KR102578876B1 - Ion-exchanged regenerative catalyst for carbon dioxide capture - Google Patents

Abstract

The present invention provides an ion-exchanged montmorillonite catalyst for carbon dioxide capture, a method for producing the same, and a method for regenerating a solvent for carbon dioxide capture using the same.
The ion-exchanged montmorillonite catalyst of the present invention exhibits material properties of excellent BET specific surface area, mesoporosity, and surface acidity, thereby efficiently optimizing the CO ₂ desorption rate at low temperature in the carbon dioxide capture and amine solvent regeneration process, thereby enabling amine solvent regeneration. Not only can the energy requirement be minimized and the catalytic activity remains constant even with repeated catalyst use, but the final catalyst material can be manufactured at a low cost because it can be obtained based on raw materials that are abundant in nature as clay minerals. Therefore, it can be useful as a technology to design the CO ₂ capture process in an energy-efficient manner in the field of carbon capture and storage.

Description

{ION-EXCHANGED REGENERATIVE CATALYST FOR CARBON DIOXIDE CAPTURE}

The present invention relates to a catalyst for carbon dioxide capture, and more specifically, to a technology for using ion-exchanged montmorillonite as a regenerative catalyst as a catalyst for carbon dioxide capture.

A significant increase in anthropogenic carbon dioxide emissions is considered a major cause of global warming and climate change. _CO2 capture and storage (CCS) is widely recognized as an important short- and medium-term measure to reduce _CO2 emissions. The main technologies currently used to separate _CO2 from flue gas streams include chemical absorption, adsorption, membrane separation and cryogenic distillation. However, the low _CO2 concentration in the flue gas (7-15%) makes physical separation techniques (adsorption, membrane filtration and cryogenics) unsuitable for large-scale _CO2 capture. CO ₂ chemical absorption into aqueous alkanol amine solutions is the most advanced commercial technology capable of processing large quantities of gas even at low CO ₂ partial pressures. However, this technology has several major drawbacks, such as energy-intensive regeneration, solvent decomposition, and equipment corrosion, making widespread large-scale application difficult.

In the thermal amine scrubbing process, CO ₂ is selectively absorbed by an aqueous amine solution at low temperature (~ 50°C), and then this CO ₂ -rich solution is thermally regenerated in a stripper column at high temperature (~ 120°C) to produce pure CO ₂ Collect streams. The regenerated solvent, i.e. CO ₂ -lean solvent, is pumped back to the absorber column for circulating use. In particular, not all the absorbed CO ₂ is desorbed in the stripper column; instead, the goal is to regenerate the amine solvent at a temperature where the reboiler heat duty is not significant, thermal decomposition of the solvent is limited, and most importantly, the absorbed CO ₂ is sufficiently discharged to induce sufficient absorption of new CO ₂ into the absorber column. Therefore, a certain amount of CO ₂ is released from the amine solution, ensuring an economically efficient process. For example, when using a benchmark 30 wt.% monoethanolamine (MEA) solvent, typically about 50% of the absorbed _CO2 is released from the stripper column and about 0.20-0.30 mol _CO2 /mol CO2-lin of MEA. The load is maintained downstream of the stripper. Unfortunately, a huge amount of thermal energy is still consumed to release half of the captured CO ₂ , mainly due to the low CO ₂ desorption kinetics. More precisely, the energy required to perform solvent regeneration accounts for approximately 70% of the total process cost. Insights into the regenerative heat duty show that only half of the total heat duty is actually spent breaking chemical bonds and releasing _CO2 , while the other half is wasted heating and evaporating the water (Qsen and Qvap).

The two basic challenges in this scenario are (i) to perform amine solvent regeneration at temperatures below 100°C, avoiding the loss of heat energy to evaporate water, and (ii) to rapidly produce a larger amount of _CO2 per unit of energy consumption. CO ₂ desorption kinetics are optimized at low temperatures to release CO 2 , which in turn results in lower regenerative heat duty (GJ/ton CO ₂ ).

Several advanced solvent formulations, such as mixed absorbents, two-phase absorbents, non-aqueous absorbents, etc., and advanced solvent regeneration methods, including microwave irradiation, electrochemical-mediated regeneration, additive introduction, etc., have been developed to address the shortcomings of energy-intensive regeneration. Several innovative approaches have been reported. For example, Chowdhury et al. developed a series of non-aqueous sorbents that have excellent CO capture performance and can be regenerated at moderate temperatures of ~80–90 °C. Pri et al. developed a soy-based solvent (SBB, amino acid mixture) and thoroughly tested its _CO2 capture performance and heat recovery compared to a benchmark MEA solution. SBB was highly effective, improving _CO2 desorption rates by 195% and reducing non-renewable energy by 37%.

