Synthesis MFI Zeolites Using Alternative Silica Source for CO₂ Capture

Abstract

In recent years, climate change has attracted the attention of the scientific community. These changes are attributed to human action, which is responsible for the emission of polluting gases, mainly through the burning of fossil fuels, deforestation, and industrial processes that are responsible for the greenhouse effect. Post-combustion CO₂ capture using solid adsorbents is a technology that is currently gaining prominence as an alternative and viable form of capture to other industrial processes used. Zeolites are adsorbents capable of capturing CO₂ selectively due to their properties such as textural properties, high surface area, and active sites. In this context, this work developed materials with a zeolite structure with an alternative low-cost silica source from beach sand, called MPI silica, to make the process eco-friendly. Crystallization time studies were carried out for materials containing MFI-type zeolites with MPI silica with a time of 15 h (ZM 15 h) and 3 days (SM 3 d), with relative crystallinities of 92.90% and 111.90%, respectively. The synthesized materials were characterized by several techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), the textural analysis of N₂ adsorption/desorption isotherms, 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 best results were for the zeolites synthesized in the basic medium, namely ZMP 3 d, ZM 10.5 h, and ZM 15 h, with capacities of 3.72, 3.10, and 3.22 mmol/g of CO₂, respectively, and in the hydrofluoric medium, namely SP 9 d, SM 3 d, and SM 6 d, with capacities of 3.94, 3.78, and 3.60 mmol/g of CO₂, respectively. The evaluation of the mathematical models indicated that the zeolites in the basic medium best fitted the Freündlich model, namely ZMP 3 d, ZM 10.5 h, and ZM 15 h, with capacities of 2.56, 1.68, and 1.87 mmol/g of CO₂, respectively. The zeolites in the hydrofluoric medium are adjusted to the Langmuir model (SP 9 d and SM 3 d) and Temkin model (SM 6 d), with capacities of 3.79, 2.23, and 2.11 mmol/g of CO₂, respectively.

1. Introduction

Climate change has been attracting increasing attention and causing concern in recent years. These changes have been attributed to human actions that have been responsible for the excessive emission of greenhouse gases into the atmosphere, with CO₂ being one of the main contributors. The CO₂ released is mainly caused by the burning of fossil fuels, deforestation, and various industrial processes [1,2]. Several technologies are used in post-combustion CO₂ capture, such as solid adsorption, liquid absorption, the calcium-looped process, membranes, and cryogenics. Post-combustion CO₂ capture is currently gaining prominence as one of the most applicable methods for preexisting sources, as it involves capturing CO₂ during the combustion process. Adsorption is a methodology seen as an alternative and viable form of CO₂ capture application [3].

Adsorbents are solid materials capable of absorbing and retaining different materials when compared to other CO₂ capture processes [3]. Solid adsorbent materials are currently considered the most promising research area for CO₂ capture applications. This is justified by the characteristics that these materials have, such as easy regeneration, material stability, high capture efficiency, and the possibility of using waste from different sources for your synthesis [4]. The adsorbents generally used in CO₂ capture are activated carbons, alumina, metal–organic frameworks (MOFs), porous silicates such as SBA-15 and MCM-41, zeolites and metal oxides (Cao and MgO), and silica [2,3,4].

Among these adsorbents, zeolites stand out, which are microporous aluminosilicate crystals with three-dimensional network structures, predominantly built from tetrahedral structures of [SiO4]⁴⁻ and [AlO4]⁵⁻ [5]. These adsorbents present interesting characteristics such as their textural properties, cation exchange capacity, structural, chemical properties, and polarity properties. In addition, they have fast absorption kinetics and regenerability, high surface area, and active sites. This allows different synthetic and natural zeolites to be widely studied for CO₂ capture [6,7]. The affinity of zeolites for CO₂ molecules occurs through the interaction between the zeolite’s electric field and the dipole and quadrupole moments of the CO₂ molecules [5].

In this context, this work contributes to the development of MFI-type materials with zeolite structures, using a low-cost silicon source called MPI obtained from beach sand, with a developed and patented methodology, BR102014025283-5, and a commercial silica source, Aerosil^®200, to promote CO₂ capture. The synthesized materials were characterized by several techniques, such as XRF, XRD, thermogravimetric analysis, FTIR, and SEM. In addition, the adsorptive mechanism involved in CO₂ capture was investigated with the application of the mathematical models of Langmuir, Freündlich, and Temkin.

2. Materials and Methods

2.1. Reagents

The reagents used in this work were silica MPI (patent: BR102014025283-5, 90.62% Si, 2.5% Al₂O₃, and 3.5% Na₂O), Ammonium Fluoride (ACS-NH₄F) (Sigma-Aldrich, St. Louis, MI, USA, ≥98%), commercial silica (Aerosil^®200, St. Louis, MI, USA), Sodium Hydroxide P.A (Vetec-Sigma-Aldrich, St. Louis, MI, USA, 99%), Tetrapropylammonium hydroxide solution—TPAOH (Sigma-Aldrich, St. Louis, MI, USA, 1.0 M in H₂O), sodium aluminate (Riedel-de Haën, Buchs, SG, Switzerland, Al₂O₃ (50–56%) and Na₂O (40–45%)), Hydrochloric Acid P.A (Synte, Diadema, SP, Brazil, 37%), Sulfuric Acid P.A (Modern Chemistry, Barueri, SP, Brazil, 97%) Tetrapropylammonium Bromide—TPABr (Sigma-Aldrich, St. Louis, MI, USA, 98%), ethyl alcohol (Modern Chemistry, Barueri, SP, Brazil, 98%), and MilliQ Water.

