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David W. Flaherty - One of the best experts on this subject based on the ideXlab platform.

  • Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts
    ACS Catalysis, 2018
    Co-Authors: Daniel T. Bregante, Nicholas E. Thornburg, Justin M. Notestein, David W. Flaherty
    Abstract:

    Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing...

  • Catalytic thiophene oxidation by groups 4 and 5 framework-substituted zeolites with hydrogen peroxide: Mechanistic and spectroscopic evidence for the effects of Metal Lewis acidity and solvent Lewis basicity
    Journal of Catalysis, 2018
    Co-Authors: Daniel T. Bregante, Ami Y. Patel, Alayna M. Johnson, David W. Flaherty
    Abstract:

    Abstract Group 4 (Ti and Zr) and 5 (Nb and Ta) atoms substituted into the *BEA zeolite framework (M-BEA) irreversibly activate hydrogen peroxide (H2O2) and form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) intermediates active for the oxidation of 2,5-dimethylthiophene (C6H8S), a model reactant representative of organosulfur species in fossil reserves and chemical weapons. Sequential oxidation pathways convert C6H8S into 2,5-dimethylthiophene oxide (C6H8SO) and subsequently into 2,5-dimethylthiophene dioxide by oxidative dearomatization. Oxidation rates measured as functions of reactant concentrations together with in situ UV–vis spectra show that all M-BEA activate H2O2 to form pools of M-OOH and M-(η2-O2), which then react with either C6H8S or H2O2 to form the sulfoxide or to decompose into H2O and O2, respectively. Turnover rates for C6H8S oxidation and H2O2 decomposition both increase exponentially with the electron affinity of the active site, which is quantitatively probed via the adsorption enthalpy for deuterated acetonitrile to active sites. C6H8S oxidation rates depend also on the nucleophilicity of the solvent used, and rates decrease in the order acetonitrile > p-dioxane ∼ acetone > ethanol ∼ methanol. In situ UV–vis spectra show that highly nucleophilic solvent molecules compete effectively for active sites, inhibit H2O2 activation and formation of reactive M-OOH and M-(η2-O2) species, and give lower turnover rates. Consequently, this work shows that turnover rates for sulfoxidation are highest when highly electrophilic active sites (i.e., stronger Lewis acids) are paired with weakly nucleophilic solvents, which can guide the design of increasingly productive catalytic systems for sulfide oxidation.

  • Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts
    2018
    Co-Authors: Daniel T. Bregante, Nicholas E. Thornburg, Justin M. Notestein, David W. Flaherty
    Abstract:

    Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C8H8 epoxidation. Values of ΔH⧧ for C8H8 epoxidation and H2O2 decomposition also decrease linearly with the ligand-to-Metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H2O2 decomposition rates, which shows that more electrophilic M-OOH and M-(η2-O2) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of ΔH⧧ for C8H8 epoxidation and adsorption enthalpies for C8H8 within the pores of *BEA and SiO2 reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C8H8 epoxidation with respect to the 5.4 nm pores of M-SiO2, while H2O2 decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H2O2. Thus, the differences in reactivity and selectivity between M-BEA and M-SiO2 materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteriathe electronegativities of the transition Metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways

Daniel T. Bregante - One of the best experts on this subject based on the ideXlab platform.

  • Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts
    ACS Catalysis, 2018
    Co-Authors: Daniel T. Bregante, Nicholas E. Thornburg, Justin M. Notestein, David W. Flaherty
    Abstract:

    Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing...

  • Catalytic thiophene oxidation by groups 4 and 5 framework-substituted zeolites with hydrogen peroxide: Mechanistic and spectroscopic evidence for the effects of Metal Lewis acidity and solvent Lewis basicity
    Journal of Catalysis, 2018
    Co-Authors: Daniel T. Bregante, Ami Y. Patel, Alayna M. Johnson, David W. Flaherty
    Abstract:

