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

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, Ti_xZr_(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 Ti_xZr_(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-Ti_0.3Zr_0.7O₂ and ZrO₂ produced EU in significant amounts, owing to their acid-base bifunctionality. Overall, EU was synthesized using CO₂-adsorbed Ti_xZr_(1−x)O₂ as a CO₂ source and as an accelerator for reacting with EDA, without requiring high-pressure or high-purity CO₂.