Acid Catalysis Mediated by Aqueous Hydronium Ions Formed by Contacting Zeolite Crystals with Liquid Water.

Zeolites are crystalline microporous aluminosilicates widely used as solid acids in catalytic routes to clean and sustainable energy carriers and chemicals from biogenic and fossil feedstocks. This study addresses how zeolites act as weak polyprotic acids and dissociate to form extra-crystalline hydronium (HO) ions in liquid water. The extent of their dissociation depends on the energy required to form the conjugate framework anions, which becomes unfavorable as the extent of dissociation increases intracrystalline charge densities because repulsive interactions ultimately preclude the detachment of all protons as catalytically relevant HO(aq) ions.

Heterolytic C-H Activation Routes in Catalytic Dehydrogenation of Light Alkanes on Lewis Acid-Base Pairs at ZrO Surfaces.

Alkane dehydrogenation is an enabling route to make alkenes useful as chemical intermediates. This study demonstrates the high reactivity of Lewis acid-base (LAB) site pairs at ZrO powders for dehydrogenation of C-C alkanes and the essential requirement for chemical treatments to remove strongly bound HO and CO titrants to avoid the high temperatures required for their desorption and the concomitant loss of active sites through sintering and annealing of ZrO crystallites. The energies and free energies of bound intermediates and transition states from density functional theory (DFT), taken together with kinetic analysis and isotopic methods, demonstrated the kinetic relevance and heterolytic character of the first C-H activation at terminal C-atoms for all alkanes with a modest activation barrier (84 kJ mol) at essentially bare Zr-O LAB site pairs.

Dynamics of Elementary Steps on Metal Surfaces at High Coverages: The Prevalence and Kinetic Competence of Contiguous Bare-Atom Ensembles.

The rate of elementary steps on densely-covered surfaces depends sensitively on repulsive interactions within dense adlayers, situations ubiquitous in practice and with kinetic consequences seldom captured by Langmuirian treatments of surface catalysis. This study develops an ensemble-based method that assesses how such repulsion influences the prevalence and kinetic competence of bare-atom ensembles of different size. Chemisorbed CO (CO*) is used as an example because it forms dense adlayers on metal nanoparticles during CO hydrogenation (CO-H) and other reactions, leading to significant repulsion that weakens the binding of CO* and kinetically-relevant transition states (TS).

Theoretical Assessment of the Mechanism and Active Sites in Alkene Dimerization on Ni Monomers Grafted onto Aluminosilicates: (Ni-OH) Centers and C-C Coupling Mediated by Lewis Acid-Base Pairs.

Ni-based solids are effective catalysts for alkene dimerization, but the nature of active centers and identity and kinetic relevance of bound species and elementary reactions remain speculative and based on organometallic chemistry. Ni centers grafted onto ordered MCM-41 mesopores lead to well-defined monomers that are rendered stable by the presence of an intrapore nonpolar liquid, thus enabling accurate experimental inquiries and indirect evidence for grafted (Ni-OH) monomers. Density functional theory (DFT) treatments presented here confirm the plausible involvement of pathways and active centers not previously considered as mediators of high turnover rates for C-C alkenes at cryogenic temperatures.

Mechanistic Connections between CO and CO Hydrogenation on Dispersed Ruthenium Nanoparticles.

Catalytic routes for upgrading CO to CO and hydrocarbons have been studied for decades, and yet the mechanistic details and structure-function relationships that control catalytic performance have remained unresolved. This study elucidates the elementary steps that mediate these reactions and examines them within the context of the established mechanism for CO hydrogenation to resolve the persistent discrepancies and to demonstrate inextricable links between CO and CO hydrogenation on dispersed Ru nanoparticles (6-12 nm mean diameter, 573 K). The formation of CH from both CO-H and CO-H reactants requires the cleavage of strong C≡O bonds in chemisorbed CO, formed as an intermediate in both reactions, via hydrogen-assisted activation pathways.

Reactivity descriptors in acid catalysis: acid strength, proton affinity and host-guest interactions.

Brønsted acids mediate chemical transformations via proton transfer to bound species and interactions between the conjugate anion and bound cationic intermediates and transition states that are also stabilized by van der Waals forces within voids of molecular dimensions in inorganic hosts. This Feature Article describes the relevant descriptors of reactivity in terms of the properties of acids and molecules that determine their ability to donate and accept protons and to reorganize their respective charges to optimize their interactions at bound states. The deprotonation energy (DPE) of the acids and the protonation energy (E) of the gaseous analogs of bound intermediates and transition states reflect their respective properties as species present at non-interacting distances.

