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Microcalorimetric study of water vapor adsorption in BaY zeolite
leading indexes got copolymers in dry condition are a specific surface and value of the times. The comparative studies have shown that porosity macromolecule, got in whiteness of n-butyl alcohol, above, than beside copolymers, synthesized with the same amount monomers to salts. It is found that true porosity increases, as from determined concentrations precipitate and DVB. Usually she does not correspond to in all intervals to volume added inert material. Porosity with increase amount precipitate and wild increases. Thereby, at copolymerisation p-tph-ATPHPHB with DVB in whitness of arctic solvents and precipitate with the following removing them with structures of the ready product manages to get macromolecules with high specific surface, but in the event of have eaten-p-tph-ATPHPHB — a greater times. The differential curves of the sharing the times also show that in whitness of 50 ob.% izo-octane and 20 mass.% DVB are formed copolymers with average diameter of the times and relatively broad distribution. When use as inert component 15,25 mass.% have eaten-p-tph-
ATPHPHB and 10 mass.% DVB appear the large times with more narrow distribution.
Existence of the opened times in the first group copolymers is proved by measurements to diffusions of the solvent, inert gas and by means of mercury porometring [5]. The locked times in polymer manages to install only by reductions to density their transverse relationships, since research outside internal surface impossible.
At fabrication macro porous copolymers of the change to their structure and arising the times possible to define visual on transparencies macromolecules. Usually, the clouding increases with increase the size of the times and differences between factor of the refraction polymeric material and ambiences, filling times.
Thereby, us on base called on experimental study are shown possibility of the syntheses and regulations characteristic new ionits for peelings of the industrial sewages and extractions ion valuable metal. The Practical application of the development can will solve many technological, economic and ecological problems of the sector of the economy of our republic.
References:
1. Ergojin E. E. Vysokopronicaemye ionits. - ALMA-ATA.: NAU^.,1979. - S. 304.
2. Martin R. B. The Chemistry ioning changed by smol. - M.: Himiya.1989. - S. 289.
3. Grigoriev G. A., Fedotova O.YA. The practical work on chemistries of a polymers. - M.: Chemistry. 2013. - S. 260.
4. Kuzin I. A. Ionits and disposits rare metallov. - M.: Nedra, 2010. - S. 325.
5. Kazinycina A. G. The Reagents for hramotografii. - M.: Himiya, 2002, - S. 196.
Rakhmatkarieva Feruza, PhD, Researcher of Institute of general and inorganic chemistry
of Uzbekistan Academy of sciences Rakhmatkariev Gairat, Dr in chemistry, Prof., Head of Laboratory of Elemental analysis of Institute of general and inorganic chemistry of Uzbekistan Academy of sciences;
E-mail: vip.gairat@gmail.com Guro Vitaly Pavlovich,
Dr in chemistry, Head of Laboratory of Non-ferrous metals of Institute of general and inorganic chemistry of Uzbekistan Academy of sciences
Microcalorimetric study of water vapor adsorption in BaY zeolite
Abstract: Differential heats and isotherm of water vapor adsorption in a zeolite BaY have been measured by Tian-Calvet-type microcalorimeter and volumetric system at 303 K. Based on the data obtained, the mechanism of H2O/Ba 2+ clusters formation and their migration from hexagonal prisms and sodalite cages into supercages is revealed. The adsorption isotherm is quantitatively reproduced by VOM theory equations.
Keywords: zeolite BaY, isotherm of adsorption, water vapor, differential heats
Introduction. Distribution and coordination of ject of numerous studies. Most of them were analyzed the various cations in the fauj asite structure was the sub- by Mortier [1]. Since information on the distribution of
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Section 8. Chemistry
Ba 2+ and barium-fluid clusters in Ba 2+ — exchanging faujasite is scarce and contradictory, though microporous materials of faujasite structure are widely involved in many industrial processes, such as molecular sieving, purification, production of detergents and catalysis [2; 3].
