Conversion of model C 6 – C 9 alkanes and straight-run gasoline over Pt(0.1%)-Fe(5%)/Al 2 O 3 catalysts promoted with various additives

Growing demand for hydrogen promotes the research devoted to the development of new catalysts for hydrocarbons processing in absence of H 2 or at its low concentration. In the present work, it was shown that during the conversion of straight-run gasoline on a zeo-lite-containing polyfunctional catalyst in a hydrogen-free environ-ment cracking, dehydrogenation, isomerization and alkylation take place due to the redistribution of H 2 between initial and formed products directly on the catalyst surface. Fine particles (≤50 Å ) localize in zeolite cavities and pores of aluminum oxide, while larger ones are on their outer surface.


Introduction
Many processes at petrochemical and chemical plants are based on the use of hydrogen or hydrogen-containing gas (HCG) [1][2][3]. Due to the rapid growth of hydrogen energy application, the shortage and cost of hydrogen are increasing [4][5][6][7][8]; therefore, reducing the consumption of HCG by developing new catalysts and technologies is urgently needed [9][10][11].
It was shown, that conversion of n-hexane over 0.3% Pt-containing zeolites of L and eryonite (E) type is influenced by the degree of ion exchange of K + . Under the same conditions (t = 400 °C), the conversion of n-hexane increases from 26.1 to 47.0% with an increase in the degree of ion exchange from 17.0% to 82.0% in L zeolite. According to NH3 adsorption, three types of acid centers with binding energy (q) equal to 100, 110 and 120 kJ/mol were determined in the catalyst. The highest activity in the nhexane isomerization reaction is typical for the centers with q = 120 kJ/mol, while the acid centers with lower binding energy also participate in hydro isomerization but with low efficiency [18]. Molecular sieve effect was shown when comparing the properties of catalysts containing L and E zeolites. On L-zeolite with wide pores (0.71 nm) isomer yield is greater than on E-zeolite with narrowpores (0.36x0.52 nm). In the latter case, the pores are not accessible to branched isomers.
In the present work, conversion of C6-C9 n-alkanes on Pt-Fe/Al2O3+HZSM catalyst modified with additives of cerium, molybdenum and phosphorus in the presence of H2 was studied. Processing of straight-run gasoline was carried out with varying content of hydrogen and in its absence.
The tatalyst was studied in the reaction of transformation of C6-C9 n-alkanes and straight-run gasoline on the laboratory flow-through unit at the temperatures varying from 280 °C up to 400 °C, the hydrogen pressure of 2MPa, the H2/feed ratio of 200/1, 50/1 and 0/1, and the volume feed rate of 5 h -1 . Hydrocarbon composition of the reaction products was analyzed on a "Chrom-4" chromatograph with a stainless steel column filled with γ-aluminum oxide by "Supelco". Argon was used as a carrier gas.

