Applied Catalysis A: General, 79 (1991) 167-180 Elsevier Science Publishers B.V., Amsterdam

167

APCAT 2176

Hydrocarbon synthesis from carbon monoxide + hydrogen on impregnated cobalt catalysts

Part III. Cobalt ( 10% ) /silica-alumina catalysts

J. Rathouskp and A. Zukal J. Heyrovsk$ Institute of Physical Chemistry and Electrochemistry, Dolejs’kova 3, 182 23 Prague 8 (Czechoslovakia), tel. (+42-2)8153945, fax. (+42-2)8584569

and

A. Lapidus and A. Krylova N.D. Zelinsky Institute of Organic Chemistry, Leninskii Prospect 47, Moscow (USSR)

(Received 18 March 1991, revised manuscript received 30 August 1991)

Abstract

The physicochemical properties of 10% Co/silica-alumina cat.&& prepared by impregnation were studied by TPR, thermoanalysis and carbon monoxide adsorption. The temperature at which the cat- alyst was pretreated was found to have a great influence on its adsorption capacity for carbon monoxide. Both reduced and unreduced samples calcined in air at relatively high temperatures adsorbed carbon monoxide, but the reduced ones adsorbed much larger amounts. The adsorption capacity tended to increase with increasing pretreatment temperature. The character of carbon monoxide TPD profiles and the proportions of the individual adsorption forms were found to depend on tbe pretreatment tem- perature and the degree of cobalt reduction. Carbon monoxide was found to be adsorbed in three forms on species originating in cobalt-support interactions, metallic cobalt and on CosO,. Cakination in air was found to cause a decrease of both cobalt reduction and the activity in hydrocarbon synthesis. While the yield of gaseous products remained constant, that of the liquid ones decreased substantially with increasing pretreatment temperature, reaching a maximum value when the degree of cobalt reduction was ca. 40-50%. The average carbon number decreased with increasing cobalt reduction. The catalytic properties of Co/silica-alumina are more similar to those of Co/SiOz than of Co/A&O,, but Co/silica- alumina catalysts are less efficient in polymerization tban either Co/SiO, or Co/Al,Ox. The hypothesis, suggesting that the adsorption centres of weakly bonded carbon monoxide were involved in the produc- tion of liquid hydrocarbons and that the cobalt oxide species act dire&y in this synthesis, was confirmed.

Keywords: cobalt/silica-alumina, hydrocarbon synthesis, synthesis gas.

INTRODUCTION

Cobalt is one of the most active metals in the Fischer-Tropsch synthesis [ 11. Various methods are employed in the preparation of supported cobalt cat- alysts, viz. impregnation of a support with cobalt carbonyl vapour or with co-

6926-866X/91/$03.50 0 1991 Elsevier Science Publishers B.V. Ah rights reserved.

168

balt nitrate solution, coprecipitation of the cobalt component, promoter and support, and physical mixing or kneading of a suitable cobalt compound (e.g. oxide, hydroxycarbonate) with the support with or without a binder. Because of its advantages (high activity, high mechanical strength) the impregnation method is often used.

The physicochemical properties of the support used, the impregnation tech- nique and the pretreatment conditions have a decisive influence on the surface state of cobalt and of cobalt oxide species formed in cobalt-support interac- tions, as well as on the course of these interactions, thus determining the cat- alytic properties. A number of investigations have therefore been devoted to the state of the cobalt surface on various supports pretreated under differing conditions, the cobalt being mostly supported on alumina [ 2-8,10-17,20-351 or on silica [9, 11-13, 17-19, 361. Titania [ll, 13, 171, magnesia [ll, 13, 37, 381, carbon [ 11-131, ZSM-5 [ 11, 39-441 and kieselguhr [45, 461 have also’ been as supports. It may be concluded that several cobalt phases are present in oxidized cobalt/alumina, viz. (i) Co304 crystallites, (ii) Co3+ ions in Co3+- AP+ oxidic crystallites or in well-dispersed surface species, (iii) surface Co2+ ions, (iv) surface or subsurface Co2+ ions in diluted or well-defined spine1 structures, or CoAl,O, crystallites [ 321. In silica supported catalysts Co,O, is the dominant cobalt phase. In several studies the effects of promoter addition [ 41,44,45,47] on the properties of cobalt catalysts have also been investigated.

