Porosity studies of isoreticular mixed-ligand metal-organic frameworks

Gcwensa, Nolwazi

Porosity studies of isoreticular mixed-ligand metal-organic frameworks

Master Thesis

2019

Abstract

The syntheses of four novel mixed-ligand metal-organic frameworks (MOFs) are reported. Isoreticular, Zn(II)-based mixed-ligand MOFs with formulae [Zn(μ2-ia)(μ2-bpe)]n·nDMF (1) and [Zn(μ2-mia)(μ2- bpe)]n·nDMF (2), where ia = isophthalate, mia = 5-methoxyisophthalate, bpe = 1,2-bis(4-pyridyl)ethane and DMF = N,N’-dimethylformamide were synthesised and characterised. Both compounds 1 and 2 exhibit sql, 2-periodic, 2D net coordination layers. Catenation of neighbouring frameworks form 2-fold interpenetrated bilayers which are interdigitated resulting in channel voids containing DMF. Experimental void calculations indicate 2′ has larger void space per unit cell than 1′; however, experimentally, 1′ showed higher water vapour and carbon dioxide 195 K sorption as well as significant hysteresis upon desorption of carbon dioxide 195 K. This hysteresis behaviour of 1′ is interchanged with 2′ for water vapour sorption at 298 K. Sorption isotherm inflection points indicate that structural changes occur, and empirical evidence point to weak bilayer···bilayer interactions in 1′ which allow the separation of the bilayers as well as the limiting effect on structural changes of the methoxy group present in 2′. Isoreticular mixed-ligand Cd(II)-based MOFs with formulae [Cd(μ2-mia)(μ2-bpe)1.5]n·n(DMF)0.5n(H2O)0.5 (3) and [Cd(μ2-nia)(μ2-bpee)1.5]n·nDMF (4), where nia = 5-nitroisophthalate and bpee = 1,2-bis(4-pyridyl)ethylene were also synthesised and characterised. Both compounds 3 and 4 exhibit sql, 2-periodic, 3D net coordination layers with disorder around a single bpe or bpee ligand. These structures are compared to published structure [Cd(bpee)1.5(nbdc)]n·nDMF (JECRAN) which is isoreticular to both MOFs. Activation of 4 and JECRAN occurs via single-crystal-to-single-crystal transformations. Potential and actual void space calculations indicate that 4′ has a larger void space than 3′ and JECROB. Liquid sorption experiments revealed that 3′ and 4′ showed affinities for different solvents. Although carbon dioxide 195 K sorption for 4′ is initially higher than for JECROB, structural changes, indicated by sorption isotherm inflection points, allow JECROB to adsorb more carbon dioxide than 4′

Keywords

Chemistry

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