Major breakthrough! Chinese scientists found a new way to turn coal into "water", creating a new milestone in the energy field

Energy plays a vital role in the development of human society.

Whether it is the mobile phones, electrical appliances, cars in our daily lives, or the aircraft engines and rocket thrusters in industrial production, they all rely on energy supply.

In my country, the main fossil energy source is still coal, accounting for nearly 70%, oil accounts for about 20%, and natural gas accounts for only 1% to 2%.

The current situation is that domestic demand for energy is greater than supply, especially the demand for liquid energy (such as olefins).

Olefins play an important role in many fields, including as fuel, chemical raw materials, fertilizer raw materials and chemical intermediates.

Therefore, developing new technologies for producing olefins is of great value to my country's economic and social development.

Scientists have raised an interesting question: Can we use our country's richer coal to produce liquid energy such as olefins?

Our country's scientific researchers have begun to put this idea into practice. They first convert coal into synthesis gas, and then further convert the synthesis gas into liquid energy such as gasoline, diesel and aromatics through the Fischer-Tropsch reaction.

3D model of ethylene, the simplest alkene. Source: Wikipedia

01

Fischer-Tropsch synthesis: the “magic kitchen” in the chemical industry

What is the “synthesis gas” we mentioned above?

In fact, it is a mixture of carbon monoxide (CO) and hydrogen (H2).

How to make this gas mixture?

Typically, we generate syngas by subjecting hydrocarbons such as coal, oil, and even biomass to chemical reactions such as partial oxidation and water-gas shift with oxidants (such as oxygen and water vapor).

This special gas has a wide range of applications in the chemical industry and is an important raw material in the process of synthesizing liquid fuels, including Fischer-Tropsch synthesis.

What is Fischer-Tropsch synthesis?

Fischer-Tropsch synthesis is a unique chemical process whose main purpose is to convert synthesis gas into liquid fuels and other valuable chemicals.

The process was first pioneered in the 1920s by two German chemists, Franz Fischer and Hans Tropsch.

We can illustrate Fischer-Tropsch synthesis with a more lifelike example: it can be imagined as a "magic kitchen".

In this kitchen, our synthesis gas is the "ingredients". After a series of chemical reactions (under the action of catalysts), we can make "delicious food" - liquid fuel.

This "magic kitchen" is able to transform simple ingredients into a variety of useful products.

For example, we can imagine carbon monoxide as a tomato and hydrogen as an egg.

Using different amounts of tomatoes and eggs and a series of different cooking methods, they can be turned into scrambled eggs with tomatoes, tomato and egg soup, or even tomato and egg pancakes.

Fischer-Tropsch synthesis plays an important role in energy diversification and efficient utilization of resources.

Especially in my country, where the resource situation is rich in coal, short of oil and little gas, this technology can convert our abundant local coal, biomass and other resources into liquid fuels, reduce our dependence on external oil, and further improve our energy security. Therefore, this reaction has very important strategic significance.

02

Catalyst: A seesaw that is hard to break through

Now that we understand the importance of the Fischer-Tropsch synthesis reaction, let’s take a look at the most important work done by scientists – improving catalysts.

A catalyst in a chemical reaction is a substance that acts as a promoter. It can speed up the reaction rate but does not participate in the reaction itself.

In the Fischer-Tropsch reaction, the catalyst type directly affects the type and distribution of products.

Not only the Fischer-Tropsch reaction, in fact, more than 85% of chemical reactions in the chemical industry rely on catalysts to increase the reaction rate.

When we are dealing with some complex reactions that can produce multiple products, we hope to obtain as many and pure target products as possible, but the activity and selectivity (selectivity represents the uniqueness of the product) of most catalyst systems will have a "seesaw effect".

At the two ends of the seesaw, one end is the activity of the reaction, and the other end is the selectivity of the reaction. If the activity increases, the selectivity will decrease, which will lead to a low yield of the target product.

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After nearly 90 years of development, the selectivity of light olefin products in the system of synthesizing light olefins from synthesis gas has been difficult to break through the theoretical limit (58%), and the catalytic system has a serious seesaw effect.

Therefore, how to break the limits and break the "seesaw" effect has always been a long-term concern of scientists in this field.

1. First-generation OXZEO catalyst: breaking the limit

How to break through the theoretical limits of low-carbon olefin product selectivity?

Academician Bao Xinhe and researcher Pan Xiulian's team from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences came up with an ingenious solution:

The active components in the catalyst are changed from traditional metals or metal carbides to a composite catalyst of metal oxides and molecular sieves - OXZEO.

The molecular sieve is a special zeolite with microscopically uniform pores and neatly arranged holes. It acts like a molecular-level sieve that can screen molecules of different sizes and shapes.

