DIFFERENTIAL ENTALPY AND ENTROPY OF AMMONIA ADSORPTION IN Са5Na3A ZEOLITE (MSS-624)

ABSTRACT

The article presents experimentally obtained values of the differential enthalpy of ammonia adsorption at 303 K in the LTA-type zeolite Са₅Na₃A (MSS-624). A correlation between the amount of ammonia adsorption and the differential enthalpy in Са₅Na₃A (MSS-624) zeolite has been established, as well as the adsorption mechanism from the initial stage to the heat of ammonia condensation and the filling of the zeolite volume with ammonia molecules. Under the experimental conditions (P=~614 mmHg), the adsorption capacity of the Са₅Na₃A (MSS-624) zeolite for ammonia was found to be approximately 10 mmol/g. It was determined that the values of differential heat of adsorption vary depending on the number of Na⁺ and Ca²⁺ cations in the zeolite structure. Ammonia molecules form tetrameric ion-molecular complexes (4NH₃:Na⁺) with sodium cations and trimeric ion-molecular complexes (3NH₃:Ca²⁺) with calcium cations in the first coordination sphere of the zeolite. Overall, the entropy change values of Са₅Na₃A (MSS-624) zeolite upon ammonia molecule adsorption are lower than the entropy values of the liquid state at the experimental temperature, with an average value of -61 J/mol·K.

АННОТАЦИЯ

В статье представлены экспериментально полученные значения дифференциальной энтальпии адсорбции аммиака при 303 К в цеолите Са₅Na₃A (MSS-624) типа LTA. Установлена ​​закономерность между величиной адсорбции аммиака и дифференциальной энтальпией в цеолите Са₅Na₃A (MSS-624), а также механизм адсорбции от начальной области до теплоты конденсации аммиака и заполнения объема цеолита молекулами аммиака. В условиях эксперимента (P=~614 мм.рт.ст.) адсорбционная емкость цеолита Са₅Na₃A (MSS-624) по аммиаку оказалась равной ~10 ммоль/г. Установлено, что значения дифференциальной теплоты адсорбции меняются в зависимости от количества катионов Na⁺ и Ca²⁺ в структуре цеолита, при этом молекулы аммиака образуют тетрамерные ионно-молекулярные комплексы 4NH₃:Na⁺ с катионами натрия и тримерные ионно-молекулярные комплексы 3NH₃:Ca²⁺ с катионами кальция в первой координационной сфере цеолита. В целом значения изменения энтропии цеолита Са₅Na₃A (MSS-624) адсорбции молекул аммиака ниже значения энтропии жидкого состояния при температуре эксперимента, а ее среднее значение составляет -61 Дж/моль×К.

Keywords: adsorption, enthalpy, free energy, entropy, isotherm, relative pressure, microcalorimeter, ammonia.

Ключевые слова: адсорбция, энтальпия, свободная энергия, энтропия, изотерма, относительное давление, микрокалориметр, аммиак.

Introduction. In the process of obtaining purified gas from natural gases, insufficient removal of additional chemical components—especially secondary gases released during industrial combustion—has led to increasing environmental problems worldwide. The combustion of natural gas with varying compositions in different countries results in the release of sulfur-containing and nitrogen-containing compounds, which are harmful to human health and plant life. Additionally, large amounts of carbon dioxide are emitted into the atmosphere, further exacerbating environmental and health concerns. As a result, the global demand for preventing these environmental issues is increasing every year. Millions of tons of synthetic zeolites are widely used to address these problems, particularly for gas purification and drying by removing unwanted components. This, in turn, necessitates the synthesis of a new generation of zeolites with enhanced sorption and catalytic properties. Moreover, research aimed at improving their selectivity characteristics plays a crucial role in the practical application of these findings.

Currently, zeolites are extensively used as adsorbents and catalysts in natural gas dehydration, petroleum refining, and petrochemical industries. The large-scale industrial application of zeolites and their use as catalysts have attracted the interest of many researchers [1]. However, before producing zeolites on a large scale, it is essential to ensure that their crystal structure is defect-free. Additionally, it is necessary to determine their adsorption behavior for various polar and nonpolar molecules, as well as to establish their fundamental thermodynamic properties.

The structural difference between LTA and LSX zeolites lies in the spatial arrangement of their cuboctahedral structural units, specifically their positioning within the framework voids [2-3]. In LTA-type zeolites, cuboctahedra form a simple cubic lattice. Each cuboctahedron is connected to six neighboring units through four-membered oxygen bridges. The voids between eight adjacent cuboctahedra create large cavities [4]. The negative charge of the zeolite crystal lattice varies according to the Si/Al ratio in its composition. To compensate for this charge fully or partly, different amounts of cations are incorporated. These cations can be alkali metals or alkaline earth metals. The type of cation present significantly influences the adsorption and catalytic properties of the zeolite. Before utilizing zeolites in various industrial applications, it is essential to study their adsorption properties.

