Heterogeneous hydrogenation of nitroaromatic compounds on gold catalysts: Influence of titanium substitution in MCM-41 mesoporous supports

A series of Ti-modified MCM-41 supports and their respective Au catalysts were prepared and characterized to study the effect of titanium into the support and on the characteristics of the Au surface species for heterogeneous hydrogenation of aromatic nitrocompounds. The systems were characterized by X-ray diffraction, Infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), diffuse reflectance UV-vis (DRUV-vis), N₂ adsorption-desorption isotherms, ICP-MS and X-ray photoelectron spectroscopy (XPS). Additionally, these catalysts were tested for the hydrogenation of nitrobenzene in a batch-type reactor using ethanol as the solvent at 25 °C. Based on a combined FT-IR and DRUV-vis was possible to conclude that the Ti(IV) is incorporated into the framework of MCM-41 and Ti-containing MCM-41 catalysts were more active than an MCM-41 unmodified catalyst. The Ti(X)-MCM-41 supports prepared with Ti 2.0 wt% incorporation showed the best catalytic performance, which was attributed to an optimal number of isolated Ti sites that increased the Au stability on the support surface. For this system, the effect of the H₂ pressure, solvent nature and recyclability of the catalyst was also studied. Finally, a quantitative structure property relationship (QSPR) model was obtained for the catalytic activities of para-substituted nitrobenzenes. The multilinear model considered two parametric descriptors: the sigma constant (σ) and the hydrophobic π-constant, which account for the electronic and hydrophobic effect of the substituents.