Alkane dehydrogenation catalysis

Pincer (mer, tridentate) phosphines have proved resistant to degradation and useful in alkane dehydrogenation catalysis. Recently, Xu et al. [118] found that a dihydrido Ir complex containing a tridentate monoaifionic aryl bis(phosphino) (PCP) pincer, 31 (R = terf-butyl), is highly active catalyst for dehydrogenation... 

A key issue for synthetic chemists is the direct and selective functionalization of alkanes under mild conditions. A major problem in C-H bond activation by molecular catalysis is the lack of a suitable reaction medium, because most organic solvents are not inert under alkane activation conditions and therefore prevent the desired reactions. In this context, dense carbon dioxide seems to be a promising reaction medium as it is miscible with organics, including organometallics, and potentially stable under alkane activation conditions. Indeed, methane carbonylation and alkane dehydrogenation by molecular catalysis have been reported using dense carbon dioxide as the reaction medium (Scheme 67). " ... 

Despite the low cost and availability of magnetite, its use in catalysis has been somewhat limited to alkene isomerisation," water gas shift chem-istry," Fischer-Tropsch synthesis and alkane dehydrogenation." However, several interesting nanoparticulate-magnetite-catalysed synthetic transformations have been reported." " ... 

Findlater M, Choi J, Goldman A, Brookhart M (2012) Alkane dehydrogenation. In Perez PJ (ed) Alkane C-H activation by single-site metal catalysis, vol 38. Springer, Netherlands, pp 113-141... 

Pincer-ligated metal complexes have displayed extraordinarily rich chemistry and have found widespread use in catalysis. Pincer complexes of numerous transition metals have been synthesized, but the most well-studied probably involve Ru, Rh, Ir, and Pd [1-7], Our group has largely focused on pincer-iridium complexes, which have shown a strong tendency toward the activation of C-H bonds. These complexes have been found to effect the oxidative addition of a variety of C-H bonds including those with sp - and sp-hybridized carbon [8-10], Most notable, however, has been the activation of C(sp )-H bonds, leading to alkane dehydrogenation [6, 7],... 

Ca.ta.lysts, A small amount of quinoline promotes the formation of rigid foams (qv) from diols and unsaturated dicarboxyhc acids (100). Acrolein and methacrolein 1,4-addition polymerisation is catalysed by lithium complexes of quinoline (101). Organic bases, including quinoline, promote the dehydrogenation of unbranched alkanes to unbranched alkenes using platinum on sodium mordenite (102). The peracetic acid epoxidation of a wide range of alkenes is catalysed by 8-hydroxyquinoline (103). Hydroformylation catalysts have been improved using 2-quinolone [59-31-4] (104) (see Catalysis). 

To illustrate how a bifunctional catalyst operates, we discuss the kinetic scheme of the isomerization of pentane [R.A. van Santen and J.W. Niemantsverdriet, Chemical Kinetics and Catalysis (1995), Plenum, New York]. The first step is the dehydrogenation of the alkane on the metal ... 

In heterogeneous metal catalysis alkanes, alkenes, and aromatics adsorbed on the metal surface rapidly exchange hydrogen and deuterium. The multiple adsorption of reactants and intermediates lowers the barriers for such exchange processes. Hydrogenation of unsaturated aliphatics and isomerisation can be accomplished under mild conditions. Catalytic dehydrogenation of alkanes to alkenes requires temperatures >200 °C, but this is because of the thermodynamics of this reaction. 

Catalytic membrane reactors are not yet commercial. In fact, this is not surprising. When catalysis is coupled with separation in one vessel, compared to separate pieces of equipment, degrees of freedom are lost. The MECR is in that respect more promising for the short term. Examples are the dehydrogenation of alkanes in order to shift the equilibrium and the methane steam reforming for hydrogen production (29,30). An enzyme-based example is the hydrolysis of fats described in the following. 

Bimetallic Pt-Sn catalysts are useful commercially, e.g., for hydrocarbon conversion reactions. In many catalysts, Pt-Sn alloys are formed and play an important role in the catalysis. This is particularly true in recent reports of highly selective oxidative dehydrogenation of alkanes [37]. In addition, Pt-Sn alloys have been investigated as electrocatalysts for fuel cells and may have applications as gas sensors. Characterization of the composition and geometric structure of single-crystal Pt-Sn alloy surfaces is important for developing improved correlations of structure with activity and/or selectivity of Pt-Sn catalysts and electrocatalysts. 

Finally, we mention supported molten metal catalysis (SMMC), in which molten metal catalysts are dispersed as nanodroplets or as thin film on the surface of porous supports. Supported salt melts provide a well-defined volume, accessible to few reactant components, with a surface that is dynamically restructuring to give access to metal cations. The supported molten salt forms a thin layer on the top of the support that is stable up to high temperatures (600 °C). Usually, the whole surface is covered, but micro- and small meso-pores are preferentially filled. Such catalysts possess very interesting properties for the oxidative dehydrogenation of light alkanes [138]. 

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. 

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