In situ synthesized Pd nanoparticles supported on B-MCM-41: an efficient catalyst for hydrogenation of nitroaromatics in supercritical carbon dioxide

Abstract

In situ synthesis of Pd nanoparticles supported on boron (B)-substituted MCM-41 (B-MCM-41) with Si/B ratio varying from 100 to 5 was carried out by hydrothermal method using H₃BO₃ as B source. The textural properties as well as thermal stability of the resultant material were investigated by XRD, TEM, FTIR and TG-DTA. Highly ordered materials were obtained depending on the Si/B ratio, which also influenced the particle size of Pd as well as dispersion. Pd/B-MCM-41 was a promising catalyst for the hydrogenation of nitrobenzene in supercritical carbon dioxide with exceptionally faster reaction rate [turnover frequency (TOF) = 5.2 × 10⁵ h⁻¹ (144 s⁻¹)] and high yield of aniline (100%). The observed reaction rate was strongly influenced by the Pd particle size related to Si/B ratio and physical properties of CO₂ such as pressure- and temperature-dependent solvent power. A comparison of catalytic activity with the Pd supported only on silica material of similar particle size inferred that the presence of even a small amount of B significantly changes the reaction rate from 70 (only Si) to 105 s⁻¹ (Si/B = 100). In addition, TOF of Pd/B-MCM-41 was high when compared with other Pd catalysts supported on Al-MCM-41 and Ga-MCM-41 obtained by a similar method, and follows the order: B (144 s⁻¹) > Ga (31.2 s⁻¹) > Al (10.2 s⁻¹). The remarkable advantage of the present catalytic system involves low metal content (∼1%), easy separation and it is successfully employed for the hydrogenation of substituted nitroaromatics, nitrile and phenol under mild reaction conditions. Furthermore, the catalyst was recyclable up to the 7th recycle without any loss of catalytic activity.