**Separation of Binary Solutions on the Basis of Zeolites**

**Separation of Binary Solutions on the Basis of Zeolites**

DOI: 10.5772/intechopen.73513

Paranuk Arambiy, Saavedra Huayta Jose Angel and Khrisonidi Vitaly Khrisonidi Vitaly Additional information is available at the end of the chapter

Paranuk Arambiy, Saavedra Huayta Jose Angel and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73513

#### **Abstract**

In this chapter, the author analyzed binary systems, ethanol + water, methanol + water and benzene + water, and an original mathematical model allowing the determination of the complete adsorption of binary systems on KaA, CaA, CaX, NaA and NaX zeolites using the Gibbs adsorption theory is proposed. The Gibbs equation and the Gibbs-Duhem equation have a number of limitations and do not take into account the properties of the investigated zeolites. Therefore, it is necessary to use the equations obtained by the author as a result of laboratory research, for the theoretical calculation and development of dehydration and concentration systems for alcohols.

**Keywords:** zeolites, binary solutions, molecular sieve properties, Van der Waals forces, surface tension of the solution

#### **1. Introduction**

The study of the processes occurring at the phase interface attracts many researchers from all over the world, and they have enormous practical and theoretical importance. A detailed description of the processes occurring at the phase interface is given in [1].

Considering these processes can be argued, the issues of adsorption of gas on adsorbents have been studied quite deeply and have an extensive theoretical and experimental basis.

But the behavior of the adsorbed liquid on the surface of solid zeolite adsorbent is very difficult to describe and the explanation is very simple - the internal structure of the liquid is much more complex than the internal structure of gases and crystals. Comparing gas and liquid, it can be asserted that the density of the liquid is many times greater than the density of the gas. Considering and comparing the molecular level, the distance between molecules in liquids is

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

so small that the properties of the liquid are largely determined by the intrinsic volume of the molecules and by the mutual attraction between them, while in gases under ordinary conditions the influence of these factors is negligible and can be neglected. Small distances in fluid molecules impose certain limitations on the mathematical models being developed and also impose serious limitations that require taking into account the polar properties and geometric parameters of the molecules [2].

not cause oxidative processes that occur during heating. The adsorption process is widely used for the recovery of waste transformer oils, for decreasing the dielectric losses of fresh transformer oils, for drying oils with zeolites, in filters for continuous regeneration of trans-

Separation of Binary Solutions on the Basis of Zeolites http://dx.doi.org/10.5772/intechopen.73513 59

*Removal of radionuclides from liquid wastes of nuclear power plants*: Synthesis of ceramic matrices by the method of sorption of radionuclides on zeolites and their subsequent conversion to feldspar allows using radionuclides to remove radionuclides from liquid wastes using a simple process scheme [3]. This method is based on the ability of synthetic zeolites with high selectivity to react with respect to Sr and Cs. Zeolites are completely iso-chemical to feldspars; moreover, the ion-exchange sorption process makes it possible to obtain zeolites of a given composition, and this process is relatively easy to control. Ion exchange on zeolites is technologically well developed and is widely used in the industry for purification of liquid waste [3].

The mechanism of the passage of molecules through the windows connecting the cavities of zeolites is complex because here are faced with the peculiarities of the forces of attraction and repulsion between individual molecules and atoms, as well as the structure of molecules and

Numerous studies have shown that molecular sieves with a small size of the connecting windows (e.g., 5 Å) adsorb paraffin hydrocarbons of normal structure, but they do not adsorb the isomers of these hydrocarbons, since they have a branched structure and cannot pass through these channels. **Figure 1** shows that isooctane cannot pass through a window of molecular

Adsorption of molecules, whose size is close to the diameter of the entrance windows of the zeolite, proceeds with the expenditure of additional energy. The possibility of deforming molecules containing two or more atoms within a small range makes it possible to adsorb

**2.1. The mechanism of the penetration of atoms and molecules through the** 

sieves with a diameter of 4.9 Å, while normal octane passes freely [4].

molecules even within a larger critical diameter, through the window size [4].

**Figure 1.** The separation of normal octane (a) and isooctane (b) into calcium containing zeolite.

former oil, and so on [3].

**windows of molecular sieves**

the structure of zeolites [4].

The properties of polar liquids depend on the interaction of the molecule with the molecule, but also on the interaction between individual parts of different molecules [2].

The first attempts to create a theory of fluid and to develop a mathematical model for analyzing the behavior of liquid molecules were based on a comparison of the liquid with mathematical models of the behavior of molecules in gas and did not yield any practical results, since they did not reflect the complexity of the interaction between molecules in the liquid [2].

The verification and application of new methods for analyzing the internal structure of liquids made it possible to establish the polarity of the molecules studied and the dielectric properties. And the use of nuclear magnetic resonance has led to the development of models for describing the behavior of molecules in a liquid [2].

It is established that the molecules of the liquid have a polarity, in addition, the attraction between them, inherent to nonpolar molecules, which manifests itself in a weak interaction between different parts carrying an electric charge. The total effect of mutual attraction of molecules is often described as the internal pressure of a liquid. For liquids other than electrolytes or weak electrolytes, the internal pressure varies from 3000 to 6000 bar under standard conditions [2]. At strong electrolytes, it can reach 10,000 bar. A large internal pressure, which is inherent in electrolyte liquids, characterizes the rest of their properties, such as considerable absorption of heat during evaporation and low compressibility [2].

## **2. Drying of liquid media**

The moisture content of organic liquids significantly changes the properties of the materials. In this regard, their dehydration is of great importance.

*Dewatering freon refrigeration oils*: The reliability and durability of airtight refrigeration machines largely depends on the purity of the refrigerants and lubricating oils. Up to 80% of the contaminants generated in freon refrigerating machines, which cause corrosion of the system, and ultimately, the combustion of the built-in electric motors is associated with the presence of moisture [3].

When drying oil with zeolites without a binder (NaA), the degree of drying, the time of the protective effect of the layer and the dynamic activity increase significantly [3].

*Dehydration of transformer oil*: Methods of dehydrating transformer oil with zeolites and oil degassing are developed on the basis of mass transfer processes. In the adsorption method, contacting the oil with zeolites is carried out at ordinary temperatures and, as a result, does not cause oxidative processes that occur during heating. The adsorption process is widely used for the recovery of waste transformer oils, for decreasing the dielectric losses of fresh transformer oils, for drying oils with zeolites, in filters for continuous regeneration of transformer oil, and so on [3].

*Removal of radionuclides from liquid wastes of nuclear power plants*: Synthesis of ceramic matrices by the method of sorption of radionuclides on zeolites and their subsequent conversion to feldspar allows using radionuclides to remove radionuclides from liquid wastes using a simple process scheme [3]. This method is based on the ability of synthetic zeolites with high selectivity to react with respect to Sr and Cs. Zeolites are completely iso-chemical to feldspars; moreover, the ion-exchange sorption process makes it possible to obtain zeolites of a given composition, and this process is relatively easy to control. Ion exchange on zeolites is technologically well developed and is widely used in the industry for purification of liquid waste [3].

#### **2.1. The mechanism of the penetration of atoms and molecules through the windows of molecular sieves**

so small that the properties of the liquid are largely determined by the intrinsic volume of the molecules and by the mutual attraction between them, while in gases under ordinary conditions the influence of these factors is negligible and can be neglected. Small distances in fluid molecules impose certain limitations on the mathematical models being developed and also impose serious limitations that require taking into account the polar properties and geometric

The properties of polar liquids depend on the interaction of the molecule with the molecule,

The first attempts to create a theory of fluid and to develop a mathematical model for analyzing the behavior of liquid molecules were based on a comparison of the liquid with mathematical models of the behavior of molecules in gas and did not yield any practical results, since they did not reflect the complexity of the interaction between molecules in the liquid [2]. The verification and application of new methods for analyzing the internal structure of liquids made it possible to establish the polarity of the molecules studied and the dielectric properties. And the use of nuclear magnetic resonance has led to the development of models

It is established that the molecules of the liquid have a polarity, in addition, the attraction between them, inherent to nonpolar molecules, which manifests itself in a weak interaction between different parts carrying an electric charge. The total effect of mutual attraction of molecules is often described as the internal pressure of a liquid. For liquids other than electrolytes or weak electrolytes, the internal pressure varies from 3000 to 6000 bar under standard conditions [2]. At strong electrolytes, it can reach 10,000 bar. A large internal pressure, which is inherent in electrolyte liquids, characterizes the rest of their properties, such as considerable

The moisture content of organic liquids significantly changes the properties of the materials.

*Dewatering freon refrigeration oils*: The reliability and durability of airtight refrigeration machines largely depends on the purity of the refrigerants and lubricating oils. Up to 80% of the contaminants generated in freon refrigerating machines, which cause corrosion of the system, and ultimately, the combustion of the built-in electric motors is associated with the

When drying oil with zeolites without a binder (NaA), the degree of drying, the time of the

*Dehydration of transformer oil*: Methods of dehydrating transformer oil with zeolites and oil degassing are developed on the basis of mass transfer processes. In the adsorption method, contacting the oil with zeolites is carried out at ordinary temperatures and, as a result, does

protective effect of the layer and the dynamic activity increase significantly [3].

but also on the interaction between individual parts of different molecules [2].

for describing the behavior of molecules in a liquid [2].

In this regard, their dehydration is of great importance.

absorption of heat during evaporation and low compressibility [2].

parameters of the molecules [2].

58 Zeolites and Their Applications

**2. Drying of liquid media**

presence of moisture [3].

The mechanism of the passage of molecules through the windows connecting the cavities of zeolites is complex because here are faced with the peculiarities of the forces of attraction and repulsion between individual molecules and atoms, as well as the structure of molecules and the structure of zeolites [4].

Numerous studies have shown that molecular sieves with a small size of the connecting windows (e.g., 5 Å) adsorb paraffin hydrocarbons of normal structure, but they do not adsorb the isomers of these hydrocarbons, since they have a branched structure and cannot pass through these channels. **Figure 1** shows that isooctane cannot pass through a window of molecular sieves with a diameter of 4.9 Å, while normal octane passes freely [4].

Adsorption of molecules, whose size is close to the diameter of the entrance windows of the zeolite, proceeds with the expenditure of additional energy. The possibility of deforming molecules containing two or more atoms within a small range makes it possible to adsorb molecules even within a larger critical diameter, through the window size [4].

**Figure 1.** The separation of normal octane (a) and isooctane (b) into calcium containing zeolite.

For example, through the windows NaA penetrates into the internal structure of ethane molecules having a critical diameter of 4 Å; penetration facilitates the thermal pulsation of the crystallite lattice. However, at some temperature characteristic of the sorbed material, the kinetic energy reserve of the molecule becomes insufficient to overcome the energy barrier; below this temperature, the substance is not sorbed by the zeolite of this type [4].

Even at a temperature of −196°C, the windows between the cavities do not contract so much that the oxygen molecules cannot shine through them. At the same time, larger molecules of nitrogen and argon do not pass through the windows at very low temperatures, as a result of

Studies have shown that even relatively small changes in the cross section of the windows connecting the surfaces lead to significant changes in the nature of the adsorption of the individual components. Both shabasite- and CaA-type screens adsorb normal hydrocarbons, but they do not adsorb isomers of hydrocarbons having a branched structure that cannot pass through the windows of these zeolites [4]. However, in crystals of both types, a certain difference in adsorption is observed, and shabasite adsorbs different normal hydrocarbons unequally [4]. The greater the molecular weight of the hydrocarbon, the slower it adsorbs the shabasite. For example, u-heptane is not adsorbed insignificantly [4]. At the same time, synthetic molecular sieves of the CaA type adsorb even a normal hydrocarbon such as tetra-

decane (n-C14H30), the hydrocarbon chain of which is twice as long as n-heptane [4].

Different types of natural and synthetic zeolites have windows of unequal dimensions [4]. The penetration of molecules through these windows depends on the properties of those ions that are located at the edges of the window. A cation that compensates for the charges of the Si─O or Al─O complex is often located at the edge of the inlet (window) and prevents the penetration of molecules whose dimensions exceed the critical diameter, that is, the diameter that allows the molecule to penetrate through the narrowed window due to the presence of

The addition of water to the dehydrated zeolites results in a pronounced change in the electrical conductivity. With an increase in the water content at a temperature of 25°C, the electrical

conductivity increases nonlinearly, which indicates preferential hydration of cations of the

In the case of type A zeolite, electrical conductivity increases with hydration until the water content is about five molecules per unit cell. This is equivalent to the hydration of four mobile sodium ions, in the vicinity of the eight-membered oxygen rings forming the entrance windows. Apparently, these sites have the highest adsorption energy. Complexes of water-sodium ions, localized in eight-membered rings, very effectively block the entrances and prevent the

Thus, it can be concluded that the circumstances confirm the results of the determination of physical adsorption, showing that in the presence of even traces of water adsorption of gases

Hydration of water molecules is accompanied by the formation of an unstable bond with sodium ions, and the localization of ions, and there is also an insignificant increase in the electrical conductivity of the sample. After the number of water molecules in each unit cell

. In the case of type A zeolite, the electrical

Separation of Binary Solutions on the Basis of Zeolites http://dx.doi.org/10.5772/intechopen.73513 61

a decrease in their cross section.

the cation [4].

same type.

**2.2. Effect of water on zeolites**

penetration of other molecules there.

of the oxygen type does not occur.

conductivity of zeolite X increases by a factor of 104

In **Figure 2** (according to R. Barreru) are curves showing how the adsorption of some gases changes with NaA-type zeolites, but as the temperature decreases. At 0°C, oxygen is only slightly adsorbed by zeolites, but at a temperature of liquid nitrogen, that is, at −196° C, approximately 12 molecules of oxygen are adsorbed in each cavity. For 1 g of zeolite, this will be about 130 cm3 of oxygen (when reduced to normal conditions). Thus, there is a process of adsorption of oxygen, on a zeolite accompanied by an increase in the adsorption capacity in oxygen with a decrease in temperature [4].

However, the adsorption of other gases (nitrogen and argon) occurs differently. As the temperature decreases, initially the adsorption of these gases increases, goes also as oxygen, but then, having reached a certain maximum, sharply decreases. The maximum adsorption of nitrogen is achieved at −120°C and argon at −150°C. At very low temperatures (−190°C and below), the adsorption of nitrogen and argon becomes quite insignificant compared to its maximum value [4].

