Fischer-Tropsch synthesis catalyzed by iron catalyst supported on porous polymers. II. Catalytic results

Reactive Polymers, 12 (1990) 187-192

187

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

F I S C H E R - T R O P S C H SYNTHESIS CATALYZED BY IRON CATALYST SUPPORTED ON P O R O U S POLYMERS II. CATALYTIC RESULTS JEAN-CLAUDE CARLU, CLAUDE CAZE *

Laboratoire de Chimie Macromol~culaire URA CNRS 351, Universitd des Sciences et Techniques de Lille Flandres Artois, 59655 Villeneuve d'Ascq Cedex (France) and FRANCIS PETIT

Laboratoire de Chimie Organique Appliqude URA CNRS 402, Ecole Nationale Supdrieure de Chimie de Lille, BP 108, 59652 Villeneuve d'Ascq Cedex (France) (Received December 23, 1988; accepted in revised form August 28, 1989)

Macroporous styrene-divinylbenzene and 4-vinylpyridine-divinylbenzene copolymers were used as supports for an iron Fischer-Tropsch catalyst. The catalytic systems were studied in batch reactions of uniformly supported catalysts, continuous reactions of uniformly supported catalysts and continuous reactions of fully heterogenised catalysts. We have shown that the particular distribution of products and selectivity observed for the corresponding homogeneous catalytic systems are in general conserved in the supported ones. The supported induces a better selectivity in alkene synthesis, and for the fully heterogenised system there is, depending on the rate of gas flow, a relatively important selectivity in the oxy product.

INTRODUCTION

In the preceding paper [1] we described a study of the distribution of a Fischer-Tropsch catalyst in porous polymeric supports by electron probe X-ray microanalysis. The catalyst was obtained by reduction of Fe(acac)3 (Hacac--pentane-2,4-dione) with Al(Et)3 in benzene solution [2,3] and in the presence of

*

To whom correspondence should be addressed,

0923-1137/90/$03.50

a polymeric support. We have shown that the distribution of the catalyst in the polymeric support depends on the method of preparation of the catalyst. Reduction of the iron before introducing the porous polymeric support gives three types of particles: metallic particles, polymer coated by the catalyst, and a uniform distribution of the catalyst in the support. When the reduction occurs after diffusion of the iron through the porous polymeric support, uniform distribution of the catalyst in the support is obtained. All the

© 1990 - Elsevier Science Publishers B.V.

188 supported catalysts studied in this paper were obtained by this last procedure. The catalysis experiments were carried out using two differents systems, the uniformly homogeneous supported catalysts and fully heterogenized catalysts. The influence of both the chemical nature and the texture of the porous polymers on the catalytic results, has been investigated,

EXPERIMENTAL

and most of the by-products of the catalyst preparation were eliminated by warming slowly to 2 0 0 ° C under a stream of syngas, which was continued for 8 h. The mixture in o,o'-terphenyl was then allowed to react with syngas.

b. Uniformly supported catalysts The supported catalysts were prepared in the same way except that polymer beads (10 g) were introduced before the reduction by AI(Et) 3.

Polymeric materials c. Fully heterogenised catalyst 4-Vinylpyridine (4VP) and styrene (ST) monomers were commercial products (Janssen) which were distilled under vacuum prior to use. Divinylbenzene (DVB) was a commercial product (Merck) containing 35% of pDVB, 15% of m-DVB, 35% of m-ethylstyrene and 15% of p-ethylstyrene. It was used without further purification, The copolymers 4-VP-DVB and S T - D V B were synthesized by suspension radical polymerization in the presence of a porogenic agent, heptane or 2-ethylhexanoic acid, respectively [4-5]. The texture of the beads was characterized in the dry state by pore volume and surface area measurements. Pore volumes were obtained using a mercury porosimeter (Carlo Erba 800). Surface areas were obtained from nitrogen adsorption/desorption following the BET method.

These were prepared by separating the beads and washing with anhydrous benzene several times before drying under vacuum at 150 o C. For catalyst P l l - H , the o, o'-terphenyl used in synthesis of the other catalysts was not added. Table 1 gives the characteristics of the catalysts used.

