Activation of propane C-h and C-C bonds by gas-phase pt atom: a theoretical study

The reaction mechanism of the gas-phase Pt atom with C(3)H(8) has been systematically investigated on the singlet and triplet potential energy surfaces at CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level. Pt atom prefers the attack of primary over secondary C-H bonds in propane. For the Pt + C(3)H(8) reaction, the major and minor reaction channels lead to PtC(3)H(6) + H(2) and PtCH(2) + C(2)H(6), respectively, whereas the possibility to form products PtC(2)H(4) + CH(4) is so small that it can be neglected. The minimal energy reaction pathway for the formation of PtC(3)H(6) + H(2), involving one spin inversion, prefers to start at the triplet state and afterward proceed along the singlet state. The optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. The C-H insertion intermediates are kinetically favored over the C-C insertion intermediates. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the more stabilizing transition state interaction ΔE(≠) (int), which is the actual interaction energy between the deformed reactants in the transition state.     

Corona treatment of isotactic polypropylene in nitrogen and carbondioxide

Corona discharge treatment of isotactic polypropylene surfaces in N₂ and CO₂ was investigated by contact angle measurements and ESCA. The electrical characteristics of the discharges as well as the influence of indirect parameters (moment of air contact and ageing time) and direct parameters (applied charge, electrical field strength and film temperature) on the surface modification were determined. These investigations showed that electrons, emitted by photo effect are the dominant charge carriers and the main cause of surface activation. The active species in the surfaces (presumably radicals) can either perform crosslinking and H-abstraction or react with the discharge gas. In the N₂-discharge the polymer radicals can only react with atomic or excited nitrogen whereas in CO₂ they also react with ground state molecules. If the samples are brought into air contact after discharge leftover radicals are oxidized by atmospheric oxygen. In addition a UV-radiation causing activation in a surface layer was found. The bulk of the polymer is not influenced by corona discharge.     

利用ESR技术研究α—蒎烯β—蒎烯光敏氧化反应机理

本文主要利用电子顺磁共振自旋捕获技术研究９,１０－二氰基蒽敏化α－蒎烯,β－蒎烯光氧化反应,提供了在乙腈中α－蒎烯和β－蒎烯的光氧化反应过程中存在超氧负离子基和单重 氧的直接证据;在四氯化碳溶剂中只捕获到＾１Ｏ２;在正己烷中没有捕获到Ｏ＾－２或＾１Ｏ２．ＥＳＲ实验结果进一步证明在乙腈中光敏氧化反应的＾１Ｏ２可能来自Ｏ＾－２和反应底物α－β,蒎烯正离子自由基之间的电荷复合。     

Borylation and silylation of C-H bonds: a platform for diverse C-H bond functionalizations

Methods that functionalize C-H bonds can lead to new approaches for the synthesis of organic molecules, but to achieve this goal, researchers must develop site-selective reactions that override the inherent reactivity of the substrates. Moreover, reactions are needed that occur with high turnover numbers and with high tolerance for functional groups if the C-H bond functionalization is to be applied to the synthesis of medicines or materials. This Account describes the discovery and development of the C-H bond functionalization of aliphatic and aromatic C-H bonds with borane and silane reagents. The fundamental principles that govern the reactivity of intermediates containing metal-boron bonds are emphasized and how an understanding of the effects of the ligands on this reactivity led us to broaden the scope of main group reagents that react under mild conditions to generate synthetically useful organosilanes is described. Complexes containing a covalent bond between a transition metal and a three-coordinate boron atom (boryl complexes) are unusually reactive toward the cleavage of typically unreactive C-H bonds. Moreover, this C-H bond cleavage leads to the formation of free, functionalized product by rapid coupling of the hydrocarbyl and boryl ligands. The initial observation of the borylation of arenes and alkanes in stoichiometric processes led to catalytic systems for the borylation of arenes and alkanes with diboron compounds (diborane(4) reagents) and boranes. In particular, complexes based on the Cp*Rh (in which Cp is the cyclopentadienyl anion) fragment catalyze the borylation of alkanes, arenes, amines, ethers, ketals, and haloalkanes. Although less reactive toward alkyl C-H bonds than the Cp*Rh systems, catalysts generated from the combination of bipyridines and iridium(I)-olefin complexes have proven to be the most reactive catalysts for the borylation of arenes. The reactions catalyzed by these complexes form arylboronates from arenes with site-selectivity for C-H bond cleavage that depends on the steric accessibility of the C-H bonds. These complexes also catalyze the borylation of heteroarenes, and the selectivity for these substrates is more dependent on electronic effects than the borylation of arenes. The products from the borylation of arenes and heteroarenes are suitable for a wide range of subsequent conversions to phenols, arylamines, aryl ethers, aryl nitriles, aryl halides, arylboronic acids, and aryl trifluoroborates. Studies of the electronic properties of the ancillary ligand on the rate of the reaction show that the flat structure and the strong electron-donating property of the bipyridine ligands, along with the strong electron-donating property of the boryl group and the presence of a p-orbital on the metal-bound atom, lead to the increased reactivity of the iridium catalysts. Based on this hypothesis, we studied catalysts containing substituted phenanthroline ligands for a series of additional transformations, including the silylation of C-H bonds. A sequence involving the silylation of benzylic alcohols, followed by the dehydrogenative silylation of aromatic C-H bonds, leads to an overall directed silylation of the C-H bond ortho to hydroxyl functionality.     

