Disentangling Reaction Processes of Zeolites within Single‐Oriented Channels

Fu, Donglong; Heijden, Onno van der; Stančiaková, Katarína; Schmidt, Joel E.; Weckhuysen, Bert M.

doi:10.1002/ange.201916596

Cited by 13 publications

(9 citation statements)

References 48 publications

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“…To elucidate the key descriptor of morphology effect over H-ZSM-5 zeolite in catalysis process, it is extremely key to differentiate diffusivities in the two-types of pore channels 13,14 . The diffusion in the two-channels system of H-ZSM-5 crystal is actually complicated due to the sensitivity to the kinetic diameter of adsorbed molecules and the topological structures of zeolites.…”

Section: Schame 1 Amentioning

confidence: 99%

Diffusion anisotropy descriptor revealing morphology effect of H-ZSM-5 zeolite for olefin catalytic cracking

Liu

Shi

Yang

et al. 2021

Preprint

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Zeolite morphology is vital in determining catalytic activity, selectivity and stability in zeolite catalysis, while quantitative description of morphology effect is great challenging but highly desirable. Herein, a descriptor to elucidate the morphology effect is proposed by revealing the diffusion anisotropy in straight and sinusoidal channels of H-ZSM-5 zeolite for olefin catalytic cracking. A series of H-ZSM-5 zeolites with similar nano-sheet morphology were precisely synthesized in which only the length in c-axis varies. It is unexpectedly demonstrated that the catalytic activity and stability can be obviously improved by employing samples with longer length in c-axis. Combining time-resolved in-situ FT-IR spectroscopy with molecular dynamic simulations, we revealed that the difference in catalytic performance can be attributed to the intracrystalline diffusive propensity in different channels. This work not only provides a clear descriptor revealing morphology effect, but also offers deep insight into design of highly effective zeolite catalysts for olefin catalytic cracking.

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Section: Schame 1 Amentioning

confidence: 99%

Diffusion anisotropy descriptor revealing morphology effect of H-ZSM-5 zeolite for olefin catalytic cracking

Liu

Shi

Yang

et al. 2021

Preprint

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“…Decreasing the crystal size of samples with comparable morphology and a/b and a/c "aspect ratios" will reduce the length of the sinusoidal and straight channels, facilitating diffusion and decreasing the extension of consecutive reactions, but will also increase the external surface exposed and the number of pore openings to the external surface, with possible consequences in product selectivity and coking 42,67,68 . The number of channel intersections will also be reduced, and this may affect the product distribution because in these crossing points the formation of aromatics will be favoured, as well as that of branched oligomers that will either crack to smaller products, or remain at the intersection leading to coke precursors 69 .…”

Section: Influence Of Diffusional Factors At Comparable Si/al Ratiosmentioning

confidence: 99%

“…This can be a direct consequence of its larger crystals, which will promote the formation of coke precursors, especially at short TOS, when the activity of the catalyst is maximum. Moreover, taking into account that deactivation in micron-sized ZSM-5 has been related to external coke formation 42 , the small external surface and small number of pore openings will also contribute to its fast deactivation. The sharp activity loss observed for MFI2000 at this contact time of 0.73 h is accompanied by a substantial drop of propene yield after 2-3 h TOS (see Figure 5a).…”

Section: Influence Of Diffusional Factors At Comparable Si/al Ratiosmentioning

confidence: 99%

“…While it is true that larger crystallites should lead to a larger number of recracking events, shorter crystallites should improve diffusion and favour catalyst life, provided that unselective reactivity at the external surface is minimized. Moreover, it is shown that other factors such as intergrowth and shape of the crystallites 41 or preferential orientation of crystals, layers or even membranes 42,43 can also have an influence on diffusion and, therefore, on reactivity. On that basis, we decided to study here the interplay of diffusional effects, dictated by zeolite crystal size and crystal morphology, with the compositional effect related with Al content, and the objective of maximizing catalyst activity, selectivity to propene and catalyst life.…”

