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Article

Zeolites Derived from Natural Kaolinite for CO2 Adsorption

by
Giorgio Celoria
1,
Federico Begni
1,
Geo Paul
1,
Enrico Boccaleri
2,3,*,
Valentino Merlo
4,
Leonardo Marchese
1 and
Chiara Bisio
1,5,*
1
Dipartimento di Scienze e Innovazione Tecnologica, Università del Piemonte Orientale A. Avogadro, Viale T. Michel 11, 15121 Alessandria, Italy
2
Dipartimento per lo Sviluppo Sostenibile e la Transizione Ecologica, Università del Piemonte Orientale A. Avogadro, Piazza S. Eusebio 5, 13100 Vercelli, Italy
3
UPO4Sustainability Center, Viale T. Michel 11, 15121 Alessandria, Italy
4
Built—Buzzi Innovation Lab and Technology, Via Restano 3, 13100 Vercelli, Italy
5
CNR-SCITEC Istituto di Scienze e Tecnologie Chimiche “Giulio Natta”, Via G. Venezian 21, 20133 Milano, Italy
*
Authors to whom correspondence should be addressed.
Processes 2024, 12(1), 194; https://doi.org/10.3390/pr12010194
Submission received: 30 December 2023 / Revised: 12 January 2024 / Accepted: 14 January 2024 / Published: 16 January 2024
(This article belongs to the Section Environmental and Green Processes)
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Abstract

:
This manuscript deals with the synthesis of different types of zeolites from natural kaolinite samples for CO2 adsorption. A zeolite A was prepared from kaolinite by means of an alkaline fusion process, followed by hydrothermal treatment, whereas a highly crystalline zeolite X was synthesized by optimizing the previously mentioned synthetic procedure. In detail, the SiO2/Al2O3 molar ratio in the preliminary mixture was modified with the addition of a secondary silicon source (sodium silicate) in order to obtain the one required for zeolites X. The physicochemical properties of the pristine clay and of the different zeolites were investigated by means of a multi-technique approach, including XRPD; SEM-EDX; 23Na, 27Al and 29Si MAS NMR spectroscopy; and N2 physisorption analysis at 77 K. Since the Si and Al molar ratios and reactivities are key parameters for the synthesis of zeolites, these aspects, primarily related to the use of a naturally occurring aluminosilicate as the raw material, have been investigated for their correlation with the physicochemical properties of the synthetic products. Moreover, by means of a custom-built volumetric apparatus, the CO2 adsorption capacity of the resulting zeolites at low gas pressures (<1 bar) and at 25 °C was assessed.

