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DOE OSTI2019

An experimental and kinetic modeling study of the oxidation of hexane isomers: Developing consistent reaction rate rules for alkanes

Alkanes are key components in gasoline, jet and diesel fuels and considerably influence the combustion behavior of these fuels because of their wide range of reactivity. An improved understanding of their combustion behavior and the development of chemical kinetic models that can accurately simulate their combustion behavior are important for the development of next-generation internal-combustion and gas-turbine engines. The current work provides here improved insight into oxidation mechanisms of a representative family of hydrocarbon fuels, specifically the hexane isomers: n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane. These isomers provide carbon “skeletons” ranging from straight-chained to highly-branched and provide a framework for the subsequent development of kinetic mechanisms for larger alkanes. New ignition delay times for the four branched hexane isomers were measured in a high-pressure shock tube and in a rapid compression machine, both at stoichiometric conditions (φ = 1), p = 15 bar and X O2 = 21% over temperatures from 600 to1300 K. These data were combined with previously published measurements under the same conditions for the remaining n-hexane isomer to provide a complete body of experimental data for kinetic modeling analysis. In addition, very recent experimental measurements of individual intermediate chemical species concentrations from all five hexane isomers in a jet-stirred reactor are also included and provide another family of data for further assessment of hexane isomer reactivity. Different reactivities were observed for each hexane isomer in each experimental facility, resulting from differences in their molecular structures. Consistent reaction rate rules have been applied to develop a combined detailed chemical kinetic model for all five hexane isomers. Kinetic model validation studies are reported to show that the current model reproduces well the ignition delay times of all five alkane isomers, as well as their variations in reactivity over a wide range of temperatures and other operating conditions. Equally important, these results show that it is not necessary to have a separate, different kinetic model for each isomer of a family of alkane fuels and that a single, coherent, integrated set of reaction rate classes and rules is sufficient to accurately describe combustion rates of combustion of straight-chain n-alkanes and branched-chain alkane fuels. This suggests strongly that a single set of reaction classes and rate rules should be sufficient to describe combustion kinetics of alkane fuels of any size and degree of branching.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Langmuir Films of n -Alkanethiol-Capped Gold Nanoparticles and n -Alkanes: Interfacial Mixing Scenarios Assessed by X-ray Reflectivity and Grazing Incidence Diffraction

A series of n-alkanes cospread with alkanethiol-stabilized gold nanoparticles (AuNP) were studied as Langmuir monolayers by synchrotron X-ray reflectivity and diffraction. Tetradecanethiol capped gold nanoparticles with core diameters close to 2 nm were used to make films at 20 °C, below the ligand order–disorder temperature. A variety of n-alkane chain lengths (Cn = C n H 2n+2 , where n = 12, 15 and 16) were tested to assess the interfacial assembly of nanoparticle films as a result of different mixing scenarios indicated in their surface pressure versus area isotherms. Synchrotron grazing incidence X-ray diffraction (GIXD) and reflectivity (XR) confirm that mixtures of n-alkane and AuNP exhibiting improved fluidity in their compression isotherm are indeed incorporating n-alkane into the AuNP ligand shell and stabilizing it at the air–water interface. The resulting films show a doubling of their correlation lengths and thus a significant improvement on their ordering, as well as increased lattice spacing that is dependent upon the n-alkane chain length. Mixtures that do not exhibit changes in their surface pressure vs area isotherm similarly show little change in the interfacial assembly of the nanoparticle films except to promote monolayer collapse and multilayer formation. Improvements to the film order are assigned to the initial formation of larger nanoparticle domains. Here, the nature of the chain length dependence and persistence of the n-alkane through compression suggest a favorable interaction with the nanoparticle ligand shell that results in the extension of methylene units of the longer alkanes beyond the thiol layer, which has implications for influencing nanoparticle interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Anaerobic Degradation of Alkanes by Marine Archaea

