337 Search Results

To meet performance requirements, the next generation of gas separation membranes will need both high gas permeability and selectivity, attainable if we could coax adsorbates to overcome Brownian motion into direction-specific diffusion down a desired axis. In this first-principles computational study, we detail how direction-specific diffusion of CO2 can be achieved in chiral hexagonal boron nitride nanotubes (hBNNT) where the chirality introduces a molecular-level “spin” on CO2 molecules to minimizes collisions and direction changes. hBNNTs with chiral rifling patterns exhibit CO2 diffusion rates faster than non-chiral tubes of comparable and larger diameters. Of the hBNNTs studied, (7,3) tubes appear tomore » be ideally sized (3.7 Å radii) and exhibit an optimal “twist rate,” enabling rapid diffusion with a predicted selectivity (CO2/N2 = 633). Calculations of two hypothetical sheet membranes prepared with aligned chiral hBBNTs have potential to surpass the Robeson upper bound for CO2.« less

Given the increasing concern around greenhouse gas emissions and the decline in the availability of fossil fuels, there is increasing global demand to develop alternate fuels for maritime transportation that are sustainable and which have lower greenhouse gas emissions. Methanol is one such alternative fuel that has garnered considerable attention given its potential to be produced by more sustainable processes and its more favorable greenhouse gas emission profile in comparison with current fossil fuels. Understanding the physical and chemical properties of methanol under a range of conditions is essential for its development as a marine fuel. In this study, wemore » seek to define physical and chemical properties of different methanol samples to simulate real-world storage conditions as these data are lacking in the literature. Several methanol samples were evaluated: nearly pure methanol; International Organization for Standardization (ISO) marine methanol (MM) grades A, B, and C; and methanol plus higher alcohols. We first evaluated all methanol samples for impurities, acetic acid content, density, and distillation range. We then characterized the effects of water absorption and found that methanol can easily absorb unacceptable water content from humid air within hours, necessitating storage conditions that prevent this process. In eight-week aging experiments at 20 degrees C and 40 degrees C in ambient air, we did not observe significant oxidation for any of the methanol samples; however, we did observe increases in acid number. We assessed the impact of contamination of methanol with water, marine gas oil (MGO), and an MGO-biodiesel mixture on density, viscosity, distillation range, and lubricity. Finally, we show that MGO contamination of methanol results in a slight increase in sooting tendency. In aggregate, our results provide an in-depth analysis of physical and chemical properties of methanol as well as the impacts of storage conditions and impurities on the properties of fuel methanol.« less

For much of the past century, carbon dioxide (CO₂) has received little attention scientifically outside of its role as a byproduct in the industrialization of the global economy. This trend has recently been upended where, due to mounting environmental concerns, CO₂ has been brought squarely into the public consciousness. This surge in activity has contributed to a once unimaginable idea now pervading the scientific community: could CO₂, a highly stable byproduct of hydrocarbon combustion, be recycled and converted back into useful chemicals and fuels? Owing to its ubiquitous nature and availability at truly massive quantities, it is thought that CO₂-basedmore » products could offer a meaningful pathway toward lowering the environmental impact of many of the top industrial products while also enhancing supply chain diversification and resilience. In this manuscript we provide a holistic review of the pathways for CO₂ conversion, the underlying chemistry and challenges involved in the transformation to products, and considerations for commercialization.« less

Microwave absorbing catalysts have the potential to electrify high-temperature thermal reactions such as the dry reforming of methane process (DRM: CO₂ + CH₄ → 2CO + 2 H₂). However, microwave catalysts present unique challenges due to their dual requirements of maintaining microwave absorption in both oxidative and reductive environments and stability across a range of temperatures in inherently non-isothermal reactors. Here, catalyst candidates from the La_0.8Sr_0.2CoO₃-La_0.8Sr_0.2NiO₃-La_0.8Sr_0.2MnO₃ perovskite systems were screened (28 total) to identify promising microwave catalysts free of noble metals for dry reforming methane. The best performing candidates met two main criteria. First, they occurred at crystal phase boundaries,more » giving rise to a pseudocubic perovskite structure. The combined use of Goldschmidt tolerance factor and octahedral tolerance factors appeared to be suitable for predicting pseudocubic perovskites. Second, they provided a balance of reducible metal sites with an irreducible metal oxide support. The best performing catalyst was found to exsolve Ni-Co alloy particles as active sites for the DRM reaction which offered superior resistance to coking for excellent reforming efficiency and stability.« less

