PHYSICOCHEMICAL CHARACTERISTICS AND CARBON DIOXIDE SORPTION PROPERTIES OF NATURAL ZEOLITES

The physicochemical characteristics of natural zeolites from the Tayzhuzgen and Shankanay deposits have been studied by BET, SEM, XRD, and FT-IR spectroscopy. Sorption properties of natural zeolites for carbon dioxide were studied in the ﬂ ow mode in the temperature range of 50-300 °C. Among the studied samples, the most active in the process of capturing CO 2 was the natural zeolite of the Tayzhuzgen deposit, at a temperature of 300 °C, the adsorption capacity for carbon dioxide was 27.4%. The high CO 2 adsorption capacity of natural zeolite from the Tayzhuzgen deposit is due to its speciﬁ c surface area and high Si/Al ratio.


Introduction
The growth of human resources, the high demand for fossil fuels due to the development of industry have led to an increase in anthropogenic emissions of carbon dioxide into the atmosphere.The largest sources of greenhouse gas emissions are primarily the combustion of coal, oil and natural gas.In this regard, many studies have been carried out and various approaches have been proposed to reduce carbon dioxide emissions.There are several technologies for capturing carbon dioxide, including physical absorption, chemical adsorption, membrane separation, etc. [1][2][3].Among them, physical absorption and chemical adsorption are the most widely used methods in CO 2 capture.However, the physical absorption method is carried out at high pressure and low temperature, which is the main disadvantage of this method [4,5].
One of the most eff ective ways to reduce the amount of carbon dioxide is its capture (adsorption), since this method is simple to perform and allows high CO 2 adsorption to be achieved [6,7].In addition, the regeneration of sorbents and the ease of temperature change during adsorption show the eff ectiveness of this method.
The literature contains adsorbents widely used to capture carbon dioxide, such as activated carbon [8], carbon materials [9] and mesoporous silica [10], zeolites [11][12], and other adsorbents [13].Among these adsorbents, zeolites are eff ective sorbents in terms of textural and structural properties and high adsorption capacity in the process of carbon dioxide capture, as well as controlled physicochemical properties [14][15].Various zeolite formulations, including synthetic zeolites and modifi ed zeolites, have been studied in carbon dioxide capture.The cost of synthetic zeolite varies from 100 to $600 per ton, depending on its composition and processing, as well as its market price [16].Natural zeolites are of great interest to researchers due to their absorbing properties of various substances.They have eff ective sorption qualities for capturing the greenhouse gas CO 2 and are also an alternative to synthetic sorbents due to their low cost, effi ciency and availability.In this regard, natural zeolites are suitable for use in CO 2 adsorption processes and are universal sorbents in adsorption and catalysis processes.
Two large zeolite deposits are known in Kazakhstan -Tayzhuzgen, located in the Tarbagatay region of the East Kazakhstan region and Shankanay, located in the Kerbulak region of the Almaty region.The Tayzhuzgen deposit is larger than Shankanay, its https://doi.org/10.18321/cpc21(2)81-87reserves are 7.1 million tons.The natural zeolites of these deposits were used in agriculture, as additives in the food of animals and poultry, as well as to increase the yield of vegetables and grains [17,18] in which the zeolites of the Tayzhuzgen and Shankanay deposits were studied as sorbents for the sorption of lead and cadmium ions from aqueous solutions.
In our work, natural zeolites from these deposits were fi rst studied as sorbents for capturing carbon dioxide.
It is known that the fl ue gases of various enterprises mainly contain carbon dioxide, nitrogen, water vapor, etc.The fl ue gas released from the chimney has a high temperature, above or slightly below 300 °C [19].In this regard, the purpose of this work is to study the sorption properties of natural zeolites of the Tayzhuzgen and Shankanay deposits in terms of carbon dioxide at high temperatures and their physicochemical characteristics.

Experimental part
The objects of study are natural zeolites of the Tayzhuzgen and Shankanay deposits, which are designated in the article as zeolite (T) and zeolite (Sh), respectively.Physico-chemical characteristics of natural zeolites were studied by X-ray phase analysis, scanning electron microscopy, Brunauer-Emmett-Taylor (BET) and FT-IR.The surface area of the studied samples was determined by the method of low-temperature nitrogen adsorption at a temperature of -196 °C on an automatic BEL Japan Inc. and thermal desorption of argon on a BELSORP-mini II instrument.Zeolite morphology was studied by scanning electron microscopy (SEM) on a JEOL JSM-6390 LA instrument with a JED 2300 energy dispersive X-ray detector radiation with CuKα radiation and a graphite monochromator on a diff racted beam.
Natural zeolites have been tested as sorbents for capturing carbon dioxide in the temperature range of 50-300 °C.Before testing, natural zeolites were mechanically crushed in a mortar and then sieved on a molecular sieve to a particle size of 0.25-0.5 mm.A small amount of water was added to the obtained samples of natural zeolites, the resulting paste was placed on a mini-form (Fig. 1), which produces granules in one technological cycle of exactly the same shape (cylindrical, Ø2.7 mm, h = 2.7 mm).The paste was calcined in a muffl e furnace at a temperature of 600 °C for 5 h at a rate of 2 °C per minute.
The sorption properties of natural zeolites were studied in a laboratory fl ow unit (Scheme 1).The installation consists of a reactor, a thermocouple, an electric furnace, a control unit and a control unit for supplying initial gases.For experiments, CO 2 was used as an adsorbate (Ikhsan Technogaz LLP, purity 99.99%), as well as He carrier gas for purging the line from the reactor to the chromatograph (Ikhsan Technogaz LLP, purity 99.99%).The reactor was made of quartz glass with an inner diameter of 10mm and a length of 25 cm.The sorbent was weighed in a volume of 2 ml and placed in a fi xed bed reactor.Adsorption of carbon dioxide on natural zeolites (T) and (Sh) was carried out in the temperature range of 50-300 °C for 30 min, desorption -at a temperature of 600 °C.

