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Cyclohexane ring opening on metal-oxide catalysts
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
227
Cyclohexane ring opening on metal-oxide catalysts L.M. Kustov a, T.V. Vasina a, O.V. Masloboishchikova a, E.G. Khelkovskaya-Sergeeva a, and P. Zeuthen b aN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, Moscow, 117334 Russia, e-mail" [email protected] bHaldor Topsoe A/S, Nymollevej 55, Lyngby, DK-2800 Denmark Ring opening of cyclohexane on various metal-containing oxide catalysts is studied in detail. Rhodium supported on neutral alumina is shown to exhibit the highest selectivity toward n-hexane, whereas the formation of side products of cracking, isomerization and dehydrogenation (CI-C4 hydrocarbons, isohexanes + methylcyclopentane, and benzene, respectively) is essentially suppressed.
1. INTRODUCTION Dearomatization of gasoline and diesel fuel is a very important problem of oil refining for the nearest future. Hydrogenation of aromatic compounds present in the fuels can be used to transfer aromatics into naphthenes but the latter can be converted again into the aromatic products and soot particulates in the engine. The most appropriate solution of the problem would be ring opening of aromatics and naphthenes into paraffins that cannot be further transformed into undesirable products. In the case of diesel fuel, ring opening occurring without skeletal isomerization can also result in the gain in the cetane number, while ring opening of C 6 - C 9 cyclic components of gasoline with simultaneous skeletal isomerization can be helpful in boosting the octane number of the motor fuel. Cyclohexane may be considered as a suitable model compound for ring opening. It is well known [1, 2] that the main transformations of cyclohexane over metal-supported catalysts involve dehydrogenation into benzene, hydroisomerization into methylcyclopentane and hydrogenolysis or cracking into low-molecular hydrocarbons, such as CI-C4 light products. Thus, although the reaction of cyclohexane ring opening theoretically could produce n-hexane with a 100% probability via single-step hydrogenolysis of one of the C-C bonds, the real pattern is much more complicated and includes the whole range of possible reactions as shown in the Scheme. The formation of n-hexane from cyclohexane was also studied in [3, 4]. The most interesting and commercially viable catalytic system based on l%Pt/H-Beta zeolite was developed by Mobil Company [5]. This catalyst exhibits rather high activity in ring opening of C6 cyclic compounds and is used in a combination with the isomerization catalyst to improve the C6 downstream feed. In this case, however, a 1 : 3 mixture of normal and isoparaffins was formed at conversions of-80% at 230-270~ under high-pressure conditions.
228 REACTION SCHEME
H2
H
G
--,,. / C 1 -C5
Another catalyst proposed by the same authors for ring opening of C6 cyclic compounds comprises a Pt/WO3/ZrO2 system [6]. The performance of this catalyst is similar to that of Pt~-Beta zeolite, but the reaction conditions are different. The directions of cyclohexane transformations should be obviously dependent on the nature of the supported metal (Pt, Ru, Rh, Ni, etc.) and on the reaction conditions. Therefore, the goal of this paper was to study the influence of different factors (nature of the metal and loading, type of the carrier, reaction temperature and pressure, space velocity, hydrogen-tohydrocarbon ratio) on ring opening of cyclohexane on oxide catalysts promoted with noble metals. 2. EXPERIMENTAL
Neutral 7-alumina, fluorinated 7-alumina, and silica were used as catalyst supports. Platinum, rhodium, ruthenium, and nickel were supported by incipient-wetness impregnation of the carrier with aqueous solutions of [Pt(NH3)6](HCO3)2, [Rh(NH3)sC1]C12, [Ru(NH3)sC1]C12, and Ni(NO3)2; the metal loading was varied from 0.5 to 3 wt. %. Prior to testing in cyclohexane ring opening, the catalysts were calcined in an air flow at 500~ for 2 h and reduced in hydrogen at 350~ for 2 h. Tests were carried out in a flow catalytic unit at T=210-400~ P = 1-5 MPa, H2 : C6 = 5-20 (vol), VHSV = 2 h -1. 3. RESULTS AND DISCUSSION
The data on the performance of oxide-based catalysts in cyclohexane ring opening under high-pressure conditions are summarized in the table. Rh-containing systems were shown to be the most active and selective catalysts for the ring opening of cyclohexane (Fig. l a). The reaction proceeds at substantially lower temperatures compared to Pt-catalysts and dehydrogenation to benzene is suppressed to a considerable
229
Table. Ring opening of cyclohexane on metal-containing oxide catalysts Catalyst
P, MPa
T, ~
Conv. %
S(n-
C6H14), CI-C5 %
l%Pt/A1203
2%Rh]AI203 1%Rh/SiO2 l%Ru/A1203
2 3 2 3 5 5 5 1
3%Ni/AI203
2
1%Pt/A1EOa-F 1%Rh/A1203
400 400 300 280 280 320 210 230 370 400
49.6 41.4 93.9 91.0 55.9 83.0 92.7 29.4 100.0 12.7 45.8
25 42 26 65 88 85 56 53 13 6
0.6 1.2 20.5 31.1 6.9 12.2 40.8 13.4 100 5.3
Yield, wt. % inC6H14 C6HI4 1.6 12.4 3.3 17.5 40.1 24.7 0.6 59.3 49.0 0.3 70.5 51.9 0.3 15.7 . . . 1.7 2.8
MCP
C6H6
2.3 3.7 4.0 -
32.7 15.7 4.6 -
2.2 4.4
8.8 33.3
.
