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.

Modern stringent environmental requirements for gasolines suggest the limitations of aromatic hydrocarbons by maintaining their high anti-knock characteristics [1-3]. One of the solutions of this problem is a conversion of straight run gasolines or natural gasoline from high-temperature dehydrocyclization to low-temperature isomerization process [1, 4-6].

The isomerization process of C₅-C₆ alkanes plays an important role in the production of modern gasolines with a low content of aromatic hydrocarbons [7-9]. The isomerization process has very high technical and economic indicators compared to other processes that increase the octane number of fuel [3-5, 10].

Isomerate is practically indispensable in the production of motor fuels that meet the latest environmental requirements. Straight-run gasoline or natural gasoline containing C₅-C₆ paraffin hydrocarbons are the main feedstocks for production of environmentally friendly high-octane gasoline components [6, 11-14]. Conversion of straight-run gasoline or natural gasoline on composite catalytic systems containing tungstated or sulfated zirconium dioxide (ZrO₂), zeolite (MOR, HZSM-5) can lead to the production of eco-friendly high-octane gasolines with limited content of aromatics [1, 15].

Experimental Part

The main objects of study were anion-modified composite catalysts containing H-zeolite, such as mordenite and anion-modified sulfated zirconium dioxide or anion-modified tungstated zirconium dioxide. Zeolites modification was carried out by decationization and dealumination, ion-impregnation of various metals such as nickel or cobalt [12]. Zeolite catalysts were promoted using metal ions of salts obtained from nickel (NiNO₃), cobalt (CoSO₄∙7H₂O) and zirconium ZrO(NO₃)₂∙2H₂O, tungsten (NH₄)₆H₂W₁₂O₄₀  as a source of active components.

The content of SO₄^2- and WO₄^2- ions was controlled using solutions with a given content of ions, and their content in the obtained samples was controlled by elemental analysis (Agilent Technologies 7700 Series ICP-MS). The amount of SO₃ and WO₃ on ZrO₂ in the final samples was 6.1 and 11.8%, respectively.

Comparative analysis of reactants and reaction products was carried out directly at the inlet and outlet of the reactor (on-line mode) and analyzed using the Perkin-Elmer Autosystem XL gas chromatograph.

Results and Discussion

Sulfated (tungstated) composite catalytic systems containing zirconium dioxide (ZrO₂), zeolite (MOR, HZSM-5) and cobalt, nickel can involve natural gasoline in isomerization process with an increasing of C₅-C₆ paraffin hydrocarbons resources [1, 16]. The composition of natural gasoline considerably affects the efficiency of the process.

Natural gasoline is a mixture consisting mainly of C₅-C₇ alkanes. Such hydrocarbon composition is quite acceptable for isomerization process of natural gasoline to increase the concentration of iso-C₅-C₆ high octane components.

The composition of natural gasoline: gaseous alkanes C₄-(5.4%), iso-C₅ (25.5%), n-C₅ (19.3%), iso-C₆ (18.2 %), n-C₆ (8.6%), C₇₊ - 22.7%. The conversion of natural gasoline was carried out at atmospheric pressure, in the temperature range of 150-200⁰C. Natural gasoline conversion over Со/HMOR/WO^2-₄ ZrO₂ catalyst is presented in table 1.

Table 1. Natural gasoline conversion over Со/HMOR/WO^2-₄ ZrO₂ catalyst: LHSV=2,5 h^-1; τ=30 min

Conversion of natural gasoline on composite catalyst which combines the properties of anion-modified zirconia and H-zeolite, leads to significant changes in the distribution of hydrocarbons. Moreover, the most important of these changes are consumption of C₇₊ alkanes (C₇₊ conversion); reduction of C_4- alkanes and accumulation of C₅-C₆ alkanes, including high-octane iso-pentane and dimethylbutanes.

C_4- consumption is observed in the temperature range of 150-200⁰C. However, the consumption of C₇₊ occurs in the range of 150-180⁰C. Moreover in this temperature range the amount of isostructural C₅-C₆ alkanes increases.

Table shows 2 the results of natural gasoline conversion on Ni/HMOR/SO^2-₄ ZrO₂ catalyst. Conversion of the natural gasoline on Ni/HMOR/SO^2-₄ ZrO₂ allows increasing of isostructural C₅-C₆ alkanes and normal C₆ components and decreasing of С₄-, С₆ and С₇ components. The increase in the content of these hydrocarbons is a consequence of a similar decrease in other hydrocarbons, especially C₇₊. Ni/HMOR/SO^2-₄ ZrO₂ can convert natural gasoline and straight-run gasoline components and considerably increase the iso-C₅, n-C₅ and iso-C₆ components which are high octane resources.

Table 2. Natural gasoline conversion over Ni/HMOR/SO^2-₄ ZrO₂ catalyst LHSV= 2 h^-1; τ=30 min; υ_H2 =30 ml/min

Table 3. Influence of temperature on natural gasoline conversion over Ni/HMOR/SO^2-₄ ZrO₂ catalyst LHSV= 2 h^-1; τ=30 min; υ_H2 =30 ml/min

Table shows 3 influences of temperature on natural gasoline conversion on Ni/HMOR/SO^2-₄ ZrO₂ catalyst. Conversion of the natural gasoline on Ni/HMOR/SO²₄ ZrO₂ enables to isomerize C₅ and C₆ alkane hydrocarbons and to involve heptane in the process. Natural gasoline undergoes significant enrichment with high octane components due to extremely low octane heptane components.

Table 3 shows that in one pass over the sulfated based composite catalytic system the research octane number of gasoline increases by 15-22 points. So, reformate compounding with the resulting mixture can be the best method for production of high-octane gasolines.

The possibility of using the H-zeolite/SO^2-₄ ZrO₂ and H-zeolite /WO^2-₄ ZrO₂ catalytic systems for the conversion of components of natural gasoline between the temperature range 150-200ºС enable to increase the concentration of isostructural C₅-C₆ hydrocarbons and involve C₇₊ alkanes in the catalytic conversion process. Furthermore, the involvement of C₇₊ hydrocarbons in the catalytic processoccurs without the formation of C₁-C₃ alkanes.

Conclusion

The results of this research showed that C_4- gaseous alkanes are consumed in the conversion process of natural gasoline by forming high molecular weight of hydrocarbons. Moreover, the conversion of natural gasoline over sulfated (tungstated) composite catalytic systems containing zirconium dioxide (ZrO₂), zeolite (MOR, HZSM-5) and cobalt, nickel can open perspective opportunity for conversion of natural gasoline and straight run gasoline from high-temperature dehydrocyclization to low-temperature isomerization process.