## 4 Chitosan as Catalytic Support
### Catalytic Metal-Chitosan Composites
Metallic aerogels were synthesized using the following steps: (i) CS-metal complexes were formed by the addition of metal salts to acidic CS solutions; (ii) NaOH was added to the resulting mixtures undervigorous stirring that resulted in hydrogel formation; (iii) the metallic hydrogels were freeze-dried; (iv) the dried metallic hydrogels were calcined at a defined temperature (800, 1200 or 1300 \({}^{\circ}\)C) for 2 h. Thus, metal NPs were formed and encapsulated in graphite shells. It was shown that the resulting metallic aerogels are highly stable, efficient and selective nanocatalysts towards the hydrogenation of different nitroarenes to amines at low catalyst loading (1.2 mol.%). Using Fe\({}_{2}\)\(\bigcap\)C-1300, more than 99% yield of target products was achieved in THF-water mixture (1:1) at 130 \({}^{\circ}\)C and 5 MPa for 48 h. This catalyst could be reused for more than 30 runs without obvious loss of activity and selectivity. These distinguished performances were attributed to the graphitic shells formed during pyrolysis, which hampered the possible aggregation of metal NPs, prevented metal leaching and increased their stability. Functionalization of CS with phosphine groups is a promising approach for the development of Pd-containing catalysts intended for cross-coupling reactions, which proceed via a homogeneous mechanism. For example, [PERSON] et al. [155] developed CS functionalized with diphenylphosphine (PPh\({}_{2}\)) and crosslinked with GA (Figure 26). CS-PPh\({}_{2}\) was used to immobilize Pd NPs with a size of about 4 nm, active in Heck cross-coupling (yield of reaction products 80-89%). The CS-PPh\({}_{2}\)-Pd catalyst could be recycled up to three times with negligible Pd-leaching. Moreover, Pd-loading was decreased down to 0.1 mol.% with no significant yield losses. Different catalytically active oxides (ZnO, MgO, Cu\({}_{2}\)O, CuO, Fe\({}_{3}\)O\({}_{4}\)) can be stabilized with CS [158; 159; 160; 133]. For example, [PERSON] et al. [133] synthesized CS-ZnO composite films containing ZnO NPs (30-32 nm) with a variation in ZnO content (5-25 wt.%). It was  
Figure 26: Scheme of CS–PPh\({}_{2}\) (based on Ref. [155]). found that CS-ZnO nanocomposite film (20 wt.%) could be a viable, effective, recyclable (for more than three uses) and heterogeneous base catalyst in the synthesis of different thiazoles with high yields (up to 93%). Catalytic systems based on CS functionalized with IC and Cu\({}_{2}\)O or CuO NPs were obtained [158] for the click reaction and CO\({}_{2}\) fixation with high efficiency. [PERSON] et al. [159] prepared a composite CS-MgO film serving as an efficient and ecofriendly basic catalyst, allowing for the synthesis of two novel series of 5-arylazo-2-hydrazonothiazoles and 2-hydrazono [1,3,4] thiadiazoles, incorporating a sulfonamide group, under microwave irradiation. Yields of target products exceeded 80% depending on started compounds. The average size of the MgO particles was found to be approximately 6-11 nm for 10 wt.% and it was found that the particles' size slightly decreased with increasing magnesia content. The CS-MgO nanocatalyst could be easily recovered and reused many times without loss in its catalytic activity. [PERSON] et al. [160] synthesized [PERSON]'s base of Fe\({}_{3}\)O\({}_{4}\)@CS with 4,4'-diselenobisbenzaldehyde (Fe\({}_{3}\)O\({}_{4}\)@CSSe) composite and used it as a catalyst for the oxidation of sulfides in the presence of H\({}_{2}\)O\({}_{2}\) at room temperature. The yield of sulfoxides was found to depend on the amount of H\({}_{2}\)O\({}_{2}\), catalyst loading and solvent nature. For example, oxidation of dibutyl sulfide was observed in asolvent-free condition with ayield of 94% in the presence of 20 mg Fe\({}_{3}\)O\({}_{4}\)@CSSe and 0.5 mL H\({}_{2}\)O\({}_{2}\). Bimetallic NPs stabilized with CS are rarely reported. One of the recent studies was reported by [PERSON] et al. [165], who synthesized CS-stabilized bimetallic iron/nickel (CS-Fe/Ni) NPs using a chemical reduction of metal precursors with NaBH\({}_{4}\) in CS solution.