Only in a few published studies have the activity and selectivity of supported cobalt catalysts been correlated with their surface state [8, 9, 13, 15-17, 34, 351. Two important recent conclusions should be pointed out. First, the ob- served variations in activity cannot be attributed to the primary structure sen- sitivity of carbon monoxide hydrogenation on cobalt catalysts [ 9, 161, e.g. it has been proved that the turnover frequency of Co/SiO, for carbon monoxide hydrogenation was invariant with cobalt dispersion in the range of 6-20% dis- persion [ 91. Secondly, temperature-programmed surface reaction studies of hydrogen with adsorbed carbon monoxide on alumina supported nickel [48- 58] and alumina-supported cobalt [ 161 provide evidence for the existence of two different reaction states for carbon monoxide methanation. Lee and Bar- tholomew [ 161 showed that the two mechanisms for carbon monoxide adsorp- tion and methanation which operated on alumina-supported cobalt were: (i) dissociation of carbon monoxide on the metal, followed by hydrogenation of .a-carbon; and (ii) spillover of carbon monoxide and hydrogen to the support where a CH,O complex was formed, followed by diffusion of the complex to metal crystallites, where it decomposed.

In the first two parts of this series [59,60], the structure of Co/Si02 and Co/Al303 catalysts prepared by impregnation was elucidated. Besides that, it was determined how the variation of the physicochemical properties of these catalysts influences their activity and selectivity in the synthesis of liquid hy- drocarbons from mixtures of carbon monoxide and hydrogen. Eventually, a

169

mechanism was proposed [60] that allowed us to explain some effects of the nature of the support used, those of the pretreatment conditions and those of the catalyst poisoning in the course of the Fischer-Tropsch synthesis.

The aim of the present contribution is to investigate the physicochemical and catalytic properties of cobalt catalysts prepared by impregnation of amor- phous silica-alumina. This support was a commercial product and was syn- thesized by co-gelling of a water-glass solution and ahnninium chloride. Since the alumina content was low, a homogeneous distribution of aluminium ions can be expected. From the survey of the literature which was available to the authors it also appeared that silica-alumina supported cobalt catalysts, to all appearance, had not been investigated very extensively.

EXPERIMENTAL

Materials

The Si02*A1203 support was a synthetic amorphous silica-alumina (AS-37, USSR, a commercial product). It contained 66.98% SiOz, 12.5% A1203, 0.20% Fez03 and 0.30% Na,O. Its specific surface area was 420 m2 g-l, pore volume 0.55 cm3 g-‘, mean pore size 6 nm.

Co/Si02.A1203 catalysts (10%) were prepared by incipient wetness impreg- nation with an aqueous solution of cobalt nitrate, followed by drying at 293 K and calcination in air for 5 h. The number after the hyphen in the catalyst designation indicates the pretreatment temperature.

Procedures

The experimental methods used have been described in detail in refs. 59,60. The thermoanalytical measurements were carried out using a Netzsch STA 409 apparatus with an argon stream at a heating rate of 10 K min-‘. TPR profiles were measured by the continuous flow technique using a thermal con- ductivity detector at a heating rate of 24 K min-l. The degree of cobalt reduc- tion was determined by a chemical method based on the measurement of the volume of hydrogen generated in the dissolution of the reduced cobalt in sul- phuric acid. The carbon monoxide uptake was determined by the pulse chro- matographic method using a thermal conductivity detector at room tempera- ture. Temperature programmed desorption of carbon dioxide was carried out by heating the catalyst in the stream of helium carrier gas from 293 to 973 K at a heating rate of 30 K min-‘. The desorbed species were detected with a TCD apparatus. Catalytic measurements were performed in a continuous flow, fixed bed reactor under atmospheric pressure. The inlet CO/H, ratio was l/2.

