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10ADSORPTION OF AMMONIA, CARBON DIOXIDE AND N-HEPTANE IN
ZEOLITE Na4,36ZSM-5
Nemat Mirzajonov Fayoz Saydullayev Gulzebo Ruzimova Yo'ldosh Yakubov
Namangan State Fergana State Tashkent City, Academy of
University University Yunusabad District sciences Republic of
302-School Uzbekistan institute
of general and inorganic chemistry
ABSTRACT
Objective. Differential heats and isotherms of ammonia, carbon dioxide and n-heptane adsorption in zeolite NaZSM-5 have been measured by Tian-Calvet-type microcalorimeter and volumetric system at 303 K.
Methods. Adsorption-calorimetric method used in this paper provides a high-precision molar thermodynamic characteristics of adsorption systems and through them to reveal the mechanism of adsorption processes occurring in the adsorbent. As a calorimeter the microcalorimeter Tian-Calvet-type, with high accuracy and stability was used.
Results. Based on the data obtained, the mechanism of ammonia, carbon dioxide and n-heptane adsorption and complexes formation in the zeolitic matrix of NaZSM-5 is revealed. The adsorption isotherms are quantitatively reproduced by VOMT theory equations.
Conclusion. The differential heats of adsorption of ammonia, carbon dioxide and n-heptane have two segments corresponding to the formation of two types of adsorption complexes with one on average. The wavelike nature of the heat of adsorption is due to the imposition of heat of interaction of adsorbate-adsorbate on the general background of heat of adsorption of adsorbate-adsorbent. Condition of ammonia, carbon dioxide and n-heptane in the zeolite matrix is solid like. The adsorption isotherms of ammonia, carbon dioxide and n-heptane in NaZSM-5 satisfactorily is described by the equations VMOT.
Keywords: Zeolite ZSM-5, isotherm of adsorption, ammonia, carbon dioxide and n-heptane, differential heats of adsorption.
Introduction. Earlier studies were performed mainly with zeolites containing alkaline cations [1-4]. However, zeolite ZSM-5 is much more effective catalyst of a number of practically important processes of oil refining and petrochemical industry than in compare with its predecessors.
There are data on the adsorption of carbon dioxide in the zeolites of ZSM-5 type, which have been prepared by various physical and chemical methods [5-7]. However, hitherto there is no data reliably reflecting energy and the mechanism of adsorption of carbon dioxide in the zeolite ZSM-5 in its hydrogen form.
The adsorbent was synthesized in Miluse (France) in a fluoride medium, the composition of the studied zeolite Na4.36ZSM-5 (Si/Al=22) is characterized by a high concentration of Na+ cations and a minimal defect content.
The differential heat of ammonia adsorption in zeolite Na4,36ZSM-5 is shown in Fig.1a. It can be seen from the figure that the heat of adsorption varies stepwise. The initial heat of adsorption linearly drops from 110 kJ/mol to 85 kJ/mol at an adsorption equal to 0.37 mmol/g, then to a = 0.75 mmol/g, ammonia is adsorbed with a heat that varies little with filling ~84 kJ/mol. Adsorption from 0.75 to 1.67 mmol/g is again accompanied by a sharp decrease in heat to 50 kJ/mol. In this area, two sites can be distinguished with a heat varying from 84 to 63 kJ/mol in the adsorption range of 0.75 -1.1 mmol/g and with a heat varying from 63 to 50 kJ/mol in the range of 1.1 - 1.67 mmol/g. Further, the step-like decrease in heat continues and we observe 2 more gentle steps on the curve: ~50 kJ/mol (from 1.67 to 2.25 mmol/g) and ~40 kJ/mol (from 2.25 to 3 mol/g).
The first three steps correlate with the adsorption of ammonia on cations Na+, located in various crystallographic positions. These cations are conventionally called Na+I and Na+II. Ammonia is adsorbed on Na+I in a ratio of 2:1, and on Na+II - 1:1. Further, this ratio is maintained and when saturated, 6 NH3 is adsorbed on Na+I, and 3 NH3 is adsorbed on Na+II.
The isotherm of ammonia adsorption on zeolite Na4,36ZSM-5 is shown in Fig.1b in semi-logarithmic coordinates. The isotherm is brought to a = 3.4 mmol/g at relative pressures P/ P ° = 0.41 (or P = 356.72 mmmHg). To describe adsorption equilibria on zeolite Na4,36ZSM-5, the most well-known model of adsorption on microporous adsorbents was used - the theory of volumetric filling of micropores (VMOT). For the ammonia - Na4,36ZSM-5 system, the equation parameters for the first term a01 = 1.54 mmol/g, E1 = 29.8 kJ/mol and p1= 8; for the second term a02 = 0.91 mmol/g, E2 = 18.33 kJ/mol and p2= 6; for the third term, a03 = 1.54 mmol/g, E3 = 9.81 kJ/mol and p3= 3; It can be seen from the figure that the calculated data are in good agreement with the experimental ones.
