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

Abstract

Abstract 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: <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"><mml:mo stretchy="false">(</mml:mo><mml:mi>M</mml:mi><mml:msub><mml:mi>g</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>0.741</mml:mn></mml:mrow></mml:msub><mml:mi>A</mml:mi><mml:msub><mml:mi>l</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>0.259</mml:mn></mml:mrow></mml:msub><mml:mo stretchy="false">(</mml:mo><mml:mi>O</mml:mi><mml:mi>H</mml:mi><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mo stretchy="false">(</mml:mo><mml:mo stretchy="false">(</mml:mo><mml:mi>F</mml:mi><mml:msup><mml:mi>e</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mi>I</mml:mi><mml:mi>I</mml:mi></mml:mrow></mml:msup><mml:mo stretchy="false">(</mml:mo><mml:mi>C</mml:mi><mml:mi>N</mml:mi><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>6</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>0.067</mml:mn></mml:mrow></mml:msub><mml:mo stretchy="false">(</mml:mo><mml:mi>C</mml:mi><mml:msub><mml:mi>O</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>0.002</mml:mn></mml:mrow></mml:msub><mml:mo stretchy="false">(</mml:mo><mml:msub><mml:mi>H</mml:mi><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi>O</mml:mi><mml:msub><mml:mo stretchy="false">)</mml:mo><mml:mrow class="MJX-TeXAtom-ORD"><mml:mn>1.05</mml:mn></mml:mrow></mml:msub><mml:mo stretchy="false">)</mml:mo></mml:math> 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.