Dehydrogenative functionalization

Olefins are formed by dehydrogenation of the n-paraffin feed over the metallic hydrogenation-dehydrogenation function and are adsorbed on the acidic surface of the catalyst as carbonium ions by proton addition. After skeletal isomerization they are desorbed as isoolefins and subsequently hydrogenated to the corresponding isoparaffins. The net result (i.e., the formation of carbonium ions) of the action of metal and acid in dual function catalysis is, on pure Friedel-Crafts type catalysts, described by the scheme ... 

Besides Ga, other metals such as Zn (11, 12) and Pt (13) have also been used in combination with ZSM-5 zeolite for C2-C4 aromatization. However, besides aromatization, Pt also catalyzes other undesired reactions, such as hydrogenolysis, hydrogenation and dealkylation that leads to excessive formation of methane and ethane, and limits the selectivity to aromatics. Therefore, Ga- and Zn-ZSM-5 catalysts are preferred over Pt-ZSM-5 except, perhaps, in the case of the more refractory ethane, in where a higher dehydrogenating function is needed to activate the reactant. The catalytic performance of Ga and Zn/ZSM-5 for propane aromatization is compared in Table 2.2. The results obtained on the purely acidic H-ZSM-5 are also included in the table. As observed, a higher conversion and yield of aromatics is obtained for the Ga/ZSM-5 catalyst. 

As stated above, the aromatization of short alkanes is carried out in presence of bifunctional catalysts, in where the dehydrogenating function is given by the metal component (Ga, Zn, Pt) and the H-ZSM-5 zeolite carries the acid sites. Although there is still some uncertainty concerning the initial activation of the alkane, probably both the metal and the zeolite acid sites are involved in this step. Metal sites can dehydrogenate the alkane to give the corresponding alkene, which can then be protonated on the Bronsted acid sites of the H-ZSM-5 zeolite to produce the carbocation. 

This suggests, for these two samples that the balance between the dehydrogenating function (GaxOy) and the acid function (H ) was almost unmodified. For sample 5%Ga A1, for which deactivation is faster, the selectivity for aromatics decreases with time on stream. This change appears to be mainly due to the decrease of the conversion rather than to a change in the balance between dehydrogenating and acid functions. It is Known that during propane aromatization CH4 is formed and that the main source of C-f comes from the first step of the reaction on the acid sites (2) (5). 

From these reaction pathways it is clear that the selectivity towards C- will decrease when the acidity decrease relatively to the dehydrogenating function. Thus a change in Ci selectivity with time on stream would reflect the change in the balance between the dehydrogenating and acid functions. In Table II we report Ci selectK/ities for the different solids as a function of time on stream. 

The heterogeneous catalysis process requires the formulation of a multifunctional catalyst which at a first approximation presents (i) acidic properties (amine adsorption, dehydration,...) and (ii) a hydro-dehydrogenating function (methanol dehydrogenation, hydrogenation of imine and enamine intermediates). 

Table II indicates that Ga A1 (exhibiting the highest acidity) experienced a significant decrease of the C- selectivity these results suggest that during the reaction of propane the deactivation of the acid function is more pronounced than that of the dehydrogenating function.

Table 6 Effect of Dehydrogenation Function of Zinc on Product Yields...

Table 7 Effect of Dehydrogenation Function on the Product Yield...

Purely basic oxide catalysts have, to some extent, a dehydrogenation function. An example is CaO in the dehydrocyclodimerization of conjugated dienes [15]. Hattori has published a review on the multi-functionality of basic catalysts in fine chemistry [16]. 

Table VI shows results obtained with a mixture of dimethyl- and ethylcyclohexanes reformed with catalysts of different acidity. The two functions of a reforming catalyst—the acidity and the hydrogenation-dehydrogenation function of the platinum—are balanced carefully for...

As explained earlier, chromia-alumina can be considered as a dual functional system, having both an acidic function and a hydrogenation-dehydrogenation function. While the present discussion is primarily concerned with the molecular configuration of the chromia dehydrogenation component, neither function can be considered entirely out of context. At the present time, however, the nature of catalyst acidity in general remains somewhat obscure, despite a considerable amount of research, and only a brief review of the acid character of chromia-alumina catalysts will be given here. 

In several instances the heteroatom is readily dislodged from the framework resulting in very active dehydrogenation centres. This is particularly so for 7n and Ga both of which can result in catalysts where the activity in hydrocarbon transformations is strongly influenced by the dehydrogenation function associated with dislodged heteroatom species. 

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