Autor/a:
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Puértolas, Begoña; Rellán-Piñeiro, Marcos; Núñez-Rico, José Luis; Amrute, Amol P.; Vidal-Ferran, Anton; López, Núria; Pérez-Ramírez, Javier; Wershofen, Stefan
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Abstract:
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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. |