**Figure 2.** Adsorption of some gases by zeolites at low temperatures. (X)—number of molecules in each cavity; (Y) temperature in degrees.

Even at a temperature of −196°C, the windows between the cavities do not contract so much that the oxygen molecules cannot shine through them. At the same time, larger molecules of nitrogen and argon do not pass through the windows at very low temperatures, as a result of a decrease in their cross section.

Studies have shown that even relatively small changes in the cross section of the windows connecting the surfaces lead to significant changes in the nature of the adsorption of the individual components. Both shabasite- and CaA-type screens adsorb normal hydrocarbons, but they do not adsorb isomers of hydrocarbons having a branched structure that cannot pass through the windows of these zeolites [4]. However, in crystals of both types, a certain difference in adsorption is observed, and shabasite adsorbs different normal hydrocarbons unequally [4]. The greater the molecular weight of the hydrocarbon, the slower it adsorbs the shabasite. For example, u-heptane is not adsorbed insignificantly [4]. At the same time, synthetic molecular sieves of the CaA type adsorb even a normal hydrocarbon such as tetradecane (n-C14H30), the hydrocarbon chain of which is twice as long as n-heptane [4].

Different types of natural and synthetic zeolites have windows of unequal dimensions [4]. The penetration of molecules through these windows depends on the properties of those ions that are located at the edges of the window. A cation that compensates for the charges of the Si─O or Al─O complex is often located at the edge of the inlet (window) and prevents the penetration of molecules whose dimensions exceed the critical diameter, that is, the diameter that allows the molecule to penetrate through the narrowed window due to the presence of the cation [4].

#### **2.2. Effect of water on zeolites**

For example, through the windows NaA penetrates into the internal structure of ethane molecules having a critical diameter of 4 Å; penetration facilitates the thermal pulsation of the crystallite lattice. However, at some temperature characteristic of the sorbed material, the kinetic energy reserve of the molecule becomes insufficient to overcome the energy barrier;

In **Figure 2** (according to R. Barreru) are curves showing how the adsorption of some gases changes with NaA-type zeolites, but as the temperature decreases. At 0°C, oxygen is only slightly adsorbed by zeolites, but at a temperature of liquid nitrogen, that is, at −196° C, approximately 12 molecules of oxygen are adsorbed in each cavity. For 1 g of zeolite, this will

adsorption of oxygen, on a zeolite accompanied by an increase in the adsorption capacity in

However, the adsorption of other gases (nitrogen and argon) occurs differently. As the temperature decreases, initially the adsorption of these gases increases, goes also as oxygen, but then, having reached a certain maximum, sharply decreases. The maximum adsorption of nitrogen is achieved at −120°C and argon at −150°C. At very low temperatures (−190°C and below), the adsorption of nitrogen and argon becomes quite insignificant compared to its

**Figure 2.** Adsorption of some gases by zeolites at low temperatures. (X)—number of molecules in each cavity; (Y)—

of oxygen (when reduced to normal conditions). Thus, there is a process of

below this temperature, the substance is not sorbed by the zeolite of this type [4].

be about 130 cm3

60 Zeolites and Their Applications

maximum value [4].

temperature in degrees.

oxygen with a decrease in temperature [4].

The addition of water to the dehydrated zeolites results in a pronounced change in the electrical conductivity. With an increase in the water content at a temperature of 25°C, the electrical conductivity of zeolite X increases by a factor of 104 . In the case of type A zeolite, the electrical conductivity increases nonlinearly, which indicates preferential hydration of cations of the same type.

In the case of type A zeolite, electrical conductivity increases with hydration until the water content is about five molecules per unit cell. This is equivalent to the hydration of four mobile sodium ions, in the vicinity of the eight-membered oxygen rings forming the entrance windows. Apparently, these sites have the highest adsorption energy. Complexes of water-sodium ions, localized in eight-membered rings, very effectively block the entrances and prevent the penetration of other molecules there.

Thus, it can be concluded that the circumstances confirm the results of the determination of physical adsorption, showing that in the presence of even traces of water adsorption of gases of the oxygen type does not occur.

Hydration of water molecules is accompanied by the formation of an unstable bond with sodium ions, and the localization of ions, and there is also an insignificant increase in the electrical conductivity of the sample. After the number of water molecules in each unit cell exceeds 16, H<sup>2</sup> O molecules occupy places with the lowest adsorption energy, forming hydrogen bonds with the anionic surface of the zeolite. As the crystals are saturated with water, the electrical conductivity of the zeolites increases; therefore, the system of channels is filled with water molecules. Some of the sodium ions remain sufficiently free to chaotically move through the channels of the zeolites. These conclusions are in agreement with the data of IR (adsorption of water) and NMR spectra.

of the cation entering the zeolite. Cations located close to the window block the entrance for molecules. For example, in cation exchange, in which two sodium cations are replaced by a single calcium cation, the input window expands; As a result, the Na zeolite has an inlet window size of 4Å, and the CaA zeolite has a size of 5Å [15]. A similar exchange in a zeolite of type X leads to a certain narrowing of the window. Considering the properties of the KA zeolite, it can be explained that, at ordinary temperatures, this kind of zeolite sorbs water very well. This property has prevented its use for the drying of unstable substances prone to

Separation of Binary Solutions on the Basis of Zeolites http://dx.doi.org/10.5772/intechopen.73513 63

If we consider the property of NaA zeolite, which is capable of sorbing most of the components of industrial gases, the critical size of the following molecules does not exceed 4Å: hydrogen sulfide, carbon disulfide, carbon dioxide, ammonia, lower diene and acetylene hydrocarbons, ethane, ethylene, propylene, organic compounds with one methyl group in molecule, as well as methane, neon, argon, krypton, xenon, oxygen, nitrogen and carbon monoxide [15]. The latter group of substances is absorbed in considerable quantities only at low temperatures. Propane and organic compounds with more than three carbon atoms in the molecule are not adsorbed by the zeolite and thus do not suppress the adsorption of the above impurities dur-

CaA zeolites adsorb hydrocarbons and alcohols only of a normal structure, and therefore, it is widely used in the processes of separation of multicomponent organic substances on a molecular sieve basis. Moreover, zeolite CaA is absorbed by methyl and ethyl mercaptans, organic compounds with the number of carbon atoms in molecule 2 (ethyl alcohol, ethylamine), diborane, and so on. Among the general-purpose zeolites of CaA type is increased resistance in a weakly acid medium, and therefore, it is used in desulfurization processes and

Zeolites of type X have rather wide entrance windows and adsorb the vast majority of the components of complex mixtures: all types of hydrocarbons are organic sulfur, nitrogen and oxygen compounds (mercaptans, thiophene, furan, quinoline, pyridine, dioxane, etc.), halogenated hydrocarbons (chloroform, carbon tetrachloride and freons), pentaborane and decaborane [15]. The use of zeolites of СаХ and NaX is based on the selectivity of adsorption and not on molecular sieve properties. With the complete replacement of the sodium cation for calcium, the zeolite of СаХ, unlike the NaX zeolite, does not adsorb aromatic hydrocarbons or their derivatives with branched radicals, e.g., 1, 3, 5β-triethylbenzene and metadichlorobenzene. This method is based on the method for identifying the zeolites of these two types and establishing the completeness of the ion exchange in the preparation of the zeolite of СаХ.

In the case when the critical diameter of the molecule is close to the diameter of the input window, the adsorption process occurs with a high activation energy and the ad-molecule must have a certain kinetic energy reserve to overcome the energy barrier. The kinetic energy of molecules rises with increasing temperature [15]. At the same time, an increase in temperature leads to an increase in the thermal pulsation of the zeolite lattice, which facilitates the penetration of the molecule into the adsorption cavity. Thus, by changing the temperature regime, it is possible to reach a point at which the adsorbent molecules begin to be absorbed

polymerization reactions.

ing drying and purification.

decarbonization of gases.

by the zeolite.

#### **2.3. Hypothetical properties of zeolites**

The first works of prediction of skeletons were carried out in the 1960s of the past century [5]. Most of these pioneering studies were performed manually. The development of computer technology and new algorithms allowed the generation of millions of hypothetical structures of zeolites [6, 7]. At present, there are two main directions for forecasting zeolite frames. One of the directions is based on the creation of hypothetical zeolites with given structural features. These hypothetical scaffolds are of great importance for functionally oriented synthesis [8, 9]. Another way of predicting zeolite frames is to create as many hypothetical structures as possible and enumerate all possible three-dimensional grids under certain topological and geometric constraints [10, 11]. Thus, in [12, 13], in order to enumerate all possible four-connected grids with a given number of unique T-atoms for all spatial groups, generation of hypothetical structures was carried out, consisting of the following basic procedures:


#### **2.4. Zeolite Molecular Sieve**

Zeolites are molecular sieves [4, 14]. Their wide application that they can be used for the separation of substances, not only on the basis of selectivity of adsorption, but also on the basis of the difference in size and shape of the molecules to be absorbed. In order to penetrate the adsorption cavity, the critical diameter of the adsorbate molecules must be smaller than the size of the entrance window [15].

The main factor determining the molecular sieve properties is the size of the entrance windows of zeolites, which depends on the location of the oxygen rings of the zeolite and on the number of oxygen atoms in the ring. The size of the input window is also affected by the size of the cation entering the zeolite. Cations located close to the window block the entrance for molecules. For example, in cation exchange, in which two sodium cations are replaced by a single calcium cation, the input window expands; As a result, the Na zeolite has an inlet window size of 4Å, and the CaA zeolite has a size of 5Å [15]. A similar exchange in a zeolite of type X leads to a certain narrowing of the window. Considering the properties of the KA zeolite, it can be explained that, at ordinary temperatures, this kind of zeolite sorbs water very well. This property has prevented its use for the drying of unstable substances prone to polymerization reactions.

exceeds 16, H<sup>2</sup>

62 Zeolites and Their Applications

basic procedures:

the whole cell.

nation of atoms are selected.

**2.4. Zeolite Molecular Sieve**

size of the entrance window [15].

(adsorption of water) and NMR spectra.

**2.3. Hypothetical properties of zeolites**

O molecules occupy places with the lowest adsorption energy, forming hydro-

gen bonds with the anionic surface of the zeolite. As the crystals are saturated with water, the electrical conductivity of the zeolites increases; therefore, the system of channels is filled with water molecules. Some of the sodium ions remain sufficiently free to chaotically move through the channels of the zeolites. These conclusions are in agreement with the data of IR

The first works of prediction of skeletons were carried out in the 1960s of the past century [5]. Most of these pioneering studies were performed manually. The development of computer technology and new algorithms allowed the generation of millions of hypothetical structures of zeolites [6, 7]. At present, there are two main directions for forecasting zeolite frames. One of the directions is based on the creation of hypothetical zeolites with given structural features. These hypothetical scaffolds are of great importance for functionally oriented synthesis [8, 9]. Another way of predicting zeolite frames is to create as many hypothetical structures as possible and enumerate all possible three-dimensional grids under certain topological and geometric constraints [10, 11]. Thus, in [12, 13], in order to enumerate all possible four-connected grids with a given number of unique T-atoms for all spatial groups, generation of hypothetical structures was carried out, consisting of the following

• Crystallographically unique T-atoms were successively placed in different positions (private and general) of the unit cell and by means of symmetry operations were generated on

• Of all the possible distributions of T-atoms in the cell, those that allow tetrahedral coordi-

• When the parameters of the unit cell were varied, optimization was performed to obtain acceptable values for the distances T–T and the angles T–T–T. At the last step, oxygen atoms were added between the bound T atoms, and the whole cell was optimized. As a result, several million hypothetical four-connected zeolite frameworks were obtained with a number

Zeolites are molecular sieves [4, 14]. Their wide application that they can be used for the separation of substances, not only on the basis of selectivity of adsorption, but also on the basis of the difference in size and shape of the molecules to be absorbed. In order to penetrate the adsorption cavity, the critical diameter of the adsorbate molecules must be smaller than the

The main factor determining the molecular sieve properties is the size of the entrance windows of zeolites, which depends on the location of the oxygen rings of the zeolite and on the number of oxygen atoms in the ring. The size of the input window is also affected by the size

of unique T-atoms ≤7, which are now presented in an online database [12, 13].

If we consider the property of NaA zeolite, which is capable of sorbing most of the components of industrial gases, the critical size of the following molecules does not exceed 4Å: hydrogen sulfide, carbon disulfide, carbon dioxide, ammonia, lower diene and acetylene hydrocarbons, ethane, ethylene, propylene, organic compounds with one methyl group in molecule, as well as methane, neon, argon, krypton, xenon, oxygen, nitrogen and carbon monoxide [15]. The latter group of substances is absorbed in considerable quantities only at low temperatures. Propane and organic compounds with more than three carbon atoms in the molecule are not adsorbed by the zeolite and thus do not suppress the adsorption of the above impurities during drying and purification.

CaA zeolites adsorb hydrocarbons and alcohols only of a normal structure, and therefore, it is widely used in the processes of separation of multicomponent organic substances on a molecular sieve basis. Moreover, zeolite CaA is absorbed by methyl and ethyl mercaptans, organic compounds with the number of carbon atoms in molecule 2 (ethyl alcohol, ethylamine), diborane, and so on. Among the general-purpose zeolites of CaA type is increased resistance in a weakly acid medium, and therefore, it is used in desulfurization processes and decarbonization of gases.

Zeolites of type X have rather wide entrance windows and adsorb the vast majority of the components of complex mixtures: all types of hydrocarbons are organic sulfur, nitrogen and oxygen compounds (mercaptans, thiophene, furan, quinoline, pyridine, dioxane, etc.), halogenated hydrocarbons (chloroform, carbon tetrachloride and freons), pentaborane and decaborane [15]. The use of zeolites of СаХ and NaX is based on the selectivity of adsorption and not on molecular sieve properties. With the complete replacement of the sodium cation for calcium, the zeolite of СаХ, unlike the NaX zeolite, does not adsorb aromatic hydrocarbons or their derivatives with branched radicals, e.g., 1, 3, 5β-triethylbenzene and metadichlorobenzene. This method is based on the method for identifying the zeolites of these two types and establishing the completeness of the ion exchange in the preparation of the zeolite of СаХ.

