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 O2- 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.
Bibliographical noteFunding Information:
The authors thank Mr. A.P.B. Sommen for the adsorption measurements and Mrs. A. Elemans-Mehring for AAS analysis (TU Eindhoven). These investigations were supported in part by Ukrainian Ministry of Education and Science and by a Spinoza grant (to R.A.v.S.) by NWO, The Netherlands.
- Base catalysis
- Hydrotalcite-like materials
- Layered double hydroxides
- Nitrile-to-amide hydrolysis