A review of CO2 catalytic regeneration research based on MEA solution

Yang, Zhiyuan; Shen, Yuhang; Yang, Haoran; Yi, Hui; Guo, Hao; Zhang, Xiaoliang

doi:10.3389/fenrg.2023.1257218

REVIEW article

Front. Energy Res., 01 September 2023

Sec. Carbon Capture, Utilization and Storage

Volume 11 - 2023 | https://doi.org/10.3389/fenrg.2023.1257218

A review of CO₂ catalytic regeneration research based on MEA solution

Zhiyuan Yang¹*

Yuhang Shen¹^†

Haoran Yang²^†

Hui Yi³

Hao Guo¹

Xiaoliang Zhang¹

¹Merchant Marine College, Shanghai Maritime University, Shanghai, China
²Shanghai Marine Diesel Engine Research Institute, Shanghai, China
³Shenzhen Huawei Offshore Shipping Transport Co., Ltd, Shenzhen, China

In recent years, the rapid increase of CO₂ emission has caused severe environmental issues. The environmental concern has made how to reduce the carbon emissions become a hot topic. Many scholars and research teams believe that the organic amine chemical absorption technology is the most favored and promising carbon capture technology due to its highly CO₂ removal effectiveness. However, it is not applied wildly in industrial environment since the desorption process energy consumption is too much, over 4 GJ/t CO₂. Many researchers report that catalysts can help to reduce the desorption energy. And it is generally assumed that four key properties of solid acid catalysts determined the performance of solid acid catalysts in the process of CO₂ desorption: the total number of acid sites; specific surface area; the ratio of Brønsted acid sites to Lewis acid sites; the amount of Brønsted acid sites. Therefore, this paper reviews the recent research on the effect of different catalysts on the energy consumption of CO₂ desorption and the progress of research on improving catalyst performance. Also, it provides views on the possible problems in practical industrial applications.

Introduction

Global warming has become an international hot issue (Montzka et al., 2011; Rhodes, 2016), and CO₂ is widely considered as a significant contributor to the global warming due to its greenhouse effect (Kerr, 2006; Barzagli and Mani, 2021; Huang et al., 2021). Human society has emitted huge amount of CO₂ since the beginning of the industrial revolution (AR5 Synthesis Report: Climate Change, 2014; Godin et al., 2021) and CO₂ concentrations have been increased from an average of 280 ppm in pre-industrial times to 410–419 ppm in 2019–2021 (Benhelal et al., 2021; Rashid et al., 2022). Usually, these anthropogenic CO₂ emissions are from industries like petrochemicals, cement, refineries, and power plants (Freund, 2003; Peters et al., 2013; Mercure et al., 2018); since the beginning of the industrial revolution, human society has developed to the point where today fossil fuels meet 85% of the global energy demand nowadays (David and Herzog, 2000; Uyanga and Idem, 2007; Yang et al., 2008; Herzog et al., 2012; Zhang et al., 2014). And the years ahead, what’s more, it is estimated that fossil fuels will remain the primary energy source for decades (Agarwal et al., 2010; Scott et al., 2013; Shah et al., 2023). And the massive consumption of fossil fuels will continue to generate large amounts of CO₂ emissions (Chen et al., 2018; Chuenphan et al., 2022). So, it will still accelerate global warming, glacier melting, seawater acidification, sea level rise and other greenhouse effect problems (Doney et al., 2009; Foster and Rohling, 2013; Xiao et al., 2016; Sun et al., 2022). As a result, how to reduce and effectively utilize CO₂ has become a hot issue for the world (Rao and Rubin, 2019; Fu et al., 2022). The current capture methods for CO₂ capture include pre-combustion capture, oxygen-enriched combustion and post-combustion capture (Schreiber et al., 2009; Zhou et al., 2010; Xia et al., 2020). Due to the low cost of the retrofit work on existing equipment (Wang et al., 2011; Zhao et al., 2013; Wang et al., 2017), post-combustion capture technology is more widely used in industry (Dou et al., 2010; Bhown and Freeman, 2011). The technology captures CO₂ from the flue gas of combustion equipment selectively (boilers and gas engines, etc.) by using chemical or physical methods (Li et al., 2016; Fu et al., 2022). It mainly includes adsorption, low-temperature distillation, membrane, biomimetic, physical, and chemical absorption technology (Davy, 2009; McGurk et al., 2017; Yousef et al., 2017; Luis Míguez et al., 2018; Liu et al., 2022). Among these methods, the chemical absorption is applied widely in industries because of its large absorption capacity, fast absorption rate (Shakerian et al., 2015), ability to handle large amounts of gas, and applicability to low CO₂ partial pressure gases (Muchan et al., 2022). Amine-based solvent is the solvent commonly used in chemical absorption methods (Rochelle, 2009), for example, monoethanolamine (MEA), butylethanolamine (BEA), methyldiethanolamine (MDEA), butyldiethanolamine (BDEA), 2-amino-2-methyl-1-propanol (AMP) (Nakrak et al., 2023), piperazine (PZ) (Carson et al., 2000; Zhao et al., 2023), listed in Table 1. Among all the amine solution, the monoethanolamine (MEA) is most used because of its relatively low cost, excellent CO₂ absorption performance and strong reactivity to CO₂ even at very low CO₂ partial pressures (Lepaumier et al., 2009; Muchan et al., 2017; Narku-Tetteh et al., 2017). However, this technology still faces three drawbacks that prevent its widespread industrial application, the relatively high energy consumption of CO₂ desorption, the degradation of amine solvents and the corrosion of equipment (Goff and Rochelle, 2004; Lepaumier et al., 2011; Liang et al., 2015; Barzagli et al., 2018; Bui et al., 2018). The high desorption energy consumption is considered the most significant drawback (Romeo et al., 2008; Lin and Rochelle, 2016; Ji et al., 2018; Herzog et al., 2022), with the CO₂ desorption process accounting for 70%–80% of the total energy consumption of the whole CCS system (Oyenekan and Gradworks, 2007; Haszeldine, 2009; Shi et al., 2018). Therefore, the key to realizing the industrial application of the chemical absorption method to capture CO₂ is to reduce the energy consumption of the desorption process of CO₂(Singh and Versteeg, 2008).