In the case when the critical diameter of the molecule is close to the diameter of the input window, the adsorption process occurs with a high activation energy and the ad-molecule must have a certain kinetic energy reserve to overcome the energy barrier. The kinetic energy of molecules rises with increasing temperature [15]. At the same time, an increase in temperature leads to an increase in the thermal pulsation of the zeolite lattice, which facilitates the penetration of the molecule into the adsorption cavity. Thus, by changing the temperature regime, it is possible to reach a point at which the adsorbent molecules begin to be absorbed by the zeolite.

*Localization of cations in the zeolite structure*: The localization of cations in the structure depends on the degree of hydration of the zeolites. Dehydration promotes the migration of cations in its structure, and it is necessary to take into account the degree of hydration of the zeolite under given conditions. In addition, protons from 100°C also show increased mobility. The location of the cation is determined not only by the degree of hydration of the zeolite, but also by the nature of the reacting substances. Thus, it was shown that in zeolites of type Y a significant number of Ni2 + and Cu2 + ions again move from small cavities to large ones when the latter are filled with olefins, ammonia, pyridine or NO molecules [16].

liquid substances adsorbed on the surface of a solid. They have a certain mobility, due to which

Separation of Binary Solutions on the Basis of Zeolites http://dx.doi.org/10.5772/intechopen.73513 65

At low pressures, adsorption increases in proportion to the increase in pressure. However, as the pressure rises, the linear relationship is broken; the amount of adsorbed gas decreases, and then, at a certain pressure, saturation appears; with further increase, the pressure of

In a number of cases, the relationship between the gas pressure and the amount of gas absorbed by the adsorbent is more complex; after reaching a certain pressure, the amount of adsorbed gas begins to increase sharply. It was found that the nature of this dependence is

In the adsorbent pores, at a certain relative pressure, the vapor becomes a liquid state and the entire internal structure begins to be filled with condensing vapor, as a result of which the amount of absorbed substance sharply increases. This phenomenon is called capillary condensation. In the fine pores of molecular sieves, capillary condensation does not occur; it can be noted only in relatively large pores formed by a binder during granulation—in a secondary porous structure [4].

Theories were advanced to explain the features of adsorption processes, and equations describing the dependence of the amount of adsorbed matter on pressure, temperature, and

The theory developed by Langmuir proceeds from the concept of the formation of a monomolecular layer of an adsorbed substance due to the action of active sites of the adsorbent.

In the potential theory advanced by Polanyi, it is assumed that the scope of the attractive

Proceeding from all of the above, the main ways to regulate the selective adsorption capacity

**1.** Change in composition in the process of crystallization. From the same starting materials, it is possible to obtain aluminosilicate porous crystals having different properties. For example, type A zeolite is formed from mixtures rich in alkalis and poor in silica, and type Y zeolite crystallizes in the region with the lowest alkalinity of the medium and the highest

**2.** Method of ion exchange. Ionic exchange can be regulated by molecular sieve properties, especially type A. Knowing the dimensions of the adsorbed molecules and zeolite windows, it is possible to select a particular cation exchange form of the zeolite to separate any mixture of gases or dissolved gases. For example, a zeolite, a spacecraft with a window size of about 3 Å, adsorbs water well, but water does not adsorb molecules of methanol, carbon dioxide, whose critical molecular diameter is greater than 3 </s>. Zeolite NaA in which

molecules of propane, hexane and other molecules with a critical diameter greater than 4 </s>. On a CaX zeolite with a window size of 8 Å, 1,3,5-triethylbenzene molecules are not adsorbed, and on NaX zeolite (the size of windows 9 Å) they are well adsorbed [17].

adsorbs methanol and carbon dioxide and does not adsorb the

forces during adsorption extends not only to the nanomolecular layer of matter.

they gradually penetrate into the solid body.

adsorption of substances does not increase [4].

other conditions were derived.

of zeolites are as follows.

concentration of silica [17].

the window size is 4 ÅA0

related to the shape and size of the adsorbent pores [4].

In NaA zeolite, the possible sites for cation localization are four-, six- and eight-membered rings and large cavities. Cations in the eight-membered rings react to the pore size due to the partial blocking of the entrance windows of the cavities [16]. In the faujasite structure, the cations are localized apparently at the centers of hexagonal prisms and on six- and fourmembered rings.

The catalytic properties of zeolites, which are studied using X-ray diffraction analysis, should be carefully considered; in fact, the localization of cautions largely depends on the conditions for pretreatment of the zeolite: the heating rate and the final temperature of the zeolite, the concentrations and the parameters for calcinations, the thickness of the layer and the possibility of reducing the cautions by hydrocarbons, and also from the possibility of separation from the vacuum lubricant used in installation.

As a rule, when analyzing the cation distribution in faujasite, the following factors are taken into account [16]:


#### **2.5. Adsorption properties of zeolites**

Considering the term adsorption can be understood this term as the ability of solids to absorb certain substances. This ability is closely related to the special properties of the surface of solids. Molecules of a gaseous or liquid substance are in contact with a solid, adsorbed by the surface of solids. The larger the surface of a solid, the more the amount of gas or liquid a given body can hold [4].

If the adsorbed substance is located for a long time on the surface of the adsorbent, the process of diffusion of adsorbed molecules inside [4] of the solid begins. Molecules of gaseous and liquid substances adsorbed on the surface of a solid. They have a certain mobility, due to which they gradually penetrate into the solid body.

*Localization of cations in the zeolite structure*: The localization of cations in the structure depends on the degree of hydration of the zeolites. Dehydration promotes the migration of cations in its structure, and it is necessary to take into account the degree of hydration of the zeolite under given conditions. In addition, protons from 100°C also show increased mobility. The location of the cation is determined not only by the degree of hydration of the zeolite, but also by the nature of the reacting substances. Thus, it was shown that in zeolites of type Y a significant number of Ni2 + and Cu2 + ions again move from small cavities to large ones when

In NaA zeolite, the possible sites for cation localization are four-, six- and eight-membered rings and large cavities. Cations in the eight-membered rings react to the pore size due to the partial blocking of the entrance windows of the cavities [16]. In the faujasite structure, the cations are localized apparently at the centers of hexagonal prisms and on six- and four-

The catalytic properties of zeolites, which are studied using X-ray diffraction analysis, should be carefully considered; in fact, the localization of cautions largely depends on the conditions for pretreatment of the zeolite: the heating rate and the final temperature of the zeolite, the concentrations and the parameters for calcinations, the thickness of the layer and the possibility of reducing the cautions by hydrocarbons, and also from the possibility of separation from

As a rule, when analyzing the cation distribution in faujasite, the following factors are taken

**1.** the need for optimal coordination; in hexagonal prisms, cations are easily coordinated with six skeleton oxygen atoms, whereas in the sodalite cavities tetrahedral coordination

**3.** the need to minimize the electrostatic energy of the system (direct repulsion of the cation-

Considering the term adsorption can be understood this term as the ability of solids to absorb certain substances. This ability is closely related to the special properties of the surface of solids. Molecules of a gaseous or liquid substance are in contact with a solid, adsorbed by the surface of solids. The larger the surface of a solid, the more the amount of gas or liquid a

If the adsorbed substance is located for a long time on the surface of the adsorbent, the process of diffusion of adsorbed molecules inside [4] of the solid begins. Molecules of gaseous and

with three skeleton oxygen atoms and one residual water molecule is possible;

**2.** the difficulty of local charge compensation with the help of multiply charged ions;

the latter are filled with olefins, ammonia, pyridine or NO molecules [16].

membered rings.

64 Zeolites and Their Applications

into account [16]:

the vacuum lubricant used in installation.

cation should be excluded);

**2.5. Adsorption properties of zeolites**

given body can hold [4].

**4.** stabilization by a crystalline field or a field of ligands;

**5.** covalence and the existence of directed bonds.

At low pressures, adsorption increases in proportion to the increase in pressure. However, as the pressure rises, the linear relationship is broken; the amount of adsorbed gas decreases, and then, at a certain pressure, saturation appears; with further increase, the pressure of adsorption of substances does not increase [4].

In a number of cases, the relationship between the gas pressure and the amount of gas absorbed by the adsorbent is more complex; after reaching a certain pressure, the amount of adsorbed gas begins to increase sharply. It was found that the nature of this dependence is related to the shape and size of the adsorbent pores [4].

In the adsorbent pores, at a certain relative pressure, the vapor becomes a liquid state and the entire internal structure begins to be filled with condensing vapor, as a result of which the amount of absorbed substance sharply increases. This phenomenon is called capillary condensation. In the fine pores of molecular sieves, capillary condensation does not occur; it can be noted only in relatively large pores formed by a binder during granulation—in a secondary porous structure [4].

Theories were advanced to explain the features of adsorption processes, and equations describing the dependence of the amount of adsorbed matter on pressure, temperature, and other conditions were derived.

The theory developed by Langmuir proceeds from the concept of the formation of a monomolecular layer of an adsorbed substance due to the action of active sites of the adsorbent.

In the potential theory advanced by Polanyi, it is assumed that the scope of the attractive forces during adsorption extends not only to the nanomolecular layer of matter.

Proceeding from all of the above, the main ways to regulate the selective adsorption capacity of zeolites are as follows.


Zeolites can manifest themselves as ions in many processes. For example, sodium in zeolite type X cannot be exchanged for alkyl ammonium cations because of the large size of the latter. Cation sieve effects in zeolites can also be caused by the fact that due to too large a size the cation cannot penetrate into small channels and cavities in the zeolite framework, or exchange cations during the synthesis of some zeolites are localized in inaccessible areas and therefore not are replaced [17].

the adsorption energy of solutions, we use the data on the surface tension of the pure compo-

*dμ*<sup>1</sup> + *G*<sup>2</sup>

are the values of the Gibbs adsorption of components; μ<sup>1</sup>

chemical potential of the components in order to relate Gibbs adsorption to the concentration

*х* \_\_2 *х*1

Surface properties of solutions are considered from the surface properties of pure liquids and strongly depend on the composition of the surface layer [20]. When a substance having a lower surface tension than a pure solvent dissolves, the surface tension of the solution decreases spontaneously, since the free energy of the system decreases. The concentration of solute in the surface layer as compared to its concentration in the solution volume increases [20]. Substances that increase the surface tension of the solution, on the contrary, are con-

Often substances that increase the surface tension of the solvent themselves have a higher surface tension in their pure form, while lowering substances have a lower surface tension than the solvent. A large surface tension means greater energy unsaturation of molecules on the surface [20]. Such molecules tend to leave the surface, since to reduce free surface energy it is more advantageous to have molecules with low energy unsaturation. Naturally, the complete separation of molecules is impeded by the loss of entropy of solution formation [20]. As a result of the action of these two factors, a composition change occurs on the surface of the solution as compared with the volume, that is, adsorption occurs [21]. For all liquid organic substances, the surface tension is less than the surface tension of water. Assuming that the total number of moles before and after adsorption remains unchanged, we find that when one component is adsorbed,

*dμ*<sup>2</sup>

tained in the surface layer in a lower concentration than those in the volume [20].

*х* \_\_2

*dμ*<sup>1</sup> + *x*<sup>2</sup>

*dμ*<sup>2</sup> (1)

Separation of Binary Solutions on the Basis of Zeolites http://dx.doi.org/10.5772/intechopen.73513

*dμ*<sup>2</sup> = 0 (2)

*dμ*<sup>2</sup> (3)

*<sup>х</sup>*1)*dμ*<sup>2</sup> (4)

, *x*<sup>2</sup> *+ x*<sup>1</sup> *=* 1. From this, it turns out:

(5)

and μ<sup>2</sup>

are the

67

nents and try to obtain the adsorption through Gibbs energy.

of the component; it is necessary to use the Gibbs-Duhem Eq. [19]

are the mole fraction of the components.

Considering the fundamental Gibbs law, we get

−*d* = *G*<sup>1</sup>

*x*<sup>1</sup>

*μ*<sup>1</sup> = −

We substitute it into the Gibbs Eq. (3) and obtain

−*d* = (*G*<sup>2</sup> − *G*<sup>1</sup>

the amount of the other is increased evenly. Then, *G*<sup>1</sup> *= G*<sup>2</sup>

*<sup>G</sup>*<sup>2</sup> <sup>=</sup> (1 <sup>−</sup> *<sup>х</sup>*2) \_\_\_ *<sup>d</sup>*

where *G1*

where *x*<sup>1</sup>

Then,

and *G2*

and *x*<sup>2</sup>

Adsorption on the surface of zeolite of polar substances, around the entrance windows, prevents the diffusion of adsorbate molecules, and prevents their movement. Thus, for example, the pre-sorption of small amounts of water vapor on zeolite type A sharply reduces the adsorption of oxygen. Effective window diameters can be adjusted to form organometallic complexes. Thus, when pyridine is treated with a copper form of zeolite X, a very strong pyridine cation complex is formed. The adsorption of molecules of gases and vapors on such a zeolite indicates a significant decrease in pore sizes due to their blocking by organometallic complexes [17].

A special place among the cation-substituted zeolites is occupied by hydrogen, or decationized, forms of zeolites. Replacement of cations of zeolite with hydrogen is one of the ways of modifying porous crystals. The hydrogen form of zeolites, unlike other forms, cannot be obtained by simple treatment of the zeolite with acids, since the latter destroy the crystal lattice, especially low-silica zeolites. Therefore, in the beginning, sodium ions are replaced by ammonium ions, then the latter is thermally decomposed, ammonia is released and a proton is formed, which ensures the neutrality of the zeolite lattice [17].

One of the methods for modifying zeolites is dealumination. Treatment of zeolite with acids leads to the dissolution of tetrahedral aluminum in the lattice. As a result, the adsorption capacity of the zeolite increases. Dealumination can also be carried out by treating the zeolite with substances that form complex compounds with aluminum ions or by treating the zeolite layer with water vapor at elevated temperatures [18]. Dealumination allows, within certain limits, to vary the ratio of silicon and aluminum-oxygen tetrahedra in the zeolite without changing its crystal lattice.

Selective adsorption on zeolites is also possible when the molecules of all components of the mixture are sufficiently small and freely penetrate into the adsorption space. Other things being equal, the exchange cations are adsorption centers and determine the specificity of the interaction during adsorption on the zeolites of molecules of different structure and electronic structure [18]. By changing the nature and size of the exchange cation, it is possible to enhance or weaken the contribution of a specific interaction to the adsorption energy. In addition to interacting with the positive charge of cations, the adsorbate molecule undergoes strong dispersion effects from other atoms forming the walls of the zeolite channels. One of the important issues of adsorption interaction on zeolites is the elucidation of the nature of active centers [18].