Catalysis A stirred batch liquid phase reactor was used for the uniformly supported catalysts. A tubular reactor (steel, length 50 cm, diameter 0.4 cm) with a fixed bed of catalyst was used for continuous catalysis. All the catalysis experiments were performed at a pressure of syngas ( C O / H 2 ratio = 1) of 20 atm and at

Preparation of the catalysts a. Homogeneous catalyst [2,3] A solution containing 8.7 × 10 -3 mol of Fe(acac)3 in 50 ml of benzene was prepared under nitrogen. Nitrogen was then replaced by H2, and 8.7 × 10- 3 mol of AI(Et)3 was added. Five minutes later, 2 g of buta-l,3-diene dissolved in benzene was introduced, and 20 re_in after the reduction, 30 g of o,o'terphenyl was added. The resulting mixture was then placed in an autoclave, and benzene

TABLE 1 Characteristics of the catalysts tested Sup- %DVB Area port a Surface

(m2/g-1)

Pore Volume

Fe content

(cm3/g-1)

(meqg-1)

P1 20 112 0.226 0.30 P9 30 174 0.143 0.38 PlO 30 135 0.176 0.37 Pll 30 184 0.693 0.42 a Pl: styrene-DVB copolymer. P9, P10 and Pll: 4VPDVB copolymers.

189 240 or 260 ° C. T h e gaseous p r o d u c t a n a l y s e d b y gas c h r o m a t o g r a p h y [6].

was

o n the c a t a l y t i c results. T h e results are rep o r t e d in T a b l e 2. T w o e x p e r i m e n t s , F2 a n d F4, were c a r r i e d o u t w i t h o u t the s u p p o r t , a n d the results o f these e x p e r i m e n t s are used as references. C o m p a r i s o n of runs 1, 2 a n d 3 shows that the velocity o f the process characterised b y the p e r c e n t a g e of C O c o n v e r t e d is a little l o w e r for the s u p p o r t e d catalysts, b u t the latter catalysts show a lower selectivity in C O 2. T h e h o m o g e n e o u s catalyst F2 gives a d i s t r i b u t i o n o f p r o d u c t s limited to the

Definitions 1. T u r n o v e r rate: (Tr) is expressed as mol o f C O t r a n s f o r m e d b y 1 tool of F e per hour. 2. Selectivity: the selectivity is e x p r e s s e d in c a r b o n units: S ( h y d r o c a r b o n ) + S ( o x y p r o duct) + C O 2 = 100% of r e c o v e r e d c a r b o n , 3. D i s t r i b u t i o n : all the d i s t r i b u t i o n of p r o d ucts cited are m o l a r distributions.

r a n g e C 1 - C 6. A n d this d i s t r i b u t i o n is conserved for the u n i f o r m l y s u p p o r t e d catalysts. T h e high selectivity in the d i s t r i b u t i o n of the p r o d u c t s is a t t r i b u t e d to the size of the particles f o r m e d in this system (40 A) [7-9]. B a l l i v e t - T k a t c h e n k o et al. have s h o w n an influence of the p o r e radii of the s u p p o r t

4. T h e c o n t a c t time (8, in s) is d e f i n e d b y the ratio o f the v o l u m e of the catalyst to the flow rate of syngas,

RESULTS AND

DISCUSSION

In all the catalytic e x p e r i m e n t s the olefins f o r m e d are a-olefins.

( a l u m i n a ) o n the d i s t r i b u t i o n of the p r o d u c t s [10], d u e p r o b a b l y to the large p o r e s of the s u p p o r t s [ 1 0 2 - 1 0 3 ]k]; such d e p e n d e n c e is n o t

Uniformly supported catalysts used in batch reactions

o b s e r v e d in o u r system. T h e c h e m i c a l n a t u r e of the s u p p o r t seems to h a v e n o significant i n f l u e n c e o n the catalytic results, as c a n b e seen if the results of r u n s 2 a n d 3 are c o m p a r e d . T h e c a p a c i t y of

W e have studied the influence of the c h e m ical n a t u r e a n d of the texture of the s u p p o r t TABLE 2

Uniformly supported catalysts used in batch reactions, t =120 rain; *t = 60 min; P = 20 atm. F2 and F4 without support Run

Catalyst T ( ° C) Conversion of CO (%) a Selectivity for CO2 b Selectivity for oxy products Selectivity for hydrocarbons Alkanes C 1-C 7 c Alkenes C 2-C 7 c C1 c C2 C3 C4 Cs C6 C7

