Selective conversion of stearic acid into high-added value octadecanedioic acid using air and transition metal acetate bromide catalyst: Kinetics, pathway and process optimization

Abstract

Liquid phase selective homogeneous catalytic oxidation of stearic acid (SA) was carried out to obtain industrially important carbon neutral high-added value octadecanedioic acid (ODDA). The oxidation was carried using air, cobalt(II)-acetate, manganese(II)-acetate and HBr catalyst in acetic acid (AcOH) solvent at an elevated temperature and pressure. SA oxidation products were analyzed by gas chromatography–mass spectrometry (GC–MS), gas chromatography (GC) and CO2 analyzer, and SA was oxidized selectively to ODDA without producing CO2 and intermediates like alcohols, aldehydes and ketones. The effect of SA loading (5–20%), pressure (2.8–5.8 barg) and temperature (353–383 K) on ODDA yield was studied by varying one variable at a time. Central composite design assisted response surface methodology was employed to find (i) the optimal design of experiments involving several combination of cobalt(II)-acetate (Co: 0–700 ppm), manganese(II)-acetate (Mn: 0–700 ppm) and HBr (Br–: 0–1144 ppm) and (ii) the most influencing variable and interaction among the variables. The synergistic effect of cobalt(II)-acetate in presence of HBr was observed and suggested that SA oxidation proceeds via bromine-bromide cycle. The elevated temperature and pressure along with reduced SA loading enhanced the yield of ODDA. The maximum ODDA yield was found to be 90.5% and corresponding optimum cobalt (II), manganese (II) and bromide concentration were 600.4, 452.2 and 1016.6 ppm, respectively, at fixed SA:AcOH-10:90, pressure-2.8 barg and temperature-383 K. Finally, SA oxidation kinetic analysis was determined based on the pseudo-first order homogeneous catalysis and found to be kinetically controlled with an average activation energy 34.55 kJ mol−1. The proposed kinetic model fitted well with the time-variant experimental SA and ODDA concentration under varying operating condition with percent average absolute deviation less than 5.0%. Keywords: Stearic acid, Octadecanedioic acid, Selective oxidation, Transition metal acetate bromide, Reaction pathway, Synergy, Kinetics, Optimization