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Totally found 28 items.

  • [期刊] Conformational inversion-topomerization processes of ethylcyclohexane and 1,2-dimethylcyclohexane: A computational investigation
    To deeply reveal the impact of the substituents and their special orientations in ring on conformational behaviors for substituted cyclohexanes, a comprehensive study of ethylcyclohexane, cis-, and trans-1,2-dimethylcyclohexanes has been carried out. All conformational structures for them were captured by the accurate ab intio method, that is, B3LYP/6-311++G(d,p) method was used for geometry optimizations, and MP2/6-311++G(d,p), G4, and CCSD(T)/6-311++G(d,p) methods were applied for the high-level single point energy refinements. Based on CCSD(T)/6-311++G(d,p) quantum results, the conformational populations of minima for these three substituted cyclohexanes were calculated by Boltzmann distribution over 300-2500 K. Additionally, the conformational inversion-topomerization pathways for them were thoroughly investigated. The complete characterization involved in their potential energy surfaces are clearly presented by three or two-dimensional schemes. (C) 2018 Elsevier Ltd. All rights reserved.
  • [期刊] Densities, Viscosities, Speeds of Sound, Bulk Moduli, Surface Tensions, and Flash Points of Binary Mixtures of Ethylcyclohexane or Methylcyclohexane with n-Dodecane or n-Hexadecane at 0.1 MPa
    This work reports physical property measurements of binary mixtures of ethylcyclohexane or methylcyclohexane with either n-dodecane or n-hexadecane at temperatures ranging from (293.15 to 333.15) K. Mixture densities fell between those of the pure components. All excess molar volumes were positive, indicating less efficient packing in the mixture systems. Mixture speeds of sound fell between those of the pure components, except in ethylcyclohexane in n-dodecane mixtures where increasing the mole fraction of ethylcyclohexane caused the speed of sound to decrease to a minimum before increasing again. Speed of sound deviations for ethylcyclohexane/n-hexadecane and methylcyclohexane/n-dodecane mixtures were not statistically different from zero, while they were positive for methylcyclohexane/n-hexadecane mixtures and negative for ethylcyclohexane/n-dodecane mixtures. Both volume expansion and compressibility influence these deviations. For all mixtures, increasing alkylcyclohexane concentrations caused the viscosity to decrease. Excess free energies of activation for viscous flow were not statistically different from zero for the mixtures studied, indicating that these mixtures were behaving ideally. Mixture surface tensions and flash points fell between the values for the pure components. Comparison of these physical properties with those of petroleum-based fuels show that ethylcyclohexane/n-hexadecane mixtures had properties that best matched those of jet fuel.
  • [期刊] Location of the Active Sites for Ethylcyclohexane Hydroisomerization by Ring Contraction and Expansion in the EUO Zeolitic Framework
    Identifying the location of the active sites in a zeolite is a current challenge, impeding the design of optimal catalysts. In this work, we identify the location of the most active sites of 1-ethylcyclohexene isomerization in the EUO framework (10 MR channels, 12 MR side pockets) thanks to DFT calculations corroborated by experiments. Skeletal isomerization of cycloalkenes is a crucial industrial reaction for the bifunctional isomerization of ethylbenzene. Ethylcyclohexene is protonated by framework protons into cyclic carbenium ions, which undergo ring contraction-expansion reactions through protonated cyclopropane (PCP) like transition states. Ab initio calculations clearly show that the acid sites located at the intersection between the channel and the pocket stabilize much less the cyclic carbenium ions involved in the reaction than 12 MR pockets and 10 MR channel sites due to stronger dispersion stabilizing interactions. This computational finding is fully confirmed experimentally by the the catalytic performances of the H-EU-1 and H-ZSM-50 zeolites in ethylcyclohexane hydroisomerization. Both zeolites possess the EUO structure but with different location of the acid sites. The ratio in turnover frequencies is quantitatively rendered by the DFT-calculated free energy profiles. Diffusion measurements reveal similar ethylcyclohexane diffusion times for the two zeolites, supporting that the difference in activity is primarily driven by the location of the active sites.
