ISSN 0965-5441, Petroleum Chemistry, 2006, Vol. 46, No. 3, pp. 182–190. © Pleiades Publishing, Inc., 2006. Original Russian Text © A.M. Akimbaeva, E.E. Ergozhin, B. Blümich, D.E. Demco, 2006, published in Neftekhimiya, 2006, Vol. 46, No. 3, pp. 204–213.
Diagnostics of Porous Structure and Assessment of Catalytic Activity of Natural Zeolite in Styrene Polymerization Reaction A. M. Akimbaevaa, 1, E. E. Ergozhina, B. Blümichb, and D. E. Demcob a
b
Bekturov Institute of Chemical Sciences, Almaty, 050010 Kazakhstan Institute for Technical Chemistry and Macromolecular Chemistry, Aachen, Germany 1 e-mail:
[email protected] Received October 17, 2006
Abstract—Physicochemical studies of natural zeolite and zeolite samples treated with different solutions of hydrochloric acid were carried out. The nitrogen adsorption technique and the BET model were used for determination of the specific surface area and the pore size distribution. It was shown that the optimal acid modification conditions facilitate the development of catalytically active forms of natural zeolite. DOI: 10.1134/S096554410603008X
Progress in petroleum refining and petrochemical industry is associated with the application of zeolitecontaining catalysts possessing high activity, thermal stability, and selectivity. Crystalline and amorphous aluminosilicates, both natural and synthetic, are active cracking, isomerization, polymerization, etc., catalysts [1–3].
properties of natural sorbents is a topical task from both the theoretical and practical aspects. The objective of this work was to study the pore structure of natural zeolite samples obtained by acid treatment and to evaluate their catalytic activity in the styrene polymerization reaction.
The results of investigations demonstrate the high capabilities of polymerization reactions on solid surfaces. In most cases, the nature of these processes is unclear and may only be guessed on the basis of mechanisms of similar reactions occurring under ordinary conditions. Advances in surface chemistry of these minerals and some experimental data concerning the modification of aluminosilicates are a prerequisite for understanding the nature of active sites and processes on their surface [4–6].
EXPERIMENTAL
Impetus for the industrial application of such processes was provided by the discovery of the high catalytic activity and selectivity of zeolites. From a practical point of view, zeolites are the most important catalysts used in the petroleum refining industry. Natural sedimentary zeolites and their modified forms can catalyze a number of chemical reactions [7, 8]. The stability of high-silica zeolites towards acids facilitates the controlling of their properties by decationation and dealumination under different acid treatment conditions. A large deposit of natural zeolites was explored in Kazakhstan (Shankhanai deposit). Taking into account the simplicity of mining, considerable reserves, and, hence, the relatively low cost of zeolites, it is of interest to study the feasibility of zeolite modification with the aim of improving their molecular-sieve properties. The investigation of adsorptive, structural, and catalytic
Natural zeolite (d = 0.05–0.10 mm) was treated with hydrochloric acid under different conditions, washed up to neutral reaction of wash water, and dried at 130°C. The thermooxidative stability of anion exchangers in air was determined by thermogravimetric analysis (TG). Calcined aluminum oxide was chosen as a standard. Thermal analysis was carried out on a Mettler Toledo Star instrument under the following conditions: a temperature interval of 50–1000°C, a heating rate of 10°C/min, a gram-sized sample mass, α-Al2O3 as a standard, and an air atmosphere. The samples subjected to study were those of air-dried zeolite tuff. The X-ray diffraction analysis (XRD) of zeolite samples was carried out on a DRON-3 X-ray diffractometer (Cu-filtered ëuäα radiation) using a PR-14 M computing device. The specimens were ground to powder and applied on a glass substrate. X-ray diffraction patterns were measured over the θ range of 3° to 30° in the continuous scanning mode by recording the scattering curve on a chart strip. 27Al and 1H NMR spectra of modified samples were obtained on a Bruker-CXP-500 spectrometer by spinning the specimens at the magic angle to the inner magnetic field at a rate of 14 kHz. The specimens were prepared by powdering them in agate mortar. The chemical shifts were measured relative to Al2é3 and tetramethylsilane as external standards. Adsorption and desorption
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Table 1. Change in clinoptilolite chemical composition after acid treatment Clinoptilolite composition, %
Sample no.
