CC BY
0MECHANISM OF AMMONIA ADSORPTION ON ZEOLITE CaA (M-34) IN THE THEORY OF VOLUME FILLING OF
MICROPORES
1Rakhmatkariyeva F., 2Kokhkharov M., 3Bakhronov Kh., 4Kholmedov Kh., 5Ganiyev A.,
6Mamadaliyev U.
1Institute of General and Inorganic Chemistry 2,6Namangan Institute of Engineering and Technology, 7 Kasansay Street, 160115, Namangan,
Uzbekistan
3,4,5Tashkent University of Information Technologies named after Muhammad al-Khwarizmi
https://doi.org/10.5281/zenodo.14001377
Abstract. This article presents the logarithmic and relative pressure values of the ammonia adsorption isotherm obtained from experiments conducted at 303 K on the synthetic zeolite CaA (M-22). The isotherm values were obtained through the utilisation of an advanced adsorption microcalorimetric method, which was conducted in conjunction with a universal high-vacuum apparatus. The adsorption isotherm was determined volumetrically within the pressure range of 10~s to 467 torr in the universal high-vacuum system. The equilibrium pressure values were used to initially calculate the differential values of Gibbs free energy, which were then used to derive a three-term mathematical equation of the sorption process based on the general equation of the volumetric filling theory of micropores (VMOT). The theoretical values were shown to correspond fully with the experimental data. It was demonstrated that the quantity of adsorption varies depending on the amount of sodium and calcium cations in the zeolite composition, as determined by the mathematical equation of the isotherm. The sorption mechanism and the regularities of ammonia molecules filling the zeolite volume from the initial to the experimental region were identified for ammonia molecule adsorption on CaA (M-22) zeolite.
Keywords: adsorption, adsorbent, pressure, isotherm, free energy, microcalorimeter, ammonia.
1 Introduction
The purification of natural gas to yield environmentally friendly gases entails the use of adsorbents to remove a range of additional inorganic and organic substances. This process is undertaken to prevent the release of harmful gases into the atmosphere, which could otherwise give rise to adverse ecological consequences. Zeolites, both natural and synthetic, are employed extensively as adsorbents. The synthesis of zeolites with high adsorption properties and the enhancement of their selectivity are of great importance, and the achievement of scientific and practical innovations through research in these areas is of significant relevance.
Zeolites are employed in a multitude of industrial sectors, including petroleum refining, gas processing, chemicals, pharmaceuticals, food processing, construction, agriculture, energy, and medicine. Naturally occurring adsorbents with sufficient drying and purification capabilities for various inorganic and organic substances are scarce. Consequently, millions of tons of
synthetic zeolites are produced on an annual basis across the globe.
One of the most frequently utilised zeolites in adsorption procedures is the A-type zeolite. However, the current challenge is the search for new approaches to the targeted synthesis of low-
modulus zeolites, due to certain drawbacks such as sensitivity to reagent concentration, high sensitivity to synthesis temperature, significant process duration, and the use of large amounts of wastewater.
All zeolites are crystalline aluminosilicates, composed of [SiO4] and [AlO4] tetrahedra and octahedra, which form regular three-dimensional frameworks through the formation of shared oxygen atoms. The presence of [AlO4] tetrahedra within the zeolite crystal lattice gives rise to a negatively charged framework. The negative charge is neutralised by cations (typically alkali and alkaline earth metals) situated within the pores and channels of the zeolite.
The categorisation of zeolites is based on their structural and compositional characteristics, as well as their adsorption and catalytic properties. This leads to the designation of specific types, such as FAU, LTA, MFI, MOR, and others [1]. The catalytic abilities and thermodynamic properties exhibited during the adsorption of molecules of varying polarities, including polar, nonpolar, and quadrupolar molecules, differ fundamentally from one another [2-7].
