SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Alkane

27

A new method for ketone enolate C-acylation is described which utilizes alkyl pentafluorophenylcarbonates, thiocarbonates, and thionocarbonates as the reactive acylating agents, and MgBr(2)·Et(2)O, DMAP, and i-Pr(2)NEt as the reagents for enolization. A wide range of ketones have been observed to undergo clean C-acylation via this protocol.

Concepts: Carbonyl, Ketone, Ketone bodies, Functional groups, Alkane, Aldehyde, Keto-enol tautomerism, Enol

25

More stringent emission requirements for nonroad diesel engines introduced with U.S. Tier 4 Final and Euro Stage IV regulations have spurred the development of exhaust aftertreatment technologies. In this study, several potential after-treatment configurations consisting of diesel oxidation catalysts (DOC), diesel particulate filters (DPF), Cu zeolite- and vanadium-based selective catalytic reduction (SCR) catalysts, and ammonia oxidation (AMOX) catalysts are evalu-ated using both Nonroad Transient (NRTC) and Steady (8-mode NRSC) Cycles in order to understand both compo-nent and system-level effects of diesel aftertreatment on emissions of polycyclic aromatic hydrocarbons (PAH) and their nitrated derivatives (nitro-PAH). Emissions are reported for four configurations including engine-out, DOC+CuZ-SCR+AMOX, V-SCR+AMOX, and DOC+DPF+CuZ-SCR+AMOX. Mechanisms responsible for the reduc-tion, and, in some cases, the formation of PAH and nitro-PAH compounds are discussed in detail, and suggestions are provided to minimize the formation of nitro-PAH compounds through aftertreatment design optimizations. Potency equivalency factors (PEFs) developed by California Environmental Protection Agency are then applied to determine the impact of aftertreatment on PAH-derived exhaust toxicity. Finally, a comprehensive set of exhaust emissions in-cluding criteria pollutants, NO2, total hydrocarbons (THC), n-alkanes, branched alkanes, saturated cycloalkanes, aro-matics, aldehydes, hopanes and steranes, and metals is provided, and the overall efficacy of the aftertreatment configu-rations is described. This detailed summary of emissions from a current nonroad diesel engine equipped with advanced aftertreatment can be used to more accurately model the impact of anthropogenic emissions on the atmosphere.

Concepts: Hydrogen, Catalytic converter, Polycyclic aromatic hydrocarbon, Alkane, Air pollution, Internal combustion engine, Diesel engine, Diesel particulate filter

24

The 2010 Deepwater Horizon disaster introduced an unprecedented discharge of oil into the deep Gulf of Mexico. Considerable uncertainty has persisted regarding the oil’s fate and effects in the deep ocean. In this work we assess the compound-specific rates of biodegradation for 125 aliphatic, aromatic, and biomarker petroleum hydrocarbons that settled to the deep ocean floor following release from the damaged Macondo Well. Based on a dataset comprising measurements of up to 168 distinct hydrocarbon analytes in 2,980 sediment samples collected within 4 y of the spill, we develop a Macondo oil “fingerprint” and conservatively identify a subset of 312 surficial samples consistent with contamination by Macondo oil. Three trends emerge from analysis of the biodegradation rates of 125 individual hydrocarbons in these samples. First, molecular structure served to modulate biodegradation in a predictable fashion, with the simplest structures subject to fastest loss, indicating that biodegradation in the deep ocean progresses similarly to other environments. Second, for many alkanes and polycyclic aromatic hydrocarbons biodegradation occurred in two distinct phases, consistent with rapid loss while oil particles remained suspended followed by slow loss after deposition to the seafloor. Third, the extent of biodegradation for any given sample was influenced by the hydrocarbon content, leading to substantially greater hydrocarbon persistence among the more highly contaminated samples. In addition, under some conditions we find strong evidence for extensive degradation of numerous petroleum biomarkers, notably including the native internal standard 17α(H),21β(H)-hopane, commonly used to calculate the extent of oil weathering.

