【作者】 董新艳；

【导师】 任其龙； 苏宝根；

【作者基本信息】 浙江大学 ， 应用化学， 2012， 博士

【摘要】 超临界CO2因其具有临界条件温和、溶解性能良好、绿色无污染、粘度小等优点而作为一种新兴的介质被广泛应用于化工领域。而溶质在超临界流体中的扩散系数是超临界萃取及分离装置的工程放大、流程设计以及工艺优化所必需的基础数据。因此,本文测定了一系列溶质在超临界CO2以及夹带不同改性剂的超临界CO2中的二元及三元无限稀释扩散系数,采用实验测定和量子化学相结合的研究手段从分子层面系统研究超临界流体的物化性质、溶质的构型和性质、改性剂的类型、以及体系中分子间作用力对溶质扩散系数的影响规律,并在此基础上构建了超临界流体中扩散系数的线性溶剂化能模型。在15～30MPa,308.15～338.15K的范围内,采用Taylor分散法测定了香芹酮和薄荷酮在超临界CO2以及含有5和10mol%乙醇的超临界CO2中的无限稀释扩散系数。研究表明,二元及三元扩散系数均随温度的升高而增大,随流体压力、密度、粘度以及乙醇摩尔分率的增加而减小。适用于二元体系的扩散系数与流体物化性质的关联方程同样适用于以乙醇为改性剂的三元体系。在21个经验模型中,Funazukuri-Ishiwata-Wakao和He-Yu-1998模型对两种溶质在超临界CO2中的扩散系数具有最好的预测效果,其平均绝对偏差(AAD)小于3.2%。此外,采用多个修正的Stokes-Einstein(SE)模型预测溶质在含有乙醇的超临界CO2中的扩散系数,预测值均大于实验值,并且AAD值随着乙醇分率的增加而增加。这主要是由于模型没有考虑溶质与乙醇缔合后形成体积增大的扩散实体。此外,Dvmond自由体积模型对溶质在CO2以及含乙醇的CO2中的扩散系数均有良好的预测效果,AAD值小于3.21%。由于不同的改性剂与溶质间不同类型和强度的相互作用力,使得溶质和改性剂形成体积增大的溶剂化聚集体,导致溶质在含有改性剂的CO2中的扩散系数小于其在纯CO2中的扩散系数。通过引入共溶剂效应参数(Ψ=三元扩散系数/二元扩散系数)可分析溶质与改性剂之间相互作用力的类型和强弱。对于没有氢键能力的改性剂,Ψ值随溶质分子体积增加而减少,主要由于溶质—改性剂间的色散作用力随溶质分子体积增加而增加；对于只具有氢键碱性的改性剂,氢键酸性高的溶质具有小的Ψ值,主要由于溶质—改性剂的氢键作用仅依赖溶质的氢键酸性并随其增加而增加；对于两性改性剂,无论改性剂的氢键酸性大于氢键碱性还是碱性大于酸性,氢键酸性高的溶质具有小的Ψ值,可以推断出溶质的氢键酸性而非氢键碱性对溶质—改性剂间氢键作用起主要贡献。由于溶质—改性剂间相互作用随着压力的增加而减弱,因此溶质的共溶剂效应参数随着压力增加而增加。此外,溶质在含有改性剂的C02中的扩散系数随着改性剂比例的增加而非线性减少,这是具有强相互作用力体系的典型现象。无氢键作用溶质在纯CO2以及含改性剂的CO2中的扩散系数主要受溶质分子体积的影响,并且构型比较线性的对位异构体在流体中扩散的最快；而对于有氢键作用溶质而言,溶质氢键官能团的邻位取代基的数目越多、链长越长,氢键官能团的空间位阻就越大,与改性剂的氢键作用就越弱,故而共溶剂效应参数就越大；对于双氢键官能团溶质,邻位同分异构体的分子内氢键会削弱溶质与C02、改性剂分子间氢键,因此其在C02及夹带具有氢键能力改性剂的C02中的扩散系数大于间位同分异构体。采用量子化学计算方法分别研究四氢呋喃和甲醇与溶质的相互作用规律,进一步深入揭示分子间相互作用力对扩散系数的影响。研究表明,只有氢键碱性的四氢呋喃与溶质氢键作用的强度仅取决于溶质氢键酸性的大小,相对于香叶醇,酸性大的间氟苯酚与四氢呋喃间形成更强的氢键作用。而无氢键酸性的1,3-二氯苯、左旋香芹酮与四氢呋喃形成弱于氢键作用的范德华作用力,并且分子体积较大且极性较大的左旋香芹酮与四氢呋喃形成更强的范德华作用力；两性改性剂甲醇分别与两性溶质(间氟苯酚、香叶醇)形成两种氢键复合体,甲醇与两性溶质(间氟苯酚、香叶醇)的氢键作用强于只具有氢键碱性的香芹酮,甲醇与1,3-二氯苯形成最弱的范德华作用力；因此,量化计算的结果可以有效解释扩散系数的实验结果：即溶质在含有四氢呋喃和甲醇的CO2中的共溶剂效应参数以间氟苯酚<香叶醇<香芹酮1<,3-二氯苯的顺序依次增加。根据23种训练集溶质在超临界CO2中以及含有10mol%四氢呋喃、甲醇的超临界CO2中扩散系数和共溶剂效应参数分别构建了LSER模型,其对12种测试集溶质在三种体系中的扩散系数均有良好的预测效果,模型总的平均绝对偏差(AAD%)分别小于2.45%,3.17%和3.40%。分析模型的回归系数可知,溶质与CO2间的色散作用力显著影响溶质在CO2中扩散系数,而溶质主要作为质子供体与四氢呋喃和甲醇间的氢键作用主要影响溶质在夹带这两种改性剂的超临界CO2中的共溶剂效应参数。更多还原

【Abstract】 Supercritical carbon dioxide (scCO2) is widely used in chemical industry because scCO2has many superior properties involving accessible critical condition, a stronger solvent power, essentially nontoxic, higher diffusivities. Diffusion coefficient is useful in design, scale-up and optimization of reactors and separators for supercritical fluid extraction or chromatography. Therefore, in this study, infinite dilution diffusion coefficients of a series of solutes in pure and cosolvent modified CO2were measured, and the effects of physicochemical properties of fluid, properties and structure of solutes, the types of cosolvent, intermolecular interaction on the diffusion coefficients in supercritical fluid were investigated systematically by means of a combination of experimental measurement and quantum chemical method from molecular level. On the basis of the above, Linear solvation energy relationship (LSER) model of diffusion coefficients was developed.The molecular diffusion coefficients of L-menthone and L-carvone in scCO2and scCO2containing5and10mol%ethanol as a modifier were measured by the Taylor dispersion method over the ranges of temperature from308.15to338.15K and pressure from15to30MPa. It was found that the diffusion coefficients increased almost linearly with temperature and decreased with the solvent pressure, density, viscosity and ethanol mole fraction. Moreover, the correlation relationships between diffusion coefficients and the temperature, pressure, viscosity, and density, which were valid in binary systems were also suitable for ternary systems of carbon dioxide containing modifier. Of several models used to predict experimental data in pure carbon dioxide, the two models of Funazukuri-Ishiwata-Wakao and He-Yu-1998were the best with the AAD less than3.2%. Furthermore, the modified Stokes-Einstein (SE) models overestimated the diffusion coefficients in ethanol modified scCO2with the AAD values increasing with the percentage of ethanol. which were probably due to the increase of the volume of solvation sphere as a true diffusion unit with the percentage of ethanol. Moreover, the free volume model of Dymond is good for predicting the experimental data in pure carbon dioxide and ethanol modified scCO2with the AAD values less than3.2%. The clustering of cosolvent molecules around the solute driven by van der Waals’ forces and hydrogen-bond interaction between solute and cosolvent resulted in solute cluster with increased effective size. Such solute cluster was considered as a true diffusion unit, leading to slower solute diffusion in ternary systems than in pure CO2. Cosolvent effect parameter, which was defined as the ratio of diffusion coefficient with and without cosolvent, respectively, was introduced to compare the strength of different types of interaction between solute and cosolvent, especially hydrogen bonding. For cosolvent without hydrogen-bond ability, the cosolvent effect parameter decreased with the increase of molecular volume and weight of the solutes, in that the strength of dispersion force with cosolvent increases with their size. For cosolvent with only HBA basicity, the solute with higher HBD acidity had smaller cosolvent effect parameter, due to the fact that the strength of hydrogen-bond interaction between the solute and cosolvent only depended on and increased with the HBD acidity of the solute. For amphiprotic cosolvent, the solute with higher HBD acidity had smaller cosolvent effect parameter, it could be inferred from which that HBD acidity rather than HBA basicity of the solute made a major contribution to the hydrogen-bond interaction between solute and cosolvent. The cosolvent effect parameter increased with pressure, because solute-cosolvent interaction decreased with pressure. Moreover, a nonlinear decrease in diffusion coefficient with the increase of cosolvent concentration was observed, which