A Review on Enhancing Solvent Regeneration in CO₂ Absorption Process Using Nanoparticles

Abstract

The employment of nanoparticles in solvents is a promising method to reduce the energy consumption during solvent regeneration. Numerous experimental and theoretical studies have been conducted to investigate the remarkable enhancement of nanoparticles. Yet, there are limited reviews on the mechanistic role of nanoparticles in enhancing the solvent regeneration performance. This review addresses the recent development on the employment of various nanoparticles, which include metals oxides, zeolites and mesoporous silicas, to enhance the mass and heat transfer, which subsequently minimize the solvent regeneration energy. The enhancement mechanisms of the nanoparticles are elaborated based on their physical and chemical effects, with a comprehensive comparison on each nanoparticle along with its enhancement ratio. This review also provides the criteria for selecting or synthesizing nanoparticles that can provide a high regeneration enhancement ratio. Furthermore, the future research prospects for the employment of nanoparticles in solvent regeneration are also recommended.

1. Introduction

Global warming stemming from anthropogenic emissions of carbon dioxide (CO₂) has been widely recognized as such a challenging problem for the past decades. Excessive global CO₂ emissions have been rapidly increasing over the years due to the increase in energy consumption along with the drastic economic growth in many countries [1]. The Paris Agreement reports that CO₂ released through the burning of fossil fuel is the dominant source of heat-trapping emissions [2]. According to the United States Environmental Protection Agency, CO₂ is the main cause of global warming and constitutes up to 80% of all greenhouse gas emissions [3,4]. The Paris Agreement also seeks to limit the rise of the global average surface temperature to below 2 °C above pre-industrial period levels this century, while aiming for 1.5 °C [2,5].

There are various CO₂ capture methods, which include pre-combustion capture, post-combustion capture and oxyfuel combustion method [6]. A post-combustion method has the advantage of retrofitting existing plants as the current technologies do not require extreme changes to it [7]. The available CO₂ capture technologies are absorption, adsorption, cryogenic and membrane separation processes [6]. The conventional technology used in separating CO₂ from flue gases in a post-combustion process is the absorption–regeneration process [8] using amine solvent [7,9]. The chemical absorption process is preferred in large-scale CO₂ capture due to its flexibility in dealing with different CO₂ concentration and feed rates.

The two key limitations regarding the chemical absorption technology are its high energy demand for the process as well as the slow reaction kinetics in capturing CO₂. The high energy requirement is due to the need for high heat in the solvent regeneration stage [10], where it constitutes up to 70% of the total CO₂ capture plant operating costs [11]. The energy consumption for solvent regeneration is also referred as reboiler heat duty. This is because the total energy that is utilized is provided by the hot steam that passes through a reboiler at the bottom of the stripping column [12]. The heat required can further be classified as the sum of three terms: the sensible heat loss ($q_{sens}$, kJ/mol CO₂), heat of evaporation ($q_{vap,H_{2}O}$, kJ/mol CO₂) and heat of absorption ($q_{abs,C{O}_2}$, kJ/mol CO₂) [10,12,13], which is displayed in Equation (1). Several studies have also taken into account the heat of condensate reflux [14]. Figure 1 shows the share of reboiler duty that contributes to the regeneration energy requirement at low CO₂ partial pressure [15]. The most viable method to reduce the CO₂ capture cost for an absorption process would be to minimize the energy consumed by lowering the reboiler heat duty or lowering the quality of extracted steam [16].

$$q_{reb}=q_{sens}+q_{vap,H_{2}O}+q_{abs,C{O}_2}$$

(1)

where $q_{sens}$ is the sensible heat required to raise the temperature of solvent to the reboiler temperature, $q_{vap,H_{2}O}$ is the heat of evaporation needed to produce the stripping steam in the reboiler, and finally, $q_{abs,C{O}_2}$ is the heat of absorption of the solvent with CO₂. This term is also the overall heat required to desorb CO₂. This is because the same amount of heat that is released in the exothermic reaction of the absorption column is needed in the stripping column to reverse the absorption process [10]. The heat of desorption is the heat that should be provided to break up the chemical bond that is formed during the absorption reaction [13]. The overall reboiler heat duty $q_{reb}$. can then be estimated by Equation (2) [10,13]. The assumptions include neglecting the flashing point of the solvent at the desorber inlet and that phase equilibrium is not usually reached in large-scale operations.

$$q_{\mathit{reb}}\approx \stackrel{q_{\mathit{sens}}}{\overbrace{\frac{c_p\left(T_{reb}-T_{feed}\right)}{∆\alpha }\frac{M_{sol}}{M_{C{O}_2}}\frac{1}{x_{solv}}}}+\stackrel{q_{\mathit{vap}},H_{2}O}{\overbrace{∆h_{vap,H_{2}O}\frac{p_{H_{2}O}}{p_{C{O}_2}}\frac{1}{M_{C{O}_2}}}}+\stackrel{q_{\mathit{abs}},{\mathit{CO}}_2}{\overbrace{\frac{∆h_{abs,C{O}_2}}{M_{C{O}_2}}}}$$

(2)

Sensible heat, $q_{sens}$, is the heat that is required to raise the temperature of the CO₂ rich solvent to the desorption temperature of the stripping column. An increase in the sensible heat would lead to an increase in the heat of regeneration. $c_p$ is the molar specific heat of the solvent, $T_{reb}$ and $T_{feed}$ are the temperatures of the desorption and absorption process. $M_{sol}$ and $M_{C{O}_2}$ represent the molar weights of the solvent and CO_2, respectively. $x_{solv}$ is the molar fraction of the solvent in the solution, and $∆\alpha$ is the difference in the CO₂ loading of the solution after absorption (rich) and after desorption (lean). The cyclic capacity (Equation (3)) represents how much CO₂ can be picked up by the solution in the absorber. This is also known as the working capacity of the solution. As seen from Equation (2), the higher the cyclic capacity, the lower the sensible heat, thus leading to the reduction in the overall reboiler heat duty.

$$Q_{cyc}=∆\alpha C_{amine}=∆\alpha \frac{M_{C{O}_2}}{M_{sol}}{x}_{sol}$$

(3)

The heat of vaporization, $q_{vap,H_{2}O}$, consists of the term $∆h_{vap,H_{2}O}$, the heat of the evaporation of water, $p_{H_{2}O}$ and $p_{C{O}_2}$, which are the partial pressures of water vapor and CO₂ in the gas phase at the desorber outlet. The heat of absorption of solvent is represented by $∆h_{abs,C{O}_2}$.

From Equation (2), it can be seen that the high energy demand is also due to the slow kinetics of CO₂ absorption, particularly in solvents such as potassium carbonate [17] and tertiary amines [18,19]. This causes the prevention of them being employed on an industrial scale as their slow kinetics counteracted their advantages. Slow reaction kinetics would mean that higher energy is required to overcome the heat of absorption of solvent and reverse the reaction for CO₂ desorption. In order to overcome this limitation, studies on the presence of the nanoparticles in a gas-liquid system to enhance the absorption process has been gaining much attention among academics and engineers. Conversely, the scope of the chemical absorption process to optimize the desorption kinetics is severely lacking.

During the past decades, numerous studies have focused on reducing the high energy consumption of the solvent regeneration process. A recently emerged technique is the employment of nanoparticles to form a nanofluid. The concept of liquid nanoparticles can potentially improve the CO₂ capture kinetics and reduce the energy consumption for the solvent regeneration [20]. Nanoparticles used can be metallic, non-metallic, oxide, carbide, ceramics, carbonic and hybrids as well as nanoscale liquid droplets [21]. The nanoparticles can greatly improve the heat and mass transfer rate between the two phases of the gas-liquid system [22]. It was concluded that the most effective phenomenon in the mass transfer enhancement caused by the employment of nanoparticles is Brownian motion, which is induced by microconvections [23]. Functional nanoparticles, such as catalytic nanoparticles [16,24], can enable further enhancements [20]. In a catalyst-aided solvent regeneration process, the nanocatalyst can enhance the desorption efficiency by increasing the CO₂ desorption rate and then decreasing the time taken for CO₂ stripping, which leads to a reduction in the energy consumption in shorter time. Transition metal oxides have been widely used, due to the catalytic properties they exhibit from the presence of defected sites on metal oxide surfaces [25]. The two types of acidity that occur on the surface are Lewis acid, which accepts an electron pair, and Bronsted acid, which donates a proton [26].

In a typical regeneration process of solvent in CO₂ capture, the desorption temperature is high, which leads to a high energy consumption. A typical temperature for aqueous amine solvent is 120–140 °C. It can be seen, that with the utilization of nanoparticles, the desorption process can be conducted at a temperature below the boiling temperature of the solvent. The transition metal oxide catalyst was able to perform amine regeneration below 105 °C [27]. The nanoparticle can enhance the heat and mass transfer, increase desorption rate, thus reducing the regeneration energy requirement. Second, the low operating temperature can reduce the sensible and vaporization heat. This is because less steam is required for the stripping column in the regeneration unit. Furthermore, when achieving a lower CO₂ loading and a faster desorption rate, the stripping column size and solvent circulation rate can be reduced. A solvent with a high cyclic capacity could also reduce the dimensions of a CO₂ capture plant, which can potentially lead to a smaller circulation flow rate as well [28]. This leads to a reduction in the energy consumption and results in capital and operational cost savings. Bhatti et al. [27] analyzed the reduction of heat duty in an amine regeneration system utilizing metal oxides. Sensible heat is the difference between the reboiler temperature $T_{reb}$ and feed temperature $T_{feed}$. Reducing the reboiler temperature in the regeneration unit can drastically reduce the sensible heat required. When this condition is achieved, less steam would be required to maintain the temperature in the stripping column and so the heat of vaporization would also decrease. Thus, the reduction in heat duty for the regeneration system occurs.

Therefore, in the energy requirement for the solvent regeneration process, both the sensible heat and heat of vaporization could potentially be reduced, due to the utilization of nanoparticles in the solvent. The heat of the reaction, on the other hand, remains constant, as in a catalytic based regeneration process the presence of catalytic material does not change the thermodynamics of a system [29]. It does, however, allow a faster desorption in a shorter time.

In recent years, various nanoparticles have been utilized in the enhancement of the absorption process for CO₂ capture. Studies on the effect of nanoparticles on CO₂ absorption processes have received significant attention [20,30,31]. However, the area of solvent regeneration using nanoparticle-based solvents has yet to be reviewed. Since the critical issue regarding the CO₂ absorption process is the solvent regeneration stage, it is crucial to evaluate the status in utilizing nanoparticles to enhance the CO₂ regeneration.

There are nanoparticles that can enhance heat and mass transfer as well as catalytic nanoparticles that can enhance the desorption rate of the solvent. Nevertheless, studies in the past did not take into consideration both the physical and chemical effects of nanoparticles on solvent regeneration for CO₂ capture application. Therefore, a comprehensive review on the principal roles and contributing factors of different nanoparticles on the solvent regeneration for CO₂ capture is substantial.

