- Email: [email protected]
Skeletal Reactions of n-Hexane over PtSiO2 (EUROPT-1)
Applied Catalysis, 43 (1988)Ll-L4 Elsevier Science Publishers B.V., Amsterdam -
Ll Printed in The Netherlands
Skeletal Reactions of n-Hexane over Pt/SiO, (EUROPT-1) Mechanism Changeover Governed by Hydrogen ZOLTAN PAAL* and XU XIAN LUN* Institute of Isotopes of the Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest (Hungary) (Received 31 May 1988, accepted 9 July 1988)
Skeletal reactions of alkanes (isomerization, aromatization, fragmentation and C,-cyclization) over metal catalysts may involve surface intermediates of various degree of dehydrogenation [l-3]. For example, two parallel aromatization pathways were reported [4], one being “direct” cyclization, the other involving open chain unsaturated intermediates [ 51. A large variety of routes were suggested for skeletal isomerization (involving or not involving C&-cyclic intermediates) [l-3,6,7]. Also, two kinds of intermediates were assumed for hydrogenolysis, based on reactions of ethane as a probe molecule [ 81. It seems obvious [9-121 that hydrogen pressure should influence the formation of these intermediates and their actual role in product formation. Recently, plotting selectivities versus conversion was suggested as a way of distinguishing between primary and secondary products. If the selectivity of a given product approaches zero when conversion approaches zero, it is likely that this product is not a primary one. The opposite is also true: a primary product should show a finite selectivity value at conversions near to zero. The method was applied for testing skeletal reactions of n-hexane over Pt/A1203 around 770 K. All product classes were found to be formed by a “direct” pathway over an acidic catalyst [ 131 whereas on a Pt-Sn/AlsOS catalyst treated with lithium, stepwise aromatization seemed to be likely, with increasing alkene selectivity at low conversions [ 141. Since there are much controversies concerning the role of metallic, acidic and bifunctional sites in these reactions [ 151, it seemed to be reasonable to study skeletal reactions of n-hexane over a well-characterized, nonacidic Pt/ SiO, (EUROPT-1) [ 161. The reactions were carried out in a static-circulation apparatus [ 171; two different hydrogen and n-hexane pressures were studied * On leave from Lanzhou Institute of Chemical Physics, Lanzhou, China.
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
L2
at a relatively low temperature: 603 K. Capillary gas chromatography was used for analysis (50 m x 0.32 mm fused silica, CP Sil5 coating). Results have been summarized in Fig. 1. At p (Hz ) = 60 Torr ( 1 Torr = 0.133 kPa) - Figs. la and b - the selectivity of alkenes increases and they tend to be the only products at low conversion values. Actually, a value near to 100% is observed in Fig. lb. All other products seem to have negligible selectivities, the value of which can be taken to be zero by extrapolation. It should be noted, that, due to the finite detection limit, errors are relatively large at the lowest conversion values, especially when the product class consists of several chemical entities, like fragments (Fig. la) or alkenes (Fig. lc ) . The role of n-hexane S,%
5%
a.
n-G
tf2=lo
b. n-c, n,- to
60 50.
50
M)
s,o,,
TR I 603 K
it
0 +
40.
-xx]
et6 Isomer
I]
olefin
*
MCP
I
Benzene
-Ea
30.
-60
zo-
40
lo-
20
v
1 5
5%
c.
SOi
lo
$10
n-c&:H*‘10.240
4o;-+r-===q< * 30
20
10
i
y
5
10
5 &*
10
15 X?L
Fig. 1. Product selectivities as a function of overall conversion. Catalyst: 10 mg EUROPT-1, timeon-stream: varying. The point in brackets in Fig. lc is an estimated value, due to the GC detection limit.