The ammonia adsorption entropy (Sd) is calculated from isotherms and differential adsorption heats according to the Gibbs-Helmholtz equation. Figure 1b shows the dependence of the change in the differential entropy of ammonia adsorption on the value of ammonia adsorption in zeolite Na4,36ZSM-5 (the entropy of liquid ammonia is taken as zero). The entropy diagram shows that the entire entropy curve is located below the entropy level of liquid ammonia and that it grows in waves with
increasing filling. In the initial region, the entropy reaches a value of ~ - 155 J/mol * K, which indicates a strong localization of ammonia at low fillings. Integral mean-value adsorption entropy (-54 J/mol*K) significantly lower entropy of liquid ammonia, which also indicates the inhibited state of ammonia in the zeolite pores.
The polyextremal dependence of the time of establishment of adsorption equilibrium on filling with numerous maxima and minima indicates the complexity of the processes occurring in the zeolite matrix. In particular, NH3/Na+ complexes shift from equilibrium positions during adsorption and migrate to new positions to saturate their coordination sphere.
The curve of differential adsorption heats (Qd) of carbon dioxide in zeolite Na4,36ZSM-5 has a stepwise character (Fig.1a). The curve starts at 58 kJ/mol and gradually decreases to the heat of condensation of carbon dioxide (27 kJ/mol). At the beginning, the differential heat curve decreases linearly from 58 kJ/mol to 49 kJ/mol at adsorption (a) 0.2 mmol/g. The second section at the level of ~ 48.5 kJ/mol extends to 0.45 mmol/g. The next stage extends to = 0.78 mmol/g with a heat decreasing from 47.8 to 44.2 kJ/mol. Further, the heat continues to decrease to 35.6 kJ/mol at 1.26 mmol/g. The subsequent adsorption of CO2 proceeds continuously with a heat of 35.1 kJ/mol to a = 1.51 mmol/g. Further, the heat decreases from 31 kJ/mol to a condensation heat of 27 kJ/mol at a = 1.69 mmol/g. With a heat equal to the heat of condensation, another 0.25 mmol/g is adsorbed.
120 -,
100
«
J 80
«
60 -
40
20
B)
1 2 a, MMOJib/r
-a
«
o
Fig.1. Differential heats (a), isotherm (b) and differential molar entropy (c) of adsorption of ammonia (1), carbon dioxide (2) and n-heptane (3) in zeolite Na4 36ZSM-5 at 303 K.
3
2
1
0
The length of these segments correlates with the number of sodium cations located in various crystallographic positions of zeolite Na4 36ZSM-5 and found earlier during ammonia adsorption. Based on the data obtained, it can be concluded that 3 CO2 molecules are coordinated around Nal cations, and 2 CO2 molecules are coordinated around Nail cations.
The isotherm of CO2 adsorption on zeolite Na4,36ZSM-5 was studied at a temperature of 303 K (Fig.lb). The equilibrium pressure at small fillings reaches P/P °
n
=9.4*10- . Low equilibrium pressures at which CO2 molecules are adsorbed indicate a strong adsorption of molecules at these fillings. it is also interesting to note a good correlation of the data, so in the region of the first high-energy stage on Qd, we also observe an inflection on the adsorption isotherm at the same filling (0.2 mmol/g). The adsorption isotherm was brought to 1.97 mmol/g at a pressure of P= 330.49 mm.Hg.
The isotherm of adsorption of carbon dioxide on zeolite Na4,36ZSM-5 is satisfactorily described by the two-term equation VMOT:
a = 1,94 • e 2201 +1,02 • e 1264 (b)
The differential molar entropy of carbon dioxide adsorption in zeolite Na4,36ZSM-5 (Fig.1b) is calculated from isotherms and differential heat of adsorption according to the Gibbs-Helmholtz equation.
The differential entropy of carbon dioxide adsorption on Na4,36ZSM-5 is noticeably lower than the entropy of liquid carbon dioxide (1v) before the adsorption of two CO2 molecules on the cation and it grows undularly with increasing filling. In the initial region, the entropy reaches a value of ~ -18 J/mol * K, which indicates a strong localization of carbon dioxide at small fillings. The mean integral entropy of adsorption is -13.41 J/mol*K and is significantly lower than the entropy of liquid carbon dioxide, which indicates a weakly localized state of CO2 molecules in the zeolite matrix.
