ISOTHERM, DIFFERENTIAL HEAT, ENTROPY AND THERMAL EQUILIBRIUM OF ADSORPTION OF CARBON DISULFIDE ON ZEOLITE LiX

ABSTRACT

The article examines the processes of adsorption of carbon disulfide on LiX zeolite, and accurately identifies the results of differential heat of adsorption, isotherm, entropy, and thermal equilibrium time, based on those obtained in a high-vacuum adsorption device. Using precise formulas, it is proved that the amount of carbon disulphide molecules adsorbed into LiX zeolite, under vacuum conditions. The authors also discuss the causes of global environmental problems, their consequences and approaches to solving these problems.

АННОТАЦИЯ

В статье рассматриваются процессы адсорбции серокись углерода на цеолит LiX, а также точно выявлены результаты дифференциальной теплоты адсорбции, изотерма, энтропия и время теплового равновесия, на основе полученных в высоковакуумном адсорбционном устройстве. С помощью точных формул доказано, количество молекул серокись углерода адсорбируемых в цеолит LiX, в вакуумных условиях. Также обсуждаются причины глобальных экологических проблем, их последствия и подходы к решению этих проблем.

Keywords:  LiX zeolite, carbon disulfide, microcalorimeter, differential heat, adsorption isotherm, entropy, thermokinetics.

Ключевые слова: Цеолит LiX, серокись углерода, микрокалориметр, дифференциальная теплота, изотерма адсорбции, энтропия, термокинетика.

Introduction: Compounds, containing sulfur in their composition negatively affect the technological process of oil and gas processing, emissions of sulfur-containing compounds into the atmosphere pollute the environment [1]. Oil and gas processing is carried out using microporous adsorbents with high sorption capacity. This is one of the most effective methods in oil refining and gas processing.

In the world, when producing environmentally friendly fuel materials from natural gas and oil, the harmful compounds contained in their composition negatively affect technological processes and product quality. In the drying processes of natural gases, in the production of fuels that meet Euro 5 standards, the purification of sulfur-containing compounds is important. This process is carried out using X-type zeolites. Therefore, the synthesis of microporous adsorbents with high sorption capacity and the achievement of new scientific and practical results is important. [2]

In the republic, the intensive development of the chemical industry, the creation of new production facilities leads to an increase in the types and volumes of products, in particular, scientific and practical results have been achieved in the development of technologies for the production of microporous adsorbents based on local raw materials [3-4].

Zeolites are skeleton aluminosilicates that have three-dimensional and interconnected cavities occupied by large Ca²⁺, Мg²⁺, K⁺, Na⁺, Li⁺, Sr²⁺, Ba²⁺ ions and water molecules that have considerable freedom of movement and the ability to dehydrate and rehydrate without destroying the structure. In total, the zeolite family includes about 40 mineral species, 1/3 of which are widely distributed [5]. Dehydrated zeolites (water is released gradually with increasing temperature) can adsorb molecules of other substances, such as COS, NH₃, NO₂, H₂S, molecules of methyl mercaptan, hydrocarbons, alcohols, organometallic and other compounds. Usually zeolites are colorless or white in color. The gloss is glassy, sometimes pearlescent. On the Earth's surface, zeolites are unstable and turn into chlorite and montmorillonite, sometimes kaolinization is observed. The origin of zeolites is hydrothermal in association with calcite, dolomite, hydro mica, and quartz [5-6]. During the formation of zeolites, a certain sequence is observed, which consists in the fact that first water-poor and silica-rich zeolites are released, and then water-rich and silica-poor zeolites.  They are widely used in the treatment of drinking water and wastewater. Synthetic zeolites are also widely used.

Zeolites include an extensive group of minerals that are essentially aqueous aluminosilicates, mainly Ca, Na and Li, Ba, Sr, K, and extremely rarely Mg and Mn    [7-9].

Cations of alkaline and alkaline-earth metals are quite mobile and can be exchanged to one degree or another for other cations. A characteristic feature of all zeolites is also the presence of water in intracrystalline channels, both in the form of isolated molecules occupying a fixed position in the lattice, and in the form of associates. In some zeolites, intracrystalline or zeolite water can be gradually and reversibly removed by heating without destroying the structure of their silicon-aluminum-oxygen framework, while in others (both in natural and synthetic zeolites), cation exchange or dehydration is accompanied by structural changes in the lattice. Significant mobility of both cations and water molecules allows for ion exchange and reversible dehydration [10, 11].

