Dufresne P.

Henriques C.

Marcilly C.

Ramõa Riberio F.

English

University of Szeged

THE INFLUENCE OF TIN ON THE ACTIVITY AND SELECTIVITY OF Sn-Pd/HY ZEOLITES, IN THE HYDROCRACKING AND HYDRO ISOMERIZATI ON OF n-HEPTANE C. HENRIQUES 3, P. DUFRESNE b, C. MARCILLY b, F. RAM0A RIBEIRO 3 GRUPO DE ESTUDOS DE CATALISE HETEROGÉNEA, Instituto Superior Técnico, Av. Rovisco Pais, 1096 Lisboa Codex, PORTUGAL (a) INSTITUT FRANÇAIS DU PETROLE, B.P.31 1 , 92 506 Rue i 1-Ma 1 ma i son Cédex, FRANCE (b) ABSTRACT The hydroc racking and hydroisomerization of n-heptane on Sn-Pd/HY zeolites have been studied, in order to understand the influence of tin on the activity and selectivity of these catalysts. When the tin content increases, we observe a decrease of the global activity, and a change of the selectivity with a decrease of the molar ratios: isomerization/ /cracking and monobranched/dibranched products. The influence of tin cannot be explained only by the poisoning of the palladium by tin. In fact, the presence of the second metal will probably induce the production of highly active hydrogen species, which diffuse towards the catalyst surface, by a spillover effect, getting in competition with the n-paraffins in the adsorption over palladium. In consequence, there is a change in the balance between the acidic and hydrogenating functions, with a decrease of the hydrogenating activity. INTRODUCTION The hydrocracking of different petroleum cuts, from light naphtas to atmospheric vacuum residue, to produce propane, butane, gasoline, jet fuels and diesel oils [1,2] uses bifunctional catalysts, some of them composed of a noble metal (palladium) on a acidic zeoli te. The hydroisomerization of n-alcanes to upgrade the octane number of the light gasoline fractions also use bifunctional catalysts. The balance between the acidic and the metallic functions has a great influen-ce on the selectivity and activity of the bifunctional catalysts [3,^]. In the last years, the use of bimetallic-acidic catalysts has had a growing interest, namely for the catalytic reforming [5,6]. Consequently, it is important the clarify the role of the second metal [7,8,9] in the behaviour of the bifunctional catalysts. The purpose of this work is to study the influence of tin on the activity and 477 selectivity of Sn-Pd/HY zeolites, in the hydrocracking and hydroisomerization of n--heptane. EXPERIMENTAL Four tin-palladium HY zeolites were prepared (with different tin contents), according to the following procedure: - Preparation of the ammonium Y zeolite, by ion exchange of the NaY with ammonium nitrate solutions; - Dilution of the NH^Y in an alumina gel (50 wt.%), extrusion and calcination in wet air at 500°C for 2 hrs; - Introduction of palladium (Pd (NHJ)J^ +), by using the technique of ion ex-change with competition [10] (the competing ion is NH i (+) ; - Calcination in dry air at 500°C, for 2 hrs, and reduction with hydrogen 2hre, at 450°C; - Introduction of tin, as described in reference [11]. The percentage loadings of tin and palladium on HY zeolite were determined by X-ray fluorescence. The metallic phase was characterized by electron probe microscopy and trans-mission electron microscopy. The n-heptane transformation was carried out in a fix-bed dynamic reactor,under the following conditions: total pressure = 60 bar, molar ratio H^/nC^ = 6.2, temper^ ture from 230 to 290°C. The liquid condensates and the gaseous effluents were analyzed by GLC (100 m squalane capillary column), with a flame-ionization detector. RESULTS Characterization of the metallic phase The metal contents of the catalysts are presented in Table 1: T a b l e 1 Metal contents of catalysts Catalyst Pd ;sn Sn/Pd (wt.l) (wt.