## 4 Chitosan as Catalytic Support
### Catalytic Metal-Chitosan Composites
[PERSON] and [PERSON] [145; 146] reported the synthesis of magnetically separable catalysts via anchoring of a CS-Schiff's base complex (CS was functionalized with 5-bromosalicylaldehyde [145] or picolinaldehyde [146]) to the surface of Fe\({}_{3}\)O\({}_{4}\) NPs (with an average size of 20 nm) with subsequent metalation with Cu(II) acetate. Fe\({}_{3}\)O\({}_{4}\)-CS-Schiff's base Cu complexes catalyzed the oxidation of sulfides to sulfoxides with 100% selectivity in all cases under green reaction conditions and excellent yields. Moreover, easy recovery and reusability up to four cycles without noticeable loss of catalytic activity was found. One of the interesting directions is the synthesis of aerogels from CS for different applications (drug delivery, biomedicine, environmental protection (purification of gases and liquids), thermal insulation and catalysis) [162; 163; 154; 164; 165; 166]. Aerogels can be obtained from different CS-containing composites consisting of other natural/synthetic polymers and inorganic materials (metals, oxides, GO, etc.). The procedure of CS-based aerogel synthesis includes several steps: (i) preparation of CS solution; (ii) gelation (a large variety of gelating agents can be used); (iii) pretreatments before drying (solvent exchange or chemical modification); (iv) drying, resulting in the formation of a porous solid (supercritical drying or freeze drying); (v) post-drying treatment (second drying step) [164]. [PERSON] et al. [164] classified the catalytic activity of CS aerogels in three categories: (i) direct organocatalysis of NH\({}_{2}\) as a Brensted base and/or hydrogen bond; (ii) use of NH\({}_{2}\) as a support of organocatalysts through chemical modifications; and (iii) supports of inorganic materials, such as metal NPs and oxides. [PERSON] et al. [153] synthesized secondary amino CS derivatives by reacting CS aerogels with arylaldehyde under heterogeneous conditions. CS aerogels were prepared in the form of beads. CS beads were obtained via common dripping of CS acidic solution into sodium hydroxide solution and then dehydrated using successive immersion in a series of water-ethanol baths of increasing alcohol concentration. Finally, CS beads were dried under supercritical CO\({}_{2}\) conditions to provide aerogel material. CS aerogels were treated with several aldehydes to reach a high degree of substitution during the timeformation process. Then, the reduction of CS-Schiff's base derivatives succeeded using NaBH\({}_{3}\)CN, and a high reduction ratio (>80%) was achieved (Figure 25). Textural analyses showed that successive heterogeneous chemical modifications to CS aerogels do not lead to a collapse of the structure and aerogels keep a surface of about 250 m\({}^{2}\)/g with a distribution of mesopores centered at 23 nm. A phosphine-based CS aerogel was used as a phosphorylated ligand in the Pd-catalyzed allyic substitution--one of the most important reactions for carbon-carbon and carbon-heteroatom bond formation. The allylation reaction between (E)-1,3-diphenyl-3-acetoxyprop-1-ene and dimethyl malonate using N,O-bis(trimethylsilyl)acetamide as  
Figure 25: Types of modified CS aerogels [153]. base was selected as a model reaction. The reaction was completed after 24 h at 20 \({}^{\circ}\)C, affording the expected allylated malonate in good 76% yield [153]. One of the recent studies on CS-based metallic aerogels was carried out by [PERSON] et al. [154], who applied an unusual approach for the fabrication of metallic aerogels--pyrolysis of xerogels derived from CS-metal (M = Fe, Co, Ni) hydrogels.