Conclusive Evidence for Two Unimolecular Pathways to Zeolite‐Catalyzed De‐alkylation of the Heptamethylbenzenium Cation

Erichsen, Marius Westgård; Mortén, Magnus; Svelle, Stian; Sekiguchi, Osamu; Uggerud, Einar; Olsbye, Unni

doi:10.1002/cctc.201500793

Cited by 11 publications

(3 citation statements)

References 32 publications

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“…Alkenes are intermediate, autocatalytic species in the MTH reaction. Reactions leading to their formation and conversion include the methylation, oligomerization, and cracking of short‐ and long‐chain alkenes, as well as dealkylation of aromatic compounds, in particular polymethylated benzene molecules . The results presented in Figure suggest that a high acid strength favors cracking of higher alkenes to form propene.…”

Section: Resultsmentioning

confidence: 99%

A Systematic Study of Isomorphically Substituted H‐MAlPO‐5 Materials for the Methanol‐to‐Hydrocarbons Reaction

et al. 2017

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Substituting metals for either aluminum or phosphorus in crystalline, microporous aluminophosphates creates Brønsted acid sites, which are well known to catalyze several key reactions, including the methanol to hydrocarbons (MTH) reaction. In this work, we synthesized a series of metal‐substituted aluminophosphates with AFI topology that differed primarily in their acid strength and that spanned a predicted range from high Brønsted acidity (H‐MgAlPO‐5, H‐CoAlPO‐5, and H‐ZnAlPO‐5) to medium acidity (H‐SAPO‐5) and low acidity (H‐TiAlPO‐5 and H‐ZrAlPO‐5). The synthesis was aimed to produce materials with homogenous properties (e.g. morphology, crystallite size, acid‐site density, and surface area) to isolate the influence of metal substitution. This was verified by extensive characterization. The materials were tested in the MTH reaction at 450 °C by using dimethyl ether (DME) as feed. A clear activity difference was found, for which the predicted stronger acids converted DME significantly faster than the medium and weak Brønsted acidic materials. Furthermore, the stronger Brønsted acids (Mg, Co and Zn) produced more light alkenes than the weaker acids. The weaker acids, especially H‐SAPO‐5, produced more aromatics and alkanes, which indicates that the relative rates of competing reactions change upon decreasing the acid strength.

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Section: Resultsmentioning

confidence: 99%

A Systematic Study of Isomorphically Substituted H‐MAlPO‐5 Materials for the Methanol‐to‐Hydrocarbons Reaction

et al. 2017

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“…Since the MTO chemistry is governed by a complex reaction network with both temporal and spatial gradients, various factors may contribute to the observed product distribution and selectivities. − Bhan and co-workers recently developed a method for analyzing transport phenomena in complex processes and discovered that diffusional constraints had the largest effect on some particular steps of the MTO reaction cycle . Hereijgers et al performed seminal work to understand the selectivity and deactivation on H-SAPO-34 .…”

Section: Introductionmentioning

confidence: 99%

Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors

et al. 2020

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The methanol-to-olefins process over H-SAPO-34 is characterized by its high shape selectivity toward light olefins. The catalyst is a supramolecular system consisting of nanometer-sized inorganic cages, decorated by Brønsted acid sites, in which organic compounds, mostly methylated benzene species, are trapped. These hydrocarbon pool species are essential to catalyze the methanol conversion but may also clog the pores. As such, diffusion of ethene and propene plays an essential role in determining the ultimate product selectivity. Enhanced sampling molecular dynamics simulations based on either force fields or density functional theory are used to determine how molecular factors influence the diffusion of light olefins through the 8-ring windows of H-SAPO-34. Our simulations show that diffusion through the 8-ring in general is a hindered process, corresponding to a hopping event of the diffusing molecule between neighboring cages. The loading of different methanol, alkene, and aromatic species in the cages may substantially slow down or facilitate the diffusion process. The presence of Brønsted acid sites in the 8-ring enhances the diffusion process due to the formation of a favorable π-complex host–guest interaction. Aromatic hydrocarbon pool species severely hinder the diffusion and their spatial distribution in the zeolite crystal may have a significant effect on the product selectivity. Herein, we unveil how molecular factors influence the diffusion of light olefins in a complex environment with confined hydrocarbon pool species, high olefin loadings, and the presence of acid sites by means of enhanced molecular dynamics simulations under operating conditions.

