Possibility of application of high silicon keolitic catalyst with thermal and mechanochemical treatment for catalytic flavoring of the propane-butane fraction

. The studies were carried out in a flow catalytic device in the stationary phase of the catalyst (catalyst volume 6 cm3), at 450-6000C, at normal atmospheric pressure (P = 0.1 MPa), was carried out under conditions in which the volumetric velocity of the initial gas mixture was 600-1000 h-1. Qualitative and quantitative composition of propane-butane fraction and reaction products were analyzed on the chromatograph "Chromatic-Crystal 5000M" under the following favorable conditions: separation of gaseous products was carried out in a column thermal conductivity detector (TCD) with a length of 3 m and a diameter of 3 mm filled with 8% NaOH / Al2O3. The separation of liquid products was carried out in a DV-1 capillary quartz column (30 m x 0.25 μm), and detection was carried out in a flame ionization detector. Check the acidity of the catalysts. The acid characteristics of the samples were studied by the method of the programmed desorption of ammonia in the automatic chemisorption analyzer “USGA-101”. X-ray phase analysis: X-ray examination of the samples (X-ray phase analysis) was performed on a diffractometer XRD-6100 (Shimadzu, Japan). Phase identification was performed based on radiographs of individual components (JSPDS card index) based on data from the literature. The purpose of this work is to study the possibility of using thermally and mechanochemically treated high-silicon zeolite catalysts in the catalytic aromatization of the propane-butane fraction and acid properties.


Introduction
Several scientists are researching the catalytic synthesis of aromatic hydrocarbons by chemical processing of propane-butane fractions [1][2][3][4][5][6]. It is known from the literature that Mo-preserving catalysts have high catalytic activity in the aromatization reaction of propanebutane fractions without the participation of oxidizers [7,8].
High-silicon zeolites are widely used as catalysts in the chemical processing of propanebutane fractions into aromatic hydrocarbons [6,7,8,9,10,11]. Chemical and thermal processing of zeolites used to increase the selectivity of aromatic hydrocarbons is effective. In the conversion of low molecular weight hydrocarbons to aromatic hydrocarbons, high silicon zeolites are modified with various metals [12,13,14,15,16,17,18]. In this study, the factors influencing the reaction rate: temperature, contact time, partial pressure of the reagents, etc. (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к were studied in the presence of a catalyst and optimal reaction conditions were determined.

Methods
The studies were carried out in a flow catalytic device in the stationary phase of the catalyst (catalyst volume 6 cm 3 ), at 450-600 0 C, at normal atmospheric pressure (P = 0.1 MPa), under the conditions of the initial gas mixture at a volume velocity of 600-1000 h -1 [20-21]. Qualitative and quantitative composition of propane-butane fraction and reaction products were analyzed on the chromatograph "Chromatic-Crystal 5000M" under the following optimal conditions: separation of gaseous products was carried out in a column thermal conductivity detector (TCD) with a length of 3 m and a diameter of 3 mm filled with 8% NaOH/Al2O3. The separation of liquid products was carried out in a DV-1 capillary quartz column (30 m x 0.25 μm), and detection was carried out in a flame ionization detector. Check the acidity of the catalysts. The acid characteristics of the samples were studied by the method of them programmed desorption of ammonia in the automatic chemisorption analyzer "USGA-101". X-ray phase analysis: X-ray examination of the samples (X-ray phase analysis) was performed on a diffractometer XRD-6100 (Shimadzu, Japan). Phase identification was performed based on radiographs of individual components (JSPDS card index) based on data from the literature. The acidic properties of the catalysts were investigated by thermogrammed desorption of ammonia. The structure and position of the active centres of the catalyst were determined by electron microscopy, and the surface area was determined by the BET method. BET equation Where p -is the adsorption pressure, Pa; P0 -saturation pressure, Pa; V-adsorption volume, cm 3 ; Vm -volume adsorbed in the molecular layer, cm 3 ; C-is equal to the constant that characterizes the heat of adsorption.

Resuits and disscussion
An important issue in the conversion of propane and butane to aromatic hydrocarbons is to increase the yield of aromatic hydrocarbons and reduce the formation of methane and ethane. Figure 1 shows the composition of the gaseous products of the aromatization reaction of propane and butane in zeolite catalysts. The aromatization reaction of propane and butanes in zeolite catalysts shows the composition of the liquid products and the catalyst yield (T = 550 0 C) in Figure 2. As can be seen from Figure 2, the main products in the catalytic conversion of propane and butane are C1-C5 gaseous alkanes, C2-C4 alkenes, and liquid aromatic hydrocarbons. The catalyst is a mixture of aromatic hydrocarbons (benzene, toluene and xylenes-BTX-fraction) and small amounts of alkylbenzenes, naphthalene and alkyl naphthalene. Gaseous products consist mainly of methane and ethane, as well as small amounts of hydrogen, C3-C5 alkanes and C2-C4 alkenes. From the above, it can be seen that the BTX fraction is formed more as a result of the catalytic conversion of propane to butane.
In the study, the possibility of using propane-butane fraction in the catalytic aromatization reaction in the presence of a catalyst (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к was considered. The results obtained are shown in the figure below ( Figure 3).

