Continuous-flow synthesis of organic urea derivatives from CO2-absorbed alkanolamines over a CeO2 catalyst

Abstract

Alkanolamines such as 2-aminoethanol (2AOL) are practically used for CO₂ capture through the formation of alkylcarbamic salts, which can serve as feedstocks for chemical synthesis. In this study, the direct and continuous conversion of alkanolamine-derived alkylcarbamic salts into organic urea derivatives was demonstrated by combining a CeO₂ catalyst with a fixed-bed flow reactor, where the corresponding amine played a significant role as a reaction solvent. The alkylcarbamic salt prepared from 2AOL and CO₂, (2-hydroxyethyl)carbamic salt (2AOL-CA), was converted into N,N′-bis(2-hydroxyethyl)urea (2LU) in up to 92% yield under the optimum conditions. The CeO₂ catalyst exhibited excellent stability and maintained 80% yield of 2LU even after 57 h of continuous operation, despite slight deactivation due to organic deposition. The flow reaction system was applicable to other alkanolamine-derived alkylcarbamic salts with different alkyl chain lengths. The investigation of contact-time dependence and substrate scope revealed the involvement of cyclic carbamates as the key intermediate in the rapid production of urea derivatives. The production of 2LU from 2AOL-CA consisted of two steps: intramolecular cyclization of 2AOL-CA via nucleophilic attack by the terminal hydroxy group to form a cyclic carbamate and a subsequent ring-opening reaction with 2AOL to yield 2LU. The extension of the alkyl chain length in alkylcarbamic salts increased the contribution of linear carbamates as intermediates, which became dominant when the alkyl chain length was four. The terminal hydroxy group of alkylcarbamic salts and the relatively high reactivity of cyclic carbamates led to such a unique two-step reaction pathway.