Ionic liquids (ILs) have shown great potential to be used as absorbents for CO2 separation owing to their unique properties such as immeasurably low vapour pressure, high thermal stability, high CO2 affinity, and tunable structure. A huge amount of researches have been carried out, and most of them focused on developing novel ILs for CO2 separation. However, high viscosity will impede the IL-based absorbents for the large-scale applications in CO2 separation.

The goal of this thesis was to develop IL-based hybrid green absorbents (i.e. choline-2-pyrrolidine-carboxylic acid/polyethylene glycol/H2O ([Cho][Pro]/PEG200/H2O) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/titanium dioxide /PEG200 ([Hmim][Tf2N]/TiO2/PEG200)) for CO2 separation, in which H2O and/or PEG200 were used as co-solvents to adjust the viscosity. A systematic study was conducted from pure to multi-component systems convering experimental measurements and theoretical modeling of thermdynmaics and kinetics, and thus the effects of co-solvents on properties, phase equilibria, and kinetics were also analyzed.

To provide reliable and systematic properties and study the effect of H2O, the properties of PEG200 and PEG200/H2O were surveyed, evaluated, and correlated. The density and viscosity of [Cho][Pro]/PEG200/H2O were measured and correlated, and the excess mole volume and excess Gibbs energy of activation were estimated to understand the molecular structure and interactions between [Cho][Pro]/PEG200 and H2O. The results showed that H2O and PEG200 could decrease the viscosity of the hybrid absorbent significantly, and H2O interacted strongly with [Cho][Pro]/PEG200.

To obtain reliable and systematic gas solibilities and analyze the effect of H2O, the solubilities of pure CO2, CH4, and N2 in PEG200 were surveyed, measured, evaluated, and described with theoretical models. The CO2, CH4, and N2 solubilities in PEG200/H2O were measured and described with the Redlich-Kwong Nonrandom-Two-Liquid (RK-NRTL) model. The CO2 solubilities in [Cho][Pro]/PEG200 and [Cho][Pro]/H2O were also surveyed, measured, evaluated, and described using the RK-NRTL model. Furthermore, CO2 solubility in the hybrid absorbent was predicted with the model and compared with the new experimental results. The further investigation showed that the formation of carbomate dominated the entire CO2 absorption process by [Cho][Pro]/PEG200/H2O, and the presence of H2O resulted in the formation of bicarbonate.

To study the kinetics systematically combined with the effect of H2O, the liquid-side mass-transfer coefficients (kL) of [Cho][Pro]/PEG200/H2O were obtained from the CO2 diffusion coefficients () and the kL values in PEG200/H2O, in which was calculated from the density and viscosity of the hybrid absorbents, while the kL values in PEG200/H2O were measured experimentally. The reaction rate constant of CO2 in the hybrid absorbents was also estimated.

Based on the systematic studies from experimental measurement and modeling, it showed that 30 wt%[Cho][Pro]/H2O + PEG200 could be promising for CO2 separation with the consideration of both thermodynamic and kinetic properties. In addition, the hybrid absorbent also showed the good recyclability.

To use the confinement as the other strategy for further developing IL-based technology, the CO2 solubility in [Hmim][Tf2N]/TiO2/PEG200 with different ratios of [Hmim][Tf2N]/TiO2 was measured, and the Henry’s constant and absorption enthalpy of CO2 in the hybrid absorbent were calculated. The CO2 solubility in the hybrid absorbent was described with the Henry's law, the contribution of the confinement effect on the CO2 solubility was quantified, and the relationship between the roughness of TiO2 and molecular parameters was established. The results showed that the mass ratio of [Hmim][NTf2]/TiO2 in the hybrid absorbent should be lower than 5.72 %, where the contribution of confinement occupied around 20 % of the total CO2 absorption capacity in [Hmim][NTf2]/T500/PEG200. 2.88 wt%[Hmim][NTf2]/T500 + PEG200 could be promising for CO2 separation on aspects of CO2 capacity and absorption enthplay, and further study will be conducted on the properties and CO2 absorption kinetics.

Developing novel hybrid ionic liquid/porous material/co-solvent absorbents with the confinement effect is essential for CO₂ separation. In this study, CO₂ solubilities in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/titanium dioxide/polyethylene glycol ([Hmim][Tf₂N]/TiO₂-PEG200) with different ratios of [Hmim][Tf₂N]/TiO₂ and various roughnesses of TiO₂ (P25 and T500) were measured and described with the Henry's law. Furthermore, the contribution of the confinement effect on the CO₂ solubility was quantified, and the relationship between the surface roughness and molecular parameters was established for predicting its contribution to the confinement effect. In addition, the hybrid absorbent was recycled by a multi-cycle experiment. The results show that the contribution of confinement effect on CO₂ absorption capacity (on mass basis) and Gibbs free energy occupy around 24.5 % and 8.12 % in [Hmim][NTf₂]/T500-PEG200 (w_{[Hmim][NTf2]/T500} = 2.88 wt%) at 308.2 K, respectively. The surface roughness can double the confinement effect. Based on the CO₂ absorption capacity and enthalpy, [Hmim][NTf₂]/T500-PEG200 (w_{[Hmim][NTf2]/T500} = 2.88 wt%) is a promising hybrid absorbent for CO₂ separation.

