Olefins are important chemical raw materials in the industry, mainly including low-carbon olefins (ethylene, propylene and butene) and long-chain olefins (C5+=), of which low-carbon olefins are the main polymer monomers, and long-chain terminal olefins (alpha olefins) are the basic raw materials for the synthesis of high-grade lubricants, high carbon alcohols, plasticizers and surfactants.
The fisher-Tropsch synthesis to olefins process is often referred to as the FTO process, where iron-based catalysts are the most common FTO catalysts, usually at temperatures above 320°C. When the reaction temperature is lower than 300 °C, the CO conversion rate is generally relatively low, and the product carbon distribution is wide (C1 to C20+ hydrocarbons).
In view of the problems of low low temperature activity and wide product distribution of traditional iron-based catalysts, a study published in Nature Nanotechnology designed and prepared a catalytic system (Na-FeCx/s-zeolite) composite MFI molecular sieve nanosheets and iron-based catalysts, which realized the efficient preparation of low-carbon olefins and C5-C10 olefins at low temperature Fischer-based catalysts.
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The authors found that even if pure silicon MFI molecular sieve nanosheets (without acid sites) were used, the catalytic activity of the iron-based catalyst could be greatly improved at low temperatures, and excellent C5-C10 alpha olefin selectivity (260 °C, 2 MPa, CO conversion rate 82.6%, olefin selectivity 74.0%, of which 81.7% of C4+ olefins are alpha olefins). Under the same conditions, the CO conversion of the Na-FeCx catalyst without zeolite molecular sieve is less than 2%.
Experimental data and theoretical calculations found that the physical mixed zeolite molecular sieve can accelerate the rapid desorption of olefin molecules from the surface of iron carbide (Fig. 1), change the reaction micro-equilibrium, thereby pulling the positive direction of the reaction, and greatly improving the catalyst activity, which is essentially different from the previous molecular sieve as an acid catalyst. The catalyst system proposed in this work will provide new multiphase catalyst design ideas for the syngas conversion process.
Figure 1. Schematic diagram of a physical mixed molecular sieve additive accelerating the desorption of olefin molecules to promote the FTO reaction process
In this work, the iron carbide catalyst (the classical catalyst for syngas to olefins, Na-FeCx) was prepared, and the catalyst evaluation (Fig 2) was carried out under the reaction conditions of 300 °C, 2 Mpa, syngas ratio of 1 (CO/H2/Ar=45/45/10), and air velocity of 2,400 ml/h/gFeCx, and the results of 25.6% CO conversion and 72.4% C1-C20+ were obtained, which were similar to the results reported in the literature. When the temperature drops to 260°C (a lower temperature for the iron-based Fischer-Tropsch reaction), the conversion of the Na-FeCx catalyst is only 1.9%.
Interestingly, when the ZSM-5 molecular sieve nanosheets with a thickness of 90-110 nm on the b-axis were physically mixed with Na-FeCx, and at 260 °C, 2 Mpa, the ratio of syngas was 1 (CO/H2/Ar=45/45/10) and the airspeed was 2,400 ml/h/gFeCx, it was found that the CO conversion rate was greatly increased to 82.5%, and the selectivity of olefins was as high as 72.0% (of which 95% were low carbon olefins and C5-C10 olefins), Methane was only 3.0% selective and the catalyst remained stable over a long test of 600 h.
In addition, the work also investigated other molecular sieves such as SSZ-13, Beta, MOR, SAPO-34, etc. under the same conditions, and found that the addition of molecular sieves can improve the CO conversion rate to varying degrees and maintain a high alkene ratio, and the best performance is the MFI molecular sieve nanosheets.
Figure 2. Performance evaluation of Na-FeCx/molecular sieve catalysts
MFI molecular sieve nanosheets were hydrothermally synthesized by urea-assisted method, and scanning electron microscopy photographs (SEM) showed that the B-axis thickness of the ZSM-5 molecular sieve nanosheets (s-ZSM-5) was 90-110 nm (Fig. 3a), while the ordinary ZSM-5 molecular sieve (n-ZSM-5) was presented as a block (Fig. 3b). Molecular sieves of both morphologies can improve CO conversion, but the product selectivity varies greatly, with Na-FeCx/s-ZSM-5 obtaining higher olefin selectivity and Na-FeCx/n-ZSM-5 exhibiting higher alkane selectivity (Fig. 3c). In addition, an investigation of the loading method of the catalyst found that the particle mixing was better than the powder mixing and the upper and lower bed filling method (Fig. 3c).
