CO₂ Adsorption on Natural Zeolites from Puebla, México, by Inverse Gas Chromatography

Abstract

The applicability of clinoptilolite zeolites in controlling the emission of greenhouse gases (GHGs) such as CO₂, the most significant GHG, is investigated herein. In this research, Mexican natural zeolites (ATN) originating from an Atzinco deposit in the state of Puebla were used. Samples of modified clinoptilolite (ATH4, ATH3, ATH2 and ATH1) were obtained from the starting material by acid treatment of various intensities. Inverse gas chromatography was used to evaluate CO₂ adsorption in clinoptilolite, natural and chemically modified. Adsorption of CO₂ was investigated in the temperature range of 433–573 K, using a TCD detector, and He as a carrier gas. The experimental CO₂ adsorption data were processed by Freundlich and Langmuir equations. The degree of interaction between CO₂ and the dealuminated clinoptilolite samples was examined through the evaluation of the isosteric enthalpy of adsorption. This calculation was made by using the Clausius–Clapeyron equation, which established the following sequence: ATH1 > ATH2 > ATH4 > ATN > ATH3. The nanoporosity of these clinoptolite zeolites from new deposit in sedimentary rocks was studied through HRADS adsorption of N₂. Simultaneously, these zeolites were, respectively, characterized by XRD, EDS, and SEM. Micropores are described by the Dubinin–Asthakov distribution. Various adsorption mechanisms that occur in these nanoporous materials at different relative pressures can be visualized. The quantitative determination of starting mineral is described as: Ca-Clinoptilolite (88.76%) >> Montmorillonite (11.11%) >> quartz (0.13%). The Si/Al molar ratio after acid treatment is: ATH4 > ATH2 > ATN > ATH3 > ATH1. The Langmuir specific surface area (A_SL) varies as follows: ATN > ATH2 > ATH4 > ATH3 > ATH1. At the same time, the V_Σ values are as follows: ATN > ATH4 > ATH3 > ATH1 > ATH2.

1. Introduction

One of the biggest environmental challenges nowadays refers to the presence of carbon dioxide (CO₂). Currently, the increase of this gas is a significant concern due to its gradual accumulation in the atmospheric concentration. This increment is the result of fossil fuels high consumption. This utilization is related to an increase in the concentration of CO₂ in the atmosphere and the global climate change. Due to this problem, it is necessary to significantly counteract CO₂ concentrations from the current levels [1]. There are several cutting-edge technologies used to reduce CO₂ concentrations. Some techniques are focused on stabilizing the CO₂ concentration in the atmosphere. For example, amine solutions for absorption, membrane separation and adsorption-based separation. These methods controlled the emission of CO₂ from various sources. There exist different materials to selectively adsorb this gas [2,3]. Among these materials, there are hydrotalcites, inorganic oxides, activated carbons, MCM, MOFs, TIFs and synthetic (NaX, FAU and LTA) and natural (ERI, CHA and HEU) zeolites [2,3]. To date, the clinoptilolite zeolite is one of the most promising zeolites to carry out more selective adsorption processes with respect to CO₂. The structural code for this zeolite is HEU, as per the International Zeolite Association, IZA [4]. This zeolite possesses remarkable adsorption characteristics, for instance, its accessibility, and purity. Additionally, it has the possibility to increase their adsorption capacity. This enhancement can be made by integrating certain cations through ion exchange, and/or via nanodeposits creation in the internal arrangement of this zeolite.

The substitution by ion exchange of Na⁺ ions in natural clinoptilolite by either Ca²⁺ or H⁺ ions improve the capacity (i.e., the accessible pore volume). Chemical treatments, via ion exchange and dealumination of zeolite result in the development of mesopores, known as secondary porosity. Clinoptilolite is a hydrophilic zeolite which structure is based on a three-dimensional porous system form by three types of channels [5]. The first two channels are two parallel channels along [001] axis: channel A with 10-member ring (0.31 × 0.75 nm) in cross-section, and channel B with 8-MR ring (0. 36 × 0.46 nm}. These A and B channels are perpendicularly intersected along [100] axis by channel C. The features of channel C are an 8-MR ring (0.28 × 0.47 nm). The channel sizes are variable due to the considerable flexibility of the framework), see Figure 1.