The correlation between the ionic degree of covalent bond comprising polymer main chain and the ionic yield due to mechanical fracture

The mechanical force to polymeric materials in vacuum at 77 K produces mechano radicals, mechano anions and mechano cations due to homogeneous and heterogeneous scissions of the covalent bonds comprising polymer main chain. The ionic degree of the covalent bond was estimated by calculating the “absolute ΔMulliken atomic charge,” which was defined as the difference between the Mulliken atomic charges of the two adjacent atoms comprising the covalent bond of the polymer main chain. The ionic yield of the covalent bond increased with increasing the absolute ΔMulliken atomic charge. The empirical formula for the ionic yield was obtained with the absolute ΔMulliken atomic charge, and indicates that the ionic yield could be estimated from its chemical structure.     

N-heterocyclic carbene gold(I) and copper(I) complexes in C-H bond activation

Environmental concerns have and will continue to have a significant role in determining how chemistry is carried out. Chemists will be challenged to develop new, efficient synthetic processes that have the fewest possible steps leading to a target molecule, the goal being to decrease the amount of waste generated and reduce energy use. Along this path, chemists will need to develop highly selective reactions with atom-economical pathways producing nontoxic byproduct. In this context, C-H bond activation and functionalization is an extremely attractive method. Indeed, for most organic transformations, the presence of a reactive functionality is required. In Total Synthesis, the "protection and deprotection" approach with such reactive groups limits the overall yield of the synthesis, involves the generation of significant chemical waste, costs energy, and in the end is not as green as one would hope. In turn, if a C-H bond functionalization were possible, instead of the use of a prefunctionalized version of the said C-H bond, the number of steps in a synthesis would obviously be reduced. In this case, the C-H bond can be viewed as a dormant functional group that can be activated when necessary during the synthetic strategy. One issue increasing the challenge of such a desired reaction is selectivity. The cleavage of a C-H bond (bond dissociation requires between 85 and 105 kcal/mol) necessitates a high-energy species, which could quickly become a drawback for the control of chemo-, regio-, and stereoselectivity. Transition metal catalysts are useful reagents for surmounting this problem; they can decrease the kinetic barrier of the reaction yet retain control over selectivity. Transition metal complexes also offer important versatility in having distinct pathways that can lead to activation of the C-H bond. An oxidative addition of the metal in the C-H bond, and a base-assisted metal-carbon bond formation in which the base can be coordinated (or not) to the metal complexes are possible. These different C-H bond activation modes provide chemists with several synthetic options. In this Account, we discuss recent discoveries involving the versatile NHC-gold(I) and NHC-copper(I) hydroxide complexes (where NHC is N-heterocyclic carbene) showing interesting Br?nsted basic properties for C-H bond activation or C-H bond functionalization purposes. The simple and easy synthesis of these two complexes involves their halide-bearing relatives reacting with simple alkali metal hydroxides. These complexes can react cleanly with organic compounds bearing protons with compatible pK(a) values, producing only water as byproduct. It is a very simple protocol indeed and may be sold as a C-H bond activation, although the less flashy "metalation reaction" also accurately describes the process. The synthesis of these complexes has led us to develop new organometallic chemistry and catalysis involving C-H bond activation (metalation) and subsequent C-H bond functionalization. We further highlight applications with these reactions, in areas such as photoluminescence and biological activities of NHC-gold(I) and NHC-copper(I) complexes.     

