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Article

Hydrothermal Synthesis of Zeolites from Volcanic Ash from Ubinas and Its Application in Catalytic Pyrolysis of Plastic Waste

by
Jonathan Almirón
1,*,
Rossibel Churata
1,
María Vargas
2,
Francine Roudet
3,
Katia Valverde-Ponce
1,
Carlos Gordillo-Andia
1 and
Danny Tupayachy-Quispe
1
1
Laboratorio de Ciencia de los Materiales, Universidad Católica de Santa María, Samuel Velarde 320, Arequipa 04013, Peru
2
General Sciences, Continental University, Av. los Incas, José Luis Bustamante y Rivero District, Arequipa 04002, Peru
3
Laboratoire de Génie Civil et Géo-Environnement, ULR 4515, Université de Lille, Villeneuve d’Ascq, F-59000 Lille, France
*
Author to whom correspondence should be addressed.
Processes 2025, 13(11), 3376; https://doi.org/10.3390/pr13113376
Submission received: 11 August 2025 / Revised: 17 October 2025 / Accepted: 17 October 2025 / Published: 22 October 2025
(This article belongs to the Section Chemical Processes and Systems)
Browse Figures
Versions Notes

Abstract

The valorization of volcanic ash as a raw material for advanced functional materials offers dual benefits for both the environment and technology. Firstly, it diverts waste from landfills, thereby reducing the environmental footprint of volcanic deposits. Secondly, it contributes to the circular economy by transforming an abundant natural residue into a high-value product. In this study, zeolites were synthesized from the ash of the Ubinas volcano via the hydrothermal method in an alkaline medium. A systematic investigation was conducted to ascertain the influence of NaOH concentration and reaction temperature on synthesis efficiency and final material properties. The crystalline phases and morphology of the products were characterized using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM), while textural and thermal properties were evaluated through the Brunauer–Emmett–Teller (BET) method and Thermogravimetric Analysis (TGA). The results revealed that both temperature and NaOH concentration significantly affected the physicochemical properties of the zeolites. Four zeolite types were obtained; among them, Zeolite Z4 (synthesized with 3 M NaOH at 150 °C) exhibited the highest adsorption capacity, with a specific surface area of 35.60 m2/g, while Zeolite Z1 (synthesized at 120 °C with 1.5 M NaOH and 27.85 m2/g) displayed superior thermal stability and crystallinity. These variations in thermal and textural properties were reflected in the catalytic pyrolysis performance of polypropylene (PP). Zeolite Z3 (synthesized at 150 °C with 1.5 M NaOH) achieved the highest gaseous product yield (80.2%), despite lacking the expected zeolitic crystalline phases. In contrast, Zeolite Z2 (synthesized at 120 °C with 3 M NaOH) yielded 57.7% gaseous products and stood out for its predominant analcime phase, characteristic of zeolitic materials. In summary, this study demonstrates that volcanic ash-derived zeolites not only enhance synthesis efficiency and functional performance but also represent a sustainable strategy for waste valorization, closing material loops and enabling the recovery of high-calorific gaseous products from plastic waste.