A relatively new innovation to overcome the energy penalty problem is the addition of catalysts to the amine solvent regeneration step to improve _CO2 desorption rates at low temperatures. It has been reported that adding metal-based catalysts can aid carbamate decomposition and release CO ₂ at low temperatures, reducing regeneration heat duty. Idem et al investigated MEA solvent regeneration using HZSM-5 and γ-Al ₂ O ₃ catalysts and demonstrated that catalyst-mediated regeneration could reduce the amine regeneration heat duty by 24%. Liang et al. synthesized excellent catalysts such as Fe-promoted SO ₄ ^2- /ZrO ₂ /MCM-41, SO ₄ ^2- /ZrO ₂ /SBA-15 and Al ₂ O ₃ /HZSM-5 and further reduced the energy penalty. This technology was expanded by optimizing key catalyst properties (mesoporosity, surface acidity, and BET specific surface area). Li et al. synthesized and investigated the catalytic performance of SO ₄ ^2- /TiO ₂ solid acid catalyst in CO ₂ -rich MEA solution at 95 °C and reported that the CO ₂ desorption rate could be improved up to ~29%. Solid metal oxides, TiO(OH) ₂ and metals, have been shown to promote carbamate decomposition at moderate temperatures. However, this approach is in its infancy and only a limited number of catalysts have been reported in the literature. Additionally, most of the reported catalysts are expensive, so any benefit gained in terms of thermal duty reduction may be offset by catalyst cost. Therefore, there is an urgent need for the development of inexpensive and abundant catalysts that can substantially optimize CO ₂ desorption at low temperatures.

The technical problem to be achieved by the present invention is to improve the CO ₂ desorption rate by minimizing energy requirements by lowering the amine regeneration temperature in the carbon capture and solvent regeneration process, and to provide the final catalyst material at low cost based on raw materials abundant in nature. It provides technology to reduce process costs by manufacturing.

The technical problem to be achieved by the present invention is not limited to the technical problem mentioned above, and other technical problems not mentioned can be clearly understood by those skilled in the art from the description below. There will be.

In order to achieve the above technical problem, according to one aspect of the present invention, an ion-exchanged montmorillonite catalyst for carbon dioxide capture is provided.

In one embodiment, the ion-exchange may be characterized in that ions contained in the montmorillonite are replaced with cations of an aqueous acid solution or an aqueous metal solution.

In one embodiment, the ion-exchange may be characterized in that Si, Al, O, Fe, Mg, Na, Ca or K ions contained in the montmorillonite are replaced with cations of H ⁺ or Zr.

In one embodiment, the ion-exchanged montmorillonite catalyst may have an element fraction selected from 0 to 7% Al, 0 to 1.2% Fe, 0 to 1% Mg, and combinations thereof.

In one embodiment, the ion-exchanged montmorillonite catalyst may have a surface area of 50 to 500 m ² /g.

In one embodiment, the ion-exchanged montmorillonite catalyst may have a pore volume of 0.15 to 0.50 cm ³ /g.

In one embodiment, the ion-exchanged montmorillonite catalyst may have an average pore diameter of 1 to 15 nm.

In one embodiment, the ion-exchanged montmorillonite catalyst may have a surface acidity of 2 mmol/g or more.

In one embodiment, the ion-exchanged montmorillonite catalyst may have a total acid site of 0.3 mmol/g or more.

According to another aspect of the invention, (a) reacting montmorillonite with an aqueous acid solution to obtain a product; and (b) filtering the product of step (a) and washing the filtrate until the pH is 5 to 9. A method for producing an ion-exchanged montmorillonite catalyst is provided.

In one embodiment, the reaction in step (a) may be performed while stirring.

In one embodiment, the acid of step (a) is hydrochloric acid, nitric acid, sulfuric acid, benzoic acid, oxalic acid, acetic acid, propionic acid, pyridinium ion, chloric acid, oxonium ion, iodic acid, oxylic acid, sulfurous acid, chlorous acid, phosphoric acid, It may be characterized as being one type selected from the group consisting of arsenic acid, chloroacetic acid, hydrogen fluoride, nitrous acid, formic acid, carbonic acid, hydrogen sulfide, hypochlorous acid, hydrogen iodide, hydrogen bromide, perchloric acid, and combinations thereof. .

In one embodiment, the aqueous metal solution of step (a) may include at least one selected from SnCl ₄ , TiCl ₄ , TiOCl ₂ , TiOSO ₄ , ZrOCl ₂ , ZrCl ₄ , ZrOSO ₄ , MnCl ₂ , and CoCl ₂ there is.

In one embodiment, the aqueous acid solution in step (a) may have a concentration of 0.5 to 10 mol/L.

In one embodiment, the montmorillonite and acid aqueous solution in step (a) may be reacted at a ratio of 1:1 to 1:100 (weight:volume).

In one embodiment, the reaction of step (a) may be performed at 70 to 110 °C.

In one embodiment, the reaction of step (a) may be performed for 1 to 20 hours.

In one embodiment, the filtration in step (b) may be performed after cooling to room temperature.

According to another aspect of the present invention, a method for regenerating a solvent for carbon dioxide capture using the ion-exchanged montmorillonite catalyst is provided.

The ion-exchanged montmorillonite catalyst of the present invention minimizes energy requirements by lowering the amine regeneration temperature in the carbon capture and solvent regeneration process, thereby improving the CO ₂ desorption rate and achieving low cost based on raw materials abundant in nature. By manufacturing the final catalyst material, it is possible to provide a technology that can reduce process costs.

The effects of the present invention are not limited to the effects described above, and should be understood to include all effects that can be inferred from the configuration of the invention described in the detailed description or claims of the present invention.

