Creation And Research Of New Fischer - Tropsch Catalyst For Clean Fuels

Coal-, natural gas-, and biomass-derived syngas (CO+H2) could be catalytically converted to ultra clear liquid fuel via Fischer-Tropsch synthesis (FTS) reaction. This process provides a very important way to deal with the recent oil crisis and environment protection. FTS catalyst is the core of this technology, and its catalytic performance could be strongly affected by the many factors, such as dispersion of active metal, promoter, support, pre-treatment, and reaction conditions. Development of novel and highly efficient FTS catalyst is becoming the most important topic in this field, which shows high academic and economic values.Conventional FTS catalysts always require high-cost and time-consuming preparation strategies and they could be improved in some aspects, such as stability and selectivity. Moreover, the relationship between the catalytic performance and structure is mostly obscure. Recently, some high-performance FTS catalysts with novel structure or new supports have been synthesized and studied, which has led to better understanding of FTS catalyst and provided new directions in this research field. In this thesis, we have prepared a series of novel high-performance FTS catalysts and optimized the preparation method, reaction condition, and promoter. Based on systematic characterizations of these catalysts, we elucidated the promotion effect and active phase of iron-based FTS catalyst, clarified the particle size effect on iron-and cobalt-based FTS catalysts, illustrated the influence of metal-support interaction on iron-catalyzed FTS reaction, and also provided some facile and general catalyst preparation method.1. Rapidly quenched skeletal FeCe FTS catalystThe skeletal FeCe catalysts (RQ FeCe) with different contents of Ce have been prepared by alkal leaching of ternary FeCeAl alloys solidified by the rapid quenching technique, and tested for the gas phase FTS performance. The improved textural and structural characteristics of the RQ FeCe catalysts probed by elemental analysis, N2physisorption, and powder X-ray diffraction confirm the structural promotion effect of Ce. It is found that the addition of Ce promoter can efficiently improve the catalytic performance of the RQ Fe catalyst in terms of activity and selectivities to long-chain products. The Fe48Ce2catalyst exhibits the highest selectivities of42.7%to C5-11products excelling the values reported on FeCe FTS catalysts prepared by wet chemistry method, which demonstrates the superiority of the metallurgical method in harnessing the beneficial effect of Ce on the Fe-based FTS catalysts.Furthermore, with the aid of XPS, temperature-programmed hydrogenation and57Fe Mossbauer spectroscopy, it is found that the addition of Ce stabilizes the Ca and Cβ as active carbonaceous species on the catalyst surface in FTS reaction. Therefore, the catalytic activity improved continuously with the increment of Ce content. Meanwhile, Ce(Ⅲ) is manifested to facilitate the formation of the Hagg carbide leading to higher C5+selectivity. And this chemical promotion effect is evidenced by the linear correlations between the content of the Hagg carbide and Ce(III) cations on the RQ FeCe catalysts as revealed by X-ray photoelectron spectroscopy.2. Study of Co@HZSM-5and Co@H-β core-shell FTS catalystA tailor-made encapsulated Co@HZSM-5catalyst with a HZSM-5zeolite shell has been directly synthesized over skeletal cobalt FTS catalyst by a hydrothermal synthesis method. Morphological and chemical analysis showed that the capsule catalyst had a core-shell structure. A compact, integral shell of HZSM-5crystallized firmly on the skeletal cobalt substrate without crack. Syngas passed through the zeolite membrane to reach the skeletal cobalt catalyst to be converted, and all hydrocarbons formed with straight chain structure must enter the zeolite channels to undergo hydrocracking as well as isomerization in this tailor-made confined reaction environment. A narrow, anti-Anderson-Schultz-Flory law product distribution was observed on these capsule catalysts. Contrary to a mechanical mixture of HZSM-5and skeletal cobalt, C12+hydrocarbons were suppressed completely on this novel capsule catalyst, and the selectivity of middle isoparaffins was considerably improved. The carbon number distribution of the products depended on the thickness of the zeolite membrane, and it was possible to selectively synthesize gasoline-range hydrocarbons from syngas directly, by simply adjusting the thickness of the zeolite membrane of the capsule catalyst. The optimized Co@HZSM-5-4d catalyst exhibits the C-5-11product selectivity of79%under523K, which is15%higher than the previously reported values.We have also prepared Co@H-β core-shell FTS catalyst