CC BY
15
PHYSICAL CHEMISTRY
ISOTHERM AND SORPTION MECHANISM OF TOLUENE ADSORPTION
ON MFI TYPE ZEOLITE
DOI - 10.32743/UniChem.2024.121.7.17891
Oybek Ergashev
Doctor of Chemical Sciences, Professor, Vice-Rector for Research and Innovation, Namangan Institute of Engineering and Technology, Uzbekistan, Namangan E-mail: okergashev 711@gmail. com
Khayot Bakhronov
Doctor of Chemical Sciences, Professor at Tashkent University of Information Technologies named after Muhammad al-Khorezmi, Uzbekistan, Tashkent E-mail: [email protected]
Nazirahon Esonkulova
Basic doctoral student, Namangan Institute of Engineering and Technology,
Uzbekistan, Namangan Email: [email protected]
Akhmadov Majidjon
Assistant at the Tashkent University of Information Technologies named after Muhammad al-Khwarizmi, Uzbekistan, Tashkent E-mail: [email protected]
Asfandyorov Marufjon
Assistant at the Tashkent University of Information Technologies named after Muhammad al-Khorakhmi, Uzbekistan, Tashkent E-mail: m. [email protected]
ИЗОТЕРМА И СОРБЦИОННЫЙ МЕХАНИЗМ АДСОРБЦИИ ТОЛУОЛА
В ЦЕОЛИТЕ ТИПА MFI
Эргашев Ойбек Каримович
д-р хим. наук, профессор, проректор по научной работе и инновациям Наманганского инженерно-технологического института,
Узбекистан, г. Наманган
Бахронов Хаёт Нурович
д-р хим. наук, профессор Ташкентского университета информационных технологий
имени Мухаммада аль-Хорезмий, Узбекистан, г. Ташкент
Библиографическое описание: ISОТHЕRM АЖ SОRPТЮN MЕСHАNISM ОF TOLTO^ АDSОRPТЮN ОN MFI ТYPЕ ZЕОLIТЕ // Universum: химия и биология : электрон. научн. журн. Еrgаshеv O. [и др.]. 2024. 7(121). URL: https://7universum.com/ru/nature/archive/item/17891
Эсонкулова Назирахон Махмудовна
базовый докторант Наманганского инженерно-технологического института,
Узбекистан, г. Наманган
Ахмадов Маджиджон Ашрафугли
ассистент
Ташкентского университета информационных технологий
имени Мухаммада аль-Хорезмий, Узбекистан, г. Ташкент
Асфандиёров Маъруфджон Мансур угли
ассистент
Ташкентского университета информационных технологий
имени Мухаммада аль-Хорезмий, Узбекистан, г. Ташкент
АВ8ТЯАСТ
тае pаpеr prеsеnts Ше isоthеrm аМ mесhаnism оf аdsоrptiоn оf tоluеnе mоlесulеs in Ше саtiоniс fоrm оf соppеr 5 mоdifiсаtiоn оf miсrоpоrоus synthеtiс zеоlitе оf MFI (Mоrdеnitе frаmеwоrk invеrtеd) typе аt а tеmpеrаturе оf 30. Тhе аdsоrptiоn isоthеrm dаtа wеrе mеаsurеd with high ассurасy аnd stаbility using а systеm соnsisting оf а Тiаnа-Саlvеt DАС-1-1А diffеrеntiаl аutоmаtiс miсrосаlоrimеtеr roupted to а univеrsаl high-vасuum instrumеnt. Тhе diffеrеntiаl Gibbs frее епе^ vаluеs wеrе саlсulаtеd frоm еquilibrium prеssurеs аМ thеrmоdynаmiс vаluеs. Bаsеd оп thе frее еnеrgy dаtа, thе аdsоrptiоn isоthеrm wаs dеsсribеd by thе trinоmiаl miсrоpоrе vоlumе filling thеоry (VMОТ) еquаtiоn. It wаs dеmоnstrаtеd tot thе thеоrеtiсаlly саlсulаtеd vаluеs соrrеspоnd to thоsе оbtаinеd еxpеrimеntаlly. Furthеrmоrе, it wаs dеtеrminеd thаt Ше vаluеs оf Ше аdsоrptiоn соеffiсiеnt in Ше first tеrm оf Ше VMOT еquаtiоn аrе twо timеs grеаtеr Шап thе numbеr оf саИош оп Ше zеоlitе. Тhе аdsоrptiоn isоthеrm оf tоluеnе in miсrоpоrоus zеоlitе MFI wаs fоund to соп^оМ tо Ше first ^е оf сlаssifiсаtiоn оf thе Brum^r isоthеrm, nаmеly tot tоluеnе mоlесulеs аrе аdsоrbеd еxсlusivеly in Ше miсrоpоrеs оf zеоlitе. Аt Ше vаluе оf rеlаtivе prеssurе P/Ps=0.142, it wаs dеtеrminеd thаt Ше sаturаtiоn соеffiсiеnt оf zеоlitе MFI in Ше саtiопiс fоrm оf Сu2+ is еquаl tо 6=68%.