By determining the fundamental thermodynamic parameters of the adsorption of polar molecules, such as water and ammonia, on LTA and LSX zeolites, the number, nature, and strength of energetically active centers at crystallographically equivalent positions can be identified [5-19]. Traditional methods such as NMR, SEM, X-ray diffraction, IR spectroscopy, and gas chromatography are insufficient for studying the thermodynamics of adsorption and determining the sorption mechanism. Investigating key thermodynamic characteristics of sorption processes—including adsorption enthalpy, Gibbs’s energy, and entropy changes—provides valuable insights and expected results.

There is a substantial body of data obtained through various physico-chemical research methods on ammonia adsorption in Са₅Na₃A (MSS-624) zeolites. However, detailed studies on the adsorption properties of Са₅Na₃A (MSS-624) zeolites in relation to different polar molecules, including ammonia, as well as comprehensive data on the key thermodynamic parameters (ΔH, ΔG, ΔS) of these systems, remain insufficient.

This article presents the experimentally obtained adsorption enthalpy of ammonia in Са₅Na₃A (MSS-624) zeolite using a microcalorimetric method, along with the theoretically calculated entropy change based on the Gibbs-Helmholtz equation. Additionally, the adsorption mechanism is discussed.

Methods and Materials. The differential adsorption enthalpy was measured using a Tian-Calvet type DAC-1-1A differential automated microcalorimeter, which was connected to a universal high-vacuum apparatus. The operational principle and characteristics of the microcalorimetric research method have been comprehensively described in previous studies [5-19]. In this adsorption study, the adsorption of ammonia on Са₅Na₃A (MSS-624) zeolite at a temperature of 303 K was investigated. The adsorption enthalpy was measured, and the entropy change was calculated using the Gibbs-Helmholtz equation based on the obtained enthalpy and Gibbs free energy values. Additionally, the adsorption mechanism was fully analyzed. The elementary unit cell composition of this zeolite is expressed as Ca₅Na₃[(AlO₂)₁₂(SiO₂)₁₂]). Based on its chemical composition and the structural arrangement of the crystal lattice, the zeolite consists of active adsorption positions S_I, S_II, and S_III. The amount of calcium cations in 1 g of zeolite is 1.9 mmol/g (total calcium amount across all positions is 2.95 mmol/g), while the amount of sodium cations is 1.1 mmol/g (total sodium amount across all positions is 1.77 mmol/g). Adsorbate molecules can be adsorbed only at the S_I and S_II positions.

Results and Discussion. The fundamental thermodynamic characteristics of ammonia adsorption on Са₅Na₃A (MSS-624) zeolite at 303 K were studied, covering the transition from the initial saturation region to the condensation enthalpy of ammonia. These characteristics include differential enthalpy, entropy change, Gibbs free energy, the work done due to gas volume change in an isothermal process, adsorption capacity, and the correlation between adsorption and energetic properties. The adsorption mechanism was analyzed in detail. The initial adsorption enthalpy values of ammonia on Са₅Na₃A (MSS-624) zeolite were found to be approximately 20-70 kJ/mol higher than those for the adsorption of various other adsorbates such as hydrogen sulfide, methyl mercaptan, carbon dioxide, and ethane [9-16]. The study confirmed that during the adsorption process, ammonia molecules interact with Na⁺ and Ca²⁺ cations located at S_I and S_II positions of the zeolite, forming various ion-molecular adsorption mechanisms.

The relationship between the differential heat of adsorption and the adsorption capacity of ammonia on Са₅Na₃A (MSS-624) zeolite is presented in Figure 1. Previous research has shown that the stepwise change in adsorption enthalpy in different synthetic zeolites corresponds to the quantity and type of cations in their structures [5-7, 10-12, 16-19]. For instance, in CaA (M-22) zeolite, ammonia molecules interact with sodium cations to form a tetramer complex 4NH₃: Na⁺, while calcium cations facilitate dimer formation 2NH₃:Ca²⁺. Additionally, two ammonia molecules initially adsorbed on calcium cations interact with each other via Van der Waals forces, leading to the formation of an adsorbent-adsorbate-adsorbate complex Са²⁺: 2NH3:2NH3, which can be expressed as the general complex 8NH₃: CaA (M-22) [9-13].

