超临界水——一种新的化学介质

水在超临界态时有着与其在常态时迥然不同的表现,近年来被开发成为一种有神奇性质的反应介质。本文介绍了超临界水的特点和超临界水中发生的化学反应。超临界水将在固体有机废弃物的焚烧处理等方面发挥其独特的作用,成为未来环境保护技术中的一枝新秀。     

超临界流体化学热力学

本文简要介绍了超临界流体的基本概念和性质，评述了化学热力学在超临界萃取、超临界态化学反应、超临界流体制备材料等方面的作用和研究进展，讨论了二氧化碳-离子液体两相体系的热力学行为。     

亚临界水脱除煤中硫的实验研究

近年来,超临界流体以其独特的性质作为溶剂或反应介质在众多领域得到应用。对于煤在超临界及亚临界水中的行为研究也越来越受到国内外学者的重视,如Kashimura等、Hu等和Cheng等对褐煤在亚临界及超临界水中的降解行为进行了研究。Timpe等报道了水热处理对年轻煤中有机硫及微量元素的脱除行为。     

孤岛渣油超临界水-合成气中悬浮床加氢裂化反应研究 Ⅰ. 催化剂的影响

利用超临界水-合成气为替代加氢氢源对孤岛渣油悬浮床加氢裂化反应进行了研究，设想利用超临界水中发生的水-气转化反应（CO+H2O→H2+CO2）为渣油加氢反应提供氢源，报道了孤岛渣油超临界水-合成气中悬浮床加氢裂化反应催化剂影响的研究结果。结果表明，催化剂在该反应中具有十分重要的作用，加入催化剂可以明显改善加氢裂化产物的分布和裂化反应产物的性质，降低裂化气体和抑制缩合生焦反应的发生。     

典型醇类物质超临界水氧化反应途径研究

采用自主设计的连续流动气封壁超临界水氧化反应装置,研究了典型醇类物质甲醇、乙醇和异丙醇在超临界水中氧化的反应途径,并归纳了醇类物质超临界水氧化反应的规律及特点。研究结果表明,甲醇超临界水氧化反应的主要中间产物为甲醛,同样条件下转化率较乙醇和异丙醇低;乙醇和异丙醇超临界水氧化反应的主要中间产物为丙酮、乙酸、乙醛和甲醇等。三种醇超临界水氧化过程中均涉及到大量活性自由基的相互作用,表现为脱氢、裂解和聚合等反应形式;产物包括碳链增长、不变、降低三种类型。总体来看,醇类物质超临界水氧化反应的趋势是向碳链降低的方向进行,即通过一系列中间产物最后生成CO2和水。     

典型醇类物质超临界水氧化反应途径研究

采用自主设计的连续流动气封壁超临界水氧化反应装置,研究了典型醇类物质甲醇、乙醇和异丙醇在超临界水中氧化的反应途径,并归纳了醇类物质超临界水氧化反应的规律及特点。研究结果表明,甲醇超临界水氧化反应的主要中间产物为甲醛,同样条件下转化率较乙醇和异丙醇低;乙醇和异丙醇超临界水氧化反应的主要中间产物为丙酮、乙酸、乙醛和甲醇等。三种醇超临界水氧化过程中均涉及到大量活性自由基的相互作用,表现为脱氢、裂解和聚合等反应形式;产物包括碳链增长、不变、降低三种类型。总体来看,醇类物质超临界水氧化反应的趋势是向碳链降低的方向进行,即通过一系列中间产物最后生成CO₂和水。     

超临界流体的共溶剂效应和混合流体研究进展

共溶剂的出现极大地拓展了超临界流体的应用范围，推动了超临界流体科学与技术的发展。本文从相行为和分子间相互作用热力学的角度，对相行为测定、量热技术、光谱技术和分子模拟等在超临界流体中共溶剂效应的研究作了综述，主要介绍超临界流体中共溶剂的作用机理和混合流体在临界点附近热力学性质研究，并对其未来发展方向进行了展望。     

超临界流体萃取金属离子在环境分析上的应用

固体和液体中的金属离子能在含有有机螯合剂的超临界二氧化碳(SC-CO2)流体中被有效地萃取出来。超临界流体萃取(SFE)金属离子的效率取决于下列因素：螯合剂在SC-CO2中的稳定性和溶解性、金属络合物在SC-CO2中的溶解度、水、pH值、压力、温度及金属离子的化学形式和基质的性质等。     

超临界水临界区域判定方法研究

对超临界水在临界区域进行合理的判定和区域划分,对于深入理解超临界水在临界过渡区域的流动和换热相关特征具有重要的作用。本文分析了超临界水从拟液态区向拟汽态区过渡的过程中,其导热系数、动力粘度、定压比热和膨胀系数等相关参数的变化规律特征,并归纳了已有超临界水在临界区域的划分判定模型。分析结果表明,在临界过渡区域,超临界水的流动特征参数和换热特征参数均会发生一系列连续剧烈的变化;只有同时考虑超临界水的膨胀特性和最大比热特性,才能更加合理地对临界区域进行划分。在此分析基础上,本工作完善了超临界水的三区分析判定模型,得到了新的超临界水在临界区域的判定划分数据,并由此拟合得到了新的超临界水分区边界计算关系式。新的计算关系式的误差范围在±0. 3℃之内,满足计算分析的要求。     