Methods and materials. The composition of the studied zeolite was Li₈₆(AlO2₂)₈₆(SiO2₂)₁₀₆. To dry and purify hydrogen sulfide, it was passed through a zeolite column. Differential molar adsorption-calorimetric studies of the adsorption of carbon disulfide in a LiX molecular sieve were performed using the device described in [12, 13]. The dissolved gases were removed by freezing the adsorbent and then pumping it out. The use of the Peltier effect method of heat flux compensation made it possible to increase the accuracy of the adsorption heat measurement by an order of magnitude. The calorimeter allows you to measure the heat released over an unlimited period of time. Adsorption measurements were carried out on a universal high-vacuum volumetric unit, which made it possible to perform adsorption measurements and dosage of adsorbate with high accuracy

 [1-4].

Results and discussion.  The size of carbon seroxy molecules is 2.7 Å. Since the size of the entrance windows of the super-cavity of type-X zeolites is 13 Å, carbon seroxide molecules are well adsorbed in it. In this case, it is necessary to describe the adsorption state of carbon desulphated due to the small size of its molecules. In synthetic zeolite LiX, the adsorption isotherm of carbon disulfide molecules at initial saturation is ln=-9.84, the relative pressure is 0.6 mm/Hg (millimeter of mercury), the atmospheric pressure is 0.00078, and the adsorption value is 0.08 mmol /g.

Figure 1. The adsorption isotherm of carbon dioxide in zeolite LiX at a temperature of 303 K. ∆-experimental value; -the value of the points calculated on the basis of the equations of MVFT

When the carbon disulfide molecules absorbed by the zeolite reach the adsorption isotherm (Figure 1) ln=-8.20, the amount of adsorption is 0.40 mmol/l. In this case, 0.4 mmol/g of carbon sulfide molecules are strongly adsorbed on the SII part of the zeolite matrix. 4.16 mmol/g of carbon disulfide molecules adsorbed on LiX zeolite shows a slight increase in the graph of the adsorption isotherm until the adsorption isotherm reaches ln=-3.0.

Initially, carbon disulfide forms molecular monocomplexes (Li (COS)) with cations due to weak bonds.

The adsorption isotherm of carbon disulfide on LiX zeolite is satisfactorily described by the three-valued equation of the micropore volume filling theory [15]

a =3.87 exp [-(A/15.76)³] +6.32 exp [- (A/6.32)³,

where: a- is the adsorption in micropores in mmol/ g, and A=RTln (P^Po/P) is the adsorption energy in kJ/mol.

Figure 2. Differential heat of adsorption of carbon disulfide on zeolite LiX at a temperature of 303 K. The horizontal dashed lines represent thermal condensation of carbon disulfide molecules at a temperature of 303 K.

Figure 2 shows a graph of changes in the differential heat (Qd, kJ/mol) based on the amount of adsorbed carbon disulfide molecules on the LiX zeolite (α, mmol/g). The heat of condensation of adsorption of carbon disulfide (∆H_v) is indicated by dashed lines.

The molecules of the initial carbon disulfide have a differential heat of adsorption of 54.03 kJ/mol, and the amount of adsorbate absorbed is 0.04 mmol/g. When the following molecules are adsorbed, carbon disulfide is absorbed with a heat of 0.16 mmol/g and a heat of 52.60 kJ/mol, and when absorbed with 0.32 mmol/g, it flows with a heat of 49.38 kJ/mol. In this part, due to the high heat of adsorption, as well as the large number of unsorbed cations in the zeolite superspaces and the low intake of carbon disulfide molecules, it takes longer to distribute metal cations to the equilibrium time. Also 0.32 mmol/g are the stages of the first stage. A high heat drop to such an adsorption value also leads to high energy under the action of carbon disulfide molecules on the walls of the silicon-aluminum frame.

At the second stage, 0.83 mmol/g of carbon disulphide molecules are sorbed, which is 1 sorbate molecule per unit cell. At this stage, the heat of adsorption decreases from 49.38 kJ/mol  to 47.27 kJ/mol. In this case, the difference in the heat of adsorption differs by 2.11 kJ/mol, and a small difference in the heat of released energy can be described by the fact that this zeolite is adsorbed at the position of the same size of the SII cavity and distributed to metal cations in equal amounts.

At the third stage, the adsorption values range from 1.15 mmol/g to 2.07 mmol/g, and the differential heat is from 47.27 kJ/mol to 43.65 kJ/mol. For sorption of 0.92 mmol/g of carbon disulfide molecules, the process proceeds with an energy difference of 3.62 kJ/mol.

At the fourth stage, adsorption occurs in the range from 2.07 mmol/g to 3.24 mmol/g. The difference between the heat of adsorption is 1.47 kJ/mol. In this case, the differential heat is from 42.18 kJ/mol to 43.65 kJ/mol.

