Dehydrogenation of alkanes into alkenes

The catalytic dehydrogenation of propane is a potential method for the production of propylene, a key chemical in the polymerization and organic synthesis industries  

Membrane Configuration Catalyst/electrode Operating mode Temperature C C) Main results Ref. 

SCYb—SrCeo.95Ybo.o503-5j BCM—BaCeo.95Mno.o503 BCY=BaCeo.85Yo.i503  

This reaction requires high temperatures (500-700 C) and low pressures (0.3-1 atm). At such high temperatures, however, the side-reaction of C3H8 thermal decomposition takes place  

Similarly, ethane can also be converted into ethylene in PCMR with enhanced yields [37, 38]. Electrical power can be co-generated with ethylene product in the fuel cell operating mode. The performance of the fuel cell MR can be improved by reducing the membrane thickness [40]. The results of propane/ethane dehydrogenation in PCMRs are also summarized in Table 6.3. 

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The ability of intermetallic compounds to absorb hydrogen reversibly can be used as a driving force in the dehydrogenation reaction of alkanes, alkenes and alcohols, which are thermodynamically unfavorable at moderate temperatures (Chetina and Lunin 1994). For example, the dehydrogenation of alkanes into alkenes at 298 K leads to an increase of the standard Gibbs free energy,... 

Dehydrocyclizatlon Pt(0) can be used for dehydrogenation of alkanes to alkenes. Ti(0) is known to absorb H2 to form a dihydride. Paquette et al.1 reasoned that a combination of the two metals could in principal effect dehydrocyclization. Indeed, cyclooctane when heated with Pt(0) and Ti(0) (1 1) adsorbed in A1203 is converted into bicyclo[3.3.0]octane (equation I). 

Finally, whilst rhenium hydride complexes have not been reported to hydrogenate alkenes, there are several reports of the dehydrogenation of alkanes in the presence of tBuCH=CH2 as an hydrogen acceptor (Scheme 6.14) [136-142]. For example, cycloalkanes are transformed catalytically into the corresponding cyclic alkene, which shows that, in principle, a Re-based catalyst could be designed. 

Oxidations by oxygen and catalysts are used for the conversion of alkanes into alcohols, ketones, or acids [54]-, for the epoxidation of alkenes [43, for the formation of alkenyl hydroperoxides [22] for the conversion of terminal alkenes into methyl ketones [60, 65] for the coupling of terminal acetylenes [2, 59, 66] for the oxidation of aromatic compounds to quinones [3] or carboxylic acids [65] for the dehydrogenation of alcohols to aldehydes [4, 55, 56] or ketones [56, 57, 62, 70] for the conversion of alcohols [56, 69], aldehydes [5, 6, 63], and ketones [52, 67] into carboxylic acids and for the oxidation of primary amines to nitriles [64], of thiols to disulfides [9] or sulfonic acids [53], of sulfoxides to sulfones [70], and of alkyl dichloroboranes to alkyl hydroperoxides [57]. 

Several examples of intermolecular C-H bond functionalization have appeared during the past decade. In addition to the oxidations reported above in Shilov-type systems, and the dehydrogenation of alkanes to make alkenes, catalytic systems have been developed to introduce functional groups into hydrocarbons. 

This class of ammoxidation reactions can be fiirther divided into three subclasses wherein R is an alkene, an aromatic, or hydrogen (H). The latter is the ammoxidation of methane to hydrogen cyanide. Alkanes are also ammoxidized to unsaturated nitriles, but the saturated alkane molecule must first undergo dehydrogenation to the corresponding alkene before the ammoxidative... 

As in hydroformylation, both linear and branched products can be obtained from RCH=CH2.The dehydrogenative silylation product, RCH=CHSiR3, is often present and can even predominate under some conditions (Eq. 9.21). The dehydrogenative path can only be explained on the modified Chalk-Harrod mechanism of Eq. 9.23, in which the alkene first inserts into the M-Si bond. 3 elimination of the intermediate alkyl now leads directly to the vinylsilane, the two H atoms thus released go on to hydrogenate the substrate leading to coproduction of alkane. As in hydrogenation, syn addition is generally observed. Apparent anti addition is due to isomerization of the intermediate metal vinyl, as we saw in Eq. 7.21, also a reaction in which initial insertion of alkyne into the M-Si bond must... 

The hydrogenation of alkenes (Section 7-7) is exothermic, with values of AH° around -80 to -120 kJ/mol (-20 to -30 kcal/mol). Therefore, dehydrogenation is endothermic and has an unfavorable (positive) value of A 77°. The entropy change for dehydrogenation is strongly favorable (AS0 = +120 J/kelvin-mol), however, because one alkane molecule is converted into two molecules (the alkene and hydrogen), and two molecules are more disordered than one. 

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