooctanoid compounds using retrievable nano magnetite-supported sulfonic acid catalyst

Magnetically separable Fe₃O₄–SO₃H nano particles have been prepared and established as an efficient catalyst for the synthesis of dihydrofuran fused cyclooctanoids via the dehydration reaction of dihydroxy cyclooctanoid heterocylces.

Palladium nanoparticles supported on agarose-functionalized magnetic nanoparticles of Fe3O4 as a recyclable catalyst for C–C bond formation via Suzuki–Miyaura, Heck–Mizoroki and Sonogashira–Hagihara coupling reactions

Palladium nanoparticles supported on agarose functionalized Fe₃O₄ catalyzed C–C bond formation reactions.

A thermally stable and easily recycled core–shell Fe2O3@CuMgAl catalyst for hydrogenolysis of glycerol

Core–shell structured magnetic Fe₂O₃@CuMgAl layered double hydroxide (LDH) catalysts were synthesized in a facile route and used in selective hydrogenolysis of glycerol.

Water-medium organic synthesis over active and reusable organometal catalysts with tunable nanostructures

Robust and reusable heterogeneous organometal catalysts open a new avenue to green chemical synthesis in water.