Water adsorption characteristics and microcalorimetric studies of MOF-5 and MOF-199 synthesized using “green” sol-gel

Abstract

In this study, types of porous metal-organic framework materials namely MOF-5 (robust framework), MOF-199 (open framework) in the non ion-exchanged condition was compared in terms of hydrophobicity-hydrophilicity of the surface. The investigations were done using microcalorimetric measurements of heat of immersions in water. MOF-5 and MOF-199 were synthesized using plant-based polymer of palm oil derived fatty alcohols (PODFA) as porous template using “green” sol-gel approach. The synthesis approach applied was environmental-friendly by eliminating the use of organic solvents and surfactants substituting with water and PODFA. X-ray diffraction patterns of MOF-199 exhibited prominent reflection peaks assigned to octahedral structure with small peak shift while MOF-5 exhibited truncated and full cubic morphology. PODFA with longer aliphatic carbon chain exhibited higher porosity and pore volume. The addition of PODFA increased the homogeneity of the cubic and octahedral single crystals typically observed in MOF-5 and MOF-199 morphology. This was due to the increased in immiscibility of metal oxide precursors and organic linkers hence allowing reactions to occur from all directions. PODFA molecules were suggested to form self-assemblies at the air-water interface governed by surface tension auto-oscillations and formed clusters on which metal oxide and organic linker precursors bind with. The water adsorption isotherms of MOF-5 and MOF-199 prepared in this study exhibited “gate-opening” effect at lower relative pressure not observed in other nanoporous framework structure made up of two-components such as organosilica materials.