Green route for carbonylation of amines by CO₂ using Sn-Ni-O bifunctional catalyst and theoretical study for finding best suited active sites

Reaction between n-alkylamine and CO₂ has gained interest due to the demand for the dialkylurea for various applications. For the first time, the tool Mathematica was used to analyze the experimental data with an idea to derive an equation which determines the best suited active sites for any given input set of dependent parameters. The equation can further be used to predict the product yield with the known values of active sites for a reaction. Among several Sn containing mixed oxides, Sn-Ni oxide (Sn-Ni-O) was found to be the better performing catalyst. The studies indicate that the formation of new defect sites when NiO and SnO₂ are in the mixed state and possibly a solid solution enhances the catalytic efficiency. There are two main reasons for improved catalytic performance; one, mixing of SnO₂ into NiO which reduces the number of holes (h⁺) localized on lattice oxygen (O²⁻+ h⁺→ O^•−) and two, smaller SnO₂ particles are dispersed on the bigger particle NiO which alters the acidic and basic active sites in the catalyst. FT-IR adsorption study with amine and CO₂ helped in deriving a plausible mechanism for this reaction. Under optimized reaction condition, Sn1.1-Ni-O-600 gave 77.3% of n-butylamine conversion and 75.7% of yield for 1,3-dibutylurea. The versatility of the catalyst was tested for carbonylation of different aliphatic and aromatic amines, diamine and hydroxy amine with CO₂.