Reactivity and Selectivity Descriptors for the Activation of C–H Bonds in Hydrocarbons and Oxygenates on Metal Oxides

C–H bond activation at lattice O atoms on oxides mediates some of the most important chemical transformations of small organic molecules. The relations between molecular and catalyst properties and C–H activation energies are discerned in this study for the diverse C–H bonds prevalent in C₁–C₄ hydrocarbons and oxygenates using lattice O atoms with a broad range of H atom abstraction properties. These activation energies determine, in turn, attainable selectivities and yields of desired oxidation products, which differ from reactants in their C–H bond strength. Brønsted-Evans–Polanyi (BEP) linear scaling relations predict that C–H activation energies depend solely and linearly on the C–H bond dissociation energies (BDE) in molecules and on the H-atom addition energies (HAE) of the lattice oxygen abstractors. These relations omit critical interactions between organic radicals and surface OH groups that form at transition states that mediate the H atom transfer, which depend on both molecular and catalyst properties; they also neglect deviations from linear relations caused by the lateness of transition states. Thus, HAE and BDE values, properties that are specific to a catalyst and a molecule in isolation, represent incomplete descriptors of reactivity and selectivity in oxidation catalysis. These effects are included here through crossing potential formalisms that account for the lateness in transition states in estimates of activation energies from HAE and BDE and by estimates of molecule-dependent but catalyst-independent parameters that account for diradical interactions that differ markedly for allylic and nonallylic C–H bonds. The systematic ensemble-averaging of activation energies for all C–H bonds in a given molecule show how strong abstractors and high temperatures decrease an otherwise ubiquitous preference for activating the weakest C–H bonds in molecules, thus allowing higher yields of products with C–H bonds weaker than in reactants than predicted from linear scaling relations based on molecule and abstractor properties. Such conclusions contradict the prevailing guidance to improve such yields by softer oxidants and lower temperatures, a self-contradictory strategy, given the lower reactivity of such weaker H-abstractors. The diradical-type interactions, not previously considered as essential reactivity descriptors in catalytic oxidations, may expand the narrow yield limits imposed by linear free energy relations by guiding the design of solids with surfaces that preferentially destabilize allylic radicals relative to those formed from saturated reactants at C–H activation transition states

Methanol Oxidative Dehydrogenation on Oxide Catalysts: Molecular and Dissociative Routes and Hydrogen Addition Energies as Descriptors of Reactivity

The oxidative dehydrogenation (ODH) of alkanols on oxide catalysts is generally described as involving H-abstraction from alkoxy species formed via O–H dissociation. Kinetic and isotopic data cannot discern between such routes and those involving kinetically-relevant H-abstraction from undissociated alkanols. Here, we combine such experiments with theoretical estimates of activation energies and entropies to show that the latter molecular routes prevail over dissociative routes for methanol reactions on polyoxometalate (POM) clusters at all practical reaction temperatures. The stability of the late transition states that mediate H-abstraction depend predominantly on the stability of the O–H bond formed, making H-addition energies (HAE) accurate and single-valued descriptors of reactivity. Density functional theory-derived activation energies depend linearly on HAE values at each O-atom location on clusters with a range of composition (H₃PMo₁₂, H₄SiMo₁₂, H₃PW₁₂, H₄PV₁Mo₁₁, and H₄PV₁W₁₁); both barriers and HAE values reflect the lowest unoccupied molecular orbital energy of metal centers that accept the electron and the protonation energy of O-atoms that accept the proton involved in the H-atom transfer. Bridging O-atoms form O–H bonds that are stronger than those of terminal atoms and therefore exhibit more negative HAE values and higher ODH reactivity on all POM clusters. For each cluster composition, ODH turnover rates reflect the reactivity-averaged HAE of all accessible O-atoms, which can be evaluated for each cluster composition to provide a rigorous and accurate predictor of ODH reactivity for catalysts with known structure. These relations together with oxidation reactivity measurements can then be used to estimate HAE values and to infer plausible structures for catalysts with uncertain active site structures

