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Author: Rajamani Pachayappan Gounder
Publisher:
ISBN:
Category :
Languages : en
Pages : 194
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Book Description
The catalytic consequences of confinement within zeolite voids were examined for several elimination (alkane cracking and dehydrogenation, alkene cracking, alkanol dehydration) and addition (alkene hydrogenation, alkylation and oligomerization) reactions catalyzed by Brønsted solid acids. These reactions are mediated by cationic transition states that are confined within voids of molecular dimensions (0.4-1.3 nm) and proceed at rates that reflect the Gibbs free energies of late ion-pairs at transition states relative to those for the relevant reactants. Ion-pair stabilities depend on electrostatic interactions between organic cations and catalyst conjugate anions and on dispersion interactions between these cations and framework oxygen atoms. The former interactions are essentially unaffected by confinement, which influences weakly Brønsted acid strength, while the latter depend strongly on the sizes and shapes of voids and the species confined within them. The catalytic effects of confinement in stabilizing ion-pairs are prevalent when transition states are measured relative to gaseous reactants, but are attenuated and in some cases become irrelevant when measured with respect to confined reactants that are similar in composition and size. Zeolite voids solvate confined species by van der Waals forces and mediate compromises in their enthalpic and entropic stabilities. Confinement is generally preferred within locations that benefit enthalpic stability over entropic freedom at low temperatures, in which free energies depend more strongly on enthalpic than entropic factors. For example, the carbonylation of dimethyl ether (400-500 K) occurs with high specificity within eight-membered (8-MR) zeolite voids, but at undetectable rates within larger voids. This specificity reflects the more effective van der Waals stabilization of carbonylation transition states within the former voids. In contrast, entropic consequences of confinement become preeminent in high temperature reactions. Alkane activation turnovers (700-800 K) are much faster on 8-MR than 12-MR protons of mordenite zeolites because the relevant ion-pairs are confined only partially within shallow 8-MR side pockets and to lesser extents than within 12-MR channels. The site requirements and confinement effects found initially for elimination reactions were also pertinent for addition reactions mediated by ion-pair transition states of similar size and structure. Ratios of rate constants for elimination and addition steps involved in the same mechanistic sequence (e.g., alkane dehydrogenation and alkene hydrogenation) reflected solely the thermodynamic equilibrium constant for the stoichiometric gas-phase reaction. These relations are consistent with the De Donder non-equilibrium thermodynamic treatments of chemical reaction rates, in spite of the different reactant pressures used to measure rates in forward and reverse directions. The De Donder relations remained relevant at these different reaction conditions because the same elementary step limited rates and surfaces remained predominantly unoccupied in both directions. Rate constants for elementary steps catalyzed by zeolitic Brønsted acids reflect the combined effects of acid strength and solvation. Their individual catalytic consequences can be extricated using Born-Haber thermochemical cycles, which dissect activation energies and entropies into terms that depend on specific catalyst and reactant properties. This approach was used to show that thermal, chemical and cation-exchange treatments, which essentially change the sizes of faujasite supercage voids by addition or removal of extraframework aluminum species, influence solvation properties strongly but acid strength only weakly. These findings have clarified controversial interpretations that have persisted for decades regarding the origins of chemical reactivity and acid strength on faujasite zeolites. Born-Haber thermochemical relations, together with Marcus theory treatments of charge transfer reaction coordinates, provide a general framework to examine the effects of reactant and catalyst structure on ion-pair transition state enthalpy and entropy. The resulting structure-function relations lead to predictive insights that advance our understanding of confinement effects in zeolite acid catalysis beyond the largely phenomenological descriptions of shape selectivity and size exclusion. These findings also open new opportunities for the design and selection of microporous materials with active sites placed within desired void structures for reasons of catalytic rate or selectivity. The ability of zeolite voids to mimic biological catalysts in their selective stabilization of certain transition states by dispersion forces imparts catalytic diversity, all the more remarkable in light of the similar acid strengths among known aluminosilicates. This offers significant promise to expand the ranges of materials used and of reactions they catalyze.
