Synthesis of LTA Zeolite from Beach Sand: A Solution for CO₂ Capture

Abstract

Emissions caused by polluting gases, such as carbon dioxide, are one of the main contributors to the generation of the greenhouse effect that leads to global warming, responsible for climate change. An alternative to mitigating these emissions is the use of adsorbents capable of capturing CO₂. Zeolites are considered one of the most effective adsorbents in gas adsorption and separation technologies due to their high specific area and pore size and, consequently, greater adsorption capacity when compared to other commonly used materials. Despite this, reagents used in syntheses as the source of silica often make obtaining these materials more expensive. Seeking to overcome this limitation, in this work, materials (for CO₂ capture) were developed with a zeolitic structure using a low-cost alternative source of silica from beach sand called MPI silica to make the synthesis process eco-friendly. The crystallization time of the materials was studied, obtaining an LTA zeolite with MPI silica in a period of 1 h (ZAM 1 h), with a relative crystallinity of 74.26%. The materials obtained were characterized using the techniques of X-ray diffraction (XRD), X-ray fluorescence (XRF), absorption spectroscopy in the infrared region with Fourier transform (FTIR), scanning electron microscopy (SEM), and thermal analysis. The evaluation of the experimental adsorption isotherms showed that the zeolite LTA Aerosil^®200 (standard zeolite) and MP had adsorption capacities of 5.25 mmol/g and 4.83 mmol/g of CO₂, respectively. The evaluation of mathematical models indicated that the LTA zeolites fit the Temkin model best and had the same trend, with calculated adsorption capacities of 3.97 mmol/g and 3.75 mmol/g, respectively.

1. Introduction

CO₂ emissions have increased significantly since the Industrial Revolution. With the use of automotive vehicles, especially in recent decades, anthropogenic quantities have been increasing through the release of enormous amounts of CO₂ from the combustion of fossil fuels. Controlling the release of CO₂ into the atmosphere is one of the most challenging and urgent environmental issues facing the world today, necessitating measures to reduce the level of CO₂ in the atmosphere [1,2]. This causes climate change, which can increase the likelihood and severity of natural disasters such as wildfires, heat waves, droughts, and storms [3]. As a result, in recent years, several strategies have been developed to reduce the amount of atmospheric CO₂. Scientific research has mainly concentrated on the development of technologies aimed at reducing CO₂ emissions. There are several post-combustion CO₂ capture technologies that include liquid absorption, membranes, cryogenics, solid adsorption, and the calcium-looping process. In this context, the adsorption method is a technology seen as an alternative and viable form for the application of CO₂ capture, being one of the most widely used technological processes currently [4]. This is justified by characteristics such as high capture efficiency, easy regeneration, material stability, and the possibility of widely using waste to produce adsorbent materials, such as zeolites from fly ash rich in silicon and aluminum [5,6]. Therefore, CO₂ capture using solid adsorbents has aroused particular interest [2]. Commonly used commercial adsorbents are available in various forms, including activated carbon, porous silicates such as SBA-15 and MCM-41, metal–organic frameworks (MOFs), zeolites, metal oxides such as CaO and MgO, activated alumina, and silica gels, and are synthesized and modified to have varying porosity and surface area properties [4,7].

Among the various adsorbents for CO₂ capture, zeolites have interesting characteristics due to their textural, structural, and chemical properties, their high cation exchange potential, and their polarity properties. Furthermore, they have a high specific area, which is desirable, as it improves the diffusion of CO₂ molecules and increases the number of surface sites available for adsorption [2,8]. Despite these advantages, some of the reagents used in the synthesis of zeolites are still expensive, for example, the silicon source [9,10]. Silicon-based materials, such as synthetic mesoporous silicas, are synthesized from a silicate solution or silane reagents and silicon alkoxides, which are expensive; therefore, it is desirable to obtain low-cost sources of silicon [11,12,13]. In this context, some works found in the literature have used alternative silica sources to replace commercial ones, such as Carvalho et al. [14] and Sales et al. [15,16].