In this paper, the current status of the employment of nanoparticles to enhance the regeneration of solvent and the differences in their physical and chemical mechanisms are discussed and analyzed. Furthermore, the selection criteria for the nanoparticles for solvent regulation is also outlined. Subsequently, a comparison summary and the future direction of using nanoparticles for solvent regeneration is highlighted in this paper.

2. Mechanisms of Solvent for CO₂ Desorption

The understanding on the reaction mechanism of solvent regeneration for CO₂ capture is crucial to explaining the role of nanoparticles on solvent regeneration. The reaction mechanism of the widely used CO₂ capture solvents, including monoethanolamine (MEA), methyldiethanolamine (MDEA), potassium carbonate (K₂CO₃) and sodium carbonate (Na₂CO₃), is briefly elaborated and discussed in the subsequent section.

2.1. Reaction Mechanism of Monoethanolamine

To date, the traditional solvents of choice that are implemented in the industry are aqueous amine solvents. These types of amine can be categorized into three groups, according to the number of hydrogen atoms that are attached to the central nitrogen [32]. For the primary amine, two H atoms are attached to the central nitrogen. The structure of MEA is seen in Figure 2.

A zwitterion mechanism is proposed to describe the reaction that occurs between the primary amine and CO₂ [33]. The zwitterion mechanism presents a two-step reaction in which a carbamate is formed, whereas the termolecular mechanism forms a reaction in only a single step. As seen in Equation (4), the CO₂ reacts with MEA (${\mathrm{RNH}}_2$) to form a zwitterion intermediate ion (${\mathrm{RNH}}_2^{+}{\mathrm{COO}}^{-}$). Then, in Equation (5), the intermediate undergoes deprotonation by a base to form a carbamate complex (${\mathrm{RNHCOO}}^{-}$). B corresponds to any species that can act as a base in the solution to abstract the proton from the zwitterion intermediate, such as ${\mathrm{H}}_2\mathrm{O},{ \mathrm{OH}}^{-},{ \mathrm{R}}_3\mathrm{N},{ \mathrm{HCO}}_3^{-},{ \mathrm{and} \mathrm{CO}}_3^{2-}$ [18].

$${\mathrm{CO}}_2+{\mathrm{RNH}}_2\leftrightarrow {\mathrm{RNH}}_2^{+}{\mathrm{COO}}^{-}$$

(4)

$${\mathrm{RNH}}_2^{+}{\mathrm{COO}}^{-}+\mathrm{B}\leftrightarrow {\mathrm{RNHCOO}}^{-}+{\mathrm{BH}}^{+}$$

(5)

The formation of carbamates that takes place leads to a high heat of absorption. This causes the regeneration energy to be higher and costly [18]. Apart from that, primary amines have the disadvantage of having a low loading capacity, which is limited to 0.5 mol CO₂/mol amine.

2.2. Reaction Mechanism of Methyldiethanolamine

For the case of a tertiary amine, all three H atoms are substituted with a hydrocarbon group. The molecular structure of MDEA is provided in Figure 3.

Donaldson and Nguyen [34] proposed a base-catalyzed mechanism where the aqueous MDEA solution would promote the hydrolysis of CO₂ to form a bicarbonate ion and a protonated amine. This also implies that tertiary amines cannot directly react with CO₂ because they lack the N-H bond that is required for the formation of carbamate. MDEA has the advantage of having a low heat of regeneration, thus lower heat duty would be required [18]. It is also beneficial since it has a higher loading capacity at 1.0 mol CO₂/mol amine.

It is reported that when CO₂ is absorbed in an aqueous MDEA solution, three reactions will occur simultaneously [35,36,37]. The reaction mechanism between CO₂ and MDEA is represented in the following reactions:

• Reaction I—CO₂ with MDEA

$$\mathrm{MDEA} +{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{k}}_{\mathrm{MDEA}}}{\leftrightarrow }{\mathrm{MDEAH}}^{+}+{\mathrm{HCO}}_3^{-}$$

(6)

$${\mathrm{r}}_{{\mathrm{CO}}_2-\mathrm{MDEA}}={\mathrm{k}}_{\mathrm{MDEA}}\left[\mathrm{MDEA}\right]\left[{\mathrm{CO}}_2\right]$$

(7)

• Reaction II—Bicarbonate formation

$${\mathrm{CO}}_2+{\mathrm{OH}}^{-}\stackrel{{\mathrm{k}}_{{\mathrm{OH}}^{-}}}{\leftrightarrow }{\mathrm{HCO}}_3^{-}$$

(8)

$${\mathrm{r}}_{{\mathrm{CO}}_2-{\mathrm{OH}}^{-}}={\mathrm{k}}_{{\mathrm{OH}}^{-}}\left[{\mathrm{OH}}^{-}\right]\left[{\mathrm{CO}}_2\right]$$

(9)

• Reaction III—CO₂ with water

$${\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{k}}_{{\mathrm{H}}_2\mathrm{O}}}{\leftrightarrow }{\mathrm{HCO}}_3^{-}+{\mathrm{H}}^{+}$$

(10)

$${\mathrm{r}}_{{\mathrm{CO}}_2-{\mathrm{H}}_2\mathrm{O}}={\mathrm{k}}_{{\mathrm{H}}_2\mathrm{O}}\left[{\mathrm{H}}_2\mathrm{O}\right]\left[{\mathrm{CO}}_2\right]$$

(11)

Among these three reactions, reaction III of CO₂ with water is very slow, thus can be neglected. The kinetics of MDEA is slower than the reaction kinetics of a primary and secondary amine because no carbamate is formed. Bicarbonate formation, on the other hand, is a slow process. However, the slow reaction between MDEA and CO₂ can be enhanced by employing activator or nanoparticles.

2.3. Reaction Mechanism of Inorganic Carbonate Solutns

The basic reaction chemistry for potassium carbonate (K₂CO₃) solvent or Sodium Carbonate (Na₂CO₃) solvent and CO₂ is represented in the overall reaction as shown in Equation (12), where M represents Na or K [38,39,40].

$${\mathrm{M}}_2{\mathrm{CO}}_3+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\leftrightarrow 2{\mathrm{MHCO}}_3$$

(12)

The aqueous carbonate solutions contain strong electrolytes; therefore, it can be assumed that the metal is only available in the form of K⁺ or Na⁺ ions, where the reaction (13) can be described in ionic terms more realistically as [40]:

$${\mathrm{CO}}_2+{\mathrm{CO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow 2{\mathrm{HCO}}_3^{-}$$

(13)

This is normally divided into two following reactions:

$${\mathrm{CO}}_2+{\mathrm{OH}}^{-}\stackrel{{\mathrm{k}}_{\mathrm{OH}}}{\leftrightarrow }{\mathrm{HCO}}_3^{-}$$

(14)

$${\mathrm{HCO}}_3^{-}+{\mathrm{OH}}^{-}\leftrightarrow {\mathrm{CO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(15)

In Equation (14), the hydration of CO₂ is in first order with respect to both the CO₂ and the ${\mathrm{OH}}^{-}$ ion. That equation is the rate determining reaction since Equation (15) is a proton transfer reaction and the rate constant is much higher than that of Equation (14) [41]. The carbonate solutions also suffer the weakness of having a slow reaction rate. Similar to MDEA, the reaction rate of this solvent can be enhanced by activator or nanoparticles.

3. Fluid Mechanics and Flow Properties of Nanoparticle-Based Solvents

It is crucial to discuss on the flow properties of nanoparticle-based solvents. The flow patterns depend on the fluid characteristics, speed of flow and the shape of the solid surface. The fluid characteristics include the viscosity, density and thermodynamic properties [20,42].

Density is one of the flow properties that should be critically evaluated as it is necessary for the detailed characterization of solvent for industrial application [43,44,45]. Increasing the density of solvent used can impact volumetric flowrate of solvent for the CO₂ absorption process, thus affecting the work input for pumps and heat exchangers [20]. On top of that, designing the absorber and desorber column diameter and packing height requires knowledge of the density of the fluid [46]. Utilizing nanoparticles in solvents increases the density of the base fluid. The following equation can be used to determine the density of nanoparticle-utilized solvent [47]:

$$\rho =\left(1-{\mathsf{\varphi }}_v\right){\rho }_w+{\mathsf{\varphi }}_v{\rho }_p$$

(16)

where ${\rho }_p$ and ${\rho }_w$ are the densities of the metal oxide particles and liquid solvent respectively. ${\mathsf{\varphi }}_v$ is the volume concentration (%) of the nanoparticles in the solvent.

The next property is viscosity, which is also another physical property of the fluid that is necessary for industrial design application. The viscosity of solvents increases in proportion to the concentration of nanoparticles in the solvents [48]. A solvent with high viscosity can cause a reduction in the absorption rate because the diffusion coefficient of CO₂ in the liquid phase decreases, thus resulting in high mass transfer resistance [49]. It is also not suitable for industrial application since a higher viscous solvent leads to a longer residence time and larger equipment size [50]. Utilizing nanoparticles into solvents can significantly increase the viscosity of the solvent [51]. This limits gas diffusion and pumping, which impacts the total cost and energy of the process [52].

Diffusivity is a transport property that is important to evaluate as it facilitates mass transfer. Due to the Brownian motion of the nanoparticles, the diffusivity of CO₂ is enhanced, which can increase the rate of CO₂ capture [23,48,53]. It has been reported that with an increase in size of the nanoparticles, the diffusivity decreases [53]. Nanoparticles of larger sizes lead to a lower level of Brownian motion [54]. This severely affects the diffusivity of gas through the solvent because the Brownian motion of nanoparticles is important to help increase the microconvection and, consequently, the diffusivity. Hence, an increase in the size of nanoparticles leads to a decrease in the Brownian motion, which also decreases the diffusivity [55,56].

4. Physical and Chemical Enhancement Mechanism

4.1. Physical Enhancement Mechanism of Nanoparticles

The physical enhancement mechanisms by nanoparticles on solvent regeneration include activation energy, surface effect and thermal conductivity enhancement effect [57,58,59,60]. The activation energy effect was proposed by Lee et al. [59] in 2015. Activation energy is the minimum energy that must be provided to result in a chemical reaction of a compound. In the regeneration process, the activation energy must be supplied so the regeneration of CO₂ gas from the solvent can occur [60]. The solvent that is blended with nanoparticles can desorb the CO₂ easily in comparison to the solvent without nanoparticles, due to the increase of activation energy. These nanoparticles in motion in the base fluid have a high activation energy and result in active liquid molecules, which leads to the clash of liquid molecules and nanoparticles. This dynamic behavior in the base fluid results in an increase in activation energy.

