| Carbon supported Pd nanocatalyst is an important alcohol oxidation catalyst, which can catalyze benzyl alcohol to benzaldehyde under mild reaction conditions. However, small Pd nanoparticles tend to aggregate during the thermal treatment, or lose during the reaction process, which lead to the decrease of activity. Thus, it is necessary to prepare Pd nanocatalysts with the feature of high dispersion and good stability. In this thesis, based on the idea of "pyrolysis embedding" and "nanospace confinement", three kinds of carbon nanomaterials including carbon nanosphere, carbon nanosheet, and tubular mesoporous carbon were selected as supports, thus to prepare carbon supported Pd catalysts with high metal dispersion, excellent activity, and reusability. These catalysts were tested in the selective oxidation of benzyl alcohol under mild reaction conditions.(1) The size-uniform polybenzoxazine-based carbon nanosphere (?180 nm) provides a good catalyst support, due to the coordination interaction between nitrogen atoms and palladium ions. Uniformly dispersed Pd nanoparticles over carbon nanospheres were obtained by "pyrolysis embedding", and the mean size was about 3 nm. The catalyst preparation conditions were investigated by varying the metal loading, loading time, thermal treatment temperatures, and so on. The reaction rate reached 21.7 mol/(L min gPd) with the selectivity to benzaldehyde of>99% under the S/M molar ratio of 1000, when the catalyst was prepared with 1 wt% Pd and the loading time of 1 h. After five runs, the catalytic activity decreased to 85% from 99%, but it could recover to 96% by calcination of the used catalyst in air.(2) The 2D nanosheet structure has a short diffuse path, which allows an easy mass transfer through the substrate. Under this consideration, the carbon nanosheets supported Pd catalysts were prepared by "pyrolysis embedding", and the obtained Pd nanoparticles with the mean size of-2.5 nm were uniformly dispersed over the support. The catalysts were prepared over the support with various nanosheet thicknesses to explore the effect of support transfer path on catalytic activities. It was found that the catalyst prepared with the support nanosheet thickness of ?9 nm exhibited the reaction rate of 32.5 mol/(L min gpd) with the selectivity to benzaldehyde of>99% under the S/M molar ratio of 1000. The reaction conditions were optimized to further improve the catalytic activity, and the reaction rate reached 42 mol/(L min gpd). After five runs, the catalytic activity decreased to 81% from 99%, but it could recover to 98%by calcination of the used catalyst in air at 200?.(3) Ordered tubular mesoporous carbon with pore channels (3-5 nm) has the "nanospace confinement" effect on the formation of palladium nanoparticles, which benefits the preparation of Pd nanoparticles with high dispersion. Compared with the three carbons (carbon nanosphere, carbon nanosheet, and mesoporous carbon) supported Pd nanoparticles, the mesoporous carbon supported Pd catalyst showed a lower activity. Gold nanoparticle has the ability of activating oxygen, and the electronic effect between gold and palladium is beneficial for improving the activity. It was found that the catalyst prepared with the Pd/Au molar ratio of 4 exhibited an excellent performance, and its conversion was 35% higher than the monometallic palladium catalyst. The obtained Pd-Au nanoparticles were uniformly dispersed in the mesoporous channels, and the mean size was about 4 nm. The bimodal mesoporous carbon (CMK-5) encapsulated Pd-Au nanoparticles exhibited the reaction rate of 28.3 mol/(L min gPd) under the S/M molar ratio of 1000, due to the rapid diffusion and mass-transfer. The study of impregnation sequences on catalytic activities reveal that a larger proportion of surface-exposed Pd atoms in the bimetallic nanoparticles has a positive effect on catalytic activity. The conversion of the used catalyst decreased to 72% from 92% after five runs, and the deactivation mechanism was attributed to the tiny amount of incompletely removed benzaldehyde adsorbed onto the active surface. Importantly, the used catalyst can be recovered to 90% by calculation at 200? in air. |
PDF Full Text Request