Indirect use of CO2: synthesis of N-substituted dicarbamates via polyurea intermediates over a Zn-based metal azolate framework

Abstract

N-substituted carbamates have become a potential raw material for the non-phosgene synthesis of isocyanates through thermal cracking. The direct synthesis of carbamates from CO₂, diamines, and alcohols without a dehydration reagent should be an ideal choice; however, the yield is very low. Herein, we report an effective route for the synthesis of N-substituted dicarbamates via the catalytic alcoholysis of polyurea intermediates, whereby polyurea could be facilely synthesized from CO₂ and diamines. A metal azolate framework composed of zinc coordinated with 5-amino-1H-tetrazole denoted as Zn(ATZ)₂ was prepared and characterized using various physicochemical techniques (ICP, BET, FTIR, XRD, TGA, XPS, SEM). The developed Zn(ATZ)₂ exhibited excellent catalytic activity for the synthesis of N-substituted dicarbamates via the alcoholysis of polyureas, and a 98% polyurea conversion with 96% diethyl hexamethylenedicarbamate (EHDC) yield could be achieved, which accomplished the indirect use of CO₂. The brilliant catalytic performance of Zn(ATZ)₂ could be attributed to the new active center after Zn species coordinated with ATZ. Moreover, the catalyst was reusable without any significant decrease in activity.