# Recovery/Reuse of Heterogeneous Supported Spent Catalysts
## 3 Methods for Recycling Heterogenous Catalysts
### Magnetic Separation
#### 3.2.5 Ionic Liquid Coated Magnetic Nanoparticles
To improve the catalytic activity, it is necessary to increase the amount of catalyst used for the catalytic process, and this could require a greater use of solvents as well as expensive recovery cost and possible toxicological concern [23]. This problem could be solved by the surface modification of the magnetic nanoparticles with different polymer grafting methods [78]. The introduction of polymer onto the MNPs increases the loading content of grafted homogenous material since more catalytic units can be attached onto magnetic support [78]. On the other hand, the polymer layer could prevent the formation of clusters in high temperature processes [79]. [PERSON] et al. developed a novel poly (ionic liquid) (PIL) coated magnetic nanoparticles catalyst, noted as n-Fe\({}_{3}\)O\({}_{4}\)/PVAm-SO\({}_{3}\)H, and used it as a magnetic heterogeneous acid catalyst for the one-pot synthesis of tri and tetrasubstituted imidazole derivatives under optimized condition in excellent yields. The catalyst was synthesized by directly grafting polyvinyl amine layer onto surface of Fe\({}_{3}\)O\({}_{4}\) without using any bridged organosilane compounds. In the following, the sulfonation treatment was carried out by covalent grafting of chlorosulfonic acid on amine groups to convert n-Fe\({}_{3}\)O\({}_{4}\)/PVAm into an efficient acid catalyst. In addition, the resulted catalyst was readily separated in a green way by using a magnetic force and reused eight times without any significant deterioration in catalytic activity. The study results show that this new acid magnetic catalyst exhibits more advantages than soluble acids [78]. Nowadays, dicationic ionic liquids (DILs), a new subclass of ILs, have attracted significant attention. Typically, these molecules contain two cationic head groups connected together with a rigid or flexible spacer, associated with two counter anions, respectively [75]. Modifying the length of the chain and the type of spacer or the cation, their physicochemical properties can be tuned [71]. In comparison to various reported monocationic ionic liquid in the literature, dicationic ionic liquids exhibit several interesting advantages such as higher thermal stability, surface activity, more extensive liquid range, and higher selectivity [75]. These ILs can be used in many applications from the \"classical\" use as solvents, catalysts, or catalytic supports in organic reactions, to more specific uses as high temperature lubricants/heat transfer fluids, including a variety of roles in analytical sciences [71]. [PERSON] and co-workers designed Brensted acidic dicationic ionic liquid immobilized on silica-coated iron oxide support with core-shell structure and applied it as a recyclable nanocatalyst under solvent-free conditions in the synthesis of pyrazole derivatives [80]. The catalyst has maintained his catalytic activity at least five times. In addition, [PERSON] demonstrated that magnetic \(\gamma\)Fe\({}_{2}\)O\({}_{3}\)@SiO\({}_{2}\) NPs-supported Bis([3-aminopropyl] triethoxysilane) dichloride bis-dicationic ionic liquid (DIL) catalyst can catalyse the Knoevenagel condensation achieving high reaction rates and yields. The synthesized nanomagnetic catalyst possesses a high superparamagnetic characteristic and can be simply recovered and reused applying an external magnetic field at least six times without loss of catalytic activity [44]. Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are a class of porous crystalline materials, highly investigated in the last twenty years. Several features of MOFs such as controllable composition, large surface areas, regular and accessible pores, and crystalline open structures allows them to encapsulate within their frameworks magnetic nanoparticles, making them promising heterogeneous catalysts or catalytic carriers of various species including ionic liquids [81]. For example, very recently, [PERSON] et al.