    Abstract Group 4 (Ti and Zr) and 5 (Nb and Ta) atoms substituted into the *BEA zeolite framework (M-BEA) irreversibly activate hydrogen peroxide (H2O2) and form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) intermediates active for the oxidation of 2,5-dimethylthiophene (C6H8S), a model reactant representative of organosulfur species in fossil reserves and chemical weapons. Sequential oxidation pathways convert C6H8S into 2,5-dimethylthiophene oxide (C6H8SO) and subsequently into 2,5-dimethylthiophene dioxide by oxidative dearomatization. Oxidation rates measured as functions of reactant concentrations together with in situ UV–vis spectra show that all M-BEA activate H2O2 to form pools of M-OOH and M-(η2-O2), which then react with either C6H8S or H2O2 to form the sulfoxide or to decompose into H2O and O2, respectively. Turnover rates for C6H8S oxidation and H2O2 decomposition both increase exponentially with the electron affinity of the active site, which is quantitatively probed via the adsorption enthalpy for deuterated acetonitrile to active sites. C6H8S oxidation rates depend also on the nucleophilicity of the solvent used, and rates decrease in the order acetonitrile > p-dioxane ∼ acetone > ethanol ∼ methanol. In situ UV–vis spectra show that highly nucleophilic solvent molecules compete effectively for active sites, inhibit H2O2 activation and formation of reactive M-OOH and M-(η2-O2) species, and give lower turnover rates. Consequently, this work shows that turnover rates for sulfoxidation are highest when highly electrophilic active sites (i.e., stronger Lewis acids) are paired with weakly nucleophilic solvents, which can guide the design of increasingly productive catalytic systems for sulfide oxidation.

  • Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts
    2018
    Co-Authors: Daniel T. Bregante, Nicholas E. Thornburg, Justin M. Notestein, David W. Flaherty
    Abstract:

    Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C8H8 epoxidation. Values of ΔH⧧ for C8H8 epoxidation and H2O2 decomposition also decrease linearly with the ligand-to-Metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H2O2 decomposition rates, which shows that more electrophilic M-OOH and M-(η2-O2) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of ΔH⧧ for C8H8 epoxidation and adsorption enthalpies for C8H8 within the pores of *BEA and SiO2 reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C8H8 epoxidation with respect to the 5.4 nm pores of M-SiO2, while H2O2 decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H2O2. Thus, the differences in reactivity and selectivity between M-BEA and M-SiO2 materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteriathe electronegativities of the transition Metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways

Nisheeth C. Desai - One of the best experts on this subject based on the ideXlab platform.

  • Zeolite Y encaged Ru(III) and Fe(III) complexes for oxidation of styrene, cyclohexene, limonene, and α-pinene: An eye-catching impact of H2SO4 on product selectivity
    Journal of Molecular Catalysis A: Chemical, 2017
    Co-Authors: Dinesh R. Godhani, Haresh D. Nakum, Digvijaysinh K. Parmar, Jignasu P. Mehta, Nisheeth C. Desai
    Abstract:

    Abstract A novel Ru(III) and Fe(III) complexes of ligands 1 and/or 2 {where 1 = 2,2′-((1E,1′E)-((azanediylbis(ethane-2,1-diyl))bis(azanylylidene))bis(methanylylidene))diphenol and 2 = 2,2′-((1E,1′E)-((azanediylbis(ethane-2,1-diyl))bis(azanylylidene))bis(methanylylidene)) bis(4-nitrophenol)} have been synthesized as ‘neat’ and zeolite Y encapsulated complexes. These catalysts are characterized by various analytical tools such as FTIR, UV–vis, elemental analysis, ICP-AES, molar conductivity, 1 H- and 13 C NMR, TGA, SEM, AAS, BET, magnetic susceptibility and powder XRD to endorse the complex formation, absence of peripheral redundant ligands and complexes, conservation of zeolite Y morphology and crystallinity, and the encapsulation of complexes without devastation in the zeolite Y framework. Out of these synthesized catalysts, 5Y is found to be a potent candidate for styrene (Conv. 76.1%, TOF: 2130 h −1 ), cyclohexene (Conv. 84.4%, TOF: 2351 h −1 ), limonene (Conv. 81.6%, TOF: 2273 h −1 ), and α-pinene (Conv. 72.6%, TOF: 2023 h −1 ) oxidation with high selectivity of respective allylic products excluding the styrene oxidation, which undergoes epoxidation only. The addition of H 2 SO 4 in an identical reaction catalyzed by 5Y not only surge the conversion up to 100% in a short time span with high TOF but also increase the selectivity of respective epoxidation products. This switchover in the selectivities could be credited to the presence of H 2 SO 4 that facilitates the heterolytic O O bond cleavage of Metal Hydroperoxide and stimulates the epoxidation over allylic oxidation. Furthermore, the results establish that the heterogeneous systems are effortlessly recovered and reused without ample drop in the activity and selectivity.

