Exploring gas adsorption properties and phase transitions of two M(tppo) metal-organic frameworks

Abstract

Separation and storage of gases is of interest to the global society in order to minimize emission of greenhouse gases and utilize these gases as resources in their pure form in the chemical industry. This work considers the porous class of materials metal-organic frameworks (MOFs) a viable option due to their adaptability and tunability.

In this project, the MOFs M(tppo) (M=Y, Zn, tppo=tris(4-carboxyphenyl)phosphane oxide) are studied on the basis of their gas adsorption abilities at low pressures. Multiple MOFs have previously been synthesised with the tppo linker, however, the single crystal-to-single crystal transformation of the Zn(tppo) MOF renders it its unique properties. The single crystal-to-single crystal transformation of Zn(tppo) from Zn4(OH)2(tppo)2(DEF)5 (1) to Zn4O(tppo)2 (3) is investigated through single-crystal X-ray diffraction and variable temperature powder X-ray diffraction and the respective gas adsorption properties of N2 at 77 K are studied. The gas adsorption results highlighted the importance of proper handling of Zn(tppo) after synthesis for the optimal uptake. The variable temperature powder X-ray diffraction (PXRD) experiments showed a discontinuous phase change from Zn4(OH)2(tppo)2 (2) to (3) at ∼ 400-430 K. Treatment at 373 K for 18 hours was shown to achieve the transformation to (3).

This work also investigates the adsorption of select hydrocarbons (CH4, C2H4, C2H6, C3H6, and C3H8) and noble gases (Ar, Kr) on Y(tppo) through adsorption isotherms and variable temperature powder X-ray diffraction.