extent. Thus, at 300~ and 3 MPa, the 1%Rh/A1203 catalyst provides the yield of n-hexane 59.3 wt.% at a selectivity of 65%. No skeletal isomerization (isohexanes, methylcyclopentane) or dehydrogenation (C6H6) products were identified under these conditions. The highest n-hexane yield of 70.5 wt.% was achieved on 2%Rh/A1203 (280~ 5 MPa) at the ring opening selectivity of 85%. The 1%Rh/SiO2 catalyst manifested lower activity in cyclohexane conversion (Fig. 1b) as compared to the l%Rh/Al203 system, and the ring opening selectivity on Rh/SiO2 did not exceed 80% at moderate yields of ring opening products at 300~ It should be also noted that the n-hexane yield on the l%Rh/SiO2 catalyst is significantly dependent on the pressure: at low pressures (up to 2 MPa), the yield is below 15%, whereas an increase in the pressure up to 5 MPa causes the enhancement in the activity and the n-hexane yield reaches 52% (320~ The temperature and pressure dependence of the n-hexane yield and selectivity for the 1%Rh/A1203 and 1%Rh/SiO2 catalysts are plotted in Fig. 1a, b. Thus, the best yield of n-hexane (50-70 wt. %) at the selectivity of 65-85% was obtained on Rh/A1203 catalysts. Dehydrogenation and cracking (the main side reactions) were shown to be almost completely suppressed at elevated pressures. Figure 2 shows the effect of the reaction temperature on the yield and selectivity of ring opening products formed on the Rh/A1203 catalyst. An increase in the reaction temperature up to 320~ results in a gradual increase in the yield of n-hexane up to -~35%. However, at 350~ crackingbecomes the predominant pathway and the yield of C1-C5 products reaches 100%. For Pt/A1203 catalysts, the main reaction route in the transformation of cyclohexane is the conversion into benzene. However, formation of aromatic hydrocarbons substantially decreases at high pressures, but still high reaction temperatures are required to provide a reasonable conversion (350-400~ Also, formation of methylcyclopentane is significant on this catalyst. For Pt supported on the neutral A1203 support, the highest n-hexane yield
230
90 80
_S (280~ it"
I-'""
I------
,..--
,,,
I--....
9
reaches 17.5 wt.%. The ring opening selectivity is 50% and n-hexane is the main product (the selectivity is 42%). The fluorination of
a
,.
70 60 50
s (300%.)
