Dehydrocyclization reactions

Powerforming is one tecnique used for aromatics chemical production. Powerforming uses a platinum catalyst to reform virgin naphthas. The principal reaction is the conversion of naphthenes in virgin naphthas to aromatics e.g., isomerization and dehydrocyclization reactions also occur in catalytic reforming. 

Aromatization of paraffins can occur through a dehydrocyclization reaction. Olefinic compounds formed by the beta scission can form a carbocation intermediate with the configuration conducive to cyclization. For example, if a carbocation such as that shown below is formed (by any of the methods mentioned earlier), cyclization is likely to occur. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

The mechanism of the dehydrocyclization reaction is not completely understood. Two alternatives were proposed (11), one which proceeds via a silaimine mechanism, exemplified by reaction (25), and one wherein ring closure occurs by nucleophilic displacement of hydrogen, reaction (26). 

MO Calculations Involving p-Hydrido Cation Intermediates Relevant to the Heptane to Toluene Dehydrocyclization Reaction... 

The cyclodecyl system is unique however in undergoing a dehydrocyclization reaction to give the 9-decalyl cation, as was shown in Scheme 1. 

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. 

So far, we have not discussed the overall reactant-product thermodynamics for these dehydrocyclization reactions (based on carbocation intermediates), and these are shown in Table III and Figure 9. 

In a laboratory scale dehydrocyclization reaction using a dual-function catalyst, Davis (8) reported that the aromatic products were o-xylene, /w-xylene and ethylbenzene in approximately equal amounts (ca. 20-30% each) and p-xylene (ca, 15%). The formation of these was assumed to be from a direct 1,6-ring closure, as sketched in the following two diagrams ... 

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... 

Although there is clear experimental evidence (8) in model dehydrocyclization reactions, using a dual function catalyst, that carbocations are... 

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). 

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... 

Hydroprocessing and special absorption techniques are utilized to remove sulfur and nitrogen from the reformer. If not removed through hydroprocessing, feedstock sulfur will be converted to H2S in the reformer. The H2S will then serve as a poison to the platinum reformer catalyst and diminish the dehydrogenation and dehydrocyclization reactions. When present, H2S can neutralize the acid sites on the catalyst diminishing the ability of the catalyst to promote isomerization, dehydrocyclization, and hydrocracking reactions. 

The hydrocarbon-type analysis of the Platformate discussed above was based on the product having an octane number of 92.9 (F-l plus 3 ml. of tetraethyllead per gallon). The aromatic content (based on charge) increases continually with increased severity. At the two highest severities, the aromatic yield (based on the charge) is in excess of the total naphthenes and aromatics present in the charge. This indicates the participation of the dehydrocyclization reaction of paraffins to form aromatics. 

As a result of the studies discussed above, a reasonably consistent picture of the kinetics and mechanism of the dehydrocyclization reaction over oxide catalysts has evolved. However, as pointed out earlier in this section, relatively few kinetic data have been reported for dehydrocyclization over platinum-alumina reforming-type catalysts. The data which have been reported include those of Hettinger and co-workers (H7), who studied the dehydrocyclization of re-heptane over platinum catalysts. These investigators found that the rate of dehydrocyclization decreased with increasing total pressure at a constant hydrogen-to-hydrocarbon ratio (Fig. 9). They also reported that the extent of dehydrocyclization was substantially greater for re-nonane than for re-heptane, which is consistent with the results obtained on oxide catalysts. In a later study of the kinetics... 

The situation is much different when an acidic support is used. First, the C8 aromatic products have a distribution that approaches an equilibrium composition. The Pt catalyst on an acidic support is both more active and produces aromatics more selectively than Pt on a nonacidic support (84). It is concluded that the bifunctional mechanism involving cyclization by the acid site followed by a bifunctional ring expansion/dehydrogenation reactions is much more rapid than the monofunctional metal catalyzed dehydrocyclization reaction. For the catalyst based on an acidic support, the tin added initially acts as a catalyst poison (Figure 5), at least during the initial 1-2 weeks of usage. 

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