Catalyst-Free Urea Synthesis from Methanol and Nitrogen via Radical-Mediated Redox Coupling on Aqueous Microdroplets

Abstract

Abstract Urea is an indispensable nitrogen fertilizer and an important feedstock for chemical synthesis. Compared to the energy-intensive Bosch–Meiser process, photocatalytic redox coupling offers a sustainable approach toward urea synthesis. However, artificial photosynthesis remains dependent on catalyst compositions and structural properties. Here, by leveraging the photocatalyst-like redox capability of aqueous microdroplets, we present a catalyst-free, radical-mediated redox coupling strategy for urea synthesis from CH3OH and N2 under ambient conditions. The system achieved an optimized urea yield rate of approximately 10.8 μg h–1 with 2 vol % CH3OH in a pure N2 atmosphere, which was comparable to some values for urea photosynthesis. Comprehensive experimental investigations and theoretical calculations elucidated a thermodynamically favorable redox coupling pathway, in which ·CHO generated from ·OH-induced CH3OH oxidation and ·NH2 produced via ·H-triggered N2 reduction played a pivotal role in urea formation. This study offers a promising platform for the green synthesis of organonitrogen compounds and provides insights into harnessing the redox chemistry of aqueous microdroplets for chemical production.