The Effect of Conformational Changes on the Catalytic Transformations of n-Alkanes

Temperature induced conformational changes of various alkanes were studied. n-Hexane was studied in order to demonstrate its ability to be involved in C₅ and C₆ ring closure reactions, while long chain alkanes, such as C₈, C₁₆ and C₂₄, were investigated with the aim to calculate their temperature dependent molecular diameter controlling their penetration ability into zeolite channels.     

Functionalization of saturated hydrocarbons

Alkanes and cycloalkanes (isobutane, butane, isopentane, isohexane, and methylcyclopentane) react with benzene or bromobenzene at 0–20 °C in the presence of RCO⁺Al₂X₇ ^– complexes (R=Me, Pr, or Ph; X=Cl or Br) to give products of the alkylacylation of arenes. The yields of alkylated aromatic ketones reach 60–87 % in 5–30 min, whereas the yields of unalkylated aromatic ketones (the competitive reaction) reach 0–40 %. The reactions of isobutane or isopentane with benzene result exclusively inpara isomers oft-BuC₆H₄COR or a mixture of Me₂(Et)CC₆H₄COR and Me(i-Pr)CHC₆H₄COR isomers (11), respectively. The reaction of isobutane with benzene also proceeds regioselectively and gives only one isomer, 2-Br-t-BuC₆H₄COR.For Part 2 seeIzv. Akad. Nauk, Ser. Khim., 1991, No 1, 105 [Bull. Acad. Sci. USSR. Div. Chem. Sci., 1991, No 1, 90 (Engl. Transl)].Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1253–1257, July, 1993.The authors express their gratitude to B. I. Bakhmutov for his assistance in interpreting the spectra.     

DFT study of the mechanism of alkane hydrogenolysis by transition metal hydrides. 1. Interaction of silica-supported zirconium hydrides with methane

Model reactions of silica-supported zirconium hydrides (Si—O—)₃ZrH and (Si—O—)₂ZrH₂ with methane, resulting in cleavage of a C—H bond in the methane molecule and the formation of (Si—O—)₃ZrCH₃ and (Si—O—)₂Zr(H)CH₃ as products were studied using the DFT approach with the PBE density functional. The processes proceed as bimolecular reactions without preliminary formation of agostic complexes. According to calculations, zirconium dihydrides (Si—O—)₂ZrH₂ are more reactive toward the methane C—H bonds than zirconium monohydrides (Si—O—)₃ZrH. The calculated activation energies of the reactions with participation of zirconium dihydrides (Si—O—)₂ZrH₂ are in better agreement with the known experimental data for the Yermakov—Basset catalytic system.     

Kinetics of oxidation of alkanes and their derivatives byn-decanepersulfonic acid

The kinetics of interaction ofn-decanepersulfonic acid with linear, branched, and substituted hydrocarbons was studied. The oxidation ofcyclo-C₆H₁₂/C₆D₁₂ occurs with a moderate kinetic isotope effect,k ^H/k ^D=2.2±0.3. A satisfactory correlation between the partial rate constants and the structure of hydrocarbons in terms of the Okamoto-Brown equation was found. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 5, pp. 822–825, May, 2000.     

Experimentally determined temperature–concentration phase diagrams of monodisperse alkanes with chains containing between 100 and 200 carbons

The temperature–concentration phase (T–c) diagrams of the uniform n-alkanes C₁₀₂H₂₀₆, C₁₂₂H₂₄₆, C₁₆₂H₃₂₆, and C₁₉₈H₃₉₈ in toluene have been determined for solution concentrations in the range 0.1 to 6% (w/w). The shorter alkanes display a “classical” behavior with the expected, strong dependence of dissolution temperature on solution concentration. The longest alkane displays a very different, “polymeric” type behavior with a concentration independent dissolution temperature (for both extended and folded chain crystals). It is argued that no current theory of polymer dissolution is able to explain this behavior. It is suggested that a locally higher concentration occurs when molecules are partially attached to a crystal either during crystallization or dissolution, and that this increased local concentration accounts for the independence of dissolution temperature on the global concentration. There are some small variations in the dissolution temperature of crystals of the same thickness grown at the same concentration, but at different temperatures. These are ascribed to differences in the stacking of the separate layers. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3188–3200, 1999     

