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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Topological effects on separation of alkane isomers in metal–organic frameworks

Polymorphism in metal–organic frameworks (MOFs) means that the same chemical building blocks (nodes and linkers) can be used to construct isomeric MOFs with different topological networks. The choice of topology can substantially impact the pore network of the MOF, changing the sizes and shapes of the pores, which has implications for adsorption and separation applications. Here, we look at the influence of topology in 38 polymorphic MOFs on the separation of linear and branched C4–C6 alkane isomers, a separation of great importance to the petrochemical industry. We find that the MOF Cu 2 (1,4-benzenedicarboxylate) in nbo topology (nbo-Cu 2 BDC) has particularly high affinity for linear alkanes due to its small pore size, which excludes the branched isomers. Upon studying this MOF in further detail, we find that it can take either of two conformations: a cubic conformation, which is typical of nbo MOFs, and a unique star conformation that contains 1D triangular and hexagonal channels. The determination of which conformation the MOF will adopt depends on steric effects between the nodes and linkers.

42 ENGINEERING↗
DOE OSTI2018

Investigation on the light alkanes aromatization over Zn and Ga modified HZSM-5 catalysts in the presence of methane

The catalytic co-aromatization of methane and paraffin-rich raffinate oil was investigated along with hexane, heptane and octane as its model compounds over zinc and gallium modified ZSM-5 zeolite catalysts. The benzene, toluene and xylene (BTX) components derived from light alkane aromatization were highly promoted with the assistance of methane. The co-existence of Zn and Ga metal species has a positive effect on the formation of BTX components, whereas the individual metal loaded catalyst resulted in the production of heavy aromatics, suggesting that zinc and gallium have a synergistic effect on the formation of BTX under the methane environment. When concerned with gaseous analysis, the introduced methane might interact with smaller intermediates and then transform into larger hydrocarbons. From the DRIFT observation, it was witnessed that the interaction between light alkane and methane occurred on the surface of the charged Zn-Ga/ZSM-5 catalyst. According to the comprehensive catalyst characterizations, the excellent catalytic performance may be closely associated with greatly dispersed metal species on the zeolite support, improved microporous characteristic, moderate Bronsted and increased Lewis acidic sites during the paraffin-rich liquid feedstock aromatization under methane environment. This research provides a promising pathway for the highly effective and profitable utilization of petrochemical resources and natural gas.

09 BIOMASS FUELS↗
DOE OSTI2022

Can CO 2 -assisted alkane dehydrogenation lead to negative CO 2 emissions?

The utilization of light alkanes to generate value-added chemicals has gained significant attention due to the U.S. shale gas boom in recent decades. For example, after methane, ethane (3-16 vol.%) and propane (up to 5 vol.%) are the most abundant hydrocarbon species in shale gas deposits. Further, these inexpensive, widely available, but often underutilized, light alkanes can be used as a feedstock to produce olefins, which are among the most important petrochemical building blocks in chemical industry. Ethylene and propylene are used in the production of polymers (e.g., polyethylene, polypropylene) and epoxides (e.g., ethylene oxide, propylene oxide), as well as intermediates in the manufacture of numerous other chemicals (e.g., ethylene glycol, ethylbenzene, acetone, isopropanol, acrylonitrile).

03 NATURAL GAS↗
DOE OSTI2020

Poison or Promoter? Investigating the Dual-Role of Carbon Monoxide in Pincer-Iridium-Based Alkane Dehydrogenation Systems via Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Pincer-ligated iridium complexes of the form [( R 4 PCP)IrL] ( R 4 PCP = κ 3 -C 6 H 3 -2,6-(XPR 2 ) 2 ; X = CH 2 , O; R = tBu, iPr) have previously been shown competent for acceptorless alkane dehydrogenation when supported on silica. It was observed by postcatalysis solid-state NMR that silica-tethered [(≡SiO- tBu 4 POCOP)Ir(C 2 H 4 )] (3-C 2 H 4 ) was converted fully to [(≡SiO- tBu 4 POCOP)Ir(CO)] (3-CO) at 300 °C. In this work, the characterization of species under dehydrogenation reaction conditions far from equilibrium between butane and butenes (approach to equilibrium Q/K eq = 0.3 at 300 °C) is performed with operando Diffuse Reflectance Infrared Fourier–Transform Spectroscopy (DRIFTS) to show the kinetics of species conversion from 3-C 2 H 4 to 3-CO. It is further found that [(≡SiO- tBu 4 POCOP)IrHCl] (3-HCl), a species considered to be a precatalyst for alkane dehydrogenation, is also fully converted to 3-CO. Here, a mechanism of decomposition is proposed that implicates surface silanol groups, while carbon monoxide acts as a “stabilizer” for the catalyst by promoting their reductive elimination and maintaining the complex in the I oxidation state.

