Dehydrodimerization Dehydrogenation

In this section, not only alkane isomerization and dehydrodimerization (equatirm 13) ate consictered, but also the dehydrogenation of alkanes to alkenes, as in diis case, two adjacent C—bonds ate replaced by a ir-type C— bond. Other C—C bond-forming reactions are also mentioned. 

Alkanes are relatively stable species thermodynamically and so many reactions of alkanes (dehydrogenation, dehydrodimerization, carbonylation) are unfavorable under ambient conditions. This means we often need to couple some favorable process with the unfavorable alkane conversion in order to drive it. We look at the details in Section III.B, but only note here that the common appearance of photochemistry in alkane chemistry can be seen as a way to drive reactions thermodynamically and to access highly reactive transition metal fragments that are kinetically competent to react with alkanes. 

Methanol dehydrogenates to methyl formate over fresh WC and P-W2C powders with selectivities higher than 90% (109,110). The dominant side reaction is the decomposition to synthesis gas. Over WC and P-W2C modified with oxygen, methanol selectively dehydrates to dimethylether at 473 K and at higher reaction temperatures, C2-C4 olefins are produced (47). Thus, the dehydrodimerization of methanol apparently requires WC sites. These sites are titrated by chemisorbed oxygen. Thus, oxygen on the surface inhibits the formation of methyl formate and introduces a surface acid function WO that catalyzes dehydration by carbenium-ion type catalysis. 

Photocatalyzed reactions have been very important in alkane activation chemistry[l-3] because, other than oxidation, most reactions of alkanes are thermodynamically unfavorable. Such is the case for alkane dehydrogenation (eq. 1), carbonylation (eq. 2) and dehydrodimerization (eq. 3), and these are three reactions that have been the best studied to date and are covered in this chapter. 

---