АННОТАЦИЯ
В статье представлены изотерма и механизм адсорбции молекул толуола в катионной форме меди 5-й модификации микропористого синтетического цеолита типа MFI ^оМепке frаmеwоrk invеrtеd) при температуре 303 К. Данные изотермы адсорбции измерены с высокой точностью и стабильностью с помощью системы, состоящей из дифференциального автоматического микрокалориметра Тианы-Кальве ДАК-1-1А, соединенного с универсальным высоковакуумным прибором. Дифференциальные значения свободной энергии Гиббса рассчитывались по равновесным давлениям и термодинамическим величинам. На основе данных свободной энергии изотерма адсорбции описывалась трехчленным уравнением теории объемного заполнения микропор (ТОЗМ). Показано, что теоретически рассчитанные значения соответствуют полученным экспериментальным путём. Также определено, что значения коэффициента адсорбции в 1-м члене уравнения ТОЗМ в 2 раза больше, чем количество катионов на цеолите. Установлено, что изотерма адсорбции толуола в микропористом цеолите MFI соответствует первому типу классификации изотермы Брунауэра, то есть молекулы толуола адсорбируются только в микропорах цеолита. При значении относительного давления P/Ps=0,142 определено, что коэффициент насыщения цеолита MFI в катионной форме Cu2+ равен 6=68%.
Kеywоrds: аdsоrptiоn, tеmpеrаturе, isоthеrm, епШа^, йее епе^, pressure, ге^^уе pressure, miсrоpоrе, соppеr, сайоп, йЬепе.
Ключевые слова: адсорбция, температура, изотерма, энтальпия, свободная энергия, давление, относительное давление, микропоры, медь, катион, толуол.
Introduction. At present, in excess 85 % of all chemical processes are conducted with the involvement of catalysts. Zeolite-based catalysts represent the largest consumer in petrochemical processes within the oil industry. In particular, zeolites possess a distinctive capacity to transform lower paraffins and olefins into high-molecular-weight products [1-7], including catalytic cracking, hydrocracking, hydroisomerisation of paraffins, hydrodearomatisation, deparaffinisation and isode-paraffinisation of fuels and oils [8-9]. The principal advantages of these catalysts are their environmental
cleanliness, chemical inertness, high chemical and thermal stability, service life, and the possibility of modifying and regenerating their physico-chemical properties. Zeolites with a selective effect are employed in a multitude of applications on a global scale. The oil and gas industry represents the most common field of application for these materials. The removal of sulfur compounds and carbon dioxide from natural gas and petroleum gas is of significant importance, as these substances cause corrosion of steel pipelines. It is also important for gas processing enterprises to clean and dry natural gas. The aforementioned issues are addressed
through the utilisation of adsorption technology, the development of novel adsorbents and catalysts, and the advancement of theoretical frameworks pertaining to the nature of active centres in dispersed substances.
Zeolites are defined as aluminosilicates with a porous crystal framework structure. The frameworks of MFI zeolites are composed of interconnected tetrahedral [SiO4] and [AlO4] units, with oxygen atoms connecting the ends of these units. One of the defining characteristics of aluminosilicates is the unique arrangement of aluminium atoms within their structure. In all aluminosilicates, aluminium and silicon atoms are arranged in the tetrahedral coordination position with respect to oxygen, and are isomorphically substituted for silicon in the general silicon-oxygen framework. The negative charge of the tetrahedron is neutralised by the introduction of various alkaline or alkaline earth cations into the zeolite cavities. The chemical composition of all synthetic zeolites can be described by the following formula:
Me2/„O-Al2O3-xSiO2->H2O (1)
The variables in question are the n-metal cation valency, the x-SiO2/Al2O3 molar ratio, and the y-number of moles of water. In place of the element Me in the formula (1), monovalent cations may be substituted with sodium, lithium, cesium, potassium, hydrogen, silver, or copper. Divalent cations may be replaced with calcium, copper, iron, barium, magnesium, or strontium, among others.
