E- Global Congress Hosted online from Dubai, U. A. E., E - Conference. Date: 28th February 2025 Website: https://eglobalcongress.com/index.php/egc ISSN (E): 2836-3612 16 | P a g e ADSORPTION ISOTHERM OF AMMONIA ON SYNTHETIC СаА (MSS-622) ZEOLITE M. Bazarbaev ¹, Z. Juraeva ², U. Mamadaliev ³ ¹Head of the Department of Biomedical Engineering, Informatics, and Biophysics, Tashkent Medical Academy, Candidate of Physical and Mathematical Sciences, Associate Professor m.bazarbaev@tma.uz ²Assistant, Department of Biomedical Engineering, Informatics, and Biophysics, Tashkent Medical Academy ziyodajurayeva1994@gmail.com ³Independent Researcher, Namangan Engineering-Technological Institute utkurbekmamadaliev53@gmail.com Abstract This article presents the experimentally obtained values of the adsorption isotherm of ammonia molecules on synthetic zeolite СаА (MSS-622) at a temperature of 303 K. The isotherm values were measured using a microcalorimeter connected to a universal high-vacuum device. The differential values of free energy were calculated based on equilibrium pressure values. A regular relationship between the adsorption amount and the energetic properties of ammonia molecules on СаА (MSS-622) zeolite was identified. Additionally, the sorption mechanism and the law of ammonia molecules filling the zeolite volume were determined, ranging from the initial adsorption region to the condensation heat region of ammonia. Under experimental conditions, the adsorption capacity of this zeolite for ammonia was found to be 10.2 mmol/g per gram of zeolite. It was established that 44% of total adsorption occurs at an equilibrium pressure of 0.33 mmHg, 53% at 1.4 mmHg, 71% at 25 mmHg, 80% at 200 mmHg, 98% at 476 mmHg, and 100% at 614 mmHg. The adsorption isotherm was reinterpreted using the volumetric micropore filling theory (VMOT) with a three-term equation, and it was demonstrated that the theoretically calculated values fully correspond to the experimental data. https://eglobalcongress.com/index.php/egc mailto:m.bazarbaev@tma.uz mailto:ziyodajurayeva1994@gmail.com mailto:utkurbekmamadaliev53@gmail.com E- Global Congress Hosted online from Dubai, U. A. E., E - Conference. Date: 28th February 2025 Website: https://eglobalcongress.com/index.php/egc ISSN (E): 2836-3612 17 | P a g e Keywords: adsorption, pressure, relative pressure, isotherm, microcalorimeter, ammonia, VMOT. Zeolites are characterized by the names MFI, MOR, FAU, and LTA, which reflect their crystal lattice structure and composition, without deviating from their highest symmetry [1]. These zeolites exhibit distinct catalytic properties and thermodynamic characteristics in the adsorption of various types of molecules, including polar, nonpolar, and quadrupolar molecules [2-7]. The СаА (M-22), СаА (M-34), Са5Na3A (MSS-624) and Са4Na4A (Horst- 50/50) zeolites contain Са2+ and Na+ cations as their primary active sites. The quantity and ratio of these cations play a crucial role in determining the adsorption capacity of sorbate molecules. By analyzing the main thermodynamic characteristics of test molecules such as water, ammonia, benzene, and carbon dioxide, it is possible to determine the number, nature, and strength of energetically active sites located in the same type of crystallographic positions [8-15]. For example, it has been established that water molecules form stepwise adsorption complexes of 22H2O/elementary cell, 28H2O/elementary cell, and 30H2O/elementary cell in CaA1, CaA2 and CaNaA1 zeolites, respectively. Similarly, carbon