£) (molar ratio) Pd HY -v i. Sn/Pd HY 0. .5 0.45 0.25 0.5-- . Sn/Pd HY 0, .8 0.45 0.38 0.8 Sn/Pd HY 2. .2 0.45 1.12 2.2 4 7 8 The character izat ion of the meta l l i c phase, by electron probe microscopy, ena-ble us to conclude that t i n remains on the external surface of the c a t a l y s t s and palladium is well d i s t r i bu ted . As shown in Figure 1, with Sn/Pd HY 0.5 t i n i s l o c a H copy zed on a thin external layer which represents about 20% of the tota l surface of the ca r r i e r . Calculated values from electron microscopy of palladium d i spers ion and average pa r t i c l e s ize were - 30% and 14 A°. Ca ta l y t i c Results ( i ) Effect of the t in content on the a c t i v i t y Figure 2 shows, for each c a t a l y s t , the n-heptane conversion as a function of the temperature. 220 240 260 280 300 T <°C) Fig. 2 - n-heptane transformation on Sn/Pd HY zeolites: conversion against temperature We can conclude that the monometallic catalyst is the most active, and the activity, decreases with the tin content. (ii) Influcence of the tin content on the selectivity In Figure 3, it is represented the molar product distribution, of the transfor mation of n-heptane at the same conversion (30%). 4 8 0 Mol. (%) 4 0 • 3 0 • 2 0 • 10-0-Pd HY 0.45 n - C 4 1.50 i - C 4 r~i n — C 1 c 2 C 3 C 4 C 5 - C 6 i - C 7 96.30 Mol.(%) 4 0 3 0 20 10 0 Sn/Pd HY 0.5 79.27 8.73 XLO 0 . 5 8 n - C 4 9 . 7 2 / > 9 1 4 i " C 4 1.34 0.94 ' C i ' " 5 7 'C3 ' C 4 C 5 ' ' C e ' i ' -C 7 Mol. (%> 4 0 3 0 20-10 0 Sn/Pd HY 0.8 73.64 Mol. (%) 4 0 y\ 0.76 n - C 4 11.12 1 £ 3 ^ > 1 1 . 7 6 i - C 4 1.61 1.10 c2 C 3 c4 C5 C 6 i - C 7 30 20 10-0 Sn/Pd HY 2.2 38.64 26.57 0.15 .2.67 n - C 4 2 ^ 7 / 2 7 . 0 0 i - C 4 2.96 2.01 n I I C1 C2 C3 C 4 C5 C g ¡-C7 rig. 3 Distribution of products produced by transformation of n-heptane on Sn/Pd HY zeolites for the same conversion We can conclude that, as tin content increases, the production of C^ isomers lecreases and that of cracked products increases. For all the catalysts, the main iracking products are butanes and propane in equimolar quantities with a very high sobutane/n-butane ratio. We observe an increase of C^ and C^ products without for-lation of C.j and C2, as tin content increases. Figure 4 shows for each catalyst, the evolution of the molar ratio of the iso-lerization/cracking products (l/C) with the conversion. It can.be remarked that in ill the catalysts, there is a decrease of the l/C ratio, as the conversion increases ly increasing the temperature. This decreasing is more pronounced for the lowest tin :ontent. For a given conversion, the l/C molar ratio decreases with the tin content. 481 Fig. k Isomerization/cracking molar ratio against the conversion for the transfor-mation of n-heptane on Sn/Pd HY zeolites In respect of the distribution of the hydroisomerization products, Figure 5 shows the evolution of the molar ratio of the monobranched/dibranched isomers (M/D) with the conversion. As the conversion increases, there is a decrease of the M/D ratio, which is more pronounced for the lowest tin content. For a given conversion, the M/D ratio decreases as the tin content increases. Conv. (%) Fig. 5 Monobranched/dibranched isomers molar ratio against the conversion for the transformation of n-heptane on Sn/Pd HY zeolites 483 11SCUSSI ON In order to understand the influence of tin on the activity and selectivity of :he bimetallic Sn/Pd HY zeolites, it is important to present the bifunctional mecha-iism [12,13] scheme for the hydroisomerization and hydrocracking of