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

Abstract

A metal organic framework [Ni_1.5(4,4′-bipy)_1.5(H₃L)(H₂O)₃]·[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]·[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 × 4.2 Å) to pore cavities (diameter: ∼12 Å). Thermogravimetric studies showed that both coordinated and lattice water molecules adsorbed in pores were removed in ultra-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 °C to form [Ni_1.5(4,4′-bipy)_1.5(H₃L)(H₂O)₃]. The ethanol adsorption isotherms for [Ni_1.5(4,4′-bipy)_1.5(H₃L)] for temperature range 20–50 °C were markedly hysteretic. The stoichiometry was [Ni_1.5(4,4′-bipy)_1.5(H₃L)]·[1.11C₂H₅OH] at p/p⁰ = 0.97 and 20 °C gave a total pore volume approximately half that of [Ni_1.5(4,4′-bipy)_1.5(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 [Ni_1.5(4,4′-bipy)_1.5(H₃L)] reveal kinetic molecular sieving occurs with very high kinetic selectivity for O₂/N₂ at 0 °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 °C. The adsorption results are discussed in terms of diffusion, thermodynamics and surface interactions in pores.