1. Introduction

In the last decades, the continuous spreading of greenhouse gases (i.e., CO2 and CH4) in the atmosphere from anthropogenic activities has raised global concerns regarding their effects on the Earth’s climate [1,2]. Due to enhancing greenhouse effects on the atmosphere, the rise in the concentration of CO2, by the increased emissions since the beginning of the 20th century, has caused an increase in global temperatures [3]. The global climate impact has led to a series of current and forthcoming adverse effects on the ecosystem, such as a rise in the sea level from melting ice, life-threatening weather events and uncontrolled climate changes [3]. Amongst the coping strategies to reduce the anthropogenic impacts on the global ecosystems to prevent irremediable compromises, researchers are even more focused on the development of efficient and cost-effective methods to reduce the amount of released CO2 [4,5]. In addition to preventing emissions by switching energy sources to renewable technologies, capture, storage and utilization of carbon dioxide (CCS/CCU) are considered key processes for the perspective of the net-zero emission scenario to be pursued by 2050 [6,7]. Within this scope, membranes [8], liquid amine-based technologies [9] and calcium looping [10] are now considered attractive technologies for the separation and storage of CO2 in post-combustion conditions. It has to be taken into account that amine-based methods suffer from corrosion issues and the chemicals and regeneration processes are known to be expensive [9], whereas for calcium looping the thermal regeneration of the Ca-based sorbents, which is performed at approximately 1000–1200 °C, is energetically demanding [10].
Among the methods for the capture of CO2, adsorption consists of a cost-effective, selective and efficient process that involves the use of a solid material that employs non-covalent interactions between the surface of the adsorbent and the adsorbate molecules [11,12]. The key aspect of adsorption is the choice of the host material that should have a high specific surface area and high pore volume but also the capacity to interact with the guest molecules by means of specific surface features, such as the ions of functional groups [13].
Thanks to the well-defined microporous structure [14], high specific surface area and pore volume [14,15], zeolites are widely employed as molecular sieves for the adsorption of small molecules, such as CO2 [16]. Zeolites are a wide category of microporous crystalline aluminosilicates based on a three-dimensional framework of interconnected silicon and aluminum tetrahedral oxide units [14,17,18]. The isomorphic substitution of SiIV with AlIII atoms introduces in the framework a negative charge located on the bridging oxygen atom between the Si and Al atoms. These negative charges are balanced by extra-framework cations (i.e., Na+, Li+, K+ and Ca2+) or protons that are usually present within the cavities of the material [14,19].
Zeolites are both natural minerals and produced by synthetic procedures. For the synthesis of these materials, the control of composition and features is now leading the industrial production of zeolites [20,21]. Zeolites are classified according to the IZA (International Zeolite Association) coding [22].
By considering the growing need for efficient and cost-effective CO2 adsorbents, the work presented here deals with the synthesis of a zeolite A and a zeolite X from a natural aluminosilicate (kaolinite). These samples were both tested as solid adsorbents for the adsorption of CO2 at low gas pressures (≤1 bar) and 25 °C. Kaolinite is a largely available natural clay, featuring a layered structure based on a silicate tetrahedral layer and an aluminium-based octahedral layer according to the so-called TO (or 1:1) structure [11,23,24,25]. Its mineralogical composition is Al2Si2O5(OH)4. The mineral is a commodity for the industry of ceramics [26] and, in this work, it is employed as a cheap and widely available Si and Al source for the preparation of zeolite samples. By following the main steps reported in the literature [27,28,29], a zeolite A was prepared through the alkaline fusion of natural kaolinite, followed by hydrothermal treatment. Briefly, alkaline fusion involves the mineralization at high temperatures in the solid state with a Si- and Al-rich precursor in the presence of an alkali activator (i.e., NaOH) to form a mixture of silicate and aluminate salts [21,30,31]. Next, the obtained mixture is usually stirred with water and hydrothermally treated to promote gel crystallization [18].
In order to prepare high-purity zeolite X with a different SiO2/Al2O3 molar ratios with respect to kaolinite, the alkaline fusion process was optimized upon the addition of a secondary silicon source (sodium silicate) within the preliminary mixture. We focused our interest on zeolites X because they are considered one of the most performant adsorbent materials for CO2 adsorption [14,16]. In fact, these types of adsorbents have proven to be particularly suitable for CO2 adsorption applications retaining up to 3 mmol·g−1 at 1 bar and 25 °C [16,32].
As reported in the literature, each zeolite requires a specific SiO2/Al2O3 molar content to be prepared [29,33]. For zeolites A and zeolites X the SiO2/Al2O3 ratio of 2 and 3.2, respectively, is needed [29,33]. Indeed, the SiO2/Al2O3 molar ratio is a crucial parameter that has to be considered during the synthesis of zeolites [29,33]; this aspect is usually not explored in the context of the alkaline fusion processes of natural clays.
In light of these considerations, the aim of this work is to modify the Si and Al molar content in the preparation mixture to prepare a specific zeolite using the alkaline fusion process with a natural aluminosilicate. Moreover, by considering that the physicochemical properties (i.e., structural, morphological and textural properties) of zeolites prepared through the alkaline fusion of natural kaolinite have been only partially explored in the literature, in this manuscript, we provide a more detailed description about this topic. Thus, we have adopted a multi-technique approach, that includes XRPD; Rietveld semiquantitative analysis; 23Na, 27Al and 29Si MAS NMR spectroscopy; SEM-EDX; and N2 physisorption analysis at 77 K, to fully characterize the obtained materials.
Moreover, the excess adsorption of CO2 in zeolite A and zeolite X was assessed at 25 °C and at low gas pressures (i.e., ≤1 bar) by means of a stainless steel custom-built volumetric apparatus. In addition, the lifetime and stability of zeolite X prepared from kaolinite were investigated over two CO2 adsorption/desorption cycles. This aspect is not usually investigated within studies regarding the synthesis of zeolites from natural clay, but it can become of crucial importance to assess the lifetime and stability of an adsorbent material [11].

2. Materials and Methods

2.1. Materials

The chemicals that were used are natural kaolinite (Si2Al2O7·2H2O, CAS: 1318-74-7, M.W. 258.16 g·mol−1, SigmaAldrich®, Darmstadt, Germany), anhydrous sodium metasilicate (Na2SiO3, CAS: 6834-92-0, M.W. 122.06 g·mol−1, Alfa Aesar®, Haverhill, MA, USA) and sodium hydroxide (NaOH, CAS: 1310-73-2, M.W. 39.99 g·mol−1, Merck KGsA, Darmstadt, Germany).

2.2. Methods

2.2.1. Zeolite A Synthesis

Following the main experimental steps described in the literature [28,34], 2 g of natural kaolinite was ground with 4 g of NaOH to form the preliminary mixture, which was transferred inside a ceramic crucible and treated at 200 °C for 4 h in a muffle furnace. Subsequently, the solid was ground again in a ceramic mortar, and it was stirred inside a 100 mL round-bottom flask with 40 mL of deionized water for 2 h at 50 °C in a thermostatic water bath. Finally, the gel was transferred inside a steel autoclave equipped with a 100 mL PTFE vessel, and it was treated at 90 °C for 17 h inside an electric furnace. Next, the solid was recovered through Büchner filtration and washed with deionized water until a neutral pH, and then the powder was dried at 100 °C overnight. At the end of the process, 2.1 g of zeolite A (hereafter named ZEO A) was obtained.

