Stability Studies Of Protic Ionic Liquids In The Treatment Of Industrial Waste Gases Containing Ammonia

NH₃ is the main cause of PM2.5,which causes environmental pollution and can also damage human health,causing serious illness and even death.NH₃ is the main cause of PM2.5,which can cause both environmental pollution and damage to human health.However,NH₃ is widely used as an important chemical raw material and is a carbon-free energy storage carrier,which has obvious advantages over hydrogen in terms of storage and transportation.Therefore,the separation and recovery of NH₃ from NH₃-containing waste gases is of great importance to reduce resource waste,reduce environmental pollution and improve resource utilisation.Protic ionic liquids（PILs）,as one type of the ionic liquids,not only have the advantages of very low vapour pressure of ionic liquids,low energy consumption,designable structure and good cyclic absorption and desorption performance,but also show better NH₃ absorption and separation capability than conventional ionic liquids due to the presence of hydrogen bond,and have become the focus of domestic and international research.In industrial applications,the stability of PILs has been found to be overestimated,with problems such as increased viscosity and impurity generation.While current research on degradation mechanisms has focused on conventional ionic liquids,systematic studies on degradation mechanisms of PILs are lacking.Therefore,we simulated the degradation process of PILs in the absorption and utilisation of industrial ammonia-containing tail gas,determined the stability of PILs and characterised the main degradation products,and deduced the degradation process of PILs,in order to provide theoretical guidance to slow down the degradation of PILs and improve their long-term stability.In this paper,three major types of PILs,imidazolium PILs,pyridinium PILs and alcohlamonium PILs,were synthesized by a one-step method.To investigate the effect of cationic modification of imidazole on stability,1-methylimidazole nitrate（[Mim][NO₃]）,1-ethylimidazole nitrate（[Eim][NO₃]）and 1-butylimidazole nitrate（[Bim][NO₃]）with different alkyl chain lengths were synthesized.To investigate the effect of anions on stability,PILs with 1-ethylimidazole as cation and chloride and tetrafluoroborate as anions were synthesised:[Eim][Cl]and[Eim][BF₄],respectively.To investigate the effect of substituent modification on the stability and degradation mechanism of pyridinium PILs,pyridine nitrates（[Py][NO₃]）,2-methylpyridine nitrates（[2-m Py][NO₃]）and 2,6-dimethylpyridine nitrates（[2,6-Dm Py][NO₃]）were synthesised.The synthetic cations were the three alcohlamonium PILs of primary,secondary and tertiary amines:2-amino-2-methyl-1-propanol nitrate（[AMP][NO₃]）,diethanolamine nitrate（[DEA][NO₃]）and N,N-Dimethyl-ethanolamine nitrate（[DMEA][NO₃]）.The PILs synthesised above were structurally characterised using IR and NMR to demonstrate the synthesis of the target ionic liquids,and moisture testing using a moisture meter indicated high purity of the PILs.Thermogravimetric TGA analysis of the thermal stability of the PILs under rapid warming conditions was also carried out,and preliminary exploration revealed that the long-term tolerance temperature of these three major classes of PILs should be less than 130°C.In addition,an NH₃ absorption device was constructed to test the NH₃ absorption performance of PILs and compared with that of conventional ionic liquids[Bmim][NO₃],and most PILs were found to have high NH₃ absorption performance.The experimental temperature of long-term degradation of PILs was determined in combination with the thermal decomposition temperature of TGA analysis,and the degradation of three types of PILs at different pressures（2 MPa/0.1 MPa）and atmospheres（O₂/N₂）was investigated,of which the longest degradation time was up to 600 h.The effect of NH₃ content of PILs on the degradation rate of PILs was also investigated.Using HPLC,the separation and quantification conditions suitable for each proton-type ionic liquid respectively were explored;the analytical methods for characterizing the degradation products of PILs and quantifying the relative content of degradation products by GC-MS were mapped.Quantitative degradation rates by HPLC revealed that for imidazole PILs,the longer the alkyl chain on the imidazole cation,the weaker the stability,while the effect of the anion on stability was greater than the effect of the cation alkyl chain length.The stability of the pyridinium PILs was:[2-m Py][NO₃]<[2,6-Dm Py][NO₃]<[Py][NO₃],with the methyl substituent on the pyridine reducing stability.The stability of the three alcohlamonium PILs was:[DEA][NO₃]<[AMP][NO₃]<[DMEA][NO₃],with the best stability when the cation was a tertiary amine.In addition,GC-MS qualitative and area-normalised quantification of[Bim][NO₃]at different pressures revealed that pressure mainly influenced the rate of degradation of PILs,with similar degradation products.For PILs with nitrate as the anion,degradation in N₂ atmosphere produced the same oxidation products as O₂ atmosphere degradation,with the main degradation processes being the same,only the degradation being less intense than in O₂ atmosphere.