On the influence of water on urea condensation reactions: a theoretical study

Abstract

The influence of water molecules on the kinetics of urea condensation reactions was studied with high-level quantum chemical methods and statistical rate theory. The study focuses on the production of biuret, triuret, and cyanuric acid from urea because of their relevance as unwanted byproducts in the urea-based selective catalytic reduction (urea-SCR) exhaust after treatment of Diesel engines. In order to characterize the potential energy surfaces and molecular reaction pathways, calculations with explicitly-correlated coupled-cluster methods were performed. It turned out that the reactions proceed via pre-reactive complexes and the inclusion of one or two water molecules into the condensation mechanisms leads to a decrease of the energy barriers. This effect is particularly pronounced in the production of biuret. Due to the pre-reactive equilibria, the rates of the overall reactions can increase or decrease by incorporating water into the mechanism, depending on the temperature and water concentration. Under the conditions of urea-SCR, the studied reactions are too slow to contribute to the observed byproduct formation.

Theoretical Study of the Thermal Decomposition of Urea Derivatives

“…9 Recently, there has been significant interest in the decomposition of urea and the subsequent formation of byproducts in the selective catalytic reduction of NO x . 10,11 Schaber et al 12 explored the thermal degradation of urea and byproduct formation from solid-sample pyrolysis. By means of TGA and DCS experiments, they provided one of the first complete reaction schemes divided into temperature-related "reaction regions": from room temperature to 190 °C, urea melts and decomposes, and from 190 to 250 °C, biuret rises to a maximum while cyanuric acid, the main byproduct from urea thermal decomposition, increases slightly.…”

Mathematical Modeling of Urea Reaction with Sulfuric Acid and Phosphoric Acid to Produce Ammonium Sulfate and Ammonium Dihydrogen Phosphate Respectively

“…This can lead to a positive trade balance by reducing the import of chemical fertilizers if the production process is performed at a large scale. However, if urea reacts with isocyanic acid, which is a product of urea decomposition, it can condensate to biuret, and eventually to triuret, and cyanuric acid, which are considered as undesirable impurities in urea-based fertilizers and also in urea-based selective catalytic reduction systems [3].…”