Pyrolysis mass spectrometry of methionine

The mechanisms of some fragmentation of methionine upon Curie-point pyrolysis have been studied by the use of deuterium labelling. They lead to products having the same nominal mass but essentially different elemental compositions compared with ions generated by electron impact on methionine. For exaple, upon pyrolysis the product with mass 116 is due to a 1,2 elimination of hydroxylamine from the title compound, whereas upon electron impact the ion with m/e 116 is generated by successive loss of a molecule of water and of a methyl radical from the molecular ion; the generated of product m/e 101 proceeds via successive loss of a molecule of water, carbon monoxide and of hydrogen upon pyrolysis, but via a one-step elimination of methanethiol upon electron impact. No evidence has been found for a diketopiperazine formation by a bimolecular process upon pyrolysis, although this type of reaction is well known for α-amino acids.     

Unsaturated ketenes: A study of their formation and rearrangement by tandem mass spectrometry and low-temperature infrared spectroscopy

A combination of collisional activation mass spectrometry and low-temperature infrared spectroscopy has been used to monitor the production and isomerization of vinyl- and methylene-ketenes. Vinyl- and 2-propenyl-ketenes were produced by flash vacuum pyrolysis of β,γ-unsaturated acid chlorides. Vinyl(carboxyl)ketenes, vinylketenes and methyleneketenes were obtained from Meldrum's acid derivatives (5-alkylidene-1,3-dioxane-4,6-diones). The formation of methyleneketenes by pyrolysis of α, β-unsaturated acid chlorides is only indicated in the mass spectrometry experiments. Carbonylcyclopropane was obtained by pyrolysis of cyclopropylcarbonyl chloride or the corresponding Meldrum's acid derivative. The methyleneketenes isomerize to vinylketenes in the gas phase, particularly under conditions involving long contact times. Carbonylcyclopropane thermally decarbonylates to allene, not methylacetylene. Molecular ions of vinylketenes are obtained via pyrolysis of either acid chlorides or Meldrum's acid derivatives. Molecular ions of alkylmethyleneketenes are obtained pure only by electron impact induced fragmentation of Meldrum's acid derivatives.     

氮掺杂石墨烯的制备及氧还原电催化性能

采用两步热解法, 用尿素掺杂氧化石墨烯(GO)得到N掺杂的还原氧化石墨烯(N-RGO), 通过控制反应温度, 制备了具有不同电催化活性的N掺杂的还原氧化石墨烯. 透射电子显微镜(TEM)和扫描电子显微镜(SEM)结果显示, 制得的氮掺杂石墨烯(nG)表面褶皱和重叠增加. X射线光电子能谱(XPS)证明, 氮元素以吡啶N、 吡咯N和石墨化的N 3种形式掺杂在石墨烯中, 最高摩尔分数为6.6%. 通过循环伏安(CV)和旋转圆盘电极(RDE)测试了nG的电化学性能, 结果表明, 在酸性电解质中对氧还原(ORR)有较高的催化活性, 起始电位在0.1 V左右, 电催化还原氧气时主要为四电子反应, 且相对商用的Pt/C催化剂有更好的电化学稳定性, 其中第一步热解温度为200℃制得的nG催化性能最好.     

A mass spectrometric study of the pyrolysis of gibberellic acid

The behaviour of gibberellic acid (GA₃) under electron impact and chemical ionization conditions has been examined. The tendency of GA₃ to undergo pyrolysis by the loss of the elements of water and carbon dioxide has been identified. Two methods of sample introduction can be used to minimize the occurrence of this pyrolysis. Accurate mass measurement allows the composition of the pyrolysis product to be determined and metastable techniques confirm the structure of this product to be epi-allogibberic acid rather than its isomer allogibberic acid. This result corrects a misconception in the literature and illustrates the advantage of metastable methods compared with accurate mass measurements, for the determination of small structural differences within a molecule.     

Application of linked scans in pyrolysis mass spectrometry of polymers

Linked scans have been used in the pyrolysis mass spectrometry of polymers containing arylene and/or heteroarylene together with vinylene units. This technique based on metastable ions allows the study of characteristic sequences in complex mixtures of degradation ions up to high mass regions. Essential information is obtained concerning the mechanism of polymerization via the Wittig reaction, the pyrolytic and electron impact induced degradation processes and the ion structures which characterize the polymers.     

