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181.
Eric K. Mayhew Constandinos M. Mitsingas Vincent D. Coburn Jacob E.G. Temme Chol-Bum M. Kweon 《Proceedings of the Combustion Institute》2021,38(4):5733-5740
The effects of blending ratio on mixtures of an alcohol-to-jet (ATJ) fuel and a conventional petroleum-derived fuel on first stage ignition and overall ignition delay are examined at engine-relevant ambient conditions. Experiments are conducted in a high-temperature pressure vessel that maintains a small flow of dry air at the desired temperature (825 K and 900 K) and pressure (6 MPa and 9 MPa) for fuel injections from a custom single-hole, axially-oriented injector, representing medium (7.5 mg) and high (10 mg) engine loading. Formaldehyde, imaged using planar laser-induced fluorescence, is measured at discrete time steps throughout the first and second stage ignition process and is used as a marker of unburned short-chain hydrocarbons formed after the initial breakdown of the fuel. The formaldehyde images are used to calculate the first stage ignition delay for each ambient and fuel loading condition. Chemiluminescence imaging of excited hydroxyl radical at 75 kHz is used to determine the overall ignition delay. At all conditions, increased volume fraction of ATJ resulted in longer, but non-linearly increasing, overall ignition delay. Across all of the blends, first stage ignition delay accounted for about 15% of the increase in overall ignition delay compared to the military's aviation kerosene, F-24, which is Jet A with additives, while extended first stage ignition duration accounted for 85% of the increase. It is observed that blends consisting of 0–60% by volume of the low cetane number ATJ fuel produced nearly identical first stage ignition delays. These results will inform the development of ignition models that can capture the non-linear effects of fuel blending on ignition processes. 相似文献
182.
Alexandra Brown Victoria Hogan Jacob Perry Renuka Manchanayakage 《Tetrahedron letters》2017,58(11):1061-1065
Chiral ionic liquids show promising applications in various different fields. A series of pyrrolidinium-based chiral ionic liquids bearing a chiral cation, a chiral anion or both was prepared in good yields using an efficient, economic and simple pathway. The chirality was introduced using (l)-lactate and (l)-menthol derivatives. The resultant chiral compounds were characterized by both spectroscopy and polarimetry. We envision that these new chiral compounds can serve as effective reaction media and chiral catalysts for asymmetric reactions, which are presently being investigated in our lab. 相似文献
183.
Bryan S. Holtzman Eric T. Roberts Nicholas S. Caminiti Jacob A. Fox Madison B. Goodstein Staci A. Hill Zitong B. Jia Isabelle N.-M. Leibler Michael L. Martini Gina M. Mendolia Sarah B. Nodder Molly S. Costanza-Robinson Richard C. Bunt 《Tetrahedron letters》2017,58(5):432-436
A nucleophile crossover experiment was used to monitor the reversibility of nucleophilic addition of benzylamine to π-allylpalladium complexes. Dppe, dppp, dppb, and PHOX showed more crossover than PPh3 and dppm in both DMF and dichloromethane. Crossover was inhibited by the addition of DBU or Cs2CO3, but much less elimination to diene side products was observed with Cs2CO3. Analysis of percent crossover vs. percent reaction completion using the PHOX ligand revealed that with added DBU or Cs2CO3 crossover only began occurring after 100% completion had been reached. 相似文献
184.
A new mathematical model is proposed for time-independent laminar flow through a rigid isotropic and consolidated porous medium of spatially varying porosity. The model is based upon volumetric averaging concepts. Explicit assumptions regarding the mean geometric properties of the porous microstructure lead to a relationship between tortuosity and porosity. Microscopic inertial effects are introduced through consideration of flow development within the pores. A momentum transport equation is derived in terms of the fluid properties, the porous medium porosity and a characteristic length of the microstructure. In the limiting cases of porosity unity and zero, the model yields the required Navier-Stokes equation for free flow and no flow in a solid, respectively. 相似文献
185.
Shock waves in saturated thermoelastic porous media 总被引:1,自引:0,他引:1
The objective of this paper is to develop and present the macroscopic motion equations for the fluid and the solid matrix, in the case of a saturated porous medium, in the form of coupled, nonlinear wave equations for the fluid and solid velocities. The nonlinearity in the equations enables the generation of shock waves. The complete set of equations required for determining phase velocities in the case of a thermoelastic solid matrix, includes also the energy balance equation for the porous medium as a whole, as well as mass balance equations for the two phase. In the special case of a rigid solid matrix, the wave after an abrupt change in pressure propagates only through the fluid. 相似文献
186.
