Molecular behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts as a function of Si/Al ratio: a quasielastic neutron scattering study

Omojola, Toyin; Silverwood, Ian P.; O’Malley, Alexander J.

doi:10.1039/d0cy00670j

Cited by 27 publications

(89 citation statements)

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“…In the 10 MR pores of zeolite MFI, the methanol movement is highly reduced where Brønsted acid sites are present, showing less hindered diffusion in the all-silica structure. It is worth noting that the localized motion of the methanol molecules in the spherical intersections of the straight and sinusoidal channels, even in all-silica zeolite MFI, is consistent with recent quasielastic neutron scattering (QENS) observations [18].…”

Section: Resultssupporting

confidence: 87%

“…The gap in our knowledge of the effect of the Brønsted acid sites on the mobility of methanol within the zeolite frameworks can be resolved by comparing the diffusivities of methanol in all-silica and acidic frameworks to quantify the influence of the Brønsted acid sites on a crucial part of the MTH process. Diffusion studies serve as a useful tool in determining and understanding the methanol-zeolite interactions, which ultimately enhances our existing knowledge of the catalytic activity, selectivity, and lifespan of the zeolites [18,19].…”

Section: Introductionmentioning

confidence: 99%

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Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

et al. 2020

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Detailed insight into molecular diffusion in zeolite frameworks is crucial for the analysis of the factors governing their catalytic performance in methanol-to-hydrocarbons (MTH) reactions. In this work, we present a molecular dynamics study of the diffusion of methanol in all-silica and acidic zeolite MFI and Beta frameworks over the range of temperatures 373–473 K. Owing to the difference in pore dimensions, methanol diffusion is more hindered in H-MFI, with diffusion coefficients that do not exceed 10×10−10 m2s−1. In comparison, H-Beta shows diffusivities that are one to two orders of magnitude larger. Consequently, the activation energy of translational diffusion can reach 16 kJ·mol−1 in H-MFI, depending on the molecular loading, against a value for H-Beta that remains between 6 and 8 kJ·mol−1. The analysis of the radial distribution functions and the residence time at the Brønsted acid sites shows a greater probability for methylation of the framework in the MFI structure compared to zeolite Beta, with the latter displaying a higher prevalence for methanol clustering. These results contribute to the understanding of the differences in catalytic performance of zeolites with varying micropore dimensions in MTH reactions.

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Section: Resultssupporting

confidence: 87%

Section: Introductionmentioning

confidence: 99%

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

et al. 2020

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“…We note that this temperature programmed surface reaction approach complement our previous studies through a reductionist approach that aimed to decouple adsorption, desorption, activity and diffusion through an individual combination of temperature programmed desorption 51 , step response 48,49 and quasi-elastic neutron scattering 52 studies to probe the formation of primary olefins from oxygenates over fresh and working ZSM-5 catalysts. In this TPSR work, we reveal scaling factors between desorption energies of DME and the barriers of formation of key intermediates (methoxy methyl species and methyl propenyl ether) involved in propylene formation over ZSM-5 catalysts and mechanistic insights into how activity can be improved by process modulation and active site engineering.…”

Section: Introductionmentioning

confidence: 81%

Mechanistic insights into the conversion of dimethyl ether over ZSM-5 catalysts: A combined temperature-programmed surface reaction and microkinetic modelling study

