氢转移反应的研究概述

氢转移反应是利用氢供体作为氢源对氢受体进行还原的一类反应,该反应与普通的氢气加氢反应相比,反应条件温和、对设备要求不高,在实验室研究和工业生产中均有广阔的应用前景。本文作者在研究氢转移反应还原芳磺酸的基础上,对氢转移反应的工艺条件、催化剂和氢供体种类、适用的反应类型等进行了较系统的总结,重点对近年来的相关文献进行了综述,表明氢转移反应是一种高效、绿色、资源化利用氢供体的氢化、氢解还原工艺。     

过氧化氢脱硫工艺技术的研究

以过氧化氢为原料脱除硫酸尾气中的二氧化硫,研究过氧化氢浓度、反应温度、反应时间对脱硫率的影响,且对反应过程中过氧化氢的分解率进行了检测。实验结果表明:过氧化氢不足量时脱硫率随过氧化氢浓度的增加而增加,过氧化氢足量时过氧化氢浓度的改变对脱硫率影响不大;在实验条件下,过氧化氢分解率较低;反应温度对脱硫率影响不明显;在SO_2进气质量浓度为4 050 mg/m~3、进气量为0.25 m~3/h的条件下,最佳工艺条件为w(H_2O_2)0.25%、反应温度70℃、反应时间1 h,此条件下脱硫率为98%。     

Kinetics of methylamine conversion on a Pd catalyst in the presence of H2

The kinetics of disproportionation of methylamine to dimethylamine and ammonia in the presence of hydrogen have been investigated over a silica-supported palladium catalyst. At a fixed partial pressure of methylamine, the reaction rate generally increases with decreasing hydrogen partial pressure, approaching a limiting or maximum value at sufficiently low pressures of hydrogen. The existence of a maximum is supported by the observation that the presence of some hydrogen appears to be necessary for the reaction to proceed at a conveniently measurable rate. At a fixed hydrogen partial pressure, the reaction rate increases with increasing methylamine partial pressure. When the methylamine partial pressure is sufficiently low or the hydrogen partial pressure is sufficiently high, the reaction order with respect to methylamine can be somewhat higher than one. At such conditions, a mechanism involving a bimolecular reaction between two partially dehydrogenated methylamine molecules on the surface appears to make a significant contribution to the overall catalytic reaction.     

Reactivities of carbon to steam and hydrogen and applications to technical gasification processes—A review

Experimental results of reactivity measurements on coal and chars to steam, hydrogen and steamhydrogen mixtures are compared and their application to technical processes is discussed. The change of surface area as a function of burn-off has a minor significance for chars made from coal with high initial accessible surface areas. In this case the reaction rate under constant steam pressure is first order with respect to the amount of carbon being gasified. The reaction rate of hydrogasification markedly decreases with burn-off, since the activation energy increases with burn-off due to the consumption of reactive carbon during the reaction. The reaction of carbon with steam hydrogen mixtures can be described as a superposition of carbon steam reaction and hydrogasification. The overall rate relative to that with hydrogen or steam alone can increase if a high amount of reactive carbon is present and immediately reacts with hydrogen. It can decrease if the inhibition of the steam carbon reaction by hydrogen predominates. Some consequences for the technical performance of coal gasification are: The overall reaction rate in a fluid bed decreases with increasing bed height, since hydrogen-steam ratios and therefore the inhibition by hydrogen increase. The overall reaction rate in a moving bed with countercurrent flow of carbon and reacting gases increases with increasing overall pressure since highly reactive carbon, formed in the pyrolysis zone, can immediately react with high partial pressures of hydrogen.     

H2对CO气相催化偶联制草酸二乙酯反应的失活机理

重点研究了氢气（H₂）对一氧化碳（CO）催化偶联反应制草酸二乙酯的影响,分别考察了不同H₂浓度、不同温度和不同空时条件下加入H₂对CO偶联反应的影响,结果发现H₂的加入使反应过程中CO转化率、草酸二乙酯选择性和空时收率明显下降,且在实验条件范围内,通入H₂浓度越高、反应温度越高,催化剂活性下降越快.研究得出,H₂气氛下CO偶联反应失活动力学方程为：-da/dt=k_dc^0.65_H2.进一步分析失活动力学方程可知,加氢反应过程中,H₂和CO吸附在同一个活性中心上,H₂在活性中心上的吸附抑制了CO在催化剂上的吸附,从而使得CO催化偶联反应生成草酸二乙酯的速率下降,导致加氢后CO转化率、草酸二乙酯选择性和空时收率降低.     

