DIFFERENTIAL ENTHALPY AND ENTROPY OF HYDROGEN SULFIDE ADSORPTION ON ZEOLITE CaA (M-22)

ABSTRACT

The paper presents experimentally obtained values of differential enthalpy of adsorption of H₂S molecules at 303 K and entropy change in synthetic zeolite CaA (M-22). The differential enthalpy of adsorption was measured using a system consisting of a Tiana-Calvet DAC-1-1A type differential automatic microcalorimeter coupled to a universal high-vacuum instrument with high accuracy and stability. Gibbs energy was calculated from differential free energy values from equilibrium pressure values. From the differential heat and Gibbs energy, the entropy change and its mean value were theoretically calculated based on the Gibbs-Helmholtz equation. A regular relationship between the adsorption value and energy properties of H₂S molecules and the sorption mechanism from the initial adsorption region to the heat of condensation region of H2S was established in CaA (M-22) zeolite, and the filling patterns of H₂S molecules in the zeolite volume were determined. It is established that the adsorption capacity of this zeolite for hydrogen sulfide under experimental conditions (P=~550 torr) is equal to ~5 mmol/g in 1 g of zeolite. It is proved that in the first coordination sphere H₂S molecules form trimer 3H₂S:Na⁺ with sodium cations and monomer 1H₂S:Ca²⁺ ion-molecular complex with Ca²⁺ cations. In general, the values of entropy change of zeolite CaA (M-22) adsorption entropy of hydrogen sulfide molecules are lower than the value of liquid state entropy at the experimental temperature, and its average value is -23 J/моль×К. It means that mobility of hydrogen sulfide molecules is close to mobility of liquid hydrogen sulfide, i.e., mobility of hydrogen sulfide molecules is limited.

АННОТАЦИЯ

В статье представлены экспериментально полученные значения дифференциальной энтальпии адсорбции молекул H₂S при 303 К и изменения энтропии в синтетическом цеолите СаА (М-22). Дифференциальную энтальпию адсорбции измеряли с помощью системы, состоящей из дифференциального автоматического микрокалориметра типа Тиана-Кальве ДАК-1-1А, соединенного с универсальным высоковакуумным прибором с высокой точностью и стабильностью. Энергия Гиббса рассчитывалась из дифференциальных значений свободной энергии от равновесных значений давления. На основе дифференциальной теплоты и энергии Гиббса изменение энтропии и ее среднее значение были теоретически рассчитаны на основе уравнения Гиббса-Гельмгольца. В цеолите СаА (М-22) установлена ​​закономерная связь между величиной адсорбции и энергетическими свойствами молекул H₂S, а также механизмом сорбции от начальной области адсорбции к области теплоты конденсации H₂S и определены закономерности заполнения молекул H₂S объема цеолита. Установлено, что адсорбционная емкость этого цеолита по сероводороду в условиях эксперимента (P=~550 мм.рт.ст.) равна ~5 ммоль/г в 1 г цеолита. Доказано, что в первой координационной сфере молекулы H₂S образуют тример 3H₂S:Na⁺ с катионами натрия и мономер 1H₂S:Ca²⁺ ион-молекулярная комплекс с катионами Ca²⁺. В целом значения изменения энтропии цеолита СаА (М-22) адсорбции молекул сероводорода ниже значения энтропии жидкого состояния при температуре эксперимента, а ее среднее значение составляет -23 Дж/моль×К. Это означает, что подвижность молекул сероводорода близка к подвижности жидкого сероводорода, то есть подвижность молекул сероводорода ограничена.

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

Ключевые слова: адсорбция, цеолит, энтальпия, свободная энергия, термодинамика, давление, микропоры, сероводород, катион, натрий, кальций.

Introduction. Synthetic A-type zeolites are important materials in the field of adsorption due to their unique microporous structure and high selectivity towards different molecules. These compounds, obtained by hydrothermal synthesis, have a regular lattice structure, which provides a high surface area and the possibility of specific capture of molecules [1-4].

Type A zeolites have a three-dimensional porous structure with a pore size of about 4 Å, which allows them to adsorb small molecules such as water, ammonia, ionic compounds and organic substances efficiently. The main components that make up zeolites are aluminosilicate lattices containing SiO⁴ and AlO⁴ tetrahedra, which gives them anionic properties and the ability to exchange cations [5-8].