Alkanes are saturated apolar hydrocarbons that range from their simplest form, methane, to high-molecular-weight compounds. Although alkanes were once considered biologically recalcitrant under anaerobic conditions, microbiological investigations have now identified several microbial taxa that can anaerobically degrade alkanes. Here we review recent discoveries in the anaerobic oxidation of alkanes with a specific focus on archaea that use specific methyl coenzyme M reductases to activate their substrates. Our understanding of the diversity of uncultured alkane-oxidizing archaea has expanded through the use of environmental metagenomics and enrichment cultures of syntrophic methane-, ethane-, propane-, and butane-oxidizing marine archaea with sulfate-reducing bacteria. A recently cultured group of archaea directly couples long-chain alkane degradation with methane formation, expanding the range of substrates used for methanogenesis. This article summarizes the rapidly growing knowledge of the diversity, physiology, and habitat distribution of alkane-degrading archaea.

Microbiology↗

DOE OSTI2021

Process for converting alkanes to olefins

A process and apparatus for converting an alkane to an olefin. In one embodiment, the process involves oxidative coupling of an alkane, e.g., methane, with an oxidant, such as air, to produce an olefin having twice the number of carbon atoms as the alkane, e.g., ethylene. In another embodiment, the process involves oxidative dehydrogenation of an alkane, e.g., ethane, with an oxidant to form an olefin having the same number of carbon atoms as the alkane, e.g., ethylene. The process involves passing a flow of the oxidant from a first flow passage through a porous medium; diffusing a flow of the alkane from a second flow passage into the porous medium; and contacting the reactant alkane and the oxidant in the presence of a catalyst within the porous medium to produce the olefin.

Weissman, Jeffrey G.↗

DOE OSTI2022

Process for converting alkanes to olefins

Weissman, Jeffrey↗

DOE OSTI2017

Electrostatic and induction effects in the solubility of water in alkanes

Experiments show that at 298 K and 1 atm pressure, the transfer free energy, μ ex , of water from its vapor to liquid normal alkanes C n H 2n+2 (n=5…12) is negative. Earlier it was found that with the united-atom TraPPE model for alkanes and the SPC/E model for water, one had to artificially enhance the attractive alkane-water cross interaction to capture this behavior. Here we revisit the calculation of μ ex using the polarizable AMOEBA and the non-polarizable Charmm General (CGenFF) forcefields. We test both the AMOEBA03 and AMOEBA14 water models; the former has been validated with the AMOEBA alkane model while the latter is a revision of AMOEBA03 to better describe liquid water. We calculate μ ex using the test particle method. With CGenFF, μ ex is positive and the error relative to experiments is about 1.5 k B T. With AMOEBA, μ ex is negative and deviations relative to experiments are between 0.25 k B T (AMOEBA14) and 0.5 k B T (AMOEBA03). Quantum chemical calculations in a continuum solvent suggest that zero point effects may account for some of the deviation. In conclusion, forcefield limitations notwithstanding, electrostatic and induction effects, commonly ignored in consideration of water-alkane interactions, appear to be decisive in the solubility of water in alkanes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Exploring novel alkane-degradation pathways in uncultured bacteria from the North Atlantic Ocean