2.2. Synthesis of Materials

MPI silica was synthesized by the methodology described by Carvalho, 2015 [8]. The synthesis of MFI-structured zeolites in a basic medium was based on the method proposed by Mignoni, (2007) [9] with some modifications for synthesis with Aerosil^®200 silica and MPI. The first phase was the synthesis of zeolites with MFI structures using Aerosil^®200 silica in the following steps:

• Ready a solution with 10 g of H₂O and 1.018 g of NaOH and stir until complete homogenization occurs.
• Add 0.257 g of sodium aluminate to this solution.
• Ready a solution with 27.277 g of H₂O and 0.133 g of TPABr and add to the solution obtained in step 1.
• Add 7.941 g of ethyl alcohol, acting as structure co-director.
• Add 5.000 g of Aerosil^®200 silica and 1.073 g of H₂SO₄
• Keep the gel under stirring for 30 min. The synthesis gel contains the following molar composition: 1 SiO₂: 0.012 Al₂O₃: 0.301 Na₂O: 0.006 TPABr: 24.927 H₂O: 2.72 C₂H₅O: 0.132 H₂SO₄.
• Place the gel in a stainless-steel autoclave and keep it in a rotating oven for the crystallization times studied at 150 °C.
• Vacuum-filter the solid obtained with distilled water.
• Dry in an oven at 80 °C for 12 h.
• Finally, calcine at 550 °C for 6 h.

The second stage was the synthesis of zeolites with an MFI structure using MPI silica, which consisted of the same steps already mentioned, but with variations in the mass, namely 1.816 g of NaOH, 0.021 g of sodium aluminate, 5.425 g of MPI silica, and 1.840 g of H₂SO_4, to maintain the molar ratio. The synthesis gel contained the following molar composition: 1 SiO₂: 0.012 Al₂O₃: 0.301 Na₂O: 0.006 TPABr: 24.927 H₂O: 2.722 C₂H₅O: 0.132 H₂SO₄.

Table 1. Description of the nomenclature of MFI-structured zeolites synthesized in the basic medium.

Nomenclature	Description
ZMP 3 d	Aerosil®200 silica in 3 days
ZM 9.5 h	MPI silica in 9.5 h
ZM 10.5 h	MPI silica in 10.5 h
ZM 11.5 h	MPI silica in 11.5 h
ZM 12 h	MPI silica in 12 h
ZM 12.5 h	MPI silica in 12.5 h
ZM 13.5 h	MPI silica in 13.5 h
ZM 15 h	MPI silica in 15 h
ZM 7.5 h	MPI silica in 7.5 h

contains the nomenclature of the synthesized zeolites. In the Supplementary Material, Figure S1 of the synthesis methodology is shown.

The synthesis of silicalite and MFI-structured zeolites in the hydrofluoric medium was based on the methodology of Vinaches (2015) [10] and consisted of the following steps:

• Ready a solution by adding 24.960 g of TPAOH and 15.000 g of water until complete homogenization.
• Then, slowly add 3.000 g of Aerosil^®200 or MPI silica and keep under constant stirring for 2 h.
• In parallel, prepare another solution containing 1.092 g of water and 0.925 g of NH₄F and keep under stirring for homogenization.
• At the end of 2 h, add the second solution to the first and continue for another 30 min. The synthesis gel for silicalite with Aerosil^®200 silica and MFI-structured zeolites with MPI silica contains the following composition: 2 SiO₂: 80 H₂O: 1 NH₄F: 1 TPAOH.
• Insert the resulting gel into a stainless-steel autoclave in a static oven and maintain at 150 °C during the crystallization time being studied.
• Vacuum-filter the resulting solid with distilled water.
• Dry the resulting solid in an oven at 80 °C.
• Finally, calcine at 550 °C for 6 h.

Table 2. Description of the nomenclature of silicalite and MFI-structured zeolites synthesized in the hydrofluoric medium.

Nomenclature	Description
SP 9 d	Aerosil®200 in 9 days
SM 3 d	MPI silica in 3 days
SM 5 d	MPI silica in 5 days
SM 6 d	MPI silica in 6 days
SM 7 d	MPI silica in 7 days
SM 9 d	MPI silica in 9 days
SM 12 d	MPI silica in 12 days

contains the nomenclature of the synthesized zeolites. In the Supplementary Materials, Figure S2 of the synthesis methodology is shown.

2.3. Caracterization of Materials

X-ray fluorescence (XRF) was obtained on a Bruker S2 Ranger (Billerica, MA, USA) device using Pd or Ag anode radiation (max. Energy, 50 W; max. voltage, 50 kV; max. current, 2 mA; and detector, XFlash^® Silicon Drift).

X-ray diffraction (XRD) was performed on Bruker D2Phaser equipment (Bruker AXS, Madison, WI, USA) 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 0.3 s, an airscatter screen module, a divergent gap of 0.6 mm, a Ni filter, a current of 10 mA, and a voltage of 30 kV. The relative crystallinity results of the synthesized materials can be calculated from the sum of the reflection intensities [11] or using the integrated area of the peaks. The data on the area of the integrated peaks in the 2θ range of 22.5 to 25.0° were used.

$$\% \mathrm{C}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{\sum \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k} \mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}{\sum \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k} \mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a} \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{p}\mathrm{a}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{n}}}\times 100$$

(1)

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

Nitrogen adsorption/desorption isotherms at 77 K were obtained to determine the textual parameters of the materials using Micromeritics ASAP 2020 equipment (Atlanta, GA, USA). The specific surface area (SBET) was calculated using the Brunauer–Emmett–Teller (BET) equation, and the pore size and volume were determined, respectively, by applying the Barrett–Joyner–Halenda (BJH) method and pressure at p/p⁰ = 0.986. Initially, a pretreatment of 60 °C for 1 h, at 5 °C/min, of approximately 0.1 g of sample was performed. Subsequently, the temperature was increased to 300 °C at 10 °C/min under vacuum for 10 h.

