Effect of physical properties of synthesized protic ionic liquid on carbon dioxide absorption rate

The concentration of carbon dioxide gas has accelerated over the last two decades which cause drastic changes in the climatic conditions. In industries, carbon capture plants use a volatile organic solvent which causes many environmental threats. So, a low-cost green absorbent has been formulated with nontoxicity and high selectivity properties for absorbing carbon dioxide gas. This paper contains the synthesis process along with the structure confirmation using 1H NMR, 13C NMR, FT-IR, and mass spectroscopy. Density, viscosity, and diffusivity are measured at different ranges with standard instruments. The kinetic studies were also conducted in a standard predefined-interface stirred cell reactor. The kinetic parameters were calculated at different parameters like agitation speeds, absorption temperature, initial concentrations of ionic liquid, and partial pressure of carbon dioxide. The reaction regime of carbon dioxide absorption is found to be in fast reaction kinetics with pseudo-first-order. The reaction rate and the activation energy of CO2 absorption are experimentally determined in the range of 299 to 333 K with different initial concentrations of ionic liquid (0.1–1.1 kmol/m3). The second-order rate constant and activation energy of carbon dioxide absorption in the synthesized ionic liquid is found to be 9.48 × 103 m3 mol−1 s−1 and 16.61 kJ mol−1 respectively. On increasing the viscosity of the reacting solvent, the diffusivity of CO2 gas molecules decreases, and thus the rate of absorption decreases. This solvent has shown great potential to absorb CO2 at a large scale.

Keywords Protic ionic liquids (PILs) · Gas-liquid interface · NMR · FTIR · Carbon dioxide absorption · Stirred cell reactor · Kinetic studies Abbreviations A Arrhenius pre-exponential factor m 3 /mol s a Volumetric gas-liquid interfacial area, m −1 B Base (water) C A The concentration of species A in the liquid phase in the foam film, k mol/m 3 C B The concentration of reactant B in the liquid phase in the storage section at time t, k mol/ m 3 C B0 The initial concentration of reactant B in the liquid stream entering the storage section, k mol/m 3 C TETAL Initial concentration of [TETA] [Lactate], k mol/m 3 CO 2 Carbon dioxide gas C 2,i CO 2 concentration at G-L interface, mol/m 3 D A The diffusion coefficient of reactant A in the liquid phase, m 2 /s D CO 2 Diffusion coefficient of carbon dioxide in the [TETA] [Lactate] solution, m 2 /s D