1 *

F2 240 9.4 33.4 5.1 61.5 86.0 14.0 52.6 22.1 14.9 5.1 3.7 1.3 0.3

2

3

P9 240 4.3 16.4 9.9 63.7 48.7 51.3 35.1 20.6 24.3 9.2 6 3.2 1.6

P1 240 3.3 10.9 5.1 84.0 48.6 52.4 40.6 23.5 19.1 9 4.7 2.2 0.9

4 *

F4 260 22.3 42.7 3.4 53.9 90.6 9.4 57.4 20.6 13.2 4.5 2.6 1.3 0.3

5

6

7

P9 260 8.6 21.6 7.3 71.1 68.2 31.7 49.3 21.3 17.2 6.7 3.6 1.5 0.4

P10 260 16.2 33.7 1.1 72.2 69.6 26.5 48.7 20.1 15.8 6.1 3.6 1.5 0.6

P11 260 15.1 23.7 4.1 72.2 73.5 26.5 52.0 20.1 15.8 6.1 3.6 1.5 0.6

consumed)/(initial CO)]Xl00. bAll selectivities are expressed in carbon units, c Molar distribution of hydrocarbons.

a [(CO

190 poly-4-vinylpyridine to form complexes with iron salts [11] does not produce any change in the catalytic result. However, we can note two important differences between the supported and unsupported catalysts: (i) a decrease of the C H 4 selectivity and (ii) an important increase of the alkene selectivity (from 14-51%). The temperature dependence of the supported P9 catalyst [obtained by comparison

TABLE 3

catalyst given by comparison of runs 1 and 4. The pore volume has no influence on the selectivity and on the distribution of the reaction products, as can be seen in Table 2~ runs 5 to 7, but support P9, which has the lowest pore volume, gave the smallest conversion of CO.

Uniformly supported catalyst (P10) used in continuous reactions.T = 260 o C Run 8 9 10 11 e(atm) 20 30 30 10 8c(s) 30 28 14 14 Tr 0.52 0.73 0.53 0.44 Selectivity forCOz a 11.6 21.0 13.5 8.7 Selectivity for oxyproducts 3.7 4.5 6.0 4.6 Selectivityfor hydrocarbons 84.7 74.5 80.5 86.6 Alkanes C1-C 7 b 52.9 59.9 51.0 46.8 Alkenes C 2-C7 b 47.1 40.1 49.0 53.2 c1 41.5 43.3 40.2 40.8 c2 26.9 26.1 25.8 26.2 C3 18.9 18.4 19.2 20.2 C4 7.8 8.2 9.0 8.6 c5 3.2 3.1 3.1 3.1 C6 1.7 0.9 2.5 1.0 C7 0 0 0.2 0.1 a All selectivities are expressed in carbon units, b Molar distribution of hydrocarbons.

Uniformly supported catalysts used in continuous reactions

Fully heterogenised catalysts used in continuous reactions

We have tested the catalyst P10 in continuous mode, and the results are reported in Table 3. Comparison of run 6 (Table 2) and run 8 (Table 3) shows the difference between static and continuous methods ( P = 20 atm, T = 260°C). We can note a decrease in the CO 2 selectivity and an increase in the alkene selectivity (37 to 47%), but the distribution of the products is unchanged. A decrease of the contact time (runs 9 and 10) causes a decrease in the turnover rate and in the CO z selectivity, but an increase in the ethylenic selectivity, This last effect could be explained by the fact that olefins have been shown to be the primary products of Fischer-Tropsch synthesis using iron catalysts [12], and they undergo secondary reactions such as hydrogenation or incorporation in the growing chain. Variation of the pressure (runs 10 and 11) gives a variation only in the turnover rate.

The results obtained are reported in Table 4. We can see that these catalysts give the same distribution of hydrocarbons as the uniformly supported catalysts in the liquid phase [comparison between run 7 (Table 2) and run 13 (Table 4)]. Two differences can be noted for these heterogeneous catalysts: (i) a decrease of the CO 2 selectivity, and (ii) an increase of the selectivity for alkenes. These are the same differences as observed previously when comparing batch and continuous use of uniformly supported catalysts. Increasing pressure or temperature (respective comparison between runs 12 and 14 and between runs 15 and 16) results only in an increase of the turnover rate. Catalyst P l l - H (runs 16 to 21) shows the same behaviour as catalyst P l l , except that there is an notable increase of the selectivity of CO 2. Runs 16 to 21 show an important feature of the catalytic system. It

of run 2 (240 o C) and run 5 (260 o C)] shows an increase of the conversion of CO and a decrease of the alkene selectivity (vs. T). This is the same behaviour as that observed for the temperature dependence of the homogeneous

