Effect of size and oxidation state of size-controlled rhodium nanoparticles on the aqueous-phase hydrodechlorination of 4-chlorophenol

Unsupported size-controlled Rh nanoparticles of different size and oxidation state were tested as catalysts models in aqueous phase hydrodechlorination (303-318 K, 1 atm) using 4-chlorophenol (4-CP) as target compound. A chemical reduction method was employed for the synthesis of the nanoparticles using methanol and poly(N-vinyl-2-pyrrolidone) (PVP) as reducing and capping agent, respectively. The size of Rh nanoparticles was in a narrow range (1.9-4.9 nm) whereas Ren+/Rh-0 ratio values were found within a wide range (0.56-3.89). High 4-CP conversion values (ca. 100%) were achieved at low Rh concentration (2.45. 10(-3) g L-1). Phenol, cyclohexanone, cyclohexanol and traces of cyclohexane were identified as reaction products. A wide range of activity values (1.7-29.4 mmol g(-1) min(-1)) were obtained, being equivalent to the measured in a previous work with unsupported Pd nanoparticles, in spite of the fact that Rh supported catalysts have generally been reported as less active than Pd ones in liquid phase hydrodechlorination. As the size of Rh nanoparticles decreased the activity increased reaching a maximum at 2.8 nm, lower size values leading to a significant decrease of activity. A remarkable dependence of activity on the Rhn+/Rh-0 ratio was found, thus a higher activity corresponded to a higher relative amount of zero-valent Rh in the nanoparticles samples. Regression models were developed in order to address the significance of nanoparticles size and oxidation state for the prediction of selectivity to cyclohexanone and cyclohexanol at varying reaction times. A crossed effect of particle size and Rhn+/Rh-0 ratio was identified as a significant factor influencing the selectivity. (C) 2013 Elsevier B.V. All rights reserved.