## 2 Preparation and Applications of Chitosan-Based Heterogeneous Catalysts
### Gold Catalyst
#### 2.1.2 Nonmetal Catalysts  
Quaternized Chitosan  
In the age of increased concerns in global warming and climate change, CO\({}_{2}\) mitigation and fixation have received a lot of attention from both economic and environmental points of view. In particular, conversion of CO\({}_{2}\) into carbonates was of remarkable interest for reducing carbon emissions as well as reusing it as alternative resources (e.g., solvents) [32,49]. Quaternary ammonium salts, such as Bu\(\ast\)NBr, Et\(\ast\)NBr, are currently in use for industrial preparation of cyclic carbonates [50], and therefore have attracted much scientific attention in the development of heterogeneous catalysts for these applications. Recent studies also revealed the use semi-clathrate hydrates in the presence of quaternary ammonia salts of TBAB for CO\({}_{2}\) capture up to 95% [51,52]. A quaternary ammonium salt-functionalized chitosan catalyst was reported to be an efficient and recyclable catalyst for synthesis of propylene carbonate from propylene oxide and carbon dioxide in 100% yield and high selectivity without any organic solvent [18]. NaOH, isopropanol, 3-chloro-2-hydroxypropyl trimethylammonium chloride were used to prepare the quaternary ammonium salts by covalently bonding on chitosan, after magnetization. The results showed that the catalysts' performance strongly depended on the leaving ability and nucleophilicity of the anions on the chitosan-supported catalysts [49]. The as-prepared catalyst could be recovered and reused for at least five times without loss of its catalytic activity. After [PERSON] _et al._ demonstrated an efficient, reversible fixing system for CO\({}_{2}\) using chitosan/ionic liquid [53], a novel approach applying quaternized chitosan as a catalyst for the cycloaddition of epoxides from reacting allyl glycidyl ether (AGE) and CO\({}_{2}\) was attempted [54]. Rather than covalently tethering a quaternized species into the chitosan's backbone, they first reported the use of quaternized chitosan from quaternization on the primary amine group inherently on the chitosan itself to form a cationic biopolymer. Furthermore, the hydroxyl groups on the chitosan would enhance the catalysis on their own. The chitosan was synthesized with NaOH, NaI and CH\({}_{3}\)I in a 1-methyl-2-pyrrolidinone solution. The quaternized chitosan showed high catalytic efficiency for most of the tested monosubstituted terminal epoxides, while AGE revealed the highest conversion and yields for the production of the corresponding cyclic carbonates with high selectivity. This approach provided a simple and easy route for synthesizing cyclic carbonates with high product yields and easy catalyst recycling. Their later study utilized microwaves to prepare quaternized chitosan catalysts for the cycloaddition of CO\({}_{2}\) and epoxides [55]. The as-prepared catalyst exhibited higher catalytic activity for the solvent-free cycloaddition of CO\({}_{2}\) and AGE, as compared to a conventional quaternized chitosan catalyst (non-microwave irradiated). Other Novel Catalysts  
[PERSON] _et al._[16] developed an efficient, cheap and recyclable chitosan-based catalyst complex for cycloaddition reactions of CO\({}_{2}\) with various epoxides. In the system, chitosan was functionalized by 1-ethyl-3-methyl imidazolium halides (with Cl or Br ions) without the addition of solvents and metal co-catalysts. The as-prepared catalyst was tested for the reaction of propylene oxide and CO\({}_{2}\) to produce propylene carbonates. The results exhibited high product yields and showed easy recyclability of the as-prepared catalyst up to five times. A possible catalytic cycle for the hydrogen bond assisted ring-opening of epoxide and activation of CO\({}_{2}\) induced by the nucleophilic tertiary nitrogen of the chitosan was also proposed. Direct synthesis of cyclic carbonates from olefins and CO\({}_{2}\) was achieved by [PERSON] _et al._[20] using a dual catalysis system with magnetic chitosan.