TABLE 1

TABLE 1. Advantages and disadvantages of solvents (Li et al., 2022).

The regeneration duty of amine process involves three parts: the desorption heat to break chemical bonds in CO₂-absorbed products, the sensible heat to raise the temperature of the CO₂-loaded solution and the latent heat to produce the water vapor (Li et al., 2015; Zhang et al., 2018c), and among which sensible heat and latent heat can be reduced by process improvement, according to existing reports (Le Moullec et al., 2014; Wang et al., 2015; Decardi-Nelson et al., 2017). However, the overall energy consumption of the MEA process can be reduced by about 30% by process improvement; the regeneration energy still accounts for more than 50% of the total energy consumption of the carbon capture system (Oyenekan and Rochelle, 2006; Rochelle, 2009). The main reason is that the temperature of the CO₂ desorption process is usually high (393K–413 K) (Finotello et al., 2008; Yu et al., 2012), which causes a large proportion of the heat to be used to boil the water in the solution (Sakwattanapong et al., 2005). In other words, the low energy utilization efficiency is attributed to the high temperature required for desorption. Therefore, if the regeneration temperature can be controlled below the boiling point of the water, the energy consumption for CO₂ desorption will be significantly reduced. Furthermore, high regeneration temperature can cause equipment corrosion (Li et al., 2016). Therefore, the key to controlling the energy consumption of chemical absorption desorption is to achieve CO₂ desorption at lower temperatures.

Research has found that absorptive thermal regeneration can be replaced by integrating the absorption and mineralization of CO₂, and this process can achieve CO₂ regeneration at 40 °C by usually using MEA solvent as the absorber and fly ash as the regeneration; however, the current development of this process is limited by the reaction rate of the CO₂ mineralization process (Ji et al., 2018; Rashid et al., 2020). In addition, adding catalysts can improve CO₂ desorption efficiency at low temperatures, reducing the temperature required for solvent regeneration (Idem et al., 2011). It has led to an increasing focus on developing new catalysts by researchers. We conducted a brief review of relevant research in recent years, as shown in Table 2. Introducing catalysts into MEA solutions makes it possible to change the proportion of sensible heat, latent heat of vaporization, and chemical reaction heat in the total heat demand effectively. Additionally, by reducing the activation energy of the reaction and increasing the effective collision frequency, the CO₂ desorption rate can be significantly increased. Therefore, MEA solutions containing catalysts exhibit faster desorption rates and lower CO₂ desorption energy consumption than blank experiments under the same experimental conditions, such as temperature and pressure. And it is attributed to the presence of Brønsted acidic sites or Lewis acidic sites in the catalyst, which effectively promote the decomposition of carbamates and the deprotonation of protonated amines in MEA solutions (Srisang et al., 2018). And the advantages of catalytic desorption are shown in Figure 1. In this paper, we present the mechanism of the promotion of the regeneration of organic amine solutions by Brønsted acid catalysts (represented by HZSM-5), Lewis acid catalysts (represented by γ-Al₂O₃) and acid-base bifunctional catalysts and highlight four characteristics that affect the catalytic performance of solid acid catalysts: the total number of acid sites, specific surface area, especially mesoporous surface area (MSA), the number of acid sites, and the number of pores. We also compared the disparities in operating conditions between laboratory settings and engineering applications. We discussed potential challenges in the engineering application of catalysts and presented various viewpoints. Finally, we concisely compared the advantages and disadvantages of different catalysts in terms of cost, stability and environmental impact. As shown in Table 3.

