Chemical diversity of zeolite catalytic sites

Lobo, Raúl F.

doi:10.1002/aic.11502

Cited by 15 publications

(12 citation statements)

References 42 publications

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“…Structural and compositional differences among active sites in zeolites reflect concomitant differences in either the acid sites or the voids that confine them (Scheme 1). Brønsted acid sites differ in composition and strength when framework Si atoms are replaced by different trivalent cations (e.g., Al, Fe, Ga, B); 9 among zeolites, which are strictly aluminosilicates, acid sites differ in local structure and geometric arrangement when Al atoms are present at different crystallographically-distinct framework tetrahedral sites (T-sites). 10,11 Intrazeolite void volumes relevant in descriptions of the catalytic behavior of these acid sites depend on both the steric constraints enforced by the crystalline framework and on the spatial requirements or structures more accurately than DFT methods alone.…”

Section: Zeolitic Active Sites: the Effects Of Void Structure On Acid...mentioning

confidence: 99%

The catalytic diversity of zeolites: confinement and solvation effects within voids of molecular dimensions

2013

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The ability of molecular sieves to control the access and egress of certain reactants and products and to preferentially contain certain transition states while excluding others based on size were captured as shape selectivity concepts early in the history of zeolite catalysis. The marked consequences for reactivity and selectivity, specifically in acid catalysis, have since inspired and sustained many discoveries of novel silicate frameworks and driven the engineering of hierarchical structures and void size to influence catalysis. The catalytic diversity of microporous voids is explored and extended here in the context of their solvating environments, wherein voids act as hosts and stabilize guests, whether reactive intermediates or transition states, by van der Waals forces. We use specific examples from acid catalysis, including activation of C-C and C-H bonds in alkanes, alkylation and hydrogenation of alkenes, carbonylation of dimethyl ether, and elimination and homologation reactions of alkanols and ethers, which involve transition states and adsorbed precursors of varying size and composition. Mechanistic interpretations of measured turnover rates enable us to assign precise chemical origins to kinetic and thermodynamic constants in rate equations and, in turn, to identify specific steps and intermediates that determine the free energy differences responsible for chemical reactivity and selectivity. These free energy differences reflect the stabilization of transition states and their relevant precursors via electrostatic interactions that depend on acid strength and van der Waals interactions that depend on confinement within voids. Their respective contributions to activation free energies are examined by Born-Haber thermochemical cycles by considering plausible transition states and the relevant precursors. These examples show that zeolite voids solvate transition states and precursors differently, and markedly so for guest moieties of different size and chemical composition, thus enabling voids of a given size and shape to provide the "right fit" for a given elementary step, defined as that which minimizes Gibbs free energies of activation. Tighter confinement is preferred at low temperatures because enthalpic gains prevail over concomitant entropic losses, while looser fits are favored at high temperatures because entropy gains offset losses in enthalpic stabilization. Confinement and solvation by van der Waals forces are not directly involved in the making or breaking of strong chemical bonds; yet, they confer remarkable diversity to zeolites, in spite of their structural rigidity and their common aluminosilicate composition. A single zeolite can itself contain a range of local void environments, each with distinct reactivity and selectivity; as a result, varying the distribution of protons among these locations within a given framework or modifying a given location by partial occlusion of the void space can extend the range of catalytic opportunities for zeolites. Taken together with theoretical to...

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Section: Zeolitic Active Sites: the Effects Of Void Structure On Acid...mentioning

confidence: 99%

The catalytic diversity of zeolites: confinement and solvation effects within voids of molecular dimensions

2013

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“…Proton‐form zeolites (H‐zeolites) are Brønsted acids and are prevalent in the petrochemical industry as solid acid catalysts for gas‐phase hydrocarbon reactions . Molecular sieves with different catalytic function can be obtained by the incorporation of other trivalent (e.g., B 3+ , Ga 3+ , and Fe 3+ ) or tetravalent (e.g., Ti 4+ , Sn 4+ , and Zr 4+ ) heteroatoms within pure‐silica frameworks to form Brønsted acid sites or Lewis acid sites, respectively, and can also be derived from the presence of extraframework species (e.g., metals, metal oxides, and metal carbides) . The catalytic diversity of these materials is quite expansive even within a class of molecular sieves defined by a single catalytic function, because of the structural differences prevalent among crystalline frameworks (∼200 in number) and among the specific void spaces that contain active sites …”

Section: Introductionmentioning

confidence: 99%

Beyond shape selective catalysis with zeolites: Hydrophobic void spaces in zeolites enable catalysis in liquid water

2013

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Zeolites confine active sites within void spaces of molecular dimension. The size and shape of these voids can be tuned by changing framework topology, which can influence catalytic reactivity and selectivity via coupled reaction‐transport phenomena that exploit differences in transport properties among reactants and/or products that differ in size and shape. The polarity and solvating properties of intrazeolite void environments can be tuned by changing chemical composition and structure, ranging from hydrophobic defect‐free pure‐silica surfaces to silica surfaces containing hydrophilic defect sites and/or heteroatoms. Here, we discuss how the polarity of zeolite voids influences catalytic reactivity and selectivity via the partitioning of reactant, product, and solvent molecules between intrazeolitic locations and external fluid phases. These findings provide a conceptual basis for developing selective catalytic processes in aqueous media using hydrophobic zeolites that are able to adsorb organic reactants while excluding liquid water from internal void spaces. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3349–3358, 2013

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“…Silicalite-1 (MFI framework) has a pure-silica structure, but does not have active sites. The incorporation of, for example, heteroatoms such as aluminum (ZSM-5) makes it catalytically active [14,15]. Nevertheless, silicalite-1 can be seen as an archetype system, of which its preparation has been characterized in great detail.…”

Section: Synthesis Of Silicalite-1 Molecular Sievesmentioning

confidence: 99%

“…Ni(en)Cl 2 Ni (HCP) 10 5 20 15 18 Scan number catalyst body is mounted on an alumina rod, which is vertically aligned to rotate on the vertical axis with minimal precession. This is mounted in a plug-flow arrangement within a quartz capillary, either side of which are heating guns.…”

Section: X-ray Diffraction Imagingmentioning

confidence: 99%

Recent Trends in Operando and In Situ Characterization: Techniques for Rational Design of Catalysts

Beale

Hofmann

Sankar

et al. 2013

Heterogeneous Catalysts for Clean Technology

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Chemical diversity of zeolite catalytic sites

Cited by 15 publications

References 42 publications

The catalytic diversity of zeolites: confinement and solvation effects within voids of molecular dimensions

The catalytic diversity of zeolites: confinement and solvation effects within voids of molecular dimensions

Beyond shape selective catalysis with zeolites: Hydrophobic void spaces in zeolites enable catalysis in liquid water

Recent Trends in Operando and In Situ Characterization: Techniques for Rational Design of Catalysts

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