2.2.2. Zeolite X Synthesis

A pure zeolite X was obtained upon the optimization of the alkaline fusion process described in Section 2.2.1. According to the literature, to prepare a zeolite X, a SiO2/Al2O3 molar ratio of 3.2 is required [29,33]. In this respect, by considering that the chemical formula of the parent kaolinite is Si2Al2O7·2H2O, to obtain the suitable Si and Al content, 0.97 g of Na2SiO3 was ground in a ceramic mortar with 2 g of kaolinite and 4 g of NaOH. Next, the mixture was treated at 200 °C for 4 h in a muffle furnace. Then, the solid was ground again, and the fine powder was stirred with 46 mL of deionized water in a 100 mL round-bottom flask at 50 °C for 2 h. The temperature was kept at 50 °C by means of a thermostatic water bath. Finally, the suspension was transferred inside a steel autoclave equipped with a 100 mL PTFE vessel, and the hydrothermal treatment of the gel was performed at 90 °C for 17 h. Subsequently, the solid was recovered through Büchner filtration and copiously washed with deionized water until a neutral pH was reached. After being dried at 100 °C overnight, 2 g of powder was obtained. This material was named ZEO X.

2.2.3. XRPD Analysis

The X-ray powder diffraction (XRPD) patterns of the samples were collected with a Bruker D8 Advance diffractometer (Karlsruhe, Germany) with Bregg–Brentano Geometry using a Cu anode X-ray source (λ = 1.5418 Å) equipped with a Ni filter and operating at 40 kV and 40 mA. The patterns were collected in a 2θ range of 5–50° 2θ, with 2θ steps of 0.02 and an acquisition time of 0.2 s/step. The anti-scatter knife was automatically synchronized, and the illumination area of the sample was kept constant at 17 mm2 all across the measurements. Semi-quantitative mineralogical analyses were conducted by the Rietveld method using the Topas 6.0 package (software was commercially supplied by Bruker AXS, Karlsruhe, Germany). The refinements were conducted in the whole 5–50° 2θ range by using a 5th-order Chebychev polynomial for the background calculation and allowing for refinement of the sample displacement; the crystal size of all structures was set freely to vary between 32 and 1000 nm.

2.2.4. SEM-EDX

The SEM micrographs of the different materials were obtained by means of a scanning electron microscope (SEM) JEOL JSM-5600 LV (Milan, Italy) operating at 15 KV with an EDS detector attachment. Before SEM analyses, the surfaces of the sample were sputtered with a 20 nm layer of platinum. For each sample, EDS measurements were performed three times in different areas, and the average results were calculated.

2.2.5. MAS-NMR Measurements

29Si, 27Al and 23Na NMR measurements were performed with a wide bore 11.75 Tesla magnet on a Bruker Avance III 500 spectrometer with operational frequencies of 99.35, 130.33 and 132.29 MHz, respectively. A sample spinning rate (magic angle spinning) of 10–15 kHz was used with a 4 mm triple resonance probe. All the 29Si MAS NMR spectra were recorded under high-power proton decoupling conditions while the 27Al MAS spectra were acquired with a large sweep width and small pulse angle (π/12) to ensure quantitative interpretation. Similarly, the 23Na MAS spectra were acquired with a π/12 pulse in the absence of 1H decoupling. For the 29Si, 27Al and 23Na NMR, relaxation delays of 60, 1 and 1 s, were used, respectively. Chemical shift referencing was performed using external standards, such as the Al(H2O)63+ ion in 1.0 M AlCl3 solution set to 0.0 ppm and TMS set to 0.0 ppm.

2.2.6. N2 Physisorption Analysis at 77 K

The N2 physisorption analyses were performed on an Autosorb 6100 (Anton Paar S.r.L., Rivoli, Italy) at 77 K in the pressure range between 1 × 10−7 and 1 P/P0 of relative pressure. Prior to the measurements, the samples were treated at 170 °C for 16 h. The specific surface area of the samples was determined by the Brunauer–Emmett–Teller (BET) multipoint method and the optimal linear range was selected by using a micropore BET assistant (IUPAC/Rouquerol method) from Kaomi 1.00 [Build:3] with Autosorb software (the obtained R2 was 0.9998). The pore size distributions were obtained through the DFT method (silica/zeolite, adsorption branch model, cylindrical pores).

2.2.7. CO2 Adsorption Measurements

A custom-built manual volumetric apparatus was employed to assess the CO2 adsorption capacity at 25 °C and up to 1 bar of gas pressure for the different materials. The snapshot and the building scheme of the volumetric apparatus are shown in Figure S1. For more details about the experimental steps to determine the excess of CO2 adsorbed see the Supporting Information.

