Towards a molecular understanding of shape selectivity

Abstract

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...

Etherification of glycerol with benzyl alcohol catalyzed by solid acids

“…In fact, the selectivity to the di and tri-benzyl-glycerol ethers was very low over the zeolite catalyst. This might be associated with the shape selectivity property 20 of zeolites, which impairs the formation of bulk transition states. Thus, the pore structure of zeolite β may not accommodate the transition state for a second or third benzylation of glycerol.…”

Discreteness-induced concentration inversion in mesoscopic chemical systems

“…Although EMREs are derived from the CME, they can be constructed from sole knowledge of the REs. The correct way of interpreting the EMREs (equations (9) and (10)) is that given a set of reactions occurring in a volume Ω,  f( ) t is the concentration prediction if we ignore molecular discreteness, and  f * ( ) t is the approximate mean concentration prediction, if we take discreteness into account. This interpretation comes from the fact that terms of order Ω 0 in the CME's volume expansion do not explicitly depend on Ω, whereas terms of order Ω − 1/2 , from which the EMREs are obtained, do depend on Ω (see equation 14 in ref.…”

“…The instantaneous solution of the EMREs is a vector of mean concentrations denoted  f * ( ) t . By setting the time derivative in the EMREs (equations (9) and (10) in Methods) to zero, one obtains an expression for the difference between the steady-state mean concentration predictions of the EMREs and REs: Figure 1 | Discreteness-induced inversion effect. The species are monomers (species 1, purple), dimers (species 2, orange), and trimers (species 3, green).…”

“…The relevance of intrinsic noise, however, is not limited to biological systems. Molecular capsules 7 , carbon nanotubes 8 and crystalline zeolites 9 are other examples of nanospaces confining chemical reactions. It has been shown that accounting for intrinsic noise leads to a considerable modification of the temperature dependence of the equilibrium constants of reactions in such confined spaces 10 .…”

Hexene catalytic cracking over 30% sapo-34 catalyst for propylene maximization: influence of reaction conditions and reaction pathway exploration

“…Most of the commercially available catalysts fail to exhibit shape selective phenomena for propylene and their acidic strength did not meet the reaction requirements. The concept of shape-selective catalytic cracking is mainly reflected in two ways (Smit and Maesen, 2008), first in the shape-selective molecular sieve that improves conversion of raw materials and second in the increase in desired product selectivity. Therefore, the catalyst, having selective pores, does not allow the higher hydrocarbon to form, which is highly desirable for olefin cracking.…”