Rate constants for the reaction of OH radicals with a series of alkenes and dialkenes at 295 ± 1 K

Using a relative rate technique, rate constants for the gas phase reactions of the OH radical with n-butane, n-hexane, and a series of alkenes and dialkenes, relative to that for propene, have been determined in one atmosphere of air at 295 ± 1 K. The rate constant ratios obtained were (propene = 1.00): ethene, 0.323 ± 0.014; 1-butene, 1.19 ± 0.06; 1-pentene, 1.19 ± 0.05; 1-hexene, 1.40 ± 0.04; 1-heptene, 1.51 ± 0.06; 3-methyl-1-butene, 1.21 ± 0.04; isobutene, 1.95 ± 0.09; cis-2-butene, 2.13 ± 0.05; trans-2-butene, 2.43 ± 0.05; 2-methyl-2-butene, 3.30 ± 0.13; 2,3-dimethyl-2-butene, 4.17 ± 0.18; propadiene, 0.367 ± 0.036; 1,3-butadiene, 2.53 ± 0.08; 2-methyl-1,3-butadiene, 3.81 ± 0.15; n-butane, 0.101 ± 0.012; and n-hexane, 0.198 ± 0.017. From a least-squares fit of these relative rate data to the most reliable literature absolute flash photolysis rate constants, these relative rate constants can be placed on an absolute basis using a rate constant for the reaction of OH radicals with propene of 2.63 × 10^?11 cm³ molecule^?1 s^?1. The resulting rate constant data, together with previous relative rate data from these and other laboratories, lead to a self-consistent data set for the reactions of OH radicals with a large number of organics at room temperature.     

Oligomerization of α-olefins by the dimeric nickel bisamido complex [Ni{1-N(PMes2)-2-N(μ-PMes2)C6H4-κN,N′,P,-κP′}]2 activated by methylalumoxane (MAO)

The reaction of Li₂[1,2-{N(PMes₂)}₂C₆H₄], formed in situ from n-BuLi and the corresponding amines, with 1 equiv. of [NiBr₂(DME)] gives [Ni{1-N(PMes₂)-2-N(μ-PMes₂)C₆H₄-κ³N,N′,P-κ¹P′}]₂ (1). After activation by methylalumoxane (MAO), 1 is a highly active catalyst in the oligomerization and isomerization of α-olefins such as ethene, propene, isobutene, 1-hexene and 1,5-hexadiene. For ethene oligomerization turnover frequencies (TOFs) range from 3000 to 79015 h⁻¹, depending on the reaction conditions. The TOF for propene oligomerization reaches 1 190 730 h⁻¹. To our knowledge, catalyst 1, activated by MAO, is the most active catalyst for the oligomerization of propene and outperforms the best known complexes for this reaction. In the reactions with 1-hexene, 1,5-hexadiene and isobutene dimerization and isomerization products were observed.     

Rate constants for the gas-phase reactions of O3 with a series of alkenes at 296 ± 2 K

The kinetics of the gas-phase reactions of O₃ with a series of alkenes have been investigated at atmospheric pressure (ca. 740 torr) of air and 296 ± 2 K, using a relative rate method in the presence of sufficient n-octane to scavenge any OH radicals generated in these reactions. Relative to k(O₃ + propene) = 1.00, the rate constants obtained were: 1-butene, 0.975 ± 0.030; 2-methylpropene, 1.14 ± 0.04; 2-methyl-1,3-butadiene (isoprene), 1.21 ± 0.02; 1,4-cyclohexadiene, 4.75 ± 0.23; cyclohexene, 7.38 ± 0.48; cis-2-butene, 12.8 ± 0.8; trans-2-butene, 21.5 ± 1.5; 2-methyl-2-butene, 42.1 ± 2.8; cyclopentene, 64.9 ± 4.3; and 2,3-dimethyl-2-butene, 123 ± 11. These relative rate constants have been placed on an absolute basis using a rate constant for the reaction of O₃ with propene of 1.01 × 10^?17 cm³ molecule^?1 s^?1 at 296 K derived from an analysis of the available literature data. The resulting rate constants then lead to a self-consistent set of room temperature data for the reactions of O₃ with these alkenes. © John Wiley & Sons, Inc.     

