Publications

2023

1. 

   Heavy oil to chemicals: multi-stage downer catalytic pyrolysis

   Xiong, Hao*, Liang, Xiaoyu, Zhang, Chenxi, Bai, Haolong, Fan, Xiaoyu,  and Wei, Fei

   CIESC Journal 2023

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   High yield of petrochemicals, such as light olefins and aromatics, is an important way for low-carbon utilization of petroleum. Based on the energy structure and technical advantages in the field of catalytic cracking in China, the development of heavy oil to directly produce chemicals (light olefins and aromatics) by catalytic cracking in zeolite catalysis under high severity gains higher profitability than the corresponding refinery with conventional FCC. It poses great challenges to the design of multi-phase reactors. Counter-current interaction between feed-oil and deep-cracking catalysts under high severity should be achieved to prevent preferential adsorption of heavy aromatics, reduce diffusion resistance in zeolitic nanopores, and boost deep cracking of short-chain alkanes. Millisecond residence time and plug-flow residence time distribution should be obtained to inhibit secondary reactions and reduce dry gas and coke production. This review focuses on the progress of discrete transport, reaction, and deactivation processes in the catalytic cracking of heavy oil directly to chemicals, as well as the progress of gas-solid heterogeneous reaction engineering and other related fields. The latest research work on the discrete adsorption-diffusion phenomenon and host-guest interactions of small molecules in zeolites is introduced, and it is proposed to use facet-selective nanosized zeolites with high resistance to coke formation and gas-solid countercurrent contact plug flow reactors to overcome the above challenges. Finally, the multi-stage downer catalytic pyrolysis (MDCPTM) developed by Tsinghua University was introduced, which uses multi-stage downers in series while achieving both millisecond-level plug-flow within one stage and counter-current gas solids interaction between each stage. In the 1 kg/h pilot plant, MDCPTM can gain excellent production distribution (51.54%(mass) yield of light olefins and 80.78%(mass) selectivity of BTEX in gasoline). With MDCPTM as the core unit, the process, i.e., heavy oil to chemicals, HOTC, is reported where the yield of the total chemicals can be up to 75%(mass), with carbon emission reduced by more than 70% compared with existing technologies.

2022

1. 

   Highly Selective Conversion of CO2 or CO into Precursors for Kerosene-Based Aviation Fuel via an Aldol–Aromatic Mechanism

   Arslan, Muhammad Tahir*, Tian, Guo*, Ali, Babar, Zhang, Chenxi,  Xiong, Hao, Li, Zhengwen, Luo, Liqiang, Chen, Xiao,  and Wei, Fei

   ACS Catal. 2022

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   The extensive emission of CO2 into the atmosphere due to the burning of fossil fuels has urged many countries to move toward decarbonized energy. Based on the concept of carbon neutrality, the European Union Emissions Trading System implemented the aviation carbon tax (ACT), which stimulated the rapid transformation in aviation fuels. To cope with the ACT and mitigate the CO2 emissions from the aviation industry, here, we present a promising route for the synthesis of precursors such as aromatics, alkyl benzenes, and naphthenes, which account for 40–50 vol % fraction of the kerosene-based aviation fuel, from waste CO2 or biomass-derived syngas. The ultra-high selectivity of single-ring aromatics (precursors for the kerosene-based aviation fuel) with a selective range of carbon chain numbers (C8–C12) was achieved (>80% in hydrocarbons at a reaction temperature of 275 \,^∘C) via the catalytic hydrogenation of CO2 or CO over a bifunctional catalyst (nano-sized ZnCr2O4/Sbx-H-ZSM-5). In situ diffuse reflectance infrared Fourier transform spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, gas chromatography–mass spectrometry, and reaction analysis revealed that the product distribution was controlled by thermodynamics and the formation of the aromatics followed an “aldol–aromatic”mechanism. The high selectivity of aromatics at the low reaction temperature (i.e., 275 \,^∘C) was attributed to the catalyst topology, the closed carbon chain aromatization reaction, and the presence of highly active oxygenated species in the hydrocarbon pool. For the first time, an asymmetric desorption behavior between the active oxygenate species and aromatic hydrocarbons over the ZnCr2O4/Sbx-H-ZSM-5 catalyst is reported with the help of integrated differential phase contrast scanning transmission electron microscopy, which made the reaction highly selective toward aromatics. This strategy gives a carbon-neutral route for the synthesis of kerosene-based aviation fuel precursors and will reduce the burden of the carbon tax on the aviation industry.

