39 Search Results

Chloroaluminate ionic liquid catalyzes the disproportionation of alkanes, a reaction readily initiated by carbenium-ion precursors such as tert-butyl chloride, resulting in equimolar amounts of isobutane and methylpentanes. The carbenium ion-AlCl4- ion-pairs stabilized by the ionic liquid are the key intermediates in two distinctive kinetic phases, i.e., a transi-ent phase (0-5 minutes) and a steady-state phase (after 5 minutes). The transient phase constitutes the majority of iso-pentane conversion and is governed by the initial carbenium ion concentration. In the steady-state phase, disproportiona-tion occurs at a considerably lower rate, affected by the carbenium ion concentration, the concentration of the ionic liq-uid, andmore » the reaction temperature. The formation of olefins observed in the 1H NMR spectra of the reacting substrates, along with the DFT calculations, suggests that dehydrochlorination of active carbenium ion-pairs reduces their concentra-tion, decreasing, in turn, the reaction rate. Kinetic modeling indicates that the transient phase is significantly controlled by the hydride transfer (kHT) and the dehydrochlorination rate constants (kDC), while the steady-state phase is additional-ly influenced by the hydrochlorination rate constant (k–DC). The overall activation energy of the reaction at the steady state, expressed as Ea,steady-state = Ea,HT – Ea,DC + Ea,–DC, was 54 kJ/mol. The reaction mechanism and the kinetics highlight the potential of Lewis acid-catalyzed conversions of hydrocarbons under remarkably mild conditions.« less

Anchoring divalent metal ions in the same zeolite framework with similar Si/Al ratio selectively as zeolite-bound M⁺² or [M⁺²-OH]⁺¹ cationic species enables critical comparison of the species’ intrinsic reactivity for industrially and fundamentally relevant reactions. H-BEA zeolites with similar Si/Al ratios but differing framework Al siting were used to anchored multiple divalent metal cations (Ni, Pd, Pt, Cr, Cu) in the zeolite micropores. State-of-the-art infrared (IR) spectroscopy, electron paramagnetic resonance (EPR) measurements, including two-dimensional pulsed HYSCORE EPR, extended X-ray absorption fine structure (EXAFS), and density functional theory (DFT) calculations together provide unambiguous evidence for the selective formation of divalent metalmore » cations as M⁺²/2Al species (for H-BEA prepared in the conventional hydroxide media), and [M⁺²OH]⁺¹/1Al species for H-BEA prepared in HF. Solid-state proton-decoupled triple-quantum magic-angle spinning (3Q MAS) NMR measurements confirmed contrasting Al distributions in the two H-BEA zeolites, which led to a contrasting divalent cation speciation. The reactivities of the two cationic species were explored for catalytic and adsorptive applications in both organometallic homogeneous and heterogeneous catalysis. This work demonstrates their divergent reactivity in ethylene dimerization, ethylene oxidation (Wacker process), selective catalytic reduction (SCR) of NO, NO adsorption, and methane oxidation. Both M⁺²/2Al and [M⁺²OH]⁺¹/1Al cations are both active for ethylene dimerization, but [M⁺²OH]⁺¹/1Al species show higher reaction rates for each Pd, Ni, Pt. [M⁺²OH]⁺¹/1Al is active for acetaldehyde formation in Wacker ethylene oxidation. A new active site for ethylene oligomerization is proposed that possesses a terminal OH group (Cr-OH) in Phillips catalysts evident by a nearly inactive isolated Cr⁺²/2Al species that contrast an active Cr─OH motif.« less

The shape of supported metal particles regulates their catalytic reactivity and is determined by the degree of wetting between the metal particle and the support surface. Flattened particles that wet support surfaces were reported in various catalytic systems, particularly in the subnanometer size regime. Such consequential metal–support wetting phenomena are poorly understood, and methods to study them on powder catalysts under realistic conditions are lacking. Here, we investigate the size-dependent wetting behaviors of Ru particles on two reducible-oxide supports, anatase TiO₂ (TiO₂-A) and CeO₂, under reducing catalytic conditions. X-ray absorption spectroscopy (XAS), low-energy ion scattering (LEIS), and density functional theorymore » (DFT) are combined to determine the shape of Ru particles. Ru particles remain three-dimensional without wetting the TiO₂-A support within the coverage range studied (0.06–0.98 Ru nm^–2). In contrast, at low coverages (<0.25 Ru nm^–2), Ru wets the CeO₂ support to form flat, disordered structures. The higher wettability of CeO₂ than TiO₂-A is attributed to oxygen vacancies in the near-surface region. The shape difference between small Ru particles or clusters on the two supports leads to drastically contrasting catalytic reactivities in polyolefin hydrogenolysis, despite similar diameters. As a result, this work highlights the implications of metal–support wetting, or cluster shape, on catalytic behaviors of small metal clusters, while establishing the foundation for future systematic studies of such a phenomenon in realistic systems, by delivering a multitechnique methodology and revealing governing fundamental principles.« less

