Thermal Stability and Surface Area Modification of Natural Zeolite Through HCl & NaCl Treatment to Apply in Pyrolysis Systems

. Pyrolysis technology relies on efficient catalyst materials to expedite the process and improve the quality of the final product. Modified natural zeolite has emerged as a promising heterogeneous catalyst in this regard. This study presents a comprehensive analysis of natural zeolite sourced from Tasikmalaya, West Java, with the primary aim of evaluating its potential as a catalyst. The samples underwent acid treatment procedures and were extensively characterized using BET, TGA, SEM-EDX, X-Ray Diffraction (XRD), and FT-IR tests. XRD results indicated that the predominant frame structure of the Indonesian raw natural zeolite was mordenite, with a Si/Al ratio ranging from 5.0 to 6.0 and a surface area of approximately 23 m²/g. Following the 1M HCl acid treatment, a significant enhancement in the Si/Al ratio to 7.8 and a considerable expansion in surface area to 125.37 m²/g were observed. Moreover, the natural zeolite demonstrated remarkable thermal stability, thus underscoring its potential as a highly suitable catalyst for various pyrolysis applications.


Introduction
Natural zeolite, a micro alumina-silicate material with a three-dimensional microporous crystal structure and large orderly arranged cavities, finds extensive use in numerous industrial processes [1].This versatile substance serves as an ion exchanger, catalyst, and absorbent, offering several advantages over other materials [2].With both acidic and basic sites, natural zeolite plays a crucial role in catalytic reactions, making it highly valuable in various applications [2].
In environmental contexts, natural zeolites demonstrate unique physical and chemical properties that make them integral to many applications [3].Rich natural zeolite reserves in regions with volcanic activity, such as Indonesia, present an opportunity for their utilization.However, the current usage primarily involves only a small portion in agricultural systems as a fertilizer mixture [4].
The alumina-silicate structure of natural zeolites comprises endless SiO₄ and AlO₄ tetrahedra, forming uniform pore sizes [5].With large specific surface areas, excellent flexibility, and well-defined micropores, these materials exhibit high chemical and physical stability [6].As molecular sieves and heterogeneous catalysts, natural zeolites find applications in various thermochemical processes, petroleum refining, and petrochemical industries [7].
The catalytic role of natural zeolite is closely tied to its physical and chemical activation process.Its network of channels and cavities facilitates efficient filtration * sunaryo@unsiq.ac.id † suyitno@uns.ac.id based on size, polarity, and shape, making it an effective adsorbent for substances like ammonia, heavy metals, pesticides, and odours [8].Furthermore, the ion exchange capabilities of zeolites, particularly with monovalent cations like ammonium ions, occur not only on the surface but also deep within their porous structure, enhancing adsorption efficiency [9].
Although natural zeolites possess attractive properties with mesoporous structures and various cations like sodium, potassium, and barium [10], their thermal stability, pore size uniformity, and catalytic activity may require enhancement through modifications [11].In pyrolysis processes, catalysts play a crucial role in promoting specific reactions that improve the quality of bio-oils [12].The use of natural zeolite as a catalyst in pyrolysis has garnered interest due to its gas adsorption, moisture absorption, and heat storage qualities [13].However, to fully utilize natural zeolite in pyrolysis processes, pre-treatments such as acid treatment are essential to improve their catalytic performance [14].
In this research, we focus on the modification of natural zeolite through acid treatment, aiming to evaluate the surface area, pore volume, pore size, Si/Al ratio, and thermal stability.This study contributes to understanding natural zeolite's potential as a catalyst for the pyrolysis process of plastic waste, which can pave the way for cost-effective and environmentally friendly recycling methods.

Technical requirements
Samples of natural zeolite obtained from the Tasikmalaya region in West Java, in their rock form, were subjected to grinding and sieving through an 80mesh sieve.The resulting zeolite powder was then mixed with distilled water using a magnetic stirrer SH-2 and washed at a ratio of 1:3.Specifically, for every 100 g of zeolite powder, it was mixed with 300 ml of distilled water and stirred for 3 hours.Subsequently, the zeolite powder was heated at 120°C for 3 hours in an electric oven (refer to Fig. 1).

Fig. 1.
Stages involved in the preparation of natural zeolite.In step (a), the crushed zeolite powder was weighed.Subsequently, in step (b), the zeolite was mixed with water in a ratio of 1:3 and stirred.Step (c) involved the sedimentation and separation process.Finally, in step (d), the zeolite was dried at 120°C for a duration of 3 hours.

Fig. 2. Stages of modification of natural zeolite
The modification process of natural zeolite involves several steps, including calcination and acid treatment, as depicted in Fig. 2. The calcination process is conducted by heating the zeolite samples at 400°C for 5 hours (referred to as A-NZ).For the acid treatment, 50 g of natural zeolite is mixed with 200 ml of 1M HCl and stirred using a magnetic stirrer at 75°C (step d).After stirring, the samples are filtered and washed with distilled water until no chloride ions are detected in the AgNO₃ solution.Subsequently, the zeolite is dried at 120°C for 3 hours and then subjected to another round of calcination at 400°C for 5 hours (step e, referred to as HNZ).The same series of steps are also performed for acid treatment with NaCl (referred to as Na-NZ).

