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The Effect of Catalyst Acidity on Product Distribution in Olefin Synthesis
H.E. Curry-Hyde and R.F. Howe (Editors), Natural Gas Conversion I1 0 1994 Elsevier Science B.V. All rights reserved.
449
The Effect of Catalyst Acidity on Product Distributioii in Olefiii Synthesis B. G. Baker and Paul Daly School of Physical Sciences, Flinders University GPO Box 2100, Adelaide, South Australia 5001, Australia.
SUMMARY Catalysts containing cobalt on acidic supports are shown to convert synthesis gas to olefins with enhanced yield in the range C,-C,. The supports, silica-alumina and tungstedalumina have strong activity for skeletal isomerization and double bond shift of C, and C, olefins. This isomerization function is retained in the cobalt Fischer-Tropsch catalysts resulting in a mix of isomeric olefins with a product distribution which peaks at C,. 1. INTRODUCTION
The conversion of synthesis gas to light olefins is recognised as an important route to high value fuel components. [1,2]. Selectivity to olefins can be achieved by some FischerTropsch catalysts but with a product distribution which follows the usual chain growth kinetics [3]. In the present work it is found that catalysts prepared by depositing cobalt on acidic supports produce an enhanced yield of light olefins with an unusual carbon number distribution. There have been numerous reports of strong departures from the Schulz-Flory equation for overall carbon number distribution and various explanations proposed [4]. In our results the distributions for alkanes and olefins are considered separately. The unusual distribution applies to the olefins only and is associated with isomerization reactions.
2. RESULTS 2.1 Acidic Catalyst Supports Two types of support for cobalt catalysts have been investigated: a commercial silicaalumina and prepared tungstedalumina. The silica-( 13%) alumina (Strem) was crushed and sieved to 125-150 iim; surface area -450m2g-I Tungstedalumina catalysts were prepared from y-alumina (Merck), 125- 150pm by adsorption from a sodium tungstate solution at 85C and at controlled pH. After washing, drying and calcining at 400C this catalyst contained 7% WO, and had surface area -80m2g-', Another series of catalysts was prepared by depositing tungsten on a heat-treated (HT) alumina made by heating y-alumina at 1200C for 30 min [3]. This catalyst contained 9% WO, and had surface area -20m2g-'. Previous studies have shown that these act as acid catalysts for the skeletal isomerization of olefins [5]. Double bond shift readily occurs at low temperatures; the branched isomers at 360C for butene and 280C for pentene. An example of
450
TABLE 1 Product distributions at C, (percent) Isomerization of 1 -pentene on silicaalumina 280C, lOOkPa
1-pentene cis-Zpentene trans-2-pentene 2 methyl-2butene pentane other
1.7 8.3 14.6 60.4 1.7 13.3
Fischer-Tropsch synthesis 260C, 800 kPa Cobalt/silica-alumina 4% CO 2% c o 4 15 22 44 15
3 13 17 60 7
the product distribution is included in Table 1. Some cracking and hydrogenation occurs particularly in the initial stages of the reaction on silica-alumina. These are shown as "other" products as in Table 1 . The isomerization reactions occur in the presence of hydrogen and water vapour, conditions similar to those encountered in the F-T synthesis. 2.2 Cobalt on silica-alumina These catalysts were prepared by direct impregnation from a solution of cobalt nitrate at pH = 4.5followed by microwave drying and calcining at 520C. Activity for the F-T reaction was measured in a fixed bed micro reactor system. Data presented in this paper is for reaction at 260C, flow 900 hr-', pressure 800 kPa. The syngas ratio H,/CO = 1. Analyses were by gas chromatography of samples taken by a heated line from a gas sampling valve. Product distributions were assessed after 40 hours of reaction. In figure l(a), results for a cobalt/silica gel catalyst are shown for comparison. The separate plots for alkenes and olefins are parallel. The overall data would therefore follow the Schulz-Flory (S-F) equation with a = 0.76 and with C, below the line. This is typical of most F-T catalysts. The olefin product is straight chain I-alkene. When silica-alumina is the support the distribution of figure l(b) and l(c) are obtained. The alkanes follow the S-F line with a = 0.61 and 0.51 respectively. The olefin distribution is curved with a maximum at C,. A mixture of isomers is formed which approaches the equilibrium distribution at C , as shown in Table 1. The selectivity to olefins and the distortion of the distribution is greatest at the lowest cobalt loading of 2 per cent. Methane, not shown in Figure 1, amounts to 20-25% of product from these catalysts. 2.3 Cobalt on tungsten-alumina The preparation of the supports was described in 2.1. The deposition of cobalt and the test conditions were as in 2.2 The results for catalysts based on y-alumina are in Figure 2. The activities at both 2% and 4% cobalt closely match those of the silica-alumina supported catalysts. The same is true of the olefin distributions and isomer compositions. This result is unexpected for two reasons.
