TY - JOUR
T1 - Thermochemistry of hydrotalcite-like phases in the MgO-Al2O 3-CO2-H2O system
T2 - A determination of enthalpy, entropy, and free energy
AU - Allada, Rama Kumar
AU - Navrotsky, Alexandra
AU - Boerio-Goates, Juliana
N1 - Copyright:
Copyright 2008 Elsevier B.V., All rights reserved.
PY - 2005/2
Y1 - 2005/2
N2 - 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 (ΔH fo) for the compounds: Mg0.69Al 0.31(OH)2.013(CO3) 0.15-0.30H2O, Mg0.74Al 0.26(OH)2(CO3) 0.13-0.39H2O, Mg0.67Al 0.33 (OH)2(CO3) 0.16-0.69H2O, and Mg0.66Al 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 (ΔHfscc); they are energetically stable by 10-20 kJ/mol. Using low-temperature adiabatic heat-capacity measurements we determined the third-law entropy (So) for the compound Mg 0.74Al0.26 (OH)2(CO3) 0.13-0.39H2O [85.58 ± 0.17 J/ (mol·K)], neglecting any configurational contributions. From our experimental data for Mg0.74Al0.26(OH) 2(CO3)0.13-0.39H2O, we calculated the free energy at 298 K (ΔGfo = -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.
AB - 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 (ΔH fo) for the compounds: Mg0.69Al 0.31(OH)2.013(CO3) 0.15-0.30H2O, Mg0.74Al 0.26(OH)2(CO3) 0.13-0.39H2O, Mg0.67Al 0.33 (OH)2(CO3) 0.16-0.69H2O, and Mg0.66Al 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 (ΔHfscc); they are energetically stable by 10-20 kJ/mol. Using low-temperature adiabatic heat-capacity measurements we determined the third-law entropy (So) for the compound Mg 0.74Al0.26 (OH)2(CO3) 0.13-0.39H2O [85.58 ± 0.17 J/ (mol·K)], neglecting any configurational contributions. From our experimental data for Mg0.74Al0.26(OH) 2(CO3)0.13-0.39H2O, we calculated the free energy at 298 K (ΔGfo = -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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U2 - 10.2138/am.2005.1737
DO - 10.2138/am.2005.1737
M3 - Article
AN - SCOPUS:14944372491
SN - 0003-004X
VL - 90
SP - 329
EP - 335
JO - American Mineralogist
JF - American Mineralogist
IS - 2-3
ER -