Preparing esters from high-amylose starch using ionic liquids as catalysts

The use of different ionic liquids (IL), mainly 1-N-alkyl-3-methylimidazolium chlorides as possible catalysts for esterification of starch is reported. A high amylose maize starch is used and the esterification is performed with the aid of different carboxylic anhydrides. These are used both as a reactant and a solvent for all reactions using a maximum of 1.2 molar equivalents (moleq) of IL per anhydro glucose unit (AGU). Highly substituted starch esters can be achieved within a 4–24 h reaction time depending on the molar ratios of ionic liquid and esterifying agent. The ionic liquids used for these reactions can easily be recovered and reused. The influence of the structure and amount of ionic liquid used in the preparation of starch acetate and starch propionate as model substances are analyzed in terms of their degree of substitution (titration), reaction rate, solubility and average molecular weight (SEC-MALLS). A study of the effects of different bases on degradation of the starch was investigated, too. We conclude that imidazolium-based ionic liquids with halogenides as counter ions can be used as catalysts for the esterification of starch and thus fulfill the definition of a recyclable catalyst that lowers activation energy.