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: C_phenol,0 = 0.33 M, C_phenol,0/C_H = 1:1 molar, W_R = 2.2 g, and space time (τ = W·Q_L^-1) = 0-147 g_cat·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 (S_DHBZ) of 100% with a yield (Y_DHBZ) 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 (S_DHBZ = 99.1% and Y_DHBZ = 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.