WATER ADSORPTION ISOTHERM AND DIFFERENTIAL HEAT OF ADSORPTION ON MICROPOROUS ADSORBENT OBTAINED FROM LOCAL KAOLIN

ABSTRACT

In this paper, the change in differential heat ( Q _d ) during adsorption of water molecules on NaA-type zeolite was analyzed from the thermodynamic point of view. The results of the study show that at the early stages of adsorption, water molecules bind with high-energy Na⁺ ions, which leads to a differential heat of about ~−70 kJ/mol. At subsequent stages of adsorption, Q _d gradually decreases to −45 kJ/mol, indicating the dominance of hydrogen bonds formed between water molecules and capillary condensation. The results obtained reveal the active role of hydrophilicity and internal porous structure of NaA zeolite in water vapor adsorption and substantiate its potential application in humidity control and selective separation processes.

АННОТАЦИЯ

В данной работе с термодинамической точки зрения анализировалось изменение дифференциальной теплоты (Q_d) при адсорбции молекул воды на цеолите типа NaA. Результаты исследования показывают, что на ранних стадиях адсорбции молекулы воды связываются с высокоэнергетическими ионами Na⁺, что приводит к дифференциальной теплоте около ~−70 кДж/моль. На последующих стадиях адсорбции Q_d постепенно уменьшается до −45 кДж/моль, что свидетельствует о доминировании водородных связей, образующихся между молекулами воды, и капиллярной конденсации. Полученные результаты раскрывают активную роль гидрофильности и внутренней пористой структуры цеолита NaA в адсорбции водяного пара и обосновывают его потенциальное применение в процессах контроля влажности и селективного разделения.

Keywords: kaolin, zeolite, water, isotherm, differential heat.

Ключевые слова: каолин, цеолит, вода, изотерма, дифференциальная теплота.

Introduction. Zeolites are among the most frequently considered objects of quantum-chemical calculations. Zeolites of type A and X are widely used in drying hydrocarbon gas, especially when it contains a sufficiently high content of acidic components [1-3].

Starting from about 38-42 molecules / el.cell , the heat of adsorption of C ₆ H ₆ by zeolite NaY changes little with filling. The authors of [4] attribute the region between 40 and 42 molecules / el.cell to adsorption of C ₆ H ₆ at the junctions between six- membered rings. In the filling region of about 38-44 molecules / el.cell , an inflection point was found, which the authors explain by the completion of the population of all 32 sites at the junctions of six -membered rings by C ₆ H ₆ molecules . In this case, the length of the region in which the heat of adsorption hardly changes (about 44 molecules / el.cell ) exactly coincides with the number of free docking sites calculated on the basis of the chemical composition of the studied sample [5-8].

In the filling range up to 50 molecules / el.cell , a remarkable similarity is observed in the course of the heats of adsorption of NH ₃ and H ₂ O on NaX [10]. Up to 10 molecules / el.cell , water is adsorbed, possibly, on defects of the crystal lattice [9]. In the filling range from 10 to 50 molecules / el.cell , the adsorbate molecules interact primarily with cations in the S _{III p}ositions [11]. However, for zeolite NaX , the penetration of H _{2 O} molecules into the zeolite cuboctahedra cannot be ruled out even at low fillings [12].

Materials and methods. The composition of the zeolite obtained for the study: Na ₁₂ (AlO ₂ ) ₁₂ (SiO ₂ ) ₁₂ . The adsorption-calorimetric technique used in this work allows us to obtain high-precision molar thermodynamic characteristics, as well as to reveal the detailed mechanism of adsorption processes occurring in adsorbents and catalysts.

Adsorption measurements and adsorbate doses were carried out using a universal adsorption device, in the working part of which only mercury valves were used, and the valves were replaced by vacuum lubrication [1]. The device allows the dosing of the adsorbate to be carried out by both gas-volume and liquid-volume methods. The calorimeter used was a DAK 1-1 calorimeter, modified to achieve high accuracy and reliability.

Results and discussion. The water adsorption behavior on NaA zeolite was studied by measuring the isotherm at constant temperature over a range of relative pressures (P/P⁰). The isotherm shows a characteristic IUPAC type I profile, indicating a microporous adsorption mechanism dominated by monolayer coverage at low relative pressures.

At P/P⁰ < 0.1, the uptake of water molecules increases sharply, which corresponds to the strong electrostatic interaction between the polar water molecules and the exposed sodium cations (Na⁺) located inside the zeolite framework. This initial steep rise is explained by chemisorption at high-energy cation sites inside the α-cells of the NaA framework.