n-heptane, pro-losed by Gu i snet [14]. nC7 mC6 dmC5 iC4+C3 n0 7 m06 ' dm05 nCy . - ' mCg • —' dmC£ " IC+-f 10 ig.6 Bifunctional transformation of n-heptane (n C7) ;m C^: methyl hexanes; d m C,-: dimethyloentanes; C +: carbocations; 0: olefins; Z: light products For high hydrogenating activity, the skeletal isomerization and the cracking of arbocations are, respectively, the rate limiting steps of the hydroisomerization and ydrocracking of n-alcanes [15,16]. For low hydrogenating activity, the dehydrogenation reactions limit the bifun-tional process, and the n-heptane transformation rate is proportional to the metal urface area [16,17] • The evolution of l/C and M/D molar ratios, with the conversion The decrease of the l/C molar ratio, as the conversion increases by increasing he temperature, can be explained by the fact that the activation energy of cracking eactions is higher than the activation energy of isomerization reactions. So, at igher temperatures, the cracking reactions become more important, and the molar atio l/C decreases. Moreover, as cracking products are obtained mainly from dibranched carbocaticns e must wait a similar behaviour for the evolution of l/C and M/D molar ratios as the onversion increases [18]. The evolution of the activity and selectivity of Pd/Sn HY zeolites with the in content The analysis of Figure 2, indicates that the activity of the catalysts, for he same temperature, decreases, as tin content increases. For the same conversion, he l/C and M/D molar ratios (Figures 4,5) also decrease as tin content increases. The bifunctional mechanism described above, offers a good explanation for the hange in the activity and selectivity with the tin content: the behaviour of the n/Pd HY zeol1 i tes; can be explained by a change in the equilibrium of the hydrogena-ing-dehydrogenating/acidic functions, caused by a decrease of the strength of the 48 3 I metal I ic funct ion. -j In fact, the decrease of the activity of the hydrogenating-dehydrogenating function, in a bifunctional catalyst, will give rise to the decrease of the forma-tion of the intermediate olefins and carbocations and consequently to a decrease of the catalyst activity. Moreover, we can also explain the decrease of the l/C and M/D molar ratios wil the decrease of the hydrogenating-dehydrogenating function, in agreement with the mechanism of n-heptane transformation on bifunctional catalysts. In fact, with a hi gher hydrogenating activity the rate of carbocation formation will be greater than carbocation skeletal isomerization and/or carbocation cracking; so we will obtain more isomers products and they will be mainly monobranched products [14]. These modifications in the catalytic behaviour of the Sn/Pd HY zeolites,by thi presence of tin, cannot be explained only by the poisoning of the palladium by tin. In fact, as referred above, only a small fraction of palladium is recovered by tin. So, we proposed that the presence of tin will induce the production of highly activi hydrogen species, easily desorbed from tin and rapidly adsorbed on palladium parti-cles, displacing the equilibrium (a) to the left: H M 2 fast (a) H u . ' H, , ' H„ 1 S G M^ - Palladium; M^ - Tin; S - Support; G - Gaz-phase; H - Adsorbed hydrogen species. So, the concentration of H u species will increase, favouring the displacemen 1 of the equilibrated reaction (b) to the left: P G + M 1 ^ = ° M l + 2 \ (b) P„ - Paraffins in gaz-phase; 0 U - Olefin adsorbed on Palladium, (j M j Therefore, there will be a decrease of the hydrogenating-dehydrogenating function, induced by the presence of tin. As we have seen above, this