Formation yields of epoxides and O(3P) atoms from the gas-phase reactions of O3 with a series of alkenes

Reactions of ozone with propene, 1-butene, cis-2-butene, trans-2-butene, 2,3-dimethyl-2-butene, and 1,3-butadiene were carried out in N₂ and air diluent at atmospheric pressure and room temperature and, by monitoring the formation of the epoxides and/or a carbonyl compound formed from the reactions of O(³P) atoms with these alkenes, the formation yields of O(³P) atoms from the O₃ reactions were investigated. No evidence for O(³P) atom formation was obtained, and upper limits to O(³P) atom formation yields of <4% for propene, <5% for 1.3-butadiene, and <2% for the other four alkenes were derived. The reaction of O₃ with 1,3-butadiene led to the direct formation of 3,4-epoxy-1-butene in (2.3 ± 0.4)% yield. These data are in agreement with the majority of the literature data and show that O(³P) atom formation is not a significant pathway in O₃—alkene reactions, and that epoxide formation only occurs to any significant extent from conjugated dienes. © 1994 John Wiley & Sons, Inc.     

Reactions of free radicals with η-allylpalladium(II) complexes: cyclohexyl radicals

Allyl palladium complexes of the types [(η³-allyl)PdCl]₂, (η³-allyl)PdCl(PPh₃) and [(η³-allyl)Pd(PPh₃)₂]Cl (allyl=C₃H₅, 1-MeC₃H₄, 2-MeC₃H₄, 1-PhC₃H₄, 2-PhC₃H₄) react with cyclohexyl radicals derived from the visible light photolysis of (c-hex)Co(DMG)₂(py). The reactions proceed via initial attack of the free radical at the metal center, followed by β-hydrogen elimination and subsequent reductive elimination of propene, 1-butene, isobutene, 3-phenylpropene and 2-phenylpropene, respectively. The 3-phenylpropene can be catalytically isomerized to the thermodynamically more stable 1-phenylpropene by either palladium metal or palladium(0) products, but the formation of 1-butene and 3-phenylpropene as primary products is unusual. A mechanism, differing in many ways from that proposed previously for analogous reactions of phenyl and trityl radicals, is proposed for the overall reaction and supported by use of the labeled cobaloxime, (2,2,6,6-D₄-c-hex)Co(DMG)₂(py).     

Laser pyrolysis studies of OH reaction rates with several butenes at 1200 K

Using a laser pyrolysis/laser fluorescence method, we have measured the rate constants for OH reacting with 1-butene, t-2-butene, isobutene, and 2,3-dimethyl-2-butene near 1200 K. The butene rate constants are large, ranging from 2.0–3.7 × 10^?11 cm^?3 s^?1, and increase with the number of allylic hydrogens. Transition-state theory considerations indicate these allylic hydrogens are easily abstracted, in contrast to prior observations on propene.     

Automatic discovery of photoisomerization mechanisms with nanosecond machine learning photodynamics simulations