The sorption properties of type A zeolites depend on their composition, porosity and adsorption conditions. The main factors affecting adsorption include [9-13]:

1. Pore size: Since the pores of zeolites have a fixed size, they exhibit high selectivity towards molecules that can pass through them. This makes zeolites ideal for gas separation and pollutant removal.

2. Cation exchange: Cations located in the zeolite channels can be exchanged with ions from solution, which affects their sorption properties. This allows the adsorption of certain molecules to be controlled, increasing the selectivity of the material.

3. Temperature and Pressure: The sorption properties of zeolites can vary with temperature and pressure, which is important to consider when designing adsorption processes in industry.

Type A synthetic zeolites are multifunctional materials with excellent sorption properties that provide them with a wide range of applications in various industries. Their selective adsorption ability makes them indispensable for purification and separation tasks, which emphasises their importance in modern technologies. Zeolite’s synthesis and modification research continues to open up new opportunities for their applications, making them relevant for future developments [14-17].

Type A synthetic zeolites are multifunctional materials with excellent sorption properties that provide them with a wide range of applications in various industries. Their selective adsorption ability makes them indispensable for purification and separation tasks, which emphasizes their importance in modern technologies. Research into the synthesis and modification of zeolites continues to open up new opportunities for their applications, making them relevant for future developments [14-17].

Synthetic zeolites of type A are a group of porous aluminosilicates with unique sorption properties. Their structure and physicochemical characteristics make them important materials for various industrial and environmental applications such as gas adsorption, wastewater treatment and catalysis. The research conducted worldwide is mainly focused on the structure of zeolites and the arrangement of cations in them. There is little information on the study of the mechanism and thermodynamic functions of the adsorption process of hydrogen sulfide molecules.

In this paper we will consider the sorption properties of type A synthetic zeolites, their structure, sorption mechanisms and applications. There are a large number of adsorption data in LTA type zeolites, which have been obtained by various physicochemical methods of investigation.

However, the data obtained by adsorption-calorimetric method are scarce, which puts on the agenda the task of further detailed adsorption properties study of zeolites of CaA (M-22) type and obtaining the main thermodynamic characteristics of these systems.

Materials and methods.

The adsorption-calorimetric method used in this work makes it possible to obtain highly accurate molar thermodynamic characteristics, as well as to reveal detailed mechanisms of adsorption processes occurring on adsorbents and catalysts. A modified DAC-1-1A thermally conductive microcalorimeter with high accuracy and stability was used as a calorimeter. We used a BARATRON B627 membrane manometer to measure equilibrium pressures. Despite its external insulating shells, it is not adiabatic because the heat released in it is introduced from the calorimetric chamber as it is released and dissipated in a large metal block. Although the temperature of the calorimetric chamber changes only slightly, the apparatus cannot be called strictly isothermal; it exhibits small changes in temperature, which are unavoidable, and serve as the basis for measurements [14-20].

In this work, the isotherm and entropy of ammonia adsorption in zeolite CaA (M-22) at a temperature of 303 K are studied. The unit cell composition of this zeolite is represented by Ca_2,975Na_1,194(SiO₂)₁₂(AlO₂)₁₂) and consists of S_I, S_II and S_III positions. According to the chemical composition, the amount of calcium cations per 1 g of zeolite is 1.89 mmol/g and the amount of sodium cations is 0.76 mmol/g.

Results and discussion. The adsorption process studies of polar, nonpolar, quadrupole and aromatic molecules are carried out in the world to determine the mechanism and thermodynamic functions, including the following priority directions: synthesis of microporous adsorbents; improvement of their composition; synthesis of synthetic zeolites and determination of their adsorption properties; enhancement of selectivity of their adsorption properties due to the exchange of cations in the composition of zeolites A, X, Y, LSX and MFI types [14-28].