The next mechanism involved in the regeneration of nanoparticles is the thermal conductivity enhancement effect on the heat transfer performance, which was proposed by Fan and Wang [61] and Keblinski et al. [62]. Due to the increase in the heat transfer by the nanoparticles, as discussed previously, the effective thermal conductivity of the nanoparticles is also improved. However, this is improved with the optimum concentration and type of nanoparticles employed. The mechanism explaining the improved thermal conductivity are Brownian motion, molecular-level layering, nature of heat transport and the effect of nanoparticle clustering [61,62]. These proposed mechanisms have not been clearly explained, but the increase in thermal conductivity upon the employment of nanoparticle has been reported by numerical and experimental studies [61,63,64]. The increase in the thermal conductivity of the nanoparticles can rapidly increase the temperature, which results in the rapid decrease of CO₂ solubility. Lee et al. [60] demonstrated the contribution of SiO₂ and Al₂O₃ nanoparticles on the regeneration enhancement of methanol solvent, in terms of heat transfer. The enhancement was reported to only be 2–3% approximately, as adding only 0.01 vol% nanoparticles could not cause a significant difference in the thermal conductivity. As supported by other studies [63,65,66], the effect of thermal conductivity enhancement by nanoparticles is not significant.

Another proposed mechanism is the surface effect, which enhances heat transfer [67]. CO₂ desorption process works similarly to the boiling process, in which more bubbles are formed upon the increase in temperature, due to Henry’s law of solubility. When nanoparticles are employed in the base fluid, they are deposited on the boiling surface due to gravity and natural convection, which then causes the change of the boiling surface properties on the surface of the heater. While the boiling process occurs at a high temperature above the saturation point, the bubble regeneration process occurs when the temperature rises above the absorption temperature. Upon the utilization of the nanoparticles, the bubble generation that takes place is easily achieved in comparison to the boiling process conditions, because of its influence on the surface characteristics. The effects that take place upon the nanoparticle deposition on bubble generating surfaces are as follows:

(1)

Increase in heat transfer surface area: As reported by Kim et al. [68], at a concentration below the critical concentration, the effective heat transfer surface area increases upon the increase in nanoparticle concentration. However, exceeding the critical concentration can cause a smooth nanoscale surface to form, which reduces the effective heat surface area.

(2)

Change in surface roughness: During the boiling process, the nanoparticles are deposited on the heating surface, which causes the change in the microstructure and topography of the heating surface. A porous layer is formed on the boiling surface, which produces a structural effect and increases wettability [67,69,70]. Therefore, more bubbles are more easily generated and desorbed from the surface. This mechanism is supported by Lee et al. [59], who studied the visualization of the CO₂ bubble generation when employing SiO₂ and Al₂O₃ nanoparticles to deionized water. The Al₂O₃ showed better bubble generation and desorption upon adding heat, in comparison to water and SiO₂ nanoparticles.

(3)

Increase in nucleation site density: In a fluid, bubbles are primarily generated at the small sites on the irregular surface (cavities, scratches, pits and cracks), which is called the “nucleation site”. As nanoparticles are deposited on the boiling surface, more nucleation sites are created. In addition to that, the floating nanoparticles, such as those on the heater surface, can also become bubble generation points. More CO₂ can be discharged as more regeneration sites are created. It has also been reported that the nucleation site density increases if the surface roughness is larger than the particle size, and the nucleation site density is reduced if the two values are similar [71].

4.2. Catalytic Enhancement Mechanism of Nanoparticles

The catalytic effects of the nanoparticles are known to improve the energy efficiency of the desorption reaction, although the theoretical level of thermodynamic energy required remains unchanged. Figure 4 shows the conditions that are generally accepted to explain the high energy consumption in a CO₂ desorption process [72,73].

The main reasons for the large heat requirement for the regeneration of solvent is the high endothermic carbamate breakdown reaction as well as the difficulty of the deprotonation of pronated amine to water. Therefore, the function of this metal oxide catalyst is to donate the Lewis acid and Bronsted Acid to the N atoms in the carbamate structure. Overall, this weakens the N-C bond and less thermal energy would be required to break up the carbamate, thus resulting in an increase in the CO₂ desorption rate [74]. This amine regeneration process can then be performed at lower temperatures which results in a decrease of the $Q_{sen}$ and $Q_{vap}$ [27].

The catalytic mechanism of metal oxides with MEA solvent has previously been proposed [24,75]. In several studies, the role of Lewis and Bronsted acid sites in the promotion mechanism of catalytic CO₂ desorption process has also been reported [24,76,77,78]. The five commercially available nanocatalyst, which are Ag₂O, Nb₂O₅, NiO, CuO and MnO₂, demonstrated a catalytic mechanism, due to the presence of acid sites over the particles [75]. Lewis acid is one of the main acidic types and is typically provided by unsaturated metal atoms. This allows it to accept a lone pair of electrons [27]. Bronsted acid, on the other hand, can donate a proton to a base [79]. This is due to the hydroxyl groups that were converted from the surface oxide of the catalyst.

The catalytic role provided by Lewis acid can be seen in reaction (17) and (18), whereas the role Bronsted acid can be seen in reaction 19 and 20.

$${\mathrm{MEAH}}^{+}+\mathrm{L}\leftrightarrow \mathrm{MEA}+\mathrm{LH}$$

(17)

$$\mathrm{LH}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{L}}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$$

(18)

$$\mathrm{MO}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{MO}·{\mathrm{H}}_2\mathrm{O}$$

(19)

$$\mathrm{MO}·{\mathrm{H}}_2\mathrm{O}\to \mathrm{MOH}·\mathrm{OH}$$

(20)

According to reactions (19) and (20) [29], the oxygen atom in the water molecule and metal atoms (Al, Si, Mo, etc.) can chemically adsorb and split the water molecule on the metal oxides’ surfaces. This forms a hydroxyl group, which behaves as a Bronsted acid site [27,75,80]. Although metal oxides have the benefit of both Lewis and Bronsted acids, they are more commonly known as Lewis acid catalysts. Their Lewis acid sites can be converted to Bronsted acid sites [29].

The literature has previously reported the higher regeneration performance of Bronsted acid in comparison to Lewis acid in amine solvents. Bronsted acid can supply protons, take part in the catalytic desorption process and is released into the solution [29]. Because of this, the CO₂ desorption rate is greatly enhanced, since the breaking down of carbamate can occur without waiting for the deprotonation reaction to occur [81]. In addition to that, the solvent regeneration process can be accelerated, since the Bronsted acid can generate protons in a tertiary amine solvent [82] and directly attack bicarbonate species [83]. An example is the study by Lai et al. [24], where the hydroxyl group on the TiO(OH)₂ nanoparticle has the ability to accept or donate protons, which accelerate any proton driven reactions. Therefore, the protonation and deprotonation reactions occur faster and, therefore, benefit both the absorption and desorption processes. The reaction that occurs between MEA and CO₂ in the presence of TiO(OH)₂ can be catalyzed. The hydroxyl group of the TiO(OH)₂ has the ability to donate or accept protons in any reactions involving protonation and deprotonation; therefore, the proton-involved reactions are accelerated.

An example of another catalyst in its catalytic regeneration mechanism is for HZSM-5 zeolite. It has been explained that HZSM-5 zeolite contains both the present of Lewis and Bronsted acids on its surfaces, which accounts for the superior catalytic performance of this zeolite [69,81]. In the mechanism, the Bronsted acid will supply a multitude of free protons for converting MEACOO− to MEACOOH even without the deprotonation step, as seen in the desorption mechanism of uncatalyzed amine. As for the Lewis Acid sites, the lone pair electrons in the O atom of the MEACOOH are attracted to the empty orbital of the Al atoms. Then the MEACOOH becomes zwitterion intermediate [77]. The NAC bond stretches and, eventually, breaks, which causes the zwitterion to become MEA and CO₂. At high regeneration temperatures, the CO₂ solubility in the aqueous solution is low and the CO₂ will then transfer to the gas phase [81,84,85,86]. Therefore, this also explains why when more HZSM-5 catalysts are present, the concentrations of available protons would increase and, therefore, allow MEACOO- to react with the protons at an increased rate. This leads to a faster desorption rate and solvent regeneration performance.

It has been reported that the physiochemical properties of catalysts can affect the CO₂ desorption performance during solvent regeneration process [29]. It is crucial to identify the parameters for future synthetization of nanoparticles. There are several factors that can impact the catalytic performance of the nanoparticles. According to past studies, there were three parameters that were reported to be essential physical properties of nanocatalysts [29]. These are: (a) mesopore surface area (MSA), (b) total surface area and (c) average pore diameter. The first property is the MSA, which represents the mesopore surface area. These are materials that contain pores between 2 and 50 nm. Smaller pores, such as those found in microporous materials (<2 nm), are inaccessible for large amine carbamate materials but is beneficial for ammonia solutions instead. The possible explanation for this is that ammonia carbamate are smaller in size than amine carbamates [87]; therefore, in this case, the total surface area is important. Besides, the average pore diameter is also crucial, as large pore size in a nanoparticle can give a better access to the interior surfaces and thus allow more available active acidic-basic sites as well as reduce the mass transfer resistance for the diffusion of the molecules through the pores of nanoparticles [88]. This is also an important property when evaluating the cyclic ability of the nanoparticle, as a larger pore size can avoid the pores being blocked with the large carbamate molecules and maintain the catalytic efficiency after several absorption-desorption cycles. The role of acidic sites (Lewis acid and Bronsted acid) of a nanoparticle is substantial and it has, in fact, been proven to be more influential than the MSA [27,75,89]. A study has shown that an increase in the Bronsted acid sites (BAS) would also have a larger positive impact to the CO₂ desorption rate in comparison with the physical properties of the catalyst, which have less impact [87]. Other studies have mentioned that the total acid sites also have a good relationship with the CO₂ desorption rate and the reduction in energy [27,75,90]. Apart from that, the acid strength of the catalyst is an important chemical property, despite limited studies regarding this aspect. An increase of the strong acid sites and acid strength of the nanoparticle would lead to an increase in the CO₂ desorption rate [91]. Another study shows that weak acid can also affect the performance of the nanoparticle [80]. The role of basic sites are also important to consider, though the studies on this area have been limited since it was first reported in 2018 [29]. The relation between the basic sites and the kinetics of CO₂ desorption has yet to be found, although even past studies have shown that an increase in the basic sites leads to an increase in the catalytic performance of the material [73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93].

5. Nanoparticle Selection Criteria

The utilization of nanoparticles has been identified as a suitable method to improve solvent regeneration rate. Yet, different nanoparticles exhibit different enhancement performances under varied operating conditions. In order to fulfil the operational and maintenance requirement, the selection criteria for nanoparticles for solvent regeneration is proposed as depicted in Figure 5.