170

RESULTS

Thermal analysis

Figs. l-3 show the results of simultaneous TG-DTA measurements of 10% Co/SiOz*A1203-293 and 10% Co/SiOz*A1203-473 and of DSC measurements of 10% Co/Si02*A1203-293.

The decomposition process of 10% Co/SiOz.A1203-293 can be divided, as seen in the DTA diagram (Fig. 1 ), into two regions: (i) up to 500 K with a maximum at 422 K and (ii) 500-approx. 600 K with a maximum at 520 K. Both processes are endothermic. The weight losses during the decomposition are very close to those observed with 10% Co/Si02-293 [59]: 19.0% up to 485 K, 13.6% in the temperature region of 485-573 K and finally 0.9% at 573-1073

0 I 7 5 I z -10 . .Ip

2” -20 -

-30 -

-40 _

- 80 5: 3

. so: P E

- 0

. -40

I I 373 573 773 g73 T (K)

Fig. 1. Simultaneous TG-DTA analysis of 10% Co/Si02-Al,O,-293 in air.

mW/mg

373 473 573 T IKI

Fig. 2. DSC of 10% Co/Si02*A1203-293 in argon:

171

I 373 573 773

g73 T IK)

Fig. 3. Simultaneous TG-DTA analysis of 10% Co/Si02*A1203-473 in air.

K. As regards the results of DSC analysis (Fig. 2), some shifts in the position of both endothermic peaks were found: the maxima are at 407 and 509 K.

The decomposition of 10% Co/SiOa*A1203-473 was found to occur in a single step with the DTA maximum at 429 K. The weight loss up to 573 K reached 9.5%; up to 1073 K it reached 11.4% (Fig. 3).

Temperature programmed reduction

The TPR profiles of 10% Co/SiOp*A120, catalysts dried or calcined at 473 or 673 K are shown in Fig. 4. Two different regions of reduction temperatures can be distinguished, namely at approximately 370-570 K and at temperatures above about 570 K. As reported in ref. 59, a reductive decomposition of cobalt nitrate takes place in the first region (Figs. 4a and 4b). This signal is absent in the TPR profile of 10% Co/SiOz*A1203-673 (Fig. 4c), which is caused by the total decomposition of cobalt nitrate during the course of calcination at 673 K. The high exothermicity of the reductive decomposition of cobalt nitrate on the surface of 10% Co/SiOz.A1203-293 causes a high local overheating, which is the reason for the strong interaction of cobalt oxides with silica-alumina. This interaction causes a broad high-temperature region with a pronounced peak at about 900 K. The reduction of various cobalt oxides originating during calcination at 673 K is the reason why the TPR profile of 10% Co/SiOz-A1203- 673 consists of several high-temperature peaks, the largest one occurring at about 700 K. The intermediate character of the TPR profile of 10% Co/ Si02*Alz03-473 corresponds to the combined action of calcination and the re- ductive decomposition of cobalt nitrate.

172

373 573 773 973 T (K)

373 573 773 973 T IK<)

373 573 773 973 T IKI

Fig. 4. TPR profiles of 10% Co/SiO,*AlzO, pretreated in air at (a) 293 K, (b) 473 K and (c) 673 K.

Carbon monoxide adsorption

The temperature of catalyst pretreatment has a great influence on its carbon monoxide adsorption capacity (Table 1) . A sample of unreduced catalyst dried at room temperature adsorbs practically no carbon monoxide. When pre- treated at a higher temperature, an unreduced catalyst sample becomes able to adsorb a certain amount of carbon monoxide (ca. 0.07408 mmol g-’ cobalt when pretreated at 673 K ). Reduced samples of this catalyst series adsorb substantially larger amounts of carbon monoxide but in this case, too, the car- bon monoxide adsorption capacity tends to increase with increasing catalyst pretreatment temperature.

