Studies on Nano Catalysts for Conversion of Carbon Oxides

概要

With the high-speed development of economy and society since the beginning of the industrial revolution, fossil fuel has been excessively consumed, which is gradually leading to serious environmental issues like global warming. The utilization of recyclable feedstock such as CO2 for making value-added products, rather than treating as a waste product from the combustion of fossil fuels, CO2 can be converting into highly value-added products such as light olefins and syngas. For synthesizing light olefins from CO2, it can be realized two primarily catalytic approaches: CO2-Fischer-Tropsch-olefins (CO2-FTO) and CO2-methanol-olefins (CO2-MTO) synthesis. However, hydrocarbons derived from FTS reaction always follow Anderson-Schulz-Flory (ASF) distribution, suggesting a wide distribution of hydrocarbons with different chain lengths, thereby leading to low selectivity for light olefins. As an alternative, direct hydrogenation CO2 over a single catalyst is suitable to selectively synthesize light olefins by coupling two sequential reactions over a hybrid catalyst. Typically, the two-step process comprises methanol synthesis step followed by methanol dehydration to hydrocarbons step. Moreover, via dry reforming of CH4 and CO2, CO2 can be transformed into syngas, which is an essential platform to convert the lower carbon resource into highvalue-added products, such as dimethyl ether (DME), liquified petrolum gas (LPG), gasoline and aromatics. Therefore, producing of high value-added chemicals and fuels from syngas has been intensively explored owing to its academic and industrial value.

As we all known, catalyst design is a vital factor in promoting the catalytic performance of catalysis reaction. Moreover, catalyst with unique structure exhibit unexpected performance in catalysis application. As typical representatives, nano-sized catalysts have attracted wide attention due to their enhanced mass transfer efficiency and specific selectivity to target products. Herein, in this thesis, we mainly concentrate on the design and application of high-performance catalysts with special structure for conversion of carbon oxide into highly value-added chemicals: (1) hybrid catalyst comprising of In2O3/ZrO2 metallic oxide and zeolite SAPO34 for directly converting CO2 into light olefins (Chapter 1); (2) optimized Ni/SiO2 supported catalyst for dry reforming of CH4 and CO2 into syngas (Chapter 2); (3) bifunctional capsule catalyst of Al2O3@Cu for direct synthesis of dimethyl ether from syngas (Chapter 3).

In chapter 1, hybrid catalyst composed of In2O3/ZrO2 metallic oxide and zeolite SAPO34 was developed and applied in direct CO2 conversion to light olefins reaction. This catalyst exhibits high light olefins selectivity of 77.59% via sub-steps tandem reactions, comprising of CO2 hydrogenation to methanol on the oxygen vacancies surface of In2O3/ZrO2 and the formed methanol simultaneously dehydrated to hydrocarbons passing through the SAPO34 zeolite channel. Moreover, by optimizing the reaction, the undesirable CO generated from the reversed water-gas shift reaction was efficiently suppressed, showing good potential leading to scale-up cause of the outstanding reaction performance and its friendly-environment properties.

In chapter 2, a series of Ni/SiO2 catalysts was fabricated by wetness impregnation method and applied in dry reforming of methane and CO2 reaction (DRM). As previous report, the nickel particle size, distribution and interaction with support are crucial to the catalytic performance in DRM. In this work, Ni particle size was controlled with altering the pH of the Ni nitrate solution via the addition of HNO3 or NH4OH. The NH4OH promoted Ni/SiO2 catalyst presents high conversions of CH4 and CO2 and excellent stability in DRM reaction, due to metallic Ni particles in small size and stronger interaction with SiO2 support. The present study provides a simple but effective way to change the metal-support interaction and metal particle size.

In chapter 3, a novel bifunctional capsule catalyst of Al2O3@10Cu was synthesized by a simple surface infiltration method and applied in direct conversion of syngas to dimethyl ether (DME). Different from the traditional capsule catalysts with the core for methanol synthesis and the shell for methanol dehydration, the Al2O3@10Cu employs an opposite strategy that using the shell for methanol synthesis while the core for methanol dehydration. The characterization results clearly demonstrate that the capsule Al2O3@10Cu possesses a more uniform Cu-Al2O3 shell structure and lower reduction temperature than other reference catalysts. Furthermore, in the direct DME synthesis from syngas, the Al2O3@10Cu with the uniform shell exhibits much higher CO conversion and DME selectivity than the conventional powder-based Cu/Al2O3-P catalyst. The present work offers a new spatially confined model for the direct DME synthesis from syngas.

Herein, three types of catalysts in nano size were successfully designed, synthesized and applied to transform the carbon oxide into high-value-added chemicals. The physical-chemical properties and catalytic performances of these efficient catalysts were also studied.

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