HCl, N
1
–
SiO2/Al2O3 SiO2
Al2O3
Na2O
K2O
CaO
MgO
Fe2O3
58.08
15.16
2.30
1.42
4.92
2.53
5.95
3.83
2
3 (20–22°C, 24 h)
58.19
15.09
1.75
1.27
4.02
2.28
5.60
3.86
3
12 (20–22°C, 24 h)
58.86
15.05
1.77
1.25
4.05
2.34
5.54
3.91
4
1.5 (96–98°C, 6 h)
69.16
15.43
1.67
1.18
2.24
1.41
4.95
4.48
5
Concentrated (96–98°C, 6 h)
79.1
6.20
1.24
1.01
0.58
0.29
0.70
12.80
isotherms of vaporized liquid nitrogen with sorbents were recorded with an ASAP-2010 instrument equipped with an automatic system for running the experiment and data processing. Styrene was distilled in a vacuum at 64°/45 mmHg; 20 0.9082; n D 1.5439.
20 ρn
Monomer polymerization was carried out in the presence of a catalyst without other initiating agents, assuming that polystyrene formation as a result of thermal polymerization was negligible under the given experimental conditions. The amount of the catalyst was varied in the range 1–20%. Styrene (or a styrene solution in a solvent) was placed into a flask equipped with a stirrer, a dropping funnel, a reflux condenser, and a thermometer. The flask was thermostated and the catalyst was added. The styrene was polymerized for a certain time at a prescribed temperature. After completion of polymerization, the resultant mixture was dissolved in benzene and centrifuged. The separated catalyst was subjected to triply repeated 10-h treatment with benzene in a Soxhlet apparatus followed by centrifugation.
The possibilities of rapid identification of natural zeolites can be significantly extended by a combined use of analytical techniques, such as XRD and thermographic methods. Taking a diffractogram makes it possible to determine the presence of heulandite-isostructural zeolites even in mixtures. An analysis of a thermogram allows one to qualitatively determine species inside this group. The low-temperature region below 400°C can be scanned for this purpose. According to XRD data, the main phase of zeolite tuff is clinoptilolite-heulandite with maxima characteristic of clinoptilolite [11]. Other phases are montmorillonite, plagioclase, and quartz. Comparison of the results with published data on monophasic clinoptilolite shows that the material in question is close to clinoptilolite in composition but has higher sodium, calcium, and iron contents (Table 1). It was established that zeolite tuff contains about 80% of zeolite of the plate form. Study showed that its Si/Al ratio is 3.83.
0
min 5 10 15 20 25 30 35 40 45 50 55 60
After centrifugation, the benzene solution of polystyrene was poured into alcohol. The precipitated polymer was dried in vacuum at 30°C to a constant weight. The viscosity of the benzene solution of this polymer fraction was measured.
1
The molecular mass of polystyrene was determined by viscometry [9] in solutions of polystyrene samples in benzene. The viscosity was determined at 20°C. The following constant values were used: K = 1.23 × 10–4, α = 0.72.
2
RESULTS AND DISCUSSION Zeolites from the Shankhanai deposit are high-silica clinoptilolite-containing rocks with impurities of quartz, feldspar, and traces of montmorillonite [10]. Their brown-red color indicates the presence of finely dispersed iron oxides. PETROLEUM CHEMISTRY
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50
150
250
350
450
550
650 °C
Fig. 1. Thermal analysis curves for the natural zeolite: (1) TG and (2) DTA.
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Vmesopor, 10–3 cm3/g
ΣVpor, 10–3 cm3/g
Vmicropor, 10–3 cm3/g
5.87
0.44
7.85
0.16
19.41
19.57
2
6.85
0.87
8.14
0.38
19.23
19.61
3
9.90
1.87
9.97
0.84
30.51
31.35
4
108.53
72.62 21.88
35.25
28.85
64.10
5
181.80 105.13 47.93
50.99
71.31 122.30
Smicropor, m2/g
1
Sample no.
