Oliver Y. Gutiérrez

Solvents can substantially alter reaction rates by modifying the thermodynamic states of substrates and the free-energy landscapes of elementary steps in acid-catalyzed reactions. Cyclohexanol dehydration within BEA-type zeolite was investigated experimentally and theoretically to elucidate solvent effects on elementary steps. Adsorbed alcohol monomers and dimers were identified in acetone, cyclohexanol, 1,4-dioxane, methanol, and decalin. Hydrated hydronium ions formed in water at Brønsted acid sites (BAS), while solvents interact with protonated cyclohexanol. Kinetic H/D isotope effects (KIE) revealed solvent-dependent rate-determining steps of C–O cleavage (KIE < 2) in acetone, 1,4-dioxane, and decalin, while C–H cleavage (KIE > 2) dominates in water and methanol. Ab initio molecular dynamics simulations showed that acetone induces dissociation of protonated cyclohexanol from BAS to form solvated cyclohexanol-acetone complexes, whereas 1,4-dioxane and decalin leave the alcohol bound to the framework, providing a molecular-level rationale for higher dehydration rates in acetone. Solvation in acetone, 1,4-dioxane, and methanol destabilizes the initial state (excess chemical potential) by 11–13 kJ/mol, compared to 6–7 kJ/mol in water and decalin. The excess chemical potential of the transition state correlates well with the free-energy barrier of the rate-determining step. The excess chemical potential of C–O cleavage transition states in acetone, 1,4-dioxane, and decalin shows greater stabilization (−21 to −11 kJ/mol) than C–H cleavage in water and methanol (−5 to −1 kJ/mol). The combined initial-state and transition-state (de)stabilization yields reaction rates up to 3 orders of magnitude higher in acetone compared to water and decalin.

Pd-catalyzed hydrogenation of benzaldehyde to benzyl alcohol in the aqueous phase is significantly enhanced relative to open-circuit conditions when H2 pressure and an external cathodic potential are applied simultaneously. This dual-mode operation affords hydrogenation rates up to 6-fold higher than those observed under open-circuit conditions. The rate enhancement exceeds the expected charge-transfer contribution by at least a factor of 2 at all investigated potentials, indicating a non-Faradaic promotion effect. Combined experimental and theoretical analyses suggest that cathodic polarization increases the quasi-equilibrium surface coverage of a hydroxy intermediate formed via a proton-coupled electron transfer step preceding the rate-determining addition of the second hydrogen atom. At more negative potentials, where this coverage saturates, further rate acceleration likely arises from modulation of the standard free energies of intermediates and transition states along the rate-determining pathway, thereby lowering the apparent standard activation free energy.

Solvent molecules in nanoporous catalysts are not passive media: they occupy pore volume, compete with reactants for active-site environments, and reshape adsorption thermodynamics. However, the structural rules that determine whether a solvent is expelled from a liquid-filled micropore or remains co-confined with a reactant are not well established. Here we show that zeolite topology switches solvent-reactant competition between two distinct regimes: solvent eviction in narrow MFI channels and solvent-reactant co-occupation in FAU supercages. Motion-sensitive in situ ² H magic-angle-spinning NMR directly resolves confined and mobile decalin populations during cyclohexanol adsorption, revealing efficient displacement of decalin from MFI but persistent co-confinement in FAU. Liquid-phase adsorption isotherms and calorimetry show that cyclohexanol adsorption in MFI is enthalpy-dominated with a large entropy penalty, whereas FAU exhibits weaker enthalpic stabilization and a smaller entropy loss. Machine-learning-enhanced free-energy simulations rationalize this topology dependence by revealing an approximately 25 kJ mol ^− 1 preference for cyclohexanol over decalin at Brønsted acid sites (BAS) in MFI, while the larger FAU cages generate a broader landscape that permits co-adsorption. These results establish pore topology as a molecular switch that controls solvent participation in liquid-filled zeolite micropores, providing a mechanistic criterion for designing catalytic environments for liquid-phase chemistry.

Lewis acidic chloroaluminate ionic liquid diluted in dichloromethane catalyzes the disproportionation of alkanes, such as isopentane, via carbenium ions in a reaction readily initiated by carbenium ion precursors such as tert-butyl chloride (TBC). The carbenium ion-AlCl₄^- ion-pairs stabilized by the ionic liquid-dichloromethane solution are the key intermediates in two distinctive kinetic regimes, i.e., a transient regime (0-5 min) and a steady-state regime (after 5 min). The transient regime constitutes the majority of isopentane conversion and is governed by the initial carbenium ion concentration. In the steady-state regime, disproportionation occurs at a considerably lower rate, affected by the carbenium ion concentration, the concentration of the ionic liquid, and the reaction temperature. The formation of alkenes observed in the ¹H NMR spectra of the reacting substrates, along with the DFT calculations, suggests that deprotonation of carbenium ion-pairs reduces their concentration, decreasing, in turn, the reaction rate. Kinetic modeling indicates that the transient regime is significantly controlled by the hydride transfer (k_HT) and the deprotonation rate constants (k_DP), while the steady-state regime is additionally influenced by the alkene protonation rate constant (k_-DP). The overall activation energy of the reaction at the steady state, expressed as E_{a,steady-state regime} = E_a,HT - E_a,DP + E_a,-DP, was 54 kJ/mol. The reaction mechanism and the kinetics highlight the potential of Lewis acid-catalyzed conversions of hydrocarbons under remarkably mild conditions.