Thermal analysis was carried out on a microbalance using the TG-209-F1-Libra equipment (Netzsch, Weimar, Germany) using an alumina crucible using 10 mg of the samples 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 carried out using Auriga equipment, manufactured by Carl Zeiss (Carl Zeiss, Oberkochen, Germany), coupled to an energy dispersive X-ray (EDS) device, model Xflash Detector 410-M, manufactured by Bruker (Bruker AXS+, Madison, WI, USA).

2.4. CO₂ Capture

CO₂ adsorption was carried out on Micromeritics ASAP 2050 equipment (Norcross, GA, USA). Analysis was carried out in the following steps:

• Pre-treatment of the sample to remove gases, and degassing, which consists of the thermal heating of the sample using a ramp of 1 °C/min up to 60 °C.
• Application of vacuum using a rate of 1.33 KPa/s from 101.31 KPa to 1.33 KPa.
• New application of vacuum up to 0.006 KPa for 10 min.
• Heating using a heating ramp at 10 °C/min to 200 °C for 10 h under vacuum.
• Application of a high vacuum of 0.0006 KPa to the sample for 1 h before starting the analysis procedure. The analysis was processed through constant pressure and temperature variation with 56 equilibrium pressures between the range of 0.66 KPa and 999.86 KPa.

2.5. Mathematical Models

The mathematical models used in this work were Langmuir, Freündlich, and Temkin, aiming to study the behavior of the mechanisms involved in the adsorptive process between the adsorbate and adsorbent in CO₂ capture. The Langmuir model (Equation (2)) suggests that the predominant adsorption mechanism indicates a homogeneous distribution on the surface. Furthermore, the monolayer has equivalent energy sites during the adsorption process, without any interaction between the molecules involved in adsorption. Equation (3) depicts the separation factor that indicates whether the adsorptive process is favorable or not for an adsorbate [12,13]. The Freündlich model (Equation (4)) indicates that the heterogeneous surface of the adsorbent is the predominant mechanism acting in the adsorption process, which occurs through multilayers [14]. The Temkin model (Equation (5)) states that the adsorptive process is physical in nature, and this acts as the predominant factor [15]. The equations used in this work are described in

Table 3. Mathematical models with their respective equations and parameters.

Equations and Parameters of Mathematical Modeling
Mathematical Model	Equations	Parameters	Units
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 Amount of adsorbed adsorbate. ² Constant that expresses the maximum adsorption of the adsorbate. ³ Pressure on equilibrium. ⁴ Langmuir constant. ⁵ Adsorption capacity. ⁶ Freündlich constant. ⁷ Constant. ⁸ Adsorption capacity.

[12,15,16,17,18,19].

The parameter q expressed in the equations corresponds to the adsorption capacity. The parameter q_max corresponds to the maximum adsorption capacity experimentally obtained. K_L is the Langmuir constant and corresponds to the binding energy; the separation factor, R_L, is related to the favorability of the system. The Freündlich parameters K_F and n refer to the behavior of the adsorbent–adsorbent system at a specific temperature and the interaction between the adsorbed molecules and the surface energy, respectively. Finally, the Temkin parameters K_T and B_T correspond to the heat of adsorption and the adsorption capacity, respectively [12,15,16,17,18,19].

3. Results and Discussion

3.1. Characterizations of MFI-Structured Zeolites in Basic Medium

The X-ray diffractograms of the MFI-structured zeolites synthesized from Aerosil^®200 and MPI silica at the studied crystallization times are shown in Figure 1. The standard used is the zeolite synthesized with commercial silica Aerosil^®200. Zeolites synthesized with MPI silica were compared with this standard to calculate the relative crystallinity, considering the ZMP 3 d as 100%. It is observed that the synthesized materials are similar, confirming the crystal structure and characteristics of the MFI-structured zeolites. The reflections for calculating crystallinity were obtained in the 2θ range from 22.5 to 25°. The reflections correspond to the planes (501), (151), (051), and (303) in MFI-type structures [20,21,22].

The results for the zeolites with MPI silica had lower crystallinity levels than ZMP 3 d, a characteristic behavior mainly when using natural materials. Thus, considering the optimization, the ideal synthesis condition in this work was 15 h, with a crystallinity of 86%. The crystallization time has an important influence on the synthesis of zeolites. Increasing synthesis time favors the formation of the MFI zeolite; however, in the long term, it will produce other undesirable crystalline phases [23]. Variations in the crystallinity of materials at high synthesis times may occur because some of the crystals present may undergo dissolution and recrystallization processes [24]. For the samples presented in Figure 1, we did not observe such an effect because we can see that the relative crystallinity increases linearly over time. The intensity of the peaks increases with the Si/Al ratio. This phenomenon is in accordance with a higher structural order due to the increasing number of equal Si-O-Si bonds in tetrahedral positions, thus decreasing the average length of the T-O-T bond [25].

