Layered double hydroxides as catalysts for aromatic nitrile hydrolysis

A series of carbonate layered double hydroxides (LDHs) with different Mg²⁺/M³⁺ (M³⁺ = Al, Ga) atomic ratios were prepared by the co-precipitation method. Extensive characterization evidenced the formation of well-crystallized hydrotalcite-like structures with homogeneously distributed M³⁺ cations for all samples. The mixed oxides obtained after calcination at 773-873 K of these LDHs, which can be considered as non-equilibrium oxide solutions, are active and highly selective catalysts in the production of several aromatic amides under mild reaction conditions by means of nitrile hydrolysis in the presence of H₂O₂. High activities were found for Mg-Al LDH with an Al/(Al + Mg) molar ratio of 0.25, for Mg-Al, Ga LDH, both calcined at 773 K, and for Mg-Ga sample with Ga/(Ga + Mg) ratio of 0.12 calcined at 873 K. The base properties of the mixed oxides, being strongly dependent on the LDH chemical composition and calcination temperature, are of key importance for the catalyst activity in nitrile hydrolysis. Brønsted basic sites of moderate strength are formed at the start of the reaction upon interaction of the O^2- species with water. These Brønsted basic sites are considered the most likely active centers. The basicity was evaluated from benzoic acid titration evaluated of partially hydrated catalysts. A good correlation was obtained between the basicity and catalyst activity. Using several benzonitrile ring substituted derivatives, it was shown that Wiberg's classical reaction mechanism is also operative when calcined LDH catalysts are used in nitrite hydrolysis. The catalysts can be regenerated by calcination achieving the same nitrile conversion as fresh catalysts.