Effect of treatment conditions of Pd-zeolite catalysts on activity and selectivity in isomerization of n-hexane

EFFECT OF TREATMENT CONDITIONS OF Pd-ZEOLITE CATALYSTS ON ACTIVITY AND SELECTIVITY IN ISOMERIZATION OF n-HEXANE* KH. M. MINACHEV, V. I. GARANIN, V. V. KHARLAMOV,L. I. PIGUZOVA and A. S. VITUKHINA N. D. Zelilxskii Institute of Organic Chemistry, U.S.S.R. Academy of Sciermes (Received 2 January 1969)

ISOMERIZATION of hydrocarbons is used to increase the octane number of fuel used in internal combustion engines and to obtain n-butane and n-pentane for synthetic rubber manufacture, etc. According to a well known view, isomerization is carried out with a bifunctional catalyst, some components of which take part in processes of dehydrogenation-hydrogenation (Pt, Pd, Ni), while others (AI~Q, A120~"Si02) promote carbonium-ionic regrouping of intermediate olefin [1]. Up to now Pt/A12Oa.HF, Pt/A1203"Si02, Ni/Al~O~.Si02, etc. have been used as isomerization catalysts for saturated hydrocarbons. More effective zeolite catalysts, containing Pt and Pd [2, 3] have recently been developed. However, in contrast to the well known bifunctional catalysts, the activation of zeolite catalysts has been little studied [4]. This paper reports a study of the dependence of the activity and selectivity of an 0.5% Pd/CaY catalyst on preliminary treatment conditions in air, nitrogen and hydrogen. EXPERIMENTAL

A palladium catalyst was prepared by ion exchange by treating powdered zeolite with CaY to give a degree of substitution of Na + by Ca 2+ ions of about 90% equivalent, with an aqueous solution of palladium tetra-aminoehloride. The amount of palladium added to the zeolite was 0.5% wt. n-Hexane was isomerized in a 'continuous apparatus at temperatures of 300-360 °, overall pressure of 30 atm, molar ratio H a : C8H14 of 3.2 and a n-hexane space velocity of 1 hr -1. Reaction products were analysed chromatographically. Three series of experiments were carried ont. In the first series the catalyst was treated, prior to the experiment, in hydrogen at different temperatures ranging from 150 to 550 °. Catalyst reduction time varied from 5 (550 °) to 10 hr (150°). In the second series of experiments the catalyst was treated * Nefteklaimiya 9, No. 6, 808-811, 1969. 227

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in air prior to r e d u c t i o n w i t h h y d r o g e n . I n t h e t h i r d series of e x p e r i m e n t s t h e c a t a l y s t was t r e a t e d in nitrogen before reduction. T h e results o b t a i n e d are shown in Figs. 1-5. DISCUSSION

F i g u r e 1 shows t h e d e p e n d e n c e of the a c t i v i t y a n d selectivity of c a t a l y s t on r e d u c t i o n t e m p e r a t u r e . Catalyst a c t i v i t y increases with increased r e d u c t i o n t e m p e r a t u r e (from 150 to 250 ° ) and t h e n begins to decrease. The m a x i m u m

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3--340; 4--360. Symbols are the same in all Figures. on this curve is, a p p a r e n t l y , due to the fact t h a t at the low t e m p e r a t u r e s of t r e a t m e n t of the c a t a l y s t in h y d r o g e n (up to 250 ° ) palladium is incompletely reduced, c o n s e q u e n t l y d e h y d r o g e n a t i n g a c t i v i t y appears lower. W h e n the % .% ! I°° 100 .

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FIG, 2. Dependence of catalyst activity and selectivity oz~ rechtetion temperature; temperature of treatment in air 380 °.

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catalyst is reduced at increased temperature (300-550 °) both the nature of zeolite acidity may change [5] and possible reduction in the dispersion of the reduced pMladium m a y result in lowering catalytic activity. %

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FIG. 3. Dependence of catalyst activity and selectivity Oll reduction temperature; temperature of treatment in nitroger~ 380°. An increase in catalyst activity during isomerization is evidence of incomplete reduction of catalyst at low temperatures Thus, the degree of con%

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FIG. 4. Dependence of catalyst activity and selectivity on the temperature of treatment in air; reduction temperature 380°. version of n-hexane to branched hexanes increased from 8 (during the first hour of catalysis) to 20% (after 5 fir) when the reduction temperature of the catalyst was 150 °. According to results previously obtained [4], crystalliza-