SBET, m2/g
Smesopor, m2/g
Table 2. Surface area and porosity of zeolite specimens
The results of thermographic investigation are given in Fig. 1. Differential thermal analysis (DTA) data show that the dehydration of clinoptilolite-containing tuff begins at the lowest temperatures, e.g., 60°C, and proceeds continuously over a broad temperature range up to 400°C. The mass loss at this temperature reaches 3.3%. The total mass loss while heating the zeolite to 700°C was 6.7%. The DTA curve of the virgin zeolite exhibits several endothermic effects with minima at 100, 160, 260, 550, and 660°C. Water release in the zeolite proceeds continuously and smoothly, as follows from the character of the mass loss (TG) curve. The character of DTA and TG curves over the entire temperature range of 50–700°C suggests that there are several forms of bound water in minerals [11]. The presence of two endothermic effects due to the release of adsorbed water indicates that active sites in clinoptilolite microcavities are inhomogeneous in energy. The heating curve of the natural zeolite shows the loss of weakly bound adsorption water and capillary-condensation water at 100°C. The energy of interaction of water molecules with potassium ions is lower, and the endothermic effect at 170°C is due to the desorption of water molecules bound to K+, as well as to the zeolite oxygen framework. The weak endothermic effects displayed by zeolite rock thermograms are due to foreign impurities. The effect at 550°C is attributed to the detachment of hydroxyl groups. As follows from the given experimental data, the initial natural zeolite exhibits a fairly high thermal stability (above 700°C). It is known that the acid treatment of natural mineral sorbents develops their pore structure. In this work, we tested different procedures of acid treatment of natural zeolite with hydrochloric acid. From the standpoint of environmental safety (simplicity of purification and regeneration of spent solutions), hydrochloric acid was chosen as a modifying acid for sorbent treatment, since waste dilute hydrochloric acid is much easier to clean of leaching products and to neutralize.
In the experiments, we varied the acid concentration, temperature, and treatment time. The effectiveness of acid treatment was appraised in terms of the total porosity of a sorbent and its specific surface area. The acid modification of the zeolite with 3 N and 12 N HCl solutions for 24 h (samples 2, 3) at room temperature showed that the silica to alumina ratio was practically the same (Table 1). More severe treatment conditions result in a sharp increase in the SiO2 : Al2O3 ratio. During modification with a 1.5 N HCl solution upon heating (sample 4), clinoptilolite undergoes decationation and dealumination without noticeable destruction of the crystal lattice in comparison to its treatment with concentrated acid under the same conditions (sample 5). During the acid treatment of the mineral, the formation of the zeolite amorphous phase and a change in its cationic composition take place (Table 1). The obtained results show that decationation and dealumination are insignificant under milder conditions of acid treatment. More severe activation conditions result in intensification of these processes, which should be considered as a parallel reaction proceeding at different rates [12]. The removal of exchangeable zeolite cations and the occupation of the free sites by protons have an effect on the cationic density of the framework and can change the size of channel windows. The removal of aluminum from the mineral framework also results in partial degradation of the zeolite crystal structure. Zeolites are systems of great practical importance. For an appropriate choice of sorbents, it is necessary to perform adsorption–desorption measurements. Such measurements are very productive in studying the porestructure morphology of sorbents. These characteristics have not been studied for natural zeolite from the Shankhanai deposit. Thus, we used the nitrogen adsorption technique and the BET model to determine the specific surface area and the pore-size distribution. The existence of macro-, meso-, and micropores on the surface of solids has an effect on their adsorption, diffusion, mechanical, capillary, and other porositydependent properties and determines many important aspects of adsorption and catalytic processes in the presence of these systems. A special role in the aforementioned processes belongs to micropores, which act as molecular sieves in solids. The presence of micropores in adsorbents and catalysts results in a substantial retardation of the transport of an adsorbate or catalyst components. Adsorption isotherms of vaporized liquid nitrogen on natural and modified zeolites are given in Fig. 2. They are typical of the BET model. The values of the specific surface area (SBET, m2/g) for the initial and leached zeolites are 5.87, 6.85, 9.90, 108.53, and 181.80 m2/g (samples 1, 2, 3, 4, and 5, respectively), depending on acid treatment conditions (Table 2). PETROLEUM CHEMISTRY
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Adsorbate volume, cm3 /g
Re
12
1
10
10
8
8
6
6
4
4
2
2
0
0
24 20
4
40
16
30
12
20
8 4
10
0
0
90 80 70 60 50 40 30 20 10 0
2
50
3
185
5
0.2
0.4
0.6 P / P0
0.8
0.2
0.4
0.6 P /P0
0.8
1.0
1.0
Fig. 2. Nitrogen adsorption–desorption isotherms on zeolite specimens: (1) natural zeolite (sample 1) and the same after treatment with (2) a 3 N HCl solution, 22–25°C (sample 2); (3) a 12 N HCl solution, 22–25°C (sample 3); (4) a 1.5 N HCl solution, 96–98°C (sample 4); and (5) concentrated HCl, 96–98°C (sample 5).
