From: CC Speech Public Welfare May 2023

Chen Xiao Department of Chemical Engineering, Tsinghua University

Video website:https://tv.sohu.com/v/dXMvMjc0MzMzOTAyLzQ0Mzc2OTU1Ni5zaHRtbA==.html

I'm Chen Xiao from the Department of Chemical Engineering, Tsinghua University. I think everyone must have seen and used this sieve as shown in the figure. It is used for sifting flour. What I have in my hand is also a sieve, but it is a sieve used to screen molecules that are invisible to our naked eyes. It is called molecular sieve.

In fact, molecular sieves can be found everywhere in our lives. There are sieves like the flour sifted in my hand, sieves eves come in a variety of colors. In our life, molecular sieve affects us everywhere. For example, in the shape of round columns, and sieves in the shape of bricks. You must have seen it before. Besides different shapes, molecular siit can separate oxygen from our air at a very low price and supply it to patients in hospitals to breathe. At the same time, it can be made into different particles to filter sediment and purify water from magnesium and calcium, thus softening our water quality. In addition, it is a very good food additive.

In fact, molecular sieves are more widely used in our industry. As shown in the picture on the left, the molecular sieve inside the device can get the gasoline we need for our cars through catalytic cracking. You may not know that 70% of the gasoline comes from the catalytic cracking of our molecular sieve, and our molecular sieve is also involved in 80% of the catalytic reaction.

In fact, our molecular sieve also plays an irreplaceable role in other fields. For example, we show the capture of carbon dioxide, the separation of carbon dioxide and the conversion of biomass, and then the treatment of automobile exhaust. Therefore, molecular sieve it in all aspects of our lives to build our green low-carbon beautiful harmonious home.

In 1930, we humans first analyzed the crystal structure of molecular sieves, and then we kept synthesizing our artificial molecular sieves. By the 1950s to the 1960s, mankind has actually been able to find many commercially significant molecular sieves, and new molecular sieves and their new structures are constantly appearing. The emergence of new molecular sieves and their new structures actually solved a huge problem in our industry, which was the difficulty of separating molecules like benzene and toluene, which are very similar in structure.

molecular sieveIn 1930, our human beings first resolved its crystal structure, and then human beings non-stop synthesis of our artificial molecular sieve. By the 1950 s to the 1960 s, in fact, mankind has been able to find a lot of commercially significant molecular sieves, and new molecular sieves and their new structures are constantly emerging. Its advent is actually a solution to our industrial benzene and toluene this structure is very similar molecules difficult to separate such a huge problem.

Just now I talked about the application of molecular sieve is very extensive, and its origin is also very long. Is it a kind of structure? In fact, the structure of molecular sieve is like our Lego, it has the primary building blocks of small squares, through our human creative hands and can build out very different animals, castles, all kinds. Same with molecular sieves. In fact, the primary structure is a silicon-oxygen tetrahedral structure composed of aluminum, silicon and phosphorus elements.-rigid silicon-oxygen tetrahedron. Then through the oxygen and oxygen between the atomic bridge connection, so that it can form a ring, prism, all kinds of topology. As shown on the left, scientists synthesized beta molecular sieve, as well as ZSM-5 molecular sieve with MFI structure. The pore size of these molecular sieves is different. They all have a common feature, that is, they have an open cage-like framework structure, and specific molecules will be able to be confined in their cages. So far, in fact, the number of topological structure types certified by professional molecular sieve institutions has reached 251, of which more than 40 kinds are natural topological structures, and the remaining 210 kinds are molecular sieves synthesized by us.

Just talked about the molecular sieve in the pore can be adsorption diffusion and transfer. How do molecules operate in the pore? In the past, we used to tell you that molecules came out of the pore by adsorption, desorption or thermal weightlessness. With the advancement of technology,X-ray diffraction can tell us the type of atomic structure of molecular sieves. However, it is still impossible to explore what kind of form the molecules look like inside the pores of the molecular sieve. However, our scientists have built such a model based on these macroscopic experimental data. It can be seen from this model that molecules enter and leave our pores very freely. When they encounter active sites at the same time, these molecules will carry out rich catalytic reactions in the pores, such as hydrogenation reaction and disproportionation reaction. And water molecules will run out of our channels. These are some of the results that scientists tell us through macroscopic experiments.