Results and discussion
The elemental composition of natural zeolites of the Tayzhuzgen and Shankanay deposits, expressed in the form of oxides, was determined using X-ray fl uorescence spectroscopy, the results are presented in Tables 1 and 2. The elemental composition of the zeolite of the Tayzhuzgen deposit is presented in a number of works by Kazakh scientists [20,21], the elemental composition of our sample diff ers slightly from these data.
It is known [22] that an important factor in the study of the structure of zeolites is the ratio of silicon oxide and aluminum (Si/Al).High-silicon zeolites   with a high Si/Al ratio have the maximum absorption eff ect.According to the data presented in Tables 1  and 2, the Si/Al ratio for the zeolite of the Tayzhuzgen deposit is 5.6, while for the zeolite of the Shankanay deposit it is 2.9.The specifi c surface area of the samples was estimated by the low temperature nitrogen adsorption (BET) method (Table 3).
As can be seen from Table 3, the specifi c surface of the natural zeolite of the Tayzhuzgen deposit is 11.12 m 2 /g, the specifi c surface of the zeolite of the Shankanay deposit is 5.61 m 2 /g.
To obtain data on the morphology of the catalyst particles, the samples were analyzed using scanning electron microscopy.Figure 2 shows SEM micrographs of natural zeolites.It is known [23] that zeolites mainly have a complex microsurface relief formed by microcrystals and aggregates, represented in most cases by a fi nely dispersed mass.On the micrographs of samples of natural zeolites from the Tayzhuzgen and Shankanay deposits, one can observe accumulations of crystals of a monometric shape.A fi nely dispersed crystalline structure is observed on the surfaces of zeolite samples.
The phase composition of natural zeolites (T) and (Sh) was studied by X-ray phase analysis, the results are shown in Figure 3.
On the diff raction patterns of zeolites, there is practically no amorphous background and broadening of peak lines, which indicates a high degree of crystallinity and homogeneity of the sample.According to XRD results, zeolites from the Tayzhuzgen and Shankanay deposits are classifi ed as clinoptilolite aluminosilicates.
Zeolites from the Tayzhuzgen and Shankanay deposits were also evaluated using FT-IR spectroscopy.FT-IR spectra of samples of natural  zeolites are shown in Fig. 4. FT-IR spectra of natural zeolites are in the range of 500-4500 cm -1 .
The spectra of both zeolites show intense peak at 1028 cm -1 , which correspond to internal deformation and antisymmetric vibrations of Si-O bonds inside SiO 4 tetrahedra.In addition, absorption bands are observed in the region of 3800-3400 cm -1 , which correspond to symmetric and antisymmetric О-Н The CO 2 sorption properties of natural zeolites from the Tayzhuzgen and Shankanay deposits were tested under dynamic conditions in a fl ow mode.Carbon dioxide adsorption was carried out at various temperatures of 50, 100, 150, and 300 °C, CO 2 desorption at 600 °C.The results obtained are shown in Figure 5.
As can be seen from Figure 5, the CO 2 sorption capacity of natural zeolites (T) and (Sh) increases with adsorption temperature.The sorption capacity of the zeolite from the Tayzhuzgen deposit increases from 22 to 27.4%, from the Shankanay deposit from 12.4 to 18.3%.The natural zeolite of the Tayzhuzgen deposit demonstrates a higher sorption capacity for CO 2 in the entire temperature range compared to the zeolite of the Shankanay deposit.

Conclusion
Thus, the physicochemical characteristics and CO 2 adsorption properties of natural zeolites from the Tayzhuzgen and Shankanay deposits were studied.
The specifi c surface area, morphology, chemical and phase composition of zeolites have been studied.It was determined that the specifi c surface of the natural zeolite of the Shankanay deposit is 5.61 m 2 /g, of the Tayzhuzgen natural zeolite -11.12 m 2 /g.A fi nely dispersed crystalline structure is observed on the surfaces of zeolite samples.It is shown that the zeolites of the Tayzhuzgen and Shankanay deposits belong to clinoptilolite aluminosilicates.The molar ratio of SiO 2 /Al 2 O 3 for the zeolite of the Shankanay deposit is 2.9, while for the Tayzhuzgen zeolite it is 5.6.It has been established that the zeolite of the Tayzhuzgen deposit has a better sorption capacity compared to the zeolite of the Shankanay deposit.
The sorption capacity for CO 2 of the zeolite of the Tayzhuzgen deposit at a temperature of 300 °C is 27.4%, while the sorption capacity of the zeolite of the Shankanay deposit is 18.3%.The high sorption capacity of the zeolite from the Tayzhuzgen deposit is due to its specifi c surface area and high SiO 2 /Al 2 O 3 ratio.
The results of the study showed that the zeolite of the Tayzhuzgen deposit can be used as a sorbent for capturing CO 2 at high temperatures.Further research will be aimed at increasing the sorption capacity of natural zeolite by modifying it with oxides of alkali and transition metals.

Fig. 1 .
Fig. 1.Mini-form for obtaining zeolites in the form of granules.

Table 1
Chemical

Table 2
Chemical

Table 3
Specifi c surface area of natural zeolites