The entire project's duration was very long, and the students were under immense pressure to graduate, once breaking down in the office and crying. The first author of the paper, doctoral student Luo Jiazhu, withstood the pressure and tried a large number of activation methods.

In the last week of the paper revision deadline, the team obtained clean macrocycle crystals through the solvent exchange method, allowing the paper to be published smoothly, said Professor Tang Hao from South China University of Technology.

In the research, the team from South China University of Technology and their collaborators found that under ambient conditions, the gated macrocycle crystals exhibited a high propylene/propane kinetic selectivity of up to 76.7, capable of rapidly adsorbing propylene with a significantly higher adsorption rate than propane, reaching adsorption equilibrium in just 6.3 minutes.

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Also under ambient conditions, the gated macrocycle crystals only needed to meet the condition of dynamic vacuum to achieve complete regeneration, demonstrating a strong advantage in low-energy regeneration.

In multiple cycles of use, the gated macrocycle crystals also showed excellent stability and regenerative ability, with thermal stability and moisture resistance superior to many traditional adsorbent materials, maintaining stability even in high-temperature and high-humidity environments.Additionally, for other small molecule gases such as ethylene, ethane, methane, carbon dioxide, and nitrogen, gated macrocyclic crystals also exhibit certain repulsion, thus providing a unique advantage in the purification of propylene in multi-component gas mixtures.

Through this research, they have addressed the high energy consumption problem of traditional separation technologies, and by studying gated macrocyclic crystals as a new type of adsorbent material, they have paved a new path for efficient and energy-saving propylene separation, providing new ideas for achieving more environmentally friendly gas purification technology.

In summary, the research team has demonstrated that gated macrocyclic crystals can achieve significant propylene/propane separation under pressure swing adsorption operations, and this kinetic sieving is due to the subtle differences in guest transport.

At the same time, this separation method has a high propylene/propane kinetic selectivity, rapid propylene kinetics, and ultra-low propylene adsorption enthalpy, and can repel ethane, ethylene, methane, carbon dioxide, and nitrogen.Through experimental structural changes and simulation studies, the separation mechanism has been well explained. This indicates that the transient motion of the gated methoxy groups on the macrocycle can amplify the subtle differences in propylene/propane transmission.

If further cost reduction and performance improvement can be achieved, this method will replace the traditional low-temperature distillation method, and thus be used for industrial-scale propylene purification, promoting the development of the petrochemical industry.

In addition, this method may also inspire more research and development of gated molecular crystals and other new adsorbent materials, further expanding the application field of gas separation materials and promoting the development of material science.

Help separation technology and separation materials usher in new applications.

Propylene is an important basic chemical raw material, and its annual output has exceeded 100 million tons. However, in industrial production, people have always been unable to separate propylene and propane with low energy consumption and high efficiency.The traditional low-temperature distillation method is indeed effective, but the high energy consumption that comes with it is daunting. With the increasing severity of global energy crises and environmental pollution issues, there is an urgent need to develop more efficient, energy-saving, and environmentally friendly separation technologies.

Therefore, this study aims to address the separation challenge of propylene and propane. By improving selectivity and adsorption rate, a more efficient separation process is achieved, and the goal of reducing energy consumption is met.

In addition, as a new type of adsorbent material, gated macrocyclic crystals are also the subject of this research topic.

This material not only has a unique gating effect but can also capture gas molecules through weak interactions.

Therefore, the research team hopes to explore the performance of gated macrocyclic crystals to open up new application fields for separation technology and materials.

Recently, the related paper was published in Chem (IF 19.1) with the title "Kinetic sieving separation of a gating macrocyclic crystal for purification of propylene."The doctoral student from South China University of Technology, Jiazhu Luo, is the first author, while the doctoral student from University of Science and Technology of China, Guokun Yang, and the associate researcher, Guozhen Zhang, serve as co-first authors [1].

Professor Hao Tang from South China University of Technology and researcher Jing Xiao, as well as Professor Banglin Chen from Fujian Normal University, act as co-corresponding authors.

"As long as one is effective, we can continue to do it."

According to Tang Hao's recollection: "At the beginning of the research, I had a discussion with my colleague, Teacher Xiao Jing, about our respective professional directions. I am engaged in the research field of chemical supramolecular chemistry, while Teacher Xiao focuses on the field of chemical engineering adsorption and separation."

Supramolecular macrocyclic crystals are a hot research direction at present, and some scholars have already been studying the adsorption and separation of organic solvents.So they began to envision: could supramolecular macrocyclic crystals be used as adsorbent materials for gas separation?

Subsequently, they conceived some functional groups. For alkenes and alkanes, these functional groups exhibit certain binding differences. In addition, they also conceived the fitting cavity size.

Based on this, Tang Hao's research group designed, synthesized, and crystallized some samples, and then took them to Xiao Jing's team for testing. Regrettably, most of the functional groups that were originally thought to be effective did not work.