Depending on the temperature of catalyst pretreatment, carbon monoxide was found to be adsorbed in various forms. Fig. 5 shows TPD profiles of carbon monoxide which was adsorbed on reduced Co/SiOz.A1,03-293 and Co/ Si02.A1203-673 catalyst samples at 293 K. For both these samples, three forms of adsorbed carbon monoxide were detected, desorbing at temperatures (I) up to 500 K, (II) from 500 to 700 K and (III) over 700 K. However, the relative amounts of all these forms were different for both samples (Table 1). While,

173

TABLE 1

Effects of the thermal pretreatment of the 10% CO/SiOx-A&O3 catalyst on carbon monoxide adsorption at 293 K

Reduction conditions: HO, 0.1 MPa, 723 K, 1 h, 3000 h-’

Pretreatment CO adsorption temperature uptake (K) (mmol/g Co)

Relative amounts of forms of CO in TPD

(%I

I II III

293 0.112 64 373 0.115 Not measured 473 0.128 Not measured 573 0.152 Not measured 673 0.151 32 673” 0.071 31 673”~~ 0.083 31 673” Not measured 31 673* Not measured 34

13 23

51 17 0 69 0 69

40 29 61 5

“Unreduced sample. bPretreatment carried out in argon. “Reduced for 45 minutes; the degree of reduction attained 45%. *Reduced for 3 h; the degree of reduction attained 80%.

on the sample pretreated by drying, carbon monoxide was predominantly ad- sorbed as the weakly bonded form I (64% ) , the sample pretreated at 673 K adsorbed carbon monoxide mostly in the more strongly bonded form II (51% ). However, when the carbon monoxide adsorption at room temperature was re- peated after TPD up to 1073 K, only the most weakly bonded form I (T_= 463 K) was detected (Fig. 5c) and the uptake of carbon monoxide was reduced to one half.

The variations of the character of the TPD profile and of the proportions of the individual carbon monoxide adsorption forms with the degree of cobalt reduction are shown in Fig. 6 and Table 1. Two adsorption states, I and III, were found for the unreduced sample pretreated at 673 K, their relative amounts being 30 and 70%, respectively (Fig. 6a, Table 1). At the same time, the char- acter of the carbon monoxide adsorption states did not depend on the manner in which the cobalt nitrate was decomposed, i.e. in air or in inert gas. After this sample had been reduced, new adsorption centres were created, on which car- bon monoxide was adsorbed in the form II. Their relative amount increased with the increasing degree of cobalt reduction. The relative content of the form I may be stated to remain approximately unchanged and to amount to ca. 30% after catalyst reduction, while the relative contents of both of the other forms changed: the relative amount of the form II increased up to 60% at 60% catalyst

373 573 773 973 373 573 773 973

T (K)

c

h 373 573 773 973 373 573 773 973

T (Kl T iKI

Fig. 5. Carbon monoxide TPD spectrum from carbon monoxide adsorption at 293 K on (a) 10% Co/Si02-Alz03-293 reduced in hydrogen at 723 K, (b) 10% Co/Si02*Ale03-673 reduced in hy- drogen at 723 K, (c) on the same catalyst as (b) but carbon monoxide was adsorbed after the first carbon monoxide TPD up to 1073 K had been finished.

Fig. 6. The variation of the character of carbon monoxide TPD profile from carbon monoxide adsorption at 293 K on 10% Co/Si02*Alz03-673 with the degree of cobalt reduction: (a) unred- uced catalysts pretreated either in air (full line) or in argon (dotted line), (b) degree of reduction 45%, (c) degree of reduction 60%.

reduction, while that of the form III decreased to only 5% at the same time (Table 1).

