HYDROGEN SULFIDE ADSORPTION ISOTHERMS IN ZEOLITE CaA (M-34)

ABSTRACT

The paper presents the adsorption isotherms of hydrogen sulfide molecules at 303 K in synthetic zeolite CaA (M-34). Adsorption isotherms were measured using a universal high-vacuum instrument with high accuracy and stability. A BARATRON B627 membrane pressure gauge was used to measure equilibrium pressures. Gibbs energy was calculated from the differential free energy values from the equilibrium pressure values. In zeolite CaA (M-34) the regular relationship between adsorption value and energy properties of hydrogen sulfide molecules was established, and also the regularities of filling of hydrogen sulfide molecules H₂S of zeolite volume were determined. It has been established that adsorption capacity of this zeolite on hydrogen sulfide under experimental conditions (P=~ 582 torr) is equal to ~6,6 mmol/g in 1 g of zeolite. It was determined that 16 % of the total amount of adsorption is at a pressure of 1 torr, 50 % part is at 40 torr, 70 % part at 300 torr.

АННОТАЦИЯ

В статье представлены изотермы адсорбции молекул сероводорода при 303 К в синтетическом цеолите СаА (М-34). Изотерма адсорбции измеряли с помощью универсальным высоковакуумным прибором с высокой точностью и стабильностью. Для измерения равновесных давлений использовано мембранный манометр BARATRON B627. Энергия Гиббса рассчитывалась из дифференциальных значений свободной энергии от равновесных значений давления. В цеолите СаА (М-34) установлена ​​закономерная связь между величиной адсорбции и энергетическими свойствами молекул сероводорода, а также определены закономерности заполнения молекул сероводорода H₂S объема цеолита. Установлено, что адсорбционная емкость этого цеолита по сероводороду в условиях эксперимента (P=~582 мм.рт.ст.) равна ~6,6 ммоль/г в 1 г цеолита. Определено, что 16 % от полной количество адсорбции составляет при давлении 1 мм.рт.ст., 50 % часть составляет при 40 мм.рт.ст., 70 % часть при 300 мм.рт.ст.

Keywords: adsorption, zeolite, enthalpy, free energy, thermodynamics, pressure, micropores, hydrogen sulfide, cation, sodium, calcium.

Ключевые слова: zeolite, adsorption, free energy, pressure, micropores, hydrogen sulfide, cation.

Introduction. Zeolites are minerals that have a crystalline structure with a three-dimensional network of channels and pores, which allows them to adsorb molecules of various substances. LTA type zeolites, in particular, have a cubic structure formed by SiO₄ and AlO₄ tetrahedra, connected by oxygen atoms. The pore size of LTA-type zeolites is approximately 5 Å, which allows them to selectively adsorb molecules whose size does not exceed this threshold. The channels have a characteristic structure of a cubic lattice, which favours high diffusion of adsorbed molecules.

Synthetic zeolites can be synthesised with different cations such as sodium, potassium, calcium and others. This allows their sorption properties to be modified depending on the requirements of a particular application [1-3].

LTA type zeolite has a high surface area (up to 800 m²/g) and significant pore volume, which makes it ideal for adsorption. It is also characterised by high mechanical strength and stability under various conditions [4-5]. Sorption processes in zeolites can be either physical or chemical. Physical sorption is based on Van-der-Waals forces, whereas chemical sorption is related to the formation of chemical bonds between adsorbent and adsorbate. LTA type zeolites have high selectivity to different molecules. This is due to both the pore size and the charge and polarity of molecules, which allows efficient separation of mixtures of gases and liquids [6-8].

The sorption properties of zeolites also depend on environmental conditions. Changes in temperature and pressure can significantly affect adsorption efficiency, which must be taken into account in process design. Adsorption isotherms such as the Bett and Freundlich isotherm are used to evaluate the sorption properties of zeolites. These isotherms help to determine the amount of a substance that can be adsorbed as a function of pressure and temperature. Zeolites are actively used in gas adsorption processes, for example, to capture CO₂, H₂S and other harmful gases. This is particularly relevant in light of global climate change and the need to reduce emissions [9-13].

