A procedure for measurement of the heat of zeolite dehydration by scanning
heating has been designed. Simultaneous data on heat flow (DSC) and mass lo
ss (TG) are required for evaluation. The heating rate depends on the experi
mental conditions (point-spread function, sample mass, crucible design, and
calorimetric reproducibility). Dehydration measurements have three advanta
ges as compared with the sorption procedure: i) one can investigate samples
with irreversible dehydration; ii) no approximation model is needed for ca
lculation 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 procedur
e. The partial molar heat of dehydration was found to depend on the water c
ontent. 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 morden
ite.
The approximation of the heat of sorption by linear regression was found to
be wrong. Detection of a 'phase transition' after this approximation has n
o meaning.