First-principles theoretical assessment of catalysis by confinement: NO-O reactions within voids of molecular dimensions in siliceous crystalline frameworks.

Density functional theory methods that include dispersive forces are used to show how voids of molecular dimensions enhance reaction rates by the mere confinement of transition states analogous to those involved in homogeneous routes and without requiring specific binding sites or structural defects within confining voids. These van der Waals interactions account for the observed large rate enhancements for NO oxidation in the presence of purely siliceous crystalline frameworks. The minimum free energy paths for NO oxidation within chabazite (CHA) and silicalite (SIL) frameworks involve intermediates similar in stoichiometry, geometry, and kinetic relevance to those involved in the homogeneous route.

Entropy-Driven High Reactivity of Formaldehyde in Nucleophilic Attack by Enolates on Oxide Surfaces.

CHOH dehydrogenation on a metal function occurs in tandem with C-C coupling of HCHO with enolates derived from alkanals or alkanones on acid-base pairs at anatase TiO surfaces with very high specificity for nucleophilic attack by enolates on HCHO over larger carbonyl molecules. The measured rate constants for enolate coupling with HCHO are >10-fold larger than for its coupling with acetone. Free energies derived from theoretical treatments of reactions between C-C bound enolates and carbonyls show that such specificity for nucleophilic attack on HCHO reflects smaller entropy losses upon formation of the transition state (TS), instead of enthalpic effects caused by weaker steric effects or the stronger electrophilic character of HCHO compared with larger carbonyls.

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces.

High CO* coverages lead to rates much higher than Langmuirian treatments predict because co-adsorbate interactions destabilize relevant transition states less than their bound precursors. This is shown here by kinetic and spectroscopic data-interpreted by rate equations modified for thermodynamically nonideal surfaces-and by DFT treatments of CO-covered Ru clusters and lattice models that mimic adlayer densification. At conditions (0.

Stability of bound species during alkene reactions on solid acids.

This study reports the thermodynamics of bound species derived from ethene, propene, -butene, and isobutene on solid acids with diverse strength and confining voids. Density functional theory (DFT) and kinetic data indicate that covalently bound alkoxides form C-C bonds in the kinetically relevant step for dimerization turnovers on protons within TON (0.57 nm) and MOR (0.

Theoretical insights into the sites and mechanisms for base catalyzed esterification and aldol condensation reactions over Cu.

Condensation and esterification are important catalytic routes in the conversion of polyols and oxygenates derived from biomass to fuels and chemical intermediates. Previous experimental studies show that alkanal, alkanol and hydrogen mixtures equilibrate over Cu/SiO and form surface alkoxides and alkanals that subsequently promote condensation and esterification reactions. First-principle density functional theory (DFT) calculations were carried out herein to elucidate the elementary paths and the corresponding energetics for the interconversion of propanal + H to propanol and the subsequent C-C and C-O bond formation paths involved in aldol condensation and esterification of these mixtures over model Cu surfaces.

Catalytic routes to fuels from C and oxygenate molecules.

This account illustrates concepts in chemical kinetics underpinned by the formalism of transition state theory using catalytic processes that enable the synthesis of molecules suitable as fuels from C and oxygenate reactants. Such feedstocks provide an essential bridge towards a carbon-free energy future, but their volatility and low energy density require the formation of new C-C bonds and the removal of oxygen. These transformations are described here through recent advances in our understanding of the mechanisms and site requirements in catalysis by surfaces, with emphasis on enabling concepts that tackle ubiquitous reactivity and selectivity challenges.

Kinetic and Mechanistic Assessment of Alkanol/Alkanal Decarbonylation and Deoxygenation Pathways on Metal Catalysts.

This study combines theory and experiment to determine the kinetically relevant steps and site requirements for deoxygenation of alkanols and alkanals. These reactants deoxygenate predominantly via decarbonylation (C-C cleavage) instead of C-O hydrogenolysis on Ir, Pt, and Ru, leading to strong inhibition effects by chemisorbed CO (CO*). C-C cleavage occurs via unsaturated species formed in sequential quasi-equilibrated dehydrogenation steps, which replace C-H with C-metal bonds, resulting in strong inhibition by H2, also observed in alkane hydrogenolysis.

Prevalence of Bimolecular Routes in the Activation of Diatomic Molecules with Strong Chemical Bonds (O2, NO, CO, N2) on Catalytic Surfaces.

