Experimental investigation of CO₂ adsorption capacities in bimetallic-doped UiO-66 and UiO-66-NH2 frameworks

Abstract

A metal-organic framework (MOF) is a crystalline material that exhibits distinctive physical and chemical properties due to the coordination between metal ions and organic ligands. These properties, including a high surface area, adjustable porosity, and the ability to easily modify the chemical composition, collectively render MOFs advantageous for adsorption and separation applications. Introducing polar groups and bimetallic components into the MOF structure could significantly enhance CO₂ adsorption performance. This article investigated the impact of integrating these two approaches on CO₂ capture. Three Lewis acid metal ions (Al³⁺, Fe³⁺, and Cu²⁺) were doped into MOFs, specifically UiO-66 and UiO-66-NH₂. A range of characterization techniques were employed to facilitate comparison and verification of the results, including N₂ adsorption, powder X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis. This study then evaluated CO₂ adsorption at varying temperatures. The findings indicated that the Fe³⁺-doped samples exhibited the best performance irrespective of the presence of amino groups. Notably, the higher the concentration of metal dopants in UiO-66, the greater the adsorption capacity. UiO-66(Zr₁ Fe₁) exhibited the highest CO₂ adsorption capacity of 2.22 mmol per gram of the modified MOF. In contrast, modified UiO-66-NH₂ exhibited the opposite trend, where the lower the metal doping level, the higher the adsorption capacity. UiO-66-NH₂(Zr₅ Fe₁) exhibited the highest adsorption capacity of 3.5 mmol/g.