Figure 3. (a) Scanning electron microscopy photographs of s-ZSM-5 and (b) n-ZSM-5 molecular sieves and (c) evaluation of their performance
In previous reports, the molecular sieve acts as a tandem catalyst to provide acid centers to participate in reactions such as cracking, aromatization, isomerization, and carbon-carbon bond coupling processes. In this work, the research team found that the product distribution of the cracking reaction and the product distribution of Na-FeCx/s-ZSM-5 were significantly different from those of Na-FeCx/s-ZSM-5 through the cracking reaction of olefin model molecules such as 1-hexene on the sieve.
In addition, the physical mixing of pure silicon MFI molecular sieve nanosheets without acid sites can greatly improve the catalytic activity of Na-FeCx under the same conditions and obtain higher olefin selectivity. The above shows that the MFI molecular sieve nanosheets in this work are not used as acid catalysts in tandem, but as an auxiliary.
In order to study the auxiliary effect of the molecular sieve, the authors mixed a certain concentration of ethylene in the reaction feed gas to investigate the effect on the reaction, and found that the Na-FeCx catalyst was greatly reduced in activity within a few hours, while the Na-FeCx/s-ZSM-5 catalyst was only slightly inactivated (Fig. 4a).
Therefore, the researchers speculate that the presence of zeolite molecular sieves contributes to the rapid desorption of olefin products formed on the surface of Na-FeCx, thereby improving catalyst activity. Infrared diffuse reflection data show that molecular sieve nanosheets can accelerate the desorption of olefin molecules from the Na-FeCx surface (Fig. 4b), supporting the above view.
Figure 4. (a) Effect of ethylene molecules mixed into the feed gas on catalyst properties. (b) Infrared diffuse spectra of ethylene molecule desorption
The authors established a theoretical model for the formation of olefin molecules on the surface of Na-FeCx and efficient transfer to the molecular sieve micropores in the combined catalytic system of iron carbide and molecular sieve, and then diffused out from the molecular sieve micropores (Fig. 5). Taking ethylene molecules as an example, in the actual process, ethylene molecules either detach from the Surface of Na-FeCx to become the final product, or resorb into the Surface of Na-FeCx, forming a dynamic equilibrium of desorption-resorption (*C2H4⇆ * + C2H4).
It should be noted that there is a free region (Region II) between na-FeCx and s-ZSM-5 components, and the ethylene molecules on the surface of Na-FeCx (Region I) are desorbed into Region II, and these free ethylene molecules are rapidly adsorbed to Phase III by adjacent molecular sieves.
Figure 5. Schematic of olefin molecules diffusing from the Surface of Na-FeCx through different regions into zeolite pores
To gain a deeper understanding of this process, the authors conducted theoretical simulation experiments to further understand the adsorption and diffusion behavior of olefin molecules on the surface of Na-FeCx. Molecular Dynamics (MD) simulations show that for the zeolite-free model, the ethylene molecules on the surface of Na-FeCx maintain a dynamic equilibrium of about 71%, and after adding the molecular sieve model, the value drops to about 48% (Fig 6a), indicating that the presence of the molecular sieve changes the desorption-resorption balance of the ethylene molecule.
In order to further study the diffusion of ethylene molecules in the pores of molecular sieve, the authors established crystal models of molecular sieve crystals with different layers. The interaction energy spectroscopy showed that the diffusion efficiency of ethylene molecules in the pores of the molecular sieve was inversely proportional to the number of adsorption sites in the pores (that is, the length of the pores of the molecular sieve).
In addition, the authors quantitatively determined the diffusion coefficient of ethylene molecules in the crystals of molecular sieves of different layers (Ds, Fig. 6b) using the slope of Mean square Displacement (MSD). The results show that the thinner the molecular sieve crystal, the shorter the residence time of the olefin molecules in the pores of the molecular sieve, which is more conducive to the continuous and rapid transfer of olefin molecules, thereby improving the reactivity and forming more olefin products.
