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Article

Comparison of the Synthesis Method of Zeolite Catalysts Based on Pozzolan, Pumice, and Ignimbrite Applied to the Sustainable Pyrolysis of Polymers

by
Luis Fernando Mamani-De La Cruz
1,
Rossibel Churata
2,
Angel Gabriel Valencia-Huaman
1,
Sandro Henry Fuentes-Mamani
1 and
Jonathan Almirón
1,*
1
Professional School of Environmental Engineering, Faculty of Process Engineering, National University of San Agustín de Arequipa, Santa Catalina Street No. 117, Arequipa 04001, Peru
2
Professional School of Materials Engineering, Faculty of Process Engineering, National University of San Agustín de Arequipa, Santa Catalina Street No. 117, Arequipa 04001, Peru
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(7), 2986; https://doi.org/10.3390/su17072986
Submission received: 16 January 2025 / Revised: 17 March 2025 / Accepted: 19 March 2025 / Published: 27 March 2025
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Abstract

:
This study aims to synthesize sustainable zeolite catalysts by taking advantage of the great abundance of natural precursors, such as pozzolana, ignimbrite, and pumice, found in the southern zone of Peru. Different methodologies were selected. On the one hand, an alkaline fusion/hydrothermal reaction with NaOH processes was utilized and, on the other hand, the hydrothermal method was employed. The characteristics of these catalysts and their application in the catalytic pyrolysis of polypropylene were evaluated. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were employed to investigate the structure and properties of the obtained catalysts. Catalytic pyrolysis experiments of polypropylene were carried out at 450 °C for 30 min with a 6% w/w zeolite catalyst. It was possible to synthesize zeolites similar to commercial zeolites such as ZSM-5 and zeolite X, with a BET surface area of up to 451.3 m2/g−1, offering the possibility of obtaining commercial products from natural materials. According to the results obtained in the pyrolytic process, method 1 (alkaline fusion/hydrothermal reaction with NaOH) presents the best results, with 94% in liquid and gaseous products. The zeolite synthesized with the pozzolan precursor was the most successful, followed by pumice.

1. Introduction

Zeolites are aluminosilicate structures characterized by their three-dimensional networks. Their crystalline and organized system is formed by tetrahedral units interconnected through oxygen, which forms strong covalent bonds with silicon [SiO4]4− and aluminum [AlO4]5− [1,2]. Zeolites are primarily microporous materials, with micropores (<2 nm) present in their three-dimensional structure. While mesopores (2–50 nm) and macropores (>50 nm) can also be found, these larger pores are less common and are not as prominently present in the synthesized material without post-synthesis treatments [3,4]. The three-dimensional structure of zeolites provides a high number of Bronsted and Lewis acid sites, offering distinctive spaces for catalytic activities [5]. Supported by their strength and acid density, they enhance the catalyst’s lifespan [6,7,8]. Additionally, their physicochemical properties become adjustable during synthesis to obtain high-rank zeolites [9]. To achieve these qualities, the raw materials for synthesis must be readily available, economical, high in Si and Al content, and possess a high yield, among other criteria [10], ensuring high production efficiency and fulfilling requirements for final applications, such as the removal of heavy metals in industrial waters with zeolite A and X [11] or ammonium removal in wastewater with activated natural zeolites [12] and synthesized from volcanic ashes [13].
On the other hand, the type of zeolite obtained depends on the natural precursor and the method of obtaining it. For example, Lee et al. synthesized Na-A and Z-S1 zeolites using the fusion/hydrothermal method from volcanic slag, achieving a crystallization efficiency level of 61.8% and reducing the crystal size to approximately 1.0 µm [14]. Novembre et al. synthesized Na-X and HS zeolites from volcanic material (Tripoli rock and ignimbrite) under hydrothermal conditions (80 °C) using alkaline silicates (NaxSiyOz) and alkaline aluminates (NaxAlyOz). [15]. Otieno et al. synthesized three types of zeolites (Na-X, Na-P, and Na-HS) from volcanic silica, demonstrating that high-purity zeolites can be obtained through the hydrothermal method. This method involves treating aluminates and silicates in alkaline media at elevated pressures and low to moderate temperatures. It is the preferred method for zeolite synthesis because it mimics the natural conditions under which zeolite minerals form in the Earth’s crust [16]. Several studies have been conducted to synthesize zeolites using the hydrothermal method, experimenting with the SiO2/Al2O3 ratio of the precursor, the temperature and duration of the hydrothermal reaction, and the type and concentration of the alkali mixture (NaOH, Na2CO3, and KOH) [14]. However, it has generally been found that the hydrothermal method for zeolite synthesis requires prolonged crystallization times. Therefore, many researchers have introduced fusion/hydrothermal methods, such as alkaline fusion/desilication with NaOH, fusion/hydrothermal reactions with Na2CO3, and alkaline fusion/hydrothermal reactions with NaOH, to improve crystallization conditions [14].
The variety of zeolites that can be obtained from volcanic precursors using different synthesis methods is remarkable, and even more noteworthy is their applicability in catalytic pyrolysis and chemical recycling. These zeolites have the potential to convert organic waste, such as used tires, plastic waste, packaging materials, and others, into liquid hydrocarbons, gases, and coal [17], reintegrating them into the industry’s value chain as substitutes for fuels and raw materials in the production of chemicals. The thermal cracking of polymers typically occurs within a temperature range of 300 to 900 °C in an inert atmosphere. However, to reduce energy costs, pyrolysis and catalysts are useful as they lower the process temperature to 250 to 600 °C, increase the yield of pyrolysis products, and even improve the quality of the obtained chemical raw materials [18]. Zeolites serve as potential catalysts due to their high acidity, appropriate selectivity, and good structural arrangement [19], aiding in breaking long carbon chains into smaller ones, forming better aromatic bonds, increasing the octane rating, etc. [20,21]. Despite limitations such as heat supply [22], the generation of heavy waxes, deactivation of synthetic zeolites used in pyrolysis, and the appropriate selection of catalysts [23], progress in the catalytic pyrolysis of petroleum-derived plastics continues. The wide variety of zeolites obtained from precursors with high Si and Al contents, as well as those already known for industrial-scale applications, such as the HZSM-5 zeolite, tend to form a higher amount of light oils, aromatic and cyclic compounds, and short chains during the pyrolysis process. This results in a broad distribution of hydrocarbons, including hydrogen (H2), methane (CH4), ethylene (C2H4), etc. [24,25,26]; by considering differentiated petroleum-based polymers such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET) [27], polyvinyl chloride (PVC), and municipal mixed plastic waste, efforts are being made to synthesize and improve the results in the catalytic pyrolysis of plastics. Cocchi et al. synthesized an HX zeolite from coal fly ash by applying catalytic cracking to a petroleum derivative, improving energy yield and the amount of light hydrocarbons (C6-C9) and achieving results that are very comparable to the commercial H-USY zeolite, an excellent heterogeneous catalyst [28]. Obali et al. synthesized mesoporous catalysts following a hydrothermal route, measuring the catalysts’ activity in the catalytic pyrolysis of PP, showing a notable reduction in the degradation temperature and activation energy to 24–28 kJ·mol−1 [29]. Additionally, Luo et al. synthesized a kaolin-based zeolite with acid modification and, by applying catalytic pyrolysis to PP, obtained aromatic oils typical of gasoline, as well as combustible gases (H2, CO, and CH4) [30].
Consequently, the present research aims to develop catalysts in a sustainable way by utilizing the great abundance of natural precursors, such as pozzolan, ignimbrite, and pumice found in the southern region of Peru, by applying differentiated methodologies of alkaline fusion/hydrothermal reactions with NaOH and the hydrothermal method. This study seeks to evaluate the synthesis of synthetic zeolites and their application in the catalytic pyrolysis of virgin polypropylene. The goal is to analyze the correlation between the structure of zeolites obtained through synthesis methods and their catalytic performance.

