Synthesis of C2+-oxygenated compounds directly from syngas

APPLIED CATALYSIS

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A: GENERAL

ELSEVIER

Applied Catalysis A: General 131 (1995) 207-214

Synthesis of C 2 +-oxygenated compounds directly from syngas P.-Z. L i n *, D.-B. L i a n g , H.-Y. L u o , C.-H. Xu, H.-W. Z h o u , S.-Y. H u a n g , L.-W. Lin Dalian Institute of Chemical Physics Academia Sinica, P.O. Box 110, Dalian 116023, China

Received 28 November 1994; revised 16 March 1995; accepted 24 May 1995

Abstract Vapor Phase synthesis of C2+-oxygenated compounds from synthesis gas was carried out on a bench-scale apparatus. A Rh-Mn/SiO2 catalyst showed an enhanced performance in terms of product selectivity and durability. Keywords: Ethanol; Hydrogenation; Acetic acid; Rhodium catalyst; Syngas

1. Introduction

The catalytic conversion of syngas to C2 +-oxygenated compounds is a topic of growing interest from both practical and mechanistic points of view. It was reported by Bhasin et al. that C2+-oxygenated compounds such as ethanol, acetaldehyde and acetic acid were selectively obtained from syngas over rhodium catalysts under high-pressure conditions [1]. Ichikawa found that a pyrolyzed rhodium cluster catalyst supported on lanthanum oxide gave ethanol in 61% selectivity [2]. At present many studies have been devoted to seeking new catalysts with high activity and selectivity to C2+-oxygenated compounds from syngas [3,4]. As a consequence of these investigations a rhodium-based catalyst was suggested as a promising candidate for the direct production of C2 +-oxygenated compounds from syngas. The effects of various supports and promoters on the catalytic activity and selectivity to the formation of C2 + -oxygenated compounds have also been studied extensively. * Corresponding author. 0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved S S D I 0 9 2 6 - 8 6 0 X ( 9 5 ) 001 42-5

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In our laboratory, we have also investigated and developed two kinds of rhodium catalysts for the production of C2 +-oxygenated compounds from syngas [ 5 ]. One is Rh-V/SiO2, and the other is Rh-Mn/SiO2. These catalysts exhibited very good activity and selectivity for the production of C2 +-oxygenated compounds from syngas at medium pressure conditions in the small scale apparatus. In order to select a promising catalyst for industrial use and to establish a process for effective production of C2 +-oxygenated compounds from syngas, bench-scale experiments were carried out. The purpose of the bench-scale experiments was as follows: • Comparison of the bench-scale results with those obtained by using a small-scale apparatus. • Selection of a catalyst for practical use by estimating the performance of the catalyst on the bench-scale apparatus. • Determination of the influence of the reaction condition parameters on the catalyst activity and selectivity. • Lifetime test of the catalyst. In the present paper, we report some interesting results of the hydrogenation of CO over a Rh-Mn/SiO2 catalyst on the bench-scale apparatus.

2. Experimental The catalysts used in the present study were prepared by impregnating silica pellets (20-40 mesh, BET area 200 m2/g, Haiyang Chemicals Plant, China) with an aqueous solution of rhodium trichloride (Beijing Institute of Chemical Engineering, China), manganese nitrate (Beijing Agents Plant, China) and small amounts of other metal nitrates as promoters, then followed by drying. The Rh content of the catalyst is 1.0 wt.-% and the R h / M n ratio by weight is 1.1. The reduction of the catalyst is carried out in the reactor in-situ before the reaction. It was performed with pure hydrogen, raising the temperature at 2 K/min up to 623 K, after which it was held constant for at least 8 h. The H2 flow rate was 40 1/h and the pressure was 0.5 MPa. The loading of catalyst is 105 g (200 ml) for each test. It was about 200 times larger in capacity than the small-scale experiments. A schematic diagram of the bench-scale apparatus is shown in Fig. 1. The reaction system consisted of a fixed bed tubular reactor with an external heating system. The tubular reactor was made of 316 L stainless steel, and was 2.0 m in length, 25 mm in internal diameter. At the end of the reduction period, the reactor was cooled to the desired temperature and the pressure of the reaction system was raised to the desired pressure. Then syngas (H2/CO = 2) was fed in to the reactor through a high pressure controller and preheater. The temperatures of reaction were measured by a thermocouple inserted in the catalyst bed. The flow rate of fresh feed syngas and recycle gas were measured by two high pressure flow rate meters separately.

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• OffGas >,¢

Recycle

Compressor

Reactor

h'eheater

A

>1 _[_'~J 11

kJ

Compressor

Separator Separator A B Fig. 1. Schematic flow diagram of the bench-scale apparatus.

