J. Chem. SOC., Faraday Trans. I, 1986, 82, 103-108 Homologation of Small Alkanes on Pt, Pd and Ni Catalysts Contribution of Intermediate Carbenes to Skeletal Isomerisation Antal Sarkany Institute of Isotopes of the Hungarian Academy of Sciences, P.O. Box 77, H- 1525 Budapest, Hungary Chain lengthening homologation of propane,n-butane,2-methylpropane,2- methylbutane and 2,2-dimethylpropane has been investigated over Pt, Pd and Ni catalysts. With both Pd and Ni the chain lengthening selectivity was observed to be commensurable to the isomerisation one. Addition of CH, onto the j?-carbon atom in n-butane and propane as well as chain lengthening of 2-methylpropane and 2-methylbutane were observed. The mechanism of the skeletal rearrangement is discussed in terms of CH, addition. _ _ _ _ _ ~ Under hydrogen-deficient conditions, the reaction of saturated hydrocarbons over transition metals has been observed to be accompanied by the formation of hydrocarbons with higher carbon number than the reactant.l The chain lengthening reaction (homo- logation) detected on a large group of metals4$ has been interpreted by the recombination of intermediate carbenes with terminal olefins uia the formation of a metallocyclobutane ring.3* At elevated temperatures the reaction of C, and C, alkanes with Ni59 ‘ 9 * and Pd4 catalysts, if the chain lengthening is not hindered by branching, results in the formation of benzene and alkylaromatics.This indicates that the addition of carbene to the non-substituted end of intermediate alk- 1 -ene is immediately followed by 1,6- dehydrocyclisation on surfaces poisoned strongly by carbonaceous deposit^.^^ In contrast to these findings, the preliminary results in our laboratory’ and those presented by O’Donohoe et aL4 have shown that the chain lengthening with small alkanes, although the selectivity is not too high and drops with temperature, yields iso-alkane and n-alkane in commensurable quantity. Moreover, the formation of methylcyclopentane from cyclopentane has been observed over some Cu-Ni/SiO,, Ag-Ni/SiO, and Au-Ni/SiO, catalysts,’ suggesting that the internal double bonds may become accessible to carbenes under certain experimental conditions.In principle, a CH, addition-abstraction mechanism, in which the CH, unit is added to a /?-carbon atom in n-alkane, might lead to the formation of the isomer of the parent hydr~carbon.~ As few data are available on the mechanism of isomerisation at low excess of H, we decided to undertake a study on the selectivity of carbene insertion onto butane, propane and 2-methylpropane over Ni, Pd and Pt catalysts.Experimental The experiments were accomplished in a glass circulation system of 0.1443 dm3 attached to a gas chromatograph. The experimental set up and the analysis of the products have already been de~cribed.~ The transformation of hydrocarbons was measured up to 20-30% conversion and the rate of the formation of the products was used for the calculation of product selectivity. The selectivity is defined in accordance with earlier publicati~ns.~q 9 9 lo The catalysts have been prepared by a conventional technique.1 wt % Pd/SiO, and 5 wt % Ni/SiO, catalysts were prepared by the method of incipient 103104 Homologation of Alkanes on Pt, Pd and Ni wetness using Pd(NH,),Cl,H,O and Ni(NO,), solutions, respectively. The catalysts were dried and calcined at 673 K for 5 h and reduced in a stream of H, at 693 K for 20 h. Prior to the catalytic and chemisorption measurements, the samples were stabilised by rt:peated oxygen-hydrogen treatments at 623 K. The dispersion, along with the experimental conditions, are presented in the tables. The preparation and characteristics of the Pt catalysts have been presented elsewhere.ll Results and Discussion The results with Pt/SiO, and Pt-black, 5 wt % Ni/SiO, and 1 wt % Pd/Al,O, catalysts are summarised in tables 1-3, respectively. Since the objective of the present study was to investigate chain lengthening, most of the hydrocarbon reactions were carried out at low H,/HC ratios. With Pt catalysts, even under these conditions, a considerable amount of 2-methylpropane was formed from n-butane and, in agreement with the previous findings,*v5 the selectivity for the formation of the next higher alkane was low.Nevertheless, both 2-methylbutane and n-pentane could be detected without any difficulty. The isomerisation selectivity