2. Materials and Methods

2.1. Materials

Zeolites were synthesized from three natural precursor materials from the province of Arequipa in Peru: pozzolan (PO), pumice (PU), and ignimbrite (IG).
The conditioning of the precursor materials involved crushing, followed by grinding in a ball mill. Additionally, sieving was conducted to obtain fine particles able to pass through a N°140 mesh sieve (106 μm) according to ASTM E11 [31]. The raw material obtained after sieving was dried in a muffle furnace at 100 °C and subsequently characterized by X-ray fluorescence (XRF) using Rigaku dispersive fluorescence equipment, model NEX QC + QuantEZ (Austin, TX, USA). Table 1 shows the chemical composition results for the three precursors; it is observed that they have high contents of silicon and aluminum oxides (>87%), which are essential components for the synthesis of zeolites [32].

2.2. Zeolite Synthesis

The synthesis process of the zeolites was carried out using three different precursor materials (PO, PU, and IG) and three methodologies: the alkaline fusion/hydrothermal reaction method with NaOH, the alkaline fusion/hydrothermal reaction method with concentrated NaOH, and the hydrothermal method. The following three synthesis methodologies were used: methodology 1 (M1), methodology 2 (M2), and methodology 3 (M3). Each methodology worked with the three precursors mentioned and the synthesized zeolites were washed until reaching a pH in the range of 8 to 9. These three synthesis conditions are based on other studies that obtained good results in the production of zeolites and pyrolysis of materials [33,34,35]. The three conditions were also applied to each precursor to observe the influence of the type of volcanic material.

2.2.1. Methodology 1: Alkaline Fusion/Hydrothermal Reaction Method with NaOH

A dry treatment with NaOH (Sigma Aldrich, St. Louis, MO, USA, 99.999%) was initiated, mixing it with each precursor material in a weight ratio of NaOH/precursor of 1.2/1. The mixture was ground in a mortar to homogenize the particle sizes. This solid mixture was calcined at 500 °C for 1 h. The resulting sample was cooled, ground again in a mortar, and mixed with distilled water in a proportion of 10 times its weight. This mixture was then subjected to magnetic stirring at room temperature for 16 h. The resultant mixture underwent hydrothermal treatment in a Teflon-lined stainless steel autoclave reactor at 90 °C for 7 h (see Table 2). The resultant mixture was filtered and washed with HCl (0.5 mol/L) from CV avantor (Ecatepec, Mexico), to slightly lower the pH to 9. It was then dried at 105 °C for 12 h. The dried material was mixed with NH4Cl (2 mol/L) also of the brand CV avantor, in a weight ratio of material/NH4Cl of 4, and stirred in a reflux system for 3 days at 70 °C. The mixture was filtered and washed 3 times, dried at 100 °C for 12 h, and finally calcined at 400 °C for 4 h.

2.2.2. Methodology 2: Alkaline Fusion/Hydrothermal Reaction Method with Concentrated NaOH

This method involved alkaline fusion followed by hydrothermal treatment. Initially, an acid treatment was applied before synthesis to remove impurities from the materials. Each material, after being crushed and sieved, was treated with an HCl (1 mol/L) solution. Specifically, 50 g of material was mixed with 250 mL of the acid solution and subjected to magnetic stirring at 90 °C for 2 h. The solid was then filtered, washed three times with deionized water, and dried at 110 °C for 24 h. The acid-treated materials were mechanically mixed with ground sodium hydroxide in a NaOH/material weight ratio of 1.2. They were then subjected to fusion in a muffle furnace at 550 °C for 1 h, cooled to room temperature, and ground to fine particles.
The molten powder was mixed with water in a proportion of 5 times its weight and stirred at room temperature for 3 h. The mixture was then placed in a Teflon-lined stainless steel autoclave and underwent hydrothermal treatment under static conditions at 90 °C for 12 h (see Table 2). Subsequently, the mixture was cooled, filtered, and washed with HCl (0.5 mol/L) to lower the pH to below 9. The filtered solid was dried at 105 °C for 12 h to remove moisture.

