Experimental, RSM modelling, and DFT simulation of CO2 adsorption on Modified activated carbon with LiOH (2024)

Characterization

The FTIR spectrum evaluates functional groups. Figure2 shows the FTIR spectrum for AC before and after LiOH modification. According to the FTIR spectrum of 24Li-AC, 3426.73, 2361.96, 1560.42, and 1161.27 cm⁻¹ peaks were related to C–OH, C-H, C≡C, and C=C bonds, respectively. The results show that the modification of the AC with LiOH has been well justified. According to the figure, it can be said that the peaks before and after active carbon modification in 2356 cm⁻¹ have a difference, which is due to higher CO₂ adsorption in the LiOH modified than unmodified AC.

Adsorbent structure properties such as surface area and pore volume were performed by BET analysis and shown in Fig.3. The BET analysis of unmodified and LiOH-AC is listed in Table.

According to the results in Table 3, surface parameters increase after AC modification. The BET surface area of AC used in this investigation was determined to be 624.55 m2/g, while after modification, the BET surface area obtained 781.84 m²/g. The reason for the increase in the modified adsorbent level is the mesoporous properties of LiOH powder. The reaction of hydroxide lithium with AC increases the adsorbent level and improves the adsorbent level.

Figure4 shows the morphological structure of AC and modified AC with 24%LiOH (24Li-AC) at 50 and 500 µm magnifications. The activated carbon surface's porosity and the adsorbent surface's uniformity with different pore sizes are specified. According to these figures, it can be concluded that the adsorbent porosity in the unmodified case is less than the modified adsorbent, which indicates that LiOH penetrates inside the AC and causes an increase in the surface area and porosity. The 24Li-AC adsorbent has highly cracked surfaces with distinct pore sizes, indicating its suitability for CO₂ adsorption.

Analysis of variance (ANOVA)

ANOVA results of RSM predicted model versus temperature, pressure, LiOH concentration, and adsorbent weight are presented in Table 4. As shown, the predicted model P-Value < 0.0001 determines the model's significance. A, B, C, D, AB, AC, BC, BD, A², and B² are significant model terms in this model. The F-value of the model performed a comparison between the model analysis and the error. The obtained Model F-value of 1607.01 indicates that the model is significant.

The R² value, which measures consistency between experimental and calculated data, is listed in Table 5. The predicted and adjusted R² values are in reasonable agreement and close to 1. Adequate Precision value was obtained at 146.0413 (greater than 4) and is desirable⁴⁹.

The empirical equation in terms of temperature (A), pressure (B), LiOH concentration (C), and adsorbent weight (D) is presented in Eq.(4).

According to the results in Table 4, the calculated data were close to the experimental data quietly, and shows that obtained empirical and equation can explain and predict the CO₂ adsorption process by unmodified and LiOH modified AC accurately.

Parameters effect on CO₂ adsorption capacity

3-D plot of CO₂ adsorption capacity and variables (temperature, pressure, LiOH concentration, and adsorbent weight) are presented in Figs.7, 8, 9, 10 and 11. CO₂ Adsorption capacity versus pressure, and LiOH concentration and temperature, and LiOH concentration are presented in Figs.5 and 7 According to these figure, High concentrations of LiOH cause the adsorbent cavities to close and adsorption capacity to decrease. CO₂ Adsorption capacity versus pressure and temperature is presented in Fig.6, which shows that according to the nature of physical adsorption, the maximum amount of adsorption capacity take place at high pressure and low temperature and the minimum amount of that take place at low pressure and high temperature. According to Figs.5 and 6, adsorption process occurs when the molecules in the gas or the liquid phase reach the solid surface and bond with adsorbent active sites. In gas bulk, the increase in pressure leads to an increase in the movement of the gas molecules to the adsorbent sites, and so, an increase in the velocity of equilibrium and adsorption reaction occurs. Therefore, pressure increasing cause to an increase in adsorption capacity (Fig.7).

Figures8 depicts the relationship between CO₂ adsorption capacity, adsorbent weight, and temperature at a LiOH concentration of 0.5 mol/L and 9 bar. Similarly, Fig.9 illustrates the relationship between CO₂ adsorption capacity, adsorbent weight, and LiOH concentration at 30 °C and 9 bar. According to these figures, by increasing the amount of adsorbent, the amount of hydroxyl salts in the adsorbent increases, and the presence of these salts only leads to an increase in adsorbent weight without contributing to the progress of the carbon dioxide adsorption process.