Titanium dioxide photocatalysis of adamantane

TiO₂ photocatalysis of adamantane in oxygen equilibrated MeCN yields 1- and 2-adamantanol and adamantanone with limited degradation and preference for functionalization at the 1 position, particularly in the presence of silver salts. Oxidation in CH₂Cl₂ is less selective. The oxidation of cyclohexane and cyclododecane is slower. In N₂-flushed solutions with Ag⁺ as a sacrificial acceptor, products from the trapping of both 1-adamantyl radical (adamantyl methyl ketone) and cation (N-adamantylacetamide) are obtained. Furthermore, alkylation of an electrophilic alkene (isopropylydenmalononitrile) has been obtained, though in a low yield.     

Insertion of singlet chlorocarbenes across C-H bonds in alkanes: Evidence for two phase mechanism

Transition states for the insertion reactions of singlet mono and dichlorocarbenes (¹CHCl and ¹CCl₂) into C-H bonds of alkanes (methane, ethane, propane and n-butane) have been investigated at MP2 and DFT levels with 6–31g (d, p) basis set. The p _π of ¹CHCl and ¹CCl₂ may interact with alkane’s filled fragment orbital of either σ or π symmetry. So chlorocarbenes insertion reactions have been investigated for both (σ/π) approaches. The σ approach has been adjudicated to be the minimum energy path over the π approach both at the MP2 and DFT levels. Mulliken, NPA and ESP derived charge analyses have been carried out along the minimal energy reaction path using the IRC method for ¹CHCl and ¹CCl₂ insertions into the primary and secondary C-H bonds of propane. The occurrence of TSs either in the electrophilic or nucleophilic phase has been identified through NBO charge analyses in addition to the net charge flow from alkane to the carbene moiety. Dedicated to our senior colleague Dr V Sethuraman on the occasion of his retirement from collegiate service     

Theoretical Study of the Hydroxyl-Radical-Initiated Degradation Mechanism,Kinetics, and Subsequent Evolution of Methyl and Ethyl Iodides in the Atmosphere

The degradation and transformation of iodinated alkanes are crucial in the iodine chemical cycle in the marine boundary layer. In this study, MP2 and CCSD(T) methods were adopted to study the atmospheric transformation mechanism and degradation kinetic properties of CH₃I and CH₃CH₂I mediated by ⋅OH radical. The results show that there are three reaction mechanisms including H-abstraction, I-substitution and I-abstraction. The H-abstraction channel producing ⋅CH₂I and CH₃C ⋅ HI radicals are the main degradation pathways of CH₃I and CH₃CH₂I, respectively. By means of the variational transition state theory and small curvature tunnel correction method, the rate constants and branching ratios of each reaction are calculated in the temperature range of 200–600 K. The results show that the tunneling effect contributes more to the reaction at low temperatures. Theoretical reaction rate constants of CH₃I and CH₃CH₂I with ⋅OH are calculated to be 1.42×10⁻¹³ and 4.44×10⁻¹³ cm³ molecule⁻¹ s⁻¹ at T=298 K, respectively, which are in good agreement with the experimental values. The atmospheric lifetimes of CH₃I and CH₃CH₂I are evaluated to be 81.51 and 26.07 day, respectively. The subsequent evolution mechanism of ⋅CH₂I and CH₃C ⋅ HI in the presence of O₂, NO and HO₂ indicates that HCHO, CH₃CHO, and I-atom are the main transformation end-products. This study provides a theoretical basis for insight into the diurnal conversion and environmental implications of iodinated alkanes.     

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.     