Sample 1 (natural zeolite) is mesoporous according to the isotherm data; it also contains some micropores, since nitrogen is adsorbed to a certain extent at low pressures (p/pS is less than 0.1). This is also indicated by the t-plot of natural sorbent (Fig. 3). The treatment of natural zeolite with a 3 N HCl solution (sample 2) leaves the surface characteristics of the sorbent practically unchanged (Table 2). However, various impurities present in the natural specimen are washed out as a result of acid activation. For example, zeolite modification with a 12 N hydrochloric acid solution (sample 3) increases the total pore volume by a factor of about 2 and the specific surface area by a factor of more than 1.5. The treatment under more severe conditions turned out to be the most effective. The porosity and the specific surface area are considerably increased by the acid activation of the mineral with a 1.5 N hydrochloric acid solution and heating (sample 4). The specific surface PETROLEUM CHEMISTRY
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area of sample 5 is as high as 181.80 m2/g. Such acid treatment increases the sorbent surface area accessible to adsorption more than 20 times. Leaching is even more effective for increasing zeolite porosity; the total pore volume in the virgin sample is 19.57 × 103 cm3 /g, whereas chemical treatment increases the zeolite porosity by a factor of 6—to 122.30 × 103 cm3 /g (sample 5). The pore size distribution was determined by the nitrogen desorption technique. Distribution curves plotted as the total pore volume versus their diameter can show which pores dominate a porous sorbent. Recall that, according to the IUPAC classification [13], macropores have an average diameter greater than 500 Å, micropores have a diameter smaller than 20 Å, and mesopores have widths between 20 and 500 Å. The differential size spectra of the initial and decationated zeolites are given in Fig. 4. As is seen, the natural zeo-
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Adsorbate volume, cm3 /g
2.0
1
2.0
1.6
1.6
1.2
1.2
0.8
0.8
0.4
0.4
0
0
3.0 2.5 2.0 1.5 1.0 0.5 0
3
60
5
2
4
35 30 25 20 15 10 5 0
1 2 3 4 5 Absorption film width, Å
50 40 30 20 10 0
1 2 3 4 5 Absorption film width, Å
Fig. 3. t-Plots of zeolite specimens: (1) natural zeolite (sample 1) and the zeolite is after its treatment with (2) a 3 N HCl solution, 22–25°C (sample 2); (3) a 12 N HCl solution, 22–25°C (sample 3); (4) a 1.5 N HCl solution, 96–98°C (sample 4); and (5) concentrated HCl, 96–98°C (sample 5).