02 PETROLEUM↗
DOE OSTI2021

Catalytic Light Alkanes Conversion through Anaerobic Ammodehydrogenation

Catalytic transformation of light alkanes could have considerable practical value, yet remains one of the most challenging areas in catalysis research due to the inertness of the C–H bond. Here, we proposed an efficient ammodehydrogenation (ADeH) catalytic system for the direct C–N linkage between light alkanes and ammonia for CH 3 CN and H 2 (CO x free) production. This breakthrough is achieved over bifunctional metal-modified HZSM-5 catalysts, through the tandem dehydrogenation–amination–dehydrogenation mechanism. We show that ethane ADeH over the Pt/HZSM-5 catalyst can be realized under atmospheric pressure at temperatures as low as 350 °C. The specific rate of CH 3 CN is ~60 μmol/(g min), and the selectivity is up to 99% under such mild conditions. The yield of CH 3 CN increases with increasing temperature; however, the selectivity decreases due to the formation of HCN, C 2 H 4 , and CH 4 . Additionally, the Pt/HZSM-5 catalyst is coke-resistant during the ADeH owing to the strong interaction between NH 3 and the acid sites of the catalyst. So, we anticipate that the proposed ADeH could be extended for the transformation of various n/iso-alkanes with tunable selectivity to alkene and nitriles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Computational Modeling of the Nature and Role of Ga Species for Light Alkane Dehydrogenation Catalyzed by Ga/H-MFI

Ga-exchanged H-MFI zeolites are highly active for the dehydrogenation of light alkanes; however, both the nature of the active gallium species and the associated dehydrogenation mechanism have been difficult to establish. In this study, we examine the activity of Ga species in Ga/H-MFI by calculating the free energy landscapes on which all reactions occur. To this end, we use a hybrid quantum mechanics/molecular mechanics model for all electronic structure calculations. Quantum chemical calculations were carried out with a range-corrected functional and a good representation of dispersive interactions. The molecular mechanics part of our approach captures the long-range effects of Coulombic and dispersive interactions due to atoms in the extended framework. The rate-determining TS (RDTS) is identified by analysis of the free energy landscape for each mechanism, using the energetic span model. Our analysis reveals that, for reduced Ga/H-MFI, univalent and divalent gallium hydrides, [GaH 2 ] + and [GaH] 2+ , respectively, are more active for ethane dehydrogenation in comparison to H + sites and Ga + sites. [GaH] 2+ sites consistently emerge as the most active sites for light alkane dehydrogenation, providing significant enthalpic stabilization to C-H cleavage TSs via alkyl and carbenium dehydrogenation routes. In contrast, carbenium-like C-H cleavage TSs occurring on Brønsted acid sites are enthalpically less favorable due to their limited electronic interactions with framework O atoms and H + sites. Activation enthalpy barriers for dehydrogenation determined using the energetic span analysis are in good agreement with those measured experimentally. Accounting for entropy of activation reveals that constrained TSs become less favorable in free energy with increasing chain length. Here, we also find that an increase in enthalpic favorability of the alkyl mechanism is observed with increasing chain length for the TS responsible for the second C-H cleavage step leading to alkene formation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Polyethylene Upcycling to Liquid Alkanes in Molten Salts under Neat and External Hydrogen Source-Free Conditions