TABLE 2

TABLE 2. A brief review of studies for catalytic CO₂ desorption process in the MEA solution.

FIGURE 1

FIGURE 1. Advantages of catalytic desorption.

TABLE 3

TABLE 3. The advantages and disadvantages of different catalysts in terms of cost, stability and environmental impact.

Solid acid catalyst

The CO₂ absorption and desorption process can be described according to the amphiphilic ion mechanism proposed by Caplow et al. (Caplow, 1968). Firstly the absorption process of tertiary amines produces amphoteric ions when reacting with CO₂, and subsequently, these amphoteric ions decompose to form carbamates and protonated amines; according to this mechanism, the regeneration process can be carried out in two main steps: the decomposition of carbamates (MEACOO⁻) and the deprotonation of protonated amines (MEAH⁺) (Afari et al., 2018; Li et al., 2023).

Zwitterion formation:

M E A + {C O}_{2} ⇋ M E A H^{+} {C O O}^{-} (1)

Carbamate formation:

{M E A H^{+} C O O}^{-} + H_{2} O ⇋ {M E A C O O}^{-} + H_{3} O^{+} (2)

Protonated amine formation:

M E A + {H_{3} O}^{+} ⇋ {M E A H}^{+} + H_{2} O (3)

Decomposition of carbamate (MEACOO-):

{M E A C O O}^{-} + {H_{3} O}^{+} ⇋ z w i t t e r i o n ⇋ M E A + {C O}_{2} (4)

Deprotonation of protonated amine (MEAH⁺):

{M E A H}^{+} + H_{2} O ⇋ M E A + H_{3} O^{+} (5)

For the whole regeneration process of MEA solution MEACOO^-decomposition process needs enough protons (H₃O⁺) as reactants to participate in the reaction; in the absence of a catalyst, deprotonation of protonated amine becomes the primary source of protons in Reaction (4), however, due to the high alkalinity of MEA which makes the deprotonation of protons from MEAH⁺ has a very high energy potential barrier, it is hard for the reaction equilibrium of Reaction (5) to toward the right side, which means that in this case, the deprotonation of MEAH⁺ becomes the most crucial rate-limiting step of the whole reaction, in addition to the fact that the MEACOO⁻ decomposition is a strongly heat-absorbing reaction with a high demand for heat load. Feng et al. (Feng et al., 2010; Du et al., 2011) proposed adjusting the amine-rich solution’s pH with weak acids (octanedioic acid, phthalic acid, oxalic acid, etc.) to provide more acid protons and promote the desorption process of CO₂. These weak acids should have the characteristic of increasing their solubility in water with increasing temperature, thereby reducing the pH value of the amine solution to promote CO₂ desorption. However, as the temperature decreases, the solubility of weak acids also decreases, allowing them to crystallize and precipitate from the amine solution. The results showed that adding weak acids could reduce the energy consumption of rich amine solution regeneration. Still, the residual weak acid dissolved in the lean amine solution reduces the CO₂ absorption performance significantly. Inspired by this method, Tontiwachwuthikul et al. proposed using solid acid catalysts to promote CO₂ desorption and reduce the energy consumption of solvent regeneration (Shi et al., 2014b; Srisang et al., 2017). Idem et al. (Idem et al., 2011) proposed adding HZSM-5 (mainly a proton donor or Brønsted acid) and γ-Al₂O₃ (mainly a solid electron acceptor or Lewis acid) as two solid acids in the amine-rich solution to reduce the regeneration load of the solvent. Liang et al. (Liang et al., 2016) further advanced this approach by developing various solid acid catalysts (including TiO₂, Al₂O₃ and HZSM-5). They demonstrated that their addition to the MEA solvent regeneration step could reduce the regeneration heat demand by up to 34%.