水/二氧化碳体系的性质及其应用研究

水和二氧化碳是地球上储量最为丰富的两种溶剂,对其性质和应用进行研究具有极其重要的意义.二者作为溶剂利用时各有优缺点,如水对极性物质有着较高的溶解能力,但对很多有机物的溶解能力较差;二氧化碳性质容易调控,对低极性物质具有较好的溶解能力,而对极性物质的溶解能力较差.如果将水和二氧化碳相结合,不仅可以克服两者作为溶剂的局限性,而且还会产生一些新的性质.本文综述了水/二氧化碳体系的性质及其在材料制备和化学反应中的应用研究.按研究体系,可分为溶解二氧化碳的水溶液、表面活性剂稳定的水/液体(或超临界)二氧化碳体系和纳米粒子稳定的水/液体(或超临界)二氧化碳体系.     

Is ammonia a better solvent than water for contact ion pairs?

The existence of a charge-transfer-to-solvent process when a KI contact ion pair (CIP) dissolved in supercritical water (SCW) is excited by UV light was confirmed by use of electronic structure calculations applied to molecular dynamics trajectories. We observed similar behavior with fluid density as that found for the KI-CIP in supercritical ammonia (SCA); nevertheless, there are some distinct features in the two supercritical solvents. First, the effect of the solvent field due to the molecules lying beyond the first solvation shell is very different in SCW compared with that observed in SCA; in SCW it actually has a destabilizing effect over the ground and excited states. Second, our results for the thermodynamic behavior of the CIP indicate that SCA is better solvent than SCW for this species. The differences found can be attributed to the solvent molecules surrounding the CIP and bridging the two ions; they shield more efficiently the ion pair from long-range solvent effects in SCA. The different behavior is partially attributed to a stronger solvent-solvent interaction in SCW than in SCA.     

Development of porosity in a char during reaction with steam or supercritical water

Two series of activated carbon have been prepared by reaction of a char (from olive stones) with supercritical water (SCW) with the objective of studying the effect of temperature and residence time on the development of porosity. The results have been compared with those obtained using the same char but with classical activation with steam. Both procedures develop porosity, but (i) the reaction rate is critical in the development of porosity for steam but not for SCW activation, and (ii) SCW activation produces a larger development of microporosity at low degrees of burnoff, whereas steam produces more meso- and macroporosity. The differences have been explained by assuming that the mechanism for the carbon-water reaction is common but the transport properties of water in the supercritical state are more favorable to facilitate the access of water to the interior of the char particles. In contrast, when steam is used for the activation of the char, the diffusion of the molecules cannot keep up with the chemical rate and, consequently, the reaction is preferentially taking place at the most accessible surface sites, thus facilitating the development of larger pores and the widening of microporosity.     

Density dependence of hydrogen bonding and the translational-orientational structural order in supercritical water: a molecular dynamics study

Molecular dynamics simulation have been performed with a wide range of densities along a near critical isotherm of supercritical water (SCW) in order to study the density dependence of the structure order and hydrogen bonding (HB). It is revealed that the translational structure order is nearly invariant while the orientational tetrahedral structure order is very sensitive to the bulk density under supercritical conditions. Meanwhile, some energetically unfavorable intermediate water dimer structures are found to appear under supercritical conditions due to the reduced energy difference and the enhanced energy fluctuation. As a consequence, a general geometrical criterion or the inclusion of a energy-based criterion instead of currently widely adopted pure r(OH)-based geometric criterion is suggested to be used in the HB statistics under supercritical conditions. It is found that the average HB number per H(2)O molecule (n(HB)) reduces with the decreasing SCW bulk density although a given pair of H(2)O molecules are shown to have a stronger ability to form a hydrogen bond under lower SCW bulk densities. Accordingly, the orientational tetrahedral structure order q decreases with the reducing bulk density under supercritical conditions. However, when the fluid is dilute with ρ ≤ 0.19ρ(c) (ρ(c) = 0.322 g/cm(3)), the energy fluctuation increases sharply and the short-range order is destroyed, signifying the supercritical fluid (SCF)-gas state transition. Accordingly, the orientational tetrahedral structure order q gets reversal around ρ = 0.19ρ(c) and approaches zero under very dilute conditions. The sensitivity of the orientational order to the density implies the microscopic origin of the significant dependence of SCF's physicochemical properties on the pressure.     

Hydrogen bond driven chemical reactions: Beckmann rearrangement of cyclohexanone oxime into epsilon-caprolactam in supercritical water

Recent experiments have shown that supercritical water (SCW) has the ability to accelerate and make selective synthetic organic reactions, thus replacing the common but environmentally harmful acid and basic catalysts. In an attempt to understand the intimate mechanism behind this observation, we analyze, via first-principles molecular dynamics, the Beckmann rearrangement of cyclohexanone oxime into epsilon-caprolactam in supercritical water, for which accurate experimental evidence has been reported. Differences in the wetting of the hydrophilic parts of the solute, enhanced by SCW, and the disrupted hydrogen bond network are shown to be crucial in triggering the reaction and in making it selective. Furthermore, the enhanced concentrations of H(+) in SCW play an important role in starting the reaction.     