f(300~
" ~ i
A1203
~
enhances
the
acidity and, as a 40 result, the cracking 3O and isomerization r,j 20 - Y (28o%) processes are predominant over the Pt/A1203-F catalysts. Although the ring 10 20 30 40 50 60 opening process is r~ _ 9 b 80 also intensified, the =" " " S (300~ 970 / yield of n-hexane under the optimum / / S (320%) 60 ring opening 50 conditions (400~ 2 / / I~~Y(320~ 40 MPa) reaches 24.7 wt. %, while the yield 30 of isohexanes is even 20 higher (40 wt. %). It is noteworthy that the conversion on the 10 20 30 40 50 60 fluorinated catalyst is P, atm about two times higher than that on the neutral Pt/A1203. Fig. 1. Influence of the pressure and reaction temperature on the Obviously, this effect cyclohexane ring opening on can be explained by (a) 1%Rh/A1203 and the progressive (b) 1%Rh/SiO2 catalysts contribution of cracking processes at elevated temperatures. The ring opening selectivity to n-hexane + isohexanes is close to 70%, but the yield of isoparaffins is twice that of the normal product. A very similar pattern was observed for Ni/A1203 catalysts, but the contribution of the cracking and hydrogenolysis reactions was even more significant as compared to the Pt/AI203 systems, while the yield of ring opening products did not exceed 3 wt. %. For the bimetallic Pt-Rh/AI203 catalysts with Pt : Rh ratios varying from 25 : 75 to 75 : 25, the yield of n-hexane is higher than on the monometallic Pt/AI203 system, but the catalyst requires higher reaction temperatures as compared with the monometallic Rh/AI203 catalyst to provide about the same yields of ring opening products. r,
I
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I
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I
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I
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I
,
I
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,
I
,,-
231
1 0 0
-
o
80-
n-C6H14 -t-i-C6H 14 -~ C1-C5 o S n-C6H14
"~ 60-~
40-
>. 1
20-
0
i
220
i
- -
260
300
1
340
T~ Fig. 2. Temperature dependences of the yields of main products in ring opening of cyclohexane on 1%Rh/A1203
The conversion of cyclohexane on a Ru/A1203 catalyst is rather high (up to 30%) at as low temperatures as 200-210~ (Fig. 3), i.e. at temperatures when both Pt- and Rh-catalysts are inactive. However, at temperatures above 230~ the Ru-containing catalysts provide exclusively the formation of the cracking products, and the temperature interval favorable for ring opening on Ru/A1203 is very narrow. The highest yield of n-hexane on this catalyst (about 12%) is achieved at-260~ Ruthenium supported on fluorinated A1203 is more active in ring opening as compared to the non-fluorinated system: the maximum in the formation of ring opening products on the acidic catalyst is attained at about 200~ and the yield of n-hexane approaches 22%.
I00
-
80- * - - conversion % - I - C1-C5
60tl) >-
40-
-" n-C6H14 o i-C6H14
20-
0~180
220
260
300
T~ Fig. 3. Performance of the 1%Ru/AI203 catalyst in cyclohexane ring opening.
232 Thus, cyclohexane transformations on noble metals supported on oxide carriers include the following processes in agreement with the scheme shown above: (1) dehydrogenation to benzene; (2) cracking and hydrogenolysis of cyclohexane and isomerization products leading to light products; (3) ring opening of cyclohexane (and MCP) resulting in the formation of n-hexane (and/or isohexanes); (4) simultaneous skeletal isomerization of n-hexane into methylpentanes and dimethylbutanes; (5) isomerization of cyclohexane to methylcyclopentane. Let us analyze this scheme with the purpose to find the factors favorable for the formation of ring-opening products, preferentially n-hexane. In addition to the nature of the metal and the carrier, as was discussed above, a very important factor that influences the reaction pattem is the reaction temperature. There must be an optimum on the dependence of the activity in ring opening versus temperature because of the interplay of the kinetic and thermodynamic factors. With decreasing temperature, the contribution of cracking and dehydrogenation processes should be suppressed. The value of the optimum temperature is obviously a function of the carrier and metal nature. Another factor affecting the catalyst performance is the hydrocarbon-to-hydrogen ratio. It is quite clear that this ratio should be high enough in order to prevent dehydrogenation of cyclohexane. On the other hand, very high hydrogen-to-substrate ratios are favorable for the occurrence of hydrogenolysis reactions. Finally, the reaction pressure seems to be a key factor determining the product distribution and other important characteristics. Indeed, by increasing the pressure, we can suppress both dehydrogenation and cracking and shift the equilibrium toward C6 paraffins. 4. CONCLUSIONS
The data obtained show that noble metals supported on oxides are efficient catalysts for ring opening of cyclic hydrocarbons. Cyclohexane can be converted with a high selectivity (up to 80-85%) into n-hexane. The use of acidic carriers causes the formation of isomeric ring-opening products and methylcyclopentane. The reaction temperature and pressure are essential parameters determining the catalyst performance. REFERENCES
1. G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, New York, Wiley, 1994. 2. Y. L. Lam, J. H. Sinfelt, J. Catal., 42 (1976) 319. 3. L. Liberman, O. V. Bragin, B. A. Kazansky, Dokl. Akad. Nauk, SSSR, 156 (1964) 5. 4. Ger. Patent No. 2 127 624 (1971). 5. US Patent No. 5 382 730 (1995). 6. US Patent No. 5 382 731 (1995).
