~ APPLIED CATALYSIS

A: GENERAL

E L S E V I E R Applied Catalysis A: General 131 (1995) 335-345

Synthesis of light alkenes from syngas on silicalite-1 supported cobalt and cobalt-manganese catalysts

Debasish Das, Gopal Ravichandran, Dipak K. Chakrabarty * Solid State Laboratory, Chemistry Department, Indian Institute of Technology Bombay 400 076, India

Received 19 April 1995; accepted 26 May 1995

Abstrac t

Fischer-Tropsch synthesis of lower hydrocarbons on silicalite-1 supported cobalt and cobalt- manganese catalysts is reported. The catalysts were prepared by impregnating the calcined support with the corresponding metal nitrate solutions. Co304 was found to be the only phase present in the manganese free catalyst, while addition of manganese favoured the formation of the mixed spinel (Co~_ xMn~)aO4 phase. Addition of manganese decreased the reduction temperature of Co304 and also presumably reduced its particle size. The catalysts showed good activity and selectivity for light hydrocarbons in the C2-C4 range, particularly propene and also showed very low water gas shift activity. Addition of manganese increased CO conversion rate at 250°C. The alkene selectivity of the Co/Sil catalyst decreased slightly by the addition of manganese.

Keywords: Fischer-tropsch synthesis; CO hydrogenation; Light alkenes; Cobalt and cobalt-manganese catalysts; Silicalite- 1

1. Introduction

Catalytic conversion of synthesis gas to lower alkenes and gasoline-range hydro- carbons using zeolite supported catalysts is of much interest [ 1-3] . Zeolites, because of their high surface area and small pore size allow high degree of metal dispersion and also influence the product selectivity due to their shape selective character. Acidity of the zeolite support also influences the product pattern. Thus, iron and iron-cobalt catalysts supported on the medium pore acidic zeolite ZSM-5 yield a high fraction of aromatics in the product [ 1,2], whereas silicalite-1 sup- ported iron and cobalt catalysts produce more alkenes [4,5]. However, most catalyst formulations are iron-based and not much has been reported on zeolite

* Corresponding author. E-mail dkc@cc.iitb.ernet.in

0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO926-860X(95)O0141-7

336 D. Das et a l . / Applied Catalysis A: General 131 (1995) 335-345

supported cobalt catalysts. Unlike iron, cobalt oxides do not carburize readily and are more stable. Although the cost of cobalt is higher than that of iron [6], this disadvantage may be compensated if a more stable and selective catalyst can be found.

Rao et al. [7] studied CO conversion reaction on Co/ZSM-5 catalysts prepared by aqueous impregnation and on a physical mixture of cobalt oxide and ZSM-5. The catalysts prepared by impregnation method yielded gasoline range hydrocar- bons with a small fraction of aromatics but the physical mixture yielded much higher fraction of aromatics. Lee and Ihm [ 8 ] also investigated CO hydrogenation reaction over Co/NaY catalysts prepared by aqueous impregnation, carbonyl com- plex impregnation and ion exchange. They observed higher activity for the catalysts prepared by solution impregnation than those prepared by other methods. A high selectivity for Ca hydrocarbons was also reported for Co/NaY catalysts derived from cobalt cluster compounds [9]. Recently Fornasari et al. [ 10] has compared CO hydrogenation activity of Y and ZSM-5 zeolite supported cobalt catalysts and observed that the hydrocarbon products are limited to a chain length of seven carbon atoms with methane as the main product. High alkene selectivity in the C2- Ca fraction was also reported on silicalite-1 supported cobalt catalyst [5 ].

Addition of manganese to iron catalysts has been known to promote the formation of light alkenes [ 11 ]. Recently, we have reported CO hydrogenation over silicalite- 1 supported Fe and Fe-Mn catalysts [12,13[ and observed that the addition of manganese reduces the particle size of the iron oxide precursor making carburiza- tion relatively difficult. The alkene selectivity was favoured by a Fe 3 + oxide phase in the catalysts. Promotional effect of manganese on cobalt catalysts supported on SiO2 and A1203 was also reported [ 14,15 ]. Selective formation of C3 hydrocarbons was also observed on precipitated Co-Mn catalysts [ 16,17]. However, there is no report on the effect of manganese on zeolite supported cobalt catalysts. In view of our earlier findings that Fe-Mn/silicalite-1 catalysts have very high selectivity for C2-C4 alkenes [ 12,13 ], it was thought pertinent to investigate CO hydrogenation on Co-Mn/silicalite- 1 catalysts.