Introduction
Carbon dioxide (CO 2 ) levels rose by 2.6 ppm in 2019, faster than the average rate for the last 10 years, which was 2.37 ppm. This has drawn a concerned mark globally. The major sectors responsible for CO 2 emission are manufacturing industries and fossil fuel-based industries (Li et al. 2019). So, there is a high demand for improving the existing mitigation methods for minimizing the CO 2 level in the atmosphere. Currently, in absorption technology, CO 2 gas is scrubbing using alkanolamines. Among alkanolamines, monoethanolamine (MEA), diethanolamine (DEA), and methyl diethanolamine (MDEA) along with some promoters are widely used. Research is ongoing to find new and eco-friendly solvents having high selectivity and absorption capacity for carbon dioxide that reacts faster and require less energy to regenerate in comparison to the existing solvents (Liu et al. 2018). The rate of CO 2 absorption and regeneration rate of CO 2 -rich solvents depend upon their chemical composition and thermophysical properties. In 2001, Brennecke et al., checked the solubility of nine gases viz., CO 2 , CH=CH, C 2 H 6 , CH 4 , Ar, O 2 , CO, H 2 , and N 2 in [C 4 mim] [PF 6 ], and the studies reveal that CO 2 has the highest solubility (Lei et al. 2014) . The ionic liquids have the greatest affinity for CO 2 gas in [C4mim] [BF 4 ] at ambient conditions (Lei, Dai, and Chen 2014). A new ionic liquid 1-propylamine-3-butyl-imidazolium tetrafluoroborate could chemically absorb CO 2 at ambient pressure (Bates et al. 2002). ILs are proven as a promising solvent for the separation of CO 2 from a mixture of gases selectively and efficiently (Costa Gomes 2007). The carbon dioxide gas can be taken up physically (Shiflett et al. 2010) or chemically in ionic liquids. The nature of the cations and anions plays a significant role in the solubility and selectivity of a gas.
The solubility of CO 2 in ionic liquids depends on the following parameters: • Effect of fluorination of the cation: The investigation established that the CO 2 is more soluble in an ionic liquid having higher fluorinated cation in comparison to others (Anderson et al. 2007). In the tested ionic liquids, the solubility of CO 2 was found to be higher in [C 8 F 13 (Ma et al. 2011). Yang et al. (2014 studied the effect of water addition in the solution of IL and MEA. In such mixtures, less energy is consumed for the regeneration of riches solvents in comparison to conventional molecular solvents (Xue et al. 2011). Mohanty et al. (2010) investigated the amino acids with different types of amine sites (primary and secondary amines) to understand the effect of the nature of amino acid anion on CO 2 absorption capacity (Mohanty et al. 2010).
The biggest limitation with the protic ionic liquids is their continuous increase in viscosity on absorbing carbon dioxide due to the formation of carbamate. So, this article covers the development of ionic liquid from amine, their structure elucidation using 1 H NMR, 13 C NMR, FTIR, and mass spectroscopy. The effect of density and viscosity on the rate of CO 2 absorption was also investigated experimentally. In earlier publications, the limited reaction kinetics of CO 2 in protic ionic liquids are available. They concluded that these ionic liquids reacted rapidly with CO 2 and were capable of absorbing a high quantity of CO 2 stoichiometrically. This article also contains detailed kinetic studies of CO 2 absorption in stirred cell reactors. The stirred cell reactor is a stirred vessel with an undisturbed (flat) gas-liquid interface, and both the phases are stirred separately. Due to which the k L (liquid − side mass transfer coefficient)value decreases in comparison to the other reactors like bubble column and jet reactor. For fluids like water and gases like CO 2 and O 2 , the k L values are lying in the range of 2-15 × 10 −3 cm/s (Gates 1985). For kinetic studies in gas-liquid reactions, stirred cell contactor is probably the most versatile reactor to employ at the lab scale. Many gas absorption studies have been taken in the stirred cells operated at the speed of 20-150 rpm without significant vortex formation. Sauchel and the group studied the absorption of ammonia in an aqueous acid solution for the manufacture of nitrogenous fertilizers (Sauchel 1960). In 1964, Sharma reported CO 2 absorption in carbonate buffers with or without a catalyst (Sharma 1964). The absorption studies of oxygen in aqueous sodium sulfite were done by Linek and the group (Linek 1966). Gupta and Sharma (1967) worked on CO 2 absorption in barium sulfide. Miller (1969) examined the absorption of C 2 H 4 in ethylene dibromide. Chaudhari and Doraiswamy (1974) used a mechanically agitated contactor for absorption of phosphine in aqueous solutions of formaldehyde and HCl. Sridharan and Sharma (1976) studied the carbon dioxide absorption rate in amines and alkanolamines dissolved in organic solvents such as isopropanol, n-butanol, cyclohexanol, aqueous diethylene glycol, toluene, and o-xylene in the stirred contactor. Oyevaar and Westerterp (1989) investigated the solubility of phosphine in aqueous solutions of sodium hypochlorite and sulfuric acid. Lahiri et al. (1981) did experimental studies on the dissolution of NO in aqueous solutions of alkaline sodium dithionite. Kucka et al. (2003) studied the absorption of CO 2 in MEA solution in a stirred cell. Jean-Mare et al. (2009) investigated the CO 2 absorption in the mixture of N-methyl diethanolamine and triethylenetetramine. Zhou et al. (2012) analyzed the CO 2 -absorption kinetics in tetramethylammonium glycinate [N 1111 ] [Gly] and 2-amino-2-methyl-1-propanol solution (Stevanovic et al. 2013). Ying and Eimer (2013) performed a kinetic study of CO 2 -absorption in aqueous MEA solution at different temperatures and concentrations of MEA. Iliuta et al. (2014) worked out CO 2 absorption in diethanolamine/ionic liquid emulsions (Ying and Eimer 2013). All these studies were performed in a stirred tank reactor. Some of the activation energies of CO 2 absorption with the stirred cell reactor at ambient condition are tabulated in Table 1.
This article is based on the study of CO 2 absorption in the transformed amine into an ionic liquid which is a novel work itself. In literature, kinetic studies on such transformed protic ionic liquid are very rarely done on stirred tank contactors with separate stirrers for both phases. Secondly, the synthesized ionic liquid is new, and its properties are not available in the literature.

