Influence of co-bivalent ions in Cu-containing LDHs and solvent on hydroxylation of benzene to phenol

Selective hydroxylation of benzene to phenol using H(2)O(2) as the oxidant was studied over copper-containing layered double hydroxides (LDHs) and their calcined oxides containing different co-bivalent ions (Mg, Zn, Ni or Co). In all the catalysts studied, the Cu/M(II) ratio was 10/90, the trivalent metal ion was aluminum and an atomic ratio of the bivalent to the trivalent metal ion was about 3.0. Catalysts were synthesized using co-precipitation under conditions of low supersaturation and were characterized by various physicochemical methods. Samples with zinc as the co-bivalent ion showed high activity whereas cobalt showed the least activity in both the as-synthesized and calcined forms. The high catalytic activity of the zinc-containing compound was attributed to the facile formation of Cu(+), the likely active species for this reaction, as confirmed through X-ray photoelectron spectroscopy (XPS) measurements. Solvent variation studies revealed that water was the best solvent and that addition of a small amount of sulfolane as the co-solvent increased the selectivity for phenol. The calcined CuZnAl oxide catalyst can be reused up to three cycles without any significant change in the yield of phenol. The retention of the dispersion of copper verified through energy dispersive X-ray analysis (EDAX) measurements was likely to be responsible for the recyclability. (C) 2011 Elsevier B.V. All rights reserved.