"The whole project cycle is very long, and the students are under a lot of pressure to graduate, and they once broke down emotionally and cried in the office. In the first work of the dissertation, Luo Jiazhu, a doctoral student, withstood the pressure and tried a large number of activation methods.
In the last week of the revision deadline, the large cyclic crystals with clean channels were obtained through solvent exchange, so that the paper could be published smoothly. Professor Tang Hao of South China University of Technology said.
Figure | Tang Hao (source: data map)
In the study, the team and collaborators at South China University of Technology found that the gated macrocyclic crystal exhibited up to 76.7 propylene/propane kinetic selectivity at room temperature, and was able to adsorb propylene rapidly, with a significantly higher adsorption rate than propane, and it only took 6.3 minutes to reach adsorption equilibrium.
Also at room temperature, the gated large-ring crystal only needs to meet the condition of dynamic vacuum to achieve complete regeneration, showing the strong advantage of low-energy regeneration.
In multiple cycles, gated macrocyclic crystals also show excellent stability and regeneration ability, and their thermal stability and moisture resistance are better than those of many traditional adsorption materials, and can remain stable even in high temperature and high humidity environments.
In addition, gated macrocyclic crystals exhibit some repulsion for other small molecule gases such as ethylene, ethane, methane, carbon dioxide, and nitrogen, thus playing a unique advantage in propylene purification of multi-component gas mixtures.
Through this research, they solved the problem of high energy consumption of traditional separation technology, and created a new path of efficient and energy-saving propylene separation by studying the new adsorption material of gated macrocyclic crystal, which provided a new idea for the realization of more environmentally friendly gas purification technology.
(Source: Chem)
Overall, the group demonstrated that gated macrocyclic crystals can achieve significant propylene/propane separations under pressure swing adsorption operations, and this kinetic sieving is due to small differences in guest transport.
At the same time, this separation method has high propylene/propane kinetic selectivity, fast propylene kinetics, and ultra-low propylene adsorption enthalpy, and can repel ethane, ethylene, methane, carbon dioxide, and nitrogen.
Through experimental structural changes and experimental simulation studies, the separation mechanism was well explained. This indicates that the transient movement of gated methoxy groups on the large ring is able to amplify small differences in propylene/propane transport.
If it can further reduce costs and improve performance, this method will replace the traditional low-temperature distillation method and can be used for industrial-scale propylene purification, helping the petrochemical industry to grow.
In addition, this method has the potential to stimulate more research and development of gated molecular crystals and other novel adsorbent materials, further expand the application field of gas separation materials, and promote the development of materials science.
Enabling new applications for separation technology and separation materials
Propylene is an important basic chemical raw material, and its annual output has exceeded 100 million tons. However, in industrial production, it has never been possible to separate propylene and propane efficiently and with low energy consumption.
While traditional low-temperature distillation is effective, the high energy consumption that comes with it is prohibitive. With the global energy crisis and the increasingly serious problem of environmental pollution, there is an urgent need to develop more efficient, energy-saving and environmentally friendly separation technologies.
Therefore, this study aims to solve the problem of separation of propylene and propane. By increasing selectivity and adsorption rates, a more efficient separation process is achieved and energy consumption is reduced.
In addition, as a new type of adsorption material, gated macrocyclic crystals are also the research object of this project.
Not only does the material have a unique gating effect, but it can also trap gas molecules through weak interactions.
Therefore, the research group hopes to explore the performance of gated macrocyclic crystals and open up new application fields for separation technology and separation materials.
日前，相关论文以《用于丙烯纯化的门控大环晶体的动力学筛分》（Kinetic sieving separation of a gating macrocyclic crystal for purification of propylene）为题发在 Chem（IF 19.1）。
Figure | Related Papers (Source: Chem)
Jiazhu Luo, a Ph.D. student at South China University of Technology, is the first author, and Guokun Yang, a Ph.D. student at the University of Science and Technology of China, and Guozhen Zhang, associate researcher, co-authored the work [1].
Prof. Hao Tang and Prof. Jing Xiao from South China University of Technology, and Prof. Banglin Chen from Fujian Normal University served as co-corresponding authors.
"As long as one works, you can continue to do it"
According to Tang Hao's recollection: "At the beginning of the research, I chatted with my colleague Xiao Jing about each other's professional directions. I am engaged in the field of chemical supramolecules, while Professor Xiao focuses on the field of chemical adsorption and separation. ”
Supramolecular macrocyclic crystals are a popular research direction at present, and some scholars have been studying the adsorption and separation of organic solvents.
So they began to think about whether supramolecular macrocyclic crystals could be used as adsorption and separation materials for gases.
Subsequently, they envisaged a number of functional groups. For olefins and alkanes, there are certain binding differences between these functional groups. In addition, they envisaged the appropriate cavity size.
Based on this, Tang Hao's research group designed, synthesized and crystallized some samples, and brought them to Xiao Jing's team for testing. Regrettably, most of the functional groups that were thought to work did not work.