#### **2.6. A mathematical model for determining the total adsorption of solutions on zeolites**

Considering the process of adsorption from solutions should be considered as a process of concentration on the surface of one of the two adsorbed components. Therefore, to determine the adsorption energy of solutions, we use the data on the surface tension of the pure components and try to obtain the adsorption through Gibbs energy.

Considering the fundamental Gibbs law, we get

$$-d\sigma = G\_1 d\mu\_1 + G\_2 d\mu\_2 \tag{1}$$

where *G1* and *G2* are the values of the Gibbs adsorption of components; μ<sup>1</sup> and μ<sup>2</sup> are the chemical potential of the components in order to relate Gibbs adsorption to the concentration of the component; it is necessary to use the Gibbs-Duhem Eq. [19]

$$\mathbf{x}\_1 d\mu\_1 + \mathbf{x}\_2 d\mu\_2 = \mathbf{0} \tag{2}$$

where *x*<sup>1</sup> and *x*<sup>2</sup> are the mole fraction of the components.

Then,

Zeolites can manifest themselves as ions in many processes. For example, sodium in zeolite type X cannot be exchanged for alkyl ammonium cations because of the large size of the latter. Cation sieve effects in zeolites can also be caused by the fact that due to too large a size the cation cannot penetrate into small channels and cavities in the zeolite framework, or exchange cations during the synthesis of some zeolites are localized in inaccessible areas and therefore

Adsorption on the surface of zeolite of polar substances, around the entrance windows, prevents the diffusion of adsorbate molecules, and prevents their movement. Thus, for example, the pre-sorption of small amounts of water vapor on zeolite type A sharply reduces the adsorption of oxygen. Effective window diameters can be adjusted to form organometallic complexes. Thus, when pyridine is treated with a copper form of zeolite X, a very strong pyridine cation complex is formed. The adsorption of molecules of gases and vapors on such a zeolite indicates a significant decrease in pore sizes due to their blocking by organometallic complexes [17].

A special place among the cation-substituted zeolites is occupied by hydrogen, or decationized, forms of zeolites. Replacement of cations of zeolite with hydrogen is one of the ways of modifying porous crystals. The hydrogen form of zeolites, unlike other forms, cannot be obtained by simple treatment of the zeolite with acids, since the latter destroy the crystal lattice, especially low-silica zeolites. Therefore, in the beginning, sodium ions are replaced by ammonium ions, then the latter is thermally decomposed, ammonia is released and a proton

One of the methods for modifying zeolites is dealumination. Treatment of zeolite with acids leads to the dissolution of tetrahedral aluminum in the lattice. As a result, the adsorption capacity of the zeolite increases. Dealumination can also be carried out by treating the zeolite with substances that form complex compounds with aluminum ions or by treating the zeolite layer with water vapor at elevated temperatures [18]. Dealumination allows, within certain limits, to vary the ratio of silicon and aluminum-oxygen tetrahedra in the zeolite without

Selective adsorption on zeolites is also possible when the molecules of all components of the mixture are sufficiently small and freely penetrate into the adsorption space. Other things being equal, the exchange cations are adsorption centers and determine the specificity of the interaction during adsorption on the zeolites of molecules of different structure and electronic structure [18]. By changing the nature and size of the exchange cation, it is possible to enhance or weaken the contribution of a specific interaction to the adsorption energy. In addition to interacting with the positive charge of cations, the adsorbate molecule undergoes strong dispersion effects from other atoms forming the walls of the zeolite channels. One of the important issues of adsorption interaction on zeolites is the elucidation of the nature of active centers [18].

**2.6. A mathematical model for determining the total adsorption of solutions on** 

Considering the process of adsorption from solutions should be considered as a process of concentration on the surface of one of the two adsorbed components. Therefore, to determine

is formed, which ensures the neutrality of the zeolite lattice [17].

not are replaced [17].

66 Zeolites and Their Applications

changing its crystal lattice.

**zeolites**

$$
\mu\_1 = -\frac{x\_2}{x\_1} d\mu\_2 \tag{3}
$$

We substitute it into the Gibbs Eq. (3) and obtain

$$-d\sigma = \left(G\_2 - G\_1 \frac{x\_2}{x\_1}\right) d\mu\_2 \tag{4}$$

Surface properties of solutions are considered from the surface properties of pure liquids and strongly depend on the composition of the surface layer [20]. When a substance having a lower surface tension than a pure solvent dissolves, the surface tension of the solution decreases spontaneously, since the free energy of the system decreases. The concentration of solute in the surface layer as compared to its concentration in the solution volume increases [20]. Substances that increase the surface tension of the solution, on the contrary, are contained in the surface layer in a lower concentration than those in the volume [20].

Often substances that increase the surface tension of the solvent themselves have a higher surface tension in their pure form, while lowering substances have a lower surface tension than the solvent. A large surface tension means greater energy unsaturation of molecules on the surface [20]. Such molecules tend to leave the surface, since to reduce free surface energy it is more advantageous to have molecules with low energy unsaturation. Naturally, the complete separation of molecules is impeded by the loss of entropy of solution formation [20]. As a result of the action of these two factors, a composition change occurs on the surface of the solution as compared with the volume, that is, adsorption occurs [21]. For all liquid organic substances, the surface tension is less than the surface tension of water. Assuming that the total number of moles before and after adsorption remains unchanged, we find that when one component is adsorbed, the amount of the other is increased evenly. Then, *G*<sup>1</sup> *= G*<sup>2</sup> , *x*<sup>2</sup> *+ x*<sup>1</sup> *=* 1. From this, it turns out:

$$\mathbf{G}\_{\mathbf{z}} = \left(\mathbf{1} - \mathbf{x}\_{\mathbf{z}}\right) \frac{d\sigma}{d\mu\_{\mathbf{z}}} \tag{5}$$

and replacing *μ*<sup>2</sup> by *c*<sup>2</sup> , we obtain an equation similar to the Gibbs adsorption equation for dilute solutions

$$\mathbf{G}\_2 = \begin{pmatrix} \mathbf{1} - \mathbf{x}\_2 \end{pmatrix} \frac{c\_2}{RT} \frac{d\sigma}{dc\_2} \tag{6}$$

Thus, the most important feature of the Gibbs method is that it allows us to give some general characteristic of the region of inhomogeneity without knowing the true course of the concentration profile.

To solve Eq. (6), it is necessary to find the value of the surface tension for a binary solution.

Determination of the surface tension of a binary solution is

$$
\sigma = \sigma\_0 + 0.049 \cdot \left(1 - \frac{c\_2}{c\_1}\right) \tag{7}
$$

where N/m is the surface tension of water; *c*<sup>1</sup> is the initial concentration of the solution, % vol.; *c*2 is the final concentration of the solution, % vol.

Since Gibbs adsorption does not allow comparison of the results with the results of the experiment, it is necessary to use the equation to determine the total adsorption. Let us single out that to determine the total adsorption (absolute adsorption), it is necessary to relate Gibbs adsorption to the experimental results [21–24].

$$a = G\_2 + \mathcal{W} \cdot \rho \tag{8}$$

The following characteristics of the investigated adsorbents are established (**Table 1**). The

35 25 0.8795 0.58 293

25 15 0.8616 0.38 293

**Adsorbent Diameter of grain, mm** *а***, total adsorption, g/g** *W***, the volume of the space in which adsorption** 

**takes place, cm3**

**Density of the solution, g/m3**

*х***2**

**—mole fraction of components**

**Temperature** *T***, К**

69

**/g**

Separation of Binary Solutions on the Basis of Zeolites http://dx.doi.org/10.5772/intechopen.73513

To check the theoretical relationships and compare them with the experimental data, it is necessary to analyze a number of binary solutions, such as ethanol + water, methanol + water, and benzene + water; we distinguish that in **Table 2** the characteristics of the pure components

For a simplified calculation, let us set the condition that adsorption in the systems under consideration occurs only for one of the components; therefore, the concentration change will be monitored by the change in the percentage of water in the solutions in question. To do this,

**OH + H2**

**O C6**

**H6 +H2 O**

**O CH3**

Zeolite СаА 0.202 0.198 0.192 Zeolite СаА 0.15 0.146 0.142 Zeolite NаА 0.176 0.172 0.167 Zeolite NаX 0.257 0.252 0.244 Zeolite KаA 0.169 0.165 0.16

volume of the solution is 200 ml, and the mass of adsorbent for adsorption is 100 g.

(60) 40 30 0.836 0.76 293

of organic solutions for theoretical calculations are listed.

**Adsorbent** *a***—total adsorption, g/g**

**Table 3.** Calculated values of total adsorption.

**C2 H5 OH + H2**

**Table 2.** Characteristics of the investigated organic substances.

Zeolite СаА 3 0.26 0.23 Zeolite СаА 0.191 0.17 Zeolite NаА 0.245 0.2 Zeolite NаX 0.304 0.292 Zeolite KаA 0.197 0.1912

**Table 1.** Characteristics of zeolite adsorbents of grades A, X [24, 25].

*с***2 —final concentration of water solution, % vol.**

**Name** *с***<sup>1</sup>**

C2 H5 OH (65%)

CH<sup>3</sup> OH (75%)

С6 Н6 **—initial** 

**% vol.**

**concentration of water,** 

where *W* is the volume of the space in which adsorption occurs, cm**<sup>3</sup>** /g; *a* is the complete adsorption of g /g; *ρ* is the density of the equilibrium phase, g/cm**<sup>3</sup>** .

Then,

H.m.t.: 
$$a = (1 - x\_2)\frac{c\_2}{RT}\frac{d\left[\sigma\_o + 0.049 \cdot \left(1 - \frac{c\_2}{C\_1}\right)\right]}{dc\_2} + \mathcal{W} \cdot \rho \tag{9}$$

Thus, Gibbs adsorption does not reflect the properties of zeolite adsorbents; therefore, it is necessary to combine the formulas (6)–(8) into a single equation for the theoretical calculation of the total adsorption of binary solutions on zeolites. With the application of the method of complete content, from the concept of the region of inhomogeneity at the interphase interface as a separate real phase of finite volume [22–24].

#### **3. Experimental part**

As a result of experimental research in the laboratory, the following hydrometer Hydrometer AHON-1 (measuring range 700–1840 kg/m<sup>3</sup> ), laboratory scales Leki 5002, calipers digital 31C628 (error) and SORBTOMETR-M were used.


**Table 1.** Characteristics of zeolite adsorbents of grades A, X [24, 25].

and replacing *μ*<sup>2</sup>

68 Zeolites and Their Applications

dilute solutions

tration profile.

*c*2

Then,

by *c*<sup>2</sup>

*<sup>G</sup>*<sup>2</sup> <sup>=</sup> (1 <sup>−</sup> *<sup>х</sup>*2) *<sup>с</sup>*

Determination of the surface tension of a binary solution is

*<sup>σ</sup>* <sup>=</sup> *<sup>σ</sup>*<sup>0</sup> <sup>+</sup> 0.049 <sup>⋅</sup> (<sup>1</sup> <sup>−</sup> *<sup>c</sup>*

where N/m is the surface tension of water; *c*<sup>1</sup>

adsorption to the experimental results [21–24].

*<sup>a</sup>* <sup>=</sup> (1 <sup>−</sup> *<sup>х</sup>*2) *<sup>с</sup>*

**3. Experimental part**

as a separate real phase of finite volume [22–24].

AHON-1 (measuring range 700–1840 kg/m<sup>3</sup>

31C628 (error) and SORBTOMETR-M were used.

is the final concentration of the solution, % vol.

, we obtain an equation similar to the Gibbs adsorption equation for

\_\_2

*<sup>c</sup>*1) (7)

/g; *a* is the complete

+ *W* ⋅ *ρ* (9)

is the initial concentration of the solution, % vol.;

.

), laboratory scales Leki 5002, calipers digital

(6)

\_\_\_2 *RT* \_\_\_ *d dc*2

Thus, the most important feature of the Gibbs method is that it allows us to give some general characteristic of the region of inhomogeneity without knowing the true course of the concen-

To solve Eq. (6), it is necessary to find the value of the surface tension for a binary solution.

Since Gibbs adsorption does not allow comparison of the results with the results of the experiment, it is necessary to use the equation to determine the total adsorption. Let us single out that to determine the total adsorption (absolute adsorption), it is necessary to relate Gibbs

*a* = *G*<sup>2</sup> + *W* ⋅ *ρ* (8)

*<sup>d</sup>*[*σ*<sup>0</sup> <sup>+</sup> 0, <sup>049</sup> <sup>⋅</sup> (<sup>1</sup> <sup>−</sup> *<sup>c</sup>*

Thus, Gibbs adsorption does not reflect the properties of zeolite adsorbents; therefore, it is necessary to combine the formulas (6)–(8) into a single equation for the theoretical calculation of the total adsorption of binary solutions on zeolites. With the application of the method of complete content, from the concept of the region of inhomogeneity at the interphase interface

As a result of experimental research in the laboratory, the following hydrometer Hydrometer

 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *dc*2

\_\_2 *c*1)]

where *W* is the volume of the space in which adsorption occurs, cm**<sup>3</sup>**

\_\_\_2 *RT*

adsorption of g /g; *ρ* is the density of the equilibrium phase, g/cm**<sup>3</sup>**


**Table 2.** Characteristics of the investigated organic substances.

The following characteristics of the investigated adsorbents are established (**Table 1**). The volume of the solution is 200 ml, and the mass of adsorbent for adsorption is 100 g.

To check the theoretical relationships and compare them with the experimental data, it is necessary to analyze a number of binary solutions, such as ethanol + water, methanol + water, and benzene + water; we distinguish that in **Table 2** the characteristics of the pure components of organic solutions for theoretical calculations are listed.

For a simplified calculation, let us set the condition that adsorption in the systems under consideration occurs only for one of the components; therefore, the concentration change will be monitored by the change in the percentage of water in the solutions in question. To do this,


**Table 3.** Calculated values of total adsorption.

when testing the mathematical model for calculating the total adsorption, we take the initial water concentration of the binary systems under consideration. The results of the calculation using formula (9) are given in **Table 3**.

[4] Sokolov VA, Torocheshnikov NS, Keltsev NV. Molecular sieves and their application.

