# Recovery/Reuse of Heterogeneous Supported Spent Catalysts
## 3 Methods for Recycling Heterogenous Catalysts
### Magnetic Separation
#### 3.2.5 Ionic Liquid Coated Magnetic Nanoparticles  
Recently, Ionic liquids (ILs)-coated MNPs have attracted growing interest as catalysts for a wide range of new industrial applications. Ionic liquids (ILs) are commonly defined as salts composed of organic cations and organic or inorganic anions with melting point below the boiling point of water. According to the nature of the cation and/or the counter anion moiety, the ionic liquid may behave as an acidic, basic, or organ catalyst [73]. Many of these ILs have been investigated in a variety of organic transformations in which ILs can act as solvents and/or catalysts [74]. They have a number of advantages such as wide liquid range and potential window, extremely low vapor pressure, high ionic conductivity, and extraordinary ability to functionalization [75]. However, their industrial applications are hampered by their high viscosity [71], high cost, large consumption, and homogeneousness (difficulty to separate) [76]. To overcome these drawbacks, supported ionic liquid catalysts (SIL) could be a valid alternative to homogeneous analogues since they maintain the excellent catalytic properties of ionic liquids, but also manifest the advantages of easy separation and recovery typical of the heterogeneous catalyst. On the basis of this awareness, different support materials, such as amorphous or ordered mesoporous silica, zeolites, polystyrene, magnetic nanoparticles, and many others have been used as carrier for loading ionic liquids [77]. Particularly, ionic liquids immobilized on the magnetic nanoparticles surface, have emerged as good catalysts because of their facile separation by an external magnet. [PERSON] et al. synthesised a new, highly active, and efficiently recoverable catalytic system modifying nano-magnetite materials with a paramagnetic halometallic-based ionic liquid (MILs), Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)@(mim)[FeCl\({}_{4}\)] (mim: methylimidazolium). This system was applied in the glycolysis of PET into BHET under conventional heating. The catalyst exhibited an excellent recycling behaviour, achieving nearly 100% yield and selectivity over twelve consecutive reaction cycles at 180 \({}^{\circ}\)C and was easily recovered with an external magnetic field avoiding tedious work-up or purification processes. Further analyses by ICP revealed that the amount of catalyst lost after each cycle was minimal and the amount of Fe in the purified product was negligible [76]. [PERSON] et al. synthesized an alkaline ionic liquid immobilized on protective copolymers coated magnetic nanoparticles ([Im][OH]/P(VBC-DVB) @MNPs). The catalyst consisted of three layers: a magnetic core, an intermediate protective shell, and an outer catalytic layer. The magnetic core, mainly composed of CoFe\({}_{2}\)O\({}_{4}\) magnetic nanoparticles (CFNPs), makes easier separation of the catalyst from reaction medium. The protective layer, formed by the copolymer of divinylbenzene (DVB) and vinyl benzyl chloride (VBC), incorporated into the catalyst, on the other hand, prevents the corrosion of magnetic nanoparticle when the catalyst is used in polar solvents, thus preserving recyclability and improving its stability. The catalytic layer, which consisted of 1-propyl-3-alkylimidazole hydroxide ionic liquid, is where the reaction takes part. This catalyst showed an excellent catalytic activity for Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate due to the large amount of OH\({}^{-}\) ions loading on the surface and a lower activation energy than of homogeneous catalyst NaOH. The catalyst was conveniently recovered upon applying an external magnetic field, and it was recycled at least six times without significant changes [23]. However, since the loading of ILs on the surface cannot be too high, catalytic activity turns out to be limited owing to the relatively low number of catalytic units compared to the large surface area of magnetic nanoparticles [71].