Special articles on zeolite chemistry and technology. Analysis of shape selectivity of methylation in toluene over HZSM-5 zeolites. Contribution of outer and inner surfaces of the crystallite.

健治, 橋本; 隆夫, 増田; 陽一, 播口

doi:10.1246/nikkashi.1989.575

Cited by 7 publications

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“…The activity and shape selectivity of zeolites are closely related to their acidic properties and diffusivities, which change with coke deposition (14)(15)(16)(17). Here, we will briefly explain how the shape selectivity is related to the above properties and will discuss about the change of it due to coke deposition.…”

Section: The Change Of Catalytic Properties Due To Coke Depositionmentioning

confidence: 99%

Deactivation of Zeolite Catalysts by Coke

Masuda

Hashimoto

1996

ACS Symposium Series

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Zeolite catalysts are widely used in conversion processes of hydrocarbons, because of their high activity and shape selectivity. The conversion rate and the shape selectivity are closely related to intracrystalline diffusion rate and acidic properties of the zeolite catalyst. Carbonaceous materials called coke deposit on the catalyst during reactions, leading to changes in catalytic properties (diffusivity and acidic properties) as well as the reaction performance. This presentation reviews the mechanisms of the deactivation of zeolite catalysts caused by coke deposition from the following points of view. (1) Characterization of coke. Examination of the location of coke formation. (2) Relationship between the changes in acidic properties, and the acid strength and the acid amount. Prediction of the reduction of diffusion rate taking account of pore structure and molecular size. (3) Development of models for predicting the change in the activity and the shape selectivity.Zeolites are solid acid catalysts which are widely used in hydrocarbon processing, such as naphtha cracking, isomerization, dispropornation and alkylation. During reactions carbonaceous materials called coke deposit on the zeolite and reduces its activity and selectivity. Coke deposited not only covers the acid sites of the catalyst, but also blocks the pores, and restrain reactants from reaching the acid sites, leading to the decrease in the apparent reaction rate (1, 2). Here, we will mainly deal with the intracrystalline diffusivity of zeolites, and will discuss the relationship between it and the change in catalyst selectivity. THE PORE STRUCTURES OF ZEOLITES AND THE MODES OF COKE DEPOSITIONZeolites are composed of crystals of alumino silicate with the size of 0.1-5 micron. The pores of HY type zeolites consist of super cages and windows (0.74 nm). ZSM-

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Section: The Change Of Catalytic Properties Due To Coke Depositionmentioning

confidence: 99%

Deactivation of Zeolite Catalysts by Coke

Masuda

Hashimoto

1996

ACS Symposium Series

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“…, Al concentration gradients from the crystallite core to its surface), all of which are influenced by synthetic and post-synthetic conditions. 5,8,9 Aluminosilicate MFI zeolites (Al-MFI) have a three-dimensional, ten-membered ring (10-MR) pore structure (∼0.55 nm pore limiting diameter), wherein external acid sites are thought to lower para -dialkylbenzene product selectivity during toluene alkylation with methanol 10–12 or ethanol 13,14 (573–873 K) because unselective toluene alkylation or dialkylbenzene isomerization pathways occur at unconfined surfaces. During butene oligomerization in Al-MFI (1.5 MPa, 573 K), initial rates correlated with the number of external acid sites, while secondary reactions ( e.g.…”

Section: Introductionmentioning

confidence: 99%

“…7 The relative amount of external H + sites (H + ext ) to internal acid sites depends on various material properties, including the bulk Si/Al ratio, crystallite size and morphology, and extent of Al zoning (i.e., Al concentration gradients from the crystallite core to its surface), all of which are influenced by synthetic and post-synthetic conditions. 5,8,9 Aluminosilicate MFI zeolites (Al-MFI) have a three-dimensional, ten-membered ring (10-MR) pore structure (∼0.55 nm pore limiting diameter), wherein external acid sites are thought to lower para-dialkylbenzene product selectivity during toluene alkylation with methanol [10][11][12] or ethanol 13,14 (573-873 K) because unselective toluene alkylation or dialkylbenzene isomerization pathways occur at unconfined surfaces. During butene oligomerization in Al-MFI (1.5 MPa, 573 K), initial rates correlated with the number of external acid sites, while secondary reactions (e.g., isomerization, cracking, dehydrocyclization) at external acid sites were suggested to increase the selectivity to lighter hydrocarbons and favor the deposition of bulky carbonaceous species.…”

Section: Introductionmentioning

confidence: 99%