Recently, Dr. Shi Run, a research associate at the Institute of Physical and Chemical Technology of the Chinese Academy of Sciences, and his team have pioneered a new low-carbon olefin preparation route under mild conditions, revealing the photocatalytic ethane oxidation dehydrogenation reaction mechanism, providing new theoretical basis for the efficient and green photocatalytic conversion and utilization of shale gas.

The production of ethylene is one of the important contents of the current petrochemical industry, which is also an important production activity related to the development of the country. At present, the production of ethylene still adopts the route of thermal catalytic naphtha cracking and partial ethane dehydrogenation.

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Therefore, in a few years, they hope to see the direct use of sunlight to drive the dehydrogenation of ethane to prepare ethylene, thereby helping to completely change the global energy pattern.

Why is it so difficult to reduce the temperature to 500°C?Ethylene is an important basic chemical raw material, mainly used for the production of ethylene oxide, polyethylene, polyvinyl chloride, dichloroethane, ethylene glycol, and vinyl acetate, among other basic organic intermediates. These raw materials can be further used to synthesize high-value chemical products such as synthetic rubber, fibers, plastics, and explosives.

Globally, more than 75% of petrochemical products and over 40% of organic chemical products are synthesized from ethylene as raw material. Therefore, the production of ethylene is a sign of the development level of a country's petrochemical industry.

In recent years, due to the rapid development of downstream products of ethylene in China, the demand for ethylene has been continuously increasing. According to statistics, in 2023, China's ethylene production capacity is about 50 million tons, surpassing the United States to become the world's largest ethylene producer.

At present, the ethylene industry is highly dependent on the petroleum steam cracking technology route. However, this process has high energy consumption, large carbon emissions, and is greatly affected by the situation of fossil resource endowment and international oil prices. Therefore, exploring a more efficient and greener ethylene production method has both academic and practical value.

Since the 21st century, with the increasing depletion of petroleum resources, countries have successively entered the "post-oil era." Shale gas is an unconventional natural gas that exists in organic-rich shale and its interlayers, with its main components being low-carbon alkanes such as methane and ethane. Currently, the global proven reserves exceed 1000 trillion cubic meters.According to the "BP World Energy Outlook 2018," China's shale gas extraction rate will grow rapidly between 2016 and 2040, and by 2040, it will become the world's second-largest shale gas producer, following the United States.

How to efficiently utilize the abundant low-carbon alkanes in shale gas has become a hot topic in the field of catalysis in recent years. Among them, the dehydrogenation of ethane to produce ethylene is a non-petroleum route that is low-cost, highly atom-economic, and environmentally friendly, and has attracted much attention.

In 2020, the ethylene output from the dehydrogenation of ethane and petroleum cracking has basically been on par. At present, the proportion of ethylene produced from ethane as raw material in North America has reached 52%, and in the Middle East, it has reached 67%.

The ethylene preparation route that fully utilizes non-petroleum resources such as shale gas has been widely recognized internationally, and its competitiveness is developing rapidly year by year.

Ethane catalytic dehydrogenation process for ethylene production: It is mainly divided into direct dehydrogenation of ethane and oxidative dehydrogenation of ethane. Among them, the direct dehydrogenation process of ethane is more mature, but this route needs to be carried out at higher temperatures (usually above 750°C). Compared with the traditional petroleum cracking route, the energy consumption and carbon emissions of this route cannot be significantly reduced.Ethane oxidative dehydrogenation refers to the introduction of oxidants (oxygen, hydrogen peroxide, etc.) into the ethane dehydrogenation reaction system, thereby establishing a new thermodynamic equilibrium, which theoretically can achieve complete conversion of ethane.

The ethane oxidative dehydrogenation process can compensate for the thermodynamic equilibrium limitations of direct ethane dehydrogenation processes and alleviate issues such as catalyst deactivation. It offers advantages in terms of equipment investment and operating costs, thus attracting widespread attention in the academic community.

However, existing research on ethane oxidative dehydrogenation is still based on the traditional thermal catalysis theory system. Even with researchers devoting a great deal of effort to the exploration of catalysts and mechanism studies, the reaction temperature for ethane oxidative dehydrogenation remains above 500°C.

Do not exaggerate for the sake of "shock".Shi Run and others, based on the research background of the team, have turned their attention to clean and sustainable low-temperature photocatalytic processes.

In fact, the initial concept of this project originated from a study on photocatalytic methane oxidation coupling published by the team in 2023 [1].

Due to the lack of systematic literature reports on photocatalytic ethane oxidative dehydrogenation, many times they can only propose some new ideas and hypotheses based on existing knowledge accumulation, design experiments, carefully verify, and analyze the obtained experimental data to ensure that every sentence and viewpoint reported is scientific.

