DIFFERENTIAL HEAT OF ADSORPTION OF O-XYLENE ON Ag ZSM-5 ZEOLITE, ENTROPY AND THERMOGENETIC

ABSTRACT

AgZSM-5 The studied unit cell of zeolite contains 1.72 cations based on its chemical composition. From the graph, the differential change of the adsorption of adsorbent and o-xylene depends on the number of cations in its content, and a regular correlation is observed. The high energy region is up to 0.3 mol·s^-1, which is AgZSM-5 fully corresponds to the amount of cations in one unit cell of AgZSM-5 It is shown that the differential molal entropy change of o-xylene in zeolite is related to the adsorption values. Molar differential entropy (S_d) of O-xylene adsorption was calculated from the isotherm and differential heats of adsorption based on the Gibbs–Helmholtz equation. The adsorption entropy change is also divided into 3 equal regions, the first region up to 0,6 mol·s^-1, the second region up to 1.2 mol·s^-1, and the third region up to 1.8 mol·s^-1.

АННОТАЦИЯ

AgZSM-5 Изучаемая элементарная ячейка цеолита содержит 1,72 катиона в зависимости от его химического состава. Из графика видно, что дифференциальное изменение адсорбции адсорбента и о-ксилола зависит от количества катионов в его составе, причем наблюдается закономерная корреляция. Высокоэнергетическая область составляет до 0,3 моль·с^-1, что полностью соответствует количеству катионов в одной элементарной ячейке AgZSM-5 Показано, что дифференциальное молярное изменение энтропии о-ксилола в цеолите связано с величинами адсорбции. Мольная дифференциальная энтропия (S_d) адсорбции о-ксилола была рассчитана из изотермы и дифференциальных теплот адсорбции на основе уравнения Гиббса–Гельмгольца. Изменение энтропии адсорбции также делится на 3 равные области: первая область до 0,6 моль·с^-1, вторая область до 1,2 моль·с^-1 и третья область до 1,8 моль·с^-1.

Keywords: Adsorption, AgZSM-5, O-xylene, differential heat, entropy

Ключевые слова: Адсорбция, AgZSM-5, О-ксилол, дифференциальная теплота, энтропия.

Introductıon. In addition to the 5th and 11th modifications of ZSM zeolite, alkanes and alkenes were studied by the authors in the adsorption process of alkanes and alkenes in zeolites of fajazite, ferrierite, and mordenite in the ultrasonic research method [1]. Based on the data of infrared spectroscopy, the activation energy for diffusion of propane and n-butane in ferrierite [2], as well as the differential heat of adsorption of C₂ -C₄ alkanes and alkenes in ZSM zeolites and silicalites with various cations were calculated isosteric method, diffusion processes in micropores n-butene diffusion evaluated using the method of comparison with energy activation results [3]. The directly experimentally observed adsorption process and structure have been found to vary depending on the type of ultrasound and adsorbed molecules [4], reflecting differences between molecules and pore sizes [5]. Such different cases are used for the interpretation of elementary adsorption processes of alkanes and alkenes in zeolites [6]. A. Ferrer studied the process of adsorption of n-butane and isobutane on MFI type zeolites (silicalite) at different temperatures using manometric and microcalorimetry methods [7]. The adsorption isotherms of both compounds are satisfactorily described by the Langmuir equation [8]. The thermodynamic quantities determined from the adsorption isotherm are consistent with the experimental data. Depending on the saturation, a flat line is first observed in the values of the adsorption enthalpy, then a small increase is observed due to the adsorbate-adsorbate interaction [9-14].

Experımental. It is important to determine the main characteristics of the adsorption of molecules (DH, DG, DS) of ortho-, meta-, para-xylols with different positions in xylols with more than two methyl groups from benzene and its mechanism [15]. It is important to study the location of adsorbed adsorbates in the pore system of the zeolite crystal lattice, determine the adsorbent-adsorbate and adsorbate-adsorbate interactions, and they affect the catalytic, adsorption, and dynamic properties of the system. In recent years, there has been increasing interest in the separation of xylene isomers using commercial ZSM-5 (including ZSM-5 Si/Al=23) zeolite membranes. However, the insufficient research presented in the literature does not allow us to draw conclusions about the unique complex interaction between the nanopores of the ZSM-5 structure and xylene molecules [16]. One of the studied adsorbates, similar in molecular structure, is para-xylene with a kinetic diameter of 0.6 nm. In each unit cell of CsZSM-5 zeolite, 8 p-xylols fill all the capacious sorption spaces of the zeolite [17]. The main thermodynamic characteristics of the adsorption of the o-xylene form of dimethylbenzene with a kinetic diameter of 0.68 nm have not been studied much. The size of the elliptical sinusoidal channel of ZSM-5 zeolite is 6.2 and 4.6 Å, and the sorption process may be in the state of strong deformation of the ZSM-5 structure [18].