Development of facile approaches to convert plastic waste into liquid fuels under neat conditions is highly desired but challenging, particularly without noble metal catalysts and an external hydrogen source. Herein, highly efficient and selective polyethylene-to-gasoline oil (branched C 6 –C 12 alkanes) conversion was achieved under mild conditions (<170 °C) using commercially available AlCl 3 -containing molten salts as reaction media and to provide catalytic sites (no extra solvents, additives, or hydrogen feeding). The high catalytic efficiency and selectivity was ensured by the abundant active Al sites with strong Lewis acidity (comparable to the Al type in acidic zeolite) and highly ionic nature of the molten salts to stabilize the carbenium intermediates. Dynamic genesis of the Al sites was elucidated via time-resolved Al K-edge soft X-ray and 27 Al NMR, confirming the tricoordinated Al 3+ as active sites and its coordination with the as-generated alkene/aromatic intermediates. Further, the carbenium formation and polyethylene chain variation was illustrated by inelastic neutron scattering (INS) and an isotope-labeling experiment. Theoretical simulations further demonstrated the successive hydride abstraction, β-scission, isomerization, and internal hydrogen transfer reaction pathway with AlCl 3 as active sites. This facile catalytic system can further achieve the conversion of robust, densely assembled, and high molecular weight plastic model compounds to liquid alkane products in the diesel range.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Mass effect on the Soret coefficient in n-alkane mixtures

We have determined the Soret coefficient of different equimolar and non equimolar n-alkane mixtures from measurements of the molecular diffusion and thermal diffusion coefficients. It is shown that equimolar mixtures behave as isotopic-like mixtures in which only the mass effect contributes to the Soret effect. In non equimolar mixtures, a small linear dependence with the molar fraction is observed. Finally, we have obtained a new correlation, which allows the determination of the Soret coefficient of n-alkane mixtures using the data of viscosity, the thermal expansion coefficient of the pure components, and the density of the equimolar mixture.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Molecular dynamics insight to phase transition in n-alkanes with carbon nanofillers

The present work aims to investigate the phase transition, dispersion and diffusion behavior of nanocomposites of carbon nanotube (CNT) and straight chain alkanes. These materials are potential candidates for organic phase change materials(PCMs) and have attracted flurry of research recently. Accurate experimental evaluation of the mass, thermal and transport properties of such composites is both difficult as well as economically taxing. Additionally it is crucial to understand the factors that results in modification or enhancement of their characteristic at atomic or molecular level. Classical molecular dynamics approach has been extended to elucidate the same. Bulk atomistic models have been generated and subjected to rigorous multistage equilibration. To reaffirm the approach, both canonical and constant-temperature, constant- pressure ensembles were employed to simulate the models under consideration. Explicit determination of kinetic, potential, non-bond and total energy assisted in understanding the enhanced thermal and transport property of the nanocomposites from molecular point of view. Crucial parameters including mean square displacement and simulated self diffusion coefficient precisely define the balance of the thermodynamic and hydrodynamic interactions. Radial distribution function also reflected the density variation, strength and mobility of the nanocomposites. It is expected that CNT functionalization could improve the dispersion within n-alkane matrix. This would further ameliorate the mass and thermal properties of the composite. Additionally, the determined density was in good agreement with experimental data. Thus, molecular dynamics can be utilized as a high throughput technique for theoretical investigation of nanocomposites PCMs.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Comparing the accuracy of high-dimensional neural network potentials and the systematic molecular fragmentation method: A benchmark study for all-trans alkanes

Many approaches, which have been developed to express the potential energy of large systems, exploit the locality of the atomic interactions. A prominent example is the fragmentation methods in which the quantum chemical calculations are carried out for overlapping small fragments of a given molecule that are then combined in a second step to yield the system’s total energy. Here we compare the accuracy of the systematic molecular fragmentation approach with the performance of high-dimensional neural network (HDNN) potentials introduced by Behler and Parrinello. HDNN potentials are similar in spirit to the fragmentation approach in that the total energy is constructed as a sum of environment-dependent atomic energies, which are derived indirectly from electronic structure calculations. As a benchmark set, we use all-trans alkanes containing up to eleven carbon atoms at the coupled cluster level of theory. These molecules have been chosen because they allow to extrapolate reliable reference energies for very long chains, enabling an assessment of the energies obtained by both methods for alkanes including up to 10 000 carbon atoms. We find that both methods predict high-quality energies with the HDNN potentials yielding smaller errors with respect to the coupled cluster reference.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Comparative study of alkali-vapour cells with alkane-, alkeneand 1-nonadecylbenzene-based antirelaxation wall coatings