Ru/C-catalyzed polyethylene (PE) and hydrocarbon hydrogenolysis under quasi-supercritical fluid of isopentane was kinetically and mechanistically investigated. PE hydrogenolysis with C-C and C-H cleavage showed zeroth order, suggesting strong adsorption of hydrocarbons. PE yielded broad product distribution of heavy (C_21-40) and diesel-range (C_11-20) hydrocarbons in the primary step of hydrogenolysis due to stochastic C-C cleavage over Ru surface. Catalytic hydrogenolysis of n-hexadecane, squalane, and light hydrocarbons such as n-pentane, iso-pentane, and n-hexane further described C-C cleavage reactivity between primary and secondary carbons, i.e., ¹C-²C and ²C-²C, which has an order of magnitude higher hydrogenolysis rate than that involving a tertiary carbon. The PE saturated Ru surface and lower C-C cleavage reactivity of tertiary carbon in iso-pentane, therefore, imited sovlent conversion during hydrogenolysis, whereas leading to selective PE conversion. Using hexadecane, we observed comparable hydrogenolysis rates between H₂ and D₂ (k _H /k _D ∼ 1), indicating the kinetically relevant step of C-C cleavage with facilitating C-H cleavage and rehydrogenation. However, the normal kinetic isotope effect between hexadecane and deuterated hexadecane (k C₁₆H₃₄ /k C₁₆D₃₄ ∼ 5) revealed that the dehydrogenation, i.e., C-H cleavage, can be kinetically involved in the hydrogenolysis kinetic. By considering the 8-fold lower H-D exchange rate with deuterated hexadecane compared to n-hexadecane, the lower rate for hydrogenolysis and H-D exchange with deuterated hexadecane can be attributed to the C-D bond dissociation energy being 3 kJ/mol higher than that of the C-H bond. Increasing H₂ pressure favors internal C-C bond cleavage over terminal one. This minimizes the formation of lower hydrocarbons, particularly methane. However, the increase in H₂ pressure increases the coverage of adsorbed hydrogen on the Ru particles due to competitive adsorption of H₂ and polyethylene, which, in turn, reduces the polyethylene conversion rates.

Chloroaluminate ionic liquids selectively transform (waste) polyolefins into gasoline-range alkanes through tandem cracking-alkylation at temperatures below 100 °C. Further improvement of this process necessitates a deep understanding of the nature of the catalytically active species and the correlated performance in the catalyzing critical reactions for the tandem polyolefin deconstruction with isoalkanes at low temperatures. Here, we address this requirement by determining the nuclearity of the chloroaluminate ions and their interactions with reaction intermediates, combining in situ ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy, in situ Raman spectroscopy, Al K-edge X-ray absorption near edge structure spectroscopy, and catalytic activity measurement. Cracking and alkylation are facilitated by carbenium ions initiated by AlCl₃-tert-butyl chloride (TBC) adducts, which are formed by the dissociation of Al₂Cl₇⁻ in the presence of TBC. The carbenium ions activate the alkane polymer strands and advance the alkylation cycle through multiple hydride transfer reactions. In situ ¹H NMR and operando infrared spectroscopy demonstrate that the cracking and alkylation processes occur synchronously; alkenes formed during cracking are rapidly incorporated into the carbenium ion-mediated alkylation cycle. The conclusions are further supported by ab initio molecular dynamics simulations coupled with an enhanced sampling method, and model experiments using n-hexadecane as a feed.

In the presence of water, hydronium ions formed within the micropores of zeolite H-BEA significantly influence the surrounding environment and the reactivity of organic substrates. The positive charge of these ions, coupled with the zeolite's negatively charged framework, results in an ionic environment that causes a strongly nonideal solvation behavior of cyclohexanol. This leads to a significantly higher excess chemical potential in the initial state and stabilizes at the same time the charged transition state in the dehydration of cyclohexanol. As a result, the free-energy barrier of the reaction is lowered, leading to a marked increase in the reaction rates. Nonetheless, there is a limit to the reaction rate enhancement by the hydronium ion concentration. Experiments conducted with low concentrations of reactants show that beyond an optimal concentration, the required spatial rearrangement between hydronium ions and cyclohexanols inhibits further increases in the reaction rate, leading to a peak in the intrinsic activity of hydronium ions. The quantification of excess chemical potential in both initial and transition states for zeolites H-BEA, along with findings from HMFI, provides a basis to generalize and predict rates for hydronium-ion-catalyzed dehydration reactions in Brønsted zeolites.

In situ Al K-edge X-ray absorption near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS) spectroscopy in conjunction with ab initio molecular dynamics (AIMD) simulations show that adsorption of 1-propanol alters the structure of the Bronsted acid site through changes in the associated aluminum-oxygen tetrahedron in zeolite H-MFI. The decreasing intensity of the pre-edge signal of the in situ Al K-edge XANES spectra with increasing 1-propanol coverage shows that Al T-sites become more symmetric as the sorbed alcohol molecules form monomers, dimers, and trimers. The adsorption of monomeric 1-propanol on Bronsted acid sites reduces the distortion of the associated Al T-site, shortens the Al-O distance, and causes the formation of a Zundel-like structure. With dimeric and trimeric alcohol clusters, the zeolite proton is fully transferred to the alcohols and the aluminum-oxygen tetrahedron becomes fully symmetric. The subtle changes in Al-K-edge XANES in the presence of sorbate structures, with the use of theory, are used to probe the local zeolite structures and provide a basis to predict the population and chemical state of the sorbed species.