227
Cyclohexane ring opening on metal-oxide catalysts L.M. Kustov a, T.V. Vasina a, O.V. Masloboishchikova a, E.G. Khelkovskaya-Sergeeva a, and P. Zeuthen b aN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp. 47, Moscow, 117334 Russia, e-mail" [email protected] bHaldor Topsoe A/S, Nymollevej 55, Lyngby, DK-2800 Denmark Ring opening of cyclohexane on various metal-containing oxide catalysts is studied in detail. Rhodium supported on neutral alumina is shown to exhibit the highest selectivity toward n-hexane, whereas the formation of side products of cracking, isomerization and dehydrogenation (CI-C4 hydrocarbons, isohexanes + methylcyclopentane, and benzene, respectively) is essentially suppressed.
1. INTRODUCTION Dearomatization of gasoline and diesel fuel is a very important problem of oil refining for the nearest future. Hydrogenation of aromatic compounds present in the fuels can be used to transfer aromatics into naphthenes but the latter can be converted again into the aromatic products and soot particulates in the engine. The most appropriate solution of the problem would be ring opening of aromatics and naphthenes into paraffins that cannot be further transformed into undesirable products. In the case of diesel fuel, ring opening occurring without skeletal isomerization can also result in the gain in the cetane number, while ring opening of C 6 - C 9 cyclic components of gasoline with simultaneous skeletal isomerization can be helpful in boosting the octane number of the motor fuel. Cyclohexane may be considered as a suitable model compound for ring opening. It is well known [1, 2] that the main transformations of cyclohexane over metal-supported catalysts involve dehydrogenation into benzene, hydroisomerization into methylcyclopentane and hydrogenolysis or cracking into low-molecular hydrocarbons, such as CI-C4 light products. Thus, although the reaction of cyclohexane ring opening theoretically could produce n-hexane with a 100% probability via single-step hydrogenolysis of one of the C-C bonds, the real pattern is much more complicated and includes the whole range of possible reactions as shown in the Scheme. The formation of n-hexane from cyclohexane was also studied in [3, 4]. The most interesting and commercially viable catalytic system based on l%Pt/H-Beta zeolite was developed by Mobil Company [5]. This catalyst exhibits rather high activity in ring opening of C6 cyclic compounds and is used in a combination with the isomerization catalyst to improve the C6 downstream feed. In this case, however, a 1 : 3 mixture of normal and isoparaffins was formed at conversions of-80% at 230-270~ under high-pressure conditions.
228 REACTION SCHEME
H2
H
G
--,,. / C 1 -C5
Another catalyst proposed by the same authors for ring opening of C6 cyclic compounds comprises a Pt/WO3/ZrO2 system [6]. The performance of this catalyst is similar to that of Pt~-Beta zeolite, but the reaction conditions are different. The directions of cyclohexane transformations should be obviously dependent on the nature of the supported metal (Pt, Ru, Rh, Ni, etc.) and on the reaction conditions. Therefore, the goal of this paper was to study the influence of different factors (nature of the metal and loading, type of the carrier, reaction temperature and pressure, space velocity, hydrogen-tohydrocarbon ratio) on ring opening of cyclohexane on oxide catalysts promoted with noble metals. 2. EXPERIMENTAL
Neutral 7-alumina, fluorinated 7-alumina, and silica were used as catalyst supports. Platinum, rhodium, ruthenium, and nickel were supported by incipient-wetness impregnation of the carrier with aqueous solutions of [Pt(NH3)6](HCO3)2, [Rh(NH3)sC1]C12, [Ru(NH3)sC1]C12, and Ni(NO3)2; the metal loading was varied from 0.5 to 3 wt. %. Prior to testing in cyclohexane ring opening, the catalysts were calcined in an air flow at 500~ for 2 h and reduced in hydrogen at 350~ for 2 h. Tests were carried out in a flow catalytic unit at T=210-400~ P = 1-5 MPa, H2 : C6 = 5-20 (vol), VHSV = 2 h -1. 3. RESULTS AND DISCUSSION
The data on the performance of oxide-based catalysts in cyclohexane ring opening under high-pressure conditions are summarized in the table. Rh-containing systems were shown to be the most active and selective catalysts for the ring opening of cyclohexane (Fig. l a). The reaction proceeds at substantially lower temperatures compared to Pt-catalysts and dehydrogenation to benzene is suppressed to a considerable
229
Table. Ring opening of cyclohexane on metal-containing oxide catalysts Catalyst
P, MPa
T, ~
Conv. %
S(n-
C6H14), CI-C5 %
l%Pt/A1203
2%Rh]AI203 1%Rh/SiO2 l%Ru/A1203
2 3 2 3 5 5 5 1
3%Ni/AI203
2
1%Pt/A1EOa-F 1%Rh/A1203
400 400 300 280 280 320 210 230 370 400
49.6 41.4 93.9 91.0 55.9 83.0 92.7 29.4 100.0 12.7 45.8
25 42 26 65 88 85 56 53 13 6
0.6 1.2 20.5 31.1 6.9 12.2 40.8 13.4 100 5.3
Yield, wt. % inC6H14 C6HI4 1.6 12.4 3.3 17.5 40.1 24.7 0.6 59.3 49.0 0.3 70.5 51.9 0.3 15.7 . . . 1.7 2.8
MCP
C6H6
2.3 3.7 4.0 -
32.7 15.7 4.6 -
2.2 4.4
8.8 33.3
.