In this study, a completely different sorption mechanism was observed during the adsorption of ammonia molecules on the Са₅Na₃A (MSS-624) zeolite. At an initial adsorption amount of 0.05 mmol/g, the differential enthalpy was approximately ~136 kJ/mol. As the adsorption amount increased, the differential enthalpy gradually decreased, forming the first step at ~1.1 mmol/g adsorption and 95 kJ/mol. With the saturation of the sorption volume, the second step was observed at 2.2 mmol/g adsorption with a decrease to ~73 kJ/mol, the third step at ~3.3 mmol/g adsorption and 68 kJ/mol enthalpy, and the fourth step at ~4.4 mmol/g adsorption and 57 kJ/mol enthalpy.

Figure 1. Differential heat of NH₃ adsorption on Са₅Na₃A (MSS-624) zeolite (Q_d). The dashed line represents the condensation heat of NH₃ at 303 K

Each stepwise adsorption increment follows a multiple of 1.1 mmol/g. Based on the chemical composition of the Са₅Na₃A (MSS-624) zeolite studied in this research, the amount of calcium cations in 1 g of zeolite is 1.9 mmol/g (with a total calcium content of 2.95 mmol/g), while the amount of sodium cations is 1.1 mmol/g (with a total sodium content of 1.77 mmol/g). The active adsorption sites of the zeolite consist of S_I, S_II, and S_III positions, but adsorbate molecules can only be adsorbed at the active sites located in the S_I and S_II positions. This implies that not all calcium and sodium cations within the zeolite fully participate in the adsorption process. In this case, how do ammonia molecules interact with calcium and sodium cations in varying proportions to form different ion-molecular mechanisms? Do ammonia molecules adsorb first onto calcium or sodium cations?

From Figure 1, the stepwise change in differential enthalpy as a function of ammonia molecule adsorption is approximately ~1.1 mmol/g. This indicates that ammonia molecules are initially adsorbed onto the sodium cations in the zeolite, where the 1.1 mmol/g amount of sodium cations acts as the primary active site in the sorption process. At an adsorption amount of 1.1 mmol/g, ammonia molecules form the monomer complex 1NH₃: Na⁺; at 2.2 mmol/g, they form the dimer 2NH₃:Na⁺; at 3.3 mmol/g, the trimer 3NH₃:Na⁺; and at 4.4 mmol/g, the tetramer 4NH₃:Na⁺, each corresponding to different values of differential enthalpy. Beyond this point, changes in differential enthalpy due to additional ammonia adsorption are no longer dependent on the 1.1 mmol/g adsorption value, meaning that ammonia molecule sorption on sodium cations is complete at 4.4 mmol/g. However, the subsequent variation in differential enthalpy during further ammonia adsorption is a multiple of 1.7 mmol/g. Additionally, the differential enthalpy values closely match the adsorption enthalpy of ammonia on CaA (M-22) and CaA (M-34) zeolites, indicating that the sorption process is now occurring on calcium cations [9-13].

As ammonia molecules saturate the sorption volume, the differential entropy initially changes linearly up to an adsorption of 6.1 mmol/g and then up to 7.8 mmol/g. The difference in adsorption amount between the adsorption at 4.4 mmol/g, where the tetramer complex is formed, and the two linear variations is 1.7 mmol/g. This value corresponds to the number of calcium cations participating in the adsorption process of Са₅Na₃A (MSS-624) zeolite. Thus, ammonia molecules form different ion-molecular complexes with calcium cations in the first coordination sphere: the monomer 1NH₃: Ca⁺² at 6.1 mmol/g adsorption with a differential enthalpy decrease to 50 kJ/mol, the dimer 2NH₃:Ca⁺² at 7.8 mmol/g adsorption with a further decrease to 37 kJ/mol, and the trimer 3NH₃:Ca⁺² at 9.5 mmol/g adsorption with an enthalpy value close to that of liquid ammonia. At an experimental pressure of 614 torr, the sorption process concludes.

Overall, ammonia molecules in Са₅Na₃A (MSS-624) zeolite form ion-molecular complexes of the type of adsorbate/zeolite 7NH₃:Са₅Na₃A (MSS-624).

The relationship between the molar differential entropy (ΔS_d) change of ammonia adsorption and sorption saturation in nanostructured Са₅Na₃A (MSS-624) zeolite is illustrated in Figure 2 (where the entropy of liquid ammonia is taken as zero). The adsorption entropy was calculated using the Gibbs-Helmholtz equation [9-13].

In general, adsorption entropy exhibits a wave-like variation corresponding to each ion-molecular complex formation mechanism and is significantly lower than the entropy of liquid ammonia, indicating restricted mobility of ammonia molecules in the zeolite. The wave-like variation of entropy corresponds well with the differential enthalpy changes at each ion-molecular complex formation stage [12, 16].

