Hydration of nitriles to amides in water by SiO2-supported Ag catalysts promoted by adsorbed oxygen atoms

A series of silica-supported silver catalysts with similar Ag loading (5 or 7 wt%) but with different preparation methods (calcination in air and reduction by H-2 or NaBH4) were prepared, and their structure was characterized by microscopy (STEM), X-ray absorption fine structure (XAFS). and CO-titration of surface oxygen atom. Ag is present as metal nanoparticle with a size range of 17-30 nm. Their surface was partially covered with oxygen atoms, and the surface coverage of the oxygen depends on the preparation condition. For hydration of 2-cyanopyridine as a test reaction, turnover frequency (TOF) per surface Ag species is estimated. TOF does not show a good correlation with Ag particle size, but it linearly increases with the coverage of the surface oxygen atoms on Ag particles. The Ag/SiO2 catalyst prepared by H-2 reduction at 700 degrees C shows the highest TOF and it acts as effective and recyclable heterogeneous catalyst for selective hydration of various nitriles to the corresponding amides. Kinetic and Raman spectroscopic studies suggest that the surface oxygen atom adjacent to Ag-0 sites plays an important role in the dissociation of H2O. (C) 2012 Elsevier B.V. All rights reserved.