Alkanes, dehydrogenation alumina-supported

Nafta reforming and alkane dehydrogenation processes are directly connected on platinum alumina based catalysts. The stability and selectivity requirements for industrial purposes induced the addition of a second metal like Sn, Ge or Re. In fact, these promoters coupled to other acidity controllers added on the support enhanced the catalytic efficiency [1-3]... 

Supported vanadium oxides have been proposed as selective catalysts in partial oxidation reactions [1] and more specifically in the oxidative dehydrogenation (ODH) of short chain alkanes [2, 3]. However, it has been observed that the catalytic behavior of these catalysts during the oxidation of alkanes depends on the vanadium loading and the acid-base character of metal oxide support. In this way, alumina-supported vanadia catalysts with low V-loading are highly active and selective during the ODH of ethane [4-7] and propane [8] but they show a low selectivity in the ODH of n-butane [4, 5, 9, 10]. 

Scheme 6.26 Proposed alkane metathesis pathway for alumina-supported alkylidyne-bisalkyl W(VI) complexes with the dual role of alumina anchoring W and catalyzing the dehydrogenation/hydrogenation steps.

Bimetallic catalysts based on platinum and tin, supported on y-alumina have become very important commercially. Platinum-tin catalysts are widely used in the dehydrogenation of alkanes. The structure of the catalyst and the role of tin have received a lot of attention. Recently Davis [1] reviewed the often contradicting literature about characterization of the bimetallic system. For the dehydrogenation reactions the main purposes with adding tin to a platinum catalyst are to increase the selectivity and stability towards coke formation. 

Supported vanadium catalysts, whereby vanadium oxide is dispersed on a support such as alumina or titania are of particular importance in, for instance, the oxidative dehydrogenation of alkanes [58-64]. Such materials have attracted considerable interest in the direct dehydrogenation of butane, where a key driver is to identify the relationship between catalytic activity and structural properties [5, 6, 65-68]. In the pure (solid) metal oxides the coordination of vanadium is well defined. However, this is not necessarily true in the case of supported catalysts. Vanadium may be present on the support surface as isolated vanadium ions dimeric or polymeric species one- and two-dimensional chains of vanadium ions ... 

As shown above, oxidized diamond exhibited considerable activity in the oxidative dehydrogenation of alkanes, hence further studies on the oxidized diamond supported catalysts were exploited. Nickel-loaded alumina is generally used for the partial oxidation of methane (reaction 5). However, carbon deposition onto the nickel is the major problem in the commercialization of this process. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

The catalytic dehydrogenation of lower alkanes was first developed more than fifty years ago using chromia/alumina systems [1]. Although there has been development of new processes [2 - 6], the catalyst technology has tended to remain with either modified chromia/alumina or modified platinum/alumina catalysts. Therefore it seemed appropriate to re-examine the possibility of using oxide systems other than chromia to effect the alkane to alkene transition. Supported vanadium pentoxide has been extensively studied for the oxidative dehydrogenation of propane to propene [7-10] but rarely for the direct dehydrogenation reaction [6]. 

The synthesis and use of immobilized iridium pincer complexes on solid supports for the transfer dehydrogenation of alkanes have also been examined [131]. Three approaches are reported (i) the post-functionalization of a Mer-rifield resin by incorporating the pincer complex (ii) the covalent bonding of the catalyst to silica via a pendant alkoxysilane linker and (iii) the adsorption of the catalyst onto y-alumina via the interaction of the phenolate group on the para-position of the pincer ligand with the Lewis-acidic sites on the alumina. The last approach showed the best activity, affording thermally robust, recyclable, and active supported ( PCP)Ir (Ir-2) and ( POCOP)Ir (Ir-13) catalysts. 

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