Optimization on adsorption process

In general, maximum CO₂ adsorption capacity is necessary for adsorbent applications in the solid sorption process, and the optimized parameter points must be determined for each of adsorbents. Therefore, maximum CO₂ adsorption capacity by LiOH modified AC, and the optimum values were obtained using desirability function value⁵⁰. Consequently, the adsorption capacity (model response) was chosen as 'maximize', and independent parameters were selected 'within the range', to achieve the highest capacity. The maximum adsorption capacity was achieved at temperature of 30 °C, pressure of 9 bar, LiOH concentration of 0.5 mol/L with adsorbent weight of 0.5 g.

Pressure effect on adsorption capacity

In order to evaluate the influence of adsorption time, 3-D plot of CO₂ adsorption capacity versus pressure, temperature and time were plotted in Fig.10 and 11. In Fig.10, the highest CO₂ adsorption capacity was obtained at a pressure of 9 bar, indicating that pressure positively affects the adsorption capacity. This trend is higher at higher pressure levels, so the equilibrium is not appreciably visible at higher pressures, and adsorption continues. The effect of pressure on improving the position of molecules in the empty places of the adsorbent and the unreacted adsorbent parts leads to an increase in the gas adsorption capacity. In general, with increasing pressure, the adsorption capacity also increases. Figure11 shows the CO₂ adsorption capacity at different temperatures: 30, 50, 70, and 90°C. The adsorption capacity increases with a decrease in temperature. It can be inferred that LiOH physically adsorbs CO₂ at lower temperatures.

According to Fig.10, the maximum adsorption capacity was detected at high pressure due to the entering of gas molecules in smaller pores with pressure increasing and low temperature with the physisorption mechanism.

Temperature effect on adsorption capacity

Comparison between AC before and after modification is presented in Figs.12 and 13 at different temperature and pressure. According to these figures, the adsorbent modification due to the presence of LiOH has led to an increase in adsorption capacity by decreasing the temperature and increasing the pressure. This increase in adsorption has been constant at almost all temperatures and pressures, and has led to an increase of approximately 25% in the modification of the carbon dioxide adsorption capacity.

Effect of LiOH concentration on adsorption capacity

The LiOH-ACs were utilized as adsorbents to investigate their performance for CO₂ adsorption. The results are indicated in Table 6 and Fig.14.

The results show that the highest adsorption capacity for CO₂ is observed when using activated carbon modified by 24% LiOH (24Li-AC). The unmodified AC can adsorb 60.02 mg/g CO₂. On the other hand, the 12%, 24%, 36%, and 48% LiOH-AC can adsorb 134.05, 154.96, 105.01, and 85.99 mg/g CO₂, respectively.

It was found that modifying AC with one molar LiOH (or up to 24%) had a positive effect and significantly increased the capacity for CO₂ adsorption. The experiment results indicated that 48Li-AC had a lower adsorption capacity. This could be attributed to the excessive concentration of LiOH solution that filled up the pores and cavities of the adsorbent. Additionally, the production of lithium salt hindered further carbon dioxide reactions. Since the 24Li-AC exhibited the best performance for CO₂ adsorption, it was selected for other studies.

Adsorption isotherm model correlation

It is crucial to identify the correct mechanisms and provide a quantitative description of thermodynamic equilibrium to optimize the design of the CO₂ capture system. So, it's essential to understand the equilibrium process to predict how adsorption will occur accurately. Therefore, the experimental equilibrium data for carbon dioxide adsorbed in modified adsorbent was investigated using Langmuir, Freundlich and Dubinin–Radushkevich isotherm models. The isotherm model parameters are given in Table 7.

CO₂ adsorption isotherms at 303 K and pressures ranging from 1 to 9 bar are illustrated in Fig.15a. The data demonstrate that higher pressures lead to increased CO₂ adsorption rates. Table 6 summarizes the experimental results along with the R² correlation coefficients for each isotherm model parameter. Utilizing nonlinear regression techniques and R² values, the effectiveness of the theoretical isotherms in describing and predicting the adsorption behavior of 24Li-AC is ranked as follows: Freundlich > Langmuir > D-R. The superior fit of the Freundlich isotherm model suggests that the modified activated carbon surface is heterogeneous with a wide range of adsorption energies. This model's parameters, the Freundlich constant and exponent, reflect this heterogeneity and energy distribution. A high Freundlich constant indicates substantial adsorption capacity, while a low exponent implies a more linear adsorption isotherm⁵¹. In summary, the Freundlich isotherm model offers critical insights into CO₂ adsorption on 24Li-AC, aiding in the optimization of their design and performance for CO₂ capture applications.