The nanoparticle’s thermal stability plays an important role in this selection process. The nanoparticle is considered to be thermally stable if it does not decompose under the influence of temperature. In this case, the solvent regeneration usually occurs at temperatures above 100 °C. One method in determining the thermal stability of the nanoparticle is to use a thermogravimetric analyser (TGA). For this selection process, any weight loss due to moisture is neglected and only the weight loss of the material due to decomposition is reported. A scale of three was used to evaluate the thermal stability and is represented as low (T < 333 °C), medium (334 °C < T < 666 °C) or high (T > 667 °C).

Next, the toxicity of chemicals. This review focuses on the acute toxicity of the chemical. Acute toxicity is defined as the adverse changes that occur either immediately or a short time after a single point or short period of exposure to a chemical [94]. The term LD₅₀ was first introduced in 1927 [95], where it defined toxicity as a “median lethal dose”, which is the dose that would kill 50% of a large animal group. The classification for the toxicity of a chemical is based on the Recommended Classification of Pesticides of the World Health Organization (WHO) [96] and can be seen in

Table 1. Toxicological classification of chemicals.

Class	LD50 for Rat (mg/kg Body Weight)
		Oral		Dermal
		Solid	Liquid	Solid	Liquid
Ia	Extremely hazardous	5 or less	20 or less	10 or less	40 or less
Ib	Highly hazardous	5–50	20–200	10–100	40–400
II	Moderately hazardous	50–500	200–2000	100–1000	400–4000
III	Slightly hazardous	Over 500	Over 2000	Over 1000	Over 4000

. The classification distinguishes between the different levels of hazards based on the toxicity of chemicals. It is also based on the acute oral and dermal toxicity in rats, since their use is the standard in toxicology. The larger the value for LD50 is, the less toxic the chemical is, and, therefore, the smaller the value, the more toxic. For this review, the LD₅₀ is retrieved from the supplier of different nanoparticles and is categorized according to the classification in Table 1. For the summarization of the toxicity of chemicals, the terms “Extreme”, “Highly”, “Moderately” and “Slightly” are implemented.

The importance of chemicals being environmentally friendly is very important, and this review is based on the LC50 of chemicals. This is because nanoparticles may lead to a severe environmental impact upon their environmental release. LC50 is very similar to LD50, where the acute toxicity is determined by the median lethal concentration that would kill 50% of a population. The aquatic toxicity classification scale is based on the U.S. Fish and Wildlife Service Research Information Bulletin [97] and the properties of nanoparticles are retrieved according to the ecotoxicology data of the chemical in question. The aquatic toxicity classification scale can be seen in

Table 2. Aquatic toxicity classification scale.

Classification	LC50 (mg/L or ppm)
Super toxic	<0.01
Extremely toxic	0.01–0.1
Highly toxic	0.1–1.0
Moderately toxic	1.0–10.0
Slightly toxic	10.0–100.0
Practically non-toxic	100.0–1000
Relatively harmless	>1000

.

The major concern regarding the employment of nanoparticles in a base fluid is its stability. The stability of nanoparticles can be defined as its ability to agglomerate and has different key aspects to it, such as core composition, shape, size and surface chemistry [98]. The most common indicators of instability that occur are sedimentation and agglomeration [20]. Agglomeration is the act of the clustering of particles. For particles that are smaller than 100 nm in size, this can cause collisions, which can result in either an attachment or repulsion [99]. Surface chemistry, which is the variation caused by surface atomic and molecular density, chemical composition and potential, also leads to aggregation [99]. The types of interaction forces between the nanoparticles can be classified into two: the van der Waals attractive force (V_A) and the repulsive force (V_R). Both are functions of the distance that is between two particles in the base fluid. When the total potential energy is zero, the particles are then considered to be stable, as they are arranged at a stable distance. According to Wang et al. [100], the nanoparticle suspensions are more unstable at high CO₂ loading. As explained in the study, when the CO₂ loading increases, an ion that is formed from the reaction of amine and CO₂ interacts with the functional group on the nanoparticle’s surface. This leads to the negative-positive reversal of Zeta potential. Additionally, when the CO₂ loading increases, the ion concentration will compress the electric double layers of nanoparticles, thus reducing the stability of the nanoparticle. Improving the stability of the nanoparticle is an important issue to tackle, especially if the nanoparticles are utilized at an industrial scale. This is because the stability of the nanoparticles would directly affect the mass transfer performance [101]. The solvent can lose its ability of heat transfer if unstable, due to the nanoparticles being prone to agglomeration. The current methods proposed to improve the stability of the nanoparticles are the use of surfactants and surface modification techniques [102]. These methods are known to be easy and economically viable in order to improve the stability of the nanoparticles, but the effects on the CO₂ absorption and desorption are still undetermined [103]. Choi et al. [57] have studied the effects of adding surfactants and concluded that they can enhance the CO₂ absorption rate but interrupt the CO₂ desorption rate. Other methods to enhance the stability, other than the addition of surfactants, are the modification of the surface, pH control of the nanoparticles and stirring and ultrasonic agitation [21]. Apart from the stability of the nanoparticle in terms of its agglomeration, it is also important to evaluate the nanoparticle’s stability in terms of its core composition. Instability, in this case, refers to the chemical composition variations or coordination number changes [99]. Oxidation is a process that leads to a reduction in the catalytic performance of nanoparticles. Another aspect of nanoparticle stability is its morphology (shape and size) [104]. The instability of this aspect comes from its cluster size, as particles in colloidal solutions usually come into contact with each other, and clusters are formed due to the unstable particle collisions [59]. This alters the thermo-physical properties of nanoparticles for its application. The effect of interparticle distance, as reported by Ono et al. [105], reported that the sample with the largest interparticle distance showed higher stability against agglomeration. Therefore, apart from the initial nanoparticle size, their distribution on a substrate’s surface can improve the stability and lifetime of a nanoparticle. The shape also affects the nanoparticle’s stability, as it is related to the conservation of the original local structure and radius of the curvature of the nanoparticles [98]. It was reported that the morphological changes caused variations in surface facet percentages, which led to a reduction in the surface energy, due to the shape variation. It is important to study the stability of the nanoparticles, in terms of their morphology, as it directly influences the physiochemical properties of the nanoparticles.

Another important criterion is the recycling ability of the nanoparticle. Different nanoparticles display different recycling abilities, which would determine the nanoparticle’s reusability or the lifetime of the solvent. For this to be achieved, nanoparticles would be selected for the further study of the absorption-regeneration cycle. In order to identify the reasons for the reduction in the catalytic performance of the material, the recycled nanoparticle would be sent for characterization so that the structural properties of the nanoparticles could be proven to have been maintained [93]. Another method is also to compare the cyclic capacity of each cycle and if there is no significant decrease, then the nanoparticle is said to possess excellent stability and can be implemented in CO₂ capture [93]. Another study reported on the amount of CO₂ released (mol) for each regeneration cycle and is an indicator regarding the particle’s stability and ability to be recycled [106]. The changes in heat duty or energy consumption reported in each cycle can also be another indicator [107]. This is also closely related to the cyclic capacity of the material.

Foaming in amine plants causes an increase in the operating costs and would reduce the separation efficiency [108]. Pure amines do not actually lead to the formation of stable foams; therefore, one or more components in the solvent must be present in order to form a persistent foam [109]. The components added into the solution can impact either the foaming tendency or foaming stability. In the case of MDEA solvent, it has a lower foaming tendency but the employment of any additives that reduce surface tension can increase foam stability. Since foam tendency is also highly correlated to the type of solvent used, this important nanoparticle selection criteria will touch on foaming stability. Nanoparticles which do not enhance foaming stability will be reported in this review.

6. Specific Nanoparticles That Enhance Solvent Regeneration

6.1. Metal Oxides

Metal oxide nanoparticles have previously been widely studied in the literature and have been used on a commercial scale due to their advantage in promoting the physical mechanism of solvent regeneration. Metal oxide nanoparticles have the advantage of both physical and chemical enhancement mechanisms [29]. In terms of their physical enhancement mechanisms, nanoparticles cause an increase in the mass transfer surface area and a decrease in mass transfer resistance at the gas-liquid interfaces of the solvent due to Brownian motion. This results in a large increase in the rate of CO₂ desorption of the solvent. Apart from that, these metal oxide nanoparticles exhibit chemical enhancement mechanisms through their catalytic behavior. This review covers the past studies on the employment of metal oxide nanoparticles in enhancing the CO₂ desorption rate.

Table 3. Summary of past studied nanoparticles and their enhancement ratio.

Solvent	Nanoparticles	Size and Concentration	Temperature	Enhancement Ratio	Ref.
Deionized Water	SiO2	0.01–0.1 vol%	20	1.078	[57]
Deionized Water	SiO2	15 nm 0.0–0.05 vol%	100	1.118	[59]
	Al2O3	45 nm 0.0–0.05 vol%	100	-
Methanol	Al2O3	20 nm, 45 nm 0, 0.01 vol%	60	-	[58]
Methanol	SiO2	15 nm 0.01 vol%	< 65	1.22	[60]
	Al2O3	45 nm 0.01 vol%	< 65	1.16
MEA	TiO2	15 nm 0.1 wt%	103	1.42	[100]
	SiO2			1.26
	Al2O3			1.21
MEA	MoO3	5 g	86	1.94	[27]
	V2O5			1.84
	Cr2O3			1.44
	TiO2			1.44
	WO3			1.44
MEA	Ag2O	10 g	70–85	1.48	[75]
	Nb2O5			1.46
	NiO			1.23
	CuO			1.30
	MnO2			1.15
Na2CO3	TiO(OH)2	17.12 Å	40–70	9.00	[38]
MEA	TiO(OH)2	17.1 Å 1–3 wt%	88	46.00	[24]
K2CO3	Cu-TiO(OH)2	0.014 vol%	373 K	-	[110]

shows the summary of nanoparticles employed in past studies along with their enhancement ratio.

6.1.1. SiO₂

Since the development of unique nanoscale particles, the industrial applications of silica nanoparticles, SiO₂, have drastically increased. Silica nanoparticles have been widely selected due to the simplicity and low-cost of their large-scale preparation, their hydrophilic nature, large specific surface area, pore volume and controlled particle size [111,112] For CO₂ capture processes, SiO₂ nanoparticles were shown to enhance the mass-transfer process and improve the absorption and desorption performance [57,59,60,100,101]. For their regeneration performance, the nanoparticles were studied in three different solvents, which were deionized water, methanol and MEA, the summary of which is depicted in

Table 4. Summary of past studies using SiO₂ nanoparticles in the regeneration of solvent.