2. 

   Atomic imaging of zeolite-confined single molecules by electron microscopy

   Shen, Boyuan*, Wang, Huiqiu*, Xiong, Hao*, Chen, Xiao, Bosch, Eric G. T., Lazić, Ivan, Qian, Weizhong,  and Wei, Fei

   Nature 2022

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   Single-molecule imaging with atomic resolution is a notable method to study various molecular behaviours and interactions1–5. Although low-dose electron microscopy has been proved effective in observing small molecules6–13, it has not yet helped us achieve an atomic understanding of the basic physics and chemistry of single molecules in porous materials, such as zeolites14–16. The configurations of small molecules interacting with acid sites determine the wide applications of zeolites in catalysis, adsorption, gas separation and energy storage17–21. Here we report the atomic imaging of single pyridine and thiophene confined in the channel of zeolite ZSM-5 (ref. 22). On the basis of integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM)23–25, we directly observe the adsorption and desorption behaviours of pyridines in ZSM-5 under the in situ atmosphere. The adsorption configuration of single pyridine is atomically resolved and the S atoms in thiophenes are located after comparing imaging results with calculations. The strong interactions between molecules and acid sites can be visually studied in real-space images. This work provides a general strategy to directly observe these molecular structures and interactions in both the static image and the in situ experiment, expanding the applications of electron microscopy to the further study of various single-molecule behaviours with high resolution.

3. 

   Modulating inherent lewis acidity at the intergrowth interface of mortise-tenon zeolite catalyst

   Wang, Huiqiu*, Shen, Boyuan*, Chen, Xiao,  Xiong, Hao, Wang, Hongmei, Song, Wenlong, Cui, Chaojie, Wei, Fei,  and Qian, Weizhong

   Nat. Commun. 2022

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   The acid sites of zeolite are important local structures to control the products in the chemical conversion. However, it remains a great challenge to precisely design the structures of acid sites, since there are still lack the controllable methods to generate and identify them with a high resolution. Here, we use the lattice mismatch of the intergrown zeolite to enrich the inherent Lewis acid sites (LASs) at the interface of a mortise-tenon ZSM-5 catalyst (ZSM-5-MT) with a 90\,^∘intergrowth structure. ZSM-5-MT is formed by two perpendicular blocks that are atomically resolved by integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). It can be revealed by various methods that novel framework-associated Al (AlFR) LASs are generated in ZSM-5-MT. Combining the iDPC-STEM results with other characterizations, we demonstrate that the partial missing of O atoms at interfaces results in the formation of inherent AlFR LASs in ZSM-5-MT. As a result, the ZSM-5-MT catalyst shows a higher selectivity of propylene and butene than the single-crystal ZSM-5 in the steady conversion of methanol. These results provide an efficient strategy to design the Lewis acidity in zeolite catalysts for tailored functions via interface engineering.

4. 

   In situ imaging of the sorption-induced subcell topological flexibility of a rigid zeolite framework

   Xiong, Hao*, Liu, Zhiqiang*, Chen, Xiao, Wang, Huiqiu, Qian, Weizhong, Zhang, Chenxi, Zheng, Anmin,  and Wei, Fei

   Science 2022

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   The crystallographic pore sizes of zeolites are substantially smaller than those inferred from catalytic transformation and molecular sieving capabilities, which reflects flexible variation in zeolite opening pores. Using in situ electron microscopy, we imaged the straight channels of ZSM-5 zeolite with benzene as a probe molecule and observed subcell flexibility of the framework. The opening pores stretched along the longest direction of confined benzene molecules with a maximum aspect change of 15%, and the Pnma space group symmetry of the MFI framework caused adjacent channels to deform. This compensation maintained the stability and rigidity of the overall unit cell within 0.5% deformation. The subcell flexibility originates mainly from the topologically soft silicon-oxygen-silicon hinges between rigid tetrahedral SiO4 units, with inner angles varying from 135\,^∘to 153\,^∘, as confirmed by ab initio molecular dynamics simulations. The pores of zeolites allow for selective adsorption of molecules based on their size and shape. The pore sizes that are calculated based on their structure are smaller than what is expected from their absorption and chemical reaction properties, which implies that the frameworks must have inherent flexibility. Xiong et al. used environmental transmission electron microscopy to image the straight channels of ZSM-5 zeolite with adsorbed benzene (see the Perspective by Willhammar and Zou). The pores stretched along the longest direction of the confined benzene molecules by up to 15%. This large change was compensated for by adjacent channels so that the overall deformation of the original unit cells was less than 0.5%. ?PDS Transmission electron microscopy reveals that benzene stretches channels of ZSM-5 zeolite and deforms empty adjacent channels.