1. Introduction

Ubinas volcano is located in the Central Volcanic Zone (ZVC) which is part of the Andean Volcanic Belt [1]. It is located 94 km north of the Moquegua region in Peru, in the General Sánchez Cerro province, Ubinas district. It is the most active volcano in Peru, with a height of 1400 m from its base at 4270 m.a.s.l. The Ubinas Volcano presented 26 eruptive episodes in the last 500 years whose rocks emitted by explosions after 2014 had an andesitic composition [2].
The mineralogical composition of volcanic ash depends on the magma chemistry and eruption conditions [3,4]. Volcanic ash is made up of pyroclastic material (<2 mm) and exhibits pozzolanic activity [3]. Generally, volcanic ash is made up of a large quantity of oxides such as SiO2, Al2O3, CaO, Fe2O3, MgO, K2O and Na2O [4].
Previous studies carried out to characterize the ashes of the Ubinas volcano determined that they are mainly rich in alumina and silica [5,6]. Likewise, research has been carried out on the use of volcanic ash for the manufacture of concrete, replacement of cement and development of geopolymers [5,6,7,8,9]. Additionally, volcanic ash has been used for zeolite synthesis [10,11,12].
Zeolites are hydrated aluminosilicate minerals with a three-dimensional framework of tetrahedral units with ion exchange and adsorption properties that allow the removal of heavy metals and contaminants from water and wastewater [10,11]. Also, zeolites have several applications in petrochemical, construction, agriculture, manufacturing of chemicals, detergents, among others [12,13,14]. Zeolites can be manufactured using different synthesis methods such as conventional hydrothermal method, microwave heating, emulsion, sol–gel, among others [13,15].
The hydrothermal method is the most widely employed technique for zeolite synthesis. It is typically conducted at relatively low temperatures, around 100 °C, and may be enhanced by alkaline fusion [16].
Various studies have demonstrated the feasibility of using different raw materials for zeolite synthesis. Terzano et al. [17] used waste from glass bottles and aluminum cans that were ground, mixed into a sodium hydroxide solution, and heated at 60 °C for one week, yielding crystalline type A zeolites with 30% efficiency. Espejel-Ayala et al. [18] synthesized type X zeolite from alum sludge combined with sodium hydroxide at elevated temperatures, followed by a hydrothermal treatment. Furthermore, they produced type A zeolite by modifying the hydrothermal process parameters and mixing alum sludge with sodium hydroxide at 550 °C for 2 h [19].
Behin et al. [20] combined coal fly ash with sodium hydroxide for 12 h at 60 °C, obtaining a Si:Al ratio of 1:1 by adding sodium aluminate. The resulting gel was exposed to microwave radiation to produce type A zeolite crystals. Kokloku et al. [21] used clays as a source of silica and alumina for the synthesis of ZSM-5 zeolites. These clays were ground, calcined, and subjected to acid leaching to increase the Si/Al molar ratio, followed by hydrothermal treatment at a crystallization temperature and time of 190 °C and 24 h, respectively. Belviso et al. [11] used ash from Mount Etna volcano with sodium hydroxide at 550 °C for 1 h. Some samples were agitated with seawater and others with distilled water, then aged hydrothermally for 4 days at 45, 60, and 70 °C, obtaining type X zeolite and sodalite. Similarly, Martínez-del-Pozo et al. [22] synthesized type X zeolite from volcanic ash via hydrothermal synthesis at 100 °C, preceded by fusion with distilled water at 550 °C.
Additionally, Collins et al. [23] report other raw materials that have been used, including mineral processing slags to produce type A and X zeolites; alumina processing residues for type A zeolites, and crushed stone and porcelain waste to obtain several types of zeolites depending on processing parameters.
Synthetic zeolites have a greater advantage over natural zeolites. Furthermore, the modification of zeolites improves their adsorption performance [10,11,15]. The production of zeolites would contribute to solving environmental problems associated with wastewater discharges and promote the circular economy [10,15]. Zeolites present unique chemical and physical properties that allow the application of these materials in the processes of adsorption of metal cations [24,25], adsorption and degradation of organic contaminants [26], gas adsorption [27] and support of various catalysts. Thus, zeolites are used as heterogeneous catalysts for the pyrolysis of plastics to obtain basic and high-quality chemical products [28,29,30].
Belrhazi et al. [31] determined that using 10% of a type P zeolite catalyst synthesized from a kaolinite clay precursor led to a reduction in temperature and activation energy until 100 °C and 118 kJ/mol, respectively, compared to thermal pyrolysis of PE without a catalyst. Similarly, Zeleke et al. [32] reported that employing a natural zeolite catalyst in spherical form increased yield and accelerated the catalytic pyrolysis process of PET from 38% to 44% and HDPE from 73% to 85%, reducing the reaction time to 15 min using a raw material: zeolite weight ratio of 10:1.
On the other hand, in a review conducted by Dong et al. [33], it was inferred that the relationship between structure and activity of zeolites used as catalysts in the catalytic pyrolysis of polyolefins influences performance, showing a direct relationship between increased acid density of the zeolite and catalytic activity due to a reduction in activation energy as total acid density increases. Additionally, pore size may affect accessibility to active sites, and the zeolite structure may influence the selectivity of isomerized hydrocarbon products.
Indeed, Han et al. [34] found that a combination of suitable acidity with reasonable pore size in Beta zeolites enhances catalytic performance and reduces the formation of secondary reactions during the pyrolysis of Kraft lignin.
Although several studies have synthesized zeolites from volcanic ash [10,11,29,35,36], the evaluation of their use as a source material for catalytic plastic pyrolysis has not yet been documented in scientific research.
It is important to mention that ash from Ubinas volcano has an advantageous combination of characteristics. First, it is one of the most abundant and recent environmental liabilities in southern Peru, with easily accessible deposits that do not require costly pretreatment processes. Second, its high proportion of silica (61%) and alumina (14%), on average, gives it a Si/Al ratio suitable for the formation of medium-acidity zeolites, particularly analcime, which is recognized for its structural stability and catalytic applications [1,6]. From an economic perspective, its use reduces dependence on synthetic or industrial precursors such as coal fly ash, which lowers acquisition and transportation costs. In addition, some studies show that Peruvian volcanic ash can be transformed into zeolites with adsorption and ion exchange properties suitable for environmental applications [9,12]. Therefore, these characteristics make Ubinas ash a competitive and sustainable local resource, whose use is in line with circular economy strategies by transforming an abundant natural waste into a high value-added material.
Likewise, Peru is a volcanically active country that offers a potential raw material. Therefore, the aim of this study was to evaluate the application of Ubinas volcanic ash using the hydrothermal method for zeolite synthesis and its use in the catalytic pyrolysis of plastic waste. Therefore, the volcanic ash from Ubinas volcano was characterized. Subsequently, four zeolites were synthesized via hydrothermal conversion using different compositions and processing parameters. Finally, these zeolites were characterized and used in the catalytic pyrolysis of polypropylene to evaluate their performance. The characterization techniques applied included X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), and the BET method.

