Dehydrogenation of linear alkanes

Among these examples, probably the most notable case is the dehydrogenation of linear alkanes to their corresponding terminal alkenes (a-olefins), this being the kinetically favored process over the production of the internal alkenes. However, the same complex slowly catalyzes an isomerization of the terminal alkene to internal alkenes, as the latter are the thermodynamic products (Scheme 13.15) [33]. 

The same cycle is followed during the reactions of linear alkanes to form linear alk-enes. Although the thermod)mamics for dehydrogenation of cyclooctene are more favorable than those for the dehydrogenation of linear alkanes, primary C-H bonds typically undergo oxidative addition faster than secondary C-H bonds, as discussed in Chapter 6. Thus, linear alkanes react faster than cyclic alkanes. However, the accumulation of a-olefin inhibits the catalytic process. An T) -olefin complex formed from the a-olefin becomes the resting state of the catalytic cycle for reactions catalyzed by the POCOP system, instead of the vinyl hydride complex that is the resting state of the PCP system. The accumulation of the olefin complex that lies off the cycle leads to a lower concentration of the iridium complexes within the cycle and slower reactions as the concentration of a-olefin product increases. 

Not only the linear Cl0-Cl8 a-olefins but also the linear C10-Cl8 olefins with internal double bonds, the so-called -v /-olefins, are of great importance in surfactant chemistry, n-a-Olefins and n-y-olefins have the same suitability for the manufacture of linear alkylbenzenes, the most important synthetic anionic surfactants, by alkylation of benzene. Nowadays medium molecular weight n- /-olefins are industrially produced by two processes the catalytic dehydrogenation of the corresponding n-alkanes [4,28] and the cometathesis of low and high molecular weight n-v /-olefins, obtained by double-bond isomerization of the isomeric n-a-olefins [29]. 

Zeolite based catalysts for linear alkylbenzene production dehydrogenation of long chain alkanes and benzene alkylation. Catal. Today, 38, 243-247. 

Linear alkanes have been successfully dehydrogenated under similar conditions. Initially, selective activation of the terminal position has been observed, leading... 

NPE). Dehydrogenation of -alkanes from petroleum (C9H20) is the source of the linear nonene. They still have 13% of the production for the major household surfactant market. 

A commercial Pt-Sn on y-Al203 catalyst showed 2-3 times higher activity in the catalytic dehydrogenation of a mixture of Ci0—CJ2 alkanes to linear monoalkenes when applied in a supercritical phase.332 The strong shift of the equilibrium under supercritical conditions is believed to be due to the high solubility of the product in supercritical fluids or the rapid desorption of alkenes from the catalyst surface. 

As for the dehydrogenation reaction, IRC calculations [67] indicate that the protolytic cracking of linear and branched alkanes follows different mechanisms. For ethane and propane the products of the reaction are methane and the proper alkoxide. For isobutane, as one follows the reaction path towards the products, the t-butyl cation decomposes into propene and a proton which restores the acid site of the zeolite. 

The dehydrogenation reaction proceeds through the simultaneous elimination of the zeolitic proton and a hydride ion from the alkane molecule, giving rise to a transition state which resembles a carbenium ion plus an almost neutral H2 molecule to be formed. For the linear alkanes, the TS decomposes into an H2 molecule and the carbenium ion correspondent alkoxide. However, for the isobutane molecule the reaction follows a different path, the TS producing isobutene and H2. Most certainly the olefin elimination is flavored to the alkoxide formation due to steric effects as the t-butyl cation approaches the zeolite framework. The same mechanism is expected to be operative for other branched alkanes. 

The protolytic cracking involves the attack of the zeolitic proton to a carbon atom of the alkane molecule and the simultaneous rupture of one its adjacent C-C bond. The carbon atom being attacked and the C-C bond being broken will be preferentially those which produce the most stable carbenium ion. As for the dehydrogenation reaction, the protolytic cracking of linear and branched alkanes also follow different mechanisms, the latter ones producing olefins instead of alkoxides. 

---