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1DIFFERENTIAL ENTHALPY AND ENTROPY CHANGES OF HYDROGEN SULFIDE ADSORPTION IN ZEOLITE CaA (M-34)
Kokhkharov M.
Namangan Institute of Engineering and Technology https://doi.org/10.5281/zenodo.14028855
Abstract. The article presents the experimentally obtained values of the differential enthalpy of adsorption of hydrogen sulfide at a temperature of 303oC in CaA (M-34) zeolite of LTA type. Enthalpy values were measured using a Tiana-Calve DAC-1-1A microcalorimeter connected to a universal high-vacuum device. The change of the differential enthalpy value of hydrogen sulfide in CaA (M-34) zeolite is legally related to the amount of adsorption. Under experimental conditions, it was found that the adsorption capacity of hydrogen sulfide for 1 gram of CaA (M-34) zeolite is equal to 6.6 mmol/g. The differential enthalpy values showed a regular step change corresponding to the amount of Na+ and Ca2+ cations in the zeolite. The differential values of the free energy - Gibbs energy - were calculated from the equilibrium values of the pressure. Based on the values of differential heat and Gibbs energy measured in direct experiment, the adsorption entropy change and its average value were theoretically calculated using the Gibbs-Helmholtz formula. The values of the entropy change of adsorption of hydrogen sulfide molecules are lower than the entropy value in the liquid state at the experimental temperature, and its average value is -20 J/mol-K, which indicates that the mobility of hydrogen sulfide molecules in the zeolite matrix is close to the mobility of hydrogen sulfide in the liquid state.
Keywords: adsorption, adsorbent, zeolite, enthalpy, free energy, entropy, microcalorimeter, hydrogen sulfide.
1 Introduction
Zeolite is a mineral that has a unique porous structure, which makes it irreplaceable in different areas, including adsorption, catalysis, and separation. One of the most known types of zeolites is LTA (Low-Temperature Adsorption), which has a specific crystalline structure and unique physical and chemical properties.
Zeolites are aluminosilicates that have a three-dimensional lattice structure formed by tetrahedra of SiO4 and AlO4. Ix unique porous structure and ion exchange ability make zeolite important for use in various technological processes. Zeolite-type LTA has cubic symmetry, which gives it a certain stability and allows forming a large network of channels. The basic element structure of LTA is a tetrahedron, consisting of one atom of silicon (Si) or aluminum (Al), surrounded by four atoms of oxygen (O). The carbon tetrahedron SiO4 and AIO4 is connected to other tetrahedra through oxygen bridges, which creates a three-dimensional lattice. The bonds between the tetrahedra determine the nature of the porous and channels and structure of the zeolite [1].
The structure of LTA includes systemic pores and channels that ensure molecule adsorption. The size of the pores and LTA is approximately 4-5 angstroms, which allows it to effectively hold small molecules. In LTA there are two basic types of channels oriented in different directions. It ensures the possibility of diffusion of the molecule through the structure and increases the efficiency of adsorption. High porosity of LTA makes it ideal for adsorbing gases and liquids, including water, ammonia, carbon dioxide, etc.
Zeolite type LTA has the ability to exchange ions, which makes it useful in various chemical processes. In the structure of LTA, sodium ions (Na+) are usually present, which can be exchanged for other ions, such as calcium (Ca2+) or magnesium (Mg2+). In contact with solutions containing other ions, sodium ions can be replaced by larger ions, which affects the properties of zeolite. Ion exchange makes LTA useful for cleaning water, catalysts and other chemical processes.
Synthesis of zeolites type LTA can be carried out with different methods, but the most common are hydrothermal methods. The basic stages of the synthesis include: 1. A mixture of starting components - starting materials are usually used, such as oxides of silicon, aluminum and alkaline metals; 2. Hydrothermal reaction - the mixture is subjected to hydrothermal exposure at elevated temperature and pressure, which creates a crystalline structure; 3. Crystallization - after the completion of the reaction, crystalline LTA is formed, which opens and dries [2].
Zeolite type LTA can be used in various areas, mainly due to its high porosity and ability to hold molecules. LTA is used for cleaning gases and liquids. For the separation of gases and cleaning processes with the release of valuable components.
Zeolite type LTA represents a very important class of materials with unique structure and properties. Their porosity, ability to ion exchange and adsorption capacity are not necessary and used in different industrial processes. Understanding the structure and characteristics of zeolite type LTA opens up new perspectives for applications in science and technology.
Adsorption is a key process in chemistry and processing materials, which has a wide application in cleaning, separation and storage of gases and liquids. This is especially relevant for zeolites of the LTA type, which, thanks to their porous structure and unique properties, can effectively adsorb various molecules, such as ammonia, water, hydrogen sulfide, and carbon dioxide [3-16].