Electrical and magnetic properties of conjugated schiff base polymers

Two new conjugated poly-Schiff bases (PPpP and PPmP) were synthesized by polycon-densation of p-phenylene diamine or m-phenylene diamine with 2,6-pyridine dicarboxal-dehyde. PPpP and PPmP can from charge transfer complexes with iodine. Maximum conductivity of PPpP-iodine complex at room temperature is 10⁻⁶ S/cm, which is 2 orders of magnitude higher than that of PPmP-iodine complex. Electronic spin resonance measurements discovered that there are stable radicals in both charge transfer complexes; and g value, line width, and spin concentration depend on doping degree. Magnetic susceptibility of charge transfer complexes of PPmP–iodine is composed of Curie magnetic susceptibility (χ_c) and Pauli magnetic susceptibility (χ_p). Its Curie constant (C), Curie spin concentration (N_c), and density of state at the Fermi level also depend on doping degree. © 1996 John Wiley & Sons, Inc.     

C-F and C-H bond activation of fluorobenzenes and fluoropyridines at transition metal centers: how fluorine tips the scales

In this Account, we describe the transition metal-mediated cleavage of C-F and C-H bonds in fluoroaromatic and fluoroheteroaromatic molecules. The simplest reactions of perfluoroarenes result in C-F oxida tive addition, but C-H activation competes with C-F activation for partially fluorinated molecules. We first consider the reactivity of the fluoroaromatics toward nickel and platinum complexes, but extend to rhenium and rhodium where they give special insight. Sections on spectroscopy and molecular structure are followed by discussions of energetics and mechanism that incorporate experimental and computational results. We highlight special characteristics of the metal-fluorine bond and the influence of the fluorine substituents on energetics and mechanism. Fluoroaromatics reacting at an ML(2) center initially yield η(2)-arene complexes, followed usually by oxidative addition to generate MF(Ar(F))(L)(2) or MH(Ar(F))(L)(2) (M is Ni, Pd, or Pt; L is trialkylphosphine). The outcome of competition between C-F and C-H bond activation is strongly metal dependent and regioselective. When C-H bonds of fluoroaromatics are activated, there is a preference for the remaining C-F bonds to lie ortho to the metal. An unusual feature of metal-fluorine bonds is their response to replacement of nickel by platinum. The Pt-F bonds are weaker than their nickel counterparts; the opposite is true for M-H bonds. Metal-fluorine bonds are sufficiently polar to form M-F···H-X hydrogen bonds and M-F···I-C(6)F(5) halogen bonds. In the competition between C-F and C-H activation, the thermodynamic product is always the metal fluoride, but marked differences emerge between metals in the energetics of C-H activation. In metal-fluoroaryl bonds, ortho-fluorine substituents generally control regioselectivity and make C-H activation more energetically favorable. The role of fluorine substituents in directing C-H activation is traced to their effect on bond energies. Correlations between M-C and H-C bond energies demonstrate that M-C bond energies increase far more on ortho-fluorine substitution than do H-C bonds. Conventional oxidative addition reactions involve a three-center triangular transition state between the carbon, metal, and X, where X is hydrogen or fluorine, but M(d)-F(2p) repulsion raises the activation energies when X is fluorine. Platinum complexes exhibit an alternative set of reactions involving rearrangement of the phosphine and the fluoroaromatics to a metal(alkyl)(fluorophosphine), M(R)(Ar(F))(PR(3))(PR(2)F). In these phosphine-assisted C-F activation reactions, the phosphine is no spectator but rather is intimately involved as a fluorine acceptor. Addition of the C-F bond across the M-PR(3) bond leads to a metallophosphorane four-center transition state; subsequent transfer of the R group to the metal generates the fluorophosphine product. We find evidence that a phosphine-assisted pathway may even be significant in some apparently simple oxidative addition reactions. While transition metal catalysis has revolutionized hydrocarbon chemistry, its impact on fluorocarbon chemistry has been more limited. Recent developments have changed the outlook as catalytic reactions involving C-F or C-H bond activation of fluorocarbons have emerged. The principles established here have several implications for catalysis, including the regioselectivity of C-H activation and the unfavorable energetics of C-F reductive elimination. Palladium-catalyzed C-H arylation is analyzed to illustrate how ortho-fluorine substituents influence thermodynamics, kinetics, and regioselectivity.     