The framework structure ofY — type faujasite is usually crystallizes in the cubic Fd3m space group. It is built by sodalite cages (also called в — cages), interconnected by double hexagonal rings (D6R, hexagonal prism). The large cavity, also called supercages or a — cages created by the arrangement of eight sodalite units. Each supercage is connected with four neighboring supercages through a single twelve — member oxygen windows (S12R). Interpretation of data obtained using neutron powder diffraction data for increasing water amount have been realized by the Rietveld technique (modified DBW and GSAS codes) [4] showed the same framework coordinates of structural Si and O, that are usually observed for the Y molecular sieve. Positions of the Ba 2+ cations were determined when using profile simulation and by considering theoretical versus experimental intensities ratios. It has been found that in dehydrated condition of the molecular sieve the best distribution of the cations being calculated for 1/8 unit cell (uc) or supercages the following: 0.8 BaI 2+ in hexagonal prisms (D6R, position I), 0.5 Ваг 2+ in the sodalite cages (position I) and 2.2 BaII 2+ in the supercages (position II).
Host/guest complexes of Ba 2+ with normal and branched hydrocarbons in the matrix of the Y molecular sieve are the most studied. However, there is too little number of papers on the adsorption of small molecules. Among them the special place is occupied by water, which due to its small size can penetrate into the soda-lite cages [5]. Review of the problem of participation of the sodalite and supercages in the adsorption process is an extremely complex task, which has not found out its final decision. An attempt to solve this problem by the method of adsorption calorimetry was undertaken [6], but unsuccessfully due to the strong dispersion of experimental points of energy data. Synchrotron — based in situ time — resolved X — ray diffraction and Rietveld analysis were used to probe the interactions between Ba-Y, FAU zeolite frameworks and H2O molecules [7]. These results provide information on the formation of double rings of hexagonal ice like clusters [(H2O)6] in the 12 ring openings of the supercage.
Objective: The present work is devoted to revealing the detailed mechanism of water adsorption in BaY molecular sieve in a wide range of filling of the pore space by means of adsorption calorimetry. Among the
precise structure — sensitive methods of investigation, adsorption calorimetry is unique [8] because it provides the most detailed information about the surface, crystal chemistry, and mechanism of host/guest clusters formation. The cation exchange plays the major role in the adsorption ofwater vapor and other small polar molecules in zeolites. Energetically uniform sites (cations) at monotypic crystallographic positions are established stoihiometrically due to the differential adsorption heats of such probe molecules as water, ammonia and methanol [5; 9; 10].
Subjects and methods. The original material for a specified sample obtaining was a NaY molecular sieve. The NaY (Si: Al=2.43) was a binder — free commercial product (Linde LZ — Y52). The crystalline structure of this material was proved by X — ray powder diffraction, and the absence of extra — framework aluminum species was examined by 27Al MAS NMR spectroscopy. NaY was subjected to repeated treatment with a solution of BaCl2 Chemical analysis showed a full exchange of Ba 2+ with Na+. The chemical composition of the fully dehydrated Ba00Al Si,0,O00/, zeolite has been verified by elemental analysis. The method of sample decomposition of synthetic zeolites with simultaneous determination of moisture content and organic impurities, followed by atomic — absorption determination (Perkin — Elmer 3030B) of exchangeable cations Na and Ba content has been developed [11]. It eliminates the influence of structure — forming elements (Al, Si) and fluctuations in the moisture content of the probes on the analysis’ results.