The dependence of both longitudinal and transverse relaxation times of ground-state magnetic polarisation in alkali atoms on the coating temperature is experimentally studied for the first time in a rubidium-vapour cell with 1-nonadecylbenzene antirelaxation coating of inner walls. The comparison of these times with the relaxation times in a caesium-vapour cell with alkane wall coatings is presented. It is found that within the studied temperature range (294 – 340K) the transverse relaxation time decreases with increasing temperature of alkene and 1-nonadecylbenzene coatings. For the alkane coating such a dependence was not explicitly found. The longitudinal relaxation time begins to decrease in all cases when passing a certain critical temperature of the coating material. It is found that the unsaturated radical structure of the coating material molecules strongly affects its antirelaxation properties. (optical pumping)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Anomaly in the Chain Length Dependence of n-Alkane Diffusion in ZIF-4 Metal-Organic Frameworks

Molecular diffusion is commonly found to slow down with increasing molecular size. Deviations from this pattern occur in some host materials with pore sizes approaching the diameters of the guest molecules. A variety of theoretical models have been suggested to explain deviations from this pattern, but robust experimental data are scarcely available. Here in this paper, we present such data, obtained by monitoring the chain length dependence of the uptake of n-alkanes in the zeolitic imidazolate framework ZIF-4. A monotonic decrease in diffusivity from ethane to n-butane was observed, followed by an increase for n-pentane, and another decrease for n-hexane. This observation was confirmed by uptake measurements with n-butane/n-pentane mixtures, which yield faster uptake of n-pentane. Further evidence is provided by the observation of overshooting effects, i.e. by transient n-pentane concentrations exceeding the (eventually attained) equilibrium value. Accompanying grand canonical Monte Carlo simulations reveal, for the larger n-alkanes, significant differences between the adsorbed and gas phase molecular configurations, indicating strong confinement effects within ZIF-4, which, with increasing chain length, may be expected to give rise to configurational shifts facilitating molecular propagation at particular chain lengths.

32 GCMC simulation↗
DOE OSTI2016

Epitaxially Self-Assembled Alkane Layers for Graphene Electronics

The epitaxially grown alkane layers on graphene are prepared by a simple drop-casting method and greatly reduce the environmentally driven doping and charge impurities in graphene. Here, multiscale simulation studies exhibit that this enhancement of charge homogeneity in graphene originates from the lifting of graphene from the SiO 2 surface toward the well-ordered and rigid alkane self-assembled layers.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Simultaneously upgrading CO 2 and light alkanes into value-added products

We will discuss recent results from our group in reacting CO 2 and light alkanes to produce syngas, olefins, aromatics, and oxygenates. We will mainly use the simultaneous upgrading of CO 2 and ethane (SU-CO 2 Et) as examples to demonstrate the feasibility and reaction pathways of these processes. We will briefly discuss synthesis methods and essential structural characterization techniques that can be applied to SU-CO 2 Et catalysts. We will then illustrate how to identify and apply distinct active sites for different upgrading processes, using a combination of kinetic studies, in situ characterization, and density functional theory (DFT) calculations. We will conclude the Perspective by pointing out challenges and potential directions in catalyst design and synthesis, as well as structural characterization and mechanistic investigations, to further advance the simultaneous upgrading of CO 2 and light alkanes.