Results and discussion
The conversion and direction of C6-C9 n-alkanes transformation on the Pt(0.1%)-Fe(5%)/Al2O3+HZSM catalyst modified with various additives is affected by the process temperature under otherwise equal conditions (PH2 = 2MPa, V = 5 h -1 , H2/feedstock ratio = 200/1). Table  1 shows the results obtained at n-nonane processing. As the temperature rises from 300 to 400 °C, the conversion of n-nonane increases from 51.2 to 93.6%.
As shown in Table 1, the olefins formed during dehydrogenation of intermediate activated surface complexes are rapidly isomerized through the carbonium ion formation and hydrogenated to isoalkanes. Due to this, only tracess of olefins are present in the products mixture. Other directions of olefins transformation probably include formation of aromatic hydrocarbons (0.1-2.7%) at temperatures of 350-400 °С. The maximum yield of C4-C8 isoalkanes (35.7%) is determined at 350 °C. The assumed mechanism of alkane conversion is possible on the active catalyst sites containing both metal and acid centers. Figure 1 presents the data on C6-C8 n-alkanes conversion on the Pt-Fe/Al2O3+HZSM catalyst promoted by different additives and the yield of isomers at different temperatures.
With the increase in temperature, conversion of nalkanes grows, and at 400 °C it reaches 96.7-97.9%. The temperature dependence of maximum yield of isomers is observed at close conversion degrees of C6-C8 n-alkanes. The curves pass through the extrema at 350-380 °C. The maximum yield (%) decreases in the following series: hexane (32.0%) > heptane (30.6%) > octane (30.3%). In the high temperature area (>350 °C) the formation of aromatic hydrocarbons (0.1-2.8%) and olefins (tracess) is observed.
Physical and chemical properties of the polyfunctional, promoted with additives, Pt (0.1%)-Fe(5%)/Al2O3+HZSM catalyst were studied by BET, XRD and electron microscopy (EM) methods. The specific surface area and total pore volume of the catalyst are 179.2 m 2 /g and 0.40 cm 3 /g, respectively. The   The EM study (120000 magnification) showed the presence of an aggregate in the Pt-Fe/Al2O3 catalyst modified with cerium, molybdenum and phosphorus, which consists of dense particles of ~200 Å size. The micro diffraction pattern is represented by two rings and can be attributed to Fe2O3 Hematite (ЈCPDS, 35-664). At low magnification, large elastic lamellar crystals with basal reflections on their bends were detected. The micro diffraction pattern can be attributed to FeOOH (JCPDS, 26-792). Small loose clusters of dispersed particles with sizes of 20 Ǻ were found in the sample; according to the micro diffraction pattern, they represent CeO2 (JCPDS, 34-394). Small loose clusters of ~100 Å particles give a diffraction pattern which can be attributed to a mixture of Pt3O4, Mo9O26 (ЈCPDS, 21-1284) and CeAlO3 (ЈCPDS, 28-260). Characteristic extensive clusters of 30-40 Å particles of Ce6O11 were detected. An aggregate of loose small particles 30-50 Å in size was found, which is a mixture of Се(MoO4)2, Се2Mo3O12 and PtO2 (ЈCPDS, 57-330). The sample also contains individual large dense crystals with cut features represented by the reflections with hexagonal arrangement, and they are related to FeMoO4, CeP, β-MoO3 (ЈCPDS, 37-1445), and PtO2 (ЈCPDS, 23-1306).
Analysis of the obtained data on dimensionality and structure of metal particles of the catalyst active phase allows us to conclude that it is possible to synthesize nanocatalysts with given composition and properties by directed selection of precursors.
Presence of phosphorus in the catalyst prevents the formation of heteroatomic Pt-Fe, Pt-Mo clusters, whereas the introduction of molybdenum into the catalyst composition in the absence of phosphorus increases the dispersion of metal particles and promotes the formation of heteroatomic highly dispersed clusters at calcination. However, it cannot be excluded that such heteroatomic clusters may be formed during the hydrogen treatment of the catalyst (350-400 °C). Metal nanoparticles are localized in the cavity and mouths of zeolite and pores of aluminum oxide. In addition, larger 100-200 Å crystals were detected by electron microscopy method on the smooth surface of zeolite. It is known from the literature [19,20] that reduction of iron (III) into iron (II) and iron (0) is observed under hydrogen treatment of air calcined Pt-Fe/Al2O3 -systems [19]. The formation of Pt-Fe clusters, which facilitates and accelerates the reduction of iron, was detected by NGRS method [20].
At straight-run gasoline hydro refining, attention was paid to the influence of H2:crude ratio on the direction of the process. The data on straight-run gasoline conversion on the Pt-Fe/Al2O3 zeolite-containing catalyst at H2:feed=50:1 are presented in Table 3. In this case, the process was carried out in the temperature range of 350-400 °C. The maximum hydro refining of straight-run gasoline occurs in these conditions. It follows from Table  3, that with H2:feed ratio decreasing to 50/1 the isomerizing activity of the catalyst decreases from 62.4 to 54.3%.  Hydrocracking with formation of C1-C3 hydrocarbons increases from 3.9 to 6.3% at 350 °C. However, at a reduced H2:feed ratio the yield of C10-C13 isoalkanes rises from 15.6 to 23.5% (350 °C). In all studied temperature range the yield of C10-C13 isoalkanes is higher than at H2:feed ratio = 200:1. The detected effect of a significant increase in the yield of aromatic hydrocarbons from 2.3 to 7.5% (Tables 2 and 3) is of particular interest. 0.3-0.5% of olefins were found in the products.
In order to elucidate more fully the role of HBG, the study of the catalyst activity in processing of straight-run gasoline in the absence of hydrogen was carried out. It was shown that the yield of С4-С9 isoalkanes at 350 °С is 62.6%, and with the rise of temperature up to 400 °С the content of isoalkanes in the product mixture falls to 39.0%. At the same time, there is a disproportionation reaction, which leads to C10-C14 isoalkanes and C10-C13 nalkanes appearance. Their yields are maximal at 350 °C (14.3% and 4.0% respectively). The most important is the formation of high-octane aromatic hydrocarbons -5.4% (350 °C) and olefins (6.7%).

Conclusions
Thus, in the absence of hydrogen the yield of C4-C9 isoalkanes is 62.6%, and heavier isoalkanes, aromatic hydrocarbons and olefins appear. Hydrocracking to C1-C3hydrocarbons increases at high temperatures (380-400 °C). At 350 °C the yield of the liquid phase, i.e. gasoline with a sufficiently high octane number is close to 100%. The analysis of the particle size of metals included in the catalyst shows that their dispersion varies within a wide range, from 20 to 200 Å. Fine particles (≤50 Å) can localize in zeolite cavities and pores of aluminum oxide, and large ones-on their outer surface. When the catalyst contacts with n-alkanes, the whole surface participates in the process, but formation of branched isomers of C6+ alkanes is possible only on the outer surface, whereas C1-C4 hydrocarbons of different structure probably appear in zeolite cavities during cracking transformation of nalkanes capable of diffusing deep into the matrix structure. Diffusion of molecules and their transformation increase with temperature, which is confirmed by the increase in cracking of n-alkanes.

Supplementary materials
No supplementary materials are available.

Funding
This research had no external funding.