Figure 1 is a schematic diagram of the Mont structure.
Figure 2 shows the characteristics evaluation results of the raw material and ion-exchanged Mont catalyst, (a) XRD pattern, (b) N ₂ adsorption-desorption isotherm, (c) NH ₃ -TPD curve, (d) Py-FTIR spectrum. am.
Figure 3 is a schematic diagram of the experimental setup used to perform solvent regeneration experiments.
Figure 4 shows the results of evaluation of MEA solvent regeneration characteristics according to the presence or absence of raw materials and ion-exchanged Mont catalyst, (a) CO ₂ desorption rate curve, (b) total amount of desorbed CO ₂ during the ramp section and the entire experiment, (c) Shows the relative heat duty of the amine solution.
Figure 5 shows a possible reaction mechanism of the ion-exchanged Mont catalyst to promote carbamate decomposition and release CO ₂ from CO ₂ -enriched MEA.
Figure 6 shows (A) the recyclability of the ion-exchanged Mont catalyst and (B) the effect of the ion-exchanged Mont catalyst on CO ₂ absorption in MEA solution.

In this specification, “acid activation” refers to a process of treating a material with an acidic substance to induce the creation of acid sites.

The present invention provides an ion-exchanged montmorillonite catalyst for carbon dioxide capture.

Montmorillonite (Mont) is a type of clay mineral. It is a monoclinic crystal. Its chemical composition is (AI, Mg) ₂ Si ₄ O ₁₀ (OH) ₂ 4H ₂ O, and it is in the form of lumpy earth. achieves It has the property of absorbing water between layers and swelling to 7 to 10 times its original volume, and has high ion exchange properties. It is a clay that is the main component of bentonite, a modified form of volcanic ash or tuff, and has special properties such as low internal friction resistance. Hardness is 1 to 1.5, specific gravity is 2 to 2.5, and refractive index is 1.48 to 1.6. It is white, yellow, light yellow, light green, green, or blue in color and has no luster.

Currently, clay minerals are considered promising catalysts because they are environmentally friendly and inexpensive. Montmorillonite is an inexpensive, abundant, naturally occurring clay composed of aluminosilicate layers and exchangeable cations (Figure 1). In the present invention, the surface acidity of montmorillonite is easily improved by a simple ion-exchange process.

The ion-exchanged montmorillonite catalyst of the present invention may have an elemental fraction selected from 0 to 25% Si, 0 to 7% Al, 0 to 1.2% Fe, 0 to 1% Mg, and combinations thereof. Montmorillonite raw material has Na, Ca, Fe, Mg, and K cations in the middle layer, and SiO ₂ and Al ₂ O ₃ are the main components, but the ion-exchanged montmorillonite catalyst mainly consists of octahedral sites and Metals such as silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), sodium (Na), calcium (Ca), and potassium (Ka) are leached from the cationic intermediate layer, changing the elemental composition ratio. . In particular, Na, Ca and K are not detected in the ion-exchanged montmorillonite catalyst. This process promotes the collapse of the layer structure, removes impurities, and modifies the structure and chemical composition of montmorillonite. Accordingly, the main textural properties of the montmorillonite catalyst, such as surface area, pore volume, mesoporosity and surface acidity, are substantially improved.

In one embodiment, the ion-exchanged montmorillonite catalyst has a surface area of 50 to 500 m ² /g, preferably 60 to 400 m ² /g, more preferably 70 to 300 m ² /g, most preferably 80 m 2 /g. It may be ~200 m ² /g.

In one embodiment, the ion-exchanged montmorillonite catalyst has a pore volume of 0.15 to 0.50 cm ³ /g, preferably 0.16 to 0.45 cm ³ /g, more preferably 0.17 to 0.40 cm ³ /g, most preferably It may be 0.18 to 0.35 cm ³ /g.

In one embodiment, the ion-exchanged montmorillonite catalyst may have an average pore diameter of 1 to 15 nm, preferably 2 to 14 nm, more preferably 3 to 13 nm, and most preferably 4 to 13 nm.

In one embodiment, the ion-exchanged montmorillonite catalyst may have a surface acidity of at least 2 mmol/g, preferably at least 3 mmol/g, more preferably at least 3.5 mmol/g, and most preferably at least 3.56 mmol/g. there is.

In one embodiment, the ion-exchanged montmorillonite catalyst may have a total acid site of at least 0.2 mmol/g, preferably at least 0.25 mmol/g, and more preferably at least 0.3 mmol/g.

The ion-exchanged montmorillonite catalyst of the present invention can significantly increase the BET specific surface area, mesoporosity and surface acidity of raw montmorillonite without noticeably destroying the clay structure in the ion-exchange process. Solvent regeneration experiments at an intermediate temperature of 86°C show that the catalyst is effective at low temperatures and can significantly increase the CO ₂ desorption rate and the amount of desorbed CO _2. Additionally, the prepared catalyst can be easily separated by simple filtration and is stable. This can be secured.

The ion-exchanged montmorillonite catalyst of the present invention includes the steps of (a) reacting montmorillonite with an aqueous acid solution or an aqueous metal solution to obtain a product; and (b) filtering the product of step (a) and washing it until the pH of the filtrate is 5 to 9.

The above manufacturing method is a process of treating montmorillonite with an acidic substance to induce the creation of acidic sites. This process promotes the collapse of the layer structure, removes impurities, and modifies the structure and chemical composition of montmorillonite.

