1-己烯热裂解基元反应选择性分析

1-己烯在873～973 K温度区间热裂解转化反应经历自由基链式反应机理, 主要产物包括C₁~C₃烷烃、C₂~C₄烯烃、丁二烯、环戊烷、环戊烯、焦炭和氢气. 各种产物的生成涉及不同的基元反应途径, 并具有不同的基元反应或然率(Reaction path probability, RPP). 提高反应温度有利于反应物分子的热均裂(链引发)和焦炭生成反应. 随着反应温度从873 K提高到973 K, 标志链式反应特征的动力学链长(KCL)从5.86缩短为5.19.     

挂式-四氢双环戊二烯热裂解产物分布研究

研究了常压下, 温度为450～650 ℃, 停留时间为15.44～4.03 s条件下的JP-10 (挂式-四氢双环戊二烯)热裂解产物分布. 在相对较长的停留时间下, JP-10热裂解转化率对温度很敏感. 热裂解主要产物为甲烷、乙烯、丙烯、丁烯、丁二烯、环戊二烯、苯、甲苯、苯乙烯、环戊烯的乙烯基取代物及C5, C6异构体(戊二烯、环戊烯的甲基取代物、环己烯、己二烯和环戊二烯甲基取代物), 其中, C1～C4、环戊二烯、苯、甲苯、乙烯基取代环戊烯为初始产物.     

乙烯在煤焦及石英砂床层上裂解实验研究

以石油炼制过程中产生的炼厂气与煤共转化利用为背景,采用小型石英管固定床反应装置,在850℃～1000℃下,对乙烯在空床、彬县煤焦以及石英砂床层上的裂解反应进行了研究。结果表明,乙烯裂解产物包括氢气、甲烷、乙烷及裂解炭,反应温度越高,裂解越彻底,生成的氢气越多;850℃～950℃时,乙烯在彬县焦上初始转化率最高,随着反应的进行逐渐降低到一个较低的平衡值,并且与在石英砂上裂解结果接近。这说明新鲜彬县煤焦对乙烯裂解呈现良好的催化作用,但随着反应进行其催化活性由于裂解生成的炭沉积在煤焦表面而逐渐丧失。1000℃时乙烯在石英砂上和空床裂解转化率均可达到94％,即在此温度下乙烯无需催化剂通过热作用即可接近完全裂解。     

吸热型碳氢燃料正癸烷热裂解机理、热沉及产物分布的理论研究

采用密度泛函理论(DFT)的B3LYP方法在6-311G(d,p)基组水平上对正癸烷裂解过程中涉及的反应物、产物及过渡态进行了几何构型优化和振动频率计算,运用B3LYP/aug-cc-pVTZ方法计算单点能并构建势能剖面图。利用TheRate程序包及Eckart校正模型计算了各反应速率常数k。采用统计热力学原理求得不同温度下的热容C_p,m^θ及熵S_{298 K}^θ,并通过设计等键反应获得了各物种的标准生成焓△_fH_{298 K}^θ。用Chemkin II程序模拟预测了产物分布,理论计算了热沉值,并讨论了温度、压力对产物分布和热沉的影响。结果表明,C-C键断裂过程是反应的初始步骤,且抽氢反应较β键断裂反应更易进行。裂解起始温度为500 ℃,反应主要发生在600~700 ℃,其主要产物为氢气、甲烷、乙烯、乙烷、丙烯和1,3-丁二烯,且产物分布随温度不同而变化。模拟计算获得正癸烷在温度600 ℃、压力2.5 MPa条件下的总热沉值为2.334 MJ/kg,对应的热裂解转化率为25.9%,该热沉值可以满足速率为5~6马赫数的飞行器的冷却要求。     

不同质量流量下正癸烷的裂解换热特性

碳氢燃料裂解吸热反应是超燃冲压发动机实现主动冷却所利用的重要手段。为探究不同燃油流量下冷却通道中的裂解换热特性,本文通过设置不同质量流量,不同出口温度探究了正癸烷的热沉、转化率、壁面温度以及结焦情况,确定了正癸烷在不同质量流量下的裂解换热特性。结果表明正癸烷在高温时化学反应对停留时间的影响减弱,停留时间对化学反应的影响得到增强。正癸烷的气相产物分布随着质量流量变化而变化,烷烃含量随质量流量增大而减少,烯烃含量随质量流量增大而增加。发现了低流量高出口温度情况下正癸烷的裂解强化换热现象,发生裂解强化换热时壁温会显著下降。同时通过裂解残液的色谱分析,得到了结焦反应迅速增强的温度与流量。     