extent. Thus, at 300~ and 3 MPa, the 1%Rh/A1203 catalyst provides the yield of n-hexane 59.3 wt.% at a selectivity of 65%. No skeletal isomerization (isohexanes, methylcyclopentane) or dehydrogenation (C6H6) products were identified under these conditions. The highest n-hexane yield of 70.5 wt.% was achieved on 2%Rh/A1203 (280~ 5 MPa) at the ring opening selectivity of 85%. The 1%Rh/SiO2 catalyst manifested lower activity in cyclohexane conversion (Fig. 1b) as compared to the l%Rh/Al203 system, and the ring opening selectivity on Rh/SiO2 did not exceed 80% at moderate yields of ring opening products at 300~ It should be also noted that the n-hexane yield on the l%Rh/SiO2 catalyst is significantly dependent on the pressure: at low pressures (up to 2 MPa), the yield is below 15%, whereas an increase in the pressure up to 5 MPa causes the enhancement in the activity and the n-hexane yield reaches 52% (320~ The temperature and pressure dependence of the n-hexane yield and selectivity for the 1%Rh/A1203 and 1%Rh/SiO2 catalysts are plotted in Fig. 1a, b. Thus, the best yield of n-hexane (50-70 wt. %) at the selectivity of 65-85% was obtained on Rh/A1203 catalysts. Dehydrogenation and cracking (the main side reactions) were shown to be almost completely suppressed at elevated pressures. Figure 2 shows the effect of the reaction temperature on the yield and selectivity of ring opening products formed on the Rh/A1203 catalyst. An increase in the reaction temperature up to 320~ results in a gradual increase in the yield of n-hexane up to -~35%. However, at 350~ crackingbecomes the predominant pathway and the yield of C1-C5 products reaches 100%. For Pt/A1203 catalysts, the main reaction route in the transformation of cyclohexane is the conversion into benzene. However, formation of aromatic hydrocarbons substantially decreases at high pressures, but still high reaction temperatures are required to provide a reasonable conversion (350-400~ Also, formation of methylcyclopentane is significant on this catalyst. For Pt supported on the neutral A1203 support, the highest n-hexane yield
230
90 80
_S (280~ it"
I-'""
I------
,..--
,,,
I--....
9
reaches 17.5 wt.%. The ring opening selectivity is 50% and n-hexane is the main product (the selectivity is 42%). The fluorination of
a
,.
70 60 50
s (300%.)