Facile preparation of nitrogen-doped graphene as a metal-free catalyst for oxygen reduction reaction

Nitrogen-doped graphene (nG) is a promising metal-free catalyst for oxygen reduction reaction (ORR) on the cathode of fuel cells. Here we report a facile preparation of nG via pyrolysis of graphene oxide with melamine. The morphology of the nG is revealed using scanning electron microscopy and transmission electron microscopy while the successful N doping is confirmed by electron energy loss spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The resulting nG shows high electrocatalytic activity toward ORR in an alkaline solution with an onset potential of -0.10 V vs. Ag/AgCl reference electrode. The nG catalyzed oxygen reduction exhibits a favorable formation of water via a four-electron pathway. Good stability and anti-crossover property are also observed, which are advantageous over the Pt/C catalyst. Furthermore, the effect of pyrolysis temperature on the structure and activity of nG is systematically studied to gain some insights into the chemical reactions during pyrolysis.     

毛竹酶解/温和酸水解木质素的快速热解研究

采用热裂解-气相色谱/质谱仪联用技术,研究毛竹酶解/温和酸水解木质素(简称EMAL)的热解特性和热解产物的分布与形成规律.以温度为重要因素,研究其对木质素快速热裂解产物的影响,并通过主要的热解产物推断热解反应途径.研究结果表明,EMAL的热解产物主要是2,3-二氢苯并呋喃、酚类、脂类和少量乙酸.热解温度对热解产物组分的相对含量有显著影响,250~400 ℃时,产物主要是2,3-二氢苯并呋喃,320 ℃时其相对含量最高,达到66.26%;400~800 ℃时,热解产物主要是酚类,600 ℃时其相对含量最高,达到62.58%;800 ℃时出现了少量的乙酸.     

Ab initio quantum-chemical study of the unimolecular pyrolysis mechanisms of acetic acid

The mechanisms of unimolecular dehydration and decarboxylation reactions occurring during the pyrolysis of acetic acid above 700°C have been investigated by ab initio methods. The atomic basis set influence as well as the electron correlation effects are analyzed by using a variety of basis sets, ranging from minimal to polarized split-valence, and by introducing the Møller-Plesset (MP) perturbation theory. With an activation barrier of 76.0 kcal mol⁻¹, the concerted dehydration process occurs via a four-centre transition state. On the other hand, the decarboxylation process could be described by two different mechanisms depending on the nature of the kinetic experiments. While in flow systems, the decarboxylation of acetic acid takes place by a concerted mechanism via a four-centre transition state with an activation energy of 77.3 kcal mol⁻, the results suggest rather a water-catalyzed concerted mechanism via a six-membered transition state for the reaction carried out in batch systems, the activation barrier amounting to 64.0 kcal mol⁻¹.     

Structural and electrochemical characterization of nanocrystalline LI[Li0.12Ni0.32Mn(0.56)]O2 synthesized by a polymer-pyrolysis route

A nanocrystalline Li[Li(0.12)Ni(0.32)Mn(0.56)]O(2) powder was synthesized via the pyrolysis of polyacrylate salt precursors prepared by in situ polymerization of the metal salts and acrylate acid. The pyrolysis mechanism of the polymeric precursor is discussed by use of thermal analysis. Layered crystalline structure of the resulting compound was characterized by powder X-ray diffraction and high-resolution transmission electron (TEM) micrographs. The results revealed that the Li[Li(0.12)Ni(0.32)Mn(0.56)]O(2) as prepared has nanosized morphology and good crystallinity even if calcined at 900 degrees C, and the electrodes made from these nanoparticles exhibited high-rate characteristics and can deliver stable discharge capacity (163 mA.h/g) with excellent capacity retention at appropriate charge-discharge voltage interval. Since this preparation method is simple and particularly suitable for preparation of mixed metal oxides, it can be used for industrial production of highly homogeneous metal oxide cathode materials of the lithium-ion batteries.     

Photoelectron spectroscopy of the [glycine x (H2O)(1,2)]- clusters: sequential hydration shifts and observation of isomers

The electron binding energies of the small hydrated amino acid anions, [glycine x (H2O)(1,2)]-, are determined using photoelectron spectroscopy. The vertical electron detachment energies (VDEs) are found to increase by approximately 0.12 eV with each additional water molecule such that the higher electron binding isomer of the dihydrate is rather robust, with a VDE value of 0.33 eV. A weak binding isomer of the dihydrate is also recovered, however, with a VDE value (0.14 eV) lower than that of the monohydrate. Unlike the situation in the smaller (n < or = 13) water cluster anions, the [Gly x (H2O)(n > or = 6)]- clusters are observed to photodissociate via water monomer evaporation upon photoexcitation in the O-H stretching region. We discuss this observation in the context of the mechanism responsible for the previously observed [S. Xu, M. Nilles, and K. H. Bowen, Jr., J. Chem. Phys. 119, 10696 (2003)] sudden onset in the cluster formation at [Gly x (H2O)5]-.     