2.2.3. Methodology 3: Hydrothermal Method for Directing ZSM-5-Type Zeolite Synthesis

Twelve grams of each material were mixed with 100 mL of NaOH (3 mol/L), applying reflux agitation at 60 °C for 3 h to form a mixed solution of silicate and aluminate. Subsequently, 31.9 g of sodium silicate (Na2SiO3) from Scharlab (Barcelona, Spain), was added to the solution at room temperature without reflux, progressively over 10 min, followed by 18.93 g of tetrapropylammonium bromide (TPABr) with 98% purity (Sigma-Aldrich, Bangalore, India), added at 0.42 g/min, and left under agitation for an additional 3 h. This step directs the formation of ZSM-5-type zeolites [36]. After agitation, the pH of the obtained gel is reduced to 11, and it undergoes hydrothermal treatment in a Teflon-lined stainless steel autoclave at 160 °C for 72 h, followed by cooling to room temperature overnight (see Table 2). The methodology continues with washing and filtering the material to reach a pH range of 7–9, followed by drying in a muffle furnace at 100 °C for 24 h to remove the organic template (TPABr). Then, the material was calcined in a furnace at 540 °C for 5 h.
To obtain protonated ZSM-5 zeolite, a reflux dissolution was carried out at 50 °C for 4 h with NH4Cl (1 mol/L) with a ratio of 30 mL of NH4Cl per gram of product. The mixture was then filtered and washed to remove chloride ions. The entire procedure was repeated, with the only variation being the agitation time, which in the second part was 6 h. The process concluded with drying the product in a muffle furnace at 100 °C overnight and then calcining it in a tubular furnace at 540 °C for 5 h.

2.3. Characterization of the Zeolites Obtained

The nine synthesized zeolites obtained by the three methodologies were subjected to Fourier-transform infrared (FTIR) analysis, performed using Perkin Elmer Frontier FT-IR/NIR equipment (Waltham, MA, USA), and with a scanning range of 4000 to 650 cm−1. To identify and characterize the crystalline phases, X-ray diffraction (XRD) tests were performed with a Rigaku Miniflex 600 X-ray diffractometer (Tokyo, Japan), using a CuKα radiation source, measuring at 40 kV and 15 mA in the 2θ range of 3–90°. The morphological characteristics of the zeolites were analyzed with a Hitachi SU8230 scanning electron microscope (Hitachi High-Tech, Tokyo, Japan), with backscattered electrons, with image magnifications of 20,000× and 25,000×. Textural characterization was performed with a Micromeritics Instrument Corporation model Gemini VII 2390 surface area analyzer (Norcross, GA, USA) using gaseous nitrogen as an adsorbent with an evacuation rate of 1000 mmHg·min−1 and an equilibration time of 5 s.

2.4. Catalytic Pyrolysis

Catalytic pyrolysis was carried out in a reactor consisting of a quartz tube 1 m in length with an internal diameter of 6 cm coupled to a horizontal tubular furnace with a room temperature adjustment capacity of up to 1200 °C. The pyrolysis conditions were set to a temperature of 450 °C for 30 min. Commercial polypropylene (PP) 5707N from SABIC (Ningbo, China) was used as the recycling material for the pyrolysis. According to the technical data sheet, the PP samples had a grain size of approximately 2 mm. These samples were placed in a rectangular crucible along with the zeolite catalyst in a proportion of 6% w/w, arranged in layers (PP as the base and the catalyst on top). The catalyst’s particle size passed through a N° 140 sieve (106 µm in diameter). They were then placed inside a quartz tube that passed through a tubular furnace to keep the chamber free of oxygen and avoid combustion reactions. N2 was used at a flow rate of 250 L·min−1. The gases generated were captured in the cooling system, consisting of a gas trap submerged in liquid nitrogen. At the end of the pyrolysis time at the established temperature, the sample was removed to cool it to room temperature and, finally, four products were obtained. These were classified into liquid products, gaseous products, waxes, and solids. The liquid fraction was then characterized using Fourier-transform infrared (FTIR) in Frontier FT-IR/NIR equipment from Perkin Elmer, with a scanning range of 4000–650 cm−1.

