The Preparation And Characterization Of Pd-based Supported Metal Catalysts With High Dispersion For CatalyticHydrogenation

The supported noble metal catalyst is one kind of essential catalytic materials, which has occupied an important position in the field of chemical engineering and petrol industry, especially for the selective hydrogenation and conversion involving various hydrocarbons. Industrially, its preparation strategies mainly include impregnation and deposition-precipitation, which easily result into the poor dispersion, unstability. worse reactivity and so on. Thus, it has been one industrial and academic center of interest to develop noble metal candidates with high dispersion for higher reactivity and stability, and lower cost.Layered Double Hydrotalcites (LDHs), one kind of materials with the intercalation structure and multiple functions, have the positive layer constructed by hydroxides containing the divalent and trivalent cations, and the negative interlayer where various anions could be located. Previous investigations have confirmed the fact that LDHs, not only as catalysts or corresponding precursors but also supports, could show excellent performances and application prospects. Thus, based on the composition and property flexibility of LDHs, this dissertation, utilizing the in-situ growth method, presents the preparation of supported Pd monometallic and RuPd bimetallic catalysts with high dispersion. And then, various characterizations as well as theoretical DFT calculation are applied for observing the microscale morphology and structure of metal particles as well as the textural and surfacial properties of catalysts. Finally, the catalytic reactivity is investigated for for the selective hydrogenation of dimethyl terephthalate (DMT) to dimethyl cyclohexane-1,4-dicarboxylate (DMCD, one important intermediate in the polymer industry). The specific investigations mainly have the following three parts, and the essential results are as follows:Firstly, the in-situ growth method makes the MgAl-LDHs in-situ growing on the surface of spherical Al2O3, and the desired modified carrier, HTC-Al2O3, is obtained after the essential calcinations and other thermal treatments. Subsequently, the Pd monometallic catalyst, viz., Pd/HTC-Al2O3, is prepared based on the impregnation. The characterization results of XRD, BET, H2-TPR/TPD, SEM-EDX and HRTEM indicates that, compared with un-modified Al2O3, the modified support HTC-Al2O3 could help Pd/HTC-Al2O3 have the higher specific surface area and lower reduction temperature as well as the Pd nanoparticles with smaller size and higher dispersion; simultaneously, it could be found that Pd is mainly located in the outer layer of HTC-Al2O3, leading to the formation of classic egg-shell structure with good uniform. The designed catalyst Pd/HTC-Al2O3 shows the excellent catalytic activity, selectivity and stability. It should be noted that, at the reaction condition, viz.,8 MPa and 180℃, Pd/HTC-Al2O3 could be operated continuously and stably for 24 h.Secondly, considering the synergetic effect in the bimetallic system, the precious Pd is partially replaced by cheaper Ru, and the desired RuPd bimetallic supported catalysts, differing in the Ru:Pd ratio, are prepared by the co-impregnation method. The characterization results of HRTEM and STEM-EDX help to confirm the presence of bimetallic nanoparticles with smaller size than that of corresponding monometallic ones, which could be ascribed to the intimate interaction between each other; simultaneously, the nanoparticle size distribution is influenced by the Ru:Pd ratio, and the average particle size reaches the smallest when the Ru:Pd ratio is 1:1. H2-TPR and H2-TPD indicate the classic bimetallic characters, and the composition has distinctive effect on the reducibility and sorptions. The total desorption amount of hydrogen species could reach the highest value when the Ru:Pd ratio is 1:1. The reaction tests reveal that bimetallic catalysts establish the superiority over monometallic Pd or Ru catalysts in the activity and selectivity under the same conditions. And, the higher pressure (8 MPa) and moderate temperature (180℃) is conducive to the excellent reactivity. Besides, as the Ru:Pd ratio is gradually modulated, the catalytic reactivity of bimetallic catalysts presents the interesting "volcano-type" tendency, leading to the optimal with the equivalent Ru and Pd. Theoretical DFT calculations demonstrate the influence of composition modulation on the d-band center, especially the presence of moderate d-band center in the system of Ru:Pd being 1:1.Finally, the modified support HTC-Al2O3 is adopted for preparing the bimetallic catalysts RuPd/HTC-Al2O3. And, the influence of injected Mg amount on the chemical and physical properties as well as the resulting reactivity is investigated. The relevant characterizations indicate that, compared with RuPd/Al2O3 prepared using un-modified Al2O3 as supports, RuPd/HTC-Al2O3 has the higher specific surface area and more pore volume. Especially, the reticular architecture in RuPd/HTc-Al2O3 constructed by crossed hydrotalcite-like layers, could be conducive to the isolation and separation of metallic sites, leading to the improvement of the interaction with supports and preventing the agglomeration and migration. Also, it should be noted that the chemical and physical properties could be regulated by the injected amount of Mg. The excess amount of Mg easily results into the more hydrotalcite microcrystallines, which bring about the the more pore blocked and surface area lossed, and consequently lead to the negative effect on the separation of metal nanoparticles. The investigation reveals that, when the amount of injected Mg is 1.0 wt%, the catalytic performance of RuPd/HTC-Al2O3 could arrive at the optimal point.