2. Materials and Methods

2.1. Analysis of Ash and Zeolite Produced

The volcanic ash employed in this work comes from Ubinas volcano, located in Moquegua (Peru). The sampling site is located at the following coordinates as latitude (297608), longitude (8196158) and altitude (4600 m above sea level). Composite sampling was conducted considering areas with large deposits of erupted material.
The selected ash samples were subjected to a grinding process in a Fritsch 6S planetary ball mill at 450 rpm for 25 min to obtain a particle size less than 75 µm.
The ground samples were used for the synthesis of zeolite through the hydrothermal conversion method in an autoclave reactor. For each sample, 10 g of volcanic ash was taken, then placed in a Teflon container with 60 mL of Sodium Hydroxide solution (NaOH 99.99% supplied by Sigma-Aldrich, headquartered in St. Louis, MO, USA) at different concentrations (1.5 M and 3 M) and temperatures (120 °C and 150 °C) for 12 h of hydro-thermal treatment (Table 1). Subsequently, the mixture was cooled, filtered, and washed with distilled water until the pH dropped below 9. The resulting solid was dried at 105 °C for 12 h to eliminate residual moisture.
X-ray fluorescence (XRF) analyses were carried out for both the ashes and the zeolites using a Spectro Xepos instrument. Subsequently, the crystalline phases were determined. To ensure precision and reliability of the data obtained during the zeolite synthesis process, a certified crystal standard was employed to calibrate the X-ray diffraction measurements. These were conducted with a Bruker D8 Advance DaVinci diffractometer with CuKα radiation (λ = 0.1542 nm), operating at 40 kV and 40 mA, with a 2θ scanning range from 10° to 80° and a scan rate of 2°/min. Crystalline phase identification was performed through database matching using the Crystallography Open Database.
The semiquantitative phase estimation was carried out using the RIR (Reference Intensity Ratio) method implemented in the “Match!” software—Phase Identification from Powder Diffraction (Crystal Impact, Germany). For each compound identified in the diffractogram, the software calculates a scale factor representing the relative contribution of the phase to the total intensity. These values are combined with the RIR coefficients from the PDF-2 database (ICDD) and subsequently normalized by dividing each scale factor by the sum of all factors and multiplying by 100, thus yielding the estimated weight fraction of each phase in the sample.
The relative crystallinity of the analcime phase was determined using characteristic reflections of this phase, based on the numerical integration of the net area of each reflection (area under the peak after background subtraction), and the sum of the six integrated areas. Furthermore, the average crystallite size of the analcime phase was estimated using the Scherrer equation:
T = K λ / ( β c o s   θ )
where T is the average crystallite size, K is the dimensionless Scherrer shape factor (value of 0.9), λ = 0.1542 nm is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM), after subtracting instrumental broadening, in radians, and θ is the Bragg angle.
Scanning Electron Microscopy (SEM) was applied with the EVO MA10 scanning electron microscope (Carl Zeiss) at 3.0 kV.
Fourier Transform Infrared (FTIR) spectroscopy measurements were performed in Attenuated Total Reflectance (ATR) mode using a Perkin Elmer Spectrum GX FT-IR spectrometer equipped with a diamond crystal for solid sample analysis. The instrument was internally validated using a certified polystyrene standard. Zeolite powders obtained from volcanic ash were oven-dried and ground to a fine particle size (<20 µm), then placed directly onto the diamond crystal. The sample was pressed using the built-in pressure system to ensure optimal optical contact. Spectra were recorded in the range of 2000–400 cm−1 with a resolution of 4 cm−1 and averaged over 10 scans. Prior to each measurement, a background spectrum was acquired in air, and the corresponding ATR correction was applied. The diamond crystal was selected due to its high mechanical strength, chemical stability, and broad spectral window, making it particularly suitable for mineral and zeolitic materials.
Thermogravimetric analysis (TGA) was carried out using a SETERAM Thermal Gravimetric Analyzer from room temperature to 800 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Brunauer–Emmett–Teller (BET) surface area measurements were conducted in triplicate at 77 K in an ASAP 2020 Micromeritics apparatus under nitrogen atmosphere. The reported values correspond to the arithmetic mean of the three measurements. Standard deviations for all measurements remained within an acceptable margin (<5%), indicating reproducibility and reliability of the data.

2.2. Catalytic Pyrolysis of Polypropylene Using Zeolites as Catalysts

Catalytic pyrolysis of polypropylene PP 5707N provided by SABIC was performed in a cylindrical quartz reactor connected to a horizontal furnace. To carry out the catalytic pyrolysis, the polypropylene was first placed in a crucible, followed by the zeolite catalyst layered on top. The pyrolysis conditions were at 450 °C for 20 min under a continuous nitrogen flow rate of 250 L/min to prevent combustion reactions. Each sample consisted of 1 g of PP and 0.1 g of zeolite. A schematic diagram of the catalytic pyrolysis system is shown in Figure 1.
Finally, the quantities of products generated were measured using a RADWAG analytical balance (model AS 220.R2) with a precision of 0.1 mg. The process was considered optimal when it yielded the greatest amount of gases and liquids.