Zeolites are also silicon-based materials. Furthermore, they normally contain aluminum. Zeolites can be defined as microporous crystalline aluminosilicate materials with a three-dimensional network structure. Predominantly, they are constructed from a tetrahedral structure of [SiO₄]⁴⁻ and [AlO₄]⁵⁻. Zeolites are characterized by properties such as adsorption, ion exchange capacity, thermal and hydrothermal stabilities, and catalysis. These unique properties are widely used in many industrial applications [1,10,17].

The adsorptive affinity of materials with zeolitic structures for CO₂ molecules is caused by the interaction between their electric fields and the dipole and quadrupole moments of CO₂. Furthermore, the presence of several active sites and the porous structure of zeolites give them unique properties for selective adsorption to CO₂ molecules [1,10]. Another characteristic of zeolites is the structural stability that enables the reversible desorption process of CO₂ molecules at higher temperatures. Furthermore, these materials can be used in several CO₂ adsorption–desorption cycles [8]. Because of that, several types of synthetic and natural zeolites have been studied in the academic sphere to promote CO₂ capture and separation in recent years [2].

In this context, this paper contributes to the development of LTA-type zeolites from a low-cost silica source called MPI obtained from beach sand with a developed and patented methodology, BR102014025283-5, and a commercial silica source, Aerosil^®200, for CO₂ capture. The materials were characterized by XRD, XRF, FTIR, SEM/EDX, and thermogravimetric analysis. The adsorption mechanism for CO₂ capture was investigated by the application of the mathematical equilibrium equations from the Langmuir, Freündlich, and Temkin models.

2. Materials and Methods

2.1. Reagents

The materials used in this work were Silica MPI (Patent: BR102014025283-5, 90.62% Si), Commercial Silica (Aerosil^®200, St. Louis, MI, USA), Sodium Hydroxide P.A (Vetec-Sigma-Aldrich, St. Louis, MI, USA, 99%), Sodium Aluminate (Riedel-de Haën, Buchs, SG, Switzerland, Al₂O₃ (50%–56%) and Na₂O (40%–45%)), P.A Hydrochloric Acid (Synth, Diadema, SP, Brazil, 37%), and MilliQ Water.

2.2. Synthesis of Materials

MPI silica was synthesized according to the methodology described by Carvalho et al. [14]. Zeolites with an LTA structure were synthesized according to the IZA (International Zeolite Association) methodology with some modifications [18]. The first step consisted of synthesizing the zeolite with Aerosil^®200 silica via the following procedure:

• A solution containing 40 g of H₂O and 0.506 g NaOH was prepared, followed by its division into two fractions of equal volumes, V1 and V2.
• To V1, 3.860 g of sodium aluminate was added and stirred until complete homogenization.
• To V2, 7.471 g of H₂O, 3.079 g of NaOH, and 2.615 g of SiO₂ were added and stirred until complete homogenization.
• The V1 solution was poured into V2 and stirred for 30 min. The synthesized gel had the following molar composition: 2 SiO₂: 1 Al₂O₃: 3.2 Na₂O: 128 H₂O.
• The formed gel was transferred to Teflon autoclaves and subjected to static crystallization at 100 °C for 1, 2, 3, and 4 h.
• Finally, the LTA zeolites obtained were dried in an oven at 60 °C for 12 h. The second stage consisted of the synthesis of zeolites with MPI silica, which followed the same steps described previously, but with variations in the weight of sodium aluminate, 3.740 g, and silica, 2.762 g, to maintain the molar proportions. The LTA zeolite synthesis flowchart is described in Figure 1.

  Table 1. Nomenclature of synthesized materials.

  Nomenclature	Description
  ZAP 2 h	LTA zeolite with Aerosil®200 silica in 2 h
  ZAM 1 h	LTA zeolite with MPI silica in 1 h
  ZAM 2 h	LTA zeolite with MPI silica in 2 h
  ZAM 3 h	LTA zeolite with MPI silica in 3 h
  ZAM 4 h	LTA zeolite with MPI silica in 4 h

  contains the names of the synthesized zeolites.