太阳能光催化制氢反应体系及其材料研究进展

光催化制氢是利用太阳能获取氢能的重要途径,是当前研究热点。长期以来,人们致力于各种新型可见光光催化制氢材料的研究并取得较大进展。反应体系的设计和选择是实现高效光催化制氢和能否走向工业化的核心问题之一,因此,近年来研究者开始对光催化制氢反应体系加大研究。光催化制氢主要有非均相光催化制氢(HPC)和光电催化制氢(PEC),不同的体系具有各自的优缺点和应用范围。重点介绍光催化制氢半反应、光催化完全分解水和光电催化分解水3种主要反应体系,分析各种反应体系的特点,阐述各个体系涉及的光催化材料的发展进程,并展望太阳能光催化制氢研究前景,其中,新型高效的PEC-PV(光伏)耦合光化学转化系统有望为光解水制氢实现工业化提供一种重要的发展途径。     

氨肟化反应中影响双氧水分解因素浅析

本文对环己酮氨肟化反应体系中引发H2O2分解的因素进行了研究,针对各种影响因素提出控制H2O2分解的优化措施。结果表明：在氨肟化反应体系中,NH3过量导致的碱性环境、H2O2过量加入或H2O2浓度过高以及氨肟化反应不完全,是引起H2O2分解的主要原因;反应温度、反应压力等操作条件的变化也可改变H2O2的分解速度;微量的锰、锌、铜、铁、镍等金属离子的存在,也会大大加速H2O2的分解。控制适当的NH3浓度及酮/H2O2摩尔比、反应条件以及降低金属离子含量有利于提高H2O2的有效利用率,这些结果可以为优化氨肟化反应工艺、降低H2O2消耗提供依据。     

The Hydrogenation of Acetylene Catalyzed by Palladium: Hydrogen Pressure Dependence

The kinetics of acetylene hydrogenation catalyzed by a clean palladium foil at high pressures are measured and yield an activation energy of 9.6±0.1 kcal/mol when using hydrogen. The rate exhibits a deuterium isotope effect such that the reaction activation energy is 9.0±0.2 kcal/mol for reaction with deuterium. The hydrogen pressure reaction order is 1.04±0.02 at 300 K with an acetylene pressure of 100 Torr and the acetylene order is −0.66 at 300 K and with a hydrogen pressure of 100 Torr. These reaction kinetics closely mimic those of supported model catalysts. In addition, it is found that the rate of benzene formation is accelerated by the addition of hydrogen to the reaction mixture. This is rationalized by proposing that hydrogen enhances the coverage of acetylene under catalytic conditions. This notion can be used to successfully calculate the hydrogen pressure dependence for acetylene hydrogenation as a function of temperature, a value which varies between ∼1.05 and 1.3 as the temperature changes from 300 to 380 K. Possible origins for this effect are discussed.     

Microkinetic analysis of isopropanol dehydrogenation over Cu/SiO2 catalyst

The reaction of isopropanol dehydrogenation to acetone over the Cu/SiO₂ catalyst was studied using the method of microkinetic analysis. This analysis provided a picture about the coverage of different surface species and their influences on the reaction rate. Specifically, the analysis reinforced the previously proposed mechanism and added some new findings for the reaction. It was found that under the normal reaction conditions, the catalyst surface is mostly covered by the isopropoxyl groups and the overall reaction rate is controlled by the elimination of α-H of the isopropoxyl groups. In addition, the elimination of the α-H is significantly affected by the availability of surface Cu sites. At the low hydrogen partial pressure, the number of bare surface Cu sites increases with the increasing of hydrogen partial pressure, leading to the increased overall reaction rate and thus the positive reaction order with respect to hydrogen. Thus, hydrogen can enhance the rate of this reaction via some kinetic factors under the specific coverage regime, although it is thermodynamically unfavorable for the reaction.     

Influence of Carbonate on the Ozone/Hydrogen Peroxide Based Advanced Oxidation Process for Drinking Water Treatment

The influence of carbonate on the ozone/hydrogen peroxide process has been investigated. Carbonate radicals, which are formed from the reaction of bicarbonate/carbonate with OH radicals, act as a chain carrier for ozone decomposition due to their reaction with hydrogen peroxide. The efficiency of bicarbonate/carbonate as a promoter for the radical-based chain reaction in presence of hydrogen peroxide has been calibrated and compared to a well-known chain promoter (methanol) and an inhibitor (tert-butanol). Relative to tert-butanol, the hydrogen peroxide induced ozone decomposition is accelerated by bicarbonate/carbonate. Relative to methanol, bicarbonate/carbonate in presence of hydrogen peroxide is less effective as a promoter under comparable experimental conditions.     