Concepts: Petroleum, Carbon, Polycyclic aromatic hydrocarbon, Benzene, Hydrocarbon, Wax, Alkane, Aromatic hydrocarbon

23

Hydrogen transfer is the major route in catalytic conversion of methanol to olefins (MTO) for the formation of nonolefinic byproducts, including alkanes and aromatics. Two separate, noninterlinked hydrogen transfer pathways have been identified. In the absence of methanol, hydrogen transfer occurs between olefins and naphthenes via protonation of the olefin and the transfer of the hydride to the carbenium ion. A hitherto unidentified hydride transfer pathway involving Lewis and Brønsted acid sites dominates as long as methanol is present in the reacting mixture, leading to aromatics and alkanes. Experiments with purely Lewis acidic ZSM-5 showed that methanol and propene react on Lewis acid sites to HCHO and propane. In turn, HCHO reacts with olefins stepwise to aromatic molecules on Brønsted acid sites. The aromatic molecules formed at Brønsted acid sites have a high tendency to convert to irreversibly adsorbed carbonaceous deposits and are responsible for the critical deactivation in the methanol to olefin process.

Concepts: Oxygen, Alcohol, Hydrogen, Hydrogenation, Alkene, Hydrocarbon, Alkane, Propane

23

The deployment of next-generation renewable biofuels can be enhanced by improving their compatibility with the current infrastructure for transportation, storage and utilization. Propane, the bulk component of liquid petroleum gas, is an appealing target as it already has a global market. In addition, it is a gas under standard conditions, but can easily be liquefied. This allows the fuel to immediately separate from the biocatalytic process after synthesis, yet does not preclude energy-dense storage as a liquid. Here we report, for the first time, a synthetic metabolic pathway for producing renewable propane. The pathway is based on a thioesterase specific for butyryl-acyl carrier protein (ACP), which allows native fatty acid biosynthesis of the Escherichia coli host to be redirected towards a synthetic alkane pathway. Propane biosynthesis is markedly stimulated by the introduction of an electron-donating module, optimizing the balance of O2 supply and removal of native aldehyde reductases.

Concepts: Metabolism, Gasoline, Petroleum, Hydrocarbon, Alkane, Natural gas, Propane, Liquefied petroleum gas

20

Electrocatalytic CO2 reduction to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO2 reduction by copper electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C-C coupled products relative to CH4 and H2 remains an impediment. Herein we report a simple yet highly selective catalytic system for CO2 reduction to C≥2 hydrocarbons on a polycrystalline Cu electrode in bicarbonate aqueous solution that uses N-substituted pyridinium additives. Selectivities of 70-80% for C2 and C3 products with a hydrocarbon ratio of C≥2/CH4 significantly greater than 100 have been observed with several additives. (13)C-labeling studies verify CO2 to be the sole carbon source in the C≥2 hydrocarbons produced. Upon electroreduction, the N-substituted pyridinium additives lead to film deposition on the Cu electrode, identified in one case as the reductive coupling product of N-arylpyridinium. Product selectivity can also be tuned from C≥2 species to H2 (∼90%) while suppressing methane with certain N-heterocyclic additives.

Concepts: Carbon dioxide, Hydrogen, Petroleum, Carbon, Hydrocarbon, Alkane, Natural gas, Methane

19

Increasing concerns about limited fossil fuels and global environmental problems have focused attention on the need to develop sustainable biofuels from renewable resources. Although microbial production of diesel has been reported, production of another much in demand transport fuel, petrol (gasoline), has not yet been demonstrated. Here we report the development of platform Escherichia coli strains that are capable of producing short-chain alkanes (SCAs; petrol), free fatty acids (FFAs), fatty esters and fatty alcohols through the fatty acyl (acyl carrier protein (ACP)) to fatty acid to fatty acyl-CoA pathway. First, the β-oxidation pathway was blocked by deleting the fadE gene to prevent the degradation of fatty acyl-CoAs generated in vivo. To increase the formation of short-chain fatty acids suitable for subsequent conversion to SCAs in vivo, the activity of 3-oxoacyl-ACP synthase (FabH), which is inhibited by unsaturated fatty acyl-ACPs, was enhanced to promote the initiation of fatty acid biosynthesis by deleting the fadR gene; deletion of the fadR gene prevents upregulation of the fabA and fabB genes responsible for unsaturated fatty acids biosynthesis. A modified thioesterase was used to convert short-chain fatty acyl-ACPs to the corresponding FFAs, which were then converted to SCAs by the sequential reactions of E. coli fatty acyl-CoA synthetase, Clostridium acetobutylicum fatty acyl-CoA reductase and Arabidopsis thaliana fatty aldehyde decarbonylase. The final engineered strain produced up to 580.8 mg l(-1) of SCAs consisting of nonane (327.8 mg l(-1)), dodecane (136.5 mg l(-1)), tridecane (64.8 mg l(-1)), 2-methyl-dodecane (42.8 mg l(-1)) and tetradecane (8.9 mg l(-1)), together with small amounts of other hydrocarbons. Furthermore, this platform strain could produce short-chain FFAs using a fadD-deleted strain, and short-chain fatty esters by introducing the Acinetobacter sp. ADP1 wax ester synthase (atfA) and the E. coli mutant alcohol dehydrogenase (adhE(mut)).