dioxide molecules form ion-molecular complexes with stepwise variations of 7CO2/elementary cell, 6CO2/elementary cell, 8CO2/elementary cell, and 9CO2/elementary cell in CaA2, CaA2, CaNaA2, and CaNaA2 zeolites, respectively [9-12, 15]. This article presents the enthalpy and isotherm results of ammonia adsorption on synthetic Са5Na3A (MSS-624) zeolite, obtained using an adsorption-calorimetric experimental method, along with the adsorption mechanism. The adsorption-calorimetric method used in this study enables the acquisition of highly accurate fundamental thermodynamic characteristics and provides a detailed understanding of the sorption mechanisms occurring in the zeolite. The apparatus is designed to measure the amount of adsorbate using both gas-volume and volumetric-liquid methods. A modified Tian- Calvet type DAC-1-1A microcalorimeter, known for its high precision and stability, was used in the study. https://eglobalcongress.com/index.php/egc E- Global Congress Hosted online from Dubai, U. A. E., E - Conference. Date: 28th February 2025 Website: https://eglobalcongress.com/index.php/egc ISSN (E): 2836-3612 18 | P a g e The adsorption of ammonia on Са5Na3A (MSS-624) zeolite at 303 K was investigated, and the adsorption isotherm was thoroughly analyzed. The elementary cell composition of this zeolite is represented as Ca5Na3[(AlO2)12(SiO2)12]). Based on its chemical composition, the amount of calcium cations in 1 g of zeolite is 2.95 mmol/g, while the amount of sodium cations is 1.77 mmol/g. The logarithmic coordinate adsorption isotherm of ammonia on Са5Na3A (MSS-624) zeolite is presented in Figure 1. At low adsorption volumes, with an adsorption amount of 0.1 mmol/g, the equilibrium relative pressure logarithm is LnP/Ps=-15.5 (P/Ps=8.1110⁻⁷, P=0.00674 torr). Here, Ps=8750 torr represents the saturation pressure of ammonia at 303 K. Due to the high relative pressure of ammonia, the experiment was conducted up to 614 torr. The adsorption isotherm reached an adsorption quantity of 10.2 mmol/g at a relative pressure of P/Ps=0.07 (or P=614 torr). Figure 1. Adsorption isotherm of ammonia on СаА (MSS-624) zeolite at 303 K. ♢–experimental data, –values from the general equation of the micropore volume filling theory (VMOT). At the initial stage of the adsorption isotherm, the cations in the zeolite matrix form strong ion-molecular complexes with ammonia molecules. The isotherm first extends toward the abscissa at P/Ps=2.510⁻⁷ (P=0.0022 torr) with an adsorption quantity of approximately 1.8 mmol/g. According to the chemical composition of the zeolite, the amount of sodium cations is 1.77 https://eglobalcongress.com/index.php/egc E- Global Congress Hosted online from Dubai, U. A. E., E - Conference. Date: 28th February 2025 Website: https://eglobalcongress.com/index.php/egc ISSN (E): 2836-3612 19 | P a g e mmol/g. This means that at ~1.8 mmol/g adsorption, ammonia molecules form 1NH3:Na+ monomer ion-molecular complexes with sodium cations in the zeolite structure. The isotherm then varies linearly up to LnP/Ps=-7 (P/Ps=0.0009 or P=7.8 torr), sequentially forming 2NH3:Na+ dimers, 3NH3:Na+ trimers, and 4NH3:Na+ tetramer ion-molecular complexes. Beyond this point, ammonia adsorption no longer depends on the number of sodium cations in the zeolite, indicating that adsorption in the first coordination sphere of sodium cations is complete. The formation of the 4NH3:Na+ tetramer ion-molecular complex is further confirmed by a relative increase in equilibrium pressure. The