LTA type zeolites are highly selective to different molecules due to the pore size and the charge nature of the cations in their structure. This allows efficient mixture separation of gases and liquids. The adsorption process in LTA zeolite usually reaches equilibrium within a short time, making it effective for various industrial processes. LTA-type zeolite is a promising material with high sorption properties, making it indispensable in various industries including environmental and industrial applications. Understanding the adsorption mechanisms and characteristics of these materials opens up new opportunities for their applications [14-19].

Synthetic zeolites also find application in catalytic processes such as hydrocarbon cracking and synthesis of organic compounds. Their porous structure enhances the activity and selectivity of the catalysts.

Type A synthetic zeolites are effectively used for the removal of heavy metal ions and organic pollutants from wastewater. Their high sorption capacity and selectivity make them ideal for this purpose. LTA type synthetic zeolites are powerful sorption materials with a wide range of applications. Their unique structure, sorption mechanisms and high selectivity make them indispensable in various industries including environmental and industrial applications. The future of research in this field is related to the development of new synthesis methods, improvement of sorption properties and expansion of zeolite applications [9-10].

The application of type A zeolites is extremely wide. They are used as drying agents, for purification of gases from harmful impurities, for separation of substances. The experimental material on the heat of adsorption of gases and vapours by zeolites accumulated to date is very extensive, but unequal, and in many cases the data obtained by different authors poorly agree with each other.

It follows from the literature review that, despite the available rather extensive experimental material on adsorption in zeolites, studies performed in a wide range of temperatures and pressures, allowing to clarify more fully the issues of adsorption interaction of polar and nonpolar substances in zeolites are insufficient. Also important are studies of adsorption in a wide range of adsorption equilibrium parameters for adsorption systems including impurity substances and zeolites accompanying natural gas. Based on the results of such studies, a number of theoretical conclusions can be drawn, in particular, about the influence of the zeolite crystal structure, the composition of ion-exchange cations, and the distribution of cations in the zeolite matrix on the adsorption thermodynamics. Consequently, it is necessary to carry out thorough studies of adsorption and adsorption energy of various substances in zeolites with different composition and structure at different temperatures.

In this paper, we review the sorption properties of synthetic A-type zeolites, their structure and sorption mechanisms and applications. There is a large amount of data on adsorption in LTA-type zeolites, which have been obtained by various physicochemical methods of investigation. However, there are few data obtained by adsorption-calorimetric method, which puts on the agenda the task of further detailed study of adsorption properties of CaA (M-34) type zeolites and obtaining the main thermodynamic characteristics of these systems.

Materials and methods. The adsorption-calorimetric method used in this work allows to obtain high-precision molar thermodynamic characteristics, as well as to reveal detailed mechanisms of adsorption processes occurring on adsorbents and catalysts. A full description of the adsorption-calorimetric method of investigation used in this work is given in the scientific articles by the authors. [20-25].

In this work, the adsorption isotherms of hydrogen sulfide in zeolite CaA (M-34) at 303 K have been studied. The unit cell composition of this zeolite is represented by Ca₉Na₃(SiO₂)₁₂(AlO₂)₁₂×27H₂O and consists of positions S_I, S_II and S_III. According to the chemical composition, the amount of calcium cations per 1 g of zeolite is 4.846 mmol/g and the amount of sodium cations is 1.616 mmol/g.

Results and discussion. Zeolites are one of the most frequently considered objects of quantum-chemical calculations. Zeolites of types A and X are widely used in hydrocarbon gas drying, especially when the content of acidic components in it is high enough [3].

In Na - forms of zeolites of type A a part of Na⁺ cations are located in the eight- membered rings of the lattice, preventing the diffusion of molecules through them. When Na⁺ is replaced by Ca²⁺, the number of cations blocking the eight-membered rings decreases both due to a decrease in the total number of cations and due to the fact that the cations of Ca²⁺ prefer, apparently, positions outside the eight-membered rings. In the case of zeolites of type A, the substitution of the first four Na⁺ cations in the unit cell (out of 12) for Ca²⁺ leads to the appearance in the lattice of such a number of free eight-membered rings, which is sufficient for at least part of the zeolite cavities to become accessible for molecules N₂ and n-alkanes.

At adsorption of CO₂ molecules, substitution of Na⁺ by Ca²⁺ leads to the increase of adsorption heat only in the region of small fillings, i.e., only some part of Ca²⁺ cations are involved in interaction with adsorbate molecules. From 40 to 60% of Ca²⁺ cations in dehydrated forms of calcium phojasites are located in cubo-octahedrons and prisms, i.e., in positions where they should be inaccessible for adsorbate molecules that do not penetrate into cubo-octahedrons.

