Estimating the standard free energy of formation of zeolites using the polymer model

The standard free energies of formation of a number of zeolites were estima ted using the polymer model developed previously for smectites by Sposito. The estimates made with the polymer model were on average within 19 kJ mol( -1) (0.28%) of the experimental values for zeolites; this compares with the values estimated by the methods of LaIglesia and Aznar and of Chermak and Rimstidt which differed from the experimental values on average by approxim ately 27 kJ mol(-1) (0.40%) and 32 kJ mol(-1) (0.47%) respectively. The val ues predicted by the polymer model for dehydrated zeolites were within 28 k J mol(-1) (0.3%) of the experimental values, whereas the other two methods generate estimates of free energies of formation values that differed on av erage by 89 kJ mol(-1) (1.28%) and 107 kJ mol(-1) (1.59%) from the experime ntal values. These comparisons indicate that the polymer model, formulated specifically for a crass of silicate minerals such as zeolites, always typi cally provides a better estimate of the standard free energy values than ge neral models for all silicate minerals which contain a number of ad hoc ass umptions. Better estimates provided by the polymer model can be attributed to the principle that the free energy changes associated with the formation of condensation polymers, such as zeolites, are related principally with t he free energy changes associated with the relative change in coordination of exchangeable cations as they transfer from reactant hydroxide sites to e xchangeable sites. (C) 1999 Elsevier Science B.V. All rights reserved.