systematic variation of the isotherm in correspondence with the active centers of the zeolite suggests the presence of a second active site—calcium cations—with a quantity of ~2.95 mmol/g. This means ammonia molecules begin to adsorb onto Са2+ cations. At a relative pressure of P/Ps=0.07 (P=614 torr) and an adsorption of 10.2 mmol/g, a 1NH3:Са2+ monomer ion- molecular complex is formed, marking the end of the adsorption process within the experimental range. From the adsorption isotherm, it is determined that ammonia molecules form a tetramer 4NH3:Nа2+ complex with sodium cations and a monomer 1NH3:Са2+ complex with calcium cations, resulting in an overall 6NH3:Са5Na3A (MSS-624) ion-molecular complex. The ammonia adsorption isotherm on СаА (MSS-624) zeolite is fully characterized by the three-term VMOT equation [15]: a=5,12exp[-(A/34,4)5] + 3,77exp[-(A/20,57)3] + 3,94exp[-(A/9,07)3] (1) Here, а is the adsorption value (mmol/g), and A=RTln(Ps/P) represents the free energy, which expresses the work (kJ/mol) performed when transferring gas to the equilibrium gas phase. From Figure 1, it is observed that the values calculated using VMOT fully correspond to the experimentally obtained adsorption amount up to 10 mmol/g. The first term of the equation represents the adsorption of ammonia molecules at the sodium cation active centers of the zeolite, the second term corresponds to adsorption at both sodium and calcium cationic centers, and the third term describes adsorption exclusively at calcium cationic centers. https://eglobalcongress.com/index.php/egc E- Global Congress Hosted online from Dubai, U. A. E., E - Conference. Date: 28th February 2025 Website: https://eglobalcongress.com/index.php/egc ISSN (E): 2836-3612 20 | P a g e Since the adsorption process follows an exponential equation, the third term of Equation 1 has almost no effect at low saturation pressure P/Ps=0.03 (P=260 torr), indicating that the process is weak and represents adsorption caused by Van der Waals interactions. CONCLUSION The adsorption isotherm of ammonia molecules on nanostructured zeolite Са5Na3A (MSS-624) was studied, and free energy values were calculated. It was determined that sodium cations in SI and SII positions form tetrameric 4NH3:Na+ ion-molecular complexes, while calcium cations form monomeric 4NH3:Na+ ion-molecular complexes. In general, ammonia molecules form 6NH3:Са5Na3A (MSS-624) ion-molecular complexes. At low saturation values of the sorption volume, the saturation coefficient was experimentally determined to be 0.44 at an equilibrium pressure of 0.33 mmHg, 0.53 at 1.4 mmHg, 0.71 at 25 mmHg, and 0.8 at 200 mmHg. The adsorption isotherm was redefined using the three-term equation of the volumetric micropore saturation theory (VMOT), and the theoretically calculated values fully matched the experimental results. REFERENCES 1. Baerlocher C., McCusker L.B., Olson D.H. “Atlas of Zeolite Framework Types”, 6th edn., Elsevier, Amsterdam, 2007: р.399. 2. Ergashev O., Bakhronov Kh., Akhmedova N., Abdullayeva Sh., Khalilov S.and Kholikov K. “Calorimetric study of methanol adsorption in LiZSM-5 and CsZSM-5 zeolites”. E3S Web of Conferences 401, 02023 (2023), https://doi.org/10.1051/e3sconf/202340102023 3. Bakhronov Kh., Ergashev O., Sultonov A., Kholmedov H., Ganievand A., Asfandiyorov M. “Basic thermodynamic characteristics and isotherm of ammonia adsorption in NaZSM-5 and LiZSM-5 zeolites”. E3S Web of Conferences 401, 02025 (2023), https://doi.org/10.1051/e3sconf/202340102025 4. Abdulkhaev