MODIFIED ZEOLITE CATALYSTS FOR EFFICIENT PROCESSING OF N-HEXANE AND GASOLINE FRACTION

In this work hydrogen-free processing (in the absence of hydrogen) and hydrotreating of n-hexane and straight-run petrol fraction on the catalysts samples “C-1” (La-ZSM-Al 2 O 3 ) and “C-2” (Ni-Mo-La-P-ZSM-Al 2 O 3 ) were studied. “C-2” catalyst has high activity in both studied processes. The conversion of n-hexane during hydrogen-free processing on the “C-2” catalyst in the range of 350-500ºC increased from 58.8 to 90.7%; the octane rating of the final product in the temperature range of 350-500 ºC was within 55.1-87.9 according to the research method (RM)


INTRODUCTION
More than 80% of the processes of chemical technology including oil refining, thanks to which countries can receive up to 30% of national income, are catalytic. [1][2][3][4] Therefore, the development of technologies for the synthesis of new effective selective stable catalysts is a promising direction worldwide, since the further intensification of production is directly related to the development of catalysts. [5][6][7][8][9][10][11][12][13] According to international standards, which are tightened from year to year the world, the content of sulfur, olefins, and aromatic hydrocarbons in gasoline should be significantly limited. 14,15 In conditions of increasing demand for the production of commercial gasoline, an urgent task is to improve the technology of catalytic purification processes, such as catalytic cracking, reforming, hydrotreating, and synthesis of new effective catalysts. The implementation of this direction is associated with the creation of new multifunctional catalysts for effective hydrotreating of petroleum gasoline fractions in one stage, including the processes of removal of sulfur, nitrogen-containing compounds, hydrogenation of unsaturated, and aromatic compounds, hydroisomerization and selective hydrocracking of n-paraffin hydrocarbons. [16][17][18][19][20][21][22] It is known that a constant deficiency of H2 is observed during oil refining. Today, a very important issue is the creation of new efficient catalysts for processing low-octane petrol fractions of oil into high-octane ones without the use of hydrogen. 13 Hydrogen-free processing of low-octane petrol fractions on zeolite-based catalysts, which have high activity and selectivity in the reactions of isomerization, dehydrogenation, and dehydrocyclization of hydrocarbons is considered one of the most perspective methods for producing high-octane motor gasoline. The issue of developing catalysts for the obtaining of high-octane gasoline from straight-run gasoline fractions without the use of hydrogen is of great theoretical and practical importance due to the growing shortage of hydrogen. 2,3,13,[23][24][25][26][27] Catalysts based on high-silica zeolites are widely used in many oil refining and petrochemical processes. Their activity and selectivity are determined by their unique properties. [28][29][30][31][32][33][34] This article presents the results of processing n-hexane and straight-run petrol on the modified zeolite-based catalysts in the absence of hydrogen and during hydrotreating.

EXPERIMENTAL
For experiments, the laboratory flow units with a fixed catalyst layer were used (Fig.-1). Composition of the initial gasoline fraction, %: alkanes С4-С10 -33.2; iso-alkanes -С4-С10 -28.8; olefins С2-С10 -0.3; aromatic hydrocarbons -4.5; naphthenic hydrocarbons -33.1. The octane rating of the initial petrol is 73.1 (RM) and 54.1 (MM). In all experiments, modified zeolite-containing catalysts were used: sample "C-1" (La-ZSM-Al2O3) and sample "C-2" (Ni-Mo-La-P-ZSM-Al2O3). The catalysts were prepared by mixing aluminum hydroxide with zeolite ZSM-5 (SiO2/Al2O3=35) and impregnating with aqueous solutions of salts of nickel nitrate, lanthanum, ammonium para-molybdate, and phosphoric acid. The procedure for the preparation of modified catalysts based on zeolites and the reaction procedure are described in our previous articles. 2,[23][24][25] The analysis of the initial compounds and reaction products was carried out by gas-liquid chromatography (GLC) on chromatographs Chromatic-Crystal 5000.2 and Agilent. The physicochemical characteristics of the synthesized catalysts were studied by BET methods on the Accusorb installation of Micromeritics manufactured in the USA. The acidic properties of zeolitecontaining catalysts have been determined by the method of thermo programmed desorption (TPD) of NH3. 21,24 The dispersion of metal particles in the catalysts was studied by Transmission Electron Microscopy (TEM) method ("EM-125K") using microdiffraction.