As the relative pressure increases to the range of 0.1 < P /P⁰ < 0.4, the adsorption rate gradually levels off, reflecting the gradual filling of the microporous structure. The adsorbed water molecules begin to form clusters through hydrogen bonds, which further stabilizes the adsorbed phase without significantly increasing the overall absorption.

Beyond P/P⁰ > 0.4, the isotherm tends to plateau, indicating that adsorption sites are close to saturation. In this region, additional adsorption is largely limited by the restrictive effects of micropores and electrostatic repulsion between adjacent water clusters.

It was found that the total adsorption capacity (Wₐ) of zeolite NaA for water under ambient conditions is in the range of 200–300 mg/g, depending on the degree of crystallinity, activation method, and integrity of the zeolite sample framework.

A typical BET adsorption isotherm analysis yields specific surface areas of 400 to 600 m²/g, and the calculated monolayer capacity (aₘ) correlates well with theoretical estimates based on the pore volume of NaA and the cross-sectional area of the water molecule.

The water vapor adsorption isotherm on NaA zeolite was measured at 303 K in the relative pressure (P/P₀) range from 0 to 0.95. The isotherm had the IUPAC type I shape, indicating strong adsorbate -adsorbent interactions and preferential filling of micropores. At low relative pressures (P/P₀ < 0.1), a sharp absorption was observed, indicating the presence of strong adsorption sites associated with Na ⁺ cations. The plateau region was reached at P/P₀ ≈ 0.3–0.4, corresponding to monolayer coverage and partial pore saturation.

Adsorption capacity of NaA zeolite (N), expressed per unit cell (UC) as the number of H₂O molecules, was investigated under controlled conditions. Figure 1 shows the corresponding adsorption isotherm at 303 K, where the adsorption was measured at a water vapor pressure of about 10⁻⁶ and at different relative pressures (p⁰), where p⁰ denotes the saturated vapor pressure of water at 303 K (4.42 kPa). The isotherm was systematically analyzed by dividing it into three distinct regions to better understand the adsorption behavior.

In the initial region, water absorption begins at ln (p/p⁰) = −16.26, which corresponds to N = 0.493 H₂O/UC. As the relative pressure increases from ln (p/p⁰) = −16.26 to −13.70, the isotherm shows an upward trend, and the adsorption value increases to N = 2.385 H₂O/UC. Then, up to ln (p/p⁰) = −10.76, the isotherm takes a more linear trajectory, indicating a transition plateau, with water adsorption increasing to N = 8.645 H₂O/UC.

Figure 1. Isotherm of water adsorption in NaA zeolite at 303 K. ∆ - experimental data, ▲ - calculated using the theory of volume filling of micropores (VOMT)

The second region covers the interval from ln (p/p⁰) = −10.40 to −8.30. Here the isotherm again bends toward the adsorption axis, and water uptake continues to increase, reaching N = 11.084 H₂O/UC. In the subsequent range, from ln (p/p⁰) = −8.30 to −5.69, the isotherm transitions to a quasi-plateau, during which adsorption levels increase from N = 11.084 to N = 16.988 H₂O/UC.

For water system - NaA zeolite the parameters of equation (3.6) for the first term are: N ₀₁ = 11 , 72 H ₂ O e.a. , E ₀₁ = 3 2 , 09 kJ/mol and n ₁ = 6 ; for the second term N ₀₂ = 6 ,0 5 H ₂ O e.a. , E ₀₂ = 17 , 97 kJ/mol and n ₂ = 8 ; for the third term N _{0 3} = 9 , 7 01 H ₂ O e.i. , E _{0 3} = 2 , 87 kJ/mol and n ₃ = 1 ;

N = 11.731exp[A/32.09) ⁶ ]+6.05exp[A/17.97) ⁸ ]+9.701exp[A/2.87) ¹ ],

Here: N is the adsorption in micro voids, (H ₂ O)/ eA; A = RTln (P °/ P – 1 H ₂ O/eA, this is the work performed to transfer steam from the surface (pressure P°) into a stable gaseous phase (pressure P).

In the final third region, the isotherm exhibits a steep rise approaching the vertical axis, which marks the onset of saturation. This behavior reflects the onset of capillary condensation and the almost complete filling of the available adsorption sites of the micropores.