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tion of the metal during the reduction of the zeolite catalyst with hydrogen is due to the "critical" water content of zeolite at the inception of reduction and to the reduction temperature of the catalyst. I t is assumed t h a t simultaneous reduction and elimination of water from zeolite by hydrogen lowers catalyst activity. After drying at 110 ° 20-24% water was normally present in the catalysts used by the authors. I t m a y therefore be expected t h a t catalyst activity would increase if, prior to reduction, it is treated in air or nitrogen to eliminate water. Experiments confirmed this assumption (Figs. 2 and 3). A comparison of results in Figs. 2 and 3 with those of Fig. 1 shows t h a t the dependence of catalyst activity on reduction temperature remains unchanged, if the catalyst is previously treated in air or nitrogen, but its activity increases. When the catalyst is treated in air or nitrogen before reduction, lowering of activity with increased reduction temperature begins at higher temperatures (350-400 °) than in the case of catalyst reduction without preliminary drying (250-300 °) (Fig. 1). This fact also indicates t h a t the presence of water i n the catalyst, apparently, promotes crystallization of palladium which begins in the presence of water at lower temperatures. From a comparison %

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FIG. 5. Dependence of catalyst activity and selectivity on the temperature of treatment in nitrogen; reduction temperature 380°. of results shown in Figs. 1-3 the following general relationship becomes clear: with an increased reduction temperature of catalyst, not only activity bu~ also selectivity* decreases. In seeking to explain this fact it should be borne in mind t h a t with an increased reduction temperature of a zeolite catalyst, not only is the dispersion of the reduced palladium altered, [4], but zeolite acidity may also change. For example, it was shown [5] t h a t with an increased temperature of treatment of decationized zeolite (¥ type) the number of Bran* "Selectivity" is defined as the ratio of hexane isomer yield to overall conversion.

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stet centres goes t h r o u g h a m a x i m u m a n d the n u m b e r of Lewis centres begins to increase at a t e m p e r a t u r e of 500 °. The effect of catalyst t r e a t m e n t in air a n d nitrogen prior to r e d u c t i o n was studied in more detail. Figures 4 a n d 5 show the d e p e n d e n c e of catMyst a c t i v i t y a n d selectivity on the t e m p e r a t u r e of t r e a t m e n t in air (Fig. 4) and nitrogen (Fig. 5). The c a t a l y s t s were reduced in 10 hr at a t e m p e r a t u r e of 380% Results indicated t h a t c a t a l y s t a c t i v i t y s o m e w h a t increased on raising t r e a t m e n t t e m p e r a t u r e from 150 to 250-300 °, t h e n r e m a i n e d constant up to 450-550 ° . No difference was observed b e t w e e n the t r e a t m e n t of the c a t a l y s t in air or nitrogen. Figures also indicate t h a t catalyst selectivity depends to a slight e x t e n t on t e m p e r a t u r e , of t r e a t m e n t of the catalysts (in air or nitrogen) before reduction. An e x p l a n a t i o n of the relations derived in the course of the t r e a t m e n t of catalysts with air and nitrogen should be sought in the v a r i a t i o n of the degree of d e h y d r a t i o n of zeolite crystals at increased t e m p e r a t u r e and v a r i a t i o n in the ratios of various t y p e s of acid eentres [6]. T h e relations observed in the v a r i a t i o n of a c t i v i t y and selectivity of palladium-zeolite catalysts are explained on the a s s u m p t i o n t h a t h e a t t r e a t m e n t of zeolite in air, nitrogen and h y d r o g e n influences b o t h the acidity of the c a t a l y s t and the condition of palladium in zeolite. F o r a more t h o r o u g h u n d e r standing of the causes of v a r i a t i o n of isomerizing a c t i v i t y and selectivity of a Pd-zeolite c a t a l y s t during activation, the h y d r o g e n a t i n g - - d e h y d r o g e n a t i n g f u n c t i o n of the c a t a l y s t (e.g. in h y d r o g e n a t i o n of u n s a t u r a t e d h y d r o c a r b o n s a n d the " a c i d " function (e.g. cracking of cumene) should be investigated). SUMMARY

l. T h e a c t i v i t y of a 0.5% P d / C a Y c a t a l y s t in isomerization of n - h e x a n e passes t h r o u g h a m a x i m u m on increasing the t e m p e r a t u r e of r e d u c t i o n from 150 to 550 ° . 2. Catalyst a c t i v i t y increases when, prior to reduction, it is t r e a t e d in air or nitrogen. 3. O p t i m u m conditions were f o u n d for t h e a c t i v a t i o n of the catalyst: t r e a t m e n t in air at 380 ° for 5 hr a n d r e d u c t i o n of the catalyst with h y d r o g e n at 250-400 ° for 10 hr. REFERENCES 1. G. A. MILLS, H. HEINEMANN, T. H. MILLIKEN and A. G. OBLAD, Industr. and

Engng. Chem. 45, 134, 1953 2. J. A. RABO, P. E. PICKERT and R. L. MAYS, Industr. and Engng. Chem. 53, 733, 1961 3. Kh. M. MINACHEV, V. I, GARANIN and Ya. L ISAKOV, Uspekhi khimii 35, 2151, 1966 4. U.S.A. Pat. 3201356, 27, 09, 1961. Chem. Abstrs. 63, 14619f, 1965 5. J. W. WARD, J. Catalysis 10, No. 1, 34, 1968 6. J. W. WARD, J. Catalysis 9, No. 3, 255, 1967