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CHEMICAL SCIENCES
DIFFERENTIAL HEATS OF WATER ADSORPTION IN ZEOLITE NH4ZSM-5 Abdulkhaev T.D.1, Kuldasheva Sh.A.2, Yakubov Yu.Yu.3 (Republic of Uzbekistan) Email: [email protected]
'Abdulkhaev Tolibjon Dolimjonovich - doctoral student, NAMANGAN ENGINEERING AND TECHNOLOGY INSTITUTE, NAMANGAN; 2Kuldasheva Shakhnoza Abdulazizovna - Doctor of Chemistry, ChiefResearcher;
3Yakubov Yuldosh Yusupboevich - Doctor of Philosophy (PhD), INSTITUTE OF GENERAL AND INORGANIC CHEMISTRY ACADEMY OF SCIENCES OF THE REPUBLIC OF UZBEKISTAN, TASHKENT, REPUBLIC OF UZBEKISTAN
Abstract: differential heats and isotherms of water adsorption in the NH4ZSM-5 zeolite were measured at 303K. The detailed mechanism of water adsorption in NH4ZSM-5 zeolite from zero filling to saturation was discovered. The isotherm of adsorption was quantitatively reproduced on the basis of VOM theory. It follows from this work that in the NH4ZSM-5 zeolite type, due to ammonium cations in the active centers, adsorption occurs almost twice as much as in zeolites without cations (silicalite 9.8 H2O / u.c.). One can also judge from the heat of adsorption and the isotherm that NH4ZSM-5 zeolite has organophilic properties. Keywords: differential heats, isotherms, NH4ZSM-5 zeolite, water, adsorption calorimetry.
ДИФФЕРЕНЦИАЛЬНЫЕ ТЕПЛОТЫ АДСОРБЦИИ ВОДЫ В ЦЕОЛИТЕ
NH4ZSM-5
Абдулхаев Т.Д.1, Кулдашева Ш.А.2, Якубов Й.Ю.3 (Республика Узбекистан)
'Абдулхаев Толибжон Долимжонович — докторант, Наманганский инженерно-технологический институт, г. Наманган;
2Кулдашева Шахноза Абдулазизовна - доктор химических наук, главный научный сотрудник; 3Якубов Йулдош Юсупбоевич - доктор философских наук (PhD), Институт общей и неорганической химии Академии наук Республики Узбекистан,
г. Ташкент, Республика Узбекистан
Аннотация: дифференциальные теплоты и изотерма адсорбции воды в цеолите NH4ZSM-5 были измерены при 303K. Раскрыт детальный механизм адсорбции воды в цеолите NH4ZSM-5 от нулевого заполнения до насыщения. Изотерма адсорбции полностью описывается уравнением ТОЗМ. Из этой работы следует, что в цеолитах типа NH4ZSM-5 за счет катионов аммония в активных центрах адсорбция происходит почти в два раза чаще, чем в цеолитах без катионов (силикалит 9,8 Н2О / у.Е.). По теплоте адсорбции и изотерме можно также судить, что цеолит NH4ZSM-5 обладает органофильными свойствами.
Ключевые слова: дифференциальные теплоты, изотерма, цеолит NH4ZSM-5, вода, адсорбционная калориметрия.
As it is known, the adsorption process forms the basis of many chemical processes and plays a significant role in the solution of many practical issues. In connection with the adsorption and active catalytic properties, the ZSM-5 type zeolites are widely used in engineering and industry. They are used in petroleum chemistry and in oil refining, and also in the process of obtaining high-quality motor oil from non-petroleum products (for example, methyl alcohol) as a catalyst. In addition, they are used for wastewater treatment for protecting the environment.
Through adducing the physicochemical, especially energy characteristics of zeolite ZSM-5, one can learn about the theoretical and practical significance of zeolite. Studying the energy properties of zeolites makes it possible to know their chemical composition and crystal structure, and this, in turn, makes it possible to use them as a reference in practical calculations and theoretical discussions. From this it follows that the classification of the thermodynamic values of the adsorption of various adsorbates to some zeolites is required.
To study the physical-chemical properties of adsorption on zeolites, first of all, it is necessary to conduct a precise measurement of the total thermodynamic properties of adsorption using the calorimetric method in a high-vacuum adsorption-microcalorimetric installation [1, p. 2117]. The installation consists of heat-resistant
glass tubes intended for high-vacuum devices, capillary microburettes and mercury closures, as well as adsorbent ampoules, measuring parts, segregations for storing adsorbents, sections for gas and steam preparation, and pump systems intended to form a vacuum.
As an object of study, defect-free zeolites of the ZSM-5 type with a high concentration of ammonium cations synthesized in a fluorine medium were selected. Initially,by an analytical balance 0.2 g. of adsorbent is weighed in the form of a powder, the dimensions of which do not exceed 1 mm., then finely granulated the zeolite under a pressure of 4 t. is re-measured on a balance. The zeolite suspended on the scales is placed in an ampoule (the ampoule also needs to be weighed). The ampoule with zeolite is placed in an even larger ampoule and is adjusted in the installation for the formation of vacuum. The installation is heated and pumped out for 8 hours at a predetermined temperature of 450°C until a vacuum is formed, i.e. until a pressure of 10-4 Pa is reached [2. p. 39-50].
As shown in the literature [3. p. 423-430], in the zeolite NH4ZSM-5 at 303 K, the differential heat and isothermal values of water adsorption are calculated. From these values, the differential molar entropy of adsorption is calculated. The isothermal value of adsorption, in turn, is compared by the equation of the theory of micropore volumetric filling (VOM).