The XRF data of the zeolites synthesized with Aerosil^®200 silica (ZMP 3 d) and MPI (ZM) are presented in

Table 4. XRF results of MFI-structured zeolites in the basic medium synthesized with Aerosil^®200 silica and MPI at the main times studied, MPI silica, and Si/Al ratio.

Chemical Composition (%)	ZMP 3 d	ZM 7.5 h	ZM 9.5 h	ZM 10.5 h	ZM 11.5 h	ZM 12 h	ZM 12.5 h	ZM 13.5 h	ZM 15 h	MPI
SiO2	90.06	92.67	93.54	94.24	94.06	94.20	94.15	93.34	90.89	90.62
Al2O3	6.72	3.07	2.48	2.43	2.50	2.49	2.52	2.67	3.88	2.25
Na2O	3.22	1.80	1.90	1.40	1.50	1.40	1.40	1.80	2.60	3.50
MgO	-	1.50	1.20	1.00	1.00	1.00	1.00	1.20	1.60	1.50
Cl	-	0.09	0.07	0.08	0.09	0.08	0.09	0.07	0.12	1.43
Others	-	0.87	0.81	0.85	0.85	0.83	0.84	1.12	0.91	0.70
Total	100	100	100	100	100	100	100	100	100	100
Si/Al	13.40	30.18	37.71	38.78	37.62	37.83	37.36	34.95	23.42	40.27

. The results indicate that the MFI-structured zeolites in the basic medium synthesized with MPI silica presented a similar composition to the main oxides obtained with ZMP 3 d, regardless of the crystallization time studied. The percentage variations would be associated with the characteristics of the alternative source of MPI silica.

The Si/Al molar ratios of ZM zeolites increase compared to zeolite with Aerosil^®200 silica, and, with the increasing crystallization time, variations in the Si/Al ratio occurred. MPI silica has a high percentage of silicon in its composition, which implies a high Si/Al ratio, which makes it suitable for the synthesis of MFI-structured zeolites that have a high Si/Al ratio, ranging from 15 to infinity. In this work, this ratio varied from 30.18 to 38.78, which is in the range of results found in the literature, such as those reported by Sivalingam and Sen, (2020) [26], who obtained a Si/Al ratio of 32.24.

The main bands of MFI-structured zeolites in the basic medium sintered with Aerosil^®200 silica (ZMP 3 d) and MPI (ZM) at the times studied were identified by FTIR, as can be seen in Figure 2. The band at 1632 cm⁻¹ is attributed to the stretching of the water molecule that was absorbed on the surface of the zeolite structure. The band at 1222 cm⁻¹ is related to the asymmetric bonds of siliceous materials in the T-O-T group of the TO4 tetrahedron. The bands at 1066 cm⁻¹ and 794 cm⁻¹ are attributed to the symmetric stretching of outer TOT bonds and the asymmetric stretching of inner TOT bonds, respectively. The band at 540 cm⁻¹ represents the vibrations from the deformation of the TO bond of the internal tetrahedral of SiO₄ and AlO_4, and the vibrations at 427 cm⁻¹ correspond to the asymmetric stretching of the two-membered rings (D5R) of the pentassyl structure of zeolites with an MFI structure and characteristics, which states the formation of its structure [27,28,29,30]. FTIR analysis thus suggests the successful formation of zeolites with MFI-type structures [31].

The SEM images of the synthesized MFI-structured zeolites in the basic medium are shown in Figure 3. The results indicate that the zeolite synthesized with Aerosi^®200 (a) ZMP 3 d silica exhibits agglomerated hexagonal clintheriform crystals with miscellaneous particle size distribution. The zeolites obtained from MPI silica at the times studied, namely (b) ZM 7.5 h, (c) ZM 9.5 h, (d) ZM 10.5 h, (e) ZM 11.5 h, (f) ZM 12 h, (g) ZM 12.5 h, and (i) ZM 15 h, also exhibit crystals with agglomerated hexagonal clintheriform morphology with varying size distributions, being smaller than those synthesized with Aerosil^®200 silica [24,32]. Furthermore, it is possible to see clusters of amorphous silica (images highlighted in red micrographs of Figure 3) with an irregular distribution that probably did not react during the synthesis or are the result of the dissolution of the tetrahedra of the zeolitic structure. The particle size does not change significantly with the extension of the crystallization time.

The results of the thermogravimetric and differential thermal curves of MFI-structured zeolites in the basic medium are presented in Figure 4, (a) with Aerosil^®200 silica (ZMP 3 d) and with MPI silica, at different times, namely (b) ZM 7.5 h, (c) ZM 9.5 h, (d) ZM 10.5 h, (e) ZM 11.5 h, (f) ZM 12 h, and at times (g) ZM 12.5 h and (h) ZM 13.5 h. Two or three mass loss events can be observed in the gravimetric curves, which vary according to the synthesis time. For ZMP 3 d, we have the first event that occurs in the range of approximately 25 °C to 229 °C with a mass loss of 5.05%, corresponding to the desorption of H₂O. The second event is in the range of approximately 229 °C to 378 °C, with a mass loss of 0.77%, and the third event is in the range of 378 °C to 526 °C, with a mass loss of 1.84%, corresponding to the degradation of the organic director TPABr [27]. Zeolites synthesized with MPI silica present similar events and mass losses to zeolites with Aerosil^®200 silica. These results indicate that calcination at 550 °C can eliminate TPABr from zeolites with an MFI structure [33].

