Controllable Synthesis Of Supported Pd-based Hierarchical Catalysts For Partial Hydrogenation Of Acetylene

Ethene industry, the foundation of the petrochemical engineering, is a most significant symbol of the level of national petrochemical industry. Polyethene production produced by the ethene polymerization is an important subset of the ethene industry. Typically, ethene obtained from thermal cracking contains 0.5-2.4% of acetylene, which could poison the catalysts used in polymerization process as well as decrease the quality of the product. Therefore, as a key process, the purification step is necessary to remove trace acetylene impurities in the stream. With the development of modern polyethene industry, a high requirement for the content of acetylene in stream is further proposed. As a result, the design and synthesis of highly performed supported catalysts for partial acetylene hydrogenation have attracted much attention.From the perspective of structural dimension, supported catalysts could be subdivided into microscopic structure, mesoscopic structure and macroscopic structure. The microscopic structure of catalysts is the electronic environment and superficial bonding, which manages the catalytic activity and selectivity. In case of the mesoscopic structure, the morphology of active component and supports porosity have a huge influence on the catalytic performance. The macroscopic structure is the stable dispersion of active component, which can be realized by the control of preparation process. Therefore, on the basis of kinetics of C2 hydrogenation and diffusion of reactive molecular, design and synergy of hierarchical structure are demanded to realize the improvement of the catalytic efficiency and atom economy.Layered double hydroxids (LDHs) is a class of two-dimensional layered inorganic material with brucite structure. As one of the novel catalytic materials, LDHs and its calcined product (LDO) offer several advantages: Cation-tunability of the brucite-like layers and anionic exchangeability, high adsorption capacity, tunable acidity and basicity of surface, uniform dispersion of cations in the layers and preferred orientation of anions as well as hybrid with other materials. These features of LDHs and LDO make it possible to prepare novel highly performed nanocatalysts for C2 hydrogenation by the structural construction from hierarchical dimension.In this paper, Pd/LDO, Pd/MgO and Pd/Al2O3 catalysts have been prepared using the impregnation method to investigate the effect of support nature on the status and electronic density of active component as well as the catalytic performance. The results of catalytic test showed that Pd/LDO possessed the highest activity and selectivity towards ethene. X-ray photoelectron spectroscopy and CO infrared spectroscopy analysis revealed the interaction between LDO support and Pd, while the acidic sites could result in the electron-deficient surface. In consequence, the preferred catalytic performance of Pd/LDO was ascribed to the high metal dispersion, high ratio of linear/bridge adsorption and low activation barrier.Supported Pd nanowire have been synthesized by one step method to study the relationship between defective structure and catalytic performance. HRTEM images showed that the twisted polycrystalline Pd nanowires were actually composed of primary cuboctahedral building units. Owing to the mismatched attachment, lattice distortion was observed at interfacial regions and the number of crystal boundaries increased with increasing length of the nanowires. Temperature programmed desorption of H2 and kinetics analysis indicated that the defect sites at crystal boundaries facilitated the activation and dissociation of hydrogen, and decreased the barrier of the reaction. Therefore, the activity of Pd nanowire catalyst with defect sites was found to increase with an increasing number of crystal boundaries, whereas the trend in the selectivity was reverse.Mesocrystals (MCs) assembled by several building units are three-dimensional nanostructure that have an ordered facet and high densities of crystalline defects. Herein, the sea urchin-like PdAg MCs have been synthesized by co-reduction method to investigate the relationship between mesoscopic structure of active component and catalytic performance. By monitoring the growth process of PdAg MCs, it was found that an oriented attachment pathway was involved, and the spontaneous self-organization of adjacent building units occured preferentially at the (100) facet. Additionally, large quantities of grain boundaries were formed during the self-organization owing to the restricted grain rotation. The result of partial hydrogenation of acetylene showed that the addition of Ag could increase the selectivity towards ethene due to the introduction of the electronic effect and geometric effect. Moreover, the abundant defect sites in PdAg MCs catalyst could enhance the activity, which overcame the weakness of activity caused by the addition of the inactive Ag component.Based on the achievement from the study on microscopic and mesoscopic structure, the hierarchical structure of supported catalysts were combined to prepare highly effective supported Pd-based catalysts with the potential of industrial application for selective acetylene hydrogenation. Bimetallic Pd-Ga/MgO-Al2O3 catalysts have been synthesized on the commercial Al2O3 support by the in situ LDHs precursor route. The bimetallic catalysts showed preferable selectivity towards ethene and comparable activity compared with monometallic Pd/MgO-Al2O3 catalyst in the partial hydrogenation of acetylene, which could be attributed to the electronic effect and/or geometric effect from the introduction of Ga. Furthermore, Pd-Ga/MgO-Al2O3 catalysts also possessed an enhanced stability owing to the net trap confinement effect of LDHs materials. Taking consideration of the activity, selectivity and stability, Pd-Ga(1:5)/MgO-Al2O3 was the optimal catalyst under our test condition.