Catalytic Conversion of Light Hydrocarbons into Aromatic Hydrocarbons over Modified Zeolite Catalysts

The process of conversion of light alkanes into aromatic hydrocarbons on zeolite catalysts, modified with Zn, La, Fe and Zr was investigated in this work. The catalysts were tested in the conversion process of propane-butane and propane-propylene mixtures at atmospheric pressure, variation of temperature and space velocity (SV). It is observed that the maximum amount of aromatic hydrocarbons (34.6%) are formed in the conversion of propane-butane mixture over Zn-La-Fe-ZSMAl2O3 catalyst at 550 oC, selectivity to aromatic hydrocarbons is 39,2%.


INTRODUCTION
Aromatic hydrocarbon are important starting materials in basic organic synthesis.They are used for manufacturing plastics, synthetic fibers, resins, various-purpose rubbers, dyes, surfactants and pharmaceutical and agricultural products.Rational use of light hydrocarbons that are component of natural, associated and refinery gases, is an actual problem.C 1 -C 4 hydrocarbons contained in these gases can be converted into aromatic hydrocarbons over zeolite catalysts.Aromatic hydrocarbons are an important feedstock for many petrochemical processes.However, so far a significant part of the light hydrocarbon are used as technological and household fuel or flared on torches, causing considerable damage to the environment.Today zeolite-containing catalysts, especially high-silica zeolites such as pentasil are widely used in the petrochemical and refining industries due to unique microporous structure and acid-base properties, capable of converting light hydrocarbons into valuable products of petrochemical synthesis.Researches of catalytic conversions of light hydrocarbon are conducted in many countries of the world.At the same time the direction of recycling process and the yield of products, generally depend on nature, conditions of preparation of the catalyst and carrying out the process.Catalytic conversions of light hydrocarbon into aromatic hydrocarbons are intensively studied on the modified pentasil-contained catalysts [1][2][3][4][5][6][7][8][9][10][11][12][13] .

Modified zeolite-containing catalysts:
Zn-La-Zr-ZSM-Al 2 O 3 , Zn-La-Co-ZSM-Al 2 O 3 , Zn-La-Fe-ZSM-Al 2 O 3 were prepared by impregnation of mixture of aluminum hydroxide and HZSM-5 zeolite with aqueous solutions of metal salts with subsequent drying at 150 o C and calcinating at 550 o C. The synthesized catalysts were tested in the conversion process of propane-butane and propanepropylene mixture in a flow-type quartz reactor with a fixed-bed catalyst at atmospheric pressure, and variation of the reaction temperature from 450 to 600 o C, SV from 140 to 500 hr -1 .The flow -type reactor contained 5 g of the catalyst with particle size of 1-2 mm sandwiched between quartz grit layers.The reaction products were cooled in receiver with a consender plased in special immersion cooler to collect the liquid fraction.
The reaction products were analyzed by chromatographic (GLC) method on the equipment "Chrom-5"with the column filled with alumina of "Supelco" firm.Liquid organic products were determined by GLS "Agilent 6890" with the capillary column 60m × 0.250mm.Liquid aromatic hydrocarbons and gaseous products are formed during the conversion of propane-butane mixture.Benzene, toluene, ethylbenzene, xylenes and the C 8+ hydrocarbons were found in the liquid products.C 1 -C 4 hydrocarbons are present in the gaseous products of reaction.
Physical and chemical characteristics of the synthesized catalysts were studied.The specific surface area and porosity of the catalysts have been measured by Brunauer-Emmett -Teller (BET) method on low-temperature adsorption of nitrogen.The acid characteristics of the catalysts were investigated by temperature-programmed desorption of ammonia 14 .