However, one macrocycle containing a naphthyl group was very effective, which means that the research can continue.

Therefore, they carried out a series of tests. Soon, Xiao Jing's team achieved a series of excellent results in adsorption and discovered many interesting pieces of information.For example, the isothermal adsorption curve of propylene and propane for the material demonstrates the characteristic of flexible adsorption. This implies that during the adsorption process, the crystalline material undergoes some structural changes.

Previously, scholars have reported that porous adaptive crystals undergo structural changes after adsorption. The changes in this crystal material seem to have similarities with it.

The results of the single-crystal experiments also seem to support the conclusion that "the crystal structure changes before and after the adsorption of gas": they found that the single crystal just crystallized out has obvious structural differences compared to the single crystal containing the adsorbed gas.

However, they soon found logical flaws in the experimental data: because the single crystal just crystallized out contains solvent molecules, it is necessary to remove these solvent molecules (i.e., "material activation") by high temperature and vacuum before the adsorption of gas.

The previous experimental data could not explain whether the crystal conformation change occurred during the activation process or during the gas adsorption process. Therefore, Tang Hao's students also made an activated single crystal for structural testing.This discovery reveals that for activated crystals and crystals adsorbed with gases, their unit cell structures are consistent and differ from the crystals that have just crystallized.

This indicates that there is a strong interaction between the solvent molecules and the macrocycles. That is, the adsorption enthalpy of the solvent molecules is sufficient to drive changes in the crystal structure.

The interaction between the gas molecules and the macrocycles is very weak, and thus insufficient to drive structural changes in the macrocycle crystals.

In fact, the extremely low adsorption enthalpy of the gas molecules also corroborates the above viewpoint. Then, how to explain the differential adsorption rates of propylene and propane becomes the next important task.The methoxy group is like a spring door, and the gas must push it open to pass through.

Tang Hao said: "The process of proposing the mechanism is a typical teacher-student collaborative process. I told the students, 'Observe along the molecular channel step by step and see if there are any groups that might block the channel.'"

For example, it's like a doctor observing the patient's intestines for polyps through a colonoscopy. Then, Luo Jiazhu quickly gave feedback, saying he saw a methoxy group in the channel.

Based on this, they proposed such a hypothesis: the methoxy group is like a spring door, and the gas must push this door open to pass through.

The molecule of propylene is relatively flat (slender), so it only needs to push the door open a little bit to get through; while the molecule of propane is relatively three-dimensional (plump), so it needs to push the door open a lot to get through.This hypothesis can explain why there is such a significant difference in the adsorption rates of two gas molecules. However, pushing the door is just an instantaneous process, so using conventional experimental methods, it is impossible to observe this transient change.

So they began to turn to molecular simulation. Tang Hao said, "This simulation is quite challenging because the construction of a channel requires several macrocyclic molecules, and each macrocyclic molecule contains many atoms."

So in the calculation, if you want to consider the positional changes of these atoms, it will put a lot of pressure on the computing power, and it is also very easy to drown out the signal changes when a single gas molecule passes through the channel.

"Fortunately, I have a younger colleague who is an expert in theoretical simulation, he is Dr. Zhang Guozhen, a research associate at the University of Science and Technology of China. He is very interested in this mechanism research, so we started to cooperate. After careful discussion, he suggested using molecular dynamics simulation, like taking continuous high-speed photos of complex molecules, to obtain the key transient structure and energy information of gas molecules migrating."

To obtain reasonable structural and energy information, they used enhanced sampling molecular dynamics simulation technology, and after several rounds of iteration and adjustment, they obtained the free energy changes and transient conformations of gas molecules passing through the gated switch.In the end, they obtained key evidence by the method of frame extraction. This demonstrates that when the gas passed through the methoxy group, the methoxy group indeed underwent significant positional and angular changes.

Moreover, as anticipated, propane does indeed need to push the gas molecules further apart in order to pass through.

After the second round of peer review, the Chem journal found a crystal expert to carefully examine the crystal data submitted by Tang Hao and his colleagues.

The expert believed that there were a small amount of residual solvents in the activated crystals, that is, on average, one solvent molecule in two macrocyclic cavities, and therefore considered that it was not truly activated. Later, the research group used various methods to activate the single crystals, thereby perfecting this study.

"As for the problem of analyzing single crystals with gas, our collaborator, Professor Chen Banglin, by combing the analysis of various gas-bearing crystals in the field of framework chemistry, elaborated on the current bottleneck in the analysis of gas-bearing crystals in the paper, thereby convincing the journal editors and reviewers, and ultimately allowing the paper to be published," said Tang Hao.In the next step, they plan to continue designing molecular structures to enhance the selectivity and adsorption rate of propylene/propane separation.

"Different macrocyclic molecules and gating groups may be explored to find combinations with superior performance, and the adsorption separation may also be extended to new systems," said Tang Hao.