L3
pressure seems to be of secondary importance: although higher p (nH) enhances alkene selectivity, the general picture remains the same. A dramatic change is observed at p(H,) ~240 Torr. Here all C6 product classes seem to have a finite ordinate intercept. The influence of n-hexane pressure on alkene selectivity is more significant here but a “direct” aromatization route seems to be induced by this increased p (H,) . Benzene selectivity is constant at higher conversions. Fragments seem to be secondary products under all conditions, due, likely, to the complex character of their intermediate, to be attached at least by two C-atoms to the surface [l-3,18], It is likely that isomers and methylcyclopentane (MCP) can be formed either via dehydrogenated intermediates or “directly”. As other experiments at otherp ( HZ) values indicate, the latter pathway is typical and the former is rather an exception. It is also seen that the relationship between MCP and isomers is not simple: MCP cannot be regarded as a direct intermediate of isoalkanes. The ratio 2-methylpentane (2MP):3methylpentane (3MP) is around 2 in all cases, indicating that no changeover in the isomerization mechanism occurred. (A predominance of 2MP was regarded as an evidence of prevailing bond shift reaction [ 191. ) We prefer to suggest that the ratio MCP to isomers depends on the “hydrogen utilization” of the common “C” surface complex [ 1,201, which, in turn, determined whether this “C” desorbs as MCP or, after taking up 2 hydrogen atoms, as isomer [ 91. The increased selectivity of MCP at higher p(nH) indicates that “crowded’ surface favours the former reaction. The drop in MCP selectivity at higher conversions indicates a secondary ring opening of ready cyclic molecules. Further experiments are necessary to elucidate the effects of other parameters, such as temperature, catalyst deactivation, etc., as well as the influence of other supports. ACKNOWLEDGEMENT
The research was supported by Grant OTKA 1309.
REFERENCES 1 2 3 4 5 6 7 8
F.G. Gault, Adv. Catal., 30 (1981) 1. Z. Pad1 and P. TQtQnyi,in G.C. Bond and G. Webb (Eds.), Catalysis (Spec. Per. Reports), The Royal Sot. Chem., London, 1982, Vol. 5, p. 80. E.H. van Broekhoven and V. Ponec, Progr. Surf. Sci., 19 (1985) 351. F.M. Dautzenberg and J.C. Platteeuw, J. Catal., 19 (1970) 41. Z. Pad1and P. Tktenyi, J. Catal., 30 (1973) 350. M.A. McKervey, J.J. Rooney and N.G. Sammann, J. Catal. 30 (1973) 330. O.E. Finlayson, J.K.A. Clarke and J.J. Rooney, J. Chem. Sot. Faraday Trans. 1,80 (1984) 345. B.S. Gudkov, L. Guczi and P. Tetinyi, J. Catal., 74 (1982) 207.
L4 9
10 11 12 13 14
15 16 17 18 19 20
Z. Pail, In Z. Pail and P.G. Menon (Eds.), Hydrogen Effects in Catalysis, Marcel Dekker, New York, 1988, p. 449. A. Frennet, in Z. Pail and P.G. Menon (Eds.), Hydrogen Effects in Catalysis, Marcel Dekker, New York, 1988, p. 399. J.B.F. Anderson, R. Burch and J.A. Cairns, J. Catal., 107 (1987) 351. J.B.F. Anderson, R. Burch and J.A. Cairns, J. Catal., 107 (1987) 364. J. Margitfalvi, P. Szedlacsek, M. Hegedtis and F. Nagy, Appl. Catal., 15 (1985) 69. E. Kern-TQlas, M. Hegedtis, S. Gobolos, P. Szedlacsek and J. Margitfalvi, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 689. Z. Pa&l,J. Catal., 105 (1987) 540. G.C. Bond and P.B. Wells, Appl. Catal., 18 (1985) 221 and subsequent four papers. H. Zimmer, Z. Pad1and P. TBt&yi, Acta Chim. Acad. Sci. Hung., 111 (1982) 513. J.R. Anderson and N.R. Avery, J. Catal., 5 (1966) 446. O.V. Bragin, Z. Karpinski, K. Matusek, Z. Pa&land P. TBt&yi, J. Catal., 56 (1979) 219. Y. Barron, G. Maire, J.M. Muller and F.G. Gault, J. Catal., 5 (1966) 428.