Prior to admission of water, the BaY sample was heated under high vacuum at 450° C for 10 h in an all — glass apparatus. The adsorption isotherm was obtained by the volumetric method, on a basis of mass difference between the amounts of water introduced into the cell and remained in its dead space at equilibrium. The details of measuring the adsorption isotherms as well as the differential heats of adsorption using a differential microcalorimeter of the Tian — Calvet type have been described elsewhere [12; 13]. The application of compensating heat flux method based on Peltier effect allowed increasing the accuracy of adsorption heat measurement more than 10 times. In addition, each experimental point at adsorption — calorimetric curves is obtained at equilibrium conditions. The molecules adsorption in zeolites especially of water, is an extremely slow process, so this advantage is decisive because the data obtained by alternative structure — sensitive methods often do fix an intermediate state of the adsorption system. After each admission of a small amount of water the heat flux
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Microcalorimetric study of water vapor adsorption in BaY zeolite
was monitored till achievement of thermodynamic equilibrium. The attainment of this state is defined to be the time at which the measured heat flux is just below the sensitivity of the instrument (1 pW).
Results and discussions. The differential heats (Qd) of water molecules adsorption per 1/8 unit cell (N) in ВаY at 303 K are presented at Figure 1. The Qd curve possesses complex and stepwise appearance. The first two steps have the same length and shape. The first one in the range of adsorption (N) from 0 to 6.26 H2O/(1/8)uc demonstrates the decrease of the heat from ~90 to 75.1 kJ/mol. The second in the range from 6.26 to 12.5 H2O/(1/8) uc decreases from 75.1 to 63.0 kJ/mol. Next, we observe two steps — from 12.5 to
15.6 H2O/(1/8) uc at the level of ~62 kJ/mol and from
15.6 to 19.5 H2O/(1/8) uc at the level of ~61 kJ/mol. The fifth step is characterized by the constancy of the heat of adsorption (~58 kJ/mol) and extends from 19.5 to 23.5 H2O/(1/8) uc. In the final stage the Qd increases smoothly, passes through a maximum (62 kJ/mol) and decreases to the heat of condensation of water (at 303 K) at N = 30 H2O/(1/8) uc. Hence, the ВаУ molecular sieve at saturation holds up to 30 water molecules at 1/8 uc. This is the same as NaY zeolite does accommodate [5].
Exchangeable cations of zeolites do create a high gradient electrostatic field and for small polar molecules present the adsorption centers of varying strength depending on the degree of coordinative unsaturation. Against the background of high energy adsorption now energy of interaction with the active sites is imposed, giving a monotonous Qd curve an stepped appearance. This differential heats of adsorption of small polar molecules must also be of discrete nature (the stepped shape of the curve Qd) and must stoichiometrically reflect the number of homogeneous active sites in the adsorbent cavities of zeolites. For zeolites along with a universal thermodynamic approach the study of adsorption systems in molecular structural aspect is also indicative. The latter direction in which clear correlations are established between the heats of adsorption and the number of homogeneous active sites in an adsorbent has been our priority over since 1969. As can be seen from Figure 1, the curve of differential heats of water adsorption in the zeolites also has a stepped appearance, where each step on the curve Qd reflects certain stage of Ва 2+ hydration. Let us consider the mechanism of water adsorption on adsorption energy of stoichiometric interrelations basis with the construction of molecular — structural model. Clear high — energy steps of equal length, 0-6.26 and 6.26-12.5 H2O/(1/8) uc indicate the adsorption of wa-
ter molecules on Ba 2+ cations, the number of which is 3.5. Consider the first step. As noted above, the distribution of cations in 1/8 uc of dehydrated ВаУ is the following: 0.5 Ba 2+ at position SI’, 2.2 Ba 2+ at position SII and 0.8 Ba 2+ at position SI. The most accessible, from the point of view of barium coordination unsaturation, to adsorbed molecules are cations at SI’ in sodalite cages, and SII in super — cages. Cations at position SI are located in the centre of the hexagonal prisms and therefore strongly shielded by the oxygen atoms, hence, its ability to adsorb is lowest. The number of water molecules adsorbed on the cations is 6.26 H2O/(1/8) uc (first step).
