Impregnation of Silica Gel with Choline Chloride-MEA as an eco-friendly adsorbent for CO2 capture

Deep eutectic solvents (DES) are a generation of ionic liquids that benefit from low cost, good stability, and environmental-friendly features. In this research, a porous silica gel was impregnated with a eutectic Choline Chloride-Monoethanolamine solvent (ChCl-MEA) to greatly improve its CO2 capture performance. In the impregnation, the weight percentages of ChCl-MEA were used in the range of 10–60 wt% at a temperature of 25 °C. The effect of ChCl-MEA loading on the structural properties of the DES-modified silica samples was studied by BET, FTIR, and TGA analyses. Investigation of the CO2 adsorption performance at different operational conditions showed that the modified silica gel with 50 wt% ChCl-MEA (Silica-CM50) presents the highest CO2 capture capacity of 89.32 mg/g. In the kinetic modeling, the fractional order model with a correlation coefficient of 0.998 resulted in the best fit with the experimental data. In addition, the isotherm data for Silica-CM50 were well-fitted with the Dual site Langmuir isotherm model with a correlation coefficient of 0.999, representing two distinct sites for the adsorption process. Moreover, the thermodynamic parameters including Enthalpy, Entropy, and Gibbs free energy at 25 °C were obtained to be − 2.770, − 0.005 and − 1.162, respectively. The results showed the exothermic, spontaneous and feasibility of the adsorption process.


Preparation of deep eutectic solvent
To prepare a deep eutectic solvent consisting of choline chloride and Monoethanolamine with a molar ratio of 1:8 (ChCl: MEA), the desired amounts of these substances were placed in a closed bath using a closed vial (Fig. 2).To homogenize the solvent, the solution is stirred at a temperature of 60-70 °C for 30 min using a heater to obtain a uniform solution.The resulting solution was left at the ambient temperature for 24 h and after that used to modify the adsorbent.

Preparation of DES impregnated silica gel
Due to the importance of using porous adsorbents in the CO 2 capture, as shown in Figs. 1 and 2. In this research silica gel were modified by the impregnation method, with eutectic solvent.First, silica gel was dried at 150 °C for 3 h in a vacuum oven to remove its moisture.After that, different weight percentages of eutectic solvent (10-60 wt%) were stirred with methanol for 30 min at room temperature.After complete dissolution, each solution was poured into a vial with silica gel and placed in ultrasonic for 5 min and then stirred at room temperature for 3 h.The methanol solvent was removed at room temperature and finally, the resulting samples were dried in an oven at 40 °C for 24 h to obtain the desired adsorbent powder.The obtained DES-modified samples were named as Silicon Varbide-Carbon-Metal x (Silica-CMx), where x refers to the weight percent of ChCl-MEA.
All procedures and formulation of the synthesis reaction are presented in Fig. 2.

Characterization methods
The presence of surface functional groups was determined using a Fourier transformed-infrared 8400S spectrometer (Shimadzu Corporation, Japan).The surface area and porosity parameters of the samples were specified using N 2 physisorption at 77 K using a Micromeritics ASAP 2020 analyzer.Brunauer, Emmet and Teller (BET) method was used to determine the surface area of the adsorbents.Pore volumes and pore size distribution were calculated by the Barret-Joyner-Halenda (BJH) method.In addition, Thermogravimetric analysis (TGA) was used for thermal analysis of the adsorbent.The surface morohology of samples was studied by scanning electron microscopy (SEM) analysis.

CO 2 adsorption experiments and setup
Carbon dioxide adsorption was performed on bare silica gel and modified silica gel at room temperature using a system consisting of a stainless-steel batch reactor, which includes inlets and outlets for gas to pass through the adsorbent surface 13 .As shown in Fig. 3, this experimental set-up was equipped with a pressure regulator and a thermocouple to adjust the pressure and desired temperature, respectively.The adsorbents, which were prepared www.nature.com/scientificreports/ in different weight percentages, were placed inside the reactor using a mesh-shaped chamber.At this stage, the temperature of the device is adjusted and the reactor door is completely closed to isolate the system so that the thermal balance between the adsorbent and the system can be achieved.As the gas enters the reactor, the adsorption operation begins.It must be ensured that the reactor outlet valve is closed at this stage.At the start of the operation, information about the pressure and temperature of the reactor can be seen by the system monitor.By checking the observed pressure drop when this value is almost constant, it

CO 2 adsorption calculations
The moles of CO 2 adsorbed into the DES-based sorbent ( n L CO 2 ) was calculated as follows: where P total is the total inner pressure of the reactor,P sat solution is the vapor pressure of the aqueous alkanolamine solution at the given temperature, R is the gas constant, T c is the critical temperature, T r is the reduced temperature, P c is the critical pressure and ω is the acentric factor, Z is the compressibility factor and calculated using Soave-Redlich-Kwong (SRK) equation of the state.Superscripts G and L refer to the gas and liquid phases, respectively 47 .