Author: Rajamani Pachayappan Gounder
Publisher:
ISBN:
Category :
Languages : en
Pages : 194
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Book Description
The catalytic consequences of confinement within zeolite voids were examined for several elimination (alkane cracking and dehydrogenation, alkene cracking, alkanol dehydration) and addition (alkene hydrogenation, alkylation and oligomerization) reactions catalyzed by Brønsted solid acids. These reactions are mediated by cationic transition states that are confined within voids of molecular dimensions (0.4-1.3 nm) and proceed at rates that reflect the Gibbs free energies of late ion-pairs at transition states relative to those for the relevant reactants. Ion-pair stabilities depend on electrostatic interactions between organic cations and catalyst conjugate anions and on dispersion interactions between these cations and framework oxygen atoms. The former interactions are essentially unaffected by confinement, which influences weakly Brønsted acid strength, while the latter depend strongly on the sizes and shapes of voids and the species confined within them. The catalytic effects of confinement in stabilizing ion-pairs are prevalent when transition states are measured relative to gaseous reactants, but are attenuated and in some cases become irrelevant when measured with respect to confined reactants that are similar in composition and size. Zeolite voids solvate confined species by van der Waals forces and mediate compromises in their enthalpic and entropic stabilities. Confinement is generally preferred within locations that benefit enthalpic stability over entropic freedom at low temperatures, in which free energies depend more strongly on enthalpic than entropic factors. For example, the carbonylation of dimethyl ether (400-500 K) occurs with high specificity within eight-membered (8-MR) zeolite voids, but at undetectable rates within larger voids. This specificity reflects the more effective van der Waals stabilization of carbonylation transition states within the former voids. In contrast, entropic consequences of confinement become preeminent in high temperature reactions. Alkane activation turnovers (700-800 K) are much faster on 8-MR than 12-MR protons of mordenite zeolites because the relevant ion-pairs are confined only partially within shallow 8-MR side pockets and to lesser extents than within 12-MR channels. The site requirements and confinement effects found initially for elimination reactions were also pertinent for addition reactions mediated by ion-pair transition states of similar size and structure. Ratios of rate constants for elimination and addition steps involved in the same mechanistic sequence (e.g., alkane dehydrogenation and alkene hydrogenation) reflected solely the thermodynamic equilibrium constant for the stoichiometric gas-phase reaction. These relations are consistent with the De Donder non-equilibrium thermodynamic treatments of chemical reaction rates, in spite of the different reactant pressures used to measure rates in forward and reverse directions. The De Donder relations remained relevant at these different reaction conditions because the same elementary step limited rates and surfaces remained predominantly unoccupied in both directions. Rate constants for elementary steps catalyzed by zeolitic Brønsted acids reflect the combined effects of acid strength and solvation. Their individual catalytic consequences can be extricated using Born-Haber thermochemical cycles, which dissect activation energies and entropies into terms that depend on specific catalyst and reactant properties. This approach was used to show that thermal, chemical and cation-exchange treatments, which essentially change the sizes of faujasite supercage voids by addition or removal of extraframework aluminum species, influence solvation properties strongly but acid strength only weakly. These findings have clarified controversial interpretations that have persisted for decades regarding the origins of chemical reactivity and acid strength on faujasite zeolites. Born-Haber thermochemical relations, together with Marcus theory treatments of charge transfer reaction coordinates, provide a general framework to examine the effects of reactant and catalyst structure on ion-pair transition state enthalpy and entropy. The resulting structure-function relations lead to predictive insights that advance our understanding of confinement effects in zeolite acid catalysis beyond the largely phenomenological descriptions of shape selectivity and size exclusion. These findings also open new opportunities for the design and selection of microporous materials with active sites placed within desired void structures for reasons of catalytic rate or selectivity. The ability of zeolite voids to mimic biological catalysts in their selective stabilization of certain transition states by dispersion forces imparts catalytic diversity, all the more remarkable in light of the similar acid strengths among known aluminosilicates. This offers significant promise to expand the ranges of materials used and of reactions they catalyze.
