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Jumaeva D.J.
Doctor of Technical Sciences, Professor, Institute of General and Inorganic Chemistry Laboratory of Colloid Chemistry and Industrial Ecology
(Uzbekistan, Tashkent)
Toirov O.Z.
Doctor of Technical Sciences, Professor, Head of Department "Electrical Machines" Tashkent State Technical University (Uzbekistan, Tashkent)
Joldasbaeva G.
Master Student of the Department "Electrical Machines" Tashkent State Technical University (Uzbekistan, Tashkent)
Darxonova Sh.
Master Student of the Department "Electrical Machines" Tashkent State Technical University (Uzbekistan, Tashkent)
Numanova Z.T.
Master Student of the Department "Chemical Technology" Namangan Institute of Engineering and Technology (Uzbekistan, Namangan)
DIFFERENTIAL HEATS OF CARBON DIOXIDE ADSORPTION ON MUSCOVITE
Abstract: the article presents data on the thermodynamic characteristics of the mineral muscovite. Adsorption isotherms, differential heats and entropies of muscovite on СО2 adsorbate have been studied. The mechanism of adsorption and complete thermodynamic characteristics of muscovite on СО2 have been revealed.
Keywords: muscovite, adsorbent, adsorbate, mechanism, heat, isotherm, mineral.
It was established from nitrogen adsorption that the hexagonal holes of the basal surface of muscovite are half occupied by K+ cations, the other half is empty. Water adsorption leads to migration of cations from neighboring layers to the basal and lateral surfaces. At the same time, K+ cations cover the unfilled half of the basal surface, and also occupy every second corner on the lateral surface [1-2].
In order to establish how this process is reversible, whether we get the initial surface after water desorption, we decided to investigate the differential heats of CO2 adsorption. Considering the relatively large size of CO2 and the fact that a linear quadrupole molecule has relatively small charges on oxygen atoms compared to a polar water molecule, it should be expected that CO2 will also selectively adsorb on cations.
The reversibility of the process will be proven if the number of K+ cations recalculated from the CO2 adsorption data is the original one. The paper considers the results of calorimetric studies of the main thermodynamic functions and the molecular mechanism of CO2 adsorption.
CO2 adsorption was studied immediately after water desorption. The sample was pumped out at a temperature of 723 K.
First of all, it should be noted that due to the high pressure of saturated vapor of CO2 (P° = 54086 mm Hg) at an experiment temperature of 303 K, we were unable to obtain a complete isotherm of CO2 adsorption on muscovite. The isotherm (Fig. 1) of carbon dioxide adsorption on muscovite is presented in semi-logarithmic coordinates.
The differential heats of CO2 adsorption on muscovite (Fig. 2) generally have a stepped form. However, the plateau formed at 30 kJ/mol is interrupted by a high
maximum (37 kJ/mol) that starts at 7 ^mol/g and ends at 14 ^mol/g. In addition, in the initial region, the heat drops sharply, starting from 50-46 kJ/mol to 30 kJ/mol at a = 3 ^mol/g. The plateau ends at 21 ^mol/g, while filling the heat drops to 27.6 kJ/mol and remains at this level up to 31 ^mol/g, after which, about 8 ^mol/g CO2 is adsorbed at the level of the heat of condensation of 27 kJ/mol. The region of inhomogeneity (0 -3 ^mol/g) at low coverage of the muscovite surface with CO2 is apparently due to the presence of defective and/or impurity cations on the surface, with which CO2 is adsorbed with increased heats.
ln(P/P°)
Fig.1. CO2 adsorption isotherm on muscovite at 303 K
Given such a high adsorption energy (50-46 kJ/mol), it can be assumed that the impurities are polyvalent cations, since monovalent Na+ or Li+ cations located in hexagonal wells are adsorbed with a heat of ~36 kJ/mol [3]. Next, adsorption occurs
on potassium cations with a constant heat of 30 kJ/mol. The reason for the appearance of the maximum is the tendency of adsorbed CO2 molecules to associate with each other [4-7]. In this case, the energy of adsorbate-absorbate interaction is additionally imposed on the energy of CO2 adsorption on muscovite, which leads to an increase in heat. Starting from 7 ^mol/g and up to 14 ^mol/g, CO2 adsorption is accompanied by the interaction of adsorbed molecules with each other. Such a high maximum on the Qd curve is explained by the fact that, first of all, CO2 molecules adsorbed on the high-energy impurity center and on the neighboring K+ cation are associated, and then on two neighboring potassium cations. On the Qd curve, these mechanisms are realized on the left and right sides of the maximum, respectively.
45 40
K
o
3
M
0> 35 30
25
0 5 10 15 20 25 30 35
a, m моль/г
Fig.2. Differential heats of CO2 adsorption on muscovite at 303 K. The dashed line is the heat of condensation at 303 K
At 21 ^mol/g, Qd decreases to 27.6 kJ/mol - to a heat somewhat higher than the heat of condensation of CO2, which at a temperature of 303K is equal to 27 kJ/mol. At this stage, the basal surface of muscovite is half occupied by monomeric CO2-cation complexes.
This result provides an answer to a very important problem - the reversibility of the process of migration of cations to the basal and lateral surfaces of muscovite during hydration.
Since, from the conducted calorimetric experiments on the adsorption of CO2 on muscovite, we found that after dehydration, 21 ^mol/g of cations remain on the surface, i.e. almost the same as before hydration - 22 ^mol/g (according to low-temperature nitrogen adsorption).
Consequently, the migration of cations to the surface of muscovite, which occurs during the adsorption of water molecules, is a reversible process, the cations that migrated from the volume of muscovite to the surface return back during desorption.
Fig.3. Molar differential entropy of C°2 adsorption in muscovite at 303 K. The dashed line is the average mole integral entropy. The entropy of liquid H2O is assumed to be zero
Figure 3 shows the molar differential entropy of CO2 adsorption on muscovite. The mean molar integral entropy is 46.9 J/mol*K. From this we can conclude that the state of CO2 on the surface of muscovite is liquid like. Based on the above results, we conclude that the heat of CO2 adsorption on muscovite at zero filling is 30 kJ/mol.
Differential heats form a plateau at a level of 30 kJ/mol, the length of which corresponds to the number of K+ cations on the basal surface (21 ^mol/g). The plateau is interrupted by a high maximum reaching ~37 kJ/mol. The reason for the maximum is the adsorbate-absorbate interaction between adsorbed CO2 molecules. The state of CO2 on the surface of muscovite is liquid like. The migration of K+ cations under the influence of water adsorption on the basal and lateral surfaces of muscovite is reversible.
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