of the highly dispersed 1 wt % Pt/SiO, sample is superior to that of Pt-black and the rupture of the C-C bond in n-butane is almost uniform with the former catalyst. Both the high isomerization selectivity and the low excess of methane formation over 1 wt % Pt/SiO, can be interpreted by the prevalence of localised Pt-hydrocarbon interactions.ll Over 5 wt % Ni/SiO, and 1 wt % Pd/Al,O, catalysts the selectivity of isomer forma- tion and that of chain lengthening were observed to be commensurable.First, we consider the selectivity data with respect to chain lengthening. The inspection of the results with n-butane in tables 2 and 3 shows that the selectivity for 2-methylbutane is roughly twice that for n-pentane, suggesting that the addition of a C, unit to a P-carbon atom is favoured under these conditions. This result is also confirmed with propane over 5 wt % Ni/SiO, catalyst. One could argue that 2-methylbutane might be formed by the immediate isomerisation of an intermediate n-pentane. This suggestion could only be accepted if the isomerisation selectivity of Ni and Pd were high enough, which is not the case as shown by the penultimate column in tables 2 and 3.The experiments with 2-methylpropane also allow us to reject this suggestion as the ratio of n-pentane over 2-methylbutane is very small (0.05 1 and 0.03 1 on Pd/Al,O, and 0.07 1 on Ni/SiO,). While the formation of 2-methylbutane could be defined by the addition of a CH, group to 2-methylpropane, n-pentane might be formed either by bond-shift skeletal isomerisation of 2-methylbutane or, more likely, by consecutive CH, addition-abstraction steps: The results with 2-methylpropane and 2-methylbutane, on the other hand, provide evidence that the presence of a methyl substituent significantly inhibits the probability of chain lengthening.On our highly dispersed Ni/SiO, sample (63% dispersion) the W h o m / Whydr ratio (the rate of homologation compared with the rate of hydrogenolysis) is observed to be 11.78 x lo-, and 3.01 x with n-butane and 2-methylpropane, re- spectively, under comparable experimental conditions. As shown by the data in table 3, the difference between the selectivity values measured with n-butane and 2- methylpropane is even less on the 1 wt % Pd/A1,0, catalyst. A further check was made with 2-methylbutane over Ni/SiO, (table 2, last two rows): the selectivity of the formation of 2-methylpentane is close to that of 3-methylpentane. This latter compound is likelyTable 1. Transformation of n-butane over 1 wt % Pt/Si02a and Pt-blackb selectivity (% ) catalyst 1 wt % Pt/SiO, 557 1.33 6.65 9.89 16.10 8.91 82.21 0.108 0.063 4.76 0.128 1 wt % Pt/SiO, 555 3.99 4.52 13.95 18.41 13.13 75.79 0.244 0.306 3.36 3.05 Pt-black 556 1.33 6.25 43.28 11.91 25.61 63.81 0.157 0.473 1.77 2.19 Pt- blac k 556 3.99 4.39 42.14 20.80 35.26 50.67 0.713 0.832 1.07 4.07 a Dispersion = 80%.B.E.T. surface area = 5.4 m2 g-l. The rate of homologue formation (iC5+nC,) is transformed to C, equivalents. b Table 2. Transformation of alkanes on a 5 wt % Ni/SiO, catalysta* 2 < selectivity c') it .~ 1ooFqso lOOKom K ydr K y d r HC T/K p(H,)/kPa C, C, C, iC, nC, iC, nC, 2M2Bu2- 2MP 3MP nH MCP B ~ Pr nBu nBu 2MPr 2,2DMPr 2,2DMPr 2MBu 2MBu 556 4.39 193.0 51.5 551 4.52 161.1 21.6 552 4.39' 291.9 21.3 556 4.39 249.3 19.7 553 4.65 454.8 12.1 513 4.65' 154.1 14.0 554 6.18' 155.6 11.1 553 4.65 113.1 9.3 ~ ____ - - - - __ 2.17 1.83 - - - - - 42.5 7.2 - 4.25 2.60 - ~ ~ 0.86 24.5 2.9 - 0.7 0.31 - - - 26.3 - 4.5 2.1 0.14 - __ - - - - - - - - - - - - - - -~~ 2.1 3.6 - 31.2 56.0 - 12.2 28.7 33.0 - 1.41 4.1 0.36 0.36 - - - - - - - - - - - 1.33 8.7 24.1 29.8 - 0.72 19.8 1.52 0.81 0.17 0.98 0.93 - 5.4 8.65 11.78 2.91 1.22 4.81 3.01 0.00 0.00 0.00 0.00 I .24 2.68 0.96 7.12 ~~ a Dispersion = 63%.Pr, nBu, 2BMPr, 2,2DMPr, 2MBu, 2M2Bu2-, 2MP, 3MP, nH, MCP and B are propane, n-butane, 2-methylpropane, 2,2-dimethylpropane, 2-methylbutane7 2-methylbut-2-ene7 2-methylpentane, 3-methylpentane, n-hexane, methylcyclopentane and benzene, respectively. Measurements with 1.33 k Pa hydrocarbon, otherwise with 3.99 k Pa.% 0 Table 3.Transformation of n-butane, 2-methylpropane and 2,2-dimethylpropane on 1 wt % Pd/Al,03 catalysta ~- selectivity (% ) nBu 524 2.66 9.84 93.6 2.6 87.8 3.21 - 3.99 4.79 64.5 3.8 78.0 8.74 - nBu 553 nBu 558 1.33 6.31 87.1 5.9 84.8 8.92 - 2MPr 523 2.66 9.57 100.2 1.5 90.4 - 5.44 2MPr 553 3.99 4.52 65.9 1.9 72.4 - 15.43 2,2DMPr 553 3.99 4.39 111.3 3.6 28.1 74.2 tr ~ - ~ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ _ ~ _ _ _ a Dispersion = 8.5%. 