The production method of the present invention includes the step of reacting montmorillonite with an aqueous acid solution or an aqueous metal solution to obtain a product (i.e., step (a)). Step (a) is a step of treating montmorillonite with an acidic substance to induce leaching of cations and creation of acid sites. The acid used in step (a) is silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), sodium (Na), calcium (Ca), and potassium (Ka) in the octahedral region of montmorillonite and the cationic intermediate layer. Any aqueous acid solution or metal aqueous solution capable of leaching metals such as hydrogen ions or metal ions can be used without limitation. Preferably, the aqueous acid solution is hydrochloric acid, nitric acid, sulfuric acid, benzoic acid, oxalic acid, acetic acid, propionic acid, Pyridinium ion, chloric acid, oxonium ion, iodic acid, oxylic acid, sulfurous acid, chlorous acid, phosphoric acid, arsenic acid, chloroacetic acid, hydrogen fluoride, nitrous acid, formic acid, carbonic acid, hydrogen sulfide, hypochlorous acid, iodination. It may be one selected from the group consisting of hydrogen, hydrogen bromide, perchloric acid, and combinations thereof, and the aqueous metal solution is SnCl ₄ , TiCl ₄ , TiOCl ₂ , TiOSO ₄ , ZrOCl ₂ , ZrCl ₄ , ZrOSO ₄ , MnCl ₂ , and CoCl It may be characterized as an aqueous solution of an inorganic metal salt containing at least one selected from ₂ . The aqueous acid solution in which the acid is dissolved or the aqueous metal solution in which the inorganic metal salt is dissolved can be used at a concentration that can induce leaching of cations and the creation of acid sites or substitution with metal ions, preferably at a concentration of 0.3 to 12 mol/ L, more preferably 0.5 to 10 mol/L, but is not limited thereto.

The reaction of step (a) may be carried out at 90 to 110° C. to induce leaching of cations and generation of acid sites, and may be carried out for an appropriate time to allow the reaction to proceed, for example, 1 to 20 hours. It can be. It may also be characterized as proceeding while stirring, and in some cases, the reaction may be carried out with strong stirring.

The production method of the present invention includes the step of filtering the product of step (a) and washing the filtrate until the pH is 5 to 9 (i.e., step (b)). Step (b) is a step of neutralization by washing the surface of the ion-exchanged montmorillonite catalyst from which cations have been leached and acid sites increased. The filtration in step (b) may be performed after cooling to room temperature. Additionally, the washing may be performed with a substance (eg, water, etc.) that can neutralize the acidic substances remaining on the surface of the ion-exchanged montmorillonite catalyst by washing them. The washed ion-exchanged montmorillonite catalyst can be used by drying and pulverizing as needed.

The present invention also provides a method for regenerating a solvent for carbon dioxide capture using the ion-exchanged montmorillonite catalyst. Using the ion-exchanged montmorillonite catalyst of the present invention, the CO ₂ desorption rate in the carbon dioxide capture process and the amine solvent regeneration process can be efficiently optimized at low temperature, thereby minimizing the energy requirements of amine solvent regeneration. Additionally, the catalytic activity was found to remain constant even with repeated catalyst use, making it useful for commercial applications in carbon capture processes.

*Hereinafter, the present invention will be described in more detail through examples. However, the following examples are for illustrating the present invention, and the scope of the present invention is not limited thereto.

<Example>

In this study, we advance this approach by preparing an abundant and inexpensive montmorillonite (Mont) clay-based catalyst using a simple ion-exchange process and evaluating its catalytic performance for CO desorption from MEA solutions. Montmorillonite is a very abundant, inexpensive, naturally occurring clay that is widely applied in adsorption and catalysis processes. It has a three-sheet lattice structure consisting of a central layer of alumina octahedra sandwiched between two layers of silica tetrahedra (Figure 1). Interlayer cations are exchangeable and the acidic properties can be altered by simple ion-exchange with acids or metals. The prepared catalyst was thoroughly characterized using XRD, BET, NH3-TPD, Pyridine-FTIR, and EDS to confirm the physicochemical transformation and efficiency induced by the ion-exchange process. Catalytic CO ₂ desorption performance was evaluated at 86 °C for CO ₂ desorption rate, total amount of CO ₂ desorbed, and relative heat duty compared to the non-catalytic reference MEA solution.

1. Preparation of catalyst

(1) Material purchase and catalyst manufacturing

Montmorillonite (Cas No. 1302-78-9), monoethanolamine (>99 %), H ₂ SO ₄ (96 %), and ZrOCl _{2.8H 2} O (reagent grade, 98%) were purchased from Sigma-Aldrich (Sigma). -Aldrich) and used without further purification.

The ion-exchanged Mont catalyst was prepared using a slightly modified method. In a typical preparation, Mont (5 g) was slowly added to 80 mL of ZrOCl _{2.8H 2} O solution (0.5 mol/L) at 80 °C with vigorous stirring. After 12 hours, the process was repeated with fresh ZrOCl _{2.8H 2} O solution (0.5 mol/L, 80 mL) for 4 hours. For H ₂ SO ₄ -mont, 5 g Mont was slowly added to H ₂ SO ₄ solution (4 mol/L) at a ratio of 1:5 (weight per volume). The reaction was carried out at 100 °C for 4 h under reflux conditions. After the ion-exchange process was completed, both samples were cooled to room temperature and washed repeatedly with deionized water until the pH of the filtrate was approximately 7. The obtained clay was dried in an oven at 110°C for 12 hours and then ground with a mortar and pestle. The prepared catalysts were named SO ₄ -Mont and Zr-Mont according to the solution used for ion-exchange.