3. Results

The structural properties of kaolinite, ZEO A and ZEO X were investigated by means of XRPD analysis. The XRPD patterns of the different materials are shown in Figure 1. XRPD patterns are indexed to identify the mineral phases present.
The XRPD pattern of the parent clay (Figure 1a) is mainly dominated by the reflections of kaolinite, thus giving an indication that the starting product has a high purity grade. In detail, the most intense reflections at about 12° and 25° 2θ are related to the (001) and (002) planes, whereas those between 20°–25° and 35°–40° 2θ are assigned to the (020), (110), (111), (0 21), (130), (130), (200), (003), (131), (131), (201), and (203) planes of kaolinite, respectively [11,35].
Moreover, the sample contains traces of quartz, whose reflection is at 27° 2θ (labelled with Q in Figure 1), and illite, chabazite and montmorillonite (marked with+) that are usually present as impurities within natural clays [36,37,38]. However, as can be derived from the results of the Rietveld semiquantitative analysis (Table S1), kaolinite is the most abundant phase (84 wt.%), whereas the other impurities represent only 16 wt.%.
The alkaline fusion process producing the sample ZEO A has led to a substantial change in the structural properties of kaolinite. As it can be derived from the results of the XRPD analysis (Figure 1b), the diffraction pattern of the product mainly contains the typical reflections of zeolite A [39,40] that were assigned by means of the DIFFRAC.EVA V2 software and PDF2 database (2004) to their corresponding Miller indexes. As it can be derived from the results of the Rietveld semiquantitative analysis (see Table S2), this phase corresponds to 80.1 wt.%, thus suggesting that the kaolinite was almost completely converted into a zeolite A. In addition, three exceedingly weak reflections at 6°, 23.2° and 26.6° 2θ (marked with asterisks) suggest that only tiny traces of a zeolite X are present, i.e., the 1.1 wt.% (see Table S2) [16,40]. A possible explanation of these results could be related to the presence of Si-rich impurities, such as quartz (SiO2), illite (Si4)(Al)2.3O10(OH)2(H2O) [41] and montmorillonite Na0.33(Al1.67Mg0.33)Si2O10(OH)2(H2O) [41] (see Figure 1a, Table 1 and Table S1), in the parent kaolinite. Since zeolites X require a higher Si content than zeolites A [29,33], it is probable that a small fraction of zeolite X could have formed. This hypothesis is corroborated by the fact that the reflections of quartz, illite and montmorillonite are not present in detectable amounts in the XRPD pattern of ZEO A (Figure 1b), thus indicating that these phases were solubilized along with kaolinite during the alkaline fusion process.
On the other hand, the XRPD pattern of ZEO X (Figure 1c) is dominated by the reflections of a zeolite X, as assigned by means of the DIFFRAC.EVA software and PDF2 database (2004) to their corresponding Miller indexes. No other reflections were observed, thus indicating that the impurities within the parent clay were mineralized along with kaolinite during alkaline fusion (see Table S2). The results of the analysis reveal that the addition of sodium silicate to increase the SiO2/Al2O3 ratio in the preliminary mixture from ca. 2 to 3.2 [29,33] is helpful to obtain a pure zeolite X.
To integrate the results of the XRPD analyses, the 27Al and 29Si MAS NMR measurements, which can reveal both crystalline and amorphous phases [42], were carried out (Figure 2). The 27Al MAS NMR spectrum of the parent kaolinite (Figure 2, left panel) is dominated by an intense signal at 3 ppm related to the Al nuclei in an octahedral coordination located within the octahedral layers of kaolinite and of montmorillonite impurities [27,38,43]. Moreover, in the tetrahedral region (i.e., between 50 and 70 ppm), two low-intensity bands (marked with asterisks) due to the Al nuclei in tetrahedral configuration within the tetrahedral layers of illite and montmorillonite impurities are visible [11,38]. The 29Si MAS NMR spectrum of kaolinite (Figure 2, right panel) is characterized by the presence of two sharp peaks at −91 and −92 ppm attributed to the Si nuclei in a tetrahedral configuration within the T-layers of kaolinite with three Si-O-Si bonds and one Si-O-Al bond formed through a non-bridging oxygen atom [Si(OSi)3(OAl)] [42,44].
The 27Al MAS NMR spectrum of ZEO A (Figure 2, left panel) shows the complete disappearance of the signal at 3 ppm due to the hexacoordinated Al nuclei, whereas a new peak at 60 ppm related to the Al nuclei in a tetrahedral coordination, typical of a zeolite A sample, appeared [45,46,47]. The shoulder at 63 ppm might be related to the presence of different tetrahedral Al species within ZEO A that might result from the slow dissolution of the parent kaolinite in alkali conditions [48].
The 29Si MAS NMR spectrum of ZEO A (Figure 2, right panel) is characterized by several signals. The most intense peak at −90 ppm corresponds to the Q4(4Al) Si nuclei in tetrahedral coordination, as expected for a zeolite A, that is characterized by four Si-O-Al bonds (i.e., [Si(OAl)4]) [45,46]. In our case, the other signals at −86 and −87 ppm are probably ascribed to other [SiO4]4− species formed during the alkali dissolution of natural kaolinite and which might have participated in zeolite germ formation [48].