Shi et al. (Shi et al., 2014b) conducted experiments to verify that two different catalysts (γ-Al₂O₃ and HZSM-5) did not exhibit any degradation effect on amine solutions during a continuous one-week test. Liang et al. (Liang et al., 2016) investigated the catalytic regeneration of CO₂-rich MEA solutions at 378 K. They found that HZSM-5, γ-Al₂O₃, and H-Y could effectively improve the desorption rate of CO₂ and reduce energy consumption during desorption. By comparing the MSA and the ratio of Brønsted acid sites to Lewis acid sites of the three catalysts, they found that higher MSA and a higher ratio of Brønsted acid sites to Lewis acid sites could enhance the catalytic performance of the catalysts. Furthermore, they explored the performance of solid acid catalysts based on the combined effect of these two factors, using the product of B/L and MSA (B/L*MSA) as a binding value. The results showed the higher the binding value of these two features, the faster the rate of catalytic CO₂ desorption, which also indirectly reflected that, compared to the Lewis acid sites on the catalyst surface, the Brønsted acid sites played a more active role. Srisang et al. (Srisang et al., 2017) experimentally investigated the role of solid acid catalysts (including HZSM-5 and γ-Al₂O₃) in reducing the thermal load of CO₂ desorption from MEA solution. They showed that HZSM-5 promoted carbamate decomposition by providing free protons and increased the absorption efficiency by 38% compared to non-catalytic conditions while reducing the thermal load by 42%. Compared to non-catalytic conditions, γ-Al₂O₃ improved absorption efficiency by 23.6% and reduced heat load by 30%. By comparing the application of HZSM-5, γ-Al₂O₃, H-Y and SiO₂- Al₂O₃ in the desorption process of CO₂, the correlation between the specific surface area of the catalyst, the ratio of Brønsted acid sites to Lewis acid sites, the total acid amount and other characteristics on the cycle capacity, absorption efficiency and thermal load of MEA absorbent was analyzed by regression. The results indicated that the acid strength had the most significant effect on reducing the heat load in the desorption of solid acid catalysts. It is followed by a larger ratio of Brønsted acid sites to Lewis acid sites for the catalytic desorption of CO₂. In addition, Liu et al. (Liu et al., 2017) investigated the impact of three different catalysts (HZSM-5, MCM-41, and SO₄^2-/ZrO₂) with varying total acid sites, mesoporous surface areas, and Brønsted and Lewis acid sites on thermal loading and CO₂ desorption rates using an intermittent reaction device. The results revealed that the catalytic performance of the three catalysts, in terms of thermal loading efficiency and CO₂ desorption rate in single and mixed amines, followed the order of HZSM-5 > MCM-41 > SO₄^2-/ZrO₂. The NH3-TPD results showed that the acidic strength of the three catalysts followed the order of SO₄^2-/ZrO₂ > HZSM-5 > MCM-41. Although HZSM-5 was moderately acidic compared to MCM-41 and SO₄^2-/ZrO₂, it exhibited the best catalytic performance among the three catalysts due to its more significant Brønsted acid site to Lewis acid site ratio. Zhang et al. (Zhang et al., 2017) experimented with studying the impact of adding two solid acid catalysts, SAPO-34 and SO₄^2-/TiO₂, to MEA solutions with high CO₂ content on the energy demand for solvent regeneration. As depicted in Figure 2, Figure 3, Figure 4, the experimental results indicate that the catalyst’s performance correlates strongly with its surface properties, which comprise the total acid amount, mesoporous surface area, Brønsted acid sites to Lewis acid sites ratio, and the interplay between these surface properties.

FIGURE 2

FIGURE 2. Influence of total acid sites on heat duty and CO₂ desorption rate (Liu et al., 2017)

FIGURE 3

FIGURE 3. Influence of mesopore surface area on heat duty and CO₂ desorption rate (Liu et al., 2017)

FIGURE 4

FIGURE 4. Influence of the ratio of Brønsted and Lewis acid sites (B/L) on heat duty and CO₂ desorption rate (Liu et al., 2017)

Brønsted acid catalysts

Brønsted acid catalysts possess multiple Brønsted acidic sites, which can provide a significant amount of H⁺ to participate in the carbamate decomposition reaction during the CO₂ desorption process (Shi et al., 2014a). On the other hand, the catalytic active sites that have lost protons can be restored by obtaining acidic protons from MEAH⁺. A proposed catalytic mechanism of the Brønsted acid catalyst for CO₂ desorption is shown in Figure 5 (Idem et al., 2011). Three active sites on the carbamate are involved in the desorption reaction of CO₂: a) the nitrogen atom, which is the crucial active center because the decomposition of the carbamate involves the dissociation of the C-N bond; b) the oxygen anion on the carbonyl group, which can serve as a carrier for the acidic proton (H⁺); c) another site located on the oxygen atom of the carbonyl group, which is a suitable attacking site for the catalyst (Idem et al., 2011; Zhang et al., 2019). With the addition of a solid acid catalyst (in the case of HZSM-5), the system can now provide protons even without the deprotonation step. MEACOO⁻ in CO₂-rich MEA solution is converted to MEACOOH after being on the surface of HZSM-5. Then, MEACOOH undergoes chemical adsorption on the catalyst surface through the bonding of O atoms and Al atoms. The H atom attached to the O atom migrates to the adjacent N atom; and this transformation converts MEACOOH into a zwitterion. As a result of the H atom transfer, the N-C bond begins to stretch and weaken, leading to the attachment of N⁺ to a second Al atom on the catalyst surface, while the N-C bond begins to break. After the bond breakage, the zwitterion becomes MEA and CO₂; since the solubility of CO₂ is low at high temperatures, CO₂ will transfer to the gas phase.

