New Cu-based catalysts supported on TiO2 films for Ullmann SnAr-type C-O coupling reactions
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New routes for the preparation of highly active TiO2-supported Cu and CuZn catalysts have been developed for CO coupling reactions. Slurries of a titania precursor were dip-coated onto glass beads to obtain either structured mesoporous or non-porous titania thin films. The Cu and CuZn nanoparticles, synthesized using a reduction by solvent method, were deposited onto calcined films to obtain a Cu loading of 2 wt¿%. The catalysts were characterized by inductively coupled plasma (ICP) spectroscopy, temperature-programmed oxidation/reduction (TPO/TPR) techniques, 63Cu nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopy (S/TEM-EDX) and X-ray photo-electron spectroscopy (XPS). The activity and stability of the catalysts obtained have been studied in the CO Ullmann coupling of 4-chloropyridine and potassium phenolate. The titania-supported nanoparticles retained catalyst activity for up to 12 h. However, catalyst deactivation was observed for longer operation times due to oxidation of the Cu nanoparticles. The oxidation rate could be significantly reduced over the CuZn/TiO2 catalytic films due to the presence of Zn. The 4-phenoxypyridine yield was 64¿% on the Cu/nonporous TiO2 at 120¿°C. The highest product yield of 84¿% was obtained on the Cu/mesoporous TiO2 at 140¿°C, corresponding to an initial reaction rate of 104 mmol¿gcat-1¿s-1. The activation energy on the Cu/mesoporous TiO2 catalyst was found to be (144±5) kJ¿mol-1, which is close to the value obtained for the reaction over unsupported CuZn nanoparticles (123±3 kJ¿mol-1) and almost twice the value observed over the catalysts deposited onto the non-porous TiO2 support (75±2 kJ¿mol-1).