Zeolite zeolite hydrogen overflow effect improves CO2 utilization rate of methane dry reforming reaction

On November 15, 2022, Beijing time, the team of Professor Xiao Fengshu and Wang Liang of Zhejiang University published an article entitled “Enhanced CO2 utilization in dry reforming of methane achieved through nickel-mediated hydrogen spillover in zeolite” in the journal Nature Catalysis Crystals”. By encapsulating Ni metal nanoparticles inside ZSM-5 zeolite molecular sieve, the team enhanced the hydrogen overflow effect in the reaction system and improved the carbon dioxide utilization rate of methane dry reforming reaction under CO2-rich conditions.

The corresponding authors of the paper are Xiao Fengshu and Wang Liang (Zhejiang University); The co-first authors are Zhu Qiuyan, Zhou Hang, and Wang Liang (Jilin University).

The South China Sea is one of the four major offshore oil and gas gathering centers in the world, with abundant natural gas, oil and other resources, and their efficient use is of great significance to China’s energy structure adjustment and national energy security. Conventional natural gas is mainly composed of methane, which also contains a certain amount of carbon dioxide. In the conventional use of natural gas, gases such as carbon dioxide need to be separated so that the efficient use of natural gas can be achieved. However, in addition to low-carbon alkanes such as methane, natural gas in the South China Sea also contains a large amount of CO2, and some natural gas contains more than 50% CO2 and some even as high as 75%. In the traditional utilization process, CO2 needs to be separated and discharged to obtain a high content of methane. Taking the Dongfang 1-1 gas field in the Yinggehai Basin of China’s South China Sea as an example, it emits about 700,000 tons of CO2 per year. Therefore, it is of great significance to develop a new path for the direct high-value utilization of CO2 in carbon-rich natural gas. At present, the most common path for converting methane from carbon-rich natural gas to carbon dioxide is the traditional dry reforming reaction, in which methane can be converted into the same molar amount of carbon dioxide. For carbon-rich natural gas with a finite amount of methane, it is expected that limited methane can be used to reduce more CO2, but the counter-water gas reaction is limited by thermodynamic equilibrium and cannot occur directly (Figure 1).

Figure 1: Thermodynamic analysis of methane dry reforming reactions.

Professor Xiao Fengshui and researcher Wang Liang of Zhejiang University cooperated with Academician He Mingyuan of East China Normal University and Chief Engineer Wu Qing of China National Offshore Oil Corporation to use nickel nanoparticles as the active center of catalytic materials based on the zeolite zeolite encapsulation metal strategy, encapsulate them inside the silicon-aluminum MFI zeolite crystal (Ni@HZSM-5, Figure 2), and further regulate the pore environment of zeolite zeolite molecular sieve. Thus, the hydrogen overflow effect of microporous zeolite is enhanced to maintain the presence of high-density active hydrogen species around nickel nanoparticles, which will facilitate the reduction of more CO2 to CO and inhibit unnecessary water gas shift side reactions. In a CO2-rich atmosphere, the methane on the catalytic material can reduce CO2 by 2.9 (methane reduces CO2 2.9 times the equivalent), which is better than the most advanced super-DRM process reported so far. The CO yield on this catalytic material can reach 3.9 molCO molCH4-1, and it can carry out long-term continuous reaction, which has certain advantages over the super-DRM process developed by predecessors. It is expected that the direct conversion and utilization of CO2-rich natural gas will be realized (Figure 3).

Figure 2: Model and structure characterization of Ni@HZSM-5 catalytic materials.

Figure 3: Test of methane dry reforming reaction performance of Ni-based catalytic materials under CO2-rich conditions.

This study reveals the key to achieving very high methane reduction in methane dry reforming reactions – improving the hydrogen overflow capacity of dry reforming catalysts. Hydrogen overflow effect is a commonly used strategy in the design of hydrogenation reaction catalysts, and the hydrogen species overflowing from the surface of metal nanoparticles have high activity, which is often used in hydrogenation reactions to improve activity and regulatory selectivity. By encapsulating Ni nanoparticles inside ZSM-5 zeolite and further regulating the microporous environment of ZSM-5 zeolite, hydrogen overflow in the reaction can be enhanced, thereby improving the methane reduction of the reaction. The reason why CO2 on Ni@HZSM-5 can continue to react with product H2 is probably by accelerating the reaction and transfer of hydrogen (i.e., hydrogen overflow effect), so that the reaction equilibrium around the active site shifts, so that more CO2 is further hydrogenated and reduced to CO.

In order to study the role of proton hydrogen in Ni@HZSM-5 in the methane dry reforming reaction, it is necessary to further explore the diffusion and conversion of hydrogen during the reaction. Both tungsten oxide discoloration experiments and semi-reaction experiments proved that there was hydrogen overflow in the reaction, and Ni@HZSM-5 had significantly stronger hydrogen overflow effect than Ni@KZSM-5 and Ni/HZSM-5. Generally, the overflowed hydrogen species have higher activity, which is conducive to CO2 reduction reaction, which should also be the key factor that Ni@HZSM-5 has stronger reducing ability than Ni@KZSM-5 and Ni/HZSM-5 in dry reforming reaction.

In this study, the hydrogen overflow effect is applied to the dry reforming reaction system under CO2-rich conditions, which maximizes the utilization of CO2 and provides a new idea for controlling CO2 emission and carbon resource conversion. This work was supported by the National Key Research and Development Program of China (2021YFA1500404) and the National Natural Science Foundation of China (U21B20101 and 21932006). (Source: Science Network)

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