Catalytic dehydrogenation of isopentane with iridium catalysts.

The catalytic activation of alkanes under mild conditions to give the corresponding alkenes and hydrogen has tremendous industrial implications because olefins are the most important starting materials for large-scale industrial processes such as olefin polymerization, dimerization, oligomerization, hydroformylation, and metathesis. In the search for a process by which olefins can be produced from alkanes without a steam cracker, iridium complexes have proved to be the best catalysts so far. An iridium catalyst with a pincer ligand was reported to give the best performance, but it had the big disadvantage that an added external olefin was required to eliminate the formed hydrogen from the equilibrium. The addition of such “sacrificial olefins” is not economic and limits the commercial value of the process. Catalysts that do not need a “sacrificial olefin” are rare, and typically they perform poorly in homogeneous solution. We found that a series of iridium complexes, in combination with phosphorus-containing compounds on silica gel as the support material, are able to activate alkanes in a fixedbed reactor to give the corresponding alkenes and hydrogen with high selectivity and high activity. As a model compound we chose isopentane, a refinery product with a high octane number of 92, which cannot be added to gasoline because of its low boiling point of 28 8C. We tested four different iridium complexes and found a drastic dependence of the activity on the number of phosphine ligands (Figure 1). The activity of H2IrCl6 for isopentane activation could be increased significantly when external PPh3 was added to the reaction mixture. In addition, the activity increased overproportionally at temperatures higher than 350 8C (see Figure 2). This behavior indicates the formation of a new catalytic species at 350 8C that is more active than the original one. In another experiment we used the phosphine-free catalyst bis(1,5-cyclooctadiene)iridium(I) tetrafluoroborate on silica gel. In addition, catalysts with Ir/P ratios of 1:4 and 1:8 ratio were synthesized. Instead of an externally added triphenylphosphine, the phosphine could also be integrated into the catalyst through functionalized silica gel (see Scheme 1). All of these catalysts were tested and compared in C–H activation experiments (Figure 3). The conversion at