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15
Ergashev Oybek Karimovich, PhD., Namangan Institute of Engineering and Technology, Uzbekistan E-mail: [email protected]
DIFFERENTIAL HEATS OF WATER ADSORPTION IN MOLECULAR SIEVES OF IODIDE SODALITE
Abstract: The detailed thermodynamic characteristics of the adsorption of water into iodide sodalite are studied. The correlation between the adsorption-energy characteristics and the crystal-chemical structure of the sodalide iodide was found, and the molecular mechanism of the adsorption of water into iodide sodalite was found throughout the entire filling region.
Keywords: iodide sodalite, isotherm, differential heat, entropy, thermokinetic, adsorption, water, adsorption calorimetry.
Introduction. It is well known that a great interest is increasing in the research of capsulation of salts and other solid particles in crystalline materials having nanometric pores, with new optical, electrical and catalytic properties of materials. In this respect, it is useful to use sodalite, which possesses sorption properties and the ability to ion-exchange reactions, like zeolites. Sodalite was studied as a suitable phase for waste [10], but its leaching behavior remained undefined. The structure of the sodalite can be described as a covalent structural unit, called a beta cell.
Sodalite and some of its forms containing various salts have been repeatedly studied by various structurally sensitive methods [1-5]. However, in this regard, not enough attention is paid to research using the calorimetric method.
Thus, in this adsorbent, molecular sieves are highly effective adsorbents and can be used as an adsorbent for heavy salts of sea and drinking water, also they can be used for water purification from oil and oil products, natural gases, etc.
In this respect, using the calorimetric method of studying the hydration mechanism of iodide sodalite is of particular importance. In studying the guest-host interaction, special attention should be paid to the adsorption-calorimetry method among precision structure-sensitive methods. This method will make it possible to evaluate the crystal chemistry, chemistry, and physics of surface parts and also to determine the active center of solid particles when investigating the mechanisms of intermolecular interactions [6].
Thus, the aim of this study is to establish a stoichiometric relationship between the precision calorimetric data of the differential heats of adsorption of water molecules on energetically homogeneous (cation) centers located on the same crystallographic positions of the sodalite which are free from iodide ions and the number of water molecules adsorbed on these centers. An iodine anion with an iodine atom localized in the center of the cavity in an anion with a nitrite anion [5]
exhibits 12-fold orientation disorder within the (4668) -sodalite cavity, which also affects the coordination of oxygen to sodium atoms.
Objective: to study isotherms and main thermodynamic characteristics of adsorption and the mechanism of adsorption of water in iodite of sodalite.
Subjects and Methods. The investigations were carried out with the help of an adsorption-calorimetric apparatus. The results obtained with this device are highly accurate. Differential heats of adsorption (Qd) are determined by the differential microcalorimetric device of the Tian-Calvet type [7]. The adsorption isotherm is determined by a volumetric method using a high vacuum apparatus. The dosage of adsorption was established by the volume-liquid method [8]. The formula of the unit cell of the studied sodalite: Na8 (AlSiO4)j2.
Results and discussion. The isotherm of adsorption of water into iodide sodalite was carried out at a temperature of 303 K, the calculated constants were also calculated from the values of a given temperature, extending up to ~ 10-6 relative pressures p/p0 (p0-pressure of water vapor, p0 (303K) = 4.24 kPa) (Figure 1).
Analysis of the isotherm of the investigated objects is carried out with the help of volumetric occupancy of micropores (VOM) by the absorbed substance. The isotherm of iodide sodalite is satisfactorily described, characterized by three-term equations by the theory ofvolumetric occupancy of micropores (VOM) [9].
а = 0.27exp[-(A/18,86)3] + 0.437exp[-(A/4,87)3] +
+ 1.87exp[-(A/0.48)], a is the amount ofadsorption in mmol/g, A=RT ln (Po / P), the adsorption energy is calculated in kJ/moles. (Po/P-trans-fer work of 1 mmol of surface steam (pressure P0) to the equilibrium gas phase (pressure P)).
Figure 1. Isotherm of adsorption of water into iodite of sodalite at 303 K.;
A - experimental data; ▲ - estimating data by VOM
Differential heats (Qd) of adsorption ofwater into iodide sodalite were measured at 303 K and are given in (Fig. 2). The intermittent line is the heat of condensation of water at 303 K (AHv = 43.5 kJ/mol). The aim of the work was to establish a stoichiometric relationship between the precision calorimet-ric data of the differential heats of adsorption of a test water molecule at energetically homogeneous centers located on the same crystallographic positions of zeolite sodalite and the number ofwater molecules adsorbed on these centers. Differential heats of adsorption of water vapor into iodite sodalite have stepped curves. It can be seen from the figure that adsorption occurs from four identical steps and in each stage the
60
amount of adsorbate is 0.4 mmol/g. In the first stage, the heat of adsorption decreases from 61.57 to 52.74 kJ mol at a = 0.4 mmol/g, the second stage wavers down from 52.74 to 51.01 kJ/mol at a = 0.81 mmol/g, the third stage also decreases in wave form and decreases from 51.01 to 49.51 at a = 1.21 mmol /g. In the case of the fourth stage, the reduction occurs from 49.51 to 47.43 at a = 1.61 mmol/g. And in the last fifth stage the amount of adsorbate is 0.2 mmol/g and the thermal value of adsorption is equated to the heat of condensation of water i.e. 43.5 kJ/mol. The stepwise dependence of Q^with respect to adsorption is justified with the adsorption of water to Na+ cations of the active center of sodalite iodide.
