Hydrogen Storage in Porous Cyanometalates:  Role of the Exchangeable Alkali Metal

Abstract

The hydrogen storage in zeolite-like hexacyanometalates with different exchangeable alkali metals within the cavities was studied. The H 2 adsorption isotherms were recorded at 75 and 85 K in order to estimate the involved adsorption heats using the isosteric method. The electric field gradient within the porous framework favors the hydrogen adsorption in the materials under study but also could lead to kinetic effects for the pore filling. Such effects were particularly pronounced for sodium among the studied compositions: Zn 3 A 2 [Fe-(CN) 6 ] 2 (A ) Na + , K + , Rb + , Cs + ) and Zn 3 [Co(CN) 6 ] 2 . For Na + , a strong interaction with the H 2 molecule takes place, where appreciable kinetic effects even at 258 K are observed. For Zn 3 [Co(CN) 6 ] 2 (rhombohedral phase) where the cavities are free of exchangeable metal and, in consequence, have a weak electric field gradient on their surface, the largest hydrogen storage capacity, close to 12 H 2 molecules per cavity (1.82% by weight), was observed. The hydrogen adsorption in these materials involves adsorption heats in the 6-8.5 kJ/mol range, following the order K > Rb > Cs ≈ Zn 3 [Co(CN) 6 ] 2 . The porous framework of this family of materials is formed by ellipsoidal cavities communicated by elliptical windows. The alkali metals are sited close to the windows. The pore accessibility and pore volume were evaluated from CO 2 adsorption isotherms recorded at 273 K. The free volume was found to be accessible to the CO 2 molecule for all of the studied compositions. According to the obtained isotherms the stabilization of the CO 2 molecule within the pores is caused by the electrostatic interaction between the electric field gradient at the cavity and the adsorbate quadrupole moment. The estimated strength for the guest-host interaction and the accessible pore volume follow the order Na > K > Rb > Cs. The largest accessible pore volume was found for Zn 3 [Co(CN) 6 ] 2 , close to 8 CO 2 molecules per cavity (28% by weight), but with the weaker guest-host interaction. The materials under study were characterized from X-ray diffraction, thermo-gravimetric, infrared, and Mo ¨ssbauer data. The obtained results shed light on the role of the electric field gradient at the cavity for the hydrogen adsorption.

Adsorption and separation of light alkane hydrocarbons by porous hexacyanocobaltates (III)

“…For zinc also, the tetrahedral coordination is possible and a zeolite-like porous solid free of vacancies results. [7,10,12] In this case, the available free volume results from ellipsoidal cavities of ca 12 × 9 × 8 Å, which remain communicated by elliptical windows of about 5 Å of larger diameter. [13] In addition to hydrogen, the porous frameworks of this series of coordination compounds have also been evaluated from H 2 O, CO 2 , N 2 , and Xe adsorption data, [7,10 -14] but remain poorly characterized for larger adsorbates like light alkane hydrocarbons.…”

“…The crystal structure, thermal stability, and information on the electronic structure for the materials under consideration are available from previous studies [7,12,13] and only a summary of their features is provided here. The XRD powder patterns and SEM images of the studied samples are available from the Supporting Information.…”

Hydrogen Adsorption on Metal Organic Framework Materials for Storage Applications

“…[34,36,38,40,50,57,58,[60][61][62]64,87,91,103,105,106,108,[119][120][121]127,129,130,145,146,149,150,156,160,[170][171][172][173][174][175][176][177][178][179][180][181][182]184,191,[199][200][201][202][203][204][205] The enthalpies for hydrogen adsorption on porous materials at cryogenic temperature...…”

“…• Carbons: 1.4-12.5 kJ mol À1 ; [40,61,62,64,87,191] • Zeolites: 5.9-18.2 kJ mol À1 ; [62,91,[202][203][204][205] • Silicas: 5.4-10.4 kJ mol À1 ; [62] • MOFs: 5.1-13.5 kJ mol À1 ; [34,36,38,50,57,58,60,[119][120][121]127,[129][130][131][132]136,137,141,[144][145][146]149,150,156,[170][171][172][173][174][175][176][177][178][179][180][181][182]…”

Structural and Gas Retention Changes Induced by Ozonization of Cobalt(II) and Manganese(II) Hexacyanocobaltates(III)

“…Transitions metal hexacyanometallates represent an attractive family of porous materials with a great diversity of topologies and pore sizes [10][11][12][13][14][15][16][17][18][19]. Some members of this family also have a high density of open metal sites on their pores walls [11], and others have exchangeable metal cations [20][21][22].…”