FIGURE 5

FIGURE 5. A possible catalytic mechanism of Brønsted acid catalyst for CO₂ desorption (Idem et al., 2011)

Zeolite is a typical Brønsted acid catalyst, which is an aluminosilicates with a three-dimensional crystalline structure composed primarily of silicon, aluminum, and oxygen atoms (Wei et al., 2015; Javad Kalbasi et al., 2018); commonly used zeolite molecular sieves are square sodium zeolite (A-type molecular sieves), octahedral zeolite (X-type, Y-type molecular sieves), mercerized zeolite (M-type molecular sieves), high-silica zeolite (ZSM-5, ZSM-11, etc.), among many zeolite molecular sieves, HZSM-5 is the most thoroughly researched molecular sieve catalyst optimized for CO₂ desorption due to its unique crystalline structure, strong dispersive force and electrostatic force, large specific surface area, uniform microporous pore size distribution, good thermal and hydrothermal stability, and a wide range of adjustable SiO₂/A1₂O₃. Wang et al. (Wang et al., 2023) compared the catalytic effects of three different Si/Al ratios of HZSM-5 zeolites (HZSM-5-25, HZSM-5-50, HZSM-5-80) on CO₂ desorption through intermittent and continuous CO₂ desorption experiments. The results showed that all three Si/Al ratios of HZSM-5 catalysts had specific catalytic effects, and the catalytic effect increased as the Si/Al ratio decreased, i.e., HZSM-5-25 > HZSM-5-50 > HZSM-5-80 > Blank. The reason was the lower the Si/Al ratio of the HZSM-5 catalyst, the stronger the acidity on its particle surface. The total acidities of the three catalysts were 2.34, 1.4, and 0.6 mmol/g, respectively. Therefore, it was believed the more substantial the acidity on the particle surface of HZSM-5, the more pronounced the promotion effect on desorption, which ranged from 9.41% to 15.75%. Bhatti et al. (Bhatti et al., 2020) synthesized a series of mesoporous HZSM-5 catalysts by removing silicon in an alkaline medium and improving the mesoporous and surface acidity of HZSM-5. The results showed that all catalysts improved the CO₂ desorption rate of MEA at lower temperatures, and the catalytic performance trend was HZ-0.7 > HZ-0.5 > HZSM-5 > HZ-0.2 > Blank. The surface acidity of the catalysts after silicon removal and recombination showed an overall trend of HZSM-5 < HZ-0.2 < HZ-0.5 < HZ-0.7. HZ-0.5 and HZ-0.7 significantly improved the CO₂ desorption rate (up to 350%–580% at 82 °C). At the same time, HZ-0.2 had lower performance due to the limited number of Brønsted acid sites, resulting in a performance gap of about 20% compared to HZSM-5.

Lewis acid catalysts

Lewis acid, also known as an electrophile reagent, γ-Al₂O₃ is a typical Lewis acid. The Al atoms in γ-Al₂O₃ have empty 3p orbitals that can accept the lone pair of electrons from N atoms, which is present in the amino ester salt. Therefore, γ-Al₂O₃ can attack the N atom in the amino ester and weaken the strength of the C-N bond, leading to the bond break and release CO₂ (Liang et al., 2016). It provides a significant reduction in the thermal load for solvent regeneration. In addition, as shown in Figure 6, the hydroxyl group in γ-Al₂O₃ can react with CO₂ in low CO₂ load solutions to form HCO₃⁻.

FIGURE 6

FIGURE 6. Catalytic desorption mechanism of γ-Al₂O₃(Liang et al., 2016)

On the one hand, HCO₃⁻ can act as a proton acceptor, accepting protons from MEAH⁺ to H₂CO₃, which can be directly thermally decomposed to form H₂O and CO₂, ( ${M E A H}^{+} + {H C O}_{3}^{-} ⇋ M E A + H_{2} C O_{3} ⇋ M E A + H_{2} O + C O_{2}$ ), on the other hand, the generated HCO₃⁻ can be considered a catalyst. Due to its higher alkalinity than water, it can decompose the protonation reaction of an amine into two steps with lower activation energies ( ${M E A H}^{+} + H_{2} O ⇋ M E A + H_{3} O^{+}$ ), providing more suitable protons for the reaction ( ${M E A C O O}^{-} + H_{3} O + ⇋ z w i t t e r i o n ⇋ M E A + {C O}_{2}$ ). Overall, because the alkalinity of HCO₃⁻ is between MEA and water, the proton can be transferred from MEAH⁺ to HCO₃⁻ and then to water, which is much easier than directly transferring it to water. Furthermore, this characteristic also indicates why γ-Al₂O₃ can exhibit good catalytic activity in the low load range (Liang et al., 2016).