3. Results

3.1. XRD

The XRD patterns of the precursors and zeolites synthesized with the three methodologies are shown in Figure 1. It can be observed that the IG precursor (Figure 1B) presents higher crystallinity compared to PO (Figure 1A) and PU (Figure 1C), which have more amorphous phases. These phases are identified through the presence of a characteristic halo or hump at 2θ between 15 and 40° [37], which would indicate the presence of thermodynamically metastable amorphous aluminosilicate structures [38]. Despite the higher crystallinity of IG, the composition of this precursor has the highest aluminum content (see Table 1), which indicates that the Si/Al ratio is lower than that of PO and PU. In this sense, Murukutti’s studies reveal that the aluminum content has a considerably crucial effect on the crystallization of zeolite particles [39]. When comparing the XRD curves of the zeolites obtained by each precursor, independently of the synthesis method used, it is observed that the width and intensity of the peak tend to decrease with the Si/Al ratio; thus, the curves of the zeolites obtained with PU and PO, with a Si/Al ratio of 9.1 and 9.4, respectively, are wider than the zeolites from IG, which have a Si/Al ratio of 5.9. These results are in agreement with the studies of Sun et al. [40].
Figure 1A shows the XRD patterns of the initial precursor PO and the zeolites PO-M1, PO-M2, and PO-M3 synthesized using the three methods. The peaks that appear for PO are mainly crystalline phases of anorthoclase and muscovite. Furthermore, it can be seen that the PO-M3 zeolite obtained from using the hydrothermal method presents peaks with greater intensities compared to PO-M1 and PO-M2, which were obtained by the alkaline fusion/hydrothermal reaction method, which can be attributed to the fact that a zeolite with greater crystallinity is achieved with method 3 [41]. Likewise, the XRD patterns show the presence of peaks associated with commercial zeolites; thus, for zeolites PO-M1 and PO-M2, there are peaks associated with zeolite X, with peaks (2θ) at 6°, 10°, 15.3°, 20°, and others [42]. Likewise, some peaks associated with zeolite A are identified for zeolite PO-M2. In the case of the PO-M3 zeolite, obtained using the hydrothermal method, peaks are present between 7° to 9° and between 22° to 25° (2θ). This is characteristic of the MFI structure and therefore also presents a similar structure to zeolite ZSM-5 [33,34]. Figure 1B shows the XRD patterns of the IG precursor and the IG-M1, IG-M2, and IG-M3 zeolites synthesized using the three methods. The IG precursor is the main crystalline phase, as well as PO, which is anorthoclase, followed by cristobalite. Zeolites IG-M1 and IG-M2 also present some peaks associated with zeolite X. IG-M3, on the other hand, presents peaks between angles that are similar to those of the zeolite ZSM-5. For the IG-M2 zeolite, some peaks associated with zeolite A are still observed. Finally, Figure 1C shows the XRD patterns of PU, which has anorthoclase and tremolite as its main crystalline phases. Likewise, the diffraction phases of the zeolites synthesized from this are also visualized. PU-M3, obtained by the hydrothermal method, is the zeolite with the least amorphous phases. The zeolites PU-M1 and PU-M2 also presented with the peaks of zeolite X [33] and zeolite A when synthesized with method 2. On the other hand, PU-M3 also has the same structure as ZSM-5, similar to PO-M3 and IG-M3. On the other hand, with the peak deconvolution method, the relative crystallinity of the synthesized zeolites was determined [43]. The crystallinities of PO-M1, PO-M2, and PO-M3 were 50, 58, and 62%, respectively. For IG-M1, IG-M2, and IG-M3, crystallinity values were 56, 55, and 62%, respectively. For PU-M1, PU-M2, and PU-M3, crystallinity values were 39, 45, and 52%, respectively. This confirms that zeolites synthesized from the PU precursor are more amorphous and that zeolites synthesized with method 3 are the most crystalline.
It is observed that only zeolites synthesized using method 3 have DRX phases similar to a ZSM5. This is because the synthesis conditions are based on other studies that obtained the same results. This corresponds to a longer reaction time and temperature, the addition of Al to improve the Si/Al ratio, and the addition of TPABr.

3.2. BET Analysis

The XRD patterns of the precursors and zeolites synthesized with the three methodologies are shown in Figure 1.
The textural properties of zeolites obtained by precursor type and methodology are summarized in Table 3. These were analyzed using the BET (Brunauer–Emmett–Teller) and BJH (Barrett–Joyner–Halenda) methods. The results show that zeolites synthesized using methodology 2 exhibit the best textural characteristics, with a higher specific surface area, greater micropore area, and greater micropore volume, qualifying them as mesoporous materials. This study is similar to that of Murukutti et al., where A- and X-type zeolites were synthesized following acid treatment, demonstrating high potential for ion exchange applications and environmental pollutant removal [39]. Methodology 1 was the least effective, providing the lowest textural properties and resulting in smaller specific surface areas and higher porosity due to larger pore sizes, which could be suitable for obtaining long polymer chains (higher molecular weight) in catalytic plastic pyrolysis. Although these are still mesopores, better control of the Si/Al ratio is needed to better guide crystal structuring [40], as larger particle sizes significantly reduce specific surface areas [44]. Methodology 3 shows interesting results, presenting medium specific surface areas compared to the other methodologies but with the smallest pore diameters overall, with PO-M3 being the smallest at 3.8 nm. These quantitative results are more closely aligned with the qualitative observations provided by SEM, where well-organized, regular structures similar to ZSM-5 are observed, corroborated by the results obtained from the BET method. This zeolite exhibits characteristics close to microporous structures due to the organic template used in M3. However, its limitation could be attributed to amorphous impurities in the precursor material [45], Additionally, the hydrothermal treatment time would be beneficial for increasing specific surface areas and forming an MFI structure, as studied by Kordatos et al. [46].
In general, zeolites synthesized by M2 and M3, regardless of the precursor, exhibit impressive characteristics, including a BET surface area of 156.5–451.3 m2·g−1, a pore volume of 0.053–0.212 cm3·g−1, and an average pore diameter of 3.8–6.0 nm. These attributes make these zeolites highly porous materials with significant surface areas and porosity, suitable for various applications including catalysis, adsorption, and separation processes. Comparing precursor types in M2 and M3, higher surface areas are achieved with PO, followed by IG and then PU. This could be due to PO having more amorphous phases according to the XRD results and IG being the precursor with the highest aluminum content and the lowest Si/Al ratio of 5.9, as these characteristics have a crucial impact on zeolite particle crystallization and surface area [37,39,47]. High crystallinity in zeolites can lead to a higher surface area and hence a higher number of active sites [42]. This is observed in the results of this study, where the zeolites synthesized from PO and IG with better crystallinity present higher specific surface areas. On the other hand, with M2, the zeolites present higher surface areas. This is due to the acid washing to remove impurities, which helps to improve the crystallinity [33]. In M1 and M3, there was no acid washing; however, the fact that there was a greater surface area with M3 is due to the fact that the length and temperature of synthesis were both greater, which allows the crystals to grow more completely, leading to greater porosity and resulting in a greater surface area [35].
On the other hand, regardless of the applied method, Table 3 shows a higher micropore volume in the synthesized PO zeolites, which is due to the high Si/Al ratio in their composition, unlike GI, which has a lower ratio [39]. Likewise, PU also presents a similar Si/Al ratio to PO, but the low micropore volume of the synthesized zeolites can be attributed to the low degree of crystallinity [40] since they present more amorphous phases compared to the other zeolites. However, a higher Si/Al ratio may lead to lower acidity, since aluminum, which can act as an acid site, is found in lower amounts [30].