The formation of ion-molecular complexes between adsorbates and adsorbents is contingent upon the number of cations present in zeolites such as FAU, LTA, MFI, MOR, and others, which serve as the primary adsorption-active sites. To illustrate, in the Na+ form of MFI zeolite, ammonia (NH3) adsorption results in the formation of an 8N№:Na+ ion-molecular complex, and 24 hydrogen (H2) molecules form a sheath around the Na+ ions in the intersection of zeolite channels [8-10].
Due to their structural characteristics, FAU-type zeolites constitute a significant component of industrial cracking catalysts. The A-type zeolite's aluminosilicate lattice is distinguished by the presence of two distinct types of polyhedral units. Despite the high adsorption capacity and hydrophilicity of LTA and FAU zeolites, their global performance in water adsorption remains suboptimal, largely due to the nature and size of the exchangeable cations. It is therefore evident that there is a pressing need to enhance these types of adsorbents [3-7].
The adsorption of molecules with a range of physicochemical properties on different cationic forms of LTA-type zeolites containing alkali and alkaline earth metal cations has been the subject of extensive study. The authors have identified a stepwise change in the adsorption isotherms and energetic characteristics as a result of stoichiometric interactions [11-14].
In A-type zeolites (M-22, M-34, MISS-624, Horst 50/50) with alkali and alkaline earth metal cations, the stepwise changes in the enthalpy of adsorption of water and CO2 molecules are attributed to stoichiometric interactions with Ca2+ and Na+ cations. To illustrate, the formation of ion-molecular complexes of water molecules in CaA1, CaA2, and CaNaA1 zeolites occurs with 22H2O/u.c., 28H2O/u.c., and 30H2O/u.c., respectively. Similarly, stepwise changes were observed for CO2 molecules in CaA1, CaA2, CaNaA1, and CaNaA2 zeolites, with 7CO2/u.c., 6CO2/UX., 8CO2/u.c., and 9CO2/u.c., respectively [15-20].
Nevertheless, a review of the literature revealed that the adsorption characteristics and mechanisms of ammonia adsorption on CaA(M-22) LTA zeolites have not been the subject of extensive investigation. This article presents the values of the ammonia adsorption isotherm on synthetic CaA(M-34) zeolite, as measured using an adsorption calorimetric experimental method. The results are discussed in accordance with the theory of volumetric filling of micropores, as well as the adsorption mechanism.
2 Testing methods
The adsorption isotherm was determined using a system comprising a Tian-Calvet type DAC-1-1A differential automated microcalorimeter connected to a universal high-vacuum apparatus. The adsorption measurements and dosage of the adsorbate quantities were conducted using the high-vacuum adsorption setup. The apparatus permits the dosing of the adsorbate to be performed using either the gas-volume or liquid-volume methods. A B627 diaphragm Baratron was utilised to quantify equilibrium pressures spanning a range from 10-5 to 0.8 torr, whereas a U-type manometer was employed for pressures exceeding 0.8 torr. The adsorption calorimetric method allows for a comprehensive investigation of the thermodynamic properties of adsorption processes occurring on nanoscale, micro-, and mesostructured adsorbents, as well as their active surface areas.
The objective of the adsorption study was to examine the adsorption of ammonia on CaA (M-22) zeolite at 303 K and to conduct a comprehensive analysis of the adsorption isotherm. The elementary unit cell composition of this zeolite is represented as Ca2,975Na1,194(SiO2)12(AlO2)12), reflecting the ratio of the constituent elements. The chemical composition of the zeolite indicates that there are 1.89 mmol/g of calcium cations and 0.76 mmol/g of sodium cations per gram.
3 Results and discussion
The adsorption isotherm of ammonia on the CaA (M-22) zeolite at 303 K was investigated through experimental means, with observations extending from low saturation regions up to a pressure of 467 torr. During the adsorption process, ammonia molecules formed ion-molecular complexes with Na+ and Ca2+ cations in the zeolite through a variety of mechanisms.
The experimental isotherm values and those calculated using each term of the general equation of the volumetric micropore saturation theory (VMOT) in logarithmic coordinates are presented in Figure 1.