Natural zeolites are elaborated adsorption structures. They are constituted from different crystalline phases. This composition is related to the materials that constitute this type of mineral. The presence of micropores (pore diameter, W < 2 nm) is related to the sole presence of the zeolite. Meanwhile, the mesopores (W = 2–50 nm), and macropores (W > 50 mm) [6,7] are associated with the presence of accompanying materials. These additional materials are usually clays and quartz. During an adsorption process in this type of material, a micropore filling is carried out at very low relative pressures [7,8]. This mechanism involves a primary filling and a secondary filling [7,8]. As the relative pressure increases, the formation of the monolayer occurs. Secondly, there is a gradual increase in relative pressure and larger pores (mesopores) are occupied. The phenomenon of capillary condensation takes place in the macropores. This event occurs when the equilibrium pressure values tend to the saturation pressure values. Consequently, the formation of the adsorbate meniscus appears.

Gas–solid chromatography (GSC) is one of the suggested methods to study microporous materials with micropores at low coverage degrees. This method is called inverse gas chromatography (IGC) when it is applied to study solid surface properties. IGC has been used extensively to study catalytic reactions and textural parameters (such as surface area and porosity). Additionally, IGC has been broadly used to analyze various types of materials [9]. IGC can be experimentally configured for finite or infinite dilution concentrations of adsorbate [10]. According to these last concepts of the IGC method, in the case of infinite dilution concentrations, a few molecules are injected into the column to approach zero surface coverage. Considering this circumstance, the lateral interactions between the adsorbed molecules on the surface can be ignored. Hence, the thermodynamic functions only change with the adsorbate–adsorbent interactions. Adsorption can be considered to occur in the linear zone of the adsorption isotherm in the Henry’s law area [11]. The attainment of the Henry’s law region is indicated by certain characteristics. Among these features, there are the symmetry of the chromatographic peaks and the constancy of the retention times. These times, measured over a significant range of the sample sizes. Therefore, the net retention volumes for a specific adsorbate are non-dependent of its gas phase concentration [12].

This study examined the adsorption capacity of natural clinoptilolite from a new deposit in Mexico. Furthermore, it also examines the modifications by decationation. Finally, it also analyzes the possibly concomitant dealumination during acid treatment. The original material and the resulting modified samples were studied using XRD, EDS, and SEM. To assess their texture properties, HRADS measurements were carried out. By using the Dubinin–Astakhov (D-A) model with the HRADS results, the micropores principal parameters (W₀ and E) were estimated.

The adsorption of CO₂ in a series of natural and decationated zeolites was studied. The adsorptions were analyzed using the Langmuir and Freundlich equations. Meanwhile, the degree of interaction between this CO₂ gas and samples of natural and decationated clinoptilolite were studied through the evaluation of the isosteric enthalpies of adsorption. Those enthalpies were analyzed with the Clausius–Clapeyron equation at zero degrees of coverage by IGC method.

2. Results

2.1. XRD

In Figure 2, the diffraction patterns of natural zeolites studied are shown. The phases observed for each zeolite sample are listed in

Table 1. Crystalline phases, XRD.

Sample	Card Number	%	Name
(Na, Ca)0.3(Al, Mg)2Si4O10(OH)2 x H2O	00-003-0015	11.110	Montmorillonite (Bentonite)
Ca3.16 Si36O72 (OH2)21.80	01-070-1859	88.761	Ca-Clinoptilolite
SiO2	01-085-0795	0.128	Quarzt

. Quantitative determination of the percentage of these phases in ATN was conducted by using the High Score Plus 3.0e computer program. Additionally, appropriately processing diffraction patterns (XRD) and the current phases atomic coordinates were also calculated. Table 1 demonstrates the results of these estimates.