2. Experimental

2.1. Catalyst preparation

Silicalite-1 was synthesized according to the method described in the literature [ 18 ]. Impregnation was carried out by adding the requisite amount of 0.45 M metal nitrate solutions to the freshly calcined support to achieve 10 wt.-% Co and 10 wt.- % of Mn loading. Impregnated catalysts were designated as Co/Sil and Co-Mn/ Sil, where Sil stands for silicalite-1. The impregnated material was dried at 120°C for 12 h and finally calcined at 450°C in air for 6 h.

D. Das et al. /Applied Catalysis A." General 131 (1995) 335-345 337

The Fe/Sil catalyst contained 10 wt.-% Fe and was the same sample used in the earlier work [ 13 ].

2.2. Catalyst characterization

X-ray diffraction patterns of the support and the catalysts were recorded in a Philips X-ray diffractometer PW 1820 with nickel filtered CuK,~ radiation at a scanning rate of 2°/min.

Fourier transform infrared spectra of the samples (as KBr disc) were recorded on a Nicolet Magna 550 spectrometer.

Temperature programmed reduction (TPR) has been studied in a conventional flow apparatus. The detailed method has been described elsewhere [ 12 ].

2.3. CO hydrogenation

Carbon monoxide hydrogenation was carried out at 21 atm in a high pressure continuous flow micro reactor ( BTRS-Jr, Autoclave Engineers, USA). The detailed reaction setup and analytical procedures were described earlier [ 12 ]. The calcined catalyst ( 1 g, particle size: 180-300 mesh) was packed between quartz wool plugs in the reactor and was reduced in-situ in a flow of hydrogen (GIISV = 800 h 1) at 450°C for 12 h prior to CO hydrogenation. The feed gas (premixed He and CO, He:CO = 3:1 ) was used without further purification.

3. Results and discussion

3.1. Catalyst characterization

Powder X-ray diffraction (XRD) pattern of the Co/Sil catalyst (Fig. 1 ) showed lines due to C0304 phase (d--2.84 and 2.43 A) in addition to those due to the silicalite support. Peukert et al. [ 5 ] earlier reported the absence of any XRD lines of cobalt oxide in Co/Sil catalyst which could be due to the low metal loading (4 wt.-%) in their samples. However, XRD pattern of the calcined Co-Mn/Sil sample (Fig. 1 ) did not show any lines due to the Co304 phase. It showed a new peak at d=2 .47 A, which is due to the mixed oxide, (COl_xMnx)304, phase. This line became more intense when the sample was calcined at higher temperatures indi- cating greater abundance of the mixed spinel phase. Formation of such mixed spinel phase was also reported for coprecipitated Co-Mn catalysts [ 20,21 ]. It was reported that this mixed phase on reduction gives Co (fcc) and MnO, and Co (fcc) on prolonged CO hydrogenation transforms to Co (bcc), which is believed to be the active phase [ 21 ].

For Fe-Mn/Sil catalysts, we have shown that the presence of manganese reduces the particles size of ~-Fe203 and it cannot be detected by XRD, although M6ssbauer

338 D. Das et al. /Applied Catalysis A: General 131 (1995) 335-345

d

t -

t -

~b 1's 2'0 z~ 3'0 3's 4'0 ~'s 20 (D~grce) -~

Fig. ]. X-ray diffraction patterns of the support and the catalysts: (A), si]icalite-]; (B), Co/Si] and (C), Co- Mn/Sil (0, C0304; ~', (Cot ,Mn,)304).

spectra confirmed its presence [ 13 ]. However, a similar straightforward conclusion cannot be drawn in the case of Co-Mn/Si l catalyst as the reduction of intensity in the XRD peaks may also be due the formation of the mixed oxide, (Co, _ xMnx)304, phase, which in turn reduces the a m o u n t of C o 3 0 4 phase in the Co-Mn/Si l catalyst.

The FT-IR spectrum (Fig. 2) of Co/Sil catalyst showed an intense band at 667 cm-~ which can be assigned to C0304 phase [ 19]. However, the other band expected at 565 cm 1 was not seen as it merges with a broad silicalite band. The FT-IR spectrum of the Co-Mn/Si l sample was found to be similar to that of Co/ Sil sample except that the band at 667 cm- ~ was of much reduced intensity. It appears that addition of manganese reduced the amount of Co304 phase in the Co- Mn/Sil sample which was possibly due to the formation of the mixed spinel phase.