lite possesses a monodisperse structure with a high maximum in the mesopore region (d ~ 30 Å). Its acid modification with 3 N hydrochloric acid under ordinary conditions (sample 2) negligibly changes the structural characteristics of the sorbent (Table 2). An increase in the acid concentration under the same conditions (entry 3) results in the development of micropores (there is a rise at d ~ 20 Å), with the surface area and the volume of micropores increasing by factors of more than 4 and 5, respectively. The differential size spectra of such samples are characterized by narrow distribution. The zeolite subjected to acid treatment with heating (sample 4) is characterized by bimodal porosity: the volume of its mesopores increased by a factor of 1.5, and the microporosity became 217 times the initial value. Further activation of natural zeolite results in the augmentation of micro- and mesoporous structure, with an extention of the range of the mesopores (sample 5). As follows from Fig. 4, the curves of size distribution of pore volume have maxima in the range 20–60 Å for
certain samples. Thus, differential distribution curves of pore volume as a function of the effective pore radius indicate that the test zeolite samples have either monoor bidisperse structure depending on the conditions of acid treatment. The adsorption and desorption isotherms of the studied specimens differ from each other, a result that is due to the phenomenon of capillary condensation. For sample 5, the portion of the curve relevant to capillary condensation has a greater slope in comparison with other parts. This fact proves the presence of pores with different radii in which gas adsorption and desorption successively take place. In this case, narrow capillaries are filled even at low relative pressures, whereas the filling of wide pores requires higher pressures. The magnitude of the specific surface area is of great importance for sorbents and catalysts. Its values calculated from the t-plot (mesopore surface) were 7.85, 8.14, 9.97, 21.88, and 47.93 m2/g for zeolite samples 1– 5, respectively, thus indicating the development of PETROLEUM CHEMISTRY
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DIAGNOSTICS OF POROUS STRUCTURE AND ASSESSMENT OF CATALYTIC ACTIVITY 0.0010 0.0008
0.0008
0.0006
0.0006
0.0004
0.0004
0.0002
0.0002
0 Derivative of pore volume, cm3 /(g Å)
0.0010
1
100
200
300
400
500
600
0
187
2
100
200
300
400
500
600
0.0024
0.0008
0.0020
3
0.0006
4
0.0016 0.0004
0.0012 0.0008
0.0002 0.0004 0
100
200
300
400
500
600
0
50
100 150 200 Pore diameter, Å
250
300
0.0024 0.0020
5
0.0016 0.0012 0.0008 0.0004 0
100
200 300 400 Pore diameter, Å
500
600
Fig. 4. Pore diameter distribution curves: (1) natural zeolite (sample 1) and the zeolite after its treatment with (2) 3 N HCl, at 22– 25°C, (3) 12 N HCl at 22–25°C, (4) 1.5 N HCl at 96–98°C, and (5) concentrated HCl at 96–98°C (samples 2–5, respectively).
microporous structure in the mineral during acid treatment. Such a change in the pore structure of zeolites activated under more severe conditions in comparison with other specimens was exclusively due to the acid treatment of the mineral, in which the action of the acid resulted in a decrease in the amount of metal oxides on the one hand and an increase in the silica content on the PETROLEUM CHEMISTRY
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other hand. As a consequence, the SiO2/Al2O3 molar ratio increased from 3.83 to 12.80 (Table 1). The process of aluminum removal by a strong inorganic acid obviously proceeds as a cation exchange for hydrogen atoms in the first step followed by the replacement of four OH groups instead of the tetrahedron [4]:
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Si O Si O Al O Si
+ H3O+ + 3HCl
O Si
Si O Si OH
H
HO Si
H
+ Al3+ + 3Cl– + H2O…
O Si
Dealumination is accompanied by an increase in the proportion of single AlO4 tetrahedrons, whose presence is associated with the existence of the strongest acid sites. It has been noted [4] that the catalyst activity as a function of the number of acid sites is described by curves with maxima. At a certain degree of dealumination, all aluminum atoms become isolated and, hence, the strength of acid sites does not increase. A further decrease in the aluminum content can take place only by removing isolated tetrahedrons, and the total activity must decrease. At the same time, the removal of aluminum by acid treatment simultaneously results in a more open structure and makes the active surface more
