**3. Modification of xylan-rich hemicellulose**

The extracted xylan-rich hemicellulose can easily be functionalized because of the presence of abundance of the hydroxyl moiety in its structure. The chemical modification of xylan provides more possibilities to tailor its properties, which aims to improve its applicability. Various xylan derivatives with better properties are successfully obtained after chemical modifications recently. Various methods to prepare xylan derivatives are discussed below.

### **3.1 Derivatization of xylan as carboxymethyl xylan (CMX)**

The carboxymethyl xylan (CMX) synthesis is carried out by using sodium chloroacetate as a carboxylate group donor to xylan. On dissolution of xylan-rich hemicelluloses in NaOH solution (i.e., under alkali treatment), NaOH reacts with the hydroxyl group of xylan and generates a strong nucleophile as alkoxide ion. The alkoxide ion from the alkali xylan attacks the chloroacetate via SN2 reaction resulting in the carboxymethylation of xylan as a product. The product obtained is neutralized with acetic acid and washed with ethanol. The resulting product is filtered, centrifuged, and dried in vacuum oven [13, 14]. The degree of carboxymethylation depends on the number of hydroxyl group substituted with carboxymethyl groups. Furthermore, the product of glycolic acid and NaCl could be generated as by-products at high concentrations of sodium chloroacetate and NaOH [15] removed in the form of extract. The involved chemical reactions are represented as **Figure 4**.

### **3.2 Derivatization of xylan as dialdehyde xylan (DAX)**

Oxidation is one of the prominently used pathways for derivatization. Oxidation of xylan is an easy and practically available method on the lab-scale now. The final

**Figure 4.** *Modification of xylan as carboxymethyl xylan (CMX).*

#### **Figure 5.**

*Chemical reactions for the formation of DAX.*

products from the oxidation of xylan called modified xylan have been proved an important role in material enhancement and functionalization.

A very well-known oxidation such as 2,2,6,6-tetramethylpiperidine 1-oxyl radical (TEMPO) oxidation used in cellulosics' material chemistry but is not pertinent to xylan, as xylan have D-xylopyranosyl residue as their building blocks, are lacking of primary hydroxyl group, which are the primary target of the TEMPO oxidation. Therefore, in oxidative xylan modification primarily sodium periodate is used, as it is easily and practically applied on the lab scale. Periodate oxidative cleavage of vicinal diol (i.e., cleavage of C2–C3 bond) is resulting in "dialdehyde polymers", which have a number of interesting applications in tissue engineering, drug delivery and as flocculating agents, and for ion-exchange separation. The resulting dialdehyde polymers exist as 2,3-hemialdal forms, which can be used for further functionalization [6, 16] particularly to prepare hydrogels. The preparation of DAX has been shown in **Figure 5**.