In Figure 3, the X-ray diffraction patterns are observed in the chemically modified zeolites (ATH1, ATH2, ATH3 and ATH4).

2.2. EDS

Table 2. Samples chemical composition (mass %), based on EDS data.

Sample	SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	TiO2	FeO	Si/Al
ATZN	59.30	7.65	1.54	6.32	5.99	2.54	0.60	1.39	7.75
ATH1	63.596	9.107	2.716	2.238	4.364	5.83	0.867	1.222	6.983
ATH2	62.484	7.935	3.46	2.518	6.028	4.939	0.784	1.556	7.874
ATH3	63.425	9.862	3.088	2.952	4.397	6.167	0.617	1.389	6.431
ATH4	61.821	7.822	3.946	2.504	5.732	4.914	0.867	1.775	7.903

shows the representative analyses of chemical composition of clinoptilolites studied in this research.

The chemical treatments to which the natural zeolite ATN are subjected allow us to establish the following sequences:

SiO₂:

ATH1 > ATH3 > ATH2 > ATH4 > ATZN; Al₂O₃: ATH3 > ATH1 > ATH2 > ATH4 > ATZN;

F₂O₃:

ATH4 > ATH2 > ATH3 > ATH1 > ATZN; CaO: ATZN > ATH3 > ATH2 > ATH4 > ATH1;

MgO:

ATH2 > ATZN > ATH4 > ATH3 > ATH1; K₂O: ATH3 > ATH1 > ATH2 > ATH4 > ATZN;

TiO₂:

ATH4 = ATH1 > ATH2 > ATH3 < ATZN; FeO: ATH4 > ATH2 > ATZN > ATH3 > ATH1 and

Si/Al:

ATH4 > ATH2 > ATZN > ATH3 > ATH1.

2.3. SEM

Micrographs of clinoptilolite samples displayed classic crystalline morphology, Figure 4.

2.4. N₂ Adsorption

Figure 5a,b shows the N₂ adsorption isotherms of the studied zeolites.

Table 3. Textural parameters of clinoptilolites from Atzinco, Puebla.

Sample	ASL m2/g	ASB m2/g	ASt m2/g	CB	Zone p/p0 BET	V∑ cm3/g	W0t cm3/g
ATN	40.50	26.17	15.54	95	0.054–0.289	0.075	0.010
ATH1	12.39	9.19	5.912	−367	0.010–0.210	0.0157	0.0058
ATH2	31.06	23.39	17.03	287	0.010–0.214	0.0156	0.0058
ATH3	15.88	10.34	6.682	610	0.01–0.314	0.0228	0.0016
ATH4	30.29	20.51	18.32	−319	0.01–0.314	0.0356	0.0012

A_SL, is the specific surface area Langmuir; A_SL is the specific surface area BET; A_St is the external surface area, t-plot; C_B is the BET constant; p/p⁰ is the range used for BET plot, V_Σ is the volume adsorbed close to saturation (p/p⁰ = 0.95), calculated as volume of liquid (Gursvitch rule) and W_0t is micropore volume, t-plot.

reports the results of the textural studies of the analyzed samples.

2.4.1. Pore Size Distribution (PSD), DA and BJH Approaches

D-A Approach

The matching nanopores size distribution was obtained from the N₂ adsorption–desorption isotherms at 77 K. The results of these calculations, Figure 6, demonstrate the uniformly distribution of the nanochannels, which diameters vary from 0.956 to 0.791 nm. This figure shows the intensity of these signals’ associates with the Al presence in the arrangement of these natural zeolites, (shown in

Table 4. Clinoptilolite zeolites micropores parameters according to the Dubinin–Astakhov equation.

