Hydrogenation of nitro-compounds over rhodium catalysts supported on poly[acrylic acid]/Al₂O₃ composites

In this report, poly[acrylic acid] gels containing Al₂O₃ were prepared by simultaneous free-radical polymerization and sol-gel chemistry using different amounts of 3-(trimethoxysilyl)propyl methacrylate (TMPM) as a compatibilizer. The hybrid materials were used as supports for a rhodium catalyst in the chemoselective hydrogenation of 3-substituted aromatic nitro-compounds. The supported rhodium catalyst was prepared by an ion-exchange process. In situ H₂ flux was used to produce active species of the catalysts. The resulting materials were characterized by infrared spectroscopy, thermogravimetric analysis, solid-state ²⁹Si and¹³ C NMR, X-ray diffraction, transmission/scanning electron microscopy, and X-ray photoelectron spectroscopy. All materials exhibited simultaneous interpenetrating hybrid network structures (SIHNs). The morphologies and physicochemical properties depended on the amount of TMPM used. The catalysts were found to be effective for the reduction of nitrobenzene in ethanol at room temperature and a hydrogen pressure of 20atm. The most active and selective catalyst was used in the hydrogenation of different 3-substituted aromatic nitro-compounds. The hydrogenation reactions displayed high conversion levels and promoted exclusive-NO₂ group reduction, resulting in the sole formation of the corresponding amino-compound, with the exception of 1,3-dinitrobenzene, in which over-hydrogenation was detected. The presence of electron-donating/electron-withdrawing substituents at the 3-position resulted in different rates of-NO₂group hydrogenation. This effect was quantified in terms of the Hammett relationship, in which the catalyst displayed a linear correlation between the substituent constant (σ_i) and the hydrogenation rate, with the exception of-OH, -NH₂, and-OCH₃ groups. One explanation for this behavior is a proposed support-substrate hydrogen bond interaction during the catalytic reaction.