The 27Al MAS NMR spectrum of ZEO X (Figure 2, left panel) contains two signals at 3 and 63 ppm. The band at 63 ppm is assigned to the 27Al nuclei in a tetrahedral coordination within the framework of zeolite X [49,50], whereas the shoulder at ca. 60 ppm might arise from different tetrahedral Al species within the sample derived from the slow dissolution of the parent kaolinite in alkali condition [48]. The presence of a low-intensity peak at 3 ppm suggests that the sample contains traces of residual hexacoordinated Al nuclei from the parent clay that were not mineralized during alkaline fusion. Crystalline phases related to possible species, consistent with this evidence, are not observed in the XRPD pattern of ZEO X (Figure 1c), probably because of their amorphous composition. The 29Si MAS NMR spectrum of ZEO X shown in Figure 2 (right panel) is characterized by different signals related to Si nuclei in tetrahedral coordination within the framework of the sample [51]. The most intense peak at −86 ppm is due to the Q4(4Al) Si nuclei [51] ([Si(OAl)4], as well as the band at −88 ppm is assigned to the Q4(3Al) Si nuclei [51] ([Si(OAl)3(OSi)]. At lower ppm, the band at −92 ppm is related to the Q4(2Al) species, hence the Si nuclei with two Si-O-Si bonds and two Si-O-Al bonds (i.e., [Si(OAl)2(OSi)2], whereas the signal at −96 ppm indicates the presence of Q4(1Al) Si nuclei that are characterized by one Si-O-Al bond and three Si-O-Si bonds, [Si(OSi)3(OAl)] [51].
By considering that NaOH was employed as a mineralizing agent for the synthesis of ZEO A and ZEO X, it is probable that both of the zeolites are in the Na+ form, hence with Na+ extra-framework cations that counterbalance the negative charges that arise from the presence of the AlIII atoms [14]. In this respect, 23Na MAS NMR spectroscopy provides an efficient tool to investigate this aspect. The 23Na MAS NMR spectra of kaolinite, ZEO A and ZEO X are shown in Figure 3.
In the 23Na MAS NMR spectrum of the pristine kaolinite (Figure 3), no signals from 23Na nuclei were observed, as natural kaolinites are usually characterized by a reduced isomorphic substitution ratio between the SiIV and AlIII atoms within the T-layers, hence the extra-framework cations that could be located in the inter-layer spaces are nearly completely absent [23,34,52].
The 23Na MAS NMR spectrum of ZEO A is characterized by a sharp peak at −1 ppm that is due to the Na nuclei balancing the zeolite A negative framework [53]. As reported in the literature, the extra-framework cations could populate three distinct lattice positions within a zeolite A (see below Figure S2A): the center of the six-rings on the threefold axis (inside the sodalite cage, Site I), in the eight-ring supercage of zeolite A (Site II) and in the four-ring site, near the entrance of the supercage (Site III) [40,54].
The 23Na MAS NMR spectrum of ZEO X (Figure 3) shows the presence of two broad signals at 1 and −1 ppm, which are both assigned to different species of Na+ extra-frameworks cations that populate different lattice positions [53]. In the case of zeolites X, a cation could be located in the sodalitic cages (site I/I’), in the hexagonal rings inside the cavities (site II/II’) and in the entrances to the supercages (site III/III’) [14] (see below Figure S2B).
The morphological properties of kaolinite, ZEO A and ZEO X samples were investigated by means of SEM microscopy. Typical micrographs of kaolinite (a), ZEO A (b) and ZEO (X) are shown in Figure 4.
The SEM micrograph of the parent clay (Figure 4a) reveals that kaolinite has a layered morphology, with clusters of lamellae of variable dimensions from 5 to 15 µm. After the alkaline fusion process and hydrothermal treatment, the particles composing ZEO A and ZEO X samples appear quite different from that of the parent sample. In fact, ZEO A is characterized by well-defined cubic-shaped crystallites of about 0.5–1 µm, whereas ZEO X is made up of cubic/octahedral crystallites of ca. 0.5–1 µm. Similar results were observed in the literature [28,55].
The chemical composition of the different materials was determined through EDX analysis (Table 1). The chemical composition of the parent kaolinite (Table 1) is typical of an aluminosilicate material. The at.% of Si is slightly higher than that of Al, as the parent clay contains quartz, illite and montmorillonite impurities
Na and K are present as extra-framework cations to counterbalance the negative charges arising from the isomorphic substitution between Si and Al atoms in the T-layers of kaolinite and montmorillonite impurities [56]. However, their at.% is low, as montmorillonite impurities are present in a small amount (see Table S1) and the T-layers of natural kaolinites are usually characterized by a low isomorphic substitution ratio [23,34,52]. For this reason, no signals related to the 23Na nuclei were observed in the 23Na MAS NMR spectrum of kaolinite (Figure 3).
The at.% of Si and Al within ZEO A is quite similar to that of the parent clay (Table 1), as no additional Si and/or Al sources were added to the preliminary mixture. As was previously described, Na+ cations are now present as extra-framework cations to counterbalance the negative charges due to the presence of Al atoms [14].
On the other hand, ZEO X is characterized by Si and Al at.% of 16.60% and 13.18%, respectively (see Table 1), with the Si/Al molar ratio of 1.26. These results are in agreement with the literature and due to the fact that sodium silicate was added. To give an example, in Z. Tahraoui’s work [40], it is reported that a Na-X zeolite has a Si/Al molar ratio of 1.23. The difference in the ZEO X sample is consistent with the signals of octahedral Al moieties in 27Al MAS NMR (Figure 2, left panel), showing that a part of the starting kaolinite, probably in amorphous form, which is commonly found after thermal treatments, was not converted during the optimized alkaline fusion. The presence of Na comes from the use of NaOH for the alkali fusion process [14].