At adsorption of H₂O by zeolite NaA on enthalpy three distinctly expressed regions are observed - the region of high (100 kJ/mol) heat of adsorption, the transition region and the region of relatively low (60 kJ/mol) heat of adsorption. It is assumed that the first two regions reflect the interaction of H₂O molecules with cations in positions S_II and S_III, and the third region corresponds to the adsorption of H₂O on cations in positions S_I [27-28]. In the case of NH₃ adsorption in zeolite NaA, as in the case of H₂O, the first NH₃ molecules interact with Na⁺ cations in positions S_II and S_III, and when one NH₃ molecule per each cation, the interaction with Na+ cations in positions S_I begins [29-30].

Figure 1 presents the variation in the differential enthalpy of hydrogen sulfide adsorption on CaA (M-22) zeolite. The stepwise change in the differential enthalpy of adsorption for various adsorbates on zeolites such as MFI, MOR, FAU, LTA, and others has been documented by several authors [14-20, 24-30].

Figure 1. The differential enthalpy of hydrogen sulfide adsorption (Q_d) on CaA (M-22) zeolite at 303 K. The dashed line represents the heat of condensation of ammonia at 303 K

In the initial region, the differential heat (enthalpy) of adsorption is approximately 91 kJ/mol. As the adsorption amount increases at the active sites of the zeolite, the differential enthalpy decreases to 56 kJ/mol at an adsorption amount of 0.76 mmol/g, forming a plateau. According to the chemical composition of the CaA (M-22) zeolite, the amount of sodium cations present is 0.76 mmol/g. Thus, the hydrogen sulfide molecules initially adsorb on the sodium cations in the zeolite. Therefore, the plateau in the enthalpy at 0.76 mmol/g corresponds to the number of sodium cations in the zeolite, indicating the formation of an adsorbate/adsorbent monomer 1H₂S:Na⁺ ion-molecular complex.

As the sorption volume of the CaA (M-22) zeolite continues to saturate, the adsorption enthalpy decreases to 50 kJ/mol at an adsorption amount of 1.52 mmol/g, forming a second plateau. This indicates that hydrogen sulfide molecules form a dimer 2H₂S:Na⁺ ion-molecular complex with sodium cations. As sodium cations form a trimer 3H₂S:Na⁺ ion-molecular complex with hydrogen sulfide molecules, the differential enthalpy increases from 50 to 52 kJ/mol. The 2 kJ/mol increase in enthalpy during the formation of the trimer 3H₂S:Na⁺ complex is due to the additional energy released from the weak van der Waals interactions between hydrogen sulfide molecules (it is known from the literature that adsorbate-adsorbate interaction energy is 2 kJ/mol). After the formation of the trimer 3H₂S:Na⁺ ion-molecular complex, the change in adsorption enthalpy is no longer dependent on the number of sodium cations in the zeolite. This suggests that the adsorption of hydrogen sulfide molecules on the initial active sites, specifically on the sodium cations, has been completed.

During the subsequent adsorption of hydrogen sulfide molecules on CaA (M-22) zeolite, the enthalpy change (including the change in adsorption entropy) varies in correlation with an adsorption amount of approximately ~1.9 mmol/g. This value corresponds to the amount of calcium cations present in the zeolite. Therefore, the next hydrogen sulfide molecules adsorb on the second active site of the zeolite, namely the calcium cations. At an adsorption amount of 3.2 mmol/g, the enthalpy decreases from 52 kJ/mol to 43 kJ/mol, forming a third plateau that remains unchanged up to an adsorption amount of 4.2 mmol/g. At 4.2 mmol/g, another plateau is formed. The difference between the adsorption amount of the trimer 3H₂S:Na⁺ and the adsorption amount at 4.2 mmol/g is 1.9 mmol/g. This indicates that hydrogen sulfide molecules are adsorbed on the calcium cations, forming a monomer ion-molecular complex 1H₂S:Ca²⁺ at an adsorption amount of 4.2 mmol/g. At an adsorption amount of approximately ~5 mmol/g, the differential enthalpy of hydrogen sulfide adsorption on the CaA (M-22) zeolite decreases to the enthalpy of liquid hydrogen sulfide at the experimental temperature, marking the end of the sorption process.

Entropy of adsorption of hydrogen sulfide is calculated from isotherms and differential enthalpies of adsorption according to the Gibbs-Helmholtz equation:         

where l is the heat of condensation, DH and DG are the enthalpy and free energy (Gibbs energy) changes during the adsorption process from the standard state to the adsorbed state.

