# Recovery/Reuse of Heterogeneous Supported Spent Catalysts
## 3 Methods for Recycling Heterogenous Catalysts
### Traditional Methods of Recovery: Filtration and Centrifugation
Recently, [PERSON] and her collaborators synthetized a highly effective heterogeneous and environmentally benign catalyst for the conversion of a variety of amines to their corresponding formamides and acetamides, which was recovered by filtering the reaction mixture and recycled seven times without any loss in activity [33]. _Centrifugation_ is a technique frequently used to separate mixtures of two or more phases on the basis of their different density. Separation of heterogeneous mixtures through sedimentation happens spontaneously due to earth gravity, but it would take a long time. Figure 2: Schematization of filtration processes: gravity, centrifugal, pressure, and vacuum. If separation is carried out by the application of a centrifugal force, this natural process is accelerated (Figure 3). According to [PERSON]'s first law, a body tends to remain in a state on rest or linear uniform motion in the absence of forces acting upon it. When the body moves with uniform speed along a circular, a force directed towards the center of the circle acts upon it. This force is called centripetal force. Changing the frame of reference, the body moving in the curve path feels an apparent force called _centrifugal force_ that has the same magnitude and dimensions as the centripetal force but points in the opposite direction. Such force is dependent on the speed of rotation in RPM and the distance of the body from the center of rotation. The centrifugal force provided by a centrifuge in combination with gravitational force cause particles to move radially away from the axis of rotation. More-dense particles of the mixture tend to move at the bottom of the tube, while less-dense particles of the mixture migrate towards the axis of the centrifuge. The solid deposited on the bottom of the tube is called \"pellet\", while the remaining solution is called the \"supermate\" or \"supernat liquid\". The supernatant liquid can be quickly decanted from the tube without disturbing the precipitate, or aspirated with a Pasteur pipette [34]. [PERSON] et al., in their recent study, prepared an efficient heterogeneous Nickel--Copper bimetallic catalyst for the synthesis of substituted amines that was successfully recovered through centrifugation and reused four times without notable decrease in its activity [35]. Figure 3: Schematic representation of the sedimentation and the centrifugation processes. However, recycling of heterogeneous catalysts using traditional methods of separation such as filtration and centrifugation is tedious and time-consuming. In addition, these separation techniques are not usually able to collect the nano-sized catalysts without weight loss. Recovery by filtration is not advisable because smaller sized nanoparticles could pass through the filter paper. On the other hand, centrifugation is uneconomic processes and, consequently, it cannot be carried out on an industrial scale. Recently, magnetic separation for catalyst recycling provided an interesting alternative to the separation methods mentioned above in terms of time, energy, and efficiency of catalysis [36; 37; 38].