Figure15b depicts the adsorption isotherms for modified activated carbon (24Li-AC) at 30°C, demonstrating the relationship between adsorption capacity (q, in mmol/g) and pressure (P, in bar). This figure offers a detailed view of how the modified activated carbon performs under varying pressure conditions. The error bars in this figure represent the standard deviation from three independent experimental measurements, providing a visual representation of the data's variability and reliability. The error bars account for potential variations in temperature readings due to the accuracy of the thermometer used in the experiments. Similarly, the error bars consider potential inaccuracies in pressure readings resulting from the precision of the pressure gauge.

Adsorption kinetic model correlation

Matching the experimental adsorption data to a set of conventional fixed models is a suitable technique for kinetic modeling due to the complexity of calculating kinetic factors and choosing the best model. Out of all the kinetic models listed in Table 8 that are used to describe the CO₂ uptake process, the first and second models are the simplest in terms of explaining the kinetics of CO₂ adsorption when compared to another models.

The kinetic model results are presented in Table 9. The parameters for each model are listed separately by temperature from 30°C to 90°C. In physical adsorption, the first-order model is suitable for predicting the behavior of CO₂ adsorption. The second-order model assumes that a reliable gas binding causes the interaction between adsorbent and adsorbate, which is more suitable for chemical adsorption when CO₂ adsorption processes involve chemical interactions and chemical bonds.

In Table 9, based on R² values, the best model for the correlation at the temperature of 30 °C is second-order and Ritch second-order. Because AC is modified by LiOH, and this modification is used to increase the rate and increase the adsorption capacity due to the chemical process. Therefore, the second-order model shows chemical interactions well in modified adsorbent. In modified adsorbents, with a temperature rise of 70 °C and 90 °C, the model is suitable for displaying the kinetic state of the Elovich equation (Table 9). The Elovich equation describes an adsorption process as a reactions group, including the release of the bulk phase, surface emission, and active catalytic levels. Also, Elovich considers effective chemical energy changes about the level of surface coating and the reduction of carbon dioxide chemical adsorption. Therefore, it is suggested that CO₂ adsorption for LiOH-AC is attributed to both chemical and physical adsorption modes (Fig.16). These observations agree with the theory that adsorption sites occupy higher levels of energy at first in adsorption systems, in decomposition chemistry before adsorption.

Adsorption thermodynamic parameters

The thermodynamic parameters and the behavior of the adsorption process can be identified by accomplishment an adsorption process at different temperatures. In engineering and environmental processes, both energy and entropy change parameters must be calculated to determine what processes will occur spontaneously. Gibbs free energy change, ΔG⁰, is an essential criterion of self-sufficiency. If the value of ΔG⁰ is negative, the reactions are performed spontaneously at a single temperature. The Gibbs free energy change (ΔG⁰), the enthalpy change (ΔH⁰) and the entropy change (ΔS⁰) are calculated using following equations:

By plotting of ln(K_d) versus 1/T, the values of ΔH⁰ and ΔS⁰ are determined from the slope and intercept of the line, respectively. The parameters ΔG⁰, ΔH⁰, and ΔS⁰ are listed in Table 9 for AC before and after LiOH modification. The negative ΔS⁰ could be could be explained by the behavior of carbon dioxide molecules during the adsorption process. This is due to the randomness of the shape of the molecules arranged on the adsorbent surface. Besides, ΔH represents the type of CO₂ adsorption process, whether in physical or chemical adsorption. ΔH⁰ in physical reactions is lower than 40 kJ/mol, while for chemical adsorption is 80 to 200 kJ/mol. Therefore, the calculated ΔH⁰ shows that the adsorption of natural CO₂ is consistent with decreasing the amount of CO₂ at high temperature (Fig.17)⁵².

According to Table 10, the value of ΔH⁰ is positive, indicating that the adsorption reaction is endothermic. Also, the positive value of ΔG⁰ decreases with increasing the temperature, which indicates that the carbon dioxide adsorption process is desirable at 30 °C relative to 50 °C.

DFT simulation results

First, the modeling and simulation related to the structure of ACis discussed and the rate of adsorption of carbon dioxide by AC is presented based on the binding energy index. In the second part of the simulation, LiOH nano-models that perform the adsorption process to perfection are introduced, and then the binding energy related to the adsorption of carbon dioxide is reported in order to can be compared performance of the AC structures and LiOH nanoclusters. And finally, we will examine the performance of carbon dioxide adsorption by hybrid systems consisting of active carbon and LiOH structures.

