Do you know the "holy grail reaction" in chemistry?

Produced by: Science Popularization China

Author: Denovo Team

Producer: China Science Expo

Speaking of natural gas, everyone must be familiar with it. Nowadays, every household cannot do without natural gas for cooking. The main component of natural gas is methane, which is the simplest hydrocarbon. As of 2023, my country's natural gas reserves will be 6.6 trillion cubic meters, accounting for 10% of the world's natural gas reserves.

Natural gas storage

(Photo source: veer photo gallery)

In the face of huge resource advantages, accelerating the development and utilization of methane is the key to achieving green and sustainable development of energy and chemical industry. In addition to being used directly as fuel, methane can also be used as a C1 resource, that is, a molecule containing one carbon atom and can be further converted, to prepare high value-added chemicals such as methanol and formic acid.

Remember when we wrote the chemical equation for the reaction of methane and oxygen in high school, we had to indicate the reaction condition as "combustion". Methane can burn in oxygen to produce water and carbon dioxide. If there is no combustion condition, can the activation and conversion of the carbon-hydrogen bonds of the methane molecule be achieved under mild conditions?

The answer is yes! This is the "holy grail reaction" in catalysis.

Reactions associated with the "holy grail" are usually extremely challenging. They may need to be carried out under very harsh conditions or need to overcome the inherent difficulties of chemical reactions, such as activation of highly stable compounds, low yields and low selectivity. These challenges make it difficult to achieve these reactions, but if they can be successfully achieved, it will bring major breakthroughs in scientific research and industrial applications.

The structure of the methane molecule

(Photo source: veer photo gallery)

Challenges of methane conversion at low temperatures

Why is it so difficult to convert methane into other useful chemicals directly using cheap oxygen at low temperatures or even at room temperature?

Let's look at the properties of methane and oxygen.

The chemical structure of methane contains four identical carbon-hydrogen bonds (CH), forming a highly symmetrical tetrahedral configuration. The bond energy of each CH3-H bond of methane is as high as 435 kJ/mol.

We can imagine the CH bond of methane as a particularly strong spring. This spring is very tight and requires a lot of force to stretch it. In chemistry, this "force" is the energy required to break the CH bond.

This high bond energy makes the CH bond of methane extremely stable in thermodynamics and extremely difficult to decompose or react under normal conditions . On the other hand, in chemical reactions, active groups are usually generated under the action of polarity (polarity refers to the phenomenon that one end of a molecule is positively charged and the other end is negatively charged), and the symmetrical structure and non-polar characteristics of methane molecules make it impossible to produce such polarity (according to the molecular configuration, molecules with symmetry planes have no polarity) and cannot provide active groups.

Therefore, the activation and conversion reaction of methane is extremely challenging and usually requires harsh conditions, such as high temperature (600-1100°C) or some "extreme molecules" such as superacids and free radicals to assist in methane activation.

Therefore, the main difficulty in achieving low-temperature activation of methane and oxygen lies in how to activate the CH bond of methane, that is, to stretch the "spring" in the CH bond.

The miracle of catalyst

Low temperature oxygen activation of methane

(Image source: Reference [3])

Scientists have come up with a good way to solve this problem. They chose to use catalysts to help activate methane at low temperatures (a catalyst is a chemical substance that does not change itself before and after the reaction, but can speed up the reaction by changing the minimum energy required to inject for the reaction to occur).

In 2023, the journal Nature Catalysis reported the use of a specific molybdenum disulfide (MoS2) catalyst at 25°C to directly convert methane and oxygen into C1 oxygenates (methanol (CH3OH), formic acid (HCOOH) and methylene glycol (HOCH2OH). Under normal temperature conditions, methane and oxygen were converted into precious C1 oxygenates, achieving a methane conversion rate of 4.2% and almost 100% C1 oxygenates.

This MoS2 is the only catalyst reported so far that can achieve room-temperature conversion of methane and oxygen.

All this is due to the unique geometry and electronic structure of the Mo site on the edge of MoS2. This Mo site has a high activation activity for oxygen in a water environment, forming a magical O=Mo=O* species. This species makes the carbon-hydrogen bond easier to break, reduces the activation energy of the CH bond of methane, and thus greatly improves the reactivity of methane, thereby achieving low-temperature activation of methane and oxygen.

This discovery will bring more possibilities for future energy utilization and environmental protection, and also give us a deeper understanding of the magical effect of catalysts.

The great strategic significance of low-temperature activation of methane

As a country rich in natural gas resources, my country has a large amount of methane reserves. Realizing the direct catalytic conversion of methane and oxygen at room temperature and converting methane in natural gas into other useful chemicals can greatly improve the utilization rate of natural gas and reduce waste. This will bring huge economic benefits to my country's energy industry, and at the same time better protect the environment and achieve sustainable development of energy.

Secondly, as a greenhouse gas, methane's "contribution" to global warming is second only to carbon dioxide. If methane can be converted into other substances, it can help us reduce greenhouse gas emissions and alleviate the pressure of global warming. This will make a huge contribution to my country's environmental protection cause and demonstrate my country's responsibility and commitment in addressing global climate change.

Based on the above discussion, the realization of direct catalytic conversion of methane and oxygen at room temperature will open an important prelude to changes in my country's energy and environmental fields, and will also bring immeasurable opportunities. By overcoming this technical difficulty, it will not only promote the optimization and greening of my country's energy structure, but also provide strong technical support for related industrial innovation, thereby accelerating my country's technological progress and leadership in the global energy and environmental protection fields.

Conclusion

On the stage of science and technology, the story of methane and oxygen continues. We believe that with the continuous progress of science, the output value of directional conversion of methane will continue to rise. Let us look forward to this beautiful future together and use the power of science to bring more surprises and convenience to our lives!

References

[1] Li, Z., Xiao, Y., Chowdhury, PR et al. Direct methane activation by atomically thin platinum nanolayers on two-dimensional metal carbides. Nat. Catal. 4, 882–891 (2021).

[2] Guo, X. et al. Direct, non-oxidative conversion of methane to ethylene, aromatics, and hydrogen. Science 344, 616–619 (2014).

[3] Mao, J., Liu, H., Cui, X. et al. Direct conversion of methane with O2 at room temperature over edge-rich MoS2. Nat. Catal. (2023).