Effect of rhenium additives to alumina-platinum catalysts on the conversion of n-heptane

EFFECT OF RHENIUM ADDITIVES TO ALUMINA-PLATINUM CATALYSTS ON THE CONVERSION OF n-HEPTANE *T A. I. ])AVLOVand M. Y~. LEVII~TER V. V. Kuibyshev Polytechnic, Kuibyshev (Received 9 November 1971)

IN ORDE~ to prepare gasoline with an octane number of 93 and more straightrun paraffin gasohnes were subjected to catalytic reforming at increased temperature and contact time. The following disadvantages normally characterize reforming of gasolines under strictly defined conditions with alumina-platinum catalysts: low yield of reforming-gasoline (76-80%), relatively rapid coking o f the catalyst and consequently, short periods between regenerations and short general service life of catalysts. The development of stable catalysts is therefore of foremost significance for reforming plants using paraffin gasoline fractions with a high degree of aromatization. Reports on the development of "rheniforming"* [1-5] pubhshed in the literature are therefore of interest; according to these reports the addition of rhenium to an alumina-platinum catalyst promotes more satisfactory dispersion of platinum and stabilization during operation. This appears likely since rhenium metal is highly heat resistant (m.p. 3170°). Considerable confusion is created by the report concerning the addition of rhenium to an aluminaplatinum catalyst which alters the selectivity of chemical reactions and reduces coke and gas yield. According to the authors, this is the reason why the apphcation of an alumina-platinum-rhenium catalyst enables the process to be carried out at pressures which are reduced compared with those of ordinary reforming and with a 3 : 1 circulation of hydrogen and accordingly at approximately 50 % higher efficiency related to the raw material [5]. Results are given in this paper concerning the effect of rhenium additives on an alumina-platinum catalyst for n-heptane conversion under impulse conditions. EXPERIMENTAL

Mixed alumina-platinum-rhenium catalysts were prepared in several stages. First non-halogenated ?-A120 3 grains (0.5-1.0 mm size) calcined at 550 ° were saturated with a solution of chloroplatinic acid with simultaneous addi* Neftokhimiya 12, No. 6, 845-848, 1972. -~ Presumably rhenium reforming--Translation Editor. 232

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tion of acetic acid in proportion of 1 ~ vol. to give a more uniform distribution of platinum according to particle diameter of alumina. The volume of chloroplatinic acid solution was more than 3 times the volume of alumina. The latter was left in the acid solution for 14-16 hr. The solution became completely discoloured in the meantime. The transparent solution was then decanted and the platinized alumina kept in a desiccator at 60-120 ° for 4 hr and after cooling to room temperature, immersed in a perrhenic acid solution. Perrhenic acid was prepared by dissolving rhenium metal in 10% nitric acid [6]. In the perrhenic acid solution the platinized alumina grains were kept at room temperature for 18-20 hr. Alumina and the platinum and rhenium compounds precipitated on it were dried at 60-140 ° (3 hr) and calcined at 360 ° (2 to 3 hr). Immediately before the experiment the compounds were reduced to metal in dry hydrogen in 3 hr at 520 °. By these methods we prepared six catalyst specimens with a total content of metal components of 0.6 % wt. calculated as alumina and different rhenium and platinum contents, respectively, o The activity of mixed alumina-platinum-rhenium catalysts was studied in a micro-catalytic pulse apparatus described previously [7]. The initial n-heptane under accepted conditions of chromatographic analysis behaved as an individual substance and its constants coincided with data in the literature. Products were analysed in a "Chrom-2" chromatograph with a flame-ioniza~ tion detector under the following conditions: the length of the stainless steel capillary column was 45 m, temperature 50 °, squalane being the liquid phase and nitrogen the carrier gas. The volume of hydrocarbon introduced in the microreactor was 0.002 ml, the amount of catalyst in the mieroreactor--80 and 240 mg, the rate of hydrogen flow at atmospheric pressure being 80 ml/min. RESULTS

Figure 1 shows the relation between yield of n-heptane conversion products, degree of n-heptane conversion and composition of the alumina-platinumrhenium catalyst. The Figure shows that on using an alumina-platinum catalyst (0.6%wt. platinum) the degree of conversion of n-heptane and the hydrocracking activity are maximal, 30.2 and 14% wt., respectively and in the ease of an alumina-rhenium catalyst (0.6% wt. rhenium), the lowest, 16.4 and 6.7

%wt. For mixed platinum-rhenium catalysts with increased rhenium content, the degree of conversion and the hydrocracking activity vary regularly (Fig. 1, curves 3, 4). The yield of n-heptane isomers under the conditions of the test (temperature 520 °, amount of catalyst in the microreactor 80 mg) is slight and does not depend practically on the content of rhenium (Fig. 1, curve 5). The yield of aromatic hydrocarbons passes through a maximum, which corresponds to a platinum content of 0.45%wt. and a rhenium content of 0.15%wt. in the catalyst (Fig. 1, curve 1).