Microbes play a significant role in the degradation of petroleum hydrocarbons in the oceans, yet little is known about the native bacteria that metabolize hydrocarbons before an oil spill. The Faroe-Shetland Channel (FSC) is a deepwater subarctic region of the North Atlantic with prominent oil production and a diverse microbial community associated with the degradation of petroleum. Here, we combine DNA-based stable-isotope probing (DNA-SIP) with metagenomics to elucidate the metabolic underpinnings of native alkane-degrading bacteria from the FSC. From two 13C n-hexadecane SIP experiments using seawater from 5 and 700 m depths in the FSC, we obtained 42 metagenome-assembled genomes (MAGs) belonging to 19 genera, including two previously overlooked hydrocarbon-degrading bacteria, Lentibacter (Alphaproteobacteria) and Dokdonia (Bacteroidetes). Diversity surveys indicated Lentibacter were dominant members of the FSC, constituting up to 17% of these communities. Many of the SIP-enriched MAGs (20/42) encoded a complete alkane oxidation pathway, including alkane monooxygenase (AlkB), rubredoxin reductase (AlkT), and rubredoxin-2 (AlkG). Fourteen Aphaproteobacteria MAGs lacked AlkG for electron transfer. Instead, they encoded novel disulfide isomerases with iron-binding cysteine motifs conserved across rubredoxins. Dokdonia lacked AlkT and AlkG, however, their central alkane-degradation catabolic pathways were complete. We describe previously unrecognized bacteria capable of hydrocarbon degradation, including the dominant genera Lentibacter, which may continuously purge hydrocarbons released from oil exploration activities in the FSC. This advances the understanding of the diversity and physiologies of alkane degradation in the North Atlantic and provides evidence of new mechanisms used to metabolize alkanes.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2025

The role of the Pt-group dehydrogenation catalyst in alkane metathesis for polyolefin deconstruction

Recent proposed approaches in the depolymerization of waste plastics employ an olefin intermediate to produce alkanes or alkenes using olefin metathesis in tandem chemistry. Here, in this study, we investigated the role of the dehydrogenation catalyst on reaction rate, kinetics, and product distribution in heterogeneous tandem dehydrogenation and olefin metathesis (alkane metathesis) of three different alkane reactants, including polyethylene. We found that many properties to which alkane dehydrogenation rates were sensitive-including metal composition, nanoparticle size, and surface doping of Re species also controlled activity in Tandem D/OM. When comparing Pd, Pt, and Pt 3 Sn 1 , supported Pd in tandem with a Re 2 O 7 olefin metathesis catalyst showed four-fold higher activity (surface area basis) compared to Pt or Pt 3 Sn 1 catalysts on the same support, mainly due to differences in the rate of hydrogenation. Catalyst preparation resulted in metal nanoparticles partially covered by ReO x , as seen from elemental mapping. Co-location of Re 2 O 7 and Pd correlated with increased rates of hydrogenation (i.e., an increase in the rate of alkane formation and simultaneous lowering of the rate of alkene formation), with a reaction order in catalyst study that further supported this conclusion. The Pd and Re 2 O 7 system displayed marked improvement compared to Pt or Pt 3 Sn 1 with Re 2 O 7 , and previous work, in the depolymerization rate of a linear polyethylene feedstock, with over 94 % reduction in polymer molecular weight in 15 h at 190 °C using less catalyst and increased reactant loadings, while keeping solvent to polymer consumption below 2.5.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Alkane biosynthesis by Aspergillus carbonarius ITEM 5010 through heterologous expression of Synechococcus elongatus acyl-ACP/CoA reductase and aldehyde deformylating oxygenase genes

In this study we describe the heterologous expression of the recently identified cyanobacterial pathway for long chain alkane biosynthesis, involving the reduction of fatty acyl-ACP to fatty aldehyde and the subsequent conversion of this into alkanes, in the filamentous fungus Aspergillus carbonarius ITEM 5010. Genes originating from Synechococcus elongatus strain PCC7942, encoding acyl-ACP/CoA reductase and aldehyde deformylating oxygenase enzymes, were successfully expressed in A. carbonarius, which lead to the production of pentadecane and heptadecane, alkanes that have not been previously produced by this fungus. Titers of 0.2, 0.5 and 2.7 mg/l pentadecane and 0.8, 1.6 and 10.2 mg/l heptadecane were achieved using glucose, Yeast malt and oatmeal media, respectively. Besides producing alkanes, we found elevated levels of internal free fatty acids and triglycerides in the alkane producing transformant. These findings can indicate that a yet unidentified, native fatty aldehyde dehydrogenase channels back the fatty aldehydes into the fatty acid metabolism, thus competing for substrate with the heterologously expressed fatty aldehyde deformylating oxygenase. These findings will potentially facilitate the future application of robust, fungal cell factories for the production of advanced biofuels from various substrates.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2015