(2) Evaluation of physical properties

X-ray diffraction (XRD) patterns were obtained using a Rigaku D/max 2200PC diffractometer equipped with a Cu sealed tube (λ=1.54178 Å), scanning at 0.5° min ^-1 in the range from 5 to 80°. Recorded at speed.

The BET (Brunauer-Emmett-Teller) specific surface area and porosity of the catalyst were measured using a nitrogen adsorption method at -200 °C using a constant volume adsorption device (Micromeritics, ASAP-2420).

Surface density and acid site strength were measured with a temperature programmed ammonia desorption device (Micromeritics, Autochem II). Approximately 0.1 g of sample was pretreated at 200 °C for 2 h in helium flow to clean the surface. The sample was then cooled to room temperature and ammonia was introduced into the reactor at 100° C. for 1 hour.

For Py-FTIR spectroscopy, free-standing wafers (11 tons cm ^-2 , 30 mg and 1 cm ² ) were pretreated at 300 °C for 2 h under vacuum (10 ^-3 mbar) in a stainless steel IR cell. Afterwards, hot pyridine vapor at 100°C was introduced into the IR cell for 2 hours until the pressure inside the IR cell reached 5 bar. Afterwards, the IR cell was connected to an infrared spectrometer, and the IR spectrum of the material was recorded in the wavelength range of 600 to 4000 cm ^-1 with an optical resolution of 8 cm ^-1 and co-addition of 32 scans.

Quantitative values of Bronsted and Lewis acid sites were calculated using equations 1 and 2.

(Equation 1)

(Equation 2)

where C is the concentration of acid sites (mmol g-1 catalyst), IA (B, L) is the integrated absorbance of the B or L band (cm-1), R is the radius of the catalyst disk (cm), W = disk weight ( mg).

(3) Regenerative performance evaluation

A 250 mL glass reactor was connected to an oil circulator to heat the amine solution. A K-type thermocouple was inserted into the reactor to record the temperature of the amine solution. A glass condenser was installed in the reactor to condense and reflux water and amine vapor. The solution was stirred during the regeneration experiment using a magnetic stirrer at 300 rpm. MEA solution regeneration was measured by heating 100 g of CO ₂ -enriched MEA solution from room temperature to about 86°C. Thereafter, the temperature was kept constant at 86°C until the CO ₂ released was less than 0.1%. In catalyst experiments, 5% by weight of each catalyst was added to 100 g of room temperature MEA solution. The released CO ₂ gas was mixed with N ₂ carrier gas (50 mL/min) through a check valve before quantitative analysis in gas chromatography. The amount of desorbed CO _{2 was calculated by integrating the CO 2 desorption} profile.

2. Material properties

(1) XRD analysis

The XRD patterns of raw material and ion-exchanged Mont catalyst are shown in Figure 2a. For raw material Mont, d001 basal gap diffraction was observed at 2θ of 7.41 ^o . For both catalysts, the d001 diffraction peak broadened and lowered the intensity, indicating layered torsion induced by the ion-exchange process that disrupted layered stacking. Different characteristic reflections assigned to the crystal lattice planes of 020, 110, 005, 200, 130 and 060 at 2θ 19.7, 26.6, 28.5, 35.3, 36.1 and 61.9 ^o , respectively, can be clearly identified for both ion-exchange catalysts. Overall, the persistence of all diffraction peaks of ion-exchanged Mont clearly confirms that the crystal structure of the raw material Mont is intact and that the ion-exchange process did not noticeably change the layer structure of Mont.

(2)N ₂ Adsorption-Desorption

The nitrogen adsorption-desorption isotherms of the raw material and ion-exchanged Mont catalyst are summarized in Figure 1b, and the calculated textural properties are summarized in Table 1. As can be seen in the table, the ion-exchange process significantly increased the surface area of the raw material Mont. The raw material Mont had a BET surface area of 24.4 m ² /g, which increased to 118.02 and 223.35 m ² /g after ion-exchange with H ₂ SO ₄ acid and Zr metal solution, respectively. The significant increase in surface area was mainly caused by removing cations from the clay and promoting porosity. Additionally, a greater amount of adsorbed-desorbed N ₂ was observed for the ion-exchanged Mont catalyst compared to the raw material Mont. In general, the N ₂ isotherm belongs to the type II profile and the hysteresis loop is type H4, indicating that N ₂ adsorption and desorption occurred on a smooth surface.

[Table 1]

(3) Temperature-programmed NH ₃ Desorption

The density and intensity of acid sites were determined using temperature-programmed _NH3 desorption and the obtained spectrum is shown in Figure 2c. The strength of the acid site was assigned as weak, moderate, or strong depending on the temperature regime of NH ₃ desorption. The raw material Mont exhibited two peaks: a small peak at ~124 ^o C due to physically adsorbed NH ₃ and another peak in the strong temperature region at 679 ^o C due to the structural Al moiety of Mont. The NH ₃ -TPD profile of the ion-exchanged Mont catalyst showed the appearance of an additional peak in the middle region, clearly indicating that the ion-exchange process improved the acidity of Mont. Additionally, the peak in the high temperature region broadened, indicating that a strong acid site was formed during the ion-exchange process.