45 1
1000
c
- 100 ul
a,
E
a,
.->
+ 0
5
u
10
1
2
3
4
5
6
7
2
3 4 5 6 7 Carbon number
I
I
I
I
I
I
2
3
4
5
6
7
Figure 1 Product distributions for the reaction of synthesis gas (CO/H, = 1) at 800 kPa, 260C on catalyst: (a) cobalt (6 %)/silica gel; (b) cobalt (4%)/silica-alumina; (c) cobalt (2 %)/silica-alumina: alkanes, 0; olefins, A.
Product distributions for the reaction of synthesis as (CO/H, = 1) at 800 H a , Figure 2 260C, on cobalt/tungsten (7 %) on y-alumina: (a) 2 % cobalt; (b) 4 % cobalt: alkanes, 0; olefins, A. Firstly, attempts to make active F-T catalysts by supporting cobalt on y-alumina were previously unsuccessful. Secondly, while the surface area of y-alumina is less than '/d that of silica-alumina, the test results suggest that the numbers of active sites are similar. The isomerization activity of tungstedy-alumina is essentially the same as for silicaalumina with the exception that the cracking and self-hydrogenation side reactions are much less. The cobalt/tungsten, HT-alumina catalysts behave differently. In Figure 3 it is seen that these catalysts have much lower selectivity to olefins and that there is only slight
452
2
3
L
5
6
7
2
3
L
5
6
7
Carbon number
Figure 3 Product distributions for the reaction of synthesis gas (CO/H, = 1) at 800 Wa, 260C, on cobalt/tungsten (9 %) on HT-alumina: (a) 1 % cobalt; (b) 2 % cobalt: alkanes, 0; olefins, A, curvature in the S-F plot. However the yields of olefins are high relative to the previous series of catalysts which had higher loadings of cobalt. Furthermore, the olefin product is fully isomerized. These results show that the increased yield of alkanes is the distinguishing feature of these catalysts. It is possible that due to the low area of the HT-alumina, even 1 percent loading of cobalt exceeds the available selective sites. 3. DISCUSSION
There have been numerous observations of deviations from the S-F equation for carbon number of distribution. References and some explanations have been cited by Inoue et al [4].It has been suggested that a convex curve, as obtained for olefins in this work, could be explained by (i) technical problems of analysing data, (ii) data taken before reaction reaches a steady state or (iii) reincorporation of products into chain propagation as a result of concentration gradients in a fixed bed reactor. None of these can be accepted as explanations of the present results. Numerous F-T catalysts have been tested under identical conditions in the same apparatus and have regular SF distributions. The product distributions of the present work were stable over a prolonged period of operation. As an experiment, 1-pentene was added to the reactant stream of a cobalt/silica-alumina catalyst. The pentene was converted to isomers but here was no change in the product at other carbon numbers showing that there is no reincorporation of product. It has been claimed that isomerization and double bond shift does not affect product distribution in F-T catalysis [6]. The present results show that acidic supports have a significant effect on the catalytic behaviour of cobalt. The distribution of alkanes, including C,, follows the S-F equation but with 0.5
453
acid site. Such a mechanism might be expected to increase the residence time and hence increase the chain growth probability. Secondly the acid site can shift the double bond. Since I-alkenes are the usual F-T product it is likely that movement away from the terminal carbon atom would interrupt the chain growth. This effect would apply from C, on. The net result is an increased yield at C, and C, with consequently lower yields of higher olefins. Alkane production is not influenced by the above effects either because the precursor is not adsorbed on an acid site or because there are cobalt sites not associated with acid sites. The site for the production of methane cannot be identified from the present results. It has been suggested that cracking of olefins is a source of some methane [6]. The experiment of adding 1-pentene, referred to above did not show any increase in methane. From the data in figures 1 and 2 it is seen that the best selectivity to olefins is achieved with the smaller loading of cobalt. This suggests that the optimum catalyst will have the number of cobalt atoms matched to the number of acid sites and the method of deposition of cobalt chosen to ensure that cobalt selectively adsorbs on these sites.
REFERENCES 1. 2. 3. 4. 5. 6.
R.A. Sheldon, Chemicals from Synthesis Gas, Reidel, Dordrecht, 1983, 71-73 B. Bussemeir, C.D. Frohning and B. Cornils, Hydrocarbon Process, 55(11) (1976) 105. B.G. Baker and N.J. Clark, Studies in Surface Science and Catalysis 31 (1987) 455. M. Inoue, T Miyake and I. Inui, J. Catalysis 105 (1987) 266. B.G. Baker and N.J. Clark, Studies in Surface Science and Catalysis 30 (1987) 483. H. Schulz, B.R. Rao and M. Elstner, Erdoel Kohle, Erdgas, Petrochem. 23 (10) (1970) 651.
