Flash vacuum pyrolysis of N-phenylbenzamide oxime and related compounds

Flash vacuum pyrolysis (FVP) of the benzamide oxime 1 at 650 °C leads to the imino-oxadiazole 10 as the major product. It is probably formed by intermolecular cycloaddition of benzonitrile oxide 11 and the diphenylcarbodiimide 12, an unexpected process to take place under FVP conditions. Intermediates 11 and 12 are themselves obtained by competitive dehydration and elimination of aniline from 1. Mixed products were obtained from FVP of the C- and N-p-tolyl analogues of 1 (5 and 6, respectively) probably owing to equilibration of the carbodiimide intermediate.     

Flash vacuum pyrolysis of azolylacroleins and azolylbutadienes

2-Aryl-5-acroleinyl-1,2,3,4-tetrazoles (1a–d) and 2-aryl-5-butadienyl-1,2,3,4-tetrazoles (1e–g) were subjected to flash vacuum pyrolysis. Acroleinyl derivatives resulted in nitrogen extrusion to give nitrilimines followed by ring closure to give the corresponding indazoles 3a–d in good yields. On the other hand, butadiene derivatives underwent ring fragmentation to give p-substituted anilines without formation of the expected indazoles. Differences between thermal behaviour of 2-(4-chlorophenyl)-5-acroleinyl-1,2,3,4-tetrazole (1c) and 1-(4-chlorophenyl)-4-acroleinyl-1,2,3-triazole (2) were studied in details. DFT calculations have been used to examine the nitrilimine and carbene nature of the intermediates involved in the thermal reactions of azolyl derivatives.     

Isomer-dependent catalytic pyrolysis mechanism of the lignin model compounds catechol,resorcinol and hydroquinone

The catalytic pyrolysis mechanism of the initial lignin depolymerization products will help us develop biomass valorization strategies. How does isomerism influence reactivity, product formation, selectivities, and side reactions? By using imaging photoelectron photoion coincidence (iPEPICO) spectroscopy with synchrotron radiation, we reveal initial, short-lived reactive intermediates driving benzenediol catalytic pyrolysis over H-ZSM-5 catalyst. The detailed reaction mechanism unveils new pathways leading to the most important products and intermediates. Thanks to the two vicinal hydroxyl groups, catechol (o-benzenediol) is readily dehydrated to form fulvenone, a reactive ketene intermediate, and exhibits the highest reactivity. Fulvenone is hydrogenated on the catalyst surface to phenol or is decarbonylated to produce cyclopentadiene. Hydroquinone (p-benzenediol) mostly dehydrogenates to produce p-benzoquinone. Resorcinol, m-benzenediol, is the most stable isomer, because dehydration and dehydrogenation both involve biradicals owing to the meta position of the hydroxyl groups and are unfavorable. The three isomers may also interconvert in a minor reaction channel, which yields small amounts of cyclopentadiene and phenol via dehydroxylation and decarbonylation. We propose a generalized reaction mechanism for benzenediols in lignin catalytic pyrolysis and provide detailed mechanistic insights on how isomerism influences conversion and product formation. The mechanism accounts for processes ranging from decomposition reactions to molecular growth by initial polycyclic aromatic hydrocarbon (PAH) formation steps to yield, e.g., naphthalene. The latter involves a Diels–Alder dimerization of cyclopentadiene, isomerization, and dehydrogenation.

Detection of reactive intermediates with synchrotron radiation and photoelectron photoion coincidence methods reveals new mechanistic insights into lignin catalytic pyrolysis. Here we focus on how the isomerism changes the conversion and product formation.     

Flash pyrolysis of Silopi asphaltite in a free-fall reactor under vacuum

Flash pyrolysis experiments on asphaltite samples were performed in a free-fall reactor under vacuum to determine the effects of pyrolysis temperature, feed rate and particle size. Maximum liquid yield of 13 wt.% was obtained in free-fall reactor under vacuum when the pyrolysis temperature was 700 °C, feed rate was 0.4 g min⁻¹ and particle sizes were between 0.075 and 0.250 mm. The liquid products obtained at various pyrolysis conditions were analyzed by gas chromatography/mass spectrometer (GC/MS) and liquid products were classified as following: C5–C10, C11–C15, C16–C20 and C20+. The amount of saturated hydrocarbons decreased while the amount of unsaturated hydrocarbons increased with increase of temperature. While percent of C5–C10 unsaturated hydrocarbons continuously increased with increase of temperature, the percent of C11–C15 unsaturated hydrocarbons increased up to 750 °C and then started to diminish. Functional group analysis of solid residue was carried out using Fourier transform infrared spectrometry (FT-IR). The proximate analysis of solid residue indicated that percent of fixed carbon and ash increased with temperature.     