was typical of the behavior of the system with strong interaction.The diffusion coefficients of non-H-bonded solutes in pure and modified supercritical carbon dioxide depended largely on molecular volume, and the para-isomer with more linear structure diffused more quickly. The large value of cosolvent effect parameter for the H-bonded solutes with more and longer-chain substituent group was probably due to the steric hindrance effect provided by substituent group in the ortho position of functional group with hydrogen-bond ability. For Di-H-bonded solute, the diffusion coefficients of ortho-isomer were larger than that of meta-isomer, which could probably be attributed to the fact that intramolecular H-bonding weakened the hydrogen-bond interaction between solute and CO2as well as between solute and cosolvent.Quantum chemical methods were employed to investigate the interaction mechanism between cosolvent (tetrahydrofuran, methanol) and solute, which can help to get insight into the effect of intermolecular interaction on diffusion coefficients. It was found that the strength of hydrogen-bond interaction between the solute and tetrahydrofuran, which had only HBA basicity, only depended on the HBD acidity of the solute. Compared to geraniol, the3-fluorophenol with higher HBD acidity formed stronger hydrogen-bond interaction with tetrahydrofuran. Since both1.3-dichlorobenzene and L-carvone had no the HBD acidity, only weak van der Waals’forces could be formed. L-carvone with larger molecular volume and higher polarity formed stronger van der Waals’ forces with tetrahydrofuran. Amphiprotic Methanol could form two kinds of hydrogen-bond complexes with amphiprotic3-fluorophenol and geraniol. Relative to L-carvone, the amphiprotic3-fluorophenol and geraniol could form stronger hydrogen-bond interaction with methanol, While, the1.3-dichlorobenzene formed the weakest van der Waals’forces with methanol. The above results from quantum chemical calculation could effectively explain why the cosolvent effect parameter in CO2containing tetrahydrofuran and containing methanol increased in the following order:3-fluorophenol<geraniol<L-carvon<1.3-dichlorobenzene.Based on the diffusion coefficients and cosolvent effect parameters of23training solutes in scCO2and CO2containing10mol%cosolvent (tetrahydrofuran, methanol). the Linear solvation energy relationship (LSER) method was used to develop predictive model. These models for the above three system were validated by making predictions for12test solutes and had good prediction accuracy with the AAD values less than2.45%,3.17%and3.40%, respectively. The coefficients of these models shown that the diffusion coefficients in CO2were strongly dependent on the dispersion force between solute and CO2. and the cosolvent effect parameters in CO2containing10mol%cosolvent (tetrahydrofuran. methanol) were mainly affected by the hydrogen-bond interaction between cosolvent and solute which served as proton donor.更多还原