[Table 2]

^{a NH} ₃ ^{-Obtained from TPD}

^{b Lewis acid sites (LAS), Brønsted acid sites (BAS) and total acid sites calculated by Py-FTIR.}

(4) Pyridine-infrared spectroscopy

The nature of the acidic site was determined by FTIR analysis of the pyridine-adsorbed species. Pyridine molecules react with hydroxyl groups present on the catalyst surface (Brønsted acid site) to form pyridinium ions (HPy species), which exhibit a characteristic peak at approximately 1540 cm ^-1 . Additionally, pyridine can interact with the coordinately unsaturated metal site of the catalyst to form LPy species (Lewis acid site). The obtained FTIR spectrum is shown in Figure 2d and quantitative values for Lewis acid moieties and Bronsted acid moieties are provided in Table 2. In the case of the raw material Mont, three prominent peaks appear at 1447, 1490, and 1592 cm ^-1 , indicating that only the Lewis acid moiety exists. For the ion-exchange catalyst, well-resolved peaks were recorded at 1440 and 1590 cm ^-1 for pyridine adsorbed on Lewis acid sites and at approximately 1540 ^-1 for pyridine adsorbed on Brønsted acid sites. The peak appearing at approximately ~1490 cm ^-1 indicates the presence of both Lewis and Bronsted acid sites.

(5) Elemental analysis

EDS (Energy Dispersive X-ray spectroscopy) was used to determine the quantitative elemental composition of the raw catalyst and the ion-exchanged Mont catalyst, and the obtained results are summarized in Table 3. The raw material Mont mainly consists of SiO ₂ and Al ₂ O ₃ octahedral sheets, with Fe ⁺³ , Mg ⁺² , Na ⁺ , Ca ⁺² and K ⁺ cations in the middle layer. For the ion-exchange catalyst, Si and Al are leached from the octahedral sheet, and Na ⁺ , Ca ^{+ 2} , and K ⁺ are completely extracted from the cationic interlayer, confirming the effectiveness of the ion-exchange process. This also indicates that the concentrations of acid and metal solutions were optimal, resulting in a strong attack on the Mont structure where exchangeable cations were replaced by H ⁺ and Zr metals. Nevertheless, significant amounts of Fe ⁺³ and Mg ⁺² remained in the ion-exchanged Mont catalyst. This may be due to the presence of insoluble quartz impurities.

[Table 3]

3. Data calculation method

C.O. ₂ Desorption rate:

The _CO2 desorption rate (mmol/min) during the regeneration experiment was calculated using Equation 1.

(One)

where Q _CO2 is the CO ₂ desorption rate (mmol/min), r _CO2 is the volumetric concentration of CO ₂ , and V _N2 N ₂ is the volumetric flow rate (mL/min).

Adsorbed CO ₂ Quantity:

The total amount of _CO2 adsorbed during the regeneration experiment was calculated by integrating the recorded _CO2 adsorption profiles.

(2)

Here, Q _CO2 is the CO ₂ desorption rate (mmol/min), and N _CO2 is the total amount of CO ₂ desorbed during the solvent regeneration experiment (mmol).

Thermal duty of solvent regeneration:

The thermal duty (KJ/mol CO ₂ ) of MEA solvent regeneration was calculated using Equation 3. Power consumption was measured using a power meter, and the amount of _CO2 emitted was calculated using Equation 2. Thermal duty was determined by dividing the power consumption by the total amount of CO ₂ emitted during the regeneration experiment.

(3)

For better comparison of catalyzed and uncatalyzed systems, relative heat duty was used as previously suggested by Liang et al. The relative heat duty of solvent regeneration was defined as Equation 4.

(4)

where RHD is the relative thermal duty (%) and HD _baseline and HD _cat are the thermal duties for the non-catalytic and catalytic MEA solutions, respectively.

4. Experimental apparatus and methods

The experimental setup used to perform the CO ₂ desorption experiments is shown in Figure 3. A 250 mL volume flat bottom glass reactor was connected to an oil circulator to heat the amine solution. A condenser was installed in the reactor to minimize vapor loss during the regeneration experiment. A K-type thermocouple and pressure gauge were installed to monitor the temperature of the amine solution and the pressure inside the reactor. The CO ₂ released during solvent regeneration was mixed with nitrogen carrier gas (50 mL/min) before passing through a check valve and then analyzed using a gas chromatograph (Agilent Technologies 7890A). The amine solution was continuously stirred at 300 rpm using a magnetic stirrer. Thermal duty of solvent regeneration was recorded using a wattmeter (Wattman HPM-100A) connected to a heated oil circulator. All pipelines were purged with nitrogen gas before and after each experiment to remove residual _CO2 .

CO ₂ was absorbed into a 30 wt.% MEA solution using 15 vol.% CO ₂ balanced N ₂ gas at 40 °C. The CO ₂ loading of the MEA solution was measured with a TOC analyzer (Analytik Jena multi N/C 3100) and found to be 0.517. For solvent regeneration experiments, 100 g of CO ₂ -rich MEA solution with and without 5% by weight catalyst was poured into the reactor at room temperature and continuously heated until the desired temperature point, i.e., 86°C, was reached. The temperature was kept constant at 86°C to enable CO ₂ release.

