Shape selectivity through entropy

We review the molecular mechanisms behind adsorption and the separations of mixtures in metal-organic frameworks and zeolites. Separation mechanisms can be based on differences in the affinity of the adsorbate with the framework and on entropic effects. To develop next-generation adsorbents, the separation efficiency of the materials needs to be improved. The performance under industrially relevant conditions largely depends on two factors: 1) the separation selectivity and 2) the pore volume capacity of the material. Enthalpic mechanisms can lead to increased selectivities, but these are mostly restricted to the low loading regime, and hence these mechanisms are unable to make use of all of the large-pore volume that a metal-organic framework can provide. Industrial processes routinely operate in the pore saturation regime. In this Review, we focus on entropic molecular separation mechanisms that are effective under these conditions and, in particular, on a recent methodology to obtain high selectivities at high pore loading.

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“…AFI with pore size of 7.3 ). [56,57] In general, the interplay of enthalpy and entropy is difficult to predict.…”

Section: Adsorptionthermodynamicsmentioning

confidence: 99%

Exploiting Large‐Pore Metal–Organic Frameworks for Separations through Entropic Molecular Mechanisms

Torres‐Knoop

Dubbeldam

2015

ChemPhysChem

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“…However, the actual process is carried out at high pressures where the zeolites are completely filled, and calculations carried out under these conditions predict product selectivities that agree with observed product distributions. In this case, the pressure-dependence of the simulation results signals that the selectivity of the process is an entropic effect: the high operating pressure favours formation of those C 6 isomers that can optimally pack inside the zeolite 19 . Subsequent desorption of C 6 , followed by readsorption and further reaction could in principle obscure this effect.…”

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confidence: 99%

“…In fact, several shape-selective transformations catalysed by zeolites have been re-analysed using the principles we have outlined above, resulting in the identification of selectivity mechanisms different from those proposed originally [19][20][21][22][23] .…”

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confidence: 99%

Towards a molecular understanding of shape selectivity

Smit

Maesen

2008

Nature

523

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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“…[31] The observed order of the sticking probabilities on unmodified H-ZSM5 (pxylene > benzene > o-xylene > toluene) agrees perfectly with the estimates by statistical thermodynamics, suggesting that overall, the sticking probabilities are strongly entropically controlled. [30,32] Sorption of the relatively rigid aromatic molecules shows that the sticking probability is highly sensitive to the number of statistically favorable orientations of the molecules during the collision with the external surface. [31] Enhancing the total number of advantageous orientations of the molecules on the surface would lead to enhanced trapping of the molecules after the collision and as a direct consequence to markedly increased sticking probabilities.…”

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confidence: 99%