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“…However, the stronger acidity results in more rapid catalyst deactivation caused by multiring aromatics that block the pore system of the zeolite . These multiring aromatics derive from the fusion of aromatic hydrocarbon pool species, which are key reaction intermediates of the MTO reaction in small‐pore zeolites . Catalyst stability is impeded by the fact that these deposits form in the outer regions of the zeolite crystals and, accordingly, prevent the access of methanol to internal regions.…”

Section: Chemical Composition Al Speciation and Textural Properties mentioning

confidence: 99%

Probing the Influence of SSZ‐13 Zeolite Pore Hierarchy in Methanol‐to‐Olefins Catalysis by Using Nanometer Accuracy by Stochastic Chemical Reactions Fluorescence Microscopy and Positron Emission Profiling

et al. 2017

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[a] Increasingly,m ethanol is considered as an important chemical intermediate between coal and natural gas feedstocks and olefinic building blocks used extensively in the chemical industry.Currently,t he methanol-to-olefins (MTO) reaction is the most important in terms of the installed capacity of all methanol-tohydrocarbon reactions. [1][2][3][4] Am icroporous silicoaluminophosphate materialw ith the chabazite (CHA) framework topology (H-SAPO-34)i st he preferred catalystc ommercially.[5] One concern is the predicted scarcity of phosphorus,w hich is contained in typical silicoaluminophosphates up to 20 wt %. [6,7] The higher acidity of SSZ-13, the isostructural phosphorous-free aluminosilicate analogue of H-SAPO-34, presentsa no pportunity to operate the MTO reactiona talower temperature and, in this way,t oi mprove process economics.[8] However,t he stronger acidityr esults in more rapid catalystd eactivation caused by multiring aromaticst hat block the pore system of the zeolite.[9] These multiring aromatics derive from the fusion of aromatic hydrocarbon pool species, which are key reactioni ntermediates of the MTOr eactioni ns mall-pore zeolites.[10] Catalyst stability is impeded by the fact that these deposits form in the outer regions of the zeolite crystalsa nd, accordingly,p reventt he accesso fm ethanolt oi nternal regions.T he introduction of mesopores within zeolite crystals is an elegant way to reduce these negative effects of diffusionl imitations and improve the catalytic performance. [11][12][13] Common and scalable approaches such as steaming and alkaline leaching have an egative effect on MTO performance if applied to SSZ-13 zeolite. [14,15] An understanding of the role of the hierarchical pore architecture of SSZ-13z eoliteso nt he catalytic performance in the methanol-to-olefins (MTO) reactioni sc rucial to guide the design of better catalysts. We investigated the influence of the space velocity on the performance of am icroporous SSZ-13 zeolite and severalh ierarchically structured SSZ-13 zeolites. Single catalytic turnovers, as recorded by nanometera ccuracy by using stochastic chemical reactions (NASCA) fluorescence microscopy verified that the hierarchical zeolitesc ontain pores larger than the 0.38 nm apertures native to SSZ-13z eolite.T he amount of fluorescent events correlated well with the additional pore volume availableb ecause of the hierarchical structuring of the zeolite. Positron emission tomography (PET) using 11 C-labeled methanolw as used to map the 2D spatial distribution of the deposits formed during the MTO reaction in the catalystb ed. We used PET imaging to demonstrate that hierarchical structuring not only improves the utilization of the availablem icroporous cages of SSZ-13 but also that the aromatic hydrocarbon pool species are involved in more turnovers beforet hey condense into larger multirings tructures that deactivate the catalyst.

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Conclusive Evidence for Two Unimolecular Pathways to Zeolite‐Catalyzed De‐alkylation of the Heptamethylbenzenium Cation

Cited by 11 publications

References 32 publications

A Systematic Study of Isomorphically Substituted H‐MAlPO‐5 Materials for the Methanol‐to‐Hydrocarbons Reaction

A Systematic Study of Isomorphically Substituted H‐MAlPO‐5 Materials for the Methanol‐to‐Hydrocarbons Reaction

Light Olefin Diffusion during the MTO Process on H-SAPO-34: A Complex Interplay of Molecular Factors

Probing the Influence of SSZ‐13 Zeolite Pore Hierarchy in Methanol‐to‐Olefins Catalysis by Using Nanometer Accuracy by Stochastic Chemical Reactions Fluorescence Microscopy and Positron Emission Profiling

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