ADSORPTION ISOTHERM OF CARBONYL SULFIDE ON KA (MSS-558) ZEOLITE

ABSTRACT

The article presents experimentally obtained values of the adsorption isotherm and the Gibbs energy of carbonyl sulfide adsorption at a temperature of 303 K on KA zeolite (MSS-558), synthesized by the hydrothermal method. The Gibbs energy was calculated from differential values of free energy based on equilibrium pressure values. A consistent relationship between the amount of adsorption and the energetic properties of carbonyl sulfide molecules was established for the KA zeolite (MSS-558). It was found that the adsorption capacity of the zeolite for carbonyl sulfide under experimental conditions (P ≈ ~506 mmHg) is approximately ~0.3 mmol/g per 1 g of zeolite. Based on the BET equation, it was shown that the monolayer adsorption capacity of carbonyl sulfide molecules on KA zeolite (MSS-558) is 0.01 mmol/g.

АННОТАЦИЯ

В статье представлены экспериментально полученные значения изотермы и энергия Гибсса адсорбции карбонилсульфида при температуре 303 К на цеолите КА (MSS-558), синтезированном гидротермальным методом. Энергия Гиббса рассчитывалась из дифференциальных значений свободной энергии от равновесных значений давления. В цеолите КА (MSS-558) установлена ​​закономерная связь между величиной адсорбции и энергетическими свойствами молекул карбонилсульфида. Установлено, что адсорбционная поверхность цеолита по карбонилсульфида в условиях эксперимента (P=~506 мм.рт.ст.) равна ~0,3 ммоль/г в 1 г цеолита. На основании уравнения БЭТ доказано, что величина монослойной адсорбции молекул карбонилсульфида на цеолите КА (MSS-558) составляет 0,01 ммоль/г.

Keywords: adsorption, isotherm, free energy, Gibbs energy, relative pressure, microcalorimeter, carbonyl sulfide.

Ключевые слова: адсорбция, изотерма, свободная энергия, энергия Гибсса, относительное давление, микрокалориметр, карбонилсульфида.

Introduction. For adsorption processes, the synthesis of nanoporous molecular sieve zeolites is not only important due to their high sorption properties, but also for their ability to adsorb molecules of various sizes. To obtain zeolites with these characteristics, it is essential to select suitable raw materials, determine the thermodynamic properties of the synthesized adsorbents, investigate the formation and localization of ion-molecular complexes within the zeolite matrix, study the thermokinetics of adsorption and ion exchange in the zeolite structure, understand the migration patterns of cations, and ultimately identify the complete molecular mechanism of zeolite adsorption.

Globally, there is a growing demand each year to purify natural gases for the production of environmentally friendly gas. This includes drying them from water vapor, removing sulfur-containing compounds, and preventing the release of carbon dioxide—a greenhouse gas—into the atmosphere. Zeolites are widely used to address these issues. The synthesis of highly adsorptive zeolites and the study of their adsorption properties play a crucial role in achieving both scientific and practical advancements.

Aluminosilicate synthetic zeolites play an important role in gas purification and petrochemical processes, particularly in solving the aforementioned issues. One of their unique features is the possibility of substituting the aluminum and silicon atoms in the aluminosilicate zeolite structure with other trivalent or pentavalent elements—such as gallium, germanium, and phosphorus—which are chemically similar. This allows for modification of their sorption and catalytic properties [1]. Furthermore, water molecules present in the internal crystal structure can evaporate at a temperature of 450°C without damaging the crystal lattice. The mobility of alkali and alkaline earth cations, along with water molecules within the zeolite structure, enables ion exchange capabilities [2–4].

Zeolites, with their porous crystal framework structure, are considered aluminosilicate adsorbents. Their frameworks are composed of interconnected tetrahedral [SiO₄] and [AlO₄] units, linked at their corners by oxygen atoms. The specific positioning of aluminum atoms within the structure is a characteristic feature of aluminosilicates. In all aluminosilicates, Al and Si atoms occupy tetrahedral coordination around oxygen atoms, with Al substituting isomorphically for Si in the silicate framework. The negative charge of the tetrahedra is balanced by various alkali or alkaline earth metal cations located within the zeolite cavities. The composition of all synthetic zeolites can be represented by the following general chemical formula:

Ме_2/nO·Al₂O₃·xSiO₂·yH₂O,

Here, n is the valency of the metal cation, x is the molar ratio of SiО₂/Аl₂O₃, and y is the number of moles of water.

One of the most widely used synthetic adsorbents is KA zeolite. Its effective pore diameter is 3 Å, which allows it to selectively adsorb only those molecules in a liquid whose dynamic diameter is less than 3 Å. Molecular sieve 3A is an excellent desiccant primarily used for water adsorption. KA zeolites are used for dehydration, drying, moisture removal, paraffin removal, drying and purification of cracked petroleum gases, dehydration of C₂ and C₃ fractions, drying and purification of ethylene, propylene, butylene, butadiene, pentane, alcohol dehydration, and purification of methane, ethane, and propane feedstocks. They are also used for purifying liquid carbon dioxide, drying dissolved acetylene, and drying and purifying water, air, gases, and polar liquids (such as methanol, ethanol), as well as for natural gas drying and purification, among others.