Proposed reaction mechanisms.
The chemical absorption reactions of carbon dioxide with primary and secondary amines are identified well in literature initially by Caplow (1968) followed by Danckwerts (1979) (Sun et al. 2017). This mechanism involves two steps: (I) First step: formation of the CO 2 amine complex (Caplow 1968). It includes two steps.
(i) Formation of zwitterion intermediate complex: (ii) De-protonation of the zwitterion (Danckwerts 1979): (1) Theoretically, one mole of carbon dioxide gas reacts with two moles of amines, i.e., 0.5 mol of CO 2 react per mole of amine, whereas in the case of a tertiary amine absorb CO 2 in a 1:1 mol ratio (Lee et al. 2007). In this research, the synthesized ionic liquid is composed of four amine groups, having two primary amines and two are secondary amines. As per the basicity rule, the secondary amines are more basic and have a higher affinity towards CO 2 than the primary amines, so the reaction takes place initially at secondary amines.

Chemicals and methodology
All chemicals and gases specified in Table 2 were used as purchased; no further purification was carried.
The purity of gases was checked by the ULTIMA-2100 series gas chromatography of Nettle make. All solutions were prepared using de-ionized water in volumetric glassware.

Synthesis of ionic liquid
Equi-molar quantities of triethylenetetramine and lactic acid were taken in a three-necked round bottom flask equipped with a reflux condenser, a pressure funnel, and a mechanical stirrer. Initially, triethylenetetramine was taken in the round bottom flask, and then lactic acid was added drop by drop using a pressure funnel with constant stirring at 120 rpm using the mechanical stirrer. As the neutralization reaction produces a lot of heat, which is equal to −9 kcal/mol, the round bottom flask was kept in an ice-water mixture. To avoid any contamination with air, the reaction mixture was placed under the N 2 atmosphere. Nitrogen is an inert gas and has not reacted to any of the components of air and the reacting mixture.
It was then stirred for several hours at room temperature. The completion of the reaction was monitored by TLC (Jeong et al. 2019). The product was a pale-yellow viscous liquid. The product was then washed with dichloromethane 2-3 times to remove impurities. Residual water is removed by vacuum heating at 80 °C for 12 h. The ionic liquid was finally stored in an airtight flask. The water content of the ionic liquid was analyzed using the TGA analysis and found to be 0.01% (w/w).