RESULTS AND DISCUSSION
The conversion of the propane-butane mixture on a Zn-La-Zr-ZSM-Al 2 O 3 catalyst was studied at a space velocity 350 hr -1 (Table 1).With an increase in temperature from 450 to 600 î Ñ the conversion of hydrocarbons (X) rises from 35.1 to 88.3%.The yield of aromatic hydrocarbons (Y) in these conditions increases from 12.3 to 24.2%.
In the process of conversion the propanebutane mixture liquid aromatic hydrocarbons and gaseous products are formed.In the liquid products benzene, toluene, ethylbenzene, xylenes and C 8 + hydrocarbons were found.In gaseous reaction products, there are C 1 -C 4 hydrocarbons.
An increase in temperature of the reaction increases from 400 to 600 o C results in the growth of benzene yield from 18.9 to 38.5% where as the yields of ethylbenzene and xylene are reduced from 17.8 to 7.5% and from 5.8 to 2.4%, respectively.The maximum yield of toluene is 62.1% at 500 o C, with further growth of temperature this value decreases to 50.0% (600 o C).
An increase in temperature leads in increased cracking with formation of C 1 -C 2 hydrocarbons.The yield of methane grows from 1.2% to 47.5%, the yield of ethane increases from 1.4% to 52.5% with temperature rise.
Conversion of propane-butane fraction over the Zn-La-Co-ZSM-Al 2 O 3 catalyst are shown in Table 2, an increase in temperature from 450 to The highest yield of aromatic hydrocarbons (34.6%) during conversion of propane-butane mixture is observed over the zeolite-containing catalyst Zn-La-Fe-ZSM-Al 2 O 3 modified with zinc, iron, lanthanum (Table 3).The maximum selectivity to the yield of aromatic hydrocarbons is 39.2% at 550 o C. As the reaction temperature grows in the range of 450-600 o C the degree of conversion increases from 75.4% to 90.2%.There were observed changes in the composition of liquid products.Thus, as process temperature increases the amount of benzene goes up from 12.7 to 44.1%, the yield of toluene and ethylbenzene decreases from 61.6 to 47.7% and from 16.7 to 5.7%, respectively.The influence of the SV (140, 350, 450, 500 hr -1 ) on the degree of conversion and the yield of products over the catalyst Zn-La-Fe-ZSM-Al 2 O 3 at 550 o C has been studied.With the increase SV of propane-butane mixture from 140 to 500 hr -1 , a decrease occurs in the yield of aromatic hydrocarbons from 49.1 to 20.7% and the degree of conversion from 88.7 to 75.8%, which connected with the reduction of time of contact of active centers of the catalyst with a feed.
With increase of feed rate from 140 to 500 hr -1 the yields of benzene and xylenes are reduced from 31.7 to 19.3% and from 8.8 to 2.3%, respectively the yield of toluene increases from 38.7 to 70.8% and ethylbenzene passes through a maximum at 350 hr -1 .Cracking is limited as the SV grows, the yields of methane and ethane are decreased from 21.1 to 4.2% and from 29.2 to 16.6%, respectively.
Stability of the Zn-La-Fe-ZSM-Al 2 O 3 catalyst was studied at conversion of propane-butane mixture (Table 4).It was observed that continuous work of the catalyst from 1 to 7 h the yield of aromatic hydrocarbons (30.0-32.5%)and the degree of conversion (81.0-78.9%)change slightly.The selectivity to the yield of aromatic hydrocarbons fluctuates within 37.4-41.2%.The qualitative composition of the liquid products remains practically unchanged.Toluene (47.0%) and benzene (36.6%) are formed in the prevailing quantity.
It should be noted that the cracking of hydrocarbons proceeds less intensively over the catalyst Zn-La-Fe-ZSM-Al 2 O 3 than over the catalysts The Zn-La-Fe-ZSM-Al 2 O 3 catalyst was tested at conversion of propane-propylene mixture.Aromatic hydrocarbons, C 1 -C 4 hydrocarbons were found in the reaction products during conversion propane-propylene mixture over this catalyst (Table 5).A significant increase in the yield of aromatic hydrocarbons is observed at conversion of propane-propylene mixture, particularly at low temperatures (450-550 o C) in comparison when using a propane-butane mixture under similar conditions.Thus, when processing propane-butane mixture on the Zn-La-Fe-ZSM-Al 2 O 3 at 450 o C the yield of aromatic hydrocarbons was 28.4%, while the processing under the same conditions a propanepropylene mixture the yield of aromatic hydrocarbons increased to 35.2%.With increase of reaction temperature in the range of 450-500 o C the yield of aromatic hydrocarbons increases from 35.2% (450 o C) to 38.4% (550 o C), but with the further growth of temperature their amount decreases to 33.9% (600 o C).The degree of conversion is 61.8% at 450 o C and it reaches 100% with temperature growth to 600 o C.
Comparison of the results obtained during processing of propane-butane and propanepropylene mixtures over the Zn-La-Fe-ZSM-Al 2 O 3 catalyst shows that the composition of the feedstock practically does not affect the conversion.The amount of aromatic hydrocarbons formed is higher when using a propane-propylene mixture.At a temperature of 550 0 C, the yield of the liquid products during processing of the propane-propylene mixture is 38.4% and the propane-butane yield is 34.6% (Table 3 and 5).
Physical and chemical characteristics of the synthesized catalysts were studied.The specific surface area and porosity of the catalysts have been measured by BET method on low-temperature adsorption of nitrogen.The specific surface of catalysts ranges within 283.5 -310.7 m 2 /g and pores dominate with d = 1.5-3.0nm (Table 6).
The acid-base characteristics of the catalysts are also essential for the conversion process of light hydrocarbons.The acidic characteristics of catalysts were studied by temperature-programmed desorption of ammonia.From the data provided in Table 7 it is seen that ammonia on the surface of the catalyst Zn-La-Fe-ZSM-Al 2 O 3 is non-uniform and adsorbed in three forms.The weakly bound ammonia is desorbed from t max = 115 o C, tightly bound from t des = 320 o C and more tightly bound ammonia is desorbed at t max = 470 o C. Their quantity is equal to 22.60; 7.80 and 4.80×10 -4 mole/g of catalyst.The total content of acidic sites is equal to 35.20×10 -4 mole/g of catalyst.
Introduction of cobalt into the composition of the catalyst changes its acidic properties.The maximum desorption of weakly bound and strongly bound forms of ammonia in the Zn-La-Co-ZSM-Al 2 O 3 shifted to the area of higher temperatures from 115 to 152 o C and from 470 to 480 o C, where as the maximum desorption of ammonia with an average binding energy is shifted to the area of lower temperatures till 310 o C.However, their content changes slightly -23.70, 8.80 and 4.00×10 -4 mol/g of catalyst, respectively.The total amount of acidic sites is equal to 36.50×10 -4 mole/g of catalyst.
Unlike the catalyst Zn-La-Co-ZSM-Al 2 O 3 and Zn-La-Fe-ZSM-Al 2 O 3 for the Zn-La-Zr-ZSM-Al 2 O 3 is characteristic presence of more strong acid centers.At a temperature of desorption of 490 0 C, the amount of stripped ammonia makes 9,4.10 -4 mol/g of the catalyst (Table 7).