Organometallic-like C-H bond activation and C-S bond formation on the disulfide bridge in the RuSSRu core complexes

C-S bond formations on the disulfide bridge in the dinuclear Ru(III) complexes have been found in the reaction with unsaturated organic molecules, such as alkenes and ketones. The reactions are initiated by the organometallic-like C-H bond activation. Through the mechanistic study of these novel reactions, the specific nature of the disulfide bridging ligand has been unveiled: (i) the double bond character of the sulfur-sulfur bond, which allows the Diels-Alder-type [4 + 2] cycloaddition reaction with butadiene to form a C(4)S(2) ring, (ii) the C-H bond activation on the S-S bond forming a C-S bond. All of the C-H activation reactions and the ensuing C-S bond formation reactions suggest that the disulfide ligand can act in a manner similar to the transition metal centers of the organometallic complexes.     

An infrared spectroscopic study of the mechanism of chloromethane conversion to higher hydrocarbons on HZSM5 catalyst

Investigation of the reaction mechanism of chloromethane on ZSM5 is a new topic. In this work an in situ FTIR technique was employed to study the conversion processes of chloromethane, the active sites on HZSM5, and the desorption state of surface species. The catalytic conversion of chloromethane to higher hydrocarbons was also studied. It is demonstrated that chloromethane can be reversibly adsorbed on acidic sites of HZSM5 at room temperature. At 100°C chloromethane is irreversibly and dissociatively adsorbed on the strong acidic sites of HZSM5, on which surface methoxyl is formed as proved by infrared characteristic C-H stretchings of-CH₃ at 2960 and 2870 cm^–1. Alkoxyls are produced and adsorbed on the catalyst surface as characterized by the infrared absorption bands of -CH₂-groups at 1460 and 2930 cm^–1. At 100°C the adsorbed methoxyl and alkoxyls are the main surface species, and a small amount of aromatics might exist as detected by a characteristic absorption band at 1510 cm^–1. Between 100 and 200°C the adsorbed surface methoxyl and alkoxyls are converted to aromatics, and the occupied OH groups partially appear. At temperature higher than 300°C the adsorbed aromatics are thermally desorbed into the gas phase. Aromatics and alkanes are the main products in catalytic conversion. These results reveal that the formation of aromatics from methoxyl and alkoxyls is easier than the desorption of aromatics from HZSM5 catalyst. An alkoxyl mechanism is proposed for the conversion of chloromethane on HZSM5 based upon the experimental results and the three assumptions: (a) The primary C-C bond is formed from surface methoxyl groups via the methoxyl group polarization and C-H bond weakening, (b) The adsorbed alkoxyls are converted to aromatics via hydrogen transfer and bond rearrangement similar to the conventional carbenium ion mechanism for the aromatization of olefins and alkanes on HZSM5. The hydrogen atoms from the aromatization stimulate the desorption of alkoxyls to alkanes. (c) At temperature higher than 300°C surface reactions and desorption of adsorbed species take place simultaneously, determining the product distribution in the catalytic conversion.     

