Alkanes dehydrocyclization

Pt-Re-alumina catalysts were prepared, using alumina containing potassium to eliminate the support acidity, in order to carry out alkane dehydrocyclization studies that paralleled earlier work with nonacidic Pt-alumina catalysts. The potassium containing Pt-Re catalyst was much less active than a similar Pt catalyst. It was speculated that the alkali metal formed salts of rhenic acid to produce a catalyst that was more difficult to reduce. However, the present ESCA results indicate that the poisoning effect of alkali in Pt-Re catalysts is not primarily due to an alteration in the rhenium reduction characteristics. 

Comparison of Alkane Dehydrocyclization Activity and Characterization Data... 

The activity data indicate that the state of Pt desired in the most active alkane dehydrocyclization is not the form that is easily detected by most characterization techniques. Thus, the catalysts where crystalline Pt is easily detected, either as the metal or the alloy, are usually not the optimum catalysts from the point of view of activity. It appears that for a catalyst utilizing an acidic alumina support, alloy formation is not desirable. Likewise, a surface tin concentration that decreases acidity to a significant extent is not desirable. 

Kushch, S.D., Fursikov, P.V., Kuyunko, N.S., Kulikov, A.V. and Savchenko, V.I. (2001) Fullerene black relationship between catalytic activity in w-alkanes dehydrocyclization and reactivity in oxidation, bromination and hydrogenolysis. Eurasian Chemico-Technol. Journal, 3, No. 2, 131-139. 

When the reactions of alkane molecules larger than the butanes or neopentane are studied, and in particular when the molecule is large enough to form a Cs or a Ce ring, the complexity of the reaction pathway is considerably increased and an important feature is the occurrence, in addition to isomerization product, of important amounts of cyclic reaction products, particularly methylcyclopentane, formed by dehydrocycliza-tion this suggests the existence of adsorbed cyclic species. The question is whether the reaction paths for dehydrocyclization and isomerization are related. There is convincing evidence that they are. Skeletal interconversions involving n-hexane, 2- and 3-methylpentane may be represented. 

In hydrocarbon reforming processes the vapour of an alkane is passed over a supported metal catalyst such as platinum on silica or alumina. Dehydrocyclization, isomerization and cracking reactions all take place to... 

Catalytic dehydrocyclization (also known as alkane reforming), discovered in 1936, is now an important industrial process that converts alkanes to aromatics (1, 2). This reaction is often shown for the prototypic conversion of heptane to toluene and four moles of hydrogen, although in model studies octanes have played a greater role since there is a larger product diversity possible. 

This introduction brings us back to the structural analogy shown earlier between the cation 1 dehydrocyclization reaction and the plausible connection between this reaction and that for a 2-octyl cation intermediate 3 (and which could include many other linear alkane carbocation systems with at least six contiguous carbons). Our initial aim therefore was to study computationally the dehydrocyclization of the cyclodecyl cation 1, to see if one could satisfactorily model this known reaction. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

We have not carried out calculations starting with secondary cations derived from the title alkanes because at a computational level, these will have ground-states and transition-states similar to heptane itself (previously discussed). This will be true even though the most stable carbocations in these branched alkanes will be the corresponding tertiary ions, which in themselves will not be directly involved in dehydrocyclization processes. However, one has to keep in mind that the thermodynamic ground-states in these real catalytic reactions will be the alkanes themselves, and in this regard secondary cations formed from n-octane or 2- (or 3-) methylheptane will not differ much in absolute energy. As shown earlier, a 1,6-closure of 2-methylheptane leads eventually to m-xylene, while 3-methylheptane has eventual routes to both o- and p-xylene. 

While a majority of laboratory-scale dehydrocyclization studies involve carefully chosen feedstocks, often a single alkane, commercial operators use a naphtha fraction consisting of a complex mixture of hydrocarbons. At least some of these will be incapable of easily undergoing direct dehydrocyclization and need to be isomerized into reactive structures if aromatics are to be formed. The work of Davis suggests that the acidity of dual function catalysts is an important added factor in these isomerizations, one which likely complements the different set of isomerizations that may be catalyzed by the platinum function. 

In contrast to this mechanism, the one proposed in our work operates direct from the oxidation state of the alkane feedstock. The same alkyl cation intermediate can lead to both alkane isomerization (an alkyl cation is widely accepted as the reactive intermediate in these reactions) and we have shown in this paper that a mechanistically viable dehydrocyclization route is feasible starting with the identical cation. Furthermore, the relative calculated barrier for each of the above processes is in accord with the experimental finding of Davis, i.e. that isomerization of a pure alkane feedstock, n-octane, with a dual function catalyst (carbocation intermediate) leads to an equilibration with isooctanes at a faster rate than the dehydrocyclization reaction of these octane isomers (8). 