Toward More Complete Descriptors of Reactivity in Catalysis by Solid Acids

Density functional theory and classical electrostatics are used to develop reactivity descriptors for catalysis by solid acids. Acid strength, as deprotonation energies (DPE), reflects the charge reorganization required to disrupt covalent OH bonds in inorganic acids and the electrostatic forces that resist the separation of protons from conjugate anions. Both charge reorganization (covalent) and electrostatic (ionic) components vary monotonically with DPE on solid acids with different heteroatoms within a given type of oxide framework, but their relative contributions differ among different acid types. Ion-pair transition states recover predominantly the ionic part of the DPE, and the extent to which they recover each component is a unique property of a transition state and thus of an acid-catalyzed reaction, independent of the acid strength or type. These fractional recoveries, together with the ionic and covalent DPE components, a unique property of a solid acid, provide a general and complete descriptor of reactivity, which we illustrate here for diverse reactions (proton shuttling, H₂O elimination, methyl shift, ring contraction) on several types of solid acids (Mo- and W-based polyoxometalate clusters with S, P, Si, Al, and Co central atoms and MFI type heterosilicates with Al, Ga, Fe, and B heteroatoms). For protons confined within small voids of heterosilicates, the transition state stabilization and reactivity depend additionally on van der Waals interactions that are unrelated to acid strength

Kinetic and Theoretical Insights into the Mechanism of Alkanol Dehydration on Solid Brønsted Acid Catalysts

Elementary steps that mediate ethanol dehydration to alkenes and ethers are determined here from rate and selectivity data on solid acids of diverse acid strength and known structure and free energies derived from density functional theory (DFT). Measured ethene and ether formation rates that differed from those expected from accepted monomolecular and bimolecular routes led to our systematic enumeration of plausible dehydration routes and to a rigorous assessment of their contributions to the products formed. H-bonded monomers, protonated alkanol dimers, and alkoxides are the prevalent bound intermediates at conditions relevant to the practice of dehydration catalysis. We conclude that direct and sequential (alkoxide-mediated) routes contribute to ether formation via S_N2-type reactions; alkenes form preferentially from sequential routes via monomolecular and bimolecular syn-E2-type eliminations; and alkoxides form via bimolecular S_N2-type substitutions. The prevalence of these elementary steps and their kinetic relevance are consistent with measured kinetic and thermodynamic parameters, which agree with values from DFT-derived free energies and with the effects of acid strength on rates, selectivities, and rate constants; such effects reflect the relative charges in transition states and their relevant precursors. Dehydration turnover rates, but not selectivities, depend on acid strength because transition states are more highly charged than their relevant precursors, but similar in charge for transition states that mediate the competing pathways responsible for selectivity

Mechanistic Evidence for Sequential Displacement–Reduction Routes in the Synthesis of Pd–Au Clusters with Uniform Size and Clean Surfaces

Bimetallic Pd–Au clusters with (Pd/Au)_at compositions of 0.5, 1.0, and 2.0 narrowly distributed in size were prepared using colloidal methods with reagents containing only C, H, and O atoms, specifically polyvinyl alcohol (PVA) as protecting species and ethanol as the organic reductant. Synthesis protocols involved contacting a solution of Au precursors with nearly monodisperse Pd clusters. The formation of Pd–Au clusters was inferred from the monotonic growth of clusters with increasing Au content and confirmed by the in situ detection of Au plasmon bands in their UV–visible spectra during synthesis. Specifically, transmission electron microscopy (TEM) showed that growth rates were proportional to the surface area of the clusters, and rigorous deconvolution and background subtraction allowed for determination of the intensity and energy of Au-derived plasmon bands. This feature emerged during initial contact between Au precursors and Pd clusters apparently because Au³⁺ species deposit as Au⁰ using Pd⁰ as the reductant in a fast galvanic displacement process consistent with their respective redox potentials. The plasmon band ultimately disappeared as a result of the subsequent slower reduction of the displaced Pd²⁺ species by ethanol and of their deposition onto the bimetallic clusters. Such displacement–reduction pathways are consistent with the thermodynamic redox tendencies of Au, Pd, and ethanol and lead to the conclusion that such triads (two metals and an organic reductant) can be chosen from thermodynamic data and applied generally to the synthesis of bimetallic clusters with other compositions. These bimetallic clusters were dispersed on mesoporous γ-Al₂O₃ supports, and PVA was removed by treatment in ozone at near-ambient temperature without any detectable changes in cluster size. The absence of strongly bound heteroatoms, ubiquitous in many other colloidal synthesis protocols, led to Al₂O₃-dispersed clusters with chemisorption uptakes consistent with their TEM-derived cluster size, thus demonstrating that cluster surfaces are accessible and free of synthetic debris. The infrared spectra of chemisorbed CO indicated that both Pd and Au were present at such clean surfaces but that any core–shell intracluster structure conferred by synthesis was rapidly destroyed by adsorption of catalytically relevant species, even at ambient temperature; this merely reflects the thermodynamic tendency and kinetic ability of an element to segregate and to decrease surface energies when it binds an adsorbate more strongly than another element in bimetallic particles

