Kinetic study of phenol hydroxylation by H2O2 in 3D Fe/SiC honeycomb monolithic reactors: Enabling the sustainable production of dihydroxybenzenes

The chemical kinetics of phenol hydroxylation by hydrogen peroxide (H2O2) to produce dihydroxybenzenes was studied using a 3D printed monolithic reactor. The monoliths were manufactured by the Robocasting technique. They consisted on honeycomb-structured Fe/SiC nanoparticles (13.5 mm in diameter and 14.8 mm in length) with triangle cell geometry and staggered interconnected channels (71 cells per cm2). The isothermal reactor was constituted by three stacked monoliths and was operated as an ideal plug flow reactor, according to the measured residence time distribution. The hydroxylation experiments were carried out at CPHENOL,0 = 0.33 M, phenol:H2O2 molar ratio 1:1, τ(space time) = 0–254 g h L-1, T = 80, 85 and 90 °C and water as unique solvent. Experimental results showed no mass transfer limitations. The best fits were obtained for H2O2 decomposition with a Langmuir-Hinshelwood-Hougen-Watson kinetic model and for phenol hydroxylation, as well as, catechol and hydroquinone production, with an Eley-Rideal kinetic model. The hydroxylation reaction mechanism underling to the developed model involved three elementary reactions: (1) adsorption of H2O2 molecules on the iron active sites, (2) chemical surface H2O2 decomposition into the hydroxyl radical species, and (3) reaction between adsorbed radical species and phenol in solution leading to the dihydroxybenzene formation and freeing the iron catalytic active sites (rds). This work contributes to the implementation of outstanding 3D Fe/SiC honeycomb monolithic reactors, with a dihydroxybenzene selectivity above 99% at 80 °C, for the sustainable production of hydroxylated aromatics.