To develop novel solvent for CO2 capture, CO2 absorption performance using the aqueous of polyethylene glycol 200 (PEG200) and choline-2-pyrrolidinecarboxylic acid ([Cho][Pro]) was studied and evaluated systematically in this work, in which the critical properties of PEG200 were estimated with group contribution method, and other thermo-physical properties were determined experimentally or taken from literatures directly and then correlated with empirical equations. The CO2 solubility in PEG200 was measured and represented with the Henry’s law and Poynting correction, while the measured CO2 solubility in PEG200/H2O was correlated with RK-NRTL model. [Cho][Pro] was used as the chemical ingredient to enhance the absorption capacity and rate of CO2 in [Cho][Pro]/PEG200/H2O, and the corresponding properties and CO2 solubility were studied. The kinetic parameters, such as enhancement factor (E), reaction rate constant (k), and activation energy (Ea) of CO2 in [Cho][Pro]/PEG200/H2O were estimated from the new experimental data measured in this work and compared with the commercialized aqueous MEA solution. The process simulation and pilot-testing based on [Cho][Pro]/PEG200/H2O will be performed in the future.

To develop polyethylene glycol 200 (PEG200) and aqueous PEG200 solutions (PEG200/H2O) as solvents for CO2 separation, in this study, the available thermo-physical properties of PEG200 and PEG200/H2O measured experimentally were surveyed, evaluated, and correlated with empirical equations. The solubility of CO2 in PEG200 was also surveyed, evaluated and described with the Henry's law with the Poynting correction, while the solubilities of CH4 and N2 in PEG200 were determined experimentally and then described with the Henry's law. The CO2, CH4 and N2 solubilities in PEG200/H2O were measured and described with the Redlich–Kwong Nonrandom-Two-Liquid (RK-NRTL) model. In addition, the performances of PEG200, PEG200/H2O and other commercialized physical solvents for CO2 separation were discussed based on the properties, and the biogas upgrading was chosen as the example to quantitatively evaluate the performances of PEG200 and PEG200/H2O with process simulation and compared with the high pressure water scrubbing (HPWS). It shows that the total energy usage and the amount of recirculated solvent for biogas upgrading can decrease by 9.1% and 26.5%, respectively, when H2O is replaced by PEG200 completely.

The development of novel absorbents is essential for improving CO2 separation technology. In this study, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide/titanium dioxide/water ([Hmim][NTf2]/TiO2-H2O) was developed to separate CO2, where the thermodynamic and kinetic experiments were conducted, and Henry's constant and the liquid-side mass-transfer coefficient were determined accordingly. Furthermore, CO2 separation performance in a bubble tower was validated. A previously proposed index named “absorption ability” (AA) was used to predict and compare the experimental results. Additionally, the cost of biogas upgrading (i.e., CO2 removal for biogas purification) using [Hmim][NTf2]/TiO2-H2O was estimated. The results showed that for the developed [Hmim][NTf2]/TiO2-based technology, the average CO2 mass-transfer rate was increased by 20.0% compared with the current commercialized technology, and the contributions from the thermodynamic and kinetic aspects were 2.5% and 17.5%, respectively. The cost of biogas upgrading was 16.6% lower. In addition, AA successfully predicted the performance of CO2 separation technologies, achieving an average relative deviation of 8.1%.

Developing novel hybrid absorbents is essential for CO₂ separation. In this study, the density and viscosity of a hybrid absorbent (choline-2-pyrrolidine-carboxylic acid/polyethylene glycol/water ([Cho][Pro]/PEG200/H₂O)) were measured experimentally, and its CO₂ solubility was also determined. The excess mole volume and excess Gibbs energy of activation of the hybrid absorbent were further estimated to understand the molecular structure and interactions between [Cho][Pro]/PEG200 and H₂O. The CO₂ solubilities in [Cho][Pro]/PEG200 and [Cho][Pro]/H₂O were analyzed and described using the Redlich–Kwong non-random-two-liquid (RK-NRTL) model. Furthermore, the CO₂ solubility in the hybrid absorbent was predicted using the RK-NRTL model and was compared with the new experimental results for verification. The effect of H₂O on the CO₂ absorption performance was further analyzed. The performance and cost of the hybrid absorbent were compared with those of other commercialized CO₂ absorbents. In addition, the recyclability of the hybrid absorbent for CO₂ separation was studied. The results of this study indicated that the hybrid absorbent could be promising for CO₂ separation.