It is possible to replace aluminium and silicon atoms in alumino-silicate zeolites with three to five valence elements, such as gallium, germanium and phosphorus, which are naturally close to them. This alters the sorption and catalytic properties of the zeolites [10]. Another distinctive feature of these zeolites is the presence of water molecules within the internal structure of the crystal. Upon heating to a temperature of 450 °C, the water molecules evaporate without compromising the integrity of the crystal lattice. This phenomenon enables ion exchange due to the mobility of alkaline and alkaline earth cations and water molecules within the zeolite [11].
The enthalpy, isotherm, entropy adsorption change of not only polar molecules, but also non-polar molecules and quadrupole, including water, ammonia, methanol, ethanol, aromatic hydrocarbons, benzene, para-, ortho- and metaxylene. The fifth modification of MFI-type zeolites with alkali metal cations exhibits a step-wise change in the thermal equilibrium time, indicating a stoichiometric interaction between the adsorbate molecules and the cations that compensate for the negative charge of the zeolite crystal lattice. As a consequence of this stoichiometric interaction, a gradual change in the basic thermodynamic parameters was observed in the formation of adsorbate/adsorbent ion-molecular complexes of varying proportions, contingent on the physico-chemical nature, charge and quantity of cations present within the zeolite. These observations were documented in [12-16].
A literature review revealed a paucity of research on the adsorption of toluene in the Cu2+ cationic form of MFI-type zeolites. This article analyses the adsorption mechanism based on the experimental results of the adsorption isotherm of toluene molecules in the form of nanostructured MFI synthetic zeolite with copper cations (Cu2+ZSM-5), as well as the values of the isotherm processed in the general equation of the volume saturation theory of micropores (VMOT). In order to describe the adsorption properties of copper cation zeolite to toluene, the isotherm of the dependence of the adsorption amount on the equilibrium relative pressure was studied. The ratio of the amount of silicon to the amount of aluminium in the Cu2+ZSM-5 type zeolite, whose adsorption properties to toluene were studied, is Si/Al=27.5, and the amount of Cu2+ cations is 0.3 mmol/g.
Materials and methods. Microcalorimetric and isosteric methods are used to determine the heat of adsorption in sorption processes. The isosteric method is a theoretical calculation method requiring the measurement of isotherms at different temperatures. However, sufficient errors can occur in the phase transition regions of the isosteres.
Another method of measuring the heat of difference, which is convenient in every respect and based on direct experience, is the calorimetric study. There are adiabatic, isothermal and heat conduction methods of calorimetric measurement. A calorimetric method with high accuracy and stability is a heat transfer method, and almost 99 % of the heat released in the sorption process is dissipated as heat energy. The improved DAC-1-1A heat transfer microcalorimeter was used in this research. Its full description can be found in the authors' works [17-18].
In this study, the isotherm of toluene adsorption on Cu2+ZSM-5 zeolite was investigated at a temperature of 303 K. The unit cell composition of zeolite is Cu2+ZSM-5 - Cu1,7[(SiO2)96,13(AlO2)3,87], according to its chemical composition. One gram of Cu2+ZSM-5 zeolite contains 0.3 mmol/g divalent copper cations. The number of copper cations present in the unit cell is 1.7, representing the presence of copper cations in 42.5% of the unit cell.
Results and discussion. The isotherm of toluene adsorption on Cu2+ZSM-5 zeolite in the equilibrium relative pressure coordinate P/Ps is presented in Fig. 1. The adsorption process was conducted up to 1.32 mmol/g adsorption at a relative pressure of P/Ps=0.704 (P=25.8 torr). At low saturations, the equilibrium pressure is equal to P/Ps=6.4x10-4 (R=0.0236 torr), which indicates strong adsorption of toluene in the initial region. In general, the adsorption isotherm is flexible at low pressures, indicating the presence of strongly interacting adsorption sites.
Figure 1 illustrates the initial region of toluene adsorption (P/Ps=6.4-10-4 or P=0.0236 torr) in comparison to the saturation region (P/Ps=0.7 or P=25.8 torr). In the figure, the adsorption isotherm at small equilibrium pressures and the isotherm from the initial area to the relative pressure of P/Ps=0.02 (P=0.68 torr) are presented to enhance the clarity of the mechanism.