via similar synthesis route. The catalytic results confirm that the C12+hydrocarbons could also be hydrocracked into C5-11products completely, which leads to the observed C5-11product selectivity of nearly79%. By comparing the FTS performances of Co@HZSM-5and Co@H-β catalysts, it is found that the larger pore size of the zeolite will shift the product distribution to long-chain hydrocarbons, and increase the initial reaction activity but decrease the stability.3. Study of one-pot-fabricated R-Fe@HZSM-5core-shell FTS catalystWe have designed a facile one-pot hydrothermal strategy to an HZSM-5-encapsulated Raney Fe catalyst (R-Fe@HZSM-5) using a commercially available Fe50Al50alloy (Fe/Al, wt/wt) as the starting material. The dual functions of tetrapropylammonium hydroxide (TPAOH) as the template for the synthesis of HZSM-5and as the base for the dealumination of the FeAl alloy are essential for the successful design of this one-pot strategy. No adventitious Al source is added, since the leached Al species can be directly utilized for zeolite synthesis. The HZSM-5crystals are expected to nucleate on the resulting Raney Fe that is porous and modified in situ by TPAOH. By using this strategy, we are able to fabricate the R-Fe@HZSM-5catalyst that is highly selective in one-step FTS of gasoline from syngas with suppressed methane formation without the aid of any promoter.On this core-shell catalyst, the C≥12hydrocarbons are completely eliminated, CH4is substantially reduced, while the gasoline-range hydrocarbons become the predominant products. We illustrate that when in combination with Raney Fe, HZSM-5is not limited to the conventionally acknowledged role of cracking/isomerization of heavy hydrocarbons. We propose that the hydrophilicity of HZSM-5is conductive to a less oxidizing chemical environment around the core, which stabilizes Hagg carbide essential for chain propagation. This work shows the prospect of selectively producing gasoline from syngas on a low-cost Fe-based catalyst. The facile synthetic strategy is extendible to other metal@zeolite catalytic materials with different core and shell functions using widely available Al-containing alloys as starting materials.4. Ordered mesoporous carbon-supported Fe/Co FTS catalystsWe have established a facile chelate-assisted one-step evaporation induced self-assembly (EISA) route for the synthesis of mesoporous carbon possessing uniform metallic nanoparticles highly dispersed in ordered meso-framework with precisely tunable sizes. Due to the distinguished properties of mesoporous carbon as support, well dispersion of iron-oxide nanoparticles and good confinement of the mesoporous carbon framework for embedded particles, the reduced Fe-C catalysts show remarkable stability and C5+selectivity, which clearly exemplifies the confinement effect of the mesoporous carbon framework on the embedded nanoparticles imparted by the unique semi-exposed nanostructure. And the particle size effect of iron-based FTS catalysts was also demonstrated. In this series of catalysts with particle size ranging from4-16nm, the catalytic performances in terms of activity and C5+product selectivity were improved by the decrease of iron particle size. Using the same method, we have also prepared of the Co-C catalysts. It is observed that the activity Co-C catalysts decrease with increment of cobalt particle size ranging from8-14nm, while the selectivity to C5+products remains the same.5. One-step synthesis of graphene-supported highly dispersed Fe FTS catalystWe demonstrate a facile one-pot hydrothermal hydrolysis-reduction (HHR) strategy that is able to fabricate sub-3nm γ-Fe2O3NPs highly dispersed on rGO, using iron(Ⅲ) acetylacetonate [Fe(acac)3] and GO as starting materials. This synthetic strategy is readily extendable to the fabrication of other metal oxide-rGO nanohybrids. The Fe-rGO nanohybrid remained highly dispersed upon reduction at723K for16h, and the FTS reaction conducted at543K for120h. Moreover, the Fe-rGO nanohybrid afforded much higher FTS activity and better selectivities to C5+and C-5-11hydrocarbons than either Fe/p-rGO fabricated using pre-reduced graphene oxide (p-rGO) instead of GO as the starting material, or Fe/AC with the commercial Vulcan X-72activated carbon as the support. The high population of the defects on rGO is believed to be the key to anchor the NPs at elevated temperatures. We also demonstrated that the nature of the carbon support strongly influences the FTS performance of the carbon-supported Fe catalysts. This work not only provides a facile synthetic strategy to thermally robust metal/metal oxide NPs on graphene, but also validates for the first time the feasibility of the graphene-related nanomaterials in very challenging high-temperature applications by proper materials design.