Synthesis of ethylene urea using carbon-dioxide-adsorbed titanium–zirconium mixed oxides

Abstract

To address the increasing global levels of CO2 emissions, it is necessary to develop efficient strategies that can convert CO2 into useful chemicals such as carbonates, ureas, and carbamates. Hybrid systems that combine direct capture and subsequent utilization of CO2 offer a unique emission-controlling pathway for achieving a carbon-neutral society. Ti-Zr mixed oxides could be ideal candidates for CO2 adsorption owing to the high specific surface areas pore sizes. Therefore, in this study, metal oxides with desirable acid-base sites that is, TixZr(1−x)O2 with different Ti/Zr ratios were synthesized using combined sol-gel and solvothermal methods and then utilized to adsorb and synthesize ethylene urea (EU), an important precursor of pharmaceuticals and agricultural products. The results indicated that the CO2 adsorption capacity of TiO2-ZrO2 mixed oxides were higher than that of TiO2 alone at 100 kPa and 25 °C, corresponding to their higher specific surface areas and pore volumes compared to their individual oxide counterparts. CO2-adsorbed TixZr(1−x)O2 was subsequently used as a CO2 source to generate EU through a reaction with ethylenediamine (EDA) and 2-propanol. Notably, samples containing higher proportions of Zr-Ti0.3Zr0.7O2 and ZrO2 produced EU in significant amounts, owing to their acid-base bifunctionality. Overall, EU was synthesized using CO2-adsorbed TixZr(1−x)O2 as a CO2 source and as an accelerator for reacting with EDA, without requiring high-pressure or high-purity CO2.