Saturation studies of carbon dioxide absorption
In all absorption experiments, pure carbon dioxide gas is used. The gas was pre-dried using infrared lamps and passed through a packed bed of 3 A° molecular sieves. The flow rate was 0.002 m 3 min − 1 . Experiments were conducted in a round bottom glass reactor with an effective volume of 500 cm 3 . The equilibrium study of carbon dioxide absorption was conducted in a 0.7 M solution of the ionic liquid at a temperature of 298.15 K and 1 atm for 70 h. The reactor was loaded with approximately 300 cm 3 of the 0.7 M ionic-liquid solution. The reactor was then sparged with nitrogen to remove all the air impurities present above the interface of the solution before each experiment. The vapor pressure of the ionic-liquid and solvent in the gas phase of the reactor was assumed to be constant, and equal to the total equilibrium pressure before the introduction of solute gas carbon dioxide to the reactor. Carbon dioxide was introduced into the reactor via a mass flow controller (MFC) and a carbon dioxide rota-meter. Absorption reached a thermodynamic equilibrium after a sufficient period, corresponding to the solubility limit of the gas in the ionic liquid at that temperature. A sample of about 1 ml of the reaction mixture was taken out at regular intervals of 5 min and weighed. The samples were then titrated with 2 M HCl solution to analyze the carbon dioxide loading at a regular interval. The experimental setup is depicted in Fig. 1.
The amount of carbon dioxide absorbed in the ionic liquid solution was calculated by the following equation by assuming ideal gas behavior:

Determination of physical properties of ionic liquids
The physical properties of the pure ionic liquid have a significant impact on gas-liquid reactions (Wei et al. 2020). The viscosity, density, diffusivity, and Henry constant of the synthesized ionic liquid play an important role in the diffusion process due to which the absorption rate of the carbon dioxide is affected (Littel et al. 1992). These properties are determined using the standard instruments experimentally and theoretically both.

Viscosity
The dynamic viscosity of the pure ionic liquid was measured at different temperatures from 273 to 388 K using a Rheometer (Make: Anton Paar, Model: MCR302 SN81193479). Watanabe et al. (Chaudhary and Bhaskarwar 2015) reported that the three main factors for the high viscosity of ionic liquids are due to its size, shape, and interaction between the anion and the cation. The interaction forces between anion and cation are coulombic forces, van der Waals forces, and hydrogen bonding. Out of these forces, the coulombic forces are responsible and in carbon dioxide absorbed ionic liquid hydrogen bonding mainly causes viscosity. The viscosity of ionic liquid limited the diffusivity of the gas molecule in the ionic liquid.

Density
The density of synthesized ionic liquid was measured from 293 to 363 K using a densitometer (Make: DE45, Model: Mettler Toledo). The density of ionic liquid depends on the length of the alkyl chain. With each increase in the -CH 2 (4) Moles of CO 2 absorbed ( ) = (760 mmHg − vapor pressure of water at ambient condition in mmHg)(Volume of displaced fluid − Volume of 2M HCl) group, the density of ionic liquid decreases which increases the free volume or sites available for CO 2 interaction.

Diffusivity D CO
The rate of diffusion plays a significant role in the rate of absorption of a gas in a liquid. So, the diffusivity of CO 2 has been determined using the Stokes-Einstein equation at various temperatures in the reactive solution.
I. Diffusivity of CO 2 in water is given by II. Diffusivity of CO 2 in aqueous amine was considered the same for amine-based ionic liquids. (Penders-van Elk et al. 2013)

Henry's law constant ( H CO
The solubility of a gas depends on the pressure exerted by a gas on the liquid surface. The pressure is different at different temperatures. So, it is also an important parameter in CO 2 absorption. The values of Henry's law constant for the CO 2 -H 2 O system at different temperatures were obtained from Sander (2015)

Stirred cell reactor
In gas-liquid absorption systems, semi-batch and countercurrent contacting schemes predominate for better efficiency and absorption (Levenspiel 1999). A double jacketed stirred cell reactor was used for the determination of the kinetics of CO 2 absorption in ionic liquid solutions. A schematic representation of the stirred cell reactor and dimensions of the reactor and its internals are shown in Fig. 2 and Table 3. Both phases were stirred separately using the different impellers to minimize the mass transfer resistance. A pair of 6-bladed-disk-turbine impellers and 4-bladed-disk-turbine impellers of stainless steel was used in the gas phase and liquid phase simultaneously. These impellers were mounted on a standard shaft. Digital temperature sensors (PT100) and pressure gauges were provided to monitor the temperature and pressure in real time for measuring CO 2 absorption. The temperature inside the reactor was maintained constant to within ±0.1 °C using a double jacket in which water is circulated continuously. A vacuum pump is also equipped with a gas-outlet port to remove the gases and volatile impurities from the reactor at the beginning and end of the absorption experiments.