Mechanism of Isobutanal–Isobutene Prins Condensation Reactions on Solid Brønsted Acids

The selectivity to 2,5-dimethyl-hexadiene isomers (2,5-DMH) via acid-catalyzed isobutanal–isobutene Prins condensation is limited by isobutene oligomerization reactions (to 2,4,4-trimethyl-pentene isomers) and by skeletal isomerization and cyclization of the primary 2,5-DMH products of Prins condensation. Experiment and theory are used here to assess and interpret acid strength effects on the reactivity and selectivity for isobutanal–isobutene Prins condensation routes to 2,5-DMH, useful as precursors to <i>p</i>-xylene. Non-coordinating 2,6-di-<i>tert</i>-butylpyridine titrants fully suppress reactivity on Keggin heteropolyacids, niobic acid, and mesoporous and microporous aluminosilicates, indicating that Prins condensation, parallel isobutene oligomerization, and secondary skeletal isomerization and cyclization of primary 2,5-DMH products occur exclusively on Brønsted acid sites. The number of titrants required to suppress rates allows site counts for active protons, a requirement for comparing reactivity among solid acids as turnover rates, as well as for the rigorous benchmarking of mechanistic proposals by theory and experiment. Kinetic and theoretical treatments show that both reactions involve kinetically relevant C–C bond formation elementary steps mediated by cationic C–C coupling transition states. Transition state charges increase with increasing acid strength for Prins condensation, becoming full carbenium-ions only on the stronger acids. Oligomerization transition state structures, in contrast, remain full ion-pairs, irrespective of acid strength. Turnover rates for both reactions increase with acid strength, but oligomerization transition states preferentially benefit from the greater stability of the conjugate anions in the stronger acids, leading to higher 2,5-DMH selectivities on weaker acids (niobic acid, aluminosilicates). These trends and findings are consistent with theoretical estimates of activation free energies for Prins condensation and oligomerization elementary steps on aluminosilicate slab and Keggin heteropolyacid cluster models. High 2,5-DMH selectivities require weak acids, which do not form a full ion-pair at transition states and thus benefit from significant stabilization by residual covalency. These trends demonstrate the previously unrecognized consequences of incomplete proton transfer at oxygen-containing transition states in dampening the effects of acid strength, which contrast the full ion-pair transition states and stronger acid strength effects in hydrocarbon rearrangements on solids acids of catalytic relevance. These mechanistic conclusions and the specific example used to illustrate them led us to conclude that reaction routes involving O-containing molecules become prevalent over hydrocarbon rearrangements on weak acids when parallel routes are accessible in mixtures of oxygenate and hydrocarbon reactants

The Roles of Entropy and Enthalpy in Stabilizing Ion-Pairs at Transition States in Zeolite Acid Catalysis