Studies on the Oxidation of cis- and trans-Pinane with molecular oxygen

The pinanes are preferably attacked at the tertiary C-H bond in 2-position, but products of the oxidative attack at the secondary C-H bonds in 3- and 4-position are also found. At 100°C cis-pinane is attacked more easily than trans-pinane (k_cis : k_trans = 6.4), the relative rates of attack at the secondary C-H bonds in positions 3 and 4 with respect to the tertiary C-H bond in 2-position were also determined (in cis-pinane k_sec: k_tert = 0.027; in trans-pinane k_sec : k_tert = 0.20). After the attack at the 2-C-H bond the radical formed can either react with oxygen to form the corresponding cis- and trans-peroxy radicals and further to give cis- and trans-2-hydroperoxy pinane or fragmentate to the monocyclic radical derived from α-terpinene, giving as final products α-terpinene hydroperoxide and the bicyclic 8-hydroperoxy 4,4,8-trimethyl 2,3-dioxabicyclo[3.3.1]nonane. The corresponding alcohols were found after reduction with sodium sulphite. The oxidation at position 2 of the pinanes delivers not only the cis- and trans-hydroperoxide but also, as shortlived intermediates, the corresponding 2-pinanyloxy radicals. These radicals fragmentate forming a carbon radical with cyclobutane structure whose oxidation products were identified. Besides fragmentation of the 2-pinanyloxy radical also an intramolecular H-transfer from the methyl group in 9-position to the oxygen of the trans-2-pinanyloxy radical takes place leading to 9-hydroperoxy trans-pinane-2-ol.     

Advances in metal chemistry of quinonoid compounds: new types of interactions between metals and aromatics

This Account presents an overview of current research activities that focus on novel types of interactions between cationic transition metal complexes and arene systems and on unprecedented quinonoid complexes which result from such interactions. When a negatively charged phenoxy group is present in a position para to the metal in a high oxidation state, intramolecular charge transfer occurs, giving the corresponding metallaquinones or quinone methide complexes. In addition, two types of interactions involving low-valent metal compounds have been observed: methylene arenium complexes which result from positive charge transfer to the aromatic ring and sigma-bonded C-H and C-C agostic complexes of cationic metals. These sigma-complexes are proposed as intermediates in metal-based bond activation processes.     

Catalytic coupling of sp2- and sp-hybridized carbon-hydrogen bonds with vinylmetalloid compounds

In the Account given herein, it has been shown that silylative coupling of olefins, well-recognized as a new catalytic route for the activation of double bond C-H bond of olefins and double bond C-Si bond of vinylsilicon compounds with ethylene elimination, can be extended over both other vinylmetalloid derivatives (double bond C-E) (where E = Ge, B, and others) as well as the activation of triple bond C-H, double bond C aryl-H, and -O-H bond of alcohols and silanols. This general transformation is catalyzed by transition-metal complexes (mainly Ru and Rh) containing or initiating TM-H and/or TM-E bonds (inorganometallics). This new general catalytic route for the activation of double bond C-H and triple bond C-H as well as double bond C-E bonds called metallative coupling or trans-metalation (cross-coupling, ring-closing, and polycondensation) constitutes an efficient method (complementary to metathesis) for stereo- and regioselective synthesis of a variety of molecular and macromolecular compounds of vinyl-E (E = Si, B, and Ge) and ethynyl-E (E = Si and Ge) functionality, also potent organometallic reagents for efficient synthesis of highly pi-conjugated organic compounds. The mechanisms of the catalysis of this deethenative metalation have been supported by equimolar reactions of TM-H and/or TM-E with initial substances and reactions with deuterium-labeled reagents.     

Theoretical studies on high-valent manganese porphyrins: Toward a deeper understanding of the energetics,electron distributions,and structural features of the reactive intermediates of enzymatic and synthetic manganese-catalyzed oxidative processes