Finally, one has to concede that gas-phase calculations are not the ideal way to model a reaction taking place on a catalyst surface. Computational chemistry developments in this area have been continuing but they are a long way from providing completely realistic models. For example, the overall kinetics for dehydrocyclizations are likely to be rate-limited by the binding of the alkane substrate to catalytically active sites. 

Rather large kH/kD isotope ratios (29) have been reported in model studies of dehydrocyclization using deuterated vs. normal alkanes, particularly when one considers the high temperatures being used, but the origin of these effects is difficult to sort out. In contrast to catalytic dehydrocyclization reactions, the dehydrocyclization reactions of the observable p-H-bridged cyclodecyl cations are much more amenable to mechanistic studies, albeit difficult because of the low temperatures involved. Examination of the dehydrocyclization transition... 

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

Stepwise Cg dehydrocyclization (aromatization) involving the gradual loss of hydrogen atoms from an alkane followed by a triene - cyclo-hexadiene ring closure step 20, 21). This can be ... 

The first metallic catalyst used for dehydrocyclization of alkanes (/) was platinum on carbon (10-40 w/w% metal). It is typically used around atmospheric pressure and temperatures not exceeding 300°C. Such catalysts are inadequate for praetical purposes. This is the reason for commercial dual-function catalysts—typically platinum on silica-alumina—having been developed 32). 

A comparison of the cyclization rates of alkanes and alkenes may help to distinguish between associative and dissociative ring closure mechanisms, just as in the case of Cg dehydrocyclization of hexane and hexenes. 

Meriasdeau, P. and Naccache, C. (1997) Dehydrocyclization of alkanes over zeolite-supported metal catalysts monofimctional or bifunctional route. Cat. Rev. Sci Eng., 39, 5-48. 

The first reaction is the isomerization from a zero-octane molecule to an alkane with 100 octane the second is the dehydrocyclization of heptane to toluene with 120 octane, while the third is the rmdesired formation of coke. To reduce the rate of cracking and coke formation, the reactor is run with a high partial pressure of H2 that promotes the reverse reactions, especially the coke removal reaction. Modem catalytic reforming reactors operate at 500 to 550°C in typically a 20 1 mole excess of H2 at pressures of 20-50 atm. These reactions are fairly endothermic, and interstage heating between fixed-bed reactors or periodic withdrawal and heating of feed are used to maintain the desired temperatures as reaction proceeds. These reactors are sketched in Figure 2-16. 

Catalytic reforming92-94 of naphthas occurs by way of carbocationic processes that permit skeletal rearrangement of alkanes and cycloalkanes, a conversion not possible in thermal reforming, which takes place via free radicals. Furthermore, dehydrocyclization of alkanes to aromatic hydrocarbons, the most important transformation in catalytic reforming, also involves carbocations and does not occur thermally. In addition to octane enhancement, catalytic reforming is an important source of aromatics (see BTX processing in Section 2.5.2) and hydrogen. It can also yield isobutane to be used in alkylation. 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

Catalytic reforming has become the most important process for the preparation of aromatics. The two major transformations that lead to aromatics are dehydrogenation of cyclohexanes and dehydrocyclization of alkanes. Additionally, isomerization of other cycloalkanes followed by dehydrogenation (dehydroisomerization) also contributes to aromatic formation. The catalysts that are able to perform these reactions are metal oxides (molybdena, chromia, alumina), noble metals, and zeolites. 

Chromia and molybdena were found to effect dehydrocyclization of alkanes under reaction conditions similar to those of aromatization of cyclohexanes.96 Because of its great practical significance in refining (hydrorefining), chromia- alumina was extensively studied in the dehydrocyclization of alkanes. 

In the dehydrocyclization of alkanes it is clear that ring closure can take place both in a metal-catalyzed reaction and as a carbocationic process. The interpretation of the reforming process proposed by Heinemann and coworkers,123 therefore, is not a complete picture of the chemistry taking place. The scheme they presented (Fig. 2.1) attributes cyclization activity solely to acidic sites. The ample evidence available since requires that metal-catalyzed C5 and C6 ring-closure possibilities be included in a comprehensive interpretation. Additionally, the metal component plays and important role in carbocationic reactions in that it generates carbocations through the formation of alkenes. 

Alkylaromatic compounds possessing a sufficiently long side chain may also undergo dehydrocyclization.216 In fact, the aromatic ring enhances this reaction alkylaromatics undergo dehydrocyclization faster than do alkanes. Depending on the side chain, condensed or isolated ring systems may be formed. 

Dehydrocyclizatlon Pt(0) can be used for dehydrogenation of alkanes to alkenes. Ti(0) is known to absorb H2 to form a dihydride. Paquette et al.1 reasoned that a combination of the two metals could in principal effect dehydrocyclization. Indeed, cyclooctane when heated with Pt(0) and Ti(0) (1 1) adsorbed in A1203 is converted into bicyclo[3.3.0]octane (equation I). 

---