f(300~
" ~ i
A1203
~
enhances
the
acidity and, as a 40 result, the cracking 3O and isomerization r,j 20 - Y (28o%) processes are predominant over the Pt/A1203-F catalysts. Although the ring 10 20 30 40 50 60 opening process is r~ _ 9 b 80 also intensified, the =" " " S (300~ 970 / yield of n-hexane under the optimum / / S (320%) 60 ring opening 50 conditions (400~ 2 / / I~~Y(320~ 40 MPa) reaches 24.7 wt. %, while the yield 30 of isohexanes is even 20 higher (40 wt. %). It is noteworthy that the conversion on the 10 20 30 40 50 60 fluorinated catalyst is P, atm about two times higher than that on the neutral Pt/A1203. Fig. 1. Influence of the pressure and reaction temperature on the Obviously, this effect cyclohexane ring opening on can be explained by (a) 1%Rh/A1203 and the progressive (b) 1%Rh/SiO2 catalysts contribution of cracking processes at elevated temperatures. The ring opening selectivity to n-hexane + isohexanes is close to 70%, but the yield of isoparaffins is twice that of the normal product. A very similar pattern was observed for Ni/A1203 catalysts, but the contribution of the cracking and hydrogenolysis reactions was even more significant as compared to the Pt/AI203 systems, while the yield of ring opening products did not exceed 3 wt. %. For the bimetallic Pt-Rh/AI203 catalysts with Pt : Rh ratios varying from 25 : 75 to 75 : 25, the yield of n-hexane is higher than on the monometallic Pt/AI203 system, but the catalyst requires higher reaction temperatures as compared with the monometallic Rh/AI203 catalyst to provide about the same yields of ring opening products. r,
I
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I
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I
i
I
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I
,
I
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o
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,,-
231
1 0 0
-
o
80-
n-C6H14 -t-i-C6H 14 -~ C1-C5 o S n-C6H14
"~ 60-~
40-
>. 1
20-
0
i
220
i
- -
260
300
1
340
T~ Fig. 2. Temperature dependences of the yields of main products in ring opening of cyclohexane on 1%Rh/A1203
The conversion of cyclohexane on a Ru/A1203 catalyst is rather high (up to 30%) at as low temperatures as 200-210~ (Fig. 3), i.e. at temperatures when both Pt- and Rh-catalysts are inactive. However, at temperatures above 230~ the Ru-containing catalysts provide exclusively the formation of the cracking products, and the temperature interval favorable for ring opening on Ru/A1203 is very narrow. The highest yield of n-hexane on this catalyst (about 12%) is achieved at-260~ Ruthenium supported on fluorinated A1203 is more active in ring opening as compared to the non-fluorinated system: the maximum in the formation of ring opening products on the acidic catalyst is attained at about 200~ and the yield of n-hexane approaches 22%.
I00
-
80- * - - conversion % - I - C1-C5
60tl) >-
40-
-" n-C6H14 o i-C6H14
20-
0~180
220
260
300
T~ Fig. 3. Performance of the 1%Ru/AI203 catalyst in cyclohexane ring opening.
232 Thus, cyclohexane transformations on noble metals supported on oxide carriers include the following processes in agreement with the scheme shown above: (1) dehydrogenation to benzene; (2) cracking and hydrogenolysis of cyclohexane and isomerization products leading to light products; (3) ring opening of cyclohexane (and MCP) resulting in the formation of n-hexane (and/or isohexanes); (4) simultaneous skeletal isomerization of n-hexane into methylpentanes and dimethylbutanes; (5) isomerization of cyclohexane to methylcyclopentane. Let us analyze this scheme with the purpose to find the factors favorable for the formation of ring-opening products, preferentially n-hexane. In addition to the nature of the metal and the carrier, as was discussed above, a very important factor that influences the reaction pattem is the reaction temperature. There must be an optimum on the dependence of the activity in ring opening versus temperature because of the interplay of the kinetic and thermodynamic factors. With decreasing temperature, the contribution of cracking and dehydrogenation processes should be suppressed. The value of the optimum temperature is obviously a function of the carrier and metal nature. Another factor affecting the catalyst performance is the hydrocarbon-to-hydrogen ratio. It is quite clear that this ratio should be high enough in order to prevent dehydrogenation of cyclohexane. On the other hand, very high hydrogen-to-substrate ratios are favorable for the occurrence of hydrogenolysis reactions. Finally, the reaction pressure seems to be a key factor determining the product distribution and other important characteristics. Indeed, by increasing the pressure, we can suppress both dehydrogenation and cracking and shift the equilibrium toward C6 paraffins. 4. CONCLUSIONS
The data obtained show that noble metals supported on oxides are efficient catalysts for ring opening of cyclic hydrocarbons. Cyclohexane can be converted with a high selectivity (up to 80-85%) into n-hexane. The use of acidic carriers causes the formation of isomeric ring-opening products and methylcyclopentane. The reaction temperature and pressure are essential parameters determining the catalyst performance. REFERENCES
1. G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, New York, Wiley, 1994. 2. Y. L. Lam, J. H. Sinfelt, J. Catal., 42 (1976) 319. 3. L. Liberman, O. V. Bragin, B. A. Kazansky, Dokl. Akad. Nauk, SSSR, 156 (1964) 5. 4. Ger. Patent No. 2 127 624 (1971). 5. US Patent No. 5 382 730 (1995). 6. US Patent No. 5 382 731 (1995).