Omojola

Veen

2021

Chemical Engineering Science

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Rates of adsorption, desorption, and surface reaction of dimethyl ether (DME) to olefins over fresh and working ZSM-5 catalysts of different Si/Al ratios (36 and 135) have been decoupled using a combination of temperature programmed surface reaction experiments and microkinetic modelling. Transient reactor performance was simulated by solving coupled 1D non-linear partial differential equations accounting for elementary steps occurring during the induction period based on the methoxymethyl mechanism on the zeolite catalyst, and axial dispersion and convection in the reactor. Propylene is the major olefin formed and scaling relations between activation energies of DME desorption and barriers of formation of methoxymethyl and methyl propenyl ether are observed. Six ensembles of sites are observed with a maximum of three adsorption/desorption sites and three adsorption/desorption/reaction sites. Barriers are generally higher over working catalysts than fresh catalysts. Activation energies of propylene formation of ca. 200 kJ mol -1 are obtained corroborating direct mechanistic proposals. File list (2) download file view on ChemRxiv TPSR of DME over ZSM-5 catalysts_ChemRXiv_Prepri... (866.48 KiB) download file view on ChemRxiv SI_TPSR of DME over ZSM-5 catalysts_ChemRXiv_Prepr... (1.06 MiB) Omojola and van Veen

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Section: Introductionmentioning

confidence: 74%

“…51,61 Higher DME self-diffusion coefficients were observed over ZSM-5 (135) compared to ZSM-5 (36). 52 The mobility of DME diffusion through ZSM-5 catalysts does not limit either the TPD or TPSR studies. We observed activation energies of DME diffusion of 0.96 and 1.33 kJ mol -1 over fresh ZSM-5 (36) and (135) catalysts, respectively.…”

Section: The Reductionist and Integrated Approach To Dto Catalysismentioning

confidence: 99%

Mechanistic Insights into the Conversion of Dimethyl Ether over ZSM-5 Catalysts: A Combined Temperature Programmed Surface Reaction and Microkinetic Modelling Study

Omojola

Veen²

2021

Preprint

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Rates of adsorption, desorption, and surface reaction of dimethyl ether (DME) to olefins over fresh and working ZSM-5 catalysts of different Si/Al ratios (36 and 135) have been decoupled using a combination of temperature programmed surface reaction experiments and microkinetic modelling. Transient reactor performance was simulated by solving coupled 1D non-linear partial differential equations accounting for elementary steps occurring during the induction period based on the methoxymethyl mechanism on the zeolite catalyst, and axial dispersion and convection in the reactor. Propylene is the major olefin formed and scaling relations between activation energies of DME desorption and barriers of formation of methoxymethyl and methyl propenyl ether are observed. Six ensembles of sites are observed with a maximum of three adsorption/desorption sites and three adsorption/desorption/reaction sites. Barriers are generally higher over working catalysts than fresh catalysts. Activation energies of propylene formation of ca. 200 kJ mol<sup>-1</sup> are obtained corroborating direct mechanistic proposals.

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Molecular behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts as a function of Si/Al ratio: a quasielastic neutron scattering study

Abstract:
Qualitative and quantitative differences are found in methanol and dimethyl ether mobility in H-ZSM-5 catalysts of varying Si/Al ratios (Brønsted acid site concentrations) using quasielastic neutron scattering.

Cited by 27 publications

References 71 publications

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

Mechanistic insights into the conversion of dimethyl ether over ZSM-5 catalysts: A combined temperature-programmed surface reaction and microkinetic modelling study

Mechanistic Insights into the Conversion of Dimethyl Ether over ZSM-5 Catalysts: A Combined Temperature Programmed Surface Reaction and Microkinetic Modelling Study

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Molecular behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts as a function of Si/Al ratio: a quasielastic neutron scattering study

Abstract: Qualitative and quantitative differences are found in methanol and dimethyl ether mobility in H-ZSM-5 catalysts of varying Si/Al ratios (Brønsted acid site concentrations) using quasielastic neutron scattering.

Cited by 27 publications

References 71 publications

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

Mechanistic insights into the conversion of dimethyl ether over ZSM-5 catalysts: A combined temperature-programmed surface reaction and microkinetic modelling study

Mechanistic Insights into the Conversion of Dimethyl Ether over ZSM-5 Catalysts: A Combined Temperature Programmed Surface Reaction and Microkinetic Modelling Study

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Abstract:
Qualitative and quantitative differences are found in methanol and dimethyl ether mobility in H-ZSM-5 catalysts of varying Si/Al ratios (Brønsted acid site concentrations) using quasielastic neutron scattering.