Results and dıscussıon. AgZSM-5 The studied unit cell of zeolite contains 1.72 cations based on its chemical composition. ZSM-5 The results of o-xylene adsorption on zeolite were obtained by the adsorption-calorimetric method in the device presented in [19]. The use of the method of compensation of heat flows using the Pelte effect, that is, the thermal effect of constant current based on the Joule-Lents law, sufficiently reduces the errors in the measurement of the heat of adsorption (up to 90% of the energy is calculated based on the Joule-Lents law, so 10% of the energy may be due to surface area calculation errors). Adsorption experimental measurements are universal high-vacuum, adsorption values are calculated using the volumetric method, with the help of which adsorption measurements and adsorbate amount measurements are performed with high accuracy.

Figure 1 shows the differential heats of adsorption (Q_d) of o-xylene on AgZSM-5 zeolite. The dependence graph of the amount of adsorption on the enthalpy value has the character of a simple mechanism.

Figure 1. Ag⁺ at 303 K , Differential heat of adsorption of o-xylene on ZSM-5 zeolites with Cu^{2 +} and Cs⁺ cations (Q_d). Dashed lines are the condensation value of o-xylene at 303 K.

The resulting step and extremum changes are related to the structural changes in the zeolite matrix as a result of the adsorbate-adsorbent interaction. From the graph, the differential change of the adsorption of adsorbent and o-xylene depends on the number of cations in its content, and a regular correlation is observed. The high energy region is up to 0.3 mol·s^-1, which is AgZSM-5 fully corresponds to the amount of cations in one unit cell of С₈ H₁₀:Ag⁺ ion-molecular complex in the first coordination sphere at an amount of 0.3 mol·s^-1 of adsorption. With saturation of the sorption volume, the differential heat sharply decreases to 65 kg·m²·s³/mol·s^-1 at an adsorption amount of 0.9 mol·s^-1, forming a dimer 2С₈H₁₀ :Ag⁺ complex. O-xylol forms p-complexes with them. Due to the coordination unsaturation of cations, the energy of adsorption changes depending on the charge of these centers. According to them, the number of centers with a certain charge can be calculated. A stepped Q_d curve in the high energy region indicates that the adsorption centers are not homogeneous.

After formation of the dimer complex, the heat decreases to 56 kg·m²·s³/mol·s^-1, which corresponds to the heat of o-xylene in the silicalite part of the lithium and cesium cationic forms of the zeolite. Therefore, the sorption process in the first coordination sphere ends with the formation of an ion-molecular mechanism in the ratio of 1:2 with the silver cations of o-xylol zeolite. The remaining molecules of o-xylene are adsorbed on silicalite (straight and sinusoidal channels), which is the non-cationic part of zeolite. The value of the differential heat does not change from 56 kg·m²·s³/mol·s^-1 to 1.2 mol·s^-1. This means the adsorption of o-xylene in the amount of 0.6 mol·s^-1 in the sinusoidal channel with a high potential area. 0.6 mmol/g o-xylene is adsorbed in the right channel of zeolite, the enthalpy decreases to the heat of condensation of o-xylene, and the process ends with complete adsorption of 1.8 mol·s^-1.

Figure 2 shows AgZSM-5 It is shown that the differential molal entropy change of o-xylene in zeolite is related to the adsorption values. Molar differential entropy of O-xylene adsorption (S_d ) was calculated from the isotherm and differential heats of adsorption based on the Gibbs-Helmholtz equation. The adsorption entropy change is also divided into 3 equal regions, the first region up to 0.6 mol·s^-1, the second region up to 1.2 mol·s^-1, and the third region up to 1.8 mmol/g. In general, the extrema in the entropy change fully correspond to the differential heat of adsorption and the isotherm change. Also, the entropy change varies parallel to the entropy change of the copper cation form of ZSM-5 zeolite. Based on the chemical composition of Cu²⁺ ZSM-5 zeolite, the amount of copper cations is equal to 0.3 mol·s^-1, as is the case with silver cations.

Figure 2. Ag ⁺ at 303 K, Entropy of o-xylene adsorption (∆S_d) on ZSM-5 zeolites with Cu ²⁺ and Cs ⁺ cations. Liquid o-xylene entropy is assumed to be zero.