2021

1. 

   Resolving atomic SAPO-34/18 intergrowth architectures for methanol conversion by identifying light atoms and bonds

   Shen, Boyuan, Chen, Xiao, Fan, Xiaoyu,  Xiong, Hao, Wang, Huiqiu, Qian, Weizhong, Wang, Yao,  and Wei, Fei

   Nat. Commun. 2021

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   The micro-structures of catalyst materials basically affect their macro-architectures and catalytic performances. Atomically resolving the micro-structures of zeolite catalysts, which have been widely used in the methanol conversion, will bring us a deeper insight into their structure-property correlations. However, it is still challenging for the atomic imaging of silicoaluminophosphate zeolites by electron microscopy due to the limits of their electron beam sensitivity. Here, we achieve the real-space imaging of the atomic lattices in SAPO-34 and SAPO-18 zeolites, including the Al–O–P atoms and bonds, by the integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). The spatial distribution of SAPO-34 and SAPO-18 domains in SAPO-34/18 intergrowths can be clearly resolved. By changing the Si contents and templates in feed, we obtain two SAPO-34/18 catalysts, hierarchical and sandwich catalysts, with highly-mixed and separated SAPO-34 and SAPO-18 lattices respectively. The reduced diffusion distances of inside products greatly improve the catalytic performances of two catalysts in methanol conversion. Based on the observed distributions of lattices and elements in these catalysts, we can have a preliminary understanding on the correlation between the synthesis conditions and structures of SAPO-34/18 intergrowth catalysts to further modify their performances based on unique architectures.

2. 

   A single-molecule van der Waals compass

   Shen, Boyuan, Chen, Xiao, Wang, Huiqiu,  Xiong, Hao, Bosch, Eric G. T., Lazić, Ivan, Cai, Dali, Qian, Weizhong, Jin, Shifeng, Liu, Xin, Han, Yu,  and Wei, Fei

   Nature 2021

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   Single-molecule imaging is challenging but highly beneficial for investigating intermolecular interactions at the molecular level1–6. Van der Waals interactions at the sub-nanometre scale strongly influence various molecular behaviours under confinement conditions7–11. Inspired by the traditional compass12, here we use a para-xylene molecule as a rotating pointer to detect the host–guest van der Waals interactions in the straight channel of the MFI-type zeolite framework. We use integrated differential phase contrast scanning transmission electron microscopy13–15 to achieve real-space imaging of a single para-xylene molecule in each channel. A good correlation between the orientation of the single-molecule pointer and the atomic structure of the channel is established by combining the results of calculations and imaging studies. The orientations of para-xylene help us to identify changes in the van der Waals interactions, which are related to the channel geometry in both spatial and temporal dimensions. This work not only provides a visible and sensitive means to investigate host–guest van der Waals interactions in porous materials at the molecular level, but also encourages the further study of other single-molecule behaviours using electron microscopy techniques.

2020

1. 

   From nanoscale discrete diffusion behavior control to macroscale coal chemical process

   Cai, Dali,  Xiong, Hao, Zhang, Chenxi,  and Wei, Fei

   CIESC Journal 2020

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   化学工业基于对化学反应和化学键的操纵,在横跨超12个数量级的空间和时间尺度上解决传递问题,为人类社会创造和生产新物质。石油的发现和利用将化学工业带入了一个全新的时代,支撑了现代社会的运行,但对中国而言,"富煤贫油少气"的资源特点决定了发展煤化工在中国有独特的战略意义。与石化行业常见的烃类裂化过程不同,现代新型煤化工依赖于小分子原料的自组装,依靠择形分子筛的控制作用高选择性制备燃料和化学品。因此,在亚纳米尺度下,即小分子在择形分子筛上的反应和传递行为不能再被视为连续,而是离散的,这导致了宏观上的拟相变失活等一系列现象。因此,需建立新的方法论理解分子筛上的离散反应和传递行为,用以认识离散尺度在十几个数量级空间和时间尺度延展后对宏观化学过程的影响。本文以纳尺度下分子筛内的离散传递现象为基础,综述了运用图论、小世界网络和先进表征手段分析分子筛内失活与传递现象的研究及发现的一系列新现象,并介绍了基于此进行的分子筛原子级精准结构调变及其对宏观煤化工过程带来的革新。面向未来,本文建立的分析分子筛内离散行为的范式将对精确调变分子筛结构、开发下一代煤化工工艺流程提供全新的思路。

2. 