Sample	E Kj mol−1	W0 cm3 g−1	n	Dp nm	Al2O3	M+
ATN	2.56	0.02	1	0.950	7.65	6.32 MgO
ATH1	3.16	0.01	1	0.890	9.107	5.83 K2O
ATH2	2.85	0.02	1	0.920	7.935	6.028 MgO
ATH3	2.73	0.01	1	0.930	9.862	6.167 K2O
ATH4	5.78	0.01	2.70	0.790	7.822	5.732 MgO

, 6th column) [13].

BJH Equation

The results when applying the BJH method in the ATN-ATH zeolites are observed in Figure 7, in which it can be observed there are predominant signals in the area of pore diameters Dp = 2167 nm (ATH2), 3033 nm (ATH4) and 3606 nm (ATN). It is also observed that these distributions become less acute depending on the chemical treatment to which this type of zeolite is subjected. Again, this effect is attributed to the generation of these types of pores during dealumination treatments. ATH1 and AH3 zeolites were not subjected to BJH analysis.

2.5. Adsorption of CO₂

The method of inverse gas chromatography (IGC) was used to estimate the adsorption isotherms based on the elution curves of adsorbate on the zeolites under study. These CO₂ adsorption isotherms were measured in a region of low adsorbate concentrations; hence, adsorption on the mesopore surface can be practically neglected. Consequently, the thermodynamics parameters that characterize the microporous structure were calculated from the CO₂ adsorption isotherms, without any correction for adsorption on the mesopore surface. CO₂ adsorption isotherms at different temperatures on ATN and ATH zeolites are presented in parts a–e in Figure 8. Additionally, the degree of interaction of CO₂ with zeolite was analyzed from the evolution of the isosteric enthalpy of adsorption -q_st, Figure 9. The values of the Freundlich constants KF and n, Henry’s constants KH, and the Langmuir monolayer capacity are shown in

Table 5. Henry (K_H, mmol g⁻¹ x mmHg⁻¹), Freundlich (K_F, mmol g⁻¹ × mmHg⁻¹) and Langmuir (am, mmol g⁻¹) parameters for the adsorption of CO₂ on modified clinoptilolite zeolites.

T/K	Sample	KF 103	n	RF	KH 103	am	RL
573	ATN	6.4	1.290	0.995	4.9	0.207	0.999
	ATH1	11.9	1.450	0.991	8.8	0.210	0.999
	ATH2	11.7	1.380	0.995	9.2	0.223	0.999
	ATH3	9.4	1.343	0.997	7.3	0.218	0.998
	ATH4	11.9	1.395	0.996	9.5	0.205	0.998
543	ATN	8.9	1.438	0.990	6.3	0.174	0.999
	ATH1	8.4	1.349	0.996	5.9	0.247	0.999
	ATH2	12.8	1.411	0.994	10	0.223	0.999
	ATH3	8.4	1.349	0.996	7.9	0.205	0.999
	ATH4	10.7	1.425	0.993	10.2	0.209	0.999
513	ATN	9.3	1.450	0.989	6.6	0.173	0.999
	ATH1	12.6	1.408	0.994	9.8	0.223	0.999
	ATH2	14.9	1.481	0.993	11.4	0.212	0.998
	ATH3	13	1.514	0.989	9.2	0.197	0.999
	ATH4	11.7	1.383	0.993	9	0.230	0.999
473	ATN	10.2	1.383	0.993	7.6	0.162	0.999
	ATH1	15.8	1.507	0.991	11.7	0.220	0.999
	ATH2	19.2	1.593	0.996	15.8	0.185	0.995
	ATH3	14.5	1.526	0.989	11.8	0.168	0.999
	ATH4	15.8	1.462	0.987	13.5	0.193	0.999
433	ATN	7.4	1.212	0.991	6.1	0.312	0.999
	ATH1	17.2	1.507	0.994	14.7	0.189	0.996
	ATH2	17.9	1.521	0.985	14.6	0.200	0.999
	ATH3	20.1	1.638	0.989	17	0.168	0.999
	ATH4	17.7	1.469	0.993	15.5	0.198	0.998

. While the isosteric enthalpies of adsorption -q_st are given in Table 6. From the measured retention volume through the column, different thermodynamic parameters can be deduced at the molecular level [14].