Diverse Bacterial Groups Contribute to the Alkane Degradation Potential of Chronically Polluted Subantarctic Coastal Sediments

We aimed to gain insight into the alkane degradation potential of microbial communities from chronically polluted sediments of a subantarctic coastal environment using a combination of metagenomic approaches. A total of 6178 sequences annotated as alkane-1-monooxygenases (EC 1.14.15.3) were retrieved from a shotgun metagenomic dataset that included two sites analyzed in triplicate. The majority of the sequences binned with AlkB described in Bacteroidetes (32 ± 13 %) or Proteobacteria (29 ± 7 %), although a large proportion remained unclassified at the phylum level. Operational taxonomic unit (OTU)-based analyses showed small differences in AlkB distribution among samples that could be correlated with alkane concentrations, as well as with site-specific variations in pH and salinity. A number of low-abundance OTUs, mostly affiliated with Actinobacterial sequences, were found to be only present in the most contaminated samples. On the other hand, the molecular screening of a large-insert metagenomic library of intertidal sediments from one of the sampling sites identified two genomic fragments containing novel alkB gene sequences, as well as various contiguous genes related to lipid metabolism. Both genomic fragments were affiliated with the phylum Planctomycetes, and one could be further assigned to the genus Rhodopirellula due to the presence of a partial sequence of the 23S ribosomal RNA (rRNA) gene. This work highlights the diversity of bacterial groups contributing to the alkane degradation potential and reveals patterns of functional diversity in relation with environmental stressors in a chronically polluted, high-latitude coastal environment. In addition, alkane biodegradation genes are described for the first time in members of Planctomycetes.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2018

Osh6p, a homologue of the oxysterol-binding protein, is involved in production of functional cytochrome P450 belonging to CYP52 family in n-alkane-assimilating yeast Yarrowia lipolytica

Highlights: • Yarrowia lipolytica has four OSBP homolog genes, OSH1, OSH3, OSH4, and OSH6. • OSH3 and OSH6 are required for growth on n-alkanes. • OSH6 is involved in the production of functional P450 of CYP52 family. • Osh6p might provide an ideal ER membrane environment for CYP52 P450 in Y. lipolytica. In this study, we investigated the role of OSH6, which encodes a homolog of the oxysterol-binding protein, in the assimilation of n-alkanes in the yeast Yarrowia lipolytica. The deletion mutant of OSH6 showed growth defects on n-alkanes of 10–16 carbons. In the deletion mutant, production of the functional cytochrome P450 was not observed. However, transcription of ALK1, encoding a major P450 belonging to the CYP52 family that plays a critical role in n-alkane hydroxylation, and further translation of its transcript were noted in the deletion mutant as well as in the wild-type strain. The phospholipid composition was altered and, the ratio of phosphatidylserine (PS) was reduced by the deletion of OSH6. Residues involved in the transport of PS and phosphatidylinositol-4-phosphate in Osh6 of Saccharomyces cerevisiae are conserved in Y. lipolytica Osh6p and substitutions of these residues resulted in a defect in the n-alkane assimilation by Y. lipolytica. From these results, we propose a hypothesis that Osh6p provides an ideal endoplasmic reticulum membrane environment for Alk proteins to have a functional conformation via lipid transport activity in Y. lipolytica.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2026

Impact of Small-Alkane Solvents on Polyolefin Hydrogenolysis over a Ruthenium Catalyst

