Rice husk the outer covering of rice grains is one

Rice husk, the outer covering of rice grains, is one of the main agricultural residues which is obtained during the milling process. Application of rice husk as an energy source for Tenatoprazole power plants, rice mills and brick factories is increasing due to its high calorific power [13]. In this combustion, rice husk ash (RHA) is produced. RHA contains a considerable amount of amorphous silica up to 80% and small proportion of impurities such as K2O, Na2O and Fe2O3[14]. In recent years, investigation on the application of TiO2-based reagents in organic reactions became an important part of our ongoing research program [15], [16], [17]. In continuation of these studies and because that RHA possesses high silica content, we were interested to investigate the possibility of the preparation of anatase-phase TiO2 over this reagent.
Experimental
Results and discussion On the basis of the information obtained from the studies on RHA/TiO2 nanocomposites, it is expected that this reagent can be used as a catalyst for the promotion of the organic reactions. So this reagent was used in the promotion of the conversion of alcohols, phenols and amines to their corresponding acetates and/or amides with acetic anhydride (Scheme 1). In the first step we focused our attention toward the optimization of the ratio of RHA to TiO2 for obtaining the highest catalytic activity. For this purpose, acetylation of 4-chlorobenzyl alcohol was studied and the best results were obtained using RHA/TiO2(30%) (Table 2). Further increase in proteinoids ratio did not improve the activity of the prepared nanocomposite. Then for obtaining the optimum reaction conditions, we studied the influence of the following factors on the acetylation of 4-chlorobenzyl alcohol with acetic anhydride: i) the amounts of the catalyst; ii) solvent or solvent-less media and iii) temperature. The obtained results clarified that the best conditions are the ones which are shown in Scheme 2. After optimization of the reaction conditions, different types of alcohols were subjected to the acetylation using this method (Table 3). Acetylation of various benzylic alcohols containing electron-withdrawing and electron-donating substituents proceeded efficiently with high isolated yields (Table 3, Entries 1–7). Primary, secondary and tertiary aliphatic alcohols were also efficiently converted to their corresponding acetates in almost quantitative yields at room temperature (Table 3, Entries 8–12). No elimination and rearrangement by-products were observed at all. Phenol and its derivatives also undergo acetylation easily using this method and their corresponding acetates can be isolated in excellent yields (Table 3, Entries 13–16). In the case of pyrogallol, higher amounts of the catalyst are needed due to high steric hindrance (Table 3, Entry 16). This method is also very useful for the acetylation of amines with acetic anhydride. All reactions are performed under mild reaction conditions in very short reaction times with high yields (Table 3, Entries 17–20).