煤焦油及其组分在超临界水中的反应特性研究

采用间歇超临界水（supercritical water, SCW）反应器,考察了三种煤焦油及其组分在SCW中的反应性。实验结果表明,沥青质裂解活性高的煤焦油在SCW中反应,轻质产物多,轻质化效果好。沥青质是轻质化反应的主要组分,它发生两极分化反应生成轻油和残焦。与常压N2热解相比,SCW可以促进沥青质转化为轻油并抑制缩合成焦的反应。由沥青质组分反应产物推断,煤焦油沥青质的芳香核主要是由1~4环的芳香族化合物构成。轻油组分在SCW中较为稳定,不足10%的轻油转化为沥青质和残焦。残焦组分在SCW中不发生反应。基于各组分的反应,探讨了煤焦油组分在SCW中的反应路径。     

Gasification reaction of organic compounds catalyzed by RuO2 in supercritical water

Nearly complete gasification of organic compounds has been achieved by stoichiometrically insufficient amounts of RuO2 in supercritical water (SCW) to provide CH4, CO2 and H2, all the hydrogen atoms of which originate from water, and the catalytic effect of RuO2 results from a redox couple of Ru(IV)/Ru(II) induced by SCW.     

超临界水中煤焦油沥青轻质化的实验研究

采用间歇超临界水（supercritical water, SCW）反应器,考察了反应温度（400℃~480℃）、停留时间(1 min~80 min)和反应压力（25 MPa~40 MPa）对煤焦油沥青轻质化的影响。与常压N2热解相比,煤焦油沥青在SCW中反应后产物中的轻油质量分数高,残焦质量分数低,表明SCW可以促进煤焦油沥青的轻质化反应进行并抑制缩合反应。沥青质是煤焦油沥青在SCW中发生反应的主要组分。在440 ℃和34 MPa时反应20 min,产物中轻油的质量分数可达原料的两倍,说明煤焦油沥青在SCW中可以发生明显轻质化反应。与温度相比,压力和停留时间的影响相对较小。     

Pyrolysis of coal-tar asphaltene in supercritical water

Pyrolysis of coal-tar asphaltene, the main active component of coal tar in supercritical water (SCW), is investigated to further understand the upgrading mechanism of coal tar. It is found that coal-tar asphaltene convert to gas, maltene and char both in N₂ and in SCW, but the conversion of coal-tar asphaltene and the yield of maltene in SCW are significant higher than those in N₂. The effect of maltene and char in coal tar on the pyrolysis of coal-tar asphaltene is also studied. The results indicate that the presence of maltene could suppress the formation of char. And the addition of char could reduce the maltene yield. The analysis of pyrolysis product indicates the aromatic nucleus of asphaltene molecule is mainly composed of 2-4 rings aromatic hydrocarbons. Based on these results the pyrolysis mechanism of asphaltene in SCW was discussed.     

超临界水中醋酸锌水解反应的分子动力学模拟

氧化锌(ZnO)是一种重要的化工原料, 超临界水热合成法制备纳米ZnO的第一步是锌盐与碱或水发生水解反应生成Zn(OH)₂, 后者接着脱水生成ZnO. 以Zn(CH₃COO)₂为原料, 直接和超临界水(SCW)反应能够制备纳米级的ZnO颗粒, 但对反应机理的探讨较少. 本研究利用分子动力学模拟超临界条件下Zn(CH₃COO)₂水解反应过程中的结构和能量变化, 发现Zn(CH₃COO)₂在SCW中容易聚集成无定形的团簇, 1个Zn²⁺平均和5个CH₃COO^-和1个H₂O配位, 形成6配位的八面体结构. 处于Zn(CH₃COO)₂团簇和SCW界面的Zn²⁺能够和更多的H₂O配位. 水解反应后系统的势能降低, 同时伴随Zn(CH₃COO)₂团簇结构的改变. 反应产物OH^-分布在Zn(CH₃COO)₂团簇内部, 富集Zn²⁺, 而CH₃COOH则分布在SCW中. 本文的工作为超临界水热合成的反应过程提供了基本的理论依据.     

Investigation of Nickel-Based Alloys Exposed to Supercritical Water Environments

Experimental tube reactors made of the nickel based alloys C-22 and Inconel 625 were exposed to supercritical water (SCW) environments. During this experiments, various inclusions and precipitates from a submicron size range to bulk scale-like layers were formed. The corrosion products and layers, located at the sub- and supercritical parts of the tubular flow reactors, were imaged and analyzed by field-emission scanning electron microscopy, combined with an energy-dispersive X-ray spectrometer (FESEM/EDX). The microanalysis was performed on metallographically prepared specimen. To obtain additional information regarding surface morphology, scanning force microscopy (SFM) and microhardness testing were applied at selected specimen.