1. Introduction

Granulated zeolites X in different cationic forms are used in large-tonnage process of adsorption drying and removal of sulfur compounds and СО₂, in gas and liquid environments of various composition. Besides, the cationic forms of zeolite X can be used for adsorption separation of mixtures of hydrocarbons. Adsorption cubic capacity and stability when used in cycles of adsorption-desorption of zeolite adsorbents are determined, first of all, by the phase purity and level of crystallinity of zeolite, contained in them, nature and content of exchangeable cations in the zeolite cavities, as well as the parameters of their porous structure.

In[1-4] described methods for producing binder-free zeolite NaХ (NaХ-BF), granules of which are crystalline aggregates . Such zeolite granules have higher values of the adsorption characteristics and mechanical strength than the granules with a binder.

The presence of cations in the cavities of the porous structure of zeolites causes the following features of zeolites as adsorbents[5]:

- the influence of the nature and content of exchangeable cations on the size of the input windows in the cavities;

-during exchange of cations Na+ for other cations change of the latter in the cavities is possible, that leads to changes of limiting volume for filling;

-the specific interaction of polar molecules with exchangeable cations at low degrees of filling of the adsorption volume.

Zeolites X are usually synthesized in the Na-form. Information in literature about obtaining other cationic forms are given only for highly dispersed and granulated zeolite X with a binder [6,7]. There is no such information for binder-free zeolite (BF-zeolite). We can assume that the basic laws of the cationic exchange for binder-free zeolite are the same. At the same time, granules of binder-free zeolite are crystalline aggregates, so they may have specific features of behavior usual for exchange process. There is also no information about influence of type and content of exchangeable cations in the BF-zeolite X on its ability to adsorb substances whose molecules differ in size and structure. According to above-mentioned, present research covers the synthesis of Н⁺, К⁺, Ca²⁺ or Mg^2+- forms of BF-zeolite X and the adsorption of molecules of Н₂О, СО₂, С₆Н₆ и н-С₇Н₁₆ by them in static and dynamic modes.

2. Experimental Part

Н^+, К⁺ , Ca²⁺ or Mg²⁺- forms of BF-zeolite X with a diameter of 1.6 mm (HNaХ-BF KNaХ-BF, СaNaХ-BF and MgNaХ-BF) were synthesized from its Na-form using ion exchange in solutions of the respective chlorides. Ion exchange experiments were carried out at 700℃, with initial concentration of salt in a solution of 70 g /l ( the ratio of the excess of second exchangeable cation to sodium), at a solution volume-to-zeolite mass ratio of 4 to 1 in an isothermal reactor of periodical operation during 2 hours with stirring. The degree of exchange of Na⁺ (αNa) for other cations regulated by the amount of exchanges (from 1 to 4) without intermediate heat treatment. HNa-form of BF-zeolite X (HNaX-BF) was obtained from BF- NH₄NaX using heat treatment after the last exchange at 450℃ during 4 hours in the open air. As a comparison with these results, we also obtained exchangeable forms of highly dispersed zeolite NaX with a particle size of 1.0 - 8.0 microns.

The chemical compositions of liquid and solid phases were analyzed using gravimetric method, as well as methods of complexometric titration and flame photometry[8].

The phase composition of the zeolites were determined using X-ray analysis on an automatic diffractometer PHILIPS PW 1800. To determine used the method of Debye - Scherrer (powder method). Analysis conditions: theta/2 theta -scanning; rotation of holder-one turn sec^-1, anode material-copper, range -5-55о/2 theta; step - 0.05°; exposure step - 2 seconds, anode voltage and current - 40 kW and 30 mA, respectively. Radiographs were identified by the known diffraction data[9].

To determine the parameters of porous structure ( pore's size and distribution radius, specific surface area) used the low-temperature nitrogen adsorption measured by static vacuum volumetric automated installation "Sorptomatic- 1900" ("Fisons")[10-12], and mercury porosimetry using «Porosimeter-2000"[13].

To determine the equilibrium adsorption cubic capacities (cm³/g) for Н₂О - А(Н₂О), С₆Н₆ - А(С₆Н₆) and n-С₇Н₁₆ - А(n-С₇Н₁₆) of obtained adsorbents used the excicatory method, which is widespread in industry.[14]

Measurements were made at 20℃. To determine A (Н₂О) in excicator added an aqueous solution of sulfuric acid, which provides P/Ps = 0,7, and pure benzine and heptane to determine A (С₆Н₆) and A (n-С₇Н₁₆) respectively. A (СО₂) was measured at concentrations of СО₂ in a binary mixture with helium, equal to 70.0 and 0.03% vol. During the experiment samples were removed from the excicator and weighed. The experiment was stopped when a constant sample weight had been obtained.

