From: http://www.flotu.tsinghua.edu.cn/info/1137/2953.htm

Recently, Wei Fei, Department of Chemical Engineering, Tsinghua University-Chen Xiao's team has made important progress in the field of in situ study of molecular adsorption and diffusion mechanisms at the sub-nanometer scale. The team used the picmi electron microscope in-situ imaging strategy to realize in-situ real-time observation of the adsorption and desorption behavior of small molecules in molecular sieves and the dynamic evolution of molecular sieve framework structure. For the first time, the team discovered the subunit topological flexible structure characteristics of rigid molecular sieves, revealed the microscopic mechanism of molecular diffusion breaking through the pore size limit, and enriched the understanding of molecular sieve shape selective catalysis and confinement effect. The work in the latest issueSciencePublished (Fig.1）。

Figure1. The current Science reports the first discovery of molecular sieve subcell topological flexibility.

Zeolite molecular sieve is a kind of rigid framework material with ordered microporous structure, which is widely used in petrochemical industry, coal chemical industry, carbon dioxide conversion, environmental treatment and gas separation and other fields. Its use of three-dimensional nano-scale channels (0.3~1.3 nm) sieves molecules of different sizes, and precisely limits the molecular motion and reaction behavior at the microscopic level, thus realizing the highly selective preparation of high value-added products. For example, by using the pore limiting effect of ZSM-5 molecular sieves, low-carbon olefins and single-ring aromatics can be obtained with high selectivity, and the formation of larger double-ring aromatics, coke and other by-products can be suppressed. This is of great significance for carbon emission reduction in chemical processes under the dual carbon target. However, in the process of practical application, it is found that the maximum molecular diameter that can be diffused or generated in the pores is often larger than the crystallographic pore size of about 0.7 angstroms, which breaks through the rigid geometric limitation of molecular sieve pores. This phenomenon has not been reasonably explained in the academic circles for a long time.

In the past six decades, due to the limitations of traditional diffraction methods and the complexity of the zeolite framework, it is difficult to observe the dynamic behavior of molecules in and out of the channels at the sub-nanometer scale, especially the phenomenon of large molecules in and out of small rigid geometric channels has been unexplained. Based on this, Wei Fei-Chen Xiao's team combined integral differential phase contrast scanning transmission electron microscopy (iDPC-STEM) with simultaneous imaging capability of light and heavy elements with in-situ atmosphere system (In-situ Atmosphere System) to build a pici in-situ electron microscope catalytic platform to observe the movement behavior of confined organic molecules and the local structure of molecular sieves in real time, at the same time, the microscopic mechanism of the adsorption, diffusion and reaction process of organic small molecules on molecular sieves is analyzed in depth (Figure 2).

Figure2. Pici in situ electron microscope observation of local severe deformation of molecular sieve pores

Wei Fei's team uses surface atomic level levelingThe dynamic adsorption and desorption behavior of benzene molecules in the ZSM-5 and the evolution of the corresponding molecular sieve pore structure were studied by ZSM-5 the transparent straight pore of the molecular sieve as the imaging window and the benzene molecule as the probe. It was found that after entering the pore, benzene molecules were aligned in a consistent orientation under the effect of spatial confinement, presenting a one-dimensional solid-like form of existence, and showed a spindle shape in the projection direction of the pore, not the symmetrical sphere assumed by molecular dynamics. For the first time, it has been observed that molecular sieve exhibits a subsingle-cell topological flexible structure of "both rigid and flexible" under the action of confined benzene molecules-its single channel can undergo a maximum deformation of 15% along the long axis of confined benzene molecules to allow benzene molecules (0.585 nm) larger than the crystallographic aperture (0.56 nm) to enter and exit. At the same time, the unique Pnma space group symmetry of MFI structure makes adjacent channels deform in opposite directions, this counteracts each other so that the structure of the entire unit cell remains rigid, and the unit cell size (~ 2 nm) changes by less than 0.5 (Figure 3). Among them, the degree of pore deformation is determined by the number of benzene molecules in the confinement domain, and the deformation is reversible in the cyclic adsorption and desorption experiment. AB initio molecular dynamics simulation (AIMD) calculations show that under the action of temperature, the molecular sieve lattice atoms undergo non-stop thermal vibration, resulting in orifice breathing effect-empty nano-channels continuously undergo flexible vibration, and the long and short axes of the pore size change alternately with time. At a certain moment, the pore channel can undergo 15% deformation along the long axis, allowing benzene molecules to enter the pore channel with the smallest cross-sectional orientation. After the benzene molecules enter, the pore is restricted by the benzene molecules to maintain the deformation state; after the benzene molecules exit from the orifice, the local flexible vibration characteristics are restored. This "rigid and flexible" phenomenon comes from the flexible connection between silicon oxygen or aluminum oxygen tetrahedra in the molecular sieve topology-the T-O-T bond angle at the tetrahedron connection can be increased from 135 ° to 153 °. Wei Fei's team refers to this phenomenon of overall rigid subcell local flexibility as subcell topological flexibility.

Figure3. The symmetry of space group brings the structural characteristics of rigid and flexible molecular sieves

This work points out that the subunit topological flexibility is the inherent structural characteristics of the interaction between porous zeolite molecular sieve materials and molecules, which solves the long-standing controversy about the diffusion of macromolecules through the pore size limit into the small pore channel, enriches the understanding of the catalytic mechanism of molecular sieve, and provides a new mechanism and experimental analysis method for the design and synthesis of new molecular sieve materials. At the same time, this work realizes the direct observation of molecular dynamic behavior and real-time evolution of skeleton structure at sub-angstrom resolution in real space, provides a research paradigm for directly observing and studying the real process of adsorption, diffusion and catalytic reaction of small molecules in confined space at the molecular scale, and establishes a solid foundation for the future in situ real-time study of material diffusion and transformation process.

The above research results"In situ real-time imaging of subcellular topological flexibility in the process of adsorption and desorption of rigid molecular sieves" (In situ imaging of the sorption-induced subcell topological flexibility of a rigid zeolite framework) was published online on April 29 in the international academic journal Science (Science). At the same time, Professor Zou Xiaodong of Stockholm University was invited to write an opinion article entitled "Adsorbent with Flexible Nanopores" (An adsorbent with flexible nanoscopic pores), highly appraising the work of Wei Fei's team and pointing out that "direct observation of the subtle interaction between zeolite host and guest molecules can better understand the complex micro-mechanism behind adsorption-desorption macro-behavior, this will help in the design and selection of adsorbents for specific applications."

The first author of the paper is Tsinghua University.Xiong Hao, a 2019-level doctoral student, and Dr. Liu Zhiqiang, Institute of Precision Measurement Science and Technology Innovation, Chinese Academy of Sciences, are co-first authors. The corresponding authors of the paper are Professor Wei Fei, Dr. Chen Xiao and Assistant Researcher Zhang Chenxi from the Department of Chemical Engineering of Tsinghua University. Other authors are Professor Qian Weizhong of Tsinghua University, 2018-level doctoral student Wang Huiqiu and researcher Zheng Anmin of the Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences.

Thesis Link:https://www.science.org/doi/10.1126/science.abn7667