Competitive Adsorption of Moisture and SO₂ for Carbon Dioxide Capture by Zeolites FAU 13X and LTA 5A

Abstract

Zeolites exhibit significant potential as porous materials for selective carbon dioxide (CO₂) capture, leveraging their distinctive adsorption properties. However, the presence of moisture (H₂O) and sulfur dioxide (SO₂) in flue gas streams can significantly affect the efficiency of CO₂ capture. This study investigates the CO₂ adsorption characteristics of zeolites FAU 5A and LTA 13X, revealing the competitive adsorption mechanism between H₂O_(g), SO₂, and CO₂. The zeolites exhibit CO₂ adsorption capacities of 93.19 mg/g and 95.80 mg/g for 5A and 13X, respectively, and demonstrate good regeneration potential. Metal cations correlated positively with CO₂ adsorption. H₂O_(g), SO₂, and CO₂ exhibit a competitive adsorption relationship, with H₂O_(g) having the highest adsorption capacity, followed by SO₂ and CO₂. Additionally, the synergistic effect of SO₂ and H₂O_(g) on CO₂ adsorption is elucidated. These findings provide valuable insights into the competitive adsorption behavior of moisture and SO₂ for CO₂ capture using zeolites LTA 5A and FAU 13X, contributing to the development of more efficient CO₂ capture processes and the design of tailored adsorbents for industrial applications.

1. Introduction

Global warming is closely linked to the rise in atmospheric greenhouse gas (GHG) concentrations, attributable to human emissions since the onset of the industrial era [1]. Carbon dioxide emissions (CO₂) are one of the primary reasons for global warming [2,3]. Technologies for carbon capture, utilization, and storage (CCUS) are anticipated to be crucial in moving the world’s energy system toward net-zero emissions [4]. A wide range of technologies, including adsorption, membrane separation, low-temperature distillation, and absorption, have been proposed and applied to capture CO₂ produced after burning [5,6,7,8,9]. Adsorption via solid sorbents is deemed an appealing alternative separation technology due to its simplicity and lack of liquid waste production [10,11]. Moreover, the regeneration of solid adsorbents is more efficient than that of liquid absorbents, primarily because CO₂ is physiosorbed on solids, whereas it is chemisorbed on liquids. Consequently, solid adsorbents generally require less energy for regeneration [12].

Zeolite is a low-cost regeneration adsorbent utilized extensively in adsorption and gas separation due to its distinctive channel structure, outstanding thermal stability, and enormous adsorption capacity [4,13,14,15,16]. Siriwardane et al. compared the CO₂ adsorption properties of 4A, 5A, 13X, and APG-II zeolites and discovered that zeolite 13X has a higher capacity for CO₂ adsorption [17]. Cavenati et al. investigated the adsorption of CO₂ and N₂ on 13X zeolite at 4 MPa pressure [18] and demonstrated that at 298 K and 4 MPa pressure, 13X zeolite can adsorb CO₂ at a rate of up to 7.5 mol/kg. The 13X and Y zeolites have a significant capability for CO₂ adsorption [19]. Marcu and Sandulescu [20] used Y-type molecular sieves to adsorb SO₂ and found that Y-type molecular sieves have good adsorption performance for SO₂ at 25–150 °C, with an adsorption capacity of up to 20–175 mg/g. It can be followed that zeolites may have good potential to treat CO₂ and SO₂ simultaneously via co-adsorption.

One of the major obstacles preventing the large-scale growth of adsorption carbon capture is the presence of moisture in the industrial flue gas [21]. Microscale or even remnants of water can be successfully removed from the gas using molecular sieves to separate the mixture. Li et al. investigated the cyclic adsorption process of 12% CO₂ + 95% RH mixtures at 30 °C and the penetration curve of CO₂ and H₂O_(g) mixes on 13X zeolites [22,23]. According to the test findings, CO₂ and moisture were competitive when absorbed by 13X zeolite, even in a vacuum. Stern et al. investigated the impact of 5A zeolite on the ability of N₂ and CO₂ to adsorb after water absorption [24] and demonstrated that zeolite molecular sieves lost their ability after absorbing water. They propose that the shift is caused by hydrate produced by water absorption clogging molecular sieve holes. Shen et al. conducted heat treatments on commercial NaX zeolites to alter the composition of the O-T-O (T = Al, Si) framework. By redistributing the Na+ ions within the O-T-O (T = Al, Si) framework, they effectively reduced the CO₂ adsorption energy and inhibited the formation of water agglomerates [25]. The flue gas, apart from water, typically contains additional contaminants. Despite pretreatment processes aimed at eliminating compounds such as siloxanes, sulfides, and halides, trace impurities persist in the gas streams entering the separation column. The impact of these trace impurities on the functional capacity and longevity of adsorbents remains insufficiently explored.

The adsorption capacity of alkaline metal oxides to CO₂ is often high, and the formed products have high strength and good wear resistance. Walton et al. investigated how modifying alkali metal cations affected the ability of X and Y zeolites to adsorb CO₂ [26]. In the ion exchange-modified X-type zeolite, the carbon dioxide adsorption capacity declines with an increase in the metal cation radius; for the modified Y-type zeolite, the order of carbon dioxide adsorption capacity is Cs < Rb ≈ K < Li ≈ Na.

This study investigated the CO₂ adsorption characteristics of the large-adsorption-capacity zeolite 13X with an S/A of 1.77 and zeolite 5A with an S/A of 1.51 using different molecular sieve particle sizes, adsorption temperatures, adsorption pressures, CO₂ concentrations, regeneration times, and H₂O_(g) contents. Moreover, the impact of alkali metal cation alteration on the CO₂ adsorption capabilities of X and A zeolites and the mechanism of competitive adsorption between SO₂/H₂O_(g)/CO₂ were primarily explored.

2. Materials and Methods

2.1. Materials

Zeolites were provided by Chinese National Pharmaceutical Group Chemical Rea-gent Co., Ltd. (Shanghai, China). In

Table 1. Chemical composition of zeolites.

Zeolite (wt. %)	Fe2O3	Al2O3	SiO2	CaO	MgO	Na2O	K2O
5A	0.97	23.75	35.75	11.84	2.08	2.95	0.30
13X	1.22	21.33	37.79	0.94	2.05	13.39	0.26

, the chemical compositions are displayed. Similar S/A values were found in the zeolite: 1.51 for 5A and 1.77 for 13X. Their cations differ significantly from one another. Zeolite 5A had a greater CaO concentration (11.84 wt%), but zeolite 13X had a higher Na₂O content (13.39 wt%).

The specific surface areas and pore size distributions of zeolites 5A and 13X were analyzed using the Brunner–Emmet–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. Zeolites 5A and 13X exhibited specific surface areas of 411 and 472 m²/g, respectively [27]. Both demonstrated an abundant microporous structure, with micropore volumes of 0.21 and 0.15 cm³g⁻¹ and an average pore diameter of 2.82 and 2.31 nm, and SEM images in Figure 1 also support the results.

2.2. Methods

2.2.1. Experimental Procedure

The test process is consistent with our previous report [24]. A schematic diagram of the experimental apparatus used in this study is shown in Figure 2. Two layers of nickel foam were used to hold the bed in place for the adsorption test, which was conducted in a U-tube with a 10 mm diameter. The measurements of the CO₂ concentration and the temperature and relative humidity make up the two primary parts of the testing system. An enhanced flue gas analyzer (MGA-5, YORK Instrument Ltd., Heilbronn, Germany) with a measurement accuracy of 1 ppm was used to detect CO₂ gas. A flue gas analyzer partly extracted the fixed bed’s exhaust gas for analysis, and the remaining gas was absorbed using a scrubber bottle filled with a sodium hydroxide solution. In addition, relative humidity and temperature were detected using temperature and humidity loggers with an accuracy of 2% RH and 0.1 °C, respectively.

2.2.2. Molecular Sieve Activation and Modification Methods

The zeolite molecular sieves were initially activated before usage to eliminate the adsorbed gas and moisture. The procedure involves placing the molecular sieve in a porcelain boat, transferring it into a muffle furnace set at a specific baking temperature, and subjecting it to a predetermined baking duration. The activated molecular sieve is rapidly removed and placed in a drying dish with water-absorbing silica gel for testing and subsequent detection to prevent it from absorbing moisture and other gases in the air.

Zeolite was altered via acid modification to evaluate the impact of cations and SiO₂/Al₂O₃ on adsorption capacity. The following were the steps: The two molecular sieves were uniformly combined with 1:10 (g/mL) solid-to-liquid ratios of hydrochloric acid and hydrofluoric acid solutions of various strengths, and they were then left to react at 30 °C for 4 h. After modification, the molecular sieve was filtered using a vacuum filter and washed with anhydrous ethanol. Subsequently, the molecular sieve was dried in a constant temperature blast oven at 100 °C and then activated at 300 °C for 2 h in a muffle furnace.

2.2.3. Distributed Water Distribution Method

By adjusting the various aeration times, the distributed water distribution system fulfills the goal of controlling gas moisture. The nitrogen valve that carries water should be closed once the required preparation for water distribution is finished. The CO₂ mixture, having the desired concentration, is introduced into the molecular sieve fixed bed to examine the adsorption characteristics when the temperature of the solid bed falls to 30 °C, which eliminates the thermal effect of the moisture adsorption process.

2.2.4. Evaluation of CO₂ Adsorption Capacity

The adsorption property of CO₂ was determined using the breakthrough curve method, and the adsorption capacity (A) and the breakthrough time (t) were used to characterize the adsorption characteristics of zeolites [28,29]. The adsorption capacity (A) was defined as in Equation (1):

$$A=KMQ{\displaystyle {\int }_{t0}^{ti}(C_0-C)dt/(m\times V_m)}$$

(1)

where A is the unit zeolite mass of the adsorption gas mass (mg/g); Q is the total gas flow rate (m³/s); m is the mass of the adsorbent (g); M is the molar mass of the adsorbed gases (g/mol); V_m is the standard molar volume (m³/mol); t₀ is the start time of adsorption (s); t_i is the arbitrary time of adsorption process that is used as a variable in the integration process (s); C₀ is the inlet CO₂ concentration (ppm); and C is the outlet CO₂ concentration at an arbitrary time during the adsorption process (ppm).

Breakthrough time (t) was defined as in Equation (2):

$$t=t_1-t_0$$

(2)

where t₀ is the start time of the experiment and t₁ is the time when the exhaust gas concentration is equal to 1% of the inlet concentration.

3. Results and Discussion

3.1. Influencing Parameters on CO₂ Adsorption Capacity

3.1.1. Effect of Molecular Sieve Particle Size

The impact of different molecular sieve particle sizes on the dynamic adsorption capacity of CO₂ was studied under the conditions of a molecular sieve of 1 g, an activation temperature of 300 °C, an activation time of 2 h, a CO₂ concentration of 10%, a mixed gas flow rate of 1 L/min, an adsorption temperature of 30 °C, an adsorption pressure of 1 atm, and a relative humidity of 24%. The initial rising slope of the integration curve in Figure 3 demonstrates that the CO₂ adsorption amount on the molecular sieves increased progressively as the particle size decreased, enhancing the CO₂ adsorption rate. When the particle size exceeds 2.8 mm, both the adsorption capacity and rate are lower compared to other sizes. This effect is attributed to the larger molecular sieves having a smaller specific surface area, which limits the adsorption rate, and to the slower diffusion rate of the adsorbate within the adsorbent, further constraining the adsorption process. Nonetheless, the maximum CO₂ adsorption capacities for the 5A and 13X molecular sieves reach 93.19 mg/g and 95.80 mg/g, respectively, at a particle size of 0.26–0.43 mm. Beyond this range, the CO₂ adsorption capacity experiences minimal variation with further decreases in particle size. Additionally, a reduction in particle size increases the resistance within the bed, thereby rendering the properties of the fixed bed more susceptible to alteration. The subsequent experiments employed a molecular sieve with a particle size ranging from 0.26 to 0.43 mm.

3.1.2. Effect of Activation Temperature and Time

The effect of activation temperature and time on the dynamic adsorption capacity of CO₂ was studied under the following conditions: a molecular sieve of 1 g, a CO₂ concentration of 10%, a mixed gas flow rate of 1 L/min, an adsorption temperature of 30 °C, an adsorption pressure of 1 atm, and a relative humidity of 24%. As depicted in Figure 4, the adsorption capacity of CO₂ initially escalates sharply with increases in activation temperature, subsequently stabilizing. Notably, at an activation temperature of 300 °C, the maximal adsorption capacities for zeolites 5A and 13X were recorded at 93.19 mg/g and 95.80 mg/g, respectively. As the activation time increased, the saturation adsorption capacity of CO₂ for both the 5A and 13X molecular sieves initially rose and then stabilized. Specifically, the capacity in 5A sieves stabilized after 1 h of activation, whereas in 13X sieves, it did so after 2 h. Consequently, to facilitate conditions for subsequent comparative analyses, an activation duration of 2 h was chosen for both types of sieves.

3.1.3. Effect of Adsorption Temperature

One of the most significant factors affecting zeolite molecular sieve adsorption is adsorption temperature [24]. The impact of fixed-bed adsorption temperature on the CO₂ adsorption capabilities of two zeolite molecular sieves was examined under the conditions of a molecular sieve dosage of 1 g, an activation temperature of 300 °C, an activation time of 2 h, a mixed gas flow rate of 1 L/min, an adsorption pressure of 1 atm, a CO₂ gas distribution concentration of 10%, and a relative humidity of 24%. Figure 5 demonstrates that as the adsorption temperature was raised from 30 °C to 100 °C, the CO₂ adsorption amount of the 5A molecular sieves decreased from 93.19 mg/g to 19.55 mg/g, a decrease of 79.01%; and the CO₂ adsorption amount of the 13X molecular sieve decreased from 95.80 mg/g to 25.78 mg/g, a reduction of 72.91%. The adsorption of CO₂ by both zeolite molecular sieves is temperature-sensitive and the molecular sieves of the 13X and 5A types are less effective at dynamic CO₂ adsorption at higher temperatures. The CO₂ gas adsorption process suffers from a rise in temperature since the adsorption process is exothermic [30]. It might be related to the nonpolar properties of CO₂, a molecule with a weak physical adsorption force and rapid desorption at high temperatures [31].

3.1.4. Effect of CO₂ Concentration

The CO₂ concentration of exhaust gases from sophisticated industrial operations is unstable [32,33]. Figure 6 illustrates the effect of varying CO₂ gas concentrations on the CO₂ adsorption of two zeolite molecular sieves under the conditions of a molecular sieve dosage of 1 g, an activation temperature of 300 °C, an activation time of 2 h, a mixed gas flow rate of 1 L/min, an adsorption temperature of 30 °C, an adsorption pressure of 1 atm, and a relative humidity of 24%. The adsorption capacities of the two molecular sieves grew as the CO₂ concentration in the mixed gas rose from 1% to 10%. The CO₂ adsorption capacities of the 5A type molecular sieve increased from 23.93 mg/g to 93.19 mg/g, and the CO₂ adsorption capacities of the 13X type molecular sieve increased from 38.63 mg/g to 95.80 mg/g. The adsorption capacities of both molecular sieves were enhanced at elevated CO₂ concentrations. On the one hand, an increase in concentration increases the likelihood that CO₂ molecules will bind to the molecular sieve’s active sites. On the other hand, an increase in concentration results in a greater concentration difference between CO₂ on the molecular sieve’s surface and the gas phase, which increases the adsorption driving force and shifts the adsorption equilibrium.

3.1.5. Effect of Acid Cations and SiO₂/Al₂O₃

Zeolite was altered via acid modification to evaluate the impact of cations and SiO₂/Al₂O₃ on adsorption capacity. Figure 7 demonstrates that HCl and HF modification induces similar trends in the SiO₂/Al₂O₃ ratios and CaO and Na₂O contents in 5A and 13X zeolites under the conditions of a molecular sieve dosage of 1 g, an activation temperature of 300 °C, an activation time of 2 h, a mixed gas flow rate of 1 L/min, an adsorption temperature of 30 °C, an adsorption pressure of 1 atm, and a CO₂ gas distribution concentration of 10%. Specifically, HF treatment reduces the SiO₂/Al₂O₃ ratios in both zeolites, with minimal changes in CaO and Na₂O levels (Figure 8). This reduction negatively affects the CO₂ penetration time, as evidenced by dynamic breakthrough curve analyses. Conversely, HCl modification increases the SiO₂/Al₂O₃ ratios while decreasing CaO and Na₂O contents in these zeolites, which, according to the dynamic breakthrough curve of CO₂, adversely impacts the penetration time due to the significant decrease in metal cation (Ca²⁺, Na⁺) levels.

3.1.6. Regeneration Potential of the Zeolites

The effect of cycle regeneration times on the CO₂ adsorption capacities in two zeolite molecular sieves was investigated using the following conditions: a molecular sieve dosage of 1 g, an adsorption temperature of 30 °C, a mixed gas flow rate of 1 L/min, a CO₂ gas concentration of 10%, an adsorption pressure of 1 atm, and a relative humidity of 24%. The molecular sieve samples underwent a cycle procedure of ‘adsorption saturation–activation desorption–adsorption saturation’. The activation desorption phase was conducted at 300 °C for 2 h. The 5A and 13X molecular sieves each completed seven adsorptive cycles during testing. Figure 9 demonstrates that after seven cycles, the saturation adsorption capacities of both molecular sieves varied slightly, maintaining their original performance levels. After activation at 300 °C for 2 h, the molecular sieve releases the adsorbed CO₂ without compromising its microstructure or pore system. This preservation of structural integrity ensures continued good adsorption performance and enables effective regeneration of the molecular sieve.

3.2. Competitive Adsorption between H₂O_(g), SO_2, and CO₂

3.2.1. Effects of Moisture Content

Under the conditions of a molecular sieve dosage of 1 g, an activation temperature of 300 °C, an activation time of 2 h, a mixed gas flow rate of 1 L/min, an adsorption temperature of 30 °C, an adsorption pressure of 1 atm, and a CO₂ gas distribution concentration of 10%, the effect of moisture on the dynamic adsorption characteristics of CO₂ was studied (Figure 10). The variation in moisture content was reflected by fluctuations in relative humidity during the experiment.

The lowest value of CO₂ concentration gradually increases during the adsorption process as relative humidity increases, and it is easier to reach adsorption saturation quickly after bed penetration. During the adsorption, H₂O_(g) fulfilled dual roles: It engaged in competitive adsorption with CO₂, thereby diminishing the adsorption capacity of CO₂. Concurrently, the adsorption of H₂O_(g) on zeolite generated substantial heat, which elevated the temperature of the fixed bed. This temperature increase, in turn, led to a reduction in CO₂ adsorption capacity.

To verify competitive adsorption, a stepwise water distribution method was used to eliminate the influence of H₂O_(g) adsorption heat. According to Figure 11, the CO₂ penetration curves on the two molecular sieves follow similar laws as the water distribution time increases. The minimum value of carbon dioxide in the adsorption process gradually increases as the moisture content increases, and carbon dioxide penetrates the fixed bed in a shorter time. It demonstrates that as the mixed water content increases, the adsorption capacity of CO₂ on the two molecular sieves gradually decreases. When the water distribution time is 60 min for the two molecular sieves, the adsorption capacity of CO₂ molecules is lost. When the relative humidity of the dynamic water distribution is much lower than that of the stepwise water distribution, the CO₂ adsorption capacity obtained via the dynamic real-time water distribution method is lower than that of the stepwise water distribution. When the relative humidity of the dynamic water distribution is 60% RH, the lowest C/C₀ value of CO₂ in the 5A molecular sieve adsorption process is 0.59, and the lowest C/C₀ value of CO₂ in the 13X molecular sieve adsorption process is 0.55. However, when the step-by-step water distribution process is used, the lowest C/C₀ value of CO₂ during the 5A molecular sieve adsorption process is 0.43 and the lowest C/C₀ value of CO₂ during the 13X molecular sieve adsorption process is 0.42. This phenomenon occurs because the step-by-step water distribution process eliminates the influence of moisture adsorption heat on CO₂ adsorption.

To demonstrate the heat effect of water vapor adsorption, water vapor with a relative humidity of 90% RH at 30 °C was introduced into a fixed bed with a molecular sieve using the nitrogen loading method. Figure 12 shows that water vapor releases a significant amount of adsorption heat during the adsorption process. The molecular sieve fixed bed reaches a maximum temperature of 69 °C at 320 s. When the flow rate is 1000 mL/min, the maximum temperature of the layer reaches 80.5 °C at 280 s, which is 50.5 °C higher than without steam. The maximum temperature of the molecular sieve fixed bed reaches 97 °C at 80 s when the flow rate is set to 2000 mL/min, which is 67 °C higher than the temperature without steam. The maximum temperature of the bed gradually increases with the increase in water vapor per unit of time, and the time when the maximum temperature appears gradually decreases. The effect of adsorption temperature on CO₂ adsorption capacity has already been discussed.

3.2.2. Synergistic Effects of SO₂ and H₂O_(g)

Varying levels of SO₂ are frequently present in industrial exhaust gases [34,35]. The effect of SO₂ on the CO₂ breakthrough curve was studied under the conditions of a molecular sieve dosage of 1 g, an activation temperature of 300 °C, an activation time of 2 h, a mixed gas flow rate of 1 L/min, an adsorption temperature of 30 °C, an adsorption pressure of 1 atm, and a 24% relative humidity, and the change in relative humidity was observed during the process (Figure 13). The SO₂ penetration times were 440 s for the 5A molecular sieve at a concentration of 4436 ppm and 510 s for the 13X molecular sieve at 3942 ppm. Both zeolite molecular sieves exhibited robust desulfurization capabilities within the mixed flue gas containing SO₂, H₂O_(g), and CO₂. However, the CO₂ adsorption was hampered in the presence of moisture and SO₂, and CO₂ only remained adsorbent for a short time. CO₂ penetration times of 80 s and 90 s were calculated for molecular sieves of types 5A and 13X, respectively, and their adsorption amounts were 15% and 18% of those without SO₂ dosing.

This phenomenon is caused by the polarity of the SO₂, H₂O_(g), and CO₂ molecules. Because the zeolite molecular sieve is composed of aluminum–oxygen tetrahedra, a metal cation is required to balance the excess negative charge of the aluminum ions. The adsorption of adsorbent molecules on zeolite molecular sieves is subject to strong adsorption forces due to the presence of metal cations. Under certain conditions, the polarity of the adsorbent molecules is related to the strength of the adsorption force. SO₂ and H₂O_(g) molecules are polar molecules while CO₂ is nonpolar. Although both SO₂ and H₂O_(g) are polar molecules, their molecular polarity is different; the SO₂ molecule’s sulfur–oxygen bond’s electronegativity difference (0.86) is smaller than the H₂O_(g) molecule’s oxygen–hydrogen bond’s (1.24), and the larger the difference in electronegativity, the more polar the chemical bond [36,37]. The SO₂ molecule’s bond angle is also higher than that of H₂O_(g). Thus, the order of affinity during adsorption between the metal cation and the adsorbent molecule is H₂O_(g) > SO₂ > CO₂, which is consistent with the findings of the experiments. This suggests that if the adsorbent molecules’ kinetic diameters are identical, the adsorbent molecules’ polarity significantly impacts the adsorption process.

4. Conclusions

This study examined the elements that affect the CO₂ adsorption capacity of the 5A and 13X zeolite molecular sieves under diverse conditions. The adsorption capacity of the 5A and 13X zeolite molecular sieves, under optimized conditions, was found to be 93.19 mg/g and 95.80 mg/g, respectively, both showcasing impressive cyclic adsorption stability during CO₂ adsorption.

During the co-adsorption process involving multiple components, a competitive adsorption relationship was identified among the three adsorbents: SO₂, H₂O_(g), and CO₂. The affinity between the adsorbent molecules and the molecular sieve was found to be directly related to the polarity of the adsorbent molecules—the higher the polarity, the stronger the affinity with the molecular sieve. The affinity on the zeolites follows the following sequence: H₂O_(g) > SO₂ > CO₂.

The application of a dynamic real-time water distribution method, coupled with a stepwise water distribution method, revealed that H₂O_(g) plays a dual role in the adsorption process. Firstly, there is competitive adsorption between CO₂ and water molecules, leading to a decrease in CO₂ adsorption capacity with an increase in water vapor content. Secondly, the adsorption of water vapor on the molecular sieve generates substantial adsorption heat, causing a rise in bed temperature and, consequently, a further decrease in CO₂ adsorption capacity.