Methodology
All experiments were conducted in the above-mentioned reactor in a semi-batch mode with the 0.5 M aqueous solution of synthesized ionic liquid. The ionic liquid solution is charged in/as a batch, and pure CO 2 gas is passed Fig. 2 Schematic representation of stirred cell contactor (adapted from Doraswamy and  (1. Gas-outlet port, 2. Gas-inlet port, 3. Gasside impeller, 4. Liquid-side impeller, 5. Liquid sample port, 6. Shaft) (Gates 2018)  continuously through it from the bottom in a semi-batch mode of process. The CO 2 -absorption experiment begins with the sparging of N 2 gas into the reactor at 2 LPM for 15 min to eliminate all residues of CO 2 or all the gaseous impurities present. Leakages of gas and liquid from the experimental setup were also tested before the conduction of experiments using soap solution. A calibrated rota-meter was used to measure the CO 2 flow rate, accompanied by a digital mass-flow controller (MFC). Initially, 500 ml of an aqueous solution of ionic liquid was charged into the reactor, and then pure CO 2 gas was introduced in it through the gas inlet port. During the experiment, both phases were stirred constantly and individually. At the beginning and end of the experiment, the temperature and pH of the liquid phase were recorded using the pre-calibrated fitted instruments. After a specific period, 1 ml of the reacted solution was sampled and tested by the Chittick apparatus (Crossno et al. 1996) for calculating CO 2 gas loading. In-flow of the CO 2 reactor and the agitation were stopped at the time the sample was drawn. The absorption runs were repeated different times until the ionic liquid solution got saturated with CO 2 gas. The absorption experiments are also conducted using the same method and by varying the different parameters. Arrhenius plot is also made by conducting the absorption studies at different temperatures, and activation energy of CO 2 absorption was calculated.

Equations and reactions involved
The gas-absorption system, i.e., a stirred cell chosen is shown schematically in Fig. 3, which illustrates the concentration profiles of carbon dioxide in gas-phase bulk, in gas film, and the film and in the bulk liquid. Let us assumed that CO 2 gas absorption in ionic liquid solution is an irreversible second-order reaction and can be represented as: A general equation for the second-order reaction, Here, A represents the CO 2 gas, B represents ionic liquid, and P is the liquid product formed after the chemical absorption reaction between A and B. For our system, the above equation can be modified as Consider the following cases of absorption Case 1: When the carbon dioxide reacts entirely in the gas-liquid interface film with film thickness (δ), the enhancement factor is calculated from the following expression (Dudukovic 1986) Fig. 3 Schematic representations of a stirred cell contactor with a flat interface, and b partial pressure and concentration profiles of CO 2 in the gas phase and liquid phase, respectively during the diffusion and chemical reaction of CO 2 in ionic liquid (fast reaction regime)

3
Case 2: When there is no change in the concentration of ionic liquid in the liquid film, the following condition will be satisfied The enhancement factor, E, is a function of the Hatta number (Ha) and the instantaneous enhancement factor (E i ) defined as Case 3: For the fast pseudo-first-order reaction regime, the following conditions should be satisfied, Ha > 2, E i /H a > 5, H a ≈ E, and H a and k ov are related by or The second-order forward reaction-rate constant, k 2 , can therefore be calculated from The governing differential equation for the diffusion and chemical reaction of carbon dioxide in the film can thus be written as where k m = k mn .C n TETAL , and the concentration of the ionic liquid in the solution remains constant throughout the film.
Then, the specific rate of CO 2 absorption can be calculated as, The rate of transfer of CO 2 from gas to liquid phase can be expressed by the rate expression for liquid film by The second-order reaction rate constant, k 2 , may then be obtained using Eq. (20).

