Designing Of Protic Ionic Liquids For Efficient NH₃ Separation

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)Designing of protic ionic liquids and the study on the relationship between the structures of imdazolium-based PILs and the NH3 absorption ability.A series of imdazolium-based PILs were 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 fractions in[Bim][NTf2]reached 2.703 mol NH3/mol IL and 0.113 g NH3/g IL,respectively and[Bim][NTf2]could be completely regenerated,while the viscosity was only 47.138 mPa-s.This solubility was about 10 times the conventional ILs.The length of the cationic side chain of the PILs had 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 showed 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]could 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 tied the interactions of the cations and anions,which increased the viscosities at the low NH3 concentrations.As the amount of NH3 absorbed on the other active sites increased,the viscosities began to decrease.(3)Designing of diprotic ILs and the study on the NH3 absorption performance and mechanism.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 quantum chemical calculations and found that once one proton atom 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 were 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 as the NH3 absorption time increased.(5)Process for the treatment of tail gas with low NH3 concentration based on protic ILs.A kind of protic absorbent composed of different 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 absorbent has been used in an absorption-desorption continuous apparatus for more than 1000 h and kept stable NH3 removal rate of over 99%.