The ammonia adsorption isotherm on CaA (M-22) zeolite is fully described by the three-term VMOT equation [21].
a=4.77exp[-(A/28.69)7] + 2.015exp[-(A/17.63)4] + 5.6exp[-(A/8.19)3] (1)
where a is the adsorption value (mmol/g), and AG=RTLnPs/P is the free energy (Gibbs energy) representing the work done to transfer gas into the equilibrium gas phase (kJ/mol).
The first term of the equation describes adsorption in the initial range up to an adsorption quantity of 4.77 mmol/g. The second and third terms do not influence the sorption process in the initial region but start to contribute at Ln(P/Ps)=-10 and Ln(P/Ps)=-6, respectively, representing the formation of adsorbate/adsorbent complexes. Based on the chemical composition of the zeolite, the amount of calcium cations in 1 gram of zeolite is 1.89 mmol/g, and the amount of sodium cations is 0.76 mmol/g. The adsorption coefficients in Equation (1) are proportional to the whole number of sodium and calcium cations in the zeolite, revealing the mechanism of the sorption processes occurring at the active sites.
From Figure 1, it is evident that the isotherm values calculated using the VMOT equation correspond well with the experimentally obtained isotherm up to an adsorption quantity of 7.2 mmol/g. The first two terms of Equation (1) describe the adsorption of ammonia molecules on the active sites of sodium and calcium cations in the zeolite. The values calculated using the first term of Equation (1) correspond to the experimental isotherm up to LnP/Ps=-9,36 (P/Ps=8,6-10-5 or P=0,75 torr) and an adsorption quantity of 3.8 mmol/g. However, the second and third terms of
Equation (1) contribute zero adsorption values up to P/Ps=8,6-10-5, confirming that sorption occurs at the initial active site of the zeolite based on the first term alone.
As the equilibrium relative pressure increases, the values calculated from the first term start to deviate from the experimental isotherm, beginning at LnP/Ps=-6 (P/Ps=0,0024 or P=21,2 torr) and an adsorption quantity of 4.7 mmol/g. The isotherm calculated based on the VMOT equation becomes parallel to the ordinate axis, indicating that the first term's contribution has ended, and adsorption at the initial active site of the zeolite is complete. At LnP/Ps=-9,36 (P/Ps=8,6-10-5 or P=0,75 torr), the second term of Equation (1) starts to increase, indicating that ammonia adsorption begins at the second active site of the zeolite.
Figure 1. Isotherm of ammonia molecule adsorption on CaA (M-22) zeolite at 303 K, plotted
in logarithmic coordinates Q- experimentally obtained isotherm values, values calculated using the VMOT
equation.
As the equilibrium relative pressure increases, the values calculated from the second term of Equation (1) intersect the ordinate axis at LnP/Ps = -3 and an adsorption quantity of approximately 2 mmol/g. Figure 1 illustrates the logarithmic relationship between the total adsorption quantity calculated based on the first and second terms of Equation (1) and relative pressure. This curve is in complete accordance with the experimental isotherm up to an adsorption quantity of 6.5 mmol/g. Furthermore, once the second term of Equation (1) becomes insignificant, the values calculated from the third term begin to increase. Consequently, at the specified relative pressure, the adsorption of ammonia on the second active centre reaches its limit, and the sorption process commences at the third active centre.
The isotherm calculated using Equation (1) is in accordance with the experimental isotherm up to an adsorption quantity of 6.9 mmol/g. This is indicative of the formation of ion-molecular complexes of ammonia with sodium and calcium cations, namely a tetramer 4NHa:Na+ at 3 mmol/g adsorption, a monomer 1NHa:Ca2+ at 4.93 mmol/g adsorption, and a dimer 2NHa:Ca2+ at 6.92 mmol/g adsorption, as identified by the authors [22-28].
Figure 2 depicts the experimental values of ammonia adsorption on CaA (M-22) zeolite as a function of relative pressure in P/Ps coordinates, alongside the isotherm values calculated based
on each term of the VMOT equation. The isotherm is consistent with Brunauer's Type I classification, indicating that ammonia molecules are adsorbed exclusively in the micropores of the zeolite.