Acidic zeolites are indispensable catalysts in the petrochemical industry because they select reactants and their chemical pathways based on size and shape. Voids of molecular dimensions confine reactive intermediates and transition states that mediate chemical reactions, stabilizing them by van der Waals interactions. This behavior is reminiscent of the solvation effects prevalent within enzyme pockets and has analogous consequences for catalytic specificity. Voids provide the “right fit” for certain transition states, reflected in their lower free energies, thus extending the catalytic diversity of zeolites well beyond simple size discrimination. This catalytic diversity is even more remarkable because acid strength is essentially unaffected by confinement among known crystalline aluminosilicates. In this Account, we discuss factors that determine the “right fit” for a specific chemical reaction, exploring predictive criteria that extend the prevailing discourse based on size and shape. We link the structures of reactants, transition states, and confining voids to chemical reactivity and selectivity.Confinement mediates enthalpy–entropy compromises that determine the Gibbs free energies of transition states and relevant reactants; these activation free energies determine turnover rates via transition state theory. At low temperatures (400–500 K), dimethyl ether carbonylation occurs with high specificity within small eight-membered ring (8-MR) voids in FER and MOR zeolite structures, but at undetectable rates within larger voids (MFI, BEA, FAU, and SiO₂–Al₂O₃). More effective van der Waals stabilization within 8-MR voids leads to lower ion-pair enthalpies but also lower entropies; taken together, carbonylation activation free energies are lower within 8-MR voids. The “right fit” is a “tight fit” at low temperatures, a consequence of how temperature appears in the defining equation for Gibbs free energy.In contrast, entropy effects dominate in high-temperature alkane activation (700–800 K), for which the “right fit” becomes a “loose fit”. Alkane activation turnovers are still faster on 8-MR MOR protons because these transition states are confined only partially within shallow 8-MR pockets; they retain higher entropies than ion-pairs fully confined within 12-MR channels at the expense of enthalpic stability. Selectivities for <i>n</i>-alkane dehydrogenation (relative to cracking) and isoalkane cracking (relative to dehydrogenation) are higher on 8-MR than 12-MR sites because partial confinement preferentially stabilizes looser ion-pair structures; these structures occur later along reaction coordinates and are higher in energy, consistent with Marcus theory for charge-transfer reactions. Enthalpy differences between cracking and dehydrogenation ion-pairs for a given reactant are independent of zeolite structure (FAU, FER, MFI, or MOR) and predominantly reflect the different gas-phase proton affinities of alkane C–C and C–H bonds, as expected from Born–Haber thermochemical cycles. These thermochemical relations, together with statistical mechanics-based treatments, predict that rotational entropy differences between intact reactants and ion-pair transition states cause intrinsic cracking rates to increase with <i>n</i>-alkane size.Through these illustrative examples, we highlight the effects of reactant and catalyst structures on ion-pair transition state enthalpies and entropies. Our discussion underscores the role of temperature in mediating enthalpic and entropic contributions to free energies and, in turn, to rates and selectivities in zeolite acid catalysis

Experimental and Theoretical Evidence for the Reactivity of Bound Intermediates in Ketonization of Carboxylic Acids and Consequences of Acid–Base Properties of Oxide Catalysts

Ketonization of carboxylic acids on metal oxides enables oxygen removal and the formation of new C–C bonds for increasing the energy density and chemical value of biomass-derived streams. Information about the surface coverages and reactivity of various bound species derived from acid reactants and the kinetic relevance of the elementary steps that activate reactants, form C–C bonds, and remove O atoms and how they depend on acid–base properties of surfaces and molecular properties of reactants is required to extend the range of ketonization catalytic practice. Here, we examine such matters for ketonization of C₂–C₄ carboxylic acids on monoclinic and tetragonal ZrO₂ (ZrO₂(m), ZrO₂(t)) materials that are among the most active and widely used ketonization catalysts by combining kinetic, isotopic, spectroscopic, and theoretical methods. Ketonization turnovers require Zr–O acid–base pairs, and rates, normalized by the number of active sites determined by titration methods during catalysis, are slightly higher on ZrO₂(m) than ZrO₂(t), but exhibit similar kinetic dependence and the essential absence of isotope effects. These rates and isotope effects are consistent with surfaces nearly saturated with acid-derived species and with kinetically limited C–C bond formation steps involving 1-hydroxy enolates formed via α-C–H cleavage in bound carboxylates and coadsorbed acids; these mechanistic conclusions, but not the magnitude of the rate parameters, are similar to those on anatase TiO₂ (TiO₂(a)). The forms of bound carboxylic acids at Zr–O pairs become more stable and evolve from molecular acids to dissociated carboxylates as the combined acid and base strength of the Zr and O centers at each type of site pair increases; these binding properties are estimated from DFT-derived NH₃ and BF₃ affinities. Infrared spectra during ketonization catalysis show that molecularly bound acids and monodentate and bidentate carboxylates coexist on ZrO₂(m) because of diversity of Zr–O site pairs that prevails on such surfaces, distinct in coordination and consequently in acid and base strengths, and that monodentate and bidentate carboxylates are the most abundant species on saturated ZrO₂ surfaces, consistent with their DFT-derived binding strengths. Theoretical assessments of free energies along the reaction coordinate show that monodentate carboxylates act as precursors to reactive 1-hydroxy enolate intermediates, while strongly bound bidentate carboxylates are unreactive spectators. Higher 1-hydroxy enolate coverages, brought forth by stabilization on the more strongly basic O sites on ZrO₂(m), account for the more reactive nature of ZrO₂(m) than TiO₂(a). These findings indicate that the elementary steps and site requirements for ketonization of C₂–C₄ carboxylic acids are similar on M–O site pairs at TiO₂ and ZrO₂ surfaces, a conclusion that seems general to other metal oxides of comparable acid–base strength