Understanding the mass-transfer kinetics of CO₂ in novel hybrid absorbents with physical and chemical contributions is essential for process design and evaluation. In this study, the liquid-side mass-transfer coefficients (k_L) and second-order reaction rate constants (k₂) of CO₂ in hybrid absorbents (namely, choline-2-pyrrolidine-carboxylic acid salt/polyethylene glycol/water ([Cho][Pro]/PEG200/H₂O)) were determined. The k_L values for the hybrid absorbents were obtained from the CO₂ diffusion coefficients (D_CO2) and the k_L values in PEG200/H₂O. The D_CO2 value was calculated from the density and viscosity of the hybrid absorbents, whereas the k_L values in PEG200/H₂O were measured experimentally. The k₂ values of CO₂ in the hybrid absorbents were estimated according to the reaction mechanism, the enhancement factor, and the k_L values, and compared with those of other commercialized absorbents. The results showed that 30 wt% [Cho][Pro]+70 wt% H₂O had the highest k_L and k₂ values at atmospheric pressure, whereas the values of k_L and k₂ of CO₂ in 30 wt% [Cho][Pro]/H₂O+PEG200 were comparable to those in diethanolamine aqueous and amino-functionalized ILs. The hybrid absorbent of [Cho][Pro]/PEG200/H₂O could be promising for CO₂ separation considering its thermodynamic and kinetic properties.

Developing immobilized-ionic liquids (ILs) sorbents is important for CO₂ separation, and prior theoretically screening ILs is desirable considering the huge number of ILs. In this study, the compressibility of ILs was proposed as a new and additional index for screening ILs, and the developed predictive theoretical model, i.e., electrolyte perturbed-chain statistical associating fluid theory, was used to predict the properties for a wide variety of ILs in a wide temperature and pressure range to provide systematic data. In screening, firstly, the isothermal compressibilities of 272 ILs were predicted at pressures ranging from 1 to 6,000 bar and temperatures ranging from 298.15 to 323.15 K, and then 30 ILs were initially screened. Subsequently, the CO₂ absorption capacities in these 30 ILs at temperatures from 298.15 to 323.15 K and pressures up to 50 bar were predicted, and 7 ILs were identified. In addition, the CO₂ desorption enthalpies in these 7 ILs were estimated for further consideration. The performance of one of the screened ILs was verified with the data determined experimentally, evidencing that the screen is reasonable, and the consideration of IL-compressibility is essential when screening ILs for the immobilized-IL sorbents.

Aqueous choline-glycine ([Cho][Gly]) solution is a potential candidate for CO₂separation owing to its excellent absorption performance and biodegradability. Moreover, the aqueous solution is easy to volatilize at high temperatures. In this work, H₂O was partially replaced with polyethylene glycol (PEG200) and the effect of PEG200 on the CO₂ separation performance in [Cho][Gly])/H₂O was investigated. The viscosity of [Cho][Gly]/H₂O/PEG200 and CO₂ solubility in the solution were determined experimentally in the temperature range 308.15–338.15 K at pressures ≤6.5 bar. Further, the measured CO₂ solubility was fitted with the reaction equilibrium thermodynamic model and the CO₂ desorption enthalpy was estimated. The regeneration performance of [Cho][Gly]/H₂O/PEG200 was also evaluated. The results revealed that [Cho][Gly]/H₂O/PEG200 has a low CO₂desorption enthalpy and high regeneration efficiency. Particularly, [Cho][Gly]/H₂O/PEG200 with 30 wt% PEG200 has a high regeneration efficiency of 95%. Owing to its physical-chemical properties and CO₂ separation performance, [Cho][Gly]/H₂O/PEG200 shows great potential as an absorbent for CO₂ separation.

CO2 capture and separation (CCS) is a key step to mitigate greenhouse gas emissions and develop renewable energy. The trade-off between the rate and efficiency in the CO2 separation process cannot be solved with the traditional process intensification. Using nano-interface to realize process intensification has been widely used in the chemical process with multi-phase transfer, and CO2 separation is one of examples. This review summarizes the research work from the establishment of CO2 transfer model at nano-interface and the resistance regulation, the acquisition of the CO2 chemical potentials at equilibrium and at the nano-interface (the driving force regulation) and the molecular simulation analysis of the interface enhancement mechanism. Based on the theoretical studies, the resistance distribution for the CO2 separation process in a real absorption tower is further analyzed and a "three-stage strengthening scheme" is proposed to decrease the investment and operating costs. © All Right Reserved.

