## 4 Chitosan as Catalytic Support
### Catalytic Metal-Chitosan Composites
The magnetic NPs (about 35 nm in diameter) were synthesized using the biomineralization process and coated with CS, followed by binding and reduction of metal ions. CS-encapsulated magnetic NPs (Fe\({}_{3}\)O\({}_{4}\)@CS) were prepared using the water-in-oil emulsion method with span-80 as surfactant. Samples of Fe\({}_{3}\)O\({}_{4}\)@CS decorated with Au and Pd NPs were synthesized using a microwave-mediated in situ reduction of Au(III) and Pd(II) ions (Figure 21). The as-synthesized hybrid nanobiomaterials were tested for several model reactions such as photocatalytic reduction of dye (MB), hydrogenation of 4-NP and Suzuki cross-coupling in aqueous solution. At ambient temperature, Fe\({}_{3}\)O\({}_{4}\)@CS-AuNPs (5.6 wt.% Au) and Fe\({}_{5}\)O\({}_{4}\)@CS-PdNPs (2.6 wt.% Pd) took only1 min under UV light to complete the reduction of MB with \(k_{\text{app}}\) of 4.0 min\({}^{-1}\) and 5.0 min\({}^{-1}\). The 4-NP hydrogenation reaction was completed within 3-14 min depending on the MeNPs on the hybrid nano-biomaterial and reaction temperature. The catalytic behavior of Fe\({}_{3}\)O\({}_{4}\)@CS-PdNPs in the C-C bond formation (Suzuki reaction) of phenylboronic acid (1.5 mmol) and \(p\)-iodoacetophenone (1 mmol) in aqueous solution was explored under various conditions. It was found that, when the Suzuki reaction was carried out in the normal laboratory condition at 50 \({}^{\circ}\)C, 98% yield (TON = 50,256 and TOF = 16,752) was found after 3 h. In addition, the hybrid nanobiomaterials demonstrated excellent recyclability and reusability without significant loss of catalytic activities [161]. [PERSON] et al. [149] reported the formation of Ag NPs in CS hydrogels. Briefly, CS solutions with the addition of crosslinking agent (formaldehyde) were poured into plastic tubes and kept for 24 h, and then the obtained hydrogels were washed with NaOH solution and water. CS hydrogels were immersed in an aqueous solution of AgNO\({}_{3}\) (10 mM) for 3 days at room temperature, allowing for the formation of Ag NPs. No reducing agents  
Figure 21: Schematic representation of synthesis of biogenic magnetic NPs (**A**), encapsulation of magnetic NPs with CS and formation of metal NPs on CS-encapsulated magnetic NPs (**B**) to form hybrid metal nanobiomaterial (Fe\({}_{3}\)O\({}_{4}\)@CS–MeNPs). Reprinted with permission from authors of [161], American Chemical Society, 2017.  
were used. Synthesized Ag-CS samples were tested in a hydrogenation of 2-NP and AO using NaBH\({}_{4}\) as a reducing agent. Both reactions proceeded at high rate constants of 0.260 min\({}^{-1}\) and 0.253 min\({}^{-1}\) for 2-NP and AO, respectively. Ultrafine nitrogen-doped TiO\({}_{2}\) NPs were formed on the surface of hollow quantized CS by electrostatic self-assembly, providing a unique core-shell structure (QCS@N-TiO\({}_{2}\)) [148]. Then, Ag NPs were deposited on the surface of QCS@N-TiO\({}_{2}\) via a simple photodeposition process. Thus, QCS@Ag-TiO\({}_{2}\) multifunctional catalytic materials were fabricated (Figure 22). QCS@Ag-TiO\({}_{2}\) allowed for the catalytic degradation of SDBS under visible light: 96.4% of SDBS conversion was achieved within 180 min. The excellent degradation characteristic was ascribed to a large number of active sites and adsorption sites on the surface of the quarternized CS, effective separation of electron-holes in N-TiO\({}_{2}\) and surface plasmon resonance and photothermal effect of Ag NPs [148]. As previously mentioned, CS-based materials can be used not only for stabilization of catalytic metal NPs, but also for metal ions. A CS-based, magnetically recoverable and recyclable catalyst, MCS-GT@Co(II), was developed for the highly efficient and green synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles [137]. The catalyst was prepared by immobilization of CS onto Fe\({}_{3}\)O\({}_{4}\) using GA as crosslinker followed by Co(II) ion immobilization (see Figure 23). Fe\({}_{3}\)O\({}_{4}\) NPs were prepared using the coprecipitation method and added to CS acid solution.