Mechanistic Insights into the Ceria-Catalyzed Synthesis of Carbamates as Polyurethane Precursors

Abstract

The methoxycarbonylation of toluenediamines with dialkyl carbonates constitutes an alternative route for the phosgene-free production of isocyanate precursors. Despite the remarkable catalytic activity of ceria in the reaction, achieving full selectivity and long-term stability still represent major challenges. Here, the mechanism of the methoxycarbonylation of the industrially-relevant 2,4-diaminotoluene (2,4-TDA) with dimethylcarbonate (DMC) along with the evolution of the property-performance interplay upon consecutive cycles are rationalized via the structural identification of reaction products, characterization tools, and density functional theory (DFT). The formation of the desired carbamates (7% mono- and 83% biscarbamate) is favored over the (111) facet, the most-abundant in the as-prepared material, and proceeds via a complex reaction mechanism that involves a broad number of isomers and multiple reaction paths. A consecutive reaction in which 2,4-TDA is converted into a monocarbamate that further reacts to the biscarbamate drives the selective path. Part of these carbamates reacts to form productive ureas, unprecedented intermediates that reversely transform into carbamates. A full product analysis enables to identify a number of side products that mostly comprise N-methylated carbamates and N-methylated ureas. Evaluation in subsequent cycles evidences the catalyst deactivation and the concomitant increase in the formation of by-products, which is linked to the increasing amount of carbon deposits along with the DMC-induced partial surface restructuring into an oxygen-defective (100) facet after six cycles. These findings highlight the challenges in the rational design of robust heterogeneous catalysts for the production of isocyanate precursors.