accessible, although it decreases the number of catalytically active sites. The nature of active sites is still under debate [4]. According to IR data, the following groups are responsible for catalytic activity: (1) hydroxyl groups that act as Brönsted acids; (2) centers with tri-coordinated aluminum, which favor the appearance of Lewis acid sites; and (3) accessible cations. The catalytic activity of zeolites with multicharged cations is usually associated with the presence of Brönsted acid sites. For example, Bierenbaum et al. [14] explain the nonmonotonic dependence of the mordenite activity on the Si/Al ratio, which is repeatedly mentioned in the literature, by the fact that dealumination facilitates the diffusion of organic compounds, whereas the number and the strength of acid sites play an insignificant role. In general, studies on dealuminated zeolites show that zeolite activity increases with an increase in the Si/Al ratio. We have found (Fig. 5) that the NMR spectra of modified zeolite specimens (samples 4 and 5) also exhibit signals at 3 and 11 ppm, which characterize aluminum atoms in the octahedral oxygen environment, along with signals of tetra-coordinated aluminum atoms (56 ppm). The decationation of the mineral increases the intensity of the signal due to octa-coordinated aluminum atoms. In fact, the majority of aluminum atoms remain in the zeolite framework in spite of decationation. An increase in the catalytic activity of the natural zeolite with a rise in the Si/Al ratio results simultaneously in the enhancement of total Brönsted acidity [4]. Thus, the amount of tetrahedrons with aluminum atoms decreases, and the number of the accessible OH groups increases. The protonation of a substrate results in the formation of a carbonium ion, which reacts with an unprotonated substrate molecule to give another carbonium ion. Most frequently, the carbonium ion is formed as a result of heterolytic C–X bond cleavage (X is an electronegative group) and protonation of the unsaturated bond by a Brönsted acid site: R–Cç=ëç2 + ç+Ä–
2
1 70
60
50
40
30
20 10 ppm
0
–10 –20 –30
Fig. 5. 27Al NMR spectra of zeolite specimens after acid treatment at 96–98°C with (1) 1.5 N HCl (sample 4) and (2) concentrated HCl (sample 5).
R–C+ç–ëç3 + Ä–.
The occurrence of both reactions is favored by polar solvents with a high ionizing power. Carbonium ions are formed on the catalyst surface as a result of monomer adsorption. Their formation during hydrocarbon adsorption on the surface of an aluminosilicate catalyst was proven by spectroscopic studies [15]. –
The walls of zeolite channels are formed by AlO 4 and SiO4 tetrahedrons. The negative charge is distributed over all Al–O bonds inside the large complex anion AlO4 and is compensated by a concentrated positive charge of cations capable of ion exchange. Such a structure is analogous to a surface that bears hydroxyl groups with protonized hydrogen (silica surface). For molecules with peripherally concentrated electron denPETROLEUM CHEMISTRY
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Table 3. Results of styrene polymerization in the presence of activated natural zeolite (HClconc, 96–98°C, 6 h) 0.5
1
5
10
15
20
2
2
2
2
2
2
Intrinsic viscosity [η]
0.05
0.01
0.10
0.18
0.23
0.24
Molecular mass
4200
9800
11010.3
24 908.2
35010.4
37142.4
15
20
24.65
54.23
56.25
63.6
Time, h
Polymer yield, %
sity, specific adsorption is observed in this case [16]. In a zeolite channel, an adsorbate molecule also experiences very strong nonspecific interaction with many atoms that form the channel walls and stronger inductive interaction due to a large effective charge of the cation (relative to the silica hydroxyl hydrogen). It was noted [17] that zeolites have an especially high adsorption energy for organic molecules with peripheral functional groups containing π bonds or atoms with lone electron pairs. In this case, stronger (donor–acceptor) interactions—e.g., between π electrons of benzene (base) and zeolite protons (Brönsted acid site)—along with the specific dispersion interaction between aluminosilicate and sorbate active fields are possible. It is likely that styrene molecules can also form such complexes. The surface structure of zeolites strongly depends on the activation conditions; therefore, their catalytic properties may be varied correspondingly. To determine whether the test zeolite specimens are useful as catalysts, we investigated styrene polymerization in their presence. The natural zeolite in question is enriched in iron ions (which are responsible for the chocolate color of the