44 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

The redox properties of Fe₃O₄ surfaces are central to many catalytic processes and enable dynamic, reduction-induced morphological restructuring during reactions. Here, we investigate the structural changes of the Fe₃O₄(001) surface during the decomposition of formic and acetic acids using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. Both acids readily deprotonate, forming ordered carboxylate overlayers on the surface. Product formation pathways involve the removal of lattice oxygen, resulting in extensive surface restructuring. For formic acid, only a modest level of surface oxygen removal (∼3%) is observed, resulting in elongated pits along the octahedral Fe rows andmore » exhibiting an aspect ratio of ∼3. In contrast, acetic acid induces more extensive reduction, with the removal of ∼20% of surface oxygen, yielding significantly larger pits while maintaining a similar aspect ratio. Repeated exposure to acetic acid further enlarges the pits, indicating preferential etching at step edges. DFT calculations reveal a mechanistic sequence in which lattice oxygen removal destabilizes adjacent Fe atoms, promoting their migration into the bulk and subsequent pit propagation and step edge formation. Together, these findings provide atomistic insights into the coupling between carboxylic acid conversion and oxide surface restructuring, underscoring the strong interplay between redox chemistry and morphological changes on catalytically active Fe₃O₄ surfaces.« less

Radioiodine released during the nuclear-fuel cycle constitutes a persistent radiological hazard. In this study, the IO₃^– uptake mechanism of CoAl LDH was resolved by combining pH-controlled sorption experiments, synchrotron XAFS, and DFT-based AIMD simulations. At pH close to 6, approximately 90% of IO₃^– was removed, and the equilibrium distribution coefficient reached about 1.7 × 10⁴ mL g^–1. EXAFS analysis indicated an average iodine–oxygen bond length of 1.81 Å and a coordination number near 3, with the fit R-factor equal to 0.002. The simulations faithfully reproduced the experimental spectrum and revealed transient proton hopping events that generated metastable I–O–H species insidemore » the interlayer, thereby confirming nitrate-to-iodate exchange as the controlling capture pathway. Atomic density profiles and radial distribution functions further showed that IO₃^– adopt an end-on orientation perpendicular to the hydroxide sheets, while water molecules mediate proton migration without disturbing the host lattice. In conclusion, the integrated experimental–computational evidence demonstrates that CoAl LDH can rapidly and selectively sequester IO₃^– under near-neutral conditions, offering atomic scale guidance for the rational engineering of layered sorbents for advanced radioactive-waste treatment.« less

Polyolefins and their chlorinated derivatives such as polyvinyl chloride (PVC) are among the most prevalent plastics in global production and waste streams. Traditional waste-to-energy methods such as incineration and pyrolysis, as well as most chemical upcycling methods for PVC utilization, require thorough, high-temperature dechlorination to prevent the release of toxic chlorinated compounds. Here, we present here a strategy for upgrading discarded PVC into chlorine-free fuel range hydrocarbons and hydrogen chloride in a single-stage process catalyzed by chloroaluminate ionic liquids. This approach offsets endothermic dechlorination and carbon-carbon bond cleavage with exothermic alkylation and hydrogen transfer by isobutane or isopentane in amore » low-temperature tandem process. The light isoalkanes are available from refinery processes and partly from recycling of the product stream. This process is suitable for handling real-world mixed and contaminated PVC and polyolefin waste streams.« less

Understanding the quantitative interactions among zeolite pore walls, Bro̷nsted acid sites, and molecules with both polar and nonpolar regions is essential for scoping out the potential of zeolites as sorbents and catalysts. Purely siliceous zeolites (MFI and Beta in the present study) are hydrophobic, whereas those containing aluminum are considered hydrophilic, preferentially adsorbing organic molecules even in aqueous environments. To characterize these interactions, we use primary alcohols of increasing molecular weight, quantifying their specific interactions in the confined pore space of the alkyl (CH_x) and OH groups. Three types of interactions were identified: (i) alkyl CH_x groups interacting with themore » zeolite pore walls (approximately 10 kJ mol⁻¹ per carbon), (ii) alcohol OH groups interacting with the pore walls (30−35 kJ mol⁻¹), and (iii) alcohol OH groups interacting with Bro̷nsted acid sites (37 kJ mol⁻¹). All three interactions were well mirrored by computational simulations. The contribution of the alkyl CH_x groups was inferred from the incremental increase in sorption enthalpy with increasing molecular weight; the interaction strength of the OH groups was determined by extrapolating the global adsorption enthalpy of the alcohols to a hypothetical OH group without an alkyl group. This value was identical to the adsorption enthalpy of water. The experiments demonstrated that only water has an adsorption enthalpy on zeolite pore walls lower than its condensation enthalpy (30−35 kJ mol⁻¹ vs 45 kJ mol⁻¹), limiting the concentration of water that can be adsorbed.« less

