Osmium(VIII) catalyzed oxidation of a sulfur containing amino acid—a kinetics and mechanistic approach

The kinetics of osmium (VIII) catalyzed oxidation of DL-methionine by hexacyanoferrate(III) (HCF) in aqueous alkaline medium at a constant ionic strength of 0.50 mol dm^?3 was studied spectrophoto-metrically. The reaction between hexacyanoferrate(III) and DL-methionine in alkaline medium exhibits 2:1 stoichiometry (2HCF:DL-methionine). The reaction is of first order each in [HCF] and [Os(VIII)], less than unit order in [alkali] and zero order for [DL-methionine]. The decrease in dielectric constant of the medium increases the rate of the reaction. The added products have no effect on the rate of reaction. The main products were identified by spot test. A free radical mechanism has been proposed. In a prior equilibrium step Os(VIII) binds to OH^? species to form a hydroxide species and reacts with [Fe(CN)₆]^3? in slow step to form an intermediate species(C₁). This reacts with a molecule of DL-methionine in a fast step to give the sulfur radical cation of methionine and yields the sulfoxide product by reacting with another molecule of [Fe(CN)₆]^3?. The rate constant of the slow step of the mechanism is calculated. The activation parameters with respect to slow step of the mechanism are evaluated and discussed.     

Kinetics of hydrolysis,acetylation, and deamination reactions on polyamide fibers in heterogeneous medium

The kinetics of reactions, namely, hydrolysis, acetylation, and deamination, on polyamide fibers has been studied at different temperatures. Rate constants and apparent activation energies of these reactions were determined. All the three reactions studied show two rates, the initial fast rate, followed by the slow one. The existence of two rates has been explained on the basis of two-phase structure of the polyamide fibers, the fast rate corresponding to the weakly hydrogen-bonded regions and the slow rate corresponding to the regions which are strongly hydrogen bonded in the regular fashion. For hydrolysis, the fast rate was 38 times faster than the slow one, while the fast rate of acetylation reaction was about 162 times that of the slow rate. The ratio between the fast and slow rates was constant at all the temperatures studied in case of the hydrolysis and acetylation reactions. This ratio, however, was found to be extremely temperature sensitive for the deamination reaction. The hydrolysis and acetylation reactions were found to be strongly temperature dependent leading to the increase in the extent of modification with increase in temperature. Deamination, on the other hand, showed negative temperature dependence giving progressive decrease in the modification with increase in temperature. All the three modifications studied observed first-order kinetics. The apparent activation energy for hydrolysis was 1.38 and 1.05 kcal/mole corresponding to the fast and slow rates, respectively. The apparent activation energy values for acetylation were 2.53 and 3.29 kcal/mole, while those for deamination reaction were ?8.28 and ?46.0 kcal/mole, respectively. The apparent activation energy values for the deamination reaction bore negative signs possibly because of the negative temperature dependence of this reaction. The apparent activation energy for slow reactions in case of acetylation and deamination was found to be higher than that of the fast rate, while the reverse was true in case of hydrolysis, showing that the acetylation and deamination reactions, being the chemical modification of polyamide fibers at the ? NH₂ groups, proceed by altogether different mechanisms than the hydrolytic breakdown of the polyamide linkage.     

微通道内催化表面气泡生长及脱离的可视化实验

采用聚二甲基硅氧烷材料(PDMS)制备矩形截面的微通道,并在微通道壁面上沉积MnO₂作为催化剂,采用高速摄影仪对通流过程中过氧化氢催化分解生成氧气气泡的过程进行了可视化实验研究,分析了反应物的浓度和流量对气泡生长速度及脱离直径的影响.结果表明:气泡在微通道内催化表面的生长及脱离过程呈周期性变化的趋势;气泡生长可以分为快速生长和缓慢生长两个阶段,当t<3 s时气泡处于快速生长阶段,催化反应主要受动力学控制,当t≥3 s时扩散控制占主要地位,气泡生长速度随反应物浓度的升高而增大;气泡脱离直径受反应物浓度影响较小,受反应物流量影响较大,而且随液相反应物Reynolds数的增大线性降低.     

