## 2 Preparation and Applications of Chitosan-Based Heterogeneous Catalysts
### Gold Catalyst  
Elaboration of gold compounds in the field of nano-chemistry showed efficient catalysis activities for carbon-carbon bond formation and other organic synthesis [41, 42]. However, the catalytic activity of gold could be considerably influenced by its dimension and interaction with supporting materials [42]. Previous studies also reported the possibility of a prepared gold nano-catalyst undergoing undesired agglomeration and precipitation before and after reactions [43]. Thus, the design of an active and selective gold-based catalyst is intended to invent an appropriate support in terms of its gold affinity, stabilization ability and large surface area [42, 44]. [PERSON] _et al._[42] proposed the use of chitosan to increase dispersion of gold, as chitosan is a naturally available polymer and can act as macromolecular ligands for chelating gold nanoparticles. Presence of spatially isolated amino groups on chitosan also prompted the capture of gold nanoparticles in the polymers. Preparation of the catalyst started by dissolving chitosan in an acid solution, followed by a gelification in basic solution to increase its surface area and porosity, and then immersed in HAuCl4 solution. Scanning electronic microscopy results showed isolated chitosan fibrils with a diameter of 8-10 nm. The resulting catalyst was then utilized for Sonogashira cross coupling (70% yield with \(\geq\)99% selectivity) and homocoupling reactions under mild conditions. The catalyst could be used three times without major loss of catalytic activities over turn-over numbers of 18 and 26 for Sonogashira and homocoupling, respectively, as the material still retained its initial porosity. A green approach to the synthesis of gold nanoparticle-chitosan catalyst by exposing chitosan in a gold salt solution (HAuCl4-3H2O or AgNO3) under thermal treatment was attempted by [PERSON] _et al._[29]. The formation of gold NPs-chitosan involved two steps: the adsorption of gold ions onto chitosan and the subsequent reduction of gold species by chitosan. In this approach, AuCl4 ions could be simultaneously reduced to zero-valent gold nanoparticles by chitosan itself without any additional reductant in the thermal reduction. Transmission electron microscopy (TEM) images confirmed the formation of metal nanoparticles in the chitosan matrix, and shortening the reaction time contributed to formation of smaller gold particles. Formation of smaller particles increased the surface roughness of the catalyst, thus promoting the chemisorption and reduction reaction of the 4-nitropenol to 4-aminophenol, although the catalyst was only effective for the first cycle. Given the fact that chitosan not only functioned as a supporting matrix for catalyst preparation, it also served as a reductant and stabilizer for the formation and dispersions of gold nanoparticles, [PERSON] _et al._[30] reported a synthesis approach to prepare magnetic gold nanoparticle-chitosan-Fe3O4 catalysts without employing any toxic reductants or capping agents. Their results verified that the Fe3O4 nanospheres were strongly embedded into the chitosan by sharing the lone electron pairs between the nitrogen atom in amine on chitosan and Fe(II) or Fe(III) on the surface of Fe3O4. This as-prepared catalyst exhibited good catalytic performance for reduction of nitro compounds with more convenient magnetic recyclability after uses. Ruthenium (Ru) catalyst was well used for the aqueous hydration of nitriles to amides [45] and hydrogenation of esters [46] in the organic chemical industries. A molecular Ru catalyst was also developed for the water oxidation in a photosystem to generate renewable energy [47]. With the inspiration of using chitosan to extract Ru nanoparticles from aqueous solutions, [PERSON] _et al._[13] proposed the direct use of unmodified chitosan as a heterogeneous support for catalytic applications, thus eliminating the involvement of tedious synthesis procedures while immobilizing the Ru metal onto a magnetic support and enhanced its reusability.