The sorption mechanism derived from the isotherm in P/Ps coordinates is corroborated by the differential enthalpy of adsorption, entropy changes, and the sorption mechanisms reflected in the logarithmic coordinates of relative pressure (Figure 2). The isotherm values calculated in accordance with Equation (1) are in accordance with the formation of adsorbate/adsorbent ion-molecular complexes involving ammonia molecules and sodium and calcium cations within the zeolite matrix. In particular, the isotherm calculated using the VMOT equation is in close agreement with the experimentally obtained isotherm up to an adsorption quantity of approximately 6.8 mmol/g.
0 0,01 0,02 0,03 0,04 0,05
P/Ps
Figure 2. Isotherm of ammonia molecule adsorption on CaA (M-22) zeolite at 303 K, plotted
in P/Ps coordinates
experimentally obtained isotherm values, values calculated using the VMOT
equation.
This is indicative of the formation of tetramer complexes 4N№:Na+ and dimer complexes 2NH3:Ca2+ with sodium and calcium cations in the zeolite structure at an adsorption level of 6.9 mmol/g.
Therefore, any additional ammonia molecules are adsorbed onto the cation-free regions of the zeolite.
Conclusion
The adsorption isotherm of ammonia molecules on the nanostructured CaA (M-22) zeolite was investigated and described using a three-term equation based on the theory of volumetric micropore filling (VMOT). The adsorption coefficients calculated using VMOT provided a comprehensive explanation of the sorption process mechanism in this zeolite. It was demonstrated that the theoretically calculated coefficients are proportional to the amounts of Na+ and Ca2+ cations present in the zeolite composition. It was established that within the initial coordination sphere, tetramer complexes of the form 4Nft:Na+ are formed with Na+ cations at the SII and SIII positions, while dimer complexes of the form 2Nft:Ca2+ are formed with Ca2+ cations. In
conclusion, the experimental findings and the isotherm values calculated by VMOT corroborate
the formation of ion-molecular complexes between ammonia molecules and metal cations within
the zeolite structure. This is exemplified by the 6NHa:Me ion-molecular mechanism observed in
CaA (M-32) zeolite.
REFERENCES
1. Baerlocher C., McCusker L.B., Olson D.H. Atlas of Zeolite Framework Types //6th edn., Elsevier, Amsterdam, 2007: p.399.
2. 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
3. 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
4. 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
5. 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: pp.120-122. https://eglobalcongress.com/index.php/egc/article/view/54/49
6. 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, http s ://econferencezone. org/index. php/ecz/arti cl e/vi ew/2137
7. 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/e3 sconf/202340102025
8. Bakhronov Kh.N., Ergashev O.K., Yakubov Y.Y. Basic energy characteristics and adsorption properties of zeolites of ZSM-5 type //Monograph. Progress Print Publishing House, Tashkent, 2022, 168 P.
9. Bakhronov Kh.N. Adsorption and energy properties of zeolites of ZSM-5 type in Li, Cs and Na- forms //Abstract for the degree of Candidate of Chemical Sciences, Tashkent, 2001: 23 p.
10. Boddenberg B., Rakhmatkariev G.U., Viets J., Bakhronov Kh.N. Statistical thermodynamics of ammonia-alkali cation complexes in zeolite ZSM-5 //Proceeding 12th International Zeolite Conference, 1998, USA, p.481-488.
11. Ergashov O.K., Kokhkharov M.H., Abdurakhmanov E.B. The Energy of Carbon Dioxide Adsorption in CaA Zeolite //Universum: Chemistry and Biology: Scientific Journal; 2019, 7(61), pp. 23-26. http s ://7universum. com/ru/nature/archive/item/7548
12. Kokhkharov M.H., Akhmedov U.K., Rakhmatkarieva F.G., Abdurakhmanov E.B. Energy of Carbon Dioxide Adsorption in Ca4Na4A Zeolites //Scientific and Technical Journal of Namangan Engineering and Technology Institute; 2020, 1(5), pp. 142-148.
