Kinetic molecular sieving, thermodynamic and structural aspects of gas/vapor sorption on metal organic framework [Ni1.5(4,40-bipyridine)1.5(H3L)- (H2O)3][H2O]7 where H6L =2,4,6-trimethylbenzene- 1,3,5-triyl tris(methylene)triphosphonic acid
作者：Xuebo Zhao,a Jon G. Bell,b Si-Fu Tang,*c Liangjun Lia and K. Mark Thomas
關鍵字：gas/vapor sorption
論文來源：期刊
具體來源：J. Mater. Chem. A, 2016, 4, 1353
發表時間：2016年
A metal organic framework [Ni1.5(4,40-bipy)1.5(H3L)(H2O)3]$[H2O]7 where H6L ? 2,4,6-trimethylbenzene-
1,3,5-triyl tris(methylene)triphosphonic acid and 4,40-bipy ? 4,40-bipyridine has been prepared. The
structures of [Ni1.5(4,40-bipy)1.5(H3L)(H2O)3]$[H2O]7 and the desolvated form [Ni1.5(4,40-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 Ni1.5(H3L) layers and 4,40-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$102}. The framework has
one-dimensional channels decorated with acidic O–H groups with irregular shape varying from narrow
windows (cross section: 4.2 4.2 °A) to pore cavities (diameter: 12 °A). Thermogravimetric studies
showed that both coordinated and lattice water molecules adsorbed in pores were removed in ultrahigh
vacuum to give [Ni1.5(4,40-bipy)1.5(H3L)]. The water vapor adsorption isotherm for [Ni1.5(4,40-
bipy)1.5(H3L)] showed that 3 coordinated and 7 pore lattice water molecules were adsorbed and the
framework structure was reformed. The desorption isotherm showed that the lattice water was easily
desorbed in vacuum at 20 C to form [Ni1.5(4,40-bipy)1.5(H3L)(H2O)3]. The ethanol adsorption isotherms
for [Ni1.5(4,40-bipy)1.5(H3L)] for temperature range 20–50 C were markedly hysteretic. The stoichiometry
was [Ni1.5(4,40-bipy)1.5(H3L)]$[1.11C2H5OH] at p/p0 ? 0.97 and 20 C gave a total pore volume
approximately half that of [Ni1.5(4,40-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 [Ni1.5(4,40-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 mol1. The corresponding activation energy for diffusion of CO2 into the
framework was 48 kJ mol1. Narrow constrictions in the porous structure of [Ni1.5(4,40-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.