Heterogeneous Wheel‐Shaped Cu20‐Polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25− Catalyst for Solvent‐Free Aerobic Oxidation of n‐Hexadecane

The selective oxidation of alkanes as a green process remains a challenging task because partial oxidation is easier to achieve with sacrificial oxidants, such as hydrogen peroxide, alkyl hydroperoxides or iodosylbenzene, than with molecular oxygen or air. Here, we report on a heterogeneous catalyst for n‐hexadecane oxidation comprised of the wheel shaped Cu₂₀‐polyoxotungstate [Cu₂₀Cl(OH)₂₄(H₂O)₁₂(P₈W₄₈O₁₈₄)]^25? anchored on 3‐aminopropyltriethoxysilane (apts)‐modified SBA‐15. The catalysts were characterized by powder X‐ray diffraction (XRD), N₂‐adsorption measurements and Fourier transform infrared reflectance (FT‐IR) spectroscopy. The heterogeneous Cu₂₀‐polyanion system catalyzed the solvent‐free aerobic oxidation of n‐hexadecane to alcohols and ketones by using air as the oxidant under ambient conditions. The catalyst exhibits an exceptionally high turn over frequency (TOF) of 20 000 h^?1 at 150 °C and is resistant to poisoning by CS₂. Moreover, it can be easily recovered and reused by filtration without loss of its catalytic activity. Possible homogeneous contributions also have been examined and eliminated. Thus, this system can use air as oxidant, which, in combination with its good overall performance and poison tolerance, raises the prospect of this type of heterogeneous catalyst for practical applications.     

Alkane Activation over Acidic Zeolites: The First Step

The heterogeneous acid‐catalyzed activation step of alkanes leading to the reaction intermediates (carbocationic or alkoxy species) was up to now the matter of a longstanding controversy. Gas chromatography and online mass spectroscopy measurements show that H₂ and methane are formed over H‐zeolites, whereas HD and CH₃D are formed over D‐zeolites as the primary products in the reaction with isobutane. These results indicate that σ‐bond protolysis by strong acid sites is the first step for hydrocarbon activation on these catalysts at mild temperatures (473 K), in analogy to the activation path occurring in liquid superacid media.     

Oxidation of alkanes and secondary alcohols to ketones with tert‐butyl hydroperoxide catalyzed by a water‐soluble ruthenium complex under solvent‐free conditions

An easily synthesized water‐soluble ruthenium complex, [C₆H₅CH₂N(CH₃)₂H]₂[Ru(dipic)Cl₃] (dipic =2,6‐pyridinedicarboxylate), as a catalyst showed high efficiency in the oxidation of alkanes and secondary alcohols to their corresponding ketones under solvent‐free and low‐catalyst‐loading conditions. This catalytic system could tolerate a variety of substrates and gave the corresponding ketones in good to excellent yields. The products were easily separated and purified due to the water solubility of the ruthenium complex.     

水热法合成MCM-41分子筛时添加辅助烃的影响

０前言中孔ＭＣＭ－４１分子筛是Ｍｏｂｉｌ公司１９９２年开发的一种具有２－１０ｎｍ孔径的新型分子筛材料。由于其独特的孔径范围和表面特性,相关研究已经成为国际分子筛研究领域的热点课题,对它的合成、表征和催化作用已进行了许多研究［１～７］。Ｍｏｂｉｌ公司的...     

A Series of Mesoporous Metal-Organic Frameworks with Tunable Windows Sizes and Exceptionally High Ethane over Ethylene Adsorption Selectivity

The NIIC-20 (NIIC stands for Nikolaev Institute of Inorganic Chemistry) is a family of five isostructural metal-organic frameworks (MOFs) based on dodecanuclear wheel-shaped carboxylate building blocks {Zn₁₂(RCOO)₁₂(glycol)₆} (glycol is deprotonated diatomic alcohol: ethylene glycol, 1,2-propanediol, 1,2-butanediol, 1,2-pentanediol or glycerol), quantitatively crystallized from readily available starting chemicals. The crystal structures contain large mesoporous cages of 25 Å connected through {Zn₁₂} rings, of which inner diameter and chemical nature depend solely on the chosen glycol. The NIIC-20 compounds feature high surface area and rarely observed inversed adsorption affinity for saturated hydrocarbon (ethane) over the unsaturated ones (ethylene, acetylene). The corresponding IAST (Ideal Adsorbed Solution Theory) adsorption selectivity factors reach as much as 15.4 for C₂H₆/C₂H₄ and 10.9 for C₂H₆/C₂H₂ gas mixtures at ambient conditions, exceeding those for any other porous MOF reported so far. The remarkable combination of high adsorption uptakes and high adsorption selectivities makes the NIIC-20 series a new benchmark of porous materials designed for ethylene separation applications.     