explains not only the decrease of the activity of the Pd/Sn HY zeolites, with the tin content,bu also the decrease of the l/C and M/D molar ratios: the occupation of an important number of palladium sites by active hydrogen species, will increase the relative acidity of the bifunctional catalysts. So, there is a change in the rate limiting step of the bifunctional mechanisn and the adsorption of the intermediate iso-olefins on the acidic sites will be favc red, with the consequent increase of the formation of dibranched carbocations and 4 8 4 their consecutive cracking. This interpretation is also supported by the results of the product distribu-tion in isoconversion conditions (Fig.3): the increasing of C^ and C^ products,with the tin content, without formation of C^ and C^, are in compliance with a mechanism where the hydrogénation of the intermediate iso-olefins has become the rate limiting step. There is an increase of the carbocations concentration, which can dimeriseand, by consecutive cracking, can originate C^ and Cg products (without the production of Cj and C ^ . CONCLUSION The decrease of the activity and the change of selectivity for the Sn/Pd HY zeolites with the tin content, on the transformation of n-heptane can be explained by a change in the balance between the acidic and metallic functions. The presence of tin will induce the production of highly active hydrogen spe-cies which diffuse towards the catalyst surface, by a spillover effect getting in competition with the n-paraffins in the adsorption over the palladium. In consequen-ce, the hydrogenating-dehydrogenating rate of n-paraffins slows down, becoming the rate limiting step for the bifunctional mechanism. REFERENCES 1. Marcilly, C., Franck, J.P., in "Catalysis by Zeolites", B.lmelik et al., Eds., Elsevier Sci. Pub. Co., Amsterdam (1980), pg. 93. 2. Jacobs, W.L., Thornton, O.P., Chem. Eng., ]]_, (25), 79 (1970). 3. Coonradt, M.L., Garwood, W.E., Ind. Eng. Chem., Proc. Res. Dev., (1), 38 (1964). 4. Ribeiro, F., Marcilly, C., Guisnet, M., Proc. S1*1 Iberoam. Symp. Catalysis, Lis-boa (1984), pg. 1237. 5. Rice, R.W., Kang Lu, J. Catal., 24, 367 (1972). 6. Sittig, M., Handbook of Catalysis Manufacture, Noyes Press Ed., Park Ridge N.Y. (1978), pg. 432. 7. Verbeek, H., Sachtler, W.M.H., J. Catal.. 42, 257 (1976). 8. Kuznetzov, B.N., Ermakov, Yu, I., Ryndin, Yu. A., Lorent, J., Duplyakin, V.K., Pentenero, A., Kinet. Katal., 22, (3), 1200 (1981). 9. Burch, R., Mitchell, A.J., Appl. Catal., 6, 121 (1983). 10. Ribeiro, F., Marcilly, C., Rev. Inst. Fr. Pétrole, 3^. ^05 (1979). 11. US PATENT 4,456,775 (June, 1984). 12. Mills, G.A., Heineman, H., Milliken, T.M., Oblad, A.G., Ind. Eng. Chem., 45, (l), 143 (1953). 13. Sinflet, J.M., Adv. Chem. Eng., 5, 37 (1964). 14. Giannetto, G., Perot, G., Guisnet, M., CNRS International Symposium, Catalysis by Acids and Bases, Sept. 1984, in press. 15. Weisz, P.B., Adv. Catal., V}, 137 (1962). 16. Guisnet, M., Perot, M., in "Zeolites Science and Technology", F. Ramoa Ribeiro et al., Eds., Martinus Nijhoff Publishers, The Hague (1984), pg. 397. 17- Ribeiro, F., Marcilly, C., Guisnet, M., J. Catalysis, 78, 267 (1982). 18. Giannetto, G., Perot, G. , Guisnet, M. , Proc. Iberoam. Synip. Catalysis, Lisboa (1984), pg. 1661. 

The influence of tin on the activity and selectivity of Sn-Pd/HY zeolites, in the hydrocracking and hydroisomerization of n-heptane

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The influence of tin on the activity and selectivity of Sn-Pd/HY zeolites, in the hydrocracking and hydroisomerization of n-heptane

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