3. Results and Discussion

3.1. Chemical Composition of Volcanic Ash

The chemical composition of the volcanic ash is shown in Table 2. This volcanic ash was used to synthetize the zeolites at different conditions (Table 1 shows the zeolites synthesized at different conditions).
The ratio of the chemical composition (%) of SiO2/Al2O3 is 4.23, suitable for obtaining zeolites with medium silica content [15].

3.2. Morphological and Phase Analysis

XRD analysis shows that the main phase formed is anorthite (Figure 2). The anorthite phase is corroborated with the revision of the COD N°CIF: 9000361. In addition, Choi et al. [37] characterized a clay in its natural state, obtaining the anorthite phase whose main peak was shown between 25° and 30°, as seen in this research. The characteristic crystals of anorthite appear in a columnar or acicular shape [38] as seen in Figure 2.

3.3. Characterization of Zeolites

Zeolites exhibited a sharp peak around 970 cm−1. As shown in Figure 3, all samples display an asymmetric stretching due to the Si–O–T bond observed between the band of 960 and 990 cm−1, where T=Si or Al [39,40,41]. Zeolites Z2 and Z4 exhibit a slight shift in this band toward lower wavenumbers (965.7 cm−1), which is associated with a higher relative incorporation of Al atoms into the framework and, consequently, with a slightly lower Si/Al ratio, in accordance with the correlation reported by Ma et al. [40]. The presence of intense and sharp peaks involves Al-OH nests formed by cation vacancies [39,42]. Likewise, Yik-Ken Ma et al. [40] determined a linear correlation between the peak shifts and the Si:Al ratio in 19 zeolite types of known composition, up to a ratio of 5. This is because a greater amount of Al atoms tends to deform the band. In addition, they determined that the shift in this band is influenced by the vibration, length and angle of the Si-O-T bond that affects the chemical environment surrounding the silicon atoms. Although Z2 and Z4 exhibit similar FTIR peaks, they differ in thermal stability; the slight shift to the right of the FTIR T–O stretching band at ≈965 cm−1 in Z2 and Z4 suggests greater incorporation of Al into the structure and a slightly lower Si/Al ratio compared to Z1 and Z3. However, thermal stability depends not only on the Si/Al ratio, but also on the distribution of cations outside the framework and the density of structural defects generated during synthesis. Although Z2 and Z4 exhibit comparable FTIR bands, in the case of Z4 synthesized under more severe alkaline conditions (3 M NaOH, 150 °C) it is likely that the strongly basic medium promoted the preferential extraction of Si (desilication) and a partial redistribution of Al, leading to both Al-enriched regions within the framework and extra-framework Al species. This combined phenomenon explains both the shift observed in the FTIR spectrum (indicating a lower apparent Si/Al ratio) and the reduced thermal stability of Z4, as the presence of Al and structural defects decreases crystallinity and promotes dehydroxylation at lower temperatures [43].
Figure 4 shows the thermogravimetric analysis (TGA) curves of volcanic ash (Z0) and the synthesized zeolites (Z1–Z4). Figure 4a depicts the differences in mass loss among the zeolites, which were generally similar, with final total mass losses of approximately 80%. In contrast, volcanic ash (Z0) exhibited negligible mass loss (~1%) up to 800 °C, indicating its high thermal stability and low volatile content. The thermal stability of zeolites is usually directly proportional to the Si/Al ratio [44]. Among the synthesized zeolites, Z4 displayed the lowest thermal stability, with significant mass loss starting at ~250 °C, whereas Z1–Z3 maintained structural integrity until approximately 450 °C. This trend suggests that Z4 has a lower Si/Al ratio, which was corroborated by FTIR analysis. Understanding these thermal stability differences is important for defining the operational temperature ranges and potential applications of these materials.
Figure 4b shows the derivative thermogravimetric (DTG) profiles for Z1–Z4, revealing three main mass loss stages: The first stage occurred between 100 and 250 °C for Z4, and 100–450 °C for Z1–Z3, corresponding to the release of water molecules from the zeolitic cavities and channels linked by hydrogen bonds [45]. Samples Z1–Z3 exhibit minor peaks near ≈250 °C and ≈350 °C; these features correspond to successive releases of physically adsorbed water and weakly bound structural water from the zeolitic cavities and channel surfaces. A similar multistep dehydration process has been reported for analcime- and faujasite-type zeolites, where weakly bound water desorbs below 300 °C, followed by the removal of molecules with stronger hydrogen bonding between 300 and 400 °C [44,45].
The high apparent total mass loss (~80%) observed in Figure 4a is attributed not solely to physisorbed and structural water but also to the progressive dehydration and dehydroxylation of amorphous aluminosilicate fractions inherited from the volcanic ash precursor. This phenomenon has been widely reported for zeolites synthesized from natural ashes and clays, where unreacted glassy phases contribute significantly to weight loss during heating [46,47,48]. As no organic structure-directing agents were employed during hydrothermal synthesis, the weight loss cannot be associated with carbonaceous residues. Instead, it originates from the removal of bound water and structural relaxation of amorphous silicate domains, which densify progressively upon heating. The residual amorphous content, evidenced by a broad halo between 20 and 35° 2θ in XRD patterns, supports this interpretation. This explanation clarifies that the high apparent loss does not indicate contamination but rather reflects the presence of amorphous aluminosilicate material intrinsic to the precursor.