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.     

Mechanisms of C—H Activation in Alkanes by Chromium-Oxo Reagents

The mechanisms of activation of a C—H bond by chromium-oxo reagents in the reactions of methane, isobutane, adamantane, and protoadamantane with CrO₂(OH)₂ were investigated by the BH&HLYP quantum-chemical method in the 6-31G^* and 6-311G^** (for Cr) basis sets. It was shown that the transition states have clearly defined biradical character and significant transfer of charge from the hydrocarbon to the electrophile.     

Synthesis and characterization of a new class of polyfluorinated alkanes: tetrakis(perfluoroalkyl)alkane

The synthesis and physico-chemical characterization of 1,1,2,2-tetrakis(perfluoroalkyl-methylene)ethane {[F(CF₂)_nCH₂]₂CH}₂ (n=6, TK6; n=8, TK8) are reported. The synthesis consists of four steps: (1) addition of allyl alcohol to a perfluoroalkyl iodide, F(CF₂)_nI (n=6,8) to give the corresponding iodo-adduct; (2) dehalogenation of the adduct by treatment with zinc in aqueous acetic acid, yielding 3-perfluoro-n-alkyl-1-propene; (3) addition of 3-perfluoro-n-alkyl-l-propene to perfluoroalkyl iodide, F(CF₂)_nI (n=6,8) to give 1,3-perfluoro-n-alkyl-2-iodo-propane; (4) coupling of 1,3-perfluoro-n-alkyl-2-iodo-propane by zinc in acetic anhydride giving the final products. TK6 and TK8 are characterized by very low surface tension values and exhibit very good properties as potential ski-waxes.     

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.     

Cyclooctane Metathesis Catalyzed by Silica‐Supported Tungsten Pentamethyl [(SiO)W(Me)5]: Distribution of Macrocyclic Alkanes

Metathesis of cyclic alkanes catalyzed by the new surface complex [(?SiO)W(Me)₅] affords a wide distribution of cyclic and macrocyclic alkanes. The major products with the formula C_nH_2n are the result of either a ring contraction or ring expansion of cyclooctane leading to lower unsubstituted cyclic alkanes (5≤n≤7) and to an unprecedented distribution of unsubstituted macrocyclic alkanes (12≤n≤40), respectively, identified by GC/MS and by NMR spectroscopies.     

Additional insights from very‐high‐resolution 13C NMR spectra of long‐chain n‐alkanes

New information has been obtained from very‐high‐resolution ¹³C NMR studies of a series of long‐chain n‐alkanes. These compounds are fundamentally important in the petroleum industry and are essential to the life of some plants, flowers, and insects. At least partial resolution of the ten different ¹³C NMR signals of n‐C₂₀H₄₂ is observed at 11.7 T for solutions in C₆D₆ or C₆D₅CD₃. A ¹³C T₁ inversion‐recovery experiment provides much more detailed information than in previous studies of long‐chain n‐alkanes, demonstrates a steady increase in the relaxation times of the ten different carbons proceeding from the middle to the end of the chain because of segmental motion, and thus confirms the assignments for the interior carbons. In contrast, there is significant overlap for the signals for C‐7 and the more interior carbons in a solution of n‐C₁₆ or longer chain alkanes in CDCl₃. Not only are the chemical shifts sensitive to the solvent used, but also the relative chemical shifts change. Signals for the interior carbons of the odd‐number alkanes in CDCl₃ are better resolved than in the spectra of their even‐number counterparts. Some mixed aromatic solvent systems give increased dispersion of the cluster of C‐6 through C‐10 signals of n‐C₂₀H₄₂, n‐C₂₁H₄₄, and n‐C₂₂H₄₆. However, none of the solvents used could even partially resolve the C‐10 and C‐11 signals of n‐C₂₁H₄₄ or n‐C₂₂H₄₆ at 11.7 T, which may result from a different distribution of conformers for n‐C₂₁H₄₄ or n‐C₂₂H₄₆ than for n‐C₂₀H₄₂ and shorter n‐alkanes. Copyright © 2013 John Wiley & Sons, Ltd.