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

The compound Mg 2+ -Al 3+ -(Fe(CN) 6 ) 4− , with a structure similar to hydrotalcite Mg 6 Al 2 (OH) 16 CO 3 ·4H 2 O, 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 N 2 , O 2 , CO 2 , and H 2 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 N 2 and O 2 at −196°C and of CO 2 at room temperature are of the Langmuir type, and the saturated amounts of N 2 , O 2 , and CO 2 adsorbed are 96.3, 65.2, and 91.8 ml (STP)/g, respectively. Adsorption isotherms of N 2 and O 2 at room temperature are of the Henry type. The amount of O 2 adsorbed is about 67% of that of N 2 . The isosteric heats of adsorption at room temperature of N 2 , O 2 , and CO 2 are 5.1, 4.8, and 11.0 kcal/mol, respectively. A dehydrated product of a Mg 2+ -Al 3+ -CO 3 2− compound does not permit adsorption of CO 2 at room temperature, but permits the adsorption of H 2 O. Hence, it has a molecular sieve effect. No adsorption of H 2 is observed in any of the compounds tested.