From Table 5, the following sequence is established for monolayer capacity (am, mmol g⁻¹) at the studied temperatures: 537 K: ATH2 > ATH3 > ATH1 > ATN > ATH4; 543 K: ATH1 > ATH2 > ATH4 > ATH3 > ATHN; 513 K: ATH4 > ATH1 > ATH2 > ATH3 > ATHN and 433 K: ATN > ATH2 > ATH4 > ATH1 > ATH3. While those minor values are obtained in the ATN sample at 473 K, the highest values correspond to the same sample at 433 K. The Langmuir methodology applies moderately well for all except the ones regarding adsorption at 573 and 473 K on ATH4, and ATH2 and ATH4 at 433 K.

Table 6. Adsorption isosteric enthalpy (-q_st, kJ mol⁻¹) of CO₂ on ATN and ATH clinoptilolites.

Sample	-qst (kJ mol−1)
ERIN [15]	18.8
ERINa2	26.2
Z17 [16]	7.7
ZSLP [17]	2.8
ATN *	2.2
ATH1 *	4.3
ATH2 *	2.7
ATH3 *	2
ATH4 *	2.6
SNN100 [18]	20
13X [19]	40
MOF-5 [20]	30

* This work.

Table 6. Adsorption isosteric enthalpy (-q_st, kJ mol⁻¹) of CO₂ on ATN and ATH clinoptilolites.

Sample	-qst (kJ mol−1)
ERIN [15]	18.8
ERINa2	26.2
Z17 [16]	7.7
ZSLP [17]	2.8
ATN *	2.2
ATH1 *	4.3
ATH2 *	2.7
ATH3 *	2
ATH4 *	2.6
SNN100 [18]	20
13X [19]	40
MOF-5 [20]	30

* This work.

3. Discussion

3.1. XRD

In Figure 2, it can be observed the natural zeolite (ATN) diffraction patterns under study. Table 1 presents the results of the estimates of the calculations of phases present. These samples contain clinoptilolite (JCPDS card 3 0427) [21] and, in smaller amounts, impurities (e.g., montmorillonite (JCPDS 29-1498), quartz (peak at ~ 27° 2θ, JCPDS 3-0427)). Figure 3 shows the X-ray diffraction patterns observed in the chemically modified zeolites (ATH1, ATH2, ATH3 and AH4). The distinguishing crystalline peaks of the clinoptilolite zeolite appear at the succeeding 2θ diffraction angles: 2θ values of 9.86°, 11.06°, 13.03°, 14.82°, 16.86°, 17.2°, 19.04°, 22.35°, 25.04°, 28.09°, 31.71°, 32.67°. It can be seen from this figure that peaks of clinoptilolite prevail. To clearly represent these signals in zeolite samples, the diffraction patterns were divided into 3 zones with a more detailed scale: zone (a) 2θ = 5–20°, (b) 2θ = 20–35°, (c) 2θ = 35–50° and, (d) in the zone of 2θ = 5–50°, the overall picture can be observed.

3.2. EDS

Representative analyses of the chemical composition of the clinoptilolites studied in this research are shown in Table 2. From these results, the parent zeolite has a Si/Al ratio > 5; such composition is typical for zeolites, which can be a result of settlings in saltwater bodies such as saline lakes and seas. It is known that amid clinoptilolite zeolites with a high Si content, that is, with a Si/Al ratio > 4, K-Clinoptilolite is the most widespread; this zeolite is dominant among the rocks produced in deep water zones. Nevertheless, Ca-Clinoptilolite and Na Clinoptilolite zeolites can be produced in a varied range of environmental factors. They can emerge in volcanic rocks, hydrothermal active systems, equally in cracks and fissures of volcanic rocks. Nonetheless, the Fe impurity is most likely Fe³⁺, which is in tetrahedral sites of crystalline framework, but its large amounts can also be associated with hematite impurities [22].