Characterization of DES-modified silica gel
FTIR spectra of the experimental samples of silica gel modified by eutectic solvent with six different weight percentages and a sample of bare silica gel were examined.An infrared spectrometer (Shimadzu Corporation, Japan) was used for spectral analysis.The FTIR spectra of the samples can be seen in Fig. 4. Peaks at 800.74 cm −1 and 1098.11cm −1 corresponded to Si-O and Si-O-Si of silica gel, respectively 48 .The peak at 3438.51 cm −1 was assigned to the N-H bonding from choline chloride 49 .The peak at 1100 cm −1 , which shifted to 1070 cm −1 , can be linked to the C-N bonding 48 .Haider et al. reported that due to the formation of hydrogen bonds in the DES, peaks have the same wavenumbers, therefore, through the formation of different types of hydrogen bonds there (1) www.nature.com/scientificreports/ is some physical interaction 50 .The presence of amine in the silica gel can be detected with the presence of the peak at 2900 cm −1 , which i intensified as the DES load is increased.Nitrogen adsorption and desorption analysis were used to show the surface properties and porosity of silica gel modified by eutectic solvent and pure silica gel.Samples were placed in a vacuum oven for resuscitation before analysis.In this analysis, the samples of pure silica gel and modified silica gel with 30, 50 and 60% have been used.Also, the analysis has been done using a Micromeritics ASAP 2020.To explain the isotherms, Brunauer, Emmett and Teller (BET) classification was provided 51,52 .According to Fig. 5, the adsorption and desorption isotherms for all the samples belong to type IV, which according to IOPAC classification indicates the mesoporous of the adsorbent samples 53 .The results of the sample analysis are given in Fig. 5. Nitrogen adsorption and desorption isotherm for the bare silica and DES-modified silica.
Table 3.According to the results, it can be seen that the specific surface area of the adsorbent is reduced and the pore size is increased with increasing the weight percentage of the eutectic solvent, which may be due to the pore blocking, pore filling, and thickening of ChCl-MEA layer onto the silica gel 25,52,54 .
Figure 6 illustrates temperature decomposition of bare silica and Silica-CM50.According to the Fig. 6, bare Silica did not display any weight loss at high temperature.That is because Silica nano-particles had high thermal stability 55 .However, by increasing temperature to 200 °C, moisture of both sample disappeared.In the TGA analysis of Silica_CM50, weight loss happens in two stages.Initially, as the temperature rises to 300 °C, the www.nature.com/scientificreports/sample's moisture evaporates.Subsequently, as the temperature climbs to 400 °C, the impregnated Choline chlo-ride_MEA decomposes and gets removed from the silica gel matrix 56 .The weight loss seen in the TGA analysis stems from the thermal breakdown of the impregnated material when exposed to higher temperatures.This decomposition results in the liberation of gases and volatile substances, leading to a reduction in the sample's 35% overall weight 38 .
Figure 7 shows the Silica-CM50 sample before exposure with CO 2 and bare Silica morphology.There were unclear changes in the sample structure after the modification with Choline Chloride-Monoethanolamine. The Silica surface was covered with ChCl: MEA, therefore, the presence of the pore on the surface of silica could not be clearly seen.