NO_x abatement from H₂ internal combustion engines (H₂-ICEs) is challenging due to high H₂O content and unburned H₂ in the exhaust. This study examines Cu-SSZ-13 SCR catalysts, focusing on the effects of high H₂O and H₂ levels on its activity and stability. High H₂O content typical of H₂-ICE exhaust hinders low-temperature SCR activity by impeding Cu migration and oxidation half cycle efficacy. H₂ slip decreases high-temperature SCR activity by reducing active Cu sites to the inactive CuI state. Combined, high H₂O and H₂ slip reduce SCR performance across all temperatures, making it less effective than in diesel applications. Additionally, agingmore » under high H₂O and H₂ contents induce a severe deterioration of Cu-SSZ-13 via CuO_x formation and dealumination, further degrading catalyst performance. This suggests Cu-SSZ-13 may not be suitable for H₂-ICE aftertreatment, especially given the ongoing development of H₂-ICE itself. Parallel efforts in H₂-ICE and catalyst development are essential to accelerate H₂-ICE deployment.« less

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.more » 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.« less

The continuous development of novel catalytic approaches is crucial for advancing efficient CO₂ hydrogenation processes. Drawing inspiration from single-atom catalysis and 2D materials, we designed a new 2D single-atom catalyst with excellent thermal stability by thermally treating Cu-adsorbed γ-AlOOH nanosheets, which yielded a Cu/γ-Al₂O₃ catalyst with high activity in the hydrogenation of CO₂-yielding methanol (CH₃OH), dimethyl ether (DME), and CO as products. The active Cu sites are monodispersed and highly stable due to their cationic oxidation state and their substitution for pentacoordinated aluminum (Al^P) sites on particle surfaces. This study demonstrates an efficient approach for achieving a high CO₂ hydrogenationmore » rate (30.45 mol mol⁻¹ h⁻¹) using a catalyst system that lacks metallic Cu centers, traditionally considered essential for H₂ dissociation, and employs what was previously thought to be an inert metal oxide (γ-Al₂O₃) for CO and CH₃OH production. Ongoing mechanistic studies aim to elucidate the synergy between cationic Cu single atoms and γ-Al₂O₃, a Lewis acid support, in facilitating hydrogen (H₂) activation and methanol formation.« less

Replacing alkaline for alkaline-earth metal hydroxide in the synthesis gel during the synthesis of siliceous SSZ-13 zeolite (Si/Al~10) yields SSZ-13 with novel, advantageous properties. Its NH₄-form ion-exchanges higher amount of isolated divalent M(II) ions than the conventional one: this is the consequence of an increased number of Al pairs in the structure induced by the +2 charge of Sr(II) cations in the synthesis gel that force two charge-compensating AlO₄– motives to reside closer together. We characterize the +2 state of Co(II) ions in these materials with infra-red spectroscopy and X-ray absorption spectroscopy measurements and show their utility for NOx pollutantmore » adsorption from ambient air: the ones derived from SSZ-13 with higher Al pair content contain more isolated cobalt(II) and, thus, perform better as ambient-air NOx adsorbers. Notably, Co(II)/SSZ-13 with an increased number of Al pairs is significantly more hydrothermally stable than its NaOH-derived analogue. Loading Pd(II) into Co-SSZ-13(Sr) produces an active NOx adsorber (PNA) material that can be used for NOx adsorption from simulated diesel engine exhaust. The critical issue for these applications is hydrothermal stability of Pd-zeolites. Pd/SSZ-13 synthesized in the presence of Sr(OH)₂ does not lose its PNA capacity after extremely harsh aging at 850 and 900 °C (10 h in 10% H₂O/air flow) and loses only ~55% capacity after hydrothermal aging at 930 °C. This can be extended to other divalent metals for catalytic applications, such as copper: we show that Cu/SSZ-13 catalyst can survive hydrothermal aging at 920 °C without losing its catalytic properties, metal dispersion and crystalline structure. Thus, we provide a new, simple, and scalable strategy for making remarkably (hydro)thermally stable metal-zeolite materials/catalysts with a number of useful applications.« less

We report the use of MIL-120 as a water-stable and cost-effective metal-organic framework (MOF) for selectively capturing CO₂ from wet flue gas. Synthesized using inexpensive and environmentally benign reagents in water, MIL-120 possesses one-dimensional pores decorated with hydroxyl-bridged Al(III) ions and benzene rings with an interstitial spacing of 4.78 Å. Carbon dioxide isotherms show steep uptake at low pressure, and the affinity of MIL-120 for CO₂ is 44 kJ mol^–1. CO₂-loading ¹³C solid-state nuclear magnetic resonance and Fourier transform infrared spectra tracking the sorption of CO₂ into MIL-120 revealed that the interplay of pore size, functionality, and dimensionality is vitalmore » for CO₂ restriction within the pores of MIL-120. Breakthrough experiments reveal that MIL-120 can capture CO₂ from dry and wet flue gas with uptake capacities of 1.215 and 1.118 mmol g^–1, respectively. Our work highlights the synthetic benefits of MIL-120 and elucidates its selective capture of CO₂ from wet flue gas.« less

Nitrosyl ions in zeolites that form via the NO + O ₂ reaction react with NH ₃ to form N ₂ and H ₂ O (SCR reaction) via formation of transient diazo N ₂ H ⁺ cation.

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increasemore » and the total number of Bronsted acid sites decreases during steaming. O₅Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al₁O₅ sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.« less