3.3. FTIR

Figure 2 shows the spectrum present in the pozzolan starting material and the respective zeolites synthesized by each method. When comparing the curve of the initial precursor with the curves of the zeolites formed, the appearance of new peaks can be observed, with one of them located between 1642–1629 cm−1. This belongs to the stretching vibrations of -OH, corresponding to the water present in the zeolite [48]. This peak is more intense in PO-M2, followed by PO-M1, and the lowest intensity can be found for PO-M3. This same trend is seen in Figure 3 for the ignimbrite precursor and zeolites with peaks between 1626–1640 cm−1 and in Figure 4 for the pumice precursor and zeolites with peaks between 1630–1639 cm−1.
Figure 2, Figure 3 and Figure 4 show the spectra present in the starting materials (precursors) and their respective zeolites synthesized by each method. The images are followed by Table 4, which shows the identification of the characteristic predominant peaks. The peaks are identified between the bands of 1070–970 cm−1, associated with the vibrations of the asymmetric stretching of Si-O-T (T = Si or Al) [47,49], which is present in all the zeolites synthesized and where it is observed that method 2, corresponding to alkaline fusion/hydrothermal reactions with concentrated NaOH, is the one with the highest intensity reflected in the peaks of the three types of zeolites (PO-M2, IG-M2, and PU-M2) for the three precursors. The band range between 795–660 cm−1 corresponds to the symmetric stretching vibrations of Si-O-T and these, in turn, are divided into two types of vibrations: vibrations related to the external bond of the tetrahedron (795–780 cm−1) and the internal vibrations of the tetrahedron (730–660 cm−1) [50].
The bands between 3405–3275 cm−1 and 1643–1625 cm−1 belong to the stretching vibrations of -OH (water) present in the zeolite. When these bands are identified in Figure 2, Figure 3 and Figure 4, it is observed that the intensities of the bands corresponding to -OH in the zeolites synthesized by method 3 are low or null, which may be due to the calcination at 540 °C for 5 h that was carried out at the end of the hydrothermal synthesis process, which eliminated almost all water, while method 2 produces bands with greater intensity, followed by method 1.
The bands between 1210 and 1219 cm−1 of method 3 are associated with the formation of zeolite type ZSM-5 and are corroborated by the other bands observed between 1052–1024 cm−1 and 793–785 cm−1 [47,51]. On the other hand, methods 1 and 2 present the bands of functional groups characteristic of H-X [52] and Na-X zeolites, respectively [52,53].
The absorption peaks near 1540 cm−1 and 1450 cm−1 are assigned to a Brönsted acid and a Lewis acid, respectively [54]. These peaks are only seen in the curves of the methodology 2 spectra, for all PO, PU, and IG precursors. The Brønsted acid sites (BAS) of the zeolite originate from the positive bridging OH group in the unit (SiOHAl) in the presence of protons, while the Lewis acid sites (LAS) are generated from various species, including those with extra distorted structures, associated structures, and aluminum structures with tetra and penta coordination [8,55]. The acidic properties of zeolite-based catalysts are determined by the acid site density, Al location, and Al distribution [56]. The cracking performance of zeolite-based catalysts in plastics is directly proportional to the BAS strength but has a discrete relationship with LAS [8,57].

3.4. SEM Analysis

The SEM analysis was obtained from various types of precursors and synthesized zeolites, which vary in morphology depending on the methodology used. Figure 5 presents the precursors pozzolan (PO, Figure 5a), ignimbrite (IG, Figure 5b), and pumice (PU, Figure 5c), showing the presence of irregular structures with vitreous columns and abundant fragmentation in all precursors. However, specifically in PU, abundant flat and stacked fragmented particles are also observed, similar to PO but in smaller quantities.
The first column of Figure 6 presents SEM images of zeolites obtained through the alkaline fusion/hydrothermal reaction method with NaOH (1 mol/L) applied to each precursor (PU, IG, and PO). It is observed that Figure 6a (PO-M1) and Figure 6c (PU-M1) show the crystalline growth of zeolite X (Faujasite group) and A (LTA group) forms, resembling rice grains and with a non-uniform and very small particle size, which may be due to the short treatment time (7 h) compared to M2 (12 h) and M3 (72 h). Some particles are covered by smaller, irregular particles, evidencing a rough surface with indistinguishable facets. A similar result was obtained by Sabatino et al. in the early hours of synthesis using the kaolinite precursor, establishing that with a longer treatment time, the structure could undergo several changes between zeolites A and X [58]. Additionally, Xu et al.’s study confirmed that in the conventional methodology, a low (non-uniform) heating rate promoted irregular nucleation, and as time passed, the formation of hydroxysodalite structures increased [59]. Figure 6b (IG-M1) evidenced the formation of zeolite X with its typical rhombohedral shape, which was replaced by sodalite (sodalite group), forming a spherical morphology integrated by thin layers with dimensions greater than 2 µm. This could be due to the SiO2 and Al2O3 content presented by ignimbrite, which has a Si/Al ratio of 5.9, promoting the formation of the hydroxysodalite phase [16,39]. The intergranular crevices in the precursor shown in Figure 5b also help to increase the surface area and structural porosity [40].
The second column of Figure 6 presents the micrographs of zeolites synthesized by the alkaline fusion/hydrothermal reaction method with concentrated NaOH (2 mol/L). Figure 6d (PO-M2) evidenced the form of the X-type zeolite, with its octahedral morphology in the manner of integrated rhombuses with an approximate dimension of 1 µm. Compared to PO-M1 using the same precursor, a difference in size and shape is observed, which could be attributed to the variation in concentrations and the longer hydrothermal treatment time following fusion, as mentioned in Murukutti et al.’s study [39]. On the other hand, the result obtained for ignimbrite in Figure 6e (IG-M2) was not as pure, as some of the crystals have the octahedral morphology of the X-type zeolite, measuring approximately 1 µm, accompanied by another pseudocubic rosette-type structure characteristic of the Na-P1 zeolite (Gismondine group), which may have formed through a heterophase nucleation mechanism during the hydrothermal treatment [60]. This could also be due to the considerable calcium concentration in the treated precursor [61]. For the structure presented in Figure 6f (PU-M2), the cubic morphology typical of A-type zeolites is observed, with a dimension of approximately 0.5 µm. Additionally, other fibrous spherical structures typical of sodalite, composed of thin nanosheets with an orderly assembly arrangement, are observed. These results were also obtained in studies by Li et al. and Zou et al., which established that with longer hydrothermal treatment times, the initial structure collapses, forming sodalite, a more thermodynamically stable structure compared to its predecessor and intermediate phases [62,63].
Finally, the third column presents zeolites obtained using methodology 3, the hydrothermal method for directing the synthesis of ZSM-5-type zeolites with a TPABr template. Figure 6g (PO-M3) shows an elongated hexagonal structure [39] of approximately 3 µm, resembling acicular crystalline aggregates typical of the ZSM-5 zeolite (MFI group), although it shares similarities with ZSM-22 and ZSM-23 structures due to similar formation phases [64,65]. Figure 6h (IG-M3) also presents a smaller hexagonal morphology of approximately 1 µm, which is characteristic of the ZSM-5 zeolite. This result is very similar to that obtained by Chareonpanich et al. and Kordatos et al., who worked with rice husk ash and a similar methodology, achieving larger and purer zeolites at higher temperatures [46,66]. In Figure 6i (PU-M3), hexagonal rod-like structures similar to those in Figure 6g are observed, but there is also a significant amount of non-zeolitized amorphous material, indicating the need for a higher alkaline concentration to improve nucleation [35], as sodium salts are more effective than hydroxide for structuring [44]. Comparing all the precursors used in this study (PO, IG, and PU) for the zeolites obtained through methodology 3, PU-M3 and IG-M3 showed a better degree of zeolitization, which could be attributed to their higher Al content and lower Si/Al ratio [40].

