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New Evidence for A Protonated Cyclopropane Mechanism in Catalytic Isomerization of n-Alkanes
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NEW EVIDENCE FOR A PROTONATED CYCLOPROPANE MECHANISM IN CATALYTIC ISOMERIZATION OF n-ALKANES Jens WEITKAMP Engler-Bunte-Institute, University of Karlsruhe, Richard-Willstatter-Allee 5, D-7500 Karlsruhe 1 , Federal Republic of Germany SUMMARY: Hydroisomerization of n-undecane at low conversions yields mainly methyldecanes. Among these, 2-methyldecane is formed at a surprisingly low rate compared to its positional isomers. Whereas a branching mechanism via classical alkyl and hydride shifts fails to explain this finding, a mechanism via protonated cyclopropanes gives a straightforward explanation. INTRODUCTION: While it is known for a long time that isomerization of alkanes on acidic catalysts proceeds via carbenium ions the precise mode of branching has been elucidated only relatively recently. From a comparison of rates of skeletal rearrangement and 13Cscrambling in Cq to C6 alkanes catalyzed by HF-SbF5’) or Pt/Si02A1203” evidence was obtained against a branching mechanism via classical alkyl and hydride shifts. Instead a mechanism via protonated cyclopropanes (PCP) intermediates was postulated. The present communication reports on experiments with a long chain alkane which give independent support for branching via PCPs. EXPERIMENTAL: n-Undecane was hydroisomerized on a 0.5 wt.-% Pt/CaY zeolite at 200 OC, a total pressure of 3.9 MPa, and a hydrocarbon partial pressure of 16 kPa. The apparatus with the fixed bed reactor which was operated in the differential regime at conversions below 5 % has been described previously along with the procedures for high resolution analysis by capillary GLC3). RESULTS AND DISCUSSION: i-Undecanes were formed from n-undecane mol/g catalyst h without any occurrence at a rate of 25 of cracked products or hydrocarbons with more than 1 1 carbon atoms. Isomerization was found to proceed in a stepwise manner, i.e. monobranched i-undecanes are primary products from which dibranched isomers form in consecutive steps. At a conversion of 1 . 3 % the Their distriyield of monobranched isomers was as high as 99.2 %. bution was 89.8 % methyldecanes, 9.1 % ethylnonanes, and 1 . 1 % 4-propyloctane. The distribution of the methyldecanes was 1 3 : 2 5 : 29 : 3 3 % for 2- : 3- : 4- : 5-methyldecane.
1405
9?--+,-k 3W 111
I-METHVLDECINE
-P+%-= I
Fig. 1 .
+YEwvmEcmE
-%%
~YElHILDECANE
H 0
Scheme for branching of n-undecyl cations via PCPs
In Fig. 1 the branching mechanism via protonated cyclopropanes is outlined for the mixture of secondary undecyl cations. Their relative concentrations are assumed to be simply given by the statistical weight of the structure, i.e. cII = cIII = cIv = cv = 2cvIIt is further assumed that the rate constants kIIAl kIIIBl kIVA, %ID equal each other and that the same holds for kIVD, Gs, the set of ringopening rate constants kAZ through kE5. A simple kinetic treatment then predicts .the following relative rates of formation : 1 : 2 : 2 : 2 for 2- : 3- : 4- : 5-methyldecane. Thus the PCP mechanism correctly predicts the low rate of formation of 2-methyldecane amounting to ca. one half of the rate of formation for its positional isomers. In contrast to this a similar kinetic treatment of the classical mechanism via alkyl and hydride shifts predicts equal relative rates of formation for the four methyldecanes which is far from reality. Even the PCP mechanism does not explain the small differences observed experimentally for the rates of formation of 3-, 4-, and 5-methyldecane and the occurrence of small amounts of ethylnonanes
h,
and propyloctane. For the moment it is speculated that the former deviation is due to some subtle structural influences on the rate constants in Fig. 1 which were neglected in the kinetic treatment while the latter discrepancy possibly reflects a consecutive formation of ethylnonyl and propyloctyl cations from methyldecyl cations during a single chemisorption step. REFERENCES 1. D.M. Brouwer and H. Hogeveen, Progr. Phys. Org. Chem., 2, 179 (1972), 2. F. Chevalier, M. Guisnet and R. Maurel, Proc. 6th Intern. Congr. Catal., Vol. 1 , p . 478, The Chemical Society, London (1977). 3. J. Weitkamp and K. Hedden, Chem.-Ing.,Techn., 47, 537 (1975).
