Solid Catalysts Designed for Carbon Dioxide Conversion

概要

For the past two centuries, carbon-rich fossil fuels, such as coal and natural gas have been utilized in promoting the development of the economic society. This has caused massive carbon dioxide emission that has resulted in a series of ecological problems such as global warming and ocean acidification. Various strategies have, therefore, been adopted in the utilization, storage and capture of carbon dioxide to mitigate the concentration of carbon dioxide in the air. Rather than a waste resource, carbon dioxide turns out to be a promising C1 building block to fabricate a series of value-added chemicals. Compared to the capture and storage of carbon dioxide, the transformation and utilization of carbon dioxide are more attractive in the long term. The utilization of carbon dioxide can provide an alternative way to convert the renewable raw material into useful chemicals or fuels and simultaneously realize the effective circulation of global carbon resources. Generally, CO2 hydrogenation to hydrocarbons takes two steps: first initial reduction of CO2 to CO through reverse water-gas shift (RWGS) reaction, and then chain propagation reaction via the Fischer-Tropsch synthesis (FTS) reaction. Thereinto, at the end of the 1920s, FTS was first mentioned by Hans Fischer and Franz Tropsch and from then on, the FTS is regarded as an alternative pathway to producing valuable fuels and chemicals from nonpetroleum sources. Thereinto, syngas (mixtures of CO and H2) as a key intermediate is a good link between carbon dioxide reactants and valuable hydrocarbons, expanding the application range of CO2 gases.

In chapter 1, we designed a bimetallic catalyst through impregnation method for catalyzing CO2 into liquid hydrocarbons. The introduction of cobalt metal increases the adsorption ability of CO2, which promotes the CO2 activation over iron species through RWGS reaction. Besides, the carbide content also increases significantly with the further introduction of cobalt metal in comparison to potassium modified one, achieving a high C-C bond coupling activity. CO2 conversion was limited by the thermodynamics, and the equilibrium conversion was generally less than 40% under normal operation conditions. To further enhance the catalytic activity of CO2 hydrogenation via ex situ water removal manner, a two-stage reactor system was developed and investigated, where the water product formed by RWGS reaction was trapped in the first ice-trap and the mixtures (including CO2, CO, light hydrocarbons) sequentially entered the second reactor. Facts proved that ex situ water removal is of great significance in enhancing catalytic activity and reducing the selectivity of CO by-product. This effectively promotional effect derived from water removal facilitates constructing a new route for improving the catalytic performance of CO2 conversion, further enhancing the yield of liquid fuel.

In chapter 2, a nitrogen functionalized carbon with embedded iron nanoparticles was developed by a simple one-pot hydrothermal synthesis process for improving CO2 hydrogenation performance. Four different nitrogenous reagents (ethylenediamine, pyridine, and diethylformamide, and pyrrolidine) were adopted to synthesized the functionalized catalysts. The characterization and catalytic performance evaluation revealed that different nitrogen sources have various effects on physical-chemical properties of catalysts. The improved CO2 hydrogenation performance over these functionalized catalysts was found to be correlated with the specific surface areas, the carbonization degree of iron species precursor, the amount of defect sites, and the content of pyridine-like nitrogen structures, which are determined by the doping nitrogen atom types. Pyrrolidine as a well-performing nitrogen source precisely regulated the physiochemical properties of the final catalyst, consequently achieving an outstanding performance.

In chapter 3, a promising Ni based mesoporous Al2O3 catalyst was prepared by one- step evaporation-induced self-assembly (EISA) method, and was employed as an efficient catalyst in combined methane dry reforming (DRM) and methane partial oxidization (POM) reaction. For comparison, a supported catalyst with ordered mesoporous Al2O3 was also prepared by sample impregnation method. The catalytic activity tests results indicated that the catalysts prepared by the one-pot method had better catalytic performance for combined methane dry reforming and methane partial oxidation reaction, ascribing to the larger exposed metal Ni surface area of Ni-MA than that of Ni/MA. Meanwhile, the catalytic performance remained stable at 750 °C for 100 h reaction. The characterization results of used catalysts indicated that combined POM with DRM could effectively suppress carbon deposition.

In chapter 4, a carbon dioxide hydrogenation to olefin process that achieves 72% selectivity for alkenes and 50.3% selectivity for C4-18 alkenes, of which formation of linear α-olefins accounts for 80%. The process is catalyzed by carbon-supported iron, commonly used in C-C coupling reactions, with multiple alkali promoters extracted from corncob. The design is based on the synergistic catalysis of mineral elements in biomass enzyme on which carbon dioxide can be directly converted into carbohydrate. The mineral elements from corncob may promote the surface enrichment of potassium, suppressing the secondary hydrogenation of alkenes on active sites. Furthermore, carburization of iron species is enhanced to form more Fe5C2 species, promoting both the reverse water-gas shift reaction and subsequent C-C coupling.

Herein, four types of catalysts with special structures were rationally designed, synthesized and applied to transform the carbon dioxide greenhouse gases into valuable chemicals or syngas. The physical-chemical properties and catalytic performances of these efficient catalysts were also studied in detail to shed new insights for solid catalyst design.

この論文で使われている画像