Mechanistic Insights into the Ceria-Catalyzed Synthesis of Carbamates as Polyurethane Precursors
| Autor: | Stefan Wershofen, Begoña Puértolas, Núria López, José Luis Núñez-Rico, Anton Vidal-Ferran, Amol P. Amrute, Marcos Rellán-Piñeiro, Javier Pérez-Ramírez |
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| Jazyk: | angličtina |
| Rok vydání: | 2019 |
| Předmět: | |
| Zdroj: | RECERCAT (Dipòsit de la Recerca de Catalunya) Recercat. Dipósit de la Recerca de Catalunya instname |
| Popis: | 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 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 us to identify a number of side products that mostly comprises N-methylated carbamates and N-methylated ureas. Evaluation in subsequent cycles evidences the catalyst deactivation and the concomitant increase in the formation of byproducts, 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. |
| Databáze: | OpenAIRE |
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