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[8] Boisen MB Jr, Gibbs GV, O'Keeffe M, Bartelmehs KL. A generation of framework structures for the tectosilicates using a molecular-based potential energy function and simulated annealing strategies. Microporous and Mesoporous Materials. 1999;**29**(3):219-266

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Kristallographie. 1969;**128**(3-6):352-370

simulated annealing. Nature. 1989;**342**(6247):260-262

Physical Chemistry Chemical Physics. 2005;**7**(23):3910

### **4. Conclusion**

The data presented show that the mathematical model developed by the author allows us to reliably estimate the total adsorption of various binary systems. When comparing theoretical values with experimental ones, it can be asserted that the values obtained fully satisfy the requirements of theoretical calculations and can be used in the design of drying apparatuses and concentrating alcoholic solutions. Let us emphasize that the accumulation of data on Gibbs adsorption will contribute to the refinement and verification of models taking into account bulk effects in adsorption solutions and will allow theoretical calculations to be carried out with high accuracy [26, 27].

Thus, the use of the approximate method of calculating absolute adsorption makes it possible to obtain more realistic information about the properties of adsorption solutions, which indicates significant structural changes in them, depending on the concentration.

### **Author details**

Paranuk Arambiy1 \*, Saavedra Huayta Jose Angel<sup>1</sup> and Khrisonidi Vitaly2

\*Address all correspondence to: rambi.paranuk@gmail.com

1 Kuban State Technological University, Krasnodar, Russia

2 Federal State Budget Education Establishment of Higher Professional Education, Maikop State Technological University, Maykop, Russia

### **References**


[4] Sokolov VA, Torocheshnikov NS, Keltsev NV. Molecular sieves and their application. Chemistry; 1964. 156 p

when testing the mathematical model for calculating the total adsorption, we take the initial water concentration of the binary systems under consideration. The results of the calculation

The data presented show that the mathematical model developed by the author allows us to reliably estimate the total adsorption of various binary systems. When comparing theoretical values with experimental ones, it can be asserted that the values obtained fully satisfy the requirements of theoretical calculations and can be used in the design of drying apparatuses and concentrating alcoholic solutions. Let us emphasize that the accumulation of data on Gibbs adsorption will contribute to the refinement and verification of models taking into account bulk effects in adsorption solutions and will allow theoretical calculations to be car-

Thus, the use of the approximate method of calculating absolute adsorption makes it possible to obtain more realistic information about the properties of adsorption solutions, which indi-

2 Federal State Budget Education Establishment of Higher Professional Education, Maikop

[1] Workshop on physical chemistry NSU. Chemical thermodynamics and kinetics. Adsorption from solutions on a solid surface: Method, Manual/O.V. Netskina. Novosibirsk: Novosibirsk

[2] Kireev VA. Course of physical chemistry. 3rd ed. recycled. and additional. M., "Chemistry". 1975. 776 p. 2. Adsorption at the solid-liquid interface. Tutorial M.: MITHT

[3] Paranuk AA, Khrisonidi VA. Industrial application of molecular sieves. Interactive

and Khrisonidi Vitaly2

cates significant structural changes in them, depending on the concentration.

\*, Saavedra Huayta Jose Angel<sup>1</sup>

\*Address all correspondence to: rambi.paranuk@gmail.com 1 Kuban State Technological University, Krasnodar, Russia

State Technological University, Maykop, Russia

State University; 2015. 17 p

them. M.V. Lomonosov, 2005

Science. 2016;**5**:51-53

using formula (9) are given in **Table 3**.

ried out with high accuracy [26, 27].

**4. Conclusion**

70 Zeolites and Their Applications

**Author details**

Paranuk Arambiy1

**References**


[19] Sokolov VA, Torocheshnikov NS, Keltsev NV. Molecular sieves and their application. Chemistry. 1964

**Chapter 5**

**Provisional chapter**

/Al2 O3

molar ratio

**Estimation of Nanoporosity of ZSM-5 Zeolites as**

**Estimation of Nanoporosity of ZSM-5 Zeolites as** 

DOI: 10.5772/intechopen.73624

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Hierarchical Materials**

**Hierarchical Materials**

Miguel Angel Hernández, A. Abbaspourrad,

Miguel Angel Hernández, A. Abbaspourrad,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Gabriela Hernández, Maria de los Angeles Velazco,

Gabriela Hernández, Maria de los Angeles Velazco,

The nanoporosity in zeolite ZSM-5 was analyzed as a function of SiO2

(MR). The internal pore structure was studied by high-resolution adsorption. Surface areas, microporous volume, characteristic energy of sorption, and pore-size distributions


The nanoporous, ordered, and three-dimensional structure of zeolites makes them materials of great practical importance in the hierarchy. The broad use of microporous zeolites (pore diameter *w* < 2 nm) makes them very important in very specific areas, such as acid catalysts and adsorbents, as well as in refining processes and the basic petrochemical industry due to their unique properties both in activity and in selectivity [1]. The great majority of zeolites possess two types of porosity: primary and secondary. Primary porosity with a well-defined

opening and widening as well as the emergence of further slit-like mesopores.

**Keywords:** ZSM-5 zeolite, sorption, nanopore measurements

sorption isotherms by the BET, Langmuir, *t*-method of de Boer,

Vitalli Petranovskii, Fernando Rojas, Roberto Portillo, Martha Alicia Salgado,

Vitalli Petranovskii, Fernando Rojas, Roberto Portillo, Martha Alicia Salgado,

Edgar Ayala and Karla Fabiola Quiroz

Edgar Ayala and Karla Fabiola Quiroz

http://dx.doi.org/10.5772/intechopen.73624

were calculated from N2

**Abstract**

αS

**1. Introduction**


#### **Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials**

DOI: 10.5772/intechopen.73624

Miguel Angel Hernández, A. Abbaspourrad, Vitalli Petranovskii, Fernando Rojas, Roberto Portillo, Martha Alicia Salgado, Gabriela Hernández, Maria de los Angeles Velazco, Edgar Ayala and Karla Fabiola Quiroz Miguel Angel Hernández, A. Abbaspourrad, Vitalli Petranovskii, Fernando Rojas, Roberto Portillo, Martha Alicia Salgado, Gabriela Hernández, Maria de los Angeles Velazco, Edgar Ayala and Karla Fabiola Quiroz

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.73624

#### **Abstract**

[19] Sokolov VA, Torocheshnikov NS, Keltsev NV. Molecular sieves and their application.

[20] Aivazov BV. Workshop on the chemistry of surface phenomena and adsorption.

[21] Tolmachev AM, Godovikova MI, Egorova TS. // Zhurn.fizich.chemii, 2005. P. 79, № 1,

[22] Paranuk AA. A mathematical model for calculating the adsorbers for drying and concentration of methanol on zeolites. Chemical and Petroleum Engineering. 2017, P. 53,

[23] Zaitsev ID, Aseev GG. Physicochemical properties of binary and multicomponent solu-

[24] Paranuk AA, Krisonidi VA, Ponomareva GV. KACO zeolite adsorption of ethyl alcohol.

[25] Tolmachev AM, Stekli F, Trubnikov OI, Kuznetsova TA. The Journal of Physical

[26] Paranuk AA, Krisonidi VA, Skhalyakho ZC, Shugalya AI. Technological scheme development of the azeotropic mix separation. Journal of Engineering and Applied Sciences.

[27] Paranuk AA, Saavedra KH, Kinenez LK. Separation of multicomponent solutions by

adsorption methods on zeolites. Exposition Oil Gas. 2015, №6(47), p. 66-67

Journal of Engineering and Applied Sciences. 2016, №11(13), p. 2876-2878

Textbook. Manual for Institutions. Higher School. 1973. 208 p

tions of inorganic substances. Ref. ed. -M: Chemistry, 1988. p. 416

Chemistry. 1964

72 Zeolites and Their Applications

№ 1-2, p. 41-43

Chemistry A. 1999;**73**(7):1267

2016, №11(13), p. 2878-2880

p. 1

The nanoporosity in zeolite ZSM-5 was analyzed as a function of SiO2 /Al2 O3 molar ratio (MR). The internal pore structure was studied by high-resolution adsorption. Surface areas, microporous volume, characteristic energy of sorption, and pore-size distributions were calculated from N2 sorption isotherms by the BET, Langmuir, *t*-method of de Boer, αS -plot of Sing, direct comparative plots of Lee, Newnham, Dubinin-Astakhov, differential adsorption curves, and nonlocal density functional theory methods. The results indicated that MR dependence in these zeolites caused structural defects through micropore opening and widening as well as the emergence of further slit-like mesopores.

**Keywords:** ZSM-5 zeolite, sorption, nanopore measurements

### **1. Introduction**

The nanoporous, ordered, and three-dimensional structure of zeolites makes them materials of great practical importance in the hierarchy. The broad use of microporous zeolites (pore diameter *w* < 2 nm) makes them very important in very specific areas, such as acid catalysts and adsorbents, as well as in refining processes and the basic petrochemical industry due to their unique properties both in activity and in selectivity [1]. The great majority of zeolites possess two types of porosity: primary and secondary. Primary porosity with a well-defined

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

size is associated with the crystalline structure of the zeolite and fundamentally depends on its structure type. Imperfections, including defects occurring during the growth of zeolite crystals, as well as defects generated by various treatments, cause secondary porosity, that is, the presence of mesopores (2 < *w* < 50 nm) and macropores (*w* > 50 nm) [2]. The secondary porosity differs from the framework porosity, since it does not directly depend on the crystalline structure of the zeolite. Taken together, these comprise the texture of a sorbent. The primary porosity is characterized by а microporous volume (*W*<sup>0</sup> ) with pore size as seen in the zeolite structure; secondary—by the pore-size distribution (PSD) and the external surface area (*AE*). These parameters are usually calculated from nitrogen sorption isotherms [3]. Many, but not all, catalysts are porous materials, in which most of the surface area is internal. Sometimes it is convenient to talk about the structure and texture of such materials. The structure is defined both by the distribution in space of atoms or ions in the material part of the catalyst and by the distribution on the surface. The texture is defined by the detailed geometry of the void space in the catalyst particles. Porosity is a concept related to texture and refers to the porous space in the material. However, with zeolites, most of the porosity is determined by the crystal structure. To accurately describe the texture of the porous catalyst, a very large number of parameters will be required. With respect to porous solids, the surface associated with the pores can be called the internal surface. Since the availability of pores can depend on the ratio of the dimensions of the channel and molecules, the extent of the accessible internal surface may depend on the size of the molecules contained in the mixture and may be different for various components of the mixture (molecular sieve effect) [4].

ZSM-5 and, its purely siliceous analog, silicalite (both have a structural code "MFI" in accordance with the IZA database) are among the most widely studied zeolites. MFI is one of the most versatile and commercially significant zeolites; it is widely used in the petroleum industry to convert methanol into complex hydrocarbons in methanol-to-gasoline processes, as well as in the alkylation of aromatic compounds and their subsequent separation [5]. The microporous network of this zeolite consists of intersecting straight and sinusoidal channels. The straight channels have pore openings defined by a cross-section of 10-member rings of 0.54–0.57 nm and sinusoidal channels by elliptic pores of 0.51–0.54 nm in cross-section. The intersections are cavities of 0.8 nm in diameter [6] (see **Figure 1**).

sites in micropores. It was shown that the shape-selective properties of zeolites may be greatly reduced due to the presence of active sites in the secondary porosity with a wide distribution of the pore diameter and on the external surface of their crystallites, so zeolites with a large

Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials

http://dx.doi.org/10.5772/intechopen.73624

75

, SS, and SZ sites,

The presence of molecules that blocks the pores of the zeolite or a partial destruction of its structure can drastically decrease its activity by reducing the microporous volume accessible for the reactants. The effect that the external surface area of ZSM-5 zeolite crystals used

**Figure 2.** Front (A), right (B), and top (C) views of a "ball and stick" ZSM-5 (MFI) model; crosscutting of zigzag channels

(D) of an "ionic radii" model; all from the IZA page: http://izasc.ethz.ch/fmi/xsl/IZA-SC/ftc\_3d.php.

outer surface area are less selective than those with fewer imperfections.

respectively.

**Figure 1.** ZSM-5 zeolite structure. The dimensions of the pore channels are in nm. I, S, and Z are SI

A detailed study on the different types of adsorption sites that constitute the structural skeleton of this zeolite was carried out by Cho et al. [7]. They classified the sorption sites into three types: (1) the SS sites located in straight channels; (2) the SZ sites located in zigzag (sinusoidal) channels; and, finally, (3) the S<sup>I</sup> sites located at the intersections (**Figure 2**). One of the most important catalytic properties of ZSM-5 is its shape selectivity. This is a consequence of its primary microporous structure and is the basis for most of its successful applications [8].

Another important parameter that allows to adjust the zeolite properties is their chemical composition, that is, their SiO2 /Al2 O3 molar ratio (MR). The amount of Al in the framework is proportional to the number of exchangeable cations, H+ among others, which affects both Lewis and Brønsted acidity. The main interactions of the sorbate molecules in the pores of the zeolite are realized through the oxygen atoms of the lattice and extra-framework cations. Microporosity and secondary porosity in zeolites and similar materials can be determined from the low- and medium-pressure regions of the sorption isotherm using various approaches [3]. The shape-selective activity of MFI can be attributed to the presence of active Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials http://dx.doi.org/10.5772/intechopen.73624 75

size is associated with the crystalline structure of the zeolite and fundamentally depends on its structure type. Imperfections, including defects occurring during the growth of zeolite crystals, as well as defects generated by various treatments, cause secondary porosity, that is, the presence of mesopores (2 < *w* < 50 nm) and macropores (*w* > 50 nm) [2]. The secondary porosity differs from the framework porosity, since it does not directly depend on the crystalline structure of the zeolite. Taken together, these comprise the texture of a sorbent.

in the zeolite structure; secondary—by the pore-size distribution (PSD) and the external surface area (*AE*). These parameters are usually calculated from nitrogen sorption isotherms [3]. Many, but not all, catalysts are porous materials, in which most of the surface area is internal. Sometimes it is convenient to talk about the structure and texture of such materials. The structure is defined both by the distribution in space of atoms or ions in the material part of the catalyst and by the distribution on the surface. The texture is defined by the detailed geometry of the void space in the catalyst particles. Porosity is a concept related to texture and refers to the porous space in the material. However, with zeolites, most of the porosity is determined by the crystal structure. To accurately describe the texture of the porous catalyst, a very large number of parameters will be required. With respect to porous solids, the surface associated with the pores can be called the internal surface. Since the availability of pores can depend on the ratio of the dimensions of the channel and molecules, the extent of the accessible internal surface may depend on the size of the molecules contained in the mixture and may be differ-

ZSM-5 and, its purely siliceous analog, silicalite (both have a structural code "MFI" in accordance with the IZA database) are among the most widely studied zeolites. MFI is one of the most versatile and commercially significant zeolites; it is widely used in the petroleum industry to convert methanol into complex hydrocarbons in methanol-to-gasoline processes, as well as in the alkylation of aromatic compounds and their subsequent separation [5]. The microporous network of this zeolite consists of intersecting straight and sinusoidal channels. The straight channels have pore openings defined by a cross-section of 10-member rings of 0.54–0.57 nm and sinusoidal channels by elliptic pores of 0.51–0.54 nm in cross-section. The

A detailed study on the different types of adsorption sites that constitute the structural skeleton of this zeolite was carried out by Cho et al. [7]. They classified the sorption sites into three

important catalytic properties of ZSM-5 is its shape selectivity. This is a consequence of its primary microporous structure and is the basis for most of its successful applications [8].