CO 2 capture capacity
To compare the CO 2 adsorption capacity of the bare silica and Silica-CM x samples, the adsorption experiments were performed at 25 °C and an initial pressure of 6 bar.As can be seen in Fig. 8, the CO 2 adsorption capacity increases with increasing DES loading on silica up to 50 wt% and then the CO 2 uptake decreases for the silica-CM60 sample.Modification of silica gel with ChCl-MEA enhances the CO 2 capacity from 31.02 mg/g to 35.77,  57 , Bare silica mainly adsorbs CO 2 through physical interactions such as van der Waals forces and electrostatic interactions.On the other hand, DES-modified silica can enhance CO 2 adsorption due to the functional groups in the DES that can chemically interact with CO 2 molecules, leading to stronger adsorption.Additionally, specific functional groups in the DES, like amino groups or hydroxyl groups, can assist in the chemisorption of CO 2 through mechanisms like hydrogen bonding or acid-base interactions.Understanding these interactions at a molecular level is crucial for elucidating the CO 2 adsorption process of DES-modified silica and improving its performance for practical applications 58 .Hence, despite a significant drop in surface area with DES loading, the higher CO 2 uptake of Silica-CM x samples could be attributed to the strong interactions between CO 2 and Silica-CM 50 .
Decrease in CO 2 adsorption capacity can be attributed to several factors.Initially, bare silica primarily adsorbs CO 2 through physical adsorption, where CO 2 molecules are attracted to the surface of the silica via weak van der Waals forces.Conversely, DES-modified silica can demonstrate improved CO 2 adsorption performance due to the presence of DES molecules, which can interact with CO 2 through specific chemical interactions like hydrogen bonding or Lewis acid-base interactions.Moreover, the presence of DES molecules on the silica surface can enhance CO 2 adsorption by offering additional adsorption sites and facilitating specific interactions with CO 2 molecules.Nevertheless, at high DES loadings, these advantages may be offset by factors such as pore blockage or competitive adsorption, resulting in a reduction in CO 2 uptake 59,60 .Figure 9 shows mechanism for CO 2 capture by Silica-CM 50.

Adsorption kinetics
Three widely applied kinetic models, including pseudo first, second, and fractional order models were applied to study the kinetic performance of the Silica-CM x samples for CO 2 capture.The kinetic models and their integrated forms after applying the boundary condition (q t = 0 at t = 0 and q t = q e at t = ∞) are written as follows: Pseudo first order 61 : Pseudo second order 62 : Fractional order 63 : where q t and q e are the adsorption capacity at the time t and at equilibrium, respectively, k 1 and k 2 are the pseudo first order and pseudo second order adsorption rate constants, respectively, and k n , m, and n are the constants for the fractional order kinetic model.To obtain kinetic information about CO 2 adsorption onto Silica-CM x samples pervious mentioned kinetic models were fitted to the experimental data.For instance, the curves generated by kinetic models as well as the experimental data for bare silica and silica-CM 50 are depicted in Fig. 10a,b.
To enhance the adsorption process, kinetic modeling was employed to specify residence time and adsorption rate.The kinetic models assist to measure adsorption mechanism and adsorption rate as well.All experimental runs were performed at 25 °C and 7 bar 64,65 .The corresponding kinetic parameters, coefficient of determination (R 2 ) for regressions and error are summarized in Table 4.The pseudo first-order model is often employed to estimate the kinetic of CO 2 physisorption on solid sorbents 66,67 .This model does not properly fit with the experimental data at the initial and final stage of adsorption, especially at higher ChCl-MEA loadings.In contrast, the pseudo-second-order model presented a better agreement with the experimental data.Other previous studies have also reported some limitations for the pseudo-first and second-order kinetic models to predict CO 2 adsorption on amine-modified silica 10,63 .The fractional order model, being put forward by Heydari-Gorji and Sayari 63 to describe CO 2 capture on polyethylenimine-impregnated mesoporous silica, offers the best description for the CO 2 adsorption behavior onto Silica-CM x samples over the entire adsorption range.The parameter m of this model refers to diffusion resistance, and n demonstrates the effect of the driving force (number of unoccupied sites) 63,68 .In general, m decreases with increasing ChCl-MEA loading because the pores blocked by excess deep eutectic solvent lead to slower CO 2 diffusion.In addition, small values of the parameter n indicate that the adsorption rate is less dependent on the driving force at higher ChCl-MEA loading 69 .In the case of CO 2 adsorption by Silica_CM 50 , both bare silica and deep eutectic solvent (DES) modified silica may fit the fractional order model better for several reasons: The adsorption of CO 2 on silica surfaces can involve multiple, simultaneous reaction pathways that are not adequately described by simple integer-order kinetics.The fractional order model allows for the representation of these complex pathways more accurately.Both bare and DES-modified silica have heterogeneous surfaces with a variety of active sites.The fractional order model can account for the different reactivities of these sites, which is not possible with integer-order models.The process of CO 2 adsorption by silica may be limited by diffusion, especially in the pores of the silica matrix.Fractional order kinetics can incorporate the effects of anomalous diffusion, which is more representative of the actual process.The interaction between CO 2 and the silica surface might involve the formation of intermediate complexes.The kinetics of these complex formations and decompositions can be better described by fractional orders due to their non-integer stoichiometry.The energy barrier for CO 2 adsorption varies across the surface of the silica.The fractional-order model can accommodate the distribution of these energy barriers better than integer-order models.These reasons contribute to the better fit of the fractional order model for describing the kinetics of CO 2 adsorption by both bare silica and DES modified silica.It's important to note that the specific reasons can vary based on the characteristics of the silica used and the conditions of the adsorption process 70 .