5. Experimental results and interpretation

(1) C.O. ₂ Desorption performance

The CO ₂ desorption rate profile of the MEA solution with 5 wt.% raw catalyst and without ion-exchange catalyst is shown in Figure 4a. The baseline non-catalytic MEA solution was unable to desorb significant amounts of CO ₂ until it reached ~75°C, after which the CO ₂ desorption rate increased significantly. This shows that the MEA solvent regeneration process is highly dependent on temperature. A maximum CO ₂ desorption rate of 0.65 mmol/min was recorded for the non-catalytic MEA solution at 87°C and 24 min. The raw material Mont slightly improved the CO ₂ desorption rate and reached the highest CO ₂ desorption value of 0.72 mmol/min at 87°C. This is ~11% higher than the non-catalytic MEA solution. This slight improvement in CO ₂ desorption rate is mainly due to the presence of a small number of Lewis acid sites. Both ion-exchange catalysts accelerated the CO ₂ desorption rate and began desorbing significant amounts of CO ₂ after ~66°C. The SO ₄ -Mont catalyst solution achieved the highest CO ₂ desorption rate value of 1.18 mmol/min at 22 minutes and 85°C, which is 100% higher than the non-catalyst solution at the same point. The Zr-Mont catalyst solution also showed similar performance, improving the CO ₂ desorption rate in the temperature rise phase and reaching a peak CO ₂ desorption rate of 1.04 mmol/min at 85° C., which was approximately ~76° C. higher than the reference solution at the same point.

In addition to optimizing the _CO2 desorption rate, the catalytic solution desorbed a larger amount of _CO2 compared to the non-catalytic MEA solution (Figure 4b). Mont, the raw material, released 13.56% of CO ₂ , while SO ₄ -Mont and Zr-Mont catalysts desorbed 44.9 and 48.6% more during the regeneration experiment. The increase in the total amount of CO ₂ desorbed from the catalyst solution was actually due to the limitation of the time allowed for regeneration (1 hour) rather than a change in thermodynamic equilibrium. Adding a catalyst to the amine regeneration system improves the rate of _CO2 desorption, resulting in more _CO2 being released per unit of thermal energy provided per unit of time, ultimately lowering the thermal duty. The relative heat duty of the catalyst solution is shown in Figure 4c. As can be seen, the raw material Mont reduced the thermal duty by about 12% compared to the non-catalytic solution, and the SO ₄ -Mont and Zr-Mont catalysts reduced the thermal duty by 31.1 and 32.7%.

Additionally, the catalyst accelerated the CO ₂ desorption rate and desorbed larger amounts of CO ₂ as well as significantly shifted CO ₂ desorption to a lower temperature region (i.e., <86°C). The uncatalyzed and raw Mont solution desorbed ~37.4% of the total desorbed CO ₂ during the temperature ramping step (≤86°C). On the other hand, the SO ₄ -Mont and Zr-Mont catalyst solutions desorbed 51.6% and 44.3% of the total desorbed CO ₂ in the ramp step, respectively. In particular, increased _CO2 desorption rates at lower temperatures allow solvents to be regenerated in smaller columns, minimizing the dimensions of reboilers, stripper columns, and lean-rich heat exchangers.

It is interesting to note that the *SO ₄ -Mont catalyst desorbed a greater amount of CO ₂ in the ramp step, whereas the Zr-Mont catalyst desorbed a greater amount of CO ₂ in the isothermal step and the overall experiment. The main reason for this trend is that the SO ₄ -Mont catalyst has a higher number of Brønsted acid sites, which is important for optimizing the CO ₂ desorption rate at low temperatures. The Zr-Mont catalyst had slightly fewer Brønsted acid sites, but was more effective in the overall experiment due to its higher total acid sites (LAS + BAS) than the SO ₄ -Mont catalyst.

(2) Catalytic mechanism

_CO2 desorption from MEA solution occurs by carbamate decomposition through zwitterion formation, as shown in Equation 5. Carbamate decomposition mainly depends on the availability of protons, which can be provided to water in the proton transfer reaction of the protonated amine (Equation 6). However, since proton transfer is difficult under basic conditions, a huge amount of heat energy is required. Due to the unviability of protons and the endothermic nature of both reactions (Equations 5 and 6), the amine solvent regeneration operation is energy intensive.

(5)

(6)

(7)

(8)

(9)

A plausible reaction mechanism for ion-exchanged Mont-promoted carbamate decomposition and subsequent CO ₂ release is shown in Figure 5. When the ion-exchanged Mont catalyst is added to the MEA solution, it provides a free proton to the system that attacks the N atom of the carbamate. Zr metal can attack the O and N atoms of carbamate and help stretch NC bonds. Involvement of the acid site converts the sp2 hybridization of the NC bond to sp3, which subsequently stretches the NC bond and decomposes at low temperature. This new catalytic pathway with lower activation energy leads to _CO2 desorption at low temperatures and minimizes the thermal energy requirements of the _CO2 desorption reaction.

Additionally, as reported in previous studies, bicarbonate and carbonate are also present in the CO 2 -rich MEA solution upon CO ₂ loading above 0.42 mol _{CO 2} /mol MEA. Carbonates and bicarbonates can also accept protons from the ion-exchanged Mont catalyst to form carbonic acid and release CO ₂ directly (Equations 8 and 9). Regeneration of the catalyst is probably achieved by a proton restoration reaction provided by the amine deprotonation reaction.