Another important parameter that allows to adjust the zeolite properties is their chemical

Lewis and Brønsted acidity. The main interactions of the sorbate molecules in the pores of the zeolite are realized through the oxygen atoms of the lattice and extra-framework cations. Microporosity and secondary porosity in zeolites and similar materials can be determined from the low- and medium-pressure regions of the sorption isotherm using various approaches [3]. The shape-selective activity of MFI can be attributed to the presence of active

sites located in straight channels; (2) the SZ sites located in zigzag (sinusoidal)

sites located at the intersections (**Figure 2**). One of the most

molar ratio (MR). The amount of Al in the framework

among others, which affects both

) with pore size as seen

The primary porosity is characterized by а microporous volume (*W*<sup>0</sup>

ent for various components of the mixture (molecular sieve effect) [4].

intersections are cavities of 0.8 nm in diameter [6] (see **Figure 1**).

/Al2 O3

is proportional to the number of exchangeable cations, H+

types: (1) the SS

74 Zeolites and Their Applications

channels; and, finally, (3) the S<sup>I</sup>

composition, that is, their SiO2

**Figure 1.** ZSM-5 zeolite structure. The dimensions of the pore channels are in nm. I, S, and Z are SI , SS, and SZ sites, respectively.

sites in micropores. It was shown that the shape-selective properties of zeolites may be greatly reduced due to the presence of active sites in the secondary porosity with a wide distribution of the pore diameter and on the external surface of their crystallites, so zeolites with a large outer surface area are less selective than those with fewer imperfections.

The presence of molecules that blocks the pores of the zeolite or a partial destruction of its structure can drastically decrease its activity by reducing the microporous volume accessible for the reactants. The effect that the external surface area of ZSM-5 zeolite crystals used

**Figure 2.** Front (A), right (B), and top (C) views of a "ball and stick" ZSM-5 (MFI) model; crosscutting of zigzag channels (D) of an "ionic radii" model; all from the IZA page: http://izasc.ethz.ch/fmi/xsl/IZA-SC/ftc\_3d.php.

for shape-selective reactions causes, was reported previously [9]. Some authors in reported works have used the α<sup>S</sup> -plot of Sing as alternative method to evaluate the external surface area and the true intra-crystalline capacity [10].

The aim of this study was to accurately describe the dependence of all the different types of ZSM-5 porosity on MR and to show which methods are best suited for measuring them in each range. This will allow us to develop an approach to the application of various existing methods of texture characterization for samples of zeolite with mixed porosity.

### **2. Methodology**

A set of ZSM-5 zeolites in their sodium form (Na-ZSM-5) with a SiO2 /Al2 O3 molar ratio (MR) varying from 30 to 120 was synthesized using a template of tetrapropylammonium bromide (TPABr) following the methodology reported by Ghiaci et al. [11]. Through the text and figures, these samples are called Z, followed by the MR value (30, 70, 95, or 120), for example, Z30 means an Na-ZSM-5 sample with an MR equal to 30. For comparison, a set of Na-ZSM-5 samples supplied by TOSOH Co., Japan, with MR 20, 23.3, and 30 were also studied. These TOSOH samples are called ZT, followed by MR value. A reference macroporous solid material required to estimate micropore volumes was obtained from the Tehuacan area in the state of Puebla, Mexico. This reference substrate was identified by X-ray powder diffraction (XRD) as α-SiO<sup>2</sup> . X-ray powder diffraction of ZSM-5 samples was obtained in the 2θ ranges of 5–50 degrees using diffractometer Bruker D8, using nickel-filtered Cu Kα (λ = 0.154 nm) radiation. Scanning Electron Microscopy images were collected from a JEOL JSM-6610LV electron microscope with tungsten filament and an electron detector operated at 20 kV. N<sup>2</sup> adsorption isotherms were measured at the boiling point of liquid N2 (76.4 K at the 2200 m altitude of Puebla City, México) in the interval of relative pressures, *p*/*p*<sup>0</sup> extending from 10−6 to 1 in an automatic volumetric adsorption system (Quantachrome AutoSorb-1C) in order to determine the textural parameters of ZSM-5 samples in addition to the evaluation of microporosity, which was analyzed through the determination of pore-size distributions calculated by the differential adsorption curves (DAC), Dubinin-Astakhov equation (D-A), and nonlocal density functional theory (NLDFT) approaches.

[101] reflections positioned at 2θ = 7.92°, 7.93°, and 8.01°, respectively, gave rise to a total peak at ~8.0. The most important difference between the standard XRD pattern and the observed for both sets of samples is the relative intensity of the various peaks, but a detailed discussion of the changes in the structure of ZSM-5 due to MR variations and synthesis conditions is beyond the scope of the present work and will be discussed elsewhere. Three peaks that appear at 2θ = 16.0°, 26.4°, and 30.9° in the ZT23.3 sample (marked with asterisks) are most

In **Figure 4**, it can be seen that the effect of the templates used during the synthesis process affects the morphology of the zeolite crystals obtained. Thus, for example, in **Figure 4(a)** and **(b)** corresponding to zeolites ZT-20 and ZT-23.3, it can be seen that the crystals obtained have lath-like shapes. In the case of the ZT-30 and ZT-23.3 zeolites, clusters of spheroidal crystals are observed where the crystals of the zeolites coexist, as seen in **Figure 4(c)** and **(d)**. Finally, the SEM images of the zeolites ZT-30 and ZT-23.3 do not exhibit a predominant or defined geometry, as seen in

sorption isotherms at 77 K for both sets of samples are shown in **Figure 5** as sorbed volume

hysteresis loops shown by ZSM-5 zeolites are of the Type H3 or H4, characteristic of capillary condensation in the slit-like pores attributed to intercrystallite adsorption within aggregates. **Table 1** gives the values of some important parameters obtained from the analysis of isotherms.

per gram of zeolite versus *p*/*p*<sup>0</sup>

Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials

http://dx.doi.org/10.5772/intechopen.73624

77

scale in the range of 105 ≤ *p*/*p*<sup>0</sup> ≤ 1. The

. **Figure 5**

probably associated with an unidentified impurity.

at standard temperature and pressure (STP) in cm3

shows the sorption isotherms using a logarithmic *p*/*p*<sup>0</sup>

**3.2. Scanning electronic microscopy**

**Figure 3.** X-ray diffraction patterns.

**Figure 4(e)** and **(f)** [13].

N2

**3.3. High-resolution adsorption**

### **3. Results and discussion**

#### **3.1. X-ray analysis**

The XRD patterns of all samples (**Figure 3**) are typical of ZSM-5 zeolites [12]. In general, all the samples showed reasonably sharp diffraction patterns, indicating good crystallinity. Please note that commercial TOSOH samples and those prepared in the laboratory are nearly identical. The main peaks appear at the following 2θ angles: 8.0°, 8.9°, 9.8°, 14.0°, 14.8°, 20.9°, 23.2°, 23.9°, 24.5°, 29.4°, and 30.0° (**Figure 3**). Most of these peaks are not resolved; usually, one peak is a superposition of several closely located reflections. For example, the [−101], [011], and

**Figure 3.** X-ray diffraction patterns.

for shape-selective reactions causes, was reported previously [9]. Some authors in reported

The aim of this study was to accurately describe the dependence of all the different types of ZSM-5 porosity on MR and to show which methods are best suited for measuring them in each range. This will allow us to develop an approach to the application of various existing

varying from 30 to 120 was synthesized using a template of tetrapropylammonium bromide (TPABr) following the methodology reported by Ghiaci et al. [11]. Through the text and figures, these samples are called Z, followed by the MR value (30, 70, 95, or 120), for example, Z30 means an Na-ZSM-5 sample with an MR equal to 30. For comparison, a set of Na-ZSM-5 samples supplied by TOSOH Co., Japan, with MR 20, 23.3, and 30 were also studied. These TOSOH samples are called ZT, followed by MR value. A reference macroporous solid material required to estimate micropore volumes was obtained from the Tehuacan area in the state of Puebla, Mexico. This reference substrate was identified by X-ray powder diffraction (XRD)

. X-ray powder diffraction of ZSM-5 samples was obtained in the 2θ ranges of 5–50

degrees using diffractometer Bruker D8, using nickel-filtered Cu Kα (λ = 0.154 nm) radiation. Scanning Electron Microscopy images were collected from a JEOL JSM-6610LV electron

automatic volumetric adsorption system (Quantachrome AutoSorb-1C) in order to determine the textural parameters of ZSM-5 samples in addition to the evaluation of microporosity, which was analyzed through the determination of pore-size distributions calculated by the differential adsorption curves (DAC), Dubinin-Astakhov equation (D-A), and nonlocal den-

The XRD patterns of all samples (**Figure 3**) are typical of ZSM-5 zeolites [12]. In general, all the samples showed reasonably sharp diffraction patterns, indicating good crystallinity. Please note that commercial TOSOH samples and those prepared in the laboratory are nearly identical. The main peaks appear at the following 2θ angles: 8.0°, 8.9°, 9.8°, 14.0°, 14.8°, 20.9°, 23.2°, 23.9°, 24.5°, 29.4°, and 30.0° (**Figure 3**). Most of these peaks are not resolved; usually, one peak is a superposition of several closely located reflections. For example, the [−101], [011], and

microscope with tungsten filament and an electron detector operated at 20 kV. N<sup>2</sup>

isotherms were measured at the boiling point of liquid N2

sity functional theory (NLDFT) approaches.

**3. Results and discussion**

**3.1. X-ray analysis**

Puebla City, México) in the interval of relative pressures, *p*/*p*<sup>0</sup>

methods of texture characterization for samples of zeolite with mixed porosity.

A set of ZSM-5 zeolites in their sodium form (Na-ZSM-5) with a SiO2


/Al2 O3

molar ratio (MR)

adsorption

(76.4 K at the 2200 m altitude of

extending from 10−6 to 1 in an

works have used the α<sup>S</sup>

76 Zeolites and Their Applications

**2. Methodology**

as α-SiO<sup>2</sup>

and the true intra-crystalline capacity [10].

[101] reflections positioned at 2θ = 7.92°, 7.93°, and 8.01°, respectively, gave rise to a total peak at ~8.0. The most important difference between the standard XRD pattern and the observed for both sets of samples is the relative intensity of the various peaks, but a detailed discussion of the changes in the structure of ZSM-5 due to MR variations and synthesis conditions is beyond the scope of the present work and will be discussed elsewhere. Three peaks that appear at 2θ = 16.0°, 26.4°, and 30.9° in the ZT23.3 sample (marked with asterisks) are most probably associated with an unidentified impurity.

#### **3.2. Scanning electronic microscopy**

In **Figure 4**, it can be seen that the effect of the templates used during the synthesis process affects the morphology of the zeolite crystals obtained. Thus, for example, in **Figure 4(a)** and **(b)** corresponding to zeolites ZT-20 and ZT-23.3, it can be seen that the crystals obtained have lath-like shapes. In the case of the ZT-30 and ZT-23.3 zeolites, clusters of spheroidal crystals are observed where the crystals of the zeolites coexist, as seen in **Figure 4(c)** and **(d)**. Finally, the SEM images of the zeolites ZT-30 and ZT-23.3 do not exhibit a predominant or defined geometry, as seen in **Figure 4(e)** and **(f)** [13].

#### **3.3. High-resolution adsorption**

N2 sorption isotherms at 77 K for both sets of samples are shown in **Figure 5** as sorbed volume at standard temperature and pressure (STP) in cm3 per gram of zeolite versus *p*/*p*<sup>0</sup> . **Figure 5** shows the sorption isotherms using a logarithmic *p*/*p*<sup>0</sup> scale in the range of 105 ≤ *p*/*p*<sup>0</sup> ≤ 1. The hysteresis loops shown by ZSM-5 zeolites are of the Type H3 or H4, characteristic of capillary condensation in the slit-like pores attributed to intercrystallite adsorption within aggregates. **Table 1** gives the values of some important parameters obtained from the analysis of isotherms.

**Figure 4.** SEM images of ZSM-5 zeolite samples with different forms and crystal sizes: (a) ZT-20, (b) ZT-23.3, (c) ZT-30, (d) ZT23.3, (e) Z-30, and (f) Z-120.

All the N2 isotherms are of Type I according to the IUPAC classification [14]. They indicate: (1) a high sorption at a very low relative pressure caused by the enhanced sorption potential of the ZSM-5 channel system and (2) formation of a monolayer at 0.1 ≤ *p*/*p*<sup>0</sup> ≤ 0.8.

> were used as reference values; α-quartz was chosen as a reference material, since adsorption on these substrata occurs similarly as on a flat surface; access to the underlying microporous structure is impeded by water molecules in the pore openings. The standard nitrogen

**ZSM-5 zeolite Si Al Na O Si/Al** Na-20 45.72 4.5 2.65 47.315 10.16 Na-23.3 47.313 3.61 2.58 46.496 13.10 Na-30 48.893 2.866 1.326 46.906 17.06 30 46.483 4.156 2.383 46.976 11.18 70 48.82 0.826 0.823 49.560 56.68 95 52.986 1.45 0.91 44.656 36.54 120 50.766 0.96 0.66 47.61 52.88

**Table 1.** ZSM-5 zeolite chemical composition (mass %, EDS).

designated by filled symbols, while samples synthesized for the present work (Z series) are designated by open symbols. The selection of symbols (circles, squares, etc.) is constant for all figures. Note that the ordinate scales are not always

sorption isotherms at 77 K. For this figure and throughout, all samples supplied by TOSOH (ZT series) are

Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials

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79

**Figure 5.** N2

the same.

