Upgrading Light Hydrocarbons: A Tandem Catalytic System for Alkane/Alkene Coupling
Light hydrocarbons, with relatively low fuel value, are abundant from several sources, including mixed alkane/alkene refinery byproduct streams. A tandem system consisting of a compatible combination of a homogeneous alkane dehydrogenation catalyst (known to be kinetically efficient but thermodynamically disfavored at low temperatures) with an olefin dimerization catalyst could effect the coupling of an alkane and alkene to produce a heavier, more valuable fuel molecule (C_nH_(2n+2) + CnH_(2n) = C_(2n)H_(4n+2)), a reaction that is thermodynamically favorable below 250 °C. We have demonstrated that coupling with a tandem homogeneous catalyst, consisting of a pincer-ligated iridium alkane dehydrogenation catalyst and an organometallic tantalum alkene dimerization catalyst; the combination couples 1-hexene/n-heptane to C_(13)/C_(14) products at temperatures ranging from 100 to 150 °C, operating with up to 90 % cooperativity. This particular combination generates alkene products rather than the desired alkanes, however, because the regioselectivity of the dimerization catalyst preferentially yields highly substituted alkenes that are not reactive towards hydrogen transfer. A complete cycle should be attainable by combining the dehydrogenation catalyst with an alternate dimerization catalyst that gives mostly linear and monosubstituted alkenes; we have synthesized a novel class of nickel-exchanged zincosilicates that exhibit the desired dimerization catalytic behavior.