Synergistic effect in the hydroxylation of phenol over CoNiAl ternary hydrotalcites

摘要

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 H_2O_2 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 Bronsted 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 deg C)with a catechol/hydroquinone ratio of 3.8.Heating of these samples even at slightly elevated temperatures (>120 deg 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.