Synthesis of Bimetallic AuPt Clusters with Clean Surfaces via Sequential Displacement-Reduction Processes

We report the synthesis of bimetallic AuPt nanoparticles (3.3–4.3 nm) of uniform size and composition using colloidal methods and reagents containing only C, H, O, and N. These clusters were dispersed onto SiO₂ and treated at low temperatures in the presence of reductants to remove all surface residues without concomitant agglomeration, thus leading to bimetallic structures suitable for mechanistic inquiries into bimetallic effects on surface reactivity. Synthesis protocols exploit and generalize galvanic displacement-reduction (GDR) processes previously used to prepare AuPd clusters; these routes promote bimetallic mixing but become more challenging for systems (e.g., AuPt) with smaller reduction potential differences and less favorable mixing enthalpies than AuPd. These hurdles are addressed here through procedural modifications that inhibit the formation of large Au-rich clusters, which compromise size and compositional uniformity. In doing so, we extend GDR techniques to endothermic alloys with elements of more similar redox properties. Higher temperatures and lower Au³⁺ precursor concentrations promoted metal mixing and inhibited homogeneous and heterogeneous nucleation. Cluster size and compositional uniformity were confirmed by UV–visible spectroscopy during and after colloid formation, transmission electron microscopy, and high-angle annular dark-field (HAADF) imaging with energy-dispersive X-ray spectroscopy (EDS). Particle-by-particle EDS analysis and HAADF imaging demonstrated the prevalence of GDR processes in AuPd bimetallic cluster assembly. These methods also showed that size-dependent intracluster diffusion during AuPt cluster formation, driven by unfavorable AuPt mixing thermodynamics, leads to Au surface enrichment, thus promoting autocatalytic Au deposition. This rigorous mechanistic comparison of AuPt and AuPd systems provides essential guidance and specific control variables and procedures for the synthesis of other bimetallic systems based on the redox potential differences and mixing thermodynamics of their two components

Transition-State Enthalpy and Entropy Effects on Reactivity and Selectivity in Hydrogenolysis of <i>n</i>‑Alkanes

Statistical mechanics and transition state (TS) theory describe rates and selectivities of C–C bond cleavage in C₂–C₁₀ <i>n</i>-alkanes on metal catalysts and provide a general description for the hydrogenolysis of hydrocarbons. Mechanistic interpretation shows the dominant role of entropy, over enthalpy, in determining the location and rate of C–C bond cleavage. Ir, Rh, and Pt clusters cleave C–C bonds at rates proportional to coverages of intermediates derived by removing 3–4 H-atoms from <i>n</i>-alkanes. Rate constants for C–C cleavage reflect large activation enthalpies (Δ<i>H</i>^⧧, 217–257 kJ mol^–1) that are independent of chain length and C–C bond location in C₄₊ <i>n</i>-alkanes. C–C bonds cleave because of large, positive activation entropies (Δ<i>S</i>^⧧, 164–259 J mol^–1 K^–1) provided by H₂ that forms with TS. Kinetic and independent spectroscopic evidence for the composition and structure of these TS give accurate estimates of Δ<i>S</i>^⧧ for cleavage at each C–C bond. Large differences between rate constants for ethane and n-decane (∼10⁸) reflect an increase in the entropy of gaseous alkanes retained at the TS. The location of C–C bond cleavage depends solely on the rotational entropies of alkyl chains attached to the cleaved C–C bond, which depend on their chain length. Such entropy considerations account for the ubiquitous, but previously unexplained, preference for cleaving nonterminal C–C bonds in <i>n</i>-alkanes. This mechanistic analysis and thermodynamic treatment illustrates the continued utility of such approaches even for hydrogenolysis reactions, with complexity seemingly beyond the reach of classical treatments, and applies to catalytic clusters beyond those reported here (0.6–2.7 nm; Ir, Rh, Pt)