Reactivity and stability of supported Pd nanoparticles during the liquid-phase and gas-phase decarbonylation of heptanoic acid

摘要

The liquid-phase and gas-phase decarbonylation of heptanoic acid over carbon- and silica-supported Pd nanoparticles was studied in a continuous-flow fixed-bed reactor at 573 K. The liquid-phase turnover frequency (TOF) under steady state conditions (>20 h) was very low at approximate to 0.00070 s(-1), presumably because of deposition of carbonaceous species and in some cases sintering of the metal particles, as revealed by H-2 chemisorption, X-ray diffraction and electron microscopy. The steady state rate was independent of the support composition, synthesis method, Pd loading (1-20 wt%), and acid concentration (0.1-6.6 M). Although the gas-phase reaction also led to deactivation of the supported Pd catalysts, extrapolation of the rate to initial time gave a TOF of approximate to 0.035 s(-1) at 573 K. The liquid- and gas-phase reactions at low conversion levels were selective towards the formation of decarbonylation products such as CO and hexenes. Higher conversion levels resulted in the subsequent conversion of the primary decarbonylation products. Post mortem analysis of the catalysts revealed that concentrated, liquid-phase heptanoic acid at 573 K severely sintered the Pd nanoparticles supported on carbon but not those supported on silica. The Pd nanoparticles were able to maintain the high dispersion on carbon when exposed to low concentrations of liquid-phase heptanoic acid or gaseous heptanoic acid at 573 K. (C) 2015 Elsevier B.V. All rights reserved.