Solvent	Size and Concentration	Temperature °C	Remarks	Ref.
Deionized water	15 nm 0.01–0.1 vol%	20	Highest regeneration rate at 0.01 vol% Desorption enhancement ratio decreases with the addition of surfactants. Three enhancement mechanisms: Activation effect, thermal conductivity effect and surface effect.	[57]
Deionized water	15 nm 0.0–0.05 vol%	100	Highest regeneration enhancement at 0.01 vol% Three enhancement mechanisms: Activation effect, thermal conductivity effect and surface effect.	[59]
Methanol	15 nm 0.01 vol%	<65	Nanoparticles’ enhancement was at 22% om in an average of 5 cycles. Three enhancement mechanisms: Activation effect, thermal conductivity effect and surface effect.	[60]
MEA	15 nm 0.1 wt%	103	Three enhancement mechanisms: Activation effect, thermal conductivity effect and surface effect.	[100]

.

Among the nanoparticles in review, the SiO₂ nanoparticle is less dense than other metal oxides. The average size of SiO₂ used in past studies were 15 nm. When the size of the nanoparticles was smaller than the surface roughness, the roughness decreased. However, when the size was larger than the surface roughness, then the nucleation site density became larger [58]. Apart from the nanoparticle powder size, its cluster size is also important, as it reflects the nanoparticle’s stability. An unstable nanoparticle would have a high nanoparticle cluster size, due to the agglomeration and sedimentation of the nanoparticle. It was also reported that the cluster size has a much greater impact on the geometric conditions of the heating surface in comparison with the nanoparticle’s particle size. The SiO₂ nanoparticles have a cluster size ranging from 680–800 nm [59], both of which are higher than that of the Al₂O₃ nanoparticle studied (330.8 nm) [60]. However, the study by Wang et al. [100] reported the cluster size of the SiO₂ nanoparticle to be 213.4 nm before absorption and 2053.5 nm after absorption, which is lower than the other two nanoparticles in the study.

The SiO₂ nanoparticle concentration is kept low at 0.01–0.1 vol% [58,59,60,100]. Higher concentrations of the nanoparticle lead to the increase in the viscosity of the solvent. However, it also been shown that when increasing the concentration, the changes in viscosity were not severe, in fact, less than 5%. It was also reported that the employment of higher concentrations contributes to an enhancement in terms of heat transfer, although it is only 2–3%. The enhancement of thermal conductivity is low for concentrations below 1.0 vol% [63,66,67]. Among the studied nanoparticles, the SiO₂ can be seen to have a low enhancement rate, and the past studies only managed to consider its physical enhancement effect in terms of mass and heat transfer mechanism. In addition to that, the thermal stability was reported in terms of its mass loss at particular temperatures and the pure silica was seen to be very stable at high temperatures, only facing very minimal mass loss [113]. However, another study did report weight loss, which may happen below 100 °C, due to the evaporation of water [114]. Subsequently, it remained almost constant up to 800 °C.

The enhancement mechanism reported in past studies include activation effect, thermal conductivity effect and surface effect. The activation effect is discussed through the collision of the SiO₂ nanoparticles and the CO₂ gas, which enhances CO₂ regeneration. Next is the thermal conductivity effect, which causes the temperature of the nanoparticle-utilized solvent to rise faster. This also enhances the regeneration performance. Lastly, the surface effect, which is discussed in regard to the bubble generation on the surface of the heater. It has been reported that the frequency of CO₂ bubble departures were between 50–160 s for SiO₂, which is faster than that of the blank test [59].

6.1.2. Al₂O₃

The $\gamma$-Al₂O₃ was first introduced as a suitable catalyst for the CO₂ desorption process [76] in 2011, which eventually became the benchmark for the other studies on the utilization of nanoparticles. The utilization of $\gamma$-Al₂O₃ in 5M MEA was able to allow the reduction of the regeneration temperature from 120–140 °C to 90–95 °C and decrease the regeneration energy requirement by 27%.

Table 5. Summary of past studies using Al₂O₃ nanoparticles on the regeneration of solvent.

Solvent	Size and Concentration	Temperature °C	Remarks	Ref.
Deionized water	45 nm 0–0.05 vol%	100	Highest regeneration rate at 0.01 vol% Desorption enhancement ratio decreases with the addition of surfactants. Three enhancement mechanisms: Activation effect, thermal conductivity effect and surface effect.	[59]
Methanol	20, 45 nm 0, 0.01 vol%	60	Highest regeneration enhancement at 0.01 vol% Three enhancement mechanisms: Activation effect, thermal conductivity effect and surface effect.	[59]
Methanol	45 nm 0.01 vol%	<65	Nanoparticle’s enhancement was at 22% om average of 5 cycles. Three enhancement mechanisms: Activation effect, thermal conductivity effect and surface effect.	[60]
MEA	15 nm 0.1 wt%	103	Three enhancement mechanisms: Activation effect, thermal conductivity effect and surface effect.	[100]
MEA	250 g	90	The heat duty for this nanoparticle was to be one of the lowest in the study (yet still higher than HZSM-5 zeolite) Increase in the cyclic capacity as the amount of catalyst increased Al2O3 was reported to work well only in a rich loading region.	[84]
Amine blend	25 g	96	Heat duty was 540.9 kJ/mol CO2, which is one of the lowest (yet still higher than HZSM-5) MSA is 173.5 m2/g B/L is 0.67	[72]

depicts the summary of the past studies on the utilization of Al₂O₃ nanoparticles in solvent regeneration [58,59,60,100], for the nanoparticles ranging from 15–45 nm. It was reported that a larger nanoparticle has a higher number of regeneration sites of 19 at 45 nm, followed by 15 sites at 20 nm and finally just 8 sites for the base solvent methanol that was used in the study. Bubbles would detach faster on the surface of nanoparticles in comparison to the base solvent when the cross-section area was bigger and had a greater number of regeneration sites. Despite the nanoparticle size being larger than that of SiO₂, as presented in the previous context, the cluster size of Al₂O₃ was reported to be smaller. Such studies reported that the cluster size of Al₂O₃/DI water nanoparticles was at 150–170 nm. Even comparing the 20 nm and 45 nm Al₂O₃, the average aggregation diameter was 306.5 nm and 169.7 nm, respectively, which shows that the 20 nm Al₂O₃ nanoparticle had a cluster size approximately 1.8 times larger than the 45 nm of the Al₂O₃ particle.

On top of that, it is more effective to analyze the surface morphology of the nanoparticle for CO₂ regeneration rather than the size or cluster size of nanoparticle. This is because, despite the fact that Al₂O₃ was reported to have a large nanoparticle size (45 nm) and smaller cluster size, the departure time of the CO₂ bubble during solvent regeneration was faster in comparison to the smaller 15 nm SiO₂ nanoparticle that had a larger cluster size. A 45 nm Al₂O₃ nanoparticle has a higher average surface roughness of 195.03 nm in comparison to a 20 nm Al₂O₃, which has an average surface roughness of 84.90 nm. The greater the surface roughness of the nanoparticle surface, the larger the number of sites of CO₂ bubbles, which is beneficial for the separation of CO₂ from the surface. The explanation of the surface morphology is much clearer than comparing the particle size and cluster size of the nanoparticles.

In terms of the concentration of the Al₂O₃ employed in previous studies, 0–0.05 vol% and 0–0.14 wt% of Al₂O₃ were blended into different solvent solutions, such as DI water, methanol and MEA. Al₂O₃ showed an interesting result, where the increase of the concentration resulted in a decrease in the regeneration performance of the nanoparticles. The reason proposed for this reduction in performance is (1) the formation of adsorbed bicarbonate and carbonate species upon the reaction of CO₂ with this metal oxide nanoparticle and (2) the difference in characteristics, such as high surface potential, according to the variation pH results in the case of CO₂ to be caught by the Al₂O₃ nanoparticle. This also indicates that it is not easy to desorb the CO₂ from Al₂O₃ in comparison to the SiO₂ nanoparticle.

It was reported that the enhancement is due to the physical enhancement mechanism, such as the activation effect, thermal conductivity effect and surface effect. The utilization of the nanoparticles leads to an increase in the dynamic motion of the molecules in the solvent. The collisions of the nanoparticles cause the gas dissolved in the solvent to be desorbed easily and in a larger volume, which enhances the regeneration performance. Next is the thermal conductivity effect, which is discussed through the increase in the thermal conductivity of $\gamma$- Al₂O₃ that can rapidly increase the temperature, which results in a rapid reduction of CO₂ solubility. The temperature difference recorded for the $\gamma$- Al₂O₃ nanoparticle utilized in methanol solvent was higher in comparison to its blank test, which shows that the contribution of the nanoparticle enhances the heat transfer, which causes an increase in the regeneration performance [60]. The last physical enhancement mechanism discussed is the surface effect model. More bubbles are generated due to the utilization of nanoparticles. Consequently, the generated bubbles desorb easily, leading to enhanced regeneration.

Apart from that, it was also reported that the Al₂O₃ nanoparticle had a catalytic effect, as seen in several studies [72,74,76,77,81,84,93]. It was explained that although metal oxides are predominantly known as Lewis acid, they can indirectly have Bronsted acid sites on their surfaces, which can donate a proton to the base. The O atoms in the water molecules and the metal oxides have strong interactions, which leads to the water molecules on the surface of the metal oxides to split. This forms a hydroxyl group, which behaves like a Bronsted acid site. This nanoparticle was also studied to mimic the role of a bicarbonate ion in low CO₂ loading, which led to the improvement in the regeneration energy of the amine solvent.

The Al₂O₃ showed a significant decrease in heat duty, such as 11.8852 Gj/ton, as seen in [84]. This is much lower than the blank test and other catalysts, apart from HZSM-5. This is supported by other examples in the literature [72,81,93]. These studies also have shown that Al₂O₃ has a higher MSA value, but a lower B/L ratio compared to other catalysts, including the HZSM-5 zeolite. The product of these two factors has been shown to be lower than HZSM-5, which also explains why HZSM-5 performs better. However, Al₂O₃ is still a good candidate, as it still performs better than other catalysts.

According to the nanoparticle selection criteria, the Al₂O₃ nanoparticle employed is reported to be unstable at high concentrations, allowing it to agglomerate. The nanoparticle is also thermally stable, as it degrades at a higher temperature than the required solvent regeneration temperature. A 4–6% weight loss was reported at 410 °C, which is the thermal degradation temperature, and a total of 23% weight loss was reported once the temperature reached 800 °C [115]. Apart from that, the Al₂O₃ nanoparticle is reusable in multiple absorption–desorption cycles. It also has shown benefits as it is a minor and reversible health hazard, making it a non-toxic chemical.

6.1.3. TiO₂

The utilization of TiO₂ nanoparticles for the process of solvent regeneration in CO₂ capture process is limited and should be improved. However, it was demonstrated that the TiO₂ nanoparticle contributed to an impressive CO₂ absorption enhancement [100,116]. With the increase in the particle loading, the viscosity of the TiO₂ employed solvent demonstrated a significant increase. This can be explained by the high density of the TiO₂ nanoparticle at 5500–6000 kg/m³, which, in fact, is the highest among all the other particles in review. This is an undesirable property as an increase in viscosity causes a reduction in the diffusion coefficient and would restrain the growth of bubbles, resulting in the reduction of the gas-liquid mass transfer rate.

