## 4 Types of Catalysts Used in Transesterification
### Nano-Catalysts  
Homogeneous and heterogeneous catalysts have their own merits and demerits, and a synergism framework of new catalytic system is urgently and critically needed. These catalysts should be dynamic and active as homogeneous catalysts are, and effective as well as easily recoverable in the way that heterogeneous catalysts are. Nano catalysts have combined point of interest and joint advantages of both homogeneous and heterogeneous catalysts. Nano catalysts permit selective and quick chemical transformation with easiness of recovery and catalyst separation with a tremendously high product yield [54]. The contact between catalyst and reactant is incremental due to the nano-size that exhibits large surface area, similarly to homogeneous catalysts. Nano-catalysts are insoluble in the way that heterogeneous catalysts are. Therefore, nano-catalysts are also easy to recover and separate without difficulty. The most technological and scientific challenge linked to the utilization of nanoparticles is to understand the atomic scale structure and composition of nano-catalysts that gives the best catalytic activity. Nano-catalysts are particles with size ranging from 1 nm to 100 nm. The nano size of catalyst affects the biodiesel yield because such catalysts offer a large surface area, thus efficiently enhancing the rate of reaction [55; 56]. Their synthesis is the main task in the field of nano-catalysis. Top-down and bottom-up technologies are two basic categories of methods for preparation of nano-catalysts. The conventional methods include solid-state synthesis, combustion, sol-gel, co-precipitation, thermal decomposition and hydrothermal etc. However, unconventional methods include spray pyrolysis, microemulsion, pyrolysis (spray and laser), sonochemical, reverse micelle, drying and biosynthesis. Figure 5 shows scheme of methods for synthesis of nanoparticles. Nanoparticle science depends on two factors: first is the union of nanoparticles to manage size and shape, second is a subatomic mechanism in order to search particular  
Figure 5: Methods for synthesis of nanoparticles. utilization of nanoparticles, especially nano-catalysis. Synthesis of nano-catalysts with controlled size is primarily achieved by employing a range of stabilizing operators, for instance polymers, ligands and surfactants etc. The selection and reaction of nano-catalysts is considerably significant because it has high impact on progress of reaction that necessarily depends on the surface area of nanoparticles as well as relevant factors, for example controlling surface morphology, structure and composition. Recyclability is the property that acts as a bottleneck for their application at large scale. Additionally, such catalyst also possesses efficient volume and surface ratio, high stability, much resistance to saponification. Nano-catalysts can be synthesized using various methods such as impregnation (CaO-Au, Li-CaO, ZrO2/C4H4O6 HK, Cs-Ca/SiO2-TiO2 and zeolite), wet impregnation Li/MgO, precipitation (MgO), deposition-precipitation (nano-sized MgO on Titanium), ball and mill method (CaO-Al2O3)glycerol nitrate combustion (TiO2-ZnO), co-precipitation (Fe/ZnO, KF/CaO-MgO), impregnation and sol-gel method (CsH2 PW12O40/Fe-SiO2) Imm-pregnation and Co-precipitation (KF/Ca-Fe3O4), sol-gel method (CaO-MgO, Sr3 Al2O6, Sr-Ti nanocomposite), Supercritical sol-gel method Cs-MgO. With a maximum control over structure and composition, the preparation of these particles is the second challenge [55]. Current nano-technological methods for synthesis and characterization of catalytic nanoparticles are highly efficient. Figure 6 shows effect of intrinsic properties of nano-sized solids on catalysis. The intrinsic properties of nano-sized materials have a considerable impact on catalysis and catalytic activity [57]. These are:  
1. Quantities such as mean lattice constant, binding energy, and atomic density are directly related to bond length. Surface relaxation and densification are induced by lattice contraction in solids having nano-sized particles;
2.