Characterization of synthesized ionic liquid
The photograph of synthesized ionic liquid shown in Fig. 4 is a pale-yellow oily appearance liquid. The structure of ionic liquid was confirmed using various instrumental techniques, viz. FT-IR, 1 H NMR, 13 C NMR, and mass spectroscopy. All the data obtained by different structure elucidation techniques are explained using the standard database library provided on the internet. The structure of the prepared ionic liquid is shown in Table 4.
The prepared ionic liquid is the result of the transformation of triethylenetetramine (TETA) to triethylenetetrammonium (TETA + ) cation and lactate anion from lactic acid.

FT-IR data
The IR spectrum of ionic liquid (TETAL) was done using the PerkinElmer infrared spectrophotometer with DTGS KBR detector, and results are explained using Fig. 5 and Table 5. For analysis, a small layer of sample is put between two KBr pallets. Before testing, KBR powder was finely powdered with a mortar and pestle and dried in a vacuum oven at 60 °C before being formed into pellets with a thickness of 0.8 mm. The wavelength range of 4000-650 cm −1 was used to obtain infrared spectra. Before comparison with the standard spectrum, all spectra were baseline corrected and normalized using the spectrum in-built feature. From FTIR studies, it is revealed that the given synthesized molecule contains both primary and secondary amine basic sites. The presence of basic sites makes it suitable for absorbing acidic gases. The molecule is showing multiple strong bands in the range of 3625.79 cm −1 for aliphatic amines (-N-CH 2 ) and 3438.49 cm −1 aliphatic alkyl chain.

NMR
Two types of NMR were done: proton NMR ( 1 H-NMR) and carbon-13 ( 13 C-NMR) to check the position and environment of different hydrogen atoms and carbon atoms in the

H-NMR
The residual proton in DMSO-d6 appeared at 2.50 ppm set as the 1 H NMR external reference. The positions and environment of different hydrogen atoms or protons present in the ionic liquid are shown in Fig. 6 and Table 6. The proton NMR of synthesized ionic liquid shows the characteristic chemical shifts which help in confirming the structure of the ionic liquid. The chemical shift at 2.34-2.72 ppm represents the protons associated with carbon and nitrogen in the structure. The broad and small peaks visible at 4.4 ppm denote the protons associated with primary and secondary amines.

C-NMR
13 C NMR spectra of ionic liquid were also obtained using Bruker 300 MHz spectrometer having N 2 as a carrier gas to finalize the position of different carbon atoms present in the ionic liquid. The different chemical shifts for various types of carbon present in DMSO-d 6 and ionic liquid are discussed below in Table 7. Figure 7 contains the 13 C chemical shifts of the ionic liquid cation relative to tetramethylsilane (TMS). The 13 C chemical shifts at 40, 45, and 50 ppm show the presence of carbon atoms in the environment of primary and secondary amines respectively. The characteristic peak at 179.38 is the confirmed peak of the carboxyl group present in the lactate ion. The carbon associated with the -OH group in lactate ion is shown at 67.38 ppm.

Mass analysis
The mass analysis of ionic liquid is done using a mass spectrometer with cation and anion moieties, quadrupole, and model: MICROTOF II. The base peak appears at 279.20 m/z, and the molecular ion peak appears at 206.1245 m/z. The molecular mass of ionic liquid was approximately 256.20 g mole −1 depicted from the mass spectrum shown in Fig. 8.

Saturation studies of carbon dioxide absorption
The experimental studies of CO 2 absorption reveal that 0.7 M aqueous solution of the ionic liquid can absorb up to 1.57 mol of carbon dioxide per mole of ionic liquid at ambient conditions as shown in Fig. 9. There is a  (Broad and small)-NH, NH 2 continuous increase in the CO 2 concentration in ionic liquid with time. After 70 h of continuous bubbling, the solution became very viscous which stops the further diffusion of CO 2 molecules in it, and it showed leveling off in the concentration of CO 2 . As per the literature, at the beginning of absorption, the carbamate formation predominated, but the end carbamate is converted into bicarbonates which are watersoluble. By wet chemistry test, it was confirmed as the end product is ammonium bicarbonate. Lower energy is required to break the bicarbonate bond in comparison to the carbamate as calculated from Gaussian 03. Thus, carbon dioxide-rich ionic liquid requires less energy for regeneration.