Figure 5 shows the N₂ adsorption/desorption isotherms for the zeolite with Aerosil^®200 silica, ZMP 3 d, and those synthesized with MPI silica, ZM 10.5 h and ZM 15 h.

Table 5. Textural properties of MFI-structured zeolites in the basic medium with Aerosil^®200 silica and MPI.

Zeolite	SBET (m2/g) 1	Sext (m2/g) 2	Smicro (m2/g) 3	Vtotal (cm3/g) 4	Vmicro (cm3/g) 5	Dpm (nm) 6
ZMP 3 d	380	79	300	0.168	0.119	0.406
ZM 10.5 h	284	111	173	0.167	0.070	0.451
ZM 15 h	299	119	180	0.162	0.073	0.458

¹ Specific area (determined through BET calculation). ² External specific area (determined through the t-Plot method). ³ Micropore area (determined through the t-Plot method). ⁴ Total pore volume (Obtained for p/p₀ = 0.99). ⁵ Micropore volume (determined through the t-Plot method). ⁶ Average pore diameter (determined through the Horvath–Kawazeo method).

contains the textural parameters. The results show that the isotherms of the materials are a combination of type I and II that have micropore characteristics and a contribution of the external area and volume between particles (typical of porous or macroporous materials) [34,35,36,37]. Furthermore, the intercrystalline pores of zeolites can be formed through aggregates of nanometric zeolitic crystals. The intercrystalline mesopores may be formed via the aggregation of nanosized zeolite crystals [38]. Another characteristic present in the isotherms is high N₂ adsorption at relatively low pressures, which can be attributed to the intrinsic micropores of the zeolites [39].

The zeolite with Aerosil^®200 silica has a larger area of 380 m²/g, a total volume of 0.168 cm³/g, and micropores of 0.119 cm³/g, which are larger than zeolites from MPI silica but have a smaller average pore diameter. The modification with silica directed the formation of mesopores, consequently causing a smaller micropore volume. The total pore volume remained almost constant, indicating that the modification did not change this parameter [40]. Usually, when the crystallization of the zeolite exceeds the aging stage and enters the crystallized stage, the value of the specific micropore area is directly related to the crystallinity; this would be the reason for the more significant number of micropores of the ZMP 3 d zeolite, followed by the ZM 15 h zeolite [41]. Comparing the zeolites from MPI silica, ZM 15 h had more significant textural parameters, except for the total pore volume, but was approximately the same. Therefore, these results of the textural parameters are consistent with the results obtained from the calculated crystallinity and phases observed in the XRD. The materials with higher percentages of more crystallinity present higher textural parameters.

3.2. Characterizations of MFI-Structured Zeolites in Hydrofluoric Medium

The X-ray diffractograms of the MFI-structured zeolites synthesized from Aerosil^®200 and MPI silica are shown in Figure 6, as well as the relative crystallinity, with SP 9 d taken as 100%. All the synthesized zeolites have the following reflections: 2θ = 7.80°, 8.86°, 14.64°, 22.93°, 23.79°, and 24.18°, which include the hkl planes (011), (200), (051), (501), (033), and (313), respectively, of the pentasil structure of MFI-type zeolites [21,24,41,42].

The results of the XRD patterns (Figure 6) indicate that most of the zeolites with MPI silica had a higher relative crystallinity than SP 9 d, with the best result for the SM 5 d zeolite, which occurs mainly when natural zeolites are used. Considering the optimization of the synthesis parameters, the ideal condition was 3 days with a relative crystallinity of 112%. Over the synthesis time, there was an increase and decrease in crystallinity, probably due to the dissolution and crystallization of the crystals [23]. This behavior was similar to the synthesis of zeolites with MFI structures in the basic medium.

The XRF analysis of silicalite with Aerosil^®200 silica (SP 9d) and zeolites with MFI structure in the hydrofluoric medium from MPI silica (SM) at the studied times are presented in

Table 6. XRF results of silicalite synthesized with standard silica and MPI in the hydrofluoric medium at the main times studied, MPI silica, and Si/Al ratio.

Chemical Composition (%)	SP 9 d	SM 3 d	SM 5 d	SM 6 d	SM 7 d	SM 9 d	SM 12 d	MPI
SiO2	100	93.3	93.19	93.91	93.18	93.52	94.35	90.62
Al2O3	-	2.27	2.26	2.13	2.50	2.23	2.12	2.25
Na2O	-	1.60	1.50	1.40	1.40	1.60	1.20	3.50
MgO	-	1.20	1.30	1.10	1.40	1.20	1.00	1.50
Cl	-	0.12	0.11	0.08	0.17	0.08	0.07	1.43
Others	-	1.51	1.64	1.38	1.35	1.37	1.26	0.70
Total	100	100	100	100	100	100	100	100
Si/Al	-	41.09	41.19	44.10	37.27	41.93	44.50	40.27

. The results indicate that the zeolites synthesized with MPI silica have compositional variations regarding those synthesized with standard silica, regardless of the crystallization time studied, but with a high percentage of silicon. The percentage variations would be associated with the characteristics of the alternative source of MPI silica. The Si/Al molar ratios of zeolites with MPI silica have higher average values than those in the basic medium.

