Weekly Achievement Award丨Nano-confined catalysis—great achievements in a very small space

The use of raw materials such as coal and petroleum coke to prepare synthesis gas (a mixture of carbon monoxide and hydrogen) and the use of synthesis gas to prepare hydrocarbon products with different numbers of carbon atoms has become an important chemical industry in modern industry. Substances with different numbers of carbon atoms have their own uses, and "low-carbon olefins" with 2-4 carbon atoms are widely used.

Compared with "high-carbon olefins", low-carbon olefins can be transported to subsequent production as high-value chemical raw materials, and used to manufacture various chemicals, drugs, plastics and other materials to serve human needs. The current mainstream method for preparing hydrocarbon products from synthesis gas is the Fischer-Tropsch process. This method, invented in 1925, can convert CO and H2 into hydrocarbons under catalysts and appropriate conditions, but is subject to the theoretical limit that the proportion of its C2-C4 hydrocarbons cannot exceed 56.7%. Nearly half of the remaining products are methane CH4, which is only worth burning, and other high-carbon olefins. In practice, the actual C2-C4 yield of Fischer-Tropsch synthesis will be even lower.

The achievements recognized by the 2020 National Natural Science First Prize, the new synthesis strategy guided by "nano-confined catalysis" proposed by Academician Bao Xinhe's team, achieved a "win-win" situation of high activity and high selectivity.

What is "nano-confined catalysis"? The popular understanding is to do extremely complex things in an extremely small space. For chemists, a space of 2 to 3 cm is already too large, and the nanoscale is often the place where they work meticulously. Although we often hear the concept of nano, such as its definition: 1 nanometer is 10-9 meters, it is still difficult to have an intuitive understanding of nano. Refer to a simple analogy: if we magnify "1 meter" to 5,200 kilometers, then "1 nanometer" will be proportionally magnified to 10-9 times of 5,200 kilometers - that is, 5.2 millimeters, which is about half the width of an adult's little finger.

“Confinement” is screening at the nanoscale

Controlling chemical reactions at the nanoscale can often achieve magical results. This is because the size of the atoms and molecules that make up our world is exactly nanoscale: for example, a water molecule is about 0.4 nanometers in size, and the distance between the carbon atom and the hydrogen atom on the same side in an ethylene molecule is only 0.25 nanometers; and as the number of carbon atoms increases, the molecular size will increase to more than a dozen or dozens of nanometers. This gives us the opportunity to increase the yield of "low-carbon olefins": if some nano-sized pores (carbon nanotubes) are made as reaction sites for the conversion of synthesis gas into olefins, if the size of these pores is very small (a few nanometers), so that only olefins with a small number of carbon atoms can exist and pass through the pores, and high-carbon olefins are not allowed to be generated in the pores, the theoretical upper limit of Fischer-Tropsch synthesis can be broken. This method of screening the generated products by regulating the nano-size of the pores is called nano "pore confinement".

Highly efficient catalytic system in nanotubes

When scientists place catalysts in these nano-sized channels, the activity of the catalyst itself is enhanced: just as people can unleash greater potential under certain pressures, in extremely narrow spaces, the environment around the catalyst changes its electronic configuration and orbital properties, thereby enhancing its catalytic efficiency and selectivity.

Academician Bao Xinhe's team successfully constructed a series of complex catalytic systems in carbon nanotubes for the efficient and precise synthesis of syngas. It is even more difficult to accurately synthesize catalytic systems, evaluate catalytic performance, and discover the scientific truth behind them at the nanoscale. But in front of excellent chemists, there are always more solutions than difficulties. More than 20 years of hard work have successfully proposed the concept of "nano-confined catalysis" and applied it in industrial demonstration, successfully realizing the highly selective synthesis of low-carbon olefins. For my country, which is short of oil and has a lot of coal, this is tantamount to adding a more efficient way to obtain chemical raw materials from coal, and opening a door to understanding the catalytic process and precisely controlling chemical reactions. In the future, this technology will continue to play its role and create greater value.

(Text: Li Cunpu, Professor of the School of Chemistry and Chemical Engineering of Chongqing University; Reviewer: Huang Xun, Associate Professor of the School of Chemistry and Chemical Engineering of Chongqing University)

China Association for Science and Technology Department of Science Popularization

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