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. This work provides a foundation for further inquiry into the contributions of H-bonded methanol and methoxy species in homologation and hydrocarbon synthesis reactions from methanol.

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. This work provides a foundation for further inquiry into the contributions of H-bonded methanol and methoxy species in homologation and hydrocarbon synthesis reactions from methanol.

The elementary steps and site requirements for the oxidation of NO on Rh and Co and the oxidation state of the catalysts were probed by isotopic tracers, chemisorption methods, and kinetic measurements of the effects of the pressures of NO, O₂, and NO₂ on turnover rates. On both catalysts, NO oxidation rates were first order in NO and O₂ and were inversely proportional to NO₂ pressure, as observed on Pt and PdO. These data implied that O₂ activation on an isolated vacancy (*) on the catalyst surfaces that were saturated with oxygen (O*) was the kinetically relevant step. Quasi-equilibrated NO–NO₂ interconversion steps established the coverage of * and O* and the chemical potential of oxygen during the catalysis. These chemical potentials set the oxidation state of Rh and Co clusters and were described by an O₂ virtual pressure, which was determined from the formalism of non-equilibrium thermodynamics. RhO₂ and Co₃O₄ were the phases that were present during NO oxidation, which had several consequences for catalysis. Turnover rates increased with increasing cluster size because the vacancies that were needed for O₂ activation were more abundant on large oxide clusters, which delocalized electrons better than small clusters. NO oxidation turnover rates on RhO₂ and Co₃O₄ were higher than expected from the oxygen-binding energy on Rh and Co metal surfaces and from the reduction potentials of Rh³⁺ and Co²⁺. These NO oxidation rates were consistent with the rates on Pt and PdO when one-electron-reduction processes, which were accessible for Rh⁴⁺ and Co³⁺ but not for Pt²⁺ and Pd²⁺, were used to describe the reactivity of RhO₂ and Co₃O₄. One-electron redox cycles caused the ¹⁶O₂–¹⁸O₂ exchange rates to be higher than the NO oxidation rates, in contrast with their analogous values on Pt and PdO, although O₂ activation on the vacancies limited NO oxidation and O₂ exchange on all of the catalysts. One-electron redox cycles allowed electron sharing between metal cations and a facile route to form vacancies on RhO₂ and Co₃O₄. This interpretation of the data highlighted the role of vacancies in kinetically relevant O₂-activation steps to explain the higher reactivity of larger metal and oxide clusters and to provide a common framework to describe NO oxidation and the active species on catalysts of practical interest.

Kinetic, isotopic, and chemical analysis methods are used to examine the identity and kinetic relevance of elementary steps and the effects of Pt cluster size on thiophene hydrodesulfurization (HDS) turnover rates. Quasi-equilibrated H₂ and H₂S heterolytic dissociation steps lead to sulfur chemical potentials given by the prevalent H₂S/H₂ ratio and to cluster surfaces with a metallic bulk, but near-saturation sulfur coverages, during steady-state catalysis. Sulfur-vacancies on such surfaces are required for η¹(S) or η⁴ thiophene adsorption modes and for H₂ and H₂S dissociation steps. H-assisted CS bond cleavage of η¹(S) thiophene and H-addition to η⁴ thiophene limits rates of direct desulfurization and hydrogenation sulfur removal pathways, respectively. These steps, their kinetic relevance, and the prevalent sulfur-saturated surfaces resemble those on Ru clusters; they are also consistent with the observed kinetic effects of reactants and products on rates, with the rapid isotopic exchange in H₂/D₂/H₂S mixtures during HDS catalysis, and with measured H₂/D₂ kinetic isotope effects. Small Pt clusters exhibit lower turnover rates, stronger inhibition by H₂S, and a greater preference for desulfurization pathways than those of large clusters. These effects reflect the prevalence of coordinatively unsaturated corner and edge sites on small clusters, which bind sulfur atoms more strongly and lead to lower densities of vacancies and to a preference for η¹(S)-bound thiophene species. Sulfur binding energies and their concomitant effects on the number of available vacancies also account for the higher turnover rates measured on Pt clusters compared with Ru clusters of similar size. These data and their mechanistic interpretation suggest that the concepts and steps proposed here apply generally to hydrogenation and direct desulfurization of organosulfur compounds. Taken together with similar observed effects of oxygen binding strength, metal identity, and cluster size for oxidation reactions of NO, hydrocarbons, and oxygenates, which also require vacancies in their respective kinetically relevant steps, these data also indicate that low reactivity of small clusters may reflect in most instances their coordinative unsaturation and the concomitant kinetic and thermodynamic preference for low vacancy concentrations on nearly saturated surfaces.

We provide kinetic and isotopic evidence for the co-homologation of linear and branched alkanes with dimethyl ether (DME) to form larger branched alkanes and isobutane on H-BEA zeolites, and for the role of adamantane as a hydride transfer co-catalyst that allows the activation of CH bonds in alkanes at low temperatures (<500 K) on Brønsted acid sites. Branched alkanes (isobutane, isopentane, and 2,3-dimethylbutane) present in equimolar mixtures with DME form the corresponding alkenes via hydride transfer to bound alkoxides (formed in DME homologation steps) and subsequent deprotonation; these alkenes, derived from the added alkanes, are then methylated to lengthen their chain by using DME-derived C₁ species, as shown by the isotopologues formed in reactions of ¹³C-DME with ¹²C-alkanes. Linear alkanes are much less reactive than branched alkanes, because of their stronger CH bonds and larger carbenium ion formation energies, which determines hydride-transfer rates to a given acceptor molecule. Adamantane increased the hydride-transfer rates to bound alkoxides from branched alkanes, and even from unreactive linear alkanes, while also increasing their extent of incorporation into DME homologation pathways; adamantane acts as a reversible hydrogen donor that mediates dehydrogenation of alkanes at low temperatures on acid sites. The co-homologation of alkanes with DME avoids the need for carbon rejection in the form of arenes to satisfy the hydrogen balance in the DME conversion to alkanes, provides a robust strategy for increasing the chain length and extent of branching in light alkanes through the selective addition of C₁ species, and mitigates the formation of unsaturated by-products ubiquitous in the homologation of DME or methanol on Brønsted acids.

O₂ reacts with propanediols via homogeneous pathways at 400–500 K. 1,2-Propanediol forms CH₃CHO, HCHO, and CO₂ via oxidative CC cleavage and acetone via dehydration routes, while symmetrical 1,3-propanediol undergoes dehydration and oxidative dehydrogenation to form, almost exclusively, acrolein (ca. 90 % selectivity). The products formed and their kinetic dependence on reactant concentrations are consistent with radical-mediated pathways initiated by O₂ insertion into CH bonds in a β position relative to oxygen atoms in diol reactants. Propagation involves β-scission reactions that form hydroxyl and hydroxyalkyl radicals. Acrolein/O₂/H₂O mixtures from the homogeneous oxidation of 1,3-propanediol form acrylic acid (with 90 % yield) in tandem reactors containing molybdenum-vanadium oxide catalysts. These data reveal the unique reactivity of diols, compared with triols and alkanols, in homogeneous oxidations, while also providing useful insight into the molecular basis for reactivity in biomass-derived oxygenates.