mineral). These ions are not removable by treatment with 3 N or 12 N hydrochloric acid under ordinary conditions (samples 2, 3). These samples do not exhibit catalytic activity in styrene polymerization (15 h, 65°C) (Table 3). The acid activation of the mineral by boiling in a concentrated acid solution leads to a more profound removal of iron ions (the zeolite takes a light color) and the appearance of catalytic activity (samples 4, 5). However, over the zeolite treated with a 1.5 N inorganic-acid solution, noticeable polymer traces appear after long-term polymerization (longer than 10 h). Thus, under comparable conditions, the reaction proceeds better (the polymer yield is higher and the reaction time is shorter) over zeolite sample 5. It is the volume of zeolite cavities available for polymer molecules that seems to determined the occurrence of reactions and the product molecular mass [18]. The dependence of the activity of the natural zeolite on the degree of decationation and dealumination is obviously explained by the deactivating action of iron ions, as well as by the accessibility of active sites. The change in the catalytic properties of the natural zeolite can also be explained by structural peculiarities. PETROLEUM CHEMISTRY
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The IR spectra of the obtained polymer and bulk polystyrene synthesized in the conventional manner are practically the same (Fig. 6). According to published data [14], noticeable styrene polymerization in the presence of acids and halide catalysts (HCl, HBr) is observed in solutions with a permittivity of more than 8, and very slight polymerization is in chloroform (D = 5.9) and butyl chloride (D = 7.3). It is believed that a higher solvent polarity favors the formation of the carbonium ion that initiates polymerization. Styrene polymerization does not occur in the low-permittivity solvents benzene and toluene (D = 2.3), presumably owing to the fact that neither HCl nor HBr form carbonium ions in the reaction with the monomer in these solvents. In solvents with low permittivity, unlike in the case of more polar solvents, ion pairs (polarized complexes) do not appear from weak acids and bases, and only addition products, e.g., hydrogen-bonded adducts, of a base to an acid are formed. The test aluminosilicate catalyst (sample 5) effects styrene polymerization in toluene (reaction time, more than 6 h; styrene : solvent volume ratio, 2 : 3). This
1 Absorbance, %
Catalyst concentration, % of the monomer mass
2
3000
2000
1500
1000
700 λ, Òm–1
Fig. 6. IR spectra of polymers: (1) bulk polystyrene and (2) low-molecular-mass polystyrene.
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AKIMBAEVA et al.
difference is presumably due to the presence of stronger acid sites whose acidity seems to be determined by the existence of strong electrostatic fields in the polar crystalline porous system of the zeolite. The formation of low-molecular-mass polystyrene is explained by the fact that the electron-acceptor properties of aluminosilicate minerals result in the formation of the carbonium radical ion at the initial stage of monomer polymerization and this ion readily dimerizes via the cationic mechanism, yielding di-, tri-, and tetramers [19]. This assumption is consistent with the known data [20] concerning the sorption of aromatic compounds on acid sites of mineral surfaces with the formation of radical ions. In summary, the acid treatment of the natural zeolite used in this work makes it possible to obtain specimens with certain structural and adsorption properties. This modification is of undoubted interest for the development of theoretical principles of controlled change in the useful properties of natural sorbents with the aim of meeting the main requirements for porous materials. The chemical and thermal stability of zeolites, as well as their ion-exchange properties, open opportunities for multiply repeated regeneration of zeolite-based catalysts with controllable properties and for the commercialization of these catalysts, as they can replace their synthetic analogues, whose manufacture is costly and requires the use of sophisticated technologies. REFERENCES 1. I. N. Slepneva, K. N. Zhdanova, L. E. Latysheva, and F. K. Shmidt, Zh. Fiz. Khim. 77, 317 (2000). 2. S. S. Safronova, L. M. Koval’, E. B. Chernov, and V. V. Bolotov, Zh. Fiz. Khim. 79, 55 (2005). 3. P. Canizares, A. Carrero, and P. Sanchez, Appl. Catal., A 190, 93 (2000).
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PETROLEUM CHEMISTRY
Vol. 46
No. 3
2006