The differential heat of water adsorption (Qd) in zeolite NH4ZSM-5 is shown in a stepwise form in the following graph (Fig. 1.). Where (Qd) consists of 6 steps: from 0 to 0.97; from 0.97 to 1.58; from 1.58 to 1.96; from 1.96 to 2.61; from 2.61 to 3.5; from 3.5 to 4.26. At the same time, at each step, you can see the stoichiometric relationship between water molecules and active centers of the zeolite. As a result of chemical analysis, 1.35 ammonium cation (NH4+) in the composition of the zeolite falls on each unit cell, i.e. (NH4+AlO2)135 (SiO2)94.65 is considered the general formula for each unit cell. It turns out that 24.6 water molecules are adsorbed in each unit cell(u.c.), which means that, on average, 18.2 adsorbates per cation. In this case, (H2O)n/(NH4+) n=18.2 constitutes clusters. As mentioned above, the adsorption heat curve consists of 6 steps; in the first step, water molecules are 100% adsorbed with active centers and form a tetra-aqua complex. At the second stage, the ratio of adsorption of water molecules with active centers is 1: 2.6; on the third stage - 1: 2.6; on the fourth stage - 1: 2.77; on the fifth step, 1: 3.8; on the sixth step -1: 3.26.
With an increase in the activity of cations located in active centers, the ratio of their adsorption also increases. For example, if in the zeolite ZSM-5 located in the Li+ active center there is an average of 32 H2O / u.c., in the zeolite NaZSM-5 34 H2O / u.c. are adsorbed, in the zeolite CsZSM-5 - 45 and H2O / u.c. In addition, the degree of adsorption also depends on the hydrophilic properties of the zeolite. The hydrophilic degree in MFI type zeolites is much higher than in other synthetic zeolites, i.e. 1.4-2.4.
120 -105 -
I
<5i 60-
30
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 a. minol/g
Fig. 1. Differential heats of Water adsorption in zeolite NH4ZSM-5 at 303 K
In this figure, it can be seen that a decrease and then an increase in the isothermal point in the initial part leads to a sharp increase in pressure due to the interference of the secondary adsorbate with primary adsorbates as a result of the reaction of ammonium cations with hydroxyl groups of water molecules of active centers in the zeolite, i.e., up to 2.1. This shows the tendency of the centers to form aquacomplexes as a result of zeolite adsorption, having numerous cations in the primary adsorption due to the presence of zeolite at the intersections of the forward and return channels of strong centers. In addition, a sharp increase in the isotherm, i.e., an increase in the curves, is the result of the formation of adsorbate-adsorbate systems in nanopores in the composition of the characterized zeolite and their interaction. The adsorption isotherm of water (fig. 2) is characterized by the equation of the theory of micropore volumetric filling (VOM) [4. p. 170, 5. p. 45-50], third order, A03 = 1.772 mmol / g, E3 = 1.74 kJ / mole N3 = 1.
8
The basic equation of the MVFT for the studied adsorption processes is given in the following form: a = ao. exp{-[A/E]n} (1)
where: a - the amount of adsorption, in mmol / g; a0 — specific adsorption in mmol / g, A = PTln (Po / P) — gas consumption per 1 mmol (kJ / mmol), E — characteristic adsorbent energy (kJ / mol); n-parameter, depending on the structure of the porous area of the adsorbent. The adsorption isotherm of NH4ZSM-5 is characterized by a three-term equation. The parameters of the first-order equation for the adsorbent water-NH4ZSM-5- A01 = 9.82 mmol / g, Ej = 16.94 kJ / mol N = 4; for the second order, A02 = 2.165 mmol / g, E2 = 4.36 kJ / mole N2 = 2; for the a =9,82 exp [-(A/16,94)4]+2,165 exp [-(A/4,36)2]+1,772 exp [-(A/1,74)1] (2).
Fig. 2. Water adsorption isotherm in zeolite NHZSM-5 at 303 K. A - experimental data; ▲ — points calculated using the
MVFT equation
As can be seen in figure 2, for the mutual coincidence of the experimental data of theoretical calculations and the values and for the full characterization of the isotherm, dividing of this equation into three isotherms is sufficient. The water adsorption isotherm in the composition of NH4ZSM-5 at saturation differs from organic adsorbates in the low value of the limiting adsorption. Although the presence of sufficiently hydrophilic centers is not observed in NH4ZSM-5, during a long adsorption time, water molecules almost completely fill all channels and parts of the NH4ZSM-5 zeolite.
References / Список литературы
1. Dubinin М.М., Rakhmatkariev G.U.,. Isirikyan A.A. // "Izvestia Akademii Nauk" USSR. Ser. chem. 1989. № 9. p. 2117 (in Russian).
2. Yakubov Y.Yu. Thermodynamics of the formation of ion-molecular complexes in zeolites HZSM-5.//Izv. diss. cand , 2017. Pp. 39-50 (in Russian).
3. Rakhmatkariev G.U. Mechanism of Adsorption of Water Vapor by Muscovite: A Model Based on Adsorption Calorimetry // Clays and Clay Minerals, 2006. Vol. 54. Pp. 423-430 (in English).
4. Dubinin M.M. Physical adsorption of gases and vapors in micropors // Progress in Syrf. membr. Sci., NY., London. Academic Press, 1975. V. Д9. Pp. 1-70 (in English).
5. Rakhmatkarieva F. G. Adsorption-calorimetric determination of the properties of ion-molecular complexes in the synthetic zeolite // Izv. diss. doc., 2018. Pp. 45-50 (in Russian).