  • [期刊] Metal/Acid Bifunctional Catalysis and Intimacy Criterion for Ethylcyclohexane Hydroconversion: When Proximity Does Not Matter
    The apparent kinetics in metal/acid bifunctional catalysis is generally strongly affected by the metal to acid site ratio and their proximity. However, these two key parameters have not been systematically investigated in the scientific literature. Such a study is provided here for bifunctional catalysts using platinum as the metallic function and EU-1 zeolite as the acidic function. Two series of bifunctional catalysts with different metal to acid sites ratios and different metal to acid site distances were prepared and tested in ethylcyclohexane hydroconversion. By increasing the metal to acid sites ratio, the catalytic activity and isomerization selectivity increased until a plateau was reached, an observation that is in agreement with the classical bifunctional mechanism. At the same time, the intimacy criterion of Weisz was evaluated: strikingly, for a given metal to acid sites ratio, activities and selectivities are not affected by their distance (up to a micrometer scale). A dual-function kinetic model was successfully applied in order to quantify the effect of the metal to acid site ratio on the catalyst activity and isomerization properties. The application of this model showed that the metal to acid sites ratio needed to reach the catalytic activity plateau is higher than the ratio needed to reach the selectivity plateau. This was interpreted as a consequence of the lower kinetic rate constant for the naphthene ring-opening reaction in comparison to the naphthene isomerization reaction.
  • [期刊] Experimental and kinetic modeling investigation on ethylcyclohexane low-temperature oxidation in a jet-stirred reactor
    In this work, the oxidation of ethylcyclohexane was studied in a jet-stirred reactor at 780 Torr, 480-780 K and equivalence ratios of 0.5, 1.0 and 2.0. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for the detection of oxidation products, including a series of reactive intermediates such as cycloalkylhydroperoxides, keto-hydroperoxides, alkenal-hydroperoxides and highly oxygenated molecules. Quantum chemistry calculations were performed to obtain ionization energies for the identification of some important intermediates and energy barriers of several important pathways. On the other hand, this work presents the first efforts on developing a low-temperature oxidation model of ethylcyclohexane. The present model can reasonably capture the low-temperature oxidation reactivities and negative temperature coefficient behaviors observed in both present and previous experimental work of ethylcyclohexane oxidation. Modeling analyses were performed to provide insight into the low-temperature oxidation chemistry of ethylcyclohexane. The two-stage O-2 addition mechanism is concluded to dominate the chain-branching process in the low-temperature region. The concerted elimination reactions of cycloalkylperoxy radical and "formally direct" chemically activated reactions of cycloalkyl+O-2 result in cycloalkenes and HO2 formation at the negative temperature coefficient region and serve as main chain-termination pathways. Compared with smaller cycloalkanes like cyclohexane and methylcyclohexane, the ethyl sidechain structure in ethylcyclohexane reduces the energy barriers of cycloalkylperoxy radical isomerization and facilitates the formation of keto-hydroperoxides which leads to more pronounced low-temperature oxidation reactivity of ethylcyclohexane. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
  • [期刊] Bubble Pressure Measurement and Prediction for n-Hexadecane and n-Eicosane plus Cyclohexane, Methylcyclohexane, and Ethylcyclohexane Binary Mixtures from 303.15 to 393.15 K
    Experimental vapor liquid equilibrium (VLE) data for long-chain n-alkane + naphthenic mixtures are scarce in the open literature. In this study, VLE data for representative binary mixtures of naphthenes with long-chain n-alkanes are presented. The selected compounds include two n-alkanes (n-hexadecane and n-eicosane) and three naphthenes (cyclohexane, methylcyclohexane, and ethylcyclohexane). The experimental data are compared with computed bubble pressures using the Peng-Robinson and PC-SAFT equations of state in order to evaluate the accuracy of predictions and to obtain regressed k(ij) values. Expected applications of these contributions include improved phase behavior model accuracy for hydrocarbon production, transport, and refining applications.
  • [期刊] Investigation of deep catalytic cracking of various model compounds of different classes of light hydrocarbons on a mesoporous catalyst based on ZSM-5 zeolite
    The catalytic cracking performance of light hydrocarbon model compounds (1-hexene, n-octane, i-octane, ethylcyclohexane and ethylbenzene) over a mesoporous catalyst based on ZSM-5 zeolite was analyzed and compared using a microscale apparatus with a fixed-bed reactor. The effects of reaction temperature and weight hourly space velocity (WHSV) on feed conversion and the yields of ethene and propene were investigated. The results showed that with increased reaction temperature, the conversion of model compounds increased monotonically, and that of 1-hexene was close to 100% above 660A degrees C; the yield of ethene plus propene of n-octane, i-octane and ethylcyclohexane increased continuously, while that of 1-hexene and ethylbenzene passed through maximum. With increased WHSV, the yield of ethene plus propene of ethylbenzene increased continuously, and that of the other model compounds decreased continuously. Through comprehensive analysis of the data, it is indicated that 1-hexene exhibited the highest cracking performance, followed by n-octane and ethylcyclohexane, whereas i-octane and ethylbenzene exhibited the lowest.