一种新型氢化反应搅拌器的改进

在化工生产过程中,氢化反应是一种较为复杂的单元过程,促进氢化反应顺利进行的因素有温度、压力、介质、酸碱度、催化剂以及物料的接触状态。从生产实际出发,剖析了氢化反应进程中固、液、气3种状态的反应物料,因搅拌能力不足致使反应速度缓慢的原因,并根据问题所在,有针对性地改变搅拌器型式,采用新型设计的组合型多层搅拌器可以使固、液、气三相能充分混合,增加了参与反应物料的接触面积,提高了气、液反应能力和催化剂的催化作用,达到加快生产反应速度的目的,缩短了反应时间,提高了产品收率。     

Effect of the rate of pulverized coal preheating on char reactivity

The effect of the preheating rate of the char particles from Kuznetsk bituminous coal on their specific reaction rate was determined. The experiments were carried out with char samples with mean diameters of 0.15 and 0.35 mm heated to temperatures of 900–1000°C. It was found that the specific surface area of the initial coal, measured by CO₂ adsorption at 273 K, for rapid and slow heat treatments increased more that two times. According to the results of thermogravimetric analysis, it was established that rapid pyrolysis in a muffle furnace led to the production of char whose maximum specific reaction rate was higher than that of the samples after slow pyrolysis.     

毕赤酵母提取液还原制备银纳米颗粒

采用毕赤酵母的提取液与银氨溶液反应制备银纳米颗粒(AgNPs),并考察了pH值对AgNPs的影响.结果表明:酸性条件下,反应速率缓慢,且生成的AgNPs呈多分散性;在一定范围内(pH值小于12.50),碱性条件有利于提高反应速率,且随着pH值的提高,AgNPs粒径分布变窄;当pH值继续提高(pH值为12.80),AgNPs发生团聚沉淀.     

Phosphate and Silica Adsorption and Desorption from Soils

Phosphate adsorption and desorption in conjunction with silica desorption, were studied on soils of two field experiments. The soil samples were taken from control plots and plots that had received a total of 14 ton/ha superphosphate over a three-year period. Initially, silica desorption was increased by P added in the past as fertilizer and that added in equilibration solutions. Subsequently, after repeated washings, the rate of silica desorption was independent of the previous phosphate treatment. The adsorbed P was completely leached from the high P fertilized samples but not from those of the control. The slow reaction of phosphate with the soil was accompanied by a release of silica to the equilibrium solution. Most of the slow reaction product was not isotopically exchangeable in 48 hr.     

Kinetics of wet oxidation of black liquor over a Pt–Pd–Ce/alumina catalyst

The catalytic oxidation of industrial wastewater from paper and pulp mills has been investigated in a slurry reactor at a temperature range 433–463 K and at pressures from 1.5 to 2.2 MPa. Adding Ce on alumina support promotes the catalytic activity for oxidation of black liquor. Pt–Pd–Ce/alumina catalyst shows a promising activity for wet catalytic oxidation of black liquor. The oxidation reaction over a Pt–Pd–Ce catalyst is characterized by an initial fast reaction step followed by a slow reaction step. The rate of total organic carbon (TOC) reduction was described by first-order kinetics with respect to TOC concentration in black liquor for both initial and later reaction steps. The activation energies were determined to be 54.53 and 50.13 kJ/mol for the initial and later oxidation steps, respectively. Comparison of the data with the generalized kinetic model was also presented.     

Subcritical Crack Growth in a Phosphate Laser Glass

The rate of subcritical crack growth in a metaphosphate Nd-doped laser glass was measured using the double-cleavage-drilled compression (DCDC) method. The crack velocity is reported as a function of stress intensity at temperatures ranging from 296 to 573 K and in nitrogen with water vapor pressures ranging from 40 Pa (0.3 mmHg) to 4.7 × 10⁴ Pa (355 mmHg). The measured crack velocities follow region I, II, and III behavior similar to that reported for silicate glasses. A chemical and mass-transport-limited reaction rate model explains the behavior of the data except at high temperatures and high water vapor pressures where crack tip blunting is observed. Blunting is characterized by an arrest in the crack growth followed by the inability to reinitiate slow crack growth at higher stresses. A dynamic crack tip blunting mechanism is proposed to explain the deviation from the reaction rate model.     