At the last fifth stage, adsorption occurs in the range from 3.24 mmol/g to 4.25 mmol/g, and the heat of adsorption decreases from 42.18 kJ/mol to 36.90 kJ/mol. The adsorption of the following carbon disulfide molecules is accompanied by sorption to protons in the LiX zeolite. Clusters consisting of three COS / Li⁺ and one COS/H⁺ complexes arranged in the form of a tetrahedron fill almost all the empty spaces of super cavities. At the last stage, the heat increases slightly and drops sharply depending on the heat of condensation of carbon disulphide at a temperature of 303 K.

In the structure of the LiX zeolite, there are 6 active centers, adsorption voids in which adsorbates are adsorbed. Basis of active centers is made by alkaline metals. In the first cavity, Li⁺ cations are located in the center of the six-membered SI oxygen rings and in the concave part of SI'. Due to the small size of this cavity, it is partially saturated with metal cations. In the third and fourth spaces, Li⁺ cations are located in the inner part of the plane of the twelve-membered oxygen rings SII, and finally, in the fifth and sixth spaces, Li⁺ cations are located opposite the four-membered oxygen rings SIII and SIII.' at the entrance to the α-space [16,17].

It can be seen that the SIII and SII cavities account for the bulk of the adsorption, since they are located inside the super cavity. Since the cations in the SI and SI` spaces are surrounded by cations of six strongly protective oxygen atoms. In total, 4.76 mmol/g of carbon disulphide molecules are adsorbed on LiX zeolite. Of these, 4.67 mmol/g of adsorbate molecules are sorbed in the SII cavity, and 0.09 mmol/g in the SIII cavity.

Figure 3. Differential entropy of adsorption of carbon disulfide on LiX zeolite at a temperature of 303K.The entropy in the liquid carbon disulfide is taken as 0. The horizontal dashed lines represent the molar integral entropy

Figure 3 shows the differential entropy of adsorption of carbon disulfide on LiX zeolite. The formula of the Gibbs-Helmholtz equation was used to calculate the differential entropy using differential heat and the value of the adsorption isotherm of carbon disulfide on zeolite LiX.

Where: λ is the heat of condensation, ∆H and G G are the change in enthalpy and free energy, T is the temperature, and Qd is the average differential heat.

 The differential entropy of adsorption initially starts at -14.99 J/mol * K. There is a slow increase in the level from -16.20 J/mol*K to -24.53 J/mol * K. In this process, carbon disulfide molecules are initially partially adsorbed on metal cations located inside a twelve-ring cavity consisting of aluminum and silicon atoms, overlapping sodalitevoids, to form zeolite[18]. After increasing to -24.53 J/mol*The entropy of adsorption increases slowly again. Then there is an increase from -24.53 J/mol*K to -27.98 J/mol * K. The adsorption entropy gradually increases to -33.49 J/mol * K, forming a wavy shape. In this case, the adsorption value is 4.01 mmol/g. After the value of 4.01 mmol/g, cases of a sharp increase in the differential entropy of adsorption are observed. After the formation of small wavy lines, carbon disulfide molecules are adsorbed in the voids of the SII zeolite matrix. Due to the large number of cations in this cavity, during cation migration and adsorption, the energy distributionis fairly uniform[19].

From the graph of molar entropy and adsorption values, it is known that 92.5% of the total adsorption amount of carbon disulfide molecules is limited in motion and the average integral differential entropy is -24.53 J/mol * K. Thus, Lix zeolite can be characterized as a very important sorbent in the calorimetric study of the adsorption of carbon disulfide molecules.

Figure 4. The specified time of thermal equilibrium of adsorption of carbon desulphated on LiX zeolite at a temperature of 303K

Figure 4 shows the time of thermal equilibrium (thermokinetics) of adsorption of the carbon disulfide molecule on the LiX zeolite. During the experiment, adsorption was performed in a microcalorimeter at two different voltages to refine the adsorption process. Initially, the device was filled with carbon disulfide molecules. After that, the carbon disulfide molecules were slowly sent to the zeolite. Adsorption of a large number of carbon disulfide molecules was carried out at a voltage of 1.171 mmV.

The equilibrium adsorption time of the carbon disulfide molecule on the LiX zeolite at the beginning is 3 hours. The adsorption equilibrium time initially takes longer to distribute adsorbates to the cations contained in the adsorbent (zeolite), i.e. adsorption, due to the small number of adsorbate molecules (carbon disulfide) [20]. After this time, the equilibrium gradually decreases. The differential heat of adsorption of carbon disulfide on LiX zeolite has a stepwise character. This state can also be observed at equilibrium. Adsorption lasts from 20 minutes to 3 hours with absorption up to 4 mmol. This indicates that adsorption occurs in zeolite super cavities. After the amount of 4 mmol/g. The adsorption equilibrium time is sharply reduced.