The textural properties of the parent kaolinite, ZEO A and ZEO X were investigated by means of N2 physisorption analysis at 77 K. The N2 adsorption–desorption isotherms of the samples and the DFT pore size distribution of ZEO X (silica/zeolite, ads. branch method, cylindrical pores) are shown in Figure 5. In Table 2 the SSABET and the pore volumes of the different materials are reported.
The N2 physisorption isotherm of kaolinite (Figure 5A) is characterized as a type III isotherm, typical of a non-porous material [57]. In this case, the host–guest interactions between the surface of the parent clay and the N2 molecules are relatively weak, and the absence of hysteresis is consistent with the almost complete lack of mesopores [57,58]. The negligible gas uptake between 0 and ca. 0.7 P/P0 suggests that the N2 molecules are clustered at the adsorption sites located on the surface of the particles and in the macropores arising from the aggregation of the clusters of lamellae [57,58]. The SSABET of the parent kaolinite corresponds to 9 m2·g−1, whereas its cumulative pore volume is 0.035 cc·g−1.
The N2 physisorption isotherm of ZEO A appears quite similar to that of kaolinite. Again, the isotherm reflects a type III isotherm, free of a hysteresis loop and typically found for non-porous materials [57,58]. This result was already observed in the literature; the Na+ extra-framework cations located in four-ring sites, near the entrances of the supercages (Site III, see Figure S2A), hinder the diffusion of the N2 molecules within the micropores [39,40,59]. Hence, no gas uptake is observed between 0 and 0.8 P/P0, and the adsorbate molecules occupy the external surfaces of the sample and the macropores due to the crystal’s aggregation [57,58]. Despite several other evidence (i.e., XRPD and MAS NMR) suggesting that the sample has an evident LTA microporous structure, N2 physisorption analysis at 77 K could not provide a correct estimation of the SSABET and the pore volume of ZEO, with results that are much lower than the real ones and correspond to 11 m2·g−1 and 0.035 cc·g−1.
The N2 physisorption isotherm of ZEO X is reported in Figure 5. Despite the first section of the curve reflecting a type I model, the presence of an H4 hysteresis loop suggests that the curve could be more compatible with a type IV(a) isotherm [57,58]. The H4 hysteresis loop is often found in the adsorption isotherms of zeolites due to aggregated crystals [57,58], and the steep N2 uptake at low relative gas pressures is attributed to enhanced host–guest interactions that occur in narrow micropores [57,58]. In fact, as it can be derived from the results of the DFT analysis (Figure 5B), ZEO X is characterized by a narrow pore size distribution centered at 7 Å. As reported in the literature, the pore size distribution of zeolites X in the Na+ form should be approximately 10 Å [60]. This difference is due to the fact that in our case N2 was used, whereas the literature result was obtained from an Ar adsorption isotherm [60]. No other peaks were observed, thus suggesting that the main contribution to the pore volume of the sample mainly comes from the micropores, which show a micropore volume of 0.19 cc·g−1, whereas those of meso- and macropore are only 0.05 cc·g−1. The SSABET of ZEO X has been assessed at 460 m2·g−1.
The excess adsorption of CO2 in the different zeolites was assessed by means of a custom-built volumetric apparatus (for more details about the building scheme and the experimental steps see Supporting Information). The CO2 excess adsorption isotherms up to ca. 1 bar of gas pressure and 298 K are shown in Figure 6.
Kaolinite has a reduced CO2 uptake, equal to 0.02 mmol/g, which is due to its low SSABET and pore volume. However, ZEO A, which has similar SSA and pore volume compared to the parent kaolinite, shows a much larger CO2 adsorption capacity of ca. 2.37 mmol of CO2 per gram of material at 25 °C and ca. 1 bar of gas pressure. The highest adsorption capacity is reached by ZEO X with a CO2 uptake of 2.60 mmol/g. The results are in agreement with those found in the literature [16,32]. Moreover, as reported in Figure S3, the CO2 adsorption measurement on ZEO X has been repeated upon thermally treating the same sample at 160 °C for 12 h under vacuum resulting in the same adsorption capacity. Thanks to the presence of a high microporous volume and narrow micropores of 7 Å (Figure 5B, Table 2), the CO2 adsorption isotherm of ZEO X shows a steep CO2 uptake between 0 and 0.3 bar. In fact, at low gas pressures and ambient temperature, micropores represent the most energetic and favorable adsorption sites for CO2 molecules [61,62]. A similar trend can be observed for ZEO A. However, the CO2 adsorption capacity of ZEO A below 0.2 bar is higher than that of ZEO X. This behavior might be related to the different pore sizes of ZEO A and ZEO X. In fact, as reported in the literature, zeolites A with Na+ extra-framework cations are characterized by an 8-ring supercage of ca. 4 Å [63]. As reported in the literature, at low gas pressure, the density of the CO2 molecules is highest in smaller pores, while at higher gas pressures larger pores show a higher adsorptive capability [61,62]. Thereby, the higher CO2 adsorption capacity of ZEO A below 0.2 bar can be explained, as ZEO X is characterized by micropores of 7 Å, whereas, based on the literature [63], it is reasonable to assume that the micropore width of ZEO A should have micropores of ca. 4 Å. As it has already been reported in the literature [14], the observed high affinity between the adsorbent and the adsorbate can be explained by considering that the permanent quadrupole moment of the CO2 molecule can give rise to non-covalent electrostatic interactions with Na+ cations [14,15]. An additional effect acting on the electropositive C atom of the CO2 molecule is its interaction with the BrØnsted oxygen (Al-O−-Si) through an acid–base mechanism [14,15].

