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

Graphical Ag/Al 2 O 3 catalytic controllable synthesis of N-monomethylated amines by using CO 2 /H 2 as C 1 source has been demonstrated in a first example. The reactivity of methylated amines was reduced by the heterolytic dissociation of H 2 over the catalyst, and hence conversion of various amines to corresponding monomethylated amines was enabled with excellent selectivity. The selective synthesis of monomethylated amines with CO 2 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 CO 2 /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/Al 2 O 3 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.