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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

Lookup NU author(s): Xuebo Zhao, Dr Jon Bell, Emeritus Professor Mark Thomas

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This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).


Abstract

A metal organic framework [Ni1.5(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 [Ni1.5(4,4’-bipy)1.5(H3L)(H2O)3]·[H2O]7 and the desolvated form [Ni1.5(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 Ni1.5(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 [Ni1.5(4,4’-bipy)1.5(H3L)]. The water vapor adsorption isotherm for [Ni1.5(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 [Ni1.5(4,4’-bipy)1.5(H3L)(H2O)3]. The ethanol adsorption isotherms for [Ni1.5(4,4’-bipy)1.5(H3L)] for temperature range 20-50°C were markedly hysteretic. The stoichiometry was [Ni1.5(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 [Ni1.5(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 [Ni1.5(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 [Ni1.5(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 (x 1000) at 30°C. The adsorption results are discussed in terms of diffusion, thermodynamics and surface interactions in pores.


Publication metadata

Author(s): Zhao X, Bell JG, Tang SF, Li LJ, Thomas KM

Publication type: Article

Publication status: Published

Journal: Journal of Materials Chemistry A

Year: 2016

Volume: 4

Pages: 1353-1365

Print publication date: 29/01/2016

Online publication date: 17/12/2015

Acceptance date: 08/12/2015

Date deposited: 21/12/2015

ISSN (print): 2050-7488

ISSN (electronic): 2050-7496

Publisher: Royal Society of Chemistry

URL: http://dx.doi.org/10.1039/c5ta08261g

DOI: 10.1039/c5ta08261g


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