The earlier onset for Z4 suggests weaker interactions between water and the framework, potentially due to higher pore accessibility or structural defects. The second stage took place between 250 and 450 °C for Z4 and 450–510 °C for Z1–Z3, attributed to the loss of structural water from the zeolite framework, including water coordinated to extra-framework cations [45,49]. This water originates from hydroxyl groups in the framework and from channels and walls that interact via hydrogen bonds and with extra-framework cations [45]. The lower onset temperature for Z4 could be related to partial framework distortion caused by displacement of extra-framework cations towards new equilibrium positions [45]. The synthesis of Z4, conducted with 3 M NaOH, higher than that used for Z3, may have altered cation concentration and distribution, affecting stability. Previous studies have reported that both the type and concentration of cations significantly influence zeolite stability, as they form an integral part of the framework [43,50]. The third stage happened above 450 °C for Z4 and above 510 °C for Z1–Z3, corresponding to minimal additional mass loss, likely related to dehydroxylation through the breakdown of hydroxyl bonds within the framework [44].
Table 3 shows the specific surface area of the synthesized zeolites whose values are relatively low, averaging 30.45 m2/g, while in previous studies the specific surface area ranges from 160 to 900 m2/g [3,15,35,51,52,53,54]. Among the samples, Zeolites Z2 and Z4 showed the highest specific surface areas with values of 32.44 and 35.6 m2/g, respectively, compared to the other ones.
Figure 5 depicts the adsorption isotherms of the synthesized zeolites. All samples have the type IV adsorption isotherms characterized by the presence of a hysteresis loop indicating that the adsorption and desorption mechanisms are different [55]. The hysteresis cycle of these isotherms corresponds to type H3. Micropores are typically filled first at relative pressures (P/P0) of 0.3, followed by mesopores filling at pressures (P/P0) around 1 [55]. Similar results were obtained by García et al. [56], who studied zeolites that went through different treatments and obtained type IV isotherms with irreversible H3 hysteresis loops. Furthermore, Figure 5 shows that zeolite Z2 and Z4 have a greater adsorption capacity that is related to their greater pore volumes (0.12 and 0.13 cm3/g, respectively) and their specific surface area obtained. This correlation confirms that adsorption is governed by the internal surface area of the zeolite [56,57].
XRD diffractograms and SEM images of the synthesized zeolites are shown in Figure 6. Phase assignments in the micrographs were based on the correlation between the crystallographic information obtained by XRD and the morphological features observed in SEM, using the typical morphology of each phase reported in the literature as a reference. Analcime crystals are identified by their euhedral, trapezohedral, or pseudocubic shapes, and their tendency to form spheroidal aggregates [58].
Calcic plagioclases (anorthite/andesine) exhibit tabular or laminar forms, with angular edges and a microlitic appearance, characteristic of feldspars in the anorthite–andesine series [59]. Finally, pyroxenes (diopside/augite) are associated with prismatic particles with well-defined edges [60].
It is important to emphasize that XRD provides global information, and although SEM micrographs allow recognition of morphologies compatible with certain crystalline phases, they do not confirm composition at the particle level. Therefore, the assignments made here should be interpreted as approximate identifications based on morphology and consistency with XRD results, rather than direct compositional determinations.
The mineral composition of the zeolites is shown in Table 4. The crystalline phases found in Z1 were analcime (H4.32Al1.806Na1.71O14.16Si4.194), sodian anorthite (Al1.55Ca0.55Na0.45O8Si2.45), and aluminan diopside (Al0.6Ca1Mg0.7O6Si1.7). For zeolite Z2, the phases were analcime, augite (Al0.1Ca0.86Fe0.25Mg0.83Mn0.01Na0.02O6), and ferroan forsterite (Fe0.2Mg1.8O4Si1). In both samples, the analcime was the dominant phase that is structurally stable and can be completely dehydrated [43]. Figure 6c,d show the diffractograms and SEM images of zeolites Z3 and Z4. The phases identified in Z3 were augite, andesine (Al1.488Ca0.491Na0.499O8Si2.506), and anorthite (Al2Ca0.94Na0.06O8Si2). For Z4, the phases included augite, andesine, analcime, and carlinite (STI2). The predominant phases were augite and andesine in Z3, and augite and analcime in Z4. Among all the zeolites, Z2 exhibited the highest analcime content.
Also, Figure 6a,b show that zeolite Z1 and Z2 are mostly composed of pseudospherical particles with smooth crystals [36]. These samples exhibited higher crystallinity at 120 °C (synthesis temperature) with more stable peaks. However, zeolite Z2 synthesized with a higher NaOH concentration had a slight reduction in crystallinity compared to Z1.
The relative crystallinity, based on the characteristic reflections of the analcime phase (2θ = 15.8°, 18.3°, 26.0°, 30.6°, 33.3°, and 35.8°) and by normalizing the total crystalline intensity of Z1 to 100%, shows that sample Z2 exhibits a relative crystallinity of 88 ± 3%. This confirms the slight reduction in crystallinity previously observed qualitatively in the diffractograms. This difference is mainly associated with a greater full width at half maximum (FWHM) of the main analcime reflections in Z2, which indicates smaller crystallite sizes.
Based on the Scherrer equation, the average crystallite size of the analcime phase corresponding to the (042) reflection located around 26° was 48 ± 5 nm for Z1 and 39 ± 4 nm for Z2. These values confirm that the increase in NaOH concentration during synthesis (from 1.5 to 3 M) promotes rapid nucleation but limits crystal growth, resulting in smaller structures and a slight reduction in long-range order. Consequently, both the integrated intensity analysis and the values calculated using the Scherrer equation confirm that zeolite Z1 exhibits the highest crystallinity, while Z2 retains an ordered structure, albeit with smaller domains, consistent with the effect of the more alkaline synthesis medium.
