## 4 Chitosan as Catalytic Support
### Catalytic Metal-Chitosan Composites
[142] developed CuO NPs embedded in porous CS membranes using a solution-casting approach with sodium TPP as a crosslinker and polyethylene glycol as a porogen. The CuO NPs incorporated in CS membranes exhibited exceptional photocatalytic activities against MO and MB. These dyes were degraded to 62-65% at 30 mg/L. CS-based porous membranes retained about 95% of theirphotodegradation activity for up to five cycles. It is noteworthy that CS-based adsorbents themselves can possess photocatalytic activity in the absence of any metals. For example, organic nanocomposite based on paddy straw (PS) incorporated with CS and crosslinked with sodium TPP [143] was used for the photocatalytic adsorption of dyes ( rhodamine B (RhB) and MG) and allowed for the removal 96.04% of RhB (10 mg/L) and 98.15% of MG (10 mg/L) for 1 h at 3.0 g/L of PS-CS, pH = 6.2 and 60 \({}^{\circ}\)C.  
[PERSON] et al. [144] prepared a polyurethane sponge (PUS) covered with CS. CS was deposited from acidic solution; no gelating agents were used. CS-PUS samples were treated with aqueous solutions of metal (Ni, Cu, Co, Ag) salts and reduced with NaBH\({}_{4}\). Resulting catalytic materials were tested in a model reaction of 4-NP hydrogenation by NaBH\({}_{4}\). It was found that Cu/CS-PUS outperformed among the other Me/CS-PUS: complete conversion of 4-NP was found for 5 min (the reaction rate constant was 0.7923 min\({}^{-1}\)). Moreover, Cu/CS-PUS catalyst was used for the reduction of different dyes (MO, AO and CR) and its high performance was observed for the CR. An electrocatalytic reduction of 4-NA was studied using CS-stabilized Ag NPs on carbon paste [147]. Ag NPs with a mean diameter of about 51 nm were formed by the reduction of silver nitrate with NaBH\({}_{4}\) in the presence of CS as a stabilizing agent. To build a voltammetric sensor (CS-Ag NPs/CPE), a carbon paste electrode (CPE) was immersed in aliquid suspension of CS-Ag NPs. The reduction overpotential of 4-NA shifted from \(-\)752.26 mV at CPE to \(-\)304 mV in CS-Ag NPs/CPE, and then showed a surface-controlled process with acatolytic rate constant of 0.125 \(\times\) 10\({}^{-3}\) L/(mol \(\times\) s). As previously mentioned, CS-containing materials are usually shaped using alkaline solutions and different gelating agents. This process requires a large amount of solvents, acids and alkalines. In the work of [PERSON] et al. [150], CS powder and thermoplastic polyurethane (TPU) were mixed to form a membrane using selective laser sintering. This method has the advantages of simple operation, fast membrane formation and low preparation cost. The shape of the hybrid CS-TPU membrane can be flexibly designed while varying the conditions of sintering. It was shown that the sintered CS-TPU composite membrane has superhydrophilic properties and can effectively adsorb Cu(II) and Pb(II) in water with maximum adsorption capacities of 19.6 mg/g and 30.4 mg/g, respectively. Pd(II) could also be adsorbed by the membrane to form palladium-loaded composite materials, which could be used as a catalyst for hydrogenation of 4-NP by sodium borohydride, allowing 96% conversion in 20 min. [PERSON] et al. [152] synthesized the series of CS-based composite hydrogels doped with Pd NPs by adding graphene oxide (GO), carbon nanotubes (CNTs) and layered double hydrotalcites (LDHs) to CS hydrogel with GA as a crosslinking agent. Pd NPs with sizes of about 2-5 nm were formed in situ during the gelation process. It is noteworthy that palladium precursor (Na\({}_{2}\)PdCl\({}_{4}\)) was added at the CS solution preparation stage, while GA and other components (GO, CNTs or LDHs) were added later. Obtained catalysts were tested in a modelled reaction of 4-NP hydrogenation and exhibited promising catalytic activity. Highest reaction rate constant (0.218 min\({}^{-1}\)) was found for CS-CNT-PdNPs. The green synthesis of magnetically separable hybrid metal nanobiomaterials (Fe\({}_{3}\)O\({}_{4}\)@CS-MeNPs; Me = Au and Pd) was reported [161].