MgO-based composites for high pressure CO2 capture: A first-principles theoretical and experimental investigation

Magnesium oxide (MgO) is an interesting material with tunable acido-basic properties. MgO-based composite sorbents (MgAl2O4, MgSiO3, and MgTiO3) have drawn much attention based on their high temperature CO2 sorption. In this study, a theoretical and experimental investigation by phonon calculations and high-pressure CO2 sorption was conducted to identify a potential candidate to achieve CO2 capture under pre-combustion conditions. The divergence of the physico-chemical properties of the various sample materials was found to be the determining factor for the enhanced CO2 sorption. From the high-pressure CO2 sorption experiment at 200 °C, MgAl2O4 shows high chemisorption capacity of CO2 compared to the other systems such as MgO, MgSiO3 and MgTiO3. However, the thermodynamic properties of MgAl2O4 for CO2 capture were found to be less favorable than those of other compounds in our phonon calculations. Thus, the carbonation of MgAl2O4, producing MgCO3 is not a favorable reaction at the experimental condition in our phonon calculations due to the formation of Al2O3 as a byproduct. On the other hand, MgO was experimentally found to have low adsorption capacity under similar conditions. Contrarily, the carbonation of MgO, which has a large number of basic sites at pre-combustion conditions and produces MgCO3, is found to be favorable in our calculations clearly defining the existence of tradeoff properties under practical CO2 sorption conditions.


INTRODUCTION
Even though coal-fired power plants have supplied the enormous energy demand, the adverse effects due to greenhouse gas emissions have led to detrimental environmental issues [1].To realize an emission-free energy supply, retrofitting energy sources such as coal-fired power plants with efficient sorption technologies is considered as a promising approach [2].The post-combustion technology has been widely established and CO 2 removal occurs via an amine scrubbing method [3].To date, amine-based CO 2 sorption has been highly studied and commercialized to a larger extent.Typically, exhaust gas contains 10-15% of CO 2 mixed with other gases such as N 2 , O 2 , traces of SOx and NOx together with 8-15% of H 2 O in the form of moisture.However, due to energy intensive operations, alternative approaches are suggested.Sorbents in the solid phase are potential alternatives for efficient CO 2 capture [4].On the other hand, pre-combustion capture method retrofitted with integrated gasification combined cycle (IGCC), produces value added CO and H 2 .It is well known that H 2 is a green source of energy and must be produced on a higher scale.However, the gasification steam subjected to the water gas shift reaction (WGS) produced a highly concentrated CO 2 , which again contributes to the greenhouse gas effect.Nevertheless, this threat can be an opportunity since the exhaust gas contains a higher concentration of CO 2 (~35-50%) and provides a chance for efficient capture.Metal oxides are candidates of interest for this technology as they afford abundant active sites, a wide operational temperature range, greater sorption capacity and replace energy-intensive sorption mechanisms [5][6][7].
Although, metal oxides are known to interact with CO 2 in flue gas streams and release pure CO 2 either by pressure or temperature swing, the extent of carbonization and de-carbonization depends not only on pressure and temperature, but also on CO 2 concentration.Among the various candidates of interest, the sorbents based on MgO have been regarded as potential alternatives because they exhibit high theoretical sorption capacity and can be used at an extensive temperature range [8].Although, theoretically they exhibit Korean J. Chem.Eng.