Photochemical reactions are widely used by academic and industrial researchers to construct complex molecular architectures via mechanisms that often require harsh reaction conditions. Photodynamics simulations provide time-resolved snapshots of molecular excited-state structures required to understand and predict reactivities and chemoselectivities. Molecular excited-states are often nearly degenerate and require computationally intensive multiconfigurational quantum mechanical methods, especially at conical intersections. Non-adiabatic molecular dynamics require thousands of these computations per trajectory, which limits simulations to ∼1 picosecond for most organic photochemical reactions. Westermayr et al. recently introduced a neural-network-based method to accelerate the predictions of electronic properties and pushed the simulation limit to 1 ns for the model system, methylenimmonium cation (CH₂NH₂⁺). We have adapted this methodology to develop the Python-based, Python Rapid Artificial Intelligence Ab Initio Molecular Dynamics (PyRAI²MD) software for the cis–trans isomerization of trans-hexafluoro-2-butene and the 4π-electrocyclic ring-closing of a norbornyl hexacyclodiene. We performed a 10 ns simulation for trans-hexafluoro-2-butene in just 2 days. The same simulation would take approximately 58 years with traditional multiconfigurational photodynamics simulations. We generated training data by combining Wigner sampling, geometrical interpolations, and short-time quantum chemical trajectories to adaptively sample sparse data regions along reaction coordinates. The final data set of the cis–trans isomerization and the 4π-electrocyclic ring-closing model has 6207 and 6267 data points, respectively. The training errors in energy using feedforward neural networks achieved chemical accuracy (0.023–0.032 eV). The neural network photodynamics simulations of trans-hexafluoro-2-butene agree with the quantum chemical calculations showing the formation of the cis-product and reactive carbene intermediate. The neural network trajectories of the norbornyl cyclohexadiene corroborate the low-yielding syn-product, which was absent in the quantum chemical trajectories, and revealed subsequent thermal reactions in 1 ns.

Photochemical reactions are widely used by academia and industry to construct complex molecular architectures via mechanisms that are often inaccessible by other means.     

高效钼基催化剂上正丁烯自歧化生成乙烯和己烯（英文）

烯烃歧化反应(又称烯烃复分解反应)是两分子烯烃通过碳-碳键断裂重排生成新烯烃分子的反应,自1964年Phillips公司的Banks等发现以来,引起了研究者的广泛关注,且在均相催化体系的发展尤为迅速;与此同时,多相烯烃歧化催化剂因其在分离简单、可循环再生利用方面的优势而在工业界崭露锋芒.多相烯烃歧化催化剂通常由活性金属组分(Re,Mo,W)分散到大比表面积的多孔载体制备而成.多相催化剂上烯烃歧化反应主要集中在乙烯和2-丁烯反歧化制丙烯反应,其中WO_3/SiO_2催化剂先后应用于Phillips公司的Triolefin Process和ABB Lummus公司的OCT工艺,低温Re系催化剂被法国石油研究院应用到Meta-4歧化工艺.同时丙烯歧化也是研究最多的反应,多数情况下被用作探针反应来研究催化剂的性能.烯烃歧化反应可以根据市场需求灵活调变产物分布,为碳四烃类的高效转化利用提供很好的途径.受国内拉动内需的政策及下游应用行业强劲需求的影响,中国液化石油气的产量逐年递增.2014年我国液化气产量约为2550万吨,其中仅有39%左右用于碳四深加工,大部分当做燃料直接烧掉.从组成来看,液化气中烯烃含量在40%-50%,可以转化为高附加值的乙烯和丙烯进一步利用.本文重点开发了一条从1-丁烯出发生产乙烯/己烯的反应路线及对应的催化剂.首先从热力学角度分析了碳四歧化反应网络中各反应路径发生的难易程度.在此基础上,以Mo/Al_2O_3为催化剂考察了Mo负载量和反应条件对产物分布的影响-.在优化的6Mo/Al_2O_3催化剂上,80°C,1.0 MPa和丁烯空速3 h1的条件下,产物中乙烯和己烯的摩尔选择性超过85%,并且在48 h内保持良好的反应稳定性.为了进一步探究催化剂结构与反应性能的关系,系统考察了催化剂载体差异对Mo物种状态和反应性能的影响.借助N2吸附,NH_3-TPD,Py-IR,H_2-TPR,UV-Vis和HRTEM等表征手段,发现催化剂反应活性与其酸密度直接相关.催化剂酸量越大,丁烯转化率越高,但副反应越多;载体适宜的酸量和较大的比表面积更有利于钼物种的分散和四配位钼物种的形成,促进目标1-丁烯自歧化制乙烯/己烯反应的发生.     