  • A hierarchical zeolite-Y hampered Metallo-ligand complexes for selective oxidation: A mechanistic point of view
    Microporous and Mesoporous Materials, 2016
    Co-Authors: Dinesh R. Godhani, Haresh D. Nakum, Digvijaysinh K. Parmar, Jignasu P. Mehta, Nisheeth C. Desai
    Abstract:

    Abstract A series of Ni 2+ and Mn 2+ complexes with ligands 1 and 2 derived from the condensation of 1-(2-hydroxyphenyl)ethan-1-one and/or 1-(5-chloro-2-hydroxyphenyl)ethan-1-one with ethane-1,2-diamine have been synthesized as neat and zeolite Y enslaved complexes. The structures of these complexes were established on the basis of various physicochemical (XRD, ICP-AES, elemental analysis, BET, SEM, magnetic measurements, and TGA) and spectroscopic studies (UV–vis and FTIR). The catalytic performance of these hybrid materials was scrutinized for the oxidation of cyclohexene, phenol, styrene, and benzene using 30% H 2 O 2 as an oxidant. Among all catalysts, 3Y ably catalyzed the cyclohexene (100%), phenol (39.2%), styrene (99.3%), and benzene (20.7%) with the higher selectivity of Cyclohex-2-en-1-one (55.6%), catechol (73.2%), benzaldehyde (87.5%), and phenol (80.7%), respectively. It has been revealed that the presence of electron-withdrawing substituents on the aromatic ring of catalyst degrades the catalytic activity and the selectivity of products. The results show that the heterogeneous systems are easily salvaged and reused without substantial fall in the activity and selectivity. Moreover, the involvement of Metal Hydroperoxide during the catalytic reaction is confirmed by the preparation, characterization, and utilization of Metal Hydroperoxide of complex 4 as catalyst over phenol oxidation without oxidant.

Justin M. Notestein - One of the best experts on this subject based on the ideXlab platform.

  • Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts
    ACS Catalysis, 2018
    Co-Authors: Daniel T. Bregante, Nicholas E. Thornburg, Justin M. Notestein, David W. Flaherty
    Abstract:

    Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing...

  • Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts
    2018
    Co-Authors: Daniel T. Bregante, Nicholas E. Thornburg, Justin M. Notestein, David W. Flaherty
    Abstract:

    Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C8H8 epoxidation. Values of ΔH⧧ for C8H8 epoxidation and H2O2 decomposition also decrease linearly with the ligand-to-Metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H2O2 decomposition rates, which shows that more electrophilic M-OOH and M-(η2-O2) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of ΔH⧧ for C8H8 epoxidation and adsorption enthalpies for C8H8 within the pores of *BEA and SiO2 reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C8H8 epoxidation with respect to the 5.4 nm pores of M-SiO2, while H2O2 decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H2O2. Thus, the differences in reactivity and selectivity between M-BEA and M-SiO2 materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteriathe electronegativities of the transition Metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways

Nicholas E. Thornburg - One of the best experts on this subject based on the ideXlab platform.

  • Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts
    ACS Catalysis, 2018
    Co-Authors: Daniel T. Bregante, Nicholas E. Thornburg, Justin M. Notestein, David W. Flaherty
    Abstract:

    Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing...

  • Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts
    2018
    Co-Authors: Daniel T. Bregante, Nicholas E. Thornburg, Justin M. Notestein, David W. Flaherty
    Abstract:

    Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO2) irreversibly activate hydrogen peroxide (H2O2) to form pools of Metal-Hydroperoxide (M-OOH) and peroxide (M-(η2-O2)) species for alkene epoxidation. The product distributions from reactions with Z-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H2O2-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η2-O2) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C8H8) epoxidation reveal that these materials first adsorb and then irreversibly activate H2O2 to form pools of interconverting M-OOH and M-(η2-O2) intermediates, which then react with styrene or H2O2 to form either styrene oxide or H2O2 decomposition products, respectively. Activation enthalpies (ΔH⧧) for C8H8 epoxidation and H2O2 decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C8H8 epoxidation. Values of ΔH⧧ for C8H8 epoxidation and H2O2 decomposition also decrease linearly with the ligand-to-Metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H2O2 decomposition rates, which shows that more electrophilic M-OOH and M-(η2-O2) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of ΔH⧧ for C8H8 epoxidation and adsorption enthalpies for C8H8 within the pores of *BEA and SiO2 reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C8H8 epoxidation with respect to the 5.4 nm pores of M-SiO2, while H2O2 decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H2O2. Thus, the differences in reactivity and selectivity between M-BEA and M-SiO2 materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteriathe electronegativities of the transition Metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways