Dynamic Intermediate-Temperature CO₂ Adsorption Performance of K₂CO₃-Promoted Layered Double Hydroxide-Derived Adsorbents

Abstract

The dynamic adsorption characteristics of K₂CO₃-promoted layered double hydroxides (LDHs)-based adsorbent, with organic and inorganic anion intercalation, were studied. MgAl–LDH, K₂CO₃/MgAl–LDH, and K₂CO₃/MgAl–LDH(C16) with varying K₂CO₃ loads were prepared and used for intermediate-temperature CO₂ sequestration. The adsorbent was thoroughly characterized using X-ray diffraction, Brunauer–Emmett–Teller, scanning electron microscopy, and Fourier Transform Infrared Spectroscopy techniques, which revealed enhanced adsorption properties of MgAl–LDH, due to K₂CO₃ promotion. Thermogravimetric CO₂ adsorption tests on the constructed adsorbent materials showed that the 12.5 wt% K₂CO₃/MgAl–LDH(C16) adsorbent with organic anion intercalation exhibited optimal adsorption activity, achieving an adsorption capacity of 1.12 mmol/g at 100% CO₂ and 350 °C. However, fixed-bed dynamic adsorption tests yielded different results; the 25 wt% K₂CO₃/MgAl–LDH prepared through inorganic anion intercalation exhibited the best adsorption performance in low-concentration CO₂ penetration tests. The recorded penetration time was 93.1 s, accompanied by an adsorption capacity of 0.722 mmol/g. This can be attributed to the faster adsorption kinetics exhibited by the 25 wt% K₂CO₃/MgAl–LDH adsorbent during the early stages of adsorption, thereby facilitating efficient CO₂ capture in low-concentration CO₂ streams. This is a conclusion that differs from previous reports. Earlier reports indicated that LDHs with organic anion intercalation exhibited higher CO₂ adsorption activity in thermogravimetric analyzer tests. However, this study found that for the fixed-bed dynamic adsorption process, K₂CO₃-modified inorganic anion-intercalated LDHs perform better, indicating their greater potential in practical applications.

1. Introduction

The substantial emissions of greenhouse gases (GHGs) contribute significantly to escalating global temperatures and climate deterioration, with CO₂ being the primary greenhouse gas, responsible for approximately 66% of global warming. Various human activities, including the burning of fossil fuels such as coal, oil, and natural gas, as well as agricultural practices, drive an accelerated surge in CO₂ levels [1,2]. These activities trigger a spectrum of extreme weather events, including climate change, sea level rise, land desertification, and agricultural decline. The exploration of CO₂ capture, utilization, and storage (CCUS) is as a crucial approach to curbing CO₂ emissions and represents a pivotal technology for carbon emission mitigation [3,4]. CCUS technology finds application primarily in traditional oil displacement and chemical utilization, exhibiting promising prospects for development [5,6].

Among various carbon reduction technologies, the sorption-enhanced water gas shift (SEWGS) is a pre-combustion CO₂ capture technology that combines water gas shift (WGS) and adsorbed CO₂ within a single container, achieving both high CO conversion and substantial CO₂ recovery [7,8,9]. The WGS process converts CO and H₂O into CO₂ and H₂, while SEWGS manipulates Le Chatelier’s principle to shift the thermodynamic equilibrium towards H₂ production, streamlining the process and enhancing energy efficiency [10,11]. The temperature range conducive to the water–gas shift reaction typically spans from 200 °C to 400 °C. Within this range, CO₂ adsorption materials exhibiting commendable performance encompass K₂CO₃-promoted LDHs, Dawsonite and MgO-based adsorbents [12]. Studies have been conducted to achieve the production of high-purity H₂ by combining these two medium-temperature CO₂ adsorbents for the SEWGS reaction carried out in a multi-layer reactor, which can be stable over 10 cycles with a long duration [13]. Dawsonite is also a promising adsorbent in the SEWGS process, with a moderate adsorption capacity (0.3–0.7 mmol/g) and a fast adsorption rate, achieving 90% absorption in 15–20 min [14]. Additionally, loading K₂CO₃ onto MgO-based adsorbents is another method to increase the adsorption capacity [15]. K₂CO₃-promoted LDH adsorbents showcase both high and consistent CO₂ adsorption capacities, coupled with rapid CO₂ adsorption kinetics at moderate temperatures, rendering them one of the most promising adsorbents for the SEWGS process [16,17].

LDHs are a class of materials consisting of a positively charged bivalent metal ion layer and a negatively charged anion layer [18,19,20]. An LDH has a unique two-dimensional (2D) layered structure that facilitates facile alteration of anions and cations, alongside a memory effect. These attributes have led to extensive research in catalytic and environmental domains, finding applications in adsorption, oxygen evolution, wastewater treatment, and CO₂ reduction, among other areas [21,22,23,24]. Moreover, LDH synthesis methods are relatively straightforward, encompassing the co-precipitation method, ion exchange method, hydrothermal method, sol-gel method, and more [17,25,26,27,28]. LDH is considered to be a material with high low-concentration CO₂ capture activity in a medium temperature range of 200–450 °C, and the factors affecting its performance include Mg/Al ratio, surface modification, etc. [29]. The performance of an LDH can be improved by adding potassium (K₂CO₃ as the source) [30,31]. In addition to LDHs, MgO-based adsorbents are also one of the commonly used adsorbents in intermediate-temperature CO₂ capture [32]. Despite the high theoretical adsorption capacity of MgO-based adsorbents, the bulk MgO exhibits sluggish kinetic reactivity, resulting in a very low CO₂ capture capacity of less than 0.1 mmol/g [33,34,35]. The CO₂ capture capabilities of MgO-based adsorbents, when promoted by alkali nitrate mixtures, have seen substantial enhancement, achieving a noteworthy 19.06 mmol/g at 325 °C [36,37]. However, subsequent studies reveal that the CO₂ adsorption performance of molten salt-modified MgO materials is notably influenced by CO₂ concentration. Specifically, CO₂ with a concentration below 20% is almost unable to be adsorbed by MgO, thereby impacting the practical viability of MgO in CO₂ separation processes.

Compared to MgO, the issue with LDH adsorbents lies in their lower adsorption capacity. Traditionally reported LDH adsorbents exhibit an adsorption capacity around 0.5 mmol/g, and, even after modification, this figure only increases to about 1 mmol/g. This limitation poses a challenge to the utilization of LDH. To address these issues, Wang et al. [38] embedded long carbon chain organic anions into LDH, resulting in a significant improvement in CO₂ capture capacity up to 1.25 mmol/g. The organic anions with extended carbon chains will produce large amounts of gaseous byproducts, such as CO₂ and H₂O, during the decomposition process, which is conducive to the formation of micropores in the mixed oxides and to improving the CO₂ capture capacity. Li et al. [39] used K₂CO₃ to promote LDHs as a precursor for the pre-combustion of a CO₂ adsorbent with a CO₂ capacity of up to 1.93 mmol/g at 300 °C. Qin et al. [40] studied the promoting effect of carbon chain length of carboxylic acid anion on CO₂ capture performance of LDH. The results show that CO₂ capture capacity increases with the increase in carbon chain length. By coating with 55 mol% (Li-Na-K)NO₃ molten salt, the CO₂ capture capacity can be increased to 3.25 mmol/g. However, all these assessments were conducted using thermogravimetric analyzer (TGA), which only evaluates the increase in weight of the adsorbent material in a CO₂ atmosphere. It does not comprehensively reflect the adsorbent’s actual performance in terms of CO₂ removal efficiency, breakthrough time, and dynamic adsorption capacity in real-life scenarios. Therefore, we performed dynamic adsorption tests on the adsorbent materials in a simulated application setting within a fixed bed.

In this contribution, the dynamic adsorption characteristics of K₂CO₃-promoted LDHs at intermediate temperature were systematically studied, and the properties of organic anion- and inorganic anion-intercalated LDH in TGA and fixed beds were compared. Comprehensive characterization was performed on all synthesized LDH materials and the impact of CO₂ concentration and K₂CO₃ loading on CO₂ capture capacity were studied. Additionally, an analysis was conducted to elucidate the differences observed between the optimal adsorbent in TGA adsorption and dynamic adsorption scenarios.

2. Results and Discussion

2.1. The CO₂ Capture Performance of Layered Double Hydroxides (LDH) and MgO Promoted with Molten Salt

The intermediate-temperature CO₂ adsorption can generally be applied to the WGS reaction or the CO₂ removal from flue gas, with temperatures typically ranging from 200–400 °C. At this temperature, commonly used adsorbents include MgO-based adsorbents and LDH-based adsorbents. For the WGS reaction, a lower temperature is favorable for promoting hydrogen production, but it also results in slow reaction rates. Considering practical application scenarios, the selected temperature for this study was 350 °C. MgAl–LDH, MgAl–LDH(C16), and molten salt-promoted MgO were synthesized for CO₂ adsorption. Figure 1a shows the adsorption properties of LDH and MgO at high concentrations of CO₂. The adsorption capacity of an organic anion intercalation LDH was found to be superior to that of inorganic carbonate ion intercalation, with MgAl–LDH(C16) achieving a capacity of 0.4 mmol/g at 100% CO₂. Figure 1b presents the adsorption performance at low CO₂ concentration, where several materials exhibited a similar trend to high concentration adsorption. MgAl–LDH(C16) achieved a capacity of 0.3 mmol/g at 10% CO₂.

As reported in the literature, MgO promoted with molten salt is commonly used for CO₂ adsorption at intermediate temperatures and exhibits favorable adsorption effects. Figure 1 illustrates the adsorption capacities of MgO under both high and low CO₂ concentration conditions. At 350 °C, the adsorption capacity of molten salt-promoted MgO was observed to be very low for both high and low CO₂ concentrations. However, the previous literature suggests that modified MgO can achieve an adsorption capacity of 12 mmol/g at 300 °C [41]. Hence, variation in temperature significantly influences the adsorption capacity of MgO, while LDHs demonstrate better performance and faster adsorption rates at 350 °C. Therefore, this paper will not further explore modified MgO.

The scanning electron microscopy (SEM) analysis was conducted to examine the morphologies of synthesized samples, and Figure 2 illustrates the morphologies and microstructure of MgAl–LDH, MgAl–LDH(C16), and modified MgO. The results show that after calcination at 400 °C, the LDH’s plate form was decomposed into nanoparticles [42]. MgAl–LDH exhibited a lamellar accumulation structure, whereas MgAl–LDH(C16) remained as lamellar nanoparticles even after calcination. Both LDHs exhibited a rose-like appearance, which is the result of selective growth along the (001) surface. In contrast, modified MgO exhibited a small spherical particle structure, which differed significantly from the sheet structure of the LDH. Furthermore, a N₂ adsorption–desorption analysis was performed to evaluate the specific surface area and pore structure of the calcined LDH, as present in Figure S1. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the samples showed a type IV isotherm and a type H3 hysteresis loop, indicating that there were slit holes and channels. The H3 ring is thought to be related to plate aggregation. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) equation and the pore volume distribution was calculated using the Barrett–Joyner–Halenda (BJH) method [43]. The analysis revealed that the specific surface area (SSA) of molten salt-promoted MgO is smaller, almost ten times less, compared to that of LDH, as shown in

Table 1. Specific surface area, pore size, and pore volume of MgAl–LDH, MgAl–LDH(C16), and modified MgO.

Samples	BET SSA (m2/g)	BJH Pore Size (nm)	BJH Pore Volume (cm3/g)
MgAl–LDH	194.29	22.52	1.09
MgAl–LDH(C16)	122.28	14.12	0.43
0.1(Li0.2Na0.14K0.52)NO3/MgO	13.11	61.16	0.2

.

Traditional tests for capturing CO₂ using solid adsorbents are typically performed using TGA. The CO₂ atmosphere in TGA is usually maintained at a constant level, and the test results only display the increase in weight of the adsorbent. This approach differs somewhat from actual conditions. However, in practical applications, CO₂ is predominantly adsorbed in a fixed or fluidized bed, which represents dynamic adsorption. In this work, the dynamic CO₂ adsorption performances of different adsorbents were studied by measuring the breakthrough curve of the fixed bed. The penetration time of the adsorbent was studied, and the adsorption capacity of the adsorbent was calculated based on the breakthrough curve. The data obtained from TGA were compared with those from the fixed bed. Figure 3 shows the typical breakthrough curve of the LDH and the molten salt-promoted MgO. Under these test conditions, MgO exhibited the shortest penetration time and the lowest adsorption capacity, which is consistent with the results obtained from TGA. The organic anion intercalation LDH demonstrated the highest adsorption capacity and longest penetration time compared to the inorganic ion intercalation, with values of 61.3 s and 0.636 mmol/g, respectively.

2.2. The CO₂ Capture Performance of Layered Double Hydroxide Promoted with K₂CO₃

Loading LDH-derived adsorbents with K₂CO₃ is an effective method for enhancing their adsorption activity. This study also investigated the impact of K₂CO₃ loading on the performance of inorganic and organic LDH-derived adsorbents. K₂CO₃ loadings of 25 wt% and 12.5 wt% were used to prepare promoted LDH adsorbents, as shown in Figure 4. The adsorption capacity of several samples in 100% CO₂ concentration was significantly enhanced by K₂CO₃ loading, which is consistent with previous studies [39]. To investigate the adsorption performance of the samples under low CO₂ concentrations, which is more representative of industrial scenarios, this study also tested the adsorption activity of each sample at a 10% CO₂ concentration. Both loadings of 12.5 wt% K₂CO₃ and 25 wt% K₂CO₃ demonstrated promotional effects on the LDH. However, the optimal loading capacity varied for different carriers. Specifically, MgAl–LDH reached its optimum loading capacity at 25 wt% K₂CO₃, while MgAl–LDH(C16) achieved its best performance at 12.5 wt% K₂CO₃. Remarkably, the adsorption capacity of 12.5 wt% K₂CO₃/MgAl–LDH(C16) reached 0.8 mmol/g at 10% CO₂, indicating that the concentration of CO₂ had minimal impact on the adsorption capacity of LDH. This is in stark contrast to MgO, as the study found that, when MgO was modified with molten salts, it exhibited an adsorption capacity exceeding 10 mmol/g in 100% CO₂ at 300 °C. However, in a low CO₂ concentration condition, the adsorption performance significantly declined. For a 10% CO₂ concentration, its adsorption capacity was less than 0.1 mmol/g [44]. This indicates that LDH-derived adsorbents have more potential for practical applications at low CO₂ concentrations.

Figure 5 shows typical breakout curves for LDHs with different K₂CO₃ loadings. It is evident that the modification of LDH with K₂CO₃ significantly enhances its CO₂ adsorption capacity, corroborating the findings from the TGA results. However, for the inorganic anion-intercalated MgAl–LDH, the penetration time and dynamic adsorption capacity were both improved, to some extent, after K₂CO₃ loading. In contrast, the improvement in adsorption capacity was not significant for organic anion-intercalated MgAl–LDH(C16) after K₂CO₃ loading. Conversely, the dynamic adsorption capacity of the material with 25 wt% K₂CO₃ loading even decreased, which may be due to an excessive K₂CO₃ load resulting in pore or surface blockage. Figure S2 explores the breakout curves of 25 wt% K₂CO₃/MgAl–LDH and 12.5 wt% K₂CO₃/MgAl–LDH(C16) at various gas flow rates. It is observed that as the flow rate increases, the penetration time decreases. This phenomenon can be attributed to the dynamic adsorption process, where a lower gas velocity allows CO₂ to linger on the adsorbent surface, resulting in a longer penetration time. The nitrogen adsorption–desorption analysis revealed that the specific surface area of both MgAl–LDH and MgAl–LDH(C16) decreased with increasing K₂CO₃ load, as shown in

Table 2. Specific surface area, pore size, and pore volume of K₂CO₃-promoted LDHs.

Samples	BET SSA (m2/g)	BJH Pore Size (nm)	BJH Pore Volume (cm3/g)
12.5 wt% K2CO3/MgAl–LDH	100.59	31.3	0.78
25 wt% K2CO3/MgAl–LDH	65.95	36.3	0.59
12.5 wt% K2CO3/MgAl–LDH(C16)	10.59	57.82	0.15
25 wt% K2CO3/MgAl–LDH(C16)	5.43	73.78	0.1

. The sample curves in Figure S3 are consistent with type IV isotherms and H3 hysteresis loops, and it can be judged that samples belong to mesoporous materials according to pore size distribution. In the dynamic adsorption experiments, 25 wt% K₂CO₃/MgAl–LDH exhibited the highest CO₂ adsorption activity, with a maximum adsorption capacity of 0.722 mmol/g and a penetration time of 93.1 s. This result is in contrast to the findings from the TGA, where 12.5 wt% K₂CO₃/MgAl–LDH(C16) achieved the highest adsorption capacity over a period of 90 min, as shown in Figure 4. The discrepancy can be explained by the short duration of the dynamic adsorption experiments, where the adsorption rate is the main influencing factor on adsorption activity rather than the saturation adsorption capacity. In comparison, 25 wt% K₂CO₃/MgAl–LDH exhibited a faster reaction rate during the initial stages of adsorption.

Figure 6 shows the X-ray diffraction (XRD) patterns of LDHs with carbonate anion intercalation and organic multi-carbon chain anion intercalation, as well as with K₂CO₃ loadings. The characteristic Bragg reflection of LDH can be clearly observed in all samples, confirming the successful formation of LDH structures. In Figure 6a, for MgAl–LDH and MgAl–LDH(C16), the 003 peak is observed at 11.3° and 2.1°, the 006 peak is observed at 22.6° and 5.8°, the 009 peak is observed at 34.5° and 6.1°, and the 012 peak of MgAl–LDH(C16) is observed at 34.9°; 003, 006, 009, and 012 can be indexed as typical hydrotalc-like structures (JCPDS 15-0087). When carbonate ions are replaced by anions in palmitic acid, the peak value of 003 shifts to a lower value, indicating that the insertion of long carbon chain organic anions increases the interlayer distance of LDH layers. All the above findings confirm the successful synthesis of MgAl–LDH and MgAl–LDH(C16), with MgAl–LDH exhibiting a basal spacing (d₀₀₃) of 8.01 Å and MgAl–LDH(C16) showing a basal spacing (d₀₀₃) of 40.89 Å; this indicates that the interlayer spacing of the material is increased by the intercalation of organic anions. In Figure 6b, it can be found that the typical characteristic peaks of hydrotalc-like compounds still exist before calcination after MgAl–LDH and MgAl–LDH(C16) impregnation of K₂CO₃, indicating that the impregnation of K₂CO₃ did not destroy the layered structure of the LDH. A series of small peaks between 2θ = 25° and 35° in carbonate sample are typical of K₂CO₃ [30]. The basal reflection of LDH is quite wide and not too strong, indicating that the layered structure has low crystallinity and may be disordered. It has been reported that the crystallinity and order of the layered structure of LDH containing organic anions are largely dependent on the properties of the anions [45].

The morphology and microstructure of MgAl–LDH and MgAl–LDH(C16) with different K₂CO₃ loadings were observed by SEM, as shown in Figure S4. MgAl–LDH exhibited a lamellar stacking structure, and its basic morphology remains unchanged after loading K₂CO₃. After calcination, MgAl–LDH(C16) still maintained a flaky nanoparticle morphology. However, when loaded with 12.5 wt% K₂CO₃, it formed a flaky clustered structure, which may have exposed more basic sites on the surface for CO₂ adsorption. At a loading capacity of 25 wt% K₂CO₃, the pores became blocked, due to excessive loading; this correlated with a decrease in the CO₂ adsorption capacity of the adsorbent at high loadings. After loading K₂CO₃, these small particles blocks the pores on the surface of the adsorbent to varying degrees, with greater pore blockage occurring as the loading load increased. Figure 7 presents SEM–EDS analysis of two adsorbents, 25 wt% K₂CO₃/MgAl–LDH and 12.5 wt% K₂CO₃/MgAl–LDH(C16). The figure illustrates the actual distribution of O, Mg, Al, and K elements on the surface of the adsorbent, showing that the four elements are evenly distributed without agglomeration. The results indicate successful loading of K₂CO₃ onto the surface of the LDH without disrupting the nanosheet structure on the adsorbent’s surface. Figure 8 provides TEM images of carbonate and palmitic acid anion-based LDH adsorbents calcined at 400 °C, captured at the same magnification. Both MgAl–LDH and MgAl–LDH(C16) exhibit a rose-like appearance. It is observed that amorphous adsorbents obscure the original brucite-like layers and aggregate within the interlayers; the brucite-like layers of 12.5 wt% K₂CO₃/MgAl–LDH(C16) collapsed and broke into smaller-sized amorphous, dispersed with black spots, which are dispersed K elements on the adsorbent’s surface.

2.3. Fit Adsorption Process and Compared by Rate Constant

To investigate the reason why the dynamic adsorption activity of 12.5 wt% K₂CO₃/MgAl–LDH(C16) is lower than that of 25 wt% K₂CO₃/MgAl–LDH, several LDH samples with different K₂CO₃ loads were analyzed, using a TGA under low concentration conditions (1% CO₂). The results are shown in Figure 9a,b. It can be seen that the adsorption capacity of several samples follows the same trend as that observed for 100% CO₂ and 10% CO₂; when the adsorption time reached 90 min, 12.5 wt% K₂CO₃/MgAl–LDH(C16) displayed optimal adsorption activity. However, during the initial stage of adsorption, the inorganic MgAl–LDH loaded with K₂CO₃ exhibited a faster adsorption rate. It was only after 20 min of reaction time that the adsorption capacity of 12.5 wt% K₂CO₃/MgAl–LDH(C16) started to fall behind. When it comes to dynamic adsorption, the process usually only lasts for 1–2 min, and the CO₂ concentration in the gas decreases to extremely low levels during this time. In such a case, 25 wt% K₂CO₃/MgAl–LDH was found to be the most effective in terms of activity.

The adsorption capacity of CO₂ was drawn according to the kinetic model, the adsorption process was fitted, the dynamic change in absorption during the reaction between the adsorbent and CO₂ was studied, and the adsorption rate of the adsorbent was further compared by the rate constant [46]. Figure 9c,d shows the dual exponential model fitted by adsorption isotherms of 25 wt% K₂CO₃/MgAl–LDH and 12.5 wt% K₂CO₃/MgAl–LDH(C16) at 1% CO₂. The model assumes the CO₂ adsorption process consists of two stages: chemisorption (k₁) and mass transfer (k₂). The former is a rapid reaction stage, while the latter reflects the slow mass transfer of CO₂ within the adsorbent. The double exponential model can be represented as y = Aexp(−k₁t) + Bexp(−k₂t) + C, where y represents the adsorption capacity, t is the adsorption time in seconds, and k₁ and k₂ are exponential constants. The kinetic parameters are summarized in

Table 3. Summary of the CO₂ capture performances of 25 wt% K₂CO₃/MgAl–LDH and 12.5 wt% K₂CO₃/MgAl–LDH(C16).

Samples	k1 (s−1)	k2 (s−1)	R2
25 wt% K2CO3/MgAl–LDH	0.0179	0.00081	0.9886
12.5 wt% K2CO3/MgAl–LDH(C16)	0.0114	0.00064	0.9954

. The experimental data show that 25 wt% K₂CO₃/MgAl–LDH has a higher k₁ value, which means that it has a higher adsorption rate and a higher adsorption amount in a short time. Also, 25 wt% K₂CO₃/MgAl–LDH has a higher k₂ value, which means it has a higher diffusion rate.

The CO₂–TPD analysis was conducted to determine the number and distribution of basic sites on the adsorbents. Figure S5 displays the CO₂–TPD characterization results for the four adsorbents. According to the previous reports, the temperature segments of (1) 50–150 °C, (2) 150–400 °C, and (3) >400 °C can be divided into the following three types of basic sites: weakly basic sites (Brønsted hydroxyl groups), moderately basic sites (Lewis acid–base centers), and strongly basic sites (Lewis bases associated with oxygen anions) [7]. From Figure S5, it can be observed that desorption peaks of MgAl–LDH and MgAl–LDH(C16) are concentrated at 100–150 °C and 500–750 °C, respectively. This is due to the desorption of bridging carbonates from weakly alkaline sites and bidentate carbonates from strongly alkaline sites, respectively. After doping carbonate, the peak of the strong alkaline site shifted to a higher temperature. The desorption peaks of 25 wt% K₂CO₃/MgAl–LDH and 12.5 wt% K₂CO₃/MgAl–LDH(C16) are concentrated at 400–900 °C, and the peaks shifted towards higher temperatures as the loading increased. It is inferred that the doping of K₂CO₃ increased the number of strongly alkaline sites, thus promoting CO₂ adsorption. Figure S6 displays the Fourier Transform Infrared Spectroscopy (FTIR) spectra of LDH and K₂CO₃-promoted LDH, allowing for the identification of the chemical components of LDH. The band near 3440 cm⁻¹ corresponds to the stretching of -OH, and 1640 cm⁻¹ is attributed to the angular deformation vibration of water molecules. The bands near 640 cm⁻¹ and 470 cm⁻¹ originate from the vibration of the Mg-O and Al-O bonds [13]. Following K₂CO₃ promotion, a new peak appeared at 860 cm⁻¹, indicating the superposition of different carbides and suggesting that potassium enhances the reaction between the sample and CO₂.

To theoretically analyze the intermediate temperature CO₂ capture performance of the LDH, in situ FTIR analysis was conducted, as shown in Figure 10. Due to the super-fast adsorption rate in pure CO₂, a lower partial pressure of CO₂ (1% CO₂) was employed. The vibrations near 1600 cm⁻¹ and 1300 cm⁻¹ in Figure 10 show the v3 asymmetric stretching of the carbonate group. This is attributed to the LDH forming bidentate carbonate when it adsorbs CO₂. The vibrations near 1080 cm⁻¹ in Figure 10b,d indicate the close interaction between the surface carbonate and potassium ions provided by potassium carbonate. In the process of CO₂ adsorption, a large amount of K₂CO₃ participated in the reaction, so that monodentate carbonates and bidentate carbonates were formed on K₂CO₃/LDH, as shown in Figure 11 [47], representing an overlap of different carbides and, hence, suggesting that potassium promotes a chemical reaction between the sample and CO₂. The band CO₂ at 1080 cm⁻¹ becomes slightly Infraredactive upon CO₂ adsorption for various carbonate species, such as unidentate and bidentate [48].

3. Materials and Methods

3.1. Synthesis of Adsorbents

LDHs with carbonate ion and organic anion intercalation were synthesized via the coprecipitation process. The following is a description of the Mg₃Al₁-CO₃ preparation process: in 100 mL deionized water, 0.075 mol Mg(NO₃)₂·6H₂O and 0.025 mol Al(NO₃)₃·9H₂O were dissolved, and 0.05 mol Na₂CO₃ was dissolved in 100 mL deionized water. The metal salt solution and NaOH (4 M) were added to the Na₂CO₃ solution drop by drop. The preparation technique for Mg₃Al₁-C16 is outlined as follows: 0.025 mol palmitic acid (C16), 0.0375 mol Mg(NO₃)₂·6H₂O, and 0.0125 mol Al(NO₃)₃·9H₂O were dissolved in 150 mL methanol at 80 °C. After all of the ingredients had been dissolved, NaOH (4 M) was added to bring the pH to 10. The resultant mixture was aged for 12 h with constant agitation before being washed with deionized water until the pH reached 7, after which it was dried in an oven at 60 °C.

Mg₃Al₁CO₃–LDH and Mg₃Al₁–LDH(C16) promoted by K₂CO₃ were produced using a wet impregnation method. The K₂CO₃ solution (in water) and LDH suspension (in alcohol) were mixed and thoroughly agitated at 80 °C before drying at 60 °C. In this article, Mg₃Al₁CO₃–LDH and Mg₃Al₁–LDH(C16) were impregnated with 12.5 and 25 wt% K₂CO₃, respectively.

3.2. Characterization of Samples

The materials’ structures were characterized by powder X-ray diffraction (XRD) using Shimadzu XRD-7000 equipment (Kyoto, Japan) in reflection mode, with Cu Kα radiation and a power of 40 kV × 40 mA (λ = 1.542 Å). Diffraction patterns were recorded from 2θ = 5–80°, with a step size of 0.02°. The materials’ morphology was determined using scanning electron microscopy (SEM, Hitachi SU8010, Tokyo, Japan). The functional groups of the LDH were identified using a Fourier transform infrared spectrometer (FT-IR, Spectrum 3, Perkin Elmer, Waltham, MA, USA). BET specific surface area, pore volume, and pore size distribution of the samples were measured using N₂ sorption/desorption studies (Builder, SSA-7000, Beijing, China). The surface basicity of the LDH was determined using temperature-programmed desorption of carbon dioxide (CO₂-TPD).

3.3. Evaluation of CO₂ Adsorption Capacity

The CO₂ adsorption capacity of adsorbents was determined using a thermogravimetric analyzer (TGA, Q50 TA Instrument, New Castle, DE, USA). LDHs were first calcined in a muffle furnace at a specific temperature (400 °C) for 5 h in air before being transported to the TGA analyzer. Prior to CO₂ adsorption, samples were calcined in situ at 400 °C for 1 h using a high purity N₂ flow (40 mL/min). The temperature was then reduced to the desired adsorption temperature, and the gas feed was changed from N₂ to CO₂, with a steady flow of high purity CO₂ (1 atm, 40 mL/min), with the CO₂ adsorption uptake measured for 90 min.

A fixed-bed reactor was utilized to test the adsorbent’s dynamic CO₂ adsorption capability. The gas volume flow rate was 40 mL/min, and the inlet concentration remained constant. Ar was used as a protective gas, and the temperature was set to 400 °C to activate the adsorbent. The system was then allowed to cool to the temperature required for analysis. The mass flow meter was then set to the desired volume flow rate in order to determine the feedstock gas composition. The gas valve was then switched to the CO₂ and N₂ mixture, and the composition of the exit gas was monitored during the adsorption trials. When the composition of the exit gas was similar to that of the feeding gas, the experiment was terminated, and the dynamic adsorption experiment was completed.

4. Conclusions

This article presents a systematic study on the dynamic adsorption characteristics of medium-temperature CO₂ adsorbents based on LDHs. The preparation and comparison of K₂CO₃/MgAl–LDH and K₂CO₃/MgAl–LDH(C16) for CO₂ adsorption performance in TGA and fixed-bed experiments were conducted. The results indicate that, in the TGA tests, 12.5 wt% K₂CO₃/MgAl–LDH(C16) exhibits higher CO₂ adsorption activity, reaching a maximum of 1.12 mmol/g. However, the adsorption performance reduced significantly as the CO₂ concentration decreased, or for the dynamic adsorption process. In the dynamic adsorption process in the fixed-bed test, 25 wt% K₂CO₃/MgAl–LDH demonstrated better CO₂ adsorption activity. In this case, it sustained a breakthrough time of 93.1 s and achieved an adsorption capacity of 0.722 mmol/g. This was mainly due to the faster initial adsorption rate of the inorganic LDH towards low-concentration CO₂. This also indicates the better application prospects of 25 wt% K₂CO₃/MgAl–LDH in practical CO₂ adsorption and separation processes.