A predictive model for the enthalpies of formation of zeolites

Abstract

To date, there is no available method for estimating the enthalpies of formation of different zeolites with identical compositions and various degrees of hydration. Fifty calorimetric data (from dissolution calorimetry in lead borate) for the enthalpies of formation of various anhydrous zeolites from the stable oxides have been obtained. A new formalism defining the enthalpies of formation from the oxides of anhydrous zeolites having a zeolite-like structure is proposed. The formalism is based (1) on a relationship between the measured enthalpies of formation of zeosils and a parameter characterizing the nature of the zeolitic framework represented by FD (defined by the number of tetrahedral atoms per 1000 Å 3), and (2) on the electronegativity difference. For a constant framework (or a same structural zeolite family), the enthalpy of formation from the oxides is the sum of the products of the molar fraction of an oxygen atom bound to any two cations multiplied by the electronegativity difference defined by the Δ HO =M z+ (zeol) between any two consecutive cations located in the extra-framework and tetrahedral sites. The enthalpy of formation of an anhydrous zeolite from the constituent oxides is governed by three major factors, which are the framework density, the Al/Si ratio and the nature of the cation. Therefore, the combination of the model tested on anhydrous zeolites with the model for estimating hydration enthalpies published elsewhere allows estimation of the enthalpies of formation of hydrated zeolites and their comparison with the numerous calorimetric measurements of 148 natural zeolites, yielding a statistical error of ±0.5%.