T., Nuridinov O., Bakhronov Kh. “Differential heats of orta- xylene in zeolite AgZSM-5”. E-Global Congress Hosted online from Dubai, U.A.E., E-Conference; 2023: рр.120-122. https://eglobalcongress.com/index.php/egc/article/view/54/49 https://eglobalcongress.com/index.php/egc https://doi.org/10.1051/e3sconf/202340102023 https://doi.org/10.1051/e3sconf/202340102025 https://eglobalcongress.com/index.php/egc/article/view/54/49 E- Global Congress Hosted online from Dubai, U. A. E., E - Conference. Date: 28th February 2025 Website: https://eglobalcongress.com/index.php/egc ISSN (E): 2836-3612 21 | P a g e 5. Abdulkhaev T., Kuldasheva Sh., Bakhronov Kh., Ganiev A. “Enthalpy and the mechanism of water adsorptionin zeolite AgZSM-5”. International Conference on Developments in Education Hosted from Saint Petersburg. - Russia; 2023: pp.81-84, https://www.econferencezone.org/index.php/ecz/article/view/2024/1877 6. Esonkulova N., Ahkmadov M., Bakhronov Kh. “Isotherm adsorption of toluene and zeolite Cu2+ZSM-5”. International Conference on Developments in Education Hosted from Delhi. -India; 2023: pp.49-52, https://econferencezone.org/index.php/ecz/article/view/2137 7. Esonkulova N., Kh.Bakhronov, Absalyamova I., Ahkmadov M. “Entropy and thermokinetics of toluene adsorption on Cu2+ZSM-5 zeolite”. International Conference on Developments in Education Hosted from Bursa. -Turkey; 2023: pp.40-44, https://econferencezone.org/index.php/ecz/article/view/2137 8. Коххаров М.Х., Ахмедов У.К., Рахматкариева Ф.Г., Абдурахмонов Э.Б. “Ca4Na4A цеолитига углерод (IV) оксиди адсорбцияси энергетикаси”. Наманган муҳандислик-технология институти илмий- техника журнали; 2020, 1(5): б.142-148 9. Kokharov M.X., Axmedov U.K., Rakhmatkarieva F.G., Abdurakhmonov E.B. “Investigation of water sorption to Са5Na3А zeolite at adsorption of micro calorimetric device”. International Journal of Advanced Research in Science, Engineering and Technology; 2020, 5(7): рр.13939-13944. 10. Абдурахмонов Э.Б., Рахматкариева Ф.Г., Якубов Й.Ю., Абдулхаев Т.Д., Худайберганов М.С. “Дифференциальные теплоты адсорбции паров бензола в цеолите LiLSX”. Universum: химия и биология; 2020, 6(72): с.60-63 11. Абдурахмонов Э.Б., Эргашев О.К. “Термокинетика адсорбции аммиака в цеолите NaLSX”. Universum: химия и биология; 2020, 8(74): с.5-8. 12. Абдурахмонов Э.Б. “Энтропия адсорбции бензола в цеолите NaLSX”. Universum: химия и биология; 2020, 8(74): с.12-15. 13. Abdurakhmonov E.B., Rakhmatkarieva F.G., Ergashev O.K. “Determination of ammonia’s adsorption properties in NaLSX zeolite by calorimetric method”. International Journal of Materials and Chemistry; 2020, 10(2): с.17-22. https://eglobalcongress.com/index.php/egc https://www.econferencezone.org/index.php/ecz/article/view/2024/1877 https://econferencezone.org/index.php/ecz/article/view/2137 https://econferencezone.org/index.php/ecz/article/view/2137 E- Global Congress Hosted online from Dubai, U. A. E., E - Conference. Date: 28th February 2025 Website: https://eglobalcongress.com/index.php/egc ISSN (E): 2836-3612 22 | P a g e 14. Abdurakhmonov E.B. “Thermodynamics of benzene adsorption in NaLSX zeolite”. International Journal of Advanced Research in Science”. Engineering and Technology; 2020, 7(10): рр. 15314-15320 15. Rakhmatkariyeva F.G., Abdurakhmonov E.B., Yakubov Y.Y. “Volumetric Analysis of Benzene Vapor Adsorption on LiLSX Zeolite in a High Vacuum Adsorption Device”. International Journal of Advanced Science and Technology; 2020, 8(29): рр. 3442-3448. https://eglobalcongress.com/index.php/egc