When CO2 is adsorbed in Na4,36ZSM-5 zeolite in the initial filling area, the process lasts an average of 50 minutes. Starting from the filling of 0.31 mmol/g, the process slows down and with the adsorption of 0.68 mmol/g reaches a maximum (2.9 hours). Further, the adsorption process is accelerated to 1.4 hours. Then, with the adsorption of 1.2 mmol / g, the 2nd maximum passes, then accelerates and with the adsorption of 1.54 mmol / g, the equilibrium is established in 30 minutes. The complex nature of the adsorption equilibrium time curve reflects the specifics of adsorption in the channels of silicalite. This is partly due to the processes of cation migration in the channels of zeolite Na4,36ZSM-5.
The differential heat of adsorption of n-heptane in defect-free zeolite Na4,36ZSM-5 was measured at a temperature of 303K. The heat of adsorption of n-heptane in zeolite Na4,36ZSM-5 at low fillings (up to 0.17 mmol/g) decreases from 107 kJ/mol to 98 kJ/mol (Fig.1a). The reason for the excessive heats at low fillings are sodium cations, with which n-heptane interacts due to the induction effect. The contribution to the total adsorption energy of the induction component is 15.5 kJ/mol.
At high fillings (more than 0.17 mmol/d) there is a slight increase in heat to 98.8 kJ/mol at adsorption of 0.33 mmol/g, which then decreases to 95.4 kJ/mol (at adsorption from 0.38 to 0.54 mmol/g). Further, the heat curve of n-heptane adsorption in zeolite Na4,36ZSM-5 increases slightly, passes through a maximum (103 kJ/mol at 0.73 mmol/g), then decreases to 90 kJ/mol at 1.15 mmol/g. At the end of the process, the heat of adsorption drops sharply to the heat of condensation of n-heptane (X = 27 kJ/mol) at an adsorption of 1.25 mmol/g.
In general, the heat of adsorption changes little with increasing filling. Its slight increase (by ~3 kJ/mol), characteristic of the adsorption of hydrocarbons in zeolite channels, is due to the interaction of adsorbed molecules with each other. The high adsorption heats of n-heptane in Na4,36ZSM-5 are due to the high potential of dispersion forces in zeolite channels.
The adsorption isotherm of n-heptane in zeolite Na4,36ZSM-5 was studied at a temperature of 303 K. For small fillings, equilibrium is established at a relative pressure
P/Po = 5 *10-4, and at saturation, adsorption reaches 1.26 mmol/g at a relative pressure P/Po = 0.58 (44.46 mm.Hg, Fig. 1b). If we assume the density of n-heptane in zeolite is the same as that of a normal liquid at the temperature of the experiment (303 K) and calculate the volume occupied by the n-heptane molecule at saturation, it turns out that
"3
n-heptane occupies ~0.185 cm /g of the sorption volume of zeolite Na4 36ZSM-5, which is about ~100% of the real volume. This result indicates the absence of noticeable amounts of amorphous phase in the sample, and that we are dealing with a well-crystallized sample.
The adsorption isotherm of n-heptane in zeolite Na4,36ZSM-5 is well described by the three-term equation of VMOT:
a=0.233exp[-(A/26.21)4]+0.399exp[-(A/15.73)5]+0.534exp[-(A/11.86)7] The differential molar entropy of n-heptane adsorption in zeolite Na4,36ZSM-5 (Sd) is calculated from the isotherm and differential adsorption heats, according to the Gibbs-Helmholtz equation (the entropy of liquid n-heptane is assumed to be zero).
The curve in the entire region of filling of zeolite channels with n-heptane is in the negative region, which indicates a dense packing of adsorbed molecules in the channels of zeolite Na4,36ZSM-5 (Fig.1b). Upon adsorption of 1.0 mmol/g, Sd decreases to -177 J/K*mol, which indicates a strong restriction of the mobility of n-heptane molecules in the saturation region. The mean integral entropy of adsorption is noticeably lower than the entropy of liquid n-heptane and is equal to -156 J/K* mol, which is significantly lower than the entropy of liquid n-heptane and indicates a solidlike state of hydrocarbon molecules in zeolite channels.
The time of establishment of the adsorption equilibrium of n-heptane in zeolite Na4,36ZSM-5, also undulates with filling, reflecting the specifics of the adsorption process. Three maxima can be distinguished on the curve. The first is due to the adsorption of n-heptane on Na+ cations. The slowing down of the kinetics at the end of the process is apparently due to the difficulty of promoting adsorbed n-heptane molecules in various zeolite segments.
Conclusion. The paper presents precision data of isotherms and complete thermodynamic characteristics of the adsorption of ammonia, carbon dioxide and n-heptane in defect-free zeolite Na4,36ZSM-5, and reveals the stepwise nature of the change in the adsorption heats of the studied systems with surface filling. A correlation was found between the adsorption-energy characteristics of the crystal chemical structure of zeolites and the molecular mechanism of adsorption of ammonia, carbon dioxide and n-heptane in zeolite Na4,36ZSM-5 in the entire filling region was revealed for the first time.
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