Author: Andreas Martin
Publisher: MDPI
ISBN: 3038422649
Category : Science
Languages : en
Pages : 1
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Book Description
This book is a printed edition of the Special Issue "Zeolite Catalysis" that was published in Catalysts
Author: Hermenegildo Garcia
Publisher: John Wiley & Sons
ISBN: 3527839267
Category : Technology & Engineering
Languages : en
Pages : 501
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Book Description
Catalysis in Confined Frameworks Understanding the synthesis and applications of porous solid catalysts Heterogeneous catalysis is a catalytic process in which catalysts and reactants exist in different phases. Heterogeneous catalysis with solid catalysts proceeds through the absorption of substrates and reagents which are liquid or gas, and this is largely dependent on the accessible surface area of the solid which can generate active reaction sites. The synthesis of porous solids is an increasingly productive approach to generating solid catalysts with larger accessible surface area, allowing more efficient catalysis. Catalysis in Confined Frameworks: Synthesis, Characterization, and Applications provides a comprehensive overview of synthesis and use of porous solids as heterogeneous catalysts. It provides detailed analysis of pore engineering, a thorough characterization of the advantages and disadvantages of porous solids as heterogeneous catalysts, and an extensive discussion of applications. The result is a foundational introduction to a cutting-edge field. Catalysis in Confined Frameworks: Synthesis, Characterization, and Applications readers will also find: An editorial team comprised of international experts with extensive experience Detailed discussion of catalyst classes including zeolites, mesoporous aluminosilicates, and more A special focus on size selective catalysis Catalysis in Confined Frameworks: Synthesis, Characterization, and Applications is an essential reference for catalytic chemists, organic chemists, materials scientists, physical chemists, and any researchers or industry professionals working with heterogeneous catalysis.
Author: Robert Ted Carr
Publisher:
ISBN:
Category :
Languages : en
Pages : 378
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Book Description
Unequivocal relations between properties of solid Brønsted acids and their catalytic function must be developed further to provide guidance for their design and application. Structure-function relations for solid Brønsted acid catalysis are developed here on Keggin polyoxometalate (POM) clusters and proton forms of zeolites because their well-defined structures permit reliable calculations of their deprotonation energies (DPE) by theory as measures of acid strength. Keggin POM clusters with W-metal atoms, but different central atoms (P, Si, Al, Co), have a wide range of acid strengths and reactivities without concomitant structural changes. Zeolites also have known structures and DPE values that are accessible to theory, however, their acid sites are located within voids of molecular dimensions, which stabilize confined reactants and transition states via van der Waals interactions. CH3OH dehydration and isomerization of C6 alkanes with different backbone structures served as probes of reactivity on these solid acids and provided illustrative examples of how reactions sense the strength and the confining environments of solid acids through the stabilities of intermediates and transition states that mediate them. Rate constants of kinetically-relevant steps in these reactions were obtained from mechanism-based interpretations of rates that were normalized as turnovers by counting the number of accessible protons with 2,6-di-tertbutyl pyridine titrations during catalysis. These rate constants were correlated with the catalyst DPE values in structure-function relations to determine how reactions "sense" the strength and the solvating environments of solid acids. Rate constants decrease exponentially with increasing DPE values on POM clusters for all probe reactions; these trends reflect predominantly higher activation energies on weaker acids because ion-pairs at transition states, a ubiquitous feature of Brønsted acid catalysis, contain less stable conjugate anions. The dependences of rate constants on DPE further suggest that activation energies change by much less than the commensurate change in DPE because the higher energy needed to deprotonate weaker acids is largely recovered at transition states via electrostatic interactions between cationic reactants and the conjugate anion. Isomerization rate constants of C6 alkanes changed similarly with DPE, in spite of large differences in their values. Cyclopropyl carbenium ions mediate each of these isomerizations at transition states of kinetically-relevant steps. Their similar charge distributions interact with conjugate anions equally via electrostatic interactions at transition states; as a result, they compensate for interactions between protons and anions equally and cause similar sensitivities to acid strength. Reactants with lower rate constants have transition state cations with less stable gas-phase analogs, however, because these are properties of non-interacting cations, they are catalyst independent and do not influence a reaction's sensitivity to DPE. Rate constants for water elimination from H-bonded alkanol intermediates are more sensitive to DPE for bimolecular CH3OH dehydration than previously reported for unimolecular butanol dehydration. Unimolecular dehydration transition states have more localized charges than bimolecular dehydration transition states where cationic charges are distributed across multiple reactant molecules. The localized cations at unimolecular dehydration transition states more closely resemble protons and are more effective at interacting with conjugate anions, causing weaker effects of DPE. The effects of DPE are weaker for CH3OH dehydration when rate constants measure transition states from reacting intermediates that are ion-pairs (than from uncharged H-bonded intermediates) because conjugate anions are present at both species and affect