Characterization of Natural Zeolite
Various characterization techniques have been employed to assess the properties of natural zeolite, both before and after the acid treatment process.These techniques encompass the Brunauer-Emmett-Teller (BET) analysis for surface area determination, Scanning Electron Microscopy with Energy Dispersive X-ray (SEM-EDX) analysis, X-Ray Diffraction (XRD) analysis, and Thermogravimetric Analysis (TGA).

BET Result
The Brunauer-Emmett-Teller (BET) method was utilised to determine the surface area and pore size of natural zeolite.The surface area of modified natural zeolite with the addition of NaCl and HCl exhibited a notable increase in both surface area and pore volume.Table 1 presents the results, showing that the HCl treatment led to an increase in surface area from 23.42 m²/g to 125.37 m²/g and in pore volume from 0.0815 cm³/g to 0.1557 cm³/g.However, it should be noted that the pore diameter decreased as a result of the treatment.

Mechanical design
The investigation involved analysing the crystal types and quantities of zeolite samples derived from natural sources, both before and after undergoing acid treatment.According to the findings depicted in Fig. 3, the natural zeolite primarily comprises the mordenite type, with trace amounts of quartz.Interestingly, the mineralogical structure remained unchanged even after acid treatment and calcination, as evidenced by the identical crystalline pattern.

SEM-EDX Results
The morphology of the natural zeolite samples observed through SEM analysis is illustrated in Fig.  2.
Notably, the SI/Al ratio experiences a significant increase to 7.83 following the acid HCl treatment.This rise in the Si/Al ratio enhances the stability of the crystal framework and reduces coke formation during catalytic cracking processes.

FTIR Results
The FT-IR spectrum was utilized to analyse the structural functional groups present in the natural zeolite samples.The investigation focused on vibration bands within the range of 400 -4000 cmˉ¹ (Figure 5).The spectra from all samples exhibited closely similar positions and intensities of absorption bands, indicating a shared overall structure containing the same functional groups among all samples.The primary band observed in all samples with the highest intensity fell within the range of 1018-1037 cmˉ¹, corresponding to the vibrations of Si-O-Si and Si-O-Al chemical structures.Additionally, three other bands displayed vibrations at lower intensities before reaching their peak in the range of 435-463 cmˉ¹, 519 -570 cmˉ¹, and 712 -795 cmˉ¹, which can be associated with different Al-O and Si-O vibrations [15].Furthermore, a broader range spanning from 3600-4000 cmˉ¹ represented vibrations of OH, NH, and CH single bond groups [16].

TGA Results
The pyrolysis process takes place within the temperature range of 30°C-600 °C, and thus, it becomes essential to examine the thermal characteristics occurring within this temperature range.The TGA characterization test for the modified natural zeolite is presented in Figure 6.As the zeolite is continuously heated, the thermogravimetric analysis curve displays a gradual decrease.

Fig. 6. TGA from modified natural zeolite
The thermal properties of natural zeolite, both before and after acid treatment, exhibit nearly identical behaviour.The mass loss rate for each modified zeolite sample is relatively minor, approximately ± 1.08%.The most significant mass loss occurs within the temperature range of 27-150 °C, which corresponds to the release of water mass from the zeolite.Additionally, between 150 and 550°C, the mass reduction can be attributed to the decomposition of organic molecules present in the natural zeolites.

Conclusions
Natural zeolite is classified as an alumina-silicate mineral within the zeolite group, boasting high crystalline properties and containing chemical elements like Na, Mg, Fe, Ca, and K.This mineral exhibits an exceptionally elevated crystalline structure, a crucial factor in its utilisation for energy applications.With their high thermal resistance, stability, ion exchange capability, adsorption properties, porosity, and substantial surface area, natural zeolites are well-suited for use as catalyst materials.
The dual characteristics of natural zeolite, exemplified by modernite, play a significant role in enhancing the composition of CH4 during the pyrolysis process or municipal solid waste gasification.Through pre-treatment and modification, the acid HCl treatment has been effective in augmenting the surface area and Si/Al ratio, further improving the characterization of natural zeolite.
To optimise energy conversion systems, comprehensive studies are essential to delve into factors such as the zeolite type, humidity, and temperature range.These efforts will contribute to maximising the benefits derived from natural zeolite in various energy applications.
We express our sincere appreciation to the Rector of Sebelas Maret University for their generous financial assistance in conducting this research.The study received full support through a PNBP grant from Universitas Sebelas Maret, Indonesia, under contract number 228/UN27.22/PT.01.03/2023, as part of the Penelitian Unggulan Terapan (PUT) grant program.

Fig. 3 .
Fig. 3. XRD test of natural zeolite before and after NaCl and HCl 4. The SEM images clearly depict clusters of large particles with irregular shapes, cubical-shaped natural pores, and well-ICIMECE 2023 https://doi.org/10.1051/e3sconf/202346501026E3S Web of Conferences 465, 01026 (2023) defined surface areas.The chemical composition of the natural zeolite includes abundant elements such as Al, Si, and O, along with smaller amounts of C, Na, Mg, K, Ca, and Fe, as shown in Table

Table 1 .
BET Result of Natural Zeolite Before and After NaCl and HCl

Table 1 .
BET Result of Natural Zeolite Before and After NaCl and HCl