Temperature programmed reduction (TPR) profiles of the calcined catalysts are given in Fig. 3. The Co/Sil sample showed a single reduction peak at 390°C which

D. Das et al, /Applied Catalysis A: General 131 (1995) 335-345 339

1200 1000 80O 600 t, OO Wovenumbcr$ (cm -1)

Fig. 2. FT-IR spectra of the catalysts: (a), silicalite-l; (b), Co/Sil and (c), Co-Mn/Sil.

is in agreement with the results reported earlier [ 22]. A shoulder was observed at about 450°C. It has been noted by Arnoldy and Moulijn [ 23 ] and also by Stranick et al. [24] that Co/AI203 catalyst contains surface cobalt phases consisting of diffused cobalt ions into the support lattice in addition to Co304 and the former reduces at about 450°C. This will account for the observed shoulder at 450°C in our TPD profile.

TPR of the Co-Mn/Sil catalyst has two distinct peaks. The low temperature peak at 370°C may be assigned to the reduction of Co304 phase. Similar lowering of reduction temperature by manganese addition was also observed for Fe-Mn/Sil samples [ 13]. Although calcined Co-Mn/Sil sample did not show any XRD lines of Co304, it was possibly present as very fine particles and was, therefore, not detected by XRD. The large high temperature peak is due to the reduction of the mixed spinel phase and was confirmed by the reduction of pure (Col xMnx)304 phase under identical conditions.

340 D. Das et al. /Applied Catalysis A: General 131 (1995) 335-345

v

c~

o

c ~

0 ¢.)

o)

o

x 2

(b)

20o 3~o 4~o s~o 660 760 aoo T e m p e r a t u r e / °C

Fig. 3. Temperature programmed reduction spectra of the catalysts: (a), Co/Sil and (b), Co-Mn/Sil.

3.2. Reac t ion s tudies

The results of CO hydrogenation on the Co/Sil catalyst obtained in this work has been compared with those on the other catalysts reported in the literature in Table 1. The Co/Sil catalyst prepared by us attained steady activity after 3-4 h of reaction after which there was no change for 18 hours. The data presented have been obtained after 6 h when the catalysts had steady catalytic activity. Results on Fe/Sil catalyst is also included for comparison. It is seen that the support exerts a strong influence on the product pattern. Thus, although silicalite and ZSM-5 have identical crystal and pore structure, Co/Sil predominantly gave CrC4 hydrocar- bons, mostly alkenes, whereas Co/ZSM-5 gave more of higher hydrocarbons. This obviously is related to the strong acidity of ZSM-5 since silicalite does not have any acidity. Bessell [22] has studied the effect of various supports on cobalt based FT catalysts and shown that the support acidity has a strong influence on product distribution. We found that Co/Sil catalyst gave more alkenes and less paraffins whereas alkenes were practically absent in case of Co/ZSM-5 catalyst. Such dif- ference was also reported by us for Fe/Sil and Fe/ZSM-5 catalysts [ 12]. Since alkenes are adsorbed strongly on acidic ZSM-5, they are likely to undergo further reaction by oligomerization as well as by cyclization. Thus, Co/ZSM-5 catalyst actually acts as a bifunctional catalyst.

Table 1 also shows that Co/Sil catalyst has better activity than the Fe/Sil giving higher CO conversion at a relatively low temperature. This Fe/Sil catalyst was earlier found to give a olefin/paraffin (O/P) ratio of 8.4 at a CO conversion rate of 8.7/zmol (g Fe) - t s ~ at 275°C [13]. However, the reaction was carried out at a different CO:H2 ratio (CO:H2 = 1:1 ) maintaining all other conditions identical. Alkene selectivity of the Co/Sil catalyst was also found to be superior to those of all other supported as well as precipitated catalysts. The low O/P ratio for precip-

D. Das et al. /Applied Catalysis A: General 131 (1995) 335-345

Table 1 CO hydrogenation over cobalt and cobalt-manganese catalysts

341

Catalyst Co/Sil Co/Sil Co/ZSM-5" Co(ppt) b Fe/Sil Co-Mn/Sil Co-Mn/Sil Co- Mn(ppt) b