Figure 1. Isotherm оf toluene аdsоrptiоn оn Сu2+ZSM-5 zеоlitе in P/Ps сооrdinаtе. ^-values оbtаinеd in the еxpеrimеnt, 4, 4, 4, 4, 4- vаlues саlсulаted in the generаl equаtiоn of the theory of vоlume saturation of micropores (VMOT)
At low pressures, the steep rise of the adsorption isotherm represents the adsorption of toluene molecules on the homogeneous, basic and primary active centre of Cu2+ZSM-5 zeolite. The adsorption isotherm rises steeply up to P/Ps=3,3-10-4 (R=0.012 torr) (Figures 1-2). The amount of adsorption at this equilibrium pressure is 0.6 mmol/g, according to the chemical composition of zeolite Cu2+ZSM-5 - Cuu[(SiO2)96,13(AlO2)3,87], which indicates that the amount of copper cations is 0.3 mmol/g. It can be observed that toluene molecules form an adsorbate/adsorbent first coordination sphere with copper cations in a ratio of 1:2. This results in the formation of an adsorbate/cation dimer, 2C7^:Cu2+ ion-molecular complex. From this complex, the equilibrium pressure begins to increase. The pressure ratio (P/Ps) was found to be 0.14 at a relative pressure of 5.2 torr, with an adsorption amount of up to 0.9 mmol/g. This isotherm exhibited a flexible change at the next 0.3 mmol/g toluene adsorption, with a pressure ratio of 0.62 at a relative pressure of 22.9 torr and a linear change up
where a is the adsorption value (mmol/g), A=RTlnPs/P is the work done in transferring 1 mmol of
to 1.21 mmol/g adsorption. This change is also a multiple of the amount of divalent copper cations in the zeolite. Regardless of whether toluene molecules are adsorbed in the first coordination sphere of Cu2+ZSM-5 zeolite or in the second coordination sphere without cations, the sorption process changes depending on the amount of Cu2+ cations in the zeolite. Consequently, the subsequent two toluene molecules form a mutual n complex bond with the copper cations in the toluene/cation dimer complex simultaneously with the negatively charged crystal lattice of the zeolite. No adsorption of toluene in the third coordination sphere was observed.
Figure 1 depicts the adsorption isotherm of toluene molecules on Cu2+ZSM-5 zeolite, calculated using the general equation of micropore volume saturation theory (VMOT) in the relative pressure coordinate. The adsorption isotherm from the initial region to the saturation state is described using the three-state VMOT equation [19]:
(2)
toluene vapor to the equilibrium gas phase (kJ/mol) or Gibbs energy.
a=0.59exp[-(A/16.66)10]+0.375exp[-(A/8.16)3]+0.242exp[-(A/3)3]
Figure 2. Isоthеrm оf tol^^ аdsоrptiоn оn ^2+ZSM-5 zеоlitе аt rеlаtivеp^ssuK P/Ps<0,02. $-vаluеs оbtаinеd in thе еxpеrimеnt, +-vаluеs саlсulаtеd in thе еquаtiоn оf thе vоlumе sаturаtiоn thеоry оf miсrоpоrеs (VMОТ)
The adsorption amount «01=0.59 mmol/g in equation (2) is approximately twice the amount of copper cations in zeolite (0.3 mmol/g). Accordingly, the VMOT theory corroborates the formation of an adsorbate/adsorbent dimer, 2C7^:Cu2+ ion-molecular complex with copper cations in a ratio of 1:2. The isotherm, calculated on the basis of equation (2), rises steeply up to the adsorption amount of 0.61 mmol/g and the relative pressure P/Ps=0.013 due to the coefficient «01=0.59 mmol/g. This confirms the adsorption in the primary active centre of the zeolite (Fig. 1). Furthermore, the second and third terms of equation (2) have no impact on the isotherm up to an adsorption amount of 0.6 mmol/g. This also corroborates the formation of a 2:1 dimer ion-molecular complex between initial toluene molecules and copper cations, which represent the primary active centre of zeolite.
At relative pressures P/Ps>0.02, the adsorption value of the first term of equation (2) remains constant, indicating that the sorption process is independent of pressure. This represents the conclusion of the sorption process in the initial active centre. From a relative pressure of P/Ps>0.02, the second term of equation (2) becomes active. This is due to the sorption of toluene in the subsequent active centre of the zeolite. The relative pressure is 0.142, with an adsorption amount of up to 0.9 mmol/g. The second term isotherm also changes almost parallel to the ordinate axis. The sum isotherm of terms 1 and 2 of the equation is in complete agreement with
the isotherm obtained in the experiment up to the relative pressure P/Ps=0.142.
At relative pressures P/Ps>0.16, the third term of equation (2) is activated. In addition to representing adsorption in non-homogeneous parts of the zeolite, the adsorption quantities «02=0.375 mmol/g and «03=0.242 mmol/g in the equation sum to «02+«03=0.617 mmol/g, which corresponds to the amount of copper cations in the zeolite (0.3 mmol/g) doubled, that is, 2 times larger than the amount of copper cations. This verifies the adsorption mechanism primarily delineated in the experimentally derived isotherm.