4. Conclusions

In this work, natural kaolinite was used as a more cost-effective and widely available source of Si and Al for the synthesis of a zeolite A (ZEO A) and a zeolite X (ZEO X) under mild conditions. ZEO A was obtained through the alkaline fusion of the parent clay in the presence of NaOH as an alkali activator, whereas a high-purity zeolite X, coded as ZEO X, was synthesized upon the addition of sodium metasilicate to the mixture along with kaolinite and NaOH to obtain the favorable SiO2/Al2O3 molar ratio for zeolites X.
The results of the XRPD measurements and the 29Si and 27Al MAS NMR measurements show that ZEO A contains traces of a zeolite X phase, which represents ca. the 1 wt.% that was probably formed due to the partial mineralization of some Si-rich impurities within the parent kaolinite (such as quartz, illite and montmorillonite), which would have slightly increased the Si content of the preliminary mixture. The results of the Rietveld semiquantitative analysis revealed that almost all the kaolinite phases within the parent clay have been successfully converted into the zeolite A phase. On the other hand, the XRPD pattern of ZEO X is dominated only by the reflections of a zeolite X, and based on the Rietveld semiquantitative analysis and the detected crystalline phases, it was determined that the zeolite X phase approached 100 wt.%. However, the results of the 27Al MAS NMR measurement revealed the presence of some residues or by-products with amorphous structures, containing octahedrally coordinated Al.
Both the morphological characterization using SEM and the elemental analysis using EDX showed clearly the conversion of the layered clay to cubic-shaped microcrystals and the incorporation of Na+ to compensate for the charge effects from the Si/Al variance. The textural properties of the different materials were investigated through N2 physisorption analysis. Kaolinite is characterized by a low SSABET, equal to 9 m2·g−1 and a pore volume limited to intercrystalline mesoporosity. A similar result, with an apparent absence of microporosity, was observed for ZEO A, as, according to the literature, the Na+ extra-framework cations lying at the entrances of the supercage hinder the diffusion of N2 molecules into the cavities of the sample. On the other hand, due to the different structure, ZEO X shows a SSABET of 460 m2·g−1, a microporous volume of 0.19 cc·g−1 and a total pore volume of 0.24 cc·g−1.
To evaluate a possible application in the field of carbon capture and storage, CO2 adsorption at 25 °C and up to 1 bar was assessed for all the synthesized materials. The excess adsorption of the parent kaolinite, ZEO A and ZEO X was investigated by means of a prototype volumetric apparatus. Kaolinite shows a very limited CO2 adsorption capacity, while the CO2 adsorption capacity of ZEO A and ZEO X were found to be 2.60 and 2.37 mmol/g, respectively, at ca. 1 bar of gas pressure and 25 °C. Moreover, it was demonstrated that the ZEO X sample can be thermally regenerated without any significant loss of its CO2 adsorption capacity.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr12010194/s1, Table S1: Rietveld semiquantitative analysis of the parent kaolinite (no external standard for amorphous phase determination was used); Table S2: Rietveld semiquantitative analysis of ZEO A and ZEO X (no external standard for amorphous phase determination was used); Figure S1: Sites that can be populated by extra-framework cations within a zeolite A, (a), and a zeolite X, (b), adapted from [14,54]; Figure S2: Snapshot, (A), and building scheme, (B), of the custom-built volumetric apparatus; Figure S3: CO2 adsorption isotherms at 25 °C of a commercial carbon performed on the volumetric apparatus (rhombus) and on the automatic system; Figure S4: CO2 adsorption isotherm on ZEO X at 25 °C and up to ca. 1 bar of gas pressure (rhombus). The same sample was thermally treated at 160 °C for 12 h under vacuum and the adsorption measurement was repeated following the same procedure (triangles). (see Ref. [64])