Co loading 10 10 9 10 c 10 10 50

(wt.-%) Pressure (atm) 21 21 21 7 21 21 21 6 T (°C) 250 275 280 220 300 250 275 220 H J C O 3 3 1 1 3 3 3 1 GHSV (h ~) 1 2 0 0 1200 na 279 1200 1200 1200 250 TOS (h) 6 6 24 122 6 6 6 275 CO conversion 20.2 48.8 56.5 43.9 37.0 33.9 44.3 42.9 (tool-%) (/xmol/g Co 12.4 29.9 22.7 22.8 29.8 s) CO2 (mol%) 3.4 12.1 na na 9.8 6.2 10.2 na Hydrocarbon distribution ( wt.-% ) Cl 17.4 27.3 24.4 19.0 23.2 19.6 24.2 8.5 C2 3.7 6.0 2.7 3.4 7.3 3.6 5.3 3.6 C2= 7.9 8.9 0.0 0.2 5.4 6.5 5.1 3.3 C3 3.7 5.0 2.7 3.9 4.4 4.6 4.0 3.7 C3= 12.0 14.7 0.8 2.1 13.8 13.2 11.6 16.1 Ca 2.2 4.9 5.4 5.7 d 5.8 5.2 4.2 10.0 d

C4= 8.8 9.2 0.8 9.5 10.2 8.0 C~ 44.4 24.1 63.0 63.6 30.4 37.0 37.6 46.9 Total C3 15.7 19.7 3.5 6.0 18.2 17.8 15.6 19.8 O / W 3.0 2.1 0.1 0.3 1.6 2.3 1.5 2.7

"Ref. [ 71. bRef. [ 17].

~Fe loading 10 wt.-%. aTotal C4. ~Alkene:alkane ratio in C2-C4 fraction, na, Not available.

itated Co catalyst was assigned to the high hydrogenation activity of the catalyst [ 17]. It can be seen from the table that under similar conditions of reaction, the Co/Sil catalyst gave a O/P ratio that was almost double of what has been observed for Fe/Sil catalyst.

Fig. 4 shows the distribution of hydrocarbon products against carbon number. A high selectivity for C3 hydrocarbons, about 20% of total hydrocarbons, was observed for Co/Sil and Fe/Sil catalysts, while Co/ZSM-5 and precipitated cobalt catalysts showed poor C3 selectivity. The ordinary Schulz-Flory product distribu- tion would predict formation of much higher amounts of C1 and C2 than C3. Thus, this high yield of C3 implies that the active Co species might be located inside the zeolite pores / cages and thereby imparts shape selectivity. Nazar et al. [ 9 ] observed a similar high selectivity for Ca hydrocarbons on NaY zeolite supported cobalt and iron catalysts. They proposed that the very small iron or cobalt clusters inside the zeolite pores are responsible for such high selectivity of Ca hydrocarbons. Fraenkel and Gates [25] also observed such high C 3 selectivity on reduced Co 2+ ion

342 D. Das et al. /Applied Catalysis A: General 131 (1995) 335-345

30-

~ 25-

20- 0 I-,

15- O ,.Q

~ 10- 0

"lJ

n~ 5-

0 o ~ ~ ~ ~ s

c a r b o n n u m b e r

Fig. 4. Hydrocarbon distribution vs. carbon number: 5 , Co/Sil (250°C); II, Co/Sil (275°C); I~1, Co/ZSM-5 (280°C); 0 , Co(ppt) (220°C) and •, Fe/Sil (300°C) (for other reaction conditions see Table 1 ).

exchanged A zeolite. They have also noted that in case of reduced Co 2 + exchanged Y zeolite the products are mainly C~ and C 4 - C 6 hydrocarbons, mostly n-butane. The observed high selectivity of C3 hydrocarbons in the present study thus may be due to the smaller pores of silicalite-1 in comparison to NaY zeolite which restricts the chain growth beyond three carbon atoms. In case of precipitated catalysts, the poor C3 selectivity may be due to the absence of any micropores in the catalyst, while the acidic sites on the ZSM-5 supported catalyst may favour secondary reactions resulting in poor C3 selectivity.

According to Nazar et al. [9], an increase in reaction temperature leads to the migration and sintering of metal clusters resulting in the formation of larger metal particles on the external zeolite surface which favours the formation of more methane. We also observed increased methane formation with rise in reaction temperature with our catalysts but XRD of the used catalysts did not show any peak due to the metal oxide phase on the zeolite surface. The increased methane formation thus could be due to the reason that formation of methane is thermodynamically more favourable at higher temperatures.

Carbon dioxide is generally formed during FT reaction by a secondary water gas shift reaction. It is important that a good FT catalyst should have low water gas shift activity. Peukert et al. [ 5 ] and McMahan et al. [ 26 ] observed very high water- gas shift activity on Co/Sil and Co/NaX catalysts (about 16% CO2 at 240°C). However, we observed only 5% CO2 formation at 250°C on our catalysts. Increase in reaction temperature from 250 to 275°C was, on the other hand, not beneficial as it produced more methane and CO2.

