Study Of Cyclohexanol Electrooxidation On Nickel Electrode

Adipic acid is an important chemical material;it has been widely used not only as an important intermediate in the manufacture of nylon but also as a plasticizer and a food additive.It is produced industrially with the oxidation of cyclohexanol and cyclohexanone using concentrated nitric acid as an oxidant.This process poses environmental constrains. The nitrous oxide emission measurably contributes to global warming,and it will do harm to our environment.Therefore,researchers are interested in the development of simple and environmentally friendly methods of its synthesis.It is well known that electron is the clearest chemical reagent because it will not bring up any pollution.In this paper synthesis of adipic acid by electrochemical oxidation of cyclohexanol was performed,which used copper as a cathodic electrode and nickel as an anodic electrode.Optimization of the electrolytic experimental condition was determined by GC and HPLC for the first time. Then,the concentration distribution of the products was investigated.It is well known,nickel is economically feasible with good electrocatalytic activity for small organic molecule oxidation.However,as far as our knowledge goes,the reaction mechanism for electrooxidation of cyclohexanol has never been appeared in the literature. We investigated the electrocatalytic behavior of NOMN electrode towards cyclohexanol electrooxidation by various electrochemical methods for the first time.In addition,as a substrate,titanium has been receiving increasing attention in recent years for its good corrosion-resistance and reasonable cost.In this paper,we firstly fabricated the supported Ni/Ti electrode by hydrothermal process and investigated its electrocatalytic activity towards cyclohexanol oxidation.The main experiment results obtained are as follows:1.Electrocatalytic oxidation of cyclohexanol was investigated with cyclic voltammograms,linear galvanic voltammograms and chronoamperometric responses on a nickel oxyhydroxide modified nickel(NOMN) electrode prepared by cycling the potential of a nickel electrode in the potential range of 0.1 V to 0.6 V(vs SCE) in alkaline solutions. It was found that cyclohexanol was oxidized by NiOOH generated with further electrochemical oxidation of nickel hydroxide during the anodic potential sweep.One of the products of the reaction between cyclohexanol and NiOOH was Ni(OH)₂ which was subsequently oxidized to NiOOH on the anode.This resulted in the appearance of a new anodic peak in cyclic voltammograms compared with the absence of cyclohexanol and this an(?)dic peak strongly depends upon potential scan rates and cyclohexanol concentrations. In addition,the presence of cyclohexanol in NaOH solutions also lead to the decrease of an(?)dic potentials in linear galvanic voltarnmetric responses and the increase of current densities in chronoamperometric curves.Results showed that the oxidation of cyclohexanol on the NOMN electrode follows the catalytic reaction mechanism.2.Titanium-supported nickel electrode(Ni/Ti) is firstly fabricated by hydrothermal process using hydrazine hydrate as a reduction agent.SEM and EDS show that the surface of Ti substrate was partly covered by nickel particles which were present as small balls with the almost uniform size.Its electrocatalytic activity towards cyclohexanol oxidation has been investigated by cyclic voltammetry(CV),chronoamperometry(CA),quasi-steady stale polarization and electrochemical impedance spectroscopy(EIS).Effects of various parameters such as potential scan rate and cyclohexanol concentration on the electrooxidation of cyclohexanol are investigated.Results show that Ni/Ti electrode acts as an efficient catalyst for the electrooxidation of cyclohexanol in basic media and the results were applied to compare with the NOMN electrode.The pseudo-steady state polarization curves show that the onset potential of cyclohexanol oxidation reaction on Ni/Ti electrode is lower than that on the NOMN electrode,which was found to be around 72mV,compared to the onset potential of approximate 98mV for the NOMN electrode in the presence of 0.2M cyclohexanol.Also at the potential of around 410mV the current density of cyclohexanol oxidation on Ni/Ti electrode is 9.9mA/cm²,which is more than six times higher than NOMN electrode(1.6 mA/cm²).In addition,the EIS data show that the charge transfer resistances at 400mV and 450mV for the NOMN electrode are 2350 and 2243Ωrespectively,which are over 8 and 14 times larger than those for the Ni/Ti electrode(290Ωand 153Ωrespectively).These results confirm that the electrocatalytic activity of Ni/Ti electrode towards cyclohexanol oxidation is higher than a nickel oxyhydroxide modified nickel electrode(NOMN).It is further confirmed that during the anodic potential sweep the electrooxidaton of cyclohexanol follows the formation of NiOOH on the electrode surface and is then catalysed by NiOOH.The rate-determining step for cyclohexanol oxidation is the reaction between the high oxidation state nickel(Ni³⁺) with the cyclohexanol adsorbed on the surface of the Ni/Ti.3.Systhesis of adipic acid by cyclohexanol electrolysis in 1M NaOH solution was carried out using copper as a cathodic electrode and nickel as an anodic electrode.The results of the electrolysis of cyclohexanol show that the optimization of experimental conditions are as follows:current density = 6mA/cm²;reaction temperature=60℃;reaction time = 16F/mol;C_NaOH=1.0 mol/L,the maximum yield of adipic acid under the condition is 42.67%.And study of the kinetics of cyclohexanol oxidation shows that the oxidation of cyclohexanol undergoes two steps:the first one is the formation of cyclohexanone and the second step is the formation of adipic acid from cyclohexanone which is the rate-determining step.It is further revealed that the formation of cyclohexanone is a fast step and formation of adipic acid is a slower one.