(Vol.40, No. 12) high sorption capacity, practical sorption capacity is within the range of 0.24 mmol/g to 2 mmol/g [9].Comparatively, modified MgO sorbents such as composites (MgO-Al 2 O 3 , MgO-TiO 2 and MgO-SiO 2 ) are believed to enhance the CO 2 sorption characteristics by virtue of altered strong basic sites (number and strength).The change in the physico-chemical properties might be induced via formation of composite structure or by surface modification of the active O 2 sites.However, subjected to high temperature and pressure, the following carbonization reactions are considered: As observed from these equations, the unmodified MgO will yield MgCO 3 .However, the carbonization of composites will also yield MgCO 3 together with TiO 2 , SiO 2 and Al 2 O 3 as a byproduct.Although theoretical estimations illustrate the formation of MgCO 3 as the main product, the sorption capacity of CO 2 , chemical potentials (T, P), the temperature (T), and the CO 2 pressure (P CO2 ) for the CO 2 capture reactions for above systems are not known.To attain high sorption capacity, it is necessary to understand its carbonization behavior under given temperature and pressure range.S1(a).It can be observed that the compositional modification has a pronounced effect on the MgO phase.Except for pristine MgO, the peaks corresponding to MgO are not identified in composites.This shows the assumed stoichiometric modification has resulted in the formation of the composite structure.However, the broader XRD peaks for MgAl 2 O 4 may be due to the presence of -Al 2 O 3 in the MgO framework.Also, it is observed that compared to MgO, MgSiO 3 , and MgTiO 3 , MgAl 2 O 4 shows amorphous nature.Further, the textural properties of the developed composites are studied by N 2 adsorption-desorption measurements as displayed in Fig. S1(b).Each composite shows the isotherms in type-IV with an H 2 -type hysteresis loop.Typically, for all the samples N 2 uptake occurs at a partial pressure >0.5.This describes the presence of higher order pores (>10 nm).The surface area follows the trend MgSiO 3 >MgAl 2 O 4 >MgTiO 3 >MgO.Whereas the pore diameter follows the trend MgTiO 3 >MgO>MgAl 2 O 4 > MgSiO 3 .The detailed textural properties are summarized in Table S1.The detailed synthesis procedure is given in the supplementary information.
To understand the thermodynamic properties of Mg-based compounds, we performed additional calculations using PHONON software [22].The displacement amplitude of non-equivalent atoms in each compound of MgO, MgAl 2 O 4 , MgSiO 3 , MgTiO 3 , MgCO 3 , Al 2 O 3 , SiO 2 , and TiO 2 was set to 0.03 Å for phonon density of states (PDOS) calculations.The approximate form of chemical potential ( 0 ) of solid-state reactants and products with CO 2 gas (suggested by Duan et al. [10][11][12][13][14]21]) is  0 (T) is Gibbs free-energy change between reactants (MgO, MgAl 2 O 4 , MgSiO 3 , MgTiO 3 , and CO 2 ) and products (MgCO 3 , Al 2 O 3 , SiO 2 , and TiO 2 ) in the CO 2 capture reactions (i.e., Eq. ( 1)-( 4)).E DFT (E ZP ) is the total (zero-point) energy difference between all compounds on the reactant side and all compounds on the product side.The reported zero-point energy value of CO 2 (0.316 eV) in literature is used in this study [21].F PH (T) and H 0 are the differences in harmonic free energy (herein, without zero-point energies) between reactants and products, and an empirical correction between calculated data and experimental data, respectively.Using the standard statistical mechanics, the Gibbs free energy (G CO2 (T)) of CO 2 was determined as expressed in Eq. ( 6).[24] December, 2023 where N a is Avogadro' s constant and h (k) is Planck' s (Boltzmann) constant. i are the vibrational frequencies of a CO 2 molecule [25].The entropy of CO 2 (S CO2 ) is calculated using the Shomate equation [26].The heat of reactions (H) can be obtained as follows: (7) where S PH (T) is the entropy difference between the solid reactant and product.Overall, the equilibrium state of chemical potentials ((T, P)=0)) can be obtained as follows: (8) To determine the empirical correction (H 0 ), our calculated and experimental data are compared for MgO and MgCO 3 , available from the HSC CHEMISTRY package in the literature [9][10][11][12][13]20,27].This correction was also used for predicting the thermodynamic properties of the other Mg-based systems without experimental data.