Viscosity
The viscosity of solvent in gas-liquid absorption reactions plays a very important role in chemical kinetics. Most of the ionic liquids are highly viscous due to which the pumping cost increases (Mota-Martinez et al. 2017). The viscosity of the ionic liquid affects the diffusivity of gas molecules. Therefore, the viscosity of the synthesized ionic liquid was measured as a function. The change in viscosity is very fast with an increase in temperature as shown in Fig. 10.
This indicates that at 308 K, its viscosity is low as water solvent. On this basis, the diffusivity constant can be taken as the same for the same concentration and temperature. All the other hydrodynamic properties which play an important role in CO 2 absorption are calculated and tabulated below in Table 8.
The viscosity, diffusivity, and Henry constant for CO 2 in different concentrations of ionic liquid are calculated using Eqs. 6 and 7 discussed above in the experimental section in detail. These parameters are important for determining the solubility of carbon dioxide in the absorbing solvent. The rate of CO 2 absorption is affected by the change in these parameters. On increasing the temperature, the viscosity, and pressure exerted by the CO 2 molecule decreases which facilitates the diffusivity of CO 2 molecules in the solution of ionic liquid. So, the effect of temperature on these parameters is calculated in above Table 8.

Density
The density of synthesized pure ionic liquid was measured using a densitometer (Model: DE45 Mettler Toledo) with a precision of ±0.005 kg/m 3 in the range of 283 to 363 K. The volume of the sample taken was 15 ml. The decrease in density is showing a linear profile for the increase in temperature as depicted in Fig. 11. The density and viscosity of the liquid phase have the greatest impact on the packing height design as well as the absorption unit's capital cost.

Influence of stirring speed
In a gas-liquid absorption reaction, the rate of gas absorption also depends on the reactor dimensions, the geometry and number of impellers, and the stirring speed. The absorption rate is directly affected by the diffusional domain for Viscosity, Pa.s Temperature, K which a large interfacial area was required. In a stirred reactor, the stirring improved the diffusion of the gas into the liquid film. The influence of the stirring speed on the liquid side mass transfer coefficient was investigated. The agitation speeds were kept relatively low to avoid disturbing the planar interface. The liquid-side mass-transfer coefficient can be represented as (Littel et al. 1992) (22) k L = f , , D CO 2 , D s , N L , D Stirred cell In Eq. (22), k L (liquid side mass transfer coefficient) is a function of ρ, density of the absorbing solution, μ, viscosity of the absorbing solution, D CO 2 , the diffusivity of CO 2 in the absorbing solution, D s ,the diameter of impeller blades in the liquid phase, N L , stirring rate in the liquid phase, D Stirred cell , the inner diameter of a stirred tank reactor. It was noticed that the measured k L a increased on increasing the stirring speed up to a certain extent, and then, it remained constant. This is due to the decrease in the liquid film thickness δ responsible for the resistance to the mass transfer of carbon dioxide gas molecules. From the experimental measurements shown in Fig. 12, it is observed that the rate of CO 2 absorption appears practically constant between 60 and 80 rpm. In this range, values of volumetric mass transfer coefficient are the same (k L a) (Fig. 13); therefore, the reaction is in the kinetic regime at this stirring speed.

