Scandium triflate catalyzed acetylation of starch at low to moderate temperatures

Scandium (III) trifluoromethane sulfonate (Sc(OTf)3) was investigated as a catalyst for the acetylation of starch in order to determine the potential for preparing new types of starch esters under mild (low to moderate temperature) conditions. At room temperature, dry granular corn starch reacts with acetic anhydride in the presence of Sc(OTf)3 to form starches highly acetylated on the granular surface only. At higher temperatures (80 °C), starch acetates of degree of substitution (DS) 1.2 and 0.40 were obtained by reaction of granular starch with acetic anhydride or acetic acid, respectively. When starch was gelatinized in acetic acid prior to reaction, acetylation with acetic anhydride was much more rapid with DS reaching 3 after 1 h at 80 °C. Acetylation was rapid at levels of Sc(OTf)3 down to 0.1 mol%. These data suggest that powerful Lewis acid catalysts such as Sc(OTf)3 may be useful in preparing novel starch esters using either carboxylic acids or anhydrides.