Ab initio study of the unimolecular pyrolysis mechanisms of monothioformic acid

The mechanisms of the possible unimolecular reactions occurring during the pyrolysis of the four tautomers (two conformers of thiol- and two conformers of thiono-) of monothioformic acid have been proposed and investigated by ab initio methods with STO-3G and 6-31G⁷ basis sets. The effects of valence electron correlation were included by Møller-Plesset (MP) perturbation theory to the fourth order at the 6-31G⁷ level. Our best results of the activation energies are given by MP4/6-31G⁷//HF/ 6-31G⁷ plus scaled zero-point energy. The barrier heights of the dehydrogenation (via a four-centre transition state) and dehydrogensuphidation (via a three-centre transition state) of thiolformic acid pyrolysis are 67.47 and 67.09 kcal mol⁻¹ respectively. The s-cis thionformic acid is dehydrated via a three-centre transition state. The activation energy of the process (81.18 kcal mol⁻¹) is much higher than the activation energy of the dehydrogenation of the s-trans form (68.83 kcal mol⁻¹) which is dehydrogenated via a four-centre transition state. These results suggest that in thionformic acid pyrolysis, the dehyrogenation of the s-cis form is more favourable than the dehydration of the s-trans form.     

Synthesis of dumbbell-shaped manganese oxide nanocrystals

Dumbbell-like homogeneous MnO nanocrystals are obtained for the first time via the pyrolysis of manganese formate in trioctylamine/oleic acid media and the orientation aggregation mechanism is proposed for this 1D growth route.     

Formation of the main gas compounds during thermal analysis and pyrolysis: Betaine and betaine monohydrate

The thermochemical behaviour of betaine and betaine monohydrate was investigated under two degradation conditions. Betaine was heated up to 700°C at 10°C min^–1 in air and nitrogen flows and the evolved gas was analysed with the combined TG-FTRIR system. The evolved gas from betaine pyrolysis at 350 and 400°C was analysed by gas chromatography using mass-selective detection (Py-GC/MSD). In addition, the electron impact mass spectra of betaine and betaine monohydrate were measured.Esterification is one of the most important pyrolytic processes involving beta- ines. Even glycine betaine can change to dimethylglycine methyl ester via intermolecular transalkylation by heating. Trimethylamine, CO₂, and glycine esters were the main degradation products. Small amounts of ester type compounds evolved both in pyrolysis and with TG-FTIR. The monohydrate lost water between 35 and 260°C while the main decomposition took place at 245-360°C. The residual carbon burnt in air to CO₂ up to a temperature 570°C.This revised version was published online in November 2005 with corrections to the Cover Date.     

Direct pyrolysis in the mass spectrometer of aromatic polysulfonates and polythiosulfonates

The thermal degradation mechanism of three aromatic polysulfonates and polythiosulfonates was investigated by direct pyrolysis in the ion source of a mass spectrometer. Thermal degradation reactions were followed directly by this method by detecting the thermal and electron impact induced fragments. The results obtained have provided evidence that sulfur dioxide extrusion from the polymer backbone takes place in these polymers above 300°C. The synthesis and molecular characterization of the polymers studied are reported in the text.     

丁醇醛和丁醇酸热解形成CO和CO2机理的密度泛函理论研究

采用密度泛函理论方法B3LYP/cc-pVTZ,对模型化合物2,3,4-羟基丁醛的脱羰基和2,3,4-羟基丁酸的脱羧基反应机理进行了量子化学理论研究。对两种模型化合物分别设计了三种热解反应途径,计算了不同温度下各热解反应途径的标准热力学及动力学参数。计算结果表明,纤维素热解过程中CO₂和CO的逸出分别与脱羧基和脱羰基反应相对应,脱羧基和脱羰基反应均为分子内氢原子转移的协同过程。脱羰基反应是吸热反应,而脱羧基反应是放热反应。饱和丁醇醛的脱羰基反应反应能垒为288.8 kJ/mol,脱水后的不饱和烯醇醛的脱羰基反应能垒增大;饱和丁醇酸的脱羧基反应能垒较高,为303.4 kJ/mol,脱水后的不饱和烯醇酸的脱羧基反应能垒明显减小,这说明脱水有利于CO₂的生成。     

Investigations of model systems of the Maillard reaction: II. Pyrolysis—gas chromatography—mass spectrometry analysis of some non-volatile products of the reaction of d-glucose and 4-chloroaniline

Low molecular weight compounds resulting from the reaction of d-glucose with 4-chloroaniline were subjected to Curie-point pyrolysis, and the cleavage products were identified by gas chromatography—mass spectrometry. Based on their structures, some general features were detected which possibly are applicable to the structural analysis of higher molecular weight Maillard compounds of this type. It is shown in one example that pyrolysis and electron impact mass spectrometry obviously prefer similar fragmentation patterns. In addition to Curie-point pyrolysis, Pyrotest pyrolysis and dry distillation were also tested but yielded less reproducible results.     