TiO₂ was also reported to have catalytic effects on enhancing the desorption performance of CO₂ capture [27]. The TiO₂ possesses Lewis acid (electron acceptor) sites on the surface of the catalyst, which is why it experiences an improved CO₂ stripping rate. This is, however, not as significant as the other catalysts studied by the author and team. In terms of its other properties, the TiO₂ is still prone to agglomeration and is thermally stable, proving its suitability to be employed for a typical regeneration temperature. According to a study [117], the TiO₂ nanoparticle powder showed two main stages of mass loss. The first being 6–8%, between 25 and 100 °C, which was due to water loss on the surface of the TiO₂. The second one was attributed to the loss of the organic matter after the rupture of the polymer chain due to the high temperature of 250 and 480 °C. This corresponded to an additional weight loss of 35%. In addition to that, it was also a recyclable nanoparticle with slight toxicity and was relatively harmless to the environment.

6.1.4. Transition Metal Oxides

Transition metal oxides have also been widely used due to their catalytic properties as they can be a good option in providing Lewis acids and Bronsted acids in the stripping column [27]. Bhatti el al. [27] studied the effects of five metal oxides, which were MoO₃, V₂O₅, Cr₂O₃, TiO₂ and WO₃, in a MEA-based solvent. It was reported that metal oxides with both types of acid (MoO₃ and V₂O₅) showed distinct effects and desorbed up to 94% and 84% of CO_2, respectively. The other three acids with only Lewis acid sites (Cr₂O₃, TiO₂ and WO₃) desorbed 44% of CO₂. The experiments were successfully run at much faster rates with 1.4–2 times greater amount of CO₂ desorbed.

This led to the introduction of five new metal oxide nanoparticles, which are Ag₂O, Nb₂O₅, NiO, CuO and MnO₂, on MEA solvents [75]. The experiment was run at a temperature as low as 80 °C, where Ag₂O and Nb₂O₅ displayed superior performance by desorbing up to 3.6 and 2.5 times greater CO₂ amounts with faster rates. The superior results from Ag₂O and Nb₂O₅ were due to the presence of a combination of acid sites: many Lewis acid sites as well as Bronsted acid sites. The remaining three nanoparticles required higher temperatures to better show their efficiencies, as only a few Lewis acid sites were present. In 2018, the researchers continued their study of ZrO₂ and ZnO metal oxide nanoparticles at a temperature range of 40–86 °C. The enhancement recorded up to 54% for the total amount of CO₂ desorbed.

Lately, in 2019, Bhatti et al. [89] continued to study the effect of Ag₂O and Ag₂CO₃ for amine solvent regeneration at a temperature of around 80 °C. The solvent used 30 wt% MEA with 5 wt% of each metal oxide nanoparticle. They reported an astounding increase in desorption rate of CO₂ up to approximately 1000%, which greatly reduced the energy consumption.

The above transition metal oxide nanoparticles are all prone to agglomeration, due to the adhesion of particles to each other by large van der Waals forces of attraction and sedimentation, in which the nanoparticles settle under the effect of gravity. Additionally, their thermal stability differs for each nanoparticle. For instance, the MoO₃ nanoparticle shows excellent thermal stability and it was reported that only 0.7% weight loss was recorded up to a temperature of 637 °C [118]. This is also almost the same as the results of another study, which had recorded a weight loss of ~1.54% up to 700 °C [119]. V₂O₅ showed weight loss in three stages, according to Xavier [120]; the first one at 250 °C that corresponded to water loss and the second was between 250 °C and 375 °C, due to the dehydroxylation of metal hydroxide and the removal of other ions. The third one shows weight loss up to 515 °C and was due to the loss of OH. The transition metal oxide nanoparticle with the highest thermal stability was Cr₂O₃ and it was reported that no considerable weight loss up to 1000 °C was observed [121]. This is similar to the Nb₂O₅, which showed a mass that remained unchanged throughout the heating process up to 1000 °C, revealing its excellent thermal stability [122]. WO₃ was reported to have a low thermal stability and showed up to 15% weight loss when the temperature rose to 250 °C [123]. The maximum weight loss, however, was reported to be at 150 °C. The next nanoparticle is Ag₂O, which showed a weight loss of about 9.85%, starting from 380 to 420 °C, due to the decomposition of this material [124]. NiO, CuO and MnO₂ reported weight loss at 207 °C [125], 190 °C [126] and 300 °C [127], respectively. Their thermal stability in descending order is Cr₂O₅, Nb₂O₅, MoO₃, Ag₂O, MnO₂, V₂O₅, NiO, CuO and WO, with the last having a poor thermal stability at 150 °C.

Among these nanoparticles, MoO₃ and V₂O₅ are reported to have superior performance since they react and dissolve into the MEA solvent [27]. For this reason, they are non-recyclable but can still be recovered by lowering the pH value of the solvent. The V₂O₅ nanoparticle is reported to be moderately hazardous in terms of its toxicological data yet is still environmentally friendly. The other nanoparticles show only slight toxicity and are relatively harmless to the aquatic environment.

6.1.5. TiO(OH)₂

TiO(OH)₂ is a fundamentally different metal oxide nanoparticle that was recently studied for the regeneration process of solvents in an absorption based process for CO₂ capture [24,38,110]. The past studies proposed a catalytic behavior instead of a heat and mass transfer enhancement. Past studies focused more on the synthesizing and characterization of the nanoparticle as well as its cyclic performances.

Table 6. Summary of past studies using TiO(OH)₂ nanoparticles in the regeneration of solvent.

Solvent	Concentration	Temperature °C	Remarks	Ref.
Sodium Carbonate Na2CO3	-	40–70	Highest regeneration enhancement up to 800% at 110 s reaction time Nanoparticles utilized for 5 cycles. Specific surface area of 807.4 m2/g	[38]
MEA	1–3 wt%	88	Highest regeneration enhancement up to 4500% and maximum desorption rate at 792 s Study demonstrated employment of nanoparticle at 50 cycles. Specific surface area of 783.2 m2/g	[24]
K2CO3	0.010 vol% 0.014 vol % (For Cu/TiO(OH)2	100	Nanoparticles were utilized for 10 cycles. The employment of Cu nanoparticle is to enhance the thermal conductivity. The study demonstrated that the improvement in thermal conductivity has a drastic effect on chemical reaction rate.	[110]

depicts the summary regarding the utilization of TiO(OH)₂ in past studies.

In 2017, Yao et al. [38] studied the employment of nanoporous TiO(OH)₂ on the regeneration of aqueous Na₂CO₃. At 65 °C, the quantity of CO₂ desorbed is approximately 800% more than the case without the presence of TiO(OH)₂. The study concluded on the increase in desorption rate upon the increase in stirring rate from 400 to 550 ppm, increase in quantity of TiO(OH)₂ added and increase in temperature (although effects gradually decreased with time). The specific surface area of the fresh and cycled nanoparticles experienced a 14% drop at 807.4 m²/g and 693.1 m²/g. The quantity of the CO₂ desorption (mmol) was also reported to be very similar, even after five absorption–desorption cycles, which demonstrates the particles’ ability to be recycled.

In 2018, Lai et al. [24] studied the employment of TiO(OH)₂ in MEA solvent, where a drastic increase in the rate of CO₂ desorption (up 4500%) was seen. The regeneration was conducted at a temperature below 100 °C at 20 wt% MEA solvent. The same team reported the utilization of TiO(OH)₂ in MDEA solvent, where the nanoparticle exhibited a stronger catalytic effect. The catalytic mechanism for TiO(OH)₂-catalyzed MEA was proposed. The highest rate achieved with the employment of TiO(OH)₂ was 0.204 mmol/s at only 792 s, whereas at the same time, the MEA solvent without the nanoparticle only achieved a rate of 0.0162 mmol/s. The study compared 50 absorption–desorption cycles, and it was reported that there was no obvious decrease in the amount of CO₂ absorbed and desorbed. The catalytic mechanism was also confirmed by observing the weak peak intensity of ${{HCO}_3}^{-}$, using RAMAN spectroscopy of the TiO(OH)₂ utilized solvent.

Another study on TiO(OH)₂ nanoparticle for enhancing solvent regeneration was by Liu et al. [110]. In their study, a nanostructured Cu/TiO(OH)₂ was prepared to enhance the desorption process of K₂CO₃ solvent solution. In the study, 0–0.014 vol% nanoparticle was employed and an increase in the desorption capacity was observed by 45%. The CO₂ desorption rate increased as the volume fraction of the nanoparticle increased but its peak performance was observed at 0.014 vol%. A further increase in the volume fraction caused the nanoparticle to agglomerate, which, in turn, affected the thermal conductivity of the nanoparticles. Its cyclic ability for 10 cycles also demonstrated great stability, where no significant changes was observed in terms of its absorption and desorption capacity at a specific time frame. The study also proved its stability using XRD, where no new crystalline phases were formed in an unwanted reaction. According to the toxicological classification, it was reported to be slightly toxic and relatively harmless to the environment.

6.2. Zeolites

Zeolites are crystalline silicates and aluminosilicates linked through the oxygen atom, which generates a three-dimensional network. They consist of channels and cavities of molecular dimensions [128]. H-type zeolite materials are capable of providing remarkable catalytic activity in the desorption of CO₂. This zeolite material can be further classified into HZ, H-mordenite and H-Beta. HZ-type zeolite is part of the medium-pore zeolite family, which consists of strong Bronsted acid sites (BAS) [81]. Such example of HZ-type zeolites that possess excellent ability to catalyze the activity of CO₂ desorption is Protonated Zeolite Socony Mobil-5 (HZSM-5 zeolite) which was reported in application by Idem et al. [76] and has also been demonstrated in other studies [72,74,77,93,129,130]. The other two types, HM and Hβ, belong to large-pore zeolite family [131]. According to Kim et al. [132] and Zhang et al. [81], HM and Hβ have superior BAS, a large-pore structure and high surface area, all of which contributes to the enhancement of the CO₂ desorption performance. It was reported [106,129] that the contribution for the desorption enhancement effect of different solid acid catalysts is due to four critical characteristics, which can be seen in Figure 6. The four critical properties are the total acid sites; the MSA, which determines the proportion of the acid sites that takes place in the catalytic reaction; the Bronsted acid to Lewis acid ratio, which determines the overriding mechanism; and finally, the number of Bronsted acid sites, which determine the number of protons provided by the catalyst for the desorption of CO₂. However, it is vital to note that none of these characteristics work independently and are responsible for the enhancement effect. It is the combined properties that affect the CO₂ desorption performance. For instance, when the Bronsted acid sites are more numerous than the Lewis Acid sites, along with a large mesoporous surface area, this would also lead to an increase in the rate of CO₂ desorption rate and decrease in heat duty.