Flash pyrolysis of polystyrene wastes in a free-fall reactor under vacuum

Plastic waste minimization and recycling are important for both economical and environmental reasons. In this flash pyrolysis study, polystyrene wastes were degraded in a free-fall reactor under vacuum to regain the monomer. A set of experiments varied the temperature between 700 and 875°C and determined its effects on the phase yields, the benzene, styrene, toluene, and naphthalene distribution of the liquid output and C1–C4 content of the gaseous output. The liquid yield maximized around 750°C and the styrene yield at 825°C. In general, operating at higher temperatures lessened the solid residue and increased the gaseous yield and total conversion. Employing waste particles in four size ranges, a second set of runs indicated that the finer the waste particles fed the higher the gaseous yield and total conversion. This recycling method can be made more promising if the feed particles are allowed more time for degradation and the removal of the primary products speeded up thereby preventing their decomposition. Ways are suggested to obviate these residence time problems.     

闪式真空热解(FVP)在有机合成中的应用

闪式真空热裂解(Flash vacuum pyrolysis,FVP)是一种反应底物在真空条件下蒸发或者升华后迅速通过较高温度的热管道发生热解反应的过程.该热裂解方法经常被运用于合成一些重要的非平面型芳香化合物,比如著名的心环烯C20H10(Corannulene),富勒烯C60等.主要针对FVP方法的发展历史、装置的基本构成、反应的基本历程以及该方法在有机合成中的实际应用等方面进行了系统的综述.相对于传统有机合成化学方法,FVP方法的优势在于可以提供更高的外界能量来帮助产物化学键的形成和更快速的冷却方式来帮助稳定反应所得到的产物,因此该方法不仅能高效、方便地合成得到一些常规有机合成方法不能轻易获得的目标化合物,还可以获得一些热力学极其不稳定的产物.当然,FVP方法也有其限制,比如对于一些在真空条件下难以挥发的化合物FVP方法就不适用了,另外,因为所有FVP反应都是在气相条件下完成,所以该方法主要适用于分子内的消除或环合反应,对于有机合成中普遍存在的双分子反应以及多分子反应也难以通过FVP方法来实现,但作为一类独特、实用的有机合成方法,FVP在有机合成中得到了较广的应用和不断地发展.     

Flash flow pyrolysis: mimicking flash vacuum pyrolysis in a high-temperature/high-pressure liquid-phase microreactor environment

Flash vacuum pyrolysis (FVP) is a gas-phase continuous-flow technique where a substrate is sublimed through a hot quartz tube under high vacuum at temperatures of 400-1100 °C. Thermal activation occurs mainly by molecule-wall collisions with contact times in the region of milliseconds. As a preparative method, FVP is used mainly to induce intramolecular high-temperature transformations leading to products that cannot easily be obtained by other methods. It is demonstrated herein that liquid-phase high-temperature/high-pressure (high-T/p) microreactor conditions (160-350 °C, 90-180 bar) employing near- or supercritical fluids as reaction media can mimic the results obtained using preparative gas-phase FVP protocols. The high-T/p liquid-phase "flash flow pyrolysis" (FFP) technique was applied to the thermolysis of Meldrum's acid derivatives, pyrrole-2,3-diones, and pyrrole-2-carboxylic esters, producing the expected target heterocycles in high yields with residence times between 10 s and 10 min. The exact control over flow rate (and thus residence time) using the liquid-phase FFP method allows a tuning of reaction selectivities not easily achievable using FVP. Since the solution-phase FFP method does not require the substrate to be volatile any more--a major limitation in classical FVP--the transformations become readily scalable, allowing higher productivities and space-time yields compared with gas-phase protocols. Differential scanning calorimetry measurements and extensive DFT calculations provided essential information on pyrolysis energy barriers and the involved reaction mechanisms. A correlation between computed activation energies and experimental gas-phase FVP (molecule-wall collisions) and liquid-phase FFP (molecule-molecule collisions) pyrolysis temperatures was derived.     

Flash vacuum pyrolysis over solid catalysts. 2. pyrazoles over hydrotalcites

Flash vacuum pyrolysis (fvp) reactions of NH-pyrazole (1) and 3,5-diphenylpyrazole (2) were investigated in the presence of anionic clays having hydrotalcite structure (HT). Solid catalysts with Mg:Al ratio equal to 2:1 containing carbonate (HT-1), nitrate (HT-2), and silicate (HT-3) as interlayer anions were employed. Between 400 and 600 degrees C, compound 1 remained almost unchanged and only unidentified volatile products were detected in small amounts. In contrast, 2 afforded benzonitrile (3) and phenylacetonitrile (4) by a ring fragmentation reaction at 450 degrees C. At a higher temperature (660 degrees C), the same products obtained in homogeneous fvp reactions, i.e., 2-phenylindene (5) and 3-phenylindene (6), were obtained showing no catalysis by HT under these conditions. Results showed that the yield is strongly dependent on the nature of the interlayer anion in the hydrotalcite structure. In comparison with reactions of 2 over zeolites, HTs exhibit selectivity for ring fragmentation reaction.     