It is known [4] that zeolite of NaA grade is one of the main adsorbents used in units of complex air purification and dehumidification. In this connection, it is important to describe its main adsorption characteristics with respect to CO₂ and H₂O in a wide range of pressures and temperatures. Considered the basic equations of adsorption theory. The available information on the properties and characteristics of zeolite NaA was generalised and supplemented with calculated data. On the basis of experimental data and the generalised theory of volumetric filling of micropores [4], the authors proposed an extended interpretation of the equation of the CO adsorption isotherm₂ by NaA zeolite from dry air. The adsorption isotherm parameters calculated by the new equation differed from the experimental values by 5%, which indicates a good convergence of the calculation results with the actual data.

In the region of filling 100-140 mol/e.y., the heat of adsorption only slightly decreases with increasing filling and the authors assume that in this case molecules H₂ O interact with Na²⁺ cations in S_II positions and are located between the previously adsorbed molecules without experiencing strong intermolecular interactions. When the last 32 molecules of H₂O are adsorbed, intermolecular repulsion becomes noticeable and the heat of adsorption drops by another ~8 kJ/mol.

The adsorption isotherm of hydrogen sulfide in zeolite CaA (M-34) in the coordinate P/P_s is presented in Fig.1. First of all, it should be noted that due to the high pressure of saturated vapour of hydrogen sulphide (P_s=17936 torr) at the experiment temperature of 303 K, we were not able to obtain the full isotherm of hydrogen sulphide adsorption in zeolite CaA (M-34). However, the isotherm of hydrogen sulfide up to saturation pressure at the experimental temperature was calculated by the VMOT equation.

In the initial adsorption region, the equilibrium relative pressure is P/P_s =1.67×10^-6. The relative pressure P/P_s =1.4×10^-4 (P=2.5 torr) corresponds to 25% (~ 616 mmol/g) of the total adsorption. This value corresponds to the formation of a monomeric 1H₂S:Na⁺ ion-molecular complex with sodium cations.

Figure 1. Hydrogen sulfide adsorption isotherm in zeolite CaA (M-34) at 303 K in P/P_s coordinates. D - experimental values of the isotherm, ♦, ♦, ♦, ♦, ♦- values calculated from the VMOT equation

Then the pressure starts to increase sharply and at the relative pressure P/P_s=0.0336 (P=582 torr) value (6.6 mmol/g) the sorption process ends.

The adsorption isotherm has a G -shape and corresponds to the description of the Branauer type 1 classification, i.e., the zeolite consists only of micropores and does not contain mesopores.

At the equilibrium relative pressure of P/P_s=2,2×10^-3 (Р=40 torr), 50% of the total adsorption amount (~3.3 mmol/g) is adsorbed. At this adsorption quantity, hydrogen sulfide forms a dimer with sodium cations, 2H₂S: Na⁺, following an ion-molecular mechanism. The subsequent hydrogen sulfide molecules are adsorbed by calcium cations.

CaA (M-34) zeolite hydrogen sulfide molecular adsorption isotherms using the three-term VMOT equation:

a=1.62exp [-(A/29.7)⁷] + 2.4exp [-(A/18.6)⁵] + 5.43exp [-(A/9.4)³] (1)

here, a is the amount of adsorption (mmol/g), and A=RTlnP_s /P, which represents the Gibbs free energy and expresses the work done (in kJ/mol) when transferring the gas into equilibrium with the gas phase.

Figure 2. Adsorption isotherm of hydrogen sulfide in zeolite CaA (M-34) at 303 K in logarithmic coordinates. D - experimental values of the isotherm, ♦, ♦, ♦, ♦, ♦- values calculated from the VMOT equation

The adsorption isotherms obtained from the experiment at relative pressures (Figure 1) and in natural logarithmic coordinates (Figure 2) fully match the isotherms calculated using VMOT up to the adsorption amount of 6.6 mmol/g (within the experimental range). Additionally, based on equation (1), the values of the hydrogen sulfide adsorption isotherms in this zeolite were calculated up to saturation pressure. It was found that the hydrogen sulfide adsorption amount in CaA (M-34) zeolite according to equation (1) is 9.5 mmol/g.

