# Recovery/Reuse of Heterogeneous Supported Spent Catalysts
## 3 Methods for Recycling Heterogenous Catalysts  
### Traditional Methods of Recovery: Filtration and Centrifugation  
In accordance with the need of green and sustainable development, a long lifetime and an easily recycling are important requirements for practical application of a catalyst in industry [31]. In gas/solid systems, the catalyst is easily recovered from the reaction medium mechanically. Instead, in liquid media reaction, a rapid and efficient method of separation is required to remove the catalyst from products in order to recycle it as many times as possible. Therefore, the appropriate separation process should be chosen based on the nature of the catalytic material, the time required, and the energy consumed. _Filtration_ and _Centrifugation_ are two of the methods traditionally employed in laboratory scale to allow handling, separation, recovery, and recycling of heterogeneous catalysts. Like all the other techniques of separation, these techniques are based on the different chemical or physical properties between the components of the reaction mixture. _Filtration_ exploits the different size of the components of the mixture to separate an undissolved solid from a liquid or a gas. The technique involves the use of a porous structure (filter/membrane) called septum that allows the fluid phase of mixture to pass through but keeps larger solid particles back. As the liquid/solid material passes through the filter, the solid material is collected and removed. In this process, the collected solid material is called \"residue\" while the fluid material more or less thoroughly separated from the solids is called \"filtrate\", \"effluent\", or \"permeate\". The permeation of the fluid phase through the filter medium is influenced by the gradient of pressure across the septum [32]. Filtration processes can be classified in accordance with driving forces that produce such difference of pressure. In most cases, the liquid is allowed to flow through the filter medium only by gravity, and the process is known as \"gravity filtration\". Generally, gravity filtration is used to remove solid impurities such as drying agent, an undesired side product, or leftover reactant from a liquid media. Since this driving force is relatively weak, the liquid phase must be of low viscosity and the mixture sufficient dilute. The difference of pressure for the \"centrifugal filtration\" is due to the centrifugal force acting on the fluid that, at high rotation speeds, can be many orders of magnitude higher than the gravitational force. The process requires more technical equipment, but, as a general rule, it is much faster and more efficient than the gravitational one. \"Vacuum  
Figure 1: The catalyst’s life cycle. filtration\" and \"pressured filtration\" usually are used in the laboratory in preference to gravity filtration. A vacuum filter operates by creating a partial vacuum on the opposite side of the filter. Therefore, the vacuum filtration technique requires a vacuum pump able to aspire the liquid through the septum. In pressure filtration, the liquid phase is driven through the filter by mechanical pressure on the feed side of the filter. Since the driving force (pressurized fluid or vacuum) is much greater than gravity, the pressure difference generated is sufficient to filter a filtrate too viscus, thus permitting higher filtration rates and a lower consumption of solvents. Processes of filtration are schematized in Figure 2.  
As in other separation processes, the separation of phases by filtration is never complete: liquid adheres to the separated solids, and the filtrate often contains some solids. Therefore, the recovery of solid catalysts after separation from reaction mixture by filtration methods very often requires further treatments such as washing, deliquoring (elimination of liquid from the filter by mechanical forces), and thermal drying [32].