Knowledge Management System Of Institute of process engineering,CAS
|Thesis Advisor||张香平, 曾少娟|
氨（NH3）是重要的基础化工原料，但其直接排放也会造成严重的污染。在化工生产中含NH3尾气的直接排放，不仅造成了资源浪费，而且对环境带来了污染。基于结构可调、蒸汽压低、热稳定性好、气体溶解度高等优点，离子液体在气体分离与净化领域受到了广泛的关注。尽管系列常规、羟基和金属等功能离子液体被用于NH3吸收，但存在常规离子液体吸收量低，功能离子液体粘度高、合成过程复杂等问题，开发吸收量高、粘度低、合成过程简单的功能离子液体是当前的发展趋势。质子型离子液体是由布朗斯特酸和布朗斯特碱通过酸碱中和形成的离子液体，在阳离子上具有可以形成氢键的质子，为NH3吸收提供了新的手段。目前质子型离子液体研究主要集中于咪唑类离子液体等，因此设计高效可循环的新型质子型离子液体是离子液体NH3分离领域研究的关键。本论文设计并合成了六元氮杂环类质子型离子液体，对离子液体的结构进行了表征，研究了离子液体吸收NH3性能、吸收机理，并提出了可逆吸收判断依据，为设计新型可逆的离子液体提供了指导。本论文的主要研究内容如下：（1）六元氮杂环类质子型离子液体的设计合成及性能评价。设计合成了5种六元氮杂环类质子型离子液体，包括吡啶双三氟甲烷磺酰亚胺盐（[Py][NTf2]）、2-甲基吡啶双三氟甲烷磺酰亚胺盐（[2-mPy][NTf2]）、2,6-二甲基吡啶双三氟甲烷磺酰亚胺盐（[2,6-2mPy][NTf2]）、哌啶双三氟甲烷磺酰亚胺盐（[Piper][NTf2]）及N-羟乙基哌啶双三氟甲烷磺酰亚胺盐（[EtOHPi][NTf2]），对以上离子液体的基础物性数据和NH3吸收性能进行了测定和评价。在吸收过程中，两种固体离子液体[Py][NTf2]及[EtOHPi][NTf2]会发生从固态到液态的相变行为，原因是NH3与离子液体之间的氢键作用破坏了离子液体自身的氢键网络；在40℃，0.10 MPa下，吡啶类和哌啶类质子型离子液体最高吸收量为3.078 mol NH3/mol IL和2.406 mol NH3/mol IL；对比吡啶类离子液体的NH3吸收量发现，吡啶类侧链上取代基对NH3吸收量影响较小；由于吡啶环上的质子氢酸性更强，吡啶类离子液体对NH3的吸收量比哌啶类离子液体高，但吡啶类离子液体在吸收饱和后及循环实验中会呈现吸收量下降的趋势，而哌啶类离子液体则保持稳定。（2）六元氮杂环类质子型离子液体吸收NH3机理研究。通过FR-IR、NMR以及量化计算对离子液体吸收NH3过程机理进行了研究，根据离子液体吸收前后的FT-IR谱图变化，说明质子型离子液体与NH3形成了氢键作用，再由吸收过程中核磁氢谱位移变化及量化计算结果，进一步证明质子氢在NH3吸收过程中起主要作用，可与NH3形成强氢键，同时羟基也可与NH3形成氢键，有利于NH3吸收；此外，[2-mPy][NTf2]解吸后红外谱图与吸收前不同，表明[2-mPy][NTf2]不能完全再生；而[EtOHPi][NTf2]解吸后红外谱图与吸收前一致，表明[EtOHPi][NTf2]可完全再生，与循环实验结果一致。（3）吡啶类质子型离子液体NH3吸收量降低原因及可逆判据。针对吡啶类质子型离子液体[2-mPy][NTf2]吸收过程中出现吸收性能逐渐减少的现象，通过ESI及核磁表征分析了下降原因。通过ESI-MS测试结果发现吸收后的[2-mPy][NTf2]-NH3体系中有NH4+生成，通过对NH3夹带气进行核磁表征发现产生了挥发性的2-甲基吡啶；进一步2-甲基吡啶、过饱和体系、[2-mPy][NTf2]的氢谱结果，发现下降原因主要是由于阳离子对应有机碱的pKa小于NH3的pKa，导致过饱和的碱性环境下质子氢被NH3剥离形成NH4+，并产生挥发性的2-甲基吡啶被气流带出，导致性能递减。而对于哌啶类质子型离子液体则不存在这种现象。基于上述结果，提出了质子型离子液体可逆吸收的判据：在吸收过饱和的碱性环境下，当阳离子对应有机碱的pKa小于NH3的pKa时，阳离子上质子氢会被NH3剥离导致NH3吸收量下降；反之，则可实现稳定可逆吸收。;Ammonia (NH3) is an important chemical raw material, but its direct emissions will also cause serious pollution. Direct emissions of NH3-containing exhaust gas in chemical production not only cause waste of resources, but also do harm to the environment. Based on the advantages of adjustable structure, low vapor pressure, good thermal stability and high gas solubility, etc., ionic liquids (ILs) have received widespread attention in the field of gas separation and purification. Although a series of conventional ILs, hydroxyl ILs and metal ILs, etc., functional ILs are used for NH3 absorption, there are problems of low absorption of conventional ILs, high viscosity of functional ILs, and complicated synthesis processes. The development of functional ILs with with high absorption, low viscosity and simple synthesis process is the current development trend. Protic ionic liquids (PILs) is a type of ILs formed by neutralization of Bronsted acid and Bronsted base, which not only has higher NH3 absorption, but also is easy to desorb, which provides a new method for NH3 absorption. Meanwhile, the current research on PILs mainly focuses on imdazolium-based PILs, etc. Therefore, designing efficient and recyclable novel PILs is the key to the research of NH3 separation by ILs. On the basis of the above background, In this thesis, novel protic ionic liquids with six-membered N-heterocyclic cations are designed and synthesized. The structure of the PILs are characterized. Besides, their NH3 absorption performance, the absorption mechanism, and reversibility criteria were studied. The results provide guidance for the design of