(Idem et al., 2011) performed CO₂ desorption experiments by adding γ-Al₂O₃ to the MEA solution, and they successfully achieved a reduction in the regeneration temperature of the CO₂-loaded MEA solution from 393K-413 K to 363K–368K, resulting in a 27% decrease in the energy required for regeneration; (Xu et al., 2020) introduced γ-Al₂O₃ into the MEA solution and investigated the regeneration of the solution at 70 °C. The results showed that adding a catalyst could reduce the regeneration energy and accelerate the desorption of CO₂ at low regeneration temperature conditions. Under this experimental condition, γ-Al₂O₃ can reduce the relative heat load by 8.1% compared to the blank case. (Akachuku et al., 2019) catalyzed the regeneration of MEA solution by adding a solid acid catalyst γ-Al₂O₃ to the absorption desorption device. The results showed that γ-Al₂O₃ could effectively improve the CO₂ conversion rate and reduce the energy demand related to CO₂ desorption at a temperature below 373 K. The desorption rate of MEA was increased by 55.8% compared with the non-catalytic process when the solution temperature was 365 K.

Metal oxide catalysts

Metal oxides or metal oxide nanoparticles have been widely used as a viable solid acid catalyst on a commercial scale. The catalytic behavior of metal oxides is attributed to their active acidic sites available through surface defects (Alivand et al., 2020), The interaction between metal oxides and water forms Brønsted acid sites by converting the oxide groups on the surface into hydroxyl groups, while Lewis acid sites are generated above the coordinatively unsaturated metal ions (Bhatti et al., 2018). Lewis acid sites and Brønsted acid sites are emerged on the surface of metal oxide, which provide the metal atoms (Lewis acid) and H⁺ (Brønsted acid) to the N atoms of carbamate to rob it of its lone pair of electrons. In this way, the configuration of N atoms will be changed from sp2 to sp3, and the N–C bond strength would also be weakened by stretching. Consequently, the carbamate would be broken up by using less thermal energy, which will lead to faster CO₂ stripping at lower temperatures (Liang et al., 2016). The proposed catalytic mechanism for CO₂ desorption of MEA sorbent with metal oxide catalysts is shown in Figure 7. Metal nanoparticles will also benefit from physical enhancement mechanisms in addition to the chemical catalytic mechanisms described above. The physical enhancement mechanism is focused around Brownian motion of nanoparticles in the solution that break down gas bubbles, increase mass transfer surface area, and decrease mass transfer resistance at gas−liquid interfaces. This results in a large increase in the CO₂ desorption rate.

FIGURE 7

FIGURE 7. Proposed catalytic mechanism for CO2 desorption of MEA sorbent with metal oxide catalysts (Bhatti et al., 2018)

(Bhatti et al., 2017) investigated the catalytic MEA solution regeneration performance of five transition metal nanoparticles oxides, including V₂O₅, MoO₃, WO₃, TiO₂, and Cr₂O₃. They researched the impact of these catalysts on amine regeneration within a temperature range of 308K–359 K. The results indicated that all the catalyst species could promote the regeneration of MEA. The trend in amine regeneration performance was found to be MoO₃ > V2O₅ > Cr₂O₃ > TiO₂ > WO₃. Catalysts with both types of acid sites (MoO₃ and V₂O₅) desorbed 94% and 84% more CO₂ than the control experiment, respectively. On the other hand, catalysts with only Lewis acid sites (Cr₂O₃, TiO₂, WO₃) desorbed nearly 44% more CO₂ than the control experiment; And they also compared the regeneration of MEA solution catalyzed by five metal oxide catalysts, Ag₂O, Nb₂O₅, NiO, CuO and MnO₂, and the results showed that all five catalysts improved the regeneration of MEA, and with Ag₂O having the best regeneration performance (Bhatti et al., 2018); Subsequently, in the Ag₂O-catalyzed MEA solution regeneration step, they found that the addition of Ag₂CO₃ catalyst to the saturated MEA solution at 80 °C effectively increased the CO₂ desorption rate by about 1000% (Bhatti et al., 2019). Xing et al. (Xing et al., 2021) compared five different transition metals (Co²⁺, Mn²⁺, Fe³⁺, Cu²⁺, and Ni²⁺). The results revealed that adding catalysts at 361 K led to increased CO₂ desorption ranging from 5% to 48%, as depicted in Figure 8.