Hydrocarbon synthesis

The results of catalytic measurements over 10% Co/SiOz.A1203 catalysts are summarised in Table 2. Though the optimum temperature of the Fischer- Tropsch synthesis (473-483 K) was found to be the same for all catalysts tested, their activity and selectivity changed, depending on the conditions of pretreatment. Calcination in air was found to cause a decrease of both cobalt reduction and the activity in hydrocarbon synthesis. When the pretreatment

175

TABLE 2

The effects of pretreatment temperature on catalytic properties of 10% Co/SiO,*Al,O, in the synthesis of hydrocarbons from synthesis gas

Sample 1 2 3 4 5

Pretreatment temperature (K) Degree of reduction (% ) Synthesis temperature (K) Catalysts activity ( X lo3 mm01 CO g-’ Co 8-l) Hydrocarbon yield (g mm3)

CI G-G C Tzal

Composition of liquid hydrocarbons in products (% ) Alkenes n-Alkanes i-Alkanes

Fractional composition of abmnes in products ( % ) C,-GUI Cn-C,, C 19+

293 373 473 573 673 69 66 64 62 50

473 483 473 483 473 4.7 4.4 4.3 4.4 3.9

20.6 25.0 21.5 22.0 21.3 17.9 20.3 20.2 20.9 16.0

106.5 95.2 87.1 75.7 159.4 145.0 140.0 128.8 118.6 96.7

10.8 10.8 12.7 12.5 12.7 56.3 55.8 42.1 37.2 36.7 32.9 33.4 45.2 50.3 50.6

69.9 71.9 73.6 73.9 69.6 28.0 25.5 24.8 24.9 28.1

2.1 2.6 1.6 1.2 2.4

temperature increased from 293 to 673 K the total hydrocarbon yield decreased lb-fold. The activity as well as the selectivity of hydrocarbon synthesis changed. While the yield of gaseous products remained constant, that of liquid products decreased l.&fold. Their percentage fell from 73% for Co/Si02*A1203-293 to 61% for Co/Si02*A1203-673. As regards the yields of n- and i-alkanes, the rel- ative content of i-alkanes was found to increase, while that of n-alkanes de- creased with increasing pretreatment temperature. These selectivity changes were probably caused by the increase of catalyst acidity in consequence of the greater interaction of the catalyst components during the pretreatment in air at higher temperatures. The fractional content of alkanes remained practically constant. The completeness of catalyst reduction is substantially influenced by the length of its duration and the temperature. Ten percent Co/Si02*A1203- 293 and 10% Co/SiOz*Alz03-673 catalysts were reduced at temperatures which corresponded to the maxima of their TPR profiles (Table 3). When the former sample was reduced at either 473 or 523 K the degree of cobalt reduction at- tained a maximum of 10%. It was practically inactive in the synthesis of hy- drocarbons from synthesis gas. In contrast to this, all four 10% Co/SiOz.Al,O,- 673 catalysts studied were active. A reduction under less severe conditions (at 603 K), gave a catalyst with a degree of cobalt reduction of 41%. This catalyst

176

TABLE 3

The effects of reduction conditions on 10% Co/SiOs~AlsOs catalyst activity and selectivity in hydrocarbon synthesis from synthesis gas

Sample 1 2 3 4 5 6 7

Pretreatment temperature (K) 293 293 673 673 673 673 673

Reduction temperature“ (K ) 473 523 603 643 723 723b 823 Degree of reduction (% ) 3.5 10 41 49 50 70 77

Synthesis temperature (K ) 523 523 483 483 473 503 523 Conversion ( % ) Very low 19 55 58 63 70 69 Hydrocarbon yield’ (g rnT3)

C, Traces 16.4 13.0 17.3 21.3 30.0 32.8

G-C, 5.5 14.0 19.7 26.0 36.9 33.4

TZll C 65.7 48.3 21.9 95.2 68.2 122.3 85.3 119.7 72.4 132.6 114.5

Composition of liquid hydrocarbons in products ( % ) Alkenes 8.1 9.2 10.1 7.8 4.5

n-Alkanes 37.2 52.1 41.6 49.7 55.3

i-Alkanes 54.7 38.7 48.3 42.5 40.2 Fractional composition of alkanes in products (I )

