1D to 3D NMR study of microporous alumino‐phosphate AlPO4‐40

From one‐ to two‐ and three‐dimensional MAS NMR solid‐state experiments involving ³¹P and ²⁷Al, we show that the structure of microporous alumino‐phosphate AlPO₄‐40 contains at least four times more sites than expected, and we attribute two types of Al_IV sites. The newly described ²⁷Al‐³¹P MQ‐HMQC opens new possibilities of describing details of three‐dimensional bounded networks. Copyright © 2009 John Wiley & Sons, Ltd.     

Selectivity of Calixarene-bonded Silica-phases in HPLC: Description of Special Characteristics with a Multiple Term Linear Equation at Two Different pH-Values

Six different calixarene-bonded phases were characterized by analyzing 36 and 26 solutes at pH 3 and 7, respectively. Dolan and Snyder's multiple term linear equation was used to correlate retention factors k' to parameters of the solutes and columns. The column parameters have been related to molecular properties of the stationary phases and new suggestions were made for the interpretation of steric selectivity. Ionic and polar interactions have been found dependent on pH value, while steric interactions are less dependent and hydrophobic interactions remain unchanged. Distinct differences of the supported interactions were confirmed between the calixarene-bonded and the common alkyl-bonded silicas. By use of the parameters, values of k' can be estimated with an average deviation of 2.50 and 7.92% at low and neutral pH-value, respectively.     

Simulated annealing in the presence of noise

In many practical optimization problems, evaluation of a solution is subject to noise, e.g., due to stochastic simulations or measuring errors. Therefore, heuristics are needed that are capable of handling such noise. This paper first reviews the state-of-the-art in applying simulated annealing to noisy optimization problems. Then, two new algorithmic variants are proposed: an improved version of stochastic annealing that allows for arbitrary annealing schedules, and a new approach called simulated annealing in noisy environments (SANE). The latter integrates ideas from statistical sequential selection in order to reduce the number of samples required for making an acceptance decision with sufficient statistical confidence. Finally, SANE is shown to significantly outperform other state-of-the-art simulated annealing techniques on a stochastic travelling salesperson problem.     

Multiple Reaction Pathways in Rhodium‐Catalyzed Hydrosilylations of Ketones

A detailed density functional theory (DFT) computational study (using the BP86/SV(P) and B3LYP/TZVP//BP86/SV(P) level of theory) of the rhodium‐catalyzed hydrosilylation of ketones has shown three mechanistic pathways to be viable. They all involve the generation of a cationic complex [L_nRh^I]⁺ stabilized by the coordination of two ketone molecules and the subsequent oxidative addition of the silane, which results in the Rh–silyl intermediates [L_nRh^III(H)SiHMe₂]⁺. However, they differ in the following reaction steps: in two of them, insertion of the ketone into the Rh? Si bond occurs, as previously proposed by Ojima et al., or into the Si? H bond, as proposed by Chan et al. for dihydrosilanes. The latter in particular is characterized by a very high activation barrier associated with the insertion of the ketone into the Si? H bond, thereby making a new, third mechanistic pathway that involves the formation of a silylene intermediate more likely. This “silylene mechanism” was found to have the lowest activation barrier for the rate‐determining step, the migration of a rhodium‐bonded hydride to the ketone that is coordinated to the silylene ligand. This explains the previously reported rate enhancement for R₂SiH₂ compared to R₃SiH as well as the inverse kinetic isotope effect (KIE) observed experimentally for the overall catalytic cycle because deuterium prefers to be located in the stronger bond, that is, C? D versus M? D.     

Structure-Activity Relationship in Ambergris-Type Woody Odorants possessing a hydronaphthalene skeleton

A series of methyl hydronaphthyl alcohols, formates, acetates, and ketones were prepared and their odour properties evaluated. Minor structural changes, even opposite to the osmophoric group, were found to have major effects on the odour. ¹³C? NMR shift assignments of the hydronaphthyl derivatives are presented.     