3.63 1.41 3.53 5.56 7.67 4.06 1.42 19.15 1.35 0.78 10.10 2.42 2.51 0.13 5.8 1 3.52 9.84 0.3 1 21.48 14.13 0.00 0.00 0.00 0.00 ~~ ~ - ~~A . Sarkany 107 to be formed by the recombination of a CH, unit with the branched end of 2-methyl- butane. All the observations in this study seem to indicate that in small alkanes the insertion of CH, to a /?-carbon atom is not hindered and at the same time the small isoalkanes are also effective in the chain lengthening.With C, and C7 n-alkanes the addition of CH, to the internal carbon atoms was observed only to a very limited extent over Ni and Pd catalysts.4* 5 9 The disagreement between the results of this paper and those reported in ref. (4) and (7) might be resolved if one accepts that with small alkanes, by virtue of their less effective carbonpoisoning activity, the surface sites are sterically crowded to a smaller extent than those with large alkanes. The small size of the incoming hydrocarbon and a less effective steric crowding might promote both the formation and the chain lengthening of an iso-alkane. One cannot rule out, however, that in the previous high temperature experiment^,^?^ with C,-C, hydrocarbons over Ni and Pd, that the rapid aromatisation and the relatively high chemical stability of benzene have obscured the real selectivity of carbene addition. The observed results afford some insight into the mechanism of isomerisation over Ni and Pd catalysts.There has been much discussion in the literature concerning the mechanism of the bond-shift type skeletal rearrangement. The experiments with caged hydrocarbons12 in excess hydrogen and deuterium labelling l7-l9 have provided strong evidence that a monoadsorbed intermediate (a-alkyl adsorbed radical) is sufficient for bond-shift skeletal rearrangement (Rooney-Samman mechanism). On the basis of the experimental results in this paper, we now have good reason to suggest that, under ‘ hydrogen deficient ’ conditions, intermediatecarbenes also contribute to the isomerisation process.Apparently, under these reaction conditions, the H,/HC ratio will decide which reaction route becomes conspicuous. In principle a large excess of hydrogen should favour the Rooney-Samman mechanism, whereas the high reaction temperatme and low excess of hydrogen might promote a reaction route viu participation of intermediate carbene. A carbene route for isomerisation has been suggested, but not proved, by Gault and Muller16 in their dehydrocyclisation studies on Pd : The elementary been discussed rearrangement, steps of the ‘ carbene metathesis ’ route (Garin-Gault mechanism) have in detail in ref. (20)-(22).The key step in this intramolecular-type as emphasised by Gault and Garin, is the rotation of an intermediate alkene in the vicinity of a reactive CH, group. However, the rotation or the migration of a surface olefin might lead to the separation of the CH, unit from the surface alkene, so that the intramolecular process might become an intermolecular one. The rotation step of the ‘carbene metathesis’ route has already been questioned by Ponec.15 Considering the results in tables 2 and 3 we suggest that the formation of isomers under these experimental conditions is part of the homologation process. A CH, addition- abstraction route (the elementary processes are not depicted) is presented :108 Homologation of Alkanes on Pt, Pd and Ni An alternative route, i.e.a CH, abstraction-addition mechanism, cannot be ruled out entirely as the chain lengthening with propane yielded both n-butane and 2-methylpropane. It should be noted, however, that in the hydrogenolysis of the C, hydrocarbons, say on Ni, the surface concentration of the C , intermediate alkene might not be high enough, owing to the propensity for deep fragmentation, whereas the interaction of the reactant C, alkane with poisoned sites would produce a sufficient amount of intermediate alk-1-lene. The last step of the above scheme, i.e. the fission of a CH, group, requires some discussion. As shown by results in table 2, starting from 2-methylpentane there is an almost equal chance of forming n-butane and 2- methylpropane. 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Paper 51432; Receked 15th March, 1985