  • [期刊] 分子筛孔结构对轻烃催化裂解性能的影响
    采用无机碱处理脱硅方法制备了具有不同孔结构的ZSM-5分子筛,通过X射线衍射(XRD)、透射电镜(TEM)、氨气程序升温脱附(NH3-TPD)、吡啶红外吸附(Py-FTIR)、N2吸附/脱附对分子筛进行了表征,将其应用于轻烃催化裂解反应考察其性能差异.结果表明,碱处理脱硅制备的微孔-介孔分子筛结合了微孔的催化性能、择形选择性和介孔的优异扩散性能.对于目的产物是低碳烯烃的烃类催化裂解反应而言,同时具有适宜微孔和介孔孔分布的分子筛体现出更优异的催化性能.同碳数的直链烷烃正辛烷和环烷烃乙基环己烷,虽然其分子结构不同,但在具有更短扩散路径和更大外表面积的4#-ZSM-5分子筛上,正辛烷和乙基环己烷催化裂解均表现出更高的反应活性稳定性和低碳烯烃产物选择性;在相同孔结构分子筛条件下,环烷烃乙基环己烷的总体反应活性低于相对应的直链烷烃正辛烷.
  • [期刊] Fungal biotransformation of short-chain n-alkylcycloalkanes
    The cycloalkanes, comprising up to 45% of the hydrocarbon fraction, occur in crude oil or refined oil products (e.g., gasoline) mainly as alkylated cyclohexane derivatives and have been increasingly found in environmental samples of soil and water. Furthermore, short-chain alkylated cycloalkanes are components of the so-called volatile organic compounds (VOCs). This study highlights the biotransformation of methyl- and ethylcyclohexane by the alkane-assimilating yeast Candida maltosa and the phenol- and benzoate-utilizing yeast Trichosporon mucoides under laboratory conditions. In the course of this biotransformation, we detected 25 different metabolites, which were analyzed by HPLC and GC-MS. The biotransformation process of methylcyclohexane in both yeasts involve (A) ring hydroxylation at different positions (C2, C3, and C4) and subsequent oxidation to ketones as well as (B) oxidation of the alkyl side chain to hydroxylated and acid products. The yeast T. mucoides additionally performs ring hydroxylation at the C1-position and (C) oxidative decarboxylation and (D) aromatization of cyclohexanecarboxylic acid. Both yeasts also oxidized the saturated ring system and the side chain of ethylcyclohexane. However, the cyclohexylacetic acid, which was formed, seemed not to be substrate for aromatization. This is the first report of several new transformation reactions of alkylated cycloalkanes for eukaryotic microorganisms.
  • [期刊] Limited surface mobility inhibits stable glass formation for 2-ethyl-1-hexanol
    Previous work has shown that vapor-deposition can prepare organic glasses with extremely high kinetic stabilities and other properties that would be expected from liquid-cooled glasses only after aging for thousands of years or more. However, recent reports have shown that some molecules form vapor-deposited glasses with only limited kinetic stability when prepared using conditions expected to yield a stable glass. In this work, we vapor deposit glasses of 2-ethyl-1-hexanol over a wide range of deposition rates and test several hypotheses for why this molecule does not form highly stable glasses under normal deposition conditions. The kinetic stability of 2-ethyl-1-hexanol glasses is found to be highly dependent on the deposition rate. For deposition at Tsubstrate = 0.90 T-g, the kinetic stability increases by 3 orders of magnitude (as measured by isothermal transformation times) when the deposition rate is decreased from 0.2 nm/s to 0.005 nm/s. We also find that, for the same preparation time, a vapor-deposited glass has much more kinetic stability than an aged liquid-cooled glass. Our results support the hypothesis that the formation of highly stable 2-ethyl-1-hexanol glasses is inhibited by limited surface mobility. We compare our deposition rate experiments to similar ones performed with ethylcyclohexane (which readily forms glasses of high kinetic stability); we estimate that the surface mobility of 2-ethyl-1-hexanol is more than 4 orders of magnitude less than that of ethylcyclohexane at 0.85 T-g. Published by AIP Publishing.
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