4. Pyrolysis Products

Figure 7 shows the distribution of the products obtained by the pyrolysis of PP with the zeolites synthesized from the three precursors and with the three methods. In the pyrolysis of PP using zeolites synthesized with M1 (alkaline fusion method/hydrothermal reaction with NaOH), the amount of solids generated was minimal, so it is not shown in the graph. A little wax was also generated, followed by gas, and oils were generated in a greater quantity. A similar result was obtained by Cocchi et al. [28] when they produced more liquid than gas using a zeolite obtained with a similar method. The best efficiency in the pyrolysis of PP was with the PO-M1 zeolite since it had an efficiency of 94.2%, as measured by the weight of oil and gas (desired products), with a low amount of wax (5.7% by weight). Following very closely is the PU-M1 zeolite, which obtained only a slightly higher amount of wax (6% by weight).
With M2 (alkaline fusion method/hydrothermal reaction with concentrated NaOH), the amount of solids generated was also minimal, so it is not shown in the graph. However, with this method, a greater amount of wax was generated for the three zeolites and, to a lesser extent, gas and oils. Of the desired products, a greater amount of gas is generated; this can be supported by the study of Cocchi et al. [28], who also obtained a higher percentage of gas in the pyrolysis of PE and PP with a zeolite X synthesized with a similar method. With this method, the best efficiency was with the PO-M2 zeolite as it produced 63.3% by weight of oil and gas, followed by IG-M2 with 56%, and finally PU-M2 with 48.3%.
With M3 (hydrothermal method for directing the synthesis of zeolite type ZSM-5), the amount of solids generated was also minimal, so it is not shown in the graph. Gases were the most generated fraction, followed by wax and, to a lesser extent, oils. Taking into account that the zeolites obtained with this method are similar to the commercial ZSM-5, it could be said that similar results were obtained in the study by Santos et al. [36] when PE and PP were subjected to pyrolysis with ZSM-5, where a higher percentage of gas than liquid was also produced under the same pyrolysis conditions. Similarly, in another study, high yields of gaseous products and light liquid products were also obtained in the pyrolysis of PP with the ZSM-5 zeolite [19]. The best efficiency was with the PU-M3 zeolite as it produced 82.3% by weight of oil and gas, followed by IG-M3 with 69.7%, and finally PO-M3 with 66.6%. These results demonstrate the usefulness of the zeolites synthesized as, compared to another study [47] with the same working conditions, the thermal pyrolysis only reached an efficiency of 59.2%. However, these results do not reach the efficiency of the commercial zeolite ZSM-5, with which 90.8% efficiency was obtained.
Comparing the synthesis methods applied, it can be seen that M1 is the most efficient in the production of oil and gas, followed by M3 and then M2. It can be understood that there are differences in the pyrolysis products of M1 and M2 with M3 as, with the latter, zeolites similar to ZSM-5 are obtained, while with M1 and M2, zeolites similar to X are obtained. When comparing methods 1 and 2, the difference in the products is notable, and better results are obtained with method 1. Despite the fact that the highest surface area values were obtained with M2, it is important to understand that in comparison with small molecule cracking reactions, the unique characteristic of the cracking of polyolefins, such as PP, lies in the obstacle caused by the polymer chain, which restricts the contact between the polymer and the active sites of the catalyst [67]. Consequently, this reaction imposes higher demands on both the concentration of the active center and the surface area value, as well as accessibility. In addition, the cracking of polyolefins depends mainly on the breaking of C–C bonds, so a higher acid density and zeolite strength are also required [8]. On the other hand, although it is true that better efficiency was obtained in the production of oils with M1, greater efficiency was obtained in the production of gases with M3. In addition, although it is true that M3 and M2 produced less oil than M1, the oil generated had a light color with a light appearance. In contrast, the M1 oils were darker and more viscous. This would suggest that more oil was obtained with M1, but possibly they are heavy oils such as tar, which is composed of condensed aromatic hydrocarbons and high-molecular-weight non-volatile linear hydrocarbon compounds that are insoluble in pentane [68]. In addition, the acidity of zeolite can be an important factor as it activates plastic molecules, which can facilitate their decomposition during pyrolysis [19]. It can also influence the formation of products during pyrolysis by favoring the formation of aromatic hydrocarbons [20,21,30]. In this study, the presence of Bronsted and Lewis acid sites was found in zeolites synthesized with M2, which would explain why the oils produced were lighter than the oils produced with M1 zeolites. This confirms the selectivity of the presence of acid sites.
Comparing the zeolites synthesized, it can be said that for M1 and M2, zeolites similar to the commercial X are obtained. With the PO precursor, better results are obtained compared to the zeolites synthesized with IG and PU. On the other hand, with method 3, where zeolites similar to ZSM-5 are obtained, it is with the PU precursor that a zeolite with better efficiency in the pyrolysis of PP is obtained, followed by IG and then PO. In all cases, the amount of solids (Char) produced is always below 0.5%, so it is negligible in the graph.

