Catalytic behavior of size-controlled palladium nanoparticles in the hydrodechlorination of 4-chlorophenol in aqueous phase

Unsupported Pd nanoparticles of controlled size were tested as catalyst in liquid-phase hydrodechlorination (303–323 K, 1 atm) using 4-chlorophenol (4-CP) as target compound. The Pd nanoparticles were synthesized by chemical reduction, using ethanol and methanol as reducing agents and poly(N-vinyl-2-pyrrolidone) (PVP) as capping agent. The size of the nanoparticles and the Pdn+/Pd0 ratio decreased with increasing alcohol concentration and PVP/Pd ratio, both being lower for ethanol medium. High 4-CP conversion values (80–100%) were achieved at low Pd concentration (2.45 × 10−3 g/ L). Phenol was the only reaction by-product detected in contrast to the previous results with supported Pd catalysts, where the active phase–support interaction in 4-CP HDC led to obtain also cyclohexanone and cyclohexanol as by-products in equivalent experimental conditions. The smaller nanoparticles showed higher activity due to the higher available surface (m2/gcat). Thus, the smaller nanoparticles synthesized in ethanol medium ranged between 2.7 and 2.8 nm and yielded activity values between 16.7 and 39.1 mmol/gcat min, whereas the smaller particles obtained in methanol medium were in the 3.1–4.2 nm range and exhibited activity values of 20.1–25.7 mmol/gcat min. However, the large nanoparticles exhibited higher activity per unit of catalyst surface, for example, 0.34–0.43 mmol/min m2 in the case of those synthesized in methanol medium. On the other hand, higher activity was observed for the nanoparticles synthesized in methanol medium when equivalent nanoparticle size was compared. Activation energy values around 100 kJ/mol were obtained for Pd nanoparticles of different characteristics, significantly higher than the value reported for supported Pd catalysts.