G-C,, 74.9 66.6 64.5 82.4 93.2

C,,-c,, 21.9 29.5 33.1 14.4 5.1 C Is+ 3.2 3.9 2.4 3.2 1.7

“Beduced for 5 h. bB.educed for 20 h.

catalyzed the hydrocarbon production with a yield of 95.2 g m-‘. The synthesis products contained 71% of the C&+ ion. When the temperature of catalyst reduction was increased to 643 K, the degree of cobalt reduction rose to 49% and the catalyst activity increased as well. The total yield of hydrocarbons attained 122 g m-‘, ca. 70% of which was liquid hydrocarbons. When the re- duction temperature was further increased (up to 823 K) the degree of cobalt reduction rose, but both the liquid hydrocarbons yields (48 g ms3) and the selectivity of their production (42% ) fell.

Lengthening the Co/SiOz.A1203-673 catalyst reduction time from 5 to 20 h at 723 K caused a substantial increase in the amount of metallic cobalt on the catalyst surface (Table 3 ) . Simultaneously, the catalyst activity rose, causing an increased yield of all synthesis products. Though the yield of liquid hydro- carbons was approximately the same for both reduction times, the selectivity of C,, hydrocarbon formation decreased from 56 to 49% when the reduction time was lengthened.

When both the temperature and the duration of catalyst reduction were in- creased, a substantial change in the composition of liquid hydrocarbons formed was observed (Table 3 ): the relative content of i-alkanes increased while that of n-alkanes decreased. The relative content of the C4-CIO fraction increased in the product composition, resulting in a decrease of the average carbon num- ber from 8.8 (at the reduction temperature of 503 K) to 7.5 (823 K).

177

DISCUSSION

The influence of the pretreatment in flowing air on the physicochemical properties and the catalytic activity of 10% Co/SiO, and 10% Co/A1203 cata- lysts has been dealt with in the preceding two parts of this series [ 59,601. The pretreatment conditions also, in many respects, determine the behaviour of the 10% Co/Si02.A1203 catalyst in the hydrocarbon synthesis from synthesis gas, as well as its adsorption properties.

As can be seen from the adsorption measurements, the reduction of cobalt nitrate, which was impregnated onto a Si03*A1203 support, occurs over a wide interval of temperatures (Fig. 4). In contrast to 10% Co/SiO, and 10% Co/ A&O3 catalysts, the increase of pretreatment temperature above 673 K does not afford only a single peak in the TPR profile. The TPR profile of the 10% Co/SiOa*A1203-673 sample is complicated and cannot be explained by the su- perposition of the TPR profiles of Co/SiO, and Co/A1203 samples. A series of interactions, which include all catalyst components, may be supposed to occur.

The variations of the catalyst reduction temperatures chosen according to their TPR profiles (Table 3) showed a substantial influence of this parameter on the yield and composition of the products of hydrocarbon synthesis. From the comparison of the results of the catalytic activity study of 10% Co/ Si02-A1203-293 and the data obtained by both TPR (Fig. 4a) and thermoan- alysis (Fig. 1) , the following conclusions may be drawn. If we assume that the first two peaks in the TPR profile of this catalyst characterize the process of cobalt reduction, the percentage of the first one in the total cobalt reduction was found to be equal to 21% while that of the other one amounted to only 2%. Thus, after the reduction of the catalyst at 523 K, the total percentage of cobalt reduced was 23%. On the other hand, the degree of cobalt reduction, which was determined by the chemical method, equals 4% (Table 3). This difference may possibly be explained by the fact that, in this temperature region, the process of cobalt nitrate thermal decomposition predominates. This conclusion is also supported by the results of thermal analysis. The degree of cobalt reduction remains low because of the extremely low reduction temperature of 473 K.

The 10% Co/SiOz*A1203-293 catalyst, which was treated with hydrogen at 573 K, when the main part of cobalt nitrate was decomposed but had not yet been reduced to a substantial degree, showed a low activity in the hydrocarbon synthesis from synthesis gas. In this case, only gaseous hydrocarbons were found to be formed over this catalyst.

