## 2 Preparation and Applications of Chitosan-Based Heterogeneous Catalysts  
### Chemical Activation and Conversion  
#### 2.1.1 Metal Catalysts  
Recent research interest in using chitosan as catalytic metal supports has been due mainly to its high sorption capacity, stability of metal anions and physical and chemical versatility [14]. Preparation of chitosan-metal-based catalysts commonly involves co-precipitation by mixing chitosan into a metal solution in an alkaline solution, in order to coagulate the chitosan into nanoparticles or other forms. Metal nanoparticles that are formed strongly bound to chitosan encourage further investigation of its catalytic performance. Gold and silver nanoparticles are of special interest because of their unusual properties compared to bulk metals [29]. Transition metal ions such as palladium and ruthenium are also available for the development of hybrid catalyst complexes, as they are constantly applied to the synthesis of organic compounds. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Catalyst** & **Synthesis Reagents \({}^{\star}\)** & **Reaction Scheme** & **Yield** & **Selectivity** & **Reference** \\ \hline Chitosan/Au & HAuCl\({}_{t}\) & & & 70\% & \(\geq\)99\% & [30] \\ \hline Chitosan-Fe\({}_{9}\)O\({}_{t}\)-Pd & FeCl\({}_{1}\)-6H\({}_{2}\)O, & FeCl\({}_{2}\)-4H\({}_{2}\)O, & & \(>\)97\% & 99\% & [5] \\ \hline  & Pd(OAc)\({}_{2}\), dimethylsulfoxide, & & & & \\ Chitosan/Pd(0) & hydrazine hydrate, digibcidyl & & & & \\  & ether polyethylene glycol & & & & \\ \hline  & Pd(OAc)\({}_{2}\), hexamethyle & & & & \\  & diisocyanate (HDI) & & & & \\ \hline  & 3-Chloro-2-hydroxypropyl & & & & \\  & armonium salt & trimethylammonium chloride, & & & \\  & isopropanol & & & \\ \hline  & & & & \\  
Palladium Catalysts  
Palladium (Pd) is a well acknowledged transition metal catalyst for carbon-carbon bond formation (e.g., Heck or Suzuki reactions) [22, 33], hydrogenation and acetalization [5] in the area of organic chemistry. The Pd catalysis usually allows selective reactions with high turnover numbers and turnover frequencies [34]. In addition, the amine groups on the chitosan-based catalyst are known to enhance the activity of Pd [35]. However, direct use of Pd as a catalyst is limited in homogeneous reaction systems due to the difficulties in separating and recycling the catalyst from the system [33]. Therefore, development of heterogeneous Pd catalysis systems is generally preferred. [PERSON] _et al._[33] prepared a chitosan-supported Pd(0) catalytic system for Suzuki cross-coupling reactions. Preparation of the catalyst started with adsorption of Pd(II) onto the chitosan beads followed by adding hydrazine hydrate (in methanol) to convert Pd(II) to Pd(0). The as-prepared catalyst was cross-linked with either glutaraldehyde or diglycidyl ether polyethylene glycol to enhance its chemical and physical stability in reactions. Its catalysis test results revealed good catalytic activity for iodo- and bromoarenes under microwave activation. Microwave energy was useful to enhance the organic reactions and has been increasingly encouraged [31, 36]. The addition of tetrabutylammonium bromide (TBAB) in the system also significantly enhanced the product yields by increasing the solubility of aryl halide for Suzuki cross-coupling in water. The as-prepared catalyst also showed high reusability after five cycles of uses due to its high stability and moderate leaching of Pd. The authors also confirmed that the randomly distributed \(\beta\)-(1,4)-linked-glucosamine and \(N\)-acetylglucosamine on chitosan made it suitable for water-compatible solid support of Pd in the Suzuki cross-coupling reaction. To increase the affinity of Pd onto chitosan, [PERSON] _et al._[37] incorporated ultrasound to assist the cross-link of Pd(II) ions onto chitosan by interacting with hexamethylene diisocyanate. The sonication from ultrasound helped the removal of air bubbles in the bulk of chitosan derivatives and the breakdown of aggregated nanospheres [38].