On the Structure of MHD Shock Waves in Diffusive-Dispersive Media

We consider the equations of compressible magnetohydrodynamics including the diffusive effects of fluid viscosity, thermal heat conductivity, electrical resistivity, and, in particular, the dispersive influence of the Hall term. The equations describe the dynamics of a plasma as it arises in astrophysical and technical applications. For a model problem we prove an existence result for shock profiles for all values of the Hall parameter. The same question for the complete six-dimensional system seems to be only solvable for small values of the Hall parameter, that is, viewing the Hall effect as a perturbation. To obtain substantial information on the complete system for all ranges of the dissipation parameters we present a careful numerical study. In particular the study confirms a well-known conjecture on the existence and bifurcation of orbits and illustrates the breaking of symmetry due to the Hall term.     

Investigation on ignition behaviors of pulverized coal particles in a tubular swirl burner

This paper intensively investigated the ignition of turbulent coal flames in a novel fully-mixed tubular swirl burner. The Nikon D300s digital camera was used to capture the statistical ignition behavior of dispersed coal particle streams in different ambiences. Meanwhile, the combustion dynamics of individual coal particles were also recorded by means of high-speed photography. Two low-rank coal samples, Hulunbel lignite and Zhundong coal, were tested in this study. The ignition delay times of coal particles in the swirl burner were compared with those in a flat-flame burner. In contrast to previous work on a laminar flat-flame burner, the current experimental results show that the turbulent ambience significantly enhances the ignition of all coal samples, which is exceptionally pronounced under high temperature and low oxygen conditions. In addition, the sensitivity analysis suggests that both the enhanced heat and mass transfer contribute to the early ignition in turbulence. The effect of elevated mass transfer coefficient turns prominent in low oxygen fraction ambience, wherein the volatile barrier effect is suppressed by the enhanced mixing process. The combined effect of turbulence favors the shifting of ignition modes to the heterogeneous-dominant region. Last but not least, the ceased volatile flame that visualized in turbulent low oxygen ambience further confirms the important role of heterogeneous ignition.     

Experimental and modeling assessment of sulfur release from coal under low and high heating rates

Coal combustion releases elevated amounts of pollutants to the atmosphere including SO_X. During the pyrolysis step, sulfur present in the coal is released to the gas phase as many different chemical species such as H₂S, COS, SO₂, CS₂, thiols and larger tars, also called SO_X precursors, as they form SO_X during combustion. Understanding the sulfur release process is crucial to the development of reliable kinetic models, which support the design of improved reactors for cleaner coal conversion processes. Sulfur release from two bituminous coals, Colombian hard coal (K1) and American high sulfur coal (U2), were studied in the present work. Low heating rate (LHR) experiments were performed in a thermogravimetric analyzer coupled with mass spectrometry (TG-MS), allowing to track the mass loss and the evolution of many volatile species (CO, CO₂, CH₄, SO₂, H₂S, COS, HCl and H₂O). High heating rate (HHR) experiments were performed in an entrained flow reactor (drop-tube reactor – DTR), coupled with MS and nondispersive infrared sensor (NDIR). HHR experiments were complemented with CFD simulation of the multidimentional reacting flow field. A kinetic model of coal pyrolysis is employed to reproduce the experiments allowing a comprehensive assessment of the process. The suitability of this model is confirmed for LHR. The combination of HHR experiments with CFD simulations and kinetic modeling revealed the complexity of sulfur chemistry in coal combustion and allowed to better understand of the individual phenomena resulting in the formation of the different SO_X precursors. LHR and HHR operating conditions lead to different distribution of sulfur species released, highly-dependent on the gas-phase temperature and residence time. Higher retention of total sulfur in char is observed at LHR (63%) when compared to HHR (37–44%), at 1273 K. These data support the development of reliable models with improved predictability.