Modeling studies on the effects of the process parameters in forced-flow chemical vapor infiltration reactor for the preparation of C/C composites

The mathematical modeling for the preparation of C/C composites from propane by F-CVI (Forced-flow chemical vapor infiltration) was studied. Experimental data were fitted with the modeling calculations and an adjusted reaction rate constant was obtained. Effects of many operation parameters such as temperature, the inlet concentration of propane, flow rate, and the initial porosity were observed. The decrease of the deposition rate due to the decrease of porosity and the depletion of propane in the middle of the preform was compensated with the increase of the lateral surface area of fibers. It was confirmed that a slow reaction rate with a low temperature and a low concentration is necessary for a uniform infiltration. As the gas flow rate and the initial porosity increase, the amount of deposition increases.     

Effects of pH and surface modification of TiO2 with SiOx on the photocatalytic degradation of a pyrimidine derivative

A pyrimidine derivative, 2-isopropyl-6-methyl-4-pyrimidinol (IMP), could be completely decomposed following pseudo-first-order kinetics by photocatalytic reaction using TiO₂. The effects of pH and surface modification of TiO₂ with SiO_x were studied. The degradation rate of IMP was fast in mild acidic condition (pH=6.3) and slower in strong acidic (pH=2.0) or basic (pH=10.0) condition. The main reason of the slow IMP degradation was different in strong acidic and basic conditions. In strong acidic condition, the slow degradation rate was explained by electrostatic repulsion. On the other hand, a little formation of hydroxyl radicals was considered as a main reason in strong basic condition. The result was supported by the experiments using SiO_x-loaded TiO₂, which has a lower isoelectric point than pure TiO₂.     

淀粉基非包膜缓释尿素肥料的制备与释放特性

以偶氮二异丁腈为引发剂,采用紫外光引发聚合方法制备玉米淀粉一丙烯酸丁酯接枝聚合物。通过改变光照引发时间、引发剂用量和反应时间等条件可得到具有不同接枝率的聚合物。不同组成的玉米淀粉一丙烯酸丁酯接枝聚合物与尿素制成的缓释尿素片均具有缓释性能,其中接枝率较高的聚合物栽体表现出更好的缓释效果。     

竹材热解动力学特性分析

采用热重分析法研究了氮气气氛下竹材的热解行为及其动力学特性,分析了升温速率和粒径对竹材热解过程及动力学参数的影响. 结果表明,竹材热解分为干燥、预热解、热解和缓慢热解４个阶段;升温速率对竹材的热失重特性有显著影响,当升温速率从40℃/min增加到100℃/min时,竹材热解出现了滞后现象,热解活化能从130.87 kJ/mol下降到73.85 kJ/mol,频率因子及反应级数单调减小;不同升温速率下计算的活化能和频率因子之间存在良好的补偿效应;当粒径大于380 mm时,竹材的热解不仅受动力学控制,受颗粒传热、传质影响也较大.     

蜡油加氢脱硫集总动力学模型研究

以减压蜡油（VGO）加氢脱硫（HDS）小试实验数据为基础,针对VGO HDS反应速率快慢,将原料中硫化物划分为快反应速率、中反应速率、慢反应速率3个集总,建立VGO HDS反应三集总动力学模型,并采用Levenberg-Marguardt算法对模型参数进行了求解。结果表明,该模型平均相对误差仅为5.18%,预测值可靠,外推性良好,模型参数计算结果符合加氢反应规律。通过模型计算,分析了液时空速、反应温度、反应压力对VGO HDS过程的影响,得到了HDS详细的反应规律,可为VGO HDS反应集总动力学研究和实验分析提供参考。     

