Hao Xiong

Selected publications

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.

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. 

   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.