(3) Catalyst stability test

The stability of the prepared catalyst was investigated by recycling all ion-exchanged catalysts and reusing them in five-cycle solvent regeneration experiments. A CO ₂ desorption profile was obtained by adding fresh catalyst to the CO ₂ -rich MEA solution and heating the solution to ∼86°C. After the experiment was completed, the catalyst was filtered to prevent amine vapor from attaching to the catalyst surface, washed repeatedly with water, and dried in an oven at 200°C for 2 hours. The average catalyst recovery was >95%, so the recovered catalyst was replenished with fresh dosage to maintain the catalyst amount at 5% by weight. Afterwards, the recovered catalyst was reused with fresh CO ₂ -rich MEA solution (100 mL) and solvent regeneration was studied under the same conditions. The catalyst was used in five periodic experiments in this way, and the catalyst regeneration performance for the total amount of desorbed CO ₂ is shown in Figure 6a. It can be clearly seen that after five cycles, the catalyst efficiency decreased by ~4% and ~7% for SO ₄ -Mont and Zr-Mont catalysts, respectively. The slight decrease in catalytic activity may be due to particle aggregation and/or regression induced by the stir bar and is consistent with published literature. Overall, it can be seen that the catalyst is stable and remains active during regeneration tests, showing its potential for use in continuous stripper columns.

(4) C.O. ₂ Influence of catalyst on adsorption performance:

Because thermal amine scrubbing is a continuous absorption/stripper process, when using catalyst regeneration techniques, it is equally important to determine whether these catalysts affect the _CO2 absorption performance of the solution. To find the answer, the present inventors tested CO ₂ absorption in 5M MEA with and without 5 wt.% of each catalyst. Feed gas (CO ₂ 15 vol.% balanced N ₂ , 40°C) was bubbled through the solution and the reactor exhaust gas was analyzed by GC. The experiment was performed until the CO ₂ composition of the exhaust gas was equal to the inlet CO ₂ composition. The concentration of degassed _CO2 as a function of time was recorded throughout the experiment. Once the amine solvent was saturated, the _CO2 loading of the solvent was also calculated using a TOC analyzer. The CO ₂ emissions curve over time is shown in Figure 6b. The results show that the presence of catalyst had no effect on CO ₂ absorption rate or absorption capacity. Identical CO ₂ -rich loadings were achieved in all solvents regardless of catalyst addition. These results are further justified by the fact that although the role of the catalyst in helping _CO2 desorption is clear, the catalyst regeneration route also requires a certain amount of thermal energy. At 40°C, the heat energy provided is much less than the energy level required to activate the catalyst. Therefore, under absorbent conditions, this catalyst does not exhibit any side effects on the CO ₂ absorption process.

In conclusion, an inexpensive montmorillonite clay catalyst was easily prepared by simple ion-exchange with H ₂ SO ₄ acid and Zr metal, and its catalytic activity in optimizing the CO ₂ desorption rate from amine solutions at low temperature was experimentally evaluated. The ion-exchange process leached Si and Al atoms from the octahedral sheets of raw material Mont and replaced most cations in the middle layer, significantly improving the BET specific surface area, surface acidity and total acidic sites. Both catalysts are effective in optimizing _CO2 desorption from MEA solutions, improving the _CO2 desorption rate by up to 196%, improving the total amount of desorbed _CO2 by 48.6%, and reducing the relative heat duty by 32.7% compared to the non-catalytic solution. reduced. Detailed characterization and experimental results showed that the role of increased BET specific surface area and total acid site was important in improving CO ₂ desorption efficiency. The stability of the catalyst was then tested by recycling and reusing the catalyst in five catalyst solvent regeneration experiments. This shows that the catalyst is completely stable and remains active during periodic use, demonstrating its scale-up ability. This study reports a new class of practical and abundant catalysts for optimizing CO ₂ desorption rates and highlights the role of catalysts and their accelerated CO ₂ desorption for efficient CO ₂ capture processes.

The scope of the present invention is indicated by the patent claims described below, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention.

Claims (4)

(a) Montmorillonite was reacted with 0.5 mol/L of an aqueous solution of at least one metal selected from ZrOCl ₂ , ZrCl ₄ and ZrOSO ₄ at 80°C, and after 12 hours, 0.5 mol/L of new ZrOCl ₂ , ZrCl ₄ and ZrOSO ₄ Obtaining a product by reacting with an aqueous solution of at least one selected metal for 4 hours; and
(b) filtering the product of step (a) and washing it until the pH of the filtrate is 5 to 9.
It contains a total acid site of 0.3 mmol/g or more, a surface area of 50 to 500 m ² /g, an average pore diameter of 4 to 13 nm, a surface acidity of 3.5 mmol/g or more, and Lewis and Bronsted acid sites. Method for producing an ion-exchanged montmorillonite catalyst for carbon dioxide capture comprising.
According to paragraph 1,
A method for producing an ion-exchanged montmorillonite catalyst for carbon dioxide capture, characterized in that the reaction in step (a) is carried out with stirring.
The ion-exchanged method of claim 1, wherein the montmorillonite and metal aqueous solution in step (a) are reacted at a ratio of 1:1 to 1:100 (weight (g): volume (ml)). Method for producing montmorillonite catalyst.
The method for producing an ion-exchanged montmorillonite catalyst for carbon dioxide capture according to claim 1, wherein the filtration in step (b) is performed after cooling to room temperature.