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.