We present here a relatively comprehensive theoretical study, based on nonlocal density functional theory calculations, of the energetics, electron distributions, and structural features of the low-lying electronic states of various high-valent intermediates of manganese porphyrins. Two classes of molecules have been examined: (a) compounds with the general formula [(P)MnX₂]⁰ (P = porphyrin; X = F, Cl, PF₆) and (b) high-valent manganese-oxo species. For [(P)Mn(PF₆)₂]⁰, the calculations reveal a number of nearly equienergetic quartet and sextet states as the lowest states, consistent with experimental results on a comparable species, [(TMP)Mn(ClO₄)₂]⁰ (TMP = tetramesitylporphyrin). In contrast, [(P)MnCl₂]⁰ and [(P)MnF₂]⁰ have a single well-defined S = 3/2 Mn(IV) ground state, again in agreement with experiment, with the three unpaired spins largely concentrated (>90%) on the manganese atom. Manganese(IV)-oxo porphyrins have an S = 3/2 ground state, with the three unpaired spins distributed approximately 2.3:0.7 between the manganese and oxygen atoms. The metal-to-oxygen spin delocalization, as measured by the oxygen spin population, for Mn^IV = O porphyrins is less than, but still qualitatively similar to, that in analogous iron(IV)-oxo intermediates, indicating that the Mn^IV = O bond is significantly weaker than the Fe^IV = O bond in an analogous molecule. Thus, the optimized metal—oxygen bond distances are 1.654 and 1.674 Å for (P)Fe^IV(O)(Py) and (P)Mn^IV(O)(Py), respectively (Py = pyridine). This is consistent with the experimental observation that Mn^IV = O stretching frequencies are over 10% lower than Fe^IV = O stretching frequencies for analogous compounds. For [(P)Mn(O)(PF₆)]⁰, [(P)Mn(O)(Py)]⁺, and [(P)Mn(O)(F)]⁰, the ground states clearly correspond to a (d_xy)² Mn(V) configuration and the short Mn–O distances of 1.541, 1.546, and 1.561 Å for the three compounds, respectively, reflect the formal triple bond character of the Mn–O interaction. Interestingly, the corresponding Mn(IV)-oxo porphyrin cation radical states are calculated to be a few tenths of an electrovolt higher than the Mn(V) ground states, suggesting that the Mn(IV)-oxo porphyrin cation radicals are not likely to exist as ground-state species.     

NO2硝化正己烷及其理论计算

对NO₂硝化正己烷的反应进行了研究,分别考察了反应温度、摩尔比和反应时间的影响。结果表明：在反应温度为120℃、正己烷与NO₂摩尔比为1∶2、反应时间为4h的反应条件下,正己烷转化率可达85.9%。通过密度泛函理论（DFT）研究了NO₂硝化正己烷的反应机理,在B3LYP/6-311++G(3df,2pd)//B3LYP/6-31G*计算水平下精确计算了三个可能反应途径的活化能（E_a）。计算结果表明：该反应决速步骤为NO₂中O原子进攻正己烷中的H原子,其中2-硝基己烷和3-硝基己烷为主要产物,且计算结果与实验结果一致。分子几何结构、原子电荷和IR振动频率的数据表明C—H键的断裂和N—H键的形成是一个协同过程,参与硝化反应的原子C(5)、H(7)、O(22)、O(23)和N(21)的分子几何参数及其原子电荷有明显的变化。     

The adsorption and activation properties of precious metal clusters toward NO: a density functional study

Adsorption and activation properties of precious metal clusters such as Ir, Pt, and Au toward NO were investigated by means of the density functional calculations. We focused on the geometrical features of model clusters such as the shape and the number of consisting atoms that could determine the ability for the adsorption and the activation of NO. We found that the order of the energetical stability of the adsorption states of NO can be described as Ir > Pt > Au. It depends on neither the shape of the pentaatomic clusters nor the number of atoms in the model clusters considered. The ability of the precious metal clusters for the activation of the N–O bond were also discussed from both vibrational and geometrical points of view. The substantial activation of the N–O bond was found on both the NO/Ir₅ and the NO/Pt₄ systems, indicating that the specific adsorption geometries enhance the ability for the activation of the N–O bond. These results indicate that the Ir cluster has the best properties for the adsorption and activation of NO. This revised version was published online in August 2006 with corrections to the Cover Date.     

