| Hydrogen peroxide as an environmentally friendly chemical is widely used in all aspects of production and life, the market of hydrogen peroxide is increase with the rapid development of the national economy, especially the increasing requirements of sustainable development with green economy. The anthraquinone process is the most important method for the preparation of hydrogen peroxide, and the hydrogenation catalyst is the key to the whole preparation process, which determines the quality, preparation cost and production efficiency of hydrogen peroxide. Compared with Ni catalyst, Pd catalyst is more widely used owing to higher activity. However,Pd is a noble metal, and the deep hydrogenation product produced by anthraquinone over-hydrogenation makes the decrease in catalytic activity.Therefore, it is of great significance to prepared anthraquinone hydrogenation catalyst with high activity and selectivity as well as stability in order to reduce noble metal consumption and improvement the catalytic performance.In view of the current Pd-based catalyst for anthraquinone hydrogenation,the active component utilization rate is low and it is easy to generated deep hydrogenation products. In this paper, the design and preparation of novel supported Pd-based catalysts were carried out to improve the activity and selectivity of catalyst for the anthraquinone hydrogenation. Moreover, the relationship between the structure and catalytic performance of Pd-based catalysts was also investigated. First, the pore structure of the catalyst support was controlled, and the influence of pore structure of the support on the dispersibility of active component and catalytic performance was discussed. Subsequently, a new method was developed to prepare Pd catalyst with high dispersion, and the effect of preparation conditions on the dispersion of Pd and the catalytic performance were investigated. In order to further improve the catalytic performance and strengthen the mass transfer effect, the bimetallic Pd-Ir alloy catalyst and the bimetallic Pd-Ir mesocrystal catalyst were prepared by co-impregnation and co-reduction method, respectively. The dispersion and defect structure of Pd active sites were controlled, and the relationship between catalyst structure and catalytic performance was also revealed. In this work, four novel Pd-based catalysts were obtained, which greatly improve the hydrogenation performance of anthraquinone and provided new ideas as well as new methods for the preparation of high efficiency anthraquinone catalyst. The detailed contents and conclusions are listed as follows.Ordered mesoporous anodic aluminum oxide (AAO) supports with different pore sizes and depths were controllable prepared by a two-step anodizing process on the A1 foil, and the prepared AAO supports with ?-ring shape were utilized to prepare the Pd/AAO@Al catalysts. The effect of pore structure on the dispersion of active component as well as the catalytic performance were investigated. Due to the confinement of AAO pore channel, the dispersion of Pd was improved. With the decreasing of AAO pore size and the increasing of pore depth, the dispersion of Pd is gradually increased. The ?-ring shaped Pd/AAO@Al catalysts with highly ordered array of cylindrical AAO pores is conductive to reducing the diffusion resistance of the reactants. The Pd/AAO@Al catalyst with 24.5 nm pore size and 18 ?m depth exhibited the best performance towords hydrogenation of anthraquinone, which is twice as high as that of the spherical Pd/Al2O3 catalyst and significant improvement in selectivity.Highly dispersed Pd catalysts were prepared by the self-reduction of PdCl42- by CoAl-LDHs, and the in-situ reduction mechanism was investigated. Moreover, the effect of in-situ reduction conditions on the dispersibility of Pd and the hydrogenation performance was also revealed.In the redox reaction between CoAl-LDHs and PdCl42-, the PdCl42- is reduced to Pd0 and the Co2+ in the CoAl-LDHs is partially oxidized to Co3+.With the decrease of the reduction temperature, the dispersion of Pd is improving, which is helpful to improve the activity. At the same time, the increase of the ratio of linear adsorption sites inhibits the formation of deep hydrogenation products. The Pd/CoAl-LDHs catalyst prepared at 0?show of the best catalytic performance, which is 1.5 times of the Pd/Al2O3 catalyst prepared by the impregnation method.Pd-Ir alloy catalysts were prepared by co-impregnation method, and the effects of geometrical and electronic synergistic of Pd-Ir alloy were investigated as well as different Pd/Ir ratios on the alloy structure and hydrogenation performance. HRTEM and kinetic studies show that the Pd-Ir alloy not only reduces the particle size of Pd active components, but also effectively reduces the energy barrier of anthraquinone hydrogenation. The introduction of Ir caused the geometrical and electronic effect on the Pd active site, which facilitates desorption of the reaction product from catalyst surface. The results of hydrogenation of anthraquinone show that the H2O2 yield of 0.75 wt% Pd-0.25 wt% Ir/Al2O3 alloy catalyst is 25.4%higher than that of 1 wt% Pd/Al2O3, and the concentration of tetrahydroanthraquinone at the same anthraquinone conversion was lower than Pd/Al2O3 when Pd-Ir alloy catalyst was used.Pd-Ir mesocrystal catalyst with three-dimensional porous were prepared by co-reduction method, and the effect of Pd-Ir mesocrystal structure on the catalytic performance was revealed. It is found that the Pd-Ir mesocrystal was self-assembled by the small-sized nanoparticle unit by monitoring the Pd-Ir mesocrystal growth process, and large quantities of grain boundary defects were formed due to the mismatch between the crystal faces during the oriented attachment. The abundant defect sites in the Pd-Ir mesocrystal catalyst are favorable for the activation and dissociation of hydrogen, thus improving the activity of the Pd-Ir mesocrystal catalyst. Moreover, the selectivity of Pd-Ir mesocrystal catalyst was also improved, which is attributed to the increase of the linear adsorption site formed by the the self-assembly of structure unit. In addition, the three-dimensional porous morphology of Pd-Ir mesocrystal is beneficial to mass transfer and increase the accessibility of the active sites, and further improve the activity and selectivity of Pd-Ir mesocrystal catalyst. |
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