# Recovery/Reuse of Heterogeneous Supported Spent Catalysts
## 3 Methods for Recycling Heterogenous Catalysts
### Magnetic Separation
#### 3.2.1 Magnetic Nanoparticles (MNPs)  
In recent years, magnetic nanoparticles (MNPs) have received considerable attention among researchers due to their wide use in the preparation of recyclable catalysts. These nanoparticles can be directly used as catalysts or as magnetic carriers for the catalytic species immobilized on their surface. Utilizing magnetic nanoparticles as catalysts or supports helps to combine the advantages of both nano-sized particles and magnetic materials. The large surface-to-volume ratio of these types of materials allows excellent loading of the homogeneous catalyst and then obtaining a magnetic catalyst with high catalytic activity. Magnetic nanoparticles (MNPs) usually exhibit a phenomenon known as superparamagnetism. Superparamagnetism is a form of magnetism that takes place when the size of ferromagnetic or ferrimagnetic nanoparticles is as low as 10-20 nm [41]. As a consequence of an externally applied magnetic field, superparamagnetic and paramagnetic materials become magnetized up to their saturation magnetization. As soon as the field goes to zero, any residual magnetic interaction disappears. In comparison with paramagnetic materials,  
Figure 4: Comparison of recovery methods: filtration, centrifugation, and magnetic separation. superparamagnetic nanoparticles have a higher magnetic susceptibility, rendering them highly sensitive to externally applied magnetic fields [42]. If nanoparticles are sufficiently small, each of them can be considered as characterized by a single magnetic domain able to act as a \"single super spin\". Unlike ferromagnetic materials, in the absence of the magnetic field, these nanoparticles randomly change the orientation of their spins even at room temperature. Instead, the ordered magnetic moments of ferromagnetic materials become disordered in response to thermal fluctuations only above the Curie Temperature (CT), or Curie point, losing their permanent magnetism. The magnetic component of MNPs provides a fast separation from the liquid phase by application of an external magnetic field and an easy redispersion in the absence of an applied magnetic field, avoiding in this way the conventional filtration, centrifugation, or other tedious workup processes. Most magnetic-supported catalysts can be reused to several runs almost keeping their initial activity [43]. When a permanent magnet is placed in contact with a beaker containing a suspension of magnetic nanomaterial, magnetic dipoles of nanoparticles point in the same direction of the field lines. As shown in Figure 5, the applied magnetic field induces nanoparticles to migrate towards the magnet. After a few minutes, all particles end up concentrated at the wall in correspondence with the magnet; in this way, the supernatant can be easily aspirated with a syringe. When the magnet is removed, the random orientation of nanoparticle is so fast that the measured magnetic moment gets down to zero quickly and nanoparticles behave as no-magnetic materials. Properties such as large surface area, relatively low preparation cost and toxicity, good stability, coercivity (the field required to bring the magnetism to zero), low Curie temperature [44], and rapid recovery make them ideal supports in the field of catalysis. Thanks to these particular properties, these materials are also applied in a variety of fields such as biotechnology/biomedicine [45], magnetic resonance imaging (MRI) [46],  
Figure 5: Schematic representation of the magnetic separation. controlled drug delivery [46], biosensors [44], hyperthermia [47], gene therapy [48], gas sensor [49], data storage [50], adsorbent materials [51], batteries [52], pigments [53], water purification [54], tissue repair [55], detoxification of biological fluids [56], and labeling and separation of cells [57]. The most common MNPs are pure metal nanoparticles (Fe, Co, Ni), metal alloys (FePt, CoPt), iron oxides (FeO, Fe\({}_{2}\)O\({}_{3}\), Fe\({}_{3}\)O\({}_{4}\)), or ferrites MFe\({}_{2}\)O\({}_{4}\) (M = Co, Mn, Cu, Zn) [24].