#### *3.3.1. External surface area*

To calculate the volume of the micropores from the sorption data, De Boer *t*-plots (thickness plots) and Harkins-Jura estimates are given in **Table 2**. An accurate estimate of these values can be influenced by the choice of the standard isotherm of a nonporous material selected to estimate the statistical thickness of the adsorbed layer (*t*) and the range of *t* values considered for the linear fitting [15] (**Figure 6**).

#### *3.3.2. Microporosity*

The total micropore volumes in cm3 g−1 for all the samples are given in **Table 2**. These values were calculated from: (1) α<sup>S</sup> -plots, (2) *t*-plots, and (3) the D-A equation (in this case, optimizing the values of the parameters *n* and *E*<sup>0</sup> ). The ratio of the micropore-filling capacity to the total sorption uptake, *W*<sup>0</sup> /*V*Σ, a parameter that somehow indicates the degree of crystallinity of the zeolite being analyzed, is also included in **Table 2** [16]. For the construction of the α<sup>S</sup> and direct comparison plots, the adsorption volumes of the α-quartz without thermal processing

**Figure 5.** N2 sorption isotherms at 77 K. For this figure and throughout, all samples supplied by TOSOH (ZT series) are designated by filled symbols, while samples synthesized for the present work (Z series) are designated by open symbols. The selection of symbols (circles, squares, etc.) is constant for all figures. Note that the ordinate scales are not always the same.

were used as reference values; α-quartz was chosen as a reference material, since adsorption on these substrata occurs similarly as on a flat surface; access to the underlying microporous structure is impeded by water molecules in the pore openings. The standard nitrogen


**Table 1.** ZSM-5 zeolite chemical composition (mass %, EDS).

All the N2

*3.3.1. External surface area*

(d) ZT23.3, (e) Z-30, and (f) Z-120.

78 Zeolites and Their Applications

*3.3.2. Microporosity*

were calculated from: (1) α<sup>S</sup>

total sorption uptake, *W*<sup>0</sup>

for the linear fitting [15] (**Figure 6**).

ing the values of the parameters *n* and *E*<sup>0</sup>

isotherms are of Type I according to the IUPAC classification [14]. They indicate: (1)

a high sorption at a very low relative pressure caused by the enhanced sorption potential of the

**Figure 4.** SEM images of ZSM-5 zeolite samples with different forms and crystal sizes: (a) ZT-20, (b) ZT-23.3, (c) ZT-30,

To calculate the volume of the micropores from the sorption data, De Boer *t*-plots (thickness plots) and Harkins-Jura estimates are given in **Table 2**. An accurate estimate of these values can be influenced by the choice of the standard isotherm of a nonporous material selected to estimate the statistical thickness of the adsorbed layer (*t*) and the range of *t* values considered

The total micropore volumes in cm3 g−1 for all the samples are given in **Table 2**. These values

the zeolite being analyzed, is also included in **Table 2** [16]. For the construction of the α<sup>S</sup>

direct comparison plots, the adsorption volumes of the α-quartz without thermal processing


/*V*Σ, a parameter that somehow indicates the degree of crystallinity of

). The ratio of the micropore-filling capacity to the

and

ZSM-5 channel system and (2) formation of a monolayer at 0.1 ≤ *p*/*p*<sup>0</sup> ≤ 0.8.


*ASL* is the Langmuir-specific surface area; *ASB* is the BET-specific surface area; *AE* is the external

The filling of macro, meso, and micropores can be proved by analyzing high-resolution α<sup>S</sup>

plots starting at low relative pressures, that is, 10−5; see **Figure 7**. There are some significant

enhancement of the sorbent-sorbate interaction in the pores of molecular dimensions, that

adsorbents having a wide range of pore sizes and results in two or more separate stages of micropore filling. **Figure 7** shows three linear ranges. Region III, with α<sup>S</sup> > 1.6, corresponds to sorption in the mesopores and adsorption on the external surface of the zeolite. Extrapolation of the line to the ordinate at *p*/*p*<sup>0</sup> = 0 allows to estimate the total microporous volume *W*<sup>0</sup>

Region II with α<sup>S</sup> = 0.6–1.6 can be a sorption in the porosity created by partial removal of the constituents of the zeolite matrix with the formation of structural defects. This type of porosity is typically developed by acid leaching. Region I, with α<sup>S</sup> < 0.25, is due to the stages of final filling of the volume of ultramicroporous elliptical sinusoidal channels (0.55 × 0.51 nm) and nearly circular straight channels (0.54 × 0.56 nm). This behavior is mainly due to the combined


is the range used for the BET plot; *CB*

Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials

/*V*Σ is the degree of crystallinity.

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, which can be explained by the



81

.

surface area; *V*Σ is the volume sorbed at *p*/*p*<sup>0</sup> = 0.95; *p*/*p*<sup>0</sup>

*-plots*

*3.3.2.1. High-resolution α<sup>S</sup>*

differences in the form of α<sup>S</sup>

**Figure 7.** *t*-plots for the N2

sorption isotherms.

is the BET constant; *dp* is average particle diameter; and *W*<sup>0</sup>

tortion of the isotherm shape is observed at a very low *p*/*p*<sup>0</sup>

is, the process of micropore filling [18]. This type of α<sup>S</sup>

**Table 2.** Adsorption structural parameters of ZSM-5 zeolites.

isotherms [17] are very similar to the adsorption isotherms of α-quartz along the adsorption branch up to a relative pressure of about 0.8. Since the same material was used as a reference for the α<sup>S</sup> direct comparison plots, similar microporous volumes were obtained from all of these methods. However, *t*-plots give slightly different results because the reference isotherm corresponds to de Boer equation.

**Figure 6.** High-resolution N2 sorption isotherms using a logarithmic *p*/*p*<sup>0</sup> scale. The graphs are divided in (a) and (b) parts with different scales due to the presence of higher values for most of the measured parameters for ZT30 and Z120 for this and the following figures.

*ASL* is the Langmuir-specific surface area; *ASB* is the BET-specific surface area; *AE* is the external surface area; *V*Σ is the volume sorbed at *p*/*p*<sup>0</sup> = 0.95; *p*/*p*<sup>0</sup> is the range used for the BET plot; *CB* is the BET constant; *dp* is average particle diameter; and *W*<sup>0</sup> /*V*Σ is the degree of crystallinity.

#### *3.3.2.1. High-resolution α<sup>S</sup> -plots*

The filling of macro, meso, and micropores can be proved by analyzing high-resolution α<sup>S</sup> plots starting at low relative pressures, that is, 10−5; see **Figure 7**. There are some significant differences in the form of α<sup>S</sup> -plots as a function of MR, mainly for Z120. A pronounced distortion of the isotherm shape is observed at a very low *p*/*p*<sup>0</sup> , which can be explained by the enhancement of the sorbent-sorbate interaction in the pores of molecular dimensions, that is, the process of micropore filling [18]. This type of α<sup>S</sup> -plot is characteristic of microporous adsorbents having a wide range of pore sizes and results in two or more separate stages of micropore filling. **Figure 7** shows three linear ranges. Region III, with α<sup>S</sup> > 1.6, corresponds to sorption in the mesopores and adsorption on the external surface of the zeolite. Extrapolation of the line to the ordinate at *p*/*p*<sup>0</sup> = 0 allows to estimate the total microporous volume *W*<sup>0</sup> . Region II with α<sup>S</sup> = 0.6–1.6 can be a sorption in the porosity created by partial removal of the constituents of the zeolite matrix with the formation of structural defects. This type of porosity is typically developed by acid leaching. Region I, with α<sup>S</sup> < 0.25, is due to the stages of final filling of the volume of ultramicroporous elliptical sinusoidal channels (0.55 × 0.51 nm) and nearly circular straight channels (0.54 × 0.56 nm). This behavior is mainly due to the combined

**Figure 7.** *t*-plots for the N2 sorption isotherms.

**Figure 6.** High-resolution N2

for the α<sup>S</sup>

80 Zeolites and Their Applications

corresponds to de Boer equation.

**Table 2.** Adsorption structural parameters of ZSM-5 zeolites.

for this and the following figures.

sorption isotherms using a logarithmic *p*/*p*<sup>0</sup>

parts with different scales due to the presence of higher values for most of the measured parameters for ZT30 and Z120

isotherms [17] are very similar to the adsorption isotherms of α-quartz along the adsorption branch up to a relative pressure of about 0.8. Since the same material was used as a reference

ZT20 287.6 224.5 13.39 0.217 0.09–0.27 −56 3.866 33.179 ZT23.3 459.5 375.4 33.32 0.390 0.05–0.17 −244 4.155 26.153 ZT30 399.4 313.9 91.27 0.812 0.05–0.19 −202 1.034 25.738 Z30 533.6 409.0 19.72 0.129 0.05–0.19 −343 1.261 68.992 Z70 491.2 349.2 47.12 0.162 0.05–0.24 −110 1.855 63.580 Z95 562.3 397.0 83.41 0.207 0.05–0.27 −107 2.085 54.106 Z120 1784 1314 279.30 0.812 0.05–0.21 −276 2.471 54.451

**Sample** *ASL* **(m2 g−1)** *ASB* **(m2 g−1)** *AE* **(m2 g−1)** *V***Σ (cm3 g−1) BET** *p***/***p***<sup>0</sup>**

these methods. However, *t*-plots give slightly different results because the reference isotherm

direct comparison plots, similar microporous volumes were obtained from all of

**range**

*C<sup>B</sup> dp* **(nm)** *W***<sup>0</sup>**

**/***V***<sup>Σ</sup>**

scale. The graphs are divided in (a) and (b)

filling of channels. However, this region is related to the filling of the ultramicropores corresponding to the narrowing and to the initial stages of channel filling. Zones of this α<sup>S</sup> -plot for Z120 appear because the substratum has mesopores, supermicropores, and uniform micropores with elliptical and nearly circular free openings. The diameter of these channels corresponds to approximately 1–3 diameters of molecules. The micropore filling regions obtained through high-resolution plots for all samples are presented in **Table 2** (**Figures 8** and **9**).

#### *3.3.3. Pore-size distributions calculated by the DAC, D-A, and NLDFT approaches*

#### *3.3.3.1. DAC method*

Calculation of pore-size distributions from desorption branches of N2 isotherms using the differential adsorption curves (DAC) [19] method yields bimodal distributions (**Figure 10**), with the thickness of the pore size of ca. 0.36 and 0.55 nm for all samples. The plots are unimodal with the pore ca. 0.36–0.40 nm. This approach correctly describes the essential qualitative features of N2 sorption in the microporous zeolites, such as ZSM-5, that is, pores in the range of 0.3–0.6 nm. The results of these estimates are shown in **Table 3**.

#### *3.3.3.2. D-A method*

The pore-size distributions obtained by the D-A method [20] are shown in **Figure 11**. The average pore diameter, seen as a maxima on the curves by this method, varies according to

**Figure 9.** Comparative plots of the N2

**Figure 10.** Micropore size distribution calculated from N2

sorption isotherms versus adsorption using an α-SiO<sup>2</sup>

sorption isotherms using the DAC approach.

reference.

Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials

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83

**Figure 8.** High resolution α<sup>S</sup> -plot for N2 sorption on ZT30 and Z120, showing (a) the ultramicropore and supermicropore volume regions and (b) the supermicropore linear region.

Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials http://dx.doi.org/10.5772/intechopen.73624 83

filling of channels. However, this region is related to the filling of the ultramicropores corre-

Z120 appear because the substratum has mesopores, supermicropores, and uniform micropores with elliptical and nearly circular free openings. The diameter of these channels corresponds to approximately 1–3 diameters of molecules. The micropore filling regions obtained through high-resolution plots for all samples are presented in **Table 2** (**Figures 8** and **9**).

ferential adsorption curves (DAC) [19] method yields bimodal distributions (**Figure 10**), with the thickness of the pore size of ca. 0.36 and 0.55 nm for all samples. The plots are unimodal with the pore ca. 0.36–0.40 nm. This approach correctly describes the essential qualitative

The pore-size distributions obtained by the D-A method [20] are shown in **Figure 11**. The average pore diameter, seen as a maxima on the curves by this method, varies according to

sorption in the microporous zeolites, such as ZSM-5, that is, pores in the range

sorption on ZT30 and Z120, showing (a) the ultramicropore and supermicropore


isotherms using the dif-

sponding to the narrowing and to the initial stages of channel filling. Zones of this α<sup>S</sup>

*3.3.3. Pore-size distributions calculated by the DAC, D-A, and NLDFT approaches*

Calculation of pore-size distributions from desorption branches of N2

of 0.3–0.6 nm. The results of these estimates are shown in **Table 3**.

*3.3.3.1. DAC method*

82 Zeolites and Their Applications

features of N2

*3.3.3.2. D-A method*

**Figure 8.** High resolution α<sup>S</sup>


volume regions and (b) the supermicropore linear region.