In summary, at 120 °C and NaOH concentrations of 1.5 and 3 M (Z1 and Z2, respectively), a controlled dissolution of Si and Al from the volcanic ash was promoted, favoring the orderly nucleation of the analcime zeolitic phase. This was reflected in XRD patterns with higher crystallinity, SEM micrographs showing pseudospherical and homogeneous crystals, and intermediate surface areas (27.9–32.4 m2/g). This behavior is consistent with findings by Breck [43] and Zhou et al. [61], who noted that temperatures near 100–120 °C promote slow but well-defined crystallization, leading to zeolites with uniform morphology and structural stability. In contrast, increasing the temperature to 150 °C under low alkalinity conditions (Z3) did not result in clear zeolitic phases; instead, andesine, anorthite, and augite were formed. This phenomenon may be attributed to insufficient initial dissolution of the ash, which failed to generate stable zeolitic nuclei before soluble species redistributed into denser aluminosilicate phases. Accordingly, SEM analysis revealed irregular and agglomerated crystals, while surface area and pore volume remained low (25.9 m2/g and 0.05 cm3/g), indicating a poorly developed microstructure. Such behavior has been previously reported in hydrothermal syntheses without thermal or alkaline pretreatment, where the lack of prior activation limits the availability of reactive Si and Al species [62,63].
Finally, the most severe synthesis condition at 150 °C and 3 M NaOH (Z4) led to a more aggressive chemical attack on the vitreous matrix, resulting in amorphous, rough, and highly porous crystals. This explains why zeolite Z4 exhibited the highest surface area (35.6 m2/g) and pore volume (0.13 cm3/g), although with reduced crystallinity and lower thermal stability, which began to degrade at 250 °C.
Liu et al. [64] reported that the crystallization process and final products obtained in NaA zeolite were influenced by the temperature and the degree of alkalinity. A similar behavior was observed in the zeolitic samples analyzed in this study, where variations in alkalinity and synthesis temperature produced differences in the phases formed.
Figure 7 depicts the distribution of the products obtained after pyrolysis of PP using zeolites as catalyst. For comparison, pyrolysis of pure PP was also conducted to contrast the effect of those containing zeolites. It is shown that there is a difference in the percentage of products obtained with the four zeolites synthesized. The highest percentage of gaseous product was obtained by zeolite Z3 (80.2%), despite the absence of identifiable zeolitic phases. However, among the zeolites that did exhibit crystalline phases, Z2 showed the best performance, producing 57.7% of gaseous products. The lowest yield was observed in Z4, with 44.4%. These findings suggest that synthesis parameters mainly temperature and alkalinity have a significant influence on the catalytic behavior and product distribution, favoring greater gas generation under certain conditions. Additionally, the catalyst loading (10 wt% relative to PP) and pyrolysis temperature (450 °C) contributed to the enhanced formation of gaseous products across all tested samples.
This is corroborated by the results from several studies which indicate that pyrolysis temperature between 450 and 750 °C lead to increased yields of gaseous products [65,66,67,68,69,70]. Similarly, the use of catalyst loading close to 10 wt% has increased the generation of gaseous products [71,72]. In particular, the synthesis of zeolite Z3 led to the formation of irregular and porous crystals, with a greater number of active sites compared to the other zeolites. This behavior is attributed to its slightly higher Si/Al ratio and the specific phases formed during synthesis. However, zeolite Z3 exhibited a relatively low specific surface area (SBET = 25.91 m2/g) and reduced pore volume (0.05 cm3/g). These characteristics, along with its structural porosity [28,73,74,75] limited the diffusion of polymer chains into the pore network, thereby hindering the conversion to liquid products. As a result, gaseous products predominated, since only the smallest polymer fragments were able to diffuse through the channels and undergo catalytic transformation [29,76], directly affecting the product distribution in the pyrolysis process. A similar study conducted by Fermanelli et al. [77] reported that, when comparing different types of zeolites, higher gaseous pyrolytic yields were obtained with materials exhibiting greater concentrations of Lewis acid sites. This enhancement was attributed to the formation of carbenium ions, which undergo β-scission and thereby promote the generation of lighter hydrocarbons. Such behavior was specifically associated with zeolite Z3. Additionally, XRD analysis of Z3 revealed that its dominant crystalline phases were andesine, anorthite, and augite. Its mineralogical composition and the surface acidity generated during synthesis were key factors in its catalytic performance. In contrast, zeolite Z4 exhibited the highest gaseous pyrolysis yield among the samples that developed expected zeolitic phases. This was attributed to its relatively high specific surface area (SBET = 25.91 m2/g) and intermediate pore volume (0.13 cm3/g), which favored gas-phase product formation.
When compared to commercial zeolites, materials such as HZSM-5, HY, and Beta exhibit high surface areas ranging from 300 to 800 m2/g and a high density of acid sites, which primarily promote the formation of liquid and aromatic fractions with yields between 60 and 70% [28,29,71]. In contrast, the zeolites synthesized from Ubinas volcanic ash showed significantly lower surface areas (25–35 m2/g) and less-defined crystalline phases. Among these, zeolite Z2 with analcime as the dominant phase has demonstrated intermediate catalytic performance (57.7% gas yield) and a greater structural stability, standing out as the most effective zeolite synthesized in this study. On the other hand, Z3, despite lacking clear zeolitic phases, achieved the highest gaseous product yield (80.2%), which is attributed to its moderate acidity and the nature of the aluminosilicate phases present. This catalytic behavior resembles that of low-cost natural zeolites or unconventional catalysts, where gas-phase product formation predominates [32,68]. In this context, although the product selectivity of volcanic zeolites differs from that reported for commercial zeolites, the findings confirm that volcanic ash can be transformed into a functional and sustainable catalyst capable of promoting the production of combustible gases from residual plastics.

