Thermal degradation of cyclodextrins

The thermal degradation of cyclodextrins and substituted beta-cyclodextrins has been studied. It is found that in inert atmosphere they all decompose in one major step (252-400 degrees C) leaving a residue (Char) which is thermally quite stable, decomposing at a low rate at higher temperature. Temperature of decomposition, char yield and thermal stability depend on the type of substituent of the cyclodextrin. On heating in air, the first degradation step is not affected whereas the char is oxidised to volatile products below 600 degrees C apart from the cases of amino and phosphate substituted cyclodextrins, which give a ceramic-like residue stable to above 800 degrees C. The charring process involves opening of the cyclodextrin rings followed by a chemical evolution similar to that of cellulose with loss of the glucosidic structure and hydroxyl groups and build up of unsaturation, carbonyl groups and aromatic structures. The kinetics and yield of the various processes, as well as the final structure of the char at high temperature, depend on the substituent of the cyclodextrin. Carbon dioxide, water, levoglucosan and furans are the major volatiles evolved from cyclodextrin degradation as from cellulose. Products deriving from substituents show that they do not simply behave as leaving groups bur rather take part in the charring. Moreover, the relative amount of CO2 and levoglucosan which are respectively related to competing charring and depolymerisation in cellulose, do not relate to the char yield of substituted cyclodextrins. (C) 2000 Elsevier Science Ltd. All rights reserved.