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|Keyword||质子型离子液体, 双活性位点离子液体, Nh3 分离,氢键|
离子液体由于极低的蒸汽压、良好的热稳定性、结构可设计及较高的气体溶解度等优点在气体分离净化领域备受关注。目前已有少量关于离子液体吸收NH3基础研究的报道，但存在物理吸收NH3的离子液体低分压下吸收能力差、化学吸收NH3的金属离子液体再生困难、功能化离子液体合成工艺较复杂且难以实现规模化制备等问题，因此设计低粘度、高NH3吸收量、可再生的新型离子液体是离子液体NH3分离领域的研究难点。基于以上背景，本论文开展了质子型离子液体和双活性位点离子液体的设计合成、离子液体结构与NH3吸收性能、质子型离子液体吸收NH3机理及NH3吸收过程物性变化的系统研究；基于复合质子型离子液体吸收剂，课题组开发了一套低分压NH3尾气净化回收工艺，为NH3尾气净化回收新技术的开发提供重要支撑。本论文的主要研究内容及成果如下：（1）新型咪唑类质子型离子液体设计合成及吸收NH3构效关系研究。合成了一系列咪唑类质子型离子液体，系统研究了阳离子侧链长度、2-H、阴离子种类以及温度压力对NH3吸收性能的影响，考察了质子型离子液体的再生性能。研究发现质子型离子液体的NH3吸收性能高于文献报道的其它非金属离子液体，在40℃，101.8 kPa下，[Bim][NTf2]的NH3摩尔吸收量和质量吸收量分别达到2.703 mol NH3/mol IL和0.113 g NH3/g IL，是常规离子液体NH3吸收量的近10倍，且可以完全再生，而粘度仅为47.138 mPa?s。质子型离子液体阳离子碳链长短对NH3吸收性能几乎无影响，2-H可以提高离子液体的NH3吸收量；阴离子对应酸的酸性越强，离子液体NH3吸收性能越高；低温、高压有利于提高离子液体的NH3吸收性能。（2）质子型离子液体吸收NH3过程物性变化及机理研究。与常规离子液体[Bmim][NTf2]和羧基功能化离子液体[HOOC(CH2)3mim][NTf2]相比，酸性比前者强、比后者弱的质子型离子液体[Bim][NTf2]表现出最高的NH3吸收量，说明离子液体酸性不是决定NH3吸收量的唯一因素。通过氘代实验中质子氘被NH3置换成质子氢、红外谱图N-H红移及咪唑环振动形式的变化，说明质子型离子液体与NH3形成较强的氢键作用，根据不同NH3浓度的离子液体核磁氢谱位移变化以及阳离子与NH3相互作用的量化计算结果，提出质子型离子液体的NH3吸收机理为质子氢可以先后与两个NH3分子形成氢键。NH3吸收过程中，[Bim][NTf2]-NH3体系密度线性下降，粘度先小幅升高后持续下降，原因是NH3分子破坏离子液体原有结构，进入到阴、阳离子之间，使离子液体阴、阳离子间距增大，因而密度降低，同时NH3分别与阴、阳离子形成的新的氢键作用提高了二者的相互作用能，使得NH3吸收初期离子液体粘度增大，随着其它位点及阴、阳离子间的NH3增多，粘度开始下降。（3）新型双质子型离子液体设计合成及高效NH3吸收研究。基于质子型离子液体质子氢可以与两分子NH3形成氢键作用，进一步设计合成了阳离子结构对称的咪唑类双质子型离子液体[2-Mim][NTf2]和[Im][NTf2]，相同条件下，其NH3摩尔吸收量分别达到3.037 mol NH3/mol IL和3.461 mol NH3/mol IL，且可以完全再生，二者的NH3吸收量较单质子型离子液体[Bim][NTf2]分别高12%和28%，但是并未实现NH3吸收量翻倍增加的效果。通过红外、核磁谱图表征及量化计算的方式考察了双质子型离子液体与单质子型离子液体NH3吸收机理的异同，结果表明，双质子型离子液体的两个质子氢分别与一分子NH3形成氢键作用后，由于NH3推电子作用的影响，只能另有一个NH3分子可以继续与离子液体形成氢键，因此咪唑类双质子型离子液体的预期NH3吸收量为3 mol NH3/mol IL。（4）双酸性离子液体和非咪唑类质子型离子液体设计合成及NH3吸收研究。基于分子量较小的碱金属离子与NH3的络合作用，开发了兼具Br?nsted和Lewis酸性的碱金属质子型离子液体，其中[2-Mim][Li(NTf2)2]的NH3吸收量在40℃，常压下可以达到7.012 mol NH3/mol IL，是双质子型离子液体的2倍以上，且可以完全再生，是一种极具潜力的NH3吸收剂；基于咪唑类质子型离子液体与NH3分子间的氢键作用，设计合成了系列吡啶类和哌啶类质子型离子液体，结果表明，相同条件下NH3吸收性能顺序为：吡啶类 > 咪唑类 > 哌啶类质子型离子液体，吡啶类质子型离子液体[2-mPy][NTf2]最高NH3吸收量为3.015 mol NH3/mol IL，但是由于吡啶阳离子与NH3分子间过强的氢键作用，可能导致质子氢剥离，导致NH3吸收量随时间增长而缓慢降低，其稳定性较咪唑类质子型离子液体差。（5）离子液体法NH3尾气分离连续评价实验。针对低分压NH3尾气，设计了一种氢键强化的复合质子型离子液体吸收剂，测定了该吸收剂的物性、NH3吸收及再生性能，通过热重、模拟NH3尾气吸收-解吸连续实验等方式，考察吸收剂稳定性，目前该吸收剂已在NH3吸收-解吸装置运行超过1000 h，NH3回收率99%以上，吸收剂性能保持稳定。本课题组开发的低分压NH3尾气净化回收工艺，既能满足NH3尾气“净化”的要求，又能实现NH3“回收”利用的目标。;Ionic liquids (ILs) are regarded as a series of new solvents in gas separation field owing to their negligible vapor pressure, high thermal stability, adjustable structures and high selectivity for different gases. Some reports have been published on the fundamental NH3 solubility data in ILs. However, there are still problems causing barriers for the applications of NH3 capture with ILs, such as the poor NH3 solubility in ILs with physical absorption ability at low NH3 partial pressure, the difficulty for NH3 desorption from metal-containing ILs and the the high cost of the systhesis of the hydroxyl-functionalized ILs. Therefore, it is of great significance to design low-cost ILs with high and reproducible NH3 absorption ability and low viscosities for NH3 separation. In this work, the designing of a series of protic ionic liquids (PILs), the NH3 solubility in these PILs, the NH3 absorption mechanism and the variations of physical properties were systematically studied. Based on the PILs absorbents, a set of process was established for the treatment of tail gas with low NH3 concentration. The main innovative work and results are as follows:(1) Study on the relationship between the structures of imdazolium-based PILs and the NH3 absorption ability. A series of imdazolium-based PILs were designed and synthesized and the effects of side chain length of the cation, 2-H, anions, temperatures and pressures on the NH3 absorption performances were systematically studied as well as the recyclability of PILs. The NH3 solubility in PILs was found to be higher than that reported in other nonmetallic ionic liquids. At 40℃ and 101.8 