Selective catalytic hydrogenolysis of polyolefins is a promising route to convert plastic waste into valuable liquid products, such as lubricants, waxes, and surfactants. However, the high viscosity of polymer melts imposes mass transfer limitations on this reaction. Solvents can mitigate these challenges, but their effects on reaction kinetics and product selectivity remain underexplored. Here, we systematically explore the effects of small n-alkanes and cycloalkanes on the hydrogenolysis of polyethylene and polypropylene over a Ru/TiO 2 catalyst. Using kinetic measurements and isotopic labeling, we show that n-octane at high mass fractions alters the mechanism from direct hydrogenation to solvent-mediated hydrogen transfer, reducing the rate of C–C bond cleavage. Longer alkanes further inhibit reactivity due to stronger surface binding. 1,4-Dimethylcyclohexane suppresses methane formation, favoring heavier products, while decalin likely forms surface-bound aromatics that poison the catalyst. Overall, alkane solvents modulate product selectivity and reduce the yield of methane byproduct, allowing for ∼35–40% selectivity to valuable C 20 -C 30 alkane products. This work highlights the complex impact of polymer–alkane mixtures on hydrogenolysis kinetics relevant to the design of commercial-scale plastic waste valorization processes.

Catalysts↗

DOE OSTI2016

Effects of Pore and Cage Topology on the Thermodynamics of n -Alkane Adsorption at Brønsted Protons in Zeolites at High Temperature

Here, Monte Carlo simulations are used to systematically investigate the effects of structural topology on the thermodynamics of n -alkanes adsorbed at Brønsted protons in zeolites having one-dimensional channel systems. In zeolites without cages, the enthalpy and entropy of adsorption (Δ H ads-H + and Δ S ads-H + ) at fixed pore-limiting diameter (PLD) generally increase (become less negative) as the ratio of the minimum to maximum channel diameter decreases, and are lowest when this ratio equals 1 (corresponding to approximately circular cross sections). The results of a change in diameter ratio on the free energy of adsorption (Δ A ads-H + ) is weak due to the changes in Δ H ads-H + and TΔ S ads-H + largely cancel. The addition of cages having a largest-cavity diameter (LCD) greater than the PLD increases both Δ H ads-H + and Δ S ads-H + . Replacing channels with cages of the same diameter does not change Δ S ads-H + significantly when the PLD is similar to the alkane length but decreases both Δ H ads-H + and Δ A ads-H + because of the greater surface area of cages relative to channels. The selectivity to adsorption via a central C-C bond vs a terminal bond when cages are absent is smallest for PLDs near the alkane length and, when cages are present, is even lower when the LCD exceeds the alkane length. This effect is attributed to more rotation of the alkane in cages vs channels. The results show that Δ A ads-H + at 773 K can be tuned by manipulating a characteristic dimension (LCD, PLD) and topology (e.g., adding cages) simultaneously, in order to circumvent the compensating changes in TΔ S ads-H + and Δ H ads-H + that occur upon changing only one structural parameter.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Efficient Sn Recovery from SnO 2 by Alkane (C x H y=2x+2 , 0 ≤ x ≤ 4) Reduction

We study the mechanism of alkane reduction of SnO 2 for efficient low-temperature recovery of Sn from SnO 2 . Based on thermodynamic simulation results, we comparatively analyze the reduction behavior and the efficiency of SnO 2 reduction by H 2 and alkanes (C x H y=2x+2 , 0 ≤ x ≤ 4). We found that alkanes (n·C x H y ) with the higher nx generally complete the reduction of SnO 2 (T 100 ) at the lower temperature. Moreover, the T 100 of the SnO 2 reduction by alkanes (n·C x H y ) was decreased from the T 100 of pure hydrogen with the same amount of hydrogen atoms (n·H y ). We found that the concentration of a gas phase product mixture, the amount of the produced solid carbon, and the T 100 complementary vary as a function of the nx and ny, the total amount of carbon and hydrogen atoms in the reducing gas phase molecules. Our results demonstrate a viability of the low temperature reduction method of SnO 2 by alkanes for efficient recovery of Sn from SnO 2 , which can be applied for Sn recovery from Sn containing industrial wastes or Sn ores with economic value added that is held by the co-produced H 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Secondary organic aerosols derived from intermediate-volatility n-alkanes adopt low-viscous phase state

