A predictive model for the enthalpies of hydration of zeolites

Abstract

A compilation of the average hydration enthalpies per mole of water of 145 diversely originating zeolites measured using different technical methods [76 data from transposed-temperature drop calorimetry (TTDC), 57 data from immersion calorimetry (IC), 6 data from phase equilibria (PE), 5 data from gas-adsorption calorimetry (GAC), and 3 data from hydrofluoric acid solution calorimetry (HF)] was generated. Statistical regressions between three parameters involving the average hydration enthalpy per mole of water {∆H hyd-W , ∆H hyd-W /(Al/Si), ln[-∆H hyd-W /(Al/Si)]}and six parameters namely:(1) the charge defined by the Al/Si ratio; (2) the ratio of the framework charge to the number of H 2 O molecules (Al/H 2 O); (3) the framework density (FD) calculated from the molecular volume of the anhydrous zeolite, FD anh , and hydrated zeolite, FD hyd ; (4) the average cation electronegativity in the exchange site characterized by parameter ∆ H O = (site A) aq ; and (5) the intracrystalline water porosity (WP) determined from the volume of liquid water that can be recovered upon thorough outgassing of the hydrated zeolite. The regressions were performed by taking into account either the nature of the measurement technique, or the nature of the zeolite family. Within the zeolites from the TTDC and IC populations (133 data), the best results were obtained with ln[-∆H hyd-W /(Al/Si)] and Al/(Al + Si). Whatever the measurement technique, considering the nature of the zeolite family having a constant framework density of the anhydrous form (129 data), the Al/(Al + Si) ratio remains the best parameter and the enthalpy of hydration can be expressed as follows: ∆H hyd-w = -(Al/Si)*e {5.491 -4.674*[Al/(Al + Si)]}This general relationship can be improved by considering the following parameters: FD anh , ∆ H O = (site A) aq , WP and a new parameter that is the product of three parameters Al/Si, ∆ H O = (site A) aq , and WP weighing the variation of the water porosity related to the nature of the cation and to the total charge of the exchange site. Therefore, an understanding of the chemical formulae and unit-cell volumes of anhydrous and hydrated zeolites is required to evaluate the enthalpy of hydration with an accuracy of ±3.25 kJ/mol H 2 O.

Thermodynamics of Hydration in Minerals: How to Predict These Entities

“…A compilation of the average hydration enthalpies per mole of water of 145 diversely originating zeolites measured using different technical methods [54] provided 76 data points from the transposed-temperature drop calorimetry (TTDC), 57 data points from the immersion calorimetry (IC), 6 data points from the phase equilibriums (PE), 5 data points from the gas-adsorption calorimetry (GAC) and 3 data points from the hydrofluoric acid calorimetry (HF).…”

“…From the selected average enthalpies of hydration of the different zeolites with their chemical formulas (145 data points), a regression equation for the enthalpy of hydration per mole of water is proposed [54]:…”

“…(17) refers to one oxygen atom. A set of internally consistent values was presented by Vieillard & Mathieu [54] and is given in Table 4.…”

“…Assuming that the unit cell volume of zeolitic water is the same as that of liquid water ( Table 4. Parameter ΔHO = M z+ (aq) of selected cations [54].…”

“…By eliminating 9 erroneous data points [54], the regression coefficient for 136 data is R2= 0.880, with a standard error of ± 3.46 kJ mol-1 for all data, regardless of the nature of the experimental data. Figure 2 displays the predicted enthalpy of hydration of zeolitic water calculated from Eqs.…”