The differential entropy of ammonia adsorption in the Са₅Na₃A (MSS-624) zeolite can be divided into six distinct regions. Adsorption amounts located below the entropy of liquid ammonia include four regions with multiples of 1.1 mmol/g and two regions with multiples of 1.89 mmol/g. Additionally, two regions with multiples of 1.7 mmol/g are positioned above the entropy of liquid ammonia.

Each region consists of well-defined steps. In the initial region, the entropy change is -226 J/mol·K. As sorption saturation increases, at an adsorption amount of 1.1 mmol/g, the entropy increases stepwise to -142 J/mol·K.

Figure 2. Entropy of NH₃ adsorption in Са₅Na₃A (MSS-624) zeolite, with dashed lines representing the average integral entropy

According to the chemical composition of the zeolite, the sodium cation content is 1.1 mmol/g, meaning that the initial ammonia molecules are adsorbed at the sodium cations in the zeolite, forming a monomeric ion-molecular complex (1NH₃: Na⁺) in the first coordination sphere. As the sorption volume becomes saturated, at an adsorption amount of approximately ~2.2 mmol/g, entropy increases stepwise to -84 J/mol·K, forming a dimeric complex (2NH₃: Na⁺).

With a linear entropy change up to -55 J/mol·K, trimeric (3NH₃: Na⁺) and tetrameric (4NH₃: Na⁺) complexes are formed at an adsorption amount of 4.4 mmol/g. The decrease in entropy variation with sorption volume saturation is associated with additional heat release due to Van der Waals interactions between ammonia molecules. This interaction restricts the mobility of ammonia molecules, which is clearly observed in the formation of trimeric (3NH₃: Na⁺) and tetrameric (4NH₃: Na⁺) complexes [20]. Entropy changes for monomeric and dimeric complex formation are -81 J/mol·K and -61 J/mol·K, respectively, whereas for the sequential formation of trimeric and tetrameric complexes, entropy change is -30 J/mol·K.

The change in entropy with respect to the amount of adsorption during the subsequent sorption of ammonia molecules does not correspond to the sodium content of 1.1 mmol/g in the zeolite structure. This indicates that the sorption process is completed when ammonia molecules form tetrameric ion-molecular complexes (4NH₃:Ca²⁺) with calcium cations in the first coordination sphere. However, from the graph, it is evident that the change in entropy with respect to the amount of adsorption corresponds to approximately ~1.7 mmol/g (Figure 2). This value matches the calcium cation content of the zeolite (1.7 mmol/g), confirming that the next stage of the sorption process occurs at the calcium cations.

At an adsorption amount of 6.1 mmol/g, entropy exhibits a linear change down to -50 J/mol·K, where ammonia molecules form monomeric ion-molecular complexes (1NH₃:Ca²⁺) in the first coordination sphere with calcium cations. At an adsorption amount of approximately ~7.8 mmol/g, entropy increases to -33 J/mol·K, forming dimeric complexes (2NH₃:Ca²⁺). Finally, at an adsorption amount of 9.5 mmol/g, entropy surpasses the entropy of liquid ammonia, forming trimeric complexes (3NH₃:Ca²⁺). This marks the completion of the sorption process at the experimental pressure of 614 mmHg.

The average entropy change is -61 J/mol·K, indicating that the mobility of ammonia molecules is significantly restricted within the zeolite structure.

Conclusion. The differential enthalpy of ammonia molecule adsorption in the nanostructured zeolite Са₅Na₃A (MSS-624) was measured using the adsorption-calorimetric research method. The entropy change was calculated using the Gibbs-Helmholtz equation based on enthalpy and Gibbs free energy values. The sorption process mechanism, as well as the pattern of ammonia molecules filling the zeolite volume, was determined in the range from low saturation to the experimental pressure (614 mmHg). At low saturation levels, ammonia molecules were found to form tetrameric (4NH₃:Na⁺) and trimeric (3NH₃:Ca²⁺) ion-molecular complexes with sodium and calcium cations, respectively, in the first coordination sphere of the zeolite at S_I and S_II positions. No ammonia molecule adsorption was observed in the second coordination sphere of the zeolite. In general, it was confirmed that ammonia molecules in Са₅Na₃A (MSS-624) zeolite form ion-molecular complexes of the type 7NH₃: Са₅Na₃A (MSS-624). The average molar integral entropy of adsorption was found to be -61 J/mol·K, indicating a loss of mobility of the adsorbed ammonia molecules within the pores of the Са₅Na₃A (MSS-624) zeolite.

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