Alkanes, dehydrogenation

As with the previously discussed methane coupling chemistry, the promise of OTMs for promoting alkane oxidative dehydrogenation reactions such as  

Somewhat similar results have been found [119] for mixed conductors based upon the fluorite structured membrane Bii jYo 381110203 vvhere per pass ethylene yields and selectivities from ethane of 56% and 80% respectively were obtained at 875 °C. Using the same materials in a fixed bed reactor gave ethylene yields and 

All of this preliminary reported work on OTMs for promoting ethane dehydrogenation to ethylene supports the merits of this strategy compared to conventional dehydrogenation and oxidative dehydrogenation technologies. 

The alternate thermal catalytic systems employ a hydrogen-acceptor like t-BuCH=CH2 to do this so that the overall reaction is given by eq. 9. 

Addition of t-BuCH2=CH2 to the reaction mixture in the photochemical system leads to an increase in the number of catalytic turnovers from 8 to 12 after 7d. and both eq. (1) and eq. (9) now take place. Preferential attack at unhindered 1° CH bonds is observed (eq. 10), and the initial kinetic products are better preserved from isomerization to the stable 1-methylcyclohexene in the presence of t-BuCH2=CH2. The figures in eq. 10 show the number of catalytic turnovers after 7d. The kinetic isotope effect for C6H12/C6D12 is 5.1 in the presence of t-BuCH2=CH2 and 7.7 in its absence, consistent with the mechanism proposed. 

Even such an apparently small change as moving to an aromatic phosphine as in 4b, L = PPh3. leads to an inactive system photochemically even though 4a and 4b are equally active thermally and both absorb at 254 nm. 

Tanaka [11], Saito [12] and Goldman [3, 13a] have reported that RhCl(CO)(PMe3)2 (5) is also an alkane dehydrogenation catalyst under photochemical conditions. Here the reactive species is probably the RhClL2 fragment formed by loss of CO (eq. 1 l)and studied in detail by Ford [14a]  

For 5 itself, reaction of the RhClL2 intermediate with alkane solvent (eq. 12) is faster than back reaction with the CO liberated in the photolysis, but for the PPh3 analogue, the CO back reaction is fast whether alkane or benzene is solvent and substrate  

---

E. Chang, Alkane Dehydrogenation andAromaticyation, Process Economics Program, Report No. 203, SRI International, Menlo Park, Calif., 1992. 

Theoretical studies of catalytic alkane-dehydrogenation reactions by [(PCP )IrH2], PCP rf-C6H3(CH2P112)2-l, 3 and [cpIr(PH3)(H)]+, suggest that they proceed through similar steps in both cases namely (i) alkane oxidation, (ii) dihydride reductive elimination, (iii) /3-II transfer from alkyl ligand to metal, (iv) elimination of olefin.402 The calculated barriers to steps (i), (ii), and (iv) are more balanced for the PCP system than for cp(PH3). 

Density Functional Studies of Iridium Catalyzed Alkane Dehydrogenation Michael B. Hall and Hua-Jun Fan... 

DENSITY FUNCTIONAL STUDIES OF IRIDIUM CATALYZED ALKANE DEHYDROGENATION... 

The thermodynamic parameters for the alkane dehydrogenation reaction are calculated for both the pincer and anthraphos iridium(III) complexes. The mechanism of the transfer reaction, and the associative, dissociative and interchange mechanisms for the acceptorless reactions are discussed and compared. As these reactions typically occur at conditions very different from STP, important corrections for high temperature, high reactant (alkane) concentration and low product (H2, olefin) concentration are important. 

The reversal of hydrogenation is also possible, as evidenced by the many iridium catalysts for alkane dehydrogenation to alkenes or arenes, though to date this area is of mainly academic interest rather than practical importance [19]. 

B. D. Chandler, L. 1. Rubinstein, andL. H. Pignolet, Alkane dehydrogenation with silica supported Pt and Pt-Au catalysts derived from phosphine ligated precursors, J. Mol. Catal. A Chem. 133,... 

Crabtree RH, Mihelcic JM, Quirk JM (1979) Iridium complexes in alkane dehydrogenation. J Am Chem Soc 101 7738... 

The success of derivatives of 1 and 2 as dehydrogenation catalysts has led to the investigation of numerous different pincer ligands for iridium-catalyzed alkane dehydrogenation. The Anthraphos pincer iridium complex (3-H2) was expected to afford even greater thermal stability (Eig. 1), and indeed, the catalyst can tolerate reaction temperatures up to 250°C [42]. The catalytic activity of 3-H2, however, is much less than that of I-H2 under comparable conditions. 

Scheme 1 Proposed mechanism of alkane dehydrogenation with acceptors...

Catalytic metathesis has been accomplished via a tandem combination of catalytic alkane dehydrogenation, olefin metathesis and subsequent olefin hydrogenation (Scheme 12.6). Two factors are crucial for this transformation ... 

The transfer (de)hydrogenation is fully reversible-that is, the iridium pincer complexes catalyze the alkane dehydrogenation as well as the alkene hydrogenation. 

Alkane dehydrogenation has been demonstrated as a suitable method for the functionalization of polyolefins such as atactic poly(l-hexene) under homogeneous conditions (Equation 12.6) [23]. 

From the outset, iridium compounds have played an important role in the better understanding of the C—H activation process, and consequently in the development of efficient alkane dehydrogenation reactions [8]. Hence, in this chapter we will review the participation of iridium complexes in the optimization of chemical processes for C—H activation which, today, have led to some highly promising... 

---