Dissociation of the strong bonds in O2, NO, CO, and N2 often involves large activation barriers on low-index planes of metal particles used as catalysts. These kinetic hurdles reflect the noble nature of some metals (O2 activation on Au), the high coverages of co-reactants (O2 activation during CO oxidation on Pt), or the strength of the chemical bonds (NO on Pt, CO and N2 on Ru). High barriers for direct dissociations from density functional theory (DFT) have led to a consensus that "defects", consisting of low-coordination exposed atoms, are required to cleave such bonds, as calculated by theory and experiments for model surfaces at low coverages.

Ionic and covalent stabilization of intermediates and transition states in catalysis by solid acids.

Reactivity descriptors describe catalyst properties that determine the stability of kinetically relevant transition states and adsorbed intermediates. Theoretical descriptors, such as deprotonation energies (DPE), rigorously account for Brønsted acid strength for catalytic solids with known structure. Here, mechanistic interpretations of methanol dehydration turnover rates are used to assess how charge reorganization (covalency) and electrostatic interactions determine DPE and how such interactions are recovered when intermediates and transition states interact with the conjugate anion in W and Mo polyoxometalate (POM) clusters and gaseous mineral acids.

Encapsulation of metal clusters within MFI via interzeolite transformations and direct hydrothermal syntheses and catalytic consequences of their confinement.

The encapsulation of metal clusters (Pt, Ru, Rh) within MFI was achieved by exchanging cationic metal precursors into a parent zeolite (BEA, FAU), reducing them with H2 to form metal clusters, and transforming these zeolites into daughter structures of higher framework density (MFI) under hydrothermal conditions. These transformations required MFI seeds or organic templates for FAU parent zeolites, but not for BEA, and occurred with the retention of encapsulated clusters. Clusters uniform in size (1.

Kinetic, spectroscopic, and theoretical assessment of associative and dissociative methanol dehydration routes in zeolites.

Mechanistic interpretations of rates and in situ IR spectra combined with density functionals that account for van der Waals interactions of intermediates and transition states within confining voids show that associative routes mediate the formation of dimethyl ether from methanol on zeolitic acids at the temperatures and pressures of practical dehydration catalysis. Methoxy-mediated dissociative routes become prevalent at higher temperatures and lower pressures, because they involve smaller transition states with higher enthalpy, but also higher entropy, than those in associative routes. These enthalpy-entropy trade-offs merely reflect the intervening role of temperature in activation free energies and the prevalence of more complex transition states at low temperatures and high pressures.

Metal-catalyzed C-C bond cleavage in alkanes: effects of methyl substitution on transition-state structures and stability.

Methyl substituents at C-C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C2-C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C-C cleavage transition states form via equilibrated dehydrogenation steps that replace several C-H bonds with C-metal bonds, desorb H atoms (H*) from saturated surfaces, and form λ H2(g) molecules. Activation enthalpies (ΔH(‡)) and entropies (ΔS(‡)) and λ values for (3)C-(x)C cleavage are larger than for (2)C-(2)C or (2)C-(1)C bonds, irrespective of the composition of metal clusters or the cyclic/acyclic structure of the reactants.

Transition-state enthalpy and entropy effects on reactivity and selectivity in hydrogenolysis of n-alkanes.

Statistical mechanics and transition state (TS) theory describe rates and selectivities of C-C bond cleavage in C2-C10 n-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 n-alkanes.

Mechanistic role of water on the rate and selectivity of Fischer-Tropsch synthesis on ruthenium catalysts.

Water increases Fischer-Tropsch synthesis (FTS) rates on Ru through H-shuttling processes. Chemisorbed hydrogen (H*) transfers its electron to the metal and protonates the O-atom of CO* to form COH*, which subsequently hydrogenates to *HCOH* in the kinetically relevant step. H2 O also increases the chain length of FTS products by mediating the H-transfer steps during reactions of alkyl groups with CO* to form longer-chain alkylidynes and OH*.

Consequences of metal-oxide interconversion for C-H bond activation during CH4 reactions on Pd catalysts.

Mechanistic assessments based on kinetic and isotopic methods combined with density functional theory are used to probe the diverse pathways by which C-H bonds in CH4 react on bare Pd clusters, Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO clusters. C-H activation routes change from oxidative addition to H-abstraction and then to σ-bond metathesis with increasing O-content, as active sites evolve from metal atom pairs (*-*) to oxygen atom (O*-O*) pairs and ultimately to Pd cation-lattice oxygen pairs (Pd(2+)-O(2-)) in PdO. The charges in the CH3 and H moieties along the reaction coordinate depend on the accessibility and chemical state of the Pd and O centers involved.