Determination by the phase shift method of the absolute rate constants of reactions of O(3P) atoms with olefins at 25°C

Technical improvements have been made in the application of the phase shift method, recently developed and used in this laboratory to measure the absolute values of the rate constants of the reactions of O(3P) atoms with ethylene, propylene, 1-butene, and isobutene. More accurate rate constants have now been obtained for these olefins and themeasurements have been also extended to three additional olefins (cis-2-butene, trimethylethylene, and tetramethylene).The mean least squares values of the rate constants of the reactions of O(³P) with olefins at 26°C, expressed as k₂ × 10^?9 (1./mole²·sec), and their standard deviations are ethylene 0.43 ±0.05, propelene 2.02 ± 0.17, 1-butene 2.40 ± 0.32, isobutene 9.85 ± 1.34, cis-2-butene 9.00 ± 1.76, trimethylethylene 31.1 ± 3.0, and tetramethylethylene 44.4 ± 4.1. A Value of (5.78 ± 0.08) × 10¹⁰l.²/mole²·sec was obtained for the rate constant of the reaction O(³P) + NO + M, with M = N₂O. Where comparison with the absolute rate constants in the literature is possible, agreement is very good, especiallywith some recent absolute values obtained by entirely different experimental techniques. Consistency with the relative rate constant of the O(³P) reactions with olefins previously determined in this laboratory, is excellent.     

Comparative study of coke deposition on catalysts in reactions with and without oxygen

Two types of catalysts, i.e. Pt/γ Al₂O₃ and Cu/Na-ZSM-5, were used to investigate the catalyst activity and amount of coke formation on the spent catalysts. The reactions of particular interest were the hydrocarbon oxidation and the SCR of NO with and without O₂. Propane and propene were used as the hydrocarbon sources. The reaction conditions were as follows: reaction temperature =170–500°C, GHSV=4,000 hr⁻¹, TOS=2 hr, feed composition depending on each reaction, but the composition of gases were fixed as HC=3,000 ppm, NO=1,000 ppm and O₂=2.5%, using He balance. It was found that both the case of Pt/γ Al₂O₃ and the case of Cu/Na-ZSM-5, propene provided higher conversion and coke deposition than propane in the presence or the absence of O₂ and/or NO. For Pt/γ Al₂O₃ catalyst, in case of the absence of oxygen reactions, the propene conversion dropped more rapidly than the propane conversion. Finally the reaction of propene gave a lower percent of hydrocarbon conversion than the reaction of propane. Additionally, propene had a higher percent selectivity of coke formation for the reaction with the absence of oxygen, but propane had a higher percent selectivity of coke formation for the reaction with the presence of oxygen. For Cu/Na-ZSM-5, in the system with absence and presence of oxygen, the addition of oxygen caused a significant change in % coke selectivity. With the presence of NO_x, the percent conversion of both propane and propene decreased and that the % coke selectivity of propane decreased, whereas that of in propene increased.     

1-Butene isomerization and metathesis over Mo/mordenite-alumina: Factors influencing product distribution and induction period

Effects of space velocity, reaction temperature and support acidity on product distribution and induction period in 1-butene isomerization and metathesis over Mo/mordenite-alumina were investigated. As revealed by the catalytic performance results, induction period and objective product were closely related to the reaction conditions. Lower space velocity led to longer induction period and higher propene yield. The optimal reaction temperature for propene production is around 150 °C and it shifted to 100 °C for ethene production. 1-Butene auto-metathesis predominated in the reaction network if the support with lower degree of sodium exchanged. And propene gradually became the dominant product upon increasing the support sodium exchange degree. 6Mo/H₁₀₀Na₀M-30Al catalyst with a support of full sodium exchange degree exhibited the highest propene yield.     

