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Thermochemistry of hydrotalcite-like phases in the MgO-Al₂O₃-CO₂-H₂O system: A determination of enthalpy, entropy, and free energy

Rama kumar Allada^1,, Alexandra Navrotsky^1,* and Juliana Boerio-Goates²

¹ Thermochemistry Facility and NEAT ORU, University of California at Davis, Davis, California 95616, U.S.A.
² Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 12345, U.S.A.

Correspondence: ^* E-mail: anavrotsky{at}ucdavis.edu

Interest in hydrotalcite-like compounds has grown due to their role in controlling the mobility of aqueous metals in the environment as well as their use as catalysts, catalyst precursors, and specialty chemicals. Although these materials have been studied in a number of contexts, little is known of their thermodynamic properties. Here we present a complete thermochemical study of hydrotalcite-like compounds of the MgO-Al₂O₃-CO₂-H₂O system. Using high-temperature oxide-melt solution calorimetry, we determined the enthalpies of formation at 298 K from the elements (H_f⁰) for the compounds: Mg_0.69Al_0.31(OH)_2.013(CO₃)_0.15·0.30H₂O, Mg_0.74Al_0.26(OH)₂(CO₃)_0.13·0.39H₂O, Mg_0.67Al_0.33(OH)₂(CO₃)_0.16·0.69H₂O, and Mg_0.66Al_0.34(OH)₂(CO₃)_0.17·0.70H₂O to be –1171.55 ± 1.81, –1165.98 ± 2.06, –1284.65 ± 1.97, and –1292.07 ± 2.05 kJ/mol, respectively. We also present the heats of formation of these materials from the single-cation hydroxides [Mg(OH)₂ and Al(OH)₃] and carbonates (MgCO₃) and water (H^scc_f); they are energetically stable by 10–20 kJ/mol. Using low-temperature adiabatic heat-capacity measurements we determined the third-law entropy (S⁰) for the compound Mg_0.74Al_0.26 (OH)₂(CO₃)_0.13·0.39H₂O [85.58 ± 0.17 J/(mol·K)], neglecting any configurational contributions. From our experimental data for Mg_0.74Al_0.26(OH)₂(CO₃)_0.13·0.39H₂O, we calculated the free energy at 298 K (G_f⁰ = –1043.08 ± 2.07 kJ/mol). Our thermodynamic studies also provide insight into the state of the interlayer water in hydrotalcite, namely the water of hydration appears to exist in a state intermediate in thermodynamic properties between that of ice and liquid water.

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