Zeolites with nanometer diffusion lengths and implications in shape selective catalysis

Bhan, Aditya; Tsapatsis, Michael

doi:10.1016/j.coche.2013.06.001

Cited by 10 publications

(8 citation statements)

References 33 publications

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“…Zeolites exclude many large/bulky molecules from accessing the internal porosity where the vast majority of active sites reside. Hence, such molecules can only be converted on the external surface of the zeolite with a low catalytic activity . Several methods have therefore been investigated to provide a larger accessible surface area for bulky molecules.…”

Section: Introductionmentioning

confidence: 99%

“…Other methods such as dual templating using surfactants and hard‐templating using polymeric spheres also yield hierarchical zeolites . Irrespective of the synthesis method used, hierarchical zeolites have generally shown higher activity in catalytic conversions of bulky reactants and longer catalyst lifetimes than their purely microporous analogues …”

Section: Introductionmentioning

confidence: 99%

“…Hence, such molecules can only be converted on the external surface of the zeolite with a low catalytic activity. [9,10] Several methods have therefore been investigated to provide a larger accessible surface area for bulky molecules. One method is to synthesize hierarchical zeolites by introducing mesopores in the microporous zeolitic structure to improve the intracrystalline diffusion rate.…”

Section: Introductionmentioning

confidence: 99%

“…[13] Irrespective of the synthesis method used, hierarchical zeolites have generally shown higher activity in catalytic conversions of bulky reactants and longer catalyst lifetimes than their purely microporous analogues. [10][11][12]16] A third method to reduce the diffusion path length and increase the concentration of easily accessible sites is to synthesize 2D zeolite nanosheets with a high aspect ratio. Choi et al [17] synthesized single unit cell thick MFI nanosheets with 2 nm thickness along the b-axis in a one-pot synthesis using an appropriately designed organic structure directing agent (SDA).…”

Section: Introductionmentioning

confidence: 99%

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Effect of Si/Al Ratio on the Catalytic Activity of Two‐Dimensional MFI Nanosheets in Aromatic Alkylation and Alcohol Etherification

et al. 2019

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Zeolite catalysts with pronounced two‐dimensional (2D) morphologies, specifically 2D MFI nanosheets composed of multilamellar stacks of MFI nanosheets, are hypothesized to offer accessibility advantages in catalytic reactions involving large, bulky molecules. The Si/Al ratio of zeolite catalysts is an important parameter determining their performance. However, the effect of Si/Al ratio on catalytic reactivity is not well understood in 2D zeolite catalysts. To provide further insight into this issue, the catalytic performance of 2D MFI nanosheet materials with different Si/Al ratios is compared using the liquid phase Friedel‐Crafts alkylation of mesitylene with benzyl alcohol & the self‐etherification of benzyl alcohol that occurs in parallel, both of which are catalyzed by Brønsted acid sites. The turnover frequency (TOF) of the catalysts is found to decrease with decreasing Si/Al ratio, with the etherification reaction being the main contributor to this trend. When the same reaction is carried out in the presence of a bulky poison, 2,6 di‐tert‐butylpyridine (DTBP), to selectively deactivate the external acid sites, only the etherification reaction of benzyl alcohol takes place in the micropores of the 2D MFI catalyst and the effectiveness factor is found to decrease with decreasing Si/Al ratio. The single component adsorption isotherms of benzyl alcohol show that the uptake normalized per acid site decreases with decreasing Si/Al ratio. Thus, increasing the density of acid sites in the micropores by decreasing the Si/Al ratio makes it more difficult for the reactant molecules to access them, as demonstrated by the decrease in TOF and the effectiveness factor. While the micropore reactions (benzene alkylation, alcohol etherification) showed a clear trend with Si/Al ratio, the reaction catalyzed on the external surface (mesitylene alkylation) did not, showing distinct differences in reactivity.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Effect of Si/Al Ratio on the Catalytic Activity of Two‐Dimensional MFI Nanosheets in Aromatic Alkylation and Alcohol Etherification

et al. 2019

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“…Critical species formed during MTO (such as aromatics) suffer from mass-transport limitations in zeolites and their diffusion rates and length scales affect rates, selectivities, and catalyst stabilities which microkinetic modeling fails to capture. [26][27][28][29][30] Here, we present a novel KMC package which models H-ZSM-5 crystals across experimentally relevant time and length scales to understand the role of transport during arene interconversion reactions (~100 reactions) previously investigated using periodic dispersion-corrected DFT methods.…”

Section: Introductionmentioning

confidence: 99%

Prediction of C6–C12 Interconversion Rates Using DFT and Zeolite-specific kinetic Monte Carlo Simulation Methods

DeLuca

Hibbitts²

2019

Preprint

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<a>This study introduces a novel kinetic Monte Carlo (KMC) simulation package which models H-ZSM-5 crystals across experimentally relevant time and length scales to understand the role of transport during arene interconversion reactions (~100 reactions). This small subset of the methanol-to-hydrocarbon (MTH) network was previously modeled using periodic, dispersion-corrected density functional theory (DFT) to determine activation barriers and reaction energies for these KMC methods. Transport of arene molecules through the straight and sinusoidal channels of MFI was modeled as site-hopping and the DFT-calculated barriers are incorporated into the KMC model to account for mass-transport limitations. Barriers of different arene molecules trend well with their effective radii, and species with a smaller effective radii diffusive more readily. A previously published maximum rate analysis of arene interconversion pathways—previously validated by experimental data—is compared to a diffusion-free KMC model to confirm the accuracy of this KMC package. The temperature and pressure dependencies of rates obtained from KMC agree well with those of maximum rate analysis on the diffusion-free model, demonstrating that KMC effectively predicts rates as well as maximum rate analysis methods commonly used in kinetic applications of DFT. Arene interconversion pathways were also analyzed on KMC models incorporating diffusion to and from interior crystal sites. These simulations suggest that large species, such as hexamethylbenzene, become trapped at 10–20% of sites, thus causing site deactivation by limiting diffusion through MFI channels and lowering overall rates of product formation. Benzene diffusion barriers are artificially varied from 20–200 kJ mol−1 and rates of benzene methylation decrease by 4-fold with diffusion barriers greater than 80 kJ mol−1; this suggests that species with diffusion barriers greater than 80 kJ mol−1 (such as penta- and hexamethylbenzene) will likely become trapped at interior sites and ultimately cause catalyst deactivation. This study serves as a proof-of-concept for a novel KMC package that expedites kinetic analysis of complex reaction pathways and introduces mass-transport limitations which are not commonly accounted for in kinetic DFT studies. This KMC package can predict the behavior of diffusion-limited species, such as penta- and hexamethylbenzene, and the mechanisms by which they are formed and eventually lead to catalyst deactivation.</a>

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Zeolite-modified electrodes for electrochemical sensing of heavy metal ions – Progress and future directions

Thatikayala

Noori

Min

2023

Materials Today Chemistry

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Zeolites with nanometer diffusion lengths and implications in shape selective catalysis

Cited by 10 publications

References 33 publications

Effect of Si/Al Ratio on the Catalytic Activity of Two‐Dimensional MFI Nanosheets in Aromatic Alkylation and Alcohol Etherification

Effect of Si/Al Ratio on the Catalytic Activity of Two‐Dimensional MFI Nanosheets in Aromatic Alkylation and Alcohol Etherification

Prediction of C6–C12 Interconversion Rates Using DFT and Zeolite-specific kinetic Monte Carlo Simulation Methods

Zeolite-modified electrodes for electrochemical sensing of heavy metal ions – Progress and future directions

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