In the study, by screening a series of oxide semiconductor light-harvesting units and transition metal catalytic units, they prepared a ZnO photocatalyst loaded with PdZn intermetallic compounds.Through this, they achieved for the first time the photocatalytic oxidative dehydrogenation of ethane to ethylene, with an ethylene formation rate of 46.4 mmol/g-1h-1 at a reaction temperature of 140°C, and an ethylene selectivity of 92.6%, which outperforms the high-temperature thermal catalytic oxidative dehydrogenation of ethane as reported in existing literature.

Simultaneously, this catalyst exhibited excellent photocatalytic oxidative dehydrogenation performance for propane and butane, and under the simulated reaction atmosphere of shale gas, it achieved a 20% ethane conversion rate and an 87% ethylene selectivity.

In-situ electron paramagnetic resonance spectroscopy, in-situ Fourier transform infrared spectroscopy, and online mass spectrometry isotope analysis revealed that the active lattice oxygen sites generated on ZnO under light irradiation can efficiently activate the inert C-H bonds of ethane.

Subsequently, the surface lattice oxygen and hydrogen atoms are removed to form oxygen vacancies, which are transferred to the photogenerated electrons on PdZn to activate oxygen and replenish the lattice oxygen, thus forming a light-enhanced lattice oxygen oxidation mechanism.

Compared with Pd nanoparticles, PdZn, due to its strong electronic interaction with the ZnO carrier, can promote the cycle of lattice oxygen removal and replenishment.It is also reported that at the beginning of guiding students to carry out this topic, Shi Run was not sure in which direction this research would develop later, and the thinking still continued the reports related to direct dehydrogenation of ethane.

In the early stage, they even observed that the photocatalytic direct dehydrogenation route of ethane had an extremely excellent reaction efficiency. However, based on existing research experience, after various investigations, the result was finally determined to be a false positive.

Due to the insufficient airtightness of the reaction device, a small amount of oxygen (air) penetrated during the reaction process, and what actually happened was the oxidative dehydrogenation of ethane. If there is no rigorous argument and a realistic attitude, and this result is always mistakenly considered as "direct dehydrogenation of ethane," the consequences will be unimaginable.

During the process of writing the paper, they had an in-depth discussion on whether to emphasize the reaction temperature in the paper.

It is generally believed that the advantage of photocatalysis is the mild reaction conditions and green sustainability, but the heat dissipation process after the material absorbs light is almost inevitable, and the temperature on the surface of the catalyst can often reach several hundred degrees Celsius under illumination.Shi Run said: "Some research groups deliberately conceal the actual reaction temperature for the so-called 'innovation', claiming it to be 'room temperature photocatalysis', which is very unrigorous."

In his and his team's work, the light source they used was an LED (Light-emitting diode, light-emitting diode) light source with very weak thermal effects, and the actual reaction temperature was around 140°C.

Despite this, they did not conceal this point, but instead conducted in-depth research on the role of temperature in catalytic reactions. Even if it cannot be called the so-called "room temperature photocatalysis", they ensured the rigor of the research results.

Finally, the research group achieved ethane oxidation dehydrogenation performance comparable to traditional thermal catalytic reactions at 500°C to 600°C under such a lower reaction temperature, which was also highly recognized by peer review experts.

"So, I think the most unforgettable thing in this research is to always ensure the authenticity and rigor of scientific research results, and not to over-package and exaggerate the research results for the so-called 'shocking' effect," said Shi Run.Ultimately, the related paper was published under the title "Photocatalytic ethylene production by oxidative dehydrogenation of ethane with dioxygen on ZnO-supported PdZn intermetallic nanoparticles" in Nature Communications[2].

Wang Pu from the Institute of Physical and Chemical Technology of the Chinese Academy of Sciences is the first author, and Associate Researcher Shi Run and Researcher Zhang Tie Rui serve as co-corresponding authors.

Subsequently, they will carry out further fine design and control of the catalyst active sites, and conduct scaling-up experiments of catalytic reactions, hoping to make new progress in the study of catalytic reaction mechanisms and applications.

Shi Run concluded by saying: "Scientific research is interesting but serious. It is interesting because we are always on the road to explore the unknown, just like the popular blind box in recent years; it is serious because I believe that most natural scientific research actually has only one answer. Even if we propose different views on the same research, it is just from different perspectives."

Like the blind men and the elephant, no matter what story is told in the end, the laws behind things are objectively existent. Therefore, he believes that researchers must conduct useful and truthful research, so as to illuminate the path for those who come after, rather than leading others astray.