Figure 2 shows the differential heat ( Q _d ) of water adsorption in zeolite NaA at 303 K. The dotted lines are the heat of water condensation at 303 K (∆ H _v = 43.5 kJ/mol). During water adsorption in zeolite NaA (starting with N = 0.24 H ₂ O / ea) the differential heat starts from ~91 kJ/mol and gradually decreases to Q _{d = 76.26 kJ/mol upon reaching} = 3.09 H ₂ O / ea. Then, in the adsorption range N = 3.09 - 6.47 H ₂ O / ea , the high-energy first small stage (75.56 kJ/mol from Q _{d = 75.26)} is formed . The next adsorption in the range from N = 6.47, at 10.35 H ₂ O / ea , forms the second small step (70.58 kJ / mol from Q _d = 75.56) . The intervals of both steps consist of 3.35 H ₂ O / ea and 3.95 H ₂ O / ea. With a wave-like state of the steps, the heat of adsorption from 75.26 kJ / mol gradually decreases to 70.58 kJ / mol. The third, in the intervals from N = 10.35 to 16.24 and from 16.24 to 21.85 H ₂ O / ea, 2 large step minima are observed. In these steps, heat adsorption at the third step decreases from 70.58 kJ/mol to 58.88 kJ/mol, at the fourth step – from 58.88 kJ/mol to 47.00 kJ/mol. At large steps, a decrease in the heat of adsorption is observed from 70.58 kJ/mol to 47.00 kJ/mol. At the end of the process, the heat is divided into two sections and the last section continues from N = 21.85 to 27.13 H ₂ O/E. In this case, the heat of adsorption decreases from 47.00 kJ/mol to 43.36 kJ/mol, and is equal to the heat of condensation. In the structure of zeolite NaA There are 3 active centers - adsorption voids, and adsorbates are adsorbed into these voids. The basis of the active centers are alkali metals.

In the first void, the Na ^{+ cations} are located in the center of the six-membered oxygen rings S _I . Due to the fact that the volume of this void is very small, it is only partially saturated with metal cations. In the second void, the Ca ²⁺ and Na ⁺ cations are located in the slightly inner side of the plane of the eight-membered oxygen rings S _II , and finally, in the third void, the Na ⁺ cation is located in the inner part of the α-void, which is located opposite the four-membered oxygen ring S _III .

As can be seen, the S _III and S _II voids located inside the supervoid account for the bulk of the adsorption. Because the cations in the S _I S _{I voids} are surrounded by strong protective cations of six oxygen atoms. In zeolite NaA , a total of 2 8 H ₂ O e.u. water molecules are adsorbed. Of these, in the S _II void 19 H ₂ O e.u. and in the S _{II I} void 8 H ₂ O e.u.

Figure 2. Differential heats of water adsorption in NaA zeolite at 303 K. The horizontal dotted line is the heat of ammonia condensation at 303 K.

The initial differential enthalpy of adsorption was found to be relatively high (~−70 kJ/mol), consistent with strong electrostatic interactions between water dipoles and framework cations. As coverage increased, the differential heat decreased to ~−45 kJ/mol, indicating weaker multilayer adsorption or cluster formation. Entropy changes were also evaluated, showing limited mobility at low coverage and a gradual increase in freedom at higher loadings, consistent with a transition from chemisorption to physisorption regimes.

NaA -type zeolites are characterized by high hydrophilicity and a regular microporous structure. They consist of an aluminosilicate skeleton balanced by Na⁺ cations, which enter strong electrostatic interactions with water molecules. For a deeper understanding of the adsorption process, it is important to determine the differential heat of adsorption (Qd), since it characterizes the intensity of the interaction of molecules with active centers on the adsorbent surface.

When water molecules are adsorbed on NaA zeolite, the differential heat release (Qd) gradually decreases. During primary adsorption, i.e., when water molecules are initially adsorbed on high-energy sites where Na⁺ cations are located, Qd is about ~−70…−75 kJ/mol. This value indicates a strong electrostatic attraction between water molecules and Na⁺ ions.

At later stages, when the internal pores of the zeolite begin to fill, hydrogen bonds begin to form between the water molecules, and the molecules are adsorbed on less active areas of the surface. As a result, the Qd value decreases to the range of -55…-45 kJ/mol. At this stage, the adsorption process occurs predominantly in the form of cluster or capillary condensation in micropores.

When analyzing the change in Qd depending on the adsorption capacity (n), a gradual decrease is observed on the adsorption curve. This decrease indicates that zeolite NaA has active centers with different energies. The initial stages of adsorption are associated with high-energy ion centers, while later stages represent a transition to low-energy centers and condensation.

Conclusions. The water adsorption isotherm of NaA zeolite shows a strong initial affinity due to framework cations followed by filling of micropores. The thermodynamic profile reflects a gradual transition from localized adsorption to condensation in pores. This knowledge is critical for applications involving humidity control and adsorption-based separation processes.

The differential heat of water adsorption on zeolite NaA distinguishes two main stages of the process: in the first stage, strong ionic bonds dominate, and in the second stage, intermolecular interactions, namely hydrogen bonds and capillary condensation, play an important role. The results of such an analysis are necessary for the design of zeolite-based desiccant materials, selective adsorbents, and heat-absorbing systems.

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