Analysis of Pyrolysis Liquid Products

Figure 8 shows the band spectra of the liquid products obtained from the pyrolysis process with the nine zeolites synthesized by the three methods, and Table 5 shows the correspondence between the observed bands and functional groups. Figure 8 shows the presence of functional groups corresponding to alkanes in the oils obtained with the nine zeolites synthesized, followed by groups of aromatic compounds, and with low-intensity bands corresponding to the functional group of alkenes. The oils obtained by M2 are those that obtained the highest presence of alkanes, according to the high intensity of the 3072 and 1650 cm−1 bands [69], and a higher presence of aromatic compounds according to the 888 cm−1 band. The latter could have been formed through cyclization and dehydrogenation reactions. The oils obtained by M3 also show the presence of aromatic compounds of a lower intensity than M2. Another difference is that methodology 2 obtained more disubstituted aromatic compounds (band 888 cm−1), whereas M3 presents more monosubstituted aromatic compounds (band 694 cm−1), with the exception of the oils obtained by the PU-M3 zeolite, which present a reduced and almost null band intensity. In this study, the gases and waxes generated were not characterized, but according to the literature, the gases produced in the pyrolysis of PP can be composed of C3H6, CH4, and H2 [70], but there is also a percentage of CO2 [71]. On the other hand, the waxes could be composed of ketones and esters since this composition was determined in another study, albeit in the pyrolysis of PET [72].

5. Conclusions

By applying the three synthesis methods, zeolites similar to commercial zeolites such as ZSM-5 and X were obtained. It was clearly noted that ZSM-5 was obtained with method 3 and zeolite X with method 1 and method 2.
The characteristics of the zeolites obtained by method 2 (alkaline fusion method/hydrothermal reaction with concentrated NaOH) are the most outstanding, with a BET surface area result of 289.6–451.3 m2·g−1, a pore volume of 0.102–0.212 cm3·g−1, and an average pore diameter of 5.6–6.0 nm.
According to the results obtained in the pyrolytic process, method 1 presents the best results, with 94% of liquid and gaseous products. The zeolite synthesized with the pozzolan precursor was the most successful, followed by pumice. Method 3 is the next in efficiency with 82%, followed by method 2, in which lower percentages of liquids and gases were obtained. However, the oils produced with method 1 could be associated with heavy oils, as opposed to methods 2 and 3, whose oils took on a light tone.
The composition of the liquid products (oils) obtained by the nine zeolites is dominated by aliphatic compounds, followed by aromatic compounds and then olefins. The aromatic compounds are characterized by a higher calorific value than the others, which means that method 2 is ideal for producing oils with potential for energy use.

