In situ characterization of the coordination sphere of CuII complexes supported on silica during the preparation of Cu/SiO2 catalysts by cation exchange

Abstract

In situ diffuse reflectance UV/visible/near-IR and EPR spectroscopies, and also XAFS, were used to study the changes in the coordination sphere of copper complexes during the different steps of preparation by cation exchange of supported Cu/SiO2 catalysts. Three different CuII precursors were used: [Cu(NH3)4(H2O)2]2+, [Cu(en)1(H2O)4]2+ and [Cu(en)2(H2O)2]2+. Before drying, the coordination sphere of the different CuII complexes remains the same as in solution, and the complexes are in electrostatic interaction with the silica support. Depending on the strength of the ligands, the CuII complexes are grafted onto the silica support as (SiO)2Cu(NH3)2(H2O)4 or (SiO)2Cu(en)1(H2O)2, or remain in electrostatic interaction with the latter as [Cu(en)2(H2O)2]2+ when the samples are dried at 25°C. Extended drying at 100°C and re-exposure to air induces weak loss of NH3 and en ligands, and some water ligands are reversibly eliminated. When the samples are calcined, the NH3 and en ligands are completely eliminated, and the same three CuII species are identified on the three samples: (SiO)2CuII(H2O)4−x, (SiO)3CuII and pairs of octahedral CuII species. On exposure to air, the three species transform into a single species: (SiO)2CuII(H2O)4.