Conclusion. CaA (M-34) nanostructured zeolites show isothermal hydrogen sulfide adsorption, and their adsorption coefficients were determined. It was discovered that in the first coordination sphere, sodium cations in positions SII and SIII of the zeolite form the dimer 2H₂S:Na⁺, while calcium cations form the monomer 1H₂S:Ca2+  ion-molecular complexes. The mechanism derived from the coefficients based on the microporous volume theory matches the experimentally obtained adsorption values, demonstrating a complete alignment with the mechanism derived from the experimental data. It has been established that adsorption capacity of this zeolite for hydrogen sulfide under experimental conditions (P=~582 torr) is equal to 6.6 mmol/g in 1 g of zeolite. It was determined that 16 % of the total amount of adsorption is at a pressure of 1 torr, 50 % part is at 40 torr. In zeolite CaA (M-34) a regular relationship between the adsorption amount and energy properties of H₂S molecules was established, as well as the sorption mechanism from the initial adsorption region to the saturation region of H₂S and the regularities of filling of H₂S molecules in the zeolite volume were determined.

References:

1. Xu X.C., Yang W.S., Liu J., Lin L.W. Synthesis of a high‐permeance NaA zeolite membrane by microwave heating //Advanced Materials. – 2000. – Vol. 12, I. 3. - PP. 195.
2. Huang A.S., Lin Y.S., Yang W.S. Synthesis and properties of A-type zeolite membranes by secondary growth method with vacuum seeding //Journal of Membrane Science. – 2004. – Vol. 245. - PP. 41.
3. Naskar M. K., Das A., Kundu D., Chatter-jee M. Emulsion-based synthesis of NaA zeolite nanocrystals and its integration towards NaA membranes //Bulletin of Materials Science. – 2011. – Vol. 34, I. 4. - PP. 651–659.
4. Majano G., Borchardt L., Mitchell S. et al. Rediscovering zeolite mechanochemistry – A pathway beyond current synthesis and modification boundaries //Microporous and Mesoporous Materials. – 2014. – Vol. 194. - PP. 106–114.
5. Prokofev V.Yu., Gordina N.E. Preparation of granulated LTA and SOD zeolites from mechanically activated mixtures of metakaolin and sodium hydroxide //Applied Clay Science. – 2014. – Vol. 101. - PP. 44–51.
6. Tsvetova E.V., Klyagina K.S., Gordina N.E., Borisova T.N. Creation of zeolite nanomaterials of various dimensions based on LTA and SOD zeolites // International scientific conference of students, graduate students and young scientists ‘Lomonosov’: conference proceedings, Moscow, 2021. - PP. 1122.
7. Gordina N.E., Borisova T.N., Afanasyeva E.E., Rumyantsev R.N. Combined approach to the synthesis of LTA membranes // IV Russian Congress on Catalysis: collection of abstracts of reports, Kazan, 2021: Institute of Catalysis Siberian Branch of the Russian Academy of Sciences. - PP. 418.
8. Borisova T.N., Afanasyeva E.E., Klyagina K.S. Comparative analysis of the surface properties of zeolite membranes of the LTA and SOD types by the NH3 desorption method // VI All-Russian Scientific Symposium ‘Physicochemistry of Surface Phenomena and Adsorption’: conference materials, Ivanovo, 2022: Federal State Budgetary Educational Institution of Higher Education Ivanovo State University of Chemical Technology. - PP. 12.
9. Cho C.H., Oh K.Y., Kim S.K. et al. Pervaporative seawater desalination using NaA zeolite membrane: mechanisms of high-water flux and high salt rejection //Journal of Membrane Science. – 2011. – Vol. 371. - PP. 226-238.
10. Swenson P., Tanchuk B., Gupta A., et al. Pervaporative desalination of water using natural zeolite membranes //Desalination. – 2012. – Vol. 285. - PP. 68–72.
11. Wang Z., Ge Q., Shao J., Yan Y. High performance zeolite LTA pervaporation membranes on ceramic hollow fibers by dipcoating - wiping seed deposition. //Journal of the American Chemical Society. – 2009. – Vol. 131. - PP. 6910.
