Kinetic molecular sieving, thermodynamic and structural aspects of gas/vapor sorption on metal organic framework [Ni1.5(4,4′-bipyridine)1.5(H3L)(H2O)3][H2O]7 where H6L=2,4,6-trimethylbenzene-1,3,5-triyl tris(methylene)triphosphonic acid

A metal organic framework [Ni-1.5(4,4'-bipy)(1.5)(H3L)(H2O)(3)center dot[H2O](7) where H6L = 2,4,6-trimethyLbenzene-1,3,5-triyl tris(methylene)triphosphonic acid and 4,4'-bipy = 4,4'-bipyridine has been prepared. The structures of [Ni-1.5(4,4'-bipy)(1.5)(H3L)(H2O)(3)center dot[H2O](7) and the desoLvated form [Ni-1.5(4,4'-bipy)(1.5)(H3L)(H2O)(3) have been determined by single crystal X-ray diffraction and the framework structures are virtually identical with the former having disordered water moLecuLes in the pores. The framework structure comprises of two-dimensional Ni-1.5(H3L) Layers and 4,4'-bipy Linkers acting as pillars with an unusual framework topology of a (3, 3, 6) net that can be denoted as: {4.62}2{63}2{68.85.102}. The framework has one-dimensional channels decorated with acidic O -H groups with irregular shape varying from narrow windows (cross section: 4.2 x 4.2 A) to pore cavities (diameter: -12 A). Thermogravimetric studies showed that both coordinated and Lattice water moLecuLes adsorbed in pores were removed in Ora high vacuum to give [Ni-1.5(4,4'-bipy)(1.5)(H3L)]. The water vapor adsorption isotherm for [Ni-1.5(4,4'bipy)(1.5)(H3L)] showed that 3 coordinated and -7 pore Lattice water moLecuLes were adsorbed and the framework structure was reformed. The desorption isotherm showed that the Lattice water was easily desorbed in vacuum at 20 degrees C to form [Ni-1.5(4,4'-bipy)(1.5)(H3L)(H2O)(3). The ethanoL adsorption isotherms for [Ni-1.5(4,4'-bipyh.5(H3L)] for temperature range 20-50 degrees C were markedly hysteretic. The stoichiometry was [Ni-1.5(4,4'-bipy)(1.5)(H3L)1[1.11C(2)H(5)OH] at p/p = 0.97 and 20 degrees C gave a total pore volume approximately half that of [Ni-1.5(4,41-bipy)15(H3L)(H2O)(3)]. The desorption isotherms show that ethanoL is strongly retained with decreasing pressure indicating a stable framework structure. The kinetic profiles for oxygen, nitrogen, carbon dioxide, and water and ethanoL vapors, can be described by Fickian, combined barrier resistance/diffusion (CBRD), and stretched exponential models for both adsorption and desorption. Gas adsorption studies for [Nit5(4,4'-bipy)(1.5)(H3L)] reveal kinetic moLecuLar sieving occurs with very high kinetic seLectivity for O-2/N-2 at O degrees C. Carbon dioxide adsorption has intermediate rates of adsorption between oxygen and nitrogen. The isosteric enthalpy for CO2 adsorption at zero surface coverage was 30.7 2.4 kJ moL(-1). The corresponding activation energy for diffusion of CO2 into the framework was -48 kJ moL(-1). Narrow constrictions in the porous structure of [Ni-1.5(4,4'-bipy)(1.5)(H3L)] give rise to kinetic moLecuLar sieving effects and do not allow adsorption of moLecuLes such as methane, which has a Larger cross-section. The seLectivity for CO2/CH4 was very high (x1000) at 30 degrees C. The adsorption results are discussed in terms of diffusion, thermodynamics and surface interactions in pores.