To assess the impact of the replacement of the cations Na⁺ by other cations on the adsorption cubic capacity of granules of BF-zeolite X in the dynamic mode was measured its adsorption cubic capacity for Н₂О - D (Н₂О), СО₂ - D (СО₂), С₆Н₆ - D (С₆Н₆) and n-С₇Н₁₆ - D (n-С₇Н₁₆) from mixtures with air in a flow adsorber at atmospheric pressure, temperature 20-25°C and the amount of the loaded adsorbent up to 150 сm³. The rate of steam or air-gas flow was 4,0 ± 0,25 dm³/min for a mixture with С₆Н₆ and n-С₇Н₁₆ at vapor concentrations of 12-15 mg/ dm³, and 1,0 ± 0,25 dm³/min for a mixture of Н₂О and СО₂ at a concentration of 13-15mg/dm³ and 200 mg/dm³, respectively. The adsorption experiment was stopped after reaching "the skip" concentration corresponding to-70℃ for Н₂О and -60℃ for С₆Н₆ and n-С₇Н₁₆. In the case of СО₂ experiment was stopped when it comes out of the adsorber.

3. Results and Discussion

Figure 1 shows the results of studying the influence of the nature of exchangeable cation and the number of treatments with Na during exchange for cations Н⁺, К⁺, Ca²⁺ or Mg²⁺ in the zeolite NaХ-BF. To compare results here stated data of similar exchanges in the samples of highly dispersed zeolite NaХ.

Presented results show that in order to achieve maximum αNa in zeolite NaХ - BF three exchange treatments are required. Any further increasing in their number has no significant impact.

Figure 1. Figure 1. Influence of the nature of exchangeable cation and number of treatments on the degree of exchange in the granular (a) and highly dispersed (b) zeolites of type X

Comparison of values of αNa in granular and highly dispersed samples with the same number of treatments, showed that the granular exchange proceeds more slowly, apparently due to diffusion limitations.

The maximum values of αNa during exchange for cations Н⁺, К⁺, Ca²⁺ or Mg²⁺ are 0.64, 0.82; 0.79 and 0.52, respectively. The discrepancy between the values is determined by difference of their own size and energy of hydration of above-mentioned ions[6, 7]. For highly dispersed zeolite NaХ αNa is 0.66, 0.87, 0.83, 0.56, respectively (Fig. 1).

X-ray analysis shows that degree of crystallinity of the highly dispersed zeolite X before exchange is close to 100.0%, and for the NaХ-BF it makes 90,0-94,0%. After an exchange of Nа⁺ cations for the above-mentioned cations, these values are almost unchanged.

The values of the mechanical crush strength of granules (butt-end crush) during of the exchange does not change and make 2.0-2.2 kg/mm².

Taking into account the fact that the characteristics of granulated zeolite's pore structure have a significant impact on the amount of material adsorbed in the dynamic mode, these characteristics of samples of zeolite NaХ-BF were measured before and after the exchange treatments. The results are shown in Table 1.

Results of determination of A (Н₂О), А(н-С₇Н₁₆) and А(С₆Н₆) for zeolite X-BF in the above-mentioned cationic forms as well as similar characteristics of the samples of powdered zeolite X for comparison are shown in Tables 2-4.

Table 1. Parameters of the porous structure of cationic forms of zeolite X-BF Cationic form of zeolite Vpor, cm3/g Roaverage, Å Sspecific, m2/g Sspecific, m2/g nitrogen mercury NaХ-BF 0,24 963 3,9 364 0,64НNaХ-BF 0,35 1250 7,9 370 0,82КNaХ-BF 0,30 1230 7,5 316 0,79СаNaХ-BF 0,29 1258 5,8 403 0,52MgNaХ-BF 0,28 1222 5,4 396

Data from Table 1 shows that the parameters of the porous structure of granules after the exchange remain unchanged.

Table 2. A (Н2О) of zeolites X in different cationic forms Cationic form of zeolite А(Н2О)cm3/g Cationic form of zeolite А(Н2О) cm3/g NaХ-BF 0,27 NaХ 0,30 0,64НNaХ-BF 0,22 0,66НNaХ 0,25 0,82КNaХ-BF 0,26 0,87КNaХ 0,29 0,79СаNaХ-BF 0,27 0,83СаNaХ 0,29 0,52MgNaХ-BF 0,28 0,56MgNaХ 0,32

Table 3. А(n-С7Н16) of zeolites X in different cationic forms Cationic form of zeolite А(n-С7Н16) cm3/g Cationic form of zeolite А(n-С7Н16) cm3/g NaХ-BF 0,28 NaХ 0,30 0,64НNaХ-BF 0,23 0,66НNaХ 0,27 0,82КNaХ-BF 0,27 0,87КNaХ 0,29 0,79СаNaХ-BF 0,29 0,83СаNaХ 0,30 0,52MgNaХ-BF 0,28 0,56MgNaХ 0,31

Table 4. А(С6Н6) of zeolites X in different cationic forms Cationic form of zeolite А(С6Н6) cm3/g Cationic form of zeolite А(С6Н6) cm3/g NaХ-BF 0,26 NaХ 0,30 0,64НNaХ-BF 0,22 0,66НNaХ 0,25 0,82КNaХ-BF 0,26 0,87КNaХ 0.28 0,79СаNaХ-BF 0,26 0,83СаNaХ 0,29 0,52MgNaХ-BF 0,27 0,55MgNaХ 0,30

It is obvious that equilibrium adsorptive cubic capacities of granular samples is 10,0-15,0% less than of powdery samples. The main reason of smaller adsorptive cubic capacity of granules is that they correspond to an aggregate of nanodimensional crystals and part of its intracrystalline space can be inaccessible for molecules of adsorbate.

