Technical analysis and research progress of CO2 hydrogenation to low-carbon olefins
At present, the hydrogenation of carbon dioxide to produce low-carbon olefins can be roughly divided into two reaction paths: Fischer-Tropsch synthesis path and methanol path. One is the reaction path with carbon monoxide as the intermediate product, that is, carbon dioxide hydrogenation first produces carbon monoxide by reverse water gas shift reaction (RWGS), and then produces low-carbon olefin (FTO) by Fischer-Tropsch synthesis reaction; The second is the reaction path with methanol as the intermediate product, that is, the hydrogenation of carbon dioxide generates methanol, which is then catalytically cracked into light olefins (MTO).
1 Fischer-Tropsch route to prepare low-carbon olefin catalyst
Carbon dioxide hydrogenation generates low-carbon olefins through Fischer-Tropsch synthesis route, and there are two main reaction processes: the first is direct reaction of carbon dioxide hydrogenation to prepare low-carbon olefins; The second one is that carbon dioxide hydrogenation firstly generates carbon monoxide through reverse water gas reaction (RWGS), and then carbon monoxide is hydrogenated through Fischer-Tropsch reaction to generate low-carbon olefins.
The second reaction process is called CO2-FTS reaction, which requires the use of double active center composite catalyst, and the catalyst has high activity for both RWGS reaction and Fischer-Tropsch chain growth reaction. Compared with the hydrogenation of carbon monoxide, the hydrogenation degree of the intermediate adsorbed on the catalyst surface is higher in the CO2-FTS reaction process, which leads to the slow adsorption speed of carbon dioxide, which in turn reduces the conversion rate of carbon dioxide, and it is easier to form a larger proportion of methane in the product.
Therefore, the main challenge of catalyst development is that the catalyst not only has high activity for RWGS reaction and FTS reaction, but also has high selectivity for value-added hydrocarbon products such as short-chain olefins and long-chain hydrocarbons.
At present, researchers have learned that Co, Ru, Ni, Fe and other components have high activity in synthesizing low-carbon olefins. The conventional Fischer-Tropsch reaction generally adopts Fe-based or Co-based catalysts, which are suitable for the hydrogenation of carbon dioxide to prepare high value-added hydrocarbon products. Fe-based and Co-based catalysts show high conversion of carbon dioxide and high selectivity of C2+ hydrocarbons in the process of carbon dioxide hydrogenation, so these catalysts are widely used in CO2-FTS reaction.
Co-based catalysts have good carbon chain growth reaction activity, and the product distribution is mainly long-chain saturated alkanes. Moreover, it has weak water gas reaction (WGS) and reverse water gas reaction (RWGS), but it is sensitive to reaction conditions, such as C/H ratio and reaction temperature ratio in raw materials.
Although Co-based catalysts have high conversion rate in carbon dioxide hydrogenation, the products are mainly methane with low added value, which is serious and the catalytic effect is not ideal. Ni-based catalysts have strong hydrogenation ability, so most of the research on Ni catalysts is about methanation of carbon dioxide, and the methane selectivity is as high as 100%. Song et al. studied the methanation of carbon dioxide over highly dispersed Ni/La2O3 catalyst. The results show that when the reaction temperature is from 208℃ to 380℃, with the increase of reaction temperature, the conversion of carbon dioxide gradually increases, and the methane selectivity reaches 100%. Ru catalyst is characterized by excellent catalytic performance. Marcin studied the thermodynamics of carbon dioxide hydrogenation on Ru/Al2O3 catalyst. The results showed that Ru had a statistically optimal distribution on Al2O3, and the activation energy reached the lowest. The products of Ru-based catalyst for carbon dioxide hydrogenation mainly concentrate on formic acid. Fe-based catalyst has a strong inhibition ability on the reaction product (methane) of carbon dioxide hydrogenation to prepare low-carbon olefins, and its advantages include: low price of catalyst, and good effect on reverse water vapor shift reaction and Fischer-Tropsch synthesis reaction. It has been widely used in the research of preparing low-carbon olefins by hydrogenation of carbon dioxide.
Compared with the above Co-based catalysts, Ni-based catalysts and Ru-based catalysts, Fe-based catalysts are considered to be the most suitable catalysts for olefin production, and the application prospects of Fe-based catalysts are more extensive.