Taking in consideration various ability of cations Ва 2+ in different crystallographic positions to interact with water and number of water molecules adsorbed, we propose the following distribution of water molecules by the adsorption centers: barium in the positions SI’ and SII forms binary clusters with water — (H2O)2/Ba 2+. For full value connection of water with barium at position SI it needs to migrate into the sodalite саge, where it can form monomeric cluster (H2O)/Ba 2+. So we obtain 0.5 * 2 = 1.0; 2.2 * 2 = 4.4 and 0.8, totally, 6.2 (H2O)/1/8 uc, that almost exactly corresponds to the number of water molecules in the first stage (6,26 (H2O)/1/8 uc). The second step, both in length and in shape, repeats the first one, hence the amount of water adsorbed on cations doubles, and at the positions SI’ and SII, we have (Н2 О)4/Ва 2+clusters. Cation migrated from position SI in the sodalite cage attaches a second water molecule and three more at an almost constant heat of adsorption of ~ 62 kJ/mol (3rd step). Sodalite cage can accommodate not more than four water molecules [5], therefore, for the formation of five — dimensional cluster (Н2 О)5/Ва 2+ it migrates to super — cage at position SII.
The most stable configuration of the cluster, apparently, is the position of central cation surrounded with four H2O molecules on the part of supercage and one coordinated H2O molecule on the part of sodalite cavity. At this stage we have three clusters of water with three barium in each supercage (2.2 + 0.8 = 3 Ва 2+) and the cluster with 0.5 Ва 2+ ion in each sodalite cage. For the formation of the barium clusters with a larger than four water molecules in supercage the position of the SII is not suitable and therefore, the clusters should migrate from their positions on other vacant centers (the 12 ring openings of the supercage), where they can be done. After high — energy levels on the curve Qd further adsorption occurs with heat changing little with the filling and ends with passing a curve through a maximum. The second half of the adsorption occurs by a mechanism
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Section 8. Chemistry
of the volumetric occupancy of micropores, resulting in the end of the process to increase the heat of adsorption due to the interaction of adsorbed molecules between
themselves and a sharp drop of heat to the heat of condensation of water at 303 K. A detailed interpretation of this stage will be forthcoming.
Fig. 1. Differential heats of adsorption, Qd, of water molecules, (N), in BaY zeolite at 303 K. The horizontal dashed line is the heat of condensation of bulk water. Top: corresponding differential molar entropy of adsorption. The entropy of liquid water is taken as zero. Dashed line — is integral mean molar entropy
Adsorption isotherm of water in the BaY zeolites is satisfactorily described by three — term equation of the theory ofvolumetric micropore occupancy (VMOT) [14].
N = 11.95 exp [-(A/29.51)10] +
+ 12.56 exp [-(A/12.62)6]+5.92 exp [-A/6.48)2]
where N is the adsorption in micropores in H2O/(l/8) uc; A= RTln (po/p) — is the adsorption energy, kJ/mol.
Using precise values of the isotherms and differential heats of adsorption, we calculated differential molar entropy of adsorption (ASd) of water on BaY according to the equation of Gibbs — Helmholtz, ASd = - (Qd --A H)/T — R ln (p/po), where po is the vapor pressure of water at saturation, po (303 К) = 4, 24 kPa (Fig. 3). The entropy of adsorption is deferred from the entropy of liquid water at the measured temperature. The whole curve is located below the level of entropy of liquid water. In the formation of di — and tetrameric clusters (up to N = 12 H2O/(l/8) uc) entropy increases from — 46 to 0 J/mol*K. A huge difference in the differential values of entropy of adsorption of the first molecules of H2O, of course, cannot be attributed entirely to the difference in the condition of the adsorbate molecules in BaY. Their part is accounted for by the Ba 2+ cations, forming clusters with water and being localized severely reduce their original mobility and migration region.