铜催化的C-H键活化生成C-O键的最新进展

芳烃酰氧类化合物一直是有机合成中一个重要的合成中间体,有许多的经典方法可以合成它,其中Cu催化的由C-H键直接转化为酰氧类化合物的方法提供了一种方便,简单,有效,廉价的途径,这篇综述仅对铜催化的C-H键活化生成C-O键,特别是生成芳烃酰氧类化合物的最新进展进行了小结。     

Upgrading of diesel fuels and mixtures of hydrocarbons by means of oxygen low pressure plasmas: a comparative study

The oxidation of n-heptane, 1-octene, toluene, cis-decahydronaphthalene, mixtures of them, 4-phenyl-1-butene, 1,2,3,4-tetrahydronaphthalene, and three commercial diesel fuels, all in the liquid phase, by means of low pressure high-voltage oxygen plasmas was studied. Oxygen pressure was 0.2 mbar, applied power was 35 watts and reaction times ranged from 1 min to 23 h. Both individually and forming part of mixtures, olefins were the most reactive with ground-state atomic oxygen, O(³P). Olefinic double bonds reacted ca. 150 times faster than C-H bonds. Products were: epoxides and aldehydes for olefins; alcohols and ketones for alkanes; phenols for aromatics. Addition of 4.7-7.8% wt of oxygen was achieved for the diesels, depending on the particular composition, those with higher content of olefins being favoured, followed by those with higher content of alkanes.     

Structure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5

The rate of propane dehydrocyclodimerization to form C₆ aromatics is limited by a sequence of irreversible dehydrogenation reactions leading to propene, higher alkenes, dienes, trienes, and aromatics. Quasi-equilibrated acid—catalyzed cracking, oligometization, and cyclization reactions of alkene intermediates occur in sequence with these dehydrogenation reactions. Each dehydrogenation reaction is in turn limited by the rate of elementary steps that dispose of H-atoms formed in C-H bond activation steps. The rate of C-H bond activation, recombinative hydrogen desorption, and propane chemical conversion have been measured from the rates of isotopic redistribution and chemical conversion during reactions of C₃H₈/C₃/D₈and D₂/C₃/H₈ mixtures on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Isotopic studies show that C-H activation steps are fast during steady-state propane dehydrocyclodimerization on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5.

Ga and Zn species increase the rates of propane chemical conversion, recombinative hydrogen desorption, and deuterium incorporation from D₂into reaction products. Disposal of hydrogen formed in C-H bond activation steps occurs by transfer of H-atoms to unsaturated species to form alkanes or to Ga and Zn species, which catalyze the recombinative desorption of H-atoms to form dihydrogen (H₂). The sequential release of several H-atoms during a propane dehydrocyclodimerization turnover limits the rate and selectivity of this reaction on H-ZSM5.

In-situ X-ray absorption studies suggest that Ga and Zn species reside at cation exchange sites as monomeric cations and that recombinative desorption involve reduction—oxidation cycles of such cations during each dehydrocyclodimerization turnover. These monomeric species form directly during exchange of Zn ions from solution onto H-ZSM5. Ga³⁺species, however, do not exchange directly from solution onto H-ZSM5, but instead form extrazeolitic Ga₂O₃ crystals. Ion exchange occurs during subsequent contact with propane or hydrogen at 700-800 K via vapor phase exchange of volatile Ga¹⁺ species.     

Reaction Mechanisms of Silicon Carbide Fiber Synthesis by Heat Treatment of Polycarbosilane Fibers Cured by Radiation: II, Free Radical Reaction

Silicon carbide (SiC) fibers were synthesized by the pyrolysis of radiation-cured polycarbosilane (PCS) fibers. The pyrolytic reaction was analyzed through free radicals by electron spin resonance (ESR). Free radicals on Si and C atoms were produced in the pyrolysis, and their yield as a function of reaction temperature depended on the oxygen content in the cured PCS fibers. There were two temperature ranges for the reactions related to Si and C atoms in PCS fibers. The radical concentration reflects the rate differences between the radical formation by the cleavage of Si-H and C-H bonds and the decay by recombination of radicals. The reaction of Si-H was strongly influenced by the oxygen content in the cured PCS fibers, and the decay rate of radicals decreased with an increase of the oxygen content in PCS fibers introduced in the curing process.