Dependence of molar differential entropy of adsorption of hydrogen sulfide (DS) in zeolite CaA (M-22) on filling is presented in Fig.2 (entropy of liquid ammonia is taken as zero). The entropy of adsorption of hydrogen sulfide in zeolite CaA (M-22) differs from the entropy of adsorption of ammonia, water and carbon dioxide in zeolites CaA (M-22), NaA and Ca₄Na₄A [27-33], which indicates the different nature and mechanism of adsorption of hydrogen sulfide, ammonia, water and carbon dioxide in these zeolites. In general, it is located below the entropy of liquid hydrogen sulfide, indicating that the mobility of hydrogen sulfide molecules in the zeolite is limited.

Figure 2. Differential molar entropy of hydrogen sulfide adsorption in zeolite CaA (M-22). The horizontal dashed line is the mean molar integral entropy

The entropy of hydrogen sulfide adsorption in CaA zeolite (M-22) starts at -97 J/mol×К. At 0.76 mmol/g, the entropy increases to -11 J/mol×K. According to the chemical composition of the zeolite, the adsorption value of 0.76 mmol/g corresponds to the amount of sodium cations, that is, hydrogen sulfide molecules adsorb on sodium cations in the zeolite and form a 1H₂S:Na⁺ ion-molecular complex in the first coordination sphere. Then at the adsorption value ~1.52 mmol/g the entropy decreases to -16 J/mol×K and a dimeric 2H₂S:Na⁺ complex is formed. When another complex is formed with each sodium cation, the hydrogen sulfide molecules entropy decreases to -40 J/ mol×K forming a trimeric 3NH₃:Na⁺ complex. Entropy at first increases and then decreases due to release of additional heat under the action of mutual Van der Waals forces of hydrogen sulphide molecules at formation of complete trimeric complex and limitation of mobility of ammonia molecules as a result of this force [28].

The entropy change as a function of the adsorption value in the subsequent sorption of hydrogen sulfide molecules does not correspond to the value of 0.76 mmol/g sodium in zeolite. It means that the process of sorption by ion-molecular mechanism of hydrogen sulfide and sodium in the first coordination sphere is completed. But it can be seen from the graph that the entropy change as a function of adsorption value corresponds to 1.89 mmol/g (Fig. 2). This value is equal to 1.89 mmol/g of calcium cations in the zeolite. Consequently, subsequent hydrogen sulfide molecules adsorb on calcium cations. When the adsorbate/cationic monomer 1H₂S:Ca²⁺ is formed in the first coordination sphere of hydrogen sulfide molecules with calcium cations, the entropy increases linearly to -23 J/mol×K at 4.2 mmol/g. Adsorption on calcium cations is terminated by a monomeric 1H₂S:Ca²⁺ ion-molecular mechanism. Further, with increasing saturation, the entropy increases to ~60 J/mol×K at ~5 mmol/g. The molar average integral entropy of adsorption is -23 J/ mol×K, indicating the retarded state of hydrogen sulfide molecules in the zeolite.

Conclusion.

As a result of adsorption-calorimetric study the differential enthalpy and calculated molar entropy of adsorption of hydrogen sulfide molecules in nanostructured zeolite CaA (M-22) were obtained. A stepwise change in the differential enthalpy and entropy of adsorption depending on the amount of sodium and calcium cations in the zeolite was found. Hydrogen sulfide molecules initially form trimeric ion-molecular complexes 3H₂S:Na⁺ in the first coordination sphere with sodium cations and monomeric ion-molecular complexes 2H₂S:Ca²⁺ with calcium cations. The molar average integral entropy of adsorption is -23 J/mol×K, indicating the loss of mobility of adsorbed hydrogen sulfide molecules in the pores of zeolite CaA (M-22). In zeolite CaA (M-22) a regular relationship between the adsorption value and energy properties of H₂S molecules, as well as the mechanism of sorption from the initial adsorption region to the region of the heat of condensation of H₂S has been established and the regularities of filling of H₂S molecules in the zeolite volume have been determined.

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