Senior Investigators: Prof. Alan Goldman (Rutgers), Prof. Maurice Brookhart (UNC-CH)

The importance of aromatic molecules cannot be overstated. Three of the “seven basic building blocks of the chemical industry” (benzene, toluene, and xylene; BTX) are aromatic.¹ These are currently obtained as by-products from reforming; as the world’s fuel supply shifts from gasoline to diesel they will be in increasingly short supply. Perhaps even more important than issues of supply are issues of selectivity. The current lack of catalyst selectivity is reflected in a mismatch between the reformate production ratio and demand (reforming gives toluene as 55% of the BTX mix, but toluene represents only 23% of BTX demand). The aromatic molecules ethylbenzene and styrene also comprise two of the top nine industrial organic chemicals (a list that also includes their precursors benzene and ethylene). As these are not major products of heterogeneously-catalyzed aromatization, styrene must be produced via two additional steps, alkylation of benzene and dehydrogenation.

Thus the potential value of selective aromatization seems great. As the world (and particularly the developed world) runs out of petroleum, and as transportation fleets convert from gasoline to diesel, it seems inevitable that Fischer-Tropsch chemistry will become more widespread (whether it is based on syngas from coal, methane, biomass or other). On one hand, this results in decreased direct production of aromatics; but conversely, F-T production offers a new feedstock with great potential, namely n-alkanes. Thus the ability to selectively convert, for example, n-octane to xylenes and/or ethylbenzene seems extremely attractive.

The potential for organometallic catalysts to effect this transformation is a possibility due to CENTC research in the Brookhart and Goldman labs with pincer-iridium dehydrogenation catalysts, where such transformations have been observed with high catalytic turn-over numbers. The mechanism for these transformations likely proceeds via dehydrogenation to yield conjugated trienes, which undergo electrocyclization to give cyclohexadienes. These are then further dehydrogenated to give the corresponding aromatics. This suggests an obvious approach toward improving the reaction efficiency: the addition of an independent or tandem catalyst (homogeneous or heterogeneous) that enhances the rate of the electrocyclization reaction.

Mechanistic studies are currently being conducted in which trienes are independently generated and then cyclized. Stereochemistry and kinetics are being studied to determine if the intermediacy of these trienes in the dehydroaromatization is consistent with their behavior as isolated species.

1. Wittcoff, H. A.; Reuben, B. G.; Plotkin, J. S., Industrial Organic Chemicals. 2nd ed.; Wiley-IEEE: 2005.