Experimental Evaluation of High-pressure CO 2 Sorption Using MgO-based Composites
The temperature of exhaust gas in pre-combustion capture technology ranges from 250-500 o C at high pressure of about 10-20 bar.MgO and its composites with Al 2 O 3 , SiO 2 and TiO 2 are the potential candidates as they meet the thermodynamic prerequisites.In this approach, the developed MgO-based composites are subjected to CO 2 sorption under high temperature (200 o C) and pressure (up to 20 bar).One of the advantages of using this approach is that the supporting materials chosen in this study have the least interaction with CO 2 and MgO primarily performs as the active phase (Eqs.( 1)-( 4)).To understand the potential of using the various developed sorbents as high temperature CO 2 sorbents, the CO 2 sorption-desorption isotherms were developed at 200 o C with an increase of the absolute pressure within the range of 0-20 bar using pure CO 2 (99.999%).In brief, a certain amount of sample was loaded in the sample holder and pretreated at 400 o C to remove the organic residual and moisture.Later, the sample was cooled to 200 o C to initiate the isotherm measurement.Initially the adsorption measure- Refer to the HSC CHEMYSTRY Package [27] in preceding research paper [10].ments were done by increasing the pressure from near vacuum to 20 bar using 100% CO 2 .The pressure was increased at regular period while ensuring equilibrium.The isotherms were established by desorption measurement by cooling the sample to room temperature followed by stepwise decrease of pressure from 20 bar to near vacuum.Like adsorption measurement, the pressure was decreased ensuring the complete desorption at that pressure.The chosen sorption conditions are similar to those outlined in the IGCC for precombustion CO 2 technology.The equilibrium CO 2 sorption-desorption isotherms for the chosen MgO and MgO-based compounds are displayed in Fig. 1.The carbon dioxide sorption uptake follows the trend MgSiO 3 >MgAl 2 O 4 >MgTiO 3 >MgO.The observed trend demonstrates that the composite formation is advantageous in order to improve the sorption capacity.This improved sorption capacity may be related to the increased basic sites in the composite structure or due to the change in the specific surface area as summarized in Table S1 (supporting information).Furthermore, CO 2 desorption isotherms were established by reversing the pressure at the same temperature (200 o C) to understand the interaction.The hysteresis loop was observed for all the samples, which appear due to chemisorbed CO 2 and without desorption by simple evacuation (PSA).However, based on the degree of interaction, the hysteresis loop was observed at a different pressure range at a constant temperature (200 o C).As observed from the equilibrium adsorption isotherms, the MgO and MgTiO 3 samples do not show significant hysteresis in the range of 10 to 20 bar; however, a minor hysteresis is observed in the lower pressure range (<10 bar).This demonstrates that the MgCO 3 formation at high pressure is reversible.Subjected to a high-pressure reaction, MgO and MgTiO 3 does not yield high CO 2 capture.However, a similar nature was observed for MgSiO 3 which shows a shift in the hysteresis to a higher pressure (~14 bar).Surprisingly, compared to other sorbents, MgSiO 3 shows high sorption capacity.Among all the samples developed, MgAl 2 O 4 shows hysteresis in the pressure range of 0-18 bar emphasizing the ability of this composite structure to adsorb the maximum amount of CO 2 even at pressures as high as 20 bar.From the hysteresis curve it was observed that the operating pressure window follows the trend MgAl 2 O 4 >MgSiO 3 >MgTiO 3 >MgO.This trend defines that among all the samples MgAl 2 O 4 could be the best candidate matching the prerequisites for IGCC coupled with the CO 2 capture by pre-combustion.Furthermore, the difference in the adsorption-desorption for the selected pressure range was calculated to understand the irreversibility of MgO-based composites as shown in Fig. 2(a).All of the samples exhibit decreasing CO 2 chemisorption signifying that the carbonation can be reversed subjected to higher pressure at 200 o C. Most importantly, MgAl 2 O 4 retains maximum CO 2 uptake at all pressure ranges, though it has a lower specific surface area than MgSiO 3 .This once again demonstrates that MgAl 2 O 4 is the best candidate among various MgO-based composites.A similar trend was observed when a temperature sweeping experiment was carried out at an ambient CO 2 pressure (Fig. S2(a), supporting information).The onset temperature for ambient pressure CO 2 sorption was set above 100 o C to avoid a contribution from physisorption.It is well known that all the composites are good adsorbents at low temperatures, where CO 2 is both physisorbed and chemisorbed.MgAl 2 O 4 gains high CO 2 sorption among all the sorbents, under high pressure analysis.It is noted that the onset temperature appears to be the same for all the composites as expected, all are good sorbents at low temperature.But their offset temperature and equilibrium sorption temperature are different.However, for a better understanding, CO 2 sorption prop-