Rate constants for the reactions of Br atoms with a series of alkanes,alkenes, and alkynes in the presence of O2

Rate constants of Br atom reactions have been determined using a relative kinetic method in a 20 l reaction chamber at total pressures between 25 and 760 torr in N₂ + O₂ diluent over the temperature range 293–355 K. The measured rate constants for the reactions with alkynes and alkenes showed dependence upon temperature, total pressure, and the concentration of O₂ present in the reaction system. Values of (6.8 ± 1.4) × 10^?15, (3.6 ± 0.7) × 10^?14, (1.5 ± 0.3) × 10^?12, (1.6 ± 0.3) × 10^?13, (2.7 ± 0.5) × 10^?12, (3.4 ± 0.7) × 10^?12, and (7.5 ± 1.5) × 10^?12 (units: cm³ s^?1) have been obtained as rate constants for the reactions of Br with 2,2,4-trimethylpentane, acetylene, propyne, ethene, propene, 1-butene, and trans-2-butene, respectively, in 760 torr of synthetic air at 298 K with respect to acetaldehyde as reference, k = 3.6 × 10^?12 cm³ s^?1. Formyl bromide and glyoxal were observed as primary products in the reaction of Br with acetylene in air which further react to form CO, HBr, HOBr, and H₂O₂. Bromoacetaldehyde was observed as an primary product in the reaction of Br with ethene. Other observed products included CO, CO₂, HBr, HOBr, BrCHO, bromoethanol, and probably bromoacetic acid.     

Combining alkali metals and zinc to harness heterometallic cooperativity in cyclic ester ring-opening polymerisation

Heterometallic cooperativity is an emerging strategy to elevate polymerisation catalyst performance. Here, we report the first heterotrimetallic Na/Zn₂ and K/Zn₂ complexes supported by a ProPhenol ligand, which deliver “best of both” in cyclic ester ring-opening polymerisation, combining the outstanding activity (Na/K) and good control (Zn₂) of homometallic analogues. Detailed NMR studies and density-functional theory calculations suggest that the Na/Zn₂ and K/Zn₂ complexes retain their heterometallic structures in the solution-state. To the best of our knowledge, the K/Zn₂ analogue is the most active heterometallic catalyst reported for rac-lactide polymerisation (k_obs = 1.7 × 10⁻² s⁻¹), giving activities five times faster than the Na/Zn₂ complex. These versatile catalysts also display outstanding performance in ε-caprolatone and δ-valerolactone ring-opening polymerisation. These studies provide underpinning methodologies for future heterometallic polymerisation catalyst design, both in cyclic ester polymerisation and other ring-opening (co)polymerisation reactions.

Cooperative heterotrimetallic Na/Zn₂ and K/Zn₂ complexes combine the excellent activities and control of the homometallic analogues, giving “best of both” in cyclic ester ring-opening polymerisation.     

IR observation of adsorption and initial reactions of olefins on Brønsted acid sites of a deuterated ZSM-5

Adsorption of linear olefins (C₂?C₄) on a deuterated H-ZSM-5 (D-ZSM-5) was studied by infrared (IR) spectroscopy. The initial interaction of the olefins with Brønsted acidic OD groups was hydrogen-bonding to form π-complexes at low temperatures. The adsorbed ethene and propene desorbed by heating under evacuation, while various reactions took place for adsorbed 1-butene; double bond migration (DBM) to 2-butene below 230 K followed by dimerization at room temperature. An unusual reaction path was deduced for DBM of 1-butene, where proton transfer from Brønsted acid sites (BAS) to the adsorbed 1-butene was not essential.     

Carbon monoxide and hydrogen (syngas) as a C1-building block for selective catalytic methylation

A catalytic reaction using syngas (CO/H₂) as feedstock for the selective β-methylation of alcohols was developed whereby carbon monoxide acts as a C1 source and hydrogen gas as a reducing agent. The overall transformation occurs through an intricate network of metal-catalyzed and base-mediated reactions. The molecular complex [Mn(CO)₂Br[HN(C₂H₄PⁱPr₂)₂]] 1 comprising earth-abundant manganese acts as the metal component in the catalytic system enabling the generation of formaldehyde from syngas in a synthetically useful reaction. This new syngas conversion opens pathways to install methyl branches at sp³ carbon centers utilizing renewable feedstocks and energy for the synthesis of biologically active compounds, fine chemicals, and advanced biofuels.