Flash vacuum pyrolysis over solid catalysts. 1. Pyrazoles over zeolites

Flash vacuum pyrolysis (fvp) reactions of 1H-pyrazole (1), 3,5-dimethylpyrazole (2), and 3,5-diphenylpyrazole (3) were carried out over zeolites. Reactions were performed using ZCOY-7, NH(4)-Y, and Na-Y zeolites. Reaction temperatures of heterogeneous reactions were lower than the corresponding temperatures in the homogeneous system, showing a catalytic effect of the zeolites. Compounds 1-3 afforded nitrogen extrusion in homogeneous fvp reactions while in the heterogeneous ones different reactions were present. Compounds 1 and 2 also afforded nitrogen extrusion; products arising from ring fragmentation were found in reactions of 2 and 3 while an isomeric imidazole was isolated in reactions of 3. Isomerization of 3 is attributed to a transition-state selectivity by the catalyst due to the relation between the size of the molecule and the cavity of the zeolite. This isomerization reaction was present only when zeolites with active Br?nsted sites were used.     

Flash vacuum pyrolysis of 1-propynoylpyrazoles: a new precursor of tricarbon monoxide

Pyrolysis of 3,5-dimethyl-1-propynoylpyrazole (1) at 640°C/0.1 torr gives 2-methyl-1$\text{H}$-pyrazolo[2,3-a]pyridin-5-one (3) with inversion of the propynoyl chain. 1-Ethynylpyrazole and tricarbon monoxide have been identified in pyrolysates formed at 700–1000°C/0.01–0.1 torr from the parent 1-propynoylpyrazole (4).     

Thermal analysis of some lignin model compounds

Compounds acting as model biphenyl and phenylcoumarane structural units in the lignin molecule (dehydrodivanillin and dehydrodiisoeugenol) were investigated by thermal analysis. The results were compared with previously obtained data on the thermal degradation of model phenylpropane monomer units of lignin. It was found that the mechanism of thermal degradation of these models and the thermal stabilities of the bonds depend on the structure. The thermal stability of the models increases in the absence of reactive functional groups in the side chain.

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Zusammenfassung Modellverbindungen mit Biphenyl und Phenylcumaran Bindungen im Lignin wie Dehydrodivanillin und Dehydrodiisoeugenol wurden einer thermoanalytischen Prüfung unterworfen und die Ergebnisse mit jenen der thermischen Zersetzung von Phenylpropan-monomeren Modellen des Lignins verglichen. Es wurde festgestellt, daß der Mechanismus der thermischen Zersetzung und die thermische Stabilität der Bindungen strukturabhängig sind. Die thermische Stabilität der Modelle nahm mit Abwesenheit reaktionsfähiger funktioneller Gruppen an der Seitenkette ab. Die Bindung vom Typ des Phenylcumarans ist thermisch am stabilsten. Das Zerreissen der Bindungen scheint durch einen Mechanismus durch freie Radikale zu erfolgen.

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Résumé Étude thermique de composés modèles pour les liaisons structurales de type diphényle et phénylcoumaranne dans la molécule de lignine (déhydrodivanilline et déhydrodiisoeugénol). Comparaison des résultats avec ceux obtenus précédemment dans le cas de la dégradation thermique de composés servant de modèles pour les unités monomères de phénylpropane dans la lignine. Détermination du mécanisme de la dégradation thermique de ces modèles et de la stabilité thermique des liaisons suivant la structure. La stabilité thermique augmente en l'absence de groupes fonctionnels réactifs dans la chaîne latérale. La liaison de type phénylcoumaranne est thermiquement la plus stable. On suppose un mécanisme avec radical libre pour la rupture des liaisons structurales.

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Flash vacuum pyrolysis (F.V.P.) of 1,2,4-benzotriazine derivatives

Flash vacuum pyrolysis of 3-methylsulfanyl-1,2,4-benzotriazine N-oxide, 3-methylsulfanyl-1,2,4-benzotriazine, and 3-phenyl-1,2,4-benzotriazine are described. The N-oxide derivative underwent deoxygenation between 500 and 600°C, whereas at higher temperatures both methylsulfanyl compounds, besides yielding the same products, also gave benzimidazole formed by an independent mechanism. Transformation of these derivatives between 600 and 750°C led to formation of a complex reaction mixture indicating the radical nature of the processes. The phenyl substituted derivative was studied between 575 and 650°C and afforded benzonitrile and traces of biphenylene.     