December, 2023
erties from experimental and theoretical characterization were correlated as displayed in Fig. 2((b) and (c)).From the experimental characterization, MgAl 2 O 4 yields high CO 2 chemisorption at 15 bar pressure, which is helpful for temperature swing CO 2 sorption.Further, Fig. 2(a) shows that all the sorbents release CO 2 at pressures 20 bar.This supports our assumption that MgAl 2 O 4 is the best candidate for high pressure CO 2 sorption.Furthermore, to understand the trend at high temperature, the upper CO 2 pressure limit for all the MgO-based composites was calculated as displayed in Fig. 2(c).Similar to Fig. 2(b), MgAl 2 O 4 shows a higher CO 2 adsorption capacity than MgO, MgSiO 3 , and MgTiO 3 .This clearly demonstrates that due to the chemisorptive nature, MgAl 2 O 4 sorbent is the best candidate for further considerations for elevated temperature CO 2 capture applications.

Structural and Thermodynamic Properties
All compounds of reactants and products, such as MgO, MgAl 2 O 4 , MgSiO 3 , MgTiO 3 , MgCO 3 , Al 2 O 3 , SiO 2 , and TiO 2 , in the reactions of ( 1)-( 4) were optimized using first-principles calculations and the corresponding lattice data are tabulated (Table 1).The calculated data including the lattice constant and the values of entropy agree with those from the experiments [28][29][30][31][32][33][34][35].The thermodynamic properties including the changes of Gibbs free energies (G 0 ) and heat of reactions (H) of each material in the reaction were obtained by phonon calculations.
MgO is a notable material to capture CO 2 as a middle-high tem- Experimental data of FactSage was obtained in preceding research paper [21].
b Experimental data of HSC Chemistry and reference data were obtained in preceding research paper [10,21].perature sorbent with common usage in thermal power stations [36].We chose a reaction of MgO with CO 2 as a reference reaction.The thermodynamic properties of CO 2 -capture reactions with MgO are summarized in Table 2. To compare calculated and experimental data of H and G, we calculated thermodynamic properties at T=300 K.In addition, we calculated turnover temperatures (i.e., boundary temperature of capture and release of CO 2 ) at various CO 2 pressures (T 1 , T 2 , T 1 bar , and T 20 bar ) for capturing CO 2 .By comparing the calculated data with experimental data in the literature for CO 2 adoption reaction of MgO at 300 K with 0.1 bar, an appropriate empirical correction was chosen as 10 kJ/mol.The turnover temperature of CO 2 at various pressures in the reference reaction of MgO with CO 2 is listed in Table 2.The post-combustion pressure (temperature) for capturing CO 2 is P=0.1 bar (T=313-333 K) [36,37] and the pre-combustion pressure (temperature) for capture is P=10-20 bar (T=523-773 K) [21].In Table 2, T 1 is a turnover temperature at the post-combustion pressure (P=0.1 bar) of CO 2 and T 2 is a pre-combustion pressure (P=10 bar) of CO 2 .The theoretically predicted turnover temperatures at both conditions of preand post-combustion pressures, including the empirical correction (10 kJ/mol), were comparable to those from the HSC Chemistry database based on experimental values.In addition, increased turnover temperatures were found with increasing CO 2 pressure.Fig. 3 shows the H and G for reactions (1)-( 4) as a function of temperature for MgO, MgAl 2 O 4 , MgTiO 3 , and MgSiO 3 .In Fig. 3

CONCLUSIONS
First-principles studies along with experimental testing were carried out to identify the practical MgO-based compound as a CO 2 sorbent.With the empirical correction value of 10 kJ/mol, we predicted the thermodynamic properties of reactions of MgO-based compounds with CO 2 .Moreover, turnover temperatures at the preand post-combustion pressures were predicted by DFT calculations.Based on our theoretical prediction, additional experimental testing examined the practical potential, such as the possible loading of CO 2 on these compounds.MgO has good thermodynamic properties for CO 2 capture compared to other compounds at the pre-combustion condition in our calculations, but it shows the lowest CO 2 adsorption capacity.Contrarily, MgAl 2 O 4 shows the highest CO 2 adsorption capacity relative to other materials as observed from the experimental investigation suggesting its potential for high temperature and pressure applications.Further investigations under simulated exhaust gas and pre-combustion conditions would help understand the CO 2 uptake characteristics of MgO-based materials.Our theoretical studies combined with experimentation will be helpful for developing suitable CO 2 adsorbent materials by screening a group of CO 2 adsorbent candidates.

Fig. 3 .
Fig. 3.The calculated (a) heats of reactions and (b) Gibbs free energy of reaction mechanisms with temperature.

Fig. 4 .
Fig. 4. Chemical potential for turnover temperatures (boundary temperature of capture and release of CO 2 ) with various CO 2 pressure for capturing CO 2 using MgO and MgO-based composites.

Table 1 . Lattice parameters of MgO, MgCO 3 , and MgO-based sorbents based on DFT calculations. E DFT is calculated total energy/formula unit. Entropy (T=300 K) and zero-point energy (E ZP ) of each material were obtained from the phonon density of state [22]
exp

Table 3 . Turnover temperatures at various range of CO 2 pressure (T 1 , T 2 , T 1 bar , and T 20 bar ) for capturing CO 2 using MgO-based sorbents. T 1 is a turnover temperature of post-combustion pressure of CO 2 at 0.1 bar and T 2 is a turnover temperature of pre-combustion pressure of CO 2 at 10 bar
capturing CO 2 with the lower turnover temperatures (at various CO 2 pressures) than the pre-combustion condition.The highest (lowest) amount of CO 2 is adsorbed on MgAl 2 O 4 (MgO) in the experiment as displayed in Fig.2(b).These results signify that a low (high) amount of CO 2 is adsorbed on the compound showing the high (low) turnover temperature.