M
■v
Ol
50
40
■
; ahv jL -L--- _A
0,5
1,5
2,5
a, mmol/g
Figure 2. Differential heat of adsorption of water adsorption in the iodite sodalite at 303 K. The horizontal dashed line is heat water condensation at 303 K
A sharp decrease in heat in the initial stages and low values of heat of adsorption in subsequent stages indicate the migration of cations to the six-membered oxygen ring of the pore center under the influence of iodide ions.
Further, the differential heats of adsorption continue along the condensation curve. In general, the heat of adsorption of water into iodide is lower than the heat of adsorption in the basic sodalite. This is due to the influence of iodide ions on the cations of the six-membered oxygen ring.
Elementary cell of iodide sodalite contains 8 cations of sodium. At each pore there are 4 cations. The presence of the four main stages indicates the adsorption of water
to cationic sodium located in the six-membered oxygen ring. The experimental determination of the position of
the water molecule adsorbed on cations and localized at various sites of the zeolite is complicated by the method of X-ray powder diffraction due to the proximity of the number of electrons of water and sodium, which makes their identification difficult. The presence of salt in the sodalite structure significantly influenced the energy of water adsorption on cations. The heat is noticeably lower than on sodalite (1-3 steps). When the second molecule is connected to the cation, it also occurs with low energy, in addition, the second molecule, only half adsorbed with pain with high heat, and the remaining 1.5 molecules of water are adsorbed with heat equal to the heat of condensation. The inclusion or defect is an equal adsorption of one water molecule at the center.
Figure 3. The differential entropy of water adsorption (ASd) in iodide at 303 K. The entropy of liquid water is taken as zero. The horizontal dashed line - mean molar integral entropy
The stepwise nature of the heat curve of adsorption is considered in connection with the stoichiometric interaction of water molecules with coordinatively unsaturated Na+ cations in the energy uniform centers of the cavities of the sodalite zeolites. To describe the hydration process, the calorimetric data were conditionally divided into five sections according to the specific points on the curve.
It is known from previous studies that in accordance of the differential heat of water with the hydration curves, the curve of the molar differential entropy (ASd) curve has a wavy character. In this case, each step of the curve of the Qd line corresponds proportionally to the entropy of adsorption. Figure 3 shows the molar differential adsorption entropy at 303 K. the entropy of liquid water is adopted as zero. The heat line corresponds of the water mid-molar integral entropy at 303 K. The heat of entropy line has a stepwise wave-like character. In general, the curves ASd are located below the entropy of liquid water. In accordance with the shape of the differential heat of adsorption curve, the entropy of adsorption of water to the sodalite, starting at 8.20 J/mol*K, falls rapidly to -21.65 at
0.31 mmol/g, then wavers down to -24.38 J/mol * K at 1.06 mmol/g followed by a wave-like growth until the saturation
of sodalite with water.The obtained values of AS, are lower
d
in comparison with the entropy of liquid water. The average molar integrated entropy is -12 J/mol*K, i.e. the motion of water molecules in the matrix of iodide sodalite characterizes the average motion of molecules between the liquid and solid state of water.
Thermometric results are given in all calorimetric studies. The obtained data indicate that the adsorption equilibrium is established in time when adsorbate molecules are absorbed on the surface of the adsorbent (Fig. 4). Figure 4 shows the time dependence of the establishment of adsorption equilibrium (t) on the adsorption ofwater. For iodide sodalite, starting from ~ 8 hours, the curve decreases wavy to 2 hours. The equilibrium of water adsorption on cations initially proceeds slowly and the equilibrium time is from 8 to 4.56 hours at N = 2H2O /ec, and remains almost constant with two waves with a maximum at 1.06 mmol/g and 1.47 mmol/g. The second wave decreases to t = 1.52 hours at 2.08 mmol/g.
Figure 4. The set-time of the adsorption equilibrium, depending on the size of the adsorption of water in the iodide of sodalite zeolite at 303K
Hence it can be seen that thermokinetic data are a good chance for considering sorption systems in the molecular structural aspect, especially in cases where heat is powerless to differentiate adsorption centers.
Conclusions. In addition to the salts that are part of the sodalite, 6 more water molecules can also be adsorbed. The differential heat of adsorption has a stepped nature of the curves and in each step the water molecule is adsorbed to 4 cations of Na+. Especially high energy is observed when adsorption of water in the amount of 0.2 mmol/g, which oc-
curs at the first stage in a ratio of 1:2. Subsequent adsorption of water proceeds by forming hydrogen bonds and without affecting cations. The process of adsorption of water occurs at 303K, close to the thermal condensation of water. The mobility of water molecules in the iodide sodalite characterizes some average between the mobility of water molecules in the liquid and solid state. The equilibrium of water adsorption on cations initially proceeds slowly and the equilibrium time is 8 to 2 hours. Then the equilibrium time is accelerated and flows for 2 hours.
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