FIGURE 8

FIGURE 8. Amount of desorbed CO₂ at 60 min and 361 K with different metals at the concentration of 0.05 mol/L (Xing et al., 2021)

Acid-base bifurcation catalysts

Among various catalysts, including zeolites, solid superacid, and metal oxide catalysts, HZSM-5 (a Brønsted acid catalyst) and γ-Al₂O₃ (a Lewis acid catalyst) have demonstrated superior catalytic performance. Some studies have reported that HZSM-5 exhibits better CO₂ desorption catalytic performance than γ-Al₂O₃ in the high CO₂ loading region of MEA solution due to its significant Brønsted acid sites, which plays a crucial role in the decomposition of amino formate. However, when the CO₂ loading is low, γ-Al₂O₃ exhibits better catalytic CO₂ desorption performance than HZSM-5, which may be attributed to the presence of prominent primary sites. Basic sites have a positive effect on the deprotonation reaction of protonated amine. Brønsted acid sites and basic sites enhance the regeneration process by different mechanisms. Therefore, combining Brønsted acid sites and primary sites to obtain a bifunctional catalyst for the MEA regeneration process in rich CO₂ gives an excellent opportunity to improve catalytic performance and further reduce energy requirements (Zhang et al., 2018b). The possible catalytic mechanism is shown in Figure 9.

FIGURE 9

FIGURE 9. A proposed catalytic mechanism for CO₂ desorption process with bifunctional catalyst (Zhang et al., 2018b).

(Zhang et al., 2018b) prepared a series of bifunctional Al₂O₃/HZSM-5 catalysts (Al-ZSM) by a combined precipitation-ultrasound method. Four different weight ratios of Al₂O₃ and HZSM-5 bifunctional catalysts (including Al-ZSM1/2, Al-ZSM1/1, Al-ZSM2/1, and Al-ZSM3/1) were studied, and they tested the catalytic performance, as shown in Figure 10. The Al-ZSM catalysts exhibited higher catalytic performance than the single Al₂O₃ and HZSM-5 catalysts during the MEA regeneration. Moreover, it can be seen from Figure 11 that the catalyst exhibited optimal performance when the Al₂O₃/HZSM-5 weight ratio was 2, and compared to the blank run, the regeneration heat load decreased by 34.2%, and the desorption coefficient increased threefold. The main reason for this improvement is that after introducing metal oxides into the zeolite, all Al-ZSM catalysts have increased Brønsted acidity, mesoporous surface area, and primary sites, which are beneficial for the CO₂ desorption process. (Zhang et al., 2020) reported a novel acid-base bifunctional catalyst system MCM-41, a mesoporous molecular sieve with a large surface area and low Brønsted acid sites. By modifying MCM-41 with three different metals (Fe, Al, Mo), the surface Brønsted and Lewis acidity sites and Lewis primary sites of the catalyst can be effectively increased. The results showed that all three catalysts could accelerate the MEA regeneration process, and the MCM-41 catalyst modified with Fe₂O₃ (MFe) exhibited catalytic performance better. Adding MFe improved the CO₂ desorption performance significantly compared to the blank run, reaching 206.3%–337.1%. A possible catalytic mechanism for this catalyst was also analyzed and is shown in Figure 12. (Lai et al., 2018) reported a novel CO₂ desorption catalyst TiO(OH)₂, which could dramatically increase the CO₂ desorption rate of a saturated solution of 5 M MEA by 4500% at a low temperature of 361 K . TiO(OH)₂ exhibited superior catalyst performance compared to conventional catalysts such as HZSM-5, notably promoting the CO₂ desorption rate of the MEA solution at low temperatures. Additionally, the authors proposed a possible catalytic mechanism for the MEA solution, illustrated in Figure 13. Based on the findings of their study, TiO(OH)₂ has promising potential as a catalyst for CO₂ desorption in MEA systems.

FIGURE 10

FIGURE 10. Catalytic CO₂ desorption performance in MEA solution at 96 °C Catalytic performance of various bifunctional catalysts (Zhang et al., 2018b).

FIGURE 11

FIGURE 11. Catalytic CO₂ desorption performance in MEA solution at 96 °C Comparison of catalytic performance the single catalysts with the worst bifunctional catalyst (Zhang et al., 2018b).

