Adsorption of N2, O2, CO2 and H2 on Hydrotalcite-Like System: Mg2+-Al3+-(Fe(CN)6)4−

Abstract

The compound Mg²⁺-Al³⁺-(Fe(CN)₆)⁴⁻, with a structure similar to hydrotalcite Mg₆Al₂(OH)₁₆CO₃ ·4H₂O, is prepared by a coprecipitation method. Chemical, thermal, and X-ray analysis of the compound lead to the formula: (Mg0.741Al0.259(OH)2)((FeII(CN)6)0.067(CO3)0.002(H2O)1.05) The compound is dehydrated at 100° to 250°C, and the adsorption of N₂, O₂, CO₂, and H₂ on it is measured at −196°C to room temperature by the volumetric method. The water is removed reversibly from 100° to 200°C, and the interstices after dehyration act as sites of adsorption. When dehydration is carried out at 150°C, the adsorptive activity reaches a maximum. Adsorption isotherms of N₂ and O₂ at −196°C and of CO₂ at room temperature are of the Langmuir type, and the saturated amounts of N₂, O₂, and CO₂ adsorbed are 96.3, 65.2, and 91.8 ml (STP)/g, respectively.Adsorption isotherms of N₂ and O₂ at room temperature are of the Henry type. The amount of O₂ adsorbed is about 67% of that of N₂. The isosteric heats of adsorption at room temperature of N₂, O₂, and CO₂ are 5.1, 4.8, and 11.0 kcal/mol, respectively. A dehydrated product of a Mg²⁺-Al³⁺-CO₃²⁻ compound does not permit adsorption of CO₂ at room temperature, but permits the adsorption of H₂O. Hence, it has a molecular sieve effect. No adsorption of H₂ is observed in any of the compounds tested.