Banerjee I Mondal D Martin J Kane RS 《Langmuir : the ACS journal of surfaces and colloids》2010,26(22):17369-17374
We report the design of antimicrobial nanocomposite films based on conjugates of multiwalled carbon nanotubes (MWNT) and protoporphyrin IX (PPIX) that are highly effective against Staphylococcus aureus (S. aureus) upon irradiation with visible light. S. aureus infections can lead to life-threatening situations, especially when caused by antibiotic-resistant strains. While the light-activated antimicrobial activity of porphyrins against such pathogens is well-known, a facile way to incorporate porphyrins into coatings may lead to their more effective use. To that end, we decided to synthesize and characterize MWNT-PPIX conjugates which combine the biocidal capacity of porphyrins with the mechanical strength of MWNTs. The conjugates could effectively deactivate S. aureus cells in solution upon irradiation with visible light. We also designed large area nanocomposite films comprised of the MWNT-PPIX conjugates that showed potent antimicrobial activity. These MWNT-PPIX conjugates represent a facile strategy for the design of antimicrobial and antifouling coatings. 相似文献
187.
Despite considerable improvements in the tolerance and efficacy of novel chemotherapeutic agents, the mortality of hematological malignancies is still high due to therapy relapse, which is associated with bad prognosis. Dietary polyphenolic compounds are of growing interest as an alternative approach, especially in cancer treatment, as they have been proven to be safe and display strong antioxidant properties. Here, we provide evidence that both resveratrol and curcumin possess huge potential for application as both chemopreventive agents and anticancer drugs and might represent promising candidates for future treatment of leukemia. Both polyphenols are currently being tested in clinical trials. We describe the underlying mechanisms, but also focus on possible limitations and how they might be overcome in future clinical use--either by chemically synthesized derivatives or special formulations that improve bioavailability and pharmacokinetics. 相似文献
188.
Garbiñe Álvarez Francisco Alcaide Oscar Miguel Laura Calvillo María Jesús Lázaro Jacob J. Quintana Juan Carlos Calderón Elena Pastor 《Journal of Solid State Electrochemistry》2010,14(6):1027-1034
This paper presents the behavior of ordered mesoporous carbon (OMC)-supported catalysts as anodes for direct methanol fuel
cells (DMFC), fed with an aqueous methanol solution. OMC samples were prepared by the nanocasting method from a polymerized
furan resin using mesoporous silica as a template. Pt and PtRu nanoparticles were supported on OMC with high dispersion, the
particle size being 2.4 nm at PtRu loading of 15 wt.%. The resulting catalysts were analyzed using carbon monoxide stripping
voltammetry, cyclic voltammetry, and chronoamperometry in three-electrode experiments and recording cell voltage vs. current
density curves in practical DMFC. It was found that PtRu-catalyzed technical electrodes exhibited good activity towards methanol
electrooxidation in half-cell experiments under fuel-cell-relevant conditions. Specifically, Pt85Ru15/OMC catalyst showed the highest catalytic enhancement compared to Pt/OMC for the steady-state electrooxidation of methanol
at 60 °C and 0.5 V, by a factor of 22 in 2-M MeOH solution. DMFC single cells yielded an open-circuit voltage of 0.625 V at
60 °C. Polarization curves indicate that DMFC with OMC-supported Pt85Ru15 catalyst at the anode exhibited the best performance. 相似文献
189.
Biswas R Maillard N Kofoed J Reymond JL 《Chemical communications (Cambridge, England)》2010,46(46):8746-8748
Fluorescence screening of a 96-membered SPOT library of histidine containing dendritic and linear peptides revealed the remarkable esterolytic activity of short histidine oligomers that show catalytic proficiencies within one order of magnitude of histidine-containing esterase peptide dendrimers. 相似文献
190.
Radical couplings of cyanopyridine radical anions represent a valuable technology for functionalizing pyridines, which are prevalent throughout pharmaceuticals, agrochemicals, and materials. Installing the cyano group, which facilitates the necessary radical anion formation and stabilization, is challenging and limits the use of this chemistry to simple cyanopyridines. We discovered that pyridylphosphonium salts, installed directly and regioselectively from C–H precursors, are useful alternatives to cyanopyridines in radical–radical coupling reactions, expanding the scope of this reaction manifold to complex pyridines. Methods for both alkylation and amination of pyridines mediated by photoredox catalysis are described. Additionally, we demonstrate late-stage functionalization of pharmaceuticals, highlighting an advantage of pyridylphosphonium salts over cyanopyridines.Cyanopyridines form dearomatized radical anions upon single-electron reduction and participate in photoredox coupling reactions. Pyridylphosphonium salts replicate that reactivity with a broader scope and increase the utility of these processes.Modern photoredox catalysis and electrochemistry have enabled new synthetic methods that proceed via open-shell intermediates.1 Under this regime, pyridine functionalization strategies have been developed where 4-cyanopyridines undergo single-electron reduction to form dearomatized radical species that couple with other stabilized radicals (Scheme 1A).2 The cyano group is critical for efficient reactivity via pyridyl radical anions; alternatives such as 4-halopyridines more readily undergo elimination to pyridyl radicals after single-electron reduction resulting in a distinct set