13. Kokharov M.X., Axmedov U.K., Rakhmatkarieva F.G., Abdurakhmonov E.B. Investigation of water sorption to Ca5Na3A zeolite at adsorption of micro calorimetric device //International Journal of Advanced Research in Science, Engineering and Technology; 2020, 5(7): pp.13939-13944.
14. Abdurakhmonov E.B., Rakhmatkarieva F.G., Yakubov Y.Y., Abdulhaev T.D., Khudaiberganov M.S. Differential heats of adsorption of benzene vapour in zeolite LiLSX //Universum: chemistry and biology; 2020, 6(72): p.60-63
15. Abdurakhmonov E.B., Ergashev O.K. Thermokinetics of ammonia adsorption in zeolite NaLSX //Universum: chemistry and biology; 2020, 8(74): pp.5-8.
16. Abdurakhmonov E.B. Entropy of benzene adsorption in NaLSX zeolite //Universum: Chemistry and Biology; 2020, 8(74): pp.12-15.
17. 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): c.17-22.
18. Abdurakhmonov E.B. Thermodynamics of benzene adsorption in NaLSX zeolite //International Journal of Advanced Research in Science". Engineering and Technology; 2020, 7(10): pp. 15314-15320
19. 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): pp. 3442-3448.
20. Abdurakhmonov E.B., Rakhmatkarieva F.G., Ergashev O.K., Ochilov G.M. Energetic Characteristics Of The Process Of Adsorption Of Benzene In Zeolites NaX And NaY //International Journal of Future Generation Communication and Networking; 2020, 4(13): pp.246-252.
21. Abdurakhmonov E.B. Mechanisms of adsorption of polar, nonpolar and quadrupole molecules in synthetic zeolites of FAU and 5A type //Abstract of Dissertation of Doctor of Chemical Sciences (DSc), Tashkent, 2020: 63 p.
22. Rakhmatkariev G.U., Isirikyan A.A. Full description of adsorption isotherm by equations of the theory of volume filling of micropores //Izv.AN USSR, Ser.chem.: 1988, 11: p.2644-2645.
23. Rakhmatkariyeva F.G., Koxxarov M.X., Bakhronov Kh.N., Isotherm of Ammonia Adsorption In Zeolite CaA (M-22) //Scientific and Technical Journal Namangan Institute of Engineering and Technology Volume 8, Issue 4, 2023 y. P.103-109.
24. Rakhmatkarieva F., Kokhkharov M., Bakhronov Kh. Adsorption of Ammonia on CaA (M-22) Zeolite Differential Heat and Mechanism //International Journal of Advanced Research in Science, Engineering and Technology Vol. 10, Issue 11, November 2023, pp.21258-21261.
25. Rakhmatkarieva F.G., Kokhkharov M.X., Ergashev O.K. Bahronov Kh.N. Absalyamova I.I. Differential entropy and isotherm of ammonia adsorption on zeolite CaA (M-22) //Uzbek Chemical Journal, 2024, No.1. - pp. 3-9.
26. Rakhmatkariyeva F.G., Kokhkharov M.Kh, Sobirzhonova S. Analysis of ammonia adsorption isotherm on zeolite CaA (M-22) in the theory of volume saturation of micropores //International Conference on Advance Research in Humanities, Applied Sciences and Education Hosted from New York, USA December, 28 th 2023, pp.160-163.
27. KoxxapoB M.X., Sobirzhonova S., Asfandiyorov M. Isotherm of ammonia adsorption in zeolite CaA (M-22) //E- Global Congress Hosted online from Dubai, U. A. E., E - Conference.
Date: 30th December 2023 Website: https://eglobalcongress.com/index.php/egc ISSN (E): 2836-3612. p. 67-72.
28. Kokhkharov M.H., Asfandiyorov M., Axmadov M., Differential heat of ammonia adsorption on zeolite LTA //Perspective studies on medicinal chemistry, bio-chemistry and biophysics in the field of medicine, International Scientific and Practical Conference, 2024, - P. 98-100.