CONCLUSION
The results shows that the catalyst Zn-La-Zr-ZSM-Al 2 O 3 has low catalytic activity during the processing of the propane-butane fraction compared to Zn-La-Co-ZSM-Al 2 O 3 and Zn-La-Fe-ZSM-Al 2 O 3 .The replacement of the modifying iron additive with cobalt results in an increase in the activity of the catalyst.The maximum activity is exhibited by the catalyst Zn-La-Co-ZSM-Al 2 O 3 , the conversion is 92.6% (500 0 C).Under equal conditions, the highest yield of aromatic compounds and selectivity are observed on catalyst Zn-La-Fe-ZSM-Al 2 O 3 : respectively 34.6% and 39.2%.
The structure and composition of the products formed during the processing of propanebutane propane-propylene mixtures indicate that modified zeolite-containing catalysts have polyfunctional properties.Several zeolite-containing Zn-La-Zr-ZSM-Al 2 O 3 , Zn-La-Co-ZSM-Al 2 O 3 and Zn-La-Fe-ZSM-Al 2 O 3 catalysts simultaneously and in parallel proceed with cracking, dehydrogenation, oligomerization, dehydrocyclization, alkylation.Cracking and dehydrogenation of the starting hydrocabons occur with the formation of intermediate activated complexes with a reduced content of carbon atoms and adsorbed olefinic structures.In the future, depending on the nature of the active center of the catalyst, various transformation directions develop with the participation of intermediate activated

Table 7 : Temperature-programmed desorption of ammonia Catalysts T max , °C The quantity of desorbed ΣNH 3 ., 10 -4 mol/g ammonia, 10 -4 mol /g of catalyst of cat.
Zn-La-Zr-Al 2 O 3 and Zn-La-Co-ZSM-Al 2 O 3 .During conversion propane-butane mixture over the Zn-La-Fe-ZSM-Al 2 O 3 catalyst as the temperature increases from 400 to 600 o C, the yield of methane grows from 9.0% to 19.1% and the yield of ethane increases from 17.7% to 25.8%.