RESULTS AND DISCUSSION
Testing the Activity of Catalysts in the Hydrogen-Free Processing of Model N-Hexane and Gasoline Fraction The results of the experiment showed that the hydrogen-free conversion of n-hexane on catalysts "C-1" and "C-2" proceeds with the formation of C1-C4 and C5-C6 iso-and n-alkanes, olefinic, aromatic, and naphthenic hydrocarbons. The degree of conversion of n-hexane on the catalyst "C-1" with an increase in temperature from 350 to 500ºC increases within 17.7-74.5% (Fig.-2). At 350ºC, the yield of light hydrocarbons ΣС1-С4 is 8.9%, ΣС5 -2.1%, ΣС4-С6 -3.5%, cyclohexane -5.1%. As the temperature rises, the qualitative and quantitative composition of the compounds formed changes. At 500ºC, the amount of C1-C4 -hydrocarbons increases from 8.9% to 45.9%, ΣC5 -from 2.1% to 5.2%, cyclohexane -from 5.1% to 16.2%, heptane- 0.8%, benzene -2.7% and octane -2.1%. The total yield of C2-C8 olefins increases from 1.8 to 23.2%, and isoalkanes -from 3.5 to 8.4% with an increase in temperature within 350-500ºC. Conversion of the original product, n-hexane, at different temperatures was (in %): 17.7 (350ºC), 45.0 (400ºC), 63.0 (450ºC), and 74.5 (500ºC). The conversion of n-hexane during hydrogen-free processing on the "C-2" catalyst in the range of 350-500ºC increases from 58.8 to 90.7%. At a temperature of 350 ºC, 11.6% of liquid-phase products are formed. As the temperature rises, the cracking direction of n-hexane transformations increases. The amount of liquid products drops to 6.7% at 450 ºC, and at 500 ºC only gaseous compounds are formed. The yield of gaseous compounds under these conditions varies from 80.8 to 100.0% (Fig.-3). C1-C10 n-paraffins, C4-C10 iso-alkanes, C6-C7 aromatic hydrocarbons, a small amount of C4-C8 olefins, and naphthenic compounds are formed. In addition, hydrogen is formed. At 350ºC, 53.6% of С5-С10-paraffins, 11.4% of С4-С10 isoalkanes, and 35.0% of aromatic hydrocarbons were found in the liquid final product. When the temperature rises to 500ºC, the yield of n-paraffin decreases to 22.8%, and aromatic hydrocarbons increases to 51.7%. The content of С4-С10 isoalkanes changes extremely with increasing temperature, reaching a maximum value (27.1%) at 400ºC, and then decreasing to 15.7% at 500ºC. Naphthenic hydrocarbons appear at 380ºC, their yield ranges from 4.5-5.4%. The yield of hydrogen increases from 0.5 to 6.5%. The octane number of the final product in the temperature range of 350-500 ºC varies from 55.1 to 87.9 according to RM, and from 61.4 to 84.9 according to MM. It was established that the process of n-hexane conversion on the studied catalysts proceeds simultaneously and in parallel in several directions: cracking, cyclization, dehydrogenation, and dehydrocyclization. In the case of hydrogenfree processing of straight-run petrol fraction on the "C-2" catalyst, with an increase in the reaction temperature from 350 to 500ºC, the yield of the liquid product decreases from 77.0 to 20.0% (Fig.-4). Under these conditions, in the final liquid product, the content of isoalkanes increases from 21.9 to 29.9%, olefins from 1.9 to 4.8%, and aromatic hydrocarbons from 4.5 to 20.7%. In addition, olefins were found in the final product (1.9-4.8%), which are probably formed as a result of the dehydrogenation of paraffin. Obviously, the presence of these compounds in the products of gasoline processing indicates the simultaneous and parallel occurrence of dehydrocyclization, dehydrogenation, isomerization, and cracking reactions. With increasing process temperature, the rate of such reactions increases significantly. The results of the experiment show that the conversion of n-hexane and gasoline fraction on the studied catalysts at the hydrogen-free processing occurs simultaneously and in parallel in several directions: cracking, cyclization, dehydrogenation, and dehydrocyclization.

Determination of Physicochemical Characteristics of Catalysts
It has been established by the BET method that the surface of the developed catalysts ranges from 201.10-252.20 m 2 /g of the catalyst. The catalysts have pores with d ≈ 1.5-3.65 nm. (Fig.-7). Studies have shown that NH3 is adsorbed on the "C-2" catalyst in three forms with Tmax equal to 155ºC, 230ºC, and the most strongly bound NH3 is desorbed at Tmax=460ºC. Their number is 16.79; 3.79, and 7.07·10 -4 mol/g of catalyst (Table-1 temperature Тmax=210ºC) is typical for the "C-2" catalyst and determines its high hydroisomerizing activity in the processes of hydrotreating of n-hexane and gasoline fractions. Metals of different oxidation states, which are fixed inside and outside the cavities of the zeolite, can be in acid sites, and the combination of two or more metals determines the multifunctionality of the catalysts. [1][2][3][4]12,34 In the study by TEM (Fig.-8)   The results obtained during the study of the hydrotreating process of the straight-run gasoline fraction on the catalysts "C-1" and "C-2" show that the conversion process proceeds by a complex mechanism. This mechanism includes hydrocracking by C-H and C-C bonds with the formation of low molecular weight nand iso-alkanes and olefins, which on the surface of the catalyst enter into secondary processes of hydrogenation, cyclization (naphthene), dehydrocyclization (aromatic hydrocarbons), alkylation, etc.

CONCLUSION
The modified zeolite-containing catalysts sample "C-1" (La-ZSM-Al2O3) and sample "C-2" (Ni-Mo-La-P-ZSM-Al2O3) were used at the hydrotreating and hydrogen-free processing of n-hexane and gasoline fraction. The hydrogen-free conversion of n-hexane on both catalysts proceeds with the formation of C1-C4 and C5-C6 iso-and n-alkanes, olefinic, aromatic, and naphthenic hydrocarbons. A comparative analysis of the processes of hydro-and hydrogen-free processing of hydrocarbon raw materials shows that during hydrogen-free processing, especially at high temperatures, the cracking direction prevails. The octane rating of the final products of the hydrotreatment process increases mainly due to an increase in the content of isoalkanes.