Author Contributions

Conceptualization, S.H.F.-M., A.G.V.-H. and L.F.M.-D.L.C.; methodology, S.H.F.-M., A.G.V.-H. and L.F.M.-D.L.C.; software, S.H.F.-M., A.G.V.-H. and L.F.M.-D.L.C.; validation, J.A. and R.C.; formal analysis, J.A.; investigation, S.H.F.-M., A.G.V.-H. and L.F.M.-D.L.C.; resources, J.A. and R.C.; data curation, R.C.; writing—original draft preparation, S.H.F.-M., A.G.V.-H. and L.F.M.-D.L.C.; writing—review and editing, J.A. and R.C.; visualization, R.C. and J.A.; supervision, J.A.; project administration, J.A.; funding acquisition, J.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financed by the Universidad Nacional de San Agustín de Arequipa-Perú with the project: “Obtención de Zeolitas a partir de Materiales Naturales de Origen Volcánico para Aplicación en Pirólisis Catalítica para el Reciclaje Químico Sostenible de Polímeros” (contract number: IBA-IB-39-2020-UNSA).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank the Universidad Nacional de San Agustín de Arequipa-Perú through Contract N°IBA-IB-39-2020-UNSA for its funding of this research. We would also like to thank Francisco Velasco, professor at the Universidad Carlos III de Madrid, for his valuable collaboration in the research.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. XRD of the PO, IG, and PU precursors and the zeolites obtained under the three studied methods (M1, M2, and M3). (A) Precursor and zeolites of pozzolan (PO); (B) precursor and zeolites of ignimbrite (IG); (C) precursor and zeolites of pumice (PU).
Figure 1. XRD of the PO, IG, and PU precursors and the zeolites obtained under the three studied methods (M1, M2, and M3). (A) Precursor and zeolites of pozzolan (PO); (B) precursor and zeolites of ignimbrite (IG); (C) precursor and zeolites of pumice (PU).
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Figure 2. FTIR of the PO precursor and the zeolites obtained with the three methodologies: PO-M1, PO-M2, and PO-M3.
Figure 2. FTIR of the PO precursor and the zeolites obtained with the three methodologies: PO-M1, PO-M2, and PO-M3.
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Figure 3. FTIR of the IG precursor and the zeolites obtained with the three methodologies: IG-M1, IG-M2, and IG-M3.
Figure 3. FTIR of the IG precursor and the zeolites obtained with the three methodologies: IG-M1, IG-M2, and IG-M3.
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Figure 4. FTIR of the PU precursor and the zeolites obtained with the three methodologies: PU-M1, PU-M2, and PU-M3.
Figure 4. FTIR of the PU precursor and the zeolites obtained with the three methodologies: PU-M1, PU-M2, and PU-M3.
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Figure 5. SEM images of the natural precursors: (a) Pozzolana (PO), (b) Ignimbrite (IG), and (c) Pumice (PU).
Figure 5. SEM images of the natural precursors: (a) Pozzolana (PO), (b) Ignimbrite (IG), and (c) Pumice (PU).
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Figure 6. SEM images of the zeolites synthesized with different methodologies: (a) PO-M1, (b) IG-M1, (c) PU-M1, (d) PO-M2, (e) IG-M2, (f) PU-M2, (g) PO-M3, (h) IG-M3, and (i) PU-M3.
Figure 6. SEM images of the zeolites synthesized with different methodologies: (a) PO-M1, (b) IG-M1, (c) PU-M1, (d) PO-M2, (e) IG-M2, (f) PU-M2, (g) PO-M3, (h) IG-M3, and (i) PU-M3.
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Figure 7. Percentage of pyrolysis products by using the synthesized zeolites of the three methods.
Figure 7. Percentage of pyrolysis products by using the synthesized zeolites of the three methods.
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Figure 8. FTIR transmittance spectra of PP pyrolysis oils from the three methods.
Figure 8. FTIR transmittance spectra of PP pyrolysis oils from the three methods.
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Table 1. Chemical Composition of Pozzolan, Pumice, and Ignimbrite.
Table 1. Chemical Composition of Pozzolan, Pumice, and Ignimbrite.
OxidePO (%)PU (%)IG (%)
SiO277.9078.6067.60
Al2O315.1014.8020.30
K2O3.653.492.29
Fe2O31.331.354.43
CaO1.301.144.22
TiO20.260.230.60
Si/Al9.109.405.90
Other0.460.3890.56
Table 2. Zeolite Synthesis Conditions.
Table 2. Zeolite Synthesis Conditions.
MethodZeolite PrecursorSample CodeMerger ConditionsHydrothermal Treatment Conditions
Temperature
(°C)		NaOH/Precursor Ratio by WeightTemperature (°C)Time (h)Liquid/Solid Ratio by WeightNaOH
(mol/L)
1PozzolanPO-M1
IgnimbriteIG-M15001.290710-
PumicePU-M1
2PozzolanPO-M2
IgnimbriteIG-M25501.290125-
PumicePU-M2
3PozzolanPO-M3
IgnimbriteIG-M3--160728.33
PumicePU-M3
Table 3. Results of the textural properties of zeolites synthesized by methodology (M1, M2, and M3).
Table 3. Results of the textural properties of zeolites synthesized by methodology (M1, M2, and M3).
ZeoliteSurface Specific AreaMicropore AreaMicropore VolumePore Diameter
m2·g−1m2·g−1cm3·g−1nm
PO-M156.83.40.00149.6
IG-M137.11.30.00048.5
PU-M1103.00.90.00008.3
PO-M2451.3408.20.2126.0
IG-M2365328.50.1705.6
PU-M2289.6196.60.1025.6
PO-M3229.4114.40.0763.8
IG-M3205.4146.90.0764.8
PU-M3156.5101.30.0535.8
Table 4. Characteristic FTIR bands of zeolites synthesized from pozzolan, ignimbrite, and pumice.
Table 4. Characteristic FTIR bands of zeolites synthesized from pozzolan, ignimbrite, and pumice.
Material/ZeoliteStretching/Bending
H-O-H		Asymmetric Stretching
Si-O-T		Symmetric Stretching
Si-O-T
cm−1cm−1cm−1cm−1cm−1
PO--1007780724
PO-M133861639993-683
PO-M234041642971-664
PO-M3-16291052785-
IG--1008792730
IG-M132781638999-710
IG-M233861640973-664
IG-M3-16261065793-
PU--996-706
PU-M1334816381003-670
PU-M234041639971-664
PU-M3-16301024787722
Table 5. Functional groups of FTIR spectra of PP pyrolysis oils.
Table 5. Functional groups of FTIR spectra of PP pyrolysis oils.
Peaks
cm−1		BondFunctional Group
3072Stretching =C-HAlkenes
2960–2955Asymmetric C-H stretching of CH3Methyl alkanes
2925–2915Asymmetric C-H stretching of CH2Methylene alkanes
2875, 2870Symmetric C-H stretching of CH3Methyl alkanes
1654–1648C=C stretching Alkenes or aromatics
1460–1455Asymmetric C-H stretching of CH3Methyl and methylene alkanes
1379–1376C-H plane bending (scissoring) of CH2Methyl alkanes
970–964C-H symmetrical bending of CH3Alkenes
888C-H bendingAromatics
795C-H out-of-plane bendingAromatics
740–728C-H out-of-plane bendingMethylene Alkanes
694C-H plane bending (Swing) of CH2Aromatics
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Mamani-De La Cruz, L.F.; Churata, R.; Valencia-Huaman, A.G.; Fuentes-Mamani, S.H.; Almirón, J. Comparison of the Synthesis Method of Zeolite Catalysts Based on Pozzolan, Pumice, and Ignimbrite Applied to the Sustainable Pyrolysis of Polymers. Sustainability 2025, 17, 2986. https://doi.org/10.3390/su17072986

AMA Style

Mamani-De La Cruz LF, Churata R, Valencia-Huaman AG, Fuentes-Mamani SH, Almirón J. Comparison of the Synthesis Method of Zeolite Catalysts Based on Pozzolan, Pumice, and Ignimbrite Applied to the Sustainable Pyrolysis of Polymers. Sustainability. 2025; 17(7):2986. https://doi.org/10.3390/su17072986

Chicago/Turabian Style

Mamani-De La Cruz, Luis Fernando, Rossibel Churata, Angel Gabriel Valencia-Huaman, Sandro Henry Fuentes-Mamani, and Jonathan Almirón. 2025. "Comparison of the Synthesis Method of Zeolite Catalysts Based on Pozzolan, Pumice, and Ignimbrite Applied to the Sustainable Pyrolysis of Polymers" Sustainability 17, no. 7: 2986. https://doi.org/10.3390/su17072986

APA Style

Mamani-De La Cruz, L. F., Churata, R., Valencia-Huaman, A. G., Fuentes-Mamani, S. H., & Almirón, J. (2025). Comparison of the Synthesis Method of Zeolite Catalysts Based on Pozzolan, Pumice, and Ignimbrite Applied to the Sustainable Pyrolysis of Polymers. Sustainability, 17(7), 2986. https://doi.org/10.3390/su17072986

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