Organocatalytic esterification of corn starches towards enhanced thermal stability and moisture resistance†
Most commercial starch esters are currently produced under harsh alkaline conditions. Triggered by environmental concerns, organocatalysis has recently emerged as a sustainable alternative offering mild reaction conditions, reduced waste formation and high selectivity. In this paper, the catalytic effect of several organic acids for the heterogeneous esterification of corn starch is compared, followed by a systematic analysis of tartaric acid-catalysed reactions on substrates with varying amylose/amylopectin ratios using a range of carboxylic acid and anhydride reagents of different chain lengths. Reaction conditions (temperature, time and catalyst concentration) were optimized and products were thoroughly characterized in terms of degree of substitution, chemical structure, morphology, moisture uptake and thermal stability. Organocatalysis proved to be competitive with conventional, alkaline methods. Our study also demonstrates that sustainability can further be improved by the direct recycling of the liquid phase. Notably, however, the tartaric acid catalyst participates in substitution reactions with starch. Despite recent claims to the opposite, reactivity of the carboxylic acid derivatives is primarily determined by carbon chain-length, while reagent type and therefore water content in the reaction mixture has a crucial effect on the final properties of the product. Our results highlight the importance of selecting appropriate synthesis methods and conditions as well as characterization techniques in order to create materials with the desired property profile according to the targeted application.