   Imaging the node-linker coordination in the bulk and local structures of metal-organic frameworks

   Shen, Boyuan, Chen, Xiao, Shen, Kui,  Xiong, Hao,  and Wei, Fei

   Nat. Commun. 2020

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   Porous metal-organic frameworks (MOFs) have shown wide applications in catalysis, gas storage and separation due to their highly tunable porosity, connectivity and local structures. However, the electron-beam sensitivity of MOFs makes it difficult to achieve the atomic imaging of their bulk and local structures under (scanning) transmission electron microscopy ((S)TEM) to study their structure-property relations. Here, we report the low-dose imaging of a beam-sensitive MOF, MIL-101, under a Cs-corrected STEM based on the integrated differential phase contrast (iDPC) technique. The images resolve the coordination of Cr nodes and organic linkers inside the frameworks with an information transfer of ~1.8Å. The local structures in MIL-101 are also revealed under iDPC-STEM, including the surfaces, interfaces and defects. These results provide an extensible method to image various beam-sensitive materials with ultrahigh resolution, and unravel the whole framework architectures for further defect and surface engineering of MOFs towards tailored functions.

2019

1. 

   Transport Phenomena in Zeolites in View of Graph Theory and Pseudo-Phase Transition

   Cai, Dali,  Xiong, Hao, Zhang, Chenxi,  and Wei, Fei

   Small 2019

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   Transport phenomena play an essential role in catalysis. While zeolite catalysis is widely applied in industrial chemical processes, its efficiency is often limited by the transport rate in the micropores of the zeolite. Experimental and theoretical methods are useful for understanding the transport phenomena on multiscale levels. Traditional diffusion models usually use a linear driving force and an isotropic continuum medium, such that transport in a hierarchical catalyst structure and the occurrence of nonlinear deactivation cannot be well understood. Due to the presence of spatial confinement and an ordered structure, some aspects of the transport in a zeolite cannot be regarded as continuum phenomena and discrete models are being developed to explain these. Graph theory and small-world networks are powerful tools that have allowed pseudo-phase transition phenomena and other nontrivial relationships to be clearly revealed. Discrete models that include graph theory can build a bridge between microscopic quantum physics and macroscopic catalyst engineering in both the space and time scales. For a fuller understanding of transport phenomena in diverse fields, several theoretical methods need to be combined for a comprehensive multiscale analysis.

2. 

   Atomic Spatial and Temporal Imaging of Local Structures and Light Elements inside Zeolite Frameworks

   Shen, Boyuan, Chen, Xiao, Cai, Dali,  Xiong, Hao, Liu, Xin, Meng, Changgong, Han, Yu,  and Wei, Fei

   Adv. Mater. 2019

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   Identifying the atomic structures of porous materials in spatial and temporal dimensions by (scanning) transmission electron microscope ((S)TEM) is significant for their wide applications in catalysis, separation and energy storage. However, the sensitivity of materials to electron beams made it difficult to reduce the electron damage to specimens while maintaining the resolution and signal-to-noise ratio. It is therefore still challenging to capture multiple images of the same area in one crystal to image the temporal changes of lattices. Usings integrated differential phase contrast (iDPC) STEM, atomic-resolution imaging of beam-sensitive zeolite frameworks is achieved with an ultralow dose of 40 e? Å?2, 2?3 orders of magnitude lower than that of conventional STEM. Based on the iDPC technique, not only the atomic 3D architecture of ZSM-5 crystals but also the changes of frameworks are observed during in situ experiments. Local structures and light-element aromatics in ZSM-5 crystals can also be revealed directly under iDPC-STEM. These results provided not only an efficient tool to image beam-sensitive materials with ultralow beam current but also a new strategy to observe and investigate the hydrocarbon pools in zeolite catalysts at the single-molecule scale.