The liquid fuels and chemicals produced by Fischer-Tropsch synthesis contain almost no toxic impurities such as sulfur. Although the traditional Fischer-Tropsch synthesis reaction route can effectively hydroconvert carbon dioxide, the product carbon chain is long and the composition is complex, and the product distribution is limited by the Anderson-Schlulz Flory(ASF) distribution law, so it is difficult to achieve high selectivity for a single target product. Fe-based catalyst will lead to the product conforming to ASF distribution, which is not conducive to the centralized preparation of a single product.
2 Catalyst for preparing low-carbon olefins by methanol route
Different from Fischer-Tropsch reaction route, methanol route is a brand-new catalytic reaction route, which can bypass Fischer-Tropsch synthesis reaction without being limited by ASF model, and the products are mainly concentrated between C2-C4 low-carbon olefins, which greatly improves the selectivity of low-carbon olefins.
The reaction route of methanol can be summarized into two main reaction steps, namely, the hydrogenation and dehydration of carbon dioxide to produce methanol, and then the reaction route of methanol to olefins (MTO). Using methanol as an intermediate can theoretically break through the theoretical limitation of ASF distribution on low-carbon olefins in Fischer-Tropsch synthesis, thus improving the selectivity of target products.
The catalyst for preparing low-carbon olefins by methanol route is usually a bifunctional catalyst composed of metal mixed oxides and zeolite. At present, ZSM-5 and SAPO-34 molecular sieve catalysts are the most widely studied and widely used in industry. ZSM-5 molecular sieve has unique MFI structure and high Brønsted acid potential density, which can not limit the production of long-chain hydrocarbons and aromatics. At the same time, due to the small molecular size of methanol, in the pore structure of ZSM-5 molecular sieve, methanol molecules diffuse rapidly and form coking on the external surface of the catalyst, thus affecting the acid strength of Brønsted acid. ZSM-5 molecular sieve has a controllable silicon-aluminum ratio in a wide range, and it is easy to be modified. The modified ZSM-5 molecular sieve has a low Si/Al ratio, which is beneficial to the improvement of propylene selectivity in the product.
SAPO-34 molecular sieve belongs to CHA configuration, and its unique pores can avoid the formation of macromolecular products, thus increasing the selectivity of low-carbon olefins. It has been widely studied in the reaction of methanol to low-carbon olefins and showed good catalytic activity. SAPO-34 molecular sieve shows high selectivity for low-carbon olefins and good thermal stability, which makes molecular sieve gradually become the focus of research on hydrogenation of carbon dioxide to low-carbon olefins.
Metal oxides such as In2O3-ZrO2, ZnO-ZrO2, ZnGa2O4, ZnAl2O4, etc. are used in the methanol production catalyzed by carbon dioxide, showing good reaction activity and sintering resistance at high temperature, and SAPO-34 molecular sieve is compounded in the MTO reaction to form a bifunctional composite catalyst.
In recent years, scholars at home and abroad have successfully applied oxide-molecular sieve combination to catalytic conversion of carbon dioxide to olefins or gasoline, and made breakthrough progress.
In 2017, Li Can et al. of Chinese Academy of Sciences compounded ZnO-ZrO2 oxide with SAPO-34 molecular sieve. The conversion rate of carbon dioxide was 12.6% and the selectivity of low-carbon olefins was 80%. In 2018, Gao Peng et al. compounded the In2O3-ZrO2 oxide with SAPO-34 molecular sieve. The conversion rate of carbon dioxide was 35.5%, and the selectivity of low-carbon olefins was 76.5%. However, by-product carbon monoxide was generated with high selectivity. In 2020, Wang Ye and others compounded spinel oxide with molecular sieve, and successfully converted carbon dioxide/carbon monoxide to produce low-carbon olefins.
Metal molecular sieve bifunctional catalysts are more and more in line with the requirements of methanol reaction route to prepare low-carbon olefins, and have attracted more and more attention. Dipping, precipitation and hydrothermal synthesis can be used to prepare bifunctional catalysts. Because of different methods, their advantages and disadvantages are also different. It is of great significance to improve the catalytic performance by constantly improving the preparation methods of catalysts.
Source：https://mp.weixin.qq.com/s/uI9Tt994K9vQ1i28IRxk7A