Mass transport in a hydrogen gas diffusion electrode

Experimental data are presented concerning the diffusion-limited current density for hydrogen oxidation in a gas diffusion electrode (GDE) under various conditions. These current densities were obtained using mixtures of hydrogen and inert gases. To elucidate the dependence of the overall mass transport coefficient on the gas phase diffusion coefficient and the liquid phase diffusion coefficient of the hydrogen, a simplified model was derived to describe the transport of hydrogen in a GDE based on literature models. The GDE consists of a hydrophobic and a hydrophilic layer, namely a porous backing and a reaction layer. The model involves gas diffusion through the porous backing of the GDE combined with gas diffusion, gas dissolution and reaction in the reaction layer of the electrode. It was found that the transport rate of hydrogen under the experimental circumstances is determined by hydrogen gas diffusion in the pores of the porous backing, as well as in the macropores of the reaction layer. Diffusion of dissolved hydrogen in the micropores of the reaction layer, through the liquid, is shown to be of little significance.     

Electrochemistry of active chromium: Part II. Three hydrogen evolution reactions on chromium in sulfuric acid

It was shown that chromium in deaerated sulfuric acid exhibits two stable corrosion potentials, depending whether the metal had previously been in contact with air or subjected to activation by cathodically evolving hydrogen. Electrochemical polarization measurements, as well as measurements of the actual metal dissolution rate at the corrosion potentials, anodic or cathodic polarization, using the analytical determination of Cr ions in the solution, weight-loss of metal, or volumes of hydrogen evolved, showed that hydrogen can evolve on chromium by three different reaction mechanisms. The first one is the electrochemical hydrogen evolution reaction from H⁺ ions at the bare chromium surface obtained by cathodic activation. This reaction and the active anodic dissolution of chromium determine one stable corrosion potential. The second reaction is the reaction of H⁺ ions on the oxidized chromium surface which, coupled with the anodic dissolution of passivated chromium determines the other stable corrosion potential. The third one is the “anomalous” or chemical reaction of chromium with water molecules and hydrogen ions whereby hydrogen is liberated. This is a potential independent reaction, occurring on the bare metal surface, and which is at pH <2 several times faster at the corrosion potential than the electrochemical hydrogen evolution reaction. The consequence is that the overall corrosion rate is several times faster than that determined by the usual electrochemical methods. The applicability of the different methods of measuring electrochemical corrosion rates and cathodic current efficiencies in chromium plating is discussed. Also, a possible role of the “anomalous” chromium dissolution in corrosion fatigue and stress corrosion cracking of stainless steels is considered.     

The influence of the hydrogen origin at the surface of Mo suboxide during the deoxygenation of carboxylic acid

This work investigates the reactivity of a molybdenum suboxide in the deoxygenation of benzoic acid in the presence of hydrogen or propylene. Major differences are highlighted with Mo₈O₂₃ as the active phase. When the reaction is performed in the presence of hydrogen as the reductant, benzoic acid is converted to benzaldehyde, toluene and benzene. If molecular hydrogen is replaced by propylene as a weaker reductant, the only product formed during the reaction is benzaldehyde. It is demonstrated that, in absence of molecular hydrogen in the reaction mixture, a large amount of benzoic acid remains irreversibly adsorbed on the catalyst surface. The comparison of the two series of catalytic tests allows to somehow discriminate how the nature and the mobility of the hydrogen atoms involved in the reaction dictate the behaviour of the catalyst.     

Kinetic aspects of self-discharge of nickel-hydrogen batteries and methods for its prevention

Results of microcalorimetric experiments, in relation to self-discharge in Ni/H₂ batteries are reviewed; the mechanism of self-discharge, as well as possible methods for its inhibition, are discussed. These studies indicate that: (i) the self-discharge is due to a direct reaction of hydrogen with the charged active material (nickel oxide); (ii) the presence of metallic nickel sinter particles does not affect the reaction rate; (iii) the reaction rate depends linearly on hydrogen pressure indicating that the reaction is first order with respect to hydrogen; (iv) the reaction rate is higher under starved-electrolyte rather than flooded electrolyte conditions, indicating that the rate is affected by a diffusion process of dissolved hydrogen; and (v) the microcalorimetric heat evolution rate correlates with that of a decrease in electrode capacity due to the self-discharge reaction. The effects of additives to the active material of the nickel electrode were tested as an approach to reduction of the intrinsic rate of self-discharge. An alternate method for minimizing this rate is by storing the hydrogen as a hydride and thereby lowering the cell operating pressure. Some alloys were thus examined for their hydriding/dehydriding characteristics.     