Author Contributions

Conceptualization, G.C., E.B. and C.B.; methodology, G.C., F.B. and G.P.; validation, G.C. and F.B.; investigation, G.C., F.B., L.M. and G.P.; resources, C.B., L.M. and E.B.; data curation, G.C., F.B. and G.P.; writing—original draft preparation, G.C., G.P. and F.B.; writing—review and editing, C.B., V.M. and E.B.; supervision, C.B. and E.B.; project administration, C.B.; funding acquisition, C.B., L.M. and E.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the SATURNO project “Organic waste and carbon dioxide transformed into fuels, fertilizers and chemicals; concrete application of circular economy” (FESR 2014/2020) technological platform (Bioeconomy I.1b.2.2_14/20_Bioeconomics—Project code 333-19), Piedmont region (2019–2022). E.B. acknowledges the Università del Piemonte Orientale founding scheme “Fondi di Ateneo per la Ricerca anno 2017” for the economic support of the project “Processi sostenibili finalizzati alla valorizzazione di materiali e materiali naturali per usi funzionali”.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRPD patterns of kaolinite (a), ZEO A (b) and ZEO X (c). Illite, chabazite and montmorillonite impurities within the parent kaolinite are marked with + symbols, whereas the zeolite X impurities within ZEO A are highlighted with * symbols.
Figure 1. XRPD patterns of kaolinite (a), ZEO A (b) and ZEO X (c). Illite, chabazite and montmorillonite impurities within the parent kaolinite are marked with + symbols, whereas the zeolite X impurities within ZEO A are highlighted with * symbols.
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Figure 2. 27Al (left panel) and 29Si MAS NMR spectra (right panel) of kaolinite, ZEO A and ZEO X. The signals related to the 27Al nuclei in tetrahedral coordination within the illite and montmorillonite impurities in the parent kaolinite are marked with ‘*’.
Figure 2. 27Al (left panel) and 29Si MAS NMR spectra (right panel) of kaolinite, ZEO A and ZEO X. The signals related to the 27Al nuclei in tetrahedral coordination within the illite and montmorillonite impurities in the parent kaolinite are marked with ‘*’.
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Figure 3. 23Na MAS NMR of kaolinite, ZEO A and ZEO X.
Figure 3. 23Na MAS NMR of kaolinite, ZEO A and ZEO X.
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Figure 4. SEM micrographs at a magnification of 2000× of kaolinite (a), ZEO A (b) and ZEO X (c).
Figure 4. SEM micrographs at a magnification of 2000× of kaolinite (a), ZEO A (b) and ZEO X (c).
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Figure 5. N2 adsorption-desorption isotherms of kaolinite (triangles), ZEO A (squares) and ZEO X (rhombus), (A) and DFT pore size distribution (silica, ads. branch model, cylindrical pores) of ZEO X (rhombus), (B).
Figure 5. N2 adsorption-desorption isotherms of kaolinite (triangles), ZEO A (squares) and ZEO X (rhombus), (A) and DFT pore size distribution (silica, ads. branch model, cylindrical pores) of ZEO X (rhombus), (B).
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Figure 6. CO2 adsorption isotherms at about 1 bar of gas pressure and 25 °C on kaolinite (triangles), ZEO A (squares) and ZEO X (rhombus). Prior to the analyses, the sample were treated at 180 °C for 17 h under vacuum (residual pressure < 10−3 mbar).
Figure 6. CO2 adsorption isotherms at about 1 bar of gas pressure and 25 °C on kaolinite (triangles), ZEO A (squares) and ZEO X (rhombus). Prior to the analyses, the sample were treated at 180 °C for 17 h under vacuum (residual pressure < 10−3 mbar).
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Table 1. EDX analysis of kaolinite, ZEO A and ZEO X. The average of three measurements are reported.
Table 1. EDX analysis of kaolinite, ZEO A and ZEO X. The average of three measurements are reported.
KaoliniteZEO AZEO X
ElementAt.%Dev.stAt.%Dev.stAt.%Dev.st
Si19.360.5419.850.3916.600.22
Al16.460.8016.590.3313.180.18
O63.411.5951.060.7658.290.49
Na0.210.1612.500.3111.930.19
K0.570.07////
Table 2. SSABET and pore volume of kaolinite, ZEO A and ZEO X.
Table 2. SSABET and pore volume of kaolinite, ZEO A and ZEO X.
SSABET [m2·g−1]Pore Volume [cc·Å−1·g−1]
V micro [<20 Å]V meso [20–500 Å]
Kaolinite9/0.035
ZEO A11/0.035
ZEO X4600.190.05
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Celoria, G.; Begni, F.; Paul, G.; Boccaleri, E.; Merlo, V.; Marchese, L.; Bisio, C. Zeolites Derived from Natural Kaolinite for CO2 Adsorption. Processes 2024, 12, 194. https://doi.org/10.3390/pr12010194

AMA Style

Celoria G, Begni F, Paul G, Boccaleri E, Merlo V, Marchese L, Bisio C. Zeolites Derived from Natural Kaolinite for CO2 Adsorption. Processes. 2024; 12(1):194. https://doi.org/10.3390/pr12010194

Chicago/Turabian Style

Celoria, Giorgio, Federico Begni, Geo Paul, Enrico Boccaleri, Valentino Merlo, Leonardo Marchese, and Chiara Bisio. 2024. "Zeolites Derived from Natural Kaolinite for CO2 Adsorption" Processes 12, no. 1: 194. https://doi.org/10.3390/pr12010194

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