Electrochemical polarization is now known to play a key role in thermochemical catalysis at solid–liquid interfaces. However, existing frameworks cannot account for why even nonpolar, nonfaradaic reactions are sensitive to interfacial polarization. In order to uncover the molecular basis of this phenomenon, we herein study the potential-dependent reaction kinetics of ethylene and trans-2-butene hydrogenation at Pt–liquid interfaces. Measurements were performed in aqueous and ortho-difluorobenzene (o-DFB) solutions, spontaneously polarizing the Pt–liquid interfaces by, respectively, varying the pH or dissolving distinct metallocene redox buffers into solution. Here, we find that at comparable mechanistic regimes, the rates of both ethylene and trans-2-butene hydrogenationmore » are maximized near the same electrochemical potential, E. Moreover, the potential-dependence, defined as $$\frac{∂ln 𝑟}{∂𝐸}$$, of trans-2-butene hydrogenation is approximately 2.2× greater than that of ethylene hydrogenation across the full potential range studied. These observations are all consistent with a model in which polarization of the Pt surface away from the local potential of zero free charge (E_PZFC) induces electrostatic organization of the polar solvent and charged ions near the interface, which impedes olefin adsorption and surface reaction because these surface reactions induce electrolyte displacement. Accordingly, interfacial polarization alters the free energy landscape and thus the rate of nonpolar heterogeneous catalysis by controlling the degree of electrostatic organization of polar and charged spectators at the interface, which do not in general need to be specifically chemisorbed onto the surface but could simply be close enough to the surface to be perturbed by the olefin adsorption. These results point toward electrochemical design handles, namely, the electrolyte, catalyst potential, and local E_PZFC of the catalyst, with which to tune interfacial catalysis of thermochemical organic transformations.« less

Heterogeneous mixtures of bio-derived oxygenates are promising feedstocks for synthetic aviation fuel (SAF), but conversion strategies for one common component—short-chain (C_5–7) internal ketones—are lacking. Previous work has shown that cyclization of ketones over H/ZSM-5 is limited by its high productivity of light paraffins. We study 4-heptanone upgrading over H/ZSM-5 and show that aromatics and olefins can be formed at high carbon yield when operating at up to 90% conversion. The yield of desirable products is not impacted by the introduction of a recycle stream of the unconverted 4-heptanone and other products with similar boiling points. We hypothesize, based on first-principlesmore » calculations, that the higher olefin yield is driven by the ease of hydrogen transfer to unreacted ketones as opposed to hydrogenating olefin products. We demonstrate how this ease of hydrogen transfer to ketones can be leveraged to enhance olefin selectivity in the conversion of methanol to olefins as well by co-feeding ketones. Olefinic products of the cyclization reaction are then oligomerized to a SAF blendstock to demonstrate an end-to-end ketone-to-SAF process facilitated by upgrading ketones over H/ZSM-5 at partial conversion with a recycle stream. The results of this work demonstrate a strategy to improve the carbon yield from bio-derived acids to SAF to over 75%, representing a relative improvement of more than 50% compared to previously reported data.« less

Addition of Ph₂SiH₂ to [(depe)₂Mo(N)][BAr^F₄] (depe = 1,2-bis(diethylphosphino)ethane, BAr^F₄ = B(3,5-(CF₃)₂C₆H₃)₄) at 60 °C generated the silyl imido molybdenum hydride complex, trans- [(depe)₂Mo(NSiHPh₂)H][BAr^F₄], a surrogate for a proposed intermediate complex in the photodriven hydrogenation to free ammonia. Irradiation of a THF solution of trans-[(depe)₂Mo(NSiHPh₂)H]- [BAr^F₄] with blue light under H₂ produced free amine along with [(depe)₂MoH₅][BAr^F₄] in 76% yield. This transformation occurred in the absence of a precious metal photocatalyst, suggesting that it was needed only for the initial addition of H₂ to the molybdenum nitride during the first N−H bond-forming step in the photodriven hydrogenation. Deuterium labeling and crossovermore » studies support concerted Si−H bond addition across the Mo≡N bond, enabled by the nucleophilicity of the nitride. Subsequent hydrogenation involves an intramolecular H migration from Mo to the imido ligand, as supported by electronic absorption spectroscopy, transient absorption spectroscopy, initial rate measurements, and deuterium kinetic isotope effect measurements. These findings provide insights into the photodriven hydrogenation of [(depe)₂Mo(N)][BAr^F₄] to ammonia and the role of the photocatalyst in this transformation.« less