Abstract. Secondary organic aerosol (SOA) derived from n-alkanes, as emitted from vehicles and volatile chemical products, is a major component of anthropogenic particulate matter, yet the chemical composition and phase state are poorly understood and thus poorly constrained in aerosol models. Here we provide a comprehensive analysis of n-alkane SOA by explicit gas-phase chemistry modeling, machine learning, and laboratory experiments to show that n-alkane SOA adopts low-viscous semi-solid or liquid states. Our study underlines the complex interplay of molecular composition and SOA viscosity: n-alkane SOA with a higher carbon number mostly consists of less functionalized first-generation products with lower viscosity, while the SOA with a lower carbon number contains more functionalized multigenerational products with higher viscosity. This study opens up a new avenue for analysis of SOA processes, and the results indicate few kinetic limitations of mass accommodation in SOA formation, supporting the application of equilibrium partitioning for simulating n-alkane SOA formation in large-scale atmospheric models.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2021

Catalytic Dehydrogenation of Alkanes by PCP–Pincer Iridium Complexes Using Proton and Electron Acceptors

Dehydrogenation to give olefins offers the most broadly applicable route to the chemical transformation of alkanes. Furthermore, transition-metal-based catalysts can selectively dehydrogenate alkanes using either olefinic sacrificial acceptors or a purge mechanism to remove H 2 ; both of these approaches have significant practical limitations. Here, we report the use of pincer-ligated iridium complexes to achieve alkane dehydrogenation by proton-coupled electron transfer, using pairs of oxidants and bases as proton and electron acceptors. Up to 97% yield was achieved with respect to oxidant and base, and up to 15 catalytic turnovers with respect to iridium, using t-butoxide as base coupled with various oxidants, including oxidants with very low reduction potentials. Mechanistic studies indicate that (pincer)IrH 2 complexes react with oxidants and base to give the corresponding cationic (pincer)IrH + complex, which is subsequently deprotonated by a second equivalent of base; this affords (pincer)Ir which is known to dehydrogenate alkanes and thereby regenerates (pincer)IrH 2 .

02 PETROLEUM↗

DOE OSTI2023

Molecular Simulations of the Chain Length Dependent Adsorption of C7‐C14 n ‐Alkanes in ZIF‐8

Recent experiments show that the diffusivities of C8 and C10 n-alkanes in ZIF-8 are higher than those of C7 and C9, respectively. We investigated this unusual ‘odd-even' effect by simulating the adsorption of C7-C14 n-alkanes in ZIF-8 using hybrid Monte Carlo molecular simulations. The resultant adsorption isotherms, guest-host energies, isosteric heats, and chain length distributions are analyzed for trends among the n-alkanes. ZIF-8 cages filled with n-alkanes are characterized using a combination of image processing and data science to quantify the differences in packing that occur with chain length. In conclusion, results indicate that packing changes drastically from C7 to C8 and from C9 to C10, which is consistent with the diffusion trends previously reported in experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Oxidation of n-alkanes using TS-1 and H 2 O 2 : Effects of chain length and solvents

The selective oxidation of n-C 8 H 18 , n-C 12 H 26 , n-C 16 H 34 , n-C 20 H 42 , and n-C 36 H 74 was studied with a goal of using these as models to provide insight into how to functionalize polyolefins. Reactions were carried out using a TS-1 catalyst and H 2 O 2 in a batch reactor with different cosolvents, including methanol, acetone, acetonitrile, methyl ethyl ketone, and methyl butyl ketone. Rates decreased with increasing alkane size, possibly due to the reduced solubility of larger alkanes into the water-rich phases. Cosolvents that promote the partitioning of alkanes in the aqueous phase increased the rates. 1 H NMR spectroscopy demonstrated that ketones were the primary products, although some alcohols also formed. There was preferential reaction at the 2 position in the alkanes, but reaction at central carbons was also observed. The results of this study suggest strategies for using this catalytic chemistry to functionalize polyolefins.

Alkane oxidation↗