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confidence: 99%

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Propene Production by Butene Cracking. Descriptors for Zeolite Catalysts

et al. 2020

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Among the possible on-purpose technologies for propene production, direct conversion of butene-rich fractions to propene represents an attractive alternative to conventional routes such as steam cracking or fluid catalytic cracking (FCC). Here we present an approach for designing an efficient ZSM-5 based catalyst for the selective cracking of butenes to propene by properly balancing diffusional and compositional effects. Instead of the large coffin-shaped ZSM-5 crystallites with very high Si/Al ratios generally reported, the optimal catalyst in terms of propene selectivity and catalyst life was found to be a ZSM-5 zeolite with squared morphology, sub-micron sized crystals (0.8 x 0.3 x 1.0 μm), and Si/Al molar ratio around 300. For this crystal conformation, the short dimensions of both, sinusoidal and straight channels, facilitate propene diffusion and reduce its consumption in consecutive reactions, limiting the formation of C5+ oligomers and aromatics and maximizing propene selectivity. Coffin-type ZSM-5 crystals, with higher diffusional restrictions than square-shaped crystals, show faster catalyst deactivation than the latter, independently of crystal size and Al content. However, among the ZSM-5 zeolite crystallites with coffin morphology, the one presenting intergrowths on the (010) face, with larger proportion of sinusoidal channels, shows lower aromatics selectivity and deactivation rate, whereas the other two, with straight channels open to the clean (010) faces, favour the formation of aromatics by direct cyclisationdehydrogenation of oligomeric intermediates.

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“…The structure-reactivity relationship between the Al and BAS proton location and catalyst activity has been well established for many zeolite-catalysed reactions such as alkane activation [134,135] or MTH reaction [136]. It is believed that a reactant confinement by the zeolite framework is the cause of this regioselective behavior [137].…”

Section: Introductionmentioning

confidence: 99%

Water in Zeolite Pores: Modelling of Water-Active Site Interactions

Stančiaková¹

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Chapter 1 pyrolysis oil in HZSM-5 zeolite. As the main cause of deactivation was dealumination due to the high water content in the reaction medium [60]. During methanol conversion the role of water is both positive and negative and depends on the reaction conditions. It has been shown that a co-feeding with water improves selectivity towards olefins and suppresses the deactivation of the catalyst due to coking [61]. De Wispelaere et al. studied the MTO procces using molecular simulations combined with in-situ microspectroscopy. They found that during the reaction water competes with methanol in the adsorption on the Brønsted acid site (BAS) as well as forms protonated water clusters with the BAS proton which results in the attenuation of all reaction steps including the reactions leading to hydrocarbon pool mechanism and subsequent catalyst deactivation [62].Another issue arises when zeolites are exposed to hot liquid water, as they are during biomass conversion in aqueous solution. Dramatic structural collapse even at temperatures as low as 473 K has been observed [63]. Contrary to the steam treatment, it has been reported that the hot liquid water attack occurs preferentially via hydrolysis of Si-O-Si bridges [64,65]. Zhang et al. studied the stability of different zeolite samples with varying Si/Al ratio, density of silanol defects, and density of BAS upon exposure to liquid water at 473 K. They observed that zeolite ZSM-5 as well as zeolite Y can be stable or unstable depending on the synthesis procedure and post-synthesis treatment. It was observed that samples dealuminated via steaming with a high density of defects suffer severe degradation [66]. The Catalyst RegenerationAn essential part of the catalytic cycle is a catalyst regeneration, especially for zeolites that suffer massive deactivation during biomass and MTH conversion due to coking [67]. The process of coke formation is reversible and the zeolite activity can be restored in a regeneration step during which is the catalyst exposed to air at high temperatures (923-973 K) leading to the complete oxidation of any coke. Iliopoulou et al. investigated the performance of a commercial Co-ZSM-5 catalyst during catalytic biomass pyrolysis in a pilot-scale fluidized bed reactor [68]. In another study, Vitolo et al. studied the behavior of HZSM-5 zeolite during the conversion of pyrolysis oil through multiple upgrading/regenerating cycles [69]. Catalyst deactivation was observed in both studies. Vitolo concluded that the primary cause of the catalyst deactivation is the loss of acid sites, which were gradually removed during the regeneration cycles due to the presence of water [69]. An irreversible catalyst deactivation due to dealumination during regeneration cycles after MTO conversion was also reported [70,71]. Considering the rapid catalyst deactivation by coking, preserving the catalyst stability after many regeneration steps without a significant irreversible deactivation is of the highest importance. Molecular Insights into Water-Zeolite Interactions...

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Disentangling Reaction Processes of Zeolites within Single‐Oriented Channels

Cited by 13 publications

References 48 publications

Diffusion anisotropy descriptor revealing morphology effect of H-ZSM-5 zeolite for olefin catalytic cracking

Diffusion anisotropy descriptor revealing morphology effect of H-ZSM-5 zeolite for olefin catalytic cracking

Propene Production by Butene Cracking. Descriptors for Zeolite Catalysts

Water in Zeolite Pores: Modelling of Water-Active Site Interactions

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