Effect of initial concentration of ionic liquid and partial pressure of CO2 gas
In this study, the pure carbon dioxide gas was bubbled in different initial concentrations of ionic liquid at 308 K and 101,325 kPa at a flow rate of 2.83 × 10 −5 m 3 s −1 . The rate of CO 2 absorption was also determined for the different partial pressure of CO 2 gas by the pressure dropping method.  As per the studies, on increasing the concentration of ionic liquid, the carbon dioxide uptake increases up to certain limit and then becomes constant. During CO 2 absorption, the solution temperature also increased by ±5°C due to an exothermic process but try to maintain it using a double jacket filled with circulated water. From the experimental data, the rate constant at different concentrations of ionic liquid is calculated using the equation is tabulated in Table 9.
It is also observed that with CO 2 uptake, the viscosity of the absorbing solution also increases, which in turn decreases the diffusivity of further CO 2 gas and thus the rate becomes constant after some time.
The experimental data are plotted in Fig. 14. The reaction rate for the studied chemical absorption reaction is found to be first-order for both initial concentration of [TETA] [Lactate] and partial pressure of CO 2 which is in good agreement with the available literature (Yuan and Rochelle 2018) (Blauwhoff et al. 1984).

Effect of temperature on absorption
The effect of temperature has also been tested on the rate of CO 2 absorption. As per the theory, the rate of reaction doubles at every 10 °C increment in temperature. Initially, with an increase in the temperature, the rate of absorption of CO 2 gas increases as shown in Fig. 15. At different temperatures, the second-order reaction rate constant, k 2 , can be calculated using the Arrhenius expression (Jamal et al. 2006) Here, A is the Arrhenius constant or pre-exponential constant (m 3 mol s −1 ), Ea represents the activation energy (kJ mol −1 ), and R represents the universal gas constant (0.008315 kJ mol −1 K −1 ) The plot of ln k 2 versus 1000/T, leads from the Arrhenius expression for the kinetic constant From the graph, The second-order reaction rate constants follow Arrhenius behavior with the activation energy of 16.61 kJ mol −1 measured for [TETA] [Lactate] which is in good agreement with similar ILs (Gurkan et al. 2013). Table 10 shows the calculated values of the second-order rate constant for CO 2 absorption at various temperatures.
The overall reaction rate expression for CO 2 absorption is therefore deduced as: As, 1.026 ≈ 1 and 1.146 ≈ 1,

Observations and conclusion
This technique of transformation proved to be better for CO 2 absorption at high temperatures. In this work, we have reported the absorption capacity of synthesized (29) R CO 2 = 9.48 × 10 3 m 3 mol −1 s −1 .C TETAL .C CO 2 ionic liquid and the effect of physicochemical properties on CO 2 absorption rate. The synthesized protic ionic liquid is easy to synthesize and also a low-cost ionic liquid. The carbon dioxide absorption capacity in an aqueous solution of ionic liquid reached up to 1.57 mol of carbon dioxide absorbed/mol of ionic liquid. This represents the very good absorption capacity to cost ratio in comparison with the benchmarked solvent, MEA (0.55 mol of CO 2 /mol of amine). The kinetic regime of carbon dioxide absorption corresponds to a fast pseudo-first-order reaction for the controlled conditions employed in this research. The second-order reaction rate constants follow Arrhenius behavior with the activation energy of 16.61 kJ mol −1 measured for [TETA] [Lactate]. The ILs studied in this work exhibit reactivity comparable to or higher than commonly used ammonium-based IL. In the aqueous [TETA] [Lactate] solution, the pseudo-first-order rate constant at different temperatures for the CO 2 absorption reaction was found to be 9.48 × 10 3 m 3 mol −1 s −1 . The physical properties like viscosity, diffusivity, and density of ionic liquid played a significant role in the rate of reaction kinetics. At last, the authors conclude the statement that the transformation of amines into ionic liquid could be the benchmark solvent for carbon dioxide absorption in the future.
Author contribution Amita Chaudhary: experiments, conceptualization, writing -original draft, data collection, and compilation, and manuscript processing. Ashok N Bhaskarwar: revising original draft of the manuscript and technical guidance Funding The authors are grateful to the CSIR-UGC, Delhi, for providing financial support to one of the authors in conducting this research work.

Competing interests
The authors declare no competing interests.