The effluent passed through a water cooled heat exchanger. Then the liquid products were condensed in the separator and unreacted gas and gas products passed through a recycle compressor into the reaction system. The recycle gas was analyzed by an on-line gas chromatography column, while the condensed liquid products were analyzed with off-line gas chromatography columns. Most of the reaction gases were recycled to the reactor after cooling. This was quite different from the small-scale experiments since the circulated gases contained considerable amounts of by-product gases. A portion of the recycled gases is purged to control the concentration of the by-products in the feed gas. To reduce the loss of syngas, the reactant gas should contain a certain concentration of the by-products such as methane, for example. In this work, the steady state was achieved at about 100 h from the start of the reaction. Hence, sampling was carried out after 100 h of reaction. Liquid samples were analyzed by collecting the sample for 12 h, while the gas products were analyzed every hour.

3. Results 3.1. Product distribution on the bench-scale experiments

The composition of C2+-oxygenated products and recycle gas on the benchscale experiments are presented in Table 1 and Table 2. These data are taken at a reaction time of 780 h in the durability test of 1000 h. Fig. 2 shows the distribution of liquid phase products and gaseous hydrocarbons from the bench-scale experiments. From Fig. 2 we can find that the Rh-Mn/SiO2

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Table 1 Composition of C2 +-oxygenated products on the bench-scale experiments ~ CHsCHO (wt.-%)

C2HsOH (wt.-%)

C3H7OH (wt.-%)

C2HsCO2CH3 (wt.-%)

C2HsCO2C2H5 (wt.-%)

C4H9OH (wt.-%)

CH3COOH (wt.-%)

19.2

34.8

3.8

4.0

4.5

3.0

30.7

"Reaction conditions: 583 K, 6.0 MPa, H2/CO = 2, SV =43 000 l/kg h. Table 2 Composition of recycle gas on the bench-scale experiments" H2 (vol.-%)

CO (vol.-%)

CO 2 (vol.-%)

CH 4 (vol.-%)

C2Ht, (vol.-%)

C3H s (vol.-%)

C4HIo (vol.-%)

C5HI2 (vol.-%)

56.11

25.79

2.94

1 1.04

2.38

1.33

0.31

0.05

"Reaction conditions: 583 K, 6.0 MPa,

H2/CO =

2, SV = 43 000 1/kg h.

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o

hydrocarbon

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4O

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5

Carbon number Fig. 2. Distribution of products of the bench-scale experiment.

catalyst produced C2 +-oxygenated compounds selectively. Methane was a major component in the gaseous products. Besides methane, hydrocarbons with a carbon number up to 5 were found, but their amount decreased sharply with the carbon number. Fig. 3 presents the relationship between the logarithmic value of the concentration of oxygenated compounds and hydrocarbons with carbon number of these compounds. From Fig. 3 we can also find that it is quite different from the Anderson-Schulz-Flory (ASF) distribution of ordinary Fischer-Tropsch synthesis for oxygenated compound production, but the distribution of hydrocarbons in the gaseous products followed the ASF distribution. According to the distribution of oxygenated and hydrocarbon products, it could be deduced that the reaction intermediate 'CHx' was formed over the catalyst surface and that the rate of chain growth was lower than that of CO insertion into the CHx species on the Rh-Mn/SiOz.

3.2. Comparison of experiment results of bench-scale and small-scale apparatus The comparison of experimental results of the bench-scale and the small scale apparatus is presented in Table 3.

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A: Gerwrd

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1

0

3

2

4

5

c~ar~iJor1 Irulnl~cl~ Fig. 3. Relation of logarithm of product concentration Table 3 Comparison

of the STY of CL + -oxygenated

Apparatus

T(K)

Ph ( MPa)

583 583

3.0 5.0

Table 4 Effect of reaction temperature ( K)

and small-scale

CO conversion” SV (ml/g 30 000 40 000

a CO conversion: single pass conversion h P, partial pressure of CO + Hz.

571 575 578 583

in bench-scale

Condition

Small-scale Bench-scale

Temperature

compounds

versus carbon number.

4.5 3.7

(g/kg

h)

314 292

of CO.

( 9%) STY C1. oxy (g/kg h)

46 53 58 64

STY C+oxy

h)

on the activity and selectivity to C,+-oxygenated

CO conversiot?

( %)

performances

220 251 256 292

compounds”

STY CH, (g/kg 52.5 53.8 59.8 76.8

h)

S C+oxy

(‘%)

57.7 53.2 58.7 52.9

’ Pressure, 6.0 MPa; SV, 40 000 ml/g h. h CO conversion: total conversion of CO

The STY (space-time yield) value of C *+-oxygenated compounds from the bench-scale experiment in Table 3 is, however, lower than that from the smallscale operations. This may be due to the difference in the experimental procedures, since sampling in the bench-scale experiments was done at the steady state of the reaction, while sampling in the small-scale case was implemented at the beginning of the reaction. 3.3. Influence of reaction conditions on activity and selectiviry The influence of reaction conditions used for the C,+-oxygenated compound production over the Rh-Mn/SiO, catalyst was studied in the bench-scale apparatus.