**Figure 9.** Comparative plots of the N2 sorption isotherms versus adsorption using an α-SiO<sup>2</sup> reference.

**Figure 10.** Micropore size distribution calculated from N2 sorption isotherms using the DAC approach.


the D-A equation are somewhat different from the microporous volumes calculated by the

 and *t* methods. The fact that these D-A microporous volumes are always larger than the volumes calculated by the other methods suggests that the uptake at low relative pressures should be corrected for mesopore adsorption. This correction will result in a lower extrapolated value of the micropore volume from the D-A equation, and better agreement with other

 and *t*) methods will be reached. It can be seen there that the D-A treatment, while overestimating somehow the pore sizes, still provides an approximate estimate of the micropore vol-

as MR increases: ZT20 > ZT23.3 > ZT30; Z30 > Z70 > Z95 > Z120 (**Table 2**). These values reflect

**Figure 12.** (a and b) NLDFT pore-size distribution showing the supermicropore region; (a′ and b′) close-up of the NLDFT

pore-size distribution showing the supermicropore region on ZSM-5 zeolites.

values obtained by the D-A method decrease

Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials

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85

αS

(α<sup>S</sup>

umes and their corresponding pore sizes. The *E*<sup>0</sup>

**Table 3.** Total micropore and mesopore volumes (*W*<sup>0</sup> , *Vmeso*, cm3 g−1) by various methods of analysis.

MR. **Table 3** lists the optimized *W*<sup>0</sup> , *n*, and *E*<sup>0</sup> values using the D-A equation. The filling of elliptical channels of ZSM-5 with a length of 1.98 nm (width 0.51 × 0.57 nm) and connected through a zigzag path with a length of 0.665 nm (width 0.54) is the main contribution to the volume adsorbed. **Figure 11** shows ZSM-5 pore-size distributions obtained by the D-A method, assuming a cylindrical microporous channel; these plots provide average diameters very similar to the width of the porous cavities of the zeolites ZSM-5 (5th column of **Table 3**). The D-A results shown in **Table 2** suggest that MR in ZSM-5 zeolites promotes the opening and widening of their micropores. Nevertheless, the microporous volumes calculated from

**Figure 11.** Micropore size distribution calculated from N2 adsorption on the ZSM-5 zeolites through D-A approach.

the D-A equation are somewhat different from the microporous volumes calculated by the αS and *t* methods. The fact that these D-A microporous volumes are always larger than the volumes calculated by the other methods suggests that the uptake at low relative pressures should be corrected for mesopore adsorption. This correction will result in a lower extrapolated value of the micropore volume from the D-A equation, and better agreement with other (α<sup>S</sup> and *t*) methods will be reached. It can be seen there that the D-A treatment, while overestimating somehow the pore sizes, still provides an approximate estimate of the micropore volumes and their corresponding pore sizes. The *E*<sup>0</sup> values obtained by the D-A method decrease as MR increases: ZT20 > ZT23.3 > ZT30; Z30 > Z70 > Z95 > Z120 (**Table 2**). These values reflect

**Sample α<sup>S</sup> DAC** *t* **D-A** *Vmeso E***<sup>0</sup>**

**Table 3.** Total micropore and mesopore volumes (*W*<sup>0</sup>

**Figure 11.** Micropore size distribution calculated from N2

MR. **Table 3** lists the optimized *W*<sup>0</sup>

84 Zeolites and Their Applications

ZT20 0.072 0.108 0.106 0.110 0.145 26.50 1 ZT23.3 0.102 0.131 0.147 0.165 0.288 20.50 1.3 ZT30 0.209 0.313 0.294 0.335 0.603 15.50 1.1 Z30 0.089 0.106 0.129 0.141 0.040 20.50 1.1 Z70 0.103 0.120 0.144 0.141 0.059 18.50 1 Z95 0.112 0.129 0.149 0.186 0.095 16 1 Z120 0.354 0.529 0.466 0.582 0.458 15.50 1

, *n*, and *E*<sup>0</sup>

elliptical channels of ZSM-5 with a length of 1.98 nm (width 0.51 × 0.57 nm) and connected through a zigzag path with a length of 0.665 nm (width 0.54) is the main contribution to the volume adsorbed. **Figure 11** shows ZSM-5 pore-size distributions obtained by the D-A method, assuming a cylindrical microporous channel; these plots provide average diameters very similar to the width of the porous cavities of the zeolites ZSM-5 (5th column of **Table 3**). The D-A results shown in **Table 2** suggest that MR in ZSM-5 zeolites promotes the opening and widening of their micropores. Nevertheless, the microporous volumes calculated from

 **(kJ mol−1)** *n*

, *Vmeso*, cm3 g−1) by various methods of analysis.

values using the D-A equation. The filling of

adsorption on the ZSM-5 zeolites through D-A approach.

**Figure 12.** (a and b) NLDFT pore-size distribution showing the supermicropore region; (a′ and b′) close-up of the NLDFT pore-size distribution showing the supermicropore region on ZSM-5 zeolites.


that the intensity of the distribution at 5.0 nm is poorly developed; however, it is represented in all distributions. A possible explanation is that the structures are not homogeneous and that it contains a significant amount of slit-like pores and pores of other irregular shapes. Based on the NLDFT method, one can get an idea about the actual widths and pore sizes of the supermi-

adsorption equation, D-A represents the Dubinin-Astakhov equation, *Vmeso* is calculated by

The obtained samples exhibit reasonable diffraction patterns, indicative of good crystallinity. The most important difference between the standard XRD pattern and those observed for both sets of samples is the relative intensity of the various peaks. The ZSM-5 samples synthesized are composed of crystals with different geometry in a range of sizes 5–10 μm. N<sup>2</sup> isotherms have been measured, starting at a relative pressure of 10−5 and up to 1. To evaluate the texture properties of ZSM-5 zeolites, BET, Langmuir, Ast, surface areas, and external surface area were used. A significant amount of micropores was found in all ZSM-5 zeolites.

in perfect agreement with the type of present pores for all the samples, that is, micropores, mesopores, and macropores. Thus, adsorption is probably the most sensitive tool for evaluating quality and structural properties of the microporous materials such as ZSM-5 zeolites. To characterize these nanomaterials, a combination of comparative methods based on reference isotherms on well-characterized ZSM-5 zeolites is recommended, as well as the results of

This work was partially supported by DGAPA-UNAM IN107817 Grant, VIEP, and the

method, DAC is direct comparison plots, *t* is the *t*-plot employing the de Boer

, *t*, and comparative plots (DAC) were used to estimate micropores in

is the characteristic energy of sorption, and *n* is the

Estimation of Nanoporosity of ZSM-5 Zeolites as Hierarchical Materials

isotherms are available.

87

http://dx.doi.org/10.5772/intechopen.73624

adsorption at 77 K are

cropore voids existing in ZSM-5 zeolites for which high-resolution N2

all zeolites. Nanopore size distributions (NSD) obtained from the N2

Academic Body "Investigación en zeolitas," CA-95 (PROMEP-SEP).

order of the sorption energy distribution from the Dubinin-Astakhov equation.

αS

is Sing's α<sup>S</sup>

**4. Conclusions**

Such methods as α<sup>S</sup>

DAC, D-A, and the NLDFT.

**Acknowledgements**

**Conflicts of interest**

The authors declare no conflict of interest.

subtracting *W*0αS from *V*Σ (**Table 2**), *E*<sup>0</sup>

DAC is the differential curves of comparison plots method, D-A represents the Dubinin-Astakhov equation, and NLDFT is the nonlocal density functional approach.

**Table 4.** Pore diameter (nm) by different methods of analysis.

the dependence of this parameter on the SiO2 /Al2 O3 ratio. Due to the crystalline nature of the zeolites, the force field created by the oxygen atoms in their structure must have symmetrical properties. Therefore, the *E*<sup>0</sup> values are affected by the aluminum content, which has changed some atoms of the zeolite framework in the channels to be displaced, as well as changing the sorption potential. From the comparison of the *E*<sup>0</sup> values obtained for these zeolites, this parameter is influenced not only by the pore sizes. Consequently, the Al content and the geometry of the pores have modified the electric field within the pores of the zeolites and thus have influenced the characteristic sorption energy.

#### *3.3.3.3. Nonlocal density functional theory method*

Nonlocal density functional theory (NLDFT) was developed to take into account pore sizes in voids of well-defined geometry [21]. With this approach, the molecules adsorbed in the pores tend to be packaged in accordance with the adhesion forces established with the substrate (i.e. attractive forces between adsorptive and adsorbent molecules) and interactions with the remaining fluid molecules. The molar density of the adsorbed phase varies as a function of pore size. The adsorption isotherm is calculated from a given pore shape (spherical, cylindrical, slit-like, etc.), and the experimental isotherm is given as the sum of a series of individual single-pore isotherms multiplied by their relative abundance over a range of pore sizes. In the present case, the microporous structure of ZSM-5 zeolite can be approximated as a bundle of parallel cylindrical pores and the nature of the adsorbent can be assumed as that of the silica. In this way, the distribution of supermicroporous zeolitic adsorbents can be calculated from highresolution adsorption isotherms. The results of the analysis of the size of the supermicropores using the NLDFT method are shown in **Figure 12** and are listed in **Table 4**. The pore-size distributions obtained from the N2 isotherms using the NLDFT cylindrical pore model yield bimodal distributions with pore size characteristics of 1.8 and 5.0 nm. It is observed from this figure that the intensity of the distribution at 5.0 nm is poorly developed; however, it is represented in all distributions. A possible explanation is that the structures are not homogeneous and that it contains a significant amount of slit-like pores and pores of other irregular shapes. Based on the NLDFT method, one can get an idea about the actual widths and pore sizes of the supermicropore voids existing in ZSM-5 zeolites for which high-resolution N2 isotherms are available.

αS is Sing's α<sup>S</sup> method, DAC is direct comparison plots, *t* is the *t*-plot employing the de Boer adsorption equation, D-A represents the Dubinin-Astakhov equation, *Vmeso* is calculated by subtracting *W*0αS from *V*Σ (**Table 2**), *E*<sup>0</sup> is the characteristic energy of sorption, and *n* is the order of the sorption energy distribution from the Dubinin-Astakhov equation.

### **4. Conclusions**

the dependence of this parameter on the SiO2

**Table 4.** Pore diameter (nm) by different methods of analysis.

the sorption potential. From the comparison of the *E*<sup>0</sup>

have influenced the characteristic sorption energy.

*3.3.3.3. Nonlocal density functional theory method*

properties. Therefore, the *E*<sup>0</sup>

is the nonlocal density functional approach.

86 Zeolites and Their Applications

butions obtained from the N2

/Al2 O3

DAC is the differential curves of comparison plots method, D-A represents the Dubinin-Astakhov equation, and NLDFT

**Sample DAC D-A NLDFT** ZT20 0.568 0.55 1.8/5.0 ZT23.3 — 0.57 1.8/5.0 ZT30 0.564 0.64 1.8 Z30 0.561 0.59 1.8/4.9 Z70 0.379/0.56 0.62 1.8/5.0 Z95 0.379/0.560 0.65 1.8/4.8 Z120 0.364/0.560/0.590 0.66 1.8/4.9

zeolites, the force field created by the oxygen atoms in their structure must have symmetrical

some atoms of the zeolite framework in the channels to be displaced, as well as changing

parameter is influenced not only by the pore sizes. Consequently, the Al content and the geometry of the pores have modified the electric field within the pores of the zeolites and thus

Nonlocal density functional theory (NLDFT) was developed to take into account pore sizes in voids of well-defined geometry [21]. With this approach, the molecules adsorbed in the pores tend to be packaged in accordance with the adhesion forces established with the substrate (i.e. attractive forces between adsorptive and adsorbent molecules) and interactions with the remaining fluid molecules. The molar density of the adsorbed phase varies as a function of pore size. The adsorption isotherm is calculated from a given pore shape (spherical, cylindrical, slit-like, etc.), and the experimental isotherm is given as the sum of a series of individual single-pore isotherms multiplied by their relative abundance over a range of pore sizes. In the present case, the microporous structure of ZSM-5 zeolite can be approximated as a bundle of parallel cylindrical pores and the nature of the adsorbent can be assumed as that of the silica. In this way, the distribution of supermicroporous zeolitic adsorbents can be calculated from highresolution adsorption isotherms. The results of the analysis of the size of the supermicropores using the NLDFT method are shown in **Figure 12** and are listed in **Table 4**. The pore-size distri-

distributions with pore size characteristics of 1.8 and 5.0 nm. It is observed from this figure

ratio. Due to the crystalline nature of the

values obtained for these zeolites, this

values are affected by the aluminum content, which has changed

isotherms using the NLDFT cylindrical pore model yield bimodal

The obtained samples exhibit reasonable diffraction patterns, indicative of good crystallinity. The most important difference between the standard XRD pattern and those observed for both sets of samples is the relative intensity of the various peaks. The ZSM-5 samples synthesized are composed of crystals with different geometry in a range of sizes 5–10 μm. N<sup>2</sup> isotherms have been measured, starting at a relative pressure of 10−5 and up to 1. To evaluate the texture properties of ZSM-5 zeolites, BET, Langmuir, Ast, surface areas, and external surface area were used. A significant amount of micropores was found in all ZSM-5 zeolites. Such methods as α<sup>S</sup> , *t*, and comparative plots (DAC) were used to estimate micropores in all zeolites. Nanopore size distributions (NSD) obtained from the N2 adsorption at 77 K are in perfect agreement with the type of present pores for all the samples, that is, micropores, mesopores, and macropores. Thus, adsorption is probably the most sensitive tool for evaluating quality and structural properties of the microporous materials such as ZSM-5 zeolites. To characterize these nanomaterials, a combination of comparative methods based on reference isotherms on well-characterized ZSM-5 zeolites is recommended, as well as the results of DAC, D-A, and the NLDFT.

### **Acknowledgements**

This work was partially supported by DGAPA-UNAM IN107817 Grant, VIEP, and the Academic Body "Investigación en zeolitas," CA-95 (PROMEP-SEP).

### **Conflicts of interest**

The authors declare no conflict of interest.

## **Author details**

Miguel Angel Hernández1 \*, A. Abbaspourrad2 , Vitalli Petranovskii3 , Fernando Rojas4 , Roberto Portillo5 , Martha Alicia Salgado5 , Gabriela Hernández6 , Maria de los Angeles Velazco7 , Edgar Ayala7 and Karla Fabiola Quiroz8

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\*Address all correspondence to: vaga1957@gmail.com

1 Zeolites Research Department, Autonomous University of Puebla, Puebla, Mexico

2 Azad University of Shahryar-Shahre Ghods, Shahre Ghods, Tehran, Iran

3 Center for Nanoscience and Nanotechnology, National Autonomous University of Mexico, Ensenada, Mexico

4 Department of Chemistry, Autonomous Metropolitan University, Mexico City, Mexico

5 Faculty of Chemistry, Autonomous University of Puebla, Puebla, Mexico

6 Department of Chemical Engineering, Autonomous Metropolitan University, Mexico City, Mexico

7 Faculty of Chemical Engineering, Autonomous University of Puebla, Puebla, Mexico

8 Interdisciplinary Professional Unit of Biotechnology of the National Polytechnic Institute, Mexico City, Mexico

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**Author details**

88 Zeolites and Their Applications

Roberto Portillo5

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6 Department of Chemical Engineering, Autonomous Metropolitan University, Mexico City,

8 Interdisciplinary Professional Unit of Biotechnology of the National Polytechnic Institute,

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**Section 3**

**Zeolites Applications**