However, one of the large rings containing naphthalene groups is very effective, which means that it can be continued.
So, they carried out a series of tests. Soon, Xiao Jing's team achieved a series of excellent results on adsorption and found a lot of interesting information.
For example, the isothermal adsorption curve of the material for propylene reflects the characteristics of flexible adsorption. This means that during the adsorption process, some structural changes occur in the crystalline material.
Previously, it has been reported that non-porous adaptive crystals will undergo structural changes after adsorption. The change in the crystal material seems to be similar to it.
The results of the single crystal experiment also seem to support the conclusion that the crystal structure changes before and after the adsorption of gas: they found that compared with the single crystal containing the adsorbed gas, the newly crystallized single crystal has a significant single crystal structure difference.
However, they soon discovered that there was a logical loophole behind the experimental data: because the single crystals that had just crystallized contained solvent molecules, they needed to be extracted by high temperatures and vacuuming before adsorbing the gas (i.e., "material activation").
However, the previous experimental data do not indicate whether crystal allosterism occurs during activation or gas adsorption. As a result, Tang Hao's students did an activated single crystal to do structural testing.
It is found that for the activated crystal and the gas adsorbed crystal, their unit cell structure is the same, and there is a difference between the crystal that has just crystallized.
This shows that there is a strong interaction between the solvent molecules and macrocyclic cycles. That is, the adsorption enthalpy of the solvent molecules, so it is enough to drive the change in crystal structure.
The interaction between the gas molecules and the macrocyclic is weak, so it is not enough to push the structural change of the macrocyclic crystal.
In fact, the extremely low adsorption enthalpy of gas molecules also supports this view. So, how do you explain the adsorption rate of propylene variability? It becomes the next big thing.
(Source: Chem)
The methoxy is a spring-loaded door that the gas has to push open if it wants to pass through
Tang Hao said: "The process of proposing the mechanism is a typical teacher-student cooperation process. I said to the students, you step along the molecular channels and see if there are any groups that might be blocking the channels. ”
For example, it's like a doctor using a colonoscopy to see if a patient has polyps in their intestines. Then, Luo Jiazhu quickly gave feedback, and he saw that there was a methoxy group in the channel.
Based on this, they hypothesized that the methoxy group was like a spring door that the gas had to push open if it wanted to pass through.
The molecules of propylene are relatively flat (thin), so you only need to push the door open a small gap to get through; The molecules of propane, on the other hand, are more three-dimensional (plump), so you need to push the door wide open to get through.
This hypothesis could explain why there is such a big difference in the adsorption rate of the two gas molecules. However, pushing the door is only an instantaneous process, so with conventional experimental methods, this transient change cannot be observed at all.
So they started to move on to molecular simulation. "This simulation is challenging because the construction of a channel requires several macrocyclic molecules, and each macrocyclic molecule contains many atoms," Tang said. ”
Therefore, if you want to consider the position changes of these atoms in the calculation, it will put a lot of pressure on the computing power, and it is also easy to drown out the signal changes when a single gas molecule passes through the channel.
"Fortunately, I have a junior who specializes in theoretical simulation, and he is Zhang Guozhen, an associate researcher at the University of Science and Technology of China. He was interested in this kind of mechanism research, so we started working together. After careful discussion, he proposed to use molecular dynamics simulations to obtain the key transient structure of gas molecule migration and the energy information of the gate as if it were a continuous high-speed photograph of a complex molecule. Tang Hao said.
In order to obtain reasonable structural and energy information, they used the molecular dynamics simulation technique of enhanced sampling, 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 the key evidence by drawing frames. This shows that when the gas passes through the methoxy group, the methoxy group does undergo obvious position and angle changes.
And, as expected, propane does need to push the gas molecules wider to get through.
(Source: Chem)
After the second round of paper review, Chem magazine found a crystal expert to carefully examine the crystal data submitted by Tang Hao and others.
The other party believed that there was a small amount of residual solvent in the activated crystal, that is, there was one solvent molecule in the two macrocyclic cavities on average, so it was not really activated, and the research team later used various methods to activate the single crystal, so as to improve the study.
"As for the analysis of single crystals with gas, our collaborator Chen Banglin explained the current analytical bottleneck of crystals with gas by sorting out the analysis of various gas-bearing crystals in the field of frame chemistry, so as to convince journal editors and reviewers, and finally let the paper be published." Tang Hao said.
In the next step, they intend to continue to design the molecular structure to improve the selectivity and adsorption rate of the propylene/propane separation.
"It is possible to explore different macrocyclic molecules and gating groups for better combinations, and to consider extending the adsorption separation to new systems." Tang Hao said.
Resources:
1.Luo J, Yang G, Zhang G, et al. Kinetic sieving separation of a gating macrocyclic crystal for purification of propylene. Chem（2024）.https://doi.org/10.1016/j.chempr.2024.06.007
2.https://authors.elsevier.com/c/1jMQh8jWHELFtp
Operation/Typesetting: He Chenlong