449
The Effect of Catalyst Acidity on Product Distributioii in Olefiii Synthesis B. G. Baker and Paul Daly School of Physical Sciences, Flinders University GPO Box 2100, Adelaide, South Australia 5001, Australia.
SUMMARY Catalysts containing cobalt on acidic supports are shown to convert synthesis gas to olefins with enhanced yield in the range C,-C,. The supports, silica-alumina and tungstedalumina have strong activity for skeletal isomerization and double bond shift of C, and C, olefins. This isomerization function is retained in the cobalt Fischer-Tropsch catalysts resulting in a mix of isomeric olefins with a product distribution which peaks at C,. 1. INTRODUCTION
The conversion of synthesis gas to light olefins is recognised as an important route to high value fuel components. [1,2]. Selectivity to olefins can be achieved by some FischerTropsch catalysts but with a product distribution which follows the usual chain growth kinetics [3]. In the present work it is found that catalysts prepared by depositing cobalt on acidic supports produce an enhanced yield of light olefins with an unusual carbon number distribution. There have been numerous reports of strong departures from the Schulz-Flory equation for overall carbon number distribution and various explanations proposed [4]. In our results the distributions for alkanes and olefins are considered separately. The unusual distribution applies to the olefins only and is associated with isomerization reactions.
2. RESULTS 2.1 Acidic Catalyst Supports Two types of support for cobalt catalysts have been investigated: a commercial silicaalumina and prepared tungstedalumina. The silica-( 13%) alumina (Strem) was crushed and sieved to 125-150 iim; surface area -450m2g-I Tungstedalumina catalysts were prepared from y-alumina (Merck), 125- 150pm by adsorption from a sodium tungstate solution at 85C and at controlled pH. After washing, drying and calcining at 400C this catalyst contained 7% WO, and had surface area -80m2g-', Another series of catalysts was prepared by depositing tungsten on a heat-treated (HT) alumina made by heating y-alumina at 1200C for 30 min [3]. This catalyst contained 9% WO, and had surface area -20m2g-'. Previous studies have shown that these act as acid catalysts for the skeletal isomerization of olefins [5]. Double bond shift readily occurs at low temperatures; the branched isomers at 360C for butene and 280C for pentene. An example of
450
TABLE 1 Product distributions at C, (percent) Isomerization of 1 -pentene on silicaalumina 280C, lOOkPa
1-pentene cis-Zpentene trans-2-pentene 2 methyl-2butene pentane other
1.7 8.3 14.6 60.4 1.7 13.3
Fischer-Tropsch synthesis 260C, 800 kPa Cobalt/silica-alumina 4% CO 2% c o 4 15 22 44 15
3 13 17 60 7
the product distribution is included in Table 1. Some cracking and hydrogenation occurs particularly in the initial stages of the reaction on silica-alumina. These are shown as "other" products as in Table 1 . The isomerization reactions occur in the presence of hydrogen and water vapour, conditions similar to those encountered in the F-T synthesis. 2.2 Cobalt on silica-alumina These catalysts were prepared by direct impregnation from a solution of cobalt nitrate at pH = 4.5followed by microwave drying and calcining at 520C. Activity for the F-T reaction was measured in a fixed bed micro reactor system. Data presented in this paper is for reaction at 260C, flow 900 hr-', pressure 800 kPa. The syngas ratio H,/CO = 1. Analyses were by gas chromatography of samples taken by a heated line from a gas sampling valve. Product distributions were assessed after 40 hours of reaction. In figure l(a), results for a cobalt/silica gel catalyst are shown for comparison. The separate plots for alkenes and olefins are parallel. The overall data would therefore follow the Schulz-Flory (S-F) equation with a = 0.76 and with C, below the line. This is typical of most F-T catalysts. The olefin product is straight chain I-alkene. When silica-alumina is the support the distribution of figure l(b) and l(c) are obtained. The alkanes follow the S-F line with a = 0.61 and 0.51 respectively. The olefin distribution is curved with a maximum at C,. A mixture of isomers is formed which approaches the equilibrium distribution at C , as shown in Table 1. The selectivity to olefins and the distortion of the distribution is greatest at the lowest cobalt loading of 2 per cent. Methane, not shown in Figure 1, amounts to 20-25% of product from these catalysts. 2.3 Cobalt on tungsten-alumina The preparation of the supports was described in 2.1. The deposition of cobalt and the test conditions were as in 2.2 The results for catalysts based on y-alumina are in Figure 2. The activities at both 2% and 4% cobalt closely match those of the silica-alumina supported catalysts. The same is true of the olefin distributions and isomer compositions. This result is unexpected for two reasons.
