| 1. |
- Polshin, Victor, et al.
(författare)
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Attaining control by design over the hydrolytic stability of Fe-TAML oxidation catalysts
- 2008
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 130:13, s. 4497-4506
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Tidskriftsartikel (refereegranskat)abstract
- The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs) ([Fe{1-X-1-2-X2C6H2-4,5-(NCOCMe2NCO)(2)CR2}(OH2)](-), 1: X-1 = X-2 = H, R-2 = Me-2 (a), R-2 = (CH2)(2) (b); X-1 = X-2 = Cl, R-2 = F-2, (c), etc.), which the proton is known to demetalate at pH < 3, are also subject to catalyzed demetalation by Bronsted acid buffer components at pH 4-9 such as H2PO4-, HSO3-, and CH3- CO2H, HO2CCH2CO2-. Buffers based on pyridine (py) and tris(hydroxymethyl)aminomethane (TRIS) are catalytically inactive. Where reactions proceed, the products are demetalated TAMLs and iron species of variable composition. Pseudo-first-order rate constants for the demetalation (k(obs)) are linear functions of the acid concentrations, and the effective second-order rate constants k(1),(eff) have a hyperbolic dependence on [H+] (k(1),eff = a(1)[H+]/(b(1)+[H+]). The rate of demetalation of 1a in H2PO4-/HPO42- buffer is appreciable, but the k(obs) values for 1b and 1c are immeasurably low, showing that the rates are strongly affected by the CR2 or "tail" fragments, which are known to potently affect the TAML basicity. The reactivities of 1 depend insignificantly on the aromatic ring or "head" group of 1. The proposed mechanism involves precoordination of the acidic buffer species followed by hydrolysis. The demetalating abilities of buffer species depend on their structures and acidities. Thus, although pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation, its 3- and 4-isomers (nicotinic and isonicotininc acids) are inactive. The difference is rationalized to result from the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in an intramolecular manner. The reaction order in picolinic acid equals one for la and two for 1a. For 1b, "inactive" pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid, suggesting that the second order arises from the axial binding of two pyridine molecules, one of which must be picolinic acid. The binding of pyridine- and imidazole-type ligands was confirmed by UV/vis equilibrium measurements and X-ray crystallography. The implications of these mechanistic findings for designing superior Fe-TAML oxidation catalysts and catalyst formulations are discussed using the results of DFT calculations.
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| 2. |
- Su, Haiyun, et al.
(författare)
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A computational study of an HCCI engine with direct injection during gas exchange
- 2006
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 147:1-2, s. 118-132
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Tidskriftsartikel (refereegranskat)abstract
- We present a new probability density function (PDF)-based computational model to simulate a homogeneous charge compression ignition (HCCI) engine with direct injection (DI) during gas exchange. This stochastic reactor model (SRM) accounts for the engine breathing process in addition to the closed-volume HCCI engine operation. A weighted-particle Monte Carlo method is used to solve the resulting PDF transport equation. While simulating the gas exchange, it is necessary to add a large number of stochastic particles to the ensemble due to the intake air and EGR streams as well as fuel injection, resulting in increased computational expense. Therefore, in this work we apply a down-sampling technique to reduce the number of stochastic particles, while conserving the statistical properties of the ensemble. In this method some of the most important statistical moments (e.g., concentration of the main chemical species and enthalpy) are conserved exactly, while other moments are conserved in a statistical sense. Detailed analysis demonstrates that the statistical error associated with the down-sampling algorithm is more sensitive to the number of particles than to the number of conserved species for the given operating conditions. For a full-cycle simulation this down-sampling procedure was observed to reduce the computational time by a factor of 8 as compared to the simulation without this strategy, while still maintaining the error within an acceptable limit. Following the detailed numerical investigation, the model, intended for volatile fuels only, is applied to simulate a two-stroke, naturally aspirated HCCI engine fueled with isooctane. The in-cylinder pressure and CO emissions predicted by the model agree reasonably well with the measured profiles. In addition, the new model is applied to estimate the influence of engine operating parameters such as the relative air-fuel ratio and early direct injection timing on HCCI combustion and emissions. The qualitative trends observed in the parametric variation study match well with experimental data in literature. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All fights reserved.
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