Carbon capture and storage (CCS) plays a critical role in achieving climate change mitigation targets, offering a pathway to decarbonize power generation, industrial processes, and heat production while addressing atmospheric CO₂ removal. While CCS technologies are technically advanced, the widespread adoption of 100 % CO₂ capture capacities such as 1 mol of CO₂/mol of material and 1 g CO₂/g storage (targeted by the DARPA, Defense Sciences Office, USA Govt.) has raised questions about the feasibility of achieving higher capture capacities. In the context of limiting global warming to 1.5°C, reaching 100 % CO₂ capture capacity is increasingly necessary, with residualmore » emissions requiring complementary carbon dioxide removal (CDR) technologies. This review exclusively focuses on the CO₂ capture capacities of various sorbents under standard conditions, using different evaluation metrics. This study explores the performance of solid and liquid sorbents under standard conditions, analyzing factors including surface area, pore structure, solvent type, and functionalization to identify materials optimized for industrial-scale CCS applications. Emerging sorbents, including ILs, MOFs, COFs, POPs, DES, RCC, hybrid materials, and reactive sorbents, offer significant potential for enhanced selectivity and energy-efficient regeneration. Through a systematic assessment of gravimetric, volumetric, and molar capacities, the study provides insights into material efficiencies and trade-offs, offering guidance on optimizing sorbent selection for specific applications. The research advances understanding of scalable CCS technologies, contributing to global efforts to achieve net-zero emissions and address the pressing challenge of climate change.« less

Intermetallic compounds serve as model catalysts for selective hydrogenation reactions, offering precise control over the active site composition(s), geometric and electronic structure. The addition of a third element to form a ternary intermetallic alters the exposed crystal facet(s), demonstrating a strategy to impart improved catalytic behavior in intermetallic catalysts. The site-specific substitution of a small fraction of Pd atoms with Au in pyrite-type PdSb₂ results in the preferential exposure of the (100) facet over the (111) facet. Electron back scattered diffraction and density functional theory calculations confirm the facet change upon the substitution of Pd with Au to form themore » ternary Pd_1−xAu_xSb₂ (0.075 ≤ x ≤ 0.25). The (100) facet demonstrates higher net alkene selectivity due to significantly weaker alkene binding compared to the (111) facet. Distinct from our prior work on chemical substitution to directly alter the active site composition, this work demonstrates the indirect modification of active sites via preferential facet exposure.« less

Reactive capture and conversion (RCC) is a process intensification approach that integrates CO₂ capture and hydrogenation within a single unit, removing the CO₂ purification and storage steps of traditional process flow schemes. This alters the catalytic step from a traditional steady-state (SS) flow process to a transient capture and conversion cycle, which could lead to product distributions distinct from those observed in conventional SS experiments. Such differences are investigated in the combined capture and hydrogenation of carbon dioxide to methanol over a ZnZrO₂ catalyst and a ZnZrO₂ + NaNO₃/Mg₃AlO_x catalytic sorbent (CS) using fixed-bed kinetic measurements, in situ diffuse reflectancemore » infrared Fourier transform spectroscopy (DRIFTS), and steady-state isotopic transient kinetic analysis-DRIFTS (SSITKA-DRIFTS). Under SS conditions, ZnZrO₂ produced methanol through sequential hydrogenation of HCOO* and CH₃O* intermediates. On the contrary, CO was attributed primarily to CO₂ dissociation at oxygen vacancies, as supported by isotopic shifts and measured reaction orders. For the CS, isotopic switching experiments suggested that monodentate carbonate species (CO₃²⁻, abbreviated as m-CO₃²⁻) act as active intermediates that can be hydrogenated to HCOO* and subsequently to CH₃O. Under RCC conditions, in situ DRIFTS and isotopic experiments reveal that m-CO₃²⁻ species formed during the CO₂ capture step follow two competing routes upon H₂ exposure: (i) direct hydrogenation to methane on the sorbent domain or (ii) migration of m-CO₃²⁻ to the ZnZrO₂ domain, where they are hydrogenated to methanol through the HCOO pathway. Overall, RCC enables carbonate hydrogenation routes not observed under SS cofeed conditions. Thus, the reaction pathways and rates during RCC can be different from operation under conventional SS conditions, and the product distribution is determined here by competition between carbonate hydrogenation on sorbent sites and migration to ZnZrO₂ for methanol synthesis.« less