kPa, the NH3 molar and mass fraction in [Bim][NTf2] reach 2.703 mol NH3/mol IL and 0.113 g NH3/g IL, respectively and [Bim][NTf2] can be completely regenerated, while the viscosity is only 47.138 mPa?s. This solubility was about 10 times the conventional ILs. The length of the cationic side chain of the PILs has little effect on the NH3 absorption performance but 2-H would increase the NH3 solubility of the both protic and conventional ILs. As for the anions’ influence, the stronger the acidity of the acids corresponding to the anions, the higher the NH3 absorption ability of the PILs. (2) Study on the physical properties variations and mechanism of PILs absorbing NH3. Taking [Bim][NTf2] as an example, the NH3 absorption mechanism in PILs was investigated. Compared with conventional IL [Bmim][NTf2] and carboxyl-functionalized IL [HOOC(CH2)3mim][NTf2], the PIL [Bim][NTf2] with moderate acidity shows the highest NH3 capacity. Through deuteration experiments, IR and NMR spectra and quantitative calculations, it was found that the protic hydrogen of the [Bim][NTf2] can form hydrogen bonds with two NH3 successively. After NH3 absorption, the densities of [Bim][NTf2]-NH3 decreased linearly as the NH3 concentrations increased and the viscosities first increased slightly and then decreased continuously. The reason may be that NH3 “squeezed” into the anion and cation of the [Bim][NTf2], resulting in an increase in the volume of the PIL and then a decrease in the density of the [Bim][NTf2]-NH3. The new hydrogen bonding between the NH3 and anion and cation ions tied the interactions of the cations and anions, which increased the viscosities at the low NH3 concentrations. As the NH3 absorbed on the other active sites increased, the viscosities began to decrease.(3) Study on the NH3 absorption performance and mechanism of diprotic ILs. Based on the PILs, two kinds of diprotic ILs [2-Mim][NTf2] and [Im][NTf2] were designed and synthesized. The NH3 capacities of the two ILs researched 3.037 mol NH3/mol IL and 3.461 mol NH3/mol IL and were about 12% and 28% higher than that of [Bim][NTf2], which was lower than the expectation of the doubling of the NH3 capacity. Also the mechanism of NH3 absorption in diprotic ILs was investigated through IR, NMR spectra and quantification calculations and found that once one proton formed hydrogen bond with two NH3, it became difficult for more than one NH3 to connect on the other proton side and the expected NH3 capacities of diprotic ILs was 3 mol NH3/mol IL.(4) Study on the absorption performances of dual acidic ILs and pyridinium and piperidinium-based PILs. Based on the complexation of NH3 with lithium ion, kinds of Br?nsted and Lewis acidic ILs were developed and NH3 capacity of [2-Mim][Li(NTf2)2] was up to 7.012 mol NH3/mol IL, which was two times that of diprotic ILs and could be totally regenerated. According to the hydrogen bonds between protic hydrogen and NH3, pyridinium and piperidinium-based PILs were also designed to compare the NH3 absorption performances. The results showed that NH3 solubility order was pyridinium > imidazolium > pyridinium based PILs. However, owing to the strong interactions between NH3 and protic hydrogen of pyridinium based PILs, the protic hydrogen might drop down from the cation the NH3 capacities of pyridinium based PILs decreased after long time NH3 absorption.(5) Establishment of process for the treatment of tail gas with low NH3 concentration based on protic ILs. A kind of protic absorbent BN composed of two ILs was designed and the physical properties, NH3 absorption and regeneration performances of the absorbent were studied as well as the physical and chemical thermal stabilities. The BN absorbent has been used in an absorption-desorption continuous apparatus for more than 1000 h and kept stable NH3 removal rate over 99%.
|尚大伟. 质子型离子液体功能调控及NH3分离研究[D]. 中国科学院大学,2018.|
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