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For comparison the variation of aromatic hy~'oearbon yield (eurw, 2) is given on reducing the eorttent of platimim ill t h e catalyst withotlt approprial e compensation with rhemum. It can he seem t h a t in this case the yieM of aromatie hydrocarbons decreases as platinum eouf.en.t, is reduced. The, presence . f :, maximum on the e u r w , (1) showing aromat,ie hydrocarbon yield apparently proves t h a t rhenium has a modifying eft'eel for a given ratio to platbmm,

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Fro. 1. Relation between product yield and the degree of n-heptane conversiort and tho composition of a l u m i n a - p l a t i n u m - r h e n i u m catalysts: / - - t o t a l yield of aromatic hydrocarbons for mixed catalysts; 2 - - t o t a l yield of aromatic hydrocarbons for alumina-plat i n u m catalysts (without rhenium additivos); 3--degree of conversion of n-heptane; 4 - - t o t a l of products of C~-C~ (inoluding benzene); 5--isoheptanes. Fro. 2. The effect of temperature on product yield and the degree of n-heptane conversion in the presence of alumina--platinum (1) and aluminas-platinum-rhenium (2) catalysts.

Figure 2 shows a coml~rison of results of the conversion of n-heptane under more controlled conditions (catalyst sample of 240 mg) on an alumina-platinum-rhenium catalyst of optimum composition (0.45%wt. platinum -F0-15 %wt. rhenium) and a conventional alumina-platinum catalyst (0.6%wt. platinum} at temperatures ranging from 400 to 500 ° .

Rhenium additives to alumina-platinum catalysts

235

With an increase in temperature an increase is observed for both catalysts in the overall degree of n-heptane conversion, the yield of aromatic hydrocarbons and products of hydrocracking (C1-C6) and coke~-losses. All conversion products remaining on the catalyst and the walls of feed pipes, which do not enter the chromatographic column are covered by the term "coke-~losses". The fact that the curve of isoheptane yield according to temperature passes through a maximum may be explained by the participation of isomers in aromatization and hydrocracLing. The passage of the curve showing alkylcyclopentane yield through a maximum supports the assumption that alkylcyclopentanes take part in aromatization as intermediate products. It can also be seen that on a mixed catalyst with the addition of rhenium the total yield of aromatic hydrocarbons and isoheptanes in the entire temperature range studied is higher and the yields of coke+losses, products of hydro-cracking (C1-Ce), alkylcyclopentanes and the degree of conversion are lower than on an alumina-platinum catalyst. All this may be explained by the more satisfactory dehydro-isomerizationproperties of a mixed catalyst, as a result of which products of Cs-dehydrocyclization of n-heptane change into six-membered naphthenes and then into aromatic hydrocarbons and thus are less involved in undesirable reactions of hydro-cracking and coking. This explanation was confirmed by results of special experiments on dehydro-isomerization of methylcyclopentane to benzene with both catalysts. Therefore, the partial replacement of platinum by rhenium, as well as reducing the cost of the catalyst, intesifies aromatizing properties and reduces the role of secondary reactions. It is significant that our conclusions concerning the modifying effect of rhenium on alumina-platinum catalysts are confirmed by results obtained by Genzel et al. in a review paper read at the VIII International Congress [8], the only difference being that the maximum activity in our case is displaced in the direction of lower gravimetric ratios of rhenium to platinum (0.25-0.30 against

0.5). SUMMARY

1. A study was made of the effect of adding rhenium to alu .mina-platinum catalysts in the conversion of n-heptane under pulse conditions at atmospheric pressure of hydrogen. 2. The aromatizing action of alumina-platinum-rhenium catalysts p a s s e s through a maximum with a rhenium content of about 25~o of the overall content of metals (0.5%wt.). 3. The addition of rhenium to an alumina-platinum catalyst reduces gas and coke formation. REFERENCES

1. USA Patent 34157737, 10. 12. 1968, RZhKhim., 2, P151, 1970 2. USA Patent 3434960, 25. 3. 1969, R Z h g h i m 10, P134, 1970

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A. [. PAVLOVand M. YE. LEVINTER

3. R. L. JACOBSON, H. E. GLUKSDAHL, C. S. MCCOY and R. V. DAVIS, Proceedings of

Division of l~efming, Acad. Press, New York 49, 504-521, 1969 4. G. D. GOULD and C. S. MCCOY, Oil and Gas, J. 68, 48, 1970 5. E. L. JACOBSON and C. S. MCCOY, Hydrocarbon Processing, 49, 1970 6. Kh. M. MINACHEV, M. A. RYASHENTSEVA a~ld B. A. EUDENKO, Izv. AN SSSR, Otd. khim. n., 1960, 1971 7. Yu, V. FOMICHEV, I. V. GOSTUNSKAYA and B. A. KAZANSKH, Izv. AN SSSR, Ser. khim., 1112, 1968 8. Ye. L. POLITTSER, V. GENZEL' and D. S. KHAIES, Sb. protsessy konversii uglevodorodov, vklyuchaya razrabotki v oblasti proizvodstva aromatiki (Processes ()f Hydrocarbon Conversion, I~Lcl,lding Those Taking Place i~l the Field of Producing Aromatic Compounds). Moscow,,, 1971