3.3. SEM

Figure 4 presents clinoptilolite crystals images at distinct magnifications. These images show that the crystals of this zeolite have different sizes and geometries. In this figure, it is possible to observe the crystals’ presence. These crystals have undefined, or irregular (anhedral) faces, which are typical of this zeolite [23,24]. Moreover, from Figure 4, it is observed the existence of a cluster of lamellar pores.

3.4. N₂ Adsorption

Regarding the specific Langmuir surface area, estimated by the A_SL method, the following sequence order is established: (A_SL) is: ATN > ATH2 > ATH4 > ATH3 > ATH1. While the V_Σ values are as follows: ATN > ATH4 > ATH3 > ATH1 > ATH2. A comparable performance is demonstrated by the total pore volume V_Σ. Meanwhile, the textural properties values of the clinoptilolite zeolites are well described with the Langmuir equation.

D-A Approach

The same Table 4 shows the optimized values of W₀, micropore volume, and the characteristic adsorption energy E₀ values, obtained through the DA equation. The results show an increase in the amount of Al leads to a greater accessibility of the nanopores. The acquired values for E₀ increase while Al₂O₃ decreases and are also affected by cations. The values found show the relationship between E₀ and the Si/Al ratio, that is, its dependence on the Al₂O₃ content.

The isomorphous substitution of Si(IV) by Al(III) in the tetrahedral units of the zeolite crystalline structure results in the appearance of an unbalanced negative charge. Additionally, it also shows the presence of nonstructural, charge-balancing exchangeable cations located into the voids of the zeolite structure. It is evident that a change in the Si/Al ratio and the kind of counter-cations altered the adsorption potential. The concentration of Al atoms and the electrostatic interaction with cations affect the electric fields within the nanopores of clinoptilolite zeolite [25]. Therefore, they affect the interaction with sorbed molecules. From the size distribution of nanopores in Figure 7, it can be said the intensity of these signals is related with the presence of Al in these zeolites arrangement, (Table 5, 6th column).

3.5. CO₂ Adsorption

In Figure 8a–e, three important points can be pointed: (i) the adsorbed quantities of CO₂ gas on ATN and ATH zeolites are comparable, (ii) the adsorption isotherms at 573 K are better described experimentally and (iii) the adsorbed amount of CO₂ decreases with temperature. Figure 9 exhibits the isosteric enthalpy of adsorption on ATN and ATH that rises with increasing surface-loading of CO₂. For all zeolites, the adsorption isosteric enthalpy reduces with growing adsorbate loading. The initial acute reduction in -q_st may be caused by the heterogeneity site. Nevertheless, a constant gradual decrease in -q_st is expected at higher adsorbate loadings. This phenomenon caused by the repellent interactions happening among the adsorbed CO₂ molecules. As a comparison, Table 6 includes values of isosteric enthalpy of adsorption of CO₂ on erionite, clinoptilolite, 13X zeolites, SNN-100 hybrid materials and metal–organic structure MOF-5.

4. Materials and Methods

4.1. Materials

The reactives HCl and AgNO₃ were purchased from Aldrich. The gases used for the textural sorption analysis of clinoptilolites (natural and modified) were ultra-high purity CO₂, N₂ and He gases (>99.999%, INFRA Corp., Puebla City, Mexico).

4.2. Methodology of Dealumination of Clinoptilolite

The zeolites employed in this work were natural zeolites found in Mexico. These samples are from Atzinco, Puebla, Mexico. The ATN tag corresponds to a natural sample that has not undergone any chemical treatment. Modified clinoptilolite samples ATH1, ATH2, ATH3 and ATH4 were prepared from the ATN precursor and treated different times (1, 2, 3 or 4) with 0.01 N solutions of the HCl at 50 °C for 6 h. This chemical treatment provokes impurities elimination and the exchange of protons in the polyvalent cations places. This alteration technique is composed by a sequence of washing cycles of the samples of natural clinoptilolite (mesh 60–80) with HCl diluted. Then, they were subject to prolonged rinses with deionized water, using AgNO₃ as indicator. A specimen named ATH1 was prepared from ATN, after the natural zeolite was treated once with HCl (H1 labeling follows the number of washings with HCl). Similarly, a sample branded as ATH2 resulted from ATN after treating twice the natural zeolite with HCl.

