No abstract

No abstract

A concurrent absorption analytical study of the ion exchange equilibria and direct microcalorimetric measurement of the heat of ion exchange absorption of K + and Ca 2+ cations for the sodium forms of two mordenites (NaM) obtained from naturally occurring samples rich in Na + and Ca 2+ cations has been carried out for the first time. The free energies and entropies of exchange have been calculated. It was shown that the free energy of exchange in the K + -NaM system is determined by the enthalpy, whereas in the Ca 2+ -NaM system it is determined by the entropy of solvation. The measured thermodynamic characteristics of exchange were analyzed taking into account the preferred localization of sorption for the cations in the structure. A memory effect in natural mordenites has been established for extra-skeletal K + and Ca 2+ cations, in the presence of which these zeolites crystallized in nature.Systematic study of the ion exchange equilibria and the thermodynamics of ion exchange for the corresponding systems is required to elucidate the high selectivity of the natural high-silicon zeolites clinoptilolite, mordenite, erionite, and shabazite to large cations of the alkali and alkaline earth metals. However even for clinoptilolite, the most studied from this point of view [1,2], the thermodynamic characteristics of ion exchange have not been studied satisfactorily and were limited to the values of the enthalpy and entropy of ion exchange for a single final degree of filling q ® 1.0 for the ion exchange absorption of a cation [3]. This explains the circumstance that data on the enthalpy of ion exchange DH for both synthetic and natural zeolites investigated have, as a rule, been obtained on the basis of an isotherm of ion exchange measured at two or three temperatures with subsequent calculation on DH using the van't Hoff equation.The hundreds of direct calorimetric measurements of the heat (enthalpy) of exchange ions in natural zeolites (see, for example, [4]) carried out by R. M. Barrer in the 60s need to be repeated because they were obtained with a low-sensitivity calorimeter which permitted determination of the heat evolved incompletely and only in the first 30 min. Since then we have investigated the heat of ion exchange on clinoptilolite [5] with a highly sensitive Tian-Calve microcalorimeter [6] and a specially cell [7] which permits the avoidance of parasite heat effects, and we have shown that the time to establish thermal equilibrium is normally 8-10 h and sometimes is achieved in a day.In the present work we have measured and analyzed the isotherms and heats of ion exchange of two samples of natural mordenite with Na + , K + , and Ca 2+ . These cations were not chosen by chance. They occur in the composition of the exchanger mordenite and equally in other natural mordenites. It can therefore be expected that study of the ion exchange equilibria and heat of ion exchange involving these cations would permit the elucidation of the memory effect in mordenite with respect to the cations in the presence of whic...

No abstract