Kinetic of Alkali Catalyzed Self-Condensation of Cyclohexanone

A kinetic model to describe the dimer formation (D) in the process of cyclohexanone self-condensation was developed, including different variables such as temperature, catalyst concentration and equilibrium concentrations. Basic catalytic self-condensation of cyclohexanone in the liquid phase was conducted in a batch reactor by using sodium hydroxide as catalyst (CNaOH values from 1.6 to 30.0 mmol/kg). The reaction temperature was varied from 127 to 149 °C; to study the equilibrium conditions, the temperature was varied from 100 to 160 °C. Cyclohexanone conversions up to 80% were reached. Dimers, trimers, and tetramers from consequent-parallel condensation reactions were identified and quantified by gas chromatography/mass spectroscopy (GC/MS). Three dimer species were found: the adduct 1′-hydroxy-[1,1′-bicyclohexyl]-2-one (D1) is first formed, and then, this compound is in situ dehydrated, yielding an isomeric mixture of reaction products, namely, 2-(1-cyclohexen-1-l)cyclohexanone (D2) and 2-cyclohexylidencyclohexanone (D3), being D2 the main dimer formed. Under all the tested experimental conditions, trimer and tetramer concentrations were obtained in traces. A lumped specie D was defined as the sum of all dimers. The effect of water on the cyclohexanone conversion and selectivity was also studied. To achieve this, several experiments were conducted at vacuum pressure to remove the formed water and at 10 bar to ensure that all the formed water remains in the reaction media. A remarkable influence of water content on the cyclohexanone conversion profiles was observed. Therefore, it had to be considered in the kinetic model. Reaction enthalpy was experimentally calculated and an endothermic reaction was determined (ΔHr° = 59.2 kJ/mol). The kinetic parameters were estimated by data fitting. The estimated activation energy of D formation was 132.6 kJ/mol. This kinetic model reproduces quite well the experimental results. Moreover, experimental data from other authors in literature can be also reasonably well predicted with the model developed.