Figure 3. The effect of temperature on the composition of gases
The relationship between the yield and temperature of semi-liquid (aromatic) hydrocarbons is shown in Figure 4. As can be seen from the figure, the amount of benzene and toluene increases with increasing temperature, while the total amounts of xylene and ethylbenzene decrease, albeit slightly. This is due to the formation of coke in the catalyst layers as the temperature rises. The acidic properties of the catalyst surface were studied by the TPD (temperatureprogrammed desorption) method. As the heat treatment time of the samples increases, the "strong" acid centres are broken down and the concentrations of the acid centres decrease. It is known that the amount of acid centres of a catalyst determines its catalytic activity, and in many cases, the amount of acid centres decreases with increasing processing time. When treated with a catalyst containing (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к corresponds to a hightemperature peak desorbed ammonia concentration (180 μmmol/g), which characterizes the desorption of "strong" acid centres. When we increase the heat treatment time for a catalyst containing (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к to 48 hours, the amount of desorbed ammonia decreases to 145 μmol / g, so the concentration of acidic centres also decreases accordingly. The results of the study of the acidic properties of the heat-treated (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к catalyst at high temperature are shown in Figure 5.

Figure 5. Acid properties of Zn-Zr / HSZ sample treated under high-temperature conditions
Where TI and TII are the maximum temperatures of the low and high-temperature peaks on the thermal desorption curves; Concentration of CI, CII, Cc-weak and strong acid centres and their sum accordingly. The use of a sufficiently simple and efficient method of modification of the catalyst, its pretreatment under high-temperature conditions, allows to target the properties of zinc and zirconium-containing zeolite and at the same time control the conversion of gaseous hydrocarbons. Figure 6 below shows the effect of temperature and duration of heat treatment time on the acidity centres of HSZ. As can be seen from Table 3, when the temperature rises from 600 0 C to 700 0 C and the duration of heat treatment increases from 4 to 24 hours, the concentration of both acid centres, mainly the "strong" acid centres, decreases monotonously.
The reason for this is the dehydroxylation of the zeolite surface, at which time 1 L-center is formed out of 2 B-centers. Thus, based on the experimental results, it can be concluded that the optimum temperature of thermal processing is 600 0 C.
The process of mechanochemical treatment of the synthesized high-silicon zeolites was carried out in a ball vibrating and planetary mill. Mechanochemical treatment allows not only to improve the method of synthesis of HSZ but also to effectively change its operational characteristics.
The crystallinity of mechanically treated zeolites was calculated chromatographically by IR spectra and the specific surface area by low-temperature adsorption of nitrogen ( Figure  7).

Fig. 7. Characteristics of thermally processed catalysts over different times
The crystallinity level and specific surface area of zeolites do not change significantly when processed in a vibrating mill for up to 48 hours. However, when treated for more than 48 hours, a decrease in the above-mentioned values is observed. When HSZ are processed in a planetary mill, the crystallinity level also decreases sharply after a few minutes. As can be seen from the table, after 5 minutes of processing, the crystallinity level is reduced to 54% and the specific surface area is reduced by about 3 times. Mechanic chemical treatment also has a significant effect on the acidic properties of the zeolite catalyst. The results obtained are presented in Figure 8. Fig. 8. Acidic properties of mechanically and chemically treated zeolite catalysts in vibratory (hours) and planetary (minutes) mills Types of I and II-acid centres, maximum Imax-peak temperature, the total concentration of CS-acid centres, activation energy of Ea-ammonia desorption.
All zeolite samples processed in a vibrating mill have 2 types of acid centres: weak acidic centres where ammonia desorption is observed at 100-320 0 C and strong acidic centres characterized by desorption of ammonia in the 320-600 0 C area.
Changes in the structure and acidity of the zeolite after mechanochemical treatment lead to changes in the catalytic properties of the catalyst. It can be seen that in all samples of the (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к catalyst a large amount of benzene-toluene-xylene fraction is formed and its share in the liquid product is more than 68%. In this case, the amount of catalyst (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к increases as the heating temperature increases. Therefore, based on experimental data obtained from the catalytic aromatization of propane, it can be concluded that the treatment of a catalyst with a content of (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к at a temperature of 650 0 C leads to an increase in the total and aromatizing activity of the catalyst during propane conversion. however, after treatment at 850 0 C, the activity of the catalyst containing (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к is significantly reduced, while maintaining the high selectivity of the catalyst relative to aromatic hydrocarbons.

Conclusion
Thus, by catalytic aromatization of propane-butane fractions, the catalytic activity of various catalysts on the reaction yield in the reaction of aromatic hydrocarbons and liquid fuels was studied. Experiments have shown that the best modifying additives are Zn, Zr and Mo. The conversion of propane starts at 450 0 C and reaches 100% when it reaches 600 0 C. Aromatic hydrocarbons are formed in sufficient quantities at 500 0 C and a maximum value of 52.5% is reached at 600 0 C. Based on the results obtained, it was proved that the conversion of butane to aromatic hydrocarbons is easier than that of propane, and at 550 0 C the conversion of butane is 100%, while the yield of aromatic hydrocarbons is 47%.
The physicochemical and texture characteristics of the catalyst containing (MoO3)x•(ZnO)y•(ZrO2)z•(В2О3)к were studied and its application in the catalytic aromatization of propane-butane fraction was investigated. As a result of experimental experiments, it was found that the acidity centres of high-silicon zeolites decrease with increasing treatment time.