Cellulose and even spruce can be converted efficiently into valuable platform chemicals via combined hydrolysis and hydrogenation in the aqueous phase. Thereby, heteropoly acids together with ...supported ruthenium catalysts show not only high activity but also remarkable selectivity to sugar alcohols reaching up to 81% yield of C(4) to C(6) sugar alcohols in only 7 h at 160 °C.
Synthetic esters are used as lubricants for applications at high temperatures, but their development can be a trial and error process. In this context, molecular dynamics simulations could be used as ...a tool to investigate the properties of new lubricants, in particular viscosity. We employ nonequilibrium molecular dynamics (NEMD) simulations to predict bulk Newtonian viscosities of a set of mixtures of two esters, di(2-ethylhexyl) sebacate (DEHS) and di(2-ethylhexyl) adipate (DEHA) at 293 and 343 K as well as equilibrium molecular dynamics (EMD) and NEMD at 393 K and compare these to experimental measurements. The simulations predict mixture densities within 5% of the experimental values, and we are able to retrieve between 99% and 75% of the experimental viscosities for all ranges of temperature. Experimental viscosities show a linear trend which we are able to capture using NEMD at low temperature and EMD at high temperature. Our work shows that, using EMD and NEMD simulations, and the workflows we developed, we can obtain reliable estimates of the viscosities of mixtures of industrially relevant ester-based lubricants at different temperatures.
The behavior of lubricants at operational conditions, such as at high pressures, is a topic of great industrial interest. In particular, viscosity and the viscosity-pressure relation are especially ...important for applications and their determination by computational simulations is very desirable. In this study, we evaluate methods to compute these quantities based on fully atomistic molecular dynamics simulations, which are computationally demanding but also have the potential to be most accurate. We used the 9,10-dimethyloctadecane molecule, main component of PAO-2 base oil as the lubricant for our tests. The methods used for the viscosity simulations are the Green-Kubo equilibrium molecular dynamics (EMD-GK) and nonequilibrium molecular dynamics (NEMD), at pressures of up to 1.0 GPa and various temperatures (40-150 degrees Celsius). We present the theory behind these methods and investigate how the simulation parameters affect the results obtained, to ensure viscosity convergence with respect to the simulation intervals and all other parameters. We show that, by using each method in its regime of applicability, we can achieve good agreement between simulated and measured values. NEMD simulations at high pressures captured zero shear viscosity successfully; while at 40 degrees Celsius, EMD-GK is only applicable to pressures up to 0.3 GPa, where the viscosity is lower. In NEMD, longer and multiply repeated simulations improve the confidence interval of viscosity, which is essential at lower pressures. Another aspect of these methods is the choice of the utilized force field for the atomic interactions. This was investigated by selecting two different commonly used force fields.
The behaviour of confined lubricants at the atomic scale as affected by the interactions at the surface-lubricant interface is relevant in a range of technological applications in areas such as the ...automotive industry. In this paper, by performing fully atomistic molecular dynamics, we investigate the regime where the viscosity starts to deviate from the bulk behaviour, a topic of great practical and scientific relevance. The simulations consist of setting up a shear flow by confining the lubricant between iron oxide surfaces. By using confined Non-Equilibrium Molecular Dynamics (NEMD) simulations at a pressure range of 0.1-1.0 GPa at 100 °C, we demonstrate that the film thickness of the fluid affects the behaviour of viscosity. We find that by increasing the number of lubricant molecules, we approach the viscosity value of the bulk fluid derived from previously published NEMD simulations for the same system. These changes in viscosity occurred at film thicknesses ranging from 10.12 to 55.93 Å. The viscosity deviations at different pressures between the system with the greatest number of lubricant molecules and the bulk simulations varied from −16% to 41%. The choice of the utilized force field for treating the atomic interactions was also investigated.
Atomistic simulations show for the first time how the variation of a lubricant's film thickness affects viscosity and at what point it eventually becomes comparable with the bulk simulation of a lubricant, at different pressures and shear regimes.
The objective of this work is the construction of a correlation between characteristics of heterogeneous catalysts, encoded in a descriptor vector, and their experimentally measured performances in ...the propene oxidation reaction. In this paper the key issue in the modeling process, namely the selection of adequate input variables, is explored. Several data-driven feature selection strategies were applied in order to obtain an estimate of the differences in variance and information content of various attributes, furthermore to compare their relative importance. Quantitative property activity relationship techniques using probabilistic neural networks have been used for the creation of various semi-empirical models. Finally, a robust classification model, assigning selected attributes of solid compounds as input to an appropriate performance class in the model reaction was obtained. It has been evident that the mathematical support for the primary attributes set proposed by chemists can be highly desirable.
Cationic complexes of the type fac-(L2)PtIVMe3(pyr-X)OTf (pyr-X = 4-substituted pyridines; L2 = diphosphine, viz., dppe = bis(diphenylphosphino)ethane and dppbz = o-bis(diphenylphosphino)benzene; OTf ...= trifluoromethanesulfonate) undergo C−C reductive elimination reactions to form L2PtIIMe(pyr-X)OTf and ethane. Detailed studies indicate that these reactions proceed by a two-step pathway, viz., initial reversible dissociation of the pyridine ligand from the cationic complex to generate a five-coordinate PtIV intermediate, followed by irreversible concerted C−C bond formation. The reaction is inhibited by pyridine. The highly positive values for ΔS ⧧ obs = +180 ± 30 J K-1 mol-1, ΔH ⧧ obs = 160 ± 10 kJ mol-1, and ΔV ⧧ obs = +16 ± 1 cm3 mol-1 can be accounted for in terms of significant bond cleavage and/or partial reduction from PtIV to PtII in going from the ground to the transition state. These cationic complexes have provided the first opportunity to carry out detailed studies of C−C reductive elimination from cationic PtIV complexes in a variety of solvents. The absence of a significant solvent effect for this reaction provides strong evidence that the C−C reductive coupling occurs from an unsaturated five-coordinate PtIV intermediate rather than from a six-coordinate PtIV solvento species.
The reaction of the platinum(II) methyl cation (N-N)Pt(CH3)(solv)+ (N-N = ArNC(Me)C(Me)NAr, Ar = 2,6-(CH3)2C6H3, solv = H2O (1a) or TFE = CF3CH2OH (1b)) with benzene in TFE/H2O solutions cleanly ...affords the platinum(II) phenyl cation (N-N)Pt(C6H5)(solv)+ (2). High-pressure kinetic studies were performed to resolve the mechanism for the entrance of benzene into the coordination sphere. The pressure dependence of the overall second-order rate constant for the reaction resulted in ΔV ⧧ = −(14.3 ± 0.6) cm3 mol-1. Since the overall second order rate constant k = K eq k 2, ΔV ⧧ = ΔV°(K eq) + ΔV ⧧(k 2). The thermodynamic parameters for the equilibrium constant between 1a and 1b, K eq = 1bH2O/1aTFE = 8.4 × 10-4 at 25 °C, were found to be ΔH° = 13.6 ± 0.5 kJ mol-1, ΔS° = −10.4 ± 1.4 J K-1 mol-1, and ΔV° = −4.8 ± 0.7 cm3 mol-1. Thus ΔV ⧧(k 2) for the activation of benzene by the TFE solvento complex equals −9.5 ± 1.3 cm3 mol-1. This significantly negative activation volume, along with the negative activation entropy for the coordination of benzene, clearly supports the operation of an associative mechanism.
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