45 1
1000
c
- 100 ul
a,
E
a,
.->
+ 0
5
u
10
1
2
3
4
5
6
7
2
3 4 5 6 7 Carbon number
I
I
I
I
I
I
2
3
4
5
6
7
Figure 1 Product distributions for the reaction of synthesis gas (CO/H, = 1) at 800 kPa, 260C on catalyst: (a) cobalt (6 %)/silica gel; (b) cobalt (4%)/silica-alumina; (c) cobalt (2 %)/silica-alumina: alkanes, 0; olefins, A.
Product distributions for the reaction of synthesis as (CO/H, = 1) at 800 H a , Figure 2 260C, on cobalt/tungsten (7 %) on y-alumina: (a) 2 % cobalt; (b) 4 % cobalt: alkanes, 0; olefins, A. Firstly, attempts to make active F-T catalysts by supporting cobalt on y-alumina were previously unsuccessful. Secondly, while the surface area of y-alumina is less than '/d that of silica-alumina, the test results suggest that the numbers of active sites are similar. The isomerization activity of tungstedy-alumina is essentially the same as for silicaalumina with the exception that the cracking and self-hydrogenation side reactions are much less. The cobalt/tungsten, HT-alumina catalysts behave differently. In Figure 3 it is seen that these catalysts have much lower selectivity to olefins and that there is only slight
452
2
3
L
5
6
7
2
3
L
5
6
7
Carbon number
Figure 3 Product distributions for the reaction of synthesis gas (CO/H, = 1) at 800 Wa, 260C, on cobalt/tungsten (9 %) on HT-alumina: (a) 1 % cobalt; (b) 2 % cobalt: alkanes, 0; olefins, A, curvature in the S-F plot. However the yields of olefins are high relative to the previous series of catalysts which had higher loadings of cobalt. Furthermore, the olefin product is fully isomerized. These results show that the increased yield of alkanes is the distinguishing feature of these catalysts. It is possible that due to the low area of the HT-alumina, even 1 percent loading of cobalt exceeds the available selective sites. 3. DISCUSSION
There have been numerous observations of deviations from the S-F equation for carbon number of distribution. References and some explanations have been cited by Inoue et al [4].It has been suggested that a convex curve, as obtained for olefins in this work, could be explained by (i) technical problems of analysing data, (ii) data taken before reaction reaches a steady state or (iii) reincorporation of products into chain propagation as a result of concentration gradients in a fixed bed reactor. None of these can be accepted as explanations of the present results. Numerous F-T catalysts have been tested under identical conditions in the same apparatus and have regular SF distributions. The product distributions of the present work were stable over a prolonged period of operation. As an experiment, 1-pentene was added to the reactant stream of a cobalt/silica-alumina catalyst. The pentene was converted to isomers but here was no change in the product at other carbon numbers showing that there is no reincorporation of product. It has been claimed that isomerization and double bond shift does not affect product distribution in F-T catalysis [6]. The present results show that acidic supports have a significant effect on the catalytic behaviour of cobalt. The distribution of alkanes, including C,, follows the S-F equation but with 0.5
453
acid site. Such a mechanism might be expected to increase the residence time and hence increase the chain growth probability. Secondly the acid site can shift the double bond. Since I-alkenes are the usual F-T product it is likely that movement away from the terminal carbon atom would interrupt the chain growth. This effect would apply from C, on. The net result is an increased yield at C, and C, with consequently lower yields of higher olefins. Alkane production is not influenced by the above effects either because the precursor is not adsorbed on an acid site or because there are cobalt sites not associated with acid sites. The site for the production of methane cannot be identified from the present results. It has been suggested that cracking of olefins is a source of some methane [6]. The experiment of adding 1-pentene, referred to above did not show any increase in methane. From the data in figures 1 and 2 it is seen that the best selectivity to olefins is achieved with the smaller loading of cobalt. This suggests that the optimum catalyst will have the number of cobalt atoms matched to the number of acid sites and the method of deposition of cobalt chosen to ensure that cobalt selectively adsorbs on these sites.
REFERENCES 1. 2. 3. 4. 5. 6.
R.A. Sheldon, Chemicals from Synthesis Gas, Reidel, Dordrecht, 1983, 71-73 B. Bussemeir, C.D. Frohning and B. Cornils, Hydrocarbon Process, 55(11) (1976) 105. B.G. Baker and N.J. Clark, Studies in Surface Science and Catalysis 31 (1987) 455. M. Inoue, T Miyake and I. Inui, J. Catalysis 105 (1987) 266. B.G. Baker and N.J. Clark, Studies in Surface Science and Catalysis 30 (1987) 483. H. Schulz, B.R. Rao and M. Elstner, Erdoel Kohle, Erdgas, Petrochem. 23 (10) (1970) 651.