The summary of all zeolite materials used in past studies to enhance CO₂ desorption performance is provided in

Table 7. Summary of past studies using zeolites nanoparticles in the regeneration of solvent.

Solvent	Quantity	Temperature °C	Remarks	Ref.
MEA	10, 30 and 60 g catalyst	70–98	HZSM-5 H-Y, $\gamma$-Al2O3 Largest MSA(B/L value is 191.9 for HZSM-5) HZSM-5 reported to have lowest heat duty	[74]
MEA	25 g (catalyst was 3–4 mm in size)	95	HZSM-5 $\gamma$-Al2O3 HZSM-5 reported to have lowest relative energy requirement	[77]
MEA	12.5 g catalyst	98	HZSM-5 Hβ HM AO Hβ showed best performance and lowest heat duty followed by HZ, AO and HM. Hβ has largest total acid sites and the highest BAS	[81]
MEA	250 g	90	HZSM-5 HY HZSM-5 showed better desorption performance than HY HZSM-5 had lower heat duty than HY B/L of HZSM-5 and HY is 1.587 and 2.3 BET surface area of HZSM-5 and HY is 414.1020 and 615.4914 m2/g, respectively.	[84]
MEA	10–70 g catalyst	96	SAPO-34 compared with SO42−/TiO2 B/L ratio is 1.4607 MSA is 146.53 m2/g 30 g SAPO-34 displayed best CO2 desorption performance and lowest heat duty Largest MSA(B/L value is by SAPO-34: 214.04)	[106]
MEA	25 g catalyst	98	HZSM-5 MCM-41 SO42−/ZrO2 MCM-41 has highest MSA followed by HZSM-5 and SO42−/ZrO2 (963.19, 151.56 and 72.53 m2/g) HZSM-5 has largest B/L ratio, followed by MCM-41 and SO42−/ZrO2 (1.5116, 0.7505, 0.5834) HZSM-5 showed best performance and lowest heat duty	[129]
DEAPA	25 g catalyst	90	SAPO-34 Catalyst was tested in DEAPA solvent in comparison to MEA B/L ratio of SAPO-34 is 1.46 MSA of SAPO-34 is 146.53 m2/g SAPO-34 5 showed best performance and lowest heat duty	[78]

. In this review, the zeolites employed in MEA solvent are taken as 30 wt% or 5 M MEA because it is considered to be the most commercially applied solvent in the CO₂ post-combustion process [133]. The reason for this selection is due to the low cost and high absorption rate.

HZSM-5 was reported to reduce the regeneration temperature of the MEA solvent from 120–140 °C to 90–95 °C [76]. This also allowed the heat duty to be drastically lowered from 3.53 MJ/kg CO₂ (based on non-catalytic performances in the pilot plant study [11]) to 1.56 MJ/kg CO₂ (based on the lab-scale study [76]). The HZSM-5 catalyst is a framework type aluminosilicate zeolite and is reported to have the best catalytic performance in 5 M MEA solution [129]. The HZSM-5 zeolite is mainly a proton donor catalyst [134] and its mechanism works by breaking down the carbamate in amine and reducing the activation enthalpy for the proton transfer [129]. The performance of HZSM-5 was reported to be excellent, with 1.10 mol of CO₂ regenerated in the first 90 min at 371 K, which is higher than the other two catalysts that were compared (MCM-41 with 1.03 mol CO₂ regenerated and SO₄²⁻/ZrO₂ with 0.94 mol CO₂ regenerated) [129]. This is a 29.41% increase from the blank test of 5 M MEA solvent.

The HZSM-5 was also compared with $\gamma$-Al₂O₃ in a amine solvent regeneration study [77]. The study focused on understanding the reasons behind the drastic reduction in energy required for the CO₂ desorption process. The absolute heat duty reported for the HZSM-5 zeolite in 5 M MEA was 26.67 MJ/kg CO₂, followed by the $\gamma$-Al₂O₃ at 29.55 MJ/kg CO₂. Both of which were lower than the heat duty for blank MEA at 42.53 MJ/kg CO₂. Therefore, there was a 37.3 reduction in the relative energy requirement for the employment of the HZSM-5 in comparison to the blank MEA. The study also reported on the amine degradation test where both catalysts were reported to not have any degradation effect on amine solvents. This is because the HZSM-5 does not have strong acid sites, which would mean it is not corrosive, but also that the operating temperature for amine regeneration is mild, which means that the HZSM-5 would not have negative degradation effects despite it potentially breaking C-H bonds at a relatively high temperature (450–500 °C).

Another study employed a different concentration of catalysts at 10 g, 30 g and 60 g of HZSM-5 in 5 M MEA [74] at 378 K. The amount of mol that CO₂ desorbs was 1.75, 1.84 and 2.04 mol CO₂, respectively. This shows a 13.64, 19.48 and 32.47% increase from the blank test, respectively. It was also reported that the product of B/L ratio to mesoporous surface area was largest for the HZSM-5 catalyst, which is at 191.9 and shows a heat duty reduction of 47.54% compared to the blank test.

Another four different Bronsted acid catalysts, H-type zeolites, namely, HZSM-5, HM, Hβ and Al₂O₃, were studied in two different amine systems in order to improve the CO₂ desorption process [81]. The Hβ catalyst was reported to have superior results in terms of the solvent regeneration and increasing the desorption performance up to 1360.8%. The relative energy requirement was also reduced by 66.1% in comparison to the blank run of MEA solvent. This increased performance by the Hβ catalyst can be explained by its large mesoporous surface area, a larger number of BAS and prominent total acid sites. Therefore, it can be deduced that the MSA, BAS and total sites all have an important effect in enhancing CO₂ desorption performance. This could explain the poor catalytic effect of the other catalysts. For instance, HZ was reported to have an 8.31% decrease from Hβ, while HZ has a larger MSA, but a smaller BAS and fewer total acid sites. This is similar to HM, which experienced a 12.61% decrease from Hβ. HM was reported to have a higher BAS and total acid sites, but the MSA was the smallest. In addition to that, the Hβ catalyst displayed remarkable stability as its cyclic capacity did not show severe reduction, the catalytic desorption performance did not show significant changes and the structure of the catalyst, analyzed through XRD, indicated that the crystalline structure was maintained throughout the four absorption–desorption cycles.

The H-Y type zeolite catalyst was also reported in a study by comparing it to the conventional HZSM-5 zeolite [74]. The MSA was reported to be 24.3 m2/g, which is much smaller than the HZSM-5. The B/L ratio, however, was significantly larger than the other two catalysts, which makes this zeolite a suitable candidate for solvent regeneration. In this study, it was found that the catalytic performance of the H-Y zeolite was drastically inferior compared to the conventional HSZM-5 zeolite, and this was probably due to the low MSA and high microporous area of the H-Y structure. This provided a very low mass transfer coefficient, which then led to low catalytic efficiency. In regard to the other factors for the nanoparticle selection criteria, the H-Y zeolite has not been widely studied in the past; therefore, there is insufficient information regarding this catalyst.

SAPO-34 was also another type of catalyst tested for the regeneration of CO₂-loaded MEA solvent [106]. The catalyst ranged from 30 g–70 g at 96 °C in a MEA solvent. The catalyst with the most amount of CO₂ desorbed in decreasing order was 30 g > 20 g > 50 g > 10 g > 60 g > 70 g at 0.91, 0.89, 0.88, 0.84, 0.82 and 0.81 mol CO₂ desorbed, respectively. This gives the 30 g employed SAPO-34 the lowest heat duty at 24.23 MJ/kg CO₂. Although the SO₄²⁻/TiO₂ showed higher total acid sites and B/L ratio, it was the SAPO-34 catalyst that showed better specific surface area and MSA. This supports the claim that these characteristics are not individually responsible, and so, the combined effects of these factors should be evaluated. Due to the higher combined value of the Bronsted/Lewis acid site ratio (B/L) and a larger mesopore surface area (MSA), the CO₂ desorption rate of SAPO-34 was reported to be higher and the heat duty lower than the other tested catalyst. Cyclic ability was also tested as it is an important characteristic in determining a good catalyst. In this study, five desorption cycles were tested, and it was reported that the amount of desorbed CO₂ did not have a significant declination. Figure 7 depicts the heat duty calculated for zeolites used in past studies, according to the studies reported in Table 7.

6.3. Mesoporous Silica

Mesoporous silicas were first introduced by scientists at the Mobil Oil company and Kuroda and co-workers, in the early 1990s, as a way to extend the utilization of zeolites [135,136,137,138]. These new synthesized materials ranged from 2–10 nm in width and were designed due to large molecules not being able to react effectively in existing zeolites [135]. The utilization of MCM-41 was reported in 2011, where it acted as a Lewis acid catalyst in the solvent regeneration process [76]. MCM-41 was already widely used but not in the CO₂ absorption process as the catalyst had a high specific surface area [139]. The study on the utilization of this catalyst should branch out into the desorption process since previous study on this area is limited.

Table 8. Summary of past studies using mesoporous silica in the regeneration of solvent.

Solvent	Quantity	Temperature °C	Remarks	Ref.
MEA	25 g	98	MCM-41 B/L ratio is 0.7505 MSA is 963.19 m2/g MCM-41 had poorer CO2 desorption performance and higher heat duty in comparison to zeolite	[129]
MEA	25 g	98	MCM-41 B/L ratio is 0.7505 MSA is 963.19 m2/g HZSM-5 showed best performance Heat Duty for the mesoporous silica and zeolite were not reported in the study.	[140]
DEAPA	25 g	90	MCM-41 and SAPO-34 Catalyst was tested on DEAPA solvent in comparison to MEA B/L ratio of MCM-41 is 0.75 MSA of MCM-41 is 171.50 m2/g MCM-41 performed best after zeolite	[78]
MEA	6.25 ± 0.01 g catalyst	98.5	SBA-15 mesoporous silica is employed MSA is 476.10 m2/g B/L ratio is 5.40 SBA-15 showed better performance than blank test but poorer performance than HZSM-5 zeolite	[107]

depicts the summary for past commercially available mesoporous silica employed in the regeneration of solvent.

The MCM-41 catalyst was compared with zeolite (HZSM-5) and sulfated metal oxide (SO₄²⁻/ZrO) at 98 °C in 5 M MEA. The CO₂ desorption performance was better than the blank test and sulfated metal oxide, yet inferior to HZSM-5. The heat duty also displayed the same result with the MCM-41 catalyst, having a heat duty of 16.9 MJ/Kg CO₂, whereas HZSM-5 was reported to have 15.6 MJ/Kg CO₂. The MSA was reported to be larger than HZSM-5, yet the B/L ratio was smaller. This implies that the Bronsted acid sites relative to the Lewis acid sites was relatively small, despite having a large MSA. Therefore, its performance was inferior to HZSM-5, which has both a large MSA and an even larger B/L. This trend also shows similar results to Zhang et al. [140].

