Measurements of heat of zeolite dehydration by scanning heating

A procedure for measurement of the heat of zeolite dehydration by scanning heating has been designed. Simultaneous data on heat flow (DSC) and mass loss (TG) are required for evaluation. The heating rate depends on the experimental conditions (point-spread function, sample mass, crucible design, and calorimetric reproducibility). Dehydration measurements have three advantages as compared with the sorption procedure: i) one can investigate samples with irreversible dehydration; ii) no approximation model is needed for calculation of the partial molar heat of dehydration; and iii) the procedure is not labor-consuming. The procedure was tested on the natural zeolites heulandite, chabazite and mordenite. The results are close to those measured by the sorption procedure. The partial molar heat of dehydration was found to depend on the water content. It increases from 50 to 87 J mol(-1) K-1 for heulandite, from 53 to 81 J mol(-1) K-1 for chabazite, and from 51 to 71 J mol(-1) K-1 for mordenite. The approximation of the heat of sorption by linear regression was found to be wrong. Detection of a 'phase transition' after this approximation has no meaning.