new reversible ionic liquids. The main research contents and results of this thesis are as follows:(1) Design, synthesis and performance evaluation of six-membered N-heterocyclic PILs. Five kinds of PILs with six-membered N-heterocyclic cations were designed and synthesized, including pyridinium bis(trifluoromethylsulfonyl)imide ([Py][NTf2]), 2-methylpyridinium bis(trifluoromethylsulfonyl)imide ([2-mPy][NTf2]), 2,6-dimethylpyridinium bis(trifluoromethylsulfonyl)imide ([2,6-2mPy][NTf2]), piperidinium bis(trifluoromethylsulfonyl)imide ([Piper][NTf2]), 2-(piperidinium1yl)ethanol bis(trifluoromethylsulfonyl)imide ([EtOHPi][NTf2]). The physicochemical properties of PILs were measured and their NH3 absorption performance were evaluated. During the NH3 absorption process, the two solid PILs will undergo a phase transition from solid to liquid, because the hydrogen bonding between NH3 and PILs destroys the own hydrogen bonding network of PILs. Under 40 °C and 0.10 MPa, the maximum NH3 capacity of pyridinium-based PILs is 3.078 mol NH3/mol IL, and the maximum NH3 capacity of piperidinium-based PILs is 2.406 mol NH3/mol IL; By comparing the NH3 capacities of pyridinium-based PILs, it was found that the substituents on the pyridine side chains had little effect on the NH3 absorption. Since the protic H on the pyridine ring are more acidic, the NH3 capacities of pyridinium-based PILs are higher than that of piperidinium-based PILs. However, Pyridinium-based PILs tend to decrease in NH3 capacity after saturation and in cycling tests, while piperidinium-based PILs can remain stable.(2) Studies on the NH3 absorption mechanism of PILs with six-membered N-heterocyclic cations. Through FR-IR, NMR and quantitative calculations, the mechanism of NH3 absorption by PILs was investigated. According to the change of FT-IR spectrum before and after absorption, it shows that PILs and NH3 form hydrogen bond. Further, the shift changes in 1H-NMR and DFT calculation results prove that protic H plays a major role in the absorption of NH3, which can form strong hydrogen bonds with NH3. The presence of hydroxyl groups are beneficial to the absorption of NH3; The FT-IR spectrum of [2-mPy][NTf2] after desorption cannot completely coincide with that of [2-mPy][NTf2] before absorption, indicating that [2-mPy][NTf2] cannot be completely regenerated.(3) Mechanism of decrease of NH3 capacity in pyridinium-based PILs and the reversible absorption criteria. Through analysis of ESI-MS, it was found that NH4+ was generated in the [2-mPy][NTf2]-NH3 systems after absorption, and 2-methylpyridine was found by 1H-NMR characterization of NH3 entrained gas. By comparing the 1H-NMR results of 2-methylpyridine, [2-mPy][NTf2]-NH3 supersaturated systems and pure [2-mPy][NTf2], it was found that the decrease was due to the pKa of the organic base corresponding to the cation was less than that of NH3, so the protic H being stripped by NH3 to form NH4+ in a supersaturated alkaline environment. Based on the above results, a criteria for the reversible absorption of six-membered N-heterocyclic PILs is proposed in combination with pKa: when the pKa of the organic base corresponding to the cation is less than the pKa of NH3, protic H will be stripped by NH3; otherwise, stable and reversible absorption can be achieved..
|李鹏飞. 六元氮杂环类质子型离子液体的设计合成及氨吸收性能研究[D]. 中国科学院大学,2020.|
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