4.3. Experimental Measurement Techniques

4.3.1. XRD

The crystalline phases were identified by using the powder X-ray diffraction (XRD) technique. The equipment utilized was diffractometer Bruker model D8 Discover (Bruker, Co., Billerica, MA, USA). This technique employed nickel-filtered Cu Kα (λ = 0.154 nm) radiation operated at 40 kV and 30 mA. The samples were pulverized in an agate mortar and placed in a sample holder, compressing the powder until it was perfectly compact. The crystalline phases were classified by means of the 2013 PDF4+ archive of the International Center for Diffraction Data (ICDD). The quantification of the percentages of the phases presents in ATN, was performed by the High Score Plus 3.0e computer program (Version 5.1, Malvern Panalytical, Cambridge, UK), properly handling the diffraction patterns (XRD) and atomic coordinates of the present phases [26].

4.3.2. SEM

Zeolites images were acquired with a JEOL, model JSM-7800F (JEOL USA, Inc., Peabody, MA, USA) high-resolution scanning electron microscope at 5 kV. The zeolites were placed on aluminum stub holders and successively coated with Au using a sputtering coater.

4.3.3. EDS

An elemental chemical analysis by X-ray energy dispersion (EDS) was performed on each of the zeolites, using an Oxford INCA energy 250+ model probe, with a resolution of 137 eV and a 10 mm² detector.

4.3.4. N₂ Adsorption

The studies of N₂ adsorption were obtained at 76.7 K. This temperature is the boiling point of liquid N₂ at the 2200-m altitude of Mexico City. The isotherms were measured in an automatic volumetric adsorption equipment (Quantachrome AutoSorb1LC).

N₂ adsorption isotherms were determined at relative pressures from p/p⁰ = 10⁻⁶ to 0.995. The pressure of saturation, p⁰, was uninterruptedly recorded during the adsorption–desorption process. Powder particle sizes, previously mesh 60–80, were obtained from all zeolites studied. Before the sorption experimentations, each portion of zeolites was exposed at 623 K during 15 h at a pressure lesser than 10⁻⁶ Torr. The relevant equations used for the calculation of the surfaces were: The Brunauer–Emmett-Teller (BET), Langmuir, and t-plots (external) in the range of p/p⁰ from 0.01 to 0.25. The total pore volume, V_∑ was calculated by using the Gurvitsch rule based on the volume adsorbed at the relative pressure p/p⁰ = 0.95, calculated as liquid volume.

4.4. Adsorption of CO₂

Chromatographic CO₂ adsorption tests were made in a GC-14A Shimadzu gas chromatograph (Shimadzu Co KK, Kyoto, Japan) prepared with a thermal conductivity detector. The glass chromatographic columns (i.d. = 5 mm, length = 50 cm) were filled with zeolites (60–80 mesh).

The incumbent GC column was maintained at a certain temperature while adsorbate pulses of different intensities were injected. The elution chromatogram of each pulse was recorded till the recorder pen got once more the baseline. Previously to the adsorption experimentation, the adsorbents were exposed in situ to a flow of He carrier gas at 573 K for 8 h.

The gas in pure form was injected to measure their corresponding retention time inside the appropriate adsorbent column. The dynamic adsorption method of gas chromatography is the technique pursued in this research to acquire the chromatographic peaks. The isosteric enthalpy of adsorption of CO₂ were calculated through Clausius–Clapeyron equation, by using sorption data monitors at two temperatures.