Structure of activated carbon

Experimental studies show that the active carbon structure can be assumed to be a fullerene composed of heptagonal and pentagonal rings⁵³. Therefore, in this research, a fullerene with the same specifications has been designed and considered as a representative of the active carbon structure in modeling’s, whose geometry is shown in Fig.18.

The above structure can be considered the smallest modeled structure for activated carbon, which consists of 28 carbon atoms and its largest diameter is about 5.33 angstroms. In this fullerene, the smallest bond is about 1.39 and the largest is about 1.56 angstroms. Figure19 shows how the electrical charge distribution of the C₂₈ structure is on the Mulliken scale⁵⁴.

Based on the Mulliken charge distribution, it can be seen that the electronegativity of the atoms of the side ring of the structure is lower than that of the carbons at the top and bottom of the structure. Therefore, the probability of carbon dioxide adsorption from the top and bottom of the C₂₈ structure is higher than from other sides. Now, using the binding energy index, the performance of carbon dioxide adsorption by AC is investigated⁵⁵. The binding energy is obtained by the following relation:

Modeling based on DFT calculations shows that the structure of C₂₈ in the optimal state adsorbs the carbon dioxide molecule to a distance of 3.3 angstroms and the corresponding binding energy is about 0.06 eV, from the side of the upper and lower face, that is, from the side of the heptagons of the structure. Of course, by applying pressure, the distance of carbon dioxide from the C₂₈ absorbent structure decreases. It should be noted that the adsorption of carbon dioxide from the around of the C₂₈ structure, near the pentagonal faces, is relatively weaker compared to its heptagonal faces. In the following, the adsorption of carbon dioxide by LiOH base structures will be investigated and finally, the issue of whether the presence of LiOH structures mixed with AC is effective in absorbing carbon dioxide.

The role of LiOH nanostructures

In this section, to investigate the role of LiOH in improving the performance of CO₂ adsorption by AC, structures with dimensions comparable to the modeled AC structure were designed. Because if the LiOH crystal was considered, it would be necessary to use periodic boundary condition calculations, while a particle modelled for AC is a free-standing structure and Gaussian functions are used for high accuracy. Anyway, in this research, both due to computational considerations and considering the effects of nanoization that increases the surface interaction of materials, LiOH nanostructures are presented. In order to model a logical free-standing structure of the LiOH crystal structure, it is necessary to consider the LiOH salt structure from different aspects. The periodic structure of LiOH crystals from several directions is shown in Fig.20.

Considering the crystal structure of LiOH, it is clear that it has a layered structure. According to the monolayer structure of LiOH, two free-standing structures were designed to express the characteristics of the LiOH crystal. One of them is in the form of a nano-square, which is designed based on a repetitive pattern in the LiOH monolayer structure⁵⁶; the other is a nano-cube, each face of which is similar to the repeating pattern in the LiOH crystal structure. Figure21 shows the designed nano-square and nano-cube structure.

While on the around of the C₂₈ structure, near the pentagonal faces, at a distance of 2.1 angstroms, the attraction force of carbon dioxide changes its place with repulsion, but nano square of LiOH at this distance optimally and stably with a binding energy of 0.11 eV absorb carbon dioxide, even this amount of binding energy appears in the nano cube at a distance of 2.4 angstroms.

The superiority of the nano-cube to the nano-square is its high symmetry, which is close to isohedral, as a result of which its performance is not dependent on the direction and acts isotropically. There is no significant difference in the binding energy between two lithium-based nanostructures. The binding energy values can be considered as the maximum value for the attraction of carbon dioxide by LiOH structures because of their small size.

Now, the adsorbent systems, which are composed of AC and LiOH, are investigated. For this work, the interaction of C₂₈ fullerene and Li₄(OH)₅ nano-square structure was investigated simultaneously with carbon dioxide. In Table 11, the results of the calculations related to the composite absorbent system consisting of AC and LiOH are given for comparison with the pure absorbent systems of AC and LiOH. Table 11 shows the results of the calculations related to the composite adsorbent system consisting of AC and LiOH. To compare the results related to the binding energy of carbon dioxide with the modelled structure of the AC and nanostructures related to LiOH, it is presented separately.

According to the data in Table 10, it can be seen that with the addition of LiOH nanostructures along with activated carbon, the binding energy of carbon dioxide adsorption increases up to two times, and the corresponding distance decreases by about 1 angstrom. The noteworthy point is that the binding energy of hybrid systems for carbon dioxide adsorption is greater than the corresponding energy for the adsorption of LiOH nanostructures.