The main groups of the silicalite synthesized with Aerosil^®200 silica (SP 9 d) and zeolites with MFI structures in the hydrofluoric medium from MPI silica (SM) were identified by FTIR, as can be seen in Figure 7. The band at 1632 cm⁻¹ refers to the -OH bond of the water absorbed on the zeolite surface. The band at 1226 cm⁻¹ is attributed to the asymmetric stretching of the Si-O-Si bond of the external SiO₄ tetrahedron. The band at 1049 cm⁻¹ arises from the asymmetric stretching of the Si–O–Si bond of the internal SiO₄ tetrahedron. The band at 782 cm⁻¹ refers to the symmetric stretching of the Si–O–Si bond of the external SiO₄ tetrahedron. The band at 537 cm⁻¹ is attributed to the vibration of the double five ring (D5R) and is characteristic of the pentasil structure of MFI-structured zeolites in the hydrofluoric medium. The intensity of this band is useful to evaluate the stability and crystallinity of the obtained samples. Similar intensities suggest that the zeolites are similar in structure, stability, and crystallinity. The band at 405 cm⁻¹ refers to the vibration of the Si-O-Si bond [43,44,45,46,47].

The SEM micrographs of silicalites synthesized with Aerosil^®200 silica (SP 9 d) and zeolites with MFI structures in the hydrofluoric medium from MPI silica (SM) are shown in Figure 8. The results indicate that the zeolite (a) SP 9 d exhibits hexagonal elongated prismatic crystals [10]. The zeolites with MFI structures with MPI silica at the studied times, namely (b) SM 3 d, (c) SM 6 d, (d) SM 9 d, and (e) SM 12 d, also exhibit crystals with a hexagonal prismatic morphology, but that are generally more rounded, that agglomerated with a varied size distribution, that have smaller particles than those synthesized with standard silica, and that have larger particles than those in the basic medium with MPI silica. A possible explanation for the morphological changes in MFI-type zeolites includes the characteristics of MPI silica, which also contain other elements in their composition, such as aluminum, and may have led to the formation of morphological characteristics more similar to those in the basic medium.

The differential thermogravimetric and thermal curves of silicalite (a) SP 9 d and zeolites with MFI structures in the hydrofluoric medium with MPI silica at different times, namely (b) SM 3 d, (c) SM 6 d, (d) SM 9 d and (e) SM 12 d, are shown in Figure 9. Most zeolites have from one to up to three mass loss events. The results indicate an event in the range approximately from 288 °C to 639 °C with a mass loss of 12.81% that corresponds to the decomposition and combustion of the organic template TPAOH and the losses of fluoride. Some zeolites synthesized with MPI silica (SM 6 d, SM 9 d, and SM 12 d) presented a first event in the range of 25 °C to 155 °C with a mass loss in the range of 0.4% to 0.58% and an event in the range of 155 °C to 247 °C with a mass loss in the range of 0.7% to 1.57%, which can be attributed to the loss of free water and water present in the zeolitic structure, respectively [10,28,33].

Figure 10 contains the N₂ adsorption/desorption isotherms for silicalite with Aerosil^®200 silica, SP 9 d, and MFI-structured zeolites in the hydrofluoric medium from MPI silica, namely SM 3 d and SM 6 d, and

Table 7. Textural properties of Aerosil^®200 silicalite with MFI structures in the hydrofluoric medium, synthesized from MPI silica.

Zeolite	SBET (m2/g) 1	Sext (m2/g) 2	Smicro (m2/g) 3	Vtotal (cm3/g) 4	Vmicro (cm3/g) 5	Dpm (nm) 6
SP 9 d	407	53	354	0.208	0.166	0.467
SM 3 d	393	173	220	0.183	0.089	0.462
SM 6 d	361	192	168	0.196	0.068	0.457

¹ Specific area (determined through BET calculation). ² External specific area (determined through the t-Plot method). ³ Micropore area (determined through the t-Plot method). ⁴ Total pore volume (obtained for p/p0 = 0.99). ⁵ Micropore volume (determined through the t-Plot method). ⁶ Average pore diameters (determined through the Horvath–Kawazeo method).

presents the textural parameters. The results show that the isotherms of the synthesized zeolites present a combination of type I and IV isotherms, which present narrow steps that are indicative of the microporous nature and capillary condensation of N₂ in the primary mesopores, possibly derived from the crystal grain boundaries [48,49,50,51].

The SP 9 d zeolite has a larger area of 407 m²/g, a total pore volume of 0.208 cm³/g, a micropore volume of 0.166 cm³/g, and an average pore diameter of 0.467 nm, which are larger compared to zeolites with MPI silica. The modification with silica, as well as those synthesized in the basic medium, led to lower mesopore formation and micropore volume. The different crystallinity values, as observed in the MFI-structured zeolite in the basic medium, may have influenced the variations in the textural parameters. Comparing the MFI-type zeolites with MPI silica, SM 3 d had a larger area and average pore diameter than SM 6 d, but had a smaller total pore volume. This variation may be related to the time variation that implies the change in the textural parameters [41].

From the characterization results, the materials synthesized with MPI silica have similar characteristics, so the main results will be used.

3.3. Application of Zeolitic Materials in CO₂ Capture

Figure 11a shows the isotherms for the MFI-structured zeolite with Aerosil^®200 silica and MPI silica. The results indicate that ZMP 3 d obtained an adsorption level of 3.72 mmol/g of CO₂, which was higher compared to ZM 15 h, with 3.22 mmol/g of CO₂ and ZM 10.5 h, with 3.10 mmol/g of CO₂. Figure 11b contains isotherms for silicalite and the MFI-structured zeolites from MPI silica. The SP 9 d zeolite had an adsorption level of 3.94 mmol/g of CO₂, which was higher compared to zeolites SM 3 d, with 3.78 mmol/g of CO₂ and SM 6 d, with 3.60 mmol/g of CO₂. The starting material, MPI silica, showed the lowest adsorption capacity of 2.29 mmol/g CO₂, indicating the advantage of using zeolites for CO₂ adsorption. All adsorption curves showed type I isotherms, according to the IUPAC classification [52,53].