191 TABLE 4 Fully heterogenised catalysts used in continuous reactions: catalyst Pll; runs 17-21 catalyst Pll-H Run T ( o C) P (atm) 8c(s) Tr Selectivity for CO 2 a Selectivity for oxyproducts Selectivity for hydrocarbons Alkanes C1-C 7 b Alkenes C2-C 7 b C1 b C2 C3 C4 C5 C6 C7

12 260 10 1.8 0.74 6.1 4.0 89.9 60.7 39.3 56.0 21.9 13.4 5.3 2.4 1 0

13 260 20 1.8 1.15 6.3 4.1 89.6 58.2 41.8 50.7 20.3 15.0 7,2 4,2 1.9 0.7

14 15 260 240 25 15 1.8 7.14 1.50 0.21 4.5 11.85 4.0 1.75 91.5 86.4 61.6 62.8 38.4 37.2 54.9 56.9 20.2 21.4 13.7 13.4 6.0 5.1 3.2 2.9 1.5 0.3 0.5 0.3

16 260 15 7.14 0.68 10.5 1.85 87.7 64.4 35.6 56.5 19.7 13.5 5.6 4.2 0.2 0.3

17 265 15 0.72 1.7 24.7 12.9 62.4 56.3 43.7 51.5 28.5 14.6 3.5 1.5 0.4 0

18 265 15 1.00 1.24 29.1 9.0 61.8 55.2 44.8 50.3 28.0 14.5 5.8 1.4 0 0

19 265 15 1.45 1.41 34.2 5.9 59.8 57.8 42.2 53.2 26.6 13.9 4.1 1.6 0.6 0

20 265 15 2.30 1.43 34.4 3.5 62.1 57.0 43.0 49.7 28.4 14.6 4.4 2.4 0.5 0

21 265 15 3.4 1.40 28.3 3.4 68.3 58.2 41.8 50.0 28.5 14.3 4.7 1.9 0.6 0

a All selectivities are expressed in carbon units, b Molar distribution of hydrocarbons.

c a n b e seen t h a t a d e c r e a s e in the c o n t a c t t i m e (to 0.72 s) i n d u c e s a significant i n c r e a s e in the selectivity of the o x y p r o d u c t s (13%) while m a i n t a i n i n g a c o n s t a n t high selectivity in alkenes (44%). T h e o x y p r o d u c t s f o r m e d are essentially alcohols ( m e t h a n o l a n d e t h a n o l u p to 90%). A p r o b a b l e e x p l a n a t i o n is t h a t the o x y a n d h y d r o c a r b o n p r o d u c t s are p r o d u c e d in parallel reactions, as h a s b e e n suggested for F i s c h e r T r o p s c h s y n t h e s e s u s i n g C o catalysts [8].

p r i n c i p a l effects of the s u p p o r t are (i) to r e d u c e the r a t e o f the h y d r o g e n a t i o n process, so i n d u c i n g a g r e a t e r selectivity in olefinic p r o d u c t s (especially a - o l e f i n p r o d u c t s ) , a n d (ii) to p r o d u c e o x y p r o d u c t s at high gas velocity rates. T e m p e r a t u r e a n d p r e s s u r e i n f l u e n c e o n l y the t u r n o v e r rate. I n the c o n t i n u o u s syst e m s the d i s t r i b u t i o n of p r o d u c t s a n d the selectivities d e p e n d o n l y o n the gas flow rate.

REFERENCES CONCLUSIONS T h e s u p p o r t e d c a t a l y s t s s t u d i e d h e r e are i m p r e g n a t e d catalysts, thus t h e r e are n o c h e m i c a l b o n d s b e t w e e n the c a t a l y s t a n d the s u p p o r t . T h e c a t a l y s t is f o r m e d b y m e t a l p a r ticles u n i f o r m l y d i s p e r s e d in the p o l y m e r i c p o r o u s b e a d s [1]. T h e s e s u p p o r t e d c a t a l y s t s c o n s e r v e the d i s t r i b u t i o n of the r e a c t i o n p r o d u c t s o b t a i n e d w i t h the a n a l o g o u s h o m o g e n e o u s system, a n d t h e y c a n b e u s e d c o n t i n u o u s l y in a fully h e t e r o g e n i z e d f o r m w i t h o u t loss of their characteristics. T h e

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