Ammoximation of ketones on titanium silicalite. A study of the reaction byproducts

In the new EniChem process cyclohexanone oxime is directly synthesized by ammoximation of cyclohexanone with ammonia and hydrogen peroxide on titanium silicalite catalyst. The ammoximation reaction is suitable for the synthesis of several oximes by reaction of the corresponding ketones with ammonia and hydrogen peroxide on titanium silicalite. New results on ketones ammoximation and on reaction byproducts are reported. The effect of some reaction parameters on the oxime yield and on byproducts formation is discussed.     

过碳酸钠生产的最佳工艺条件研究

以碳酸钠和双氧水为原料制备过碳酸钠,考察了双氧水浓度、反应温度、反应时间、物料配比等因素对产品收率的影响。结果表明,当反应温度为15℃,双氧水浓度为30%,反应时间0.5 h,反应物碳酸钠和双氧水的摩尔比为1∶1.5~1∶1.6,稳定剂为硅酸钠,用量为0.5%,过碳酸钠的收率大于85%。     

环氧化天然橡胶制备反应动力学

采用适当提高反应温度、减小甲酸和过氧化氢用量的方法制备不同环氧化程度（Ｂ）的环氧化天然橡胶（ＥＮＲ）,并利用质量作用定理,从ＥＮＲ的制备基本反应式推导出ＥＮＲ的制备反应动力学方程,讨论了Ｂ与反应温度和时间及甲酸、过氧化氢用量的关系。结果表明,用过氧甲酸对ＮＲ进行环化改性制备ＥＮＲ的反应为二级反应,对ＮＲ和过氧化氢的反应则为一级反应。求出的反应速率常数ｋ４为１１２５×１０－４ｄｍ３·（ｍｏｌ·ｓ）－１,活化能为８２６７４ｋＪ·ｍｏｌ－１;Ｂ与过氧化氢用量之间为线性关系     

盐酸双氧水体系合成氯代甲苯

用盐酸-双氧水替代液氯进行芳环的亲电氯化。用盐酸和双氧水的混合物作为氯源和甲苯反应,合成氯代甲苯,考察了反应温度、反应物料比、投料方式、反应时间对产物收率的影响。结果表明,适宜的合成条件为:甲苯、盐酸、双氧水的摩尔比为1∶8∶1,反应温度为室温,反应时间8 h,双氧水滴加速率5¹0 mL/h,一氯甲苯的收率可达91%。     

Titanium-magnesium catalysts for propylene polymerization: The effect of donors

The chain transfer reaction with hydrogen at propylene polymerization over Ti-Mg catalysts (TMCs) of composition TiCl₄/D₁/MgCl₂-AlEt₃/D₂ is studied in a wide hydrogen concentration range. A two-step mechanism of this reaction is suggested. This mechanism accounts for the fractional order of the reaction with respect to hydrogen concentration. Constants of chain transfer reaction with hydrogen are determined for TMC with different donors: 1,3-diether or dibutyl phthalate as D₁ and tetraethoxysilane or dicyclopentyldimethoxysilane as D₂. In propylene polymerization over the TMCs, the length of the polymer chain is mainly determined by the ratio of the propylene and hydrogen concentrations because the propagation and chain transfer rate constants are comparable. The rate constant of chain transfer with hydrogen at ethylene polymerization is significantly (more than one order of magnitude) less, and higher hydrogen concentrations are required for attaining the same degree of polymerization. The results of this study might be helpful in simulation of industrial polymerization processes and in control of the polymer molar mass.     

Free radical chemistry of coal liquefaction: role of molecular hydrogen

Model compounds containing the types of carboncarbon bonds thought to be present in coal were pyrolyzed in the presence of tetralin and molecular hydrogen at 450 °C. The relative rates of conversion of the model structures are predictable from the bond dissociation energies of the compounds. Conversion of dibenzyl in the presence of both tetralin and molecular hydrogen or in the presence of hydrogen alone proceeds along two parallel reaction paths. Toluene is produced by a thermal cracking reaction in which the rate-controlling step is the thermal cleavage of the β-bond in dibenzyl. Benzene and ethylbenzene are produced by a hydrocracking reaction. The rate of the hydrocracking reaction is directly proportional to the hydrogen pressure. The strong bond in diphenyl is hydrocracked in a system containing both molecular hydrogen and a source of free radicals. These studies on model coal structures offer firm evidence that molecular hydrogen can participate directly in free radical reactions under coal liquefaction conditions. Under some conditions molecular hydrogen can compete with a good donor solvent to stabilize the thermally produced free radicals. Molecular hydrogen can also promote some hydrocracking reactions in coal liquefaction that do not occur to an appreciable extent in the presence of only donor.