of coupling processes.3 We aimed to show that pyridylphosphonium salts could replicate the reactivity of cyanopyridines and allow a broader set of inputs into dearomatized pyridyl radical coupling reactions.4Open in a separate windowScheme 1Expansion of radical coupling reactions to complex pyridines.Cyanopyridines have facilitated pyridine alkylation, allylation, and alkenylation reactions providing access to valuable building blocks for medicinal and agrochemical programs.5 The cyano group is essential for these methods, but a problem arises when applying this chemistry to complex pyridines, such as those found in pharmaceutical and agrochemical candidates. These structures are often devoid of pre-installed functional groups, and it is often challenging to install a cyano group from C–H precursors regioselectively.6 We envisioned pyridylphosphonium salts, regioselectively constructed from the C–H bonds of a diverse set of pyridines, could serve as alternatives to cyanopyridines.7 Herein, we report couplings between alkyl BF3K salts and preliminary studies of carboxylic acids and amines with pyridylphosphonium salts, including late-stage functionalization of complex pyridine-containing pharmaceuticals using this strategy.Recently, we reported a radical coupling reaction between a boryl-stabilized cyanopyridyl radical and a boryl-stabilized pyridylphosphonium radical.7a The intermediate radicals arose via an unusual inner-sphere process that would be difficult to extend to other coupling reactions. A significant advance would be to show that pyridylphosphonium salts could function more generally as radical anion precursors and mimic the reactivity of cyano-pyridines. In particular, showing their viability in photoredox and electrochemical processes would translate to numerous synthetic transformations. To demonstrate this principle, we envisioned a redox-neutral alkylation reaction (Scheme 1B) via a radical coupling between radical zwitterion I, formed through single-electron reduction of a pyridylphosphonium salt (Eredp/2 = −1.51 V vs. SCE) and benzyl radical II, resulting from single-electron oxidation of a BF3K salt (Ered = +1.10 V vs. SCE for a primary benzylic salt).8 Loss of triphenylphosphine from dearomatized intermediate (III) would furnish the alkylated pyridine product. Notably, the redox events could invert, where the photocatalyst oxidizes the BF3K salt first and reduces the pyridylphosphonium salt second, broadening the scope of amenable photocatalysts.We began our investigation by examining a series of photocatalysts for the coupling reaction of phosphonium salt 1a, formed with complete regioselectivity for the 4-position from 2-phenylpyridine, and benzylic BF3K salt 2a under irradiation from a 455 nm Kessil light (Scheme 1B are potentially interchangeable.1b The Adachi-type photocatalyst 3DPAFIPN improved the yield to 77% with a further increase to 82% after increasing the reaction concentration (entries 3 and 4). Adding 2,6-lutidine, previously shown as an effective additive for photoredox cross-coupling reactions of BF3K salts by the Molander group,9 had no impact on the yield of 2-phenylpyridine salt 1a (entry 5) and the [Ir(ppy)2(dtbbpy)]PF6 catalyst was marginally less efficient under the same conditions (entry 6). We observed that 2,6-lutidine did substantially improve the yield when isomeric 3-Ph salt 1b was employed (entries 7 and 8); without 2,6-lutidine, the crude 1H NMR indicates significant amounts of decomposition occurred, including 3-phenylpyridine, and the 4- vs. 2-position product ratio was 3 : 1. This outcome suggests that protiodephosphination and non-selective Minisci-type pathways can occur under these conditions. With 2,6-lutidine, the crude reaction pathway is cleaner, and the 4- vs. 2-position ratio improved to 8 : 1. At this point, we have not established the role of 2,6-lutidine, although it is conceivable that it reacts with BF3 produced as the reaction progresses. In 2-substituted systems, steric hindrance around the pyridine N-atom of the salt would deter BF3-coordination, whereas, in 3-substituted systems, such as salt 1b, coordination is more likely and may have a deleterious effect on the reaction (vide infra). Given the structural variation of pyridines that we anticipated applying to this process and how those structures could impact boron speciation during the reaction, we elected to use 2,6-lutidine as an additive in all subsequent reactions.10Optimization of pyridine alkylation, photocatalyst data and effect of BF3·OEt2 as an additivea
Open in a separate windowaConditions: 1a (1.0 equiv.), 2a (2.0 equiv.), photocatalyst (2 mol%), additive (3.0 equiv.), rt.bYields determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as internal standard.cIsolated yield on 0.50 mmol scale.dIsolated yield on 2.00 mmol scale.e3 : 1 4- vs. 2-regioisomeric ratio determined from the crude 1H NMR.f8 : 1 4- vs. 2-regioisomeric ratio determined from the crude 1H NMR.gUsed 365 nm LEDs instead of 455 nm Kessil light for 89 h.hAll redox potentials reported vs. SCE and all values compiled from previous literature reports.1iCounterion omitted in structure for simplicity.We conducted a series of further experiments to explore the effect of light and photocatalyst type on the reaction (†).11 Furthermore, a photocatalyst with a redox potential window misaligned with the redox events in Scheme 1B, [Mes-Acr]BF4, is also competent (entry 11). An energy transfer mechanism was considered based on entry 9, but the low triplet state energies for [Mes-Acr]BF4 make this pathway unlikely (12–14Employing the optimized conditions, we investigated the scope of pyridylphosphonium salts in this coupling process (