In the Supplementary Materials, Tables S1 and S2 show the results obtained regarding the CO₂ adsorption capacity of materials at low pressures of up to 20 KPa and the maximum pressure reached, 1000 kPa, at a temperature of 23 °C.

Table 8. Works found in the literature used in CO₂ capture.

Adsorbent	Conditions	Adsorbed Capacity (mmol/g)	References
ZMP 3 d	23 °C	3.72	This work
ZM 10.5 h	23 °C	3.10	This work
ZM 15 h	23 °C	3.22	This work
SP 9 d	23 °C	3.94	This work
SM 3 d	23 °C	3.78	This work
SM 6 d	23 °C	3.60	This work
MPI	23 °C	2.29	This work
Zeolite LTA	25 °C	3.30	[6]
Zeolite LTA	25 °C	4.48	[54]
Zeolite A	25 °C	2.50	[55]
ZSM-5/Silicalite-1 (Sil-ZSM-A)	25 °C	2.00	[56]
ZSM-5 (Na[Co] ZSM-5)	25 °C	5.33	[52]
ZSM-5	25 °C	2.76	[57]
zeolite Y (ZY and PDY-7): Hierarhical	25 °C	4.50, 5.40	[35]
silica	25 °C	0.749	[58]
Al-MCM-41/PEI	75 °C	1.27	[59]
Coal	25 °C	2.03	[13]

shows some of the materials in the literature used for CO₂ capture with their respective adsorptive capacities. Some of the results found are inferior, similar, or better compared to those obtained in this work. On the other hand, this work uses a low-cost source that causes modifications in the characteristics of the studied zeolites, as observed in the morphology, which can be explored and modified to improve the CO₂ adsorptive capacity of these materials.

The evaluation of the adsorption mechanisms involving the adsorbent and adsorbate was obtained from the adjustments of the Langmuir, Freündlich, and Temkin linear mathematical models, shown in Figure 12, Figure 13 and Figure 14.

Table 9. Parameters and coefficients of the determination of the Langmuir, Freündlich, and Temkin isothermal mathematical models for MPI silica and MFI-structured zeolites.

Mathematical Model	Parameters	Adsorbent
		MPI	ZMP 3 d	ZM 10.5 h	ZM 15 h	SP 9 d	SM 3 d	SM 6 d
	qm (mmol/g)	0.9955	2.5634	1.6815	1.8775	3.7979	2.2391	2.1191
Langmuir	KL (1/KPa)	0.0073	0.0445	0.0222	0.0291	0.0014	0.0083	0.0183
	R2	0.9876	0.8003	0.8429	0.8040	0.9999	0.9703	0.8831
	RL	0.0018	0.0030	0.0059	0.0045	0.0853	0.0157	0.0072
	KF (mmol/g)·(1/KPa)1/n	0.0285	0.5144	0.1632	0.2081	0.0264	0.0861	0.1637
Freündlich	1/n	0.4807	0.2229	0.3262	0.3123	0.5967	0.4441	0.3593
	R2	0.9864	0.9862	0.9913	0.9854	0.9311	0.9538	0.9735
	KT (1/KPa)	0.0299	0.3598	0.0827	0.1083	0.0370	0.0580	0.0817
Temkin	β	0.3082	0.4355	0.4040	0.4360	0.6341	0.5681	0.5209
	BT (KJ/mmol)	7960.5	5634.1	6073.9	5628.1	3869.8	4319.4	4710.8
	R2	0.8116	0.9846	0.9415	0.9670	0.9568	0.9669	0.9753

presents the parameters obtained. The results indicate that the highest adsorptive capacities of each type of adsorbent (qm) were for those synthesized with Aerosil^®200 silica 2.56 mmol/g CO₂ (ZMP 3 d) and 3.79 mmol/g CO₂ (SP 9 d). Regarding those synthesized with MPI silica, the best result was for SM 3 d, with a capacity of 2.23 mmol/g CO₂ and ZM 15 h, with a capacity of 1.87 mmol/g CO₂. MPI silica had the lowest adsorption capacity, with 0.99 mmol/g CO₂. All synthesized materials have the separation factor R_L between 0 and 1, indicating that they are favorable to CO₂ adsorptive processes.

MPI silica, Figure 12, obtained the best fit for the Langmuir model, with a correlation coefficient of R² = 0.9876, meaning that the predominant adsorption mechanism has a homogeneous distribution on the surface. Monolayer adsorption contributes to a restricted amount of adsorbent sites that are energetically equivalent. Furthermore, there is no interaction between the adsorbed molecules [12,13]. The Freündlich model was the second-best fit, with a correction coefficient of R² = 0.9864, higher K_F values indicate a greater affinity for an adsorbate and a specific temperature. The value of 1/n was greater than 0 and less than 1 (0 < 1/n < 1), indicating that the adsorptive process involved is favorable, which is in agreement with the results of the separation factor R_L [17,18]. The MPI silica did not fit the Temkin model, with a coefficient of R² = 0.8116