In the OXZEO system, carbon monoxide molecules are adsorbed onto the surface of metal oxides, and then the CO bonds are “cut” to form oxygen atoms and carbon atoms on the surface;

The hydrogen in the gas phase reacts with the surface carbon atoms to form a hydrocarbon intermediate, which then enters the pores of the molecular sieve that can "sieve molecules" and begins to grow in a chain of carbon atoms.

This process cleverly utilizes the restrictiveness of the molecular sieve pores. By adjusting the pore size of the molecular sieve, the type of reaction products can be precisely controlled, thus breaking the selectivity limit of synthesizing light olefins from synthesis gas.

The reaction process of syngas to olefins catalyzed by OXZEO

Source: Science Magazine, 2016

This breakthrough research result allows the selectivity of light olefins to reach as high as 80% when the carbon monoxide conversion rate reaches 17%, successfully breaking through the theoretical limit of 58%.

At the same time, this catalytic system abandons the traditional high water consumption and high energy consumption path, subverts the Fischer-Tropsch synthesis route invented by German scientists in the 1920s, and in principle creates a new way to convert coal into synthesis gas in one step with low water consumption (no water circulation in the reaction and no wastewater discharge).

The research results were published in the journal Science in 2016. By 2020, the team completed an industrial trial with an annual output of 1,000 tons of light-carbon olefins in the factory, confirming the feasibility of the process in scientific principles and practical processes, further promoting the development of light-carbon olefin product preparation technology, and providing more reliable and efficient technical means for the production of green energy and chemicals.

2. Next-generation OXZEO catalyst: Going beyond ourselves

The first generation of OXZEO catalysts broke the centuries-old theoretical limit of 58% for light olefins, but the conversion rate of the reactant carbon monoxide was only 17%.

In order to solve the seesaw problem of activity and selectivity in the synthesis gas to olefins reaction system, Academician Bao Xinhe's team continued to conduct in-depth research, striving to develop catalysts that can simultaneously improve activity and selectivity.

They found that the root cause of the limitations of the seesaw effect lies in the fact that current molecular sieves not only catalyze the main reaction (carbon-carbon "hand-in-hand" conversion to produce light olefins), but also simultaneously catalyze two side reactions (light olefins combine with other substances to produce low-value alkanes, and light olefin groups "hand in hand" to produce large molecular olefins).

This common active center is like the fulcrum of a "seesaw". Once the conversion rate increases, the selectivity will decrease accordingly, making it difficult to increase both the conversion rate and the selectivity at the same time, ultimately leading to a lower yield of light olefins.

To solve this problem, they optimized the first-generation OXZEO catalyst and prepared a microporous molecular sieve based on metal germanium ions (GeAPO-18) based on the original molecular sieve.

This new type of molecular sieve weakens the acidity, effectively inhibits the self-polymerization of low-carbon olefins to form large molecules, and the possibility of combining with other atoms, achieves the complete separation of active centers, and reduces the occurrence of side reactions.

This optimization is like transforming the original "seesaw" model with one fulcrum into two independent "wings", so that the two processes of the formation of the initial reaction intermediate and the subsequent chain growth of carbon atoms occur at independent sites, allowing the reaction to "fly freely".

Under optimized reaction conditions, this new catalyst achieves an astonishing single-pass carbon monoxide conversion rate of 85% while maintaining a light olefin selectivity of greater than 80% (up to 83%), and achieves a light olefin yield (yield refers to the ratio of actual output to theoretical output) of 48%, an internationally optimal level, more than double that of the first-generation OXZEO catalyst.

This major breakthrough was published online in the journal Science on the 19th of this month.

Activity-selectivity trade-off in OXZEO syngas to light olefins process

Source: Science Magazine, 2023

Academician Bao Xinhe and his team successfully expanded the design thinking of OXZEO catalysts and initially built an innovative technology platform for direct conversion of coal to synthesis gas.

They have achieved the directional synthesis of a series of high-value chemicals and fuels, leading the new development direction of water-saving, energy-saving and efficient coal chemical industry.

This groundbreaking achievement completely overturned the Fischer-Tropsch route that the coal chemical industry has adhered to for more than 90 years, and successfully solved the "seesaw" problem of simultaneously improving activity and selectivity in traditional catalytic reactions.

This reaction process will not only significantly reduce the water and energy consumption of coal chemical industry, but also is praised by the industry as a "milestone breakthrough" in the field of coal conversion.

Conclusion

This new process will undoubtedly have a profound impact on the development and application of coal and natural gas in the chemical industry.

It has opened a new chapter in coal chemical industry and promoted the development of the entire field towards a more efficient, green and sustainable direction.

Chinese scientists are on the road to bring new vitality to the energy sector.

(Original title: Important breakthrough in the development of coal-to-olefin catalysts! In addition to direct burning, coal has greater uses)

Produced by | Science Popularization China

Author｜Denovo Popular Science Writer

Producer｜China Science Expo

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