03 NATURAL GAS↗
DOE OSTI2023

Metal‐Organic Frameworks for C6 Alkane Separation

Abstract The separation of alkane isomers is an important yet challenging process in the petrochemical industry. Being a crucial step to produce premium gasoline components as well as optimum ethylene feed, the current industrial separation by distillation is extremely energy intensive. Adsorptive separation based on zeolite is limited by insufficient adsorption capacity. Metal‐organic frameworks (MOFs) hold enormous promise as alternative adsorbents due to their diverse structural tunability and exceptional porosity. Precise control of their pore geometry/dimensions has led to superior performance. In this minireview, we highlight the recent progresses in developing MOFs for the separation of C6 alkane isomers. Representative MOFs are reviewed based on their separation mechanisms. Emphasis is put on the material design rationale for achieving optimal separation capability. Finally, we briefly discuss the existing challenges, possible solutions, and future directions of this important field.

Xie, Feng↗
DOE OSTI2025

Abstraction of Hydride from Alkanes and Dihydrogen by the Perfluorotrityl Cation

Abstract Lewis acids play a central role in a large variety of chemical transformations. The reactivity of the strongest Lewis acids is typically studied in the context of affinity towards hard bases, such as fluoride or oxygenous species. Carbocations can be viewed as soft Lewis acids, possessing significant affinity for softer bases, such as hydride. This work presents the ambient‐temperature isolation of salts of the perfluorotrityl cation ((C 6 F 5 ) 3 C + or F 15 Tr + ) in combination with halogenated carborane anions. The F 15 Tr + cation exhibits remarkable hydride affinity, illustrated by the observation of hydride abstraction from dihydrogen, and of the rapid abstraction of hydride from −CH 2 −groups in alkanes. Theoretical studies support the favorability of hydride abstraction from dihydrogen, and indicate that the hydride abstraction from alkanes proceeds via a concerted hydride transfer process that is sensitive to steric effects.

Leong, Derek W. [Department of Chemistry Texas A&a↗
DOE OSTI2025

Abstraction of Hydride from Alkanes and Dihydrogen by the Perfluorotrityl Cation

Abstract Lewis acids play a central role in a large variety of chemical transformations. The reactivity of the strongest Lewis acids is typically studied in the context of affinity towards hard bases, such as fluoride or oxygenous species. Carbocations can be viewed as soft Lewis acids, possessing significant affinity for softer bases, such as hydride. This work presents the ambient‐temperature isolation of salts of the perfluorotrityl cation ((C 6 F 5 ) 3 C + or F 15 Tr + ) in combination with halogenated carborane anions. The F 15 Tr + cation exhibits remarkable hydride affinity, illustrated by the observation of hydride abstraction from dihydrogen, and of the rapid abstraction of hydride from −CH 2 −groups in alkanes. Theoretical studies support the favorability of hydride abstraction from dihydrogen, and indicate that the hydride abstraction from alkanes proceeds via a concerted hydride transfer process that is sensitive to steric effects.

Leong, Derek W. [Department of Chemistry Texas A&a↗
DOE OSTI2017

Understanding Brønsted-Acid Catalyzed Monomolecular Reactions of Alkanes in Zeolite Pores by Combining Insights from Experiment and Theory

Acidic zeolites are effective catalysts for the cracking of large hydrocarbon molecules into lower molecular weight products required for transportation fuels. However, the ways in which the zeolite structure affects the catalytic activity at Brønsted protons are not fully understood. One way to characterize the influence of the zeolite structure on the catalysis is to study alkane cracking and dehydrogenation at very low conversion, conditions for which the kinetics are well defined. To understand the effects of zeolite structure on the measured rate coefficient (k app ), it is necessary to identify the equilibrium constant for adsorption into the reactant state (K ads-H + ) and the intrinsic rate coefficient of the reaction (k int ) at reaction temperatures, since k app is proportional to the product of K ads-H + and k int . We show that K ads-H + cannot be calculated from experimental adsorption data collected near ambient temperature, but can, however, be estimated accurately from configurational-bias Monte Carlo (CBMC) simulations. Finally, using monomolecular cracking and dehydrogenation of C 3 –C 6 alkanes as an example, we review recent efforts aimed at elucidating the influence of the acid site location and the zeolite framework structure on the observed values of k app and its components, K ads-H + and k int .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