12. Prokofev V.Yu., Gordina N.E., Borisova T.N. et al. Study of the kinetics of adsorption and desorption of water vapor on lowsilica zeolites //Petroleum Chemistry. – 2020. – V. 60, I. 4. - PP. 550.
13. Vaičiukynienė D., Kantautas A., Vaitkevičius V., et al. Effects of ultrasonic treatment on zeolite NaA synthesized from by-product silica //Ultrasonics Sonochemistry. – 2015. – Vol. 27. - PP. 515–521.
14. Borisova, T.N., Klyagina K.S., Gordina N.E., Maksimova L.S. Study of the surface properties of zeolite membranes of the LTA and SOD type // VI All-Russian Scientific Symposium Physicochemistry of Surface Phenomena and Adsorption: conference materials, Ivanovo, 2022: Federal State Budgetary Educational Institution of Higher Education Ivanovo State University of Chemical Technology. - PP. 26.
15. Gordina N.E., Borisova T.N., Klyagina K.S. et al. Investigation of NH₃ desorption kinetics on the LTA and SOD zeolite membranes //Membranes. –2022. – Vol. 12. - PP. 147.
16. Klepel, O., Hunger B. Temperature-programmed desorption (TPD) of carbon dioxide on alkali-metal cation-exchanged faujasite type zeolites //Journal of Thermal Analysis and Calorimetry. – 2005. – Vol. 80. - PP. 201–206.
17. Borisova T.N., Afanasyeva E.E., Gordina N.E. Mechanochemical synthesis of adsorbents based on LTA and SOD zeolites for natural gas drying processes // International Conference ‘Innovative Development of the Oil and Gas Industry, Modern Energy and Their Actual Problems’: collection of abstracts, Moscow, 2020. - PP. 191–193.
18. Domínguez-Domínguez, S., Berenguer-Murcia A., Morallón E., LinaresSolano A., Cazorla-Amorós D. Zeolite LTA/carbon membranes for air separation //Microporous and Mesoporous Materials. – 2008. – Vol. 115, I. 1–2. - PP. 51-60.
19. Tsvetova E.V., Gordina N.E., Kolobkova A.E., Borisova T.N. Study of the properties of type A zeolite membrane in the pervaporation process // Scientific and practical conference ‘Actual problems of chemical technology’: materials of the international conference, Tashkent, 2021. - PP. 637–638.
20. Ergashev O.K., Kokharov M.Kh., Abdurakhmonov E.B. Adsorption energetics of carbon dioxide in zeolite CaA //Universum: chemistry and biology: scientific journal. – No. 7(61). ‘Moscow Center for Continuous Mathematical Education’, 2019. - PP. 40. – Electronic version of the printed publication. - PP. 23-27.
21. Rakhmatkariev G.U., Rakhmatkarieva F.G. Ion-molecular complexes in nanostructured zeolite NaA //Universum: chemistry and biology: scientific journal. - No. 7 (61). ‘Moscow Center for Continuous Mathematical Education’, 2019. - PP. 40. - Electronic version of the printed publication. - PP. 31-38.
22. Kokharov M.H., 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 Vol. 7, Issue 5, May 2020. - PP. 13939-13944.
23. Rakhmatkarieva F., Kokhkharov M., Bakhronov Kh. Adsorption of Ammonia on СаА (М-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.
24. Ergashev O.K., Bakhronov Kh.N., Rakhmatkarieva F.G., Esonkulova N.M., Akhmadov M.A., Rakhmonov O.K. Isotherm and thermokinetics of toluene adsorption on Cu²⁺ZSM-5 zeolite //Scientific and technical journal of  FerPI. -2023. - Vol. 27. Special issue, -No. 10. - PP. 114-118.
25. Bakhronov Kh.N., Ergashev O.K., Esonkulova N.M., Rakhmatkarieva F.G., Absalyamova I.E. Differential heats and entropy of toluene adsorption on Cu²⁺ZSM-5 zeolite //Scientific Bulletin of Fergu State University. - N. 2. - 2023. - PP. 59-64.
26. Arefyev Ya. B., Pashchenko K. P., Karatun O. N. Quantum-chemical modeling of the process of hydrocarbon gas drying on zeolites. All-Russian scientific conference ‘Processing of hydrocarbon raw materials. Complex solutions’ (Levinter readings), Samara, 24-25 October, 2006. - PP.140-141.
27. Lavrenchenko G.K., Kopytin A.V., Shvets S.G. Description of adsorption characteristics of zeolite NaA for conditions of drying and cleaning air from CO2 in an input-distribution device. Technical gases N5, 2005. - PP. 45-55.