Figure 6. (a) Molecular Dynamics (MD) simulation results. (b) Determination of the diffusion coefficient of ethylene molecules in the crystals of molecular sieves of different layers
By mixing the appropriate molecular sieve material, the Research Group made the Na-FeCx catalyst exhibit efficient low-temperature catalytic activity in the FTO process and optimized the product distribution. The composite catalytic system of iron carbide and molecular sieve reported in this work is fundamentally different from the metal/metal oxide + molecular sieve system reported in the previous literature. Experimental data and theoretical studies have shown that the molecular sieve is not used as an acid catalyst, but changes the desorption-resorption balance of olefin molecules on the surface of iron carbide, and reasonable control of the morphology and pore environment of the molecular sieve can accelerate the desorption of olefin molecules from the Surface of Na-FeCx, which is conducive to the continuous and efficient performance of syngas on the surface of Na-FeCx.
In addition, future research should also have a deeper understanding of the synergy between metal carbides and molecular sieves, providing more new ideas for the design of synthetic and efficient multiphase catalysts for the syngas conversion process.
bibliography
[1] Wang, C., Fang, W., Liu, Z. et al. Fischer–Tropsch synthesis to olefins boosted by MFI zeolite nanosheets. Nat. Nanotechnol. (2022). https://doi.org/10.1038/s41565-022-01154-9
Author: Contributed by the team
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Research team
Corresponding author Wang Liang: Researcher of the "Hundred Talents Program" of Zhejiang University and doctoral supervisor. His research interests include nano and porous catalytic materials and their applications in carbon resource conversion and synthesis of fine chemicals. Published more than 80 papers as newsletter/first author (including Science, Nature Catal., J. Am. Chem. Soc.,Nature Commun.,Angew. Chem., ACS Catal., etc.). He was awarded the 2016 International Conference on Catalysis Young Scientist Award, the 2017 China Catalysis Rookie Award, the 2021 Chinese Chemical Society Youth Chemistry Award, the National Natural Science Foundation of China (2018) and the Zhejiang Natural Science Foundation Jieqing Project (2017), and was selected as the second level of 151 talents in Zhejiang Province.
Corresponding author Zheng Anmin: Doctoral supervisor, researcher of the Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences, leader of the magnetic resonance computing simulation team of energy materials, and head of the "Hubei Provincial Innovation Group". He is mainly engaged in nuclear magnetic resonance experiments and multi-scale theoretical simulation of the structural properties of solid functional materials. Based on corresponding authors in Science, Nature Protocols, PNAS, Sci. Adv.,J. Am. Chem.,Soc., Angew. Chem. and other important international journals have published more than 100 SCI papers. He has been funded by the National Science Foundation for Outstanding Youth, the National Outstanding Youth Fund and the Outstanding Member of the Youth Science and Technology Promotion Association of the Chinese Academy of Sciences.
WeChat public account of the research group: ZhengAM Lab
Corresponding author Xiao Fengshou: Qiushi Distinguished Professor of Zhejiang University, doctoral supervisor. His research interests include the synthesis characterization and catalytic properties of microporous zeolites, the synthesis characterization and catalytic properties of mesoporous zeolite catalytic materials, the synthesis characterization and catalytic properties of porous organic polymer catalytic materials, the efficient catalytic conversion of biomass, and environmental catalysis. In Science, Nat. Catal.,Chem,JACS,Angew. He has published papers in high-level magazines such as Chem as a corresponding author, and he has cited more than 25,000 times, and has won many awards and honors such as the National Science Foundation for Outstanding Youth, the Second Prize of Natural Science of the Ministry of Education, the Outstanding Young Teacher of the Ministry of Education, the Cross-century Excellent Talent, the 2021 China Molecular Sieve Achievement Award, the Thomson Scientific Outstanding Research Award, and the First Prize of Zhejiang Provincial Technological Invention.
Research Group Webpage: http://www.chem.zju.edu.cn/xiaofs/
(Co-authors) Wang Chengtao, Fang Wei, Liu Zhiqiang
Thesis information
Published the journal Nature Nanotechnology
Published July 11, 2022
论文标题Fischer-Tropsch synthesis to olefins boosted by MFI zeolite nanosheets
(DOI:https://doi.org/10.1038/s41565-022-01154-9)
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