正辛烷热裂化和催化裂化生成甲烷反应机理

采用脉冲微反装置,在反应温度为550~650℃,低转化率(小于15%)下,研究了正辛烷在石英砂和ZRP分子筛上的热裂化和催化裂化反应,分析了甲烷的生成机理。结果表明,正辛烷热裂化时,乙烯、丙烯和正丁烯是初始产物,甲烷由4种反应路径生成。当反应温度为600℃时,甲基自由基攻击碳链端部C-H键生成甲烷。中部C-H键脱氢形成的辛基自由基在端部C-C键断裂的活化能较高,仅在高温下生成甲烷。正辛烷在ZRP分子筛上主要发生质子化裂化反应,正构烷烃占有相当比重,甲烷由质子化裂化步骤生成。热裂化与质子化裂化对甲烷贡献的对比可知,当反应温度低于600℃时,甲烷由质子化裂化反应生成;在高温下,热裂化反应决定甲烷选择性。     

水蒸气存在时Mo/HZSM-5催化剂上的甲烷芳构化反应性能

研究了水蒸气存在条件下Mo/HZSM-5沸石分子筛催化剂上的甲烷芳构化反应行为,发现水蒸气的引入可以明显地降低甲烷芳构化反应的起始温度,从而在较为温和的条件下实现甲烷的活化.适量水蒸气的加入可以在一定程度上改善Mo/HZSM-5催化剂的稳定性,过量水蒸气的引入则会抑制甲烷芳构化反应.在反应温度为973 K时,引入适量的水蒸气对芳构化反应产物的分布没有明显影响在低温条件下的甲烷芳构化反应过程中检测到有乙烯生成,该结果支持了甲烷芳构化反应可能经历了乙烯这一中间产物的机理.实验结果还表明,水蒸气对催化剂上的积炭量没有明显的影响.     

正癸烷热解的小规模化学动力学机理模型

正癸烷是目前常用的吸热型燃料的替代组分, 但是其热解机理的研究迄今还很少, 且现有的少数几个机理由于规模庞大使用不便. 本文首先构建了一个包含33种组分和75个基元反应的正癸烷热解动力学机理模型(Mech33); 随后, 在该机理的基础上进一步通过灵敏度分析得到影响主要热裂解组分生成的速率控制步, 并采用局部平衡和稳态假设对Mech33机理简化得到了规模更小的、仅包含22种组分和59步反应动力学机理模型(Mech22). 在较宽的温度和压力范围内对流动反应器及激波管中正癸烷热解过程进行了数值模拟, 并与实验数据进行了对比, 结果表明, Mech33和Mech22两个动力学机理模型都能够很好地描述正癸烷热裂解过程,并准确预测主要热裂解产物的浓度分布, 为进一步实现化学反应与计算流体力学(CFD)耦合的工程计算提供了有价值的动力学机理模型.     

加拿大合成原油瓦斯油裂解反应规律与产物生成机理

利用小型固定流化床实验装置研究了加拿大合成原油重瓦斯油(HGO)和轻瓦斯油(LGO)的催化裂解性能和热裂解性能。HGO和LGO催化裂解总低碳烯烃(乙烯+丙烯+丁烯)产率在660℃附近达到最大值,分别为33.8%和35.6%。HGO和LGO热裂解反应程度很大,700℃的转化率分别为66.7%和76.3%。HGO热裂解总低碳烯烃的产率在680℃达到最大值27.9%。通过对比分析催化裂解与热裂解气体产物产率的比值发现,催化剂的加入促进了乙烯和液化气的生成,同时抑制了甲烷和乙烷的生成。研究结果揭示了小分子烃类的生成机理,甲烷和乙烷主要是自由基反应的产物,乙烯和液化气是自由基反应和正碳离子反应的共同产物。     

ZSM-5上甲醇制烯烃反应中低碳烯烃的成因

考察和比较了573—823K范围内HZSM-5上甲醇、C_2—C_(12)直链烯烃(除C_7~=为庚烯-2外,其它均为α-烯烃),以及C_6—C_(12)正构烷烃的转化产物分布,用TPSR技术追踪了甲醇转化反应的历程。结果表明甲醇转化过程中较长链烯烃(或其前体)的裂解对最终产物分布有很大影响。高于723K时,低碳烯烃,特别是乙烯,主要源于这些较长链烯烃的次级裂解。且温度愈高,愈有利于生成乙烯。甲烷含量在623K附近有一极大值。证实甲烷为甲醇转化的第一个烃类产物,极化了的表面(+)CH_3是产生初始C-C键的活性中间物种。讨论了甲醇转化的机理。     