4. Conclusions

The zeolites obtained from volcanic ash from the Ubinas were synthesized with variations in hydrothermal treatment (different temperature and NaOH concentration) and these have an influence on the structural, morphological and textural properties obtained. FTIR spectra showed that zeolite Z2 and Z4 may possess slightly lower Si:Al ratios, as evidenced by sharper and more intense peaks associated with the formation of Al–OH nests. It is worth mentioning that the values obtained were not significantly high, despite a consistent trend being observed and corroborated by complementary characterization techniques. Thermogravimetry analysis determined that all zeolites had an average mass loss of around 80%. However, zeolite Z4 showed reduced thermal stability, with degradation beginning around 250 °C, compared to the other samples whose thermal stability remained up to 450 °C. This behavior further supports the inference that Z4 has a lower Si:Al ratio, given that thermal stability is generally proportional to this ratio. BET analysis revealed that zeolites Z2 and Z4 had a higher specific surface area with values of 32.44 and 35.6 m2/g, respectively. They also had a higher adsorption capacity with pore volumes of 0.12 and 0.13 cm3/g, respectively. SEM and XRD analysis revealed that zeolite Z1 and Z2 had higher crystallinity and the formation of smooth crystals with analcime identified as the dominant phase. In contrast, zeolites Z3 and Z4 exhibited rough and irregular crystals with augite and andesine as the main phase for Z3, and augite and analcime for Zeolite Z4. Notably, Z3 did not exhibit the expected zeolitic phases, suggesting that a pretreatment step may be necessary prior to hydrothermal synthesis.
In the catalytic pyrolysis of PP, the highest catalytic yield of 80.2% of gaseous product was obtained by zeolite Z3, and the lowest yield of 44.4% of gaseous product by zeolite Z4. The differences in pyrolytic product distribution among the zeolites are primarily attributed to their structural configuration, phase composition, and porosity, which govern the accessibility of polymer chains to active sites. In this context, a higher density of acid sites favors the formation of lighter hydrocarbons, even in materials with lower specific surface area and pore volume, as observed in Z3. In contrast, the other samples (Z1, Z2, and Z4), which exhibited well-defined zeolitic crystalline phases, achieved intermediate catalytic yields averaging around 49.7%, with Z2 standing out at 57.7% gaseous product. These findings highlight the potential of Z2 for applications in pyrolytic waste valorization.
The characteristics of the zeolites produced under different synthesis conditions allow the application and potential uses of these materials in different sectors and industries.