FIGURE 12

FIGURE 12. A possible catalytic CO₂ desorption mechanism in MEA solution over MFe catalyst (Zhang et al., 2020)

FIGURE 13

FIGURE 13. A possible catalytic CO₂ desorption mechanism in MEA solution over TiO(OH)₂ catalyst (Lai et al., 2018)

As mentioned above, the excellent catalytic effect of acid-base bifunctional catalysts is mainly attributed to their large mesopore area and the possession of some acidic and basic active sites at the same time, and the improvement of the mesopore area and the number of acid-base sites is often realized during the preparation of the catalysts. For example, the prepared Al-ZSM bifunctional catalyst realized the conversion of part of the microporous structure into a mesoporous structure while retaining the prominent acidic sites possessed by HZSM-5 and also increased, to a certain extent, the basic sites on the surface of HZSM-5; for the preparation of MFe catalysts, firstly, MCM-41, which is a kind of MFe catalyst with an ordered mesoporous structure and a large specific surface area. However, it is limited by the smaller ratio of Brønsted acid sites to Lewis acid sites and the number of Brønsted acid sites, which leads to the relatively low catalytic CO₂ desorption performance of MCM-41, the partial replacement of silicon ions in MCM-41 by Fe³⁺ and the formation of Fe-O-Si bonds by oxygen ions linking to the framework. Typically, the Fe³⁺ centers formed upon reaction with water can be considered as Lewis acid sites for accepting electrons, and the resulting hydroxyl groups are regarded as Brønsted acid sites for releasing protons.

Summary and outlook

The chemical absorption method based on MEA solution that is widely used in industrial production for CO₂ removal, carbon capture system operation high energy consumption is considered the most crucial reason hindering its development. However, many researchers have made many efforts in process optimization, but the ideal desorption energy consumption is still a certain distance; this is mainly because the MEA solution regeneration process temperature is very high, resulting in large part of the energy being used for the vaporization of water in solution. The Brønsted acid site and Lewis acid site contained in a solid acid catalyst can effectively change the CO₂ desorption reaction pathway so that the reaction can proceed at a lower temperature, thereby significantly reducing the energy consumption of amine solution regeneration; this paper mainly introduces the promotion mechanism and promotion effect of several different solid acid catalysts (including Brønsted acid catalyst, Lewis acid catalyst, metal oxide catalyst, and acid-base bifunctional catalyst) in the process of CO₂ desorption. It is not difficult to find that the introduction of a solid acid catalyst can control the regeneration temperature of MEA solution below 373K, which can effectively reduce the heat load of 10%–50% in the process of CO₂ desorption, and the desorption amount of CO₂ can be increased by up to 4500%.

Different techniques were used to characterize the physicochemical properties of the catalyst surface, and the results showed that the catalytic performance of the catalysts is usually influenced by the total number of acidic sites on the surface; the ratio of Brønsted to Lewis acid sites; the amount of Brønsted acid sites and the specific surface area, especially the mesoporous surface area. Therefore, it is important to develop new and efficient solid acid catalysts with a high number of acid sites, a large ratio of Brønsted acid sites to Lewis acid sites, a high amount of Brønsted acid sites, and a large mesoporous surface area or to improve the catalytic performance of existing solid acid catalysts by improving the above physicochemical properties.

At this stage, most studies on catalyst performance are based on intermittent laboratory reactors. Although these experimental studies show that the addition of catalysts effectively reduces the energy consumption of the amine solution regeneration process, the industrial carbon capture process is often a continuous absorption and desorption process. Firstly, the CO₂ desorption promotion effect in this continuous process is limited to some extent by the contact time between the solution and the catalyst; secondly, the amount of catalyst and the size of the regeneration equipment need to be considered because of the large amount of solution circulation in the industrial carbon capture process; finally, the decay of the catalytic effect of the catalyst in this continuous use process, i.e., the lifetime of the catalyst, is also a key consideration.

Industrial regeneration towers usually enhance CO₂ desorption by solid packing. Some studies have shown that acidic catalytic packing can lower the regeneration temperature and thus reduce the sensible heat and latent heat of vaporization in the regeneration process. However, there are few studies in this area and no clear results on the catalytic effect of acidic catalytic packing in regeneration towers compared to the catalyst immersion reaction process in the laboratory stage.

Finally, we compared the advantages and disadvantages of different catalysts in terms of cost, stability and environmental impact of catalyst desorption of CO2. We found that the preparation process of catalysts with better performance (including catalytic effect and stability) is often more complicated, which leads to the environmental impact caused by the consumption of large quantities of chemicals in the preparation process, as well as higher production costs, so how to ensure the performance of the catalysts on the premise of achieving low-pollution and low-cost preparation of catalysts is also a primary research direction in the future.

Author contributions

ZY: Formal analysis, Funding acquisition. YS: Accessing information, Writing–Original draft and Editing. HaY: Supervision, Resources, Review. HuY: Data curation, Review. HG: Polishing. XZ: Proof.

Funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This paper is funded by the entrusted projects for enterprises and institutions (H20210252).

Conflict of interest

HuY was employed by Shenzhen Huawei Offshore Shipping Transport Co., Ltd.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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References

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