正癸烷与二甲苯在超临界压力下的热裂解

采用连续流动装置对正癸烷和二甲苯在超临界压力下的热裂解对比研究. 用气相色谱和色质联用仪对其气相产物和液相产物进行分析, 计算气相产物产率和裂解转化率, 并运用计算化学方法获得正癸烷和二甲苯不同化学键的键能, 从实验和理论上分析其裂解反应的难易程度和裂解规律. 实验结果表明, 在4 MPa和650、700、750 ℃条件下, 正癸烷比二甲苯更容易裂解, 正癸烷裂解产物以C₁-C₃小分子的烃类和氢气为主, 而二甲苯裂解产物主要为乙苯、甲苯和其它芳香类化合物; 键能计算结果表明, 正癸烷碳链骨架的C－C键能和C－H键能均较小, 裂解反应的诱发步骤应该是C－C键断裂, 而二甲苯苯环上C－C和C－H键能均较大, 裂解诱发步骤应该是侧链甲基脱氢反应. 因此正癸烷裂解反应以C－C键断裂和脱氢反应为主, 二甲苯裂解主要发生侧链甲基C－C键断裂和脱氢反应, 而芳环则比较稳定, 理论计算键能分析与裂解实验结果一致.     

Effects of fuel additives on the thermal cracking of n-decane from reactive molecular dynamics

Thermal cracking of n-decane and n-decane in the presence of several fuel additives are studied in order to improve the rate of thermal cracking by using reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field. From MD simulations, we find the initiation mechanisms of pyrolysis of n-decane are mainly through two pathways: (1) the cleavage of a C-C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction to form an H radical and the corresponding decyl radical. Another pathway is the H-abstraction reactions by small radicals including H, CH(3), and C(2)H(5). The basic reaction mechanisms are in good agreement with existing chemical kinetic models of thermal decomposition of n-decane. Quantum mechanical calculations of reaction enthalpies demonstrate that the H-abstraction channel is easier compared with the direct C-C or C-H bond-breaking in n-decane. The thermal cracking of n-decane with several additives is further investigated. ReaxFF MD simulations lead to reasonable Arrhenius parameters compared with experimental results based on first-order kinetic analysis. The different chemical structures of the fuel additives greatly affect the apparent activation energy and pre-exponential factors. The presence of diethyl ether (DEE), methyl tert-butyl ether (MTBE), 1-nitropropane (NP), 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexaoxonane (TEMPO), triethylamine (TEA), and diacetonediperodixe (DADP) exhibit remarkable promoting effect on the thermal cracking rates, compared with that of pure n-decane, in the following order: NP > TEMPO > DADP > DEE (～MTBE) > TEA, which coincides with experimental results. These results demonstrate that reactive MD simulations can be used to screen for fuel additives and provide useful information for more comprehensive chemical kinetic model studies at the molecular level.     

Tricritical phenomena in quasi-ternary mixtures of water + n-decane + n-undecane + tert-butanol

Coexistence curves for the quasi-ternary system of water + n-decane + n-undecane + tert-butanol have been determined by measurements of the refractive index in three coexisting liquid phases. The binary mixtures of n-decane + n-undecane constructed the quasi-pure components in which the mass fraction beta of n-decane controls the approach to the tricritical point. The coexistence curves can be fitted to Scott's extended theory and can be extrapolated to a tricritical point at (44.1 +/- 0.3) degrees C and beta = 0.77 +/- 0.02 corresponding to an average n-alkane-carbon number of 10.19 +/- 0.02. The nonclassical critical amplitude ratio has been confirmed to be 4-5% smaller than the classical value, which is consistent with Fisher's prediction.     

Surface properties of surfactant-free oil droplets dispersed in water studied by confocal fluorescence microscopy

The fluorescence spectrum of dye molecules, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyron (DCM), dissolved in surfactant-free n-decane droplets (average diameters of approximately 300 and approximately 2000 nm) dispersed in water was measured by a confocal microscope. The fluorescence spectra for 300- and 2000-nm droplets are found to exhibit a peak at 640 and 625 nm, respectively, and the peak red shifts with a decrease in the droplet diameter (solvatochromic shift of DCM molecules). It is concluded that (1) DCM molecules are located in a polar surface region of n-decane droplets and (2) the polarity increases with decreasing the droplet diameter.     