Conclusion. The heat of adsorption of the sulfur oxide carbon molecule on the LiX zeolite has a stepwise form, where all stages form one - and multidimensional adsorption clusters (COS)n / Li⁺ in the LiX zeolite matrix. The adsorption isotherm is represented by the two-level MVFT equation. The final sorption volume of LiX zeolite is 4.81 mmol/g of carbon disulfide molecules. The average integral entropy of adsorption of adsorbate molecules on LiX zeolite is -24.53 J/mol * K. Carbon disulfide molecules are strongly adsorbed in the solid state in zeolite super cavities. The adsorption equilibrium time is initially long. Gradually, the equilibrium time continues at the same rate, and then sharply reduces to a few minutes.

References:

1. Manovyan A. K. Technology of primary processing of oil and natural gas. 2001. PP. 568.
2. Bekirov T. M. Primary processing of natural gases. 1987. PP. 256.
3. Murin V. I. Vol.  II. Technology of processing natural gas and condensate: Handbook. Moscow: LLC Nedra-Biznestsentr, 2002, Part 1. PP. 517.
4. Nikolaev V. V., Busygina N. V., Busygin I. G. Basic processes of physical and physico-chemical gas processing.1998. PP. 184.
5. Strizhov I. N. Gas production / I. N. Strizhov, I. E. Khodanovich.  Moscow-Izhevsk: Institute of Computer Research. 2003. PP. 376.
6. Zibert G. K. Preparation and processing of hydrocarbon gases and condensate. Zibert G. K., Sedykh A.D. Kashitsky Yu.A., Mikhailov N. V., Demin V. M. Technologies and equipment: A reference guide. Moscow: JSC Nedra-Biznestsentr, 2001. PP. 316.
7. Process gas purification / Edited by T. A. Semenova and I. L. Leites, Moscow: Khimiya. 1977. PP. 243.
8. Busygina N. V., Busygin I. G. Technology of processing natural gas and gas condensate. Orenburg: Gazpromneft. 2002. PP. 432.
9. Rudin M. G., Somov V. E., Fomin A. S. Pocket reference book of an oil refiner. Moscow: Tsniiteneftekhim, 2004. PP. 336.
10. Sorkin Ya. G. Features of processing of sulfur oils and environmental protection. 1975. PP. 295.
11. Katz, D. L., Cornell, D. L., Kobayashi, R. L. Guide to Natural Gas production, transport and processing. Translated from English under the general editorship of Ph. D. Yu. P. Korotaev, G. Ponomareva, Moscow: Nedra. 1965. PP. 676.
12. Straus V. Industrial gas cleaning. 1981. PP. 616.
13. Koul A. L., Rizenfeld F. S. Gas cleaning / A. L. Koul, Moscow: Nedra. 1968. PP. 391.
14. Campbell D. M. Cleaning and processing of natural gases / D. M. Campbell. Moscow: Nedra. 1977. PP. 133.
15. Domulajanov, Ibragimjon Khadzhimukhamedovich, Nigora Namanzhanovna Dehkanova, and Nodirabegim Bakhtierjon Kizi Zhamoliddinova. ‘The impact of air pollution on public health.’ Universum: chemistry and Biology 6-1 (96) (2022). PP. 19-23.
16. Ismailov, Muminjon Yusupovich, and Nigora Namanzhanovna Dehkanova. ‘Production and purification of naphthenic acids by synthetic method.’ Universum: chemistry and Biology 2-1 (104) (2023). PP. 54-58.
17. Abduraxmonovna, Abdurazakova Iqbolxon. ‘Kolloid eritmalarning tibbiyotdagi ahamiyati.’ Science and innovation ideas in modern education 1.9 (2023).
18. Abduraxmonovna, Abdurazakova Iqbolxon. ‘Biogen elementkarning odam organizmidagi biologic faolligi.’ Science and innovation ideas in modern education 1.9 (2023).
19. Abdullaev, S. S. ‘A new multilayer technology for squeezing leather semi-finished products.’ Doctor of Economics, Professor YUV Fedorova Doctor of Philosophy of Pedagogical Sciences (PhD), Associate Professor, Muxammadiein KB Doctor of Sociological Sciences, Associate Professor TV Smirnova (2023): 3.
20. Abdullaev, S. S. ‘Rational solution of the issue of processing off-balance ore into concentrated nitrogen-phosphate fertilizers using cyclic method.’ Economy and society 5-1 (108) (2023): 4-9.