Addressing hysteresis and slow equilibration issues in cavity-based calculation of chemical potentials.

In this paper, we explore the strengths and weaknesses of a cavity-based method to calculate the excess chemical potential of a large molecular solute in a dense liquid solvent. Use of the cavity alleviates some technical problems associated with the appearance of (integrable) divergences in the integrand during alchemical particle growth. The excess chemical potential calculated using the cavity-based method should be independent of the cavity attributes. However, the performance of the method (equilibration time and the robustness) does depend on the cavity attributes. To illustrate the importance of a suitable choice of the cavity attributes, we calculate the partition coefficient of pyrene in toluene and heptane using a coarse-grained model. We find that a poor choice for the functional form of the cavity may lead to hysteresis between growth and shrinkage of the cavity. Somewhat unexpectedly, we find that, by allowing the cavity to move as a pseudo-particle within the simulation box, the decay time of fluctuations in the integrand of the thermodynamic integration can be reduced by an order of magnitude, thereby increasing the statistical accuracy of the calculation.BP ICA

Experimental and modeling studies of the micro-structures of opposed flow diffusion flames: Methane

The micro-structure of an atmospheric pressure, opposed flow, methane diffusion flame has been studied using heated micro-probe sampling and chemical kinetic modeling. Mole fraction profiles of major products as well as trace aromatic, substituted aromatic, and polycyclic aromatic hydrocarbons (PAH up to C{sub 16}H{sub 10}, e.g. pyrene) were quantified by direct gas chromatography/mass spectrometry (GC/MS) analysis of samples withdrawn from within the flame without any pre-concentration. Mole fractions range from 0.8 to 1.0 {times} 10{sup {minus}7}. The experimental measurements are compared to results from a newly-developed chemical kinetic model that includes chemistry for the production and consumption of aromatics and PAH species. The model predictions are in reasonable agreement with the experimental data for the major species profiles and for the peak concentrations of many of the trace aromatics and PAH species. 36 refs

Experimental and Modeling Investigation of Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Premixed Ethylene Flame

Experimental and detailed chemical kinetic modeling has been performed to investigate aromatic and polyaromatic hydrocarbon formation pathways in a rich, sooting, ethylene-oxygen-argon premixed flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.5 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer (GC/MS) technique. Measurements were made in the flame and post-flame zone for a number of low molecular weight species, aliphatics, aromatics and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-aromatic fused rings. The modeling results show the key reaction sequences leading to aromatic and polycyclic aromatic hydrocarbon growth involve the combination of resonantly stabilized radicals. In particular, propargyl and 1-methylallenyl combination reactions lead to benzene and methyl substituted benzene formation, while polycyclic aromatics are formed from cyclopentadienyl radicals and fused rings that have a shared C{sub 5} side structure. Naphthalene production through the reaction step of cyclopentadienyl self-combination and phenanthrene formation from indenyl and cyclopentadienyl combination were shown to be important in the flame modeling study. The removal of phenyl by O{sub 2} leading to cyclopentadienyl formation is expected to play a pivotal role in the PAH or soot precursor growth process under fuel-rich oxidation conditions

Quantum chemical study of the catalytic oxidative coupling of methane

Oxidative coupling of methane reaction pathways on MgO and lithium-modified MgO were theoretically studied using the semiempirical MNDO-PM3 molecular orbital method. The surface of the MgO catalyst was modeled by a Mg9O9 molecular cluster containing structural defects such as edges and corners. Lithium-promoted magnesia was simulated by isomorphic substitution of Mg2+ by Li+; the excess negative charge of the cluster was compensated by a proton connected to a neighboring O2- site. Heterolytic adsorption of methane was found to be directly related to the coordination number of both the lattice oxygen and the metal sites. Energetically the most favorable site pair was Mg-3c-O-3c with a neighboring Li-4c site present. Various sequential oxygen and methane adsorption pathways were explored resulting in CH3OH formation with lower energy barriers for the Li-modified MgO cluster as compared to unmodified MgO

Partial oxidation of methane on the SiO2 surface - A quantum chemical study

Reaction pathways for methane partial oxidation (MPO) on silica were theoretically investigated using the semiempirical MOPAC-PM3 molecular orbital method. The surface of SiO2 was modeled by a helical Si6O18H12 molecular cluster that also exhibits a strained siloxane bridge defect. First, a bond energy analysis was performed on the silica cluster with isolated 3- and 4-coordinated Si surface atoms. Calculated bond dissociation energies for Si-H, SiO-H, and SI-OH were comparable to H-CH3, H-OH, and O-O. In the second phase, elementary reactions around the bridge structure were studied. The facile ring-opening reaction with water, which reconstitutes a pair of vicinal hydroxyls, was found both thermodynamically and kinetically favored, in good agreement with the experiment and other theoretical methods. Activation of methane by the lattice bridge oxygen was thermodynamically unfavorable with high activation energy. On the other hand, the computational results also confirmed the important role adsorbed or "activated" oxygen plays in an MPO reaction, and indicated the likely formation of methanol as an intermediate in formaldehyde production

Selective oxidation of propylene to propylene oxide using combinatorial methodologies

Direct oxidation of propylene by oxygen to propylene oxide (PO) has been studied through the application of the techniques of combinatorial catalysis. Catalytic materials containing single and binary metal components were prepared by impregnating standard gamma-Al2O3 pellets. In the first stage, 34 single component catalytic materials at three different metal loading levels were prepared and screened for PO activity and selectivity using array channel microreactors and mass spectrometry. Experiments were conducted at a GSHV of 20,000 h(-1), 10 1 kPa pressure and over a temperature range of 200-350degreesC. Following a matrix inversion technique to deconvolute the mass spectrometric intensity measurements, signals that were directly attributable to PO were calculated. From these determinations, the elements Rh, Mn, and Mo were the most PO active single metal catalysts on gamma-Al2O3. For acetone (AT) Rh, Pb, and Ir were somewhat effective, while Cu, Mn, and W favored some acrolein (AL) formation. In the second step, catalytic materials containing binary combinations of metals were prepared using a variety of strategies. However, the binary catalytic materials that exhibited the highest PO production levels always contained Rh. The binary combinations that exhibited superior PO production levels were Rh-V, Rh-Cr, Rh-Sn, Rh-In, Rh-Mo, and Rh-Sm, albeit substantial CO2 formation. On the other hand, Rh-Ag, Rh-Zn, and Rh-Cr combinations were significant leads with regard to high PO and low CO2 production. These findings call for the undertaking of detailed secondary screening studies to confirm the primary screening results reported here and to obtain information on the durabilities of these catalytic materials