All over the world, not only natural adsorbents, but synthetically produced zeolites are widely used in order to obtain clean gas from natural gases, to purify them from various additives, especially organic substances, and to prevent environmental problems caused by the release of toxic gases into the atmosphere, and the need for them is increasing year by year. Synthesizing zeolites with high adsorption and catalytic properties, and achieving not only scientific, but also practical innovations based on the results of scientific and practical research on increasing the level of their selectivity is of great importance.
Zeolites are widely used in oil and gas processing, pharmaceuticals, food, construction, agriculture, energy, medicine and many other industries. Natural adsorbents and various components based on them, which have the properties of cleaning from various inorganic and organic substances, as well as sufficient drying from water, are very rare. Therefore, synthetic zeolites are produced in very high volumes every year all over the world.
LTA type zeolites are one of the most widely used zeolites in adsorption processes. Due to disadvantages such as the use of a large amount of wastewater, high sensitivity to the temperature of the synthesis process, the concentration of reagents, and the significant duration of the process, the search for new approaches to the targeted synthesis of low-modulus zeolites is currently an urgent problem.
However, it was found out from the analysis of the literature that the properties of enthalpy and entropy of hydrogen sulfide adsorption in LTA zeolite type CaA(M-34) and adsorption mechanisms have not been sufficiently studied. This article presents the experimental results of
the change of the differential enthalpy (heat) and entropy of the adsorption of hydrogen sulfide in CaA(M-34) synthetic zeolite obtained by the adsorption-calorimetric experimental method depending on the amount of adsorption.
2 Testing methods
The adsorption-calorimetric method, used in this work, allows obtaining high-precision molecular thermodynamic characteristics, as well as revealing detailed mechanisms of adsorption processes, lubricants, adsorbents, and catalysts. A full description of the adsorption-calorimetric research method used in the work is given in the scientific articles of the authors [17-24].
In this work, the isothermal adsorption of zeolite CaA (M-34) in zeolite CaA (M-34) at a temperature of 303oC was studied. The composition of the elementary cell of this zeolite is represented by Ca9Na3(SiO2)12(AlO2)12)-27H20 and the isotopic position Si, Sii and Siii. According to the chemical composition, the amount of calcium cations per 1 g of zeolite is 4.846 mmol/g, the amount of sodium cations is 1.616 mmol/g.
3 Results and discussion
in this adsorption study, the differential heat of adsorption of hydrogen sulfide molecules on CaA (M-34) zeolite at 303 oC was measured, and Gibbs energy (free energy), enthalpy, and entropy change were calculated from the adsorption isotherm at equilibrium pressure.
Figure 1 shows the graph of the differential enthalpy of hydrogen sulfide adsorption on CaA (M-34) zeolite. It is known that the differential enthalpy of adsorption of adsorbates of different nature in synthetic zeolites changes stepwise depending on its active centers, including the amount of cations considered as the main active centers [6-16]. The differential heat of adsorption in the primary region is ~93 kJ/mol. As the amount of adsorption increases, the enthalpy at 0.64 mmol/g decreases smoothly to 65 kJ/mol. The differential heat produces a peak at 1.6 mmol/g adsorption and 64 kJ/mol. This value is equal to the number of sodium cations in zeolite (1.616 mmol/g). Therefore, hydrogen sulfide molecules are initially adsorbed on sodium cations in the zeolite. Therefore, the formation of a step in the enthalpy at the adsorption amount of 1.6 mmol/g corresponds to the number of sodium cations in the zeolite, that is, the adsorbate/adsorbent in the ratio 1/1 forms a monomer 1H2S:Na+ ion-molecular complex, and the differential heat decreases to 50 kJ/mol.
10 ~i-1-1-1-1-1-1-1-1-1-1-1-1—
0 1 2 3 4 5 6
a, mmol/g
Figure 1. Differential heat of adsorption (Qd) of hydrogen sulfide on zeolite CaA (M-34) at 303 oC. The dashed line is the heat of condensation of hydrogen sulfide at 303 oC.
As the zeolite becomes saturated, the enthalpy first increases to 52 kJ/mol at an adsorption amount of 3.2 mmol/g, and then decreases to 44 kJ/mol, that is, hydrogen sulfide molecules form a dimer 2H2S:Na+ ion-molecular complex with sodium cations. In the formation of 3H2S:Na+, the differential enthalpy decreased from 48 kJ/mol to 28 kJ/mol initially, and the adsorption of hydrogen sulfide molecules on the sodium cations in the zeolite was completed. When all sodium cations of hydrogen sulfide form a dimer 2H2S:Na+ ion-molecular complex, the partial increase in enthalpy (2 kJ/mol) corresponds to the release of additional energy (2 kJ/mol) as a result of the weak Van der Waals interaction of hydrogen sulfide molecules.
In the subsequent adsorption of hydrogen sulfide molecules, the differential enthalpy of adsorption is 6 mmol/g, and the heat of condensation of hydrogen sulfide at the experimental temperature is reduced to 20 kJ/mol. Since the measurement range of the device is up to ~600 torr pressure as described above, the experiment was carried out up to 6.6 mmol/g adsorption amount at 582 torr pressure.
