## 6 Challenges, Future Perspectives and Recommendations  
Challenges in utilization of homogeneous catalysts have been elaborately addressed in the literature as extensive and exhaustive research has already been conducted in this aspect. However, due to significant ongoing studies, heterogeneous nanocatalysis appears \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Inmobilization** & **Fat/Oil** & **Lipase** & **Reaction Conditions** & **Yield (\%)** \\ \hline \multirow{8}{*}{**Entrapment**} & \multirow{3}{*}{**Palm**} & \multirow{3}{*}{_Candida ragosa_} & Immob. Lipase (1%), 15: EtOH/Oil molar ratio, & \multirow{3}{*}{85/70} \\  & & & 35 \({}^{\circ}\)C 1 h; Immob. Lipase (1%), 14:1 MeOH/Oil molar ratio, & & \\  & & & & molar ratio, 37 \({}^{\circ}\)C, 1 h & \\  & & & & Immob. Lipase (4.75\%), 12:1 EtOH/Oil molar & & \\  & Soybean & & & ratio, 35 \({}^{\circ}\)C, 1 h; Immob. Lipase (4.75\%), 7.5:1 & 65/67 \\  & & _Pseudomonas capsaicin_ & MeOH/Oil molar ratio, 35 \({}^{\circ}\)C, 1 h & & \\  & Waste cooking & & & Immob. Lipase (1%), 5:1 MeOH/Oil molar ratio, & & \\  & oil \({}^{\ast}\)/soybean \({}^{\text{b}}\) & & & 43 \({}^{\circ}\)C, 36 h & 67 \({}^{\ast}\)/68 \({}^{\text{b}}\) \\  & Grease and tallow & & & 4:1 EtOH/Oil molar ratio, 50 \({}^{\circ}\)C, 24 h & 95 \\ \hline \multirow{8}{*}{**Physical**} & \multirow{3}{*}{**Waste edible oil**} & Lipozyme TLIM & Immob. Lipase (4%), 4:1 MeOH/Oil molar ratio, & \multirow{3}{*}{95} \\  & Olive husk & _Rhizomucar michei_ & & 2:1 EtOH/Oil molar ratio, 37 \({}^{\circ}\)C, 8 h & & \\  & Cotton seed & _Candida ragosa_ & Immob. Lipase (4%), 12:1 MeOH/Oil molar ratio, & & \\  & Jatropha & _Pseudomonas capsaicin_ & Immob. Lipase (10%), 4:1 EtOH/Oil molar ratio, & & \\  & & & 40 \({}^{\circ}\)C, 8 h & & \\ \hline \multirow{8}{*}{**Covalent**} & \multirow{3}{*}{**Jatropha**} & \multirow{3}{*}{_Candida cylindracosa_} & Immob. Lipase (40%), MeOH 40 \({}^{\circ}\)C, 48 h & 99 \\  & Jatropha & _Rhizomucas oryzae_ & & 30 \({}^{\circ}\)C, 4 h, MeOH & & \\  & Olive & _Thermomyces lanuginosus_ & & 6:1 MeOH/Oil molar ratio, 25 \({}^{\circ}\)C, 24 h & 93 \\  & & _Pseudomonas_ & & 18:1 EtOH/Oil molar ratio, 45 \({}^{\circ}\)C, 48 h/9:1 & \\  & Palm/Babassu & _florescens/Thermomyces_ & EtOH/Oil molar ratio, 45 \({}^{\circ}\)C, 48 h & \(>\)93 \\  & & _lumpsinus_ & & \\ \hline \multirow{8}{*}{**Cross linking**} & \multirow{3}{*}{**Jatropha**} & \multirow{3}{*}{_Burtholderia capsaicin_} & Immob. Lipase (52.5\%), 10:1 EtOH/Oil molar & \multirow{3}{*}{100} \\  & & & ratio, 35 \({}^{\circ}\)C, 24 h & & \\ \cline{1-1}  & Canola & _Thermomyces lanuginosus_ & & 6:1 MeOH/Oil molar ratio, 40 \({}^{\circ}\)C, 24 h & 90 \\ \cline{1-1}  & _Madhusic indica_ & _Pseudomonas capsaicin_ & Immob. Lipase (10%), 4:1 EtOH/Oil molar ratio, & & \\ \cline{1-1}  & Olive & _Photobacterium lipidvicticum_ & & 40 \({}^{\circ}\)C, 25 h & & \\ \hline \hline \end{tabular} Note: a (waste cooking oil) and b (Soybean).  
\end{table} 
Table 10: Lipase mobilization techniques for biodiesel synthesis [136]. to be an advanced and emerging field. The main challenges associated with heterogeneous catalysts include: instability due to short life span along with low reaction rate, sensitivity to FFA and water resulting in saponification (solid base catalysts), leaching and product contamination (solid acid catalysts). For high performance of nano-catalysts require high energy and severe conditions to operate in ordinary reaction time because under mild reaction conditions, these catalysts take relatively more reaction time. In case of biocatalysts, the methanol presence causes inhibition of lipase (enzymatic transesterification). In future work, the following aspects need to be addressed:  
1. Tremendous efforts should be made in order to exploit the ways to enhance kinetic rate of heterogeneous nano-catalysts, especially CaO because it has high catalytic activity, long life span and mild operating conditions;
2. Biomass or waste derived catalysts should be synthesized in order to utilize and reuse the superfluous matter.