| In recent years, the synthesis of high-quality liquid fuels from syngas derived from coal, natural gas, biomass, etc., via Fischer-Tropsch (FT) route has received much attention due to the complex situation of the worldwide petroleum market and the increasingly growing global environmental problem. However, the FT products, which are controlled by the Anderson-Schulz-Flory (ASF) polymerization kinetics, are nonselective to any specific hydrocarbons. Therefore, development of a high-performance catalyst for one-step selectively synthesizing the desired group of hydrocarbons is important in both theoretical studies and practical applications. However, it is still a challenging work in the domain of FT synthesis.The abundance, environmental compatibility, adjustable acidity, layered structure and exchangeable cations make montmorillonite (MMT) a potentially good solid acid for controlling the product distribution of FT synthesis. However, few works by using pillared MMT as a support of Co have been reported for FT synthesis. Thus, in this dissertation, multifunctional catalysts were designed and prepared by effectively intergrating FT metals of Co and Ru with an MMT, in which its structural and acidic properties are tailored in large scales by different methods, i.e., interlayer cation exchange, pillaring with metal oxides, and template directed pillaring of metal oxides. Results indicate that our strategy on the catalyst design and the method for preparing the catalyst are effective. With a similar CO conversion and CH4selectivity to a typical Co/SiO2FT catalyst, the optimal catalyst showed a high selectivity to C5-C20liquid fuels (>60%) and very low selectivity to C21+hydrocarbons (<4%). The activity and the selectivity of FT synthesis were qantitatively correlated with the structure, acidity, reduction behavior, and the synergistic effect between different catalytic functions of the catalyst or MMT, which are characterized by XRD, physical/chemical adsorption, and microscopic techniques, etc. The mechanistic relationship between the thus mentioned factors and FT performance of the catalyst, especially the key factor on selectively controlling the FT product distribution, was determined, and a reasonable model for the optimal multifunctional catalyst was proposed. The experiment and main conclusions are summarized as follows: (1) By using natural MMT as a matrix, a series of pillared MMT (PILC) by different oxides (Al-PILC, Si-PILC, and Zr-PILC) and mixed oxides (Si/Al-PILC, Si/Zr-PILC, Si/Ti-PILC, and Al/Zr-PILC) were successfully prepared. Co/PILC catalysts prepared by the incipient impregnation method were comparatively studied for FT synthesis. Results indicate that the basal spaing, surface area, porous structure, and surface acidity of the PILC are significantly influenced by the cation exchange capacity (CEC) of MMT and the nature and composition of the pillaring oxides. The surface area of PILC prepared by the MMT with a higher CEC was larger than that prepared by the MMT with a lower CEC. The surface area of Si/Al-PILC, Si/Zr-PILC, and Si/Ti-PILC was higher than the single oxide pillared MMT of Si-PILC. Moreover, the amount of total acidic sites and medium to strong acidic aites was significantly increased as a result of the introduction of the second pillaring oxide. The secondary reactions, i.e., cracking, hydrocracking, and isomerization of FT products over the acidic sites, were occurred with different extents due to the varied of amount and strength of the acidic site over PILC. This results in that the product selectivity over Co/PILC was significantly deviated from the ASF distribution of FT synthesis over the typical catalyst of Co/SiO2. Among the Co/PILC catalysts investigated, the optimal catalyst of Co/Si/Zr-PILC was obtained with a high selectivity to C5-C20liquid fuels (64.2%) and a very low selectivity to C21+hydrocarbons (5.1%).(2) Without using a large amount of alkyl amine as a co-surfactant, porous clay heterostructures (PCH) interclated with SiO2directed by a single template of cetyltrimethylammonium bromide (CTAB) was successfully prepared. Moreover, a highly ordered PCH is synthesized in the medium of supercritical CO2(scCO2), and a cooperative mechanism is reasonably revealed for the formation of PCH. Following the proposed mechanism, ordered PCH, which is directed by a Gemini surfactant and quaternary ammonium cation with polyoxyethylene group chain, respectively, was successfully prepared under mild conditions.(3) A new method for the one-step, facile, and well controlled synthesis of PCH was proposed based on the properties of scCO2, i.e. low viscosity, high diffusivity, and low surface tension. The method was demonstrated by using H+-MMT as a matrix, CTAB as a structure-directing agent, tetraethyl orthosilicate (TEOS) as the precursor of S1O2, a higherly ordered PCH with large basal spacing and narrowed pore size distribution was obtained. Importantly, the method provides a new strategy for the preparation of intercalated layer materials.(4) The synthesized PCH was comparatively investigated as support of Co for FT synthesis in a fixed-bed reactor under the conditions of1.0MPa,235℃, H2/CO=2, and W/F=5.02g h/mol. Results indicate that the FT product selectivity is significantly influenced by the synergistic effect between the acidity and porous structure of PCH. Secondary reaction of the FT hydrocarbons catalyzed by the acidic PCH can be reasonably expected. However, the selectivity of iso-paraffins over Co/PCH was very low, indicating that the isomerization of the FT hydrocarbons is less occurred as a result of the lower reaction temperature and lower acidity of PCH. In contrast, cracking or hydrocracking of long-chain FT hrdrocarbons was significantly over all the Co/PCH catalysts. Moreover, the degree of cracking and hydrocracking of the long-chain FT hydrocarbons was increased not only with the increase of the total acidity of PCH, but also was closely related to the diffusional behavior of the FT product. According to the spillover mechanism of hydrocracking reactions, the hydrocracking of long-chain FT hydrocarbons over Co/G12-2-12-PCH can be reasonably attributed to that the Co active site is closer to the acidic sites.(5) The3wt.%Ru/G12-2-12-PCH catalyst was investigated for the FT synthesis under the conditions of1.0MPa,260℃, H2/CO=2, and W/F=9.33g h/mol. Results indicate that the selectivity of C5-C12was43.26%, and the ratio of iso-paraffins to normal paraffins was2.33. This confirms that the lower degree of isomerization over Co/PCH catalysts is really due to the the lower reaction temperature and lower acidity of the PCH. Moreover, this result provides a general guidance for optimizing the FT catalyst with an improved gasoline ranged iso-paraffins.(6) By ion-exchanging the natural Na-MMT with K+, Mg2+, Ca2+,, Fe3+, Ni2+Cu2+, Zn2+, Ce3+and Zr4+, respectively, MMTs with different interlayer cations was obtained. The effect of the MMT interlayer cations on the performance of Co/MMT catalysts for FT synthesis was comparatively investigated under the conditions of1.0MPa,235℃, H2/CO=2, and W/F=5.02g h/mol. Results indicate that the textural properties of the MMTs are strongly dependent on the interlayer cations and the CO conversion and product distribution over Co/MMT are significantly affected by the promotional effect of the confined cations. |
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