3D-Printed Fe/γ-Al2O3 Monoliths from MOF-Based Boehmite Inks for the Catalytic Hydroxylation of Phenol.

The synthesis of dihydroxybenzenes (DHBZ), essential chemical reagents in numerous industrial processes, with a high degree of selectivity and yield from the hydroxylation of phenol is progressively attracting great interest in the catalysis field. Furthermore, the additive manufacturing of catalysts to produce 3D printed monoliths would provide additional benefits to enhance the DHBZ synthesis performance. Herein, 3D cellular Fe/γ-Al2O3 monoliths with a total porosity of 88% and low density (0.43 g·cm-3) are printed by Robocasting from pseudoplastic Fe-metal-organic frameworks (Fe-MOF)-based aqueous boehmite inks to develop catalytic monoliths containing a Fe network of dispersed clusters (≤5 µm), nanoclusters (<50 nm), and nanoparticles (∼20 nm) into the porous ceramic skeleton. The hydroxylation of phenol in the presence of hydrogen peroxide is carried out at different reaction temperatures (65-85 °C) in a flow reactor filled with eight stacked 3D Fe/γ-Al2O3 monoliths and with the following operating conditions: Cphenol,0 = 0.33 M, Cphenol,0/CH2O2,0 = 1:1 molar, WR = 2.2 g, and space time (τ = W·QL-1) = 0-147 gcat·h·L-1. The scaffolds present a good mechanical resistance (∼1 MPa) to be employed in a catalytic reactor and do not show any cracks or damage after the chemical reaction. DHBZ selectivity (SDHBZ) of 100% with a yield (YDHBZ) of 32% due to the presence of the Fe network in the monoliths is reported at 85 °C, which represents an improved synthesis performance as compared to that obtained by using the conventional Enichem process and the well-known titanium silicalite-1 catalysts (SDHBZ = 99.1% and YDHBZ = 29.6% at 80 °C). This printing strategy allows manufacturing novel 3D structured catalysts for the synthesis of critical chemical compounds with higher reaction efficiencies.

Structured Reactors Based on 3D Fe/SiC Catalysts: Understanding the Effects of Mixing.

The application of structured reactors provides a number of advantages in chemical processes. In this paper, two different three-dimensional (3D) Fe/SiC catalysts with a square cell geometry have been manufactured by Robocasting: monoliths (D = 14 and H = 15 mm) and meshes (D = 24 and H = 2 mm) and studied in the catalytic phenol oxidation by hydrogen peroxide (H2O2) for the sustainable production of dihydroxybenzenes (DHBZ). The fluid dynamics, catalytic performance, reaction rates, external mass transport limitation, and catalyst stability have been compared in three different reactors, monolithic fixed-bed reactor, multimesh fixed-bed reactor, and monolithic stirrer reactor, at selected operating conditions. The results show that the mechanical stirring of the 3D Fe/SiC monoliths avoids the external mass transfer limitation caused by the presence of oxygen bubbles in the channels (produced from the HO x · species in autoscavenging radical reactions). In addition, the backmixing has a positive effect on the efficient consumption of H2O2 but an adverse effect on the phenol selectivity to DHBZ since they are overoxidized to tar products at longer contact times. On the other hand, the wall porosity, and not the backmixing, affects the susceptibility of the 3D Fe/SiC catalyst to the Fe leaching, as occurs in the mesh structures. In conclusion, the monoliths operating under plug-flow and external mass transfer limitation in the monolithic fixed-bed reactor (MFB) provide an outstanding phenol selectivity to DHBZ and catalyst stability.

Kinetics of imidazolium-based ionic liquids degradation in aqueous solution by Fenton oxidation.

In the last few years, several works dealing with Fenton oxidation of ionic liquids (ILs) have proved the capability of this technology for their degradation, achieving complete ILs removal and non-toxic effluents. Nevertheless, very little is known about the kinetics of this process, crucial for its potential application. In this work, the effect of several operating conditions, including reaction temperature (50-90 °C), catalyst load (10-50 mg L-1 Fe3+), initial IL concentration (100-2000 mg L-1), and hydrogen peroxide dose (10-200% of the stoichiometric amount for the complete IL mineralization) on 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) oxidation has been investigated. Under the optimum operating conditions (T = 90 °C; [Fe3+]0 = 50 mg L-1; [H2O2]0 = 100% of the stoichiometric amount), the complete removal of [C4mim]Cl (1000 mg L-1) was achieved at 1.5-min reaction time. From the experimental results, a potential kinetic model capable to describe the removal of imidazolium-based ILs by Fenton oxidation has been developed. By fitting the proposed model to the experimental data, the orders of the reaction with respect to IL initial concentration, Fe3+ amount and H2O2 dose were found to be close to 1, with an apparent activation energy of 43.3 kJ mol-1. The model resulted in a reasonable fit within the wide range of operating conditions tested in this work.