2.3. Characterization of Materials

X-ray diffraction (XRD) was performed on Bruker D2Phaser equipment equipped with a Lynxeye detector, copper radiation (CuKα, λ = 1.5406 Å), a 2θ range of 3°–50°, a step of 0.01°, an acquisition time of 0.1 s, a central gap of 1 mm, a divergent gap of 0.6 mm, a Ni filter, a current of 10 mA, and a voltage of 30 kV. The relative crystallinity of the synthesized zeolites was calculated by summing the reflection intensities using Equation (1) [19].

$$\% \mathrm{C}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{\mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k}\mathrm{s} \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{o}\mathrm{f} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{X}\mathrm{R}\mathrm{D}}{\mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k}\mathrm{s} \mathrm{I}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{y} \mathrm{o}\mathrm{f} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{p}\mathrm{a}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{n} \mathrm{X}\mathrm{R}\mathrm{D}}}\times 100$$

(1)

X-ray fluorescence (XRF) was performed on a Bruker S2 Ranger device using Pd or Ag anode radiation with a max. energy of 50 W, a max. voltage of 50 kV, a max. current of 2 mA, and an XFlash^® Silicon Drift detector.

Fourier transform infrared absorption (FTIR) spectroscopy was performed on IRAffinity-1 equipment with total reflectance attenuation (RTA) from Shimadzu (Barueri, Brazil). All spectra were obtained at room temperature and in a wavenumber range of 450–4000 cm⁻¹.

Thermal analysis was carried out on a microbalance using TG-209-F1-Libra equipment (Netzsch, Weimar, Germany) with an alumina crucible using 10 mg of the sample with a continuous heating rate of 10 °C/min in oxygen purge gas at a flow rate of 20 mL/min.

Scanning electron microscopy (SEM) was performed using Auriga equipment, manufactured by Carl Zeiss, coupled to an energy dispersive X-ray (EDS) device, model Xflash Detector 410-M, manufactured by Bruker (Billerica, MA, USA).

2.4. CO₂ Capture

CO₂ adsorption was carried out on Micromeritics ASAP 2050 equipment (Norcross, GA, USA). The samples underwent thermal pretreatments that consisted of a ramp of 1 °C/min up to 60 °C; a first application of vacuum at a rate of 1.33 KPa/s from 101.31 KPa to 1.33 KPa; a second application of vacuum up to 0.006 KPa for 10 min; heating with a ramp of 10 °C/min up to 200 °C for 10 min under vacuum; and finally, a high vacuum of 0.0006 KPa for one hour before starting the analysis.

After carrying out this treatment and before starting the analysis, the samples were subjected to a high vacuum of 0.0006 KPa for 60 min. The analysis process occurred through pressure variation at a constant temperature, with 56 equilibrium pressures observed between 0.66 KPa and 999.86 KPa.

2.5. Mathematical Models

The mathematical models used to evaluate the behavior of the adsorption mechanism between the adsorbate and adsorbent involved in CO₂ capture were Langmuir, Freündlich, and Temkin. According to the Langmuir model (Equation (2)), the predominant adsorption mechanism involves a homogeneous distribution across the entire surface, with contribution from a monolayer of energetically equivalent sites, without the occurrence of interactions between adsorbed molecules. Equation (3) is the separation factor that indicates the favorability of the adsorptive process [20,21].

The Freündlich model (Equation (4)) indicates that the predominant mechanism is adsorption on the heterogeneous surface of the adsorbent through multilayers [22]. The Temkin model (Equation (5)) indicates that the predominant factor in adsorption is physical in nature [23]. The equations used in this work are contained in

Table 2. Mathematical equations and parameters.

