Origin of Stability in Branched Alkanes

The potential origins of stability in branched alkanes are investigated, paying close attention to two recent hypotheses: geminal steric repulsion and protobranching. All alkane isomers through C₆H₁₄ along with heptane and octane were investigated at the MPW1B95/6‐311++G(d,p) level. Their geminal steric repulsion, total steric repulsion, and orbital interactions were evaluated by using natural bond orbital analysis. All measures of steric repulsion fail to explain the stability of branched alkanes. The extra stability of branched alkanes and protobranching, in general, is tied to stabilizing geminal σ→σ* delocalization, particularly of the type that involves adjacent C? C bonds and, thus, preferentially stabilizes branched alkanes. This picture is corroborated by valence bond calculations that attribute the effect to additional ionic structures (e.g., CH₃⁺ :CH₂ :CH₃^? and CH₃:^? CH₂: CH₃⁺ for propane) that are not possible without protobranching.     

Influence of steric and intramolecular inductive effects on the variable trends in R-X (R=Alkyl) bond dissociation energy

The question “why are the variation trends of R-X bond dissociation energy different?” is answered. The R-X bond dissociation energy (BDE) may be influenced by three main factors: the C-X intrinsic bond energy, the 1,3 geminal repulsion, and the intramolecular charge-induced dipole. In the presence of atom X, the variation trend of BDE in R-X (R= Me, Et, i-Pr, t-Bu) is dominated by two factors, the 1,3 geminal repulsion and the intramolecular charge-induced dipole. The former decreases the R-X BDE, and the latter either increases or decreases the R-X BDE. For the series of R-X with the R-C bond (such as R-Me, R-CH == CH², R-C≡CH, and R-CN), the 1,3 geminal repulsion decreases the R-X BDE, and the variation trends of R-C BDE decrease from Me to t-Bu. As regards the series of R-X (such as R-H, R-BH², and R-SiH²) in which the electronegativity of atom X is smaller than that of the carbon atom, the above two factors decrease the R-X BDE, and the variation trends of the R-X BDE decrease from Me to t-Bu. As to the series of R-X (such as R-F, R-OH, R-Cl, R-Br, R-I, and R-NH²) in which the electronegativity of atom X is larger than that of the carbon atom, the 1,3 geminal repulsion decreases the R-X BDE, while the intramolecular charge-induced dipole increases the R-X BDE. In this case, the variation trends of R-X BDE depend on the competition of the two factors. As a result, some of them (e. g., R-F, R-OH) increase from Me to t-Bu, some (e. g., R-I) decrease from Me to t-Bu, and some (e. g., R-Br) change very little.     

高熔点蜡中正、异构烷烃碳数分布的高温气相色谱法

选用高温气相色谱仪ＨＰ５８９０Ⅱ分析高熔点蜡可达到Ｃ_（５５）左右,将每个碳数中的正、异构烷烃完全分离,数据经ＢＡＳＩＣ程序处理,得到分布直方图。分析速度快并可减少样品组分的歧化作用。     

Recent Progress in Commercial and Novel Catalysts for Catalytic Dehydrogenation of Light Alkanes

Catalytic dehydrogenation of light alkanes can effectively produce olefins and hydrogen. Even though Pt and CrO_x‐based catalysts are widely applied in industry, research to improve the activity and stability of these catalysts continued. This review summarizes important achievements obtained in recent years, focusing on the development of supports, promoters and preparation methods of Pt and CrO_x‐based catalysts, which mainly aimed to improve the dispersion of the active species and to enhance coke resistance. Furthermore, the high cost of Pt‐based catalysts and environmental problems encountered with CrO_x‐based catalysts have spurred the development of alternative catalysts. The dehydrogenation performances and characteristics of promising alternative VO_x‐, modified Ni‐ and Sn‐based catalysts are also reviewed. Comparison with the catalytic reforming process of naphtha further probes the necessity of catalyst acidity in these two different processes. The choice of the dehydrogenation reactor is discussed, and future perspectives and research directions are indicated.     

Equilibrium Adsorption of Light Alkanes in Silicalite-1 by the Inertial Microbalance Technique

The equilibrium adsorption of the light alkanes methane, ethane, propane, n-butane, and i-butane in silicalite-1 has been investigated using the TEOM technique. Either a conventional or a dual-site Langmuir isotherm appropriately describes the equilibrium data. Good agreement with the literature data determined by other techniques indicates the TEOM is a reliable technique. The adsorption of i-butane in silicalite-1 reveals the discrete preferential molecular siting, implying a discrete-dual-structural heterogeneity for light alkanes in silicalite-1.     

The Uptake and Assembly of Alkanes within a Porous Nanocapsule in Water: New Information about Hydrophobic Confinement

In Nature, enzymes provide hydrophobic cavities and channels for sequestering small alkanes or long‐chain alkyl groups from water. Similarly, the porous metal oxide capsule [{Mo^VI₆O₂₁(H₂O)₆}₁₂{(Mo^V₂O₄)₃₀(L)₂₉(H₂O)₂}]^41? (L=propionate ligand) features distinct domains for sequestering differently sized alkanes (as in Nature) as well as internal dimensions suitable for multi‐alkane clustering. The ethyl tails of the 29 endohedrally coordinated ligands, L, form a spherical, hydrophobic “shell”, while their methyl end groups generate a hydrophobic cavity with a diameter of 11 Å at the center of the capsule. As such, C₇ to C₃ straight‐chain alkanes are tightly intercalated between the ethyl tails, giving assemblies containing 90 to 110 methyl and methylene units, whereas two or three ethane molecules reside in the central cavity of the capsule, where they are free to rotate rapidly, a phenomenon never before observed for the uptake of alkanes from water by molecular cages or containers.     