Before the saturation, the entropy goes through a deep minimum, — 42 J/mol*K at N = 28 H2O/(l/8) uc,
which lower than the entropy of crystalline water. Such low values of entropy, as noted above, partly refer to water associates belonging to different clusters in supercages. But the real state of water molecule in BaY generally manifests the integral mean molar entropies of adsorbed water which is on — 18.7 J/mol*K less than the entropy ofliquid water, this indicates highly hindered state of motion of the water molecules in the cages of zeolite.
Conclusion. The heat of adsorption of water in the zeolite have a stepwise appearance and every step meets the stoichiometric formation of polymeric adsorption clusters (Н2 0)n/Ba 2+ in the matrix of BaY zeolite. In the process of adsorption Ba 2+ cations migrate from six — membered oxygen prisms (SI) into sodalite — cages and then into the super — cages and localizes at position SII. The first half of the adsorption process is completed with the formation of 3.0 [(H2O)4/Ba 2+] clusters in supercages at position SII and 0.5 [(Н2 0)4/Ba 2+] clusters in sodalite cages at position SI’ The second half of the process proceeds according to the mechanism of volumetric occupancy of micropores (VOM). Adsorption isotherm is satisfactorily described by the equation of the theory of VOM. BaY holds up to 30 water molecules per 1/8 uc. The state of water in the matrix of the zeo-loite BaY is highly hindered of motion of the water molecules (entropy is on — 18.7 J/mol*K less than the entropy of liquid water).
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Microcalorimetric study of carbon dioxide adsorption in BaY zeolite
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6. Moise, J. C., Bellat, J. P. ets. Adsorption of water vapor on X and Yzeolites exchanged with barium. Microporous and Mesoporous Materials. 2001, 43, 91-101.
7. Xianqin Wang, Jonathan C. Hanson, Ja Hun Kwak, Janos Szanyi, and Charles, H. F. Cation Movements during Dehydration and NO2 Desorption in a Ba-Y, FAU Zeolite: An in Situ Time - Resolved X - ray Diffraction Study. J. Phys.Chem. C. 2013,11, 3915-3922.
8. Isirikyan, A. A., Rakhmatkariev, G. U. Energy aspect ofvapor adsorption by A, X and ZSM - 5 zeolites. Proc. 5th Conf. Appl. Chem. unit aperations and processes. Balaton, Hungary, Sept. 3-7, 1989, 1, 61-67.
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13. Rakhmatkariev, G. U. Mechanism of Adsorption of Water Vapor by Muscovite: A Model Based on Adsorption Calorimetry. Clays and Clay Minerals. 2006, 54. 423-430.
14. Rakhmatkariev, G. U., Isirikyan, A. A. Complete description of the adsorption isotherm by the equations of the volumetric micropore occupancy theory. Izv. AN SSSR, Ser. chem. 1988, (11), 2644-2645.
15. Boddenberg, B., Rakhmatkariev, G. U., Viets J. and Bakhranov Kh. N. Statistical thermodynamics of ammonia-alkali cation complexes in zeolite ZSM-5. Proceedings of the 12th International Zeolite Conference. July 5-10. 1998, Baltimore, Maryland, USA. 481-488.
Rakhmatkarieva Feruza, PhD, Researcher of Institute of general and inorganic chemistry
of Uzbekistan Academy of sciences Rakhmatkariev Gairat, Dr in chemistry, Prof., Head of Laboratory of Elemental analysis of Institute of general and inorganic chemistry of Uzbekistan Academy of sciences;
E-mail: vip.gairat@gmail.com Guro Vitaly Pavlovich,
Dr in chemistry, Head of Laboratory of Non-ferrous metals of Institute of general and inorganic chemistry of Uzbekistan Academy of sciences
Microcalorimetric study of carbon dioxide adsorption in BaY zeolite
Abstract: Differential heats and isotherm of carbon dioxide adsorption in a zeolite BaY have been measured by Tian-Calvet-type microcalorimeter and volumetric system at 303 K. Based on the data obtained, the mechanism of
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