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Table 5 Effect of reaction pressure on the activity and selectivity to C,, -oxygenated compounds~ P (MPa)

CO conversion (%)

STY C,+ oxy (g/kg h)

STY CH4 (g/kg h)

SC2~oxy (%)

4.5 6.0 7.0

59 64 74

253 291 319

78.5 76.8 70.9

48.1 52.9 57.3

~'P, total reaction pressure; temperature, 583 K: SV, 40 000 ml/g h. Table 6 Effect of space velocity on the activity and selectivity to C:. oxygenated compound production ;' SV (ml/g h)

CO conversion (%)

STY C2+oxy (g/kg h)

S C2+oxy (%)

2.7.104 3.5.104 4.5.104

46 43 43

245 258 283

70 64 59

Pressure, 6.0 MPa: temperature, 578 K.

2.50

200

1 O0

I

I

I

I

I

[

)

t

I 1O0

t ;200

:3(1(I

I ,t00

I 500

i 6()0

t 700

I 800

I

150

..-...,

80 v

:>-, 60

40 GO 20

I 900

1 1000

R e a c t . i o n rl'ilI*e (]l)') Fig. 4. Durability of Rh-Mn/SiO 2 catalyst.

The experimental data of CO conversion, STY and selectivity to C2 +-oxygenated compounds under different reaction conditions are presented in Table 4, Table 5 and Table 6. The experimental results indicate that, when the reaction temperature was elevated, the CO conversion, the production of C2 +-oxygenated compounds and the formation of methane increased, while the selectivity to Cz+-oxygenated corn-

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P.-. Linet al. /Applied Catalysis A: General 131 (1995) 207-214

213

pounds remained almost constant. It can also be found that when the total reaction pressure was raised, the conversion of CO and the STY of C2 +-oxygenated compounds increased significantly, but the methane formation decreased. This implied that selectivity to C2 +-oxygenated compounds increased with reaction pressure.

3.4. Test of durabili~ of the Rh-Mn/Si02 catalyst with reaction time The change in catalyst performance with reaction time was measured with the bench-scale apparatus. The measurement was carried out continuously for more than 1000 h. After the first 200 h, the reaction pressure was elevated from 3.0 to 6.0 MPa. Then, most of the measurements were carried out at a pressure of 6.0 MPa and a reaction temperature of 583 K, when a steady state of reaction was achieved. The activity and selectivity to C2 +-oxygenated compounds were found to be quite stationary throughout the whole durability test. In the last 50 h, the reaction pressure was increased to 7.0 MPa to observe the effect of elevated pressure on the activity and C2 + -oxygenates selectivity. The results of the catalyst durability test are shown in Fig. 4. The performance of the Rh-Mn/SiO2 catalyst in the stationary state on the bench-scale experiment is presented as follows: Total reaction pressure(MPa) 6.0 Reaction temperature (K) 583 2 H2/CO in the fresh feed gas 240 Fresh feed gas (l/h) Recycle ratio 19 62 Total conversion of CO (%) 290.6 STY of C2 + -oxygenates (g / kg h) 54.2 Selectivity (%) Carbon balance (%) 99.5

4. Conclusions

• The Rh-Mn/SiO2 catalyst used in our bench-scale experiments exhibited high activity and selectivity for the production of C2 +-oxygenated compounds in CO hydrogenation. The STY of C2+-oxygenated compounds was 290 g/kg h (29 g / g Rh h) at 583 K of reaction temperature and 6.0 MPa of total reaction pressure. • In the oxygenated compounds produced, ethanol, acetaldehyde and acetic acid were the most abundant compounds. This implies that high selectivity to C2 +oxygenated compounds is a unique feature of this newly developed catalyst. • Comparing the results of CO hydrogenation over the Rh-Mn/SiO2 catalyst of the bench-scale experiments with the small-scale operations, we observed that the scale-up effect in the reactor as well as in the catalyst preparation was not very pronounced.

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• Influence of reaction conditions on the Rh-Mn/SiO2 catalyst on the bench-scale experiments showed that the formation of both the desirable C2+-oxygenated compounds and the methane by-product increased with temperature. On the other hand, the production of C2+-oxygenated compounds increased with reaction pressure, while the formation of methane decreased with pressure. • The performance of the Rh-Mn/SiO2 catalyst was very stable, and no deterioration of the catalyst activity was observed during the lifetime test of more than 1000 h.

References [ 1] [2] [3] [4] [5]

M.M. Bhasin, J. Catal., 54 (1978) 120. M. Ichjkawa, Bull. Chem. Soc. Jpn., 51 (1978) 2273. T. Fukushima, J. Phys. Chem., 89 (1985) 4470. P.R. Watson and G.A. Somorjai, J. Catal., 72 ( 1981 ) 347. H. Luo, J. Catal., 145 (1994) 232