Ll Printed in The Netherlands
Skeletal Reactions of n-Hexane over Pt/SiO, (EUROPT-1) Mechanism Changeover Governed by Hydrogen ZOLTAN PAAL* and XU XIAN LUN* Institute of Isotopes of the Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest (Hungary) (Received 31 May 1988, accepted 9 July 1988)
Skeletal reactions of alkanes (isomerization, aromatization, fragmentation and C,-cyclization) over metal catalysts may involve surface intermediates of various degree of dehydrogenation [l-3]. For example, two parallel aromatization pathways were reported [4], one being “direct” cyclization, the other involving open chain unsaturated intermediates [ 51. A large variety of routes were suggested for skeletal isomerization (involving or not involving C&-cyclic intermediates) [l-3,6,7]. Also, two kinds of intermediates were assumed for hydrogenolysis, based on reactions of ethane as a probe molecule [ 81. It seems obvious [9-121 that hydrogen pressure should influence the formation of these intermediates and their actual role in product formation. Recently, plotting selectivities versus conversion was suggested as a way of distinguishing between primary and secondary products. If the selectivity of a given product approaches zero when conversion approaches zero, it is likely that this product is not a primary one. The opposite is also true: a primary product should show a finite selectivity value at conversions near to zero. The method was applied for testing skeletal reactions of n-hexane over Pt/A1203 around 770 K. All product classes were found to be formed by a “direct” pathway over an acidic catalyst [ 131 whereas on a Pt-Sn/AlsOS catalyst treated with lithium, stepwise aromatization seemed to be likely, with increasing alkene selectivity at low conversions [ 141. Since there are much controversies concerning the role of metallic, acidic and bifunctional sites in these reactions [ 151, it seemed to be reasonable to study skeletal reactions of n-hexane over a well-characterized, nonacidic Pt/ SiO, (EUROPT-1) [ 161. The reactions were carried out in a static-circulation apparatus [ 171; two different hydrogen and n-hexane pressures were studied * On leave from Lanzhou Institute of Chemical Physics, Lanzhou, China.
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
L2
at a relatively low temperature: 603 K. Capillary gas chromatography was used for analysis (50 m x 0.32 mm fused silica, CP Sil5 coating). Results have been summarized in Fig. 1. At p (Hz ) = 60 Torr ( 1 Torr = 0.133 kPa) - Figs. la and b - the selectivity of alkenes increases and they tend to be the only products at low conversion values. Actually, a value near to 100% is observed in Fig. lb. All other products seem to have negligible selectivities, the value of which can be taken to be zero by extrapolation. It should be noted, that, due to the finite detection limit, errors are relatively large at the lowest conversion values, especially when the product class consists of several chemical entities, like fragments (Fig. la) or alkenes (Fig. lc ) . The role of n-hexane S,%
5%
a.
n-G
tf2=lo
b. n-c, n,- to
60 50.
50
M)
s,o,,
TR I 603 K
it
0 +
40.
-xx]
et6 Isomer
I]
olefin
*
MCP
I
Benzene
-Ea
30.
-60
zo-
40
lo-
20
v
1 5
5%
c.
SOi
lo
$10
n-c&:H*‘10.240
4o;-+r-===q< * 30
20
10
i
y
5
10
5 &*
10
15 X?L
Fig. 1. Product selectivities as a function of overall conversion. Catalyst: 10 mg EUROPT-1, timeon-stream: varying. The point in brackets in Fig. lc is an estimated value, due to the GC detection limit.