Adsorption isotherms
The adsorption isotherms can be used to describe the nature of the CO 2 -adsorbent interactions.With that end in view, the experimental equilibrium data were fitted according to the most common isotherm models: Langmuir, (8) where b and q m are Langmuir parameters representing the affinity constant (1/bar) and the maximum monolayer uptake capacity (mg/g).P is the equilibrium CO 2 partial pressure (bar) and q e refers to the amount of gas adsorbed at this pressure (mg/g).Dual site Langmuir predicts the adsorption over a heterogeneous surface, assuming two distinct adsorption sites would be available to gas molecules: where the subscripts refer to sites 1 and 2. Freundlich isotherm is the earliest empirical model describing the adsorption process.Freundlich equation can be applied to study the multilayer adsorption of gas molecules on heterogeneous surfaces 72 : ( 14) q e = q m bP 1 + bP where k is the Freundlich constant and n is an indicator of the surface heterogeneity.The accuracy of each model was assessed by coefficient of determination (R 2 ) and an average relative error (ARE): where q exp and q mod denote the experimental and model predicted values of adsorption uptakes, respectively, n is the number of experimental data.To find out the temperature influence on the CO 2 uptake capacity, the equilibrium isotherm data were measured for the Silica-CM 50 sample, which exhibited substantially higher CO 2 capacity value among all studied samples.Figure 13 shows the adsorption isotherms of CO 2 on Silica-CM 50 at 25, 40 and 60 °C and 7 bar; the fitting of isotherm data with Dual site Langmuir model is illustrated as well.In the studied range of pressure, the maximum CO 2 capture is 89.32 mg/g, obtained at 25 °C.As can be seen from Fig. 11, at higher temperatures, the CO 2 adsorption capacity decreases, indicating the exothermic nature of the adsorption process.This could be because with rising temperature, the interaction between CO 2 molecules and sorbent surface becomes weak and the desorption process is initialized.
To figure out the surface characteristics of Silica-CM 50 , Langmuir, Dual site Langmuir, and Freundlich isotherm models have been fitted to the obtained equilibrium data and the corresponding parameters along with the R 2 and error values are listed in Table 5. Langmuir model showed a significant deviation from the experimental data which could be because this model assumes a monolayer CO 2 uptake and does not consider the surface ( 16)  www.nature.com/scientificreports/heterogeneity.Freundlich isotherm follows the experimental data more accurately.However, the higher R 2 and lower error values confirm that the dual-site Langmuir model fits best with the isotherm data.This reflects the heterogeneous nature of the surface which is supported by values greater than one obtained for the n parameter in Freundlich isotherms.Also, the affinity constant b indicates a reducing trend with temperature increasing, verifying the exothermic nature of the CO 2 uptake process 73,74 .

Adsorption thermodynamic
To find out the effects of adsorption temperature on DES-modified adsorbent, CO 2 adsorption curves of Silica-CM50 at various adsorption temperatures are tested, as appeared in Fig. 12.It can be seen that by raising the temperature from 298 to 343 K, a decrease in the uptake ability of CO 2 can be observed.The thermodynamic parameters are obtained from the adsorption experiments at 298, 313, 328 and 343 K and 5 bar.Thermodynamic parameters, including ΔG, ΔS and ΔH, are used to evaluate the thermodynamic feasibility of the process between gas molecules and Silica-CM50 and to confirm the nature of the uptake process.The laws of thermodynamics shared with the experimentally attained adsorption data obtained from the Langmuir isotherm can be used to evaluate the thermodynamic limitations according to Eqs. ( 18) and ( 19): The ΔS and ΔH were determined from the intercept and slope of the linear variation of ln K d against 1/T as shown in Fig. 13, with Eq. ( 20):  ΔG is used to understand the spontaneity of the adsorption process, which is also a vital factor.The negative ΔG values were acquired for CO 2 at the three above-mentioned temperatures, reflecting the gas uptake was thermodynamically spontaneous and favourable at low temperatures.The positive or negative ΔS values lead to an increase or a decrease in the randomness at the gas-solid interface during the adsorption process, respectively.The negative ΔS value (0.00541 kj/mol K) demonstrates the decrease in entropy during the adsorption process, which indicates the high order of adsorbent molecules in the adsorption process.
ΔH is a critical parameter to examine whether the adsorption process is exothermic or endothermic according to its negative or positive value.The negative ΔH value (− 2.774 kJ/mol) indicated the exothermic nature of CO 2 uptake on this synthesized sample, which was the explanation for the reduction in uptake amount at higher temperatures.These results can be attributed to the higher temperatures, the interaction between the adsorbent surface and gas molecules becomes weak and the results in the initialization of the desorption process 68 .The summary of the calculated thermodynamic parameters is listed in Table 6.

