Highly Selective Monomethylation of Amines with CO2/H2 via Ag/Al2O3 as a Catalyst

The selective synthesis of monomethylated amines with CO2 is particularly challenging because the formation of tertiary amines is thermodynamically more favorable. Herein, a new strategy for the controllable synthesis of N-monomethylated amines from primary amines and CO2/H-2 is explored. First-principle calculations reveal that the dissociation of H-2 via an heterolytic route reduces the reactivity of methylated amines and thus inhibit successive methylation. In situ DRIFTS proves the process of formation and decomposition of ammonium salt by secondary amine reversible binding with H+ on the Ag/Al2O3 catalyst, thereby reducing its reactivity. Meanwhile, the energy barrier for the rate-determining step of monomethylation was much lower than that of overmethylation (0.34 eV vs. 0.58 eV) means amines monomethylation in preference to successive methylation. Under optimal reaction conditions, a variety of amines were converted to the corresponding monomethylated amines in good to excellent yields, and more than 90 % yield of product was obtained.