Equations and Parameters of Mathematical Modeling
Model	Equation	Parameter	Unit
Langmuir	${\displaystyle \frac{\mathrm{P}}{{\mathrm{q}}_{\mathrm{e}}}}={\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{K}}_{\mathrm{L}}}}+{\displaystyle \frac{\mathrm{P}}{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}}$	1 qe, 2 qmax, 3 P e 4 KL	KPa/g, mmol/g and KPa and 1/KPa	(2)
	${\mathrm{R}}_{\mathrm{L}}={\displaystyle \frac{1}{1+{\mathrm{K}}_{\mathrm{L}}\mathrm{P}}}$	3 P e 4 KL	KPa and 1/KPa	(3)
Freündlich	$\log {\mathrm{q}}_{\mathrm{e}}=\mathrm{l}\mathrm{o}\mathrm{g}{\mathrm{k}}_{\mathrm{F}}+{\displaystyle \frac{1}{\mathrm{n}}}\mathrm{l}\mathrm{o}\mathrm{g}\mathrm{P}$	1 qe, 3 P, 5 KF e 6 n	mmol/g, KPa (mmol/g)·(1/KPa)1/n and Const.	(4)
Temkin	${\mathrm{q}}_{\mathrm{e}}=\beta \mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{t}}+\beta \mathrm{l}\mathrm{n}\mathrm{P}$	1 qe, 3 P, 7 β e 8 Kt	mmol/g, Const. and 1/KPa	(5)

¹ The amount of adsorbed adsorbate. ² A constant that expresses the maximum adsorption of the adsorbate. ³ Pressure at equilibrium. ⁴ The Langmuir constant. ⁵ The adsorption capacity. ⁶ The Freündlich constant. ⁷ A constant. ⁸ Adsorption capacity.

[20,24,25,26,27].

Among the parameters in the equations, q corresponds to the adsorption capacity and q_max to the experimentally calculated maximum adsorption capacity. The parameters K_F and n in the Freündlich equation are the empirical constants related to the adsorbent–adsorbent system at a specific temperature and indicate the affinity of the adsorbate molecule to the adsorbent surface and the lateral interaction between the adsorbed molecules and the energetic heterogeneity of the surface, respectively. The K_L parameter in the Langmuir equation is related to binding energy, and R_L refers to the favorable adsorptive process of the system. And, finally, the Temkin K_T and B_T parameters correspond to the adsorption capacity and the heat of adsorption [20,22,23,24,25,26,27].

3. Results and Discussion

The X-ray diffractogram patterns of the LTA zeolites synthesized from Aerosil^®200 and MPI silica at the crystallization times studied are shown in Figure 2, as well as the crystallinities obtained, taking ZAP 2 h as 100%. It is observed that the synthesized materials are similar, confirming the crystalline structure and characteristics of LTA zeolite [14,28,29]. The reflections chosen to calculate crystallinity were 2θ = 7.36°, 10.32°, 12.61°, 16.23°, 21.76°, 24.13°, 30.04°, and 34.2°, which are assigned to the hkl planes (200), (220), (222), (420), (442), (622), (642), (644), and (664), respectively.

The zeolites synthesized from MPI silica (ZAM) achieved lower crystallinity levels than ZAP 2 h, which occurs mainly when using natural materials. Taking atomization into account, the ideal synthesis condition in this work was 1 h, with a relative crystallinity of 74.26%. The tendency is for the crystallization time to have an important influence on the hydrothermal synthesis of zeolites. The increase in synthesis time favors the formation of LTA zeolite; however, in the long term, it will produce other undesirable crystalline phases, resulting in impure LTA zeolites [30]. This variation in crystallinity occurs due to successive dissolutions and recrystallizations of the tetrahedral sheets of the LTA zeolite, which are observed as variations in the intensities of the peaks in the diffractogram [31].

Table 3. Chemical composition of LTA zeolites with Aerosil^®200 silica (ZAP) and MPI silica (ZAM) at the times studied and the Si/Al ratio.

Chemical Composition (%)	ZAP	ZAM 1 h	ZAM 2 h	ZAM 3 h	ZAM 4 h	MPI
SiO2	46.80	47.73	45.36	47.61	47.70	90.62
Al2O3	37.13	35.89	35.48	35.34	35.69	2.25
Na2O	16.07	14.50	15.05	15.00	14.60	3.50
MgO	-	1.30	3.50	1.40	1.40	1.50
Cl	-	0.20	0.22	0.21	0.19	1.43
Others	-	0.38	0.39	0.44	0.42	0.7
Total	100	100	100	100	100	100
Si/Al	1.26	1.32	1.27	1.34	1.33	40.27

presents the chemical composition of the synthesized materials, as well as the MPI silica used as a silicon source. A similar composition is observed for the synthesized LTA zeolites. Those that used the MPI silica source contained other elements, such as MgO and Cl, coming from the MPI silica itself. These other elements may be responsible for the variations in the crystallinity and intensity of the crystalline phases of zeolites, as observed in the XRD diffractogram. The Si/Al molar ratios obtained are near to 1.00.

