Green Synthesis of Urea from Carbon Dioxide and Ammonia Catalyzed by Ultraporous Permanently Polarized Hydroxyapatite

Abstract The sustainable synthesis of urea from ammonia (NH 3 ) and carbon dioxide (CO 2 ) using ultraporous permanently polarized hydroxyapatite (upp‐HAp) as catalyst has been explored as an advantageous CO 2 ‐revalorization strategy. As the simultaneous activation of N 2 and CO 2 (single‐step) demands an increase of the reaction conditions, we have re‐visited the industrial two‐step Bazarov reaction. upp‐HAp has been designed as a stable multifunctional catalyst capable of promoting both CO 2 and NH 3 adsorption for their subsequent C−N bond formation. Herein we report the synthesis of 1 mmol/g cat of urea with a selectivity of 97 % under strictly mild conditions (95–120 °C and 1 bar of CO 2 ; without applying any electrical currents or UV irradiation) which represents an efficiency of ~2 % and ~30 % with respect to the NH 3 and CO 2 content, respectively. The study of the NH 3 content, products adsorbed in the catalyst, presence of intermediates and temperature of the reaction allows unveiling the great potential of upp‐HAp as a green catalyst for sustainable Bazarov reactions. Results suggest that the double‐step approach could be more advantageous for both synthesizing urea and as a CO 2 ‐revalorization strategy, which in turn promotes the development of specific technologies for the independent synthesis of green NH 3 .