Simulation studies of the structure and energetics of sorbed molecules in high-silica zeolites. 1. Hydrocarbons

Titiloye, James O.; Parker, Stephen C.; Stone, F. S.; Catlow, C. Richard A.

doi:10.1021/j100163a029

Cited by 107 publications

(93 citation statements)

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“…TAP reactor data at low coverages show that over , sites are relatively populated with active sites distributed within the zeolite. On increasing the Si/Al ratio, merging of the sites occurs, leading to isolated sites preferentially located on the straight channels of the ZSM-5 catalyst [61].…”

Section: Nature Of Binding Sitesmentioning

confidence: 99%

Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

et al. 2017

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The desorption of methanol and dimethyl ether has been studied over fresh and hydrocarbon-occluded ZSM-5 catalysts with Si/Al ratios of 25, 36 and 135 using a temporal analysis of products reactor. The catalysts were characterized by XRD, SEM, N 2 physisorption and pyridine FT-IR. The crystal size increases with Si/Al ratio from 0.10 to 0.78 µm. The kinetic parameters were obtained using the Redhead method and a plug flow reactor model with coupled convection, adsorption and desorption steps. ZSM-5 catalysts with Si/Al ratios of 25 and 36 exhibit three adsorption sites (low, medium, and high temperature sites), while there is no difference between medium and high temperature sites at a Si/Al ratio of 135. Molecular adsorption on the low temperature site and dissociative adsorption on the medium and high temperature sites give a good match between experiment and the plug flow reactor model. The DME desorption activation energy was systematically higher than that of methanol. Adsorption stoichiometry shows that methanol and DME form clusters onto the binding sites. When non-activated re-adsorption is accounted for, a local equilibrium is reached only on the low and medium temperature binding sites. No differences were observed, other than in site densities, when extracting the kinetic parameters for fresh and activated ZSM-5 catalysts at full coverage. Graphical AbstractElectronic supplementary material The online version of this article (https://doi

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Section: Nature Of Binding Sitesmentioning

confidence: 99%

Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

et al. 2017

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“…In order to simplify the analysis, only zeolites that possess straight channels are considered in this work. Although previous studies have demonstrated that the shape traced by the channel path (e.g., straight, sinusoidal) influences alkane adsorption behavior, [19][20][21][42][43][44] zeolites with channels that do not trace straight paths have been omitted because of their small number in the database. However, even among zeolites having only straight channels, the atoms that comprise the channel openings may connect to form circles, ovals, or other shapes, and these shapes are expected to influence the adsorption thermodynamics.…”

Section: Effects Of Channel Diameter and Cross-section For Zeolites Wmentioning

confidence: 99%

“…Several studies have been focused on measuring [16][17][18] or computing [19][20][21][22] thermodynamic adsorption properties and Henry constants for alkane adsorption and then interpreting the results based on a qualitative assessment of the level of confinement. Lower (i.e., more negative) values for the enthalpies and entropies of adsorption are generally associated with more confining features such as small pores, while higher values are associated with less confining features such as large cages.…”

Section: Introductionmentioning

confidence: 99%

Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

et al. 2017

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Monte Carlo simulations are used to systematically investigate the effects of structural topology on the thermodynamics of n-alkanes adsorbed at Brønsted protons in zeolites having one-dimensional channel systems. In zeolites without cages, the enthalpy and entropy of adsorption (∆H ads-H+ and ∆S ads-H+ ) at fixed pore-limiting diameter (PLD) generally increase (become less negative) as the ratio of the minimum to maximum channel diameter decreases, and are lowest when this ratio equals 1 (corresponding to approximately circular cross-sections). The effect of a change in diameter ratio on the free energy of adsorption (∆A ads-H+ ) is weak because the changes in ∆H ads-H+ and T∆S ads-H+ largely cancel. The addition of cages having a largestcavity diameter (LCD) greater than the PLD increases both ∆H ads-H+ and ∆S ads-H+ . Replacing channels with cages of the same diameter does not change ∆S ads-H+ significantly when the PLD is similar to the alkane length, but decreases both ∆H ads-H+ and ∆A ads-H+ because of the greater surface area of cages relative to channels. The selectivity to adsorption via a central C-C bond vs. a terminal bond when cages are absent is smallest for PLDs near the alkane length, and when cages are present, is even lower when the LCD exceeds the alkane length. This effect is attributed to more rotation of the alkane in cages vs. channels. The results show that ∆A ads-H+ at 773 K can be tuned by manipulating a characteristic dimension (LCD, PLD) and topology (e.g., adding cages) simultaneously, in order to circumvent the compensating changes in T∆S ads-H+ and ∆H ads-H+ that occur upon changing only one structural parameter.3

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“…With other alkane combinations, azeotropic behavior of the adsorbate phase was also encountered. Systematic exploration of binary mixtures in the carbon range C 5 -C 22 led to the identification of the underlying mechanisms.…”

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confidence: 99%

Molecular Competition Effects in Liquid‐Phase Adsorption of Long‐Chain n‐Alkane Mixtures in ZSM‐5 Zeolite Pores

et al. 2003

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Zeolite adsorbents play a vital role in molecular separation technology, industrial catalysis and pollution control. In spite of its relevance to these areas, few fundamental studies have thus far been devoted to the adsorption of mixtures. The complexity of the molecular filling patterns of zeolite micropores directed earlier investigations, rather, into the adsorption of a single compound. Herein, we report the investigation of the adsorption of binary mixtures of n-alkanes with chain lengths up to C 22 from the liquid phase on ZSM-5 zeolite, and the discovery of unexpected selectivities depending on subtle differences in the number of carbon atoms. In some n-alkane mixtures, the uptake of the lighter molecule was favored, whereas in other mixtures the heavier molecule was preferred. With other alkane combinations, azeotropic behavior of the adsorbate phase was also encountered. Systematic exploration of binary mixtures in the carbon range C 5 -C 22 led to the identification of the underlying mechanisms.Intriguing effects in the adsorption of single components in microporous environments, such as zeolites were found recently. For example, ferrierite behaves as a 1D pore system for long-chain n-alkane molecules (> C 5 ), whereas only shorter molecules can exploit its full 2D pore system. [1][2][3][4] With ferrierite and ZSM-22, the siting of dibranched alkane seems to depend subtly on their size. At low coverage the dibranched alkanes adsorb in parallel to the outer surface of the zeolite and as coverage increases, additional molecules adsorb in such a manner that propyl and butyl groups point into the pores.[5] Other studies focused on the relationship between pore size and energy of interaction of pure substances. [6,7] Efforts have been made in the study of adsorption of hydrocarbons on ZSM-5 and Silicalite-1. These zeolites have the MFI framework topology are among the most studied zeolites in the field of adsorption, diffusion and catalysis.[8] The pore system of these zeolites comprises linear channels, with a free pore diameter of 5.6 5.3 , that are

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Simulation studies of the structure and energetics of sorbed molecules in high-silica zeolites. 1. Hydrocarbons

Cited by 107 publications

References 0 publications

Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

Effects of Pore and Cage Topology on the Thermodynamics of n-Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

Molecular Competition Effects in Liquid‐Phase Adsorption of Long‐Chain n‐Alkane Mixtures in ZSM‐5 Zeolite Pores

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