The results of FT reaction on Co-Mn/Si l and precipitated Cc>-Mn catalysts are also shown in Table 1. It appears that with the addition of manganese to Co/Sil catalysts CO conversion was increased two-fold at 250°C, whereas at 275°C it

D. Das et al. /Applied Catalysis A: General 131 (1995) 335-345

60-

343

20- ro

o ~ ~

T i m e o n s t r e a m / h

Fig. 5. CO conversion vs. time on stream plots: (D, Co/Sil 250°C; 0 , Co/Sil 275°C; [Z, Co--Mn/Sil 250°C and I , Co-Mn/Si1275°C.

remained almost constant. Also on the basis of the cobalt content of the catalysts, it was seen that the Co-Mn/Sil catalyst was much more active than the precipitated catalyst.

The alkene selectivity of the manganese promoted Co/Sil catalyst was found to be quite comparable to that of the precipitated Co-Mn catalyst, although a much higher Hz:CO ratio was used with the former catalyst. Thus, Co-Mn/Sil catalyst was more selective to alkenes even at a H2"CO ratio of 3. Comparison with the unpromoted Co/Sil catalyst show that unlike precipitated catalyst, addition of manganese increased the hydrogenation activity of the catalyst as indicated by a drop in the O/P ratio.

Fig. 5 shows CO conversion as a function of time on stream. The catalytic activity initially increased for 3-4 h on stream after which there was almost no change in activity. On the contrary, we found earlier that the Fe/Sil catalysts attain stable activity very quickly within an hour [ 13 ]. This is not unexpected as unlike iron, cobalt does not carburize readily by a synthesis gas mixture. This indicates that the catalytically active species are formed more slowly on Co catalysts than on Fe catalysts.

Fig. 6 shows the FT-IR spectrum of the Co/Sil and Co-Mn/Sil samples (after reaction at 250°C). It can be seen that an additional pair of bands appear at 2924 and 2850 c m - 1, which are assigned to asymmetric and symmetric C-H stretching vibrations in - - C H 2 - species [ 27 ]. The band at 2924 c m - 1 has also a shoulder near 2970 c m - 1 which can be assigned to asymmetric C-H vibration in -CH3 species [ 27]. A similar pair of bands was also observed in case of used Fe/SiO2 catalysts [ 28 ]. These infrared bands characteristic of -CHz-and -CHa species disappear on calcining the used catalysts in air suggesting complete burn off of the carbonaceous deposits. However, no infrared bands due to aromatic ring C=C stretching vibration has been noticed. In this way, the IR spectrum of the used Co/Sil catalyst resembles that of Co/SiO2, but is different from Co/TiO 2 and Co/AI203 [28]. In the used

344 D. Das et al. /Applied Catalysis A: General 131 (1995) 335-345

(o)

(b)

(c)

p-

3O00 2500 Wav~numbers(crn 4)

Fig. 6. FT-IR spectra of the catalysts: (a), calcined Co/Sil; (b), calcined Co-Mn/Sil; (c) and (d), (a) and (b) after use at 250°C.

Co-Mn/Si l catalyst the intensity of these two peaks increases many-fold showing greater abundance of hydrocarbon species on the surface which is in agreement with the higher activity of the Co-Mn/Si l catalyst.

4. Conclusions

1. C o 3 0 4 w a s found to be the only phase present in Co/Sil catalysts, while in manganese promoted catalysts the presence of the mixed spinel phase, ( C o I _ x M n x ) 304 , w a s observed.

2. Addition of manganese facilitates the reduction of Co304 and also presumably reduces its particle size.

3. Both Co/Sil and Co-Mn/Si l catalysts showed low water gas shift activity and good CO conversion activity and selectivity for alkenes in the C2-C4 range. The observed high selectivity for C3 hydrocarbons was possibly due to formation of small cobalt oxide particles inside the zeolite pores. Alkene selectivity of Co/Sil catalyst was found to be higher than that of Fe/Sil catalyst.

D. Das et al. /Applied Catalysis A: General 131 (1995) 335-345 345

4. With the addition of manganese to Co/Sil catalyst CO conversion increases at 250°C, however, at 275°C there was no further increase.

5. Addition of manganese slightly reduces the alkene selectivity of the Co/Sil catalysts.

Acknowledgements

We gratefully acknowledge the financial support from Department of Science and Technology, New Delhi. Thanks are also due to RSIC, Bombay for recording the FT-IR spectra.

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