To determine the volumetric sorption characteristics of KA zeolite, most studies have focused on water adsorption [5–10]. However, the surface sorption properties have not been sufficiently studied. This article presents the adsorption isotherms and Gibbs energy values obtained by an adsorption microcalorimetric method for carbonyl sulfide on hydrothermally synthesized KA (MSS-558) zeolite, as well as the proposed adsorption mechanism.

Methodology and Materials. The adsorption isotherm was measured using a universal high-vacuum apparatus. The operating principle and specifications of the apparatus are described in detail in the authors’ previous works [11–15]. In this adsorption study, the adsorption of carbonyl sulfide on KA (MSS-558) zeolite was investigated at a temperature of 303 K. The Gibbs energy was calculated from equilibrium pressure values, and the adsorption mechanism was thoroughly analyzed.

The unit cell composition of this zeolite is represented as K_4.63Al_10.62H₇₂Na₆O_91.32Si_25.38. Based on its chemical formula, the amount of potassium cations in 1 gram of the zeolite is equivalent to 1 mmol/g. KA (MSS-558) zeolite was synthesized via the hydrothermal method in a special autoclave using chemically pure oxides [16].

Results and Discussion. The main thermodynamic characteristics of carbonyl sulfide adsorption on KA (MSS-558) zeolite at 303 K were investigated. These include the adsorption isotherm, Gibbs free energy, work performed during the isothermal process as gas volume changes, the correlation between adsorption amount and energetic parameters, the adsorption process mechanism, and the migration of potassium cations in the zeolite structure during the adsorption of carbonyl sulfide molecules.

The adsorption isotherm of carbonyl sulfide on KA (MSS-558) zeolite is shown in Figure 1. At low saturation levels, the equilibrium relative pressure is P/P_s=8×10^-5 (P=0.9 torr), indicating relatively weak adsorption of carbonyl sulfide molecules on the zeolite. The adsorption isotherm reaches an adsorption capacity of 0.3 mmol/g at a relative pressure of P/P_s=0.045 (P=506 torr). The effective pore diameter of the KA zeolite is 3 Å, which is approximately equal to the size of the carbonyl sulfide molecule (~3 Å). The adsorption isotherm corresponds to Type II according to the Brunauer classification. Therefore, based on the above values and the Brunauer isotherm distribution, it can be concluded that carbonyl sulfide molecules are adsorbed on the external surface of the zeolite.

Figure 1. Adsorption isotherm of carbonyl sulfide molecules on KA (MSS-558) zeolite

Up to an adsorption amount of 0.066 mmol/g, the relative pressure (P/P_s=0.004, P=48 torr) increases slowly. This indicates that the adsorption of carbonyl sulfide molecules on the surface of KA (MSS-558) zeolite occurs with relatively strong intermolecular interactions. The isotherm shows a linear increase up to an adsorption amount of 0.1 mmol/g and a relative pressure of P/P_s=0.0177 (P=200 torr). Starting from P/P_s=0.0177, the equilibrium relative pressure increases sharply, and the adsorption process completes at a relative pressure of P/P_s=0.045 (P=506 torr) and an adsorption amount of 0.3 mmol/g.

The overall view of the adsorption isotherm of carbonyl sulfide molecules on KA (MSS-558) zeolite in BET coordinates is presented in Figure 2. The nearly linear change in the isotherm up to an adsorption amount of 0.1 mmol/g corresponds to a linear trend in the relative pressure region.

Figure 2. BET plot of the adsorption isotherm of carbonyl sulfide molecules on KA (MSS-558) zeolite

Based on the chemical composition of KA (MSS-558) zeolite (K_4.63Al_10.62H₇₂Na₆O_91.32Si_25.38), the amount of potassium cations in 1 gram of zeolite is 1 mmol/g. The linear variation of the isotherm in both relative pressure and BET coordinates is proportional to the potassium content of the zeolite. This is associated with the migration of potassium cations to the zeolite surface and the formation of ion–molecular complexes with carbonyl sulfide molecules.

Figure 3 presents the linear region of the adsorption isotherm of carbonyl sulfide on KA (MSS-558) zeolite, calculated using the BET equation, from the initial relative pressure value P/P_s=0.00008 to P/P_s=0.0016. From Figure 3, it was determined that the monolayer adsorption amount of carbonyl sulfide molecules is a_m=0.01 mmol/g.

Figure 3. BET plot of the adsorption isotherm of carbonyl sulfide molecules on KA (MSS-558) zeolite

The adsorption isotherm of carbonyl sulfide on KA (MSS-558) zeolite at low pressure values demonstrates that the interaction between the adsorbent and adsorbate is sufficiently strong. This indicates the formation of donor–acceptor bonds between the carbonyl sulfide molecules and the potassium cations in the zeolite.

CONCLUSION

The adsorption isotherm of carbonyl sulfide molecules on the nanostructured KA (MSS-558) zeolite was measured using the adsorption-calorimetric method. Within the pressure range from low saturation levels up to the experimental pressure (506 torr), the thermodynamics of the sorption process on the zeolite surface, as well as the regularity of surface coverage by carbonyl sulfide molecules, were determined. It was found that up to an adsorption amount of 0.1 mmol/g, the carbonyl sulfide molecules form sufficiently strong interactions with the zeolite surface. The isotherm of the physical adsorption, which is governed by van der Waals forces, was interpreted based on the BET equation, and it was proven that the monolayer adsorption capacity is equal to 0.01 mmol/g.

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