Therefore, the adsorption entropy change in them consists of a parallel line, that is, the amount of adsorption in each ion-molecular mechanism is equal to each other (0.3 mol·s^-1 and 0.6 mol·s^-1). If the entropy change sharply increases from the minimum value up to the adsorption value of 0.3 mol·s^-1, that is, until the formation of the 1С₈H₁₀:Ag⁺ complex, then the entropy changes up to the adsorption value of 0.6 mol·s^-1, that is, when the 2С₈H₁₀:Ag⁺ ion-molecular complex is formed. little changes. But in the cesium form of this zeolite (according to its chemical composition, the amount of cesium cation is equal to 0.577 mol·s^-1), the entropy change of o-xylene adsorption also corresponds to the form of silver and copper cations in the initial region (up to 0.6 mol·s^-1). Despite the fact that the amount of cesium cations is almost two times less, the cesium cationic form of zeolite forms 1С₈H₁₀:2Cs⁺ ion-molecular complexes sandwiched with o-xylene molecules [7,8]. Next, o-xylene molecules are adsorbed on the non-cationic part of zeolite with cesium cation, that is, on silicalite. Is -242 kg·m²·s²/mol·s^-1·K at an adsorption amount of 0.07 mol·s^-1, and the o-xylene molecules are highly localized. Corresponding to the saturation of the sorption volume, the entropy change ×increases sharply up to -35 kg·m²·s²/mol·s^-1·K and does not change up to 0.6 mol·s^-1. At this value of entropy and adsorption, o-xylene molecules form dimer 2С₈H₁₀:Ag⁺ ion-molecular mechanism in the first coordination sphere with silver cations^. In the subsequent adsorption of o-xylene molecules in the second coordination sphere, the entropy change initially increases to -8 kg·m²·s²/mol·s^-1·K at 0.74 mol·s^-1 adsorption, and as the sorption volume is filled with o-xylene molecules, the entropy change decreases linearly and reaches its second level at 1.2 mol·s^-1 the minimum is equal to -30 kg·m²·s²/mol·s^-1·K. After the adsorption amount of 1.2 mol·s^-1, the entropy increases to the entropy of liquid o-xylene, and the sorption process ends.

Figure 3. Ag⁺ at 303 K, Thermal equilibrium time of o-xylene adsorption on ZSM-5 zeolites with Cu²⁺ and Cs⁺ cations.

The average entropy change in the formation of the dimer 2С₈H₁₀:Ag⁺ ion-molecular complex in the first coordination sphere is -137 kg·m²·s²/mol·s^-1·K. This indicates that the mobility of o-xylene molecules is extremely limited, that is, the o-xylene molecules are in an inhibited state, and the adsorbate is firmly located in the zeolite matrix.

It follows that the average entropy change up to 0.6 mol·s^-1 adsorption is -137 kg·m²·s²/mol·s^-1·K and the average entropy change after 0.6 mol·s^-1 adsorption is -18 kg·m²·s²/mol·s^-1·K. ×Adsorption of the complex with ion-molecular mechanism in ratio: 1, and then silicalite of zeolite in straight and zigzag channels is evident.

A general description of the adsorbate state in zeolite is the molar average integral entropy of adsorption, which ×is -40 kg·m²·s²/mol·s^-1·K in o-xylene. This indicates that the position of o-xylol in the silver cation ZSM-5 matrix is inhibited.

Of the amount of o-xylene adsorption on the AgZSM-5 zeolite on the time of thermal stability is given in Fig. 3 and fully corresponds to the energetic characteristics. In the initial area, the equilibrium time at the adsorption amount of 0.07 is 10.7 hours, and ~the average value of the thermal equilibrium time up to the adsorption amount of 0.3 mol·s^-1 ~is 10 hours. Based on the above analysis, the amount of silver cations is equal to 0.3 mol·s^-1 based on the chemical composition of zeolite, that is, the time for establishing thermal equilibrium in the formation of monomer 1С₈H₁₀:Ag⁺ ion-molecular mechanism is extremely slow and it ~is equal to 10 hours on average. This is due to the fact that silver cations are dispersed throughout the zeolite matrix and migration of these cations to the intersection of straight and zigzag channels of zeolite during the formation of the monomer 1С₈H₁₀:Ag⁺ ion-molecular mechanism, which means that this process takes a long time. It is also associated with complications in the movement of large molecules of o-xylene through channels with relatively small dimensions.

After the adsorbate-adsorbent ratio mechanism of 1:1, the thermal equilibrium time decreases sharply to 2 hours at an adsorption amount of 0.6 mol·s^-1. The equilibrium time of subsequent adsorption of o-xylene molecules initially increases to 4.5 hours at an adsorption amount of 0.9 mol·s^-1, and decreases to 50 minutes at an adsorption amount of 1.2 mol·s^-1. The sorption heat equilibrium time of subsequent o-xylene molecules decreases to almost zero, and the sorption process ends. In general, the silver cation of o-xylene in ZSM-5 zeolite is higher than the thermal equilibrium time of cesium and divalent copper cation models. Naturally, this is due to the fact that the interaction between the silver cation and the o-xylene molecule is weak compared to the aforementioned cations [20 ].

Conclusions. Ag ZSM-5 zeolite were studied and sorption mechanisms were determined. Ag ZSM-5 zeolite is extremely slow (in 10-12 hours) until the formation of a monomer ion-molecular mechanism at the intersection of the straight and sinusoidal channels of the zeolite. At the intersections of zeolite channels, benzene and o-xylene molecules are in different proportions with silver cations, that is, benzene trimer 3C₆H₆:Ag⁺ and it was proved that o-xylol dimer 2C₆H₆:Ag⁺ forms ion-molecular complexes.

On average, 5.2 benzene molecules and 3.5 o-xylene molecules were adsorbed by silver cations in each unit cell of Ag ZSM-5 zeolite. Adsorption isotherms were described using the three-state micropore volume saturation theory (MHTN) equation.

From the average entropy values, it was determined that the mobility of benzene and o-xylene molecules is limited, and their state is highly localized and in the solid state.

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