Synergistic effect in the hydroxylation of phenol over CoNiAl ternary hydrotalcites

Hydroxylation of phenol was carried out over a series of CoNiAl ternary hydrotalcites (HTs) having a (Co+Ni)/Al atomic ratio close to 2.6 and a Co:Ni atomic ratios ranging from 1:0 to 0:1 using H2O2 as oxidant and water as solvent. Both end members of this series, namely CoAl-HT and NiAl-HT, showed negligible conversion of phenol, while cooperative catalytic behavior was noted when both elements were present together. However, activity of the catalysts varied with Co/Ni atomic composition, wherein the activity decreased with a decrease in the concentration of cobalt. A variation in the activity with the crystallinity of the materials was noted wherein a highly ordered material showed maximum activity and dropped with a decrease in the orderliness of the layered structure. Temperature-programmed reduction (TPR) of the fresh samples substantiated the activity trend by exhibiting a decrease in the reduction maximum with a decrease in the concentration of cobalt. Probably an optimal configuration of sites involving cobalt, nickel, and Brønsted basic hydroxyl groups is necessary for promoting the reaction. Among the catalysts studied, catalyst with a Co/Ni atomic ratio of 1:5 showed maximum conversion of phenol (14.2%, substrate:oxidant=2:1, 65 °C) with a catechol/hydroquinone ratio of 3.8. Heating of these samples even at slightly elevated temperatures (>120 °C in air for 5 h) led to complete loss in the activity necessitating the requirement of a well-ordered network. Prior to catalytic studies, the samples were characterized by various physicochemical techniques for structure–activity relationships.