Catalytic Urea Synthesis from Ammonium Carbamate Using a Copper(II) Complex: A Combined Experimental and Theoretical Study

Abstract

The synthesis of urea fertilizer is currently the largest CO 2 conversion process by volume in the industry. In this process, ammonium carbamate is an intermediate en route to urea formation. We determined that the tetraammineaquacopper(II) sulfate complex, [Cu(NH 3 ) 4 (OH 2 )]SO 4 , catalyzed the formation of urea from ammonium carbamate in an aqueous solution. A urea yield of up to 18 ± 6% was obtained at 120 °C after 15 h and in a high-pressure metal reactor. No significant urea formed without the catalyst. The urea product was characterized by Fourier transform infrared (FT-IR), powder X-ray diffraction (PXRD), and quantitative 1 H{ 13 C} NMR analyses. The [Cu(NH 3 ) 4 (OH 2 )]SO 4 catalyst was then recovered at the end of the reaction in a 29% recovery yield, as verified by FT-IR, PXRD, and quantitative UV−vis spectroscopy. A precipitation method using CO 2 was developed to recover and reuse 66 ± 3% of Cu(II). The catalysis mechanism was investigated by the density functional theory at the B3LYP/6-31G** level with an SMD continuum solvent model. We determined that the [Cu(NH 3 ) 4 ] 2+ complex is likely an effective catalyst structure. The study of the catalysis mechanism suggests that the coordinated carbamate with [Cu(NH 3 ) 4 ] 2+ is likely the starting point of the catalyzed reaction, and carbamic acid can be involved as a transient intermediate that facilitates the removal of an OH group. Our work has paved the way for the rational design of catalysts for urea synthesis from the greenhouse gas CO 2 .

Dual-Silicon-Doped Graphitic Carbon Nitride Sheet: An Efficient Metal-Free Electrocatalyst for Urea Synthesis

“…To address these issues, people have tried electrocatalytic N 2 fixation under ambient conditions; − however, the complicated postprocessing procedure for combining hitherto obtained NH 3 and CO 2 from another source makes the overall urea synthesis less efficient. ,, Very recently, Chen et al have creatively introduced the concept of simultaneous electrocatalytic coupling of N 2 and CO 2 in the presence of H 2 O to synthesize urea under ambient conditions using an electrocatalyst comprising of PdCu alloyed nanoparticles on TiO 2 nanosheets. It has been demonstrated that the coupling reaction proceeds through the formation of C–N bond where spontaneous CO insertion takes place in between the activated molecular N 2 .…”

Oxovanadium(V)-Catalyzed Synthesis of Unsymmetrical Ureas by Activation of Carbon Dioxide under Ambient Pressure

“…Ureas are regarded as the most prominent carbonyl compounds widely used in pesticides, herbicides, and the raw material of resins . In particular, ureas used in pharmaceuticals and agrochemicals often have unsymmetrical structures [daimuron, linuron, and thidiazuron (Figure )].…”

Implications of the Pore Size of Graphitic Carbon Nitride Monolayers on the Selectivity of Dual-Boron Atom Catalysts for the Reduction of N2 to Urea and Ammonia: A Computational Investigation

“…Recently, Jiao and co-workers devised a successful strategy and demonstrated that C–N bonds can be formed during the coelectrolysis of CO and NH 3 molecules . Similarly, other interesting works have explored the possibility of coupling CO and NH 3 by means of electrocatalysis. − However, apart from the synthesis challenges, NH 3 as the nitrogen-containing feedstock in such a process suffers from separation and purification from aqueous media for subsequent urea production. − This has shifted the focus toward direct electrocoupling of carbon dioxide and nitrogen molecules by avoiding the NH 3 synthetic process for urea synthesis. For example, Kayan and Köleli achieved a urea formation rate of 31.8 μg h –1 cm –2 over polyaniline and polypyrrole-coated platinum electrodes by a simultaneous electroreduction of CO 2 and N 2 gases at a high pressure of 60 bar.…”