The main bands of LTA zeolites synthesized with Aerosil^®200 and MPI silica were identified by Fourier transform infrared spectroscopy (Figure 3). The broad absorption band ranging from 3671 cm⁻¹ to 293 cm⁻¹ and the band at 1628 cm⁻¹ can be attributed to the stretching of the OH group of adsorbed water molecules. The bands at 1363 cm⁻¹ and 1219 cm⁻¹ can be attributed to the asymmetric stretching vibrations of the Al-OH bond. The vibration bands at 977 cm⁻¹ and 663 cm⁻¹ correspond to the Si-O-Si and Si-O-Al asymmetric stretching of the TO₄ internal tetrahedral structure (where T = Si or Al) of the primary building unit, respectively [1,32,33,34].

The band at 547 cm⁻¹ is complex and connected to the symmetric stretching vibrations of Si-O-Si bonds and the bending vibrations of O-Si-O, which is known as the D4R bending vibration band (external vibration of the four double rings of the zeolite structure) and represents the secondary building unit in the LTA zeolite structure. The band at 463 cm⁻¹ indicates the bending vibration of O-Si-O. The range from 800 cm⁻¹ to 400 cm⁻¹ can be considered the fingerprint region for LTA zeolites. Generally, the absorption bands within the range of 420 cm⁻¹ to 500 cm⁻¹ are related to the T bending of the O-T single bond of the vibration mode (T = Al, Si), and those in the range of 950 cm⁻¹ to 1250 cm⁻¹ are related to the stretching vibration mode of the T-O-T single bond. The bands in the region from 650 cm⁻¹ to 500 cm⁻¹ are related to the presence of double rings (D4R and D6R) in the structures of zeolite materials [1,32,33,34].

The SEM micrographs of the materials are presented in Figure 4. The results indicated that the ZAP 2 h zeolites and those synthesized with MPI silica presented a morphology of cubic crystals with flat surfaces and well-defined edges that are characteristic of LTA zeolites [33,34]. In standard LTA, cubic crystals of different sizes and smaller than the other edges are observed. For materials obtained with MPI silica, crystals of different sizes are observed. Over time, the crystals and their sizes become more uniform.

The thermogravimetric and differential thermal curves of the materials are shown in Figure 5. The first event in the range of 25 °C to 214 °C, with a mass loss of 18.29% to 19.06%, is attributed to the removal of water and other volatile components present in the sample. The second event in the range of approximately 214 °C to 465 °C, with a mass loss of 2.97% to 3.15%, corresponds to the removal of water molecules adsorbed on the structure and dehydration, involving the formation of hydrate complexes with exchangeable cations [35,36]. These results indicate good thermal stability and similar characteristics for the zeolites obtained.

Since the materials synthesized using MPI silica have similar properties, we only used the ZAM 1 h material for CO₂ capture.

Application of Zeolites in CO₂ Capture

The adsorption isotherms for Aerosil^®200 and MPI silica are presented in Figure 6. The results showed that the ZAP 2 h zeolite presented the highest adsorption capacity with 5.25 mmol/g of CO₂. ZAM 1 h had an adsorption capacity of 4.83 mmol/g of CO₂, which is relatively close to that of commercial silica zeolite. The starting material, MPI silica, had the lowest adsorption capacity of 2.29 mmol/g of CO₂, indicating the gain in using zeolites for the adsorption of carbon dioxide.

Table 4. CO₂ adsorption capacities at low pressure and high pressure at 23 °C.