Desorption analysis
Adsorbent regeneration and activation are the most important issues in the design of adsorbents for practicable applications 75 .The regeneration studies was performed at 25 °C and 9 bar.The feasibility of regeneration was determined by desorption studies for the adsorbent using nitrogen gas. Figure 14 shows the adsorption   www.nature.com/scientificreports/capacity (q) diagram after 5 steps of CO 2 adsorption and the nitrogen gas was used to reduce the adsorbent with each adsorption step.The decrease in silica_CM50's ability to regenerate for CO 2 capture could be due to several reasons.The silica may deteriorate or disintegrate from frequent exposure to high temperatures during regeneration, leading to a decrease in surface area and pore volume, which affects CO 2 adsorption.Deactivation of active sites by CO 2 molecules and performance decline caused by material degradation from repeated adsorption-regeneration cycles may also play a role in reducing efficiency over time.

Conclusion
The primary objective of this investigation is to analyze the CO 2 adsorption capabilities of untreated silica gel and silica_CMx with varying ratios for CO 2 capture.Initially, silica gel was impregnated with Choline chloride_MEA in various proportions in order to accomplish this goal.Our research team observed through experimentation that silica_CM50 demonstrated a superior CO 2 adsorption capacity in comparison to untreated silica gel.The distinguishing feature of our study is the utilization of the impregnation technique with Choline chloride_MEA, which facilitated the enhancement of the CO 2 adsorption characteristics of the silica gel.This novel methodology differentiates our study from prior investigations in the realm of CO 2 capture utilizing silica-based materials.Also, by examining the structural properties of the adsorbent, it was founded that the confinement of DES reduced the BET surface area.The presence of surface functional groups was determined using Fourier transformed-infrared 8400S spectrometer and Thermogravimetric analysis was performed for samples.In addition, experiments were performed to investigate isothermal and kinetic models of the adsorption process.
It was observed that the isotherm model is consistent with the dual Langmuir isotherm model, which refers to heterogeneous adsorption.The study of kinetic models showed that the fractional order model had the best fit with the adsorption data.The thermodynamic parameters indicated the exothermic nature of CO 2 uptake on this synthesized sample which was the explanation for the reduction in uptake amount at higher temperature.Silica-CM50 may not be the top performer in CO 2 adsorption compared to other materials, but it has notable advantages as a CO 2 adsorbent.These include a high adsorption capacity of 89.32 mg/g at 25 °C, selectivity for CO 2 , stability under various conditions, ease of regeneration for multiple cycles of CO 2 capture, and the ability to be produced in large quantities at a reasonable cost.Evaluating its potential for cost-effective CO 2 capture should consider these strengths.Further research may reveal more benefits and optimization strategies for Silica-CM50 in CO 2 capture applications.

Figure 2 .
Figure 2. Formulation of the synthesis reaction.

Figure 4 .Figure 5 .
Figure 4. FTIR spectra for pure silica gel and DES-modified silica with six different weight percent.

Figure 12 .
Figure12.Effect of temperature on uptake of CO 2 on Silica-CM50 at different temperatures at 5 bar.

Figure 14 .
Figure 14.CO 2 adsorption capacity of bare silica and DES-modified silica after 5 adsorption and regeneration step.

Table 1 .
Solubility of CO 2 in different DESs at different molar ratios, temperatures and pressures.

Table 2 .
CO 2 adsorption capacity at different samples.

Table 3 .
BET Analysis Results for the bare silica and DES-modified silica samples.

Table 4 .
Results of CO 2 adsorption kinetic models from experimental data of bare silica gel and DESmodified silica gel with different ratios.

Table 5 .
Results of CO 2 adsorption isotherm models from experimental data of Silica-CM50.