The results, represented in tables 2-4, show that during transition from NaX- BF to KNaX-BF, CaNaX-BF or MgNaX-BF with maximum degrees of exchange, there were no considerable changes in the values of А(Н₂О), А(С₆Н₆) and А(н-С₇Н₁₆).

Table 5 represents the results of the research of СО₂ adsorption on the zeolite X in different cationic forms. It is clear that when the СО₂ concentration is equal to 70,0%, the values of maximum CO₂ adsorptive cubic capacities for all cationic forms of zeolites X are also similar (table 5).

Table 5. А (СО2) of zeolites X in different cationic forms

The obtained results are explained by the fact that in the researched conditions volumetric filling of microcellular intracrystalline space of zeolite X-BF takes place. In this case А(Н₂О), А(С₆Н₆), А(n-С₇Н₁₆) and А(СО₂) (СО₂ concentration = 70,0%.) are mainly defined by its volume and by sizes of entrance windows in cavity. In transition from one cationic form, studied in this work, to another the indicated characteristics in zeolite X practically do not change.

The nature and content of the cation in zeolite Х-BF become apparent in case of small degrees of filling. In the present work such results were obtained for СО₂. From the data of table 5 it is clear that NaX-BF, for which the lower surface acidity is characteristic, adsorb CO₂ when its concentration is equal to 0,03%, that is 1,5-2 times more than other exchange forms of the same zeolites.

Table 6. А (СО2) of zeolites X in different cationic forms Cationic form of zeolite D(Н2О) mg/cm3 D(n-С7Н16) mg/cm3 D(С6Н6) mg/cm3 D(CO2) mg/cm3 NaХ-BF 159 90 88 55 0,64НNaХ-BF 109 65 62 24 0,82КNaХ-BF 139 89 87 54 0,79CaNaХ-BF 143 80 88 49 0,52MgNaХ-BF 138 82 80 48

Table 6 contains the results of the research concerning the influence of type and content of exchange cations in zeolite Х-BF on its adsorptive cubic capacities (mg/sm³) of H₂O – D(H₂O) , n-C₇H₁₆ – D(n-C₇H₁₆), C₆H₆ - D(C₆H₆) and CO₂ – D(CO₂) in running adsorber.

It is obvious that in flow reactor with high speeds of gas stream the differences in values of D(Н₂О), D(С₆Н₆), D(n-С₇Н₁₆) and D(CO₂) for all cationic forms of zeolites X do not exceed experimental error.

Other results were obtained while studying adsorption on decationed form of zeolite X-BF with the degree of exchange of cations Na⁺ for Н⁺ equal to 0,6. The data in tables 2-4 show that adsorptive cubic capacities of НNaХ-BF, especially in flow regime, are considerably reduce. The maximum decrease typical for CO₂ because decating results in the increase of zeolite surface acidity. We can also suppose that removal from the intracrystalline space of cations, that are able to interact with the molecules of adsorbate, reduce the diffusion rate of the latter in the cavity of zeolite.

4. Conclusions

Ion exchange of cations Na⁺ for cations Н⁺, К⁺, Ca²⁺ or Mg²⁺ in granular zeolite binder-free NaХ was studied. This study showed that degree of exchange after first processing and final degrees of exchange are 10,0- 15,0% lower than exchange in fine-grained samples because of lower cations’ diffusion rate and inaccessibility of the part of intracrystalline space in granules, which correspond to integrated crystalline aggregates. It was found out that in granules the maximum values of the degrees of exchange of Na⁺ for Н⁺, К⁺ , Ca²⁺ or Mg²⁺ are different: 0,64; 0,82; 0,79; 0,52 accordingly. After exchange high degree of crystallinity and parameters of porous structure of granules are invariable.

It was found out that granular zeolite Х without binding material in different cationic forms has maximum adsorptive cubic capacities of Н₂О, СО₂, С₆Н₆ and n-С₇Н₁₆ that are 10,0-15,0% lower than maximum adsorptive cubic capacities of fine-grained samples because of inaccessibility of the part of intracrystalline space in granules. The research showed that in zeolite Х-BF during the transition from Na-form to K, Ca or Mg-form with maximum degrees of exchange and in condition of 20ºC and high concentrations of adsorbates there is no significant changes of equilibrium adsorptive cubic capacities of Н₂О, СО₂, С₆Н₆ and n-С₇Н₁₆. If the СО₂ concentration is 0,03% - because of its minimal acidity 1,5-2 times more СО₂ adsorbs on Na-form of the zeolites X than on other exchange forms of the same zeolites. It was also found out that production of decationed form of zeolite Х-BF results in reduction of its adsorption property. For HNa-forms with the degrees of exchange of 0,6 adsorptive cubic capacities of Н₂О and СО₂ in flow reactor reduce for 1,5 and 2,0 times accordingly.