Data obtained on the effects of reduction time and temperature on the ac- tivity and selectivity of 10% Co/Si02.A1203 catalyst in the synthesis of hydro- carbon from synthesis gas confirmed the suggestion, proposed in ref. 60, that the formation of liquid products over cobalt catalysts occurred with the partic- ipation of both oxidic and metallic forms of the active component. The creation

178

and preservation of a certain relation between both components is therefore necessary for the preparation of an active catalyst.

In spite of the fact that the catalysts investigated here exhibit a complex reduction profile, the adsorption of carbon monoxide on reduced samples oc- curs only in three forms (Fig. 6). When comparing the profiles of carbon mon- oxide TPD of unreduced and reduced samples of Co/Si02*A1203-673, the form II, which was found to be absent in the TPD profile of the former samples, may be suggested to characterize carbon monoxide adsorption on metallic cobalt while both the other ones can be ascribed to adsorption on the oxide compo- nent of the catalyst. The form III represents carbon monoxide adsorbed on centres which are being reduced and thus destroyed in the course of catalyst treatment in hydrogen. The decrease of the relative amount of this form with increasing degree of cobalt reduction confirms this conclusion (Fig. 6, Table 1). On the basis of adsorption studies reported in ref. 59, we can suggest that the form III corresponds to the carbon monoxide adsorption on Co,O,, which was formed on the surface of the catalyst during calcination in flowing air.

The most weakly bonded form of adsorbed carbon monoxide (form I) may be ascribed to the centres that change only little during the course of catalyst reduction. These centres may correspond to cobalt-aluminium compounds, which are difficult to reduce [ 321, or to the support itself.

As regards the catalytic properties, Co/SiO,*Al,O, catalysts are more simi- lar to Co/SiO, catalysts than to Co/A&O3 ones. Thus, the activity of both Co/ SiO, and Co/SiO,* Al,O, samples decreased substantially with increasing pre- treatment temperature. Yields of liquid hydrocarbons were reduced by one half in the interval of pretreatment temperatures studied. However, with the Co/ SiOz*A1203 catalyst the liquid hydrocarbon yields were 1.3-1.5 times greater than with Co/SiO,. At the same time, the selectivity of liquid hydrocarbon production over Co/SiOz* AlzO, catalysts dropped from 73% (for samples pre- treated at 293 K) to 61% (at 673 K), while over both Co/SiO, and Co/A1203 this parameter did not change to any great degree.

Over Co/SiOp*A1203 catalysts, a much greater proportion of i-alkanes is formed than over both Co/SiO, and Co/A&O, catalysts (Table 2); this may be explained by a greater extent of isomerization reactions, resulting from the higher acidity of the Si02-A1203 support. The isomerization activity of the SiO,*Al,O, support was found to be increased by the action of pretreatment in the flow of air so that the fraction of i-alkanes in liquid products formed over Co/SiOz*A1203-673 reached 50%.

As regards its polymerization capability, Co/SiOz*A1203 catalysts are less efficient than both Co/SiO, and Co/A120,: over the former catalysts, C,-C,, hydrocarbons are mainly formed (70% ) while over both of the other catalysts the content of this fraction of n-alkanes does not exceed 40% [ 601.

179

CONCLUSION

In this study, the catalytic activity and selectivity in the Fischer-Tropsch synthesis on impregnated Co/SiOP*A1203 catalysts, which were pretreated by drying or calcination in air, have been related to their carbon monoxide ad- sorption properties, especially to the proportions of different forms of carbon monoxide adsorption. The results were found to confirm the hypotheses pro- posed in ref. 60 about the participation of adsorption centres of weakly bonded carbon monoxide in the production of liquid hydrocarbons from synthesis gas and about the direct participation of the oxidic component of supported cobalt catalysts in the synthesis of liquid hydrocarbons.

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