Alkane hydrocracking: shape selectivity or kinetics?

Maesen, Theo L. M.

doi:10.1016/j.jcat.2003.07.003

Cited by 48 publications

(29 citation statements)

References 41 publications

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“…When this metal functionality is absent (as in catalytic cracking) or defective, then hydrogen subtraction and addition can occur through several different pathways so that the reaction network becomes more complex. In such situations, it becomes commensurately more difficult to identify shape selectivity unambiguously 51 . The original explanation 6 for the shape selectivity associated with zeolite catalysis is simple and intuitive: the pores, or rather the active sites within the pores, exclusively process the molecules that fit inside.…”

Section: Zeolites As Industrial Catalystsmentioning

confidence: 99%

Towards a molecular understanding of shape selectivity

Smit

Maesen

2008

Nature

528

437

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Shape selectivity is a simple concept: the transformation of reactants into products depends on how the processed molecules fit the active site of the catalyst. Nature makes abundant use of this concept, in that enzymes usually process only very few molecules, which fit their active sites. Industry has also exploited shape selectivity in zeolite catalysis for almost 50 years, yet our mechanistic understanding remains rather limited. Here we review shape selectivity in zeolite catalysis, and argue that a simple thermodynamic analysis of the molecules adsorbed inside the zeolite pores can explain which products form and guide the identification of zeolite structures that are particularly suitable for desired catalytic applications.Z eolites are microporous mineral materials that have found wide use in industry since the late 1950s, with one of their most important applications being chemical catalysis. They are particularly important as cracking catalysts in oil refining. One of their defining features-apart from being solid catalysts that are easy to recycle-is that the shape, or topology, of the internal pore structure of a zeolite can strongly affect the selectivity with which particular product molecules are formed in chemical transformations catalysed by the zeolite. Here, we will argue that this shape selectivity can be explained by very simple thermodynamic analyses that consider the impact of zeolite topology on the free energy landscape; that is, on the free energies of formation of the various molecules involved in the catalysed reactions.The analyses presented here are simple and straightforward, yet have become feasible only relatively recently as advances in molecular simulation techniques have started to provide access to the thermodynamic data underpinning them. After a short introduction of zeolites and their use as catalysts, we will therefore also briefly outline the developments in simulation capabilities that give access to the thermodynamic information crucial for our understanding of zeolite catalysis. We then show how the free-energy-landscape approach can elucidate the molecular-level mechanism(s), giving rise to shape selectivity in a number of simple yet industrially important processes. We conclude this review by outlining the crucial issues that need to be addressed to take the free-energy-landscape approach to the next stage, where the combined use of simulations and thermodynamic analysis might have profound implications for how we screen and develop zeolite-based catalysts. Zeolites as industrial catalystsZeolites are crystalline aluminosilicates with a three-dimensional framework that consists of nanometre-sized channels and cages and imparts high porosity and a large surface area to the material. The basic structural unit of all zeolite frameworks consists of a silicon or aluminium atom tetrahedrally coordinated to four oxygen atoms. Any zeolite built of silica and oxygen only is neutral, but replacing Si 41 by Al 31 creates a negative charge on the framework. All such framework ch...

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Section: Zeolites As Industrial Catalystsmentioning

confidence: 99%

Towards a molecular understanding of shape selectivity

Smit

Maesen

2008

Nature

528

437

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“…The extent of thermodynamic equilibration between the dimethylalkanes during hydrocracking can be assessed by comparing the n-alkane distribution in the experimental hydrocracking product slate with the distribution calculated from a mixture of aa-and ag-dimethylalkane at thermodynamic equilibrium (403). For example, in the case of hydrocracking of n-decane hydroconversion, the calculated dimethyloctane hydrocracking product slate (assuming that there is no preferential cracking) contains 8, 11, 16, and 15 mol per 100 mol C 10 hydrocracked of C 3 , n-C 4 , n-C 5 , and n-C 6, respectively (Scheme 9) (407). Similarly, calculation of a thermodynamically equilibrated mixture of V. M. Akhmedov and S. H. Al-Khowaiter aag-trimethylheptanes gives 38, 32, and 30% of 2,4,4-and 3,3,5-trimethylheptane, respectively (403).…”

Section: Recent Advances and Future Aspects In N-alkanesmentioning

confidence: 99%

“…In the absence of thermodynamic equilibrium hydroisomerization kinetics dictate the composition of the hydrocracking product slate (407). The evaluation of this kinetic effect shows the probability of formation of aag-branched isomers dependents on the proximity of the methyl groups to the center of the alkane.…”

Section: Recent Advances and Future Aspects In N-alkanesmentioning

confidence: 99%

Recent Advances and Future Aspects in the Selective Isomerization of High n‐Alkanes

2007

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“…Impressive progress toward this goal has been made during the past two decades, resulting in a much better understanding of the various factors contributing to the rates of catalyzed reactions. For example, it is now possible to predict adsorption isotherms and diffusion coefficients for various molecules and mixtures of molecules in zeolites of different framework structure and pore size using the methods of statistical mechanics and molecular dynamics. , Likewise quantum chemical calculations, particularly those based on density functional theory (DFT), have advanced to a stage where it is possible to analyze complex reaction pathways and to calculate rate coefficients for elementary processes. − In a number of instances, it has been demonstrated that overall reaction kinetics determined using such rate coefficients correctly describe observed rates and changes with catalyst composition. − These successes suggest that by combining different theoretical methods, it should be possible to make quantitative predictions of reaction kinetics for systems involving the combined effects of reaction kinetics, adsorption, and diffusion. The initial efforts in this direction are encouraging, and, hence, it is anticipated that theoretical investigation of the combined effects of adsorption thermodynamics, diffusion, and reaction kinetics on the rates of catalyzed reactions will continue to be a fertile area for future research.…”

Section: Introductionmentioning

confidence: 99%

Theoretical Simulation of n-Alkane Cracking on Zeolites

et al. 2010

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The kinetics of alkane cracking in zeolites MFI and FAU have been simulated theoretically from first principles. The apparent rate coefficient for alkane cracking was described as the product of the number of alkane molecules per unit mass of zeolite that are close enough to a Brønsted-acid site to be in the reactant state for the cleavage of a specific C-C bond and the intrinsic rate coefficient for the cleavage of that bond. Adsorption thermodynamics were calculated by Monte Carlo simulation and the intrinsic rate coefficient for alkane cracking was determined from density functional theory calculations combined with absolute rate theory. The effects of functional, basis set, and cluster size on the intrinsic activation energy for alkane cracking were investigated. The dependence of the apparent rate coefficient on the carbon number for the cracking of C 3 -C 6 alkanes on MFI and FAU determined by simulation agrees well with experimental observation, but the absolute values of the apparent rate coefficients are a factor of 10 to 100 smaller than those observed. This discrepancy is attributed to the use of a small T5 cluster representation of the Brønsted-acid site. Limited calculations for propane and butane cracking on MFI reveal that significantly better agreement between prediction and observation is achieved using a T23 cluster for both the apparent rate coefficient and the apparent activation energy. The apparent rate coefficients for alkane cracking are noticeably larger for MFI than FAU, in agreement with recent findings reported in the experimental literature.

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Alkane hydrocracking: shape selectivity or kinetics?

Cited by 48 publications

References 41 publications

Towards a molecular understanding of shape selectivity

Towards a molecular understanding of shape selectivity

Recent Advances and Future Aspects in the Selective Isomerization of High n‐Alkanes

Theoretical Simulation of n-Alkane Cracking on Zeolites

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