Introduction. The main consumers of hydrocarbon raw materials are engines, the demand for which has been increasing in recent years. There are two main reasons which arise the search for additional sources of hydrocarbon raw materials, alternative ones and moreover the need to protect the environment from emissions of motor gases. The modern requirements to gasoline require decrease of toxic aromatic hydrocarbons in order to maintain high octane number and to improve the environmental performance of motor gasolines [1].

In terms of solutions, it is proposed to subject normal and weakly branched paraffin hydrocarbons to low temperature isomerization process [2]. In world practice, several modifications of the isomerization process are used, which differ in the catalysts used and the process conditions. The use of new modificated catalysts enable to increase the high octane components of modern gasolines, so that the involvement of saturated hydrocarbons to the process contributes to the solution of this problem.

Natural gasoline is the mixture of C₅-C₇ and C₇₊ paraffins, that have certain difficulties. For instance, the presence of C₇₊ n-paraffins can lead to the formation of undesirable gaseous products due to the hydrocracking or hydrogenolysis of branched isomeric structural hydrocarbons [3]. In order to maintain the high efficiency of isomerization process catalyst, it is necessary to limit the content of C_{7 +} paraffins in the feedstock. Furthermore, due to the importance of C₅-C₆ izomers which have high-octane number, it is important to increase the amount of normal and weakly branched paraffin hydrocarbons.

Experimental Part

The multicomponent catalysts consist of А(γ-Al₂O₃) or HMOR zeolite (SiO₂/Al₂O₃=17) and anion SO₄^2- modified sulfated zirconia (SZ). The components of the composite catalyst were prepared by modifying the initial MOR zeolite with cobalt, nickel or zirconium. The modification of the original zeolites was carried out by decationization, dealumination, ion-impregnation of various metals, impregnation with a solution of a sulfating agent (NH₄)₂SO₄, based on the number of ions.

As a active components ZrO (NO₃)₂∙ 2H₂O and (NH₄)₄W₅O₁₇ ∙ 2.5H₂O salts were used so as to prepare catalysts. For the synthesis of catalysts zirconium dioxide gel was obtained by hydrolysis of ZrOCl₂ with a solution of 25% ammonia at pH = 8-9 [3]. The deposition of the salts on the H-form of the zeolite was carried out by impregnation for 24 hours with further evaporation, drying, mixing with a binder component Al₂O₃ (25% of the catalyst mass) and subsequent heat treatment at different temperatures during 4 hours. The new prepared composite catalyst is 65 wt.% of Al₂O₃ or zeolite, 15% SZ and the rest is binder. The main features of the reaction resulting products have been done be means of «Auto System XL, Perkin Elmer» chromatography provided with the relevant computer program.

Results and Discussion

Catalytic isomerization process is one of the predominant processes in modern oil refining that is used in order to obtain high-octane isocomponents for motor fuels. The process should be carried out in such a way as to maintain a minimum yield of aromatics and olefins, which is achieved by selecting the catalyst and the process conditions.

In addition, important argument for the inclusion of an isomerization unit in the oil refining scheme is an increase in the octane stock of the entire gasoline stream, which makes it possible to reduce the “rigidity” of the reforming process. The latter leads to an increase in the yield of reformate and the concentration of aromatic hydrocarbons simultaneously decrease in commercial gasolines [4,5].

All components of the original natural gasoline over the Ni/MOR/SZ undergo considerable amount of changes. The content of the iso-C₅, n-C₅, and iso-C₆ components in the catalyst increases, while the content of the remaining components decreases. Among the changes noted, it is necessary especially to note a decrease in the concentration of hydrocarbons C₄, C₆ and ∑C_{7 +}. Changes in the distribution of hydrocarbons before and after contact with the catalyst are depend on temperature and process.

Table 1 – Conversion of natural gasoline over the Ni/MOR/S; WHSV= 2 h^-1; υ_Н2 =30 ml/min

As can be seen from the table 1, the contact of natural gasoline with the Ni/MOR/SZ catalyst allows not only isomerization of the C₅ and C₆ alkanes, but also the involvement of the heptane to the process. The conversion of this component in the temperature range of 150-200⁰C is around 40-49%, while the products of this conversion are only high-octane alkanes iso-C₅, iso-C₆ and n-C₅. It also should be noted that the C₆ components of natural gasoline are also involved in the formation of these components. In other words it is obvious that natural gasoline is significantly enriched in higher-octane components in one pass over the catalyst due to extremely declining of low-octane heptane components. The noted changes in the distribution of hydrocarbons contribute to a natural change in the octane quality of the mixture. The evaluation of the octane characteristics of the feedstock, presented in Table 1, indicates that in one pass over the catalyst, the RON of gasoline can increase by 17-23 points. Thus, compounding of reformate with the obtained fraction can become a perspective method for producing high-octane gasolines that meet modern requirements.

Conclusions

The possibility of using Н-zeolite/SO₄^2-(WO₄^2-)ZrO₂ catalytic systems for the conversion of hydrocarbon mixtures of natural gasoline at low temperatures of 140-200⁰С and the accumulation of C₅-C₆ alkanes are formed as a result of isomerization, decomposition of bimolecular intermediate and as well as the involvement of C_{7 +} alkanes to the process without the formation of undesirable gaseous alkanes can open new avenue for using such catalytic systems for isomerization processing (isoforming) of stabilized gasoline containing significant amounts of C_{7 +} alkanes.

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