their stabilities similarly. Zeolites are significantly weaker acids than Keggin POM clusters according to their DPE values, yet their reactivities fall within the range of POM clusters for these probe reactions. Larger alkane isomerization rate constants are measured on zeolite BEA than are predicted from its DPE value because significant van der Waals forces stabilize confined cyclopropyl carbenium ions at transition states and overcompensate for any additional entropy loss caused by confinement. Transition state solvation reduces isomerization activation energies because they are measured with respect to gas-phase reactants that are unconfined. Confinement of acid sites within the channels of BEA favors alkyl shift reactions over those that change the degree of hydrocarbon branching and also favor reactions that have less branched transition states. Confinement preferentially stabilizes those transition state cations that best interact with zeolite channel walls via van der Waals contacts. The effects of confinement are weaker for CH3OH dehydration when bimolecular transition states are measured with respect to intermediates where both CH3OH reactants are confined than intermediates where one of the CH3OH is unconfined in the gas-phase. These relations demonstrate how fundamental properties of solid acids such as their acid strengths and their confining environments, influence stabilities of relevant intermediates and transition states, and by inference influence reactivity, according to their charges and the sizes of confined species. The effects of acid strength are strongest when uncharged reactive intermediates form transition state cations that interact weakly with conjugate anions because of their diffuse charges. The effects of acid strength weaken as transition states become more similar to a proton or as reacting intermediates also become ion-pairs. The effects of confinement are determined by van der Waals stabilization of transition states; these effects are most pronounced when reactants or reacting intermediates are unconfined. The success of these relations indicates the importance of using well-defined acids whose properties can be assessed unambiguously, counting the number of active sites directly during reactions, and interpreting reactivity as chemical events. The effects of composition on the DPE values and reactivities of Keggin clusters are investigated further using density functional theory (DFT) because their well-defined structures permit reliable calculations of their properties by theoretical methods. DPE values are dissected into energy terms that reflect covalent and electrostatic interactions between protons and anions by using thermochemical cycles. Similar thermochemical cycles describing interaction energies between conjugate anions and organic cations indicate how catalyst composition influences reactivity through the stabilities of transition states and intermediates, specifically shown here for CH3OH dehydration. Central atoms of Keggin clusters influence the densities of delocalized electrons in anions, which determine their electrostatic interactions with cations, while addenda atoms influence both covalent and electrostatic interactions between ions. Central atoms influence the stabilization of protons and organic cations because they both interact with the delocalized electrons. The charge distributions of cations determine how strongly changes in the anionic distribution affect electrostatic interactions. Protons are the cation that is most sensitive to changes in the anion because of their localized charges and close proximities to anions. Addenda atoms influence the stabilities protons much more strongly than ion-pair transition states or intermediates, because the latter have much weaker covalent interactions with anions than the former. As a results, solid acids with different covalent contributions to OH bonds cannot be compared directly using DPE values as the descriptor for acid strength in structure-function relations, because ion-pair transition states do not recover covalent interactions that must be overcome to deprotonate the catalyst. H-atom addition energies (HAE), which are also accessible for Keggin clusters from DFT, probe the local abilities of catalysts to accept H-atoms and electrons. HAE values are accurate descriptors of alkane and alkanol oxidative dehydrogenation (ODH) reactions, because H-atom addition and kinetically-relevant H-abstraction steps in ODH reactions both transfer electrons to unoccupied metal atom orbitals, the energies of which are consequential for ODH rates and HAE values.
Author: Arthur W. Chester
Publisher: Springer Science & Business Media
ISBN: 140209678X
Category : Science
Languages : en
Pages : 372
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Book Description
The idea for putting together a tutorial on zeolites came originally from my co-editor, Eric Derouane, about 5 years ago. I ?rst met Eric in the mid-1980s when he spent 2 years working for Mobil R&D at our then Corporate lab at Princeton, NJ. He was on the senior technical staff with projects in the synthesis and characterization of new materials. At that time, I managed a group at our Paulsboro lab that was responsible for catalyst characterization in support of our catalyst and process development efforts, and also had a substantial group working on new material synthesis. Hence, our interests overlapped considerably and we met regularly. After Eric moved back to Namur (initially), we maintained contact, and in the 1990s, we met a number of times in Europe on projects of joint interest. It was after I retired from ExxonMobil in 2002 that we began to discuss the tutorial concept seriously. Eric had (semi-)retired and lived on the Algarve, the southern coast of Portugal. In January 2003, my wife and I spent 3 weeks outside of Lagos, and I worked parts of most days with Eric on the proposed content of the book. We decided on a comprehensive approach that ultimately amounted to some 20+ chapters covering all of zeolite chemistry and catalysis and gave it the title Zeolite Chemistry and Catalysis: An integrated Approach and Tutorial.