Is there a possibility that we can see these molecules running out of the pore? The answer is yes, of course. This requires the help of a special microscope to see the beating process of molecules. The course of the microscope is also a long process. If the advent of the optical microscope to solve our understanding of the microscopic world. If it is the first leap, then the advent of the electron microscope is actually a leap into the second. Electron microscope uses electrons as the transmission medium. As we all know, the transmission wavelength of electrons is much shorter than that of our visible light, so its resolution ability is much better than that of our microscope, so we can unlock bacteria, cells,This kind of structure of DNA. With the advancement of our technology, the advent of spherical aberration correctors, we can further see these microscopic atomic structures. In addition, we can add some external environment such as light, electricity, force, gas and liquid to see a process of change in the structure of these atoms under such stimulation. That is, the advent of our spherical aberration corrector has brought our modern electron microscopy technology into a pico era. What concept? Equivalent to a resolution level where we can see one millionth of our hair cast.

Just now we talked about our molecular sieve, which is a porous material, so that molecules can enter and exit. However, there is a problem in our electron microscope. When our high-energy electron beam and our porous material work together, it will inevitably cause some damage to our material. Therefore, the picture we just got is an amorphous structure just like the bottom left picture. This is not its real structure, in fact it is because our electron beam has broken its structure.

With the advancement of our technology, what has been developed is called integral differential phase contrast imaging, which is the one I wrote in the figure.iDPC-STEM technology can well solve the problem that our porous materials are easily damaged in the electron beam. In Senior high school, we all studied the periodic table of elements, and the elements on the periodic table can be shown very clearly in this diagram. So we used this technology to first study another porous material that is very similar in structure to our molecular sieves, which is called a metal organic framework. Why should we use it to verify the feasibility of our method in the first place? Because its structure contains heavy metal elements such as iron and chromium, it may be more clearly presented. Sure enough, we got one of the highest resolution images in the world right now. From this picture, we can see that its characteristic pore structure is a cage structure of 29 angstroms and 34 angstroms, respectively. This is due to its very high resolution, which is currently the highest level of resolution in the world. So, we will use this technology in the molecular sieve that we are more interested in.

Just talked about molecules being able to get in and out of our pores. What exactly does the molecule look like in the pores of our molecular sieves? Before we were all macroscopic experiments, or we imagined that it might be a form of free rotation similar to Brownian motion. We don't know what it's like to see, we don't know what it's like. So, we designed a very clever experiment, which actually originated from our ancient compass. The compass can clearly indicate our south and north. If we put a molecule that matches our pore very well into our pore, it is possible that it can indicate a change in stress around us. Our molecules go in and out of the tunnel because we have an invisible force field here, so its molecules can stay in it.

Why did you choose the p-xylene molecule? Because the p-xylene molecule and the size of our pore are very matched. It has two opposing methyl groups, and the opposing methyl groups are able to maintain its rotating state standing in our channel. Sure enough, our molecules in the pores are really completely different from what we thought before. It has only four very comfortable states to stay here, and these four states can fully reflect a change in a force field in its pore, and the molecule keeps rotating in these four directions, Even ran out of the pore. This is like putting four magnets in the four directions of our compass, and the molecules can only rotate back and forth in these four directions.

Just now I said that the molecule is static and it rotates in the channel. Is there a possibility that we can observe such a process of molecular sieve? What is the form of molecular entrance and exit channels? Of course, it is possible. We transfer the gas for this industrial application through the control cabinet into our electron microscope. Let's look at the changes caused by our molecules entering our pores in this electron microscope. This is amazing. It's not like what we thought. It goes in and out or spins freely. It's not like this. The first thing that catches the eye is a circular channel of its very round ten-membered ring, as shown on the right. When our molecule goes in, it will support this holeA huge deformation of 15%. This was completely unimaginable before and completely subverted our cognition.