Concepts: Protein, Alcohol, Fatty acid, Fatty acid metabolism, Carboxylic acid, Ester, Alkane, Unsaturated fat

14

Abandoned oil and gas wells provide a potential pathway for subsurface migration and emissions of methane and other fluids to the atmosphere. Little is known about methane fluxes from the millions of abandoned wells that exist in the United States. Here, we report direct measurements of methane fluxes from abandoned oil and gas wells in Pennsylvania, using static flux chambers. A total of 42 and 52 direct measurements were made at wells and at locations near the wells (“controls”) in forested, wetland, grassland, and river areas in July, August, October 2013 and January 2014, respectively. The mean methane flow rates at these well locations were 0.27 kg/d/well, and the mean methane flow rate at the control locations was 4.5 × 10(-6) kg/d/location. Three out of the 19 measured wells were high emitters that had methane flow rates that were three orders of magnitude larger than the median flow rate of 1.3 × 10(-3) kg/d/well. Assuming the mean flow rate found here is representative of all abandoned wells in Pennsylvania, we scaled the methane emissions to be 4-7% of estimated total anthropogenic methane emissions in Pennsylvania. The presence of ethane, propane, and n-butane, along with the methane isotopic composition, indicate that the emitted methane is predominantly of thermogenic origin. These measurements show that methane emissions from abandoned oil and gas wells can be significant. The research required to quantify these emissions nationally should be undertaken so they can be accurately described and included in greenhouse gas emissions inventories.

Concepts: Carbon dioxide, Hydrocarbon, Alkane, Natural gas, Methane, Greenhouse gas, Flux, Propane

11

A one-step, gas-phase photothermocatalytic process for the synthesis of hydrocarbons, including liquid alkanes, aromatics, and oxygenates, with carbon numbers (Cn) up to C13, from CO2 and water is demonstrated in a flow photoreactor operating at elevated temperatures (180-200 °C) and pressures (1-6 bar) using a 5% cobalt on TiO2 catalyst and under UV irradiation. A parametric study of temperature, pressure, and partial pressure ratio revealed that temperatures in excess of 160 °C are needed to obtain the higher Cn products in quantity and that the product distribution shifts toward higher Cn products with increasing pressure. In the best run so far, over 13% by mass of the products were C5+ hydrocarbons and some of these, i.e., octane, are drop-in replacements for existing liquid hydrocarbons fuels. Dioxygen was detected in yields ranging between 64% and 150%. In principle, this tandem photochemical-thermochemical process, fitted with a photocatalyst better matched to the solar spectrum, could provide a cheap and direct method to produce liquid hydrocarbons from CO2 and water via a solar process which uses concentrated sunlight for both photochemical excitation to generate high-energy intermediates and heat to drive important thermochemical carbon-chain-forming reactions.

Concepts: Oxygen, Ultraviolet, Temperature, Thermodynamics, Hydrocarbon, Alkane, Pressure, Gas

8

Carbon-carbon (C-C) bonds form the backbone of many important molecules, including polymers, dyes and pharmaceutical agents. The development of new methods to create these essential connections in a rapid and practical fashion has been the focus of numerous organic chemists. This endeavour relies heavily on the ability to form C-C bonds in the presence of sensitive functional groups and congested structural environments. Here we report a chemical transformation that allows the facile construction of highly substituted and uniquely functionalized C-C bonds. Using a simple iron catalyst, an inexpensive silane and a benign solvent under ambient atmosphere, heteroatom-substituted olefins are easily reacted with electron-deficient olefins to create molecular architectures that were previously difficult or impossible to access. More than 60 examples are presented with a wide array of substrates, demonstrating the chemoselectivity and mildness of this simple reaction.

Concepts: Chemical reaction, Functional group, Alkyne, Alkene, Organic chemistry, Hydrocarbon, Alkane, Aldehyde