4.5. Calculation Methods

The adsorption mechanisms of the zeolites studied in this work have been previously reported [15,16,17,25]. The microporosity of the natural zeolites used in this work was studied with the Dubinin–Polany theory, in the range of p/p⁰ = [10⁻⁶–0.2] and with high resolution t plots in the range of p/p⁰ = [10⁻⁶–0.8]. The Dubinin–Astakhov (D-A) method was used, which is based on the following equation:

$$W=W_0\exp \left[-{\left(\frac{A}{\beta E_0}\right)}^n\right]$$

(1)

where W is the microporous volume that has been filled at a given p/p⁰ and W₀ is the total volume of the microporous space. In turn -A = RT ln(p⁰/p) is the negative of the isothermal molar Gibbs free energy. β is called a scaling factor and E₀ is the characteristic energy of the adsorption system. This term reflects the influence of the nature of the substrate on the adsorbed amount.

The experimental data of the adsorption of CO₂ in the zeolites were treated by the adsorption models of Freundlich and Langmuir in their linear form. The Freundlich adsorption equation can be written as:

a = K_F + p^1/n

(2)

where a is the adsorbed amount (mmol g⁻¹), K_F is the Freundlich adsorption constant, and n is an exponential factor. From gas adsorption data at low pressures, it is possible to evaluate the Henry constants (K_H) at different temperatures for the series of adsorbent–adsorptive pairs employed in this work according to the following expression [27]:

K_H = lim_p→0 (a/a_mp)

(3)

where a represents the amount adsorbed on the solid walls at pressure p, while a_m is the monolayer capacity evaluated from the Langmuir equation:

θ = a/a_m = Kp/1 + Kp

(4)

where K a_m = K_H, is something that can be tested graphically by plotting 1/a versus 1/p:

1/a = 1/a_m + 1/a_mKp

(5)

The enthalpy isosteric of adsorption, -q_st (kJ mol⁻¹), at different adsorbate loadings can be evaluated from the adsorption isotherms data through a Clausius–Clapeyron type equation [27]:

[∂ ln p/∂T]_a = -q_st a/RT²

(6)

where p and T are the equilibrium pressure and temperature at a given adsorbate loading (a).

5. Conclusions

In this work, the experimental CO₂ data were obtained by IGC. Afterwards, the data were treated through the equations of Freundlich and Langmuir. Meanwhile, the degree of interaction between CO₂ and the clinoptilolite samples studied was analyzed with the isosteric enthalpy of adsorption. This enthalpy was studied by applying the Clausius–Clapeyron equation.

From the initial results which establishes the following sequence: ATH1 > ATH2 > ATH4 > ATN > ATH3. The initial sharp decrease in -q_st may be attribute to the site heterogeneity. Though, a constant gentle reduction in -q_st is anticipated at higher adsorbate loadings due to repellent interactions between adsorbed CO₂ molecules. Previously, the nanoporosity of natural clinoptilolite zeolites from different locations in Atzinco, Puebla, Mexico was studied through HRADS adsorption of N₂. Laterally, these natural zeolites have been conveniently characterized by XRD, EDS, and SEM. The quantitative determination showed that natural tuff contains Ca-Clinoptilolite (88.76%) >> Montmorillonite (11.11%) >> quartz (0.13%). The SiO₂/Al₂O₃ molar ratio for the starting material is 7.75; after acid treatment, this ratio forms a jumping curve: ATH4 > ATH2 > ATN > ATH3 > ATH1. SEM images of these zeolites indicate the presence of crystals with different sizes and morphology, protruding anhedral crystals and lamellar pores. Porosity studies indicate the formation of type VI adsorption isotherms for ATN zeolite, while for the ATH zeolites it describes by mixed type I–IV isotherms with hysteresis cycles of type H3, which are characteristic of lamellar pores. The Langmuir specific surface area (A_SL) is: ATN > ATH2 > ATH4 > ATH3 > ATH1. While the V_Σ values are as follows: ATN > ATH4 > ATH3 > ATH1 > ATH2.