Why? Because in our impression, molecular sieve is a rigid framework type. Just now we talked about a silicon-oxygen tetrahedron, which is a ten-membered ring connected by oxygen bridge. But this experiment tells us that it did happen.15% of the huge deformation, but on the whole we have not seen any macro deformation. Through our continuous experiments, we can see that the original molecule will cause a huge deformation after entering the pore. That, in turn, as the molecules slowly come out of our pore, the pore slowly returns to its circular state. In other words, the structure of the pore determines a dominant conformation in which our molecule enters, and it can be seen that it has these conformational relationships. At the same time, the guest molecule will affect the structure of our pore after entering, that is to say, the interaction between the guest molecule and the skeleton is the behavior of the interaction between the subject and the object.

Scientists always like to ask why? When we do research, we also like to ask why it can undergo such a large deformation. From the video on the left, we can see that the silicon-oxygen tetrahedron, which was originally thought to be very rigid, has actually occurred at the position of the oxygen bridge it connects.A huge tensile deformation of 135 degrees to 153 degrees. This is the problem! The original molecular sieve in the overall maintenance of rigidity at the same time, in fact, it has a local nearly ten degrees of tension.

We said that molecular sieves, like Lego, can come up with different topologies. In fact, it can not only build out different topological structures, but also build out the structure of these molecular sieves in different forms. As shown in these three pictures, the left picture shows this mortise and tenon joint structure inspired by Chinese mortise and tenon joint structure. And in order to reduce the transfer and diffusion of our molecules in the pore, and designed this sandwich structure and multi-level pore structure. These three structures have actually shown very excellent performance, we can not help but ask its microstructure is the same? Sure enough, completely different. Our mortise and tenon structure is very transparent, but there will be orderly oxygen vacancies at its interface. And this orderly creation of oxygen vacancies causes an enhancement of the lewy acids in our skeleton. This Lewy acid is actually an acidic site for one of its reactions. And its enhancement will further promote a conversion and selectivity of our syngas to olefin products.

The sandwich structure and multi-level pore structure just mentioned can also be seen, the original sandwich structure has undergone obvious phase separation. For example, among us it isAEI-that is, different topologies. The topology of the AEI and the topology of the CHA are different connections that make up our sandwich structure. And this multi-stage pore structure can be seen that it does not occur phase separation, but the two phases are very uniformly mixed together. And it is precisely because of its microstructure differences, resulting in a huge difference in our macro performance.

I just talked about the acidic site. Is there a possibility for this acidic site? Can we see it directly through the image? That requires us to design it more finely. We first useHeating at 200°C for one hour removes one of the effects of the van der Waals force field just mentioned, leaving only the hydrogen bond effect. This hydrogen bond actually corresponds to our acidic site, that is, its reactive center. After we first put the pyridine molecule in it, we did see this very clear benzene ring molecule that looks exactly like our textbook. This is also the first time we have used our transmission electron microscope to observe small molecules. Then you can see that the six atoms in the six-membered ring actually grow like this, and the nitrogen in these six atoms can completely correspond to our acid sites one by one. We later found through statistics that this acidic site is actually different from the random distribution we thought before. It will have a probability distribution. In other words, this molecule also has its specific reaction hobby, not that I can catalyze any arbitrary site.

Further, just now we can see the acid sites. So how can we better design molecular sieve catalysts under this blessing? So we chose this oxide and our molecular sieve coupling to carry out our catalytic reaction. Sure enough, we actually reached1+1>2 effect. That is, after we couple the oxide with the molecular sieve, we can use carbon dioxide, which we think is not very favorable and not very friendly, through our coupled composite catalyst, to produce the aviation kerosene needed by our aircraft in one step. In other words, we have not only solved our environmental problems, but also prepared the energy materials we need.

In fact, I said so many interesting things just now, which actually passed through us.Five years, even the accumulation of our previous seniors for a long time. So doing scientific research is actually boring, but it's actually very interesting. Because through these interesting results, we can make our due contribution to our scientific research or to our country.

In fact, our direction was very, very unpopular before, and not many people did it. But through our research, we slowly make the direction that we seem colder slowly heat up. It also allows the researchers and scientists in our small field to develop in a direction that we think is very interesting, constantly chasing exploration. We also hope to use our hands and eyes to tell you what molecular sieve looks like and how it works. We also hope that we can design better catalytic effects and better catalysts. We also hope that our research results can better serve the society. Thank you all!