NEW EVIDENCE FOR A PROTONATED CYCLOPROPANE MECHANISM IN CATALYTIC ISOMERIZATION OF n-ALKANES Jens WEITKAMP Engler-Bunte-Institute, University of Karlsruhe, Richard-Willstatter-Allee 5, D-7500 Karlsruhe 1 , Federal Republic of Germany SUMMARY: Hydroisomerization of n-undecane at low conversions yields mainly methyldecanes. Among these, 2-methyldecane is formed at a surprisingly low rate compared to its positional isomers. Whereas a branching mechanism via classical alkyl and hydride shifts fails to explain this finding, a mechanism via protonated cyclopropanes gives a straightforward explanation. INTRODUCTION: While it is known for a long time that isomerization of alkanes on acidic catalysts proceeds via carbenium ions the precise mode of branching has been elucidated only relatively recently. From a comparison of rates of skeletal rearrangement and 13Cscrambling in Cq to C6 alkanes catalyzed by HF-SbF5’) or Pt/Si02A1203” evidence was obtained against a branching mechanism via classical alkyl and hydride shifts. Instead a mechanism via protonated cyclopropanes (PCP) intermediates was postulated. The present communication reports on experiments with a long chain alkane which give independent support for branching via PCPs. EXPERIMENTAL: n-Undecane was hydroisomerized on a 0.5 wt.-% Pt/CaY zeolite at 200 OC, a total pressure of 3.9 MPa, and a hydrocarbon partial pressure of 16 kPa. The apparatus with the fixed bed reactor which was operated in the differential regime at conversions below 5 % has been described previously along with the procedures for high resolution analysis by capillary GLC3). RESULTS AND DISCUSSION: i-Undecanes were formed from n-undecane mol/g catalyst h without any occurrence at a rate of 25 of cracked products or hydrocarbons with more than 1 1 carbon atoms. Isomerization was found to proceed in a stepwise manner, i.e. monobranched i-undecanes are primary products from which dibranched isomers form in consecutive steps. At a conversion of 1 . 3 % the Their distriyield of monobranched isomers was as high as 99.2 %. bution was 89.8 % methyldecanes, 9.1 % ethylnonanes, and 1 . 1 % 4-propyloctane. The distribution of the methyldecanes was 1 3 : 2 5 : 29 : 3 3 % for 2- : 3- : 4- : 5-methyldecane.
1405
9?--+,-k 3W 111
I-METHVLDECINE
-P+%-= I
Fig. 1 .
+YEwvmEcmE
-%%
~YElHILDECANE
H 0
Scheme for branching of n-undecyl cations via PCPs
In Fig. 1 the branching mechanism via protonated cyclopropanes is outlined for the mixture of secondary undecyl cations. Their relative concentrations are assumed to be simply given by the statistical weight of the structure, i.e. cII = cIII = cIv = cv = 2cvIIt is further assumed that the rate constants kIIAl kIIIBl kIVA, %ID equal each other and that the same holds for kIVD, Gs, the set of ringopening rate constants kAZ through kE5. A simple kinetic treatment then predicts .the following relative rates of formation : 1 : 2 : 2 : 2 for 2- : 3- : 4- : 5-methyldecane. Thus the PCP mechanism correctly predicts the low rate of formation of 2-methyldecane amounting to ca. one half of the rate of formation for its positional isomers. In contrast to this a similar kinetic treatment of the classical mechanism via alkyl and hydride shifts predicts equal relative rates of formation for the four methyldecanes which is far from reality. Even the PCP mechanism does not explain the small differences observed experimentally for the rates of formation of 3-, 4-, and 5-methyldecane and the occurrence of small amounts of ethylnonanes
h,
and propyloctane. For the moment it is speculated that the former deviation is due to some subtle structural influences on the rate constants in Fig. 1 which were neglected in the kinetic treatment while the latter discrepancy possibly reflects a consecutive formation of ethylnonyl and propyloctyl cations from methyldecyl cations during a single chemisorption step. REFERENCES 1. D.M. Brouwer and H. Hogeveen, Progr. Phys. Org. Chem., 2, 179 (1972), 2. F. Chevalier, M. Guisnet and R. Maurel, Proc. 6th Intern. Congr. Catal., Vol. 1 , p . 478, The Chemical Society, London (1977). 3. J. Weitkamp and K. Hedden, Chem.-Ing.,Techn., 47, 537 (1975).