Electronic/steric effects in hydrogenation of nitroarenes over the heterogeneous Pd@BEA and Pd@MWW catalysts
Abstract
Hydrogenation of nitroarenes is a catalytic reaction of high interest owing to the importance of the resulting aromatic amines in the chemical industry. Up to date, various metal@zeolite catalysts have been reported for this transformation.
Herein, Pd@Beta and Pd@MWW catalysts were synthesized by ion exchange with Pd (NH3)(4)(NO3)(2) and characterized by XRD, nitrogen sorption, ICP-OES, electron microscopy and FTIR(CO). Structural and textural analysis proved no significant changes of the zeolites after the Pd precursor was introduced.
STEM images proved the uniform distribution of metal species in zeolite crystals. Average size of Pd NPs is 2.6 nm and 1.7 nm for Beta and MWW zeolites, respectively.
The resulting Pd@Beta catalyst exhibited higher catalytic activity compared to Pd@MWW in the hydrogenation of nitroarenes. The electronic/steric effects of substrates and products were also investigated for both catalysts.
Nitroarenes with electron-donating groups exhibit higher initial reaction rates than nitroarenes containing electron-withdrawing groups. The nitroarenes bearing functionalized groups in the ortho-position are harder to hydrogenate than corresponding meta- and para- isomers because of the steric hindrance effect.