Metal chloride-catalyzed acetylation of starch: Synthesis and characterization

Abstract

Acylation of polysaccharides is a commercially important reaction and is usually performed in a process involving the polysaccharide, an acid anhydride, and an inorganic acid. As an alternative to inorganic acid, many catalysts, including some metal chlorides, have been previously reported as catalysts. In this work, we took a more comprehensive look at several metal chlorides to observe trends and reactivities among them, particularly relating to reaction temperature, time, and amount of acetic anhydride used. Iodine was also included for comparison. Almost all the metal chlorides studied were found to be active as catalysts for the acetylation of starch under suitable reaction conditions. However, each metal chloride had a somewhat different reactivity with a different optimal temperature needed for satisfactory reactions to take place. The molecular weight of the starch acetate products decreased in all cases observed. The reactivity trends among the metal halides seemed to correlate both with the ease of complexation between the halide and the substrate and with the acidity of the metal chloride. Characterization was achieved through ¹³C NMR, FT-IR, thermogravimetric analysis and size exclusion chromatography.

Keywords:

• Acetylation
• esterification
• Lewis acids
• metal chlorides
• starch

Additional information

Funding

Thanks are due to Karl Vermillion for NMR data, and Jason Adkins for the SEC and thermal analysis data. Atanu Biswas and Roselayne Ferro Furtado thank Brazilian National Council for Scientific and Technological Development (CNPq, process no. 405506/2013-9) for support of this work through the Science without Borders program. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.