Author: Gadi Rothenberg
Publisher: John Wiley & Sons
ISBN: 3527808906
Category : Science
Languages : en
Pages : 356
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Book Description
This introductory textbook covers all aspects of catalysis. It also bridges computational methods, industrial applications and green chemistry, with over 600 references. The book is aimed at chemistry and chemical engineering students, and is suitable for both senior undergraduate- and graduate-level courses, with many examples and hands-on exercises. The author, a renowned researcher in catalysis, is well known for his clear teaching and writing style (he was voted "lecturer of the year" by the chemistry students). Following an introduction to green chemistry and the basics of catalysis, the book covers the principles and applications of homogeneous catalysis, heterogeneous catalysis and biocatalysis. Each chapter includes up-to-date industrial examples, that demonstrate how catalysis helps our society reach the goals of sustainable development. Since its publication in 2008, Catalysis: Concepts and Green Applications has become the most popular textbook in catalysis. This second edition is updated with the latest developments in catalysis research in academia and industry. It also contains 50 additional exercises, based on the suggestions of students and teachers of chemistry and chemical engineering from all over the world. The book is also available in the Chinese language (https://detail.tmall.com/item.htm?spm=a212k0.12153887.0.0.4e60687dUTEDKm&id=619581126247). Additional teaching material (original figures as PowerPoint lecture slides) is freely available in the Supplementary Material.
Author: Denise Barthomeuf
Publisher: Springer Science & Business Media
ISBN: 146845787X
Category : Science
Languages : en
Pages : 424
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Book Description
Low dimensionality is a multifarious concept which applies to very diversified materials. Thus, examples of low-dimensional systems are structures with one or several layers, single lines or patterns of lines, and small clusters isolated or dispersed in solid systems. Such low dimensional features can be produced in a wide variety of materials systems with a broad spectrum of scientific and practical interests. These features, in turn, induce specific properties and, particularly, specific transport properties. In the case of zeolites, low dimensionality appears in the network of small-diameter pores of molecular size, extending in one, two or three di mensions, that these solids exhibit as a characteristic feature and which explains the term of "molecular sieves" currently used to name these ma terials. Indeed, a large number of industrial processes for separation of gases and liquids, and for catalysis are based upon the use of this low dimensional feature in zeolites. For instance, zeolites constitute the first class of catalysts employed allover the world. Because of the peculiarity and flexibility of their structure (and composition), zeolites can be adapted to suit many specific and diversified applications. For this reason, zeolites are presently the object of a large and fast-growing interest among chemists and chemical engineers.
Author: Mario L. Occelli
Publisher: CRC Press
ISBN: 9780824700799
Category : Science
Languages : en
Pages : 368
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Book Description
Reviews recent accomplishments in the field of fluid cracking catalysts (FCC). Discusses the development of more specialized and effective catalysts and processes as well as the modification of current technology to meet future challenges in fuel refining. Written by nearly 50 internationally recognized experts from academia and industry.
Author: Christian Marcilly
Publisher:
ISBN:
Category : Catalysis
Languages : en
Pages : 456
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Book Description
Author: Jiří Čejka
Publisher: Royal Society of Chemistry
ISBN: 1788011562
Category : Science
Languages : en
Pages : 547
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Book Description
Covering the breadth of zeolite chemistry and catalysis, this book provides the reader with a complete introduction to field, covering synthesis, structure, characterisation and applications. Beginning with the history of natural and synthetic zeolites, the reader will learn how zeolite structures are formed, synthetic routes, and experimental and theoretical structure determination techniques. Their industrial applications are covered in-depth, from their use in the petrochemical industry, through to fine chemicals and more specialised clinical applications. Novel zeolite materials are covered, including hierarchical zeolites and two-dimensional zeolites, showcasing modern developments in the field. This book is ideal for newcomers who need to get up to speed with zeolite chemistry, and also experienced researchers who will find this a modern, up-to-date guide.