A broadly applicable catalytic process for the selective β-methylation of alcohols is presented using syngas (CO/H₂) directly as a C1 building block and the shown manganese complex in the presence of a base as the catalytic system.     

Development of a detailed pyrolysis mechanism for C1–C4 hydrocarbons under a wide range of temperature and pressure

A model of core mechanism of hydrocarbon pyrolysis with good predictive ability is crucial to the development of active cooling technology for advanced aeroengines. In this work, a detailed core kinetic model of pyrolysis of C₁–C₄ hydrocarbon fuels is developed through the combination of a series of potential energy surfaces and validated against a series of experimental results. The kinetic model contains 103 species and 1290 reactions, and most of the kinetic and thermochemical parameters are compiled from recent highly accurate quantum chemical calculations without modification. The pressure-dependent rate constants are considered for the dissociation/association reactions, isomerization reactions, and chemically activated reactions. Simulation results for various alkanes (methane, ethane, propane, n-butane, isobutane), alkenes (ethylene, propene, 1-butene, 2-butene, isobutene, allene, 1,3-butadiene), and alkynes (acetylene, propyne, vinylacetylene) indicate that the major product distributions at various temperatures (800-2300 K) and pressures (0.8-10 atm) can be predicted well by the developed core kinetic model. Thus, the developed pyrolysis mechanism for C₁–C₄ hydrocarbons can be used as a cornerstone to develop the pyrolysis mechanisms of larger hydrocarbon fuels and thus support the development of thermal management in advanced aeroengines.     

Visible light driven deuteration of formyl C–H and hydridic C(sp3)–H bonds in feedstock chemicals and pharmaceutical molecules

Deuterium labelled compounds are of significant importance in chemical mechanism investigations, mass spectrometric studies, diagnoses of drug metabolisms, and pharmaceutical discovery. Herein, we report an efficient hydrogen deuterium exchange reaction using deuterium oxide (D₂O) as the deuterium source, enabled by merging a tetra-n-butylammonium decatungstate (TBADT) hydrogen atom transfer photocatalyst and a thiol catalyst under light irradiation at 390 nm. This deuteration protocol is effective with formyl C–H bonds and a wide range of hydridic C(sp³)–H bonds (e.g. α-oxy, α-thioxy, α-amino, benzylic, and unactivated tertiary C(sp³)–H bonds). It has been successfully applied to the high incorporation of deuterium in 38 feedstock chemicals, 15 pharmaceutical compounds, and 6 drug precursors. Sequential deuteration between formyl C–H bonds of aldehydes and other activated hydridic C(sp³)–H bonds can be achieved in a selective manner.

A selective hydrogen deuterium exchange reaction with formyl C–H bonds and a wide range of hydridic C(sp³)–H bonds has been achieved by merging tetra-n-butylammonium decatungstate photocatalyst and a thiol catalyst under 390 nm light irradiation.     

Kinetics of oxidation of lower olefins with Fe(III) aqua ions in the presence of the Pd/ZrO2/SO4 precatalyst in a chloride-free system

The kinetics of oxidation of ethene, propene, and 1-butene with Fe(III) aqua ions to the corresponding carbonyl compounds in the presence of the 1% Pd/ZrO₂/SO₄ precatalyst in aqueous perchloric acid at 40–80°C was studied. The oxidation rate increases in the order C₂H₄ < C₄H₈ < C₃H₆ and with increasing catalyst weight and in the acid and Fe(III) concentrations; it is independent of the olefin pressure. The ethene oxidation rate is described by the Michaelis-Menten equation. In the case of 1-butene, the reaction is accompanied by migration of the double bond with the formation of 2-butene.