Adsorbent	Pressure (KPa)	Adsorbed Capacity (mmol/g)
ZAP 2 h	0.69	0.55
	1.27	0.82
	2.09	1.10
	2.67	1.26
	3.40	1.43
	4.72	1.67
	6.87	1.95
	10.09	2.24
	13.35	2.44
	20.04	2.72
	999.27	5.25
ZAM 1 h	0.70	0.60
	1.33	0.88
	1.98	1.11
	2.68	1.29
	3.44	1.46
	4.82	1.69
	6.84	1.93
	10.13	2.20
	13.22	2.37
	20.00	2.62
	992.85	4.83
MPI	0.67	0.03
	1.41	0.07
	2.10	0.10
	2.99	0.13
	3.82	0.16
	4.97	0.18
	6.94	0.22
	10.16	0.27
	13.64	0.30
	20.16	0.36
	992.75	2.29

contains results for the CO₂ adsorption capacity of the materials at low pressures up to 20 KPa and at the maximum pressure reached, 1000 kPa, at a temperature of 23 °C. The results show the influence of pressure in increasing the CO₂ adsorption capacity.

Table 5. Previous work found in the literature on materials used in CO₂ capture.

Adsorbent	Conditions	Adsorbed Capacity (mmol/g)	References
ZAP 2 h	23 °C	5.25	This work
ZAM 1 h	23 °C	4.83	This work
MPI	23 °C	2.29	This work
zeolite LTA (Na-e CaLTA)	25 °C	3.10, 3.30	[2]
zeolite LTA spherical (LTA-P1)	25 °C	4.48	[37]
ZSM-5 (Na [Co] ZSM-5)	25 °C	5.33	[38]
zeolite MOR	25 °C	3.20	[17]
zeolite Y (ZY and PDY-7: hierarchical)	25 °C	4.50, 5.40	[10]
SAPO-34	24 °C	3.00, 3.40	[39]
Coal	0.00 °C, 25 °C	5.00, 2.03	[21]

presents the materials found in the literature used to capture CO₂ with their respective adsorption capacities. Some studies have results that are inferior to, similar to, or better than those obtained in this work. On the other hand, this study used a low-cost source that promotes new variations in the characteristics of zeolites, as observed in the morphology, which can still be explored with modifications to these materials to improve adsorption capacity.

A mathematical adjustment study was performed using the linear Langmuir, Freündlich, and Temkin models to evaluate the relationship between the adsorbate and adsorbent (Figure 7).

Table 6. Parameters and coefficients of determination of the Langmuir, Freündlich, and Temkin isothermal mathematical models for MPI silica and synthesized LTA zeolites.

Models	Parameters
		MPI	ZAP 2 h	ZAM 1 h
Langmuir	qm (mmol/g)	0.9955	3.9793	3.7509
	KL (1/KPa)	0.0054	0.0215	0.2446
	R2	0.9876	0.9662	0.9604
	RL	0.0024	0.0006	0.0005
Freündlich	KF (mmol/g)·(1/KPa)1/n	0.0752	0.3458	0.8759
	1/n	0.4807	0.2359	0.2189
	R2	0.9864	0.9144	0.9132
Temkin	KT (1/KPa)	0.0224	0.2927	0.2105
	β	0.3082	0.6157	0.5493
	BT (KJ/mmol)	7960.5	3985.4	4466.6
	R2	0.8116	0.9928	0.9937

presents the parameters obtained by the mathematical models. The results indicated that the highest adsorptive capacity (qm) was for zeolite synthesized with Aerosil^®200 silica: 3.97 mmol/g of CO₂. The one synthesized with MPI silica had an adsorption capacity of 3.75 mmol/g of CO₂. MPI silica had the lowest adsorption capacity, with 0.99 mmol/g of CO₂. All synthesized materials have a separation factor, R_L, between 0 and 1, indicating that they are favorable for CO₂ adsorption processes.

MPI silica had the best fit to the Langmuir model, with a correlation coefficient R² = 0.9876, indicating that the predominant adsorption mechanism involves a homogeneous distribution across the entire surface. Monolayer adsorption acts on a restricted number of adsorbent sites that are energetically equivalent. Furthermore, there is no interaction between the adsorbed molecules [20,21]. The second-best fit was to the Freündlich model, with a correction coefficient R² = 0.9864. Higher K_F values mean greater affinity for a specific adsorbate at a specific temperature. The value of 1/n was greater than 0 and less than 1 (0 < 1/n < 1), showing that the adsorption process is favorable, in agreement with the separation factor R_L [25,26]. The MPI silica did not fit the Temkin model, with a coefficient R² = 0.8116.