The change of the molar differential entropy of the adsorption of hydrogen sulfide molecules on CaA (M-34) zeolite at a temperature of 303oC from the region of small saturations to the heat of condensation of hydrogen sulfide was calculated based on the Gibbs-Helmholtz formula, the existence of a regular relationship between the entropy change of the adsorption amount from the initial region to the saturation pressure, the mechanism of the sorption process, it was found that the average value of the molar differential entropy of hydrogen sulfide molecule adsorption in this zeolite is lower than the adsorption entropy of water and ammonia molecules. This means that the mobility of hydrogen sulfide molecules in the zeolite matrix is relatively limited. During the adsorption process, it was shown that hydrogen sulfide molecules interact with sodium and calcium cations in Sii and Siii positions of zeolite to form ion-molecular complexes with different nH2S:Me ratios.
The dependence of the change in molar differential entropy (ASa) of adsorption of hydrogen sulfide molecules on CaA (M-34) nanostructured zeolite on adsorption saturation is presented in Fig. 1 (the entropy of liquid hydrogen sulfide is taken as zero). The adsorption entropy was calculated by the following Gibbs-Helmgols formula:
where X represents the heat of condensation, AH and AG represent the adsorption enthalpy and free energy, i.e., the change in Gibbs energy, during the transition from the standard state to the adsorbed state.
In general, the adsorption entropy changes according to each generated ion-molecular mechanism, and it is below the entropy value of liquid hydrogen sulfide, which indicates that the mobility of hydrogen sulfide molecules in the zeolite matrix is limited. The stepwise change in entropy and the formation of each ion-molecular complex corresponds to the differential enthalpy. Entropy change in the initial area at the amount of adsorption of 0.12 mmol/ha is equal to -102 J/mol-K. At the adsorption amount of 0.4 mmol/g, the entropy increases from -102 J/mol-K to -66 J/mol-K to form the first step. With the saturation of the sorption volume, it forms the second step, changing from -66 J/mol-K to -41 J/mol-K with a partial tilt up to the adsorption amount of 3.2 mmol/g. This value is 2 times greater than the amount of Na+ cation in zeolite (~1.6 mmol/g). Therefore, the initial hydrogen sulfide molecules are adsorbed on Na+ cations contained in zeolite,
AS
AH -AG T
(Qa-X) + RT lnPs / P
(1)
a
T
that is, hydrogen sulfide molecules form an adsorbate/adsorbent dimer 2H2S:Na+ ion-molecular complex with Na+ cations at a step change value of 3.2 mmol/g of adsorption.
50
a 10
"3 S
<1
-70 -110
Figure 2. Molal differential entropy change (Qd) of hydrogen adsorption on CaA (M-34) zeolite at 303oC. The dashed line is the mean entropy change.
The entropy increases to 11 J/mol-K at 4.8 mmol/g adsorption, forming a step with the formation of a trimer 3H2S:Na+ ion-molecular complex in the adsorbate/adsorbent ratio 3:1, and the sorption of hydrogen sulfide on sodium cations ends. The entropy increases to 61 J/mol-K due to the subsequent adsorption of hydrogen sulfide molecules in the cation-free part, and the sorption process is fully completed at 6.6 mmol/g adsorption.
The average entropy is equal to -20 J/mol-K, which partially limits the mobility of hydrogen sulfide molecules.
Conclusion.
In the adsorption-calorimetric study, the main thermodynamic characteristics of the adsorption of hydrogen sulfide molecules on CaA (M-34) nanostructured zeolite were investigated. The differential enthalpy of adsorption was determined to change stepwise depending on the amount of sodium cations in the zeolite. At the Sii and Siii positions of the zeolite, sodium cations first form trimer 3H2S:Na+ ion-molecular complexes, then it was found that hydrogen sulfide molecules are adsorbed in the cation-free part of the zeolite. Gibbs free energy was determined from the differential enthalpy of adsorption of H2S molecules in CaA (M-34) nanostructured zeolite and the values of thermodynamic equilibrium pressure, and the molar differential entropy change and its average value were calculated based on the Gibbs-Helmholtz equation. It was found that the change of molar differential entropy is linearly dependent on the amount of Na+ cations in the zeolite. The values of the entropy change of adsorption of hydrogen sulfide molecules are lower than the entropy value of the liquid state at the experimental temperature, its average value is -20 J/mol-K. This indicates that the mobility of hydrogen sulfide molecules in the zeolite matrix is close to the mobility of hydrogen sulfide in the liquid state.
The results of the adsorption-calorimetric research obtained on the basis of the experiment allow to obtain the main thermodynamic functions of the studied systems, which are necessary for the development of theoretical concepts of chemical and physical adsorption in synthetic zeolites
of the LTA type, including CaA, as well as in the calculation of sorption technology processes and
devices in practice.
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