Dehydrocyclization catalysts

Dehydrocyclization refers to the conversion of feed paraffins into alkylcyclohexane and alkylcyclopentane naphthenes. These, in turn, are subsequently converted by isomerization and dehydrogenation into aromatics. Dehydrocyclization is controlled by both acid and platinum functions and is the most sensitive indicator of catalyst selectivity. 

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. 

Catalytic reformers are normally designed to have a series of catalyst beds (typically three beds). The first bed usually contains less catalyst than the other beds. This arrangement is important because the dehydrogenation of naphthenes to aromatics can reach equilibrium faster than the other reforming reactions. Dehydrocyclization is a slower reaction and may only reach equilibrium at the exit of the third reactor. Isomerization and hydrocracking reactions are slow. They have low equilibrium constants and may not reach equilibrium before exiting the reactor. 

The second and third reactors contain more catalyst than the first one to enhance the slow reactions and allow more time in favor of a higher yield of aromatics and branched paraffins. Because the dehydrogenation of naphthenes and the dehydrocyclization of paraffins are highly endothermic, the reactor outlet temperature is lower than the inlet temperature. The effluent from the first and second reactors are reheated to compensate for the heat loss. 

Supported bimetallic Re—Pt catalysts are important in selective reforming of petroleum. It is believed that sulhding the catalyst before use gives ReS units which act as inert diluents to reduce the size of a local ensemble of platinum atoms. Selectivity for desirable dehydrocyclization and isomerization reactions... 

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. 

According to this scheme, the catalyst serves primarily to promote dehydrogenation. Cyclization of the hexatriene was shown years ago (JJ.) to occur thermally in the gas phase at temperatures well below these dehydrocyclization conditions. Thus, the overall reaction is projected to be the combination of several catalytic dehydrogenation steps and a non-catalytic cyclization step. This projection implies that the design of the catalytic reactor may be important in order to optimize the ratio of void space for cyclization and catalyst space for dehydrogenation. 

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. 

Fig. 12. Variation with average platinum particle diameter of the initial rate of reaction (isomerization plus dehydrocyclization) of n-hexane (- -) and 2-methylpentane (-O-) over ultrathin film catalysts at 275°C. Hydrogen/reactant hydrocarbon, 10/1 total reactant pressure 100 Torr.

Muller (ISO) has recently shown that the dehydrocyclization of 2,2,4,4-tetramethylpentane to 1,1,3,3-tetramethylcyclopentane occurs on thick polycrystalline platinum film catalysts with a rate that is comparable to the formation of 1,1,3-trimethylcyclopentane from 1,2,2-trimethyl-pentane. As Muller points out, reactions (10)—(12) cannot occur from 2,2,4,4-tetramethypentane, and it is clear that either these mechanisms are inadequate, or at least there must be an alternative mechanism available. Muller suggests mechanism (14) which requires two adjacent platinum sites. 

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... 

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... 

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. 

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). 

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