The MFI-structured zeolites in the basic medium, Figure 13, had the best fit to the Freündlich model, with R² = 0.9862, R² = 0.9913, and R² = 0.9854 for ZMP 3 d, ZM 10.5 h, and ZM 15 h, respectively, indicating that the predominant mechanism is the adsorption on the heterogeneous surface of the adsorbent through multilayers [14]. The value of 1/n was greater than 0 and less than 1, indicating that the adsorptive process involved is favorable. Higher K_F values indicate a greater affinity for an adsorbate at a specific temperature. These zeolites showed a good fit to the Temkin model, with the correlation coefficients R² = 0.9846, R² = 0.9415, and R² = 0.9670, respectively, indicating that a physical nature acts in the adsorption process. The value of the Temkin constant (BT) was also lower than 8000 Kj·mmol⁻¹, indicating that there is a contribution of physical nature in the adsorptive process. The Langmuir model did not fit the zeolites ZMP 3 d, ZM 10.5 h, and ZM 15 h, with coefficients of R² = 0.8003, R² = 0.8429, and R² = 0.8040, respectively.

Silicalite SP 9 d and zeolites with MFI structures in the hydrofluoric medium with MPI silica SM 3 d, Figure 14, obtained the best fit for the Langmuir model, with correlation coefficients of R² = 0.9999 and R² = 0.9703, respectively. This indicates that the predominant adsorption mechanism presents a homogeneous distribution across the surface, with a monolayer formation in a restricted number of adsorbent sites that are energetically equivalent. Furthermore, there is no interaction between the adsorbed molecules [12,13]. The Temkin model also had a good fit for these zeolites with the coefficients R² = 0.9568 and R² = 0.9669, respectively, indicating the contribution of the physisorption process in the adsorption mechanism, since the value of the Temkin constant (BT) was also less than 8000 Kj·mmol⁻¹. For Freündlich, the value of 1/n was greater than 0 and less than 1, indicating that the adsorption process is favorable [18].

The SM 6 d zeolite, Figure 14, had the best fit for the Temkin model, with a coefficient of R² = 0.9753, indicating that the predominant adsorptive mechanism is physical adsorption [15]. The second-best fit was for the Freündlich model, with R² = 0.9735, indicating a greater contribution of the monolayer and multilayer in the adsorptive process. Furthermore, n values greater than 1 indicate that adsorption is favorable. The Langmuir model did not fit the experimental data for this material, with a coefficient R² = 0.8831 [12,18].

4. Conclusions

MPI silica obtained from beach sand was successfully used in the eco-friendly synthesis of zeolitic materials with MFI structures, being applied in CO₂ capture.

The MFI-structured zeolites in the basic medium synthesized from MPI silica have pure and crystalline phases, with the best synthesis time for ZM 15 h with a relative crystallinity of 92.90%. Zeolites have a similar chemical composition with the main elements, Si, Al, and Na, in their structure that is similar to ZMP 3 d, with variations in other elements due to MPI silica. The zeolites with MPI silica have higher silicon–aluminum ratios than ZMP 3 d, but are consistent with values found in the literature. The FTIR results showed that zeolites have the characteristic bands of the MFI structure. The SEM analyses indicate that the zeolites have agglomerated hexagonal clintform morphology with a varied particle size distribution. Zeolites from MPI silica have smaller particle sizes than ZMP 3 d, and this can be associated with the characteristics of the silicon source. The TG/DTG results indicate the good thermal stability of the molecular sieves. The textural parameters were characteristic of zeolites with MFI structures, with a larger specific area for ZMP 3 d, at 380 m²/g.

The MFI-type zeolites in the hydrofluoric medium synthesized with MPI silica have pure and crystalline phases, with the best results for the SM 3 d zeolite with a relative crystallinity of 111.90% about SP 9 d. The zeolites from MPI silica have a different chemical composition from SP 9 d due to the presence of elements from MPI silica, mainly Al and Na. The Si/Al molar ratios are, on average, higher than the MFI-type zeolites in the basic medium. The FTIR analyses indicate that the zeolites contain all the characteristic bands of MFI-type zeolites. The SEM results show that the MFI-type zeolites have the characteristic morphology of hexagonal primes much smaller than the SP 9 d zeolite. A possible explanation for the morphological changes is probably the characteristics of the silicon source, which also contained other elements in its composition, such as aluminum, and may have led to the formation of morphological characteristics more similar to those synthesized in the basic medium. The largest specific area was for silicalite, at 407 m²/g.

The results of the adsorption study indicate that SP 9 d and SM 3 d in the hydrofluoric medium had the highest adsorption capacities with 3.94 mmol/g CO₂ and 3.78 mmol/g CO₂, respectively. And the MFI-structured zeolites in the basic medium, namely ZMP 3 d and ZM 15 h, showed good results with capacities of 3.72 mmol/g CO₂ and 3.22 mmol/g CO₂, respectively. Thus, the synthesized zeolites had a higher capacity regarding MPI silica, which obtained an adsorption capacity of 2.29 mmol/g CO₂.

Mathematical modeling indicated that the MFI-structured zeolites in the basic medium had a better fit to the Freündlich model, indicating that the predominant mechanism was the adsorption on the heterogeneous surface of the adsorbent through multilayers. Silicalite and the MFI-structured zeolites in the hydrofluoric medium synthesized from the MPI silica SM 3 d fit better to the Langmuir model, indicating that the adsorption mechanism, monolayer adsorption, acts strongly on a restricted number of sites and that there is no interaction between the adsorbed molecules, while SM 6 d fits better to the Temkin model.