In this work, aqueous pentaethylenehexamine (PEHA) was studied as a solvent for CO2 removal to produce purified bio-syngas from biomass gasification, but also as a first step towards negative carbon emissions applying carbon capture and storage (CCS) technologies. Capture of CO2 was tested both with synthetic gas (labscale) and real syngases from the pilot-scale gasifier fed with a wide range of forest-based biomasses. The results showed that the effects of the components other than CO2 and the impurities from the real syngas on the performance of PEHA for CO2 removal are negligible. Combined with previous research results from labtesting with pure CO2 absorption, the aqueous PEHA was shown to be a promising solvent for CO2 removal from syngas. PEHA was also tested as a biomass pre-treatment agent to improve gasification behavior, however, no significant improvement could be identified during the tests performed in this study.

In this work, an aqueous pentaethylenehexamine (PEHA) solution was studied for CO₂ removal from bio-syngas for the first time. Firstly, pure CO₂ absorption in aqueous PEHA solution under different conditions was conducted, and 20 wt% PEHA solution was identified as the best option. Secondly, the capture of CO₂ was tested with synthetic syngas from a gas cylinder, and the species other than CO₂ showed a negligible impact on CO₂ removal. Finally, to evaluate the practical feasibility of using aqueous PEHA solution on the downstream CO₂ capture, the pilot experiments of gasification with boreal forest-based biomasses were designed to provide real syngas with a realistic distribution in composition for further testing. The results showed that the operating conditions and the type of feedstocks affected the distribution in the bio-syngas composition. Among these feedstocks, at the optimal oxygen supply, using spruce needles generated the highest yields of CO and H₂ and, meanwhile, gave rise to similar yields of other gases such as CO₂, CH₄, etc. The influence of the species other than CO₂ for CO₂ removal was negligible. Additionally, aqueous PEHA solution was tested as a biomass pretreatment agent, showing that no significant changes could be identified by the ultimate analysis (except for increased nitrogen content), but the yields of CO were affected negatively. On the other hand, when using the pretreated biomass by the aqueous PEHA solution, the NH₃ concentration in bio-syngas reached to the highest (4000 parts per million), which slightly affected the CO₂ absorption capacity and initial absorption rate of 20 wt% PEHA solution in a positive way.

Supported ionic liquid (IL) sorbents for CO₂ capture were prepared by impregnating tetramethylammonium glycinate ([N₁₁₁₁][Gly]) into four types of porous materials in this study. The CO₂ adsorption behavior was investigated in a thermogravimetric analyzer (TGA). Among them, poly(methyl methacrylate) (PMMA)-[N₁₁₁₁][Gly] exhibits the best CO₂ adsorption properties in terms of adsorption capacity and rate. The CO₂ adsorption capacity reaches up to 2.14 mmol·g^{− 1} sorbent at 35 °C. The fast CO₂ adsorption rate of PMMA-[N₁₁₁₁][Gly] allows 60 min of adsorption equilibrium time at 35 °C and much shorter time of 4 min is achieved at 75 °C. Further, Avrami's fractional-order kinetic model was used and fitted well with the experiment data, which shows good consistency between experimental results and theoretical model. In addition, PMMA-[N₁₁₁₁][Gly] remained excellent durability in the continuous adsorption–desorption cycling test. Therefore, this stable PMMA-[N₁₁₁₁][Gly] sorbent has great potential to be used for fast CO₂ adsorption from flue-gas.

Ionic liquids (ILs) attract high interest as lubricants or lubricant additives due to their special physicochemical characteristics. CO₂ is a widely distributed gas. In many situations, its influence on lubricants cannot be avoided. In this work, three ILs are synthesized from choline and amino acids of glycine, l-proline and lysine, respectively. The influence of CO₂ absorption on their solubility and lubricity is investigated. In general, it is interesting to find that the solubility is decreased and their lubricity is obviously improved after absorbing CO₂, which is strongly related to their functional group (amino group) interactions with CO₂. The formation of carbamate groups greatly increases the viscosity resulting in less solid contacts, and strengthens the interfacial adhesion between ILs and solids.

A new mixed solvent system that consists of cholinium glycinate ([Cho][Gly]) and aqueous N-methyldiethanolamine (MDEA) solution was developed in this work to serve as CO2 absorbent. The equilibrium absorption was carried out to investigate the effect of solution composition, pressure and temperature on CO2 absorption performance. The effect of CO2 absorption on the viscosity of the aqueous solutions was studied, and the regeneration efficiency of the aqueous solutions was also investigated. The results showed that the CO2 absorption loading decreased with increasing [Cho][Gly] concentration and temperature, and the absorption loading strongly depended on CO2 partial pressure. The reactivity of MDEA was significantly enhanced with the addition of [Cho][Gly]. The aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) showed an optimal CO2 absorption and high regeneration efficiency. Furthermore, the CO2 absorption mechanism in the aqueous [Cho][Gly]-MDEA solution was explored by 13C Nuclear Magnetic Resonance (NMR), which indicated that the CO2 absorption in the aqueous [Cho][Gly]-MDEA solution was zwitterion mechanism.