Соnсlusiоn. In the Cu2+ZSM-5 nanostructured microporous synthetic zeolite, toluene molecules are adsorbed at high energy values up to a relative pressure of P/Ps=0.142. The total adsorption amount is approximately 1.32 mmol/g. In the first coordination sphere, a dimer ion-molecular complex with a ratio of 2C7H8:Cu2+ is formed, and this complex is located at the intersection of the straight and zigzag channels of the zeolite. A 0.6 mmol/g toluene molecule forms a n complex with two copper cations. In the third coordination sphere, namely the adsorption of toluene molecules by toluene molecules, no adsorption of zeolite was observed. A total adsorption amount of 68 % corresponds to the relative pressure P/Ps=0.142. Toluene molecules occupy 70 % of the micropores of the zeolite. This represents a 38 % increase in the volume occupied by toluene in the cation-free form of zeolite. At a relative pressure value of P/Ps=0.142, the saturation coefficient of MFI zeolite in the Cu2+ cationic form is equal to 68 %.
Rеfеrеnсеs:
1. Weitkamp J., Raichle A., Traa Y. //Appl. Catal. A Gen. - 2001. - V. 222. - P. 277-297.
2. Wei Y., Liu Z., Wang G., Qi Y., Xu L., Xie P., He Y. //Stud. Surf. Sci. Catal. - 2005. - V. 158. - P. 1223-1230.
3. Song J., Ma H., Tian Z., Yan L., Xu Z., Liu Q., Qu W. Song, J. // Appl. Catal. A Gen. - 2015. - V. 492. - P. 31-37.
4. Ahuja R., Punji B., Findlater M., Supplee C., Schinski W., Brookhart M., Goldman A. //Nat. Chem. - 2011. - V. 3. -P. 167-171.
5. Su X., Zan W., Bai X., Wang G., Wu W. //Catal. Sci. Technol. - 2017. - V. 7. - P. 1943-1952.
6. Zeng C., Hu Q. //Chin.J. Chem. Eng. - 2019. - V. 27. - P. 2606-2614.
7. Minachev X., Dergachev A. //Uspexi ximii. - 1990. - T. 59. - N. 9. -S. 1522-1554. [In Russian].
8. Ivanova I., Ponomareva O., Andriako Ye., Nesterenko N. //Energeticheskaya politika. - 2021. - N. 6. - S. 68-79. [In Russian].
9. Travkina O., Agliullin M., Kutepov B. //Kataliz v promtishlennosti. - 2021. - T. 21. - N. 5. - S. 297-307. [In Russian].
10. Corma A., Kumar D. Micro- and Meso-Porous Materials as Catalysts //New Trends in Material Chemistry. -1997. -vol. 498. -PP.403-408.
11. Smit Dj. //Ximiya syeolitov i kataliz na seolitax. -Moskva. -«Mir». -1980. -S.11-14. [In Russian].
12. Dubinin M.M., Raxmatkariyev G.U., Isirikyan A.A., //Izv. AN SSSR. Ser. Xim.,-1989. -N. 11.-S.2633-2635. [In Russian].
13. Dubinin M.M., Raxmatkariyev G.U., Isirikyan A.A., // Izv. AN SSSR., Ser. Xim. - 1989.-№12.-S.2862-2864. [In Russian].
14. Bakhronov Kh., Ergashev O. //Scientific and technical" journal of Namangan institute of engineering and technology, -Issue 4, -Vol 6, -2021, -PP.80-86
15. Rakhmatkariev G.U. //Clays and Clay Minerals, 2006, vol. 54, P. 423-430.
16. Ito Hiromitsu, liyarna Taku, Hamasaki Atom, Ozeki Sumio, Yamazaki Satoshi //Chem. Lett. -2012. -41, -N. 10. -PP. 7279-7281.
17. Ergashev O.K., Baxronov X.N., Raxmatkariyeva F.G., Esonkulova N.M., Axmadov M.A., Raxmonov O.K. // Nauchno-texnicheskiy jurnal FerPI. -2023. -T.27. Spes.vipusk, -N.10. -S.114-118. [In Russian].
18. Baxronov X.N., Ergashev O.K., Esonkulova N.M., Raxmatkariyeva F.G., Absalyamova I.E. //Nauchniy vestnik FerGU. -N. 2. -2023. -S.59-64. [In Russian].
19. Raxmatkariyev G.U., Isirikyan A.A. //Izv.AN SSSR, Ser.xim. -1988. -N. 11. -S.2644-2645. [In Russian].