低碳烷烃选择氧化金属氧化物催化剂的结构与其催化性能的关系

基于金属氧化物催化剂的低碳烷烃选择氧化是催化研究的一个热点和难点, 而认识催化剂的结构与其催化性能之间的关系对于获得高效催化剂, 解决低碳烷烃选择氧化反应中存在的问题至关重要. 本文围绕催化剂的结构对其催化性能的影响, 概述了V-P-O、V-Sb-O、Mo-V-Te-Nb-O和V-Mg-O等几类典型的氧化物催化剂体系在这方面的研究进展. 结合我们开发的Re-Sb-O催化剂体系, 分析了多功能中心、活性位分离、两相协同效应等方面对金属氧化物催化剂的催化性能的影响. 这些认识将有助于设计合成性能优异的催化剂及实现低碳烷烃的选择氧化活化和定向转化.     

CO_2选择氧化低碳烷烃的研究进展

本文以低碳烷烃的选择催化氧化反应为对象 ,对几种 CO2 选择氧化低碳烷烃的反应工艺进行了归纳总结 ,重点分析讨论了催化氧化反应中 CO2 作为氧化剂的作用机制 ,并提出了研究展望。     

Positronium Conversion Reactions for Testing the Electron Delocalization in 3d Complexes Caused by Ligands

The rate constants of ortho- into para-positronium conversion reactions promoted by paramagnetic 3d complexes were found to be linearly correlated with the delocalization, of unpaired metal electrons caused by ligands. It is shown here that s, usually obtained by UV/Vis absorption spectroscopy, may also be deduced from the correlations mentioned above.     

Catalytic Properties of Heteropolyacid-Modified ZrO2 in the Partial Oxidation of Lower Alkanes by Nitric Oxide

The partial oxidation of C₃ and C₄ hydrocarbons by nitric oxide, in particular, their oxidation to alcohols, occurs on zirconium dioxide modified by various heteropolyacids (HPA). The catalyst activity depends on the amount of deposited HPA, while the reaction selectivity relative to the partial oxidation products correlates with the acid properties of the surface, in particular, with the concentration of weak acid sites.     

Electron Delocalization in a New Mixed-Valence σ-Bridged Biferrocenium Cation

The physical properties of mixed-valence 1,3-diferrocenyl-propan-1-one (1) are reported. The Mössbauer spectrum of mixed-valence cation 1 at 300 K shows two doublets, one for a Fe¹¹ metallocene (ΔE_Q = 2.192 mm s^?1) and the other for a Fe^III metallocene (ΔE_O = 0.179 mm s^?1) Thus, the thermal electron-transfer rate in 1 is less than the time scale of the Mössbauer technique (～10⁷ s^?1). Furthermore, the NMR, electrochemical IR, and near-IR data conclusively indicate that there is no significant interaction between the two ferrocenyl moieties.     

Pd/NbOPO4 Multifunctional Catalyst for the Direct Production of Liquid Alkanes from Aldol Adducts of Furans

Great efforts have been made to convert renewable biomass into transportation fuels. Herein, we report the novel properties of NbO_x‐based catalysts in the hydrodeoxygenation of furan‐derived adducts to liquid alkanes. Excellent activity and stability were observed with almost no decrease in octane yield (>90 % throughout) in a 256 h time‐on‐stream test. Experimental and theoretical studies showed that NbO_x species play the key role in C? O bond cleavage. As a multifunctional catalyst, Pd/NbOPO₄ plays three roles in the conversion of aldol adducts into alkanes: 1) The noble metal (in this case Pd) is the active center for hydrogenation; 2) NbO_x species help to cleave the C? O bond, especially of the tetrahydrofuran ring; and 3) a niobium‐based solid acid catalyzes the dehydration, thus enabling the quantitative conversion of furan‐derived adducts into alkanes under mild conditions.     

Theoretical Study of Electron Density Delocalization in the Excited States of Transition Metal Complexes

This paper presents a theoretical study of electron density delocalization effects over an electron-accepting ligand in metal-to-ligand charge-transfer (MLCT) complexes in the excited states, where the ligand is 4,4'-X₂-2,2'-bpy (X = H, NH₂, CH₃, Ph, Cl, CO₂Et, NO₂, bpy = 2,2'-bipyridine) or terpy (2,2':6',2'-terpyridine). Optimal geometry calculations are performed for neutral ligand molecules and their radical anions modeling the state of the ligands during MLCT excitations. Spin density distribution over atoms in the radical anions is used as a measure of the degree of delocalization. The role of spin density distribution in excitation-induced changes of geometrical parameters of the ligands is considered.     

Significance of Photosensitized Oxidation of Alkanes During the Photochemical Degradation of Petroleum Hydrocarbon Fractions in Seawater

Abstract

A crude oil hydrocarbon fraction was photooxidized as a surface film on seawater under simulated environmental conditions. After irradiation, gas chromatography and gas chromatography/mass spectrometry analyses allowed to demonstrate the presence of relatively high quantities of compounds deriving from the photosensitized oxidation of n-alkanes, pristane and phytane, being among the non-acidic photoproducts of this fraction. The results suggest that the photochemical degradation of alkanes should be considered in studies of the fate of petroleum crudes in the marine environment.     

Hyperconjugation Is the Source of Helicity in Perfluorinated n‐Alkanes

Hyperconjugative, steric, and electrostatic effects were evaluated as possible sources of the helicity in linear perfluorinated alkanes through analysis of natural bond orbitals and classical electrostatics. Contrary to previous rationalizations, which indicate dominating steric or electrostatic effects, this analysis indicates that hyperconjugative stabilization through σ_CC→σ*_CF interactions are the underlying driving force for the origin of the observed helicity in perfluoroalkanes.