木质素二聚体模型化合物热解机理的量子化学研究

β-O-4连接是木质素主体结构单元之间的主要联结方式。采用密度泛函理论方法B3LYP,在6-31G (d, p)基组水平上,对β-O-4型木质素二聚体模型化合物(1-愈创木基-2-(2-甲氧基苯氧基)-1,3丙二醇)的热解反应机理进行了研究。提出了三种热解反应途径:C_β-O键均裂的后续反应、C_α-C_β键均裂的后续反应以及协同反应。对各种反应的反应物、产物、中间体和过渡态的结构进行了能量梯度全优化,计算了各热解反应途径的标准动力学参数。分析了各种主要热解产物的形成演化机理以及热解过程中温度对热解机理的影响。计算结果表明,C_β-O键的均裂反应和协同反应路径(1)和(3)是木质素二聚体热解过程中主要的反应路径,而C_α-C_β键的均裂反应和协同反应路径(2)和(5)是主要的竞争反应路径;热解的主要产物是酚类化合物如愈创木酚、1-愈创木基-3-羟基丙酮、3-愈创木基-3-羟基丙醛、愈创木基甲醛和乙烯等。在热解低温阶段协同反应是热解过程中的主要反应形式,而在高温阶段自由基均裂反应是热解过程的主要反应形式。     

Flash vacuum pyrolysis of azo and nitrosophenols: new routes towards hydroxyarylnitrenes and their reactions

Flash vacuum pyrolysis of phenylazonaphthols and nitrosonaphthols at 700°C and 0.02 Torr yielded quinoline, isoquinoline, indene and naphthols (and aniline only from the phenylazo derivatives). Similar FVP of p-nitroso and p-phenylazophenol gave pyridine. Also, FVP of phenanthraquinonemonophenylhydrazone and monooxime gave phenathridine and fluorenone. The formation of the heterocyclic system was assumed to involve nitrene and azatropone intermediates.     

木质素降解模型化合物愈创木酚及苯酚原位加氢制备环己醇

以Raney Ni为催化剂,研究了甲醇水相重整制氢与木质素降解模型化合物愈创木酚／苯酚加氢的耦合反应.考察了反应前冷压、反应温度、反应时间、物料配比等条件对木质素降解模型化合物原位加氢反应性能的影响,并对影响机制进行了讨论.结果表明,在反应温度为220 ℃、反应前冷压0 MPa(表压)、物料比水／甲醇／模型化合物为20∶5∶0.8的条件下,反应7 h后愈创木酚转化率与环己醇选择性分别达99.00％和93.74％,反应12 h后苯酚的转化率与环己醇选择性分别达90.50％和99.29％.采用原位加氢反应,木质素降解的酚类模型化合物转化率和选择性明显优于外部供氢反应的转化率和选择性,同时,避免了外部供氢反应存在的氢气制备、储存、传输及加氢条件苛刻等问题,为木质素解聚产物制备化工品提供了新思路与实验基础.     

Flash vacuum thermolysis of spirocyclohexadienones

In an attempt to prepare short-bridged hydroxymetacyclophanes $\text{1b-d}$, the spirocyclohexadienones $\text{2b-d}$ were pyrolyzed by flash vacuum thermolysis (FVT). Instead of $\text{1b-d}$, variable amounts of 4-(5-hexenyl)phenol ($\text{4b}$), β-hydroxybenzocycloalkenes ($\text{5b-d}$) and 4-(trans-1-alkenyl) phenols ($\text{6c-d}$) were obtained. The formation of these products is explained by invoking cleavage of a spiro bond in $\text{2}$ under formation of the intermediate diradical $\text{3}$ which, depending on the length of the aliphatic chain and on the temperature, has several pathways open for isomerization to spin-paired products.     

Thermophysical properties of model compounds of the lignin structural unit

Russian Chemical Bulletin - Sixteen lignin related aromatic compounds formed by the thermal degradation of lignin and being potentially valuable products of the processing of renewable raw...     

Flash vacuum pyrolysis (fvp) of some hexahydroquinazolin-4(1H)-ones

Some cis/trans-2-thioquinazolin-4-ones and their 2,4-dione analogs were subjected to flash vacuum pyrolysis. The cis- and trans-thio compounds reacted at lower temperatures than the cis- and trans-dioxo analogs, showing a lower thermal stability. All of these compounds afforded similar reactions: ring opening to the corresponding iso(thio)cyanate, the loss of H and NCS to form three isomeric cyclohexadienes and then aromatization to form the corresponding benzamide. The cis-dioxo compound also underwent a competitive retro Diels-Alder (RDA) reaction to form 3-phenylpyrimidine-2,4(1H,3H-dione(3-phenyluracil)) and butadiene. Kinetic measurements of the ring opening reaction supported a concerted β-elimination as the most probable mechanism.