The LTA zeolites ZAP 2 h and ZAM 1 h fit the Temkin model best, with correlation coefficients R² = 0.9928 and R² = 0.9937, respectively. The Temkin constant (B_T) values were lower than 8000 Kj.mmol⁻¹, indicating that the predominant factor in adsorption is physical in nature [23]. In relation to the Freündlich model, higher K_F values mean greater affinity for a specific adsorbate at a specific temperature, whereas the 1/n values were all greater than 0 and less than 1 (0 < 1/n < 1) for adsorbents, indicating that the adsorption process is favorable [25,26]. For the Langmuir model, there was also a good fit, with correlation coefficients of R² = 0.9662 and R² = 0.9604, respectively, indicating that the adsorption sites are uniformly distributed over the entire adsorbent surface, that the adsorption capacity is limited only by the monolayer, and that there are no interactions between adsorbed molecules [20,26].

Although the calculated theoretical adsorption capacity is lower than those obtained from the experimental isotherm curves, the order of the best capacities for each material type remained the same. Modeling is important for understanding how the adsorption process occurs and which factors are predominant.

The results presented indicate that the synthesized zeolites and MPI silica are favorable for the CO₂ adsorption process, as shown by the separation factor R_L, and adsorption occurs via a mechanism that acts predominantly when we have a better fit to the Temkin or Langmuir or Freundlich model. The structural and microporous differences and specific surface areas of the zeolites can affect the adsorption mechanism and the adsorption capacity [40]. Thus, it is verified that there is variation in fit to the mathematical model that may indicate that there are several mechanisms acting concomitantly, which explains why an adsorbent fits better to one model but is very close to fitting another.

Another factor that may influence the adsorption capacity is the crystallization time, which interferes with the relative crystallinity. According to [41], temperature and aging time in the synthesis of zeolites influence the development of the micropore network, which leads to increased crystallinity when the aging temperature is raised. This implies an improvement in the CO₂ adsorption capacity of zeolites compared to MPI silica, which is amorphous. In this work, the aging temperature, time, and crystallization temperature were kept fixed, and the crystallization time of the zeolites was varied according to the methodology, which influenced the crystallinity and, probably, the CO₂ adoption capacity and best-fitting mathematical model.

4. Conclusions

MPI silica from beach sand was successfully applied in the eco-friendly synthesis of a zeolitic material, LTA zeolite, used for CO₂ capture.

The X-ray diffractogram patterns indicated that the synthesized zeolites have a similar crystalline structure to the LTA zeolite. The zeolites synthesized with MPI silica achieved lower crystallinity rates than ZAP 2 h, with the ideal synthesis condition being a time of 1 h with a relative crystallinity of 74.26%. LTA zeolites have a similar chemical composition, with Si, Al, and Na as the main elements present. In zeolites used with MPI silica, the presence of other elements, such as MgO and Cl, originating from MPI silica is observed. The Si/Al molar ratios obtained are approximately equal to 1.00. FTIR analyses showed the presence of all characteristic bands of LTA zeolite. The micrographs indicated the characteristic morphology of cubic crystals with well-defined edges of different sizes, and with longer times, the crystal size became uniform. Thermal analysis showed good thermal stability and similar characteristics of the synthesized zeolites.

The experimental results of the adsorptive study indicated that the synthesized LTA zeolite ZAP 2 h and ZAM 1 h had adsorption capacities of 5.25 mmol/g of CO₂ and 4.83 mmol/g of CO₂, respectively. Mathematical modeling indicated that the LTA zeolites best fit the Temkin model, with physical adsorption being the preponderant adsorption mechanism for this adsorbent and capacities of 3.97 mmol/g of CO₂ and 3.75 mmol/g of CO₂, respectively, indicating the same trend in the experimentally obtained results. The results of this work indicate the importance of obtaining zeolites from a low-cost silica source and optimizing synthesis parameters to obtain eco-friendly and lower-cost materials capable of adsorbing CO₂ and contributing to mitigating emissions of this polluting gas.