Ionic Liquid-Mediated Urea Pyrolysis to Cyanuric Acid: Experimental Protocol and Mechanistic Insights

Cyanuric acid (CA) is a critical precursor preventing the photodegradation of chlorine in water. So mainly, CA is utilized in the industry as a chlorine stabilizer. Compared to urea, the higher cost of CA necessitates an economic protocol to produce. As an alternative to current methods, we proposed dimethyl ammonium hydrosulfate [dmaH][HSO4] ionic liquid (IL)-mediated urea pyrolysis to CA for the first time. The reaction was optimized based on changing parameters: time, catalyst loading, solvent, and temperature. The optimized method does not require solvent utilization; IL acts as a solvent/catalyst system to produce CA in an ∼70% yield at 220 °C in 30 min. The recycle test of the IL catalyst shows that it can be reused four times without the loss of catalytic activity. The IL catalytic activities on the urea pyrolysis reaction are studied using density functional theory (DFT). The reaction profile of urea conversion to CA is calculated to follow biuret, triuret, and triuret cyclization steps, with extrusion of ammonia in each step. The IL-free reaction profile is also included to observe the IL role in the reaction. Considering experimental observation of isocyanic acid (ICA) as one of the urea pyrolysis products, we also tested CA formation possibilities via ICA trimerization. The calculations show that ICA formation (TS-ICA, ΔG‡ = 27.1 kcal/mol) is 14.1 kcal lower than biuret formation (TS1-IL, ΔG‡ = 41.2 kcal/mol). Further mechanistic studies imply that urea pyrolysis to CA follows a straightforward ICA trimerization route.