L3
pressure seems to be of secondary importance: although higher p (nH) enhances alkene selectivity, the general picture remains the same. A dramatic change is observed at p(H,) ~240 Torr. Here all C6 product classes seem to have a finite ordinate intercept. The influence of n-hexane pressure on alkene selectivity is more significant here but a “direct” aromatization route seems to be induced by this increased p (H,) . Benzene selectivity is constant at higher conversions. Fragments seem to be secondary products under all conditions, due, likely, to the complex character of their intermediate, to be attached at least by two C-atoms to the surface [l-3,18], It is likely that isomers and methylcyclopentane (MCP) can be formed either via dehydrogenated intermediates or “directly”. As other experiments at otherp ( HZ) values indicate, the latter pathway is typical and the former is rather an exception. It is also seen that the relationship between MCP and isomers is not simple: MCP cannot be regarded as a direct intermediate of isoalkanes. The ratio 2-methylpentane (2MP):3methylpentane (3MP) is around 2 in all cases, indicating that no changeover in the isomerization mechanism occurred. (A predominance of 2MP was regarded as an evidence of prevailing bond shift reaction [ 191. ) We prefer to suggest that the ratio MCP to isomers depends on the “hydrogen utilization” of the common “C” surface complex [ 1,201, which, in turn, determined whether this “C” desorbs as MCP or, after taking up 2 hydrogen atoms, as isomer [ 91. The increased selectivity of MCP at higher p(nH) indicates that “crowded’ surface favours the former reaction. The drop in MCP selectivity at higher conversions indicates a secondary ring opening of ready cyclic molecules. Further experiments are necessary to elucidate the effects of other parameters, such as temperature, catalyst deactivation, etc., as well as the influence of other supports. ACKNOWLEDGEMENT
The research was supported by Grant OTKA 1309.
REFERENCES 1 2 3 4 5 6 7 8
F.G. Gault, Adv. Catal., 30 (1981) 1. Z. Pad1 and P. TQtQnyi,in G.C. Bond and G. Webb (Eds.), Catalysis (Spec. Per. Reports), The Royal Sot. Chem., London, 1982, Vol. 5, p. 80. E.H. van Broekhoven and V. Ponec, Progr. Surf. Sci., 19 (1985) 351. F.M. Dautzenberg and J.C. Platteeuw, J. Catal., 19 (1970) 41. Z. Pad1and P. Tktenyi, J. Catal., 30 (1973) 350. M.A. McKervey, J.J. Rooney and N.G. Sammann, J. Catal. 30 (1973) 330. O.E. Finlayson, J.K.A. Clarke and J.J. Rooney, J. Chem. Sot. Faraday Trans. 1,80 (1984) 345. B.S. Gudkov, L. Guczi and P. Tetinyi, J. Catal., 74 (1982) 207.
L4 9
10 11 12 13 14
15 16 17 18 19 20
Z. Pail, In Z. Pail and P.G. Menon (Eds.), Hydrogen Effects in Catalysis, Marcel Dekker, New York, 1988, p. 449. A. Frennet, in Z. Pail and P.G. Menon (Eds.), Hydrogen Effects in Catalysis, Marcel Dekker, New York, 1988, p. 399. J.B.F. Anderson, R. Burch and J.A. Cairns, J. Catal., 107 (1987) 351. J.B.F. Anderson, R. Burch and J.A. Cairns, J. Catal., 107 (1987) 364. J. Margitfalvi, P. Szedlacsek, M. Hegedtis and F. Nagy, Appl. Catal., 15 (1985) 69. E. Kern-TQlas, M. Hegedtis, S. Gobolos, P. Szedlacsek and J. Margitfalvi, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, p. 689. Z. Pa&l,J. Catal., 105 (1987) 540. G.C. Bond and P.B. Wells, Appl. Catal., 18 (1985) 221 and subsequent four papers. H. Zimmer, Z. Pad1and P. TBt&yi, Acta Chim. Acad. Sci. Hung., 111 (1982) 513. J.R. Anderson and N.R. Avery, J. Catal., 5 (1966) 446. O.V. Bragin, Z. Karpinski, K. Matusek, Z. Pa&land P. TBt&yi, J. Catal., 56 (1979) 219. Y. Barron, G. Maire, J.M. Muller and F.G. Gault, J. Catal., 5 (1966) 428.