Temperature dependence of particle-particle adherence forces in ice and clathrate hydrates

Particle-particle pulloff adherence forces were measured as a function of temperature in the ice/n-decane/ice and tetrahydrofuran (THF) hydrate/n-decane/THF hydrate systems using a newly developed micromechanical testing technique. Experiments using approximately 200 microm radius particles were performed at atmospheric pressure over the temperature range 263-275 K. The ice and hydrate particles displayed very similar behavior. While the measured adherence forces had significant variation, the shapes of the cumulative force distribution curves were similar among the different sets of experiments. The measured adherence forces distributions shifted to lower force values as the temperature was decreased from the solid melting temperature. The observed forces and trends were explained by the capillary cohesion of rough surfaces, with the capillary bridging liquid being stabilized below its freezing point by the negative curvature of the bridging liquid/n-decane interface.     

Formation of uniform ferrocenyl-terminated monolayer covalently bonded to Si using reaction of hydrogen-terminated Si(111) surface with vinylferrocene/n-decane solution by visible-light excitation

Electrochemically active self-assembled monolayers (SAM) have been successfully fabricated with atomic-scale uniformity on a silicon (Si)(111) surface by immobilizing vinylferrocene (VFC) molecules through Si-C covalent bonds. The reaction of VFC with the hydrogen-terminated Si (H-Si)(111) surface was photochemically promoted by irradiation of visible light on a H-Si(111) substrate immersed in n-decane solution of VFC. We found that aggregation and polymerization of VFC was avoided when n-decane was used as a solvent. Voltammetric quantification revealed that the surface density of ferrocenyl groups was 1.4×10(-10) mol cm(-2), i.e., 11% in substitution rate of Si-H bond. VFC-SAMs were then formed by the optimized preparation method on n-type and p-type Si wafers. VFC-SAM on n-type Si showed positive photo-responsivity, while VFC-SAM on p-type Si showed negative photo-responsivity.     

Thermal diffusion and molecular diffusion values for some alkane mixtures: a comparison between thermogravitational column and thermal diffusion forced rayleigh scattering

In the present work we studied the thermal diffusion behavior of n-decane in various alkanes by thermogravitational column (TC) technique and the thermal diffusion forced Rayleigh scattering (TDFRS) method. The investigated lighter alkanes compared to n-decane are n-pentane, n-hexane, n-heptane, n-octane, and the heavier ones are n-tetradecane, n-pentadecane, n-hexadecane, n-heptadecane, n-octadecane, and n-eicosane. The binary mixture n-decane/ n-pentane we investigated at several different concentrations; all other mixtures were only investigated at a mass fraction of 50%. Even for the volatile n-pentane/ n-decane mixture the deviations between the thermal diffusion coefficients determined by the different methods agreed within the error bars. Typically the agreement between the two methods was in the order of 5%. In comparison to recently published TC and TDFRS data we found deviations in the order of 30% up to 40%. We analyze and discuss the possible reasons for the discrepancies for the present and the past publications.     

Kinetics study of OH radical reactions with n-octane, n-nonane, and n-decane at 240-340 K using the relative rate/discharge flow/mass spectrometry technique

The kinetics of the reactions of hydroxyl radical with n-octane (k1), n-nonane (k2), and n-decane (k3) at 240-340 K and a total pressure of approximately 1 Torr has been studied using relative rate combined with discharge flow and mass spectrometer (RR/DF/MS) technique. The rate constant for these reactions was found to be positively dependent on temperature, with an Arrhenius expression of k1 = (2.27 +/- 0.21) x 10(-11)exp[(-296 +/- 27)/T], k2 = (4.35 +/- 0.49) x 10(-11)exp[(-411 +/- 32)/T], and k3 = (2.26 +/- 0.28) x 10(-11)exp[(-160 +/- 36)/T] cm3 molecule(-1) s(-1) (uncertainties taken as 2sigma), respectively. Our results are in good agreement with previous studies at and above room temperature using different techniques. Assuming that the reaction of alkane with hydroxyl radical is the predominant form for loss of these alkanes in the troposphere, the atmospheric lifetime for n-octane, n-nonane, and n-decane is estimated to be about 43, 35, and 28 h, respectively.