For neural network potentials (NNPs) to gain widespread use, researchers must be able to trust model outputs. However, the blackbox nature of neural networks and their inherent stochasticity are often deterrents, especially for foundation models trained over broad swaths of chemical space. Uncertainty information provided at the time of prediction can help reduce aversion to NNPs. In this work, we detail two uncertainty quantification (UQ) methods. Readout ensembling, by finetuning the readout layers of an ensemble of foundation models, provides information about model uncertainty, while quantile regression, by replacing point predictions with distributional predictions, provides information about uncertainty within themore » underlying training data. We demonstrate our approach with the MACE-MP-0 model, applying UQ to the foundation model and a series of finetuned models. The uncertainties produced by the readout ensemble and quantile methods are demonstrated to be distinct measures by which the quality of the NNP output can be judged.« less

Understanding the role of surface structure and hydroxylation in catalytic reactions on metal oxide surfaces is important for developing a mechanistic insight into the complex interface processes. Here, we investigate the reactivity of formic acid on reconstructed Fe₃O₄(001) using a combination of X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, temperature-programmed reaction spectroscopy, low energy electron diffraction, and electronic structure calculations. We find that formic acid initially dissociates at low temperatures (< 80 K) into bidentate formate and a hydroxyl up to an initial dosed coverage of two HCOOH per Fe₃O₄(001) unit cell. At higher temperatures (> 450 K), formate largelymore » decomposes along the dehydration pathway, producing CO and H₂O, with dehydrogenation to CO₂ being a minority side reaction. As a first step, water formation leads to surface oxygen extraction via the Mars-van Krevelen mechanism. Computational studies reveal formate embedded in oxygen vacancies as a key intermediate in the CO formation mechanism. CO formation proceeds via two reaction pathways with desorption that peaks at 530 K on the hydroxyl-rich surface and 560 K on the hydroxyl-deficient surface. Atomic hydrogen coadsorption experiments and ab initio calculations reveal that the presence of surface hydroxyls reduces the CO formation barrier. Furthermore, these results highlight the complex interactions between substrate and intermediate species occurring during reactions on metal oxide surfaces.« less

The spin of diffuse electrons has been proposed in the literature as qubit for quantum hardware applications. Here we provide the first investigation of the thermal stability for a newly reported family of materials with diffuse electrons. This material has a diamond-like grid of Li⁺ centers bridged by diamine chains NH₂(CH₂)_nH₂N of varying carbon length. The tetracoordinated lithium-amine center is surrounded by one diffuse electron solvated by the N–H bonds. Previous work has demonstrated the tunability of the electronic structure of this material, with short chain lengths producing a metallic material and longer a semiconductor. Density functional theory-based ab initiomore » molecular dynamics simulations are employed to characterize the thermal stability and melting point of the crystalline material. Calculations show that the thermal stability ranges from 100 to 220 K, primarily depending on the carbon chain length, with longer chains increasing the stability. Melting of the material is characterized by dissociation of the diamine coordination and formation of disordered clumps of undercoordinated Li-diamine centers. These melting points are well above temperatures used in typical quantum computing applications. The computational study provides insight into avenues for the future development of similar materials and the improvement of their stability.« less

The stability and activity of supported single-atom catalysts (SACs) represent critical yet opposing factors limiting our ability to explore and exploit their unique properties. Here, this study demonstrates the operation of a switchable catalyst that is activated in the presence of surface intermediates and reverts back to a stable but inactive form when the reaction is completed. We employ atomically defined Rh-Fe₃O₄(001) catalysts to demonstrate how structurally stable Rh, bound in surface octahedral Fe sites, gets destabilized to form highly active Rh adatoms and small clusters. Conversion of formic acid, leading initially to surface formate and hydroxyl species, is employedmore » as a model reaction to probe the dynamics of such processes. We find that surface hydroxyl recombination to water through the Mars van Krevelen mechanism reduces Rh coordination, triggering its conversion to active Rh adatoms. Since such lattice oxygen exchange is observed in many acid-base and redox chemistries, the process can be broadly applicable to controlling the activation of the range of SACs.« less

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. Crackingmore » 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.« less