吸氧多层薄膜内脱氧树脂层的传质与反应模型

制备了多种配方的脱氧树脂薄膜,测定了铁粉含量、铁粉粒度、卤化物、偶联剂、填充剂及调湿时间等因素对脱氧树脂薄膜吸氧性能的影响,建立了脱氧树脂薄膜吸氧过程的传质与反应模型.该模型表明,当M^1/2＞3时,吸氧过程为快速反应,吸氧速率与膜厚无关.当M^1/2＜0.3时,吸氧过程为慢速反应,吸氧速率与D_e无关,膜内氧气浓度均匀,且等于气固界面浓度.采用多种配方脱氧树脂薄膜的吸氧实验表明,该模型与实验结果吻合良好.     

乙基叔戊基醚的合成及反应动力学研究

用国产NKC－29型强酸性阳离子交换树脂为催化剂，在常液相条件下用乙醇和叔戊醇为原料合成了乙基叔戊基醚。讨论了温度，催化剂浓度，反应物初始摩尔比等参数对反应速度以及反应选择性的影响。结果表明：反应温度升高，将加快反应速度，但是当温度过高将加速叔戊醇的分解，使反应的选择性降低，催化剂浓度与产物的初始生成速率成线性关系，表明反应为动力学控制步骤：增加乙醇与叔戊醇的初始摩尔比，可有效控制叔戊醇分解为水与异戊烯的副反应。与另一种国产S－54型树脂催化剂比较的结果显示：NKC－29的催化活性及选择性优于S－54。根据实验结果并考虑水对催化剂的阻碍效应，建立了该反应的动力学模型，并得到了相应的模型参数，即反应速度常数和水的阻碍系数。实验结果与模型计算值吻合得较好。     

OSCILLATORY STATES AND SLOW ACTIVITY CHANGES DURING THE OXIDATION OF HYDROGEN BY PALLADIUM

Sustained oscillations of the reaction rate were observed during the oxidation of hydrogen on a palladium wire whose temperature was maintained constant by a constant temperature anemometer. This oscillatory behavior was observed only within a region of operating conditions under which the activity of the wire responded very slowly to sudden changes in the limiting reactant's concentration. The slow activity changes and oscillations are probably caused by slow surface oxidation and reduction. The oscillatory states are independent of the linear gas velocity for sufficiently high velocities, indicating that they are not caused by the coupling between chemical and transport rate processes.     

Influence of Various Reduction Reagents on the Morphological Properties of Ag Nanoparticles@Silk Fiber Prepared Using Sonochemical Method

Silk fiber-containing Ag nanoparticles were prepared through chemical reduction under ultrasound irradiation. The influence of reducing reagents on the morphological properties of the Ag nanoparticles-silk fiber was studied. The size of the metallic nanoparticles vary significantly with the type of reducing reagent used in the synthesis. A strong reducing reagent promotes a fast reaction rate and favors the formation of smaller nanoparticles. A weak reducing reagent induces a slow reaction rate and favors relatively larger particles. The products were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD).     

Resination and decarboxylation reactions in fusion synthesis of ca-resinates

The reaction steps in fusion synthesis of Ca-resinates include a fast, irreversible resination reaction and a slow decarboxylation of rosin acids. The kinetics of the reactions was studied separately in a laboratory scale batch reactor. The kinetic data were obtained by acid number titrations and FTIR measurements. Resination was observed to be a fast irreversible reaction in the temperature range 190 to 235°C. Rate equations for the decarboxylation reaction were derived based on plausible mechanistic reaction steps of free and solvating rosin acids. The parameters of the rate equations were estimated for syntheses at different reaction temperatures (265 to 285°C) and the kinetic model was successfully applied to predict the experimental kinetics of rosin acid decarboxylation.     

氯与植物纤维原料中木素的反应速度及反应性质的探讨

<正> 在氯化法制造纸浆的过程中,氯气与经过碱液处理的植物纤维原料作用,使其中的木素成为氯化木素,以便用稀碱除去.本工作的目的是寻求在氯化法的条件下氯与木素的反应速度,并试图明确该反应的性质.