Author Contributions

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

Funding

This research received no external funding.

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.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

BETBrunauer–Emmett–Teller
XRDX-ray diffraction
FTIRFourier Transform Infrared Spectroscopy
SBETSpecific surface area
SEMScanning Electron Microscopy
TGAThermogravimetric Analysis
XRFX-ray Fluorescence Spectrometry

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Figure 1. Catalytic pyrolysis system.
Figure 1. Catalytic pyrolysis system.
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Figure 2. XRD spectrum and SEM images of volcanic ash.
Figure 2. XRD spectrum and SEM images of volcanic ash.
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Figure 3. FTIR spectra of the zeolitic samples.
Figure 3. FTIR spectra of the zeolitic samples.
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Figure 4. TGA curves (a) of zeolites Z0 to Z4 and DTG curves (b) of zeolites Z1 to Z4.
Figure 4. TGA curves (a) of zeolites Z0 to Z4 and DTG curves (b) of zeolites Z1 to Z4.
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Figure 5. N2 adsorption isotherms in synthesized zeolites Z1 to Z4.
Figure 5. N2 adsorption isotherms in synthesized zeolites Z1 to Z4.
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Figure 6. XRD spectra and SEM images of the zeolitic samples: (a) Z1 (×1000), (b) Z2 (×1000), (c) Z3 (×5000) and (d) Z4 (×1000).
Figure 6. XRD spectra and SEM images of the zeolitic samples: (a) Z1 (×1000), (b) Z2 (×1000), (c) Z3 (×5000) and (d) Z4 (×1000).
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Figure 7. Percentage of products obtained from catalytic pyrolysis of PP: (a) Z1 + PP, (b) Z2 + PP, (c) Z3 + PP, (d) Z4 + PP and (e) thermal pyrolysis of PP.
Figure 7. Percentage of products obtained from catalytic pyrolysis of PP: (a) Z1 + PP, (b) Z2 + PP, (c) Z3 + PP, (d) Z4 + PP and (e) thermal pyrolysis of PP.
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Table 1. Coding of the zeolites synthesized at different conditions.
Table 1. Coding of the zeolites synthesized at different conditions.
N°CodeT (°C)[NaOH] (M)t (h)
0Z0 (volcanic ash)---
1Z11201.512
2Z2120312
3Z31501.512
4Z4150312
Table 2. Chemical composition of volcanic ash Z0.
Table 2. Chemical composition of volcanic ash Z0.
OxidesAl2O3CaOFe2O3K2OMgOMnONa2OP2O5SiO2TiO2LOI*
%14.585.176.232.153.150.082.940.3061.691.112.34
Note: LOI*: loss on ignition.
Table 3. Specific surface values of synthesized zeolites.
Table 3. Specific surface values of synthesized zeolites.
N°CodeSBET (m2/g)Pore Volume (cm3/g)
0Z02.060.008
1Z127.850.04
2Z232.440.12
3Z325.910.05
4Z435.600.13
Table 4. Mineral composition of the zeolites.
Table 4. Mineral composition of the zeolites.
Type of ZeolitePercentage in Weight (wt%)
AnalcimeAnorthiteDiopsideAugiteForsteriteAndesineCarlinite
Z1671716----
Z282--144--
Z3-38-23-39-
Z466--15-136
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Almirón, J.; Churata, R.; Vargas, M.; Roudet, F.; Valverde-Ponce, K.; Gordillo-Andia, C.; Tupayachy-Quispe, D. Hydrothermal Synthesis of Zeolites from Volcanic Ash from Ubinas and Its Application in Catalytic Pyrolysis of Plastic Waste. Processes 2025, 13, 3376. https://doi.org/10.3390/pr13113376

AMA Style

Almirón J, Churata R, Vargas M, Roudet F, Valverde-Ponce K, Gordillo-Andia C, Tupayachy-Quispe D. Hydrothermal Synthesis of Zeolites from Volcanic Ash from Ubinas and Its Application in Catalytic Pyrolysis of Plastic Waste. Processes. 2025; 13(11):3376. https://doi.org/10.3390/pr13113376

Chicago/Turabian Style

Almirón, Jonathan, Rossibel Churata, María Vargas, Francine Roudet, Katia Valverde-Ponce, Carlos Gordillo-Andia, and Danny Tupayachy-Quispe. 2025. "Hydrothermal Synthesis of Zeolites from Volcanic Ash from Ubinas and Its Application in Catalytic Pyrolysis of Plastic Waste" Processes 13, no. 11: 3376. https://doi.org/10.3390/pr13113376

APA Style

Almirón, J., Churata, R., Vargas, M., Roudet, F., Valverde-Ponce, K., Gordillo-Andia, C., & Tupayachy-Quispe, D. (2025). Hydrothermal Synthesis of Zeolites from Volcanic Ash from Ubinas and Its Application in Catalytic Pyrolysis of Plastic Waste. Processes, 13(11), 3376. https://doi.org/10.3390/pr13113376

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