乙基乙酰胺热解反应机理的MO研究

对于烷基乙酰胺的初始热解反应机理, 通常认为与酯类的热解反应相类似。Maccoll和Nagra通过对该热解反应的动力学研究, 认为两者存在不同。差异之一, 烷基乙酰胺存在两种可能的热解途径【参见本文(129页)前述反应方程(1),(2)】。而在酯类热解反应中(2)的活化能过高, 且四元环过渡态极不稳定。差异之二, 极性溶剂(比如乙酸)对酰胺热解反应的催化作用不明显, 而对酯类等气相热解反应的催化作用是十分显著的。为此, 我们用MINDO/3分子轨道法对乙基乙酰胺的初始热解反应进行了较全面的研究。用能量梯度法对此反应的反应物、中间体和生成物的平衡几何构型进行了全优化。(如图1所示)用极小能量途径法分别寻找反应(1)和反应(2)的初始过渡态, 继而用Powell法全优化过渡态的几何构型, 计算所得的过渡态TS1、TS2和TS3分别见图2a, 图3a和图4a。为了确证这些过渡态, 进行了振动分析研究, 结果表明这些过渡态的力常数矩阵的诸本征值中均只有一个负值, 且虚振动模式展示了走向各自的反应物和生成物的趋势, (如图2b,图3b和图4b所示)。它们的总能量及反应(1)和反应(2)的活化能列于表1. 对整个热解反应(1)作了内禀反应坐标(IRC)理论分析, 反应历程见图5所示. 与IRC相应的总能偶极矩以及部分关键的键长和原子净电荷变化一并列于表2.本文研究结果表明, 在乙基乙酰胺的初始反应中主反应即反应(1)与酯类反应相类似, ...     

Time-Resolved Electron Paramagnetic Resonance Study of Photoionization of Tyrosine Anion in Aqueous Solution

Abstract— Photoionization of the amino acid tyrosine in basic water was studied by time-resolved electron paramagnetic resonance (TREPR) at X-band (9.5 GHz). Photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction, J, is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen only upon addition of the electron acceptor 2-bro-mo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic acid). This channel is also found to be monophotonic.     

Producing fuel and specialty chemicals from the slow pyrolysis of poultry DAF skimmings

The production of bio-oil via the slow pyrolysis of dissolved air flotation (DAF) skimmings from poultry processing is described. The raw DAF skimmings were characterized for physicochemical properties and for thermal behavior (TGA). The bio-oil was produced in a batch pyrolysis system at varying temperatures between 400 and 700 °C to study the effect of temperature on product yield. The fatty acids in the bio-oil produced displayed a high degree of saturation that caused the bio-oil to have poor cold flow properties (high cloud point and viscosity) so a solvent extraction scheme was devised to extract a bio-oil fraction rich in unsaturated fatty acids that could be further esterified into a bio-diesel and fatty nitriles that could be further processed into surfactants. This ethyl acetate-soluble fraction demonstrated much improved cold flow properties as well as lower water content and a higher HHV. The esterification of this soluble fraction was performed using methanol and sulfuric acid as an acid catalyst and the formation of fatty acid methyl esters was verified using GC/MS and FT-IR.     

TG-FTIR Study on Corn Straw Pyrolysis-influence of Minerals

In order to study the effect of minerals on biomass pyrolysis, experiments on pyrolysis of corn straw with different pretreatment methods were performed by using a thermogravimetric analyzer（TGA） coupled with a Fourier transform infrared （FTIR） spectrometer. The pretreatment methods included water washing and acid washing. The experimental results show that acid washing can remove almost all K^＋ and 78% of Ca^2＋ , while water washing only removes most of K ^＋. The existence of K^＋ and Ca^2＋ obviously favors the formation of compounds containing carbonyl groups and CO2 , but it will decrease the yields of compounds containing C-O-C groups. However, the formation of H2O, CO and CH4 are slightly affected by the removal of inorganic ions. With regard to the structure of the metal ions-adsorbed cellulose characterized by IR analysis, it can be considered that there is an ＂ion force＂ between metal ions and cellulosic biomass. The results of thermal kinetic analysis show that this force can make the reaction activation energy of the biomass pyrolysis decrease. A new mechanism is proposed for explaining the effect of inorganic ions on cellulose pyrolysis.