The employment of the same catalyst was reported in a different study but in a different amine solvent; 3-(diethylamino)propylamine (DEAPA) [78]. The results from comparing SAPO-34 zeolite and the blank amine solvent similar to previous studies, where the MCM-41 benefited over the blank test and SO₄²⁻/TiO₂ but was poorer in comparison to the zeolite catalyst. Another material is SBA-15 that was compared with sulfated SBA-15 materials [107]. The CO₂ desorption performance of the non-optimized SBA-15 was reported to be lower than the blank MEA test and the employment of HZSM-5. The heat duty of SBA-15 was 49.92, followed by 51.92 and 61.60 MJ/kg CO₂ for HZSM-5 and the blank MEA test.

Mesoporous silica was reported to aggregate, which affected its long term stability [141]. It has lower hydrothermal stability in comparison to zeolite [142,143]; although, its stability is high enough to be heated to at least 850 °C [143], which indicates it is suitable to undergo the temperature for the regeneration of solvent in a CO₂ capture process. The morphology of MCM-41 and SBA-15 was also reported to be intact after five cycles of the CO₂ capture process, which indicates good regeneration and reuse of the nanoparticle [144].

Figure 8 depicts the type of mesoporous silica previously studied for the regeneration of solvent along with their respected heat duties (MJ/kg CO₂). MCM-41 has a significantly lower heat duty in comparison to SBA-15, yet when it was compared to zeolite, it was found to be inferior.

7. Summary of Nanoparticles According to Selection Criteria

Table 9. Summary of metal oxide nanoparticles according to selection criteria.

Nanomaterial	Agglomeration/Sedimentation	Thermal Stability (°C)	Recycling Ability	Foaming	Toxicity **a	Environmentally Friendly **b	Desorption Enhancement
	Metal Oxides
SiO2	Yes	High	Reusable	Yes	Slight	Relatively Harmless	Low
Al2O3	Yes	Medium	Reusable	Yes	Slight	Relatively Harmless	Low
TiO2	Yes	Low	Reusable	Yes	Slight	Relatively Harmless	Medium
MoO3	Yes	Medium	Non-reusable	N/A	Slight	Relatively Harmless	High
V2O5	Yes	Low	Non-reusable	N/A	Moderately Hazardous	Relatively Harmless	Medium
Cr2O5	Yes	High	Reusable	N/A	Slight	N/A	Medium
WO3	Yes	Low	Non-reusable	N/A	Slight	N/A	Medium
Ag2O	Yes	Medium	Non-reusable	N/A	Slight	N/A	Medium
Nb2O5	Yes	High	Reusable	N/A	Slight	Relatively Harmless	Medium
NiO	Yes	Low	Reusable	N/A	Slight	N/A	Medium
CuO	Yes	Low	Non-reusable	Yes	Moderately Hazardous	N/A	Medium
MnO2	Yes	Low	Reusable	N/A	Slight	N/A	Low
Ti(OH)2	Yes	Medium	Reusable	N/A	Slight	Relatively Harmless	Very high

**^a Classification standard by: World Health Organization Acute Hazard Rankings (Toxicity). **^b Classification standard by: U.S. Fish and Wildlife Service Research Information Bulletin #84–78, 1984. Thermal stability and desorption enhancement column uses a three-point scale system. N/A = not available

and

Table 10. Summary of zeolite and mesoporous silica nanoparticles according to selection criteria.

Nanomaterial	Agglomeration/Sedimentation	Thermal Stability	Recycling Ability	Foaming	Toxicity **a	Environmentally Friendly **b	Heat Duty (MJ/Kg CO2)
Zeolites
HZSM-5	N/A	High	Reusable	N/A	Slight	N/A	(0.25–5.1) **c
Hβ	No	N/A	Reusable	N/A	Slight	N/A	0.21–0.44
H-mordenite	N/A	N/A	N/A	N/A	Slight	N/A	0.08–0.26
H-Y	N/A	N/A	N/A	N/A	Slight	N/A	5.3
SAPO-34	N/A	N/A	Reusable	N/A	Slight	N/A	0.34–0.83
Mesoporous Silica
MCM-41	Yes	High	Reusable	N/A	Slight	N/A	(1.7–3.8) **d
SBA-15	Yes	Low	Reusable	N/A	Moderately Hazardous	N/A	0.51

^**a Classification standard by: World Health Organization Acute Hazard Rankings (Toxicity). ^**b Classification standard by: U.S. Fish and Wildlife Service Research Information Bulletin #84–78, 1984. **^c: 1.1, 0.7, 1.5, 5.1, 2.0, 3.8, 2.9, 0.70, 0.25, 0.25, 0.30, 0.87 MJ/Kg mol CO_2. **^d 3.8, 1.8, 2.3, 1.7 MJ/Kg mol CO_2. N/A = not available

depict the summary of the nanoparticle selection criteria for metal oxides, zeolites and mesoporous silica, as discussed in the earlier section. In summary, a nanoparticle with a good dispersion stability is preferred as it can lead to a stable performance when being used in multiple absorption–desorption cycles. It has been reported that the employment of surfactants can be effective in certain nanoparticles, such as Al₂O₃-water nanoparticles [30] and SiO₂-water nanoparticles [57]. The thermal stability should also be sufficiently high in order for it to withstand the high regeneration temperature of the solvent. As for recycling ability, a reusable catalyst should be chosen as it does not either degrade or take part in the reaction which causes the lifetime of the nanoparticle in the solvent to be shorter and also to decrease the efficiency of the desorption process. Certain nanoparticles may lead to foaming or stabilize foaming upon the addition to liquid solvents; therefore, the risk of foaming stability should be evaluated further. The toxicity and the effect on the environment are also important criteria. Finally, the analysis on the selection criteria has exhibited the enhancement effect and heat duty reduction for the metal oxide nanoparticles, zeolites and mesoporous silicas, respectively.

8. Perspective and Future Directions

The intensive energy consumption of the solvent regeneration process is one of the crucial factors that influences the techno-economics of the existing CO₂ absorption process. Various studies and reviews have been conducted on the employment of additives in solvent to enhance absorption performance. However, comprehensive reviews on enhancing the regeneration/desorption performance of a solvent are limited. After reviewing the potential of nanoparticles on solvent regeneration in an earlier section, the proposed future directions in this field of research can be concluded as demonstrated in

Table 11. Summary of metal oxides, zeolite and mesoporous silica nanoparticles according to the selection criteria.

Potential Direction
Metal Oxides	Synthesizing the nanoparticles with modified surfaces or adding surfactants can improve the stability of nanoparticles. Certain metal oxides should improve on their recyclability (such as MoO3, V2O5). Further studies on the catalytic effect of certain metal oxide nanoparticles (SiO2, TiO2). To evaluate and reduce the foaming tendency of the nanoparticles on solvents. Quantitively evaluate the regeneration energy required and compare the conventional absorption process. A model explaining the relationship between both the physical and catalytic enhancement mechanisms should be developed.
Zeolites	Searching for a potential alternative to dispersing nanoparticles in solvent that can increase the stability. To evaluate and reduce the foaming tendency of zeolites on solvents. To evaluate the thermal stability of the zeolites.
Mesoporous Silica	To evaluate and reduce the foaming tendency of zeolites on solvent. To improve the desorption enhancement of mesoporous silica by modifying the catalyst as a hybrid nanoparticle.

.

(1)

The existing studies on the regeneration performance of nanoparticles is still limited. The employment of different types of nanoparticles has been widely studied, though many of them do not focus on desorption performance. For instance, the employment of Fe₃O₄ and CNT nanoparticles has been reported to have a better absorption performance than SiO₂ and Al₂O₃ at lower concentrations [145]. However, the desorption performance has yet to be investigated for these nanoparticles.

(2)

The stability of a nanoparticle is an important characteristic in the application of the CO₂ separation process. Long term stability for nanoparticles is considered to be an issue for practical applications as different nanoparticles may require different stability methods. Identifying an easy and low-cost method to improve the stability should be considered. It has been reported that the addition of surfactants could further improve the stability, however their effects on the desorption rate should be further investigated. Modifying the surface, for instance, on Fe₃O₄, is one way to improve its stability [146]. However, its effect on the desorption performance of CO₂ capture should be further discussed.

(3)

The enhancement factors that affect the regeneration performance of nanoparticles have been explained in terms of the size, concentration and type in the current review. More factors should be considered, such as the gas flow rate and the gas concentration. Apart from that, the physical and chemical properties of transformed nanoparticles should be properly discussed. The density, viscosity and other thermodynamic properties are important to further evaluate the overall performance of CO₂ capture.

(4)

Since nanoparticles exhibit both catalytic and physical effects on the desorption performance, the relationship between the two mechanisms should be properly discussed. Metal oxide nanoparticles have been widely discussed as having both effects. However, this is not the case for zeolite and mesoporous silica.

(5)

Apart from that, the reduction in the heat duty of using nanoparticles has been discussed. Future research should quantitively evaluate the regeneration energy requirement and its feasibility for these nanoparticles to be implemented in large scale applications.

9. Conclusions

Solvent-based CO₂ absorption processes are widely used in industries to separate CO₂ from pre-combustion and post-combustion processes. Despite this, it is generally agreed that the high energy penalty comes from the solvent regeneration process. Various nanoparticles have been reviewed to improve the CO₂ regeneration on the performance of solvents. It can be concluded that:

• The high energy requirement in the desorption process is due to the high sensible heat and heat of vaporization. Therefore, to reduce it, the regeneration of the solvent should be achieved at a lower temperature and the higher the amount of CO₂ desorbed is desirable so that the cyclic capacity can be higher. It is important to note that this reduction in energy occurs on the assumption that equilibrium is not reached.
• Metal oxides can demonstrate both physical and chemical enhancement mechanisms that improve the heat and mass transfer of the solvent and provide catalytic behavior. However, zeolites and mesoporous silica have only been reported to provide a chemical enhancement mechanism.
• The nanoparticles do not change the thermodynamic properties of the solvent but are able to reduce the energy in a shorter time frame, due to the improvement of the rate of CO₂ desorption.
• The physical and chemical properties of the synthesized nanoparticles play a vital role in evaluating the CO₂ desorption performance. The acid and basic sites should be evaluated along with the other physical factors, such as MSA, average pore diameter and total surface area.
• The nanoparticle selection criteria have been discussed according to factors that can improve the regeneration of solvent.
• TiO(OH)₂ has the highest enhancement ratio among all the nanoparticles in review.

The ability for the catalyst to lower the heat of vaporization and sensible heat can result in lower energy consumption and operating costs. The lower operating temperature can mitigate the challenges of a CO₂ absorption process, such as corrosion and solvent degradation.